Scientific Output
Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.
Below, you can either scroll through the complete list of our annually published material, or search for a specific author or term via the free text search to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.
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2023 • 175 Atomic cluster expansion for Pt–Rh catalysts: From ab initio to the simulation of nanoclusters in few steps
Liang, Yanyan and Mrovec, Matous and Lysogorskiy, Yury and Vega-Paredes, Miquel and Scheu, Christina and Drautz, Ralf
Journal of Materials Research 38 5125 – 5135 (2023)Abstract: Insight into structural and thermodynamic properties of nanoparticles is crucial for designing optimal catalysts with enhanced activity and stability. In this work, we present a semi-automated workflow for parameterizing the atomic cluster expansion (ACE) from ab initio data. The main steps of the workflow are the generation of training data from accurate electronic structure calculations, an efficient fitting procedure supported by active learning and uncertainty indication, and a thorough validation. We apply the workflow to the simulation of binary Pt–Rh nanoparticles that are important for catalytic applications. We demonstrate that the Pt–Rh ACE is able to reproduce accurately a broad range of fundamental properties of the elemental metals as well as their compounds while retaining an outstanding computational efficiency. This enables a direct comparison of atomistic simulations to high-resolution experiments. Graphical abstract: [Figure not available: see fulltext.]. © 2023, The Author(s).view abstract doi: 10.1557/s43578-023-01123-5 2023 • 174 Catalytic effects of molybdate and chromate–molybdate films deposited on platinum for efficient hydrogen evolution
Diaz-Morales, O. and Lindberg, A. and Smulders, V. and Anil, A. and Simic, N. and Wildlock, M. and Alvarez, G.S. and Mul, G. and Mei, B. and Cornell, A.
Journal of Chemical Technology and Biotechnology 98 1269-1278 (2023)BACKGROUND: Sodium chlorate (NaClO3) is extensively used in the paper industry, but its production uses strictly regulated highly toxic Na2Cr2O7 to reach high hydrogen evolution reaction (HER) Faradaic efficiencies. It is therefore important to find alternatives either to replace Na2Cr2O7 or reduce its concentration. RESULTS: The Na2Cr2O7 concentration can be significantly reduced by using Na2MoO4 as an electrolyte co-additive. Na2MoO4 in the millimolar range shifts the platinum cathode potential to less negative values due to an activating effect of cathodically deposited Mo species. It also acts as a stabilizer of the electrodeposited chromium hydroxide but has a minor effect on the HER Faradaic efficiency. X-ray photoelectron spectroscopy (XPS) results show cathodic deposition of molybdenum of different oxidation states, depending on deposition conditions. Once Na2Cr2O7 was present, molybdenum was not detected by XPS, as it is likely that only trace levels were deposited. Using electrochemical measurements and mass spectrometry we quantitatively monitored H2 and O2 production rates. The results indicate that 3 μmol L−1 Na2Cr2O7 (contrary to current industrial 10–30 mmol L−1) is sufficient to enhance the HER Faradaic efficiency on platinum by 15%, and by co-adding 10 mmol L−1 Na2MoO4 the cathode is activated while avoiding detrimental O2 generation from chemical and electrochemical reactions. Higher concentrations of Na2MoO4 led to increased oxygen production. CONCLUSION: Careful tuning of the molybdate concentration can enhance performance of the chlorate process using chromate in the micromolar range. These insights could be also exploited in the efficient hydrogen generation by photocatalytic water splitting and in the remediation of industrial wastewater. © 2023 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI). © 2023 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).view abstract doi: 10.1002/jctb.7345 2023 • 173 Electrochemical decarboxylation of acetic acid on boron-doped diamond and platinum-functionalised electrodes for pyrolysis-oil treatment
Ashraf, Talal and Rodriguez, Ainoa Paradelo and Mei, Bastian Timo and Mul, Guido
Faraday Discussions 247 254 – 269 (2023)Electrochemical decarboxylation of acetic acid on boron-doped-diamond (BDD) electrodes was studied as a possible means to decrease the acidity of pyrolysis oil. It is shown that decarboxylation occurs without the competitive oxygen evolution reaction (OER) on BDD electrodes to form methanol and methyl acetate by consecutive reaction of hydroxyl radicals with acetic acid. The performance is little affected by the applied current density (and associated potential), concentration, and the pH of the solution. At current densities above 50 mA cm−2, faradaic efficiencies (FEs) of 90% towards the decarboxylation products are obtained, confirmed by in situ electrochemical mass spectrometry (ECMS) investigation showing only small amounts of oxygen formed by water oxidation. Using platinum-modified BDD electrodes, it is shown that selectivity to ethane, the Kolbe product, strongly depends on the shape and geometry of the platinum particles. Using nano-thorn-like Pt particles, a faradaic efficiency of approx. 40% towards ethane can be obtained, whereas 3D porous platinum nanoparticles showed high selectivity towards the OER. Using thin platinum layers, a high FE of >70% towards ethane was obtained, which is thickness-independent at layer thicknesses above 20 nm. Comparison with other substrates revealed that BDD is an ideal support for Pt functionalisation, giving advantages of stability and high-value-product formation (ethane and methanol). In short, this work provides guidelines for electrode fabrication in the context of the electrochemical upgrading of biomass feedstocks by acid decarboxylation. © 2023 The Royal Society of Chemistry.view abstract doi: 10.1039/d3fd00066d 2023 • 172 Standardizing OER Electrocatalyst Benchmarking in Aqueous Electrolytes: Comprehensive Guidelines for Accelerated Stress Tests and Backing Electrodes
Zlatar, Matej and Escalera-López, Daniel and Rodríguez, Miquel Gamón and Hrbek, Tomáš and Götz, Carina and Mary Joy, Rani and Savan, Alan and Tran, Hoang Phi and Nong, Hong Nhan and Pobedinskas, Paulius and Briega-Martos, Va...
ACS Catalysis 13 15375 – 15392 (2023)The scarcity of iridium, needed to catalyze the sluggish oxygen evolution reaction (OER), hinders large-scale hydrogen production with proton exchange membrane water electrolyzers (PEMWEs). Crucial steps require reducing its loading while improving its overall activity and stability. Despite knowledge transfer challenges, cost and time constraints still favor aqueous model systems (AMSs) over real devices for the OER electrocatalyst testing. During AMS testing, benchmarking strategies such as accelerated stress tests (ASTs) aim at improving catalyst lifetime estimation compared to constant current loads. This study systematically evaluates a commercial Ir catalyst by modifying both AST parameters and the employed backing electrodes to examine their impact on activity-stability relationships. A comprehensive set of spectroscopy and microscopy techniques, including in situ inductively coupled plasma mass spectrometry, is employed to monitor Ir and backing electrode modifications. Our findings demonstrate that the choice of both lower potential limit (LPL) in ASTs and backing electrode significantly influences the estimation of Ir-based electrocatalysts’ activity and stability. Unique degradation mechanisms, such as passivation, redeposition on active sites, and contribution to the OER, were observed for different backing electrodes at varying LPLs. These results emphasize the importance of optimizing parameters and electrode selection in ASTs to accurately assess the electrocatalyst performance. Furthermore, they establish the foundation for developing relevant standardized test protocols, enabling the cost-effective development of high-performance catalysts for PEMWE applications. © 2023 The Authors. Published by American Chemical Society.view abstract doi: 10.1021/acscatal.3c03880 2022 • 171 A single-Pt-atom-on-Ru-nanoparticle electrocatalyst for CO-resilient methanol oxidation
Poerwoprajitno, A.R. and Gloag, L. and Watt, J. and Cheong, S. and Tan, X. and Lei, H. and Tahini, H.A. and Henson, A. and Subhash, B. and Bedford, N.M. and Miller, B.K. and O’Mara, P.B. and Benedetti, T.M. and Huber, D.L. and Z...
Nature Catalysis 5 231-237 (2022)Single Pt atom catalysts are key targets because a high exposure of Pt substantially enhances electrocatalytic activity. In addition, PtRu alloy nanoparticles are the most active catalysts for the methanol oxidation reaction. To combine the exceptional activity of single Pt atom catalysts with an active Ru support we must overcome the synthetic challenge of forming single Pt atoms on noble metal nanoparticles. Here we demonstrate a process that grows and spreads Pt islands on Ru branched nanoparticles to create single-Pt-atom-on-Ru catalysts. By following the spreading process by in situ TEM, we found that the formation of a stable single atom structure is thermodynamically driven by the formation of strong Pt–Ru bonds and the lowering of the surface energy of the Pt islands. The stability of the single-Pt-atom-on-Ru structure and its resilience to CO poisoning result in a high current density and mass activity for the methanol oxidation reaction over time. [Figure not available: see fulltext.] © 2022, The Author(s), under exclusive licence to Springer Nature Limited.view abstract doi: 10.1038/s41929-022-00756-9 2022 • 170 Electrophoretic Deposition of Platinum Nanoparticles using Ethanol-Water Mixtures Significantly Reduces Neural Electrode Impedance
Ramesh, V. and Giera, B. and Karnes, J.J. and Stratmann, N. and Schaufler, V. and Li, Y. and Rehbock, C. and Barcikowski, S.
Journal of the Electrochemical Society 169 (2022)Platinum electrodes are critical components in many biomedical devices, an important example being implantable neural stimulation or recording electrodes. However, upon implantation, scar tissue forms around the electrode surface, causing unwanted deterioration of the electrical contact. We demonstrate that sub-monolayer coatings of platinum nanoparticles (PtNPs) applied to 3D neural electrodes by electrophoretic deposition (EPD) can enhance the electrode?s active surface area and significantly lower its impedance. In this work we use ethanol-water mixtures as the EPD solvent, in contrast to our previous studies carried out in water. We show that EPD coating in 30 vol.% ethanol improves the device?s electrochemical performance. Computational mesoscale multiparticle simulations were for the first time applied to PtNP-on-Pt EPD, revealing correlations between ethanol concentration, electrochemical properties, and coating homogeneity. Thereto, this optimum ethanol concentration (30 vol.%) balances two opposing trends: (i) the addition of ethanol reduces water splitting and gas bubble formation, which benefits surface coverage, and (ii) increased viscosity and reduced permittivity occur at high ethanol concentrations, which impair the coating quality and favoring clustering. A seven-fold increase in active surface area and significantly reduced in vitro impedance of the nano-modified neural stimulation electrode surfaces highlight the influence of ethanol-water mixtures in PtNP EPD. © 2022 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.view abstract doi: 10.1149/1945-7111/ac51f8 2021 • 169 Comparing Direct and Pulsed-Direct Current Electrophoretic Deposition on Neural Electrodes: Deposition Mechanism and Functional Influence
Ramesh, V. and Rehbock, C. and Giera, B. and Karnes, J.J. and Forien, J.-B. and Angelov, S.D. and Schwabe, K. and Krauss, J.K. and Barcikowski, S.
Langmuir 37 9724-9734 (2021)Electrophoretic deposition (EPD) of platinum nanoparticles (PtNPs) on platinum-iridium (Pt-Ir) neural electrode surfaces is a promising strategy to tune the impedance of electrodes implanted for deep brain stimulation in various neurological disorders such as advanced Parkinson's disease and dystonia. However, previous results are contradicting as impedance reduction was observed on flat samples while in three-dimensional (3D) structures, an increase in impedance was observed. Hence, defined correlations between coating properties and impedance are to date not fully understood. In this work, the influence of direct current (DC) and pulsed-DC electric fields on NP deposition is systematically compared and clear correlations between surface coating homogeneity and in vitro impedance are established. The ligand-free NPs were synthesized via pulsed laser processing in liquid, yielding monomodal particle size distributions, verified by analytical disk centrifugation (ADC). Deposits formed were quantified by UV-vis supernatant analysis and further characterized by scanning electron microscopy (SEM) with semiautomated interparticle distance analyses. Our findings reveal that pulsed-DC electric fields yield more ordered surface coatings with a lower abundance of particle assemblates, while DC fields produce coatings with more pronounced aggregation. Impedance measurements further highlight that impedance of the corresponding electrodes is significantly reduced in the case of more ordered coatings realized by pulsed-DC depositions. We attribute this phenomenon to the higher active surface area of the adsorbed NPs in homogeneous coatings and the reduced particle-electrode electrical contact in NP assemblates. These results provide insight for the efficient EPD of bare metal NPs on micron-sized surfaces for biomedical applications in neuroscience and correlate coating homogeneity with in vitro functionality. © 2021 American Chemical Society.view abstract doi: 10.1021/acs.langmuir.1c01081 2021 • 168 General Efficacy of Atomically Dispersed Pt Catalysts for the Chlorine Evolution Reaction: Potential-Dependent Switching of the Kinetics and Mechanism
Lim, T. and Kim, J.H. and Kim, J. and Baek, D.S. and Shin, T.J. and Jeong, H.Y. and Lee, K.-S. and Exner, K.S. and Joo, S.H.
ACS Catalysis 11 12232-12246 (2021)The electrochemical chlorine evolution reaction (CER) is a key anodic reaction in the chlor-alkali process for Cl2production, on-site generation of ClO-, and Cl2-mediated electrosynthesis. Although Ru-based mixed metal oxides have long been used as CER catalysts, they suffer from a selectivity problem due to the competing oxygen evolution reaction. To overcome this shortcoming, we have developed a new CER catalyst composed of atomically dispersed Pt-N4sites on carbon nanotubes (Pt1/CNT). In this study, we demonstrate that the catalytically active Pt-N4sites can be constructed from H2PtCl6·6H2O and an ionic liquid via a bottom-up approach and a Pt-porphyrin-driven top-down method. Both catalysts exhibit excellent CER activity and remarkable selectivity, demonstrating the general efficacy of Pt1/CNT for the CER. The electrochemical and in situ X-ray absorption spectroscopy analyses reveal that Pt1/CNT catalysts show a reaction order of ∼1.8 in the low overpotential regime, where the Volmer step is reconciled with the rate-determining step (RDS). Interestingly, in the high overpotential region, the CER over Pt1/CNT proceeds with a lower reaction order and the RDS switches to the Heyrovský step. These unprecedented kinetic insights are clearly distinguished from the oxide-based CER catalysts with the opposite sequence of the RDS. © 2021 American Chemical Societyview abstract doi: 10.1021/acscatal.1c03893 2021 • 167 Investigation of an atomic-layer-deposited Al2O3 diffusion barrier between Pt and Si for the use in atomic scale atom probe tomography studies on a combinatorial processing platform
Li, Y. and Zanders, D. and Meischein, M. and Devi, A. and Ludwig, A.
Surface and Interface Analysis 53 727-733 (2021)In order to enable the application of atomic probe tomography combinatorial processing platforms for atomic-scale investigations of phase evolution at elevated temperatures, the pre-sharpened Si tip of 10–20 nm in diameter must be protected against interdiffusion and reaction of the reactive Si with a film of interest by a conformal coating on the Si tip. It is shown that unwanted reactions can be suppressed by introducing a 20-nm-thick intermediate Al2O3 layer grown by atomic layer deposition (ALD). As a representative case, Pt is chosen as a film of interest, as it easily forms silicides. Whereas without the ALD coating diffusion/reactions occur, with the protective film, this is prevented for temperatures up to at least 600°C. The effectiveness of the Al2O3 layer serving as a diffusion barrier is not limited to a sharpened Si tip but works generally for all cases where a Si substrate is used. © 2021 The Authors. Surface and Interface Analysis published by John Wiley & Sons Ltd.view abstract doi: 10.1002/sia.6955 2021 • 166 On-chip electrocatalytic NO sensing using ruthenium oxide nanorods
Tanumihardja, E. and Paradelo Rodríguez, A. and Loessberg-Zahl, J.T. and Mei, B. and Olthuis, W. and van den Berg, A.
Sensors and Actuators, B: Chemical 334 (2021)Online, on-chip measurement of nitric oxide (NO) in organ-on-chip devices is desired to study endothelial (dys)function under dynamic conditions. In this work, ruthenium oxide (RuOx) is explored as an amperometric NO sensor and its suitability for organ-on-chip applications. For testing purposes, diethylamine NONOate was used as chemical NO donor. The NONOate's NO generation and electrochemical oxidation of generated NO were confirmed by real-time electrochemical/mass-spectrometry. Using RuOx nanorods electrodes, we show that NO oxidation occurred at a lower onset potential (+675 mV vs. Ag/AgCl) than on bare Pt electrode (+800 mV vs. Ag/AgCl). Due to NO adsorption on the RuOx surface, NO oxidation also delivered a higher current density (33.5 nA.μM−1. cm-2) compared to bare Pt (19.6 nA.μM−1. cm-2), making RuOx nanorods a favourable electrode for NO sensing applications. The RuOx electrode's suitability for organ-on-chip applications was successfully tested by using the electrode to detect a few micromolar concentration of NO generated by endothelial cell culture. Overall, the RuOx nanorods proved to be suitable for organ-on-chip studies due to their high sensitivity and selectivity. Our chip-integrated electrode allows for online NO monitoring in biologically relevant in vitro experiments. © 2021 The Author(s)view abstract doi: 10.1016/j.snb.2021.129631 2020 • 165 Effective size separation of laser-generated, surfactant-free nanoparticles by continuous centrifugation
Kohsakowski, S. and Seiser, F. and Wiederrecht, J.-P. and Reichenberger, S. and Vinnay, T. and Barcikowski, S. and Marzun, G.
Nanotechnology 31 (2020)High-power, nanosecond, pulsed-laser ablation in liquids enables the continuous synthesis of highly pure colloidal nanoparticles (NPs) at an application-relevant scale. The gained mass-weighted particle size distribution is however often reported to be broad, requiring post treatment like centrifugation to remove undesired particle size fractions. To date, available centrifugation techniques are generally discontinuous, limiting the throughput and hindering economic upscaling. Hence, throughout this paper, a scalable, continuously operating centrifugation of laser-generated platinum NPs in a tubular bowl centrifuge is reported for the first time. To that end, using a 121 W ns-laser, the continuous production of a colloidal suspension of NPs, yet with broad particle size distribution has been employed, yielding productivities of 1-2 g h-1 for gold, silver, and platinum. The power-specific productivities (Au: 18 mg h-1 W-1, Pt: 13 mg h-1 W-1, Ag: 8 mg h-1 W-1, Ni: 6 mg h-1 W-1) are far higher than reported before. Subsequent downstream integration of a continuously operating tubular bowl centrifuge was successfully achieved for Pt NPs allowing the removal of undesired particle size with high throughput. By means of a systematic study of relevant centrifugation parameters involved, effective size optimization and respective size sharpness parameters for a maximum Pt NP diameter of 10 nm are reported. The results of the experimental centrifugation of laser-generated Pt NPs were in excellent agreement with the theoretically calculated cut-off diameter. After centrifugation with optimized parameters (residence time of 5 min; g-force of 38,454 g), the polydispersity indices of the Pt NPs size distributions were reduced by a factor of six, and high monodispersity was observed. © 2019 IOP Publishing Ltd.view abstract doi: 10.1088/1361-6528/ab55bd 2020 • 164 Enhanced antibacterial performance of ultrathin silver/platinum nanopatches by a sacrificial anode mechanism
Abuayyash, A. and Ziegler, N. and Meyer, H. and Meischein, M. and Sengstock, C. and Moellenhoff, J. and Rurainsky, C. and Heggen, M. and Garzón-Manjón, A. and Scheu, C. and Tschulik, K. and Ludwig, Al. and Köller, M.
Nanomedicine: Nanotechnology, Biology, and Medicine 24 (2020)The development of antibacterial implant surfaces is a challenging task in biomaterial research. We fabricated a highly antibacterial bimetallic platinum (Pt)/silver(Ag) nanopatch surface by short time sputtering of Pt and Ag on titanium. The sputter process led to a patch-like distribution with crystalline areas in the nanometer-size range (1.3–3.9 nm thickness, 3–60 nm extension). Structural analyses of Pt/Ag samples showed Ag- and Pt-rich areas containing nanoparticle-like Pt deposits of 1–2 nm. The adhesion and proliferation properties of S. aureus on the nanopatch samples were analyzed. Consecutively sputtered Ag/Pt nanopatches (Pt followed by Ag) induced enhanced antimicrobial activity compared to co-sputtered Pt/Ag samples or pure Ag patches of similar Ag amounts. The underlying sacrificial anode mechanism was proved by linear sweep voltammetry. The advantages of this nanopatch coating are the enhanced antimicrobial activity despite a reduced total amount of Ag/Pt and a self-limited effect due the rapid Ag dissolution. © 2019 Elsevier Inc.view abstract doi: 10.1016/j.nano.2019.102126 2020 • 163 Metal Complexes of Donor-functionalized Fluorinated β-Ketoiminates – Synthesis, Structure, and CVD Application
Kaiser, K. and Ganesamoorthy, C. and Wölper, C. and Schulz, S.
Zeitschrift fur Anorganische und Allgemeine Chemie 646 681-691 (2020)Six alkali metal complexes of partly-fluorinated, donor-functionalized β-ketoiminate ligands [L1Li (1), L1Na (2), L1K (3), L1Cs (4), L1 = OC(CF3)CHC(CH3)NCH2CH2OCH3; L2Li (5), L2Na (6), L2 = OC(CF3)CHC(CH3)NCH2CH2N(CH3)2] were prepared and structurally characterized. Reactions of L1Li with PtCl2 gave the homoleptic Pt complex L12Pt (7), which was characterized spectroscopically and by single-crystal X-ray diffraction and whose promising application as CVD precursor (chemical vapor deposition) is shown. Polycrystalline, pure Pt films were grown at 500 °C on SiO2@Si(100) substrates at 10–3 mbar and characterized by XRD, SEM, AFM, EDX and XPS. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.view abstract doi: 10.1002/zaac.201900239 2020 • 162 Photocatalytic hydrogen production by photo-reforming of methanol with one-pot synthesized Pt-containing TiO2 photocatalysts
Chung, Y.-H. and Han, K. and Lin, C.-Y. and O'Neill, D. and Mul, G. and Mei, B. and Yang, C.-M.
Catalysis Today 356 95-100 (2020)Functionalization of semiconductors by metallic nanoparticle is considered to be one of the most effective procedure to improve photocatalytic hydrogen production. Photodeposition is frequently used for functionalization but particle sizes and dispersions are still difficult to control. Here, Pt functionalization is achieved in a one-pot synthesis. The as-prepared samples are compared to reference materials prepared by conventional photodeposition and our results confirm that small and well-dispersed nanoparticles with superior stability are obtained by one-pot synthesis. The enhanced stability is attributed to a limited leaching of Pt nanoparticles during illumination likely caused by the preferable interaction of small, well dispersed Pt nanoparticles with the TiO2 support material. In addition, our results demonstrate that Na-residues are detrimental for the photocatalytic performance and washing in acidic solution is mandatory to effectively reduce the sodium contamination. © 2019 Elsevier B.V.view abstract doi: 10.1016/j.cattod.2019.07.042 2019 • 161 Bimetallic silver-platinum nanoparticles with combined osteo-promotive and antimicrobial activity
Breisch, M. and Grasmik, V. and Loza, K. and Pappert, K. and Rostek, A. and Ziegler, N. and Ludwig, Al. and Heggen, M. and Epple, M. and Tiller, J.C. and Schildhauer, T.A. and Köller, M. and Sengstock, C.
Nanotechnology 30 (2019)Bimetallic alloyed silver-platinum nanoparticles (AgPt NP) with different metal composition from Ag10Pt90 to Ag90Pt10 in steps of 20 mol% were synthesized. The biological effects of AgPt NP, including cellular uptake, cell viability, osteogenic differentiation and osteoclastogenesis as well as the antimicrobial activity towards Staphylococcus aureus and Escherichia coli were analyzed in comparison to pure Ag NP and pure Pt NP. The uptake of NP into human mesenchymal stem cells was confirmed by cross-sectional focused-ion beam preparation and observation by scanning and transmission electron microscopy in combination with energy-dispersive x-ray analysis. Lower cytotoxicity and antimicrobial activity were observed for AgPt NP compared to pure Ag NP. Thus, an enhanced Ag ion release due to a possible sacrificial anode effect was not achieved. Nevertheless, a Ag content of at least 50 mol% was sufficient to induce bactericidal effects against both Staphylococcus aureus and Escherichia coli. In addition, a Pt-related (≥50 mol% Pt) osteo-promotive activity on human mesenchymal stem cells was observed by enhanced cell calcification and alkaline phosphatase activity. In contrast, the osteoclastogenesis of rat primary precursor osteoclasts was inhibited. In summary, these results demonstrate a combinatory osteo-promotive and antimicrobial activity of bimetallic Ag50Pt50 NP. © 2019 IOP Publishing Ltd.view abstract doi: 10.1088/1361-6528/ab172b 2019 • 160 Durability study of platinum nanoparticles supported on gas-phase synthesized graphene in oxygen reduction reaction conditions
Bertin, E. and Münzer, A. and Reichenberger, S. and Streubel, R. and Vinnay, T. and Wiggers, H. and Schulz, C. and Barcikowski, S. and Marzun, G.
Applied Surface Science 467-468 1181-1186 (2019)Ligand-free platinum nanoparticles were prepared by pulsed laser ablation in liquids (PLAL) and employed as a benchmarking catalyst to evaluate the durability of a new gas-phase synthesized graphene support in oxygen reduction conditions. Raman measurements showed that the graphene, as compared to Vulcan, was almost defect free. Transmission electron microscopy and initial electrochemically active surface area measurements confirmed good dispersion of the catalysts on both supports. During durability tests, graphene supported Pt nanoparticles showed much better ECSA retention (75% on graphene as compared to 38% on Vulcan), ultimately retaining a higher ECSA than a commercial sample subjected to the same procedure. © 2018 Elsevier B.V.view abstract doi: 10.1016/j.apsusc.2018.10.061 2019 • 159 First PEM fuel cell based on ligand-free, laser-generated platinum nanoparticles
Kohsakowski, S. and Streubel, R. and Radev, I. and Peinecke, V. and Barcikowski, S. and Marzun, G. and Reichenberger, S.
Applied Surface Science 467-468 486-492 (2019)For the first time, surfactant-free platinum nanoparticles (Pt-NPs) prepared by pulsed laser ablation in liquids were employed in a real proton exchange membrane fuel cell (PEMFC). Laser-generated Pt-NPs show a larger size in comparison to a commercial Pt/C catalyst, being 6.6 nm for the laser-based and 3.8 nm for the reference, respectively. The laser-based Pt/C electrocatalyst was synthesized by colloidal deposition of laser-generated Pt-NPs onto carbonic Vulcan as support. The durability tests showed a much better stability of the laser-based catalyst in the electrochemical active surface area (ECSA) with an ECSA retention of 88% of its initial area. This better durability is probably caused by a reduced Pt dissolution rate which is higher for the reference catalyst containing NPs smaller than 3 nm. The occurrence of Pt dissolution is further indicated by X-ray photoelectron spectroscopy (XPS) of the electrodes after electrocatalytic testing showing no Pt signal in case of the reference catalyst. Performance tests of both PEMFC show a 20% higher mass-specific power density for the laser-based proton exchange membrane fuel cell. © 2018 Elsevier B.V.view abstract doi: 10.1016/j.apsusc.2018.10.145 2019 • 158 Impact of Preparation Method and Hydrothermal Aging on Particle Size Distribution of Pt/γ-Al 2 O 3 and Its Performance in CO and NO Oxidation
Ogel, E. and Casapu, M. and Doronkin, D.E. and Popescu, R. and Störmer, H. and Mechler, C. and Marzun, G. and Barcikowski, S. and Türk, M. and Grunwaldt, J.-D.
Journal of Physical Chemistry C (2019)The influence of the preparation method and the corresponding particle size distribution on the hydrothermal deactivation behavior at 600-800 °C and performance during CO/NO oxidation was systematically investigated for a series of Pt/Al 2 O 3 catalysts. Representative conventional (incipient wetness impregnation) and advanced preparation methods (flame spray pyrolysis, supercritical fluid reactive deposition, and laser ablation in liquid) were selected, which generated samples containing narrow and homogeneous but also heterogeneous particle size distributions. Basic characterization was conducted by inductively coupled plasma-optical emission spectrometry, N 2 physisorption, and X-ray diffraction. The particle size distribution and the corresponding oxidation state were analyzed using transmission electron microscopy and X-ray absorption spectroscopy. The systematic study shows that oxidized Pt nanoparticles smaller than 2 nm sinter very fast, already at 600 °C, but potential chlorine traces from the catalyst precursor seem to stabilize Pt nanoparticles against further sintering and consequently maintain the catalytic performance. Samples prepared by flame spray pyrolysis and laser ablation showed a superior hydrothermal resistance of the alumina support, although, due to small interparticle distance in case of laser synthesized particles, the particle size distribution increases considerably at high temperatures. Significant deceleration of the noble metal sintering process was obtained for the catalysts containing homogeneously distributed but slightly larger Pt nanoparticles (supercritical fluid reactive deposition) or for particles deposited on a thermally stable alumina support (flame spray pyrolysis). The correlations obtained between Pt particle size distribution, oxidation state, and catalytic performance indicate different trends for CO and NO oxidation reactions, in line with their structure sensitivity. © 2019 American Chemical Society.view abstract doi: 10.1021/acs.jpcc.8b11065 2019 • 157 Impact of Preparation Method and Hydrothermal Aging on Particle Size Distribution of Pt/γ-Al2O3 and Its Performance in CO and NO Oxidation
Ogel, E. and Casapu, M. and Doronkin, D.E. and Popescu, R. and Störmer, H. and Mechler, C. and Marzun, G. and Barcikowski, S. and Türk, M. and Grunwaldt, J.-D.
Journal of Physical Chemistry C 123 5433-5446 (2019)The influence of the preparation method and the corresponding particle size distribution on the hydrothermal deactivation behavior at 600-800 °C and performance during CO/NO oxidation was systematically investigated for a series of Pt/Al2O3 catalysts. Representative conventional (incipient wetness impregnation) and advanced preparation methods (flame spray pyrolysis, supercritical fluid reactive deposition, and laser ablation in liquid) were selected, which generated samples containing narrow and homogeneous but also heterogeneous particle size distributions. Basic characterization was conducted by inductively coupled plasma-optical emission spectrometry, N2 physisorption, and X-ray diffraction. The particle size distribution and the corresponding oxidation state were analyzed using transmission electron microscopy and X-ray absorption spectroscopy. The systematic study shows that oxidized Pt nanoparticles smaller than 2 nm sinter very fast, already at 600 °C, but potential chlorine traces from the catalyst precursor seem to stabilize Pt nanoparticles against further sintering and consequently maintain the catalytic performance. Samples prepared by flame spray pyrolysis and laser ablation showed a superior hydrothermal resistance of the alumina support, although, due to small interparticle distance in case of laser synthesized particles, the particle size distribution increases considerably at high temperatures. Significant deceleration of the noble metal sintering process was obtained for the catalysts containing homogeneously distributed but slightly larger Pt nanoparticles (supercritical fluid reactive deposition) or for particles deposited on a thermally stable alumina support (flame spray pyrolysis). The correlations obtained between Pt particle size distribution, oxidation state, and catalytic performance indicate different trends for CO and NO oxidation reactions, in line with their structure sensitivity. © 2019 American Chemical Society.view abstract doi: 10.1021/acs.jpcc.8b11065 2019 • 156 Interaction-driven spin-orbit effects and Chern insulating phases in corundum-based 4d and 5d oxide honeycomb lattices
Köksal, O. and Pentcheva, R.
Journal of Physics and Chemistry of Solids 128 301-309 (2019)Using density functional theory calculations with a Hubbard U, we explore topologically nontrivial phases in X2O3 honeycomb layers with X= 4d and 5d cation inserted in the band insulator α-Al2O3 along the [0001]-direction. Several promising candidates for quantum anomalous Hall insulators (QAHI)are identified. In particular, for X = Tc and Pt spin-orbit coupling (SOC)opens a gap of 54 and 59 meV, respectively, leading to Chern insulators (CI)with C = −2 and −1. The nature of different Chern numbers is related to the corresponding spin textures. The Chern insulating phase is sensitive to the Coulomb repulsion strength: X = Tc undergoes a transition from a CI to a trivial metallic state beyond a critical strength of Uc=2.5 eV. A comparison between the isoelectronic metastable FM phases of X = Pd and Pt emphasizes the intricate balance between electronic correlations and SOC: while the former is a trivial insulator, the latter is a Chern insulator. In addition, X = Os turns out to be a FM Mott insulator with an unpaired electron in the t2g manifold where SOC induces an unusually high orbital moment of 0.34 μB along the z-axis. Parallels to the 3d honeycomb corundum cases are discussed. © 2018 Elsevier Ltdview abstract doi: 10.1016/j.jpcs.2018.01.049 2019 • 155 Platinum nanoparticles supported on reduced graphene oxide prepared in situ by a continuous one-step laser process
Haxhiaj, I. and Tigges, S. and Firla, D. and Zhang, X. and Hagemann, U. and Kondo, T. and Nakamura, J. and Marzun, G. and Barcikowski, S.
Applied Surface Science 469 811-820 (2019)A large research emphasis is still placed on improvement of production routes of nanosized materials with enhanced catalytic properties. Here we developed a continuous process for generation of platinum (Pt) nanoparticles supported on reduced graphene oxide (rGO) in situ via pulsed laser ablation in liquid (PLAL) dispersion of rGO. This in situ PLAL technique is a single step procedure that allows the synthesis of heterogeneous catalysts with a simultaneous control of particle size and mass loading. By this method, Pt particles with mean particle diameters around 2.5 nm and in a regime of 3–4 nm have been produced in ethanol and saline water, respectively, and adsorbed on rGO with up to 50 wt%. Both inorganic and organic solvents used during in situ synthesis lead to production of CO tolerant Pt/rGO catalysts, which are relevant for fuel cell applications due to the remarkably low CO desorption temperatures around 65–80 °C. © 2018 Elsevier B.V.view abstract doi: 10.1016/j.apsusc.2018.10.257 2019 • 154 Thermally stable iridium contacts to highly doped p-In0:53Ga0:47As for indium phosphide double heterojunction bipolar transistors
Brahem, M. and Mogilatenko, A. and Stoppel, D. and Berger, D. and Hochheim, S. and Rentner, D. and Ostermay, I. and Reiner, M. and Boppel, S. and Nosaeva, K. and Weimann, N.
Microelectronic Engineering 215 (2019)We report on surface pretreatment for ohmic contacts to p-doped In0.53Ga0.47 As with improved thermal stability. It is found that the cleaning of In0.53Ga0.47 As surface by ammonium sulfide or sulfuric acid offers the optimum surface treatment prior to metal deposition. Contacts using an iridium contact layer and palladium diffusion barrier were fabricated and compared to a conventional platinum-based contact Pt/Ti/Pt/Au. Pt-based metal stack suffered from void formation and high reactivity with the semiconductor when annealed at 240 °C for a few hours, as examined by transmission electron microscopy. As a result, the Pt-based stack exhibited strong deterioration of the resistivity. On the other hand, the Ir contact maintained its integrity during thermal stress. The improved contact exhibited a void and reaction-free microstructure and offered stable resistivity values with annealing. © 2019 Elsevier B.V.view abstract doi: 10.1016/j.mee.2019.111017 2019 • 153 Tuning the magnetic anisotropy of niptmnga by substitution and epitaxial strain
Herper, H.C. and Grunebohm, A.
IEEE Transactions on Magnetics 55 (2019)Large magnetocrystalline anisotropy (MCA) is of high technical relevance, in particular for magnetic actuators, permanent magnets, and memory devices with high density. Large MCAs have been reported for the low temperature L10 phase of Ni2MnGa. Both, Mn and Pt substitution can stabilize this phase at and above room temperature. Despite the larger spin-orbit coupling in the heavy 5d-element Pt, it has been reported that Pt substitution may result in degeneration of the MCA. In this paper, we study the MCA for a combination of epitaxial strain and Mn and Pt substitution based on density functional theory methods. We show that one can stabilize both large uniaxial and easy-plane anisotropies depending on the value of strain. In particular, small changes of the applied strain may allow to switch between low- and high-anisotropy states or even switch the direction of the easy-axis magnetization direction. © 1965-2012 IEEE.view abstract doi: 10.1109/TMAG.2018.2856461 2018 • 152 Antibacterial Efficacy of Sacrifical Anode Thin Films Combining Silver with Platinum Group Elements within a Bacteria-Containing Human Plasma Clot
Abuayyash, A. and Ziegler, N. and Gessmann, J. and Sengstock, C. and Schildhauer, T.A. and Ludwig, Al. and Köller, M.
Advanced Engineering Materials 20 (2018)Silver (Ag) dots arrays (64 and 400 dots per mm2) are fabricated on a continuous platinum (Pt), palladium (Pd), or iridium (Ir) thin film (sacrifical anode systems for Ag) and for comparison on titanium (Ti) film (non-sacrifical anode system for Ag) by sputter deposition and photolithographic patterning. The samples are embedded within a tissue-like plasma clot matrix containing Staphylococcus aureus (S. aureus), cultivated for 24 h. Bacterial growth is analyzed by fluorescence microscopy. Among platinum group sacrifical anode elements and a dense Ag sample, only the high Ag ion releasing Ag–Ir system is able to inhibit the bacterial growth within the adjacent plasma clot matrix. This study demonstrates that the antibacterial efficiency of Ag coatings is reduced under tissue-like conditions. However, the new sacrificial anode based Ag–Ir system can overcome this limitation. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimview abstract doi: 10.1002/adem.201700493 2018 • 151 Effect of Pt and Au current collector in LiMn2O4 thin film for micro-batteries
Trócoli, R. and Dushina, A. and Borhani-Haghighi, S. and Ludwig, Al. and La Mantia, F.
Nanotechnology 29 (2018)The crystal orientation and morphology of sputtered LiMn2O4 thin films is strongly affected by the current collector. By substituting Pt with Au, it is possible to observe in the x-ray diffraction pattern of LiMn2O4 a change in the preferential orientation of the grains from (111) to (400). In addition, LiMn2O4 thin films deposited on Au show a higher porosity than films deposited on Pt. These structural differences cause an improvement in the electrochemical performances of the thin films deposited on Au, with up to 50% more specific charge. Aqueous cells using thin film based on LiMn2O4 sputtered on Au or Pt as the cathode electrode present a similar retention of specific charge, delivering 85% and 100%, respectively, of the initial values after 100 cycles. The critical role of the nature of the substrate used in the morphology and electrochemical behaviour observed could permit the exploration of similar effects for other lithium intercalation electrodes. © 2017 IOP Publishing Ltd.view abstract doi: 10.1088/1361-6528/aa9e33 2018 • 150 Investigation of Carbon Nanofiber-supported Electrocatalysts with Ultra-low Platinum Loading for the Use in PEM Fuel Cells
Podleschny, P. and Rost, U. and Muntean, R. and Marginean, G. and Heinzel, A. and Peinecke, V. and Radev, I. and Muhler, M. and Brodmann, M.
Fuel Cells 18 586-593 (2018)The present study aims to investigate gas diffusion electrodes with ultra-low platinum loading and increased durability, prepared by pulsed electrodeposition process, applicable for polymer electrolyte membrane fuel cells (PEMFC). Testing was performed both, in situ in a PEMFC test bench while prepared GDEs were compared to anodes and cathodes with commercially available catalysts by Johnson Matthey (JM), as well as ex situ regarding electrochemical properties and catalyst layer structure. High and stable performance of developed electrodes was achieved, while the Pt catalyst loading of investigated anodes and cathodes was reduced to 10 µgPt cm−2. The catalyst deposition was achieved via pulsed electrodeposition process from H2PtCl6-containing electrolyte on an oxygen plasma-pretreated corrosion-stable carbon nanofiber (CNF) support. In situ performance tests show a similar operation behavior of the compared anodes, while activation losses of investigated cathodes are high due to the limited amount of catalyst material. However, Pt/CNF_c cathodes show significantly higher power densities than cathodes prepared with JM catalyst. Membrane electrode assemblies containing developed Pt/CNF_a anodes with 10 µgPt cm−2 reached a power density of 0.525 W cm−2 at a cell potential of 0.65 V, which was similar to anodes with commercially available HiSPEC 2000 catalysts by JM. Accelerated stress tests (AST) revealed that Pt/CNF_a anodes preserve their performance, whereas the commercial catalyst degraded severely. Cyclic voltammetry (CV) indicated a high electrochemical active surface area of Pt/CNF_a anodes. Moreover, the electrode characteristics, analyzed via electrochemical impedance spectroscopy after AST, showed marginal anode degradation. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimview abstract doi: 10.1002/fuce.201700220 2018 • 149 Local Activities of Hydroxide and Water Determine the Operation of Silver-Based Oxygen Depolarized Cathodes
Botz, A. and Clausmeyer, J. and Öhl, D. and Tarnev, T. and Franzen, D. and Turek, T. and Schuhmann, W.
Angewandte Chemie - International Edition 57 12285-12289 (2018)Local ion activity changes in close proximity to the surface of an oxygen depolarized cathode (ODC) were measured by scanning electrochemical microscopy (SECM). While the operating ODC produces OH− ions and consumes O2 and H2O through the electrocatalytic oxygen reduction reaction (ORR), local changes in the activity of OH− ions and H2O are detected by means of a positioned Pt microelectrode serving as an SECM tip. Sensing at the Pt tip is based on the pH-dependent reduction of PtO and obviates the need for prior electrode modification steps. It can be used to evaluate the coordination numbers of OH− ions and H2O, and the method was exploited as a novel approach of catalyst activity assessment. We show that the electrochemical reaction on highly active catalysts can have a drastic influence on the reaction environment. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheimview abstract doi: 10.1002/anie.201807798 2018 • 148 Synthesis and biological characterization of alloyed silver-platinum nanoparticles: From compact core-shell nanoparticles to hollow nanoalloys
Grasmik, V. and Breisch, M. and Loza, K. and Heggen, M. and Köller, M. and Sengstock, C. and Epple, M.
RSC Advances 8 38582-38590 (2018)Bimetallic nanoparticles consisting of silver and platinum were prepared by a modified seeded-growth process in water in the full composition range in steps of 10 mol%. The particles had diameters between 15-25 nm as determined by disc centrifugal sedimentation (DCS) and transmission electron microscopy (TEM). Whereas particles with high platinum content were mostly spherical with a solid silver core/platinum shell structure, mostly hollow alloyed nanoparticles were observed with increasing silver content. The internal structure and the elemental distribution within the particles were elucidated by high-resolution transmission electron microscopy (HRTEM) in combination with energy-dispersive X-ray spectroscopy (EDX). The particles were cytotoxic for human mesenchymal stem cells (hMSC) above 50 mol% silver. This was explained by dissolution experiments where silver was only released at and above 50 mol% silver. In contrast, platinum-rich particles (less than 50 mol% silver) did not release any silver ions. This indicates that the presence of platinum inhibits the oxidative dissolution of silver. © 2018 The Royal Society of Chemistry.view abstract doi: 10.1039/c8ra06461j 2017 • 147 Antibacterial activity of microstructured sacrificial anode thin films by combination of silver with platinum group elements (platinum, palladium, iridium)
Köller, M. and Bellova, P. and Javid, S.M. and Motemani, Y. and Khare, C. and Sengstock, C. and Tschulik, K. and Schildhauer, T.A. and Ludwig, Al.
Materials Science and Engineering C 74 536-541 (2017)Five different Ag dots arrays (16 to 400dots/mm2) were fabricated on a continuous platinum, palladium, or iridium thin film and for comparison also on titanium film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Staphylococcus aureus (S. aureus) were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards . S. aureus was induced by Ag dot arrays on each of the platinum group thin film (sacrificial anode system for Ag) in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag). Among platinum group elements the Ir-Ag system exerted the highest antibacterial activity which was accompanied by most advanced dissolution of the Ag dots and Ag ion release compared to Ag dots on Pt or Pd. © 2016 Elsevier B.V.view abstract doi: 10.1016/j.msec.2016.12.075 2017 • 146 High surface area, amorphous titania with reactive Ti3+ through a photo-assisted synthesis method for photocatalytic H2 generation
Zywitzki, D. and Jing, H. and Tüysüz, H. and Chan, C.K.
Journal of Materials Chemistry A 5 10957-10967 (2017)Amorphous titania-based photocatalysts are synthesized using a facile, UV-light mediated method and evaluated as photocatalysts for hydrogen evolution from water/methanol mixtures. The photocatalysts are prepared through the direct injection of a titanium alkoxide precursor into a water/methanol mixture, with subsequent hydrolysis, condensation, and polycondensation to form TiOx(OH)y species under UV-irradiation. The resulting amorphous titania materials exhibit an overall higher hydrogen evolution rate compared to a crystalline TiO2 reference (P25) on a molar basis of the photocatalyst due to their highly porous structure and high surface area (∼500 m2 g-1). The employed titanium alkoxide precursor did not play a major role in affecting the hydrogen evolution rate or the catalyst surface and morphology. A blue coloration, which is associated with the formation of Ti3+ species, was observed in the amorphous titania but not in P25 upon light irradiation and is enabled by the porous and disordered structure of the amorphous photocatalyst. The Ti3+ species are also used to reduce protons to H2 in the absence of light irradiation or reduce Pt2+ to form Pt nanoparticles. These Pt nanoparticles are smaller and better dispersed on the photocatalyst compared to particles prepared using conventional photodeposition, leading to higher H2 evolution rates. The results show that the direct injection method is a facile approach for the preparation of high surface area titania photocatalysts containing Ti3+ species with good photocatalytic activity for the production of H2. © 2017 The Royal Society of Chemistry.view abstract doi: 10.1039/c7ta01614j 2017 • 145 Laser synthesis, structure and chemical properties of colloidal nickel-molybdenum nanoparticles for the substitution of noble metals in heterogeneous catalysis
Marzun, G. and Levish, A. and Mackert, V. and Kallio, T. and Barcikowski, S. and Wagener, P.
Journal of Colloid and Interface Science 489 57-67 (2017)Platinum and iridium are rare and expensive noble metals that are used as catalysts for different sectors including in heterogeneous chemical automotive emission catalysis and electrochemical energy conversion. Nickel and its alloys are promising materials to substitute noble metals. Nickel based materials are cost-effective with good availability and show comparable catalytic performances. The nickel-molybdenum system is a very interesting alternative to platinum in water electrolysis. We produced ligand-free nickel-molybdenum nanoparticles by laser ablation in water and acetone. Our results show that segregated particles were formed in water due to the oxidation of the metals. X-ray diffraction shows a significant change in the lattice parameter due to a diffusion of molybdenum atoms into the nickel lattice with increasing activity in the electrochemical oxygen evolution reaction. Even though the solubility of molecular oxygen in acetone is higher than in water, there were no oxides and a more homogeneous metal distribution in the particles in acetone as seen by TEM-EDX. This showed that dissolved molecular oxygen does not control oxide formation. Overall, the laser ablation of pressed micro particulate mixtures in liquids offers a combinational synthesis approach that allows the screening of alloy nanoparticles for catalytic testing and can convert micro-mixtures into nano-alloys. © 2016 Elsevier Inc.view abstract doi: 10.1016/j.jcis.2016.09.014 2017 • 144 Localized Synthesis of Conductive Copper-Tetracyanoquinodimethane Nanostructures in Ultrasmall Microchambers for Nanoelectronics
Xing, Y. and Sun, G. and Speiser, E. and Esser, N. and Dittrich, P.S.
ACS Applied Materials and Interfaces 9 17271-17278 (2017)In this work, the microfluidic-assisted synthesis of copper-tetracyanoquinodimethane (Cu-TCNQ) nanostructures in an ambient environment is reported for the first time. A two-layer microfluidic device comprising parallel actuated microchambers was used for the synthesis and enabled excellent fluid handling for the continuous and multiple chemical reactions in confined ultrasmall chambers. Different precautions were applied to ensure the reduction state of copper (Cu) for the synthesis of Cu-TCNQ charge-transfer compounds. The localized synthesis of Cu and in situ transformation to Cu-TCNQ complexes in solution were achieved by applying different gas pressures in the control layer. Additionally, various diameters of the Cu-TCNQ nano/microstructures were obtained by adjusting the concentration of the precursors and reaction time. After the synthesis, platinum (Pt) microelectrode arrays, which were aligned at the microchambers, could enable the in situ measurements of the electronic properties of the synthesized nanostructures without further manipulation. The as-prepared Cu-TCNQ wire bundles showed good conductivity and a reversible hysteretic switching effect, which proved the possibility in using them to build advanced nanoelectronics. © 2017 American Chemical Society.view abstract doi: 10.1021/acsami.7b01664 2017 • 143 Optimizing in Vitro Impedance and Physico-Chemical Properties of Neural Electrodes by Electrophoretic Deposition of Pt Nanoparticles
Koenen, S. and Rehbock, C. and Heissler, H.E. and Angelov, S.D. and Schwabe, K. and Krauss, J.K. and Barcikowski, S.
ChemPhysChem 18 1108-1117 (2017)Neural electrodes suffer from an undesired incline in impedance when in permanent contact with human tissue. Nanostructures, induced by electrophoretic deposition (EPD) of ligand-free laser-generated nanoparticles (NPs) on the electrodes are known to stabilize impedance in vivo. Hence, Pt surfaces were systematically EPD-coated with Pt NPs and evaluated for impedance as well as surface coverage, contact angle, electrochemically active surface area (ECSA) and surface oxidation. The aim was to establish a systematic correlation between EPD process parameters and physical surface properties. The findings clearly reveal a linear decrease in impedance with increasing surface coverage, which goes along with a proportional reduction of the contact angle and an increase in ECSA and surface oxidation. EPD process parameters, prone to yield surface coatings with low impedance, are long deposition times (40–60 min), while high colloid concentrations (>250 μg mL−1) and electric field strengths (>25 V cm−1) should be avoided due to detrimental NP assemblage effects. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheimview abstract doi: 10.1002/cphc.201601180 2017 • 142 Peptide Cross-linkers: Immobilization of Platinum Nanoparticles Highly Dispersed on Graphene Oxide Nanosheets with Enhanced Photocatalytic Activities
Mizutaru, T. and Marzun, G. and Kohsakowski, S. and Barcikowski, S. and Hong, D. and Kotani, H. and Kojima, T. and Kondo, T. and Nakamura, J. and Yamamoto, Y.
ACS Applied Materials and Interfaces 9 9996-10002 (2017)For exerting potential catalytic and photocatalytic activities of metal nanoparticles (MNPs), immobilization of MNPs on a support medium in highly dispersed state is desired. In this Research Article, we demonstrated that surfactant-free platinum nanoparticles (PtNPs) were efficiently immobilized on graphene oxide (GO) nanosheets in a highly dispersed state by utilizing oligopeptide β-sheets as a cross-linker. The fluorenyl-substituted peptides were designed to form β-sheets, where metal-binding thiol groups and protonated and positively charged amino groups are integrated on the opposite sides of the surface of a β-sheet, which efficiently bridge PtNPs and GO nanosheet. In comparison to PtNP/GO composite without the peptide linker, the PtNP/peptide/GO ternary complex exhibited excellent photocatalytic dye degradation activity via electron transfer from GO to PtNP and simultaneous hole transfer from oxidized GO to the dye. Furthermore, the ternary complex showed photoinduced hydrogen evolution upon visible light irradiation using a hole scavenger. This research provides a new methodology for the development of photocatalytic materials by a bottom-up strategy on the basis of self-assembling features of biomolecules. © 2017 American Chemical Society.view abstract doi: 10.1021/acsami.6b16765 2017 • 141 Poly(benzoxazine) as an Immobilization Matrix for Miniaturized ATP and Glucose Biosensors
Ziller, C. and Lin, J. and Knittel, P. and Friedrich, L. and Andronescu, C. and Pöller, S. and Schuhmann, W. and Kranz, C.
ChemElectroChem 4 864-871 (2017)The reproducible immobilization of enzymes represents a key requirement in developing sensitive and fast-responding amperometric microbiosensors. A microbiosensor for the respective detection of glucose and adenosine-5′-triphosphate (ATP) is presented by using a poly(benzoxazine) derivative for the entrapment of the enzymes glucose oxidase (GOD) and hexokinase (HEX) at platinum (Pt) microelectrodes (MEs). For glucose, a sensitivity of 123.05±10.78 pA/mM (n=5) was obtained, which shows twice as high sensitivity compared to microbiosensors that use electrophoretic paints as the immobilization matrix for the same size ME (radius: 25 μm). For the determination of ATP, a sensitivity of 48.47±5.12 pA/μM and a signal-to-noise ratio of 40 at physiological pH values were obtained. Apart from their enhanced sensitivity, a significant improvement of these sensors is related to their improved mechanical stability. The applicability of these poly(benzoxine)-based microbiosensors for ATP detection was demonstrated with measurements at receptor protein tyrosine phosphatase zeta (PTPRζ) osteoblastic cells during mechanical stimulation. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheimview abstract doi: 10.1002/celc.201600765 2017 • 140 Promoting Photocatalytic Overall Water Splitting by Controlled Magnesium Incorporation in SrTiO3 Photocatalysts
Han, K. and Lin, Y.-C. and Yang, C.-M. and Jong, R. and Mul, G. and Mei, B.
ChemSusChem 10 4510-4516 (2017)SrTiO3 is a well-known photocatalyst inducing overall water splitting when exposed to UV irradiation of wavelengths <370 nm. However, the apparent quantum efficiency of SrTiO3 is typically low, even when functionalized with nanoparticles of Pt or Ni@NiO. Here, we introduce a simple solid-state preparation method to control the incorporation of magnesium into the perovskite structure of SrTiO3. After deposition of Pt or Ni@NiO, the photocatalytic water-splitting efficiency of the Mg:SrTiOx composites is up to 20 times higher compared to SrTiO3 containing similar catalytic nanoparticles, and an apparent quantum yield (AQY) of 10 % can be obtained in the wavelength range of 300–400 nm. Detailed characterization of the Mg:SrTiOx composites revealed that Mg is likely substituting the tetravalent Ti ion, leading to a favorable surface–space–charge layer. This originates from tuning of the donor density in the cubic SrTiO3 structure by Mg incorporation and enables high oxygen-evolution rates. Nevertheless, interfacing with an appropriate hydrogen evolution catalyst is mandatory and non-trivial to obtain high-performance in water splitting. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheimview abstract doi: 10.1002/cssc.201701794 2017 • 139 Role of Dissolved and Molecular Oxygen on Cu and PtCu Alloy Particle Structure during Laser Ablation Synthesis in Liquids
Marzun, G. and Bönnemann, H. and Lehmann, C. and Spliethoff, B. and Weidenthaler, C. and Barcikowski, S.
ChemPhysChem 18 1175-1184 (2017)The role of molecular oxygen dissolved in the solvent is often discussed as being an influential factor on particle oxidation during pulsed laser ablation in liquids. However, the formation of the particles during laser synthesis takes place under extreme conditions that enable the decomposition of the liquid medium. Reactive species of the solvent may then affect particle formation due to a chemical reaction in the reactive plasma. Experimental results show a difference between the role of dissolved molecular oxygen and the contribution from the oxygen in water molecules. Using a metallic Cu target in air-saturated water, laser ablation led to 20.5 wt % Cu, 11.5 wt % Cu2O, and 68 wt % CuO nanoparticles, according to X-ray diffraction results. In contrast to particles obtained in air-saturated water, no CuO was observed in the colloid synthesized in a Schlenk ablation chamber in completely oxygen-free water. Under these conditions, less-oxidized nanoparticles (25 wt % Cu and 75 wt % Cu2O) were synthesized. The results show that nanoparticle oxidation during laser synthesis is mainly caused by reactive oxygen species from the decomposition of water molecules. However, the addition of molecular oxygen promotes particle oxidation. Storage of the Cu colloid in the presence of dissolved oxygen leads, due to aging, to nanostructures with a higher oxidation state than the freshly prepared colloid. The XRD pattern of the sample prepared in air-saturated acetone showed no crystalline phases, which is possibly due to small crystallites or low particle concentration. Concentration of the particles by centrifugation showed that in the large fraction (>20 nm), even less oxidized nanoparticles (46 wt % Cu and 54 wt % Cu2O) were present, although the solubility of molecular oxygen is higher in acetone than in water. The nanoparticles in acetone were stable due to a Cu-catalyzed graphite layer formed on their surfaces. The influence of the solvent on alloy synthesis is also crucial. Laser ablation of PtCu3 in air-saturated water led to separated large CuO and Pt-rich spherical nanoparticles, whereas homogeneous PtCu3 alloy nanoparticles were formed in acetone. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheimview abstract doi: 10.1002/cphc.201601315 2017 • 138 The Space Confinement Approach Using Hollow Graphitic Spheres to Unveil Activity and Stability of Pt-Co Nanocatalysts for PEMFC
Pizzutilo, E. and Knossalla, J. and Geiger, S. and Grote, J.-P. and Polymeros, G. and Baldizzone, C. and Mezzavilla, S. and Ledendecker, M. and Mingers, A. and Cherevko, S. and Schüth, F. and Mayrhofer, K.J.J.
Advanced Energy Materials 7 (2017)The performance of polymer electrolyte fuel cells is strongly correlated to the electrocatalytic activity and stability. In particular, the latter is the result of an interplay between different degradation mechanisms. The essential high stability, demanded for real applications, requires the synthesis of advanced electrocatalysts that withstand the harsh operation conditions. In the first part of this study, the synthesis of oxygen reduction electrocatalysts consisting of Pt-Co (i.e., Pt5Co, Pt3Co, and PtCo) alloyed nanoparticles encapsulated in the mesoporous shell of hollow graphitic spheres (HGS) is reported. The mass activities of the activated catalysts depend on the initial alloy composition and an activity increase on the order of two to threefold, compared to pure Pt@HGS, is achieved. The key point of the second part is the investigation of the degradation of PtCo@HGS (showing the highest activity). Thanks to pore confinement, the impact of dissolution/dealloying and carbon corrosion can be studied without the interplay of other degradation mechanisms that would induce a substantial change in the particle size distribution. Therefore, impact of the upper potential limit and the scan rates on the dealloying and electrochemical surface area evolution can be examined in detail. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimview abstract doi: 10.1002/aenm.201700835 2016 • 137 Barrierless growth of precursor-free, ultrafast laser-fragmented noble metal nanoparticles by colloidal atom clusters - A kinetic in situ study
Jendrzej, S. and Gökce, B. and Amendola, V. and Barcikowski, S.
Journal of Colloid and Interface Science 463 299-307 (2016)Unintended post-synthesis growth of noble metal colloids caused by excess amounts of reactants or highly reactive atom clusters represents a fundamental problem in colloidal chemistry, affecting product stability or purity. Hence, quantified kinetics could allow defining nanoparticle size determination in dependence of the time. Here, we investigate in situ the growth kinetics of ps pulsed laser-fragmented platinum nanoparticles in presence of naked atom clusters in water without any influence of reducing agents or surfactants. The nanoparticle growth is investigated for platinum covering a time scale of minutes to 50 days after nanoparticle generation, it is also supplemented by results obtained from gold and palladium. Since a minimum atom cluster concentration is exceeded, a significant growth is determined by time resolved UV/Vis spectroscopy, analytical disc centrifugation, zeta potential measurement and transmission electron microscopy. We suggest a decrease of atom cluster concentration over time, since nanoparticles grow at the expense of atom clusters. The growth mechanism during early phase (<1. day) of laser-synthesized colloid is kinetically modeled by rapid barrierless coalescence. The prolonged slow nanoparticle growth is kinetically modeled by a combination of coalescence and Lifshitz-Slyozov-Wagner kinetic for Ostwald ripening, validated experimentally by the temperature dependence of Pt nanoparticle size and growth quenching by Iodide anions. © 2015.view abstract doi: 10.1016/j.jcis.2015.10.032 2016 • 136 Electrochemical bromination of organosulfur containing species for the determination of the strength of garlic (A. sativum)
Hall, E.M. and Tschulik, K. and Batchelor-McAuley, C. and Compton, R.G.
Food Chemistry 199 817-821 (2016)The extraction by ethyl acetate and subsequent electrochemical detection of organosulfur containing molecules from garlic is demonstrated. The electrochemical results first evidence the high sensitivity of the process towards the model compound propyl disulfide. Through the in situ formation of bromine at a platinum electrode the propyl disulfide can be readily detected at concentrations as low as 12.5 μM. Second, the work focuses on the detection of organosulfur from fresh garlic samples. Extraction of the organosulfur 'flavour' molecules is achieved with ethyl acetate. Addition of this extract to the electrochemical cell results in an analytically useful signal allowing the voltammetric peak height to be successfully correlated with the garlic strength, as measured using an organoleptic tasting panel. © 2015 Elsevier Ltd. All rights reserved.view abstract doi: 10.1016/j.foodchem.2015.12.086 2016 • 135 Electrophoretic deposition of ligand-free platinum nanoparticles on neural electrodes affects their impedance in vitro and in vivo with no negative effect on reactive gliosis
Angelov, S.D. and Koenen, S. and Jakobi, J. and Heissler, H.E. and Alam, M. and Schwabe, K. and Barcikowski, S. and Krauss, J.K.
Journal of Nanobiotechnology 14 (2016)Background: Electrodes for neural stimulation and recording are used for the treatment of neurological disorders. Their features critically depend on impedance and interaction with brain tissue. The effect of surface modification on electrode impedance was examined in vitro and in vivo after intracranial implantation in rats. Electrodes coated by electrophoretic deposition with platinum nanoparticles (NP; <10 and 50 nm) as well as uncoated references were implanted into the rat's subthalamic nucleus. After postoperative recovery, rats were electrostimulated for 3 weeks. Impedance was measured before implantation, after recovery and then weekly during stimulation. Finally, local field potential was recorded and tissue-to-implant reaction was immunohistochemically studied. Results: Coating with NP significantly increased electrode's impedance in vitro. Postoperatively, the impedance of all electrodes was temporarily further increased. This effect was lowest for the electrodes coated with particles <10 nm, which also showed the most stable impedance dynamics during stimulation for 3 weeks and the lowest total power of local field potential during neuronal activity recording. Histological analysis revealed that NP-coating did not affect glial reactions or neural cell-count. Conclusions: Coating with NP <10 nm may improve electrode's impedance stability without affecting biocompatibility. Increased impedance after NP-coating may improve neural recording due to better signal-to-noise ratio. © 2016 Angelov et al.view abstract doi: 10.1186/s12951-015-0154-9 2016 • 134 High temperature stability study of carbon supported high surface area catalysts—Expanding the boundaries of ex-situ diagnostics
Polymeros, G. and Baldizzone, C. and Geiger, S. and Grote, J.P. and Knossalla, J. and Mezzavilla, S. and Keeley, G.P. and Cherevko, S. and Zeradjanin, A.R. and Schüth, F. and Mayrhofer, K.J.J.
Electrochimica Acta 211 744-753 (2016)The performance of proton-exchange membrane fuel cells (PEMFCs) is defined by the equally important parameters of the intrinsic activity and stability of the electrocatalysts. This work focuses on the stability of carbon supported high surface area oxygen reduction reaction catalysts at potentials and temperatures similar to the operating conditions of PEMFCs. The catalysts used for this investigation consist of Pt nanoparticles of the same particle size supported on two types of carbon support having different textural properties, i.e., Vulcan and Hollow Graphitic Spheres (HGS). A broad toolbox of characterization techniques is utilized at 60 °C in order to resolve the contribution of the different degradation mechanisms, namely nanoparticle coalescence, metal dissolution and the corrosion of carbon support, to the total active surface area loss. The results obtained by investigating the impact of temperature, potential treatment and catalyst layer morphology on the aging behavior lead to a deeper understanding of the aging mechanisms and their interrelation at application-relevant conditions. Moreover, the previously reported improved performance of the Pt/HGS catalyst is confirmed also under higher temperatures. The experimental approach introduced in this work, highlights new challenges for high-temperature degradation investigations with supported PEMFC catalyst. © 2016 Elsevier Ltdview abstract doi: 10.1016/j.electacta.2016.06.105 2016 • 133 Improved photoelectrochemical performance of electrodeposited metal-doped BiVO4 on Pt-nanoparticle modified FTO surfaces
Gutkowski, R. and Peeters, D. and Schuhmann, W.
Journal of Materials Chemistry A 4 7875-7882 (2016)The recombination of photogenerated electron-hole pairs is one of the main limiting factors of photoelectrocatalysts absorbing in the visible part of the solar spectrum. Especially for BiVO4 the slow electron transport to the back contact facilitates charge recombination. Hence, thin layers have to be used to obtain higher photocurrents which are concomitantly only allow low absorption of the incident light. To address this limitation we have modified FTO substrates with Pt-nanoparticles before electrodepositing BiVO4. The Pt-nanoparticles decrease the overpotential for the electrodeposition of BiVO4, but more importantly they provide the basis for decreased charge recombination. Electrodeposited Mo-doped BiVO4 on Pt-nanoparticle modified FTO exhibits a substantially decreased recombination of photogenerated charge carriers during frontside illumination. Simultaneous co-doping of BiVO4 with two different metals leads to a substantial enhancement of the incident-photon-to-current efficiency (IPCE) during light driven oxygen evolution reaction. Highest IPCE (>30% at 1.2 V vs. RHE) values were obtained for Mo/Zn- and Mo/B-doped BiVO4. © 2016 The Royal Society of Chemistry.view abstract doi: 10.1039/c6ta01340f 2016 • 132 In Situ Hydrocracking of Fischer-Tropsch Hydrocarbons: CO-Prompted Diverging Reaction Pathways for Paraffin and α-Olefin Primary Products
Duyckaerts, N. and Trotuş, I.-T. and Swertz, A.-C. and Schüth, F. and Prieto, G.
ACS Catalysis 6 4229-4238 (2016)The single-step production of wax-free liquid hydrocarbons from syngas (H2 + CO) via integration of Fischer-Trospch (FT) and hydrocracking catalysts represents an attractive approach toward process intensification in compact gas-to-liquid technologies. Despite current, intensive efforts on the development of hybrid (multifunctional) catalysts to this end, not much is known about the reactivity of different FT primary products on hydrocracking catalysts under syngas. Using model compounds, the individual and collective reactivities of n-paraffin and α-olefin FT primary products were systematically studied on a Pt/nano-H-ZSM-5 hydrocracking catalyst under H2 (standard hydrocracking) and syngas (in situ hydroprocessing) atmospheres. Under H2, both reactants show indistinguishable reactivity as rapid olefin hydrogenation precedes hydrocracking. Under syngas, however, inhibition of (de)hydrogenation functionalities by CO poisoning of metal sites leads to a notable divergence of the reaction pathways for n-paraffins and α-olefins. Under these conditions, α-olefins showed enhanced reactivity, as an initial dehydrogenative activation step is not required, and contributed to moderate secondary cracking, likely via enhanced competitive adsorption on the acid sites. Besides, CO poisoning restored the intrinsic activity of the zeolite for the oligomerization of short-chain (α-)olefins, providing an additional net chain-growth pathway, which contributes to reducing the overall yield to undesired gas (C4-) hydrocarbons. These findings emphasize the key role of not only the chain-length distribution, but also the olefinic content of the FT primary hydrocarbons for the ultimate product distribution, and suggest guidelines for the design of multifunctional catalysts for the single-step synthesis of liquid hydrocarbons from syngas. © 2016 American Chemical Society.view abstract doi: 10.1021/acscatal.6b00904 2016 • 131 In Situ Investigations of Laser-Generated Ligand-Free Platinum Nanoparticles by X-ray Absorption Spectroscopy: How Does the Immediate Environment Influence the Particle Surface?
Fischer, M. and Hormes, J. and Marzun, G. and Wagener, P. and Hagemann, U. and Barcikowski, S.
Langmuir 32 8793-8802 (2016)Pulsed laser ablation in liquid (PLAL) has proven its usefulness as a nanoparticle (NP) synthesis method alternative to traditional chemical reduction methods, where the absence of any molecular ligands or residual reactants makes laser-generated nanoparticles ideal reference materials for charge-transfer experiments. We synthesized additive-free platinum nanoparticles by PLAL and in-situ characterized their interaction with H2O, sodium phosphate buffer, and sodium citrate as well as a TiO2 support by X-ray absorption fine structure (XAFS), i.e., X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Differences in the white-line intensity among the colloidal particles in the three liquids indicate that the respective NP-solvent interaction varies in strength. The ions added ex situ diffuse through the particles' electric double layer and interact electrostatically with the Stern plane. Consequently, these ions weaken the interaction of the functional OH groups that are bound to the partially oxidized platinum surfaces and cause their partial reduction. Comparing XAFS spectra of laser-generated Pt NPs in citrate with wet-chemically synthesized ones (both ligand-covered) indicates different types of Pt-O bonds: a Pt(IV)O2 type in the case of wet-chemical NPs and a Pt(II)O type in the case of laser-generated NPs. A comparison of unsupported laser-generated platinum NPs in H2O with TiO2-supported ones shows no white-line intensity differences and also an identical number of Pt-O bonds in both cases. This suggests that in the deposition process at least part of the double-layer coating stays intact and that the ligand-free Pt particle properties are preserved in the TiO2-supported Pt particles, relevant for heterogeneous catalysis. © 2016 American Chemical Society.view abstract doi: 10.1021/acs.langmuir.6b00685 2016 • 130 Interplay of the Open Circuit Potential-Relaxation and the Dissolution Behavior of a Single H2 Bubble Generated at a Pt Microelectrode
Karnbach, F. and Yang, X. and Mutschke, G. and Fröhlich, J. and Eckert, J. and Gebert, A. and Tschulik, K. and Eckert, K. and Uhlemann, M.
Journal of Physical Chemistry C 120 15137-15146 (2016)The dissolution behavior of a single H2 bubble electrochemically generated at a Pt microelectrode in 1 M H2SO4 was studied. The open circuit potential (OCP) relaxation after the polarization end was recorded and correlated with the dissolved H2 concentration at the interface electrode/electrolyte/gas. Simultaneously, the shrinking of the bubble was followed optically by means of a high speed camera. In addition, analytical modeling and numerical simulations for the bubble dissolution were performed. Three characteristic regions are identified in the OCP and the bubble radius transients: (i) slow relaxation and shrinking, (ii) transition region, and (iii) a long-term slowed down dissolution process. The high supersaturation after polarization remains longer than theoretically predicted and feeds the bubble in region (i). This reduces the dissolution rate of the bubble which differs significantly from that of nonelectrochemically produced bubbles. Numerical multispecies simulations prove that oxygen and nitrogen dissolved in the electrolyte additionally influence the bubble dissolution and slow down its shrinkage compared to pure hydrogen diffusion. In region (iii), a complete exchange of hydrogen gas with nitrogen and oxygen has occurred in the gas bubble. © 2016 American Chemical Society.view abstract doi: 10.1021/acs.jpcc.6b02305 2016 • 129 Local Platinum Environments in a Solid Analogue of the Molecular Periana Catalyst
Soorholtz, M. and Jones, L.C. and Samuelis, D. and Weidenthaler, C. and White, R.J. and Titirici, M.-M. and Cullen, D.A. and Zimmermann, T. and Antonietti, M. and Maier, J. and Palkovits, R. and Chmelka, B.F. and Schüth, F.
ACS Catalysis 6 2332-2340 (2016)Combining advantages of homogeneous and heterogeneous catalysis by incorporating active species on a solid support is often an effective strategy for improving overall catalyst performance, although the influences of the support are generally challenging to establish, especially at a molecular level. Here, we report the local compositions, and structures of platinum species incorporated into covalent triazine framework (Pt-CTF) materials, a solid analogue of the molecular Periana catalyst, Pt(bpym)Cl2, both of which are active for the selective oxidation of methane in the presence of concentrated sulfuric acid. By using a combination of solid-state 195Pt nuclear magnetic resonance (NMR) spectroscopy, aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS), important similarities and differences are observed between the Pt-CTF and Periana catalysts, which are likely related to their respective macroscopic reaction properties. In particular, wide-line solid-state 195Pt NMR spectra enable direct measurement, identification, and quantification of distinct platinum species in as-synthesized and used Pt-CTF catalysts. The results indicate that locally ordered and disordered Pt sites are present in as-synthesized Pt-CTF, with the former being similar to one of the two crystallographically distinct Pt sites in crystalline Pt(bpym)Cl2. A distribution of relatively disordered Pt moieties is also present in the used catalyst, among which are the principal active sites. Similarly XAS shows good agreement between the measured data of Pt-CTF and a theoretical model based on Pt(bpym)Cl2. Analyses of the absorption spectra of Pt-CTF used for methane oxidation suggests ligand exchange, as predicted for the molecular catalyst. XPS analyses of Pt(bpym)Cl2, Pt-CTF, as well as the unmodified ligands, further corroborate platinum coordination by pyridinic N atoms. Aberration-corrected high-angle annular dark-field STEM proves that Pt atoms are distributed within Pt-CTF before and after catalysis. The overall results establish the close similarities of Pt-CTF and the molecular Periana catalyst Pt(bpym)Cl2, along with differences that account for their respective properties. (Figure Presented). © 2016 American Chemical Society.view abstract doi: 10.1021/acscatal.5b02305 2016 • 128 Making the hydrogen evolution reaction in polymer electrolyte membrane electrolysers even faster
Tymoczko, J. and Calle-Vallejo, F. and Schuhmann, W. and Bandarenka, A.S.
Nature Communications 7 (2016)Although the hydrogen evolution reaction (HER) is one of the fastest electrocatalytic reactions, modern polymer electrolyte membrane (PEM) electrolysers require larger platinum loadings (∼0.5-1.0 mg cm-2) than those in PEM fuel cell anodes and cathodes altogether (∼0.5 mg cm-2). Thus, catalyst optimization would help in substantially reducing the costs for hydrogen production using this technology. Here we show that the activity of platinum(111) electrodes towards HER is significantly enhanced with just monolayer amounts of copper. Positioning copper atoms into the subsurface layer of platinum weakens the surface binding of adsorbed H-intermediates and provides a twofold activity increase, surpassing the highest specific HER activities reported for acidic media under similar conditions, to the best of our knowledge. These improvements are rationalized using a simple model based on structure-sensitive hydrogen adsorption at platinum and copper-modified platinum surfaces. This model also solves a long-lasting puzzle in electrocatalysis, namely why polycrystalline platinum electrodes are more active than platinum(111) for the HER.view abstract doi: 10.1038/ncomms10990 2016 • 127 Pilot-scale synthesis of metal nanoparticles by high-speed pulsed laser ablation in liquids
Streubel, R. and Bendt, G. and Gökce, B.
Nanotechnology 27 (2016)The synthesis of catalysis-relevant nanoparticles such as platinum and gold is demonstrated with productivities of 4 g h-1 for pulsed laser ablation in liquids (PLAL). The major drawback of low productivity of PLAL is overcome by utilizing a novel ultrafast high-repetition rate laser system combined with a polygon scanner that reaches scanning speeds up to 500 m s-1. This high scanning speed is exploited to spatially bypass the laser-induced cavitation bubbles at MHz-repetition rates resulting in an increase of the applicable, ablation-effective, repetition rate for PLAL by two orders of magnitude. The particle size, morphology and oxidation state of fully automated synthesized colloids are analyzed while the ablation mechanisms are studied for different laser fluences, repetition rates, interpulse distances, ablation times, volumetric flow rates and focus positions. It is found that at high scanning speeds and high repetition rate PLAL the ablation process is stable in crystallite size and decoupled from shielding and liquid effects that conventionally occur during low-speed PLAL. © 2016 IOP Publishing Ltd.view abstract doi: 10.1088/0957-4484/27/20/205602 2016 • 126 Selective Methane Oxidation Catalyzed by Platinum Salts in Oleum at Turnover Frequencies of Large-Scale Industrial Processes
Zimmermann, T. and Soorholtz, M. and Bilke, M. and Schüth, F.
Journal of the American Chemical Society 138 12395-12400 (2016)Direct catalytic methane functionalization, a "dream reaction", is typically characterized by relatively low catalyst activities. This also holds for the n2-(2,2′-bipyrimidyl)dichloroplatinum(II) [(bpym)PtCl2, 1] catalyst which oxidizes methane to methyl bisulfate in sulfuric acid. Nevertheless, it is arguably still one of the best systems for the partial oxidation of methane reported so far. Detailed studies of the dependence of activity on the SO3 concentration and the interplay with the solubility of different platinum compounds revealed potassium tetrachloroplatinate (K2PtCl4) as an extremely active, selective, and stable catalyst, reaching turnover frequencies (TOFs) of more than 25,000 h-1 in 20% oleum with selectivities above 98%. The TOFs are more than 3 orders of magnitude higher compared to the original report on (bpym)PtCl2 and easily reach or exceed those realized in commercial industrial processes, such as the Cativa process for the carbonylation of methanol. Also space-time-yields are on the order of large-scale commercialized processes. © 2016 American Chemical Society.view abstract doi: 10.1021/jacs.6b05167 2016 • 125 Self-supporting hierarchical porous PtAg alloy nanotubular aerogels as highly active and durable electrocatalysts
Liu, W. and Haubold, D. and Rutkowski, B. and Oschatz, M. and Hübner, R. and Werheid, M. and Ziegler, C. and Sonntag, L. and Liu, S. and Zheng, Z. and Herrmann, A.-K. and Geiger, D. and Terlan, B. and Gemming, T. and Borchardt, L...
Chemistry of Materials 28 6477-6483 (2016)Developing electrocatalysts with low cost, high activity, and good durability is urgently demanded for the wide commercialization of fuel cells. By taking advantage of nanostructure engineering, we fabricated PtAg nanotubular aerogels (NTAGs) with high electrocatalytic activity and good durability via a simple galvanic replacement reaction between the in situ spontaneously gelated Ag hydrogel and the Pt precursor. The PtAg NTAGs have hierarchical porous network features with primary networks and pores from the interconnected nanotubes of the aerogel and secondary networks and pores from the interconnected thin nanowires on the nanotube surface, and they show very high porosities and large specific surface areas. Due to the unique structure, the PtAg NTAGs exhibit greatly enhanced electrocatalytic activity toward formic acid oxidation, reaching 19 times higher metal-based mass current density as compared to the commercial Pt black. Furthermore, the PtAg NTAGs show outstanding structural stability and electrochemical durability during the electrocatalysis. Noble metal-based NTAGs are promising candidates for applications in electrocatalysis not only for fuel cells, but also for other energy-related systems. © 2016 American Chemical Society.view abstract doi: 10.1021/acs.chemmater.6b01394 2016 • 124 Size-dependent adhesion energy of shape-selected Pd and Pt nanoparticles
Ahmadi, M. and Behafarid, F. and Cuenya, B.R.
Nanoscale 8 11635-11641 (2016)Thermodynamically stable shape-selected Pt and Pd nanoparticles (NPs) were synthesized via inverse micelle encapsulation and a subsequent thermal treatment in vacuum above 1000 °C. The majority of the Pd NPs imaged via scanning tunneling microscopy (STM) had a truncated octahedron shape with (111) top and interfacial facets, while the Pt NPs were found to adopt a variety of shapes. For NPs of identical shape for both material systems, the NP-support adhesion energy calculated based on STM data was found to be size-dependent, with large NPs (e.g. ∼6 nm) having lower adhesion energies than smaller NPs (e.g. ∼1 nm). This phenomenon was rationalized based on support-induced strain that for larger NPs favors the formation of lattice dislocations at the interface rather than a lattice distortion that may propagate through the smaller NPs. In addition, identically prepared Pt NPs of the same shape were found to display a lower adhesion energy compared to Pd NPs. While in both cases, a transition from a lattice distortion to interface dislocations is expected to occur with increasing NP size, the higher elastic energy in Pt leads to a lower transition size, which in turn lowers the adhesion energy of Pt NPs compared to Pd. © 2016 The Royal Society of Chemistry.view abstract doi: 10.1039/c6nr02166b 2016 • 123 Structure-Activity-Stability Relationships for Space-Confined PtxNiy Nanoparticles in the Oxygen Reduction Reaction
Mezzavilla, S. and Baldizzone, C. and Swertz, A.-C. and Hodnik, N. and Pizzutilo, E. and Polymeros, G. and Keeley, G.P. and Knossalla, J. and Heggen, M. and Mayrhofer, K.J.J. and Schüth, F.
ACS Catalysis 6 8058-8068 (2016)This study focuses on the synthesis and electrochemical performance (i.e, activity and stability) of advanced electrocatalysts for the oxygen reduction reaction (ORR), made of Pt-Ni nanoparticles embedded in hollow graphitic spheres (HGS). The mechanism of the confined space alloying, that is, the controlled alloying of bimetallic precursors with different compositions (i.e., Pt3Ni, PtNi, and PtNi3) within the HGS mesoporous shell, was examined in detail. It was found that the presence of platinum during the reduction step, as well as the application of high annealing temperatures (at least 850°C for 3.5h in Ar), are necessary conditions to achieve the complete encapsulation and the full stability of the catalysts. The evolution of the activity, the electrochemical surface area, and the residual alloy composition of the Pt-Ni@HGS catalysts was thoroughly monitored (at the macro- and nanoscale level) under different degradation conditions. After the initial activation, the embedded Pt-Ni nanoparticles (3-4 nm in size) yield mass activities that are 2- to 3.5-fold higher than that of pure Pt@HGS (depending on the alloy composition). Most importantly, it is demonstrated that under the normal operation range of an ORR catalyst in PEM-FCs (potential excursions between 0.4 and 1.0 VRHE) both the nanoparticle-related degradation pathways (particle agglomeration) and dealloying phenomena are effectively suppressed, irrespectively of the alloy composition. Thus, the initial enhanced activity is completely maintained over an extended degradation protocol. In addition, owing to the peculiar configuration of the catalysts consisting of space-confined nanoparticles, it was possible to elucidate the impact of the dealloying process (as a function of alloy composition and severity of the degradation protocols) separately from other parallel phenomena, providing valuable insight into this elusive degradation mechanism. (Graph Presented). © 2016 American Chemical Society.view abstract doi: 10.1021/acscatal.6b02221 2015 • 122 Bidentate NHC pyrozolate ligands in luminescent platinum(ii) complexes
Naziruddin, A.R. and Galstyan, A. and Iordache, A. and Daniliuc, C.G. and Strassert, C.A. and De Cola, L.
Dalton Transactions 44 8467-8477 (2015)A bidentate C^N donor set derived from an N-heterocyclic carbene (NHC) precursor linked to a trifluoromethyl (CF3) functionalized pyrazole ring is described for the first time. The ligands have been employed to prepare four new phosphorescent complexes by the coordination of platinum(ii) centres bearing cyclometalated phenyl-pyridine/triazole-pyridine chelates. The electronic and steric environments of these complexes were tuned through the incorporation of suitable substituents in the phenyl-pyridine/triazole-pyridine ligands, wherein the position of the phenyl-ring substituent (a CF3 group) also directs the selective adoption of either a trans or a cis configuration between the CNHC and the Cphenyl donor atoms. Molecular structures obtained by X-ray diffraction for three of the complexes confirm a distorted square-planar configuration around the platinum centre, and DFT calculations show that the substituents have a significant influence on the energies of the frontier orbitals. Moreover, a platinum(ii) complex featuring the new bidentate NHC^pyrazolate ligand and a bulky adamantyl functionalized pyridine-triazole luminophore was observed to be highly emissive and exhibiting a sky-blue luminescence (λEm = 470 nm) with photoluminescence quantum yields as high as 50% in doped PMMA matrices. A complete photophysical investigation of all of the complexes in solution as well as in the solid state is herein reported. © 2015 The Royal Society of Chemistry.view abstract doi: 10.1039/c4dt03651d 2015 • 121 Carbon Monoxide-Induced Stability and Atomic Segregation Phenomena in Shape-Selected Octahedral PtNi Nanoparticles
Ahmadi, M. and Cui, C. and Mistry, H. and Strasser, P. and Roldan Cuenya, B.
ACS Nano 9 10686-10694 (2015)The chemical and morphological stability of size- and shape-selected octahedral PtNi nanoparticles (NP) were investigated after different annealing treatments up to a maximum temperature of 700 °C in a vacuum and under 1 bar of CO. Atomic force microscopy was used to examine the mobility of the NPs and their stability against coarsening, and X-ray photoelectron spectroscopy to study the surface composition, chemical state of Pt and Ni in the NPs, and thermally and CO-induced atomic segregation trends. Exposing the samples to 1 bar of CO at room temperature before annealing in a vacuum was found to be effective at enhancing the stability of the NPs against coarsening. In contrast, significant coarsening was observed when the sample was annealed in 1 bar of CO, most likely as a result of Ni(CO)4 formation and their enhanced mobility on the support surface. Sample exposure to CO at room temperature prior to annealing led to the segregation of Pt to the NP surface. Nevertheless, oxidic PtOx and NiOx species still remained at the NP surface, and, irrespective of the initial sample pretreatment, Ni surface segregation was observed upon annealing in a vacuum at moderate temperature (T < 300 °C). Interestingly, a distinct atomic segregation trend was detected between 300 and 500 °C for the sample pre-exposed to CO; namely, Ni surface segregation was partially hindered. This might be attributed to the higher bonding energy of CO to Pt as compared to Ni. Annealing in the presence of 1 bar CO also resulted in the initial surface segregation of Ni (T < 400 °C) as long as PtOx and NiOx species were available on the surface as a result of the higher affinity of Ni for oxygen. Above 500 °C, and regardless of the sample pretreatment, the diffusion of Pt atoms to the NP surface and the formation of a Ni-Pt alloy are observed. © 2015 American Chemical Society.view abstract doi: 10.1021/acsnano.5b01807 2015 • 120 Catalytic hydrodeoxygenation of guaiacol over platinum supported on metal oxides and zeolites
Hellinger, M. and Carvalho, H.W.P. and Baier, S. and Wang, D. and Kleist, W. and Grunwaldt, J.-D.
Applied Catalysis A: General 490 181-192 (2015)Hydrodeoxygenation of guaiacol over Pt-based catalysts was studied as a representative for phenolic compounds in pyrolysis oil. Screening of various Pt-based catalysts supported on different oxides and using different preparation methods resulted in 1%Pt/SiO2 and platinum supported on zeolites, such as 1% Pt/H-MFI-90, as the most promising catalysts in a temperature range up to 200 ° C. Thereby conversions of 86% and 100% were received, respectively. Particularly, selectivities to cyclohexane above 90% were achieved for 1% Pt/H-MFI-90. X-ray absorption near edge structure (XANES) uncovered that mild reduction temperatures were sufficient for the reduction of 1%Pt/SiO2 (up to 150°C) and 1%Pt/H-MFI-90 (up to 40°C) while 1%Pt/Al2O3 required a higher temperature of at least 320 °C. The average particle size obtained for Pt/SiO2 was 2-3 nm as unraveled by scanning transmission electron microscopy (STEM) and extended X-ray absorption fine structure (EXAFS). The deoxygenation ability of the catalysts was improved if the Pt particles were deposited on an acidic H-MFI zeolite (>130 μmol acid sites per gram) as support. 1%Pt/SiO2 showed the highest selectivity towards deoxygenation at 50 °C, whereas for 1% Pt/H-MFI-90 temperatures of about 150 °C were required to achieve a high selectivity to cyclohexane. For the latter catalyst a longer reaction time was beneficial to maximize the selectivity towards cyclohexane. The hydrogen pressure did not have significant influence on the reaction rate. The results are in agreement with a hydrodeoxygenation mechanism over Pt/zeolite catalysts at temperatures up to 200 °C that comprises hydrogenation in the first step and acid catalyzed dehydration combined with hydrogenation in the second step. © 2014 Elsevier B.V. All rights reserved.view abstract doi: 10.1016/j.apcata.2014.10.043 2015 • 119 Detection of 2D phase transitions at the electrode/electrolyte interface using electrochemical impedance spectroscopy
Tymoczko, J. and Colic, V. and Bandarenka, A.S. and Schuhmann, W.
Surface Science 631 81-87 (2015)The capacitance of the electric double layer, CDL, formed at the electrode/electrolyte interface is generally determined by electrochemical impedance spectroscopy (EIS). However, CDL values obtained using EIS data often depend on the ac frequency of the potential perturbation used in EIS. The reasons for the observed frequency dispersions can be various, and hence extracting valuable information about the status of the electrified interface is not possible with the required certainty. In this work, using well-understood electrochemical systems, namely Pt(111) electrodes in contact with a series of acidic sulfate ions containing electrolytes, we provide strong evidence that 2D phase transitions in the adsorbate layers and, in general, structural effects at the electrode/electrolyte interface are in many cases responsible for the frequency dispersion of the double layer capacitance. These empirical findings open new opportunities for the detection and evaluation of 2D phase transition processes and other structural effects using EIS, even in presence of simultaneously occurring electrochemical processes. However, further theoretical elaboration of this effect is necessary. © 2014 Elsevier B.V. All rights reserved.view abstract doi: 10.1016/j.susc.2014.04.014 2015 • 118 Effect of substitution on elastic stability, electronic structure and magnetic property of Ni-Mn based Heusler alloys: An ab initio comparison
Roy, T. and Gruner, M.E. and Entel, P. and Chakrabarti, A.
Journal of Alloys and Compounds 632 822-829 (2015)First-principles density functional theory based calculations have been used to predict the bulk mechanical properties of magnetic shape memory Heusler alloy Ni2MnGa substituted by copper (Cu), platinum (Pt), palladium (Pd) and manganese (Mn) at the Ni site. The elastic constants of Ni2MnGa alloy with and without substitution are calculated. We analyze and compare in detail the bulk mechanical properties for these alloys, in particular, the ratio between the calculated bulk and shear modulii, as well as the Poisson's ratio and Young's modulii. This analysis further based on an empirical relation, indicates that Pt2MnGa may inherently be the least brittle material, among the above-mentioned alloys. Interesting difference has been observed between the shear modulii calculated from Voigt's and Reuss's method. This has been explained in terms of the values of the tetragonal shear constant C′ of the materials. Study of Heisenberg exchange coupling parameters and Curie temperature as well as density of states of the materials shows the effect of substitution at the Ni site on the magnetic and electronic properties, respectively. © 2015 Elsevier B.V. All rights reserved.view abstract doi: 10.1016/j.jallcom.2015.01.255 2015 • 117 Energy transfer in argon atom - Surface interactions studied by Pt-SiO2-Si thin film chemoelectronic devices
Scheele, M. and Nedrygailov, I.I. and Hasselbrink, E. and Diesing, D.
Vacuum 111 137-141 (2015)The energy transferred from a heated platinum surface to an adjacent argon gas of several mbar pressure is studied. The cooling effects during accommodation of the argon atoms when colliding with the surface can be monitored by current changes in a chemoelectronic device, in this case consisting of silicon, silicon-oxide and platinum. A numerical heat flow model and the experimentally observed temperature dependence of the conduction in the device show that the observed signals are due to a cooling of the whole device. Temperature gradients and accompanying thermoelectric effects play only a minor role. © 2014 Published by Elsevier Ltd.view abstract doi: 10.1016/j.vacuum.2014.09.017 2015 • 116 Enhanced stability of multilayer graphene-supported catalysts for polymer electrolyte membrane fuel cell cathodes
Marinkas, A. and Hempelmann, R. and Heinzel, A. and Peinecke, V. and Radev, I. and Natter, H.
Journal of Power Sources 295 79-91 (2015)Abstract One of the biggest challenges in the field of polymer electrolyte membrane fuel cells (PEMFC) is to enhance the lifetime and the long-term stability of PEMFC electrodes, especially of cathodes, furthermore, to reduce their platinum loading, which could lead to a cost reduction for efficient PEMFCs. These demands could be achieved with a new catalyst support architecture consisting of a composite of carbon structures with significant different morphologies. A highly porous cathode catalyst support layer is prepared by addition of various carbon types (carbon black particles, multi-walled carbon nanotubes (MWCNT)) to multilayer graphene (MLG). The reported optimized cathodes shows extremely high durability and similar performance to commercial standard cathodes but with 89% lower Pt loading. The accelerated aging protocol (AAP) on the membrane electrode assemblies (MEA) shows that the presence of MLG increases drastically the durability and the Pt-extended electrochemical surface area (ECSA). In fact, after the AAP slightly enhanced performance can be observed for the MLG-containing cathodes instead of a performance loss, which is typical for the commercial carbon-based cathodes. Furthermore, the presence of MLG drastically decreases the ECSA loss rate. The MLG-containing cathodes show up to 6.8 times higher mass-normalized Pt-extended ECSA compared to the commercial standard systems. © 2015 Elsevier B.V.view abstract doi: 10.1016/j.jpowsour.2015.06.126 2015 • 115 Evaluation of the Electrochemical Stability of Model Cu-Pt(111) Near-Surface Alloy Catalysts
Tymoczko, J. and Calle-Vallejo, F. and Čolić, V. and Schuhmann, W. and Bandarenka, A.S.
Electrochimica Acta 179 469-474 (2015)Better understanding of the factors responsible for the long-term stability of electrocatalysts is of increasing importance for the development of new generations of efficient electrode materials relevant for sustainable energy provision. Therefore, experiments with model, often single-crystal catalytic surfaces are of significance for fundamental electrochemistry and technological applications. Among model electrocatalysts, near-surface alloys (NSAs) of Pt with Cu, Ni and other metals formed via electrochemical deposition and thermal annealing have shown remarkable properties, demonstrating high activity towards a number of important reactions, including the oxygen reduction reaction (ORR) and CO oxidation. However, relatively little is known about the electrochemical stability and mechanisms of degradation of model NSAs. In this work, we employ a simple electrochemical approach, supported by density functional theory calculations, to evaluate the stability of Cu-Pt(111) NSAs in 0.1 M HClO4. Our results show that ∼30% of the Cu atoms initially incorporated into the second atomic layer of Pt are lost within the first 2000 cycles performed between 0.05 V and 1.0 V (RHE). After 5000 cycles, ca. half of the Cu atoms initially placed in the second atomic layer still remained in the subsurface region. The dissolution of Cu has a substantial impact on the measured shift in the average OH-binding energy for the catalyst surface and, consequently, on the ORR activity. Interestingly, after dissolution of Cu from NSAs, voltammetric features, which are characteristic to the Pt(111) facets, are partially restored suggesting the formation of NSA and Pt(111) domains in the resulting surface. © 2015 Elsevier Ltd. All rights reserved.view abstract doi: 10.1016/j.electacta.2015.02.110 2015 • 114 Finding optimal surface sites on heterogeneous catalysts by counting nearest neighbors
Calle-Vallejo, F. and Tymoczko, J. and Colic, V. and Vu, Q.H. and Pohl, M.D. and Morgenstern, K. and Loffreda, D. and Sautet, P. and Schuhmann, W. and Bandarenka, A.S.
Science 350 185-189 (2015)A good heterogeneous catalyst for a given chemical reaction very often has only one specific type of surface site that is catalytically active. Widespread methodologies such as Sabatier-type activity plots determine optimal adsorption energies to maximize catalytic activity, but these are difficult to use as guidelines to devise new catalysts. We introduce "coordination-activity plots" that predict the geometric structure of optimal active sites. The method is illustrated on the oxygen reduction reaction catalyzed by platinum. Sites with the same number of first-nearest neighbors as (111) terraces but with an increased number of second-nearest neighbors are predicted to have superior catalytic activity. We used this rationale to create highly active sites on platinum (111), without alloying and using three different affordable experimental methods.view abstract doi: 10.1126/science.aab3501 2015 • 113 General Method for the Synthesis of Hollow Mesoporous Carbon Spheres with Tunable Textural Properties
Mezzavilla, S. and Baldizzone, C. and Mayrhofer, K.J.J. and Schüth, F.
ACS Applied Materials and Interfaces 7 12914-12922 (2015)A versatile synthetic procedure to prepare hollow mesoporous carbon spheres (HMCS) is presented here. This approach is based on the deposition of a homogeneous hybrid polymer/silica composite shell on the outer surface of silica spheres through the surfactant-assisted simultaneous polycondensation of silica and polymer precursors in a colloidal suspension. Such composite materials can be further processed to give hollow mesoporous carbon spheres. The flexibility of this method allows for independent control of the morphological (i.e., core diameter and shell thickness) and textural features of the carbon spheres. In particular, it is demonstrated that the size of the pores within the mesoporous shell can be precisely tailored over an extended range (2-20 nm) by simply adjusting the reaction conditions. In a similar fashion, also the specific carbon surface area as well as the total shell porosity can be tuned. Most importantly, the textural features can be adjusted without affecting the dimension or the morphology of the spheres. The possibility to directly modify the shell textural properties by varying the synthetic parameters in a scalable process represents a distinct asset over the multistep hard-templating (nanocasting) routes. As an exemplary application, Pt nanoparticles were encapsulated in the mesoporous shell of HMCS. The resulting Pt@HMCS catalyst showed an enhanced stability during the oxygen reduction reaction, one of the most important reactions in electrocatalysis. This new synthetic procedure could allow the expansion, perhaps even beyond the lab-scale, of advanced carbon nanostructured supports for applications in catalysis. © 2015 American Chemical Society.view abstract doi: 10.1021/acsami.5b02580 2015 • 112 How electrophoretic deposition with ligand-free platinum nanoparticles affects contact angle
Heinemann, A. and Koenen, S. and Schwabe, K. and Rehbock, C. and Barcikowski, S.
Key Engineering Materials 654 218-223 (2015)Electrophoretic deposition of ligand-free platinum nanoparticles has been studied to elucidate how wettability, indicated by contact angle measurements, is linked to vital parameters of the electrophoretic deposition process. These parameters, namely the colloid concentration, electric field strength and deposition time, have been systematically varied in order to determine their influence on the contact angle. Additionally, scanning electron microscopy has been used to confirm the homogeneity of the achieved coatings. © (2015) Trans Tech Publications, Switzerland.view abstract doi: 10.4028/www.scientific.net/KEM.654.218 2015 • 111 Influence of the alkali metal cations on the activity of Pt(1 1 1) towards model electrocatalytic reactions in acidic sulfuric media
Tymoczko, J. and Colic, V. and Ganassin, A. and Schuhmann, W. and Bandarenka, A.S.
Catalysis Today 244 96-102 (2015)The impact of the alkali metal cations (Li+, Na+, K+, Rb+, Cs+) on the catalytic activity of Pt(1 1 1) electrodes towards model reactions (oxygen reduction, oxygen evolution, hydrogen evolution and hydrogen oxidation) in sulfuric acid has been evaluated. In contrast to essentially monotonic activity trends (i.e. from Li+ to Cs+) reported in the literature for alkaline media, the nature of the cations influences the activity of the Pt electrodes largely non-monotonously in the presence of SO4 2- ions. This is in certain cases due to the specifically adsorbing (bi)sulfate anions which make interactions between electrolyte components and reaction intermediates very complex. Surprisingly, the activity of the Pt(1 1 1) electrodes towards all investigated electrocatalytic reactions was substantially higher in Rb+ ions containing electrolytes. © 2014 Elsevier B.V. All rights reserved.view abstract doi: 10.1016/j.cattod.2014.07.007 2015 • 110 Light Induced H2 Evolution from a Biophotocathode Based on Photosystem 1 - Pt Nanoparticles Complexes Integrated in Solvated Redox Polymers Films
Zhao, F. and Conzuelo, F. and Hartmann, V. and Li, H. and Nowaczyk, M.M. and Plumeré, N. and Rögner, M. and Schuhmann, W.
Journal of Physical Chemistry B 119 13726-13731 (2015)We report on a biophotocathode based on photosystem 1 (PS1)-Pt nanoparticle complexes integrated in a redox hydrogel for photoelectrocatalytic H2 evolution at low overpotential. A poly(vinyl)imidazole Os(bispyridine)2Cl polymer serves as conducting matrix to shuttle the electrons from the electrode to the PS1-Pt complexes embedded within the hydrogel. Light induced charge separation at the PS1-Pt complexes results in the generation of photocurrents (4.8 ± 0.4 μA cm-2) when the biophotocathodes are exposed to anaerobic buffer solutions. Under these conditions, the protons are the sole possible electron acceptors, suggesting that the photocurrent generation is associated with H2 evolution. Direct evidence for the latter process is provided by monitoring the H2 production with a Pt microelectrode in scanning electrochemical microscopy configuration over the redox hydrogel film containing the PS1-Pt complexes under illumination. © 2015 American Chemical Society.view abstract doi: 10.1021/acs.jpcb.5b03511 2015 • 109 Oxygen-plasma-functionalized carbon nanotubes as supports for platinum-ruthenium catalysts applied in electrochemical methanol oxidation
Chetty, R. and Maniam, K.K. and Schuhmann, W. and Muhler, M.
ChemPlusChem 80 130-135 (2015)Multiwalled carbon nanotubes (CNTs) functionalized by oxygen plasma were used as a support for platinum-ruthenium nanoparticles for electrochemical methanol oxidation. The influence of plasma treatment time on the electrocatalytic activity was investigated by cyclic voltammetry, CO stripping voltammetry, and chronoamperometry. The electrocatalysts were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results showed that oxygen plasma treatment led to the formation of -CO and -COO groups on the CNT surface. Platinum-ruthenium nanoparticles dispersed with an optimum plasma treatment time of 30 min exhibited the maximum catalytic activity towards methanol oxidation. The rationale for the high catalytic activity is discussed. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.view abstract doi: 10.1002/cplu.201402192 2015 • 108 Shape-Selection of Thermodynamically Stabilized Colloidal Pd and Pt Nanoparticles Controlled via Support Effects
Ahmadi, M. and Behafarid, F. and Holse, C. and Nielsen, J.H. and Roldan Cuenya, B.
Journal of Physical Chemistry C 119 29178-29185 (2015)Colloidal chemistry, in combination with nanoparticle (NP)/support epitaxial interactions is used here to synthesize shape-selected and thermodynamically stable metallic NPs over a broad range of NP sizes. The morphology of three-dimensional palladium and platinum NPs supported on TiO2(110) was investigated using scanning tunneling microscopy. Well-defined Pd and Pt NPs were synthesized via inverse micelle encapsulation. The initially spherical NPs were found to become faceted and form an epitaxial relationship with the support after high-temperature annealing (e.g., 1100 °C). Shape selection was achieved for almost all Pd NPs, namely, a truncated octahedron shape with (111) top and interfacial facets. The Pt NPs were however found to adopt a variety of shapes. The epitaxial relationship of the NPs with the support was evidenced by the alignment of the cluster's edges with TiO2(110)-[001] atomic rows and was found to be responsible for the shape control. The ability of synthesizing thermally stable shape-selected metal NPs demonstrated here is expected to be of relevance for applications in the field of catalysis, since the activity and selectivity of NP catalysts has been shown to strongly depend on the NP shape. © 2015 American Chemical Society.view abstract doi: 10.1021/acs.jpcc.5b09980 2015 • 107 Simulations of the Eutectic Transformations in the Platinum–Carbon System
Monas, A. and Bloembergen, P. and Dong, W. and Shchyglo, O. and Steinbach, I.
International Journal of Thermophysics 36 3366-3383 (2015)In this paper, we present the simulation of the eutectic phase transitions in the Pt–C system, in terms of both freezing and melting, using the multi-phase-field model. The experimentally obtained heat-extraction and -injection rates associated with the induction of freezing and melting are converted into the corresponding rates for microstructure-scale simulations. In spite of the extreme differences in the volume fractions of the FCC–Pt-rich phase on the one hand and graphite (C) on the other, satisfactory results for the kinetics of solidification and melting have been obtained, involving reasonable offsets in temperature, inducing freezing and melting, with respect to the equilibrium eutectic temperature. For freezing in the simulations, the needle/rod-like morphology, as experimentally observed, was reproduced for different heat extraction rates. The seemingly anomalous peak characterizing the simulated freezing curves is ascribed to the speed up of the solidification process due to the curvature effect. Similarly, a peak is observed in the experimental freezing curves, also showing up more clearly with increasing freezing rates. Melting was simulated starting from a frozen structure produced by a freezing simulation. The simulations reproduce the experimental melting curves and, together with the simulated freezing curves, help to understand the phase transition of the Pt–C eutectic. Finally, the effect of metallic impurities was studied. As shown for Au, impurities affect the morphology of the eutectic structure, their impact increasing with the impurity content, i.e., they can act as modifiers of the structure, as earlier reported for irregular eutectics. © 2015, Springer Science+Business Media New York.view abstract doi: 10.1007/s10765-015-1999-8 2015 • 106 Stability of Dealloyed Porous Pt/Ni Nanoparticles
Baldizzone, C. and Gan, L. and Hodnik, N. and Keeley, G.P. and Kostka, A. and Heggen, M. and Strasser, P. and Mayrhofer, K.J.J.
ACS Catalysis 5 5000-5007 (2015)We provide a comprehensive durability assessment dedicated to a promising class of electrocatalysts for the oxygen reduction reaction (i.e., porous platinum nanoparticles). The stability of these nanoengineered open structures is tested under two accelerated degradation test conditions (ADT), particularly selected to mimic the potential regimes experienced by the catalyst during the operative life of a fuel cell (i.e., load cycles (up to 1.0 V<inf>RHE</inf>) and start-up cycles (up to 1.4 V<inf>RHE</inf>)). To understand the evolution of the electrochemical performance, the catalyst properties are investigated by means of fundamental rotating disc electrode studies, identical location-transmission electron microscopy (IL-TEM) coupled with electron energy loss spectroscopy chemical mapping (IL-EELS), and post-use chemical analysis and online highly sensitive potential resolved dissolution concentration monitoring by scanning flow cell inductively coupled plasma-mass spectrometry (SFC-ICP-MS). The experimental results on the nanoporous Pt revealed distinctive degradation mechanisms that could potentially affect a wide range of other nanoengineered open structures. The study concludes that, although providing promising activity performance, under the relevant operational conditions of fuel cells, the nanoporosity is only metastable and subjected to a progressive reorganization toward the minimization of the nanoscale curvature. The rate and pathways of this specific degradation mechanism together with other well-known degradation mechanisms like carbon corrosion and platinum dissolution are strongly dependent on the selected upper limit potential, leading to distinctly different durability performance. © 2015 American Chemical Society.view abstract doi: 10.1021/acscatal.5b01151 2015 • 105 Sterically Hindered Luminescent PtII-Phosphite Complexes for Electroluminescent Devices
Mydlak, M. and Yang, C.-H. and Polo, F. and Galstyan, A. and Daniliuc, C.G. and Felicetti, M. and Leonhardt, J. and Strassert, C.A. and De Cola, L.
Chemistry - A European Journal 21 5161-5172 (2015)PtII complexes with one bulky, sterically demanding, tertiary phosphite ancillary ligand and a coordinating chromophore are herein presented. The phosphite ligand, tris(2,4-di-tert-butylphenyl) acts as a bidentate ligand coordinating the platinum ion through the central phosphorus atom and a cyclometalating carbon atom of one of the substituents. The two free phenoxy moieties lie above and below the coordination plane, leading to steric hindrance that avoids aggregation and provides solubility in organic solvents. The other two coordination sites on the central metal ion are occupied by a chromophoric ligand, which is responsible for the energy of the luminescent excited state. This separation of functions, on the two coordinated ligands, allows the use of a wider range of luminophores with good luminescent properties, maintaining the control of the intermolecular interactions with the non-chromophoric ligand. Based on this approach we were able to achieve a bright deep blue emission (λ= 444 nm, Φem=0.38) from a complex with a tailored ligand, which was then used for the fabrication of an electroluminescent device. In addition commercially available luminophores were also employed to synthesize green emitters. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.view abstract doi: 10.1002/chem.201405839 2014 • 104 Ab Initio Predicted Impact of Pt on Phase Stabilities in Ni-Mn-Ga Heusler Alloys
Dutta, B. and Hickel, T. and Entel, P. and Neugebauer, J.
Journal of Phase Equilibria and Diffusion 35 695-700 (2014)The paper discusses the stabilization of the martensite in Ni2MnGa at finite temperatures that is caused by the substitution of Ni by Pt. For this purpose a recently developed ab initio based formalism employing density functional theory is applied. The free energies of the relevant austenite and martensite phases of Ni1.75Pt0.25MnGa are determined incorporating quasiharmonic phonons and fixed-spin magnons. In addition the dependence of the transition temperatures on the Pt concentration is investigated. Though our results are in qualitative agreement with estimates based on ground-state energies, they clearly demonstrate that a proper treatment of finite temperature contributions is important to predict the martensitic transition quantitatively. © 2014, ASM International.view abstract doi: 10.1007/s11669-014-0342-6 2014 • 103 Adsorption of Colloidal Platinum Nanoparticles to Supports: Charge Transfer and Effects of Electrostatic and Steric Interactions
Marzun, G. and Streich, C. and Jendrzej, S. and Barcikowski, S. and Wagener, P.
Langmuir 30 11928-11936 (2014)Adsorption of colloidal nanoparticles to surfaces and supports is a convenient approach to heterogeneous catalysts, polymer additives, or wastewater treatment. We investigated the adsorption efficiency of laser-generated and initially ligand-free platinum nanoparticles to TiO<inf>2</inf> supports as a function of pH, ionic strength, and ligand surface coverage. The nanoparticle adsorption is dominantly controlled by electrostatic interactions: if the pH of the suspension is between the isoelectric point of the nanoparticles and the support, nanoparticles are adsorbed and transfer a net charge to the support. This charge-driven adsorption is not affected by steric repulsion due to various ligands attached to the nanoparticle surface. In addition to electrostatic interactions, colloidal stability given by moderate ionic strengths and pH values above the isoelectric point of nanoparticles are prerequisites for colloidal deposition. (Chemical Equation Presented). © 2014 American Chemical Society.view abstract doi: 10.1021/la502588g 2014 • 102 Atomic imaging of carbon-supported Pt, Pt/Co, and Ir@Pt nanocatalysts by atom-probe tomography
Li, T. and Bagot, P.A.J. and Christian, E. and Theobald, B.R.C. and Sharman, J.D.B. and Ozkaya, D. and Moody, M.P. and Tsang, S.C.E. and Smith, G.D.W.
ACS Catalysis 4 695-702 (2014)Atom probe tomography (APT) has been used to characterize commercially prepared Pt, Pt/Co alloy, and Ir@Pt core-shell nanoparticles supported on high-surface-area carbon black. Concentration profiles and 3D atom maps revealing the detailed internal structures and compositions of Pt, Pt/Co alloy, and Ir@Pt core-shell particles have been generated, and the distribution of trace impurity elements, including Na and Cl, has been examined. The observation of retained Na on the support, especially in the Pt nanoparticle system, indicates a more rigorous washing procedure is required. In the Pt/Co alloyed carbon-supported nanoparticle system, a marked variation in both compositions and particle sizes is observed. In the case of Ir@Pt, significant intermixing of the Ir core and Pt shell atoms takes place, which would be very difficult to measure by other techniques. All such observations will likely impact the catalytic performance of these materials. We envisage that the single nanoparticle analysis capability of APT, providing atomic-scale structures and chemical mapping, can also act as a means of quality control, identifying differences in the final product compared with the intended specification. Although the catalytic activity of these nanoparticles was not part of current study, the detailed information offered by such studies will permit knowledge-based improvements in nanoscale catalyst preparation methods and will also provide new ways of investigating structure and activity relationships at the nanometer scale. © 2014 American Chemical Society.view abstract doi: 10.1021/cs401117e 2014 • 101 Carbon monoxide-assisted size confinement of bimetallic alloy nanoparticles
Cui, C. and Gan, L. and Neumann, M. and Heggen, M. and Roldan Cuenya, B. and Strasser, P.
Journal of the American Chemical Society 136 4813-4816 (2014)Colloid-based chemical synthesis methods of bimetallic alloy nanoparticles (NPs) provide good monodispersity, yet generally show a strong variation of the resulting mean particle size with alloy composition. This severely compromises accurate correlation between composition of alloy particles and their size-dependent properties. To address this issue, a general CO adsorption-assisted capping ligand-free solvothermal synthesis method is reported which provides homogeneous bimetallic NPs with almost perfectly constant particle size over an unusually wide compositional range. Using Pt-Ni alloy NPs as an example, we show that variation of the reaction temperature between 160 and 240 °C allows for precise control of the resulting alloy particle bulk composition between 15 and 70 atomic % Ni, coupled with a constant mean particle size of ∼4 nm. The size-confining and Ni content-controlling role of CO during the nucleation and growth processes are investigated and discussed. Data suggest that size-dependent CO surface chemisorption and reversible Ni-carbonyl formation are key factors for the achievement of a constant particle size and temperature-controlled Ni content. To demonstrate the usefulness of the independent control of size and composition, size-deconvoluted relations between composition and electrocatalytic properties are established. Refining earlier reports, we uncover intrinsic monotonic relations between catalytic activity and initial Ni content, as expected from theoretical considerations. © 2014 American Chemical Society.view abstract doi: 10.1021/ja4124658 2014 • 100 Carbon-based yolk-shell materials for fuel cell applications
Galeano, C. and Baldizzone, C. and Bongard, H. and Spliethoff, B. and Weidenthaler, C. and Meier, J.C. and Mayrhofer, K.J.J. and Schüth, F.
Advanced Functional Materials 24 220-232 (2014)The synthesis of yolk-shell catalysts, consisting of platinum or gold-platinum cores and graphitic carbon shells, and their electrocatalytic stabilities are described. Different encapsulation pathways for the metal nanoparticles are explored and optimized. Electrochemical studies of the optimized AuPt, @C catalyst revealed a high stability of the encapsulated metal particles. However, in order to reach full activity, several thousand potential cycles are required. After the electrochemical surface area is fully developed, the catalysts show exceptionally high stability, with almost no degradation over approximately 30 000 potential cycles between 0.4 and 1.4 VRHE. Encapsulation of noble metals in graphitic hollow shells by hard templating is explored as a means for stabilizing fuel cell catalysts. Small platinum particles can be encapsulated, but the achievable loading is too small. Encapsulation of Au-Pt yolk-shell particles allows higher loading, and with such cores, stable catalysts could be produced. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/adfm.201302239 2014 • 99 Characterisation of non-uniform functional surfaces: Towards linking basic surface properties with electrocatalytic activity
Maljusch, A. and Henry, J.B. and Tymoczko, J. and Bandarenka, A.S. and Schuhmann, W.
RSC Advances 4 1532-1537 (2014)Functional materials, particularly heterogeneous catalysts, are often non-uniform at a microscopic level making their detailed characterisation extremely complex. This complexity inhibits the design and implementation of novel functional materials as such characterisation is a key to understanding interfaces for heterogeneous catalysis. We demonstrate that a combination of Scanning Kelvin Probe (SKP) and Scanning Electrochemical Microscopy (SECM) experiments made over the same sample surface using an integrated SKP-SECM system provides a powerful and robust tool to link basic surface properties with the observed electrocatalytic activity. As the SKP-response can be accurately assessed using modern quantum chemical approaches to benchmark analytical signals for different surface structures with varying compositions, application of an integrated SKP-SECM system can offer valuable insight into the origin of the observed electrocatalytic activity. As model objects, we used Pt(111)-like thin films modified with sub-monolayer and monolayer amounts of Cu atoms located at the electrode surface and in the sub-surface region. The exact position of the Cu atoms relative to the topmost Pt layer greatly affects basic surface properties and governs the electrocatalytic activity of the surface towards various reactions, i.e. the oxygen reduction reaction. SKP-SECM appeared to be a very sensitive tool to monitor those changes as a function of the spatial coordinates. © 2014 The Royal Society of Chemistry.view abstract doi: 10.1039/c3ra45845h 2014 • 98 Composition-Dependent Oxygen Reduction Activity and Stability of Pt-Cu Thin Films
Schuppert, A.K. and Topalov, A.A. and Savan, A. and Ludwig, Al. and Mayrhofer, K.J.J.
ChemElectroChem 1 358-361 (2014)Catalyst considerations: Pt-Cu alloys are prepared as a thin-film material library with a composition gradient. By using a scanning flow cell coupled to on-line mass spectrometry, this library can be screened over to measure the activity towards the oxygen reduction reaction as well as the time-resolved dissolution of both alloy components in parallel. This results in comprehensive insights into the composition-dependent performance of the Pt-Cu system. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/celc.201300078 2014 • 97 Confined-space alloying of nanoparticles for the synthesis of efficient PtNi fuel-cell catalysts
Baldizzone, C. and Mezzavilla, S. and Carvalho, H.W.P. and Meier, J.C. and Schuppert, A.K. and Heggen, M. and Galeano, C. and Grunwaldt, J.-D. and Schüth, F. and Mayrhofer, K.J.J.
Angewandte Chemie - International Edition 53 14250-14254 (2014)The efficiency of polymer electrolyte membrane fuel cells is strongly depending on the electrocatalyst performance, that is, its activity and stability. We have designed a catalyst material that combines both, the high activity for the decisive cathodic oxygen reduction reaction associated with nanoscale Pt alloys, and the excellent durability of an advanced nano-structured support. Owing to the high specific activity and large active surface area, the catalyst shows extraordinary mass activity values of 1.0 AmgPt -1. Moreover, the material retains its initial active surface area and intrinsic activity during an extended accelerated aging test within the typical operation range. This excellent performance is achieved by confined space alloying of the nanoparticles in a controlled manner in the pores of the support. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/anie.201406812 2014 • 96 Dose-dependent surface endothelialization and biocompatibility of polyurethane noble metal nanocomposites
Hess, C. and Schwenke, A. and Wagener, P. and Franzka, S. and Laszlo Sajti, C. and Pflaum, M. and Wiegmann, B. and Haverich, A. and Barcikowski, S.
Journal of Biomedical Materials Research - Part A 102 1909-1920 (2014)Surface pre-endothelialization is a promising approach to improve the hemocompatibility of implants, medical devices, and artificial organs. To promote the adhesive property of thermoplastic polyurethane (TPU) for endothelial cells (ECs), up to 1 wt % of gold (Au) or platinum (Pt) nanoparticles, fabricated by pulsed laser ablation in polymer solution, were embedded into the polymer matrix. The analysis of these nanocomposites showed a homogenous dispersion of the nanoparticles, with average diameters of 7 nm for Au or 9 nm for Pt. A dose-dependent effect was found when ECs were seeded onto nanocomposites comprising different nanoparticle concentrations, resulting in a fivefold improvement of proliferation at 0.1 wt % nanoparticle load. This effect was associated with a nanoparticle concentration-dependent hydrophilicity and negative charge of the nanocomposite. In dynamic flow tests, nanocomposites containing 0.1 wt % Au or Pt nanoparticles allowed for the generation of a confluent and resistant EC layer. Real-time polymerase chain reaction quantification of specific markers for EC activation indicated that ECs cultivated on nanocomposites remain in an inactivated, nonthrombogenic and noninflammatory state; however, maintain the ability to trigger an inflammatory response upon stimulation. These findings were confirmed by a platelet and leukocyte adhesion assay. The results of this study suggest the possible applicability of TPU nanocomposites, containing 0.1 wt % Au or Pt nanoparticles, for the generation of pre-endothelialized surfaces of medical devices. © 2013 Wiley Periodicals, Inc.view abstract doi: 10.1002/jbm.a.34860 2014 • 95 Electrochemical nanoprobes for single-cell analysis
Actis, P. and Tokar, S. and Clausmeyer, J. and Babakinejad, B. and Mikhaleva, S. and Cornut, R. and Takahashi, Y. and López Córdoba, A. and Novak, P. and Shevchuck, A.I. and Dougan, J.A. and Kazarian, S.G. and Gorelkin, P.V. and...
ACS Nano 8 875-884 (2014)The measurement of key molecules in individual cells with minimal disruption to the biological milieu is the next frontier in single-cell analyses. Nanoscale devices are ideal analytical tools because of their small size and their potential for high spatial and temporal resolution recordings. Here, we report the fabrication of disk-shaped carbon nanoelectrodes whose radius can be precisely tuned within the range 5-200 nm. The functionalization of the nanoelectrode with platinum allowed the monitoring of oxygen consumption outside and inside a brain slice. Furthermore, we show that nanoelectrodes of this type can be used to impale individual cells to perform electrochemical measurements within the cell with minimal disruption to cell function. These nanoelectrodes can be fabricated combined with scanning ion conductance microscopy probes, which should allow high resolution electrochemical mapping of species on or in living cells. © 2013 American Chemical Society.view abstract doi: 10.1021/nn405612q 2014 • 94 Infrared emitting nanostructures for highly efficient microhotplates
Müller, L. and Käpplinger, I. and Biermann, S. and Brode, W. and Hoffmann, M.
Journal of Micromechanics and Microengineering 24 (2014)A highly emissive Si-based microhotplate based on self-organizing nanostructures is presented. The silicon was structured by a self-masking deep reactive ion etching process resulting in needle-like non-periodical microstructures. Evaporated platinum settles in a kind of glancing angle deposition as well-defined nanocrystals on the silicon microstructures. Finite-difference time-domain simulation allowed the evaluation of the ideal platinum thickness for maximized infrared absorption and emission. We measured the hemispherical spectral transmittance and reflectivity of the fabricated surfaces and found the hemispherical spectral absorbance to be up to 0.97 in the investigated wavelength range. To demonstrate the advantages of these micro-nano-structures, we present the fabrication and characterization of a thermal infrared hotplate-emitter. With integrated Pt-on-Si-needles, the emitter shows a 2.6 times higher IR emission without wavelength-dependent interference patterns as compared to an uncoated Si-based emitter at the same membrane temperature. © 2014 IOP Publishing Ltd.view abstract doi: 10.1088/0960-1317/24/3/035014 2014 • 93 Multimetallic aerogels by template-free self-assembly of Au, Ag, Pt, and Pd nanoparticles
Herrmann, A.-K. and Formanek, P. and Borchardt, L. and Klose, M. and Giebeler, L. and Eckert, J. and Kaskel, S. and Gaponik, N. and Eychmüller, A.
Chemistry of Materials 26 1074-1083 (2014)Nanostructured, porous metals are of great interest for material scientists since they combine high surface area, gas permeability, electrical conductivity, plasmonic behavior, and size-enhanced catalytic reactivity. Here we present the formation of multimetallic porous three-dimensional networks by a template-free self-assembly process. Nanochains are formed by the controlled coalescence of Au, Ag, Pt, and Pd nanoparticles in aqueous media, and their interconnection and interpenetration leads to the formation of a self-supporting network. The resulting noble-metal-gels are transformed into solid aerogels by the supercritical drying technique. Compared to previously reported results, the technique is facilitated by exclusion of additional destabilizers. Moreover, temperature control is demonstrated as a powerful tool, allowing acceleration of the gelation process as well as improvement of its reproducibility and applicability. Electron microscopy shows the nanostructuring of the network and its high porosity. XRD and EDX STEM are used to investigate the alloying behavior of the bimetallic aerogels and prove the control of the alloying state by temperature induced phase modifications. Furthermore, the resulting multimetallic aerogels show an extremely low relative density (<0.2%) and a very high surface area (>50 m2/g) compared to porous noble metals obtained by other approaches. Electrically conductive thin films as well as hybrid materials with organic polymers are depicted to underline the processability of the materials, which is a key factor regarding handling of the fragile structures and integration into device architectures. Owing to their exceptional and tunable properties, multimetallic aerogels are very promising materials for applications in heterogeneous catalysis and electrocatalysis, hydrogen storage, and sensor systems but also in surface enhanced Raman spectroscopy (SERS) and the preparation of transparent conductive substrates. © 2013 American Chemical Society.view abstract doi: 10.1021/cm4033258 2014 • 92 Oxygen-deficient titania as alternative support for Pt catalysts for the oxygen reduction reaction
Zhao, A. and Masa, J. and Xia, W.
Journal of Energy Chemistry 23 701-707 (2014)Insufficient electrochemical stability is a major challenge for carbon materials in oxygen reduction reaction (ORR) due to carbon corrosion and insufficient metal-support interactions. In this work, titania is explored as an alternative support for Pt catalysts. Oxygen deficient titania samples including TiO<inf>2-x</inf> and TiO<inf>2-x</inf>N<inf>y</inf> were obtained by thermal treatment of anatase TiO<inf>2</inf> under flowing H<inf>2</inf> and NH<inf>3</inf>, respectively. Pt nanoparticles were deposited on the titania by a modified ethylene glycol method. The samples were characterized by N<inf>2</inf>-physisorption, X-ray diffraction and X-ray photoelectron spectroscopy. The ORR activity and long-term stability of supported Pt catalysts were evaluated using linear sweep voltammetry and chronoamperometry in 0.1 mol/L HClO<inf>4</inf>. Pt/TiO<inf>2-x</inf> and Pt/TiO<inf>2-x</inf>N<inf>y</inf> showed higher ORR activities than Pt/TiO<inf>2</inf> as indicated by higher onset potentials. Oxygen deficiency in TiO<inf>2-x</inf> and TiO<inf>2-x</inf>N<inf>y</inf> contributed to the high ORR activity due to enhanced charge transfer, as disclosed by electrochemical impedance spectroscopy studies. Electrochemical stability studies revealed that Pt/TiO<inf>2-x</inf> exhibited a higher stability with a lower current decay rate than commercial Pt/C, which can be attributed to the stable oxide support and strong interaction between Pt nanoparticles and the oxygen-deficient TiO<inf>2-x</inf> support. © 2014 Dalian Institute of Chemical Physics, the Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.view abstract doi: 10.1016/S2095-4956(14)60202-3 2014 • 91 Platinum-cobalt bimetallic nanoparticles in hollow carbon nanospheres for hydrogenolysis of 5-hydroxymethylfurfural
Wang, G.-H. and Hilgert, J. and Richter, F.H. and Wang, F. and Bongard, H.-J. and Spliethoff, B. and Weidenthaler, C. and Schüth, F.
Nature Materials 13 293-300 (2014)The synthesis of 2,5-dimethylfuran (DMF) from 5-hydroxymethylfurfural (HMF) is a highly attractive route to a renewable fuel. However, achieving high yields in this reaction is a substantial challenge. Here it is described how PtCo bimetallic nanoparticles with diameters of 3.6 ± 0.7 nm can solve this problem. Over PtCo catalysts the conversion of HMF was 100% within 10 min and the yield to DMF reached 98% after 2 h, which substantially exceeds the best results reported in the literature. Moreover, the synthetic method can be generalized to other bimetallic nanoparticles encapsulated in hollow carbon spheres. © 2014 Macmillan Publishers Limited.view abstract doi: 10.1038/nmat3872 2014 • 90 Position of Cu atoms at the Pt(111) electrode surfaces controls electrosorption of (H)SO4 (2)- from H2SO4 electrolytes
Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
ChemElectroChem 1 (2014)Selective positioning of monolayer amounts of foreign atoms at the surface and subsurface regions of metal electrodes is a promising way to fine-tune the properties of the electrode/ electrolyte interface. The latter is critical as it largely governs the adsorption of electrolyte components and reaction intermediates and, therefore, controls many key electrocatalytic processes. Using model Pt(111) single-crystal electrodes, we demonstrate how the relative position of Cu atoms at the surface drastically changes the adsorption energies for (bi)sulfate anions. Our measurements involve pseudomorphic overlayers of Cu on Pt(111) as well as Cu-Pt(111) surface and sub-surface alloys, where Cu atoms were located either in the first or in the second atomic layers of Pt, respectively. In the case of Cu- Pt(111) surface alloys, specific adsorption of the anions starts earlier compared to the unmodified Pt(111) surface. In contrast, placing Cu atoms into the second atomic layer weakens the binding between the surface and the anions. Surprisingly, Cu pseudomorphic overlayers do not reveal any specific adsorption of (bi)sulfates (within the region of the overlayer stability). Taking into account that electrified interfaces between Pt(111) electrodes and sulfate-containing electrolytes often play the role of benchmark systems in fundamental physico-chemical and, particularly, electrocatalytic studies, our findings demonstrate a promising and relatively easy route of tuning the properties of these interfaces. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/celc.201300107 2014 • 89 Pressure-dependent effect of hydrogen adsorption on structural and electronic properties of Pt/γ-Al2O3 nanoparticles
Mistry, H. and Behafarid, F. and Bare, S.R. and Roldan Cuenya, B.
ChemCatChem 6 348-352 (2014)Understanding the interaction of hydrogen with subnanometer platinum nanoparticles (NPs) under industrially relevant conditions is of great importance to heterogeneous catalysis. In this work, we investigate the pressure-dependent changes in hydrogen coverage on size- and shape-selected Pt/γ-Al2O3 NPs by in situ X-ray absorption near-edge structure (XANES) analysis. Difference XANES calculations revealed an increase in the H/Pt ratio from 1.9 to 2.5 upon increasing the hydrogen pressure from 1 to 21 bar at room temperature (1 bar=100 kPa). In addition, extended X-ray absorption fine structure measurements of the local geometrical structure showed changes in Pt - Pt bond length and coordination number, revealing a morphological transformation in the NPs from a 2 D to a 3 D shape under increasing H2 pressure at room temperature. Such shape evolution leads to a decrease in the NP-support contact area and is thus expected to affect the NP stability against coarsening. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/cctc.201300783 2014 • 88 Role and evolution of nanoparticle structure and chemical state during the oxidation of NO over size- and shape-controlled Pt/γ-Al2O 3 catalysts under operando conditions
Lira, E. and Merte, L.R. and Behafarid, F. and Ono, L.K. and Zhang, L. and Roldan Cuenya, B.
ACS Catalysis 4 1875-1884 (2014)The structure and chemical state of size-selected Pt nanoparticles (NPs) supported on γ-Al2O3 were studied during the oxidation of NO using X-ray absorption near-edge spectroscopy and extended X-ray absorption fine-structure spectroscopy measurements under operando conditions. The data revealed the formation of PtOx species in the course of the reaction that remained present at the maximum temperature studied, 350 °C. The PtOx species were found in all samples, but those with the smallest NPs showed the highest degree of oxidation. Moreover, NO-induced nanoparticle redispersion was observed at temperatures below 150 °C for all catalysts studied. Catalytic tests showed activity toward the oxidation of NO for all samples. Nevertheless, the catalyst with the smallest NPs was found to be the least active, which is explained by a more extensive formation of PtOx species in this catalyst and their detrimental contribution to the oxidation of NO. © 2014 American Chemical Society.view abstract doi: 10.1021/cs500137r 2014 • 87 Shape-dependent catalytic oxidation of 2-butanol over Pt nanoparticles supported on γ-Al2O3
Mistry, H. and Behafarid, F. and Zhou, E. and Ono, L.K. and Zhang, L. and Roldan Cuenya, B.
ACS Catalysis 4 109-115 (2014)This study illustrates the effect of nanoparticle (NP) shape on the reactivity of size-selected Pt/γ-Al2O3 nanocatalysts for 2-butanol oxidation. Nanoparticles similar in size [transmission electron microscopy (TEM) diameter of ∼1 nm] but with different shapes were prepared via encapsulation in inverse micelles. The NP shape was resolved by combining information extracted from extended X-ray absorption fine structure spectroscopy (EXAFS) data, TEM, and modeling. A correlation was observed between the average first nearest neighbor coordination number of atoms at the NP surface and their catalytic activity. In particular, the NPs with the largest number of weakly coordinated surface atoms (i.e., edges and corners) were found to be the least active for the total oxidation of 2-butanol. This result highlights that not only size but also shape control must be achieved to tailor the catalytic properties of nanoscale materials. © 2013 American Chemical Society.view abstract doi: 10.1021/cs400888n 2013 • 86 Application of SECM in tracing of hydrogen peroxide at multicomponent non-noble electrocatalyst films for the oxygen reduction reaction
Dobrzeniecka, A. and Zeradjanin, A. and Masa, J. and Puschhof, A. and Stroka, J. and Kulesza, P.J. and Schuhmann, W.
Catalysis Today 202 55-62 (2013)The redox competition mode of scanning electrochemical microscopy (RC-SECM) was used to study the electrocatalytic activity of three different non-noble metal O2 reduction catalysts at a pH value of 7.4, namely; multi-walled carbon nanotubes (MWCNTs), cobalt protoporphyrin (CoP) and a composite of MWCNTs/CoP. The collection efficiency of a scanning electrochemical microscopy (SECM) tip for the H2O2 generated by the reduction of O2 at the catalyst layer was almost 100%. Consequently, SECM experiments in a combined redox competition and generator/collector mode could be applied for the determination of the number of electrons exchanged during O2 reduction, leading to improved understanding of the intrinsic features of catalyst activity. This approach avoids the typical limitations encountered with rotating ring disk electrode (RRDE) voltammetry, notably, the variation of the quantity of H2O2 in the proximity of the electrode with the speed of electrode rotation or the chemical decomposition of reaction intermediates on the Pt ring, which often introduce inconsistencies and errors in the measured values of the number of exchanged electrons. It is commonly assumed that the O2 reduction reaction on most non-noble metal catalysts proceeds via formation of H2O 2 as an intermediate. The follow-up reaction of H2O 2, typically chemical decomposition or electrochemical reduction, influences the overall number of electrons exchanged during O2 reduction. In this study, we have confirmed by comparing the rate of electrochemical reduction of H2O2 using rotating disk electrode (RDE) measurements with its rate of chemical decomposition studied using a positioned SECM tip, that for the MWCNTs/CoP catalyst, chemical decomposition is predominantly determining the overall number of exchanged electrons per O2 molecule. © 2012 Elsevier B.V.view abstract doi: 10.1016/j.cattod.2012.03.060 2013 • 85 Atomic engineering of platinum alloy surfaces
Li, T. and Bagot, P.A.J. and Marquis, E.A. and Edman Tsang, S.C. and Smith, G.D.W.
Ultramicroscopy 132 205-211 (2013)A major practical challenge in heterogeneous catalysis is to minimize the loading of expensive platinum group metals (PGMs) without degrading the overall catalytic efficiency. Gaining a thorough atomic-scale understanding of the chemical/structural changes occurring during catalyst manufacture/operation could potentially enable the design and production of "nano-engineered" catalysts, optimized for cost, stability and performance. In the present study, the oxidation behavior of a Pt-31 at% Pd alloy between 673-1073. K is investigated using atom probe tomography (APT). Over this range of temperatures, three markedly different chemical structures are observed near the surface of the alloy. At 673. K, the surface oxide formed is enriched with Pd, the concentration of which rises further following oxidation at 773. K. During oxidation at 873. K, a thick, stable oxide layer is formed on the surface with a stoichiometry of PdO, beneath which a Pd-depleted (Pt-rich) layer exists. Above 873. K, the surface composition switches to enrichment in Pt, with the Pt content increasing further with increasing oxidation temperature. This treatment suggests a route for tuning the surfaces of Pt-Pd nanoparticles to be either Pd-rich or Pt-rich, simply by adjusting the oxidation temperatures in order to form two different types of core-shell structures. In addition, comparison of the oxidation behavior of Pt-Pd with Pt-Rh and Pd-Rh alloys demonstrates markedly different trends under the same conditions for these three binary alloys. © 2012.view abstract doi: 10.1016/j.ultramic.2012.10.012 2013 • 84 Complex magnetic ordering as a driving mechanism of multifunctional properties of Heusler alloys from first principles
Entel, P. and Siewert, M. and Gruner, M.E. and Herper, H.C. and Comtesse, D. and Arróyave, R. and Singh, N. and Talapatra, A. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Albertini, F. and Righi, L. and Chernenko, V.A.
European Physical Journal B 86 (2013)First-principles calculations are used to study the structural, electronic and magnetic properties of (Pd, Pt)-Mn-Ni-(Ga, In, Sn, Sb) alloys, which display multifunctional properties like the magnetic shape-memory, magnetocaloric and exchange bias effect. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior arising from competing ferro- and antiferromagnetic interactions with increasing number of Mn excess atoms in the unit cell. This information allows to optimize, for example, the magnetocaloric effect by using the strong influence of compositional changes on the magnetic interactions. Thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. We present guidelines of how to improve the functional properties. For Pt-Ni-Mn-Ga alloys, a shape memory effect with 14% strain can be achieved in an external magnetic field. © 2013 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.view abstract doi: 10.1140/epjb/e2012-30936-9 2013 • 83 Correlating catalytic methanol oxidation with the structure and oxidation state of size-selected pt nanoparticles
Merte, L.R. and Ahmadi, M. and Behafarid, F. and Ono, L.K. and Lira, E. and Matos, J. and Li, L. and Yang, J.C. and Cuenya, B.R.
ACS Catalysis 3 1460-1468 (2013)We have investigated the structure and chemical state of size-selected platinum nanoparticles (NPs) prepared by micelle encapsulation and supported on γ-Al2O3 during the oxidation of methanol under oxygen-rich reaction conditions following both oxidative and reductive pretreatments. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectroscopy measurements reveal that in both cases, the catalyst is substantially oxidized under reaction conditions at room temperature and becomes partially reduced when the reactor temperature is raised to 50 C. Reactivity tests show that at low temperatures, the preoxidized catalyst, in which a larger degree of oxidation was observed, is more active than the prereduced catalyst. We conclude that the differences in reactivity can be linked to the formation and stabilization of distinct active oxide species during the pretreatment. © 2013 American Chemical Society.view abstract doi: 10.1021/cs400234h 2013 • 82 Direct methane oxidation over Pt-modified nitrogen-doped carbons
Soorholtz, M. and White, R.J. and Zimmermann, T. and Titirici, M.-M. and Antonietti, M. and Palkovits, R. and Schüth, F.
Chemical Communications 49 240-242 (2013)Nitrogen-doped carbons derived from biomass precursors were modified with Pt2+ and successfully tested as solid catalysts in the direct oxidation of methane in fuming sulfuric acid. Remarkably, the catalytic performance was found to be substantially better than the Pt-modified Covalent Triazine Framework (CTF) system previously reported, although deactivation is more pronounced for the biomass derived catalyst supports. © 2013 The Royal Society of Chemistry.view abstract doi: 10.1039/c2cc36232e 2013 • 81 Discrete and polymeric heteronuclear constructs derived from triangular 2,2′-bipyrazine complexes of cis-a2PtII (with a = NH3 or a2 = en)
Galstyan, A. and Sanz Miguel, P.J. and Weise, K. and Lippert, B.
Dalton Transactions 42 16151-16161 (2013)cis-[Pta2(H2O)2]2+ (with a = NH3 or a2 = en) and 2,2′-bipyrazine (2,2′-bpz) react in water preferentially to cyclic, triangular complexes of composition [{cis-Pta2(2,2′-bpz-N4,N4′)}3]6+ (a = NH3, 1a; a2 = en, 1b). In their C3-symmetric conformation (all bridging pyrazine rings adopt cisoid orientations with respect to the central C2-C2′ bond), 1a and 1b provide three pairs of N1,N1′ donor sites each, which are capable of chelating additional (hetero)metal ions. The latter can in turn bridge to other N1,N1′ sites in an intermolecular fashion or simply complete their coordination spheres with other ligands (water, anions). Four previously not observed structural variants of heteronuclear (PtII, CuII; PtII, Ag I; PtII, CdII) constructs have now been isolated by us and are reported here: a decanuclear Pt6Cu4 complex, [{(en)Pt(2,2′-bpz)}3]2Cu 4(H2O)6(NO3)20· 11H2O (2) with the appearance of a paddle-wheel, a dodecanuclear Pt6Cu6 capsule with a Cu2+ ion in its interior and additional Cu2+ ions in its periphery giving a total composition of [{(en)Pt(2,2′-bpz)}3]2Cu11(NO 3)34(H2O)18·3H2O (3), as well as two coordination polymers, {[{(en)Pt(2,2′-bpz)} 3]Cd2(H2O)7}(SO4) 5·{[Cd(H2O)6](SO4)} ·15H2O (4) and [{cis-(NH3)2Pt(2, 2′-bpz)}3]Ag(SiF6)3(BF 4)·7H2O (5). The structures of these four compounds are dominated by host-guest interactions between the triangular metal vases of 1a and 1b and anions, respectively, as well as hydrogen bonding involving anions and water molecules and electrostatics. In aqueous solution considerable dissociation of the heteronuclear compounds 2-5 into the triangular Pt complexes 1 and the heterometal ions takes place. Preliminary AFM (atomic force microscopy) studies reveal that the +6 cations of 1a have a strong tendency to interact with double-stranded DNA with formation of condensed DNA states. © 2013 The Royal Society of Chemistry.view abstract doi: 10.1039/c3dt51589c 2013 • 80 Electrochemically deposited Pd-Pt and Pd-Au codeposits on graphite electrodes for electrocatalytic H2O2 reduction
Nagaiah, T.C. and Schäfer, D. and Schuhmann, W. and Dimcheva, N.
Analytical Chemistry 85 7897-7903 (2013)Improved electrocatalytic activity and selectivity for the reduction of H2O2 were obtained by electrodepositing Pd-Pt and Pd-Au on spectrographic graphite from solutions containing salts of the two metals at varying ratio. The electrocatalytic activity of the resulting binary codeposits for H2O2 reduction was evaluated by means of the redox-competition mode of scanning electrochemical microscopy (SECM) and voltammetric methods. In a potential range from 0 to-600 mV (vs. Ag/AgCl/3 M KCl) at pH 7.0 in 0.1 M phosphate citrate buffer, the electrocatalytic activity of both Pd-Pt and Pd-Au codeposits was substantially improved as compared with the identically deposited single metals suggesting an electrocatalytic synergy of the codeposits. Pd-Pt and Pd-Au codeposits were characterized by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Codepositing with Au caused a change of hedgehog-like shaped Pd nanoparticles into cauliflower-like nanoparticles with the particle size decreasing with increasing Au concentration. Codepositing Pd with Pt caused the formation of oblong structures with the size initially increasing with increasing Pt content. However, the particle size decreases with further increase in Pt concentration. The improved electrocatalytic capability for H2O2 reduction of the Pd-Pt electrodeposits on graphite was further demonstrated by immobilizing glucose oxidase as a basis for the development of an interference-free amperometric glucose biosensor. © 2013 American Chemical Society.view abstract doi: 10.1021/ac401317y 2013 • 79 Graphene as catalyst support: The influences of carbon additives and catalyst preparation methods on the performance of PEM fuel cells
Marinkas, A. and Arena, F. and Mitzel, J. and Prinz, G.M. and Heinzel, A. and Peinecke, V. and Natter, H.
Carbon 58 139-150 (2013)The reduction of the platinum amount for efficient PEM (polymer electrolyte membrane) fuel cells was achieved by the use of graphene/carbon composites as catalyst support. The influences of the carbon support type and also of the catalyst preparation method on the fuel cell performance were investigated with electrochemical, spectroscopic and microscopic techniques. Using pure graphene supports the final catalyst layer consists of a dense and well orientated roof tile structure which causes strong mass transport limitations for fuels and products. Thus the catalysts efficiency and finally the fuel cell performance were reduced. The addition of different carbon additives like carbon black particles or multi-walled carbon nanotubes (MWCNT) destroys this structure and forms a porous layer which is very efficient for the mass transport. The network structure of the catalyst layer and therefore the performance depends on the amount and on the morphology of the carbon additives. Due to optimizing these parameters the platinum amount could be reduced by 37% compared to a commercial standard system. © 2013 Elsevier Ltd. All rights reserved.view abstract doi: 10.1016/j.carbon.2013.02.043 2013 • 78 Interaction effects and transport properties of Pt capped Co nanoparticles
Ludwig, Ar. and Agudo, L. and Eggeler, G. and Ludwig, Al. and Wieck, A.D. and Petracic, O.
Journal of Applied Physics 113 (2013)We studied the magnetic and transport properties of Co nanoparticles (NPs) being capped with varying amounts of Pt. Beside field and temperature dependent magnetization measurements, we performed δΜ measurements to study the magnetic interactions between the Co NPs. We observe a transition from demagnetizing towards magnetizing interactions between the particles for an increasing amount of Pt capping. Resistivity measurements show a crossover from giant magnetoresistance towards anisotropic magnetoresistance. © 2013 American Institute of Physics.view abstract doi: 10.1063/1.4789422 2013 • 77 Long-range segregation phenomena in shape-selected bimetallic nanoparticles: Chemical state effects
Ahmadi, M. and Behafarid, F. and Cui, C. and Strasser, P. and Cuenya, B.R.
ACS Nano 7 9195-9204 (2013)A study of the morphological and chemical stability of shape-selected octahedral Pt0.5Ni0.5 nanoparticles (NPs) supported on highly oriented pyrolytic graphite (HOPG) is presented. Ex situ atomic force microscopy (AFM) and in situ X-ray photoelectron spectroscopy (XPS) measurements were used to monitor the mobility of Pt0.5Ni0.5 NPs and to study long-range atomic segregation and alloy formation phenomena under vacuum, H2, and O2 environments. The chemical state of the NPs was found to play a pivotal role in their surface composition after different thermal treatments. In particular, for these ex situ synthesized NPs, Ni segregation to the NP surface was observed in all environments as long as PtOx species were present. In the presence of oxygen, an enhanced Ni surface segregation was observed at all temperatures. In contrast, in hydrogen and vacuum, the Ni outward segregation occurs only at low temperature (< 200-270 C), while PtOx species are still present. At higher temperatures, the reduction of the Pt oxide species results in Pt diffusion toward the NP surface and the formation of a Ni-Pt alloy. A consistent correlation between the NP surface composition and its electrocatalytic CO oxidation activity was established. © 2013 American Chemical Society.view abstract doi: 10.1021/nn403793a 2013 • 76 Polyoxometalate-stabilized, water dispersible Fe2Pt magnetic nanoparticles
Seemann, K.M. and Bauer, A. and Kindervater, J. and Meyer, M. and Besson, C. and Luysberg, M. and Durkin, P. and Pyckhout-Hintzen, W. and Budisa, N. and Georgii, R. and Schneider, C.M. and Kögerler, P.
Nanoscale 5 2511-2519 (2013)Magnetic Fe2Pt core-shell nanoparticles with 2 nm cores were synthesized with a monolayer coating of silicotungstate Keggin clusters. The core-shell composition is substantiated by structural analysis performed using high-resolution scanning transmission electron microscopy (HR-STEM) and small angle X-ray scattering (SAXS) in a liquid suspension. The molecular metal oxide cluster shell introduces an enhanced dispersibility of the magnetic Fe-Pt core-shell nanoparticles in aqueous media and thereby opens up new routes to nanoparticle bio-functionalization, for example, using pre-functionalized polyoxometalates. © 2013 The Royal Society of Chemistry.view abstract doi: 10.1039/c3nr33374d 2013 • 75 Preparation of thin film Cu-Pt(1 1 1) near-surface alloys: One small step towards up-scaling model single crystal surfaces
Henry, J.B. and Maljusch, A. and Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
Electrochimica Acta 112 887-893 (2013)A method for the preparation of Pt(1 1 1) like thin films and thin film Cu-Pt(1 1 1) near-surface alloys (where Cu is preferentially located in the subsurface region) is reported in detail. Cyclic voltammograms of the resultant Pt(1 1 1)-like thin films in 0.1 M HClO4 demonstrate characteristic "butterfly" peaks attributed to the disorder/order phase transition in the adsorbed*OH layer, typical for a large Pt(1 1 1) crystals. Modification of the film surface with a monolayer of Cu and subsequent annealing in a reducing Ar/H2(5%) atmosphere, the voltammograms resemble those obtained for Cu-Pt(1 1 1) near-surface alloys prepared on commercial bulk single crystals. This method shows promise for the fabrication of extended (1 1 1)-type alloy surfaces of Pt and its alloys and can additionally be used to up-scale model objects for wider industrial and laboratory applications. © 2012 Elsevier Ltd.view abstract doi: 10.1016/j.electacta.2012.11.139 2013 • 74 Pt-Cu alloys as catalysts for the oxygen reduction reaction - A thin-film study of activity and stability
Schuppert, A.K. and Topalov, A.A. and Savan, A. and Ludwig, Al. and Mayrhofer, K.J.J.
ECS Transactions 58 587-592 (2013)Critical factors for the commercial application of fuel cells are the high costs and the limited stability of Pt catalysts. In order to improve the activity and material efficiency, Pt-alloys with nonnoble metals play an essential role. However, stability remains a critical factor for this type of catalysts. In order to understand the dissolution of Pt-alloys and eventually improve their performance, we therefore analyze a Pt-Cu thin-film alloy with varying composition using a combinatorial screening approach coupled to online analytics. © The Electrochemical Society.view abstract doi: 10.1149/05801.0587ecst 2013 • 73 Structural mimicking of inorganic catalyst supports with polydivinylbenzene to improve performance in the selective aerobic oxidation of ethanol and glycerol in water
Richter, F.H. and Meng, Y. and Klasen, T. and Sahraoui, L. and Schüth, F.
Journal of Catalysis 308 341-351 (2013)Many forms of polymers have been prepared and studied as polymeric catalyst support for metal nanoparticles and solid acid catalysts. The nanocasted mesoporous polydivinylbenzene (PDVB)-supported platinum catalysts presented here are distinguished by their customized mesoporosity and bulk morphology that are comparable to typical carbon-and alumina-supported powdered catalysts. Platinum nanoparticles are deposited on PDVB at loadings between 1 wt% and 9 wt% and a mean size between 2.7 nm and 6.2 nm, dependent on the synthesis method. Bifunctional catalysts containing platinum and acidic functionality are prepared by gas-phase sulfonation of the Pt/PDVB catalysts. The PDVB-supported catalysts are active for the oxidation of ethanol with molecular oxygen in water with up to 94% yield of acetic acid. In the analogous oxidation of glycerol, up to 60% yield of glyceric acid is reached with the bifunctional catalyst, and the polymer-supported catalysts feature lower formation of unidentified side products than Pt/C and Pt/Al2O3. Altogether, we find the polymers to be more active than the alumina and more selective than the carbon supports and thus overall have optimized performance. © 2013 Elsevier Ltd. All rights reserved.view abstract doi: 10.1016/j.jcat.2013.08.014 2013 • 72 The constant phase element reveals 2D phase transitions in adsorbate layers at the electrode/electrolyte interfaces
Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
Electrochemistry Communications 27 42-45 (2013)Using one of the most understood and well-characterized electrochemical systems, Pt(111) surface in contact with H2SO4, we provide evidences that specific adsorption, 2D phase transitions in the adsorbate layers and, in general, structural effects in the double layer are largely responsible for the so-called frequency dispersion of the double layer. The results also show promise that parameters of the constant phase element (which is used in impedance spectroscopy to account for the frequency dispersion) obtained as a function of the electrode potential can be reasonably used to detect 2D phase transitions at the electrode/electrolyte interfaces. This would provide a better insight into the interface, increasing the impact of measurements made by electrochemical impedance spectroscopy. © 2012 Elsevier B.V. All rights reserved. All rights reserved.view abstract doi: 10.1016/j.elecom.2012.11.001 2013 • 71 The influence of the residual growth catalyst in functionalized carbon nanotubes on supported Pt nanoparticles applied in selective olefin hydrogenation
Chen, P. and Chew, L.M. and Xia, W.
Journal of Catalysis 307 84-93 (2013)The influence of the residual growth catalyst on the reducibility and catalytic activity of Pt nanoparticles supported on oxygen- and nitrogen-functionalized CNTs (OCNTs and NCNTs) was systematically investigated. It was found that the presence of the residual growth catalyst significantly influenced the oxygen and nitrogen functionalization of CNTs, which consequently altered the reducibility of the supported Pt nanoparticles. Pt nanoparticles on NCNTs showed a higher stability against sintering in reducing atmosphere at 200 C and 400 C than those on OCNTs. On NCNTs, Pt was in a higher oxidation state and was not as easily reducible as on OCNTs. In hydrogenation catalysis, removing the residual growth catalyst is essential for the supported Pt catalyst to achieve a better performance. Compared with Pt on OCNTs, Pt on NCNTs was less active, but more selective in olefin hydrogenation due to the poisoning effect of the surface nitrogen species. © 2013 Elsevier Inc. All rights reserved.view abstract doi: 10.1016/j.jcat.2013.06.030 2013 • 70 Towards the understanding of sintering phenomena at the nanoscale: Geometric and environmental effects
Behafarid, F. and Roldan Cuenya, B.
Topics in Catalysis 56 1542-1559 (2013)One of the technologically most important requirements for the application of supported metal nanoparticles (NPs) to the field of heterogeneous catalysis is the achievement of thermally and chemically stable systems under reaction conditions. For this purpose, a thorough understanding of the different pathways underlying coarsening phenomena is needed. In particular, in depth knowledge must be achieved on the role of the NP synthesis method, geometrical features of the NPs (size and shape), initial NP dispersion on the support (interparticle distance), support pre-treatment (affecting its morphology and chemical state), and reaction environment (gaseous or liquid medium, pressure, temperature). This study provides examples of the stability and sintering behavior of nanoscale systems monitored ex situ, in situ, and under operando conditions via transmission electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray absorption fine-structure spectroscopy. Experimental data corresponding to physical-vapor-deposited and micelle-synthesized metal (Pt, Au) NPs supported on TiO<inf>2</inf>, SiO<inf>2</inf> and Al<inf>2</inf>O <inf>3</inf> will be used to illustrate Ostwald-ripening and diffusion coalescence processes. In addition, the role of the annealing environment (H<inf>2</inf>, O<inf>2</inf>, water vapor) on the stability of NPs will be discussed. © 2013 Springer Science+Business Media New York.view abstract doi: 10.1007/s11244-013-0149-4 2012 • 69 A first-principles investigation of the compositional dependent properties of magnetic shape memory heusler alloys
Siewert, M. and Gruner, M.E. and Hucht, A. and Herper, H.C. and Dannenberg, A. and Chakrabarti, A. and Singh, N. and Arróyave, R. and Entel, P.
Advanced Engineering Materials 14 530-546 (2012)The interplay of structural and magnetic properties of magnetic shape memory alloys is closely related to their composition. In this study the influence of the valence electron concentration on the tetragonal transformation in Ni 2Mn 1 + xZ 1 - x (Z = Ga, In, Sn, Sb) and Co 2Ni 1 + xGa 1 - x is investigated by means of ab initio calculations. While the type of magnetic interaction is different for the two series, the trends of the total energy changes under a tetragonal transformation are very similar. We find that tetragonal structures become energetically preferred with respect to the cubic one as the valence electron concentration e/a is increased regardless of the system under consideration. In particular, the energy difference between the austenite and martensite structures increases linearly with e/a, which is in part responsible for the linear increase of the matensite transformation temperature. The substitution of nickel by platinum increases even further the transformation temperature. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/adem.201200063 2012 • 68 A quick method for the preparation of Pt(111)-like thin films
Maljusch, A. and Henry, J.B. and Schuhmann, W. and Bondarenko, A.S.
Electrochemistry Communications 16 88-91 (2012)A simple and quick method for forming Pt(111)-like thin films on Si/Ti substrates for electrochemical and/or electrocatalytic experiments is reported. This method involves physical vapour deposition followed by flame annealing, electrochemical cleaning and a short heat treatment under a controlled atmosphere. Careful selection of the substrate, surface preparation and cooling atmosphere allows production of Pt thin films which show voltammetry features typical of large Pt(111) single crystal electrodes in 0.1 M HClO 4. This technique promises a method for the production of Pt(111) type surfaces on a larger scale. © 2011 Elsevier B.V. All rights reserved.view abstract doi: 10.1016/j.elecom.2011.12.004 2012 • 67 Characterization of oxidation and reduction of Pt-Ru and Pt-Rh-Ru alloys by atom probe tomography and comparison with Pt-Rh
Li, T. and Bagot, P.A.J. and Marquis, E.A. and Tsang, S.C.E. and Smith, G.D.W.
Journal of Physical Chemistry C 116 17633-17640 (2012)Pt-based alloys containing Rh and Ru are effective catalysts in a range of applications, including pollution control and low-temperature fuel cells. As the Pt group metals are generally rare and expensive, minimizing the loading of them while also increasing the efficiency of catalyst materials is a continual challenge in heterogeneous catalysis. A smart method to "nanoengineer" the surface of the nanocatalyst particles would greatly aid this goal. In our study, the oxidation of a Pt-8.9 at. % Ru alloy between 773 and 973 K and the oxidation and oxidation/reduction behavior of a Pt-23.9 at. % Rh-9.7 at. % Ru alloy at 873 K for various exposure times were studied using atom probe tomography. The surface of the Pt-Ru alloy is enriched with Ru after oxidation at 773 K, whereas it is depleted in Ru at 873 K, and at 973 K. The surface oxide layer vanishes at higher temperatures, leaving behind a Pt-rich surface. In the case of the Pt-Rh-Ru alloy, oxidation initiates from the grain boundaries, forming an oxide with a stoichiometry of MO 2. As the oxidation time increases, this oxide evolves into a twophase nanostructure, involving a Rh-rich oxide phase (Rh, Ru) 2O 3 and a Ru-rich oxide phase (Ru, Rh)O 2. When this two-phase oxide is reduced in hydrogen at low temperatures, separate Rh-rich and Ru-rich nanoscale regions remain. This process could, therefore, be useful for synthesizing complex island structures on Pt-Rh-Ru nanoparticle catalysts. © 2012 American Chemical Society.view abstract doi: 10.1021/jp304359m 2012 • 66 Coarsening phenomena of metal nanoparticles and the influence of the support pre-treatment: Pt/TiO 2(110)
Behafarid, F. and Roldan Cuenya, B.
Surface Science 606 908-918 (2012)One of the technologically most important requirements for the application of oxide-supported metal nanoparticles (NPs) in the fields of molecular electronics, plasmonics, and catalysis is the achievement of thermally stable systems. For this purpose, a thorough understanding of the different pathways underlying thermally-driven coarsening phenomena, and the effect of the nanoparticle synthesis method, support morphology, and degree of support reduction on NP sintering is needed. In this study, the sintering of supported metal NPs has been monitored via scanning tunneling microscopy combined with simulations following the Ostwald ripening and diffusion-coalescence models. Modifications were introduced to the diffusion-coalescence model to incorporate the correct temperature dependence and energetics. Such methods were applied to describe coarsening phenomena of physical-vapor deposited (PVD) and micellar Pt NPs supported on TiO 2(110). The TiO 2(110) substrates were exposed to different pre-treatments, leading to reduced, oxidized and polymer-modified TiO 2 surfaces. Such pre-treatments were found to affect the coarsening behavior of the NPs. No coarsening was observed for the micellar Pt NPs, maintaining their as-prepared size of ~ 3 nm after annealing in UHV at 1060 °C. Regardless of the initial substrate pre-treatment, the average size of the PVD-grown NPs was found to increase after identical thermal cycles, namely, from 0.5 ± 0.2 nm to 1.0 ± 0.3 nm for pristine TiO 2, and from 0.8 ± 0.3 nm to 1.3 ± 0.6 nm for polymer-coated TiO 2 after identical thermal treatments. Although no direct real-time in situ microscopic evidence is available to determine the dominant coarsening mechanism of the PVD NPs unequivocally, our simulations following the diffusion-coalescence coarsening route were in significantly better agreement with the experimental data as compared to those based on the Ostwald-ripening model. The enhanced thermal stability of the micellar NPs as compared to the PVD clusters might be related to their initial larger NP size, narrower size distribution, and larger interparticle distances. © 2012 Elsevier B.V. All rights reserved.view abstract doi: 10.1016/j.susc.2012.01.022 2012 • 65 Competition between ordering, twinning, and segregation in binary magnetic 3d-5d nanoparticles: A supercomputing perspective
Gruner, M.E. and Entel, P.
International Journal of Quantum Chemistry 112 277-288 (2012)The benefit of massively parallel supercomputers for technologically relevant applications in the field of materials science is demonstrated at the example of first-principles total energy calculations of magnetic binary transition metal nanoparticles containing up to 1415 3d and 5d transition metal atoms. The simulations, which take into account structural optimizations without symmetry constraints, reveal the size-dependent evolution of the energetic order of single crystalline and multiply twinned Fe-Pt nanoparticles up to 4 nm in diameter, which are discussed as promising building blocks for future ultra-high density data recording media. Although at small diameters, multiply twinned morphologies are preferred, we can show that an energetic crossover to a single crystalline, ordered arrangement can be expected at diameters around four nanometers. The comparison with Co-Pt indicates that the contributions of the interfaces in multiply twinned structures are of similar importance as the surface and cannot be neglected especially for small particle sizes. The results imply that for Co-Pt particles segregation of Pt to the surface and the formation of a Pt-depleted subsurface layer is also dominant for nanometer-sized single crystalline particles and may help to stabilize particles with partial L10 order, whereas for Fe-Pt multiple twinning is the most important equilibrium mechanism for small particle sizes. Hybrid combinations of the most favorable ordering motifs, that is, L10-type ordering in the particle core in combination with segregation in the outer shells, may thus lead to highly stable morphologies, which could dominate the growth process. © 2011 Wiley Periodicals, Inc.view abstract doi: 10.1002/qua.23254 2012 • 64 Complete and partial oxidation of methane on ceria/platinum silicon carbide nanocomposites
Frind, R. and Borchardt, L. and Kockrick, E. and Mammitzsch, L. and Petasch, U. and Herrmann, M. and Kaskel, S.
Catalysis Science and Technology 2 139-146 (2012)We have studied the catalytic activity of CeO 2/Pt-SiC composites in the total and partial oxidation as well as the dry reforming of methane. The composites were synthesized by an in situ functionalization strategy with variation in ceria and platinum contents and processing conditions. The impact of composition and pyrolysis temperature on the specific surface area and catalytic activity of the composite materials is studied. All catalysts have a high activity in the partial oxidation and dry reforming of methane close to the thermodynamic equilibrium composition. In the complete oxidation of methane, the T 10% was lowered by 443 K compared to the non-catalyzed reaction.view abstract doi: 10.1039/c1cy00311a 2012 • 63 Degradation mechanisms of Pt/C fuel cell catalysts under simulated start-stop conditions
Meier, J.C. and Galeano, C. and Katsounaros, I. and Topalov, A.A. and Kostka, A. and Schüth, F. and Mayrhofer, K.J.J.
ACS Catalysis 2 832-843 (2012)This manuscript investigates the degradation of a Pt/Vulcan fuel cell catalyst under simulated start-stop conditions in an electrochemical half-cell. Identical location transmission electron microscopy (IL-TEM) is used to visualize the several different degradation pathways occurring on the same catalyst material under potential cycling conditions. The complexity of degradation on the nanoscale leading to macroscopic active surface area lossis demonstrated and discussed. Namely, four different degradation pathways at one single Pt/Vulcan aggregate are clearly observed. Furthermore, inhomogeneous degradation behavior for different catalyst locations is shown, and trends in degradation mechanisms related to the platinum particle size are discussed in brief. Attention is drawn to the vast field of parameters influencing catalyst stability. We also present the development of a new technique to study changes of the catalyst not only with 2D projections of standard TEM images but also in 3D. For this purpose, identical location tomography (IL-tomography) is introduced, which visualizes the 3D structure of an identical catalyst location before and after degradation. © 2012 American Chemical Society.view abstract doi: 10.1021/cs300024h 2012 • 62 Electrochemical oxidation of size-selected pt nanoparticles studied using in situ high-energy-resolution X-ray absorption spectroscopy
Merte, L.R. and Behafarid, F. and Miller, D.J. and Friebel, D. and Cho, S. and Mbuga, F. and Sokaras, D. and Alonso-Mori, R. and Weng, T.-C. and Nordlund, D. and Nilsson, A. and Roldan Cuenya, B.
ACS Catalysis 2 2371-2376 (2012)High-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) has been applied to study the chemical state of ∼1.2 nm size-selected Pt nanoparticles (NPs) in an electrochemical environment under potential control. Spectral features due to chemisorbed hydrogen, chemisorbed O/OH, and platinum oxides can be distinguished with increasing potential. Pt electro-oxidation follows two competitive pathways involving both oxide formation and Pt dissolution. © 2012 American Chemical Society.view abstract doi: 10.1021/cs300494f 2012 • 61 Enhanced electrocatalytic stability of platinum nanoparticles supported on a nitrogen-doped composite of carbon nanotubes and mesoporous titania under oxygen reduction conditions
Masa, J. and Bordoloi, A. and Muhler, M. and Schuhmann, W. and Xia, W.
ChemSusChem 5 523-525 (2012)Cheers for titania: An N-doped composite of carbon nanotubes (CNTs) and mesoporous TiO 2 is used as support for Pt nanoparticles applied in the oxygen reduction reaction. The composite Pt/N-TiO 2-CNT shows a higher stability than Pt particles on carbon black or N-doped CNTs, as indicated by accelerated stress tests of up to 2000 cycles. The enhanced stability is attributed to strong interactions between TiO 2 and Pt and a higher corrosion resistance of TiO 2 as well as CNTs. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/cssc.201100643 2012 • 60 Formation of highly ordered alloy nanoparticles based on precursor-filled latex spheres
Manzke, A. and Plettl, A. and Wiedwald, U. and Han, L. and Ziemann, P. and Schreiber, E. and Ziener, U. and Vogel, N. and Weiss, C.K. and Landfester, K. and Fauth, K. and Biskupek, J. and Kaiser, U.
Chemistry of Materials 24 1048-1054 (2012)An experimental approach is presented, allowing the preparation of substrate supported, hexagonally arranged metallic alloy nanoparticles with narrow size distributions, well-defined interparticle distances, and controlled chemical composition. The method is based on miniemulsion polymerization and isotropic plasma etching. Polystyrene (PS) and poly(methyl methacrylate) (PMMA) colloids-in the present study containing Fe- and Pt-precursor complexes in a predefined ratio-are deposited onto hydrophilic Si/SiO 2 substrates by dip-coating, forming a highly ordered monolayer. Contrary to colloidal lithography, here, precursor-filled polystyrene colloids serve as carriers for the alloy forming elements. After reactive ion etching and annealing, hexagonally ordered arrays of crystalline FePt nanoparticles are formed exhibiting the desired 1:1 Fe-Pt ratio, as revealed by detailed analysis after each preparation step. Formation of stoichiometric binary alloy FePt nanoparticles is confirmed by determining magnetic hysteresis loops, as well as applying aberration-corrected high-resolution transmission electron microscopy. © 2012 American Chemical Society.view abstract doi: 10.1021/cm203241p 2012 • 59 How citrate ligands affect nanoparticle adsorption to microparticle supports
Wagener, P. and Schwenke, A. and Barcikowski, S.
Langmuir 28 6132-6140 (2012)Residual ligands from colloidal synthesis of nanoparticles influence adsorption of nanoparticles to supports and may complicate fabrication of nanoparticle-decorated microparticles. In this work, we studied the adsorption of completely ligand-free metal nanoparticles and controlled ligand-functionalized nanoparticles to chemically inert microparticle supports. Adsorption of ligand-free silver nanoparticles to barium sulfate microparticle supports is a quantitative, nonreversible process following Freundlich adsorption isotherm. However, adsorption efficiency is very sensitive to ligand concentration applied during laser-based synthesis of silver nanoparticles: exceeding a specific threshold concentration of 50 μmol/L citrate equal to a nanoparticle ligand surface coverage of about 50%, results in an almost complete prevention of nanoparticle adsorption because of electrosteric repulsion by ligand shell. Laser-based synthesis of nanoparticle-decorated microparticles is demonstrated with a variety of metal nanoparticles (Ag, Au, Pt, Fe) and supporting microparticles (calcium phosphate, titanium dioxide, barium sulfate) with application potential in heterogeneous catalysis or biomedicine where ligand control offers extra value, like enhanced catalytic activity or biocompatibility. © 2012 American Chemical Society.view abstract doi: 10.1021/la204839m 2012 • 58 Influence of Cs + and Na + on specific adsorption of *oH, *o, and *h at platinum in acidic sulfuric media
Berkes, B.B. and Inzelt, G. and Schuhmann, W. and Bondarenko, A.S.
Journal of Physical Chemistry C 116 10995-11003 (2012)The influence of Cs + and Na + on the adsorption of *H, *OH, and *O (where * denotes adsorbed species) at polycrystalline Pt in acidic sulfuric media has been investigated. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry, and electrochemical nanogravimetry were used (i) to elucidate the models of the interface between polycrystalline Pt and the electrolytes in a wide range of electrode potentials and (ii) to resolve contributions originating from adsorbed *H, (bi)sulfate, *OH, and *O as well as the cations to the overall interface status. Using impedance analysis it was possible to separate at least two adsorption processes: (bi)sulfate and hydrogen adsorption. The nanogravimetry additionally resolves the contribution from Cs +. Specific adsorption of Cs + at Pt surface significantly affects hydrogen adsorption, while it has almost no effect on the dynamics of SO 4 2- adsorption. Specifically adsorbed alkali cations, however, are desorbed by the onset of *OH(*O) adsorption at the Pt surface. Nevertheless, the cations likely remain in the close proximity to the surface, probably in the second H 2O layer, and largely contribute to the formation of the *OH and *O adsorbed species originating from the surface water. © 2012 American Chemical Society.view abstract doi: 10.1021/jp300863z 2012 • 57 Magnetic anomaly and dielectric tunability of (Sr,Mn)TiO 3 thin films
Tkach, A. and Okhay, O. and Wu, A. and Vilarinho, P.M. and Bedanta, S. and Shvartsman, V.V. and Borisov, P.
Ferroelectrics 426 274-281 (2012)Polycrystalline Sr 0.98Mn 0.02TiO 3 thin films are prepared by sol-gel spin-coating method on Si/SiO 2/TiO 2/Pt substrates. Their dielectric permittivity, polarization, and magnetization are investigated as a function of temperature (from 10 to 300 K), electric and magnetic fields. Temperature dependences of the magnetization M ZFC-FH(T), real part of the dielectric permittivity (T), and dissipation factor tan(T) of Sr 0.98Mn 0.02TiO 3 films show anomalies around 41-45 K, implying an interrelation between polar and magnetic order. Butterfly-like dc bias dependences of the real part of the dielectric permittivity (E dc), as well as slim polarization and magnetization hysteresis loops are observed, suggesting that Sr 0.98Mn 0.02TiO 3 thin films belong to multiglass systems. © Taylor & Francis Group, LLC.view abstract doi: 10.1080/00150193.2012.672038 2012 • 56 Nano pinstripes: TiO 2 nanostripe formation by nanoparticle-mediated pinning of step edges
Behafarid, F. and Cuenya, B.R.
Journal of Physical Chemistry Letters 3 608-612 (2012)The present scanning tunneling microscopy study describes the high-temperature growth of TiO 2 nanostripes with tunable width, orientation, and spacing, mediated by thermally stable micellar Pt and Au NPs deposited on TiO 2(110). This phenomenon could not be explained by spillover effects but is based on the preferential stabilization of [11̄0] step edges on TiO 2(110) by the metal NPs. Contrary to the behavior of physical-vapor-deposited NPs, which are known to move toward step edges upon annealing, our micellar NPs remain immobile up to 1000 °C. Instead, the mobility of TiO 2 step edges toward the micellar NPs, where they become stabilized, is observed. Our findings are relevant to the technological application of nanostructured materials in the fields of catalysis, molecular electronics, and plasmonics. © 2012 American Chemical Society.view abstract doi: 10.1021/jz300022c 2012 • 55 Noninvasive measurement and control of the temperature of Pt nanofilms on Si supports
Nedrygailov, I.I. and Hasselbrink, E. and Diesing, D. and Dasari, S.K. and Hashemian, M.A. and Karpov, E.G.
Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 30 (2012)A direct, noninvasive thermometry method based on the temperature dependence of the in-plane electrical resistance R(T) of 20 nm Pt films on Si-based semiconducting substrates is presented. At the calibration stage, the entire sample is slowly heated or cooled by external means. For moderately doped semiconductors, the R(T) dependence is closely linear at low temperatures (T 300 K), when the nanofilm metal conductance dominates, followed by a maximum and a subsequent decrease attributed to an increasing substrate conductance at higher temperatures. The position of the maximum depends on the Pt-substrate interface conditions, in particular, on the presence of an oxide layer, and may vary from 350 to 500 K. With the R(T) dependence measured in equilibrium one can derive T(R), which provides a highly accurate method for noninvasive measuring and controlling the temperature of the nanofilm with a direct resistive heating technique. Accuracy of the present method for dynamical measurement is shown to be significantly better than that of a standard approach using a Pt resistance temperature detector attached to the nanofilm for heating rates of 1-7 K/s. © 2012 American Vacuum Society.view abstract doi: 10.1116/1.3696973 2012 • 54 Rapid and surfactant-free synthesis of bimetallic Pt-Cu nanoparticles simply via ultrasound-assisted redox replacement
Sun, Z. and Masa, J. and Xia, W. and König, D. and Ludwig, Al. and Li, Z.-A. and Farle, M. and Schuhmann, W. and Muhler, M.
ACS Catalysis 2 1647-1653 (2012)The synthesis of bimetallic nanoparticles (NPs) with well-defined morphology and a size of <5 nm remains an ongoing challenge. Here, we developed a facile and efficient approach to the design of bimetallic nanostructures by the galvanic replacement reaction facilitated by high-intensity ultrasound (100 W, 20 kHz) at low temperatures. As a model system, Pt-Cu NPs deposited on nitrogen-doped carbon nanotubes (NCNTs) were synthesized and characterized by spectroscopic and microscopic techniques. Transmission electron microscopy (TEM) inspection shows that the mean diameter of Pt-Cu NPs can be as low as ≈2.8 nm, regardless of the much larger initial Cu particle size, and that a significant increase in particle number density by a factor of 35 had occurred during the replacement process. The concentration of the Pt precursor solution as well as of the size of the seed particles were found to control the size of the bimetallic NPs. Energy dispersive X-ray spectroscopy performed in the scanning TEM mode confirmed the alloyed nature of the Pt-Cu NPs. Electrochemical oxygen reduction measurements demonstrated that the resulting Pt-Cu/NCNT catalysts exhibit an approximately 2-fold enhancement in both mass- and area-related activities compared with a commercial Pt/C catalyst. © 2012 American Chemical Society.view abstract doi: 10.1021/cs300187z 2012 • 53 Scaling potential of local redox processes in memristive SrTiO 3 thin-film devices
Dittmann, R. and Muenstermann, R. and Krug, I. and Park, D. and Menke, T. and Mayer, J. and Besmehn, A. and Kronast, F. and Schneider, C. M. and Waser, R.
Proceedings of the IEEE 100 1979-1990 (2012)In this work, we address the following question: Where do the resistive switching processes take place in memristive thin-film devices of the single crystalline model material Fe-doped SrTiO 3? We compare resistive switching induced by the tip of the atomic force microscope on the bare thin-film surface with the switching properties observed in memristive devices with Pt top electrode. In order to close the gap between these two approaches, we combine conductive-tip atomic force microscopy with a delamination technique to remove the top electrode of Fe-doped SrTiO 3 metal-insulator-metal (MIM) structures to gain insight into the active switching interface with nanoscale lateral resolution. This enables us to prove the coexistence of a filamentary and area-dependent switching process with opposite switching polarities in the same sample. The spatially resolved analysis by transmission electron microscopy and photoelectron spectromicroscopy gives us some hints that the two switching types take place in device regions with different defect density and significant stoichiometry difference. © 1963-2012 IEEE.view abstract doi: 10.1109/JPROC.2012.2188771 2012 • 52 Stability of platinum nanoparticles supported on SiO2/Si(111): A high-pressure X-ray photoelectron spectroscopy study
Porsgaard, S. and Merte, L.R. and Ono, L.K. and Behafarid, F. and Matos, J. and Helveg, S. and Salmeron, M. and Roldan Cuenya, B. and Besenbacher, F.
ACS Nano 6 10743-10749 (2012)The stability of Pt nanoparticles (NPs) supported on ultrathin SiO 2 films on Si(111) was investigated in situ under H2 and O2 (0.5 Torr) by high-pressure X-ray photoelectron spectroscopy (HP-XPS) and ex situ by atomic force microscopy (AFM). No indication of sintering was observed up to 600 C in both reducing and oxidizing environments for size-selected Pt NPs synthesized by inverse micelle encapsulation. However, HP-XPS revealed a competing effect of volatile PtOx desorption from the Pt NPs (∼2 and ∼4 nm NP sizes) at temperatures above 450 C in the presence of 0.5 Torr of O2. Under oxidizing conditions, the entire NPs were oxidized, although with no indication of a PtO2 phase, with XPS binding energies better matching PtO. The stability of catalytic NPs in hydrogenation and oxidation reactions is of great importance due to the strong structure sensitivity observed in a number of catalytic processes of industrial relevance. An optimum must be found between the maximization of the surface active sites and metal loading (i.e., minimization of the NP size), combined with the maximization of their stability, which, as it will be shown here, is strongly dependent on the reaction environment. © 2012 American Chemical Society.view abstract doi: 10.1021/nn3040167 2012 • 51 Synthesis of an improved hierarchical carbon-fiber composite as a catalyst support for platinum and its application in electrocatalysis
Kundu, S. and Nagaiah, T.C. and Chen, X. and Xia, W. and Bron, M. and Schuhmann, W. and Muhler, M.
Carbon 50 4534-4542 (2012)A hierarchical carbon-fiber composite was synthesized based on carbon cloth (CC) modified with primary carbon microfibers (CMF) and subsequently secondary carbon nanotubes (CNT), thus forming a three-dimensional hierarchical structure with high BET surface area. The primary CMFs and the secondary CNTs are grown with electrodeposited iron nanoparticles as catalysts from methane and ethylene, respectively. After deposition of Pt nanoparticles by chemical vapor deposition from (trimethyl)cyclopentadienylplatinum, the resulting hierarchical composite was used as catalyst in the electrocatalytic oxygen reduction (oxygen reduction reaction, ORR) as specific test reaction. The modification of the CC with CMFs and CNTs improved the electrochemical properties of the carbon composite as revealed by electrochemical impedance measurements evidencing a low charge transfer resistance for redox mediators at the modified CC. X-ray photoelectron spectroscopy measurements were carried out to identify the chemical state and the surface atomic concentration of the Pt catalysts deposited on the hierarchical carbon composites. The ORR activity of Pt supported on different composites was investigated using rotating disk electrode measurements and scanning electrochemical microscopy. These electrochemical studies revealed that the obtained structured catalyst support is very promising for electrochemical applications, e.g. fuel cells. © 2012 Elsevier Ltd. All rights reserved.view abstract doi: 10.1016/j.carbon.2012.05.037 2012 • 50 Thin-film Cu-Pt(111) near-surface alloys: Active electrocatalysts for the oxygen reduction reaction
Henry, J.B. and Maljusch, A. and Huang, M. and Schuhmann, W. and Bondarenko, A.S.
ACS Catalysis 2 1457-1460 (2012)A simple method is presented for the formation of thin films of Cu-Pt(111) near-surface alloys (NSA). In these thin films, the solute metal (Cu) is preferentially located in the second platinum layer and protected by a Pt surface layer. The NSA-films act as active and fairly stable electrocatalysts for the reduction of oxygen with the activity and stability which approach those for bulk single crystalline Pt-alloy surfaces and ∼5 times more active than state-of-the-art Pt thin films. © 2012 American Chemical Society.view abstract doi: 10.1021/cs300165t 2012 • 49 Toward highly stable electrocatalysts via nanoparticle pore confinement
Galeano, C. and Meier, J.C. and Peinecke, V. and Bongard, H. and Katsounaros, I. and Topalov, A.A. and Lu, A. and Mayrhofer, K.J.J. and Schüth, F.
Journal of the American Chemical Society 134 20457-20465 (2012)The durability of electrode materials is a limiting parameter for many electrochemical energy conversion systems. In particular, electrocatalysts for the essential oxygen reduction reaction (ORR) present some of the most challenging instability issues shortening their practical lifetime. Here, we report a mesostructured graphitic carbon support, Hollow Graphitic Spheres (HGS) with a specific surface area exceeding 1000 m2 g-1 and precisely controlled pore structure, that was specifically developed to overcome the long-term catalyst degradation, while still sustaining high activity. The synthetic pathway leads to platinum nanoparticles of approximately 3 to 4 nm size encapsulated in the HGS pore structure that are stable at 850 C and, more importantly, during simulated accelerated electrochemical aging. Moreover, the high stability of the cathode electrocatalyst is also retained in a fully assembled polymer electrolyte membrane fuel cell (PEMFC). Identical location scanning and scanning transmission electron microscopy (IL-SEM and IL-STEM) conclusively proved that during electrochemical cycling the encapsulation significantly suppresses detachment and agglomeration of Pt nanoparticles, two of the major degradation mechanisms in fuel cell catalysts of this particle size. Thus, beyond providing an improved electrocatalyst, this study describes the blueprint for targeted improvement of fuel cell catalysts by design of the carbon support. © 2012 American Chemical Society.view abstract doi: 10.1021/ja308570c 2012 • 48 Transition metal loaded silicon carbide-derived carbons with enhanced catalytic properties
Borchardt, L. and Hasché, F. and Lohe, M.R. and Oschatz, M. and Schmidt, F. and Kockrick, E. and Ziegler, C. and Lescouet, T. and Bachmatiuk, A. and Büchner, B. and Farrusseng, D. and Strasser, P. and Kaskel, S.
Carbon 50 1861-1870 (2012)Carbide-derived carbons (CDC) with incorporated transition metal nanoparticles (∼2.5 nm) were prepared using a microemulsion approach. Time-consuming post synthesis functionalization of the carbon support material can thus be avoided and nanoparticle sizes can be controlled by changing the microemulsion composition. This synthesis strategy is a technique for the preparation of highly porous carbon materials with a catalytically active component. In particular we investigated the integration of ruthenium, palladium, and platinum in a concentration ranging from 4.45 to 12 wt.%. It was found that the transition metal has a considerable influence on sorption properties of resulting nanoparticle-CDC composite materials. Depending on the used metal salt additive the surface area and the pore volume ranges from 1480 m 2/g and 1.25 cm 3/g for Pt to 2480 m 2/g and 2.0 cm 3/g for Ru doped carbons. Moreover, members of this material class show impressive properties as heterogeneous catalysts. The liquid phase oxidation of tetralin and the partial oxidation of methane were studied, and electrochemical applications were also investigated. Primarily Pt doped CDCs are highly active in the oxygen reduction reaction, which is of great importance in present day fuel cell research. © 2012 Elsevier Ltd. All rights reserved.view abstract doi: 10.1016/j.carbon.2011.12.036 2011 • 47 A "directed" approach toward a cationic molecular square containing four isonicotinamidate ligands and (4 + 2) (en)PtII metal entities
Galstyan, A. and Sanz Miguel, P.J. and Lippert, B.
Inorganica Chimica Acta 374 453-460 (2011)The use of 4-cyanopyridine (4-CNpy) and 3-cyanopyridine (3-CNpy) as ditopic ligands with 180° and 120° directionalities, respectively, for the construction of molecular architectures with the 90° metal fragments (en)PtII and (en)PdII in water is hampered by the ease with which these ligands undergo hydrolysis to isonicotinamide (4-C(O)NH 2py) and nicotinamide (3-C(O)NH2py). As described in this article, out of six X-ray structurally characterized complexes (1-6), only a single one (1) reveal coordination of the unchanged ligand (4-CNpy) to (en)PtII. Nevertheless also the hydrolysis products are of interest in the context of obtaining discrete metallacyclic compounds: Thus, (en)Pt II and 4-C(O)NH2py form a hexanuclear complex, [PF 6{(en)Pt}6(4-C(O)NHpy)4](NO3) 7·10H2O (2), in which the anionic isonicotinamidate ligands function as tridentate, bridging ligands to produce a hybrid between a metallasquare and a 2-floor open box. The resulting cation with a +8 charge accommodates a single hexafluorophosphate anion in its interior. Adjacent cations of 2 pack in such a way as to develop Pt4 chains as typically seen in "platinum blues" and their [PtII]4 precursors. © 2011 Elsevier B.V. All rights reserved.view abstract doi: 10.1016/j.ica.2011.02.041 2011 • 46 A guideline for atomistic design and understanding of ultrahard nanomagnets
Antoniak, C. and Gruner, M.E. and Spasova, M. and Trunova, A.V. and Römer, F.M. and Warland, A. and Krumme, B. and Fauth, K. and Sun, S. and Entel, P. and Farle, M. and Wende, H.
Nature Communications 2 (2011)Magnetic nanoparticles are of immense current interest because of their possible use in biomedical and technological applications. Here we demonstrate that the large magnetic anisotropy of FePt nanoparticles can be significantly modified by surface design. We employ X-ray absorption spectroscopy offering an element-specific approach to magnetocrystalline anisotropy and the orbital magnetism. Experimental results on oxide-free FePt nanoparticles embedded in Al are compared with large-scale density functional theory calculations of the geometric- and spin-resolved electronic structure, which only recently have become possible on world-leading supercomputer architectures. The combination of both approaches yields a more detailed understanding that may open new ways for a microscopic design of magnetic nanoparticles and allows us to present three rules to achieve desired magnetic properties. In addition, concrete suggestions of capping materials for FePt nanoparticles are given for tailoring both magnetocrystalline anisotropy and magnetic moments. © 2011 Macmillan Publishers Limited. All rights reserved.view abstract doi: 10.1038/ncomms1538 2011 • 45 A novel automated electrochemical ascorbic acid assay in the 24-well microtiter plate format
Intarakamhang, S. and Leson, C. and Schuhmann, W. and Schulte, A.
Analytica Chimica Acta 687 1-6 (2011)Automatic ascorbic acid (AA) voltammetry was established in 24-well microtiter plates. The assay used a movable assembly of a pencil rod working, an Ag/AgCl reference and a Pt counter electrode with differential pulse voltammetry (DPV) for concentration-dependent current generation. A computer was in command of electrode (z) and microtiter plate (x, y) positioning and timed potentiostat operation. Synchronization of these actions supported sequential approach of all wells and subsequent execution of electrode treatment procedures or AA voltammetry at defined intervals in a measuring cycle. DPV in well solutions offered a linear current/concentration range between 0.1 and 8.0mM, a sensitivity of about 1μAmM-1 AA, and a detection limit of 50μM. When used with a calibration curve or standard addition, automated voltammetry of samples with added known amounts of AA demonstrated good recovery rates. Also, the assay achieved the accurate determination of the AA content of vitamin C tablets, a fruit juice and an herbal tea extract. Robotic AA voltammetry has the advantage of conveniently handling multiple samples in a single measuring run without the continuous attention of laboratory personnel. It is a good option when the goal is cost-effective AA screening of sample libraries and has potential for applications in health care and the food processing, cosmetic and pharmaceutical industries. © 2010 Elsevier B.V.view abstract doi: 10.1016/j.aca.2010.11.023 2011 • 44 Activation/inhibition effects during the coelectrodeposition of PtAg nanoparticles: Application for ORR in alkaline media
Schwamborn, S. and Stoica, L. and Schuhmann, W.
ChemPhysChem 12 1741-1746 (2011)PtAg bimetallic nanoparticles for oxygen reduction reaction (ORR) in alkaline media were prepared by pulse electrodeposition (PED). During PED the reduction of Ag + ions predominates, thus an increased Ag content in the co-deposit is accomplished. The mechanism for this anomalous co-deposition was elucidated by potential pulse experiments, which revealed that nuclei formation mainly occurs via the reduction of Pt 2+ ions. The growth of the particles is diffusion controlled leading to the formation of a Ag shell covering a PtAg alloyed region. However, the shell is not growing homogeneously on the PtAg alloy. Hence, regions of the PtAg alloy are exposed, which exhibit an enhanced ORR activity compared to a pure Ag surface. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/cphc.201100029 2011 • 43 Al-RUB-41: A shape-selective zeolite catalyst from a layered silicate
Yilmaz, B. and Müller, U. and Tijsebaert, B. and Vos, D.D. and Xie, B. and Xiao, F.-S. and Gies, H. and Zhang, W. and Bao, X. and Imai, H. and Tatsumi, T.
Chemical Communications 47 1812-1814 (2011)A new zeolite catalyst, Al-RUB-41, was synthesized for the first time. It was tested as a catalyst in methanol amination, and showed a shape-selective performance that results in a highly favorable product distribution. The shape-selective nature was also evidenced by using Pt-Al-RUB-41 as a bifunctional catalyst for decane hydroconversion. With its unique pore architecture and remarkable shape-selective character, Al-RUB-41 presents a significant commercial potential in industrial catalysis. © 2011 The Royal Society of Chemistry.view abstract doi: 10.1039/c0cc03895d 2011 • 42 Applications of an energy-dispersive pnCCD for X-ray reflectivity: Investigation of interdiffusion in Fe-Pt multilayers
Abboud, A. and Send, S. and Hartmann, R. and Strüder, L. and Savan, A. and Ludwig, Al. and Zotov, N. and Pietsch, U.
Physica Status Solidi (A) Applications and Materials Science 208 2601-2607 (2011)A frame store pn-junction CCD (pnCCD) detector was applied to study thermally induced interdiffusion in Fe/Pt thin film multilayers (MLs) in a temperature range between 300 and 585K. Based on the energy resolution of the detector the reflectivity was measured simultaneously in a spectral range between 8keV< E< 20keV including the Pt L-edge energies close to 11.5keV. Above T=533K we find a strong drop of intensities at 1st and 2nd order ML Bragg peak interpreted by mutual interdiffusion. Considering a simulated model of interdiffusion it has been found that the concentration of iron that diffuses into the platinum sub layers is higher than that of platinum into iron. The time dependence of inter diffusion was also calculated in the range of 533-568K and was described by the Arrhenius equation D(T)=D 0exp(-H a/k BT). The activation energy for the MLs used [Fe 1.7nm/Pt 2nm] 50 was found to be 0.94±0.22eV. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/pssa.201184268 2011 • 41 CeO2/Pt catalyst nanoparticle containing carbide-derived carbon composites by a new in situ functionalization strategy
Kockrick, E. and Borchardt, L. and Schrage, C. and Gaudillere, C. and Ziegler, C. and Freudenberg, T. and Farrusseng, D. and Eychmüller, A. and Kaskel, S.
Chemistry of Materials 23 57-66 (2011)A new class of CeO2/Pt nanostructures containing highly porous carbide-derived carbon composites was obtained for the first time using a polymer precursor strategy and subsequent ceramization. The catalytic transition metal compounds were incorporated into polymeric polycarbosilane structures using an inverse microemulsion method in precisely tunable nanoscale particle sizes. Porous ceramic and carbon composites were obtained by pyrolysis and subsequent chlorination processes. The adsorption properties of nonoxidic ceramic intermediates can be adjusted by the pyrolysis temperatures from mainly microporous to meso- and macroporous materials, respectively. These pore structures remain during the chlorination process confirmed by comparative nitrogen physisorption and small-angle X-ray scattering investigations. The specific surface areas significantly increase up to 1774 m2/g after selective silicon removal. In comparison to unsupported CeO2/Pt nanoparticle structures, the particle sizes and dispersion of the active metal compounds of composite structures remain during pyrolysis and chlorination process studied by electron microscopy methods. Ceramic and carbonaceous composites show catalytic activity and stability in selective methane oxidation. In contrast to the SiC composites, the CDC materials promote the formation of carbon monoxide and hydrogen in reforming reactions at higher temperatures, a conversion pathway important for the generation of synthetic fuels. © 2010 American Chemical Society.view abstract doi: 10.1021/cm102376b 2011 • 40 Characterization of oxidation and reduction of a platinum-rhodium alloy by atom-probe tomography
Li, T. and Marquis, E.A. and Bagot, P.A.J. and Tsang, S.C. and Smith, G.D.W.
Catalysis Today 175 552-557 (2011)An active challenge in heterogeneous catalysis is to minimize the quantities of the expensive platinum group metals used without causing degradation of the overall catalytic efficiency in a chemical reaction. To achieve this goal, a thorough atomic-scale understanding of these materials under reactive conditions is required. This will enable the design and production of "nano-engineered" catalysts, optimised for cost, stability and performance. In this study, the oxidation and reduction behaviour of a Pt-Rh alloy between 873 and 1073K was investigated by atom-probe tomography (APT). Detailed observations of the concentration profiles at the oxide/metal interfaces show that the growth of Rh2O3 oxide is limited by diffusion of Rh in the alloy. By varying the oxidation conditions, it was possible to calculate the activation energy for Rh diffusion in Pt-Rh as 236 ± 41 kJ/mol, together with diffusion coefficients for Rh for a range of temperatures. Reduction of the oxide phase left a thin, almost pure, layer of the most reactive (and expensive) element, Rh, on the surface of the specimen, suggesting a simple route for engineering the formation of the core-shell structure Pt-Rh nanoparticles. © 2011 Elsevier B.V. All rights reserved.view abstract doi: 10.1016/j.cattod.2011.03.046 2011 • 39 Designing shape-memory Heusler alloys from first-principles
Siewert, M. and Gruner, M.E. and Dannenberg, A. and Chakrabarti, A. and Herper, H.C. and Wuttig, M. and Barman, S.R. and Singh, S. and Al-Zubi, A. and Hickel, T. and Neugebauer, J. and Gillessen, M. and Dronskowski, R. and Entel, P.
Applied Physics Letters 99 (2011)The phase diagrams of magnetic shape-memory Heusler alloys, in particular, ternary Ni-Mn-Z and quarternary (Pt, Ni)-Mn-Z alloys with Z = Ga, Sn, have been addressed by density functional theory and Monte Carlo simulations. Finite temperature free energy calculations show that the phonon contribution stabilizes the high-temperature austenite structure while at low temperatures magnetism and the band Jahn-Teller effect favor the modulated monoclinic 14M or the nonmodulated tetragonal structure. The substitution of Ni by Pt leads to a series of magnetic shape-memory alloys with very similar properties to Ni-Mn-Ga but with a maximal eigenstrain of 14. © 2011 American Institute of Physics.view abstract doi: 10.1063/1.3655905 2011 • 38 Easily made and handled carbon nanocones for scanning tunneling microscopy and electroanalysis
Sripirom, J. and Noor, S. and Köhler, U. and Schulte, A.
Carbon 49 2402-2412 (2011)Well-formed carbon nanocones at the ends of micrometer-diameter carbon fibers (CFs) were fashioned into functional tips for scanning tunneling microscopy (STM) and miniaturized voltammetric sensors. Sharpening of single graphite filaments was achieved by simple DC electrochemical etching in 0.1 N NaOH. Operated as STM tips, pointed CFs resolved in air the contour and surface morphology of a nanoscopic Au line pattern and imaged in vacuum a Si (1 1 1) surface with clear atomic resolution. Subjecting already etched CFs to tip-sparing insulation with electrodeposited paint produced conical carbon ultramicroelectrodes (UMEs) with effective radii down to about 900 nm. Comparative cyclic voltammetry trials in alkaline, neutral and acidic solutions showed that the conical carbon UME's had a wider practical potential window for electroanalytic applications than, for instance, Pt disk UMEs. The CF-based conical sensors described here are exceptionally easy to make with simple laboratory equipment and perform well in STM topography imaging and voltammetry. The inherent simplicity of sensor production widens the field of potential users, and offers clear advantages over existing types of UMEs, in particular those based on carbon nanotubes, which are especially hard to handle in an optical microscope setting. © 2011 Elsevier Ltd. All rights reserved.view abstract doi: 10.1016/j.carbon.2011.02.007 2011 • 37 High-throughput characterization of Pt supported on thin film oxide material libraries applied in the oxygen reduction reaction
Schäfer, D. and Mardare, C. and Savan, A. and Sanchez, M.D. and Mei, B. and Xia, W. and Muhler, M. and Ludwig, Al. and Schuhmann, W.
Analytical Chemistry 83 1916-1923 (2011)Thin film metal oxide material libraries were prepared by sputter deposition of nanoscale Ti/Nb precursor multilayers followed by ex situ oxidation. The metal composition was varied from 6 at.% Nb to 27 at.% Nb. Additionally, thin wedge-type layers of Pt with a nominal thickness gradient from 0 to 5 nm were sputter-deposited on top of the oxides. The materials libraries were characterized with respect to metallic film composition, oxide thickness, phases, electrical conductivity, Pt thickness, and electrochemical activity for the oxygen reduction reaction (ORR). Electrochemical investigations were carried out by cyclic voltammetry using an automated scanning droplet cell. For a nominal Pt thickness >1 nm, no significant dependence of the ORR activity on the Pt thickness or the substrate composition was observed. However, below that critical thickness, a strong decrease of the surface-normalized activity in terms of reduction currents and potentials was observed. For such thin Pt layers, the conductivity of the substrate seems to have a substantial impact on the catalytic activity. Results from X-ray photoelectron spectroscopy (XPS) measurements suggest that the critical Pt thickness coincides with the transition from a continuous Pt film into isolated particles at decreasing nominal Pt thickness. In the case of isolated Pt particles, the activity of Pt decisively depends on its ability to exchange electrons with the oxide layer, and hence, a dependence on the substrate conductivity is rationalized. © 2011 American Chemical Society.view abstract doi: 10.1021/ac102303u 2011 • 36 Highly active metal-free nitrogen-containing carbon catalysts for oxygen reduction synthesized by thermal treatment of polypyridine-carbon black mixtures
Xia, W. and Masa, J. and Bron, M. and Schuhmann, W. and Muhler, M.
Electrochemistry Communications 13 593-596 (2011)A straight-forward method for the synthesis of metal-free catalysts for oxygen reduction by thermal treatment of a mixture of poly(3,5-pyridine) with carbon black in helium is reported. The catalyst was characterized by X-ray diffraction and photoelectron spectroscopy, cyclic voltammetry and rotating disk electrode measurements. The new catalyst exhibited remarkable activity similar to Pt-based catalysts in alkaline media. © 2011 Elsevier B.V. All Rights Reserved.view abstract doi: 10.1016/j.elecom.2011.03.018 2011 • 35 Integrated scanning kelvin probe-scanning electrochemical microscope system: Development and first applications
Maljusch, A. and Schönberger, B. and Lindner, A. and Stratmann, M. and Rohwerder, M. and Schuhmann, W.
Analytical Chemistry 83 6114-6120 (2011)The integration of a scanning Kelvin probe (SKP) and a scanning electrochemical microscope (SECM) into a single SKP-SECM setup, the concept of the proposed system, its technical realization, and first applications are presented and discussed in detail. A preloaded piezo actuator placed in a grounded stainless steel case was used as the driving mechanism for oscillation of a Pt disk electrode as conventionally used in SECM when the system was operated in the SKP mode. Thus, the same tip is recording the contact potential difference (CPD) during SKP scanning and is used as a working electrode for SECM imaging in the redox-competition mode (RC-SECM). The detection of the local CPD is established by amplification of the displacement current at an ultralow noise operational amplifier and its compensation by application of a variable backing potential (V b) in the external circuit. The control of the tip-to-sample distance is performed by applying an additional alternating voltage with a much lower frequency than the oscillation frequency of the Kelvin probe. The main advantage of the SKP-SECM system is that it allows constant distance measurements of the CPD in air under ambient conditions and in the redox-competition mode of the SECM in the electrolyte of choice over the same sample area without replacement of the sample or exchange of the working electrode. The performance of the system was evaluated using a test sample made by sputtering thin Pt and W films on an oxidized silicon wafer. The obtained values of the CPD correlate well with known data, and the electrochemical activity for oxygen reduction is as expected higher over Pt than W. © 2011 American Chemical Society.view abstract doi: 10.1021/ac200953b 2011 • 34 Length-scale modulated and electrocatalytic activity enhanced nanoporous gold by doping
Wang, X. and Frenzel, J. and Wang, W. and Ji, H. and Qi, Z. and Zhang, Z. and Eggeler, G.
Journal of Physical Chemistry C 115 4456-4465 (2011)In the present paper, we have investigated the dealloying of Pt- and/or Pd-doped Al2Au intermetallic compounds and the formation of ultrafine nanoporous Au (np-Au) alloys through a chemical dealloying strategy. The microstructural characterization confirms that these doping atoms enter into crystal lattices of the precursors and then transmit into the as-obtained np-Au, both existing in the form of solid solutions. When dealloying in the 20 wt % NaOH solution is performed, a certain amount of Pt and/or Pd addition shows a superior refining effect and the ligament/channel sizes of the as-doped np-Au can be facilely modulated below 10 nm. When dealloying in the 5 wt % HCl solution is performed, however, the anticoarsening capacity of Pt doping is more remarkable compared with that of Pd doping. In addition, the amount of doping also has an important influence on the ligament resistance to coarsening. Apart from causing the refinement of ligaments/channels, the introduction of Pt and/or Pd into np-Au has generated novel bi- or trimetallic functionalized nanoporous alloys. These as-doped np-Au alloys with an appropriate amount of Pt and/or Pd exhibit excellent electrocatalytic activities toward methanol and formic acid oxidation and will find promising applications in the catalysis-related areas. © 2011 American Chemical Society.view abstract doi: 10.1021/jp110011w 2011 • 33 Local electrocatalytic induction of sol-gel deposition at Pt nanoparticles
Schwamborn, S. and Etienne, M. and Schuhmann, W.
Electrochemistry Communications 13 759-762 (2011)Electrochemically-assisted deposition of sol-gel materials can be locally confined at Pt nanoparticles. Pt nanoparticles have been locally deposited on glassy carbon surfaces by pulse electrodeposition using a droplet cell. Upon applying a potential for electrochemically-assisted deposition, the formed sol-gel film mirrors the region of the glassy carbon surface previously modified with Pt nanoparticles. © 2011 Elsevier B.V. All rights reserved.view abstract doi: 10.1016/j.elecom.2011.02.030 2011 • 32 Modeling proton transfer to charged silver electrodes
Wilhelm, F. and Schmickler, W. and Nazmutdinov, R. and Spohr, E.
Electrochimica Acta 56 10632-10644 (2011)Density functional theory (DFT) and molecular dynamics (MD) techniques are used to study proton transfer from an aqueous solution to an Ag(1 1 1) surface. DFT is applied to study Ag-water and Ag-hydronium interactions as well as proton transfer for small systems based on the cluster model. The data gained are then used to adjust an empirical Ag-water interaction potential and to reparametrize an empirical valence-bond (EVB) model, which has been successfully applied for the study of proton transfer to a Pt(1 1 1) surface before. Employing these force fields in MD simulations enables dynamic modeling of the electrolyte-metal interface on a scale large enough to give realistic results. Results from a MD trajectory study on Ag(1 1 1) are reported and compared to the analogous study for platinum. Low discharge rates on Ag(1 1 1) are observed, and the potential range for hydrogen evolution can be estimated. The different behavior relative to Pt(1 1 1) can be traced to features of the respective potential energy surfaces and to the different structural properties of the aqueous/metallic interfaces. © 2011 Elsevier Ltd. All rights reserved.view abstract doi: 10.1016/j.electacta.2011.04.036 2011 • 31 Nanoepitaxy using micellar nanoparticles
Behafarid, F. and Roldan Cuenya, B.
Nano Letters 11 5290-5296 (2011)The shape of platinum and gold nanoparticles (NPs) synthesized via inverse micelle encapsulation and supported on TiO 2(110) has been resolved by scanning tunneling microscopy. Annealing these systems at high temperature (∼1000 °C) and subsequent cooling to room temperature produced ordered arrays of well-separated three-dimensional faceted NPs in their equilibrium state. The observed shapes differ from the kinetically limited shapes of conventional physical vapor deposited NPs, which normally form two-dimensional flat islands upon annealing at elevated temperatures. The initial NP volume was found to provide a means to control the final NP shape. Despite the liquid-phase ex situ synthesis of the micellar particles, the in situ removal of the encapsulating ligands and subsequent annealing consistently lead to the development of a well-defined epitaxial relationship of the metal NPs with the oxide support. The observed epitaxial relationships could be explained in terms of the best overlap between the interfacial Pt (or Au) and TiO 2 lattices. In most cases, the ratio of {100}/{111} facets obtained for the NP shapes resolved clearly deviates from that of conventional bulklike Wulff structures. © 2011 American Chemical Society.view abstract doi: 10.1021/nl2027525 2011 • 30 Nanoscaled alloy formation from self-assembled elemental Co nanoparticles on top of Pt films
Han, L. and Wiedwald, U. and Biskupek, J. and Fauth, K. and Kaiser, U. and Ziemann, P.
Beilstein Journal of Nanotechnology 2 473-485 (2011)The thermally activated formation of nanoscale CoPt alloys was investigated, after deposition of self-assembled Co nanoparticles on textured Pt(111) and epitaxial Pt(100) films on MgO(100) and SrTiO3(100) substrates, respectively. For this purpose, metallic Co nanoparticles (diameter 7 nm) were prepared with a spacing of 100 nm by deposition of precursor-loaded reverse micelles, subsequent plasma etching and reduction on flat Pt surfaces. The samples were then annealed at successively higher temperatures under a H2 atmosphere, and the resulting variations of their structure, morphology and magnetic properties were characterized. We observed pronounced differences in the diffusion and alloying of Co nanoparticles on Pt films with different orientations and microstructures. On textured Pt(111) films exhibiting grain sizes (20-30 nm) smaller than the particle spacing (100 nm), the formation of local nanoalloys at the surface is strongly suppressed and Co incorporation into the film via grain boundaries is favoured. In contrast, due to the absence of grain boundaries on high quality epitaxial Pt(100) films with micron-sized grains, local alloying at the film surface was established. Signatures of alloy formation were evident from magnetic investigations. Upon annealing to temperatures up to 380 °C, we found an increase both of the coercive field and of the Co orbital magnetic moment, indicating the formation of a CoPt phase with strongly increased magnetic anisotropy compared to pure Co. At higher temperatures, however, the Co atoms diffuse into a nearby surface region where Pt-rich compounds are formed, as shown by element-specific microscopy. © 2011 Han et al.view abstract doi: 10.3762/bjnano.2.51 2011 • 29 Oxygen chemisorption, formation, and thermal stability of Pt oxides on Pt nanoparticles supported on SiO2/Si(001): Size effects
Ono, L.K. and Croy, J.R. and Heinrich, H. and Roldan Cuenya, B.
Journal of Physical Chemistry C 115 16856-16866 (2011)The changes induced in the structure and chemical state of size-selected Pt nanoparticles (NPs) supported on ultrathin SiO2 films upon exposure to oxygen have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), in situ X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). For low atomic oxygen exposures, chemisorbed oxygen species were detected on all samples. Exposure to higher atomic oxygen coverages at room temperature leads to the formation and stabilization of PtOx species (PtO2 and PtO). On all samples, a two-step thermal decomposition process was observed upon annealing in ultrahigh vacuum: PtO2 → PtO → Pt. For NPs in the 2-6 nm range, the NP size was found to affect the strength of the O binding. Contrary to the case of Pt(111), where no oxides were detected above 700 K, 10-20% PtO was detected on the NP samples via XPS at the same temperature, suggesting the presence of strongly bound oxygen species. In addition, for identical atomic oxygen exposures, decreasing the NP size was found to favor their ability to form oxides. Interestingly, regardless of whether the desorption of chemisorbed oxygen species or that of oxygen in PtOx species was considered, our TPD data revealed higher O2-desorption temperatures for the Pt NPs as compared with the Pt(111) surface. Furthermore, a clear size-dependent trend was observed, with an increase in the strength of the oxygen bonding with decreasing NP size. © 2011 American Chemical Society.view abstract doi: 10.1021/jp204743q 2011 • 28 Partial oxidation of methane on Pt-supported lanthanide doped ceria-zirconia oxides: Effect of the surface/lattice oxygen mobility on catalytic performance
Sadykov, V.A. and Sazonova, N.N. and Bobin, A.S. and Muzykantov, V.S. and Gubanova, E.L. and Alikina, G.M. and Lukashevich, A.I. and Rogov, V.A. and Ermakova, E.N. and Sadovskaya, E.M. and Mezentseva, N.V. and Zevak, E.G. and Veni...
Catalysis Today 169 125-137 (2011)Partial oxidation of methane into syngas at short contact times (5-15 ms) was studied in both steady-state and transient modes at temperatures up to 850 °C in realistic feeds (CH4 content up to 20%, CH 4/O2 = 2) with a minimum impact of mass and heat transfer for structured catalysts carrying Pt/Ln0.3Ce0.35Zr 0.35O2-y (Ln = La, Pr, Gd) as thin layers on walls of corundum channel substrates. Oxygen mobility and reactivity of the active phase were characterized by oxygen isotope heteroexchange, temperature-programmed O2 desorption and CH4 reduction, isothermal pulse reduction by methane with wide variation of CH4 concentrations and TAP pulse studies. Experimental data point towards a selective oxidation of methane into syngas via a direct route with oxygen-assisted methane activation. This mechanistic feature is related to the strong Pt-support interaction stabilizing highly dispersed oxidic Pt species less active in CH4 and syngas combustion than metallic Pt clusters. Support activates O2 molecules and supplies active oxygen species to Pt sites. A high rate of oxygen diffusion on the surface and in the bulk of the support and Pt-support oxygen spillover stabilizes Pt in a well dispersed partially oxidized state while preventing coking at high concentrations of CH4 in the feed. © 2010 Elsevier B.V. All rights reserved.view abstract doi: 10.1016/j.cattod.2010.10.098 2011 • 27 Probing the Pt Surface for Oxygen Reduction by Insertion of Ag
Schwamborn, S. and Bron, M. and Schuhmann, W.
Electroanalysis 23 588-594 (2011)We report on the probing of the Pt surface for oxygen reduction reaction (ORR) by insertion of Ag. Therefore, PtAg bimetallic nanoparticles were prepared by pulse electrodeposition. In a second step, Ag was electro-dissolved in acidic media from the particles under formation of Pt skeleton. The ORR activity of these Pt skeleton depends on two factors: (1) on the surface properties of the Pt-shell and (2) on the electronic as well as geometric influences of the remaining Ag in the particle core. By varying the conditioning procedure prior to measuring the ORR activity, we were able to differentiate between these two effects. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/elan.201000510 2011 • 26 Radiofrequency driven and low cost fabricated microhollow cathode discharge for gaseous atomic emission spectrometry
Meyer, C. and Heming, R. and Gurevich, E.L. and Marggraf, U. and Okruss, M. and Florek, S. and Franzke, J.
Journal of Analytical Atomic Spectrometry 26 505-510 (2011)The current research presents a microhollow cathode discharge (MHCD) used as an analytical microplasma gas detector combining the advantages of a hollow cathode geometry in a miniaturized system offering atmospheric pressure operation. The plasma is driven by a homemade resonant radiofrequency generator (f = 1-10 MHz) reducing the electrode sputtering by a factor of 6.5 compared to common direct current operation leading to an extension of the lifetime of the microplasma chip of the same range. This paper aims further for the development of a novel low priced and therefore replaceable MHCD chip exchanging formerly used Pt electrodes by thin Cu electrodes. The analytical performance of the low cost Cu-MHCD with lifetime enhancing radiofrequency operation is demonstrated by atomic emission spectrometry with halogenated hydrocarbons with the Cl emission line at 912.114 nm. This leads to an excellent detection limit of 15 ppb v/v gaseous Cl in He making this microplasma chip suitable for lab on a chip application. © 2011 The Royal Society of Chemistry.view abstract doi: 10.1039/c0ja00216j 2011 • 25 Segregation and stability in surface alloys: PdxRu 1-x/Ru(0001) and PtxRu1-x/Ru(0001)
Bergbreiter, A. and Hoster, H.E. and Behm, R.J.
ChemPhysChem 12 1148-1154 (2011)The stability of PdRu/Ru(0001) and PtRu/Ru(0001) surface alloys and the tendency for surface segregation of Pd and Pt subsurface guest metals in these surface alloys is studied by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Atomic resolution STM imaging and AES measurements reveal that upon overgrowing the surface alloys with a 1-2 monolayer Ru film and subsequent annealing to the temperatures required for initial surface alloy formation, the Ru-covered Pd (Pt) atoms float back to the outermost layer. The lateral distribution of these species is also essentially identical to that of the initial surface alloys, before overgrowth by Ru. In combination, this clearly demonstrates that the surface alloys represent stable surface configurations, metastable only towards entropically favored bulk dissolution, and that there is a distinct driving force for surface segregation of these species. Consequences of these data on the mechanism for surface alloy formation are discussed. Floating in PtRu (PdRu) surface alloys on Ru(0001): The PtRu (PdRu)monolayer surface alloy layer is covered with the substrate metal Ru by means of physical vapour depositon. Subsequent annealing to temperatures necessary for surface alloy formation reconstitutes the original Pt (Pd) amount as well as the original atom distribution of the initial equilibrated alloy layer (see picture). Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/cphc.201001087 2011 • 24 Simultaneous acquisition of impedance and gravimetric data in a cyclic potential scan for the characterization of nonstationary electrode/electrolyte interfaces
Berkes, B.B. and Maljusch, A. and Schuhmann, W. and Bondarenko, A.S.
Journal of Physical Chemistry C 115 9122-9130 (2011)Simultaneous acquisition of electrochemical impedance spectroscopy and quartz crystal microbalance (EIS-EQCM) data in cyclic electrode potential scans was used to characterize nonstationary underpotential deposition (UPD) of atomic layers of Ag on Au and Cu on Pt. Both EIS and EQCM data sets complemented each other in the elucidation of interface models and the investigation of different aspects of the interfacial dynamics. EIS-EQCM provided an opportunity to monitor coadsorption and competitive adsorption of anions during the Ag and Cu UPD using (i) the electrode mass change, (ii) adsorption capacitances, and (iii) double-layer capacitances. Kinetic information is available in the EIS-EQCM through the charge transfer resistances and apparent rate coefficients. The latter expresses the rate of UPD into the partially covered electrode surface. The apparent rate coefficients for the Ag UPD were determined to vary from 0.15 to 0.45 cm/s which is between the standard constant rates k0 of Ag bulk deposition on Ag reported previously for different Ag surfaces. Cu UPD on Pt and Ag UPD on Au contributed differently into a resonance resistance ?R(E) available from the EQCM data sets. Spontaneous surface alloying between Ag and Au during the Ag UPD continuously increased the ?R, while the Cu overlayer formation on Pt as well as experiments without Ag+ and Cu 2+ in the solution did not change this parameter significantly. The EIS-EQCM appeared to be a promising tool for an improved characterization and understanding of nonstationary electrochemical interfaces. © 2011 American Chemical Society.view abstract doi: 10.1021/jp200755p 2011 • 23 Stoichiometry of alloy nanoparticles from laser ablation of PtIr in acetone and their electrophoretic deposition on PtIr electrodes
Jakobi, J. and Menéndez-Manjón, A. and Chakravadhanula, V.S.K. and Kienle, L. and Wagener, P. and Barcikowski, S.
Nanotechnology 22 (2011)Charged Pt-Ir alloy nanoparticles are generated through femtosecond laser ablation of a Pt9Ir target in acetone without using chemical precursors or stabilizing agents. Preservation of the target's stoichiometry in the colloidal nanoparticles is confirmed by transmission electron microscopy (TEM)-energy-dispersive x-ray spectroscopy (EDX), high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM)-EDX elemental maps, high resolution TEM and selected area electron diffraction (SAED) measurements. Results are discussed with reference to thermophysical properties and the phase diagram. The nanoparticles show a lognormal size distribution with a mean Feret particle size of 26nm. The zeta potential of - 45mV indicates high stability of the colloid with a hydrodynamic diameter of 63nm. The charge of the particles enables electrophoretic deposition of nanoparticles, creating nanoscale roughness on three-dimensional PtIr neural electrodes within a minute. In contrast to coating with Pt or Ir oxides, this method allows modification of the surface roughness without changing the chemical composition of PtIr. © 2011 IOP Publishing Ltd.view abstract doi: 10.1088/0957-4484/22/14/145601 2011 • 22 Structural and magnetic properties of ternary Fe1-xmnxpt nanoalloys from first principles
Gruner, M.E. and Entel, P.
Beilstein Journal of Nanotechnology 2 162-172 (2011)Background: Structural and magnetic properties of binary Mn-Pt and ternary Fe1-xMnxPt nanoparticles in the size range of up to 2.5 nm (561 atoms) have been explored systematically by means of large scale first principles calculations in the framework of density functional theory. For each composition several magnetic and structural configurations have been compared. Results: The concentration dependence of magnetization and structural properties of the ternary systems are in good agreement with previous bulk and thin film measurements. At an intermediate Mn-content around x = 0.25 a crossover between several phases with magnetic and structural properties is encountered, which may be interesting for exploitation in functional devices. Conclusion: Addition of Mn effectively increases the stability of single crystalline L10 particles over multiply twinned morphologies. This, however, compromises the stability of the ferromagnetic phase due to an increased number of antiferromagnetic interactions. The consequence is that only small additions of Mn can be tolerated for data recording applications. © 2011 Gruner and Entel.view abstract doi: 10.3762/bjnano.2.20 2011 • 21 Structure, chemical composition, and reactivity correlations during the in situ oxidation of 2-propanol
Paredis, K. and Ono, L.K. and Mostafa, S. and Li, L. and Zhang, Z. and Yang, J.C. and Barrio, L. and Frenkel, A.I. and Cuenya, B.R.
Journal of the American Chemical Society 133 6728-6735 (2011)Unraveling the complex interaction between catalysts and reactants under operando conditions is a key step toward gaining fundamental insight in catalysis. We report the evolution of the structure and chemical composition of size-selected micellar Pt nanoparticles (∼1 nm) supported on nanocrystalline γ-Al2O3 during the catalytic oxidation of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were found to be the active species for the partial oxidation of 2-propanol (< 140 °C), while the complete oxidation (>140 °C) is initially catalyzed by oxygen-covered metallic Pt nanoparticles, which were found to regrow a thin surface oxide layer above 200 °C. The intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of the Pt oxide species due to the production of reducing intermediates and the blocking of O2 adsorption sites on the nanoparticle surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of 2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions. © 2011 American Chemical Society.view abstract doi: 10.1021/ja200178f 2011 • 20 Synthesis of bifunctional Au/Pt/Au core/shell nanoraspberries for in situ SERS monitoring of platinum-catalyzed reactions
Xie, W. and Herrmann, C. and Kömpe, K. and Haase, M. and Schlücker, S.
Journal of the American Chemical Society 133 19302-19305 (2011)The synthesis of bifunctional Au/Pt/Au nanoraspberries for use in quantitative in situ monitoring of platinum-catalyzed reactions by surface-enhanced Raman scattering (SERS) is presented. Highly convolved SERS spectra of reaction mixtures can be decomposed into the contributions of distinct molecular species by multivariate data analysis. © 2011 American Chemical Society.view abstract doi: 10.1021/ja208298q 2010 • 19 4D shearforce-based constant-distance mode scanning electrochemical microscopy
Nebel, M. and Eckhard, K. and Erichsen, T. and Schulte, A. and Schuhmann, W.
Analytical Chemistry 82 7842-7848 (2010)4D shearforce-based constant-distance mode scanning electrochemical microscopy (4D SF/CD-SECM) is designed to assess SECM tip currents at several but constant distances to the sample topography at each point of the x,y-scanning grid. The distance dependent signal is achieved by a shearforce interaction between the in-resonance vibrating SECM tip and the sample surface. A 4D SF/CD-SECM measuring cycle at each grid point involves a shearforce controlled SECM tip z-approach to a point of closest distance and subsequent stepwise tip retractions. At the point of closest approach and during the retraction steps, pairs of tip current (I) and position are acquired for various distances above the sample surface. Such a sequence provides x,y,I maps, that can be compiled and displayed for each selected data acquisition distance. Thus, multiple SECM images are obtained at known and constant distances above the sample topography. 4D SF/CD-SECM supports distance-controlled tip operation while continuous scanning of the SECM tip in the shear-force distance is avoided. In this way, constant-distance mode SECM imaging can be performed at user-defined, large tip-to-sample distances. The feasibility and the potential of the proposed 4D SF/CD-SECM imaging is demonstrated using on the one hand amperometric feedback mode imaging of a Pt band electrode array and on the other hand the visualization of the diffusion zone of a redox active species above a microelectrode in a generator/collector arrangement. © 2010 American Chemical Society.view abstract doi: 10.1021/ac1008805 2010 • 18 Chemical trends in structure and magnetism of bimetallic nanoparticles from atomistic calculations
Gruner, M.E.
Journal of Physics D: Applied Physics 43 (2010)By means of large scale first-principles calculations in the framework of density functional theory, structure and magnetism of 561 atom nanoparticles are compared in order to obtain a systematic picture of the evolution with respect to a change in the constitutional elements. The investigation comprises ordered and disordered, cuboctahedral, icosahedral and decahedral morphologies of composition A265B296, where A is one of Mn, Fe and Co and B is Pt and, additionally, with A = Fe and B = Ni, Pd, Pt, Ir and Au. Fe-Ir and Fe-Pd and Co-Pt exhibit in comparison with Fe-Pt an increased tendency to form multiply-twinned structures and prefer segregation of the heavier element to the surface. The latter trend also applies to Fe-Au, where, on the other hand, icosahedral and crystalline motifs are very close in energy. Only in Mn-Pt the formation of multiply-twinned structures is effectively suppressed. The combinations with reduced valence electron concentration, Mn-Pt and Fe-Ir, exhibit a strong preference for antiferromagnetic spin order. The structural and magnetic trends are tentatively related to the change in features in the element and site-resolved electronic density of states. © 2010 IOP Publishing Ltd.view abstract doi: 10.1088/0022-3727/43/47/474008 2010 • 17 Core-shell morphologies of FePt and CoPt nanoparticles: An ab initio comparison
Gruner, M.E.
Journal of Physics: Conference Series 200 (2010)Large-scale first principles calculations for near-stoichiometric FePt and CoPt clusters with up to 923 atoms are presented including unconstrained structural relaxations. The energetic order and magnetic properties of 561 atom FePt and CoPt nanoparticles are evaluated with emphasis on segregated morphologies with Pt covered surfaces. The results imply that for CoPt particles segregation of Pt to the surface and the formation of a Pt depleted subsurface layer is dominant also for nanometer-sized single crystalline particles and may help to stabilize particles with partial L10 order, while for FePt multiple twinning is the most important mechanism at small particle sizes. The mixed (100) surfaces of the fully L10 ordered FePt and CoPt isomers exhibit a characteristic reconstruction. © 2010 IOP Publishing Ltd.view abstract doi: 10.1088/1742-6596/200/7/072039 2010 • 16 Development of molecular and solid catalysts for the direct low-temperature oxidation of methane to methanol
Palkovits, R. and von Malotki, C. and Baumgarten, M. and Müllen, K. and Baltes, C. and Antonietti, M. and Kuhn, P. and Weber, J. and Thomas, A. and Schüth, F.
ChemSusChem 3 277-282 (2010)The direct low-temperature oxidation of methane to methanol is demonstrated on a highly active homogeneous molecular catalyst system and on heterogeneous molecular catalysts based on polymeric materials possessing ligand motifs within the material structure. The N-(2-methylpropyl)-4,5-diazacarbazolyl-dichloro-platinum(II) complex reaches significantly higher activity compared to the well-known Periana system and allows first conclusions on electronic and structural requirements for high catalytic activity in this reaction. Interestingly, comparable activities could be achieved utilizing a platinum modified poly(benzimidazole) material, which demonstrates for the first time a solid catalyst with superior activity compared to the Periana system. Although the material shows platinum leaching, improved activity and altered electronic properties, compared to the conventional Periana system, support the proposed conclusions on structure-activity relationships. In comparison, platinum modified triazine-based catalysts show lower catalytic activity, but rather stable platinum coordination even after several catalytic cycles. Based on these systems, further development of improved solid catalysts for the direct low-temperature oxidation of methane to methanol is feasible. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.view abstract doi: 10.1002/cssc.200900123 2010 • 15 Discharge cavitation during microwave electrochemistry at micrometre-sized electrodes
Rassaei, L. and Nebel, M. and Rees, N.V. and Compton, R.G. and Schuhmann, W. and Marken, F.
Chemical Communications 46 812-814 (2010)Microwave induced activation of electrochemical processes at microelectrodes (ca. 0.8 m diameter) immersed in aqueous electrolyte media is shown to be driven by (i) continuous stable cavitation (giving rise to Faradaic current enhancements by up to three orders of magnitude) and (ii) transient discharge cavitation on the s timescale (giving rise to cathodic plasma current spikes and more violent surface erosion effects). © 2010 The Royal Society of Chemistry.view abstract doi: 10.1039/b920154h 2010 • 14 Electrochemical synthesis of core-shell catalysts for electrocatalytic applications
Kulp, C. and Chen, X. and Puschhof, A. and Schwamborn, S. and Somsen, C. and Schuhmann, W. and Bron, M.
ChemPhysChem 11 2854-2861 (2010)A novel electrochemical method to prepare platinum shells around carbon-supported metal nanoparticles (Ru and Au) by pulsed electrodeposition from solutions containing Pt ions is presented. Shell formation is confirmed by characteristic changes in the cyclic voltammograms, and is further evidenced by monitoring particle growth by transmission electron microscopy as well as by energy-dispersive analysis of X rays (EDX). Scanning electrochemical microscopy and EDX measurements indicate a selective Pt deposition on the metal/carbon catalyst, but not on the glassy carbon substrate. The thus prepared carbon-supported core-shell nanoparticles are investigated with regard to their activity in electrocatalytic oxygen reduction, which demonstrates the applicability of these materials in electrocatalysis or sensors. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.view abstract doi: 10.1002/cphc.200900881 2010 • 13 Fabrication of two-dimensional Au@FePt core-shell nanoparticle arrays by photochemical metal deposition
Härtling, T. and Uhlig, T. and Seidenstücker, A. and Bigall, N.C. and Olk, P. and Wiedwald, U. and Han, L. and Eychmüller, A. and Plettl, A. and Ziemann, P. and Eng, L.M.
Applied Physics Letters 96 (2010)In this report, we experimentally demonstrate that single platinum nanoparticles exhibit the necessary catalytic activity for the optically induced reduction of H [AuCl4] complexes to elemental gold. This finding is exploited for the parallel Au encapsulation of FePt nanoparticles arranged in a self-assembled two-dimensional array. Magnetic force microscopy reveals that the thin gold layer formed on the FePt particles leads to a strongly increased long-term stability of their magnetization under ambient conditions. © 2010 American Institute of Physics.view abstract doi: 10.1063/1.3425670 2010 • 12 First-principles study of the structural stability of L11 order in Pt-based alloys
Dannenberg, A. and Gruner, M.E. and Entel, P.
Journal of Physics: Conference Series 200 (2010)We investigate in the framework of density functional theory the structural and electronic properties of stoichiometric L11 ordered transition metal alloys of Pt and the 3d transition metals Mn, Fe, Co, Ni and Cu. A marked dependence of the energy difference between L11 and L10 structure on the valence electron concentration is encountered, with the L1 1 order being the preferred structure for CuPt, whereas the other alloys favor the L10 arrangement. The changes of the electronic density of states on composition are well represented within a rigid-band-picture, while the transition from L10 to L11 order is accompanied by a characteristic redistribution of the minority spin states around the Fermi level. © 2010 IOP Publishing Ltd.view abstract doi: 10.1088/1742-6596/200/7/072021 2010 • 11 Formation and thermal stability of platinum oxides on size-selected platinum nanoparticles: Support effects
Ono, L.K. and Yuan, B. and Heinrich, H. and Roldan Cuenya, B.
Journal of Physical Chemistry C 114 22119-22133 (2010)This article presents a systematic study of the formation and thermal stability of Pt oxide species on sizeselected Pt nanoparticles (NPs) supported on SiO2, ZrO2, and TiO2 thin films. The studies were carried out in ultrahigh vacuum (UHV) by temperature-dependent X-ray photoelectron spectroscopy (XPS) measurements and ex situ transmission electron microscopy and atomic force microscopy. The NPs were synthesized by inverse micelle encapsulation and oxidized in UHV at room temperature by an oxygen plasma treatment. For a given particle size distribution, the role played by the NP support on the stability of Pt oxides was analyzed. PtO2 species are formed on all supports investigated after O2-plasma exposure. A two-step thermal decomposition (PtO2 → PtO → Pt) is observed from 300 to 600 K upon annealing in UHV. The stability of oxidized Pt species was found to be enhanced on ZrO2 under annealing treatments in O2. Strong NP/support interactions and the formation of Pt-Ti-O alloys are detected for Pt/TiO2 upon annealing in UHV above 550 K but not under an identical treatment in O2. Furthermore, thermal treatments in both environments above 700 K lead to the encapsulation of Pt by TiOx. The final shape of the micellar Pt NPs is influenced by the type of underlying support as well as by the post-deposition treatment. Spherical Pt NPs are stable on SiO2, ZrO2, and TiO 2 after in situ ligand removal with atomic oxygen at RT. However, annealing in UHV at 1000 K leads to NP flattening on ZrO2 and to the diffusion of Pt NPs into TiO2. The stronger the nature of the NP/support interaction, the more dramatic is the change in the NP shape (TiO2 > ZrO2 > SiO2). © 2010 American Chemical Society.view abstract doi: 10.1021/jp1086703 2010 • 10 Identification of magnetic properties of few nm sized FePt crystalline particles by characterizing the intrinsic atom order using aberration corrected S/TEM
Biskupek, J. and Jinschek, J.R. and Wiedwald, U. and Bendele, M. and Han, L. and Ziemann, P. and Kaiser, U.
Ultramicroscopy 110 820-825 (2010)Hard-magnetic nanomaterials like nanoparticles of FePt are of great interest because of their promising potential for data storage applications. The magnetic properties of FePt structures strongly differ whether the crystal phases are face centered cubic (fcc) or face centered tetragonal (fct). We evaluated aberration corrected HRTEM, electron diffraction and aberration corrected HAADF-STEM as methods to measure the chemical degree of order S that describes the ordering of Pt and Fe atoms within the crystals unit cells. S/TEM experiments are accompanied by image calculations. The findings are compared with results obtained from X-ray diffraction on a FePt film. Our results show that STEM is a reasonable fast approach over HRTEM and electron diffraction to locally determine the chemical degree of order S. © 2010 Elsevier B.V.view abstract doi: 10.1016/j.ultramic.2010.02.043 2010 • 9 Magnetic hardness of Fe60Pt40 nanoparticles controlled by surface chemistry
Serantes, D. and Spasova, M. and Baldomir, D. and Farle, M. and Salgueirino, V.
Chemistry of Materials 22 4103-4110 (2010)Fe60Pt40 nanoparticles stabilized by oleic acid/oleylamine or tetramethylammonium hydroxide and self-assembled in 3D dispersions permit a detailed analysis of the competition of surface, finitesize effects and magnetic interparticle interactions which controls the collective macroscopic magnetic behavior. Temperature dependent magnetometry demonstrates that for FePt nanoparticles with identical size distribution but different surface chemistry, substantial differences of the effective magnetic anisotropy exist and can be understood by comparison with different theoretical models. Finally, a model yielding quantitative data for the competing intrinsic magnetic parameters of complex core-shell nanoparticles is derived. © 2010 American Chemical Society.view abstract doi: 10.1021/cm1010967 2010 • 8 Optimization of mesh-based anodes for direct methanol fuel cells
Chetty, R. and Scott, K. and Kundu, S. and Muhler, M.
Journal of Fuel Cell Science and Technology 7 0310101-0310119 (2010)Platinum based binary and ternary catalysts were prepared by thermal decomposition onto a titanium mesh and were evaluated for the anodic oxidation of methanol. The binary Pt:Ru catalyst with a composition of 1:1 gave the highest performance for methanol oxidation at 80° C. The effect of temperature and time for thermal decomposition was optimized with respect to methanol oxidation, and the catalysts were characterized by cyclic voltammetry, linear sweep voltammetry, scanning electron microscopy, X-ray diffraction studies, and X-ray photoelectron spectroscopy. The best catalyst was evaluated in a single fuel cell, and the effect of methanol concentration, temperature, and oxygen/air flow was studied. The mesh-based fuel cell, operating at 80°C with 1 mol dm 3 methanol, gave maximum power densities of 38 mWcm -2 and 22 mWcm -2 with 1 bar (gauge) oxygen and air, respectively. © 2010 by ASME.view abstract doi: 10.1115/1.3117605 2010 • 7 Platinum nanoparticles: The crucial role of crystal face and colloid stabilizer in the diastereoselective hydrogenation of cinchonidine
Schmidt, E. and Kleist, W. and Krumeich, F. and Mallat, T. and Baiker, A.
Chemistry - A European Journal 16 2181-2192 (2010)The preparation of stable metal nanoparticles requires a strong interaction between the (organic) stabilizer and the metal surface that might alter the catalytic properties. This behavior has been described as "poisoning" since the stabilizer normally decreases the catalytic activity due to site blocking. Here we show a striking influence of the stabilizer on the selectivity in the hydrogenation of cinchonidine (CD) over poly(acrylic acid) (PAA)-stabilized Pt nanoparticles with well-defined shape distributions. In the hydrogenation of the heteroaromatic ring of cinchonidine in toluene, the diastereomeric excess of the (S)-hexahydrocinchonidine increased upon increasing Pt{111}/Pt{100} ratio, but this distinct shape selectivity was observed only after the oxidative removal of PAA at 473 K. The use of the as-prepared nanoparticles inverted the major diastereomer to R, and this isomer was formed also in acetic acid. This striking change in the diastereoselectivity indicates that poly(acrylic acid), which remains on the Pt surface after preparation, interacts with CD during hydrogenation almost as strongly as the solvent acetic acid. The PAA stabilizer plays a dual role: it allows one to control the size and shape of the nanoparticles during their synthesis, and it affects the rate and diastereoselectivity of the hydrogenation of CD probably through a "surface-localized acidification". © 2010 Wiley-VCH Verlag GmbH & Co. KGaA,.view abstract doi: 10.1002/chem.200902517 2010 • 6 Proton transfer to charged platinum electrodes. A molecular dynamics trajectory study
Wilhelm, F. and Schmickler, W. and Spohr, E.
Journal of Physics Condensed Matter 22 (2010)A recently developed empirical valence bond(EVB) model for proton transfer on Pt(111) electrodes (Wilhelm et al 2008 J. Phys. Chem. C 112 10814) has been applied in molecular dynamics(MD) simulations of a water film in contact with a charged Pt surface. A total of seven negative surface charge densities σ between - 7.5 and - 18.9νCcm-2 were investigated. For each value of σ, between 30 and 84 initial conditions of a solvated proton within a water slab were sampled, and the trajectories were integrated until discharge of a proton occurred on the charged surfaces. We have calculated the mean rates for discharge and for adsorption of solvated protons within the adsorbed water layer in contact with the metal electrode as a function of surface charge density. For the less negative values of σ we observe a Tafel-like exponential increase of discharge rate with decreasing σ. At the more negative values this exponential increase levels off and the discharge process is apparently transport limited. Mechanistically, the Tafel regime corresponds to a stepwise proton transfer: first, a proton is transferred from the bulk into the contact water layer, which is followed by transfer of a proton to the charged surface and concomitant discharge. At the more negative surface charge densities the proton transfer into the contact water layer and the transfer of another proton to the surface and its discharge occur almost simultaneously. © 2010 IOP Publishing Ltd.view abstract doi: 10.1088/0953-8984/22/17/175001 2010 • 5 Pt-Ag catalysts as cathode material for oxygen-depolarized electrodes in hydrochloric acid electrolysis
Maljusch, A. and Nagaiah, T.C. and Schwamborn, S. and Bron, M. and Schuhmann, W.
Analytical Chemistry 82 1890-1896 (2010)Pt-Ag nanoparticles were prepared on a glassy carbon (GC) surface by pulsed electrodeposition and tested using cyclic voltammetry and scanning electrochemical microscopy (SECM) with respect to their possible use as catalyst material for oxygen reduction in 400 mM HCl solution. For comparison, a Pt catalyst was investigated under similar conditions. The redox competition mode of scanning electrochemical microscopy (RC-SECM) was adapted to the specific conditions caused by the presence of Cl ions and used to visualize the local catalytic activity of the Pt-Ag deposits. Similarly prepared Pt deposits were shown to dissolve underneath the SECM tip. Pt-Ag composites showed improved long-term stability toward oxygen reduction as compared with Pt even under multiple switching off to open-circuit potential in 400 mM HCl. © 2010 American Chemical Society.view abstract doi: 10.1021/ac902620g 2010 • 4 Shape-dependent catalytic properties of Pt nanoparticles
Mostafa, S. and Behafarid, F. and Croy, J.R. and Ono, L.K. and Li, L. and Yang, J.C. and Frenkel, A.I. and Cuenya, B.R.
Journal of the American Chemical Society 132 15714-15719 (2010)Tailoring the chemical reactivity of nanomaterials at the atomic level is one of the most important challenges in catalysis research. In order to achieve this elusive goal, fundamental understanding of the geometric and electronic structure of these complex systems at the atomic level must be obtained. This article reports the influence of the nanoparticle shape on the reactivity of Pt nanocatalysts supported on γ-Al2O3. Nanoparticles with analogous average size distributions (∼0.8-1 nm), but with different shapes, synthesized by inverse micelle encapsulation, were found to display distinct reactivities for the oxidation of 2-propanol. A correlation between the number of undercoordinated atoms at the nanoparticle surface and the onset temperature for 2-propanol oxidation was observed, demonstrating that catalytic properties can be controlled through shape-selective synthesis. © 2010 American Chemical Society.view abstract doi: 10.1021/ja106679z 2010 • 3 Solving the structure of size-selected Pt nanocatalysts synthesized by inverse micelle encapsulation
Roldan Cuenya, B. and Croy, J.R. and Mostafa, S. and Behafarid, F. and Li, L. and Zhang, Z. and Yang, J.C. and Wang, Q. and Frenkel, A.I.
Journal of the American Chemical Society 132 8747-8756 (2010)The structure, size, and shape of γ-Al2O 3-supported Pt nanoparticles (NPs) synthesized by inverse micelle encapsulation have been resolved via a synergistic combination of imaging and spectroscopic tools. It is shown that this synthesis method leads to 3D NP shapes even for subnanometer clusters, in contrast to the raft-like structures obtained for the same systems via traditional deposition-precipitation methods. Furthermore, a high degree of atomic ordering is observed for the micellar NPs in H2 atmosphere at all sizes studied, possibly due to H-induced surface reconstruction in these high surface area clusters. Our findings demonstrate that the influence of NP/support interactions on NP structure can be diminished in favor of NP/adsorbate interactions when NP catalysts are prepared by micelle encapsulation methods. © 2010 American Chemical Society.view abstract doi: 10.1021/ja101997z 2010 • 2 Theoretical investigation of the Pt3Al ground state
Chauke, H.R. and Minisini, B. and Drautz, R. and Nguyen-Manh, D. and Ngoepe, P.E. and Pettifor, D.G.
Intermetallics 18 417-421 (2010)The deleterious low-temperature tetragonal phases in prototypical Pt-based superalloys have variously been reported as taking the tI16-U3Si (DOc), tI16-Ir3Si (DOc′) and tP16-Pt3Ga structure-types in contrast to the high-temperature cubic cP4-Cu3Au (L12) phase. We have investigated the relative stability of these four structure-types at absolute zero by using density functional theory. We find that the ground state of stoichiometric Pt3Al is tP16-Pt3Ga and that the other three lattices are mechanically unstable at absolute zero. Experiments are needed to measure the internal displacement parameters of these three competing tetragonal phases. © 2009 Elsevier Ltd. All rights reserved.view abstract doi: 10.1016/j.intermet.2009.08.016 2010 • 1 Tuning adsorption via strain and vertical ligand effects
Hoster, H.E. and Alves, O.B. and Koper, M.T.M.
ChemPhysChem 11 1518-1524 (2010)We report on the structure and electrochemical adsorption properties of well-defined pseudomorphic Pt mono-and multilayers on Ru(0001). These act as model surfaces for Pt(111) with slightly decreased affinity to adsorbed hydrogen (Had) and hydroxyl (OHad). In cyclic voltammograms, this is reflected in more negative/positive potential regions for the reversible adsorption of upd-Had/OHad, respectively, compared to Pt(111). For upd-Had, we show that the corresponding trends can be predicted with high accuracy by density functional theory (DFT). In particular, the upd-Had onset regions can be precisely simulated using the Had adsorption energies from DFT, the layer thickness distribution from STM, and the base voltammogram of Pt(111) as reference.© 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.view abstract doi: 10.1002/cphc.200900500