Prof. Dr. Bastian Mei

Technical Chemistry
Ruhr-Universität Bochum


  • A high-purity gas–solid photoreactor for reliable and reproducible photocatalytic CO2 reduction measurements
    Moustakas, N.G. and Klahn, M. and Mei, B.T. and Pougin, A. and Dilla, M. and Peppel, T. and Ristig, S. and Strunk, J.
    HardwareX 15 (2023)
    Reactions between a gas phase and a solid material are of high importance in the study of alternative ways for energy conversion utilizing otherwise useless carbon dioxide (CO2). The photocatalytic CO2 reduction to hydrocarbon fuels like e.g., methane (CH4) is such a potential candidate process converting solar light into molecular bonds. In this work, the design, construction, and operation of a high-purity gas–solid photoreactor is described. The design aims at eliminating any unwanted carbon-containing impurities and leak points, ensuring the collection of reliable and reproducible data in photocatalytic CO2 reduction measurements. Apart from the hardware design, a detailed experimental procedure including gas analysis is presented, allowing newcomers in the field of gas–solid CO2 reduction to learn the essential basics and valuable tricks. By performing extensive blank measurements (with/without sample and/or light) the true performance of photocatalytic materials can be monitored, leading to the identification of trends and the proposal of possible mechanisms in CO2 photoreduction. The reproducibility of measurements between different versions of the here presented reactor on the ppm level is evidenced. © 2023 The Authors
    view abstract10.1016/j.ohx.2023.e00448
  • Carbon monoxide separation: past, present and future
    Ma, X. and Albertsma, J. and Gabriels, D. and Horst, R. and Polat, S. and Snoeks, C. and Kapteijn, F. and Eral, H.B. and Vermaas, D.A. and Mei, B. and de Beer, S. and van der Veen, M.A.
    Chemical Society Reviews (2023)
    Large amounts of carbon monoxide are produced by industrial processes such as biomass gasification and steel manufacturing. The CO present in vent streams is often burnt, this produces a large amount of CO2, e.g., oxidation of CO from metallurgic flue gasses is solely responsible for 2.7% of manmade CO2 emissions. The separation of N2 from CO due to their very similar physical properties is very challenging, meaning that numerous energy-intensive steps are required for CO separation, making the CO separation from many process streams uneconomical in spite of CO being a valuable building block in the production of major chemicals through C1 chemistry and the production of linear hydrocarbons by the Fischer-Tropsch process. The development of suitable processes for the separation of carbon monoxide has both industrial and environmental significance. Especially since CO is a main product of electrocatalytic CO2 reduction, an emerging sustainable technology to enable carbon neutrality. This technology also requires an energy-efficient separation process. Therefore, there is a great need to develop energy efficient CO separation processes adequate for these different process streams. As such the urgency of separating carbon monoxide is gaining greater recognition, with research in the field becoming more and more crucial. This review details the principles on which CO separation is based and provides an overview of currently commercialised CO separation processes and their limitations. Adsorption is identified as a technology with the potential for CO separation with high selectivity and energy efficiency. We review the research efforts, mainly seen in the last decades, in developing new materials for CO separation via ad/bsorption and membrane technology. We have geared our review to both traditional CO sources and emerging CO sources, including CO production from CO2 conversion. To that end, a variety of emerging processes as potential CO2-to-CO technologies are discussed and, specifically, the need for CO capture after electrochemical CO2 reduction is highlighted, which is still underexposed in the available literature. Altogether, we aim to highlight the knowledge gaps that could guide future research to improve CO separation performance for industrial implementation. © 2023 The Royal Society of Chemistry
    view abstract10.1039/d3cs00147d
  • 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 (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 abstract10.1002/jctb.7345
  • Light-driven urea oxidation for a wearable artificial kidney
    Vollenbroek, J.C. and Rodriguez, A.P. and Mei, B.T. and Mul, G. and Verhaar, M.C. and Odijk, M. and Gerritsen, K.G.F.
    Catalysis Today 419 (2023)
    For the development of a wearable artificial kidney (WAK) that uses a small dialysate volume that is continuously regenerated, it is essential that urea, one of the main uremic retention solutes, is removed. Despite advances in sorbent technology or electro-oxidation no safe, efficient and selective method for urea removal has been reported that allows miniaturization of the artificial kidney to wearable proportions. Here we have developed a flow cell for light-driven, photo-electrocatalytic (PEC) urea removal for use in a WAK. We use a photo-active material (hematite) coated with a catalyst (NiOOH) as working electrode for selective urea oxidation and a silver-chloride (AgCl) cathode. The use of the AgCl counter electrodes eliminates the need for an external bias voltage, and allows operation under light illumination only. Using LED illumination (460 nm) we show that urea is selectively oxidized over chloride. N2 formation is confirmed by gas-phase analysis of the headspace of the sample vial, using mass spectrometry. Other nitrogen containing products include nitrite but importantly ammonia and nitrate are not detected. Using the PEC concept a urea removal rate of 2.5 μmol/cm2h (or 0.15 mg/cm2h) has been achieved. Extrapolating our results to an upscaled system, a surface area of 0.5 m2 would enable efficient removal of the daily produced amount of urea (∼300 mmol) urea within 24 h, when driven by LED illumination only. © 2023 The Authors
    view abstract10.1016/j.cattod.2023.114163
  • Advanced oxidation processes for removal of organics from cooling tower blowdown: Efficiencies and evaluation of chlorinated species
    Saha, P. and Wang, Y. and Moradi, M. and Brüninghoff, R. and Moussavi, G. and Mei, B. and Mul, G. and H. M. Rijnaarts, H. and Bruning, H.
    Separation and Purification Technology 278 (2022)
    One of the major challenges in reusing cooling tower blowdown water (CTBD) utilizing membrane processes is its remaining organic compounds, e.g., humic substances leading to biofouling. Besides, the possible abundance of chloride in CTBD imposes the concern of the formation of chlorinated by-products. To choose a pre-treatment process for the studied CTBD composition, various advanced oxidation processes (AOPs), including electrooxidation (EO), photocatalytic degradation (PCD), heat-activated persulfate oxidation (PS), UVC/vacuum UV (UVC/VUV), and UVC processes, were evaluated and compared based on two main targets: i) highest removal and mineralization of the organics, especially humic substances; and ii) lowest formation of chlorinated by-products including adsorbable organic halides and oxychlorides. All the processes were conducted in the natural condition of the real CTBD, while solution pH was monitored. Based on results of chemical oxygen demand, total organic carbon, dissolved organic carbon, UV254 absorbance, liquid-chromatography–organic carbon detection (LC-OCD), and fluorescence excitation-emission matrices (FEEM), it is concluded that PS leads to complete removal of organic compounds along with the lowest formation of low molecular weight organic acids and organic neutrals. FEEM and LC-OCD data also indicated that EO, PCD, and UVC/VUV processes brought about substantial removal of organic compounds and broke down the humic substances into low molecular weight building blocks and organics. Besides, EO exhibited the highest AOX and oxychlorides formation, while these were limited when using the other AOPs. Summarizing, PS, PCD, and UVC/VUV were efficient processes for the degradation and mineralization of organics without generating significant amounts of chlorinated by-products. © 2021 The Author(s)
    view abstract10.1016/j.seppur.2021.119537
  • Carbon-nitrogen bond formation on Cu electrodes during CO2 reduction in NO3- solution
    Krzywda, P.M. and Paradelo Rodríguez, A. and Benes, N.E. and Mei, B.T. and Mul, G.
    Applied Catalysis B: Environmental 316 (2022)
    We demonstrate by Raman Spectroscopy that simultaneous reduction of NO3- and CO2 on Cu surfaces leads to formation of Cu-C[tbnd]N–like species, showing Raman bands at 2080 and 2150 cm−1 when associated with reduced or oxidized Cu surfaces, respectively. Furthermore Cu-C[tbnd]N–like species are soluble, explaining vast restructuring of the Cu surface observed after co-electrolysis of CO2 and nitrate. Oxidation of deposited Cu-C[tbnd]N–like species results in the formation of NO. Cu-C[tbnd]N–like species do not form in electrolytes containing i) NH4+ and CO2, or ii) NO3- and HCOO-, suggesting these likely originate from Cu-CO, the commonly accepted intermediate in electrochemical reduction of CO2, and Cu-NHx species, previously identified in the literature as intermediate towards C-N bond formation. The implications of the previously unresolved formation of Cu-C[tbnd]N–like species for the development of electrodes and processes for electrochemical formation of carbon-nitrogen bonds, including urea, amines or amides, are briefly discussed. © 2022 The Authors
    view abstract10.1016/j.apcatb.2022.121512
  • Controlled Synthesis of Chromium-Oxide-Based Protective Layers on Pt: Influence of Layer Thickness on Selectivity
    Worsley, M. and Smulders, V. and Mei, B.
    Catalysts 12 (2022)
    Chromium-oxyhydroxide (CrxOyHz)-based thin films have previously been shown in photocatalysis and industrial chlorate production to prevent unwanted reduction reactions to occur, thereby enhancing the selectivity for hydrogen evolution and thus the overall process efficiency. Here, a highly reproducible synthesis protocol was developed to allow for the electrodeposition of CrxOyHz-based thin films with controlled thickness in the range of the sub-monolayer up to (>4) multilayer coverage. Electrodeposited CrxOyHz coatings were electrochemically characterized using voltammetry and stripping experiments, allowing thickness-dependent film selectivity to be deduced in detail. The results are discussed in terms of mass transport properties and structure of the electrodeposited chromium oxyhydroxide films. It is shown that the permeation of diatomic probe molecules, such as O2 and CO, was significantly reduced by films as thin as four monolayers. Importantly, it is shown that the prepared thin film coatings enabled prolonged hydrogen oxidation in the presence of CO (up to 5 vol.%), demonstrating the benefits of thin-film-protected electrocatalysts. In general, this study provides insight into the synthesis and use of thin-film-protected electrodes leading to improvements in (electro)catalyst selectivity and durability. © 2022 by the authors.
    view abstract10.3390/catal12101077
  • Design and validation of a low-cost open-source impedance based quartz crystal microbalance for electrochemical research
    Horst, R.J. and Katzourakis, A. and Mei, B.T. and de Beer, S.
    HardwareX 12 (2022)
    The quartz crystal microbalance (QCM) measurement technique is utilized in a broad variety of scientific fields and applications, where surface and interfacial processes are relevant. However, the costs of purchasing QCMs is typically high, which has limited its employment in education as well as by scientists in developing countries. In this article, we present an open-source QCM, built on the OpenQCM project, and using an impedance-based measurement technique (QCM-I), which can be built for <200 euro. Our QCM allows for simultaneous monitoring of the frequency change and dissipation, such that both soft and rigid materials can be characterized. In addition, our QCM measurements can be combined with simultaneous electrochemical measurement techniques (EQCM-I). We demonstrate the validity of our system by characterizing the electrodeposition of a rigid metallic film (Cu) and by the electropolymerization of aniline. Finally, we discuss potential improvements to our system. © 2022 The Author(s)
    view abstract10.1016/j.ohx.2022.e00374
  • Effect of Electrolyte and Electrode Configuration on Cu-Catalyzed Nitric Oxide Reduction to Ammonia
    Krzywda, P.M. and Paradelo Rodríguez, A. and Benes, N.E. and Mei, B.T. and Mul, G.
    ChemElectroChem 9 (2022)
    Reduction of nitric oxide was investigated using Cu electrodes in acid and neutral pH conditions. Analysis of Cu discs in stagnant electrolyte by Electrochemical Mass Spectrometry (EC-MS), revealed the favorable formation of ammonia (and hydrogen) in acidic electrolyte, while N2O and N2 are formed in significant quantities at neutral conditions. Additional performance evaluation of Cu electrodes in hollow fiber geometry, was performed using 10 vol % NO in Ar supplied through the porous electrode structure and off-line determination of ammonia by 1H NMR spectroscopy. The pH dependent performance of the Cu hollow fiber is in agreement with EC-MS data at low gas flow rates, showing the highest ammonia selectivity in acidic conditions. However, at relatively high gas flow rates, almost 90 % faradaic efficiency and a NH3 production rate of 400 μmol h−2 cm−2 were obtained in neutral electrolyte at −0.6 V vs RHE, likely due to enhanced availability of NO at the electrode surface, suppressing the hydrogen evolution reaction. This approach shows conversion of waste NO gas to valuable green fertilizer components is possible. © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH
    view abstract10.1002/celc.202101273
  • Electroreduction of NO3− on tubular porous Ti electrodes
    Krzywda, P.M. and Paradelo Rodríguez, A. and Cino, L. and Benes, N.E. and Mei, B.T. and Mul, G.
    Catalysis Science and Technology (2022)
    Inefficient fertilizer use in agriculture causes nitrate runoff, polluting rivers and streams. This pollution can be mitigated by partially converting nitrate into ammonia - rebalancing the composition to ammonium nitrate, and allowing recycling of fertilizer. Here, we present efficient electrochemical conversion of nitrate (50 mM) to ammonia in acidic electrolyte using tubular porous Ti electrodes. A high faradaic efficiency (FE) of 58% and partial current density to ammonia of −33 mA cm−2 at −1 V vs. RHE were achieved in the absence of inert gas purge. Additionally, we reveal that hydroxylamine is formed, as well as NO and N2O by spontaneous decomposition of nitrite, as has been determined by EC-MS analysis. The effective increase in local mass transport by introducing a flow of inert gas exiting the wall of the hollow fiber electrode results in an unprecedently high partial current density to ammonia of ∼−75 mA cm−2, while maintaining a faradaic efficiency to ammonia of up to 45%. This concept facilitates nitrate conversion at high FE even at low concentrations, and holds promise for development to practical scale if electrochemical potential and exiting gas flow rate are well controlled. © 2022 The Royal Society of Chemistry.
    view abstract10.1039/d2cy00289b
  • Photocatalytic nanocomposite membranes for environmental remediation
    Golmohammadi, M. and Sabbagh Alvani, A.A. and Sameie, H. and Mei, B. and Salimi, R. and Poelman, D. and Rosei, F.
    Nanotechnology 33 (2022)
    We report the design and one-pot synthesis of Ag-doped BiVO4 embedded in reduced graphene oxide (BiVO4:Ag/rGO) nanocomposites via a hydrothermal processing route. The binary heterojunction photocatalysts exhibited high efficiency for visible light degradation of model dyes and were correspondingly used for the preparation of photocatalytic membranes using polyvinylidene fluoride (PVDF) or polyethylene glycol (PEG)-modified polyimide (PI), respectively. The surface and cross-section images combined with elemental mapping illustrated the effective distribution of the nanocomposites within the polymeric membranes. Photocatalytic degradation efficiencies of 61% and 70% were achieved after 5 h of visible light irradiation using BiVO4:Ag/rGO@PVDF and BiVO4:Ag/rGO@PI (PEG-modified) systems, respectively. The beneficial photocatalytic performance of the BiVO4:Ag/rGO@PI (PEG-modified) membrane is explained by the higher hydrophilicity due to the PEG modification of the PI membrane. This work may provide a rational and effective strategy to fabricate highly efficient photocatalytic nanocomposite membranes with well-contacted interfaces for environmental purification. © 2022 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/ac8682
  • Study on the Effect of Electrolyte pH during Kolbe Electrolysis of Acetic Acid on Pt Anodes
    Nordkamp, M.O. and Mei, B. and Venderbosch, R. and Mul, G.
    ChemCatChem 14 (2022)
    Kolbe already discovered in 1849 that electrochemical oxidative decarboxylation of carboxylic acids is feasible and leads to formation of alkanes and CO2, via alkyl radical intermediates. We now show for Pt electrodes that Kolbe electrolysis of acetic acid is favored in electrolytes with a pH similar to, or larger than the pKa of acetic acid, suppressing the formation of O2. However extended duration of electrolysis of acetate at basic pH results in loss of Faradaic efficiency to ethane, compensated by the formation of methanol. This change in selectivity is likely caused by the dissolution of CO2 near the electrode-electrolyte interface, resulting in enlarged concentration of bicarbonate/carbonate. On the positively polarized, and oxidized Pt surface, these anions seem to inhibit homocoupling of methyl radicals to ethane. These results demonstrate that reaction selectivity in acetic acid (acetate) oxidation using oxidized Pt electrodes is determined by the pH and the anionic composition of the electrolyte. © 2022 The Authors. ChemCatChem published by Wiley-VCH GmbH.
    view abstract10.1002/cctc.202200438
  • The effect of buoyancy driven convection on the growth and dissolution of bubbles on electrodes
    Sepahi, F. and Pande, N. and Chong, K.L. and Mul, G. and Verzicco, R. and Lohse, D. and Mei, B.T. and Krug, D.
    Electrochimica Acta 403 (2022)
    Enhancing the efficiency of water electrolysis, which can be severely impacted by the nucleation and growth of bubbles, is key in the energy transition. In this combined experimental and numerical study, in-situ bubble evolution and dissolution processes are imaged and compared to numerical simulations employing the immersed boundary method. We find that it is crucial to include solutal driven natural convection in order to represent the experimentally observed bubble behaviour even though such effects have commonly been neglected in modelling efforts so far. We reveal how the convective patterns depend on current densities and bubble spacings, leading to distinctively different bubble growth and shrinkage dynamics. Bubbles are seen to promote the convective instability if their spacing is large (≥4 mm for the present conditions), whereas the onset of convection is delayed if the inter-bubble distance is smaller. Our approach and our results can help devise efficient mass transfer solutions for gas evolving electrodes. © 2021 The Authors
    view abstract10.1016/j.electacta.2021.139616
  • CrOx-Mediated Performance Enhancement of Ni/NiO-Mg:SrTiO3in Photocatalytic Water Splitting
    Han, K. and Haiber, D.M. and Knöppel, J. and Lievens, C. and Cherevko, S. and Crozier, P. and Mul, G. and Mei, B.
    ACS Catalysis 11 (2021)
    By photodeposition of CrOxon SrTiO3-based semiconductors doped with aliovalent Mg(II) and functionalized with Ni/NiOxcatalytic nanoparticles (economically significantly more viable than commonly used Rh catalysts), an increase in apparent quantum yield (AQYs) from ∼10 to 26% in overall water splitting was obtained. More importantly, deposition of CrOxalso significantly enhances the stability of Ni/NiO nanoparticles in the production of hydrogen, allowing sustained operation, even in intermittent cycles of illumination.In situelemental analysis of the water constituents during or after photocatalysis by inductively coupled plasma mass spectrometry/optical emission spectrometry shows that after CrOxdeposition, dissolution of Ni ions from Ni/NiOx-Mg:SrTiO3is significantly suppressed, in agreement with the stabilizing effect observed, when both Mg dopant and CrOxare present. State-of-the-art electron microscopy and energy-dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) analyses demonstrate that upon preparation, CrOxis photodeposited in the vicinity of several, but not all, Ni/NiOxparticles. This implies the formation of a Ni-Cr mixed metal oxide, which is highly effective in water reduction. Inhomogeneities in the interfacial contact, evident from differences in contact angles between Ni/NiOxparticles and the Mg:SrTiO3semiconductor, likely affect the probability of reduction of Cr(VI) species during synthesis by photodeposition, explaining the observed inhomogeneity in the spatial CrOxdistribution. Furthermore, by comparison with undoped SrTiO3, Mg-doping appears essential to provide such favorable interfacial contact and to establish the beneficial effect of CrOx. This study suggests that the performance of semiconductors can be significantly improved if inhomogeneities in interfacial contact between semiconductors and highly effective catalytic nanoparticles can be resolved by (surface) doping and improved synthesis protocols. © 2021 The Authors. Published by American Chemical Society
    view abstract10.1021/acscatal.1c03104
  • Electrochemical preparation of defect-engineered titania: Bulk doping versus surface contamination
    Brüninghoff, R. and Paradelo Rodríguez, A. and Jong, R.P.H. and Sturm, J.M. and Breuer, U. and Lievens, C. and Jeremiasse, A.W. and Mul, G. and Mei, B.
    Applied Surface Science 539 (2021)
    Defect-engineered or substoichiometric TiOx is of interest for use in photo- and electrocatalytic processes both as active material and catalyst support. Electrochemical doping of TiO2 via cathodic polarization is an appealing preparation method and frequently employed. Here, we explored the electrochemical preparation of TiOx in an undivided cell using iridium-based (iridium mixed-metal-oxide) and boron doped diamond (BDD) counter electrodes. Cyclic voltammetry and impedance spectroscopy revealed superior charge transfer properties of crystalline TiOx electrodes prepared with BDD (TiOx-BDD). It is shown that the electrochemical properties correlate well with intensities of the H-signals determined using Time of Flight - Secondary Ion Mass Spectrometry (ToF-SIMS). Thus, it is concluded that electrochemical preparation using BDD causes favourable H+ intercalation and/or H diffusion into the sub-surface layers of TiOx. Our extensive analysis using a combination of electrochemical and surface characterization (LEIS and XPS) techniques, additionally suggests that cathodic deposition of Ir, originating from the Ir-based counter electrode, present in sub-ppm concentrations only results in less-efficient doping. Instead in the presence of sub-ppm level Ir contamination hydrogen evolution is favoured during cathodic polarization. The results presented within this study highlight the necessity to use inherently stable counter electrodes for electrochemical preparation and reveal the pronounced influence of trace contamination in electrochemistry in general and the doping mechanism of TiOx electrodes in particular. © 2020 Elsevier B.V.
    view abstract10.1016/j.apsusc.2020.148136
  • Electroconvective Instability in Water Electrolysis: An Evaluation of Electroconvective Patterns and Their Onset Features
    Pande, N. and Wood, J.A. and Mul, G. and Lohse, D. and Mei, B.T. and Krug, D.
    Physical Review Applied 16 (2021)
    In electrochemical systems, an understanding of the underlying transport processes is required to aid in their better design. This includes knowledge of possible near-electrode convective mixing that can enhance measured currents. Here, for a binary acidic electrolyte in contact with a platinum electrode, we provide evidence of electroconvective instability during electrocatalytic proton reduction. The current-voltage characteristics indicate that electroconvection, visualized with a fluorescent dye, drives current densities larger than the diffusion transport limit. The onset and transition times of the instability do not follow the expected inverse-square dependence on the current density, but, above a bulk-reaction-limited current density, are delayed by the water dissociation reaction, that is, the formation of H+ and OH- ions. The dominant size of the electroconvective patterns is also measured and found to vary with the diffusion length scale, confirming previous predictions on the size of electroconvective vortices. © 2021 American Physical Society.
    view abstract10.1103/PhysRevApplied.16.034021
  • Facet-Dependent Surface Charge and Hydration of Semiconducting Nanoparticles at Variable pH
    Su, S. and Siretanu, I. and van den Ende, D. and Mei, B. and Mul, G. and Mugele, F.
    Advanced Materials 33 (2021)
    Understanding structure and function of solid–liquid interfaces is essential for the development of nanomaterials for various applications including heterogeneous catalysis in liquid phase processes and water splitting for storage of renewable electricity. The characteristic anisotropy of crystalline nanoparticles is believed to be essential for their performance but remains poorly understood and difficult to characterize. Dual scale atomic force microscopy is used to measure electrostatic and hydration forces of faceted semiconducting SrTiO3 nanoparticles in aqueous electrolyte at variable pH. The following are demonstrated: the ability to quantify strongly facet-dependent surface charges yielding isoelectric points of the dominant {100} and {110} facets that differ by as much as 2 pH units; facet-dependent accumulation of oppositely charged (SiO2) particles; and that atomic scale defects can be resolved but are in fact rare for the samples investigated. Atomically resolved images and facet-dependent oscillatory hydration forces suggest a microscopic charge generation mechanism that explains colloidal scale electrostatic forces. © 2021 The Authors. Advanced Materials published by Wiley-VCH GmbH
    view abstract10.1002/adma.202106229
  • Mixed Chromate and Molybdate Additives for Cathodic Enhancement in the Chlorate Process
    Smulders, V. and Gomes, A.S.O. and Simic, N. and Mei, B. and Mul, G.
    Electrocatalysis 12 (2021)
    The economic viability of the electrochemical chlorate process depends on toxic chromate to induce cathodic selectivity to hydrogen and mitigate reduction of hypochlorite or chlorate. In this study, it is shown that performance of a pilot plant for chlorate production can be sustained when a 1000-fold reduction in chromate concentration is compensated by addition of molybdate. Laboratory measurements employing a Quartz Crystal Microbalance suggest growth of a nanometre-thick hybrid Mo–Cr-oxide film to induce cathodic selectivity. An optimized energy efficiency for pilot plant operation was obtained using 0.8 mM molybdate and 27 μM chromate, balancing formation of an effective oxide layer and undesired Mo-induced decomposition of hypochlorite to oxygen in solution. Refinement at the pilot scale level is expected to further optimize the energy consumption, thereby increasing safety aspects and the economic viability of chlorate production. Graphical Abstract: [Figure not available: see fulltext.] © 2021, The Author(s).
    view abstract10.1007/s12678-021-00666-7
  • Mo-doped ZnV2O6/reduced graphene oxide photoanodes for solar hydrogen production
    Sameie, H. and Alvani, A.A.S. and Mei, B.T. and Salimi, R. and Poelman, D. and Rosei, F.
    Electrochimica Acta 382 (2021)
    We report the fabrication and characterization of molybdenum (Mo)-doped ZnV2O6/reduced graphene oxide (rGO) composite and its use as photoanode for photoelectrochemical (PEC) hydrogen production. Compared to pure ZnV2O6, Mo ions act as electron donor in the ZnV2O6:Mo lattice increasing charge carrier concentration and subsequently mobility in the bulk by the polaron transport. We measured the hole transfer efficiency for the pure and Mo-doped ZnV2O6 electrodes and revealing a substantial increase from 16 to 25%. The mechanism of enhanced photoactivity of Mo-doped ZnV2O6 was studied by density functional theory calculations. Moreover, electrochemical impedance spectroscopy measurements show that graphene modification improves carrier separation and transfer across the electrode/electrolyte interface. Therefore, the combination of the two strategies triggers a synergistic enhancement in PEC performance in terms of incident photon-to-current efficiency, which is 17% at 370 nm, being 4.5- and 3.6-times greater than those of pristine ZnV2O6 and ZnV2O6:Mo photoanodes, respectively. With photocurrent onset potentials of 0.6 V and photocurrent densities of 2.07 mA/cm2 at 1.23 V vs. RHE, ZnV2O6:Mo/rGO photoanodes are of interest for the design of high performance PEC visible-light-induced water-splitting devices. © 2021
    view abstract10.1016/j.electacta.2021.138333
  • 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 abstract10.1016/j.snb.2021.129631
  • Principles and Limitations of Photoelectrochemical Fuel Generation
    Mei, B. and Wenderich, K.
    Heterogeneous Photocatalysis: From Fundamentals to Applications in Energy Conversion and Depollution (2021)
    Photoelectrochemical (PEC) cells could play a pivotal role in meeting future energy demands. In PEC cells, semiconductors are used to convert solar energy directly into chemical energy, i.e. to drive thermodynamically uphill reactions to produce valuable products such as fuels. The “classic” application of PEC cells is to produce hydrogen through water splitting, although production of other chemicals is possible. In this chapter, the basics of photoelectrocatalysis are discussed. The thermodynamic requirements of the semiconductor will be touched upon first. Then, the properties of the semiconductor/electrolyte interface will be discussed, both under dark conditions and under illumination. Afterward, different PEC device configurations, device efficiencies, and limitations of the PEC devices themselves will be presented. We will also briefly touch upon PEC cells used for different applications than “classic” water splitting and the corresponding theoretical limitations. Finally, recent approaches to enhance the catalytic activity or to improve the stability of PEC cells by surface modification are described. © 2021 WILEY-VCH GmbH, Boschstr.
    view abstract10.1002/9783527815296.ch4
  • Selective Electrochemical Oxidation of H2O to H2O2Using Boron-Doped Diamond: An Experimental and Techno-Economic Evaluation
    Wenderich, K. and Nieuweweme, B.A.M. and Mul, G. and Mei, B.T.
    ACS Sustainable Chemistry and Engineering 9 (2021)
    Selective water oxidation to hydrogen peroxide has emerged as an economically attractive replacement for oxygen in electrochemical hydrogen production by water splitting. Here, boron-doped diamond (BDD) is shown to be a promising anode material for anodic H2O2 formation. Faradaic efficiencies of up to 31.7% at 2.90 V versus the reference hydrogen electrode and a current density of 39.8 mA cm-2 were observed, corresponding to a H2O2 production rate of 3.93 μmol min-1 cm-2. A techno-economic evaluation based on the experimentally obtained values demonstrates that the corresponding levelized cost of hydrogen (LCH) is significant ($62.0 kg-1). Particularly, the current market price of BDD limits its implementation as a selective water oxidation anode for H2O2 generation. The sensitivity analysis however suggests that the LCH can be significantly improved by either decreasing the anode cost or increasing the current density. Both approaches are in fact feasible to allow for cost-effective electrochemical H2 production and even competition with H2 obtained from steam methane reforming. This study will guide ongoing research efforts toward BDD development and implementation of selective water oxidation to hydrogen peroxide. © 2021 The Authors. Published by American Chemical Society.
    view abstract10.1021/acssuschemeng.1c01244
  • Synergy of ferroelectric polarization and oxygen vacancy to promote CO2 photoreduction
    Yu, H. and Chen, F. and Li, X. and Huang, H. and Zhang, Q. and Su, S. and Wang, K. and Mao, E. and Mei, B. and Mul, G. and Ma, T. and Zhang, Y.
    Nature Communications 12 (2021)
    Solar-light driven CO2 reduction into value-added chemicals and fuels emerges as a significant approach for CO2 conversion. However, inefficient electron-hole separation and the complex multi-electrons transfer processes hamper the efficiency of CO2 photoreduction. Herein, we prepare ferroelectric Bi3TiNbO9 nanosheets and employ corona poling to strengthen their ferroelectric polarization to facilitate the bulk charge separation within Bi3TiNbO9 nanosheets. Furthermore, surface oxygen vacancies are introduced to extend the photo-absorption of the synthesized materials and also to promote the adsorption and activation of CO2 molecules on the catalysts’ surface. More importantly, the oxygen vacancies exert a pinning effect on ferroelectric domains that enables Bi3TiNbO9 nanosheets to maintain superb ferroelectric polarization, tackling above-mentioned key challenges in photocatalytic CO2 reduction. This work highlights the importance of ferroelectric properties and controlled surface defect engineering, and emphasizes the key roles of tuning bulk and surface properties in enhancing the CO2 photoreduction performance. © 2021, The Author(s).
    view abstract10.1038/s41467-021-24882-3
  • Electrochemically Induced pH Change: Time-Resolved Confocal Fluorescence Microscopy Measurements and Comparison with Numerical Model
    Pande, N. and Chandrasekar, S.K. and Lohse, D. and Mul, G. and Wood, J.A. and Mei, B.T. and Krug, D.
    Journal of Physical Chemistry Letters 11 (2020)
    Confocal fluorescence microscopy is a proven technique, which can image near-electrode pH changes. For a complete understanding of electrode processes, time-resolved measurements are required, which have not been achieved previously. Here we present the first measurements of time-resolved pH profiles with confocal fluorescence microscopy. The experimental results compare favorably with a one-dimensional reaction-diffusion model; this holds up to the point where the measurements reveal three-dimensionality in the pH distribution. Specific factors affecting the pH measurement such as attenuation of light and the role of dye migration are also discussed in detail. The method is further applied to reveal the buffer effects observed in sulfate-containing electrolytes. The work presented here is paving the way toward the use of confocal fluorescence microscopy in the measurement of 3D time-resolved pH changes in numerous electrochemical settings, for example, in the vicinity of bubbles. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acs.jpclett.0c01575
  • German Catalysis Society Meeting: Exploring the Various Facets of Catalysis
    Titus, J. and Harth, F. and Eckert, R. and Mei, B.T. and Franken, T.
    ChemCatChem 12 (2020)
    view abstract10.1002/cctc.201902185
  • Industrial feasibility of anodic hydrogen peroxide production through photoelectrochemical water splitting: a techno-economic analysis
    Wenderich, K. and Kwak, W. and Grimm, A. and Kramer, G.J. and Mul, G. and Mei, B.
    Sustainable Energy and Fuels 4 (2020)
    Photoelectrochemical (PEC) water splitting is a promising approach to drive green, carbon-free production of hydrogen (H2). In ‘classic’ water splitting, oxygen (O2) is formed at the anode as a by-product. It has been suggested that substitution of anodic O2production with hydrogen peroxide (H2O2) could increase the financial attractiveness of PEC water splitting. Here, we present a techno-economic analysis of a photoelectrochemical H2/H2O2process. Specifically, we model photoelectrochemical farms with industrially relevant production capacities. Two scenarios are considered: (i) a theoretical scenario with an optimal solar-to-hydrogen (STH) efficiency of 27.55% and (ii) a literature-based state-of-the-art scenario with an STH efficiency of 10.1%. When applying an averaged market value of $0.85 kg−1for H2O2, the analysis reveals a negative levelized cost of hydrogen (LCH) for scenario (i),i.e.$6.45 kg−1, and for scenario (ii) an LCH of $6.19 kg−1. Our results imply that these values are superior to the LCH of ‘classic’ PEC water splitting (ca.$10 kg−1), while the negative value for scenario (i) even outcompetes the LCH of steam methane reforming ($1.4 kg−1). We predict that significant reduction in the LCH can be realized within the PEC community when future research is aimed at enhancing the stability of the photoanode and optimizing the STH efficiency for anodic H2O2formation. This manuscript clearly demonstrates the financial benefits of value-added product formation, such as hydrogen peroxide, over O2formation. In a broader context, our analysis verifies that further research on valuable commodity chemicals at the anode in water splitting and CO2reduction should be stimulated in the future to facilitate implementation of emerging, cost-intensive technologies. © The Royal Society of Chemistry 2020.
    view abstract10.1039/d0se00524j
  • 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 (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 abstract10.1016/j.cattod.2019.07.042
  • Photocatalytic oxidation of propane using hydrothermally prepared anatase-brookite-rutile tio2 samples. An in situ drifts study
    Cano-Casanova, L. and Mei, B. and Mul, G. and Lillo-Ródenas, M.Á. and Román-Martínez, M.C.
    Nanomaterials 10 (2020)
    Photocatalytic oxidation of propane using hydrothermally synthesized TiO2 samples with similar primary crystal size containing different ratios of anatase, brookite and rutile phases has been studied by measuring light-induced propane conversion and in situ DRIFTS (diffuse reflectance Fourier transform infrared spectroscopy). Propane was found to adsorb on the photocatalysts, both in the absence and presence of light. The extent of adsorption depends on the phase composition of synthesized titania powders and, in general, it decreases with increasing rutile and brookite content. Still, the intrinsic activity for photocatalytic decomposition of propane is higher for photocatalysts with lower ability for propane adsorption, suggesting this is not the rate-limiting step. In situ DRIFTS analysis shows that bands related to adsorbed acetone, formate and bicarbonate species appear on the surface of the photocatalysts during illumination. Correlation of propane conversion and infrared (IR) data shows that the presence of formate and bicarbonate species, in excess with respect to acetone, is composition dependent, and results in relatively low activity of the respective TiO2. This study highlights the need for precise control of the phase composition to optimize rates in the photocatalytic oxidation of propane and a high rutile content seems to be favorable. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano10071314
  • Selective photocatalytic oxidation of cyclohexanol to cyclohexanone: A spectroscopic and kinetic study
    Karimi Estahbanati, M.R. and Feilizadeh, M. and Babin, A. and Mei, B. and Mul, G. and Iliuta, M.C.
    Chemical Engineering Journal 382 (2020)
    In this work, spectroscopic and kinetic studies were performed on photocatalytic oxidation of cyclohexanol to cyclohexanone. The photocatalytic experiments were performed according to a three-level full factorial design and the rate of cyclohexanone production was determined by HPLC analysis. In situ ATR-FTIR analysis of the photocatalytic reaction revealed that cyclohexanol can be selectively converted to cyclohexanone, without the formation of significant amounts of carbonates and carboxylates. A reaction mechanism based on different steps from charge separation to cyclohexanone molecule formation is proposed. The results were utilized to determine the kinetic parameters (with the help of genetic algorithm) and validate the model. The developed kinetic model illustrates that the rate of cyclohexanone production increases as a power function with respect to the light intensity and decreases as an exponential function with respect to time. An excellent selectivity of cyclohexanone was confirmed by spectroscopic and chromatographic studies. This study demonstrates that photocatalysis can be a promising technology for formation of cyclohexanone from cyclohexanol. © 2019 Elsevier B.V.
    view abstract10.1016/j.cej.2019.122732
  • Ag-Functionalized CuWO4/WO3 nanocomposites for solar water splitting
    Salimi, R. and Sabbagh Alvani, A.A. and Mei, B.T. and Naseri, N. and Du, S.F. and Mul, G.
    New Journal of Chemistry 43 (2019)
    Ag-Functionalized CuWO4/WO3 heterostructures were successfully prepared via a polyvinyl pyrrolidone (PVP)-assisted sol-gel (PSG) route. Thin films prepared via electrophoretic deposition were used as photoanodes for photoelectrochemical (PEC) water splitting. Compared to pristine CuWO4 and WO3 films, a significant enhancement of the photocurrent (3-4 times) at the thermodynamic potential for oxygen evolution (0.62 V vs. Ag/AgCl, pH 7) was obtained for the Ag-functionalized CuWO4/WO3 photoanodes. The obtained enhancement is shown to be derived from a synergic contribution of heterostructure formation (CuWO4/WO3) and improvements of light utilization by Ag-induced surface plasmon resonance (SPR) effects. Accordingly, a photocurrent of 0.205 mA cm-2 at 0.62 V vs. Ag/AgCl under neutral conditions (without hole scavengers) under front-side simulated AM1.5G illumination was achieved. A detailed analysis of the obtained PEC data alongside performed impedance measurements suggests that charge seperation is significantly improved for the prepared Ag-functionalized CuWO4/WO3 photoanodes. Our work offers beneficial insights to design new plasmonic metal/heterostructured nanocomposites for energy conversion applications. © 2019 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
    view abstract10.1039/c8nj05625k
  • Comparative Analysis of Photocatalytic and Electrochemical Degradation of 4-Ethylphenol in Saline Conditions
    Brüninghoff, R. and Van Duijne, A.K. and Braakhuis, L. and Saha, P. and Jeremiasse, A.W. and Mei, B. and Mul, G.
    Environmental Science and Technology 53 (2019)
    We evaluated electrochemical degradation (ECD) and photocatalytic degradation (PCD) technologies for saline water purification, with a focus on rate comparison and formation and degradation of chlorinated aromatic intermediates using the same non-chlorinated parent compound, 4-ethylphenol (4EP). At 15 mA·cm-2, and in the absence of chloride (0.6 mol·L-1 NaNO3 was used as supporting electrolyte), ECD resulted in an apparent zero-order rate of 30 μmol L-1·h-1, whereas rates of ∼300 μmol L-1·h-1 and ∼3750 μmol L-1·h-1 were computed for low (0.03 mol·L-1) and high (0.6 mol·L-1) NaCl concentration, respectively. For PCD, initial rates of ∼330 μmol L-1·h-1 and 205 μmol L-1·h-1 were found for low and high NaCl concentrations, at a photocatalyst (TiO2) concentration of 0.5 g·L-1, and illumination at λmax ≈ 375 nm, with an intensity ∼0.32 mW·cm-2. In the chlorine mediated ECD approach, significant quantities of free chlorine (hypochlorite, Cl2) and chlorinated hydrocarbons were formed in solution, while photocatalytic degradation did not show the formation of free chlorine, nor chlorine-containing intermediates, and resulted in better removal of non-purgeable hydrocarbons than ECD. The origin of the minimal formation of free chlorine and chlorinated compounds in photocatalytic degradation is discussed based on photoelectrochemical results and existing literature, and explained by a chloride-mediated surface-charge recombination mechanism. © 2019 American Chemical Society.
    view abstract10.1021/acs.est.9b01244
  • Correlating the short-time current response of a hydrogen evolving nickel electrode to bubble growth
    Pande, N. and Mul, G. and Lohse, D. and Mei, B.
    Journal of the Electrochemical Society 166 (2019)
    Gas evolving electrochemical reactions induce bubble formation and growth at surfaces of electrodes. To study one such situation, hydrogen evolution on nickel electrodes, short time chronoamperometric experiments were performed in combination with in-situ microscopy. The entire electrode of 3.14 mm2 was imaged with confocal microscopy and the current response of the electrode then correlated to the observed bubble growth features. Somehow counterintuitively, first a 2–3% increase in current was observed consistently when a bubble grows close to the electrode on the edge of the electrode holder, made of a polymer. This is argued to be due to the removal of surface attached gas from the electrode. Next, we observe a consecutive regime of decreasing current, in which large bubbles accumulate on the surface. Interestingly, when these surface attached bubbles coalesce, a steep change in current is observed, which is accompanied by a burst of small bubbles nucleating on the surface previously occupied by the large bubble. These phenomena are qualitatively discussed on the basis of existing literature, and implications for improvements for electrodes on which gases are produced, are outlined. © The Author(s) 2019.
    view abstract10.1149/2.0191910jes
  • Electrochemical formation of Cr(III)-based films on Au electrodes
    Smulders, V. and Simic, N. and Gomes, A.S.O. and Mei, B. and Mul, G.
    Electrochimica Acta 296 (2019)
    In electrochemical production of sodium chlorate from brine solutions, an intriguing function of sodium (di)chromate is to inhibit cathodic reduction of oxychlorides, while maintaining effective reduction of water to form hydrogen. Using an electrochemical Quartz Crystal Microbalance (eQCM) and a Rotating Ring Disk Electrode (RRDE; Au disk, Pt ring), we analyzed the deposition of reduced Cr-species formed from reduction of CrVIO4 2− on Au electrodes. Generally, the current induced by reduction of CrVIO4 2− is significantly larger than the accumulated amount of weight deposited on the Au electrode. Deconvolution of the reductive peak reveals two processes that can be differentiated by varying rotation speed. We therefore propose soluble CrVO4 3− is formed by reduction of CrVIO4 2−, followed by consecutive reduction of CrVO4 3− to primarily soluble CrIII(OH)4 -. Simultaneously, reduction of CrVO4 3− also leads to the formation of a monolayer of CrIII(hydr)oxide. This monolayer significantly inhibits the further reduction of CrVIO4 2−, but allows the film to reach a maximum thickness of approximately 1.85 nm by reduction of surface adsorbed CrVO4 3− and/or de-hydroxylation of CrIII(OH)4 -. The observation that limitation of film growth is due to film-induced inhibition of reduction of CrVIO4 2−, and significant solubility of CrIII(OH)3 in the form of CrIII(OH)4 -, will aid in the search of a non-toxic chrome-free alternative for inhibition of cathodic reduction of oxychlorides and selective hydrogen evolution in the chlorate process. © 2018
    view abstract10.1016/j.electacta.2018.11.057
  • Electrochemical oxidation of H2S on polycrystalline Ni electrodes
    Yang, J. and Smulders, V. and Smits, J.J.T. and Mei, B.T. and Mul, G.
    Journal of Applied Electrochemistry 49 (2019)
    Abstract: We have evaluated the applicability of Ni anodes in electrochemical conversion of H2S to form sulfur (polysulfides) and H2. Two different electrolytes containing sulfide were evaluated: a buffered solution of Na2HPO4 at pH 9.2, and a NaOH solution at pH 13. At pH 9.2, deposition of sulfur on the Ni anode was observed, resulting in a significant decrease in electrochemical performance. The composition, morphology, and thickness of the sulfur deposit, as determined by Raman spectroscopy and SEM, was found to strongly depend on the applied potential, and ranged from dense S8 films to highly porous spherical sulfur structures. Oxidation of the anode was also observed by conversion of Ni to NiS2. The formation of the sulfur film was prevented by performing the reaction at pH 13 in NaOH in the range of − 1.0 V to + 0.6 V versus Hg/HgO. It is proposed that at these highly basic pH values, sulfur is dissolved in the electrolyte in the form of polysulfides, such as S2 2− or S8 2−. When using Ni anodes some oxygen evolution was observed at the anode, in particular at pH 13, resulting in a Faradaic efficiency for sulfur removal of ~ 90%. Graphic Abstract: [Figure not available: see fulltext.]. © 2019, The Author(s).
    view abstract10.1007/s10800-019-01334-x
  • Heterodimetallic Ferrocenyl Dithiophosphonate Complexes of Nickel(II), Zinc(II) and Cadmium(II) as Sensitizers for TiO2-Based Dye-Sensitized Solar Cells
    Ajayi, T.J. and Ollengo, M. and le Roux, L. and Pillay, M.N. and Staples, R.J. and Biros, S.M. and Wenderich, K. and Mei, B. and van Zyl, W.E.
    ChemistrySelect 4 (2019)
    The formation, characterization, and dye sensitized solar cell application of nickel(II), zinc(II) and cadmium(II) ferrocenyl dithiophosphonate complexes were investigated. The multidentate monoanionic ligand [S2PFc(OH)]− (L1) was synthesized. The reaction between metal salt precursors and L1 produced Ni(II) complexes of the type [Ni{S2P(Fc)(OH)}2] (1) (molar ratio 1:2), and a tetranickel(II) complex of the type [Ni2{S2OP(Fc)}2]2 (2) (molar ratio (1:1). It also produced a Zn(II) complex [Zn{S2P(Fc)(OH)}2]2 (3), and a Cd(II) complex [Cd{S2P(Fc)(OH)}2]2 (4). Complexes 1–4 were characterized by 1H and 31P NMR, FTIR and elemental analysis, and complexes 1 and 2 were additionally analyzed by X-ray crystallography. The first examples of dye-sensitized solar cells (DSSCs) co-sensitized with ferrocenyl dithiophosphonate complexes 1–4 are reported. Co-sensitization with the ruthenium dye N719, produced the dye materials (3)-N719 (η=8.30%) and (4)-N719 (η=7.78%), and they were found to have a better overall conversion efficiency than the pure Ru N719 dye standard (η=7.14%) under the same experimental conditions. The DSSCs were characterized using UV/vis, cyclic voltammetry, electrochemical impedance spectroscopy (EIS), photovoltaic- (I−V curves), and performing incident photon-to-current efficiency (IPCE) measurements. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/slct.201900622
  • In situ formed vanadium-oxide cathode coatings for selective hydrogen production
    Endrődi, B. and Smulders, V. and Simic, N. and Wildlock, M. and Mul, G. and Mei, B. and Cornell, A.
    Applied Catalysis B: Environmental 244 (2019)
    Electrode selectivity towards hydrogen production is essential in various conversion technologies for renewable energy, as well as in different industrial processes, such as the electrochemical production of sodium chlorate. In this study we present sodium metavanadate as a solution additive, inducing selective cathodic formation of hydrogen in the presence of various other reducible species such as hypochlorite, chlorate, oxygen, nitrate, hydrogen-peroxide and ferricyanide. During electrolysis a vanadium-oxide coating forms from the reduction of sodium metavanadate, explaining the observed enhanced selectivity. The hydrogen evolution reaction proceeds without significantly altered kinetics on such in situ modified electrode surfaces. This suggests that the reaction takes place at the interface between the electrode surface and the protective film, which acts as a diffusion barrier preventing the unwanted species to reach the electrode surface. © 2018 The Authors
    view abstract10.1016/j.apcatb.2018.11.038
  • Integration of Molybdenum-Doped, Hydrogen-Annealed BiVO 4 with Silicon Microwires for Photoelectrochemical Applications
    Milbrat, A. and Vijselaar, W. and Guo, Y. and Mei, B. and Huskens, J. and Mul, G.
    ACS Sustainable Chemistry and Engineering 7 (2019)
    H-BiVO 4-x :Mo was successfully deposited on microwire-structured silicon substrates, using indium tin oxide (ITO) as an interlayer and BiOI prepared by electrodeposition as precursor. Electrodeposition of BiOI, induced by the electrochemical reduction of p-benzoquinone, appeared to proceed through three stages, being nucleation of particles at the base and bottom of the microwire arrays, followed by rapid (homogeneous) growth, and termination by increasing interfacial resistances. Variations in charge density and morphology as a function of spacing of the microwires are explained by (a) variations in mass transfer limitations, most likely associated with the electrochemical reduction of p-benzoquinone, and (b) inhomogeneity in ITO deposition. Unexpectedly, H-BiVO 4-x :Mo on microwire substrates (4 μm radius, 4 to 20 μm spacing, and 5 to 16 μm length) underperformed compared to H-BiVO 4-x :Mo on flat surfaces in photocatalytic tests employing sulfite (SO 3 2- ) oxidation in a KPi buffer solution at pH 7.0. While we cannot exclude optical effects, or differences in material properties on the nanoscale, we predominantly attribute this to detrimental diffusion limitations of the redox species within the internal volume of the microwire arrays, in agreement with existing literature and the observations regarding the electrodeposition of BiOI. Our results may assist in developing high-efficiency PEC devices. © Copyright © 2019 American Chemical Society.
    view abstract10.1021/acssuschemeng.8b05756
  • Time-Dependent Photoluminescence of Nanostructured Anatase TiO2 and the Role of Bulk and Surface Processes
    Brüninghoff, R. and Wenderich, K. and Korterik, J.P. and Mei, B.T. and Mul, G. and Huijser, A.
    Journal of Physical Chemistry C 123 (2019)
    TiO2 is one of the most attractive photocatalysts, with applications in water splitting, wastewater treatment, and air purification. Understanding the fundamentals of the functioning of TiO2 requires knowledge of the nature and dynamics of photo-induced excitons and charge carriers. Although photoluminescence (PL) spectroscopy can provide important fundamental insights, photophysical mechanisms are still under debate. To address this problem, the aim of the present work is to investigate the evolution of the PL spectrum in time of nanostructured anatase TiO2 thin films and the nature of associated processes, at room temperature and in aqueous media closely resembling photocatalytic conditions. We show that the PL spectrum of commonly used nanostructured anatase TiO2 thin films in aqueous media is time-dependent, with pH-dependent broadening at the low energy side of the spectrum in time. By global analysis of the spectrotemporal PL behavior and the effect of addition of NaCl at neutral and mildly acidic conditions, we show that this spectral development is due to an increasing contribution of processes sensitive to surface termination relative to bulk processes to the PL in time. The time-dependent PL spectrum and dynamics can be assigned to the recombination of mobile electrons populating the conduction band or shallow traps with immobile hole polarons in deep traps and motion of electrons from the nanoparticle bulk toward the depletion layer/surface in ca. 1 ns. This directionality likely plays an important role in the photocatalytic performance of nanostructured anatase TiO2 and effects of ions such as chloride in aqueous media. Control of the directional motion of electrons and suppression of surface charge recombination via surface engineering show promise to further increase the photocatalytic activity. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acs.jpcc.9b06890
  • Driving Surface Redox Reactions in Heterogeneous Photocatalysis: The Active State of Illuminated Semiconductor-Supported Nanoparticles during Overall Water-Splitting
    Mei, B. and Han, K. and Mul, G.
    ACS Catalysis 8 (2018)
    Materials used for photocatalytic overall water splitting (POWS) are typically composed of light-absorbing semiconductor crystals, functionalized with so-called cocatalytic nanoparticles to improve the kinetics of the hydrogen and/or oxygen evolution reactions. While function, quantity, and protection of such metal(oxide) nanoparticles have been addressed in the literature of photocatalysis, the stability and transients in the active oxidation-state upon illumination have received relatively little attention. In this Perspective, the latest insights in the active state of frequently applied cocatalysts systems, including Pt, Rh/Cr2O3, or Ni/NiOx, will be presented. While the initial morphology and oxidation state of such nanoparticles is a strong function of the applied preparation procedure, significant changes in these properties can occur during water splitting. We discuss these changes in relation to the nature of the cocatalyst/semiconductor interface. We also show how know-how of other disciplines such as heterogeneous catalysis or electro-catalysis and recent advances in analytical methodology can help to determine the active state of cocatalytic nanoparticles in photocatalytic applications. © 2018 American Chemical Society.
    view abstract10.1021/acscatal.8b02215
  • Overcoming cathode poisoning from electrolyte impurities in alkaline electrolysis by means of self-healing electrocatalyst films
    Barwe, S. and Mei, B. and Masa, J. and Schuhmann, W. and Ventosa, E.
    Nano Energy 53 (2018)
    The performance of electrolyzers for hydrogen production is strongly influenced by electrolyte impurities having either a positive or negative impact on the activity of electrocatalysts. We show that cathode deactivation by zinc impurities present in the electrolyte can be overcome by employing catalyst immobilization based on self-assembled and self-healing films. During electrolysis zinc impurities deposit as dendritic films on the cathode electrode increasing the overpotential for the hydrogen evolution reaction (HER), however, continuous self-assembling and self-healing of HER catalyst films subsequently mask the zinc dendrites restoring the advantageous HER overpotential. Zn electrolyte impurities are turned from having a negative to a positive impact leading to an enhanced performance of the cathode due to the increase in surface area caused by the growth of the Zn dendrites. © 2018 Elsevier Ltd
    view abstract10.1016/j.nanoen.2018.09.045
  • Pathways to electrochemical solar-hydrogen technologies
    Ardo, S. and Fernandez Rivas, D. and Modestino, M.A. and Schulze Greiving, V. and Abdi, F.F. and Alarcon Llado, E. and Artero, V. and Ayers, K. and Battaglia, C. and Becker, J.-P. and Bederak, D. and Berger, A. and Buda, F. and Chinello, E. and Dam, B. and Di Palma, V. and Edvinsson, T. and Fujii, K. and Gardeniers, H. and Geerlings, H. and Hashemi, S.M. and Haussener, S. and Houle, F. and Huskens, J. and James, B.D. and Konrad, K. and Kudo, A. and Kunturu, P.P. and Lohse, D. and Mei, B. and Miller, E.L. and Moore, G.F. and Muller, J. and Orchard, K.L. and Rosser, T.E. and Saadi, F.H. and Schüttauf, J.-W. and Seger, B. and Sheehan, S.W. and Smith, W.A. and Spurgeon, J. and Tang, M.H. and Van De Krol, R. and Vesborg, P.C.K. and Westerik, P.
    Energy and Environmental Science 11 (2018)
    Solar-powered electrochemical production of hydrogen through water electrolysis is an active and important research endeavor. However, technologies and roadmaps for implementation of this process do not exist. In this perspective paper, we describe potential pathways for solar-hydrogen technologies into the marketplace in the form of photoelectrochemical or photovoltaic-driven electrolysis devices and systems. We detail technical approaches for device and system architectures, economic drivers, societal perceptions, political impacts, technological challenges, and research opportunities. Implementation scenarios are broken down into short-term and long-term markets, and a specific technology roadmap is defined. In the short term, the only plausible economical option will be photovoltaic-driven electrolysis systems for niche applications. In the long term, electrochemical solar-hydrogen technologies could be deployed more broadly in energy markets but will require advances in the technology, significant cost reductions, and/or policy changes. Ultimately, a transition to a society that significantly relies on solar-hydrogen technologies will benefit from continued creativity and influence from the scientific community. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c7ee03639f
  • PH-Dependence in facet-selective photo-deposition of metals and metal oxides on semiconductor particles
    Guo, Y. and Siretanu, I. and Zhang, Y. and Mei, B. and Li, X. and Mugele, F. and Huang, H. and Mul, G.
    Journal of Materials Chemistry A 6 (2018)
    Facet-engineering and the deposition of co-catalysts lead to significant improvement in efficiency of semiconductors in photocatalytic applications. Here, we demonstrate, using the specific example of bismuth-oxy-bromide (BiOBr) particles, that facet-selective, photo-induced reductive or oxidative deposition of co-catalysts onto plate-like semiconductor particles is strongly pH-dependent. High resolution atomic force microscopy and spectroscopy measurements demonstrate that the effect of pH is caused by a reversal of the surface charge of the [001] facets upon increasing pH from 3 to 9 (isoelectric point ≈5), while the side facets become increasingly negatively-charged. We discuss the effect of facet-surface-charge on particle distributions by band-bending, favoring either electron transfer and metal deposition, or hole transfer and metal-oxide deposition. This finding opens up new ways to design highly effective, photocatalytic composite architectures, containing spatially separated catalytic particles of multiple compositions. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8ta00781k
  • Photocatalytic activity of ZnV 2 O 6 /reduced graphene oxide nanocomposite: From theory to experiment
    Sameie, H. and Sabbagh Alvani, A.A. and Naseri, N. and Rosei, F. and Mul, G. and Mei, B.T.
    Journal of the Electrochemical Society 165 (2018)
    A nanocomposite of ZnV 2 O 6 with hierarchical flower-like structure hybridized with reduced graphene oxide (rGO) was fabricated using a facile hydrothermal approach. The structure, morphology, optical and electronic properties were explored using comprehensive analytical techniques. The results revealed that the rGO sheets were decorated with the in situ-formed ZnV 2 O 6 nanoparticles yielding a well-combined composite structure. The photocatalytic activity of as-prepared ZnV 2 O 6 /rGO hybrids is 2.48 times larger than that of pristine ZnV 2 O 6 for the degradation of Rhodamine B (RhB). In parallel to the experimental results, the basic mechanisms of interfacial interaction, charge transfer/separation and subsequently their influence on the photocatalytic activity were theoretically studied by first-principles calculations. The photocatalytic enhancement is attributed to efficient interfacial electron transfer from ZnV 2 O 6 to rGO, leading to a prolonged lifetime of photoinduced charge carriers. We anticipate that these results will lead to new insights in the judicious design of graphene-based semiconductor photocatalysts. © 2018 The Electrochemical Society.
    view abstract10.1149/2.0601807jes
  • Towards sustainable chlorate production: The effect of permanganate addition on current efficiency
    Endrődi, B. and Sandin, S. and Smulders, V. and Simic, N. and Wildlock, M. and Mul, G. and Mei, B.T. and Cornell, A.
    Journal of Cleaner Production 182 (2018)
    Sodium dichromate is an essential solution additive for the electrocatalytic production of sodium chlorate, assuring selective hydrogen evolution. Unfortunately, the serious environmental and health concerns related to hexavalent chromium mean there is an urgent need to find an alternative solution to achieve the required selectivity. In this study sodium permanganate is evaluated as a possible alternative to chromate, with positive results. The permanganate additive is stable in hypochlorite-containing solutions, and during electrolysis a thin film is reductively deposited on the cathode. The deposit is identified as amorphous manganese oxide by Raman spectroscopic and X-ray diffraction studies. Using different electrochemical techniques (potentiodynamic measurements, galvanostatic polarization curves) we demonstrate that the reduction of hypochlorite is suppressed, while the hydrogen evolution reaction can still proceed. In addition, the formed manganese oxide film acts as a barrier for the reduction of dissolved oxygen. The extent of hydrogen evolution selectivity in hypochlorite solutions was quantified in an undivided electrochemical cell using mass spectrometry. The cathodic current efficiency is significantly enhanced after the addition of permanganate, while the effect on the anodic selectivity and the decomposition of hypochlorite in solution is negligible. Importantly, similar results were obtained using electrodes with manganese oxide films formed ex situ. In conclusion, manganese oxides show great promise in inducing selective hydrogen evolution, and may open new research avenues to the rational design of selective cathodes, both for the chlorate process and for related processes such as photocatalytic water splitting. © 2018 Elsevier Ltd
    view abstract10.1016/j.jclepro.2018.02.071
  • Beyond Water Splitting: Efficiencies of Photo-Electrochemical Devices Producing Hydrogen and Valuable Oxidation Products
    Mei, B. and Mul, G. and Seger, B.
    Advanced Sustainable Systems 1 (2017)
    view abstract10.1002/adsu.201600035
  • Overcoming the Instability of Nanoparticle-Based Catalyst Films in Alkaline Electrolyzers by using Self-Assembling and Self-Healing Films
    Barwe, S. and Masa, J. and Andronescu, C. and Mei, B. and Schuhmann, W. and Ventosa, E.
    Angewandte Chemie - International Edition 56 (2017)
    Engineering stable electrodes using highly active catalyst nanopowders for electrochemical water splitting remains a challenge. We report an innovative and general approach for attaining highly stable catalyst films with self-healing capability based on the in situ self-assembly of catalyst particles during electrolysis. The catalyst particles are added to the electrolyte forming a suspension that is pumped through the electrolyzer. Particles with negatively charged surfaces stick onto the anode, while particles with positively charged surfaces stick to the cathode. The self-assembled catalyst films have self-healing properties as long as sufficient catalyst particles are present in the electrolyte. The proof-of-concept was demonstrated in a non-zero gap alkaline electrolyzer using NiFe-LDH and NixB catalyst nanopowders for anode and cathode, respectively. Steady cell voltages were maintained for at least three weeks during continuous electrolysis at 50–100 mA cm−2. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201703963
  • Oxidative photo-deposition of chromia: Tuning the activity for overall water splitting of the Rh/CrO: X co-catalyst system
    Menze, J. and Mei, B. and Weide, P. and Muhler, M.
    Journal of Materials Chemistry A 5 (2017)
    Employing an oxidative photodeposition of CrOx the well-known Rh/CrOx co-catalyst system was prepared on different semiconductors. These photocatalysts showed up to 25% higher overall water splitting activities compared with conventionally prepared materials. The enhancement is attributed to a favorable selective deposition of CrOx caused by charge-directed deposition. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7ta04924b
  • 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 (2017)
    SrTiO3 is a well-known photocatalyst inducing overall water splitting when exposed to UV irradiation of wavelengths &lt;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, Weinheim
    view abstract10.1002/cssc.201701794
  • Solid electrolyte interphase (SEI) at TiO2 electrodes in li-ion batteries: Defining apparent and effective SEI based on evidence from X-ay photoemission spectroscopy and scanning electrochemical microscopy
    Ventosa, E. and Madej, E. and Zampardi, G. and Mei, B. and Weide, P. and Antoni, H. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    ACS Applied Materials and Interfaces 9 (2017)
    The high (de)lithiation potential of TiO2 (ca. 1.7 V vs Li/ Li+ in 1 M Li+) decreases the voltage and, thus, the energy density of a corresponding Li-ion battery. On the other hand, it offers several advantages such as the (de)lithiation potential far from lithium deposition or absence of a solid electrolyte interphase (SEI). The latter is currently under controversial debate as several studies reported the presence of a SEI when operating TiO2 electrodes at potentials above 1.0 V vs Li/Li+. We investigate the formation of a SEI at anatase TiO2 electrodes by means of X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). The investigations were performed in different potential ranges, namely, during storage (without external polarization), between 3.0-2.0 V and 3.0-1.0 V vs Li/Li+, respectively. No SEI is formed when a completely dried and residues-free TiO2 electrode is cycled between 3.0 and 2.0 V vs Li/Li+. A SEI is detected by XPS in the case of samples stored for 6 weeks or cycled between 3.0 and 1.0 V vs Li/Li+. With use of SECM, it is verified that this SEI does not possess the electrically insulating character as expected for a "classic" SEI. Therefore, we propose the term apparent SEI for TiO2 electrodes to differentiate it from the protecting and ef fective SEI formed at graphite electrodes. © 2016 American Chemical Society.
    view abstract10.1021/acsami.6b13306
  • Transient Behavior of Ni@NiOx Functionalized SrTiO3 in Overall Water Splitting
    Han, K. and Kreuger, T. and Mei, B. and Mul, G.
    ACS Catalysis 7 (2017)
    Transients in the composition of Ni@NiOx core-shell co-catalysts deposited on SrTiO3 are discussed on the basis of state-of-the-art continuous analysis of photocatalytic water splitting, and post-XPS and TEM analyses. The formation of excessive hydrogen (H2:O2 ≫ 2) in the initial stages of illumination demonstrates oxidation of Ni(OH)2 to NiOOH (nickel oxyhydroxide), with the latter catalyzing water oxidation. A disproportionation reaction of Ni and NiOOH, yielding Ni(OH)2 with residual embedded Ni, occurs when illumination is discontinued, which explains repetitive transients in (excess) hydrogen and oxygen formation when illumination is reinitiated. (Chemical Equation Presented). © 2017 American Chemical Society.
    view abstract10.1021/acscatal.6b03662
  • Back-Illuminated Si-Based Photoanode with Nickel Cobalt Oxide Catalytic Protection Layer
    Bae, D. and Mei, B. and Frydendal, R. and Pedersen, T. and Seger, B. and Hansen, O. and Vesborg, P.C.K. and Chorkendorff, I.
    ChemElectroChem 3 (2016)
    The front cover artwork is provided by Ib Chorkendorff, Dowon Bae, and co-workers at the Technical University of Denmark (Denmark). The image highlights a time-dependent improvement in the water oxidation reaction of a back-side illuminated c-Si-based photoanode protected by a co-sputtered thin film of NiCoOX with iron incorporation. Read the full text of the Article at 10.1002/celc.201500554. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201600566
  • Surface Structure and Photocatalytic Properties of Bi2WO6 Nanoplatelets Modified by Molybdena Islands from Chemical Vapor Deposition
    Dittmer, A. and Menze, J. and Mei, B. and Strunk, J. and Luftman, H.S. and Gutkowski, R. and Wachs, I.E. and Schuhmann, W. and Muhler, M.
    Journal of Physical Chemistry C 120 (2016)
    We report on a novel route of preparing molybdena-modified bismuth tungstates and their successful application in the photocatalytic oxygen evolution reaction and the oxidation of glycerol. Hierarchically assembled monocrystalline Bi2WO6 nanoplatelets with a specific surface area of 10 m2/g were obtained applying a hydrothermal synthesis method using Na2WO4 and Bi(NO3)3 as precursors, followed by a solvent-free chemical vapor deposition method using Mo(CO)6, resulting in highly dispersed molybdena species. Extensive characterization using X-ray photoelectron spectroscopy, low-energy ion scattering, and Raman spectroscopy showed that microcrystalline MoO3 islands were formed on the bismuth tungstate surface that grew in height and lateral dimension with increasing loading. Correspondingly, the molybdena-modified materials were found to have favorable photocatalytic and photoelectrochemical properties in the oxygen evolution reaction and the selective oxidation of glycerol. © 2016 American Chemical Society.
    view abstract10.1021/acs.jpcc.6b07007
  • Tailoring Mixed-Halide, Wide-Gap Perovskites via Multistep Conversion Process
    Bae, D. and Palmstrom, A. and Roelofs, K. and Mei, B. and Chorkendorff, I. and Bent, S.F. and Vesborg, P.C.K.
    ACS Applied Materials and Interfaces 8 (2016)
    Wide-band-gap mixed-halide CH3NH3PbI3-XBrX-based solar cells have been prepared by means of a sequential spin-coating process. The spin-rate for PbI2 as well as its repetitive deposition are important in determining the cross-sectional shape and surface morphology of perovskite, and, consequently, J-V performance. A perovskite solar cell converted from PbI2 with a dense bottom layer and porous top layer achieved higher device performance than those of analogue cells with a dense PbI2 top layer. This work demonstrates a facile way to control PbI2 film configuration and morphology simply by modification of spin-coating parameters without any additional chemical or thermal post-treatment. © 2016 American Chemical Society.
    view abstract10.1021/acsami.6b01246
  • Cocatalyst Designing: A Regenerable Molybdenum-Containing Ternary Cocatalyst System for Efficient Photocatalytic Water Splitting
    Busser, G.W. and Mei, B. and Weide, P. and Vesborg, P.C.K. and Stührenberg, K. and Bauer, M. and Huang, X. and Willinger, M.-G. and Chorkendorff, I. and Schlögl, R. and Muhler, M.
    ACS Catalysis 5 (2015)
    Earth-abundant materials are required to facilitate upscaling of renewable hydrogen generation. Here, the synthesis of a novel oxidic ternary cocatalyst containing molybdenum, chromium, and copper, which has been found to be highly active in the overall photocatalytic splitting of water over gallium oxide, is described. With the noble metal-free system, requiring hydrogen evolution rates comparable to that of the well-established Rh<inf>x</inf>Cr<inf>2-x</inf>O<inf>3</inf>/Ga<inf>2</inf>O<inf>3</inf> water splitting cocatalyst is achieved. Although the stability of the as-prepared ternary cocatalyst system appeared to be poor, the cocatalyst can be easily regenerated and stabilized by an oxygen treatment under ambient conditions. Furthermore, higher MoO<inf>x</inf> loadings were found to be more active and showed improved stability. By means of careful characterization using X-ray-based spectroscopy and TEM, the function of the individual cocatalyst compounds was closely examined, suggesting synergetic interactions of molybdena and chromia stabilizing CuO against photoreduction. Although stability issues should be further addressed, this work highlights that multicomponent systems, which are well-studied in industrial processes for heterogeneous reactions and commonly used in various other fields of research, can be used in solar water splitting. In particular, molybdena-containing materials are discovered as a new class of earth-abundant cocatalysts for overall water-splitting. © 2015 American Chemical Society.
    view abstract10.1021/acscatal.5b01428
  • Crystalline TiO2: A Generic and Effective Electron-Conducting Protection Layer for Photoanodes and -cathodes
    Mei, B. and Pedersen, T. and Malacrida, P. and Bae, D. and Frydendal, R. and Hansen, O. and Vesborg, P.C.K. and Seger, B. and Chorkendorff, I.
    Journal of Physical Chemistry C 119 (2015)
    Stabilizing efficient photoabsorbers for solar water splitting has recently shown significant progress with the development of various protection layers. Suitable protection layers for tandem devices should be conductive, transparent, and stable in strongly acidic or alkaline solutions. This paper shows that under certain conditions n-type semiconductors, such as TiO2, can be used as protection layers for Si-based photoanodes. It also provides evidence that even in a photoanode assembly TiO2 is conducting only electrons (not holes as in p-type protection layers), and therefore TiO2 can be described as a simple ohmic contact. This renders n-type semiconductors, such as TiO2, to be versatile and simple protection layers, which can be used for photoanodes and as previously shown for photocathodes. The ohmic behavior of n-type TiO2 in a Si/TiO2-photoanode assembly is demonstrated under dark and illuminated conditions by performing the oxygen evolution reaction (OER) and using the Fe(II)/Fe(III) redox couple. These measurements reveal that the performance of the Si/TiO2-photoanode assembly is strongly dependent on the TiO2/electrolyte interaction. Finally, the conditions and requirements that make TiO2 generally applicable for photoanode assemblies, and thus for protecting tandem devices, are outlined and quantitatively shown by band diagram calculations. The results presented here provide the understanding required for the design of highly efficient and stable photoelectrochemical water splitting devices. © 2015 American Chemical Society.
    view abstract10.1021/acs.jpcc.5b04407
  • Scalability and feasibility of photoelectrochemical H2 evolution: the ultimate limit of Pt nanoparticle as an HER catalyst
    Kemppainen, E. and Bodin, A. and Sebok, B. and Pedersen, T. and Seger, B. and Mei, B. and Bae, D. and Vesborg, P.C.K. and Halme, J. and Hansen, O. and Lund, P.D. and Chorkendorff, I.
    Energy and Environmental Science 8 (2015)
    The recent surge in investigating electrocatalysts for the H2 evolution reaction is based on finding a cheap alternative to Pt. However platinum's excellent catalytic activity means very little catalyst needs to be used. The present study combines model experiments with numerical modeling to determine exactly how little catalyst is needed. Specifically we investigate ultra-low Pt loadings for use in photoelectrochemical H2 evolution using TiO2-Ti-pn+Si photocathodes. At a current density of 10 mA cm-2, we photocathodically evolve H2 at +465, +450, +350 and +270 mV vs., RHE at Pt loadings of 1000, 200, 50, and 10 ng cm-2 corresponding to HER overpotentials of η1000ng = 32 mV, η200ng = 46 mV, η50ng = 142 mV, and η10ng = 231 mV. To put this in perspective, if 30% of the world's current annual Pt production was used for H2 evolution catalysis, using a loading of 100 ng cm-2 and a current of 10 mA cm-2 would produce 1 TWaverage of H2. The photoelectrochemical data matched the modeling calculations implying that we were near the fundamental maximum in performance for our system. Furthermore modeling indicated that the overpotentials were dominated by mass transfer effects, rather than catalysis unless catalyst loadings were less than 1000 ng cm-2. © The Royal Society of Chemistry.
    view abstract10.1039/c5ee02188j
  • Enhanced photocatalytic degradation rates at rutile TiO2 photocatalysts modified with redox co-catalysts
    Neubert, S. and Pulisova, P. and Wiktor, C. and Weide, P. and Mei, B. and Guschin, D.A. and Fischer, R.A. and Muhler, M. and Beranek, R.
    Catalysis Today 230 (2014)
    Nanocrystalline rutile TiO2 powders were modified with small amounts of CuOx and FeOx clusters by impregnation and drying. The modified rutile samples exhibited drastically enhanced photocatalytic degradation of 4-chlorophenol under UV + vis (λ &gt; 320 nm) irradiation. The reaction rates were increased by the factor of 7 and 4 for the optimized TiO2(R)-CuOx and TiO2(R)-FeO x samples containing 0.12 wt.% Cu and 0.13 wt.% Fe, respectively. The visible light (λ &gt; 455 nm) activity in 4-CP degradation was negligible. Photopotential transient measurements have confirmed that amorphous CuOx and FeOx clusters deposited at the surface of rutile TiO2 act as efficient co-catalysts for oxygen reduction by photogenerated electrons, which leads to improved charge separation and diminished recombination. This study shows that simple modification of TiO 2 photocatalysts with redox co-catalysts based on metal oxides is a promising strategy for enhancing the photocatalytic activity in degradation of aqueous pollutants. © 2013 Elsevier B.V.
    view abstract10.1016/j.cattod.2013.11.046
  • Iron-treated NiO as a highly transparent p-type protection layer for efficient Si-based photoanodes
    Mei, B. and Permyakova, A.A. and Frydendal, R. and Bae, D. and Pedersen, T. and Malacrida, P. and Hansen, O. and Stephens, I.E.L. and Vesborg, P.C.K. and Seger, B. and Chorkendorff, I.
    Journal of Physical Chemistry Letters 5 (2014)
    Sputter deposition of 50 nm thick NiO films on p+-n-Si and subsequent treatment in an Fe-containing electrolyte yielded highly transparent photoanodes capable of water oxidation (OER) in alkaline media (1 M KOH) with high efficiency and stability. The Fe treatment of NiO thin films enabled Si-based photoanode assemblies to obtain a current density of 10 mA/cm2 (requirement for &gt;10% efficient devices) at 1.15 V versus RHE (reversible hydrogen electrode) under red-light (38.6 mW/cm2) irradiation. Thus, the photoanodes were harvesting ∼80 mV of free energy (voltage), which places them among the best-performing Si-based photoanodes in alkaline media. The stability was proven by chronoamperometry at 1.3 V versus RHE for 300 h. Furthermore, measurements with electrochemical quartz crystal microbalances coupled with ICP-MS showed minor corrosion under dark operation. Extrapolation of the corrosion rate showed stability for more than 2000 days of continuous operation. Therefore, protection by Fe-treated NiO films is a promising strategy to achieve highly efficient and stable photoanodes. © 2014 American Chemical Society.
    view abstract10.1021/jz501872k
  • Photodeposition of copper and chromia on gallium oxide: The role of co-catalysts in photocatalytic water splitting
    Busser, G.W. and Mei, B. and Pougin, A. and Strunk, J. and Gutkowski, R. and Schuhmann, W. and Willinger, M.-G. and Schlögl, R. and Muhler, M.
    ChemSusChem 7 (2014)
    Split second: The photocatalytic activity of gallium oxide (β-Ga 2O3) depends strongly on the co-catalysts CuOx and chromia, which can be efficiently deposited in a stepwise manner by photoreduction of Cu2+ and CrO42-. The water-splitting activity can be tuned by varying the Cu loading in the range 0.025-1.5 wt %, whereas the Cr loading is not affecting the rate as long as small amounts (such as 0.05 wt %) are present. Chromia is identified as highly efficient co-catalyst in the presence of CuOx: it is essential for the oxidation of water. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201301065
  • Protection of p+-n-Si photoanodes by sputter-deposited Ir/IrOx thin films
    Mei, B. and Seger, B. and Pedersen, T. and Malizia, M. and Hansen, O. and Chorkendorff, I. and Vesborg, P.C.K.
    Journal of Physical Chemistry Letters 5 (2014)
    Sputter deposition of Ir/IrOx on p+-n-Si without interfacial corrosion protection layers yielded photoanodes capable of efficient water oxidation (OER) in acidic media (1 M H2SO4). Stability of at least 18 h was shown by chronoamperomety at 1.23 V versus RHE (reversible hydrogen electrode) under 38.6 mW/cm2 simulated sunlight irradiation (λ &gt; 635 nm, AM 1.5G) and measurements with quartz crystal microbalances. Films exceeding a thickness of 4 nm were shown to be highly active though metastable due to an amorphous character. By contrast, 2 nm IrOx films were stable, enabling OER at a current density of 1 mA/cm2 at 1.05 V vs. RHE. Further improvement by heat treatment resulted in a cathodic shift of 40 mV and enabled a current density of 10 mA/cm2 (requirements for a 10% efficient tandem device) at 1.12 V vs. RHS under irradiation. Thus, the simple IrOx/Ir/p+-n-Si structures not only provide the necessary overpotential for OER at realistic device current, but also harvest ∼100 mV of free energy (voltage) which makes them among the best-performing Si-based photoanodes in low-pH media. © 2014 American Chemical Society.
    view abstract10.1021/jz500865g
  • Reliable benchmark material for anatase TiO2 in Li-ion batteries: On the role of dehydration of commercial TiO2
    Madej, E. and La Mantia, F. and Mei, B. and Klink, S. and Muhler, M. and Schuhmann, W. and Ventosa, E.
    Journal of Power Sources 266 (2014)
    Commercially available anatase TiO2 nanoparticles (ca. 15-20 nm particle size) were investigated as negative electrode material for Li-ion batteries. Despite the high initial specific charge of 200 mAh g-1 at 0.5C, the pristine commercial TiO2 failed to retain the reversible capacity upon cycling, keeping only 23% of the initial value after 80 cycles. X-ray photoelectron spectroscopy (XPS) results together with electrochemical data suggest that the failure in cyclability is of kinetic nature as the loss in specific charge is not completely irreversible. Thermogravimetry analysis revealed that the pristine TiO2 contained a significant amount of TiO(OH)2 (ca. 8%) which can be easily removed by dehydration when annealing in air above 250 °C. Air-annealing of TiO2 at 300 °C resulted in a remarkable improvement in cyclability retaining 83% of initial specific charge after 80 cycles at 0.5C. No further improvement in cyclability was observed for TiO2 annealed at 450 °C suggesting that the dehydration of TiO(OH)2 was the primary source of the improvement. Knowing the role of dehydration of TiO2 allows obtaining a reliable benchmark material via simple air-annealing and becomes a key factor when developing advanced materials from commercial TiO2. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2014.05.018
  • Strategies to design efficient silica-supported photocatalysts for reduction of CO2
    Hamdy, M.S. and Amrollahi, R. and Sinev, I. and Mei, B. and Mul, G.
    Journal of the American Chemical Society 136 (2014)
    The photocatalytic reduction of CO2 by water vapor to produce light hydrocarbons was studied over a series of catalysts consisting of variable loading of Ti incorporated in TUD-1 mesoporous silica, either modified by ZnO nanoparticles or isolated Cr-sites. Unexpectedly, the performance of ZnO-Ti-TUD-1 and Cr-Ti-TUD-1 was inferior to the parent Ti-TUD-1. An explanation can be found in experiments on the photocatalytic degradation of a mixture of hydrocarbons (i.e., CH4, C2H4, C 2H6, C3H6, and C3H 8) under the same illumination conditions. Ti-TUD-1 exhibits the poorest activity in hydrocarbon degradation, while ZnO-Ti-TUD-1 and Cr-Ti-TUD-1 showed very significant degradation rates. This study clearly demonstrates the importance of evaluating hydrocarbon conversion over photocatalysts active in converting CO2 to hydrocarbons (in batch reactors). © 2013 American Chemical Society.
    view abstract10.1021/ja410363v
  • Surface-modified TiO2 photocatalysts prepared by a photosynthetic route: Mechanism, enhancement, and limits
    Neubert, S. and Ramakrishnan, A. and Strunk, J. and Shi, H. and Mei, B. and Wang, L. and Bledowski, M. and A. Guschin, D. and Kauer, M. and Wang, Y. and Muhler, M. and Beranek, R.
    ChemPlusChem 79 (2014)
    Surface-modified TiO2 photocatalysts were synthesized by a photosynthetic route involving visible-light-induced (l&gt; 455 nm) activation of benzene and toluene at the surface of TiO2 leading to the formation of carbonaceous polymeric deposits. IR spectroscopic and photoelectrochemical experiments showed that the mechanism of the photosynthetic reactions involves intra-bandgap surface states at TiO2 related to surface OH groups interacting with adsorbed aromatic molecules. The photosynthesized surface-modified TiO2 materials exhibited enhanced activity, relative to pristine TiO2, in photocatalytic degradation (and complete mineralization) of 4-chlorophenol. The improvement was pronounced particularly under visible-light (l&gt;455 nm) irradiation with the relative initial photodegradation rate enhanced by a factor of four. The surface-modified photocatalysts exhibited good stability under the operating conditions, and the optimum carbon content was approximately 0.5 wt%. Mechanistic studies showed that the enhanced visible-light photodegradation of 4-chlorophenol is due to modified surface-adsorption properties that facilitate formation of a surface complex between titania and 4-chlorophenol, rather than due to any sensitizing effect of the carbonaceous deposits. The study highlights the importance of considering the interaction between pollutant molecules and the photocatalyst surface in heterogeneous photocatalysis, and possibly opens up a route for photosynthesis of further surface- modified photocatalysts with tuned surface properties. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cplu.201300277
  • Ammonia-annealed TiO2 as a negative electrode material in Li-Ion batteries: N doping or oxygen deficiency?
    Ventosa, E. and Xia, W. and Klink, S. and Mantia, F.L. and Mei, B. and Muhler, M. and Schuhmann, W.
    Chemistry - A European Journal 19 (2013)
    Improving the chemical diffusion of Li ions in anatase TiO2 is essential to enhance its rate capability as a negative electrode for Li-ion batteries. Ammonia annealing has been used to improve the rate capability of Li4Ti5O12. Similarly, ammonia annealing improves the Li-ion storage performance of anatase TiO2 in terms of the stability upon cycling and the Crate capability. In order to distinguish whether N doping or oxygen deficiencies, both introduced upon ammonia annealing, are more relevant for the observed improvement, a systematic electrochemical study was performed. The results suggest that the creation of oxygen vacancies upon ammonia annealing is the main reason for the improvement of the stability and C-rate capability. © 2013 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/chem.201302306
  • Beneficial effect of Nb doping on the photoelectrochemical properties of TiO2 and TiO2-polyheptazine hybrids
    Mei, B. and Byford, H. and Bledowski, M. and Wang, L. and Strunk, J. and Muhler, M. and Beranek, R.
    Solar Energy Materials and Solar Cells 117 (2013)
    Nb-doped TiO2 in pure anatase form prepared by spray drying exhibits enhanced photoelectrochemical performance both in its bare form (under UV irradiation) and when used as an electron collector in TiO 2-polyheptazine hybrid photoanodes for water photooxidation under visible (λ&gt;420 nm) light. The optimum Nb-doping concentration was 0.1 at%, and the enhancement of photocurrents was found to be chiefly due to enhanced mobility of electrons in Nb-doped TiO2. Accordingly, the beneficial effect of Nb doping on photocurrent generation in hybrid photoanodes was pronounced particularly at longer irradiation wavelengths and lower bias potentials. © 2013 Elsevier B.V.
    view abstract10.1016/j.solmat.2013.05.024
  • Effect of Sn surface states on the photocatalytic activity of anatase TiO2
    Oropeza, F.E. and Mei, B. and Sinev, I. and Becerikli, A.E. and Muhler, M. and Strunk, J.
    Applied Catalysis B: Environmental 140-141 (2013)
    The influence of surface Sn-doping on the photocatalytic properties of anatase TiO2 has been investigated in samples prepared by a grafting route using Sn(IV) tert-butoxide as Sn precursor. The grafting procedure leads to the formation of isolated Sn(IV) sites on the surface of anatase TiO2 powders as gauged by structural characterisation based on XRD, Raman spectroscopy and XAS. Studies of the surface reduction based on TPR experiments and XPS provide the conditions for a selective reduction of surface Sn(IV) to the divalent oxidation state. Electronic structure characterisation based on valence band XPS and DRS shows that there is a slight widening of the band gap upon Sn(IV)-grafting, but Sn(II) related states emerge at the top of the main valence band upon reduction at temperatures up to 350°C, and this induces visible light absorption. Grafting of TiO2 with Sn(IV) increases the formation rate of OH radicals on the surface of the material. Reduction of the Sn(IV)-grafted TiO2 to form surface Sn(II) brings about substantial increase of the photocatalytic efficiency for the methylene blue degradation under irradiation with λ≥320nm compared with Sn(IV)-grafted and pure anatase TiO2. This observation is explained based on a surface hole trapping by the Sn(II)-related surface states which lie above the top of the main valence band and can therefore act as trapping sites for holes produced under photoexcitation. © 2013 Elsevier B.V.
    view abstract10.1016/j.apcatb.2013.03.043
  • Evidence for metal-support interactions in Au modified TiO x/SBA-15 materials prepared by photodeposition
    Mei, B. and Wiktor, C. and Turner, S. and Pougin, A. and Van Tendeloo, G. and Fischer, R.A. and Muhler, M. and Strunk, J.
    ACS Catalysis 3 (2013)
    Gold nanoparticles have been efficiently photodeposited onto titanate-loaded SBA-15 (Ti(x)/SBA-15) with different titania coordination. Transmission electron microscopy shows that relatively large Au nanoparticles are photodeposited on the outer surface of the Ti(x)/SBA-15 materials and that TiOx tends to form agglomerates in close proximity to the Au nanoparticles, often forming core-shell Au/TiOx structures. This behavior resembles typical processes observed due to strong-metal support interactions. In the presence of gold, the formation of hydrogen on Ti(x)/SBA-15 during the photodeposition process and the performance in the hydroxylation of terephthalic acid is greatly enhanced. The activity of the Au/Ti(x)/SBA-15 materials is found to depend on the TiOx loading, increasing with a larger amount of initially isolated TiO4 tetrahedra. Samples with initially clustered TiOx species show lower photocatalytic activities. When isolated zinc oxide (ZnOx) species are present on Ti(x)/SBA-15, gold nanoparticles are smaller and well dispersed within the pores. Agglomeration of TiOx species and the formation of Au/TiO x structures is negligible. The dispersion of gold and the formation of Au/TiOx in the SBA-15 matrix seem to depend on the mobility of the TiOx species. The mobility is determined by the initial degree of agglomeration of TiOx. Effective hydrogen evolution requires Au/TiOx core-shell composites as in Au/Ti(x)/SBA-15, whereas hydroxylation of terephthalic acid can also be performed with Au/ZnO x/TiOx/SBA-15 materials. However, isolated TiOx species have to be grafted onto the support prior to the zinc oxide species, providing strong evidence for the necessity of Ti-O-Si bridges for high photocatalytic activity in terephthalic acid hydroxylation. © 2013 American Chemical Society.
    view abstract10.1021/cs400964k
  • Influence of photodeposited gold nanoparticles on the photocatalytic activity of titanate species in the reduction of CO2 to hydrocarbons
    Mei, B. and Pougin, A. and Strunk, J.
    Journal of Catalysis 306 (2013)
    Photodeposition of Au nanoparticles was found to enhance the rate of photocatalytic CO2 reduction to short hydrocarbons over titanate species in SBA-15. Product formation from contaminants was ruled out experimentally by a thorough pre-cleaning of the samples and the leak-tight design of the fully metal-sealed gas-phase photoreactor. Without Au, an active carbon pool was observed to accumulate on the catalyst, and higher hydrocarbons were formed. Infrared spectroscopy was used to identify formaldehyde/ paraformaldehyde as major compounds of the carbon pool. The results obtained in the absence of gas-phase water indicate that adsorbed water in the mesopores is sufficient to achieve CO2 reduction and convert CO2 mainly to methane. These results contribute to the knowledge-based improvement of photocatalysts for CO2 reduction. © 2013 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.jcat.2013.06.027
  • TiO2(B)/anatase composites synthesized by spray drying as high performance negative electrode material in Li-ion batteries
    Ventosa, E. and Mei, B. and Xia, W. and Muhler, M. and Schuhmann, W.
    ChemSusChem 6 (2013)
    The power of spray-dried TiO2 in LIBs: TiO2(B)/ anatase is synthesized by spray drying and investigated as negative electrode material in Li-ion batteries. It exhibits excellent Li-ion storage performances, especially at high charge/discharge rates. The presence of the β phase of TiO2 improves Li-ion diffusivity. Additionally, the scalable synthesis method also allows for Nb-doping, which assists in the maintenance of the electronic conductivity as the thickness of film increases. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201300439
  • Trace metal residues promote the activity of supposedly metal-free nitrogen-modified carbon catalysts for the oxygen reduction reaction
    Masa, J. and Zhao, A. and Xia, W. and Sun, Z. and Mei, B. and Muhler, M. and Schuhmann, W.
    Electrochemistry Communications 34 (2013)
    We show in this study that the presence of trace metal residues in some supposedly metal-free catalysts for oxygen reduction, at concentrations which are difficult to detect using conventional methods such as XPS and EDX, can profoundly promote the ORR activity of the catalysts. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elecom.2013.05.032
  • Universal method for protein immobilization on chemically functionalized germanium investigated by ATR-FTIR difference spectroscopy
    Schartner, J. and Güldenhaupt, J. and Mei, B. and Rögner, M. and Muhler, M. and Gerwert, K. and Kötting, C.
    Journal of the American Chemical Society 135 (2013)
    Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy allows a detailed analysis of surface attached molecules, including their secondary structure, orientation, and interaction with small molecules in the case of proteins. Here, we present a universal immobilization technique on germanium for all oligo-histidine-tagged proteins. For this purpose, new triethoxysilane derivates were developed: we synthesized a linker-silane with a succinimidyl ester as amine-reactive headgroup and a matrix-silane with an unreactive ethylene glycol group. A new methodology for the attachment of triethoxysilanes on germanium was established, and the surface was characterized by ATR-FTIR and X-ray photoelectron spectroscopy. In the next step, the succinimidyl ester was reacted with aminonitrilotriacetic acid. Subsequently, Ni2+ was coordinated to form Ni-nitrilotriacetic acid for His-tag binding. The capability of the functionalized surface was demonstrated by experiments using the small GTPase Ras and photosystem I (PS I). The native binding of the proteins was proven by difference spectroscopy, which probes protein function. The function of Ras as molecular switch was demonstrated by a beryllium trifluoride anion titration assay, which allows observation of the "on" and "off" switching of Ras at atomic resolution. Furthermore, the activity of immobilized PS I was proven by light-induced difference spectroscopy. Subsequent treatment with imidazole removes attached proteins, enabling repeated binding. This universal technique allows specific attachment of His-tagged proteins and a detailed study of their function at the atomic level using FTIR difference spectroscopy. © 2013 American Chemical Society.
    view abstract10.1021/ja400253p
  • Enhanced performance of surface-modified TiO2 photocatalysts prepared via a visible-light photosynthetic route
    Ramakrishnan, A. and Neubert, S. and Mei, B. and Strunk, J. and Wang, L. and Bledowski, M. and Muhler, M. and Beranek, R.
    Chemical Communications 48 (2012)
    Benzene can be activated by visible light (λ &gt; 455 nm) in the presence of TiO2, which leads to formation of carbonaceous polymeric deposits on the titania surface. These photosynthesized surface-modified materials exhibit enhanced photoactivity in degradation of phenolic compounds, particularly under visible light irradiation. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2cc34243j
  • Optimizing the deposition of hydrogen evolution sites on suspended semiconductor particles using on-line photocatalytic reforming of aqueous methanol solutions
    Busser, G.W. and Mei, B. and Muhler, M.
    ChemSusChem 5 (2012)
    The deposition of hydrogen evolution sites on photocatalysts is a crucial step in the multistep process of synthesizing a catalyst that is active for overall photocatalytic water splitting. An alternative approach to conventional photodeposition was developed, applying the photocatalytic reforming of aqueous methanol solutions to deposit metal particles on semiconductor materials such as Ga2O3 and (Ga0.6Zn0.4)(N 0.6O0.4). The method allows optimizing the loading of the co-catalysts based on the stepwise addition of their precursors and the continuous online monitoring of the evolved hydrogen. Moreover, a synergetic effect between different co-catalysts can be directly established. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201200374
  • Tuning the acid/base and structural properties of titanate-loaded mesoporous silica by grafting of zinc oxide
    Mei, B. and Becerikli, A. and Pougin, A. and Heeskens, D. and Sinev, I. and Grünert, W. and Muhler, M. and Strunk, J.
    Journal of Physical Chemistry C 116 (2012)
    Mesoporous silica (SBA-15) loaded with TiO x species was synthesized by anhydrous grafting of titanium isopropoxide, and a novel procedure for the preparation of ZnO x/SBA-15 materials by grafting of Zn(acac) 2 was explored. The TiO x/SBA-15 and ZnO x/SBA-15 materials as well as subsequently prepared bifunctional ZnO x- and TiO x-containing SBA-15 materials were characterized in depth by combining N 2 physisorption measurements, UV-vis, X-ray photoelectron and X-ray absorption spectroscopy, and CO 2 and NH 3 temperature-programmed desorption experiments. The characterization results confirmed a close proximity of ZnO x and TiO x in the subsequently grafted materials. Because of strong interactions between the Zn precursor and the SiO 2 surface, the order of the ZnO x and TiO x grafting steps affected the amount of Ti-O-Zn bonds formed in the materials. When ZnO x is present in SBA-15, subsequently grafted TiO x is higher coordinated and more Ti-O-Zn bonds are formed compared to SBA-15 in which TiO x was introduced first, indicating strong interactions between the Ti precursor and ZnO x. While all TiO x and ZnO x-containing samples exhibit a large amount of acidic sites, ZnO x present as isolated species or small clusters in SBA-15 significantly improves the CO 2 adsorption capacity by introducing basic sites. In the subsequently grafted samples the amount of acidic and basic sites is found to be unaffected by the order in which the two transition metals are introduced. © 2012 American Chemical Society.
    view abstract10.1021/jp301908c
  • 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 (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 abstract10.1021/ac102303u
  • The synthesis of Nb-doped TiO2 nanoparticles by spray drying: An efficient and scalable method
    Mei, B. and Sánchez, M.D. and Reinecke, T. and Kaluza, S. and Xia, W. and Muhler, M.
    Journal of Materials Chemistry 21 (2011)
    Nb-doped TiO2 nanoparticles were prepared by a continuous spray drying process using ammonium niobate (V) oxalate and titanium oxysulfate as water-soluble precursors. The structural and electronic properties were investigated using thermogravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Nb was found to be mainly incorporated as Nb5+ into the TiO2 lattice resulting in a charge compensation by Ti vacancies. The characterization results indicate that Nb was homogeneously distributed within the titania lattice, and that the surface segregation of Nb, which is commonly observed for Nb-doped TiO 2, was significantly less pronounced. The high homogeneity and the lower extent of surface segregation originate from the efficient atomization of homogeneous precursor solutions and the fast evaporation of the solvent in the spray drying process. As a result, the ion mobility is diminished and spheres of well-mixed precursor materials are formed. Using the continuous spray drying process followed by a controlled heat treatment, the phase composition, the crystal size and the surface area of the Nb-doped TiO2 nanoparticles are easily adjustable. © The Royal Society of Chemistry 2011.
    view abstract10.1039/c1jm11431j
  • TiO 2 coating of high surface area silica gel by chemical vapor deposition of TiCl 4 in a fluidized-bed reactor
    Xia, W. and Mei, B. and Sánchez, M.D. and Strunk, J. and Muhler, M.
    Journal of Nanoscience and Nanotechnology 11 (2011)
    TiO 2 was deposited on high surface area porous silica gel (400 m 2g -1) in a fluidized bed reactor. Chemical vapor deposition was employed for the coating under vacuum conditions with TiCl 4 as precursor. Nitrogen physisorption, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and UV-vis spectroscopy were applied to characterize the obtained TiO 2-SiO 2 composites with different Ti loadings up to 5 wt%. Only a slight decrease in the specific surface area was detected at low Ti loadings. At a Ti loading of 2 wt%, TiO 2 was found to be highly dispersed on the SiO 2 surface likely in form of a thin film. At higher Ti loadings, two weak reflections corresponding to anatase TiO 2 were observed in the diffraction patterns indicating the presence of crystalline bulk TiO 2. High resolution XPS clearly distinguished two types of Ti species, i.e., Ti-O-Si at the interface and Ti-O-Ti in bulk TiO 2. The presence of polymeric TiOx species at low Ti loadings was confirmed by a blue shift in the UV-vis spectra as compared to bulk TiO 2. All these results point to a strong interaction between the TiO 2 deposit and the porous SiO 2 substrate especially at low Ti loadings. Copyright © 2011 American Scientific Publishers All rights reserved.
    view abstract10.1166/jnn.2011.5107
  • Gas-phase synthesis of gradient catalyst libraries consisting of nanoparticles supported on high surface area porous substrates
    Xia, W. and Mei, B. and Muhler, M.
    Nanoscience and Nanotechnology Letters 2 (2010)
    Despite the advances in high throughput experimentation in recent years the synthesis of realistic catalyst libraries especially gradient catalyst libraries remains as a challenge in material science. Recently, we have developed a method for the synthesis of gradient catalyst libraries consisting of nanoparticles supported on high surface area porous substrates. Chemical vapor deposition (CVD) was employed as a gas-phase method for the synthesis. The method made use of the lateral concentration profile of the precursor-loaded carrier gas stream during CVD, resulting in concentration profile of the deposits on porous substrates. In this report, high surface area materials of both powders (e.g., silica) and bulk composites (e.g., hierarchical carbon structures) were successfully employed as substrates for the deposition of single metal or bimetallic catalyst libraries. The synthesis was achieved by controlling the flow behavior of the effluent precursor stream. The resulting effusion cone led to a radial deposition gradient on the substrate. Different from thin film-type model catalyst libraries, the obtained catalysts can be tested under realistic reaction conditions. Methanol oxidation was studied as a test reaction using scanning mass spectrometry. Copyright © 2010 American Scientific Publishers.
    view abstract10.1166/nnl.2010.1046

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