Prof. Dr. Kristina Tschulik
- 3D atomic-scale imaging of mixed Co-Fe spinel oxide nanoparticles during oxygen evolution reaction
Xiang, W. and Yang, N. and Li, X. and Linnemann, J. and Hagemann, U. and Ruediger, O. and Heidelmann, M. and Falk, T. and Aramini, M. and DeBeer, S. and Muhler, M. and Tschulik, K. and Li, T.
Nature Communications 13 (2022)The three-dimensional (3D) distribution of individual atoms on the surface of catalyst nanoparticles plays a vital role in their activity and stability. Optimising the performance of electrocatalysts requires atomic-scale information, but it is difficult to obtain. Here, we use atom probe tomography to elucidate the 3D structure of 10 nm sized Co2FeO4 and CoFe2O4 nanoparticles during oxygen evolution reaction (OER). We reveal nanoscale spinodal decomposition in pristine Co2FeO4. The interfaces of Co-rich and Fe-rich nanodomains of Co2FeO4 become trapping sites for hydroxyl groups, contributing to a higher OER activity compared to that of CoFe2O4. However, the activity of Co2FeO4 drops considerably due to concurrent irreversible transformation towards CoIVO2 and pronounced Fe dissolution. In contrast, there is negligible elemental redistribution for CoFe2O4 after OER, except for surface structural transformation towards (FeIII, CoIII)2O3. Overall, our study provides a unique 3D compositional distribution of mixed Co-Fe spinel oxides, which gives atomic-scale insights into active sites and the deactivation of electrocatalysts during OER. © 2022, The Author(s).
view abstract 10.1038/s41467-021-27788-2
- Operando electrochemical SERS monitors nanoparticle reactions by capping agent fingerprints
Wonner, K. and Murke, S. and Alfarano, S.R. and Hosseini, P. and Havenith, M. and Tschulik, K.
Nano Research (2022)Nanomaterials are frequently employed in daily life goods, including health, textile, and food industry. A comprehensive picture is lacking on the role of the capping agents, added ligand molecules, in case of nanoparticle reactions and degradation in aqueous solutions, like surface waters or biofluids. Here, we aim to elucidate the capping agent influence on nanoparticle reactivity probing two commonly employed capping agents citrate and polyvinylpyrrolidone (PVP). Their influence on silver nanoparticle (AgNP) transformation is studied, which is particularly important due to its application as an antimicrobial agent. We induce oxidation and reduction processes of AgNPs in halide solutions and we monitor the associated transformations of particles and capping agents by spectro-electrochemical surface-enhanced Raman spectroscopy (SERS). Raman bands of the capping agents are used here to track chemical changes of the nanoparticles under operando conditions. The sparingly soluble and non-plasmon active silver salts (AgBr and AgCl) are formed under potential bias. In addition, we spectroscopically observe plasmon-mediated structural changes of citrate to cis- or trans-aconitate, while PVP is unaltered. The different behavior of the capping agents implies a change in the physical properties on the surface of AgNPs, in particular with respect to the surface accessibility. Moreover, we showcase that reactions of the capping agents induced by different external stimuli, such as applied bias or laser irradiation, can be assessed. Our results demonstrate how SERS of capping agents can be exploited to operando track nanoparticle conversions in liquid media. This approach is envisaged to provide a more comprehensive understanding of nanoparticle fates in complex liquid environments and varied redox conditions. [Figure not available: see fulltext.] © 2021, The Author(s).
view abstract 10.1007/s12274-021-3999-2
- Unexpectedly High Capacitance of the Metal Nanoparticle/Water Interface: Molecular-Level Insights into the Electrical Double Layer
Azimzadeh Sani, M. and Pavlopoulos, N.G. and Pezzotti, S. and Serva, A. and Cignoni, P. and Linnemann, J. and Salanne, M. and Gaigeot, M.-P. and Tschulik, K.
Angewandte Chemie - International Edition 61 (2022)The electrical double-layer plays a key role in important interfacial electrochemical processes from catalysis to energy storage and corrosion. Therefore, understanding its structure is crucial for the progress of sustainable technologies. We extract new physico-chemical information on the capacitance and structure of the electrical double-layer of platinum and gold nanoparticles at the molecular level, employing single nanoparticle electrochemistry. The charge storage ability of the solid/liquid interface is larger by one order-of-magnitude than predicted by the traditional mean-field models of the double-layer such as the Gouy–Chapman–Stern model. Performing molecular dynamics simulations, we investigate the possible relationship between the measured high capacitance and adsorption strength of the water adlayer formed at the metal surface. These insights may launch the active tuning of solid–solvent and solvent–solvent interactions as an innovative design strategy to transform energy technologies towards superior performance and sustainability. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
view abstract 10.1002/anie.202112679
- Unravelling Anion Solvation in Water-Alcohol Mixtures by Single Entity Electrochemistry
Saw, E.N. and Kanokkanchana, K. and Amin, H.M.A. and Tschulik, K.
ChemElectroChem (2022)Single entity electrochemistry is employed to gain insights into ion solvation in solvent mixtures. To this end, the time required for the oxidation of individual indicator nanoparticles to sparingly soluble products is used to probe ionic diffusion, and hence gain new insights into the solvation properties of solvent mixtures. Herein, water-ethanol or water-methanol mixtures of different compositions are analyzed following this new approach, using silver nanoparticle oxidation in the presence of chloride and iodide as a complementary indicator reaction. For increasing concentrations of the bulkier alcohol molecules in the mixtures with water, an increasing content of alcohol molecules in the halide's solvation shell is detected by the observation of hindered halide diffusion. The extent of this solvent replacement is shown to scale with the charge density of the ions and the experimental results are rationalized with respect to literature-derived thermodynamic data, highlighting the ability of single entity electrochemistry to explore solvation in solvent mixtures. © 2022 The Authors. ChemElectroChem published by Wiley-VCH GmbH.
view abstract 10.1002/celc.202101435
- A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols
Najafishirtari, S. and Friedel Ortega, K. and Douthwaite, M. and Pattisson, S. and Hutchings, G.J. and Bondue, C.J. and Tschulik, K. and Waffel, D. and Peng, B. and Deitermann, M. and Busser, G.W. and Muhler, M. and Behrens, M.
Chemistry - A European Journal (2021)Selective oxidation of higher alcohols using heterogeneous catalysts is an important reaction in the synthesis of fine chemicals with added value. Though the process for primary alcohol oxidation is industrially established, there is still a lack of fundamental understanding considering the complexity of the catalysts and their dynamics under reaction conditions, especially when higher alcohols and liquid-phase reaction media are involved. Additionally, new materials should be developed offering higher activity, selectivity, and stability. This can be achieved by unraveling the structure–performance correlations of these catalysts under reaction conditions. In this regard, researchers are encouraged to develop more advanced characterization techniques to address the complex interplay between the solid surface, the dissolved reactants, and the solvent. In this mini-review, we report some of the most important approaches taken in the field and give a perspective on how to tackle the complex challenges for different approaches in alcohol oxidation while providing insight into the remaining challenges. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
view abstract 10.1002/chem.202102868
- Design Strategies for Electrocatalysts from an Electrochemist's Perspective
Linnemann, J. and Kanokkanchana, K. and Tschulik, K.
ACS Catalysis 11 (2021)The aim to produce highly active, selective, and long-lived electrocatalysts by design drives major research efforts toward gaining fundamental understanding of the relationship between material properties and their catalytic performance. Surface characterization tools enable to assess atomic scale information on the complexity of electrocatalyst materials. Advancing electrochemical methodologies to adequately characterize such systems was less of a research focus point. In this Review, we shed light on the ability to gain fundamental insights into electrocatalysis from a complementary perspective and establish corresponding design strategies. These may rely on adopting the perceptions and models of other subareas of electrochemistry, such as corrosion, battery research, or electrodeposition. Concepts on how to account for and improve mass transport, manage gas bubble release, or exploit magnetic fields are highlighted in this respect. Particular attention is paid to deriving design strategies for nanoelectrocatalysts, which is often impeded, as structural and physical material properties are buried in electrochemical data of whole electrodes or even devices. Thus, a second major approach focuses on overcoming this difference in the considered level of complexity by methods of single-entity electrochemistry. The gained understanding of intrinsic catalyst performance may allow to rationally advance design concepts with increased complexity, such as three-dimensional electrode architectures. Many materials undergo structural changes upon formation of the working catalyst. Accordingly, developing "precatalysts"with low hindrance of the electrochemical transformation to the active catalyst is suggested as a final design strategy. ©
view abstract 10.1021/acscatal.0c04118
- Direct Detection of Surface Species Formed on Iridium Electrocatalysts during the Oxygen Evolution Reaction
BalaKrishnan, A. and Blanc, N. and Hagemann, U. and Gemagami, P. and Wonner, K. and Tschulik, K. and Li, T.
Angewandte Chemie - International Edition 60 (2021)The effect of surface orientations on the formation of iridium oxide species during the oxygen evolution reaction (OER) remains yet unknown. Herein, we use a needle-shaped iridium atom probe specimen as a nanosized working electrode to ascertain the role of the surface orientations in the formation of oxide species during OER. At the beginning of electrolysis, the top 2–3 nm of (024), (026), (113), and (115) planes are covered by IrO−OH, which activates all surfaces towards OER. A thick subsurface oxide layer consisting of sub-stoichiometric Ir−O species is formed on the open (024) planes as OER proceeds. Such metastable Ir−O species are thought to provide an additional contribution to the OER activity. Overall, this study sheds light on the importance of the morphological effects of iridium electrocatalysts for OER. It also provides an innovative approach that can directly reveal surface species on electrocatalysts at atomic scale. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
view abstract 10.1002/anie.202106790
- Editorial overview: Physical and nano electrochemistry 2021 rising stars at the horizon of nanoscale electrochemistry
Current Opinion in Electrochemistry 29 (2021)
view abstract 10.1016/j.coelec.2021.100815
- Interface Sensitivity in Electron/Ion Yield X-ray Absorption Spectroscopy: The TiO2-H2O Interface
Van Spronsen, M.A. and Zhao, X. and Jaugstetter, M. and Escudero, C. and Duchoň, T. and Hunt, A. and Waluyo, I. and Yang, P. and Tschulik, K. and Salmeron, M.B.
Journal of Physical Chemistry Letters 12 (2021)To understand corrosion, energy storage, (electro)catalysis, etc., obtaining chemical information on the solid-liquid interface is crucial but remains extremely challenging. Here, X-ray absorption spectroscopy (XAS) is used to study the solid-liquid interface between TiO2 and H2O. A thin film (6.7 nm) of TiO2 is deposited on an X-ray-transparent SiNx window, acting as the working electrode in a three-electrode flow cell. The spectra are collected based on the electron emission resulting from the decay of the X-ray-induced core-hole-excited atoms, which we show is sensitive to the solid-liquid interface within a few nm. The drain currents measured at the working and counter electrodes are identical but of opposite sign. With this method, we found that the water layer next to anatase is spectroscopically similar to ice. This result highlights the potential of electron-yield XAS to obtain chemical and structural information with a high sensitivity for the species at the electrode-electrolyte interface. © 2021 The Authors. Published by American Chemical Society.
view abstract 10.1021/acs.jpclett.1c02115
- Nanoparticle impact electrochemistry
Azimzadeh Sani, M. and Tschulik, K.
Frontiers of Nanoscience 18 (2021)This chapter presents recent developments in the use of nanoparticle impact techniques to detect and characterize individual nanoparticles. We initially present the four major nanoparticle impact techniques utilized to date: blocking impacts of electrochemically inactive nanoparticles on an ultramicroelectrode (UME); capacitive impacts of nanoparticles on a potentiostated UME; transformative electrochemical conversion of nanoparticles at a polarized UME; and catalytic nanoparticle impacts through amplification of a faradaic current. Some key uses of nanoparticle impacts are reviewed, including nanoparticle detection; characterization of various metallic, metal oxide, metal halide, polymer, and organic nanoparticles; and electrocatalysis investigations. Practical instrumentation and analysis considerations for nanoparticle impact experiments are summarized. Hereafter, recent, novel experimental configurations used for nanoparticle impact electrochemistry are highlighted, including microjet systems, fast-scan cyclic voltammetry, and the combination of impact experiments with optical methods as well as nanopipette and nanogap systems. © 2021 Elsevier Ltd
view abstract 10.1016/B978-0-12-820055-1.00007-1
- Single co3o4 nanocubes electrocatalyzing the oxygen evolution reaction: Nano-impact insights into intrinsic activity and support effects
Liu, Z. and Corva, M. and Amin, H.M.A. and Blanc, N. and Linnemann, J. and Tschulik, K.
International Journal of Molecular Sciences 22 (2021)Single-entity electrochemistry allows for assessing electrocatalytic activities of individual material entities such as nanoparticles (NPs). Thus, it becomes possible to consider intrinsic electrochemical properties of nanocatalysts when researching how activity relates to physical and structural material properties. Conversely, conventional electrochemical techniques provide a normal-ized sum current referring to a huge ensemble of NPs constituting, along with additives (e.g., bind-ers), a complete catalyst-coated electrode. Accordingly, recording electrocatalytic responses of single NPs avoids interferences of ensemble effects and reduces the complexity of electrocatalytic pro-cesses, thus enabling detailed description and modelling. Herein, we present insights into the oxygen evolution catalysis at individual cubic Co3O4 NPs impacting microelectrodes of different support materials. Simulating diffusion at supported nanocubes, measured step current signals can be analyzed, providing edge lengths, corresponding size distributions, and interference-free turnover frequencies. The provided nano-impact investigation of (electro-)catalyst-support effects contra-dicts assumptions on a low number of highly active sites. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
view abstract 10.3390/ijms222313137
- Stripping away ion hydration shells in electrical double-layer formation: Water networks matter
Alfarano, S.R. and Pezzotti, S. and Stein, C.J. and Lin, Z. and Sebastiani, F. and Funke, S. and Hoberg, C. and Kolling, I. and Ma, C.Y. and Mauelshagen, K. and Ockelmann, T. and Schwaab, G. and Fu, L. and Brubach, J.-B. and Roy, P. and Head-Gordon, M. and Tschulik, K. and Gaigeot, M.-P. and Havenith, M.
Proceedings of the National Academy of Sciences of the United States of America 118 (2021)The double layer at the solid/electrolyte interface is a key concept in electrochemistry. Here, we present an experimental study combined with simulations, which provides a molecular picture of the double-layer formation under applied voltage. By THz spectroscopy we are able to follow the stripping away of the cation/anion hydration shells for an NaCl electrolyte at the Au surface when decreasing/increasing the bias potential. While Na+ is attracted toward the electrode at the smallest applied negative potentials, stripping of the Cl2 hydration shell is observed only at higher potential values. These phenomena are directly measured by THz spectroscopy with ultrabright synchrotron light as a source and rationalized by accompanying molecular dynamics simulations and electronic-structure calculations. © 2021 National Academy of Sciences. All rights reserved.
view abstract 10.1073/pnas.2108568118
- Thermal Detection of Glucose in Urine Using a Molecularly Imprinted Polymer as a Recognition Element
Caldara, M. and Lowdon, J.W. and Rogosic, R. and Arreguin-Campos, R. and Jimenez-Monroy, K.L. and Heidt, B. and Tschulik, K. and Cleij, T.J. and Diliën, H. and Eersels, K. and van Grinsven, B.
ACS Sensors 6 (2021)Glucose bio-sensing technologies have received increasing attention in the last few decades, primarily due to the fundamental role that glucose metabolism plays in diseases (e.g., diabetes). Molecularly imprinted polymers (MIPs) could offer an alternative means of analysis to a field that is traditionally dominated by enzyme-based devices, posing superior chemical stability, cost-effectiveness, and ease of fabrication. Their integration into sensing devices as recognition elements has been extensively studied with different readout methods such as quartz-crystal microbalance or impedance spectroscopy. In this work, a dummy imprinting approach is introduced, describing the synthesis and optimization of a MIP toward the sensing of glucose. Integration of this polymer into a thermally conductive receptor layer was achieved by micro-contact deposition. In essence, the MIP particles are pressed into a polyvinyl chloride adhesive layer using a polydimethylsiloxane stamp. The prepared layer is then evaluated with the so-called heat-transfer method, allowing the determination of the specificity and the sensitivity of the receptor layer. Furthermore, the selectivity was assessed by analyzing the thermal response after infusion with increasing concentrations of different saccharide analogues in phosphate-buffered saline (PBS). The obtained results show a linear range of the sensor of 0.0194–0.3300 mM for the detection of glucose in PBS. Finally, a potential application of the sensor was demonstrated by exposing the receptor layer to increasing concentrations of glucose in human urine samples, demonstrating a linear range of 0.0444–0.3300 mM. The results obtained in this paper highlight the applicability of the sensor both in terms of non-invasive glucose monitoring and for the analysis of food samples. © 2021 The Authors. Published by American Chemical Society
view abstract 10.1021/acssensors.1c02223
- Electrochemical dealloying as a tool to tune the porosity, composition and catalytic activity of nanomaterials
Rurainsky, C. and Manjón, A.G. and Hiege, F. and Chen, Y.-T. and Scheu, C. and Tschulik, K.
Journal of Materials Chemistry A 8 (2020)Electrochemical dealloying as a post-Treatment can greatly improve the catalytic activity of nanoparticles. To date, selecting suitable conditions to reach desired porosity, composition and catalytic activity is based on trial-And-error-Attempts, due to insufficient understanding of the electrochemically induced morphological and compositional changes of the nanoparticles. These changes are elucidated here by combining electrochemistry with identical location electron microscopy analyses and linking them to the electrocatalytic properties of the obtained nanocatalysts. Using AgAu alloy nanoparticles and the hydrogen evolution reaction as a model system, the influence of cyclic voltammetry parameters on the catalytic activity upon electrochemical dealloying is investigated. Increasing the number of cycles initially results in a decreased Ag content and a sharp improvement in activity. Additional dealloying increases the nanoparticle porosity, while marginally altering their composition, due to surface motion of atoms. Since this is accompanied by particle aggregation, a decrease in catalytic activity results upon extensive cycling. This transition between porosity formation and particle aggregation marks the optimum for nanocatalyst post-production. The gained insights may aid speeding up the development of new materials by electrochemical dealloying as an easy-To-control post-processing route to tune the properties of existing nanoparticles, instead of having to alter usually delicate synthesis routes as a whole. © The Royal Society of Chemistry.
view abstract 10.1039/d0ta04880a
- Enhanced antibacterial performance of ultrathin silver/platinum nanopatches by a sacrificial anode mechanism
Abuayyash, A. and Ziegler, N. and Meyer, H. and Meischein, M. and Sengstock, C. and Moellenhoff, J. and Rurainsky, C. and Heggen, M. and Garzón-Manjón, A. and Scheu, C. and Tschulik, K. and Ludwig, Al. and Köller, M.
Nanomedicine: Nanotechnology, Biology, and Medicine 24 (2020)The development of antibacterial implant surfaces is a challenging task in biomaterial research. We fabricated a highly antibacterial bimetallic platinum (Pt)/silver(Ag) nanopatch surface by short time sputtering of Pt and Ag on titanium. The sputter process led to a patch-like distribution with crystalline areas in the nanometer-size range (1.3–3.9 nm thickness, 3–60 nm extension). Structural analyses of Pt/Ag samples showed Ag- and Pt-rich areas containing nanoparticle-like Pt deposits of 1–2 nm. The adhesion and proliferation properties of S. aureus on the nanopatch samples were analyzed. Consecutively sputtered Ag/Pt nanopatches (Pt followed by Ag) induced enhanced antimicrobial activity compared to co-sputtered Pt/Ag samples or pure Ag patches of similar Ag amounts. The underlying sacrificial anode mechanism was proved by linear sweep voltammetry. The advantages of this nanopatch coating are the enhanced antimicrobial activity despite a reduced total amount of Ag/Pt and a self-limited effect due the rapid Ag dissolution. © 2019 Elsevier Inc.
view abstract 10.1016/j.nano.2019.102126
- Enhanced dissolution of silver nanoparticles in a physical mixture with platinum nanoparticles based on the sacrificial anode effect
Breisch, M. and Loza, K. and Pappert, K. and Rostek, A. and Rurainsky, C. and Tschulik, K. and Heggen, M. and Epple, M. and Tiller, J.C. and Schildhauer, T.A. and Köller, M. and Sengstock, C.
Nanotechnology 31 (2020)A strategy to reduce implant-related infections is the inhibition of the initial bacterial implant colonization by biomaterials containing silver (Ag). The antimicrobial efficacy of such biomaterials can be increased by surface enhancement (nanosilver) or by creating a sacrificial anode system for Ag. Such a system will lead to an electrochemically driven enhanced Ag ion release due to the presence of a more noble metal. Here we combined the enlarged surface of nanoparticles (NP) with a possible sacrificial anode effect for Ag induced by the presence of the electrochemically more noble platinum (Pt) in physical mixtures of Ag NP and Pt NP dispersions. These Ag NP/Pt NP mixtures were compared to the same amounts of pure Ag NP in terms of cell biological responses, i.e. the antimicrobial activity against Staphylococcus aureus and Escherichia coli as well as the viability of human mesenchymal stem cells (hMSC). In addition, Ag NP was analyzed by ultraviolet-visible (UV-vis) spectroscopy, cyclic voltammetry, and atomic absorption spectroscopy. It was found that the dissolution rate of Ag NP was enhanced in the presence of Pt NP within the physical mixture compared to a dispersion of pure Ag NP. Dissolution experiments revealed a fourfold increased Ag ion release from physical mixtures due to enhanced electrochemical activity, which resulted in a significantly increased toxicity towards both bacteria and hMSC. Thus, our results provide evidence for an underlying sacrificial anode mechanism induced by the presence of Pt NP within physical mixtures with Ag NP. Such physical mixtures have a high potential for various applications, for example as antimicrobial implant coatings in the biomedicine or as bactericidal systems for water and surface purification in the technical area. © 2019 IOP Publishing Ltd.
view abstract 10.1088/1361-6528/ab4e48
- Implications of resistance and mass transport limitations on the common Tafel approach at composite catalyst thin-film electrodes
Blanc, N. and Rurainsky, C. and Tschulik, K.
Journal of Electroanalytical Chemistry 872 (2020)The use of rotating disk electrodes modified with multicomponent catalyst inks is common practice in electrocatalysis. In this work, we present a numerical model to simulate the effect of altered mass transport and conductivity inside a catalyst film, consisting of catalytically active nanoparticles and an inert binder material. Implications for the classical Tafel analysis are evaluated at different combinations of film resistances and mass transport properties. We show that in some cases, linear Tafel-like voltammetric responses may result, which do not contain actual kinetic information and might therefore be misleading and cause of erroneous catalyst activity evaluation. © 2020 Elsevier B.V.
view abstract 10.1016/j.jelechem.2020.114345
- Metal-Rich Chalcogenides for Electrocatalytic Hydrogen Evolution: Activity of Electrodes and Bulk Materials
Siegmund, D. and Blanc, N. and Smialkowski, M. and Tschulik, K. and Apfel, U.-P.
ChemElectroChem 7 (2020)Metal-rich chalcogenides composed of highly abundant elements recently emerged as promising catalysts for the electrocatalytic hydrogen evolution reaction (HER). Many of these materials benefit from a high intrinsic conductivity as compared to their chalcogen-rich congeners, greatly reducing the necessity for conductive additives or sophisticated nanostructuring. Herein, we showcase the high potential of metal-rich transition-metal chalcogenides for the electrocatalytic hydrogen formation by summarizing the recent progress achieved with M9S8 (pentlandite type) and M3S2 (heazlewoodite type) based materials, which represent the most frequently applied compositions for this purpose. By a detailed electrochemical comparison of bulk as well as pellet electrodes of metal-rich Fe4.5Ni4.5S8, we also aim at raising awareness in the community for the inherent differences in catalytic properties of the materials themselves and those of the fabricated electrodes, a point that is often disregarded in reports on HER-catalyst systems. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
view abstract 10.1002/celc.201902125
- Operando Studies of the Electrochemical Dissolution of Silver Nanoparticles in Nitrate Solutions Observed With Hyperspectral Dark-Field Microscopy
Wonner, K. and Rurainsky, C. and Tschulik, K.
Frontiers in Chemistry 7 (2020)Since nanoparticles are frequently used in commercial applications, there is a huge demand to obtain deeper insights into processes at the nanoscale. Especially, catalysis, chemical and electrochemical reaction dynamics are still poorly understood. Thus, simultaneous and coupled opto-and spectro-electrochemical dark-field microscopy is used to study in situ and operando the electrochemically driven dissolution mechanism of single silver nanoparticles in the presence of nitrate ions as non-complexing counter-ions, herein. Hyperspectral imaging is used to probe the intrinsic localized surface plasmon resonance of individual silver nanospheres before, during and after their electrochemical oxidation on a transparent indium tin oxide (ITO) electrode. Furthermore, optical video imaging was performed for additional information. Based on the complete loss of spectral information and intensity, a dissolution of the particles during the reaction was concluded. This way it is revealed that the dissolution of individual particles proceeds over several seconds, indicating a hindrance by the nitrate ions. Only electrochemical analysis does not provide this insight as the measured current does not allow distinguishing between successive fast dissolution of one particle after another or slow dissolution of several particles in a concerted manner. For comparison, experiments were performed in the presence of chloride ions. It was observed that the silver chloride formation is an instantaneous process. Thus, it is possible to study and define the reaction dynamics on the single nanoparticle level in various electrochemical systems and electrolyte solutions. Accordingly, operando opto- and spectro-electrochemical studies allow us to conclude, that the oxidation of silver to solvated silver cations is a kinetically slow process, while the oxidation to silver chloride is fast. We propose this approach as a new method to study electrocatalyst materials, their transformation and degradation under operando conditions. © Copyright © 2020 Wonner, Rurainsky and Tschulik.
view abstract 10.3389/fchem.2019.00912
- Ultrafast Construction of Oxygen-Containing Scaffold over Graphite for Trapping Ni2+into Single Atom Catalysts
Liu, Z. and Li, S. and Yang, J. and Tan, X. and Yu, C. and Zhao, C. and Han, X. and Huang, H. and Wan, G. and Liu, Y. and Tschulik, K. and Qiu, J.
ACS Nano 14 (2020)Ultrafast construction of oxygen-containing scaffold over graphite for trapping Ni2+ into single atom catalysts (SACs) was developed and presented by a one-step electrochemical activation technique. The present method for Ni SACs starts with graphite foil and is capable of achieving ultrafast preparation (1.5 min) and mass production. The defective oxygen featuring the strong electronegativity enables primarily attracting Ni2+ ions and stabilizing Ni atoms via Ni-O6 coordination instead of conventional metal-C or metal-N. In addition, the oxygen defects for trapping are tunable through altering the applied voltage or electrolyte, further altering the loading of Ni atoms, indicative of enhanced oxygen evolution activity. This simple and ultrafast electrochemical synthesis is promising for the mass and controllable production of oxygen-coordinated Ni SACs, which exhibit good performance for oxygen evolution reaction. © 2020 American Chemical Society.
view abstract 10.1021/acsnano.0c04210
- Intrinsic Activity of Oxygen Evolution Catalysts Probed at Single CoFe2O4 Nanoparticles
El Arrassi, A. and Liu, Z. and Evers, M.V. and Blanc, N. and Bendt, G. and Saddeler, S. and Tetzlaff, D. and Pohl, D. and Damm, C. and Schulz, S. and Tschulik, K.
Journal of the American Chemical Society 141 (2019)Identifying the intrinsic electrocatalytic activity of nanomaterials is challenging, as their characterization usually requires additives and binders whose contributions are difficult to dissect. Herein, we use nano impact electrochemistry as an additive-free method to overcome this problem. Due to the efficient mass transport at individual catalyst nanoparticles, high current densities can be realized. High-resolution bright-field transmission electron microscopy and selected area diffraction studies of the catalyst particles before and after the experiments provide valuable insights in the transformation of the nanomaterials during harsh oxygen evolution reaction (OER) conditions. We demonstrate this for 4 nm sized CoFe2O4 spinel nanoparticles. It is revealed that these particles retain their size and crystal structure even after OER at current densities as high as several kA·m-2. The steady-state current scales with the particle size distribution and is limited by the diffusion of produced oxygen away from the particle. This versatilely applicable method provides new insights into intrinsic nanocatalyst activities, which is key to the efficient development of improved and precious metal-free catalysts for renewable energy technologies. © 2019 American Chemical Society.
view abstract 10.1021/jacs.9b04516
- Piece by Piece—Electrochemical Synthesis of Individual Nanoparticles and their Performance in ORR Electrocatalysis
Evers, M.V. and Bernal, M. and Roldan Cuenya, B. and Tschulik, K.
Angewandte Chemie - International Edition 58 (2019)The impact of individual HAuCl4 nanoreactors is measured electrochemically, which provides operando insights and precise control over the modification of electrodes with functional nanoparticles of well-defined size. Uniformly sized micelles are loaded with a dissolved metal salt. These solution-phase precursor entities are then reduced electrochemically—one by one—to form nanoparticles (NPs). The charge transferred during the reduction of each micelle is measured individually and allows operando sizing of each of the formed nanoparticles. Thus, particles of known number and sizes can be deposited homogenously even on nonplanar electrodes. This is demonstrated for the decoration of cylindrical carbon fibre electrodes with 25±7 nm sized Au particles from HAuCl4-filled micelles. These Au NP-decorated electrodes show great catalyst performance for ORR (oxygen reduction reaction) already at low catalyst loadings. Hence, collisions of individual precursor-filled nanocontainers are presented as a new route to nanoparticle-modified electrodes with high catalyst utilization. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
view abstract 10.1002/anie.201813993
- Single entity electrochemistry for the elucidation of lithiation kinetics of TiO 2 particles in non-aqueous batteries
Löffler, T. and Clausmeyer, J. and Wilde, P. and Tschulik, K. and Schuhmann, W. and Ventosa, E.
Nano Energy 57 (2019)In battery research, the development of analytical techniques is of key importance for determining intrinsic properties of active materials ultimately dictating the battery performance. We report the application of nano-impact electrochemistry to gain insight into the intrinsic properties of commercial battery materials i.e. TiO 2 particles in non-aqueous media. Potentiostatic lithiation measurements do not only provide qualitative information about the rate-limiting step in the lithiation process, but also demonstrate that nano-impact electrochemistry is a suitable technique in non-aqueous media in complete absence of oxygen and water. Our results reveal that the intrinsic lithiation rate of individual TiO 2 particles is not – as generally assumed – determined by interfacial ion transfer kinetics, mobility of ion and/or electrons in the bulk of the particle, but by the solid-solid electron transfer. These findings have important implications for future studies of fundamental properties of battery materials considering that charge transfer in battery electrodes does not always obey Butler-Volmer kinetics. © 2018 Elsevier Ltd
view abstract 10.1016/j.nanoen.2018.12.064
- Single entity electrochemistry for the elucidation of lithiation kinetics of TiO2 particles in non-aqueous batteries
Löffler, T. and Clausmeyer, J. and Wilde, P. and Tschulik, K. and Schuhmann, W. and Ventosa, E.
Nano Energy 57 (2019)In battery research, the development of analytical techniques is of key importance for determining intrinsic properties of active materials ultimately dictating the battery performance. We report the application of nano-impact electrochemistry to gain insight into the intrinsic properties of commercial battery materials i.e. TiO2 particles in non-aqueous media. Potentiostatic lithiation measurements do not only provide qualitative information about the rate-limiting step in the lithiation process, but also demonstrate that nano-impact electrochemistry is a suitable technique in non-aqueous media in complete absence of oxygen and water. Our results reveal that the intrinsic lithiation rate of individual TiO2 particles is not – as generally assumed – determined by interfacial ion transfer kinetics, mobility of ion and/or electrons in the bulk of the particle, but by the solid-solid electron transfer. These findings have important implications for future studies of fundamental properties of battery materials considering that charge transfer in battery electrodes does not always obey Butler-Volmer kinetics. © 2018 Elsevier Ltd
view abstract 10.1016/j.nanoen.2018.12.064
- The electrochemical dissolution of single silver nanoparticles enlightened by hyperspectral dark-field microscopy
Wonner, K. and Evers, M.V. and Tschulik, K.
Electrochimica Acta 301 (2019)Nowadays, silver nanoparticles are extensively employed in several branches of industry and medicine. Hence, those particles are amongst the most studied class of nanomaterials, yet their reactivity and in particular their reactivity in biological systems is still poorly understood. This discrepancy leads to a huge demand for further insights into the reaction dynamics of electrochemical reactions. For this purpose, coupled opto- and spectro-electrochemical dark-field microscopy is used herein to study the electrochemical oxidation and dissolution process of individual silver nanoparticles in thiocyanate solutions. It is observed, that upon electrochemical oxidation of silver, a silver thiocyanate complex is formed. This is indicated by a change of both the measured plasmon resonance frequency and scattering intensity, simultaneous to the detection of an oxidative current. Subsequently, this silver pseudo-halide is chemically converted to a silver thiocyanate complex with higher solubility in the presence of high thiocyanate concentrations. This follow-up reaction is only detectable thanks to in situ spectroscopy as no current is associated with this chemical conversion but a distinct change in the spectroscopy response of the individual particles is seen. We were thus able to reveal that the total conversion of silver nanoparticles in the presence of thiocyanate is a multi-step process which lasts much longer than the electrochemical response suggests. © 2019 Elsevier Ltd
view abstract 10.1016/j.electacta.2019.01.129
- Deciphering the Surface Composition and the Internal Structure of Alloyed Silver–Gold Nanoparticles
Grasmik, V. and Rurainsky, C. and Loza, K. and Evers, M.V. and Prymak, O. and Heggen, M. and Tschulik, K. and Epple, M.
Chemistry - A European Journal 24 (2018)Spherical bimetallic AgAu nanoparticles in the molar ratios 30:70, 50:50, and 70:30 with diameters of 30 to 40 nm were analyzed together with pure silver and gold nanoparticles of the same size. Dynamic light scattering (DLS) and differential centrifugal sedimentation (DCS) were used for size determination. Cyclic voltammetry (CV) was used to determine the nanoalloy composition, together with atomic absorption spectroscopy (AAS), energy-dispersive X-ray spectroscopy (EDX) and ultraviolet-visible (UV/Vis) spectroscopy. Underpotential deposition (UPD) of lead (Pb) on the particle surface gave information about its spatial elemental distribution and surface area. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) were applied to study the shape and the size of the nanoparticles. X-ray powder diffraction gave the crystallite size and the microstrain. The particles form a solid solution (alloy) with an enrichment of silver on the nanoparticle surface, including some silver-rich patches. UPD indicated that the surface only consists of silver atoms. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
view abstract 10.1002/chem.201800579
- Electrochemical C-H Cyanation of Electron-Rich (Hetero)Arenes
Hayrapetyan, D. and Rit, R.K. and Kratz, M. and Tschulik, K. and Gooßen, L.J.
Chemistry - A European Journal (2018)A straightforward method for the electrochemical C-H cyanation of arenes and heteroarenes that proceeds at room temperature in MeOH, with NaCN as the reagent in a simple, open, undivided electrochemical cell is reported. The platinum electrodes are passivated by adsorbed cyanide, which allows conversion of an exceptionally broad range of electron-rich substrates all the way down to dialkyl arenes. The cyanide electrolyte can be replenished with HCN, opening opportunities for salt-free industrial C-H cyanation. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/chem.201802247
- Evaluation of the intrinsic catalytic activity of nanoparticles without prior knowledge of the mass loading
Löffler, T. and Wilde, P. and Öhl, D. and Chen, Y.-T. and Tschulik, K. and Schuhmann, W.
Faraday Discussions 210 (2018)The quantitative characterisation of electrocatalytic properties of nanoparticle catalyst materials is so far only performed for layers typically comprising additionally conducting additives and binders. We propose a method enabling the evaluation of intrinsic catalytic activity of nanoparticles based on the diffusion-limited steady-state current. In a step-after-step process, the influence of coverage on kinetic and diffusion limited current is evaluated to highlight the challenges of sub-monolayer electroanalysis. Conclusions are used to point out strategies and their limitations for qualitative and quantitative comparison of intrinsic catalytic properties. Particularly, the impact of coverage, electrode geometry, altered diffusion profile for nanoparticles and the catalyst activity and selectivity are discussed. Fundamental information about electrochemical sub-monolayer nanoparticle analysis is provided. © 2018 The Royal Society of Chemistry.
view abstract 10.1039/c8fd00029h
- Metal-free trifluoromethylthiolation of arenediazonium salts with Me4NSCF3
Bertoli, G. and Exner, B. and Evers, M.V. and Tschulik, K. and Gooßen, L.J.
Journal of Fluorine Chemistry 210 (2018)A metal-free entry to the pharmaceutically meaningful substrate class of trifluoromethyl thioethers has been developed starting from widely available arenediazonium salts and commercially available Me4N+SCF3 −. This reaction proceeds within one hour at 0 °C and is applicable to a wide range of functionalized substrates. © 2018 Elsevier B.V.
view abstract 10.1016/j.jfluchem.2018.03.011
- Nano Impact Electrochemistry: Effects of Electronic Filtering on Peak Height, Duration and Area
Kanokkanchana, K. and Saw, E.N. and Tschulik, K.
ChemElectroChem 5 (2018)Nano impact electrochemistry is used to measure a transient signal while a nanoparticle (NP) hits an electrode due to its motion in a solution. A variety of information can be obtained from this current pulse, yet its accurate measurement is challenging due to its short duration (μs to s) and small amplitude (≤10 nA). A typically used low bandwidth low-pass filter can improve the signal-to-noise ratio, but it may cost severely in the accuracy of the data. Here, we demonstrate the effects of electronic filters by using generated current impulses with duration from 125 μs to 8 ms. Initially, a system dedicated to measure short and low current impulses was employed. There, an 8th order Bessel filter was used and the effect of varying the cut-off frequency between 50 Hz and 20 kHz on the impulse response is studied. Even though the charge is generally conserved by the filter, amplitude and duration of the pulse vary greatly in dependence of the cut-off frequency. In comparison, the response of widely used potentiostats was tested and significant deviations of the measured signal from the input were detected. Supported by destructive nano impact experiments with Ag NPs in KCl(aq), we show how the filtering affects the experimentally determined size of Ag NPs and Cl− diffusion coefficient, using impact charges and duration, respectively. As a result, we suggest a general guideline to researchers for accurate electrochemical nano impact measurements, in particular with respect to current peak duration analysis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
view abstract 10.1002/celc.201800738
- Nano-Impact Electrochemistry: Effects of Electronic Filtering on Peak Height, Duration, and Area
Kanokkanchana, K. and Saw, E.N. and Tschulik, K.
ChemElectroChem 5 (2018)Invited for this month's cover picture is the group of Prof. Dr. Kristina Tschulik from Ruhr University Bochum (Germany). The cover picture shows the alteration of a current signal caused by the electronic filtering implemented in potentiostats. These filter effects are uncovered and a simple method is provided for electrochemists to test and validate their experimental setup. This is particularly important for transient single-entity electrochemistry, which requires low current measurements at high time resolution. Read the full text of the Article at 10.1002/celc.201800738. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
view abstract 10.1002/celc.201801201
- Simultaneous Opto- and Spectro-Electrochemistry: Reactions of Individual Nanoparticles Uncovered by Dark-Field Microscopy
Wonner, K. and Evers, M.V. and Tschulik, K.
Journal of the American Chemical Society 140 (2018)Despite the frequent use of silver nanoparticles in consumer products and medical treatments, their reactivity and degradation in aqueous suspensions are still under debate. Here we elucidate this reactivity by an in situ opto- and spectro-electrochemical approach. Using dark-field microscopy coupled to a spectrophotometer and to an electrochemical cell, redox reactions of individual silver nanoparticles are studied in the presence of chloride. The intensity and spectral position of the plasmon resonance of an individual particle are tracked simultaneously in real time during cyclic voltammetry. They both change almost instantaneously with the detected current in a chemically reversible way. Thus, it is evidenced that the intensity decrease of the optical signal at the silver peak position is caused by the reversible formation of silver chloride and not by dissolution of silver. Moreover, at large positive potentials, further transformation to silver oxide or chlorite is revealed spectroscopically, although the electrochemical current is hidden by water and chloride oxidation. Thus, the combination of electrochemistry with dark-field microscopy and hyperspectral imaging is introduced as a new tool for real-time analysis of (electro-)chemical reactions of nanoparticles on a single-entity level. Copyright © 2018 American Chemical Society.
view abstract 10.1021/jacs.8b02367
- Single Nanoparticle Growth from Nanoparticle Tracking Analysis: From Monte Carlo Simulations to Nanoparticle Electrogeneration
Brasiliense, V. and Noël, J.-M. and Wonner, K. and Tschulik, K. and Combellas, C. and Kanoufi, F.
ChemElectroChem 5 (2018)By scrutinizing the trajectory of individual nanoparticles (NPs) in solution, NP tracking analysis (NTA) allows sizing individual NPs and providing meaningful complementary information to single NP electrochemistry. Herein, a model is developed to extend NTA to allow dynamic NP sizing and to analyze the kinetics of growth of NPs in solution. Interpreting the NP trajectories as scaled Brownian motion, Monte Carlo simulations produce stochastic trajectories of growing NPs (under diffusion-controlled growth). These trajectories are grounds for determining a strategy to estimate the growth parameters of individual NPs from the time evolution analysis of the mean square displacement (MSD) curves. In particular, we evaluate the accuracy and precision of the parameter estimates from MSD analysis. In addition, the strategy is illustrated to depict the homogeneous electrosynthesis of silver NPs from the oxidation of a sacrificial Ag ultramicroelectrode (UME) in Fe2+ solution. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
view abstract 10.1002/celc.201800742
- Spannung im Dunkelfeld
Evers, M.V. and Wonner, K. and Tschulik, K.
Nachrichten aus der Chemie 66 (2018)
view abstract 10.1002/nadc.20184080520
- Time-resolved impact electrochemistry - A new method to determine diffusion coefficients of ions in solution
Saw, E.N. and Blanc, N. and Kanokkanchana, K. and Tschulik, K.
Electrochimica Acta 282 (2018)Diffusion is often the rate-limiting factor of reactions in condensed phase. Thus, knowing the diffusion coefficient is key in numerous aspects ranging from drug release to steering of reactions in both homogeneous liquid phase and electrochemical reactions. Cyclic voltammetry at macro electrodes and chronoamperometry at micro electrodes are well-established methods to determine the diffusion coefficients of redox-active species dissolved in a solution. However, if the formal potentials of the redox species are outside of the potential window of the solvent, then these methods cannot be readily applied. Here we demonstrate a new concept to determine the diffusion coefficient of ions to overcome this limitation. We use their reaction with a well-defined amount of a redox-active indicator substance, which is confined in a nanoparticle suspended in a solution containing the species of interest. Employing transformative nanoparticle impact analysis, the diffusion-limited reaction of an indicator nanoparticle with the species of interest is initiated and followed by chronoamperometry. Measuring the time it takes to fully convert the indicator particle enables the determination of the diffusion coefficient of interest. This concept is demonstrated for variety of (pseudo-)halides in aqueous solution using Ag nanoparticles as redox indicator. Using chloride as an example, is further shown that this new methodology can be applied to study effects of temperature and viscosity on the diffusion coefficients. Given the multitude of nanoparticles that may serve as electrochemical redox indicator, this approach can be used to determine the diffusion coefficients for a large variety of species in different liquid environments. © 2018 Elsevier Ltd
view abstract 10.1016/j.electacta.2018.06.013
- A Unified Interdisciplinary Approach to Design Antibacterial Coatings for Fast Silver Release
El Arrassi, A. and Bellova, P. and Javid, S.M. and Motemani, Y. and Khare, C. and Sengstock, C. and Köller, M. and Ludwig, Al. and Tschulik, K.
ChemElectroChem (2017)The increasing number of surgical treatments performed per year requires novel approaches to inhibit implant-associated infections, caused by multi-antibiotic resistant bacteria. Silver ions (Ag+) are known for their effective antimicrobial activity. Therefore, a system that efficiently and locally releases the minimum required amount of Ag+ directly after the surgical treatment is in high demand. Herein we study electrochemically, microbiologically, microscopically and spectroscopically sacrificial Ag anode coatings for antibacterial implant applications. It is found that Ag dot arrays deposited on noble metals (Pd, Ir) release Ag+ much faster than continuous Ag thin films. The Ag+ release qualitatively scales with the difference of standard potentials between Ag and the noble metal. Furthermore, with higher numbers of Ag dots, the total amount of released Ag+ increases, while the release efficiency declines. Notably, an efficient killing of Staphylococcus aureus bacteria was seen for coatings containing as little as 23ng of Ag per mm2. Thus, the use of sacrificial Ag anodes as highly efficient antibacterial coating materials is evaluated. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/celc.201700247
- Antibacterial activity of microstructured sacrificial anode thin films by combination of silver with platinum group elements (platinum, palladium, iridium)
Köller, M. and Bellova, P. and Javid, S.M. and Motemani, Y. and Khare, C. and Sengstock, C. and Tschulik, K. and Schildhauer, T.A. and Ludwig, Al.
Materials Science and Engineering C 74 (2017)Five different Ag dots arrays (16 to 400dots/mm2) were fabricated on a continuous platinum, palladium, or iridium thin film and for comparison also on titanium film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Staphylococcus aureus (S. aureus) were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards . S. aureus was induced by Ag dot arrays on each of the platinum group thin film (sacrificial anode system for Ag) in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag). Among platinum group elements the Ir-Ag system exerted the highest antibacterial activity which was accompanied by most advanced dissolution of the Ag dots and Ag ion release compared to Ag dots on Pt or Pd. © 2016 Elsevier B.V.
view abstract 10.1016/j.msec.2016.12.075
- Time-resolved impact electrochemistry for quantitative measurement of single-nanoparticle reaction kinetics
Saw, E.N. and Kratz, M. and Tschulik, K.
Nano Research (2017)Single-nanoparticle electrochemistry has been established as a tool to characterize various nanomaterials based on the charge passed during their random impact at an electrode. Here it is demonstrated that the duration and shape of the resulting current peak can be used to quantify the reaction kinetics on a single-particle basis. Both the chemical rate constant and reaction mechanism for oxidation of single nanoparticles in different electrolytes can be determined directly from the duration of the current signal recorded in high-speed, highsensitivity current measurements. Using 29-nm-sized Ag particles in four different electrolytes as a proof of concept for this general approach, hitherto inaccessible insights into single-particle reactivity are provided. While comparable rate constants were measured for the four electrolytes at low overpotentials, transport-limited impacts at high overpotentials were found to depend strongly on the type and quantity of anions present in solution. [Figure not available: see fulltext.] © 2017 Tsinghua University Press and Springer-Verlag GmbH Germany
view abstract 10.1007/s12274-017-1578-3
- Detection of individual nanoparticle impacts using etched carbon nanoelectrodes
Clausmeyer, J. and Wilde, P. and Löffler, T. and Ventosa, E. and Tschulik, K. and Schuhmann, W.
Electrochemistry Communications 73 (2016)A rapid and reliable nanofabrication route produces electrodes with beneficial properties for electrochemistry based on stochastic nanoparticle collision events. Carbon nanoelectrodes are etched to expose conical carbon tips which present an increased surface area for the detection of nanoparticle impacts. The tuneable electrode size as well as the conical geometry allow to increase the observed particle impact frequency while maintaining low background noise. Moreover, anodic particle coulometry for the sizing of silver nanoparticles shows that the detected impacts are representative of the polydisperse particle population. © 2016
view abstract 10.1016/j.elecom.2016.11.003
- Electrochemical bromination of organosulfur containing species for the determination of the strength of garlic (A. sativum)
Hall, E.M. and Tschulik, K. and Batchelor-McAuley, C. and Compton, R.G.
Food Chemistry 199 (2016)The extraction by ethyl acetate and subsequent electrochemical detection of organosulfur containing molecules from garlic is demonstrated. The electrochemical results first evidence the high sensitivity of the process towards the model compound propyl disulfide. Through the in situ formation of bromine at a platinum electrode the propyl disulfide can be readily detected at concentrations as low as 12.5 μM. Second, the work focuses on the detection of organosulfur from fresh garlic samples. Extraction of the organosulfur 'flavour' molecules is achieved with ethyl acetate. Addition of this extract to the electrochemical cell results in an analytically useful signal allowing the voltammetric peak height to be successfully correlated with the garlic strength, as measured using an organoleptic tasting panel. © 2015 Elsevier Ltd. All rights reserved.
view abstract 10.1016/j.foodchem.2015.12.086
- Electrochemistry at single bimetallic nanoparticles - using nano impacts for sizing and compositional analysis of individual AgAu alloy nanoparticles
Saw, E.N. and Grasmik, V. and Rurainsky, C. and Epple, M. and Tschulik, K.
Faraday Discussions 193 (2016)The increasing interest in producing bimetallic nanoparticles and utilizing them in modern technologies sets the demand for fast and affordable characterization of these materials. To date Scanning Transmission Electron Microscopy (STEM) coupled to energy dispersive X-ray spectroscopy is usually used to determine the size and composition of alloy nanoparticles, which is time-consuming and expensive. Here electrochemical single nanoparticle analysis is presented as an alternative approach to infer the particle size and composition of alloy nanoparticles, directly in a dispersion of these particles. As a proof of concept, 14 nm sized Ag0.73Au0.27 alloy nanoparticles are analyzed using a combination of chronoamperometric single nanoparticle analysis and cyclic voltammetry ensemble studies. It is demonstrated that the size, the alloying and the composition can all be inferred using this approach. Thus, the electrochemical characterization of single bimetallic alloy nanoparticles is suggested here as a powerful and convenient complement or alternative to TEM characterization of alloy nanoparticles. © The Royal Society of Chemistry.
view abstract 10.1039/c6fd00112b
- Electrochemistry of single nanoparticles: general discussion
Albrecht, T. and MacPherson, J. and Magnussen, O. and Fermin, D. and Crooks, R. and Gooding, J. and Hersbach, T. and Kanoufi, F. and Schuhmann, W. and Bentley, C. and Tao, N. and Mitra, S. and Krischer, K. and Tschulik, K. and Faez, S. and Nogala, W. and Unwin, P. and Long, Y. and Koper, M. and Tian, Z. and Alpuche-Aviles, M.A. and White, H. and Brasiliense, V. and Kranz, C. and Schmickler, W. and Stevenson, K. and Jing, C. and Edwards, M.
Faraday Discussions 193 (2016)
view abstract 10.1039/c6fd90068b
- Exploring the mineral-water interface: Reduction and reaction kinetics of single hematite (α-Fe2O3) nanoparticles
Shimizu, K. and Tschulik, K. and Compton, R.G.
Chemical Science 7 (2016)In spite of their natural and technological importance, the intrinsic electrochemical properties of hematite (α-Fe2O3) nanoparticles are not well understood. In particular, particle agglomeration, the presence of surface impurities, and/or inadequate proton concentrations are major obstacles to uncover the fundamental redox activities of minerals in solution. These are particularly problematic when samples are characterized in common electrochemical analyses such as cyclic voltammetry in which nanoparticles are immobilized on a stationary electrode. In this work, the intrinsic reaction kinetics and thermodynamics of individual hematite nanoparticles are investigated by particle impact chronoamperometry. The particle radius derived from the integrated area of spikes recorded in a chronoamperogram is in excellent agreement with electron microscopy results, indicating that the method provides a quantitative analysis of the reduction of the nanoparticles to the ferrous ion. A key finding is that the suspended individual nanoparticles undergo electrochemical reduction at potentials much more positive than those immobilized on a stationary electrode. The critical importance of the solid/water interface on nanoparticle activity is further illustrated by a kinetic model. It is found that the first electron transfer process is the rate determining step of the reductive dissolution of hematite nanoparticles, while the overall process is strongly affected by the interfacial proton concentration. This article highlights the effects of the interfacial proton and ferrous ion concentrations on the reductive dissolution of hematite nanoparticles and provides a highly effective method that can be readily applied to study a wide range of other mineral nanoparticles. © 2016 The Royal Society of Chemistry.
view abstract 10.1039/c5sc03678j
- Ferrocene Aryl Derivatives for the Redox Tagging of Graphene Nanoplatelets
Yang, M. and Batchelor-Mcauley, C. and MoreiraGonçalves, L. and Lima, C.F.R.A.C. and Vyskočil, V. and Tschulik, K. and Compton, R.G.
Electroanalysis 28 (2016)Aryl derivatives of ferrocene are used for the modification of graphitic surfaces. Stronger adsorption is utilized as a route to enable the electrochemically tagging of new carbon materials. Model experiments are reported on an EPPG electrode where the adsorption of 1-(biphen-4-yl)ferrocene is found to be a factor of ca. 5 times more thermodynamically favorable than the underivatised form. Two further derivatives were also studied and the voltammetric responses of this class of ferrocenes are found to be significantly influenced by ion-pairing. Finally, the successful use of these new materials for the modification and redox tagging of graphene nanoplatelets is demonstrated. © 2016 WILEY-VCH Verlag GmbH & Co.
view abstract 10.1002/elan.201500590
- From single cells to single molecules: general discussion
Gooding, J. and Magnussen, O. and Fermin, D. and Crooks, R. and Kanoufi, F. and Schuhmann, W. and Nichols, R. and Schmickler, W. and Tao, N. and Chen, S. and Actis, P. and Page, A. and Tschulik, K. and Faez, S. and Edwards, M. and Johnson, R. and Nogala, W. and Kranz, C. and Eikerling, M. and Unwin, P. and Thomas, B. and Prabhakaran, V. and Clausmeyer, J. and Vincent, K. and Koper, M. and Tian, Z. and Mount, A. and Alpuche-Aviles, M.A. and White, H. and Ewing, A. and Higgins, S. and Baker, L. and Zhan, D. and Ulstrup, J. and Bohn, P.W. and Lemay, S.
Faraday Discussions 193 (2016)
view abstract 10.1039/c6fd90066f
- Interplay of the Open Circuit Potential-Relaxation and the Dissolution Behavior of a Single H2 Bubble Generated at a Pt Microelectrode
Karnbach, F. and Yang, X. and Mutschke, G. and Fröhlich, J. and Eckert, J. and Gebert, A. and Tschulik, K. and Eckert, K. and Uhlemann, M.
Journal of Physical Chemistry C 120 (2016)The dissolution behavior of a single H2 bubble electrochemically generated at a Pt microelectrode in 1 M H2SO4 was studied. The open circuit potential (OCP) relaxation after the polarization end was recorded and correlated with the dissolved H2 concentration at the interface electrode/electrolyte/gas. Simultaneously, the shrinking of the bubble was followed optically by means of a high speed camera. In addition, analytical modeling and numerical simulations for the bubble dissolution were performed. Three characteristic regions are identified in the OCP and the bubble radius transients: (i) slow relaxation and shrinking, (ii) transition region, and (iii) a long-term slowed down dissolution process. The high supersaturation after polarization remains longer than theoretically predicted and feeds the bubble in region (i). This reduces the dissolution rate of the bubble which differs significantly from that of nonelectrochemically produced bubbles. Numerical multispecies simulations prove that oxygen and nitrogen dissolved in the electrolyte additionally influence the bubble dissolution and slow down its shrinkage compared to pure hydrogen diffusion. In region (iii), a complete exchange of hydrogen gas with nitrogen and oxygen has occurred in the gas bubble. © 2016 American Chemical Society.
view abstract 10.1021/acs.jpcc.6b02305
- Reactions at the nanoscale: general discussion
Hersbach, T. and MacPherson, J. and Magnussen, O. and Crooks, R. and Higgins, S. and Fermin, D. and Kanoufi, F. and Schuhmann, W. and Nichols, R. and Mitra, S. and Schmickler, W. and Tschulik, K. and Bartlett, P. and Faez, S. and Nogala, W. and Eikerling, M. and Kranz, C. and Unwin, P. and Koper, M. and Lemay, S. and Mount, A. and Ewing, A. and Tian, Z. and White, H. and Chen, S. and Clausmeyer, J. and Krischer, K.
Faraday Discussions 193 (2016)
view abstract 10.1039/C6FD90067D
- Single Nanoparticle Voltammetry: Contact Modulation of the Mediated Current
Li, X. and Batchelor-Mcauley, C. and Whitby, S.A.I. and Tschulik, K. and Shao, L. and Compton, R.G.
Angewandte Chemie - International Edition 55 (2016)The cyclic voltammetric responses of individual palladium-coated carbon nanotubes are reported. Upon impact - from the solution phase - with the electrified interface, the nanoparticles act as individual nanoelectrodes catalyzing the hydrogen-oxidation reaction. At high overpotentials the current is shown to reach a quasi-steady-state diffusion limit, allowing determination of the tube length. The electrochemical response of the individual nanotubes also reveals the system to be modulated by the electrical contact between the electrode and carbon nanotube. This modulation presents itself as fluctuations in the recorded Faradaic current. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/anie.201509017
- The Electrochemical Characterization of Single Core-Shell Nanoparticles
Holt, L.R. and Plowman, B.J. and Young, N.P. and Tschulik, K. and Compton, R.G.
Angewandte Chemie - International Edition 55 (2016)We report the direct solution-phase characterization of individual gold-core silver-shell nanoparticles through an electrochemical means, with selectivity achieved between the core and shell components based on their different redox activities. The electrochemically determined core-shell sizes are in excellent agreement with electron microscopy-based results, successfully demonstrating the electrochemical characterization of individual core-shell nanoparticles. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/anie.201509008
- Advancing from Rules of Thumb: Quantifying the Effects of Small Density Changes in Mass Transport to Electrodes. Understanding Natural Convection
Ngamchuea, K. and Eloul, S. and Tschulik, K. and Compton, R.G.
Analytical Chemistry 87 (2015)Understanding mass transport is prerequisite to all quantitative analysis of electrochemical experiments. While the contribution of diffusion is well understood, the influence of density gradient-driven natural convection on the mass transport in electrochemical systems is not. To date, it has been assumed to be relevant only for high concentrations of redox-active species and at long experimental time scales. If unjustified, this assumption risks misinterpretation of analytical data obtained from scanning electrochemical microscopy (SECM) and generator-collector experiments, as well as analytical sensors utilizing macroelectrodes/microelectrode arrays. It also affects the results expected from electrodeposition. On the basis of numerical simulation, herein it is demonstrated that even at less than 10 mM concentrations and short experimental times of tens of seconds, density gradient-driven natural convection significantly affects mass transport. This is evident from in-depth numerical simulation for the oxidation of hexacyanoferrate (II) at various electrode sizes and electrode orientations. In each case, the induced convection and its influence on the diffusion layer established near the electrode are illustrated by maps of the velocity fields and concentration distributions evolving with time. The effects of natural convection on mass transport and chronoamperometric currents are thus quantified and discussed for the different cases studied. © 2015 American Chemical Society.
view abstract 10.1021/acs.analchem.5b01293
- Are Nanoparticles Spherical or Quasi-Spherical?
Sokolov, S.V. and Batchelor-Mcauley, C. and Tschulik, K. and Fletcher, S. and Compton, R.G.
Chemistry - A European Journal 21 (2015)The geometry of quasi-spherical nanoparticles is investigated. The combination of SEM imaging and electrochemical nano-impact experiments is demonstrated to allow sizing and characterization of the geometry of single silver nanoparticles. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/chem.201500807
- Bifunctional redox tagging of carbon nanoparticles
Poon, J. and Batchelor-McAuley, C. and Tschulik, K. and Palgrave, R.G. and Compton, R.G.
Nanoscale 7 (2015)Despite extensive work on the controlled surface modification of carbon with redox moieties, to date almost all available methodologies involve complex chemistry and are prone to the formation of polymerized multi-layer surface structures. Herein, the facile bifunctional redox tagging of carbon nanoparticles (diameter 27 nm) and its characterization is undertaken using the industrial dye Reactive Blue 2. The modification route is demonstrated to be via exceptionally strong physisorption. The modified carbon is found to exhibit both well-defined oxidative and reductive voltammetric redox features which are quantitatively interpreted. The method provides a generic approach to monolayer modifications of carbon and carbon nanoparticle surfaces. © The Royal Society of Chemistry 2015.
view abstract 10.1039/c4nr06058j
- Capping agent promoted oxidation of gold nanoparticles: cetyl trimethylammonium bromide
Plowman, B.J. and Tschulik, K. and Young, N.P. and Compton, R.G.
Physical Chemistry Chemical Physics 17 (2015)Capping agents, key for nanoparticle stability, may hugely influence chemical behaviour. We show that differently capped gold nanoparticles, with either citrate or cetyl trimethylammonium bromide (CTAB) capping agents, show qualitatively different electron transfer properties. Specifically through cyclic voltammetry and nanoimpact studies the CTAB promoted dissolution of gold nanoparticles is shown, highlighting the active role which capping agents can play in charge transfer. This journal is © the Owner Societies.
view abstract 10.1039/c5cp05146k
- Core-Shell Nanoparticles: Characterizing Multifunctional Materials beyond Imaging - Distinguishing and Quantifying Perfect and Broken Shells
Tschulik, K. and Ngamchuea, K. and Ziegler, C. and Beier, M.G. and Damm, C. and Eychmueller, A. and Compton, R.G.
Advanced Functional Materials 25 (2015)Core-shell nanoparticles (NPs) are amongst the most promising candidates in the development of new functional materials. Their fabrication and characterization are challenging, in particular when thin and intact shells are needed. To date no technique has been available that differentiates between intact and broken or cracked shells. Here a method is presented to distinguish and quantify these types of shells in a single cyclic voltammetry experiment by using the different electrochemical reactivities of the core and the shell material. A simple comparison of the charge measured during the stripping of the core material before and after the removal of the shell makes it possible to determine the quality of the shells and to estimate their thickness. As a proof-of-concept two multifunctional examples of core-shell NPs, Fe<inf>3</inf>O<inf>4</inf>@Au and Au@SnO<inf>2</inf>, are used. This general and original method can be applied whenever core and shell materials show different redox properties. Because billions of NPs are probed simultaneously and at a low cost, this method is a convenient new screening tool for the development of new multifunctional core-shell materials and is hence a powerful complementary technique or even an alternative to the state-of-the-art characterization of core-shell NPs by TEM. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/adfm.201501556
- Diffusional impacts of nanoparticles on microdisc and microwire electrodes: The limit of detection and first passage statistics
Eloul, S. and Kätelhön, E. and Batchelor-McAuley, C. and Tschulik, K. and Compton, R.G.
Journal of Electroanalytical Chemistry 755 (2015)We derive approximate expressions for the average number of diffusive impacts/hits of nanoparticles on microdisc and microwire electrodes for the case where the impact leads to the loss of the nanoparticles from solution either via irreversible adsorption or complete electro-dissolution. The theory can also be applied to sub-micrometre size electrodes (nano-electrodes). The resulting equations can be utilised to analyse the number of impacts and its variance in the 'nano-impact' experiment. We also provide analytical expressions for the first passage time of an impact for dilute nanoparticle solutions in the continuum limit of Fickian diffusion. The expressions for the first passage times are used to estimate the lower limit of detection in ultra-dilute nanoparticle solutions for typical nano-impact experiments, and show the advantage of using microwire electrodes in ultra-dilute solutions or solutions containing larger nano-particles. © 2015 Elsevier B.V. All rights reserved.
view abstract 10.1016/j.jelechem.2015.07.042
- Diffusional nanoimpacts: The stochastic limit
Eloul, S. and Kätelhön, E. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
Journal of Physical Chemistry C 119 (2015)The probability expressions for the average number of diffusional impact events on a surface are established using Fick's diffusion in the limit of a continuum flux. The number and the corresponding variance are calculated for the case of nanoparticles impacting on an electrode at which they are annihilated. The calculations show the dependency on concentration in the limit of noncontinuous media and small electrode sizes for the cases of linear diffusion to a macroelectrode and of convergent diffusion to a small sphere. Using random walk simulations, we confirm that the variance follows a Poisson distribution for ultradilute and dilute solutions. We also present an average "first passage time" for the ultradilute solutions expression that directly relates to the lower limit of detection in ultradilute solutions as a function of the electrode size. The analytical expressions provide a straightforward way to predict the stochastics of impacts in a "nanoimpact" experiment by using Fick's second law and assuming a continuum dilute flux. Therefore, the study's results are applicable to practical electrochemical systems where the number of particles is very small but much larger than one. Moreover, the presented analytical expression for the variance can be utilized to identify effects of particle inhomogeneity in the solution and is of general interest in all studies of diffusion processes toward an absorbing wall in the stochastic limit. © 2015 American Chemical Society.
view abstract 10.1021/acs.jpcc.5b03210
- Electrochemical detection of single E. coli bacteria labeled with silver nanoparticles
Sepunaru, L. and Tschulik, K. and Batchelor-McAuley, C. and Gavish, R. and Compton, R.G.
Biomaterials Science 3 (2015)A proof-of-concept for the electrochemical detection of single Escherichia coli bacteria decorated with silver nanoparticles is reported. Impacts of bacteria with an electrode - held at a suitably oxidizing potential - lead to an accompanying burst of current with each collision event. The frequency of impacts scales with the concentration of bacteria and the charge indicates the extent of decoration. © The Royal Society of Chemistry.
view abstract 10.1039/c5bm00114e
- In Situ Detection of Particle Aggregation on Electrode Surfaces
Ngamchuea, K. and Tschulik, K. and Eloul, S. and Compton, R.G.
ChemPhysChem 16 (2015)Partially blocked electrodes (PBEs) are important; many applications use non-conductive nanoparticles (NPs) to introduce new electrode functionalities. As aggregation is a problem in NP immobilization, developing an in situ method to detect aggregation is vital to characterise such modified electrodes. We present chronoamperometry as a method for detection of NP surface aggregation and semi-quantitative sizing of the formed aggregates, based on the diffusion limited current measured at PBEs as compared with the values calculated numerically for different blocking feature sizes. In contrast to voltammetry, no approximations on electrode kinetics are needed, making chronoamperometry a more general and reliable method. Sizing is shown for two modification methods. Upon drop casting, significant aggregation is observed, while it is minimized in electrophoretic NP deposition. The aggregate sizes determined are in semi-quantitative agreement with ex situ microscopic analysis of the PBEs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/cphc.201500168
- In situ nanoparticle sizing with zeptomole sensitivity
Batchelor-Mcauley, C. and Ellison, J. and Tschulik, K. and Hurst, P.L. and Boldt, R. and Compton, R.G.
Analyst 140 (2015)We present the basis for an entirely new approach to in situ nanoparticle sizing. Nanoparticles containing just 12 zeptomoles (1 zeptomole = 10-21 moles) of silver, are detected via in situ particle coulometry. These stochastic charge measurements correspond to the transfer of only 7000-8000 electrons, yielding direct information relating to the individual nanoparticle volumes. The resulting particle size distribution (average equivalent radius 5 nm) obtained via nanoparticle coulometry is in excellent correspondence with that attained from TEM analysis. Moreover, the measurable particle size limit by this electrochemical method is shown to be significantly below that of more common optical nanoparticle tracking techniques, and as such can be viewed as a potential disruptive nano-technology. © 2015 The Royal Society of Chemistry.
view abstract 10.1039/c5an00474h
- Influence of Adsorption Kinetics upon the Electrochemically Reversible Hydrogen Oxidation Reaction
Lin, C. and Jiao, X. and Tschulik, K. and Batchelor-Mcauley, C. and Compton, R.G.
Journal of Physical Chemistry C 119 (2015)The hydrogen oxidation reaction was studied at bright polycrystalline platinum microelectrodes. A smaller steady-state current was observed in experiment as compared to that anticipated for a diffusion limited process. To facilitate physical insight into this system, a simulation model based on the Tafel-Volmer mechanism for the hydrogen oxidation reaction was developed. Under conditions of reversible electron transfer, the adsorption kinetics k<inf>a</inf> and k<inf>d</inf> are found to have distinctly different influences upon the voltammetry responses. Correspondence between the simulated and the experimental voltammograms is found, confirming the decrease of the steady-state current is caused by the slow adsorption process. The combined adsorption parameter k<inf>a</inf>γmax2 on the Tafel-Volmer mechanism was approximately 5.0 × 10-4 m s-1, where γ <inf>max</inf> (mol m-2) is the maximum surface coverage of adsorption hydrogen atoms. © 2015 American Chemical Society.
view abstract 10.1021/acs.jpcc.5b04293
- Magnetic control: Switchable ultrahigh magnetic gradients at Fe
3O 4nanoparticles to enhance solution-phase mass transport
Ngamchuea, K. and Tschulik, K. and Compton, R.G.
Nano Research 8 (2015)Enhancing mass transport to electrodes is desired in almost all types of electrochemical sensing, electrocatalysis, and energy storage or conversion. Here, a method of doing so by means of the magnetic gradient force generated at magnetic-nanoparticle-modified electrodes is presented. It is shown using Fe<inf>3</inf>O<inf>4</inf>-nanoparticle-modified electrodes that the ultrahigh magnetic gradients (>108 T·m–1) established at the magnetized Fe<inf>3</inf>O<inf>4</inf> nanoparticles speed up the transport of reactants and products at the electrode surface. Using the Fe(III)/Fe(II)-hexacyanoferrate redox couple, it is demonstrated that this mass transport enhancement can conveniently and repeatedly be switched on and off by applying and removing an external magnetic field, owing to the superparamagnetic properties of magnetite nanoparticles. Thus, it is shown for the first time that magnetic nanoparticles can be used to control mass transport in electrochemical systems. Importantly, this approach does not require any means of mechanical agitation and is therefore particularly interesting for application in micro- and nanofluidic systems and devices. [Figure not available: see fulltext.] © 2015, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
view abstract 10.1007/s12274-015-0830-y
- Magnetostatic nearest neighbor interactions in a Co48Fe52 nanowire array probed by in-field magnetic force microscopy
Vock, S. and Tschulik, K. and Uhlemann, M. and Hengst, C. and Fähler, S. and Schultz, L. and Neu, V.
Journal of Applied Physics 118 (2015)The magnetization behavior of nanowires embedded in an array is influenced by the sum of the dipolar fields produced by all surrounding nanowires. These magnetostatic interactions largely modify the array properties and thus complicate the reconstruction of the ensemble averaged behavior of the individual nanowires, such as the intrinsic switching field distribution. Simply correcting the shearing of the hysteresis in a mean-field approach does not account for the locally fluctuating demagnetizing field, which originates from the individual magnetization configuration in the close surrounding of each nanowire. We present an in-field Magnetic Force Microscopy study of electrochemically produced Co48Fe52 nanowires, in which the influence of the magnetic nearest neighbor configuration on the switching behavior of the individual embedded nanowires is clearly detected. Based on this finding, a statistical evaluation method of nearest neighbor histograms is proposed, which potentially allows to judge the strength of the local magnetostatic interactions against the magnitude of the intrinsic switching field distribution. © 2015 AIP Publishing LLC.
view abstract 10.1063/1.4937275
- Metal-halide Nanoparticle Formation: Electrolytic and Chemical Synthesis of Mercury(I) Chloride Nanoparticles
Bartlett, T.R. and Batchelor-Mcauley, C. and Tschulik, K. and Jurkschat, K. and Compton, R.G.
ChemElectroChem 2 (2015)Mercury(I) chloride (Hg<inf>2</inf>Cl<inf>2</inf>) nanoparticles (NPs) are synthesised for the first time by using two different techniques. First, particles are formed by implosion of a calomel nanolayer, induced by partial electrolysis at a mercury hemisphere microelectrode. The resulting NPs are then characterised by the nanoimpact method, demonstrating the first time metal chloride NPs have been sized by this technique and showing the ability to form and study NPs insitu. Second, Hg<inf>2</inf>Cl<inf>2</inf> NPs are synthesised by using the precipitation reaction of Hg<inf>2</inf>(NO<inf>3</inf>)<inf>2</inf> with KCl. The NPs are characterised on both mercury and carbon microelectrodes and their size is found to agree with TEM results. Sizable studies: Mercury(I) chloride (Hg<inf>2</inf>Cl<inf>2</inf>) nanoparticles (NPs) are synthesised for the first time by using two different techniques. First, particles are formed by implosion of a calomel nanolayer, induced by partial electrolysis at a mercury hemisphere microelectrode. Second, Hg<inf>2</inf>Cl<inf>2</inf> NPs are synthesised by the precipitation reaction between Hg<inf>2</inf>(NO<inf>3</inf>)<inf>2</inf> and KCl. The NPs are characterised on both mercury and carbon microelectrodes by using the nanoimpact method and their size is found to agree with TEM results. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/celc.201402401
- Molecular-scale hybridization of clay monolayers and conducting polymer for thin-film supercapacitors
Zhao, J. and Xu, S. and Tschulik, K. and Compton, R.G. and Wei, M. and O'Hare, D. and Evans, D.G. and Duan, X.
Advanced Functional Materials 25 (2015)Development of electrode materials with well-defined architectures is a fruitful and profitable approach for achieving highly-efficient energy storage systems. A molecular-scale hybrid system is presented based on the self-assembly of CoNi-layered double hydroxide (CoNi-LDH) monolayers and the conducting polymer (poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate), denoted as PEDOT:PSS) into an alternating-layer superlattice. Owing to the homogeneous interface and intimate interaction, the resulting CoNi-LDH/PEDOT:PSS hybrid materials possess a simultaneous enhancement in ion and charge-carrier transport and exhibit improved capacitive properties with a high specific capacitance (960 F g-1 at 2 A g-1) and excellent rate capability (83.7% retention at 30 A g-1). In addition, an in-plane supercapacitor device with an interdigital design is fabricated based on a CoNi-LDH/PEDOT:PSS thin film, delivering a significantly enhanced energy and power output (an energy density of 46.1 Wh kg-1 at 11.9 kW kg-1). Its application in miniaturized devices is further demonstrated by successfully driving a photodetector. These characteristics demonstrate that the molecular-scale assembly of LDH monolayers and the conducting polymer is promising for energy storage and conversion applications in miniaturized electronics. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/adfm.201500408
- Nanoparticle Capping Agent Dynamics and Electron Transfer: Polymer-Gated Oxidation of Silver Nanoparticles
Tanner, E.E.L. and Tschulik, K. and Tahany, R. and Jurkschat, K. and Batchelor-McAuley, C. and Compton, R.G.
Journal of Physical Chemistry C 119 (2015)Capping agent-controlled stability of nanoparticles tailors them for different applications, but the associated particle-solvent dynamics are poorly understood. Herein, previously unseen capping agent-gated nanoparticle redox activity is observed for poly(ethylene glycol)-coated silver nanoparticles. This is revealed by stochastic nanoparticle stripping, probing one individual nanoparticle at a time, from an ensemble of surface-immobilized nanoparticles. Thus, new and previously inaccessible understanding is gained on the crucial role of capping agent dynamics on nanoparticle reactivity. © 2015 American Chemical Society.
view abstract 10.1021/acs.jpcc.5b05789
- Non-Invasive Probing of Nanoparticle Electrostatics
Tschulik, K. and Cheng, W. and Batchelor-Mcauley, C. and Murphy, S. and Omanović, D. and Compton, R.G.
ChemElectroChem 2 (2015)Electrostatic interactions between surface-charged nanoparticles (NPs) and electrodes studied using existing techniques unavoidably and significantly alter the system being analyzed. Here we present a methodology that allows the probing of unperturbed electrostatic interactions between individual NPs and charged surfaces. The uniqueness of this approach is that stochastic NP impact events are used as the probe. During a single impact, only an attomole of the redox species reacts and is released at the interface during each sensing event. As an example, the effect of electrostatic screening on the reduction of negatively charged indigo NPs at a mercury microelectrode is explored at potentials positive and negative of the potential of zero charge. At suitable overpotentials fully driven electron transfer is seen for all but very low (<0.005M) ionic strengths. The loss of charge transfer in such dilute electrolytes is unambiguously shown to arise from a reduced driving force for the reaction rather than a reduced population of NPs near the electrode, contradicting popular perceptions. Electrostatics were found not to significantly affect the reactivity of the studied NPs. Importantly, the presented technique is general and can be applied to a wide variety of NPs, including metals, metal oxides and organic compounds. Not what you might think: A new and non-invasive technique to probe the electrostatic interaction between surface-charged nanoparticles and a charged metal/solution interface shows that electrostatic effects are insignificant in all but very dilute electrolytes. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/celc.201402285
- Reversible or Not? Distinguishing Agglomeration and Aggregation at the Nanoscale
Sokolov, S.V. and Tschulik, K. and Batchelor-McAuley, C. and Jurkschat, K. and Compton, R.G.
Analytical Chemistry 87 (2015)Nanoparticles are prone to clustering either via aggregation (irreversible) or agglomeration (reversible) processes. It is exceedingly difficult to distinguish the two via conventional techniques such as dynamic light scattering (DLS), nanoparticle tracking analysis (NTA), or electron microscopy imaging (scanning electron microscopy (SEM), transmission electron microscopy (TEM)) as such techniques only generally confirm the presence of large particle clusters. Herein we develop a joint approach to tackle the issue of distinguishing between nanoparticle aggregation vs agglomeration by characterizing a colloidal system of Ag NPs using DLS, NTA, SEM imaging and the electrochemical nanoimpacts technique. In contrast to the conventional techniques which all reveal the presence of large clusters of particles, electrochemical nanoimpacts provide information regarding individual nanoparticles in the solution phase and reveal the presence of small nanoparticles (<30 nm) even in high ionic strength (above 0.5 M KCl) and allow a more complete analysis. The detection of small nanoparticles in high ionic strength media evidence the clustering to be a reversible process. As a result it is concluded that agglomeration rather than irreversible aggregation takes place. This observation is of general importance for all colloids as it provides a feasible analysis technique for a wide range of systems with an ability to distinguish subtly different processes. © 2015 American Chemical Society.
view abstract 10.1021/acs.analchem.5b02639
- Single graphene nanoplatelets: Capacitance, potential of zero charge and diffusion coefficient
Poon, J. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
Chemical Science 6 (2015)Nano-impact chronoamperometric experiments are a powerful technique for simultaneously probing both the potential of zero charge (PZC) and the diffusion coefficient (D<inf>0</inf>) of graphene nanoplatelets (GNPs). The method provides an efficient general approach to material characterisation. Using nano-impact experiments, capacitative impacts can be seen for graphene nanoplatelets of 15 μm width and 6-8 nm thickness. The current transient features seen allow the determination of the PZC of the graphene nanoplatelet in PBS buffer as -0.14 ± 0.03 V (vs. saturated calomel electrode). The diffusion coefficient in the same aqueous medium, isotonic with many biological conditions, for the graphene nanoplatelets is experimentally found to be 2 ± 0.8 × 10-13 m2 s-1. This quick characterisation technique may significantly assist the application of graphene nanoplatelets, or similar nano-materials, in electronic, sensor, and clinical medicinal technologies. © The Royal Society of Chemistry 2015.
view abstract 10.1039/c5sc00623f
- The effect of insulator nano-sheath thickness on the steady state current at a micro-disc electrode
Ellison, J. and Eloul, S. and Batchelor-Mcauley, C. and Tschulik, K. and Salter, C. and Compton, R.G.
Journal of Electroanalytical Chemistry 745 (2015)Abstract The relative size of the insulating sheath to electrode area at a micro-disc electrode can lead to significant perturbations in the steady state current observed. A minimum, constant steady state current value is realised once the sheath thickness is greater than twice the radius Δl>2rd. However, as the sheath thickness decreases below this value, the observed current increases. In this paper a theoretical model is presented, allowing for the accurate determination of the outer sheath thickness. The effects of an ultra-thin sheath on steady state currents are demonstrated experimentally and these results are shown to accurately fit with the simulated model developed. Therefore, the model presented here can be used to determine the size of a sheath of unknown thickness. Furthermore, it allows these size effects on the steady state current to be explored. © 2015 Elsevier B.V. All rights reserved.
view abstract 10.1016/j.jelechem.2015.02.027
- The fate of nano-silver in aqueous media
Plowman, B.J. and Tschulik, K. and Walport, E. and Young, N.P. and Compton, R.G.
Nanoscale 7 (2015)Silver nanoparticles offer highly attractive properties for many applications, however concern has been raised over the possible toxicity of this material in environmental systems. While it is thought that the release of Ag+ can play a crucial role in this toxicity, the mechanism by which the oxidative dissolution of nano-silver occurs is not yet understood. Here we address this through the electrochemical analysis of gold-core silver-shell nanoparticles in various solutions. This novel method allows the direct quantification of silver dissolution by normalisation to the gold core signal. This is shown to be highly effective at discriminating between silver dissolution and the loss of nanoparticles from the electrode surface. We evidence through this rigorous approach that the reduction of O<inf>2</inf> drives the dissolution of nano-silver, while in the presence of Cl- this dissolution is greatly inhibited. This work is extended to the single nanoparticle level using nano-impact experiments. © The Royal Society of Chemistry.
view abstract 10.1039/c5nr02995c
- The subtleties of the reversible hydrogen evolution reaction arising from the nonunity stoichiometry
Jiao, X. and Batchelor-Mcauley, C. and Kätelhön, E. and Ellison, J. and Tschulik, K. and Compton, R.G.
Journal of Physical Chemistry C 119 (2015)The proton/hydrogen redox couple underpins the electrochemical sciences; however, the nonunity stoichiometry of the reaction leads to distinct voltammetric complications. This Article provides a joint analytical, numerical, and experimental investigation into the reversible hydrogen evolution reaction at a platinum microelectrode. Literature obscurities and nuances are highlighted and corrected, allowing the presentation of an holistic overview of the electrochemical reaction at the reversible limit. Under such conditions, it is demonstrated, first, how the reaction may be misinterpreted as being irreversible and, second, that the transfer coefficient for the reversible (Nernstian) hydrogen evolution reaction is equal to 2. Importantly, the use of the reversible hydrogen electrode (RHE) as a reference potential in voltammetric experiments is critically evaluated. © 2015 American Chemical Society.
view abstract 10.1021/acs.jpcc.5b01864
- Ultra-small Palladium Nanoparticle Decorated Carbon Nanotubes: Conductivity and Reactivity
Li, X. and Batchelor-McAuley, C. and Tschulik, K. and Shao, L. and Compton, R.G.
ChemPhysChem 16 (2015)Carbon nanotubes decorated with ultra-small metal nanoparticles are of great value in catalysis. We report that individual multiwalled carbon nanotubes decorated with ultra-small palladium nanoparticles can be detected by using the nano-impacts method. The high conductivity and reactivity of each decorated carbon nanotube is directly evidenced; this is achieved through studying the proton-reduction reaction for the underpotential deposition of hydrogen onto the nanoparticles decorated on the carbon nanotube walls. The reductive spikes from current amplification are analyzed to estimate the approximate length of the decorated carbon nanotubes, revealing that the decorated carbon nanotubes are electroactive along its entire length of several micrometers. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/cphc.201500404
- A critical evaluation of the interpretation of electrocatalytic nanoimpacts
Ly, L.S.Y. and Batchelor-Mcauley, C. and Tschulik, K. and Kätelhön, E. and Compton, R.G.
Journal of Physical Chemistry C 118 (2014)The kinetics of the proton reduction reaction is studied on a variety of gold surfaces including both macro (r0 = 1.0 mm) and micro (r 0 = 4.6 μm) electrodes, as well as gold nanoparticles (r NP = ∼10 nm). For the gold nanoparticles, two complementary methodologies of study are used. First the particles are investigated as part of an ensemble response in an array (k0 ∼ 7 × 10-8 m s-1). Second, the rate is recorded stochastically at individually impacting nanoparticles (k0 ∼2 × 10-9 m s -1). This apparent decrease in reaction rates on transitioning from the ensemble to individual nanoparticles is understood in terms of the differing connectivity of the nanoparticles to the electrode surface. During the course of the individual catalytic impacts, or "pulses", the recorded current is found to be highly variable; this variability is interpreted as originating from the nanoscopic motion of the particle above the electrode interface. © 2014 American Chemical Society.
view abstract 10.1021/jp504968j
- A proof-of-concept - Using pre-created nucleation centres to improve the limit of detection in anodic stripping voltammetry
Toh, H.S. and Batchelor-Mcauley, C. and Tschulik, K. and Damm, C. and Compton, R.G.
Sensors and Actuators, B: Chemical 193 (2014)Anodic stripping voltammetry is a much-utilised method for trace metal analysis. We provide a simple proof-of-concept technique to improve the sensitivity of the method, which is illustrated by the detection of silver cations. This approach requires an electrode pre-treatment, which involves drop casting a metal nanoparticle suspension and oxidising the nanoparticles, leaving small metal nuclei on the electrode surface. In turn, the small metal nuclei act as nucleation sites for subsequent metal deposition when used to interrogate target solutions. In particular, the pre-treatment increases the amount of deposited metal in a given amount of time. Silver nitrate concentrations from 30 nM to 1 μM were tested and at silver ion concentration of 300 nM, the pre-treated electrode gave a signal, which was 40 times larger than the untreated electrode. The larger signal leads to the enhancement of sensitivity and a lowering of the detection limit of anodic stripping voltammetry without introducing other organic molecules, metals or impurities. © 2013 Elsevier B.V.
view abstract 10.1016/j.snb.2013.11.120
- Chemical interactions between silver nanoparticles and thiols: A comparison of mercaptohexanol against cysteine
Toh, H.S. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
Science China Chemistry 57 (2014)The interaction between citrate capped silver nanoparticles and two different thiols, mercaptohexanol (MH) and cysteine, was investigated. The thiols interacted with silver nanoparticles in a significantly contrasting manner. With MH, a sparingly soluble silver(I) thiolate complex AgSRm (Rm = -(CH2)6OH) was formed on the silver nanoparticle surface. Cyclic voltammograms and UV-vis spectra were used to infer that the AgSRm complex on the nanoparticle surface undergoes a phase transition to give a mixture of AgSRm and Ag2S-like complexes. In contrast, when silver nanoparticles were exposed to cysteine, the citrate capping agent on the silver nanoparticles was replaced by cysteine to give cysteine capped nanoparticles. As cysteine capped nanoparticles form, the electrochemical data displayed a decrease in oxidative peak charge but the UV-vis spectra showed a constant signal. Therefore, cysteine capped nanoparticles were suggested to have either inactivated the silver surface or else promoted detachment from the electrode surface. © 2014 Science China Press and Springer-Verlag Berlin Heidelberg.
view abstract 10.1007/s11426-014-5141-8
- Diffusional transport to and through thin-layer nanoparticle film modified electrodes: Capped CdSe nanoparticle modified electrodes
Hepburn, W.G. and Batchelor-Mcauley, C. and Tschulik, K. and Barnes, E.O. and Kachoosangi, R.T. and Compton, R.G.
Physical Chemistry Chemical Physics 16 (2014)We present a simple and general theoretical model which accounts fully for the influence of an electrode modifying non-electroactive layer on the voltammetric response of a diffusional redox probe. The layer is solely considered to alter the solubilities and diffusion coefficients of the electroactive species within the thin layer on the electrode surface. On this basis it is demonstrated how, first, the apparent electrochemical rate constant can deviate significantly from that measured at an unmodified electrode. Second, depending on the conditions within the layer the modification of the electrode may lead to either apparent 'negative' or 'positive' electrocatalytic effects without the true standard electrochemical rate constant for the electron transfer at the electrode surface being altered. Having presented the theoretical model three experimental cases are investigated, specifically, the reductions of ruthenium(iii) hexaamine, oxygen and boric acid on a gold macro electrode with and without a multi-layer organic capped nanoparticle film. In the latter case of the reduction of boric acid the voltammetric reduction is found to be enhanced by the presence of the organic layer. This result is interpreted as being due to an increase in the solubility of the analyte within the non-electroactive layer and not due to an alteration of the standard electrochemical rate constant. This journal is © the Partner Organisations 2014.
view abstract 10.1039/c4cp02440k
- Electrochemical detection and characterisation of polymer nanoparticles
Zhou, X.-F. and Cheng, W. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
Electroanalysis 26 (2014)We report the detection and characterisation of polymer nanoparticles using electrochemistry using poly(N-vinylcarbazole) nanoparticles (PVK NPs) as a model system. These were synthesised using the reprecipitation method. The number of electrons (n=2) transferred per PVK monomer was characterised by drop-casting method. Sticking and sensing experiments were then conducted, which involve PVK nanoparticle immobilisation on the electrode surface and subsequent oxidative sensing, to enable rapid detection of polymer nanoparticles in aqueous solution. It is shown for the first time, that using this "stick and sense" method, polymer nanoparticles in aqueous solution can be immobilised, preconcentrated and quantified. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/elan.201300438
- Electrochemical detection of glutathione using a poly(caffeic acid) nanocarbon composite modified electrode
Lee, P.T. and Ward, K.R. and Tschulik, K. and Chapman, G. and Compton, R.G.
Electroanalysis 26 (2014)A modified glassy carbon electrode was prepared through electropolymerization of caffeic acid in the presence of either carbon nanotubes or nano-carbon drop cast onto the electrode surface. The voltammetric behaviour of the electrode was characterized using the ortho-quinone moiety on the caffeic acid unit and the surface loading optimized for current response. The nanocomposite mediated electrode was used for the sensitive detection of glutathione at concentrations as low as 500nM. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/elan.201300486
- Electrochemical observation of single collision events: Fullerene nanoparticles
Stuart, E.J.E. and Tschulik, K. and Batchelor-Mcauley, C. and Compton, R.G.
ACS Nano 8 (2014)Individual fullerene nanoparticles are detected and sized in a non-aqueous solution via cathodic particle coulometry where the direct, quantitative reduction of single nanoparticles is achieved upon collision with a potentiostated gold electrode. This is the first time that the nanoparticle impact technique has been shown to work in a non-aqueous electrolyte and utilized to coulometrically size carbonaceous nanoparticles. Contrast is drawn between single-nanoparticle electrochemistry and that seen using nanoparticle ensembles via modified electrodes. © 2014 American Chemical Society.
view abstract 10.1021/nn502634n
- Electrochemical quantification of iodide ions in synthetic urine using silver nanoparticles: A proof-of-concept
Toh, H.S. and Tschulik, K. and Batchelor-Mcauley, C. and Compton, R.G.
Analyst 139 (2014)Typical urinary iodide concentrations range from 0.3 μM to 6.0 μM. The conventional analytical method is based on the Sandell-Kolthoff reaction. It involves the toxic reagent, arsenic acid, and a waiting time of 30 minutes for the iodide ions to reduce the cerium(iv) ions. In the presented work, an alternative fast electrochemical method based on a silver nanoparticle modified electrode is proposed. Cyclic voltammetry was performed with a freshly modified electrode in presence of iodide ions and the voltammetric peaks corresponding to the oxidation of silver to silver iodide and the reverse reaction were recorded. The peak height of the reduction signal of silver iodide was used to plot a calibration line for the iodide ions. Two calibration plots for the iodide ions were obtained, one in 0.1 M sodium nitrate (a chloride-ion free environment to circumvent any interference from the other halides) and another in synthetic urine (which contains 0.2 M KCl). In both of the calibration plots, linear relationships were found between the reduction peak height and the iodide ion concentration of 0.3 μM to 6.0 μM. A slope of 1.46 × 10-2 A M-1 and a R2 value of 0.999 were obtained for the iodide detection in sodium nitrate. For the synthetic urine experiments, a slope of 3.58 × 10-3 A M-1 and a R2 value of 0.942 were measured. A robust iodide sensor with the potential to be developed into a point-of-care system has been validated. This journal is © the Partner Organisations 2014.
view abstract 10.1039/c4an00741g
- Electrochemical studies of silver nanoparticles: A guide for experimentalists and a perspective
Tschulik, K. and Batchelor-Mcauley, C. and Toh, H.-S. and Stuart, E.J.E. and Compton, R.G.
Physical Chemistry Chemical Physics 16 (2014)This perspective summarises four different electrochemical techniques that have been established and frequently used to characterize various properties of silver nanoparticles. These are based on drop casting (I), in situ nanoparticle sticking and stripping (II), transfer sticking and stripping (III) or nanoparticle impacts (IV). The specific characteristics of the different methodologies are explained and contrasted with each other with the focus being on the respective benefits and limitations together with essential insights for experimentalists. © 2014 the Owner Societies.
view abstract 10.1039/c3cp54221a
- Gold electrodes from recordable CDs for the sensitive, semi-quantitative detection of commercial silver nanoparticles in seawater media
Stuart, E.J.E. and Tschulik, K. and Lowinsohn, D. and Cullen, J.T. and Compton, R.G.
Sensors and Actuators, B: Chemical 195 (2014)We report the use of homemade disposable gold electrodes fabricated from commercial recordable CDs for the detection and quantification of silver nanoparticles from a consumer product in a seawater sample. The "CDtrode" is immersed in a seawater sample containing silver nanoparticles for a certain amount of time during which the silver nanoparticles adsorb onto the CDtrode surface under open circuit conditions. The CDtrode is then transferred to an aqueous electrolyte and oxidative stripping is used to determine the amount of silver nanoparticles that have become stuck to the electrode surface. Depending on immersion time and silver nanoparticle concentration, up to a full monolayer coverage of silver nanoparticles on the CDtrode surface has been achieved. © 2014 Elsevier B.V.
view abstract 10.1016/j.snb.2014.01.040
- Improving the rate of silver nanoparticle adhesion to 'sticky electrodes': Stick and strip experiments at a DMSA-modified gold electrode
Stuart, E.J.E. and Tschulik, K. and Ellison, J. and Compton, R.G.
Electroanalysis 26 (2014)The immobilisation of nanoparticles from solution at a solid surface followed by anodic stripping voltammetry is a simple technique allowing the analysis of nanoparticle concentrations and identity. We report that the modification of gold electrodes with meso-2,3-dimercaptosuccinic acid (DMSA) shows a useful increase in the adsorption rate of silver nanoparticles on a gold substrate showing that the chemical modification of the electrode is analytically advantageous. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/elan.201300452
- Magnetic field templated patterning of the soft magnetic alloy CoFe
Karnbach, F. and Uhlemann, M. and Gebert, A. and Eckert, J. and Tschulik, K.
Electrochimica Acta 123 (2014)The ongoing miniaturization of multifunctional electronic consumer products demands for cost-efficient production of functional metal and alloy structures. Recently, an electrochemical method of structuring by means of magnetic gradient fields has been introduced for metal deposition. Here, not only the structured deposition of the metals Co and Fe is presented, but it is further demonstrated that this method can be applied to structure alloys. This is shown for the industrially highly relevant magnetic CoFe alloy. Deposits with maximum layer thickness in regions of high magnetic gradients are formed and the chemical composition of the alloy is constant across the structure. Electrochemical quartz crystal microbalance studies revealed that in applied magnetic gradient fields the current efficiency for alloy deposition is significantly increased with respect to that for hydrogen reduction in the additive free sulphate based electrolyte used in this study. © 2014 Elsevier Ltd.
view abstract 10.1016/j.electacta.2014.01.055
- Magnetic separation of paramagnetic ions from initially homogeneous solutions
Yang, X. and Tschulik, K. and Uhlemann, M. and Odenbach, S. and Eckert, K.
IEEE Transactions on Magnetics 50 (2014)The concentration change in an initially homogeneous paramagnetic solution is studied interferometrically upon applying sufficiently strong inhomogeneous magnetic fields. For this purpose, five different magnetic field configurations are analyzed. Clear evidence is provided that an enrichment of paramagnetic ions occurs in the field gradient. In particular, we show that the shape of this enrichment layer maps the spatial distribution of the magnetic field gradient force. © 1965-2012 IEEE.
view abstract 10.1109/TMAG.2014.2324284
- Magnetic vortex observation in FeCo nanowires by quantitative magnetic force microscopy
Vock, S. and Hengst, C. and Wolf, M. and Tschulik, K. and Uhlemann, M. and Sasvári, Z. and Makarov, D. and Schmidt, O.G. and Schultz, L. and Neu, V.
Applied Physics Letters 105 (2014)An approach is presented that allows quantifying the three dimensional magnetization pattern of a magnetic nanoobject from measured two dimensional Magnetic Force Microscopy (MFM) data. This is based on a MFM deconvolution approach, which quantitatively determines the effective surface charges, on a micromagnetic calculation of the total magnetic charges at and below the sample surface, and on a projection of the lower lying charges onto the sample surface for a comparison of the such obtained effective surface charges with the experimentally determined ones. Thus, by making use of the depth sensitivity of MFM and by applying a quantitative contrast analysis, we are able to reconstruct the inhomogeneous magnetization state at the end of individual cylindrical Fe52Co48 nanowires arranged in a triangular array. As a result, we prove the existence of a magnetic vortex state at their ends. © 2014 AIP Publishing LLC.
view abstract 10.1063/1.4900998
- Nanoparticle impacts reveal magnetic field induced agglomeration and reduced dissolution rates
Tschulik, K. and Compton, R.G.
Physical Chemistry Chemical Physics 16 (2014)Superparamagnetic nanoparticles (NPs) are used in a variety of magnetic field-assisted chemical and medical applications, yet little of their fate during magnetic field interrogation is known. Here, fundamental and new insights in this are gained by cathodic particle coulometry. This methodology is used to study individual Fe3O4 NPs in the presence and absence of a magnetic field. It is first noticed that no major NP agglomeration occurs in the absence of a magnetic field even in a suspension of high ionic strength. In contrast, a significant magnetic field-induced agglomeration of NPs is observed in a magnetic field. A second new finding is that the dissolution of Fe 3O4 NPs is strongly inhibited in a magnetic field. This is explained as a result of the magnetic field gradient force trapping the released Fe2+ ions near the surface of a magnetized Fe 3O4 NP and thus hindering the mass-transport controlled NP dissolution. Consequently, fundamental magnetic field effects are measured and quantified on both the single NP scale and in suspension and two novel effects are discovered. This journal is © the Partner Organisations 2014.
view abstract 10.1039/c4cp01618a
- Nanoparticle-Impact Experiments are Highly Sensitive to the Presence of Adsorbed Species on Electrode Surfaces
Kätelhön, E. and Cheng, W. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
ChemElectroChem 1 (2014)We theoretically and experimentally investigate the influence of partial surface blocking on the electrochemistry of nanoparticles impacting at an electrode. To this end, we introduce an analytical model for the adsorption of single blocking molecules on the electrode and calculate the resulting fractional electrode coverage. We find that even small amounts of adsorbed molecules can fully suppress detection of impacts of nanoparticles while the electrode characteristics in the detection of electroactive molecules hardly change. Our findings are supported by experimental data on the indigo nanoparticle electroreduction at a carbon microelectrode (radius 5.5μm) in aqueous solution. We find that nanoimpacts are fully suppressed in the presence of acetone at concentrations of 250nm, which have a negligible effect on the electrode kinetics of the Fe(CN)3-/4- 6 couple. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/celc.201402014
- Planar diffusion to macro disc electrodes—what electrode size is required for the Cottrell and Randles-Sevcik equations to apply quantitatively?
Ngamchuea, K. and Eloul, S. and Tschulik, K. and Compton, R.G.
Journal of Solid State Electrochemistry 18 (2014)Simulations and experiments are reported which investigate the size of a macro disc electrode necessary to quantitatively show the chronoamperometric or voltammetric behaviour predicted by the Cottrell equation or the Randles-Sevcik equation on the basis of exclusive one-dimensional diffusional mass transport. For experimental time scales of several seconds, the contribution of radial diffusion is seen to be measurable even for electrodes of millimetres in radius. Recommendations on the size of macro electrodes for quantitative study are given and should exceed 4 mm radius in aqueous solution. © 2014, Springer-Verlag Berlin Heidelberg.
view abstract 10.1007/s10008-014-2664-z
- Retarding the corrosion of iron by inhomogeneous magnetic fields
Sueptitz, R. and Tschulik, K. and Uhlemann, M. and Eckert, J. and Gebert, A.
Materials and Corrosion 65 (2014)The influence of a magnetic field yielding high magnetic flux densities and high flux density gradients on the free corrosion behavior of iron in a low concentrated acidic solution with and without chloride ions is studied by long time exposure experiments and electrochemical impedance spectroscopy (EIS). A decrease of the corrosion rate in electrode surface regions of high magnetic flux density is detected. This decrease of the dissolution rate is significantly stronger in the presence of chloride ions. The observed effects are discussed on the basis of the magnetically induced forces acting on the ions present in the solution, the surface coverage fraction of adsorbed species, and the stability of these adsorbed species. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/maco.201206890
- Simultaneous electrochemical and 3D optical imaging of silver nanoparticle oxidation
Batchelor-Mcauley, C. and Martinez-Marrades, A. and Tschulik, K. and Patel, A.N. and Combellas, C. and Kanoufi, F. and Tessier, G. and Compton, R.G.
Chemical Physics Letters 597 (2014)The oxidation of AgNPs at a thin-film gold electrode is simultaneously investigated via digital holography and electrochemistry. The use of holography allows, for the first time, the 3D visualization of the electrochemical interfacial region at a relatively high acquisition rate. It is demonstrated how the coupling of these two techniques provides complementary chemical information. The ensemble response of the oxidation of surface-adsorbed silver nanoparticles to AgCl is monitored electrochemically, whereas this process is difficult to observe optically. Conversely, the subsequent chemical dissolution of individual AgCl nanocrystals can be tracked optically due to the associated decrease in the scattered light intensity. © 2014 Elsevier B.V. All rights reserved.
view abstract 10.1016/j.cplett.2014.02.007
- Strong negative nanocatalysis: Oxygen reduction and hydrogen evolution at very small (2 nm) gold nanoparticles
Wang, Y. and Laborda, E. and Tschulik, K. and Damm, C. and Molina, A. and Compton, R.G.
Nanoscale 6 (2014)The electron transfer kinetics associated with both the reduction of oxygen and of protons to form hydrogen at gold nanoparticles are shown to display strong retardation when studied at citrate capped ultra small (2 nm) gold nanoparticles. Negative nanocatalysis in the hydrogen evolution reaction (HER) is reported for the first time. This journal is © the Partner Organisations 2014.
view abstract 10.1039/c4nr03850a
- The strong catalytic effect of Pb(II) on the oxygen reduction reaction on 5 nm gold nanoparticles
Wang, Y. and Laborda, E. and Plowman, B.J. and Tschulik, K. and Ward, K.R. and Palgrave, R.G. and Damm, C. and Compton, R.G.
Physical Chemistry Chemical Physics 16 (2014)Citrate-capped gold nanoparticles (AuNPs) of 5 nm in diameter are synthesized via wet chemistry and deposited on a glassy carbon electrode through electrophoresis. The kinetics of the oxygen reduction reaction (ORR) on the modified electrode is determined quantitatively in oxygen-saturated 0.5 M sulphuric acid solution by modelling the cathode as an array of interactive nanoelectrodes. Quantitative analysis of the cyclic voltammetry shows that no apparent ORR electrocatalysis takes place, the kinetics on AuNPs being effectively the same as on bulk gold. Contrasting with the above, a strong ORR catalysis is found when Pb2+ is added to the oxygen saturated solution or when the modified electrode is cycled in lead alkaline solution such that lead dioxide is repeatedly electrodeposited and stripped off on the nanoparticles. In both cases, the underpotential deposition of lead on the gold nanoparticles is found to be related to the catalysis. This journal is © the Owner Societies 2014.
view abstract 10.1039/c3cp55306j
- The surface energy of single nanoparticles probed via anodic stripping voltammetry
Neumann, C.C.M. and Batchelor-McAuley, C. and Tschulik, K. and Toh, H.S. and Shumbul, P. and Pillay, J. and Tshikhudo, R. and Compton, R.G.
ChemElectroChem 1 (2014)
view abstract 10.1002/celc.201300062
- The use of cylindrical micro-wire electrodes for nano-impact experiments; Facilitating the sub-picomolar detection of single nanoparticles
Ellison, J. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
Sensors and Actuators, B: Chemical 200 (2014)Electrochemical impact experiments can be used to detect and size single nanoparticles in suspension and at low concentrations. This is generally performed using a micro-disc working electrode; however, for the first time we report the use of cylindrical micro-wire electrodes for nanoparticle impact experiments. These electrodes provide much enhanced detection limits; specifically decreasing the concentration of nanoparticles measurable by over two orders of magnitude. In addition, the use of micro-wire electrodes reduces the shielding effect due to absorption of particles to the insulating sheath that surrounds a micro-disc electrode. Micro-wire electrodes are fabricated and their electrochemical response analysed via cyclic voltammetry experiments using molecular species. This provides a theoretical framework which is used to calculate the reduced concentration of nanoparticles required for an impact experiment at a micro-cylinder electrode in comparison to the micro-disc. Experimentally, it is demonstrated that impact experiments on the micro-cylinder electrodes can indeed be used for accurate characterisation of ultra-low concentrations (≈0.1 pM) of silver nanoparticles. © 2014 Elsevier B.V.
view abstract 10.1016/j.snb.2014.03.085
- Use of the capping agent for the electrochemical detection and quantification of nanoparticles: CdSe quantum dots
Hepburn, W.G. and Batchelor-Mcauley, C. and Tschulik, K. and Kachoosangi, R.T. and Ness, D. and Compton, R.G.
Sensors and Actuators, B: Chemical 204 (2014)The electrochemical detection of organic capped CdSe nanoparticles is achieved down to the highly dilute concentration of 15 pM. Herein, electrode modification is undertaken either via a simple and fast adsorption methodology, or by direct dropcasting of the material. Importantly, the adsorption of the CdSe nanoparticles is evidenced at higher surface coverages by the direct measurement of the cadmium reduction signal. A lower analytical detection limit for the CdSe nanoparticles is enabled by the enhancement of the diffusional borax reduction signal on a gold electrode modified with the quantum dots. The presence of a non-electroactive layer on an electrode has been shown to alter the apparent electrochemical rate constant via modifying the solubility and mass-transport of an electroactive species adjacent to the electrochemical interface. In the present case the origin of the enhanced rate of reduction for borax is ascribed as being due to the presence of the non-electroactive organic capping agent. Hence, due to the ubiquitous nature of capping agents within the field of nano-chemistry, the methodology represents a facile and generally applicable detection route. © 2014 Elsevier B.V.
view abstract 10.1016/j.snb.2014.07.111
- 'Sticky electrodes' for the detection of silver nanoparticles
Tschulik, K. and Palgrave, R.G. and Batchelor-Mcauley, C. and Compton, R.G.
Nanotechnology 24 (2013)Detection and quantification of nanoparticles in environmental systems is a task that requires reliable and affordable analytical methods. Here an approach using a cysteine-modified 'sticky' glassy carbon electrode is presented. The electrode is immersed in a silver nanoparticle containing electrolyte and left in this suspension without an applied potential, i.e. under open circuit condition, for a variable amount of time. The amount of silver nanoparticles immobilized on the electrode within this sticking time is then determined by oxidative stripping, yielding the anodic charge and thus the amount of Ag nanoparticles sticking to the electrode surface. When using a cysteine-modified glassy carbon electrode, significant and reproducible amounts of silver nanoparticles stick to the surface, which is not the case for unmodified glassy carbon surfaces. Additionally, proof-of-concept experiments are performed on real seawater samples. These demonstrate that also under simulated environmental conditions an increased immobilization and hence improved detection of silver nanoparticles on cysteine-modified glassy carbon electrodes is achieved, while no inhibitive interference with this complex matrix is observed. © 2013 IOP Publishing Ltd.
view abstract 10.1088/0957-4484/24/29/295502
- A disposable sticky electrode for the detection of commercial silver NPs in seawater
Cheng, W. and Stuart, E.J.E. and Tschulik, K. and Cullen, J.T. and Compton, R.G.
Nanotechnology 24 (2013)The ability to perform efficient and affordable field detection and quantification of nanoparticles in aquatic environmental systems remains a significant technical challenge. Recently we reported a proof of concept of using 'sticky' electrodes for the detection of silver nanoparticles (Tschulik et al 2013 Nanotechnology 29 295502). Now a disposable electrode for detection and quantification of commercial Ag nanoparticles in natural seawater is presented. A disposable screen printed electrode is modified with cysteine and characterized by sticking and stripping experiments, with silver nanoparticle immobilization on the electrode surface and subsequent oxidative stripping, yielding a quantitative determination of the amount of Ag nanoparticles adhering to the electrode surface. The modified electrode was applied to natural seawater to mimic field-based environmental monitoring of Ag NPs present in seawater. The results demonstrated that commercial Ag NPs in natural seawater can be immobilized, enriched and quantified within short time period using the disposable electrodes without any need for elaborate experiments. © 2013 IOP Publishing Ltd.
view abstract 10.1088/0957-4484/24/50/505501
- A kinetic study of oxygen reduction reaction and characterization on electrodeposited gold nanoparticles of diameter between 17 nm and 40 nm in 0.5 M sulfuric acid
Wang, Y. and Laborda, E. and Ward, K.R. and Tschulik, K. and Compton, R.G.
Nanoscale 5 (2013)Kinetic and mechanistic studies of the oxygen reduction reaction (ORR) in oxygen saturated 0.5 M sulfuric acid at 298 K at a gold macroelectrode and at an electrodeposited gold nanoparticle-modified glassy carbon electrode are reported. The conditions of electrodeposition are optimized to obtain small nanoparticles of diameter from 17 nm to 40 nm. The mechanism and kinetics of ORR on the gold macroelectrode are investigated and compared with those obtained for nanoparticle-modified electrodes. The mechanism for this system includes two electron and two proton transfers and hydrogen peroxide as the final product. The first electron transfer step corresponding to the reduction of O2 to O2 - is defined as the rate determining step. No significant changes are found for the nanoparticles here employed: electron transfer rate constant (k0) is k0,bulk = 0.30 cm s -1 on the bulk material and k0,nano = 0.21 cm s -1 on nanoparticles; transfer coefficient (α) changes from αbulk = 0.45 on macro-scale to αnano = 0.37 at the nano-scale. © The Royal Society of Chemistry 2013.
view abstract 10.1039/c3nr02340k
- Analysis of the electrolyte convection inside the concentration boundary layer during structured electrodeposition of copper in high magnetic gradient fields
König, J. and Tschulik, K. and Büttner, L. and Uhlemann, M. and Czarske, J.
Analytical Chemistry 85 (2013)To experimentally reveal the correlation between electrodeposited structure and electrolyte convection induced inside the concentration boundary layer, a highly inhomogeneous magnetic field, generated by a magnetized Fe-wire, has been applied to an electrochemical system. The influence of Lorentz and magnetic field gradient force to the local transport phenomena of copper ions has been studied using a novel two-component laser Doppler velocity profile sensor. With this sensor, the electrolyte convection within 500 μm of a horizontally aligned cathode is presented. The electrode-normal two-component velocity profiles below the electrodeposited structure show that electrolyte convection is induced and directed toward the rim of the Fe-wire. The measured deposited structure directly correlates to the observed boundary layer flow. As the local concentration of Cu2+ ions is enhanced due to the induced convection, maximum deposit thicknesses can be found at the rim of the Fe-wire. Furthermore, a complex boundary layer flow structure was determined, indicating that electrolyte convection of second order is induced. Moreover, the Lorentz force-driven convection rapidly vanishes, while the electrolyte convection induced by the magnetic field gradient force is preserved much longer. The progress for research is the first direct experimental proof of the electrolyte convection inside the concentration boundary layer that correlates to the deposited structure and reveals that the magnetic field gradient force is responsible for the observed structuring effect. © 2013 American Chemical Society.
view abstract 10.1021/ac302601t
- Capacitance performance of cobalt hydroxide-based capacitors with utilization of near-neutral electrolytes
Fedorov, F.S. and Linnemann, J. and Tschulik, K. and Giebeler, L. and Uhlemann, M. and Gebert, A.
Electrochimica Acta 90 (2013)Conventional alkaline solutions used for capacitive performance of electrodeposited cobalt hydroxides have a number of disadvantages as they are corrosive, environmentally unfriendly and provide a small working potential range. In this study, the capacitive properties of electrodeposited cobalt hydroxide/oxide were investigated in 1 M Na2SO4 solution with pH 5.5 by means of cyclic voltammetry, galvanostatic charging/discharging experiments and electrochemical impedance spectroscopy. The capacitance of the cobalt hydroxide/oxide was demonstrated to have high values of 141 F g -1 at scan rate 8 mV s-1 in this 1 M Na2SO 4 solution. The anodic potential range is extended by 0.8-1.3 V vs. Ag/AgCl. A good cyclic stability and reversibility were observed. © 2012 Elsevier Ltd.
view abstract 10.1016/j.electacta.2012.11.123
- Coulometric sizing of nanoparticles: Cathodic and anodic impact experiments open two independent routes to electrochemical sizing of Fe3O4 nanoparticles
Tschulik, K. and Haddou, B. and Omanović, D. and Rees, N.V. and Compton, R.G.
Nano Research 6 (2013)Anodic particle coulometry (APC) is a recently established method of sizing individual metal nanoparticles by oxidising them during their impact on a micro electrode. Here it is demonstrated that the application of APC can be extended to sizing of metal oxide nanoparticles, such as Fe3O4 magnetite nanoparticles. Additionally, a new route to electrochemical nanoparticle sizing is introduced-cathodic particle coulometry (CPC). This method uses the reduction of impacting nanoparticles, e.g., metal oxide nanoparticles, and is demonstrated to yield correct size information for Fe3O4 nanoparticles. The combination of these two independent electrochemical methods of nanoparticle sizing, allows for purely electrochemical sizing of single nanoparticles and simultaneous verification of the obtained results. © 2013 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
view abstract 10.1007/s12274-013-0361-3
- Electrochemical deposition of Co(Cu)/Cu multilayered nanowires
Fedorov, F.S. and Mönch, I. and Mickel, C. and Tschulik, K. and Zhao, B. and Uhlemann, M. and Gebert, A. and Eckert, J.
Journal of the Electrochemical Society 160 (2013)Electrochemical deposition in alumina templates is proved as a promising method for production of Co(Cu)/Cu nanowires showing high giant magnetoresistance (GMR). This study discusses the deposition of multilayered structures in highly ordered alumina templates in dependence on diffusion (Cu) or kinetically controlled (Co) behavior. Results show a high impact of spherical diffusion on the enhancement ofcurrent density of the diffusion controlled component compared todepositionof thin films on planar electrodes. To achieve a separation of the layers and to decrease the amount of Cu in the Co layer the deposition potential of Co was shifted toward more negative potentials. Lithographic structuring of the template surface was carried out to allow a 4-point measurement of the resistance. A high GMR of about 12% was obtained for [Co(Cu) (9 nm)/Cu (11 nm)]380 multilayered nanowires with high accuracy and reproducibility. © 2012 The Electrochemical Society.
view abstract 10.1149/2.006302jes
- Electrochemical detection of chloride levels in sweat using silver nanoparticles: A basis for the preliminary screening for cystic fibrosis
Toh, H.S. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
Analyst 138 (2013)Cystic fibrosis is a common disease which has an associated characteristic symptom of high sweat chloride content. Thus, chloride ion quantification in sweat is important towards the screening of cystic fibrosis. Electrochemical methods, being cost effective and convenient, can be exploited for this. The electrochemical oxidation of silver nanoparticles in the absence of chloride ions gives one voltammetric signal related to the oxidation of silver to silver ions. The presence of chloride ions in the solution causes the appearance of an additional signal at a lower potential which is related to the oxidation of silver to silver chloride. This signal has a peak height which correlates linearly with the concentration of chloride ions from 2 mM to 40 mM when the electrochemical experiments are performed on silver nanoparticle modified screen printed electrodes. Thus, reliable quantification was found to be achievable. Furthermore, chloride ion levels of diluted synthetic sweat samples are measured accurately with the modified electrodes. Thus, the detection of the chloride ion concentration with a silver nanoparticle modified electrode provides a proof-of-concept for a point-of-care system for preliminary screening of cystic fibrosis. © 2013 The Royal Society of Chemistry.
view abstract 10.1039/c3an00843f
- Electrochemical detection of commercial silver nanoparticles: Identification, sizing and detection in environmental media
Stuart, E.J.E. and Tschulik, K. and Omanović, D. and Cullen, J.T. and Jurkschat, K. and Crossley, A. and Compton, R.G.
Nanotechnology 24 (2013)The electrochemistry of silver nanoparticles contained in a consumer product has been studied. The redox properties of silver particles in a commercially available disinfectant cleaning spray were investigated via cyclic voltammetry before particle-impact voltammetry was used to detect single particles in both a typical aqueous electrolyte and authentic seawater media. We show that particle-impact voltammetry is a promising method for the detection of nanoparticles that have leached into the environment from consumer products, which is an important development for the determination of risks associated with the incorporation of nanotechnology into everyday products. © 2013 IOP Publishing Ltd.
view abstract 10.1088/0957-4484/24/44/444002
- Electrochemical micromachining of passive electrodes
Sueptitz, R. and Dunne, P. and Tschulik, K. and Uhlemann, M. and Eckert, J. and Gebert, A.
Electrochimica Acta 109 (2013)The electronic model describing the electrochemical micromachining (ECMM) of passive electrodes uti-lizing the transpassive dissolution is discussed. Numerical simulations are performed on a machiningmodel circuit using measured electrochemical properties of the model system which consisted of a tung-sten tool electrode, a 1 M H2SO4electrolyte and a stainless steel work piece electrode. The results of thesesimulations were verified by performing machining experiments applying the same model system. For apassive stainless steel electrode it is shown that it can be treated like an actively dissolving electrode withhigh reaction overpotential. The efficiency of the machining process can be enhanced by polarizing thesteel work piece electrode close to the transpassive potential region. Three different ways of achievingthis polarization are discussed: by polarizing the work piece electrode only, by polarizing both electrodesand by adding oxidizing species to the electrolyte solution. © 2013 Elsevier Ltd. All rights reserved.
view abstract 10.1016/j.electacta.2013.07.139
- Get more out of your data: A new approach to agglomeration and aggregation studies using nanoparticle impact experiments
Ellison, J. and Tschulik, K. and Stuart, E.J.E. and Jurkschat, K. and Omanović, D. and Uhlemann, M. and Crossley, A. and Compton, R.G.
ChemistryOpen 2 (2013)Anodic particle coloumetry is used to size silver nanoparticles impacting a carbon microelectrode in a potassium chloride/citrate solution. Besides their size, their agglomeration state in solution is also investigated solely by electrochemical means and subsequent data analysis. Validation of this new approach to nanoparticle agglomeration studies is performed by comparison with the results of a commercially available nanoparticle tracking analysis system, which shows excellent agreement. Moreover, it is demonstrated that the electrochemical technique has the advantage of directly yielding the number of atoms per impacting nanoparticle irrespective of its shape. This is not true for the optical nanoparticle tracking system, which requires a correction for the nonspherical shape of agglomerated nanoparticles to derive reasonable information on the agglomeration state. © 2013 The Authors.
view abstract 10.1002/open.201300005
- Nanocarbon Paste Electrodes
Lowinsohn, D. and Gan, P. and Tschulik, K. and Foord, J.S. and Compton, R.G.
Electroanalysis 25 (2013)The electrochemical behaviour of carbon paste electrodes prepared using nanocarbon and mineral oil was investigated and the results contrasted with different carbon and carbon pastes electrodes. The composition of carbon paste was studied by performing cyclic voltammetry performed in 0.1M KCl solution in the presence of 4.0mM Ru(NH3)6Cl3, a well-characterized redox system commonly used to test the electrode behaviour. After optimisation of the paste composition, the sensors chosen were tested for the analysis and characterization of three different systems: Ru(NH3)6 3+/2+, FcCH2OH/FcCH2OH+ and acetaminophen. The ability to obtain high quality voltammetry from the nanocarbon electrode was demonstrated and simulation of the voltammetry allowed the extraction of electrode kinetic parameters with high precision. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/elan.201300364
- Nanoparticle impacts show high-ionic-strength citrate avoids aggregation of silver nanoparticles
Lees, J.C. and Ellison, J. and Batchelor-Mcauley, C. and Tschulik, K. and Damm, C. and Omanovic̈, D. and Compton, R.G.
ChemPhysChem 14 (2013)Quantitative analytical detection and sizing of silver nanoparticles is achieved by applying the new electrochemical method nanoparticle coulometry. For the first time, tri-sodium citrate is used as both an electrolyte and a nanoparticle stabilizing agent, allowing the individual particles to be addressed. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/cphc.201300796
- Nanotoxicity - An electrochemist's perspective
Batchelor-McAuley, C. and Tschulik, K. and Compton, R.G.
Portugaliae Electrochimica Acta 31 (2013)This article highlights the fundamental role of mass-transport for interfacial reactions. First, the dissolution of particulate CaCO3 is discussed demonstrating how the dimensions of the dissolving particle can 'switch' the reaction mechanism from being diffusion to surface controlled. Second, the influence of mass-transoprt on electrochemical reactions is considered, specifically considering how electrode modification can alter the observed voltammetric response in the absence of changing the electrochemical mechanism or the rate of electron transfer. Finally, these observations on the chemically controlling role of mass-transport are concluded by considering nanoparticle toxicity and how 'size effects' may be exhibited even in the absence of altered thermodynamics or interfacial kinetics of the reactions involved.
view abstract 10.4152/pea.201305249
- Optical velocity measurements of electrolytic boundary layer flows influenced by magnetic fields
König, J. and Neumann, M. and Mühlenhoff, S. and Tschulik, K. and Albrecht, T. and Eckert, K. and Uhlemann, M. and Weier, T. and Büttner, L. and Czarske, J.
European Physical Journal: Special Topics 220 (2013)Magnetic fields are applied to electrically conducting fluids in order to influence electrochemical processes through the magnetohydrodynamic effect. Various phenomena, e. g. on electrodeposited metal layers, which can be attributed to forced convections were observed. To provide information about acting forces, the laser Doppler velocity profile sensor was applied to measure the transition layer of a Lorentz force influenced flow over a backward-facing step and the velocity boundary layer during copper deposition. With this sensor, the electrolyte convection within < 500 μm of the front of an electrode is measured with a spatial resolution down to 15 μm. The interaction of buoyancy, Lorentz and magnetic field gradient forces is studied by measuring the velocities down to 10 μm in front of the cathode. Inside the concentration boundary layer, complex electrolyte convection is induced, which varies not only in time but also in its structure, depending on the forces present and their influence over time. In inhomogeneous magnetic field configurations, the magnetic field gradient force dominates the velocity boundary layer at steady state and transports electrolyte toward regions of high magnetic gradients, where maximum deposit thicknesses are found. In this way, the measurements confirm the predicted influence of the magnetic field gradient force on the structuring of copper deposits. © 2013 EDP Sciences and Springer.
view abstract 10.1140/epjst/e2013-01798-x
- Performance of silver nanoparticles in the catalysis of the oxygen reduction reaction in neutral media: Efficiency limitation due to hydrogen peroxide escape
Neumann, C.C.M. and Laborda, E. and Tschulik, K. and Ward, K.R. and Compton, R.G.
Nano Research 6 (2013)The electrocatalytic activity for oxygen reduction reaction (ORR) at neutral pH of citrate-capped silver nanoparticles (diameter = 18 nm) supported on glassy carbon (GC) is investigated voltammetrically. Novelly, the modification of the substrate by nanoparticles sticking to form a random nanoparticle array and the voltammetric experiments are carried out simultaneously by immersion of the GC electrode in an air-saturated 0.1 M NaClO4 solution (pH = 5.8) containing chemically-synthesized nanoparticles. The experimental voltammograms of the resulting nanoparticle array are simulated with homemade programs according to the two-proton, two-electron reduction of oxygen to hydrogen peroxide where the first electron transfer is rate determining. In the case of silver electrodes, the hydrogen peroxide generated is partially further reduced to water via heterogeneous decomposition. Comparison of the results obtained on a silver macroelectrode and silver nanoparticles indicates that, for the silver nanoparticles and particle coverages (0.035%-0.457%) employed in this study, the ORR electrode kinetics is slower and the production of hydrogen peroxide larger on the glassy carbon-supported nanoparticles than on bulk silver. [Figure not available: see fulltext.] © 2013 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
view abstract 10.1007/s12274-013-0328-4
- Structured electrodeposition in magnetic gradient fields
Uhlemann, M. and Tschulik, K. and Gebert, A. and Mutschke, G. and Fröhlich, J. and Bund, A. and Yang, X. and Eckert, K.
European Physical Journal: Special Topics 220 (2013)Electrodeposition in superimposed magnetic gradient fields is a new and promising method of structuring metal deposits while avoiding masking techniques. The magnetic properties of the ions involved, their concentrations, the electrochemical deposition parameters, and the amplitude of the applied magnetic gradient field determine the structure generated. This structure can be thicker in regions of high magnetic field gradients. It can also be free-standing or inversely structured. The complex mechanism of structured electrodeposition of metallic layers in superimposed magnetic gradient fields was studied by different experimental methods, by analytical methods and by numerical simulation and will be discussed comprehensively. © 2013 EDP Sciences and Springer.
view abstract 10.1140/epjst/e2013-01814-3
- The anodic stripping voltammetry of nanoparticles: Electrochemical evidence for the surface agglomeration of silver nanoparticles
Toh, H.S. and Batchelor-McAuley, C. and Tschulik, K. and Uhlemann, M. and Crossley, A. and Compton, R.G.
Nanoscale 5 (2013)Analytical expressions for the anodic stripping voltammetry of metallic nanoparticles from an electrode are provided. First, for reversible electron transfer, two limits are studied: that of diffusionally independent nanoparticles and the regime where the diffusion layers originating from each particle overlap strongly. Second, an analytical expression for the voltammetric response under conditions of irreversible electron transfer kinetics is also derived. These equations demonstrate how the peak potential for the stripping process is expected to occur at values negative of the formal potential for the redox process in which the surface immobilised nanoparticles are oxidised to the corresponding metal cation in the solution phase. This work is further developed by considering the surface energies of the nanoparticles and its effect on the formal potential for the oxidation. The change in the formal potential is modelled in accordance with the equations provided by Plieth [J. Phys. Chem., 1982, 86, 3166-3170]. The new analytical expressions are used to investigate the stripping of silver nanoparticles from a glassy carbon electrode. The relative invariance of the stripping peak potential at low surface coverages of silver is shown to be directly related to the surface agglomeration of the nanoparticles. © 2013 The Royal Society of Chemistry.
view abstract 10.1039/c3nr00898c
- Clarifying the mechanism of reverse structuring during electrodeposition in magnetic gradient fields
Tschulik, K. and Cierpka, C. and Mutschke, G. and Gebert, A. and Schultz, L. and Uhlemann, M.
Analytical Chemistry 84 (2012)Deviating from the common expectation, magnetoelectrochemical structuring during deposition of diamagnetic ions was demonstrated, very recently. To achieve this, electrochemically inert paramagnetic ions have to be added to the electrolyte and the deposition has to be performed in a magnetic gradient field. A reverse structuring occurs, yielding thinner deposits near high gradient regions. In this paper we aim to clarify the mechanism of this reverse structuring. Potentiodynamic and potentiostatic investigations were performed, including measurements of the deposited mass with an electrochemical quartz crystal microbalance (EQCM). The convection of the electrolyte was studied in situ by astigmatism particle tracking velocimetry (APTV). It was revealed that during the reverse structuring a convection is induced in the electrolyte, which is directed away from the working electrode in regions of high magnetic gradients. Due to this additional convection, the overall deposition rate is increased, whereby it is locally reduced in regions of high magnetic gradients. The mechanism for reverse structuring is discussed in detail. Also, the influence of all relevant magnetic forces is addressed. © 2012 American Chemical Society.
view abstract 10.1021/ac2029612
- Electrochemical quartz crystal microbalance study of the Fe-Ga co-deposition
Iselt, D. and Tschulik, K. and Oswald, S. and Pohl, D. and Schultz, L. and Schlörb, H.
Journal of the Electrochemical Society 159 (2012)The electrochemical co-deposition of iron and gallium from a simple aqueous electrolyte was investigated by means of the electrochemical quartz crystal microbalance technique. The results reveal that alloy deposition occurs at potentials more positive than the deposition potential of single gallium. At the same time, large amounts of hydroxides are chemically precipitated due to a pH increase caused by strong hydrogen evolution. If the pH increase is compensated by applying potential pulses, these hydroxides are re-dissolved and metallic alloy films with low oxygen content are directly accessible. XPS and TEM investigations confirm the formation of an Fe100-xGax alloy (x = 20 ± 4 at.%). © 2012 The Electrochemical Society.
view abstract 10.1149/2.028207jes
- Electrodeposition of separated metallic structures in superimposed magnetic gradient fields
Tschulik, K. and Cierpka, Ch. and Uhlemann, M. and Gebert, A. and Schultz, L.
ECS Transactions 41 (2012)Structuring of Cu deposits was reported to occur during electrodeposition in magnetic gradient fields, recently. This work aims on increasing the knowledge about the convection-induced structuring mechanism. Therefore, in-situ velocity measurements by Astigmatism Particle Tracking Velocimetry were performed. The information gained by this study is used to choose pulse-reverse-plating parameters suitable for deposition of separated Cu structures. Additionally, it is demonstrated that the structure shape can be changed by varying the superimposed magnetic field gradients. The structure height can be adjusted by the number of pulse-reverse-plating cycles. The structuring mechanism is discussed for potentiostatic as well as for pulse-reverse-plating conditions with respect to the acting magnetic forces. ©The Electrochemical Society.
view abstract 10.1149/1.3696825
- Enrichment of paramagnetic ions from homogeneous solutions in inhomogeneous magnetic fields
Yang, X. and Tschulik, K. and Uhlemann, M. and Odenbach, S. and Eckert, K.
Journal of Physical Chemistry Letters 3 (2012)Applying interferometry to an aqueous solution of paramagnetic manganese ions, subjected to an inhomogeneous magnetic field, we observe an unexpected but highly reproducible change in the refractive index. This change occurs in the top layer of the solution, closest to the magnet. The shape of the layer is in accord with the spatial distribution of the largest component of the magnetic field gradient force. It turns out that this layer is heavier than the underlying solution because it undergoes a Rayleigh-Taylor instability upon removal of the magnet. The very good agreement between the magnitudes of buoyancy, associated with this layer, and the field gradient force at steady state provides conclusive evidence that the layer formation results from an enrichment of paramagnetic manganese ions in regions of high magnetic field gradient. © 2012 American Chemical Society.
view abstract 10.1021/jz301561q
- Micropatterning of Fe-based bulk metallic glass surfaces by pulsed electrochemical micromachining
Sueptitz, R. and Tschulik, K. and Becker, C. and Stoica, M. and Uhlemann, M. and Eckert, J. and Gebert, A.
Journal of Materials Research 27 (2012)A new technique for micropatterning Fe-based bulk metallic glass surfaces is reported. The transpassive dissolution process is utilized for a defined localized material removal when using a pulsed electrochemical micromachining process. By applying submicrosecond pulses between a work piece and a tool electrode, microholes of high aspect ratio and depth of up to 100 μm can be machined into the bulk glassy Fe65.5Cr4Mo4Ga4P12C5B5.5 alloy. Two potential electrolytes are identified for the machining process. For these electrolytes, different reaction mechanisms are discussed. The possibility of machining more complex structures is demonstrated for the most promising electrolyte, a methanolic H2SO4solution. The impact of the process parameters, pulse length and pulse voltage, on the machining gap and the surface quality of the machined structures is evaluated. © Materials Research Society 2012.
view abstract 10.1557/jmr.2012.347
- 3D3C velocity field measurements during the electrochemical copper deposition by long-range microscopic astigmatism PTV
Cierpka, C. and Tschulik, K. and Segura, R. and Uhlemann, M. and Kähler, C.J.
Technisches Messen 78 (2011)Using the astigmatism particle tracking method in connection with a long-range microscope enables highly temporal and spatial resolved measurements of the three dimensional velocity field in a fluid volume by a single camera. Thus, the complex interplay of magnetic field gradient force and the Lorentz force induced convective effects was ana- lyzed experimentally for the first time. © Oldenbourg Wissenschaftsverlag.
view abstract 10.1524/teme.2011.0132
- Effect of high gradient magnetic fields on the anodic behaviour and localized corrosion of iron in sulphuric acid solutions
Sueptitz, R. and Tschulik, K. and Uhlemann, M. and Schultz, L. and Gebert, A.
Corrosion Science 53 (2011)The influence of high gradient magnetic fields on the anodic dissolution of iron in sulphuric acid solutions and the localization of the corrosion attack is investigated by means of potentiodynamic and potentiostatic polarization experiments and subsequent surface profile analysis. A localization of the material loss is observed in every potential region of the anodic Fe dissolution except from the passive region. The impact of the magnetic field on the anodic current density and the localization of the corrosion attack are explained by the action of the Lorentz force and the magnetic field gradient force. © 2011 Elsevier Ltd.
view abstract 10.1016/j.corsci.2011.05.070
- Effect of magnetization state on the corrosion behaviour of NdFeB permanent magnets
Sueptitz, R. and Tschulik, K. and Uhlemann, M. and Katter, M. and Schultz, L. and Gebert, A.
Corrosion Science 53 (2011)The corrosion behaviour of uncoated NdFeB permanent magnets in the non-magnetized and magnetized state was comparatively investigated in H2SO4 solutions by potentiodynamic and potentiostatic polarization experiments. Depth profiles of the corroded surfaces were recorded and an effect of magnetization on the localization of the corrosion attack was identified. The anodic behaviour is discussed on the basis of previously reported results on the corrosion of neodymium and iron and on basis of the magnetic forces acting on the electrochemical system. © 2011 Elsevier Ltd.
view abstract 10.1016/j.corsci.2011.05.022
- Electrodeposition of separated 3D metallic structures by pulse-reverse plating in magnetic gradient fields
Tschulik, K. and Sueptitz, R. and Uhlemann, M. and Schultz, L. and Gebert, A.
Electrochimica Acta 56 (2011)We present a new technique for electrodeposition of separated three-dimensional metallic structures solely by the superposition of magnetic gradient fields. Separate columns and stripes of Cu are generated by pulse reverse plating controlled by tailored magnetic field gradients. It is demonstrated that structures of μm-dimension are accessible with this technique and that the height of deposited structures can be adjusted by the number of plating cycles. The experimental observations are explained by the action of the magnetic field gradient force on the paramagnetic Cu 2+-ions during the deposition and the dissolution part of the cycle. Additional influences of the Lorentz force are also discussed. © 2011 Elsevier Ltd. All rights reserved.
view abstract 10.1016/j.electacta.2011.03.051
- How to obtain structured metal deposits from diamagnetic ions in magnetic gradient fields?
Tschulik, K. and Yang, X. and Mutschke, G. and Uhlemann, M. and Eckert, K. and Sueptitz, R. and Schultz, L. and Gebert, A.
Electrochemistry Communications 13 (2011)Electrodeposition of Bi in magnetic gradient fields was performed from two different electrolytes. The first electrolyte contained only diamagnetic Bi 3+-ions; the second one additionally contained electrochemically inert paramagnetic Mn2+-ions. While homogeneous Bi deposits were obtained from the former, structured Bi layers were derived from the latter. The structured deposits show an inverse correlation between deposit thickness and superimposed magnetic field gradient. Minima of film thickness are observed in regions of maximum magnetic gradients. This demonstration of magneto-electrochemical structuring by deposition of diamagnetic ions is discussed considering the acting magnetic forces. Several possibilities explaining the structuring mechanism are presented. © 2011 Elsevier B.V. All rights reserved.
view abstract 10.1016/j.elecom.2011.06.007
- In situ analysis of three-dimensional electrolyte convection evolving during the electrodeposition of copper in magnetic gradient fields
Tschulik, K. and Cierpka, C. and Gebert, A. and Schultz, L. and Kähler, C.J. and Uhlemann, M.
Analytical Chemistry 83 (2011)A novel three-dimensional particle tracking velocimetry technique was used to examine the flow during electrodeposition of Cu. For the first time electrode-normal, circumferential, and radial velocities were spatially resolved during deposition in superimposed low and high magnetic gradient fields. In this way the complex interaction of magnetic field gradient force and Lorentz force induced convective effects could be measured and analyzed. Magnetic field gradient force induced electrolyte flow was detected only in high gradient magnetic fields, and it was found to be directed toward regions of gradient maxima. Since this electrode-normal flow causes enhanced transport of Cu 2+ ions from the bulk electrolyte to those regions of the working electrode where maxima of magnetic gradients are present, a structured deposit is formed during diffusion-limited electrodeposition. Lorentz force driven convection was observed during deposition in the low and the high magnetic gradient experiments. The overall fluid motion and the convection near the working electrode were determined experimentally and discussed with regard to the acting magnetic forces and numerical simulations. © 2011 American Chemical Society.
view abstract 10.1021/ac102763m
- Magnetic field effects on the active dissolution of iron
Sueptitz, R. and Tschulik, K. and Uhlemann, M. and Schultz, L. and Gebert, A.
Electrochimica Acta 56 (2011)The impact of superimposed magnetic fields on the behavior of iron in 0.05 M H2SO4 at low anodic polarization was investigated by means of potentiostatic polarization measurements. Significant magnetic field effects were observed even though the active dissolution reaction in the investigated potential region is formally charge transfer controlled. The current density can be enhanced or reduced dependent on the magnetic field to electrode configuration. The results are discussed in terms of the magnetic field impact on the surface pH value during the anodic dissolution. Our findings are likely to have important consequences for the life-time prediction of ferromagnetic components in electromagnetic devices and for future studies on magneto-electrodeposition processes. © 2011 Elsevier Ltd. All Rights Reserved.
view abstract 10.1016/j.electacta.2011.04.126
- Electrocrystallisation of metallic films under the influence of an external homogeneous magnetic field - Early stages of the layer growth
Koza, J.A. and Mogi, I. and Tschulik, K. and Uhlemann, M. and Mickel, C. and Gebert, A. and Schultz, L.
Electrochimica Acta 55 (2010)The effects of a uniform magnetic field on the early stages of Ag, Fe and CoFe alloys electrocrystallisation have been investigated. It was found for Fe and CoFe alloys, irrespective of the applied parameters, that early stages of the layer growth can be characterised by a nucleation and 3D diffusion controlled growth. The influence of the deposition parameters on the nucleation behaviour was studied on the basis of the Sharifker-Mostany (SM) model. A modification to the existing model has been proposed in order to model alloy systems. It is reported that a magnetic field superposed parallel to the electrode surface has a significant influence on the early stages of Fe and CoFe alloys growth. The growth of the nuclei is enhanced by the Lorentz-force-driven convection, while the nucleation processes remain unaffected. The hydrodynamic origin of these phenomena is confirmed by independent rotating disk electrode (RDE) investigations. Moreover, the proposed mechanism of a magnetic field influence on the 3D diffusion controlled growth is supported by a microscopic investigation of Ag deposits. It was found that Ag deposits obtained without a magnetic field superposition are characterised by a relatively large number of small 3D growth centres, whereas the deposits obtained in a field show fewer 3D centres but their size is greatly increased. © 2010 Elsevier Ltd. All rights reserved.
view abstract 10.1016/j.electacta.2010.06.026
- Impact of magnetic field gradients on the free corrosion of iron
Sueptitz, R. and Tschulik, K. and Uhlemann, M. and Gebert, A. and Schultz, L.
Electrochimica Acta 55 (2010)The influence of magnetic fields oriented parallel and perpendicular to the electrode surface on the corrosion behaviour of differently shaped iron samples in low concentrated sulphuric acid solutions has been studied. It is demonstrated, that the relative sample-to-magnet configuration, which determines the magnetic flux density distribution in front of the electrode surface, is decisive for the free corrosion activity. In a configuration generating low magnetic flux density gradients the Lorentz force driven micro-convection leads to an anodic shift of the free corrosion potential. In contrast, a configuration yielding high magnetic flux density gradients causes a cathodic potential shift and leads to a suppression of the corrosion reaction. These effects are discussed on the basis of the Lorentz force and the magnetic field gradient force acting on the partial reaction steps during the corrosion process. © 2010 Elsevier Ltd. All rights reserved.
view abstract 10.1016/j.electacta.2010.04.039
- Magnetoelectrochemical surface structuring electrodeposition of structured metallic layers in magnetic gradient fields
Tschulik, K. and Koza, J.A. and Uhlemann, M. and Gebert, A. and Schultz, L.
ECS Transactions 25 (2010)Structured copper layers were prepared by superimposition of inhomogeneous magnetic fields during the electrodeposition process. Self-made magnetic field templates were utilized for this purpose in order to demonstrate that even moderate magnetic flux densities applied at the working electrode are sufficient for this new structuring technique. The structuring effect was demonstrated for use of magnetic stray fields of magnetized Fe-wires as well as of permanent NdFeB-magnets. It was found that maxima of deposit thickness result in regions of maximum magnetic flux density. On the contrary, thickness minima are observed where the magnetic flux density at the working electrode is low. The results are discussed with respect to the acting magnetic forces and their influence on the deposition process. ©The Electrochemical Society.
view abstract 10.1149/1.3422508
- On the action of magnetic gradient forces in micro-structured copper deposition
Mutschke, G. and Tschulik, K. and Weier, T. and Uhlemann, M. and Bund, A. and Fröhlich, J.
Electrochimica Acta 55 (2010)In order to shed more light on the role of magnetic gradient forces and Lorentz forces on the deposition pattern found recently at copper electrodes, experiments and numerical simulations have been performed in a simple geometry that consists of a single small cylindrical permanent magnet which is placed behind the cathode. The cylinder axis coincides with the magnetization direction and points normal to the electrode surface. The electrode is oriented vertically which allows a separate discussion of the influence of both forces. Experiments and numerical simulations are found to give very good qualitative agreement with respect to the deposition pattern. Our analysis clearly shows that the major influence is due to the action of the magnetic gradient force. Numerical simulations prove that the separate action of the Lorentz force does not reproduce the deposition structure. A detailed analytical discussion of the motion forced by the different magnetic forces in superposition with natural convection is given. © 2010 Elsevier Ltd All rights reserved.
view abstract 10.1016/j.electacta.2010.08.046
- Studies on the patterning effect of copper deposits in magnetic gradient fields
Tschulik, K. and Sueptitz, R. and Koza, J. and Uhlemann, M. and Mutschke, G. and Weier, T. and Gebert, A. and Schultz, L.
Electrochimica Acta 56 (2010)Electrochemical Cu deposition was performed from electrolytes containing 0.1-1 M CuSO4 in a well-defined magnetic gradient field. Patterned deposits resulted in all cases showing a strong correlation to this gradient field. It was observed that with increasing Cu2+ concentration the structuring effect decreases in terms of differences in topography and morphology. An explanation of this effect is presented based on local convection induced by the magnetic field gradient force which is dependent on the concentration gradient established during the deposition. Superimposed effects of Lorentz force driven convection were observed for high Cu2+ concentrations, and their influence on the deposition process was discussed. © 2010 Elsevier Ltd.
view abstract 10.1016/j.electacta.2010.08.080
- Studies regarding the homogeneity range of the zirconium phosphide telluride Zr2+δPTe2
Tschulik, K. and Hoffmann, S. and Fokwa, B.P.T. and Gilleßen, M. and Schmidt, P.
Solid State Sciences 12 (2010)The phosphide tellurides Zr2+δPTe2 (0 ≤ δ ≤ 1) can be synthesized from the elements in a solid state reaction or by thermal decomposition of Z. Zr2PTe2 decomposes under release of Te2(g) + P4(g) forming the homogeneity range Zr2+δPTe2. The growth of single crystals of Zr 2+δPTe2 succeeded by chemical vapour transport using iodine as transport agent from 830 °C in direction of higher temperatures up to 900 °C. Zr2+δPTe2 crystallizes in the rhombohedral space group R3m (no. 166) with lattice parameters a = 383(1)...386(1) pm and c = 2935(4)...2970(4) pm for δ = 0...1, respectively. Single crystal data have been determined for Zr 2.40(2)PTe2 with lattice parameters a = 385.24(4) pm and c = 2967.8(4) pm. The electronic structure and chemical bonding in Zr 2+δPTe2 was investigated by the linear muffin-tin orbital (LMTO) method. Both Zr2PTe2 and Zr 3PTe2 show non-vanishing DOS values at the Fermi level (EF) indicating metallic character. According to COHP bonding analyses, mainly the heteroatomic Zr-P and Zr-Te bonds are responsible for the structural stability of Zr3PTe2. The new Zr2-Te bond, which is not present in Zr2PTe2, is stronger than Zr1-Te and is thought to be responsible for the stability of phases having Zr in excess.
view abstract 10.1016/j.solidstatesciences.2010.08.022