Prof. Dr. Dina Fattakhova-Rohlfing

Institute of Energy and Climate Research
University of Duisburg-Essen/Forschungszentrum Jülich

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  • Low temperature sintering of fully inorganic all-solid-state batteries – Impact of interfaces on full cell performance
    Ihrig, M. and Finsterbusch, M. and Tsai, C.-L. and Laptev, A.M. and Tu, C.-H. and Bram, M. and Sohn, Y.J. and Ye, R. and Sevinc, S. and Lin, S.-K. and Fattakhova-Rohlfing, D. and Guillon, O.
    Journal of Power Sources 482 (2021)
    One of the necessary prerequisites to advance the electrochemical performance of Li7La3Zr2O12 (LLZ) based all-solid-state lithium batteries is the manufacturing of dense composite cathodes from cathode active material (CAM) and the LLZ ceramic solid electrolyte. However, free co-sintering of LLZ and CAM mixtures requires temperatures above 1000 °C which often leads to decomposition and secondary phase formation, especially for high energy CAMs. In our study we present a completely dry processing route which is fast, free of any sintering additives and coatings and suitable to fabricate dense mixed cathodes, pure LLZ separators and multilayers of the two. Through application of high mechanical pressure during Field-Assisted Sintering we were able to reduce the sintering temperature down to 675–750 °C with dwell times as low as 10 min, while still obtaining 95% theoretical density for LCO/LLZ mixtures. The low sintering temperature is suitable for high energy CAMs, but leads to a significant effect of surface impurities, especially from powder handling in air, and affects the crystallinity of the CAM/LLZ interface. In the present paper we investigate the impact of resulting interfaces on the ionic conductivity, the interfacial impedance and the cycling stability of produced cells and propose the optimization strategy. © 2020 Elsevier B.V.
    view abstract10.1016/j.jpowsour.2020.228905
  • An aminotetracyanocyclopentadienide system: Light-induced formation of a thermally stable cyclopentadienyl radical
    Nimax, P.R. and Zoller, F. and Blockhaus, T. and Küblböck, T. and Fattakhova-Rohlfing, D. and Sünkel, K.
    New Journal of Chemistry 44 (2020)
    Crystals of the aminotetracyanocyclopentadienyl radical were obtained from the reaction of CaCl2 with Ag[C5(CN)4(NH2)] and recrystallization in MeOH, performed in sunlight. The radical was characterized by X-ray diffraction, EPR and UV Vis spectroscopy as well as by cyclovoltammetry and DFT calculations. © 2019 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
    view abstract10.1039/c9nj04354c
  • Black phosphorus–arsenic alloys for lithium ion batteries
    Luxa, J. and Bouša, D. and Zoller, F. and Fattakhova-Rohlfing, D. and Sofer, Z.
    FlatChem 19 (2020)
    Phosphorus and arsenic belong to the 5th group of elements – so-called pnictogens. These materials are among the most intensively studied nanomaterials with layered structure. In this contribution we report the synthesis of arsenic – black phosphorus alloys. Two samples with various black phosphorus and arsenic content together with pure black phosphorus were exfoliated using shear force milling. Extensive analyses have revealed the successful synthesis of AsP alloys with good crystallinity and composition close to that of the intended value. Testing these materials for lithium ion batteries (LIBs) shows that there is a huge capacity loss after the initial charge/discharge cycles. Such a drop was attributed to a delithiation of the lithium rich phase and a loss of proper electrical contact. After the initial capacity loss, the Coulombic efficiencies in the subsequent cycles reached 90–99%. Moreover, both of the alloys exhibited higher capacity than pure black phosphorus sample, indicating that alloying with arsenic is an advantageous technique. The results of this work show the fundamental charge storage capabilities of AsP alloys a can serve as a starting point for the synthesis of advanced materials based on AsP alloys. © 2019 Elsevier B.V.
    view abstract10.1016/j.flatc.2019.100143
  • Cellulose Nanocrystal-Templated Tin Dioxide Thin Films for Gas Sensing
    Ivanova, A. and Frka-Petesic, B. and Paul, A. and Wagner, T. and Jumabekov, A.N. and Vilk, Y. and Weber, J. and Schmedt Auf Der Günne, J. and Vignolini, S. and Tiemann, M. and Fattakhova-Rohlfing, D. and Bein, T.
    ACS Applied Materials and Interfaces 12 (2020)
    Porous tin dioxide is an important low-cost semiconductor applied in electronics, gas sensors, and biosensors. Here, we present a versatile template-assisted synthesis of nanostructured tin dioxide thin films using cellulose nanocrystals (CNCs). We demonstrate that the structural features of CNC-templated tin dioxide films strongly depend on the precursor composition. The precursor properties were studied by using low-temperature nuclear magnetic resonance spectroscopy of tin tetrachloride in solution. We demonstrate that it is possible to optimize the precursor conditions to obtain homogeneous precursor mixtures and therefore highly porous thin films with pore dimensions in the range of 10-20 nm (ABET = 46-64 m2 g-1, measured on powder). Finally, by exploiting the high surface area of the material, we developed a resistive gas sensor based on CNC-templated tin dioxide. The sensor shows high sensitivity to carbon monoxide (CO) in ppm concentrations and low cross-sensitivity to humidity. Most importantly, the sensing kinetics are remarkably fast; both the response to the analyte gas and the signal decay after gas exposure occur within a few seconds, faster than in standard SnO2-based CO sensors. This is attributed to the high gas accessibility of the very thin porous film. © 2020 American Chemical Society.
    view abstract10.1021/acsami.9b11891
  • Dendrite-tolerant all-solid-state sodium batteries and an important mechanism of metal self-diffusion
    Tsai, C.-L. and Lan, T. and Dellen, C. and Ling, Y. and Ma, Q. and Fattakhova-Rohlfing, D. and Guillon, O. and Tietz, F.
    Journal of Power Sources 476 (2020)
    Inhibition of dendrite growth in all-solid-state lithium batteries (ASSLBs) has long been a challenge to the field. In the present study, the conditions for dendrite growth for a similar but less mature technology, all-solid-state sodium batteries (ASSNBs), are investigated. By simply sticking sodium metal to Na3.4Zr2(SiO4)2.4(PO4)0.6 (NZSP) ceramic pellets and without applying external pressure during operation, the critical current density of Na/NZSP/Na symmetric ASSNBs reaches 9 mA cm−2 at 25 °C. The cells can be stably operated at an areal capacity of 5 mAh cm−2 (per half cycle, with 1.0 mA cm−2) at 25 °C for 300 h in a galvanostatic cycling measurement without any dendrite formation. The results outperform the existing ASSLBs and ASSNBs, and also go beyond satisfying the requirements for practical applications. The influence of the high metal self-diffusion coefficient on the dendritic plating/stripping is regarded as the most likely reason for the high dendrite tolerance of ASSNBs. A mathematical model based on Fick's second law was applied as a first approximation to illustrate this influence. Full ASSNBs were fabricated with infiltrated Na3V2(PO4)3 (NVP) as the cathode and can be stably operated with a capacity of 0.60 mAh cm−2 at high rate of 0.5 mA cm−2 at room temperature. © 2020 Elsevier B.V.
    view abstract10.1016/j.jpowsour.2020.228666
  • Efficient OER Catalyst with Low Ir Volume Density Obtained by Homogeneous Deposition of Iridium Oxide Nanoparticles on Macroporous Antimony-Doped Tin Oxide Support
    Böhm, D. and Beetz, M. and Schuster, M. and Peters, K. and Hufnagel, A.G. and Döblinger, M. and Böller, B. and Bein, T. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 30 (2020)
    A multistep synthesis procedure for the homogeneous coating of a complex porous conductive oxide with small Ir nanoparticles is introduced to obtain a highly active electrocatalyst for water oxidation. At first, inverse opal macroporous Sb doped SnO2 (ATO) microparticles with defined pore size, composition, and open-porous morphology are synthesized that reach a conductivity of ≈3.6 S cm−1 and are further used as catalyst support. ATO-supported iridium catalysts with a controlled amount of active material are prepared by solvothermal reduction of an IrOx colloid in the presence of the porous ATO particles, whereby homogeneous coating of the complete outer and inner surface of the particles with nanodispersed metallic Ir is achieved. Thermal oxidation leads to the formation of ATO-supported IrO2 nanoparticles with a void volume fraction of ≈89% calculated for catalyst thin films based on scanning transmission electron microscope tomography data and microparticle size distribution. A remarkably low Ir bulk density of ≈0.08 g cm−3 for this supported oxide catalyst architecture with 25 wt% Ir is determined. This highly efficient oxygen evolution reaction catalyst reaches a current density of 63 A gIr −1 at an overpotential of 300 mV versus reversible hydrogen electrode, significantly exceeding a commercial TiO2-supported IrO2 reference catalyst under the same measurement conditions. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201906670
  • Freestanding LiFe0.2Mn0.8PO4/rGO nanocomposites as high energy density fast charging cathodes for lithium-ion batteries
    Zoller, F. and Böhm, D. and Luxa, J. and Döblinger, M. and Sofer, Z. and Semenenko, D. and Bein, T. and Fattakhova-Rohlfing, D.
    Materials Today Energy 16 (2020)
    Freestanding electrodes for lithium ion batteries are considered as a promising option to increase the total gravimetric energy density of the cells due to a decreased weight of electrochemically inactive materials. We report a simple procedure for the fabrication of freestanding LiFe0.2Mn0.8PO4 (LFMP)/rGO electrodes with a very high loading of active material of 83 wt%, high total loading of up to 8 mg cm−2, high energy density, excellent cycling stability and at the same time very fast charging rate, with a total performance significantly exceeding the values reported in the literature. The keys to the improved electrode performance are optimization of LFMP nanoparticles via nanoscaling and doping; the use of graphene oxide (GO) with its high concentration of surface functional groups favoring the adhesion of high amounts of LFMP nanoparticles, and freeze-casting of the GO-based nanocomposites to prevent the morphology collapse and provide a unique fluffy open microstructure of the freestanding electrodes. The rate and the cycling performance of the obtained freestanding electrodes are superior compared to their Al-foil coated equivalents, especially when calculated for the entire weight of the electrode, due to the extremely reduced content of electrochemically inactive material (17 wt% of electrochemically inactive material in case of the freestanding compared to 90 wt% for the Al-foil based electrode), resulting in 120 mAh g−1 electrode in contrast to 10 mAh g−1 electrode at 0.2 C. The electrochemical performance of the freestanding LFMP/rGO electrodes is also considerably better than the values reported in literature for freestanding LFMP and LMP composites, and can even keep up with those of LFP-based analogues. The freestanding LFMP/rGO reported in this work is additionally attractive due to its high gravimetric energy density (604 Wh kg−1 LFMP at 0.2C). The obtained results demonstrate the advantage of freestanding LiFe0.2Mn0.8PO4/rGO electrodes and their great potential for applications in lithium ion batteries. © 2020 Elsevier Ltd
    view abstract10.1016/j.mtener.2020.100416
  • How photocorrosion can trick you: A detailed study on low-bandgap Li doped CuO photocathodes for solar hydrogen production
    Kampmann, J. and Betzler, S. and Hajiyani, H. and Häringer, S. and Beetz, M. and Harzer, T. and Kraus, J. and Lotsch, B.V. and Scheu, C. and Pentcheva, R. and Fattakhova-Rohlfing, D. and Bein, T.
    Nanoscale 12 (2020)
    The efficiency of photoelectrochemical tandem cells is still limited by the availability of stable low band gap electrodes. In this work, we report a photocathode based on lithium doped copper(ii) oxide, a black p-type semiconductor. Density functional theory calculations with a Hubbard U term show that low concentrations of Li (Li0.03Cu0.97O) lead to an upward shift of the valence band maximum that crosses the Fermi level and results in a p-type semiconductor. Therefore, Li doping emerged as a suitable approach to manipulate the electronic structure of copper oxide based photocathodes. As this material class suffers from instability in water under operating conditions, the recorded photocurrents are repeatedly misinterpreted as hydrogen evolution evidence. We investigated the photocorrosion behavior of LixCu1-xO cathodes in detail and give the first mechanistic study of the fundamental physical process. The reduced copper oxide species were localized by electron energy loss spectroscopy mapping. Cu2O grows as distinct crystallites on the surface of LixCu1-xO instead of forming a dense layer. Additionally, there is no obvious Cu2O gradient inside the films, as Cu2O seems to form on all LixCu1-xO nanocrystals exposed to water. The application of a thin Ti0.8Nb0.2Ox coating by atomic layer deposition and the deposition of a platinum co-catalyst increased the stability of LixCu1-xO against decomposition. These devices showed a stable hydrogen evolution for 15 minutes. © The Royal Society of Chemistry .
    view abstract10.1039/c9nr10250g
  • Nanocellulose-Mediated Transition of Lithium-Rich Pseudo-Quaternary Metal Oxide Nanoparticles into Lithium Nickel Cobalt Manganese Oxide (NCM) Nanostructures
    Zehetmaier, P.M. and Zoller, F. and Beetz, M. and Plaß, M.A. and Häringer, S. and Böller, B. and Döblinger, M. and Bein, T. and Fattakhova-Rohlfing, D.
    ChemNanoMat 6 (2020)
    We report the syntheses of various compounds within the pseudo-quaternary system of the type LiwNixCoyMnzOδ (δ≤1) (pre-NCMs). Four different compositions of this compound were realized as ultrasmall crystalline nanoparticles of 1–4 nm diameter using low-temperature solvothermal reaction conditions in tert-butanol at only 170 °C. All of the pre-NCMs crystallize in the rock-salt structure and their lithium content is between 20% and 30% with respect to the complete metal content. By adjusting the lithium content to 105% stoichiometry in the solvothermal reaction, the pre-NCMs can easily react to the respective Li(NixCoyMnz)O2 (NCM) nanoparticles. Furthermore, nanosized desert-rose structured NCMs were obtained after addition of nanocellulose during the synthesis. By using the mixed metal monoxides as precursor for the NCMs, cation mixing between lithium and nickel is favored and gets more pronounced with increasing nickel content. The cation mixing effect compromises good electrochemical capacity retention, but the desert-rose structure nevertheless enables enhanced stability at high power conditions, especially for NCM333. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cnma.201900748
  • Recycling strategies for ceramic all-solid-state batteries—part i: Study on possible treatments in contrast to li-ion battery recycling
    Schwich, L. and Küpers, M. and Finsterbusch, M. and Schreiber, A. and Fattakhova-Rohlfing, D. and Guillon, O. and Friedrich, B.
    Metals 10 (2020)
    In the coming years, the demand for safe electrical energy storage devices with high energy density will increase drastically due to the electrification of the transportation sector and the need for stationary storage for renewable energies. Advanced battery concepts like all-solid-state batteries (ASBs) are considered one of the most promising candidates for future energy storage technologies. They offer several advantages over conventional Lithium-Ion Batteries (LIBs), especially with regard to stability, safety, and energy density. Hardly any recycling studies have been conducted, yet, but such examinations will play an important role when considering raw materials supply, sustainability of battery systems, CO2 footprint, and general strive towards a circular economy. Although different methods for recycling LIBs are already available, the transferability to ASBs is not straightforward due to differences in used materials and fabrication technologies, even if the chemistry does not change (e.g., Li-intercalation cathodes). Challenges in terms of the ceramic nature of the cell components and thus the necessity for specific recycling strategies are investigated here for the first time. As a major result, a recycling route based on inert shredding, a subsequent thermal treatment, and a sorting step is suggested, and transferring the extracted black mass to a dedicated hydrometallurgical recycling process is proposed. The hydrometallurgical approach is split into two scenarios differing in terms of solubility of the ASB-battery components. Hence, developing a full recycling concept is reached by this study, which will be experimentally examined in future research. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/met10111523
  • V(III)-Doped Nickel Oxide-Based Nanocatalysts for Electrochemical Water Splitting: Influence of Phase, Composition, and Doping on the Electrocatalytic Activity
    Böhm, D. and Beetz, M. and Kutz, C. and Zhang, S. and Scheu, C. and Bein, T. and Fattakhova-Rohlfing, D.
    Chemistry of Materials 32 (2020)
    Doped nickel oxide-based compounds are attracting great interest as very efficient and abundant catalysts and were thoroughly investigated as battery materials in the past. However, there is still no clear understanding of the influence of dopants on the complex dynamic character of their chemically and potentially driven transformations. We have developed a synthesis procedure enabling the controlled formation of nanosized nickel hydroxide and nickel oxide polymorphs substituted with vanadium(III) [V(III)] ions and further investigated their structure-activity correlation for electrochemical water oxidation. This work therefore primarily focuses on an in-depth structural characterization of the homogeneously doped nanosized α- and β-Ni(OH)2 polymorphs. It could be shown that concentrations of 10 at. % V(III) and higher can effectively inhibit a spontaneous phase transformation known as chemical aging of the turbostratic α-phase to the more crystalline β-Ni(OH)2 phase in neutral aqueous media. The Fe-impurity-biased electrocatalytic activity determined for α-/β-Ni1-xVx(OH)2 showed only a minor increase of 10% oxygen evolution reaction (OER) activity for an 1 at. % doped nonaged sample resembling the α-phase, while a 5 at. % V(III)-doped sample chemically aged over 24 h led to a doubled OER activity versus the undoped reference which transformed into β-Ni(OH)2 over that period of time. ©
    view abstract10.1021/acs.chemmater.0c02851
  • A garnet structure-based all-solid-state Li battery without interface modification: Resolving incompatibility issues on positive electrodes
    Tsai, C.-L. and Ma, Q. and Dellen, C. and Lobe, S. and Vondahlen, F. and Windmüller, A. and Grüner, D. and Zheng, H. and Uhlenbruck, S. and Finsterbusch, M. and Tietz, F. and Fattakhova-Rohlfing, D. and Buchkremer, H.P. and Guillon, O.
    Sustainable Energy and Fuels 3 (2019)
    The development of high-performance Li 7 La 3 Zr 2 O 12 (LLZO)-based all-solid-state lithium batteries (SSLB) is usually hampered by highly resistive interfaces due to the need for sintering at elevated temperatures to form ionic diffusion paths through the grains. Many strategies have been proposed to solve the problem but the achievements have been limited. Herein, a new design principle is introduced, based on co-sintering crystalline LCO and Ta-substituted LLZO instead of using the more reactive Li-Co-O precursors and Al-substituted LLZO, which allows the fabrication of high specific areal density and low cell area resistance without the interface modification of LLZO-based SSLB. Detailed studies using micro-Raman and EDS mapping revealed that the well-sintered interfaces are free from detrimental secondary phases. To demonstrate that a true bulk-type SSLB can be constructed by this straightforward strategy, the material loading for a composite positive electrode was increased to about 10 times that in previous reports, which resulted in a high areal capacity of 1.63 mA h cm -2 (i.e. 110 mA h g -1 ) when discharged with a current density of 50 μA cm -2 . It also allows one to discharge the fabricated SSLB at a very high current density of 500 μA cm -2 at 50 °C due to the minimized cell areal resistance. The new fabrication strategy for the LLZO-based SSLB paves the way for achieving SSLB with high safety and energy density. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8se00436f
  • Carbon-templated conductive oxide supports for oxygen evolution catalysis
    Hufnagel, A.G. and Häringer, S. and Beetz, M. and Böller, B. and Fattakhova-Rohlfing, D. and Bein, T.
    Nanoscale 11 (2019)
    We present a novel route for the preparation of supported IrO2 catalysts for the oxygen evolution reaction in proton exchange membrane electrolyzers. It uses carbon soot as a nanostructure template, which is sequentially coated with a conductive niobium-doped titanium oxide (NTO) layer and an ultrathin, highly pure IrO2 catalyst layer by atomic layer deposition (ALD). The NTO acts as an oxidation-stable conductor between the metal current distributor and the catalyst. The highly controlled film growth by ALD enables the fabrication of electrodes with a very low noble metal loading. Nonetheless, these electrodes exhibit very high catalytic activity and good stability under cyclic and constant load conditions. At an IrO2 content of less than 10 percent by mass of the oxide material and an area-based Ir content of 153 μg cm-2, the nanostructured NTO/IrO2 electrode achieves an oxygen evolution current density of 1 mA cm-2 at an overpotential of ∼250 mV, which is significantly lower than the reported values for particulate NTO/IrO2 catalysts. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9nr03013a
  • Ceramics for electrochemical storage
    Arinicheva, Y. and Wolff, M. and Lobe, S. and Dellen, C. and Fattakhova-Rohlfing, D. and Guillon, O. and Böhm, D. and Zoller, F. and Schmuch, R. and Li, J. and Winter, M. and Adamczyk, E. and Pralong, V.
    Advanced Ceramics for Energy Conversion and Storage (2019)
    In this chapter, after having introduced the basics of electrochemical storage and types of secondary batteries, detailed focus is given on: (i) anode ceramic materials, (ii) cathode active materials, and (iii) separators and solid electrolytes. Chemistries of interest are based on lithium and sodium, covering both current commercial applications as well as technologies under development such as solid-state batteries. © 2020 Elsevier Ltd. All rights reserved.
    view abstract10.1016/B978-0-08-102726-4.00010-7
  • Flexible freestanding MoS2-based composite paper for energy conversion and storage
    Zoller, F. and Luxa, J. and Bein, T. and Fattakhova-Rohlfing, D. and Bouša, D. and Sofer, Z.
    Beilstein Journal of Nanotechnology 10 (2019)
    The construction of flexible electrochemical devices for energy storage and generation is of utmost importance in modern society. In this article, we report on the synthesis of flexible MoS2-based composite paper by high-energy shear force milling and simple vacuum filtration. This composite material combines high flexibility, mechanical strength and good chemical stability. Chronopotentiometric charge-discharge measurements were used to determine the capacitance of our paper material. The highest capacitance achieved was 33 mF·cm-2 at a current density of 1 mA·cm-2, demonstrating potential application in supercapacitors. We further used the material as a cathode for the hydrogen evolution reaction (HER) with an onset potential of approximately -0.2 V vs RHE. The onset potential was even lower (approximately -0.1 V vs RHE) after treatment with n-butyllithium, suggesting the introduction of new active sites. Finally, a potential use in lithium ion batteries (LIB) was examined. Our material can be used directly without any binder, additive carbon or copper current collector and delivers specific capacity of 740 mA·h·g-1 at a current density of 0.1 A·g-1. After 40 cycles at this current density the material still reached a capacity retention of 91%. Our findings show that this composite material could find application in electrochemical energy storage and generation devices where high flexibility and mechanical strength are desired. © 2019 Zoller et al.
    view abstract10.3762/bjnano.10.147
  • Nanosized Lithium-Rich Cobalt Oxide Particles and Their Transformation to Lithium Cobalt Oxide Cathodes with Optimized High-Rate Morphology
    Zehetmaier, P.M. and Cornélis, A. and Zoller, F. and Böller, B. and Wisnet, A. and Döblinger, M. and Böhm, D. and Bein, T. and Fattakhova-Rohlfing, D.
    Chemistry of Materials (2019)
    We report the formation of crystalline dispersible LixCo1-xOy (with y 1) nanoparticles with an unusual rock-salt phase containing â¼15 at. % Li in the crystalline structure. This is the first time that this composition was formed at temperatures as low as 150 °C under conditions of a solvothermal process, although it is referred to as a higherature metastable phase in a very limited number of known publications. The Li0.15Co0.85Oy nanoparticles of 2-3 nm size completely transform to higherature LiCoO2 (HT-LCO) nanoparticles at 560 °C in the presence of slightly overstoichiometric amounts of Li source. The presence of lithium in the CoO lattice slows down the kinetics of its phase transformation, enabling to obtain very small HT-LCO nanocrystals during the subsequent calcination. The HT-LCO particles formed after this transformation have an elongated shape with a mean size of about 17 × 60 nm, which is targeted as an optimum size for battery applications. An attractive feature of the Li0.15Co0.85Oy nanoparticles is their high dispersibility enabling their assembly into different nanostructures with optimized morphology. Open porous HT-LCO electrodes prepared via self-assembly of Li0.15Co0.85Oy nanoparticles and Pluronic F127 as a structure-directing agent demonstrate very good performances at high current densities representing short charge/discharge times below 10 min. Even at a charge/discharge time of 72 s (50C), 50% of the theoretical capacity has been preserved. After 250 cycles at a charge/discharge time of 6 min (10C), over 60% of the initial discharge capacity was retained. © 2019 American Chemical Society.
    view abstract10.1021/acs.chemmater.9b02231
  • Sn-Doped Hematite for Photoelectrochemical Water Splitting: The Effect of Sn Concentration
    Zhang, S. and Hajiyani, H. and Hufnagel, A.G. and Kampmann, J. and Breitbach, B. and Bein, T. and Fattakhova-Rohlfing, D. and Pentcheva, R. and Scheu, C.
    Zeitschrift fur Physikalische Chemie (2019)
    Hematite-based photoanodes have been intensively studied for photoelectrochemical water oxidation. The n-type dopant Sn has been shown to benefit the activity of hematite anodes. We demonstrate in this study that Sn-doped hematite thin films grown by atomic layer deposition can achieve uniform doping across the film thickness up to at least 32 mol%, far exceeding the equilibrium solubility limit of less than 1 mol%. On the other hand, with the introduction of Sn doping, the hematite crystallite size decreases and many twin boundaries form in the film, which may contribute to the low photocurrent observed in these films. Density functional theory calculations with a Hubbard U term show that Sn doping has multiple effects on the hematite properties. With increasing Sn4+ content, the Fe2+ concentration increases, leading to a reduction of the band gap and finally to a metallic state. This goes hand in hand with an increase of the lattice constant. ©2019 Walter de Gruyter GmbH, Berlin/Boston 2019.
    view abstract10.1515/zpch-2019-1482
  • Tin Oxide Based Nanomaterials and Their Application as Anodes in Lithium-Ion Batteries and Beyond
    Zoller, F. and Böhm, D. and Bein, T. and Fattakhova-Rohlfing, D.
    ChemSusChem 12 (2019)
    Invited for this month′s cover is the Minireview by the groups of Prof. Fattakhova-Rohlfing and Prof. Bein at Forschungszentrum Jülich (FZJ), University of Duisburg-Essen (UDE), and Ludwig-Maximilians-Universität München (LMU), on tin oxide-based anodes for alkali (lithium, sodium, and potassium) ion batteries. The image shows the lithiation, sodiation, and potassiation of a SnO2/graphene composite material represented by a SnO2 unit cell chain-linked to an underlying graphene sheet. The cross-section of a battery depicts a branched SnO2-based nanostructure undergoing conversion and alloying reactions with alkali ions, affecting its nano- and microstructure, as indicated by the volume and textural changes. The Minireview itself is available at 10.1002/cssc.201901487. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201902421
  • Electron-Blocking and Oxygen Evolution Catalyst Layers by Plasma-Enhanced Atomic Layer Deposition of Nickel Oxide
    Hufnagel, A.G. and Henß, A.-K. and Hoffmann, R. and Zeman, O.E.O. and Häringer, S. and Fattakhova-Rohlfing, D. and Bein, T.
    Advanced Materials Interfaces 5 (2018)
    A plasma-enhanced atomic layer deposition (ALD) process is presented, capable of producing thin conformal films of nickel(II) oxide (NiO) on various substrates. Nickelocene (NiCp2) is used as an inexpensive metal precursor with oxygen plasma as the oxidant. The film growth rate saturates with both nickel precursor and plasma exposure. An ALD window is observed between 225 and 275 °C. Linear growth is achieved at 250 °C with a growth rate of 0.042 nm per cycle. The thickness is highly uniform and the surface roughness is below 1 nm rms for 52 nm thick films on Si(100). Substrates with aspect ratios up to 1:10 can be processed. As-deposited, the films consist of polycrystalline, cubic NiO, and are transparent over the entire visible range with an optical bandgap of 3.7 eV. The films consist of stoichiometric NiO and contain ≈1% of carbon impurities. Two promising applications of these films are showcased in renewable energy conversion and storage devices: The films are pinhole-free and exhibit excellent electron blocking capabilities, making them potential hole-selective contact layers in solar cells. Also, high electrocatalytic activity of ultrathin NiO films is demonstrated for the alkaline oxygen evolution reaction, especially in electrolytes containing Fe3+. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/admi.201701531
  • Making Ultrafast High-Capacity Anodes for Lithium-Ion Batteries via Antimony Doping of Nanosized Tin Oxide/Graphene Composites
    Zoller, F. and Peters, K. and Zehetmaier, P.M. and Zeller, P. and Döblinger, M. and Bein, T. and Sofer, Z. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 28 (2018)
    Tin oxide-based materials attract increasing attention as anodes in lithium-ion batteries due to their high theoretical capacity, low cost, and high abundance. Composites of such materials with a carbonaceous matrix such as graphene are particularly promising, as they can overcome the limitations of the individual materials. The fabrication of antimony-doped tin oxide (ATO)/graphene hybrid nanocomposites is described with high reversible capacity and superior rate performance using a microwave assisted in situ synthesis in tert-butyl alcohol. This reaction enables the growth of ultrasmall ATO nanoparticles with sizes below 3 nm on the surface of graphene, providing a composite anode material with a high electric conductivity and high structural stability. Antimony doping results in greatly increased lithium insertion rates of this conversion-type anode and an improved cycling stability, presumably due to the increased electrical conductivity. The uniform composites feature gravimetric capacity of 1226 mAh g−1 at the charging rate 1C and still a high capacity of 577 mAh g−1 at very high charging rates of up to 60C, as compared to 93 mAh g−1 at 60C for the undoped composite synthesized in a similar way. At the same time, the antimony-doped anodes demonstrate excellent stability with a capacity retention of 77% after 1000 cycles. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201706529
  • Oriented Films of Conjugated 2D Covalent Organic Frameworks as Photocathodes for Water Splitting
    Sick, T. and Hufnagel, A.G. and Kampmann, J. and Kondofersky, I. and Calik, M. and Rotter, J.M. and Evans, A. and Döblinger, M. and Herbert, S. and Peters, K. and Böhm, D. and Knochel, P. and Medina, D.D. and Fattakhova-Rohlfing, D. and Bein, T.
    Journal of the American Chemical Society 140 (2018)
    Light-driven water electrolysis at a semiconductor surface is a promising way to generate hydrogen from sustainable energy sources, but its efficiency is limited by the performance of available photoabsorbers. Here we report the first time investigation of covalent organic frameworks (COFs) as a new class of photoelectrodes. The presented 2D-COF structure is assembled from aromatic amine-functionalized tetraphenylethylene and thiophene-based dialdehyde building blocks to form conjugated polyimine sheets, which π-stack in the third dimension to create photoactive porous frameworks. Highly oriented COF films absorb light in the visible range to generate photoexcited electrons that diffuse to the surface and are transferred to the electrolyte, resulting in proton reduction and hydrogen evolution. The observed photoelectrochemical activity of the 2D-COF films and their photocorrosion stability in water pave the way for a novel class of photoabsorber materials with versatile optical and electronic properties that are tunable through the selection of appropriate building blocks and their three-dimensional stacking. © 2017 American Chemical Society.
    view abstract10.1021/jacs.7b06081
  • Why Tin-Doping Enhances the Efficiency of Hematite Photoanodes for Water Splitting—The Full Picture
    Hufnagel, A.G. and Hajiyani, H. and Zhang, S. and Li, T. and Kasian, O. and Gault, B. and Breitbach, B. and Bein, T. and Fattakhova-Rohlfing, D. and Scheu, C. and Pentcheva, R.
    Advanced Functional Materials 28 (2018)
    The beneficial effects of Sn(IV) as a dopant in ultrathin hematite (α-Fe 2 O 3 ) photoanodes for water oxidation are examined. Different Sn concentration profiles are prepared by alternating atomic layer deposition of Fe 2 O 3 and SnO x . Combined data from spectrophotometry and intensity-modulated photocurrent spectroscopy yields the individual process efficiencies for light harvesting, charge separation, and charge transfer. The best performing photoanodes are Sn-doped both on the surface and in the subsurface region and benefit from enhanced charge separation and transfer. Sn-doping throughout the bulk of the hematite photoanode causes segregation at the grain boundaries and hence a lower overall efficiency. Fe 2 O 3 (0001) and terminations, shown to be dominant by microstructural analysis, are investigated by density functional theory (DFT) calculations. The energetics of surface intermediates during the oxygen evolution reaction (OER) reveal that while Sn-doping decreases the overpotential on the (0001) surface, the Fe 2 O 3 orientation shows one of the lowest overpotentials reported for hematite so far. Electronic structure calculations demonstrate that Sn-doping on the surface also enhances the charge transfer efficiency by elimination of surface hole trap states (passivation) and that subsurface Sn-doping introduces a gradient of the band edges that reinforces the band bending at the semiconductor/electrolyte interface and thus boosts charge separation. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201804472
  • Black Magic in Gray Titania: Noble-Metal-Free Photocatalytic H2 Evolution from Hydrogenated Anatase
    Liu, N. and Zhou, X. and Nguyen, N.T. and Peters, K. and Zoller, F. and Hwang, I. and Schneider, C. and Miehlich, M.E. and Freitag, D. and Meyer, K. and Fattakhova-Rohlfing, D. and Schmuki, P.
    ChemSusChem 10 (2017)
    ‘Black’ TiO2—in the widest sense, TiO2 reduced by various treatments—has attracted tremendous scientific interest in recent years because of some outstanding properties; most remarkably in photocatalysis. While the material effects visible light absorption (the blacker, the better), black titania produced by high pressure hydrogenation was recently reported to show another highly interesting feature; noble-metal-free photocatalytic H2 generation. In a systematic investigation of high-temperature hydrogen treatments of anatase nanoparticles, TEM, XRD, EPR, XPS, and photoelectrochemistry are used to characterize different degrees of surface hydrogenation, surface termination, electrical conductivity, and structural defects in the differently treated materials. The materials’ intrinsic activity for photocatalytic hydrogen evolution is coupled neither with their visible light absorption behavior nor the formation of amorphous material, but rather must be ascribed to optimized and specific defect formation (gray is better than black). This finding is further confirmed by using a mesoporous anatase matrix as a hydrogenation precursor, which, after conversion to the gray state, even further enhances the overall photocatalytic hydrogen evolution activity. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201601264
  • Dual absorber Fe2O3/WO3 host-guest architectures for improved charge generation and transfer in photoelectrochemical applications
    Müller, A. and Kondofersky, I. and Folger, A. and Fattakhova-Rohlfing, D. and Bein, T. and Scheu, C.
    Materials Research Express 4 (2017)
    In this study the influence of the spatial distribution and of different nanostructures of WO3 and Fe2O3 in the dual absorber system Fe2O3/WO3 was systematically investigated for the first time. WO3 was applied as a scaffold and/or as a surface treatment to mesoporous Fe2O3 films. Both approaches strongly increased the performance compared to the individual photoabsorbers. By combining a host guest architecture with a surface treatment, current densities of about 0.7 mA cm-2 at 1.23 V versus reversible hydrogen electrode under AM 1.5 illumination with an incident photon-to-current effciency of 17% at 350 nm were achieved without the use of further catalysts. We could identify several beneficial interactions between Fe2O3 and WO3. WO3 strongly scatters visible light, resulting in increased absorption by Fe2O3 and higher current densities. We also determined a cathodic shift in the onset potential to 0.8 V and increased transfer rates of up to 88%. This combination of beneficial effects proves the viability of the presented device architecture.
    view abstract10.1088/2053-1591/aa570f
  • In Situ Study of Degradation in P3HT-Titania-Based Solid-State Dye-Sensitized Solar Cells
    Song, L. and Wang, W. and Pröller, S. and Moseguí González, D. and Schlipf, J. and Schaffer, C.J. and Peters, K. and Herzig, E.M. and Bernstorff, S. and Bein, T. and Fattakhova-Rohlfing, D. and Müller-Buschbaum, P.
    ACS Energy Letters 2 (2017)
    The degradation of poly(3-hexylthiophene) (P3HT)-titania-based solid-state dye-sensitized solar cells (ssDSSCs) is studied to better understand device aging mechanisms. The correlation of temporal evolution between P3HT crystallite structures and device performance is discussed for the first time using in situ measurements. For comparison, two types of mesoporous titania photoanodes with different pore sizes are prepared. Grazing incidence wide-angle X-ray scattering is used in situ under continuous solar illumination to obtain information about the impact of pore size on P3HT crystalline order and on temporal evolution of the P3HT crystallites. The development of the photovoltaic characteristics is explored in parallel. The lattice constants, crystal sizes, and volume fraction of crystalline P3HT in the large-pore active layer remain stable over 30 min, while the volume fraction of crystalline P3HT decreases in the small-pore active layer. Thus, the pore size of titania photoanodes is important for the stability of P3HT-titania-based ssDSSCs. © 2017 American Chemical Society.
    view abstract10.1021/acsenergylett.7b00117
  • In situ study of spray deposited titania photoanodes for scalable fabrication of solid-state dye-sensitized solar cells
    Song, L. and Wang, W. and Körstgens, V. and Moseguí González, D. and Löhrer, F.C. and Schaffer, C.J. and Schlipf, J. and Peters, K. and Bein, T. and Fattakhova-Rohlfing, D. and Roth, S.V. and Müller-Buschbaum, P.
    Nano Energy 40 (2017)
    Spray coating, a cost-effective and scalable technique, has been employed for fabricating titania films for solid-state dye-sensitized solar cells (ssDSSCs). The spray deposition of films is inherently based on kinetic processes with great complexity, which poses great challenges in its understanding. In the present work, the kinetics of the structure evolution of deposited films are investigated by in situ grazing-incidence small-angle x-ray scattering during spray deposition. The spray-solution is prepared via a polystyrene-block-polyethylene oxide (PS-b-PEO) template assisted sol-gel synthesis. It is turned into nanostructured titania/PS-b-PEO composite films via spray deposition. The information about nanostructure length scales of the composite film is obtained in real-time and in situ, revealing the morphological evolution during the spray deposition. The resulting mesoporous titania films serve as photoanodes of ssDSSCs, which couple with the solution-cast hole transport layer to form the active layers. The well working ssDSSCs demonstrate the successful use of spray deposition as a large-scale manufacturing process for photoanodes. © 2017 Elsevier Ltd
    view abstract10.1016/j.nanoen.2017.08.023
  • Nonagglomerated Iron Oxyhydroxide Akaganeite Nanocrystals Incorporating Extraordinary High Amounts of Different Dopants
    Fominykh, K. and Böhm, D. and Zhang, S. and Folger, A. and Döblinger, M. and Bein, T. and Scheu, C. and Fattakhova-Rohlfing, D.
    Chemistry of Materials 29 (2017)
    Dispersible nonagglomerated akaganeite (β-FeOOH) nanocrystals doped with various elements in different oxidation states such as Co(II), Ni(II), V(III), Ti(IV), Sn(IV), Si(IV), and Nb(V) were prepared using a microwave-assisted solvothermal synthesis in tert-butanol. The doping elements could be incorporated in very high concentrations of up to 20 at.â€"%, which is attributed to the kinetic control of the phase formation during the solvothermal reaction, together with the extremely small crystal size, which can stabilize the unusual structural compositions. The particle morphology is mostly anisotropic consisting of nanorods â1/44 nm in width and varying length. Depending on the doping element, the length ranges from â4 nm, resulting in an almost-spherical shape, to 90 nm, giving the highest aspect ratio. The particles are perfectly dispersible in water, giving stable colloidal dispersions that can be deposited on different substrates to produce thin films 35-250 nm thick. In addition, films up to 30 μm thick, consisting of interconnected mesoporous spheres, can be prepared in situ during the reactions. The nanostructures assembled from akaganeite nanocrystals are stable up to high temperatures of >400 °C. They can be transformed to hematite (α-Fe2O3) by heating between 480 °C and 600 °C without losing the morphology, which can be used for the fabrication of doped hematite nanostructures. The tunable properties of the doped akaganeite nanoparticles make them excellent candidates for a wide range of applications, as well as versatile building blocks for the fabrication of doped hematite nanomorphologies. © 2017 American Chemical Society.
    view abstract10.1021/acs.chemmater.7b01611
  • Rock Salt Ni/Co Oxides with Unusual Nanoscale-Stabilized Composition as Water Splitting Electrocatalysts
    Fominykh, K. and Tok, G.C. and Zeller, P. and Hajiyani, H. and Miller, T. and Döblinger, M. and Pentcheva, R. and Bein, T. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 27 (2017)
    The influence of nanoscale on the formation of metastable phases is an important aspect of nanostructuring that can lead to the discovery of unusual material compositions. Here, the synthesis, structural characterization, and electrochemical performance of Ni/Co mixed oxide nanocrystals in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is reported and the influence of nanoscaling on their composition and solubility range is investigated. Using a solvothermal synthesis in tert-butanol ultrasmall crystalline and highly dispersible Ni x Co1− x O nanoparticles with rock salt type structure are obtained. The mixed oxides feature non-equilibrium phases with unusual miscibility in the whole composition range, which is attributed to a stabilizing effect of the nanoscale combined with kinetic control of particle formation. Substitutional incorporation of Co and Ni atoms into the rock salt lattice has a remarkable effect on the formal potentials of NiO oxidation that shift continuously to lower values with increasing Co content. This can be related to a monotonic reduction of the work function of (001) and (111)-oriented surfaces with an increase in Co content, as obtained from density functional theory (DFT+U) calculations. Furthermore, the electrocatalytic performance of the Ni x Co1− x O nanoparticles in water splitting changes significantly. OER activity continuously increases with increasing Ni contents, while HER activity shows an opposite trend, increasing for higher Co contents. The high electrocatalytic activity and tunable performance of the nonequilibrium Ni x Co1− x O nanoparticles in HER and OER demonstrate great potential in the design of electrocatalysts for overall water splitting. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201605121
  • Investigation of the pH-Dependent Impact of Sulfonated Polyaniline on Bioelectrocatalytic Activity of Xanthine Dehydrogenase
    Sarauli, D. and Borowski, A. and Peters, K. and Schulz, B. and Fattakhova-Rohlfing, D. and Leimkühler, S. and Lisdat, F.
    ACS Catalysis 6 (2016)
    We report on the pH-dependent bioelectrocatalytic activity of the redox enzyme xanthine dehydrogenase (XDH) in the presence of sulfonated polyaniline PMSA1 (poly(2-methoxyaniline-5-sulfonic acid)-co-aniline). Ultraviolet-visible (UV-vis) spectroscopic measurements with both components in solution reveal electron transfer from the hypoxanthine (HX)-reduced enzyme to the polymer. The enzyme shows bioelectrocatalytic activity on indium tin oxide (ITO) electrodes, when the polymer is present. Depending on solution pH, different processes can be identified. It can be demonstrated that not only product-based communication with the electrode but also efficient polymer-supported bioelectrocatalysis occur. Interestingly, substrate-dependent catalytic currents can be obtained in acidic and neutral solutions, although the highest activity of XDH with natural reaction partners is in the alkaline region. Furthermore, operation of the enzyme electrode without addition of the natural cofactor of XDH is feasible. Finally, macroporous ITO electrodes have been used as an immobilization platform for the fabrication of HX-sensitive electrodes. The study shows that the efficient polymer/enzyme interaction can be advantageously combined with the open structure of an electrode material of controlled pore size, resulting in good processability, stability, and defined signal transfer in the presence of a substrate. © 2016 American Chemical Society.
    view abstract10.1021/acscatal.6b02011
  • Nanostructured Antimony-Doped Tin Oxide Layers with Tunable Pore Architectures as Versatile Transparent Current Collectors for Biophotovoltaics
    Peters, K. and Lokupitiya, H.N. and Sarauli, D. and Labs, M. and Pribil, M. and Rathouský, J. and Kuhn, A. and Leister, D. and Stefik, M. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 26 (2016)
    Nanostructured transparent conducting oxide (TCO) layers gain increasing importance as high surface area electrodes enabling incorporation of functional redox species with high loading. The fabrication of porous TCO films, namely, antimony-doped tin oxide (ATO), is reported using the self-assembly of preformed ATO nanocrystals with poly(ethylene oxide-b-hexyl acrylate) (PEO-b-PHA) block copolymer. The high molar mass of the polymer and tunable solution processing conditions enable the fabrication of TCO electrodes with pore sizes ranging from mesopores to macropores. Particularly notable is access to uniform macroporous films with a nominal pore size of around 80 nm, which is difficult to obtain by other techniques. The combination of tunable porosity with a large conducting interface makes the obtained layers versatile current collectors with adjustable performance. While all the obtained electrodes incorporate a large amount of small redox molecules such as molybdenum polyoxometalate, only the electrodes with sufficiently large macropores are able to accommodate high amounts of bulky photoactive photosystem I (PSI) protein complexes. The 11-fold enhancement of the current response of PSI modified macroporous ATO electrodes compared to PSI on planar indium tin oxide (ITO), makes this type of electrodes promising candidates for the development of biohybrid devices. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201602148
  • Nanostructured Ternary FeCrAl Oxide Photocathodes for Water Photoelectrolysis
    Kondofersky, I. and Müller, A. and Dunn, H.K. and Ivanova, A. and Štefanić, G. and Ehrensperger, M. and Scheu, C. and Parkinson, B.A. and Fattakhova-Rohlfing, D. and Bein, T.
    Journal of the American Chemical Society 138 (2016)
    A sol-gel method for the synthesis of semiconducting FeCrAl oxide photocathodes for solar-driven hydrogen production was developed and applied for the production of meso- and macroporous layers with the overall stoichiometry Fe0.84Cr1.0Al0.16O3. Using transmission electron microscopy and energy-dispersive X-ray spectroscopy, phase separation into Fe- and Cr-rich phases was observed for both morphologies. Compared to prior work and to the mesoporous layer, the macroporous FeCrAl oxide photocathode had a significantly enhanced photoelectrolysis performance, even at a very early onset potential of 1.1 V vs RHE. By optimizing the macroporous electrodes, the device reached current densities of up to 0.68 mA cm-2 at 0.5 V vs RHE under AM 1.5 with an incident photon-to-current efficiency (IPCE) of 28% at 400 nm without the use of catalysts. Based on transient measurements, this performance increase could be attributed to an improved collection efficiency. At a potential of 0.75 V vs RHE, an electron transfer efficiency of 48.5% was determined. © 2016 American Chemical Society.
    view abstract10.1021/jacs.5b08040
  • Photoelectrochemistry: Zinc Ferrite Photoanode Nanomorphologies with Favorable Kinetics for Water-Splitting (Adv. Funct. Mater. 25/2016)
    Hufnagel, A.G. and Peters, K. and Müller, A. and Scheu, C. and Fattakhova-Rohlfing, D. and Bein, T.
    Advanced Functional Materials 26 (2016)
    view abstract10.1002/adfm.201670155
  • Porous Transparent Conductors: Nanostructured Antimony-Doped Tin Oxide Layers with Tunable Pore Architectures as Versatile Transparent Current Collectors for Biophotovoltaics (Adv. Funct. Mater. 37/2016)
    Peters, K. and Lokupitiya, H.N. and Sarauli, D. and Labs, M. and Pribil, M. and Rathouský, J. and Kuhn, A. and Leister, D. and Stefik, M. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 26 (2016)
    view abstract10.1002/adfm.201670239
  • Spray Deposition of Titania Films with Incorporated Crystalline Nanoparticles for All-Solid-State Dye-Sensitized Solar Cells Using P3HT
    Song, L. and Wang, W. and Körstgens, V. and Moseguí González, D. and Yao, Y. and Minar, N.K. and Feckl, J.M. and Peters, K. and Bein, T. and Fattakhova-Rohlfing, D. and Santoro, G. and Roth, S.V. and Müller-Buschbaum, P.
    Advanced Functional Materials 26 (2016)
    Spray coating, a simple and low-cost technique for large-scale film deposition, is employed to fabricate mesoporous titania films, which are electron-transporting layers in all-solid-state dye-sensitized solar cells (DSSCs). To optimize solar cell performance, presynthesized crystalline titania nanoparticles are introduced into the mesoporous titania films. The composite film morphology is examined with scanning electron microscopy, grazing incidence small-angle X-ray scattering, and nitrogen adsorption-desorption isotherms. The crystal phase and crystallite sizes are verified by X-ray diffraction measurements. The photovoltaic performance of all-solid-state DSSCs is investigated. The findings reveal that an optimal active layer of the all-solid-state DSSC is obtained by including 50 wt% titania nanoparticles, showing a foam-like morphology with an average pore size of 20 nm, featuring an anatase phase, and presenting a surface area of 225.2 m2 g-1. The optimized morphology obtained by adding 50 wt% presynthesized crystalline titania nanoparticles yields, correspondingly, the best solar cell efficiency of 2.7 ± 0.1%. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201504498
  • Tunable dielectric properties of KTaO3 single crystals in the terahertz range
    Skoromets, V. and Kadlec, C. and Němec, H. and Fattakhova-Rohlfing, D. and Kužel, P.
    Journal of Physics D: Applied Physics 49 (2016)
    Electric-field tunability of the dielectric properties of potassium tantalate single crystal was studied by terahertz spectroscopy in a broad temperature range (40-250 K). Complex-valued terahertz transmission spectra of samples were measured with an external electric field perpendicular to the sample surface and parallel to the terahertz wave-vector. We found that the ferroelectric soft mode hardening is fully responsible for the observed electric-field-induced changes in the spectra and no signature of a central mode was detected. We determined the anharmonic properties of the soft-mode potential in the mean field approximation. The observed behavior was compared with that previously reported for SrTiO3 single crystals. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/49/6/065306
  • Zinc Ferrite Photoanode Nanomorphologies with Favorable Kinetics for Water-Splitting
    Hufnagel, A.G. and Peters, K. and Müller, A. and Scheu, C. and Fattakhova-Rohlfing, D. and Bein, T.
    Advanced Functional Materials 26 (2016)
    The n-type semiconducting spinel zinc ferrite (ZnFe2O4) is used as a photoabsorber material for light-driven water-splitting. It is prepared for the first time by atomic layer deposition. Using the resulting well-defined thin films as a model system, the performance of ZnFe2O4 in photoelectrochemical water oxidation is characterized. Compared to benchmark α-Fe2O3 (hematite) films, ZnFe2O4 thin films achieve a lower photocurrent at the reversible potential. However, the oxidation onset potential of ZnFe2O4 is 200 mV more cathodic, allowing the water-splitting reaction to proceed at a lower external bias and resulting in a maximum applied-bias power efficiency (ABPE) similar to that of Fe2O3. The kinetics of the water oxidation reaction are examined by intensity-modulated photocurrent spectroscopy. The data indicate a considerably higher charge transfer efficiency of ZnFe2O4 at potentials between 0.8 and 1.3 V versus the reversible hydrogen electrode, attributable to significantly slower surface charge recombination. Finally, nanostructured ZnFe2O4 photoanodes employing a macroporous antimony-doped tin oxide current collector reach a five times higher photocurrent than the flat films. The maximum ABPE of these host–guest photoanodes is similarly increased. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201600461
  • Zintl clusters as wet-chemical precursors for germanium nanomorphologies with tunable composition
    Bentlohner, M.M. and Waibel, M. and Zeller, P. and Sarkar, K. and Müller-Buschbaum, P. and Fattakhova-Rohlfing, D. and Fässler, T.F.
    Angewandte Chemie - International Edition 55 (2016)
    [Ge9]4- Zintl clusters are used as soluble germanium source for a bottom-up fabrication of Ge nanomorphologies such as inverse opal structures with tunable composition. The method is based on the assembly and oxidation of [Ge9]4- clusters in a template mold using SiCl4, GeCl4, and PCl3 leading to Si and P-containing Ge phases as shown by X-ray diffraction, Raman spectroscopy, and energy-dispersive X-ray analysis. [Ge9]4- clusters are retained using ethylenediamine (en) as a transfer medium to a mold after removal of the solvent if water is thoroughly excluded, but are oxidized to amorphous Ge in presence of water traces. 1H NMR spectroscopy reveals the oxidative deprotonation of en by [Ge9]4-. Subsequent annealing leads to crystalline Ge. As an example for wet-chemical synthesis of complex Ge nanomorphologies, we describe the fabrication of undoped and P-doped inverse opal-structured Ge films with a rather low oxygen contents. The morphology of the films with regular volume porosity is characterized by SEM, TEM, and grazing incidence small-angle X-ray scattering. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201508246
  • Charge transport in Sb-doped SnO2 nanoparticles studied by THz spectroscopy
    Skoromets, V. and Němec, H. and Peters, K. and Fattakhova-Rohlfing, D. and Kužel, P.
    IRMMW-THz 2015 - 40th International Conference on Infrared, Millimeter, and Terahertz Waves (2015)
    Using terahertz spectroscopy we investigate the charge transport in Sb-doped SnO2 nanoparticles pressed into pellets. Doped and undoped samples are shown to have different mechanisms of intrinsic conductivity. We systematically study the influence of annealing on intra- and inter-nanoparticle charge transport. © 2015 IEEE.
    view abstract10.1109/IRMMW-THz.2015.7327718
  • Conductivity Mechanisms in Sb-Doped SnO2 Nanoparticle Assemblies: DC and Terahertz Regime
    Skoromets, V. and Němec, H. and Kopeček, J. and Kužel, P. and Peters, K. and Fattakhova-Rohlfing, D. and Vetushka, A. and Müller, M. and Ganzerová, K. and Fejfar, A.
    Journal of Physical Chemistry C 119 (2015)
    Assemblies of undoped and antimony-doped tin oxide nanoparticles synthesized via a nonaqueous sol-gel procedure, pressed into pellets, and annealed under various conditions were investigated using time-domain terahertz spectroscopy, scanning electron microscopy, atomic force microscopy, and dc conductivity measurements. Combination of these methods made it possible to resolve the conductivity limitations imposed by intrinsic properties of the material and by the morphology of the samples. Percolation of the nanoparticles was confirmed in all samples. The undoped samples exhibit a weak hopping conductivity, whereas bandlike conduction of charges partially confined in the nanoparticles dominates in the doped samples. The conductivity of nanoparticles and their connectivity can be greatly controlled during the sample preparation, namely by the calcination temperature and by the order of technological steps. A substantial increase of the conductivity inside nanoparticles and of the charge transport between them is achieved upon calcination at 500 °C. © 2015 American Chemical Society.
    view abstract10.1021/acs.jpcc.5b05091
  • Electron collection in host-guest nanostructured hematite photoanodes for water splitting: The influence of scaffold doping density
    Kondofersky, I. and Dunn, H.K. and Müller, A. and Mandlmeier, B. and Feckl, J.M. and Fattakhova-Rohlfing, D. and Scheu, C. and Peter, L.M. and Bein, T.
    ACS Applied Materials and Interfaces 7 (2015)
    Nanostructuring has proven to be a successful strategy in overcoming the trade-off between light absorption and hole transport to the solid/electrolyte interface in hematite photoanodes for water splitting. The suggestion that poor electron (majority carrier) collection hinders the performance of nanostructured hematite electrodes has led to the emergence of host-guest architectures in which the absorber layer is deposited onto a transparent high-surface-area electron collector. To date, however, state of the art nanostructured hematite electrodes still outperform their host-guest counterparts, and a quantitative evaluation of the benefits of the host-guest architecture is still lacking. In this paper, we examine the impact of host-guest architectures by comparing nanostructured tin-doped hematite electrodes with hematite nanoparticle layers coated onto two types of conducting macroporous SnO2 scaffolds. Analysis of the external quantum efficiency spectra for substrate (SI) and electrolyte side (EI) illumination reveals that the electron diffusion length in the host-guest electrodes based on an undoped SnO2 scaffold is increased substantially relative to the nanostructured hematite electrode without a supporting scaffold. Nevertheless, electron collection is still incomplete for EI illumination. By contrast, an electron collection efficiency of 100% is achieved by fabricating the scaffold using antimony-doped SnO2, showing that the scaffold conductivity is crucial for the device performance. © 2015 American Chemical Society.
    view abstract10.1021/am5078667
  • Guided in situ polymerization of MEH-PPV in mesoporous titania photoanodes
    Minar, N.K. and Docampo, P. and Fattakhova-Rohlfing, D. and Bein, T.
    ACS Applied Materials and Interfaces 7 (2015)
    Incorporation of conjugated polymers into porous metal oxide networks is a challenging task, which is being pursued via many different approaches. We have developed the guided in situ polymerization of poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) in porous titania films by means of surface functionalization. The controlled polymerization via the Gilch route was induced by an alkoxide base and by increasing the temperature. The selected and specially designed surface-functionalizing linker molecules mimic the monomer or its activated form, respectively. In this way, we drastically enhanced the amount of MEH-PPV incorporated into the porous titania phase compared to nonfunctionalized samples by a factor of 6. Additionally, photovoltaic measurements were performed. The devices show shunting or series resistance limitations, depending on the surface functionalization prior to in situ polymerization of MEH-PPV. We suggest that the reason for this behavior can be found in the orientation of the grown polymer chains with respect to the titania surface. Therefore, the geometry of the anchoring via the linker molecules is relevant for exploiting the full electronic potential of the conjugated polymer in the resulting hybrid composite. This observation will help to design future synthesis methods for new hybrid materials from conjugated polymers and n-type semiconductors to take full advantage of favorable electronic interactions between the two phases. © 2015 American Chemical Society.
    view abstract10.1021/acsami.5b01262
  • Interaction of Fructose Dehydrogenase with a Sulfonated Polyaniline: Application for Enhanced Bioelectrocatalysis
    Sarauli, D. and Wettstein, C. and Peters, K. and Schulz, B. and Fattakhova-Rohlfing, D. and Lisdat, F.
    ACS Catalysis 5 (2015)
    We report on efficient bioelectrocatalysis of the redox enzyme fructose dehydrogenase (FDH) upon its interaction with the sulfonated polyaniline PMSA1 (poly(2-methoxyaniline-5-sulfonic acid)-co-aniline). This interaction has been monitored in solution and on the surface of planar and macroporous indium tin oxide (ITO) electrodes by UV - vis and cyclic voltammetric measurements. Moreover, an enhancement of the catalytic activity for fructose conversion induced by a structural change of sulfonated polyaniline PMSA1 caused by the presence of Ca2+ ions is observed. An entrapment of the Ca2+-bound polymer and enzyme inside the pores of macroporous ITO electrodes leads to a significantly increased (∼35-fold) bioelectrocatalytic signal in comparison to that of a flat ITO and allows the fabrication of highly efficient electrodes with good stability. (Figure Presented) © 2015 American Chemical Society.
    view abstract10.1021/acscatal.5b00136
  • Iron-doped nickel oxide nanocrystals as highly efficient electrocatalysts for alkaline water splitting
    Fominykh, K. and Chernev, P. and Zaharieva, I. and Sicklinger, J. and Stefanic, G. and Döblinger, M. and Müller, A. and Pokharel, A. and Böcklein, S. and Scheu, C. and Bein, T. and Fattakhova-Rohlfing, D.
    ACS Nano 9 (2015)
    Efficient electrochemical water splitting to hydrogen and oxygen is considered a promising technology to overcome our dependency on fossil fuels. Searching for novel catalytic materials for electrochemical oxygen generation is essential for improving the total efficiency of water splitting processes. We report the synthesis, structural characterization, and electrochemical performance in the oxygen evolution reaction of Fe-doped NiO nanocrystals. The facile solvothermal synthesis in tert-butanol leads to the formation of ultrasmall crystalline and highly dispersible Fe<inf>x</inf>Ni<inf>1-x</inf>O nanoparticles with dopant concentrations of up to 20%. The increase in Fe content is accompanied by a decrease in particle size, resulting in nonagglomerated nanocrystals of 1.5-3.8 nm in size. The Fe content and composition of the nanoparticles are determined by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy measurements, while Mössbauer and extended X-ray absorption fine structure analyses reveal a substitutional incorporation of Fe(III) into the NiO rock salt structure. The excellent dispersibility of the nanoparticles in ethanol allows for the preparation of homogeneous ca. 8 nm thin films with a smooth surface on various substrates. The turnover frequencies (TOF) of these films could be precisely calculated using a quartz crystal microbalance. Fe<inf>0.1</inf>Ni<inf>0.9</inf>O was found to have the highest electrocatalytic water oxidation activity in basic media with a TOF of 1.9 s-1 at the overpotential of 300 mV. The current density of 10 mA cm-2 is reached at an overpotential of 297 mV with a Tafel slope of 37 mV dec-1. The extremely high catalytic activity, facile preparation, and low cost of the single crystalline Fe<inf>x</inf>Ni<inf>1-x</inf>O nanoparticles make them very promising catalysts for the oxygen evolution reaction. © 2015 American Chemical Society.
    view abstract10.1021/acsnano.5b00520
  • Nanocellulose-assisted formation of porous hematite nanostructures
    Ivanova, A. and Fominykh, K. and Fattakhova-Rohlfing, D. and Zeller, P. and Döblinger, M. and Bein, T.
    Inorganic Chemistry 54 (2015)
    We report the formation of porous iron oxide (hematite) nanostructures via sol-gel transformations of molecular precursors in the confined space of self-organized nanocrystalline cellulose (NCC) used as a shape-persistent template. The obtained structures are highly porous α-Fe2O3 (hematite) morphologies with a well-defined anisotropic porosity. The character of the porous nanostructure depends on the iron salt used as the precursor and the heat treatment. Moreover, a postsynthetic hydrothermal treatment of the NCC/iron salt composites strongly affects the crystal growth as well as the porous nanomorphology of the obtained hematite scaffolds. We demonstrate that the hydrothermal treatment alters the crystallization mechanism of the molecular iron precursors, which proceeds via the formation of anisotropic iron oxyhydroxide species. The nanocellulose templating technique established here enables the straightforward fabrication of a variety of mesoporous crystalline iron oxide scaffolds with defined porous structure and is particularly attractive for the processing of porous hematite films on different substrates. © 2014 American Chemical Society.
    view abstract10.1021/ic502446f
  • Nanocellulose-Templated Porous Titania Scaffolds Incorporating Presynthesized Titania Nanocrystals
    Ivanova, A. and Fravventura, M.C. and Fattakhova-Rohlfing, D. and Rathouský, J. and Movsesyan, L. and Ganter, P. and Savenije, T.J. and Bein, T.
    Chemistry of Materials 27 (2015)
    Nanocrystalline cellulose (NCC) is an abundant biogenic nanomaterial with unique properties that enables the efficient synthesis of mesoporous crystalline titania. We significantly enhance the photocatalytic activity of titania thin films by introducing solvothermally synthesized preformed anatase nanoparticles into a sol-gel based biotemplated titania scaffold. The resulting dual source titania thin films containing different amounts of preformed crystalline species were investigated by time-resolved microwave conductivity (TRMC) measurements and tested in the photocatalytic conversion of 4-chlorophenol. The gradual addition of preformed nanoparticles leads to a consistent increase of the mean size of titania crystalline domains, whereas the porosity of the composite is well-preserved due to the shape-persistent nature of the NCC template. Microwave conductivity studies establish increased photoconductivity of the films containing preformed anatase nanoparticles in comparison to that of films made without the nanoparticles. The synergistic features of the dual source titania, namely the improved crystalline properties brought by the preformed nanocrystals in combination with the high surface area provided by the NCC-templated sol-gel titania, result in a very high photocatalytic activity of the films in the photocatalytic decomposition of 4-chlorophenol. In quantitative terms, the dual source titania films prepared with 75% nanoparticles exhibit a first order degradation rate constant of 0.53 h-1 (1.47 × 10-4 sec-1), which strongly outperforms the activity of commercial P90 nanopowder showing a rate constant of 0.17 h-1 (0.47 × 10-4 sec-1) under the same conditions. © 2015 American Chemical Society.
    view abstract10.1021/acs.chemmater.5b00770
  • Ultrasmall Co3O4 Nanocrystals Strongly Enhance Solar Water Splitting on Mesoporous Hematite
    Feckl, J.M. and Dunn, H.K. and Zehetmaier, P.M. and Müller, A. and Pendlebury, S.R. and Zeller, P. and Fominykh, K. and Kondofersky, I. and Döblinger, M. and Durrant, J.R. and Scheu, C. and Peter, L. and Fattakhova-Rohlfing, D. and Bein, T.
    Advanced Materials Interfaces 2 (2015)
    The synthesis of crystalline, nonagglomerated, and perfectly dispersible Co3O4 nanoparticles with an average size of 3-7 nm using a solvothermal reaction in tert-butanol is reported. The very small size and high dispersibility of the Co3O4 nanoparticles allow for their homogeneous deposition on mesoporous hematite layers serving as the photoactive absorber in the light-driven water splitting reaction. This surface treatment leads to a striking photocurrent increase. While the enhancement of hematite photoanode performance by cobalt oxides is known, the preformation and subsequent application of well-defined cobalt oxide nanoparticles are novel and allow for the treatment of arbitrarily complex hematite morphologies. Photoelectrochemical and transient absorption spectroscopy studies show that this enhanced performance is due to the suppression of surface electron-hole recombination on time scales of milliseconds to seconds. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/admi.201500358
  • Water-dispersible small monodisperse electrically conducting antimony doped tin oxide nanoparticles
    Peters, K. and Zeller, P. and Stefanic, G. and Skoromets, V. and Němec, H. and Kužel, P. and Fattakhova-Rohlfing, D.
    Chemistry of Materials 27 (2015)
    We describe the fabrication of crystalline electrically conducting antimony-doped tin oxide (ATO) nanoparticles highly dispersible in polar solvents such as water and ethanol without any stabilizing agents. Nonagglomerated monodisperse ATO nanoparticles with different doping levels are obtained by a facile solvothermal reaction in tert-butanol, leading to the formation of monodisperse nanocrystals with a size of about 3 nm directly after synthesis. Electrical conductivity of ATO nanoparticles strongly increases due to the substitutional doping with antimony, reaching 6.8 × 10-2 S cm-1 for the as-synthesized nanoparticles prepared with 3-5 mol % Sb. This increase stems from transition from hopping in the undoped samples to band-like conduction in the doped samples as revealed by terahertz (THz) spectroscopy measurements describing transport on nanometer distances. The dc conductivity of the doped nanoparticles increases by about 3 orders of magnitude up to 62 S cm-1 after annealing in air at 500 °C. The electrical conductivity, crystallinity, small size, and high dispersibility in polar solvents make the obtained ATO nanoparticles promising building blocks for the direct assembly of more complex conducting architectures using polymer templates that could be damaged in organic solvents. We illustrate the benefits of the water-dispersible ATO nanoparticles by their assembly to periodic macroporous electrodes using poly(methyl methacrylate) (PMMA) beads as the porosity templates. Aqueous dispersion of ATO nanoparticles can be directly combined with PMMA beads that are easily removed by calcination, enabling a facile deposition of 3D-macroporous ATO electrodes featuring optical transparency and a large periodically ordered conducting interface. © 2015 American Chemical Society.
    view abstract10.1021/cm504409k
  • 3D-electrode architectures for enhanced direct bioelectrocatalysis of pyrroloquinoline quinone-dependent glucose dehydrogenase
    Sarauli, D. and Peters, K. and Xu, C. and Schulz, B. and Fattakhova-Rohlfing, D. and Lisdat, F.
    ACS Applied Materials and Interfaces 6 (2014)
    We report on the fabrication of a complex electrode architecture for efficient direct bioelectrocatalysis. In the developed procedure, the redox enzyme pyrroloquinoline quinone-dependent glucose dehydrogenase entrapped in a sulfonated polyaniline [poly(2-methoxyaniline-5-sulfonic acid)-co-aniline] was immobilized on macroporous indium tin oxide (macroITO) electrodes. The use of the 3D-conducting scaffold with a large surface area in combination with the conductive polymer enables immobilization of large amounts of enzyme and its efficient communication with the electrode, leading to enhanced direct bioelectrocatalysis. In the presence of glucose, the fabricated bioelectrodes show an exceptionally high direct bioelectrocatalytical response without any additional mediator. The catalytic current is increased more than 200-fold compared to planar ITO electrodes. Together with a high long-term stability (the current response is maintained for >90% of the initial value even after 2 weeks of storage), the transparent 3D macroITO structure with a conductive polymer represents a valuable basis for the construction of highly efficient bioelectronic units, which are useful as indicators for processes liberating glucose and allowing optical and electrochemical transduction. © 2014 American Chemical Society.
    view abstract10.1021/am5046026
  • Atomic-Layer-Deposited Aluminum and Zirconium Oxides for Surface Passivation of TiO2 in High-Efficiency Organic Photovoltaics
    Vasilopoulou, M. and Georgiadou, D.G. and Soultati, A. and Boukos, N. and Gardelis, S. and Palilis, L.C. and Fakis, M. and Skoulatakis, G. and Kennou, S. and Botzakaki, M. and Georga, S. and Krontiras, C.A. and Auras, F. and Fattakhova-Rohlfing, D. and Bein, T. and Papadopoulos, T.A. and Davazoglou, D. and Argitis, P.
    Advanced Energy Materials 4 (2014)
    The reduction in electronic recombination losses by the passivation of surfaces is a key factor enabling high-efficiency solar cells. Here a strategy to passivate surface trap states of TiO<inf>2</inf> films used as cathode interlayers in organic photovoltaics (OPVs) through applying alumina (Al<inf>2</inf>O<inf>3</inf>) or zirconia (ZrO<inf>2</inf>) insulating nanolayers by thermal atomic layer deposition (ALD) is investigated. The results suggest that the surface traps in TiO<inf>2</inf> are oxygen vacancies, which cause undesirable recombination and high electron extraction barrier, reducing the open-circuit voltage and the short-circuit current of the complete OPV device. It is found that the ALD metal oxides enable excellent passivation of the TiO<inf>2</inf> surface followed by a downward shift of the conduction band minimum. OPV devices based on different photoactive layers and using the passivated TiO<inf>2</inf> electron extraction layers exhibit a significant enhancement of more than 30% in their power conversion efficiencies compared to their reference devices without the insulating metal oxide nanolayers. This is a result of significant suppression of charge recombination and enhanced electron extraction rates at the TiO<inf>2</inf>/ALD metal oxide/organic interface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/aenm.201400214
  • Covalent immobilization of redox protein within the mesopores of transparent conducting electrodes
    Müller, V. and Rathousky, J. and Fattakhova-Rohlfing, D.
    Electrochimica Acta 116 (2014)
    Redox protein cytochrome c was immobilized at high electrochemically accessible loading on mesoporous films of antimony doped tin oxide (ATO) exhibiting high conductivity, transparency and a large surface area. The grafting was achieved by covalent attachment of the protein to the electrode surface. Alternatively, cytochrome c was effectively adsorbed on the ATO electrode surface due to strong electrostatic interaction between the positively charged cytochrome c and the negatively charged ATO surface. The amount of electrochemically addressable cytochrome c is proportional to the specific surface area, reaching up to 440 pmol/cm2 and 600 pmol/cm2 for covalently attached and adsorbed protein, respectively, for the 370 nm thick films. The covalently attached protein exhibits substantially higher stability towards leaching than the adsorbed one. The combination of transparent conducting porous electrode matrix with the electroactive proteins is promising for the development of efficient bio-optoelectronic devices and for in situ spectroelectrochemical studies of biomolecules. © 2013 Elsevier Ltd.
    view abstract10.1016/j.electacta.2013.10.136
  • Macroporous indium tin oxide electrode layers as conducting substrates for immobilization of bulky electroactive guests
    Liu, Y. and Peters, K. and Mandlmeier, B. and Müller, A. and Fominykh, K. and Rathousky, J. and Scheu, C. and Fattakhova-Rohlfing, D.
    Electrochimica Acta 140 (2014)
    Macroporous indium tin oxide (ITO) electrodes with a defined uniform pore size were prepared via direct co-assembly of ultra-small indium tin hydroxide (nano-ITOH) nanoparticles and poly(methyl methacrylate) (PMMA) beads. The use of nano-ITOH nanoparticles enables a facile large-scale fabrication of homogeneous crack-free coatings with good adhesion to the substrate, good optical quality and tunable thickness, which easily transform at 400 °C into crystalline ITO with similar morphology. Macroporous ITO films exhibit reasonably high electric conductivity of 4.0 ± 0.3 S cm-1 and open interconnected pores with a uniform size of ca. 300 nm, which makes them suitable conducting platforms for immobilization of bulky redox species or for deposition of functional electroactive layers. Deposition of functional semiconducting layers on the walls of the porous ITO scaffold was shown for titanium dioxide, which penetrates the ITO framework as shown by transmission electron microscopy (TEM) analysis of a cross-section. The obtained layers were used as conducting substrates for the immobilization of the heme proteins cytochrome c and hemoglobin, which demonstrate a direct electron transfer to the macroporous ITO electrode. Surface coverage of cytochrome c adsorbed on the macroporous ITO electrode is more than 12 times higher than on a planar one, reaching 400 pmol cm-2 for a film of ca. 360 nm in thickness. Hemoglobin adsorbed on the macroporous ITO electrodes also demonstrates a noticeably high surface coverage of ca. 160 ± 20 pmol cm-2, which is roughly 7-10 times higher than the theoretical value for monolayer coverage. © 2014 Elsevier Ltd.
    view abstract10.1016/j.electacta.2014.05.046
  • Tailoring the morphology of mesoporous titania thin films through biotemplating with nanocrystalline cellulose
    Ivanova, A. and Fattakhova-Rohlfing, D. and Kayaalp, B.E. and Rathouský, J. and Bein, T.
    Journal of the American Chemical Society 136 (2014)
    The tunable porosity of titania thin films is a key factor for successful applications in photovoltaics, sensing, and photocatalysis. Here, we report on nanocrystalline cellulose (NCC) as a novel shape-persistent templating agent enabling the straightforward synthesis of mesoporous titania thin films. The obtained structures are highly porous anatase morphologies having well-defined, narrow pore size distributions. By varying the titania-to-template ratio, it is possible to tune the surface area, pore size, pore anisotropy, and dimensions of titania crystallites in the films. Moreover, a post-treatment at high humidity and subsequent slow template removal can be used to achieve pore widening; this treatment is also beneficial for the multilayer deposition of thick films. The resulting homogeneous transparent films can be directly spin- or dip- coated on glass, silicon, and transparent conducting oxide (TCO) substrates. The mesoporous titania films show very high activity in the photocatalytic NO conversion and in the degradation of 4-chlorophenol. Furthermore, the films can be successfully applied as anodes in dye-sensitized solar cells. © 2014 American Chemical Society.
    view abstract10.1021/ja411292u
  • Thick titania films with hierarchical porosity assembled from ultrasmall titania nanoparticles as photoanodes for dye-sensitized solar cells
    Feckl, J.M. and Haynes, A. and Bein, T. and Fattakhova-Rohlfing, D.
    New Journal of Chemistry 38 (2014)
    Thin mesoporous titania films prepared by surfactant templating feature some of the highest light conversion efficiencies per thickness ratios as anodes in dye-sensitized solar cells (DSCs). However, the fabrication of thicker films required for sufficient light absorption is very challenging using this approach, often resulting in cracking and delamination of the films. Here we present a simple and scalable method to prepare thick mesoporous titania photoanodes via a surfactant-directed assembly of crystalline ultra-small TiO2 nanoparticles in combination with phase separation due to ethyl cellulose added to the coating solutions. Along with increasing film thickness, the ethyl cellulose introduces an interpenetrating macropore network into the films, leading to the formation of hierarchical porous films with bimodal porosity, with the smaller mesopores resulting from the structure-directing agent, Pluronic F127. In this way, films of up to 2 μm per layer without delamination can be produced, exhibiting a high surface area of 130 m 2 g-1, about twice the value of films based on standard TiO2 nanoparticle paste. The preparation of multilayer films by a sequential spin-coating and calcination procedure enables the production of films with an overall thickness of up to 10 μm in only 5 steps, which showed high efficiencies of 7.7% in dye-sensitized solar cells. © 2014 The Partner Organisations.
    view abstract10.1039/c3nj00972f
  • Three-dimensional titanium dioxide nanomaterials
    Fattakhova-Rohlfing, D. and Zaleska, A. and Bein, T.
    Chemical Reviews 114 (2014)
    view abstract10.1021/cr500201c
  • Tin doping speeds up hole transfer during light-driven water oxidation at hematite photoanodes
    Dunn, H.K. and Feckl, J.M. and Müller, A. and Fattakhova-Rohlfing, D. and Morehead, S.G. and Roos, J. and Peter, L.M. and Scheu, C. and Bein, T.
    Physical Chemistry Chemical Physics 16 (2014)
    Numerous studies have shown that the performance of hematite photoanodes for light-driven water splitting is improved substantially by doping with various metals, including tin. Although the enhanced performance has commonly been attributed to bulk effects such as increased conductivity, recent studies have noted an impact of doping on the efficiency of the interfacial transfer of holes involved in the oxygen evolution reaction. However, the methods used were not able to elucidate the origin of this improved efficiency, which could originate from passivation of surface electron-hole recombination or catalysis of the oxygen evolution reaction. The present study used intensity-modulated photocurrent spectroscopy (IMPS), which is a powerful small amplitude perturbation technique that can de-convolute the rate constants for charge transfer and recombination at illuminated semiconductor electrodes. The method was applied to examine the kinetics of water oxidation on thin solution-processed hematite model photoanodes, which can be Sn-doped without morphological change. We observed a significant increase in photocurrent upon Sn-doping, which is attributed to a higher transfer efficiency. The kinetic data obtained using IMPS show that Sn-doping brings about a more than tenfold increase in the rate constant for water oxidation by photogenerated holes. This result provides the first demonstration that Sn-doping speeds up water oxidation on hematite by increasing the rate constant for hole transfer. © the Partner Organisations 2014.
    view abstract10.1039/c4cp03946g
  • Tuning the crystallinity parameters in macroporous titania films
    Mandlmeier, B. and Minar, N.K. and Feckl, J.M. and Fattakhova-Rohlfing, D. and Bein, T.
    Journal of Materials Chemistry A 2 (2014)
    Although macroporous titania scaffolds are used for many different applications, not much is known about the importance of the synthesis strategy on the resulting materials' properties. We present a comparative study on the influence of different colloidal titania precursors for direct co-deposition with poly(methyl methacrylate) (PMMA) beads on the properties of the resulting macroporous scaffolds after calcination. The colloidal titania precursors for the film assembly differ in their size and initial crystallinity, ranging from amorphous sol-gel clusters to already crystalline pre-formed particles of 4 nm, 6 nm and 20 nm in size, as well as a combination of sol-gel and nanoparticle precursors in the so-called 'Brick and Mortar' approach. The type of the precursor greatly influences the morphology, texture and the specific crystallinity parameters of the macroporous titania scaffolds after calcination such as the size of the crystalline domains, packing density of the crystallites in the macroporous walls and interconnectivity between the crystals. Moreover, the texture and the crystallinity of the films can be tuned by post-synthesis processing of the films such as calcination at different temperatures, which can be also preceded by a hydrothermal treatment. The ability to adjust the porosity, the total surface area and the crystallinity parameters of the crystalline macroporous films by selecting suitable precursors and by applying different post-synthetic treatments provides useful tools to optimize the film properties for different applications. This journal is © the Partner Organisations 2014.
    view abstract10.1039/c3ta13165c
  • Ultrasmall dispersible crystalline nickel oxide nanoparticles as high-performance catalysts for electrochemical water splitting
    Fominykh, K. and Feckl, J.M. and Sicklinger, J. and Döblinger, M. and Böcklein, S. and Ziegler, J. and Peter, L. and Rathousky, J. and Scheidt, E.-W. and Bein, T. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 24 (2014)
    Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time, and it is shown that they are promising candidates as catalysts for electrochemical water oxidation. Using a solvothermal reaction in tert-butanol, very small nickel oxide nanocrystals can be made with sizes tunable from 2.5 to 5 nm and a narrow particle size distribution. The crystals are perfectly dispersible in ethanol even after drying, giving stable transparent colloidal dispersions. The structure of the nanocrystals corresponds to phase-pure stoichiometric nickel(ii) oxide with a partially oxidized surface exhibiting Ni(iii) states. The 3.3 nm nanoparticles demonstrate a remarkably high turn-over frequency of 0.29 s-1 at an overpotential of g = 300 mV for electrochemical water oxidation, outperforming even expensive rare earth iridium oxide catalysts. The unique features of these NiO nanocrystals provide great potential for the preparation of novel composite materials with applications in the field of (photo)electrochemical water splitting. The dispersed colloidal solutions may also find other applications, such as the preparation of uniform hole-conducting layers for organic solar cells. Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time using a solvothermal reaction in tert-butanol. These nanocrystals can be prepared with sizes tunable from 2.5 to 5 nm and are highly efficient catalysts for electrochemical oxygen generation. © 2014 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201303600
  • Charge transport in TiO2 Films with complex percolation pathways investigated by time-resolved terahertz spectroscopy
    Nemec, H. and Zajac, V. and Rychetsky, I. and Fattakhova-Rohlfing, D. and Mandlmeier, B. and Bein, T. and Mics, Z. and Kuzel, P.
    IEEE Transactions on Terahertz Science and Technology 3 (2013)
    The depolarization fields play an important role in terahertz experiments on nanostructured samples with complex nanoparticle morphologies and percolation pathways. Namely, their effects can hide or distort peculiarities of nanoscopic charge transport in the spectra measured on these structures. We calculate the local fields for a large number of percolated and non-percolated two-dimensional model structures by numerical solving of Maxwell equations in the quasi-static limit. The results strongly suggest that in a broad family of structures a simple effective medium approximation model can be applied to characterize the effective response. The model consists in an equivalent circuit composed of a resistance accounting for the percolated chains with an additional parallel RC-branch describing the non-percolated part. The physical meaning of this model is discussed in the frame of the Bergman spectral representation of effective medium. We show a recipe for the retrieval of a response connected to the depolarization fields and to the nanoscale transport mechanisms from transient terahertz spectra. Finally, we use the model to interpret our THz photoconductivity spectra in various TiO2 films with nanofabricated percolation pathways. © 2011-2012 IEEE.
    view abstract10.1109/TTHZ.2013.2255555
  • Electric-field-tunable defect mode in one-dimensional photonic crystal operating in the terahertz range
    Skoromets, V. and Němec, H. and Kadlec, C. and Fattakhova-Rohlfing, D. and Kužel, P.
    Applied Physics Letters 102 (2013)
    A one-dimensional photonic crystal possessing an electric-field-tunable defect mode in the lowest forbidden band is demonstrated. The compact photonic structure consists of two symmetric Bragg mirrors made of alternate quarter-wave layers of SiO2 and CeO2 separated by a defect layer of an incipient ferroelectric SrTiO3 with electrodes transparent for terahertz radiation on its both sides. The applied bias is then perpendicular to the layer and modifies the in-plane dielectric function, which is probed by the transverse terahertz wave. The observed tunable behavior is in agreement with the model of the ferroelectric soft mode behavior in SrTiO3 single crystals. The defect-mode frequency tunability is proportional to that of the soft mode: we achieved a relative tunability of 6.5% at 105 K under an electric bias of 60 kV/cm. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4809821
  • Erratum: A novel method to measure diffusion coefficients in porous metal-organic frameworks (Physical Chemistry Chemical Physics (2010) 12 (8093-8098) DOI: 10.1039/B927601G)
    Zybaylo, O. and Shekhah, O. and Wang, H. and Tafipolsky, M. and Schmid, R. and Johannsmann, D. and Wöll, C.
    Physical Chemistry Chemical Physics 15 (2013)
    view abstract10.1039/c3cp90155f
  • Erratum: Highly soluble energy relay dyes for dye-sensitized solar cells (Physical Chemistry Chemical Physics (2013) 15 (11306-11312) DOI: 10.1039/C3CP51018B)
    Margulis, G.Y. and Lim, B. and Hardin, B.E. and Unger, E.L. and Yum, J.-H. and Feckl, J.M. and Fattakhova-Rohlfing, D. and Bein, T. and Grätzel, M. and Sellinger, A. and McGehee, M.D.
    Physical Chemistry Chemical Physics 15 (2013)
    view abstract10.1039/c3cp90155f
  • Highly soluble energy relay dyes for dye-sensitized solar cells
    Margulis, G.Y. and Lim, B. and Hardin, B.E. and Unger, E.L. and Yum, J.-H. and Feckl, J.M. and Fattakhova-Rohlfing, D. and Bein, T. and Grätzel, M. and Sellinger, A. and McGehee, M.D.
    Physical Chemistry Chemical Physics 15 (2013)
    High solubility is a requirement for energy relay dyes (ERDs) to absorb a large portion of incident light and significantly improve the efficiency of dye-sensitized solar cells (DSSCs). Two benzonitrile-soluble ERDs, BL302 and BL315, were synthesized, characterized, and resulted in a 65% increase in the efficiency of TT1-sensitized DSSCs. The high solubility (180 mM) of these ERDs allows for absorption of over 95% of incident light at their peak wavelength. The overall power conversion efficiency of DSSCs with BL302 and BL315 was found to be limited by their energy transfer efficiency of approximately 70%. Losses due to large pore size, dynamic collisional quenching of the ERD, energy transfer to desorbed sensitizing dyes and static quenching by complex formation were investigated and it was found that a majority of the losses are caused by the formation of statically quenched ERDs in solution. © 2013 the Owner Societies.
    view abstract10.1039/c3cp51018b
  • Assembly of mesoporous indium tin oxide electrodes from nano-hydroxide building blocks
    Liu, Y. and Štefanić, G. and Rathouský, J. and Hayden, O. and Bein, T. and Fattakhova-Rohlfing, D.
    Chemical Science 3 (2012)
    We describe the elaboration of nanostructured transparent conducting indium tin oxide (ITO) materials that is based on controlled self-assembly of ultra-small indium tin hydroxide nanoparticles. We developed a strategy for preparing nanosized, nearly spherical and highly dispersible nanoparticles of indium tin hydroxide ("nano-hydroxides"), which can be assembled into regular mesoporous architectures directed by a commercially available Pluronic polymer. The assembled structures are easily transformed into conducting crystalline mesoporous ITO films by a mild heat treatment at 300 °C. The resulting ITO layers feature a regular mesoporosity with a mesostructure periodicity of about 13 ± 2 nm, high surface area of 190 m2 cm-3, porosity of 44% and electrical conductivity up to 9.5 S cm -1. The ITO films can accommodate large amounts of redox-active molecules and serve as efficient conducting electrodes with a very high surface area. The perfect dispersibility of nano-hydroxides without any stabilizing agents, their preferential interaction with the hydrophilic part of amphiphilic molecules leading to their self-assembly, and a facile transformation of the assembled nano-hydroxides into crystalline ITO with similar morphology make the nano-hydroxides very attractive building blocks for the elaboration of nanostructured ITO materials. We believe that the nano-hydroxides can become universal building blocks for the fabrication of crystalline ITO materials with arbitrary nano-morphologies. © The Royal Society of Chemistry 2012.
    view abstract10.1039/c2sc20042b
  • Multilayered high surface area "brick and mortar" mesoporous titania films as efficient anodes in dye-sensitized solar cells
    Szeifert, J.M. and Fattakhova-Rohlfing, D. and Rathouský, J. and Bein, T.
    Chemistry of Materials 24 (2012)
    The "brick and mortar" approach is employed to synthesize thick surfactant-templated mesoporous titanium dioxide films of up to 10 μm thickness using multilayer deposition. The films exhibit very high surface areas scaling linearly with the thickness, and roughness factors of up to 1600 cm 2/cm 2 can be reached. For the first time, surfactant-derived mesoporous titanium dioxide films of such a large thickness and surface area can be prepared without serious cracking, delamination, or deterioration of the porous structure. The mesopores are rather large (12 nm), and stacking many layers does not affect their size or accessibility, which is shown by krypton and dye adsorption experiments. Applied in dye-sensitized solar cells, the films feature a high power conversion efficiency of over 7% already at thicknesses below 4 μm due to their high surface area and dye adsorption. © 2012 American Chemical Society.
    view abstract10.1021/cm202218w
  • Nanoscale porous framework of lithium titanate for ultrafast lithium insertion
    Feckl, J.M. and Fominykh, K. and Döblinger, M. and Fattakhova-Rohlfing, D. and Bein, T.
    Angewandte Chemie - International Edition 51 (2012)
    Mesoporous lithium titanate has been prepared with a titanate morphology that leads to the fastest insertion of lithium. It features a gravimetric capacity of about 175 mAhg -1 and delivers up to 73% of the maximum capacity at up to 800 C (4.5 s) without deterioration over 1000 cycles. A key feature is a fully crystalline interconnected porous framework composed of spinel nanocrystals of only a few nanometers in size. Scale bar: 10 nm. Copyright © 2012 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/anie.201201463
  • Surface functionalization of mesoporous antimony doped tin oxide by metalorganic reaction
    Müller, V. and Haase, F. and Rathousky, J. and Fattakhova-Rohlfing, D.
    Materials Chemistry and Physics 137 (2012)
    Electrically conducting mesoporous antimony doped tin oxide was functionalized by the metalorganic reaction with several Grignard reagents. This fast and efficient grafting approach enables a direct connection of the metal atoms with the organic functionalities avoiding the formation of insulating Si-O linkers, which is of special interest for the interfacial charge transfer processes. Using this approach we introduced vinyl, allyl and phenyl groups into the pores of mesoporous ATO, which was confirmed by IR spectroscopy, TGA and nitrogen sorption measurements. We obtained a high loading of organic groups corresponding to about 50-60% of the monolayer surface coverage. The obtained mesoporous inorganic-organic hybrids can serve as a platform for incorporation of electrochemically active species. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.matchemphys.2012.09.008
  • A facile synthesis of mesoporous crystalline tin oxide films involving a base-triggered formation of sol-gel building blocks
    Fried, D.I. and Ivanova, A. and Müller, V. and Rathousky, J. and Smarsly, B.M. and Fattakhova-Rohlfing, D.
    Nanoscale 3 (2011)
    We have developed a new facile procedure for manufacturing crystalline thin films of SnO2 with a uniform mesoporous architecture and full crystallinity of the walls. The procedure is based on the evaporation-induced self-assembly (EISA) of prehydrolyzed tin oxide precursor directed by a commercially available Pluronic polymer. The formation of the tin oxide precursor, which can be self-assembled into a mesoporous structure, is achieved by an addition of ammonium hydroxide to a tin tetrachloride solution. The relative concentration of ammonium hydroxide as well as the duration and temperature of the hydrolysis reaction influence significantly the properties of hydrolyzed tin oxide species and the mesostructure assembled from them. The films coated from these precursor solutions and calcined at 300 °C to 400 °C exhibit a well-developed worm-like porosity with a wall to wall distance of ca. 18 nm, a surface area of up to 50 cm2 cm-2 (corresponding to 55 ± 5 m2 g-1), and high crystallinity. © 2011 The Royal Society of Chemistry.
    view abstract10.1039/c0nr00872a
  • All-inorganic core-shell silica-titania mesoporous colloidal nanoparticles showing orthogonal functionality
    Cauda, V. and Szeifert, J.M. and Merk, K. and Fattakhova-Rohlfing, D. and Bein, T.
    Journal of Materials Chemistry 21 (2011)
    Colloidal mesoporous silica (CMS) nanoparticles with a thin titania-enriched outer shell showing a spatially resolved functionality were synthesized by a delayed co-condensation approach. The titania-shell can serve as a selective nucleation site for the growth of nanocrystalline anatase clusters. These fully inorganic pure silica-core titania-enriched shell mesoporous nanoparticles show orthogonal functionality, demonstrated through the selective adsorption of a carboxylate-containing ruthenium N3-dye. UV-Vis and fluorescence spectroscopy indicate the strong interaction of the N3-dye with the titania-phase at the outer shell of the CMS nanoparticles. In particular, this interaction and thus the selective functionalization are greatly enhanced when anatase nanocrystallites are nucleated at the titania-enriched shell surface. © 2011 The Royal Society of Chemistry.
    view abstract10.1039/c0jm04528d
  • Antimony doped tin oxide nanoparticles and their assembly in mesostructured film
    Müller, V. and Rasp, M. and Stefanic, G. and Günther, S. and Rathousky, J. and Niederberger, M. and Fattakhova-Rohlfing, D.
    Physica Status Solidi (C) Current Topics in Solid State Physics 8 (2011)
    Mesoporous transparent conducting films of antimony-doped tin oxide (ATO) were prepared by self-assembly of crystalline ATO nanoparticles, which enables to obtain a fully crystalline frameworks with a sufficient electric conductivity. Such frameworks are promising as transparent electrodes with a high surface area, as shown for ferrocene molecules covalently immobilized within a conducting mesoporous matrix. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssc.201000129
  • Formation of interpenetrating hierarchical titania structures by confined synthesis in inverse opal
    Mandlmeier, B. and Szeifert, J.M. and Fattakhova-Rohlfing, D. and Amenitsch, H. and Bein, T.
    Journal of the American Chemical Society 133 (2011)
    Hierarchical periodic titania nanostructures composed of a macroporous crystalline scaffold and mesoporous titania were prepared by confined synthesis. The strategy for the generation of these hierarchical structures involves preparation of inverse opal titania layers and subsequent filling of the interstitial macroporous voids with surfactant-containing titania precursors to obtain a mesostructured titania phase using the surfactant Pluronic P123. The formation of mesostructure in the confined space of the macroporous scaffold upon thermal treatment was investigated with in situ grazing incidence small-angle X-ray scattering (GISAXS). The macroporous scaffold strongly influences the mesostructure assembly and leads to much larger structural parameters of the formed mesostructure, this effect becoming more pronounced with decreasing pore size of the macroporous host. Furthermore, the inverse opal scaffold acts as a stabilizing matrix, limiting the shrinkage of the mesopores upon heating. This effect is coupled with an enhanced crystallization of the mesophase, which is attributed to the crystalline walls of the macroporous host. Sorption measurements of the final hierarchical titania structure of 5 μm thickness show that the porous system is fully accessible, has a high total surface area of 154 m2/g, and has an average mesopore size of 6.1 nm, which is about 20% larger than the pore size of 5.1 nm for the reference mesoporous film obtained on a flat substrate. These hierarchical structures were implemented as anodes in dye-sensitized solar cells (DSCs), showing a conversion efficiency of 4% under one sun illumination, whereas the calcined macroporous scaffold alone shows an efficiency of only 0.4%. © 2011 American Chemical Society.
    view abstract10.1021/ja204667e
  • Tuning of dielectric properties of SrTiO3 in the terahertz range
    Skoromets, V. and Kadlec, F. and Kadlec, C. and Němec, H. and Rychetsky, I. and Panaitov, G. and Müller, V. and Fattakhova-Rohlfing, D. and Moch, P. and Küel, P.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    Tuning of the dielectric permittivity spectra of strontium titanate (SrTiO3) single crystals in an external electric field is investigated between 90 and 300 K by means of terahertz time-domain spectroscopy. Application of the electric bias leads to an appreciable tuning of the permittivity observed up to room temperature both in the parallel and perpendicular directions to the bias field. The observed behavior is interpreted in terms of soft-mode hardening due to the anharmonic character of its potential. No additional low-frequency relaxation mode was observed. A weak temperature dependence of the anharmonic coefficients was found in agreement with previously published low-temperature data. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.174121
  • Tuning the conduction mechanism in niobium-doped titania nanoparticle networks
    Němec, H. and Mics, Z. and Kempa, M. and Kužel, P. and Hayden, O. and Liu, Y. and Bein, T. and Fattakhova-Rohlfing, D.
    Journal of Physical Chemistry C 115 (2011)
    Networks of niobium-doped TiO2 anatase nanoparticles with variable doping concentrations were investigated by time-domain terahertz spectroscopy and microwave impedance spectroscopy. A detailed description of their electromagnetic response is proposed; the model takes into account the depolarization fields of inhomogeneous samples and allows us to understand the conductive and dielectric response of individual nanoparticles. We find that electron hopping is the dominating contribution to the conductivity at terahertz frequencies and that the dielectric losses of TiO2 nanoparticles are enhanced in comparison with bulk anatase. The conductive properties of nanoparticles can be tuned via synthesis conditions and thermal posttreatment. In particular, annealing at elevated temperatures improves the nanoparticle crystallinity, reduces the density of structural defects, and enhances the conductive percolation of the network. The developed model of the conduction processes can be helpful for interpretation of charge transport in other semiconducting nanoscale materials. © 2011 American Chemical Society.
    view abstract10.1021/jp200822y
  • electrochemistry

  • energy conversion

  • energy conversion

  • nanostructured materials

  • solar water splitting

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