Amorphous Transition Metal Oxides Research Articles

Highly efficient catalytic oxidation of toluene by amorphous/microcrystalline mixed-phase MnO2

Amorphous transition metal oxides exhibit considerable potential in the catalytic oxidation of volatile organic compounds (VOCs) owing to their distinctive surface properties. In this study, we achieved the modulation of the crystalline structure of manganese oxides by employing various C3 alcohols (alcohols with three carbons) with distinct hydroxyl types during the reduction process with potassium permanganate. Subsequently, a series of characterization techniques were employed to assess the catalytic performance of the prepared MnO2 in the oxidation of toluene. Notably, the MnO2-G obtained from glycerol presented excellent catalytic activity (T90 = 219 °C), stability, and water resistance. The unique amorphous/microcrystalline state of MnO2-G exposed more phase boundaries, and the unsaturated coordination structure facilitated the generation of additional defective sites, favoring the adsorption and activation of oxygen. Furthermore, the mixed-crystalline state induced the elevated Mn3+ content on the surface of MnO2-G, leading to elongation of the Mn-O bond due to the pronounced Jahn-Teller effect of Mn3+. Consequently, the synergistic effects of the mixed crystalline state contributed to the efficient catalytic oxidation of toluene at relatively low temperatures. Additionally, the conversion pathway of toluene on MnO2-G was further revealed by in situ DRIFTS.

Crystalline and amorphous Fe2O3 nanocubes grown on electrospun carbon nanofibers for lithium-ion batteries and lithium-sulfur batteries: A comparative study

Amorphous transition metal oxides have recently received particular research interests in electrochemical energy storage. However, there is still a lack of direct comparisons between amorphous materials and their crystalline counterparts. Here, we demonstrate the rational synthesis of crystalline and amorphous Fe2O3 nanocubes uniformly grown on carbon nanofibers (denoted as CNFs@C-Fe2O3 and CNFs@A-Fe2O3, respectively) for lithium-ion batteries (LIBs) and lithium-sulfur batteries (LSBs). In such a structure, the Fe2O3 nanocubes possess strong interfacial bonding with CNFs, which can ensure rapid electron transportation. Besides, these Fe2O3 nanocubes are highly porous, which can effectively alleviate the volume change, enlarge the surface area, increase active sites and facilitate ion diffusion. When employed as freestanding anode for LIBs, the CNFs@C-Fe2O3 electrode delivers much improved lithium ion storage performance compared to that of CNFs@A-Fe2O3. When evaluated as interlayers for LSBs, instead, the batteries with CNFs@A-Fe2O3 exhibit better rate performance cycling stability than that of with CNFs@C-Fe2O3. Moreover, theoretical calculations elucidate the amorphous Fe2O3 has stronger adsorption ability toward the soluble lithium polysulfides. This work would provide new insights into the reasonably development of crystalline and amorphous transition metal oxides toward electrochemical energy storage.

Emerging Amorphous to Crystalline Conversion Chemistry in Ca-Doped VO2 Cathodes for High-Capacity and Long-Term Wearable Aqueous Zinc-Ion Batteries.

Amorphous transition metal oxides have attracted significant attention in energy storage devices owing to their potentially desirable electrochemical properties caused by abundant unsaturated dangling bonds. However, the amorphization further amplifies the shortcoming of the poor intrinsic electronic conductivity of the metal oxides, resulting in unsatisfying rate capability and power density. Herein, freestanding amorphous Ca-doped V2 O5 (a-Ca-V2 O5 ) cathodes are successfully prepared via in situ electrochemical oxidation of Ca-doped VO2 nanoarrays for wearable aqueous zinc-ion batteries. The doping of Ca and construction of freestanding structure effectively uncover the potential of amorphous V2 O5 , which can make full use of the abundant active sites for high volumetric capacity and simultaneously achieve fast reaction kinetics for excellent rate performance. More importantly, the introduction of Ca can notably reduce the formation energy of VO2 according to theoretical calculation results and realizes amorphous to crystalline reversible conversion chemistry in the charge/discharge procedure, thereby facilitating the reversible capacity of the newly developed a-Ca-V2 O5 . This work provides an innovative design strategy to construct high-rate capacity amorphous metal oxides as freestanding electrodes for low-cost and high-safe wearable energy-storage technology.

Engineering amorphous SnO2 nanoparticles integrated into porous N-doped carbon matrix as high-performance anode for lithium-ion batteries

A facile in-situ preparation strategy is proposed to anchor amorphous SnO2 nanoparticles into the porous N-doped carbon (NC) matrix to fabricate amorphous composite powders (am-SnO2@p-NC), which feature the hierarchically interconnected and well interlaced porous configurations by employing polyvinylpyrrolidone as the soft template. The morphology regulation of the porous structure is precisely realized by adjusting the content of the template and the relationship between structural evolution and electrochemical performance of composite powders is accurately described to explore the optimal template dosage. The results indicate that the am-SnO2@p-NC-50 % composite electrode can deliver the improved lithium storage capacity and cycling performance when the content of the template is controlled at 0.500 g, in which the initial discharge specific capacity is about 1557.6 mAh/g and the reversible value retains at 841.5 mAh/g after 100 cycles at 100 mA/g. Meanwhile, the discharge specific capacity of 869.8 mAh/g is exhibited for the am-SnO2@p-NC-50 % composite electrode after 60 cycles when the current density is recovered from 2000 to 100 mA/g. Moreover, the Li+ ions diffusion coefficient up to about 5.5 × 10−12 cm2/s is calculated from galvanostatic intermittent titration technique tests, which can be partly ascribed to the conductive NC substrate that provides the high electronic conductivity, and partly to the highly porous structure that shortens Li+ ions transfer pathways and guarantees the fast reaction kinetics. Therefore, the hierarchically porous engineering of carbon networks to confine amorphous transition metal oxide nanoparticles is of great significance in the development of high-performance anode materials for lithium-ion batteries.

CO2‐Induced Two‐Dimensional Amorphous TiO2 and Its Excellent Film‐Forming Properties

AbstractFabrication of amorphous transition metal oxides (TMOs) is necessary to explore their functionality for potential applications, while it is challenging for this type of inorganic materials. Finding an ideal and efficient route for their amorphization is in anticipation. Herein, as a typical non‐layered TMO, two‐dimensional amorphous TiO2(B) has been successfully fabricated with the assistance of supercritical CO2 (SC CO2). Depending on multi‐level theoretical simulations, it demonstrates that the defect regions in TiO2(B) are favorable for CO2 to exert its amorphization effect. Therefore, it provides a new insight into understanding the microscopic mechanism of SC CO2 to functionalize non‐layered TMOs. Further, stable optically transparent thin films based on the as‐prepared samples were conveniently obtained, and their excellent optical stability is also identified.

Amorphous VO2 : A Pseudocapacitive Platform for High-Rate Symmetric Batteries.

Among the various VO2 polymorphs, the layered compound, VO2 (B), has been the most widely investigated lithium-ion battery electrode material. For sodium-ion electrodes, however, an amorphous solid may be more advantageous as a result of the open framework to facilitate ion insertion and the ability to tolerate volumetric changes. Herein, it is shown that the Na+ insertion properties of amorphous VO2 (a-VO2 ) are superior to those of crystalline VO2 (B). Amorphous VO2 exhibits a linear voltage characteristic over a 3 V range (4.0 to 1.0V vs Na/Na+ ) leading to a reversible capacity as high as 400 mAh g-1 and rapid redox kinetics, which is attributed to its pseudocapacitive nature. The linear voltage characteristic over 3 V affords the opportunity of fabricating a symmetric Na-ion battery in which the a-VO2 material serves as both the positive electrode and the negative electrode. Such a symmetric battery offers safer operation in terms of overcharging, overdischarging, polarity reversal, high charge/discharge current abuse, and long-term usage. The results suggest that amorphous transition metal oxides may offer advantageous attributes for rapid, safe, and energy-dense storage.

Generalized Synthetic Strategy for Amorphous Transition Metal Oxides‐Based 2D Heterojunctions with Superb Photocatalytic Hydrogen and Oxygen Evolution

Abstract2D amorphous transition metal oxides (a‐TMOs) heterojunctions that have the synergistic effects of interface (efficiently promoting the separation of electron−hole pairs) and amorphous nature (abundant defects and dangling bonds) have attracted substantial interest as compelling photocatalysts for solar energy conversion. Strategies to facilely construct a‐TMOs‐based 2D/2D heterojunctions is still a big challenge due to the difficulty of preparing individual amorphous counterparts. A generalized synthesis strategy based on supramolecular self‐assembly for bottom–up growth of a‐TMOs‐based 2D heterojunctions is reported, by taking 2D/2D g‐C3N4 (CN)/a‐TMOs heterojunction as a proof‐of‐concept. This strategy primarily depends on controlling the cooperation of the growth of supramolecular precursor and the coordinated covalent bonds arising from the tendency of metal ions to attain the stable configuration of electrons, which is independent on the intrinsic character of individual metal ion, indicating it is universally applicable. As a demonstration, the structure, physical properties, and photocatalytic water‐splitting performance of CN/a‐ZnO heterojunction are systematically studied. The optimized 2D/2D CN/a‐ZnO exhibits enhanced photocatalytic performance, the hydrogen (432.6 µmol h−1 g−1) and oxygen (532.4 µmol h−1 g−1) evolution rate are 15.5 and 12.2 times than bulk CN, respectively. This synthetic strategy is useful to construct 2D a‐TMOs nanomaterials for applications in energy‐related areas and beyond.

General anion-exchange reaction derived amorphous mixed-metal oxides hollow nanoprisms for highly efficient water oxidation electrocatalysis

High cost precious metal oxides (RuO2 and IrO2) electrocatalysts are required to overcome the sluggish kinetics of the oxygen evolution reaction (OER), which severely restricts the scalable application. Herein, we present a versatile anion-exchange based strategy to fabricate various amorphous transition metal oxide hollow nanoprisms with FeCo-MIL-88B template, which exhibit highly efficient OER catalysis in alkaline media. Impressively, a low overpotential of 294 mV at 10 mA cm−2 with an ultrasmall Tafel slope of 45.1 mV dec-1 and decent durability can be achieved. More importantly, the underlying origin is unraveled using the in-situ Raman spectroscopy measurement combined with the ex-situ X-ray photoelectron spectroscopy anlysis, where the electrochemically transformed metal hydroxide and oxyhydroxide species could be the real active species. This work provides a universal strategy to construct transition metal oxide hollow nanostructures with enhanced OER activity, which could push forward the advance of noble metal free electrocatalytic water splitting.

CO2-assisted fabrication of two-dimensional amorphous transition metal oxides.

This Frontier presents the recent developments and applications of two-dimensional (2D) amorphous transition metal oxides (TMOs) obtained by using supercritical CO2. CO2 molecules can affect the crystal transformation of layered materials and allow diffusive atomic disordering during the exfoliation process. If amorphous structures are introduced into TMOs, the strong localization tail states that exist in the band gap of TMOs can effectively promote chemical reactions. We also discuss the future challenges to be overcome for this class of supercritical CO2 and 2D amorphous materials.

Diffusion of oxygen in amorphous tantalum oxide

The switching in resistive random-access memory (RRAM) relies on the migration of ions in sputtered amorphous transition metal oxides to both create and annihilate conductive filaments. A comprehensive understanding of ionic diffusion in these materials is therefore critical for optimizing future RRAM devices. While predicting diffusion barriers in crystalline materials is straightforward, the complex energy landscape in disordered materials makes analysis of the effective diffusion barrier inherently more difficult. Using classical molecular dynamics, we examine oxygen diffusion at various temperatures in amorphous tantala $({\mathrm{Ta}}_{2}{\mathrm{O}}_{5})$, a leading material for RRAM applications. We compare the predicted activation energies for self-diffusion to those from isotope tracer diffusion experiments. We find a diffusion activation energy of 1.55--1.60 eV, which is higher than that measured in isotopic diffusion experiments $(1.2\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.16em}{0ex}}\mathrm{eV})$ on amorphous ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$. However, the predicted values are comparable to the activation energy $(1.60\ifmmode\pm\else\textpm\fi{}0.18\phantom{\rule{0.16em}{0ex}}\mathrm{eV})$ determined from secondary ion mass spectroscopy analysis of isotopic diffusion experiments of sintered L-${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ samples. We find that this difference could be due to issues of deposited film porosity, film stoichiometry, or else a preference for the empirical potentials to form nanocrystalline regions.

Disentangling the intricate atomic short-range order and electronic properties in amorphous transition metal oxides

Solid state materials with crystalline order have been well-known and characterized for almost a century while the description of disordered materials still bears significant challenges. Among these are the atomic short-range order and electronic properties of amorphous transition metal oxides [aTMOs], that have emerged as novel multifunctional materials due to their optical switching properties and high-capacity to intercalate alkali metal ions at low voltages. For decades, research on aTMOs has dealt with technological optimization. However, it remains challenging to unveil their intricate atomic short-range order. Currently, no systematic and broadly applicable methods exist to assess atomic-size structure, and since electronic localization is structure-dependent, still there are not well-established optical and electronic mechanisms for modelling the properties of aTMOs. We present state-of-the-art systematic procedures involving theory and experiment in a self-consistent computational framework to unveil the atomic short-range order and its role for the electronic properties. The scheme is applied to amorphous tungsten trioxide aWO3, which is the most studied electrochromic aTMO in spite of its unidentified atomic-size structure. Our approach provides a one-to-one matching of experimental data and corresponding model structure from which electronic properties can be directly calculated in agreement with the electronic transitions observed in the XANES spectra.

Anti-biofouling function of amorphous nano-Ta2O5 coating for VO2-based intelligent windows

From environmental and health perspectives, the acquisition of a surface anti-biofouling property holds important significance for the usability of VO2 intelligent windows. Herein, we firstly deposited amorphous Ta2O5 nanoparticles on VO2 film by the magnetron sputtering method. It was found that the amorphous nano-Ta2O5 coating possessed a favorable anti-biofouling capability against Pseudomonas aeruginosa as an environmental microorganism model, behind which lay the mechanism that the amorphous nano-Ta2O5 could interrupt the microbial membrane electron transport chain and significantly elevate the intracellular reactive oxygen species (ROS) level. A plausible relationship was established between the anti-biofouling activity and physicochemical nature of amorphous Ta2O5 nanoparticles from the perspective of defect chemistry. ROS-induced oxidative damage gave rise to microbial viability loss. In addition, the amorphous nano-Ta2O5 coating can endow VO2 with favorable cytocompatibility with human skin fibroblasts. This study may provide new insights into understanding the anti-biofouling and antimicrobial actions of amorphous transition metal oxide nanoparticles, which is conducive to expanding their potential applications in environmental fields.

Linear topology in amorphous metal oxide electrochromic networks obtained via low-temperature solution processing

Amorphous transition metal oxides are recognized as leading candidates for electrochromic window coatings that can dynamically modulate solar irradiation and improve building energy efficiency. However, their thin films are normally prepared by energy-intensive sputtering techniques or high-temperature solution methods, which increase manufacturing cost and complexity. Here, we report on a room-temperature solution process to fabricate electrochromic films of niobium oxide glass (NbOx) and 'nanocrystal-in-glass' composites (that is, tin-doped indium oxide (ITO) nanocrystals embedded in NbOx glass) via acid-catalysed condensation of polyniobate clusters. A combination of X-ray scattering and spectroscopic characterization with complementary simulations reveals that this strategy leads to a unique one-dimensional chain-like NbOx structure, which significantly enhances the electrochromic performance, compared to a typical three-dimensional NbOx network obtained from conventional high-temperature thermal processing. In addition, we show how self-assembled ITO-in-NbOx composite films can be successfully integrated into high-performance flexible electrochromic devices.

Optical absorption and small-polaron hopping in oxygen deficient and lithium-ion-intercalated amorphous titanium oxide films

Optical absorption in oxygen-deficient and Li+-ion inserted titanium oxide films was studied in the framework of small-polaron hopping. Non-stoichiometric TiOy films with 1.68 ≤ y ≤ 2.00 were deposited by reactive DC magnetron sputtering and were subjected to electrochemical intercalation of Li+-ions and charge-balancing electrons to obtain LixTiOy films with 0.12 ≤ x ≤ 0.34. Dispersion analysis was applied to calculate the complex dielectric function ε(ℏω) ≡ ε1(ℏω) + iε2(ℏω) from numerical inversion of optical transmittance and reflectance spectra; a superposition of Tauc-Lorentz and Lorentz oscillator models was used for this purpose. Data on ε2(ℏω) were employed to calculate the optical conductivity and fit this property to a small-polaron model for disordered systems with strong electron-phonon interaction and involving transitions near the Fermi level. The introduction of oxygen vacancies and/or Li+ insertion yielded band gap widening by ∼0.20–0.35 eV, and both processes induced similar low-energy optical absorption. The small-polaron-based analysis indicated increases in the Fermi level by ∼0.15–0.3 eV for sub-stoichiometric and/or Li+-inserted films. This suggests the existence of polaronic Ti3+ states in the lower part of the conduction band arising from transfer of electrons from oxygen vacancies and/or inserted Li+ species. The present article is a sequel to an earlier paper on oxygen-deficient and/or Li+-inserted amorphous WOy thin films and forms part of a comprehensive investigation of optical absorption in amorphous transition metal oxides with different valence states of the metallic ions.

Facile synthesis of amorphous Ni(OH)2 for high-performance supercapacitors via electrochemical assembly in a reverse micelle

The specific and unique amorphous nickel hydroxide nanostructures were synthesized via a simple and straightforward electrochemical way. By simply tuning the soft templates and applying a suitable current density, a specific and unique microstructure is obtained. The diameter of the Ni(OH)2 nano-sphere is greatly influenced by the current density, the sample obtained at 1 mA•cm−2 is about 200nm while the size of that assembled at 6 mA•cm−2 is less than 20nm. However, the Ni(OH)2 film prepared at the current density of 2 mA•cm−2 showed the best electrochemical performance for the highly porous surface morphology which contributed to the uniform morphology obtained at this current density. The maximum specific capacitance can be achieved as high as 1460F•g−1 in 2M NaOH at the scan rate of 2mV•s−1. Such impressive electrochemical properties can be ascribed to the highly porous microstructures and disorder of the amorphous phase which was considered for the better accommodation of repeated volume changes associated with doping-undoping process. The high specific capacitance and remarkable rate capability of amorphous transition metal oxides nanostructures show broad prospect for potential applications in energy conversion and storage devices.

Optically Transparent Flexible IGZO TFTs Fabricated with a Selective Wet-Etch Process

The emerging transparent thin-film transistor (TFTs) technology has recently received great attention in the active-matrix organic light-emitting diode (AMOLED) flexible displays. This TFT technology also has the potential to enable enhanced functionality in conventional large-area electronics applications ranging from smart windows and heads-up displays to flexible sensor arrays. During the past few years, amorphous transition-metal oxides have been suggested to be one of the best candidates in this application. Amorphous InGaZnO (IGZO) TFTs are the most promising transparent-metal oxide semiconductor due to its high optical transparency and field-effect mobility (μFE) in TFT devices compared to conventional amorphous silicon (a-Si:H). The amorphous structure is more suitable for large-area processes than other polycrystalline metal-oxide TFTs. These unique advantages have resulted in the rapid development of IGZO TFT technology. Given the relatively poor selectivity to common semiconductor etchants, IGZO TFTs are typically fabricated using lift-off processes to define the electrical contacts. However, the success of the emerging transition-metal oxide materials system for flexible electronics requires compatibility with conventional TFT microfabrication manufacturing processes. Flexible IGZO TFTs having 85% optical transparency in the visible regime were fabricated on polyethylene napthalate substrates using a selective wet etching process to eliminate the need for lift-off processing. The devices were processed directly onto the plastic platform at a maximum temperature of 150°C. The TFT structure consisted of a sputtered 100 nm aluminum doped zinc oxide (AZO) gate electrode, a 300nm SiNx gate dielectric deposited using plasma enhanced chemical vapor deposition (PECVD) method at 150ºC, and a 50nm IGZO layer deposited using RF sputtering at room temperature. A 150 nm AZO source/drain (S/D) electrodes were deposited by using RF sputtering at 150ºC and patterned wet etching process in diluted HCl solution having a selectivity of 11 between AZO and IGZO. The channel width and length of the studied TFTs were 200 and 100 µm, respectively. The fabricated TFTs exhibited a field-effect mobility of ~10 cm2/V.sec, threshold voltage of ~5.0 V, a sub-threshold swing of 0.8 V/decade, and an Ion/Ioff ratio of 107. No current crowding behavior was observed for the TFTs at the low drain-source voltage (VD) regime. The I-V characteristics showed excellent ohmic contacts can be fabricated using AZO S/D electrodes through a wet etching process and promises a more conventional approach to large-area flexible electronics fabrication. The highly selective etching process provides a means to fabricate IGZO based circuits with low processing complexity that will enable system-on-“plastic” integration of flexible transparent flexible displays. In addition, the effect of long-term electrical bias on the electrical stability of the devices will also be presented.

A Study of Li Based Solid State Battery with Nanoscale Transition Metal Oxide Coatings

1. Introduction – Solid state batteries overcome many of the essential drawbacks of liquid electrolyte batteries, such as liquid electrolyte and leakage issues [1]. Another major issue is the degradation of the Li based cathode from continuous cycling due to a change in its microstructure [2]. Nanoscale coatings are thought to prevent the degradation of the cathode by acting as a protective layer on its surface [3, 4]. This paper studies the performance of lithium cobalt oxide (LCO) based cathodes with and without protective coatings of Al2O3 and TiO2. 2. Fabrication procedures – Fabrication procedures entail the following stages: (i) Making the liquid lithium lanthanum tantalite (LLTO) electrolyte by a sol gel technique from its precursors and then vaporizing the liquid by exposing it to elevated temperature. The paste left behind is thoroughly dried and finely ground and is included as the solid state electrolyte. (ii) The cathode is made by thoroughly grinding lithium cobalt oxide (LCO) powders and then making slurry by mixing them thoroughly with PTFE binder, carbon powder, and alcohol. Pasted on to a stainless steel foil, this whole system acts as the cathode. (iii) The Atomic Layer Deposition (ALD) technique is used to deposit thin films (~6 nm) on the surface of the LCO cathodes to compare them with the pristine cathodes. (iv) The anode includes Li metal foil. Once this is complete, the three separate components are assembled into a coin cell. 3. Experimental steps and results – The characterization procedures are as follows:(i) XRD and SEM study to check the phase of the solid state LLTO electrolyte, pristine LCO cathode, and cathodes with Al2O3 and TiO2 coatings and the surface morphology.Figure 1 shows the XRD data of the LCO cathode with TiO2 deposition. The main Al2O3 peaks are seen and no TiO2 peaks are obtained, indicating the amorphous nature of the deposition.(ii) TEM and XPS analysis to study the nature and actual thickness of the coating. This will give an idea about the actual depth of the deposition as well as the nature of electronic bonding at the surface.From Figure 3 it may be seen that reduction potential shifts more to the right for the TiO2 coated cathode. The greater ΔV value exhibited by the coated cathode indicates greater energy needed for the reduction reaction owing to the deposition, and TiO2 has the highest ΔV value.Figure 2 represents the XPS data of the LCO cathode coated with Al2O3, showing clearly the Al 2s and 2p and the O 1s peaks.(iii) Cyclic voltammetry is performed within a voltage range of 3 V to 4.5 V and the applied current is 5 mA/s.(iv) Cyclical performance of the cell.Figure 4 shows that open circuit voltages are obtained of slightly less than 3 V for a current density of 10 mA/cm2, compared with 3.259 V and 3.312 V, respectively, for i = 30 mA/cm2 and 40 mA/cm2 for the LCO pristine cathode. 4. Conclusion – Amorphous transition metal oxide coatings have been tried out. XPS data have shown the formation of the thin film. A rightward shift in the reduction potential demonstrated by the CV data indicates greater energy necessary which is a measure of the protective nature of the coating. Future work includes studies of the cycling performance of the cell with the coated cathodes.

Stoichiometry determined exchange interactions in amorphous ternary transition metal oxides: Theory and experiment

Amorphous transition metal oxides exhibit exotic transport and magnetic properties, while the absence of periodic structure has long been a major obstacle for the understanding of their electronic structure and exchange interaction. In this paper, we have formulated a theoretical approach, which combines the melt-quench approach and the spin dynamic Monte-Carlo simulations, and based on it, we explored amorphous Co0.5Zn0.5O1−y ternary transition metal oxides. Our theoretical results reveal that the microstructure, the magnetic properties, and the exchange interactions of Co0.5Zn0.5O1−y are strongly determined by the oxygen stoichiometry. In the oxygen-deficient sample (y > 0), we have observed the long-range ferromagnetic spin ordering which is associated with the non-stoichiometric cobalt-rich region rather than metallic clusters. On the other hand, the microstructure of stoichiometric sample takes the form of continuous random networks, and no long-range ferromagnetism has been observed in it. Magnetization characterization of experimental synthesized Co0.61Zn0.39O1−y films verifies the relation between the spin ordering and the oxygen stoichiometry. Furthermore, the temperature dependence of electrical transport shows a typical feature of semiconductors, in agreement with our theoretical results.

Solar Water Splitting By Transition Metal Oxide – Silicon Photoanodes Prepared under Cathodic Electrochemical Conditions

Photo-assisted water splitting represents a pivotal strategy for realization of the hydrogen economy of the future. In this approach, the preparation of stable and efficient photoanodes is challenging due to the susceptibility of sensitive semiconductors to photocorrosion. Here, we demonstrate a metalorganic electrochemical approach by which amorphous transition metal oxides are formed on silicon under cathodic electrochemical conditions. This approach makes anti-corrosive interlayers potentially superfluous and realizes an oxide-silicon heterostructure with enhanced activity for the light-induced oxygen evolution reaction in alkaline electrolytes. Exemplifying analysis of electronic and optical properties is provided for a nickel oxide / silicon heterojunction. Interface engineering is demonstrated by formation of nanoscopic Au-Si heterocontacts acting as rectifying nanoemitters for light-induced holes toward the electrocatalytic nickel oxide layer.

Water Oxidation by Amorphous Cobalt‐Based Oxides: Volume Activity and Proton Transfer to Electrolyte Bases

Water oxidation in the neutral pH regime catalyzed by amorphous transition-metal oxides is of high interest in energy science. Crucial determinants of electrocatalytic activity were investigated for a cobalt-based oxide film electrodeposited at various thicknesses on inert electrodes. For water oxidation at low current densities, the turnover frequency (TOF) per cobalt ion of the bulk material stayed fully constant for variation of the thickness of the oxide film by a factor of 100 (from about 15 nm to 1.5 μm). Thickness variation changed neither the nanostructure of the outer film surface nor the atomic structure of the oxide catalyst significantly. These findings imply catalytic activity of the bulk hydrated oxide material. Nonclassical dependence on pH was observed. For buffered electrolytes with pKa values of the buffer base ranging from 4.7 (acetate) to 10.3 (hydrogen carbonate), the catalytic activity reflected the protonation state of the buffer base in the electrolyte solution directly and not the intrinsic catalytic properties of the oxide itself. It is proposed that catalysis of water oxidation occurs within the bulk hydrated oxide film at the margins of cobalt oxide fragments of molecular dimensions. At high current densities, the availability of a proton-accepting base at the catalyst-electrolyte interface controls the rate of water oxidation. The reported findings may be of general relevance for water oxidation catalyzed at moderate pH by amorphous transition-metal oxides.