Structure Of Transition Metal Oxides Research Articles

The electronic structure of transition metal oxides for oxygen evolution reaction

In this review article, we summarise the key electronic features of transition metal oxides that govern their OER catalytic properties, and how such electronic descriptors are applied for OER electrocatalysts design.

Construction of hierarchical structure of Co3O4 electrode based on electrospinning technique for supercapacitor

The design and synthesis of hierarchical micro-nano structures of transition metal oxides have played an essential role in the supercapacitor field. In this work, in situ three-dimensional construction of nanoporous cobalt oxide (Co3O4) has been derived from the metal-organic framework (MOF) distributed evenly in electrospun polyacrylonitrile nanofibers. Due to large specific surface area and network architectures, the as-synthesized Co3O4 electrode notably presents a high specific capacitance of 970 F/g at a current density of 1 A/g. Besides, the as-obtained electrode exhibits a high energy density of 54.6 Wh/kg at a power density of 360.6 W/kg and maintains a capacitance retention of 77.5% after 5000 cycles at 6 A/g. Therefore, this method paves a way to produce the nanoporous MOF-derived Co3O4 network architecture as advanced electrodes materials, which shows an application potential for the energy storage industry.

Universality in the electronic structure of 3d transition metal oxides

Electronic structure of strongly correlated transition metal oxides (TMOs) is a complex phenomenon due to competing interaction among the charge, spin, orbital and lattice degrees of freedom. Often individual compounds are examined to explain certain properties associated with these compounds or in rare cases few members of a family are investigated to define a particular trend exhibited by that family. Here, with the objective of generalization, we have investigated the electronic structure of three families of compounds, namely, highly symmetric cubic mono-oxides, symmetry-lowered spinels and less explored asymmetric olivine phosphates, through density functional calculations. From the results we have developed empirical hypotheses involving electron hopping, electron-lattice coupling, Hund's rule coupling, strong correlation and d-band filling. These hypotheses, classified through the point group symmetry of the transition metal - oxygen complexes, can be useful to understand and predict the electronic and magnetic structure of 3d TMOs.

Strong spin-orbit effects in transition metal oxides with tetrahedral coordination

To prove that spin-orbit coupling can play a relevant role in determining the magnetic structure of transition metal oxides with tetrahedral coordination, we investigate the d1 Mott insulator KOsO4, combining density functional theory calculations and the exact diagonalization approach. We find that the interplay between crystal field, strong spin-orbit coupling, electronic correlations and structural distortions brings the system towards an antiferromagnetic phase, characterized by a non-vanishing orbital angular momentum and anisotropy among the in-plane and the out-of-plane antiferromagnetic correlations. We also show that, due to the peculiar interplay between spin-orbit coupling, Hund's coupling and hopping connectivity the system is on the verge of developing short range ferromagnetic correlations marked by strong directionality.

Enhancing Catalysis through Substitute-Driven Redox Tuning

Electrocatalysts enable many energy conversion and storage technologies. Tuning the electronic structure of electrocatalytic transition metal oxides and complexes by introducing substitute metals—which possess a different electronegativity than the parent metals—presents a rational strategy for enhancing performance. Kuznetsov et al. show a unifying concept regarding how the inductive effects of substitute metals alter the molecular orbital structure of transition metal oxides and complexes. Such tuning leads to increased battery capacity and higher catalytic activity.

Two-step growth mechanism of supported Co3O4-based sea-urchin like hierarchical nanostructures

Supported 3D hierarchical nanostructures of transition metal oxides exhibit enhanced photocatalytic performances and long-term stability under working conditions. The growth mechanisms crucially determine their intimate structure, that is a key element to optimize their properties. We report on the formation mechanism of supported Co3O4 hierarchical sea urchin-like nanostructured catalyst, starting from Co-O-B layers deposited by Pulsed Laser Deposition (PLD). The particles deposited on the layer surface, that constitute the seeds for the urchin formation, have been investigated after separation from the underneath deposited layer, by X-ray diffraction, X-ray absorption spectroscopy and scanning electron microscopy. The comparison with PLD deposited layers without O and/or B indicates a crucial role of B for the urchin formation that (i) limits Co oxidation during the deposition process and (ii) induces a chemical reduction of Co, especially in the particle core, in the first step of air annealing (2 h, 500 °C). After 2 h heating Co oxidation proceeds and Co atoms outdiffuse from the Co fcc particle core likely through fast diffusion channel present in the shell and form Co3O4 nano-needles. The growth of nano-needles from the layer beneath the particles is prevented by a faster Co oxidation and a minimum fraction of metallic Co. This investigation shows how diffusion mechanisms and chemical effects can be effectively coupled to obtain hierarchical structures of transition metal oxides.

Universality and difference of electronic and spin transport properties at the irradiated oxide surface

The ion irradiation technique is utilized to modify the surface structure of transition metal oxides (TMOs), including SrTiO3 and TiO2. After low-energy Ar+ ion bombardment, oxygen vacancies are introduced into the TMO surface layer, and their resistance decreases with increasing irradiation time. For the two oxides, the temperature-dependent resistivity exhibits different conducting behaviors. Due to the generation of an electron-doped quasi-2D surface layer, the voltage signals of the inverse Rashba–Edelstein effect are extracted in the ion-irradiated TMO with ferromagnetic permalloy films, and the spin transport parameters are respectively calculated from the ferromagnetic resonance measurement data.

Novel synthesis and shape-dependent catalytic performance of Cu–Mn oxides for CO oxidation

Transition metal oxides with large specific surface area are attractive for high-activity catalysts, and hierarchical structures of transition metal oxides with porous feature possess the structural advantage in the transfer of gaseous reactant and product. In this work, porous Cu–Mn oxides with high surface area were successfully obtained through low-temperature coprecipitation method in alcohol/water solvent and then post-annealing. The addition of alcohol showed great influences on the shape and catalytic performances for CO oxidation. Dumbbell-like Cu–Mn oxide particles with splitting ends displayed high catalytic activity and a complete conversion of CO was achieved at 45°C, suggesting a shape-dependent catalytic activity. The oxidative activity was attributed to a combination of factors including specific surface area, active surface oxygen species and Mn(IV) cations. The results may supply a new thought to design high-performance Cu–Mn oxide catalysts.

Defect Engineering, Electronic Structure, and Catalytic Properties of Perovskite Oxide La0.5 Sr0.5 CoO3-δ.

The electronic structures of transition metal oxides play a crucial role in the physical and chemical properties of solid materials. Defect engineering is an efficient way to regulate the electronic structure and improve the performance of materials. Here, we develop a defect engineering route that is implemented by controlling the topochemical reactions between cobalt perovskite and urea to optimize the electronic structure of La0.5 Sr0.5 CoO3-δ (LSCO). Urea pyrolysis is able to increase the oxygen defect concentration and cause octahedral distortions. Furthermore, we can distinctly observe that the introduction of oxygen vacancies narrows the hybridization orbital between O 2p and Co 3d and optimizes the O p-band center near the Fermi level by X-ray absorption spectroscopy, which greatly improves the catalytic activity of CO oxidation and photocatalytic water splitting. These results highlight the relationship between oxygen defects, electronic structure, and catalytic activity of perovskite LSCO, and demonstrate a rational approach to defect design and reveal the importance of anion redox chemistry for the structures and properties of perovskite oxides.

Physics of Ultrathin Films and Heterostructures of Rare-Earth Nickelates

The electronic structure of transition metal oxides featuring correlated electrons can be rationalized within the Zaanen-Sawatzky-Allen framework. Following a brief description of the present paradigms of electronic behavior, we focus on the physics of rare-earth nickelates as an archetype of complexity emerging within the charge transfer regime. The intriguing prospect of realizing the physics of high-Tc cuprates through heterostructuring resulted in a massive endeavor to epitaxially stabilize these materials in ultrathin form. A plethora of new phenomena unfolded in such artificial structures due to the effect of epitaxial strain, quantum confinement, and interfacial charge transfer. Here we review the present status of artificial rare-earth nickelates in an effort to uncover the interconnection between the electronic and magnetic behavior and the underlying crystal structure. We conclude by discussing future directions to disentangle the puzzle regarding the origin of the metal-insulator transition, the role of oxygen holes, and the true nature of the antiferromagnetic spin configuration in the ultrathin limit.

Role of Metal-Lithium Oxide Interfaces in the Extra Lithium Capacity of Metal Oxide Lithium-Ion Battery Anode Materials

Lithium storage capacities beyond the stoichiometric limit of conversion have been reported for the transition metal oxides MnO, CoO, NiO, CuO, and beyond the alloying limit for the main group metal oxide SnO. This work examines the molecular mechanisms responsible for this extra capacity using first principles plane wave density functional theory with periodic boundary conditions. Energies of the optimized structures were employed to construct first discharge curves. Analysis of lithiated structures of transition metal oxides with moderate lithium content revealed the formation of two different phases within the electrode material, namely bulk metal and lithium oxide, Li2O. At higher lithium content, the interfacial region between the two phases was found to expand, giving rise to “free volume” for additional lithium storage. MnO had the highest free volume in the interfacial region for lithium storage, followed by CoO, NiO, CuO, and SnO. Of particular interest was CuO, which upon lithiation, formed linear and branched chains of Cu atoms that percolated throughout a large portion of the system. This behavior suggests that CuO may better maintain conductivity through the anode material for more cycles than the other transition metal oxides examined.

Semi-Metallic TiO2 Nanotubes: Growth, Properties and Applications

Self organized nanotubular structures of transition metal oxides grown on their metallic substrates, especially titanium, have attracted great scientific and technological interest due to the possibility to exploit their functional properties (such in photo-catalysis, electro-catalysis, solar energy conversion, DSSCs and as host for Li storage) in nanotubular morphology. In contrast to the conventional way to grow anodic TiO2 nanotubes, which is an optimized electrochemical treatment of Ti in fluoride containing electrolyte (for an overview see [1]) this work reports on the growth of nanotubes by so-called RBA (rapid breakdown anodization).[2] This nanotube growth-process is in comparison extremely fast (seconds instead of hours) and seems to be initiated by a pit. This leads to surface coatings consisting of dense packed bundles of nanotubes with very high aspect ratios (tube length up to 50 micrometer, tube diameter of ~40 nanometer). In addition, this method provides a new route for an insitu electrochemical doping during the growth of TiO2 nanotubular layers with various transition metals to adjust the electronic levels in the material which is important for charge transfer reactions through the semiconductor/electrolyte interface [3]. The second part of the presentation will demonstrate that alternatively the electronic and optical properties [4] of these TiO2 nanotubes can be significantly modified, showing semi-metallic behavior by a high temperature reduction treatment in acethylene or ammonia creating a suitable platform for highly conductive surface area electrodes which is important i.e. sensing, methanol oxidation or supercap applications [5]

Formation of nickel cobalt sulfide ball-in-ball hollow spheres with enhanced electrochemical pseudocapacitive properties.

While the synthesis of hollow structures of transition metal oxides is well established, it is extremely challenging to fabricate complex hollow structures for mixed transition metal sulfides. Here we report an anion exchange method to synthesize a complex ternary metal sulfides hollow structure, namely nickel cobalt sulfide ball-in-ball hollow spheres. Uniform nickel cobalt glycerate solid spheres are first synthesized as the precursor and subsequently chemically transformed into nickel cobalt sulfide ball-in-ball hollow spheres. When used as electrode materials for electrochemical capacitors, these nickel cobalt sulfide hollow spheres deliver a specific capacitance of 1,036 F g(-1) at a current density of 1.0 A g(-1). An asymmetric supercapacitor based on these ball-in-ball structures shows long-term cycling performance with a high energy density of 42.3 Wh kg(-1) at a power density of 476 W kg(-1), suggesting their potential application in high-performance electrochemical capacitors.

Yolk bishell Mn(x)Co(1-x)Fe2O4 hollow microspheres and their embedded form in carbon for highly reversible lithium storage.

The yolk-shell hollow structure of transition metal oxides has many applications in lithium-ion batteries and catalysis. However, it is still a big challenge to fabricate uniform hollow microspheres with the yolk bishell structure for mixed transition metal oxides and their supported or embedded forms in carbon microspheres with superior lithium storage properties. Here we report a new approach to the synthesis of manganese cobalt iron oxides/carbon (MnxCo1-xFe2O4 (0 ≤ x ≤ 1)) microspheres through carbonization of Mn(2+)Co(2+)Fe(3+)/carbonaceous microspheres in N2, which can be directly applied as high-performance anodes with a long cycle life for lithium storage. Furthermore, uniform hollow microspheres with a MnxCo1-xFe2O4 yolk bishell structure are obtained by annealing the above MnxCo1-xFe2O4/carbon microspheres in air. As demonstrated, these anodes exhibited a high reversible capacity of 498.3 mAh g(-1) even after 500 cycles for Mn0.5Co0.5Fe2O4/carbon microspheres and 774.6 mAh g(-1) over 100 cycles for Mn0.5Co0.5Fe2O4 yolk bishell hollow microspheres at the current density of 200 mA g(-1). The present strategy not only develops a high-performance anode material with long cycle life for lithium-ion batteries but also demonstrates a novel and feasible technique for designed synthesis of transition metal oxides yolk bishell hollow microspheres with various applications.

Studies of the Atomic and Crystalline Characteristics of Ceramic Oxide Nano Powders after Bio field Treatment

Transition metal oxides (TMOs) have been known for their extraordinary electrical and magnetic properties. In the present study, some transition metal oxides (Zinc oxide, iron oxide and copper oxide) which are widely used in the fabrication of electronic devices were selected and subjected to biofield treatment. The atomic and crystal structures of TMOs were carefully studied by Fourier transform infrared (FT-IR) spectroscopy and X-ray diffraction (XRD) studies. XRD analysis reveals that biofield treatment significantly changed the lattice strain in unit cells, crystallite sizes and densities in ceramics oxide powders. The computed molecular weight of the treated samples exhibited significant variation. FT-IR spectra indicated that biofield treatment has altered the metal-oxygen bond strength. Since biofield treatment significantly altered the crystallite size, lattice strain and bond strength, we postulate that electrical and magnetic properties in TMOs (transition metal oxides) can be modulated by biofield treatment.

Impact of lattice distortion and electron doping on α-MoO3 electronic structure.

Band structure of transition metal oxides plays a critical role in many applications such as photo-catalysis, photovoltaics, and electroluminescent devices. In this work we report findings that the band structure of MoO3 can be significantly altered by a distortion in the octahedral coordination structure. We discovered that, in addition to epitaxial type of structural strain, chemical force such as hydrogen inclusion can also cause extended lattice distortion. The lattice distortion in hydrogenated MoO3 led to a significant reduction of the energy gap, overshadowing the Moss-Burstein effect of band filling. Charge doping simulations revealed that filling of conduction band drives the lattice distortion. This suggests that any charge transfer or n-type electron doping could lead to lattice distortion and consequentially a reduction in energy gap.

Interfaces and Nanostructures of Functional Oxide Octahedral Framework Structures

In the past decade, the rich physics exhibited by solid interfaces combining octahedral framework structures of transition metal oxides has fascinated the materials science community. However, the behavior of these materials still elude the current understanding of classical semiconductor and metal epitaxy. The reason for that is rooted in the surprising versatility of linked coordination units to adapt to a dissimilar substrate and the strong sensitivity of correlated oxides to external perturbations. The confluence of atomic control in oxide thin film epitaxy, state of the art high spatial resolution characterization techniques, and electronic structure computations, has allowed in recent years to obtain first insights on the underlying microscopic mechanisms governing the epitaxy of these fascinating materials. Here, we shortly review these mechanisms and highlight their potential in the design of novel nanostructures with enhanced functionalities.

Ultrafast Anodic Growth of TiO2-Nanotubes with Semi-Metallic Properties

Self organized nanotubular structures of transition metal oxides grown on their metallic substrates, especially titanium [1], have attracted great scientific and technological interest due to the possibility to exploit their functional properties (such in photo-catalysis, electro-catalysis, solar energy conversion and as host for Li storage) in a nanotubular morphology. In contrast to the conventional way to grow anodic TiO2 nanotubes, which is an optimized electrochemical treatment of Ti in fluoride containing electrolyte (for an overview see [2]) this work reports on the growth of nanotubes by so-called RBA (rapid breakdown anodization).[3] This nanotube growth-process is in comparison extremely fast (seconds instead of hours) and leads to surface coatings consisting of dense packed bundles of nanotubes with very high aspect ratios (tube length up to 50 micrometer, tube diameter of ~40 nanometer). In addition, this method provides a new route for an insitu electrochemical doping during the growth of TiO2 nanotubular layers with various transition metals to adjust i.e. the photocatalytic activity [4]. The second part of the presentation will demonstrate that alternatively the electronic and optical properties [5] of these TiO2 nanotubes can be significantly modified, showing semi-metallic behavior by a high temperature reduction treatment in acethylene or ammonia creating a suitable platform for highly conductive surface area electrodes which is important i.e. sensing applications. [6] Literature: [1] V. Zwilling, M. Aucouturier, E. Darque-Ceretti, Electrochim. Acta, 45, 921 (1999).[2] S. Berger, R. Hahn, P. Roy, P. Schmuki Phys. Status Solidi B247, 10, 2424 (2010).[3] R. Hahn, J. M. Macak, P. Schmuki, Electrochem. Commun.,9, 947 (2007).[4] M. Stark, R. Hahn, M.S. Killian, P. Schmuki Catal. Sci. Technol., 3, 1765 (2013).[5] R. Hahn, F. Schmidt-Stein, J. Salonen, S. Thiemann, Y.Y. Song, J. Kunze,V.-P. Lehto, P Schmuki, Angewandte Chemie - International Edition 48 (39), 7236 (2009).[6] R. Hahn et al submitted

General Synthesis of Multi‐Shelled Mixed Metal Oxide Hollow Spheres with Superior Lithium Storage Properties

AbstractComplex hollow structures of transition metal oxides, especially mixed metal oxides, could be promising for different applications such as lithium ion batteries. However, it remains a great challenge to fabricate well‐defined hollow spheres with multiple shells for mixed transition metal oxides. Herein, we have developed a new “penetration–solidification–annealing” strategy which can realize the synthesis of various mixed metal oxide multi‐shelled hollow spheres. Importantly, it is found that multi‐shelled hollow spheres possess impressive lithium storage properties with both high specific capacity and excellent cycling stability. Specifically, the carbon‐coated CoMn2O4 triple‐shelled hollow spheres exhibit a specific capacity of 726.7 mA h g−1 and a nearly 100 % capacity retention after 200 cycles. The present general strategy could represent a milestone in design and synthesis of mixed metal oxide complex hollow spheres and their promising uses in different areas.

General synthesis of multi-shelled mixed metal oxide hollow spheres with superior lithium storage properties.

Complex hollow structures of transition metal oxides, especially mixed metal oxides, could be promising for different applications such as lithium ion batteries. However, it remains a great challenge to fabricate well-defined hollow spheres with multiple shells for mixed transition metal oxides. Herein, we have developed a new "penetration-solidification-annealing" strategy which can realize the synthesis of various mixed metal oxide multi-shelled hollow spheres. Importantly, it is found that multi-shelled hollow spheres possess impressive lithium storage properties with both high specific capacity and excellent cycling stability. Specifically, the carbon-coated CoMn2O4 triple-shelled hollow spheres exhibit a specific capacity of 726.7 mA h g(-1) and a nearly 100 % capacity retention after 200 cycles. The present general strategy could represent a milestone in design and synthesis of mixed metal oxide complex hollow spheres and their promising uses in different areas.