Properties Of Transition Metal Oxides Research Articles

Origin of the orbital polarization of Co2+ in La2CoTiO6 and ( LaCoO3)1+(LaTiO3)1 : A DFT+U and DFMT study

The unequal electronic occupation of localized orbitals (orbital polarization), and associated lowering of symmetry and degeneracy, play an important role in the properties of transition metal oxides. Here, we examine systematically the underlying origin of orbital polarization, taking as exemplar the 3$d$ manifold of Co$^{2+}$ in a variety of spin, orbital and structural phases in the double perovskite La$_2$CoTiO$_6$ and the (001) superlattice (LaCoO$_3$)$_1$+(LaTiO$_3$)$_1$ systems. Superlattices are of specific interest due to the large experimentally observed orbital polarization of their Co cations. Based on first principles calculations, we find that robust and observable orbital polarization requires symmetry reduction through the lattice structure; the role of local electronic interactions is to greatly enhance the orbital polarization.

Constrained density functional theory plus the Hubbard U correction approach for the electronic polaron mobility: A case study of TiO2

The formation and migration of polarons have important influences on physical and chemical properties of transition metal oxides. Density functional theory plus the Hubbard U correction (DFT+U) and constrained density functional theory (cDFT) are often used to obtain the transfer properties for small polarons. In this work we have implemented the cDFT plus the Hubbard U correction method in the projector augmented wave (PAW) framework, and applied it to study polaron transfer in the bulk phases of TiO2. We have confirmed that the parameter U can have significant impact on theoretical prediction of polaronic properties. It was found that using the Hubbard U calculated by the cDFT method with the same orbital projection as used in DFT+U, one can obtain theoretical prediction of polaronic properties of rutile and anatase phases of TiO2 in good agreement with experiment. This work indicates that the cDFT+U method with consistently evaluated U is a promising first-principles approach to polaronic properties of transition metal oxides without empirical input.

Tuning electromagnetic absorption properties of transition metal oxides by hydrogenation with nascent hydrogen

• Zn/HCl system was used for controlled hydrogenation of transition metal oxides. • Electrical conductivity and dielectric property correlate to hydrogenation degree. • The EMA performance of transition metal oxides can be tuned by proton hydrogenation. • Hydrogenation effect on conductivity was revealed by First-principle calculations. Transition metal oxides (TMO) show great potential in various applications, but are rarely be reported as good electromagnetic absorption (EMA) materials, owing to their low electrical conductivity. In this work, we present a modified Zn/HCl system for controlled hydrogenation of a series of TMO, and investigate the relationship between EMA property and electronic structure. It is discovered that the EMA performance of TMO is improved remarkably by this proton hydrogenation strategy. It is revealed that EMA property of TMO is closely associate with its hydrogenation degree, and can be efficiently tuned by hydrogenation controlling. First-principle calculations and experimental results suggest that the variation of band structure resulting in increased delocalized electrons, narrowed bandgap and elevated electrical conductivity is thought to be the main reason for the enhanced EMA performance of hydrogenated TMO. This research expands the application scope of hydrogenated TMO and provides a new idea for the design of high-performance EMA materials.

The electro-catalytic activity of nanosphere strontium doped zinc oxide with rGO layers screen-printed carbon electrode for the sensing of chloramphenicol

Sorting out several toxic threats related to the nominal dosage of drugs, chloramphenicol (CAP) was chosen as a growing emergence of detection. Combined properties of transition metal oxides and rare earth metals were utilized for detection of CAP. Herein, strontium doped zinc oxide with reduced graphene oxide (Sr-ZnO@rGO) was synthesized and employed as an electrochemical sensor on account of good electrical conductivity, large surface area and the electroreduction towards CAP with screen printed carbon electrode (SPCE) as a fabrication electrode. Besides, Sr-ZnO@rGO was synthesized via hydrothermal method that offers good morphologies, surface area, the temperature-controlled process and further implied. In addition, their structural, morphological and elemental composition were investigated with x-ray diffraction spectroscopy (XRD), x-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM) analysis. Electrochemical sensing analysis was carried out with cyclic voltammetry and linear sweep voltammetry techniques utilizing (Sr-ZnO@rGO/SPCE). The redox response of chloramphenicol was investigated with the limit of detection 0.131 μ mol L-1 with a linear range of 0.190 to 410.84 μ mol L-1 and 653.06 to 2847.14μ mol L-1 was estimated. The sensitivity was about 1.0554 μA μM-1cm−2. The better performance of Sr-ZnO@rGO/SPCE proved to be an efficient electrochemical sensor for the detection of chloramphenicol with the real sample analysis with milk and powdered milk samples.

Óxidos nanoestructurados de metales de transición con aplicaciones en catálsisis

Se presenta un resumen de los factores que afectan de manera significativa a las propiedades catalíticas de los óxidos de metales de transición. Entre estos factores podemos mencionar el tamaño de partícula, su forma, su composición química, la interacción metal-soporte o la interacción catalizador-reactivo. Recientes investigaciones han demostrado que es posible controlar algunos de estos factores mediante una síntesis bien planeada y ejecutada, una caracterización profunda, y estudios teóricos de los catalizadores, que permitan conocer las estructuras electrónicas y geométricas del catalizador (como átomos simples, nanoaglomerados o nanopartículas) y correlacionarlas con su comportamiento en diferentes reacciones.

Propagation Control of Octahedral Tilt in SrRuO3 via Artificial Heterostructuring.

Bonding geometry engineering of metal–oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilt of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin‐film lattice parameters. In this study, a method to selectively engineer the octahedral bonding geometries is proposed, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering is developed using atomically designed SrRuO3/SrTiO3 (SRO/STO) superlattices. In particular, the propagation of RuO6 octahedral tilt within the SRO layers having identical thicknesses is systematically controlled by varying the thickness of adjacent STO layers. This leads to a substantial modification in the electromagnetic properties of the SRO layer, significantly enhancing the magnetic moment of Ru. This approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties.

Negative thermal expansion of Ca2RuO4 with oxygen vacancies**Project supported by the National Natural Science Foundation of China (Grant Nos. 11874328 and 11574276). The SXRD experiments were performed at the BL02B2 and BL04B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; proposal Nos. 2019A1167, 2019A1095, and 2019A1340). We also acknowledge the help of Beamline Scientists Dr.

Oxygen vacancies have a profound effect on the magnetic, electronic, and transport properties of transition metal oxides but little is known about their effect on thermal expansion. Herein we report the effect of oxygen defects on the structure formation and thermal expansion properties of the layered perovskite Ca2RuO4 (CRO). It is shown that the CRO containing excess oxygen crystallizes in a metallic L-CRO phase without structure transition from 100 K to 500 K and displays a normal thermal expansion behavior, whereas those with oxygen vacancies adopt at room temperature an insulating S-CRO phase and exhibit an enormous negative thermal expansion (NTE) from 100 K to about 360 K, from where they undergo a structure transition to a high temperature metallic L-CRO phase. Compared to the L-CRO containing excess oxygen, the S-CRO structure has increasingly large orthorhombic strain and distinctive in-plane distortion upon cooling. The in-plane distortion of the RuO6 octahedra reaches a maximum across 260 K and then relaxes monotonically, providing a structure evidence for the appearance of an antiferromagnetic orbital ordering in the paramagnetic phase and the Ag phonon mode suppression and phase flip across the same temperature found recently. Both the L- and S-CRO display an antiferromagnetic ordering at about 150–110 K, with ferromagnetic ordering components at lower temperature. The NTE in S-CRO is a result of a complex interplay among the spin, orbital, and lattice.

Ordering of oxygen vacancies in LaBaCo2O6-δ epitaxial films

Structure change resulting from oxygen vacancies can strongly influence the chemical and physical properties of transition metal oxides, leading to new functionalities for novel electronic devices. In this work, by annealing LaBaCo2O6-δ epitaxial films in different atmospheres, e.g., O2, N2 and H2, we deduce that the concentration of oxygen vacancies and the stress in the film lattice imposed by the substrate play an important role in the ordering of oxygen vacancies in the LaBaCo2O6-δ epitaxial films. Moreover, the structure changes resulting from the vacancy ordering have a direct influence on the electrical and optical properties of the epitaxial films.

Multi-heteroatom-doped dual carbon-confined Fe3O4 nanospheres as high-capacity and long-life anode materials for lithium/sodium ion batteries

The lithium/sodium-ion storage properties of transition metal oxides often undergo startling volume variation and poor electrical conductivity. Herein, N, P and S doped dual carbon-confined Fe3O4 nanospheres (Fe3O4@C@G) are prepared by the multi-heteroatom-doped dual carbon-confined strategy. The first carbon layer results from multi-heteroatom-containing polymer derived N, P and S doped carbon to form Fe3O4@doped carbon core-shell nanostructure. And the second carbon layer results from the further encapsulated reduced graphene oxide (rGO) to form Fe3O4@doped carbon@graphene 3D architecture (Fe3O4@C@G). As expected, the resulting Fe3O4@C@G can be served as the universal anode materials towards lithium/sodium-ion batteries (LIBs/SIBs). Interestingly, Fe3O4@C@G delivers higher reversible capacity of 919 mAh g−1 at 0.1 A g−1 for LIBs. As for SIBs, Fe3O4@C@G also shows a high reversible capacity of 180 mAh g−1 after 600 cycles at 0.1 A g−1. Furthermore, the electrochemical reaction kinetics in LIBs/SIBs are investigated and Li+ full cells are also assembled to demonstrate its practical application.

Reversible phase switching between antiferromagnetic SrCoO2.5 and ferromagnetic SrCoO3−δ by a flexible solid-state electrolyte gate

Manipulation of oxygen vacancies (VO) in transition metal oxides (TMOs) can largely alter their physical and chemical properties, such as electrical conductivity, magnetic state, optical band-gap, and electrocatalytic reactivity. Many experimental and theoretical works have been conducted to study the formation/annihilation of VO and its corresponding effect on the properties in TMOs. In this paper, a solid-state approach to modulate the oxygen stoichiometry in high quality SrCoOx epitaxial thin films was demonstrated. Dependence of the magnetic and electrical conducting properties on VO was investigated. Room temperature reversible phase switching between brownmillerite antiferromagnetic insulating SrCoO2.5 and perovskite ferromagnetic metallic SrCoO3−δ was achieved by electric-field induced oxygen non-stoichiometry. This room temperature reversible phase switching indicates that SrCoOx thin films are a promising candidate for practical applications in resistive random access memory and spintronic devices.

SYNTHESIS OF MESOPOROUS NANOPARTICLES VIA MICROWAVE-ASSISTED METHOD FOR PHOTOCATALYTIC DEGRADATION OF PHENOL DERIVATIVES

Mesoporous transition metal oxides have gained attention widely since they possess both optical and electronic properties of transition metal oxides especially for photocatalytic degradation application. In this research work, mesoporous titania nanoparticles (MTN) and mesoporous zinc oxide nanoparticles (MZN) were successfully synthesized using microwave (MW)-assisted method to degrade phenol derivatives under visible light irradiation. The microwave sintering effect on the surface of these modified structures was studied to relate with their photocatalytic performance. The characterization results indicated that MW-assisted method was mainly contributed in generating Ti3+ site defects (TSD) and oxygen vacancies (OV) in MTN while for MZN contained only OV as one of the strategies in light-absorption modification for TiO2 and ZnO to enhance their photoactivity. MTN also showed the degradation of 2-chlorophenol was up to 97% while degradation of phenol by MZN was up to 87%.

Genetic algorithm prediction of pressure-induced multiferroicity in the perovskite PbCoO3

It was well known that pressure can have profound effects on the structural, magnetic, and electronic properties of transition metal oxides. However, it is a challenge both experimentally and theoretically to determine the exact structural and magnetic order of a complicated oxide system under pressure. To this end, we develop a global optimization approach based on the genetic algorithm by taking into account explicitly both structural and magnetic degrees of freedom. With this approach, we investigate the effect of pressure on the structure, magnetism, and charge ordering of the recently synthesized perovskite oxide $\mathrm{PbCo}{\mathrm{O}}_{3}$, which is an interesting system since both Pb and Co sites can change the valence states. At low pressure, we predict that $\mathrm{PbCo}{\mathrm{O}}_{3}$ can order in two multiferroic phases (i.e., $R3c$ and $Pna{2}_{1}$). Both phases are ferroelectric with a large electric polarization due to unusual coupling between the ferroelectric mode and other antipolar modes. Both phases display weak ferromagnetism. The ferroelectric polarization and weak ferromagnetism in these two phases are coupled to each other, suggesting that they may be used to realize the electric field control of magnetism. At higher pressure, the valence transition takes place from $\mathrm{P}{\mathrm{b}}^{4+}\mathrm{C}{\mathrm{o}}^{2+}{\mathrm{O}}_{3}$ to the $A$-site and $B$-site charge-ordering state $\mathrm{P}{\mathrm{b}}^{2+}{\mathrm{Pb}}_{3}^{4+}{\mathrm{Co}}_{2}^{2+}{\mathrm{Co}}_{2}^{3+}{\mathrm{O}}_{12}$ accompanied with an unusual enhancement of the band gap. In this work, we not only provide a general powerful method to determine the spin-orbital-charge ordering of intriguing correlated systems, but also predict that $\mathrm{PbCo}{\mathrm{O}}_{3}$ may be a multiferroic material under low pressure.

X‐Ray Writing of Metallic Conductivity and Oxygen Vacancies at Silicon/SrTiO3 Interfaces

AbstractTunable electronic properties of transition metal oxides and their interfaces offer remarkable functionalities for future devices. The interest in these materials has been boosted with the discovery of a 2D electron gas (2DEG) at SrTiO3 (STO)‐based interfaces. For the majority of these systems, oxygen vacancies play a crucial role in the emergence of interface conductivity, ferromagnetism, and high electron mobility. Despite its great importance, controlling the density and spatial distribution of oxygen vacancies in a dynamic way remains extremely challenging. Here, lithography‐like writing of a metallic state at the interface between SrTiO3 and amorphous Si using X‐ray irradiation is reported. Using a combination of transport techniques and in operando photoemission spectroscopy, it is revealed in real time that the X‐ray radiation induces transfer of oxygen across the interface leading to the on‐demand formation of oxygen vacancies and a 2DEG in STO. The formed 2DEG stays stable in ambient conditions as the interface oxygen vacancies are stabilized by the capping of Si. The study provides a fundamental understanding of X‐ray‐induced redox reactions at the SrTiO3‐based interfaces and in addition shows the potential of X‐ray radiation for patterning stabile conductive pathways for future oxide‐based electronic devices.

Ordering of Oxygen Vacancies in LaBaCo <sub>2</sub>O <sub>6-δ</sub> Epitaxial Films

Structure change resulting from oxygen vacancies can strongly influence the chemical and physical properties of transition metal oxide, leading to new functionalities for novel electronic devices. In this work, by annealing the LaBaCo2O6-δ epitaxial films in different atmospheres, e.g. O2, N2 and H2, we deduce that the concentration of oxygen vacancies and the stress in the film lattice imposed by the substrate play an important role in the ordering of oxygen vacancies in the LaBaCo2O6-δ epitaxial films. Moreover, the structure changes resulting from the vacancy ordering have a direct influence on the electrical and optical properties of the epitaxial films.

Electrochemical and Photoelectrochemical Properties of Nickel Oxide (NiO) With Nanostructured Morphology for Photoconversion Applications.

The cost-effective production of chemicals in electrolytic cells and the conversion of the radiation energy into electrical energy in photoelectrochemical cells (PECs) require the use of electrodes with large surface area, which possess either electrocatalytic or photoelectrocatalytic properties. In this context nanostructured semiconductors are electrodic materials of great relevance because of the possibility of varying their photoelectrocatalytic properties in a controlled fashion via doping, dye-sensitization or modification of the conditions of deposition. Among semiconductors for electrolysers and PECs the class of the transition metal oxides (TMOs) with a particular focus on NiO interests for the chemical-physical inertness in ambient conditions and the intrinsic electroactivity in the solid state. The latter aspect implies the existence of capacitive properties in TMO and NiO electrodes which thus act as charge storage systems. After a comparative analysis of the (photo)electrochemical properties of nanostructured TMO electrodes in the configuration of thin film the use of NiO and analogs for the specific applications of water photoelectrolysis and, secondly, photoelectrochemical conversion of carbon dioxide will be discussed.

Engineering Transport in Manganites by Tuning Local Nonstoichiometry in Grain Boundaries.

Interface-dominated materials such as nanocrystalline thin films have emerged as an enthralling class of materials able to engineer functional properties of transition metal oxides widely used in energy and information technologies. In particular, it has been proven that strain-induced defects in grain boundaries of manganites deeply impact their functional properties by boosting their oxygen mass transport while abating their electronic and magnetic order. In this work, the origin of these dramatic changes is correlated for the first time with strong modifications of the anionic and cationic composition in the vicinity of strained grain boundary regions. We are also able to alter the grain boundary composition by tuning the overall cationic content in the films, which represents a new and powerful tool, beyond the classical space charge layer effect, for engineering electronic and mass transport properties of metal oxide thin films useful for a collection of relevant solid-state devices.

Nanoscale oxygen ion dynamics in SrFeO2.5+δ epitaxial thin films

A variety of functional properties in transition metal oxides are often underpinned by oxygen vacancies. While the oxygen vacancy concentration and arrangements are well-known to have strong influence on the physical properties of oxides, the oxygen dynamics in oxides—including oxygen ion incorporation and movements during redox reactions—remain elusive. Performing conductive AFM studies of epitaxial thin films of oxygen-deficient SrFeO2.5 treated by air-annealing at various temperatures, we observe oxidation-induced enhancement of local electronic conduction on the higher terraces near the outer step edges at which oxygen ions are preferably incorporated and diffuse into the films. We also show that the local conduction can be reversibly controlled by electric-field-induced redox reactions at room temperature. These results highlight the importance of the nanoscale oxygen dynamics in redox reactions in SrFeO2.5 films.

Promotional Effect of Gold on the WGS Activity of Alumina-Supported Copper-Manganese Mixed Oxides

The water-gas shift (WGS) reaction is a well-known industrial process used for the production of hydrogen. During the last few decades, it has attracted renewed attention due to the need for high-purity hydrogen for fuel-cell processing systems. The aim of the present study was to develop a cost-effective and catalytically efficient formulation that combined the advantageous properties of transition metal oxides and gold nanoparticles. Alumina-supported copper- manganese mixed oxides were prepared by wet impregnation. The deposition-precipitation method was used for the synthesis of gold catalysts. The effect of the Cu:Mn molar ratio and the role of Au addition on the WGS reaction’s performance was evaluated. Considerable emphasis was put on the characterization of the as-prepared and WGS-tested samples by means of a number of physicochemical methods (X-ray powder diffraction, high-resolution transmission electron microscopy, electron paramagnetic resonance, X-ray photoelectron spectroscopy, and temperature-programmed reduction) in order to explain the relationship between the structure and the reductive and WGS behavior. Catalytic tests revealed the promotional effect of gold addition. The best performance of the gold-promoted sample with a higher Cu content, i.e., a Cu:Mn molar ratio of 2:1 might be related to the beneficial role of Au on the spinel decomposition and the highly dispersed copper particle formation during the reaction, thus, ensuring the presence of two highly dispersed active metallic phases. High-surface-area alumina that was modified with a surface fraction of Cu–Mn mixed oxides favored the stabilization of finely dispersed gold particles. These new catalytic systems are very promising for practical applications due to their economic viability because the composition mainly includes alumina (80%).

Atomic Level Engineering of Structural and Functional Properties of Transition Metal Oxides

Metal-oxygen bonds are responsible for a broad spectrum of functional properties of transition-metal oxides, and engineering the bonds is a key for exploring their properties. Artificial oxide heterostructures with chemically abrupt interfaces have provided a platform for engineering bonding geometries that could lead to emergent phenomena not seen in bulk oxides. In this paper, we demonstrate that interfacially engineering oxygen displacement is a good route for exploring structural and functional properties of oxides. Our high-resolution annular bright-field scanning transmission electron microscopy observations revealed that interfacial oxygen displacement determines metal-oxygen bond angles in entire heterostructures and the magnitude of the displacements can be adjusted by controlling propagations of the oxygen octahedral rotations across the heterointerface. We also show that by heterostructuring an itinerant ferromagnet SrRuO3 with Ca0.5Sr0.5TiO3 (0–4 monolayers thick) grown on a GdScO3 substrate, structural phase and magnetic anisotropy of SrRuO3 can be controlled through the interfacial engineering of its oxygen displacement.

Kinetic energy classification and smoothing for compact B-spline basis sets in quantum Monte Carlo.

Quantum Monte Carlo calculations of defect properties of transition metal oxides have become feasible in recent years due to increases in computing power. As the system size has grown, availability of on-node memory has become a limiting factor. Saving memory while minimizing computational cost is now a priority. The main growth in memory demand stems from the B-spline representation of the single particle orbitals, especially for heavier elements such as transition metals where semi-core states are present. Despite the associated memory costs, splines are computationally efficient. In this work, we explore alternatives to reduce the memory usage of splined orbitals without significantly affecting numerical fidelity or computational efficiency. We make use of the kinetic energy operator to both classify and smooth the occupied set of orbitals prior to splining. By using a partitioning scheme based on the per-orbital kinetic energy distributions, we show that memory savings of about 50% is possible for select transition metal oxide systems. For production supercells of practical interest, our scheme incurs a performance penalty of less than 5%.