Functional Transition Metal Oxide Research Articles

Oxygen vacancy induced conductivity in Sr n+1Ti n O3n+1/SrTiO3 heterostructures

SrTiO3-based heterostructures have attracted much attention due to the abundant properties compared to single components. Here, we fabricate oxide heterostructure of layed perovskite/perovskite Sr n+1Ti n O3n+1/SrTiO3 and investigate the n value-dependent and thickness-dependent conductivity. X-ray diffraction peaks and reflective high energy electron diffraction indicate good film quality. For films of n=6, the heterostructures are conductive, and the conductivity is better for thicker film. On the contrary, heterostructures with films of n=1, 2, 3, 4, ∞ are insulating. Conductive atomic force microscopy results and surface conductivity tests manifest that the oxygen vacancy induced conductive layer exists near the interface between film and substrate. This work provides feasible method to modulate the transport properties of functional transition metal oxides.

Facile synthesis of fine 1D VO2 and its supercapacitance as a binder free electrode

Functional transition metal oxides (TMOs) have gained significant attention for use as electrode materials in electrochemical energy storage devices because to the fascinating surface activities they provide. One dimensional (1D) VO2 is one of several TMOs whose behavior has not been thoroughly studied due to a lack of effective synthesis methods. Here, a novel chemical route is optimized to obtain fine 1D VO2 after a two-step hydrothermal process. The monoclinic structure of 1D VO2 with fine morphologies with diameters mostly in the range of 10–100 nm is confirmed by XRD, XPS, EDS, SEM, and HRTEM investigations. As a consequence, the best-in-class binder free electrode material for supercapacitance was achieved. A promising capacity of 615.8F/g at 2 A/g is achieved in 3 electrode system with 1 M Na2SO4 electrolyte. Energy storage device characteristics were tested by fabricating a 2-electrode symmetric supercapacitor to account a high energy density of 122.3 Wh/kg at 2160 W/kg close to sodium/potassium ion batteries.

Graphene-based nanocomposite cathodes architecture with palladium and α-MnO2 for high cycle life lithium-oxygen batteries

High-efficiency electrocatalysts of palladium and α-MnO2 nanowires supported on reduced graphene oxide (rGO) sheets are developed through an effective process to enhance the electrochemical performance of current lithium-oxygen batteries. Palladium is known as an oxygen evolution reaction (OER) electrocatalyst in Li–O2 cathode to reduce the charge overpotential and exhibit stable cycling performance. On the other hand, MnO2 is an attractive, functional transition metal oxide catalyst in Li–O2 batteries due to its low cost, high catalytic activity, and good oxygen reduction behavior. This study integrates the synergic effects of α-MnO2 nanowires and palladium nanoparticles by decorating on graphene sheets to improve cyclability and capacity to obtain highly efficient performance of Li–O2 cells. As-prepared rGO/Pd/α-MnO2 hybrid nanocomposite cathode indicates an initial discharge capacity of 7500 mA h g−1 and stable cycle life for 50 cycles at a limited capacity of 800 mA h g−1. As a result, although the polarization of the cell dramatically decreases and stable capacity behavior is observed with the contribution of α-MnO2 and Pd catalysts, the limited stable cycle life of 50 is obtained due to the consumption of lithium metal which causes total capacity failure after 60 cycles.

Effective medium theory to the description of plasmonic resonances: Role of Au and Ti nanoparticles embedded in MoO3 thin films

The growing interest in functional transition metal oxides for efficient energy consumption or in the bio-sensing process; indicates that is necessary to develop a new theoretical method that describes experiments. This article presents a new theoretical methodology to characterize molybdenum trioxide (MoO3) thin films doped with resonant gold – nanoparticles (Au – NPs) and non-resonant titanium – nanoparticles (Ti – NPs). The modulation of surface plasmon resonance (SPR) and the implications in the MoO3 transmittance spectrum is described by applying an effective medium theory. The transmittance modulation was modified by variating three parameters, the radius of the NPs, the concentration of the NPs as well as the variation of the MoO3 thin films thickness. It was found that the nanoparticles concentration is the most important parameter in the transmittance modulation. Additionally, the orthorhombic and monoclinic structure of MoO3 was studied, from which it was obtained that the monoclinic structure of the MoO3 doped with Au – NPs favors the reduction in the transmittance values in the visible region which is associated with the increase of the SPR signal. Similar analyses are performed for non-resonant nanoparticles such as Ti, where it was found that optical modulation is not as marked as the case of gold nanoparticles.

New approaches for achieving more perfect transition metal oxide thin films

This perspective considers the enormous promise of epitaxial functional transition metal oxide thin films for future applications in low power electronic and energy applications since they offer wide-ranging and highly tunable functionalities and multifunctionalities, unrivaled among other classes of materials. It also considers the great challenges that must be overcome for transition metal oxide thin films to meet what is needed in the application domain. These challenges arise from the presence of intrinsic defects and strain effects, which lead to extrinsic defects. Current conventional thin film deposition routes often cannot deliver the required perfection and performance. Since there is a strong link between the physical properties, defects and strain, routes to achieving more perfect materials need to be studied. Several emerging methods and modifications of current methods are presented and discussed. The reasons these methods better address the perfection challenge are considered and evaluated.

(LaCoO3)n/(SrCoO2.5)n superlattices: Tunable ferromagnetic insulator

Ferromagnetic insulators have great potential for spintronic applications. For such applications, it is essential to find materials with a robust and controllable ferromagnetic insulating phase. However, because ferromagnetism in functional transition-metal oxides is usually coupled to metallicity, ferromagnetic insulators are very rare and independent control of their magnetic and electrical properties is difficult. In this study, the electrical, magnetic, and optical properties of ${(\mathrm{LaCo}{\mathrm{O}}_{3})}_{n}/{(\mathrm{SrCo}{\mathrm{O}}_{2.5})}_{n}$ superlattice films are investigated for the manipulation of the ferromagnetic insulating phase. While the superlattices remain insulating irrespective of the periodicity $n$, the electronic structure and magnetic state vary drastically. Superlattices with large periodicities $n$ of 10 and 20 show a ferromagnetic transition at a critical temperature ${T}_{\mathrm{C}}$ of $\ensuremath{\sim}80\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. With decreasing periodicity and increasing interface density of the superlattices, system with $n=4$ becomes almost nonmagnetic, while in systems with $n=2$ and 1, a reentrant ferromagnetic phase is observed at ${T}_{\mathrm{C}}$ of $\ensuremath{\sim}180$ and $\ensuremath{\sim}225\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, respectively. Optical spectroscopy reveals that the fine control of the magnetic ground state is achieved by the modified electronic structure associated with the spin-state transition. Our results suggest an important design principle to create and manipulate the ferromagnetic insulating properties of Co-based oxide thin films.

Structure and magnetic properties of highly oriented LaBaCo2O5+δ films deposited on Si wafers with Pt/Ti buffer layer.

Fabrication of highly crystalline oxide films onto silicon wafers has long been a critical obstacle for integrating multi-functional oxides into silicon-based technology. Herein, Pt/Ti is used as a buffer layer for the integration of highly oriented crystalline LaBaCo2O5+δ (LBCO) thin films onto silicon via pulsed laser deposition. LBCO films are highly (00l) oriented with smooth and sharp LBCO/Pt interfaces. The highly oriented LBCO films exhibit a high magnetic transition temperature (TC) and large coercive field (HC) with superparamagnetism over those deposited on single crystal substrates. What is more, the metallic-like behavior with enhanced magnetoresistance is also observed. The opportunity of using a Pt/Ti buffer layer as the growth template opens an alternative route for integrating functional transition metal oxides with tunable magnetic properties into Si-based technology.

Oxygen defect-induced localized surface plasmon resonance at the WO3-x quantum dot/silver nanowire interface: SERS and photocatalysis.

Oxygen defects play a crucial role in a variety of functional transition metal oxides, ranging from photocatalytic materials to photoelectric devices. Tungsten oxide (WO3-x) is a type of transition metal oxide that has rich substoichiometric compositions and possesses oxygen defects. These oxygen defects determine the photon-electron interactions in the WO3-x structures. Therein, WO3-x quantum dots (QDs) exhibit fast carrier-transport for photon-electron interactions due to their strong quantum-size effects. Here, we report the use of non-stoichiometric WO3-x QDs, as a model material, in combination with silver nanowires (Ag NWs) to study photon-electron interactions on the nanoscale. We demonstrate that the incident photon-to-electron conversion efficiency can be increased by 8.5% and that the dye photodegradation performance was improved by 40% in a WO2.72 QD@Ag NW (WO2.72 QDs supported on AgNWs) composite compared to those of individual WO2.72 QDs under simulated AM 1.5G light. Furthermore, the WO3-x QD@Ag NW composite exhibits both photocatalytic activity and surface-enhanced Raman scattering (SERS) features, and the WO3-x QDs can be switched between a "photocatalytic state" and a "SERS state" by changing the stoichiometric ratio. The synergistic effects are ascribed to the "plasmonic state" of WO2.72 QDs upon light irradiation. This work provides new insight into the design of highly efficient transition metal oxide/plasmonic metal nanocomposites for photoelectric devices.

Potential‐Specific Structure at the Hematite–Electrolyte Interface

AbstractThe atomic‐scale structure of the interface between a transition metal oxide and aqueous electrolyte regulates the interfacial chemical reactions fundamental to (photo)electrochemical energy conversion and electrode degradation. Measurements that probe oxide–electrolyte interfaces in situ provide important details of ion and solvent arrangements, but atomically precise structural models do not exist for common oxide–electrolyte interfaces far from equilibrium. Using a novel cell, the structure of the hematite (α‐Fe2O3) ()–electrolyte interface is measured under controlled electrochemical bias using synchrotron crystal truncation rod X‐ray scattering. At increasingly cathodic potentials, charge‐compensating protonation of surface oxygen groups increases the coverage of specifically bound water while adjacent water layers displace outwardly and became disordered. Returning to open circuit potential leaves the surface in a persistent metastable state. Therefore, the flux of current and ions across the interface is regulated by multiple electrolyte layers whose specific structure and polarization change in response to the applied potential. The study reveals the complex environment underlying the simplified electrical double layer models used to interpret electrochemical measurements and emphasizes the importance of condition‐specific structural characterization for properly understanding catalytic processes at functional transition metal oxide–electrolyte interfaces.

Atomically Resolved Short-Range Order at the Nanoscale in the Ca-Mn-O System.

The elucidation of the reaction mechanisms involving redox processes in functional transition-metal oxides, which usually start in areas of very few nanometers in size, is yet a challenge to be satisfactorily achieved. Atomically resolved HAADF and EELS have provided both chemical and structural information at the nanoscale, which reveal the preservation of short-range cationic order in areas of 2-3 nm length as the driving force behind the reversibility of the Ca2Mn3O8-Ca2Mn3O5 redox process. Oxygen evolution is accommodated by cationic diffusion along the Ca and Mn layers of the cation-deficient Ca2Mn3O8 delafossite related structure, whereas Mn remains octahedrally coordinated.