Oxide Vacancies Research Articles

Tuning the local electronic structure of oxygen vacancies over copper-doped zinc oxide for efficient CO2 electroreduction

Oxygen vacancies in metal oxides can serve as electron trap centers to capture CO2 and lower energy barriers for the electrochemical CO2 reduction reaction (CO2RR). Under aqueous electrolytes, however, such charge-enriched active sites can be occupied by adsorbed hydrogen (H∗) and lose their effectiveness for the CO2RR. Here, we develop an efficient catalyst consisting of Cu-doped, defect-rich ZnO (Cu–ZnO) for the CO2RR, which exhibits enhanced CO Faradaic efficiency and current density compared to pristine ZnO. The introduced Cu dopants simultaneously stabilize neighboring oxygen vacancies and modulate their local electronic structure, achieving inhibition of hydrogen evolution and acceleration of the CO2RR. In a flow cell test, a current density of more than 45 ​mA ​cm−2 and a CO Faradaic efficiency of > 80% is obtained for a Cu–ZnO electrocatalyst in the wide potential range of −0.76 ​V to −1.06 ​V vs. Reversible Hydrogen Electrode (RHE). This work opens up great opportunities for dopant-modulated metal oxide catalysts for the CO2RR.

Tailoring Surface Oxygen Vacancies in Tungsten Oxides for Surface Plasmon Resonance-Enhanced Near-Infrared Photoreduction of Cr(VI)

Tailoring Surface Oxygen Vacancies in Tungsten Oxides for Surface Plasmon Resonance-Enhanced Near-Infrared Photoreduction of Cr(VI)

Cationic Interstitials: An Overlooked Ionic Defect in Memristors.

Metal oxide-based memristors are promising candidates for breaking through the limitations in data storage density and transmission efficiency in traditional von Neumann systems, owing to their great potential in multi-state data storage and achievement of the in-memory neuromorphic computing paradigm. Currently, the resistive switching behavior of those is mainly ascribed to the formation and rupture of conductive filaments or paths formed by the migration of cations from electrodes or oxygen vacancies in oxides. However, due to the relatively low stability and endurance of the cations from electrodes, and the high mobility and weak immunity of oxygen vacancies, intermediate resistance states can be hardly retained for multilevel or synaptic resistive switching. Herein, we reviewed the memristors based on cationic interstitials which have been overlooked in achieving digital or analog resistive switching processes. Both theoretical calculations and experimental works have been surveyed, which may provide reference and inspiration for the rational design of multifunctional memristors, and will promote the increments in the memristor fabrications.

(Keynote) Roles of Oxygen Vacancies in Metal Oxide Photocatalysts and Electrocatalysts

This talk concerns with the roles of oxygen vacancies in various metal oxide photocatalysts and electrocatalysts. A low level of oxygen vacancies in titanium oxide can induce shallow and deep sub-band gap Ti(III) states below the conduction band, which increases light absorption but not enhance photocatalytic activity. A high level of oxygen vacancies in metal oxides may generate an abundance of states near Fermi level and induce surface plasmon resonance.

Revealing the surface structure-performance relationship of interface-engineered NiFe alloys for oxygen evolution reaction

NiFe alloys are among the most promising electrocatalysts for oxygen evolution reaction (OER). However, a comprehensive study is yet to be done to reveal the surface structure-performance relationship of NiFe alloys. In particular, the role of the ultrathin surface oxide layer, which is unavoidable for pure NiFe alloys, is always neglected. Herein, a series of NiFe alloys with different Ni/Fe ratios are fabricated. It is found that different Ni/Fe ratios lead to significant differences in surface composition and structure of the NiFe alloys, and thus affect their catalytic performance. Then, the oxide/metal interface of the Ni4Fe1 alloy is tailored by adjusting the hydrogenation temperature to further understand the surface structure-activity relationship, and the optimal OER performance is achieved at the oxide/metal interfaces that have suitable surface Fe/Ni ratio and an appropriate amount of oxygen vacancies. In-situ Raman characterization shows that the Ni4Fe1 alloy with well-tailored oxide/metal interface facilitates the formation of active species. Density functional theory calculations demonstrate that the ultrathin surface oxide layers are responsible for the high catalytic activity of the NiFe alloys, and that the quantity of oxygen vacancies in the surface oxides affects the adsorption energy of O* and thus to a great extent determines the catalytic activity.

Identification of Active Sites in HCHO Oxidation over TiO2-Supported Pt Catalysts

The cooperative catalysis between noble metals and oxygen vacancies in reducible oxides plays an important role in many heterogeneous catalytic reactions, but definite identification of active sites remains challenging. Herein, we report an experimental identification of active sites in HCHO oxidation over TiO2-supported Pt catalysts by means of microscopic and spectroscopic techniques. Pt/TiO2–x catalysts with tunable oxygen vacancy concentrations were designed and fabricated based on hydrogen spillover. A combination of aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy allowed us to directly observe the spatial distribution of oxygen vacancies and probe its correlation with the catalytic activity. In situ diffuse reflectance infrared Fourier transform spectroscopy in combination with the online gas chromatography technique was employed to investigate the functionalities of Pt and oxygen vacancies in HCHO oxidation. Our study revealed that HCHO oxidation occurred at the Pt/TiO2–x interface via a cooperative mechanism: oxygen vacancies served as active sites for chemically absorbing HCHO molecules and the adjacent Pt site dissociatively activated O2.

Structure and the enhanced ferromagnetism in single phase Sr4Fe5CoO13-δ ceramic

Oxides with superstructure have attracted special attention for their great tolerance in structure and composition regulation, which enables them to have great potential in energy conversion devices, electromagnetic regulation devices, spintronic devices, and so on. Here, we successfully prepared a superstructure oxide Sr4Fe5CoO13-δ via combustion method. X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectra (XAS) analysis suggest that Co dopant could improve the amount of oxygen vacancies in oxides. Unlike the thermal activated electrical conducting behavior of Sr4Fe6O13-δ, Sr4Fe5CoO13-δ demonstrates an electrical behavior in line with the Mott's variable range hopping (VRH) model at low temperatures, implying the localization of electrons. Sr4Fe5CoO13-δ shows a ferromagnetic property with a high Curie temperature of 771 K, which should be attributed to the new formed Dzyaloshinskii-Moriya (D-M) interaction of Fe–O–Co, the increased lattice distortion and the increased amount of oxygen vacancies induced by Co dopant. These properties render it a broadened working temperature range and may contribute to explore high temperature spintronic devices.

NiCo2O4 nanoparticles anchored on reduced graphene oxide with enhanced catalytic activity towards the reduction of p-Nitrophenol in water

We report a simple two-step method for the synthesis of reduced graphene oxide supported NiCo2O4 nanoparticles (RGNCO) as a potential active catalyst for the reduction of p-nitrophenol with NaBH4. Various characterization techniques were employed to investigate the morphology, structure and surface composition of the catalyst. The results confirm successful anchoring of NiCo2O4 on the graphene sheets and suggests the presence of oxygen vacancies. The as prepared RGNCO catalyst exhibited good stability and excellent catalytic property with an activity factor of 356.5 min−1 g−1, which is greater than the highest value reported in literature for NiCo2O4 based catalysts. The improved catalytic performance is due to the synergistic effect of reduced graphene oxide and oxygen vacancy rich NiCo2O4 nanoparticles. The effect of different reaction parameters like temperature, quantity of catalyst, concentration of reactants, and presence of different anions on the catalytic property were investigated in detail. The surface catalysed p-nitrophenol reduction using RGNCO catalyst was successfully modelled using the Langmuir-Hinshelwood mechanism. The ability of the RGNCO catalyst to reduce a relatively large volume of PNP solution (20 ml) with an activity factor comparable to that of noble metal-based catalysts signifies the novelty of the material and makes it a potential candidate for practical environmental catalytic applications.

On the Interplay Between Oxygen Vacancies and Small Polarons in Manganese Iron Spinel Oxides.

Ternary spinel oxides are promising materials due to their potentially versatile properties resulting from the disorder inherent in their crystal structure. To fully unlock the potential of these materials, a deeper understanding of their electronic structures, both as pristine and defective crystals, is required. In the present work, we investigate the effects of oxygen vacancies on the electronic structure and charge transport properties of the ternary spinel oxide Mn x Fe3-x O4, modeled on epitaxial thin films of the material, using density functional theory + U (DFT + U). The formation energy of a single oxygen vacancy in the spinel cell is found to be large and unaffected by changes in stoichiometry, in agreement with experimental results. We find that the immediate vicinity of the vacancy has a marked impact on the formation energy. In particular, Mn cations are found to be preferred over Fe as sites for charge localization around the vacancy. Finally, we examine the charge transport in the defective cell using the formalism of Marcus theory and find that the activation barrier for electron small-polaron hopping between sites not adjacent to the vacancy is significantly increased, with a large driving force toward sites that reside on the same (001) plane as the vacancy. Hence, vacancies delay charge transport by increasing the activation barrier, attributed to a rearrangement of vacancy-released charge on the cations immediately neighboring the vacancy site. These results highlight the impact of oxygen vacancies on charge transport in spinel oxides.

Oxygen vacancies activated porous MnO/graphene submicron needle arrays for high-capacity lithium-ion batteries

Engineering oxygen vacancies (Ovs) in submicron transition metal oxides (TMOs) is a promising way to fabricate high-capacity anodes for lithium-ion batteries (LIBs). Herein, oxygen vacancies activated porous MnO/graphene submicron needle arrays (Ovs-MnO/G NAs) are directly grown on flexible nanoporous Cu–Mn substrate by chemical vapour deposition and hydrogen etching method, and their lithium-ion diffusion and storage mechanisms and morphology evolution are systematically investigated at submicron scale. It can be concluded that these modification strategies synergistically guarantee the fast charge transfer, effective lithium-ion diffusion and storage behaviour, and electrode structural stability. Thus, the optimal Ovs-MnO/G NAs electrode deliver a high reversible capacity of 7.7 mAh cm−2 after 100 cycles at 0.2 mA cm−2 and outstanding cycling performance of 6.12 mAh cm−2 after 500 cycles at 1.0 mA cm−2. Furthermore, the Ovs-MnO/G NAs//NCM 523 full cells also achieve high average capacities of 6.74 mAh cm−2 and cycling stability of 3.88 mAh cm−2 after 100 cycles at 0.2C, indicating its practical application value.

Modeling of electrochemical oxide film growth-a PDM refinement

The Point Defect Model (PDM) is known for over 40 years and has brought deeper insight to the understanding of passivity. During the last decades it has seen several changes and refinements, and it has been widely used to analyze growth kinetics of different alloys. Nevertheless, the model has been based on still unconfirmed assumptions, as constant and potential independent electric field strength. To overcome this limitation, we introduce a Refined PDM (R-PDM) in which we replace those assumptions by using additional equations for charge distribution including new physically valid boundary conditions based on considering finite dimensions for the defects by introduction of two defect layer at the film boundaries and by calculating the potential drop at the surface of the film towards the solution over the compact double layer. The calculations by the R-PDM show that the original PDM assumptions are only valid for very specific parameter combinations of oxide film growth and vacancies transport and cannot generally be taken for granted. We believe our findings of electric field and potential drop dependency on the external potential to pave the way for a more realistic description of passive layer formation.

Effect of the oxidizing capacity of ceria-based support on the conversion of methane to syngas

Ceria-based solid solutions have been proposed as catalytic supports for the conversion of methane to syngas. Control of oxygen vacancies in vacancy-rich oxides represents a promising way to stable catalysts with improved activity.

Oxide Ion and Proton Conductivity in a Family of Highly Oxygen-Deficient Perovskite Derivatives.

Functional oxides showing high ionic conductivity have many important technological applications. We report oxide ion and proton conductivity in a family of perovskite-related compounds of the general formula A3OhTd2O7.5, where Oh is an octahedrally coordinated metal ion and Td is a tetrahedrally coordinated metal ion. The high tetrahedral content in these ABO2.5 compositions relative to that in the perovskite ABO3 or brownmillerite A2B2O5 structures leads to tetrahedra with only three of their four vertices connected in the polyhedral framework, imparting a potential low-energy mechanism for O2- migration. The low- and high-temperature average and local structures of Ba3YGa2O7 (P2/c, a = 7.94820(5) Å, b = 5.96986(4) Å, c = 18.4641(1) Å, and β = 91.2927(5) ° at 22 °C) were determined by Rietveld and neutron pair distribution function (PDF) analysis, and a phase transition to a high-temperature P1121/a structure (a = 12.0602(1) Å, b = 9.8282(2) Å, c = 8.04982(6) Å, and γ = 107.844(3)° at 1000 °C) involving the migration of O2- ions was identified. Ionic conductivities of Ba3YGa2O7.5 and compositions substituted to introduce additional oxide vacancies and interstitials are reported. Most phases show proton conductivity at lower temperatures and oxide ion conductivity at high temperatures, with Ba3YGa2O7.5 retaining proton conductivity at high temperatures. Ba2.9La0.1YGa2O7.55 and Ba3YGa1.9Ti0.1O7.55 appear to be dominant oxide ion conductors, with conductivities an order of magnitude higher than that of the parent compound.

Ordered Oxygen Vacancies in the Lithium-Rich Oxide Li4CuSbO5.5, a Triclinic Structure Type Derived from the Cubic Rocksalt Structure.

Li-rich rocksalt oxides are promising candidates as high-energy density cathode materials for next-generation Li-ion batteries because they present extremely diverse structures and compositions. Most reported materials in this family contain as many cations as anions, a characteristic of the ideal cubic closed-packed rocksalt composition. In this work, a new rocksalt-derived structure type is stabilized by selecting divalent Cu and pentavalent Sb cations to favor the formation of oxygen vacancies during synthesis. The structure and composition of the oxygen-deficient Li4CuSbO5.5□0.5 phase is characterized by combining X-ray and neutron diffraction, ICP-OES, XAS, and magnetometry measurements. The ordering of cations and oxygen vacancies is discussed in comparison with the related Li2CuO2□1 and Li5SbO5□1 phases. The electrochemical properties of this material are presented, with only 0.55 Li+ extracted upon oxidation, corresponding to a limited utilization of cationic and/or anionic redox, whereas more than 2 Li+ ions can be reversibly inserted upon reduction to 1 V vs Li+/Li, a large capacity attributed to a conversion reaction and the reduction of Cu2+ to Cu0. Control of the formation of oxygen vacancies in Li-rich rocksalt oxides by selecting appropriate cations and synthesis conditions affords a new route for tuning the electrochemical properties of cathode materials for Li-ion batteries. Furthermore, the development of material models of the required level of detail to predict phase diagrams and electrochemical properties, including oxygen release in Li-rich rocksalt oxides, still relies on the accurate prediction of crystal structures. Experimental identification of new accessible structure types stabilized by oxygen vacancies represents a valuable step forward in the development of predictive models.

Conductivities and transport properties of Ca(Zr/Hf)0.9Sc0.1O2.95

Doped CaZrO3 proton conductors are promising materials for electrochemical hydrogen sensors, which have already been used in commercialized hydrogen sensors for aluminum melts. Hafnium and Zirconium possess similar properties but their influences on conductive properties of CaZrO3 proton conductors have not been well investigated. In this study, CaZr0.9Sc0.1O2.95 and CaHf0.9Sc0.1O2.95 were prepared by solid-state reaction, and their conductivities and transport properties were systematically investigated by defect equilibria model. Total conductivities of CaZr0.9Sc0.1O2.95 and CaHf0.9Sc0.1O2.95 reached 4.70 × 10−5−1.69 × 10−3 S·cm−1 and 4.83 × 10−6−9.71 × 10−4 S·cm−1 under humid air and 400 °C−800 °C. Their total standard molar hydration enthalpies were estimated to −59.1 kJ/mol and −56.9 kJ/mol, respectively. Conductivities of grain interiors were higher than those of total conductivities, and activation energies of protons were lower than oxide vacancies and holes. Transport numbers showed protonic conduction of Ca(Zr/Hf)0.9Sc0.1O2.95 to always dominate under humid air at 400 °C−700 °C. Protonic transport numbers of CaZr0.9Sc0.1O2.95 and CaHf0.9Sc0.1O2.95 were estimated to 0.41 and 0.44 at 800 °C, respectively. Meanwhile, protonic transport number of grain interiors was higher than that of total sample. Therefore, grain interior could block the transfer of oxide vacancies and holes. In sum, these findings look promising for application on electrochemical hydrogen sensors.

Al-Modified CuO/Cu2O for High-Temperature Thermochemical Energy Storage: from Reaction Performance to Modification Mechanism.

Next-generation concentrated solar power plants with high-temperature energy storage requirements stimulate the pursuit of advanced thermochemical energy storage materials. Copper oxide emerges as an attractive option with advantages of high energy density and low cost. But its easy sinterability limits its reversibility and cyclic stability performance. In this work, aluminum-doped copper oxides are synthesized and evaluated via thermogravimetric analysis. The reversibility of Cu-Al oxides reaches 99.5% in the first redox cycle and maintains 81.1% of the initial capacity after 120 cycles. The Al element can modify the CuO particle surface in the form of CuAl2O4, which separates the copper oxide particles from each other during redox cycles to avoid agglomeration and participates in the redox reaction. Through DFT analysis, the introduction of Al is found to increase the formation energy of copper vacancies in copper oxides, which helps avoid the sintering problem and thus improves the oxidation rate. This study provides a generalizable operational mechanism of element doping and can serve as a guideline for the optimization of high-performance materials in thermochemical energy storage.

Oxygen Vacancy Injection as a Pathway to Enhancing Electromechanical Response in Ferroelectrics.

Since their discovery in late 1940s, perovskite ferroelectric materials have become one of the central objects of condensed matter physics and materials science due to the broad spectrum of functional behaviors they exhibit, including electro-optical phenomena and strong electromechanical coupling. In such disordered materials, the static properties of defects such as oxygen vacancies are well explored but the dynamic effects are less understood. In this work, the first observation of enhanced electromechanical response in BaTiO3 thin films is reported driven via dynamic local oxygen vacancy control in piezoresponse force microscopy (PFM). A persistence in peizoelectricity past the bulk Curie temperature and an enhanced electromechanical response due to a created internal electric field that further enhances the intrinsic electrostriction are explicitly demonstrated. The findings are supported by a series of temperature dependent band excitation PFM in ultrahigh vacuum and a combination of modeling techniques including finite element modeling, reactive force field, and density functional theory. This study shows the pivotal role that dynamics of vacancies in complex oxides can play in determining functional properties and thus provides a new route toward- achieving enhanced ferroic response with higher functional temperature windows in ferroelectrics and other ferroic materials.

The role of oxygen vacancies in metal oxides for rechargeable ion batteries

Rechargeable ion batteries are one of the most reliable energy storage technologies for the applications ranging from small portable devices and electric vehicles to renewable energy integration and large-scale stationary energy storage. In the roadmap of developing and understanding new electrode materials for rechargeable ion batteries, oxygen vacancies, known as defects in metal oxides, have shown a high impact on the final electrochemical performance of the oxides. The present review aims to summarise the synthesis methods and characterisation techniques of oxygen vacancies as well as some of the most recent and exciting progress made to understand the role of oxygen vacancies in the electrochemical performance of Li-, Na-, K- and Zn-ion batteries. This review discusses not only the role of oxygen vacancies directly in electrode materials and indirectly in the coating layers on electrode materials, but also the synergistic role of oxygen vacancies interplaying with other contributors such as carbonaceous materials, doping, amorphisation, structural transformation, nanostructuring and functional coating. Finally, perspectives are given to stimulate new ideas and open questions to facilitate the further development of oxygen deficient electrode materials in energy research landscape.

Highly stable and selective CoxNiyTiO3 for CO2 methanation: Electron transfer and interface interaction

CO2 methanation with Sabatier reaction is an effective way to convert the greenhouse gases into valuable industrial product. In this work, we designed and developed a stable and effective Co modified NiTiO3 catalyst. The best catalyst exhibits high CO2 conversion (86 %) and high methane selectivity (nearly 100 %), which can work continuously for 16 h without significant deactivation at 350 ℃. As evidenced by various characterization methods, like XPS, H2-TPD and CO2-TPR, Co doping is found to influence the electron distribution of the catalyst, change the size of metal particle and the interaction of metal and support, which in turn has positive impact on methanation performance, while NiTiO3 support improves the ability to capture CO2 because of its abundant basic oxide and oxygen vacancies after reduction. Cobalt modification proved to be a potential way to construct Ti-O-Ni interfacial structure, and increase the hydrogen activation sites and carbon dioxide activation site simultaneously. This work may provide useful information for designing and optimizing multi-metal modified catalyst for CO2 methanation.

Facile Removal of Bulk Oxygen Vacancy Defects in Metal Oxides Driven by Hydrogen-Dopant Evaporation.

Oxygen vacancy is a common defect in metal oxides that causes appreciable damage to material properties and performance. Removing bulk defects of oxygen vacancy (VO) typically needs harsh conditions such as high-temperature annealing. Supported by first-principles simulations, we propose an effective strategy of removing VO bulk defects in metal oxides by evaporating hydrogen dopants. The hydrogen dopants not only lower the migration barrier of VO but also push VO away due to their repulsive interaction. The coevaporation mechanism was supported by a neural networks potential-based molecular dynamics simulation, which shows that the migration of hydrogen dopants from inside to surface at 400 K promotes the migration of VO as well. Our proof-of-concept study suggests an alternative and efficient way of modulating oxygen vacancies in metal oxides via reversible hydrogen doping.