Oxygen Vacancies In Oxide Research Articles

Unraveling the influence of oxygen vacancies in MoOx catalysts on CO2 hydrogenation

Oxygen vacancies in oxides are core parameters determining the catalytic performance in various reactions. This study investigated the influence of oxygen vacancies of MoOx catalysts on the reverse water gas shift (RWGS) reaction to gain a deep insight into the structure–function relationship. A distinct volcano-shaped trend was observed among MoOx catalysts with different amounts of oxygen vacancies. Through comprehensive characterizations and steady-state kinetic analysis, an optimal concentration of oxygen vacancies was identified to not only facilitate the adsorption and activation of CO2 but also restrain the formation of by-product CH4. In-situ characterizations combined with DFT calculations uncovered a formate-involved reaction pathway over the oxygen vacancies structured MoOx catalyst, with the hydrogenation of formate identified as the rate-determining step. These findings not only showcase the potential of Mo-based oxides in CO2 hydrogenation reactions but also provide fundamental insights into the role of oxygen vacancies in shaping catalytic behavior.

Tuning Oxygen Vacancies in Oxides by Configurational Entropy.

Tuning surface oxygen vacancies is important for oxide catalysts. Doping elements with different chemical valence states or different atomic radii into host oxides is a common method to create oxygen vacancies. However, the concentration of oxygen vacancies in oxide catalysts is still limited to the amount of foreign dopants that can be tolerated (generally less than 10% atoms). Herein, a principle of engineering the configurational entropy to tune oxygen vacancies was proposed. First, the positive relationship between the configuration entropy and the formation energy of oxygen vacancies (Eov) in 16 model oxides was estimated by a DFT calculation. To verify this, single binary oxides and high-entropy quinary oxides (HEOs) were prepared. Indeed, the concentration of oxygen vacancies in HEOs (Oβ/α = 3.66) was higher compared to those of single or binary oxides (Oβ/α = 0.22-0.75) by O1s XPS, O2-TPD, and EPR. Interestingly, the reduction temperatures of transition metal ions in HEOs were generally lower than that in single-metal oxides by H2-TPR. The lower Eov of HEOs may contribute to this feature, which was confirmed by in situ XPS and in situ XRD. Moreover, with catalytic CO/C3H6 oxidation as a model, the high-entropy (MnCuCo3NiFe)xOy catalyst showed higher catalytic activity than single and binary oxides, which experimentally verified the hypothesis of the DFT calculation. This work may inspire more oxide catalysts with preferred oxygen vacancies.

Insight into the Role and Evidence of Oxygen Vacancies in Porous Single-Crystalline Oxide to Enhance Catalytic Activity and Durability

Introducing and stabilizing oxygen vacancies in oxide catalysts is considered to be a promising strategy for improving catalytic activity and durability. Herein, we quantitatively create oxygen vacancies in the lattice of porous single-crystalline β-Ga 2 O 3 monoliths by reduction treatments and stabilize them through the long-range ordering of crystal lattice to enhance catalytic activity and durability. The combination analysis of time-of-flight neutron powder diffraction and extended x-ray absorption fine structure discloses that the preferential generation of oxygen vacancy tends to occur at the site of tetrahedral coordination oxygen ions (O III sites), which contributes to the formation of unsaturated Ga–O coordination in the monoclinic phase. The oxygen vacancies are randomly distributed in lattice even though some of them are present in the form of domain defect in the PSC Ga 2 O 3 monoliths after the reduction treatment. The number of oxygen vacancies in the reduced monoliths gives 2.32 × 10 13 , 2.87 × 10 13 , and 3.45 × 10 13 mg −1 for the Ga 2 O 2.952 , Ga 2 O 2.895 , and Ga 2 O 2.880 , respectively. We therefore demonstrate the exceptionally high C 2 H 4 selectivity of ~100% at the C 2 H 6 conversion of ~37% for nonoxidative dehydrogenation of C 2 H 6 to C 2 H 4 . We further demonstrate the excellent durability even at 620 °C for 240 h of continuous operation.

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.

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.

Oxygen vacancy induced phase and conductivity transition of epitaxial BaTiO3−δ films directly grown on Ge (001) without surface passivation

The heterogeneous epitaxial system of BaTiO3/Ge (BTO/Ge) is of great interest for both fundamental research and device applications, thanks to its quasi-lattice-matching feature and the integration of functional oxides on semiconductors. Currently, the heteroepitaxial growth of crystalline BTO films on Ge includes the utilization of ultrahigh vacuum tools and complex surface passivation pre-treatment as well as careful control of oxygen partial pressure during the growth. Meanwhile, oxygen vacancies in oxides strongly impact their structural and electrical properties. Here, we report a facile method to directly grow single crystalline BTO films on Ge using pulsed laser deposition. The strict control of oxygen partial pressure ensures a sharp interface with an atom-to-atom registry and also leads to the oxygen-deficient characteristics of BTO. The epitaxial relationship of BTO and Ge is [110] BTO (001)//[100] Ge (001). Detailed crystallographic studies on BTO films with different thicknesses show that, for the films with a thickness less than 20 nm, BTO shows a mixture of tetragonal and cubic phases due to the oxygen vacancies and the strain from the Ge substrate and the cubic phase eventually dominates as the film thickness increases. Such oxygen-deficient BTO films reveal conducting characteristics rather than dielectric properties. The oxygen vacancies can be partly “cured” after a low temperature annealing process. These results not only demonstrate the possibility to directly grow single crystalline oxides on semiconductors without surface passivation but also highlight the importance of oxygen vacancies and lattice strain on the crystallographic and electrical properties of BTO films.

Tuning Oxygen Vacancies of Oxides to Promote Electrocatalytic Reduction of Carbon Dioxide

Tin oxide (SnOx) has emerged as a promising metal oxide catalyst for the CO2 reduction reaction (CO2RR) into value-added chemicals such as formic acid/formate. However, the improvement of SnOx cata...

The origin of the exceptionally low activation energy of oxygen vacancy in tantalum pentoxide based resistive memory

It is well known that collective migrations of oxygen vacancies in oxide is the key principle of resistance change in oxide-based resistive memory (OxRAM). The practical usefulness of OxRAM mainly arises from the fact that these oxygen vacancy migrations take place at relatively low operating voltages. The activation energy of oxygen vacancy migration, which can be inferred from the operational voltage of an OxRAM, is much smaller compared to the experimentally measured activation energy of oxygen, and the underlying mechanism of the discrepancy has not been highlighted yet. We ask this fundamental question in this paper for tantalum oxide which is one of the most commonly employed oxides in OxRAMs and try the theoretical answer based on the first-principles calculations. From the results, it is proven that the exceptionally large mobility of oxygen vacancy expected by the switching model can be well explained by the exceptionally low activation barrier of positively charged oxygen vacancy within the two-dimensional substructure.

Tuning Oxygen Vacancy Diffusion through Strain in SrTiO3 Thin Films.

Understanding diffusion of oxygen vacancies in oxides under different external stimuli is crucial for the design of ion-based electronic devices, improvement of catalytic performance, and so forth. In this manuscript, using an external electric field produced by an atomic force microscopy tip, we obtain the room-temperature diffusion coefficient of oxygen-vacancies in thin films of SrTiO3 under compressive/tensile epitaxial strain. Tensile strain produces a substantial increase of the diffusion coefficient, facilitating the mobility of vacancies through the film. Additionally, the effect of tip bias, pulse time, and temperature on the local concentration of vacancies is investigated. These are important parameters of control in the production and stabilization of nonvolatile states in ion-based devices. Our findings show the key role played by strain for the control of oxygen vacancy migration in thin-film oxides.

Understanding the Coexistence of Two Bipolar Resistive Switching Modes with Opposite Polarity in Pt/TiO2/Ti/Pt Nanosized ReRAM Devices

Redox-type resistive random access memories based on transition-metal oxides are studied as adjustable two-terminal devices for integrated network applications beyond von Neumann computing. The prevailing, so-called, counter-eight-wise (c8w) polarity of the switching hysteresis in filamentary-type valence change mechanism devices originates from a temperature- and field-controlled drift-diffusion process of mobile ions, predominantly oxygen vacancies in the switching oxide. Recently, a bipolar resistive switching (BRS) process with opposite polarity, so-called, eight-wise (8w) switching, has been reported that, especially for TiO2 cells, is still not completely understood. Here, we report on nanosized (<0.01 μm2) asymmetric memristive cells from 3 to 6 nm thick TiO2 films by atomic layer deposition, which reveal a coexistence of c8w and 8w switching in the same cell. As important characteristics for the studied Pt/TiO2/Ti/Pt devices, the resistance states of both modes are nonvolatile and share one common state; i.e., the high-resistance state of the c8w mode equals the low-resistance state of the 8w-mode. A transition between the opposite hysteresis loops is possible by voltage control. Specifically, 8w BRS in the TiO2 cells is a self-limited low-energy nonvolatile switching process. Additionally, the 8w reset process enables the programming of multilevel high-resistance states. Combining the experimental results with data from simulation studies allows to propose a model, which explains 8w BRS by an oxygen transfer process across the Pt/TiO2 Schottky interface at the position of the c8w filament. Therefore, the coexistence of c8w and 8w BRS in the nanoscale asymmetric Pt/TiO2/Ti/Pt cells is understood from a competition between drift/diffusion of oxygen vacancies in the oxide layer and an oxygen exchange reaction across the Pt/TiO2 interface.

Atomic structure, electronic states, and optical properties of epitaxially grown β-Ga2O3 layers

β-Ga2O3 epitaxial layers of different thicknesses were grown and characterized by X-ray photoelectron (XPS) and optical reflectance spectroscopies. The XPS electronic structure mapping and the valence band spectra demonstrate the relationship between the time of deposition of the β-Ga2O3 epitaxial layers and the number of defects. Counterintuitively, the thinnest films (8 nm) fabricated within 2 min were almost defect-free in contrast to the thickest ones formed at 10 and 30 min. Density functional theory modeling predicted rather high (several eV) formation energies of all types of defects in bulk β-Ga2O3 and the energetic favorability of the formation of the interstitial gallium defects instead of the standard oxide oxygen vacancies on the surface. This uniqueness of β-Ga2O3 could be explained by the small ionic radii of gallium and appeared to be the cause of the increase in the number of defects with the time of growth.

Electronic properties of MoS2/MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts.

In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations.

Microwave Combustion for Modification of Transition Metal Oxides

The introduction of oxygen vacancy for transition metal oxide is an effective method to tune their conductivity. Here, microwave combustion method is used to quickly synthesize reduced metal oxides and graphene hybrid composite in several minutes. Among these as‐synthesized composites, the reduced orthorhombic Nb2O5 and graphene oxide (rNb2O5/rGO) composite demonstrates great electrochemical performance with a reversible specific capacity of 726.2 C g−1 at 2 mV s−1 in organic electrolyte as well as a good capacity retention of 87% after 3000 cycles at 10 A g−1. It is believed that this strategy can be a common route to quickly produce oxygen vacancy in oxides and satisfy much more application requirements even for the possibility of industrialization.

Identification of vacancy defect complexes in transparent semiconducting oxides ZnO, In2O3 and SnO2

Positron annihilation spectroscopy, when combined with supporting high-quality modeling of positron states and annihilation in matter, is a powerful tool for detailed defect identification of vacancy-type defects in semiconductors and oxides. Here we demonstrate that the Doppler broadening of the positron annihilation radiation is a very sensitive means for observing the oxygen environment around cation vacancies, the main open-volume defects trapping positrons in measurements made for transparent semiconducting oxides. Changes in the positron annihilation signal due to external manipulation such as irradiation and annealing can be correlated with the associated changes in the sizes of the detected vacancy clusters. Our examples for ZnO, In2O3 and SnO2 demonstrate that oxygen vacancies in oxides can be detected directly using positron annihilation spectroscopy when they are complexed with cation vacancies.

The Origin of Oxygen Vacancies Controlling La2/3Sr1/3MnO3 Electronic and Magnetic Properties

Mixed‐valence manganites gain increasing attentions thanks to their extraordinary properties including half‐metallicity and colossal magnetoresistive response, rendering them ideal candidate for oxide spintronics applications. Oxygen vacancies in oxides have been approved to be important functional defects and are effective to manipulating their multifunctional properties. To gain a deep insight into the roles of oxygen vacancies on regulating the atomic structure and electronic properties of the mixed‐valence manganites, two high‐quality epitaxial La2/3Sr1/3MnO3 films around a critical point (without/with oxygen vacancies) were designed and fabricated. From the experiments and theoretical calculations, it was found that the oxygen vacancies induce a weakening of Mn–O–Mn hybridized bond and an increase of concentration of Mn3+ ions, impair the double exchange between Mn3+ and Mn4+, and therefore lead to the transition from metal to insulator and the degraded magnetic properties. Our finding demonstrates a practical approach to tune the magnetic and transport properties of oxide thin films by precisely controlling the oxygen vacancies for high performance spintronics applications.

Detection of Oxygen Vacancies in Oxides by Defect-Dependent Cataluminescence.

Oxygen vacancies can control a number of distinct properties of oxides. However, rapid and simple detection of oxygen vacancies is a great challenge owing to their elusive species and highly diluted contents. In this work, we have discovered that cataluminescence (CTL) intensity in diethyl ether oxidation reaction on the surface of TiO2 nanoparticles is proportional to the content of oxygen vacancies. The oxygen vacancy-dependent diethyl ether CTL is attributed to the fact that abundant chemisorbed O2 in oxygen vacancies could facilitate its contact reaction with chemisorbed diethyl ether molecules, resulting in an obvious improvement of CTL intensity. Therefore, diethyl ether CTL can be employed as a simple probe for oxygen vacancies in TiO2 nanoparticles. Its feasibility is validated by detecting the CTL intensity of diethyl ether on the surface of TiO2 with variable oxygen vacancies by metal ion-doped TiO2 nanoparticles (Cu, Fe, Co, and Cr) and hydrogen-treated TiO2 nanoparticles at different temperatures. The content of oxygen vacancies by the present CTL probe is in good agreement with that obtained by conventional X-ray photoelectron spectroscopy (XPS) technique. The superior properties of the developed CTL probe over already-developed methods include fast response, easy operation, low cost, long-term stability, and simple configuration. We believe that the oxygen vacancy-sensitive CTL probe has a great potential in distinguishing oxygen vacancies in oxides.

Cost-effective personal radiation dosimetry

Deep understanding of physical properties of the materials under the inuence of radiation exposure is vital for the effective design of dosimeter devices. Detection of radiation is based on the fact that both the electrical and the optical properties of the materials undergo changes upon the exposure to ionizing radiation. It is believed that radiation causes structural defects (called colour centres or oxygen vacancies in oxides) leading to change in their density on the exposure to radiation. Thin lm technology is considered as cost-effective alternative for a broad range of sensors. However, it is especially attractive for metal oxide lms with melting point below 2000 ‐ C, as a wide range of lms with mixed composition can be produced. The inuence of radiation depends on both the dose and the parameters of the lms including their thickness: the degradation is more severe for the higher dose and the thinner lms. This paper reports on gamma radiation sensing properties of thermally evaporated NbO2 thin lms. These lms were deposited at different deposition rate and pressure. It was experimentally conrmed that the manufacturing parameters of the lms affected their gamma radiation sensitivity.

Theoretical Formation Energy of Oxygen-Vacancies in Oxides

Formation energies of neutral and charged oxygen vacancies in MgO, ZnO, Al2O3, In2O3 and SnO2 have been calculated by a first principles plane-wave pseudopotential method. Two kinds of polymorphs, i.e., an ordinary phase and a high-pressure or an hypothetical negative pressure phase, have been chosen in order to see the effects of crystal structure. Supercells composed of 54 to 96 atoms were employed, and structural relaxation around the vacancy within second nearest neighbor distances was taken into account. Defect levels were obtained from the difference in total energies of the neutral and charged supercells that contain a vacancy. Ionization energies of the vacancy were calculated as the difference in the bottom of the conduction band and the defect levels. They are found to be proportional to band-gaps with a factor of approximately 0.5, which are prohibitively large for the n-type conduction.

Could the Oxygen Vacancy in Oxides of the Perovskite Family Be a Dipole Center?

AbstractThe lattice polarization in oxides of the perovskite family is demonstrated to give rise to symmetry breaking of one‐electron orbitals at F centers. This can provide an explanation of recent experiments on paraelectric KTaO3 where, by the second harmonic generation method, approximately 1018 cm−3 dipole centers were found, the concentration of which depends on reduction and oxidation processes.

Breaking of symmetry of one-electron orbitals at oxygen vacancies in perovskite-type oxides

Some general relationships have been derived in terms of the unrestricted Hartree-Fock method that define the conditions leading to the breaking of symmetry of one-electron orbitals at oxygen vacancies in oxides of the perovskite family. Numerical calculations of the critical values of relevant parameters have been carried out. Experimental data are discussed in the light of the results obtained.