Effect Of Oxygen Vacancy Concentration Research Articles

Effect of oxygen vacancy concentration on the resistive random access memory characteristics of NiOx (x = 1, 0.97, 0.94) thin films

Effect of oxygen vacancy concentration on the resistive random access memory characteristics of NiOx (x = 1, 0.97, 0.94) thin films

Boosting piezocatalytic performance of perovskite BiFeO3 with controlled oxygen vacancies

Piezocatalysis, which is capable of directly converting mechanical energy into chemical energy, has been identified as a promising strategy for environmental wastewater remediation and clean energy production. Regrettably, the present piezocatalysis is still in the early stage and the piezocatalytic activity is still far from ideal. Thus, exploring and discovering more efficient catalyst design strategies remains a challenge. In this work, a novel and simple method was employed to synthesize a piezocatalytic BiFeO3 (BFO) using metal-organic frameworks (MOFs) as precursors, and the MOFs derived BFO (MOF-BFO) has excellent piezocatalytic activity. The maximum reaction rate constant (k) for rhodamine B (RhB) can reach 1.70 × 10−2 min−1, which is about 4.7 times that of the sample annealed at 700 °C (BFO-700, 0.36 × 10−2 min−1) and about 10.3 times that of the sample annealed at 450 °C (BFO-450, 0.17 × 10−2 min−1) using conventional one pot co-precipitation method. The MOF-BFO catalyst is also suitable for degradation of other typical dyes, including methylene blue (MB) and methyl orange (MO), dramatically increasing its utility against complex wastewater components. What's more, the analysis indicates that the concentration of oxygen vacancy (OV) may be the primary factor influencing the piezocatalytic performance. Thus, the effect of OV concentration on piezocatalytic activity is further studied, which acts like a double-edged sword, manifesting that creating moderate OV is crucial for regulating piezocatalytic performance. This study not only demonstrates the potential of MOF-BFO as a candidate material for piezocatalytic applications, but also offers insights for the development of efficient piezocatalysts in future.

Modification of BCZT piezoelectric ceramics by modulating oxygen vacancies in different sintering atmospheres

In this work, we regulated the oxygen vacancy concentration by sintering BCZT + 0.1 mol % Y + 0.1 mol % Nb [BCZT-0.1 mol % (Y + Nb)] ceramics under air, oxygen, argon, and vacuum atmospheres and examined the effect of oxygen vacancy concentration on phase composition, crystal structure, microstructure, and electrical properties. X-ray photoelectron spectrometry and X-ray diffraction Rietveld refinement showed that the air- and oxygen-sintered ceramic samples had lower oxygen vacancy concentrations, and the oxygen octahedral lattice distortion in the samples was stable when the R-O-T phases coexisted, forming a multiphase coexistence structure with more spontaneous polarization directions. The oxygen-sintering atmosphere is important in oxygen compensation, which promotes the formation of dense microstructure, thereby increasing the Curie temperature (TC). Oxygen vacancy formation energy was low in the argon and vacuum atmospheres, and some oxygen vacancies increased the mass transfer effect and grain growth. Nevertheless, excess oxygen vacancies increased the leakage current during polarization resulting in poor polarity, which prevents directional deflection of domains and degrades performance. The vacuum-sintered sample was calcined in 1000 °C air for oxygen supplementation sintering, and as the sintering time increased, the oxygen vacancy concentration decreased, the grains grew further, the density increased, and the piezoelectric properties improved. The oxygen-sintered BCZT-0.1 mol % (Y + Nb) samples had good comprehensive electrical properties (d33 = 547 pC/N, kp = 55%, εr = 5355, Pr = 10.63 µC/cm2, EC = 1.89 kV/cm, tan δ = 0.02, and TC = 102 °C).

A Memristor-Based Leaky Integrate-and-Fire Artificial Neuron With Tunable Performance

Artificial neurons have received extensive attention as an important part of neuromorphic computing. Recently, tremendous efforts have been made on the memristor-based neurons, while the regulation of performance of such neurons and its underlining mechanism has been rarely studied. In this work, we propose an artificial neuron device based on Ag/TaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> /Si, which exhibits good threshold switching characteristics (on-off ratio above 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> ) along with good device stability and cycling stability. The Leaky Integrate-and-Fire (LIF) neuron model is successfully simulated without additional circuitry, including leaky integrated firing and refractory periods. In addition, the effect of oxygen vacancy concentration on the performance of artificial neurons is investigated, and the results showed that an increase of oxygen vacancies can significantly reduce the threshold voltage of neuron activation, the holding voltage and the probability of refractory period. This work provides a simple and effective strategy for the development of artificial neurons with tunable properties.

Effect of coloured TiO2 with different oxygen vacancy concentrations on the photocatalytic oxidation of gaseous mercury

ABSTRACT To develop a facile method to prepare an efficient photocatalyst for removing element Hg0 with highly toxic, the coloured TiO2 were prepared through calcining ethanol impregnation method at mild temperature, which is economical and simple and conducive to large-scale industrial promotion. The as-prepared samples were characterised by XRD, SEM, TEM, BET, XPS, EPR, UV-vis etc. and evaluated for the effect of oxygen vacancy concentration on the photocatalytic activity through the removal of zero-valent (Hg0) mercury. The photocatalytic mercury removal efficiency of T200 reached 62.3%, 3 times higher than that of P25. And the related mechanism explanation is proved.

Effect of oxygen vacancy concentration on the photocatalytic hydrogen evolution performance of anatase TiO2: DFT and experimental studies

Oxygen vacancies (OVs) are important for changing the geometric and electronic structure as well as the chemical properties of anatase TiO2. In this work, we performed a density functional theory (DFT) calculation on the electronic structure and catalytic performance of anatase TiO2 (101) with different numbers of OVs. A comparison of the measured XRD results with the simulated ones of TiO2 demonstrates that OVs can cause changes in the crystal structure. The changes in the electronic structure (Mulliken charges, band structure, and partial density of states) and water splitting on TiO2 (101) surfaces were investigated as a function of oxygen vacancy concentration. The results show that the introduction of OVs forms impurity levels below the conduction band of Ti 3d orbitals, through which electrons can gradually transit from VB to CB. However, when oxygen vacancy concentration is too high, the maximum electron transition energy increases and the promotion effect of OVs on water splitting is weakened. This work would provide more enlightenment and information for the design of defective TiO2 with higher photocatalytic activity.

Modified polyacrylamide gel synthesis of CeO2 nanoparticles by using cerium sulfate as metal source and its optical and photoluminescence properties

Developing a novel method to synthesize the highly dispersed nanosemiconductor materials is the major challenge in the expand the application field of luminescent materials. A modified polyacrylamide gel method has been used to synthesize the CeO2 nanoparticles by using cerium sulfate as metal source. The method makes full use of exothermic reaction of cerium sulfate in the precursor solution to polymerize acrylamide and methylene diacrylamide to obtain gel. The phase purity, particle size, optical properties, and photoluminescence properties of CeO2 nanoparticles demonstrate the strongly dependent behavior of acrylamide and methylene diacrylamide (AMMD) content. The synergistic effect of oxygen vacancy concentration, impurity functional group, and surface adsorbed oxygen leads to high photoluminescence performance of CeO2 nanoparticles prepared with the AMMD content of 0.3.

Dynamical Insights into Oxygen Diffusion in BaTiO3 and SrTiO3

Oxygen diffusion is a key process in BaTiO3 and SrTiO3 that determines many of their important electronic properties. Despite the importance of oxygen diffusion in these systems, there is still currently significant variation in the oxygen diffusion coefficients for BaTiO3 and SrTiO3 reported in the literature from both experimental and computational methods. Using molecular dynamics simulations, the effect of oxygen vacancy concentration on the oxygen diffusion properties is investigated for these perovskite materials. Oxygen diffusion coefficients and activation energies for both materials are calculated over a temperature range of 900–1500 K. Oxygen vacancy charge compensation is achieved by the reduction of Ti4+ to Ti3+ ions, in agreement with experimental results for undoped BaTiO3 and SrTiO3. The findings from this study yield an accurate reference point for the calculation of oxygen diffusion in these materials and also the application of molecular dynamics in studying such phenomena.

Influence of strontium content on the oxygen surface exchange kinetics and oxygen diffusion in La1–xSrxCoO3–δ oxides

Oxygen surface exchange kinetics between the gas phase oxygen and La1–xSrxCoO3–δ (x = 0, 0.3, 0.4, 0.6) oxides at 0.67 kPa oxygen pressure in the 600–850°°С range were investigated. The mechanism within the two-step model including two consecutive stages, oxygen dissociative adsorption and oxygen adatom incorporation into the oxide lattice, was analysed. The oxygen diffusion coefficient (D), the rate of heterogeneous oxygen exchange (rH) and the elementary rates of this process — dissociative adsorption (ra) and incorporation (ri) — were determined. The effect of oxygen vacancy concentration, strontium segregation on the surface, temperature and oxygen pressure on rH, ra and ri are discussed.

Kinetics of Furfural Hydrogenation over Bimetallic Overlayer Catalysts and the Effect of Oxygen Vacancy Concentration on Product Selectivity

AbstractFurfural hydrogenation over silica‐supported Co@Pd and Cu@Pd bimetallic overlayer catalysts was studied in this work. Both overlayer catalysts showed enhanced reactivity compared to pure Pd in furfural hydrogenation. Langmuir‐Hinshelwood kinetic study revealed that this enhanced reactivity could be attributed to the decreased hydrogen surface coverage on catalyst surface of overlayer catalysts compared to pure Pd during reaction, releasing more available active sites for surface reaction or furfural adsorption to take place. The effect of oxygen vacancy concentration on furfural hydrogenation product selectivity was also studied. It is proposed that oxygen vacancies were crucial for both furfural hydrodeoxygenation and aromatic ring hydrogenation, depending on the concentration of oxygen vacancies. This work provides new perspective for tuning the furfural hydrogenation product selectivity by altering the oxygen vacancy concentration.

Petroleum like biodiesel production by catalytic decarboxylation of oleic acid over Pd/Ce-ZrO2 under solvent-free condition

The Ce/Zr ratio of Pd/Ce-ZrO2 catalysts was systematically changed in order to investigate the effect of oxygen vacancy concentration on their decarboxylation activityunder solvent-free conditions for potential sustainable petroleum like biodiesel production. Pd/Ce0.5Zr0.5O2 exhibited the highest catalytic activity from all other tested catalysts because it contained the highest oxygen vacancy concentration and Pd dispersion, as shown by the X-ray photoelectron spectroscopy, Raman spectroscopy, and CO-chemisorption data. A catalyst deactivation study also showed that both the Pd dispersion and the oxygen vacancy concentration influences the catalytic activity.The catalyst deactivation was found to occur mainly due to Pd sintering, decreases in the BET surface area and Pd dispersion, and partially due to the loss of oxygen vacancies.

Synthesis, dielectric properties, and influences oxygen vacancies have on electrical properties of Na1/2Bi1/2Cu3Ti4O12 ceramics prepared by a urea combustion method

The influences of oxygen vacancies on the dielectric and electrical properties of Na1/2Bi1/2Cu3Ti4O12 ceramics prepared using a urea combustion method were investigated via an annealing treatment in an O2 atmosphere. Interestingly, a single Na1/2Bi1/2Cu3Ti4O12 phase was successfully prepared using a low calcination temperature of 800 °C. High dielectric permittivity (e′) and dense ceramic microstructure were achieved by sintering compacted powders at a low temperature of 980 °C. e′ and the loss tangent (tanδ) were decreased by annealing in an O2 atmosphere. This was associated with the oxygen vacancy concentration at grain boundaries (GBs). A decrease in low-frequency tanδ was caused by enhancement of GB resistance, which was due to filling oxygen vacancies at GBs. A slight decrease in e′ was attributed to the reduction in GB capacitance. Through use of an annealing process, the conduction activation energy at GBs was increased, whereas the conduction activation energy inside the grains remained unchanged. These results clearly indicated the effect of oxygen vacancy concentration at the GBs on the giant dielectric response and correlated GB response.

Insight into the Effect of Oxygen Vacancy Concentration on the Catalytic Performance of MnO2

Oxygen vacancies (OVs) are important for changing the geometric and electronic structures as well as the chemical properties of MnO2. In this study, we performed a DFT+U calculation on the electronic structure and catalytic performance of a β-MnO2 catalyst for the oxygen reduction reaction (ORR) with different numbers and extents of OVs. Comparing those results with the experimental XRD analysis, we determined that OVs produce a new crystalline phase of β-MnO2. Changes in the electronic structure (Bader charges, band structure, partial density of states, local density of states, and frontier molecular orbital), proton insertion, and oxygen adsorption in β-MnO2 (110) were investigated as a function of the bulk OVs. The results show that a moderate concentration of bulk OVs reduced the band gap, increased the Fermi and HOMO levels of the MnO2 (or MnOOH), and elongated the O–O bond of the adsorbed O2 and coadsorbed O2 with H. These changes substantially increase the conductivity of MnO2 for the catalysis of ...

Effect of oxygen vacancy concentration on the absorption spectra of Dy2Ti2O7 crystal

Dy2Ti2O7 (DTO) crystal with magneto-optical effect has been grown by Czochralski method. Two DTO wafers with different oxygen vacancy concentrations were prepared by annealing in oxygen or ammonia atmospheres. The effect of oxygen vacancy concentration on the absorption spectra of DTO crystal was investigated by first-principles calculation based on the density functional theory. It is found that the absorption loss of DTO annealed in oxygen atmosphere is about twice as high as that of DTO annealed in ammonia atmosphere in the wavelength of 380–580nm at room temperature. Based on their identical Faraday rotation angle, the magneto-optical figures of merit of DTO annealed in ammonia atmosphere are about twice as large as that of DTO in oxygen atmosphere in the 380–580nm range. The corresponding results reveal that oxygen vacancy concentration plays a significant role in the absorption spectra of DTO crystal, and DTO with high oxygen vacancy concentration may be a promising material for magneto-optical devices especially in the 400–700nm range.

Effect of annealing on electrical responses of electrode and surface layer in giant-permittivity CuO ceramic

The effect of heat treatments on the electrical responses of the electrode and surface layer in a giant-permittivity CuO ceramic is investigated. It is found that the giant low-frequency relative permittivity of the CuO ceramic can be tuned by annealing in Ar and O 2—it can be reduced by annealing in Ar, and then it can be enhanced up to the initial value by annealing in O 2. The results indicate to the effect of oxygen vacancy concentration on the giant dielectric properties of the CuO ceramic. Interestingly, three sets of dielectric relaxations are observed in the O 2–annealed sample, which can be assigned as the effects of outmost surface layer, electrode, and grain boundary. Our results reveal that the giant low-frequency dielectric response in the CuO ceramic is associated with both of the interfacial polarization at the sample–electrode interface resulted from a non-Ohmic electrode contact and the outmost surface layer-inner part interface.

The effect of oxygen vacancy concentration on the elastic modulus of fluorite-structured oxides

The elastic modulus of fluorite-structured pure ceria, gadolinium doped ceria (GDC) and yttria stabilized zirconia (YSZ) with different levels of oxygen vacancy concentration were measured by the nanoindentation technique. The samples were heat treated at 800 °C under an oxygen partial pressure in the range of 0.21 to 10 − 25 atm for 15 h until equilibrium was reached at a sufficient depth. The defect concentrations were conserved to room temperature by fast cooling. The results reveal that the elastic modulus of both pure ceria and GDC decreases drastically in samples heat treated at very low partial pressures of oxygen. However, the elastic modulus change for YSZ, within the studied range, is insignificant. An analysis of the experimental results suggests that an increase in the point defect content weakens the attractive forces between atoms in fluorite-structured oxides.

Dielectric properties of multilayered SrTiO3 thin films with graded oxygen vacancy concentration

SrTiO3 (STO) thin films were homo-epitaxially grown on Nb-doped STO substrates at varying oxygen pressures, and the effect of oxygen vacancy concentration on the dielectric properties of the STO thin films was studied and is presented herein. Although the STO thin films with low oxygen vacancy concentration demonstrated low zero-bias permittivity, low dielectric tunability, and high dielectric dissipation, the STO thin films, however, could withstand a large electric field. While the STO thin films with high oxygen vacancy concentration exhibited reduced dielectric loss and high dielectric tunability, they exhibited a low breakdown electric field. In order to make use of the respective advantages of the STO thin films with different oxygen vacancy concentrations, a trilayered structure was obtained by varying the oxygen pressure during deposition and combining one thin STO layer with large oxygen vacancy concentration sandwiched by two STO thin films with low oxygen vacancy concentration. The microstructure and dielectric properties of the trilayer were then studied. X-ray diffraction analysis indicated that the trilayer was a relaxed STO multilayer formed by two STO thin layers with different lattice parameters. An improved optimization of high tunability and low loss was achieved in the relaxed trilayer.

Modeling Optical Changes in Perovskite Capacitor Materials Due to dc‐Field Degradation

The depth sensitivity of spectroscopic ellipsometry (SE) to the presence of oxygen vacancies (VO••), in perovskite titanates, was examined over the wavelength range from 250 to 750 nm. To determine the effect of oxygen vacancy concentration, reference optical properties of barium titanate (BaTiO3) and Fe‐doped SrTiO3 were determined for oxidized and reduced samples. These reference data were used to model the sensitivity of SE to changes due to degradation. Using these models, it was found that SE could detect reduced layers of 4 Å for BaTiO3 and an 8 Å layer for Fe‐doped SrTiO3. From the models, concentration gradients were resolvable by SE with an average diffusion depth (√Dt) of 10 Å for BaTiO3 and 50 Å for Fe‐doped SrTiO3. These models also predict that SE should be able to discern different degradation mechanisms through the way the optical properties change during degradation.

Influence of oxygen vacancy concentration on electrical resistivity in EuO1−x

The electrical properties of EuO1−x magnetic semiconductors were studied to determine the conduction mechanism. We considered the effect of oxygen vacancy concentration on the carrier concentration, the relaxation time and the electrical resistivity. These quantities were calculated as functions of temperature and external magnetic field and the results compared with experimental data for thin films of EuO with oxygen vacancies.