Concentration Of Oxygen Defects Research Articles

System of excitons bound at oxygen centers and stimulated emission of CdS crystals

The paper presents the results of a study of the optical properties of CdS on the basis of the band anticrossing theory (BAC) with the involvement of the wider source data to the analysis results. Depending on the growth conditions, the presence and change in the concentration of oxygen and intrinsic point defects, which determine the composition of the crystals, have been taken into account. Data from studies of the structural features of crystals have also been used. The concept of nonuniform distribution of oxygen isoelectronic centers in the CdS volume due to their predominant segregation in layers of stacking faults (SFs) and also on dislocations has been introduced. To analyze the optical data, the capabilities of the method for constructing band models, which collect extensive and multilateral information about specific samples have been used. A refined model multizone of [Formula: see text] with SFs, which determines the spectrum of edge emission, has been presented. The nature of the edge emission A2B6 crystals with a small deviation from stoichiometry (CdS type) was explained as a spectrum of bound excitons at oxygen centers in SF layers. The article gives recommendations for the diagnostics of CdS crystals, suitable for creating stable luminescent systems or lasers.

High pressure and Ti promote oxygen vacancies in perovskites for enhanced thermoelectric performance

The electrical properties of the perovskite oxide thermoelectric material SrTiO3(STO) should be improved in order to realize its thermoelectric application due to its intrinsic insulation characteristics. The bulk STO materials usually optimize their electrical properties by annealing in reducing environment after synthesis. However, this method inevitably leads to the gradient distribution of oxygen defect concentration in materials. In this study, the combination of High Temperature and High Pressure (HPHT) method is used, and Ti powder is added to optimize the electrical properties of STO materials. HPHT provides a closed synthesis environment. In addition, with the strong oxygen-capturing ability of Ti at high temperature, the semi-conductivity of materials is performed. The thermoelectric properties of the synthesized materials under HPHT are studied using XRD, XPS, SEM, and TEM. The properties and micro-morphology of the synthesized samples with different Ti contents are investigated. The results show that, with the increase of the Ti content, the oxygen defect concentration increases and the electrical properties are significantly improved. Simultaneously, the sample synthesized using the HPHT method changes its micro-morphology and reduces the thermal conductivity of the material. The increase of oxygen vacancy concentration can also effectively reduce the thermal conductivity. Under the collaborative optimization of HPHT while adding Ti, the maximum obtained zT value at 20 wt% addition is 0.19 @ 973 K. The density functional theory calculation also shows that, compared with pure STO, the band gap of STO with oxygen vacancy decreases, which increases the conductivity. Therefore, a high pressure and the addition of Ti powder provide a fast, simple and efficient method for preparing oxygen-deficient perovskite oxide materials, which is of great significance for improving its thermoelectric properties.

Defect-engineered TiO2 nanocrystals for enhanced lithium-ion battery storage performance

TiO2 nanocrystals containing oxygen defects were successfully prepared by a hydrothermal method with reductive sintering in a 5% H2 + 95% Ar mixed atmosphere. The high-resolution transmission electron microscopy and X-ray diffraction results showed that the prepared samples were all anatase TiO2. X-ray photoelectron spectroscopy analysis showed that the reductive sintering converted Ti4+ to Ti3+. The Ti3+/Ti4+ molar ratio of the TiO2 nanocrystals increased with increasing sintering time. The results of the electron paramagnetic resonance spectroscopy analysis showed that the reduced and sintered TiO2 nanocrystals produced strong signals at g = 2.0010 and 1.9478, corresponding to the presence of oxygen defects and Ti3+, respectively. Furthermore, stronger signals were generated with increasing sintering time, indicating a progressively higher concentration of oxygen defects. The TiO2 nanocrystalswere used as anode materials for lithium-ion batteries. One sample, H-TiO2-5, containing a moderate concentration of oxygen defects, presented a higher lithium ion diffusion coefficient (D = 5.1 × 10-13 cm2s−1) compared to the other TiO2 samples. This sample had a Ti3+/Ti4+ molar ratio of 0.179 and excellent cycling stability after 100 cycles. The discharge capacity was 124 mAh.g−1 after 500 cycles at a current density of 1C, which means that it has good cyclic stability.

Multidimensional In2O3/In2S3 heterojunction with lattice distortion for CO2 photoconversion

Photocatalytic CO2 reduction to sustainably product of fuels is a potential route to achieve clean energy conversion. Unfortunately, the sluggish charge transport dynamics and poor CO2 activation performance result in a low CO2 conversion efficiency. Herein, we develop a multidimensional In2O3/In2S3 (IO/IS) heterojunction with abundant lattice distortion structure and high concentration of oxygen defects. The close contact interfaces between the junction of the two phases ensure undisturbed transmission of electrons with high-speed. The increased free electron concentration promotes the adsorption and activation of CO2 on the catalyst surface, leaving the key intermediate *COOH at a lower energy barrier. The perfect combination of the band matching oxide and sulfide effectively reduces the internal energy barrier of the CO2 reduction reaction. Furthermore, the lattice distortion structure not only provides additional active sites, but also optimizes the kinetics of the reaction through microstructural regulation. Remarkably, the optimal IO/IS heterojunction exhibits superior CO2 reduction performance with CO evolution rate of 12.22 μmol g−1 h−1, achieving about 4 times compared to that of In2O3 and In2S3, respectively. This work emphasizes the importance of tight interfaces of heterojunction in improving the performance of CO2 photoreduction, and provides an effective strategy for construction of heterojunction photocatalysts.

Mesoporous LaCoO3 perovskite oxide with high catalytic performance for NOx storage and reduction

A mesoporous LaCoO3 perovskite oxide (LaCoO3-Meso) with three-dimensionally ordered helical interwoven structure was synthesized by a nano-casting method using KIT-6 as the hard template. The obtained LaCoO3-Meso with high surface area was tested for its catalytic performance in the NOx storage and reduction (NSR) reaction and compared with a sample synthesized by the conventional sol-gel method. The LaCoO3-Meso showed a significant advantage for NOx storage, with a NOx storage capacity 2 times higher than the regular sample. LaCoO3-Meso also exhibited improved NSR catalytic performance in the 150–450 °C temperature range, especially within 350–400 °C, where the NOx conversion was raised for 40%. The results of X-ray photoelectron spectroscopy and X-ray absorption fine structure measurements suggested the presence of a high concentration of oxygen defects on the LaCoO3-Meso surface. Further results provided by temperature programmed reduction and temperature programmed desorption indicated that the oxygen defects not only increase the amount of trapped NOx, but also improve the low-temperature redox performance of the catalyst. The lower stability of NOx species adsorbed on oxygen defects promotes the NOx release step in the NSR reaction and benefits the regeneration of storage sites.

Enhanced defect emission of TiO2-doped transparent glass-ceramics embedding ZnO quantum dots with optimized heat-treatment schedule

This work mainly concentrated on the structural and optical properties of TiO 2 -doped transparent glass-ceramics embedding ZnO quantum dots (QDs). The results show that the concentration of non-bridging oxygen and related defects in the glasses increases and thermal parameters decreases by the introduction of TiO 2 . The surface defect concentration of ZnO QDs is greatly improved and the size of ZnO QDs is reduced by heat treatment below the T g and the introduction of TiO 2 . The green emission of glasses is mainly derived from oxygen vacancy defects and the yellow emission of glass-ceramics primarily originated from oxygen interstitial defects which show a tendency of enhancing first and then reducing with the increase of TiO 2 content. Among them, the optimal content for the strongest emission intensity of TiO 2 -doped glasses and glass-ceramics was determined to be 1.0 mol%, and the relative exceeds 10 times and 5.4 times that of TiO 2 -free glasses and glass-ceramics, respectively. The average size of ZnO QDs (∼2.1 nm) remarkably close to its exciton Bohr radius is successfully prepared by the optimal heat treatment condition (630 °C/8 h, below the T g ). The results show that these glass-ceramics embedding ZnO QDs are good yellow emission materials and have application prospects in WLEDs.

Negative Effects of Annealed Seed Layer on the Performance of ZnO-Nanorods Based Nitric Oxide Gas Sensor.

Nitric oxide (NO) is a toxic gas, which is dangerous for human health and causes many respiratory infections, poisoning, and lung damage. In this work, we have successfully grown ZnO nanorod film on annealed ZnO seed layer in different ambient temperatures, and the morphology of the nanorods sensing layer that affects the gas sensing response to nitric oxide (NO) gas were investigated. To acknowledge the effect of annealing treatment, the devices were fabricated with annealed seed layers in air and argon ambient at 300 °C and 500 °C for 1 h. To simulate a vertical device structure, a silver nanowire electrode covered in ZnO nanorod film was placed onto the hydrothermal grown ZnO nanorod film. We found that annealing treatment changes the seed layer’s grain size and defect concentration and is responsible for this phenomenon. The I–V and gas sensing characteristics were dependent on the oxygen defects concentration and porosity of nanorods to react with the target gas. The resulting as-deposited ZnO seed layer shows better sensing response than that annealed in an air and argon environment due to the nanorod morphology and variation in oxygen defect concentration. At room temperature, the devices show good sensing response to NO concentration of 10 ppb and up to 100 ppb. Shortly, these results can be beneficial in the NO breath detection for patients with chronic inflammatory airway disease, such as asthma.

Role of pO2 and film microstructure on the memristive properties of La2NiO4+δ/LaNiO3−δ bilayers

In mixed-conducting LaNiO3−δ/La2NiO4+δ bilayers the deposition conditions determine the film microstructure and the oxygen defect concentration at the interface (and ) building up an electronic barrier which induces a memristive behavior.

Materials AIILnInO4 with Ruddlesden-Popper Structure for Electrochemical Applications: Relationship between Ion (Oxygen-Ion, Proton) Conductivity, Water Uptake, and Structural Changes.

In this paper, the review of the new class of ionic conductors was made. For the last several years, the layered perovskites with Ruddlesden-Popper structure AIILnInO4 attracted attention from the point of view of possibility of the realization of ionic transport. The materials based on Ba(Sr)La(Nd)InO4 and the various doped compositions were investigated as oxygen-ion and proton conductors. It was found that doped and undoped layered perovskites BaNdInO4, SrLaInO4, and BaLaInO4 demonstrate mixed hole-ionic nature of conductivity in dry air. Acceptor and donor doping leads to a significant increase (up to ~1.5–2 orders of magnitude) of conductivity. One of the most conductive compositions BaNd0.9Ca0.1InO3.95 demonstrates the conductivity value of 5 × 10−4 S/cm at 500 °C under dry air. The proton conductivity is realized under humid air at low (<500 °C) temperatures. The highest values of proton conductivity are attributed to the compositions BaNd0.9Ca0.1InO3.95 and Ba1.1La0.9InO3.95 (7.6 × 10−6 and 3.2 × 10−6 S/cm correspondingly at the 350 °C under wet air). The proton concentration is not correlated with the concentration of oxygen defects in the structure and it increases with an increase in the unit cell volume. The highest proton conductivity (with 95−98% of proton transport below 400 °C) for the materials based on BaLaInO4 was demonstrated by the compositions with dopant content no more that 0.1 mol. The layered perovskites AIILnInO4 are novel and prospective class of functional materials which can be used in the different electrochemical devices in the near future.

Surface properties of chlorophyll-a sensitized TiO2 nanorods for dye-sensitized solar cells applications

Dye-sensitized solar cells (DSSCs) based on TiO2 nanorods have recently achieved improvement by tuning surface polarity with the help of chlorophyll-a dye. Additionally, we have successfully explored the effects of surface free energy, oxygen defect concentration, micro strain, surface morphology, chemical composition, photoluminescence, and surface properties of chlorophyll-a sensitized TiO2 nanorods. Sensitization on TiO2 nanorods has been tested with photoelectrochemical performance, and the resulting surface shows a superhydrophobicity with CA = 154 ± 30 while oxygen defect is reduced to 2.7228 × 1021 cm−3. The surface free energy has been successfully studied with the help of Owens-Wendt-Rabel-Kaelble (OWRK) and the wetting envelope model. A sensitized system of chlorophyll-a and TiO2 nanorods for the DSSC application presents a maximum of 3.69 times PEC improvement under 1000 W light illumination.

Oxygen defect, electron transfer and photocatalytic activity of Ag/CeO2/SBA-15 hybrid catalysts

Ag/CeO2/SBA-15 hybrid photocatalysts were synthesized and their catalytic activities were investigated in the photodegradation of Congo red (CR) under different conditions. The results of X-ray diffraction (XRD) analysis, Rietveld refinement, X-ray photoelectron spectroscopy (XPS) analysis, and DRS-UV–vis characterization confirmed that oxygen defects were created in CeO2 layer and its concentration decreased on increasing of ceria loading of the catalysts. Ag0/Ag+ and Ce3+/Ce4+ couples coexisted in the Ag/CeO2/SBA-15 catalysts, resulting from the electrons transferring from Ag0 nanoparticles (AgNPs) to Ce4+ around the oxygen defects in CeO2. The values of surface Ag0/Ag+ and Ce3+/Ce4+ ratios were correlated well with the variation of oxygen defect concentration and the photocatalytic activity. The catalysts with the lower CeO2 content, i.e. 5–10 wt%, exhibited the better photocatalytic activity due to the formation of a greater number of oxygen defects and more electrons transferring between AgNPs and CeO2. CR could be photodegraded with a higher conversion under acidic than basic condition, following the first order kinetics. A four-stage reaction mechanism consisting of the primary, secondary, aromatic ring opening, and mineralization steps was proposed. This work also confirms that the combination of computer simulation technique for the crystalline structure refinement with the catalytic experiments can gain new insights into the reaction mechanism and help to better understand the photocatalytic behaviors.

Fabrication of ZnO Ceramics with Defects by Spark Plasma Sintering Method and Investigations of Their Photoelectrochemical Properties

ZnO, as an important semiconductor material, has attracted much attention due to its excellent physical properties, which can be widely used in many fields. Notably, the defects concentration and type greatly affect the intrinsic properties of ZnO. Thus, controllable adjustment of ZnO defects is particularly important for studying its photoelectric properties. In this work, we fabricated ZnO ceramics (ZnO(C)) with different defects through spark plasma sintering (SPS) process by varying sintering temperature and using reduction environment. The experimental results indicate that the changes of color and light absorption in as-prepared ZnO originate from the different kinds of defects, i.e., oxygen vacancies (VO), interstitial zinc (Zni), and Zinc vacancies (VZn). Moreover, with the increase in calcination temperature, the concentration of oxygen defects and interstitial zinc defects in the ceramics increases gradually, and the conductivity of the ceramics is also improved. However, too many defects are harmful to the photoelectrochemical properties of the ceramics, and the appropriate oxygen defects can improve the utilization of visible light.

The Influence of the Chemical Potential on Defects and Function of Perovskites in Catalysis.

A Sm-deficient Sm0.96MnO3 perovskite was prepared on a gram scale to investigate the influence of the chemical potential of the gas phase on the defect concentration, the oxidation states of the metals and the nature of the oxygen species at the surface. The oxide was treated at 450°C in nitrogen, synthetic air, oxygen, water vapor or CO and investigated for its properties as a catalyst in the oxidative dehydrogenation of propane both before and after treatment. After treatment in water vapor, but especially after treatment with CO, increased selectivity to propene was observed, but only when water vapor was added to the reaction gas. As shown by XRD, SEM, EDX and XRF, the bulk structure of the oxide remained stable under all conditions. In contrast, the surface underwent strong changes. This was shown by AP-XPS and AP-NEXAFS measurements in the presence of the different gas atmospheres at elevated temperatures. The treatment with CO caused a partial reduction of the metals at the surface, leading to changes in the charge of the cations, which was compensated by an increased concentration of oxygen defects. Based on the present experiments, the influence of defects and concentration of electrophilic oxygen species at the catalyst surface on the selectivity in propane oxidation is discussed.

Effect of carbon, oxygen, and intrinsic defects on hydrogen-related donor concentration in proton irradiated n-type silicon

We investigated the effect of the concentration of carbon, oxygen, and irradiation-induced intrinsic defects on hydrogen-related donor (HD) concentration. Several n-type silicon wafers having different carbon and oxygen concentrations were irradiated with 2 MeV protons, subsequently annealed at 300–400 °C, and analyzed by spreading resistance profiling. The HD concentration had no correlation with carbon and oxygen concentration. Additionally, the HD concentration showed a strong increasing linear dependence with proton-irradiation dose at 350 and 400 °C and a square root dependence at 300 °C. In the decay process of HD concentration at 400 °C, fast- and slow-decay components were observed regardless of wafer type. Our results show that the HD formation is based on the interactive process of irradiation-induced intrinsic defects and hydrogen, rather than hydrogen-catalyzed thermal double donor formation. Magnetic-field-applied Czochralski (m:Cz) wafers with 300 mm diameter, which are critical for the production scaling of power devices, have a relatively higher oxygen concentration than conventional floating-zone wafers. Our results further suggest that controlling the intrinsic defect formation, rather than oxygen impurity concentration, is more important in realizing designed doping profiles with high accuracy and reproducibility for next-generation power devices using large-diameter m:Cz wafers as a standard starting material.

Enhanced gas sensing response for 2D α-MoO3 layers: Thickness-dependent changes in defect concentration, surface oxygen adsorption, and metal-metal oxide contact

• Synthesis of α- MoO 3 2D layers and thin films via pulsed laser deposition technique. • 2D layers show high gas sensing performance towards NO 2 as compared to the thin film. • p-type gas sensing behavior observed in 2D layers in contrast to n-type gas sensing behavior in thin film. • Sensor behavior is explained based on enhanced surface activity, oxygen vacancies, and metal- metal oxide contacts. In this study, α-MoO 3 two-dimensional (2D) layers and thin films were synthesized using pulsed laser deposition technique. X-ray diffraction and Raman spectroscopy confirm the formation of anisotropic α-MoO 3 . Atomic Force Microscopy images show the evolution of morphology from 2D layers to oriented crystallite growth with increase in film thickness. Temperature and gas concentration dependent, gas sensing response has been observed to be quite different in 2D layers in comparison to thin film sample. 2D α-MoO 3 layers (∼ 6 nm) show higher response of about 25 % at a lower temperature (100 °C); They exhibit lower detection limits (up to 100 ppb) and selectivity towards NO 2 gas. Also, 2D layers show p-type gas sensing response and nonlinear current-voltage (I–V) characteristics of metal- metal oxide junctions, in complete contrast to thin film sample, which shows an n-type gas sensing response and linear I–V characteristics. The results have been explained on the basis of lower oxygen defect concentration, enhanced oxygen adsorption at the surface, and metal- α-MoO 3 contact favoring hole conduction.

Influence of vacancy defect on the structural and optical properties of hexagonal barium titanate ceramics: Experimental and theoretical analysis

This study aims to investigate the properties of the metastable hexagonal barium titanate and its corresponding phase evolution with annealing treatment. Specifically, the metastable hexagonal (h) phase, its defect structure, and the tetragonal (t) one were investigated from both the first-principles computational and the experimental perspectives. The powders prepared using self-propagating high-temperature synthesis were consolidated by spark plasma sintering (SPS) under vacuum conditions. The ceramics were subjected to an annealing treatment at 800 and 1200℃ to restore the oxygen stoichiometry and initiate phase transformation. The unannealed sample had about 80 wt% of the h-BaTiO3 phase and 8.4% oxygen defect concentration. Our first-principles calculations showed that oxygen vacancy induced spin-polarized solution with half-metallic behavior.

Controlled generation and tuning the oxygen defects in nanofibers of Ca2Fe2O5 toward high and stable Li-ion battery anode

In recent years, oxygen-defect (vacancies) and nanostructured-based materials have been fabricated and applied as electrode materials for lithium-ion batteries (LIBs). However, the individual role of oxygen defects and nanostructuring on the improvement of Li-storage performance is still not clear. In addition, the technique which generates the control amount of oxygen-defects as well as maintains the nanostructured morphology of the electrode materials is highly desirable. Hence, in this work, we have attempted to create the oxygen defects in Ca2Fe2O5 (C2FONF) nanofibers via electrospinning technique and subsequent heat-treatment on as-spun nanofibers such as 700 °C (C2FONF-700), 800 °C (C2FONF-800), and 900 °C (C2FONF-900). All the as-prepared samples are characterized by XRD, FE-SEM, BET, XPS, and I-V characteristics. From the quantification of oxygen defect, it is observed that C2FONF-900 sample consists of the highest number of oxygen defects. As an LIB anode, C2FONF-900 exhibits superior reversible capacity (530 (±10) mAh g−1 at 50 mAg−1 up to 100 cycles), cyclability (370 (±10) mAh g−1 at 1C: 500 mAg−1 up to 100 cycles). Further, the rate capability of C2FONF-900 is found better than the C2FONF-700 and C2FONF-800. Furthermore, the study of capacitive/diffusion-controlled process showed the higher contribution of capacitive capacity in the C2FONF-900 electrodes, which is responsible for their better rate capability. In this study, we have shown that the Li-storage properties of iron-based oxides can be easily improved by tuning their concentration of oxygen defects while maintaining similar fibric nanostructure.

Structural design and synthesis of new MOO3-x interlayer bi-functional nanomaterials for enhanced up-conversion luminescence properties

A novel imaging materials based on bi-functional Fe3O4@MOO3-x@YF3:Yb/Er nanoparticles (NPs) with strong up-conversion luminescence and magnetic properties was designed and synthesized by inlaying MOO3-x with localized surface plasmon resonance (LSPR) and ferromagnetic property in the bi-functional Fe3O4@YF3:Yb/Er NPs. The morphology, structure and properties of Fe3O4@MOO3-x@YF3:Yb/Er NPs are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), photoluminescence (PL) spectra, UV–Vis-NIR spectrophotometer and superconducting quantum interference devices (SQUID). It was found that the experimental conditions (reaction temperature, reaction time, the mass ratio of core:shell and oxygen pressure in heat treatment) have a certain effect on the oxygen defect concentration of the MOO3-x interlayer. XRD also showed the crystal structure of Fe3O4, Fe3O4@MOO3-x and Fe3O4@MOO3-x@YF3:Yb/Er NPs. The SEM and TEM results showed that the average grain size of the prepared NPs were 200 nm. The SQUID and PL results showed that the Fe3O4@MOO3-x@YF3:Yb/Er NPs have stronger magnetism (14.3 emu/g) and excellent up-conversion luminescence performance (the emission peak at 525 nm is nearly 20 times higher than the corresponding emission intensity of the Fe3O4@YF3:Yb/Er). The Fe3O4@MOO3-x@YF3:Yb/Er NPs can be easily used to magnetic resonance and fluorescence dual-mode image-guided visual delivery drug and also increase the accurate diagnosis and treatment effect of malignant tumor.

Oxygen ion conductivity studies of bismuth and bismuth-calcium co-doped ThO2

A series of trivalent (Bi3+) doped and divalent (Ca2+) co-doped ThO2 samples i.e., Th0.50-xCaxBi0.50O2-δ (x = 0.00, 0.05, 0.10, 0.15 and 0.20) have been synthesized by citrate-nitrate solution combustion route and investigated in the context of oxygen ion conductivity and dielectric relaxation phenomena. The Rietveld refinement of the Powder X-ray diffraction data confirmed monophasic fluorite structures (S.G. Fm3‾m) for calcium concentrations up to 20 mol %. The optical band gap decreased with the increase in Ca2+ concentration up to 10 mol %. In contrast, the defect band's intensity in the Raman spectra increased due to oxygen vacancies on divalent addition. The quantitative aspect of oxygen vacancy and defect concentration was derived from Raman spectra. The crystallographic index application was further employed to interpret the optimum doping concentration to maximize oxide ion conductivity. Remarkably high oxide ion conductivity (~10−3 S/cm) was observed for Bi3+ doped (50 mol %), and Bi3+ (50 mol %)-Ca2+ (10 mol %) co-doped ThO2 samples at 773 K. The Nyquist plot exhibited grain contribution for low dopant levels. Both grain and grain boundary contribution were present in the higher dopant concentrations. Conductivity, dielectric, and modulus properties of doped and co-doped samples have been compared, from which 10 mol % of Ca2+-doping was identified to be the optimum concentration.

Initial electrical properties of tantalum oxide resistive memories influenced by oxygen defect concentrations

The initial electrical properties of tantalum oxide resistive memory were investigated using four metal electrodes (TiN, Ti, Ta, and Al) and two kinds of tantalum oxide with different amounts of intrinsic oxygen vacancies. The initial resistance depended on the electrode material. This indicated that oxygen scavenging by the electrodes contributed to a reduction in the resistance. However, the resistance change depended on the intrinsic oxygen vacancy concentration introduced during the tantalum oxide deposition. The forming voltage also depended on the electrode metal. For the device with an Al electrode, a clear aluminum oxide layer was identified at the electrode–insulator interface, which was hypothesized to be the origin of the high forming voltage. All factors concerning the oxygen vacancies, i.e. intrinsic vacancies introduced via film deposition and extrinsic vacancies caused by the electrode scavenging effect, influenced the initial state of tantalum oxide, and thus, its switching performance as a resistive memory.