Influence Of Oxygen Vacancies Research Articles

Combined Experimental and Theoretical Insights into the Synergistic Effect of Cerium Doping and Oxygen Vacancies in BaZrO3−δ Hollow Nanospheres for Efficient Photocatalytic Hydrogen Production

The long-standing debate over the influence of oxygen vacancies and various dopants has been the center point in perovskite-based compounds for their photocatalytic applications. Hydrothermally synthesized cerium-doped BaZrO3 (BZO) hollow nanospheres have been systematically studied by experimental and theoretical calculations to understand the effect of cerium doping and oxygen vacancies on the photocatalytic properties. Compounds synthesized by a template-free route were composed of hollow nanospheres generated by Ostwald ripening of spherical nanospheres, which were formed by agglomeration of nanoparticles. The high alkaline condition and high temperature during the hydrothermal condition may lead to the formation of local disorders and oxygen vacancies in the compounds, confirmed by ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) analysis, and density functional theoretical (DFT) calculations. Combination of oxyge...

Structural and electrical properties of ZnCo2O4 spinel synthesized by sol-gel combustion method

In this present study, we have discussed about the synthesis of spinel type ZnCo2O4 by sol-gel combustion method using glycine as a chelating agent. Structural and morphological verifications are carried out using XRD, SEM and the functional group confirmation by using FT-IR. The temperature dependent dielectric relaxation and conduction mechanism have been analyzed by using dielectric measurements, impedance spectroscopy and electric modulus studies in the frequency range of 50 Hz to 5 MHz between the temperatures 303 K and 593 K. Influence of oxygen vacancy on the behaviour of dielectric relaxation is due to the hopping mechanism between Co2+ and Co3+ sites due to the structural defect in the material. XPS measurement confirms the different oxidation state in Co-site and the existence of oxygen vacancy in the material. The conductivity of the material is studied with Jonscher Power law.

Influence of oxygen vacancy on the response properties of TiO2 ultraviolet detectors

In this paper, the effect of sputtering distance between target and substrate (DTS) on the structural, morphological and optical properties of TiO2 films were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed that crystallite size and surface morphology were strongly dependent on DTS. The X-ray photoelectron spectroscopy (XPS) results showed that obvious three peaks including C1S, Ti2p and O1s could be clearly observed and the binding energy was 285, 458 and 531 eV, respectively. For each sample, four peaks near 386, 462, 527 and 595 nm in photoluminescence (PL) spectra were observed and originated from intrinsic recombination, F center, F+ center and electrons transferred from 3d Ti3+ states to the deep levels OH−, respectively. Finally, the photoresponse properties of UV detectors based on TiO2 films were analyzed and the TiO2 UV detector prepared at a DTS of 80 mm showed high responsivity of 2.02 × 10−4 A/W and excellent photosensitivity of 32.25, also, exhibited a fast response with a rise time (τr) of 0.98 s and fall time (τf) of 3.14 s.

Oxygen vacancy modulation of {010}-dominated TiO2 for enhanced photodegradation of Sulfamethoxazole

Oxygen vacancy modulation of metal oxide photocatalysts has triggered an explosion of interest for the unprecedented and superior photoactivities. Herein, the influence of oxygen vacancies on the activity of {010}-faceted TiO2 for photodegradating SMX was evaluated. Experimental studies indicated that the formation of oxygen vacancies was highly dependent with the thermal treatment atmosphere. Vacuum treatment was favorable for the generation of surface hydroxyl groups, which contributed to the interfacial extraction of holes. In contrast, air calcinations benefited the formation of surface defects. The enhanced charge separation resulted in the remarkably improved photoactivity for SMX degradation.

Oxygen Vacancy Enhanced Gas-Sensing Performance of CeO2/Graphene Heterostructure at Room Temperature.

Oxygen vacancies (Ov) as the active sites have significant influences on the gas sensing performance of metal oxides, and self-doping of Ce3+ in CeO2 might promote the formation of oxygen vacancies. In this work, hydrothermal process is adopted to fabricate the composites of graphene and CeO2 nanoparticles, and the influences of oxygen vacancies as well as Ce3+ ions on the sensing response to NO2 are studied. It is found that the sensitivity of the composites to NO2 increases gradually, as the proportion of Ce3+ relative to all of the cerium ions is increased from 14.6% to 50.7% but decreases after that value. First-principles calculations illustrate that CeO2 becomes metallic at the Ce3+ proportion of <50.7%, the chemical potential of electrons on surface decreases, and the Fermi level shifts upward due to the existence of low-electronegativity Ce3+ ions, resulting in reduced Schottky barrier height (SBH) at the CeO2/graphene interface, enhanced interfacial charge transfer, and high gas sensing performance. However, deep energy level will be induced at the Ce3+ proportion of >50.7%, and the Fermi level is pinned at the interface. As a result, the density of free electrons is reduced, leading to increased SBH and poor gas sensing response. It demonstrates that an appropriate concentration of oxygen vacancies in CeO2 is needed to enhance the gas sensing performance to NO2.

Influence of oxygen vacancies on the structural, dielectric, and magnetic properties of (Mn, Co) co-doped ZnO nanostructures

We analysed the variation and effect of oxygen vacancies on the structural, dielectric and magnetic properties in case of Mn (4%) and Co (1, 2 and 4%) co-doped ZnO nanoparticles (NPs), synthesized by chemical precipitation route and annealed at 750 °C for 2 h. From the XRD, the calculated average crystallite size increased from15.30 ± 0.73 nm to 16.71 ± 012 nm, when Co content is increased from 1 to 4%. Enhancement of dopants (Mn, Co) introduced more and more oxygen vacancies to ZnO lattice confirmed from EDX and XPS. The high-temperature annealing leads to reduction of the dielectric properties due to enhancement in grain growth (large grain volume and lesser number of grain boundaries) with the incorporation of Co and Mn ions into the ZnO lattice. The electrical conductivity of the Mn doped and (Mn, Co) co-doped ZnO samples were enhanced due to increase in the volume of conducting grains and charge density (liberation of trapped charge carriers in oxygen vacancies and free charge carriers at higher frequencies). The Mn-doped and (Mn, Co) co-doped ZnO NPs show ferromagnetic (FM) behaviour. The saturation and remnant magnetizations (Ms and Mr) elevates from (0.235 to 1.489) × 10−2 and (0.12 to 0.27) × 10−2 emu/g while Coercivity (Hc) reduced from 97 to 36 Oe with enhancement in the concentration of dopants in ZnO matrix. Oxygen vacancies were found to be the main reason for room-temperature ferromagnetism (RTFM) in the doped and co-doped ZnO NPs. The results show that the enhanced dielectric and magnetic properties of Mn doped and (Mn, Co) co-doped ZnO is strongly correlated with the concentration of oxygen vacancies. The observed enhanced RTFM, dielectric properties and electrical conductivity makes TM doped ZnO nanoparticles suitable for spintronics, microelectronics and optoelectronics based applications.

Influences of oxygen vacancies on the enhanced nonlinear optical properties of confined ZnO quantum dots

Confining ZnO quantum dots (QDs) into a solid state matrix to form nanocomposite materials is an important mean to design nonlinear optical devices. In this paper, ZnO confined in Al2O3 matrix are synthesized by pulsed laser deposition method and rapid thermal annealing technique. The substantial oxygen vacancies are generated in the N2-annealed ZnO QDs. Magnetic measurements illustrate that the ferromagnetic nature of ZnO QDs confined in Al2O3 matrix can be due to the oxygen vacancies. The determined two-photon absorption in N2-annealed ZnO QDs is an instantaneous nonlinear process and is enhanced almost 2 times compared to that of O2-annealed ZnO QDs. The enhancement of optical nonlinearities achieved in confined ZnO QDs with substantial oxygen vacancies is important for the practical applications of ZnO nanostructures in such as optical limiting devices and all-optical switching elements.

Influence of oxygen vacancies in ALD HfO2-x thin films on non-volatile resistive switching phenomena with a Ti/HfO2-x/Pt structure

We report a modulation of oxygen vacancies profile in atomic layer deposition (ALD) HfO2-x thin films by reducing oxidant pulse time (0.7 s–0.1 s) and study its effect on resistive switching behavior with a Ti/HfO2-x/Pt structure. Hf 4f spectra of x-ray photoelectron microscopy (XPS) and depth profile confirm varied oxygen vacancies profiles by shifts of binding energies of Hf 4f5/2 and Hf 4f7/2 main peaks and its according HfO2-x sub-oxides for each device. The ultraviolet photoelectron spectroscopy (UPS) confirms different electron affinity (χ) of HfO2 and HfO2-x thin films, implying that barrier height at Ti/oxide interface is reduced. Current transport mechanism is dictated by Ohmic conduction in fully oxidized HfO2 thin films - Device A (0.7 s) and by Trap Filled Space Charge Limited Conduction (TF-SCLC) in less oxidized HfO2-x thin films - Device B (0.3 s) and Device C (0.1 s). A switching mechanism related to the oxygen vacancies modulation in Ti/HfO2-x/Pt based resistive random access memory (RRAM) devices is used to explain carefully notified current transport mechanism variations from device-to-device. A proper endurance and long-time retention characteristics of the devices are also obtained.

Influence of Oxygen Vacancies on the Frictional Properties of Nanocrystalline Zinc Oxide Thin Films in Ambient Conditions.

Oxygen vacancy is the most studied point defect and has been found to significantly influence the physical properties of zinc oxide (ZnO). By using atomic force microscopy (AFM), we show that the frictional properties on the ZnO surface at the nanoscale greatly depend on the amount of oxygen vacancies present in the surface layer and the ambient humidity. The photocatalytic effect (PCE) is used to qualitatively control the amount of oxygen vacancies in the surface layer of ZnO and reversibly switch the surface wettability between hydrophobic and superhydrophilic states. Because oxygen vacancies in the ZnO surface can attract ambient water molecules, during the AFM friction measurement, water meniscus can form between the asperities at the AFM tip-ZnO contact due to the capillary condensation, leading to negative dependence of friction on the logarithm of tip sliding velocity. Such dependence is found to be a strong function of relative humidity and can be reversibly manipulated by the PCE. Our results indicate that it is possible to control the frictional properties of ZnO surface at the nanoscale using optical approaches.

Influence of oxygen vacancies on surface charge potential and transportation properties of Al-doped ZnO nanostructures produced via atomic layer deposition

Abstract High quality Al-doped ZnO (AZO) films are advantageous for many applications. Their properties can be tuned by controlling the doping concentration and intrinsic defect density. In this work, high-quality AZO films have been synthesized using the atomic layer deposition (ALD) technique. Throughout the doping cycle and post-annealing treatments under various atmospheres, the oxygen-related vacancies were controlled. The effect of oxygen vacancies on the charge transportation and surface potential were studied. The O 1s X-ray photoelectron spectrometry (XPS) spectra of the AZO film were deconvoluted into three components related to the O 2 − species, corresponding to the oxygen in the ZnO lattice ( O L ); oxygen vacancies or defects ( O V ); and chemisorbed or dissociated ( O C ) oxygen species. In the case of the as-deposited films and films annealed under various atmospheres, a co-relationship between the O V and mobility ( μ ) can be determined. In the case of the N 2 -annealed film, the O V fraction is at its maximum value, while the other components are at their minimum values. Among the as-deposited films, the maximum O V fraction is obtained when a Al:Zn ratio ( R Al/Zn ) of 7% is used. When R Al/Zn = 7%, the μ value of the as-deposited AZO film is enhanced from 12.1 cm 2 V −1 s −1 ( R Al/Zn = 3%), to 18.5 cm 2 V −1 s −1 . It shows its potential application as photoanode. The Hall Effect and the XPS analysis of the film reviled a co-relationship between O V and μ with the Al-doping concentration or post annealing atmosphere. Kelvin probe atomic force microscopy (KPFM) was used to evaluate the surface charge potentials of the films. The N 2 -annealed AZO film with R Al/Zn = 3% ( AZO-3-N 2 ) exhibited the maximum negative potential (−115.79 mV); however, the film with R Al/Zn = 7% ( AZO-7-N 2 ) exhibited the maximum positive potential (797.23 mV). Hence, the properties of these films may directly pertain to the bit readout signal and reliability of charge storage and memory applications.

Hydrothermal oxidation synthesis of rod‐like ZnO and the influence of oxygen vacancy on photocatalysis

Rod‐like ZnO nanoparticles (NPs) were synthesized from zinc powder by a simple hydrothermal oxidation method. The presence of acetylacetone could promote the oxidation reaction of Zn and the formation of hexagonal nanorods. Then, the as‐prepared samples were annealed in N2 , O2 , and air atmosphere at 550°C for 2 h to control the number of oxygen vacancies in the samples. The samples were characterized by X‐ray diffraction, scanning electron microscopy, X‐ray photoelectron spectroscopy, and UV–vis spectroscopy. The correlation between the oxygen vacancies and the photocatalytic activity was investigated. The results reveal that the annealing process alters the samples’ bandgap and number of the oxygen vacancies, thereby improving the photocatalytic activity. The enhancement of photodegradation efficiency arising from the appropriate content of oxygen vacancies is discussed.

The coupled effects of oxygen defect and crystallographic orientation on the electromechanical properties of BaTiO 3 nanowires

Abstract Influence of oxygen vacancies on electromechanical properties of individual BaTiO 3 (BTO) nanowires (NWs) is investigated, using molecular dynamics simulations. The simulations were performed for defected 0–4% oxygen vacancy defects) tetragonal NWs with axial directions of [001] and [110]. Results show an increase in spontaneous polarization and piezoelectric constant of the individual NWs due to increasing oxygen vacancy concentration, and a decrease in yield stress and Young's modulus. It seems that in individual BTO NWs the softening effect of the oxygen vacancies overcomes the pinning effect and results in enhancement of piezoelectric constant and spontaneous polarization. Results also show that yield stress and Young's modulus of the NWs with axial direction of [001] are higher than those for NWs with axial direction of [110] while it is reverse for spontaneous polarization and piezoelectric constant.

Influence of oxygen vacancy on electric structure and optical properties of pure and N-doped Sr2M2O7(M = Nb, Ta)

We have investigated the electronic structure and optical properties of the pure and N-doped Sr2M2O7(M=Nb,Ta) systems with the oxygen vacancy by the HSE06 functional. For the undoped compounds, we find that the oxygen vacancy induces magnetism in both Sr2Nb2O6.75 and Sr2Ta2O6.75, in which the magnetic moments are 0.76 μB and 0.34 μB on the Nb and Ta atoms, respectively. Furthermore, the oxygen vacancy gives rise to defect electronic states near the bottom of the conduction band, which shifts the absorption border into the visible light area. For the N-doped systems, we obtain magnetic moments of 0.53 μB and 0.52 μB on the N atom for Sr2Nb2O6.75N0.25 and Sr2Ta2O6.75N0.25 respectively. Nitrogen doping induces impurity N 2p states in the band gap and thus produces visible light absorption behavior. With the introduction of the oxygen vacancy in Sr2Nb2O6.75N0.25 and Sr2Ta2O6.75N0.25, the magnetism is weakened and the impurity N 2p electronic states in the band gap disappear. There exist defect Nb 4d and Ta 5d states that lie just below the Fermi level near the conduction band for Sr2Nb2O6.5N0.25 and Sr2Ta2O6.5N0.25 respectively. This makes the absorption edge further red-shifted.

Influence of oxygen vacancies on the photoresponse of β-Ga2O3/SiC n–n type heterojunctions

β-Ga2O3/4H-SiC n–n type heterojunctions have been fabricated by depositing high quality β-Ga2O3 films on c-axis orientation n-type 4H-SiC substrates using laser molecular beam expitaxy. The influences of oxygen vacancies on the junction performances are investigated. It is found that the existence of oxygen vacancies degrades the rectifying and photoresponse properties of the heterojunctions. A large rectification ratio of 1900, a high 254 nm ultraviolet photosensitivity of 6308% and a zero response of 365 nm ultraviolet have been achieved by reducing oxygen vacancies. It is supposed that the oxygen vacancies in Ga2O3 affect the depletion layer of the n–n junction greatly.

Experimental and ab initio study of the hyperfine parameters of ZnFe 2 O 4 with defects

We present a combined Mossbauer and ab initio study on the influence of oxygen-vacancies on the hyperfine and magnetic properties of the ZnFe 2O4 spinel ferrite. Samples with different degree of oxygen-vacancies were obtained from zinc ferrite powder that was thermally treated at different temperatures up to 650 ∘C under vacuum.Theoretical calculations of the hyperfine parameters, magnetic moments and magnetic alignment have been carried out considering different defects such as oxygen vacancies and cation inversion. We show how theoretical and experimental approaches are complementary to characterize the local structure around Fe atoms and interpret the observed changes in the hyperfine parameters as the level of defects increases.

Lattice Defects Induce Multiferroic Responses in Ce, La‐Substituted BaFe0.01Ti0.99O3 Nanostructures

Single‐phase multiferroic Ba(Fe0.67Ce0.33)0.01Ti0.99O3 (BFTO:Ce) and Ba(Fe0.67La0.33)0.01Ti0.99O3 (BFTO:La) nanostructures were synthesized by a hydrothermal method (180°C/48 h). Rietveld refinement of X‐ray diffraction could confirm crystalline phase and lattice deformation by Ce, La into BFTO. The Ce and La doping induce nanoaggregation‐type BFTO nanostructural product due to their ionic size effect and chemical behavior with OH− ions. Raman active modes show tetragonal phase and defects due to vacancies in the BFTO lattice. Photoluminescence spectrum involves multiple visible emissions due to defects/vacancies. The observed ferroelectric polarization is enhanced due to shape/size effect of nanoparticles, lattice distortion, and filling of d orbital in the perovskite BaTiO3. The room‐temperature magnetic behavior is described due to antiferromagnetic interactions that strengthen by Ce and La doping. The zero‐field cooling and field cooling magnetic measurement at 500 Oe indicates antiferromagnetic to ferromagnetic transition. Dynamic magnetoelectric coupling was investigated, and maximum longitudinal magnetoelectric coefficient is 62.65 and 49.79 mV/cmOe, respectively, measured for BFTO:Ce and BFTO:La. The magnetocapacitance measurements induce negative values that described in terms of magnetoresistance and magnetic phase transition effects. The influence of oxygen vacancy on multiferroicity is evaluated by valance states of O ions.

Strong interaction between ferroelectric polarization and oxygen vacancy in BiFeO&lt;sub&gt;3&lt;/sub&gt; thin film capacitors

Influence of oxygen vacancy (VO••) distribution on the polarization properties of SrRuO3(SRO)/BiFeO3(BFO)/SRO capacitors was investigated. Our thin-film experiments and density function theory (DFT) calculations reveal that a strong attractive interaction between VO•• and the bottom BFO/SRO interface gives rise to an VO••-rich defective layer for as-deposited capacitors. The distribution with the VO••-rich layer is gradually broken by an electrical training and finally becomes symmetric, leading to a polarization-hysteresis change from a markedly rounded to a typical ferroelectric loop. This study demonstrates that defect engineering utilizing VO•• is an effective approach to control the polarization-related properties of BFO in capacitor form.

Evidence of oxygen vacancy and possible intermediate gap state in layered α-MoO3 single-crystal nanobelts

Multilayered meso-structured MoO3 nanobelts have been synthesized by thermally oxidizing a molybdenum chip in a reduced oxygen atmosphere, with a view to disclosing the existence of oxygen vacancy and understanding the mechanism behind the influence of oxygen vacancy on the electronic structure of molybdenum oxides. Based on the measurements from X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM) and transmission electron microscope (TEM), it is found that the as-grown sample is single-crystal α-MoO3 with a (001) preferred orientation, which shows an irregular belt-like morphology being composed of some ~20nm single-crystal thin layers. The present sample includes a lot of oxygen vacancies in the lattice, as evidenced by the considerably reduced coordination number of the central Mo atoms from X-ray absorption spectra (XAS) as well as the red shift of the main Raman peaks. The existence of the oxygen vacancies are further tested by the photoluminescence (PL) results as the main emission peak shows an obvious red shift with the corresponding optical band gap reduced to 2.3eV. Very importantly, an extra emission positioned at 738nm (1.68eV) is believed to originate from the recombination of the electrons from the intermediate band (IB) to the valence band (VB), and the formation of the IB in the gap is also caused by oxygen-ion 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.

Influence of oxygen vacancies on the EPIR effect in Nd0.7Sr0.3MnO3 ceramics

Nd0.7Sr0.3MnO3 polycrystalline ceramic samples (NSMO) were synthesized by solid state reaction with a post annealing for parts of samples under different partial pressures of oxygen at 1350 °C for 12 h. XRD shows that all the samples are in single phase with a perovskite structure. The electrical transport measurement implies that the oxygen concentration has an important influence on the electrical pulse induced resistance, i.e. the EPIR-effect, for NSMO samples. For the oxygen-enriched sample, which probably contains a smaller amount of oxygen vacancies, the I–V characteristic is linear with a very small resistance, and the EPIR-effect is not detectable within the experimental accuracy. However, after sintered and annealed in reducing atmospheres, an EPIR effect appears and even becomes stronger and more stable if the sintering atmosphere is more reducing than that of normal air condition. Besides, the I–V becomes nonlinear and the initial resistance increases with increasing deficiency of oxygen, which means that the oxygen vacancies and their deep level trap states play an important role for the EPIR effect of NSMO. In this manuscript, an oxygen vacancy trapped state model is put forward to explain the relation between oxygen concentration and the EPIR effect.