Terms Of Oxygen Vacancies Research Articles

Fabrication of transparent polycrystalline Y2Ti2O7 pyrochlore via direct solid-state reaction sintering

Transparent Y2Ti2O7 pyrochlore was fabricated via the direct solid-state reaction of Y2O3 and TiO2 by hot pressing at 1400 °C under 20 MPa in vacuum, followed by annealing at 700–1000 °C for 4–10 h in air. A small amount of excess Y2O3 was added to the stoichiometric composition to promote densification via the formation of more oxygen vacancies. The black-colored samples obtained after hot pressing exhibited transparency upon annealing owing to the restoration of oxygen ions in the oxygen vacancies, where the transmittance depended significantly on the annealing conditions. The highest transmittances of 55.1 and 74% were obtained at 800 and 2400 nm, respectively, for the Y2Ti2O7 annealed at 800 °C for 8 h. The bandgap energy was 3.75 eV for the transparent Y2Ti2O7, while that of the black Y2Ti2O7 without annealing was 0.29 eV due to the defect level formed by the oxygen vacancies. A model was proposed to explain the transparency behavior of Y(2+2X)Ti2O(7-X) owing to the variations in the atmosphere and excess Y2O3 in terms of oxygen vacancies and ions present in a unit cell.

Structural, photoluminescence, antibacterial and biocompatibility features of zinc incorporated hydroxyapatite nanocomposites

The development of bone-implant materials with enhanced bioactivity and biocompatibility has become important in the field of orthopedics. This preparation of nanocomposites of pure Hydroxyapatite (HAp) nanopowder, Hydroxyapatite-Zinc (HAp-Zn), Hydroxyapatite-reduced Graphene oxide (HAp-rGO) and Hydroxyapatite-Zinc-reduced Graphene Oxide (Hap-Zn-rGO) were accomplished using the sol–gel method. The effect of Zn and rGO on the structural, morphological, optical, antimicrobial property and biocompatibility of hydroxyapatite nanopowders are studied. The structural characterization analysed through XRD. The pure HAp and HAp-rGO exhibited hexagonal structure and incorporation of Zn in the host lattice changed this structure into orthorhombic. The average crystallite size ranges from 37 to 51 nm for various nanocomposites. The incorporation of rGO and Zn ions decreased the overall crystallite size. The presence of functional groups in the samples is analyzed and confirmed using FTIR analysis. The SEM photos revealed the structures and morphology of the nanoparticles. The chemical compositions are investigated using Energy Dispersive X-ray analysis (EDX), and all the ratios are in good agreement with the stoichiometry ratio. The photoluminescence emission peaks received around 468 nm and 385 nm and the reasons are discussed in terms of oxygen vacancies and some electronic transition between states. The optical band gap values were redshifted due to the incorporation of rGO with HAp. The in vitro antibacterial activities and biocompatibility of the synthesized pure hydroxyapatite and HAp-Zn-rGO nanocomposite are investigated using Kirby-Bauer antibacterial testing and MTT assay, respectively. The results show that zinc incorporated hydroxyapatite nanocomposites are more toxic against all bacterial strains and did not show any toxicity against living tissues.

Multiple nano-filaments based efficient resistive switching in TiO2 nanotubes array influenced by thermally induced self-doping and anatase to rutile phase transformation

In this paper, the impact of thermally induced self-doping and phase transformation in TiO2 based resistive random-access memory (ReRAM) is discussed. Instead of a thin film, a vertically aligned one-dimensional TiO2 nanotube array (TNTA) was used as a switching element. Anodic oxidation method was employed to synthesize TNTA, which was thermally treated in the air at 350 °C followed by further annealing from 350 °C to 650 °C in argon. Au/TiO2 nanotube/Ti resistive switching devices were fabricated with porous gold (Au) top electrode. The x-ray diffraction results along with Raman spectra evidently demonstrate a change in phase of crystallinity from anatase to rutile, whereas photoluminescence spectra revealed the self-doping level in terms of oxygen vacancies (OV) and Ti interstitials (Tii) as the temperature of thermal treatment gets increased. The electrical characterizations establish the bipolar and electroforming free resistive switching in all the samples. Among those, the ReRAM sample S3 thermally treated at 550 °C displayed the most effective resistive switching properties with R OFF/R ON of 102 at a read voltage of −0.6 V and a SET voltage of −2.0 V. Moreover, the S3 sample showed excellent retention performance for over 106 s, where stable R OFF/R ON ≈ 107 was maintained throughout the experiment.

Transparent boron-doped zinc oxide films for antibacterial and magnetic applications

B-doped ZnO thin films have been synthesized by a sol–gel dip-coating deposition technique. Deposition conditions were optimized to achieve highly ordered thin films. XRD patterns confirm that B ions are inserted into the ZnO; lattice and the crystallite size decrease from 25.83 to 20.94 nm as the amount of B increases to optimal value (9 at.wt%). Synthesized B-doped ZnO thin films have ordered hexagonal wurtzite structures and granular morphology with high specific surface area. Optical transmittance spectra display transparency in the visible region. The band gap of the films decreases from 3.89 to 3.04 eV with the increase in B concentration. Band gap lowering is due to increase of defect level with the increase in B dopant percentage. The origin of the ferromagnetism is explained in terms of oxygen vacancies and Zn interstitials. Dielectric constant increases while DC conductivity and AC conductivity decrease with the increase in B doping. Antibacterial activity increases with the increase in B doping percentage but lower than undoped ZnO due to large crystallite size of B-doped ZnO nanostructures.

Defect-Engineered MoO2 Nanostructures as an Efficient Electrocatalyst for Oxygen Evolution Reaction

This article presents the experimental and theoretical insights on the defect engineered MoO2 nanostructures in terms of oxygen vacancy and OH− occupancy towards the oxygen evolution reaction (OER)...

Argon Annealing and Oxygen Purity Affect Structural and Critical Parameters of YBCO Copper Oxide System

We report here the effects of argon annealing on the structural and critical parameters of oxygenated copper oxide system with different oxygen purities (Hp 99.99% and Lp 93%). It is found that the structure of the samples maintains orthorhombic single phase independent of both oxygen purity and annealing. The mean field temperature Tcmf is increased by annealing from 92 to 94 K for Lp sample, but it is decreased from 93 to 90 K for Hp sample. Similar behavior is obtained for crossover temperatures To. Further, the coherence length and interlayer coupling are decreased by annealing for both samples, and their values are higher for Lp samples than those for Hp samples. The excess conductivity analysis reveals two different exponents corresponding to crossover temperature for each plot: The first exponent is obtained in the normal field region at a temperature range of ln e (0 ≥ ln e ≥ − 2), and their values are 1.50, 1.53, 1.62 and 1.47 for both samples, in which the order parameter dimensionality (OPD) is one dimension, while the second exponent is obtained in the mean field region at a temperature range of ln e (− 2 ≥ ln e ≥ − 4), and their values are 0.39, 0.59, 0.56 and 0.41 for both samples, in which the OPD is 3D. Although the critical temperatures are decreased by annealing for both samples, the critical fields and critical currents are increased by annealing for both samples and their values are higher for Hp sample than those for Lp sample. Our results are discussed in terms of oxygen vacancies and concentration of carriers which are produced by annealing and oxygen purity for the considered samples.

Electrical, photoluminescence and ferromagnetic characterization of pure and doped ZnO nanostructures

The structural, electrical, optical, and magnetic properties of Zn0.95M0.05O samples (M = Zn, Ni, Cu, Fe, and Mn) are investigated. X-ray diffraction pattern revealed pure hexagonal wurtzite structure for all samples with an (210) additional peak formed only at 2 ~ 43.5° for Ni sample. The average crystalline diameter decreased from 20 nm for ZnO to 11.35 nm by the dopants, while the lattice and U parameters are nearly unchanged. Furthermore, the nonlinear coefficient and breakdown field are increased from 370.37 and 21.91 for ZnO up to 2291.67 and 55.96 by the dopants. The electrical conductivity of the upturn region is also increased by the dopants as compared to ZnO. There are two different values of the energy bandgap for each sample. The first gap is called fundamental gap Egh and its value above 3 eV, while the second gap is called optical gap EgL and its value below 2.6 eV. On the other hand, the UV band edges of photoluminescence intensity are 394, 402, 372, 390, and 404 nm, for Zn, Ni, Cu, Fe, and Mn samples. The Green shift is only recorded for Cu sample at 562 nm corresponding to 2.21 eV energy gap. The exciton energy, loss factor tan δ, and (N/m*) parameter are generally decreased by the dopants and follow the order Ni, Cu, Fe, Mn, and Zn, while lattice dielectric constant and inter-atomic distance are increased and follow the orders Zn, Mn, Fe, Cu, and Ni, and Zn, Ni, Cu, Mn, and Fe, respectively. Magnetization curves revealed clear room temperature ferromagnetism (RTFM) for Mn, Ni, and Fe samples, while they are showed weakly RTFM for Zn and Cu samples. Saturated magnetization Ms, remnant magnetization Mr, magnetization width, magnetic moment, coercivity of the field, and magneto-crystalline anisotropy factor are also affected by the dopants. Our results are discussed in terms of oxygen vacancies, valance state, and recombination of carriers, exhibited by the dopants to ZnO, rather than individual RTFM.

On the correlation between electrical, optical and magnetic properties of Zn1−xPrxO nanoparticles

We report here the structure, electrical, optical and magnetic properties of Zn1−xPrxO nanoparticles with various x values (0.00 ≤ x ≤ 0.50). X-ray diffraction pattern revealed wurtzite structure for all samples with an additional unknown peak formed at 2Ɵ = 28° for Pr = 0.00, 0.05, 0.10 samples. The cell parameters and dislocation density are increased with increasing Pr up to 0.30, followed by a decrease at Pr = 0.50. While a little bit decrease in the average grain size and an increase in the ZnO bond length are obtained. The nonlinear coefficient β, breakdown field EB and barrier voltage Vab are gradually shifted to lower values as Pr increases. They are decreased from 31.3, 2333 V cm−1 and 0.005 V for ZnO to 8.9, 750 V cm−1 and 0.001 V for 0.50 of Pr. The electrical conductivity versus temperature shows two conduction mechanisms and it is generally increased by increasing Pr and temperature. The conduction mechanisms are obtained at a temperatures width of (300–540 K) and (540–620 K), respectively. The activation energy Ea is decreased from 0.42 eV to 0.13 eV by 0.50 of Pr in the first region, while it is increased from 0.42 eV to 0.93 eV in the second region. The optical band gap Eg and exaction photon energy Eex are decreased by increasing Pr up to 0.50. They are decreased from 3.1 eV and 3.4 eV for ZnO to 1.62 eV and 2.79 eV by 0.50 of Pr, while Urbach energy Eu increased from 0.32 eV to 1.13 eV. Magnetization curves revealed room temperature ferromagnetism (RTFM) as Pr increases. The coercivity of the field Hc, saturated and remnant magnetization, magnetic moment are also increased by Pr up to 0.50. Our results are discussed in terms of oxygen vacancies, the density of states and local spin-polarized electrons exhibited by Pr as a dopant to ZnO.

Tailoring the efficiency of an active catalyst for CO abatement through oxidation reaction: The case study of samarium-doped ceria

In the present study, for the first time we tailored organic chemistry traditional synthesis tools towards preparation of CexSm1-xO2 (x = 0.2, 0.5, 0.8, 1) fluorite-type mixed oxides which are used for CO abatement. In this study, microwave radiation has been used as stand-alone heating method (conventional microwave), in the presence of oxalic acid acting as complexing agent (OX), or it has been coupled with simultaneous air cooling (AC), reflux (RE) and on/off cycles of the magnetron power (C1 and C5) in the so called enhanced microwave synthesis (EMW) approach. The EMW methods were applied in the case of Ce0.5Sm0.5O2 composition and enlightened different aspects of the rational design of its morphology (flower-like, rod-shaped, random aggregates), as well as the engineering of its oxygen vacant sites. CO oxidation reaction, in the presence and absence of H2, was used as a model probe reaction towards the evaluation of the catalysts. Different characterization techniques, namely, XRD, BET, ICP-MS, Raman, SEM, FTIR, TEM, XPS, H2-TPR, and CO2-TPD, have been employed to understand the synthesis −structure-properties relationship of the catalysts. Interestingly, the EMW synthesis conditions affect the crystalline structure of the catalysts (XRD), and the crystal growth, giving rise to CexSm1-xO2 particles with a crystallite size in the range of 8–26 nm. A rational manipulation of the morphology of the CexSm1-xO2 was achieved under the different EMW synthesis conditions and reported herein for the first time in such a systematic way, where the Ce0.5Sm0.5O2 catalyst was crystallized in octahedral rods (RE), random aggregates (C1 and C5), and elongated flower-like shapes (AC). The co-presence of Ce3+ and Sm3+ in the metal oxides (XPS studies) facilitates the presence of oxygen vacant sites, as confirmed by the Raman spectra (550–590 cm‐1), the mobility of which, though, differs as has been proved by H2-TPR, following the order: Ce0.5Sm0.5 (RE) > Ce0.5Sm0.5 (AC) > Ce0.5Sm0.5 (C1). A tensile lattice strain in the 0.6–2.4% range of the cubic CexSm1-xO2 lattice was found to be strongly correlated with the CO oxidation performance. Two of the enhanced microwave methods, RE and AC, led to catalysts with improved catalytic performance, lowering the T50 by 130 °C (or 27%) and 30 °C (or 6%), respectively, compared to the Ce0.5Sm0.5O2 (OX). The superior performance is discussed in terms of oxygen vacancies, oxygen mobility, lattice strain and CO2-affinity of the surfaces involved.

Towards spin-polarized two-dimensional electron gas at a surface of an antiferromagnetic insulating oxide

The surfaces of transition-metal oxides with the perovskite structure are fertile grounds for the discovery of novel electronic and magnetic phenomena. In this article, we combine scanning transmission electron microscopy (STEM) with density functional theory (DFT) calculations to obtain the electronic and magnetic properties of the (001) surface of a ( LaFe O 3 ) 8 / ( SrFe O 3 ) 1 superlattice film capped with four layers of LaFe O 3 . Simultaneously acquired STEM images and electron-energy-loss spectra reveal the surface structure and a reduction in the oxidation state of iron from F e 3 + in the bulk to F e 2 + at the surface, extending over several atomic layers, which signals the presence of oxygen vacancies. The DFT calculations confirm the reduction in terms of oxygen vacancies and further demonstrate the stabilization of an exotic phase in which the surface layer is half metallic and ferromagnetic, while the bulk remains antiferromagnetic and insulating. Based on the calculations, we predict that the surface magnetism and conductivity can be controlled by tuning the partial pressure of oxygen.

Effect of Ho3+doping on the electric, dielectric, ferromagnetic properties and TC of BiFeO3 ceramics

Multiferroic Bi1−xHoxFeO3 (x=0, 0.05, 0.1, 0.15, 0.2) ceramics have been prepared by rapid liquid phase sintering method. The effect of Ho doping on the structure, electrical, dielectric, ferromagnetism properties and TC of BiFeO3 ceramics is studied. The result shows that all the peaks for Bi1−xHoxFeO3 samples can be indexed according to the crystal structure of pure BiFeO3 by XRD. When x=0.15 and 0.2, the samples consist of two phases including rhombohedral and orthorhombic. Ho doping BiFeO3 enhanced the electrical properties with lower leakage current density. This dielectric behavior of Bi1−xHoxFeO3 ceramics varies with content x and frequency, which might be understood in terms of oxygen vacancy, the displacement of Fe3+ ions and lattice phase transition. The magnetic moment of Bi1−xHoxFeO3 ceramics varies with temperature from 300 to 1000K at 5kOe. It shows that the TN of BiFeO3 changes slightly from 638K to 632K with Ho3+ doping. The magnetic Curie temperature of Bi1−xHoxFeO3 will reduce from 970K to 890K with increasing Ho3+ content, depending mainly on the Fe–O–Fe super-exchange and magnetic structure of relative stability. By measuring the magnetic hysteresis loops, all the samples exhibit weak ferromagnetic behavior under 890K and paramagnetism above 890K. It evidences that the ferromagnetic phase transition of Bi1−xHoxFeO3 samples occurs at 890K.

Growth and Properties of $a$-Axis Oriented Thin Films of Infinite-Layer $\hbox{Sr}_{1 - x}\hbox{La}_{x} \hbox{CuO}_{2}$

A-axis oriented, electron-doped, Sr1-xLaxCuO2 thin films were grown on LaAlO3 and YAlO3 substrates by magnetron sputtering. X-ray diffraction analysis revealed that in an as-grown state, the a-axis oriented films grew with the long-c phase, which is related to oxygen vacancies in the CuO2 planes, unlike the c-axis oriented films. Another feature of our a -axis oriented films is that the films contain 90° rotated domains from in-plane X-ray diffraction and planer transmission electron microscopy image. We discuss features of a-axis oriented films in terms of oxygen vacancies in the CuO2 planes.

FERROMAGNETISM IN MECHANICALLY MILLED PURE SnO2

Pure SnO 2 based wide band gap semiconductors were prepared by mechanical alloying method by using steel and tungsten carbide vials. They were further annealed at 900°C. The XRD patterns could be refined by using P42/ mnm space group with typical lattice parameters a = b = 4.7322 Å and c = 3.1848 Å. The as milled powders obtained from both the vials exhibit room temperature ferromagnetism (FM) without any transition element doping. However upon annealing, the FM was destroyed in one of the samples. The observed FM is explained in terms of oxygen vacancy and defects induced electrons and exchange interaction between them. The ferromagnetic transition temperature obtained from the temperature variation of magnetization was found to be 915 K. The initial magnetization data could be analyzed in terms of bound magnetic polaron model. The resonance field shift in electron spin resonance spectrum is explained in terms of observed ferromagnetism.

Oxygen purity effect oxygen deficiency by argon heat treatment on Y:123 superconductors

Argon heat treatments at 450°C for different times of 0, 12, 24, 36, 48 and 60h on the two Y:123 oxygenated samples with different oxygen purity (Hp 99.99% and Lp 93%) are reported. The results of X-ray diffraction, oxygen deficiency and resistivity measurements are presented in details. Furthermore, the superconducting critical temperature Tc, normal resistivity ρ300, residual resistivity ρ0, resistivity slope dρ/dT and width of transition ΔTc are extracted from resistivity data. It is found that both c-parameter and oxygen deficiency increase by increasing annealing time, while OD and effective Cu valance decrease. But, the relative increase/decrease is more in Lp samples than that of Hp samples. Although Tc is decreased by annealing time for the two samples, the relative decrease in Tc is more in Lp samples when compared with those of Hp samples. Our results are discussed in terms of oxygen vacancies and hole carriers which are produced by annealing for the considered samples.

Optical and ferroelectric properties of ruthenium-doped BaTiO3 nanocubes

Structural, optical, dielectric and ferroelectric properties of Ru-doped BaTi1−xRuxO3(x = 0.01, 0.03, 0.05, 0.07, 0.10) nanocubes have been investigated. The average size of nanocube obtained from Rietveld refinement is in the range 35–60 nm. The incorporation of Ru leads to a mixed tetragonal and cubic crystalline phases in doping concentration range 5–7%. Optical band gap changes from 3.2 to 2.74 eV with increase in Ru concentration. Replacement of Ti with Ru quenches band-edge emission and enhances defect emission intensity. Optical absorption and emission spectra have been interpreted in terms of oxygen vacancy. Ruthenium 4d orbital plays a crucial role to improve dielectric constant and diminish ferroelectric Curie temperature. The combined effect of Ru dopant and size gives rise to complex variation of Curie temperature.

Disappearance of Griffiths Phase in Polycrystalline Sample La0.75Ca0.15MnO3−Δ with Controlling Oxygen Vacancy

The Griffiths phase has been investigated in terms of oxygen vacancy in polycrystalline samples La0.75Ca0.15MnO3 (LCMO-1) and La0.75Ca0.15MnO3−Δ (LCMO-2). The latter of the two samples is prepared by annealing the former (with existence of Griffiths phase) in N2 atmosphere. Experimental results demonstrate that for the latter sample Griffith’s phase disappears (according to its obeying Curie–Weiss law in the temperature dependence of inverse susceptibility) due to redistribution of magnetic disorder between Mn3+ and Mn4+ ions induced by the increment of crystal lattice structure parameters. XRD patterns of the two samples show an increase of crystal lattice structure parameters. X-ray absorption fine structure data manifest that the degree of structural disorder in LCMO-2 sample is higher than that of LCMO-1, indicating that local structural disorder in this system is not the key role to affect the existence of Griffiths phase, while magnetic distribution/disorder is an important character.

Phase transformation of titania gels highly doped with Fe in different sintering atmospheres

The effect of sintering atmosphere on the phase transformation of high contents of Fe-doped titania gels was studied. Experimental results show that the sintering atmosphere has less influence in the transformation temperature of the gel to anatase. Sintering in an oxidizing atmosphere causes precipitation of the Fe2O3 phase while sintering in a reducing atmosphere promotes the transformation of the gel to anatase, causes the Fe ions to dissolve entirely into the titania crystals, and enhances the transformation temperature of anatase to rutile. Fe-doping inhibits the transformation of the gel to anatase while promoting the transformation of anatase to rutile and decreasing the mean crystal size of the titania crystals during sintering in oxidizing or reducing atmospheres. Moreover, the action mechanism of the sintering atmosphere and Fe-doping are discussed in terms of oxygen vacancies, ionic valence and substitutions.

Role of growth temperature and oxygen partial pressure on the structural and electrical properties of pulsed laser deposited La 1− xSr xnO 3− δ thin films

The paper presents the fabrication and characterization of La 0.65Sr 0.35MnO 3− δ (LSMO) polycrystalline thin films deposited directly on Si (1 0 0) substrates using pulsed laser deposition technique. Various deposition parameters like substrate temperature and oxygen partial pressure have been varied systematically to obtain stoichiometric, crack-free films with smooth surface morphology having nearly monodisperse grain size distribution. The substrate temperature variation from 600 to 800 °C had profound effects on the microstructure and topography of the deposited film, with optimum result being obtained at 700 °C. The variation of partial pressure of oxygen controls the deposition kinetics as well as the stoichiometry of the film in terms of oxygen vacancy, which influences the magnetic and electrical transport properties of the manganate films. The microstructure and crystallinity of the deposited films have been studied using X-ray diffraction, scanning electron microscopy and atomic force microscopy. A correlation between the oxygen stoichiometry and micro-structural and transport properties of the deposited films has been obtained.

Support Effects in the Gold‐Catalyzed Preferential Oxidation of CO

AbstractThe study of support effects on the gold‐catalyzed preferential oxidation of carbon monoxide in the presence of hydrogen (PROX reaction) is possible only with careful control of the gold particle size, which is facilitated by the application of the direct anionic exchange method. Catalytic evaluation of thermally stable gold nanoparticles, with an average size of around 3 nm on a variety of supports (alumina, titania, zirconia, or ceria), clearly shows that the influence of the support on the CO oxidation rate is of primary importance under CO+O2 conditions and that this influence becomes secondary in the presence of hydrogen. The impact of the support surface structure on the oxidation rates, catalyst selectivity, and catalyst activation/deactivation is investigated in terms of oxygen vacancies, oxygen mobility, OH groups, and surface area on the oxidation rates, catalyst selectivity and catalyst activation/deactivation. It allows the identification of key morphological and structural features of the support to ensure high activity and selectivity in the gold‐catalyzed PROX reaction.

Electronic and magnetic properties of Co-doped ZnO diluted magnetic semiconductor

The effect of low level Co doping (5%) on polycrystalline ZnO samples has been investigated to correlate the observed changes in their magnetic state vis à vis changes in their electronic properties. Rietveld refinement of the XRD patterns confirms single phase crystallization of the samples in the wurtzite type lattice , with no evidence of any secondary phases. The as-synthesized Co-doped sample shows a paramagnetic (PM) state, however, when hydrogenated for ∼6 h, it shows a strong ferromagnetic (FM) ordering. The magnetic moment suppressed significantly when hydrogen ions were evaporated by heating and the sample turned completely paramagnetic upon long heating in air. The Co 2p X-ray photoelectron spectroscopy (XPS) results show that the substituted Co ions are in 2+ oxidation state that incorporate at Zn 2+ sites and no evidence of metallic Co is observed upon hydrogenation. The XRD refinement results and the O 1s XPS results show clear evidence of oxygen depletion upon hydrogenation, followed by a complete regain upon their long heating in air. A plausible explanation for the observed room temperature ferromagnetism (RTFM) is presented in terms of oxygen vacancies, in the framework of bound magnetic polarons model. Our results evidence that the FM ordering can be switched between “on” and “off” by introducing (upon hydrogenation) or removing (by re-heating), respectively, the oxygen vacancies in the Co-doped ZnO matrix.