Contribution Of Oxygen Vacancies Research Articles

Resistive switching and synaptic simulation behaviors of epitaxial ZnO/Nb-doped SrTiO3 heterojunction controlled by the substrate orientation

Zinc oxide (ZnO) films were grown epitaxially on Nb-doped SrTiO3 (NSTO) substrates with different orientations using laser pulse deposition to create the (112¯0)ZnO/(100)NSTO, (0002)ZnO/(110)NSTO and (0002)ZnO/(111)NSTO heterojunctions. Epitaxial ZnO films on the (100) NSTO had two orthorgonal domains, resembling the fourfold symmetry of the (100) NSTO substrate. This good matching between the grown ZnO film and NSTO substrate resulted in a lower growth rate, smoother and denser surface, and a higher density of interface defect states. The interface state density was approximately 4.4 × 1012 eV−1cm−2, which was notably higher than that of the other two heterojunctions. Electrical transport studies revealed that ZnO/(100)NSTO heterojunction demonstrated bipolar resistive switching and negative differential resistance (NDR) due to a higher density of donor interface states from contributions of oxygen vacancies, while both the ZnO/(110)NSTO and ZnO/(111)NSTO heterojunction exhibited typical rectifying behavior. The preliminary investigation into synaptic behaviors suggested that (112¯0) ZnO/(100)NSTO heterojunction holds significant potential in brain-inspired neuromorphic computing systems.

Tuning of photoluminescence properties: Impact of Pr-doping in SrTiO3 crystallites

Materials having deep blue emission characteristics with a CIEy chromaticity value less than 0.1 are highly desired. Here we incorporated Pr3+ into the Sr2+ site of the Sr1-x PrxTiO3 (x=0.05, 0.075, 0.10, 0.125, 0.15, 0.20) to quench the typical red photoluminescence (PL) emission in Pr-doped compounds and tune it to a blue-emitting system. XRD analysis and structural refinement demonstrate transition from cubic to tetragonal symmetry at Pr concentrations of, x≥0.1. The contribution of oxygen vacancies in emission mechanism was investigated through XPS analysis. High-resolution FE-SEM analysis of morphological features revealed the creation of tiny grains with the appearance of micro pebbles. Under 380 nm UV illumination a narrow-band blue emission peak FWHM of 19.3 nm was obtained at 428 nm for x=0.05. CIE plots were used to examine the quality of emission peaks and samples show CIEy less than 0.1. This blue emission is crucial for lighting and display applications.

Elucidating the role of oxygen vacancies on the electrical conductivity of β-Ga2O3 single-crystals

The contribution of oxygen vacancies (VO) to the electrical conductivity of unintentionally doped β-Ga2O3 has been a topic of recent debate. Here, we use a combination of Hall measurements and Raman spectroscopy on as-grown and O2-annealed β-Ga2O3 crystals to investigate the role of VO on electrical conductivity. The annealed samples show a significant decrease in carrier concentration. By comparing the relative Raman shift of individual modes with theoretically calculated contributions of oxygen sites to these modes, we verify the marked reduction of VO in annealed β-Ga2O3 crystals. Furthermore, the IR modes in β-Ga2O3, usually hidden by free carrier absorption, are clearly seen in the annealed sample. The reduction of band tail states as well as free carrier absorption in the annealed samples provides additional evidence for reduced carrier concentration related to VO, making them a key determinant of electrical conductivity in β-Ga2O3.

Oxygen vacancy promoted heterogeneous Fenton-like degradation of sulfamethazine by chlorine-incorporated micro zero-valent iron

Herein, we report an oxygen vacancies (OVs)-rich micro zero-valent iron (mZVI) interface for selective and efficient H2O2 degradation by mechanical chloride (Cl) doping. With the contribution of OVs, electron transfer process was sped up due to the charge unbalance of mZVI, and abundant adsorbed ferrous was produced. The resultant Cl-ZVIBM/H2O2 system increased the yield of •OH for threefold, without a significant increase in the subordinate reactive oxygen species, and the degradation rate for sulfamethazine was enhanced by 13 folds. The catalytic reactivity of as-prepared material was maintained for more than 230 days in dry air. The performance of as-prepared catalyst to practical water remediation was evaluated, and it demonstrated excellent results. The study shed light to improve the utilization efficiency of H2O2 by an OVs-rich iron-based catalyst using a simple mechanochemistry method of doping halogen elements.

Oxygen tracer diffusion analysis and observation of domain structure in quenched (Bi0.5Na0.5)TiO3 ceramics

Lead-free ferroelectric and piezoelectric ceramics, (Bi0.5Na0.5)TiO3 [BNT] ceramics were prepared by controlling the quenching rate. The quenching effects mainly caused an increase in lattice distortion 90-α and T d, contribution of electrical properties and ordered structural phase transitions. In this study, we analyzed oxygen tracer diffusion and examined domain structures for quenched BNT ceramics, and then we proposed a mechanism for the quenching effect. As a result, the diffusion coefficients D of 18O tracer for the OF and quenched samples were 2.5 × 10–11 and 1.8 × 10–11 cm2 s−1, respectively, and there were no significant differences in oxygen vacancies after quenching. The correlation between the quenching effect and oxygen vacancies was also examined in BNT with hard and soft dopants. However, the contribution of oxygen vacancies was small thus, the correlation between the quench effect and oxygen vacancies was low. On the other hand, the domain structure of BNT ceramics was observed and the domain size of OF-BNT was around 20 nm. This is a typical domain size for BNT ceramics. In contrast, the domain size of the quenched BNT was 40 nm. Thus, quenching increased the domain size and decreased their density. Consequently, we propose that the domain structure is strongly correlated with the quench-induced increase in lattice distortion and T d, its contribution to electrical properties, and ordered structural phase transitions.

Insights into the Contribution of Oxygen Vacancies on Co2 Activation for Dry Reforming of Methane Over Ceria-Based Solid Solutions

Insights into the Contribution of Oxygen Vacancies on Co2 Activation for Dry Reforming of Methane Over Ceria-Based Solid Solutions

Engineering Multidefects on Cex Si1- x O2- δ Nanocomposites for the Catalytic Ozonation Reaction.

Herein, it is shown that by engineering defects on Cex Si1- x O2- δ nanocomposites synthesized via flame spray pyrolysis, oxygen vacancies can be created with an increased density of trapped electrons, enhancing the formation of reactive oxygen species (ROSs) and hydroxyl radicals in an ozone-filled environment. Spectroscopic analysis and density functional theory calculations indicate that two-electron oxygen vacancies (OV 0 ) or peroxide species, and their degree of clustering, play a critical role in forming reactive radicals. It is also found that a higher Si content in the binary oxide imposes a high OV 0 ratio and, consequently, higher catalytic activity. Si inclusion in the nanocomposite appears to stabilize the surface oxygen vacancies as well as increase the reactive electron density at these sites. A mechanistic study on effective ROSs generated during catalytic ozonation reveals that the hydroxyl radical is the most effective ROS for organic degradation and is formed primarily through H2 O2 generation in the presence of the OV 0 . Examining the binary oxides offers insights on the contribution of oxygen vacancies and their state of charge to catalytic reactions, in this instance for the catalytic ozonation of organic compounds.

Structural, optical, antimicrobial and ferromagnetic properties of Zn1−xLaxO nanorods synthesized by chemical route

Nanoparticles of Zn1−xLaxO (x = 0, 0.01, 0.03 and 0.05) were successfully synthesized by a cost-effective chemical method. The influences of La on the microstructural, optical, antimicrobial and magnetic properties of these ZnO nanoparticles were investigated. XRD studies revealed the presence of hexagonal wurtzite phase of ZnO without any impurity. The samples possess nanorod morphology as confirmed by FESEM and TEM analysis. The band gap of ZnO nanorods increased from 3.25 to 3.27 eV with increasing La concentration from 0% to 5%. The presence of various defects in our samples were identified by PL spectroscopy. Magnetic studies exposed the signature of ferromagnetism in all the samples at 300 K. The magnetization of ZnO nanorods is significantly enhanced by La doping. A maximum values of magnetization was observed for 1% La doped ZnO sample which is ~6 times more than for a pure ZnO nanorods. The contributions of oxygen vacancies towards the observed ferromagnetism were confirmed. The origin of ferromagnetism was enlightened on the basis of results of a BMP model. In vitro bioassays showed that synthesized nanorods exhibited narrow-spectrum antibacterial activity against Staphylococcus aureus that compared well with commercial ZnO, nanoparticles (Sigma). Furthermore, the synthesized nanorods when tested for toxicity against Caenorhabditis elegans, but show only low toxicity at 100 µg/mL. Toxicity in zebrafish was also limited. Based on their multifunctional properties, we believe that the synthesized ZnO nanorods could be beneficial for optical device applications in the UV range, spintronic devices, as well as for biomedical purposes.

Colossal dielectric response in erbium iron garnet ceramics

Polycrystalline Er3Fe5O12 ceramic sample was synthesized by a solid-state reaction technique. The dielectric properties of the sample were experimentally studied. A colossal dielectric constant more than 103 was observed over a wide temperature and frequency range. The dielectric relaxation in the sample was identified by the measurement of permittivity modulus, impedance spectroscopy, and conductivity in detail. At relatively low temperatures (T < 600 K), the dielectric relaxation under high frequencies (f ≥ 50 kHz) is triggered by the dipolar effect accompanied by the Fe2+ and Fe3+ charge transitions, while that under low frequencies (f ≤ 50 kHz) is mainly be associated with the Maxwell–Wagner effect. Moreover, a low-frequency dielectric relaxation at high temperatures (T ≥ 600 K) is probably evolving from the contribution of oxygen vacancies and grain boundaries, which is also suggested to be the origin of the colossal dielectric constant.

Impedance spectrum and magnetic properties of BiFe0.95Ti0.05O3 ceramics

Multiferroic BiFe0.95Ti0.05O3 (BFTO) powders were synthesized by sol–gel method, and then compacted by spark plasmas sintering (SPS) at 650 °C. Analysis of structure and morphology suggested the formation of a distorted rhombohedral in BFTO ceramics with a homogeneous distribution in grain sizes. BFTO ceramics revealed electric homogenous properties verified from the plots of complex impedance versus frequency. The contribution of oxygen vacancies in the behaviors of dielectric and ac conductivity was analyzed. At high temperature (473 K), the dielectric relaxation and ac conductivity were mainly attributed to the motion of oxygen vacancies. Doping of Ti4+ in BiFeO3 ceramics showed a high remanent magnetization due to the structural evolution and the broken of the spiral magnetic ordering in BiFeO3 system.

Post-annealing effects on excitonic states and defects in nanostructured Ti-doped ZnO thin films

We study the effects of post-annealing treatments on excitonic states and defects of nanostructured Ti-doped ZnO thin films grown by room-temperature magnetron sputtering technique. Here, the as-grown samples were annealed by varying the annealing environment, time, and temperature. We found that the higher excitonic emission was at 600 °C under N2 environment for 30 min. The excitonic states are analyzed by using spectroscopic ellipsometry, photoluminescence, and ultraviolet-visible absorption spectroscopy. Through critical point analysis of the complex dielectric function from spectroscopic ellipsometry, the energy of exciton is redshifted from 3.49 eV to 3.36 eV and the increase of excitonic transition is observed upon the annealing treatment. The weak excitonic transition in the as-grown sample is mainly due to the oxygen vacancies that promote electronic screening effect. The contribution of oxygen vacancies is confirmed by the presence of green luminescence (∼2.5 eV) and space-charge-limited current regime from electrical properties measurements. This work is very important for tunable optoelectronic devices based on the doped ZnO system.

Intrinsic and extrinsic doping contributions in SnO2 and SnO2:Sb thin films prepared by reactive sputtering

SnO2 thin films are considered cheap transparent electrodes, interesting for scalable electronic applications, with intrinsic n-type conductivity related to donor defects like tin interstitials and oxygen vacancies. Besides, SnO2 allows n-type extrinsic doping by pentavalent cations such as Ta or Sb. In the present work, SnO2 and SnO2:Sb films were deposited on unheated soda lime glass substrates by reactive DC sputtering from Sn and Sn:Sb(5%) targets. Their structural, optical and electrical properties have been analyzed comparatively for the layers prepared at different oxygen partial pressures in the sputtering atmosphere, as-grown and after heating in air at several temperatures. Sb doping is proven effective to improve the electrical conductivity of sputtered tin dioxide samples. The contribution of oxygen vacancies is found dependent on the reactive deposition environment, in both pure and Sb-doped films, but subsequent heating can change the proportion and nature of the donor defects. For operation at temperatures above 400 °C, SnO2:Sb shows the best electrical performance and good thermal behavior is also achieved with pure SnO2 layers by adjusting the oxygen content in the sputtering atmosphere.

Ambient Electrosynthesis of Ammonia on a Core-Shell-Structured Au@CeO2 Catalyst: Contribution of Oxygen Vacancies in CeO2.

Electrosynthesis of NH3 through the N2 reduction reaction (NRR) under ambient conditions is regarded as promising technology to replace the industrial energy- and capital-intensive Haber-Bosch process. Herein, a room-temperature spontaneous redox approach to fabricate a core-shell-structured Au@CeO2 composite, with Au nanoparticle sizes below about 10 nm and a loading amount of 3.6 wt %, is reported for the NRR. The results demonstrate that as-synthesized Au@CeO2 possesses a surface area of 40.7 m2 g-1 and a porous structure. As an electrocatalyst, it exhibits high NRR activity, with an NH3 yield rate of 28.2 μg h-1 cm-2 (10.6 μg h-1 mg-1 cat. , 293.8 μg h-1 mg-1 Au ) and a faradaic efficiency of 9.50 % at -0.4 V versus a reversible hydrogen electrode in 0.01 m H2 SO4 electrolyte. The characterization results reveal the presence of rich oxygen vacancies in the CeO2 nanoparticle shell of Au@CeO2 ; these are favorable for N2 adsorption and activation for the NRR. This has been further verified by theoretical calculations. The abundant oxygen vacancies in the CeO2 nanoparticle shell, combined with the Au nanoparticle core of Au@CeO2 , are electrocatalytically active sites for the NRR, and thus, synergistically enhance the conversion of N2 into NH3 .

Cover Feature: Ambient Electrosynthesis of Ammonia on a Core–Shell‐Structured Au@CeO2 Catalyst: Contribution of Oxygen Vacancies in CeO2 (Chem. Eur. J. 23/2019)

The energy intensive Haber–Bosch process could potentially be replaced by the electrocatalytic N2 reduction reaction (NRR) to produce NH3 if high-efficiency electrocatalysts can be developed. This work reports an electrocatalytic N2 reduction reaction to produce NH3 using a core-shell structure Au@CeO2 electrocatalyst synthesized by a room-temperature spontaneous redox approach using Ce3+ and AuCl4−1 as reaction precursors. The experimental results and theoretical calculations validate that the abundant oxygen vacancies in shell layer CeO2 nanoparticles are electrocatalytically active sites for the N2 adsorption and activation, coupling with small-sized Au nanoparticles in the core layer of Au@CeO2, synergistically enhancing its NRR performance. More information can be found in the Full Paper by M. Han, H. Zhang et al. on page 5904.

Simultaneous regulation of photoabsorption and ferromagnetism of NaTaO3 by Fe doping

NaTa1-xFexO3 (0 ≤ x ≤ 0.40) nanocubes were synthesized by a relatively low temperature hydrothermal method, using Ta2O5, FeCl3 and NaOH as the precursors. The UV–vis diffuse reflectance spectra showed that NaTa1-xFexO3 had significant visible-light-absorbing capability, and the absorption edge of NaTaO3 shifted to longer wavelength with the increase of Fe dopants. Moreover, NaTa1-xFexO3 exhibited room-temperature ferromagnetism when Fe3+ occupied Ta5+ sites in NaTaO3 crystal lattice. The ferromagnetism is mainly attributed to the superexchange interactions between doped Fe3+, rather than the contribution of oxygen vacancies caused by Fe doping. Therefore, Fe doping can simultaneously regulate the optical and magnetic properties of NaTaO3 semiconductor, which will enable its potential applications in multifunctional optical-electronics and optical-spintronics devices.

Thermodynamic Stabilities, Electronic Properties, and Optical Transitions of Intrinsic Defects and Lanthanide Ions (Ce3+, Eu2+, and Eu3+) in Li2SrSiO4.

Geometric structures, electronic properties, thermodynamic stabilities, and optical transitions of intrinsic defects (vacancies and antisite defects) and lanthanide ions (Ce3+, Eu2+, and Eu3+) in Li2SrSiO4 (LSSO) host are studied by theoretical calculations combined with hybrid density functional theory, the multireference configuration interaction method, and empirical models. Calculations on the defect formation energies and the ab initio simulations of 4f → 5d electronic transitions for Ce3+ ions determine the most possible charge-compensation mechanism and accurately identify excitation bands in experimental spectra for LSSO:Ce3+ phosphors. On the basis of previously reported experimental spectra of Ce3+- and Eu3+-doped LSSO phosphors as well as a series of empirical models developed by Dorenbos, the locations of the 4f ground states and the lowest 5d excited states of Ln3+ and Ln2+ ions in the host (illustrated by the host-referred binding energy scheme, i.e., the HRBE scheme) are obtained, which is useful for the investigation of the electron-transfer and spectroscopic properties in lanthanide-doped LSSO. Moreover, thermodynamic and optical transition energy levels related to intrinsic defects and lanthanide ions (with various charge states) are derived from total energy calculations. The mechanisms of thermoluminescence (TL) and long-lasting luminescence (LLL) in LSSO:Eu2+,Dy3+ phosphors and especially the contributions of oxygen vacancies ( VO) and Dy3+ dopants are then interpreted. The aim of this study is thus to deeply understand the mechanisms of charge compensation, TL, and LLL in lanthanide-doped phosphors from theoretical calculations and analyses.

Contributions of oxygen vacancies and titanium interstitials to band-gap states of reduced titania

The spectroscopic fingerprints of the point defects of titanium dioxide remain highly controversial. Seemingly indisputable experiments lead to conflicting conclusions in which oxygen vacancies and titanium interstitials are alternately referred to as the primary origin of the Ti $3d$ band-gap states. We report on experiments performed by electron energy loss spectroscopy whose key is the direct annealing of only the very surface of rutile ${\mathrm{TiO}}_{2}(110)$ crystals and the simultaneous measurement of its temperature via the Bose-Einstein loss/gain ratio. By surface preparations involving reactions with oxygen and water vapor, in particular, under electron irradiation, vacancy- and interstitial-related band-gap states are singled out. Off-specular measurements reveal that both types of defects contribute to a unique charge distribution that peaks in subsurface layers with a common dispersive behavior.

Dielectric Properties in the Microwave Range of K0.5Na0.5NbO3 Ceramics

Dielectric properties of a potassium sodium niobate (KNN) system in the microwave range up to GHz have rarely been studied. Since K0.5Na0.5NbO3 is the most common and typical type of KNN materials, non-doped K0.5Na0.5 NbO3 ceramics were synthesized at different temperatures (1080°C, 1090°C, 1100°C, and 1110°C) by a traditional solid reaction method for further characterization and analysis. The ceramics were in perovskite phase with orthorhombic symmetry. A small quantity of second phase was found in the 1110°C sintered specimen, which resulted from the volatilization of alkali oxides as the temperature increased. The complex permittivity was measured for the first time in the microwave range (8.2–12.4 GHz) and in the temperature range from 100°C to 220°C, and the effects of annealing on the dielectric properties were studied. The results indicate that the complex permittivity of KNN ceramics over the microwave range increases mainly due to high bulk density and the additional dielectric contributions of oxygen vacancies at high temperature.

Contribution of oxygen vacancies to the giant dielectric response in Sm1.5Sr0.5NiO4−δ ceramics

The dielectric properties of Sm1.5Sr0.5NiO4−δ ceramics with different concentrations of oxygen vacancies were characterized. The ceramics with lower concentration of oxygen vacancies were prepared by directly sintering the sol–gel derived powders in air, while the higher one could be obtained by annealing the as-sintered ceramics in the flow of nitrogen. The post-densification annealing in the flow of nitrogen decreased the dielectric constant at low temperature and increased it at high temperature, while the dielectric loss increased in overall temperature range. The activation energy of low-temperature dielectric relaxation decreased with increasing the concentration of oxygen vacancies, and so did that of bulk electrical resistances although the values of resistances increased, while the activation energy of electrical resistances for grain boundary increased though the values of resistances decreased. The giant dielectric response in the as-sintered Sm1.5Sr0.5NiO4−δ ceramics should be mainly attributed to the small polaronic hopping process, while that of annealed ceramics should be directly linked to the oxygen vacancies.

Enhanced Photoluminescence Effect of Poly(L-lactide) Biodegradable Polymer with Functionalized Carbon Nanotubes

Functionalization of Multiwalled Carbon Nanotubes (MWCNTs) for biological applications was considered. For this purpose, the pristine MWCNTs (pMWCNTs) are covalently aminated using p-amino benzoic acid and phosphoric acid without shortening of pMWCNTs. The grafted amine groups on the sidewall of pMWCNTs are useful as initiator for grafting of polymer chains on the sidewall of aminated MWCNTs (MWCNT-NH2). The PLLA homopolymer chains grafted covalently from the sidewall of aminated MWCNTs successfully. The FT-IR spectra revealed that the amine groups and the PLLA chains grafted form the sidewall of pMWCNTs strongly. The fluoroscopy test results shows that the pMWCNTs and aminated MWCNTs have not any significant photoluminescence (PL) effect at 350 nm excitation. The MWCNT-g-PLLAs exhibit an extremely strong visible emission at 404 nm and 429 nm. These results may be ascribed to the contribution of oxygen vacancies and defects at the end chain of grafted PLLA chains on the sidewall of MWCNTs that created by Sn atoms of initiator during polymerization on the sidewall of MWCNTs. The oxygen vacancies usually act as radiative centers in the PL process. In addition, The MWCNT-g-PLLAs enhance the PL of neat PLLA. The PL of composites slightly diminishes with blue shift during in-vitro degradation. As mentioned above, the MWCNT-g-PLLAs are the functionalized MWCNTs that applicable in the field of medicine. Due to biocompatible and biodegradable molecular groups on the sidewall of MWCNTs, it is predictable that the MWCNT-g-PLLAs will pass all exams for its application in medicine.