Oxygen Vacancy Levels Research Articles

Variable dimensional structure and interface design of In2O3/rGO nanocomposites with oxygen vacancy for enhancing NO2 sensing performance

Rational interface design is an efficient strategy to enhance gas sensing performance. Herein, the large-scale heterogeneous interfaces consisting of porous In2O3 sheets with reduced graphene oxide (rGO) modification were constructed by convenient face-to-face contacts. Heterogeneous In2O3 nanorods were employed as a counterpart to explore the role of the interface between rGO and In2O3. The morphology, surface area, chemical surface state, adsorption heat, activation energy, etc., were investigated comprehensively. The results indicated that 0.5% rGO/h-In2O3-sheet and 0.5% rGO/hc-In2O3-rod enhanced 3.88-fold and 1.66-fold in the response than corresponding pristine ones, respectively. 0.5% rGO/h-In2O3-sheet with extensive interface achieved superior sensing performance. Simultaneously, the response and recovery times of hybrids were shortened significantly, owing to the low activation energy. Moreover, 0.5% rGO/h-In2O3-sheet exhibited an ultra-high sensor response of 1397 to 1 ppm NO2 with a detection limit as low as 5 ppb at 62.5 ℃. It also possessed excellent selectivity, repeatability and long-term stability. The decoration of rGO on the In2O3 sheets dramatically manipulated the oxygen vacancy, band-gap, and Fermi level and enhanced the efficiency of gas and carriers exchange and transportation, eventually boosting the sensing performance.

β particles induced directional inward migration of oxygen vacancies: Surface oxygen vacancies and interface oxygen vacancies synergistically activate PMS

In this work, a high-energy β electron beam was applied to promote the generation and directional inward migration of surface oxygen defects. The valence band potential changed to a high potential due to the transfer of surface oxygen vacancies on BiVO4/Bi2O3−x. Interface oxygen vacancies have significant advantages in inducing the formation of oxygen vacancy levels and reducing the band gap energy. Surface oxygen vacancy and vacancy levels can prevent the recombination of photogenerated carriers. The electrons enriched in the surface oxygen vacancies can remarkably activate PMS with O−O bonds to degrade ciprofloxacin (CIP). The degradation kinetic constant and CIP mineralization rate of BiVO4/Bi2O3−x-450/PMS were 2.09 and 1.51 times higher than those of BiVO4/Bi2O3−x/PMS, respectively. BiVO4/Bi2O3−x-450 with a lower relative content of surface oxygen vacancies (24.70%) exhibited a stronger activation ability. Obviously, the change of band structure caused by interface oxygen vacancies is the reason for increasing photoactivation efficiency of PMS.

Natural Diatomite Supported Zirconium-Doped TiO2 with Tailoring Band Structure for Enhanced Visible-Light Photocatalytic Properties.

The development of economically applicable, highly efficient and low cost photocatalytic materials has always been a challenge. In this work, we report a zirconium doped TiO/diatomite (ZrTD) composite with enhanced visible light-induced photocatalytic activity. The as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, UV–VIS diffused reflectance spectroscopy, high-performance liquid chromatography-mass spectrometry, photoluminescence and X-ray photoelectron spectroscopy, respectively. The optimal doping ratio of zirconium into TiO was obtained at 3% (3%ZrTD composite), and the degradation rate constant of which tetracycline (TC) is up to around 8.65 times higher that of zirconium doped TiO. In addition, zirconium doping introduces the impurity levels of Zr 3d and oxygen vacancies into the lattice of TiO, resulting in broadening the light absorption range, reducing the band gap, and improving the separation efficiency of photogenerated electron-hole pairs, thus endowing with visible light photocatalytic properties. Moreover, both the photogenerated holes (h) and superoxide (•O) radicals are responsible for the degradation process of TC, and a possible degradation pathway and the corresponding intermediate products of TC by ZrTD composite are also proposed in detail.

Structure and defects-related optical properties of highly (002)-oriented zinc oxide thin films

We report on the highly (002)-oriented ZnO thin films deposited on glass substrates at different substrate temperatures using the recently developed pulsed laser deposition (PLD) facility at our institution. Effects of the substrate temperature on the structural, compositional, morphological, and optical properties of the ZnO thin films are studied. The XRD studies revealed the deposition of high-quality ZnO thin films with preferred hexagonal wurtzite structure along the (002) plane above 200 °C substrate temperatures. An increase in the crystallite size estimated using the Scherrer formula from 9.58 nm to 13.25 nm of the deposited thin films reflects the enhancement of the crystallinity with the increasing substrate temperature. Scanning electron microscope (SEM) micrographs revealed a smooth surface morphology with small grains for the thin film deposited at room temperature. The thin films deposited at high substrate temperatures showed agglomeration of nanoparticles resulting in nano rod-like structures. The optical band gap energy was found to be increased with an increase in the crystallinity of the thin films. In the photoluminescence (PL) spectra; near band edge (NBE) emissions were observed in all of the deposited thin films. The deep-level emissions of ZnO thin films were found to be highly dependent on the substrate temperature during film deposition. The thin films grown at 500 °C showed the visible emission in blue, green, and yellow-orange regions, which corresponded to zinc vacancy (VZn), oxygen vacancy (VO), and oxygen interstitial (Oi) defect levels, respectively. These crystal defects in the deposited ZnO thin films are responsible for the deep level emission linked to the visible emission.

(Keynote) Roles of Oxygen Vacancies in Metal Oxide Photocatalysts and Electrocatalysts

This talk concerns with the roles of oxygen vacancies in various metal oxide photocatalysts and electrocatalysts. A low level of oxygen vacancies in titanium oxide can induce shallow and deep sub-band gap Ti(III) states below the conduction band, which increases light absorption but not enhance photocatalytic activity. A high level of oxygen vacancies in metal oxides may generate an abundance of states near Fermi level and induce surface plasmon resonance.

Selective formation of reactive oxygen species in peroxymonosulfate activation by metal-organic framework-derived membranes: A defect engineering-dependent study

Defect engineering is an effective way to unveil relationship within structures and catalytic activities of transition metal oxides. Herein, a novel strategy has been developed for in situ generation and controlling oxygen vacancy (Ov) levels in a host lattice by varying oxygen pressure during calcination of zeolitic imidazolate framework-67 (ZIF-67) membranes. The as-prepared NFZ-5 membrane with the largest Ov content (δ, 0.912) gave the highest 1O2 production (98.3%) and PMS activated BPA degradation kinetics (k = 0.11 min−1). Advanced characterization and density functional theory (DFT) calculations have revealed the pivotal role of Ov in modifying surface chemistry of the catalytic membrane via enhancing the number of Lewis acid sites. These Lewis acid sites have facilitated the chemisorption of peroxymonosulfate (PMS) onto membrane, and the resulting reactive intermediate complexes have altered the electron transfer direction between PMS and the catalyst.

The Influence of Repetitive UIS on Electrical Properties of Advanced Automotive Power Transistors

This paper investigates a degradation of three types of automotive power MOSFETs through repetitive Unclamped Inductive Switching (UIS) test typically used to evaluate the avalanche robustness of power devices. It is not uncommon in switching applications that greater than the planned voltage for voltage spikes can occur, so even the best electronic designs may encounter frequent avalanche events. Hence, there is a need to analyse the impact of repetitive avalanching on the electrical performance of power transistors. This article focused on the shift of main electrical parameters: on-resistance R_{ON}, breakdown voltage V_{BR}, threshold voltage V_{TH}, and corresponding characteristics, as well as capacitances. Analysis proved that DMOS transistors are less vulnerable to repetitive avalanching. The most impacted parameter was on-resistance R_{DSon}, where a 14 % increase was observed after 6 x 10^7 stress pulses. The parameters shift is attributed to hot carrier injection in the space charge region of blocking PN junction and involves mainly defects generation/activation in the drain side region of the gate oxide. For the TrenchMOS transistor, a significant shift of I-V curves was observed with considerable impact on the R_{ON} where an increase of 22 % was observed. The trench corner is verified to be the mainly degraded region by Synopsys Technology Computer Aided Design (TCAD) simulations. Degradation of drain-gate capacitance C_{DG} and input capacitance C_{in} was observed in all three types of analysed structures. DLTS was used to verify the generation/activation of defects invoked by stress. An increase of DLTS signal corresponding to energy levels of oxygen vacancies and impurities in SiO_2 and on interfaces were detected on stressed samples.

Activation of peroxymonosulfate by MnO2 with oxygen vacancies: Degradation of organic compounds by electron transfer nonradical mechanism

Recently, ecofriendly and low toxicity manganese-based materials are receiving increased attention for the activation of peroxymonosulfate (PMS). The surface property of manganese-based materials has strong effect on its activation performance. In this study, MnO2 with differing levels of oxygen vacancies (Vo-MnO2) were prepared and used for PMS activation. The effects of oxygen vacancies (Vo), activator dosage, PMS concentration, and the initial pH on para-chloroaniline (PCA) degradation were studied. In these studies, Vo-MnO2 showed good stability and reusability for PMS activation. In addition, a series of experiments revealed the PMS activation mechanism. Interestingly, •OH, SO4•- and 1O2 were not primarily responsible for PCA degradation. PCA was degraded by the reactive complex formed by PMS and Vo-MnO2. Based on the characterization results and DFT calculations, the introduction of Vo in MnO2 changed the charge distribution of Mn atoms and enhanced PMS adsorption. This is more favorable to form the reactive complex in electron transfer mechanism. Besides, Vo increases the Mn3+ content in MnO2, which enhances oxidation ability of reactive complex and the efficiency of electron transfer. The degradation efficiency of pollutants in the Vo-MnO2/PMS system has been improved significantly. This study highlights the pivotal roles of Vo in electron transfer mechanism and provides a new method for the practical application in wastewater treatment.

Preparation of Li2MnO3 nanowires with structural defects as high rate and high capacity cathodes for lithium-ion batteries

Li2MnO3, as a member of Li-rich and Mn-rich (LMR) layered oxides, has been considered as an appealing candidate for cathode preparation due to its high capacity and high voltage. Nevertheless, its commercialized application has been hindered by its demerits of poor ratability and capacity decay. Meanwhile, structural defects have been proven to influence the electrochemical activities of Li2MnO3. Herein, Li2MnO3 nanowires with varying numbers of structural defects were prepared and measured as cathode materials. rLMO-2 nws with the highest level of oxygen vacancies and the largest amount of Mn3+ were found to exhibit higher specific capacities, higher capacity retention rates, and better rate performances. In order to demonstrate the superiority of nanowire materials with large surface areas, the performance of nanowires materials was compared with that of nanobelt materials. As LIB cathodes within the voltage window of 2.0–4.8 V and at the rate of 0.1C, R-LMO-30 nbs were found to reach the specific capacities of 98.7, 119.1, 137.7, and 146.8 mAh g−1 in the 2nd, 5th, 10th, and 20th cycles, respectively, while rLMO-2 nws delivered the relatively high values of 170.7, 189.4, 196.4, and 162.3 mAh g−1, respectively. The rLMO-2 nws sample also showed an enhanced rate capacity. The superior electrochemical properties of the rLMO-2 sample were associated with the nanowire structure’s large surface area, which could not only activate the Li2MnO3 phase quite easily but also enhance electron and ion transportation properties. This research revealed the influences of morphological control on the formation of surface structural defects in Li-rich layered oxides. And we believe our research findings will be helpful in providing a feasible strategy to enhance the electrochemical performance of Li-rich and Mn-rich layered oxides materials.

Structural, electrical, and luminescence properties of (0001) ZnO epitaxial layers grown on c-GaN/sapphire templates by pulsed laser deposition technique

A systematic study of growth, structural, electrical, and luminescence properties of zinc oxide (ZnO) layers grown on c-oriented GaN/sapphire templates by the pulsed laser deposition technique is carried out. A thorough high-resolution x-ray diffraction study reveals that c-ZnO films with high crystalline quality can be grown under certain growth conditions. Screw and edge dislocation densities in these films are found to be as low as 7×108 and 3×1010cm−2, respectively. All layers are found to be unintentionally n-type with ∼1019cm−3 electron concentration and mobility as high as ∼50 cm2 V−1 s−1. Temperature and excitation intensity dependent photoluminescence (PL) studies as functions of the growth conditions are carried out to identify the transition processes behind various luminescence features found in these samples. At low temperatures, PL spectra are marked by sharp neutral donor bound excitonic transitions, their phonon replicas, and two broad luminescence bands at 2.2 and 2.9 eV. These broad bands are attributed to transitions from the (2+/0) oxygen vacancy (VO) and (2+/+/0) zinc-interstitial (Zni) levels, respectively, to the valence band. Thermal energy needed to depopulate these defects is found to be 11 and 385 meV, respectively, for the (2+/0) VO and (2+/+/0) Zni levels. Low temperature PL spectra for the samples grown with relatively high oxygen pressures are featured by the Zn-vacancy (VZn) related neutral acceptor bound excitonic transition, its phonon replicas, and a broad band at 2.75 eV. This band diminishes with increasing temperature and, instead, another broad feature appears at ∼2.1 eV. Our study attributes the 2.75 eV band to transition from the conduction band to (0/−) VZn levels and the 2.1 eV feature to the transition between (−/2−) VZn levels and the valence band. It has been found that all the defect related features can be minimized by adjusting the growth conditions.

Optoelectronic Functionality of BiFeO3–SrTiO3 Interface

AbstractOne effective way to extend the functional degree of freedom for semiconductors is to introduce structural defects (e.g., surface or interface) and chemical defects as they commonly exist and modify the properties of the entity. Here, the optoelectronic properties of bismuth ferrite thin films and their tuning by the interface between the film and the strontium titanate substrate are reported. The defects that have been demonstrated, especially oxygen vacancies, are of paramount importance in the photoelectric properties of a film‐substrate system. A detailed analysis of the oxygen vacancy levels supports the role of the interface in carrier transport. These results provide a new strategy to design the optoelectronic device via usual defect doping and interface coupling.

Temperature dependence of ZnO crystals from ion-beam-induced luminescence

The effect of temperature (300-100 K) on the luminescence behavior of ZnO crystals irradiated with 2 MeV H+ were reported by ion-beam-induced luminescence (IBIL). Compared with constant-temperature IBIL spectra, the fluence mainly affects the luminous intensity, and the peak position shift is mainly affected by the temperature. For deep-band emissions (DBEs) at temperature ranges of 300–200 K, three emission bands exist in the red, orange-red, and green regions. These three emission bands illustrate different temperature dependencies with decreasing temperature: red and orange-red emission bands are not sensitive to temperature and redshift to green emission bands. The origins of three emission bands are the radiative recombination of deeply trapped holes with shallowly trapped electrons at the Oi, transition from the Zni to Oi level and transition related to oxygen vacancy levels. For DBE of 200–100 K, the orange-red emission band is not sensitive to temperature and has the same origin as the high-temperature section. The temperature dependence of UV band emission associated with recombination of excitons is caused by the temperature-induced band-gap shrinkage. The abnormal movement with temperature of violet emission band associated with Zni results from competition between localization effect at Zni and the band-gap shrinkage.

Photo-Detectivity Modulation in Complementary Light-to-Frequency Conversion Circuits via Oxygen Vacancy Controlled Amorphous Indium–Gallium–Zinc Oxide Films

Herein, light-to-frequency conversion circuits (LFCs) using n-type amorphous indium-gallium-zinc oxide thin film transistors (a-IGZO TFTs) and p-type single-walled carbon nanotube (SWNT) TFTs are proposed as circuit level photodetector. The photoresponse of a-IGZO TFTs under visible spectrum is adjusted with control of annealing temperature condition, which is closely related with oxygen vacancy level, validated by X-ray photoelectron spectroscopy (XPS) and band diagrams in the TFTs. With optimized annealing temperature for a-IGZO TFTs, LFCs are successfully demonstrated by photosensitive complementary inverters and their 3-stage ring oscillators with power and wavelength dependency of light. This result is expected to be applicable to the newly conceived internet-of-things (IoT) sensor systems including interactive displays.

Co-doped magnetic N-TiO2-x/rGO heterojunction@cellulose nanofibrous flakelet for enhanced photocatalytic oxidation and facile separation: Efficient charge separation and self-floatability

Visible-light-absorbing N-TiO2-x/rGO@cellulose nanohybrid photocatalyst with magnetism and self-floatability was fabricated via a facile in-situ nanocomposite strategy. The co-doping levels of nitrogen and oxygen vacancies formed bandgap states highly broadening the visible-light harvesting. The N-TiO2-x/rGO heterojunction remarkably enhanced the interfacial charge separation for excellent photocatalytic activity. Bisphenol A (10 mg·L−1) was photodegraded practically (96.2%) by this nanohybrid within 60 min through the synergistic oxidation via persulfate activation. Radical trapping experiments and EPR measurement revealed that the photo-induced superoxide radical (·O2–) and hydroxyl radical (·OH) were the predominant reactive oxidizing species. The nanohybrids were stably floating on aqueous solutions for an unprecedented long period of time (up to 5 months) and can be reused without difficult separation or photocatalytic attenuation. Contact angle measurement and thermodynamic model were combined to reveal the floatable nature. This work is therefore of interest for inexpensive and scalable manufacturing of multi-featured visible-light photocatalyst towards environmental remediation applications.

Post-Plasma Catalysis for Trichloroethylene Abatement with Ce-Doped Birnessite Downstream DC Corona Discharge Reactor

Trichloroethylene (TCE) removal was investigated in a post-plasma catalysis (PPC) configuration in nearly dry air (RH = 0.7%) and moist air (RH = 15%), using, for non-thermal plasma (NTP), a 10-pin-to-plate negative DC corona discharge and, for PPC, Ce0.01Mn as a catalyst, calcined at 400 °C (Ce0.01Mn-400) or treated with nitric acid (Ce0.01Mn-AT). One of the key points was to take advantage of the ozone emitted from NTP as a potential source of active oxygen species for further oxidation, at a very low temperature (100 °C), of untreated TCE and of potential gaseous hazardous by-products from the NTP. The plasma-assisted Ce0.01Mn-AT catalyst presented the best CO2 yield in dry air, with minimization of the formation of gaseous chlorinated by-products. This result was attributed to the high level of oxygen vacancies with a higher amount of Mn3+, improved specific surface area and strong surface acidity. These features also allow the promotion of ozone decomposition efficiency. Both catalysts exhibited good stability towards chlorine. Ce0.01Mn-AT tested in moist air (RH = 15%) showed good stability as a function of time, indicating good water tolerance also.

Role of oxygen vacancy defects in piezoelectric thermal stability characteristics of Mn-doped (K,Na,Li)NbO3 piezoceramics

Piezoelectric thermal stability characteristics of (K,Na,Li)NbO3 ceramics were investigated with a particular focus on the role of oxygen vacancy defects, by excluding the lattice distortion or microstructural effects with the help of a low level of Mn doping (less than 1.0 mol%). Benefiting from the reduction of intrinsic oxygen vacancy defects, controlled Mn doping (0.25 mol%) could achieve excellent ferroelectric/piezoelectric activity and superior thermal aging resistance. Beyond this doping level, however, the defect dipoles combined with extrinsic oxygen vacancies, although advantageous to the hardening effect, deteriorated the piezoelectric thermal stability and aging resistance including room-temperature properties. The thermal dissociation and migration of oxygen-vacancy-associated defect dipoles were considered mainly responsible for negating the ability to restore the initial poled domain state with defect-dipole-induced polarization. Results demonstrated the importance of keeping the intrinsic and extrinsic oxygen vacancy levels as low as possible for lead-free (K,Na)NbO3 systems with more thermally stable domain structure.

Significantly improved dielectric and piezoelectric properties by defects in PbNb2O6-based piezoceramics

PbNb2O6 (PN)-based piezoceramics are considered important materials used in high temperatures. Whereas, the main problem of PN-based ceramic is its low electrical performance such as piezoelectric constant (d33), severely restricting its practical applications. Here, based on the principle of regulating structure and performance via defect engineering, a new way of artificially tailoring point defects by A-site nonstoichiometry is implemented in this study. The results show that the PN-based ceramics with A-site deficiency exhibit significantly enhanced dielectric and piezoelectric properties. Particularly, the relative dielectric permittivity (εr) increased by 165% (from 197 to 523) and the d33 increased by 177% (from 30 to 83 pC/N) while maintaining a high Curie temperature (TC ~ 543 °C), and the resultant superior comprehensive performance is nearly highest in PN-based ceramic system. The X-ray photoelectron spectrometer and Raman spectra reveal that A-site deficiency leads to a decreased level of oxygen vacancies and NbO6 octahedral distortion as well as disorderly motions of A-site cations, which further causes the markedly decreased domain size exhibited in piezoresponse force microscopy (PFM) images. Consequently, the improved domain structure gives rise to improved dielectric and piezoelectric properties. The results of this work provide a simple and effective strategy to improve the dielectric and piezoelectric properties while keeping a high TC in the PN-based ceramics system, meeting the urgent demands of high-temperature applications.

Formation of quasi-two-dimensional electron gas at the interface of ZnO/Ga2O3 heterostructure

A two-dimensional electron gas (2DEG) was formed at the interface of an ultrathin ZnO/Ga2O3 heterostructure deposited by atomic layer deposition (ALD) at 200 °C on a SiO2/Si substrate. It is found that the sheet resistance decreases steeply to ∼104 Ω/□ as the thickness of the ZnO layer increases to a certain extent, and an optimal thickness of the top layer is determined. An ∼50-nm-thick β-phase Ga2O3 layer was formed after annealing at 800 °C, and a 12-nm-thick amorphous ZnO layer was observed by x-ray diffraction and transmission electron microscopy. The oxygen defects were analyzed by x-ray photoelectron spectroscopy. The defect energy levels of oxygen vacancies in Ga2O3 and ZnO thin films were calculated by first-principles simulation, and oxygen vacancies were generated at the interface of the ZnO/Ga2O3 thin film heterostructure to form 2DEG, which is demonstrated experimentally and theoretically. An assumption is proposed that the oxygen vacancy plays a role of the donor of free electrons at the ZnO/Ga2O3 interface because of the small energy level between the oxygen vacancy defect level and conduction band minimum (CBM) of the ZnO layer, which could easily trigger the charge transfer in the interface region. The 2DEG can be realized on the partially ordered microstructure of the Ga2O3 layer. With the assistance of the ALD technique, the thickness of the ZnO/Ga2O3 heterostructure can be as thin as ∼65 nm, which is favorable for devices of stack or ultrathin structures. Moreover, the low temperature deposition by ALD can be adopted for flexible or stretchable devices.

Ab-Initio Calculations of Oxygen Vacancy in Ga2O3 Crystals

Abstract Gallium oxide β-Ga2O3 is an important wide-band gap semiconductor. In this study, we have calculated the formation energy and transition levels of oxygen vacancies in β-Ga2O3 crystal using the B3LYP hybrid exchange-correlation functional within the LCAO-DFT approach. The obtained electronic charge redistribution in perfect Ga2O3 shows notable covalency of the Ga-O bonds. The formation of the neutral oxygen vacancy in β-Ga2O3 leads to the presence of deep donor defects with quite low concentration. This is a clear reason why oxygen vacancies can be hardly responsible for n-type conductivity in β-Ga2O3.

Engineering Crystallinity and Oxygen Vacancies of Co(II) Oxide Nanosheets for High Performance and Robust Rechargeable Zn–Air Batteries

AbstractEfficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes highly rely on the rational design and synthesis of high‐performance electrocatalysts. Herein, comprehensive characterizations and density functional theory (DFT) calculations are combined to verify the important roles of the crystallinity and oxygen vacancy levels of Co(II) oxide (CoO) on ORR and OER activities. A facile and controllable vacuum‐calcination strategy is utilized to convert Co(OH)2 into oxygen‐defective amorphous‐crystalline CoO (namely ODAC‐CoO) nanosheets. With the carefully controlled crystallinity and oxygen vacancy levels, the optimal ODAC‐CoO sample exhibits dramatically enhanced ORR and OER electrocatalytic activities compared with the pure crystalline CoO counterpart. The assembled liquid and quasi‐solid‐state Zn–air batteries with ODAC‐CoO as cathode material achieve remarkable specific capacity, power density, and excellent cycling stability, outperforming the benchmark Pt/C+IrO2 catalysts. This study theoretically proposes and experimentally demonstrates that the simultaneous introduction of amorphous structures and oxygen vacancies could be an effective avenue towards high‐performance electrocatalytic ORR and OER.