Control Of Oxygen Vacancies Research Articles

Efficient WO3−x nanoplates photoanode based on bidentate hydrogen bonds and thermal reduction of ethylene glycol

In this study, an efficient WO3−x nanoplates photoanode was generated based on bidentate hydrogen bonds and in subsequent thermal reduction of ethylene glycol (EG). An appropriate number of controllable oxygen vacancies (Ov) was generated in-situ on the surface of the WO3 nanoplates without deep defects by bidentate hydrogen bonds. Density functional theory (DFT) calculations indicate that the distance between two alcoholic hydrogens (5.124 Å) in EG matches that of the diagonal oxygens (5.483 Å) in the WO3 (002) surface, which allows EG to combine through the most stable bidentate hydrogen bonds with O–H intervals of approximately 2.5 Å. Diagonal oxygens are captured directly from the surface, leaving Ov owing to the special hydrogen-bond structure and moderate reducibility of EG under appropriate thermal conditions. The photocurrent density of the WO3−x nanoplates improves considerably to 2.07 from the 0.91 mA cm−2 of pristine WO3 with the introduction of Ov, which demonstrates the superior surface reaction kinetics from the reduced holes-to-water resistance and increase in surface injection efficiency. DFT calculations of the oxygen evolution reaction reveal that surface Ov could substantially decrease the reaction energy barrier for a lower overpotential of 0.494 V compared to that of WO3 (1.037 V), which is consistent with the reduction in the Tafel slope from 412 to 243 mV dec−1. Therefore, this study provides an innovative method to obtain an efficient WO3 photoanode based on the treatment of EG.

Holey engineered 2D ZnO-nanosheets architecture for supersensitive ppm level H2 gas detection at room temperature

Conquering the issue of room temperature H2 detection at ppm/ppb level and their fundamental sensing mechanism are vitally needed for the development of highly sensitive/selective sensing devices. Herein, a strategy is proposed to synthesize 2D ZnO holey nanosheets by engineering the tunable pore/hole size with controlled oxygen vacancies using the annealing process for H2 detection at room temperature. 2D ZnO holey nanosheets annealed at 400 °C shows a highly porous network owing to its high surface area, more channels for gas diffusion, and mass transport that exhibits improved gas-sensing performance. ZnO@400 sensor exhibits maximum response of ∼115 (20 times more than ZnO@800 sample) towards 100 ppm of H2 at room temperature. The sensor response (recovery) times of the order ∼9(6) secs recorded to be fastest for ZnO@400 sensor as compared to ZnO@600 (∼19(13) secs) and ZnO@800 (∼27(20) secs) sensor respectively. Further, the ZnO@400 sensor also displays superior repeatability and stability of ∼97−99% after 45 days. The involved gas sensing mechanism has also been verified by carrying out XPS measurements before and after H2 exposure at room temperature that helps to complement the theoretical justification about room temperature metallization effect. Thus, 2D ZnO holey nanosheets turn out to be a pivotal strategy to improve the gas sensing performance due to the synergetic effect of highly porous network and large specific surface areas of 2D nanosheets. The present approach proves to be one of the best methodologies to alleviate the restacking issue of the 2D nanosheets by opening up the inaccessible surfaces.

Controlling oxygen vacancies of WOx suboxides by ZnWO4 nanophase hybridization

Substoichiometric tungsten oxide (WOx, 2 < x < 3) nanomaterials are important nanoscale semiconductor materials with broad applications in optoelectronic devices; however a controllable modification of their physical properties remains challenging task. Here, we report an alternative green synthesis of hybrid nanostructures composed by WOx and ZnWO4 phases in ethanol via a solvothermal process using ZnO and WCl6 as the precursors. The results indicate that the formation of ZnWO4 leads to the structural conversion of WOx from nanorods to nanoparticles due to the effect of ZnWO4 on the side growth of WOx nanostructures. Moreover, the surface morphology tailored by ZnWO4 and crystalline phase transformation induce different photoluminescence (PL) emission from pure WOx nanostructures and WOx/ZnWO4 hybrid nanostructures due to the increased number of oxygen vacancies.

The self-compensation mechanism in barium titanate ceramics with colossal permittivity

Based on adjusting the relationship between polarization and charge carriers, barium titanate (BaTiO3) ceramics with colossal permittivity and high insulation resistivity are prepared by solid-state reaction approach. Modified by reducing atmosphere (0.1%H2/N2) and donor-ions doping, different polarization modes are discussed via treatment with impedance analysis and dielectric data, then the relationship between dielectric properties and charge carriers is established. It is illustrated that the interfacial polarization and hopping polarization can be adjusted along with different charge compensation modes by changing Nb5+ content from 0.5 mol% to 3 mol%, corresponding to colossal permittivity and insulation resistivity, respectively. Thus, the approach provides an adjustable mechanism for balancing the colossal permittivity and insulation resistivity in BaTiO3 ceramics. The colossal permittivity can be achieved by controlling oxygen vacancies, and the distribution of oxygen vacancies is further adjusted by the donor-ions. Meanwhile, the self-compensation is found in the grains to improve the grain resistance. On this basis, single-layer microchip capacitors of BaTiO3-based ceramics are available in the future.

Impact of oxygen vacancy occupancy on piezo-catalytic activity of BaTiO3 nanobelt

BaTiO3 nanobelt featuring controlled oxygen vacancies, namely BTO-OV-X (X represents time (h) of vacuum heat-treatment), were prepared to systematically investigate the effect of oxygen vacancies occupancy on pizeocatalytic performance for degradation of organic pollutant. Remarkably, the creation of oxygen vacancies on BaTiO3 can mediate the piezocatalytic activity. The piezocatalytic activity exhibited a volcano-type trend with increasing oxygen vacancies. Results from first-principles density functional theoretical (DFT) calculations and O2-temperature programmed desorption (O2-TPD) measurement indicated that the presence of oxygen vacancies could efficiently adsorb and activate O2 on the surface of BaTiO3 nanobelt and consequently enhance piezocatalytic activity. Importantly, with the aid of piezoresponse force microscopy (PFM) measurement, the intrinsic reason of volcano-type trend of oxygen vacancy-activity was well unveiled. This work reveals the effect and mechanism of oxygen vacancy occupancy on enhancing piezocatalytic activity of BaTiO3 nanobelt and will shed light on design of efficient piezocatalysts in the future.

Joule self-heating effects and controlling oxygen vacancy in La0.8Ba0.2MnO3 ultrathin films with nano-sized labyrinth morphology

Electric current induced Joule heating effects have been investigated in La0.8Ba0.2MnO3 ultrathin films deposited on a LaAlO3(001) single crystal substrate with a smaller lattice constant by using the sol–gel method. By applying moderate bias currents (∼10 mA), it is found that Joule self-heating simply gives rise to a temperature deviation between the thermostat and the test sample, but the intrinsic ρ(T) relationship measured at a low current (0.1 mA) changes a little. However, it is noteworthy that the low-temperature transport behavior degrades from the metallic to the insulating state after applying higher bias currents (&amp;gt;31 mA) in vacuum. Furthermore, the metallic transport can be recovered by placing the degraded film in air. The results clearly suggest that the oxygen vacancy in the La0.8Ba0.2MnO3 films is controllable in different atmospheres, particularly with the aid of the Joule self-heating. According to the SEM images, we attribute the controlled oxygen vacancy to the nano-sized labyrinth pattern of the films, where the large surface-to-volume ratio plays a crucial role.

Optical Defects Engineering of Metal Oxide Nanosheets Toward Exceptional Chemiresistor

Metal oxide nanosheets (NSs) exhibit intriguing physicochemical properties such as high specific surface area, high fraction of reaction sites, thus leading to promising performance in various applications including energy storage, catalysis, and gas sensing. [1] Beyond the 2D metal oxide NSs, defect engineering and/or control of non-stoichiometry, that is, control of metal or oxygen vacancies, are a few of the powerful strategies to generate abundant active sites to enhance gas sensing performance. [2] Precise control of the heat treatment, ramping rate control or reducing heat treatment as examples, is an effective way to generate the oxygen vacancies, but is time-consuming which requires high thermal-budget. Building-up of hetero-junctions is another powerful method to boost up the sensing performance. [3] However, it has been a challenge to prepare thin-layered heterogeneous metal oxide NSs with abundant active defect sites.An optical engineering to generate controllable oxygen vacancies and new crystal phases on various metal oxide nanosheets (NSs) is introduced via graphene oxide (GO) templating route followed by intense pulsed light (IPL) irradiation. Negatively charged GO flakes and metal ions readily self-assemble to form GO@M (where M = Sn2+, Co2+, Ti4+, and Zn2+) and polycrystalline metal oxide NSs, such as SnO2, Co3O4, TiO2, and ZnO, are successfully synthesized after calcination. Upon a few flashes of IPL irradiation (< 20 ms), a rapid increase in temperature (1200 °C) has induced evolution of defect sites and new crystal phases. As a representative, optically treated Pt-SnO2 NSs showed unparalleled formaldehyde (HCHO) response, excellent selectivity, low detection limit (100 ppb), and fast response speed (< 10 s) assisted by the synergistic effects of the Pt catalysts, numerous oxygen vacancies, and hetero-junctions formed by flash-induced partial reduction of n-type SnO2 to p-type SnO. This defects engineering method could greatly strengthen the design of various heterogeneous metal oxide NSs and their potential application toward various fields with the optimal customized physicochemical properties.

Improved Performance of Acetone Gas Sensing through Oxygen Vacancy Formation of ZnO Nanoparticles

Introduction Recently, interest in gas sensors have been increasing in different fields of not only industry and medical applications but also development of smart city and detection of indoor air pollutants. In particular, acetone is a volatile organic compound (VOC) used as a biomarker to indicate various diseases in human exhalation, or has been importantly measured as one of the indicator gases for indoor pollution measurement [1]. Unlike industrial sensors that need to detect relatively high concentration gas, the detection of acetone for biomarkers requires highly sensitive sensing materials which can react with the gas quickly.Metal oxide semiconductor (MOS) have been widely used as gas sensors because of their thermal and chemical stability and natural abundance. A lot of researches for detecting acetone gas using metal oxide semiconductor, for instance, WO3, SnO2 and ZnO through diverse method and morphology control, have been studied in recent [2]. Among them, ZnO is in the spotlight for various reasons due to their facile size and structure control, gas reactive surface and appropriate band gap. Many papers associated gas sensing of ZnO were published in multiple ways as metal doping and quantum dot control [3, 4]. In this study, oxygen vacancy was produced by simple H2O2 surface treatment of commercial ZnO, and the amount of oxygen vacancy by the degree of surface treatment was quantified. In addition, through evaluating acetone sensing performance according to the amount of oxygen vacancy, optimal vacancy amount showing significantly superior sensitivity was found. Chemical calculation described the reason of the outstanding response [5-8]. Further information will be discussed in detail. Method Surface oxygen vacancy is generated by a variety of H2O2 treatment with 400 ˚C annealing. ZnO controlled oxygen vacancy was reacted with 10 ppm acetone at 300 – 500 ˚C, and confirm response of the material. Response of varied temperature was confirmed through three or more repeated sensor measurements, and the measurement was performed at various concentration ranging from 0.01 ppm to 10 ppm under optimized temperature conditions. And the performance was identified under various humidity conditions and long-term stability. Results and Conclusions When H2O2 was treated on commercial ZnO, ZnO2 was produced. ZnO with oxygen vacancy was formed after 400 ˚C annealing on the obtained ZnO2. Treatment with high concentrated H2O2 results in more defects, breaking crystallinity and reducing the XRD peak sharpness. Therefore, we found optimal concentration conditions that create oxygen vacancy around the surface. The deficient oxygen and lattice oxygen peaks were deconvoluted from the O 1s peaks of the XPS spectrum and the vacancy to lattice ratio was obtained. Also, the formation of oxygen vacancy could be confirmed by the peak shift of Zn 2p3/2. The oxygen vacanced ZnO had a high sensitivity of 1692.1 (Fig. 1). This results showed the world’s best sensitivity compared with previous reported outcomes such as commercial ZnO (S=107), SnO2 nanorod arrays (S=511) and In-doped ZnO quantum dots (S=424). This study shows the possibility of commercialization through a simple surface treatment method that can be widely applied to a wide range of metal oxides, and it is widely applicable to systems of diverse fields where low concentrations of acetone have to be fairly sensitive.

Enhanced photocatalytic oxidizing ability via adjusting the band-edge position and oxygen defect concentration of bismuth phosphate

Abstract Oxide defect engineering exert huge impact on semiconductor photocatalysis, which not only can adjust the band-gap structure but also can act as electron trap for photoinduced carriers separation. In this work, the photoactivity oxidation capability of bismuth phosphate were enhanced by the fine-tuning of lattice parameters with different oxygen defect concentrations. The non-linear changes of lattice parameters calculated from XRD suggested that the lattice oxygen missed during thermal treatment. XPS, Raman, EPR, HRTEM, PL and UV–vis diffuse reflectance spectroscopy also identified that the types and concentrations of defect changed with various temperatures. The photo-carriers separation efficiency and oxidizability are increased via controllable defect density to adjusting lattice parameters that can attract photogenerated electrons and lower valance band position, respectively. Benefiting from the higher separation efficiency of photogenerated carriers and stronger oxidation capability, the BiPO4 with the smallest lattice parameter exhibited reinforced photocatalytic removal efficiency of methylene blue, better repeatability and stability, which is 3.2 times as high as that of pristine BiPO4. The findings, controlling oxygen vacancy concentration to regulate the lattice parameters, could be a promising strategy for enhancing photoactivity oxidizing ability of photocatalysts.

Intercalation-type electrodes of copper–cobalt oxides for high-energy-density supercapacitors

A series of CuxCo3-xO4 nanostructures (CCO) with a controllable surface morphology and oxygen vacancy density are directly grown on charge-collecting substrates via an electrochemical deposition process and subsequently used as intercalation-type electrodes for high-energy-density supercapacitors (SCs). The capacitive performance of the CCOs is strongly dependent on the rate of oxygen diffusion via intercalation in oxygen-deficient CCOs. The nanopetal electrode of Cu0.95Co2.05O4 calcined at 200 °C (CCO-200) exhibits a capacitance of ~1890 F g−1 at a scan rate of 10 mV s−1, whereas it decreases gradually with increasing sintering temperature. A symmetric CCO-200@stainless steel SCs exhibits a capacitance and an energy density as high as ~865.50 F g−1 and 108.18 W h kg−1, respectively, at a current density of 1.25 A g−1 with high durability for 10,000 cycles. The enhanced capacitive performance is attributed to the high electrical conductivity and high oxygen diffusion rate in CCO-200.

An organometallic chemistry-assisted strategy for modification of zinc oxide nanoparticles by tin oxide nanoparticles: Formation of n-n heterojunction and boosting NO2 sensing properties

Here, we report an organometallic chemistry-assisted method for modification of zinc oxide (ZnO) nanoparticles by tin oxide (SnO2) nanoparticles, providing a novel and efficient approach for preparation of metal oxides-based heterojunction. The SnO2/ZnO heterojunctions were prepared by modification of ZnO nanoparticles with (CH3)2SnCl2 through organometallic chemistry reaction firstly, and subsequently thermal treatment in air atmosphere. The combined characterizations indicate the successful formation of ZnO/SnO2 heterojunctions with controllable surface oxygen vacancy concentrations by optimizing organometallic chemistry reactions. Most importantly, ZnO-Sn-0.75-based NO2 sensor delivers the response of 14.3 toward 0.5 ppm NO2 at 190 °C with response time of 100 s and recovery time of 101 s. It is also found that ZnO-Sn-0.75 sample exhibits better NO2 sensing performances than ZnO nanoparticles and other ZnO-Sn samples (ZnO-Sn-0.50, and ZnO-Sn-1.0). The excellent sensing performances of ZnO-Sn-0.75 are attributed to the synergistic effect n-n heterojunction and controllable surface oxygen species. The present work opens a generalized avenue for facile, cheap and mass production of transition metal oxides-based heterojunctions for various applications.

Selective activation of memristive interfaces in TaOx-based devices by controlling oxygen vacancies dynamics at the nanoscale

The development of novel devices for neuromorphic computing and non-traditional logic operations largely relies on the fabrication of well controlled memristive systems with functionalities beyond standard bipolar behavior and digital ON–OFF states. In the present work we demonstrate for Ta2O5-based devices that it is possible to selectively activate/deactivate two series memristive interfaces in order to obtain clockwise or counter-clockwise multilevel squared remanent resistance loops, just by controlling both the electroforming process and the (a)symmetry of the applied stimuli, and independently of the nature of the used metallic electrodes. Based on our thorough characterization, analysis and modeling, we show that the physical origin of this electrical behavior relies on controlled oxygen vacancies electromigration between three different nanoscopic zones of the active Ta2O5−x layer: a central one and two quasi-symmetric interfaces with reduced TaO2−h(y) layers. Our devices fabrication process is rather simple as it implies the room temperature deposition of only one CMOS compatible oxide—Ta-oxide—and one metal, suggesting that it might be possible to take advantage of these properties at low cost and with easy scability. The tunable opposite remanent resistance loops circulations with multiple—analogic—intermediate stable states allows mimicking the adaptable synaptic weight of biological systems and presents potential for non-standard logic devices.

Ultra-fast synthesis of water soluble MoO3-x quantum dots with controlled oxygen vacancies and their near infrared fluorescence sensing to detect H2O2.

We report a facile method for the synthesis of water soluble MoO3-x quantum dots (QDs) at room temperature by injecting hydrochloric acid and mercaptosuccinic acid into ammonium molybdate solution within 5 seconds. The optical properties and oxygen vacancy concentration of the QDs could be well controlled by the competitive absorption of a carboxyl group and sulfhydryl group of the ligand with QDs by coordination interaction. The obtained QDs could be used as near infrared region (NIR) fluorescence probes to detect hydrogen peroxide with a low detection limit (3 nM).

In situ growth of porous TiO2 with controllable oxygen vacancies on an atomic scale for highly efficient photocatalytic water splitting

In situ regulation of oxygen vacancies of porous TiO2 at atomic scale with promoting photocatalytic efficiency.

Controlled oxygen vacancy engineering on In2O3−x/CeO2−y nanotubes for highly selective and efficient electrocatalytic nitrogen reduction

Introducing and adjusting the oxygen vacancies (VO) of transition metal oxides has been proposed as a significant and effective way to tackle the sluggish nitrogen reduction reaction (NRR) in the electrocatalysis process.

Oxygen Vacancy Concentration: Effect of Controlled Oxygen Vacancy on H2‐Production through the Piezocatalysis and Piezophototronics of Ferroelectric R3C ZnSnO3 Nanowires (Adv. Funct. Mater. 5/2020)

Oxygen Vacancy Concentration: Effect of Controlled Oxygen Vacancy on H₂‐Production through the Piezocatalysis and Piezophototronics of Ferroelectric R3C ZnSnO₃ Nanowires (Adv. Funct. Mater. 5/2020)

Self-assembled CoTiO3 nanorods with controllable oxygen vacancies for the efficient photochemical reduction of CO2 to CO

CoTiO3 with oxygen vacancies was used as a cocatalyst for the photochemical reduction of CO2 to CO. The bond length of the double bond between C and O in CO2 increases from 1.17 Å to 1.25 Å in the activation step.

Effect of Controlled Oxygen Vacancy on H2‐Production through the Piezocatalysis and Piezophototronics of Ferroelectric R3C ZnSnO3 Nanowires

AbstractThis study is the first to demonstrate that ferroelectric R3c LiNbO3‐type ZnSnO3 nanowires (NWs), through the piezocatalysis and piezophototronic process, demonstrate a highly efficient hydrogen evolution reaction (HER). The polarization and electric field curves indicate that ZnSnO3 NWs exhibit typical ferroelectric hysteresis loops. Time‐resolved photoluminescence spectra reveal that the relaxation time increases with the increasing concentration of oxygen vacancies. Moderated 3H‐ZnSnO3 NWs (thermally annealed for 3 h in a hydrogen environment) have the longest extended carrier lifetime of approximately 8.3 ns. The piezoelectricity‐induced HER, via the piezocatalysis process (without light irradiation), reaches an optimal H2‐production rate of approximately 3453.1 µmol g−1 h−1. Through the synergistic piezophototronic process, the HER reaches approximately 6000 µmol g−1 in 7 h. Crucially, the mechanical force–induced spontaneous polarization functions as a carrier separator, driving the electron and hole in opposite directions in ferroelectric ZnSnO3 NWs; this separation reduces the recombination rate, enhancing the redox process. This theoretical analysis indicates that the photocatalytic and piezocatalytic effects can synergistically enhance piezophototronic performance through capitalizing on well‐modulated oxygen vacancies in ferroelectric semiconductors. This study demonstrates the essential role of this synergy in purifying water pollutants and converting water into hydrogen gas through the piezophototronic process.

Controlling oxygen vacancies and enhanced visible light photocatalysis of CeO2/ZnO nanocomposites

Oxygen vacancy in semiconductors plays crucial roles in the efficiency enhancement of photocatalysis. In this paper, a facial method to control oxygen vacancy concentration of CeO2/ZnO nanocomposites was developed by varying the ratio of air:N2 in the process of calcination. Detailed analysis based on X-ray photoelectron spectroscopy (XPS) revealed that the oxygen vacancy concentration in CeO2/ZnO nanostructures was richer than that in unmixed ZnO, and reached its maximum value when the ratio of air:N2 in calcination atmosphere was 2:8 or 4:6. The photocatalytic performance was investigated by the degradation of Rhodamine B (RhB) under visible light irradiation. The CeO2/ZnO nanoparticles with air:N2 = 2:8 showed the highest photocatalytic activities owing to the highest oxygen vacancy concentration. The transmission electron microscopy (TEM) results indicated that an intimate contact between ZnO and CeO2 was formed. It was possible that the improved photocatalytic activity was caused by the synergistic effect of CeO2/ZnO heterojunction and oxygen vacancy in CeO2/ZnO nanocomposites.

Oxygen‐vacancy‐controlled magnetic properties with magnetic pole inversion in BiFeO3‐based multiferroics

AbstractOxygen vacancies which are generally present in ferrite oxide may significantly affect their magnetic properties. A comprehensive understanding of the relationship between oxygen vacancies and magnetism is of great importance. In this work, we report an oxygen vacancy concentration dependence of magnetism in a single‐phase multiferroic BiFeO3 (BFO)‐based system. The BiFeO3‐DyFeO3 (BDFO) solid solution is synthesized with controlled oxygen vacancies and is characterized by X‐ray powder diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), and AC impedance spectroscopy. The magnetic properties, especially magnetic pole inversion, are found to be highly dependent on oxygen vacancy concentration. The oxygen vacancies generate a Weiss molecular field on the Dy3+ ions in the magnetic field range of 570‐1000 Oe depending on the oxygen vacancy concentration, and result in a residual net magnetization by breaking the balance between the two nearly antiparallel spin lattices of Fe3+ ions. This work demonstrates an effective way to control oxygen vacancies and thereby the magnetic properties and sheds light on the relationship between oxygen vacancies and magnetism in BFO‐based multiferroics.