High Density Of Oxygen Vacancies Research Articles

Electronic Properties of W’ Twin Walls in Ferroelastic BiVO4

AbstractTopological defects in ferroic materials can exhibit intrinsic properties that differ from the bulk. Here， structural and electronic variations of non‐prominent (W’) ferroelastic twin domain walls are investigated in BiVO4, a widely investigated photocatalytic material. Using aberration‐corrected scanning transmission electron microscopy (STEM), a kink configuration of the sharp ferroelastic twin wall with an altered electronic structure is revealed. Nanoscale conductivity measurements by conductive atomic force microscopy (c‐AFM) show higher conductivity at twin walls compared to non‐conductive bulk domains. Electronic structure investigation by electron energy loss spectroscopy (EELS) shows a higher density of oxygen vacancies and possible polaron accumulation at the wall. These findings reveal the electronic properties of BiVO4 domain walls, which are interesting for nanoscale‐engineered catalytic concepts of BiVO4 and materials design for photochemistry‐relevant applications.

Formation and energetics of head-to-head and tail-to-tail domain walls in hafnium zirconium oxide

180∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document} domains walls (DWs) of head-to-head/tail-to-tail (H–H/T–T) type in ferroelectric (FE) materials are of immense interest for a comprehensive understanding of the FE attributes as well as harnessing them for new applications. Our first principles calculation suggests that such DW formation in hafnium zirconium oxide (HZO) based FEs depends on the unique attributes of the HZO unit cell, such as polar-spacer segmentation. Cross pattern of the polar and spacer segments in two neighboring domains along the polarization direction (where polar segment of one domain aligns with the spacer segment of another) boosts the stability of such DWs. We further show that low density of oxygen vacancies at the metal-HZO interface and high work function of metal electrodes are conducive for T–T DW formation. On the other hand, high density of oxygen vacancy and low work function of metal electrode favor H–H DW formation. Polarization bound charges at the DW get screened when band bending from depolarization field accumulates holes (electrons) in T–T (H–H) DW. For a comprehensive understanding, we also investigate their FE nature and domain growth mechanism. Our analysis suggests that a minimum thickness criterion of domains has to be satisfied for the stability of H–H/T–T DW and switching of the domains through such DW formation.

Origin of the High Density of Oxygen Vacancies at the Back Channel of Back-Channel-Etched a-InGaZnO Thin-Film Transistors.

This study reveals the pronounced density of oxygen vacancies (Vo) at the back channel of back-channel-etched (BCE) a-InGaZnO (a-IGZO) thin-film transistors (TFTs) results from the sputtered deposition rather than the wet etching process of the source/drain metal, and they are distributed within approximately 25 nm of the back surface. Furthermore, the existence and distribution depth of the high density of Vo defects are verified by means of XPS spectra analyses. Then, the mechanism through which the above Vo defects lead to the instability of BCE a-IGZO TFTs is elucidated. Lastly, it is demonstrated that the device instability under high-humidity conditions and negative bias temperature illumination stress can be effectively alleviated by etching and thus removing the surface layer of the back channel, which contains the high density of Vo defects. In addition, this etch method does not cause a significant deterioration in the uniformity of electrical characteristics and is quite convenient to implement in practical fabrication processes. Thus, a novel and effective solution to the device instability of BCE a-IGZO TFTs is provided.

Engineering Pd-oxygen vacancy interface for enhanced hydrodeoxygenation of hexahydrophthalic anhydride to hexahydrophthalide

The development of chemical recyclable polyester based on hexahydrophthalide offers a solution to the plastic pollution issue. However, it remains a challenge to construct a promising catalyst route for monomers synthesis and clarifying the binary interactions between the support and active sites. In this work, we reported Pd/ZrO2 catalysts with different crystal structures by metal-support interaction and oxygen vacancy (Ov) regulation for the hydrodeoxygenation of hexahydrophthalic anhydride (HHPA) toward the continuous synthesis of hexahydrophthalide. The Pd/m-ZrO2 catalyst showed the highest conversion of HHPA (93.1 %) and the superior selectivity to hexahydrophthalide (96.1 %), a low apparent activation energy (Ea = 75 kJ/mol), and great stability under 200 °C, 3 MPa H2. Detailed experimental studies indicated that Pd/m-ZrO2 with the higher oxygen vacancy density, and the stronger interaction between the metal and the support, which results in the formation of more Pd-O-Zr structures with excellent catalytic performance. This work opens up a new pathway for the related acid anhydride hydrodeoxygenation reactions to recyclable lactones through catalytic engineering methods.

Stabilized Li-Rich Layered Oxide Cathode by a Spontaneously Formed Yb and Oxygen-Vacancy Rich Layer on the Surface.

Li-rich layered oxides (LLOs) are among the most promising cathode materials with high theoretical specific capacity (>250 mAh g-1 ). However, capacity decay and voltage hysteresis due tostructural degradation during cycling impede the commercial application of LLOs. Surface engineering and element doping are two methods widely applied tomitigate the structural degradation. Here, it is found that trace amount lanthanide element Yb doping can spontaneously form a surficial Yb-rich layer with high density of oxygen vacancy on the LLO-0.3% Yb (Li1.2 Mn0.54 Co0.13-x Ybx Ni0.13 O2 where x = 0.003) cathodes, which mitigating lattice oxygen loss and the non-preferred layered-to-spinel-to-rock salt tri-phase transition. Meanwhile, there are also some Yb ions doped into the lattice of LLO, which enhance the binding energy with oxygen and stabilize the lattice in grain interior during cycling. The dual effects of Yb doping greatly mitigate the structure degradation during cycling, and facilitate fast diffusion of lithium ions. As a result, the LLO-0.3% Yb sample achieves significantly improved cycling stability, with a capacity retention of 84.69% after 100 cycles at 0.2 C and 84.3% after 200 cycles at 1 C. These finding shighlight the promising rare element doping strategy that can have both surface engineering and doping effects in preparing LLO cathodes with high stability.

Well-Designed morphology regulated ZIF-67 derived 0D/1D Co3O4@TiO2 NWs integrated with in-situ grown Ni/Co-Active metals for Low-Temperature driven CO2 methanation

Well-designed morphology regulated MOF-templated Co3O4 over titanium dioxide nanowires (TiO2 NWs) boasting intimate interfacial contact for highly efficient CO2 methanation was synthesized via a facile in-situ grown approach. The synergistic effect of 0D Co3O4 with reducible 1D TiO2 NWs and the role of Ni/Co-active metals were systematically investigated for CO2 methanation. The performance comparison of the Ni dispersed in-situ grown Co3O4 on TiO2 NWs (Insitu-10N7CT) and its mechanically assembled counterpart (M−10N7CT) was further explored. The in-situ grown ZIF-67 acts as a sacrificial template to produce Co3O4 with high dispersion of Ni active sites to enhance CH4 production. By regulating the morphology, 0D Co3O4 promoted a superior methanation performance for Insitu-10N7CT (XCO2 = 97.42 %, SCH4 = 99.29 %) compared to M−10N7CT (XCO2 = 91.45 %, SCH4 = 98.42) due to intimate interfacial contact between the Ni, Co, and Ti species. Comparatively, the MOF-derived composites displayed much higher CH4 yield than uncalcined MOF nanocomposites. This is because the high reaction temperature caused a rapid collapse of the ZIF-67 dodecahedral structure, leading to a lower methanation performance. Insitu-10N7CT exhibited higher reducibility, generating low valence oxidation/metallic states of Ni-Co-Ti, leading to the high density of oxygen vacancies (Ov). An optimal reaction temperature of 350 °C and gas hourly space velocity of 9,400 mL gcat-1h−1 was revealed, suggesting an efficient reactant contact and CO2 conversion into CH4. The Insitu-10N7CT composite exhibited durability up to 100-hour time-on-stream due to the gradual reduction of Co3O4. Thus, the novel MOF-templated ternary nanocomposite revealed a facile method for regulating the structure of MOF derivatives while boosting strong interfacial interactions for efficient renewable fuel production.

Tailored monolithic catalyst and reactor by direct photo-curing 3D printing for oxidative desulfurization of fuels

Structured monolithic catalysts have attracted growing attention in the petrochemical industry due to significant advantages of reusability, easy scale-up, and high heat and mass transfer rate. 3D printing offers enormous flexibility in accurately regulating the configuration of monolithic catalysts to realize the strengthening of the reaction process. However, this remains challenging in terms of reliable manufacturing, high resolution, and the distribution of active sites. Here, we report a direct photo-curing 3D printing based method for manufacturing precise monolithic catalysts for oxidative desulfurization of fuels. Numerous molybdenum oxide nanoparticles with a high density of oxygen vacancies are spatially distributed in three-dimensional Al2O3 by 3D printing technology and post-treatment, thus favoring the catalytic reaction. This 3D-MoOx/Al2O3 exhibits excellent catalytic performance with a 99.2 % conversion of dibenzothiophene. Furthermore, this tailored catalyst is 3D-printed to manufacture a monolithic agitating impeller as a stirring reactor, which plays a dual role in mixing and catalysis. The study provides an effective strategy for precisely manufacturing structured catalysts with excellent catalytic activity and can be readily applied to a range of chemical reactions.

Simple and efficient strategy for α-MnO2/C by in-situ synthesis and its performance for degrading urea process wastewater

A simple and low-cost method for the synthesis of α-MnO2/C catalyst was proposed, in which KMnO4 was reduced in-situ by cellulose in urea solution. The heterogeneous catalytic degradation of urea wastewater were carried out over α-MnO2/C. The effect of KMnO4-urea solution concentration on the structure and performance of the catalysts was investigated. The results indicate that δ-MnO2 tends to curl and collapse into tunnel structure of α-MnO2 as the concentration of KMnO4-urea solution increases, leading to α-MnO2 is evenly distributed on carbon carrier. When the concentration of KMnO4-urea solution is appropriate, the catalyst has not only a large specific surface area but also high oxygen vacancy density, which is the key to be able to completely degrade the urea process wastewater. The reaction kinetics show that the catalytic hydrolysis of urea behaves as a pseudo-first-order reaction. A possible catalytic mechanism of urea hydrolysis over α-MnO2/C was proposed. In addition, the reusability experiments show that the α-MnO2/C catalyst has a good recycling ability.

Self-supported iridium-ruthenium oxides catalysts with enriched phase interfaces for boosting oxygen evolution reaction in acid

Developing highly efficient low-noble-metal loading electrocatalysts for acidic oxygen evolution reaction (OER) remains a great challenge. Herein, we developed a novel self-supported IrmRunOx/Ti electrode exhibiting an amorphous film encapsulated with a small number of crystalline nanoparticles for efficient acidic OER. The as-synthesized Ir0.95Ru0.05Ox/Ti electrode exhibits an excellent OER performance with an overpotential of 200 mV at 10 mA cm−2, a Tafel slope of 46.8 mV dec-1, and robust stability of 192 h in 0.1 M HClO4. The enriched heterogeneous interfaces between the amorphous phase and crystalline phase enhance the stability and activity of the solid solution oxides. The high density of oxygen vacancies on the surface, together with the strong electronic interaction contribute to boosting the OER. This work provides some valuable insights into developing efficient Ir-based catalysts for acidic water splitting.

Performance improvement of a tunnel junction memristor with amorphous insulator film

This study theoretically demonstrated the oxygen vacancy (VO2+)-based modulation of a tunneling junction memristor (TJM) with a high and tunable tunneling electroresistance (TER) ratio. The tunneling barrier height and width are modulated by the VO2+-related dipoles, and the ON and OFF-state of the device are achieved by the accumulation of VO2+ and negative charges near the semiconductor electrode, respectively. Furthemore, the TER ratio of TJMs can be tuned by varying the density of the ion dipoles (Ndipole), thicknesses of ferroelectric-like film (TFE) and SiO2 (Tox), doping concentration (Nd) of the semiconductor electrode, and the workfunction of the top electrode (TE). An optimized TER ratio can be achieved with high oxygen vacancy density, relatively thick TFE, thin Tox, small Nd, and moderate TE workfunction.

Metastable pitting corrosion behavior and characteristics of passive film of laser powder bed fusion produced Ti–6Al–4V in NaCl solutions with different concentrations

This work investigated the metastable pitting corrosion mechanism of laser powder bed fusion (LPBF) produced Ti–6Al–4V. The passive films of samples were produced by potentiostatic polarization in NaCl solutions with different concentrations, and electrochemical measurements were employed to understand the influence of Cl- on the characteristics of passive films. The frequency of pitting nucleation and the pit dimension increase with increased Cl- concentration. The attack of Cl- promotes the dissolution of passive films. A higher density of oxygen vacancies is produced in passive film because of the ingressive Cl-, resulting in the condensation of voids and pitting corrosion.

Surface anchored atomic cobalt-oxide species coupled with oxygen vacancies boost the CO-production yield of Pd nanoparticles

The high density of oxygen vacancies and adjacent Pd domains promote CO2 activation and H2 dissociation, respectively.

Thermal effects on TiN/Ti/HfO2/Pt memristors charge conduction

TiN/Ti/HfO2/Pt resistive switching devices have been fabricated, measured, and modeled. After programming the devices in the low resistance state, the current–voltage characteristic below the reset switching voltage was measured at different temperatures (from 90 to 350 K). A weak but complex temperature dependence was obtained for several voltage regimes. These memristors belong to a wider set known as valence change memories, whose conductance is determined by the formation of conductive filaments (CFs) linked to a high density of oxygen vacancies in a dielectric sandwiched between two metal electrodes. This usually leads to ohmic conduction in the low resistance state. However, a non-linear current dependence has been also observed in the measured devices, in addition to symmetric current–voltage curves for positive and negative biases in the 0–0.6 V voltage range. Three different thermal dependences have been considered for explaining the whole set of experimental data. Two of them are linked to ohmic filamentary conduction; the CF shows a conductivity enhancement due to thermally activated mechanisms at low temperatures; on the contrary, a CF conductivity degradation is observed at the higher temperatures. Finally, an additional slightly higher value for the non-linear current component as the temperature rises has also been taken into account. A semiempirical compact model has been implemented including these conduction mechanisms and their corresponding temperature dependences, the device has been simulated in LT-Spice and the experimental currents have been correctly reproduced.

Tuning the photoconductivity of Co3O4 thin films by defect engineering

Light-stimulated phenomena, such as photoconductivity, allow access to the role played by structural and point defects of semiconductor oxides. We report on a systematic study of the room temperature photoconductivity of polycrystalline Co3O4 thin films grown by spray pyrolysis. Using different excitation wavelengths, we access the optical absorption edges related to Co2+ or Co3+. Structural and morphological characteristics, as well as their relation with the type of substrate, are also investigated. As-grown Co3O4 exhibits a high density of oxygen vacancies (VO), which can be modified by implementing defect engineering via annealing in different atmospheres. Regardless of the atmosphere, the XPS measurements reveal a similar drop in VO induced by thermal annealing. In addition, Co3+ ions are reduced to Co2+ on the films’ surface. The response time is tens of seconds for optical excitation above the Co3O4 absorption and increases for excitations closer to the Co2+ edge. The absence of saturation profile in as-grown Co3O4 indicates the influence of deep trap defect centers. In contrast, annealing inhibits the persistence effects of red excitations. The oxidizing atmosphere provides long response and recovery times, while annealing in Ar produces fast responses. The reduction in Vo on the film surfaces indicates that oxygen vacancies are not responsible for the deep traps.

Catalytic oxidation of toluene over Co-Cu bimetallic oxides derived from CoyCu3−y-MOF-74

As a precursor material, CoyCu3−y-MOF-74 has been synthesized by using a cost-effective and environmentally friendly mechanochemical method. It had a uniform nano-morphology and a controllable stoichiometric composition. By calcinating the CoyCu3−y-MOF-74 material, a bimetallic Co-Cu oxide catalytic material, with the formula M-CoyCu3−yOx, could, thereafter, be prepared. It was later on used for catalytic oxidation of toluene. The effect of calcination temperature and Co/Cu molar ratio on the redox properties, catalytic activity, and structural properties of the catalyst, were also systematically studied. The M-Co2Cu1Ox catalyst showed a complete toluene conversion of 1000 ppm toluene at 225 ℃ and with a under weight hour space velocity of 30,000 ml/ (g h). The corresponding conversion temperature for 50% and 90% of the original amount of M-Co2Cu1Ox (i.e., T50 and T90) were 202 and 220 ℃, respectively. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and electron paramagnetic resonance (EPR) results also showed a high density of oxygen vacancies in M-Co2Cu1Ox. Also, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments indicated that benzaldehyde and benzoic acid were the main intermediates formed during the catalytic oxidation of toluene. In addition, the M-Co2Cu1Ox catalyst did also show an excellent durability and good water resistance by exposing it to a 5% water vapor for 24 h. Thus, the green and facile mechanical synthesis of the MOF-74 precursor has been proven to be very beneficial for the design and synthesis of a mixed metal oxide catalyst with a high toluene catalytic oxidation efficiency.

Enhanced resistive switching performance of hafnium oxide-based devices: Effects of growth and annealing temperatures

Although hafnium oxide-based resistive switching (RS) devices have been explored extensively, extremely high operating voltages and currents hinder their application in low power operations. In this study, the growth temperature and annealing conditions have been varied to significantly improve the characteristics of hafnium oxide-based RS devices. Deposited using a rf magnetron sputtering at different temperatures, HfOx films are subsequently subjected to vacuum annealing to create a high density of oxygen vacancies. It is observed that the crystallinity, defect density, and RS device performance of these films are influenced by these thermal treatments. The HfOx film annealed at 450 ºC exhibited bipolar RS with the substantial improvement in the device performance, such as electroforming, set/reset voltages, and operating currents as low as ~1 V, 0.5/− 0.5 V, and ~1 µA, respectively, in addition to large ON/OFF ratio> 103, good endurance (>103 cycles), and retention capability (>104 s). The localized conductive filaments composed of oxygen vacancies are found to be responsible for the RS behavior in Al/HfOx/FTO devices, and the simple vacuum annealing approach considerably increases the proportion of oxygen vacancies. With the insights into the influence of annealing temperatures on the switching parameters, this work paves the way for the design of low-power devices for data storage applications.

MgO as promoter for electrocatalytic activities of Co3O4–MgO composite via abundant oxygen vacancies and Co2+ ions towards oxygen evolution reaction

Oxygen evolution reaction (OER) is known as bottleneck problem during the water splitting process due to high energy barrier and non-availability of efficient nonprecious electrocatalysts. The cobalt oxide (Co3O4) in the spinel phase has limited OER activity and stability in the alkaline media. For this purpose, we have carried out the synthesis of Co3O4–MgO (CM) composite by wet chemical method and it offers abundant oxygen vacancies and Co2+ concentration for the efficient OER reaction. The effect of different amounts of MgO on the OER activity of Co3O4 was also studied. Despite inactivity of MgO towards OER, it creates high density of oxygen vacancies and favored the formation Co2+ ions at the surface, thus accelerated the OER kinetics. The physical studies were performed to investigate the morphology, crystalline structure, surface information and chemical composition using several analytical techniques. The optimized CM-0.1 composite produced an overpotential of 274 mV at 10 mAcm−2 which is lower in value than the pristine Co3O4. The significant enhancement in the OER activity was verified by the large value of electrochemical active surface area values 12.8 μFcm−2 and the low charge transfer resistance of 45.96 Ω for the optimized CM-0.1 composite. The use of abundance materials for the synthesis of CM composite revealed an enhanced OER performance, suggesting the dynamic role of MgO, therefore it could be used for improving the electrochemical properties of extended range of metal oxides for specific application especially energy conversion and storage devices.

Effect of W concentration in the organized Ti-W alloy oxide nanotubes array on the photoelectrocatalytic properties and its application in the removal of endocrine disruptors using real water matrix

To evaluate the tungsten content on the photoelectrocatalytic properties of self-organized nanotubes grown by anodization on Ti-xW (wt%) alloys, where x is 0.5, 2.5 or 5.0, this work investigated the phase transformation of TiO2, the tungsten oxidation states in the oxide layers and their photoelectrocatalytic performance in the degradation of estrone (E1) and 17α-ethinylestradiol (EE2). These films were employed in photocatalytic and photoelectrocatalytic removals of hormones from synthetic and real water matrices. In synthetic water, E1 and EE2 removals, reached efficiencies of 92% and 71%, respectively, after 2 min under UV–Vis photoelectrocatalysis using NT/Ti-5.0W as catalyst. About 30 times longer was needed to degrade 77% of both hormones from the real water matrix due to the presence of other high organic charge. The high performance of the NT/Ti-5.0W was associated with the combination of doping (W-doped TiO2) and WO3 (W6+) heterojunction. However, this electrode had its stability compromised under long degradation times, mainly under visible light, due to a WO3 leaching process. As for NT/Ti-0.5W, the substitutional W-doped TiO2 contributed to its greater stability and efficiency for a long time. The low performance of NT/Ti-2.5W was justified by high density of oxygen vacancy and unfavorable position of its adsorption bands.

Trace Al component in ε-(AlxGa1-x)2O3 alloy films and film-based solar-blind photodetectors

ε-(AlxGa1-x)2O3 alloy thin films containing Al fractions of 0, 2.9 at%, and 4.7 at% with ε-Ga2O3 templates were fabricated on (0001) c-sapphire by tin-assisted PLD technique. Sn is a surface-mediated surfactant to stabilize the formation of epsilon phase. XRD reveals that the peak position of ε-(AlxGa1-x)2O3 shifts to a higher diffraction angle and that ε-(AlxGa1-x)2O3 is accompanied by a deterioration of crystal quality when the Al composition increases from 0 to 4.7 at%. With Al content increasing, XPS demonstrates the appearance of more aluminum atoms bound to oxygen atoms and higher oxygen vacancy density in ε-(AlxGa1-x)2O3 films. The optical bandgap for ε-(AlxGa1−x)2O3 thin films with various Al components are 4.97 eV, 5.09 eV and 5.16 eV, respectively. The solar-blind photodetectors were fabricated on ε-(AlxGa1-x)2O3 alloy thin films with various Al fractions. The photo-to-dark current ratio, responsiveness, detectivity and response time (rise time/decay time) for ε-(AlxGa1-x)2O3 (x = 2.9 at%) PDs were 2.7 × 103, 4.2 A/W, 1.23 × 1013 Jones and 2.09 s/0.49 s.

Interface engineered Sb2O3/W18O49 heterostructure for enhanced visible-light-driven photocatalytic N2 reduction

The direct photochemical N2 reduction reaction boasts the potential for clean and sustainable NH3 manufacturing, yet achieving this remains a significant challenge. To activate and cleave the nonpolar N≡N bond and accelerate the transport and separation of charge carriers during photocatalysis, the design and facile synthesis of active, robust, cheap, and earth-abundant photocatalysts is crucial. Here, we demonstrate efficient and stable visible light fixation of N2 to NH3 by constructing a novel heterostructure consisting of two-dimensional Sb2O3 nanosheets and one-dimensional W18O49 nanowires. A remarkable NH3 production rate of up to 731 μgNH3 h−1 gcat.−1 is attained in visible light over the nanocomposite without cocatalyst, 5.5-fold and 3.4-fold improvement over that of discrete Sb2O3 and W18O49 respectively, which also significantly exceeds many previous reported photocatalytic systems. The superior photocatalytic performance can be attributed to the unique structure of the composites, which provides ample exposed surface catalytic sites with a high density of oxygen vacancies, promoting the adsorption and activation of N2 molecules to form *NNH. Equally importantly, the interface between Sb2O3 and W18O49 greatly facilitates charge carrier migration and separation. Further density functional theory calculations manifest that the interfacial metal sites on Sb2O3/W18O49 serve as the main active centers which possess a strong donating electron ability to the empty N2 antibonding orbitals to trigger the following NH3 synthesis. This work provides a simple and effective method to modify metal oxides for enhancing N2 photofixation.