Decrease Of Oxygen Vacancies Research Articles

Defect engineering of charge transport and photovoltaic effect in BiFeO3 films

Bismuth ferrite (BiFeO3) is an attractive multiferroic material, extensively explored in photoferroelectric investigations. However, its applications are hindered by the high leakage current, requiring precise control of charge transport properties. Defect engineering has emerged as a promising strategy to address this issue: controlling the defect chemistry, particularly oxygen vacancies, is key to tuning the electrical properties. This study investigates the influence of 5% ▪ - and 2% ▪ -doping on the dark and light-induced charge transport properties of polycrystalline BiFeO3 films. Our results demonstrate that ▪ reduces dark conductivity by decreasing oxygen vacancy concentration with no change in the physical nature of the charge transport mechanism. In contrast, ▪ modifies the charge transport mechanism, increasing low-field (E < 100kVcm-1) dark conductivity while drastically reducing high-field (E > 250kVcm-1) dark conductivity. This tuning of the defect chemistry is also key to enhance the photovoltages of the bulk photovoltaic effect in BiFeO3. High photoinduced electric fields up to 7kVcm-1 and low photoconductivity values are obtained with ▪ -doping, while high short-circuit photocurrent values are obtained with ▪ -doping.

Structure regulation and performance optimization mechanism of Sr0.7Bi0.2TiO3-based energy storage ceramics based on charged defect design engineering

The linear-like relaxor ferroelectric Sr0.7Bi0.2TiO3 with regulable microstructure offers a new platform to reveal the essential mechanism of energy storage properties improvement and develop advanced pulse capacitors. Herein, Li with relatively weak volatility accompanied by Bi was introduced in Sr0.7Bi0.2TiO3 to form a charged defect and increase the maximum polarization (Pmax) while avoiding the deterioration of dielectric breakdown strength (BDS). The occupation of Li and the formation of Li-Bi defect dipole polar were analyzed using first principle calculations. A unique local superlattice induced by the lattice distortion was first observed in related systems. The Pmax was enhanced due to the increased size and number of polar structures. The breakdown behaviour and local electric field distribution were analyzed using numerical simulation. The BDS was improved due to the decreased oxygen vacancies, high thermal conductivity and enhanced electrical insulation. The excellent and stable energy storage performance was achieved in different temperatures, frequencies and cycles. Meanwhile, the optimized sample possesses excellent and stable charge–discharge properties, especially fatigue resistance (105 cycles under 25 °C and 150 °C). After the process optimization, the sample shows a higher energy storage density of 3.44 J/cm3 with a high efficiency of 93.74 % at 345 kV/cm due to the further enhanced BDS. In this work, the performance improvement mechanism was studied in-depth, providing a new strategy for optimizing linear-like lead-free relaxor ferroelectric energy storage properties.

Insights into in situ surface reconstruction in cobalt perovskite oxides for enhanced catalytic activity

An depth understanding of the fundamental interactions between surface termination and catalytic activity is crucial to prompt the properties of functional perovskite materials. The elastic energy due to size mismatch and electrostatic attraction of the charged Sr dopant by positively charged oxygen vacancies induced inert A-site surface enrichment rearrangement for perovskites. Lower temperatures could reduce A-site enrichment, but it is difficult to form perovskite crystals. La0.8Sr0.2CoO3-δ (LSCO) as a model perovskite oxide was modified with additive urea to reduce the crystallization temperature, and suppress Sr segregation. The LSCO catalysts with 600 °C annealing temperature (LSCO-600) exhibited a 19.4-fold reaction reactivity of toluene oxidation than that with 800 °C annealing temperature (LSCO-800). Combined surface-sensitive and depth-resolved techniques for surface and sub-surface analysis, surface Sr enrichment was effectively suppressed due to decreased oxygen vacancy concentration and smaller electrostatic driving force. DFT calculations and in-situ DRIFTs spectra well revealed that tuning the surface composition/termination affected the intrinsic reactivity. The catalyst surface with lower Sr enrichment could easily adsorb toluene, cleave, and decompose benzene rings, thus contributing to toluene degradation to CO2. This work demonstrates a green and efficient way to control surface composition and termination at the atomic scale for higher catalytic activity.

Stabilizing atomic Ru species in conjugated sp2 carbon-linked covalent organic framework for acidic water oxidation

Suppressing the kinetically favorable lattice oxygen oxidation mechanism pathway and triggering the adsorbate evolution mechanism pathway at the expense of activity are the state-of-the-art strategies for Ru-based electrocatalysts toward acidic water oxidation. Herein, atomically dispersed Ru species are anchored into an acidic stable vinyl-linked 2D covalent organic framework with unique crossed π-conjugation, termed as COF-205-Ru. The crossed π-conjugated structure of COF-205-Ru not only suppresses the dissolution of Ru through strong Ru-N motifs, but also reduces the oxidation state of Ru by multiple π-conjugations, thereby activating the oxygen coordinated to Ru and stabilizing the oxygen vacancies during oxygen evolution process. Experimental results including X-ray absorption spectroscopy, in situ Raman spectroscopy, in situ powder X-ray diffraction patterns, and theoretical calculations unveil the activated oxygen with elevated energy level of O 2p band, decreased oxygen vacancy formation energy, promoted electrochemical stability, and significantly reduced energy barrier of potential determining step for acidic water oxidation. Consequently, the obtained COF-205-Ru displays a high mass activity with 2659.3 A g−1, which is 32-fold higher than the commercial RuO2, and retains long-term durability of over 280 h. This work provides a strategy to simultaneously promote the stability and activity of Ru-based catalysts for acidic water oxidation.

Decreased Accumulation of SnO2 Particles Results from Sodium Citrate Dispersant Assisted Chemical Bath Deposition for High Quality Perovskite Solar Cells

Electron transport layer (ETL) is a crucial part for perovskite solar cells (PSCs). However, leakage current and oxygen vacancies result from little pores grown from sol‐gel method would inevitably deteriorate the performance of ETL. In contrast, thin films prepared by chemical bath deposition (CBD) could significantly restrain the little cracks. Here, SnO2 ETL are prepared by optimized CBD process, sodium citrate (SC) is explored as an auxiliary reagent to disperse ETL, thus enhances the smoothness and conductivity of the ETL, resulting in a pore‐free flat surface morphology and decreased oxygen vacancy, which is favorable for electron transport and deposition of perovskite (PVK). The PVK film fabricated on the oxygen vacancy free substrate demonstrates high quality and decreased defect density, thus effectively prevents the non‐radiative recombination of carriers. As a result, the short‐circuit current (JSC) and open circuit voltage (Voc) of the PSC devices have been obviously improved, leading to increased photoelectric conversion efficiency from 21.03% to 22.04%. Meanwhile, the long‐term and moisture stability have been promoted simultaneously.

Effect of grain boundary segregation and oxygen vacancy annihilation on aging resistance of cobalt oxide-doped 3Y-TZP ceramics for biomedical applications

Abstract This study aims to investigate the diffusion stabilization process of nano-Co2O3 during the non-precursor transformation of 3Y-TZP. 3Y-TZP was set as the control group, and the experimental groups were 0.1–0.3 mol% nano-Co2O3-doped 3Y-TZP. The samples were prepared by the ball milling process, isostatic cool pressing, and sintering. All samples were hydrothermally treated at 134°C and 2 bar for different time periods. The resistance to low-temperature degradation of nano-Co2O3-doped 3Y-TZP was analyzed by X-ray diffraction. The microstructure of zirconia ceramic samples was determined by scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and electron paramagnetic resonance studies. The addition of nano-Co2O3 into 3Y-TZP resulted in higher hydrothermal aging resistance than 3Y-TZP. The addition of 0.2 mol% nano-Co2O3 dopants resulted in the highest hydrothermal aging resistance among nano-Co2O3-doped 3Y-TZP ceramics. The grain sizes of 3Y-0.2Co are smaller than those in the control group. With the increase of cobaltous oxide doping contents, the segregation of Co3+ ions at the crystal boundary increased. The content of oxygen vacancies on the surface of the sample increased with the increase of the Co2O3 doping content. The oxygen vacancy concentrations of 3Y-0.2Co increased obviously after aging. 3Y-0.1Co, 3Y-0.3Co, and the control showed decreased oxygen vacancy concentrations after aging. Trivalent element doping of 3Y-TZP effectively improved the aging resistance of 3Y-TZP. The addition of 0.2 mol% nano-Co2O3 resulted in the highest hydrothermal aging resistance. Improved aging resistance is attributed to the nano-Co2O3 doping resulting in the 3Y-TZP grain size inhibition, grain boundary segregation of cobalt ions, and oxygen vacancy maintenance. This work is expected to provide an effective reference for the development and application of budget dental materials by regulating grain boundary engineering.

Exploring the impact of thermal oxidation temperature on photoelectrochemical water splitting capabilities of Ga2O3[formula omitted] modified TiO2

This paper reports the influence of thermal oxidation of Ga2O3 thin films deposited on titanium foil substrates via RF magnetron sputtering. The samples were characterized and subjected to photoelectrochemical analysis to assess their photocurrent density response. XRD spectra disclose Ti, TiO2), and Ga2O3 crystal phases. EDX analysis indicates that oxidation temperature reduces oxygen vacancies. Kubelka-Munk technique is used to estimate the optical band gap energies of the TiO2/Ga2O3. The band gap energy decreases from 2.39 to 1.99 and 2.14 eV for TG500 and TG600, whereas TG800 and TG700 increase to 2.98 and 3.09 eV. The TG500 disclosed the highest photocurrent density response of 1382 μAcm−1 at 1.2 V vs. Ag/AgCl in 0.5 M NaOH electrolyte, attributed to decreased oxygen vacancies and band gap. With the strong photocurrent response, stability, and repeatability, the samples may be useful in photocatalytic water-splitting applications for green hydrogen production.

Understanding the impact of Bi stoichiometry towards optimised BiFeO3 photocathodes: structure, morphology, defects and ferroelectricity.

BiFeO3 thin films have been widely studied for photoelectrochemical water splitting applications because of its narrow bandgap and good ferroelectricity which can promote the separation of photo-generated charges. Bismuth is well known as a volatile element and excess bismuth is usually added into the precursor to compensate the loss of bismuth during heat treatment, but the amount of excess bismuth required and how excess bismuth will affect PEC performance have not been clearly studied. Herein, self-doped Bi1+x FeO3 thin films are prepared via simple chemical solution deposition method with excess bismuth from 0-30% in the precursor. The loss of bismuth after annealing is confirmed by EDX and XPS. Multiple factors were investigated and it was found that non-stoichiometric Bi resulted in changes of structure, morphology, defects, electronic properties and PEC performance. An enhanced photocurrent is observed in bismuth-rich BiFeO3 films, which can be ascribed to the larger grain size, decreased oxygen vacancies, lattice distortion and supported charge separation. Moreover, the photocathodic performance can be further enhance by ferroelectric poling. Our work indicates that deficient bismuth should be carefully avoided during heat treatment and moreover, a slight excess of Bi is beneficial for PEC performance. Therefore, we offer a simple way to enhance PEC performance of BiFeO3-based ferroelectric materials through careful control of their stoichiometry.

Influence of iron content on water uptake and charge transport in BaCe0.6Zr0.2Y0.2−xFexO3−δ triple-conducting oxides

Higher iron content in BaCe0.6Zr0.2Y0.2−xFexO3−δ system leads to decreased oxygen vacancy concentration and diminished proton uptake, as well as lower electronic and oxygen ionic conductivity.

High-entropy perovskite oxides for direct solar-driven thermochemical CO2 splitting

The global energy crisis and climate change have fueled interest in solar-driven thermochemical CO2 splitting as a potential solution. However, conventional materials like CeO2 encounter limitations attributable to their low solar absorptivity and CO yield (even at high reduction temperatures), exerting a detrimental influence on both solar energy utilization and process efficiency. This study proposes high-entropy A-site doped perovskites as a potential solution for efficient solar thermochemical CO2 splitting. The increased configurational entropy enhances the solar thermal CO2 splitting cycles by decreasing oxygen vacancy formation energy and lattice oxygen migration energy barrier. This is supported by experimental results, where Sm0.25Sr0.25Ca0.25La0.25MnO3 achieved a maximum CO yield of 764.76 μmol g−1 with low temperature difference between the reduction and oxidation steps, marking an important progress in solar thermal CO2 splitting cycles. The study demonstrates the potential of high-entropy perovskites for direct solar energy harvesting with high spectrum absorption coefficients and sustainable CO2 utilization.

Modulation of electric and magnetic properties in 0.70BiFeO3-0.30BaTi0.9O3 ceramics with controlled oxygen vacancies and Fe2+/Fe3+ ratio

The physical properties of transition-metal oxides are closely related to defect concentrations. Herein, the modulation of oxygen vacancies and Fe2+/Fe3+ ratio on the electric and magnetic properties in Ti non-stoichiometry 0.70BiFeO3-0.30BaTi0.9O3 ceramics were investigated systematically. As the concentration of oxygen vacancies decreases, grain size increases and the perovskite unit cell with rhombohedral phase becomes more distorted. The maximum dielectric constant gradually increases and the relaxation factor decreases with decreasing oxygen vacancy concentration. The enhancement of ferroelectricity is consistent with the degree of the distortion of the perovskite lattice, and the reduction of internal bias field comes from the reduction of the defect dipoles in the ceramics. At low oxygen partial pressure, Fe2+ defects are preferred and form the double exchange interaction between Fe2+ and Fe3+ ions, which will enhance the magnetization.

In situ studies on the industrial production process for molybdenum dioxide, MoO2

AbstractIn the industrial production of molybdenum the reduction of molybdenum trioxide with hydrogen is a two‐step process. In order to get a deeper understanding of the chemistry involved, in situ studies were performed on the thermal behavior of molybdenum trioxide and the first reduction step to molybdenum dioxide. MoO3 shows a very strong anisotropy for thermal expansion (linear expansion coefficients α in 10−4 K−1: 1.32(4) for a, 5.5(1) for b, −0.246(5) for c). Upon heating in air, preferred orientation in XRPD patterns decreases and the color changes from grey to yellow, probably caused by decreasing oxygen vacancy concentration. The reduction of MoO3 by hydrogen was investigated via in situ X‐ray powder diffraction for varying heating rate, hydrogen flow and potassium content. The formation of the intermediate γ‐Mo4O11 shows a higher rate for increasing potassium content; it does not play a role whether potassium is present in the starting material MoO3 or in MoO2 formed in the reaction. The high‐temperature modification of K2MoO4 was found to crystallize in the α‐K2SO4 type (P63/mmc, a=6.3463(2) Å, c= 8.1331(2) Å). Heating up MoO3 in vacuum (1 Pa) with the same T,t protocol as above yields MoO2 even without the presence of hydrogen.

A co-doping strategy to achieve high energy storage performance in BiFeO3-based ceramics

In this work, (Bi0.65-0.65xBa0.35-0.35xNdx)(Fe0.65-0.65xTi0.35-0.35xNbx)O3+x (abbreviated as: BFBT-xNN, x = 0, 0.04, 0.08 and 0.12) ceramics were fabricated through a solid-state reaction method, aiming to obtain high-performance capacitor materials. BFBT ceramics have attracted increasing attention as potential energy storage materials because of their excellent dielectric and ferroelectric properties. However, two handicaps limit the large improvement of their energy storage performance (ESP). One is the large hysteresis of their polarization (P)-electric field (E) loops; the other is the low dielectric breakdown strength (Eb), which has a close correlation with the inevitable defects present in BF-based ceramics. Herein, we designed a co-doping strategy to overcome the two handicaps in order to achieve high ESP in BFBT ceramics. In this strategy, the trivalent rare earth Nd3+ ions and high valence Nb5+ ions are simultaneously introduced at the perovskite A- and B-sites of BFBT ceramics, which is called NN co-doping. The NN co-doping enhances the relaxor properties of BFBT ceramics due to the enhanced composition and charge fluctuations and promotes the formation of the Bi-rich phases, which contribute to the appearance of a slim P-E loop. Meanwhile, the Eb of BFBT ceramics is enhanced, which is ascribed to the significantly decreased oxygen vacancy concentration, the reduced grain size and the Bi-rich phases. Benefitting from these factors, a high recoverable energy storage density (Wrec) of 3.64 J/cm3 and energy storage efficiency (η) of 88% are simultaneously obtained under an enhanced Eb of 330 kV/cm in BFBT-0.08NN ceramics. Moreover, the ESP of BFBT-0.08NN ceramics shows good thermal stability (30–150 °C) and charge-discharge properties. These results indicate that BFBT-0.08NN ceramics could be a promising capacitor material.

The impact of oxygen vacancy defect density on the nonlinearity and short-term plasticity of TiO2-based exponential selector

The readout margin of the one selector-one RRAM crossbar array architecture is strongly dependent on the nonlinearity of the selector device. In this work, we demonstrated that the nonlinearity of Pt/TiO2/Pt exponential selectors increases with decreasing oxygen vacancy defect density. The defect density is controlled by modulating the sputtering pressure in the oxide deposition process. Our results reveal that the dominant conduction mechanisms of the Pt/TiO2/Pt structure transit from Schottky emission to Poole–Frenkel emission with the increase of sputtering pressure. Such transition is attributed to the rise of oxygen vacancy concentration. In addition, the short-term plasticity feature of the Pt/TiO2/Pt selector is shown to be enhanced with a lower defect density. These results suggest that low defect density is necessary for improved exponential selector performances.

Thermal-strain driving sharp metal-to-insulate transition and island-grain growth of solution-derived NdNiO3 epitaxial thin films

An ultra-sharp metal-to-insulate transition (MIT) of 1.24 K−1 in the epitaxial perovskite NdNiO3 thin films was derived by the chemical solution deposition on the LaAlO3 substrates. The thermal strains from shrink, grain growth, and thermal expansion coefficient misfit play a key role in the film microstructure and electrical properties. The originally theoretical in-plane compressive epitaxial strain changes into a tensile one caused by the thermal driving force. It relaxes with improved grain growth via decreased oxygen vacancies with increasing annealing temperature, while the concurrently enhanced tensile strain from the thermal expansion coefficient misfit between the films and the substrate leads to the destabilization of Ni3+ and the higher MIT temperature. Nevertheless, too much higher tensile strain gives rise to island-grain growth in the films, leading to the weak and even disappeared MIT.

In-situ composition development of Zn/In-doped Ga2O3 nanowire with ultrahigh responsivity and long-term stability for deep-UV photodetector

High-performance deep ultraviolet photodetectors (UV PDs) based on Ga2O3 have been applied widely in civil and military exploration. The incident photon-to-electron conversion efficiency of Ga2O3-based UV PDs could be improved by doping metal or non-metal elements in the lattice. Herein, multi-metallic oxide of InZn co-doped Ga2O3 (InZn-Ga2O3) nanowires is synthesized via the in-situ chemical vapor deposition (CVD) method. The favorable 25%InZn-Ga2O3 exhibits a maximum responsivity of 440.42 A W−1 and a peak external quantum efficiency (EQE) of 2.15 × 105% under 254 nm light illumination at 10 V, which are 1.19 × 104 times higher than that for pure Ga2O3, indicating its enhanced detection ability for deep-UV light and high conversion efficiency from photons to electrons. A high UV/visible rejection ratio of 7.3 × 105 is also obtained, implying a high sensitivity for UV light. Moreover, the responsivity of the InZn-Ga2O3 UV PD remains unchanged after a long period of 8000 s testing. The superior performances of InZn-Ga2O3 UV PD are attributed to the improved crystal quality with decreased oxygen vacancies and elevated conductivity, which facilitate the separation and transfer of photoinduced electron-hole carriers. The DFT calculation and UV–vis spectra results reveal that the InZn-Ga2O3 possesses a narrowed band gap with enhanced light absorption and a decreased work function for the fast escape of electrons. In addition, the ohmic contact between InZn-Ga2O3 and Ag electrode accelerates the transfer and diffusion of photogenerated electrons. The achievements in our work present a multi-metallic oxide nanowire with high performance and an innovative mechanism analysis method for the applications of optoelectronic devices.

Enhanced energy storage density and efficiency in La(Mg2/3Ta1/3)O3-doped BiFeO3 based ceramics

High-performance dielectric capacitor materials are required for the miniaturization of electronic devices. To this end, (1-x)(0.65BiFeO3-0.35BaTiO3)-xLa(Mg2/3Ta1/3)O3 (BFBT-xLMT, x = 0, 0.06,0.10, 0.14 and 0.18) ceramics were fabricated by a solid-state method. The experimental results indicate that LMT doping can well improve the energy storage density (Wrec) and efficiency (η) of BFBT ceramics. Especially, the optimal energy storage performance (ESP) (Wrec=4.92 J/cm3, η = 85%) was obtained in BFBT-0.14LMT ceramics under 390 kV/cm. The enhancement of ESP of BFBT-0.14LMT ceramics can be ascribed to the decreased hysteresis of polarization (P) - electric field (E) loops resulting from the aggravation of the composition and charge disorder in the system, and the improved breakdown strength (Eb) which is related to the reduced grain size, the widened band gap (Eg) and the decreased oxygen vacancy concentration. Additionally, the thermal stability of ESP of BFBT-0.14LMT ceramics is also enhanced compared with pure BFBT ceramics. Meanwhile, BFBT-0.14LMT ceramics exhibit a fast discharge rate (t0.9 = 152 ns) under 180 kV/cm. These results indicate that BFBT-0.14LMT ceramics could be a promising dielectric capacitor material. In general, our study reveals that LMT is good dopant to enhance the Eb and ESP of BF-based ceramics.

Integrated atmospheric microplasma with ultrasonic spray pyrolysis deposition of aluminum-doped zinc oxide

Aluminum-doped zinc oxide (AZO) has electrical conductivity and visible light transmittance similar to those of indium tin oxide that is used in modern display devices at a lower price. Thus, many studies regarded it as an alternative material for transparent conductive oxide coating. This study has integrated atmospheric microplasma with an ultrasonic spray pyrolysis deposition system to prepare economically and rapidly AZO using zinc chloride and aluminum chloride as precursors. The low electrical resistivity of the coating prepared in this system is proposed to be due to free radicals and reactive oxygen. They are generated by the atmospheric plasma that is adsorbed to the layer, thereby decreasing oxygen vacancy and free carrier concentration. Vacuum annealing heat treatment was used to improve the properties of the coating; the resistivity of argon-annealed AZO was [Formula: see text]–cm with 80% transmittance, thus meeting the transparent conducting oxide requirements.

Phase Character and Dielectric Properties of Amphoteric Holmium‐Doped Ba0.90Ca0.10(Ti0.95Zr0.05)2O5 Polycrystalline Ceramics

Amphoteric rare‐earth ion dopants can improve ferroelectric ceramic electrical performance because of their amphoteric ion radius and valences. Herein, Ho3+‐doped Ba0.90Ca0.10(Ti0.95Zr0.05)2O5 ceramics are sintered at 1230 °C with as‐synthesized nanoparticles. The effects of various holmium contents (x) on the phase, structure, dielectric, and piezoelectric characteristics are investigated. For low x (&lt;1.5 mol%), Ho3+ occupies the Ba2+ or/and Ca2+ sites, leading to high dielectric properties due to decreased oxygen vacancies, while the electrical properties weaken with further increase in x. The ferroelectricity and piezoelectricity also deteriorate when x &gt; 1.5 mol% because of the re‐emerged oxygen vacancies and interrupted long‐range ordering with large compositional fluctuations. The defect chemistry mechanism at different x contents is deduced in accordance with the phase and electrical properties. The activation energy initially elevates to 1.56 eV and then decreases to 1.35 eV with increasing x. Results indicate that the ceramics are promising candidates for lead‐free ferroelectric ceramics.

Improving the performance of organic light-emitting devices by employing oxygen plasma treatment on indium-tin-oxide surfaces

ITO was modified by an oxygen plasma treatment, and its surface characteristics were evaluated by ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, and contact angle technologies. The plasma discharge boosted the ITO work function without inducing surface damage, attributed to eliminating carbon contaminants instead of decreasing oxygen vacancy, and improved the surface morphology, confirmed by the smaller root-mean-square. Besides, the plasma treatment increased the surface wettability evaluated by the surface energy, which improved the adhesion between organic molecular and ITO surfaces and was beneficial for realizing high-efficiency devices. The prepared white device based on the oxygen-plasma-treated ITO performed better than the reference white device with a pristine ITO anode, demonstrating higher current efficiencies and power efficiency over a wide range of current densities. The improved efficiency is ascribed to alleviating leakage current density and reducing operating voltages.