Oxygen Vacancy Defects Research Articles

Eu-doped ZnO coatings prepared by spray pyrolysis for photocatalytic applications

The ZnO-Eu2O3 coatings immobilized on stainless steel foil were synthesized by a spray pyrolysis method using an inorganic salt, Zn(CH3COO)2·2H2O, and Eu2O3 nanopowder with different Zn/Eu ratios. The coatings were investigated by SEM, XRD, XPS, DRS, and photoluminescence analysis, and their photoactivity was studied in the degradation of two pollutants, lindane and methyl orange. The ZnO-Eu2O3 coatings were more photoactive than coatings containing only ZnO. Among the ZnO-Eu2O3 coatings, the photocatalyst with 2 wt% Eu2O3 showed the highest photoactivity for both pollutants. The incorporation of Eu3+ ions into the ZnO crystal lattice causes the appearance of oxygen vacancy defects and better charge separation in the ZnO-Eu2O3 coating. When the Eu2O3 concentration exceeds 2 wt%, the photoactivity decreases as additional centers for electron-hole pair recombination are created. It is shown that the main factor influencing the photoactivity of Eu-doped ZnO composite coatings is the amount of Eu3+ incorporated into the ZnO lattice.

High-temperature hydrogen sensor based on MOFs-derived Mn-doped In2O3 hollow nanotubes

Developing high-temperature hydrogen (H2) sensors with fast response speed is urgently demanded in harsh application environments, especially for chemical industries and the aerospace field. Herein, we have reported a facile strategy to synthesize Mn-doped In2O3 hollow nanotubes (Mn-In2O3) by solvothermal and annealing route using In-MOFs as precursors. The experimental results indicate that the obtained products possess hollow nanotube structures with plenty of holes and Mn doping greatly boosts the gas-sensing performance of In2O3-based sensors towards H2. In particular, the responses of 3 mol% Mn-In2O3 are 2.57 and 2.3 towards 50 ppm H2 at 360 °C and 400 °C, respectively, which are much higher than those of bare In2O3 hollow nanotubes. Besides, the sensor based on 3 mol% Mn-In2O3 exhibits a low limit of detection (25 ppb), excellent selectivity, rapid response/recovery speed (∼4 and ∼15 s@20 ppm), and excellent stability at high temperature (360 °C). Such enhancement of H2-sensing properties can be put down to the hollow structure derived from In-MOFs and abundant oxygen vacancy defects produced by Mn doping. The Mn-In2O3 hollow nanotubes could be regarded as promising materials for selectively detecting H2 in a wide range of concentrations.

The Effect of Oxygen Vacancy Defects on the Structure and Electrochemical Behaviors of LiMn0.65Fe0.35PO4 Cathode

The presence of oxygen vacancy defects significantly impacts the crystal structure and electrochemical attributes of phosphate cathodes. In this investigation, LiMn0.65Fe0.35PO4 materials with varying levels of oxygen vacancy defects were synthesized via hydrogen plasma-induced reduction. It was observed that the content of oxygen vacancy defects on the crystal surface increased proportionately with the rise in hydrogen (H2) flow rate. Notably, the LMFP-3 sample, prepared with an H2 flow rate of 10 ml min−1, demonstrated superior electrochemical performance, characterized by a 159.7 mAh g−1 discharge capacity at 0.1 C and a remarkable 99.8% capacity retention at 5 C after 200 cycles. This enhancement in electrochemical performance is attributed to the improved intrinsic conductivity of the LiMn0.65Fe0.35PO4 material due to the presence of oxygen vacancy defects. However, it is important to note that an excessively high H2 flow rate can lead to the formation of Fe2P impurities, which hinder lithium ion (Li+) diffusion. Furthermore, theoretical calculations conducted using density functional theory provide a rational explanation for the observed improvement in electronic conductivity. The introduction of oxygen vacancy defects results in a significant reduction in the Band gap, which is highly beneficial for enhancing the intrinsic conductivity of the LiMn0.65Fe0.35PO4 materials.

UV and X-ray induced photochromic material based on defect state exchanges

Inorganic photochromic (PC) materials are gaining increasing recognition as promising candidates for applications in optical storage and information display. However, conventional photochromic materials exhibit sluggish coding rates and a deficiency in intelligent response for writing and erasing under multiple excitation sources. This deficiency contributes to suboptimal readout/decoding efficiency and storage security. Here, we show a Ba2NaNb5O15:Er3+ photochromic material with photoluminescence reversible modulation that can fast respond to both ultraviolet light and 532 nm laser within about 1 s, which accompanied by thermal bleaching. The PC mechanism which is based on defect state exchanges between oxygen vacancy defects and color centers is attributed to the structural flexibility of Nb-O systems and oxygen vacancy increased under irradiation. More importantly, it shows differential PC decay behavior in response to X-ray and UV light due to the difference in the number of trapped carriers. Therefore, a simple strategy based on different self-recovery levels induced by double excitation is constructed to develop an encrypted optical memory model. This fast and differential excitation source response behavior hopefully stimulates the development of new encrypted optical memory.

Quenching-induced oxygen vacancy engineering boosts photocatalytic activities of CaTiO3

Oxygen vacancies (VOs) play a pivotal role in promoting the photocatalytic activities of perovskite oxides. Herein, we proposed a simple and effective strategy of introducing both surface oxygen vacancies and bulk single electron trapped oxygen vacancies (SETOVs) into CaTiO3 by ethanol quenching. The purchased CaTiO3 was preheated at 800 °C and then the hot CaTiO3 was quenched in ethanol instantly. The ethanol-quenched QE-CaTiO3 delivers much improved photocatalytic activities towards both RhB degradation and hydrogen evolution under visible light irradiation by 6.5 and 65.2 times, respectively, in comparison with the pristine CaTiO3. Both the bulk SETOVs and surface VOs enhance light absorption, and the surface VOs serve as photocatalytic active sites. DFT calculations further reveal the narrowed band gap and accumulated charge density near the Fermi level in QE-CaTiO3, which help the visible light absorption, facilitate charge carriers generation and migration, and promote photocatalytic activities. Our findings provide an efficient and easily scalable approach to engineering oxygen vacancy defects in photocatalysts and other catalysts.

Unveiling the NIR modulation performance enhancement of VO2 endowed by oxygen vacancy elimination

Vanadium dioxide has emerged as a promising material for smart windows owing to the temperature-responsive variable near-infrared (NIR) transmittance. Yet, the poor NIR modulation ability challenges its efficiency in thermal management. In this study, by meticulously controlling the oxygen vacancy content at a low level, VO2 nanoparticles with excellent NIR modulation performance are achieved. Oxygen vacancy (VO) defects elimination leads to a remarkable decrease of reflectance in the monoclinic (M) phase, dramatically enhancing the near-infrared contrast of VO2 by 154 %. Density functional theory (DFT) calculations reveal that VO elimination favors low refractive index in the NIR region. The optimized experiment is carried out to prepare VO2 nanoparticles with low defects and high crystallinity. It shows the best NIR transmittance contrast at 1500 nm (ΔT1500 nm) of 24.4 %, simultaneously keeping a high luminous transmittance (Tlum) of 79.7 %. This study is believed to provide valuable guidance for the current defect and thermochromic performance study of VO2.

Novel bilayer 2D V2O5 as a potential catalyst for fast photodegradation of organic dyes

Two-dimensional (2D) materials have recently drawn interest in various applications due to their superior electronic properties, high specific surface area, and surface activity. However, studies on the catalytic properties of the 2D counterpart of V2O5 are scarce. In the present study, the catalytic properties of 2D V2O5 vis-à-vis bulk V2O5 for the degradation of methylene blue dye are discussed for the first time. The 2D V2O5 catalyst was synthesized using a modified chemical exfoliation technique. A massive increase in the electrochemically active surface area of 2D V2O5 by one order of magnitude greater than that of bulk V2O5 was observed in this study. Simultaneously, ~ 7 times increase in the optical absorption coefficient of 2D V2O5 significantly increases the number of photogenerated electrons involved in the catalytic performance. In addition, the surface activity of the 2D V2O5 catalyst is enhanced by generating surface oxygen vacancy defects. In the current study, we have achieved ~ 99% degradation of 16 ppm dye using the 2D V2O5 nanosheet catalysts under UV light exposure with a remarkable degradation rate constant of 2.31 min−1, which is an increase of the order of 102 from previous studies using V2O5 nanostructures and nanocomposites as catalysts. Since the enhanced photocatalytic activity emerged from the surface and optical properties of the catalyst, the current study shows great promise for the future application of 2D V2O5 in photo- and electrocatalysis.

Size-Dependent Oxygen Vacancy of Iron Oxide Nanoparticles.

Prior research has highlighted the reduction of iron oxide nanoparticle (IONPs) sizes to the "ultra-small" dimension as a pivotal approach in developing T1-MRI contrast agents, and the enhancement in T1 contrast performance with the reducing size is usually attributed to the increased specific surface area and weakened magnetization. Nonetheless, as the size decreases, the variation in surface defects, particularly oxygen vacancy (VO) defects, significantly impacts the T1 imaging efficacy. In this study, the VO on IONPs is meticulously investigated through XPS, Raman, and EPR spectroscopy. As the nanoparticle size decreased, the VO concentration rose initially but subsequently declined, with the peak concentration observed in the size of 8.27nm. Further insights gained from synchrotron XAS analysis and DFT calculations indicate that both surface tension and phase transition in IONPs contribute to alterations in the Fe─O bond length, thereby influencing the VO formation energy across varying nanoparticle sizes. The MRI tests reveal that the VO in IONPs serve as pivotal sites for the attachment of water molecules to iron ions, and IONPs with fewer VO exhibited a deterioration in T1-MRI contrast effects. This research may provide a deeper understanding of the relationship between T1 contrast performance and the size of IONPs.

High‐Efficiency and Stable Perovskite Solar Cells via Buried Interface Modification with Multi‐Functional Phosphorylcholine Chloride

AbstractThe electron transport layer (ETL), perovskite layer, hole transport layer, and electrode layer collectively constitute the perovskite solar cells (PSCs). Each of these layers plays a critical role in the performance of devices. However, there are mismatches in crystal structure and energy levels between ETL materials and the perovskite layer, resulting in numerous defects at their interface. In this study, multifunctional organic molecule called phosphorylcholine chloride is designed to modify the interface between SnO2 and perovskite layer. This modification serves to both reduce oxygen vacancy defects in the SnO2 and passivate defects in the perovskite layer. Consequently, the conductivity and electron mobility of SnO2 are improved, and the higher‐quality of perovskite film is obtained. Ultimately, the optimized PSC device achieves an impressive champion power conversion efficiency (PCE) of 24.34% with minimal hysteresis index. Even after 1200 h of ambient exposure at 25 °C and 25% relative humidity without encapsulation, the device maintains an impressive 91.44% of its initial efficiency. Additionally, a PCE of 22.38% is attained in flexible PSCs. This research will pave the way for the development of low interface defects, high stability as well as high PCE perovskite solar cells.

Reduction-Assisted Calcination Enhances the Photocatalytic Activity of ZnO and WO3 Nanoparticles in Biomass Conversion.

Rising energy needs and environmental issues have prompted the creation of effective and affordable photocatalysts for converting biomass. Utilizing abundant biomass, oxidation of 5-hydroxymethylfurfural (HMF) emerges as a method for generating high-value chemicals from biomass, offering an alternative to fossil fuels. We synthesized defect-engineered metal oxides (ZnO and WO3) by calcination with NaBH4 as a reducing agent. Atomic-level analyses identified oxygen vacancy defects induced by the reduction of metal ions within the metal oxide nanoparticles. Further analysis showed an unchanged band gap but an up to 4-fold increase in current density. This enhancement is attributed to the trapping of electrons in defect sites created during the calcination process. The formation of new electron donor states hindered photogenerated electron-hole recombination, enhancing the photocatalytic efficiency of the metal oxide. The photocatalytic degradation yield of HMF was over 95%, and the selective organic products 2,5-diformylfuran (DFF) and 2,5-furandicarboxylic acid (FDCA) were obtained without byproducts. Kinetic studies demonstrated that the photocatalytic conversion reaction rates were accelerated by up to 3.5-fold. Improved photocatalytic activity for HMF oxidation was achieved by introducing oxygen vacancy defects upon the reduction of metal ions within the metal oxides. Our results provide a promising approach for designing efficient photocatalysts.

Thermal stable Pt clusters anchored by K/TiO2–Al2O3 for efficient cycloalkane dehydrogenation

Catalytic dehydrogenation of cycloalkanes is considered a valuable endothermic process for alleviating the thermal barrier issue of hypersonic vehicles. However, conventional Pt-based catalysts often face the severe problem of metal sintering under high-temperature conditions. Herein, we develop an efficient K2CO3-modified Pt/TiO2–Al2O3 (K–Pt/TA) for cycloalkane dehydrogenation. The optimized K–Pt/TA showed a high specific activity above 27.9 mol·mol−1·s−1(H2/Pt), with toluene selectivity above 90.0% at 600 °C with a high weight hourly space velocity of 266.4 h−1. The introduction of alkali metal ions could generate titanate layers after high-temperature hydrogen reduction treatment, which promotes the generation of oxygen vacancy defects to anchored Pt clusters. In addition, the titanate layers could weaken the surface acidity of catalysts and inhibit side reactions, including pyrolysis, polymerization, and isomerization reactions. Thus, this work provides a modification method to develop efficient and stable dehydrogenation catalysts under high-temperature conditions.

Fe2TiO5 nanoparticle-based novel gas sensor with high response to ethanol and acetone

This paper presents a novel Fe2TiO5 sensing material for ethanol and acetone detection. Pure Fe2TiO5 nanomaterial was synthesized by a combined solvothermal and calcination method using ferrous sulfate as precursor and urea as a surfactant. The microstructure and surface morphology indicate that Fe2TiO5 belongs to the orthorhombic crystal structure with a Bbmm space group and is composed of loose aggregates of irregular nanoparticles (20–30nm) and abundant pores. X-ray photoelectron spectroscopy analysis indicates that the Fe2TiO5 aggregates contain mixed valence states of Fe2+ and Fe3+ and abundant oxygen vacancies. Examination of gas sensing properties suggests that Fe2TiO5 is an excellent gas-sensitive material. Particularly, at a low optimum operating temperature (240 °C), the gas response of the Fe2TiO5 aggregates to 100 ppm ethanol and acetone is as high as 84.1 and 220.9, respectively. The linear relationship between log(S-1) and logC indicates that Fe2TiO5 can accurately detect ethanol and acetone over a wide concentration range. The gas-sensing performance of the novel Fe2TiO5 nanomaterial can be explained based on the synergistic effect of oxygen vacancy defects and the characteristic surface morphology.

Construction of hollow sphere MnOX with abundant oxygen vacancy for accelerating VOCs degradation: Investigation through operando spectroscopycombined with on-line mass spectrometry

To clarify the key role of oxygen vacancy defects on enhancing the oxidative activity of the catalysts, metal–organic frameworks (MOFs) derived MnOX catalysts with different morphologies and oxygen vacancy defects were successfully prepared using a facile in-situ self-assembly strategy with different alkali moderators. The obtained morphologies included three-dimensional (3D) triangular cone stacked MnOX hollow sphere (MnOX-H) and 3D nanoparticle stacked MnOX nanosphere (MnOX-N). Compared to MnOX-N, MnOX-H exhibited higher activity for the oxidation of toluene (T90 = 226 °C). This was mainly due to the large number of oxygen vacancy defects and Mn4+ species in the MnOX-H catalyst. In addition, the hollow structure of MnOX-H not only facilitated toluene adsorption and activation of toluene and also provided more active sites for toluene oxidation. Reaction mechanism studies showed that the conversion of toluene to benzoate could be realized over MnOX-H catalyst during toluene adsorption at room temperature. In addition, abundant oxygen vacancy defects can accelerate the activated oxidation of toluene and the formation of oxidation products during toluene oxidation.

Metal organic framework-derived porous Ni-doped In2O3 for highly sensitive and selective detection to hydrogen at low temperature

Achieving high sensitivity and selective detection at low temperature is of great significance for hydrogen leakage monitoring and early warning. As a very promising hydrogen sensitive material, In2O3 still faces great challenges in achieving high-performance hydrogen sensing at low temperatures due to inherent defects such as low specific surface area and limited active sites. Here, we prepared Ni-doped In2O3 mesoporous nanomaterials using metal organic framework (MOF) as sacrificial templates. The prepared Ni-doped In2O3 exhibited mesoporous structures with small diameters (2–3 nm), which could be used as effective molecular sieves for the selective detection of hydrogen. The high specific surface area (72.0 m²/g) provided numerous active sites for surface redox reactions, resulting in a remarkable hydrogen sensing response. Notably, gas sensitivity tests demonstrated that the Ni-doped In2O3 gas sensor achieved optimal hydrogen detection performance at 100°C. Particularly, the 3 at% Ni-doped In2O3 nanomaterials exhibited a high response of 12.5–100 ppm hydrogen, along with favorable response/recovery time and a practical limit of detection down to 5 ppm. Furthermore, the Ni-doped In2O3 sensor exhibited excellent reversibility, reproducibility, selectivity and long-term stability. In addition, simulation calculations based on molecular dynamics (MD) and density functional theory (DFT) show that molecular sieve effect is the main reason for the enhanced selectivity, and Ni doping induced oxygen vacancy (OV) defects could enhance gas adsorption. This work could provide technical support for leak detection and warning during hydrogen use, storage and transportation in the future.

Electrochemical sensing platform using cerium tungstate for highly sensitive sensing of chromium in water

Chromium, in some forms and large amounts, can be hazardous to human health and the environment. In particular, hexavalent chromium Cr(VI) and its derivatives are the most poisonous forms of Cr, whose toxicity depends on its oxidation state. Herein, cerium tungstate (CeW) is demonstrated as an efficient sensing material for Cr(VI) detection in aqueous medium. Two different surfactants are used to hydrothermally synthesize flake-like and sphere-like morphology. The flake-like morphology shows excellent Cr(VI) detection ability owing to its larger surface area, oxygen vacancy defects, and electrochemically active surface area. The sensitivity and limit of detection (LoD) value of the CeW nanoflakes are found to be 0.71 μA/ppb and 6.1 ppb, respectively. The LoD is lower than the WHO recommended standard value, which suggests that the synthesized material can be used for determining Cr(VI) concentration in domestic and potable water. The presence of different ions, such as Fe3+, Zn2+, Cu2+, Hg2+and Pb2+in the medium is found not interfere the Cr(VI) detection ability of CeW nanoflakes and thereby enhancing its practical applicability in Cr(VI) detection.

Vacancy Mediated Electrooxidation of 5‐Hydroxymethyl Furfuryl Using Defect Engineered Layered Double Hydroxide Electrocatalysts

AbstractElectrochemical biomass oxidation coupled with hydrogen evolution offers a promising route to generate value‐added chemicals and clean energy. The complex adsorption behavior of 5‐hydroxymethyl furfural (HMF) and hydroxyl ions (OH−) on the electrocatalyst surface during HMF electrooxidation reaction (HMFOR) necessitates an in‐depth understanding of active sites available for adsorption. Herein, oxygen vacancy (VO) defects are introduced in NiFe layered double hydroxide (LDH) using Ce dopants to manipulate electronic structure. Synchrotron‐based HE‐XRD and XAS indicate negligible VO in La‐doped NiFe while Ce doping leads to VO defects due to flexible Ce redox (Ce3+↔ Ce4+). The VO‐rich Ce‐NiFe exhibits higher Faradic efficiency of ≈90% to produce 2,5‐furan dicarboxylic acid (FDCA), far greater than ≈60% for NiFe VO in Ce‐NiFe act as alternative active sites for OH− adsorption, hence reducing adsorption competition for the same metal sites. DFT calculation results corroborate experimental findings by showcasing that the presence of VO in Ce‐NiFe manipulates the adsorption energies and facilitates the chemical adsorption OH− in VO to improve HMFOR. In situ HE‐XRD derived pair distribution function coupled to RMC simulations confirm OH− trapping in VO and HMF adsorption on metal centers as evident by interlayer distance evolution. Taken together, this work showcases routes for dual‐site electrocatalyst design for improved biomass electrooxidation.

Synthesis of VO‐TiO2/VC‐g‐C3N4 Heterojunction for Improving the Photocatalytic Activity by Introducing Defect Sites

AbstractDefect engineering is a proven strategy for enhancing the performance of photocatalysts. Herein, we present a new method for preparing heterojunction photocatalytic materials, VO‐TiO2/VC‐g‐C3N4, with enhanced activity. The presence of defect sites and a heterojunction structure in the catalyst gives rise to exceptional charge separation efficiency and light absorption capabilities. The synergistic effect of the heterojunction interface and activated defect sites, which act as active sites, significantly enhanced the overall photocatalytic performance of VO‐TiO2/VC‐g‐C3N4. The catalyst VO‐TiO2/VC‐g‐C3N4 exhibits superior photocatalytic activity in the NO degradation reaction: displaying 3.5‐ and 2.2 times higher degradation efficiency than VO‐TiO2 and VC‐g‐C3N4, respectively. Moreover, the nitrogen fixation performance of VO‐TiO2/VC‐g‐C3N4 is 3.8‐ and 3.1 times higher than that of VO‐TiO2 and VC‐g‐C3N4, respectively. Electron paramagnetic resonance (EPR) spectroscopy capture experiments revealed that 1O2 and ⋅ OH are the primary active species in the photocatalytic reaction. The collaboration of carbon vacancy defects, oxygen vacancy defects, and the heterojunction structure created additional active sites for VO‐TiO2/VC‐g‐C3N4. The insights gained from this study offer valuable guidance for the efficient design and synthesis of VO‐TiO2/VC‐g‐C3N4 heterojunction photocatalysts, and lay a solid foundation for further research and development of such systems for large‐scale environmental remediation.

Exploring tungsten-oxygen vacancy synergy: Impact on leakage characteristics in Hf0.5Zr0.5O2 ferroelectric thin films

The recent discovery of ferroelectric properties in HfO2 has sparked significant interest in the fields of nonvolatile memory and neuromorphic computing. Yet, as device scaling approaches sub-nanometer dimensions, leakage currents present a formidable challenge. While tungsten (W) electrodes are favored over traditional TiN electrodes for their superior strain and interface engineering capabilities, they are significantly hampered by leakage issues. In this study, we elucidate a positive feedback mechanism attributable to W electrodes that exacerbates oxygen vacancy defects, as evidenced by density functional theory computations. Specifically, intrinsic oxygen vacancies facilitate the diffusion of W, which, in turn, lowers the formation energy of additional oxygen vacancies. This cascade effect introduces extra defect energy levels, thereby compromising the leakage characteristics of the device. We introduce a pre-annealing method to impede W diffusion, diminishing oxygen vacancy concentration by 5%. This reduction significantly curtails leakage currents by an order of magnitude. Our findings provide a foundational understanding for developing effective leakage suppression strategies in ferroelectric devices.

Mechanism and simulation analysis of high electric field of NaNbO3 − based energy storage ceramics based on defect engineering design

Ceramic materials possessing high polarization and substantial breakdown electric fields represent a principal strategy for enhancing the performance of pulse power systems. To augment the energy storage capabilities of ceramic materials, numerous studies have suggested a variety of specific control methods. However, reports on the vacancy defects arising during the regulatory process and the relationship between these vacancy defects and energy storage performance are scarce. This paper introduces an optimal quantity of oxygen vacancy defects into the prepared NN–based ceramics for characterization and analysis, aiming to elucidate the impact of oxygen vacancy defects and their concentration on the structure and energy storage properties of ceramics. Through managing the content of oxygen vacancies, the 0.83NN–0.17SNS ceramic achieved a high energy storage density (Wrec) of 6.27 J/cm3 and an energy storage efficiency (η) of 82.79 % at 705 kV/cm. This work presents a novel approach to regulating the energy storage performance, offering a new method for optimizing the performance of lead-free energy storage materials.

LiGa5O8: Fe3+: A novel and super long near-infrared persistent material

The development of near-infrared persistent materials has become a popular research area owing to their diverse applications. However, the efficient emitters for these materials remains limited. Cr3+ is one of the strongest candidates, since it displays better performance with long duration. Fe3+ is a potential nontoxic alternative although its performance is not as good as Cr3+. In this study, a series of novel Fe3+-activated near-infrared long afterglow phosphors of LiGa5O8 have been prepared. Photoluminescence and persistence luminescence spectroscopy have been collected to investigate its optical properties. Furthermore, the trap characteristics of the samples were systematically analyzed by collecting thermoluminescence signals. The results indicate that there is an intense near-infrared emission centered at 675 nm and a long afterglow duration of more than 28 h for the optimized sample. Trap analysis indicates that oxygen vacancy defects are not the primary reasons for the extraordinarily long afterglow in these samples. Instead, the interaction of other trap types, such as antisite defects, probably contribute to the excellent afterglow properties of the material.