Role Of Surface Oxygen Vacancies Research Articles

Promoted oxygen adsorption on porous CeO2 cubes with abundant oxygen vacancies for efficient gaseous formaldehyde removal

Photocatalytic degradation stands as a promising method for eliminating gas-phase pollutants, with the efficiency largely hinging on the capture of photogenerated electrons by oxygen. In this work, we synthesized a porous CeO2 single crystal cube with abundant oxygen vacancies as photocatalyst, employing urea as a pore-forming agent and for gas-phase formaldehyde degradation. Compared with the CeO2 cubes without pores, the porous ones were superior in specific surface area, akin to conventional CeO2 nanoparticles. The photocatalytic degradation for gas-phase formaldehyde on porous CeO2 cubes was significantly accelerated, of which degradation rate is 3.3 times and 2.1 times that of CeO2 cubes without pores and CeO2 nanoparticles, respectively. Photoelectric tests and DFT calculations revealed that this enhancement stemmed from facilitated oxygen adsorption due to pronounced oxygen vacancies. Consequently, the capture of photoelectrons by oxygen was promoted and its recombination with holes was suppressed, along with an accelerated generation of curial free radicals such as ·OH. This work reveals the pivotal role of surface oxygen vacancies in promoting adsorbed oxygen, proposing a viable strategy to enhance the photocatalytic degradation efficiency for gas-phase pollutants.

Effective prediction of SnO2 conduction band edge potential: The key role of surface oxygen vacancies.

Several theoretical studies at different levels of theory have attempted to calculate the absolute position of the SnO2 conduction band, whose knowledge is key for its effective application in optoelectronic devices such us, for example, perovskite solar cells. However, the predicted band edges fall outside the experimentally measured range. In this work, we introduce a computational scheme designed to calculate the conduction band minimum values of SnO2, yielding results aligned with experiments. Our analysis points out the fundamental role of encompassing surface oxygen vacancies to properly describe the electronic profile of this material. We explore the impact of both bridge and in-plane oxygen vacancy defects on the structural and electronic properties of SnO2, explaining from an atomistic perspective the experimental observables. The results underscore the importance of simulating both types of defects to accurately predict SnO2 features and provide new fundamental insights that can guide future studies concerning design and optimization of SnO2-based materials and functional interfaces.

Enhanced solar-driven water splitting performance using oxygen vacancy rich ZnO photoanodes

This work reports a facile two-step method of producing highly efficient ZnO photoanodes for photoelectrochemical (PEC) water splitting under solar light conditions and describes the role of surface oxygen vacancies (VO) in their enhanced PEC performance. The photoanode fabrication involves post-growth oxidation of a metallic Zn layer, which produces a nanostructured ZnO film consisting of ∼50 nm diameter nanorods containing a high concentration of VO defects. The PEC activity of the ZnO films is investigated by studying water oxidation in an aqueous electrolyte under simulated solar illumination. The relationship of PEC and charge transfer characteristics of the ZnO photoanodes with ionized surface VO defects is established using cathodoluminescence, X-ray photoemission, voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The combined results show that the photoanode fabricated in this work possesses a high surface density of ionized VO states that facilitate the effective transportation of holes for water oxidation. It is found that the photoanode exhibits an exceptional photocurrent density of 1.14 mA/cm2 at 1.23 VRHE, being one of the best performances reported in the literature for ZnO-based photoelectrodes so far. Our results demonstrate a simple, low-cost method for fabricating highly efficient VO rich ZnO-based PEC photoanodes that is suitable for large scale production.

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Conversion of La2Ti2O7 to LaTiO2N via ammonolysis: a first-principles investigation.

Perovskite oxynitrides are, due to their reduced band gap compared to oxides, promising materials for photocatalytic applications. They are most commonly synthesized from {110} layered Carpy-Galy (A2B2O7) perovskites via thermal ammonolysis, i.e. the exposure to a flow of ammonia at elevated temperature. The conversion of the layered oxide to the non-layered oxynitride must involve a complex combination of nitrogen incorporation, oxygen removal and ultimately structural transition by elimination of the interlayer shear plane. Despite the process being commonly used, little is known about the microscopic mechanisms and hence factors that could ease the conversion. Here we aim to derive such insights via density functional theory calculations of the defect chemistry of the oxide and the oxynitride as well as the oxide's surface chemistry. Our results point to the crucial role of surface oxygen vacancies in forming clusters of NH3 decomposition products and in incorporating N, most favorably substitutionally at the anion site. N then spontaneously diffuses away from the surface, more easily parallel to the surface and in interlayer regions, while diffusion perpendicular to the interlayer plane is somewhat slower. Once incorporation and diffusion lead to a local N concentration of about 70% of the stoichiometric oxynitride composition, the nitridated oxide spontaneously transforms to a nitrogen-deficient oxynitride. Since anion vacancies are crucial for the nitrogen incorporation and diffusion as well as the transformation process, their concentration in the precursor oxide is a relevant tuning parameter to optimize the oxynitride's synthesis and properties.

Role of Surface Oxygen Vacancies and Oxygen Species on CuO Nanostructured Surfaces in Model Catalytic Oxidation and Reductions: Insight into the Structure-Activity Relationship Toward the Performance.

Defect engineering, such as modification of oxygen vacancy density, has been considered as an effective approach to tailor the catalytic performance on transition-metal oxide nanostructured surfaces. The role of oxygen vacancies (OV) on the surface of the as-prepared, zinnia-shaped morphology of CuO nanostructures and their marigold forms on calcination at 800 °C has been investigated through the study of model catalytic reactions of reduction of 4-nitrophenol and aerobic oxidation of benzyl alcohol. The OV on the surfaces of different morphologies of CuO have been identified and quantified through Rietveld analysis and HRTEM, EPR, and XPS studies. The structure-activity relationships between surface oxygen vacancies (OV) and catalytic performance have been systematically investigated. The enhanced catalytic performance of the cubic CuO nanostructures compared to their as-prepared forms has been attributed to the formation of surface oxygen species on the reactive and dominant (110) surface that has low oxygen vacancy formation energy. The mechanistic role of surface oxygen species in the studied reactions has been quantitatively correlated with the catalytic activity of the different morphological forms of the CuO nanostructures.

Tuning the selectivity of cerium oxide for ethanol dehydration to ethylene

Ethylene is a prized raw material in the chemical industry, as well as a valued monomer to produce important commodities. At present, ethylene is mainly produced via the cracking of hydrocarbons, but there is an increasing interest in exploiting renewable biomass-derived feedstocks. The catalytic dehydration of bioethanol is a sustainable alternative route to produce ethylene. Ceria-based catalysts are active for ethanol dehydration to ethylene at low-temperature, but their selectivity is limited by the undesired formation of acetaldehyde. Partially reduced ceria can promote the dehydration of primary alcohols into alkenes over their dehydrogenation to aldehydes. This work provides computational insight into the role of surface oxygen vacancies in the selectivity of ceria. Based on this, it is proposed that the selectivity of ceria toward ethylene can be tuned by substituting Ce4+ with La3+ cations. This is tested using both density functional theory (DFT) calculations and temperature-programmed desorption (TPD) experiments to quantify the impact of doping ceria with lanthanum on the production of ethylene and acetaldehyde. The La-doped ceria sample is found to be more active and selective. This insight can guide the rational design of new ceria-based catalysts for an ethanol dehydration reaction with high selectivity to ethylene.

Oxygen Vacancy-Governed Opposite Catalytic Performance for C3H6 and C3H8 Combustion: The Effect of the Pt Electronic Structure and Chemisorbed Oxygen Species.

Revealing the role of engineered surface oxygen vacancies in the catalytic degradation of volatile organic compounds (VOCs) is of importance for the development of highly efficient catalysts. However, because of various structures of VOC molecules, the role of surface oxygen vacancies in different catalytic reactions remains ambiguous. Herein, a defective Pt/TiO2-x catalyst is proposed to uncover the different catalytic mechanisms of C3H6 and C3H8 combustion via experiments and theoretical calculations. The electron transfer, originated from the oxygen vacancy, facilitates the formation of reduced Pt0 species and simultaneously interfacial chemisorbed O2, thus promoting the C3H6 combustion via efficient C═C cleavage. The reduced Pt nanoparticles facilitate the robust chemisorption of bridging dimer O22- (Pt-O-O-Ti) species. This chemisorbed oxygen inhibits the C3H8 combustion by depressing C3H8 adsorption. This work offers insights for the rational design of highly efficient catalysts for activating the C═C bond in alkene or C-H bond in alkane.

Role of surface oxygen vacancies in zinc oxide/graphitic carbon nitride composite for adjusting energy band structure to promote visible-light-driven photocatalytic activity

Introducing oxygen vacancies is an effective way to promote photocatalytic performance of catalysts. Numerous explorations on metallic oxides and their composites were aimed at promoting visible light efficiency by narrowing bandgap and enhancing charge separations. In this work, we designed and synthesized the zinc oxide/graphitic carbon nitride (ZnO/g-C3N4) composites with diverse content of surface oxygen vacancies. We investigated the possible correlations between the surface defects and energy band structure using ultraviolet–visible (UV–vis) absorption spectra, Mott-Schottky plots and X-ray photoelectron spectroscopy. The results show that the surface oxygen vacancies in ZnO/g-C3N4 were responsible for the narrowing of the bandgap derived from the lower conduction band of ZnO. Composites of ZnO/g-C3N4 with more surface oxygen vacancies had significantly increased transient photocurrent with higher photocatalytic activity under visible light. The improved photocatalytic activity was demonstrated through photocatalytic organic degradation kinetics. Moreover, the introduction of surface oxygen vacancies in composite was originated from the formation of Zn-C bonds at heterojunctions between ZnO and g-C3N4.

Regulating activation pathway of Cu/persulfate through the incorporation of unreducible metal oxides: Pivotal role of surface oxygen vacancies

In this paper, we surprisingly found that the incorporation of unreducible metal oxides MxOy (M = Mg, Zn, Ca, Ba, Al) onto CuO hybrid magnetic nano ferric oxide (Cu@Fe3O4) may alter the reaction pathway in persulfate activation, and increase the reaction rate constant. The activation of peroxymonosulfate (PMS) by Cu@Fe3O4 led to a classic sulfate radical based oxidation process (SR-AOP) with an acetaminophen (ACE) degradation rate constant of 0.004 min−1, while 1O2-dominated nonradical oxidation process was disclosed in CuM@Fe3O4 with wildly fluctuated reaction rate constants from 0.003 to 0.242 min−1. Mechanism studies indicated that singlet oxygen (1O2) derived from the direct oxidation of superoxide anions radicals (O2−) or the recombination of O2− was the main reactive oxygen species (ROS) in CuM@Fe3O4/PMS system. A series of characterization experiments (pHpzc tests, XPS, H2-TPR, et al.) and DFT calculation disclosed that the addition of an unreducible metal M yielded many positive effects: (1) the formation of surface oxygen vacancies (OV) raised the zero point charge (pHpzc) of CuM@Fe3O4, thus enhanced the adsorption and activation of PMS; (2) promoting the generation of a new Cu species (Cu3+) on the surface of CuM@Fe3O4, which then participated in the generation of 1O2. The different reducibility of Cu3+ led to differences in the catalytic properties of CuM@Fe3O4. In addition, the effects of various water matrix species and the results of reusability experiment, mineralization experiment, and ecotoxicity test exhibited that CuM@Fe3O4/PMS system possessed excellent practical application value.

Simple strategy for the construction of oxygen vacancies on α-MnO2 catalyst to improve toluene catalytic oxidation

A strategy simple, safe and suitable for large scale production of α-MnO2 catalyst with high activity in VOCs oxidation is crucial for its application. The catalytic reactivity of α-MnO2 catalyst is largely related with its oxygen vacancy. Herein, we report effective construction of oxygen vacancies on α-MnO2 through simply adjusting precipitation temperature of a redox precipitation process. The key role of surface oxygen vacancies in toluene oxidation and the formation of different amount and distribution of the oxygen vacancies over the α-MnO2 catalysts were revealed by characterizations together with DFT calculations. The best catalyst (α-MnO2-60) exhibited significantly improved catalytic activity of α-MnO2 catalyst in toluene oxidation (T90 = 203 ℃) and excellent water resistance. The richest surface oxygen vacancies of α-MnO2-60 contributed to its best catalytic activity, despite of its relatively lower specific surface area. This work may provide a new perspective for the rational design of high efficient VOCs catalysts.

Birnessite as a Highly Efficient Catalyst for Low-Temperature NH3-SCR: The Vital Role of Surface Oxygen Vacancies

Birnessite was synthesized by a simple hydrothermal method and used for low temperature selective catalytic reduction of NO with NH3 (NH3-SCR). Physicochemical characterizations showed that the exp...

Role of Surface Oxygen Vacancies in Intermediate Formation on Mullite-type Oxides upon NO Adsorption

Identifying the nature and reactivity of surface intermediate species is critical to understanding the fundamental reaction pathways of NO oxidation on mullite-type oxide catalysts. Using in situ F...

Synergy between Defects, Photoexcited Electrons, and Supported Single Atom Catalysts for CO2 Reduction

Dispersed atomic catalysts can achieve high catalytic efficiency and have the potential to enable chemical transformation of inert molecules like CO2. The effect of surface defects on photocatalytic reduction of CO2 using supported single atom catalysts however requires clarification. Using density functional theory and experimental techniques, we have investigated the role of surface oxygen vacancies (Ov) and photoexcited electrons on supported single atom Cu catalysts and CO2 reduction. Adsorption of Cu was strong to the TiO2 surface, and charges of the Cu atoms were highly dependent on whether surface defects were present. Cu atoms with Ov aided in the adsorption of activated bent CO2, which is key to CO2 reduction. Our results also show that CO2 dissociation (CO2* → CO* + O*), which is a proposed initial step of CO2 reduction to hydrocarbon products, occurs very readily for a single Cu atom in an Ov, with barriers of ∼0.19 eV. Such low barriers do not occur with Cu over a stoichiometric surface. Furth...

Rational construction of oxygen vacancies onto tungsten trioxide to improve visible light photocatalytic water oxidation reaction

Defect engineering is a promising strategy to enhance light absorption and charge separation of photocatalysts. Herein, we simply tailor the quantity and distribution of oxygen vacancies, as one of typical defects, on surface or bulk of thermal-treated WO3 in the different H2 concentration. The quantity of bulk oxygen vacancies on WO3 consistently rises with the increased H2 concentration, while that of surface oxygen vacancies presents a volcano-type variation. The sample of WO3-H20, thermal-pretreated in 20% H2, contains the largest amount of surface oxygen vacancies. Our results show that both surface and bulk oxygen vacancies on WO3 can promote the visible light photocatalytic activity in water splitting, however, in different ways. Bulk oxygen vacancies mainly promote the visible light harvesting and slightly restrain the electrons and holes recombination by narrowing band gap energy (Eg), while surface oxygen vacancies significantly increase the charge-carriers separation efficiency by lowering valence band edge (VBE). Compared with the light absorption, the separation of electrons and holes is more critical in photocatalytic oxygen evolution over WO3, revealing the more decisive role of surface oxygen vacancies than bulk oxygen vacancies. Expectedly, WO3-H20 shows the highest charge-carriers separation efficiency and visible light photocatalytic performance. Our work provides a new insight into designing of efficient defect-engineered semiconductors for the related solar light utilization processes.

Role of Surface Oxygen Vacancies and Lanthanide Contraction Phenomenon of Ln(OH)3 (Ln = La, Pr, and Nd) in Sulfide-Mediated Photoelectrochemical Water Splitting.

Herein, we report the role of surface oxygen vacancies and lanthanide contraction phenomenon on HS– anion adsorption and desorption in the sulfide-mediated photoelectrochemical water splitting of Ln(OH)3 (Ln = La, Pr, and Nd). The Ln(OH)3 were synthesized via a solvothermal route using ethylenediamine as the solvent. The surface defects are characterized by Raman, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and high-resolution transmission electron microscopy analyses. The photoelectrochemical water-splitting behavior of Ln(OH)3 enriched with surface oxygen vacancies has been examined in a 1 M Na2S solution under illumination conditions. La(OH)3 exhibited a highly stable and saturated current density of ∼26 mA/cm2 at 0.8 V (vs Ag/AgCl). Similarly, the hydroxides of Pr and Nd demonstrated current densities of 18 and 14 mA/cm2, respectively, at 0.8 V (vs Ag/AgCl). A reduction trend in the saturated current densities from La to Nd indicates the lanthanide contraction phenomenon, where the basicity decreases in the same order. The results also demonstrate that the surface adsorption of the HS– anion in the active sites of the surface oxygen vacancies played a vital role in enhancing the photoelectrochemical water-splitting behavior of Ln(OH)3. The stability of Ln(OH)3 was examined after 4 h of chronoamperometry studies at 0.8 V (vs Ag/AgCl) and analyzed using X-ray diffraction, Fourier transform infrared, Raman, and EPR and XPS analyses. The results show that the Ln(OH)3 exhibited excellent stability by demonstrating their phase purity after photoelectrochemical water splitting. We propose Ln(OH)3 as highly stable photoelectrochemical water-splitting catalysts in highly concentrated sulfide-based electrolytes and anticipate Ln(OH)3 systems to be explored in a major scale for the production of H2 as an ecofriendly process.

Oxygen vacancy engineering of Bi2O3/Bi2O2CO3 heterojunctions: Implications of the interfacial charge transfer, NO adsorption and removal

Efficient enrichment of targeted gaseous pollutants and fast diffusion rates of charge carriers are essential for the photocatalytic removal of nitric oxides at ambient concentration levels. Here we demonstrate that the construction of nano-structured Bi2O3/Bi2O2CO3 heterojunctions with oxygen vacancies, increasing the photocatalytic NO removal activity, durability and selectivity for final products nitrate formation. Combining the experimental and density-functional theory calculations, it was elucidated that the presence of surface oxygen vacancies not only work as adsorption sites of low concentration NO, but also offer an intimate and integrated structure between surface defects and the light-harvesting heterojunctions, which can facilitate solar energy conversion and charge carrier transfer (more than 2 times). Control experiments with pristine Bi2O3/Bi2O2CO3 also confirmed the crucial role of surface oxygen vacancies on the improvement of NO adsorption and removal ability during the photocatalytic degradation process. We explain the enhanced removal of NO through the synergistic effect of oxygen vacancy and heterojunction, which not only guaranteed the generation of more OH radicals, but also provided another route to produce hydrogen peroxide. Our findings may provide an opportunity to develop a promising catalyst for air pollution control.

Oxygen vacancies on the surface of HxWO3−y for enhanced charge storage

The dominant role of surface oxygen vacancies (OVs) in improving energy storage is underscored by comparing the OV-mediated charge storage performance of surface and bulk defective HxWO3−y with that of WO3 and bulk defective e-HxWO3−y.

In Operando Self‐Healing of Perovskite Electrocatalysts: A Case Study of SrCoO3 for the Oxygen Evolution Reaction

Perovskites are promising catalysts for oxygen evolution reactions (OER); among them, SrCoO3 is one of the best for these reactions. We study the O* intermediates and the role of surface oxygen vacancies of SrCoO3 during OER. A self-healing mechanism is proposed in which O* are incorporated into the surface to recover the redox capabilities of the material.

Density functional theory study of the NO2-sensing mechanism on a WO3 (0 0 1) surface: the role of surface oxygen vacancies in the formation of NO and NO3

ABSTRACTThe trapping and detection of nitrogen oxide with tungsten trioxide has become a popular research topic in recent years. Knowledge of the complete reaction mechanism for nitrogen oxide adsorption is necessary to improve detector performance. In this work, we used density functional theory (DFT) calculations to study the adsorption characteristics and electron transfer of nitrogen dioxide on an oxygen-deficient monoclinic WO3 (0 0 1) surface. We observed different reactions of NO2 on slabs with different O- and WO-terminated WO3 (0 0 1) surfaces with oxygen vacancies. Our calculations show that the bridging oxygen atom on an oxygen defect on an O-terminated WO3 (0 0 1) surface is the active site where an NO2 molecule is oxidised into nitrate and is adsorbed onto the surface. On a WO-terminated (0 0 1) surface, one of the oxygen atoms from the NO2 molecule fills the oxygen vacancy, and the resulting NO fragment is adsorbed onto a W atom. Both of these adsorption models can cause an increase in the electrical resistance of WO3. We also calculated the adsorption energies of NO2 on slabs with different oxygen-deficient WO3 surfaces.