Presence Of Surface Oxygen Vacancies Research Articles

Significantly enhanced photocatalytic performance of the W-Zr-MOF-NH2@TpTt-COF heterostructure for tetracycline removal by defects engineering

A hollow heterojunction of W-Zr-MOF-NH2@TpTt-COF (WZT) was fabricated by combining etched W-Zr-MOF-NH2 with TpTt-COF. WZT, synthesized through acid etching and hydrothermal methods, showed better tetracycline degradation than pure W-Zr-MOF-NH2 and pure TpTt-COF due to its larger specific surface area and more active sites. The surface of WZT contains abundant oxygen vacancies (OVs) that capture electrons, promoting the separation of photogenerated e--h+ pairs and increasing WZT’s photocatalytic activity. WZT (1:1, mass) showed 87% degradation of tetracycline under 300 W Xenon lamp irradiation. The results of cycling experiments demonstrated the high stability and reusability of WZT (1:1), making it feasible for production. The excellent photocatalytic degradation of tetracycline by WZT (1:1) originates from the formation of heterojunctions and the presence of surface OVs, which reduces the strong photogenerated e--h+ complexation efficiency. Possible photocatalytic degradation mechanisms are proposed while demonstrating the formation of Z-scheme heterojunction in WZT, which aligned with the experimental results of electron spin resonance (ESR). The tetracycline oxidative decomposition process involves the participation of •OH, 1O2, •O2-, e-, and h+ as proven by ESR and radical trapping experiments. This work proposes a research idea to prepare high-performance photocatalysts with MOF@COF type, featuring OVs-rich and Z-scheme heterojunctions.

Unraveling the molecular and growth mechanism of colloidal black In2O3-x.

Black metal oxides with varying concentrations of O-vacancies display enhanced optical and catalytic properties. However, direct solution syntheses of this class of materials have been limited despite being highly advantageous given the different synthetic handles that can be leveraged towards control of the targeted material. Herein, we present an alternate colloidal synthesis of black In2O3-x nanoparticles from the simple reaction between In(acac)3 and oleyl alcohol. Growth studies by PXRD, TEM, and STEM-EDS coupled to mechanistic insights from 1H, 13C NMR revealed the particles form via two paths, one of which involves In0. We also show that variations in the synthesis atmosphere, ligand environment, and indium precursor can inhibit formation of the black In2O3-x. The optical spectrum for the black nanoparticles displayed a significant redshift when compared to pristine In2O3, consistent with the presence of O-vacancies. Raman spectra and surface analysis also supported the presence of surface oxygen vacancies in the as-synthesized black In2O3-x.

Electro-Oxidation Reaction of Methanol over La2-xSrxNi1-y(Mn/Fe/Co)yO4+δ Ruddlesden-Popper Oxides.

Solution combustion-synthesized Ruddlesden-Popper oxides La1.4Sr0.6Ni0.9(Mn/Fe/Co)0.1O4+δ were explored for the methanol electro-oxidation reaction. With optimal doping of Sr2+ in the A site and Co2+ in the B site, Ni3+ with t2g6 configuration in La1.4Sr0.6Ni0.9Co0.1O4+δ exhibited a tetragonal distortion with compression in axial bonds and elongation in equatorial bonds. This structural modification fostered an augmented overlap of orbitals with axial O 2p orbitals, leading to a heightened density of states at the Fermi level. Consequently, this facilitated not only elevated electrical conductivity but also a noteworthy reduction in the charge transfer resistance. These effects collectively contributed to the exceptional methanol oxidation activity of La1.4Sr0.6Ni0.9Co0.1O4+δ, as evidenced by an impressive current density of 21.4 mA cm-2 and retention of 95% of initial current density even after 10 h of prolonged reaction. The presence of Ni3+ further played a pivotal role in the creation of NiOOH, a crucial intermediate species, facilitated by the presence of surface oxygen vacancies. These factors synergistically enabled efficient methanol oxidation. In summary, our present study not only yields substantial insights but also paves the way for a novel avenue to fine-tune the activity of Ruddlesden-Popper oxides for the successful electro-oxidation of methanol.

Advancement of visible light-driven photocatalytic degradation of dimethyl sulfide by Zn doped rGO/TiO2

In this study, visible light-driven photocatalytic degradation of dimethyl sulfide (DMS) by nanocomposite photocatalyst was achieved through doping TiO2 with reduced graphene oxide (rGO) and various contents of Zn via solvothermal method. Doping of Zn led to formation of oxygen defects and presence of Ti3+ which further promoted photocatalytic activity. After modification, formation of lower level band gap between VB and CB reduced required energy for electron excitation and accelerated light energy utilization. Besides, the presence of surface oxygen vacancies enhanced photocatalytic activity via adsorption of exoteric oxygen. The best DMS conversion (almost 50 %) was performed by 0.1rGO/Zn1TiO2 under typical indoor environment at 60 % RH. Declining of relative humidity to below 10 % improved photocatalytic degradation of DMS due to lower competitive adsorption of water vapor. The best fitting by Langmuir-Hinshelwood kinetics model 4 also supported the occurrence of competitive adsorption with water vapor. Moreover, detection of generated intermediates was used in development of proposed reaction mechanism. Thus, the discussion on DMS photocatalytic degradation over Zn/rGO/TiO2 could be comprehensively conducted from the preparation, characterization, stability test, proposed reaction mechanism, and kinetic study.

Water-repelling behavior of the 1-D hematite nano-network.

Hematite is an attractive material used as electron transport layer in perovskite-based solar cells. Being hydrophilic in nature it attracts moisture, which can be damaging for perovskite layers. Therefore, it is important to make hematite moisture repelling, which can be beneficial for applications in solar cells or for protecting iron surfaces from further rust formation. In this work, we demonstrate that the systematic irradiation of nanostructured hematite with low-energy argon ions (Ar+) at various ion fluences can change the surface wettability as well as promote the formation of junctions between nanorods. The nano-welded network of the irradiated hematite turns out to be hydrophobic. Using TRI3DYN simulations, ion-induced surface roughening, the presence of surface oxygen vacancies, and joining between adjacent nanorods are predicted. Furthermore, the water-repelling behavior of the irradiated nano-network is evaluated using density functional theory (DFT) simulations by determining the interaction of water molecules with the surface. The interconnected hematite nano-network also shows a notable improvement in electrical conductivity.

Interaction of Hydrogen with Cu-Modified Cerium Oxide Surfaces

We investigate the interaction between molecular hydrogen and ultrathin epitaxial CeO2 films modified with a 2% concentration of Cu atoms using X-ray photoemission spectroscopy (XPS) during thermal reduction cycles in H2. The XPS measurements are combined with density functional theory calculations to obtain further insight into the observed modifications of the film surface. Our results show that the presence of Cu atoms decreases the barrier for H2 dissociation in comparison to that on pure ceria surfaces, leading to the formation of surface OH groups after exposure to H2. Moreover, surface oxygen vacancies are generated already at mild temperatures (470 K), most likely via water formation and desorption. The presence of surface oxygen vacancies and hydroxyls contributes to the observed large increase in surface Ce3+ concentration with increasing reduction temperature. In spite of these atomic scale modifications, the surface morphology observed by scanning tunneling microscopy remains substantially unchanged on the length scale of tens of nm.

Acetone and Toluene Gas Sensing by WO3: Focusing on the Selectivity from First Principle Calculations.

Sensitivity and selectivity are the two major parameters that should be optimized in chemiresistive devices with boosted performances towards Volatile Organic Compounds (VOCs). Notwithstanding a plethora of metal oxides/VOCs combinations that have been investigated so far, a close inspection based on theoretical models to provide guidelines to enhance sensors features has been scarcely explored. In this work, we measured experimentally the sensor response of a WO3 chemiresistor towards gaseous acetone and toluene, observing a two orders of magnitude higher signal for the former. In order to gain insight on the observed selectivity, Density Functional Theory was then adopted to elucidate how acetone and toluene molecules adsorption may perturb the electronic structure of WO3 due to electrostatic interactions with the surface and hybridization with its electronic structure. The results of acetone adsorption suggest the activation of the carbonyl group for reactions, while an overall lower charge redistribution on the surface and the molecule was observed for toluene. This, combined with acetone’s higher binding energy, justifies the difference in the final responses. Notably, the presence of surface oxygen vacancies, characterizing the nanostructure of the oxide, influences the sensing performances.

Pyrochlores for Advanced Oxygen Electrocatalysis.

Fuel cells (FCs), water electrolyzers (WEs), unitized regenerative fuel cells (URFCs), and metal-air batteries (MABs) are among the emerging electrochemical technologies for energy storage, fuel (H2), oxidant (O2), and clean energy production. Their commercial applications are hindered by the low oxygen reduction reaction/oxygen evolution reaction (ORR/OER) bifunctional activity (for URFCs and MABs), OER selectivity (brine electrolysis in seawater and Martian environments), and high cost of the benchmark electrocatalysts (OER: RuO2, IrO2 and ORR: Pt/C) which affects the performance and affordability of the devices. Low-cost electrocatalysts with highly symmetric ORR/OER bifunctional activity and high OER selectivity are crucial for large-scale FC, WE, URFC, and MAB application. Recent studies have revealed that tuning the structure of pyrochlore oxides provides a pathway to enhancing OER and ORR activity over a wide range of pH. Pyrochlore oxides commonly contain a cubic A2B2O7-x structure with two types of tetrahedrally coordinated O atoms containing (1) A-O-A and (2) A-O-B types with a cationic radii mismatch of rA/rB > 1.5 and propensity toward oxygen vacancy formation. The variety of pyrochlore oxides and their tunable properties make them attractive for a wide spectrum of applications. Among all the metal oxides, Ru-based pyrochlores (e.g., Pb2Ru2O7-x) exhibit the best bifunctional oxygen electrocatalytic activity, i.e., low bifunctionality index (BI), in alkaline medium. Furthermore, pyrochlores exhibit high OER selectivity in brine electrolytes due to the presence of surface oxygen vacancies, making them suitable for space applications (brine electrolysis on Mars) and coastal hydrogen generation. Their bifunctional activity and selectivity can be further amplified by (1) substituting "A" and "B" sites of pyrochlores (AA'BB'O7-x), (2) tuning metal oxidation states of A and B by varying synthesis conditions, and (3) modulating oxygen vacancy concentration, each of which yield favorable structural and electronic variations. In recent years, research on the synthesis and understanding of pyrochlores has significantly enhanced their viability, offering a new horizon in the quest for economical and active electrocatalysts. However, an account that focuses on critical developments in this field is still lacking.In this Account, we focus on the recent development of a variety of pyrochlore electrocatalysts to understand intrinsic structure-activity-selectivity-stability relationships in these materials. Recent developments and applications of pyrochlore-based electrocatalysts are discussed under the following headings: (1) modulation of crystal and electronic structure of pyrochlores, (2) structure-activity-stability relationships of different pyrochlores for OER and ORR, (3) development of OER-selective pyrochlores for brine electrolysis, and (4) the application of pyrochlores in electrochemical devices. Finally, we highlight some unaddressed issues such as the precise identification of active sites, which can be addressed in the future through advanced in situ and ex situ characterization techniques coupled with the density functional theory-based analyses. This Account provides foundational understanding to guide the comprehensive development of highly active, selective, stable and low-cost structurally engineered pyrochlores for high performance electrochemical devices.

Resolving the Effect of Oxygen Vacancies on Co Nanostructures Using Soft XAS/X-PEEM.

Improving both the extent of metallic Co nanoparticle (Co NP) formation and their stability is necessary to ensure good catalytic performance, particularly for Fischer–Tropsch synthesis (FTS). Here, we observe how the presence of surface oxygen vacancies (Ovac) on TiO2 can readily reduce individual Co3O4 NPs directly into CoO/Co0 in the freshly prepared sample by using a combination of X-ray photoemission electron microscopy (X-PEEM) coupled with soft X-ray absorption spectroscopy. The Ovac are particularly good at reducing the edge of the NPs as opposed to their center, leading to smaller particles being more reduced than larger ones. We then show how further reduction (and Ovac consumption) is achieved during heating in H2/syngas (H2 + CO) and reveal that Ovac also prevents total reoxidation of Co NPs in syngas, particularly the smallest (∼8 nm) particles, thus maintaining the presence of metallic Co, potentially improving catalyst performance.

Computational Study of Zn Single-Atom Catalysts on In2O3 Nanomaterials for Direct Synthesis of Acetic Acid from CH4 and CO2

The direct conversion of methane and carbon dioxide will be a promising way to alleviate the greenhouse gas effect and reduce the dependency on traditional fossil fuels. In this study, the catalytic transformation of methane and carbon dioxide to acetic acid with a 100% atomic economy effect over the zinc single-atom catalyst supported on In2O3 nanomaterial with surface oxygen vacancy (Zn1/In2O3–x) was investigated by using density functional theory calculations, including dissociative adsorption of methane, C–C bond coupling, acetate species protonation, and desorption of acetic acid. The results indicate that the Zn single atom is beneficial to the dissociation of methane and the stabilization of methyl by forming the Zn–CH3 species. Besides, carbon dioxide can be adsorbed on the surface of the catalyst and activated further owing to the presence of surface oxygen vacancy. The facile C–C coupling of co-adsorbed methyl and carbon dioxide is realized by the synergistic mode combining the Zn single atom and surface oxygen vacancy following the Langmuir–Hinshelwood mechanism, in which a Zn–C–C moiety is formed in the transition state. The subsequently formed acetate species is bonded with Zn strongly and undergoes a protonation process to form acetic acid which also interacts strongly with the Zn single atom, resulting in desorption of acetic acid being the rate-determining step of the whole reaction. The insights in this work would be useful to clarify the reaction mechanism of producing acetic acid from methane and carbon dioxide over the Zn1/In2O3–x single-atom catalyst, which could provide a sufficient way to design efficient catalysts for methane and carbon dioxide conversion.

Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces.

Utilizing CO2 as a sustainable carbon source to form valuable products requires activating it by active sites on catalyst surfaces. These active sites are usually in or below the nanometer scale. Some metals and metal oxides can catalyze the CO2 transformation reactions. On metal oxide-based catalysts, CO2 transformations are promoted significantly in the presence of surface oxygen vacancies or surface defect sites. Electrons transferable to the neutral CO2 molecule can be enriched on oxygen vacancies, which can also act as CO2 adsorption sites. CO2 activation is also possible without necessarily transferring electrons by tailoring catalytic sites that promote interactions at an appropriate energy level alignment of the catalyst and CO2 molecule. This review discusses CO2 activation on various catalysts, particularly the impacts of various structural factors, such as oxygen vacancies, on CO2 activation.

On the ensemble requirement of fully selective chemical looping methane partial oxidation over La-Fe-based perovskites

The dynamic nature of active sites on La-Fe-based perovskites in chemical looping methane oxidation has been studied. According to experimental observations, the reduction of the oxygen carrier is divided into three stages occurring at different active ensembles where the Fe cations are in different coordination environments. The Mars van Krevelen mechanism formulated by microkinetic analysis describes well the effect of oxygen vacancy on the catalytic performance of LaFeO3. CO2 is not produced by CO oxidation but rather is a primary product of methane oxidation, and the presence of surface oxygen vacancies would dramatically increase the overall energy barrier for total combustion, thus decreasing the selectivity toward CO2. Hence, the Fe coordination environment (and hence the oxygen vacancy concentration) is the key parameter governing the catalyst selectivity, in the sense that methane oxidation can vary from total combustion on the O-rich surface to fully selective partial oxidation on the O-deficient surface.

Enhancing the Catalytic Activity of Lanthanum-Ceria Fluorite for Methane Conversion in SOFC

A2B2O7 compounds have unique properties, such as good thermal stability, oxygen mobility, ionic conductivity, etc. Such compounds with tailored compositions exhibit good catalytic properties in a variety of high-temperature reactions, such as reform and oxidative coupling of methane (OCM). An effective catalyst for OCM must have electrophilic oxygen mobility and alkalinity on its surface. A typical A2B2O7 compound that contains a trivalent rare earth metal at site A, such as La, can provide the necessary surface alkalinity for a good OCM catalyst. Previous studies show that La2Ce2O7 (LCO) catalysts perform well in OCM due to the selective mobile oxygen sites, suitable alkaline sites, and high thermal stability. Doping LCO with Ca increases the alkalinity, which can considerably increase selectivity for ethylene in the OCM reaction. We have studied calcium-doped lanthanum ceria materials as a catalytic layer in solid oxide fuel cells reactors for methane conversion to ethylene. We have explored the combustion synthesis to obtain homogeneous La2-xCaxCe2O7 powders with x = 0, 0.25, and 0.50 (LCO, LCa25, and LCa50, respectively). X-ray diffraction (XRD) showed that the calcium addition in LCO formed a single-phase solid solution. The crystalline structure of the synthesized materials is the disordered fluorite-type, as indicated by both XRD and Raman analysis. Raman spectroscopy data evidenced the presence of surface oxygen vacancies on all materials, which may benefit the OCM reaction. SEM images of as-prepared powders obtained evidenced a similar microstructure composed of porous sponge-like agglomerated particles with irregular shape, expected from the combustion synthesis (Fig. 1). Ca-doping changed the sintering behavior by reducing the onset of shrinkage during the sintering process. Impedance spectroscopy data showed increased ionic conductivity with increasing Ca-doping. Such an increase is more evident at high measuring temperature (800 oC) and indicates enhanced oxygen mobility that are expected to contribute to OCM reaction.Figure 1: Scanning electron microscope micrographs of La2-xCaxCe2O7 (x = 0, 0.25, and 0.50) as-prepared powders obtained by combustion method. Figure 1

Unveiling the acetone sensing mechanism by [formula omitted] chemiresistors through a joint theory-experiment approach

The acetone sensing mechanism by WO3 was investigated through a combined experimental and theoretical approach. The γ−monoclinic WO3 powder was synthesized by a template-mediated sol-gel method and characterized on structural, surface, morphological and optical points of view. A thin film of WO3 was deposited on interdigitated Au electrodes by hot-spray method and tested at 300 ∘C (while applying a bias of 1.0 V) for acetone gas sensing, both in presence and absence of oxygen in the gas carrier. Interestingly, the absence of oxygen had no significant effect on the sensor response intensity but it dramatically increased the recovery times (from 120 s to 2700 s). In order to explain these experimental results, by means of ab initio density functional theory calculations, we modeled a defective γ−WO3 surface structure and simulated the adsorption of acetone and oxygen molecules on top of it. We unprecedentedly evidenced that, in presence of surface oxygen vacancies, both acetone adsorption and its oxidation reaction can occur. However, their contribution to the sensor response strictly depends on the inert/oxidative atmosphere present in the sensing chamber, which in turn strongly affects the surface oxygen population. Our findings can either be the guidelines for future studies aimed at delineating the possible reaction products or pave the way for the engineering of tailored nanomaterials having specific surface features and enhanced sensing properties.

Thermodynamic and Kinetic Influence of Oxygen Vacancies on the Solar Water Oxidation Reaction of α-Fe2O3 Photoanodes.

To reveal the role of oxygen vacancies in the solar water oxidation of α-Fe2O3 photoanodes, the kinetic and thermodynamic properties that are closely related to the water oxidation reaction of the α-Fe2O3 photoanode containing oxygen vacancies were investigated. Compared with the pristine α-Fe2O3 photoanode, the presence of surface oxygen vacancies can improve the water oxidation activity and stability of the α-Fe2O3 photoanode simultaneously, but the bulk oxygen vacancies have a negative effect on the water oxidation performance of the α-Fe2O3 photoanode. In thermodynamics, our investigations revealed that the presence of surface oxygen vacancies narrows the space charge region width of the α-Fe2O3 photoanode, which could boost the charge separation and transfer efficiency of the α-Fe2O3 photoanode during water oxidation. Because the surface property and hydrophilicity of α-Fe2O3 are modified owing to the presence of surface oxygen vacancies, the water oxidation kinetics of the α-Fe2O3 photoanode with surface oxygen vacancies is obviously boosted. Our findings in the present work provide comprehensive understanding of the thermodynamic and kinetic differences for α-Fe2O3 photoanodes with and without oxygen vacancies for solar water oxidation.

Construction of 2D/2D TiO2/g-C3N4 nanosheet heterostructures with improved photocatalytic activity

Constructing two-dimensional (2D) heterostructure is an effective method to obtain highly efficient photocatalysts. In this work, 2D/2D TiO2/g-C3N4 heterostructures were constructed via in-situ growth of TiO2 nanosheets on the surface of g-C3N4 nanosheets by a facile hydrothermal co-assembly technique. The increased dispersion is favorable to expose more vacancies, which is beneficial to the enhancement of visible light absorption. This unique 2D/2D heterostructure exhibits ultrathin thickness and larger interfacial contact area, which results in shorter migration distance of photo-generated carriers to the surface and faster participation in photocatalytic water splitting. As expected, the optimized TiO2/g-C3N4 nanocomposites showed a higher photocatalytic hydrogen evolution performance under visible light irradiation than that of pristine g-C3N4 and TiO2 nanosheets. The improved performance can be attributed to the ultrathin nanosheet structure, the intimate interfacial contact and the presence of surface oxygen vacancies. More appealingly, this environment-friendly 2D/2D TiO2/g-C3N4 nanohybrid would attract more attention in water splitting.

Surface-oxygen vacancy defect-promoted electron-hole separation of defective tungsten trioxide ultrathin nanosheets and their enhanced solar-driven photocatalytic performance

Defective WO3 ultrathin surface-engineered nanosheets are fabricated by a solvothermal and low-temperature surface hydrogenation reduction strategy. The obtained defective WO3 ultrathin nanosheets with thicknesses of ∼4 nm possess a relatively large surface area of ∼25 m2 g−1. After surface engineering, the bandgap is narrowed to ∼2.48 eV due to the presence of surface oxygen vacancies, which further enhance the visible light absorption. The defective WO3 ultrathin nanosheets exhibit excellent solar-driven photocatalytic degradation performance for the complete degradation of the highly-toxic metribuzin herbicide (∼100%). The first-order rate constant (k) of the defective WO3 ultrathin nanosheets is ∼3 times higher than that of the pristine one. This can be ascribed to the formation of suitable surface-oxygen vacancy defects that promote the separation of photogenerated electron-hole pairs, and the two-dimensional ultrathin structure facilitating the surface engineering as well as furnishing a large number of surface active sites. Moreover, the defective WO3 ultrathin nanosheets exhibit high stability because the photocatalytic activity remains almost unchanged after 10 cycles, making them favorable for practical applications. This work offers new insights into the fabrication of other high-performance ultrathin nanosheet oxide photocatalysts for environmental applications.

Role of Molecular Structure on Controlling the Dynamic Equilibrium of the Ecosystem Present in Shallow Trench Isolation (STI) Chemical Mechanical Planarization (CMP) Slurry Formulations

Chemical Mechanical Planarization (CMP) has emerged as a critical process step for achieving angstrom-level uniformity in advanced integrated circuit manufacturing and has ultimately lead to the extension of Moore’s Law. This work specifically looks at Shallow Trench Isolation (STI) CMP, which is used in the electrical isolation of the active components in an integrated circuit by actively removing oxide overburden and stopping on Si3N4 to achieve global planarity and limit defectivity. This planarity is achieved through the synergistic balance of an applied mechanical force and the incorporation of a colloidal dispersion (slurry) containing ceria (CeO2) nanoparticles, rheology modifiers, and stability additives. In addition to the aforementioned additives, a main initiative of STI slurry formulation development has been centered around oxide rate control and selectivity enhancement. Current research has emphasized the addition of compounds that contain carboxylic acid, amine, and amino acid functionality. While the mechanism behind these additives is still of great debate, results have shown the importance of additive surface adsorption and nanoparticle surface redox properties. More specifically, in order to achieve optimal performance, the slurry additive must not impede the presence of surface oxygen vacancies on the nanoparticle, which are known to facilitate high oxide material removal rate (MRR). Slurry additives with redox activity as well as antioxidant properties show the highest MRR due to an increase in the surface oxygen vacancies and a decrease in the non-productive surface interactions. A suite of dynamic, analytical techniques such as atomic force microscopy (AFM), contact angle measurements, pre/post characterization of particle properties (size and zeta potential), coefficient of friction (COF), and UV-Vis spectroscopy, were employed. Results indicate a strong correlation between additive structure, slurry macro-environment, and overall CMP performance.

Effects of H2O2 generation over visible light-responsive Bi/Bi2O2−xCO3 nanosheets on their photocatalytic NOx removal performance

The photocatalytic removal of gaseous NOx is commonly accompanied by secondary pollution, which necessitates the development of highly efficient nanostructured catalysts with a decreased propensity to toxic intermediate production. Herein, we describe the synthesis of plasmonic Bi/Bi2O2−xCO3 and demonstrate the presence of surface oxygen vacancies therein, revealing that the maximal NOx removal efficiency of Bi/Bi2O2−xCO3 under visible light irradiation reached 50.5% and exceeded that of a commercial photocatalyst, while the production of toxic NO2 as a by-product was completely suppressed (the selectivity reached up to 98%). In-situ introduction of plasmonic Bi on the surface of Bi2O2−xCO3 promoted the generation of H2O2 by capturing electrons from the defect states of Bi2O2−xCO3 via the two-electron reduction of O2 and thus inhibited NO2 production (as confirmed by scavenger experiments), additionally broadening the light absorption range of the above photocatalyst. Moreover, surface oxygen vacancies in Bi–O layers provided a channel for electron transfer between Bi and Bi2O2−xCO3, which resulted in increased charge separation efficiency (maximum photocurrent = 1.1 μA cm−2, 14.5 times higher than that of pristine Bi2O2CO3). Furthermore, the toxicity assessment authenticated good biocompatibility of Bi/Bi2O2−xCO3. Thus, this study sheds light on the possible roles of H2O2 in NOx degradation and provides an efficient surface engineering strategy to prepare highly reactive and selective photocatalysts.

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.