Effect Of Surface Oxygen Vacancies Research Articles

Synergistic effect of surface oxygen vacancies and hydroxyl groups on Cu-doped TiO2 photocatalyst for hydrogen evolution

The intricate nature of the titanium dioxide (TiO2) photocatalytic system, influenced by Cu doping, often yields conflicting outcomes. This study addresses this complexity by examining the impact of Cu ion doping in nanostructured TiO2 to augment its photocatalytic efficacy in hydrogen generation from ethanol-water mixtures. The improved performance is primarily attributed to the generation of oxygen vacancies (Ovac) within the TiO2 lattice. By adjusting the thermal processing parameters, the chemical state of Cu is modulated to optimize photocatalytic performance. Solid-state nuclear magnetic resonance (SSNMR) and density functional theory (DFT) calculations confirm that the presence of Ovac and hydroxyl groups (OH−) surface species not only facilitates local spatial charge separation but also enhances light absorption, thereby improving photocatalytic hydrogen evolution reaction (HER) rate up to 20.2 mmol h−1 g−1 under UV light. Furthermore, a proposed mechanism involves the participation of Ovac and OH− species, supported by Mott-Schottky (M − S) plots and ultraviolet photoelectron spectroscopy (UPS), revealing that Cu–TiO2 exhibits a lower flat band potential (Efb) and smaller work function compared to TiO2. This research underscores the pivotal role of Cu ions and Ovac:OH− in advancing the photocatalytic activity of TiO2-based crystals, imparting noteworthy implications for sustainable hydrogen generation.

CO2 hydrogenation to linear α-olefins on FeCx/ZnO catalysts: Effects of surface oxygen vacancies

Selective formation of hydrocarbons from CO2 hydrogenation via the FTS pathway has been attracting interest. Oxygen vacancies (Ov) on the catalyst surface show a subtle role during CO2 activation. Herein, the effects of Ov on FeCx/ZnO catalysts are presented for CO2 hydrogenation to linear α-olefins (LAOs). ZnO with surface Ov of varied concentrations was firstly obtained using heated H2-treatment and used to support active FeCx phases. FeCx/ZnO with appropriate surface Ov achieved CO2 conv. of 48.86% with LAOs sel. of 49.38% and C1 product below 10%. Characterizations confirmed that the Ov concentration exhibited a positive correlation with the interaction between the iron species and ZnO. With H2-treatment of ZnO at 500–600 °C, FeCx/ZnO-T showed 26–33% Ov in all the oxygen species, which increased the surface C/H ratio and promoted the formation of the key intermediate HCOO− and CHx species for chain growth. Excessive Ov accelerated secondary hydrogenation and increased CH4.

Selective Hydrogenation of Propyne to Propene Promoted by Synergistic Effect of Surface Oxygen Vacancies and Hydride Species on Ceria

Selective Hydrogenation of Propyne to Propene Promoted by Synergistic Effect of Surface Oxygen Vacancies and Hydride Species on Ceria

Coupling of surface oxygen vacancies and structural engineering on ZnIn2S4/Bi2MoO6 for efficient norfloxacin degradation

The synergistic effects of surface oxygen vacancies (OVS) and ultrathin 2D structures offer promising prospects for improving photoelectric conversion efficiency by enhancing the transport efficiency of photogenerated charge carriers. Herein, we designed a unique honeycomb structure composite of Bi2MoO6-OVS/ZnIn2S4, contains both ultrathin 2D ZnIn2S4 nanosheets and a rich content of OVS. This composite exhibited excellent efficiency in the photocatalytic degradation of norfloxacin (NFX). The ultrathin ZnIn2S4 nanosheets as "electron collectors", trapping electrons and accelerating carrier transport. Density functional theory calculations confirmed the significant utilization efficiency of photoexcited charge carriers using 20%-BM@ZIS (Bi2MOO6-OVS proportion: 20%) by introducing OVS. In the NFX degradation experiment, 20%-BM@ZIS achieved a 97.1% conversion rate of NFX, which was 4.201, 2.092, and 3.375 times higher than the pristine Bi2MoO6, Bi2MoO6-OVS, and ZnIn2S4, respectively. The combination of OVS and ultrathin 2D materials is expected to shed new light upon the eco-friendly and rational design of efficient materials for converting solar light into chemical energy and also providing an effective solution for the efficient degradation of norfloxacin.

Water adsorption on rutile titanium dioxide (110): Theoretical study of the effect of surface oxygen vacancies and water flux in the steady state case

The aim purpose of the present work is highlighting the impact of surface oxygen vacancies and H2O flux on the behavior of water adsorption at the rutile titanium dioxide (110). Therefore, a theoretical model, based on molecular and dissociation mechanisms at different surface atomic sites, was formulated in a system of partial differential coupled equations. The proposed model used to study, in an atomic scale, this complex phenomenon of adsorption governed by several factors including surface vacancies defects and water flux. The findings of the solution of the system of equations in the steady state case, presented in this paper, strongly indicated that the rate coverage of surface oxygen vacancies has an important role in the dissociation of H2O as well as the flux which is a key factor in the behavior of water adsorption on the TiO2 (110) and the rate coverage of OH groups.

Impact of oxygen vacancy on piezo-photocatalytic catalytic activity of barium titanate

Piezo-photocatalysis driven by available vibration energy and light irradiation is an attractive strategy to address the environmental issues and energy crisis, which highly relies on the design of high-performance piezoelectric-based catalysts. In this work, photo-responsive piezoelectric barium titanate (BaTiO3) nanoparticles with surface oxygen vacancies have been successfully synthesized and used as the prototype to explore the underlying effects of surface oxygen vacancies on the photocatalysis and piezoelectric catalysis. It is found that the surface oxygen vacancies can efficiently promote the light absorption and charge separation, which, however, slightly impair piezoelectric property, fractionally reducing the piezoelectric catalytic activity. As a result, the surface oxygen vacancies play a double-edged sword role in mediating the photocatalytic and piezoelectric activities. Specifically, the surface oxygen vacancies prompt a 3.5 times enhancement of photocatalytic activity, and cause a 1.5 times reduction of piezoelectric catalytic performance. More importantly, the surface oxygen vacancies display the positive effect on the coupled piezo-photocatalytic catalytic performance, which originates from the balance of two competing effects of surface oxygen vacancies on the photocatalysis and piezoelectric catalysis. This research reveals the influence and mechanism of surface oxygen vacancies on the piezo-photocatalytic activity of BaTiO3.

Enhanced photocatalytic NO removal with the superior selectivity for NO2−/NO3− species of Bi12GeO20-based composites via a ball-milling treatment: Synergetic effect of surface oxygen vacancies and n-p heterojunctions

Bi12GeO20-based composites Bi12GeO20-Bi2S3 (BGS) were successfully constructed through a facile ball-milling method using sulfur powder for the first time. Systematical analyses verified the in-situ generation of n-p heterojunctions with surface oxygen vacancies (OVs). These composites showed reinforced photocatalytic removal of NO at ppb level under visible light with high selectivity for NO2−/NO3− species, avoiding the generation of toxic NO2 as far as possible. Especially, the best candidate BGS0.1 possessed 46% NO removal with 96% selectivity for NO2−/NO3− species that were much higher than those by Bi12GeO20, mainly relevant to the enhanced visible-light absorption, synergistic effect of heterojunctions containing surface OVs to promote charge carriers segregation and reactive radicals formation, and suitable phase composition with proper band structures. The effect of heterojunctions with surface OVs over band structures and reaction paths was demonstrated by density functional theory (DFT) calculation. DRIFTS and FT-IR spectra were recorded to deduce NO conversion routes. Eventually, a preliminary photocatalysis mechanism of these robust composites was conjectured in a Z-scheme manner basing experimental and analytical results. This study might pave roads for in-situ construction of sillenite-based composites with surface OVs by a mechanochemical approach with satisfactory photocatalytic NO treatment under visible light.

Highly efficient photoelectrocatalytic degradation of cefotaxime sodium on the MoSe2/TiO2 nanotubes photoanode with abundant oxygen vacancies

Novel multifunctional photoanode on basis of MoSe2 nanoparticles modified TiO2 nanotubes(denoted as MoSe2/TiO2 NTs) were successfully prepared by facile electrochemical methods namely anodization followed by cyclic voltammetry method using titanium foil as substrate. Compared to TiO2 NTs, the MoSe2/TiO2 NTs display excellent photoeletric properties, including more abundant surface oxygen vacancies, enhanced visible-light absorption, improved photocurrent response, reduced fluorescence intensity and decreased charge transfer resistance. These enhancements should be attributed to efficient spatial separation and rapid transfer of photo-induced electrons and holes on the intimate contact interface between MoSe2 nanoparticles and TiO2 NTs. In the photo-electric synergistic catalytic system, the as-prepared MoSe2/TiO2 NTs play double roles as both photoanode and catalyst. Under simulated solar light illumination, the MoSe2/TiO2 NTs photoanode shows superior photoelectrocatalytic activity in the degradation of cefotaxime sodium at low applied bias voltage. Moreover, a possible mechanism of the transfer pathway of photo-induced charge carriers on the MoSe2/TiO2 NTs photoanode is proposed based on the synergistic effect of surface oxygen vacancies and direct Z-scheme heterojunction.

Modulating oxygen vacancies on bismuth-molybdate hierarchical hollow microspheres for photocatalytic selective alcohol oxidation with hydrogen peroxide production

Photocatalytic selective oxidation of alcohols into high value-added carbonyl compounds accompanied by producing hydrogen peroxide (H2O2) is undoubtedly a more efficient solar energy conversion strategy with high atom economy. Herein, we have developed an efficient photocatalyst of bismuth-molybdate (Bi2MoO6) hierarchical hollow microspheres with tunable surface oxygen vacancies (OVs) for promoting the photocatalytic selective alcohol oxidation with H2O2 production. The effect of surface OVs on the photocatalytic efficiency is studied systematically by comparing the performance of different photocatalysts. The benzaldehyde and H2O2 production rates over the OV-rich Bi2MoO6 photocatalyst reach up to 1310 and 67.2 μmol g−1 h−1, respectively, which are 2.3 and 4.0 times those generated from the OV-poor Bi2MoO6 hollow microspheres. The roles of various active radicals in the photocatalytic reaction are probed by a series of controlled experiments and in situ ESR measurements, revealing that both superoxide radical (•O2-) and carbon-centered radical are the key active intermediates. The introduction of surface OVs on Bi2MoO6 hollow microspheres accelerates the separation and transfer of photo-generated charge carriers as well as enhances the adsorption and activation of reactant molecules, thereby greatly promoting the photocatalytic selective oxidation of alcohols along with H2O2 production. This work not only demonstrates a facile strategy for the preparation of high-efficiency photocatalysts by simultaneous modulations of morphology and surface defects, but also offers insight into developing the dual-functional photocatalytic reactions for the full utilizations of photoinduced electrons and holes.

Decoupling the effect of Ni particle size and surface oxygen deficiencies in CO2 methanation over ceria supported Ni

The study describes decoupling the effect of surface oxygen vacancies and Ni particle size in the CO2 methanation over CeO2 supported Ni nanoparticles. The availability of surface oxygen vacancies with the interaction between Ni and CeO2 served to appreciably enhance the catalytic capacity for activation of CO2, generating higher conversion rates, larger selectivity to CH4 and lower activation energies relative to Ni/SiO2. A structure-sensitivity in the reaction over Ni/CeO2 was established without the interference of different concentrations of surface oxygen vacancies, where the turnover frequencies of CO2 decreased with increasing the Ni nanoparticle sizes (8−21 nm). The TPSR and Operando FT-IR analysis demonstrated that formate was the critical intermediate for CH4 formation. The CO formation with smaller nickel size resulted from the carboxyl species. In addition, the long-term evaluations and TGA measurements revealed small Ni nanoparticles suffered a temporary loss of activity due to the carbon deposition.

Formaldehyde Oxidation on Pd/TiO2 Catalysts at Room Temperature: The Effects of Surface Oxygen Vacancies

High reduction temperature generally induces the agglomeration of supported noble metals. Howerve, we found that high temperature reduction did not induce Pd particles sintering but improved Pd dispersion. Multiple methods were further carried out to illuminate the abnormal phenomenon. The results indicated more surface oxygen defects and diffusion of Pd particles were simultaneously induced by high temperature reduction. During diffusion process of Pd particles, they were trapped by the oxygen defects because of the strong metal-support interaction, which led to improvement of Pd dispersion on the Pd/TiO2-450R catalyst. In addition, more surface oxygen vacancies on the Pd/TiO2-450R catalyst resulted in more active sites of H2O activation to form abundant surface OH groups which further enhanced adsorbed O2 activation and mobility, and then opening a more effective pathway for HCHO oxidation, which result in its high activity of Pd/TiO2-450R for ambient HCHO oxidation.

A theoretical study on CO2 electrolysis through synergistic manipulation of Ni/Mn doping and oxygen vacancies in La(Sr)FeO3

To improve the catalytic activity of La(Sr)FeO3 based perovskites (LSF) for CO2 reduction in solid oxide electrolysis cell (SOEC), CO2 adsorption and reduction reaction mechanism were investigated on 12 surface models describing the effects of surface oxygen vacancies and Ni/Mn doping (25% and 50% surface cation doping ratios). In particular, a phase diagram was established to find the most stable LSF structure under SOEC operating conditions. These were carried out using Density Functional Theory (DFT) + U calculations. A microkinetic model was then developed to simulate polarization curves and compared with the experimental data of pure LSF. Ni-Mn double doping with 2 surface oxygen vacancies of LSF was identified as the most effective electrocatalysts. This is attributed to fine tuning O affinity by Ni, Mn and Fe in B-site (catalytic active site), as indicated by the Bader charge analysis. Experimental studies for this material have yet to be reported in the literature.

Synergistic effect of surface oxygen vacancies and interfacial charge transfer on Fe(III)/Bi2MoO6 for efficient photocatalysis

Novel Fe(III) clusters grafted Bi2MoO6 nanosheets with surface oxygen vacancies (denoted as F/BMO-SOVs) heterostructured composite have been firstly fabricated via a reliable calcination process combined with impregnation approach. The surface oxygen vacancies (SOVs) in Bi2MoO6 were formed due to controlled calcination process. The presence of Fe (III) clusters was confirmed by HRTEM, XPS, and UV–vis DRS. Under visible light irradiation, the optimum molar ratio of 15% F/BMO-SOVs achieved 93.4% degradation efficiency of phenol within 180 min, representing nearly 80 times higher activity than the pure Bi2MoO6, confirmed by both absorption spectrum and TOC measurement. The dramatically enhanced photocatalytic activity is attributed to the synergistic effect between the SOVs, Fe(III) clusters and Bi2MoO6, which not only narrows the band gap, improving the visible light response ability, but also facilitates the direct interfacial charge transfer (IFCT) from the SOVs to the surface Fe(III) clusters, greatly promoting the efficient separation of photogenerated electron-hole pairs. According to the trapping experiments and ESR measurements results, ·O2−, ·OH, and h+ all participated in the phenol photodegradation process over F/BMO-SOVs. Thus, this work not only provides a synergistic effect between SOVs, Fe(III) clusters and Bi2MoO6 involving an IFCT process, but also proposes an efficient approach to fabricating highly active photocatalysts in environmental remediation and solar fuel synthesis.

Ag/Bi2MoO6-x with enhanced visible-light-responsive photocatalytic activities via the synergistic effect of surface oxygen vacancies and surface plasmon

In this study, a heterostructured Ag/Bi2MoO6-x photocatalyst was rationally designed and successfully fabricated via the deposition of plasmonic silver nanoparticles onto the surface of Bi2MoO6 with surface oxygen vacancy (denoted as Bi2MoO6−x). Bi2MoO6−x (Abbr. BMO6-x was first synthesized via a solvothermal synthesis and calcination process. The plasmonic silver nanoparticles were then loaded onto the surface of BMO6−x using a simple photoreduction process to form Ag/BMO6−x composite. Surface oxygen vacancies (SOVs) in BMO6−x were confirmed by electron paramagnetic resonance (EPR) spectrum. The structures of BMO6−xand Ag/BiMoO6−x) were characterized using high-resolution transmission electron microscopy, powder X-ray diffraction, and X-ray photoelectron spectroscopy. Under visible light irradiation, sample Ag/BMO6−x exhibits a highest visible-light-responsive photocatalytic performance compared to those of pure-Bi2MoO6 (BMO), BMO6−x and Ag/BMO for the degradation of rhodamine B (RhB), which is attributed predominantly to the synergistic effect of SOVs and Ag surface plasmonic resonance (SPR) on the surface of Bi2MoO6−x leading to the efficient separation and migration of photogenerated electrons/holes and hence broadening light responsive region. The significant improvement of the migration and separation of photogenerated electrons/holes in the Ag/BMO6−x was evidenced by photoluminescence spectra, time-resolved fluorescence decay, photocurrent, and electrochemical impedance spectrum. The ESR with spin-trap technique and reactive species trapping experiments confirm that the mainly active species O2− and h+ are playing key roles in the RhB photodegradation process over Ag/BMO6−x. This study not only provides an understandable synergistic effect of SOVs and SPR Ag but also pioneers a new approach for fabricating a series of highly catalytically active metal-semiconductor photocatalysts with surface atom defects.

A nanosheet-like α-Bi2O3/g-C3N4 heterostructure modified by plasmonic metallic Bi and oxygen vacancies with high photodegradation activity of organic pollutants

Abstract Bi/α-Bi2O3 nanoparticles rich in oxygen vacancies in the surface and subsurface loading on g-C3N4 nanosheets were realized via a calcination-photoreduction technique, during which the emergence of oxygen vacancies and the generation of metallic Bi from α-Bi2O3 decomposition were achieved simultaneously. The co-occurrence of Bi nanoparticles and oxygen vacancies was favorable for the consumption and separation of the photogenerated electrons and holes. The three-component heterojunction photocatalyst (Bi/α-Bi2O3/g-C3N4, labeled as BBC) exhibited a remarkably high photocatalytic activity under the visible light irradiation, the degradation rate reached almost 90.2% for Tetracycline (TC) and 95.6% for RhB after 180 min and 90 min reaction respectively, far outperforming other decent photocatalysts under the same conditions. The enhanced photocatalytic activity was attributed to the synergistic effect of surface oxygen vacancies (OVs) and Bi nanoparticles. Furthermore, a probable photocatalytic mechanism for degrading the target contaminants by BBC photocatalysts was tentatively proposed.

Hydrogenation-induced surface oxygen vacancies in BiFeO3 nanoparticles for enhanced visible light photocatalytic performance

In recent years, the favorable effect of surface oxygen vacancies on photocatalytic activity has been observed in various metal oxides. In this work, surface oxygen vacancies, for the first time, were introduced into the synthesized BiFeO3 (BFO) nanoparticles by a high pressure hydrogenation process. The formation of surface oxygen vacancies was verified by a variety of characterization techniques. Compared to the pristine BFO, the hydrogenated BFO with surface oxygen vacancies exhibited much higher photocatalytic activities for methyl orange under visible light irradiation. The enhanced photocatalytic activities of the hydrogenated BFO photocatalyst could be attributed to the increased optical absorption, the efficient charge transfer and separation as well as the suppressed recombination of photogenerated electron-hole pairs derived from the induced surface oxygen vacancies in the hydrogenated BFO.

Defect-rich ZnO nanosheets of high surface area as an efficient visible-light photocatalyst

A facile ultra-rapid solution method was developed to fabricate ZnO nanosheets with tunable BET surface area and rich oxygen-vacancy defects. The addition of 1molL−1 Na2SO4 led to an increase of BET surface area of ZnO nanosheets from 6.7 to 34.5m2/g, through an electrostatic-controlled growth and self-assembly mechanism. Detailed analysis based on Raman scattering, room-temperature photoluminescence, X-ray photoelectron spectroscopy and electron spin resonance revealed that the as-prepared ZnO nanosheets were rich in oxygen-vacancies. Increased BET surface area led to a further increase of surface oxygen-vacancy concentration. The rich oxygen-vacancies promoted the visible-light absorption of the ZnO nanosheets, leading to high photocurrent responses and photocatalytic activities towards the degradation of rhodamine B (apparent rate constants, k=0.0179min−1) under visible-light illumination (λ>420nm), about 13 and 11 times higher, respectively than that of ZnO nanoparticles with few oxygen defects. In addition, the high-surface-area ZnO nanosheets could be effectively hybridized with Ag3PO4 nanoparticles, resulting in a further enhancement of the visible-light photocatalytic performance (k=0.0421min−1). This increase in performance was attributed to the increased visible-light absorption as well as the energy level matching, the latter leading to efficient charge transfer between oxygen-vacancy-rich ZnO nanosheet and Ag3PO4, suggesting a synergistic effect of surface oxygen vacancies and Ag3PO4 coupling.

A DFT Study of the Structures of Aux Clusters on a CeO2(111) Surface

Studying the structures of metal clusters on oxide supports is challenging due to their various structural possibilities. In the present work, a simple rule in which the number of Au atoms in different layers of Au(x) clusters is changed successively is used to systematically investigate the structures of Au(x) (x=1-10) clusters on stoichiometric and partially reduced CeO(2)(111) surface by DFT calculations. The calculations indicate that the adsorption energy of a single Au atom on the surface, the surface structure, as well as the Au-Au bond strength and arrangement play the key roles in determining Au(x) structures on CeO(2)(111). The most stable Au(2) and Au(3) clusters on CeO(2)(111) are 2D vertical structures, while the most stable structures of Au(x) clusters (x>3) are generally 3D structures, except for Au(7). The 3D structures of large Au(x) clusters in which the Au number in the bottom layer does not exceed that in the top layer are not stable. The differences between Au(x) on CeO(2)(111) and Mg(100) were also studied. The stabilizing effect of surface oxygen vacancies on Au(x) cluster structures depends on the size of Au(x) cluster and the relative positions of Au(x) cluster and oxygen vacancy. The present work will be helpful in improving the understanding of metal cluster structures on oxide supports.

Oxygen-Vacancy-Stabilized Positively Charged Au Nanoparticles on CeO2(111) Studied by Reflection−Absorption Infrared Spectroscopy

To examine the active form of Au on Au/CeO2 catalysts in the low-temperature water−gas shift (WGS) reaction, the effect of surface oxygen vacancies on CeO2(111) on the charged state of Au nanoparticles (or clusters) deposited at 85 K was studied by combination of structural investigation by scanning tunneling microscopy (STM) and charged state estimation by reflection−absorption infrared spectroscopy (RAIRS) using CO molecule as a probe. On a stoichiometric CeO2(111) without oxygen vacancies, only neutral Au species, which gave a ν(CO) peak at ∼2100 cm−1 for bound CO, were observed independent of preannealing temperatures up to 400 K. Introduction of surface oxygen vacancies by the electron stimulated desorption (ESD) process showed the majority of Au deposited at 85 K was positively charged, giving ν(CO) peaks at approximately 2130 and 2145 cm−1 bound to positively charged Au at different degree. Thermal stability measurements for the species in vacuum revealed that less positively charged Au species rem...

Effect of Surface Oxygen Vacancy on Pt Cluster Adsorption and Growth on the Defective Anatase TiO2(101) Surface

The effect of surface oxygen vacancies on the adsorption and clustering of the Pt adatoms over the defective anatase TiO2(101) surface has been studied using density functional theory slab calculations. The surface oxygen vacancy site was found to be the most active site for a single Pt adatom with an adsorption energy of 4.87 eV. As such, this site may act as a nucleation center for particle growth on the defective anatase TiO2(101) surface. The pathways for forming the two stable Pt2 adsorption configurations from a single Pt adatom in the oxygen vacancy site and another from the neighboring bridging 2cO sites involve the diffusion of the second Pt adatom out of the bridging 2cO site. The transition states for the dimer formation were located at Pt adatom diffusing out of the bridging 2cO site. Furthermore, we found that the bond-breaking step determines the barrier height for the Pt adatom diffusion, which is ∼1 eV. Among five stable Pt3 adsorption structures at the oxygen vacancy site on the defective anatase TiO2(101), the most stable structure is triangular with all three Pt atoms interacting directly with the surface atoms. The possible pathways for Pt adatom diffusion to form the most stable Pt3 configuration were analyzed. The barriers for Pt adatom diffusion in Pt3 formation were predicted to be similar to that of the Pt2 formation due to similar transition state structures. The barrier height indicates that clustering will be kinetically hindered at low temperature.