Creation Of Surface Oxygen Vacancies Research Articles

In-situ synthesis of WO3–x/MoO3–x heterojunction with abundant oxygen vacancies for efficient photocatalytic reduction of CO2

Photocatalytic conversion of CO2 into valuable fuels is considered to be a promising approach for developing renewable and sustainable energy. However, due to poor light harvesting, low charge-separation efficiency and insufficient reaction sites on the surface of photocatalyst, the overall conversion efficiency is facing great challenges. Herein, we reported a novel WO3−x/MoO3−x heterojunction photocatalyst with abundant oxygen vacancies via a facile in-situ solvothermal process to tackle all of the aforementioned issues. Due to the well-matched band gap between WO3−x and MoO3−x, the resultant WO3−x/MoO3−x heterojunction can not only extend the optical response to overlap the NIR region, but also greatly promote the separation of electron-hole pairs. Meanwhile, the improvement of specific surface area and the creation of surface oxygen vacancies endow WO3−x/MoO3−x heterojunction with enhanced CO2 adsorption and activation capacities. As a result, WO3−x/MoO3−x heterojunction exhibits a significant improvement in the photoreduction activity of CO2. And its CO productivity is 40.2 μmol·g−1·h−1, which is 9.5 times higher than that of the pristine MoO3−x nanosheet. This strategy might provide a novel way to improve the comprehensive performance of photocatalysts and develop renewable energy sources.

Oxygen vacancies in nano-sized TiO2 anatase nanoparticles

Anatase nanoparticles containing surface oxygen vacancies (VO) and Ti3+ are of great importance for applications in photocatalysis, batteries, catalysis, sensors among other uses. The properties of VO and their dependence on the size of nanoparticles are of great research interest and could allow obtaining advanced functional materials. In this work, a complete set of oxygen vacancies in an anatase nanoparticle of size 1.1 nm was investigated and compared to those of a twice larger nanoparticle, having the same shape and surface hydroxylation pattern. It turned out that the decrease in the size of the anatase nanoparticle strongly facilitated creation of surface oxygen vacancies and Ti3+. After their creation, oxygen vacancies undergo three transformation paths — (1) small repulsion of surrounding Ti cations with retention of the vacancy, (2) transfer of oxygen anion, leading to the movement of oxygen vacancy to a more stable position, and (3) collapse of oxygen vacancy accompanied by structure deformation towards Magneli-like phase.

Silver and manganese co-doped titanium oxide aerogel for effective diclofenac degradation under UV-A light irradiation

The photodegradation of diclofenac from aqueous medium under UV-A light condition has been essayed using co-doped with manganese and silver TiO2 photocatalysts. X-ray diffraction, Raman spectroscopy, Scanning electron microscopy, Transmission electron microscopy, High resolution transmission electron microscopy, Energy dispersive X-ray, N2 adsorption-desorption measurements at 77 K, UV–Visible diffuse reflectance, X-ray photoelectron spectroscopy and photoluminescence analyses were used to characterize the materials. A high surface specific area of 165 m2/g, low band gap energy (2.7 eV) and effective charge separation were obtained for the 0.5%Ag-0.6%Mn/TiO2 photocatalyst. The results of the photocatalytic experiments showed that co-doping improved the photocatalytic activity of TiO2. In particular, TiO2 co-doped with 0.6 mol% Mn and 0.5 mol% Ag exhibited the maximum diclofenac removal (86%) after 4 h under UV-A light irradiation. The photodegradation rates followed a first order kinetics according to the Langmuir-Hinshelwood model, being the highest apparent rate of 0.0064 min−1. The photocatalytic performance appears as related to the specific surface area, the low optical band gap energy, the creation of surface oxygen vacancies and the efficient separation of photogenerated electron-hole pairs.

Hierarchical 0D/2D Co3O4 hybrids rich in oxygen vacancies as catalysts towards styrene epoxidation reaction

Great efforts have been devoted to the developing of simple, efficient and stable heterogeneous catalysts for the styrene epoxidation reaction (SER). Metal oxides can be of industrial importance by offering an economic and green route for selectively converting styrene into styrene oxide (SO). Herein, by treating the pristine porous 2D Co3O4 sheets with NaBH4 solution, a novel hierarchical structure, i.e., 0D Co3O4 nanoparticles decorated on 2D porous Co3O4 sheets, was obtained. This simple solution reduction strategy not only realizes the morphology evolution, but also induces the modification of the valence states of metal ions and the simultaneous generation of surface oxygen vacancies. The hierarchical 0D/2D Co3O4 hybrids rich in oxygen vacancies (OV-Co3O4) exhibit a much better SER performance than the Co3O4 sheets (P-Co3O4), with the yield of SO more than doubled. The excellent catalytic performance of the OV-Co3O4 can be ascribed to the synergistic effects regarding the hierarchical porous structure, the modification of surface chemical composition and the creation of surface oxygen vacancies.

Synthesis in a Glovebox: Utilizing Surface Oxygen Vacancies To Enhance the Atomic Dispersion of Palladium on Ceria for Carbon Monoxide Oxidation and Propane Combustion

Metal dispersion strongly influences the catalytic performance of metal/oxide catalysts, and the surface oxygen vacancies on oxides are capable of strongly interacting with supported metals and enhancing metal dispersion. However, direct utilization of the surface oxygen vacancies on oxides to synthesize highly dispersed metal/oxide catalysts remains as a challenge because of the instability of the surface oxygen vacancies on oxides under ambient conditions. Herein we report a novel approach of “Synthesis in a Glovebox” in which the creation of surface oxygen vacancies on oxides and the subsequent catalyst synthesis are both carried out in a glovebox with O2 concentration lower than 0.1 ppm in order to stabilize the surface oxygen vacancies. Such an approach was applied to synthesize Pd/CeO2 catalysts. Surface oxygen vacancies were created on CeO2 by H2-reduction treatment at various temperatures, and Pd(NO3)2 aqueous solutions were then impregnated onto the reduced supports. It was found that Pd cations ...

Enhancing Electrocatalytic Activity of Perovskite Oxides by Tuning Cation Deficiency for Oxygen Reduction and Evolution Reactions

Development of cost-effective and efficient electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of prime importance to emerging renewable energy technologies. Here, we report a simple and effective strategy for enhancing ORR and OER electrocatalytic activity in alkaline solution by introducing A-site cation deficiency in LaFeO3 perovskite; the enhancement effect is more pronounced for the OER than the ORR. Among the A-site cation deficient perovskites studied, La0.95FeO3-δ (L0.95F) demonstrates the highest ORR and OER activity and, hence, the best bifunctionality. The dramatic enhancement is attributed to the creation of surface oxygen vacancies and a small amount of Fe4+ species. This work highlights the importance of tuning cation deficiency in perovskites as an effective strategy for enhancing ORR and OER activity for applications in various oxygen-based energy storage and conversion processes.

Photocatalytic and antimicrobial activities of fluorine doped TiO2-carbon nano cones and disc composites

Carbon nano cone and disc-fluorine-co-doped-titanium dioxide nanocomposites were synthesized by the solid state method. The physical and chemical properties of the prepared nanocomposites were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and field emission-scanning electron microscopic (FE-SEM) techniques. In addition, a detailed study of antimicrobial activity and photocatalytic activity of the prepared nanoparticles were tested for its efficiency. Carbon cone and disc-doping into TiO2 showed not only the improvement in UV light absorption, but also in the creation of surface oxygen vacancies. Fluorine-doping produced several beneficial effects, including the formation of surface oxygen vacancies and the enhancement of Ti3+ ions. XRD analysis showed the formation of TiO2 anatase phase in all cases. FE-SEM image, Fig. 3d revealed irregular shapes of carbon cone and disc. The photocatalytic tests indicated that the carbon cone and disc-fluorine-co-doped TiO2 nanocubes have high UV light activity in degradation of methyl orange, which could be attributed to the creation of oxygen vacancies, increase the concentration of •OH radicals in the solution, which is beneficial for improving the photodegradation rate of organic compounds. The carbon cone and disc-fluorine-TiO2 nanocomposites act as a strong antibacterial agent.

Mn-Doped CeO2: DFT+U Study of a Catalyst for Oxidation Reactions

Fil: Garcia Pintos, Delfina. Universidad de Buenos Aires. Facultad de Ingenieri­a. Departamento de Ingenieri­a Quimica; Argentina;

Effect of carbon and nitrogen co-doping on characteristics and sonocatalytic activity of TiO2 nanotubes catalyst for degradation of Rhodamine B in water

Carbon and nitrogen co-doped titanium dioxide nanotubes (CN co-doped TiO2 NTs) catalysts were successfully synthesized using a hydrothermal method at N:Ti molar ratios between 0.1 and 1.0. Transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), X-ray photoelectron spectroscope (XPS), UV–Vis diffuse reflectance spectroscope (UV–Vis DRS) and surface area analyzer were used to characterize the synthesized catalysts. Urea doping at N:Ti=0.3 led to the highest sonocatalytic activity with an apparent-first-order rate constant of 0.0226min−1 as compared to 0.0098min−1 for the un-doped TiO2 NTs. The high performance was associated with the synergistic effects between the doped C and N atoms, higher surface area and lower band gap energy. It was also ascribed to the creation of surface oxygen vacancies due to the formation of Ti3+ species and better crystallinity of anatase phase as compared to the un-doped TiO2 NTs. Most of the nitrogen atoms were bonded to oxygen atoms in the interstitial sites (TiON and TiNO), but only minority of them could substitute the sites of oxygen atoms (NTiO). C atoms evidently presented in the form of TiC and a complex carbonate species on the catalyst surface.

Photocatalytic CO2 Reduction with H2O on TiO2 Nanocrystals: Comparison of Anatase, Rutile, and Brookite Polymorphs and Exploration of Surface Chemistry

CO2 photoreduction with water vapor has been studied on three TiO2 nanocrystal polymorphs (anatase, rutile, and brookite) that were engineered with defect-free and oxygen-deficient surfaces, respectively. It was demonstrated that helium pretreatment of the as-prepared TiO2 at a moderate temperature resulted in the creation of surface oxygen vacancies (VO) and Ti3+ sites on anatase and brookite but not on rutile. The production of CO and CH4 from CO2 photoreduction was remarkably enhanced on defective anatase and brookite TiO2 (up to 10-fold enhancement) as compared to the defect-free surfaces. Defective brookite was photocatalytically more active than anatase and rutile, probably because of a lower formation energy of VO on brookite. The results from in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses suggested that (1) defect-free TiO2 was not active for CO2 photoreduction since no CO2– is generated, and (2) CO2 photoreduction to CO possibly underwent different reaction pathways on oxygen-deficient anatase and brookite via different intermediates (e.g., CO2– on anatase; CO2– and HCOOH on brookite). The combined DRIFTS and photoactivity studies reported in this paper have provided new insights to the role of surface defects in CO2 photoreduction on TiO2 nanocrystals, and revealed significant information on the much less studied but promising brookite phase.

Characterization study on the promoting effect of F-doping V2O5/TiO2 SCR catalysts

V2O5/TiO2 catalysts were prepared in the presence of (NH4)2TiF6 with controlled fluorine to titanium molar ratios (RF/Ti × 100) (0, 0.5, 1, 1.35 and 2) for selective catalytic reduction of nitrogen oxides (NOx) by NH3 (SCR process). It was found that the NOx conversion over F-doping V2O5/TiO2 was greatly improved compared with pure V2O5/TiO2. The effects of F-doping on the structure of the catalysts were systematically examined using XRD, Raman spectrum, BET, TEM, H2-TPR,NO-TPD, XPS and EPR. The XPS results showed that F ions were incorporated into the TiO2 lattice. The high catalytic activity was ascribed to several beneficial effects produced by F-doping – preventing of grain growth, inhibiting of phase transformation from anatase to rutile, higher reducibility of good dispersion of vanadia species, creation of oxygen vacancies, and increasing active sites. It was interesting to point out that the better catalytic activity of F-doped V2O5/TiO2 was achieved by the creation of surface oxygen vacancies.

Visible-Light-Driven Catalytic Properties and First-Principles Study of Fluorine-Doped TiO<sub>2</sub> Nanotubes

Improving the photocatalytic activity and the utilization of visible light of TiO2 is the most important research topics in the photocatalytic field. To improve the photocatalytic activity of TiO2, we used chemical vapor deposition (CVD) to dope TiO2 nanotubes with fluorine. Scanning electron microscopy (SEM) images showed that the annealing temperature significantly affected the morphological integrity of TiO2 nanotubes. Upon annealing at 550 and 700 °C, the structure of F-doped TiO2 nanotubes suffered from an observable disintegration of morphological integrity. X-ray diffraction (XRD) results indicated that the F impurity retarded the anatase-rutile phase transition. Fluorine was successfully doped into TiO2 by CVD, as indicated by the X-ray photoelectron spectroscopy (XPS) results. F-doped TiO2 nanotubes showed higher photocatalytic activity. First-principles calculations suggested that the F 2p states were located in the lower-energy range of valence band (VB) and less mixed with O 2p states. It thus contributed little to the reduction of the optical band gap. This is consistent with the finding that the band gap of F-doped TiO2 is very close to that of undoped TiO2. Therefore, the higher catalytic activity of F-doped TiO2 should be attributed to the creation of surface oxygen vacancies upon F-doping, which enhances surface acidity and increases the amount of Ti3+ ions.

Enhanced photocatalytic activity of transition metal ions Mn 2+, Ni 2+ and Zn 2+ doped polycrystalline titania for the degradation of Aniline Blue under UV/solar light

Anatase TiO 2 was doped with divalent transition metal ions like Mn 2+, Ni 2+ and Zn 2+ and characterized by various analytical techniques. Powder X-ray diffraction revealed stabilization of anatase phase for Ni 2+ and Zn 2+ doped samples, while phase transformation from anatase to rutile was promoted due to Mn 2+ inclusion. The rutile fraction increased with Mn 2+ concentration due to the creation of surface oxygen vacancies. All the doped catalysts showed red shift in the band gap absorption to the visible region. The photocatalytic activities of these catalysts were evaluated in the degradation of Aniline Blue (AB) under UV/solar light. Among the photocatalysts, Mn 2+ (0.06 at.%)–TiO 2 showed enhanced activity, which is attributed to the synergistic effect in the bicrystalline framework of anatase and rutile. Further the unique half filled electronic structure of Mn 2+ serves as a shallow trap for the charge carriers to enhance the photocatalytic activity. An insight to the mechanism of interfacial charge transfer in the mixed phase of anatase and rutile is explored, taking into consideration the theories of previous models.

Effects of the structure of ceria on the activity of gold/ceria catalysts for the oxidation of carbon monoxide and benzene

Using precipitated cerium hydroxide dried at 100 °C as the support, highly dispersed ceria-supported gold catalyst was prepared. Compared with similarly prepared catalysts supported on low-surface area ceria, gold on the high-surface area support showed more resistance to sintering and was more active toward the oxidation of CO and benzene. The oxidation of benzene was very dependent on the structure of ceria. Temperature-programmed reduction showed that the reducibility of surface oxygen was higher for high-surface area ceria. It is proposed that the creation of surface oxygen vacancies by ceria surface reduction promoted oxygen adsorption. Active oxygen formed by dissociation of the adsorbed oxygen is the active species for benzene oxidation, and the dissociation is promoted by gold nanoparticles. When present together in the reactant mixture, benzene inhibited the oxidation of CO, but CO enhanced the oxidation of benzene. Moisture had a promoting effect on both CO and benzene oxidation.

Origin of visible-light-driven photocatalysis: A comparative study on N/F-doped and N–F-codoped TiO 2 powders by means of experimental characterizations and theoretical calculations

An overall comparative study was carried out on N-doped, F-doped, and N–F-codoped TiO 2 powders (NTO, FTO, NFTO) synthesized by spray pyrolysis in order to elucidate the origin of their visible-light-driven photocatalysis. The comparisons in their experimentally obtained characteristics were based on the analysis of XPS, UV–Vis, PL, NH 3-TPD and ESR spectra. The comparisons in their theoretically predicted properties were based on the analysis of the calculated electronic structures. As the results, N-doping into TiO 2 resulted in not only the improvement in visible-light absorption but also the creation of surface oxygen vacancies. F-doping produced several beneficial effects including the creation of surface oxygen vacancies, the enhancement of surface acidity and the increase of Ti 3+ ions. Doped N atoms formed a localized energy state above the valence band of TiO 2, whereas doped F atoms themselves had no influence on the band structure. The photocatalytic tests indicated that the NFTO demonstrated the highest visible-light activity for decompositions of both acetaldehyde and trichloroethylene. This high activity was ascribed to a synergetic consequence of several beneficial effects induced by the N–F-codoping.

Nitrogen and Fluorine Roles in Visible-Light-Driven Anion-Doped TiO2 Photocatalysis

ABSTRACTThis paper describes the synthesis of N-doped, F-doped, and N-F-codoped TiO2 powders (NTO, FTO, NFTO) by spray pyrolysis (SP). An overall comparative study was carried out on these anion-doped powders in order to elucidate the roles of N and F played in visible-light (Vis)-driven photocatalysis. The comparisons in their characteristics were based on the analysis of UV-Vis, PL, NH3-TPD and ESR spectra. As the results, N-doping into TiO2 resulted in both the improvement of Vis absorption and the creation of surface oxygen vacancies (OVs). F-doping produced several beneficial effects including the creation of surface OVs, the enhancement of surface acidity and the increase of Ti3+ ions. The photocatalytic tests indicated that the NFTO demonstrated the highest Vis activity for decomposition of both acetaldehyde and trichloroethylene. This high activity was ascribed to a synergetic consequence of doped N and F.

Visible-light-driven photocatalysis on fluorine-doped TiO 2 powders by the creation of surface oxygen vacancies

F-doped TiO 2 (FT) powders were prepared by spray pyrolysis from an aqueous solution of H 2TiF 6. These FT powders demonstrated very high visible-light (Vis) photocatalytic activities for decompositions of both acetaldehyde and trichloroethylene, although their UV–Vis spectra indicated that F-doping had less effect on the optical absorption property of bulk TiO 2. However, photoluminescence (PL) spectra revealed that two kinds of oxygen vacancies ( F and F + centers) were formed by F-doping. The appearance of Vis photocatalytic activity for FT powders was ascribed to the creation of these oxygen vacancies.

Fluorine-doped TiO 2 powders prepared by spray pyrolysis and their improved photocatalytic activity for decomposition of gas-phase acetaldehyde

F-doped TiO 2 (FTO) powders were synthesized by spray pyrolysis (SP) from an aqueous solution of H 2TiF 6. The resulting FTO powders possessed spherical particles with a rough surface morphology and a strong surface acidity. The fluorine concentrations in the FTO powders calculated from XPS spectra significantly depended on SP temperature and ranged from 2.76 to 9.40 at.%. The FTO powder prepared at SP temperature of 1173 K demonstrated the highest photocatalytic activity for the decomposition of gas-phase acetaldehyde under both ultraviolet (UV) and visible light (vis) irradiations, and it was higher than that of commercial P 25. This high photocatalytic activity was ascribed to several beneficial effects produced by F-doping: enhancement of surface acidity, creation of oxygen vacancies, and increase of active sites. It was interesting to point out that the vis photocatalytic activity of FTO powders was achieved by the creation of surface oxygen vacancies rather than the improvement of optical absorption property of bulk TiO 2 in vis region.

Induced zinc desorption from zinc oxide during exposure to a reducing agent

CH3CH2OD (deuterated ethanol) pulses (5 × 10−4 ML, 20 ms) are offered to polycrystalline ZnO under ultra high vacuum conditions. The reaction products are monitored with a rate of 1000 spectra per second by a time of flight mass spectrometer. D2O and CO2, both including lattice oxygen, are observed in desorption. The creation of surface oxygen vacancies destabilizes zinc atoms nearby which appear also in desorption. The rate constant ka (inverse relaxation time) increases below 900 K. Above this temperature ka decreases. The decrease may be due to bulk oxygen diffusing to the surface and refilling the surface oxygen vacancies.