Novel dual heterojunction between MoS2 and anatase TiO2 with coexposed {101} and {001} facets

This work prepared an anatase TiO₂ by controlling coexposed {101} and {001} facets. The anatase TiO₂ was composited with MoS₂, and the photocatalytic behavior was studied. The relationship between the exposed ratio of {001} facets and the enhancement proportion by the composite was compared. An appropriate exposed ratio of {001} facets, instead of a high ratio, can cause the highest enhancement proportion for photocatalytic activity. This finding confirmed that exposed {001} facets are not the main factors for improving photocatalytic activity. Dual heterojunction was observed to form between MoS₂ and anatase TiO₂ with coexposed {101} and {001} facets, which is a crystal‐face heterojunction between {101} and {001} facets and a semiconductor heterojunction between MoS₂ and {001} facets of TiO₂. In this dual heterojunction, photo‐induced electrons in TiO₂ will flow into {101} facets of TiO₂, instead of MoS₂, whereas the photo‐induced holes in TiO₂ will flow into MoS₂ to reach a high separation efficiency. This finding further revealed the elevating mechanism of constructing heterojunction based on coexposed crystal faces.     

Synthesis of uniform anatase TiO2 microspheres with exposed {001} facets: Without HF utilization and Its photocatalytic selectivity

Uniform anatase TiO₂ microspheres with a high percentage of exposed {001} facets have been successfully synthesized by a one-step hydrothermal strategy in the presence of H₂O₂ and (NH₄)₂CO₃ solution. The as-synthesized anatase TiO₂ microspheres with exposed {001} facets exhibited higher photocatalytic selectivity for degradation of methylene blue and methylene orange mixed solution under UV light irradiation at room temperature than anatase TiO₂ microspheres without {001} facets exposing and P25 TiO₂.     

Synthesis and photocatalytic activity of ring-like anatase TiO2 with {001} facts exposed

Developing photocatalysts with specific morphology and good photocatalytic activities promises good opportunities to discover the geometry-dependent properties. In the present work, ring-like anatase TiO₂ with dominant {001} facets exposed were successfully synthesized via a one-pot solvothermal process of tetrabutyl titanate and hydrofluoric acid solution at 180 °C for 8 h. We found that hydrofluoric acid plays an important role in the formation of ring-like TiO₂. The morphology and microstructure were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Brunauer–Emmett–Teller N₂ gas adsorption–desorption isotherms. The photocatalytic activity was evaluated by photocatalytic oxidation degradation of methylene blue aqueous solution under UV light. Results showed that ring-like TiO₂ with {001} facets exposed exhibited an excellent photocatalytic activities due to its unique structure: Nanosheets with hole.     

Photocatalytic oxidation of toluene to benzaldehyde over anatase TiO2 hollow spheres with exposed {001} facets

A new series of anatase TiO₂ hollow structures were prepared by a facile hydrothermal process. When the hydrothermal time was increased from 20 min to 72 h, the resulting TiO₂ solid spheres gradually transformed into TiO₂ hollow spheres with higher surface crystallinity and exposed {001} facets. The as-prepared TiO₂-72 h sample exhibited the highest activity comparing to other TiO₂-based samples and commercial product Degussa P-25 towards the selective photocatalytic oxidation of toluene to benzaldehyde. Such great photocatalytic performance was mainly attributed to enhanced UV-adsorption and better charge separation efficiency due to higher surface crystallinity of TiO₂-72 h.     

Nb,F-codoped TiO2 hollow spheres with high visible light photocatalytic activity

Nb, F-codoped TiO₂ hollow spheres (NFTSs) were successfully synthesized via a hydrothermal method with niobium oxide, hydrofluoric acid, and tetrabutyl titanate. The obtained spheres were hollow, with a diameter of about 2 μm, and the sphere wall was made up of nanorods arranged close together. The NFTSs presented anatase phase with more {001} facets exposed, which could be mainly attributed to F ions which were preferentially adsorbed on the {001} facets during the crystal growth process. Ti³⁺ states in NFTSs were increased due to Nb, F-codoping, resulting in the decrease of the band gap and the red shift of the absorption edge of the NFTSs. The NFTSs exhibited 20.1% higher photocatalytic speed compared to P25 on the degradation of methylene orange.     

In situ synthesis of ultrathin 2-D TiO2 with high energy facets on graphene oxide for enhancing photocatalytic activity

Two kinds of TiO₂ with novel structures, interpenetrating anatase TiO₂ tablets (IP-TiO₂), and overlapping anatase TiO₂ nanosheets (OL-TiO₂) with exposed {0 0 1} facets, are synthesized. The graphene oxide (GO) supported ultrathin TiO₂ nanosheets (OL-TiO₂/GO) is also prepared by one-pot hydrothermal method. The microscopic feature, morphology, phase, and nitrogen adsorption–desorption isotherms are characterized. The performance of photocatalytic degradation of methyl blue is also measured. Compared with IP-TiO₂, the OL-TiO₂ with GO possess higher photocatalytic efficiency. The GO can improve the photocatalytic property by increasing specific surface area, accelerating the separation of electron–hole pairs, as well as extending the electron life. The growth process of TiO₂ nanosheets on graphene oxide layers probably follows a step-growth mechanism with F⁻ as morphology controlling agent. The steps on the surface can improve the photocatalytic activity further due to the increase of dangling bonds of 5-coordinated Ti (Ti_5c) which are considered to be the active sites in the photocatalytic reaction.     

Alkaline‐Earth Metal Ca and N Codoped TiO2 with Exposed {001} Facets for Enhancing Visible Light Photocatalytic Activity

Alkaline‐earth metal Ca and N codoped TiO₂ sheets with exposed {001} facets were obtained through a one‐step hydrothermal process. The codoped TiO₂ appears as microsheets with length of 1–2 μm and thickness of 100–200 nm. The X‐ray diffractometer and X‐ray photoelectron spectroscopy results confirm that Ca and N codoped TiO₂ has higher crystallinity than N‐doped TiO₂, as well Ca, N atoms were successfully codoped into TiO₂ as interstitial Ca and interstitial N or an O–Ti–N structure, respectively. Compared with N monodoped, further alkaline‐earth Ca codoped has little influence on the energy bands of TiO₂ except slightly elevating the conduction band edge at a value of 0.02 eV. The hydroxyl radicals (?OH) producing and photocatalytic experiment shows that Ca and N codoped can effectively decrease the generation of recombination centers, and enhance separation efficiency of photo‐induced electrons and holes as well as the photocatalytic activity of TiO₂. The codoped photocatalyst has the highest photocatalytic activity when Ca doped ratio reach 0.48%. Excess Ca doped will weaken the crystallization of anatase TiO₂, form charge center, produce new recombination centers and finally reduce the photocatalytic activity of TiO₂.     

Anatase TiO2 nanocrystals with exposed {001} facets on graphene sheets via molecular grafting for enhanced photocatalytic activity

Owing to their extensive practical applications and fundamental importance, the controllable synthesis of well-faceted anatase TiO(2) crystal with high percentage of reactive facets has attracted increasing attention. Here, nano-sized anatase TiO(2) sheets mainly dominated by {001} facets had been prepared on graphene sheets by using a facile solvothermal synthetic route. The percentage of {001} facets in TiO(2) nanosheets was calculated to be ca. 64%. The morphologies, structural properties, growth procedures and photocatalytic activities of the resultant TiO(2)/graphene nanocomposites were investigated. In comparison with commercial P25 and pure TiO(2) nanosheets, the composite exhibited significant improvement in photocatalytic degradation of the azo dye Rhodamine B under visible light irradiation. The enhancement of photocatalytic activity and stability was attributed to the effective charge anti-recombination of graphene and the high catalytic activity of {001} facets.     

Hydrothermal preparation of Ag-TiO2 nanostructures with exposed {001}/{101} facets for enhancing visible light photocatalytic activity

Nano-composite materials of Ag nanoparticles dispersed TiO₂ nanocubes with exposed {001}/{101} crystal faces were fabricated mainly via a flexible one-step method of hydrothermal treatment with different content of Ag from 1 up to 3 mol%. Prepared photocatalysts were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscopy, UV–vis absorption spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. These analysis was carried out for understanding the contribution of different content of silver for enhancing the photocatalytic activity of TiO₂ nanocubes. Prepared silver nanoparticles had small particle size and grafted to the {101} crystal face of TiO₂ with the role of template control agent and linking agent. The photocatalytic performance of Ag-TiO₂ nanocubes were researched via Rhodamine B dye removal under visible light irradiation ( ≧420 nm). Ag-TiO₂ composite materials with the content of 2 mol% Ag showed the best photocatalytic activity for degradation of Rhodamine B, which was five times more than bare TiO₂ and associated with the localized surface plasmon resonance (LSPR) propelled effect. The mechanism by which silver enhanced the photocatalytic activity of TiO₂ was also demonstrated.     

An HREM study of the WO3/TiO2 monolayer catalyst system. Proposals for the overlayer structure

High resolution electron microscopy (HREM) has been used to characterise the WO₃/TiO₂ (anatase) catalyst system. Comparisons between pure samples of TiO₂ in the uncalcined and calcined states with titania in the catalyst (loaded with 10 wt% WO₃) indicate that the tungsten oxide overlayer preserves the surface roughness of the support (as observed in the pure uncalcined material). The calcination of pure anatase results in significant grain growth and surface smoothing. In the electron microscope, the tungsten oxide overlayer is revealed as 2D pseudo-hexagonal shaped clusters which appear to be epitaxially related to the support. After noting that the anatase support predominantly exposes {112}, {011}, {110} and (001) type facets we have combined this information with structural data on the overlayer derived from a previous EXAFS and XANES study by Hilbrig et al. that reported the presence of WO₄ species and six-coordinate WO₅ groups. By considering the arrangement of terminating oxygen atoms on each of the aforementioned anatase surfaces, we suggest ways in which the WO _x species may be linked together to form the tungsten oxide overlayer. This approach has led us to conclude that, with the exception of the (001) surface, the overlayer may consist of WO₄ dimers rather than chains of linked WO₅ groups terminated by WO₄ species.     

CO dissociation activities of clean Pt {430}, {320}, {520}, {310}, {410} and {210}: a comparison with the predictions of Banholzer's orbital symmetry model

Banholzer et al. used an orbital symmetry model to predict the activity of various Pt faces for the dissociation of adsorbed NO. The predictions were later extended to the adsorption system CO/Pt. Our experimentally observed activities for the dissociation of CO on clean Pt {430}, {320}, {520}, {310}, {410} and {210} do not follow these predictions. However, the measured activities agree well with the predictions of the C₆ atomic site model.     

Nitrogen-doped anatase titania nanorods with reactive {101} + {010} facets exposure produced from ultrathin titania nanosheets for high photocatalytic performance

To obtain high performance catalysts for the photooxidation of organic pollutants in water, nitrogen-doped TiO₂ nanorods with {101} + {010} facets exposure were successfully synthesized by using ultrathin two-dimensional titania nanosheets as precursor. The nitrogen-doped one-dimensional nanorods had a uniform size within 100 nm. The crystal structure of nitrogen-doped TiO₂ nanorods (anatase) didn't alter even though subjected from morphology transformation, however, their crystallinity improved tremendously, and the exposure and proportion of active facets changed, which synergistically boosted the catalytic efficiency. This integrated technology including modification, morphology and phase control might be indicative for fabrication of advanced photocatalysts to remedy environmental pollutant and produce clean energy in the future.     

On the Structure Sensitivity of Direct NO Decomposition over Low-Index Transition Metal Facets

We present a study of the dissociative chemisorption of NO, O₂, and N₂ over close-packed, stepped, kinked, and open (fcc {111}, {211}, {311}, {532}, {100}, and {110}) transition metal facets using density functional theory (DFT). The offset of the Brønsted–Evans–Polanyi (BEP) relations suggest that the {111} surface is the least reactive, and that the {110} surface is the most reactive. This observation is verified by establishing volcano-relations based on mean-field microkinetic models for each facet, showing that the maximum rate over {110} is 4 orders of magnitude larger than the maximum {111} rate. The ordering of the maximum activity over the facets is: {110} > {100} ~ {532} > {311} ~ {211} > {111}, which is in general agreement with the offset in the BEP relations. We show that the top-point location and shape of the volcano relations are approximately independent of facet. This observation lends credibility to the approach of analyzing trends in catalytic reactivity over a single low-index facet, and assuming the experimentally observed activity trends are qualitatively well-described by such a single-facet analysis. Our study suggests that a key element for generally obtaining quantitative agreement between theory and experiments is for the simulations to address in detail the propensities of the various types of active sites. Finally, we show that the ordering of NO decomposition rates among metals and facets is essentially unaltered when using BEP- and scaling relations in the microkinetics instead of explicit DFT calculations for each elementary reaction step, and that using a “universal” BEP relation introduces no significant additional qualitative error in trend prediction.     

Preparation and photocatalytic performance of TiO2/PbTiO3 fiber composite enhanced by external force induced piezoelectric field

In this paper, anatase TiO₂ nanoparticles (14 nm) were coated on the surface of piezoelectric PbTiO₃ single crystal fibers by a secondary hydrothermal method, thus a p-n junction was formed at the interface between TiO₂ and PbTiO₃. The PbTiO₃ fibers were exposed to an external force (by ultrasound) to generate a dynamic piezoelectric electric field in the crystal, which incessantly separates the carriers and improves the photocatalytic efficiency of TiO₂. When the ultrasonic frequency is 40 kHz, the TiO₂/PbTiO₃ fibers have the highest photocatalytic degradation efficiency (96%) for methylene orange (MO), which is up to 60% improvement compared to TiO₂ nanoparticles.     

Facile one-step synthesis of BiOCl/BiOI heterojunctions with exposed {001} facet for highly enhanced visible light photocatalytic performances

Flowers-like structure BiOCl/BiOI heterojunctions with exposed {001} facets have been prepared by a facile one-step hydrothermal method. The 70% BiOCl/BiOI heterojunction exhibited the highest photocatalytic activity. The highly enhanced visible light photocatalytic performance was mainly ascribed to the synergistic effect of {001} crystal facets, sensitization of RhB, increasing photo absorption and heterojunctions. The synergistic effect of coupled system could open up new opportunities to develop high-performance photocatalysts for degradation of organic pollutants in water.     

Combustion Synthesis of BiOCl with Tunable Percentage of Exposed {001} Facets and Enhanced Photocatalytic Properties

Bismuth oxychloride (BiOCl) was synthesized by a simple combustion method. The structures, morphologies, absorbance, optical, and photocatalytic properties of the samples were studied by X‐ray powder diffraction, scanning electron microscopy, transmission electron microscopy, ultraviolet–visible spectrophotometry, and photoluminescence. The experimental results show that BiOCl microsheet with high percentage of exposed {001} facets was obtained. It is interesting to note that the thickness of BiOCl sheets, in other words, the percentage of exposed {001} facets can be modulated through changing the added amount of ammonium chloride. The sample with the thinnest BiOCl sheets shows the best photodegradation performance of Rhodamine B under ultraviolet light irradiation, which can be attributed to the cooperative effect between the high percentage of exposed {001} facets and high‐specific surface area. Moreover, the corresponding influencing mechanism on the percentage of exposed {001} facets of BiOCl was discussed.     

Influence of surface orientation on the photochemical reactivity of CaTiO3

The orientations and surface structures of ferroelastic CaTiO₃ crystals within a polycrystalline ceramic were measured by electron backscatter diffraction and atomic force microscopy (AFM) respectively. Surfaces annealed at 1250°C in air are made up of a combination of facets with orientations near {100}, {110}, and {111} (indexed with reference to the cubic perovskite cell). The samples were immersed in an aqueous AgNO₃ solution and illuminated to photochemically reduce silver. Based on the amount of silver reduced on different surfaces, it was determined that surfaces with a {111} orientation had the greatest photocathodic reactivity, those with a {110} orientation had the least photocathodic reactivity, and surfaces with the {100} orientation had intermediate reactivity. While the {110} surfaces were essentially inert for photocathodic reduction, they were active for the photoanodic oxidation of Mn²⁺. Ferroelastic domains were observed to enhance and retard the photochemical reduction of silver on spatially alternating domains on 20% of the grains. However, domains had no influence on the reactivity of the other 80% and are therefore secondary to the surface orientation in determining the reactivity.     

Ag3PO4形貌和晶面对Ag/Ag3PO4等离子体催化剂光催化还原CO2的影响

采用简单的离子交换-光还原法制备了3种表面暴露的晶面分别为单一{110}、{100}和{111}晶面的Ag/Ag₃PO₄等离子体光催化剂,利用X射线衍射、扫描电子显微镜、紫外-可见漫反射光谱、X射线光电子能谱和比表面积测试等技术分别对3种样品的晶相组成、微观形貌和吸光度等进行了表征。所制备催化剂用于可见光照射下光催化还原气相中的CO₂生成碳氢化合物,考察了催化剂暴露的晶面与催化还原CO₂活性的关系,并探讨Ag/Ag₃PO₄光催化还原CO₂的机理。研究表明,{111}晶面暴露的四面体Ag/Ag₃PO₄具有最大的光催化还原CO₂生成CH₃OH的光量子效率、能量投入产出比和转换数。CO₂可通过CO₂ → ·CO₂^- → HCOOH → HCHO → CH₃OH途径还原。     

Facet engineered TiO2 hollow sphere for the visible-light-mediated degradation of antibiotics via ligand-to-metal charge transfer

Efficient removal of tetracycline (TC) under visible-light irradiation over TiO₂-based photocatalysts remains a challenge based on the fact that the reported photocatalytic systems still suffer from weak visible-light absorption and/or inefficient charge separation. Herein, we constructed {101} and {001} facets co-exposed TiO₂ hollow sphere (001-HT) via a gentle NaF treatment, in which the hollow mesoporous feature can trap incident light for a long time to improve photons efficiently. Meanwhile, the as-formed facet heterojunction significantly facilitates the charge separation. As a result, the 001-HT exhibits a high removal rate (~90.1%) of TC under visible-light irradiation, beyond the values of many reported TiO₂-based photocatalysts. Most importantly, we further expound the ligand-to-metal charge transfer mechanism towards TiO₂-assisted degradation of TC under visible-light irradiation, which effectively clarifies the confusion about the origin of pure TiO₂ visible-light activity towards TC degradation because both TiO₂ and TC do not exhibit any visible-light catalytic activity. Therefore, this work provides a new insight in revealing the mechanism of visible-light-mediated TC degradation over pure TiO₂ photocatalyst.     

Morphology-controlled synthesis of TiO2 nanostructures for environmental application

Uniform free standing TiO₂ nanostructure with different shapes was extensively prepared by a template assisted hydrothermal method by controlling the pH and using Ti[OCH(CH₃)₂]₄ as the titanium source. {101} and {200} planes of TiO₂ were found to be the preferentially exposed active crystal planes for both nanospheres and nanostrips TiO₂. The newly synthesised TiO₂ materials were found to be very efficient for the photocatalytic degradation of organic pollutants in industrial wastewater.