Effects of anatase TiO2 morphology and surface fluorination on environmentally relevant photocatalytic reduction and oxidation reactions

Aiming at clarifying the interplay on TiO₂ photoactivity between particle morphology and surface fluorination, the photocatalytic performance of anatase nanocrystals, characterized by a pseudo-spherical shape or a nanosheet structure, is investigated in both a reduction and an oxidation reaction, either in the absence or in the presence of added fluoride anions. Cr(VI) photocatalytic reduction is strongly favored by a large exposure of anatase {001} facets; however, surface fluorination leads in this case to a morphology-independent photoactivity decrease, due to the decreased adsorption of the reaction substrate. More interestingly, a beneficial synergistic effect between the platelet-like anatase morphology and TiO₂ surface fluorination is clearly outlined in Rhodamine B photocatalytic degradation, possibly resulting from the intrinsic ability of fluorinated {001} anatase facets of boosting ^?OH radical mediated oxidation paths, due to their larger amount of surface –OH groups, as revealed using Fourier-transform infrared spectroscopy.     

Synthesis of {100} Facet Dominant Anatase TiO2 Nanobelts and the Origin of Facet‐Dependent Photoreactivity

Sword‐like anatase TiO₂ nanobelts exposed with 78 % clean {100} facets were synthesized and the facet‐dependent photoreactivity of anatase TiO₂ was investigated. By quantitative comparison with the reference {001} facets, the {100} facets possessed about ten‐times higher active sites density than that on {001} facets, resulting in higher photoreaction efficiency. After the active sites density normalization, the {100} and {001} facets exhibited distinct wavelength‐dependent photocatalytic performance, attributed to the anisotropic electronic structures in TiO₂ crystals.     

Facet-Control versus Co-Catalyst-Control in Photocatalytic H2 Evolution from Anatase TiO2 Nanocrystals

Titanium dioxide (TiO₂) and, in particular, its anatase polymorph, is widely studied for photocatalytic H₂ production. In the present work, we examine the importance of reactive facets of anatase crystallites on the photocatalytic H₂ evolution from aqueous methanol solutions. For this, we synthesized anatase TiO₂ nanocrystals with a large amount of either {001} facets, that is, nanosheets, or {101} facets, that is, octahedral nanocubes, and examined their photocatalytic H₂ evolution and then repeated this procedure with samples where Pt co-catalyst is present on all facets. Octahedral nanocubes with abundant {101} facets produce >4 times more H₂ than nanosheets enriched in {001} facets if the reaction is carried out under co-catalyst-free conditions. For samples that carry Pt co-catalyst on both {001} and {101} facets, faceting loses entirely its significance. This demonstrates that the beneficial role of faceting, namely the introduction of {101} facets that act as electron transfer mediator is relevant only for co-catalyst-free TiO₂ surfaces.     

Enhanced and suppressed effects of ionic liquid on the photocatalytic activity of TiO2

The effects of a room temperature ionic liquid, 1-butyl-3-methylimidazolium terafluoroborate ([Bmim]BF₄), on the photocatalytic performance of Degussa P25 TiO₂ were investigated. Also, the photocatalysis mechanism was systematically analyzed by conducting different reactive radical trapping experiments. The results showed that photogenerated electrons were the main reactive species involved in the photocatalytic degradation of methyl orange (MO), while ?OH radicals and photogenerated holes played an important role in the photocatalytic decomposition of rhodamine B (RhB). The addition of ionic liquid (IL) could slightly enhance the photocatalytic degradation rate of MO because adsorption of [Bmim]⁺ ions on the TiO₂ surface not only enhanced traping and transfer of photogenerated electrons, but also facilitated adsorption of negatively charged MO. On the contrary, IL suppressed the degradation rate of RhB because [Bmim]⁺ on the TiO₂ surface not only hindered the access of positively charged RhB to TiO₂, but also restricted the diffusion of positively charged holes to the TiO₂/solution interface.     

Graphite-like carbon deposited anatase TiO<Subscript>2</Subscript> single crystals as efficient visible-light photocatalysts

Graphite-like carbon deposited single-crystal anatase TiO₂ with exposed {001} facets was fabricated through a two-step solvothermal process by using glucose as carbon source. The physicochemical properties of the as-prepared samples were investigated by X-ray diffraction, Brunauer-Emmett-Teller, transmission electron microscopy, Raman, UV–vis diffuse reflectance spectra, electrochemical impedance spectroscopy and surface photovoltage spectroscopy. These results demonstrated that graphite-like carbon layers were deposited on the surface of TiO₂ single-crystal nanosheets with exposed highly reactive {001} facets via the dehydration of glucose during the process of hydrothermal treatment. The loading of the graphite-like carbon layers could effectively extend the light absorption edge of the single-crystal anatase TiO₂ nanosheets to visible light region and accelerate the separation of photo-generated electrons and holes, contributing an excellent visible-light driven photocatalytic performance to the graphite-like carbon deposited single-crystal anatase TiO₂ nanosheets for the degradation of methyl orange.     

具有丰富(001)面的锐钛矿TiO2晶体的无氟合成

采用一种全新的方法实现了无氟、在阳离子聚合物参与下合成具有丰富(001)面的锐钛矿TiO₂晶体(TiO₂-P), 并通过X射线衍射、扫描电镜、透射电镜、紫外-可见光谱等手段对样品进行了表征. 光催化测试显示, 该样品对甲基橙的降解具有比常规锐钛矿TiO₂和商业P25更好的活性, 这可归因于TiO₂-P具有大量的暴露(001)晶面.     

锐钛矿(001)与(101)晶面在光催化反应中的作用

采用水热法制备了(001)和(101)晶面暴露的单晶锐钛矿TiO₂颗粒. 利用光还原沉积贵金属(Au, Ag, Pt)和光氧化沉积金属氧化物(PbO₂, MnO_x)的方法研究了暴露的锐钛矿(001)和(101)晶面在光催化中的作用. 通过透射电子显微镜(TEM)、扫描电子显微镜(STM)、能量色散X射线光谱仪(EDX)和X射线光电子能谱(XPS)的表征, 发现发生光还原反应生成的贵金属粒子主要沉积在暴露的锐钛矿(101)晶面上, 而发生光氧化反应产生的金属氧化物颗粒主要沉积在暴露的锐钛矿(001)晶面上. 此结果表明光激发产生的电子与空穴主要并分别分布在单晶锐钛矿TiO₂的(101)与(001)晶面上, 并在其上参与光催化还原反应和氧化反应. 同时也表明暴露的不同晶面对光生电荷具有分离效应. 基于本研究可以认为同时暴露分别进行氧化和还原反应的晶面可以有效促进光催化反应.     

Photocatalytic degradation of dimethyl phthalate ester using novel hydrophobic TiO2 pillared montmorillonite photocatalyst

TiO₂ pillared montmorillonites were prepared by introducing Ti⁴⁺ into a layer of montmorillonite modified with or without cetyltrimethylammonium bromide. The components and texture of the prepared composites were characterized by thermogravimetric analysis, X-ray powder diffraction and scanning electron misroscopy. The adsorption and photocatalytic degradation performance of a model environmental endocrine disruptor, dimethyl phthalate ester, were investigated using this newly prepared hydrophobic TiO₂ pillared montmorillonite photocatalyst. The adsorption of dimethyl phthalate ester from water varied from 9% to 28% on the prepared hydrophobic photocatalyst. Although the experimental results showed that the photocatalytic activity of the hydrophobic photocatalyst was slightly lower than that of hydrophilic one, electron spin resonance verified that hydroxyl radicals were also generated in hydrophobic TiO₂ pillared montmorillonite photocatalyst under UV irradiation. To elucidate the decomposition mechanism of dimethyl phthalate ester, 12 main photocatalytic intermediates were identified during the photocatalytic degradation process, and a plausible degradation mechanism was also proposed.     

Degradation of terbuthylazine by Fe/TiO2 with visible-light-driven photo-Fenton catalytic activity

In order to improve the catalytic activity of Fenton catalyst, a composite catalyst, Fe/TiO₂, with both visible-light photocatalytic and Fenton-like catalytic activities was synthesized via a brief solvothermal process. The XRD and SEM results indicated that Fe was dispersed homogeneously on the surface of TiO₂ in the form of Fe₂O₃, and the loading of Fe did not have significant effects on the particle size and morphology of TiO₂. The EDS results showed that the loading content of Fe was about 1.4 wt%. The photocatalytic results showed that the prepared Fe/TiO₂ composite catalyst had excellent catalytic behaviors for terbuthylazine degradation under visible-irradiation with H₂O₂ assistance, the degradation ratio reached up to 90% after 120 min. The reinforced degradation performance were primarily attributable to the introduction of carrier TiO₂, which expanded visible response range by H₂O₂ adsorption, and accelerated the cycle of Fe (Ⅱ)/Fe (Ⅲ). The fluorescent spectroscopy results revealed that the degradation process of terbuthylazine involved the generation and participation of active species such as hydroxyl radicals and superoxide radicals. This study is expected to provide a visual approach for designing a novel photo-Fenton catalyst to jointly utilize both photocatalytic and Fenton activities, which can be better applied to the actual use of organics purification in wastewater.     

Enhancing the Photocatalytic Activity of Anatase TiO2 by Improving the Specific Facet‐Induced Spontaneous Separation of Photogenerated Electrons and Holes

Recently, it has been proven that directional flow of photogenerated charge carriers occurs on specific facets of TiO₂ nanocrystals. Herein, we demonstrate that the photocatalytic activity of anatase TiO₂ nanocrystals in both photoreduction and photooxidation processes can be enhanced by selectively depositing Pt nanoparticles on the {101} facets, which strengthens spontaneously surface‐induced separation between photogenerated electrons and holes in the photocatalysis process. An optimal ratio of the oxidative {001} facets to the reductive {101} facets exists with regard to the photocatalysis of the faceted TiO₂ nanocrystals, and this is crucial for balancing the recombination and redox reaction rates of photogenerated electrons and holes. The present work might help us gain deeper insight into the relation between the specific surface of semiconductor photocatalysts and their photocatalytic activities and provides us with a new route to design photocatalysts with high photocatalytic activity.     

锐钛矿型多孔TiO2薄膜的溶解法制备及性能表征

在具有锐钛矿晶粒的TiO₂溶胶中加入苯丙乳液粒子，使用该混合液浸渍提拉涂膜，然后利用甲苯将薄膜中的苯丙乳液粒子溶解去除，并通过重复涂膜，在室温下获得了具有良好多孔性的锐钛矿型TiO₂薄膜。考察了多孔薄膜的表面形貌、光学性能、吸附性能和光催化性能。结果表明：随薄膜涂膜次数的增加，TiO₂多孔薄膜的吸光度增大，透光率减小，光吸收边波长向长波方向移动。罗丹明B在TiO₂多孔薄膜上的吸附量随涂膜次数的增加先升高，后降低；多次涂膜会在薄膜中产生半封闭的孔洞，经过长时间的毛细渗透等作用能进一步增加薄膜对罗丹明B的吸附。TiO₂多孔薄膜通过吸附+光催化氧化的模式快速分解罗丹明B，其活性主要受到薄膜在光催化反应初期的吸附能力的影响。此外，TiO₂的负载量、光的利用效率、以及光生电荷迁移及其分离等也是影响薄膜光催化活性的因素。     

Active Sites for Adsorption and Reaction of Molecules on Rutile TiO2(110) and Anatase TiO2(001) Surfaces (cited: 1)

The reactivity of specific sites on rutile TiO₂(110)-(1×1) surface and anatase TiO₂(001)-(1×4) surface has been comparably studied by means of high resolution scanning tunneling microscopy. At the rutile TiO₂(110)-(1×1) surface, we find the defects of oxygen vacancy provide distinct reactivity for O₂ and CO₂ adsorption, while the terminal fivefold-coordinated Ti sites dominate the photocatalytic reactivity for H₂O and CH₃OH dissociation. At the anatase TiO₂(001)-(1×4) surface, the sixfold-coordinated terminal Ti sites at the oxidized surface seem to be inert in both O₂ and H₂O reactions, but the Ti-rich defects which introduce the Ti³⁺ state into the reduced surface are found to provide high reactivity for the reactions of O₂ and H₂O. By comparing the reactions on both rutile and anatase surfaces under similar experimental conditions, we find the reactivity of anatase TiO₂(001) is actually lower than rutile TiO₂(110), which challenges the conventional knowledge that the anatase (001) is the most reactive TiO₂ surface. Our findings could provide atomic level insights into the mechanisms of TiO₂ based catalytic and photocatalytic chemical reactions.     

Prolonging charge-separation states by doping lanthanide-ions into {001}/{101} facets-coexposed TiO2 nanosheets for enhancing photocatalytic H2 evolution

Ultrathin TiO₂ nanosheets with coexposed {001}/{101} facets have attracted considerable attention because of their high photocatalytic activity. However, the charge-separated states in the TiO₂ nanosheets must be extended to further enhance their photocatalytic activity for H₂ evolution. Herein, we present a successful attempt to selectively dope lanthanide ions into the {101} facets of ultrathin TiO₂ nanosheets with coexposed {001}/{101} facets through a facile one-step solvothermal method. The lanthanide doping slightly extended the light-harvesting region and markedly improved the charge-separated states of the TiO₂ nanosheets as evidenced by UV-vis absorption and steady-state/transient photoluminescence spectra. Upon simulated sunlight irradiation, we observed a 4.2-fold enhancement in the photocatalytic H₂ evolution activity of optimal Yb³⁺-doped TiO₂ nanosheets compared to that of their undoped counterparts. Furthermore, when Pt nanoparticles were used as cocatalysts to reduce the H₂ overpotential in this system, the photocatalytic activity enhancement factor increased to 8.5. By combining these results with those of control experiments, we confirmed that the extended charge-separated states play the main role in the enhancement of the photocatalytic H₂ evolution activity of lanthanide-doped TiO₂ nanosheets with coexposed {001}/{101} facets.     

Graphitic-C<Subscript>3</Subscript>N<Subscript>4</Subscript> quantum dots decorated {001}-faceted TiO<Subscript>2</Subscript> nanosheets as a 0D/2D composite with enhanced solar photocatalytic activity

Zero-dimensional (0D)/two-dimensional (2D) heterojunctions have attracted great attention in photocatalysis due to their superior interfacial effects. In this work, 0D g-C₃N₄ quantum dots (CNQDs) were firstly used to modify {001}-faceted 2D TiO₂ nanosheets by a simple solvothermal method. During the controlled growth of TiO₂ nanosheets with exposed reactive {001} facets, the CNQDs can be simultaneously assembled on the surface of TiO₂ nanosheets in a highly dispersive way. The 0D/2D composite containing only 0.5% of CNQDs shows the optimized solar photocatalytic activity for the degradation of rhodamine B and 4-chlorophenol. More importantly, the 0D/2D composite exhibits a better solar photocatalytic activity than the bulk g-C₃N₄/TiO₂ nanosheets composite. This improvement can be ascribed to the close interfacial contact and strong interaction between the highly dispersed CNQDs and the TiO₂ nanosheets, which could lead to efficient separation of the photogenerated electron–hole pairs, provide more catalytic active sites, and enhance the absorption of solar light. The 0D/2D composite also shows good stability for its practical applications.     

The roles of hydroxyl radicals, photo-generated holes and oxygen in the photocatalytic degradation of humic acid

This article is aimed at studying on the roles of the hydroxyl radicals, photo-generated holes, and oxygen in the photocatalytic degradation of humic acid (HA) in acid and alkaline conditions. The results indicate that hydroxyl radicals?? scavenger alone can inhibit the photocatalytic degradation process completely in alkaline condition, which implies that photo-generated holes cannot directly degrade the organic matter in alkaline condition. Moreover, the reaction sites between hydroxyl radicals and HA is on the TiO₂ surface in acid condition. But in alkaline condition, hydroxyl radicals diffuse and react with HA in the solution. The generation of hydroxyl radicals almost stops and the photocatalytic degradation is inhibited seriously without oxygen, which illustrates that oxygen plays an important role in the photocatalytic degradation of HA.     

Surface morphology and active sites of TiO<Subscript>2</Subscript> for photoassisted catalysis

The main aim of this work is to discriminate the closely related adsorption and catalytic degradation processes that occur during a photocatalytic reaction. Very high-surface-area TiO₂ and Pd-doped TiO₂ were synthesized by microwave-assisted hydrothermal synthesis and used for degradation of methylene blue as a model pollutant dye. Thorough structural, morphological, and surface analyses of the synthesized catalysts were conducted to investigate key material properties that influence adsorption and catalytic performance. The adsorption capacity of the catalysts was determined by fitting adsorption data using the Langmuir isotherm model, and the photocatalytic activity of the synthesized samples was evaluated by periodically measuring the concentration of methylene blue as it was photocatalytically degraded under ultraviolet (UV) light. The results indicated that noble-metal incorporation compromised adsorption but favored catalytic performance.     

EPR and kinetic investigation of free cyanide oxidation by photocatalysis and ozonation

Oxidation of free cyanide in aqueous suspensions of three commercial TiO₂ specimens, with different anatase crystal size, has been carried out in a batch photoreactor by simultaneously applying ozonation and photocatalysis. Dissolved ozone participates both in homogeneous and catalytic reactions with cyanide; the extents of these two processes are comparable to that of the photodegradation with oxygen. The reactivity results are well described by the Langmuir-Hinshelwood kinetic model, providing the values of the kinetic and equilibrium adsorption constants for the catalytic and photocatalytic reactions contributing to cyanide oxidation. The cyanide concentration decreases faster with time for catalysts with increasing anatase crystal size, being more marked under UV irradiation. EPR studies on gaseous ozone adsorption on the three samples in the dark have shown stronger ozone interactions with Ti⁴⁺ and O^2? ions of the samples with largest anatase crystal size, leading to the formation of significant signals of Ti³⁺ and _s O^??O₂ radicals than with the anatase with the lowest crystal size, where ozone was mainly adsorbed on water molecular arrangements covering its surface. The hampering of the ozone and/or cyanide adsorption by the water molecular arrangements covering the surface of the catalyst with the lowest crystal size would justify the low cyanide degradation rate observed for this sample.     

Photocatalytic characteristics of TiO2 supported on SiO2

The photocatalytic decomposition of acetaldehyde was carried out on TiO₂/SiO₂. The presence of a support (SiO₂) in TiO₂/SiO₂ helped to promote the efficiency of the photocatalyst. The silica support enhanced the effective surface area of TiO₂ and adsorption of acetaldehyde on TiO₂/SiO₂. TiO₂/SiO₂ synthesized from Ti(SO₄)₂ showed promoting effect on acetaldehyde decomposition. The XPS results revealed that TiO₂/SiO₂ prepared with Ti(SO₄)₂ generated SO ₄ ^2? sites on the TiO₂ surface. The increased acidity could promote the adsorption of acetaldehyde and photocatalytic degradation of acetaldehyde. The sulfate ion seemed to generate the bifunctional sites (acid sites and photoactive sites) and promoted the acetaldehyde decomposition on TiO₂/SiO₂.     

Synthesis of Cs2.5H0.5PW12O40/TiO2 Nanocomposites with Dominant TiO2 {001} Facets and Related Photocatalytic Properties

TiO₂ nanosheets with dominant {001} facets, coupled with Cs_2.5H_0.5PW₁₂O₄₀, were successfully synthesized by a one‐step hydrothermal reaction. The photocatalytic activity of nanocatalysts was evaluated by the degradation of Rhodamine B under UV light irradiation. The results showed that both the addition of Cs_2.5H_0.5PW₁₂O₄₀ and the exposed {001} facets of TiO₂ have a positive effect on the photocatalytic activity. The improved photoactivity of nanocomposites in comparison with that of TiO₂ nanosheets could be attributed to the synergistic effect between Cs_2.5H_0.5PW₁₂O₄₀ and TiO₂ which facilitates the separation of photo‐induced hole‐electron pairs.     

掺氮TiO2可见光降解有机污染物的比较研究

用溶胶-凝胶法制备了不同掺杂量的N/TiO₂复合纳米粉末, 采用X射线衍射(XRD)、扫描透镜(TEM)、紫外-可见反射吸收光谱(UV-vis)对催化剂进行了初步表征. 通过X射线光电子能谱(XPS)、元素分析仪(EA)测定其含氮量. XPS分析结果显示TiO₂晶格中的氧被氮原子取代, N/TiO₂表面存在Ti^3＋离子; 紫外-可见反射吸收光谱测得不同掺杂量的N/TiO₂的禁带宽度(E_g), 推测在TiO₂价带上方生成了由N诱导产生的中间带, 当氮、钛摩尔比为0.0880时N/TiO₂的E_g最小, 为2.50 eV. 在可见光下, 以酸性桃红(SRB)和无色小分子对氯苯酚(4-CP)作为可见光活性实验的探针反应, 确定了最佳掺杂比为n_N/n_Ti＝0.0880. 结果表明, 最佳掺杂量下N/TiO₂能显著降解SRB和4-CP, 通过测定ESR, IR, TOC, COD, 重点比较了TiO₂在掺杂N前后在降解SRB和4-CP时的差异, 包括氧化物种、矿化率、最终产物等, 证明在可见光下, N/TiO₂的降解机理为电子从独立的N 2p轨道激发到Ti 3d轨道, 产生羟基自由基等氧化物种, 达到降解有机物的目的.