Influence of crystallite structure of nanometer TiO2 catalysts on photocatalyst reaction

25 samples of nanometer TiO₂ were prepared through modifying the preparation conditions including the concentrations of Ti(SO₄)₂ and NH₄HCO₃, the pH value at the end of precipitation operation, and sintering temperature and time by methods of orthogonal design. The relationships between their catalytic activities of photocatalytic degradation of sodium dodecyl benzene sulfonate (SDBS) and the normal crystallite size, lattice-strain and X-ray diffraction intensity of 10 crystal faces were analyzed. It was discovered that the photocatalytic degradation reaction of SDBS follows first-order kinetics. Crystal faces of catalysts’ (101) have greater influence than other crystal faces on the reaction. The photocatalytic reaction needs more perfect (101) crystal faces with less lattice-strain. Smaller normal crystallite size and greater specific surface area of (101) crystal face are better for increasing reaction rate. The photocatalytic reaction is mainly proceeded on the (101) crystal face, comparatively, amorphous TiO₂ has lower catalytic activity.     

Synthesis of nanosized TiO2/SiO2 particles using microwave processes and their photocatalytic activity on the decomposition of orange II

The nanosized titania and TiO₂/SiO₂ particles were prepared by the microwave-hydrothermal method. The effect of physical properties TTIP/TEOS ratio and calcination temperature has been investigated. The major phase of the pure TiO₂ particle is of the anatase structure, and a rutile peak was observed above 800°C. In TiO₂/SiO₂ particles, however, no significant rutile phase was observed, although the calcination temperature was 900°C. No peaks for the silica crystal phase were observed at either silica/titania ratio. The crystallite size of TiO₂/SiO₂ particles decreases as compared to pure TiO₂ at high calcination temperatures. The TiO₂/SiO₂ particles show higher activity on the photocatalytic decomposition of orange II as compared to pure TiO₂ particles.     

转晶纳米TiO2的制备及其声催化活性的研究

近几年兴起的半导体多相光催化技术在有机废水处理方面受到了人们的普遍关注[1,2]。众多的半导体光催化剂材料中,TiO2因其化学性质稳定和催化效率高而倍受青睐[3,4]。在紫外光的照射下,TiO2与H2O产生具有极强氧化能力的HO·自由基,最终可使废水中的有机物完全降解生成CO2,H2O以     

Syntheses of TiO2(B) nanowires and TiO2 anatase nanowires by hydrothermal and post-heat treatments

TiO₂(B) nanowires and TiO₂ anatase nanowires were synthesized by the hydrothermal processing in 10 M NaOH aq. at 150 °C followed by the post-heat treatment at 300-800 °C. As-synthesized Na-free titanate nanowires (prepared by the hydrothermal treatment and repeated ion exchanging by HCl (aq.) were transformed into TiO₂(B) structure with maintaining 1-D morphology at 300-500 °C, and further transformed into anatase structure at 600-800 °C with keeping 1-D shape. At 900 °C, they transformed into rod-shaped rutile grains. Microstructure of these 1-D TiO₂ nanomaterials is reported.     

Synthesis of titanate, TiO2 (B), and anatase TiO2 nanofibers from natural rutile sand

Titanate nanofibers were synthesized by hydrothermal method (150 °C for 72 h) using natural rutile sand as the starting materials. TiO₂ (B) and anatase TiO₂ (high crystallinity) nanofibers with the diameters of 20-100 nm and the lengths of 10-100 μm were obtained by calcined titanate nanofibers for 4 h at 400 and 700 °C (in air), respectively. The samples characterized by XRD, SEM, TEM, SAED, HRTEM, and BET surface area. This synthesis method provides a simple route to fabricate one-dimensional nanostructured TiO₂ from low cost material.     

Thermal and hydrothermal stability of flame hydrolytically synthesized SiO2/TiO2 mixed oxides

The phase stability of the two TiO₂ modifications (anatase and rutile) in fumed SiO₂/TiO₂ nano-composites (0–24.8 wt-% silica) under thermal and hydrothermal conditions was investigated by X-ray powder diffraction, transmission electron microscopy (TEM) and gas adsorption methods (BET). The results show that the phase transformation from anatase to rutile type of structure and the growth of anatase crystallites are significantly retarded by mixing small amounts of SiO₂ into TiO₂, while the specific surface area is maintained. The SiO₂/TiO₂-composites reveal a remarkable shift in the anatase to rutile transformation temperature from approx. 500 °C (pure TiO₂) to approx. 1000 °C (samples with SiO₂ contents of more than 10%). The rate of phase transformation from anatase to rutile is enhanced under hydrothermal conditions compared to conventional thermal treatment, e.g. pure titania (AEROXIDE^® TiO₂ P25) annealed under hydrothermal conditions (100 g/m³ absolute humidity, 4 h at 600 °C) had a rutile content of 85%, while the same specimens annealed in absence of humidity contained only 46% rutile. However, the difference in rate of phase transformation became less pronounced when the silica content in SiO₂/TiO₂-composites was further increased.TEM results showed that the surface of the anatase crystallites was covered with silica. This averts coalescence of anatase crystallites and keeps them under a critical size during the annealing process. When the crystal domains grew larger, a rapid conversion to rutile took place. The critical size of anatase crystallites for the phase transformation was estimated to be 15–20 nm.     

Hydrothermal synthesis of mesostructured nanocrystalline TiO2 in an ionic liquid–water mixture and its photocatalytic performance

Anatase mesostructured TiO₂ nanocrystalline was prepared in a mixture of 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIM⁺BF₄⁻) ionic liquid and water by a low temperature hydrothermal method. The obtained materials were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and N₂ adsorption–desorption. The existence of BMIM⁺BF₄⁻ enhanced the polycondensation and crystallization rate, which encouraged the formation of anatase crystal. The TiO₂ particles were thermally very stable and thus resistant to anatase-rutile phase transformation during calcination at high temperatures. The anatase TiO₂ showed high photocatalytic activity in the degradation of p-chlorophenol than that of the commercially available TiO₂, Degussa P25. After 2 h reaction under the UV-irradiation of 250 W, the removing rate of p-chlorophenol was up to 96.3%.     

二氧化钛与针铁矿复合光催化材料的制备与光催化性能

以四氯化钛为钛源,针铁矿(α-FeOOH)为载体,采用水解沉淀法制备了金红石相二氧化钛(Ti₂O)与α-FeOOH的复合光催化材料,并采用X射线衍射、扫描电子显微镜、透射电子显微镜、X射线能量散射谱和X射线光电子能谱对样品进行了表征.结果表明,低温下,金红石相Ti₂O包覆于α-FeOOH表面,并形成复合结构;较高温下,铁离子进入金红石相Ti₂O晶格,并形成铁掺杂金红石相Ti₂O纳米管;中温下,样品兼有复合和掺杂两者特征.在室温下以甲基橙为降解对象,采用钨灯+氘灯(波长200~800nm)为光源,对样品的光催化活性进行了测试.结果表明,样品对甲基橙的光催化降解效果良好;与纯α-FeOOH和金红石相Ti₂O相比,不同结构样品的光催化活性均有所提高,其中,复合兼掺杂型样品的光催化活性最高.由此可见,与α-FeOOH复合和铁掺杂是提高Ti₂O光催化活性的有效途径.     

Pulsed laser-induced oxygen deficiency at TiO2 surface: Anomalous structure and electrical transport properties

We have studied pulsed laser-induced oxygen deficiencies at rutile TiO₂ surfaces. The crystal surface was successfully reduced by excimer laser irradiation, and an oxygen-deficient TiO_2−δ layer with 160 nm thickness was formed by means of ArF laser irradiation at 140 mJ/cm² for 2000 pulses. The TiO_2−δ layer fundamentally maintained a rutile structure, though this structure was distorted by many stacking faults caused by the large oxygen deficiency. The electrical resistivity of the obtained TiO_2−δ layer exhibited unconventional metallic behavior with hysteresis. A metal–insulator transition occurred at 42 K, and the electrical resistivity exceeded 10⁴ Ω cm below 42 K. This metal–insulator transition could be caused by bipolaronic ordering derived from Ti–Ti pairings that formed along the stacking faults. The constant magnetization behavior observed below 42 K is consistent with the bipolaronic scenario that has been observed previously for Ti₄O₇. These peculiar electrical properties are strongly linked to the oxygen-deficient crystal structure, which contains many stacking faults formed by instantaneous heating during excimer laser irradiation.     

Preparation and characterization of SiO2/TiO2 composite microspheres with microporous SiO2 core/mesoporous TiO2 shell

SiO₂/TiO₂ composite microspheres with microporous SiO₂ core/mesoporous TiO₂ shell structures were prepared by hydrolysis of titanium tetrabutylorthotitanate (TTBT) in the presence of microporous silica microspheres using hydroxypropyl cellulose (HPC) as a surface esterification agent and porous template, and then dried and calcined at different temperatures. The as-prepared products were characterized with differential thermal analysis and thermogravimetric (DTA/TG), scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen adsorption. The results showed that composite particles were about 1.8 μm in diameter, and had a spherical morphology and a narrow size distribution. Uniform mesoporous titania coatings on the surfaces of microporous silica microspheres could be obtained by adjusting the HPC concentration to an optimal concentration of about 3.2 mmol L⁻¹. The anatase and rutile phase in the SiO₂/TiO₂ composite microspheres began to form at 700 and 900 °C, respectively. At 700 °C, the specific surface area and pore volume of the SiO₂/TiO₂ composite microspheres were 552 and 0.652 mL g⁻¹, respectively. However, at 900 °C, the specific surface area and pore volume significantly decreased due to the phase transformation from anatase to rutile.     

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.     

Preparation of uniform rhodamine B-doped SiO2/TiO2 composite microspheres

Uniform rhodamine B (RB)-doped SiO₂/TiO₂ composite microspheres with catalytic and fluorescent properties were prepared by an easy and economical method in this paper. The composite microspheres were built up with well-dispersed silica particles as the cores, RB as both the doped agent and stabilizer, and the TiO₂ shells were obtained through the hydrolysis of TiCl₄ in water bath. The morphology and structure of the particles were characterized by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). The characterization results indicate that composite particles are all in spherical shape and have a narrow size distribution. The composite particles calcined above 500 °C reveal clear crystalline reflection peaks of the rutile TiO₂ which exhibits well catalytic property. The photocatalytic experiment was carried out in order to examine the catalytic property of composite microspheres. The fluorescent property of particles was also investigated. Dye-leakage test indicates that RB molecules entrapped in the composite particles by this method are stable inside the particles.     

SnO2/TiO2纳米管复合光催化剂的性能及失活再生

基于微波水热法和微乳液法合成SnO₂/TiO₂纳米管复合光催化剂. 通过X射线衍射（XRD）、配有能量色散X射线光谱仪（EDX）的透射电镜（TEM）和电化学手段对光催化剂进行表征. 以甲苯为模型污染物,考察光催化剂在紫外光（UV）和真空远紫外光（VUV）下的性能及失活再生. 结果表明,SnO₂/TiO₂纳米管复合光催化剂形成三元异质结（锐钛矿相TiO₂（A-TiO₂）/金红石相TiO₂（R-TiO₂）、A-TiO₂/SnO₂和R-TiO₂/SnO₂异质结）,促使光生电子-空穴对的有效分离,提高光催化活性. SnO₂/TiO₂表现出最佳的光催化性能,UV和VUV条件下的甲苯降解率均达100%,CO₂生成速率（k₂）均为P25的3倍左右. 但由于UV光照矿化能力不足,中间产物易在催化剂表面累积. 随着UV光照时间的增加,SnO₂/TiO₂逐渐失活,20 h 后k₂由138.5 mg·m^-3·h^-1下降到76.1 mg·m^-3·h^-1. 利用VUV再生失活的SnO₂/TiO₂,过程中产生的·OH、O₂^-·、O（¹D）、O（³P）、O₃等活性物质可氧化吸附于催化剂活性位的难降解中间产物,使催化剂得以再生,12 h后k₂恢复到143.6 mg·m^-3·h^-1. UV和VUV的协同效应使UV降解耦合VUV再生成为一种可持续的光催化降解污染物模式.     

Controlled crystallization of titanium dioxide particles in the presence of poly(vinyl alcohol) from peroxytitanic acid

Crystallization of TiO₂ particles from peroxytatanic acid in the presence of poly(vinyl alcohol) (PVA) via a hydrothermal process was investigated. The crystal structure and morphology of TiO₂ were significantly affected by pH and PVA dosage. High acidic condition without PVA led to the formation of rutile TiO₂, while addition of PVA resulted in the formation of anatase TiO₂. Much amount of PVA addition, and neutral or high pH circumstance were favorable for formation of anatase TiO₂ particles. The pH of solution also affected morphology of TiO₂ particles produced. From TEM images, it was observed that in pH 5.5 solution rod-shaped clusters and in pH 8.5 solution nanoparticles aggregates composed of 20–30 nm TiO₂ particles were formed.     

Novel three-dimensional dandelion-like TiO2 structure with high photocatalytic activity

Pure rutile phase crystalline TiO₂ powder with novel 3D dandelion-like structure was synthesized by using a facile hydrothermal method with TiCl₃ as the main starting material. In such a 3D structure, the nanometer-scale construction elements aggregate together and form a micrometer-scale artificial unit. The typical 3D dandelion structure has an average diameter of 1.5-2 μm and is packed radially by nanorods with [001] preference growth direction. Each individual nanorod is hundreds of nanometers in length, and tens of nanometers in diameter. The obtained 3D dandelion-like TiO₂ powder has a high photocatalytic activity, which is equivalent to that of the commercial available P25 titania powder. Mechanisms of the formation of the dandelion-like structure were also discussed. A different oxidation process of Ti(III) to Ti(IV) during hydrothermal was suggested.     

Photoactivity and phase stability of ZrO2-doped anatase-type TiO2 directly formed as nanometer-sized particles by hydrolysis under hydrothermal conditions

Anatase-type TiO₂ doped with 4.7 and 12.4 mol% ZrO₂ that were directly precipitated as nanometer-sized particles from acidic precursor solutions of TiOSO₄ and Zr(SO₄)₂ by simultaneous hydrolysis under hydrothermal conditions at 200°C, showed higher photocatalytic activity than pure anatase-type TiO₂ for the decomposition of methylene blue. The crystallite growth and the phase transformation from anatase-type to rutile-type structure caused by heating at high temperature were retarded by doping ZrO₂ into TiO₂. The anatase-type TiO₂ doped with ZrO₂ showed high phase stability and maintained anatase-type structure even after heating at 1000°C for 1 h.     

Low temperature CO oxidation on Au/TiO2 sol-gel catalysts

A comparative study on Au/TiO₂catalysts prepared by impregnation with HAuCl₄of commercial TiO₂ or by impregnation of sol-gel derived TiO₂has been carried out during CO oxidation. Specific surface areas and mean Au particle of 49 and 74 m²/g and 35 and 25 Å were obtained for impregnated commercial TiO₂ and sol-gel preparations, respectively. XRD patterns shown that in sol-gel derived TiO₂ only anatase phase was identified, while in commercial TiO₂ anatase and rutile phases co-exist. Titania support effect on Au activity for the oxidation of CO has been observed. The light-off during the reaction on Au/TiO₂initiates at 50°C, whereas for commercial impregnated TiO₂ catalyst the light-off initiates at 200°C.     

Hierarchical chlorine-doped rutile TiO2 spherical clusters of nanorods: Large-scale synthesis and high photocatalytic activity

In this study, we report the synthesis of hierarchical chlorine-doped rutile TiO₂ spherical clusters of nanorods photocatalyst on a large scale via a soft interface approach. This catalyst showed much higher photocatalytic activity than the famous commercial titania (Degussa P25) under visible light (λ>420 nm). The resulting sample was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), nitrogen adsorption, X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy, ¹H solid magic-angle spinning nuclear magnetic resonance (MAS-NMR) and photoluminescence spectroscopy. On the basis of characterization results, we found that the doping of chlorine resulted in red shift of absorption and higher surface acidity as well as crystal defects in the photocatalyst, which were the reasons for high photocatalytic activity of chlorine-doped TiO₂ under visible light (λ>420 nm). These hierarchical chlorine-doped rutile TiO₂ spherical clusters of nanorods are very attractive in the fields of environmental pollutants removal and solar cell because of their easy separation and high activity.     

TiO2/Gd2O3纳米粉体的制备、表征及光催化活性

利用酸催化的溶胶-凝胶法制备了纯TiO₂和Gd³⁺(0.5wt%)掺杂的TiO₂纳米粉体，采用XRD、BET、XPS、紫外-可见漫反射谱(DRS)和表面光电压谱(SPS)等技术进行了表征；以亚甲基蓝(MB)的光催化降解为探针反应，评价了其光催化活性；探讨了Gd³⁺掺杂对TiO₂纳米粉体的光催化活性的影响机制。结果表明，TiO₂/Gd₂O₃纳米粒子对MB溶液的光催化活性提高到纯TiO₂的1.5倍。掺杂Gd³⁺可以强烈抑制TiO₂由锐钛矿相向金红石相的转变；阻碍TiO₂晶粒的生长；提高高温组织稳定性，改善粉体的表面织构特性；形成光生电子的浅势捕获陷阱，抑制e^-/h⁺复合，这些因素共同作用最终导致TiO₂/Gd₂O₃纳米粉体的光催化活性明显提高。XPS分析结果证实，掺杂Gd³⁺导致粉体的表面羟基含量降低。由于产生了量子尺寸效应，复合粉体的紫外吸收带边蓝移，光的吸收能力略有降低。     

Synthesis of bimodal porous structured TiO2 microsphere with high photocatalytic activity for water treatment

In this work bimodal structured titanium dioxide (TiO₂) microsphere has been prepared from commercial TiO₂ powder and nano-sized titania gel via sol–gel spray-coating technique. Crystallization and transformation behavior of titania gel were investigated. The results revealed that the crystallization and transformation of anatase particles were substantially affected by the concentration of solvent and calcination temperature. Anatase crystallite size of 10 nm was obtained at mole ratio of solvent/precursor 50/1 and calcination temperature of 450 °C. The prepared nano-sized titania gel was embedded within the core (commercial TiO₂, P25) during the spraying process. The prepared TiO₂ microsphere was characterized using X-ray diffractometer (XRD), scanning electron microscope (SEM), field emission electron microscope (FESEM) and micropore analysis. The photocatalytic activity was monitored by following the degradation of phenol with activity benchmarked against commercial P25 (Degussa). The increase of photocatalytic activity of TiO₂ microsphere was attributed to the nano-sized anatase crystallite which has been incorporated into the TiO₂ microsphere.