Growth and Phase Transformation of Nanometer-Sized Titanium Oxide Powders Produced by the Precipitation Method

We report an in situ TEM investigation of the growth and transformation in nanometer-sized titania powders. The powders were produced through precipitation of titanium tetrachloride under different pH conditions. The initial phase of the produced powders was amorphous or was a mixture of anatase and brookite according to the pH conditions. During calcination, the anatase particles grew and transformed into rutile. The transformation temperature increased with increasing pH value. In situ TEM observations showed that the anatase particles were absorbed into rutile, and then rutile particles grew by coalescence. Furthermore, small pores were observed to form in samples prepared with high pH from the effects of hydroxyl ions and zeta potential. Pore formation increased the surface area, which delayed the transformation and nucleation of rutile. This explains the difference of growth and transformation of titania powders produced under different pH conditions during calcination.     

Synthesis of Nitrogen-doped Titania by Solvothermal Reactions in Alcohols

Nitrogen-doped titania nanoparticles were obtained by the homogeneous precipitation in hexamethylenetetramine- titanium trichloride-alcohol aqueous solutions at 90℃ followed by heating at 190℃. Anatase, rutile and brookite were obtained, where the crystallite size, specific surface area and color greatly changed as 5~50 nm, 20~200 m2/g and light gray to yellow, depending on the solvent and pH. The products after calcination were yellow, indicating doping with nitrogen ion. All colored titania showed photocatalytic activity under visible light irradiation for the oxidative decomposition of nitrogen monoxide in air. Especially, the nanoparticles of anatase type nitrogen-doped titania obtained using methanol aqueous solution showed excellent photocatalytic activity.     

Effect of brookite phase on the anatase–rutile transition in titania nanoparticles

Nanosized TiO₂ powders were prepared from the precipitation in the TiCl₄ precursor under various pH values. The prepared titania existed in the form of nanocrystalline anatase with some brookite, which was evidenced by X-ray diffraction analysis and Raman spectroscopy. The average crystallite sizes of the TiO₂ particles heat treated at 450 °C for 2 h are in the range of 7–9 nm. The lattice constant c of anatase increased with increasing the synthesized pH value, whereas the volume fraction of the brookite phase increased with decreasing the synthesized pH value. The beginning and ending temperatures for the anatase–rutile transformation were found to decrease with increasing the volume fraction of the brookite phase. The brookite phase in the powder is responsible for enhancing the anatase–rutile transition.     

Tailoring high-surface-area nanocrystalline TiO2 polymorphs for high-power Li ion battery electrodes

The crystallization and morphology of brookite and anatase titania (TiO₂) were controlled using the urea-mediated hydrolysis/precipitation route in the presence of the Ti³⁺ ions. Without the strong complexing agents and the non-hydrothermal conditions, simple alterations to the urea concentration led to the synthesis from brookite nanorods to anatase nanoflowers at a low temperature below 100 °C, whereas the BET specific surface area evolved from 102 to 268 m² g⁻¹, respectively. A possible formation mechanism was also proposed for these TiO₂ nanostructures. The excellent reversible capacity and rate capability were achieved for the anatase nanoflowers because of the small crystallite size and significantly large surface area.     

Photocatalytic degradation of methanol on titania and titania–silica aerogels prepared by non-alkoxide sol–gel route

Titania and titania–silica aerogels were prepared by alkoxide or non-alkoxide sol–gel route and subsequent supercritical drying with carbon dioxide at low temperature. The resulting aerogels having high surface area and mesoporosity were used as photocatalysts for gas phase methanol degradation reaction. Photocatalytic degradation reactions were carried out on titania and titania–silica aerogels, and commercial Degussa P-25 titania. The photocatalytic activities of titania and titania–silica aerogels were higher than that of the P-25. While the conversion of methanol degradation over the P-25 catalyst was only 50–60%, that for the titania aerogel was observed to be above 98% due to the higher specific surface area and the well developed mesoporous structure. In spite of lower titania contents, much higher surface area and high dispersion of titania of titania–silica aerogel gave rise to the high photocatalytic activity in comparison to those of titania aerogels. Moreover, titania–silica aerogel was also used for the photodegradation and adsorption hybrid system. It was observed that the high removal efficiency for methanol was caused by the combination of higher catalytic activity and adsorption capacity.     

TiO_2纳米晶和薄膜的制备及微观性质研究

以钛酸四丁酯为前驱体,采用溶胶-凝胶法制备了TiO2纳米晶和薄膜。分别用X射线衍射(XRD)和差热-热重分析(DTA-TGA)对不同温度热处理的样品进行了表征。XRD分析表明:经过热处理的样品,锐钛矿相TiO2纳米晶平均粒子尺寸为5 3～32 4nm,其薄膜的平均粒子尺寸为6 7～29 6nm,但经相同温度热处理的TiO2纳米晶平均粒子尺寸普遍较薄膜样品大,此外,TiO2薄膜的相转变温度比纳米晶约高150℃,且其锐钛矿相(101)面上具有一定的择优取向,取向因子为0 53;DTA-TGA结果表明,360～500℃,TiO2凝胶从非晶态-锐钛矿相的转变是逐渐转变的过程;实验所制备样品的比表面积可达72 57m2/g,呈现出巨大的表面效应;同时光生载流子能在极短的时间内(<20ps)跃迁到TiO2纳米颗粒表面,有利于电子和空穴的分离。     

Mesoporous characteristics of crystalline indium-titania synthesized by the sol-gel route

Mesoporous nanocrystalline titanium dioxide with narrow pore size distribution was prepared by the sol-gel technique. During synthesis, the gels were doped with indium salt incorporation in the early gelation stage. The presence of indium had an evident influence on the phase transition of TiO₂ and the nucleation process. Crystal sizes were estimated by X-ray diffraction (XRD). Mesoporous structures in indium-doped titania were maintained after heat treatment at 400 and 600 °C for 4 h, exhibiting significant thermal stability. The brookite phase was observed in samples that underwent phase transition. In this paper, we have studied the relationship between textural properties and phase transition, discussing the possible role of average crystal size.     

Photocatalytic performance of highly transparent and mesoporous molybdenum-doped titania films fabricated by templating cellulose nanocrystals

In this paper, the synthesis of mesoporous Mo-doped titania films templated by cellulose nanocrystals (CNCs) and their photocatalytic performance are reported for the first time. The prepared titania composite precursors containing the CNCs and molybdenum chloride were spin-coated on indium tin oxide (ITO) glass substrate, followed by calcining at 400?°C for 1?h. X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscope (SEM), and UV–vis spectrometer were employed to characterize the phase composition, pore structure, morphology, and optical property of the titania films in relation to CNCs templating and Mo doping. Photocatalytic performances of the titania films were also evaluated on the photodegradation of trichloroethylene under a fluorescent light source. The Mo-doped titania films with CNCs templating were highly transparent and mesoporous, exhibiting only anatase phase, high specific surface areas ranging in 135.4 – 149.0?m²/g, and small crystallite sizes of 9.5 – 11.1?nm. The results indicate that Mo ions were successfully doped by substituting for Ti ions in the titania lattice. The Mo doping stabilized the anatase phase and also increased the surface area of the CNCs-templated titania film while decreasing the mean pore width. Notably, the visible light absorption capacity and photocatalytic activity of the CNCs-templated titania films doped with Mo were dramatically greater than those of the pure and the CNCs-templated titania films, which is ascribed to the decreased recombination rate of photoexcited charges and the increased surface area with aids of the CNCs templating and the Mo doping.     

Morphology of titania coatings on silica gel

Control of the hydrolysis and condensation of soluble precursors of titanium is shown to yield oxide coatings of well-defined morphology. A ``smooth' coating, consisting of patches of titania on silica, causes only a small increase in the surface area, from 130 m<sup>2</sup>/g for the uncoated silica gel to 154 m<sup>2</sup>/g. On the other hand, the ``rough' coating, which consists of 3 nm diameter titania particles dispersed over the silica surface, results in an increase in the BET surface area to 350 m²/g. The specific titania surface area was determined via isopropanol dehydration activity and was found to be comparable to the BET surface area indicating that the titania phase is responsible for the increase in surface area. Small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) provide definitive evidence for the coating morphology while X-ray photoelectron spectroscopy (XPS) was used to determine the dispersion of the titania. The supported titania maintains its surface area upon calcination at temperatures up to 873 K while unsupported titania looses much of its surface area. Also demonstrated in this work is a novel preparation method using titanium bis-ammonium lactato dihydroxide (TALH), a water soluble precursor. This revised version was published online in July 2006 with corrections to the Cover Date.     

Visible-light-induced photocatalytic activity of TiO2−xNy prepared by solvothermal process in urea–alcohol system

Nitrogen-doped anatase, rutile and brookite titania photocatalyst TiO_2−xN_y which can be excited by visible light were prepared by mixing aqueous TiCl₃ solutions with urea ((NH₂)₂CO) and various type of alcohols followed by solvothermal treatment at 190 °C. The phase composition, crystallinity, microstructure and specific surface area of titania powders greatly changed depending on the pH and type of solvents. Violet, yellowish and grayish TiO_2−xN_y with excellent visible light absorption and photocatalytic activity were prepared. The TiO_2−xN_y powders prepared in urea–methanol solution showed excellent photocatalytic ability for the oxidative destruction of nitrogen monoxide under irradiation of visible light λ > 510 nm.     

Effect of calcination temperature and addition of silica, zirconia, alumina on the photocatalytic activity of titania

Nanophase titania was prepared by sol-gel method and spray pyrolysis. We tried to elucidate the relationship between the photoactivity and the crystallite size of anatase phase. To better understand the changes in the bulk and the surface of titania as the calcination temperature is changed, EPR and photoluminescence analysis were carried out. The effect of the secondary metal oxide embedded into titania matrix on the photoactivity was also investigated. It was found that the photoactivity of titania has a linear relationship to the crystallite size. For the analysis of EPR and photoluminescence for pure titania, the increase of photoactivity with increasing the calcination temperature is due to the formation of surface active sites such as O^- as well as the increase of crystallinity resulting from the removal of bulk defects. For silica/titania mixed oxide, it was found that the improvement of the thermal stability of anatase phase is important to enhance the photoactivity of titania because the prepared catalyst was calcined at a higher temperature than 700 °C without forming rutile phase. It was also concluded that the simultaneous increase of the surface area and the crystallinity promises to improve the photoactivity achieved by increasing the content of silica up to 60%. By the analysis of EPR and photoluminescence, it was found that the embedding of silica into titania matrix suppresses the formation of Ti³⁺ and produces a new active site of Ti-O-Si, which easily interacts with the oxygen. In the investigation of zirconia/titania and alumina/titania mixed oxide, it was found that the increase of the surface OH is essential to positively affect of the improved thermal stability on the photoactivity.     

Synthesizing and Comparing the Photocatalytic Properties of High Surface Area Rutile and Anatase Titania Nanoparticles

Rutile titania nanocrystalline particles with high specific surface areas were directly prepared by thermal hydrolysis of titanium tetrachloride aqueous solution. The as-prepared rutile titania powder was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller surface area analysis, and Fourier transform Raman and IR spectroscopies. Neither anatase nor amorphous titania could be detected in this titania powder by XRD, Raman spectroscopy, and high-resolution TEM. In the phenol degradation reaction, the rutile titania powder with an initial crystalline size of 7 nm was found to have higher photocatalytic activity than that of anatase titania with the same specific surface area. The rutile titania powders calcined at 300° and 450°C also showed a relatively high photocatalytic property. The high activity of the as-prepared rutile titania was attributed to the abundance of hydroxy groups in the powder, as was proven by thermogravimetric analysis data, which provided more active sites for the degradation reaction.     

Influence of temperature schedules on particle size and crystallinity of titania synthesized by vapor-phase oxidation route

Crystalline TiO₂ particles were produced in a tubular flow reactor by chemical vapor synthesis using titanium tetrachloride as a starting material in oxygen containing atmospheres. The dependence of particle size, morphology and crystalline phase of titania on temperature schedules including the reactor temperature, the oxygen preheated temperature and the product cooling measure were explored. It is found that there are two opposite effects of temperature on particle size and crystalline phase content. The particle size distribution, SEM and TEM of resulting powders show that the grain size is controlled by the relative magnitudes of the nucleation rate and growth rate, both of them being subject to the temperature schedules. XRD indicates that particles crystalline phase is predominately anatase and the rutile content increment is not consistent with temperature increase. Anatase titania can be converted to rutile by addition of crystal modifier AlCl₃. The element analysis by EDS shows that Al enriches on the particle outer surface.     

Monodispersed Spherical Particles of Brookite-Type TiO2: Synthesis, Characterization, and Photocatalytic Property

Phase-pure brookite of high crystallinity, which was classically obtained via hydrothermal treatment, has been synthesized under ambient pressure at 70°C via reacting a mixed solution of urea and titanium (III) chloride (instead of the widely used titanium (IV) compounds). The resultant particles are monodispersed spheres (∼154 nm) composed of brookite nanocrystals (∼25 nm), which are stable in terms of phase purity and morphology up to ∼500°C, above which a direct transition to rutile occurred. The as-made powder has a high specific surface area of ∼41.2 m²/g, which rapidly decreased to ∼9.7 m²/g after transforming to rutile at 700°C. The brookite powder shows good catalytic property for the decomposition of acetaldehyde under UV radiation.     

Brookite and anatase nanomaterial polymorphs of TiO2 synthesized from TiCl3

Synthesis of anatase and brookite was achieved under mild conditions in aqueous solution. In addition, mixtures of brookite and rutile as well as mixtures of the anatase and rutile polymorphs were observed at different temperatures. It was observed that temperatures above 80 °C produced anatase and brookite phases, exclusively. The samples prepared using the acetate synthesis showed on average bandgap around 2.95–3.0 eV and brookite was observed. Whereas the samples prepared using the sulfate synthesis showed and bandgap from 3.1 to 3.2 eV and anatase was observed. In addition, the average grain size of the brookite and anatase phase synthesized at 100 °C were 9.7 and 12 nm, respectively, as determined from XRD.     

Drying processes for preparation of titania aerogel using supercritical carbon dioxide

Supercritical carbon dioxide drying was performed for the preparation of titania aerogels from sol–gel routes. The conditions of supercritical carbon dioxide drying were 313–323 K and 7.8–15.5 MPa. The solvents in titania wet gels obtained from the sol–gel routes were replaced by acetone. The titania aerogels obtained from supercritical carbon dioxide drying form needle-like structures. In supercritical carbon dioxide drying, the extraction rates of acetone from the wet gels were measured by using an on-line Fourier transform infrared spectroscope. It was found that the titania aerogels with lower cohesion were induced from the formations of homogenous phase for carbon dioxide + acetone system and the lower extraction rates of acetone. Furthermore, titania films were prepared by the depositions of the titania aerogels on ITO-coated PET substrates. The needle-like aerogels with lower cohesion derive the titania film with high surface area.     

Dopants for synthesis of stable bimodally porous titania

Bimodally porous titania powders doped with alumina, zirconia, and silica were made by wet precipitation from organometallic precursors (for Al/Ti=0.05-0.4, and Zr/Ti=Si/Ti=0.1). Doping retards not only the anatase-to-rutile phase transformation, but also the crystallite growth of titania. So it was used to control the powder phase composition and pore structure at high temperatures. The extent of the retarding effect on pore structure and phase transformation increased with increasing alumina concentration. The effectiveness of these dopants follows the order of: zirconia>silica>alumina. The dopants also reduce the loss of surface area of the calcined powders by decreasing the sintering and phase transformation rates. All powders exhibited bimodal pore size distributions (PSD) with fine intra-particle pores (1–4 nm) and larger inter-particle pores (10–120 nm). However, the intra-particle pores of the pure titania disappeared at 600°C, while the bimodal PSD of doped titania was maintained up to 750°C.     

Phase content controlled TiO2 nanoparticles using the MicroJetReactor technology

In this paper, a method for the continuous preparation of nanoscaled titania with controlled phase content is presented. The method bases on the MicroJetReactor technology. The synthesis process was carried out by using the hydrolysis of titanium tetraethylate (TET). Synthesis with flow rates to 14 ml/min are implemented, and temperatures are varied between 20 and 210 °C. Particle size distribution measurements by dynamic light scattering (DLS) show monomodal particle size distributions from 1 to 10 nm, stable for more than 24 h. There is no correlation between hydrolysis temperature and the particle size distributions.XRD (X-ray diffractometry) investigations showed, that crystal structures of anatase, brookite, rutile and an amorphous content can be detected in all samples. Quantitative analysis using the Rietveld refinement shows a significant effect of the synthesis temperature on the phase content. The relative phase content of anatase can be raised from 40 wt% up to 75 wt%, accompanied by a loss of all other phases.     

A method for measuring the titania surface area on mixed oxides of titania and silica

We have used temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) of 2-propanol to characterize several silica, titania, and silica-supported titania samples. Upon evacuation, most of the 2-propanol desorbed intact from the silica samples. This is in contrast to the results on titania and on silica-supported titania, where a significant amount remained on the surface following evacuation, with a fraction of this reacting to propene and water. The coverages of 2-propanol are approximately proportional to the titania surface area, corresponding to between 2.4 and 6.1×10¹⁸ molecules/m² of titania, depending on the form of the titania. The results suggest that selective adsorption of 2-propanol may be useful for determining the surface area of titania in titania-silicates.     

Preparation, Characterization of Sulfur-Doped Nanosized TiO2 and Photocatalytic Degradation of Methylene Blue Under Visible Light

Sulfur-doped TiO₂ catalysts were prepared by a mechanochemical method. The prepared catalysts exhibited photocatalytic activity in methylene blue degradation under visible light. The catalyst structure has been characterized using UV–visible spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry analysis (TGA) as well as Fourier transform infrared spectroscopy (FT-IR). UV–visible spectroscopy revealed that the absorption edge of the doped TiO₂ was red-shifted compared with bare TiO₂. XRD patterns suggested that the brookite phase became more prevalent with increasing ball milling duration. In addition, surface sulfate species were detected by FT-IR, XPS and TGA. We deduce the rise of catalytic activity is due to the synergetic effect between the brookite phase and the anatase phase that would probably retard the electron–hole recombination. On the other hand, methylene blue was found to be N-demethylated during the irradiation thus giving rise to blue-shifting of peak at 664 nm in UV–visible spectroscopy.