Synthesis and photocatalytic activity for water-splitting reaction of nanocrystalline mesoporous titania prepared by hydrothermal method

Nanocrystalline mesoporous TiO₂ was synthesized by hydrothermal method using titanium butoxide as starting material. XRD, SEM, and TEM analyses revealed that the synthesized TiO₂ had anatase structure with crystalline size of about 8 nm. Moreover, the synthesized titania possessed a narrow pore size distribution with average pore diameter and high specific surface area of 215 m²/g. The photocatalytic activity of synthesized TiO₂ was evaluated with photocatalytic H₂ production from water-splitting reaction. The photocatalytic activity of synthesized TiO₂ treated with appropriate calcination temperature was considerably higher than that of commercial TiO₂ (Ishihara ST-01). The utilization of mesoporous TiO₂ photocatalyst with high crystallinity of anatase phase promoted great H₂ production. Furthermore, the reaction temperature significantly influences the water-splitting reaction.     

Improvement of the Visible‐Light Photocatalytic Performance of TiO2 by Carbon Mesostructures

An improvement in the photodegradation performance for dyes due to interaction between carbon and titania in a self‐assembled mesoporous C? TiO₂ composite catalyst, even for the difficult degradation of azo dyes, is reported herein. The dye removal process involves adsorption of the dye from water by the mesoporous carbon–titania, followed by photodegradation on the separated dye‐loaded solid. Such adsorption–catalysis cycles can be carried out more than 80 times without discernible loss of photocatalytic activity or the anatase content of the composite. In each run, about 120 mg dye per g catalyst can be degraded. The mesoporous carbon–titania catalyst also exhibits a high capacity for converting methyl orange in aqueous solution under visible light. Characterization by XRD, TEM, and N₂ sorption techniques has revealed that the self‐assembled composite catalyst has an ordered mesostructure, uniform mesopores (4.3 nm), a large pore volume (0.30 cm³ g^?1), and a high surface area (348 m² g^?1). The pore walls are composed of amorphous carbon and anatase nanoparticles of size 4.2 nm, which are well dispersed and confined. X‐ray photoelectron spectroscopy (XPS), surface photovoltage spectroscopy (SPS), and UV/Vis absorption results indicate doping of carbon into the anatase lattice and a change in the bandgap of the semiconductor. The synergistic improvement in the composite catalyst can be attributed to the following features: (1) carbon doping of the anatase lattice modifies its bandgap and enhances its activity under visible light; (2) confinement within carbon pore walls prevents aggregation of tiny anatase nanoparticles, improving their activity and stability; (3) the mesopores provide a confined space for photocatalysis; and (4) the strong adsorption ability of porous carbon for organic substances ensures that large quantities can be processed and inhibits further diffusion of the adsorbed organic substances, thereby enhancing the mineralization on anatase.     

The effects of the crystallization rate of the mesoporous TiO2 on the stability of the mesoporous structure after reflux

A simple synthetic method was employed to prepare mesoporous titania with anatase crystalline walls and high photocatalytic activity. The properties and structures of mesoporous titania were characterized by means of low angle and wide angle X-ray diffraction (XRD), Fourier transform (FT)-IR spectra, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and N₂ adsorption–desorption. The characteristic results clearly show that crystallization rate of the mesoporous titania affects the stability of the mesoporous structure after reflux, and that the anatase crystal in the mesoporous wall of mesoporous titania can stabilize the mesoporous structure. The photocatalytic activity of titania powder was evaluated from an analysis of the photodegradation of methyl orange under UV irradiation. The results indicate that the titania powder with mesoporous structure shows the highest photocatalytic activity.     

Novel synthesis of high-surface-area ordered mesoporous TiO2 with anatase framework for photocatalytic applications

Ordered mesoporous TiO₂ materials with an anatase frameworks have been synthesized by using a cationic surfactant cetyltrimethylammonium bromide (C₁₆TMABr) as a structure-directing agent and soluble peroxytitanates as Ti precursor through a self-assembly between the positive charged surfactant S⁺ and the negatively charged inorganic framework I^? (S⁺I^? type). The low-angle X-ray diffraction (XRD) pattern of the as-prepared mesoporous TiO₂ materials indicates a hexagonal mesostructure. XRD and transmission electron microscopy results and nitrogen adsorption–desorption isotherms measurements indicate that the calcined mesoporous TiO₂ possesses an anatase crystalline framework having a maximum pore size of 6.9 nm and a maximum Brunauer–Emmett–Teller specific surface area of 284 m² g^?1. This ordered mesoporous anatase TiO₂ also demonstrates a high photocatalytic activity for degradation of methylene blue under ultraviolet irradiation.     

Visible light photocatalyst: iodine-doped mesoporous titania with a bicrystalline framework

Iodine-doped (I-doped) mesoporous titania with a bicrystalline (anatase and rutile) framework was synthesized by a two-step template hydrothermal synthesis route. I-doped titania with anatase structure was also synthesized without the use of a block copolymer as a template. The resultant titania samples were characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infrared, nitrogen adsorption, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-visible absorption spectroscopy. Both I-doped titania samples, with and without template, show much better photocatalytic activity than commercial P25 titania in the photodegradation of methylene blue under the irradiation of visible light (>420 nm) and UV-visible light. Furthermore, I-doped mesoporous titania with a bicrystalline framework exhibits better activity than I-doped titania with anatase structure. The effect of rutile phase in titania on the adsorptive capacity of water and surface hydroxyl, and photocatalytic activity was investigated in detail. The excellent performance of I-doped mesoporous titania under both visible light and UV-visible light can be attributed to the combined effects of bicrystalline framework, high crystallinity, large surface area, mesoporous structure, and high visible light absorption induced by I-doping.     

Aerosol-assisted fabrication of mesoporous titania spheres with crystallized anatase structures and investigation of their photocatalitic properties

We demonstrate practical aerosol-assisted approach to synthesize spherical mesoporous titania particles with high surface areas. Scanning electron microscopy observation of the spray-dried products clearly shows spherical morphology. To remove surfactants and enhance crystallinity, the spray-dried products are calcined under various temperatures. The crystalline structures inside the particles are carefully detected by wide-angle XRD measurements. With increase of the calcination temperatures, anatase crystal growth proceeds and transformation from anatase to rutile is occurred. The effect of various calcination temperatures on the mesostructures is also studied by using N₂ adsorption desorption isotherms. The mesoporous titania particles calcined at 350, 400, and 500 °C exhibit type IV isotherms with a capillary condensation step and shows a hysteresis loop, which is a characteristic of mesoporous materials. The reduction in the surface areas and the pore volumes is confirmed by increasing the calcination temperatures, while the average pore diameters are increased gradually. This is attributed to the distortion of the mesostructures due to the grain growth of the anatase phase and the transformation to the rutile phase during the calcination process. As a preliminary experimental photocatalytic activity, oxidative decomposition of acetaldehyde under UV irradiation is examined. The mesoporous titania calcined at 400 °C shows the highest photocatalytic activity, due to both high surface area and well-developed anatase crystalline phase.     

New method to synthesize mesoporous titania by photodegradation of surfactant template

Mesoporous titania has been successfully synthesized by photodegradation removal of cetyltrimethylammonium bromide as the surfactant, after slow hydrolyzation of titanium(IV) isopropoxide. Fourier transform infrared spectra proved that photodegradation has successfully decreased the peak areas of the alkyl groups from the template. The nitrogen adsorption analysis showed that the pore size and the specific surface area of the mesoporous titania were 3.7 nm and 203 m² g⁻¹, respectively, proving the mesoporosity of the titania obtained with the existence of the interparticle mesoporosity which was confirmed by transmission electron microscopy. Based on X-ray diffraction results, the mesoporous titania obtained was in the form of crystalline anatase phase. Furthermore, results from the diffuse reflectance ultra violet-visible spectra showed that the composition of tetrahedral titanium(IV) was more than the octahedral titanium(IV). When the mesoporous titania obtained was used as a catalyst in the oxidation of styrene, an improvement in the conversion of styrene (38%) was observed when compared to those obtained using Degussa P25 TiO₂ (14%) as the catalyst.     

Reduced Graphene Oxide‐Supported TiO2 Fiber Bundles with Mesostructures as Anode Materials for Lithium‐Ion Batteries

Although the synthesis of mesoporous materials is well established, the preparation of TiO₂ fiber bundles with mesostructures, highly crystalline walls, and good thermal stability on the RGO nanosheets remains a challenge. Herein, a low‐cost and environmentally friendly hydrothermal route for the synthesis of RGO nanosheet‐supported anatase TiO₂ fiber bundles with dense mesostructures is used. These mesostructured TiO₂‐RGO materials are used for investigation of Li‐ion insertion properties, which show a reversible capacity of 235 mA h g^?1 at 200 mA g^?1 and 150 mA h g^?1 at 1000 mA g^?1 after 1000 cycles. The higher specific surface area of the new mesostructures and high conductive substrate (RGO nanosheets) result in excellent lithium storage performance, high‐rate performance, and strong cycling stability of the TiO₂‐RGO composites.     

Hierarchically Organized Silica–Titania Monoliths Prepared under Purely Aqueous Conditions

Hierarchically organized silica–titania monoliths were synthesized under purely aqueous conditions by applying a new ethylene glycol‐modified single‐source precursor, such as 3‐[3‐{tris(2‐hydroxyethoxy)silyl}propyl]acetylacetone coordinated to a titanium center. The influence of the silicon‐ and titanium‐containing single‐source precursor, the novel glycolated organofunctional silane, and the addition of tetrakis(2‐hydroxyethyl)orthosilicate on the formation of the final porous network was investigated by SEM, TEM, nitrogen sorption, and SAXS/WAXS. In situ SAXS measurements were performed to obtain insight into the development of the mesoporous network during sol–gel transition. IR‐ATR, UV/Vis, XPS, and XAFS measurements showed that up to a Si/Ti ratio of 35:1, well‐dispersed titanium centers in a macro‐/mesoporous SiO₂ network with a specific surface area of up to 582 m² g^?1 were obtained. An increase in Ti content resulted in a decrease in specific surface area and a loss of the cellular character of the macroporous network. With a 1:1 Si/Ti ratio, silica–titania powders with circa 100 m² g^?1 and anatase domains within the SiO₂ matrix were obtained.     

Acetic Anhydride as an Oxygen Donor in the Non-Hydrolytic Sol–Gel Synthesis of Mesoporous TiO2 with High Electrochemical Lithium Storage Performances

An original, halide-free non-hydrolytic sol–gel route to mesoporous anatase TiO₂ with hierarchical porosity and high specific surface area is reported. This route is based on the reaction at 200 °C of titanium(IV) isopropoxide with acetic anhydride, in the absence of a catalyst or solvent. NMR spectroscopic studies indicate that this method provides an efficient, truly non-hydrolytic and aprotic route to TiO₂. Formation of the oxide involves successive acetoxylation and condensation reactions, both with ester elimination. The resulting TiO₂ materials were nanocrystalline, even before calcination. Small (about 10 nm) anatase nanocrystals spontaneously aggregated to form mesoporous micron-sized particles with high specific surface area (240 m² g⁻¹ before calcination). Evaluation of the lithium storage performances shows a high reversible specific capacity, particularly for the non-calcined sample with the highest specific surface area favouring pseudo-capacitive storage: 253 mAh g⁻¹ at 0.1 C and 218 mAh g⁻¹ at 1 C (C=336 mA g⁻¹). This sample also shows good cyclability (92 % retention after 200 cycles at 336 mA g⁻¹) with a high coulombic efficiency (99.8 %). Synthesis in the presence of a solvent (toluene or squalane) offers the possibility to tune the morphology and texture of the TiO₂ nanomaterials.     

Structurally Stabilized Organosilane‐Templated Thermostable Mesoporous Titania

Structurally thermostable mesoporous anatase TiO₂ (m‐TiO₂) nanoparticles, uniquely decorated with atomically dispersed SiO₂, is reported for the first time. The inorganic Si portion of the novel organosilane template, used as a mesopores‐directing agent, is found to be incorporated in the pore walls of the titania aggregates, mainly as isolated sites. This is evident by transmission electron microscopy and high‐angle annular dark field scanning transmission electron microscopy, combined with electron dispersive X‐ray spectroscopy. This type of unique structure provides exceptional stability to this new material against thermal collapse of the mesoporous structure, which is reflected in its high surface area (the highest known for anatase titania), even after high‐temperature (550 °C) calcination. Control of crystallite size, pore diameter, and surface area is achieved by varying the molar ratios of the titanium precursor and the template during synthesis. These mesoporous materials retain their porosity and high surface area after template removal and further NaOH/HCl treatment to remove silica. We investigate their performance for dye‐sensitized solar cells (DSSCs) with bilayer TiO₂ electrodes, which are prepared by applying a coating of m‐TiO₂ onto a commercial titania (P25) film. The high surface area of the upper mesoporous layer in the P25–m‐TiO₂ DSSC significantly increases the dye loading ability of the photoanode. The photocurrent and fill factor for the DSSC with the bilayer TiO₂ electrode are greatly improved. The large increase in photocurrent current (ca. 56 %) in the P25–m‐TiO₂ DSSC is believed to play a significant role in achieving a remarkable increase in the photovoltaic efficiency (60 %) of the device, compared to DSSCs with a monolayer of P25 as the electrode.     

Synthesis of Hollow Mesoporous TiO2 Microspheres with Single and Double Au Nanoparticle Layers for Enhanced Visible‐Light Photocatalysis

A facile method was used to prepare hollow mesoporous TiO₂ and Au@TiO₂ spheres using polystyrene (PS) templates. Au nanoparticles (NPs) were simultaneously synthesized and attached on the surface of PS spheres by reducing AuCl₄^? ions using sodium citrate which resulted in the uniform deposition of Au NPs. The outer coating of titania via sol‐gel produced PS@Au@TiO₂ core–shell spheres. Removing the templates from these core–shell spheres through calcination produced hollow mesoporous and crystalline Au@TiO₂ spheres with Au NPs inside the TiO₂ shell in a single step. Anatase spheres with double Au NPs layers, one inside and another outside of TiO₂ shell, were also prepared. Different characterization techniques indicated the hollow mesoporous and crystalline morphology of the prepared spheres with Au NPs. Hollow anatase spheres with Au NPs indicated enhanced harvesting of visible light and therefore demonstrated efficient catalytic activity toward the degradation of organic dyes under the irradiation of visible light as compared to bare TiO₂ spheres.     

Sorption of Eu(III) by amorphous titania, anatase and rutile

Sorption of Eu(III), an analogue of trivalent actinides (Am, Cm), by amorphous titania as well as different crystalline phases of titania, namely anatase and rutile, have been studied as a function of pH, using ¹⁵⁴Eu (half life?=?8.8 yrs, E_???=?123,247?keV) as a radiotracer. The objective of this study was to investigate the effect of the crystalline phase of the titania on their sorption behaviour towards the metal ion. Amorphous titania was prepared by organic route and was converted into anatase and rutile by heating at elevated temperatures based on the differential thermal analysis studies. Eu(III) sorption by all forms of titania rises sharply with the pH of the suspension, with the sorption edge shifting to higher value in the order; amorphous?<?anatase?<?rutile. However, the normalization of the sorption data to the surface area of the sorbents resulted in the overlapping of the sorption curves for amorphous and anatase phases, with the data being higher for rutle in the lower pH region, indicating the effect of the crystal phase on sorption behaviour of Eu(III).     

Study of photocatalytic activity of Cd-doped mesoporous nanocrystalline TiO2 prepared at low temperature

Mesoporous nanocrystalline Cd-doped titania was firstly prepared at low temperature by a modified sol–gel method, using dodecylamine as a template. The template could be easily removed by refluxing samples in nitric acid ethanol solution. The Fourier transform infrared spectrometer (FT-IR), low-angle and wide-angle X-ray diffraction (XRD), N₂ adsorption–desorption, transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), and UV–visible diffuse reflectance spectroscopy were used for the characterization of catalysts. The characteristic results clearly showed that Cd²⁺ ions were doped into the titania lattice, and the mesoporous architecture of Cd-doped TiO₂ was composed of mixed-phase crystal textures of anatase and brookite. The samples displayed high visible-light photocatalytic activity for photodegradating 2,4-dichlorophenol (2,4-DCP) solution. The high activities of samples were attributed to the bicrystalline framework, large BET surface area, small crystallite size, and Cd-doping.     

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.     

锐钛相虫蛀状介孔二氧化钛的表征

0IntroductionMesostructuredtitania(TiO2)hasattractedagreatdealofattention,inviewofcontrolleddelivery,catalytic,photocatlytic,orenergyconversionapplica-tions犤1犦.Theuseoftitaniumisopropoxidebis-acetylace-tonateprecursors,combinedwithalkylphosphatean-ionicsurfactantsastemplate,ledtothefirstdocu-mentedpuremesoporousTiO2犤2犦.Insubsequentyears,moresyntheticstrategieshavebeendevelopedusingavarietyofstructure-directingagents,forexample,car-boxylicacids犤3犦,alkylamine犤4犦,aminium犤5犦,blockcop-o…     

含 TiO2(B) 介孔氧化钛材料的制备、特性和应用

 综述了近年来本课题组依据材料化学工程研究思想, 对含 TiO2(B)(一种比金红石和锐钛矿相结构更松散的氧化钛晶型) 介孔氧化钛材料在制备、结构和性能方面所取得的研究进展. 该介孔材料由二钛酸钾经水合、离子交换和热处理得到, 具有良好原子尺度晶格匹配界面特征的锐钛矿和 TiO2(B) 核壳结构. 研究表明, 该介孔材料在兼备高比表面积、高晶化孔壁和高热稳定性的同时, 还表现出良好的纳米颗粒担载稳定性, 在光催化、油品加氢精制、药物载体、固体酸催化和电化学电容器等方面已凸显出良好的应用潜力和推广价值. 目前该新型含 TiO2(B) 介孔氧化钛材料已经实现低成本、规模化制备.     

Spiky Mesoporous Anatase Titania Beads: A Metastable Ammonium Titanate‐Mediated Synthesis

A complex titania nanostructure of monodisperse spiky mesoporous anatase beads composed of anatase nanocrystals with diameters of less than 15 nm in the core and much larger hollow‐cone shaped spikes on the surface was fabricated using a facile solvothermal process in the presence of ammonia. This proceeded through a controllable phase transformation from an amorphous titania to a metastable amorphous titania/ammonium titanate core‐shell structure then finally to anatase titania. The size of the spiky anatase nanostructures can be increased from approximately 55×100 nm to 160×410 nm (square edge×length) by increasing the ammonia concentration used in the solvothermal treatment step from 2.2 to 17.4 wt. %. Such hollow‐cone shaped nanostructures, as revealed by HRTEM characterization, are single crystals elongated along the c axis of the tetragonal anatase titania. The resultant spiky titania beads have high surface areas of up to 112 m²g^?1 and pore diameters and pore volumes that vary depending on the ammonia concentration and solvothermal treatment time. The morphological evolution and crystallization process of the spiky titania beads was investigated using SEM and XRD techniques. A metastable amorphous titania/ammonium titanate core‐shell structure evolved from the smooth amorphous precursor beads producing a “fluffy” titanate intermediate, on further heating the final spiky mesoporous titania beads were clearly observed. This titanate‐phase‐mediated approach allows control over the size of the nanocrystals in the core of the bead, as well as the anatase spikes on the surface, and thereby, tuning of the surface area and porosity of the resultant products. The spiky mesoporous titania beads have been used to prepare working electrodes for dye‐sensitized solar cells achieving a solar to electric power conversion efficiency of 10.30 %, indicating their potential for application in the photovoltaic field. Such complex titania nanostructures would have a number of other possible applications, such as photocatalysis, lithium ion batteries, and catalysis.     

The effect of F-doping and temperature on the structural and textural evolution of mesoporous TiO2 powders

Mesoporous F⁻-doped TiO₂ powders were prepared by hydrolysis of titanium tetraisopropoxide (TTIP) in a mixed NH₄F-H₂O solution. Effects of F⁻ ion content and calcination temperatures on the phase composition and porosity of mesoporous titania were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and BET surface areas. The results showed the BET surface area (S_BET) of the pure and doped powders dried at 100°C ranged from 260 to 310 m²/g as determined by nitrogen adsorption. With increasing calcination temperatures, the S_BET values of the calcined titania powders decreased due to the increase in crystalline size. The pore size distribution was bimodal with fine intra-particle pore and larger inter-particle pore as determined by nitrogen adsorption isotherms. The peak pore diameter of intra-particle pore increases with increasing F⁻ ion content. At 700°C, all the titania powders exhibit monomodal pore size distributions due to the complete collapse of the intra-particle pores. The crystallization of anatase was obviously enhanced due to F⁻-doping at 400°C and 500°C. Moreover, with increasing F⁻ ion concent, F⁻ ions not only suppressed the formation of brookite phase at low temperature, but also prevented phase transition of anatase to rutile at high temperature.     

Application of synthesized mesoporous titania microspheres for arsenic [(III) and (V)] uptake studies

The present work describes the application of radiotracer technique for studying uptake of arsenic on titanium hydroxide, commercial titanium dioxide (TiO₂) powder (anatase) and synthesized mesoporous titania beads in acidic, neutral, and alkaline conditions. Sol–gel templating method was used to prepare titania–polysaccharide composites, with different polymer contents. Mesoporous titania was obtained by heat treatment of the composite beads in a controlled environment. The synthesis process was optimized, using thermogravimetry analysis. X-ray diffraction patterns confirmed the formation of anatase pure phase titania (TiO₂) at 700 °C in different environments, and scanning electron microscopy studies confirmed uniform pore size distribution. The effect of surface area, polymer content and pH on uptake of arsenic(III) and (V) on the synthesized titania beads was also investigated. Arsenic(V) was found to be retained quantitatively on the titania beads synthesized from 0.8% polymer content titania–polymer composite precursor in neutral to alkaline conditions. Details of the results obtained are discussed.