Preparation And Performance Studying Of The TiO₂-Based Nano-Structured Materials

TiO2-based semiconductor nano-materials have a broad application prospects in such fields as photocatalysis, photoelectric conversion. The mesoporous TiO2 materials have many benefits, for instance, well-defined pore structure, high specific surface area, tunable pore size and mesostructure, high photo-electronic conversion ability. At present, much work is focused on the controlling of microstructure of the mesoporous TiO2 materials. However, few efforts are devoted in the thermal stability of the mesostructured TiO2 materials. The layered H2Ti4O9 is a special kind of TiO2 material with crystal waters between its layers. Like the mesoporous TiO2 materials, the layered H2Ti4O9 has high specific surface area, which permits it to apply in catalysts and carriers. Generally, the preparation of layered H2Ti4O9 is carried out through soft-chemical methods. These methods have some drawbacks, for instance, an excessively long reaction time is required, additionally, much organic materials are needed to help to exfoliate the layers.Using triblock copolymer Pluronic F127 (EO106PO70EO106) as the templating agent, Ti(OBun)4 as the inorganic precusror, mesostructured TiO2 thin films with large mesopores (7.4nm), thick pore walls (10-13nm), and high thermal stability (up to 600oC) were constructed through sol-gel combined with evaporation-induced self-assembly (EISA) methods for the first time. The long hydrophilic PEO block and hydrophobic PPO block of the F127 are the premises for the formation of thick pore wall and large pore size in the TiO2 network. In the synthesis process, anhydrous and strong acidic media, and acetylacetone were taken to controll the hydrolysis/condensation of the precursor, which favored the the formation of small Ti-O oligomers. The controlling of aging conditions, such as relative humidity (about 45%RH), aging time (about 24 h) can make the Ti-O oligomers and F127 micells assembly adequately. The cooperation of three factors above contributed to the formation of mesoporous TiO2 with thick walls, which deliveres the high thermal stability of the TiO2 mesostructure.We have investigated systematically the effect of different calcination temperatures on the mesostructure, photocatalytic activity, and photo-induced hydrophilicity of the mesoporous TiO2 thin films. The results showed that when the calcination temperatures increased from 450℃to 600℃, the synthesized samples have the pore diameters more than 7 nm, indicated that the excellent thermal stability of the obtained TiO2 mesostructure. At 700℃, the mesostructure was destructed. The analysis results indicated that the destruction of the mesoporous TiO2 thin films was induced by the cooperation of dynamics and thermodynamics effect, which resulted in the grain's growth and rearrangement and the contraction of the mesostrcuture. Hydrophilic performance measurement indicated that the synthesized samples had good hydrophilicity in spite of the absence of light irradiation, the smallest contact angle with water was 22.5 o, which was induced by the roughness and the infiltration (nanowicking) of the mesoporous TiO2 thin film surface. Photo-induced experiment indicated that the sample calcined at 500℃had the best photocatalytic activity and photo-induced hydrophilicity, the contact angle was 9.5 o, due to the suitable grain size of 10.2 nm favored the separation and transmission of electron-hole pairs.The mesoporous TiO2 materials with different pore sizes range from 5.4 to 9.1nm (calcined at 500℃) were prepared through changing the mass of templating agent but the precursor was kept constant. The obtained mesoporous TiO2 materials has the pore size of 9.1 nm was the largest pore size for the mesostructured TiO2 materials calcined at 500℃reported up to now. When more templating agent F127 is used, relatively less Ti-O oligomers assembly with F127 micelles in the solution, so the mesoporous TiO2 with thin walls can be formed, accordingly a larger pore mesostructure will be fabricated. Contrarily, if fewer F127 is adopted, much Ti-O oligomers react with the micelles, so the mesoporous TiO2 with thick walls and small pores can be constructed. In such a way, the TiO2 mesostructure can be adjusted through changing the ratio of templating agent and inorganic precursor. Photocatalytic experiments indicated that the synthesized mesoporous TiO2 with pore size of 7.4 nm and 9.1nm exhibited the better photocatalytic activity than the commercial P25 TiO2.Taking the thiourea as the doping resource, the N/S codoped mesoporous TiO2 were prepared for the first time. It can be found that the addition of the thiourea had twofold roles. Alternatively, changing the mesostrcuture, the NH4+ ion yielded by the thiourea can swell the pore size of obtained mesoporous TiO2, the largest pore size of the obtained mesostructured TiO2 is 12.4 nm, on the other hand, doping N and S to the mesoporous TiO2 materials. The UV-vis reflection spectra showed that the absorption edges of the samples were expanded to about 550 nm at most and the band gap can be decreased to 2.25 eV. The photocatalytic degradation of methyl orange (MO) results indicated that the (N,S)-codoped samples had good photocatalytic performance both in visible light (λ>400nm) and UV light (λ<400nm). Some samples showed the better photocatalytic ability than commercial P25 TiO2. Taking layered K2Ti4O9 as the raw materials, the H2Ti4O9 crystallite nanosheets were prepared by using the cooperation of ball milling and ionic exchange strategies. The obtained sample is mainly composed of single layered nanosheete H2Ti4O9, which has the lateral size less than 50 nm, and specific surface area more than 240 m2·g-1. Additionally, the TiO2/H2Ti4O9 composite was prepared through electrostatic interplay between the TiO2 nanopowder and H2Ti4O9 nanosheets. The photocatalytic results indicated that the TiO2/H2Ti4O9 composition had a higher activity than the commercial P25 TiO2 in the photocatalysis degradating methyl orange (MO). But the H2Ti4O9 exhibited a lower photocatalytic activity. The compound effect of TiO2 and H2Ti4O9 avoids agglomerating of TiO2 and H2Ti4O9 in the solution, which benefited the good photocatalytic activity.