TiO₂ (rutile and anatase) deposited on ordered mesoporous SiO₂: Effect of pore size on photocatalytic activity

The aim of the study is to show how the pore size of mesoporous SiO₂ host structures can affect the physical and photocatalytic properties of the impregnated host structure with TiO₂ nanoparticles. The first phase of the investigation involved preparing mesoporous silica structures at two different hydrothermal temperatures (70°C and 130°C) which resulted in achieving different pore sizes in each sample. The synthesized samples were then impregnated with TiO₂ nanoparticles (rutile and ana-tase) and finally heat treated at two different temperatures (400°C and 800°C). The prepared materials were characterized by X-ray diffraction, small-angle X-ray scattering, N₂ adsorption–desorption isotherm measurements, transmission electron microscopy and UV–Vis spectroscopy. After calcination of impregnated samples at a low temperature (400°C), the sample with higher surface area (365 m²/g) showed a photoactivity half as big as the sample with lower surface area (329 m²/g). This phenomenon is due to the effect of pore size on localization of TiO₂ nanoparticles (rutile and anatase) inside the pores and consequently on photocatalytic properties. In fact, since large rutile crystals can enter the channels of samples with large pores, coexistence of rutile and anatase crystals can lead to more photocatalytic efficiency. All of the samples indicated higher photoactivity with increased calcination temperature from 400°C to 800°C due to an increasing degree of crystallinity. Interestingly, the sample with larger pores retained a higher surface area and pore volume compared with the sample having smaller channels at elevated temperatures (800°C) owing to the existence of open and accessible pores.