The aim of the study is to show how 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 structure at two different hydrothermal temperatures (70 and 130°C) which results in achieving different pore sizes in each sample. Then, the synthesized samples were impregnated with TiO2 nanoparticles (rutile and anatase) and finally heat treated at two different temperatures (400 and 800°C). The prepared materials were characterized by X-ray diffraction (XRD), Small angle X-ray scattering (SAXS), N2 adsorption–desorption isotherm measurements, transmission electron microscopy (TEM) and UV-vis spectroscopy. After calcination of impregnated samples at low temperature (400°C), surprisingly the sample with higher surface area (365 m2/g) showed photoactivity half as big as the sample with lower surface area (329 m2/g). The 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 channels of sample with large pores, coexistence of rutile and anatase crystals leads to more photocatalytic efficiency. All of the samples indicated higher photoactivity with increased calcination temperature from 400 to 800°C due to an increasing degree of crystallinity. Interestingly, the sample with lager pores retained surface area and pore volume more compared to the sample with smaller channels at elevated temperature (800°C) owing to the existence of open and accessible pores.
