Relationship between Synthesis Conditions and Photocatalytic Activity of Nanocrystalline TiO2

The degradation efficiency of methylene blue by TiO 2 nanoparticles, which were synthesized under different synthesis conditions (i.e., molar ratio of water and titanium tetraisopropoxide (TTIP), pH, and calcination temperature) in a sol-gel process, was systematically investigated. The results showed that increasing the molar ratio of water and TTIP led to the enhanced photocatalytic activity of TiO 2 nanoparticles, which were likely attributed to the increased specific surface area of TiO 2 nanoparticles synthesized with high molar ratio. The results were supported by the relative increase in the size of interaggregated pores of the aggregated TiO 2 nanoparticles. The best photocatalytic activity of TiO 2 nanoparticles was observed at acidic synthesis conditions; however, the results were not consistent with physical properties for the crystallinity and the crystallite size of TiO 2 nanoparticles but rather explained by the presence of abundant hydroxyl groups and water molecules existing on the surface of TiO 2 under acidic synthesis environments. Furthermore, methylene blue degradation experiments revealed that the photocatalytic activity of TiO 2 nanoparticles was maximized at the calcination temperature of 700°C. The trend was likely due to the combined effect of the anatase crystallinity which showed the highest value at 700°C and the crystallite size/specific surface area which did not excessively increase up to 700°C.