All reagents were analytical grade (Shanghai Chemical Reagent Factory of China, Shanghai, China), which were used without further purification. TiO2 nanoparticles were prepared by the hydrothermal method from Yu et al.
In a typical synthesis process, 0.02 mM of tetrabutyl titanate (TBOT) was firstly dissolved in30 mL of absolute alcohol.
The resulting TBOT alcoholic solution was drop-wise added into 120 mL of NH4HF2 aqueous solution under magnetic stirring.
The nominal atomic ratio of fluorine to titanium was 0.5.
The pH value of the solution was ca. 3.5.
The resulting solution was transferred to a 200 mL Teflon-lined autoclave, which was then maintained at 150◦C for 10 h.
After cooling this solution to room temperature, the precipitate was filtered, rinsed with distilled water and ethanol three times, and dried in a vacuum oven at 60◦C for 8 h.