Effect of phase structures on the formation rate of hydroxyl radicals on the surface of TiO2

To study the relationship between the phase structures of TiO2 and the photoinduced hydroxyl radicals (OH), TiO2 nanocrystallines were synthesized by a hydrolysis-precipitate method using tetrabutylorthotitanate (TBOT) as precursor, and then calcined at 450, 600, 700, 800 and 900 °C for 2 h, respectively. The calcined samples were characterized by X-ray diffraction and N2 sorption. The formation rate of OH on the surface of UV-illuminated TiO2 was detected by the photoluminescence (PL) technique using terephthalic acid as a probe molecule. The results show that with increasing calcined temperatures, the amorphous (Am) TiO2 precursor begins to turn into anatase (A) at 450 °C and rutile (R) phase appears at 600 °C, which is completely turned into the rutile phase at 900 °C. The BET specific surface areas of the catalyst decrease as the calcined temperatures increase. TiO2 sample calcined at 600 °C, with a mixed phase of anatase and rutile, shows the highestOH formation rate, and the order of the OH formation rate is as follows: A+R>A>R>Am. Phase structures of TiO2 play a more important role than specific surface areas in the OH formation rate. Two phase structure of anatase and rutile with a proper ratio is beneficial to the OH formation due to decrease of the combination rate of photo-generated electrons and holes. Our experimental result implies that the mixed phase of anatase and rutile can markedly enhance the photocatalytic activity of TiO2.