Effect of the P/Ti Ratio on the Visible-Light Photocatalytic Activity of P-Doped TiO2

P-doped TiO2 nanoparticles were synthesized by the sol-gel method with various H3PO4 amounts. The samples were calcinated at different temperature and charactered by XPS, ICP, XRD, SEM, Raman, FTIR, and UV-vis methods, so that the formation process of phosphate species could be inspected. The XRD results show that P species hinder the particle growth of anatase and increase the anatase-to-rutile phase transformation temperature to more than 900 degrees C, and a new titanyl phosphate, Ti5O4(PO4)(4), was observed in P-doped TiO2 when calcined at 1000 degrees C. The UV-vis results indicate that the P species is likely to have two different states, leading to the variety of visible-light photocatalytic activity and band gap energy of P-doped TiO2. One state is the separated phase. In this state, the P/Ti ratio is very low so that the P species is surrounded by TiO2. The separated phase of P species introduces oxygen into TiO2 lattice and hence causes a red-shift of the adsorption band edge of anatase, leading to the increased visible-light photocatalytic activity of P-doped TiO2. The other state is the congregated phase. It appears at the micro region where the ratio of P/Ti is high enough to make the TiO2 clusters isolated by P species. The congregated phase of P species acts as the interface phase between TiO2 clusters and strongly retards the crystal growth of anatase, resulting in the widened band gap of P-doped TiO2. Furthermore, a possible mechanism was also proposed to explain the formation of the two phases during the sol-gel process. The results indicate that in order to improve the visible-light photocatalytic activity of P-doped TiO2 the percentage of separated phase in P species needs to be enhanced.