Surface modification of TiO₂ by phosphate: Effect on photocatalytic activity and mechanism implication

Phosphate modified TiO₂ photocatalysts were prepared by phosphoric acid treatment before or after TiO₂ crystallization. Substrates with different structures were chosen to explore the photocatalytic activity of asmodified TiO₂ under UV irradiation. It was found that the effect of phosphate modification is definitely attributed to the surface-bound phosphate anion, and the modification by phosphate can affect both the rates and pathways of photocatalytic reactions, which are of great dependence on the structures and properties of substrates. The degradation of substrates (such as 4-chloropehenol, phenol, and rhodamine B) with weak adsorption on the pure TiO₂ was markedly accelerated by phosphate modification, while substrates (such as dichloroacetic acid, alizarin red, and catechol) with strong adsorption exhibited a much lower degradation rate in the phosphate modified system. A much higher amount of hydroxyl radical was produced in phosphate modified system. All of the experimental results imply that phosphate modification largely accelerates the hydroxyl radical attack, but hinders the direct hole oxidation pathway. A common operating mechanism for the phosphate modification, which can be applicable to other inert anions, is also discussed from the viewpoint of an anion-induced negative electrostatic field in the surface layer of TiO₂ and the hydrogen bond between modification anion and H₂O molecule.