Effect of Fluoride Doping and Adsorption on the Photocatalytic Activity of TiO2

It has been reported that bulk doping or surface modification of TiO2 with fluoride ions can enhance its photocatalytic activity for degradation of organic compounds in water. The effect of the former is ascribed to enhanced separation of photogenerated charge carriers through the surface-formed Ti3+ species, whereas that of the latter is ascribed to enhanced desorption of hydroxyl radicals through the interfacial fluoride ions. However, the difference in activity between two modified catalysts has not been investigated. In this work, different fluoride-doped samples were hydrothermally prepared from butyl titanate and NH4F. Their photocatalytic activities after addition of NaF or AgNO3 to the aqueous suspension were evaluated using phenol degradation as a model reaction. All the fluoride ions in the oxide lattices and in the outer and inner Helmholtz double layers of TiO2 were positive to phenol degradation, but the magnitude of their influences followed a decreasing order. Moreover, phenol degradation in the presence of both NaF and AgNO3 was much faster than the sum of their individual rates. These results indicate that combination of conduction band electron reduction and valence band hole oxidation is an effective way to improve the quantum yield of TiO2 photocatalysis.