Effects of TiO₂ Surface Fluorination on Photocatalytic Reactions and Photoelectrochemical Behaviors

The formation of surface fluorides on TiO₂ (F-TiO₂), which can be easily attained by a simple addition of F^- to aqueous TiO₂ suspensions, uniquely affects both photocatalytic reactions and photoelectrochemical behaviors. The fluoride adsorption is favored at acidic pH and greatly reduces the positive surface charge on TiO₂ by replacing ≡Ti-OH₂⁺ by ≡Ti-F species. Effects of surface fluorination on the photocatalytic reactivities are very different depending on the kind of substrates to be degraded. F-TiO₂ is more effective than pure TiO₂ for the photocatalytic oxidation of Acid Orange 7 and phenol, but less effective for the degradation of dichloroacetate. It is proposed that the OH radical mediated oxidation pathways are enhanced on F-TiO₂, whereas the hole transfer mediated oxidations are largely inhibited due to the hindered adsorption (or complexation) of substrates on F-TiO₂. As for the photocatalytic reduction, the dechlorination of trichloroacetate is much reduced on F-TiO₂. The photocurrents collected in TiO₂ suspensions, which are mediated by electron shuttles (methyl viologen or ferric ions), and short-circuit photocurrents generated on an illuminated TiO₂/Ti electrode are also markedly reduced in the presence of F^-. The surface ≡Ti-F group seems to act as an electron-trapping site and to reduce interfacial electron transfer rates by tightly holding trapped electrons due to the strong electronegativity of the fluorine. Finally, elementary charge transfer processes on F-TiO ₂ and their implications to photocatalytic reaction pathway are discussed.