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−TiO2 and their implications to photocatalytic reaction pathway are discussed.