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 dropTi-OH2+ by dropTi-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 dropTi-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.