Identifying the Site-Dependent Photoactivity of Anatase TiO2(001)-(1x4) Surface

We here report our investigation on the photocatalytic reactions of methanol on the anatase TiO2(001)-(1X4) surface using scanning tunneling microscopy and temperature-programmed desorption. Our results show that in the reduced surface the Ti-pair defect sites are photoinactive, but in the reoxidized surface the partially oxidized defect sites are photoactive to dissociate methanol and methoxy to formaldehyde. The perfect (1x4) lattice sites are photoactive, even though they are inactive in thermally driven reaction. The photocatalytic reaction of methanol (CD3OH) at the perfect lattice sites is evidenced by the conversion of CD3OH to various other deuterated methanol (CD3OD and CD2HOH) and a small amount of formaldehyde (CD2O) after ultraviolet light irradiation, in contrast with no such conversion reaction in the nonirradiated sample. The production of CD3OD and CD2HOH can be attributed to the reverse reaction between the produced CD2O and the H or D adatoms during or after ultraviolet light irradiation, which also leads to the small amount of formaldehyde in final products. The identification of the site-dependent photocatalytic reactions of methanol, in addition to our recently characterized site-dependent thermally driven reactions, may provide an insightful understanding about the activity and photoactivity of the anatase TiO2(001)-(1X4) surface.