Semiconductor photocatalysis .20. Role of surface in the photoreduction of carbon dioxide catalysed by colloidal ZnS nanocrystallites in organic solvent

Colloidal solutions of monodispersed ZnS nanocrystallites, ZnS-DMF, ZnS-AN and ZnS-MeOH, have been prepared by reacting zinc perchlorate with H2S in N,N-dimethylformamide, acetonitrile or methanol, respectively. In a ZnS-DMF (hexagonal nanocrystallites, average size ca. 2 nm) solution, carbon dioxide undergoes effective photoreduction in the presence of triethylamine as a sacrificial electron donor, under UV light (lambda > 290 nm) irradiation, giving formate and CO. Formate is formed exclusively when ZnS-DMF is prepared stoichiometrically. The system shows a blue emission at ca. 325 nm which is attributed to emission from the conduction band or the shallow electron-trap sites of ZnS-DMF. The strength and the lifetime of the blue emission were enhanced by the addition of CO2 to the system, which can be explained by assuming the stabilization of photoformed electrons in the condition band or in shallow electron-trap sites via the adsorptive interaction of a CO2 molecules on the ZnS-DMF surface. The addition of a zinc ion to the system changes the product distribution without losing efficiency or changing emission behaviour: i.e. the competitive formation of CO with formate and the appearance of a red emission at ca. 460 nm under continuous light excitation can be observed, which may be ascribed to the formation of surface sulfur vacancies. Theoretical molecular orbital calculations using a density-functional method support the preferential adsorptive interaction of a CO2 molecule with a Zn atom in the vicinity of a sulfur vacancy on hexagonal ZnS.