Surface Characteristics of ZnS Nanocrystallites Relating to Their Photocatalysis for CO2 Reduction1

Hexagonal ZnS nanocrystallites (ZnS−DMF(OAc); ca. 2 nm in diameter) prepared in N,N-dimethylformamide (DMF) using Zn(CH3COO)2·2H2O (Zn(OAc)2·2H2O) as the Zn2+ source and H2S as the sulfur source catalyzed selective photoreduction of CO2 to HCOO- in the presence of triethylamine as an electron donor. When excess zinc acetate was added to the system, the efficiency increased while still keeping the product selectivity. The photocatalytic behavior of ZnS−DMF(OAc) is in contrast to that of ZnS−DMF(ClO4) prepared using Zn(ClO4)2·6H2O as the Zn2+ source, where both HCOO- and CO were produced, especially when excess zinc perchlorate was added into the system. FT-IR analysis of the ZnS−DMF(OAc) system revealed the presence of SH groups on the surface, explaining the gradual growth of the size with the addition of excess zinc acetate into the system. Extended X-ray absorption fine structure analysis revealed the correlation between the photocatalysis and the microscopic surface structure change of the ZnS nanocrystallites induced by the addition of Zn2+ to the nanocrystallite systems. The intimately interacting acetate ions to Zn atoms should prevent the formation of sulfur vacancies as catalytic sites of CO production, contributing to the enhanced photocatalytic activity for production of HCOO- due to the formation of the DMF-coordinated nanocrystallites.