Boosting up the Low Catalytic Activity of Silver for H₂ Production on Ag/TiO₂ Photocatalyst: Thiocyanate as a Selective Modifier

Noble metal cocatalysts like Pt have been widely employed as an essential ingredient in many kinds of photocatalytic materials for solar hydrogen production. The high material cost of Pt is the biggest limitation. Silver is far less expensive but much less active than Pt and Au as a hydrogen evolving catalyst. Here we demonstrate a new strategy to boost up the activity of silver in Ag/TiO₂ for photocatalytic H₂ production via forming a simple surface complexation of thiocyanate (SCN^-) on silver. The addition of thiocyanate in the suspension of Ag/TiO₂ markedly enhanced the photocatalytic production of H₂ by about 4 times. Thiocyanate was not consumed at all during the photoreaction, which ruled out the role of thiocyanate as an electron donor. Such a positive role of thiocyanate was not observed with bare TiO₂, Pt/TiO₂, and Au/TiO₂. The selective chemisorption of thiocyanate on silver was confirmed by the analyses of Raman spectroscopy and spot-profile energy-dispersive spectroscopy. In the presence of thiocyanate, the overpotential for water reduction on Ag/TiO₂ electrode was slightly reduced, and the interfacial charge transfer resistance on Ag/TiO₂ (measured by electrochemical impedance spectroscopy) was significantly decreased, whereas other electrode systems (bare TiO₂, Au/TiO₂, and Pt/TiO₂) showed the opposite effect of thiocyanate. These results indicate that the adsorption of thiocyanate on Ag facilitates the transfer of photogenerated electrons on the Ag/TiO₂ electrode. It is proposed that the formation of Ag-SCN surface complex enhances the interfacial electron transfer rate and facilitates the reduction of protons on Ag/TiO₂.