Synergistic Effect of Hydrogenation and Thiocyanate Treatments on Ag-Loaded TiO₂ Nanoparticles for Solar-to-Hydrogen Conversion

Abstract

H2 evolution rate enhanced by Ag-loading on 25 nm TiO2 anatase nanoparticles (denoted as Ag/TiO2), Ag-loaded on hydrogenated TiO2 NPs (Ag/H:TiO2), as well as by the treatment of both NPs with potassium thiocyanate (KSCN) solution have been systematically investigated in conjunction with quantum-chemical calculations and XANES and EXAFS analyses with synchrotron radiation. We have observed a cumulative enhancement effect of these fabrication processes on solar to hydrogen (STH) conversion using a simulating light source. Ag/TiO2 shows an enhanced visible absorption with 4–5 time increase in H2 evolution over that of TiO2 or H:TiO2 prepared under mild hydrogenation conditions, while Ag/H:TiO2 exhibits an even greater UV–visible absorption, similar to that of AgSCN/H:TiO2, with 3.1 times higher STH than that of Ag/TiO2. The treatment of Ag/TiO2 and Ag/H:TiO2 NPs with 0.1 mM KSCN solution further increases their STHs by 3.6 and 2.8 times, respectively. Optimization of KSCN concentration up to 0.2 mM gave [H2] production rate rise to 2.75 mmol h–1 g–1 under Xe lamp illumination for the AgSCN/H:TiO2 system, which has also been tested for its durability, showing a notable robustness. The observed synergistic effect of TiO2 hydrogenation and SCN treatment of the Ag/H:TiO2 NPs has been corroborated by the results of quantum chemical elucidation of H2 production mechanism and the photocatalytic effects of Ag/H:TiO2 and AgSCN/H:TiO2 NPs revealed by appearances of new sub-band states within the TiO2 bandgap, as well as by the result of XANES and EXAFS analyses which support the electron-pulling effect of the SCN group attached to Ag. Finally, we have also compared the efficacies of H2, HCOOH, and CH3OH as hydrogenation sources at 300 °C and the efficacies of CH3OH, C2H5OH, and sucrose as sacrificial agent to facilitate the separation of the electron from the hole.