Selective adsorption of thiocyanate anions on Ag-modified g-C3N4 for enhanced photocatalytic hydrogen evolution

Abstract

Silver-modified semiconductor photocatalysts typically exhibit enhanced photocatalytic activity toward the degradation of organic substances. In comparison, their hydrogen-evolution rates are relatively low owing to poor interfacial catalytic reactions to producing hydrogen. In the present study, thiocyanate anions (SCN−) as interfacial catalytic active sites were selectively adsorbed onto the Ag surface of g-C3N4/Ag photocatalyst to promote interfacial H2-evolution reactions. The thiocyanate-modified g-C3N4/Ag (g-C3N4/Ag-SCN) photocatalysts were synthesized via photodeposition of metallic Ag on g-C3N4 and subsequent selective adsorption of SCN− ions on the Ag surface by an impregnation method. The resulting g-C3N4/Ag-SCN photocatalysts exhibited considerably higher photocatalytic H2-evolution activity than the g-C3N4, g-C3N4/Ag, and g-C3N4/SCN photocatalysts. Furthermore, the g-C3N4/Ag-SCN photocatalyst displayed the highest H2-evolution rate (3.9 μmol h−1) when the concentration of the SCN− ions was adjusted to 0.3 mmol L−1. The H2-evolution rate obtained was higher than those of g-C3N4 (0.15 μmol h−1) and g-C3N4/Ag (0.71 μmol h−1). Considering the enhanced performance of g-C3N4/Ag upon minimal addition of SCN− ions, a synergistic effect of metallic Ag and SCN− ions is proposed—the Ag nanoparticles act as an effective electron-transfer mediator for the steady capture and rapid transportation of photogenerated electrons, while the adsorbed SCN− ions serve as an interfacial active site to effectively absorb protons from solution and promote rapid interfacial H2-evolution reactions. Considering the present facile synthesis and its high efficacy, the present work may provide new insights into preparing high-performance photocatalytic materials.