Enhanced Photoinduced-Stability and Photocatalytic Activity of CdS by Dual Amorphous Cocatalysts: Synergistic Effect of Ti(IV)-Hole Cocatalyst and Ni(II)-Electron Cocatalyst

CdS is one of the most well-known and important visible-light photocatalytic materials for water splitting to produce hydrogen energy. Owing to its serious photocorrosion property (poor photoinduced stability), however, CdS photocatalyst can unavoidably be oxidized to form S-0 by its photogenerated holes, causing an obviously decreased photocatalytic performance. In this study, to improve the photoinduced stability of CdS photocatalyst, amorphous TiO2 (referred to as Ti(IV)) as a hole cocatalyst was successfully loaded on the CdS surface to prepare Ti(IV)/CdS photocatalysts. It was found that the resultant Ti(IV)/CdS photocatalyst exhibited an obviously enhanced photocatalytic stability, namely, its deactivation rate clearly decreased from 37.9% to 13.5% after five cycles of photocatalytic reactions. However, its corresponding photocatalytic activity only showed a very limited increase (ca. 37.4%) compared with the naked CdS. To further improve its photocatalytic performance, the amorphous Ni(II) as an electron cocatalyst was subsequently modified on the Ti(IV)/CdS surface to prepare the dual amorphous-cocatalyst modified Ti(IV) Ni(II)/CdS photocatalyst. In this case, the resultant Ti(IV) Ni(II)/CdS photocatalyst not only exhibited a significantly improved photocatalytic activity and stability, but also could maintain the excellent photoinduced stability of CdS surface structure. Based on the experimental results, a synergistic effect of dual amorphous Ti(IV) Ni(II) cocatalysts is proposed, namely, the amorphous Ti(IV) works as a hole-cocatalyst to rapidly capture the photogenerated holes from CdS surface, causing the less oxidation of surface lattice S2- ions in CdS, while the amorphous Ni(II) functions as an electron-cocatalyst to rapidly transfer the photogenerated electrons and then promote their following interfacial H-2-evolution reaction. Compared with the traditional noble metal cocatalysts (such as Pt and RuO2), the present amorphous Ti(IV) and Ni(II) cocatalysts are apparently low-cost, nontoxic, and earth-abundant, which can widely be applied in the design and development of highly efficient photocatalytic materials.