A novel Cr-doped CdS/ZnO nanocomposite for efficient photocatalytic hydroxylation of benzene to phenol

In this study, Cr-doped CdS/ZnO (Cr-CdS/ZnO) nanocomposites were synthesized for the first time and systematically characterized using different physicochemical methods, such as XRD, SEM, HRTEM, UV-Vis DRS, XPS, PL, EIS and EPR, etc. The photocatalytic activity of the Cr-CdS/ZnO nanocomposites was evaluated for the hydroxylation of benzene to phenol using H2O2 as the oxidant under visible-light irradiation. The impacts of various reaction conditions, including the role of catalyst dosage, solvents and volume ratio of benzene to H2O2 were specifically investigated. Owing to the similar lattice structure and well-matched band positions of CdS and ZnO, generation of electrons and holes by CdS-captured photons, as well as separation and imigration of electrons to ZnO were all realized. Furthermore, Cr-doping favored the formation of a new trapping level in the band gap of the Cr-CdS/ZnO composite, which would not only enhance visible-light absorption capacity, but also inhibit the rapid recombination of photo-generated electron-hole pairs. As a result, an excellent phenol selectivity of 98% with a phenol yield of 11.1% was achieved, which was higher than those of the pristine Cr-CdS, Cr-ZnO and CdS/ZnO. In the end, a rational mechanism for enhanced photocatalytic activity of the synthesized composites under visible-light irradiation was also proposed. The visible-light absorption and remarkably improved photocatalytic performance for benzene hydroxylation enable the potential of Cr-CdS/ZnO nanocomposites for the application of photocatalytic green synthesis of fine chemicals.