Direct Z-scheme heterojunction nanocomposite for the enhanced solar H2 production

Abstract The low-cost and scalable Cu and Zn doped TiO 2 (CuZn-TiO 2 ) photocatalyst was synthesized by simple hydrothermal method. The heterojunction formed by ZnO, CuO, Cu 0 and Cu 2 O with TiO 2 in 0.5 wt% CuZn-TiO 2 photocatalyst effectively suppressed the electron-hole (e − -h + ) recombination and enhanced the charge transfer efficiency of TiO 2 . The heterojunctions formed between metal oxides in nanocomposite material were confirmed from the overlapped lattice fringes in TEM images. XPS confirmed the presence of Ti 4+ , Zn 2+ , Cu 2+ , Cu +1 and Cu 0 in 0.5 wt% CuZn-TiO 2 . In DRS-UV–vis spectra, co-doping of Zn and Cu showed an appreciable red shift for 0.5 wt% CuZn-TiO 2 due to the formation of impurity energy levels by heterojunctions between semiconductors (S-S) like TiO 2 , ZnO, CuO and Cu 2 O and also between semiconductors - metallic Cu metal (S-M). Additionally, 0.5 wt% Cu-TiO 2 showed greater red shift than 0.5 wt% Zn-TiO 2 , this revealed the contribution of Cu doping in the effective utilization of solar light. PL spectra clearly revealed the role of ZnO, Cu 2 O, CuO and Cu 0 on suppressing the e − -h + recombination of TiO 2 . The mesoporous nature of 0.5 wt% CuZn-TiO 2 was confirmed from the N 2 adsorption-desorption study. For the first time, direct Z-scheme along with heterojunction double charge transfer mechanism were proposed for the better performances of CuZn-TiO 2 nanocomposite. Among the synthesized materials 0.5 wt% CuZn-TiO 2 showed 3.5 fold time improved H 2 production than TiO 2 due to effective e − -h + separation at S-S and S-M heterojunctions and the better utilization of visible light in the solar spectrum.