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

Abstract

The low-cost and scalable Cu and Zn doped TiO2 (CuZn-TiO2) photocatalyst was synthesized by simple hydrothermal method. The heterojunction formed by ZnO, CuO, Cu0 and Cu2O with TiO2 in 0.5 wt% CuZn-TiO2 photocatalyst effectively suppressed the electron-hole (e−-h+) recombination and enhanced the charge transfer efficiency of TiO2. The heterojunctions formed between metal oxides in nanocomposite material were confirmed from the overlapped lattice fringes in TEM images. XPS confirmed the presence of Ti4+, Zn2+, Cu2+, Cu+1 and Cu0 in 0.5 wt% CuZn-TiO2. In DRS-UV–vis spectra, co-doping of Zn and Cu showed an appreciable red shift for 0.5 wt% CuZn-TiO2 due to the formation of impurity energy levels by heterojunctions between semiconductors (S-S) like TiO2, ZnO, CuO and Cu2O and also between semiconductors - metallic Cu metal (S-M). Additionally, 0.5 wt% Cu-TiO2 showed greater red shift than 0.5 wt% Zn-TiO2, this revealed the contribution of Cu doping in the effective utilization of solar light. PL spectra clearly revealed the role of ZnO, Cu2O, CuO and Cu0 on suppressing the e−-h+ recombination of TiO2. The mesoporous nature of 0.5 wt% CuZn-TiO2 was confirmed from the N2 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-TiO2 nanocomposite. Among the synthesized materials 0.5 wt% CuZn-TiO2 showed 3.5 fold time improved H2 production than TiO2 due to effective e−-h+ separation at S-S and S-M heterojunctions and the better utilization of visible light in the solar spectrum.