Fig. 3

Control experiments with the ZnO and Cu₂O nanoparticles and mechanistic study of the ZnO-Cu₂O catalysts. TEM images of a ZnO spheres and b Cu₂O nanocubes. The scale bars represent 50 nm. c Amount of CH₄ production using ZnO-Cu₂O (black) catalysts, and ZnO (red) and Cu₂O (blue) nanoparticles under the conditions with catalyst amount fixed to 19 mg, pH = 7.4, and λ > 200 nm. d Photoresponse data of the ZnO-Cu₂O catalyst deposited on a FTO electrode at a potential of −0.45 V vs. Ag/AgCl in a phosphate buffer by the irradiation of UV–visible (black) and visible (red) light using a cutoff filter (λ > 425 nm). e Band alignment and proposed electron transfer mechanism of the ZnO-Cu₂O hybrid catalysts