Photocatalytic H₂ production and CO₂ reduction on Cu, Ni-doped ZnO: effect of metal doping and oxygen vacancies

Cu, Ni-doped ZnO materials were prepared through a fast and simple nital-solution method. XRD and SEM studies confirmed the obtention of the hexagonal brucite structure of ZnO with hexagonal truncated-pyramid morphology. A slight decrease in the cell parameters of ZnO:Cu and ZnO:Ni was observed due to the incorporation in the crystal lattice of Cu²⁺ and Ni²⁺. The absorption spectra of ZnO was red shifted in the doped samples. The formation of oxygen vacancies was confirmed through Raman and XPS spectroscopy, and it was found that the metal doping increased the oxygen vacancies in ZnO, being ZnO:Ni the sample that exhibited the higher density of oxygen vacancies. The photocatalytic activity of the samples was evaluated in the H₂ production and the reduction of CO₂ under UV light. ZnO:Cu and ZnO:Ni samples showed enhanced activity compared to pure ZnO in both photoreduction reactions. ZnO:Ni exhibited the highest photocatalytic activity (224 μmol g⁻¹ h⁻¹ H₂, 29 μmol g⁻¹ h⁻¹ CH₃OH), adjudicated to the major oxygen vacancies observed in this sample, which improved the charge separation and reduced the recombination of the photogenerated electron–hole pairs.