Effect of Cu and N co-doping in ZnO crystals through thermal processing on energy-level distribution

Metal- and nonmetal-doped ZnO samples, such as N-doped ZnO (NZO), Cu-doped ZnO (CZO) and Cu and N co-doped ZnO (CNZO) crystals, were synthesized and analysed. These ZnO samples were prepared by mixing precursors, such as zinc acetate, ammonia (A), ammonium acetate (AA) and copper acetate, at various proportions through thermal treatment. Results showed that the morphology, crystalline size and light absorption of the Cu- and N-doped ZnO samples were affected by different dopants. The bandgap energy (Eg) of the doped ZnO was determined and became narrow, which was attributed to the presence of N and Cu dopants using UV–vis spectral analysis. The energy bands, including the conduction band minimum (Ec) and valence band maximum (Ev), of the doped ZnO were determined using linear sweep voltammetry and UV–vis data analysis. Both Ec and Ev values for CNZO reduced in the presence of Cu and N in ZnO. The degree of energy-level variations among the doped ZnO samples was discussed and explained according to the experimental analysis.