Sunlight driven photocatalytic activity of Cu2O–ZnO hetero-photocatalyst for the degradation of methylene blue: A combined experimental and theoretical study

ZnO, Cu2O, and p-type Cu2O/n-type ZnO hetero-photocatalysts were prepared at low cost with high yield by using a one-step hydrothermal method. X-ray diffraction confirmed the successful synthesis of hexagonal wurtzite ZnO and cubic phase Cu2O crystal structures, while Fourier transform infrared spectroscopy affirmed the coexistence of both oxides in the Cu2O–ZnO heterogeneous photocatalyst. Transmission electron microscopy revealed different sizes of ZnO nanoparticles stuck to spherical Cu2O particles, establishing well-defined heterojunction interfaces. Optical studies showed bandgap energies of 3.12 eV for ZnO and 2.0 eV for Cu2O, whereas the Cu2O–ZnO heterostructure exhibited a reduced bandgap of 2.46 eV with enhanced visible-light absorption. Density functional theory calculations supported a direct-to-indirect bandgap transition with a reduction in the optical bandgap and confirmed efficient charge separation at the heterointerface. Under optimal conditions (pH ∼7 and catalyst dosage of 30 mg/l), both ZnO and Cu2O catalysts show ∼45% photocatalytic degradation efficiency in 110 min of sunlight exposure in degrading methylene blue (MB) dye, whereas the Cu2O–ZnO hetero-photocatalyst achieved ∼98.08% degradation. The improved photocatalytic degradation efficiency (98.08%) of our best sample (Cu2O–ZnO hetero-photocatalyst) was attributed to synergistic interfacial charge transfer, as validated by experimental and theoretical analyses. The effects of MB concentration (30, 40, 50, and 60 ppm) and Cu2O–ZnO hetero-photocatalyst dosage (20, 30, and 40 mg/l) were also studied. The Cu2O–ZnO catalyst retained at 89.73% efficiency after six cycles, demonstrating excellent stability and reusability. These combined studies might help realize the potential application of Cu2O–ZnO catalysts as efficient sunlight-driven photocatalysts for the purification of wastewater.