A p–n Junction-Assisted Z-Scheme Cu2O–SnO2/g-C3N4 Heterojunction for Highly Efficient Visible-Light Photocatalysis

In semiconductor photocatalysis, the strong redox ability of a wide band gap and the broad light absorption range of a narrow band gap are a pair of irreconcilable trade-offs. By combining semiconductors with different band gap widths and band edge positions, both of these advantages can be achieved simultaneously. A ternary Cu2O-SnO2/g-C3N4 (CuSnCN) composite photocatalyst was prepared through the hydrothermal and calcination method. Structural analyses confirm the successful integration of truncated octahedral Cu2O, spherical SnO2, and layered g-C3N4, which extends the visible-light response to 650 nm. The optimized system achieves a remarkable 96.58% degradation of methyl orange (80 min, 0.2 g/L catalyst, pH = 3) through dual heterojunction synergies: p–n junctions (Cu2O/g-C3N4 and SnO2/g-C3N4) and Z-scheme charge transfer (SnO2/Cu2O), with •O2−/h+ identified as the dominant reactive species. This work establishes a tunable heterojunction platform for the elimination of multiple pollutants through engineered radical-generation pathways.