Constructing the Band Alignment of Graphitic Carbon Nitride (g-C₃N₄)/Copper(I) Oxide (Cu₂O) Composites by Adjusting the Contact Facet for Superior Photocatalytic Activity

g-C3N4/Cu2O p–n junction composites with different crystal facets were prepared by a simple one-step route, which showed strong facet-dependent photoactivity, where the degradation rate value k for 20% C3N4–Cu2O truncated cubes composites with {100} facets heterojunction was ∼2.5 times higher than that of pure Cu2O truncated cubes. Meanwhile, k value for 20% C3N4–Cu2O octahedra with {111} facets heterojunction exceeds that of pure Cu2O octahedra by a factor of ∼1.9. p–n heterojunctions with type II energy alignment are determined by XPS analysis. Larger band energy offset (0.96 eV for ΔECBO, 1.73 eV for ΔEVBO) was observed in 20% C3N4–Cu2O truncated cubes composites compared with that of 20% C3N4–Cu2O octahedra composites (0.85 eV for ΔECBO, 1.64 eV for ΔEVBO). The bigger band offset means stronger driving force of the electron transfer between Cu2O truncated cubes with {100} facets and C3N4, indicating band alignment of the heterojunction was facet-dependent, the properly larger band offsets between Cu2O truncated cubes and C3N4 result in the stronger driving force to the transfer of electron and hole in C3N4–Cu2O truncated cubes composites, which is beneficial to the photocatalytic performance. The study provides a new prospect for the rational design of highly efficient photocatalyst in terms of the facet-dependent composites.