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-C 3 N 4 /Cu 2 O 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% C 3 N 4 –Cu 2 O truncated cubes composites with {100} facets heterojunction was ∼2.5 times higher than that of pure Cu 2 O truncated cubes. Meanwhile, k value for 20% C 3 N 4 –Cu 2 O octahedra with {111} facets heterojunction exceeds that of pure Cu 2 O 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 Δ E CBO, 1.73 eV for Δ E VBO ) was observed in 20% C 3 N 4 –Cu 2 O truncated cubes composites compared with that of 20% C 3 N 4 –Cu 2 O octahedra composites (0.85 eV for ΔE CBO, 1.64 eV for ΔE VBO ). The bigger band offset means stronger driving force of the electron transfer between Cu 2 O truncated cubes with {100} facets and C 3 N 4, indicating band alignment of the heterojunction was facet-dependent, the properly larger band offsets between Cu 2 O truncated cubes and C 3 N 4 result in the stronger driving force to the transfer of electron and hole in C 3 N 4 –Cu 2 O 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.