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As a kind of metal-free organic semiconductor photocatalyst, g-C 3 N 4 has been widely explored for use in photocatalysis. However, the low quantum yield, small absorption range, and poor conductivity limit its large-scale application. Introducing another kind of semiconductor, particularly an oxide semiconductor, can result in some unexpected properties, such as an improved change transfer, enhanced light absorption, and better conductivity. In this work, CuO/g-C 3 N 4 is successfully prepared through an impregnation and post-calcination method. A series of measurements support the formation of an organic-inorganic hybrid p-n heterojunction at the CuO (p-type) and g-C 3 N 4 (n-type) interface. Furthermore, the photoactivity of the composite is evaluated via photocatalytic NO removal and the visible degradation of rhodamine B (RhB). Results show that the photocatalytic properties of CuO/g-C 3 N 4 are almost twice as high as those of g-C 3 N 4 . In comparative tests, the photocatalytic degradation performance of Mix-CuO/g-C 3 N 4 (the mixture of CuO and g-C 3 N 4 nanosheets prepared by mechanically mixing) is even lower than that of pure g-C 3 N 4 . The degradation of RhB is only 19.7% under visible light after 30 min of irradiation. The improvement in the photoactivity of CuO/g-C 3 N 4 results from the built-in electric field close to the formed p-n heterojunction, which switches the electron transfer mechanism from a double-charge transfer mechanism to a Z-scheme mechanism. In addition, the formed p-n heterojunction favors charge transfer, and thus the photocatalytic performance is significantly improved., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)