Determining the Charge‐Transfer Direction in a p–n Heterojunction BiOCl/g‐C3N4 Photocatalyst by Ultrafast Spectroscopy

Very recently, the type-II heterostructure composed of p-type BiOCl and n-type g-C3N4 has received much attention due to its high efficiency of charge separation in photocatalysis. However, there exists an awkward ambiguity about the transfer direction of photogenerated charge carriers in the BiOCl(p)/g-C3N4(n) system. In several previous reports, although it seems not an issue in terms of organic photodegradation applications, misinterpretation of charge transfer direction and hence misjudgment of the redox sites in the hybrid system would be a serious problem for other photocatalytic reactions, especially for half reactions, to which the determination of the redox sites is crucial. To solve this annoying problem, we carefully interrogate this topical heterojunction system formed between p-type BiOCl and n-type g-C3N4 by means of ultrafast transient absorption spectroscopy. Our experimental results reveal that the photogenerated electrons are transferred, on a time scale of a few picoseconds, from the conduction band of BiOCl to that of g-C3N4, not on the contrary. This work enables elimination of the confusing ambiguity in using two opposite pictures to describe p-n heterojunction-based photocatalysis mechanism, thereby providing instructive information for rational design of photocatalytic nanosystems.