Direct Z-scheme Cs₂O-Bi₂O₃-ZnO heterostructures for photocatalytic overall water splitting

In this work, a direct Z-scheme Cs₂O-Bi₂O₃-ZnO heterostructure without any electron mediator is fabricated by a simple solution combustion route. Cs₂O is chosen as a sensitizer to expand the light absorption range, and in addition, its conduction band minimum (CBM) and valence band maximum (VBM) positions are suitable to construct a direct Z-scheme system with ZnO and Bi₂O₃. Structural and elemental analyses show clear evidence for heterostructure formation. The Z-scheme charge carrier migration pathway in Cs₂O-Bi₂O₃-ZnO is confirmed by high resolution XPS and ESR studies. The fabricated heterostructure exhibits a good ability to split water to H₂ and O₂ under simulated sunlight irradiation without any sacrificial agents or co-catalysts and has excellent photostability. The apparent quantum efficiency of the optimized Cs₂O-Bi₂O₃-ZnO heterostructure reaches up to 0.92% at 420 nm. The excellent efficiency of this fabricated heterostructure is attributed to the efficient charge carrier separation, the high redox potential of the CBM and VBM benefiting from a direct Z-scheme charge carrier migration pathway and the extended light absorption range.