Strong Facet Effects on Interfacial Charge Transfer Revealed through the Examination of Photocatalytic Activities of Various Cu2O-ZnO Heterostructures

Confirming the photocatalytic inactivity of Cu2O nanocubes through the formation of Au-decorated-Cu2O heterostructures, spiky ZnO nanostructures are grown on Cu2O cubes, octahedra, and rhombic dodecahedra to demonstrate that charge transfer across semiconductor heterojunctions is also strongly facet dependent. Unintended CuO formation in the growth of ZnO on perfect Cu2O cubes makes them slightly active toward methyl orange photodegradation. Under optimal ZnO growth conditions without CuO presence, Cu2O cubes remain inactive, while rhombic dodecahedra show an enhanced photocatalytic activity due to better charge transfer according to normal Cu2O-ZnO band alignment. Surprisingly, photocatalytically active Cu2O octahedra become inactive after ZnO deposition. An extensive interfacial microscopic examination reveals preferential formation of the ZnO (101) planes on the {111} surfaces of Cu2O octahedra, while different ZnO lattice planes are observed to deposit on Cu2O cubes and rhombic dodecahedra. The photocatalytic inactivity of ZnO-decorated Cu2O octahedra is explained in terms of an unfavorable band alignment arising from an unusual degree of band bending for the ZnO {101} face relative to the band energy of the Cu2O {111} surface. The efficiency of charge transfer across semiconductor heterojunctions strongly depends on the band edge energies of the contacting planes.