Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated Cu ₂ O Polyhedra Arising from Tunable Interfacial Band Alignment

ZnS particles were grown over Cu ₂ O cubes, octahedra, and rhombic dodecahedra for examination of their facet-dependent photocatalytic behaviors. After ZnS growth, Cu ₂ O cubes stay photocatalytically inactive. ZnS-decorated Cu ₂ O octahedra show enhanced photocatalytic activity, resulting from better charge carrier separation upon photoexcitation. Surprisingly, Cu ₂ O rhombic dodecahedra give greatly suppressed photocatalytic activity after ZnS deposition. Electron paramagnetic resonance spectra agree with these experimental observations. Time-resolved photoluminescence profiles provide charge-transfer insights. The decrease in the photocatalytic activity is attributed to an unfavorable band alignment caused by significant band bending within the Cu ₂ O(110)/ZnS(200) plane interface. A modified Cu ₂ O-ZnS band diagram is presented. Density functional theory calculations generating plane-specific band energy diagrams of Cu ₂ O and ZnS match well with the experimental results, showing that charge transfer across the Cu ₂ O(110)/ZnS(200) plane interface would not happen. This example further illustrates that the actual photocatalysis outcome for semiconductor heterojunctions cannot be assumed because interfacial charge transfer is strongly facet-dependent.