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.