Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated CuO Polyhedra Arising from Tunable Interfacial Band Alignment.

ZnS particles were grown over CuO cubes, octahedra, and rhombic dodecahedra for examination of their facet-dependent photocatalytic behaviors. After ZnS growth, CuO cubes stay photocatalytically inactive. ZnS-decorated CuO octahedra show enhanced photocatalytic activity, resulting from better charge carrier separation upon photoexcitation. Surprisingly, CuO 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 CuO(110)/ZnS(200) plane interface. A modified CuO-ZnS band diagram is presented. Density functional theory calculations generating plane-specific band energy diagrams of CuO and ZnS match well with the experimental results, showing that charge transfer across the CuO(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.