Inactive Cu₂O Cubes Become Highly Photocatalytically Active with Ag₂S Deposition

Previously, Cu₂O cubes have been shown to remain photocatalytically inert toward methyl orange degradation even after surface decoration with ZnO, ZnS, CdS, and Ag₃PO₄ nanostructures. Surprisingly, when Ag₂S nanoparticles are lightly deposited on Cu₂O cubes as seen through scanning electron microscopy (SEM) images, the heterostructures become highly photocatalytically active. X-ray diffraction (XRD) patterns show mainly Cu₂O diffraction peaks due to lightly deposited Ag₂S, but Ag₂S peaks can emerge with increased Ag₂S deposition. X-ray photoelectron spectroscopy (XPS) analysis also supports Ag₂S formation on Cu₂O crystals. The Ag₂S-deposited Cu₂O octahedra and rhombic dodecahedra show the expected activity enhancement. Electron paramagnetic resonance (EPR) measurements, as well as electron, hole, and radical scavenger tests, all confirmed the emergence of photocatalytic activity from the Ag₂S-Cu₂O cubes. Photoluminescence lifetimes are shortened after Ag₂S deposition. Electrochemical impedance measurements revealed a large decrease in charge transfer resistance for Cu₂O cubes after the Ag₂S deposition. Unexpectedly, the separately synthesized Ag₂S particles are also photocatalytically inactive. No specific lattice planes of Ag₂S are formed directly over the {100} face of Cu₂O. Diffuse reflectance and ultraviolet photoelectron spectral data were used to construct band diagrams of different Cu₂O crystals and Ag₂S nanoparticles. A Z-scheme charge transfer mechanism may be involved at the heterojunction interface to promote charge carrier separation. However, to explain the sudden appearance of photocatalytic activity from the Ag₂S-deposited Cu₂O cubes, a large change in the {100} surface band bending after Ag₂S deposition should be used. This work illustrates that an unusual photocatalytic outcome is possible to semiconductor heterojunctions, where two photocatalytically inert components can become highly active when joined together.