Inactive CuO Cubes Become Highly Photocatalytically Active with AgS Deposition.

Previously, CuO cubes have been shown to remain photocatalytically inert toward methyl orange degradation even after surface decoration with ZnO, ZnS, CdS, and AgPO nanostructures. Surprisingly, when AgS nanoparticles are lightly deposited on CuO cubes as seen through scanning electron microscopy (SEM) images, the heterostructures become highly photocatalytically active. X-ray diffraction (XRD) patterns show mainly CuO diffraction peaks due to lightly deposited AgS, but AgS peaks can emerge with increased AgS deposition. X-ray photoelectron spectroscopy (XPS) analysis also supports AgS formation on CuO crystals. The AgS-deposited CuO 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 AgS-CuO cubes. Photoluminescence lifetimes are shortened after AgS deposition. Electrochemical impedance measurements revealed a large decrease in charge transfer resistance for CuO cubes after the AgS deposition. Unexpectedly, the separately synthesized AgS particles are also photocatalytically inactive. No specific lattice planes of AgS are formed directly over the {100} face of CuO. Diffuse reflectance and ultraviolet photoelectron spectral data were used to construct band diagrams of different CuO crystals and AgS 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 AgS-deposited CuO cubes, a large change in the {100} surface band bending after AgS 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.