Facet-dependent photocatalytic properties of Cu2O crystals probed by using electron, hole and radical scavengers

Abstract

To understand the photocatalytic inactivity of Cu₂O nanocubes and the superior photocatalytic activity of Cu₂O rhombic dodecahedra compared to that of Cu₂O octahedra, electron and hole scavengers, as well as hydroxyl and superoxide anion radical scavengers, were introduced in the photodegradation of methyl orange using these Cu₂O crystals as the photocatalysts. Scavenger results suggest that photogenerated electrons and holes experience a large barrier at the {100} faces of cubes preventing charge migration to the particle surfaces, leading to the photocatalytic inactivity of Cu₂O cubes. For Cu₂O rhombic dodecahedra, both photogenerated electrons and holes are efficiently utilized to produce radicals, giving them excellent photocatalytic activity. In contrast, the photocatalytic activity of Cu₂O octahedra mostly comes from photoexcited electrons migrating to the particle surfaces to yield superoxide anion radicals and hydroxyl radicals, while holes are largely unavailable for the photo-oxidation reaction. Ultraviolet photoelectron spectroscopy (UPS) and reflectance spectra were used to construct conventional band diagrams for these particle shapes, but such band diagrams cannot account for the large face-specific photocatalytic properties of Cu₂O nanocrystals. A modified band diagram presenting different degrees of band bending at various crystal surfaces is more useful to explain the observed facet-dependent phenomena.