Plasmon-induced charge separation (PICS)^1-3 involves electron transfer from plasmonic nanoparticles of Au, Ag, or Cu to a semiconductor, typically TiO₂.^1-4 The electron transfer is caused by external photoelectric effects (including hot carrier injection) or interfacial electron transition.³ We have reported that oxidation of Ag to Ag⁺ occurs at the Ag nanoparticle surface in PICS of Ag-TiO₂ systems,⁵ and this reaction occurs preferentially at the sites where optical near field is localized.^6-8 This can be explained in terms of ejection of energetic holes from the nanoparticles as Ag⁺ ions.^8,9 This possibility has been further investigated on the basis of other anodic reactions. For a system with isolated Au nanoparticles on TiO₂, the interface mode may be more important than the full-surface mode. However, plasmon coupling between two nanoparticles, which enhances full-surface mode but not necessarily enhances the interface mode, increases the PICS efficiency. This could also indicate that not only injection of electrons from Au nanoparticles into TiO₂ but also ejection of holes from Au nanoparticles play an important role in those PICS systems.

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