Plasmon-induced hot electron transfer has attracted much attention as a novel strategy for the solar energy conversions. However, the solar energy conversion efficiency is limited by the insufficient absorption on monolayer of metallic nanoparticles. To solve this problem, in the present study, we apply the principle of strong coupling to plasmonic water splitting induced by the plasmon-excited electron transferring into wide-bandgap semiconductor on a Au nanoparticle (Au-NP)/TiO2 thin-film/Au-film (ATA) photoanode.
Strong coupling between the Fabry-Pérot nanocavity mode of the TiO2 thin-film/Au-film and the localized surface plasmon mode of the Au-NPs is induced when their resonant frequencies overlap. To increase the coupling strength in this strong coupling regime, a key feature is partially inlaying of Au-NPs into the TiO2 nanocavity by several nanometers. Under a three-electrode system measurement with a saturated calomel electrode (SCE) as a reference electrode, a Pt wire as a counter electrode and an electrolyte of KOH (0.1 mol/dm3), we demonstrated that the action spectrum of incident photon to current conversion efficiency (IPCE) exhibited two bands, which almost corresponds to the absorption spectrum of ATA. The IPCE of ATA is extraordinarily enhanced as compared to that of Au-NPs/TiO2 photoanode. Most importantly, under the strong coupling regime, the internal quantum efficiency (IQE) of the photocurrent generation is also enhanced at the strong coupling wavelengths. The increase in IQE implies the possibility of increasing the generation of hot electrons due to the strong coupling. The plasmon-induced water splitting using a two-electrode system is also discussed.