(Keynote) Dynamics of Electron Transfer in Enhanced Water Splitting Under Modal Strong Coupling Conditions
Authors: Misawa Hiroaki 
Published: December 2020

Metallic nanoparticles such as gold (Au) and silver (Ag) nanoparticles shows light absorption and scattering at the arbitrary wavelength of visible and near-infrared regions based on localized surface plasmon resonances (LSPRs). LSPRs which are collective oscillations of conductive electrons give rise to the enhancement of near-field and are expected as a light harvesting optical antenna for light energy conversion devices We have successfully developed the plasmon-induced artificial photosynthesis systems such as water splitting and ammonia synthesis systems as well as solid-state plasmonic solar cells based on the principle of plasmon-induced charge separation between gold nanoparticles (Au-NPs) and semiconductor.[1]-[6] Recently, the plasmon-induced charge separation has received considerable attention as a novel strategy for solar energy conversion.[7],[8] However, in general Au-NPs loaded semiconductor photoelectrode, the insufficient absorption limits its solar energy conversion efficiency. Aiming at the enhancement of light absorption, in the present study, we apply the principle of modal strong coupling to plasmonic water splitting using Au-NPs/titanium dioxide (TiO2) thin-film/Au-film (ATA) photoelectrode.[9],[10] Furthermore, the dynamics of the hot electron transfer under the modal strong coupling conditions are further studied by transient reflection spectroscopy.

Modal strong coupling between the Fabry–Pérot nanocavity mode of the TiO2thin-film/Au-film and the LSPR mode of the Au-NPs is induced when their resonant frequencies overlap. To increase the coupling strength in this modal strong coupling regime, a key feature of the ATA photoelectrode 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/TiO2photoanode. Most importantly, under the modal strong coupling conditions, the internal quantum efficiency (IQE) of the photocurrent generation is also enhanced at the wavelengths of strong coupling induced hybrid branches. The increase in IQE implies the possibility of increasing the generation of hot electrons in the modal strong coupling regime. The plasmon-induced water splitting using a two-electrode system was also quantitatively measured. To understand the IQE enhancement under the strong coupling conditions, the dynamics of the hot electrons transfer from Au-NPs to TiO2 were further explored by transient measurement using a visible pump/infrared probe (at 3500 nm) femtosecond transient reflection spectrometers. An ultrafast hot electrons injection from Au-NPs to TiO2 was observed. Interestingly, under the strong coupling conditions, the hot electrons excited in the ATA photoelectrode that fabricated on Al film instead of Au film exhibited a much longer lifetime than that on Au film. The contributions for the long lifetime of the injected electrons to TiO2 are also discussed.

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