(Invited) Enhanced Photochemical Reactions Under Modal Strong Coupling Conditions
Authors: Misawa Hiroaki 
Published: May 2020

Metallic nanoparticles such as gold (Au) and silver (Ag) 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 due to their spectrum tunability. 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 the semiconductor photoelectrode.[1]-[7] Previously, the plasmon-induced charge separation has received considerable attention as a novel strategy for solar energy conversion.[8],[9] However, for the monolayer of Au-NPs on the semiconductor the insufficient absorption limited its solar energy conversion efficiency.
Recently, we reported Au-NPs/TiO2/Au-film photoanode with a modal strong coupling between Fabry–Pérot nanocavity (FP-nanocavity) mode and LSPR of Au-NPscan enhance water splitting reaction.[10] In particular, it should be noted that in addition to the absorption increment, the internal quantum efficiency (IQE) is enhanced under strong couplingconditions.
Additionally, we investigated the efficiency of hot-electron transfer under modal strong coupling conditions by monitoring the photocurrent generated at a plasmonic photoanode. We explored the effect of the modal strong coupling on the incident photon-to-current conversion efficiency (IPCE) and IQE in the presence of triethanolamine (TEOA) as a sacrificial electron donor to accelerate the surface reaction enough.[11] The absorptionspectrum showed distinct dual bands,which corresponded to the strong-coupling-induced splitting ofenergy levels into upper and lower branches. The IPCE was dramaticallyenhanced as the TEOA concentration increased, and finally, theIPCE reached a maximum of ca. 4%. Additionally, both hybrid modes formed bythe modal strong coupling contributed to the hot-electron transfers and photocurrentgeneration in the presence of TEOA because the IPCE action spectracan be separated into two peaks. Furthermore, the integrated IQE, which was obtained for wavelengths from 500 to 800 nm, was enhanced by approximately 5 times upon the addition of 1 vol% of TEOA and reached 3%.

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