Recently, we reported that the modal strong coupling between a Fabry–Pérot (F-P) nanocavity mode and a localized surface plasmon resonance (LSPR) facilitates water splitting reactions.^[5] We used a Au-NPs/TiO₂/Au-film (ATA) structure as a photoanode. The TiO₂/Au-film component of this photoanode acts as F-P nanocavity. The light absorption of the ATA was promoted by the optical hybrid modes based on the strong coupling formation across a broad range of wavelengths, followed by a hot electron transfer to TiO₂. We observed an 11-fold increase in the incident photon-to-current conversion efficiency (IPCE) with respect to a photoanode structure without Au-film. Importantly, the internal quantum efficiency (IQE) was enhanced 1.5 times under a strong coupling over that under uncoupled conditions.

We speculated that the coupling strength of the modal strong coupling affects the hot electron transfer efficiency. To increase the coupling strength, we developed a photoanode consisting of Au-Ag alloy nanoparticles /TiO₂/Au-film (AATA).^[6] It was expected that the splitting energy of the modal strong coupling system increase with increasing LSPR oscillator strength by mixing Ag in the nanoparticle. The AATA structure formed under modal strong coupling with a large splitting energy of 520 meV, which can be categorized into the ultrastrong coupling regime. The AATA photoanode showed a 4.0% maximum IPCE obtained at 580 nm, and the IQE was 4.1%. Additionally, the highly efficient hot-electron injection on AATA was directly observed by transient absorption measurements. Furthermore, hybrid mode-induced water oxidation using AATA structures was performed with a Faraday efficiency of more than 70% for O₂ evolution.

We have applied the concept of photoanodes with modal strong coupling to photocathodes as well. As a semiconductor for constructing F-P nanocavity, we employed p-type nickel oxide (NiO) which is capable of transferring holes generated in Au-NPs and fabricated a photocathode consisting of Au-NP/NiO/Pt-film (ANP).^[7] The ANP structure can absorb visible light over a wide wavelength range from 500 nm to 850 nm due to a hybrid mode based on the modal strong coupling. The IPCE based on H₂evolution through water/proton reduction by hot electrons reached 0.2% at 650 nm and 0.04% at 800 nm, which was significantly larger than that of Au-NPs on NiO without Pt-film.

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5. X. Shi, K. Ueno, T. Oshikiri, Q. Sun, K. Sasaki, H. Misawa, Nat. Nanotechnol. 2018, 13, 953-958.
6. Y. Suganami, T. Oshikiri, X. Shi, H. Misawa, Angew. Chem. Int. Ed. 2021, 60, 18438-18442.
7. T. Oshikiri, H. Jo, X. Shi, and H. Misawa, Chem. Eur. J. 2022, published online, DOI: 10.1002/chem.202200288