We developed a photoanode consisting of Au‐Ag alloy nanoparticles (NPs), a TiO2 thin film and a Au film (AATA) under modal strong coupling conditions with a large splitting energy of 520 meV, which can be categorized into the ultrastrong coupling regime. We fabricated a photoanode under ultrastrong coupling conditions to verify the relationship between the coupling strength and photoelectric conversion efficiency and successfully performed efficient photochemical reactions. The AATA photoanode showed a 4.0 % maximum incident photon‐to‐current efficiency (IPCE), obtained at 580 nm, and the internal quantum efficiency (IQE) was 4.1 %. These results were attributed to the high hot‐electron injection efficiency due to the larger near‐field enhancement and relatively negative potential distribution of the hot electrons. Furthermore, hybrid mode‐induced water oxidation using AATA structures was performed, with a Faraday efficiency of more than 70 % for O2 evolution.

Plasmonic Au/TiO2 thin film works as a stable water oxidation photoanode. Here we show that site-selective deposition of nanosized CoO x as a water oxidation cocatalyst at the edge of Au nanoparticles on the TiO2 film improves the photoelectrochemical water oxidation activity. The nanosized CoO x was deposited by a photoassisted electrochemical method onto the Au/TiO2 thin film. The CoO x loading amount was controllable by changing the amount of electric charge that flowed during the deposition, which influenced the photoelectrochemical performance. Under visible light, the optimized CoO x /Au/TiO2 thin film generated stable photoanodic current, which was ∼3 times higher than that obtained using Au/TiO2.

We developed a photoanode consisting of Au‐Ag alloy nanoparticles (NPs), a TiO2 thin film and a Au film (AATA) under modal strong coupling conditions with a large splitting energy of 520 meV, which can be categorized into the ultrastrong coupling regime. We fabricated a photoanode under ultrastrong coupling conditions to verify the relationship between the coupling strength and photoelectric conversion efficiency and successfully performed efficient photochemical reactions. The AATA photoanode showed a 4.0 % maximum incident photon‐to‐current efficiency (IPCE), obtained at 580 nm, and the internal quantum efficiency (IQE) was 4.1 %. These results were attributed to the high hot‐electron injection efficiency due to the larger near‐field enhancement and relatively negative potential distribution of the hot electrons. Furthermore, hybrid mode‐induced water oxidation using AATA structures was performed, with a Faraday efficiency of more than 70 % for O2 evolution.

Strong coupling between a localized surface plasmon resonance (LSPR) at the surface of metal nanoparticles (NPs) and a Fabry–Pérot (FP) nanocavity can facilitate photochemical reactions. It is very interesting and critical to study the enhancement mechanism of plasmon-induced chemical reactions under plasmon–nanocavity strong coupling to further improve the photochemical reaction efficiency. In this study, we fabricated a LSPR–FP nanocavity strong coupling photoelectrode composed of Au–Ag alloy NPs, titanium dioxide (TiO2), and a Au film as a working electrode to investigate the mechanism of water oxidation enhancement under plasmon–nanocavity strong coupling conditions. In situ electrochemical surface-enhanced Raman spectroscopy measurements were performed to detect the intermediate species of water oxidation under a series of electrochemical potentials. The Au–O and Au–OH stretching vibrations related to the intermediates of water oxidation were investigated. Compared with the Au–Ag alloy NPs/TiO2 structure without strong coupling, the surface-enhanced Raman spectroscopy signal of the Au–O stretching vibration on the strong coupling electrode exhibited a more negative onset potential, indicating that more efficient water oxidation occurred on it. This efficient water oxidation in the strong coupling photoelectrode was considered a result of the quantum coherence between the Au–Ag alloy NPs through the nanocavity.

Plasmon-induced hot electron transfer at the metal/semiconductor interface has attracted considerable attention as a novel mechanism to promote artificial photosynthesis under visible and near-infrared irradiation.[1-4] However, a single layer of gold nanoparticles (Au-NPs) cannot efficiently harvest light. 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. Y. Nishijima, K. Ueno, Y. Kotake, K. Murakoshi, H. Inoue, H. Misawa, J. Phys. Chem. Lett. 2012, 3, 1248-1252. Y. Zhong, K. Ueno, Y. Mori, X. Shi, T. Oshikiri, K. Murakoshi, H. Inoue, H. Misawa, Angew. Chem. Int. Ed. 2014, 53, 10350-10354. T. Oshikiri, K. Ueno, H. Misawa, Angew. Chem. Int. Ed. 2016, 55, 3942-3946. M. Okazaki, Y. Suganami, N. Hirayama, H. Nakata, T. Oshikiri, T. Yokoi, H. Misawa, K. Maeda, ACS Appl. Energy Mater. 2020, 3, 5142–5146. X. Shi, K. Ueno, T. Oshikiri, Q. Sun, K. Sasaki, H. Misawa, Nat. Nanotechnol. 2018, 13, 953-958. Y. Suganami, T. Oshikiri, X. Shi, H. Misawa, Angew. Chem. Int. Ed. 2021, 60, 18438-18442. T. Oshikiri, H. Jo, X. Shi, and H. Misawa, Chem. Eur. J. 2022, published online, DOI: 10.1002/chem.202200288