In-Situ Surface-Enhanced Raman Scattering Observation of Intermediate Species at Plasmon-Induced Water Oxidation Process Using Au Nanostructures on Titanium Oxide Single Crystal

Recent attempts to use localized surface plasmon resonance (LSPR) for photoexcitation of molecules suggests the possibility to modify the excitation processes and the selection rule of electrons [1, 2].  Among several choices of the applications of LSPR, the system of Au‑nanostructured titanium dioxide (TiO₂) is one of promising approaches for plasmon‑enhanced photoenergy conversion under illumination of visible and near infrared light . It has been suggested that photo-induced electron injection from the Au structure into the TiO₂ conduction band occurs by localized surface plasmons excitation. In this system, Au‑nanostructured TiO₂ enabled efficient water oxidation owing to the excitation of multiple electrons and holes generated by near‑infrared light, and the evolution of O₂ via four‑electron oxidation of water. In this work, size‑controlled Au‑nanostructures was fabricated on a TiO₂ single crystalline by means of angle‑resolved nanosphere lithography (AR‑NSL) to measure surface‑enhanced Raman scattering (SERS) from the intermediate species at water oxidation process at the electrode‑electrolyte interface under near-infrared light irradiation. SERS spectra were obtained from the size‑controlled Au nano‑dimer (Au‑NSL) arrays on a single crystalline TiO₂ immersed in 0.1 M NaF solution. For comparison, we also measured SERS spectra from Au‑NSL on indium tin oxide (ITO). Observed spectral feature at ~580 cm^‑1 can be attributed to Au‑O stretching vibrations of adsorbates. This wavenumber suggests that adsorbed hydroxide anion with Au‑O bonding are produced as intermediate of water oxidation reaction at Au/TiO₂. It should be noteworthy that Au‑O vibration was observed at more negative potential on Au/TiO₂ than that of Au/ITO. Negative shift is attributed to the water oxidation on Au/TiO₂ is assisted by electrons and holes generated by plasmon‑induced excitation.

References

[1] M. Takase et al., Nature Photonics, 7, 550 (2013).

[2] F. Nagasawa, M. Takase, H. Nabika, K. Murakoshi, "Depolarization of Surface-Enhanced Raman Scattering Photons from a Small Number of Molecules on Metal Surfaces", in Vibrational Spectroscopy at Electrified Interfaces, Eds. C. Korzeniewski, B. Braunschweig and A. Wieckowski: Wiley, NY (2013).