Active Intermediates in Plasmon-Induced Water Oxidation at Au Nanodimer Structures on a Single Crystal of TiO₂

The photoinduced water-splitting reaction is a promising approach for converting sustainable sunlight into chemical energy. However, the high overpotential for traditional metal catalysts to drive oxygen evolution limits the conversion efficiency. Plasmonic metal–semiconductor systems can potentially overcome this problem, as hot electrons generated by localized surface plasmon resonance are rapidly injected into the semiconductor and hot holes are concentrated to oxidize water. The intermediates and pathways of such plasmon-induced water oxidation are currently unknown. Herein, taking Au-nanostructured TiO2 as a prototype, we investigated distinct spectral characteristics of water oxidation intermediates at electrochemical potentials close to the flat-band potential of TiO2 using in situ electrochemical surface-enhanced Raman spectroscopy. Au–O stretching vibrations are attributed to the initial oxidized-state intermediate Au(OH)3 at pH 0 or 13 and the highest-energy intermediate AuOOH at pH 7. The existence of AuOOH provides the first robust experimental evidence for the accelerated four-electron multistep reaction via plasmon-assisted water oxidation.