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

Abstract

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.

Plasmonic Photoelectrochemistry: In View of Hot Carriers

“…[ 7,11 ] Taking plasmonic Au/TiO 2 heterostructures as an example, it was commonly believed that active sites for hot‐electron‐induced reactions were on TiO 2 surfaces and active sites for hot‐hole‐driven reactions were on Au surfaces. [ 11a,b,12 ] However, recent studies suggested that active sites for hot‐hole‐driven reactions were at Au/TiO 2 interfaces. [ 2d,7,11c ] Such a discrepancy calls for more in‐depth mechanistic studies to verify physical locations of those active sites.…”

Water Oxidation under Modal Ultrastrong Coupling Conditions Using Gold/Silver Alloy Nanoparticles and Fabry–Pérot Nanocavities

“…Such plasmonic metal NPs not only serve as light‐harvesting units but also promote plasmon‐induced hot electron and hot hole (hot carrier) generation, [8, 9] and the charge separation of these hot carriers occurs at the metal/semiconductor interface, followed by reduction/oxidation reactions [10–12] . For example, gold nanoparticle (Au NP)‐loaded titanium dioxide (TiO 2 ) has been shown to function as a stable photoanode for water oxidation under irradiation by light with wavelengths longer than 550 nm in the presence of an electrical or chemical bias without noticeable loss of activity [13–17] . However, the LSPR of single‐layer Au NPs on a TiO 2 substrate cannot realize sufficient broadband light absorption, and the quantum yield of the water oxidation reaction is still low [18] …”

“…[ 10 , 11 , 12 ] For example, gold nanoparticle (Au NP)‐loaded titanium dioxide (TiO 2 ) has been shown to function as a stable photoanode for water oxidation under irradiation by light with wavelengths longer than 550 nm in the presence of an electrical or chemical bias without noticeable loss of activity. [ 13 , 14 , 15 , 16 , 17 ] However, the LSPR of single‐layer Au NPs on a TiO 2 substrate cannot realize sufficient broadband light absorption, and the quantum yield of the water oxidation reaction is still low. [18] …”

Plasmon-induced electron injection into the large negative potential conduction band of Ga2O3for coupling with water oxidation

“…20 In addition, Murakoshi et al also reported that TiO 2 was beneficial for water oxidation by studying the intermediate species during plasmon-induced oxygen evolution. 21 Therefore, improving the hole-trapping ability at the interface by the surface states of TiO 2 is an efficient way to enhance the water oxidation efficiency.…”