Mechanistic Insights on Plasmon-Driven Photocatalytic Oxidative Coupling of Thiophenol Derivatives: Evidence for Steady-State Photoactivated Oxygen

The plasmonic electron oscillations in optically excited metallic nanoparticles result in surface confinement of photon energy over much longer time scales in comparison to the unconfined photons traveling at the speed of light, thereby producing an enormous buildup of photon intensity and highly concentrated energetic hot electrons at the nanoparticle surfaces. While the plasmonic hot electrons can be harnessed to drive unconventional photocatalytic molecular transformations at the nanoparticle–molecule interfaces, a set of fundamentally important issues concerning detailed reaction mechanisms still remain poorly understood. Here we use surface-enhanced Raman scattering (SERS) as a unique time-resolving and molecular finger-printing tool to spectroscopically resolve the complex kinetics and underlying pathways of plasmon-driven oxidative coupling of thiophenol derivatives chemisorbed on the surfaces of optically excited plasmonic Ag nanostructures. A hybrid suprananostructure composed of an SiO₂ bead densely decorated with Ag nanocubes was used as both the SERS substrate and the plasmonic photocatalyst under near-infrared excitations. Through deliberately designed time-resolved single-particle SERS measurements, we have been able to pinpoint the effects of excitation power, local-field enhancement, interfacial oxygen abundance, molecular structures, and photothermal heating on the kinetics and yields of the hot electron-driven oxidative coupling reactions. Our time-resolved SERS results provide compelling experimental evidence for the steady-state photoactivated oxygen, revealing that the chemical transformations of thiophenol derivatives rather than the photo-activation of interfacial oxygen constitute the kinetic bottlenecks along the multistep reaction pathways, while the overall reaction rates are dynamically maneuvered by the photoactivated oxygen at its steady-state concentrations.