Metal-Adsorbate Interactions Modulate Molecular Reactivity in Plasmon-Driven Photocatalytic Reactions

Molecular adsorbates on the surfaces of optically excited metallic nanostructures may undergo intriguing plasmon-mediated photocatalytic transformations that are mechanistically distinct from the catalytic reactions under thermal conditions. The energy distribution of the plasmonic hot carriers, the local electric field enhancements, and the plasmonic photothermal heating are all crucial factors that may profoundly influence the kinetic enhancement of the plasmon-driven photoreactions. While the photoexcitation and relaxation of localized plasmons involve ultrafast photophysical processes that occur on the timescales from fs to ns, the plasmon-driven photochemical reactions have been observed to be kinetically much slower, strongly suggesting that the rate-limiting kinetic bottle-neck of plasmon-driven photocatalysis is associated with the interfacial chemical transformations of molecular adsorbates rather than the plasmonic hot carrier or resonance energy transfer processes. Here we use plasmon-driven reductive coupling of nitrophenyl derivative adsorbates on Ag nanoparticle surfaces as a model reaction system to investigate how the chemical nature of the metal-adsorbate interactions influences the molecular reactivity and the reaction kinetics. We systematically compare the transforming behaviors of three nitrophenyl derivatives, including nitrothiophenol, nitrophenylisocyanide, nitrophenylacetylene, which chemisorb to the Ag surfaces using different functional groups. We use surface-enhanced Raman scattering as an in situ spectroscopic tool with unique time-resolving and molecular finger-printing capabilities to fine-resolve the mechanistic and kinetic features of the plasmon-driven coupling reactions. Fast reaction rates and high reaction yields are observed when the molecular adsorbates interact with Ag through σ-donation, whereas the reactions become significantly slower if the metal-adsorbate interactions are dominated by π-back donation. The experimental observations are further corroborated by density functional theory calculations.