Origin of Superlinear Power Dependence of Reaction Rates in Plasmon-Driven Photocatalysis: A Case Study of Reductive Nitrothiophenol Coupling Reactions

The superlinear dependence of the reaction rate on the power of the excitation light, which may arise from both thermal and nonthermal effects, has been a hallmark of plasmon-driven photocatalysis on nanostructured metal surfaces. However, it remains challenging to distinguish and quantify the thermal and nonthermal effects because even slight uncertainties in measuring the local temperatures at the active surface sites may lead to significant errors in assessing thermal and nonthermal contributions to the overall reaction rates. Here we employ surface-enhanced Raman scattering as a surface-sensitive in situ spectroscopic tool to correlate detailed kinetic features of plasmon-mediated molecular transformations to the local temperatures at the active sites on photocatalyst surfaces. Our spectroscopic results clearly reveal that the superlinearity in the power dependence of the reaction rate observed in a plasmon-driven model reaction, specifically the reductive coupling of para-nitrothiophenol adsorbates on Ag nanoparticle surfaces, originates essentially from photothermal heating rather than nonthermal plasmonic effects.