Reply to the ‘Comment on “Thermal effects – an alternative mechanism for plasmon-assisted photocatalysis”’ by P. Jain, Chem. Sci., 2020, 11, DOI: 10.1039/D0SC02914A

Abstract

In his Comment to our paper “Thermal effects – an alternative mechanism for plasmon-assisted photocatalysis”, Jain correctly points out that using an Arrhenius fit to the reaction rate is not enough to distinguish thermal from non-thermal effects.

Light-Off in Plasmon-Mediated Photocatalysis

“…Mechanistically, the LSPR phenomenon may influence a chemical process on a metal nanoparticle surface in three distinctly different ways, which, in principle, may manifest themselves individually or in concert: (i) photothermal heat generation, (ii) optical near-field enhancement, and (iii) direct hot-charge carrier generation in the metal and their injection into surface-adsorbed reactant species. − Ever since the hot-carrier mechanism was first proposed, one of the most important and critical questions has been how to distinguish it from the photothermal one. As a consequence, being able to appropriately answer this question has developed into a key effort in the field, spurred significantly by a recent controversy on data interpretation in key studies, − and discussions of experimental procedures for distinguishing photothermal from hot-carrier reaction enhancement processes. , …”

Dual‐Mode Operando Raman Spectroscopy and Upconversion Thermometry for Probing Thermal Contributions to Plasmonic Photocatalysis

“…Amid these advances, a major debate has emerged surrounding the relative contributions of thermal versus non‐thermal effects to the observed plasmonic enhancement. [ 5–13 ] Excitation of the LSPR initially gives rise to non‐thermal hot electrons that can initiate chemical reactions through several proposed mechanisms largely involving energy transfer from the hot electrons to molecules adsorbed on the nanoparticle surface. [ 14 ] Subsequent decay of the LSPR ultimately leads the hot electrons to thermalize with the nanoparticle lattice.…”

“…Given the strong Arrhenius temperature dependence of chemical reactions, a surface temperature rise will also increase the reaction rate, making thermal and non‐thermal contributions difficult to disentangle. [ 8–13 ] A non‐thermal mechanism provides clear advantages for performing reactions that traditionally require high temperatures at milder conditions and achieving high levels of product selectivity. [ 13 ] However, a thermal mechanism also offers the possibility of generating spatially tailored temperature profiles and replacing conventional heat sources with sunlight.…”

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

“…Plasmon-driven photocatalysis has emerged as a paradigm-shifting approach toward the solar-to-chemical energy conversion, enabling us to harness nanoscale light–matter interactions as a unique leverage to kinetically modulate interfacial molecular transformations on nanostructured metal surfaces with selectively controlled reaction outcomes. − Unambiguous elucidation of detailed mechanisms underpinning plasmonic photocatalysis, however, has often been challenging due to strong interplay among multiple plasmon-derived nonthermal and thermal effects over a broad distribution of time scales. ,,,, A widely adopted experimental strategy for mechanistic studies involves exploration of the relationship between the reaction rate and the excitation power, which delivers highly informative messages concerning the underlying reaction mechanisms. ,,− The rates of plasmon-driven photocatalytic reactions have been observed to be linearly proportional to the excitation power under moderate continuous wave (CW) illumination but may switch to a superlinear dependence when the excitation power exceeds certain threshold values or under illumination by pulsed lasers due to multiphoton absorption ,,, and plasmon-induced activation energy reduction. ,, Such superlinear power dependence is a unique feature of plasmonic photocatalysis, ,, fundamentally distinct from the sublinear power dependence commonly observed in conventional semiconductor-based photocatalysis. Another singular characteristic of plasmon-driven photocatalysis is that the reaction rate increases exponentially with the working temperature, whereas the rate of a semiconductor-driven photocatalytic reaction typically goes down at elevated temperatures. ,,, Whether the superlinearity of power dependence observed in plasmon-driven photocatalysis originates primarily from the hot carrier-related nonthermal effects or the plasmonic photothermal heating has been a vigorously debated open question well-worthy of in-depth investigations. ,,,− …”