Thermal effects – an alternative mechanism for plasmon-assisted photocatalysis

Abstract

Recent experiments claimed that the enhancement of catalytic reaction rates occurs via the reduction of activation barriers driven by non-equilibrium ("hot") electrons in plasmonic metal nanoparticles. These experiments place plasmonic photo-catalysis as a promising path for enhancing the efficiency of various chemical reactions. Here, we argue that what appears to be photo-catalysis is in fact thermo-catalysis, driven by the well-known plasmon-enhanced ability of illuminated metallic nanoparticles to serve as heat sources. Specifically, we point to some of the most important papers in the field, and show that a simple theory of illumination-induced heating can explain the extracted experimental data to remarkable agreement, with minimal to no fit parameters. We further show that any small temperature difference between the photocatalysis experiment and a control experiment performed under uniform external heating is effectively amplified by the exponential sensitivity of the reaction, and very likely to be interpreted incorrectly as "hot" electron effects.

Plasmonic metal nanostructures: concepts, challenges and opportunities in photo-mediated chemical transformations

“…However, this popular explanation has been questioned recently ( Sivan et al., 2019a , 2019b , 2020 ; Baffou et al., 2020 ; Dubi et al., 2020 ). Sivan et al.…”

“…Photothermal effects and energetic charge carriers from nonradiative plasmon decay are accounted for the observed desirable reaction results at illuminated PMNs. However, the individual contributions of photothermal effects and energetic charge carriers in PMCRs are still under vigorous debate ( Zhou et al., 2018 , 2019 ; Sivan et al., 2019a , 2020 ; Baffou et al., 2020 ; Dubi et al., 2020 ; Jain et al, 2020 ; Sivan et al., 2020 ). In addition, the specific mechanism by which energetic charge carriers interact with molecules that undergo chemical reactions needs to be further elucidated ( Kale et al., 2013 ; Brongersma et al., 2015 ; Mubeen et al., 2013 ).…”

Increase in CO2 reduction rate via optical near-field effect

“…Several reports have been published on the thermal effects in metallic nanoparticles. 29,30 The temperature increase can be described as follows: 31…”

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

“…The key point is that, as our theoretical work demonstrated, 4 two points seem to contradict the idea that "hot" electrons somehow contribute to the reaction rates in the papers we discuss in ref. 1. The rst is that only a tiny fraction of the illumination power ($10 À7 to 10 À10 ) actually goes to generating hot electrons, and the second is that the number of "hot" electrons nevertheless increases by many orders of magnitude, but reaction rates only rise by a moderate 1-2 orders of magnitude.…”

“…5 Moreover, in ref. 1 we evaluate the photothermal conversion coefficient independently from both tting to the data ( Fig. 5) and from any direct calculation (Section II of the SI to ref.…”