Photothermal Effect, Local Field Dependence, and Charge Carrier Relaying Species in Plasmon-Driven Photocatalysis: A Case Study of Aerobic Nitrothiophenol Coupling Reaction

Abstract

Optical excitation of plasmonic electron oscillations confined by metallic nanoparticles provides a unique means of driving unconventional photocatalytic transformations of molecular adsorbates on the nanoparticle surfaces. Photothermal heating, local-field enhancement, and hot carrier generation have been identified as three major plasmon-induced photophysical effects, all of which are directly relevant to plasmon-driven photocatalysis. However, delineation of the contribution of each effect has long been challenging due to the strong synergy among the three effects and the mechanistic complexity of plasmon-driven molecular transformations. Aiming at unambiguously elucidating the photothermal effect, local-field dependence, and hot carrier channeling mechanisms that underpin plasmondriven photocatalysis, we conducted a detailed case study on the aerobic reductive coupling of p-nitrothiophenol chemisorbed on Ag nanocube surfaces under near-infrared excitations. We used surface-enhanced Raman scattering (SERS) as a plasmon-enhanced, molecular fingerprinting spectroscopic tool to track the plasmon-driven structural evolution of molecular adsorbates in real time, based on which we were able to correlate the molecule-transforming kinetics with local-field intensities and photothermal heating at the nanoparticle surfaces. The information extracted from the time-resolved SERS results allowed us not only to clarify several controversial issues regarding the photothermal effect and local field dependence but also to unravel a unique function of surface-adsorbed molecular oxygen as an interfacial charge carrier relaying cocatalyst that works in conjunction with the plasmonic Ag photocatalysts to mediate the multistep coupling reaction.

Simple experimental procedures to distinguish photothermal from hot-carrier processes in plasmonics

“…In the case of a photochemical process in plasmonics, such as near-field enhancement of photochemical reactions or hot-charge-carrier-assisted redox reactions at the nanoparticle surface, the rate η 92 , 2016 93 , 2017 26,94 , 2018 23,24,41,42,56,84,95 , 2019 22,23,27,38,[48][49][50][96][97][98][99] , 2020 [100][101][102][103] photons and therefore to the incident light power impinging on the sample. This assumption holds true for CW illumination under moderate light power and may deviate toward a super-linear dependence for very high power 31 or under fs-pulsed laser illumination due to multiphoton absorption [32][33][34][35] .…”

Decoding the kinetic limitations of plasmon catalysis: the case of 4-nitrothiophenol dimerization

“…Consequently, many publications use quasi-single molecular kinetics to extract a reaction rate of dimerization. [20,23,25,36] However, to be the limiting step, the activation by energetic electrons must be the slowest process in the reaction. This is rather unlikely, as the thermalization time of energetic electrons, which is the timescale in which the activation must occur, is in the order of a few fs.…”

“…[12] Examples range from the dissociation of H2, [11,13] via the reduction H2O [14] and of CO2, [15] to complex organic N-N or C-C coupling reactions. [16][17][18][19][20][21][22][23][24][25] The latter reaction type is particularly interesting as it forms the basis for true photosynthetic capabilities.…”

“…In order to advance the discussion on the role of photoheating in plasmon-driven chemistry, we investigated the light-intensity dependence of the reaction rate of a deliberately complex multistep reaction: the photodimerization of 4-nitrothiophenol (4NTP) to 4-,4'mercaptoazobenzne (DMAB) in the presence of gold nanoparticles. As this reaction has become somewhat the model instance for plasmon induced coupling reactions, [17][18][19][20][21][22]24,25,30] it is of no surprise that several studies have already approached the influence photoheating in the past.…”

“…[17,[20][21][22] On the one hand, Golubev et al demonstrated that under reaction conditions that do not cause a reduction of 4NTP upon illumination, the reduction could be started by additionally increasing the temperature. [21] Moreover, Zhang et al presented a superlinear increase of the reaction rate on temperature, [20] which can be interpreted as sign of a photothermal effect. On the other hand, seemingly opposing results were published by Keller and Frontiera, who measured the photoheating by ultrafast Raman thermometry.…”