Session: 11-58-01: Nanoscale Thermal Transport

Paper Number: 119760

119760 - Dual-Mode Operando Thermometry and Reaction Monitoring for Probing Thermal Contributions to Plasmonic Photocatalysis 

Many experimental demonstrations have shown that chemical reaction rates can be strongly enhanced when the reactions are performed on the surfaces of plasmonic nanostructures, a phenomenon known as plasmonic photocatalysis. However, the relative contribution of hot electron versus thermal effects to the observed enhancement remains sharply contested. In particular, isolating the role of local laser-induced heating of the plasmonic surface remains challenging. Operando thermometry techniques can thus play a critical role in elucidating the physical mechanisms behind plasmonic photocatalysis if high-fidelity thermometers with the requisite chemical inertness, thermal stability, and spatial resolution can be identified. Here, we demonstrate that a single near-infrared (808 nm) laser can simultaneously photocatalyze the dimerization of 4-nitrothiophenol (4-NTP) to 4,4’-dimercaptoazobenze (DMAB) and excite NaYF₄:Nd³⁺,Yb³⁺,Er³⁺ upconverting nanoparticles (UCNPs) that serve as luminescent thermometers. The UCNP emission is anti-Stokes shifted to the visible wavelength range, while the 4-NTP Raman signal, which we use to monitor the chemical reaction, remains much closer to the excitation wavelength. The two signals thus naturally separate in the spectral domain, allowing simultaneous yet separate operando thermometry and reaction monitoring. 

 

We use this dual-mode operando thermometry and reaction monitoring approach to study the photocatalyzed dimerization of 4-NTP on plasmonic substrates that consist of silver-coated silicon nanopillars. After adsorbing 4-NTP molecules onto the nanopillar surface, we drop cast colloidal dispersions containing UCNPs approximately 35 nm in diameter onto the same surface. Following this co-deposition of 4-NTP and UCNPs, we can simultaneously detect both the 4-NTP Raman and UCNP luminescence signals at any location on the substrate. We show that the same range of excitation intensity can both excite the UCNP thermometers and progressively photocatalyze the 4-NTP dimerization, and we record temperature rises exceeding 40 K at the maximum excitation intensity we employ. To quantify the reaction progress, we use the intensity ratio of a Raman peak corresponding to one of the N=N stretching modes of DMAB versus a Raman peak corresponding to the NO₂ symmetric stretching mode of 4-NTP. Measurements performed at different excitation intensities and across different locations on a plasmonic substrate show a clear correlation of the reaction progress with the surface temperature rise. However, when we use an excitation intensity too low to photocatalyze the reaction and raise the sample temperature as high as 340 K using a thermal stage, the reaction never occurs. While these results indicate that the photocatalysis mechanism cannot be purely thermal, further experiments in which we use the lowest excitation intensity that can photocatalyze reaction and subsequently raise our thermal stage to 320 K and then 340 K show that heating can in fact enhance the reaction. We thus conclude that although heating alone cannot catalyze the reaction, local surface heating cannot be ruled out as an integral component of the photocatalysis process.

Presenting Author: Andrea Pickel University of Rochester

Presenting Author Biography: Andrea Pickel joined the Department of Mechanical Engineering at the University of Rochester as an Assistant Professor in July 2019. She received her Ph.D. in Mechanical Engineering from the University of California, Berkeley in May 2019, where she was supported by a National Science Foundation (NSF) Graduate Research Fellowship and a UC Berkeley Chancellor’s Fellowship. She received her B.S. in Mechanical Engineering with University and College Honors from Carnegie Mellon University in 2014. Her current research focuses on harnessing the unique properties of luminescent materials to develop nanothermometry techniques for challenging operating environments. Andrea is the recipient of a 2020 American Chemical Society Petroleum Research Fund (ACS PRF) Doctoral New Investigator Award, a 2021 University of Rochester Furth Fund Award, and a 2022 NSF CAREER Award. Her teaching contributions have been recognized with the 2023 G. Graydon Curtis ’58 and Jane W. Curtis Award for Non-Tenured Faculty Teaching.

Authors:

Andrea Pickel University of Rochester