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

Abstract Operando thermometry can help resolve open questions about the importance of thermal contributions to plasmonic photocatalysis, but identifying high‐fidelity thermometers with the requisite chemical inertness, thermal stability, and spatial resolution remains challenging. Here, it is demonstrated that a single near‐infrared laser can simultaneously excite upconverting nanoparticles (UCNPs) that serve as luminescent thermometers and photocatalyze the dimerization of 4‐nitrothiophenol (4‐NTP), which is employed as a model reaction. Due to its large anti‐Stokes shift, the UCNP thermometry signal naturally separates from the 4‐NTP Raman signal, which is used to monitor the chemical reaction, in the spectral domain. The surface temperature rise of plasmonic substrates under varying illumination intensity is systematically correlated with the reaction progress. Temperature rises exceeding 40 K are recorded at the maximum intensity used, yet lower intensities combined with external heating to achieve the same temperature rise are shown to catalyze the reaction less effectively. Furthermore, measurements performed using equivalent external heating and an intensity too low to photocatalyze the reaction display no evidence of the reaction occurring. By providing high‐fidelity operando surface temperature measurements, this method offers a valuable tool for elucidating thermal contributions to plasmonic photocatalysis.