Understanding the Interplay of Thermal and Laser Intensity Effects on Plasmonic Photocatalysis Using Upconverting Nanoparticle Thermometry

Numerous experimental studies have demonstrated that chemical reaction rates can be significantly increased with plasmonic photocatalysis. However, the relative contribution of hot electron versus thermal effects to the observed enhancement is still an open-ended question. Isolating the role of the laser induced heating of the plasmonic surface remains challenging. Operando thermometry techniques can 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. Our previous work demonstrated that a single near-infrared (808 nm) laser can simultaneously excite NaYF₄:Nd^3+,Yb³⁺,Er³⁺ upconverting nanoparticles (UCNPs) that serve as luminescent thermometers and photocatalyze the dimerization of 4-nitrothiophenol (4-NTP) to 4,4’-dimercaptoaazobenze (DMAB) [1]. The UCNP and 4-NTP Raman signals naturally separate in the spectral domain, allowing simultaneous yet separate operando thermometry and reaction monitoring. To quantify the reaction progress, we used 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.
Previously, we utilized silver-coated silicon nanopillars as the plasmonic substrate and measured an increase in the surface temperature that was correlated with the reaction progress, but because this laser induced temperature rise is inherently coupled with the laser intensity, disentangling laser intensity and heating effects is very difficult. Separate experiments in which a thermal stage was used to raise the sample temperature and a laser intensity too low to photocatalyze the reaction was used to probe the molecules show no evidence of the reaction occurring. However, once a laser intensity high enough to photocatalyze the reaction is applied, additional external heating further enhances the reaction, underscoring the complicated coupling between laser intensity and temperature. To determine if heating is an integral component of the photocatalysis process, there are two major next steps. The first is to see how generalizable our results are for different reactions. Our recent work analyzed the conversion of 4-NTP to DMAB, a well-known model reaction, while another closely related reaction is the conversion of 4-aminothiophenol (4-ATP) to DMAB. Though seemingly chemically similar, temperature has been shown to have different effects on 4-ATP compared to 4-NTP. By repeating our experiment with 4-ATP instead of 4-NTP, we can further analyze the role of temperature.
Furthermore, if we could keep the plasmonic structure optically the same while changing its heat dissipation properties, we could maintain a constant laser intensity while changing only the laser induced temperature rise in a controlled manner. One way of achieving this goal is to alter the substrate's thermal conductivity. As opposed to the nanopillars, this study focuses on how heat dissipation can be changed by creating gold nanoislands (AuNIs) on different optically transparent substrates with varying thermal conductivities. By varying the thickness and annealing time of the deposited Au film, we were able to tune the absorption peak of the plasmonic AuNIs to be closer to the excitation wavelength of the UCNPs. Meanwhile, comparing the chemical reaction on otherwise identical plasmonic AuNI samples with different substrate thermal conductivities allows us to investigate the role of laser heating independently from laser intensity.

[1] Ye, Z., Bommidi, D. K., and Pickel, A. D. (2023). Dual-Mode Operando Raman Spectroscopy and Upconversion Thermometry for Probing Thermal Contributions to Plasmonic Photocatalysis. Advanced Optical Materials, 11(21), 2300824.