Probing the Role of Hot Carriers and Photothermal Effects in Plasmonic Photocatalysis with Ultrafast Surface-Enhanced Raman Spectroscopy

Abstract

Plasmonic materials convert light into chemical energy, which is used to drive chemical reactions with greater efficiency and selectivity than traditional catalysts. To enhance the efficacy of these plasmon-driven processes, it is critical to determine the underlying mechanisms that occur once plasmons are excited by light. However, the manner in which these materials convert light into chemical energy is poorly understood due to the fast time scales of energy partitioning into various plasmon decay pathways, including hot carrier generation, localized heating, and enhanced electromagnetic fields. To that end, I have developed ultrafast surface-enhanced Raman spectroscopy (SERS) as a technique to probe the fundamental interactions between plasmons and molecules to elucidate a possible mechanism for plasmon-driven photochemistry. Initial studies using ultrafast SERS examined plasmon-molecule interactions to probe how hot electron generation upon plasmon decay affects adsorbed molecules. The observed photophysical response of 4-nitrobenzenethiol (4-NBT) adsorbed to aggregated gold nanoparticles allowed for the quantification of charge delocalization across the plasmonic substrate. The surprisingly large number of charges available is promising for hot electron driven plasmonic processes. Later studies focus on the modification of the ultrafast SERS instrument for ultrafast nanoscale Raman thermometry measurements. The ability to measure the localized heating of adsorbates on plasmonic materials is essential for understanding the contribution of localized heating in plasmonic photocatalysis. Follow-up studies examine the effect of the local environment upon plasmonic heating of adsorbed molecules. The local environment is an important consideration as catalysts are commonly stabilized by various catalytic support materials. Finally, I characterize the plasmonic properties of a new non-noble plasmonic material, consisting of copper selenide nanoparticles, by measuring their SERS enhancement factor. Additionally, copper selenide nanoparticles are promising photocatalysts as shown by their ability to dimerize 4-NBT in the presence of light. New plasmonic materials, like copper selenide nanoparticles, are exciting earth-abundant and cost-effective alternatives to the more traditional noble metal photocatalysts. Herein, I show that ultrafast SERS provides a unique approach to examine plasmon-molecule interactions on the picosecond time scale of chemical reactivity. This technique has the potential to determine the underlying processes that directly influence plasmon-mediated catalysis and to provide insights for the development of more efficient plasmonic photocatalysts.