Label-free tracking of photocatalysis on a single nanoparticle

Nanocatalysis is a rapidly growing field where nanoparticles are exploited to catalyze industrially relevant chemical reactions. The successful design of these heterogeneous catalysts is hindered by our limited understanding of the chemical dynamics occurring in the working environment of a catalyst. An example is the limited mechanistic insights available on the photocatalytic reduction of CO2 to hydrocarbons and other value-added chemicals. Monitoring catalysts in action—in situ investigation—can elucidate potential reaction intermediates and pathways, especially under the action of light. Additionally, by probing catalysis at the level of individual active nanoparticles one-at-a-time, interparticle heterogeneity in catalytic activity, information otherwise masked in ensemble measurements can be extracted and utilized for designing better catalysts. To close the knowledge gap in CO2 reduction (CO2RR) studies, we have developed a sensitive and versatile in-situ surface enhanced Raman scattering (SERS)-based chemical imaging technique that has single-nanoparticle-level spatial resolution and a 100-ms time-resolution and can probe aqueous phase reactions. Using this technique, we studied plasmon-excitation assisted CO2RR on individual Ag nanoparticle (NP) catalysts in CO2-saturated water. This in-situ investigation of Ag NP catalysts at the single-nanoparticle level captured a rich array of C1 and C2+ surface species formed in the CO2RR. The catalog of species abundant with multi-carbon compounds, such as butanol, discovered in this study, hints the favorability of kinetically challenging C–C coupling on a plasmonically excited Ag surface. Another advancement in this work was the use of isotope labeling in the single-molecule-level sensitive nanoscale probing of a NP surface. This, in turn allowed the confirmation that detected surface species are the intermediates and products of the catalytic reaction—CO2RR—rather than spurious contaminants. Additionally, by cataloging the surface species profile on individual NPs of a population, we found that catalytic behavior fluctuates from NP to NP. Conventionally, the interparticle heterogeneity in catalytic activity is attributed to factors such as structural inhomogeneities or differences in the local environment. However, in our study, the fluctuations of the species were observed both across the population and in time. We infer that the fluctuations in large part represent noise resulting from the inherent stochasticity of chemical events probed on the level of individual NPs. Ultimately, this stochasticity is manifested as a NP-to-NP variation of the species profile. We were also able to exploit the NP-to-NP variation to determine pathways responsible for the formation of multicarbon species on the surface of Ag. This work exemplifies the power of nanoscale probing for revealing the otherwise hidden molecular-level behavior of a complex catalytic system. The surface chemical knowledge made accessible by our approach here will guide the future modeling and engineering of active and selective catalysts.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste