Mapping the Non-Thermal and Photothermal Energy Landscape in Plasmonic Photocatalysts

Abstract

Catalysts are critical to the production of chemical precursors and specialty products at immense scales—from commercial fertilizers to plastics, fuels, and pharmaceuticals. These chemical products have drastically improved the quality of life on our planet. However, even with selective and reaction-specific catalysts, manufacturing these materials requires a massive thermal energy demand that is traditionally sourced from non-renewable fossil fuels. As the negative effects of the fossil fuel economy on climate change mount, it has become pertinent to design more sustainable and highly selective chemical conversion routes.

To this end, a promising class of photocatalysts known as plasmonic metal nanoparticles offer the unique ability to utilize light to drive chemical transformations at their surfaces. The motivation behind using these materials in place of conventional thermal catalysts is to employ sunlight to drive chemistry, thereby supplementing or displacing the combustion of fossil fuels for heat. Leveraging the interactions between plasmonic metal nanoparticles and light represents a promising step toward carbon-neutral chemical transformations.

Over the past decade, plasmonic photocatalysts have proven promising in driving a variety of essential chemical reactions. Despite recent advances in the design and implementation of plasmonic catalysts, there remain gaps in our knowledge concerning the mechanisms that drive chemistry and enable reaction rate enhancements under illumination. To date, various reaction rate enhancement mechanisms have been invoked, including several charge-carrier (electron or hole) driven processes as well as light-induced photothermal heating of the bulk photocatalyst. Discerning the relative roles of these processes remains elusive but pertinent—because each may have fundamentally different consequences for the rational design of reaction-specific catalysts.

In this dissertation, we first describe the significance of photoreactor design in enabling the assessment of photothermal heating in plasmonic catalysts. We introduce a new, patent-pending photoreactor that allows for the direct, in-operando measurement of thermal and photothermal temperature profiles in plasmonic catalyst beds. We employ this reactor to characterize photothermal heating in plasmonic catalysts with unprecedented accuracy.

Furthermore, we evaluate the influence of nanoparticle density and clustering on local and nonlocal rate enhancement mechanisms. We find that even as nonlocal contributions (i.e., photothermal heating) increase, localized contributions to plasmon-mediated reaction rates are non-negligible and account for ~50% of the measured photo-rates in our model system of CO oxidation over a Ag-based catalyst. Pairing these observations with other experimental approaches, we investigate photothermal heating at localized sites of interest including catalyst support sites, nanoparticle sites, and within adsorbates at the surface of these nanoparticles.

Finally, we touch on the concept of hybrid plasmonics—materials that interface plasmonic metal nanoparticles with non-plasmonic but more catalytically functional metal sites. We compare how light absorption changes when we couple plasmonic and non-plasmonic metals in different hybrid geometries. Using a combination of computational and experimental characterization, we identify a new family of hybrid plasmonics (single atom or dilute surface alloy plasmonics) that should be ideal for increasing total light absorption and utilization by the non-plasmonic, catalytically active sites.