Untangling Thermal and Nonthermal Effects in Plasmonic Photocatalysis

Abstract

Plasmonic photocatalysis exploits the strong light-matter interactions of small metal nanoparticles and offers a sustainable route for the synthesis of fuels such as hydrocarbons and ammonia (NH3) from light. Illumination of tailored plasmonic photocatalysts or traditionally thermal catalysts with intrinsic plasmonic properties leads to several photo-physical effects including: (1) generation of hot carriers, (2) photothermal heating, and (3) local enhancement of the electric field. Demonstrations of the excitation of localized surface plasmon resonances in plasmonic photocatalysis have shown enhanced reaction rates and improved product selectivity at reduced temperatures which alleviates several problems found in thermally-driven processes. While the injection of hot carriers from the metal nanoparticles is usually proposed as the dominant mechanism, the contribution of plasmon-induced heating must not be neglected. To understand the underlying mechanism in these plasmon-driven processes, the intertwined thermal and nonthermal effects from light must be untangled. This dissertation summarizes our efforts in establishing theoretical and experimental techniques to accurately distinguish between thermal and nonthermal contributions. The intrinsic plasmonic and catalytic properties of supported rhodium (Rh) and ruthenium (Ru) catalysts are investigated in two model reactions of plasmonic photocatalysis: carbon dioxide (CO2) hydrogenation and NH3 synthesis.We begin with an overview of plasmonic photocatalysis (Chapter 1). Fundamental background along with key challenges and opportunities of this emerging field is discussed. In alignment with recent demonstrations, our initial observation of reaction rate enhancement and photo-induced product selectivity in CO2 hydrogenation on Rh-c/Al2O3 is attributed to the unique hot carrier mediated process. The fervent debate on whether these observed hot carrier effects can be simply explained by plasmonic photothermal heating motivated our quest to determine the true role of light. For an improved understanding of the nonthermal contribution in plasmonic photocatalysis, we must first address the effects of photothermal heating. Chapter 2 describes a multi-thermocouple strategy to monitor the dynamic thermal profiles of the catalyst under reaction conditions. Using this method, we derive an effective thermal contribution for illuminated conditions to understand how photogenerated carriers enhance the nonthermal reaction rate for CO2 methanation on Rh/TiO2 photocatalysts. We investigate the effect of support and reaction order for CO2 and H2 to gain insight on the mechanism of nonthermal reactions.This approach is then applied to investigate the plasmonic properties of a traditionally thermal Ru-based catalyst for NH3 synthesis in Chapter 3. It is shown that weak and broadly absorbing plasmonic properties of Ru enables the opportunity to investigate photothermal heating effects. Light can be used to produce controlled thermal gradients within the catalyst to achieve a balance between thermodynamics and kinetics. Due to the contrast in thermal profiles produced under dark thermal and illuminated conditions, residual effects related to the photothermal heating may be misrepresented as nonthermal effects. We experimentally capture both photothermal and thermal contributions via indirect illumination of the catalyst and confirm that this system is dominated by photothermal effects. We then employ this indirect illumination technique in plasmon-enhanced CO2 methanation on a Rh/TiO2 photocatalyst to verify the proposed hot-carrier mediated process from Chapter 2. The extracted nonthermal methane (CH4) production rate has a linear dependence on the top surface temperature, distinctly different from an exponential dependence for thermal catalysis. Interestingly, the apparent quantum efficiency from the nonthermal contribution has no dependence on light intensity but maintains a linear dependence on top surface temperatures between 200 and 300 oC. However, past a threshold temperature of ~350 °C, heat begins to affect the light-driven reaction negatively as the reverse reaction of CH4 reforming is also enhanced by illumination.Throughout the debate over the dominant mechanism in plasmonic photocatalysis, the observation of product selectivity cannot be explained due to thermal effects. Chapter 5 revisits our initial investigation of plasmon-enhanced CO2 hydrogenation with our newly established experimental techniques to attest that plasmon-induced product selectivity in the nonthermal reaction occurs through a hot carrier mechanism. Under illumination, the rate of CH4 is significantly enhanced and this photo-selectivity can be attributed to selective activation of specific reaction intermediates by photo-generated hot electrons. The strategies by which we discriminate thermal and nonthermal contributions and extract the reaction rate and efficiency of hot carrier driven reactions may be applied universally to any explorations of plasmonic photocatalysis. Our analysis affirms that plasmonic behavior provides new control over the catalytic behaviors of metal nanostructures when the mechanism is thermal, nonthermal, or both.