Harnessing the Full Solar Spectrum: Leveraging Both Thermal and Nonthermal Pathways in Plasmonic Catalysis for Net-Zero Carbon Fuel Production

Abstract

Plasmonic photocatalysis offers a promising approach for driving chemical transformations using light under mild conditions. Upon light illumination, collective oscillations of free electron oscillations, known as localized surface plasmon resonance (LSPR), form on the surface of plasmonic nanoparticles. The decay of LSPR generates energetic electron–hole pairs, or hot carriers, which can open new reaction pathways and enhance catalytic activity and selectivity. Two primary mechanisms contribute to this process: electronic excitation (nonthermal) and photothermal heating. This dissertation investigates plasmonic photocatalysts for CO2 hydrogenation and NH3 synthesis by harnessing broadband light energy, with a particular focus on understanding and exploiting the photothermal and nonthermal contributions.Chapter 1 provides an overview of plasmonic photocatalysis, introducing the fundamental principles of LSPR, common plasmonic materials including metals and semiconductors, mechanisms of plasmon-induced chemical reactions, and representative plasmonic catalysts for CO2 reduction and N2 fixation. Emphasis is placed on the challenges of distinguishing electronic excitation from photothermal effects, and various experimental strategies, such as thermocouple calibration, infrared thermography, and a cover/uncover technique, are critically examined. In Chapter 2, a mass-dependent methodology is developed to quantitatively separate thermal and nonthermal pathways during CO2 methanation over Rh/TiO2 catalysts. A new metric, overall light effectiveness (OLE), is introduced to evaluate light utilization efficiency of catalysts. The study reveals cooperative and competitive interactions between nonthermal and photothermal effects depending on reaction conditions, providing design guidelines for optimizing light-driven catalysis. Chapter 3 builds on this framework by integrating broadband solar absorber, TiN, with Rh/TiO2 catalysts to extend light harvesting into the visible and near-infrared regions. By tuning material composition and reaction conditions, the study demonstrates that photothermal energy can be strategically introduced to complement UV- and blue-driven nonthermal processes, reducing the need for external heating and improving overall efficiency. Chapter 4 shifts focus to ammonia synthesis under ambient pressure using an earth-abundant plasmonic semiconductor, Mo2N/MoO2-x. The catalyst exhibits high activity under visible light. Both the mass-dependent method and a modified cover/uncover approach are employed to probe the light-induced mechanisms, confirming the coexistence of nonthermal and photothermal pathways. The chapter also explores the influence of catalyst composition on activity and highlights the self-activation capability of MoO3 under reaction conditions. Overall, this dissertation provides both methodological and conceptual advances in plasmonic photocatalysis. The OLE metric and associated experimental protocols offer a general framework for disentangling competing light-induced mechanisms across a variety of systems. These insights contribute to the rational design of next generation plasmonic catalysts for sustainable chemical production.