Plasmonic Photocatalysis: Is there a limit in total light enhancement?

Abstract

Plasmonic photocatalysis presents a novel approach for controlling heat and light, enabling reactions to take place under milder conditions. When metal nanoparticles are excited by specific wavelengths, they generate hot carriers. These generated hot carriers would decay and initiate reactions through both nonthermal and photothermal pathways. 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. Separating these effects remains challenging, and few studies have thoroughly explored their combined effects on overall light enhancement. In this dissertation, we summarize the efforts in establishing theoretical and experimental techniques to accurately distinguish between thermal and nonthermal contributions. A new parameter was proposed to study the overall light enhancement and to explore the limit of plasmonic behaviors under high temperatures. We begin with an overview of plasmonic photocatalysis (Chapter 1). We discuss this emerging field's fundamental background, key challenges, and opportunities. The debate on whether plasmonic photothermal heating can explain these observed hot carrier effects drives us to determine the proper role of light. Some common experimental techniques are discussed, along with their applications in separating the nonthermal and photothermal light effects. Precisely measuring the surface temperature is crucial for distinguishing between the two light effects. Previous experiments primarily used a thermal couple or IR camera to measure the local temperature, which would bring uncertainties towards the temperature measured. A novel Raman thermometry method was developed to better understand the role of photothermal behaviors in the system and validate our previous temperature measurement method in CO2 methanation on Rh/TiO2 photocatalysts (Chapter 2). We then developed a new catalyst thickness-dependent method to isolate thermal and non-thermal contributions (Chapter 3). This new method did not involve monitoring surface temperature to obtain the same thermal gradient in the catalyst bed. This method was more profound as the top surface was kept at the same height, and the bottom photothermal layers were gradually replaced by inert support. Both light effects and their contributions towards the overall light enhancement could be extracted. A new benchmark was developed by considering the enhancement from both light effects as a whole part, called overall light effectiveness. The results showed a plateau of light effectiveness under high heating temperatures and high light intensities. It shows a generalizable potential in the design of catalyst systems with optimum heating and light illumination combinations, especially with broadband light illumination such as sunlight, for achieving the most economical light-to-matter conversion in plasmonic catalysis. We then focused on increasing the light effectiveness by enhancing broadband light absorption capabilities for plasmonic photocatalysts under white light illumination (Chapter 4). Replacing parts of the catalyst with a solar absorber can significantly improve the total reaction rate under mild heating conditions with less catalyst. Through careful comparison of reaction conditions and systematic optimization of the contributions from photothermal and nonthermal effects, we demonstrate a substantial enhancement in broadband light absorption capacity and overall light effectiveness, paving the way for advanced plasmonic photocatalysts with greater efficiency and practical applicability using solar light as the energy source. Lastly, several factors were identified as limits to plasmonic photocatalytic behaviors (Chapter 5). High-temperature plasmonic behavior has captured our attention since many previous studies indicated a sublinear relationship between light intensity and enhanced production rates at elevated temperatures. By combining the heat-controlled reactor with transient absorption spectroscopy, we could investigate the relationship between electron dynamics and electron temperatures at various surrounding temperatures. In general, we explored thermal and nonthermal effects, proposing a novel approach to distinguish between them. We then introduced a new benchmark that can be applied to each photocatalytic system to quantify the overall effectiveness of light enhancement. By demonstrating the sublinear relationship between the light-enhanced production rate and light intensity, we researched the possible factors that may cause such a plateau and suggested techniques to investigate plasmonic behavior at high temperatures. Ultimately, we believe an optimal reaction condition would provide the highest overall light enhancement and a better way of utilizing energy.