Manipulating Plasmon-Generated Hot Carriers for Photocatalysis

Abstract

Localized surface plasmon resonance (LSPR), a distinctive optoelectronic property of plasmonic nanocrystals, arises from the collective oscillation of conduction electrons in resonance with incident light. The excitation of LSPR confines incident light near the surface of plasmonic nanocrystals and amplifies the local electric field. Moreover, the frequency of LSPR is highly tunable in the visible and near-IR regions, allowing plasmonic nanocrystals to efficiently absorb and scatter light across the solar spectrum. Such a property makes plasmonic nanocrystals promising candidates for utilizing solar irradiation to drive chemical reactions, a process known as plasmonic photocatalysis. Upon the resonant excitation of LSPR, energetic hot electrons and holes are generated via the nonradiative decay of LSPR in plasmonic nanocrystals. Those hot carriers can be transferred into the molecular orbitals of adsorbed reactants, enabling chemical transformations at the surface of nanocrystals. However, during the charge transport within plasmonic nanocrystals, hot carriers rapidly relax into lower-energy states. As a result, their energy is often dissipated to the lattice as heat, increasing the local temperature rather than directly contributing to chemical reactions─posing a fundamental challenge to achieving efficient solar-to-chemical energy conversion using plasmonic nanocrystals.

To address this challenge, our group has developed multiple strategies to control the lifetime, energy level, and spatial distribution of plasmon-generated hot carriers to enhance the photocatalytic activity of Au nanocrystals. To extend the lifetime of hot carriers to match the slow kinetics of chemical reactions, Au nanocrystals were attached to an n-type semiconductor to form a heterojunction. This structure was found to prolong the lifetime of hot electrons through efficient spatial separation of hot electrons and holes, facilitated by the Schottky barrier at the metal/semiconductor interface. In parallel, decorating Au nanocrystals with redox-active molecules was shown to extend the lifetime of hot holes. Those hot holes were chemically stabilized and trapped within the bonds of the redox-active species, allowing them to participate in subsequent chemical reactions. Furthermore, a direct correlation between the activity of hot-electron-driven reduction reactions and the size of plasmonic nanocrystals, as well as between hot-hole-driven oxidation reactions and the wavelength of incident light, was established. Those observations demonstrated that energy levels of hot carriers involved in chemical reactions can be manipulated by tuning the size of nanocrystals and the wavelength of light. Moreover, positively charged molecules with facet-selective adsorption on Au nanocrystals were found to stabilize the plasmon-generated hot electrons, enabling control over the spatial distribution of hot carriers. Manipulating plasmon-generated hot carriers not only enhances the kinetics of plasmon-driven chemical reactions─such as oxygen evolution, hydrogen evolution, and nanocrystal growth─but also introduces new reaction pathways in those chemical processes, paving the way for highly efficient plasmonic photocatalysis.