Hot carrier multiplication in plasmonic photocatalysis

Light-induced hot carriers derived from the surface plasmons of metal nanostructures have been shown to be highly promising agents for photocatalysis. While both nonthermal and thermalized hot carriers can potentially contribute to this process, their specific role in any given chemical reaction has generally not been identified. Here, we report the observation that the H₂–D₂ exchange reaction photocatalyzed by Cu nanoparticles is driven primarily by thermalized hot carriers. The external quantum yield shows an intriguing S-shaped intensity dependence and exceeds 100% for high light intensities, suggesting that hot carrier multiplication plays a role. A simplified model for the quantum yield of thermalized hot carriers reproduces the observed kinetic features of the reaction, validating our hypothesis of a thermalized hot carrier mechanism. A quantum mechanical study reveals that vibrational excitations of the surface Cu–H bond is the likely activation mechanism, further supporting the effectiveness of low-energy thermalized hot carriers in photocatalyzing this reaction.

Metal nanostructures that couple strongly to light and exhibit a driven, resonant oscillation of their free electrons, known as a localized surface plasmon resonance (LSPR), have attracted considerable recent interest for their role in light-driven chemical transformations (1). Both monometallic (2–5) and “antenna-reactor” multimetallic (6–9) plasmonic photocatalysts have been shown to be superior to traditional thermocatalysts in terms of activity, selectivity, and stability for a number of fundamental and industrially significant reactions. Chemical reactions involving adsorbates electronically (1) or vibrationally (10) activated by the energetic electrons or holes resulting from LSPR decay provide a widely accepted mechanism in plasmon-induced photochemistry (11, 12). Hot carriers (HCs) have been shown to reduce activation barriers and facilitate molecular rearrangements.The nonradiative decay of a surface plasmon results in the generation of two types of HCs: 1) the initial nonthermal HCs first generated by Landau-like damping, and 2) thermalized HCs, i.e., lower energy HCs created during relaxation and carrier multiplication through electron-electron interaction (13). The distribution of these relaxed HCs can be described phenomenologically by a high-effective-temperature Fermi-Dirac distribution (14), which is why they are referred to as thermalized HCs. Nonthermal HCs are typically regarded as the primary mediator of most HC-induced chemical processes, primarily because of their relatively long lifetime compared to HCs in extended metals and their higher energies relative to thermalized HCs. Nevertheless, the potential contribution of thermalized HCs is never excluded. Avanesian and Christopher have investigated the effect of thermalized HCs theoretically and shown that the contribution of thermalized HCs increases with the decrease of charge transfer barrier (15). However, experimental research highlighting the contribution of thermalized HC and exploring the corresponding kinetic behavior is still lacking.Here, we report a plasmonic photocatalysis system showing singular behavior where high–effective-temperature HCs appear to be the dominant contributor. Specifically, Cu nanoparticles grown onto a mixed metal oxide support photocatalyze H₂ and D₂ dissociation, detected by the formation of HD under laser illumination. The contribution of HCs was extracted from the total photocatalytic rate by subtracting the photothermally induced reaction rate, determined from the measured thermocatalytic rate together with simulated temperature distributions. The HC-mediated reaction rate exhibits a unique S-shape intensity dependence and an external quantum yield (EQY) greater than 1. An adiabatic model was applied to calculate the quantum yield of the thermalized HCs, which reproduces the quantum yield of the HC-mediated reaction rate and supports the observed above-unity quantum yield. First-principles quantum mechanical calculations identified the rate-determining step (the desorption of the diatomic molecule) and showed that HC activation to enable excited-state, lower-barrier HD desorption is likely achieved through sequential vibrational excitations of the surface Cu–H bond, which can be driven readily by low-energy HCs.