Multi-electron redox reactions, although central to artificial photosynthesis, are kinetically sluggish. Amidst the search for synthetic catalysts for such processes, plasmonic nanoparticles have been found to catalyse multi-electron reduction of CO under visible light. This example motivates the need for a general, insight-driven framework for plasmonic catalysis of such multi-electron chemistry. Here, we elucidate the principles underlying the extraction of multiple redox equivalents from a plasmonic photocatalyst. We measure the kinetics of electron harvesting from a gold nanoparticle photocatalyst as a function of photon flux. Our measurements, supported by theoretical modelling, reveal a regime where two-electron transfer from the excited gold nanoparticle becomes prevalent. Multiple electron harvesting becomes possible under continuous-wave, visible-light excitation of moderate intensity due to strong interband transitions in gold and electron-hole separation accomplished using a hole scavenger. These insights will help expand the utility of plasmonic photocatalysis beyond CO reduction to other challenging multi-electron, multi-proton transformations such as N fixation.

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The realization of common materials transformations in nanocrystalline systems is fostering the development of novel nanostructures and allowing a deep look into the atomistic mechanisms involved. Galvanic corrosion is one such transformation. We studied galvanic replacement within individual metal nanoparticles by using a combination of plasmonic spectroscopy and scanning transmission electron microscopy. Single-nanoparticle reaction trajectories showed that a Ag nanoparticle exposed to Au³⁺ makes an abrupt transition into a nanocage structure. The transition is limited by a critical structural event, which we identified by electron microscopy to comprise the formation of a nanosized void. Trajectories also revealed a surprisingly strong nonlinearity of the reaction kinetics, which we explain by a model involving the critical coalescence of vacancies into a growing void. The critical void size for galvanic exchange to spontaneously proceed was found to be 20 atomic vacancies.