Using plasmonically generated carriers as redox equivalents
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troduction Ever since the discovery of surface-enhanced Raman scattering (SERS) on nanostructured metals, there has been much interest in photochemistry on such surfaces.1,2 For nanostructures of coinage metals Cu, Ag, and Au, visible-light irradiation induces collective electronic excitations (i.e., localized surface plasmon resonances [LSPRs]), resulting in enhanced electric fields on the surface. In the presence of these collective electronic excitations and strong electric fields, chemistry is expected to be altered and chemical reaction rates enhanced. These expectations have reached fruition in recent years. Several studies have found that the LSPR excitation of metal nanoparticles triggers chemical reactions on the surface of the nanoparticle. As a classic example, the visible-light excitation of LSPRs of Ag nanoparticles activates the dissociation of adsorbed O2 into reactive O–,3,4 a reaction that otherwise has a negligible rate. There are several such examples of chemical reactions that are either accelerated by LSPR excitation (plasmon-enhanced chemistry or plasmonic catalysis) or driven solely by LSPR excitation (plasmon-driven chemistry), including H2 dissociation,5,6 C2H4 and C3H6 epoxidation,3,4,7
and reduction of p-nitrothiophenol.8,9 The large light absorption cross sections of coinage metal nanoparticles on plasmon resonance coupled with their inherent catalytic attributes add to the appeal and richness of plasmonic catalysis and motivate the exploration and exploitation of plasmonic metal nanoparticles as photocatalysts (without the involvement of lightabsorbing semiconductors).
Role of hot carriers When the influence of photothermal heating, invariably an outcome of LSPR excitation, is properly accounted for,10 the enhanced chemical reactivity can be attributed to the activation of adsorbates by either electric fields11 or excited carriers generated by LSPR excitation. While both mechanisms have been proposed, the role of excited carriers has received more concrete treatment, especially in the context of bond-dissociation reactions such as O2 or H2 splitting. Optical excitation of LSPRs in metal nanostructures induces a cascade of carrier dynamics, which is known from ultrafast spectroscopy,12–17 as depicted in Figure 1. Briefly, the collective electronic excitation undergoes damping on the time scale
Sungju Yu, Department of Chemistry, University of Illinois at Urbana-Champaign, USA; [email protected] Varun Mohan, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, USA; [email protected] Prashant K. Jain, Department of Chemistry, Materials Research Laboratory, Department of Physics, and Beckman Institute of Advanced Science and Technology, University of Illinois at UrbanaChampaign, USA; [email protected] doi:10.1557/mrs.2019.293
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