Abstract
Plasmonic metal nanoparticles enhance chemical reactions on their surface when illuminated with light of particular frequencies. It has been shown that these processes are driven by excitation of localized surface plasmon resonance (LSPR). The interaction of LSPR with adsorbate orbitals can lead to the injection of energized charge carriers into the adsorbate, which can result in chemical transformations. The mechanism of the charge injection process (and role of LSPR) is not well understood. Here we shed light on the specifics of this mechanism by coupling optical characterization methods, mainly wavelength-dependent Stokes and anti-Stokes SERS, with kinetic analysis of photocatalytic reactions in an Ag nanocube–methylene blue plasmonic system. We propose that localized LSPR-induced electric fields result in a direct charge transfer within the molecule–adsorbate system. These observations provide a foundation for the development of plasmonic catalysts that can selectively activate targeted chemical bonds, since the mechanism allows for tuning plasmonic nanomaterials in such a way that illumination can selectively enhance desired chemical pathways.
Highlights
Plasmonic metal nanoparticles enhance chemical reactions on their surface when illuminated with light of particular frequencies
The role of localized surface plasmon resonance (LSPR) in this process is to increase the rates of charge excitation, and the energetic electron distribution associated with this mechanism is very similar to the one obtained if a high-intensity laser was illuminated on an extended metal structure
The SERS platforms used in our studies contained small aggregate clusters of silver (Ag) nanocubes with the cube length of B75 nm dispersed on a silicon substrate as shown in the optical micrograph in the inset of Fig. 2
Summary
Plasmonic metal nanoparticles enhance chemical reactions on their surface when illuminated with light of particular frequencies. It has been proposed that the interaction of LSPR with energetically accessible adsorbate orbitals can yield a charge injection process, where energized charge carriers (electrons or holes, formed by the plasmon decay) transiently populate otherwise unpopulated electronic states (orbitals), centred on the adsorbate molecule[7,8,9] In this process, the adsorbate (the entire adsorbate–nanoparticle system) is moved to a different potential energy surface and forces are induced on atoms in the adsorbate. If indirect charge transfer plays the dominant role the steady-state concentration of energetic electrons is the highest close to the Fermi level (that is, similar to Fermi–Dirac electron distribution) and the electrondriven chemical transformation would preferentially proceed through the adsorbate orbitals closest to the Fermi level This mechanism does not offer much opportunity to tune the chemical pathways by tuning the plasmonic properties of nanostructures since LSPR would only enhance the rates of electron excitation without impacting the electron distribution. The direct mechanism offers an opportunity to affect chemical selectivity by creating plasmonic nanostructures with the LSPR characteristics that enhance the rates of targeted electronic transitions and the rates of chemical transformations that proceed through only those specific transitions
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