Fe³⁺to Fe²⁺Conversion by Plasmonically Generated Hot Electrons from Ag Nanoparticles: Surface-Enhanced Raman Scattering Evidence

Abstract

We have demonstrated using surface-enhanced Raman scattering (SERS) of cyanide that “hot” electrons can be plasmonically generated from nanostructured Ag and/or Au substrates. For this, we first fabricated poly(ethylenimine) (PEI)-capped Ag and Au nanoparticle films onto glass slides. Cyanide was then adsorbed (via the carbon lone-pair electrons) onto the films to obtain the NC/Ag and NC/Au systems. Subsequently, Fe3+ or Fe2+ ions were bound to the pendant nitrogen atoms to obtain the corresponding Fe3+/NC/Ag and Fe3+/NC/Au systems or the Fe2+/NC/Ag and Fe2+/NC/Au systems. All these systems were stable under laser light illumination at 632.8 nm, with CN stretching bands at 2159 and 2143 cm–1 for the Fe3+/NC/Ag and Fe2+/NC/Ag systems, respectively, and at 2180 and 2158 cm–1 for the Fe3+/NC/Au and Fe2+/NC/Au systems, respectively. Under the laser light illumination at 514.5 nm, the Fe3+/NC/Ag system was gradually converted to the Fe2+/NC/Ag system, with the CN stretching band shifting from 2159 to 2143 cm–1. This Fe3+ to Fe2+ conversion is due to the hot electrons plasmonically generated from the PEI-capped Ag nanoparticle film. Furthermore, it appears as though the generation of hot electrons is an efficient process, because Fe3+ to Fe2+ conversion was facile although the Fe3+/NC/Ag system was buried in ice at the liquid N2 temperature (77 K). In turn, the infeasible oxidation of Fe2+ to Fe3+ is due to the formation of so-called “hot” holes, which if generated, would be reactive only to the species in contact with the Ag nanoparticles. Because the PEI-capped Au film was not SERS-active at 514.5 nm, the generation of hot electrons with excitation occurring at shorter wavelengths could not be examined, although the Fe3+/NC/Au system could be converted to the Fe2+/NC/Au system (or vice versa) by a brief contact with a mild solution of borohydride (or permanganate).