Proving the Dual Electronic Structure of Charged Metal-Molecule Interfaces: SERS of Cyanide Adsorbed on a Nanostructured Silver Electrode
2020
Experimental
and theoretical calculations confirm, for the first
time, the existence of two different kinds of electronic structures
of a surface complex formed by a particular molecule bonded to charged
metal electrodes, clusters, or nanoparticles. Surface-enhanced Raman
scattering (SERS) of cyanine adsorbed on a silver electrode shows
three regions, which are selected by the electrode potential and characterized
by the differentiated response of the vibrational wavenumbers of the
ν(CN) stretching band to the electrode potential. The combination
between the experimental SERS and DFT calculations has allowed for
relating the three regions to chemisorbed (C-hybrid) and physisorbed
(P-hybrid) surface complexes, where cyanide is bonded through the
carbon on top of a single silver atom of the surface and to bidentate
species, respectively. The electrode potential selects one or another
type of electronic structure of the surface complex, which are of
different natures and with a differentiated response to the applied
potential. The electric potential tunes smoothly the wavenumbers,
bond energies, and injected charges of the P-hybrid at more negative
potentials than that of the zero charge of the electrode, but the
very strong C-hybrid prevents significant changes of these properties
at positive excesses of charge. The existence of the dual electronic
structure of metal-molecule interfaces might require reinterpreting
experiments that are usually discussed by resorting to, for instance,
the reorientation of the adsorbate, the formation of complexes with
different stoichiometries, the existence of nonequivalent local sites
on the surface, or to instrumental artifacts.
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