Surface-Enhanced Raman
Scattering Study of the Kinetics of Self-Assembly of Carboxylate-Terminated <i>n</i>-Alkanethiols on Silver
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Abstract
Adsorption of 11-mercaptoundecanoic acid (MUA) on silver
from methanol and aqueous solutions was monitored <i>in situ</i> by surface-enhanced Raman scattering (SRES) spectroscopy. While
adsorption of MUA from methanol is a one-step formation of a thiol-bound
monolayer, SERS spectra reveal that monolayer formation from aqueous
solution involves interactions of both carboxylate and thiol groups
of MUA with the silver surface. Several Raman scattering bands, including
the ν(C–S), ν<sub>s</sub>(COO<sup>–</sup>), and ν(C–C), were used to investigate the evolution
of the structure of adsorbed MUA on silver surfaces. The time-dependent
profiles of these bands for assembly from aqueous solution indicate
a multistep process, which is initiated by the binding of both carboxylate
and thiol groups to silver, producing a mixture of gauche and trans
conformations. In a subsequent step, the COO–Ag interactions
are displaced by stronger S–Ag bonds, leading to ordering of
the resulting monolayer with formation of a complete SAM with all-trans
conformations. The results also showed that the adsorption process
depended strongly on the solution pH and surface potential of the
metal. These factors can significantly affect the participation and
displacement of −COO<sup>–</sup> during self-assembly
of MUA from aqueous solution