Surface plasmons (SP’s) or surface plasmon polaritons are longitudinal electromagnetic waves which propagate at the interface between a metal and a dielectric. They are localized in the interface and are sensitive to the optical properties of the materials comprising the interfacial region. As such, the SP present a unique probe to study the electrochemical interface and double layer [[1],[2]].Typical plasmonic metals/electrodes are Au and Ag thin films, which have been a subject of studies where the SPs have been used to probe the electrochemical processes such as adsorption and underpotential deposition [1,[3],[4]]. The most recent developments indicate that the SPs can be used to enhance certain electro-catalytic and photocatalytic reactions [[5]].

In this report, we present the results investigating the catalyst monolayer modification effect on plasmon resonance of Au thin films. The experiments are conducted using an attenuated total reflection arrangement in which p-polarized light is incident through a prism onto a thin (-400 A) Au film deposited on the mica substrate. (Kretschmann configuration [[6]]). The angle of incidence on the base of the prism is greater than the critical angle for the prism: electrolyte interface and SPs are excited by the evanescent wave generated under these conditions of total internal reflection. The excitation is manifested by a decrease in the intensity of reflected light. The Pd monolayer on Au were electrodeposited using procedure reported by Kolbe et al [[7],[8]], while the Pt submonolayer/monolayers were deposited on Au using Surface Limited Redox Replacement (SLRR) of Pb UPD monolayer [[9]]. The plasmon resonance frequency of Au(hkl), Pd_ML/Au(hkl) and Pt_ML/Au(hkl) and Pt_SML/Au(hkl) were investigated as a function of the applied potential in the range between -0.24 V to 0.4 V vs. SCE. The comparative analysis, between Au and catalyst monolayer modified Au(hkl) samples indicate that SP resonant frequency change is induced by both; surface charge and adsorption of H on the catalyst monolayer surface in the UPD region. The latter effect is quantified using subtractive analysis between the Au and catalyst modified Au surface and solutions with different pH. In conclusion, we present a simple model describing the catalyst monolayer effect on plasmon resonance of Au films.

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