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Interfacial chemical effect evidenced on SERS spectra of polyaniline thin films deposited on rough metallic supports
Author(s) -
Baibarac M.,
Mihut L.,
Louarn G.,
Mevellec J. Y.,
Wery J.,
Lefrant S.,
Baltog I.
Publication year - 1999
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/(sici)1097-4555(199912)30:12<1105::aid-jrs507>3.0.co;2-3
Subject(s) - polyaniline , delocalized electron , materials science , polarizability , raman spectroscopy , molecule , metal , polymer , raman scattering , chemical physics , chemical engineering , nanotechnology , chemistry , organic chemistry , composite material , optics , polymerization , physics , metallurgy , engineering
Surface‐enhanced Raman scattering (SERS) has proved to be an effective technique for studying the structural properties of conducting polymer thin films. The enhancement process has a twofold origin, electromagnetic and chemical. The electromagnetic enhancement, which is the dominant mechanism in SERS generation, consists in the excitation of localized and delocalized surface plasmons (SPs) in the metallic support of the thin film. The Raman emission of the adsorbed molecules on the metal surface (the most efficient being Ag, Au and Cu) is due to the intense evanescent electromagnetic field located at the interface between the metal and the surrounding medium. The second enhancing mechanism for SERS is of chemical origin, involving the formation of new chemical bonds between the molecules and the metal surface, with the polarizability thus becoming considerably higher than that of the free molecules. This mechanism is as a rule accompanied by a metal–molecule or molecule–metal charge transfer, which partly accounts for the success of the SERS studies on conducting polymers. Unfortunately, these studies have revealed that the chemical effects at the polymer–metal interface varied substantially depending on the various types of polymer and metallic support. In this context, polyaniline containing two different entities (a reduced and an oxidized state) in its repeating units exhibits specific alterations in its SERS spectra depending on the type of metallic support. This paper presents new results concerning the structure of emeraldine‐base and emeraldine‐salt polyaniline thin films deposited on rough Ag and Au supports. The effect on the SERS spectra of the polymer–metal and polymer–ambient interface chemical reactions is also described. The presence of an interface compound depending on the oxidizing properties of the metallic support has a strong influence on the SERS spectra, no matter how the PAN films were deposited on the support (whether by solvent evaporation or by an electrochemical process, i.e. cyclic voltammetry). When an emeraldine base is doped with HSO 4 − ions, it turns into an emeraldine salt, which exhibits a disordered state in its macromolecular chain, leading to a modified profile of the ∼1162 cm −1 Raman line which is associated with the CH bond of the quinoid ring. The Lorentzian profile is altered by the addition of a Gaussian profile component. For a rough Ag support, the SERS spectra show that the transformation of emeraldine base films into emeraldine salt films is a reversible process. Copyright © 1999 John Wiley & Sons, Ltd.