Vibrational properties of the electrochemically synthesized polyindole/single-walled carbon nanotubes composite

Abstract

Electrochemical polymerization of indole in a LiClO₄/CH₃CN solution on a single-walled carbon nanotubes (SWNTs) film was studied by cyclic voltammetry, Raman scattering and FTIR spectroscopy. Comparing the cyclic voltammograms recorded on a blank Pt electrode with those obtained when carbon nanotubes films were previously deposited onto the Pt electrode, a down-shift of the indole reduction peak potential in the latter case was observed. Raman spectroscopy studies indicate that the electrochemical deposition of polyindole (PIN) onto the SWNT film results in a breaking of SWNT bundles into individual tubes. A covalent functionalization of SWNTs with PIN in the doped state is demonstrated by FTIR spectroscopy, when an increase in the intensity of the absorption band at 1045 cm⁻¹ is observed. Besides, Raman and FTIR studies performed on samples electrochemically prepared and thereafter post-chemically reacted with an NH₄OH solution, indicate both a roping process of individual tubes with PIN as a binding agent and a strong steric hindrance effect.

Introduction

Polyindole (PIN) is an electroactive polymer, which has received a significant attention in the past several years [1], [2], [3], [4], [5], [6], [7]. This macromolecular compound is a good candidate for applications in various areas, such as electronics, electrocatalysis, active materials for anodes of batteries, anticorrosion coatings and pharmacology. Up to now, PIN was synthesized both by means of chemical and electrochemical ways. The advantages of electrosynthesized PIN samples consist in a good thermal stability [8], a high redox activity [9], a slow degradation rate in comparison with those of polyaniline and polypyrrole [10] and air-stable electrical conductivity close of 0.1 S cm⁻¹ in the doped state [11]. These properties make this polymer a tempting component in the achievement of composite materials based on PIN and single-walled carbon nanotubes (SWNTs). According to a recent paper [12], the synthesis of a composite with desired properties requires knowledge regarding the interaction between the host polymer matrix and the guest carbon nanoparticles. Among the most used experimental methods proving the functionalization of carbon nanotubes with different functional groups, molecules or polymers are studies by X-ray diffraction, measurements of conductivity and photoconductivity, photoluminescence, thermogravimetric analysis, determination of their mechanical properties, absorption in UV–VIS–NIR range, IR spectroscopy and Raman light scattering. In this paper, to investigate the interaction type between SWNTs and PIN, IR spectroscopy and Raman light scattering have been used, since these methods have proved their usefulness in the field of conducting polymers and carbon nanotubes. For conducting polymers, the most important contribution of IR and Raman spectroscopy has consisted in the demonstration of the doping process [13], [14]. For carbon nanotubes, the two experimental techniques have definitively contributed to the characterization of SWNTs, but also of double-walled carbon nanotubes (DWNTs), multi-walled carbon nanotubes (MWNTs) as well as the evidence of the n- or p-doping process, oxidation or reduction reactions, breaking of SWNTs bundles into individual tubes and more recently the separation of metallic and semiconducting SWNTs [15], [16], [17]. In the case of composites based on carbon nanotubes and conducting polymers, the two methods have often been used to elucidate the type of interaction between the two constituents [12]. Two basic processes are often reported in the case of the conducting polymers/carbon nanotubes composites: either the polymer functionalizes SWNTs and/or conducting polymer is doped with SWNTs, the latter resulting from a charge transfer between the two constituents [12], [18], [19]. Depending on the synthesis method, the functionalization of SWNTs with conducting polymers can be covalent or non-covalent [20], [21]. An answer concerning the interaction type between PIN and SWNTs is intended to be shown in this paper by complementary studies of Raman scattering and ATR-IR spectroscopy.

Despite a large number of publications related to the electrochemical polymerization mechanisms of indole, the identification of the polymerization sites has been a subject of intense debate. Different possible structures of PIN were reported as follows: 1,3; 2,3; 1,1-3,3 and 2,2-3,3 [22]. At present time, the last structure is rather accredited, namely when the macromolecular chain is characterized by a 3,3′-linkage of the repeating units. In un-doped state, PIN can exist in the reduced (aromatic) or oxidized (quinoid) state, forms known under the name of enediamine (Scheme 1a) and diimine (Scheme 1b), respectively. The molecular structure of the composite based on un-doped PIN and SWNTs, is also studied in this paper.