Hybrid organic–inorganic materials based on poly(o-phenylenediamine) and polyoxometallate functionalized carbon nanotubes

Abstract

The chemical polymerization of o-phenylenediamine (OPD) on single-walled carbon nanotubes (SWCNTs) in the presence of phosphomolybdic acid (H₃PMo₁₂O₄₀ xH₂O) has been studied by surface enhanced resonant Raman scattering (SERRS) spectroscopy. One demonstrates that an organic–inorganic hybrid composite of the type poly(o-phenylenediamine)/polyoxometallate-functionalized SWCNTs is produced by the chemical interaction between polyoxometallate-functionalized SWCNTs and poly(o-phenylenediamine) (POPD) doped with [H₂PMo₁₂O₄₀]⁻ ions. According to TEM investigations, a result of the chemical interaction of SWCNT with H₃PMo₁₂O₄₀ xH₂O is the formation into the composite mass of tube fragments of shorter length, which behave like closed shell fullerenes since Raman fingerprint is given by lines situated at 240–275 and 1450–1472 cm⁻¹. The chemical polymerization of OPD on SWCNTs achieved in the absence of H₃PMo₁₂O₄₀ xH₂O leads to a covalent functionalization of the wall side of the tubes, which is revealed in Raman spectra, recorded at the excitation wavelength of 514 nm, by an enhancement of the lines associated with the tangential vibrational modes of SWCNTs. Using FTIR spectroscopy, significant hindrance steric effects are evidenced in the POPD/polyoxometallate-functionalized SWCNT composite.

Introduction

In the last years, polyoxometallates have received a special attention due to their ability to give rise to chemisorption on metals, glassy carbon electrodes, powders and fibers [1], [2], [3], [4]. Applications in the electrochemical supercapacitors field were reported since 2005, when hybrid materials formed from polyoxometallates dispersed in conducting polymers were synthesized by cyclic voltammetry [5]. According to Ref. [5], it has been shown that hybrid materials such as polyaniline (PANI)/H₄SiW₁₂O₄₀, PANI/H₃PW₁₂O₄₀ and PANI/H₃PMo₁₂O₄₀ display a combined activity of its organic and inorganic components, storing and releasing charges in solid-state electrochemical capacitor cells achieving a value of 120 F g⁻¹ and a good cyclability, i.e. beyond 1000 cycles. In the same year, multi-layers of H₃PMo₁₂O₄₀ xH₂O and multi-walled carbon nanotubes (MWCNTs) modified by poly(diallyl-dimethylammonium chloride) were prepared by the Langmuir Blodgett method to be used as electrodes in electrochemistry [6]. In order to improve the specific capacitance of carbon nanostructures, a first composite material based on conducting polymers (such as polyaniline and poly(3,4-ethylenedioxythiophene)), H₃PMo₁₂O₄₀ xH₂O and MWCNTs was reported in 2006. The interest for these new materials is justified by multiple applications in the area of electrocatalysis, amperometric sensors and biosensors [7]. In 2006, a new catalyst support based on polyoxometallate-modified carbon nanotubes having good potential applications in direct methanol fuel cells was reported by Pan et al. [8]. The assembly of a magnetic polyoxometallate encompassing a single cobalt ion and its isostructural diamagnetic zinc on SWCNTs provides new application in the nanocircuits field [9]. A new concept for the preparation of multifunctional compounds by the use of SWCNTs and counteranions such as Br⁻, ${\text{PF}}_6^{-}$, ${\text{PF}}_4^{-}$ is reported by Zhang et al., showing that polyoxometallates may be deliberately coupled via imidazolium groups [10]. As recently reported, polyoxometallates are promising antitumor drugs and exhibiting a very strong interaction with the carbon nanotube surface [11]. However in 2008, a more detailed TEM analysis of the chemical interaction of MWCNTs with H₃PMo₁₂O₄₀ xH₂O indicated that a prolonged exposure of MWCNTs to H₃PMo₁₂O₄₀ xH₂O induces a tube shortening [12]. Taking into account all these studies, a question arises if from the chemical interaction of H₃PMo₁₂O₄₀ xH₂O with SWCNTs results functionalized or shortened nanotubes. In this work, the answer to this question as well as implications for the synthesis of hybrid materials based on poly(o-phenylenediamine) (POPD) and heteropolyacids functionalized SWCNTs are shown. We use the surface enhanced resonant Raman scattering (SERRS) spectroscopy to elucidate the molecular structure of the obtained composite. Regarding the interactions between components, it has often been suggested that in conducting polymers/carbon nanotubes (CPs/CNTs) composites, either the CPs functionalize the CNTs or the CPs are doped with CNTs [13], [14], [15]. Recently, resonant anti-Stokes Raman scattering studies together with the Stokes studies have proved to be valuable tools for revealing the charge transfer between the constituents of composite materials [16], [17]. Using the resonant anti-Stokes and Stokes Raman spectroscopy, we demonstrate that the chemical polymerization of OPD onto SWCNTs in the presence and absence of H₃PMo₁₂O₄₀ xH₂O yields in composites of the type POPD/polyoxometallate-functionalized SWCNTs and POPD covalently functionalized SWCNTs, respectively.