In situ resonance Raman spectroelectrochemistry of polypyrrole doped with dodecylbenzenesulfonate
Introduction
Over the past 20 years, polypyrrole has been one of the most widely studied electronically conducting polymers. This is due in part to its ease of preparation and relative long-term stability [1]. Much of the early research involved polypyrrole films doped with small anions (Cl−, ClO4−, etc) that were incorporated into the film on oxidation and expelled on reduction. However, little information can be obtained from cyclic voltammograms of the films, which show large capacitive components in the oxidised state and are relatively non-conducting in the reduced state [2].
In the past decade, much research has been carried out into polypyrrole films doped with larger anionic species such as polypyrrole/dodecylsulphate (PPy/DS) [3], [4], [5], [6] and polypyrrole/dodecylbenzenesulfonate (PPy/DBS) [7], [8], [9], [10]. These films display smoother morphology than the cauliflower-like appearance of PPy/Cl− or PPy/ClO4− films under SEM [11]. Also, cyclic voltammograms show much smaller capacitive currents and more negative formal potentials [8]. The differences between the films can be attributed to the fact that the large anions are unable to diffuse out of the film and in order to maintain charge neutrality in the polymer film, cations from the electrolyte, as well as anions, take part in the redox reaction. The cations are incorporated in the film on reduction and expelled on oxidation as has been shown by ECQM and probe beam deflection studies [9], [10], [12].
PPy/DBS films have received much attention due to their interesting behaviour during redox cycling. On ITO electrodes, the film has been reported as showing a reproducible colour change from a light blue in the oxidised state to a clear yellow in the reduced state [7]. In addition, it also displays an increase in volume of approximately 35% when cycled from its oxidised to its reduced form [13]. These properties give PPy/DBS many potential applications such as in movable electrochromic pixels or micro-actuators [14], [15].
A number of studies have been carried out into the Raman spectroelectrochemistry of polypyrrole films doped with small anions. Furukawa et al. characterised the Raman vibrational modes of polypyrrole at different stages of reduction ex situ and assigned the bands to the neutral, radical cation and dicationic species [16]. Son and Rajeshwar investigated the changes in the Raman bands in situ as a PPy film was cycled in an aqueous KCl solution [17]. Liu et al. used cyclic voltammetry combined with surface-enhanced Raman spectroscopy (CV-SERS) to investigate the doping/undoping and structural changes of a PPy film in aqueous KNO3 [18] while Buckowska and Jackowska used in situ Raman to study PPy and polythiophene films in different electrolytic media [19].
Relatively little investigation has been carried out into the Raman spectroscopy of polypyrrole doped with larger anionic species. Choi and Tachikawa used in situ Raman spectroelectrochemistry to characterise a PPy-copper phthalocyanine tetrasulfonate film [20]. Jenden et al. examined PPy films doped with p-toluene sulfonate and DS using near-IR excitation [21]. Recently, Girotto et al. characterised PPy films doped with DS and indigo carmine with Raman spectroscopy and XRD [22]. However to our knowledge, no Raman spectroscopy has been carried out on PPy/DBS films.
In this paper, in situ Raman spectroelectrochemistry was performed on PPy/DBS films in aqueous NaClO4 using different excitation lines (632.8, 514.5 and 457.9 nm). The changes in intensity of the Raman bands during redox cycling are discussed in terms of resonance of the neutral polymer, radical cation and dication species with a particular excitation line with reference to changes in the UV–vis spectrum.
Section snippets
Apparatus and reagents
All solutions were prepared using analytical grade reagents and distilled and deionised water. Pyrrole (99%, Aldrich) was distilled prior to use. Sodium dodecylbenzenesulfonate (Aldrich) was used as received while sodium perchlorate (Riedel-de Haën) was stored in a desiccator.
The spectroelectrochemical cell consisted of a covered polyethylene petri dish with holes in the top to allow a nitrogen line and the electrodes to be placed in solution. The working electrode was a one-sided platinum
Cyclic voltammetry of PPy/DBS film
Fig. 1 shows the cyclic voltammogram obtained for a PPy/DBS film cycled repeatedly at 10 mV s−1 in 0.1 M NaClO4. It is similar to those reported previously in the literature [8], [9]. As can be seen there is a strong reduction peak at −0.6 V in the initial negative scan followed by a broader oxidation band at −0.45 V on the positive scan. In the second cycle, the reduction peak shifts to −0.5 V and is considerably smaller, while the oxidation peak increases slightly but is otherwise unchanged.
Conclusion
In situ resonance Raman spectroelectrochemistry was found to be a powerful tool in analysing the evolution of the neutral, cationic and dicationic species in a PPy/DBS film as it was redox cycled in deaerated NaClO4. It is possible to select the species of interest by choosing a laser excitation line that results in resonance with that species, greatly increasing the intensity of the bands due to that species, relative to those that are due to off-resonance species.
With 457.9 nm excitation, the
Acknowledgements
K.C. thanks DIT for a scholarship. K.C. and J.C. thank FOCAS for use of the Raman spectrometer. FOCAS is funded by the National Development Plan 2000–2006.
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