J. Phys. Chem. zyxwvut 1993,97, zyxwvu 81 13-8115 8113 Enhanced Rate of Redox Conversion of Polyaniline Films Induced by the Incorporation of Platinum Microparticles R. Kostecki, M. Ulmann, and J. Augustynski' Dspartement de Chimie Minirale, Analytique et Appliquie, UniuersitB de Gen.?ve, Switzerland D. J. Strike and M. Koudelka-Hep Institut de Microtechnique, Universitb de Neuchiftel, Switzerland Received: April 26, 1993; In Final Form: June 21, 1993 Relatively thick (in the range of 50 pm) electrochemically grown polyaniline films, with and without incorporated platinum microparticles, have been characterized by means of ac impedance measurements. In particular, resistance changes for both kinds of films were followed in situ (in 1 M aqueous zyxw H2S04 solution) as a function of potential. Rather unexpectedly, the rate of redox conversion of such polyaniline films is found to increase significantly in the presence of less than 100 pg cm-2 of electrodeposited metal particles dispersed inside the polymer layer. This letter describessome new observationsconcerningchanges in the conductivity behavior of polyaniline films, induced by their modification with dispersed metal (Pt) microparticles. Electro- active polyaniline films, with the nominal structure similar to emeraldine,' can be electrochemically formed on metal or semiconductor surfaces using, for example, a potential cycling procedure.2 The oxidized, electronically conducting form of polyaniline is reversibly transformed into the reduced, isolating form by changing its electrochemicalpotential to negative values (below ca. -0.1 V vs SCE in 1 M H2S04). The electronically conducting film of polyaniline may still undergo a second insulating transition following upon a positive shift of potential above ca. 0.7-0.8 V.3,4 In contrast with the former, reductive transformation, the latter one leads progressively to irreversible changes in physicochemical properties of polyaniline. One of the currently employed means of modifying conducting polymer films consists in incorporating metal microparticles. It is known that the polyaniline films including, for example, dispersed Pt particles exhibit enhanced electrocatalytic activity toward such reactions as oxidation of methanol536 or formic In contrast, much less attention (if any) has been paid to other than electrocatalytic properties of such composite materials. The present report focuses on the significantincrease of the conversion rate between conducting (oxidized) and poorly conducting (reduced) state of the polyaniline films brought about by the deposition inside the film of Pt microparticles. These findings are particularly important in view of the potential application of polyaniline in electrochromic displays3 and in electrochemical sensors9 and may throw some additional light on the mechanism of charge transfer in this polymer. The changes in conductivity of the films were followed in situ by means of impedance measurements. Experimental Section The polyaniline films were formed on planar gold electrodes of ca. 0.18 cm2 exposed geometric area. The films were grown electrochemically by cycling continuously, at 50 mV s-l, the electrodepotential between41 8 and 0.745 Vvs SCE. To initiate the deposition the anodic limit of the first scan was extended to 0.855 V. The electropolymerization was carried out in 2.2 M solution of sulfuric acid (Romil, analytical grade) containing 0.5 M dm-3 of freshly distilled aniline (Fluka). Platinum microparticleswere incorporated into the polyaniline films electrochemically from a 3.9 zyxwvuts X 10-3 M solution of 0022-3654/93/2097-8 1 13$04.00/0 hexachloroplatinicacid in 1 M HzSO4. The oxidized polyaniline films were first soaked in the latter solution for 20 min to allow the anion exchange, then rinsed with distilled water, transferred to 1 M HzSO4 solution and cycled between -0.2 and 0.625 V. Most of the electrochemical measurements were carried out at 25 OC in 1 M HzSO4 kept under nitrogen. A large platinum counterelectrode (placed close to the polyaniline electrode) was used. Ac measurements employed a Solartron 1286 electro- chemical interface and a Solartron 1260 impedance analyzer. Both instruments were controlled by a PCXT personal computer. The frequency response of the polyaniline electrodeswas in general analysed over the frequency range 1 Hz to 20 kHz. The surface and the cross section of polyaniline films were examined by scanning electron microscopy (SEM) and energy- dispersive X-ray spectroscopy (EDX). Results and Discussion Electrochemical polymerization of aniline begins with the formation of a dense polymer layer, extending up to 100-200 nm from the substrate, the further growth resulting in loosely packed deposit with more or less fibrous structure. Because of the inhomogeneous structure of thicker coatings, apparent through their optical characteristics,1°most of previous impedancestudies areconcerned withvery thin polyaniline films.4Jl-*S On the other hand, in the present work we used relatively thick >10 pm films of the polymer, more directly relevant to most of the potential applications (sensors, batteries). Figure 1 shows a typical cyclic voltammogram for a ca. 45- pm-thick polyaniline film (the thickness of a reduced, dry film determined using an Alpha-step 200 surface profilometer from Tencor Instruments). The shape of such voltammograms, with the characteristic hysteresis between anodic (switching on) and cathodic (switching off) portions, remained practically unchanged after incorporation of ca. 100 pg cm-2 of Pt particles.16 Figure 2 shows an SEM micrograph of a ca. 40-wm-thick polyaniline film containing about 70 zyx pg of Pt/cmz. The polyaniline fibrils (ca. 300 nm in diameter) are partly covered with Pt particles and aggregates of different sizes. There is, in particular, a large amount of nearly spherical metal particles, close to 50 nm in diameter. To appreciate the distribution of platinum inside the polyaniline matrix, the cross section of the film was examined by EDX. In addition to the main C and Pt signals, weaker signals due to 0, S, and C1 were also found. The latter signals may reasonably be assigned to S042- ions and to zyxwvutsrqpo 0 1993 American Chemical Society