Dichroism in thin polyaniline films

aNa-4686/wml0 + o.00 M-M

Ek~hImlco Act&Vd. 39,No. 2 pi 173-177,1994 RintedinGRatkiti

DICHROISM IN THIN POLYANILINE FILMS A. M. FUNT~KOV,M. D. Lwt* and V. V. Vnrurr~ A. N. Frumkin Institute of Electrochemistry. Leninski prospect 31, Moscow 117071, Russia (Received 7 April 1993) Ahstra~-The elcctro-optical behaviour of a platinum or gold electrode covered with thin polyaniline (PANI) Sims has been studied using electrore&tance (ER) and differential rellectance (DR) techniques. Depending on the electrosynthesis procedure a drastic polarization selectivity in the red part of the optical spectra has been observed. This effect was shown to reflect a great extent of the orientation of PAN1 chains perpendicular to the electrode surface. Key words: conducting polymers, electrochemical doping, optical anisotropy, electror&ctance, polarixation selectivity. In this work, differential reflectance (DR) and ER spectra from a thin (cu. 30nm thick) PANI fihn, deposited according to different electrochemial proand discussed semicedures, are presented quantitatively.

INTRODUCTION In the past years, polyaniline (PANI) lihns have attracted much attention because of a number of useful properties such as high electronic and ionic conductance in the doped state[l-31, electrochromic behaviour[4-61, considerable e!Iiciency as gasseparating membranes[7$ Resides, this polymer is easily processable and different kinds of materials can be coated with or impregnated from its solution in NMP[S]. Though anisotropy of conductivity is not an unusual phenomenon among the conductive polymers owing to their relatively long macromolecular chains, only a few papers have appeared on the anisotropy of their optical characteristics[9, lo]. One can reasonably assume that the appearance of the anisotropic behaviour of such tihns might be in close connection with the procedure of their preparation. Two basic directions, namely normal and parallel to the electrode surface are considered using polarized light. In the case of a monolayer of adsorbed molecules on a metal-solution interface drastic polarization selectivity of electroreflectance (ER) was observed by several authors[ll, 121. The variation of the dielectric function was shown to be controlled by the Stark effect. For the PANI-coated electrodes the dielectric function of the tilm is expected to vary considerably during the course of the electrochemical doping. Nevertheless, optical absorption measurements were as a rule carried out for relatively thick PANI films, typically of the order O.l-lpm using non-polarized light. The ER technique takes advantage of recording the absorption spectra in polarized light. Thus, the electrochemical deposition of PANI flhns onto an inert metallic or semiconductor surface allows us to observe dependence of the ER response on the thickness of the 6lm which can be varied uniformly, that is without the corresponding change in morphology and orientation of the macromolecules (very thin films) or, in contrast, non-uniformly (thicker fihns).

EXPERIMENTAL PAN1 6hns were deposited onto a Pt or Au electrode which were prior polished with a 1~ diamond powder. An aqueous solution containing 0.1 M aniline and 0.5 M H,SO, was typically used for PANI deposition. ER measurements were performed following the procedure described elsewhere[ 131. DR spectra were recorded as a difIerence between the reflected light from the PANIcoated electrode (R) and the bare one (R,,) using digital oscilloscope with the subsequent calculation of the DR response: (R - RJR,, = (Z - Z,)/Z,. ER and DR spectra were recorded in the range of wavelengths between 300 and 900nm in s- and p polarized light. The angle of the incidence, 6, was equal to 60”. Two methods of electrochemical deposition of PANI 6hns were used, namely cyclic voltammetric or the potentiostatic one. The former consisted of cycling the potential between -0.15 and 0.8OV as referred to a saturated Ag/Ag.Cl electrode at a potential scan rate 20 mV s- i, whereas constant potential E = 0.8V was maintained during the course of the potentiostatic deposition. Cyclic voltammograms and the capacity of the film-coated electrodes were measured at different stages of the deposition process. The amplitude of modulation of the electrode potential was equal to 5 mV.

IWSULTS

l Author to whom correspondence should be addressed at CENG/DRFMC/SESAM/EM, 85X 38041 GrenobleCedex, France.

AND DISCUSSION

Figure 1 shows typical cyclic voltammogram (cu) and the corresponding differential capacity of the 30nm thick PAN1 flhn deposited onto a Pt electrode using continuous potential sweeping Quadrature component was less by a factor varying from 3 to 5.

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A. M. FUNTIKOV et al.

174

Potential/mV

Fig. 1. Cyclic voltammetric curve (SOmVs-‘) and the corresponding dependence of the diffenmtial capacity on the electrode potential for the PANI-coated electrode ca. 39 mn thick.

PANI lilms deposited onto Optically Transparent Electrode (OTE) or gold[4, IO]. Two absorption bands, namely at L = 45Onm and in the red part of the spectra are clearly seen. In contrast, s-polarized light gives rise to only single absorption band in the blue part of the spectra (see Fig. 2b). Polarization selectivity in the red part of the spectra is expressed even more clearly in the ER spectra as is evidenced from Fig. 3a, b. The corresponding ER response is very small in the case of s-polarized light and in addition reveals practically no dependence on the electrode potent&l. Since modulation of the elec-

in both the current and differential capacity when the potential is shifted towards more positive values can be explained in terms of the insulator-metal transition. The quasi-equilibrium character of doping and dedoping of PANI manifests itself in a rather remarkable hysteresis of both curves as well as in some differences between the shape of the peaks on cm and the corresponding capacitive curves. The DR spectra in p-polarized light for the same film are shown in Fig 2a. These spectra seem to be in reasonable agreement with the absorption spectra of The increase

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Dichroism in thin polyaniline films

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trode potential in this case does not produce any appcciable effect on the value of the absorption, the observed ER response ~811thus be ascriw to the variation of the refractive index rather than the variation of the extinction coeflSent. Drastic polarization selectivity was observed only for rather thin fihns, up to 30nm thick. When the thickness of the lilm was further increased the polarization selectivity disappeared and as a result absorption bands in the red part of the spectra clearly appeared in s-polarized light. The intensity of

the absorption band was comparable with the comsponding intensity in ppolarized light. Thus s&ctivity disappeared completely when the thickness of the film was increased by 15-20%. It should be emphasized that PAM films deposited potcntiostatically onto the electrode surface lost the polarization selectivity when their thickness reached the value ca. 3mn. Figure 4a, b shows the corresponding DR spectra in s- and p-polarized light, respectively (compare with Fig 2a, b for the lilm of the same thickness deposited in cu regime).

176

A. M. FIJNTIKOVet al.

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Drastic polarization selectivity for the DR and ER spectra can be explained on the basis of the threelayer model [14]. For an isotropic film the latter predicts comparatively equal variation of the dielectric function in s- and p-polar&d light when the angle of incidence is close to 60”. Drastic polarization selectivity can only be expected when the variation of the normal component of the dielectric function, A& with the potential is much larger than the corresponding variation of the tangential component, A&. In linear approximation (er d 4 L, d is the thickness of the tilm) the following expressions are valid [15]:

Here 0 is the angle of incidence of the light beam, e, and c?, the dielectric functions of the solution and the

metal, respectively. If l$.l % le^r,1, the DR spectra should have a rather drastic polarization selectivity. Figure 3c shows ER spectra for the very thin film (cc. 3nm thick) which can be described by equation (1). The shape of the spectrum in p-polarized light differs from that of a thicker film (compare with Fig. 3a, b at E = 0.4V) since in the latter case ER response is determined not only by the variation of e,, but also by the full dielectric function. In other words, ER response is dependent on the doping level. Drastic anisotropy of the dielectric function, more precisely of the coellicient of extinction, can be accounted for by the fact that macromolecules of PANI are oriented perpendicular to the electrode surface and hence the optical transition moment in the red part of the spectra coincides with this direction. For such a behaviour one would expect that the electronic transition between the molecules should proceed strictly along the polymer chain. In contrast, in the blue part of the spectra, the optical transition moment corresponding to the intramolecular excitation is apparently directed at a definite angle to PANI chains. It is of interest to compare this conclusion with the direct quantum-chemical calculations.

177

Dichroism in thin polyanilinc films CONCLUSION Application of the DR and ER techniques for studying the electro-optical behaviour of thin PAN1 films reveal their strictly oriented structure. It was concluded that macromolecules of PAN1 are oriented perpendicular to the electrode surface. In this case a tangential component of the dielectric function in the red part of the spectra is practically a real value and hence the optical transition moment is strictly parallel to the direction of the macromolecular chains. The break of the selectivity for the thicker film can be ascribed to increasing disorder caused by accumulation of different defects during the deposition of the film. Cyclic potential sweeping regime for the film preparation was shown to provide much larger orientation of PAN1 macromolecules as oompared to the fdms of the same thickness deposited potentiostatically.

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