J. Electroad. Chem., 354 (1993) 325-329 Elsevier Sequoia S.A., Lausanne
JEC 02863PN Preliminary Note
conducting polymer films
D.C. Read, P.A. Christensen
and A. Hamnett
Department of Chemistry, The University of Newcastle, Newcastle upon Tyne, NE1 7RV, (UK) (Received 26 March 1993)
It is now a familiar experiment in electrochemical laboratories to grow a film of polypyrrole or polythiophene electrochemically and subsequently to cycle its potential between limits corresponding to oxidised and neutral forms of the film. If this experiment is carried out under normal conditions, the resultant film is usually relatively translucent in the neutral form but highly coloured in the oxidised form. It is now common ground that the origin of this colour lies in the formation of energy levels lying between the highest occupied and lowest unoccupied energy levels of the neutral form [ll. These energy levels originate from local distortions in the geometry of the chain caused by the formation of localised charge carriers, or polarons. Movement of the charge carriers in these types of polymers thus involves the coupling of electron hopping and chain vibrations, a process that leads not only to a substantially enhanced oscillator strength for certain IR vibrations, but also to the appearance of one or more intermediate energy levels to which electronic transitions can take place from the occupied +-system. As oxidation proceeds, the density of these intermediate electronic states increases, giving rise to the very dark colour normally associated with the oxidised film. It is clear that one method whereby the optical absorption might be reduced in the visible region would be to fabricate a polymer with a much reduced energy gap between highest occupied (H.O.) and lowest unoccupied (L.U.) states in the neutral form. Intermediate states then generated on oxidation of the chain would, of necessity, lie between these levels, with the result that optical transitions to these states would lie in the infrared. This strategy has most successfully been applied by Wudl , who adapted an hypothesis by Brtdas  to verify that the fused-ring monomer benzo[c]thiophene, or isothionaphthene did indeed have a very small energy separation between H.O. and L.U. levels, and was indeed 0022-0728/93/$06.00
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transparent in the oxidised form. However, this film is coloured in the neutral form, and benzo[c]thiophene is far from straightforward to synthesise. An alternative approach is to consider the possible role of film morphology in the optical properties of the system. It is already known that the properties of polypyrrole can be modified by the incorporation of large surfactant anions [41. It is also known that polypyrrole grown chemically at low temperatures on a polymeric substrate also shows novel optical properties and enhanced electronic conductivity . The rationale for our approach derives from the belief that the underlying physics of both these observations is the generation of local order. The basis of the polymerisation method we have discovered is the electrochemical induction of order: by employing a combination of low temperature and potential cycling, we have been able to initiate the growth of polymeric pyrrole and thiophene in completely novel transparent forms. EXPERIMENTAL
A standard three-electrode cell was employed, which was thermostatted at - 15°C. The cell incorporated a platinum counter electrode, a saturated calomel reference electrode (SCE) via a salt bridge, and an indium-tin-oxide coated glass slide or a platinum foil as the working electrode. The electrolyte was acetonitrile, freshly distilled under nitrogen from CaH,, with 0.1 M tetrabutylammonium tetrafluoroborate (TBAT, recrystallised from ethyl acetate) as the supporting salt. All transfers of electrolyte were carried out with standard Schlenk-line techniques to minimise water contamination, and we describe in detail the preparation of polythiophene. The cell was filled with the acetonitrile/TBAT electrolyte and cooled in the thermostatted bath. When the cell temperature had stabilised at -WC, 0.2M thiophene was injected into the cell and mixed thoroughly with the electrolyte. The potential of the working electrode was then cycled between -0.5 and + 1.5V for 10 minutes. Following this pretreatment, the positive limit of the scan was increased in 0.05V steps to + 1.65V, each time allowing the cyclic voltammogram (C.V.) to equilibrate, and the faradaic current to fall to zero. The positive limit was then increased in O.OlV steps; after each step, the CV was again allowed to stabilise, until electrochemical growth of the polymer just began. After growth had been initiated, the positive potential limit of the cycle was increased only when necessary to maintain growth at the absolute minimum, until the required thickness had been obtained. A polythiophene film grown in this way is transparent in the visible region of the spectrum in both the neutral (insulating) and oxidised (conducting) forms; this characteristic being retained on electrochemical cycling of the film at room temperature. In fact, colour only develops in the film when it is repeatedly washed with ethanol. Transparent polypyrrole has also been grown by this method, although the precise condit.ions remain to be optimised in this case.
Figures l-3 compare the electrochemical, UV-Vis and IR characteristics of polythiophene films grown by conventional cycling techniques and at - 15°C by the process described above. The CVs for the two films are very similar, with essentially identical charges under the two anodic waves. However, the UV-Vis spectra of the two films grown on ITO-glass are dramatically different, as can be seen in Fig. 2. The IR spectra of the two types of film also differ substantially, as can be seen in Fig. 3. The spectra are presented in normalised difference form, as discussed elsewhere 161,such that peaks pointing down represent gains in absorbance at 1.2V cf OV. The fact that there is a large shift in the low-energy electronic absorption band, which is centred at 2000 cm-’ in the low-temperature film, is of particular interest. This absorption is associated with the distortion of the chain on oxidation, as polarons form, and the substantial shift in this peak is clear evidence that, as we indicated above, the low-temperature film has a significantly different morphology. The conductivity of the low-temperature form of the film is confirmed by the appearance of strong IRAV bands in the mid-IR region; in fact the strength of
Polythlophens CU comparison
0 50 40 3Q E \
1 E 1 U us.SCE
Fig. 1. C.V. showing the growth of polythiophene on an indium-doped tin oxide-coated glass electrode immersed in 0.2M thiophene in O.lM tetra-n-butyl ammonium tetrafluoroborate/acetonitrile electrolyte, at - 15°C (LT) and +25”C CRT). The scan rate was 100 mV s-l, and the charges under the positive sweeps of the two voltammograms between 0.5 V and 1.5 V are 2.10 mC rtO.05 mC. The growth was stopped after these voltammograms were collected.
0.25 E x & Y)
Fig. 2. UV-Visible spectra collected in-situ in transmittance mode of the two films depicted in Fig. 1. In both cases, the spectrum was taken at the end of the growth process with the film in the fully oxidised form, ie. at 1.2V vs. SCE, and still immersed in the growth solution. The spectra were ratioed to the reference spectrum taken of the clean electrode in the growth solution at either -15°C or + 25°C. PaiyLhmphene
Fig. 3. In-situ Fourier transform infrared spectra collected of polythiophene films grown at - 15°C (LT) and + 25°C (RT) on a platinum disc electrode. The spectra were recorded at room temperature, in the base electrolyte, and with the films in the fully oxidised form, ie. at 1.2V vs. SCE. The details of the methodology involved in obtaining in-situ FTIR spectra are given elsewhere .
these bands suggests an enhanced electronic conductivity for the low-temperature film, in agreement with Techagumpuch et al [Sl. CONCLUSIONS
We have reported the discovery of a novel route to the formation of conducting transparent films from inexpensive monomers at low temperature. Furthermore, once these films have been grown, they retain their novel properties even at room temperature. This work is the subject of an international patent number PCT/GB93/00094. ACKNOWLEDGEMENTS
DR wishes to thank SERC for a studentship and we acknowledge support from SERC and the University of Newcastle.
REFERENCES 1 H. Yashima, M. Kobayashi, K.-B. Lee, D. Chung, A.J. Heeger, and F. Wudl, J. Electrochem. Sot., 134 (1987) 46. 2 T.A. Skotheim, Handbook of Conducting Polymers, Marcel Dekker, New York, 1986. 3 J.L. Bredas, R. Themans, J.M. Andre, A.J. Heeger and F. Wudl, Synth. Met., 11 (1985) 343. 4 L.F. Warren, and D.P. Anderson, J. Electrochem. Sot., 134 (1987) 101. 5 A. Techagumpuch, H.S. Nalwa and S. Miyata, in T.A. Skotheim (Ed.) Electroresponsive Molecular and Polymeric Systems, Vol. 2, Marcel Dekker, New York, 1991. 6 D.A. Chesher, P.A. Christensen and A.J. Hamnett, Chem. Sot. Farad. Trans., 89 (1993) 303.