652
J . Phys. Chem. 1984, 88, 652-654
A similar analysis has also been applied to a BSA solution at the isoelectric pH with a satisfactory result. The magnitude of the Hamaker constant A obtained from the fitting seems to be also of the right order of magnitude. In the process we extracted S ( 0 ) and discussed the small-0 behavior of the structure factor f o r a system of particles w i t h i n attractive interaction.
Acknowledgment. We are grateful to the Biology Department
of BNL and to the NCSASR for use of their respective spectrometers in this work. This research is supported by the Sloan Fund for the Basic Research at MIT, by the Petroleum Research Fund, administered by the American Chemical Society, and by the National Science Foundation. Registry No. Lithium dodecyl sulfate, 2044-56-6; lithium chloride, 7447-41-8.
I n Situ Investigations of the 3-Methylthiophene Polymer with Attenuated Total Reflection Fourier Transform Infrared Spectroscopyt Helmut Neugebauer, Gerhard Nauer, Adolf Neckel, Institut fur Physikalische Chemie der Uniuersitat Wien. A - 1090 Wien, Austria
Gerard Tourillon, Francis Garnier,* and Pbilippe Lang C.N.R.S.,E.R.241, Laboratoire de Photochimie Solaire, 94320- Thiais, France (Received: August 18, 1983)
In situ F U R spectra of thin poly(3-methylthiophene) films, electrochemicallygrafted onto a Pt electrode, have been recorded. The doped conducting state shows a very large broad band in the near IR, associated to the creation of free carriers. The spectrum of the electrochemically undoped state reveals a structure corresponding to an a-a' coupling of the thiophene units. The intensity increase and the energy shift observed for some bands when going to the doped state are explained by a vibronic coupling. The reversibility of the doping-undoping cycles is characterized by the occurrence and disappearance of the dopant absorption bands.
Introduction Much scientific work has been focussed on organic conducting polymers in the past few years. The first investigations on polyacetylene demonstrated the possibility of enhancing the electrical conductivity by several orders of magnitude by doping the polymer with oxidizing or reducing agents.' Inspired by these initial findings, investigations were carried out on several other organic polymer systems with similar behavior.2 These newly described systems display several interesting characteristics. Of particular interest is the suggestion that polyacetylene could be used as a material for electrodes of rechargeable b a t t e r i e ~ . ~However, the practical application of polyacetylene is impeded by its high sensitivity to oxygen which induces an irreversible degradation of its conducting properties. Other polymers are currently thought to overcome these difficulties. Polymerized heterocyclic aromatics, such as p~lythiophenes~ and polypyrr~les,~ seem to present a possible solution. These substances can be produced by electrochemical polymerization on appropriate substrates. The polymers based on heterocyclic aromatics show, as does polyacetylene, semiconducting properties in the neutral form and metallic conductivity in the oxidized (cationic) form.4 In order to improve the properties of the polymeric substances it is necessary to obtain a better understanding of the formation process and the doping mechanism in connection with the electrical properties. Spectroscopic investigations in the infrared spectral range can yield information about the molecular structure and the strength of the chemical bonds. Special techniques (in situ infrared spectroscopy with attenuated total reflection (ATR)6,7)allow the study of spectral changes occurring during electrochemical reactions of the polymer directly on the surface of a transparent electrode in contact with an electrolyte. In this paper we present some investigations on the electrochemical doping of the 3methylthiophene polymer using Fourier transform infrared (FTIR) spectroscopy. These investigations are made in order to get a 'Presented in part at the 1l t h Bunsenkolloquium on 'In Situ Investigations of Electrode Processes", Oct. 1, 1982, Vienna, Austria.
0022-3654/84/2088-0652$01.50/0
deeper insight into the doping and undoping process.
Experimental Section The spectroelectrochemical cell has been installed as a flow cell, as described in previous papem6q7 After improving the sealing of the cell, we found it possible to work with an evacuated spectrometer (Bruker IFS 113 V FTIR spectrometer, Karlsruhe, FRG) to eliminate instabilities caused by flushing the instrument with inert gas. To improve the signal/noise ratio of the infrared spectra a number of interferograms were added before performing the Fourier transformation. The electrochemical equipment consisted of a potentiostat (Jaissle 60 TB, Neustadt, FRG), a sweep generator (PP RI HiTek Instruments, High Wycombe, GB), and a X-Y plotter (Bryans 29000 A3, Surrey, GB). The transparent working electrode was R germanium crystal with a thin evaporated gold layer, and the reference electrode was a saturated calomel electrode (SCE) in an aqueous solution; both were placed in the vessel with the electrolyte and separated by a diaphragm. All potential values in this paper refer to this electrode. The polymerizations were performed in a 0.4 M solution of 3methylthiophene in acetonitrile with 0.1 M tetrabutylammonium perchlorate or tetrabutylammonium tetrafluoroborate as supporting electrolyte. A potential of +1.5 V/SCE was applied on the germanium crystal to start the polymerization. The polymer (1) H. Shirakawa, E. Louis, A. MacDiarmid, C. Chiang, and A. J. Heeger, J . Chem. SOC.,Chem. Commun., 578 (1977). (2) Summary in G. Wegner, Angew. Chem., 93, 352 (1981). (3) P. J. Nigrey, D. Maclnnes, Jr., D. P. Nairns, and A. G. MacDiarmid, J . Electrochem. Soc., 128, 1651 (1981). (4) G. Tourillon and F. Garnier, J . Electroanal. Chem., 135, 173 (1982). ( 5 ) A. F. Diaz, K. K. Kanazawa, and G. P. Gardini, J . Chem. Soc., Chem. Commun., 635 (1979). (6) H. Neugebauer, G . Nauer, N. Brinda-Konopik, and G . Gidaly, J . Elecfroanal. Chem., 122, 381 (1981). (7) H. Neugebauer, G. Nauer, N. Brinda-Konopik, and R. Kellner, Fresenius' Z . Anal. Chem., 314, 266 (1983).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 4, 1984 653
Letters
t
current density 0.1 m A l c r n 2
00 + i s potential 4.0v Figure 2. Potentiodynamic current density/potential curve of a 3methylthiophene polymer film on a germanium crystal with an evaporated gold layer. Electrolyte solution as in Figure 1, sweep range 0.0 to 1.0 V, sweep rate 0.002 V/s. The numbered vertical bars indicate the potentials at which ATR-FTIR spectra (each spectrum originating from 20 interferograms) were recorded.
WRYENUMBERS C t l - 1
Figure 1. Infrared spectrum of neutral 3methylthiophene polymer. (a) ATR-FTIR spectrum of a film on the surface of a reflection element in contact with the electrolyte solution (0.1 M tetrabutylammonium perchlorate, 0.4 M 3-methylthiophene in acetonitrile). The smoothed and baseline-corrected spectrum originates from 200 added interferograms. The spectrum of the electrolyte solution was subtracted. (b) Transmission spectrum obtained by using a KBr pellet. The polymer was formed in a lithium perchlorate containing electrolyte, reduced to its neutral form, rinsed, and dried.
I S p e c t r u m number
I
was produced in the oxidized form as an adherent film on the electrode surface. When a suitable film thickness was reached (controlled by the infrared absorption of the polymer), the oxidized form was reduced to the neutral form by applying a potential of 0.0 V/SCE on the working electrode; this process can be repeated frequently.
Results and Discussion The polymer has been characterized by different techniques. With scanning and transmission microscopy, it was shown that the polymer has a fibrillar structure: similar to that observed with polyacetylene. The absorption spectrum of the polymer in its neutral (semiconducting) state9 shows a maximum at 480 nm, corresponding to a highly conjugated polymer chain. In its oxidized (metallic) state a very broad band appears between 600 and 2000 nm, due to the existence of free carriers in the conducting state. This change in the visible range of the absorption spectrum corresponds to a change of the color of the polymer from red (neutral state) to blue (oxidized state).1° The average doping level, determined by elemental microanalysis, is 25-30%.9 The following structures have been proposed: Me
Me
Me
Me
neutral state. ceniiconducting
'
L'
J nn
oxidized state. nictallic X - = CIO;. BF4 -
In Figure 1a the infrared spectrum within the ranges 800-1 800 and 2800-3 100 cm-I for the neutral 3-methylthiophene polymer, produced in a tetrabutylammonium perchlorate containing electrolyte, is displayed. The smoothed a n d baseline-corrected spectrum was calculated from 200 added interferograms. The (8) G. Tourillon and F. Gamier, J . Polym. Sci., in press. (9) G.Tourillon and F. Gamier, J . Phys. Chem., 87, 2289 (1983). (IO) F. Gamier, G . Tourillon, M. Gazard, and J. C. Dubois, J . Elecfroanal. Chem., 148, 299 (1983).
URVENUVBERS Cn-1
0
Figure 3. ATR-FTIR spectra recorded during the oxidation of a 3methylthiophene polymer in contact with an electrolyte solution containing tetrabutylammonium perchlorate. The spectrum numbers correspond to the numbers in Figure 2. The spectra are smoothed but not baseline corrected. The spectrum of the electrolyte solution was subtracted.
spectrum of the electrolyte solution was subtracted and therefore infrared bands of the electrolyte do not appear. The transmission spectrum ranging from 600 to 900 cm-', given in Figure 1b, was obtained by using a KBr pellet. This technique has to be used in this spectral range because the germanium crystal absorbs strongly below 800 cm-'. Some bands of the neutral polymer (Figure 1) lie at the same position as bands of monomeric 3-meth~lthiophene.~ I Nevertheless, it is impossible at this time to assign all bands to particular vibrations in the polymer. It is hoped that an interpretation using normal coordinate analysis not only for the monomer but also for the dimer and possibly for a model polymer can be provided in the future. The potential, which is necessary for further polymerization on the film surface, lies about 0.7 V/SCE more anodically than the oxidation potential of the polymer film (about +0.6 V). Therefore the film can be oxidized and reduced without further polymerization. Figure 2 shows a current density/potential curve derived for a polymer film in the potential range between 0.0 and 1 .O V/SCE with a 0.002 V/s sweep rate. During the oxidation of the film, infrared spectra (each spectrum originating from 20 added interferograms) were recorded consecutively in potential regions of about 0.03 V. The spectra, which are numbered, are shown in Figure 3. The number characterizing each individual spectrum in Figure 3 reappears in Figure 2. In Figure 2, this number defines the potential, marked by a vertical bar, at which each spectrum was recorded. The spectra were smoothed but not baseline corrected. The spectrum of the electrolyte solution was again eliminated by subtraction. Spectrum 1 represents the (11) A. Hidalgo, J . Phys. Radium., 16, 366 (1955).
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The Journal of Physical Chemistry, Vol. 88, No. 4, 1984 7
Letters
I
1600
1400 I200 UPVENUtIBERS CB-1
IO00
8 0
Figure 4. ATR-FTIR spectra recorded during the oxidation of 3methythiophene polymer in contact with an electrolyte solution containing tetrabutylammonium tetrafluoroborate. The other experimental conditions were identical with those used in Figure 3.
spectrum of the neutral form on another scale. In the potential range between +0.3 and +0.8 V/SCE, some intense bands associated with the oxidized form of the polymer occur. The intensity of the bands of the neutral form at 1688, 1657, 1512, 1445, and 922 cm-I are not significantly affected by the oxidation process. The band near 1377 cm-', the double band at 1204 and 1182 cm-', and the bands at 1032 and 827 cm-' containing a shoulder grow with increasing degree of oxidation. Very large additional absorption bands (half-bandwidth 100-1 50 cm-') appear at about 1300, 1160,1100, and 975 cm-I. These bands have no counterparts in the neutral form. The strong intensity of the bands growing during oxidation may be explained by a vibronic intensity enhancement effect. Similar results have been obtained during the doping of polyacetylene by Rabolt et al.,'* who attributed these phenomena to the coupling of the skeletal backbone vibrations with the ?r-electron charge oscillations along the chain. The broad bands at about 1300 and 975 cm-I are shifted to lower wavenumbers during oxidation. Likewise the maximum of the band occurring in the region of 1100 cm-I shifts with increasing oxidation to wavenumbers as low as 1075 cm-'. This shift could be possibly due to the presence of two or more overlapping bands, whose intensities increase at different rates during oxidation, thus creating the observed shift. This band, or one of the overlapping bands, is definitely associated with the perchlorate anion (ClO,), which diffuses into the film during the oxidation process and which exhibits a band near 1100 cm-' in s o l ~ t i o n . ' ~ (12) J. F. Rabolt, T. C. Clarke, and G. B. Street, J . Chem. Phys., 71,4614 (1979). (13) S. D. Ross, "Inorganic Infrared and Raman Spectra", McGraw-Hill, London, 1972.
00 LOO0
3500
3wo
2500
2000
!500
1000
WAVENUMBERS CH-1
Figure 5. ATR-FTIR spectrum of the oxidized form of 3-methylthiophene polymer on the surface. of a reflection element in contact with an electrolyte solution containing tetrabutylammonium perchlorate. The smoothed spectrum originates from 200 added interferograms. The spectrum of the electrolyte solution was subtracted.
Figure 4 shows infrared spectra obtained during the oxidation of a 3-methylthiophene polymer film generated and oxidized in an electrolyte solution containing tetrabutylammonium tetrafluoroborate instead of tetrabutylammonium perchlorate. The other experimental conditions were identical with those previously described. Instead of the band at 1100 cm-' a band appears at 1060 cm-I during oxidation. It can be attributed to the tetrafluoroborate anion (BF4-).I3 From Figure 4 it seems likely that the strong band at 1160-1 130 cm-I consists also in this case of two bands which increase at different rates during the oxidation process. The rest of the spectrum shows the same behavior for both applied anions. In addition to the previously mentioned increasing bands, a further, extremely broad band beginning at 1600 cm-' and extending to 4000 cm-' appears during oxidation (Figure 5 ) . Such a behavior is generally attributed to electronic transitions associated with the presence of free carriers in these high-conducting materials. Further investigations with other monomers and substrate materials are now being carried out to obtain more information about the structure and the normal vibration modes of organic conducting polymers.
Acknowledgment. We gratefully acknowledge the support given by the "Fonds Zur Forderung der wissenschaftlichen Forschung". We also express our grateful thanks to Prof. R. Kellner, Institut fur Analytische Chemie der Technischen Universitat Wien, for his kind permission to use the FTIR equipment. The authors benefit by valuable discussion with Prof. N. Brinda-Konopik. Registry No. Poly(3-methylthiophene) (homopolymer), 84928-92-7.
