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In-situ FTIR studies on electrochemical polymerization of polythiophene
,~vnthetic Metals, 28 (1989) C859 C862
C859
IN-SITU FTIR STUDIES ON ELECTROCHEMICALPOLYMERIZATION OF POLYTHIOPHENE
A. Ivaska Department of Analytical Chemistry, /kbo Akademi, SF-20500 Turku (Finland) M.Koponen, P.Passiniemi and J-E. Osterholm Neste Oy, Corporate R & D, Technology Centre, SF-06850 Kulloo (Finland)
ABSTRACT We report in-situ FTIR-studies on electrochemical polymerization of polythiophene using both external and internal reflection techniques. Complex IR spectra containing information about the monomer, solvent, undoped and doped polymer are observed.
INTRODUCTION Rather little is known about the electrochemical polymerization mechanisms of conducting polymers. Because polythiophene (PTh) can be produced electrochemically one can utilize in-situ spectroscopic methods to study the polymerization kinetics and to gain an understanding of the polymerization mechanism /1/. Neugebauer et al. have utilized in-situ FTIR using external reflection techniques in studying the electrochemical behavior of PTh /2, 3/. We have studied the same system also with internal reflection techniques /4/. Hillman et al. have recently published an in-situ study of the polymerization kinetics of thiophene at the very beginning of the polymerization process /5/. They used UV-visible spectroscopy in the transmission mode. The experimental time used was up to a few seconds. According to their results polymers of short chain length are formed at the beginning of the polymerization process. Only after longer times of polymerization long chain polymers start to dominate. The polymerization follows instantaneous nucleation kinetics with a three dimensional continuation. In this work we present results on in-situ FTIR studies of long time polymerizations of PTh at potentistatic conditions. The experimental time ranges from several minutes to hours.
EXPERIMENTAL Electrochemical polymerization of thiophene was studied in three different solutions. As solvents propylene carbonate (PC), acetonitrile (AN) and bentzonitrile (BN) were used. The
0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
C860
electrolyte salt was in all three cases 0.1 M LiBF 4 and the m o n o m e r concentration 0.5 M thiophene. The chemicals used were PA grade. Before making the solutions the lithium salt was v a c u u m dried, thiophene was distilled and solvents were kept several days over molecular sieves. The final solutions were kept over molecular sieves and stored in argon atmosphere. The electrochemical cell used was slight modification o f the one described in the literature / 1 / . In most of the experiments the external reflection technique was used in recording the IR spectra. A n o t h e r type of cell was used in some internal reflection technique experiments (so called A T R spectra). The working electrode onto which PTh was polymerized was a Pt-disc. The counter and reference electrodes used were lithium in PC as well as a l u m i n i u m in all the solvents. The potential of the working electrode was controlled by a m i n i - c o m p u t e r connected to a h o m e - m a d e digital potentiostat. The IR spectra were recorded by a Nicolet SX 60 FTIR spectrometer. At every potential 64 interferograms were s u m m e d before conversion to the IR spectrum. The spectra were recorded with a resolution of 4 cm -1. The polymerization potential was 4.7 V vs. the reference electrode. In the case of the Li reference electrode this potential corresponds to that used by Hillman / 5 / . In the case of the AI reference electrode the working electrode potential was 1.3 V higher thus accelerating the polymerization kinetics. All the experiments were done at room temperature. Because of the long polymerization times the thickness of the polymer films varied between 3 ~ m and 50f~m.
R E S U L T S A N D DISCUSSION The IR s p e c t r u m at the beginning of the polymerization served as a reference spectrum and was subtracted f r o m the subsequent spectra. The IR spectra obtained in these experiments were very complex by nature. T h e y contain information of the m o n o m e r , solvent, polymer and doped polymer. However, the influence of the m o n o m e r spectra was negligible. Solvent spectra, on the contrary, were changing significantly. This was partly due
to the diminishing of the absorbing solution thickness during the
polymerization ( - - d e c r e a s i n g solvent peaks, e.g. ca 3050 cm -1 in BN, see Fig.l) and partly due to the increase of specifically absorbed solvent ( w i n c r e a s i n g solvent peaks, e.g. 2312 cm 1 in BN). A broad absorbance band extending from ca 1500 cm -1 to higher wave n u m b e r s was observed to increase during polymerization. This band is due to the electronic transition between the valence band and the lowest bipolaron state of polythiophene and is characteristic to highly doped polymers.
C861
',
~
,
•v~,:,
>,.
T
~n3:~
: :~r i H!'
,'[
:~sc
'TI,,
LI "i!'F r~'
:: ;,
Fig. 1. Polymerization of thiophene in BN.
An increasing absorbance band appears also at 1230-1260 cm q (Fig. 1). The same band is f o u n d in doped polythiophene and is obviously due to incorporation of the dopant anion in the polymer. This band starts to appear a few minutes after initation of the polymerization process indicating that some degree of polymerization is required before doping can proceed (Fig. 2).
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~m
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Fig. 2. Absorbance in the region 1240-1220 cm q as function o f time. Absorbance in this region is due to doping. BN as solvent.
Essentially the same type o f I n - s p e c t r a were obtained regardless of the solvent used. From the complex nature o f the spectra obtained in this study we conclude that in order to get more reliable information about polymerization kinetics and m e c h a n i s m s m u c h shorter polymerization times are required. In s u m m a r y , we feel that the external reflection technique used in this stude is a powerful tool in characterizing conducting polymers.
C862
REFERENCES 1 A. Bewick and S. Pons, in R.J.H. Clark and R.E. Hester (eds.), Advances in Infrared and Raman Spectroscopy, Vol. 12, Wiley Heyden, London, 1985, p. I. 2 H. Neugebauer, G. Nauer, A. Neckel, G. Tourillon, F. Gamier and P.J. Lang, J.Phys.Chem., 88 (1984) 652. 3 R. Kellner, H. Neugebauer, G. Nauer and D.G. Camerun, Pr0c.Svmp.on 5th International Conference on Fourier and Computerized Infrared Spectroscopy, Ottawa, 24-28 June, 1985, Vol. 553, SPIE, p. 12. 4 A. Ivaska et al.,'submitted for publication in Microchimlca Acta. 5 A.R. Hillman and E.F. Mallen, J.ElectroanaI.Chem., 243 (1988) 403.
C859
IN-SITU FTIR STUDIES ON ELECTROCHEMICALPOLYMERIZATION OF POLYTHIOPHENE
A. Ivaska Department of Analytical Chemistry, /kbo Akademi, SF-20500 Turku (Finland) M.Koponen, P.Passiniemi and J-E. Osterholm Neste Oy, Corporate R & D, Technology Centre, SF-06850 Kulloo (Finland)
ABSTRACT We report in-situ FTIR-studies on electrochemical polymerization of polythiophene using both external and internal reflection techniques. Complex IR spectra containing information about the monomer, solvent, undoped and doped polymer are observed.
INTRODUCTION Rather little is known about the electrochemical polymerization mechanisms of conducting polymers. Because polythiophene (PTh) can be produced electrochemically one can utilize in-situ spectroscopic methods to study the polymerization kinetics and to gain an understanding of the polymerization mechanism /1/. Neugebauer et al. have utilized in-situ FTIR using external reflection techniques in studying the electrochemical behavior of PTh /2, 3/. We have studied the same system also with internal reflection techniques /4/. Hillman et al. have recently published an in-situ study of the polymerization kinetics of thiophene at the very beginning of the polymerization process /5/. They used UV-visible spectroscopy in the transmission mode. The experimental time used was up to a few seconds. According to their results polymers of short chain length are formed at the beginning of the polymerization process. Only after longer times of polymerization long chain polymers start to dominate. The polymerization follows instantaneous nucleation kinetics with a three dimensional continuation. In this work we present results on in-situ FTIR studies of long time polymerizations of PTh at potentistatic conditions. The experimental time ranges from several minutes to hours.
EXPERIMENTAL Electrochemical polymerization of thiophene was studied in three different solutions. As solvents propylene carbonate (PC), acetonitrile (AN) and bentzonitrile (BN) were used. The
0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
C860
electrolyte salt was in all three cases 0.1 M LiBF 4 and the m o n o m e r concentration 0.5 M thiophene. The chemicals used were PA grade. Before making the solutions the lithium salt was v a c u u m dried, thiophene was distilled and solvents were kept several days over molecular sieves. The final solutions were kept over molecular sieves and stored in argon atmosphere. The electrochemical cell used was slight modification o f the one described in the literature / 1 / . In most of the experiments the external reflection technique was used in recording the IR spectra. A n o t h e r type of cell was used in some internal reflection technique experiments (so called A T R spectra). The working electrode onto which PTh was polymerized was a Pt-disc. The counter and reference electrodes used were lithium in PC as well as a l u m i n i u m in all the solvents. The potential of the working electrode was controlled by a m i n i - c o m p u t e r connected to a h o m e - m a d e digital potentiostat. The IR spectra were recorded by a Nicolet SX 60 FTIR spectrometer. At every potential 64 interferograms were s u m m e d before conversion to the IR spectrum. The spectra were recorded with a resolution of 4 cm -1. The polymerization potential was 4.7 V vs. the reference electrode. In the case of the Li reference electrode this potential corresponds to that used by Hillman / 5 / . In the case of the AI reference electrode the working electrode potential was 1.3 V higher thus accelerating the polymerization kinetics. All the experiments were done at room temperature. Because of the long polymerization times the thickness of the polymer films varied between 3 ~ m and 50f~m.
R E S U L T S A N D DISCUSSION The IR s p e c t r u m at the beginning of the polymerization served as a reference spectrum and was subtracted f r o m the subsequent spectra. The IR spectra obtained in these experiments were very complex by nature. T h e y contain information of the m o n o m e r , solvent, polymer and doped polymer. However, the influence of the m o n o m e r spectra was negligible. Solvent spectra, on the contrary, were changing significantly. This was partly due
to the diminishing of the absorbing solution thickness during the
polymerization ( - - d e c r e a s i n g solvent peaks, e.g. ca 3050 cm -1 in BN, see Fig.l) and partly due to the increase of specifically absorbed solvent ( w i n c r e a s i n g solvent peaks, e.g. 2312 cm 1 in BN). A broad absorbance band extending from ca 1500 cm -1 to higher wave n u m b e r s was observed to increase during polymerization. This band is due to the electronic transition between the valence band and the lowest bipolaron state of polythiophene and is characteristic to highly doped polymers.
C861
',
~
,
•v~,:,
>,.
T
~n3:~
: :~r i H!'
,'[
:~sc
'TI,,
LI "i!'F r~'
:: ;,
Fig. 1. Polymerization of thiophene in BN.
An increasing absorbance band appears also at 1230-1260 cm q (Fig. 1). The same band is f o u n d in doped polythiophene and is obviously due to incorporation of the dopant anion in the polymer. This band starts to appear a few minutes after initation of the polymerization process indicating that some degree of polymerization is required before doping can proceed (Fig. 2).
W ......
~e~
1240
12~1}
~m
1
/
d O0
149
; 45
'co
]30
&o
}41
~so
1037
1334
1631
19. 29
~so
gTO
~so
~9o
rIME (MIN)
DATA POINTS
Fig. 2. Absorbance in the region 1240-1220 cm q as function o f time. Absorbance in this region is due to doping. BN as solvent.
Essentially the same type o f I n - s p e c t r a were obtained regardless of the solvent used. From the complex nature o f the spectra obtained in this study we conclude that in order to get more reliable information about polymerization kinetics and m e c h a n i s m s m u c h shorter polymerization times are required. In s u m m a r y , we feel that the external reflection technique used in this stude is a powerful tool in characterizing conducting polymers.
C862
REFERENCES 1 A. Bewick and S. Pons, in R.J.H. Clark and R.E. Hester (eds.), Advances in Infrared and Raman Spectroscopy, Vol. 12, Wiley Heyden, London, 1985, p. I. 2 H. Neugebauer, G. Nauer, A. Neckel, G. Tourillon, F. Gamier and P.J. Lang, J.Phys.Chem., 88 (1984) 652. 3 R. Kellner, H. Neugebauer, G. Nauer and D.G. Camerun, Pr0c.Svmp.on 5th International Conference on Fourier and Computerized Infrared Spectroscopy, Ottawa, 24-28 June, 1985, Vol. 553, SPIE, p. 12. 4 A. Ivaska et al.,'submitted for publication in Microchimlca Acta. 5 A.R. Hillman and E.F. Mallen, J.ElectroanaI.Chem., 243 (1988) 403.