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An in situ infrared study of the electrochemical doping-undoping process of poly(1-naphthol) film
Synthetic Metals, J9 (1990) 109-115
Short
109
Communication
An in situ infrared study of the electrochemical d o p i n g - u n d o p i n g p r o c e s s o f p o l y ( 1 - n a p h t h o l ) film Minh-Chau P h a m and J a m a l Moslih Inst~tut de Topologie et de D y n a m i q u e des Systeb,nes, CNRS (URA 34), Universitd Paris 7, 1 rue Guy de la Brosse, 75005 Paris (France)
(Received March 29, 1990; in revised form June 15, 1990; accepted June 20, 1990)
Abstract The electrochemical doping-undoping process of poly(1-naphthol) film is studied by potential-step experiments. Infraxed spectra are recorded simultaneously. Spectral changes show clearly the structural modifications: oxygen atoms are positively charged during oxidation associated with counterion diffusion from the electrolyte. Reversible behaviour is observed during reduction.
Introduction
Recently, we have shown that an electroactive and conducting polymer film, poly(1 -naphthol) [poly(NAP- 1) ], is obtained by electrochemical oxidation of 1-naphthol in acetonitrile solution. The polymer structure and the electropolymerization mechanism have been elucidated by XPS and i n situ IR spectroscopy [1, 2]. In this paper we report preliminary results of an in situ IR investigation of the doping-undoping mechanism of this polymer.
E x p e r i m e n t a l , r e s u l t s and d i s c u s s i o n
The poly(NAP-1) film used was prepared in the spectroelectrochemical cell by scanning the electrode for 6 potential cycles between 0.2 and 1.3 V versus Ag/AgC1 in an acetonitrile solution containing 0.1 M 1-naphthol and 0.1 M LiAsF6. The working electrode was a platinum-coated germanium prism. Details concerning the in s i t u MIRFrIRS (multiple internal reflection Fourier transform infrared spectroscopy) method and the spectroelectrochemical cell were published elsewhere [3, 4]. The film is electroactive as shown by the cyclic voltammograms of a poly(NAP-1) Pt-coated electrode in 0.1 M LiAsF6-CHsCN solution (Fig. l(a), 0379-6779/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
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111 (b)). Peak currents increase linearly with increasing scan speed indicating that the charge t r a ns por t kinetics of the film are fast (Fig. l(c)). The poly(NAP-1) structure consists of a series of alternating naphthylene and furan rings [1]. The neutral f or m has structure (a) and the oxidized form could be p r e s e n t e d by formula (b).
0
(a) Poly(NAP-1) neutral undoped polymer
®
0a) Poly(NAP-1) oxidized form
The s p ectr um of this film in the dry state is present ed in Fig. 2. It is worth recalling the bands related to the polymer structure [1, 2]. For naphthylene rings, C---C~ stretch is seen at 1590 cm -1 and C - C stretch between m o n o m e r i c moieties at 1124 cm -1. For furan rings, C--C stretch is observed at 1573 cm -1, asymmetric vibration of the C - O - C linkage at 1290 cm -1 and symmetric vibration of the same bond at ( 1 0 7 1 - 1 0 5 9 ) cm -1 [5]. A band at 1270 cm -1 is related to C--O stretch of the --OH groups at the ends of the chains. + The film is partially oxidized as indicated by the C~=O----C band at ( 1 2 1 4 - 1 2 2 4 ) cm -1 which is associated with a band at 700 cm -1 due to AsF6- counterions. To investigate the electrochemical d o p i n g - u n d o p i n g process and confirm the structure p r e s e n t e d for the oxidized form, a double potential-step experiment was p e r f o r m e d on a film-coated Pt electrode. An electrode coated with the above film was s t e ppe d between 1.3 V and 0.6 V versus Ag/AgC1 in an acetonitrile solution containing 0.1 M NBu4C104 as electrolyte. I n s i t u MIRF'rIRS s pe c t r a w e r e r e c o r d e d during this experiment. To obtain the transmittance difference spectra presented, we applied the 'subtraction meth o d ' to the three 'energy curves' Bo =f(A), B~ = g (A) and B2 = h (A) corresponding respectively to the system S (film-coated Ge/Pt in the spectroelectrochemical cell) without solution (background); the system S with solution (NBu4C104+CHaCN) before polarization; and the system S with solution on polarization at an indicated potential. From the spect rum of (S + solution) before polarization, we subtracted that of S without solution; the resulting one is the r e f er e nc e spectrum. Subsequently, for each s pect r um r e c o r d e d at an indicated potential, the re fer en ce s p e c t r u m was subtracted. Thus each transmittance difference spec-
112
L~ 1290 [ 1270
700
1071
112t
N~VENUHBERS
Fig. 2. E x situ internal reflection IR spectrum of the poly(NAP-1) film.
t r u m p r e s e n t e d is the s p e c t r u m of the film at the potential indicated. Each s p e c t r u m was obtained using 500 interferometers (time duration ~ 130 s). T h ey wer e successively recorded, one after anot her during the double potentialstep experiment. $1, $3, $5 were r e c o r d e d at 1.3 V and $2, $4, $6 at 0.6 V (Fig. 3). Spectrum $1 related to the film when s t e p p e d at 1.3 V shows that the doping process by C104- ions occurs. A strong band related+to C104- is seen at 1096 cm -1. The band at 1228 cm -~ assigned to C = O f C stretch of the oxidized f or m is very intense. A band at 1546 cm -~ can be attributed to C = C stretch be t w een the m o n o m e r i c moieties. Consequently, the C - C stretch between m o n o m e r s seen at 1137 cm -~ is weak. The - O H groups at the ends of the chains are oxidized in quinone form as shown by the C = O stretch at 1664 cm -~. The undoping process is p e r f o r m e d when the potential is s t e p p e d to 0.6 V ( s p ectr u m $2). The bands at 1546, 1228 and 1096 cm -1 are now v e ry weak, while the 1137 c m - ~ band is strengthened. The oxidation reaction o f --OH in quinone groups is not reversible because the 1664 cm -1 band remains steady afterwards. E x c e p t for the quinone formation, the d o p i n g - u n d o p i n g process is quite reversible as shown in s pe c t r a Ss-S4 and $5-$6 (Fig. 3). Nevertheless, after multiple potential steps, C104- ions seem to remain in the pol ym er matrix
113
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(c) Fig. 3. Potential-step MIRFrIRS s p e c t r a from a poly(NAP-1)-coated Pt electrode in 0.1 M NBt~C10~CH3CN solution. The potential is indicated o n each spectrum. All spectra (500 scm'~, 130 s) were normalized to the reference s p e c t r u m of the system before polarization.
even at the reduced state as indicated by a quite important intensity of the C104- band in spectrum $8. The C104- band persists even after a relaxing period of c. 30 min at 0.6 V. A more thorough investigation of the doping-undoping process of poly(NAP-1) film is under study. I n s i t u IR data can be correlated with that from the probe beam deflection technique to reveal the presence of ion pairs in the polymer matrix. These results will be discussed in a forthcoming paper.
Acknowledgement We wish to thank Mrs Monique Simon for technical assistance in MIRF'rIRS experiments.
References 1 M. C. Pham, J. Moslih and P. C. Lacaze, J. Electroanal. Chem., 278 (1990) 415. 2 M. C. Pham, J. Mosl/h and P. C. Lacaze, J. Electrochem. Soc., in press.
115 3 M. C. Pham, F. Adami, P. C. Lacaze, J. P. Doucet and J. E. Dubois, J. ElectroanaL Chem., 201 (1986) 413. 4 M. C. Pham, F. Adami and J. E. Dubois, J. Electrochem. Soc., 134 (1987) 2166. 5 (a) A. H. J. Cross, S. G. E. Stevens and T. H. E. Watts, J. Appl. Chem., 7 (1957) 562; (b) A. R. Katrizky and J. M. Lagowski, J. Chem. Soc., (1959) 657; (c) H. W. Thompson and R. B. Temple, Trans. Faraday Soc., 41 (1945) 27.
Short
109
Communication
An in situ infrared study of the electrochemical d o p i n g - u n d o p i n g p r o c e s s o f p o l y ( 1 - n a p h t h o l ) film Minh-Chau P h a m and J a m a l Moslih Inst~tut de Topologie et de D y n a m i q u e des Systeb,nes, CNRS (URA 34), Universitd Paris 7, 1 rue Guy de la Brosse, 75005 Paris (France)
(Received March 29, 1990; in revised form June 15, 1990; accepted June 20, 1990)
Abstract The electrochemical doping-undoping process of poly(1-naphthol) film is studied by potential-step experiments. Infraxed spectra are recorded simultaneously. Spectral changes show clearly the structural modifications: oxygen atoms are positively charged during oxidation associated with counterion diffusion from the electrolyte. Reversible behaviour is observed during reduction.
Introduction
Recently, we have shown that an electroactive and conducting polymer film, poly(1 -naphthol) [poly(NAP- 1) ], is obtained by electrochemical oxidation of 1-naphthol in acetonitrile solution. The polymer structure and the electropolymerization mechanism have been elucidated by XPS and i n situ IR spectroscopy [1, 2]. In this paper we report preliminary results of an in situ IR investigation of the doping-undoping mechanism of this polymer.
E x p e r i m e n t a l , r e s u l t s and d i s c u s s i o n
The poly(NAP-1) film used was prepared in the spectroelectrochemical cell by scanning the electrode for 6 potential cycles between 0.2 and 1.3 V versus Ag/AgC1 in an acetonitrile solution containing 0.1 M 1-naphthol and 0.1 M LiAsF6. The working electrode was a platinum-coated germanium prism. Details concerning the in s i t u MIRFrIRS (multiple internal reflection Fourier transform infrared spectroscopy) method and the spectroelectrochemical cell were published elsewhere [3, 4]. The film is electroactive as shown by the cyclic voltammograms of a poly(NAP-1) Pt-coated electrode in 0.1 M LiAsF6-CHsCN solution (Fig. l(a), 0379-6779/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
110
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111 (b)). Peak currents increase linearly with increasing scan speed indicating that the charge t r a ns por t kinetics of the film are fast (Fig. l(c)). The poly(NAP-1) structure consists of a series of alternating naphthylene and furan rings [1]. The neutral f or m has structure (a) and the oxidized form could be p r e s e n t e d by formula (b).
0
(a) Poly(NAP-1) neutral undoped polymer
®
0a) Poly(NAP-1) oxidized form
The s p ectr um of this film in the dry state is present ed in Fig. 2. It is worth recalling the bands related to the polymer structure [1, 2]. For naphthylene rings, C---C~ stretch is seen at 1590 cm -1 and C - C stretch between m o n o m e r i c moieties at 1124 cm -1. For furan rings, C--C stretch is observed at 1573 cm -1, asymmetric vibration of the C - O - C linkage at 1290 cm -1 and symmetric vibration of the same bond at ( 1 0 7 1 - 1 0 5 9 ) cm -1 [5]. A band at 1270 cm -1 is related to C--O stretch of the --OH groups at the ends of the chains. + The film is partially oxidized as indicated by the C~=O----C band at ( 1 2 1 4 - 1 2 2 4 ) cm -1 which is associated with a band at 700 cm -1 due to AsF6- counterions. To investigate the electrochemical d o p i n g - u n d o p i n g process and confirm the structure p r e s e n t e d for the oxidized form, a double potential-step experiment was p e r f o r m e d on a film-coated Pt electrode. An electrode coated with the above film was s t e ppe d between 1.3 V and 0.6 V versus Ag/AgC1 in an acetonitrile solution containing 0.1 M NBu4C104 as electrolyte. I n s i t u MIRF'rIRS s pe c t r a w e r e r e c o r d e d during this experiment. To obtain the transmittance difference spectra presented, we applied the 'subtraction meth o d ' to the three 'energy curves' Bo =f(A), B~ = g (A) and B2 = h (A) corresponding respectively to the system S (film-coated Ge/Pt in the spectroelectrochemical cell) without solution (background); the system S with solution (NBu4C104+CHaCN) before polarization; and the system S with solution on polarization at an indicated potential. From the spect rum of (S + solution) before polarization, we subtracted that of S without solution; the resulting one is the r e f er e nc e spectrum. Subsequently, for each s pect r um r e c o r d e d at an indicated potential, the re fer en ce s p e c t r u m was subtracted. Thus each transmittance difference spec-
112
L~ 1290 [ 1270
700
1071
112t
N~VENUHBERS
Fig. 2. E x situ internal reflection IR spectrum of the poly(NAP-1) film.
t r u m p r e s e n t e d is the s p e c t r u m of the film at the potential indicated. Each s p e c t r u m was obtained using 500 interferometers (time duration ~ 130 s). T h ey wer e successively recorded, one after anot her during the double potentialstep experiment. $1, $3, $5 were r e c o r d e d at 1.3 V and $2, $4, $6 at 0.6 V (Fig. 3). Spectrum $1 related to the film when s t e p p e d at 1.3 V shows that the doping process by C104- ions occurs. A strong band related+to C104- is seen at 1096 cm -1. The band at 1228 cm -~ assigned to C = O f C stretch of the oxidized f or m is very intense. A band at 1546 cm -~ can be attributed to C = C stretch be t w een the m o n o m e r i c moieties. Consequently, the C - C stretch between m o n o m e r s seen at 1137 cm -~ is weak. The - O H groups at the ends of the chains are oxidized in quinone form as shown by the C = O stretch at 1664 cm -~. The undoping process is p e r f o r m e d when the potential is s t e p p e d to 0.6 V ( s p ectr u m $2). The bands at 1546, 1228 and 1096 cm -1 are now v e ry weak, while the 1137 c m - ~ band is strengthened. The oxidation reaction o f --OH in quinone groups is not reversible because the 1664 cm -1 band remains steady afterwards. E x c e p t for the quinone formation, the d o p i n g - u n d o p i n g process is quite reversible as shown in s pe c t r a Ss-S4 and $5-$6 (Fig. 3). Nevertheless, after multiple potential steps, C104- ions seem to remain in the pol ym er matrix
113
S~-~.-1.3 V
<2:C3
S7= = O,6V
ED
5 c~ t 1661.
7~O
IBEd
'~gD
~360
ti?e l. iDO ~4r~VENUrqBERS
97D
~I~CJ
51D
(a) P,.] 03
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• O.GV
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~) Fig. 3.
(continued)
114 [D (D
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z
t
t
t~ tD
WAVENiJMBEBS
(c) Fig. 3. Potential-step MIRFrIRS s p e c t r a from a poly(NAP-1)-coated Pt electrode in 0.1 M NBt~C10~CH3CN solution. The potential is indicated o n each spectrum. All spectra (500 scm'~, 130 s) were normalized to the reference s p e c t r u m of the system before polarization.
even at the reduced state as indicated by a quite important intensity of the C104- band in spectrum $8. The C104- band persists even after a relaxing period of c. 30 min at 0.6 V. A more thorough investigation of the doping-undoping process of poly(NAP-1) film is under study. I n s i t u IR data can be correlated with that from the probe beam deflection technique to reveal the presence of ion pairs in the polymer matrix. These results will be discussed in a forthcoming paper.
Acknowledgement We wish to thank Mrs Monique Simon for technical assistance in MIRF'rIRS experiments.
References 1 M. C. Pham, J. Moslih and P. C. Lacaze, J. Electroanal. Chem., 278 (1990) 415. 2 M. C. Pham, J. Mosl/h and P. C. Lacaze, J. Electrochem. Soc., in press.
115 3 M. C. Pham, F. Adami, P. C. Lacaze, J. P. Doucet and J. E. Dubois, J. ElectroanaL Chem., 201 (1986) 413. 4 M. C. Pham, F. Adami and J. E. Dubois, J. Electrochem. Soc., 134 (1987) 2166. 5 (a) A. H. J. Cross, S. G. E. Stevens and T. H. E. Watts, J. Appl. Chem., 7 (1957) 562; (b) A. R. Katrizky and J. M. Lagowski, J. Chem. Soc., (1959) 657; (c) H. W. Thompson and R. B. Temple, Trans. Faraday Soc., 41 (1945) 27.