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A conducting polymeric thin film from the cathodic electropolymerization of a fluorocarbon monomer
Synthetic Metals, 20 (1987) 387 - 391
387
Short Communication
A Conducting Polymeric Thin Film from the Cathodic Electropolymerization of a Fluorocarbon Monomer JEFFREY G. EAVES, HUGH S. MUNRO and DAVID PARKER* Department o f Chemistry, University of Durham, South Road, Durham DH1 3LE (U.K.)
(Received and accepted April 23, 1987)
Thin films of polypyrrole [1] and polythiophene [2] are conventionally formed by anodic oxidation to give anion-doped conducting polymeric materials. Although the cathodic electrosynthesis of redoxconducting films has been extensively studied [3], there is little evidence for the formation of electronically conducting films by such methods. The synthesis of conducting fluorocarbon polymers has attracted considerable interest in recent years, spurred by the suggestion that such systems may exhibit enhanced environmental stability with respect to their hydrocarbon analogues [4]. Following the preliminary observations of Silvester and Chambers [ 5], we hereby report further details of the electropolymerization of perfluorocyclopentene and the characterization of the conducting cationdoped polymeric films obtained. The initial sweep cyclic voltammogram of perfluorocyclopentene, 1, at a gold disc electrode in a 0.1 M solution of nBu~NC104 in dimethylformamide revealed an irreversible reduction a t - - 1 . 2 6 V and a 'nucleation loop' F F
NR4
F2
(i)
F2
F
F2F 2
F2F 2
(2)
upon reversal of the scan. The crossover points observed (a and b in Fig. 1) are caused by the nucleation overpotential required to deposit the polymer phase on the foreign electrode. Such phenomena have been observed in the anodic electrodeposition of pyrroles and thiophenes [6 - 8]. Chronoamperometric experiments on the same solution gave a sharply rising transient, which passed through a limiting value before falling away towards a steadystate current at longer times. Typical i - t curves, obtained in DMF or CHsCN, *Author to whom correspondence should be addressed. 0378-6779/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
388
1 m A c m -2
j
-1.5 V
Fig. 1. Single sweep cyclic voltammogram of perfluorocyclopentene at a gold microelectrode (second scan, v = 50 m V s-1, 0.1 M nBu4NC1Oa in DMF).
0 0
S
10
'
J
15
tls 5
10
15
fls
0
30.
I/mAcro
i l mAcro-2
•
60.
(a)
(b)
Fig. 2. Current-time curves obtained for the cathodic deposition from perfluorocyclopentene solutions in (a) E t d q C 1 0 4 / D M F (b) Et4NC1Oa/CH3CN, f o l l o w i n g a 0 to - - 1 . 7 V potential step.
are shown in Fig. 2: a reasonable fit to a linear i - t 2 relation was found for the rising portion. The current beyond the peak gave a linear i - t - 1 / 2 plot indicative o f film growth that is limited by diffusional transport of monomer to the cathode. Such results are consistent with instantaneous nucleation of the polymer phase and three-dimensional growth subject to limiting diffusion control. The longer time for the current to peak in CH3CN compared to DMF suggests that the solvent has a significant effect on the deposition process. For subsequent cyclic voltammetric scans, the crossover features disappear and the film may be observed to thicken. The background current level between 0 and --1.0 V increases with cycle number and may be associated with non-Faradaic charging of the growing film {Fig. 3). A quasi-reversible wave fractionally negative of 0 V is apparent, but this feature is apparently
389
I
i Fig. 3. Cyclic voltammetric deposition from perfluorocyclopentene solution (~ = 50 mV s-l, 0,1 M in perfluorocyclopentene and 0,1 M in nBu4NC104/DMF).
not retained for a deposit grown in an identical manner and then observed in fresh electrolyte in the absence of m o n o m e r {Fig. 4). The redox reaction observed in Fig. 3 may be related to the oxidation of the polymer to a neutral state, followed by reduction on the reverse scan to the conducting state, accompanied by colour change, with incorporation of the dopant tetraalkylammonium cation. Air-state electrode deposits may also be formed by controlled potential electrolysis at --1.5 V of solutions of perfluorocyclopentene, 1, in DMF saturated with Et~q+C104 - , nBu,N+C10~ or nHexSq+I - , as reported earlier [5]. After washing and drying, only samples deposited from nHex~N+I - were
50rows I O/+mAcm -z
7
-13V
Fig. 4. Cyclic voltammogram for perfluorocyclopentene deposit in inert solvent and electrolyte (9 = 50 mV s-1, 0.1 M nBu4NC104 in DMF).
390 black; those grown in Et4NC104 appeared blue in transmitted light but bronze in reflected light, while films grown in nBu4NC104 were either a lustrous blue or bronze colour according to the film thickness. The a m o u n t of material deposited on the electrode increased with the alkyl chain length of the supporting electrolyte cation. From nHex4N+I - , deposits up to ~ 1 mm thick may be grown, while with Et4NC104 good quality films up to several /am thick could be formed. A scanning electron micrograph of a typical film grown in Et4NC104 is shown in Fig. 5, in which the uneven topography is clearly discernible. The deposit was sufficiently electronically conducting not to require gold coating in order to obtain the micrograph, suggestive of high electronic conductivity. The film appeared to be composed of two distinct 'layers': one appears smooth and amorphous, while the other consist of microcrystalline fibres that may be responsible for the electronic charge transfer within the film, since the observed emission current was a m a x i m u m on these fibrous regions. By depositing a film grown onto a gold-coated glass slide and measuring its thickness, film conductivities of 10 -3 S cm -1 were typically measured. Such a value probably represents a lower limit of the film conductivity. Elemental microanalysis of washed and dried films of similar thickness consistently gave a carbon-to-fluorine ratio of unity, independent of the solvent or electrolyte used. This suggests that three fluorine atoms are lost per monomer, assuming that all of the carbon from perfluorocyclopentene m o n o m e r is retained in the deposit. The calculated ratio of dopant cation is one for every 23 matrix carbon atoms (i.e., 4/5 monomers). Such a dopant ratio is similar to that observed with electrochemically reduced PTFE [9], in which it is assumed that a graphite intercalation c o m p o u n d is formed. In the
Fig. 5. Scanning electron micrograph of the deposit formed from Et4NC104 solution.
391 case o f p o l y m e r derived f r o m p e r f l u o r o c y c l o p e n t e n e , it is unlikely t h a t such a s t r u c t u r e occurs. In t h e E S C A spectra o f the d e p o s i t e d film, the Cls s p e c t r a s h o w e d peaks c o n s i s t e n t w i t h CF2, CF and q u a t e r n a r y c a r b o n envir o n m e n t s [10]. In a semi-fulvene s t r u c t u r e such as 2, an e l e c t r o n m a y be delocalized along t h e c o n j u g a t e d b a c k b o n e . Such a s t r u c t u r e , w h i c h is sensitive to f u r t h e r loss o f F - , c o u l d be f o r m e d b y the initial reductive f o r m a t i o n o f a radical anion, w h i c h m a y rapidly lose fluoride [11, 12] to generate a reactive radical t h a t m a y dimerize, be f u r t h e r r e d u c e d and subs e q u e n t l y eliminate m o r e F - . The t e t r a a l k y l a m m o n i u m c a t i o n m a y serve n o t o n l y as d o p a n t b u t also m a y aid in the r e m o v a l of F - . A p o l y m e r i c m a t r i x c o u l d t h e n be established. Definitive p r o o f o f the s t r u c t u r e o f these c o n d u c t i n g d e p o s i t s awaits f u r t h e r e x p e r i m e n t a l analysis and the p o s t u l a t e d s t r u c t u r e s h o u l d be regarded o n l y as a w o r k i n g h y p o t h e s i s . We t h a n k t h e S.E.R.C. and Research C o r p o r a t i o n for s u p p o r t .
References 1 A. F. Diaz, K. K. Kanazawa and G. P. Gardini, J. Chem. Soe. Chem. Commun., (1980) 635. 2 F. Garnier and G. Tourillon, J. Electroanal. Chem. Interfacial Electrochem., 148 (1983) 299. 3 H. D. Abruna, P. Denisevich, M. Umana, T. J. Meyer and R. W. Murray, J. Am. Chem. Soc., 103 (1981) 1; J. M. Calvert, R. H. Schmehl, B. P. Sullivan, J. S. Facci, T. J. Meyer and R. W. Murray, Inorg. Chem., 22 (1983) 2151. 4 T. Yamaba, K. Tanaka, H. Terama, K. Fukui, H. Shirakawa and S. Ikeda, Synth. Met., 1 (1979) 321. 5 M. J. Silvester, Ph.D. Thesis, University of Durham, 1981. 6 M. G. Cross, D. Walton, N. J. Moise, R. J. Mortimer, D. R. Rosseinsky and D. J. Simmonds, J. Electroanal. Chem. Interfacial Electrochem., 189 (1985) 389. 7 A. J. Downard and D. Pletcher, J. Electroanal. Chem. Interfacial Electrochem., 206 (1986) 147. 8 A. J. Downard and D. Pletcher, J. Electroanal. Chem. Interfacial Electrochem., 206 (1986) 139. 9 D. J. Barker, D. M. Brewis, R. H. Dahm and L. R. J. Joy, Polymer, 19 (1978) 856. 10 J. G. Eaves, Ph.D. Thesis, University of Durham, 1986. 11 S. F. Campbell, R. Stephens and J. C. Tatlow, Tetrahedron, 20 (1966) 2997. 12 M. M. Ahmad and W. J. Feast, Mol. Cryst. Liq. Cryst., 118 (1985) 417.
387
Short Communication
A Conducting Polymeric Thin Film from the Cathodic Electropolymerization of a Fluorocarbon Monomer JEFFREY G. EAVES, HUGH S. MUNRO and DAVID PARKER* Department o f Chemistry, University of Durham, South Road, Durham DH1 3LE (U.K.)
(Received and accepted April 23, 1987)
Thin films of polypyrrole [1] and polythiophene [2] are conventionally formed by anodic oxidation to give anion-doped conducting polymeric materials. Although the cathodic electrosynthesis of redoxconducting films has been extensively studied [3], there is little evidence for the formation of electronically conducting films by such methods. The synthesis of conducting fluorocarbon polymers has attracted considerable interest in recent years, spurred by the suggestion that such systems may exhibit enhanced environmental stability with respect to their hydrocarbon analogues [4]. Following the preliminary observations of Silvester and Chambers [ 5], we hereby report further details of the electropolymerization of perfluorocyclopentene and the characterization of the conducting cationdoped polymeric films obtained. The initial sweep cyclic voltammogram of perfluorocyclopentene, 1, at a gold disc electrode in a 0.1 M solution of nBu~NC104 in dimethylformamide revealed an irreversible reduction a t - - 1 . 2 6 V and a 'nucleation loop' F F
NR4
F2
(i)
F2
F
F2F 2
F2F 2
(2)
upon reversal of the scan. The crossover points observed (a and b in Fig. 1) are caused by the nucleation overpotential required to deposit the polymer phase on the foreign electrode. Such phenomena have been observed in the anodic electrodeposition of pyrroles and thiophenes [6 - 8]. Chronoamperometric experiments on the same solution gave a sharply rising transient, which passed through a limiting value before falling away towards a steadystate current at longer times. Typical i - t curves, obtained in DMF or CHsCN, *Author to whom correspondence should be addressed. 0378-6779/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
388
1 m A c m -2
j
-1.5 V
Fig. 1. Single sweep cyclic voltammogram of perfluorocyclopentene at a gold microelectrode (second scan, v = 50 m V s-1, 0.1 M nBu4NC1Oa in DMF).
0 0
S
10
'
J
15
tls 5
10
15
fls
0
30.
I/mAcro
i l mAcro-2
•
60.
(a)
(b)
Fig. 2. Current-time curves obtained for the cathodic deposition from perfluorocyclopentene solutions in (a) E t d q C 1 0 4 / D M F (b) Et4NC1Oa/CH3CN, f o l l o w i n g a 0 to - - 1 . 7 V potential step.
are shown in Fig. 2: a reasonable fit to a linear i - t 2 relation was found for the rising portion. The current beyond the peak gave a linear i - t - 1 / 2 plot indicative o f film growth that is limited by diffusional transport of monomer to the cathode. Such results are consistent with instantaneous nucleation of the polymer phase and three-dimensional growth subject to limiting diffusion control. The longer time for the current to peak in CH3CN compared to DMF suggests that the solvent has a significant effect on the deposition process. For subsequent cyclic voltammetric scans, the crossover features disappear and the film may be observed to thicken. The background current level between 0 and --1.0 V increases with cycle number and may be associated with non-Faradaic charging of the growing film {Fig. 3). A quasi-reversible wave fractionally negative of 0 V is apparent, but this feature is apparently
389
I
i Fig. 3. Cyclic voltammetric deposition from perfluorocyclopentene solution (~ = 50 mV s-l, 0,1 M in perfluorocyclopentene and 0,1 M in nBu4NC104/DMF).
not retained for a deposit grown in an identical manner and then observed in fresh electrolyte in the absence of m o n o m e r {Fig. 4). The redox reaction observed in Fig. 3 may be related to the oxidation of the polymer to a neutral state, followed by reduction on the reverse scan to the conducting state, accompanied by colour change, with incorporation of the dopant tetraalkylammonium cation. Air-state electrode deposits may also be formed by controlled potential electrolysis at --1.5 V of solutions of perfluorocyclopentene, 1, in DMF saturated with Et~q+C104 - , nBu,N+C10~ or nHexSq+I - , as reported earlier [5]. After washing and drying, only samples deposited from nHex~N+I - were
50rows I O/+mAcm -z
7
-13V
Fig. 4. Cyclic voltammogram for perfluorocyclopentene deposit in inert solvent and electrolyte (9 = 50 mV s-1, 0.1 M nBu4NC104 in DMF).
390 black; those grown in Et4NC104 appeared blue in transmitted light but bronze in reflected light, while films grown in nBu4NC104 were either a lustrous blue or bronze colour according to the film thickness. The a m o u n t of material deposited on the electrode increased with the alkyl chain length of the supporting electrolyte cation. From nHex4N+I - , deposits up to ~ 1 mm thick may be grown, while with Et4NC104 good quality films up to several /am thick could be formed. A scanning electron micrograph of a typical film grown in Et4NC104 is shown in Fig. 5, in which the uneven topography is clearly discernible. The deposit was sufficiently electronically conducting not to require gold coating in order to obtain the micrograph, suggestive of high electronic conductivity. The film appeared to be composed of two distinct 'layers': one appears smooth and amorphous, while the other consist of microcrystalline fibres that may be responsible for the electronic charge transfer within the film, since the observed emission current was a m a x i m u m on these fibrous regions. By depositing a film grown onto a gold-coated glass slide and measuring its thickness, film conductivities of 10 -3 S cm -1 were typically measured. Such a value probably represents a lower limit of the film conductivity. Elemental microanalysis of washed and dried films of similar thickness consistently gave a carbon-to-fluorine ratio of unity, independent of the solvent or electrolyte used. This suggests that three fluorine atoms are lost per monomer, assuming that all of the carbon from perfluorocyclopentene m o n o m e r is retained in the deposit. The calculated ratio of dopant cation is one for every 23 matrix carbon atoms (i.e., 4/5 monomers). Such a dopant ratio is similar to that observed with electrochemically reduced PTFE [9], in which it is assumed that a graphite intercalation c o m p o u n d is formed. In the
Fig. 5. Scanning electron micrograph of the deposit formed from Et4NC104 solution.
391 case o f p o l y m e r derived f r o m p e r f l u o r o c y c l o p e n t e n e , it is unlikely t h a t such a s t r u c t u r e occurs. In t h e E S C A spectra o f the d e p o s i t e d film, the Cls s p e c t r a s h o w e d peaks c o n s i s t e n t w i t h CF2, CF and q u a t e r n a r y c a r b o n envir o n m e n t s [10]. In a semi-fulvene s t r u c t u r e such as 2, an e l e c t r o n m a y be delocalized along t h e c o n j u g a t e d b a c k b o n e . Such a s t r u c t u r e , w h i c h is sensitive to f u r t h e r loss o f F - , c o u l d be f o r m e d b y the initial reductive f o r m a t i o n o f a radical anion, w h i c h m a y rapidly lose fluoride [11, 12] to generate a reactive radical t h a t m a y dimerize, be f u r t h e r r e d u c e d and subs e q u e n t l y eliminate m o r e F - . The t e t r a a l k y l a m m o n i u m c a t i o n m a y serve n o t o n l y as d o p a n t b u t also m a y aid in the r e m o v a l of F - . A p o l y m e r i c m a t r i x c o u l d t h e n be established. Definitive p r o o f o f the s t r u c t u r e o f these c o n d u c t i n g d e p o s i t s awaits f u r t h e r e x p e r i m e n t a l analysis and the p o s t u l a t e d s t r u c t u r e s h o u l d be regarded o n l y as a w o r k i n g h y p o t h e s i s . We t h a n k t h e S.E.R.C. and Research C o r p o r a t i o n for s u p p o r t .
References 1 A. F. Diaz, K. K. Kanazawa and G. P. Gardini, J. Chem. Soe. Chem. Commun., (1980) 635. 2 F. Garnier and G. Tourillon, J. Electroanal. Chem. Interfacial Electrochem., 148 (1983) 299. 3 H. D. Abruna, P. Denisevich, M. Umana, T. J. Meyer and R. W. Murray, J. Am. Chem. Soc., 103 (1981) 1; J. M. Calvert, R. H. Schmehl, B. P. Sullivan, J. S. Facci, T. J. Meyer and R. W. Murray, Inorg. Chem., 22 (1983) 2151. 4 T. Yamaba, K. Tanaka, H. Terama, K. Fukui, H. Shirakawa and S. Ikeda, Synth. Met., 1 (1979) 321. 5 M. J. Silvester, Ph.D. Thesis, University of Durham, 1981. 6 M. G. Cross, D. Walton, N. J. Moise, R. J. Mortimer, D. R. Rosseinsky and D. J. Simmonds, J. Electroanal. Chem. Interfacial Electrochem., 189 (1985) 389. 7 A. J. Downard and D. Pletcher, J. Electroanal. Chem. Interfacial Electrochem., 206 (1986) 147. 8 A. J. Downard and D. Pletcher, J. Electroanal. Chem. Interfacial Electrochem., 206 (1986) 139. 9 D. J. Barker, D. M. Brewis, R. H. Dahm and L. R. J. Joy, Polymer, 19 (1978) 856. 10 J. G. Eaves, Ph.D. Thesis, University of Durham, 1986. 11 S. F. Campbell, R. Stephens and J. C. Tatlow, Tetrahedron, 20 (1966) 2997. 12 M. M. Ahmad and W. J. Feast, Mol. Cryst. Liq. Cryst., 118 (1985) 417.