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Electrosynthesis, electrochemical behavior and structure of poly[bis(phenoxyphosphazene)]-polypyrrole doped composite film
Synthetic Metals 106 Ž1999. 121–127 www.elsevier.comrlocatersynmet
Electrosynthesis, electrochemical behavior and structure of polywbis žphenoxyphosphazene/x-polypyrrole doped composite film M.A. de la Plaza a , M.C. Izquierdo a b
a,)
, E. Sanchez de la Blanca b, I. Hernandez Fuentes ´ ´
1
Departamento Ciencias y Tecnicas Fisicoquımicas. Facultad de Ciencias. U.N.E.D. 28040, Madrid, Spain ´ ´ Departamento Quımica Fısica I, Facultad de Quımica. UniÕersidad Complutense, 28040, Madrid, Spain ´ ´ ´ Received 1 March 1999; received in revised form 11 May 1999; accepted 21 June 1999
Abstract The aim of this work was to improve the mechanical properties of polypyrrole, PPy, by its electropolymerization onto a cast film of polywbisŽphenoxyphosphazene.x, PBPP, in order to form a PBPP–PPy composite, with NaClO4 as electrolyte and acetonitrile as solvent. The influence of the electrosynthesis method Žgalvanostatic or potentiostatic. on the polymerization efficiency and specific charge storage capacity was studied. The presence of PBPP increased the polymerization efficiency and decreased the specific charge storage capacity for composites generated potentiostatically, but its influence was less marked for composites obtained galvanostatically. With both methods the incorporation of PBPP improved the mechanical properties, but the electrochemical stability decreased. The effects of current density, thickness of PBPP and electropolymerization time were studied. The specific charge storage capacity increased with PBPP thickness. The composite was studied by Fourier transform infrared ŽFTIR.. In the high frequency region the FTIR spectra showed the absorption tail characteristic of conducting polymers in both PPy and in the composites. In the low frequency region, bands attributed to PPy, PBPP and ClO4y were analyzed in order to confirm the electrochemical results. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Polypyrrole-polywbisŽphenoxyphosphazene.x; Electropolymerization; Electrochemical studies; FTIR spectroscopy
1. Introduction Polypyrrole ŽPPy. has been extensively studied because of its high stability in ambient conditions and easy electrogeneration, which make it a good candidate for many applications, e.g., as chemical sensor, as biosensor w1x, in battery electrodes and for protection against oxidation and corrosion. Composites have been obtained by incorporation of electrically inactive polymers, in order to improve the mechanical properties of PPy, since although doped PPy shows good conductivity and stability, its brittleness raises problems for large scale industrial applications w2–6x. In the present work we use the two-step method, in which the electrode is first coated with a thin cast film of
) Corresponding author. Tel.: q34-1-398-7379; fax: q34-1-398-6697; E-mail: [email protected] 1 Deceased.
an inert host polymer and then a film of polypyrrole doped with NaClO4 , ŽPPyrClO4 . is grown on it electrochemically w7x. During the electropolymerization the monomer and the electrolyte diffuse across the porous film of inert polymer and the former reacts at the metal electrode surface. A good candidate for the inert matrix is polywbisŽphenoxyphosphazene.x, PBPP, whose properties depend strongly on the nature of the side group. Different materials can be obtained: flexible films, fiber-forming and colorless films, elastomers with very low glass transition temperatures, and even semicrystalline materials w8–10x. The polymer films were formed by galvanostatic or potentiostatic electrooxidation of pyrrole on the metal through the inert matrix of PBPP, using perchlorate as dopant and acetonitrile as solvent. The influence of deposition current density, PBPP film thickness, and deposition time on the electrochemical behaviour and structure of these composite ŽPBPP–PPy.
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 2 4 - 1
M.A. de la Plaza et al.r Synthetic Metals 106 (1999) 121–127
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films was studied, and the composites were compared with PPyrClO4 films obtained under similar conditions w11x.
2. Experimental PPyrClO4 and PBPP–PPy rClO4 films were obtained by electropolymerization of pyrrole, Py, at constant current density, i d , or constant potential, Ed , through inert PBPP matrices w16x. Both the electropolymerization and the electrochemical characterization of the films were carried out in a one compartment electrochemical cell with three electrodes ŽMetrohm.. The working electrodes were platinum sheets of 2.4 or 4.0 cm2 surface area Žboth sides.. Before each electrodeposition the working electrode was polished with 0.3 mm alumina ŽBuehler Micropolish. and 1 mm diamond paste ŽStruers., cleaned in acetone in an ultrasonic bath, washed with water and hot concentrated sulfuric acid, and finally rinsed copiously with Milli-Q water and dried before weighing. The Pt electrodes were modified with thin PBPP fims, obtained by casting from PBPP toluene solutions. The average thickness of each PBPP film, d PBPP , was determined by weighing, using the density of bulk PBPP. A platinum sheet of 8 cm2 surface area and an AgrŽ10y2 M Agq . electrode were used as counter and reference electrodes, respectively. Py ŽAldrich Chemie, 99%. was vacuum-distilled before use and stored in a refrigerator. Acetonitrile, MeCN ŽCarlo ˚ Erba of HPLC quality. was dried over a Merck 3 A molecular sieves. This solvent was chosen for its property of swelling PBPP, so that the pyrrole can be electrochemically polymerized by using electrolyte solutions containing 0.1 M NaClO4 ŽAldrich Chemie 99%.. Pyrrole concentra-
tion was 0.1 M. Before each experiment the solution was deoxygenated by bubbling dry nitrogen for 10 min. All experiments were run under nitrogen atmosphere and at room temperature. After polymerization, the film was washed with acetonitrile, vacuum-dried for 48 h at room temperature, weighed and the film stripped from the substrate. The PPyrPBPP mass ratio of each composite, m PPyrm PBBB , was determined by weighing; the thickness of PPy films could not be calculated from their weight because of the probable influence of the method and conditions of synthesis on PPy density. Likewise, an estimation of thickness by SEM was not attempted because of uncertainty as to the level of penetration of PPy into the inert matrix w12,13x. Electrochemical synthesis and cyclic voltammetry were carried out with a 273A PAR potentiostat–galvanostat connected to a Pentium PC and 7475A HP Plotter. PBPP–PPy rClO4 composites were synthetized potentiostatically Žfilm A. at 850 mV, a slightly higher potential than that of monomer oxidation w14x, and galvanostatically Žfilms B, C, D, E and F. at current densities of 2.5 and 4.2 mA cmy2 and different polymerization times ŽTable 1.. The polymerization charges were obtained by integration of chronoamperograms and from the time at constant c.d., respectively. Characterization of these films was carried out in 0.1 M NaClO4 MeCN solutions in the range y0.5 to 1.5 V ŽAgrAgq. . Vibrational spectra in the 4000–400 cmy1 region were registered with a Fourier transform infrared ŽFTIR. spectrophotometer, Nicolet Magna 750, at room temperature. The PBPP spectrum was obtained from a thick film, and the spectra of doped PPy and composites were obtained with the KBr disk technique. In all cases the background Žair or KBr. was subtracted.
3. Results and discussion 3.1. Electrochemical study A preliminary electrochemical study was made of each component of the composite. Fig. 1a shows cyclic voltammograms ŽCVs. of a PtrPBPP electrode with d PBBB s 33.8 mm and of bare platinum in the same electrolyte Ž0.1 M
Table 1 FILM and COMP
i d ŽmA cmy2 .
Ed ŽV.
td Žmin.
M PPy Žmg.
d PBPP Žmm.
m PPyrm PBPP
PE Ž10y4 g Cy1 .
j ŽC mgy1 .
PPy P PPyG A B C D E F
– 4.2 – 2.5 2.5 4.2 4.2 4.2
0.85 – 0.85 – – – – –
10 10 10 10 105 10 10 10
0.8 1.9 2.1 3.1 22.3 2.1 2.4 1.1
– – 5.7 1.9 3.1 1.6 3.1 20.3
– – 1.9 5.2 22.3 7 4 0.3
4.3 7.5 20.7 20.6 14.1 8 9.5 4.3
0.65 0.20 0.15 0.23 0.05 0.24 0.33 0.74
M.A. de la Plaza et al.r Synthetic Metals 106 (1999) 121–127
Fig. 1. CVs of: Ža. Pt Ž-. and PtrPBPP with d PB PP s 33.8 mm Ž – – – .; Žb. PPyrClO4 films generated galvanostatically Ž . and potentiostatically Ž – – – .. Electrolyte: 0.1M NaClO4 qMeCN. Sweep rate, 5 mV sy1 .
NaClO4 q MeCN.. The peak at 1 V can be attributed to residual water, since the distilled acetonitrile contained 0.01% water Žwrw., as determined by Karl Fischer analysis. The lower currents with the PBPP film reveal its insulating character. Fig. 1b shows CVs of PPyrClO4
123
films obtained: ŽI. at a current density of 4.2 mA cmy2 for 10 min ŽPPyG . and II. at a potential of 0.850 V ŽPPy P . for the same time. The polymerization efficiency, PE, is higher at constant current density. Both films show an anodic peak at 0.67 V, which may be attributed to a ClO4y doping process. Electrodeposition of the PBPP–PPyrClO4 composites started with the diffusion of PPy through the PBPP matrix, which was swollen by the solvent. Then electropolymerization of the monomer commenced at the electrode edges, spreading towards the centre until the electrode became completely covered by a black PPy film w15x. The synthesis conditions and several parameters of all composites and films are given in Table 1. The specific charge storage capacity, j , is the anodic charge in a positive linear sweep per milligram of polymer in the film. It corresponds to the number of positive charges stored along the polymer chains and the corresponding ClO4y ions that enter the polymer to retain electroneutrality. With the potentiostatic method, a comparison of film ŽPPy P . and composite ŽA. shows that the presence of PBPP increases the polymerization efficiency by nearly five times, while the specific charge storage capacity decreases to less than one-half. On the contrary, when the polymerization is carried out galvanostatically Žfilm PPyG and composite D., the presence of PBPP has little effect on either polymerization efficiency or specific charge storage capacity. In potentiostatically generated composites, the peak separation, D Ep , increased with scan rate ŽFig. 2a.. The total oxidation charge, Qa , was higher than the total reduction charge, Qc , at all rates and for all composites, with QarQc s 1.2. Values higher than unity indicate a deep oxidation level in the PPy chains, i.e., a great counter ion penetration, and an irregular dedoping process. The slope
Fig. 2. Ža. CV of PBPP–PPyrClO4 composite ŽA. at different potential scan rates, in 0.1M NaClO4 q MeCN. Žb. Log i p vs. log Õ b plots. Empty and full symbols correspond to cathodic and anodic peaks, respectively.
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M.A. de la Plaza et al.r Synthetic Metals 106 (1999) 121–127
Fig. 3. Ža. CV of PBPP–PPyrClO4 composite ŽF. at different potential scan rates, in 0.1M NaClO4 qMeCN. Žb. Log i p vs. log Õ b plots. Empty and full symbols correspond to cathodic and anodic peaks, respectively.
of the log–log plot of the anodic and cathodic current density, i p , of the voltammograms as a function of scan rate, Õ, was close to unity, indicating a pseudo capacitive process ŽFig. 2b.. The influence of the potential scan rate, Õ, on a composite ŽF in Table 1. made up of a 3.05 mm layer of PPy, generated by application of 4.2 mA cmy2 for 10 min, over a 20.3 mm layer of PBPP, is shown in Fig. 3a. There is an anodic peak at about 0.5 V, and a wide reduction peak. In all cases the oxidation charge was higher than the reduction charge. Again the slope of the log–log plot of the anodic and cathodic current density as a function of Õ was close to 1, as corresponds to a pseudocapacitive surface process ŽFig. 3b.. The influence of the inert matrix thickness, PBPP, on the electrochemical properties of the composites formed at
4.2 mA cmy2 and 10 min Žfilms D, E and F., was studied. The anodic peak shifted positively with increasing inert matrix thickness Ž1.6, 3.1 and 20.3 mm, respectively., due to the increasing resistance of the PBPP insulating film. The specific charge storage capacity increased markedly with increasing thickness. With a similar thickness Ž1.9–1.6 mm. of PBPP Žfilms B and D, respectively., an increase of 1.7 times in the galvanostatic current density the storage capacity remained constant, but the polymerization efficiency decreased to less than one half. The anodic peak potential decreased with increasing galvanostatic current density, and the anodic peaks became better defined. Finally, with a constant polymerization c.d. of 2.5 mA cmy2 and a similar thickness of PBPP Ž1.9 and 3.1 mm., the specific charge storage capacity decreased by 78% as the deposition time was increased from 10 min Žcomposite B. to 105 min Žcomposite C.. This large decrease can be attributed to a decrease of accessibility to anions with increasing thickness of PPy films or to a higher degradation of PPy. However, the total charge storage capacity of composite C was 1.1 C, i.e., 57% larger than that of 0.70 C of composite B, since the total amount of PPy in the former was 7.2 times larger than that in the later. For composite C, the log–log plot of peak current densities as a function of potential sweep rate had a slope close to 0.5, indicating control by diffusion. 3.2. Fourier transform infrared (FTIR) study For FTIR analysis the films were withdrawn from the solution under potential control, with the potential at the positive sweep limit, i.e., at the potential of maximum doping. The FTIR spectrum of a PPyrClO4y film ŽFig. 4. shows a pronounced monotonic increase in absorbance at
Fig. 4. FTIR spectrum of doped PPyrClO4 .
M.A. de la Plaza et al.r Synthetic Metals 106 (1999) 121–127
frequencies above 1700 cmy1 due to free carrier absorption, as is characteristic of the conductive state Žmetallic system. w17x. The band at 3400 cmy1 corresponds to NH stretching Ž n NH ., and those at 2920 and 2860 cmy1 to aliphatic CH stretching Ž n CH .. The aromatic CH stretching bands at around 3100 cmy1 were completely buried by the free-carrier absorbance in this doped film. The region below 1700 cmy1 showed characteristic PPy bands w14,18,19x, together with absorption bands of perchlorate ion at about 1100 and 617 cmy1 . The 1100 cmy1 band was broadened because of overlapping of the NH bending vibration with the perchlorate vibration. The so-called doping-induced bands, originated by a doping-induced lower symmetry of the polymer, can be seen at about 1160 and 900 cmy1 w20x. Fig. 5 shows the FTIR spectrum of PBPP. The absorbance at frequencies above 1700 cmy1 did not increase, since the film is not an electronic conductor. Bands due to aromatic Ž3100–3000 cmy1 . and aliphatic Ž3000–2800 cmy1 . CH stretching were visible. The 1700–400 cmy1 region shows characteristic bands at 1490 cmy1 Ž n P 5 N ., at 1250 cmy1 Ž n P – N ., at 1208 cmy1 Ž n ŽP – O. . asym at 945 cmy1 Ž n ŽP – O. sym . and at 1162 cmy1 w n ŽC–N.x w10x. The FTIR spectrum of a PBPP–PPyrClO4 composite synthesized at 0.85 V for 10 min ŽA. is shown in Fig. 6a. In the high frequency region the absorption tail characteristic of conducting polymers can be seen, with superimposed bands attributed to n NH and n ŽCH . aliph. As in doped PPy a very broad band at 1700–400 cmy1 was observed, probably due to ŽNH . . . N. vibration of very strong hydrogen bonds. The spectra of solids containing OH . . . O ˚ show many of these hydrogen bonds shorter than 2.60 A broad bands w21x. Vibrational bands due to PPy Ž1560, 1165, and 1030., to PBPP Ž1281, 1200 and 930 cmy1 . and to ClO4y Ž1121, 1109 and 625 cmy1 . on top of the broad
125
absorption band were clearly visible. Finally, the presence of a C5O group, indicating ring rupture, was shown by a band at 1716 cmy1 , in the same way as in the film grown at constant current density. The FTIR spectrum of PBPP–PPyrClO4 composite C, synthesized at 2.5 mA cmy2 for 105 min, is shown in Fig. 6b. An absorption tail at energies above 1700 cmy1 , typical of conducting polymers, is similar to that in the doped PPyrClO4 film Žsee Fig. 4.. The bands attributed to n NH and n ŽCH . aliph were not totally buried by this absorption. The presence of ClO4y ion in the composite was evidenced by very strong bands at about 1100 cmy1 and bands at about 630 cmy1 . Characteristic PPy bands appear at 1630 and 1540 cmy1 . The presence of PBPP was not very clear, probably due to the strong ClO4y absorption. The shoulder at about 1713 cmy1 may be attributed to a n C 5 O vibration, which could indicate the presence of a pyrrolidinone C5O group as observed by us w13,22x and by other authors w23x in doped PPy films. Fig. 6c shows the spectrum of a PBPP–PPyrClO4 composite synthesized at 4.2 mA cmy2 for 10 min ŽF.. As in the other composite films, it shows the tail of the NIR band, along with the bands of n NH and n ŽCH . aliph more deeply buried than in the two above spectra. As in the previous case, the low frequency region Ž1700–400 cmy1 . was dominated by the strong bands attributed to ClO4y ions centered at about 1100 and 625 cmy1 and by the bands attributed to PPy vibrations Ž1630 and 1397 cmy1 .. The weak band at about 1720 cmy1 could correspond to a n C 5 O vibration as in the other cases. FTIR spectra confirmed the electrochemical results. Thus, the bands of PPy and PBPP–PPy were more clearly visible in composite A, generated potentiostatically, than
Fig. 5. FTIR spectrum of PBPP.
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M.A. de la Plaza et al.r Synthetic Metals 106 (1999) 121–127
Fig. 6. FTIR spectra of: Ža. composite A; Žb. composite C and Žc. composite F.
in composites C and F, generated galvanostatically, indicating that the former contained less ClO4y, since the ClO4y band at about 1100 cmy1 had a much lower relative intensity in composite A than in composites C and F.
Acknowledgements This work has been supported by the Spanish DGICYT under ŽProject PB-920188.. The authors would like to thank the ‘‘Centro de espectroscopıa’’ ´ of Universidad Complutense of Madrid ŽSpain., for the use of its facilities.
4. Conclusions References Oxidation–reduction in the PBPP–PPyrClO4 composite was slow Žnon-Nernstian., as shown by the increase of D EŽox – red. with scan rate. The anodic charge was higher than the cathodic charge in all cases. For composites generated potenciostatically, the presence of PBPP increased the polymerization efficiency and decreased the specific charge storage capacity, while for composites obtained galvanostatically the presence of PBPP had little or no effect on these two parameters. The incorporation of PBPP improved in all cases the mechanical properties, making the material more manageable and less fragile. The FTIR spectra of all the composites showed the absorption tail in the high frequency region Ž4000–1700 cmy1 . characteristic of conducting polymers. The relative intensity of the PBPP bands was higher in the FTIR spectrum of the potentiostatically generated composite ŽA. than in the galvanostatically generated composites ŽC and F., indicating a lower relative amount of ClO4y in the first case.
w1x H. Korri-Youssoufi, F. Garnier, P. Srivastava, P. Godillot, A. Yassar, J. Am. Chem. Soc. 119 Ž1997. 7388. w2x M. Morita, I. Hashida, Makromol. Chem. 193 Ž1992. 235. w3x H.T. Chiu, J.S. Lin, J.N. Shiau, J. Appl. Electrochem. 22 Ž1992. 522. w4x C.M. Elliot, A.B. Kopelove, W.J. Albery, Z. Chen, J. Phys. Chem. 95 Ž1991. 1743. w5x M.A. De Paoli, R.J. Waltman, A.F. Diaz, J. Chem. Soc. Chem. Commun., 1984, 1015. w6x M.I. Sanchez de Pinto, H.T. Mishima, B.A. Lopez de Mishima, J. ´ ´ Appl. Electrochem. 27 Ž1997. 831. w7x A.F. Diaz, J.C. Lacroix, New. J. Chem. 12 Ž1988. 171. w8x M. Kojima, W. Kluge, J.H. Magill, Macromolecules 17 Ž1984. 1421. w9x T.P. Russell, D.P. Anderson, R.S. Stein, C.R. Desper, J.J. Beres, N.S. Schneider, Macromolecules 17 Ž1984. 1795. w10x Y.W. Chen-Yang, J.J. Hwang, F.H. Chang, Macromolecules 33 Ž1997. 3825. w11x T.F. Otero, M.J. Gonzalez-Tejera, J. Electroanal. Chem. 410 Ž1996. ´ 69. w12x A.F. Dıaz, ´ J.I. Castillo, J.A. Logan, W.Y. Lee, J. Electroanal. Chem. 129 Ž1981. 115. w13x M.A. De la Plaza, M.J. Gonzalez-Tejera, E. Sanchez de la Blanca, ´ ´
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w14x w15x w16x w17x w18x
J.R. Jurado, I. Hernandez-Fuentes, Polymer International 38 Ž1995. ´ 395. D.A. Kaplin, S. Qutubuddin, Polymer 36 Ž1995. 1275. Y. Kang, M.H. Lee, S.B. Rhee, Synthetic Metals 47 Ž1992. 157. P. Delahay, New Instrumental Methods in Electrochemistry, Interscience Publishers, New York, 1954. J. Lei, W. Liang, C.R. Martin, Synthetic Metals 48 Ž1992. 301. G.B. Street, T.C. Clarke, M. Krombi, K. Kanazawa, V. Lee, P. Pfluger, J.C. Scott, G. Weiser, Polymer 23 Ž1. Ž1987. 117.
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w19x H. Kato, O. Nishikawa, S. Honna, H. Kokado, J. Phys. Chem. 95 Ž1991. 6014. w20x B. Tian, G.J. Zerbi, J. Chem. Phys. 92 Ž1990. 3892. w21x Ph. Colomban, A. Gruger, A. Novak, A. Regis, J. Mol. Struct. 317 ´ Ž1994. 261. w22x M.J. Gonzalez-Tejera, M.A. de la Plaza, E. Sanchez de la Blanca, I. ´ ´ Hernandez-Fuentes, Polymer International 31 Ž1993. 45. ´ w23x S. Dong, J. Ding, Synthetic Metals 20 Ž1987. 119.
Electrosynthesis, electrochemical behavior and structure of polywbis žphenoxyphosphazene/x-polypyrrole doped composite film M.A. de la Plaza a , M.C. Izquierdo a b
a,)
, E. Sanchez de la Blanca b, I. Hernandez Fuentes ´ ´
1
Departamento Ciencias y Tecnicas Fisicoquımicas. Facultad de Ciencias. U.N.E.D. 28040, Madrid, Spain ´ ´ Departamento Quımica Fısica I, Facultad de Quımica. UniÕersidad Complutense, 28040, Madrid, Spain ´ ´ ´ Received 1 March 1999; received in revised form 11 May 1999; accepted 21 June 1999
Abstract The aim of this work was to improve the mechanical properties of polypyrrole, PPy, by its electropolymerization onto a cast film of polywbisŽphenoxyphosphazene.x, PBPP, in order to form a PBPP–PPy composite, with NaClO4 as electrolyte and acetonitrile as solvent. The influence of the electrosynthesis method Žgalvanostatic or potentiostatic. on the polymerization efficiency and specific charge storage capacity was studied. The presence of PBPP increased the polymerization efficiency and decreased the specific charge storage capacity for composites generated potentiostatically, but its influence was less marked for composites obtained galvanostatically. With both methods the incorporation of PBPP improved the mechanical properties, but the electrochemical stability decreased. The effects of current density, thickness of PBPP and electropolymerization time were studied. The specific charge storage capacity increased with PBPP thickness. The composite was studied by Fourier transform infrared ŽFTIR.. In the high frequency region the FTIR spectra showed the absorption tail characteristic of conducting polymers in both PPy and in the composites. In the low frequency region, bands attributed to PPy, PBPP and ClO4y were analyzed in order to confirm the electrochemical results. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Polypyrrole-polywbisŽphenoxyphosphazene.x; Electropolymerization; Electrochemical studies; FTIR spectroscopy
1. Introduction Polypyrrole ŽPPy. has been extensively studied because of its high stability in ambient conditions and easy electrogeneration, which make it a good candidate for many applications, e.g., as chemical sensor, as biosensor w1x, in battery electrodes and for protection against oxidation and corrosion. Composites have been obtained by incorporation of electrically inactive polymers, in order to improve the mechanical properties of PPy, since although doped PPy shows good conductivity and stability, its brittleness raises problems for large scale industrial applications w2–6x. In the present work we use the two-step method, in which the electrode is first coated with a thin cast film of
) Corresponding author. Tel.: q34-1-398-7379; fax: q34-1-398-6697; E-mail: [email protected] 1 Deceased.
an inert host polymer and then a film of polypyrrole doped with NaClO4 , ŽPPyrClO4 . is grown on it electrochemically w7x. During the electropolymerization the monomer and the electrolyte diffuse across the porous film of inert polymer and the former reacts at the metal electrode surface. A good candidate for the inert matrix is polywbisŽphenoxyphosphazene.x, PBPP, whose properties depend strongly on the nature of the side group. Different materials can be obtained: flexible films, fiber-forming and colorless films, elastomers with very low glass transition temperatures, and even semicrystalline materials w8–10x. The polymer films were formed by galvanostatic or potentiostatic electrooxidation of pyrrole on the metal through the inert matrix of PBPP, using perchlorate as dopant and acetonitrile as solvent. The influence of deposition current density, PBPP film thickness, and deposition time on the electrochemical behaviour and structure of these composite ŽPBPP–PPy.
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 2 4 - 1
M.A. de la Plaza et al.r Synthetic Metals 106 (1999) 121–127
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films was studied, and the composites were compared with PPyrClO4 films obtained under similar conditions w11x.
2. Experimental PPyrClO4 and PBPP–PPy rClO4 films were obtained by electropolymerization of pyrrole, Py, at constant current density, i d , or constant potential, Ed , through inert PBPP matrices w16x. Both the electropolymerization and the electrochemical characterization of the films were carried out in a one compartment electrochemical cell with three electrodes ŽMetrohm.. The working electrodes were platinum sheets of 2.4 or 4.0 cm2 surface area Žboth sides.. Before each electrodeposition the working electrode was polished with 0.3 mm alumina ŽBuehler Micropolish. and 1 mm diamond paste ŽStruers., cleaned in acetone in an ultrasonic bath, washed with water and hot concentrated sulfuric acid, and finally rinsed copiously with Milli-Q water and dried before weighing. The Pt electrodes were modified with thin PBPP fims, obtained by casting from PBPP toluene solutions. The average thickness of each PBPP film, d PBPP , was determined by weighing, using the density of bulk PBPP. A platinum sheet of 8 cm2 surface area and an AgrŽ10y2 M Agq . electrode were used as counter and reference electrodes, respectively. Py ŽAldrich Chemie, 99%. was vacuum-distilled before use and stored in a refrigerator. Acetonitrile, MeCN ŽCarlo ˚ Erba of HPLC quality. was dried over a Merck 3 A molecular sieves. This solvent was chosen for its property of swelling PBPP, so that the pyrrole can be electrochemically polymerized by using electrolyte solutions containing 0.1 M NaClO4 ŽAldrich Chemie 99%.. Pyrrole concentra-
tion was 0.1 M. Before each experiment the solution was deoxygenated by bubbling dry nitrogen for 10 min. All experiments were run under nitrogen atmosphere and at room temperature. After polymerization, the film was washed with acetonitrile, vacuum-dried for 48 h at room temperature, weighed and the film stripped from the substrate. The PPyrPBPP mass ratio of each composite, m PPyrm PBBB , was determined by weighing; the thickness of PPy films could not be calculated from their weight because of the probable influence of the method and conditions of synthesis on PPy density. Likewise, an estimation of thickness by SEM was not attempted because of uncertainty as to the level of penetration of PPy into the inert matrix w12,13x. Electrochemical synthesis and cyclic voltammetry were carried out with a 273A PAR potentiostat–galvanostat connected to a Pentium PC and 7475A HP Plotter. PBPP–PPy rClO4 composites were synthetized potentiostatically Žfilm A. at 850 mV, a slightly higher potential than that of monomer oxidation w14x, and galvanostatically Žfilms B, C, D, E and F. at current densities of 2.5 and 4.2 mA cmy2 and different polymerization times ŽTable 1.. The polymerization charges were obtained by integration of chronoamperograms and from the time at constant c.d., respectively. Characterization of these films was carried out in 0.1 M NaClO4 MeCN solutions in the range y0.5 to 1.5 V ŽAgrAgq. . Vibrational spectra in the 4000–400 cmy1 region were registered with a Fourier transform infrared ŽFTIR. spectrophotometer, Nicolet Magna 750, at room temperature. The PBPP spectrum was obtained from a thick film, and the spectra of doped PPy and composites were obtained with the KBr disk technique. In all cases the background Žair or KBr. was subtracted.
3. Results and discussion 3.1. Electrochemical study A preliminary electrochemical study was made of each component of the composite. Fig. 1a shows cyclic voltammograms ŽCVs. of a PtrPBPP electrode with d PBBB s 33.8 mm and of bare platinum in the same electrolyte Ž0.1 M
Table 1 FILM and COMP
i d ŽmA cmy2 .
Ed ŽV.
td Žmin.
M PPy Žmg.
d PBPP Žmm.
m PPyrm PBPP
PE Ž10y4 g Cy1 .
j ŽC mgy1 .
PPy P PPyG A B C D E F
– 4.2 – 2.5 2.5 4.2 4.2 4.2
0.85 – 0.85 – – – – –
10 10 10 10 105 10 10 10
0.8 1.9 2.1 3.1 22.3 2.1 2.4 1.1
– – 5.7 1.9 3.1 1.6 3.1 20.3
– – 1.9 5.2 22.3 7 4 0.3
4.3 7.5 20.7 20.6 14.1 8 9.5 4.3
0.65 0.20 0.15 0.23 0.05 0.24 0.33 0.74
M.A. de la Plaza et al.r Synthetic Metals 106 (1999) 121–127
Fig. 1. CVs of: Ža. Pt Ž-. and PtrPBPP with d PB PP s 33.8 mm Ž – – – .; Žb. PPyrClO4 films generated galvanostatically Ž . and potentiostatically Ž – – – .. Electrolyte: 0.1M NaClO4 qMeCN. Sweep rate, 5 mV sy1 .
NaClO4 q MeCN.. The peak at 1 V can be attributed to residual water, since the distilled acetonitrile contained 0.01% water Žwrw., as determined by Karl Fischer analysis. The lower currents with the PBPP film reveal its insulating character. Fig. 1b shows CVs of PPyrClO4
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films obtained: ŽI. at a current density of 4.2 mA cmy2 for 10 min ŽPPyG . and II. at a potential of 0.850 V ŽPPy P . for the same time. The polymerization efficiency, PE, is higher at constant current density. Both films show an anodic peak at 0.67 V, which may be attributed to a ClO4y doping process. Electrodeposition of the PBPP–PPyrClO4 composites started with the diffusion of PPy through the PBPP matrix, which was swollen by the solvent. Then electropolymerization of the monomer commenced at the electrode edges, spreading towards the centre until the electrode became completely covered by a black PPy film w15x. The synthesis conditions and several parameters of all composites and films are given in Table 1. The specific charge storage capacity, j , is the anodic charge in a positive linear sweep per milligram of polymer in the film. It corresponds to the number of positive charges stored along the polymer chains and the corresponding ClO4y ions that enter the polymer to retain electroneutrality. With the potentiostatic method, a comparison of film ŽPPy P . and composite ŽA. shows that the presence of PBPP increases the polymerization efficiency by nearly five times, while the specific charge storage capacity decreases to less than one-half. On the contrary, when the polymerization is carried out galvanostatically Žfilm PPyG and composite D., the presence of PBPP has little effect on either polymerization efficiency or specific charge storage capacity. In potentiostatically generated composites, the peak separation, D Ep , increased with scan rate ŽFig. 2a.. The total oxidation charge, Qa , was higher than the total reduction charge, Qc , at all rates and for all composites, with QarQc s 1.2. Values higher than unity indicate a deep oxidation level in the PPy chains, i.e., a great counter ion penetration, and an irregular dedoping process. The slope
Fig. 2. Ža. CV of PBPP–PPyrClO4 composite ŽA. at different potential scan rates, in 0.1M NaClO4 q MeCN. Žb. Log i p vs. log Õ b plots. Empty and full symbols correspond to cathodic and anodic peaks, respectively.
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Fig. 3. Ža. CV of PBPP–PPyrClO4 composite ŽF. at different potential scan rates, in 0.1M NaClO4 qMeCN. Žb. Log i p vs. log Õ b plots. Empty and full symbols correspond to cathodic and anodic peaks, respectively.
of the log–log plot of the anodic and cathodic current density, i p , of the voltammograms as a function of scan rate, Õ, was close to unity, indicating a pseudo capacitive process ŽFig. 2b.. The influence of the potential scan rate, Õ, on a composite ŽF in Table 1. made up of a 3.05 mm layer of PPy, generated by application of 4.2 mA cmy2 for 10 min, over a 20.3 mm layer of PBPP, is shown in Fig. 3a. There is an anodic peak at about 0.5 V, and a wide reduction peak. In all cases the oxidation charge was higher than the reduction charge. Again the slope of the log–log plot of the anodic and cathodic current density as a function of Õ was close to 1, as corresponds to a pseudocapacitive surface process ŽFig. 3b.. The influence of the inert matrix thickness, PBPP, on the electrochemical properties of the composites formed at
4.2 mA cmy2 and 10 min Žfilms D, E and F., was studied. The anodic peak shifted positively with increasing inert matrix thickness Ž1.6, 3.1 and 20.3 mm, respectively., due to the increasing resistance of the PBPP insulating film. The specific charge storage capacity increased markedly with increasing thickness. With a similar thickness Ž1.9–1.6 mm. of PBPP Žfilms B and D, respectively., an increase of 1.7 times in the galvanostatic current density the storage capacity remained constant, but the polymerization efficiency decreased to less than one half. The anodic peak potential decreased with increasing galvanostatic current density, and the anodic peaks became better defined. Finally, with a constant polymerization c.d. of 2.5 mA cmy2 and a similar thickness of PBPP Ž1.9 and 3.1 mm., the specific charge storage capacity decreased by 78% as the deposition time was increased from 10 min Žcomposite B. to 105 min Žcomposite C.. This large decrease can be attributed to a decrease of accessibility to anions with increasing thickness of PPy films or to a higher degradation of PPy. However, the total charge storage capacity of composite C was 1.1 C, i.e., 57% larger than that of 0.70 C of composite B, since the total amount of PPy in the former was 7.2 times larger than that in the later. For composite C, the log–log plot of peak current densities as a function of potential sweep rate had a slope close to 0.5, indicating control by diffusion. 3.2. Fourier transform infrared (FTIR) study For FTIR analysis the films were withdrawn from the solution under potential control, with the potential at the positive sweep limit, i.e., at the potential of maximum doping. The FTIR spectrum of a PPyrClO4y film ŽFig. 4. shows a pronounced monotonic increase in absorbance at
Fig. 4. FTIR spectrum of doped PPyrClO4 .
M.A. de la Plaza et al.r Synthetic Metals 106 (1999) 121–127
frequencies above 1700 cmy1 due to free carrier absorption, as is characteristic of the conductive state Žmetallic system. w17x. The band at 3400 cmy1 corresponds to NH stretching Ž n NH ., and those at 2920 and 2860 cmy1 to aliphatic CH stretching Ž n CH .. The aromatic CH stretching bands at around 3100 cmy1 were completely buried by the free-carrier absorbance in this doped film. The region below 1700 cmy1 showed characteristic PPy bands w14,18,19x, together with absorption bands of perchlorate ion at about 1100 and 617 cmy1 . The 1100 cmy1 band was broadened because of overlapping of the NH bending vibration with the perchlorate vibration. The so-called doping-induced bands, originated by a doping-induced lower symmetry of the polymer, can be seen at about 1160 and 900 cmy1 w20x. Fig. 5 shows the FTIR spectrum of PBPP. The absorbance at frequencies above 1700 cmy1 did not increase, since the film is not an electronic conductor. Bands due to aromatic Ž3100–3000 cmy1 . and aliphatic Ž3000–2800 cmy1 . CH stretching were visible. The 1700–400 cmy1 region shows characteristic bands at 1490 cmy1 Ž n P 5 N ., at 1250 cmy1 Ž n P – N ., at 1208 cmy1 Ž n ŽP – O. . asym at 945 cmy1 Ž n ŽP – O. sym . and at 1162 cmy1 w n ŽC–N.x w10x. The FTIR spectrum of a PBPP–PPyrClO4 composite synthesized at 0.85 V for 10 min ŽA. is shown in Fig. 6a. In the high frequency region the absorption tail characteristic of conducting polymers can be seen, with superimposed bands attributed to n NH and n ŽCH . aliph. As in doped PPy a very broad band at 1700–400 cmy1 was observed, probably due to ŽNH . . . N. vibration of very strong hydrogen bonds. The spectra of solids containing OH . . . O ˚ show many of these hydrogen bonds shorter than 2.60 A broad bands w21x. Vibrational bands due to PPy Ž1560, 1165, and 1030., to PBPP Ž1281, 1200 and 930 cmy1 . and to ClO4y Ž1121, 1109 and 625 cmy1 . on top of the broad
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absorption band were clearly visible. Finally, the presence of a C5O group, indicating ring rupture, was shown by a band at 1716 cmy1 , in the same way as in the film grown at constant current density. The FTIR spectrum of PBPP–PPyrClO4 composite C, synthesized at 2.5 mA cmy2 for 105 min, is shown in Fig. 6b. An absorption tail at energies above 1700 cmy1 , typical of conducting polymers, is similar to that in the doped PPyrClO4 film Žsee Fig. 4.. The bands attributed to n NH and n ŽCH . aliph were not totally buried by this absorption. The presence of ClO4y ion in the composite was evidenced by very strong bands at about 1100 cmy1 and bands at about 630 cmy1 . Characteristic PPy bands appear at 1630 and 1540 cmy1 . The presence of PBPP was not very clear, probably due to the strong ClO4y absorption. The shoulder at about 1713 cmy1 may be attributed to a n C 5 O vibration, which could indicate the presence of a pyrrolidinone C5O group as observed by us w13,22x and by other authors w23x in doped PPy films. Fig. 6c shows the spectrum of a PBPP–PPyrClO4 composite synthesized at 4.2 mA cmy2 for 10 min ŽF.. As in the other composite films, it shows the tail of the NIR band, along with the bands of n NH and n ŽCH . aliph more deeply buried than in the two above spectra. As in the previous case, the low frequency region Ž1700–400 cmy1 . was dominated by the strong bands attributed to ClO4y ions centered at about 1100 and 625 cmy1 and by the bands attributed to PPy vibrations Ž1630 and 1397 cmy1 .. The weak band at about 1720 cmy1 could correspond to a n C 5 O vibration as in the other cases. FTIR spectra confirmed the electrochemical results. Thus, the bands of PPy and PBPP–PPy were more clearly visible in composite A, generated potentiostatically, than
Fig. 5. FTIR spectrum of PBPP.
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Fig. 6. FTIR spectra of: Ža. composite A; Žb. composite C and Žc. composite F.
in composites C and F, generated galvanostatically, indicating that the former contained less ClO4y, since the ClO4y band at about 1100 cmy1 had a much lower relative intensity in composite A than in composites C and F.
Acknowledgements This work has been supported by the Spanish DGICYT under ŽProject PB-920188.. The authors would like to thank the ‘‘Centro de espectroscopıa’’ ´ of Universidad Complutense of Madrid ŽSpain., for the use of its facilities.
4. Conclusions References Oxidation–reduction in the PBPP–PPyrClO4 composite was slow Žnon-Nernstian., as shown by the increase of D EŽox – red. with scan rate. The anodic charge was higher than the cathodic charge in all cases. For composites generated potenciostatically, the presence of PBPP increased the polymerization efficiency and decreased the specific charge storage capacity, while for composites obtained galvanostatically the presence of PBPP had little or no effect on these two parameters. The incorporation of PBPP improved in all cases the mechanical properties, making the material more manageable and less fragile. The FTIR spectra of all the composites showed the absorption tail in the high frequency region Ž4000–1700 cmy1 . characteristic of conducting polymers. The relative intensity of the PBPP bands was higher in the FTIR spectrum of the potentiostatically generated composite ŽA. than in the galvanostatically generated composites ŽC and F., indicating a lower relative amount of ClO4y in the first case.
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