- Email: [email protected]
Electrochemical process of formation of an insulating polypyrrole film
201
Journal of Electroanalytical Chemistry, 372 (1994) 201-207
Electrochemical
process of formation of an insulating polypyrrole film
Tetsuya Osaka, Toshiyuki Momma, Shin-ichi Komaba and Himeko Kanagawa Department of Applied Chemistry, School of Science and Engineering, Waseda University, Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Shinjuku-ku, Tokyo 169 (Japan)
Sadako Nakamura Department of Chemical and Biological Science, Faculty of Science, Japan Women’s University, Mejirodai, Bunkyo-ky
Tokyo 112 (Japan)
(Received 6 September 1993; in revised form 17 December 1993)
Abstract
Insulating and electroinactive polypyrrole (PPy(NaOH)) film can be synthesized by electropolymerization from an NaOH aqueous solution containing pyrrole at a highly positive potential. The synthesis of this electroinactive PPy(NaOH) was investigated using in situ UV and X-ray photoelectron spectroscopy, and was compared with the irreversible oxidation (“overoxidation”) of electroactive PPy under a highly positive potential. Overoxidation made the PPy inactive. However, the formation of PPy (NaOH) started above 0.5 V vs. Ag/AgCl, and the “overoxidation” began simultaneously in the polymerizing solution. Overoxidation with nucleophilic hydroxide ions caused a structural change in the PPy molecule, which destroyed the original P-conjugated system.
1. Introduction
Electropolymerization is an interesting method for the fabrication of conducting polymer films [l-3] for applications as electrochromic devices [4,51, ion-selective electrodes [6-81 and rechargeable polymer batteries [9,101. In these cases, the key function of the conducting polymers is their electroactivity, which is produced with the formation of a polaron, bipolaron and soliton as a result of the process of doping counter ions into the polymer matrix [ll]. Among the various electrochemically polymerized films, the electroactive polypyrrole (PPy) film has the great advantage of stability; however, application of an anomalously positive potential sometimes decreases the conductivity and electroactivity of PPy, owing to irreversible oxidation (“overoxidation”) [12-151. We have previously reported that an insulating polypyrrole (PPy(NaOH)) film was deposited electrochemically from an aqueous pyrrole solution containing sodium hydroxide at 1.5 V vs. Ag/AgCl; this film became quite electroinactive in the as-prepared condition and its thickness stopped increasing automatically at a certain low value (ca. 0.2 pm) under potentiostatic 0022-0728/94/$7.00 SSDI 0022-0728(94)03298-H
conditions 116,171. It was also possible to deposit insulating PPy films from sodium carbonate or sodium bicarbonate aqueous solution [18]. Utilizing these insulating PPy films, we have fabricated metal/insulating PPy/metal (MIM) type switching devices for large-scale flat panel displays 1193, and amperometric pH sensor devices for a very small volume test solution [20]. This kind of film also has potential for use as protection against corrosion of commodity metals. In the present paper, we discuss the synthesis of insulating PPy(NaOH), using in situ UV-visible spectroscopy and X-ray photoelectron spectroscopy (XPS), compared with the overoxidation process of active PPy in aqueous media. 2. Experimental
details
Reagent grade pyrrole (Wake Pure Chemicals Industries, Ltd.), NaOH (Kant0 Chemical Co., Inc.) and NaCl (Kokusan Chemical Works, Ltd.) were used without further purification. Aqueous solutions of 0.25 ma1 dmp3 pyrrole containing either 0.01 mol dme3 NaOH(pH 12.0) or 0.2 mol dmm3 NaCl as supporting electrolyte were used for electrochemical deposition of the inactive PPy(NaOH) and the active PPy(NaC1) films 0 1994 - Elsevier Science S.A. All rights reserved
202
T. Osaka et al. / Formation of insulating polypyrrole film
respectively. Deionized and distilled water was used throughout all experiments. For the electrochemical polymerization and measurements, a one-component cell was assembled with a conducting indium tin oxide (ITO) coated glass plate as a working electrode, a Pt counter-electrode, and an Ag/AgCl,,,,, reference electrode. The electrolyte solutions were deoxygenated by bubbling with argon prior to all electrochemical polymerizations and measurements. In order to take the optical absorption spectra of PPy in situ, the potential of the PPy/ITO electrode was controlled in an electrochemical UV cell (pathlength 1 cm) made of quartz glass. The PPy(NaC1) was prepared galvanostatically (current density 0.4 mA cme2, amount of charge 0.05 C cmp2> on an IT0 glass electrode (8 X 11 mm2). After the preparation of this active PPy(NaCl), the electrode was rinsed with an electrolyte solution prior to electrochemical optical measurements. Another electroinactive PPy(NaOH) film was deposited by the potential sweep method (from 0 to 1.3 V vs. Ag/AgCl) at 1 mV s-* in the electrochemical UV cell, in which the change in the absorbance was measured during polymerization. For the XPS measurements, the active PPy(NaC1) prepared galvanostatically and the inactive PPy(NaOH) formed potentiostatically at 1.5 V were rinsed in water and dried with flowing nitrogen gas. These PPy films were analyzed using XPS (JEOL JPS-90SQ, target Al, acceleration voltage 10 kV, gun current 20 mA1. Monochromatic Al Ka was used as incident X-rays. The background pressure during the measurements was ca. lo-’ Pa.
64“: 8 e P
0 X .
.-
2o-2-4 -6 I ’ -0.6
I -0.3
I 0
c 0.1
E / V vs. Ag/AgCI Fig. 1. A cyclic voltammogram of electroactive PPy(NaCI) on an IT0 glass electrode in a 0.2 mol dmm3 NaCl aqueous solution at 100 mV S-l.
shows in situ UV-visible spectra of the active Ppy(NaC1) under various applied potentials. The wavelength of the absorption peak in the spectra of active PPy is related to the band transition [ll]. A strong absorption peak at h = 390 nm appears in the spectrum of PPy(NaC1) in the undoped state at -0.6 V, which is attributed to the a-r* transition. However, a very broad peak around 800 nm appears in the spectrum of the oxidized PPy(NaC1) at an anodic potential of more than -0.2 V, this is caused by bipolaron (BP) formation on doping with chloride anions [ll]. Polarons and bipolarons can form along the PPy chain by
3. Results and discussion 3.1. Electrochemical PPy(NaCI)
optical
studies
of
electroactive
The electroactive PPy(NaC1) was prepared galvanostatically from an aqueous solution containing pyrrole and NaCl. A current density of 0.4 mA cmp2 was used to load the electrode with a charge of 50 mC cmm2 for the formation of PPy(NaC1). During polymerization, the electrode potential was held at an almost constant value around +0.6 V. The resulting PPy(NaC1) film had a thickness of about 0.2 pm. Figure 1 shows a cyclic voltammogram of the PPy(NaC1) in a 0.2 mol dmp3 NaCl aqueous solution. A pair of redox peaks appears around - 0.1 V, owing to the reversible doping of chloride anions into the PPy matrix. Continuous potential cycling did not change the loci of the voltammograms, indicating that the PPy(NaC1) has high activity under the applied potential in the range between - 0.6 V and +0.3 V. Figure 2
Applied potential I V vs. Ag/AgCl ‘0.4 0.2 0
ollr 300
-0.4 -0.6 400
500
600
700
800
Wavelength I nm Fig. 2. In situ UV-visible absorption spectra of electroactive PPy(NaCI) on an IT0 glass electrode in a 0.2 mol dmm3 NaCl aqueous solution at various applied potentials.
203
T. Osaka et al. / Formation of insulating polypyrrole film
removing electrons from the r-conjugated system, and the phenomenon of formation of polaron or bipolaron levels in the band structure is revealed in the optical absorption spectra, as seen in Fig. 2. In order to confirm the reversibility of the spectral change, the absorbance of two typical wavelengths, which correspond to the r-conjugated system and bipolaron formation, was measured continuously. Figure 3(a) shows the change in absorbance of two typical wavelengths, 390 nm (r--71*) and 760 nm (BP), which corresponds to the applied potential sweeping between -0.6 V and +0.3 V at 2 mV s-l as shown in Fig. 3(b). The absorbance at 390 nm (r-r*) becomes maximum at -0.6 V in the reduced state, and minimum at +0.3 V in the oxidized state. In contrast, the absorbance at 760 nm (BP) is the reverse. The change in absorbance at the typical wavelengths was very similar on each scan. This indicates that the change in band structure of the active PPy is reversible. 3.2. Inactivation
and
overoxidation
of
electroactive
PpY(NaC1)
Figure 4 shows cyclic voltammograms of the active PPy(NaC1) between -0.6 V and + 1.5 V, where the potential range is extended to the anodic side. In comparison with the redox peaks around -0.1 V, a 1.d
8
2
2
w ? BP(760 nm)
0 ; f
0.3 0
f -0.3 > . -0.6 W
0
10
20 t I
30
40
min.
Fig. 3. (a) Absorbance at 390 nm and 760 nm, and (b) sweep potential curve for a reversible redox process of electroactive PPy(NaCI) on an IT0 glass electrode in a 0.2 mol dmm3 NaCl aqueous solution at 2 mV s-l.
I
’ -0.6
I
I
I
I
0
0.5
1.0
1.5
E I V vs. AgiAgCl Fig. 4. Cyclic voltammograms for the overoxidation of electroactive PPy(NaCI) on an IT0 glass electrode in a 0.2 mol dmm3 NaCl aqueous solution at 2 mV s-l. The numbers in the figure refer to the scan cycle.
broadened anodic peak appears around + 1.0 V. This anodic current is observed only in the first scan, indicating that the irreversible oxidation (“overoxidation”) of active PPy(NaC1) occurs under an anomalous anodic potential in NaCl aqueous solution. After the anodic current flows, almost no current of the redox reaction and overoxidation is observed in subsequent scans. Thus it is suggested that the active PPy(NaC1) is inactivated by a large anodic current. The change in absorbance of the PPy(NaC1) during a potential sweep in such an extended range is shown in Fig. 5. Only in the initial scan up to +0.3 V is a similar tendency observed as in. Fig. 3. Higher than +0.5 V, however, the absorbance of the two wavelengths changes anomalously. This suggests that the overoxidation accompanying the large anodic current changes the band structure of polypyrrole in a different way from the reversible redox reaction. During the positive scan from +0.5 V to + 1.5 V, there are three stages: first, the absorbance at 390 nm increases up to +0.7 V; second, from + 0.7 V to + 1.0 V the absorbance at 760 nm begins to decrease; third, at over + 1.0 V the absorbances at 390 nm and 760 nm become constant and decrease slowly. Therefore, the applied potentials of + 0.5 V, + 0.7 V and + 1.0 V are turning points of stages in overoxidation. The details will be checked later. In subsequent scans, the change in absorbance which appeared in the first scan was not observed. This indicates that the spectrum of active PPy(NaC1) is changed irreversibly by the overoxidation process. In particular, at -0.6 V in the second reduced state, the absorbance at 390 nm decreases more than the first one. The original r-conjugated system along the active PPy(NaC1) chain seems to be destroyed by overoxidation on application of a higher potential. The absorbance at 760 nm also decreases in the second positive scan, indicating that the bipolaron of PPy can hardly be formed in the inactivated state.
T. Osaka et al. / Formation of insulating polypyrrole film
204
3.3. Deposition of electroinactivePPy(NaOH) As reported previously, PPy(NaOH) synthesized electrochemically from an NaOH aqueous solution containing pyrrole monomer is electroinactive and insulating in the as-prepared condition. The growth of the PPy(NaOH) film stopped automatically at ca. 0.2 pm after 60 min under potentiostatic conditions of 1.5 V [16-181. Figure 6 shows cyclic voltammograms in the polymerizing solution for PPy(NaOH) in the potential range between -0.3 V and + 1.8 V at 1 mV s-l. Only in the first positive scan did two sharp anodic peaks appear around +0.7 V and + 1.0 V, and no redox current is shown. After the first cyclic scan, i.e. after deposition of the thin PPy(NaOH) film, no redox peak appears in the voltammograms. The cyclic voltammograms of PPy(NaOH) deposited from the NaOH polymerizing solution do not show redox peaks in a 0.2 mol dmm3 NaCl aqueous solution. However, the cyclic voltammogram of PPy(NaCl) in a polymerizing solution over a wide range shows an anodic current in the positive scan and the growth rate does not decrease as is seen for the PPy(NaOH).
I
I
20
1
40 t I min.
I
I
60
80
Fig. 5. (a) Absorbance at 390 nm and 760 nm, and (b) sweeping potential curve for the overoxidation of electroactive PPy(NaCl) on an IT0 glass electrode in a 0.2 mol dme3 NaCl aqueous solution at 2 mV s-l.
Ll -0.3
I
I
I
I
I
0
0.5
1.0
1.5
1.8
E I V vs. AgiAgCl Fig. 6. Cyclic voltammograms on an IT0 glass electrode in a 0.01 mol drne3 NaOH aqueous solution containing 0.25 mol dmm3 pyrrole at 1 mV s-l. The numbers in the figure refer to the scan cycle.
In order to investigate the two peaks of the cyclic voltammograms in Fig. 6, PPy(NaOH) was deposited by a potential sweep method from 0 to + 1.3 V at 1 mV s-l in the electrochemical UV cell. The change in absorbance at 390 nm during the potential sweep in NaOH polymerizing solution was observed. In Fig. 7(a), the absorbance at 390 nm (7r-r*> is plotted against the charge passed during the potential sweep. In the case of deposition of active PPy(NaCI) at a constant potential of +0.65 V, the absorbance increases linearly after an initial slow increase as shown in Fig. 7(b). The linear relationship suggests uniform growth of the polypyrrole film. However, the gradual increase in absorbance at the initial stage might be due to the formation of oligomers which do not have such a long r-conjugated system as the polymer because the absorption of the oligomer at 390 nm is weaker than that of the polymer. In Fig. 7(a), the linear relationship appears up to a charge passed of 7 x lop2 C cm-2 as well as that of PPy(NaC0. In the region over + 1.00 V, however, the slope becomes lower, suggesting that the anodic current promotes both the polymerization of pyrrole and the destruction of the original r-conjugated system in this region. The potential at which the slope changes corresponds to that of the second anodic peak in Fig. 6. Judging from the appearance of the overoxidation peak at + 1.0 V in Fig. 4, the first peak in the initial positive scan in Fig. 6 corresponds to the polymerization reaction of pyrrole, and the overoxidation then begins to occur at the second peak. The charge passed in the second peak is more than ten times as large as that of the first peak. Considering the estimation by Beck et al. [13] that about five electrons per pyrrole unit are consumed for the exhaustive oxidation, the large second peak includes polymerization in addition
T. Osaka et al. / Formation of insulating polypyrrole film
0.4
205
a)
0.3 8 c e 8 2
0.2
5
10
4
15
8
Q / x10-2 C cm-2 Fig. 7. Dependence of the absorbance at 390 nm on the charge passed on an IT0 glass electrode (a) in a 0.01 mol dm-3 NaOH aqueous solution containing 0.25 mol dme3 pyrrole during potential sweep at 1 mV s- ’ from 0 to + 1.3 V vs. Ag/AgCI, and (b) in a 0.2 mol dmW3 NaCl aqueous solution containing 0.25 mol dmm3 pyrrole at +0.65 V vs. Ag/AgCl.
to complete overoxidation. As a result, the PPy(NaOH) has a very low conductivity compared with inactivated polypyrrole [15,16,18]. 3.4. X-ray photoelectron spectroscopy PpY(NaOH) and PpY(NaC1)
analysis
Thus it seems that the difference in oxygen content correlates with the inactivity and low conductivity of the PPy(NaOH).
of
For XPS analysis, the inactive PPy(NaOH1 and the active PPy(NaC1) were prepared potentiostatically at 1.5 V for 60 min and galvanostatically at 0.4 mA cm-* with 200 mC cm-* on IT0 electrodes (10 x 10 mm*> respectively. Although the insulating PPy(NaOH) sample might become charged up during the measurements, no conducting treatment was adopted. Figure 8 shows the XPS spectra of inactive PPy(NaOH) and active PPy(NaC1). In both spectra, almost the same intensity ratios of C 1s and N 1s peaks around 285 eV and 400 eV are clearly observed. This suggests that the insulating PPy(NaOH) also consists mainly of carbon and nitrogen of pyrrole units as well as the active PPy(NaC1). Since the Na 1s peak at 1072 eV is not observed in either spectrum, impurities such as NaCl and NaOH from electrolyte solutions are removed from the films by rinsing after the polymerization. The clear Cl 2p and Cl 2s peaks at 199 eV and 270 eV for the active PPy(NaC1) originate from the dopant anion. From the clear appearances of the 0 1s peak at 531 eV and O,, Auger peak at 976 eV for the inactive PPy(NaOH), the PPy(NaOH) combines with oxygen atoms and the oxygen content is not due to doping by hydroxide anions [21]. The very small 0 1s peak for Ppy(NaC1) might be due to molecular oxygen interacting with PPy r-electrons or nitrogen atoms [22,23].
a) PPy(NaOH)
N 1s
b) PPy(NaC1)
1000
800
600
N 1s I
400
200
Binding energy i eV Fig. 8. XPS spectra over a wide scan of (a) inactive PPy(NaOH), and (b) active PPy(NaC1) using monochromatic Al Ka radiation.
T. Osaka et al. / Formation of insulating polypyrrole film
N 1s a) PPy(NaOH)
AI_._._-
I\ b) PPy(NaCI)
294
290
286
282
405
Binding energy I eV
401
397
393
389
Binding energy / eV
Fig. 9. C 1s XPS spectra of (a) inactive PPy(NaOH), and (b) active PwNaCI) using monochromatic Al Ka radiation.
Fig. 10. N 1s XPS spectra of (a) inactive PPy(NaOH), and (b) active PPytNaCI) using monochromatic Al Ka radiation.
Although the ratios of the C Is peak to the N 1s peak of both PPy are almost equal, the two peaks display quite different shapes, as shown in Figs. 9 and 10. In Fig. 9, the active PPy(NaC1) has a shoulder in the C 1s peak at 283.5 eV, which corresponds to /3carbon in active polypyrrole [24]. However, no shoulder is seen in the C 1s peak for the inactive PPy(NaOH), suggesting that the carbonyl group on the P-carbon exists in inactive PPy(NaOH). The result agrees well
with that already confirmed by FTIR spectra [16,181. In Fig. 10, the PPy(NaOH) shows a core peak and two small peaks. These two small peaks seem to be due to the covalent binding of hydroxide anion with the polypyrrole chain and deprotonation at the N position by overoxidation [23,25]. The overoxidation mechanism for active polypyrrole was proposed by Beck et al. [13]. In their mechanism, the p-carbon of the pyrrole unit was attacked by a nucleophilic hydroxide ion under the application of a
J-Jd”j=+--$ H
H
Anode Potential Scheme 1.
-q$--Q Hv
H
-w
T. Osaka et al. / Formation of insulating polypyrrole film
highly positive potential as shown in Scheme 1. As a result, the carbonyl group is introduced at the P-carbon position. The nucleophilic attack occurs irreversibly at an active site of the positive charged bipolaron. It seems that these structures in the scheme correspond to the three stages of overoxidation shown in Fig. 5. The increase in the absorbance at 390 nm (r-r*) in the first stage suggests that the original r-conjugated system is reproduced by the formation of 3-hydroxylpyrrole units from +0.5 V to t-O.7 V. The decrease in the absorbance of 760 nm (BP) in the second stage suggests that its r-conjugated system is destroyed by the formation of pyrrolinone units with carbonyl group. In the third stage, further deprotonation occurs at the N position and a hydroxyl group is introduced at the P’-carbon. Owing to the formation of the carbonyl group, the positive charged bipolaron cannot exist at this position and the oxygen atom which combined with the polymer chain has a high electronegativity. Consequently, inactivated PPy has a lower conductivity than undoped PPy. The same mechanism could be adopted for the formation of inactive PPy(NaOH). We already confirmed such a structure of the pyrrole unit in the inactive PPy(NaOH) by FTIR measurements [16,18]. These XPS results also support clearly this mechanism. The difference between the inactive PPy(NaOH) and the inactivated PPy(NaC1) might be the oxidation state. After so great an oxidation, the PPy(NaOH) film becomes quite electroinactive and insulating. 4. Conclusions The active PPy(NaC1) was oxidized irreversibly under a highly positive potential in an aqueous solution containing no monomer. The inactivation process of active PPy(NaC1) was detected in the optical spectrum at 390 nm (a-r*) and 760 nm (BP), where the r-conjugated system of the active PPy(NaC1) was destroyed in three stages. The insulating and inactive PPy(NaOH) was synthesized from an aqueous solution containing monomer and sodium hydroxide, in which the polymerization of pyrrole and the overoxidation of polypyrrole occurred simultaneously. The inactivation was due to the formation of a carbonyl group by attack of a nucleophilic hydroxide ion.
201
Acknowledgments This work was supported financially by the Grantin-Aid for Scientific Research on Priority Areas, the Ministry of Education, Science and Culture. References 1 A.F. Diaz and K.K. Kanazawa, J. Chem. Sot., Chem. Commun., (1979) 635. 2 K.K. Kanazawa, A.F. Diaz, R.H. Gill, J.F. Kwak, J.A. Logan, J.F. Robolt and G.B. Street, J. Chem. Sot., Chem. Commun., (1983) 854. B.J. Feldman, P. Burgmayer and R.W. Murray, J. Am. Chem. Sot., 107 (1985) 872. A. Kitani, J. Yano and K. Sasaki, J. Electroanal. Chem., 209 (1986) 227. H.T. Chiu, J.S. Lin and J.N. Shiau, J. Appl. Electrochem., 22 (1992) 522. S. Dong, Z. Sun and Z. Lu, Analyst, 113 (1988) 1525. T. Okada, H. Hayashi, K. Hiratani, H. Sugihara and N. Koshizaki, Analyst, 116 (1991) 923. Q. Pei and R. Qian, Synth. Met., 45 (1991) 35. R.J. Waltman, A.F. Diaz and J. Bargon, J. Electrochem. Sot., 131 (1984) 740. T. Osaka, T. Nakajima, K. Shiota and B.B. Owens, in S. Subbarao, V.R. Koch, B.B. Owens and W.H. Smyrl (Eds.) Rechargeable Lithium Batteries, PV 90-5, The Electrochemical Society Softbound Proceedings Series, Pennington, NJ, 1990, p. 170. 11 J.L. Bredas and G.B. Street, Act. Chem. Res., 18 (1985) 309. 12 T.F. Otero, R. Tejada and AS. Elola, Polymer, 28 (1987) 651. 13 F. Beck, P. Braun and M. Oberst, Ber. Bunsenges. Phys. Chem., 91 (1987) 967. 14 P. Novak, B. Rasch and W. Vielstich, J. Electrochem. Sot., 138 (1991) 3300. 15 J.B. Schlenoff and H. Xu, J. Electrochem. Sot., 139 (1992) 2397. 16 T. Osaka, T. Fukuda, K. Ouchi and T. Nakajima, Denki Kagaku, 12 (1991) 1019. 17 T. Osaka, T. Fukuda, K. Ouchi and T. Momma, Thin Solid Films, 215 (1992) 200. 18 T. Osaka, T. Momma and H. Kanagawa, Chem. Lett., (1993) 649. 19 T. Nakajima, F. Matsushima, T. Fukuda and T. Osaka, Denki Kagaku, 12 (1991) 1074. 20 T. Osaka, T. Fukuda, H. Kanagawa, T. Momma and S. Yamauchi, Sens. Actuators B, 13 (1993) 205. 21 F. Beck, P. Braun and F. Schloten, J. Electroanal. Chem., 267 (1989) 141. 22 Y. Li and R. Qian, Synth. Met., 28 (1989) C127. 23 P. Pfluger, M. Krounbi and G.B. Street, J. Chem. Phys., 78 (1983) 3212. 24 G.B. Street, in T.A. Skotheim (Ed.), Handbook of Conducting Polymers, Vol. 1, Marcel Decker, New York, 1986, p. 265. 25 H. Miinstedt, Polymer, 27 (1986) 899.
Journal of Electroanalytical Chemistry, 372 (1994) 201-207
Electrochemical
process of formation of an insulating polypyrrole film
Tetsuya Osaka, Toshiyuki Momma, Shin-ichi Komaba and Himeko Kanagawa Department of Applied Chemistry, School of Science and Engineering, Waseda University, Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Shinjuku-ku, Tokyo 169 (Japan)
Sadako Nakamura Department of Chemical and Biological Science, Faculty of Science, Japan Women’s University, Mejirodai, Bunkyo-ky
Tokyo 112 (Japan)
(Received 6 September 1993; in revised form 17 December 1993)
Abstract
Insulating and electroinactive polypyrrole (PPy(NaOH)) film can be synthesized by electropolymerization from an NaOH aqueous solution containing pyrrole at a highly positive potential. The synthesis of this electroinactive PPy(NaOH) was investigated using in situ UV and X-ray photoelectron spectroscopy, and was compared with the irreversible oxidation (“overoxidation”) of electroactive PPy under a highly positive potential. Overoxidation made the PPy inactive. However, the formation of PPy (NaOH) started above 0.5 V vs. Ag/AgCl, and the “overoxidation” began simultaneously in the polymerizing solution. Overoxidation with nucleophilic hydroxide ions caused a structural change in the PPy molecule, which destroyed the original P-conjugated system.
1. Introduction
Electropolymerization is an interesting method for the fabrication of conducting polymer films [l-3] for applications as electrochromic devices [4,51, ion-selective electrodes [6-81 and rechargeable polymer batteries [9,101. In these cases, the key function of the conducting polymers is their electroactivity, which is produced with the formation of a polaron, bipolaron and soliton as a result of the process of doping counter ions into the polymer matrix [ll]. Among the various electrochemically polymerized films, the electroactive polypyrrole (PPy) film has the great advantage of stability; however, application of an anomalously positive potential sometimes decreases the conductivity and electroactivity of PPy, owing to irreversible oxidation (“overoxidation”) [12-151. We have previously reported that an insulating polypyrrole (PPy(NaOH)) film was deposited electrochemically from an aqueous pyrrole solution containing sodium hydroxide at 1.5 V vs. Ag/AgCl; this film became quite electroinactive in the as-prepared condition and its thickness stopped increasing automatically at a certain low value (ca. 0.2 pm) under potentiostatic 0022-0728/94/$7.00 SSDI 0022-0728(94)03298-H
conditions 116,171. It was also possible to deposit insulating PPy films from sodium carbonate or sodium bicarbonate aqueous solution [18]. Utilizing these insulating PPy films, we have fabricated metal/insulating PPy/metal (MIM) type switching devices for large-scale flat panel displays 1193, and amperometric pH sensor devices for a very small volume test solution [20]. This kind of film also has potential for use as protection against corrosion of commodity metals. In the present paper, we discuss the synthesis of insulating PPy(NaOH), using in situ UV-visible spectroscopy and X-ray photoelectron spectroscopy (XPS), compared with the overoxidation process of active PPy in aqueous media. 2. Experimental
details
Reagent grade pyrrole (Wake Pure Chemicals Industries, Ltd.), NaOH (Kant0 Chemical Co., Inc.) and NaCl (Kokusan Chemical Works, Ltd.) were used without further purification. Aqueous solutions of 0.25 ma1 dmp3 pyrrole containing either 0.01 mol dme3 NaOH(pH 12.0) or 0.2 mol dmm3 NaCl as supporting electrolyte were used for electrochemical deposition of the inactive PPy(NaOH) and the active PPy(NaC1) films 0 1994 - Elsevier Science S.A. All rights reserved
202
T. Osaka et al. / Formation of insulating polypyrrole film
respectively. Deionized and distilled water was used throughout all experiments. For the electrochemical polymerization and measurements, a one-component cell was assembled with a conducting indium tin oxide (ITO) coated glass plate as a working electrode, a Pt counter-electrode, and an Ag/AgCl,,,,, reference electrode. The electrolyte solutions were deoxygenated by bubbling with argon prior to all electrochemical polymerizations and measurements. In order to take the optical absorption spectra of PPy in situ, the potential of the PPy/ITO electrode was controlled in an electrochemical UV cell (pathlength 1 cm) made of quartz glass. The PPy(NaC1) was prepared galvanostatically (current density 0.4 mA cme2, amount of charge 0.05 C cmp2> on an IT0 glass electrode (8 X 11 mm2). After the preparation of this active PPy(NaCl), the electrode was rinsed with an electrolyte solution prior to electrochemical optical measurements. Another electroinactive PPy(NaOH) film was deposited by the potential sweep method (from 0 to 1.3 V vs. Ag/AgCl) at 1 mV s-* in the electrochemical UV cell, in which the change in the absorbance was measured during polymerization. For the XPS measurements, the active PPy(NaC1) prepared galvanostatically and the inactive PPy(NaOH) formed potentiostatically at 1.5 V were rinsed in water and dried with flowing nitrogen gas. These PPy films were analyzed using XPS (JEOL JPS-90SQ, target Al, acceleration voltage 10 kV, gun current 20 mA1. Monochromatic Al Ka was used as incident X-rays. The background pressure during the measurements was ca. lo-’ Pa.
64“: 8 e P
0 X .
.-
2o-2-4 -6 I ’ -0.6
I -0.3
I 0
c 0.1
E / V vs. Ag/AgCI Fig. 1. A cyclic voltammogram of electroactive PPy(NaCI) on an IT0 glass electrode in a 0.2 mol dmm3 NaCl aqueous solution at 100 mV S-l.
shows in situ UV-visible spectra of the active Ppy(NaC1) under various applied potentials. The wavelength of the absorption peak in the spectra of active PPy is related to the band transition [ll]. A strong absorption peak at h = 390 nm appears in the spectrum of PPy(NaC1) in the undoped state at -0.6 V, which is attributed to the a-r* transition. However, a very broad peak around 800 nm appears in the spectrum of the oxidized PPy(NaC1) at an anodic potential of more than -0.2 V, this is caused by bipolaron (BP) formation on doping with chloride anions [ll]. Polarons and bipolarons can form along the PPy chain by
3. Results and discussion 3.1. Electrochemical PPy(NaCI)
optical
studies
of
electroactive
The electroactive PPy(NaC1) was prepared galvanostatically from an aqueous solution containing pyrrole and NaCl. A current density of 0.4 mA cmp2 was used to load the electrode with a charge of 50 mC cmm2 for the formation of PPy(NaC1). During polymerization, the electrode potential was held at an almost constant value around +0.6 V. The resulting PPy(NaC1) film had a thickness of about 0.2 pm. Figure 1 shows a cyclic voltammogram of the PPy(NaC1) in a 0.2 mol dmp3 NaCl aqueous solution. A pair of redox peaks appears around - 0.1 V, owing to the reversible doping of chloride anions into the PPy matrix. Continuous potential cycling did not change the loci of the voltammograms, indicating that the PPy(NaC1) has high activity under the applied potential in the range between - 0.6 V and +0.3 V. Figure 2
Applied potential I V vs. Ag/AgCl ‘0.4 0.2 0
ollr 300
-0.4 -0.6 400
500
600
700
800
Wavelength I nm Fig. 2. In situ UV-visible absorption spectra of electroactive PPy(NaCI) on an IT0 glass electrode in a 0.2 mol dmm3 NaCl aqueous solution at various applied potentials.
203
T. Osaka et al. / Formation of insulating polypyrrole film
removing electrons from the r-conjugated system, and the phenomenon of formation of polaron or bipolaron levels in the band structure is revealed in the optical absorption spectra, as seen in Fig. 2. In order to confirm the reversibility of the spectral change, the absorbance of two typical wavelengths, which correspond to the r-conjugated system and bipolaron formation, was measured continuously. Figure 3(a) shows the change in absorbance of two typical wavelengths, 390 nm (r--71*) and 760 nm (BP), which corresponds to the applied potential sweeping between -0.6 V and +0.3 V at 2 mV s-l as shown in Fig. 3(b). The absorbance at 390 nm (r-r*) becomes maximum at -0.6 V in the reduced state, and minimum at +0.3 V in the oxidized state. In contrast, the absorbance at 760 nm (BP) is the reverse. The change in absorbance at the typical wavelengths was very similar on each scan. This indicates that the change in band structure of the active PPy is reversible. 3.2. Inactivation
and
overoxidation
of
electroactive
PpY(NaC1)
Figure 4 shows cyclic voltammograms of the active PPy(NaC1) between -0.6 V and + 1.5 V, where the potential range is extended to the anodic side. In comparison with the redox peaks around -0.1 V, a 1.d
8
2
2
w ? BP(760 nm)
0 ; f
0.3 0
f -0.3 > . -0.6 W
0
10
20 t I
30
40
min.
Fig. 3. (a) Absorbance at 390 nm and 760 nm, and (b) sweep potential curve for a reversible redox process of electroactive PPy(NaCI) on an IT0 glass electrode in a 0.2 mol dmm3 NaCl aqueous solution at 2 mV s-l.
I
’ -0.6
I
I
I
I
0
0.5
1.0
1.5
E I V vs. AgiAgCl Fig. 4. Cyclic voltammograms for the overoxidation of electroactive PPy(NaCI) on an IT0 glass electrode in a 0.2 mol dmm3 NaCl aqueous solution at 2 mV s-l. The numbers in the figure refer to the scan cycle.
broadened anodic peak appears around + 1.0 V. This anodic current is observed only in the first scan, indicating that the irreversible oxidation (“overoxidation”) of active PPy(NaC1) occurs under an anomalous anodic potential in NaCl aqueous solution. After the anodic current flows, almost no current of the redox reaction and overoxidation is observed in subsequent scans. Thus it is suggested that the active PPy(NaC1) is inactivated by a large anodic current. The change in absorbance of the PPy(NaC1) during a potential sweep in such an extended range is shown in Fig. 5. Only in the initial scan up to +0.3 V is a similar tendency observed as in. Fig. 3. Higher than +0.5 V, however, the absorbance of the two wavelengths changes anomalously. This suggests that the overoxidation accompanying the large anodic current changes the band structure of polypyrrole in a different way from the reversible redox reaction. During the positive scan from +0.5 V to + 1.5 V, there are three stages: first, the absorbance at 390 nm increases up to +0.7 V; second, from + 0.7 V to + 1.0 V the absorbance at 760 nm begins to decrease; third, at over + 1.0 V the absorbances at 390 nm and 760 nm become constant and decrease slowly. Therefore, the applied potentials of + 0.5 V, + 0.7 V and + 1.0 V are turning points of stages in overoxidation. The details will be checked later. In subsequent scans, the change in absorbance which appeared in the first scan was not observed. This indicates that the spectrum of active PPy(NaC1) is changed irreversibly by the overoxidation process. In particular, at -0.6 V in the second reduced state, the absorbance at 390 nm decreases more than the first one. The original r-conjugated system along the active PPy(NaC1) chain seems to be destroyed by overoxidation on application of a higher potential. The absorbance at 760 nm also decreases in the second positive scan, indicating that the bipolaron of PPy can hardly be formed in the inactivated state.
T. Osaka et al. / Formation of insulating polypyrrole film
204
3.3. Deposition of electroinactivePPy(NaOH) As reported previously, PPy(NaOH) synthesized electrochemically from an NaOH aqueous solution containing pyrrole monomer is electroinactive and insulating in the as-prepared condition. The growth of the PPy(NaOH) film stopped automatically at ca. 0.2 pm after 60 min under potentiostatic conditions of 1.5 V [16-181. Figure 6 shows cyclic voltammograms in the polymerizing solution for PPy(NaOH) in the potential range between -0.3 V and + 1.8 V at 1 mV s-l. Only in the first positive scan did two sharp anodic peaks appear around +0.7 V and + 1.0 V, and no redox current is shown. After the first cyclic scan, i.e. after deposition of the thin PPy(NaOH) film, no redox peak appears in the voltammograms. The cyclic voltammograms of PPy(NaOH) deposited from the NaOH polymerizing solution do not show redox peaks in a 0.2 mol dmm3 NaCl aqueous solution. However, the cyclic voltammogram of PPy(NaCl) in a polymerizing solution over a wide range shows an anodic current in the positive scan and the growth rate does not decrease as is seen for the PPy(NaOH).
I
I
20
1
40 t I min.
I
I
60
80
Fig. 5. (a) Absorbance at 390 nm and 760 nm, and (b) sweeping potential curve for the overoxidation of electroactive PPy(NaCl) on an IT0 glass electrode in a 0.2 mol dme3 NaCl aqueous solution at 2 mV s-l.
Ll -0.3
I
I
I
I
I
0
0.5
1.0
1.5
1.8
E I V vs. AgiAgCl Fig. 6. Cyclic voltammograms on an IT0 glass electrode in a 0.01 mol drne3 NaOH aqueous solution containing 0.25 mol dmm3 pyrrole at 1 mV s-l. The numbers in the figure refer to the scan cycle.
In order to investigate the two peaks of the cyclic voltammograms in Fig. 6, PPy(NaOH) was deposited by a potential sweep method from 0 to + 1.3 V at 1 mV s-l in the electrochemical UV cell. The change in absorbance at 390 nm during the potential sweep in NaOH polymerizing solution was observed. In Fig. 7(a), the absorbance at 390 nm (7r-r*> is plotted against the charge passed during the potential sweep. In the case of deposition of active PPy(NaCI) at a constant potential of +0.65 V, the absorbance increases linearly after an initial slow increase as shown in Fig. 7(b). The linear relationship suggests uniform growth of the polypyrrole film. However, the gradual increase in absorbance at the initial stage might be due to the formation of oligomers which do not have such a long r-conjugated system as the polymer because the absorption of the oligomer at 390 nm is weaker than that of the polymer. In Fig. 7(a), the linear relationship appears up to a charge passed of 7 x lop2 C cm-2 as well as that of PPy(NaC0. In the region over + 1.00 V, however, the slope becomes lower, suggesting that the anodic current promotes both the polymerization of pyrrole and the destruction of the original r-conjugated system in this region. The potential at which the slope changes corresponds to that of the second anodic peak in Fig. 6. Judging from the appearance of the overoxidation peak at + 1.0 V in Fig. 4, the first peak in the initial positive scan in Fig. 6 corresponds to the polymerization reaction of pyrrole, and the overoxidation then begins to occur at the second peak. The charge passed in the second peak is more than ten times as large as that of the first peak. Considering the estimation by Beck et al. [13] that about five electrons per pyrrole unit are consumed for the exhaustive oxidation, the large second peak includes polymerization in addition
T. Osaka et al. / Formation of insulating polypyrrole film
0.4
205
a)
0.3 8 c e 8 2
0.2
5
10
4
15
8
Q / x10-2 C cm-2 Fig. 7. Dependence of the absorbance at 390 nm on the charge passed on an IT0 glass electrode (a) in a 0.01 mol dm-3 NaOH aqueous solution containing 0.25 mol dme3 pyrrole during potential sweep at 1 mV s- ’ from 0 to + 1.3 V vs. Ag/AgCI, and (b) in a 0.2 mol dmW3 NaCl aqueous solution containing 0.25 mol dmm3 pyrrole at +0.65 V vs. Ag/AgCl.
to complete overoxidation. As a result, the PPy(NaOH) has a very low conductivity compared with inactivated polypyrrole [15,16,18]. 3.4. X-ray photoelectron spectroscopy PpY(NaOH) and PpY(NaC1)
analysis
Thus it seems that the difference in oxygen content correlates with the inactivity and low conductivity of the PPy(NaOH).
of
For XPS analysis, the inactive PPy(NaOH1 and the active PPy(NaC1) were prepared potentiostatically at 1.5 V for 60 min and galvanostatically at 0.4 mA cm-* with 200 mC cm-* on IT0 electrodes (10 x 10 mm*> respectively. Although the insulating PPy(NaOH) sample might become charged up during the measurements, no conducting treatment was adopted. Figure 8 shows the XPS spectra of inactive PPy(NaOH) and active PPy(NaC1). In both spectra, almost the same intensity ratios of C 1s and N 1s peaks around 285 eV and 400 eV are clearly observed. This suggests that the insulating PPy(NaOH) also consists mainly of carbon and nitrogen of pyrrole units as well as the active PPy(NaC1). Since the Na 1s peak at 1072 eV is not observed in either spectrum, impurities such as NaCl and NaOH from electrolyte solutions are removed from the films by rinsing after the polymerization. The clear Cl 2p and Cl 2s peaks at 199 eV and 270 eV for the active PPy(NaC1) originate from the dopant anion. From the clear appearances of the 0 1s peak at 531 eV and O,, Auger peak at 976 eV for the inactive PPy(NaOH), the PPy(NaOH) combines with oxygen atoms and the oxygen content is not due to doping by hydroxide anions [21]. The very small 0 1s peak for Ppy(NaC1) might be due to molecular oxygen interacting with PPy r-electrons or nitrogen atoms [22,23].
a) PPy(NaOH)
N 1s
b) PPy(NaC1)
1000
800
600
N 1s I
400
200
Binding energy i eV Fig. 8. XPS spectra over a wide scan of (a) inactive PPy(NaOH), and (b) active PPy(NaC1) using monochromatic Al Ka radiation.
T. Osaka et al. / Formation of insulating polypyrrole film
N 1s a) PPy(NaOH)
AI_._._-
I\ b) PPy(NaCI)
294
290
286
282
405
Binding energy I eV
401
397
393
389
Binding energy / eV
Fig. 9. C 1s XPS spectra of (a) inactive PPy(NaOH), and (b) active PwNaCI) using monochromatic Al Ka radiation.
Fig. 10. N 1s XPS spectra of (a) inactive PPy(NaOH), and (b) active PPytNaCI) using monochromatic Al Ka radiation.
Although the ratios of the C Is peak to the N 1s peak of both PPy are almost equal, the two peaks display quite different shapes, as shown in Figs. 9 and 10. In Fig. 9, the active PPy(NaC1) has a shoulder in the C 1s peak at 283.5 eV, which corresponds to /3carbon in active polypyrrole [24]. However, no shoulder is seen in the C 1s peak for the inactive PPy(NaOH), suggesting that the carbonyl group on the P-carbon exists in inactive PPy(NaOH). The result agrees well
with that already confirmed by FTIR spectra [16,181. In Fig. 10, the PPy(NaOH) shows a core peak and two small peaks. These two small peaks seem to be due to the covalent binding of hydroxide anion with the polypyrrole chain and deprotonation at the N position by overoxidation [23,25]. The overoxidation mechanism for active polypyrrole was proposed by Beck et al. [13]. In their mechanism, the p-carbon of the pyrrole unit was attacked by a nucleophilic hydroxide ion under the application of a
J-Jd”j=+--$ H
H
Anode Potential Scheme 1.
-q$--Q Hv
H
-w
T. Osaka et al. / Formation of insulating polypyrrole film
highly positive potential as shown in Scheme 1. As a result, the carbonyl group is introduced at the P-carbon position. The nucleophilic attack occurs irreversibly at an active site of the positive charged bipolaron. It seems that these structures in the scheme correspond to the three stages of overoxidation shown in Fig. 5. The increase in the absorbance at 390 nm (r-r*) in the first stage suggests that the original r-conjugated system is reproduced by the formation of 3-hydroxylpyrrole units from +0.5 V to t-O.7 V. The decrease in the absorbance of 760 nm (BP) in the second stage suggests that its r-conjugated system is destroyed by the formation of pyrrolinone units with carbonyl group. In the third stage, further deprotonation occurs at the N position and a hydroxyl group is introduced at the P’-carbon. Owing to the formation of the carbonyl group, the positive charged bipolaron cannot exist at this position and the oxygen atom which combined with the polymer chain has a high electronegativity. Consequently, inactivated PPy has a lower conductivity than undoped PPy. The same mechanism could be adopted for the formation of inactive PPy(NaOH). We already confirmed such a structure of the pyrrole unit in the inactive PPy(NaOH) by FTIR measurements [16,18]. These XPS results also support clearly this mechanism. The difference between the inactive PPy(NaOH) and the inactivated PPy(NaC1) might be the oxidation state. After so great an oxidation, the PPy(NaOH) film becomes quite electroinactive and insulating. 4. Conclusions The active PPy(NaC1) was oxidized irreversibly under a highly positive potential in an aqueous solution containing no monomer. The inactivation process of active PPy(NaC1) was detected in the optical spectrum at 390 nm (a-r*) and 760 nm (BP), where the r-conjugated system of the active PPy(NaC1) was destroyed in three stages. The insulating and inactive PPy(NaOH) was synthesized from an aqueous solution containing monomer and sodium hydroxide, in which the polymerization of pyrrole and the overoxidation of polypyrrole occurred simultaneously. The inactivation was due to the formation of a carbonyl group by attack of a nucleophilic hydroxide ion.
201
Acknowledgments This work was supported financially by the Grantin-Aid for Scientific Research on Priority Areas, the Ministry of Education, Science and Culture. References 1 A.F. Diaz and K.K. Kanazawa, J. Chem. Sot., Chem. Commun., (1979) 635. 2 K.K. Kanazawa, A.F. Diaz, R.H. Gill, J.F. Kwak, J.A. Logan, J.F. Robolt and G.B. Street, J. Chem. Sot., Chem. Commun., (1983) 854. B.J. Feldman, P. Burgmayer and R.W. Murray, J. Am. Chem. Sot., 107 (1985) 872. A. Kitani, J. Yano and K. Sasaki, J. Electroanal. Chem., 209 (1986) 227. H.T. Chiu, J.S. Lin and J.N. Shiau, J. Appl. Electrochem., 22 (1992) 522. S. Dong, Z. Sun and Z. Lu, Analyst, 113 (1988) 1525. T. Okada, H. Hayashi, K. Hiratani, H. Sugihara and N. Koshizaki, Analyst, 116 (1991) 923. Q. Pei and R. Qian, Synth. Met., 45 (1991) 35. R.J. Waltman, A.F. Diaz and J. Bargon, J. Electrochem. Sot., 131 (1984) 740. T. Osaka, T. Nakajima, K. Shiota and B.B. Owens, in S. Subbarao, V.R. Koch, B.B. Owens and W.H. Smyrl (Eds.) Rechargeable Lithium Batteries, PV 90-5, The Electrochemical Society Softbound Proceedings Series, Pennington, NJ, 1990, p. 170. 11 J.L. Bredas and G.B. Street, Act. Chem. Res., 18 (1985) 309. 12 T.F. Otero, R. Tejada and AS. Elola, Polymer, 28 (1987) 651. 13 F. Beck, P. Braun and M. Oberst, Ber. Bunsenges. Phys. Chem., 91 (1987) 967. 14 P. Novak, B. Rasch and W. Vielstich, J. Electrochem. Sot., 138 (1991) 3300. 15 J.B. Schlenoff and H. Xu, J. Electrochem. Sot., 139 (1992) 2397. 16 T. Osaka, T. Fukuda, K. Ouchi and T. Nakajima, Denki Kagaku, 12 (1991) 1019. 17 T. Osaka, T. Fukuda, K. Ouchi and T. Momma, Thin Solid Films, 215 (1992) 200. 18 T. Osaka, T. Momma and H. Kanagawa, Chem. Lett., (1993) 649. 19 T. Nakajima, F. Matsushima, T. Fukuda and T. Osaka, Denki Kagaku, 12 (1991) 1074. 20 T. Osaka, T. Fukuda, H. Kanagawa, T. Momma and S. Yamauchi, Sens. Actuators B, 13 (1993) 205. 21 F. Beck, P. Braun and F. Schloten, J. Electroanal. Chem., 267 (1989) 141. 22 Y. Li and R. Qian, Synth. Met., 28 (1989) C127. 23 P. Pfluger, M. Krounbi and G.B. Street, J. Chem. Phys., 78 (1983) 3212. 24 G.B. Street, in T.A. Skotheim (Ed.), Handbook of Conducting Polymers, Vol. 1, Marcel Decker, New York, 1986, p. 265. 25 H. Miinstedt, Polymer, 27 (1986) 899.