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Preparation of polyaniline films doped with methylene blue-bound Nafion and the electrochromic properties of the resulting films
97
J. Electroanal. Chem., 281 (1990) 97-107 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preparation of polyaniline films doped with methylene blue-bound Nafion and the electrochromic properties of the resulting films Susumu Kuwabata, Kazuki Mitsui and Hiroshi Yoneyama Department of Applied Chemistry, Osaka 565 (Japan)
Faculty
l
of Engineering, Osaka University, Yamada-oka 2-1, Suita,
(Received 3 August 1989; in revised form 3 November 1989)
ABSTRACT The preparation of polyaniline films doped with cationic methylene blue has been accomplished by the electro-oxidation of aniline in the presence of dissolved Nafion and methylene blue. The amount of methylene blue in the film could be varied by changing the relative concentration of methylene blue to Nafion in the deposition bath of polyanihne. The incorporation behaviour of methylene blue in polyaniline films is discussed in terms of the electrostatic binding of methylene blue with the sulphonate groups of Nafione in the deposition bath. The resulting films showed electrochemical responses of both polyaniline and methylene blue.
INTRODUCTION
Polyaniline (PAn) films prepared by electrochemical polymerization of aniline exhibit stable redox reactions [l-6], and several applications, for example to electrochromic [7-91 and electronic [lO,ll] devices, have been attempted using the redox properties. Recently, modification of the electrocatalytic activity of polyaniline by incorporating cobalt phthalocyanine tetrasulphonates [12] and heteropolyanions [13] has been reported. These functionalizations have been achieved successfully by utilizing the property that electrolyte anions are incorporated into the polyaniline films during the course of the polymerization of aniline to compensate the positive charges induced in the polymer. However, a procedure for the incorporation of cationic functional molecules into polyaniline films has not yet been devised.
l
To whom correspondence should be addressed.
0022-0728/90/$03.50
0 1990 Elsevier Sequoia S.A.
98
In this paper, the preparation of polyaniline films doped with cationic methylene blue is described. The strategy was to polymerize aniline in the presence of methylene blue and Nafion, with the latter being in excess. Under such conditions, a fraction of the sulphonate groups of Nafion are bound electrostatically with methylene blue, and the rest are free and can work as the charge compensator of positive charges induced in polyaniline. The electrochemical and electrochromic properties of the resulting films are also described. EXPERIMENTAL
Indium-tin-oxide coated glass (ITO) and Pt plates polished with alumina (1 pm) were used as the electrode substrates for the absorption spectra and electrochemical measurements, respectively. An electrical lead wire of the IT0 was attached to its round rim with silver-epoxy and fixed with epoxy resin. The Pt plate was mounted in a Teflon electrode holder having an exposed area of 1 cm2. Aldrich Nafion 117 solution was used without dilution as the supporting electrolyte solution for preparing the deposition baths. This solution was a mixture of a lower aliphatic alcohol and water (9 : 1) in which 5 wt% of the hydrogen form of Nafion (molar mass 1100 g) was dissolved. The concentration of sulphonic acid groups was 39.7 mM, and the pH of the solution was 1.7, giving ca. 50% dissociation of sulphonic acid groups. The deposition bath was prepared by dissolving 5 mM aniline and various amounts of methylene blue (MB) in the Nafion solution. The anodic polymerization to give polyaniline films doped with methylene blue-bound Nafion, which will be denoted in this paper as PAn/MB-Nafion, was carried out galvanostatically in a one-compartment cell with a Pt counter-electrode under N, atmosphere. The current density chosen was 40 PA cmp2, and the quantity of electricity used was 12 mC cm -2 for the electrochemical measurements and 36 mC cme2 for the other measurements. The preparation of the Nafion-coated Pt electrode containing methylene blue in the Nafion layer (MB-Nafion) has been described elsewhere [14] in detail. The amount of Nafion coated on the Pt electrode mol cmW2 of sulphonate substrate was 2.0 ~1 cmp2, which contained 7.9 x lo-’ groups, and the film thickness was 0.5 pm in the dry state. The incorporation of methylene blue into the Nafion layer in this case was performed by immersion in a 1 mM methylene blue solution for 30 min. Electrochemical measurements were carried out using a potentiostat (Hokuto Denko HA-301), a function generator (Hokuto Denko HB-101) and an X-Y recorder (Yokogawa Model 3077). A saturated calomel electrode (SCE) served as the reference electrode. Spectroelectrochemical studies were carried out using a quartz cell (20 X 10 X 40 mm) equipped with an IT0 electrode coated with PAn/MB-Nafion, a Pt counterelectrode and a SCE. The cell was set in a UV-visible spectrophotometer (Shimadzu MPS-5000), and absorption spectra were measured in situ under polarization. The electrochromic responses of PAn/MB-Nafion and MB-Nafion were investigated by measuring the changes in reflectance during the application of potential pulses of
99
0.5 and 0 V (vs. SCE). For this purpose, the films coated on irradiated with a 500 W xenon lamp (Ushio Electric UXL-SOOD-0), the intensity of the reflected light caused by the application of measured at 660 nm using a monochrometer (JASCO CT-25N) and RESULTS
Pt plates were and changes in potentials were a PbS detector.
AND DISCUSSION
Electrostatic
binding between methylene
blue and Nafion
The electrostatic binding of methylene blue with sulphonate groups of Nafion was studied by measuring the changes in the absorption spectra of methylene blue dissolved in n-propanol with addition of the Aldrich Nafion solution. The results obtained are shown in Fig. 1. The absorption spectrum of methylene blue taken in the absence of Nafion showed a sharp peak at 660 nm and a weak hump at 610 nm which are assigned to the absorption bands of monomeric and dimeric methylene blue, respectively [15-171. The intensity of these bands decreased significantly with an increase in the concentration of sulphonate groups of Nafion, while a new band assigned to trimeric methylene blue appeared and then increased at 550 nm, and simultaneously the colour of the solution turned from blue to purple. The phenomena observed are regarded as “metachromasia”, which has been reported for changes in the colour of several kinds of cationic dye solutions such as methylene blue, phenosafranine and pyronine G upon addition of anionic polyelectrolytes such as polystyrene sulphonate and polyvinyl sulphate [l&23]. The cationic dye molecules were concentrated in the polyelectrolytes due to electrostatic binding with anionic substituent groups of the polymers, resulting in the formation of dimeric or trimeric dyes.
0.8 3 5 Kl
0.6
B 9
0.4 0.2 n ”
400
so0
600
700
Wavelength / nm Fig. 1. Changes in the absorption spectra of 1.0 X 10V5 M methylene blue dissolved in n-propanol with addition of the Nafion solution. Concentration of sulphonate groups of Nafion/M: 0 (a); 5.0 X 10e6 (b); 8.0~10-~ (c); 1.0X10K5 (d); 2.0~10~’ (e); 5.0X10F5 (0.
100
If a binding equilibrium is established between the cationic dyes and anionic polyelectrolytes, the ratio (r) of the anionic substituent groups of the polyelectrolyte containing cationic dyes to all the anionic substituent groups is given by the equation [22-241 l/r = l/nKc,,
+ l/n
where cr,,, is the concentration of free cationic dyes dissolved in the solution, and K and n are the intrinsic binding constant and the maximum number of cationic dye molecules capable of binding to an anionic substituent group of the polyelectrolyte, respectively. r in eqn. (1) is given by
r= (cnl- Cfnl>/Cs where c, is the total concentration of free and bound methylene blue and c, is that of methylene blue-bound and -free sulphonate groups of Nafion dissolved in the solution. The concentration of free methylene blue (cr,,,) can be obtained from the apparent molecular absorption coefficient of monomeric methylene blue at 660 nm (eeeO) by using the equation [22-241 <660 Cfm
=
-
c6*6O
Gxl-
cm Ga
where eg*60is the absorption coefficient of monomeric methylene blue obtained when all the methylene blue molecules are bound to Nafion, and c&, is that in the absence of Nafion. The value of cl60 was estimated to be 89 800 M-’ cm-’ from the absorption spectrum shown in Fig. 1 (curve a). The value of ez60 was found to be 2570 M-i cm-’ from extrapolation of the c660 vs. c,/c, relation to c,/c, = 0 [22,23], as shown in Fig. 2. The cfm values of methylene blue solutions containing various concentrations of Nafion were then obtained from eqn. (3) using the ehCO
0
0.2
0.4
0.6
0.8
1.0
cm its
Fig. 2. Relationship Nafion.
between
ee6,, and c,/c,
of n-propanol
containing
1.0~ 10m5 M methylene
blue and
101
3
‘i L 2
0
0
5 ro-5c‘,,/,-l
lo
Fig. 3. Relationship between l/r eqns. (2) and (3).
and l/c,
l5
obtained from c&0 of the spectra shown in Fig. 1 using
values deduced from each spectrum shown in Fig. 1, and the values of r were determined from eqn. (2) using the cy, values determined. Figure 3 shows that eqn. (1) holds between l/r and l/c,, indicating the establishment of a binding equilibrium between methylene blue and the sulphonate groups of Nafion. The values of n and K were then estimated to be 1.02 and 5.57 x 105, respectively. It is recognized from the n value determined that methylene blue molecules are bound to sulphonate groups of Nafion in the ratio of 1 : 1. The binding constant is then formulated as K = c,,/cfscfm ( = 5.57 x lo5 M-r) (4) where c,, is the concentration of methylene blue-bound sulphonate groups of Nafion in the solution, and cyS is that of methylene blue-free sulphonate groups. Preparation
of polyaniline
films doped with methylene
blue-bound Nafion
Polyaniline films are prepared by the electro-oxidation of aniline in highly acidic solutions [1,2,25]. It was found that the original Aldrich Nafion solution had a sufficiently high acidity to deposit smooth polyaniline films on electrode substrates such as Pt, glassy carbon and IT0 electrodes, as judged from the electrolysis results of 5 mM aniline in the Nafion solution. In this case, the sulphonate groups of Nafion should serve as the charge compensator of the positive charges of polyaniline [26]. Polyaniline films were prepared on IT0 electrodes from Aldrich Nafion solutions containing 5 mM aniline and 0, 1, 3 and 10 mM methylene blue, and the absorption spectra of the resulting films were obtained as shown in Fig. 4. The spectrum of the polyaniline films prepared in the absence of methylene blue (Fig. 4a) was almost the same as that of polyaniline films prepared in a conventional electrolyte solution
102 I
_I 500
400
600
Wavelength
700
/ nrn
Fig. 4. Absorption spectra of PAn/MB-Nafion films prepared from Nafion aniline and 0 (a), 1.0 (b), 3.0 (c) and 10.0 mM (d) methylene blue.
solutions
containing
5 mM
such as chloride and perchlorate solutions [1,6,7], whereas the polyaniline films prepared in the presence of methylene blue exhibited definite absorptions at 660 and 610 nm, which are assigned to monomeric and dimeric methylene blue, respectively. The intensity of these bands increased with increasing concentration of methylene blue in the deposition bath. As already shown above, methylene blue was bound to the sulphonate groups of Nafion, but a fraction of the sulphonate groups of Nafion remained free from binding with methylene blue, because the concentration of sulphonate groups was 39.7 mM, i.e. greater than the concentration of methylene blue dissolved. It is then thought that the observed incorporation of methylene blue in the polyaniline films occurs with the methylene blue-free sulphonates of Nafion acting as a charge compensator. The concentrations of monomeric methylene blue (I,,,) and dimeric methylene blue (I,,) in the polyaniline films, and their sum (I, = I,,, + 2I’,), were determined by using the equations r
= m
cd 660A 610 Go&o
4lOA660 x 1o-3
- Goe:10
(5)
and r
=
“A '610
“A 660-c660
610
d
x
1o-3
(6)
E?10c:60 - Egm60E:10
which were derived in our previous paper [14] to estimate the methylene blue concentration incorporated in Nafion-modified electrodes. In eqns. (5) and (6) ego and E&, are the molar extinction coefficients at 610 and 660 nm of monomeric methylene blue dissolved in aqueous solution, respectively; zgdioand CL, are those of dimeric methylene blue; and A,,, and A,, are the absorbances at 610 and 660 nm obtained after correction of the absorbance due to polyaniline. The values of rm, r, and I, are listed in Table 1 together with c,,,/c, for three different concentrations of methylene blue.
103 TABLE 1 Concentration of methylene blue and the r values a of the PAn/MB-Nafion c, /mM
lo9 r,/mol
1.0 3.0 10.0
1.3 4.3 15
cm-2
lo9 &/mol
cm-*
1.5 0.73 4.5
lo9 r,/mol
films
cm-’
1.6 5.8 24
C,/CSb
r
0.025 0.072 0.24
0.021 0.076 0.252
a Relative ratio of the sulphonate groups bound with MB to all sulphonate groups of Nafion incorporated in the PAn/MB-Nafion films, estimated using eqn. (8). b c, = 39.7 mM.
Judging from the electrode potential (ca. 0.5 V vs. SCE) monitored in the course of the electropolymerization of aniline, the polyaniline films prepared must be in the emeraldine salt form in which the molar ratio of doped electrolyte anions to aniline units is ca. 0.5 [27-301. Since the polymerization reaction in this case is given by
l0.5n A-
the concentration rA
of the doped anions in the polymer film (I,.-)
is given by
_AsE!LOZP
(7) F 2.5n . F where F and Q are the Faraday constant and the quantity of electricity consumed in the polymerization, respectively. In the present study, the polyaniline films were prepared using Q = 36 mC cm-2, so that IA- of the sulphonate groups of Nafion must be 7.5 x lop8 mol cm- *. The total concentration of sulphonate groups of Nafion incorporated in the prepared polyaniline films must be the sum of IA- and I,. Then the ratio of the methylene blue-bound sulphonate groups of Nafion to the total amount of sulphonate groups in the polyaniline films is given by r=
q/(r,-+ r,)
(8)
The values of r estimated by this equation are given in Table 1. It is noteworthy from the table that the r values obtained are very close to the relative ratio of the concentration of methylene blue to that of sulphonate groups of Nafion (cm/c,) in the deposition baths of polyaniline, indicating that almost all the methylene blue molecules in the deposition bath are bound to the sulphonate groups of Nafion. This is quite natural judging from the very large binding constant (5.57 X lo5 M-‘) obtained in the relatively dilute concentrations of methylene blue and sulphonate groups of Nafion. Electrochemical and electrochromic methylene blue-bound Nafion
properties
of the polyaniline
films
doped
with
Cyclic voltammograms of polyaniline films doped with Nafion (PAn/Nafion) and doped with MB-bound Nafion (PAn/MB-Nafion) were taken in 1 M HCl
3 E vs. SCE /V Fig. 5. Cyclic voftammograms of PAn/Nafion (a) and PAn/MB-N~ion (b-d) taken in 1.0 M HCI solution. dE/dt = 100 mV s-‘, PAn/MB-Nafion films were prepared from Nafion solutions containing 5 mM aniline and 1.0 (b), 3.0 (c) and 10.0 mM (d) methylene blue.
solution. As Fig. 5 shows, a couple of redox waves due to polyaniline alone are observed at PAn/Nafion, whereas at ~An/MB-Nafion another couple of anodic and cathodic waves due to the redox reactions of methylene blue appeared at 0.27 and 0.23 V (vs. SCE), respectively. The peak currents due to this redox reaction increased with an increase in the quantity of methylene blue incorporated in the film, indicating that the redox reactions of methylene blue can take place effectively even in the polya~~ne films. The absorption spectra of PAn/MB-Nafion prepared from a deposition bath containing 10 mM methylene blue were measured in situ under polarization at several potentials in 1 M HCl solution. The results are shown in Fig. 6. The spectrum taken at 0 V (vs. SCE), where all the methylene blue must be reduced to leu~o-methylene blue, resembles that of a reduced poly~iline film f&6,7], and the film was yellow. When the electrode potential was shifted positively, the absorption bands of monomeric and dimeric methylene blue became very intense, and the colour of the film gradually changed to blue. Figure 7 shows plots of the potentials applied to the electrode against log(I’&P,,,), where PMB and PLMs denote the concentrations of methylene blue and leuco-methylene blue, respectively. FMe is the sum of I’, and 2I’,, and was estimated from the absorbance at 610 and 660 nm using eqns. (5) and (6). A,,, and A,,, used in these equations were obtained by subtracting the absorbance at 610 and 660 nm of PAn/Nafion from that of PAn/MB-Nafion which was taken at the same potential, while the concentration of leuco-methylene blue was obtained by subtracting P,, from I”,. A linear relationship is established between the two, and the slope is 30.1 mV, indicating that the Nernstian relation prevails in the methylene blue/leuco-methylene blue couple incorporated in the polyaniline film.
105
0
400
500
600
700
Wwelength / nm Fig. 6. Absorption spectra of a PAn/MB-Nafion film polarized at 0.5 (a), 0.4 (b), 0.3 (c), 0.25 (d), 0.2 (e), 0.15 (f), 0.1 (g) and 0 V (vs. SCE) (h) in 1.0 M HCI. The PAn/MB-Nafion film was prepared from a Nafion solution containing 5 mM aniline and 10 mM methylene blue.
It was reported in our previous paper [14] that methylene blue-incorporated Nafion modified electrodes (MB-Nafion) exhibit an electrochromic response due to the redox reaction of the incorporated methylene blue. However, the colour changes in that case were relatively slow; it took more than 3 min to complete the colour change. In contrast, the colour change occurred rather rapidly at PAn/MB-Nafion. Figure 8 shows the electrochromic responses of PAn/MB-Nafion of 0.7 pm thickness and MB-Nafion of 0.5 pm thickness, obtained at 660 nm. In the case of
0.35
> . ii i W
0.25-
0.20-
I -2
I
I
I
I
-1
0
1
2
IOg(rMB/r,,B Fig. 7. Relationship the film.
between
)
log( rMa/rLMa)
in the PAn/MB-Nafion
film and the potential
applied
to
106
0.5v
a
A s T aI 2 v E cz
I
I
I
0
1
2 Time/
t ov I
I
I
3
4
5
s
o.sv
5 i
: * ov
I
I
0
I
I 100
I
I
200
I
I
300
Time/ s Fig. 8. Response behaviour of the reflectance changes at 660 nm of PAn/MB-Nafion (a) and MB-Nafion (b), obtained by potential pulse application of 0.5 and 0 V (vs. SCE) in 1.0 M HCl. The film thickness of PAn/MB-Nafion was 0.7 pm and that of MB-Nafion was 0.5 pm in the dry state, as determined by observations with a scanning electron microscope.
PAn/MB-Nafion, the colouring and bleaching took place within 1 and 2 s, respectively, with the involvement of redox reactions of both polyaniline and the incorporated methylene blue of monomeric and dimeric forms. The rate of the electrochromic response was remarkably higher at PAn/MB-Nafion than at MBNafion. The observed difference in the electrochromic responses does not result from the difference in film thickness, because PAn/MB-Nafion was slightly thicker than MB-Nafion, but the response rate was higher at the former film. Rather the fast response of PAn/MB-Nafion must result from the fact that the polyaniline matrices provided conducting networks available for the redox reactions of methylene blue in Nafion. According to the result obtained at PAn/MB-Nafion, the rate of bleaching was lower than that of colouring. This must be due to a decrease in the conductivity of the polyaniline matrices by reduction of the film when bleaching occurred.
107 ACKNOWLEDGEMENT
This work was supported by a Grant-in-Aid for Scientific Research, No. 63470066, from the Ministry of Education, Science and Culture. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
A.F. Diaz and J.A. Logan, J. Electroanal. Chem., 111 (1980) 111. T. Kobayashi, H. Yoneyama and H. Tamura, J. Electroanal. Chem., 177 (1984) 281. A. Kitani, J. Izumi, J. Yano, Y. Hiromoto and K. Sasaki, Bull. Chem. Sot. Jpn., 57 (1984) 2254. E.M. Genies and C. Tsintavis, J. Electroanal. Chem., 195 (1985) 109. E.M. Genies and C. Tsintavis, J. Electroanal. Chem., 200 (1986) 127. T. Hirai, S. Kuwabata and H. Yoneyama, J. Chem. Sot., Faraday Trans. 1, 85 (1989) 969. T. Kobayashi, H. Yoneyama and H. Tamura, J. Electroanal. Chem., 161 (1984) 419. A. Kitani, J. Yano and K. Sasaki, J. Electroanal. Chem., 209 (1986) 227. M. Kaneko, H. Nakamura and T. Shimomura, Makromol. Chem., Rapid Commun., 8 (1987) 179. E.W. Paul, A.J. Ricco and M.S. Wrighton, J. Phys. Chem., 89 (1985) 1441. S. Chao and MS. Wrighton, J. Am. Chem. Sot., 109 (1987) 6627. R. Jiang and S. Dong, J. Electroanal. Chem., 246 (1988) 101. G. Bidan, E.M. Genies and M. Lapkowski, J. Chem. Sot., Chem. Commun., (1988) 533. S. Kuwabata, J. Nakamura and H. Yoneyama, J. Electroanal. Chem., 261 (1989) 363. C.N. Lewis, 0. Golds&mid and J. Bigeleisen, J. Am. Chem. Sot., 65 (1943) 1150. D.R. Lemin and T. Vickerstaff, Trans. Faraday Sot., 43 (1947) 491. W. Spencer and J.R. Sutter, J. Phys. Chem., 83 (1979) 1573. A.R. Peacocke and J.N.H. Sherrett, Trans. Faraday Sot., 52 (1956) 261. J.S. Moore, G.O. Phillips and D.M. Power, J. Chem. Sot. A, (1970) 1155. P.J. Bat& J.B. Lawton and G.O. Phillips, J. Phys. Chem., 76 (1972) 688. M. Shirai and M. Tanaka, J. Polym. Sci., Polym. Chem. Ed., 14 (1976) 343. M. Shirai, T. Nagatsuka and M. Tanaka, Makromol. Chem., 178 (1977) 37. M. Shirai, Y. Murakami and M. Tanaka, Makromol. Chem., 178 (1977) 2141. I.M. Klotz, F.M. Walker and R.B. Pivan, J. Am. Chem. Sot., 68 (1946) 1486. E.M. Genies and M. Lapkowski, J. Electroanal Chem., 236 (1987) 189. T. Hirai, S. Kuwabata and H. Yoneyama, J. Electrochem. Sot., 135 (1988) 1132. A.G. MacDiarmid, J.C. Chiang, A.F. Richter and A.J. Epstein, Synth. Met., 18 (1987) 285. W.S. Huang, B.D. Humphrey and A.G. MacDiarmid, J. Chem. Sot., Faraday Trans. 1, 82 (1986) 2385. R.J. Cushman, P.M. McManus and S.C. Yang, J. Electroanal. Chem., 291 (1986) 335. R.J. Cushman, P.M. McManus and S.C. Yang, Makromol. Chem., Rapid Commun., 8 (1987) 69. L. Michaelis and S. Granick, J. Am. Chem. Sot., 63 (1941) 1636. C.Y. Tzung, Anal. Chem., 67 (1967) 390.
J. Electroanal. Chem., 281 (1990) 97-107 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preparation of polyaniline films doped with methylene blue-bound Nafion and the electrochromic properties of the resulting films Susumu Kuwabata, Kazuki Mitsui and Hiroshi Yoneyama Department of Applied Chemistry, Osaka 565 (Japan)
Faculty
l
of Engineering, Osaka University, Yamada-oka 2-1, Suita,
(Received 3 August 1989; in revised form 3 November 1989)
ABSTRACT The preparation of polyaniline films doped with cationic methylene blue has been accomplished by the electro-oxidation of aniline in the presence of dissolved Nafion and methylene blue. The amount of methylene blue in the film could be varied by changing the relative concentration of methylene blue to Nafion in the deposition bath of polyanihne. The incorporation behaviour of methylene blue in polyaniline films is discussed in terms of the electrostatic binding of methylene blue with the sulphonate groups of Nafione in the deposition bath. The resulting films showed electrochemical responses of both polyaniline and methylene blue.
INTRODUCTION
Polyaniline (PAn) films prepared by electrochemical polymerization of aniline exhibit stable redox reactions [l-6], and several applications, for example to electrochromic [7-91 and electronic [lO,ll] devices, have been attempted using the redox properties. Recently, modification of the electrocatalytic activity of polyaniline by incorporating cobalt phthalocyanine tetrasulphonates [12] and heteropolyanions [13] has been reported. These functionalizations have been achieved successfully by utilizing the property that electrolyte anions are incorporated into the polyaniline films during the course of the polymerization of aniline to compensate the positive charges induced in the polymer. However, a procedure for the incorporation of cationic functional molecules into polyaniline films has not yet been devised.
l
To whom correspondence should be addressed.
0022-0728/90/$03.50
0 1990 Elsevier Sequoia S.A.
98
In this paper, the preparation of polyaniline films doped with cationic methylene blue is described. The strategy was to polymerize aniline in the presence of methylene blue and Nafion, with the latter being in excess. Under such conditions, a fraction of the sulphonate groups of Nafion are bound electrostatically with methylene blue, and the rest are free and can work as the charge compensator of positive charges induced in polyaniline. The electrochemical and electrochromic properties of the resulting films are also described. EXPERIMENTAL
Indium-tin-oxide coated glass (ITO) and Pt plates polished with alumina (1 pm) were used as the electrode substrates for the absorption spectra and electrochemical measurements, respectively. An electrical lead wire of the IT0 was attached to its round rim with silver-epoxy and fixed with epoxy resin. The Pt plate was mounted in a Teflon electrode holder having an exposed area of 1 cm2. Aldrich Nafion 117 solution was used without dilution as the supporting electrolyte solution for preparing the deposition baths. This solution was a mixture of a lower aliphatic alcohol and water (9 : 1) in which 5 wt% of the hydrogen form of Nafion (molar mass 1100 g) was dissolved. The concentration of sulphonic acid groups was 39.7 mM, and the pH of the solution was 1.7, giving ca. 50% dissociation of sulphonic acid groups. The deposition bath was prepared by dissolving 5 mM aniline and various amounts of methylene blue (MB) in the Nafion solution. The anodic polymerization to give polyaniline films doped with methylene blue-bound Nafion, which will be denoted in this paper as PAn/MB-Nafion, was carried out galvanostatically in a one-compartment cell with a Pt counter-electrode under N, atmosphere. The current density chosen was 40 PA cmp2, and the quantity of electricity used was 12 mC cm -2 for the electrochemical measurements and 36 mC cme2 for the other measurements. The preparation of the Nafion-coated Pt electrode containing methylene blue in the Nafion layer (MB-Nafion) has been described elsewhere [14] in detail. The amount of Nafion coated on the Pt electrode mol cmW2 of sulphonate substrate was 2.0 ~1 cmp2, which contained 7.9 x lo-’ groups, and the film thickness was 0.5 pm in the dry state. The incorporation of methylene blue into the Nafion layer in this case was performed by immersion in a 1 mM methylene blue solution for 30 min. Electrochemical measurements were carried out using a potentiostat (Hokuto Denko HA-301), a function generator (Hokuto Denko HB-101) and an X-Y recorder (Yokogawa Model 3077). A saturated calomel electrode (SCE) served as the reference electrode. Spectroelectrochemical studies were carried out using a quartz cell (20 X 10 X 40 mm) equipped with an IT0 electrode coated with PAn/MB-Nafion, a Pt counterelectrode and a SCE. The cell was set in a UV-visible spectrophotometer (Shimadzu MPS-5000), and absorption spectra were measured in situ under polarization. The electrochromic responses of PAn/MB-Nafion and MB-Nafion were investigated by measuring the changes in reflectance during the application of potential pulses of
99
0.5 and 0 V (vs. SCE). For this purpose, the films coated on irradiated with a 500 W xenon lamp (Ushio Electric UXL-SOOD-0), the intensity of the reflected light caused by the application of measured at 660 nm using a monochrometer (JASCO CT-25N) and RESULTS
Pt plates were and changes in potentials were a PbS detector.
AND DISCUSSION
Electrostatic
binding between methylene
blue and Nafion
The electrostatic binding of methylene blue with sulphonate groups of Nafion was studied by measuring the changes in the absorption spectra of methylene blue dissolved in n-propanol with addition of the Aldrich Nafion solution. The results obtained are shown in Fig. 1. The absorption spectrum of methylene blue taken in the absence of Nafion showed a sharp peak at 660 nm and a weak hump at 610 nm which are assigned to the absorption bands of monomeric and dimeric methylene blue, respectively [15-171. The intensity of these bands decreased significantly with an increase in the concentration of sulphonate groups of Nafion, while a new band assigned to trimeric methylene blue appeared and then increased at 550 nm, and simultaneously the colour of the solution turned from blue to purple. The phenomena observed are regarded as “metachromasia”, which has been reported for changes in the colour of several kinds of cationic dye solutions such as methylene blue, phenosafranine and pyronine G upon addition of anionic polyelectrolytes such as polystyrene sulphonate and polyvinyl sulphate [l&23]. The cationic dye molecules were concentrated in the polyelectrolytes due to electrostatic binding with anionic substituent groups of the polymers, resulting in the formation of dimeric or trimeric dyes.
0.8 3 5 Kl
0.6
B 9
0.4 0.2 n ”
400
so0
600
700
Wavelength / nm Fig. 1. Changes in the absorption spectra of 1.0 X 10V5 M methylene blue dissolved in n-propanol with addition of the Nafion solution. Concentration of sulphonate groups of Nafion/M: 0 (a); 5.0 X 10e6 (b); 8.0~10-~ (c); 1.0X10K5 (d); 2.0~10~’ (e); 5.0X10F5 (0.
100
If a binding equilibrium is established between the cationic dyes and anionic polyelectrolytes, the ratio (r) of the anionic substituent groups of the polyelectrolyte containing cationic dyes to all the anionic substituent groups is given by the equation [22-241 l/r = l/nKc,,
+ l/n
where cr,,, is the concentration of free cationic dyes dissolved in the solution, and K and n are the intrinsic binding constant and the maximum number of cationic dye molecules capable of binding to an anionic substituent group of the polyelectrolyte, respectively. r in eqn. (1) is given by
r= (cnl- Cfnl>/Cs where c, is the total concentration of free and bound methylene blue and c, is that of methylene blue-bound and -free sulphonate groups of Nafion dissolved in the solution. The concentration of free methylene blue (cr,,,) can be obtained from the apparent molecular absorption coefficient of monomeric methylene blue at 660 nm (eeeO) by using the equation [22-241 <660 Cfm
=
-
c6*6O
Gxl-
cm Ga
where eg*60is the absorption coefficient of monomeric methylene blue obtained when all the methylene blue molecules are bound to Nafion, and c&, is that in the absence of Nafion. The value of cl60 was estimated to be 89 800 M-’ cm-’ from the absorption spectrum shown in Fig. 1 (curve a). The value of ez60 was found to be 2570 M-i cm-’ from extrapolation of the c660 vs. c,/c, relation to c,/c, = 0 [22,23], as shown in Fig. 2. The cfm values of methylene blue solutions containing various concentrations of Nafion were then obtained from eqn. (3) using the ehCO
0
0.2
0.4
0.6
0.8
1.0
cm its
Fig. 2. Relationship Nafion.
between
ee6,, and c,/c,
of n-propanol
containing
1.0~ 10m5 M methylene
blue and
101
3
‘i L 2
0
0
5 ro-5c‘,,/,-l
lo
Fig. 3. Relationship between l/r eqns. (2) and (3).
and l/c,
l5
obtained from c&0 of the spectra shown in Fig. 1 using
values deduced from each spectrum shown in Fig. 1, and the values of r were determined from eqn. (2) using the cy, values determined. Figure 3 shows that eqn. (1) holds between l/r and l/c,, indicating the establishment of a binding equilibrium between methylene blue and the sulphonate groups of Nafion. The values of n and K were then estimated to be 1.02 and 5.57 x 105, respectively. It is recognized from the n value determined that methylene blue molecules are bound to sulphonate groups of Nafion in the ratio of 1 : 1. The binding constant is then formulated as K = c,,/cfscfm ( = 5.57 x lo5 M-r) (4) where c,, is the concentration of methylene blue-bound sulphonate groups of Nafion in the solution, and cyS is that of methylene blue-free sulphonate groups. Preparation
of polyaniline
films doped with methylene
blue-bound Nafion
Polyaniline films are prepared by the electro-oxidation of aniline in highly acidic solutions [1,2,25]. It was found that the original Aldrich Nafion solution had a sufficiently high acidity to deposit smooth polyaniline films on electrode substrates such as Pt, glassy carbon and IT0 electrodes, as judged from the electrolysis results of 5 mM aniline in the Nafion solution. In this case, the sulphonate groups of Nafion should serve as the charge compensator of the positive charges of polyaniline [26]. Polyaniline films were prepared on IT0 electrodes from Aldrich Nafion solutions containing 5 mM aniline and 0, 1, 3 and 10 mM methylene blue, and the absorption spectra of the resulting films were obtained as shown in Fig. 4. The spectrum of the polyaniline films prepared in the absence of methylene blue (Fig. 4a) was almost the same as that of polyaniline films prepared in a conventional electrolyte solution
102 I
_I 500
400
600
Wavelength
700
/ nrn
Fig. 4. Absorption spectra of PAn/MB-Nafion films prepared from Nafion aniline and 0 (a), 1.0 (b), 3.0 (c) and 10.0 mM (d) methylene blue.
solutions
containing
5 mM
such as chloride and perchlorate solutions [1,6,7], whereas the polyaniline films prepared in the presence of methylene blue exhibited definite absorptions at 660 and 610 nm, which are assigned to monomeric and dimeric methylene blue, respectively. The intensity of these bands increased with increasing concentration of methylene blue in the deposition bath. As already shown above, methylene blue was bound to the sulphonate groups of Nafion, but a fraction of the sulphonate groups of Nafion remained free from binding with methylene blue, because the concentration of sulphonate groups was 39.7 mM, i.e. greater than the concentration of methylene blue dissolved. It is then thought that the observed incorporation of methylene blue in the polyaniline films occurs with the methylene blue-free sulphonates of Nafion acting as a charge compensator. The concentrations of monomeric methylene blue (I,,,) and dimeric methylene blue (I,,) in the polyaniline films, and their sum (I, = I,,, + 2I’,), were determined by using the equations r
= m
cd 660A 610 Go&o
4lOA660 x 1o-3
- Goe:10
(5)
and r
=
“A '610
“A 660-c660
610
d
x
1o-3
(6)
E?10c:60 - Egm60E:10
which were derived in our previous paper [14] to estimate the methylene blue concentration incorporated in Nafion-modified electrodes. In eqns. (5) and (6) ego and E&, are the molar extinction coefficients at 610 and 660 nm of monomeric methylene blue dissolved in aqueous solution, respectively; zgdioand CL, are those of dimeric methylene blue; and A,,, and A,, are the absorbances at 610 and 660 nm obtained after correction of the absorbance due to polyaniline. The values of rm, r, and I, are listed in Table 1 together with c,,,/c, for three different concentrations of methylene blue.
103 TABLE 1 Concentration of methylene blue and the r values a of the PAn/MB-Nafion c, /mM
lo9 r,/mol
1.0 3.0 10.0
1.3 4.3 15
cm-2
lo9 &/mol
cm-*
1.5 0.73 4.5
lo9 r,/mol
films
cm-’
1.6 5.8 24
C,/CSb
r
0.025 0.072 0.24
0.021 0.076 0.252
a Relative ratio of the sulphonate groups bound with MB to all sulphonate groups of Nafion incorporated in the PAn/MB-Nafion films, estimated using eqn. (8). b c, = 39.7 mM.
Judging from the electrode potential (ca. 0.5 V vs. SCE) monitored in the course of the electropolymerization of aniline, the polyaniline films prepared must be in the emeraldine salt form in which the molar ratio of doped electrolyte anions to aniline units is ca. 0.5 [27-301. Since the polymerization reaction in this case is given by
l0.5n A-
the concentration rA
of the doped anions in the polymer film (I,.-)
is given by
_AsE!LOZP
(7) F 2.5n . F where F and Q are the Faraday constant and the quantity of electricity consumed in the polymerization, respectively. In the present study, the polyaniline films were prepared using Q = 36 mC cm-2, so that IA- of the sulphonate groups of Nafion must be 7.5 x lop8 mol cm- *. The total concentration of sulphonate groups of Nafion incorporated in the prepared polyaniline films must be the sum of IA- and I,. Then the ratio of the methylene blue-bound sulphonate groups of Nafion to the total amount of sulphonate groups in the polyaniline films is given by r=
q/(r,-+ r,)
(8)
The values of r estimated by this equation are given in Table 1. It is noteworthy from the table that the r values obtained are very close to the relative ratio of the concentration of methylene blue to that of sulphonate groups of Nafion (cm/c,) in the deposition baths of polyaniline, indicating that almost all the methylene blue molecules in the deposition bath are bound to the sulphonate groups of Nafion. This is quite natural judging from the very large binding constant (5.57 X lo5 M-‘) obtained in the relatively dilute concentrations of methylene blue and sulphonate groups of Nafion. Electrochemical and electrochromic methylene blue-bound Nafion
properties
of the polyaniline
films
doped
with
Cyclic voltammograms of polyaniline films doped with Nafion (PAn/Nafion) and doped with MB-bound Nafion (PAn/MB-Nafion) were taken in 1 M HCl
3 E vs. SCE /V Fig. 5. Cyclic voftammograms of PAn/Nafion (a) and PAn/MB-N~ion (b-d) taken in 1.0 M HCI solution. dE/dt = 100 mV s-‘, PAn/MB-Nafion films were prepared from Nafion solutions containing 5 mM aniline and 1.0 (b), 3.0 (c) and 10.0 mM (d) methylene blue.
solution. As Fig. 5 shows, a couple of redox waves due to polyaniline alone are observed at PAn/Nafion, whereas at ~An/MB-Nafion another couple of anodic and cathodic waves due to the redox reactions of methylene blue appeared at 0.27 and 0.23 V (vs. SCE), respectively. The peak currents due to this redox reaction increased with an increase in the quantity of methylene blue incorporated in the film, indicating that the redox reactions of methylene blue can take place effectively even in the polya~~ne films. The absorption spectra of PAn/MB-Nafion prepared from a deposition bath containing 10 mM methylene blue were measured in situ under polarization at several potentials in 1 M HCl solution. The results are shown in Fig. 6. The spectrum taken at 0 V (vs. SCE), where all the methylene blue must be reduced to leu~o-methylene blue, resembles that of a reduced poly~iline film f&6,7], and the film was yellow. When the electrode potential was shifted positively, the absorption bands of monomeric and dimeric methylene blue became very intense, and the colour of the film gradually changed to blue. Figure 7 shows plots of the potentials applied to the electrode against log(I’&P,,,), where PMB and PLMs denote the concentrations of methylene blue and leuco-methylene blue, respectively. FMe is the sum of I’, and 2I’,, and was estimated from the absorbance at 610 and 660 nm using eqns. (5) and (6). A,,, and A,,, used in these equations were obtained by subtracting the absorbance at 610 and 660 nm of PAn/Nafion from that of PAn/MB-Nafion which was taken at the same potential, while the concentration of leuco-methylene blue was obtained by subtracting P,, from I”,. A linear relationship is established between the two, and the slope is 30.1 mV, indicating that the Nernstian relation prevails in the methylene blue/leuco-methylene blue couple incorporated in the polyaniline film.
105
0
400
500
600
700
Wwelength / nm Fig. 6. Absorption spectra of a PAn/MB-Nafion film polarized at 0.5 (a), 0.4 (b), 0.3 (c), 0.25 (d), 0.2 (e), 0.15 (f), 0.1 (g) and 0 V (vs. SCE) (h) in 1.0 M HCI. The PAn/MB-Nafion film was prepared from a Nafion solution containing 5 mM aniline and 10 mM methylene blue.
It was reported in our previous paper [14] that methylene blue-incorporated Nafion modified electrodes (MB-Nafion) exhibit an electrochromic response due to the redox reaction of the incorporated methylene blue. However, the colour changes in that case were relatively slow; it took more than 3 min to complete the colour change. In contrast, the colour change occurred rather rapidly at PAn/MB-Nafion. Figure 8 shows the electrochromic responses of PAn/MB-Nafion of 0.7 pm thickness and MB-Nafion of 0.5 pm thickness, obtained at 660 nm. In the case of
0.35
> . ii i W
0.25-
0.20-
I -2
I
I
I
I
-1
0
1
2
IOg(rMB/r,,B Fig. 7. Relationship the film.
between
)
log( rMa/rLMa)
in the PAn/MB-Nafion
film and the potential
applied
to
106
0.5v
a
A s T aI 2 v E cz
I
I
I
0
1
2 Time/
t ov I
I
I
3
4
5
s
o.sv
5 i
: * ov
I
I
0
I
I 100
I
I
200
I
I
300
Time/ s Fig. 8. Response behaviour of the reflectance changes at 660 nm of PAn/MB-Nafion (a) and MB-Nafion (b), obtained by potential pulse application of 0.5 and 0 V (vs. SCE) in 1.0 M HCl. The film thickness of PAn/MB-Nafion was 0.7 pm and that of MB-Nafion was 0.5 pm in the dry state, as determined by observations with a scanning electron microscope.
PAn/MB-Nafion, the colouring and bleaching took place within 1 and 2 s, respectively, with the involvement of redox reactions of both polyaniline and the incorporated methylene blue of monomeric and dimeric forms. The rate of the electrochromic response was remarkably higher at PAn/MB-Nafion than at MBNafion. The observed difference in the electrochromic responses does not result from the difference in film thickness, because PAn/MB-Nafion was slightly thicker than MB-Nafion, but the response rate was higher at the former film. Rather the fast response of PAn/MB-Nafion must result from the fact that the polyaniline matrices provided conducting networks available for the redox reactions of methylene blue in Nafion. According to the result obtained at PAn/MB-Nafion, the rate of bleaching was lower than that of colouring. This must be due to a decrease in the conductivity of the polyaniline matrices by reduction of the film when bleaching occurred.
107 ACKNOWLEDGEMENT
This work was supported by a Grant-in-Aid for Scientific Research, No. 63470066, from the Ministry of Education, Science and Culture. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
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