Scan rate =lOO mV/s. Numbers in circles refer to the number of cycles. Figure 3 shows the FT-IR spectrum of the polymer obtained by anodic oxidation of PAAB ...
161
J. Elecrroanal. Chem., 315 (1991) 161-174 Elsevier Sequoia S.A., Lausanne
JEC 01680
Oxidative polymerization acetonitrile
of p-aminoazobenzene
in
A new electroactive polymer Hamid A. Abd El-Rahman Tokuda **
l, Takeo
Oh&a,
Department of Electronic Chemistry, Graduate Nagatsuta, Midori-ku, Yokohama 227 (Japan) (Received
25 March
Fusao Kitamura and Koichi
School at Nagatsuta,
Tokyo
Institute
of Technology,
1991; in revised form 28 May 1991)
Abstract The oxidative polymerisation of p-aminoazobenzene (PAAB) on glassy carbon in acetonitrile was studied by cyclic voltammetry, the potential step method, and SEM and FT-IR spectroscopy. The presence of a base-like pyridine was found to be essential for the polymerization process. The prepared polymer (poly( p-aminoazobenzene), abbreviated as poly-PAAB) films of thicknesses up to several pm were found to be very stable and insoluble in aqueous solutions and in common organic solvents, except dimethyl formamide and tetrahydrofuran. The films are electroactive in acidic solutions but electroinactive in acetonitrile. The electroactivity of these films is attributed to a proton+ electron addition/ elimination reaction at -NHsites. The capacitive current contribution in the potential region where the films are electroactive is larger than is usually observed for the conventional polyaniline films. The concentration of the electroactive sites in poly-PAAB was found to be about twenty times lower than that of polyaniline while the diffusion coefficient for the charge transport process within the films was found to be four times lower. The azo groups presented in the films were found to be reduced irreversibly in acetonitrile and aqueous acidic solutions. The prolonged reduction of the azo groups was found to lead to the loss of the electroactivity and the slow degradation of the films.
INTRODUCTION
Among the modified electrodes subject to intensive studies recently, those resulting from oxidative polymerization of aromatic amines on a suitable substrate, e.g., * l
On leave from the Department of Chemistry, Faculty * To whom correspondence should be addressed.
0022-0728/91/$03.50
0 1991 - Elsevier Sequoia
of Science,
Cairo University,
S.A. All rights reserved
Gisa, Egypt.
162
platinum, glassy carbon, basal-plane pyrolytic graphite occupy a distinctive position [l-12]. Electroactive polymeric films of these compounds can be formed readily either in aqueous acid solutions or in non-aqueous media containing a base. Due to such electroactivity, many studies on the use of these films in rechargeable batteries, electrochromic display devices, ion-selective electrodes and pH modulators have been made [13-181. Of the aromatic amines, the simplest, aniline, seems to be the most intensively investigated compound [1,19-271. Polymeric films of the aniline derivatives containing a redox group which works at potentials substantially far from that of the electroactive center, i.e., -NHsite, are of interest. The ability of the azo group to participate in metal chelation and the sharp color change upon its electrochemical reduction to the hydrazo form [28] make aniline derivatives containing such a group good candidates for electropolymerization. In the same sense the redox behavior of polypyrroles containing naphthoquinone and benzoquinone groups has recently been studied [29]. In the present paper, the oxidative polymerization of p-aminoazobenzene in acetonitrile containing pyridine, the structure, the morphology, and the electroactivity of the new polymeric film are reported. @=++2 p-aminoazobenzene EXPERIMENTAL
Glassy carbon disc electrodes (GC-20, Tokai Carbon) having a geometric surface area of 0.07 cm* were used for the electropolymerization of p-aminoazobenzene (PAAB, extra pure, Tokyo Kasei) in acetonitrile (AN, reagent grade and doubly distilled, Kanto Chemical) containing pyridine (analytical grade, Kanto Chemical) and NaClO, (analytical grade, Kanto Chemical ) as a supporting electrolyte. The polymerization solutions were composed of 0.05 M PAAB + 0.05 M pyridine + 0.1 M NaClO, in AN. Conventional three-electrode, one compartment electrochemical cells and auxiliary platinum wire electrodes were used. Potentials in AN were measured versus a Ag/AgClO,/O.l M NaClO, reference electrode while potentials in aqueous solutions were measured versus a saturated calomel electrode (WE). Aqueous acidic chloride solutions were prepared from analytical grade HCl and NaCl (Kant0 Chemical) and doubly distilled water. For cyclic voltammetry and constant-potential electrolysis a polarization unit (TTR PS-07, Toho Technical Research), a potentiostat (HECS 311B, Huso Electrochemical System) and a D-72C X-Y recorder (Riken Denshi) were employed. Potential-step chronoamperometry (PSCA) and chronocoulometry (PSCC) were conducted by using a computer-controlled electrolysis system (CYSY-1, Cypress System Inc.). Measurements of the film thickness using a surface texture measuring instrument (Surfcom 550A, Tokyo Seimitsu) and scanning electron microscopy (SEM, JEOL JSMTlOO) were made on polymeric films deposited on indium-tin
163
oxide coated glass electrodes (ITO, Matsuzaki Shinku Himaku, surface resistance is ca. 10 Q/cm*). For Fourier transform IR absorption spectroscopy (FT-IR, ETSA20B/D, Digilab Inc.) the films deposited on different substrates (GC, IT0 and basal-plane pyrolytic graphite) were collected separately, washed thoroughly with AN and dried in air for several days before recording their FT-IR spectra in KBr pellets. All electrochemical experiments were run at a temperature of 20°C under N2.
RESULTS
AND
DISCUSSION
Polymeric film formation
Poly-PAAB films were formed on GC either by constant-potential electrolysis or by potential-sweep electrolysis in AN containing 0.05 M PAAB + 0.05 M pyridine + 0.1 M NaClO,. The potential was kept between 0.9 and 1.2 V in the constant potential electrolysis and was swept in range from -0.05 to 1.0 or 1.2 V. No significant differences were observed in the characteristics of the polymerized films. Typical cyclic voltammograms (CVs) for oxidative polymerization of PAAB in AN are shown in Fig. 1. The anodic current starts to increase at - 480 mV, with the formation of soluble products falling from the electrode surface, until the peak at - 860 mV is reached, then it decreases. The large anodic peak at 860 mV corresponds to the oxidation of the -NH, group [2,21]. During the next cycles the peak current diminishes substantially, the formation of soluble products stops and a polymeric film is formed. In the constant-potential electrolysis, the formation of soluble products occurs also during the early stages of the process. In the absence of pyridine, however, the formation of soluble products predominates and no polymeric films could be detected. Stability, morphology and IR spectra of poly-PAAB
The poly-PAAB films formed were found to be very stable and highly insoluble in aqueous solutions. They are also insoluble in common organic solvents, e.g., ethanol, methanol, acetone, diethyl ether, ethyl acetate, dioxane, benzene, hexane, dimethyl sulfoxide, AN, pyridine, but they are soluble in dimethyl formamide and terahydrofuran. The color of these films, after formation and drying, is yellow (thin films) to yellowish brown (thick films) but it changes reversibly with the solution pH in a manner similar to the monomer, PAAB, i.e. it is red in acidic and yellow in alkaline solutions. The vertical sectional profiles of the films (monitored by SEM) show that they are continuous and have a reasonably smooth surface and the thickness is fairly uniform over the whole surface. The scanning electron micrograph of poly-PAAB films deposited on IT0 electrodes (Fig. 2) shows that the films are porous and of the fibrous type.
164
20
15
10 : E 0 2 \ s5
0
-5
I
0.0
I
I
0.2
I
I
1
0.4
E / V vs. Rg IQgClO,
I
0.6
I
I
0.6
I
I
1.0
IO. 1 M ClO;l
Fig. 1. Cyclic voltammograms for oxidative polymerization of PAAB on a glassy carbon acetonitrile (0.05 M PAAB+0.05 M pyridine+O.l M NaClO,). Scan rate =lOO mV/s. circles refer to the number of cycles.
electrode Numbers
in in
Figure 3 shows the FT-IR spectrum of the polymer obtained by anodic oxidation of PAAB on IT0 electrodes in comparison with that of the monomer dye. The spectra of the polymers obtained by using the other substrates were identical to that of ITO. The IR spectral assignments of the vibration modes of the polymer and the monomer are given in Table 1. The peaks at 3050, 1600, 1508, 940-920, 860-840, 770 and 695 cm-’ are characteristic of the various vibration modes of the C-H and C-C bonds of the aromatic nuclei [2,19,30,31]. The absorption at 3500-3250 cm-’ corresponds to the stretching of the N-H bond. The IR spectrum of the polymer exhibits a single broad peak due to such a vibration mode in agreement with previous data for polyaniline [2,19] while the monomer shows three peaks. The peaks at 1310-1235 cm-’ are attributed to the stretching of the C-N bonds of the secondary amines while that at 1625 cm-’ corresponds to the stretching of C=N bonds [30,31]. Thus the polymerization seems to proceed via formation of C-N=C and C-NH-C bonds as reported previously for polyaniline [19]. The azo group in both the polymer and the monomer, manifests itself by the peak(s) at 1435-1455
165
10 pm Fig. 2. Scanning electron mV, on an IT0 electrode
micrograph of a poly-PAAB film deposited at a constant potential; E = 1200 (The amount of charge passed = 2.0 C cm-‘). Film thickness = 7.0 pm.
(a)
4000
3000
2000
1000
1500 Wave number/
Fig. 3. Fourier transform IR spectra of poly-PUB on IT0 electrodes by anodic oxidation at constant
400
cd (a) and PAAB (b). The polymer potential; E = 1200 mV.
films were deposited
166 TABLE 1 Assignments acetonitrile.
of the IR data of poly-PAAB
Vibration mode a
by oxidative
polymerization
of PAAB
in
Wave number/cm-’
(N-W, (C-H), (C-C), CC=% (C-N), (N=N), ClO,-
(C-H),,
(C-C),, a The subscripts respectively.
films formed
s, bl
Poly-PAAB
PAAB
3440-3350 3050 1600 1508 1625 1300 1235 1455-1440 1130 1090 630 940-920 860-840 170 695
3500 3050 1600 1508 1625 1310 1240 1450-1435
and b2 refer to the stretching
940-920 840 110 695 and out-of-plane
bending
vibration
modes,
cm-’ which corresponds to the stretching vibration mode of such a group [19,30,31]. Finally the peaks at 1130, 1090 and 630, cm-‘, which are present only for the polymer, are attributed to the presence of Cloy ions [2,19,30,31]. Electroactivity
of poly-PAAB
films
Figure 4 shows the electroactive response of poly-PAAB films in 1.0 M HCl. The reversible redox peaks at 500-550 mV are attributed to a proton + electron addition/elimination reaction at -NHsites in the polymeric films. For an aniline derivative polymer, such redox peaks are expected at potentials close to the redox peaks of polyaniline and polyaniline derivatives [19-271. The redox potential of this reaction is located - 300 mV more positive than that reported for polyaniline films [13,20]. The films do not suffer any degradation or loss in the electroactivity by repetition of potential sweeps in the potential region between - 200 and 800 mV vs. SCE and the electroactivity is almost unaffected by drying the films even for several weeks or dipping them in aqueous or non-aqueous media. In AN, however, the films are not electroactive. The effect of potential scan rate, v, on the CVs of poly-PAAB deposited on GC electrodes in acidic chloride solutions of differing pH, [Cl-] = 1.0 M, was examined in the range of v from 5 mV/s to 1.0 V/s. A typical result is shown in Fig. 5. The corrected peak current, i,, was obtained by subtracting the capacitive and other background currents from the apparent peak current. i, varies linearly with v up to
167
I
1
I
1
0.4
E / V
I
1
vi.”
1
0.8
I
I
1.0
SCE
Fig. 4, Cyclic voltammograms of glassy carbon electrodes, covered by poly-PAAB films (a) and bare (b) in 1.0 M HCI. Scan rate = 100 mV/s. Films were formed at a constant potential of 1200 mV for 10 min in acetonitrile containing 0.05 M PAAR + 0.05 M pyridine + 0.1 M NaGlO,.
u = 200 mV/s (see Fig. 6) indicating that the electrode reactions of the films are phenomenologically equivalent to that of a surface-attached redox species [3,20,32]. However, at u > 200 mV/s linear i,-0 i” behavior (i.e. linear diffusion) prevails. As can be seen in Fig. 7, the peak potential shifts to less positive values and the peak current decreases as the solution pH increases consistent with the requirements of a proton + electron ad~tion/e~~ation reaction at -NH- sites in acidic solutions. The electroactive responses of the films become less pronounced as the pH increases and they are hard to analyze at pH > 3. The eleetroactivity of the films was found to respond reversibly to the change in the solution pH. A formal redox potential, E o at pH = 0 was estimated as E 0 = ((E,,a f E&/2), where E,, and E,, are the anodie and cathodic peak potentials at low scan rates, respectively. It was found that E o = 520 mV vs. SCE at pH = 0 and dE O/dpH = - 58 mV/pH. The latter value supports Nernstian behavior, i.e., a 1: 1 proton : electron reversible process. The capacitive current contribution, i,, where the films are electroactive, i.e., in the potential region of the redox peaks, is comparatively larger than that usually observed for polyaniline films (see Figs. 4, 5 and 7). To make the comparison, the value @ = j~/(~~)~~~~~~~,which expresses the relative contribution of the capacitive
168
80
r
a
40 ': E 0 a 0 y \ --J -40
0.4
0.2
0.6
0.8
E / ‘J vs. SCE Fig. 5. Cyclic voltammograms of glassy carbon electrodes covered by poly-PAAB in 1.0 M HCl at different scan rates; (a) 100, (b) 70, (c) 50 and (d) 30 mV/s. Films were formed at a constant potential; E = !XKl mV for 10 min in acetonitrile containing 0.05 M PAAB+0.05 M pyridine+O.l M NaCIO.,.
a 3 \ %
Fig. 6. Dependence of anodic Details are shown in Fig. 5.
and cathodic
peak currents
(i,,,
and
i,.,)
on potential
scan rate (u).
169
a =i \ -
0
-40 I
I
I
0.2 E
/
‘J
I
v”,‘:
I
0.6
SCE
Fig. 7. Cyclic voltammograms of glassy carbon electrodes covered by poly-PAAB films in acidic chloride solutions of different pHs; (a) 0.00, (b) 0.30, (c) 1.30 and (d) 2.42. Ionic strength = 1.0 M. Scan rate = 100 mV/s. Films were formed by 10 cycles from - 50 mV to 900 mV at 20 mV SK’ in acetonitrile containing 0.05 M PAAB +O.O5M pyridine+O.l M NaCIO.,.
current to the apparent peak current, was estimated. In 1.0 M HCl 0 value for poly-PAAB equals 0.77 f 0.01 while for polyaniline; 0 = 0.22 f 0.03 as estimated from published CV data [13,20]. It is generally assumed that the capacitive currents are created by an over-doping of the polymer which gives rise to the existence of two types of counter-anion attachment sites along the polymer backbone [33,34]. Strongly bound anions are connected to the faradaic processes, while superficially fixed anions are responsible for the capacitive effects. According to Kobayashi et al. [20], the capacitive current is associated with a slow insertion/elimination process of the anionic species. Furthermore, it has recently been concluded that polyaniline has two types of sites, protic (acidic) sites responsible for the faradaic processes and aprotic (basic) sites responsible for the capacitive processes [26]. For poly-PAAB, the protonation of the azo group in acidic solutions is accompanied by insertion of superficially fixed anionic species which may contribute to the observed large capacitive current. Typical potential-step choronoamperometric responses for the oxidation and reduction of poly-PAAB films in 1.0 M HCl are shown in Fig. 8. From these responses, charge, Q, vs. square root of time curves were obtained automatically with the aid of the computer. The initial part of the Q-t’/2 curves (t < 1 ms) is connected with the charging of the film and the double layer. The diffusion
170
0
2
4
(time)‘12/
6
n-15.“~ .60
0
20
40
60
80
time I ms
Fig. 8. Chronoamperometric responses for glassy carbon electrodes covered by poly-PAAB films in 1.0 M HCI. The potential was stepped from 150 mV to 700 mV (a) and from 700 mV to 150 mV (c). Films were formed as in Fig. 5. Inset: charge vs. square root of time curves.
coefficient, D, for the charge transport process within the films was estimated from the slopes of the linear part (t = 5-50 ms) of these curves according to the equation [35]: Q = Qdl + 2nFc” ( Dt/n)“2
(1)
where Qd, is the double layer charge, n is number of electrons involved in the process, F is the Faraday constant, and co is the concentration of the electroactive sites. co was calculated as c ’ = T/d; where r is the surface excess and d is the film thickness. In the present study, the thickness was estimated for the dry films. Naturally, the thickness of the dry film is expected to be slightly lower than the solvated “swollen” film in the test electrolyte. r was obtained from the peak charge of CV of the films in acidic chloride solutions of pH = 0 and [Cl-] = 1.0 M at low scan rates. The peak charge corresponding to the faradaic processes only was estimated after determining plausible base-lines for the anodic and cathodic peaks and it was found that the peak charge of the oxidation process is equal to that of the reduction process. For the films formed at a constant potential of 900 mV for 10 min (Figs. 5 and 8), r = 5.2 nmol cmP2 and d = 1.5 pm, i.e., co = 3.4 X lop5 mol cm-3. The
171
4s
n
3-
a
a
2“: ;
l-
\ ‘s
o-
-1 -
-2 ’ -1.6
I -1.2
I
I -0.8
I
I -0.4
I
I 0
E / V vs. Ag IRgClO, (0.1 MClO;, Fig. 9. Cyclic voltammogmms of glassy carbon electrodes covered by poly-PUB films (curves b) in acetonitrile containing 0.1 M NaClO,. Curves a are CVs for bare glassy carbon electrodes in acetonitrile containing 0.05 M PAAB +0.05 M pyridine+O.l M NaClO,. Numbers in circles refer to the number of cycles. Films were formed by 30 cycles from 0 mV to 900 mV at 100 mV/s in acetonitrile containing 0.05 M PAAB + 0.05 M pyridine + 0.1 M NaCIO.,.
concentration of the electroactive sites in poly-PAAB films is lower than those of polyaniline: co = 7.3 x 10e4 mol cm-3 [36], poly(N-ethylaniline): co = 3.3 x 10e4 mol cme3, poly(o-phenylenediamine): co = 7.8 X low4 cmp3, poly(N-methylaniline): c o = 10.1 X lop4 mol cme3 [7] and poly(l-pyrenamine): co = 9.7 x lop4 mol cm- 3 [37]. The average D value for many poly-PAAB film samples (anodic and cathodic processes) was found to be 1.08 X lo- * cm2 s- ‘. For the polymeric films of the related aromatic amines, D values in the range of 1.5 X 10-i’ - 4.2 x lo-’ cm2 [7]. S -’ were reported The redox response of the ato group Figure 9 shows the redox behavior of the azo groups included in the poly-PAAB films in AN in comparison with the redox behavior of that group in the monomer. The reduction peak at - 1140 mV (monomer) and - 1250 mV (polymer) seems to correspond to the formation of the dianion in one two-electron step [28,38]. In the polymer case, the peak at - 430 mV, corresponding to the oxidation of the dianion, is absent, i.e. the reduction of the azo group in the polymeric film proceeds irreversibly. On the next cycles, the peak potentials in the monomer case shift
172
-0.3’ -0.6
I
I -0.6
t
I -0.4
E/Vvs.
I
I -0.2
I
I 0.0
SCE
Fig. 10. Cyclic voltammograms of glassy carbon electrodes covered by poly-PAAB (curves (b)) in 1.0 M HCl. Curves (a) are CVs for bare glassy carbon electrodes in 1.0 M HCl containing 0.01 M PAAB. Numbers in circles refer to the number of cycles. Films were formed as described in Fig. 9.
considerably to higher potentials due to the deposition of the reduced species on the electrode surface, but they are unaffected in the polymer case. These results show that while the parent molecules can be generated by oxidation of the dianions [38], the reduced polymer seems to be involved in a subsequent chemical reaction which leads to the degradation of the polymer as will be seen later. In acidic chloride solutions the azo groups in both the polymer and the monomer are reduced irreversibly (see Fig. 10) in agreement with the known four-electron irreversible reaction with cleavage of the nitrogen-nitrogen bond for p-hydroxyand paminoazobenzenes [39-411. Prolonged reduction of the azo group in AN or acidic chloride solutions was found to lead to the gradual loss of the electroactivity observed at 520-550 mV and the slow degradation of the films. This indicates that the azo groups, or at least some of these groups, are included in the main polymeric chains not as side chains. According to the results obtained, the structure of the polymer is assumed to be as Structure I (Fig. 11). In analogy with polyaniline derivatives, the polymerization may also be propagated through the ortho-positions [21], Structure II (Fig. 12).
Fig. 11. Structure 1. Result of propagation of the polymerization by coupling in the para-position in the phenylazo group.
173
Fig. 12. Structure
II. Result of propagation
of the polymerization
by coupling
in the ortho-position.
Other possibilities like coupling in the meta-positions [19] or cross-linking may be also involved. ACKNOWLEDGEMENTS
The present work was supported by a Grant-in-aid for Scientific Research from the Ministry of Education, Science and Culture, Japan. One of the authors (H.A.A) is a participant in UNESCO I.P.U. Course in Chem. and Chem. Eng. at Tokyo Institute of Technology. The authors wish to thank Prof. N. Oyama at Tokyo University of Agriculture and Technology for his valuable advice and Mr. H. Miyamoto for technical assistance in the FT-IR spectroscopic and SEM measurements. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
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