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On the nature of redox processes in the cyclic voltammetry of polypyrrole nitrate in aqueous solutions
267
Joumal of Electroanatytical Chemistry, 362 (1993) 267-272
JECO2W
On the nature of redox processes in the cyclic voltarnmetry of polypyrrole nitrate in aqueous solutions Yongfang Li and Renyuan Qian Institute of Chemistry, Academia Sinica, Beijing loo080 fChid (Received 24 July 1992; in revised form 23 April 19931
Abstract The counter-anion doping states and redox processes of polypyrrole nitrate (PMNO;)) were studied by elemental analysis, cyclic voltammetry and spectroelectroche.mical measurements. Two counter-agion doping sites, the positively charged conjugated chain of PPy (site 1) and the protonated pyrrole (py) unit’(site 21, were dbtinguished. It was found that the counter-anions in site 2 are removed with the &pro@nattionof the py unit in a aentral or basic aqueous solution, while those in site 1 are exchanged with OHin a basic solution. Two redox processes of PPyfNO,) in aqueous so]utions, corresponding to the two doping sites, were observed. The meclmnisms of the redox processe s in acidic, neutral and basic solutions are discussed.
1. Introduction Conducting polypyrrole (PPy) has aroused great interest over the past decade owing to its ease of preparation in both organic [1,2] and aqueous [3-51 solutions, its good stability in its oxidized conducting state [6] and its unusual electrochemical properties. In cyclic. voltammetric studies of the reversible redox processes of PPy a “capacitive” current is generally observed, which has been shown to be a true faradaic current [7,8]. When de-insertion of the counter-anions in PPy is .difficult, insertion of cations from the electrolyte solution also contributes to the cathodic current [g-111. In neutral and basic aqueous solutions two reduction processes were observed [121, presumably related to two doping sites of the PPy chain as was first suggested by Tanguy and coworkers [13,14]. However, the nature of the two doping sites remains ambiguous. The present study of polypyrrole nitrate (Ppy(N0;)) attempts to clarify the relationship between the two doping sites
and the redox processes involved on the basis of the results of elemental analysis, absorption spectrometry and cyclic voltammetry.
glass plate (IT01 electrode under a constant potential of 0.7 V/SCE in an aqueous solution of 0.1 M pyrrole and 0.2 M NaNO, at pH 3-4. The acidic solution pH was adjusted by adding 0.1 M HNO, solution. The electrode area was 20 mm* and the total charges consumed during polymerization were 50 mC for the Pt electrode and 10 mC for the IT0 electrode which was used for optical spectrum measurement. Cyclic voltammetry was carried out using an EG&G PAR model 174A polarographic analyser and a PAR model 175 universal programmer with a Pt plate counter-electrode and a saturated calomel reference electrode (SCE) at a scan rate of 20 mV/s. In-situ visible-near IR WIS-NIR) absorption spectra were recorded with a Nanometrics Nanospec/lO VIS-NIR microspectrophotometer. The sodium content in PPy was analyzed using atomic absorption spectrometry. All electrode potential values are reported relative to SCE. All experiments were performed at room temperature. 3. Results and discussion
3.1. Two hping sites in PPy(N0;) 2. Experimental
In order to examine the stab%@ of the counter-anions in Pqr(NO,) dipped in .O.SM NaNO, solutions at
PPy(NO; 1 films were electrochemically polymerized on a Pt plate or an indium tin oxide coated conducting
different pH values, elemental an&& was carried out for the PPy fihus after immersion in the solution for 15
0022-0728/93/56.O0
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1993 - Elsetier Sequoia S.A. All rights reserved
Y. Li, R. Qian / Redoxprocessesof poiypyrrolenitrate
268
TABLE 1. The results of elemental analysis of PPy(NO,) soaking in 0.5 M NaNO, aqueous solution for 15 h Solution pH
(Starting PPy) pH3 pH7 pH 42 1 M NaOH
after
Mole ratio (C = 4)
designated site 1, is a traditional doping site, i.e. the positively charged conjugated chain: NO;
H
NO, a
Ob
3.18 3.17 2.92 3.08 2.90
0.22 0.22 0.11 0.015 0.00
0.61 0.65 0.78 0.90 1.02
a N in pyrrole units taken as unity. b Calculated from balance.
h. The results are shown in Table 1. It is clear that the counter-anion content remains unchanged after treatment in an acidic solution, half the counter-anions are lost in a neutral solution and all of them are removed in a basic solution. This agrees with the results obtained for PPy treated in buffer solutions [El. It was also found that the pH values of the neutral or basic solutions in which PPy was immersed decreased appreciably. In addition, the absorption peak at ca. 460 run disappeared when the PMNO,) film was immersed in the neutral solution for 6 h (Fig. 1). The 460 nm peak has been assigned to the absorption of a protonated PPy chain [16]. Therefore the dedoping of counter-anions in neutral solution is accompanied by the deprotonation of the PPy chain. All these observations suggest the existence of two different doping sites in Ppy(NO;J. One of the sites,
The other doping site, designated site 2, is related to a protonated Py unit on the PPy chain, which is easily dedoped in contact with a solution at pH 2 7. It is difficult at present to determine the exact structure of doping site 2, but some possible structures can be discussed. One possibility is protonation on the P-C of the py unit:
Jp&T~ NO;
Site 2
*
The XI-I, absorption in the IR spectra of PPy [5] supports this proposal. Another possibility is the -C=O structure [15,17] of the py unit. Since half the counteranions are lost in neutral sol&ion, we believe that the number of counter-anions doped in site 2 of PMNO;) is equal to the number doped in site 1. It should be noted that the structure of site 2 partially disrupts the conjugated PPy chain. Therefore it will decrease its conductivity. The conductivity of the PPy(NO;) film was measured as ca. 11 S cm-‘. It changed to ca. 3 S cm-’ after .treatment in neutral solution, i.e. a decrease of a factor of 3-4. However, dedoping of counter-aniohs from the conjugated chain leads to a decrease in conductivity by 5-6 orders of magnitude [ 181. In the rest of this paper PPy(NO,) is expressed as PPG)+NO;PP(II)+HNO; , where WI)+ represents the structure of doping site 1 and PP(II)‘H represents the protonated structure of doping site 2. The structure change of PPy(NO;) in a neutral solution can be expressed as PP( I) + NO;PP( II) + HNO,
-
PP(I)+NO;PP(II)
0.0
III
t200
1600
Wawtengtt~/nm Fig. 1. WS-NIR absorption spectra of PpyfNO<) soaked in 0.5 M NaNO, aqueous solutions at (a) pH 3, (b) pH 7 and (c) pH 12.
+HNO,
(1)
The dedoping of counter-anions of PPy in an alkaline solution is well known [19-211. Further deprotonation of the PPy chain resulting in a quinoicl structure of the -NH group, as in the case of polyaniline (PAN) treated in an alkaline solution, has been suggested [22]. The VIS-NIR absorption spectrum of PPy(N0;) treated with NaOH solution is shown in Fig. l(c). Fairly strong absorption in the near IR region, peaking at 750 run, was observed accompanied by the disappearance of the absorption at, 460 mu. However, in the
Y Li, R Qian / Redoxprocessesof polypyrrolenitrate
CL28V
CLOV
1.0
.O.IV
8
6
-0.2v
3
-0.3v
0
2
, 0.5
I
-0.4v
.I, -0.5v -0.6V 100’;; -0:sv
0.c
400
1
600 Wawlength/nm
Fig. 2. In-situ absorption spectra of PPy(NO;)
in aqueous 0.5 M
NaNO, solution (pH 3) at various potentials.
case of FAN no pronounced absorption peak was observed in the NIR region 1231. Consequently, it seems likely that after NaOH solution treatment the PPy chains are still in the positively charged state &d the counter-anions are OH-. This was first proposed by the present authors to explain the electrochemical redox activity of PPy in NaOH solution [20]. The structural changes are PP(I)+NO;PP(II)+HNO;
+2OH-
PP(1) + OH-PP(I1)
269
tion peak at ca. 460 nm, which is due to the protonated PPy chain, gradually shifts to ca. 550 run. It is still present after the potential is reduced to -0.2 V (see broken curve in Fig. 2). In contrast with Fig. l(b), it can be seen that a valley in the absorption spectrum of PPy after deprotonation in a neutral solution is located at ca. 550 nm. This means that the absorption at ca. 550 nm is evidence for the doped state in site 2. Therefore we believe that only doping site 1 of PPy is reduced in the first stage of reduction. As the potential is decreased further from -0.3 V, the absorption at ca. 550 run drops rapidly and the spectral change falls in stage 2 of the reduction, where the isosbestic point exists at 480 nm. Therefore it is suggested that doping site 2 of PPy is reduced in the second stage. In conclusion, we suggest that two reduction processes for PPy, corresponding to the two doping sites, exist between 0.28 V and -0.8 V in an acidic aqueous solution: 0.28 V to -0.3 V PP(I)+NO;PP(II)+HNO;
+ e-
PP( I)“PP( II) + HNO,
PP(I)“PP(II)+HNO;
+ e-
3.2. In-situ spectroelectrochemical analysis in an acidic solution The electrochemical behavior of PPy in an acidic solution has been studied earlier [5,12]. The anion de-insertion-re-insertion mechanism has generally been accepted for the reduction-re-oxidation of PPy. Here, this mechanism is combined with the two-doping-site concept. Figure 2 shows the in-situ absorption spectra of PPy(NO;) reduced at various potentials in aqueous 0.5 M NaNO, solution (pH 3). The potential of the as-prepared PPy film is 0.28 V. As the potential is decreased, i.e. the film is reduced, the absorption in the NIR region decreases, while that at ca. 400 nm gradually increases, as has been observed before [12,24]. The interesting phenomenon here is that two reduction stages, stage 1 from 0.28 V to -0.3 V and stage 2 from - 0.3 V to - 0.8 V, can be distinguished on the basis of the isosbestic points in the spectra. Two isosbestic points at 430 nm and 610 nm are observed in stage 1 of the reduction and the absorption between the two isosbestic points changes very little. The strong absorp-
(3)
+ NO,
(4)
-
PP(I)“PP(II)H (2)
+ NO;
-0.3 V to -0.8 V
-
+ H,O + 2NO;
-
Genies and Pemaut [25] proposed two one-electron processes for the reduction of PPy. Obviously, the above expressions support their assumption. 3.3. Rehx processes in a neutral aqueous solution Two reduction processes in the potential range from 0.3 V to -0.9 V can be distinguished in the cyclic
III -1.0
1
I
I
I
I1
-0.5
Potential/V
I
I
I
I
0.0
vs. SCE
Fig. 3. Cyclic vol tammogmms of Pqr(NO; ) iii aqueous 0.5 M NaNO, solutions at (a) pH 7 and (b) pH 3.
270
Y. Li, R. @an / Redoxpmcesses of potypyrrolenitmte TABLE 2. Elemental composition of PPy after reduction at diierent potentials in neutral aqueous 0.5 M solution Sample conditions
Mole ratios (C = 4)
Ppy(NO; 1 as prepared Reduced at - 0.6 V Reduced at - 1.0 V
H
N
NO;
Na
3.18 3.18 3.14
1 0.99 0.99
0.22 0 0
0.00036 0.0019 0.0316
V to -0.5 V should come from site 1 of the PPy chain. Therefore the reduction reaction in the potential range 0.3 V to -0.5 V can be represented as PP(I)+NO;PP(II) Fig. 4. Cyclic voltammograms of Ppy(NO, ) in neutral aqueous 0.5 M TsONa solution: - - - first cycle; * . . . . . second cycle.
voltammograms of PPy(N0;) in neutral aqueous solution (Fig. 3(a)). The total amount of charge consumed in the preparation of the PPy(NO;) film is 50 mC. Based on the integration of the reduction current in Fig. 3(a), a total reduction charge of 6.1 mC was consumed. The charge in the potential range of 0.3 V to -0.5 V is 3.1 mC and that in the range -0.5 V to -0.9 V is 3.0 mC. The two reduction processes consumed approximately equal amounts of charge. In addition, it should be noted that the total reduction charge of PPy(N0;) in the neutral solution is the same as that in an acidic solution, as shown in Fig. 3. In order to examine the mechanism of the two reduction processes, the effect of solution anions and cations on the cyclic voltammograms was investigated. Figure 4 shows the cyclic voltammograms of PPy(NO;) in a neutral TsONa solution containing the large anion TsO-. In this case, the re-oxidation current is much smaller than the current of the first reduction scan (solid curve>. Comparison of Fig. 3(a) and Fig. 4 shows that the first reduction scan is independent of solution anions, but the re-oxidation peak current and the reduction peak current in the subsequent cycles are quite different for different anions in the neutral solutions. The first redox processes disappeared from the second scan in TsONa solution (dotted curve in Fig. 4) because the large TsO- ions cannot be re-inserted into PPy(NO;) film in the re-oxidation scan [12]. The results of elemental analysis of PPy after reduction are listed in Table 2. It can be seen that the counter-anions are all dedoped after reduction at -0.6 V. Therefore we can relate the first reduction process in the potential range from 0.3 V to -0.5 V to the de-insertion of the counter-anions. Since only the counter-anions in site 1 remain in a neutral solution, as discussed previously, the reduction process in the potential range 0.3
+ e-
PP( I)“PP( II) + NO;
(5)
The reduction process in the potential range -0.5 V to -0.9 V is little affected by the nature of the anions in the solution. However, it was found to be strongly affected by the nature of the cations present, as shown in Fig. 5. The reduction peak around -0.8 V disappeared in NiCl,, CaCl, or BaCl, solutions. In addition, the sodium content of PPy increased substantially after reduction at - 1.0 V in neutral aqueous NaNO, solution, as shown in Table 2. Therefore Na+ cation insertion is involved in the second reduction process of PPy in the potential range -0.5 V to - 0.9 V. Moreover, bivalent cations cannot be inserted in the reduction of PPy(N0; ). To check the effect of water on the second reduction process, cyclic voltammetry of polypyrrole which had been treated in neutral aqueous solution was performed in an organic solution (0.2 M NaClO, in propylene carbonate (PC) solution). It was found that the reduction current was very weak over the whole potential range from 0.2 V to -0.9 V. Therefore water also plays a very important role in the reduction of PPy(N0;) after treatment in neutral aqueous solution. Figure 6 shows the in-situ absorption spectra of Ppy(N0;) in neutral aqueous solution at different I-
I
Potential/V
vs. SCE
Fii. 5. Cyclic voltammograms of PPg(N0; ) in neutral aqueous 0.5 M Nii, solution.
Y L.i,R Qian / Redox processesof poiy~rrole nitrate
,_/-
I*
I
271
I
I
II
-1.0
I
I
I
I
I
-0.5
I
I
II
I
0.0
Potentiol/V vs. SCE
f.! ::
Fig. 7. Cyclic voltammograms of Ppy
$
4” 0
0.
I
400
I
1
I
I
600
600 Wovdength/ nm
Fig. 6. In-situ absorption spectra of Ppy(NO,) in neutral aqueous 0.5 M NaNO, solution at (a) 0.0 V/SCE, (b) - 0.2 V/SCE, (c) - 0.4 V/SCE and (d) - 0.9 V/SCJL
potentials. The changes of absorption with potential in the NIR region and at ca. 400 nm are similar to those in acidic solution. At -0.9 V the absorption spectrum in the neutral solution is almost the same as that in an acidic solution (see Fig. 2). The neutral polypyrrole Ppy” was obtained at -0.9 V in the acidic solution, and so the same neutral Ppy” should be produced at -0.9 V in the neutral solution. Moreover, the charge consumed in the reduction from -0.5 V to -0.9 V coincided with the concentration of site 2 in PMNO;). Therefore insertion of Na+ cation is probably accompanied by the hydrogenation of site 2 in the potential range -0.5 V to -0.9 V in a neutral solution: PP(I)“PP(II)
+ e-+ H,ONa+
indicate that there are also two reduction peaks in the potential range 0.0 V to -0.9 V, similar to the cyclic voltammograms in the neutral solutions except that the broad weak reduction peak in the potential range 0.2 V to -0.5 V shifted to more negative potentials. As in the neutral solutions, the reduction peak at -0.6 V to -0.9 V almost disappears in the basic divalent cation solution, as shown in Fig. 7 (solid curve). After treatment in basic solution, PPy loses electrochemical activity in 0.2 M NaClO, + PC solution in the potential range from 0.2 V to - 1.0 V, which means that water is important for its reduction. Therefore the same mechanism of reaction as in the neutral solution, i.e. insertion of hydrated cations into site 2 of PPy, was assumed for the reduction in the potential range -0.6 V to - 0.9 v. In the potential range 0.0 to -0.6 V, the reduction current is independent of the nature of the cation but it is related to the OH- anions in solution. Figure 8 shows the cyclic voltammograms of PPy(BS-) in 0.2 M NaBS solutions at pH 7 and pH 12 (BS- represents benzenesulfonate). It can be seen that the reduction peak at around -0.5 V appears only in the basic solution and it increases with the increase of solution
-
PP(I)“PP(II)HOH-Na+
(6)
where H,ONa+ is the hydrated sodium cation in the solution. Reaction (8) can be understood as follows. The real reactant in the reaction is water and Na+ acts as a catalyst. Since the reduction reaction takes place at quite negative potentials, the hydrated cation Na+ easily approaches site 2, so that water is taken into PPy for the reduction. After the reduction reaction, the OH- anion produced and the Na+ cation are probably adsorbed on PPy film.
V \ \
/’ \ / \ / \ I /I \ ./’
,
/
/’
I/-
I
25pA
-0.5
0.0
Potential/V vs. SCE
3.4. Redox processes in a basic solution The cyclic voltammograms of PPy(NO;) in the basic NaCl solutions, as shown in Fig. 7 (broken curve),
Fig. 8. Cyclic voltammograms of PPy(BS- ) (20 mC charge consumed in polymerization) in aqueous 0.2 M NaBS solutions at pH 7 (solid curve) and pH 12 (broken curve).
Y Li, R Qian / Redox pmcesses of polypyrrok nitrate
272
pH values. Therefore we believe that the counter-anions in site 1 were replaced by OH- anions in the basic solution. Then the reduction process is accompanied by the de-insertion of OH-: O.OVto -0.6V PP(I)+OH-PP(II)
+ e- PPy(I)‘PPy(II) + OH-
(7)
Acknowiedepnent
This work was supported by the National Natural Science Foundation, China. References 1 A.F. Diaz, K.K Kanazawa and G.P. Gardhti, J. Chem. Sot. Chem. Commun., (1979) 635. 2 EM. Genies, G. Bidan and A.F. Diaz, J. Electroanal Chem., 149 (1983) 101. 3 S. Asavapiriyanont, G.K. Chandler, G.A. Gunawardena and D. PIetcher, J. Electroanal Chem., 177 (1984) 229. 4 W. Wernet, M. Monkenbusch and G. Wegner, Mol. Cryst. Liq. tryst., 118 (1985) 193. 5 R. Qian and J. Qiu, Polym. J., 19 (1987) 157. 6 K.K. Kanazawa, A.F. Diaz, R.H. Geiss, W.D. Gill, J.F. Kwak, J.A. Logan, J.F. Rabolt and G.B. Street, J. Chem. Sot. Chem. Commun., (1979) 854.
7 J. He&e, M. Dietrich and J. Mortensen, Makromol. Chem. Macromol. Symp., 8 (1987) 73. 8 B. Yan, J. Yang, Y. Li and R. Qian, Synth. Met., 58 (1993) 17. 9 T. Shimidzu, A. Ohtani, T. Iyoda and K. Honda, J. Electroanal. Chem., 224 (1987) 123. 10 E.W. Tsai, G.W. Jang and K. Rajeshwar, J. Chem. Sot. Chem. Commun., (1987) 1776. 11 Q.X. Zhou, C.J. Kolaskie and L.L. Miller, J. Electroanal. Chem., 223 (1987) 283. 12 Y. Li and R. Qian, Synth. Met., 28 (1989) C127. 13 J. Tauguy, N. Mermilliod and M. Hoclet, Synth. Met., 18 (1987) 7. 14 J. Tauguy and N. Mermilliod, Synth. Met., 21 (1987) 129. 1.5 R. Qian, Q. Pei and Z. Huang, Makromol. Chem., 129 (1991) 1263. 16 Q. Pei and R. Qian, Synth. Met., 45 (1991) 35. 17 K. Hyodo and A.G. MacDiarmid, Synth. Met., 11 (1985) 167. 18 B.J. Feldman, P. Burgmayer and R.W. Murray, J. Am. Chem. Sot., 107 (1985) 872. 19 W. Wernet and G. Wegner, Makromol. Chem., 188 (1987) 1465. 20 Y. Li and R. Qian, Synth. Met., 26 (1988) 139. 21 0. Inganas, R. Erlandsson, C. Nylander and I. Lundstrom, J. Phys. Chem. Solids, 45 (1984) 427. 22 P. Pfluger, M. Krounbi and G.B. Street, J. Chem. Phys., 78 (1987) 2642. 23 Y. Li, B. Yan, J. Yang, Y. Cao and R. Qian, Synth. Met., 25 (1988) 79. 24 E.M. Genies, J.M. Pernaut, C. Santier, A.A. Syed and C. Tsintavis, Electronic Properties of Polymers and Related Compounds, Springer Series on Solid State Science, Vol. 63, Springer-Verlag, 1985, p. 211. 25 E.M. Genies and J.M. Pernaut, J. Electroanal. Chem., 191(1985) 111.
Joumal of Electroanatytical Chemistry, 362 (1993) 267-272
JECO2W
On the nature of redox processes in the cyclic voltarnmetry of polypyrrole nitrate in aqueous solutions Yongfang Li and Renyuan Qian Institute of Chemistry, Academia Sinica, Beijing loo080 fChid (Received 24 July 1992; in revised form 23 April 19931
Abstract The counter-anion doping states and redox processes of polypyrrole nitrate (PMNO;)) were studied by elemental analysis, cyclic voltammetry and spectroelectroche.mical measurements. Two counter-agion doping sites, the positively charged conjugated chain of PPy (site 1) and the protonated pyrrole (py) unit’(site 21, were dbtinguished. It was found that the counter-anions in site 2 are removed with the &pro@nattionof the py unit in a aentral or basic aqueous solution, while those in site 1 are exchanged with OHin a basic solution. Two redox processes of PPyfNO,) in aqueous so]utions, corresponding to the two doping sites, were observed. The meclmnisms of the redox processe s in acidic, neutral and basic solutions are discussed.
1. Introduction Conducting polypyrrole (PPy) has aroused great interest over the past decade owing to its ease of preparation in both organic [1,2] and aqueous [3-51 solutions, its good stability in its oxidized conducting state [6] and its unusual electrochemical properties. In cyclic. voltammetric studies of the reversible redox processes of PPy a “capacitive” current is generally observed, which has been shown to be a true faradaic current [7,8]. When de-insertion of the counter-anions in PPy is .difficult, insertion of cations from the electrolyte solution also contributes to the cathodic current [g-111. In neutral and basic aqueous solutions two reduction processes were observed [121, presumably related to two doping sites of the PPy chain as was first suggested by Tanguy and coworkers [13,14]. However, the nature of the two doping sites remains ambiguous. The present study of polypyrrole nitrate (Ppy(N0;)) attempts to clarify the relationship between the two doping sites
and the redox processes involved on the basis of the results of elemental analysis, absorption spectrometry and cyclic voltammetry.
glass plate (IT01 electrode under a constant potential of 0.7 V/SCE in an aqueous solution of 0.1 M pyrrole and 0.2 M NaNO, at pH 3-4. The acidic solution pH was adjusted by adding 0.1 M HNO, solution. The electrode area was 20 mm* and the total charges consumed during polymerization were 50 mC for the Pt electrode and 10 mC for the IT0 electrode which was used for optical spectrum measurement. Cyclic voltammetry was carried out using an EG&G PAR model 174A polarographic analyser and a PAR model 175 universal programmer with a Pt plate counter-electrode and a saturated calomel reference electrode (SCE) at a scan rate of 20 mV/s. In-situ visible-near IR WIS-NIR) absorption spectra were recorded with a Nanometrics Nanospec/lO VIS-NIR microspectrophotometer. The sodium content in PPy was analyzed using atomic absorption spectrometry. All electrode potential values are reported relative to SCE. All experiments were performed at room temperature. 3. Results and discussion
3.1. Two hping sites in PPy(N0;) 2. Experimental
In order to examine the stab%@ of the counter-anions in Pqr(NO,) dipped in .O.SM NaNO, solutions at
PPy(NO; 1 films were electrochemically polymerized on a Pt plate or an indium tin oxide coated conducting
different pH values, elemental an&& was carried out for the PPy fihus after immersion in the solution for 15
0022-0728/93/56.O0
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1993 - Elsetier Sequoia S.A. All rights reserved
Y. Li, R. Qian / Redoxprocessesof poiypyrrolenitrate
268
TABLE 1. The results of elemental analysis of PPy(NO,) soaking in 0.5 M NaNO, aqueous solution for 15 h Solution pH
(Starting PPy) pH3 pH7 pH 42 1 M NaOH
after
Mole ratio (C = 4)
designated site 1, is a traditional doping site, i.e. the positively charged conjugated chain: NO;
H
NO, a
Ob
3.18 3.17 2.92 3.08 2.90
0.22 0.22 0.11 0.015 0.00
0.61 0.65 0.78 0.90 1.02
a N in pyrrole units taken as unity. b Calculated from balance.
h. The results are shown in Table 1. It is clear that the counter-anion content remains unchanged after treatment in an acidic solution, half the counter-anions are lost in a neutral solution and all of them are removed in a basic solution. This agrees with the results obtained for PPy treated in buffer solutions [El. It was also found that the pH values of the neutral or basic solutions in which PPy was immersed decreased appreciably. In addition, the absorption peak at ca. 460 run disappeared when the PMNO,) film was immersed in the neutral solution for 6 h (Fig. 1). The 460 nm peak has been assigned to the absorption of a protonated PPy chain [16]. Therefore the dedoping of counter-anions in neutral solution is accompanied by the deprotonation of the PPy chain. All these observations suggest the existence of two different doping sites in Ppy(NO;J. One of the sites,
The other doping site, designated site 2, is related to a protonated Py unit on the PPy chain, which is easily dedoped in contact with a solution at pH 2 7. It is difficult at present to determine the exact structure of doping site 2, but some possible structures can be discussed. One possibility is protonation on the P-C of the py unit:
Jp&T~ NO;
Site 2
*
The XI-I, absorption in the IR spectra of PPy [5] supports this proposal. Another possibility is the -C=O structure [15,17] of the py unit. Since half the counteranions are lost in neutral sol&ion, we believe that the number of counter-anions doped in site 2 of PMNO;) is equal to the number doped in site 1. It should be noted that the structure of site 2 partially disrupts the conjugated PPy chain. Therefore it will decrease its conductivity. The conductivity of the PPy(NO;) film was measured as ca. 11 S cm-‘. It changed to ca. 3 S cm-’ after .treatment in neutral solution, i.e. a decrease of a factor of 3-4. However, dedoping of counter-aniohs from the conjugated chain leads to a decrease in conductivity by 5-6 orders of magnitude [ 181. In the rest of this paper PPy(NO,) is expressed as PPG)+NO;PP(II)+HNO; , where WI)+ represents the structure of doping site 1 and PP(II)‘H represents the protonated structure of doping site 2. The structure change of PPy(NO;) in a neutral solution can be expressed as PP( I) + NO;PP( II) + HNO,
-
PP(I)+NO;PP(II)
0.0
III
t200
1600
Wawtengtt~/nm Fig. 1. WS-NIR absorption spectra of PpyfNO<) soaked in 0.5 M NaNO, aqueous solutions at (a) pH 3, (b) pH 7 and (c) pH 12.
+HNO,
(1)
The dedoping of counter-anions of PPy in an alkaline solution is well known [19-211. Further deprotonation of the PPy chain resulting in a quinoicl structure of the -NH group, as in the case of polyaniline (PAN) treated in an alkaline solution, has been suggested [22]. The VIS-NIR absorption spectrum of PPy(N0;) treated with NaOH solution is shown in Fig. l(c). Fairly strong absorption in the near IR region, peaking at 750 run, was observed accompanied by the disappearance of the absorption at, 460 mu. However, in the
Y Li, R Qian / Redoxprocessesof polypyrrolenitrate
CL28V
CLOV
1.0
.O.IV
8
6
-0.2v
3
-0.3v
0
2
, 0.5
I
-0.4v
.I, -0.5v -0.6V 100’;; -0:sv
0.c
400
1
600 Wawlength/nm
Fig. 2. In-situ absorption spectra of PPy(NO;)
in aqueous 0.5 M
NaNO, solution (pH 3) at various potentials.
case of FAN no pronounced absorption peak was observed in the NIR region 1231. Consequently, it seems likely that after NaOH solution treatment the PPy chains are still in the positively charged state &d the counter-anions are OH-. This was first proposed by the present authors to explain the electrochemical redox activity of PPy in NaOH solution [20]. The structural changes are PP(I)+NO;PP(II)+HNO;
+2OH-
PP(1) + OH-PP(I1)
269
tion peak at ca. 460 nm, which is due to the protonated PPy chain, gradually shifts to ca. 550 run. It is still present after the potential is reduced to -0.2 V (see broken curve in Fig. 2). In contrast with Fig. l(b), it can be seen that a valley in the absorption spectrum of PPy after deprotonation in a neutral solution is located at ca. 550 nm. This means that the absorption at ca. 550 nm is evidence for the doped state in site 2. Therefore we believe that only doping site 1 of PPy is reduced in the first stage of reduction. As the potential is decreased further from -0.3 V, the absorption at ca. 550 run drops rapidly and the spectral change falls in stage 2 of the reduction, where the isosbestic point exists at 480 nm. Therefore it is suggested that doping site 2 of PPy is reduced in the second stage. In conclusion, we suggest that two reduction processes for PPy, corresponding to the two doping sites, exist between 0.28 V and -0.8 V in an acidic aqueous solution: 0.28 V to -0.3 V PP(I)+NO;PP(II)+HNO;
+ e-
PP( I)“PP( II) + HNO,
PP(I)“PP(II)+HNO;
+ e-
3.2. In-situ spectroelectrochemical analysis in an acidic solution The electrochemical behavior of PPy in an acidic solution has been studied earlier [5,12]. The anion de-insertion-re-insertion mechanism has generally been accepted for the reduction-re-oxidation of PPy. Here, this mechanism is combined with the two-doping-site concept. Figure 2 shows the in-situ absorption spectra of PPy(NO;) reduced at various potentials in aqueous 0.5 M NaNO, solution (pH 3). The potential of the as-prepared PPy film is 0.28 V. As the potential is decreased, i.e. the film is reduced, the absorption in the NIR region decreases, while that at ca. 400 nm gradually increases, as has been observed before [12,24]. The interesting phenomenon here is that two reduction stages, stage 1 from 0.28 V to -0.3 V and stage 2 from - 0.3 V to - 0.8 V, can be distinguished on the basis of the isosbestic points in the spectra. Two isosbestic points at 430 nm and 610 nm are observed in stage 1 of the reduction and the absorption between the two isosbestic points changes very little. The strong absorp-
(3)
+ NO,
(4)
-
PP(I)“PP(II)H (2)
+ NO;
-0.3 V to -0.8 V
-
+ H,O + 2NO;
-
Genies and Pemaut [25] proposed two one-electron processes for the reduction of PPy. Obviously, the above expressions support their assumption. 3.3. Rehx processes in a neutral aqueous solution Two reduction processes in the potential range from 0.3 V to -0.9 V can be distinguished in the cyclic
III -1.0
1
I
I
I
I1
-0.5
Potential/V
I
I
I
I
0.0
vs. SCE
Fig. 3. Cyclic vol tammogmms of Pqr(NO; ) iii aqueous 0.5 M NaNO, solutions at (a) pH 7 and (b) pH 3.
270
Y. Li, R. @an / Redoxpmcesses of potypyrrolenitmte TABLE 2. Elemental composition of PPy after reduction at diierent potentials in neutral aqueous 0.5 M solution Sample conditions
Mole ratios (C = 4)
Ppy(NO; 1 as prepared Reduced at - 0.6 V Reduced at - 1.0 V
H
N
NO;
Na
3.18 3.18 3.14
1 0.99 0.99
0.22 0 0
0.00036 0.0019 0.0316
V to -0.5 V should come from site 1 of the PPy chain. Therefore the reduction reaction in the potential range 0.3 V to -0.5 V can be represented as PP(I)+NO;PP(II) Fig. 4. Cyclic voltammograms of Ppy(NO, ) in neutral aqueous 0.5 M TsONa solution: - - - first cycle; * . . . . . second cycle.
voltammograms of PPy(N0;) in neutral aqueous solution (Fig. 3(a)). The total amount of charge consumed in the preparation of the PPy(NO;) film is 50 mC. Based on the integration of the reduction current in Fig. 3(a), a total reduction charge of 6.1 mC was consumed. The charge in the potential range of 0.3 V to -0.5 V is 3.1 mC and that in the range -0.5 V to -0.9 V is 3.0 mC. The two reduction processes consumed approximately equal amounts of charge. In addition, it should be noted that the total reduction charge of PPy(N0;) in the neutral solution is the same as that in an acidic solution, as shown in Fig. 3. In order to examine the mechanism of the two reduction processes, the effect of solution anions and cations on the cyclic voltammograms was investigated. Figure 4 shows the cyclic voltammograms of PPy(NO;) in a neutral TsONa solution containing the large anion TsO-. In this case, the re-oxidation current is much smaller than the current of the first reduction scan (solid curve>. Comparison of Fig. 3(a) and Fig. 4 shows that the first reduction scan is independent of solution anions, but the re-oxidation peak current and the reduction peak current in the subsequent cycles are quite different for different anions in the neutral solutions. The first redox processes disappeared from the second scan in TsONa solution (dotted curve in Fig. 4) because the large TsO- ions cannot be re-inserted into PPy(NO;) film in the re-oxidation scan [12]. The results of elemental analysis of PPy after reduction are listed in Table 2. It can be seen that the counter-anions are all dedoped after reduction at -0.6 V. Therefore we can relate the first reduction process in the potential range from 0.3 V to -0.5 V to the de-insertion of the counter-anions. Since only the counter-anions in site 1 remain in a neutral solution, as discussed previously, the reduction process in the potential range 0.3
+ e-
PP( I)“PP( II) + NO;
(5)
The reduction process in the potential range -0.5 V to -0.9 V is little affected by the nature of the anions in the solution. However, it was found to be strongly affected by the nature of the cations present, as shown in Fig. 5. The reduction peak around -0.8 V disappeared in NiCl,, CaCl, or BaCl, solutions. In addition, the sodium content of PPy increased substantially after reduction at - 1.0 V in neutral aqueous NaNO, solution, as shown in Table 2. Therefore Na+ cation insertion is involved in the second reduction process of PPy in the potential range -0.5 V to - 0.9 V. Moreover, bivalent cations cannot be inserted in the reduction of PPy(N0; ). To check the effect of water on the second reduction process, cyclic voltammetry of polypyrrole which had been treated in neutral aqueous solution was performed in an organic solution (0.2 M NaClO, in propylene carbonate (PC) solution). It was found that the reduction current was very weak over the whole potential range from 0.2 V to -0.9 V. Therefore water also plays a very important role in the reduction of PPy(N0;) after treatment in neutral aqueous solution. Figure 6 shows the in-situ absorption spectra of Ppy(N0;) in neutral aqueous solution at different I-
I
Potential/V
vs. SCE
Fii. 5. Cyclic voltammograms of PPg(N0; ) in neutral aqueous 0.5 M Nii, solution.
Y L.i,R Qian / Redox processesof poiy~rrole nitrate
,_/-
I*
I
271
I
I
II
-1.0
I
I
I
I
I
-0.5
I
I
II
I
0.0
Potentiol/V vs. SCE
f.! ::
Fig. 7. Cyclic voltammograms of Ppy
$
4” 0
0.
I
400
I
1
I
I
600
600 Wovdength/ nm
Fig. 6. In-situ absorption spectra of Ppy(NO,) in neutral aqueous 0.5 M NaNO, solution at (a) 0.0 V/SCE, (b) - 0.2 V/SCE, (c) - 0.4 V/SCE and (d) - 0.9 V/SCJL
potentials. The changes of absorption with potential in the NIR region and at ca. 400 nm are similar to those in acidic solution. At -0.9 V the absorption spectrum in the neutral solution is almost the same as that in an acidic solution (see Fig. 2). The neutral polypyrrole Ppy” was obtained at -0.9 V in the acidic solution, and so the same neutral Ppy” should be produced at -0.9 V in the neutral solution. Moreover, the charge consumed in the reduction from -0.5 V to -0.9 V coincided with the concentration of site 2 in PMNO;). Therefore insertion of Na+ cation is probably accompanied by the hydrogenation of site 2 in the potential range -0.5 V to -0.9 V in a neutral solution: PP(I)“PP(II)
+ e-+ H,ONa+
indicate that there are also two reduction peaks in the potential range 0.0 V to -0.9 V, similar to the cyclic voltammograms in the neutral solutions except that the broad weak reduction peak in the potential range 0.2 V to -0.5 V shifted to more negative potentials. As in the neutral solutions, the reduction peak at -0.6 V to -0.9 V almost disappears in the basic divalent cation solution, as shown in Fig. 7 (solid curve). After treatment in basic solution, PPy loses electrochemical activity in 0.2 M NaClO, + PC solution in the potential range from 0.2 V to - 1.0 V, which means that water is important for its reduction. Therefore the same mechanism of reaction as in the neutral solution, i.e. insertion of hydrated cations into site 2 of PPy, was assumed for the reduction in the potential range -0.6 V to - 0.9 v. In the potential range 0.0 to -0.6 V, the reduction current is independent of the nature of the cation but it is related to the OH- anions in solution. Figure 8 shows the cyclic voltammograms of PPy(BS-) in 0.2 M NaBS solutions at pH 7 and pH 12 (BS- represents benzenesulfonate). It can be seen that the reduction peak at around -0.5 V appears only in the basic solution and it increases with the increase of solution
-
PP(I)“PP(II)HOH-Na+
(6)
where H,ONa+ is the hydrated sodium cation in the solution. Reaction (8) can be understood as follows. The real reactant in the reaction is water and Na+ acts as a catalyst. Since the reduction reaction takes place at quite negative potentials, the hydrated cation Na+ easily approaches site 2, so that water is taken into PPy for the reduction. After the reduction reaction, the OH- anion produced and the Na+ cation are probably adsorbed on PPy film.
V \ \
/’ \ / \ / \ I /I \ ./’
,
/
/’
I/-
I
25pA
-0.5
0.0
Potential/V vs. SCE
3.4. Redox processes in a basic solution The cyclic voltammograms of PPy(NO;) in the basic NaCl solutions, as shown in Fig. 7 (broken curve),
Fig. 8. Cyclic voltammograms of PPy(BS- ) (20 mC charge consumed in polymerization) in aqueous 0.2 M NaBS solutions at pH 7 (solid curve) and pH 12 (broken curve).
Y Li, R Qian / Redox pmcesses of polypyrrok nitrate
272
pH values. Therefore we believe that the counter-anions in site 1 were replaced by OH- anions in the basic solution. Then the reduction process is accompanied by the de-insertion of OH-: O.OVto -0.6V PP(I)+OH-PP(II)
+ e- PPy(I)‘PPy(II) + OH-
(7)
Acknowiedepnent
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