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Effect of preparation conditions on the two doping structures of polypyrrole
ELSEVIER
Synthetic
Metals94
( 1998) 127-129
Effect of preparation conditions on the two doping structures of polypyrrole Yongfang Institute
of Chemistry,
Chinese
Li *, Gufeng He Academy
of Sciences.
Beijing
100080,
China
Abstract The effect of electrolyte concentration of the polymerization solution and temperature on the two doping structures, conjugatedchaindoped with counter-anions ( 1) and proton-acid doping structure (2), of polypyrrole nitrate films was investigated via elemental analysis and cyclic voltammetry. It was found that the doping degree of 1 incrcascd with increase of NaNO, concentration in its polymerization solution and with decrease of temperature. 2 is easily formed in low electrolyte concentration and at higher temperature. 0 1998 Elsevier Science S.A. All rights Keywords:
reserved. Doping;
Polypyrrole
and derivatives
1. Introduction Conducting polypyrrole (PPy) has attracted much attention in the past two decades due to its good stability and high conductivity [ 1,2]. The properties and structure of PPy films are very sensitive to the polymerization conditions [ 2-61. Recently, Li et al. [7-91 found that there are two doping structures,oxidized conjugated chain doped with counter-
anions (1) and proton-acid doping structure (2), in PPy(NO,-) films:
Jlp&+
...... -$-&&&
1
2
1 is widely acceptedin the literature, but 2 is hardly mentioned. Actually, 2 alsoexists in polypyrrole films prepared with other counter-anions[ 91. The proposalof P-C protonation in 2 comesfrom the fact that the pK, value of pyrrole protonated at the P-C position is - 5.9, while that at the N position is - 10, which meansthat the possibility of protonation at P-C is 10000 timeshigher than that at the N position [ 101. The conductivity
and mechanical
properties
are greatly
related to the doping degreeand the ratio of the two doping structures,and the dopingdegreeof the two dopingstructures * Corresponding
author
0379-6779/98/$19.00 Pnso379-6779(97)04158-1
0
1998 Elsevier
Science S.A. All rights reserved.
dependsonthe natureof the anionsusedin thepolymerization [9]. Thus, it is very important to make clear the effect of polymerization conditions on the two doping structuresfor the preparationof high quality PPy films. 2. Experimental The electropolymerization of pyrrole wasperformedin pH 3, 0.1 mol I- ’ pyrrole, 0.2-0.4 mol I- ’ NaNO, aqueous solution. Two stainless-steelelectrodeswere usedas anode andcathodein the preparationof PPy film for theconductivity andtensile strength measurements; the thickness of the films was in the range of 8-12 km. For the cyclic voltammetry, PPy films were depositedon a Pt disc electrode.The cyclic voltammetry was performed with an EG&G PAR model 174A polarographic analyserand model 173 universal programmerin a three-electrodecell with a Pt plate asa counter electrode and a saturatedcalomel electrode (SCE) as the referenceelectrode. NaNO, aqueoussolutionswere usedas the electrolyte, the pH value of which wasregulatedby 1 mol I-’
HNO,
solution.
Potentials
are relative
to SCE unless
otherwisestated. 3. Resultsand discussion Aqueous solutionsof pH 3, 0.1 mol 1-l pyrrole with various concentrations
of NaNO,
(0.2, 1 .O, 2.0 and 4.0 mol I - ’ )
were usedin the electrochemicalpolymerization of pyrrole. The conductivity and tensile strength of PPy(N03-) films
Y. Li, G. He /Synthetic
128 Table I Effect of NaNO, concentration the PPy (NO, ) tihns prepared
on the conductivity and tensile strength from aqueous solutions
Property
NaNO,
Conductivity (S cm- ‘) Tensile strength ( MPa)
concentration
Met&
127-129
of
(mol I- ‘)
0.2
I.0
2.0
4.0
13.8 5.7
19.6 9. I
20.8 12.3
22.3 14.3
prepared from the solutions are listed in Table 1. The conductivity and tensile strength were enhanced with increase of NaNO, concentration. Especially when the concentration increased from 0.2 to 1.O mol 1~ ‘, the conductivity and tensile strength jumped from 13.8 S cm-’ and 5.7 M Pa to 16.9 S cm-’ and 9.1 M Pa, respectively, i.e., an increase of more than 50%. The improvement of the conductivity and tensile strength of PPy( NO3 ~ ) from higher NaNO, concentration solutions may be related to the changes of the two doping structures. The doping degree of the PPy( N03-) films was investigated by elemental analysis, as shown in Table 2. The mole ratio of NO?- in PPy (NO, ) is the total doping degree of 1 and 2. The total doping degree of the films from 1 mol I - ’ or higher concentrations of NaNO, (0.29-0.32) is much higher than that from 0.2 mol I- ’ NaN03 solution (0.24)) in agreement with the high conductivity and strong tensile strength of the PPy(NO,-) films from the higher concentration solutions. Two reduction processes of PPy(N03-), corresponding to its two doping structures, have been studied by cyclic voltammetry in a neutral aqueous solution [ 71. The reduction process in the potential range from 0.3 to - 0.6 V corresponds to the reduction of 1, and that from - 0.6 to - I .O V is from the reduction of 2. Thus, the larger the area of the reduction current peak in the cyclic voltammogram from 0.3 to -0.6 V, the higher is the doping degree of 1, and, similarly, from - 0.6 to - 1.O V for the doping degree of 2. Fig. 1 shows the cyclic voltammograms of PPy(NO,-) films in a neutral 0.5 mol 1-l NaNO, aqueous solution. It can be seen that the current values of the two reduction peaks depend on NaNO, concentrations used in the polymerization solutions. With the increase of NaNO, concentration, the Table 2 Results of elemental analysis of PPy films prepared from pH 3,0. I mol I-’ pyrrole aqueous solutions with different concentrations of NaNO, NaNO,
Mole
concentration (mol I-‘)
C
H
N
NO, -
0”
0.2 I .o 2.0 4.0
4 4 4 4
3.39 3.36 3.32 3.26
I I I I
0.24 0.29 0.32 0.32
0.74 0.52 0.49 0.54
’ As balance.
94 (1998)
ratios
I
I
I
-1.0
III
Ill
I1 0.0
-0.5
Potential
1 ( 3
IV vs. SCE
Fig. I, Cyclic voltammograms of PPy(NO,) tilms in neutral 0.5 mol I- ’ NaNO, aqueous solution at a scan rate of 20 mV s I. The tilms were prepared from pH 3, 0.1 mol I- ’ pyrrole aqueous solutions with various NaNO, concentrations of (a) 0.2; (b) 0.5: (c) I .O mol I- ‘.
reduction current peak in the potential range from 0.3 to - 0.6 V increased, while that from - 0.6 to - I .O V was weakened. The values of the doping degree of 1 (y,) and 2 (y2) were calculated by integration of the corresponding reduction current, based on the total doping degree of the PPy(N03-) film. They are: y, =0.13, y2=0.11 for PPy(N03-, 0.2 M) which represents the PPy(NOi- ) film prepared from a 0.2 mol I-’ NaNO, solution;y, =0.16, y2 =0.09 for PPy(N03-, 0.5 M); and y, =0.21, yl=0.08 for PPy(N03-, 1.0 M). Thus, it can be concluded that the doping degree of 1 increased when the PPy( NO,-) film was prepared in the solution with higher NaNO, concentration, which is responsible for their higher values of conductivity and tensile strength. Polymerization temperature also affects the conductivity of the as-prepared PPy( N03- ) films. For the films prepared in pH 3, 0.1 mol I-’ pyrrole, 1 mol I-’ NaNO, aqueous solution at 30, 15, 0 and - lO”C, their conductivity values are 9.2, 19.4,41.1 and 24 S cm- ‘, respectively, as shown in Table 3. The doping degree values of the two doping structures, obtained from elemental analysis and cyclic voltammograms in neutral solution, are also listed in Table 3. The conductivity and doping degree of 1 increased when the polymerization temperature decreased from 30 to O”C, while they reduced when the temperature decreased further to O”C, probably due to the lower mass transportation at too low temperature. In contrast, the doping degree of 2 decreased with decrease of temperature. Table 3 Effect of polymerization temperature on the conductivity and doping degree of PPy films prepared in pH 3,0.1 mol I ’ pyrrole, I mol I ’ NaNO, Property
Conductivity (S cm- ’ ) Doping degree of 1 Doping degree of 2
Polymerization
temperature
(“C)
30
I5
0
- IO
9.2 0.15 0.1 I
19.6 0.2 I 0.08
41.1 0.26 0.06
24 0.24 0.06
Y. Li, G. He/Svnthetic
Met&
94 (1998)
127-129
129
4. Conclusions
References
Polypyrrole films with higher conductivity and stronger mechanical property can be prepared from aqueous solutions with higher concentration of electrolyte salt and at lower temperature. The good polypyrrole films have a higher doping degree for 1 and lower doping degree for 2.
[ 1 1 A.F. Diaz, K.K. Kanazawa, G.P. Gardini, J. Chem. Sot., Chem. Commun. (1979) 635. [21 M. Salmon, A.F. Diaz, A.J. Logan, M. Krounbi, J. Bargon, Mol. Cryst. Liq. Cryst. 83 (1982) 265. 131 R. Qian, J. Qiu, Polym. J. 19 (1987) 153. 141 L.F. Warren, D.P. Anderson, J. Electrochem. Sot. 134 (1987) 101. [51 J. Ouyang, Y. Li, Synth. Met. 79 (1996) 121. [61 J. Ouyang, Y. Li, Polymer 38 ( 1997) 1971. [71 Y. Li, R. Qian, J. Electroanal. Chem. 362 ( 1993) 267. [Sl Y. Li, R. Qian, K. Imaeda, H. Inokuchi, Polym. J. 26 ( 1994) 535. 191 Y. Li, J. Ouyang, J. Yang, Synth. Met. 74 (1995) 49. [ 101 R.A. Jones (ed.), Pyrrole. Part 1. The Synthesis and the Physical and Chemical Aspects of the Pyrrole Ring, Wiley, New York, 1990, pp.
Acknowledgements This work was supported by the National Natural Science Foundation of China, Grant No. 29474182.
305-325.
Synthetic
Metals94
( 1998) 127-129
Effect of preparation conditions on the two doping structures of polypyrrole Yongfang Institute
of Chemistry,
Chinese
Li *, Gufeng He Academy
of Sciences.
Beijing
100080,
China
Abstract The effect of electrolyte concentration of the polymerization solution and temperature on the two doping structures, conjugatedchaindoped with counter-anions ( 1) and proton-acid doping structure (2), of polypyrrole nitrate films was investigated via elemental analysis and cyclic voltammetry. It was found that the doping degree of 1 incrcascd with increase of NaNO, concentration in its polymerization solution and with decrease of temperature. 2 is easily formed in low electrolyte concentration and at higher temperature. 0 1998 Elsevier Science S.A. All rights Keywords:
reserved. Doping;
Polypyrrole
and derivatives
1. Introduction Conducting polypyrrole (PPy) has attracted much attention in the past two decades due to its good stability and high conductivity [ 1,2]. The properties and structure of PPy films are very sensitive to the polymerization conditions [ 2-61. Recently, Li et al. [7-91 found that there are two doping structures,oxidized conjugated chain doped with counter-
anions (1) and proton-acid doping structure (2), in PPy(NO,-) films:
Jlp&+
...... -$-&&&
1
2
1 is widely acceptedin the literature, but 2 is hardly mentioned. Actually, 2 alsoexists in polypyrrole films prepared with other counter-anions[ 91. The proposalof P-C protonation in 2 comesfrom the fact that the pK, value of pyrrole protonated at the P-C position is - 5.9, while that at the N position is - 10, which meansthat the possibility of protonation at P-C is 10000 timeshigher than that at the N position [ 101. The conductivity
and mechanical
properties
are greatly
related to the doping degreeand the ratio of the two doping structures,and the dopingdegreeof the two dopingstructures * Corresponding
author
0379-6779/98/$19.00 Pnso379-6779(97)04158-1
0
1998 Elsevier
Science S.A. All rights reserved.
dependsonthe natureof the anionsusedin thepolymerization [9]. Thus, it is very important to make clear the effect of polymerization conditions on the two doping structuresfor the preparationof high quality PPy films. 2. Experimental The electropolymerization of pyrrole wasperformedin pH 3, 0.1 mol I- ’ pyrrole, 0.2-0.4 mol I- ’ NaNO, aqueous solution. Two stainless-steelelectrodeswere usedas anode andcathodein the preparationof PPy film for theconductivity andtensile strength measurements; the thickness of the films was in the range of 8-12 km. For the cyclic voltammetry, PPy films were depositedon a Pt disc electrode.The cyclic voltammetry was performed with an EG&G PAR model 174A polarographic analyserand model 173 universal programmerin a three-electrodecell with a Pt plate asa counter electrode and a saturatedcalomel electrode (SCE) as the referenceelectrode. NaNO, aqueoussolutionswere usedas the electrolyte, the pH value of which wasregulatedby 1 mol I-’
HNO,
solution.
Potentials
are relative
to SCE unless
otherwisestated. 3. Resultsand discussion Aqueous solutionsof pH 3, 0.1 mol 1-l pyrrole with various concentrations
of NaNO,
(0.2, 1 .O, 2.0 and 4.0 mol I - ’ )
were usedin the electrochemicalpolymerization of pyrrole. The conductivity and tensile strength of PPy(N03-) films
Y. Li, G. He /Synthetic
128 Table I Effect of NaNO, concentration the PPy (NO, ) tihns prepared
on the conductivity and tensile strength from aqueous solutions
Property
NaNO,
Conductivity (S cm- ‘) Tensile strength ( MPa)
concentration
Met&
127-129
of
(mol I- ‘)
0.2
I.0
2.0
4.0
13.8 5.7
19.6 9. I
20.8 12.3
22.3 14.3
prepared from the solutions are listed in Table 1. The conductivity and tensile strength were enhanced with increase of NaNO, concentration. Especially when the concentration increased from 0.2 to 1.O mol 1~ ‘, the conductivity and tensile strength jumped from 13.8 S cm-’ and 5.7 M Pa to 16.9 S cm-’ and 9.1 M Pa, respectively, i.e., an increase of more than 50%. The improvement of the conductivity and tensile strength of PPy( NO3 ~ ) from higher NaNO, concentration solutions may be related to the changes of the two doping structures. The doping degree of the PPy( N03-) films was investigated by elemental analysis, as shown in Table 2. The mole ratio of NO?- in PPy (NO, ) is the total doping degree of 1 and 2. The total doping degree of the films from 1 mol I - ’ or higher concentrations of NaNO, (0.29-0.32) is much higher than that from 0.2 mol I- ’ NaN03 solution (0.24)) in agreement with the high conductivity and strong tensile strength of the PPy(NO,-) films from the higher concentration solutions. Two reduction processes of PPy(N03-), corresponding to its two doping structures, have been studied by cyclic voltammetry in a neutral aqueous solution [ 71. The reduction process in the potential range from 0.3 to - 0.6 V corresponds to the reduction of 1, and that from - 0.6 to - I .O V is from the reduction of 2. Thus, the larger the area of the reduction current peak in the cyclic voltammogram from 0.3 to -0.6 V, the higher is the doping degree of 1, and, similarly, from - 0.6 to - 1.O V for the doping degree of 2. Fig. 1 shows the cyclic voltammograms of PPy(NO,-) films in a neutral 0.5 mol 1-l NaNO, aqueous solution. It can be seen that the current values of the two reduction peaks depend on NaNO, concentrations used in the polymerization solutions. With the increase of NaNO, concentration, the Table 2 Results of elemental analysis of PPy films prepared from pH 3,0. I mol I-’ pyrrole aqueous solutions with different concentrations of NaNO, NaNO,
Mole
concentration (mol I-‘)
C
H
N
NO, -
0”
0.2 I .o 2.0 4.0
4 4 4 4
3.39 3.36 3.32 3.26
I I I I
0.24 0.29 0.32 0.32
0.74 0.52 0.49 0.54
’ As balance.
94 (1998)
ratios
I
I
I
-1.0
III
Ill
I1 0.0
-0.5
Potential
1 ( 3
IV vs. SCE
Fig. I, Cyclic voltammograms of PPy(NO,) tilms in neutral 0.5 mol I- ’ NaNO, aqueous solution at a scan rate of 20 mV s I. The tilms were prepared from pH 3, 0.1 mol I- ’ pyrrole aqueous solutions with various NaNO, concentrations of (a) 0.2; (b) 0.5: (c) I .O mol I- ‘.
reduction current peak in the potential range from 0.3 to - 0.6 V increased, while that from - 0.6 to - I .O V was weakened. The values of the doping degree of 1 (y,) and 2 (y2) were calculated by integration of the corresponding reduction current, based on the total doping degree of the PPy(N03-) film. They are: y, =0.13, y2=0.11 for PPy(N03-, 0.2 M) which represents the PPy(NOi- ) film prepared from a 0.2 mol I-’ NaNO, solution;y, =0.16, y2 =0.09 for PPy(N03-, 0.5 M); and y, =0.21, yl=0.08 for PPy(N03-, 1.0 M). Thus, it can be concluded that the doping degree of 1 increased when the PPy( NO,-) film was prepared in the solution with higher NaNO, concentration, which is responsible for their higher values of conductivity and tensile strength. Polymerization temperature also affects the conductivity of the as-prepared PPy( N03- ) films. For the films prepared in pH 3, 0.1 mol I-’ pyrrole, 1 mol I-’ NaNO, aqueous solution at 30, 15, 0 and - lO”C, their conductivity values are 9.2, 19.4,41.1 and 24 S cm- ‘, respectively, as shown in Table 3. The doping degree values of the two doping structures, obtained from elemental analysis and cyclic voltammograms in neutral solution, are also listed in Table 3. The conductivity and doping degree of 1 increased when the polymerization temperature decreased from 30 to O”C, while they reduced when the temperature decreased further to O”C, probably due to the lower mass transportation at too low temperature. In contrast, the doping degree of 2 decreased with decrease of temperature. Table 3 Effect of polymerization temperature on the conductivity and doping degree of PPy films prepared in pH 3,0.1 mol I ’ pyrrole, I mol I ’ NaNO, Property
Conductivity (S cm- ’ ) Doping degree of 1 Doping degree of 2
Polymerization
temperature
(“C)
30
I5
0
- IO
9.2 0.15 0.1 I
19.6 0.2 I 0.08
41.1 0.26 0.06
24 0.24 0.06
Y. Li, G. He/Svnthetic
Met&
94 (1998)
127-129
129
4. Conclusions
References
Polypyrrole films with higher conductivity and stronger mechanical property can be prepared from aqueous solutions with higher concentration of electrolyte salt and at lower temperature. The good polypyrrole films have a higher doping degree for 1 and lower doping degree for 2.
[ 1 1 A.F. Diaz, K.K. Kanazawa, G.P. Gardini, J. Chem. Sot., Chem. Commun. (1979) 635. [21 M. Salmon, A.F. Diaz, A.J. Logan, M. Krounbi, J. Bargon, Mol. Cryst. Liq. Cryst. 83 (1982) 265. 131 R. Qian, J. Qiu, Polym. J. 19 (1987) 153. 141 L.F. Warren, D.P. Anderson, J. Electrochem. Sot. 134 (1987) 101. [51 J. Ouyang, Y. Li, Synth. Met. 79 (1996) 121. [61 J. Ouyang, Y. Li, Polymer 38 ( 1997) 1971. [71 Y. Li, R. Qian, J. Electroanal. Chem. 362 ( 1993) 267. [Sl Y. Li, R. Qian, K. Imaeda, H. Inokuchi, Polym. J. 26 ( 1994) 535. 191 Y. Li, J. Ouyang, J. Yang, Synth. Met. 74 (1995) 49. [ 101 R.A. Jones (ed.), Pyrrole. Part 1. The Synthesis and the Physical and Chemical Aspects of the Pyrrole Ring, Wiley, New York, 1990, pp.
Acknowledgements This work was supported by the National Natural Science Foundation of China, Grant No. 29474182.
305-325.