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The influence of dopant species on transport properties in as-grown polypyrrole films prepared by electrochemical method
ELSEVIER
Synthetic
Metals
69 (1995) 345-346
The influence of dopant species on transport properties in as-grown polypyrrole electrochemical
films prepared by
method
S. Masubuchi’, S. Kazama’, R. Matsushita* and T. Matsuyam$ ‘Department of Physics, Chuo University, Bunkyo-ku, Tokyo, 112 Japan 2Research Reactor Institute, Kyoto University, Kumatori, Osaka, 590-04 Japan Abstract A transport study was made on electrochemically prepared polypyrrole (PPy) films to elucidate the influence of dopant species on the electrical transport properties. We present the experimental results on as-grown PPy films prepared with several anion species such as CIO,., PF,~. BF,., TsO- and HSO,. The conductivities of these films at room temperature were between 50-200 S/cm. The temperature dependence exhibited a semiconductor-like behaviour over the whole range between 1.6-300K, with the only exception of PF,- doped PPy, which changed to a metal-like dependence at 8K. At 300K, the absolute thermoelectric power S(T) takes very small values (3.7pViK) like ordinary metals. Characteristic behaviour was observed in their temperature dependence. S(T) is linearly dependent on temperature below about 230K but becomes nearly constant above 230K. The Voltage-Shorted-Compaction (VSC) resistivity (pvs,-) showed a metallic temperature dependence with its slope definitely changed at 230K, corresponding well to the result of S(T), which implies a transition between metallic states of different nature. Introduction Polypyrrole (PPy) is one of the most familiar conducting polymers having a n-conjugated skeleton. Tough and smooth PPy film with high conductivity (>lOOScm) can be easily obtained by the electrochemical polymerization [ 11. On investigating the transport properties in conducting polymers, we always suffer from difficulties inherent to the higher order structure. Resistivity (conductivity) is significantly influenced by complicated morphology of the polymer itself. The transport process in conducting polymers involves various processes such as the transport within crystallized regions (intra- and inter-chain), the one between the segregated crystallized regions, etc. The observed resistance includes contributions from the both regions. The temperature dependence of the resistivity is dominated by the transport process between the crystallized regions through hopping and/or tunneling, resulting in a semiconducting temperature dependence. The intrinsic properties and the different contribution from different kind of dopant are thus obscured in the microscopic measurements. In this work, a series of experiments such as the ordinary fourterminal and VSC resistivity and thermoelectric power were made for as-grown PPy polymerized with several kinds of anion species with an aim to approach the intrinsic nature of PPy. Experimental Platinum plates were used for anode and cathode in a onecomponent cell. Pyrrole was distilled under a reduced pressure and stored in an argon atmosphere. Electrolytes such as Bu,NCIO,, BQNPF,, Et,NBF,, Et,NTsO and Bu,NHS04 were thoroughly degassed by pumping to remove oxygen. Propylene-carbonate was pumped out for one hour before use. A solution of propylene-
0379~6779/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02478-H
carbonate containing 1 ~01%of water dissolving 0.06 M of pyrrole and 0.06 M of an appropriate electrolyte was prepared in the cell under an argon flow. Electrochemical synthesis was carried out with a constant current density of typically 0. I n&cm2 for 40 hours at -20°C according to Yamaura et al. [2]. After the completion of polymerization, residual solvent in the film was rinsed out with acetonitrile. The as-grown film was dried up by pumping for one hour and was stored in an argon atmosphere at -20°C until further use. The conductivity of these films at room temperature were between 50 and 200 S/cm depending on the polymerization condition and anion species. The conductivity was measured with the conventional fourterminal method by applying a direct current typically of lOuA at temperatures between 1.6-300K. The VSC measurements were made by brushing the film with a silver paste as thin as possible. Absolute thermoelectric power S(T) was measured with a temperature difference across the sample being set to be within 0.5K at the measuring temperatures above 10K and within 5% of the temperature below 10K. S(T) was measured between 4.2.300K [3]. Results and Discussion Temperature dependence of resistivity, p,,(T), in PPy doped with ClO,-, BF;, HS04- and TsO- exhibited semiconductor-like behaviour over the whole temperature ranges between 1.6 and 300K. In contrast, p&T) of PF,- doped PPy showed a turnover to the metal-like dependence below 8K. Such a behaviour in p&T) has been reported by several groups particularly in PF,- doped PPy 141. Table I summarizes the conductivity (a) at 280K. The resistance ratio, p,=pDc(4.2K)/p,,-(280K), is also listed in Table 1. A sample with larger cr takes smaller value of pr In spite of the small difference in cr of nearly threefold, pr changes more than two orders of magnitude. The discrepancy in cr and pr have been considered to
346
S. Masubuchi
et al. I Synthetic Metals 69 (1995) 345-346
h
0.9
g
0.8
2
0.7
E
0.6
z
0.5 0.4
I
-2 ]
100
0
I
I
200
300
VW
T(K)
Fig. 1 Temperature
dependence
Table 1 : Conductivities dopant species
of thermoelectric
power.
at 280K for PPy doped with differing
come from the morphology of the polymer, as discussed in our previous papers [5]. The temperature dependence can be interpreted in terms of a metallic transport within the crystallized regions that are interrupted by semiconducting and/or insulating regions. The observed resistance can be represented by the sum of a metallic resistance R,(T) originated from the crystallized regions and a semiconducting one R,(T). The different u and pr would be caused by the difference in the ratio of R,(T) to R,(T), since the degree of polymerization and the crystallinity are affected by the conditions for synthesis. Fig 1 shows the temperature dependence of the absolute thermoelectric power S(T) for PPy doped with the same dopants as in Table.1. The values of S(T) at 280K were between 3-7 pV/K. The behaviour of S(T) can be classified into two types. Type- 1: S(T) is proportional to temperature throughout the entire temperature range, approaching towards -0.5 uV/K as T - OK. This type of behaviour is observed in PF,- and TsO- doped PPy, indicating a metallic character. Type-2: As can be seen typically in PPy doped with ClOq, BF; and HS04 at high temperatures, S(T) changes at a characteristic temperature Tc=210, 230 and 250K, respectively. S(T) is nearly constant above Tc and S(T) exhibits similar dependence as in Type-1 below Tc with the exception of HS04doped PPy at low temperatures. The same observation around Tc has been reported by Maddison et al. in TsO- doped PPy [6]. In HSO, doped PPy, a drastic increase in S(T) was observed below 15K like that of semiconductors. Fig.2 displays the temperature dependence of the VSC resistivity, p&T) (=R(T)/R(280K)), for PPy doped with BF,-, ClO; and HSO,. The data are normalized at 280K. All the sample show a metallic temperature dependence in a wide temperature range. p&T) for both BF; and ClO; doped PPy behaves peculiarly at high temperatures: p “s,-(T) decreases linearly with temperature down to 230K. It changes sharply at 230K and becomes to follow a T”s-law down to temperatures as low as 30K. The observation is in accordance with the results in S(T) and clearly indicates that a transition between metallic states of different nature occurs around
Fig. 2 Temperature dependence normalized at 280K.
of pvs,-(T).
The data are
230K in PPy doped with BFiand ClO,-. For HSOA doped PPy, on the other hand, the temperature dependence of pvs,-(T) is different: pvsc(T) decreases with a decrease in temperature following a T “? law down to 1_5K,without any obvious change at 230K. A turning over around 15K was observed corresponding to the upward change in S(T). The finding indicates that some transition (M-I or M-S etc.) takes place around 15K like in polythiophene [3]. It has been demonstrated that the temperature dependence of the intrinsic resistivity for heavily doped PA [7] and as-grown polythiophene (PI) [5] was definitely metallic at high temperatures following a law of T ” (nr2). The intrinsic resistivity in PPy have shown a weaker dependence than that in PA and PT. This discrepancy between PPy and other conducting polymers might come from the difference in crystallinity among them, since the crystallinity in PPy is lower than that in PA and PI. As for the transition around 23OK, it is observed only in ClO,, BF,- and HSO; doped PPy. The mechanism of the transition is yet to be clarified at present. It may be related to a molecular motion since the transition takes place at such a high temperature. Acknowledgement This work was partially supported by the Instituted of Science and Engineering of Chuo University. References Handbook of Conducting Polymers, edited by T. Skotheim (Dekker, New York, 1986). M. Yamaura, T. Hagiwara and K. Iwata, Synrh. Met., 26 (1988) 209. S. Masubuchi, S. Kazama, K. Mizoguchi, M. Honda, K. Kume, R. Matsushita and T. Matsuyama, Synth. Met.. 55-57 ( 1993) 4962. 4. K. Sato, M. Yamaura, T. Hagiwara, K. Murata and M. Tokumoto, Synth. Met., 40 (1991) 35; M. Reghu, C. 0. Yoon, D. Moses and A. J. Heeger, Synth. Met., 64 (1994) 53. 5. S. Masubuchi, S. Kazama, K. Mizoguchi, F. Shimizu, K. Kume, R. Matsushita and T. Matsuyama, Synth. Met., 55-57 (1993) 4866. 6. D. S. Maddison, R. B. Roberts and J. Unsworth, Synth. Met., 33 (1989) 281. 7. S. Masubuchi, K. Mizoguchi, K. Mizuno, K. Kume and H. Shirakawa, Synth. Met., 22 (1987) 41.
Synthetic
Metals
69 (1995) 345-346
The influence of dopant species on transport properties in as-grown polypyrrole electrochemical
films prepared by
method
S. Masubuchi’, S. Kazama’, R. Matsushita* and T. Matsuyam$ ‘Department of Physics, Chuo University, Bunkyo-ku, Tokyo, 112 Japan 2Research Reactor Institute, Kyoto University, Kumatori, Osaka, 590-04 Japan Abstract A transport study was made on electrochemically prepared polypyrrole (PPy) films to elucidate the influence of dopant species on the electrical transport properties. We present the experimental results on as-grown PPy films prepared with several anion species such as CIO,., PF,~. BF,., TsO- and HSO,. The conductivities of these films at room temperature were between 50-200 S/cm. The temperature dependence exhibited a semiconductor-like behaviour over the whole range between 1.6-300K, with the only exception of PF,- doped PPy, which changed to a metal-like dependence at 8K. At 300K, the absolute thermoelectric power S(T) takes very small values (3.7pViK) like ordinary metals. Characteristic behaviour was observed in their temperature dependence. S(T) is linearly dependent on temperature below about 230K but becomes nearly constant above 230K. The Voltage-Shorted-Compaction (VSC) resistivity (pvs,-) showed a metallic temperature dependence with its slope definitely changed at 230K, corresponding well to the result of S(T), which implies a transition between metallic states of different nature. Introduction Polypyrrole (PPy) is one of the most familiar conducting polymers having a n-conjugated skeleton. Tough and smooth PPy film with high conductivity (>lOOScm) can be easily obtained by the electrochemical polymerization [ 11. On investigating the transport properties in conducting polymers, we always suffer from difficulties inherent to the higher order structure. Resistivity (conductivity) is significantly influenced by complicated morphology of the polymer itself. The transport process in conducting polymers involves various processes such as the transport within crystallized regions (intra- and inter-chain), the one between the segregated crystallized regions, etc. The observed resistance includes contributions from the both regions. The temperature dependence of the resistivity is dominated by the transport process between the crystallized regions through hopping and/or tunneling, resulting in a semiconducting temperature dependence. The intrinsic properties and the different contribution from different kind of dopant are thus obscured in the microscopic measurements. In this work, a series of experiments such as the ordinary fourterminal and VSC resistivity and thermoelectric power were made for as-grown PPy polymerized with several kinds of anion species with an aim to approach the intrinsic nature of PPy. Experimental Platinum plates were used for anode and cathode in a onecomponent cell. Pyrrole was distilled under a reduced pressure and stored in an argon atmosphere. Electrolytes such as Bu,NCIO,, BQNPF,, Et,NBF,, Et,NTsO and Bu,NHS04 were thoroughly degassed by pumping to remove oxygen. Propylene-carbonate was pumped out for one hour before use. A solution of propylene-
0379~6779/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02478-H
carbonate containing 1 ~01%of water dissolving 0.06 M of pyrrole and 0.06 M of an appropriate electrolyte was prepared in the cell under an argon flow. Electrochemical synthesis was carried out with a constant current density of typically 0. I n&cm2 for 40 hours at -20°C according to Yamaura et al. [2]. After the completion of polymerization, residual solvent in the film was rinsed out with acetonitrile. The as-grown film was dried up by pumping for one hour and was stored in an argon atmosphere at -20°C until further use. The conductivity of these films at room temperature were between 50 and 200 S/cm depending on the polymerization condition and anion species. The conductivity was measured with the conventional fourterminal method by applying a direct current typically of lOuA at temperatures between 1.6-300K. The VSC measurements were made by brushing the film with a silver paste as thin as possible. Absolute thermoelectric power S(T) was measured with a temperature difference across the sample being set to be within 0.5K at the measuring temperatures above 10K and within 5% of the temperature below 10K. S(T) was measured between 4.2.300K [3]. Results and Discussion Temperature dependence of resistivity, p,,(T), in PPy doped with ClO,-, BF;, HS04- and TsO- exhibited semiconductor-like behaviour over the whole temperature ranges between 1.6 and 300K. In contrast, p&T) of PF,- doped PPy showed a turnover to the metal-like dependence below 8K. Such a behaviour in p&T) has been reported by several groups particularly in PF,- doped PPy 141. Table I summarizes the conductivity (a) at 280K. The resistance ratio, p,=pDc(4.2K)/p,,-(280K), is also listed in Table 1. A sample with larger cr takes smaller value of pr In spite of the small difference in cr of nearly threefold, pr changes more than two orders of magnitude. The discrepancy in cr and pr have been considered to
346
S. Masubuchi
et al. I Synthetic Metals 69 (1995) 345-346
h
0.9
g
0.8
2
0.7
E
0.6
z
0.5 0.4
I
-2 ]
100
0
I
I
200
300
VW
T(K)
Fig. 1 Temperature
dependence
Table 1 : Conductivities dopant species
of thermoelectric
power.
at 280K for PPy doped with differing
come from the morphology of the polymer, as discussed in our previous papers [5]. The temperature dependence can be interpreted in terms of a metallic transport within the crystallized regions that are interrupted by semiconducting and/or insulating regions. The observed resistance can be represented by the sum of a metallic resistance R,(T) originated from the crystallized regions and a semiconducting one R,(T). The different u and pr would be caused by the difference in the ratio of R,(T) to R,(T), since the degree of polymerization and the crystallinity are affected by the conditions for synthesis. Fig 1 shows the temperature dependence of the absolute thermoelectric power S(T) for PPy doped with the same dopants as in Table.1. The values of S(T) at 280K were between 3-7 pV/K. The behaviour of S(T) can be classified into two types. Type- 1: S(T) is proportional to temperature throughout the entire temperature range, approaching towards -0.5 uV/K as T - OK. This type of behaviour is observed in PF,- and TsO- doped PPy, indicating a metallic character. Type-2: As can be seen typically in PPy doped with ClOq, BF; and HS04 at high temperatures, S(T) changes at a characteristic temperature Tc=210, 230 and 250K, respectively. S(T) is nearly constant above Tc and S(T) exhibits similar dependence as in Type-1 below Tc with the exception of HS04doped PPy at low temperatures. The same observation around Tc has been reported by Maddison et al. in TsO- doped PPy [6]. In HSO, doped PPy, a drastic increase in S(T) was observed below 15K like that of semiconductors. Fig.2 displays the temperature dependence of the VSC resistivity, p&T) (=R(T)/R(280K)), for PPy doped with BF,-, ClO; and HSO,. The data are normalized at 280K. All the sample show a metallic temperature dependence in a wide temperature range. p&T) for both BF; and ClO; doped PPy behaves peculiarly at high temperatures: p “s,-(T) decreases linearly with temperature down to 230K. It changes sharply at 230K and becomes to follow a T”s-law down to temperatures as low as 30K. The observation is in accordance with the results in S(T) and clearly indicates that a transition between metallic states of different nature occurs around
Fig. 2 Temperature dependence normalized at 280K.
of pvs,-(T).
The data are
230K in PPy doped with BFiand ClO,-. For HSOA doped PPy, on the other hand, the temperature dependence of pvs,-(T) is different: pvsc(T) decreases with a decrease in temperature following a T “? law down to 1_5K,without any obvious change at 230K. A turning over around 15K was observed corresponding to the upward change in S(T). The finding indicates that some transition (M-I or M-S etc.) takes place around 15K like in polythiophene [3]. It has been demonstrated that the temperature dependence of the intrinsic resistivity for heavily doped PA [7] and as-grown polythiophene (PI) [5] was definitely metallic at high temperatures following a law of T ” (nr2). The intrinsic resistivity in PPy have shown a weaker dependence than that in PA and PT. This discrepancy between PPy and other conducting polymers might come from the difference in crystallinity among them, since the crystallinity in PPy is lower than that in PA and PI. As for the transition around 23OK, it is observed only in ClO,, BF,- and HSO; doped PPy. The mechanism of the transition is yet to be clarified at present. It may be related to a molecular motion since the transition takes place at such a high temperature. Acknowledgement This work was partially supported by the Instituted of Science and Engineering of Chuo University. References Handbook of Conducting Polymers, edited by T. Skotheim (Dekker, New York, 1986). M. Yamaura, T. Hagiwara and K. Iwata, Synrh. Met., 26 (1988) 209. S. Masubuchi, S. Kazama, K. Mizoguchi, M. Honda, K. Kume, R. Matsushita and T. Matsuyama, Synth. Met.. 55-57 ( 1993) 4962. 4. K. Sato, M. Yamaura, T. Hagiwara, K. Murata and M. Tokumoto, Synth. Met., 40 (1991) 35; M. Reghu, C. 0. Yoon, D. Moses and A. J. Heeger, Synth. Met., 64 (1994) 53. 5. S. Masubuchi, S. Kazama, K. Mizoguchi, F. Shimizu, K. Kume, R. Matsushita and T. Matsuyama, Synth. Met., 55-57 (1993) 4866. 6. D. S. Maddison, R. B. Roberts and J. Unsworth, Synth. Met., 33 (1989) 281. 7. S. Masubuchi, K. Mizoguchi, K. Mizuno, K. Kume and H. Shirakawa, Synth. Met., 22 (1987) 41.