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Electrical, magnetic, and structural properties of chemically and electrochemically synthesized polypyrroles
Synthetic Metals 117 (2001) 45±51
Electrical, magnetic, and structural properties of chemically and electrochemically synthesized polypyrroles J. Jooa,*, J.K. Leea, J.S. Baecka, K.H. Kimb, E.J. Ohc, J. Epsteind a
Department of Physics and Center for Electro and Photo Responsive Molecules, Korea University, Seoul 136-701, South Korea b Department of Chemistry, Yonsei University, Seoul 120-749, South Korea c Department of Chemistry, Myongji University, Yong-In 449-728, South Korea d Department of Physics and Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA
Abstract We report the results of temperature-dependent dc conductivity (sdc(T)), EPR (electron paramagnetic resonance) magnetic susceptibility, and XPS (X-ray photoelectron spectroscopy) experiments for chemically and electrochemically synthesized polypyrrole (PPy) samples. For chemically synthesized dodecylbenzenesulfonic acid (DBSA) or naphthalenesulfonic acid (NSA) doped PPy samples (PPy-DBSA or PPyNSA, respectively) soluble in m-cresol solvent, dc conductivity and its temperature dependence show the strong localization behavior, while those of electrochemically synthesized PPy doped with hexa¯uorophosphate (PPy-PF6) are in the critical regime. Pauli susceptibility (wp) and the density of states are obtained from the temperature dependence of EPR magnetic susceptibility. The density of states of chemically synthesized PPy is one-third of that of electrochemically synthesized PPy. From the analysis of the area ratio of carbon 1s XPS peak, the disorder effect due to interchain links or side chains of PPy-DBSA (m-cresol) samples is 22%, while that of PPy-PF6 samples is 33%. This result indicates that the percent of interchain links or side chains of chemically synthesized PPy-DBSA (m-cresol) samples is reduced by 10% compared to that of electrochemically synthesized PPy-PF6 samples. We analyze that the side chains or interchain links of chemically synthesized PPy samples are relatively reduced due to the synthesis method using large size dopants, and subsequently the interchain interaction weakens. The results of EPR experiments of PPy-DBSA (m-cresol) samples with different doping levels are discussed. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polypyrrole; dc conductivity; Magnetic susceptibility; X-ray photoelectron spectroscopy
1. Introduction The chemical doping in conjugated polymers such as polyacetylene, polyaniline (PAN), or polypyrrole (PPy) induces an insulator-metal transition similar to that of conventional (inorganic) semiconductors [1]. There are fundamental distinctions between two systems. While conventional semiconductors have three-dimensional (3D) structures, the morphological unit of conducting polymers is a quasi one-dimensional conjugated polymer chains, with covalent bonding along the chains and weak bonding between chains. Anisotropic conductivity, dielectric constant, and thermoelectric power have been reported for conducting polymers [1,2]. The dopant ions in conducting polymers are positioned interstitially between polymer chains, while the dopants in conventional semiconductors generally are substituted directly in the host lattice. The interchain hopping with the intrachain charge diffusion has * Corresponding author. Tel.: 82-2-3290-3103; fax: 82-2-927-3292. E-mail address: [email protected] (J. Joo).
been emphasized for the charge transport mechanism in conducting polymers [3±6]. Doped polyacetylene and PAN have been considered as quasi 1D structure, in which the effects of interchain links (connected the nearest polymer backbones) or side chains (connected one polymer backbone) on physical properties have not been reported. However, for polypyrrole, P¯uger and co-workers reported the existence of 33% of interchain links or side chains through 2,3 positions in pyrrole ring based upon X-ray photoelectron spectroscopy (XPS) study of electrochemically synthesized PPy-PF6 samples, while most of pyrrole units are linked at the 2,5 positions to form straight chains [7,8]. It is noted that temperature dependence of dc conductivity (sdc(T)) of highly conducting PPy samples doped with hexa¯uorophosphate (PPy-PF6) is different from that of PAN doped with camphorsulfonic acid (CSA) prepared in m-cresol solvent (PAN-CSA (m-cresol)), although sdc of two systems at room temperature (RT) is in the range of 200±400 S/cm [6,9±11]. The slope of dsdc(T)/ dT of PAN-CSA (m-cresol) samples is negative from RT to 200 K, while that of PPy-PF6 materials is negative below
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J. Joo et al. / Synthetic Metals 117 (2001) 45±51
30 K [6,9±11]. That is, temperature ranges to demonstrate the intrinsic metallic behavior are reversed in two systems. For heavily perchlorate (ClO4) doped stretched oriented polyacetylene, sdc(RT) is 104±105 S/cm, and its intrinsic metallic behavior, i.e. dsdc T=dT < 0 is observed in temperature range from RT to 200 K, which is similar to that of PAN-CSA (m-cresol) samples [12]. The structure and the charge transport of doped PPy have been studied for electrochemically synthesized PPy samples only. Recent progress in chemical synthesis using various dopants and organic solvents has contributed to the development of soluble PPy [13,14]. The synthesis method using relatively large dopants such as dodecylbenzenesulfonic acid (DBSA) or naphthalenesulfonic acid (NSA) reduces the interchain interaction of polymer chains resulting in the increase of the solubility in various organic solvents [13,14]. For chemically synthesized soluble PAN materials, the charge transport properties vary with dopants and solvents used [15]. The interchain interaction and the conformational change of polymer chains are important for the conductivity and its temperature in the PAN systems. In this report, the results of the temperature dependent dc conductivity, the density of states, and XPS experiments are compared for chemically synthesized PPy-DBSA (m-cresol) and PPy-NSA (m-cresol) samples and electrochemically synthesized PPy-PF6 samples. The results of EPR experiments for chemically synthesized PPy-DBSA (m-cresol) samples with different doping levels are discussed. We observe that Pauli susceptibility and the density of states of PPy-DBSA (m-cresol) samples increase as the doping level increases, which indicates that the system becomes more highly conducting. From XPS experiments, the percent of interchain links or side chains of PPy-DBSA (m-cresol) samples is reduced by 10% compared to that of PPy-PF6 samples. The synthesis method using relatively large dopants results in weak interchain interaction, low sdc, and strong localization, re¯ecting the insulating state of soluble PPy systems. However, electrochemically synthesized PPy-PF6 samples with the relatively high concentration of interchain links or side chains show critical or metallic behavior. From the temperature dependence of the magnetic susceptibility, we obtain that the density of states of electrochemically synthesized PPy samples is higher than that of chemically synthesized PPy materials. We conclude that the synthesis method using relatively large dopants for PPy samples plays an important role for controlling the degree of interchain links or side chains, the solubility, and the interchain interaction.
1 0 0). Four thin gold wires (0.05 mm thick and 99% pure gold) were attached in parallel on the sample surface by conducting graphite paint (Acheson Electrodag 502) for better electrical contact. The EPR spectra were obtained by using a Bruker Instruments ESP300 (X-band) electron paramagnetic resonance spectrometer with TE102 resonant cavity. The samples were put into an EPR tube (Wilmad 707), and pumped under 10ÿ5 Torr for 4 h. The magnetic susceptibility of the system was estimated from the EPR integrated intensities calibrated against a Li±LiF crystal. The XPS data were measured using a VG ESCALAB MKII spectrometer (Mg Ka X-ray source, 1253.6 eV photons). All binding energies were referenced by C(1s) neutral carbon peak at 284.6 eV to compensate for surface charging effects. The area ratios corrected by the sensitivity factor were used for quantitative analysis of the XPS data. For synthesis of soluble PPy, distilled pyrrole monomer (0.225 mol) and dopant (0.075 mol of DBSA or NSA) were dissolved in distilled water under magnetic stirring. Fig. 1 shows the chemical structures of DBSA, NSA, and PF6 dopants. For an oxidant, 0.05 mol of (NH4)2S2O8 was dissolved in distilled water. The solution containing oxidant was poured into the solution of pyrrole and dopant under magnetic stirring. Polymerization was carried out for 30 h at 58C in air. PPy powder was ®ltered and washed with methanol, and dried under dynamic vacuum for 48 h. Free standing ®lm were cast from homogeneous solution of doped PPy dissolved in m-cresol solvent. Solubility conditions were determined by dopants and solvent used, etc. However, when small dopant was used, such as sulfosucci-
2. Experimental A four-probe method was used for measuring dc conductivity in order to eliminate the effect of contact resistance. The measured temperature range for sdc was from 300 to 10 K using a Janis closed-cycle refrigerator system (CCS-
Fig. 1. Schematic chemical structure of: (a) one-dimensional polypyrrole (aromatic and quinoid forms) and (b) dodecylbenzenesulfonic acid (DBSA), naphthalenesulfonic acid (NSA), and hexafluorophosphate (PF6) dopants.
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nic acid, the solubility of PPy powder in organic solvents was reduced resulting in dif®culty to make free standing ®lms. Experimentally, PPy powder with relatively large dopants was easily soluble in organic solvents. For synthesis of electrochemical PPy-PF6 samples, 99% pure pyrrole was used [16]. Acetronitrile (AN) solvent was passed into alumina powder at 4008C, and tetraethylammonium hexa¯uorophosphate (TEAPF6) was used for an electrolyte. The anodic oxidation of pyrrole was carried out in an electrolyte bath including TEAPF6 (0.1 mol), pyrrole (0.2 mol), and AN solvent. The materials were synthesized at 1.21 V and for 5 h by using EG & G PAR model 270A potentiostat/galvanostat. The samples were washed by using the mixture of water and AN solvent, and dried in vacuum oven for 72 h. Both chemically and electrochemically prepared samples studied here were in the form of free standing ®lm of 60 and 200 mm thickness, respectively. The mass density of chemically synthesized PPy-DBSA (m-cresol) and electrochemically synthesized PPy-PF6 was 1.2 and 0.9 g/cm3, respectively. 3. Experimental results and discussion Temperature dependence of dc conductivity (sdc(T)) for chemically synthesized PPy-DBSA and PPy-NSA samples soluble in m-cresol or NMP solvents and electrochemically synthesized PPy-PF6 samples is compared in Fig. 2. For soluble PPy samples, the sdc(RT) is 10ÿ1 S/cm. The sdc(RT) of the PPy-PF6 samples studied here is 50 S/ cm, much higher than that of the chemically synthesized PPy samples. The three-dimensional (3D) variable range hopping (VRH) model [17] provides the best ®tting for sdc(T) of chemically synthesized PPy samples: " # T0 1=4 ÿ1=2 exp ÿ : (1) sdc T s0 T T Here T0 16/(kB D(EF) L3), D(EF) is the density of states at the Fermi level, and L is the localization length [17]. The slope T0 is estimated to be 9:1 107 and 3:0 106 K for PPy-DBSA (m-cresol) and PPy-NSA (m-cresol) samples, respectively. From the results of D(EF) obtained from EPR experiments, the localization length L is estimated as 4 and Ê for PPy-NSA (m-cresol) and PPy-DBSA (m-cresol) 2 A samples, respectively, indicating that the charge is strong localized in soluble PPy systems. The interchain links or side chains of PPy systems provide the extra doping centers in addition to the polymer backbones. These might contribute to the 3D VRH of the poorly conducting PPy materials. The charge transport properties of soluble PPy materials also vary with solvents used due to the screen effects of the polar solvents or the interactions between the solvents, the dopants, and the polymer backbones [14]. Fig. 2 (b) shows the sdc(T) of PPy-DBSA and PPy-NSA samples prepared in NMP solvents. The results demonstrate that sdc
Fig. 2. Comparison of temperature dependence of dc conductivity of: (a) PPy-DBSA (m-cresol) and PPy-NSA (m-cresol) samples, (b) PPy-DBSA (NMP) and PPy-NSA (NMP) samples, based on the three-dimensional variable range hopping model and (c) PPy-PF6 materials based on the power law.
and its temperature dependence of soluble PPy samples vary with the solvents and dopants used. A power law provides the best fitting for sdc(T) of electrochemically synthesized PPy-PF6 samples: sdc T / T b ;
(2)
where 1=3 < b < 1. As shown in Fig. 2 (c), the exponent b is estimated as 0.5, which implies that the materials are in the critical regime in the insulator-metal transition, or close to the metallic regimes [9]. Table 1 compares the charge transport properties of chemically and electrochemically synthesized PPy samples studied here. The effects of the dopant size of the electrochemically synthesized PPy samples on sdc(T) were reported by Sato et al. [19]. They observed that the system becomes more highly conducting state as the size of the dopants decreases. One can determine the insulating, critical, or metallic state of the materials from the slope of temperature dependence of the reduced activation energy de®ned as [9,18] d ln sdc T : (3) W T log10 d ln T
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J. Joo et al. / Synthetic Metals 117 (2001) 45±51
Table 1 Comparison of the charge transport properties of electrochemically synthesized PPy-PF6 materials and chemically synthesized PPy-DBSA (m-cresol) and PPy-NSA (m-cresol) samples Materials
sdc(RT)/sdc(100 K) Model Constants
Electrochemically synthesized
Chemically synthesized
PPy-PF6
PPy-DBSA(m-cresol)
2 sdc(T ) / T b b 0.5
Fig. 3 compares the W(T) of the PPy materials studied here. The slope of soluble PPy samples is negative, i.e. W increases as T decreases, which indicates that the systems are in the insulating regime. The slope of the W(T) plot of PPy-PF6 samples is estimated to be 0 or even positive indicating that the systems are in the critical or the metallic regime. Fig. 4 presents w±T of PPy-DBSA (m-cresol), PPy-NSA (m-cresol), and PPy-PF6 samples as a function of temperature. The density of states, D(EF), was obtained from the measurement of susceptibility (w) through EPR experiments. Considering w wp wc where wp (Pauli susceptibility) is independent of temperature and wc (Curie susceptibility) / I/ T, the slope of the straight line of T provides the wp. Using the relation wp 2mB 2 D(EF) where mB is the Bohr magneton, and the density of states, D(EF), is estimated as 0.072 states/(eV ring), 0.107 states/(eV ring), and 0.333 states/ (eV ring) for PPy-DBSA (m-cresol), PPy-NSA (m-cresol), and PPy-PF6 samples, respectively. The results imply that the electrochemically synthesized PPy-PF6 with its relatively small dopant is in highly conducting states compared to that of the doped soluble PPy materials. It is noted that Curie constant of wc obtained from the y-intercept in Fig. 4 is 9:77 10ÿ4 , 9:72 10ÿ4 , and 1:74 10ÿ4 emu K/mol ring for PPy-PF6, PPy-NSA (m-cresol), and PPy-DBSA (m-cresol) samples, respectively.
3
10 sdc(T ) / T ÿ1/2 exp[(To/T )1/4] Ê T0 9.1 107 K, L 2 A
PPy-NSA(m-cresol) 2 101 sdc(T ) / T ÿ1/2 exp[(To/T )1/4] Ê T0 3.0 106 K, L 4 A
The density of states D(EF) of soluble PPy systems varies with the doping levels. Fig. 5 shows the variation of the D(EF) and the wp of PPy-DBSA (m-cresol) samples as a function of doping levels. The concentration of dopants of the samples is 0.075, 0.057, 0.038 and 0.028 mol with 0.225 mol pyrrole monomer. The D(EF) and the wp increases as the doping level increases. The results demonstrate that the system becomes more highly conducting with increasing doping level. This is similar to that of other conducting polymers such as doped polyacetylene and polyaniline [1,2]. It is expected that the conductivity of the system increases with increasing doing level, because conductivity is controlled by the concentration of conduction electrons and mobility. Fig. 6 shows the carbon 1s (C1s) XPS core level spectra of electrochemically synthesized PPy-PF6 and chemically synthesized PPy-DBSA (m-cresol) samples at RT. A standard line shape analysis with Gaussian ®tting shows that the C1s main peak for PPy-PF6 samples is decomposed into three lines as shown in Fig. 6 (a). The line due to the pyrrole b carbons is centered at 284.33 eV, while that of the a carbons is positioned at 285.27 eV. There is 0.94 eV energy splitting between two lines, which agrees with the observed energy separation in pyrrole monomer [7,20]. The C1s spectrum is not symmetric on the higher binding energy
Fig. 3. Temperature dependence of reduced activation energy W(T) of PPy-DBSA (m-cresol) and PPy-PF6 samples.
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Fig. 4. w±T vs. T for PPy-DBSA (m-cresol) (solid diamonds), PPy-NSA (m-cresol) (solid triangles), and PPy-PF6 (circles).
side of the peak. This asymmetry was assigned to ``disorder effects'' such as interchain links, side chains, or chain terminations [7,8]. The measured area ratio of the disorder effects to the sum of three peaks assigned to a, b sites and the disorder effects is 0.33, in agreement with the report of P¯uger and Street [7]. For the chemically synthesized PPyDBSA (m-cresol), the C1s main peak is decomposed into four lines including the covalent bonded carbon in the DBSA dopant, which corresponds to the lowest binding energy centered at 284.66 eV, as shown in Fig. 6 (b). The line due to the pyrrole b carbons is centered at 285.47 eV, while that of the a carbons is positioned at 286.37 eV. The overall shift of the peaks of the a, b carbons and the disorder effects as compared to those of electrochemically prepared PPy samples originate from the effects of charging. However, the 0.9 eV energy splitting between the a and the b carbons is maintained here. The measured area ratio of the disorder effects assumed due to side chains, interchain links, or chain
Fig. 5. Comparison of the density of states (D(EF)) (left-hand scale) and the Pauli spin susceptibility (wp) (right-hand scale) as a function of doping level for soluble PPy-DBSA (m-cresol) samples.
terminations is 22%, which is a one-third reduction as compared to that of electrochemically synthesized PPy-PF6 samples. The XPS results support the reduction of the 2,3 coupling modes mainly due to the synthesis method using large dopants and a resulting relatively weak bonding between the chains. Based on the analysis of the area ratio of phosphorous (or sulfur) to nitrogen, we obtain the existence of one dopant per three pyrrole rings. The error in the XPS curve ®tting is 5%. Fig. 7 compares the nitrogen 1s (N1s) XPS core level spectra of PPy materials at RT. The N1s signal originates
Fig. 6. Carbon 1s XPS core level spectrum of: (a) PPy-PF6 and (b) PPyDBSA (m-cresol) samples. The top line through the data represents the best fit obtained from the superposition of the Gaussian peaks of the lower lines.
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J. Joo et al. / Synthetic Metals 117 (2001) 45±51
Fig. 7. Nitrogen 1s XPS core level spectrum of: (a) PPy-PF6 and (b) PPyDBSA (m-cresol) samples.
from the nitrogen atoms of the pyrrole rings of the polymer backbone because of the lack of nitrogen in the dopants used. The spectrum of the N1s of electrochemically synthesized PPy-PF6 samples is asymmetric, while that of chemically synthesized PPy-DBSA (m-cresol) is relatively symmetric, as shown in Fig. 7. For PPy-PF6 samples, the higher binding energy peak is separated from the neutral nitrogen peak by 2.53 eV as shown in Fig. 7 (a). This shoulder on the higher energy side is assigned to an electrostatic effect of the nearest PF6ÿ counter ion [7]. This electrostatic effect is not observed for chemically synthesized PPy-DBSA (m-cresol) samples. Comparing the size of the counter ions, the electric ®eld due to the large size DBSAÿ counter ion may be negligible. This implies that the interchain interaction of soluble PPy systems with the DBSA dopants is relatively weak compared to that of electrochemically synthesized PPy-PF6 samples. Experimentally, the effects of interchain links or side chains on physical properties have not been reported for doped polyacetylene and PAN, which have been considered as quasi 1D structures. The chemically and electrochemi-
Fig. 8. Schematic chemical structure of interchain links or side chains through 2,3 coupling modes of polypyrrole. Inset: pyrrole monomer with the conventional labeling of atom positions shown.
J. Joo et al. / Synthetic Metals 117 (2001) 45±51
cally synthesized PPy systems can be considered as quasi network systems through the 2,3 coupling modes as shown in Fig. 8, which are supported by the 3D VRH for sdc(T) and the disordered effect obtained from XPS experiments. The 2,3 coupling modes might increase the mechanical strength, and provide extra doping centers, which induce the 3D VRH and relatively high density of states for poorly conducting soluble PPy samples. The interchain links or side chains through the 2,3 coupling modes induce the reduction of the solubility of PPy in organic solvents and the variation of the conjugation length. Theoretical study by Prigodin and coworkers for a random network model of metallic wires demonstrated the insulator-metal transition which can be determined by the concentration of the junctions such as interchain links or side chains in conducting polymers [21,22]. For soluble PPy systems, the reduction of the interchain links or side chains encourages charge localization. 4. Conclusion Electrical, magnetic, and structural properties for chemically and electrochemically synthesized PPy samples are compared. From the results of sdc(T), the density of states, and XPS experiments, chemically synthesized PPy systems are in an insulating state associated with the reduction of the interchain links or side chains and the weaker interchain interaction. We observe that the density of states of PPyDBSA (m-cresol) samples increases as the doping level increases. The electrochemically synthesized PPy-PF6 materials have higher concentration of interchain links or side chains, and are in the critical or the metallic regime. The density of states of electrochemically prepared PPy-PF6 samples is higher than that of soluble PPy materials. The synthesis method using relatively large dopants for soluble PPy samples controls the solubility, the structure, and the charge transport mechanism.
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Acknowledgements This work was supported in part by the CRM-KOSEF (1998). References [1] J.E. Mark (Ed.), Physical Properties of Polymers Handbook, Chapter 34, AIP Press, New York, 1996. [2] H.S. Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers, Vol. 1, 4, Wiley, New York, 1996. [3] Z.H. Wang, C. Li, A.J. Epstein, E.M. Scherr, A.G. MacDiarmid, Phys. Rev. Lett. 66 (1991) 1745. [4] E.M. Conwell, H.A. Mizes, Synth. Met. 65 (1994) 203. [5] M.I. Salkola, S.A. Kivelson, Phys. Rev. B 50 (1994) 13962. [6] J. Joo, Z. Oblakowski, G. Du, J.P. Pouget, E.J. Oh, J.M. Weisinger, Y. Min, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 49 (1994) 2977. [7] P. Pfluger, G.B. Street, J. Chem. Phys. 80 (1984) 544. [8] M.G. Kanatzidis Chem. Engi. News, December 3 (1990) 36. [9] M. Reghu, Y.O. Yoon, D. Moses, A.J. Heeger, Y. Cao, Phys. Rev. B 48 (1993) 17685. [10] R.S. Kohlman, J. Joo, Y.Z. Wang, J.P. Pouget, H. Kaneko, T. Ishiguro, A.J. Epstein, Phys. Rev. Lett. 74 (1995) 773. [11] T. Ishiguro, H. Kaneko, Y. Nogami, H. Ishimoto, H. Nishiyama, J. Tsukamoto, A. Takahashi, M. Yamaura, T. Hagiwara, K. Sato, Phys. Rev. Lett. 69 (1992) 660. [12] Y.W. Park, E.S. Choi, D.S. Suh, Synth. Met. 96 (1998) 81. [13] J.Y. Lee, D.Y. Kim, C.Y. Kim, Synth. Met. 74 (1995) 103. [14] J. Joo, J.K. Lee, J.K. Hong, J.S. Baeck, W.P. Lee, A.J. Epstein, K.S. Jang, J.S. Suh, E.J. Oh, Macromolecules 31 (1998) 479. [15] J. Joo, H.G. Song, Y.C. Chung, J.S. Baeck, S.K. Jeong, J.S. Suh, E.J. Oh, J. Kor. Phys. Soc. 30 (1997) 230. [16] J.K. Lee, H.G. Song, J. Joo, G.J. Lee, C.H. Yo, K.H. Kim, S.K. Song, K.S. Jang, E.J. Oh, Sae Mulli (New Phys.) 38 (1998) 16. [17] N.F. Mott, E. Davis, Electronic Processes in Non-Crystalline Materials, Clarendon Press, Oxford, 1979. [18] A.G. Zabrodskii, K.N. Zinoveva, Sov. Phys. JETP 59 (1984) 425. [19] K. Sato, M. Yamaura, T. Hagiwara, K. Murata, M. Tokumoto, Synth. Met. 40 (1991) 35. [20] L. Atanasoska, K. Nao, W.H. Smyrl, Chem. Mater. 4 (1992) 988. [21] V.N. Prigodin, K.B. Efetov, Phys. Rev. Lett. 70 (1993) 2932. [22] A.N. Samukhin, V.N. Prigodin, L. Jastrabik, A.J. Epstein, Phys. Rev. B 58 (1998) 11354.
Electrical, magnetic, and structural properties of chemically and electrochemically synthesized polypyrroles J. Jooa,*, J.K. Leea, J.S. Baecka, K.H. Kimb, E.J. Ohc, J. Epsteind a
Department of Physics and Center for Electro and Photo Responsive Molecules, Korea University, Seoul 136-701, South Korea b Department of Chemistry, Yonsei University, Seoul 120-749, South Korea c Department of Chemistry, Myongji University, Yong-In 449-728, South Korea d Department of Physics and Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA
Abstract We report the results of temperature-dependent dc conductivity (sdc(T)), EPR (electron paramagnetic resonance) magnetic susceptibility, and XPS (X-ray photoelectron spectroscopy) experiments for chemically and electrochemically synthesized polypyrrole (PPy) samples. For chemically synthesized dodecylbenzenesulfonic acid (DBSA) or naphthalenesulfonic acid (NSA) doped PPy samples (PPy-DBSA or PPyNSA, respectively) soluble in m-cresol solvent, dc conductivity and its temperature dependence show the strong localization behavior, while those of electrochemically synthesized PPy doped with hexa¯uorophosphate (PPy-PF6) are in the critical regime. Pauli susceptibility (wp) and the density of states are obtained from the temperature dependence of EPR magnetic susceptibility. The density of states of chemically synthesized PPy is one-third of that of electrochemically synthesized PPy. From the analysis of the area ratio of carbon 1s XPS peak, the disorder effect due to interchain links or side chains of PPy-DBSA (m-cresol) samples is 22%, while that of PPy-PF6 samples is 33%. This result indicates that the percent of interchain links or side chains of chemically synthesized PPy-DBSA (m-cresol) samples is reduced by 10% compared to that of electrochemically synthesized PPy-PF6 samples. We analyze that the side chains or interchain links of chemically synthesized PPy samples are relatively reduced due to the synthesis method using large size dopants, and subsequently the interchain interaction weakens. The results of EPR experiments of PPy-DBSA (m-cresol) samples with different doping levels are discussed. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Polypyrrole; dc conductivity; Magnetic susceptibility; X-ray photoelectron spectroscopy
1. Introduction The chemical doping in conjugated polymers such as polyacetylene, polyaniline (PAN), or polypyrrole (PPy) induces an insulator-metal transition similar to that of conventional (inorganic) semiconductors [1]. There are fundamental distinctions between two systems. While conventional semiconductors have three-dimensional (3D) structures, the morphological unit of conducting polymers is a quasi one-dimensional conjugated polymer chains, with covalent bonding along the chains and weak bonding between chains. Anisotropic conductivity, dielectric constant, and thermoelectric power have been reported for conducting polymers [1,2]. The dopant ions in conducting polymers are positioned interstitially between polymer chains, while the dopants in conventional semiconductors generally are substituted directly in the host lattice. The interchain hopping with the intrachain charge diffusion has * Corresponding author. Tel.: 82-2-3290-3103; fax: 82-2-927-3292. E-mail address: [email protected] (J. Joo).
been emphasized for the charge transport mechanism in conducting polymers [3±6]. Doped polyacetylene and PAN have been considered as quasi 1D structure, in which the effects of interchain links (connected the nearest polymer backbones) or side chains (connected one polymer backbone) on physical properties have not been reported. However, for polypyrrole, P¯uger and co-workers reported the existence of 33% of interchain links or side chains through 2,3 positions in pyrrole ring based upon X-ray photoelectron spectroscopy (XPS) study of electrochemically synthesized PPy-PF6 samples, while most of pyrrole units are linked at the 2,5 positions to form straight chains [7,8]. It is noted that temperature dependence of dc conductivity (sdc(T)) of highly conducting PPy samples doped with hexa¯uorophosphate (PPy-PF6) is different from that of PAN doped with camphorsulfonic acid (CSA) prepared in m-cresol solvent (PAN-CSA (m-cresol)), although sdc of two systems at room temperature (RT) is in the range of 200±400 S/cm [6,9±11]. The slope of dsdc(T)/ dT of PAN-CSA (m-cresol) samples is negative from RT to 200 K, while that of PPy-PF6 materials is negative below
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30 K [6,9±11]. That is, temperature ranges to demonstrate the intrinsic metallic behavior are reversed in two systems. For heavily perchlorate (ClO4) doped stretched oriented polyacetylene, sdc(RT) is 104±105 S/cm, and its intrinsic metallic behavior, i.e. dsdc T=dT < 0 is observed in temperature range from RT to 200 K, which is similar to that of PAN-CSA (m-cresol) samples [12]. The structure and the charge transport of doped PPy have been studied for electrochemically synthesized PPy samples only. Recent progress in chemical synthesis using various dopants and organic solvents has contributed to the development of soluble PPy [13,14]. The synthesis method using relatively large dopants such as dodecylbenzenesulfonic acid (DBSA) or naphthalenesulfonic acid (NSA) reduces the interchain interaction of polymer chains resulting in the increase of the solubility in various organic solvents [13,14]. For chemically synthesized soluble PAN materials, the charge transport properties vary with dopants and solvents used [15]. The interchain interaction and the conformational change of polymer chains are important for the conductivity and its temperature in the PAN systems. In this report, the results of the temperature dependent dc conductivity, the density of states, and XPS experiments are compared for chemically synthesized PPy-DBSA (m-cresol) and PPy-NSA (m-cresol) samples and electrochemically synthesized PPy-PF6 samples. The results of EPR experiments for chemically synthesized PPy-DBSA (m-cresol) samples with different doping levels are discussed. We observe that Pauli susceptibility and the density of states of PPy-DBSA (m-cresol) samples increase as the doping level increases, which indicates that the system becomes more highly conducting. From XPS experiments, the percent of interchain links or side chains of PPy-DBSA (m-cresol) samples is reduced by 10% compared to that of PPy-PF6 samples. The synthesis method using relatively large dopants results in weak interchain interaction, low sdc, and strong localization, re¯ecting the insulating state of soluble PPy systems. However, electrochemically synthesized PPy-PF6 samples with the relatively high concentration of interchain links or side chains show critical or metallic behavior. From the temperature dependence of the magnetic susceptibility, we obtain that the density of states of electrochemically synthesized PPy samples is higher than that of chemically synthesized PPy materials. We conclude that the synthesis method using relatively large dopants for PPy samples plays an important role for controlling the degree of interchain links or side chains, the solubility, and the interchain interaction.
1 0 0). Four thin gold wires (0.05 mm thick and 99% pure gold) were attached in parallel on the sample surface by conducting graphite paint (Acheson Electrodag 502) for better electrical contact. The EPR spectra were obtained by using a Bruker Instruments ESP300 (X-band) electron paramagnetic resonance spectrometer with TE102 resonant cavity. The samples were put into an EPR tube (Wilmad 707), and pumped under 10ÿ5 Torr for 4 h. The magnetic susceptibility of the system was estimated from the EPR integrated intensities calibrated against a Li±LiF crystal. The XPS data were measured using a VG ESCALAB MKII spectrometer (Mg Ka X-ray source, 1253.6 eV photons). All binding energies were referenced by C(1s) neutral carbon peak at 284.6 eV to compensate for surface charging effects. The area ratios corrected by the sensitivity factor were used for quantitative analysis of the XPS data. For synthesis of soluble PPy, distilled pyrrole monomer (0.225 mol) and dopant (0.075 mol of DBSA or NSA) were dissolved in distilled water under magnetic stirring. Fig. 1 shows the chemical structures of DBSA, NSA, and PF6 dopants. For an oxidant, 0.05 mol of (NH4)2S2O8 was dissolved in distilled water. The solution containing oxidant was poured into the solution of pyrrole and dopant under magnetic stirring. Polymerization was carried out for 30 h at 58C in air. PPy powder was ®ltered and washed with methanol, and dried under dynamic vacuum for 48 h. Free standing ®lm were cast from homogeneous solution of doped PPy dissolved in m-cresol solvent. Solubility conditions were determined by dopants and solvent used, etc. However, when small dopant was used, such as sulfosucci-
2. Experimental A four-probe method was used for measuring dc conductivity in order to eliminate the effect of contact resistance. The measured temperature range for sdc was from 300 to 10 K using a Janis closed-cycle refrigerator system (CCS-
Fig. 1. Schematic chemical structure of: (a) one-dimensional polypyrrole (aromatic and quinoid forms) and (b) dodecylbenzenesulfonic acid (DBSA), naphthalenesulfonic acid (NSA), and hexafluorophosphate (PF6) dopants.
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47
nic acid, the solubility of PPy powder in organic solvents was reduced resulting in dif®culty to make free standing ®lms. Experimentally, PPy powder with relatively large dopants was easily soluble in organic solvents. For synthesis of electrochemical PPy-PF6 samples, 99% pure pyrrole was used [16]. Acetronitrile (AN) solvent was passed into alumina powder at 4008C, and tetraethylammonium hexa¯uorophosphate (TEAPF6) was used for an electrolyte. The anodic oxidation of pyrrole was carried out in an electrolyte bath including TEAPF6 (0.1 mol), pyrrole (0.2 mol), and AN solvent. The materials were synthesized at 1.21 V and for 5 h by using EG & G PAR model 270A potentiostat/galvanostat. The samples were washed by using the mixture of water and AN solvent, and dried in vacuum oven for 72 h. Both chemically and electrochemically prepared samples studied here were in the form of free standing ®lm of 60 and 200 mm thickness, respectively. The mass density of chemically synthesized PPy-DBSA (m-cresol) and electrochemically synthesized PPy-PF6 was 1.2 and 0.9 g/cm3, respectively. 3. Experimental results and discussion Temperature dependence of dc conductivity (sdc(T)) for chemically synthesized PPy-DBSA and PPy-NSA samples soluble in m-cresol or NMP solvents and electrochemically synthesized PPy-PF6 samples is compared in Fig. 2. For soluble PPy samples, the sdc(RT) is 10ÿ1 S/cm. The sdc(RT) of the PPy-PF6 samples studied here is 50 S/ cm, much higher than that of the chemically synthesized PPy samples. The three-dimensional (3D) variable range hopping (VRH) model [17] provides the best ®tting for sdc(T) of chemically synthesized PPy samples: " # T0 1=4 ÿ1=2 exp ÿ : (1) sdc T s0 T T Here T0 16/(kB D(EF) L3), D(EF) is the density of states at the Fermi level, and L is the localization length [17]. The slope T0 is estimated to be 9:1 107 and 3:0 106 K for PPy-DBSA (m-cresol) and PPy-NSA (m-cresol) samples, respectively. From the results of D(EF) obtained from EPR experiments, the localization length L is estimated as 4 and Ê for PPy-NSA (m-cresol) and PPy-DBSA (m-cresol) 2 A samples, respectively, indicating that the charge is strong localized in soluble PPy systems. The interchain links or side chains of PPy systems provide the extra doping centers in addition to the polymer backbones. These might contribute to the 3D VRH of the poorly conducting PPy materials. The charge transport properties of soluble PPy materials also vary with solvents used due to the screen effects of the polar solvents or the interactions between the solvents, the dopants, and the polymer backbones [14]. Fig. 2 (b) shows the sdc(T) of PPy-DBSA and PPy-NSA samples prepared in NMP solvents. The results demonstrate that sdc
Fig. 2. Comparison of temperature dependence of dc conductivity of: (a) PPy-DBSA (m-cresol) and PPy-NSA (m-cresol) samples, (b) PPy-DBSA (NMP) and PPy-NSA (NMP) samples, based on the three-dimensional variable range hopping model and (c) PPy-PF6 materials based on the power law.
and its temperature dependence of soluble PPy samples vary with the solvents and dopants used. A power law provides the best fitting for sdc(T) of electrochemically synthesized PPy-PF6 samples: sdc T / T b ;
(2)
where 1=3 < b < 1. As shown in Fig. 2 (c), the exponent b is estimated as 0.5, which implies that the materials are in the critical regime in the insulator-metal transition, or close to the metallic regimes [9]. Table 1 compares the charge transport properties of chemically and electrochemically synthesized PPy samples studied here. The effects of the dopant size of the electrochemically synthesized PPy samples on sdc(T) were reported by Sato et al. [19]. They observed that the system becomes more highly conducting state as the size of the dopants decreases. One can determine the insulating, critical, or metallic state of the materials from the slope of temperature dependence of the reduced activation energy de®ned as [9,18] d ln sdc T : (3) W T log10 d ln T
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J. Joo et al. / Synthetic Metals 117 (2001) 45±51
Table 1 Comparison of the charge transport properties of electrochemically synthesized PPy-PF6 materials and chemically synthesized PPy-DBSA (m-cresol) and PPy-NSA (m-cresol) samples Materials
sdc(RT)/sdc(100 K) Model Constants
Electrochemically synthesized
Chemically synthesized
PPy-PF6
PPy-DBSA(m-cresol)
2 sdc(T ) / T b b 0.5
Fig. 3 compares the W(T) of the PPy materials studied here. The slope of soluble PPy samples is negative, i.e. W increases as T decreases, which indicates that the systems are in the insulating regime. The slope of the W(T) plot of PPy-PF6 samples is estimated to be 0 or even positive indicating that the systems are in the critical or the metallic regime. Fig. 4 presents w±T of PPy-DBSA (m-cresol), PPy-NSA (m-cresol), and PPy-PF6 samples as a function of temperature. The density of states, D(EF), was obtained from the measurement of susceptibility (w) through EPR experiments. Considering w wp wc where wp (Pauli susceptibility) is independent of temperature and wc (Curie susceptibility) / I/ T, the slope of the straight line of T provides the wp. Using the relation wp 2mB 2 D(EF) where mB is the Bohr magneton, and the density of states, D(EF), is estimated as 0.072 states/(eV ring), 0.107 states/(eV ring), and 0.333 states/ (eV ring) for PPy-DBSA (m-cresol), PPy-NSA (m-cresol), and PPy-PF6 samples, respectively. The results imply that the electrochemically synthesized PPy-PF6 with its relatively small dopant is in highly conducting states compared to that of the doped soluble PPy materials. It is noted that Curie constant of wc obtained from the y-intercept in Fig. 4 is 9:77 10ÿ4 , 9:72 10ÿ4 , and 1:74 10ÿ4 emu K/mol ring for PPy-PF6, PPy-NSA (m-cresol), and PPy-DBSA (m-cresol) samples, respectively.
3
10 sdc(T ) / T ÿ1/2 exp[(To/T )1/4] Ê T0 9.1 107 K, L 2 A
PPy-NSA(m-cresol) 2 101 sdc(T ) / T ÿ1/2 exp[(To/T )1/4] Ê T0 3.0 106 K, L 4 A
The density of states D(EF) of soluble PPy systems varies with the doping levels. Fig. 5 shows the variation of the D(EF) and the wp of PPy-DBSA (m-cresol) samples as a function of doping levels. The concentration of dopants of the samples is 0.075, 0.057, 0.038 and 0.028 mol with 0.225 mol pyrrole monomer. The D(EF) and the wp increases as the doping level increases. The results demonstrate that the system becomes more highly conducting with increasing doping level. This is similar to that of other conducting polymers such as doped polyacetylene and polyaniline [1,2]. It is expected that the conductivity of the system increases with increasing doing level, because conductivity is controlled by the concentration of conduction electrons and mobility. Fig. 6 shows the carbon 1s (C1s) XPS core level spectra of electrochemically synthesized PPy-PF6 and chemically synthesized PPy-DBSA (m-cresol) samples at RT. A standard line shape analysis with Gaussian ®tting shows that the C1s main peak for PPy-PF6 samples is decomposed into three lines as shown in Fig. 6 (a). The line due to the pyrrole b carbons is centered at 284.33 eV, while that of the a carbons is positioned at 285.27 eV. There is 0.94 eV energy splitting between two lines, which agrees with the observed energy separation in pyrrole monomer [7,20]. The C1s spectrum is not symmetric on the higher binding energy
Fig. 3. Temperature dependence of reduced activation energy W(T) of PPy-DBSA (m-cresol) and PPy-PF6 samples.
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Fig. 4. w±T vs. T for PPy-DBSA (m-cresol) (solid diamonds), PPy-NSA (m-cresol) (solid triangles), and PPy-PF6 (circles).
side of the peak. This asymmetry was assigned to ``disorder effects'' such as interchain links, side chains, or chain terminations [7,8]. The measured area ratio of the disorder effects to the sum of three peaks assigned to a, b sites and the disorder effects is 0.33, in agreement with the report of P¯uger and Street [7]. For the chemically synthesized PPyDBSA (m-cresol), the C1s main peak is decomposed into four lines including the covalent bonded carbon in the DBSA dopant, which corresponds to the lowest binding energy centered at 284.66 eV, as shown in Fig. 6 (b). The line due to the pyrrole b carbons is centered at 285.47 eV, while that of the a carbons is positioned at 286.37 eV. The overall shift of the peaks of the a, b carbons and the disorder effects as compared to those of electrochemically prepared PPy samples originate from the effects of charging. However, the 0.9 eV energy splitting between the a and the b carbons is maintained here. The measured area ratio of the disorder effects assumed due to side chains, interchain links, or chain
Fig. 5. Comparison of the density of states (D(EF)) (left-hand scale) and the Pauli spin susceptibility (wp) (right-hand scale) as a function of doping level for soluble PPy-DBSA (m-cresol) samples.
terminations is 22%, which is a one-third reduction as compared to that of electrochemically synthesized PPy-PF6 samples. The XPS results support the reduction of the 2,3 coupling modes mainly due to the synthesis method using large dopants and a resulting relatively weak bonding between the chains. Based on the analysis of the area ratio of phosphorous (or sulfur) to nitrogen, we obtain the existence of one dopant per three pyrrole rings. The error in the XPS curve ®tting is 5%. Fig. 7 compares the nitrogen 1s (N1s) XPS core level spectra of PPy materials at RT. The N1s signal originates
Fig. 6. Carbon 1s XPS core level spectrum of: (a) PPy-PF6 and (b) PPyDBSA (m-cresol) samples. The top line through the data represents the best fit obtained from the superposition of the Gaussian peaks of the lower lines.
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J. Joo et al. / Synthetic Metals 117 (2001) 45±51
Fig. 7. Nitrogen 1s XPS core level spectrum of: (a) PPy-PF6 and (b) PPyDBSA (m-cresol) samples.
from the nitrogen atoms of the pyrrole rings of the polymer backbone because of the lack of nitrogen in the dopants used. The spectrum of the N1s of electrochemically synthesized PPy-PF6 samples is asymmetric, while that of chemically synthesized PPy-DBSA (m-cresol) is relatively symmetric, as shown in Fig. 7. For PPy-PF6 samples, the higher binding energy peak is separated from the neutral nitrogen peak by 2.53 eV as shown in Fig. 7 (a). This shoulder on the higher energy side is assigned to an electrostatic effect of the nearest PF6ÿ counter ion [7]. This electrostatic effect is not observed for chemically synthesized PPy-DBSA (m-cresol) samples. Comparing the size of the counter ions, the electric ®eld due to the large size DBSAÿ counter ion may be negligible. This implies that the interchain interaction of soluble PPy systems with the DBSA dopants is relatively weak compared to that of electrochemically synthesized PPy-PF6 samples. Experimentally, the effects of interchain links or side chains on physical properties have not been reported for doped polyacetylene and PAN, which have been considered as quasi 1D structures. The chemically and electrochemi-
Fig. 8. Schematic chemical structure of interchain links or side chains through 2,3 coupling modes of polypyrrole. Inset: pyrrole monomer with the conventional labeling of atom positions shown.
J. Joo et al. / Synthetic Metals 117 (2001) 45±51
cally synthesized PPy systems can be considered as quasi network systems through the 2,3 coupling modes as shown in Fig. 8, which are supported by the 3D VRH for sdc(T) and the disordered effect obtained from XPS experiments. The 2,3 coupling modes might increase the mechanical strength, and provide extra doping centers, which induce the 3D VRH and relatively high density of states for poorly conducting soluble PPy samples. The interchain links or side chains through the 2,3 coupling modes induce the reduction of the solubility of PPy in organic solvents and the variation of the conjugation length. Theoretical study by Prigodin and coworkers for a random network model of metallic wires demonstrated the insulator-metal transition which can be determined by the concentration of the junctions such as interchain links or side chains in conducting polymers [21,22]. For soluble PPy systems, the reduction of the interchain links or side chains encourages charge localization. 4. Conclusion Electrical, magnetic, and structural properties for chemically and electrochemically synthesized PPy samples are compared. From the results of sdc(T), the density of states, and XPS experiments, chemically synthesized PPy systems are in an insulating state associated with the reduction of the interchain links or side chains and the weaker interchain interaction. We observe that the density of states of PPyDBSA (m-cresol) samples increases as the doping level increases. The electrochemically synthesized PPy-PF6 materials have higher concentration of interchain links or side chains, and are in the critical or the metallic regime. The density of states of electrochemically prepared PPy-PF6 samples is higher than that of soluble PPy materials. The synthesis method using relatively large dopants for soluble PPy samples controls the solubility, the structure, and the charge transport mechanism.
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