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Facile charge carrier adjustment for improving thermopower of doped polyaniline
Polymer 54 (2013) 1136e1140
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Polymer journal homepage: www.elsevier.com/locate/polymer
Facile charge carrier adjustment for improving thermopower of doped polyaniline Hong Wang, Liang Yin, Xiong Pu, Choongho Yu* Department of Mechanical Engineering, Materials Science and Engineering Program, Texas A&M University, College Station, TX 77843, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2012 Received in revised form 15 December 2012 Accepted 16 December 2012 Available online 21 December 2012
We report a facile method of improving thermopower of doped polyaniline (PANI). It is believed that the electrical properties were altered by controlling the concentration of charge carriers with camphor sulfonic acid (CSA) and ammonium hydroxide. With CSA doping followed by drying and annealing at temperatures below w100 C, the electrical conductivity was increased up to w5700 S/m with a thermopower value, w14 mV/K at room temperature. The power factor reached w1 mW/m-K2 after optimizing the drying and annealing temperature, which is 5e1000 times larger than those of other doped PANI without stretching. This thermopower of PANIeCSA films was monotonically improved in proportion to the time period of the ammonium hydroxide reaction, resulting in w115 mV/K at room temperature, which is 1.2e10 times higher than those of other doped PANI. The intensity changes of Fourier transform infrared spectra before and after the de-doping process with ammonium hydroxide indicates the thermopower increase is likely to be from the reduction in the charge carrier concentration. This method is a facile and convenient way of adjusting carrier concentrations, which are crucial to find the optimum doping level for high performance thermoelectrics. We believe further studies to improve crystallinity of doped PANI may dramatically raise the electrical conductivity in addition to the large thermopower, so as to obtain a high performance PANI-containing polymer composite for thermoelectric applications. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Thermopower Polyaniline Charge carrier
1. Introduction Thermoelectric (TE) devices are of great interest for direct conversion of heat to electricity or vice versa with simple leg-type structures without mechanically moving components or hazardous working fluids. Their applications span various fields including energy recovery from waste heat and solid state cooling [1]. Despite of the great advantages of TE devices, the relatively low efficiency has been a hurdle for wide use, ZT ¼ S2sT/k, where S, s, T, and k are thermopower (or the Seebeck coefficient), electrical conductivity, absolute temperature, and thermal conductivity, respectively. While the strong correlation between these parameters is the main reason for the relatively low efficiency, it has been demonstrated that low thermal conductivity is achievable by scattering phonons without significantly hampering electrical transport [2]. Compared to recent intense efforts to suppress phonon transport, systematic studies to achieve high S and s are relatively lacking despite the
* Corresponding author. Tel.: þ1 979 862 1073; fax: þ1 979 845 3081. E-mail address: [email protected] (C. Yu). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.12.038
importance of the electrical properties. For instance, an increase of thermopower may lead to a large ZT due to the square in the numerator of ZT (i.e., the power factor S2s). In particular, it is very important to improve thermopower of organic materials (e.g., polymers) and their composites since this type of materials typically have low thermal conductivity (i.e., not much room to suppress k further). Conducting polymers have great potentials as TE materials because of decent electrical conductivity with low thermal conductivity. Furthermore, polymers have additional advantages, compared with inorganic materials, such as low density and easy synthesis process with a low cost [3e9]. It was reported that the electrical conductivity of air-stable conducting polymers can reach w104 S/m (e.g., poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)). This value is still one order of magnitude lower than those of the current state-of-the-art TE materials containing Bi, Te, and Sb. This electrical conductivity can be further improved to w105 S/m with conducting additives like carbon nanotubes [3,4,8,10]. Nevertheless, it has been rarely studied to control thermopower of polymers. In particular, typical thermopower of doped polymers is very low, compared to those of inorganic TE
H. Wang et al. / Polymer 54 (2013) 1136e1140
semiconductors. For example, polyaniline (PANI) has been studied for thermoelectric applications, but the thermopower of doped PANI without stretching lies in the range of only 10e40 mV/K at room temperature even with various dopants [11e13] and doping conditions [14e16]. Only Mateeva et al. reported a relatively high PANI thermopower value close to 90 mV/K by sacrificing electrical conductivity (w0.1 S/m) considerably [17]. Here, we report a facile method to improve the thermopower of doped PANI. The PANI was doped with camphor sulfonic acid (CSA) and then the carrier concentration was adjusted by using ammonium hydroxide. Scheme 1 suggests the experimental process that we have used in this study. Emeraldine base PANI (Scheme 1a) has two benzenoid and two quinoid rings connected in series with an angle of w140 (at the center) [18]. Upon CSA doping, two protons are bound to the quinoid, resulting in an increase of electrical conductivity (Scheme 1b). When the CSA-doped PANI is reacted with NH4OH, CSA is removed from the PANI backbone (de-doping) (Scheme 1c). The de-doping process will decrease the carrier concentration and thereby increase thermopower, as described below. With a proper reaction time, thermopower was measured to be as high as 115 mV/K at room temperature, which is 1.2e10 times higher than other doped PANI [11e16,19]. Prior to controlling thermopower, the condition for synthesizing CSA-doped PANI was optimized by adjusting drying and annealing temperatures. We used X-ray diffraction (XRD) to identify the crystallinity of PANI structure since high crystallinity will provide better electrical conductivity. Fourier transform infrared spectroscopy (FTIR) revealed that the reaction of CSA-doped PANI films with ammonium hydroxide has decreased the doping level of CSA, which is likely to be responsible for the large increase of thermopower. Further work may produce a high performance PANI-containing polymer composite with both high thermopower and electrical conductivity for the thermoelectric applications.
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2. Experimental 2.1. Materials Emeraldine base PANI [(C6H4NH)2(C6H4N)]2n with 99.9% purity and a molecular weight of w50,000 (Sigma Aldrich), and CSA with 99% purity (Alfa Aesar) and m-cresol with 99% purity (Alfa Aesar) were used for synthesizing CSA-doped PANI. 2.2. Sample preparation First, a solution containing 1-wt% PANIeCSA was prepared by mixing 0.5-g PANI and 0.64-g CSA in 35-mL m-cresol. The solution was sonicated by using a pen-type sonicator with 20 W for 30 min, and then an additional 20-h sonication in a bath-type sonicator was performed to make the solution homogeneous. Subsequently, the PANIeCSA solution was filtered using polytetrafluoroethylene filters whose pore size is 0.45 mm. The filtered solution was dropped on glass substrates and dried at various temperatures until the solution was fully dried (typically for w5 days at room temperature and w20 h at higher temperatures). The film thickness was measured to be w25 mm with 1 mL of the solution on 2 2 cm2 glass substrates. The dried films were then annealed at different temperatures for 2 h in vacuum (w0.1 mmHg) to improve electrical conductivity. The reaction of the CSA-doped PANI films with ammonium hydroxide was performed by passing argon gas through a 15-M ammonium hydroxide solution to a glass flask where the samples were placed. 2.3. Characterization X-ray diffraction (XRD) analysis was carried out on Philips PW1710 automatic X-ray diffractometer with Cu-Ka radiation A). Fourier transform infrared spectroscopy attenuated (l ¼ 1.5404 total reflectance (FTIReATR) spectra were acquired using Nicolet 380 (Thermo Fisher Scientific) in conjunction with ATR accessory (AVATAR OMNI Sampler, Germanium crystal) under an ambient condition. Electrical conductivity and thermopower were measured at room temperature along the in-plane direction of the films with a Keithley 2000 Multimeter and Labview (National instruments, Austin, TX). A four-probe currentevoltage measurement method was employed to accurately determine the electrical conductivity. For each data point, four or more samples were tested. The error (E) was calculated according to the following relation:
" E ¼
n X
#0:5 ðXi Xm Þ2 =n
i¼1
and Xm ¼
n X
Xi =n
i¼1
where Xi and n are experimental data and the number of samples, respectively. 3. Results and discussion 3.1. Crystallinity of PANI doped with CSA at different drying and annealing temperature
Scheme 1. (a) Emeraldine base PANI, (b) CSA-doped PANI, (c) CSA-doped PANI after an ammonium hydroxide treatment.
A PANI doped with CSA in m-cresol was obtained by using Cao and Heeger’s method [20]. A ground mixture of PANI and CSA powders was added into m-cresol. The mixture solution was sonicated for 30 min with a pen-type sonicator and then 20 h with a bath-type sonicator. The homogeneous solution was dropped on glass substrates, and subsequent drying and annealing at different temperatures formed CSA-doped PANI films. The crystallinity of the samples was evaluated by XRD, as depicted in Fig. 1. PANI has three
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H. Wang et al. / Polymer 54 (2013) 1136e1140
Fig. 1. (a) XRD of CSA-doped PANI films dried at 25 C, 50 C, 70 C, and 90 C, and (b) CSA-doped PANI films dried at 50 C and subsequently annealed at 50 C, 100 C, 150 C, and 200 C.
peaks at 15.3 , 20.6 , and 25.6 , which are typical PANI crystal peaks [18,21,22]. The related d-spacing values for the peaks at 15.3 , 20.6 and 25.6 are 5.6 Ǻ, 4.2 Ǻ and 3.5 Ǻ, respectively. The dspacing values of 5.6 Ǻ and 4.2 Ǻ are the side-by-side interaction distance of PANI molecules and face-to-face pep stacking distance between phenyl rings, respectively. The d-spacing value of 3.5 Ǻ is attributed to the distance between two nitrogen atoms coordinated with two CSA anions [18]. The intensity increase towards low angles is due to amorphous glass substrates [23e25]. A crystallinity percentage (Ap) of the PANI films was estimated using the equation: Ap ¼ Ac/(Ac þ Aa) [22,26], where Ac is the area under the crystalline peaks and Aa is from amorphous peak areas at low angles appeared below w13 . The crystallinity percentage was calculated to be 62%, 60%, 53%, and 50% for films dried at 25 C, 50 C, 70 C, and 90 C, respectively. The decent crystallinity may come from the ultrasonic process for 20 h with m-cresol, which has been reported to improve the crystallinity [27]. These results suggest that slow drying is better to improve crystallinity of PANI. The crystallinity percentages were increased to 69% and 70%, respectively for the samples annealed at 50 C and 100 C for 2 h (the sample was dried at 50 C prior to the annealing.). For the samples annealed at 150 C and 200 C, the crystallinity percentages were decreased to 44% and 40%, respectively.
The CSA-doped PANI films synthesized with 50 C drying and 100 C annealing processes were treated with ammonium hydroxide and then their electrical conductivity and thermopower values were measured as a function of reaction time, as depicted in Fig. 3. After 10-min reactions with ammonium hydroxide, thermopower was raised to w46 mV/K, which was continuously improved with longer reactions. The highest thermopower was measured to be w115 mV/K at room temperature, which is the highest value from doped PANI without stretching to our best knowledge, with an ammonium hydroxide treatment for 48 h. The high thermopower could be maintained while the electrical conductivity can be increased by adding carbon nanotubes in the PANI, as shown in our past experimental results [3,4,8]. This value was maintained within 10% even after air exposure for two weeks. On the other hand, the electrical conductivity was dropped after the reaction with ammonium hydroxide. This inversely proportional correlation between thermopower and electrical conductivity is typical when the number of carriers is reduced. Since the electrical conductivity was decreased to a small value, w2 S/m after the 48-h reaction, further experiments with longer reaction time periods were not performed.
3.2. Electrical property characterization of CSA-doped PANI films
In order to understand the changes in the electrical conductivity and thermopower after the ammonium hydroxide treatments, FTIR was performed. Fig. 4 shows the FTIR spectra of CSA-doped PANI films before and after the treatment for 20 h. The peaks from the CSA-doped PANI were observed at 1610 cm1, 1494 cm1, 1324 cm1, and 1166 cm1, which can be attributed to C]C stretching of quinoid ring, C]C stretching of benzenoid ring, CeN stretching, and the vibration mode of ]NeHþ structure [28,29]. After the reaction with NH4OH for 20 h, the peaks were red-shifted to 1621 cm1, 1515 cm1, 1326 cm1, and 1170 cm1, which is an indication of a de-doping process [30]. In other words, upon the removal of protons, PANI becomes more stable (i.e., in a lower energy state). The intensity ratio of quinoid to benzenoid in NH4OH-treated CSA-doped PANI, compared to CSA-only PANI, was decreased, which would be from the changes in the quinoid with the ammonium hydroxide treatment, as shown in Scheme 1. The peak at 1170 cm1, which corresponds to ]NeHþ, is related to doping level [30]. The intensity reduction after the NH4OH treatment indicates that the carrier concentration has been decreased, which is responsible for the large reduction in electrical conductivity.
Fig. 2 shows the electrical conductivity and thermopower of CSA-doped PANI films dried and annealed at different temperatures. The electrical conductivities of the films dried at 50 C and room temperature were similar, w3700 S/m, whereas the electrical conductivity was decreased down to w1700 S/m upon increasing the drying temperature, as shown in Fig. 2a. The thermopower of the CSA-doped PANI was kept to be relatively constant, w16 mV/K (Fig. 2b), regardless of the drying temperature. After the CSA-doped PANI films dried at 50 C were annealed at 50 C, 100 C, and 150 C in a vacuum environment, the electrical conductivities were improved to w5700 S/m (Fig. 2c). However, the annealing process at 200 C reduced the electrical conductivity. The increase of electrical conductivity may be attributed to the better crystalline structures and the evaporation of moisture inside of the film since polar water molecules might quench charge carriers in the films. However, a high annealing temperature, like 200 C, deteriorates PANI crystallinity, as evident in the XRD results (Fig. 1b), and thereby suppresses electrical conductivity. The annealing temperature has negligible influence on thermopower.
3.3. FTIR properties of PANIeCSA with and without ammonium hydroxide treatment
H. Wang et al. / Polymer 54 (2013) 1136e1140
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Fig. 2. Electrical conductivity and thermopower of CSA-doped PANI films dried (a, b) and annealed (c, d) at different temperatures.
a
b
Fig. 3. Electrical conductivity (a) and thermopower (b) of CSA-doped PANI films treated by ammonium hydroxide for various time periods.
Fig. 4. FTIR spectra of CSA-doped PANI film and CSA-doped PANI film treated with ammonium hydroxide for 20 h.
Fig. 5. The correlation between the power factor (S2s) and the charge carrier concentration. The charge mobility was assumed to be 0.5 cm2 V1 s1.
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H. Wang et al. / Polymer 54 (2013) 1136e1140
3.4. Charge carrier concentration
Acknowledgement
The charge carrier concentration may be estimated by the following relation, s ¼ nem, where n, e, and m respectively represent charge carrier concentration, elementary charge, and charge mobility. Assuming that the charge mobility of PANI is constant and 0.39e0.5 cm2 V1 s1 at room temperature [31,32], the numbers of carriers for s ¼ 5700 and 2 S/m are estimated to be w7 1020 and w2 1017 cm3. The carrier concentrations for s ¼ w5700 S/m are relatively high, which suggest that it is necessary to have a higher mobility in order to obtain a higher electrical conductivity. In general, a higher mobility is obtained by improving crystallinity. Fig. 5 shows the relationship between the power factor and the charge carrier concentration. The highest power factor obtained in this paper is w1 mW/m-K2, which is 5e1000 times higher than the reported values from other acid-doped PANI without stretching [32,33].
The authors gratefully acknowledge financial supports from the US Air Force Office of Scientific Research (FA9550-09-1-0609) under the auspices of Dr. Charles Lee, II-VI foundation, and the Pioneer Research Center Program through the National Research Foundation of Korea (2011-0001645) and funded by the Ministry of Education, Science and Technology.
4. Conclusions In summary, we report a facile method of controlling the electrical properties of PANI. The electrical conductivity was first improved by doping CSA, and then the number of charge carriers was reduced by ammonium hydroxide so as to increase thermopower. The thermopower was monotonically increased as a function of the time period of the ammonium hydroxide reaction, resulting in w115 mV/K at room temperature, which is 1.2e10 times higher than those of other doped PANI. The power factor was calculated to be as high as w1 mW/m-K2, which is 5e1000 times larger than those of other PANI samples without stretching. The change in the electrical properties can be attributed to the reduction in the number of charge carriers due to the ammonium hydroxide reaction (dedoping) process. We observed that the intensity of FTIR spectra associated with the carrier concentration was largely altered after the de-doping reaction. According to XRD results, the crystallinity of CSA-doped PANI was estimated to be as high as 70%, and drying and/ or annealing below w100 C improved PANI crystallinity. We believe it is feasible to further improve the crystallinity of PANI, which is likely to result in a higher charge mobility and thereby a higher electrical conductivity. More importantly, the electrical conductivity can be considerably improved by adding conductive additives such as carbon nanotubes. Our facile and convenient way of adjusting carrier concentration will be of great help to find the optimum electrical transport properties.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
Bell LE. Science 2008;321:1457e561. Majumdar A. Science 2004;303:777e8. Yu C, Choi K, Yin L, Grunlan JC. ACS Nano 2011;5:7885e92. Yu C, Kim YS, Kim D, Grunlan JC. Nano Lett 2008;8:4428e32. Yu C, Murali A, Choi K, Ryu Y. Energy Environ Sci 2012;5:9481e6. Dubey N, Leclerc M. J Polym Sci Part B Polym Phys 2011;49:467e75. Toshima N. Macromol Symp 2002;186:81e6. Kim D, Kim Y, Choi K, Grunlan JC, Yu C. ACS Nano 2010;4:513e23. Kim YS, Kim D, Martin KJ, Yu C, Grunlan JC. Macromol Mater Eng 2010;295: 431e6. Ryu Y, Yu C. J Mater Chem 2012;22:6959e64. Li J, Tang X, Li H, Yan Y, Zhang Q. Synth Met 2010;160:1153e8. Yan H, Toshima N. Chem Lett 1999:1217e8. Yan H, Ohta T, Toshima N. Macromol Mater Eng 2001;286(3):139e42. Yakuphanoglu F, Senkal BF, Sarac A. J Electron Mater 2008;37:930e4. Sakkopoulos S, Vitoratos E, Dalas E, Pandis G, Tsamourast D. J Phys Condens Matter 1992;4:2231e7. Park YW, Lee YS, Park C, Shacklette LW, Baughman RH. Solid State Commun 1987;63:1063e6. Mateeva N, Niculescu H, Schlenoff J, Testardi LR. J Appl Phys 1998;83(6): 3111e7. Pouget JP, Jozefowicz ME, Epstein AJ, Tang X, MacDiamid AG. Macromolecules 1991;24:779e89. Yoon CO, Kim JH, Sung HK, Lee H. Synth Met 1997;84:789e90. Cao Y, Heeger AJ. Synth Met 1992;52:193e200. Lee K, Cho S, Park SH, Heeger AJ, Lee CW, Lee SH. Nature 2006;441:65e8. Varma SJ, Xavier F, Varghese S, Jayalekshmi S. Polym Int 2012;61:743e8. Yin L, Yu C. J Solid State Chem 2012;187:58e63. Park J, Ryu Y, Kim H, Yu C. Nanotechnology 2009;20(105608):105601e8. Yu C, Park J. J Solid State Chem 2010;183:2268e73. Sinha S, Bhadra S, Khastgir D. J Appl Polym Sci 2009;112:3135e40. Liu H, Hu XB, Wang JY, Boughton RI. Macromolecules 2002;35:9414e9. Amado FDR, Rodrigues MAS, Bertuol DA, Bernardes AM, Ferreira JZ, Ferreira CA. J Memb Sci 2009;330:227e32. Kunteppa H, Roy AS, Koppalkar AR, Prasad MVNA. Phys B 2011;406:3997e 4000. Tang J, Jing X, Wang B, Wang F. Synth Met 1988;24:231e8. Holland ER, Pomfret SJ, Adams PN, Monkman AP. J Phys Condens Matter 1996; 8:2991e3002. Liu J, Sun J, Gao L. Nanoscale 2011;3:3616e9. Yao Q, Chen L, Zhang W, Liufu S, Chen X. ACS Nano 2010;4:2445e51.
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Facile charge carrier adjustment for improving thermopower of doped polyaniline Hong Wang, Liang Yin, Xiong Pu, Choongho Yu* Department of Mechanical Engineering, Materials Science and Engineering Program, Texas A&M University, College Station, TX 77843, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2012 Received in revised form 15 December 2012 Accepted 16 December 2012 Available online 21 December 2012
We report a facile method of improving thermopower of doped polyaniline (PANI). It is believed that the electrical properties were altered by controlling the concentration of charge carriers with camphor sulfonic acid (CSA) and ammonium hydroxide. With CSA doping followed by drying and annealing at temperatures below w100 C, the electrical conductivity was increased up to w5700 S/m with a thermopower value, w14 mV/K at room temperature. The power factor reached w1 mW/m-K2 after optimizing the drying and annealing temperature, which is 5e1000 times larger than those of other doped PANI without stretching. This thermopower of PANIeCSA films was monotonically improved in proportion to the time period of the ammonium hydroxide reaction, resulting in w115 mV/K at room temperature, which is 1.2e10 times higher than those of other doped PANI. The intensity changes of Fourier transform infrared spectra before and after the de-doping process with ammonium hydroxide indicates the thermopower increase is likely to be from the reduction in the charge carrier concentration. This method is a facile and convenient way of adjusting carrier concentrations, which are crucial to find the optimum doping level for high performance thermoelectrics. We believe further studies to improve crystallinity of doped PANI may dramatically raise the electrical conductivity in addition to the large thermopower, so as to obtain a high performance PANI-containing polymer composite for thermoelectric applications. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Thermopower Polyaniline Charge carrier
1. Introduction Thermoelectric (TE) devices are of great interest for direct conversion of heat to electricity or vice versa with simple leg-type structures without mechanically moving components or hazardous working fluids. Their applications span various fields including energy recovery from waste heat and solid state cooling [1]. Despite of the great advantages of TE devices, the relatively low efficiency has been a hurdle for wide use, ZT ¼ S2sT/k, where S, s, T, and k are thermopower (or the Seebeck coefficient), electrical conductivity, absolute temperature, and thermal conductivity, respectively. While the strong correlation between these parameters is the main reason for the relatively low efficiency, it has been demonstrated that low thermal conductivity is achievable by scattering phonons without significantly hampering electrical transport [2]. Compared to recent intense efforts to suppress phonon transport, systematic studies to achieve high S and s are relatively lacking despite the
* Corresponding author. Tel.: þ1 979 862 1073; fax: þ1 979 845 3081. E-mail address: [email protected] (C. Yu). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.12.038
importance of the electrical properties. For instance, an increase of thermopower may lead to a large ZT due to the square in the numerator of ZT (i.e., the power factor S2s). In particular, it is very important to improve thermopower of organic materials (e.g., polymers) and their composites since this type of materials typically have low thermal conductivity (i.e., not much room to suppress k further). Conducting polymers have great potentials as TE materials because of decent electrical conductivity with low thermal conductivity. Furthermore, polymers have additional advantages, compared with inorganic materials, such as low density and easy synthesis process with a low cost [3e9]. It was reported that the electrical conductivity of air-stable conducting polymers can reach w104 S/m (e.g., poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)). This value is still one order of magnitude lower than those of the current state-of-the-art TE materials containing Bi, Te, and Sb. This electrical conductivity can be further improved to w105 S/m with conducting additives like carbon nanotubes [3,4,8,10]. Nevertheless, it has been rarely studied to control thermopower of polymers. In particular, typical thermopower of doped polymers is very low, compared to those of inorganic TE
H. Wang et al. / Polymer 54 (2013) 1136e1140
semiconductors. For example, polyaniline (PANI) has been studied for thermoelectric applications, but the thermopower of doped PANI without stretching lies in the range of only 10e40 mV/K at room temperature even with various dopants [11e13] and doping conditions [14e16]. Only Mateeva et al. reported a relatively high PANI thermopower value close to 90 mV/K by sacrificing electrical conductivity (w0.1 S/m) considerably [17]. Here, we report a facile method to improve the thermopower of doped PANI. The PANI was doped with camphor sulfonic acid (CSA) and then the carrier concentration was adjusted by using ammonium hydroxide. Scheme 1 suggests the experimental process that we have used in this study. Emeraldine base PANI (Scheme 1a) has two benzenoid and two quinoid rings connected in series with an angle of w140 (at the center) [18]. Upon CSA doping, two protons are bound to the quinoid, resulting in an increase of electrical conductivity (Scheme 1b). When the CSA-doped PANI is reacted with NH4OH, CSA is removed from the PANI backbone (de-doping) (Scheme 1c). The de-doping process will decrease the carrier concentration and thereby increase thermopower, as described below. With a proper reaction time, thermopower was measured to be as high as 115 mV/K at room temperature, which is 1.2e10 times higher than other doped PANI [11e16,19]. Prior to controlling thermopower, the condition for synthesizing CSA-doped PANI was optimized by adjusting drying and annealing temperatures. We used X-ray diffraction (XRD) to identify the crystallinity of PANI structure since high crystallinity will provide better electrical conductivity. Fourier transform infrared spectroscopy (FTIR) revealed that the reaction of CSA-doped PANI films with ammonium hydroxide has decreased the doping level of CSA, which is likely to be responsible for the large increase of thermopower. Further work may produce a high performance PANI-containing polymer composite with both high thermopower and electrical conductivity for the thermoelectric applications.
1137
2. Experimental 2.1. Materials Emeraldine base PANI [(C6H4NH)2(C6H4N)]2n with 99.9% purity and a molecular weight of w50,000 (Sigma Aldrich), and CSA with 99% purity (Alfa Aesar) and m-cresol with 99% purity (Alfa Aesar) were used for synthesizing CSA-doped PANI. 2.2. Sample preparation First, a solution containing 1-wt% PANIeCSA was prepared by mixing 0.5-g PANI and 0.64-g CSA in 35-mL m-cresol. The solution was sonicated by using a pen-type sonicator with 20 W for 30 min, and then an additional 20-h sonication in a bath-type sonicator was performed to make the solution homogeneous. Subsequently, the PANIeCSA solution was filtered using polytetrafluoroethylene filters whose pore size is 0.45 mm. The filtered solution was dropped on glass substrates and dried at various temperatures until the solution was fully dried (typically for w5 days at room temperature and w20 h at higher temperatures). The film thickness was measured to be w25 mm with 1 mL of the solution on 2 2 cm2 glass substrates. The dried films were then annealed at different temperatures for 2 h in vacuum (w0.1 mmHg) to improve electrical conductivity. The reaction of the CSA-doped PANI films with ammonium hydroxide was performed by passing argon gas through a 15-M ammonium hydroxide solution to a glass flask where the samples were placed. 2.3. Characterization X-ray diffraction (XRD) analysis was carried out on Philips PW1710 automatic X-ray diffractometer with Cu-Ka radiation A). Fourier transform infrared spectroscopy attenuated (l ¼ 1.5404 total reflectance (FTIReATR) spectra were acquired using Nicolet 380 (Thermo Fisher Scientific) in conjunction with ATR accessory (AVATAR OMNI Sampler, Germanium crystal) under an ambient condition. Electrical conductivity and thermopower were measured at room temperature along the in-plane direction of the films with a Keithley 2000 Multimeter and Labview (National instruments, Austin, TX). A four-probe currentevoltage measurement method was employed to accurately determine the electrical conductivity. For each data point, four or more samples were tested. The error (E) was calculated according to the following relation:
" E ¼
n X
#0:5 ðXi Xm Þ2 =n
i¼1
and Xm ¼
n X
Xi =n
i¼1
where Xi and n are experimental data and the number of samples, respectively. 3. Results and discussion 3.1. Crystallinity of PANI doped with CSA at different drying and annealing temperature
Scheme 1. (a) Emeraldine base PANI, (b) CSA-doped PANI, (c) CSA-doped PANI after an ammonium hydroxide treatment.
A PANI doped with CSA in m-cresol was obtained by using Cao and Heeger’s method [20]. A ground mixture of PANI and CSA powders was added into m-cresol. The mixture solution was sonicated for 30 min with a pen-type sonicator and then 20 h with a bath-type sonicator. The homogeneous solution was dropped on glass substrates, and subsequent drying and annealing at different temperatures formed CSA-doped PANI films. The crystallinity of the samples was evaluated by XRD, as depicted in Fig. 1. PANI has three
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Fig. 1. (a) XRD of CSA-doped PANI films dried at 25 C, 50 C, 70 C, and 90 C, and (b) CSA-doped PANI films dried at 50 C and subsequently annealed at 50 C, 100 C, 150 C, and 200 C.
peaks at 15.3 , 20.6 , and 25.6 , which are typical PANI crystal peaks [18,21,22]. The related d-spacing values for the peaks at 15.3 , 20.6 and 25.6 are 5.6 Ǻ, 4.2 Ǻ and 3.5 Ǻ, respectively. The dspacing values of 5.6 Ǻ and 4.2 Ǻ are the side-by-side interaction distance of PANI molecules and face-to-face pep stacking distance between phenyl rings, respectively. The d-spacing value of 3.5 Ǻ is attributed to the distance between two nitrogen atoms coordinated with two CSA anions [18]. The intensity increase towards low angles is due to amorphous glass substrates [23e25]. A crystallinity percentage (Ap) of the PANI films was estimated using the equation: Ap ¼ Ac/(Ac þ Aa) [22,26], where Ac is the area under the crystalline peaks and Aa is from amorphous peak areas at low angles appeared below w13 . The crystallinity percentage was calculated to be 62%, 60%, 53%, and 50% for films dried at 25 C, 50 C, 70 C, and 90 C, respectively. The decent crystallinity may come from the ultrasonic process for 20 h with m-cresol, which has been reported to improve the crystallinity [27]. These results suggest that slow drying is better to improve crystallinity of PANI. The crystallinity percentages were increased to 69% and 70%, respectively for the samples annealed at 50 C and 100 C for 2 h (the sample was dried at 50 C prior to the annealing.). For the samples annealed at 150 C and 200 C, the crystallinity percentages were decreased to 44% and 40%, respectively.
The CSA-doped PANI films synthesized with 50 C drying and 100 C annealing processes were treated with ammonium hydroxide and then their electrical conductivity and thermopower values were measured as a function of reaction time, as depicted in Fig. 3. After 10-min reactions with ammonium hydroxide, thermopower was raised to w46 mV/K, which was continuously improved with longer reactions. The highest thermopower was measured to be w115 mV/K at room temperature, which is the highest value from doped PANI without stretching to our best knowledge, with an ammonium hydroxide treatment for 48 h. The high thermopower could be maintained while the electrical conductivity can be increased by adding carbon nanotubes in the PANI, as shown in our past experimental results [3,4,8]. This value was maintained within 10% even after air exposure for two weeks. On the other hand, the electrical conductivity was dropped after the reaction with ammonium hydroxide. This inversely proportional correlation between thermopower and electrical conductivity is typical when the number of carriers is reduced. Since the electrical conductivity was decreased to a small value, w2 S/m after the 48-h reaction, further experiments with longer reaction time periods were not performed.
3.2. Electrical property characterization of CSA-doped PANI films
In order to understand the changes in the electrical conductivity and thermopower after the ammonium hydroxide treatments, FTIR was performed. Fig. 4 shows the FTIR spectra of CSA-doped PANI films before and after the treatment for 20 h. The peaks from the CSA-doped PANI were observed at 1610 cm1, 1494 cm1, 1324 cm1, and 1166 cm1, which can be attributed to C]C stretching of quinoid ring, C]C stretching of benzenoid ring, CeN stretching, and the vibration mode of ]NeHþ structure [28,29]. After the reaction with NH4OH for 20 h, the peaks were red-shifted to 1621 cm1, 1515 cm1, 1326 cm1, and 1170 cm1, which is an indication of a de-doping process [30]. In other words, upon the removal of protons, PANI becomes more stable (i.e., in a lower energy state). The intensity ratio of quinoid to benzenoid in NH4OH-treated CSA-doped PANI, compared to CSA-only PANI, was decreased, which would be from the changes in the quinoid with the ammonium hydroxide treatment, as shown in Scheme 1. The peak at 1170 cm1, which corresponds to ]NeHþ, is related to doping level [30]. The intensity reduction after the NH4OH treatment indicates that the carrier concentration has been decreased, which is responsible for the large reduction in electrical conductivity.
Fig. 2 shows the electrical conductivity and thermopower of CSA-doped PANI films dried and annealed at different temperatures. The electrical conductivities of the films dried at 50 C and room temperature were similar, w3700 S/m, whereas the electrical conductivity was decreased down to w1700 S/m upon increasing the drying temperature, as shown in Fig. 2a. The thermopower of the CSA-doped PANI was kept to be relatively constant, w16 mV/K (Fig. 2b), regardless of the drying temperature. After the CSA-doped PANI films dried at 50 C were annealed at 50 C, 100 C, and 150 C in a vacuum environment, the electrical conductivities were improved to w5700 S/m (Fig. 2c). However, the annealing process at 200 C reduced the electrical conductivity. The increase of electrical conductivity may be attributed to the better crystalline structures and the evaporation of moisture inside of the film since polar water molecules might quench charge carriers in the films. However, a high annealing temperature, like 200 C, deteriorates PANI crystallinity, as evident in the XRD results (Fig. 1b), and thereby suppresses electrical conductivity. The annealing temperature has negligible influence on thermopower.
3.3. FTIR properties of PANIeCSA with and without ammonium hydroxide treatment
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Fig. 2. Electrical conductivity and thermopower of CSA-doped PANI films dried (a, b) and annealed (c, d) at different temperatures.
a
b
Fig. 3. Electrical conductivity (a) and thermopower (b) of CSA-doped PANI films treated by ammonium hydroxide for various time periods.
Fig. 4. FTIR spectra of CSA-doped PANI film and CSA-doped PANI film treated with ammonium hydroxide for 20 h.
Fig. 5. The correlation between the power factor (S2s) and the charge carrier concentration. The charge mobility was assumed to be 0.5 cm2 V1 s1.
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3.4. Charge carrier concentration
Acknowledgement
The charge carrier concentration may be estimated by the following relation, s ¼ nem, where n, e, and m respectively represent charge carrier concentration, elementary charge, and charge mobility. Assuming that the charge mobility of PANI is constant and 0.39e0.5 cm2 V1 s1 at room temperature [31,32], the numbers of carriers for s ¼ 5700 and 2 S/m are estimated to be w7 1020 and w2 1017 cm3. The carrier concentrations for s ¼ w5700 S/m are relatively high, which suggest that it is necessary to have a higher mobility in order to obtain a higher electrical conductivity. In general, a higher mobility is obtained by improving crystallinity. Fig. 5 shows the relationship between the power factor and the charge carrier concentration. The highest power factor obtained in this paper is w1 mW/m-K2, which is 5e1000 times higher than the reported values from other acid-doped PANI without stretching [32,33].
The authors gratefully acknowledge financial supports from the US Air Force Office of Scientific Research (FA9550-09-1-0609) under the auspices of Dr. Charles Lee, II-VI foundation, and the Pioneer Research Center Program through the National Research Foundation of Korea (2011-0001645) and funded by the Ministry of Education, Science and Technology.
4. Conclusions In summary, we report a facile method of controlling the electrical properties of PANI. The electrical conductivity was first improved by doping CSA, and then the number of charge carriers was reduced by ammonium hydroxide so as to increase thermopower. The thermopower was monotonically increased as a function of the time period of the ammonium hydroxide reaction, resulting in w115 mV/K at room temperature, which is 1.2e10 times higher than those of other doped PANI. The power factor was calculated to be as high as w1 mW/m-K2, which is 5e1000 times larger than those of other PANI samples without stretching. The change in the electrical properties can be attributed to the reduction in the number of charge carriers due to the ammonium hydroxide reaction (dedoping) process. We observed that the intensity of FTIR spectra associated with the carrier concentration was largely altered after the de-doping reaction. According to XRD results, the crystallinity of CSA-doped PANI was estimated to be as high as 70%, and drying and/ or annealing below w100 C improved PANI crystallinity. We believe it is feasible to further improve the crystallinity of PANI, which is likely to result in a higher charge mobility and thereby a higher electrical conductivity. More importantly, the electrical conductivity can be considerably improved by adding conductive additives such as carbon nanotubes. Our facile and convenient way of adjusting carrier concentration will be of great help to find the optimum electrical transport properties.
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