- Email: [email protected]
polyaniline composites
Journal of Solid State Chemistry 196 (2012) 309–313
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Electrical and thermal properties of graphite/polyaniline composites Shawn E. Bourdo a,n, Brock A. Warford b, Tito Viswanathan b a b
Center for Integrative Nanotechnology Sciences, University of Arkansas at Little Rock, 2801 South University Avenue, Little Rock, AR 72204, USA Department of Chemistry, University of Arkansas at Little Rock, 2801 South University Avenue, Little Rock, AR 72204, USA
a r t i c l e i n f o
abstract
Article history: Received 19 March 2012 Received in revised form 20 June 2012 Accepted 24 June 2012 Available online 1 July 2012
A composite of a carbon allotrope (graphite) and an inherently conducting polymer, polyaniline (PANI), has been prepared that exhibits an electrical conductivity greater than either of the two components. An almost 2-fold increase in the bulk conductivity occurs when only a small mass fraction of polyaniline exists in the composite (91% graphite/ 9% polyaniline, by mass). This increase in dc electrical conductivity is curious since in most cases a composite material will exhibit a conductivity somewhere between the two individual components, unless a modification to the electronic nature of the material occurs. In order to elucidate the fundamental electrical properties of the composite we have performed variable temperature conductivity measurements to better understand the nature of conduction in these materials. The results from these studies suggest a change in the mechanism of conduction as the amount of polyaniline is increased in the composite. Along with superior electrical properties, the composites exhibit an increase in thermal stability as compared to the graphite. & 2012 Elsevier Inc. All rights reserved.
Keywords: Composites Conducting polymers Electrical properties X-ray diffraction Thermogravimetric analysis
1. Introduction Inherently conducting polymers (ICPs) have been at the forefront of research initiatives for the last several decades due to many unique chemical, optical, and electrical properties. When ICPs are combined with carbonaceous materials, improved thermal and electrical properties may be observed [1–3]. In our lab we have produced such a material that exhibits an electrical conductivity greater than the two components of the composite: polyaniline (PANI) and graphite (G). This observation was curious since, in many instances, the combination of materials with differing electrical properties will produce a composite with electrical conductivity between that of the two individual components. There have only been two such reports on this curious observation for G–PANI composites, both by our group. Previous studies led to the hypothesis that a charge transfer occurs between the graphite and PANI components of the composite that results in superior electrical conductivity and increased crystallinity [4,5]. Our focus of the present report is to investigate the mechanism of conduction through variable temperature conductivity measurements. Several methods have been employed to improve the characteristics of ICPs and ICP composites. There have been reports of graphite oxide–polyaniline composites [6,7] and graphite nanosheet–polyaniline composites [8,9], but neither has shown
n
Corresponding author. Fax: þ1 501 683 7601. E-mail address: [email protected] (S.E. Bourdo).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.06.038
conductivities higher than the two components. Other groups have focused on producing highly linear polymers by modifying reaction conditions for the synthesis of PANI. There are recent articles discussing production of metallic type conducting polyaniline by the self-stabilized dispersion polymerization (SSDP) method [10,11]. We had previously observed a dramatic increase in the conductivity of G–PANI composites [4,5] and now here the characterization of a composite where the synthetic approach combines the use of graphite nanosheets [8,9,12] and a modified SSDP method (where the graphite serves as a dispersed organic media). The reaction media and conditions for synthesis of composite materials should be amenable to the individual constituents of the composite (solvent, temperature, etc.). For this procedure we have utilized a common media for graphite dispersion [8,9,12] and self-stabilized dispersion polymerization of aniline [10,11], namely the organic dispersant/solvent, isopropanol. The concept of self-stabilized dispersion polymerization technique is ideally suited for these composites since the reaction is carried out in a heterogeneous biphasic medium to prevent undesirable side reactions. The ICP compatibility with graphite surface would promote it to adhere and align parallel to the surface of the graphite resulting in a uniform composite sample. Four composite samples were characterized by thermogravimetric analysis to determine the ratio of graphite to PANI, followed by electrical characterization. These results were then compared with samples of pristine PANI and graphite. At certain ratios of graphite to ICP the bulk conductivity is higher than the two components alone, and it is this observation that we have
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S.E. Bourdo et al. / Journal of Solid State Chemistry 196 (2012) 309–313
investigated further through variable temperature conductivity studies. These studies provided some insight to the conduction mechanism of these materials. Along with superior electrical properties, the composites of graphite and polyaniline exhibited increased thermal stability. Both components have a plethora of potential applications and therefore further investigation could lead to use of these composites in any number of technologies. Touted applications that could benefit from the properties of these composites exist in a wide range of fields, including electromagnetic interference (EMI) shielding [13–17], low power rechargeable batteries [2,3,18], electrostatic dissipation (ESD) for anti-static textiles [13], electronic devices [19], resistive heating [20], and supercapacitors [21].
2. Experimental 2.1. Synthesis Graphite nanosheets were prepared according to methods in references [8,9] and [12].In a typical preparation of stock expanded graphite 50 g of natural flake graphite (Graphite Sales, Inc.) was placed into a 4 L beaker with a stir bar. The beaker was placed on a stir plate, 500 mL of concentrated H2SO4 added and the mixture stirred until the graphite appeared well dispersed. To this mixture 50 mL of 30% H2O2 was added slowly but all at once. The mixture bubbled and gas was given off immediately as the volume level appeared to double within about 30–60 s. The volume then steadily decreased while the mixture continued to bubble for approximately 15 min. The mixture was stirred for approximately 2 h at which time it was diluted by 2 successive additions of 250 ml deionized water; the mixture was allowed to cool between additions. The mixture was filtered by vacuum through a sintered glass funnel. The solids were dried in an oven at 100 1C overnight. Briefly, a certain mass of graphite was placed into a porcelain dish and placed in a furnace at 900 1C for 15 min that yielded expanded graphite. The expanded graphite was dispersed in isopropyl alcohol by sonication for 1–2 h to break the graphite into smaller graphite ‘‘nanosheets’’. A solution of 2 mL of aniline in 120 mL 1 M HCl was then added to a reaction vessel. The vessel was placed on a cold plate stirrer and cooled to approximately 15 1C. While the reaction mixture was cooling, a 3.93 g sample of sodium persulfate (Na2S2O8) was dissolved in 40 mL of 1 M HCl and chilled on ice. Once the reaction mixture had reached 15 1C or below, the oxidant solution was added dropwise over a period of about 40 min. After at least 4 h, the reaction was vacuum filtered through Whatman 4 filter paper and washed with deionized water until the filtrate was colorless. The wet cake was then washed with 50 mL of 1 M HCl. A portion of this wetcake labeled as sample ‘‘A’’ was dried under vacuum for at least 24 h and used for electrical characterization. Another portion was labeled as sample ‘‘B’’ and was washed with 10 mL of 1 M NH4OH. The ‘‘B’’ sample was then transferred to an Erlenmeyer flask and stirred with at least 100 mL of 0.1 M NH4OH overnight. The solids were filtered, washed again with 10 mL of 1 M NH4OH and then with deionized water until the filtrate was at neutral pH. The solids were then dried in a vacuum oven for at least 24 h. The ‘‘B’’ sample was used to determine mass ratio of graphite to PANI using thermogravimetric analysis. 2.2. Characterization 2.2.1. Room temperature conductivity Room temperature DC bulk conductivity measurements were made on pressed pellets (13 mm diameter using 15000 lbs of force,
o1 mm thick) with an Alessi four-point conductivity probe. A Keithley 617 programmable electrometer and a Keithley 224 programmable current source were used for the measurement of voltage as a function of current. Four measurements were taken and an average has been reported. 2.2.2. Temperature dependent conductivity Temperature dependent DC bulk conductivity measurements were made on pressed pellets (13 mm diameter using 15000 lbs of force, o1 mm thick) using a four-point probe assembly inside a Janis SVT Cryostat System. The temperature was regulated using a Lakeshore 321 Temperature Controller. The electrical measurements were made by sourcing current with a Keithely 2400 Sourcemeter and measuring the voltage with a Keithely 2182 A Nanovoltmeter. Electrical measurements were made at varied intervals, every 51 from 5 K to 100 K then every 101 from 100 K to 300 K. After each step increase, the temperature was allowed to stabilize for 1 min before a measurement was taken. 2.2.3. X-ray diffraction analysis X-ray diffraction patterns were obtained on pressed pellets of the composites similar to those used for the conductivity studies. Typically, powdered samples were loaded into a 13 mm pellet die and compressed with a force of 15,000 lbs using a hydraulic press. The X-ray diffraction patterns of the pellets were measured on a Bruker AXS D8 Discover, with a GADDS 2D counter. Copper K alpha line was used as excitation source and the X-ray tube ran at 40 kV and 35 mA. 2.2.4. Thermal analysis Thermogravimetric measurements were performed under airflow of 150 mL/min using a Mettler Toledo TG50 connected to a TC15 controller. The temperature was increased at a rate of 10 1C/ min. Analysis was performed using STARe software. Composite samples were powdered and masses of approximately 100 mg were used for the analysis. 70 mL alumina crucibles were used for the analysis.
3. Results and discussion Four composite samples were characterized along with pristine PANI and graphite. The weight ratio of G to PANI determined by thermogravimetric analysis in Composites 1, 2, 3, and 4 are 31/ 69, 72/28, 85/15, and 91/9, respectively. Fig. 1 displays the room temperature dc conductivity measurements determined by the four-point collinear probe method. G–PANI Composite 4 had the highest conductivity, and both Composites 3 and 4 displayed a higher conductivity than pristine graphite. Transport studies are crucial to explaining the conductivity characteristics of materials. Temperature dependent direct current conductivity studies can be very useful in probing the conduction mechanism of a material [10,22,23]. This type of analysis has broadened our understanding of why an increase in conductivity is observed for this system. By varying the temperature and measuring electrical conductivity, one can gain useful information regarding the mechanism of conduction for a material: metallic or semiconducting. The electrical conductivity of metals and semiconductors varies with temperature in a directly opposite manner. The electrical conductivity of a metal increases as the temperature decreases, while the conductivity of a semiconductor decreases with decrease in temperature. The mobility of electrons within any material directly influences the electrical properties of the material. For metals, electrons are highly mobile within the band structure of the material and do not have any thermal barriers
S.E. Bourdo et al. / Journal of Solid State Chemistry 196 (2012) 309–313
Fig. 1. Room temperature dc electrical conductivity values for PANI, graphite, and four G–PANI composites: red circle—pristine PANI, green triangle—31% G/69% PANI, blue triangle–72% G/28% PANI, aqua diamond—85% G/15% PANI, magenta triangle—91% G/9% PANI and black square—graphite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to overcome in order to be mobile. When the temperature is decreased, the crystal lattice of the metal becomes much more rigid (less vibrations) and it is for this reason that metallic conduction increases as temperature decreases. The electrons are able to move throughout the lattice with fewer impediments due to decreased vibrational lattice distortions. For semiconductors/insulators, thermal barriers must be overcome for an electron to be mobile within the crystal lattice. At 0 K the valence band is completely filled with electrons, and the conduction band is completely void of electrons. As the temperature increases these electrons gain more energy and can then become mobile charge carriers. The mobility of charge in semiconductors generally follows a variable range hopping mechanism (VRH) in which electrons will ‘‘hop’’ between localized states of similar energies, even if there is a shorter path along a chain (as in ICPs) or plane (as in graphite) [24]. It is for these reasons that we have studied the temperature dependent conductivity of G–PANI composites to better understand the mechanism of conduction in these unique materials. The investigation into the temperature dependent conductivity of these materials reinforces the claim of charge transfer and furthers the discussion into the nature of the conduction mechanism of the composites. Results from temperature dependent studies (Fig. 2) have shown dramatic change in conductivities upon temperature variation. The figure above displays that the composites exhibit a greater dependence on temperature than graphite alone. This implies that the charge carriers are more localized in the composites than in pristine graphite and are closer to that of a Fermi glass (insulator) than graphite, which is a semimetal. Several publications report the ratio of conductivity (or resistivity) values at ambient temperature (300 K) to the values at low temperatures (5 K and lower) as a measure of the degree of disorder and an effective means of qualitatively evaluating temperature dependent conductivity data. Conductance ratios, sr ¼ s (300 K)/ s (5 K), of less than 2 indicate conduction in the metallic region and imply a highly ordered sample. For 2o sr o6 the sample is said to lie in the critical region, and for 6o sr, the sample is said to be in the insulating region (a Fermi glass) [22,23,25]. Therefore by taking the ratio of conductivity values one can gain insight into the degree of disorder and type of conduction that dominates a specific
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Fig. 2. Temperature dependence of dc conductivity for several G–PANI composites of graphite (G) and HCl-doped polyaniline (PANI): red circle—pristine PANI, green triangle—31% G/69% PANI, blue triangle—72% G/28% PANI, aqua diamond—85% G/15% PANI, magenta triangle—91% G/9% PANI and black square—graphite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Values of room temperature dc conductivities and conductance ratios, s (300 K)/ s (5 K), for HCl doped composites. Sample
PANI Composite Composite Composite Composite Graphite
1 2 3 4
% Graphite (w/w)
Room temperature dc conductivity (S/cm)
Conductivity ratio
0 31 72 85 91 100
4.5 40 250 570 880 460
2.70 105 1.90 107 260 15 5.0 1.4
material. Table 1 displays the room temperature dc conductivity values and the conductance ratios for the graphite, PANI and the G–PANI composites. While pristine graphite exhibits a conductivity ratio, sr ¼ s (300 K)/ s (5 K), that lies in the metallic region (sr o2) the composite with highest conductivity exhibits a conductivity ratio that lies in the critical region (2o sr o6). The G–PANI composite samples with sr greater than pristine graphite still exhibit room temperature conductivities above that of pristine graphite. This is contrary to the initial expectations of the composite. It was thought that since higher room temperature conductivity was observed, the material might exhibit electronic conduction closer to that of a metal. The analysis actually describes a disordered material with higher temperature dependence than graphite (or PANI) alone. Even though a more disordered material exists, it is thought that the disorder results from an increase in localized charge carriers throughout the bulk material. These localized charge carriers conduct electricity through a 3-dimensional variable range hopping (VRH) mechanism [14]. Another implication of these results is that the G–PANI composite behaves more like a semiconductor than a semi-metal as a result of charge transfer between the graphite and PANI. As the charge transfer takes place between graphite and PANI, localized charge carriers would be formed within the graphite crystallites, and may result in these electronically ‘‘disordered’’ regions. Even though a more ‘‘disordered’’ system seems to exist
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as a result of the charge transfer, more carriers are present that result in an increase in electrical conductivity. This enhancement in conductivity results from increased charge mobility in the direction perpendicular to the graphite planes [26]. X-ray diffractograms are displayed in Fig. 3 and provide more insight to the effect PANI has on the graphite crystal structure due to the electronic interaction at the atomic level. While the electronic nature of the composites becomes ‘‘disordered’’, the graphitic structure was observed to become more crystalline as evidenced by a dramatic increase in the intensity of the peaks at 261, which correspond to the (002) plane of the graphite unit cell (c-axis) [27,28]. It must be noted that while an increase in electronic disorder is observed from the temperature dependent conductivity studies, this does not necessarily correlate to structural disorder.
Fig. 3. X-ray diffraction patterns of black—graphite, green—Composite 4, blue—Composite 3, aqua—Composite 2, magenta—Composite 1 and red—PANI. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The diffraction patterns show an increased intensity for certain lattice parameters, however, no ‘‘major’’ structural changes, such as those that would result in shifts in the XRD peaks, in the graphite crystal lattice has been observed. The experimentally observed increase of intensity for the diffraction peak at 261 can be attributed to the charge transfer that occurs between the PANI and graphite crystallites, resulting in relaxation of the atomic positions within the graphene planes and a higher degree of structural ordering in the normal direction to the graphene planes [28]. In order to determine relative mass percents of the composites, the thermograms were analyzed. Mass differences observed in the thermograms were taken from the point of minimal mass loss per time (from the derivative curves) corresponding to PANI and graphite burnoff. The temperature for PANI burnoff was generally between 150 1C and 600 1C; graphite burnoff was between 600 1C and 900 1C. The analysis was performed on thermograms displayed in Fig. 4 using the STARe Software. While the thermal analysis was necessary for determining the ratio of components in the composite, information regarding thermal stability was also gathered through this characterization. Analysis of the thermogram derivatives, presented in Fig. 4, along with analysis of the thermograms for onset of mass loss gives a good comparison for thermal stability. Fig. 4 displays several samples along with graphite and PANI for comparisons. The samples that are shown to be more thermally stable than graphite alone (line drawn for emphasis) are Composites 2–4 with graphite-to-PANI ratios of 72/28, 85/15, and 91/9, respectively. In order to exemplify the increase in thermal stability of the composites, we have determined the onset of mass loss for the PANI component and graphite component of the samples. As expected the onset of mass loss for PANI is drastically increased in the composite, but what is more interesting is the increase for the onset for graphite degradation. Three out of four of the composite
Fig. 4. Thermal properties of samples: left—thermograms and top right—derivatives of thermograms (black—graphite, green—Composite 4, blue—Composite 3, aqua—Composite 2, magenta—Composite 1 and red—PANI); bottom right—thermal analysis of samples (black squares—temperature of maximum rate of mass loss and blue triangles—temperature of onset of mass loss). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
S.E. Bourdo et al. / Journal of Solid State Chemistry 196 (2012) 309–313
samples exhibit an onset of mass loss at þ20 1C as compared to graphite alone.
4. Conclusion We have shown that an increase in conductivity of G–PANI composites occurs at a certain ratio of graphite to polyaniline. This observation has been previously correlated with structural investigations [4] and has now been correlated with electron transport studies. Temperature dependent studies confirm a change in electronic nature of the G–PANI composites. Since the conductivity ratio is a measure of the localized charge carriers (electronic disorder), the study indicates a transition from more ordered system (pristine graphite) to a less ordered system (G– PANI composite), while the G–PANI composite has been determined to have a higher electrical conductivity than pristine graphite. Even though a more ‘‘disordered’’ electronic system is present in the composites as compared to the graphite, an increased structural order in the c-axis of the graphite crystal lattice has been observed. The charge transfer interaction between guest and host in graphite systems is said to be enhanced when an acceptor type compound are present [29]. This charge transfer between PANI and graphite results in enhanced electrical conductivity greater than the individual components, which may allow for these composites to be utilized in a variety of applications.
Acknowledgments We gratefully acknowledge support from the US Department of Energy (Grant no. DE-FG 36-06 GO 86072). References [1] J.K. Avlyanov, S. Dahman, in: B.R. Hsieh, Y. Wei (Eds.), Semiconducting Polymers (ACS Symposium Series 735), ACS Publishing, Washington, DC, 1999, pp. 270–279.
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[2] B. Veeraraghavan, J. Paul, B. Haran, B. Popov, J. Power Sources 109 (2002) 377–387. [3] H. Karami, M.F. Mousavi, M. Shamsipur, J. Power Sources 117 (2003) 255–259. [4] S.E. Bourdo, Z. Li, A.S. Biris, F. Watanabe, T. Viswanathan, I. Pavel, Adv. Funct. Mater. 18 (2008) 432–440. [5] S.E. Bourdo, T. Viswanathan, Carbon 43 (2005) 2983–2988. [6] P. Xiao, M. Xiao, P. Liu, K. Gong, Carbon 38 (2000) 623–628. [7] P. Liu, K. Gong, Carbon 37 (1999) 706–707. [8] X.S. Du, M. Xiao, Y.Z. Meng, J. Polym. Sci., Part B: Polym. Phys. 42 (2004) 1972–1978. [9] X.S. Du, M. Xiao, Y.Z. Meng, Eur. Polym. J. 40 (2004) 1489–1493. [10] K. Lee, S. Cho, S.H. Park, A.J. Heeger, C.W. Lee, S.H. Lee, Nature 441 (2006) 65–68. [11] S.H. Lee, D.H. Lee, K. Lee, C.W. Lee, Adv. Funct. Mater. 15 (2005) 1495–1500. [12] Y. Meng, in: Y.W. Mai, Z.-Z. Yu (Eds.), Polymer Nanocomposites, CRC Press, Boca Raton, FL, 2006, pp. 510–539. [13] H.H. Kuhn, A.D. Child, in: T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, 2nd ed.,Marcel Dekker, New York, 1998, pp. 993–1013. [14] D.D.L. Chung, Carbon 39 (2001) 279–285. [15] D.D.L. Chung, J. Mater. Sci. 37 (2002) 1475–1489. [16] C.Y. Lee, H.G. Song, K.S. Jang, E.J. Oh, A.J. Epstein, J. Joo, Synth. Met. 102 (1999) 1346–1349. [17] J. Joo, A.J. Epstein, Appl. Phys. Lett. 65 (1994) 2278–2280. [18] A.J. Heeger, J. Phys. Chem. B 105 (2001) 8475–8491. [19] A.G. MacDiarmid, Rev. Mod. Phys. 73 (2001) 701–712. [20] B. Liu, S. Bourdo, E. Dervishi, B. Berry, H. Kim, A. Bhattacharyya, T. Viswanathan, Proceedings of SPIE—The International Society for Optical Engineering 5388 (Industrial and Commercial Applications of Smart Structures Technologies), 2004, pp. 355–363. [21] Kai Zhang, Li Zhang, X.S. Zhao, Jishan Wu, Chem. Mater. 22 (2010) 1392–1401. [22] R. Menon, C.O. Yoon, D. Moses, A.J. Heeger, in: T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, 2nd ed.,Marcel Dekker, New York, 1998, pp. 27–84. [23] R. Menon, C.O. Yoon, D. Moses, A.J. Heeger, Y. Cao, Phys. Rev. B: Condens. Matter Mater. Phys. 48 (1993) 17685–17694. [24] S. Elliot, The Physics and Chemistry of Solids, John Wiley and Sons, New York, 1998, p. 770. [25] M. Ghosh, A. Barman, A.K. Meikap, S.K. De, S. Chatterjee, Phys. Lett. A 260 (1999) 138–148. [26] N. Bartlett, B.W. McQuillan, in: M.S. Whittingham, A.J. Jacobson (Eds.), Intercalation Chemistry, Academic Press, New York, 1982, pp. 19–50. [27] Odd E. Andersson, B.L.V. Prasad, Hirohiko Sato, Toshiaki Enoki, Yoshihiro Hishiyama, Yutaka Kaburagi, Masanori Yoshikawa, Shunji Bandow, Phys. Rev. B 58 (1998) 16387–16395. [28] Z.Q. Li, C.J. Lu, Z.P. Xia, Y. Zhou, Z. Luo, Carbon 45 (2007) 1686–1695. [29] T. Enoki, J. Phys. Chem. Solids 65 (2004) 103–108.
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Electrical and thermal properties of graphite/polyaniline composites Shawn E. Bourdo a,n, Brock A. Warford b, Tito Viswanathan b a b
Center for Integrative Nanotechnology Sciences, University of Arkansas at Little Rock, 2801 South University Avenue, Little Rock, AR 72204, USA Department of Chemistry, University of Arkansas at Little Rock, 2801 South University Avenue, Little Rock, AR 72204, USA
a r t i c l e i n f o
abstract
Article history: Received 19 March 2012 Received in revised form 20 June 2012 Accepted 24 June 2012 Available online 1 July 2012
A composite of a carbon allotrope (graphite) and an inherently conducting polymer, polyaniline (PANI), has been prepared that exhibits an electrical conductivity greater than either of the two components. An almost 2-fold increase in the bulk conductivity occurs when only a small mass fraction of polyaniline exists in the composite (91% graphite/ 9% polyaniline, by mass). This increase in dc electrical conductivity is curious since in most cases a composite material will exhibit a conductivity somewhere between the two individual components, unless a modification to the electronic nature of the material occurs. In order to elucidate the fundamental electrical properties of the composite we have performed variable temperature conductivity measurements to better understand the nature of conduction in these materials. The results from these studies suggest a change in the mechanism of conduction as the amount of polyaniline is increased in the composite. Along with superior electrical properties, the composites exhibit an increase in thermal stability as compared to the graphite. & 2012 Elsevier Inc. All rights reserved.
Keywords: Composites Conducting polymers Electrical properties X-ray diffraction Thermogravimetric analysis
1. Introduction Inherently conducting polymers (ICPs) have been at the forefront of research initiatives for the last several decades due to many unique chemical, optical, and electrical properties. When ICPs are combined with carbonaceous materials, improved thermal and electrical properties may be observed [1–3]. In our lab we have produced such a material that exhibits an electrical conductivity greater than the two components of the composite: polyaniline (PANI) and graphite (G). This observation was curious since, in many instances, the combination of materials with differing electrical properties will produce a composite with electrical conductivity between that of the two individual components. There have only been two such reports on this curious observation for G–PANI composites, both by our group. Previous studies led to the hypothesis that a charge transfer occurs between the graphite and PANI components of the composite that results in superior electrical conductivity and increased crystallinity [4,5]. Our focus of the present report is to investigate the mechanism of conduction through variable temperature conductivity measurements. Several methods have been employed to improve the characteristics of ICPs and ICP composites. There have been reports of graphite oxide–polyaniline composites [6,7] and graphite nanosheet–polyaniline composites [8,9], but neither has shown
n
Corresponding author. Fax: þ1 501 683 7601. E-mail address: [email protected] (S.E. Bourdo).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.06.038
conductivities higher than the two components. Other groups have focused on producing highly linear polymers by modifying reaction conditions for the synthesis of PANI. There are recent articles discussing production of metallic type conducting polyaniline by the self-stabilized dispersion polymerization (SSDP) method [10,11]. We had previously observed a dramatic increase in the conductivity of G–PANI composites [4,5] and now here the characterization of a composite where the synthetic approach combines the use of graphite nanosheets [8,9,12] and a modified SSDP method (where the graphite serves as a dispersed organic media). The reaction media and conditions for synthesis of composite materials should be amenable to the individual constituents of the composite (solvent, temperature, etc.). For this procedure we have utilized a common media for graphite dispersion [8,9,12] and self-stabilized dispersion polymerization of aniline [10,11], namely the organic dispersant/solvent, isopropanol. The concept of self-stabilized dispersion polymerization technique is ideally suited for these composites since the reaction is carried out in a heterogeneous biphasic medium to prevent undesirable side reactions. The ICP compatibility with graphite surface would promote it to adhere and align parallel to the surface of the graphite resulting in a uniform composite sample. Four composite samples were characterized by thermogravimetric analysis to determine the ratio of graphite to PANI, followed by electrical characterization. These results were then compared with samples of pristine PANI and graphite. At certain ratios of graphite to ICP the bulk conductivity is higher than the two components alone, and it is this observation that we have
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S.E. Bourdo et al. / Journal of Solid State Chemistry 196 (2012) 309–313
investigated further through variable temperature conductivity studies. These studies provided some insight to the conduction mechanism of these materials. Along with superior electrical properties, the composites of graphite and polyaniline exhibited increased thermal stability. Both components have a plethora of potential applications and therefore further investigation could lead to use of these composites in any number of technologies. Touted applications that could benefit from the properties of these composites exist in a wide range of fields, including electromagnetic interference (EMI) shielding [13–17], low power rechargeable batteries [2,3,18], electrostatic dissipation (ESD) for anti-static textiles [13], electronic devices [19], resistive heating [20], and supercapacitors [21].
2. Experimental 2.1. Synthesis Graphite nanosheets were prepared according to methods in references [8,9] and [12].In a typical preparation of stock expanded graphite 50 g of natural flake graphite (Graphite Sales, Inc.) was placed into a 4 L beaker with a stir bar. The beaker was placed on a stir plate, 500 mL of concentrated H2SO4 added and the mixture stirred until the graphite appeared well dispersed. To this mixture 50 mL of 30% H2O2 was added slowly but all at once. The mixture bubbled and gas was given off immediately as the volume level appeared to double within about 30–60 s. The volume then steadily decreased while the mixture continued to bubble for approximately 15 min. The mixture was stirred for approximately 2 h at which time it was diluted by 2 successive additions of 250 ml deionized water; the mixture was allowed to cool between additions. The mixture was filtered by vacuum through a sintered glass funnel. The solids were dried in an oven at 100 1C overnight. Briefly, a certain mass of graphite was placed into a porcelain dish and placed in a furnace at 900 1C for 15 min that yielded expanded graphite. The expanded graphite was dispersed in isopropyl alcohol by sonication for 1–2 h to break the graphite into smaller graphite ‘‘nanosheets’’. A solution of 2 mL of aniline in 120 mL 1 M HCl was then added to a reaction vessel. The vessel was placed on a cold plate stirrer and cooled to approximately 15 1C. While the reaction mixture was cooling, a 3.93 g sample of sodium persulfate (Na2S2O8) was dissolved in 40 mL of 1 M HCl and chilled on ice. Once the reaction mixture had reached 15 1C or below, the oxidant solution was added dropwise over a period of about 40 min. After at least 4 h, the reaction was vacuum filtered through Whatman 4 filter paper and washed with deionized water until the filtrate was colorless. The wet cake was then washed with 50 mL of 1 M HCl. A portion of this wetcake labeled as sample ‘‘A’’ was dried under vacuum for at least 24 h and used for electrical characterization. Another portion was labeled as sample ‘‘B’’ and was washed with 10 mL of 1 M NH4OH. The ‘‘B’’ sample was then transferred to an Erlenmeyer flask and stirred with at least 100 mL of 0.1 M NH4OH overnight. The solids were filtered, washed again with 10 mL of 1 M NH4OH and then with deionized water until the filtrate was at neutral pH. The solids were then dried in a vacuum oven for at least 24 h. The ‘‘B’’ sample was used to determine mass ratio of graphite to PANI using thermogravimetric analysis. 2.2. Characterization 2.2.1. Room temperature conductivity Room temperature DC bulk conductivity measurements were made on pressed pellets (13 mm diameter using 15000 lbs of force,
o1 mm thick) with an Alessi four-point conductivity probe. A Keithley 617 programmable electrometer and a Keithley 224 programmable current source were used for the measurement of voltage as a function of current. Four measurements were taken and an average has been reported. 2.2.2. Temperature dependent conductivity Temperature dependent DC bulk conductivity measurements were made on pressed pellets (13 mm diameter using 15000 lbs of force, o1 mm thick) using a four-point probe assembly inside a Janis SVT Cryostat System. The temperature was regulated using a Lakeshore 321 Temperature Controller. The electrical measurements were made by sourcing current with a Keithely 2400 Sourcemeter and measuring the voltage with a Keithely 2182 A Nanovoltmeter. Electrical measurements were made at varied intervals, every 51 from 5 K to 100 K then every 101 from 100 K to 300 K. After each step increase, the temperature was allowed to stabilize for 1 min before a measurement was taken. 2.2.3. X-ray diffraction analysis X-ray diffraction patterns were obtained on pressed pellets of the composites similar to those used for the conductivity studies. Typically, powdered samples were loaded into a 13 mm pellet die and compressed with a force of 15,000 lbs using a hydraulic press. The X-ray diffraction patterns of the pellets were measured on a Bruker AXS D8 Discover, with a GADDS 2D counter. Copper K alpha line was used as excitation source and the X-ray tube ran at 40 kV and 35 mA. 2.2.4. Thermal analysis Thermogravimetric measurements were performed under airflow of 150 mL/min using a Mettler Toledo TG50 connected to a TC15 controller. The temperature was increased at a rate of 10 1C/ min. Analysis was performed using STARe software. Composite samples were powdered and masses of approximately 100 mg were used for the analysis. 70 mL alumina crucibles were used for the analysis.
3. Results and discussion Four composite samples were characterized along with pristine PANI and graphite. The weight ratio of G to PANI determined by thermogravimetric analysis in Composites 1, 2, 3, and 4 are 31/ 69, 72/28, 85/15, and 91/9, respectively. Fig. 1 displays the room temperature dc conductivity measurements determined by the four-point collinear probe method. G–PANI Composite 4 had the highest conductivity, and both Composites 3 and 4 displayed a higher conductivity than pristine graphite. Transport studies are crucial to explaining the conductivity characteristics of materials. Temperature dependent direct current conductivity studies can be very useful in probing the conduction mechanism of a material [10,22,23]. This type of analysis has broadened our understanding of why an increase in conductivity is observed for this system. By varying the temperature and measuring electrical conductivity, one can gain useful information regarding the mechanism of conduction for a material: metallic or semiconducting. The electrical conductivity of metals and semiconductors varies with temperature in a directly opposite manner. The electrical conductivity of a metal increases as the temperature decreases, while the conductivity of a semiconductor decreases with decrease in temperature. The mobility of electrons within any material directly influences the electrical properties of the material. For metals, electrons are highly mobile within the band structure of the material and do not have any thermal barriers
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Fig. 1. Room temperature dc electrical conductivity values for PANI, graphite, and four G–PANI composites: red circle—pristine PANI, green triangle—31% G/69% PANI, blue triangle–72% G/28% PANI, aqua diamond—85% G/15% PANI, magenta triangle—91% G/9% PANI and black square—graphite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to overcome in order to be mobile. When the temperature is decreased, the crystal lattice of the metal becomes much more rigid (less vibrations) and it is for this reason that metallic conduction increases as temperature decreases. The electrons are able to move throughout the lattice with fewer impediments due to decreased vibrational lattice distortions. For semiconductors/insulators, thermal barriers must be overcome for an electron to be mobile within the crystal lattice. At 0 K the valence band is completely filled with electrons, and the conduction band is completely void of electrons. As the temperature increases these electrons gain more energy and can then become mobile charge carriers. The mobility of charge in semiconductors generally follows a variable range hopping mechanism (VRH) in which electrons will ‘‘hop’’ between localized states of similar energies, even if there is a shorter path along a chain (as in ICPs) or plane (as in graphite) [24]. It is for these reasons that we have studied the temperature dependent conductivity of G–PANI composites to better understand the mechanism of conduction in these unique materials. The investigation into the temperature dependent conductivity of these materials reinforces the claim of charge transfer and furthers the discussion into the nature of the conduction mechanism of the composites. Results from temperature dependent studies (Fig. 2) have shown dramatic change in conductivities upon temperature variation. The figure above displays that the composites exhibit a greater dependence on temperature than graphite alone. This implies that the charge carriers are more localized in the composites than in pristine graphite and are closer to that of a Fermi glass (insulator) than graphite, which is a semimetal. Several publications report the ratio of conductivity (or resistivity) values at ambient temperature (300 K) to the values at low temperatures (5 K and lower) as a measure of the degree of disorder and an effective means of qualitatively evaluating temperature dependent conductivity data. Conductance ratios, sr ¼ s (300 K)/ s (5 K), of less than 2 indicate conduction in the metallic region and imply a highly ordered sample. For 2o sr o6 the sample is said to lie in the critical region, and for 6o sr, the sample is said to be in the insulating region (a Fermi glass) [22,23,25]. Therefore by taking the ratio of conductivity values one can gain insight into the degree of disorder and type of conduction that dominates a specific
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Fig. 2. Temperature dependence of dc conductivity for several G–PANI composites of graphite (G) and HCl-doped polyaniline (PANI): red circle—pristine PANI, green triangle—31% G/69% PANI, blue triangle—72% G/28% PANI, aqua diamond—85% G/15% PANI, magenta triangle—91% G/9% PANI and black square—graphite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Values of room temperature dc conductivities and conductance ratios, s (300 K)/ s (5 K), for HCl doped composites. Sample
PANI Composite Composite Composite Composite Graphite
1 2 3 4
% Graphite (w/w)
Room temperature dc conductivity (S/cm)
Conductivity ratio
0 31 72 85 91 100
4.5 40 250 570 880 460
2.70 105 1.90 107 260 15 5.0 1.4
material. Table 1 displays the room temperature dc conductivity values and the conductance ratios for the graphite, PANI and the G–PANI composites. While pristine graphite exhibits a conductivity ratio, sr ¼ s (300 K)/ s (5 K), that lies in the metallic region (sr o2) the composite with highest conductivity exhibits a conductivity ratio that lies in the critical region (2o sr o6). The G–PANI composite samples with sr greater than pristine graphite still exhibit room temperature conductivities above that of pristine graphite. This is contrary to the initial expectations of the composite. It was thought that since higher room temperature conductivity was observed, the material might exhibit electronic conduction closer to that of a metal. The analysis actually describes a disordered material with higher temperature dependence than graphite (or PANI) alone. Even though a more disordered material exists, it is thought that the disorder results from an increase in localized charge carriers throughout the bulk material. These localized charge carriers conduct electricity through a 3-dimensional variable range hopping (VRH) mechanism [14]. Another implication of these results is that the G–PANI composite behaves more like a semiconductor than a semi-metal as a result of charge transfer between the graphite and PANI. As the charge transfer takes place between graphite and PANI, localized charge carriers would be formed within the graphite crystallites, and may result in these electronically ‘‘disordered’’ regions. Even though a more ‘‘disordered’’ system seems to exist
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as a result of the charge transfer, more carriers are present that result in an increase in electrical conductivity. This enhancement in conductivity results from increased charge mobility in the direction perpendicular to the graphite planes [26]. X-ray diffractograms are displayed in Fig. 3 and provide more insight to the effect PANI has on the graphite crystal structure due to the electronic interaction at the atomic level. While the electronic nature of the composites becomes ‘‘disordered’’, the graphitic structure was observed to become more crystalline as evidenced by a dramatic increase in the intensity of the peaks at 261, which correspond to the (002) plane of the graphite unit cell (c-axis) [27,28]. It must be noted that while an increase in electronic disorder is observed from the temperature dependent conductivity studies, this does not necessarily correlate to structural disorder.
Fig. 3. X-ray diffraction patterns of black—graphite, green—Composite 4, blue—Composite 3, aqua—Composite 2, magenta—Composite 1 and red—PANI. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The diffraction patterns show an increased intensity for certain lattice parameters, however, no ‘‘major’’ structural changes, such as those that would result in shifts in the XRD peaks, in the graphite crystal lattice has been observed. The experimentally observed increase of intensity for the diffraction peak at 261 can be attributed to the charge transfer that occurs between the PANI and graphite crystallites, resulting in relaxation of the atomic positions within the graphene planes and a higher degree of structural ordering in the normal direction to the graphene planes [28]. In order to determine relative mass percents of the composites, the thermograms were analyzed. Mass differences observed in the thermograms were taken from the point of minimal mass loss per time (from the derivative curves) corresponding to PANI and graphite burnoff. The temperature for PANI burnoff was generally between 150 1C and 600 1C; graphite burnoff was between 600 1C and 900 1C. The analysis was performed on thermograms displayed in Fig. 4 using the STARe Software. While the thermal analysis was necessary for determining the ratio of components in the composite, information regarding thermal stability was also gathered through this characterization. Analysis of the thermogram derivatives, presented in Fig. 4, along with analysis of the thermograms for onset of mass loss gives a good comparison for thermal stability. Fig. 4 displays several samples along with graphite and PANI for comparisons. The samples that are shown to be more thermally stable than graphite alone (line drawn for emphasis) are Composites 2–4 with graphite-to-PANI ratios of 72/28, 85/15, and 91/9, respectively. In order to exemplify the increase in thermal stability of the composites, we have determined the onset of mass loss for the PANI component and graphite component of the samples. As expected the onset of mass loss for PANI is drastically increased in the composite, but what is more interesting is the increase for the onset for graphite degradation. Three out of four of the composite
Fig. 4. Thermal properties of samples: left—thermograms and top right—derivatives of thermograms (black—graphite, green—Composite 4, blue—Composite 3, aqua—Composite 2, magenta—Composite 1 and red—PANI); bottom right—thermal analysis of samples (black squares—temperature of maximum rate of mass loss and blue triangles—temperature of onset of mass loss). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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samples exhibit an onset of mass loss at þ20 1C as compared to graphite alone.
4. Conclusion We have shown that an increase in conductivity of G–PANI composites occurs at a certain ratio of graphite to polyaniline. This observation has been previously correlated with structural investigations [4] and has now been correlated with electron transport studies. Temperature dependent studies confirm a change in electronic nature of the G–PANI composites. Since the conductivity ratio is a measure of the localized charge carriers (electronic disorder), the study indicates a transition from more ordered system (pristine graphite) to a less ordered system (G– PANI composite), while the G–PANI composite has been determined to have a higher electrical conductivity than pristine graphite. Even though a more ‘‘disordered’’ electronic system is present in the composites as compared to the graphite, an increased structural order in the c-axis of the graphite crystal lattice has been observed. The charge transfer interaction between guest and host in graphite systems is said to be enhanced when an acceptor type compound are present [29]. This charge transfer between PANI and graphite results in enhanced electrical conductivity greater than the individual components, which may allow for these composites to be utilized in a variety of applications.
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