Langmuir 2006, 22, 1729-1734
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Encapsulation of Polyanilines into Graphite Oxide Rabin Bissessur,* Peter K. Y. Liu, Wade White, and Stephen F. Scully Department of Chemistry, UniVersity of Prince Edward Island, Charlottetown, Prince Edward Island, Canada C1A 4P3 ReceiVed October 9, 2005. In Final Form: December 9, 2005 Herein we report on the intercalation of polyaniline, poly(2-ethylaniline), and poly(2-propylaniline) into graphite oxide. This was achieved by taking advantage of the exfoliation/reconstruction properties of the layered host. The resulting intercalates were characterized by powder X-ray diffraction and thermogravimetric analysis.
Introduction Graphite oxide (GO) is a layered structure that has attracted considerable attention in recent years.1-4 It is obtained through the reaction of graphite with strong oxidants such as potassium permanganate in concentrated sulfuric acid.5 Similar to graphite, graphite oxide has a lamellar structure; however, the latter is hydrophilic and therefore amenable to the intercalation of polar guest species such as poly(ethylene oxide)6 and poly(furfuryl alcohol).7 The intercalation of electrically conductive polymers such as polyaniline, polypyrrole, and polythiophene into GO is of interest because the resulting nanocomposites could possess physical properties derived synergistically from both components, such as enhancement in electrical conductivity and mechanical strength. The intercalation of polyaniline (PANI) into GO was previously reported by both the Matsuo8 and Gong9 groups, using analogous methodologies. A colloidal suspension of GO was first obtained by treatment with a dilute solution of sodium hydroxide. Aniline monomer was then added to the colloidal suspension followed by the slow addition of an aqueous solution of an oxidizing agent. The Gong group used iron(III) chloride as the oxidant, whereas Matsuo et al. used ammonium peroxydisulfate. The product obtained by the Matsuo group showed strong evidence of intercalation from XRD measurements; however, Gong’s product did not. In light of this, Gong et al. re-explored the intercalation of PANI into GO10 and used a modification of their previously reported procedure. An intercalation compound of aniline into GO was prepared and then treated with ammonium peroxydisulfate, resulting in the formation of the desired compound. XRD measurements support the formation of aniline/GO and PANI/GO phases.10 The Matsuo group also re-explored the intercalation of PANI into GO, by using a different synthetic route.11 This time, an intercalation compound of n-hexylamine into GO was prepared. Further treatment of this compound with an NMP solution of PANI afforded (PANI)xGO through an ion exchange reaction. * To whom correspondence should be addressed. Phone: (902) 5660510. Fax: (902) 566-0632. E-mail:
[email protected]. (1) Cassagneau, T.; Gue´rin, F.; Fendler, J. H. Langmuir 2000, 16, 7318. (2) Cassagneau, T.; Fendler, J. H. AdV. Mater. 1998, 10, 877. (3) Ding, R.; Hu, Y.; Gui, Z.; Zong, R.; Chen, Z.; Fan, W. Polym. Degrad. Stab. 2003, 81, 473. (4) Kovtyukhova, N. I.; Olliver P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771. (5) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (6) Matsuo, Y.; Tahara, K.; Sugie, Y. Carbon 1997, 35, 113. (7) Kyotani, T.; Moriyama, H.; Tomita, A. Carbon 1997, 35, 1185. (8) Higashika, S.; Kimura, K.; Matsuo, Y.; Sugie, Y. Carbon 1999, 37, 351. (9) Liu, P.; Gong, K. Carbon 1999, 37, 701. (10) Xiao, P.; Xiao, M.; Liu, P.; Gong, K. Carbon 2000, 38, 623. (11) Matsuo, Y.; Higashika, S.; Kimura, K.; Miyamoto, Y.; Fukutsuka, T.; Sugie, Y. J. Mater. Chem. 2002. 12, 1592.
While the intercalation of polyaniline into GO has been examined to a certain extent, there are no reports on the insertion of substituted polyanilines. Therefore, we have decided to investigate the intercalation of poly(2-ethylaniline) (PEA) and poly(2-propylaniline) (PPA) into GO, in addition to that of the parent polymer polyaniline for direct comparison. Furthermore, our technique of intercalation differs from the previously reported methods. In this paper we demonstrate that polyanilines can be directly inserted into GO without preparation of precursor intercalation compounds. Experimental Section Materials. Aniline, 2-ethylaniline, 2-propylaniline, and ammonium peroxydisulfate were purchased from Aldrich. The aniline was dried over KOH and then distilled under reduced pressure. The other reagents were used as received without further purification. Synthesis of Polymers. Aniline, 2-ethylaniline, and 2-propylaniline were dissolved in 1 M HCl and then cooled to 0 °C. Ammonium peroxydisulfate (1.15 equiv) was then added to the monomer solutions. The reaction mixture was allowed to stir at ice temperature, with the exception of that for the polymerization of 2-propylaniline, which was stirred at room temperature. The reaction time varied, depending on the monomer, ranging from 1.5 h for aniline to 3.5 h for 2-ethylaniline and overnight for 2-propylaniline. The reaction mixtures were filtered under reduced pressure and washed with 1 M HCl. The dark blue products obtained were allowed to dry under reduced pressure and then freeze-dried. Synthesis of GO. The preparation of graphite oxide was adapted from ref 5. Concentrated sulfuric acid (230 mL) was transferred to a 6 L Erlenmeyer flask and was cooled to 0 °C. Graphite powder (10 g) was then added to the flask, with mechanical stirring. KMnO4 (30 g) was slowly added to the dark mixture such that the temperature was not allowed to surpass 20 °C. The reaction mixture was then allowed to cool to 2 °C and stirred at room temperature for 30 min. Deionized water (230 mL) was slowly added to the reaction vessel, whereupon a rapid rise in temperature was again observed, and care was exercised to keep the temperature below 98 °C. The reaction mixture was then allowed to stir for 15 min, and an additional 1.4 L of deionized water was quickly added, immediately followed by hydrogen peroxide (100 mL). The resultant mixture was allowed to stand overnight; the GO particles settled to the bottom, and the excess liquid was decanted. The remaining liquid and solid particles were transferred to dialysis tubes (MW cutoff 3500), and dialysis was performed until no precipitate of BaSO4 could be detected upon addition of an aqueous solution of barium chloride. The contents of the dialysis bags were transferred to a Schlenk flask, and the water was removed under reduced pressure. The resulting gel-like material was then freeze-dried to a fine brown powder. Elemental analyses of GO gave the following results, from which the composition C8H3.5O5.1 was calculated: C, 47.16; H, 1.72; O, 40.27.
10.1021/la0527339 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/14/2006
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Table 1. Major Vibrations (cm-1) of the Polymers band
PANI
PEA
PPA
N-H alkyl C-H aromatic C-H bend benzene C-H stretch quinone C-H stretch benzene C-N quinone C-N alkyl C-N
3424 none 2919 1648 1460 1296 1114 none
3324 2955 nonea 1590 1491 1211 1163 none
∼3200a 2955 nonea 1585 1499 1213 1161 none
a
The band is too broad to assign an exact wavenumber.
Intercalation of Polyanilines into Graphite Oxide. To graphite oxide (0.1 g, 6 × 10-4 mol) was added 11 mL of deionized water, and the resulting suspension was sonicated for 30 min. To 1 equiv of the polymer (PANI, PEA, or PPA) was added 40 mL of NMF, and the suspension was sonicated for 1/2 h. The polymer colloid was added to the GO suspension, and the reaction mixture was sonicated for 30 min. It was then allowed to stir overnight at room temperature. Five drops of concentrated HCl solution was added to the reaction mixture, which was then stirred for another day, followed by heating at 60 °C for 90 min. The reaction mixture was then allowed to sit overnight, without any disturbance, and thereafter centrifuged for 30 min. The supernatant liquid was decanted, and the wet product was freeze-dried for 3 days. Instrumentation. Infrared spectra were obtained on a PerkinElmer 1600 FTIR series instrument. Powder X-ray diffraction (XRD) was run on a Bruker AXS D8 Advance diffractometer equipped with a graphite monochromator, variable divergence and antiscatter slits, and a scintillation detector. Cu KR radiation (λ ) 1.542 Å) was used, and the data collection was carried out in air, at room temperature, using a scan range of 2-60°. Solid samples were pressed as thin pellets and run on a nondiffracting silicon substrate. The aqueous suspension of exfoliated graphite oxide was run on a plastic substrate, whereas restacked graphite oxide was run as a thin film on a glass substrate.
Figure 1. Powder pattern of PPA.
Thermogravimetric analysis (TGA) was performed on a TA 500 instrument using a heating rate of 5 °C/min.
Results and Discussion FTIR spectroscopy confirms that the polymers have been successfully synthesized. The major vibrations of the polymers are summarized in Table 1. Polyaniline exhibits a strong, broad vibration at 3424 cm-1 which is indicative of the secondary amine in the polymer backbone. The band at 2919 cm-1 is due to stretching of the C-H bonds in the phenyl rings. The CdC bands for the benzene and quinone groups are visible at 1648 and 1460 cm-1, respectively. The vibrational bands at 1296 and 1114 cm-1 are from the presence of C-N bonds in the benzene and quinone rings, respectively. The infrared spectrum of PEA shows a broad absorption band at 3324 cm-1 which is again typical of a secondary amine. The alkyl C-H stretch is observed at 2955 cm-1, which is well within the range for this type of vibration. The strong peaks at 1590 and 1491 cm-1 are ascribed to the benzene and quinone CdC bends, respectively. The peaks at 1211 and 1163 cm-1 are due to C-N bonds to benzene and quinone groups, respectively. The FTIR spectrum of PPA shows a weak, broad band above 3000 cm-1 which is indicative of a N-H stretch of a secondary amine. The alkyl C-H stretch is observed at 2955 cm-1. The benzene CdC bend is observed at 1585 cm-1, and the vibration for the quinone CdC bend can be seen at 1499 cm-1. Vibration from C-N bonds can be observed at 1213 and 1161 cm-1, corresponding to the benzene and quinone moieties, respectively. Powder X-ray diffraction shows that the polymers are amorphous. The diffractograms consist of a single broad peak, which clearly indicates a disordered structure. As an illustration the powder pattern of PPA is depicted in Figure 1.
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Table 2. Comparison of the Thermal Stability of the Polymers in Air versus a Nitrogen Atmosphere thermal stability (°C)
thermal stability (°C)
polymer
air
N2
polymer
air
N2
PANI PMA
377
471
PEA PPA
364 317
375 400
Table 3. Electrical Conductivity Values of the Polymers polymer
electrical conductivity(S/cm)
PANI PEA PPA
1 × 101 3 × 10-4 1 × 10-4
Since the polymers are not completely soluble in NMF, we could not determine their molecular weight and molecular weight distribution by gel permeation chromatography. However, we believe the molecular weight of the polymers to be high. Thermogravimetric analyses were performed on the polymers to gain insight into their decomposition characteristics and to determine their thermal stabilities both in compressed air and under a nitrogen atmosphere. Under compressed air, PANI starts to show a gradual weight loss up to 377 °C. By that temperature, a total weight loss of 24% was recorded. This initial weight loss is assigned to loosely bound water molecules and the evolution of dopant HCl. From 377 to 600 °C, a major weight loss of 73.87% was observed which corresponds to the decomposition of the polymer backbone. Beyond 600 °C, the remaining carbon residue (2.345%) was stable up to 1000 °C. The other polyaniline derivatives show similar decomposition patterns. In air, PEA shows an initial weight loss of 29.80%. The material was stable up to 364 °C. Beyond that temperature, a major weight loss of 70.23% was observed up to 600 °C. The remaining residue (0.23%) was found to be stable up to 1000 °C. In the case of PPA, an initial weight loss of 17.42% was observed up to 317 °C, followed by a major weight loss of 81.43%
Figure 2. XRD of pristine GO.
till 600 °C was reached. The residue left (1.43%) was found to be stable up to 1000 °C. The thermal stabilities of the polymers were also studied under a nitrogen atmosphere. In general, it was found that the polymers were more stable under nitrogen flow than under compressed air. The results are tabulated in Table 2. The electrical conductivities of the polymers were determined by using the van der Pauw technique as described in ref 12. The conductivity of PANI was found to be 10 S/cm, the highest value among the polymers studied in this work. This is within the range of values that has been reported by other workers.13 However, compared to PANI, PEA shows a dramatic decrease in conductivity by a factor of 3 × 104. An even further decrease in conductivity was observed for PPA. These results are tabulated in Table 3. Powder X-ray diffraction confirms the formation of graphite oxide (Figure 2). The interlayer spacing of our synthesized graphite oxide is 8.3 Å, which is within the range of values that have been previously reported.14 The interlayer spacing of GO would depend on the method of preparation as well as on the number of layers of water molecules in the gallery space of the material.14 The presence of water in our GO system was confirmed by performing pyrolysis mass spectrometry. From thermogravimetric analysis the water content in the GO was found to be 9.08% (Figure 3). Using this information and in addition to the data from elemental analysis, the number of moles of water in the GO was determined to be 0.91. We therefore conclude that GO as synthesized in our laboratory has a layer of water molecules in its van der Waals gap. Fourier transform infrared spectroscopy confirms that the oxidation of graphite to graphite oxide was successful. The spectrum of GO shows a strong OH peak at 3364 cm-1 and other C-O functionalities, COOH at 1727.8 cm-1 and COC/COH in the range 1383.3-1065.0 cm-1. The spectrum also depicts a peak at 1618.3 cm-1 which corresponds to the remaining sp2 character.
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Figure 3. TGA of pristine GO.
Figure 4. XRD of (a) exfoliated GO and (b) restacked GO.
Graphite oxide was found to exfoliate readily in water. The powder pattern of exfoliated GO does not reveal any diffraction peaks, suggesting complete separation of the GO platelets (Figure 4a). Once exfoliated, the GO layers can be reconstructed even without the presence of polymers. [The procedure involved in making reconstructed graphite oxide is the same as for the (12) Bissessur, R.; White, W.; Dahn, D. C. Mater. Lett. 2006, 60, 248.
formation of polyaniline/GO intercalates, with the exception that once the layers are exfoliated they are allowed to restack without the addition of polymer solutions. Both pristine GO and restacked GO have a similar interlayer spacing value of 8.3 Å.] Figure 4b depicts the powder pattern of restacked GO, which is similar to that of pristine GO. Powder X-ray diffraction confirms the successful intercalation of PANI, PEA, and PPA into GO (Figure 5). For instance, the PANI/GO system shows an interlayer spacing value of 9.68 Å. With respect to GO this corresponds to an increase in the interlayer spacing of only 1.38 Å. However, we should bear in mind that GO has a layer of water molecules between its sheets, and it is reasonable to assume that the intercalation process is driven by entropy, with the interlayer water molecules being replaced by the incoming polymer. Therefore, a correction factor of 2.8 Å corresponding to the dimension of the water molecule must be added to the increase in interlayer spacing value to obtain the net expansion of the GO sheets. For polyaniline the net expansion is 4.18 Å, which would suggest that a monolayer of the polymer is lying parallel with respect to the GO layers. A similar arrangement was observed for polyaniline intercalated in MoS2.15 The proposed structural model is consistent with the XRD data, and is reasonable because of the favorable face-to-face π-stacking interaction between the polymer and GO host. It is interesting to note that the interlayer spacing of the intercalated phase increases with the size of the alkyl group on the polymer backbone, clearly demonstrating a steric effect. On the basis of the net expansion of the layers as listed in Table 4, the PEA/GO and PPA/GO systems would also consist of a monolayer of the polymer sandwiched parallel to the GO lamellae. Since the powder patterns show only a few well-defined diffraction peaks, an indepth structural characterization of these compounds is not possible. (13) Genie`s, E. M.; Boyce, A.; Lapowkski, M.; Tsintavis, C. Synth. Met. 1990, 139, 182. (14) Liu, Z.-H.; Wang, Z.-M.; Yang, X.; Ooi, K. Langmuir 2002, 18, 4926. (15) Kanatzidis, M. Bissessur, R.; DeGroot, D. C. Schindler, J. L.; Kannewurf; C. R. Chem. Mater. 1993, 5, 595.
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Figure 6. TGA of PANI/GO. Table 5. Average Crystallite Size of the Intercalates intercalate
average crystallitesize (Å)
intercalate
averagecrystallite size (Å)
GO PANI/GO
211 99
PEA/GO PPA/GO
36 24
Table 6. Compositions of the Polymer/GO Systems
Figure 5. XRD of (a) PANI/GO, (b) PEA/GO, and (c) PPA/GO. Table 4. Summary of the X-ray Data net interlayer spacing expansion (Å) (Å) intercalate
net interlayer spacing expansion (Å) (Å) intercalate
GO PANI/GO
PEA/GO PPA/GO
8.30 9.68
4.18
9.80 10.73
4.30 5.23
From the XRD data, the average crystallite size of the intercalates was determined by using the Scherrer equation.16 Graphite oxide itself has a crystallite size of 211 Å. However, the intercalates show a considerable decrease in crystallite size which is ascribed to the improper restacking of the layers after exfoliation. (16) Bissessur, R.; Haines R. I.; Bru¨ning R. J. Mater. Chem. 2003, 13, 44.
guest species
stoichiometry of the intercalate
PANI PEA PPA
(H2O)1.1(NMF)0.52(PANI)1.0GO (H2O)1.3(NMF)0.36(PEA)0.59GO (H2O)1.2(NMF)0.56(PPA)0.82GO
It is also interesting to note that, as the size of the alkyl group on the polymer backbone increases, the crystallite size of the intercalate decreases even further, suggesting that the presence of longer alkyl chains in the gallery space results in a higher degree of disorder. These results are summarized in Table 5. The stoichiometry of the intercalates were determined by performing thermogravimetric analysis in air up to 700 °C. As an example, the thermogram of PANI/GO is shown in Figure 6. The TGA data show an initial weight loss which is attributed to the loss of co-inserted water molecules. The next weight loss is assigned to the liberation co-intercalated NMF. These assignments are reasonable since water was used to exfoliate the GO layers and NMF to disperse the polymer, and the co-insertion of these solvent molecules is inevitable. The next weight loss, also observed in the TGA of pure GO, corresponds to the destruction of carbonyl groups that are present in the layered host.14 Thereafter, a weight loss corresponding to the combustion of the carbon skeleton of the lamellar system is observed, and is assumed to be complete at 516 °C. This assumption is based on the TGA data of pure GO (Figure 3). The final weight loss in the thermogram of PANI/GO is attributed to the decomposition of the intercalated polymer. This assumption is verified by performing a TGA experiment on the pure polymer. The other intercalates show similar patterns. The calculated compositions are listed in Table 6. The calculated loading of the polymers is reasonable considering that a mole ratio of polymer to GO of 1:1 was used in the reaction. The loading of PANI is the highest, and this is understandable since the solubility of the polymer is the lowest in NMF. The lower loadings for PEA and PPA are due to the fact that some of these polymers might have been lost during the centrifuging/washing process due to their increased solubility in NMF.
Conclusions We have shown that polyanilines can be directly inserted into GO by using the synthetic procedure as outlined in this paper.
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The preparation method involves the use of conditions that favor complete intercalation of the polymers into GO, as shown by XRD data. In this paper we have used a mole ratio of polymer to GO of 1:1 for all of the polymers, for direct comparison. However, we are currently preparing samples with various polymer:GO ratios since we expect differences in physical properties such as mechanical strength and electrical conductivity.17 Furthermore, our direct method of intercalation is invaluable since polymers of very large molecular weight could potentially be intercalated. The synthetic methodology for the insertion could (17) Bissessur, R.; Liu, P. K. Y. Work in progress.
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be extended to other conductive polymers such as polypyrrole18 and a variety of other molecules such as organic dyes, liquid crystals, and cluster molecules. Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), Atlantic Innovation Fund of Canada (AIF), and UPEI for financial support. LA0527339 (18) Bissessur, R.; Liu, P. K. Y. Submitted for publication to Synth. Met.
