reassembling method

Carbon 43 (2005) 2564–2570 www.elsevier.com/locate/carbon

Synthesis of poly(aniline-co-o-anisidine)-intercalated graphite oxide composite by delamination/reassembling method Gengchao Wang *, Zhenyu Yang, Xingwei Li, Chunzhong Li Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, PR China Received 7 January 2005; accepted 9 May 2005 Available online 24 June 2005

Abstract Aniline/o-anisidine copolymer (P(An-co-oAs))-intercalated GO composite was synthesized by the delamination/reassembling method in N-methyl-2-pyrrolidone (NMP) solvent, and was characterized by FTIR, XRD, DSC, SEM, TGA, conductivity and cyclic voltammetry. XRD and FTIR spectra indicate that the P(An-co-oAs) exists as a monolayer of outstretched chains in the gallery of GO due to the hydrogen bonds between –NH, @N– and –OCH3 groups of P(An-co-oAs) and the oxygen functional groups of the GO layers. The results of thermal analysis show that no de-intercalation of P(An-co-oAs) from GO occurred during heating. Its electrical conductivity has reached 1.9 · 10 2 S/cm, which is by 3 orders of magnitude higher than that of GO. The intercalation of P(An-co-oAs) also has a pronounced effect on the stabilization of electrochemical response in relation to the GO matrix and P(An-co-oAs).
2005 Elsevier Ltd. All rights reserved. Keywords: Graphite oxide; Intercalation; Thermal analysis; Electrochemical properties; Electrical properties

1. Introduction Conducting polymers intercalated in layered inorganic solids, such as metal oxides, metal dichalcogenides, FeOCl and layered metal phosphates, etc., are candidates for the cathode materials of rechargeable lithium ion batteries [1–7]. These composites have shown higher capacity and better reversibility than that of the pristine layered materials. The intercalation of high molecular weight conducting polymers would make the intercalated compounds more stable, which is favorable for the active materials of Li+ secondary batteries. Graphite oxide (GO) is a typical pseudo-two-dimensional solid in bulk form, with strong covalent bonding within the layers. Some functional groups, such as *

Corresponding author. Tel.: +86 21 64253527; fax: +86 21 64251372. E-mail address: [email protected] (G. Wang). 0008-6223/$ - see front matter
2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.05.008

hydroxyl, carboxyl, and other groups, bound to carbon sheets in GO lamellae make graphite oxide hydrophilic and give rise to a rich intercalation chemistry. It has been reported that some polar organic molecules and polymers, such as organic ammonium ions [8], alcohols [9] poly(ethylene oxide) (PEO) [10,11], poly(vinyl alcohol) (PVA) [12], poly(furfuryl alcohol) (PFA) [13] and poly(diallyldimethylammonium chloride) (PDDA) [14], etc., can be easily inserted by different methods between its lamellae to form intercalated GO composites. However, some oil-soluble polymers such as polyaniline (PAn), poly(vinyl acetate) (PVAc), etc., would be difficult to insert into GO to form intercalated GO compounds. The polyaniline-intercalated GO have been synthesized by an exfoliation/adsorption process [15– 18] and by exchange reactions with surfactants with hydrophobic alkyl chains [19]. However, it has never been reported that the large polyaniline and aniline/derivant copolymers molecules are inserted into GO in

G. Wang et al. / Carbon 43 (2005) 2564–2570

polar organic solvent by a delamination/reassembling method [20]. Herein, we report the preparation of the P(An-co-oAs)intercalated GO composite, which is synthesized by a delamination/reassembling method in N-methyl-2-pyrrolidone (NMP) solvent. The synthesized method we used is very simple by comparison with others, and the composite has good electrical conductivity and electrochemical properties.

2. Experimental 2.1. Materials Aniline (An) and o-anisidine (oAs) of analytical grade were purchased from Shanghai Chemical Reagent Corp., and distilled under vacuum prior to use. Natural graphite powder (30–50 lm) was first treated by 5% HCl twice, then filtered, washed with de-ionized water thoroughly, and dried at 110 C for 24 h. All other reagents were received as analytical grade and were used without further purification. 2.2. Preparation of graphite oxide (GO) Graphite oxide (GO) was synthesized from natural graphite powder by the Hummers–Offeman method [21]. Graphite powder (5 g) and sodium nitrate (2.5 g) was placed in cold (0 C) concentrated H2SO4 (120 ml). KMnO4 (15 g) was added gradually with stirring and cooling, so that the temperature of the mixture was not allowed to reach 20 C. The mixture was then stirred at 35 C for 30 min, de-ionized water (250 ml) was slowly added to cause an increase in temperature to 98 C, and this temperature was held for 15 min. The reaction was terminated by addition of a large amount of de-ionized water (500 ml) and 30% H2O2 solution (50 ml). The mixture was filtered, washed successively with 5% HCl aqueous solution completely until sulfate could not be detected with BaCl2, and dried under vacuum at 50 C for 48 h. 2.3. Synthesis of poly(aniline-co-o-anisidine) An amount of 6.5 g (70.0 mmol) of aniline and 3.7 g (30.0 mmol) of o-anisidine were injected in 200 ml of 1 M HCl (aq.) with constant stirring. 23.9 g (105.0 mmol) of ammonium persulfate (dissolved in 100 ml de-ionized water) were dropped into the above solution. The polymerization was allowed to proceed for 24 h at 5 C. Reaction mixture was filtered under gravity, and washed with de-ionized water, afterwards dried at 60 C for 48 h under vacuum to obtain a fine green powder. Obtained green powder was immersed into 10% ammonia solution for 24 h for deprotona-

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tion. After drying, an aniline/o-anisidine copolymer (P(An-co-oAs)) powder was obtained. 1H NMR spectroscopy of this P(An-co-oAs) powder showed a molar ratio aniline/o-anisidine of 0.64/0.36. 2.4. Synthesis of poly(aniline-co-o-anisidine)-intercalated GO An amount of 0.5 g GO was put into 20 ml of NMP and sonicated for 60 min, which provided a colloidal solution. Mixed with the above colloidal solution and NMP solution (10 ml) of P(An-co-oAs) (0.1 g), and sonicated for 30 min. After stirring for 24 h, the precipitate was filtered, and washed with NMP solution completely until the filtrate was colorless. The solid product was protonated in 1 M HCl solution for 24 h. The product was dried under vacuum at 60 C for 48 h. From the elemental analysis of nitrogen, the P(An-co-oAs) content in P(An-co-oAs)-intercalated GO composite was calculated to be 15.6 wt% (C: 57.81%, H: 3.32%, N: 2.06%, O: balance). 2.5. Measurements X-ray diffraction (XRD) patterns were obtained with a Rigaku D/Max 2550 VB/PC X-ray diffractometer using Cu Ka radiation. The diffraction data were recorded for 2h angles between 3 and 50. FTIR spectra were recorded from KBr pellets using a Nicolet MagnaIR550 spectrometer. Differential Scanning Calorimetry (DSC) thermograms were taken on a NETZSCH DSC 200PC thermal analyzer, operating under nitrogen atmosphere at a heating rate of 10 C/min. Gaseous products formed by heating of the GO composite to 180 C were analyzed using a HP6890 gas chromatograph equipped with a thermal conductivity detector (TCD). Thermogravimetric analyses (TGA) were preformed on a TGA 2050 thermogravimetric analyzer from room temperature to 600 C at a heating rate of 10 C/min under nitrogen. Cyclic voltammetry experiments were carried out with a ZF potentiostat/galvanostat. A conventional three-electrode system was used, consisting of a working electrode, a platinum sheet auxiliary electrode and an Ag/AgCl reference electrode. The working electrodes were prepared from active material (GO or P(An-co-oAs)-intercalated GO), carbon black (VULCAN XC72, Cabot Co.) and Kynar Flex 2820 (75:15:10 wt%) were mixed in NMP and the slurry was spread onto a platinum sheet. After evaporation of the solvent, the working electrode was heated at 100 C for 4 h prior to their introduction to the glove box. The electrode had a surface area of 1 cm2 and contained about 3 mg of active material. Supporting electrolyte was 1 M LiPF6 in dimethyl carbonate/ethyl methyl carbonate/ethylene carbonate (1/1/1, w/w). The experiments were carried out at room temperature and in an inert

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atmosphere by bubbling N2 through the solution. The electrical conductivity of GO and P(An-co-oAs)-intercalated GO pellets was determined by the four-probe method using compressed pellets and a SX 1934 fourprobe instrument.

3. Results and discussion

Intensity (A.U.)

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a

b

3.1. Structure of P(An-co-oAs)-intercalated GO The FTIR spectra of the GO, P(An-co-oAs) and P(An-co-oAs)-intercalated GO composite are given in Fig. 1, respectively. Fig. 1 shows that absorption peaks of GO (3400 cm 1, 1732 cm 1 and 1630 cm 1) and P(An-co-oAs) (1591 cm 1 and 1496 cm 1) all appear in the spectra of P(An-co-oAs)-intercalated GO. From Fig. 1, it can be seen that the C=O stretching mode in GO also shifts to lower wavenumbers (from 1732 cm 1 to 1716 cm 1). This indicates that hydrogen bonds are formed between the carboxyl groups at the edges of the GO layers and the -NH groups of P(An-co-oAs). Moreover, hydrogen bonds can also form between the hydroxyl and epoxide groups of the GO on one side, and –NH, @N– and –OCH3 groups of the polymer on the other side. It also is found that C@C stretching modes (1577 cm 1 and 1463 cm 1) for the quinoid and benzenoid rings in P(An-co-oAs) shift to lower wavenumbers in the P(An-co-oAs)-intercalated GO composite. This is because the P(An-co-oAs) chains change from curly chains to outstretched chains by the action of hydrogen bonds and electrostatic interactions, which improves the conjugation degree of the P(An-co-oAs) chains.

a

Transmittance (A.U.)

1732 1630

1463 1716 1577

b

10

20

30 2θ (°)

40

50

Fig. 2. X-ray diffraction patterns of (a) GO, (b) P(An-co-oAs)intercalated GO, and (c) P(An-co-oAs).

Fig. 2 shows the X-ray diffraction patterns of the GO, P(An-co-oAs)-intercalated GO composite and P(An-cooAs), respectively. The strongest peak at 2h = 14.1 corresponds to the (0 0 1) diffraction peak of GO, which is a typical layered material (see Fig. 2, trace a). The 2h value corresponds to an interlayer spacing (Ic) of 0.628 nm. Upon intercalation, the (0 0 1) diffraction peak for P(Anco-oAs)-intercalated GO composite shifts to a lower angle, and the Ic of P(An-co-oAs)-intercalated GO increased to 1.11 nm from 0.628 nm of GO (Fig. 2, trace b). The 0.48 nm interlayer expansion is the result of removing one layer of H2O (approximately 0.28 nm) and inserting a single layer of P(An-co-oAs). Therefore, the net expansion due to an intercalated monolayer of P(An-co-oAs) corresponds to 0.76 nm and is comparable to that found in most conjugated polymer-intercalated composites in which parallel polymer chains lie ordered between the host sheets [1,22]. The XRD result also indicates that the chains of P(An-co-oAs) with extended-chain conformation exist in a dimensionally confined environment (1.11 nm), thus P(An-co-oAs)intercalated GO is a GO/nano-P(An-co-oAs) composite. We think that the driving force of reassembling should be hydrogen bond formation between –NH, @N– and –OCH3 groups of P(An-co-oAs) and the oxygen functional groups of the GO layers. A schematic illustration for the formation of the P(An-co-oAs)-intercalated GO composite is shown in Fig. 3. 3.2. Thermal analysis

c 1591 1496

4000

c

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Fig. 1. FTIR spectra of (a) GO, (b) P(An-co-oAs)-intercalated GO, and (c) P(An-co-oAs).

Fig. 4 shows the DSC curves of GO and P(Anco-oAs)-intercalated GO composite. In the DSC curve of GO, a sharp exothermic peak is observed at 204 C (Fig. 4, trace a), attributing to the decomposition of the oxygen functional groups in the GO and the formation of CO, CO2, H2O, and carbon [23,24]. From

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Fig. 3. Schematic illustration for formation of P(An-co-oAs)-intercalated GO.

lated GO composite after heating at 180 C for 30 min. It can be seen that Ic of the P(An-co-oAs)-intercalated GO composite did not change significantly after heating to 180 C, which indicates that no de-intercalation of P(An-co-oAs) from GO occurred. However, the (0 0 1) diffraction peak is considerably broadened, indicating an increase of disorder in this composite. This is because CO and CO2 are evolved by the GO composite on heating to 180 C, which is confirmed by gas chromatography analysis of the gaseous products, and the remaining defect carbon layers are left between the polymer layers. Fig. 6 shows the SEM images of GO and

EXO

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Fig. 4. DSC curves of (a) GO, and (b) P(An-co-oAs)-intercalated GO.

a

Intensity (A.U.)

Fig. 4, trace b, it is found that the DSC curve of the P(An-co-oAs)-intercalated GO composite differs from that of GO. It has only a small exothermic peak at 174 C. This is because the action of hydrogen bonds between P(An-co-oAs) and oxygen functional groups of GO has decreased the extent of destruction of the oxygen functional groups of GO, and the endothermic nature of dissociation of the hydrogen bonds between P(Anco-oAs) and GO also has weakened the intensity of the exothermic peak. The X-ray diffraction measurement can confirm whether the P(An-co-oAs) chains deintercalated from the GO layer during decomposition of GO. Fig. 5 gives X-ray diffraction patterns of the P(An-co-oAs)-interca-

b 10

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30

40

50

2θ (°)

Fig. 5. X-ray diffraction patterns of the P(An-co-oAs)-intercalated GO (a) not heated, and (b) heated at 180 C for 30 min.

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3.3. Electrical and electrochemical properties In this paper, the synthesized pristine GO (dark color) is a semi-conducting material. The intercalation of P(An-co-oAs) in the GO interlayer space generates the hybrid material in which two different types of lowdimensional electrical conductors coexist at a nanometer level in a dimensionally constrained environment. There are two types of charge carriers in the material, small

100 c

90 Weight (%)

P(An-co-oAs)-intercalated GO after rapid heating to 300 C. It can be seen that the morphology of the P(An-co-oAs)-intercalated GO differs entirely from that of pristine GO after fast heating to 300 C. The P(An-co-oAs)-intercalated GO composite has a closegrained sheet morphology (Fig. 6b). On the other hand, the GO has a fluffy flaky appearance with a thickness of the sheets less than 0.2 lm (Fig. 6a). These results also indicate that at a high temperature of 300 C the no deinsertion of P(An-co-oAs) from GO occurred. TGA curves of the GO, the P(An-co-oAs)-intercalated GO composite, and P(An-co-oAs) are given in Fig. 7. Fig. 7 shows that the GO and the P(An-cooAs)-intercalated GO composite have different weight loss. The weight loss of GO up to 150 C is 11%, which is attributed to the removal of water from the GO. However, the weight loss of P(An-co-oAs)-intercalated GO composite up to ca. 150 C is only 4%, which could be due to adsorbed water or co-intercalated NMP. This indicates that the intercalation of P(An-co-oAs) has decreased the water content between the GO layers. The weight loss around 200 C owes to the decomposition of the oxygen functional groups in the GO layer. In this stage, GO loses 32% of its weight, and the P(Anco-oAs)-intercalated GO composite only 18%. It implies that the intercalation of the P(An-co-oAs) has improved the thermal stability of GO. In the last weight loss stage, the weight loss of GO can be attributed to the combustion of its carbon skeleton [25]. However, the weight loss of P(An-co-oAs)-intercalated GO composite above 500 C should be due to the decomposition of P(An-co-oAs), as shown in Fig. 7, trace c.

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600

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Fig. 7. TGA curves of (a) GO, (b) P(An-co-oAs)-intercalated GO, and (c) P(An-co-oAs).

400 GO P(An-co-oAs)-intercalated GO P(An-co-oAs)

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Fig. 8. Cyclic voltammograms of GO (dash line), P(An-co-oAs) (dot line), and P(An-co-oAs)-intercalated GO (solid line): v = 20 mV/s. Electrolytic solution: 1 M LiPF6 in DMC/EMC/DC (1/1/1, w/w).

polarons (electrons) found on the GO lattice, and massive polarons located on the P(An-co-oAs) backbone [26]. The electrical conductivity in the composite would depend on the relative mobility of two different kinds of carriers.

Fig. 6. SEM images of (a) GO and (b) P(An-co-oAs)-intercalated GO after rapid heating to 300 C.

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The insertion of P(An-co-oAs) has a pronounced effect on the conductivity of the P(An-co-oAs)-intercalated GO composite as has been observed in most polyaniline-intercalated composites [1,2,22]. The room temperature conductivity changed from 4.8 · 10 5 S cm 1 to 1.9 · 10 2 S cm 1 with introduction of P(Anco-oAs) into the GO structure.

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Fig. 8 shows the cyclic voltammograms for GO, P(An-co-oAs), and P(An-co-oAs)-intercalated GO. Apparently, cyclic voltammograms of P(An-co-oAs)intercalated GO exhibit two stronger oxidation peaks and only one reduction peak. The redox potential and intensity of these peaks entirely differ from those of GO and P(An-co-oAs). Obviously, they are associated with other reactions. The increase of electrochemical response of P(An-co-oAs)-intercalated GO is attributed to a higher occupancy of the Li+ ions in the reduction sites and a more remarkable redox response of the P(An-cooAs), suggesting that the organic and inorganic components are simultaneously reduced/oxidized [27]. Inorganic–organic hybrid structure of P(An-co-oAs)intercalated GO led to a surprising stabilization of the electrochemical response over several cycle (30), as shown in Fig. 9. During the Li+ insertion/deinsertion process, changes in volume and lattice parameters occur in the GO matrix, contributing to electrochemical instability of GO in consecutive cycles. The intercalation/deintercalation of counter-anions in the electrochemical cycles produces a structural variety of P(An-co-oAs), resulting in electrochemical instability of P(An-cooAs). The cause of stabilization of the electrochemical response in the P(An-co-oAs)-intercalated GO is explained by the acting of P(An-co-oAs) chains as pillars between GO sheets, with a resulting increased interlamellar separation, thus decreasing the structural changes in the samples caused by solvated Li+ on insertion/ deinsertion.

0 -100

Acknowledgements

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We are grateful for the financial support from the National Natural Science Foundation of China (20236020), the Shanghai Municipal Science and Technology Commission (0352nm052, 04DZ14002) and the Development Project of Shanghai Priority Academic Discipline.

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Fig. 9. Stability measurement of (a) GO, (b) P(An-co-oAs), and (c) P(An-co-oAs)-intercalated GO: v = 20 mV/s. Electrolytic solution: 1 M LiPF6 in DMC/EMC/DC (1/1/1, w/w).

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