- Email: [email protected]
Chemically modified graphene sheets produced by the solvothermal reduction of colloidal dispersions of graphite oxide
CARBON
4 6 ( 2 0 0 8 ) 1 9 9 4 –1 9 9 8
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Chemically modified graphene sheets produced by the solvothermal reduction of colloidal dispersions of graphite oxide C. Nethravathi, Michael Rajamathi* Materials Research Group, Department of Chemistry, Saint Joseph’s College, 36 Lalbagh Road, Bangalore 560 027, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Chemically modified graphene sheets are obtained through solvothermal reduction of col-
Received 27 December 2007
loidal dispersions of graphite oxide in various solvents. Reduction occurs at relatively low
Accepted 18 August 2008
temperatures (120–200 °C). Reaction temperature, the self-generated pressure in the sealed
Available online 26 August 2008
reaction vessel and the reducing power of the solvent influences the extent of reduction of graphite oxide sheets to modified graphene sheets. Ó 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Graphene sheets are planar monolayers of sp2-bonded carbon atoms tightly packed into a two-dimensional honeycomb lattice. These are the basic building blocks of graphitic materials of all dimensionalities [1]. Two-dimensional conducting graphene sheets with extraordinary electronic properties have gained interest as a potent material in high electron mobility applications [2] and to serve as a nanofiller in composites that can be used in many applications [3,4]. Brodie [5] reported the production of individual graphene sheets by exfoliation of graphite in 1859. Since then many routes have been developed to synthesize the metastable two-dimensional graphene sheets. Synthesis routes like chemical exfoliation by intercalation/de-intercalation of guest species in graphite lattice [6], sheets grown epitaxially by chemical vapour deposition (CVD) of hydrocarbons on metal substrates [7,8] and thermal decomposition on SiC [9] are unreliable due to low yield and the use of very high temperature, inert atmospheres and sophisticated instruments. Thermal and chemical reduction of graphite oxide (GO) could be used as low temperature routes to obtain graphene sheets. Though graphene sheets have been obtained on thermal/chemical reduction of GO in composites containing GO [10–12] only recently attempts have been made to synthesize
them thermally [13] using GO as precursor and chemically starting from aqueous colloidal dispersion of GO [14,15]. GO is derived on oxidation of neutral graphite. It is characterized to be a lamellar solid with unoxidized aromatic regions and aliphatic regions containing phenolic, carboxyl and epoxide groups as a result of oxidation [16]. Thus the GO platelets are strongly hydrophilic and dispersible in water to form monolayer colloidal dispersions [17]. Alkylamineintercalated GO delaminates to form colloidal dispersions in organic solvents such as alcohols [18]. Delamination of layered solids leads to highly dispersed phases, which forms the basis for the synthesis of layered composites and nanomaterials with unique properties [19,20]. GO can be delaminated in water and polar protic solvents such as alcohols and the resulting colloidal dispersions are expected to consist of two-dimensional unilamellar/multilamellar GO sheets depending on the nature of interactions between the GO layers and the solvent. Reduction of these dispersed GO layers results in graphene sheets [15]. Solvothermal reactions have been widely employed in nanomaterial synthesis [21]. Solvothermal reactions due to their unique features such as very high self-generated pressure inside the sealed reaction vessel (autogenous pressure) and containment of volatile products are well suited for the preparation of metastable phases. It would be interesting to
* Corresponding author: Fax: +91 80 22245831. E-mail address: [email protected] (M. Rajamathi). 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.08.013
CARBON
see if GO colloidal dispersions could be reduced under solvothermal conditions. Here we report solvothermal synthesis of graphene sheets using colloidal dispersions of GO and alkylamine-modified GO in water and organic solvents (which may also function as reducing agents).
heated at 80 °C in vacuum to remove adsorbed solvent. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM 840A microscope by mounting the sample on conducting carbon tape and sputter coating with gold.
3. 2.
Experimental
2.1.
Synthesis of GO
The method due to Hummers and Offeman [22] was adopted to prepare GO from graphite powder. About 1 g of graphite powder was added to 23 ml of cooled (0 °C) concentrated H2SO4. About 3 g of KMnO4 was added gradually with stirring and cooling, so that the temperature of the mixture was maintained below 20 °C. The mixture was then stirred at 35 °C for 30 min. About 46 ml of distilled water was slowly added to cause an increase in temperature to 98 °C and the mixture was maintained at that temperature for 15 min. The reaction was terminated by adding 140 ml of distilled water followed by 10 ml of 30% H2O2 solution. The solid product was separated by centrifugation, washed repeatedly with 5% HCl solution until sulphate could not be detected with BaCl2, then washed 3–4 times with acetone and dried in an air oven at 65 °C overnight.
2.2.
1995
4 6 ( 2 0 0 8 ) 1 9 9 4 –1 9 9 8
Synthesis of alkylamine-intercalated GO
Results and discussion
In all the cases, a black solid precipitates at the end of the solvothermal reaction. This may be attributed to the reduction of hydrophilic GO sheets to hydrophobic graphene sheets, resulting in increased incompatibility with polar solvents. The pXRD pattern of the as prepared GO is compared with those of graphene obtained on solvothermal reduction of the colloidal dispersion of GO in different solvents in Fig. 1. Compared to the parent GO, the solvothermally reduced samples show a decrease in the d002 value. The basal spacing decreases ˚ of the pristine GO to 3.5–3.8 A ˚ in the reduced from 9.0 A samples. These decreased values confirm the decomposition of the GO sheets to graphene sheets. Though there is a decrease in the interlayer spacing, the obtained basal spacing ˚ . The highis higher than that of well-ordered graphite, 3.35 A er basal spacing may be due to the presence of residual oxygen and hydrogen indicating incomplete reduction of GO sheets to graphene. Hence the graphene sheets obtained here are not pure but chemically modified to different extents. However, for simplicity we hereafter refer to them as graph-
Octylamine-intercalated GO (GO-OA) was prepared by the method due to Matsuo et al. [23], which involves grinding GO powder with octylamine (in the mole ratio 1:4) for 0.5 h in the presence of small amounts of n-hexane. The product obtained was washed repeatedly with acetone and dried in air at 65 °C overnight.
2.3.
d 3.8 Å
Solvothermal reduction of colloidal dispersions c
2.4.
3.7 Å
Relative Intensity
Colloidal dispersions of GO were subjected to solvothermal reduction in different solvents (water, ethanol, 1-butanol, ethylene glycol) at temperatures ranging from 80 to 200 °C for varying durations ranging from 4 to 48 h. In each case 100 mg of GO was dispersed in 50 ml of solvent by sonication for 0.5 h and the brown colloidal dispersion was transferred to a stainless steel autoclave and sealed and heated in oven. GOOA (100 mg) was dispersed in 50 ml of 1-butanol by sonication and the colloidal dispersion was subjected to solvothermal treatment as described above. In all the cases the product obtained after solvothermal treatment was washed with acetone followed by water and then dried in a hot air oven at 65 °C.
b
3.6 Å
a
9.0 Å
Characterization
All the samples were characterized by powder X-ray diffraction (pXRD) using a Bruker D8 Advance Diffractometer (Cu Ka radiation, 2° 2h per min), infrared (IR) spectroscopy using a Nicolet IR200 FTIR spectrometer (KBr pellets, 4 cm 1 resolution). C, H and N content analysis of the samples were carried out using a ThermoFinnigan FLASH EA 1112 CHNS analyzer. Prior to the elemental analysis the samples were
10
20
30
40
50
60
2θ (degrees) Fig. 1 – pXRD patterns of pristine GO (a), graphite obtained on solvothermal reduction of GO in water (b), butanol (c) and ethylene glycol (d) at 200 °C.
1996
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4 6 ( 2 0 0 8 ) 1 9 9 4 –1 9 9 8
skeletal vibration
Transmittance (%)
of graphene
b
The C=O stretching
o
3 C - OH
O-H bending O-H stretching a 4000
3500
3000
2500
2000
1500
1000
500
Wavenumber(cm-1) Fig. 2 – IR spectra of pristine GO (a) and graphite obtained on solvothermal reduction of GO in water at 200 °C (b).
Table 1 – Conditions under which the reduction of colloidal dispersions of GO was carried out and the observed basal spacing of the reduced products Precursor
Solvent
Temperature (°C)
˚) d (A
GO GO GO GO GO GO GO GO-OA
Water Water Ethylene glycol Ethylene glycol Ethanol Ethanol 1-Butanol 1-Butanol
200 180 180 120 180 120 160 160
3.59 3.71 3.68 3.68 3.67 3.87 3.83 3.83
ene sheets. The 0 0 2 reflection in these samples is very broad suggesting that the samples are very poorly ordered along the stacking direction. This is an indication that these samples comprise largely free graphene sheets. Fig. 2 shows the IR spectra of GO and the reduction product obtained by solvothermal reaction in water at 220 °C. In GO we observe a strong and broad absorption at 3400 cm 1 due to O–H stretching vibration. The C@O stretching of COOH groups situated at edges of GO sheets is observed at 1726 cm 1. The absorptions due to the O–H bending vibration, epoxide groups and skeletal ring vibrations are observed around 1622 cm 1. The absorption at 1393 cm 1 may be attributed to tertiary C–OH groups [24]. The IR spectrum of the solvothermal product (Fig. 2b) confirms reduction of GO sheets. Here the absorption due to the C@O group (1726 cm 1) is decreased very much in intensity and absorptions at 1622 and 1393 cm 1 are absent. A new absorption band that appears at 1560 cm 1 may be attributed to the skeletal vibration of the graphene sheets. The various solvothermal reactions carried out and the basal spacing of the products obtained in each case are listed in Table 1. The solvothermal reduction of GO sheets to graphene in water occurs at 180 °C, which is much lower temperature than what is reported (300 °C) by Matsuo and Sugie for dry thermal reduction of GO to carbon [13]. According to an earlier study by Scholz and Boehm GO loses 27% of oxygen on heating upto 180 °C and a further 10% above 180 °C [25]. The fact that reduction occurs at a relatively low temperature in water under solvothermal conditions is interesting because water is not a good reducing agent and is not known to assist decarboxylation and elimination of phenolic OH groups. Therefore the low reduction temperature points to the fact that in solvothermal reactions besides temperature, the autogenous pressure developed inside the sealed autoclave also contributes to the reduction of GO sheets to graphene sheets. In order to explore the possibilities of decreasing the operating temperatures of reduction of colloidal dispersion of GO, we studied the solvothermal reactions in different solvents – ethanol, ethylene glycol and 1-butanol – that can also function as reducing agents. It is clear, from the results presented in Table 1 that with increasing reducing power of the solvent the temperature required for reduction of the colloidal disper-
Table 2 – Composition analysis of the chemically modified graphene obtained by solvothermal reduction of colloidal dispersions of GO under different conditions Precursor Solvent
GO GO GO GO GO GO GO GO-OA
Water Water Ethylene glycol Ethylene glycol Ethanol Ethanol 1-Butanol 1-Butanol
Temperature (°C) Mass loss when heated under vacuum (%) Mass percentage
200 180 180 120 180 120 160 160
3.3 3.5 8.4 8.4 8.3 8.7 8.8 8.9
C
H
N
Oa
93.4 89.1 91.6 89.0 90.8 87.2 86.0 79.1
0 0.3 0.3 0.3 0.4 0.4 0.6 1.2
0 0 0 0 0 0 0 3.1b
6.6 10.6 8.1 10.7 8.8 12.4 13.4 16.6
a The oxygen percentage was calculated by subtracting the sum of percentages of C, H and N from 100. b The small percentage of nitrogen could be due to adsorbed amine or its decomposed products.
Nominal formula
C21O C21O1.86H0.85 C21O1.41H0.83 C21O1.97H0.85 C21O1.53H1.1 C21O2.2H1.2 C21O2.5H1.8 C21O3.3H3.8N0.7
CARBON
4 6 ( 2 0 0 8 ) 1 9 9 4 –1 9 9 8
sion of GO decreases. The reducing power of the solvents is in the order ethylene glycol > ethanol > 1-butanol. The observed minimum temperatures for reduction in these solvents are 120, 120 and 160 °C. Though the minimum reduction temper-
b
Relative Intensity
4.1 Å
a
15.8 Å
10
20
30
40
50
60
2θ (degrees) Fig. 3 – pXRD patterns of as prepared GO-OA (a) and graphite obtained on solvothermal reduction of GO-OA in butanol at 160 °C (b).
1997
ature is same for glycol and ethanol the basal spacings of the ˚ , respecreduced products in these solvents are 3.7 and 3.9 A tively, suggesting better reduction in the case of ethylene glycol. The carbon contents of the reduced products in ethanol and ethylene glycol are 87.2% and 89.0% (Table 2) and this further confirms that the reduction is better in glycol. In all the solvents the extent of reduction increased with increase in reaction temperature. In all the cases, optimum duration of reduction of colloidal dispersion of GO to graphene sheets was found to be 16 h. When the reaction is carried out for duration of less than 8 h, the reduction is incomplete with the precursor GO still being present at the end of the reaction. There is no appreciable improvement in the extent of reduction when the reaction is carried out for any amount of time greater than 16 h. In order to see how important the pressure in the conversion of GO sheets to graphene sheets is we carried out the reaction in ethylene glycol at 120 °C in an open vessel. There is only partial reduction – we observe two broad reflections in ˚ indicating that the pXRD pattern (not shown) at 7 and 3.8 A GO is still present in the sample. This suggests that the pressure plays an important role in the conversion of GO sheets to graphene sheets. Long chain amine modified GO have been shown to give colloidal dispersions due to delamination in 1-butanol [17]. In order to see if better dispersion in the solvent would lead to easier reduction we carried out solvothermal reaction of GO-OA dispersed in 1-butanol. The minimum temperature at which reduction occurs in this case is same as that in the case of the dispersion of the unmodified GO in 1-butanol (Table 1). However, the observed basal spacing of the reduced product in ˚ while that in the case of GO is 3.8 A ˚ the case of GO-OA is 4.1 A (Fig. 3a and b), which suggests that the reduction is better in the case of GO. The carbon content of the reduced products in the case of GO-OA is 79.1% and that in the case of GO is 86.0% (Table 2). This further confirms that GO is reduced better than GO-OA. This implies that the extent of dispersion may not be an important factor in the reduction of GO.
Fig. 4 – Scanning electron microscopy (SEM) images of graphite obtained on solvothermal reduction of GO in water (a), ethanol (b) and butanol (c) at 200 °C.
1998
CARBON
4 6 ( 2 0 0 8 ) 1 9 9 4 –1 9 9 8
SEM images of solid obtained on solvothermal reduction of dispersions of GO in water, ethanol and 1-butanol are shown in Fig. 4a–c, respectively. In all the cases aggregates of crumpled sheets of graphene are observed, similar to what has been reported in the literature [15].
4.
Conclusions
Solvothermal reduction is a convenient method to convert GO sheets to graphene sheets. Besides temperature, autogenous pressure and the reducing power of the solvents influence the extent of reduction of GO to graphene. The temperatures required for solvothermal reduction even in a non-reducing solvent (water) is relatively low and it is further lowered when reducing solvents are employed. As solvothermal reactions have been extensively used in synthesis of nanomaterials, this work opens up new avenues to one-pot synthesis of graphite-based composites in various solvents, where the nanomaterial synthesis and reduction of GO sheets to graphene sheets could occur simultaneously.
Acknowledgements This work was funded by DST, New Delhi. C.N. thanks CSIR, New Delhi for the award of Senior Research Fellowship. We thank UGC, New Delhi for having provided IR spectrometer through CPE scheme, Department of Chemistry, Bangalore University for providing pXRD facility and Department of Organic Chemistry, I.I.Sc., Bangalore for carrying out CHNS analysis.
R E F E R E N C E S
[1] Geim AK, Novoselov KS. The rise of graphene. Nature 2007;6:183–91. [2] Kotov NA. Materials science. Carbon sheet solutions. Nature 2006;442(7100):254–5. [3] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 2006;442:282–6. [4] Cassagneau T, Guerin F, Fendler JH. Preparation and characterization of ultrathin films layer-by-layer self-assembled from graphite oxide nanoplatelets and polymers. Langmuir 2000;16:7318–24. [5] Brodie BC. On the atomic weight of graphite. Philos Trans R Soc Lon 1859;149:249–59. [6] Chung DDL. Exfoliation of graphite. J Mater Sci 1987;22(12):4190–8. [7] Land TA, Michely T, Behm RJ, Hemminger JC, Comsa G. STM investigation of single layer graphite structures produced on Pt (1 1 1) by hydrocarbon decomposition. Surf Sci 1992;264:261–70.
[8] Nagashima A, Nuka K, Itoh H, Ichinokawa T, Oshima C, Otani S. Electronic states of monolayer graphite formed on TiC (1 1 1) surface. Surf Sci 1993;291:93–8. [9] Berger C, Song Z, Li T, Li X, Ogbazghi AY, Feng R, et al. Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics. J Phys Chem B 2004;108:19912–6. [10] Matsuo Y, Tahara K, Sugie Y. Structure and thermal properties of poly (ethylene oxide)-intercalated graphite oxide. Carbon 1997;35:113–20. [11] Morishige K, Hamada T. Iron oxide pillared graphite. Langmuir 2005;21:6277–81. [12] Kotov NA, Dekany I, Fendler JH. Ultrathin graphite oxidepolyelectrolyte composites prepared by self assembly: transition between conductive and non-conductive states. Adv Mater 1996;8:637–41. [13] Matsuo Y, Sugie Y. Preparation, structure and electrochemical property of pyrolytic carbon from graphite oxide. Carbon 1998;36:301–3. [14] Bourlinos AB, Gournis D, Petridis D, Szabo T, Szeri A, Dekany I. Graphite oxide: chemical reduction to graphite and surface modification with primary aliphatic amines and amino acids. Langmuir 2003;19:6050–5. [15] Stankovich S, Dikin D, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45:1558–65. [16] Lerf A, He H, Forster M, Klinowski J. Structure of graphite oxide revisited. J Phys Chem B 1998;102:4477–82. [17] Croft RC. Lamellar compounds of graphite. Quartz Rev 1960;14(1):11–3. [18] Nethravathi C, Rajamathi M. Delamination, colloidal dispersion and reassembly of alkylamine intercalated graphite oxide in alcohols. Carbon 2006;44:2635–41. [19] Nethravathi C, Ravishankar N, Shivakumara C, Rajamathi M. Nanocomposites of a-hydroxides of nickel and cobalt by delamination and co-stacking: enhanced stability of a-motifs in alkaline medium and electrochemical behaviour. J Power Sources 2007;172:970–4. [20] Venugopal BR, Shivakumara C, Rajamathi M. A composite of layered double hydroxides obtained through random co-stacking of layers from MgAl and CoAl LDHs by delamination-restacking: thermal decomposition and reconstruction behavior. Solid State Sci 2007;9:287–94. [21] Rajamathi M, Seshadri R. Oxide and chalcogenide nanoparticles from hydrothermal/solvothermal reactions. Curr Opin Solid State Mater Sci 2002;6:337–45. [22] Hummers WS, Offeman RE. Preparation of graphite oxide. J Am Chem Soc 1958;80:1339. [23] Matsuo Y, Higashika S, Kimura K, Miyamoto Y, Fukutsuka T, Sugie Y. Synthesis of polyaniline intercalated layered materials via exchange reaction. J Mater Chem 2002;12:1592–6. [24] Szabo T, Berkesi O, Forgo P, Josepovits K, Sanakis Y, Petridis D, et al. Evolution of surface functional groups in a series of progressively oxidized graphite oxides. Chem Mater 2006;18:2740–9. [25] Scholz W, Boehm HP. Thermal decomposition of graphite oxide. Naturwissenschaften 1964;51:160.
4 6 ( 2 0 0 8 ) 1 9 9 4 –1 9 9 8
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Chemically modified graphene sheets produced by the solvothermal reduction of colloidal dispersions of graphite oxide C. Nethravathi, Michael Rajamathi* Materials Research Group, Department of Chemistry, Saint Joseph’s College, 36 Lalbagh Road, Bangalore 560 027, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Chemically modified graphene sheets are obtained through solvothermal reduction of col-
Received 27 December 2007
loidal dispersions of graphite oxide in various solvents. Reduction occurs at relatively low
Accepted 18 August 2008
temperatures (120–200 °C). Reaction temperature, the self-generated pressure in the sealed
Available online 26 August 2008
reaction vessel and the reducing power of the solvent influences the extent of reduction of graphite oxide sheets to modified graphene sheets. Ó 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
Graphene sheets are planar monolayers of sp2-bonded carbon atoms tightly packed into a two-dimensional honeycomb lattice. These are the basic building blocks of graphitic materials of all dimensionalities [1]. Two-dimensional conducting graphene sheets with extraordinary electronic properties have gained interest as a potent material in high electron mobility applications [2] and to serve as a nanofiller in composites that can be used in many applications [3,4]. Brodie [5] reported the production of individual graphene sheets by exfoliation of graphite in 1859. Since then many routes have been developed to synthesize the metastable two-dimensional graphene sheets. Synthesis routes like chemical exfoliation by intercalation/de-intercalation of guest species in graphite lattice [6], sheets grown epitaxially by chemical vapour deposition (CVD) of hydrocarbons on metal substrates [7,8] and thermal decomposition on SiC [9] are unreliable due to low yield and the use of very high temperature, inert atmospheres and sophisticated instruments. Thermal and chemical reduction of graphite oxide (GO) could be used as low temperature routes to obtain graphene sheets. Though graphene sheets have been obtained on thermal/chemical reduction of GO in composites containing GO [10–12] only recently attempts have been made to synthesize
them thermally [13] using GO as precursor and chemically starting from aqueous colloidal dispersion of GO [14,15]. GO is derived on oxidation of neutral graphite. It is characterized to be a lamellar solid with unoxidized aromatic regions and aliphatic regions containing phenolic, carboxyl and epoxide groups as a result of oxidation [16]. Thus the GO platelets are strongly hydrophilic and dispersible in water to form monolayer colloidal dispersions [17]. Alkylamineintercalated GO delaminates to form colloidal dispersions in organic solvents such as alcohols [18]. Delamination of layered solids leads to highly dispersed phases, which forms the basis for the synthesis of layered composites and nanomaterials with unique properties [19,20]. GO can be delaminated in water and polar protic solvents such as alcohols and the resulting colloidal dispersions are expected to consist of two-dimensional unilamellar/multilamellar GO sheets depending on the nature of interactions between the GO layers and the solvent. Reduction of these dispersed GO layers results in graphene sheets [15]. Solvothermal reactions have been widely employed in nanomaterial synthesis [21]. Solvothermal reactions due to their unique features such as very high self-generated pressure inside the sealed reaction vessel (autogenous pressure) and containment of volatile products are well suited for the preparation of metastable phases. It would be interesting to
* Corresponding author: Fax: +91 80 22245831. E-mail address: [email protected] (M. Rajamathi). 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.08.013
CARBON
see if GO colloidal dispersions could be reduced under solvothermal conditions. Here we report solvothermal synthesis of graphene sheets using colloidal dispersions of GO and alkylamine-modified GO in water and organic solvents (which may also function as reducing agents).
heated at 80 °C in vacuum to remove adsorbed solvent. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM 840A microscope by mounting the sample on conducting carbon tape and sputter coating with gold.
3. 2.
Experimental
2.1.
Synthesis of GO
The method due to Hummers and Offeman [22] was adopted to prepare GO from graphite powder. About 1 g of graphite powder was added to 23 ml of cooled (0 °C) concentrated H2SO4. About 3 g of KMnO4 was added gradually with stirring and cooling, so that the temperature of the mixture was maintained below 20 °C. The mixture was then stirred at 35 °C for 30 min. About 46 ml of distilled water was slowly added to cause an increase in temperature to 98 °C and the mixture was maintained at that temperature for 15 min. The reaction was terminated by adding 140 ml of distilled water followed by 10 ml of 30% H2O2 solution. The solid product was separated by centrifugation, washed repeatedly with 5% HCl solution until sulphate could not be detected with BaCl2, then washed 3–4 times with acetone and dried in an air oven at 65 °C overnight.
2.2.
1995
4 6 ( 2 0 0 8 ) 1 9 9 4 –1 9 9 8
Synthesis of alkylamine-intercalated GO
Results and discussion
In all the cases, a black solid precipitates at the end of the solvothermal reaction. This may be attributed to the reduction of hydrophilic GO sheets to hydrophobic graphene sheets, resulting in increased incompatibility with polar solvents. The pXRD pattern of the as prepared GO is compared with those of graphene obtained on solvothermal reduction of the colloidal dispersion of GO in different solvents in Fig. 1. Compared to the parent GO, the solvothermally reduced samples show a decrease in the d002 value. The basal spacing decreases ˚ of the pristine GO to 3.5–3.8 A ˚ in the reduced from 9.0 A samples. These decreased values confirm the decomposition of the GO sheets to graphene sheets. Though there is a decrease in the interlayer spacing, the obtained basal spacing ˚ . The highis higher than that of well-ordered graphite, 3.35 A er basal spacing may be due to the presence of residual oxygen and hydrogen indicating incomplete reduction of GO sheets to graphene. Hence the graphene sheets obtained here are not pure but chemically modified to different extents. However, for simplicity we hereafter refer to them as graph-
Octylamine-intercalated GO (GO-OA) was prepared by the method due to Matsuo et al. [23], which involves grinding GO powder with octylamine (in the mole ratio 1:4) for 0.5 h in the presence of small amounts of n-hexane. The product obtained was washed repeatedly with acetone and dried in air at 65 °C overnight.
2.3.
d 3.8 Å
Solvothermal reduction of colloidal dispersions c
2.4.
3.7 Å
Relative Intensity
Colloidal dispersions of GO were subjected to solvothermal reduction in different solvents (water, ethanol, 1-butanol, ethylene glycol) at temperatures ranging from 80 to 200 °C for varying durations ranging from 4 to 48 h. In each case 100 mg of GO was dispersed in 50 ml of solvent by sonication for 0.5 h and the brown colloidal dispersion was transferred to a stainless steel autoclave and sealed and heated in oven. GOOA (100 mg) was dispersed in 50 ml of 1-butanol by sonication and the colloidal dispersion was subjected to solvothermal treatment as described above. In all the cases the product obtained after solvothermal treatment was washed with acetone followed by water and then dried in a hot air oven at 65 °C.
b
3.6 Å
a
9.0 Å
Characterization
All the samples were characterized by powder X-ray diffraction (pXRD) using a Bruker D8 Advance Diffractometer (Cu Ka radiation, 2° 2h per min), infrared (IR) spectroscopy using a Nicolet IR200 FTIR spectrometer (KBr pellets, 4 cm 1 resolution). C, H and N content analysis of the samples were carried out using a ThermoFinnigan FLASH EA 1112 CHNS analyzer. Prior to the elemental analysis the samples were
10
20
30
40
50
60
2θ (degrees) Fig. 1 – pXRD patterns of pristine GO (a), graphite obtained on solvothermal reduction of GO in water (b), butanol (c) and ethylene glycol (d) at 200 °C.
1996
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4 6 ( 2 0 0 8 ) 1 9 9 4 –1 9 9 8
skeletal vibration
Transmittance (%)
of graphene
b
The C=O stretching
o
3 C - OH
O-H bending O-H stretching a 4000
3500
3000
2500
2000
1500
1000
500
Wavenumber(cm-1) Fig. 2 – IR spectra of pristine GO (a) and graphite obtained on solvothermal reduction of GO in water at 200 °C (b).
Table 1 – Conditions under which the reduction of colloidal dispersions of GO was carried out and the observed basal spacing of the reduced products Precursor
Solvent
Temperature (°C)
˚) d (A
GO GO GO GO GO GO GO GO-OA
Water Water Ethylene glycol Ethylene glycol Ethanol Ethanol 1-Butanol 1-Butanol
200 180 180 120 180 120 160 160
3.59 3.71 3.68 3.68 3.67 3.87 3.83 3.83
ene sheets. The 0 0 2 reflection in these samples is very broad suggesting that the samples are very poorly ordered along the stacking direction. This is an indication that these samples comprise largely free graphene sheets. Fig. 2 shows the IR spectra of GO and the reduction product obtained by solvothermal reaction in water at 220 °C. In GO we observe a strong and broad absorption at 3400 cm 1 due to O–H stretching vibration. The C@O stretching of COOH groups situated at edges of GO sheets is observed at 1726 cm 1. The absorptions due to the O–H bending vibration, epoxide groups and skeletal ring vibrations are observed around 1622 cm 1. The absorption at 1393 cm 1 may be attributed to tertiary C–OH groups [24]. The IR spectrum of the solvothermal product (Fig. 2b) confirms reduction of GO sheets. Here the absorption due to the C@O group (1726 cm 1) is decreased very much in intensity and absorptions at 1622 and 1393 cm 1 are absent. A new absorption band that appears at 1560 cm 1 may be attributed to the skeletal vibration of the graphene sheets. The various solvothermal reactions carried out and the basal spacing of the products obtained in each case are listed in Table 1. The solvothermal reduction of GO sheets to graphene in water occurs at 180 °C, which is much lower temperature than what is reported (300 °C) by Matsuo and Sugie for dry thermal reduction of GO to carbon [13]. According to an earlier study by Scholz and Boehm GO loses 27% of oxygen on heating upto 180 °C and a further 10% above 180 °C [25]. The fact that reduction occurs at a relatively low temperature in water under solvothermal conditions is interesting because water is not a good reducing agent and is not known to assist decarboxylation and elimination of phenolic OH groups. Therefore the low reduction temperature points to the fact that in solvothermal reactions besides temperature, the autogenous pressure developed inside the sealed autoclave also contributes to the reduction of GO sheets to graphene sheets. In order to explore the possibilities of decreasing the operating temperatures of reduction of colloidal dispersion of GO, we studied the solvothermal reactions in different solvents – ethanol, ethylene glycol and 1-butanol – that can also function as reducing agents. It is clear, from the results presented in Table 1 that with increasing reducing power of the solvent the temperature required for reduction of the colloidal disper-
Table 2 – Composition analysis of the chemically modified graphene obtained by solvothermal reduction of colloidal dispersions of GO under different conditions Precursor Solvent
GO GO GO GO GO GO GO GO-OA
Water Water Ethylene glycol Ethylene glycol Ethanol Ethanol 1-Butanol 1-Butanol
Temperature (°C) Mass loss when heated under vacuum (%) Mass percentage
200 180 180 120 180 120 160 160
3.3 3.5 8.4 8.4 8.3 8.7 8.8 8.9
C
H
N
Oa
93.4 89.1 91.6 89.0 90.8 87.2 86.0 79.1
0 0.3 0.3 0.3 0.4 0.4 0.6 1.2
0 0 0 0 0 0 0 3.1b
6.6 10.6 8.1 10.7 8.8 12.4 13.4 16.6
a The oxygen percentage was calculated by subtracting the sum of percentages of C, H and N from 100. b The small percentage of nitrogen could be due to adsorbed amine or its decomposed products.
Nominal formula
C21O C21O1.86H0.85 C21O1.41H0.83 C21O1.97H0.85 C21O1.53H1.1 C21O2.2H1.2 C21O2.5H1.8 C21O3.3H3.8N0.7
CARBON
4 6 ( 2 0 0 8 ) 1 9 9 4 –1 9 9 8
sion of GO decreases. The reducing power of the solvents is in the order ethylene glycol > ethanol > 1-butanol. The observed minimum temperatures for reduction in these solvents are 120, 120 and 160 °C. Though the minimum reduction temper-
b
Relative Intensity
4.1 Å
a
15.8 Å
10
20
30
40
50
60
2θ (degrees) Fig. 3 – pXRD patterns of as prepared GO-OA (a) and graphite obtained on solvothermal reduction of GO-OA in butanol at 160 °C (b).
1997
ature is same for glycol and ethanol the basal spacings of the ˚ , respecreduced products in these solvents are 3.7 and 3.9 A tively, suggesting better reduction in the case of ethylene glycol. The carbon contents of the reduced products in ethanol and ethylene glycol are 87.2% and 89.0% (Table 2) and this further confirms that the reduction is better in glycol. In all the solvents the extent of reduction increased with increase in reaction temperature. In all the cases, optimum duration of reduction of colloidal dispersion of GO to graphene sheets was found to be 16 h. When the reaction is carried out for duration of less than 8 h, the reduction is incomplete with the precursor GO still being present at the end of the reaction. There is no appreciable improvement in the extent of reduction when the reaction is carried out for any amount of time greater than 16 h. In order to see how important the pressure in the conversion of GO sheets to graphene sheets is we carried out the reaction in ethylene glycol at 120 °C in an open vessel. There is only partial reduction – we observe two broad reflections in ˚ indicating that the pXRD pattern (not shown) at 7 and 3.8 A GO is still present in the sample. This suggests that the pressure plays an important role in the conversion of GO sheets to graphene sheets. Long chain amine modified GO have been shown to give colloidal dispersions due to delamination in 1-butanol [17]. In order to see if better dispersion in the solvent would lead to easier reduction we carried out solvothermal reaction of GO-OA dispersed in 1-butanol. The minimum temperature at which reduction occurs in this case is same as that in the case of the dispersion of the unmodified GO in 1-butanol (Table 1). However, the observed basal spacing of the reduced product in ˚ while that in the case of GO is 3.8 A ˚ the case of GO-OA is 4.1 A (Fig. 3a and b), which suggests that the reduction is better in the case of GO. The carbon content of the reduced products in the case of GO-OA is 79.1% and that in the case of GO is 86.0% (Table 2). This further confirms that GO is reduced better than GO-OA. This implies that the extent of dispersion may not be an important factor in the reduction of GO.
Fig. 4 – Scanning electron microscopy (SEM) images of graphite obtained on solvothermal reduction of GO in water (a), ethanol (b) and butanol (c) at 200 °C.
1998
CARBON
4 6 ( 2 0 0 8 ) 1 9 9 4 –1 9 9 8
SEM images of solid obtained on solvothermal reduction of dispersions of GO in water, ethanol and 1-butanol are shown in Fig. 4a–c, respectively. In all the cases aggregates of crumpled sheets of graphene are observed, similar to what has been reported in the literature [15].
4.
Conclusions
Solvothermal reduction is a convenient method to convert GO sheets to graphene sheets. Besides temperature, autogenous pressure and the reducing power of the solvents influence the extent of reduction of GO to graphene. The temperatures required for solvothermal reduction even in a non-reducing solvent (water) is relatively low and it is further lowered when reducing solvents are employed. As solvothermal reactions have been extensively used in synthesis of nanomaterials, this work opens up new avenues to one-pot synthesis of graphite-based composites in various solvents, where the nanomaterial synthesis and reduction of GO sheets to graphene sheets could occur simultaneously.
Acknowledgements This work was funded by DST, New Delhi. C.N. thanks CSIR, New Delhi for the award of Senior Research Fellowship. We thank UGC, New Delhi for having provided IR spectrometer through CPE scheme, Department of Chemistry, Bangalore University for providing pXRD facility and Department of Organic Chemistry, I.I.Sc., Bangalore for carrying out CHNS analysis.
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