- Email: [email protected]
Microwave exfoliation of a graphite intercalation compound
Letters to the Editor / Carbon 45 (2007) 1364–1369
[3]
[4]
[5]
[6]
single-wall carbon nanohorns for fuel cell application. Physica B 2002;323:124–6. Murata K, Hashimoto A, Yudasaka M, Kasuya D, Kaneko K, Iijima S. The use of charge transfer to enhance the methane-storage capacity of single-walled, nanostructured carbon. Adv Mater 2004:1520–2. Kasuya D, Yudasaka M, Takahashi K, Kokai F, Iijima S. Selective production of single-wall carbon nanohorn aggregates and their formation mechanism. J Phys Chem B 2002;106:4947–51. Yamaguchi T, Bandow S, Iijima S. Synthesis of carbon nanohorn particles by simple pulsed arc discharge ignited between pre-heated carbon rods. Chem Phys Lett 2004;389:181–5. Fan J, Yudasaka M, Kasuya D, Azami T, Yuge R, Imai H, et al. Micrometer-sized graphitic balls produced together with single-wall carbon nanohorns. J Phys Chem B 2005;109:10756–9.
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[7] Kasuya D, Kokai F, Takahashi K, Yudasaka M, Iijima S. Formation of C60 using CO2 laser vaporization of graphite at room temperature. Chem Phys Lett 2001;337:25–30. [8] Kasuya D, Yudasaka M. Growth mechanisms of carbon nanotubes and nanohorns. New Diamond 2004;73:2–7. [9] Kawai T, Miyamoto Y, Sugino O, Koga Y. Nanotube and nanohorn nucleation from graphitic patches: tight-binding molecular-dynamics simulations. Phys Rev B 2002;66:33404–14. [10] Utsumi S, Miyawaki J, Tanaka H, Hattori Y, Itoi T, Ichikuni N, et al. Opening mechanism of internal nanoporosity of single-wall carbon nanohorn. J Phys Chem B 2005;109:14319–24.
Microwave exfoliation of a graphite intercalation compound Eduardo H.L. Falcao a,b, Richard G. Blair a, Julia J. Mack a, Lisa M. Viculis a, Chai-Won Kwon c, Michael Bendikov a, Richard B. Kaner a,c, Bruce S. Dunn c, Fred Wudl a,* a
Department of Chemistry and Biochemistry, and Exotic Materials Institute, University of California, Los Angeles, CA 90095, United States b Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara, CA 93106, United States c Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095, United States Received 23 June 2006; accepted 11 January 2007 Available online 1 February 2007
The search for nanostructured materials has propelled much of the recent research on graphite intercalation compounds (GICs) and their exfoliation [1–4]. Exfoliation occurs when the graphene layers are forced apart [5], which may be caused by the sudden vaporization or decomposition of the intercalated species. Considerable research has also focused on methods to produce exfoliated graphites (EGs) that are simpler than the traditional heating of acid-based GICs, if possible at lower temperatures [6] or without using inorganic acids [7]. Several groups have also used microwave (MW) irradiation to produce EGs [8–10]. We report here the microwave-exfoliation of a potassium-THF graphite co-intercalation compound. Unlike previously reported MW-EGs [8,10], we did not use H2SO4 or SO3 to prepare the GIC. Being much more volatile than sulfuric acid, we expected that the sudden heating of the THF-containing residue GIC [7] would result in a higher degree of exfoliation and increased surface area, hence providing an effective way to obtain EG without using inorganic acids. Graphite (Cornerstone natural flakes or Aldrich synthetic graphite powder) and potassium were heated
*
Corresponding author. Fax: +1 310 825 0767. E-mail address: [email protected] (F. Wudl).
0008-6223/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.01.018
(210 °C) in an evacuated, flame-sealed glass tube to afford the golden-colored KC8 [4]. In a glove box, KC8 was transferred to flasks containing either dry THF or a THF solution of 18-crown-6 (18C6, Aldrich), soaked for 4–24 h and sonicated (outside the glove box) for about 1 h to co-intercalate THF [7]. We used a 1:4 ratio between the GIC estimated amount of K and 18C6, and 0.5–7 mL of THF per mmol of K. Although the 18C6 was used to ease the cointercalation of THF by stabilizing K+ ions, no difference between the 18C6 solution and pure THF was noticed in the exfoliation efficiency. The resulting black solid was filtered, dried (70 °C) and transferred to glass tubes. These tubes were heated at high power in a commercial MW oven. A 6-cm aluminum disk was placed at the oven center to concentrate the microwaves [4,11]. Few seconds of irradiation would turn the residue graphite compound red-hot and cause a remarkable volume expansion, due to the violent expulsion of the volatile species from the interlayer spaces of the GIC and consequent layer separation [5]. The characteristic accordion-like structures of EGs are clearly seen in the SEM image of a MW-EG sample (obtained with a Hitachi 4100 microscope) shown in Fig. 1. The macroscopic volume of the MW-exfoliated samples increases 6–10 times, as seen in the Fig. 1 insert. Control experiments using a flame to heat the residue compound would generally result in a less pronounced expansion.
1368
Letters to the Editor / Carbon 45 (2007) 1364–1369
100
Weight (%)
80 60
(c)
(a)
(b) 40 20 0 0
Intensity
12000
8000
(c) 20
25
30
(b) 4000
(a) 0
0
10
20
30
40
50 60 2 (theta)
70
80
90 100
Fig. 2. Powder XRD patterns of graphite (a), THF co-intercalated GIC (b), and MW-EG (c). Insert: Expansion of XRD patterns within the range 20–30°.
Using open or evacuated tubes did not affect the process noticeably. Fig. 2 shows powder XRD results (CuKa radiation). Graphite showed the very strong 002 line at 26.55°. Spectra of co-intercalated samples sometimes exhibited the same line (though much weaker), but they always featured a distinctive, broader peak around 24.4°. We attributed its appearance to an increased interlayer spacing due to weaker van der Waals interaction caused by solvent co-intercalation. From Bragg’s equation, the d-spacing in these ˚ . After MW heating, the samples varied from 3.7 to 3.8 A lower 2h peak always disappeared, and the 26.55° peak usually increased in intensity, as seen in Fig. 2. As stated above, the sudden and intense temperature increase leads to the vaporization and violent expulsion of solvent, causing exfoliation [6], which explains the volume increase and the disappearance of the lower 2h peak. The BET surface areas (SA, determined with a Micromeritics ASAP 2100) exhibited a general trend: after
800
1000
co-intercalation, the SA decreases slightly but increases significantly with MW exfoliation. For MW-EGs prepared from the Cornerstone graphite, in particular, SAs higher than 50 m2/g were attained (in comparison with less than 5 m2/g for the pristine graphite). TGA results (performed under air flow in a Perkin– Elmer TGA7) are shown in Fig. 3. Weight loss for graphite starts at about 700 °C, when the sample progressively reacts with O2 to form CO2. The residue compound, on the other hand, loses about 30 wt% in several steps below 150 °C, due to release of co-intercalated solvent not eliminated during the drying at 70 °C. Sample burning causes the next 60–80 wt% loss. This occurs in the 400–600 °C range because of the lowered thermal stability due to the reduced van der Waals interaction (as suggested by the increased d-spacing observed with the XRD) [4]. The increased SA of the MW-EG samples (expected to render the material more reactive) causes 80 wt% to be lost in the 400–600 °C range. Since all solvent was expelled during the MW heating, no loss occurred below 150 °C. The remaining material (20%) for both the co-intercalated 3.5 2.5
3.0
Voltage (V)
16000
400 600 Temperature (°C)
Fig. 3. Thermogravimetric analysis of pure graphite (a), THF co-intercalated GIC (b), and MW-exfoliated GIC (c).
2.5
Voltage (V)
Fig. 1. SEM of a MW-exfoliated sample showing the accordion-like structures of EG. Insert: test tubes showing samples at several stages of the experiment. From left: pure graphite, golden-colored KC8, THF cointercalated GIC, and MW-exfoliated sample. Initial amount of graphite in each sample was 0.3 g. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
200
2.0 1.5 1.0 0.5 0.0 0
2.0
10
20
30
40
Q (mA.h/g)
1.5 1.0 0.5 0.0 0
200
400
600
800
1000
Q (mA h/g) Fig. 4. Galvanostatic cycling of a MW-EG sample. Insert: electrochemical results for a graphite sample.
Letters to the Editor / Carbon 45 (2007) 1364–1369
and MW-exfoliated samples burns above 800 °C and presumably corresponds to domains that did not undergo any intercalation. Graphite and other carbonaceous materials are widely used as anodes in lithium ion batteries [12], which led us to investigate the electrochemical properties of the MWEG. A typical three-electrode cell (MW-EG on a steel mesh as working electrode, Li foils as counter and reference electrodes) was used. Fig. 4 shows the galvanostatic profile of the MW-EG sample. Electrolyte decomposition and formation of the solid electrolyte interface accounts for the plateau in the 0.9–0.8 V region and the high irreversible capacity of the first discharge [13]. From the next two cycles, the reversible capacity of the MW-EG was estimated as Qrev 130 mA h/g. Natural graphite tested under similar conditions (Fig. 4 insert) shows an expected poor performance, with Qrev 10 mA h/g, as we used a material not tailored for electrochemical applications [14]. Although low in comparison with the ideal reversible capacity of graphite, 372 mA h/g [12,13], higher values would possibly result if better starting materials are used. In summary, we prepared exfoliated graphite from a residue potassium-THF co-intercalation compound using MW irradiation. The procedure is simple and effective, affording an EG which is free from acid residue, and shows increased surface area and electrochemical lithium capacity. Acknowledgements Work supported by the Office of Naval Research (MURI grant no. N00014-01-1-0757).
1369
References [1] Viculis LM, Mack JJ, Kaner RB. A chemical route to carbon nanoscrolls. Science 2003;299(5611):1361-1. [2] Shioyama H, Akita T. A new route to carbon nanotubes. Carbon 2003;41(1):179–81. [3] Chen G, Weng W, Wu D, Wu C, Lu J, Wang P, Chen X. Preparation and characterization of graphite nanosheets from ultrasound powdering technique. Carbon 2004;42(4):753–9. [4] Viculis LM, Mack JJ, Mayer OM, Hahn HT, Kaner RB. Intercalation and exfoliation routes to graphite nanoplatelets. J Mater Chem 2005;15(9):974–8. [5] Chung DDL. Exfoliation of graphite. J Mater Sci 1987;22(12): 4190–8. [6] Zhang Z, Lerner MM. Preparation, characterization, and exfoliation of graphite perfluorooctanesulfonate. Chem Mater 1996;8(1):257–63. [7] Inagaki M, Muramatsu K, Maeda Y, Maekawa M. Production of exfoliated graphite from potassium-graphite-tetrahydrofuran ternary compounds and its applications. Synth Met 1983;8(3–4):335–42. [8] Kwon OY, Choi SW, Park KW, Kwon YB. The preparation of exfoliated graphite by using microwave. J Ind Eng Chem 2003;9(6):743–7. [9] Du XS, Xiao M, Meng YZ. Synthesis and characterization of polyaniline/graphite conducting nanocomposites. J Polym Sci B: Polym Phys 2004;42(10):1972–8. [10] Tryba B, Morawski AW, Inagaki M. Preparation of exfoliated graphite by microwave irradiation. Carbon 2005;43(11):2417–9. [11] Blair RG. Control and optimization of solid-state displacement reactions. Los Angeles CA USA, University of California, Los Angeles, Ph.D. Dissertation, 2004, p. 52–3. [12] Nishi Y. Lithium ion secondary batteries; past 10 years and the future. J Power Sources 2001;100(1–2):101–6. [13] Dahn JR. Phase diagram of LixC6. Phys Rev B 1991;44(17):9170–7. [14] Aurbach D, Teller H, Levi E. Morphology/behavior relationship in reversible electrochemical lithium insertion into graphitic materials. J Electrochem Soc 2002;149(10):A1255–66.
[3]
[4]
[5]
[6]
single-wall carbon nanohorns for fuel cell application. Physica B 2002;323:124–6. Murata K, Hashimoto A, Yudasaka M, Kasuya D, Kaneko K, Iijima S. The use of charge transfer to enhance the methane-storage capacity of single-walled, nanostructured carbon. Adv Mater 2004:1520–2. Kasuya D, Yudasaka M, Takahashi K, Kokai F, Iijima S. Selective production of single-wall carbon nanohorn aggregates and their formation mechanism. J Phys Chem B 2002;106:4947–51. Yamaguchi T, Bandow S, Iijima S. Synthesis of carbon nanohorn particles by simple pulsed arc discharge ignited between pre-heated carbon rods. Chem Phys Lett 2004;389:181–5. Fan J, Yudasaka M, Kasuya D, Azami T, Yuge R, Imai H, et al. Micrometer-sized graphitic balls produced together with single-wall carbon nanohorns. J Phys Chem B 2005;109:10756–9.
1367
[7] Kasuya D, Kokai F, Takahashi K, Yudasaka M, Iijima S. Formation of C60 using CO2 laser vaporization of graphite at room temperature. Chem Phys Lett 2001;337:25–30. [8] Kasuya D, Yudasaka M. Growth mechanisms of carbon nanotubes and nanohorns. New Diamond 2004;73:2–7. [9] Kawai T, Miyamoto Y, Sugino O, Koga Y. Nanotube and nanohorn nucleation from graphitic patches: tight-binding molecular-dynamics simulations. Phys Rev B 2002;66:33404–14. [10] Utsumi S, Miyawaki J, Tanaka H, Hattori Y, Itoi T, Ichikuni N, et al. Opening mechanism of internal nanoporosity of single-wall carbon nanohorn. J Phys Chem B 2005;109:14319–24.
Microwave exfoliation of a graphite intercalation compound Eduardo H.L. Falcao a,b, Richard G. Blair a, Julia J. Mack a, Lisa M. Viculis a, Chai-Won Kwon c, Michael Bendikov a, Richard B. Kaner a,c, Bruce S. Dunn c, Fred Wudl a,* a
Department of Chemistry and Biochemistry, and Exotic Materials Institute, University of California, Los Angeles, CA 90095, United States b Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara, CA 93106, United States c Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095, United States Received 23 June 2006; accepted 11 January 2007 Available online 1 February 2007
The search for nanostructured materials has propelled much of the recent research on graphite intercalation compounds (GICs) and their exfoliation [1–4]. Exfoliation occurs when the graphene layers are forced apart [5], which may be caused by the sudden vaporization or decomposition of the intercalated species. Considerable research has also focused on methods to produce exfoliated graphites (EGs) that are simpler than the traditional heating of acid-based GICs, if possible at lower temperatures [6] or without using inorganic acids [7]. Several groups have also used microwave (MW) irradiation to produce EGs [8–10]. We report here the microwave-exfoliation of a potassium-THF graphite co-intercalation compound. Unlike previously reported MW-EGs [8,10], we did not use H2SO4 or SO3 to prepare the GIC. Being much more volatile than sulfuric acid, we expected that the sudden heating of the THF-containing residue GIC [7] would result in a higher degree of exfoliation and increased surface area, hence providing an effective way to obtain EG without using inorganic acids. Graphite (Cornerstone natural flakes or Aldrich synthetic graphite powder) and potassium were heated
*
Corresponding author. Fax: +1 310 825 0767. E-mail address: [email protected] (F. Wudl).
0008-6223/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.01.018
(210 °C) in an evacuated, flame-sealed glass tube to afford the golden-colored KC8 [4]. In a glove box, KC8 was transferred to flasks containing either dry THF or a THF solution of 18-crown-6 (18C6, Aldrich), soaked for 4–24 h and sonicated (outside the glove box) for about 1 h to co-intercalate THF [7]. We used a 1:4 ratio between the GIC estimated amount of K and 18C6, and 0.5–7 mL of THF per mmol of K. Although the 18C6 was used to ease the cointercalation of THF by stabilizing K+ ions, no difference between the 18C6 solution and pure THF was noticed in the exfoliation efficiency. The resulting black solid was filtered, dried (70 °C) and transferred to glass tubes. These tubes were heated at high power in a commercial MW oven. A 6-cm aluminum disk was placed at the oven center to concentrate the microwaves [4,11]. Few seconds of irradiation would turn the residue graphite compound red-hot and cause a remarkable volume expansion, due to the violent expulsion of the volatile species from the interlayer spaces of the GIC and consequent layer separation [5]. The characteristic accordion-like structures of EGs are clearly seen in the SEM image of a MW-EG sample (obtained with a Hitachi 4100 microscope) shown in Fig. 1. The macroscopic volume of the MW-exfoliated samples increases 6–10 times, as seen in the Fig. 1 insert. Control experiments using a flame to heat the residue compound would generally result in a less pronounced expansion.
1368
Letters to the Editor / Carbon 45 (2007) 1364–1369
100
Weight (%)
80 60
(c)
(a)
(b) 40 20 0 0
Intensity
12000
8000
(c) 20
25
30
(b) 4000
(a) 0
0
10
20
30
40
50 60 2 (theta)
70
80
90 100
Fig. 2. Powder XRD patterns of graphite (a), THF co-intercalated GIC (b), and MW-EG (c). Insert: Expansion of XRD patterns within the range 20–30°.
Using open or evacuated tubes did not affect the process noticeably. Fig. 2 shows powder XRD results (CuKa radiation). Graphite showed the very strong 002 line at 26.55°. Spectra of co-intercalated samples sometimes exhibited the same line (though much weaker), but they always featured a distinctive, broader peak around 24.4°. We attributed its appearance to an increased interlayer spacing due to weaker van der Waals interaction caused by solvent co-intercalation. From Bragg’s equation, the d-spacing in these ˚ . After MW heating, the samples varied from 3.7 to 3.8 A lower 2h peak always disappeared, and the 26.55° peak usually increased in intensity, as seen in Fig. 2. As stated above, the sudden and intense temperature increase leads to the vaporization and violent expulsion of solvent, causing exfoliation [6], which explains the volume increase and the disappearance of the lower 2h peak. The BET surface areas (SA, determined with a Micromeritics ASAP 2100) exhibited a general trend: after
800
1000
co-intercalation, the SA decreases slightly but increases significantly with MW exfoliation. For MW-EGs prepared from the Cornerstone graphite, in particular, SAs higher than 50 m2/g were attained (in comparison with less than 5 m2/g for the pristine graphite). TGA results (performed under air flow in a Perkin– Elmer TGA7) are shown in Fig. 3. Weight loss for graphite starts at about 700 °C, when the sample progressively reacts with O2 to form CO2. The residue compound, on the other hand, loses about 30 wt% in several steps below 150 °C, due to release of co-intercalated solvent not eliminated during the drying at 70 °C. Sample burning causes the next 60–80 wt% loss. This occurs in the 400–600 °C range because of the lowered thermal stability due to the reduced van der Waals interaction (as suggested by the increased d-spacing observed with the XRD) [4]. The increased SA of the MW-EG samples (expected to render the material more reactive) causes 80 wt% to be lost in the 400–600 °C range. Since all solvent was expelled during the MW heating, no loss occurred below 150 °C. The remaining material (20%) for both the co-intercalated 3.5 2.5
3.0
Voltage (V)
16000
400 600 Temperature (°C)
Fig. 3. Thermogravimetric analysis of pure graphite (a), THF co-intercalated GIC (b), and MW-exfoliated GIC (c).
2.5
Voltage (V)
Fig. 1. SEM of a MW-exfoliated sample showing the accordion-like structures of EG. Insert: test tubes showing samples at several stages of the experiment. From left: pure graphite, golden-colored KC8, THF cointercalated GIC, and MW-exfoliated sample. Initial amount of graphite in each sample was 0.3 g. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
200
2.0 1.5 1.0 0.5 0.0 0
2.0
10
20
30
40
Q (mA.h/g)
1.5 1.0 0.5 0.0 0
200
400
600
800
1000
Q (mA h/g) Fig. 4. Galvanostatic cycling of a MW-EG sample. Insert: electrochemical results for a graphite sample.
Letters to the Editor / Carbon 45 (2007) 1364–1369
and MW-exfoliated samples burns above 800 °C and presumably corresponds to domains that did not undergo any intercalation. Graphite and other carbonaceous materials are widely used as anodes in lithium ion batteries [12], which led us to investigate the electrochemical properties of the MWEG. A typical three-electrode cell (MW-EG on a steel mesh as working electrode, Li foils as counter and reference electrodes) was used. Fig. 4 shows the galvanostatic profile of the MW-EG sample. Electrolyte decomposition and formation of the solid electrolyte interface accounts for the plateau in the 0.9–0.8 V region and the high irreversible capacity of the first discharge [13]. From the next two cycles, the reversible capacity of the MW-EG was estimated as Qrev 130 mA h/g. Natural graphite tested under similar conditions (Fig. 4 insert) shows an expected poor performance, with Qrev 10 mA h/g, as we used a material not tailored for electrochemical applications [14]. Although low in comparison with the ideal reversible capacity of graphite, 372 mA h/g [12,13], higher values would possibly result if better starting materials are used. In summary, we prepared exfoliated graphite from a residue potassium-THF co-intercalation compound using MW irradiation. The procedure is simple and effective, affording an EG which is free from acid residue, and shows increased surface area and electrochemical lithium capacity. Acknowledgements Work supported by the Office of Naval Research (MURI grant no. N00014-01-1-0757).
1369
References [1] Viculis LM, Mack JJ, Kaner RB. A chemical route to carbon nanoscrolls. Science 2003;299(5611):1361-1. [2] Shioyama H, Akita T. A new route to carbon nanotubes. Carbon 2003;41(1):179–81. [3] Chen G, Weng W, Wu D, Wu C, Lu J, Wang P, Chen X. Preparation and characterization of graphite nanosheets from ultrasound powdering technique. Carbon 2004;42(4):753–9. [4] Viculis LM, Mack JJ, Mayer OM, Hahn HT, Kaner RB. Intercalation and exfoliation routes to graphite nanoplatelets. J Mater Chem 2005;15(9):974–8. [5] Chung DDL. Exfoliation of graphite. J Mater Sci 1987;22(12): 4190–8. [6] Zhang Z, Lerner MM. Preparation, characterization, and exfoliation of graphite perfluorooctanesulfonate. Chem Mater 1996;8(1):257–63. [7] Inagaki M, Muramatsu K, Maeda Y, Maekawa M. Production of exfoliated graphite from potassium-graphite-tetrahydrofuran ternary compounds and its applications. Synth Met 1983;8(3–4):335–42. [8] Kwon OY, Choi SW, Park KW, Kwon YB. The preparation of exfoliated graphite by using microwave. J Ind Eng Chem 2003;9(6):743–7. [9] Du XS, Xiao M, Meng YZ. Synthesis and characterization of polyaniline/graphite conducting nanocomposites. J Polym Sci B: Polym Phys 2004;42(10):1972–8. [10] Tryba B, Morawski AW, Inagaki M. Preparation of exfoliated graphite by microwave irradiation. Carbon 2005;43(11):2417–9. [11] Blair RG. Control and optimization of solid-state displacement reactions. Los Angeles CA USA, University of California, Los Angeles, Ph.D. Dissertation, 2004, p. 52–3. [12] Nishi Y. Lithium ion secondary batteries; past 10 years and the future. J Power Sources 2001;100(1–2):101–6. [13] Dahn JR. Phase diagram of LixC6. Phys Rev B 1991;44(17):9170–7. [14] Aurbach D, Teller H, Levi E. Morphology/behavior relationship in reversible electrochemical lithium insertion into graphitic materials. J Electrochem Soc 2002;149(10):A1255–66.