- Email: [email protected]
Hollow graphene spheres self-assembled from graphene oxide sheets by a one-step hydrothermal process
CARBON
389
5 6 (2 0 1 3) 3 8 3–39 1
Hollow graphene spheres self-assembled from graphene oxide sheets by a one-step hydrothermal process Jianyun Cao a, Yaming Wang a,*, Ping Xiao b, Yingchun Chen b, Yu Zhou a, Jia-Hu Ouyang a, Dechang Jia a a b
Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin 150001, China Materials Science Centre, School of Materials, University of Manchester, Manchester M1 7HS, UK
A R T I C L E I N F O
A B S T R A C T
Article history:
We developed a one-step hydrothermal method to assemble graphene oxide (GO) sheets
Received 15 October 2012
into hollow graphene spheres (HGSs), using only a GO/H2SO4 aqueous suspension as the
Accepted 26 December 2012
starting material. Scanning electron microscope, focused ion beam scanning electron
Available online 3 January 2013
microscope and transmission electron microscope images show that the as-prepared HGSs vary from 1 to 3 lm in diameter and have a hollow interior structure. The as-prepared HGSs show a high capacitance of 207 F g 1, as well as good rate capability and cycling stability when used as electrode materials for supercapacitors. Ó 2013 Elsevier Ltd. All rights reserved.
As a two-dimensional (2D) nanomaterial prepared from natural graphite, graphene oxide (GO), could be thought as ‘‘soft’’ 2D macromolecules for self-assembly to achieve various novel structures [1,2]. Hollow carbon spheres have attracted great attention due to the wide applications including catalyst support, drug-delivery, electrode materials for energy storage devices and substrate for developing core/shell structure composites [3,4]. Thus, the recent attentions have been focused on the possibility of assembly of 2D GO sheets into three-dimensional hollow structures, such as hollow graphene oxide spheres [4], hollow graphene microspheres [5] and hollow graphene capsules [6]. Herein, we report a one-step self-assemble hydrothermal method (Fig. 1a) using a GO/H2SO4 aqueous suspension as the precursor to prepare hollow graphene spheres (HGSs), in which the GO sheets act as 2D macromolecules and H2SO4 as promoting agent. The as-prepared HGSs exhibit a high specific capacitance of 207 F g 1 as well as good rate capability and cycle stability when used as electrode material for supercapacitors. GO was prepared by the modified hummers’ method (see Supplementary material). The as-prepared monolayer GO sheets (0.727 nm in thickness) are several hundred nanometers in size as revealed by atomic force microscopy (AFM) images (Fig. 1b and Fig. S1). A certain amount of concentrated H2SO4 (98%) was added to the GO aqueous suspension and the final concentration of H2SO4 is 1 M, immediate agglomeration of GO sheets was observed (Fig. S2) after H2SO4 was added. This is because of the small zeta potential ( 7.8 mV) of GO
in acidic media caused by the protonation of the O and COO groups [7]. After being homogenized by stirring for several minutes, a total of 30 mL GO/H2SO4 aqueous suspension was transferred to a 40 mL Teflon-lined autoclave and heated at 160 °C for 10 h, through which the GO sheets were selfassembled to HGSs. Then the HGSs were collected by filtration, washed with distilled water for several times and dried at 80 °C in air for 10 h. The scanning electron microscope (SEM) image (Fig. 1c) shows that the as-prepared HGSs vary from 1 to 3 lm in diameter. The interior hollow structure of the HGSs was demonstrated both by a focused ion beam scanning electron microscope (FIB-SEM) and a transmission electron microscope (TEM). The inset of Fig. 1c shows a FIB-SEM image of split HGSs (also see Fig. S3), which clearly revealed the interior hollow structure of the HGSs. The TEM image of HGSs (inset of Fig. 1d) confirmed the hollow structure of the HGSs, and the corresponding high resolution TEM (HRTEM) image shows that the shell thickness is less than 2 nm with layered graphene structure. The TEM image of a broken hollow sphere (Fig. S4) also confirmed the sphere shell was assembled from the graphene oxide sheets. Fig. 1e shows the C1s spectrum of GO and HGSs, the C1s spectrum confirms the removal of most of C–OH, C@O and C(O)O groups after hydrothermal treatment. We also found that the other strong acids, such as H3PO4 and HCl, can also promote the formation of HGSs under hydrothermal conditions (Fig. S5), and the control experiment performed without adding of any acids shows that a graphene
* Corresponding author: Fax: +86 451 86413922. E-mail address: [email protected] (Y. Wang). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.12.075
CARBON
(a)
5 6 ( 2 0 1 3 ) 3 8 3 –3 9 1
(a) -1
Current (A g )
1 M H2SO4 o
160 C, 10 h Hydrothermal
(c)
(b)
6 4 2 0 -2 -4 -6
100 mV s-1
10 mV s-1
0.0
0.2 0.4 0.6 Potential (V)
5 nm
Capacitance (F g )
Relative Intensity
(d)
(e) GO
-C =O -C O O -
C -O sp 3 C
2
sp C
H G Ss 296 292 288 284 B inding Energy (eV )
280
Fig. 1 – Self-assembled HGSs. (a) Schematic illustration of one-step hydrothermal self-assembly of HGSs. Insert photographs: as-prepared GO/H2SO4 aqueous suspension (left) and black precipitate containing self-assembled HGSs after hydrothermal process (right). (b) AFM image of GO sheets. (c) SEM image of HGSs. The inset is a FIB-SEM image shows the interior hollow structure (scale bar: 500 nm). (d) HRTEM image of the HGSs shell. The inset is a TEM image of HGSs (scale bar: 1 lm). (e) C1s XPS spectrum of GO and HGSs.
hydrogel structure (Fig. S6) was formed as the previous study [2]. Since the polymerization and carbonization of glucose solution under hydrothermal condition could be acid-catalyzed [8], and this process is accompanied with formation of gaseous products (CO2, CO and H2), thus these bubbles serve as templates for construction of hollow carbonaceous spheres. GO sheets share similar functional groups (–OH and C@O) with glucose molecules, so we suspect that bubbles might be generated in the GO/H2SO4 aqueous suspension under hydrothermal conditions, and these bubbles might serve as templates for construction of HGSs. Driven by the minimization of interfacial energy, GO sheets may prefer to aggregate around the liquid/gas interface [9,10], and finally HGSs form. However, in the neutral GO suspension, although bubbles might also be generated, the GO sheets prefer to randomly disperse in water instead of aggregating around the liquid/ gas interface due to their strong hydrophilicity and electrostatic repulsion effect resulting from the high zeta potential ( 42.8 mV), and finally assemble to graphene hydrogel structure after the hydrothermal process [2]. The HGSs can find extensive potential applications in the fields of catalyst support, drug-delivery and energy storage devices. Here we provide an example of HGSs using as electrode material for supercapacitors. The electrochemical measurements were carried out in a two electrode cell and 6 M KOH aqueous solution was used as electrolyte. Fig. 2a shows the cyclic voltammetry (CV) curves of the HGSs cell at various
-1
200 150 100
-1
0.2A g
0.4 0.2
-1
0.5 A g
0.0
-1
1Ag
0
100 200 300 400 500 Time (s)
0
200 400 600 800 1000 Cycle number
(d) 250
-1
Capacitance (F g )
5 µm
0.6
-0.2
0.8
(c) 250 200 nm
(b) 0.8
50 mV s-1
Voltage (V)
390
0
1 2 3 4 5 Current density (A g-1)
200 150 100 50 0
Fig. 2 – Electrochemical characterization of HGSs. (a) CV curves of HGSs at various scan rates. (b) Galvanostatic charge and discharge curves of HGSs at different current densities. (c) Capacitance of HGSs at various discharge current densities. (d) Capacitance versus cycle number of HGSs at galvanostatic charge and discharge current density of 1 A g 1.
scan rates, the rectangular shape of the CV curves shows a capacitive behavior of the HGSs. The galvanostatic charge and discharge curves of the HGSs cells at various current densities are shown in Fig. 2b, the specific capacitance calculated from the discharge curves is 207 F g 1 at a discharge current density of 0.2 A g 1, and the capacitance slightly decreases to 163 F g 1 while the current density increases to 5 A g 1, indicating a good rate capability (Fig. 2c). The specific capacitance of the HGSs (193 F g 1 at 1 A g 1) is higher than that of the graphene hydrogel prepared by similar hydrothermal process (160 F g 1 at 1 A g 1) [2]. The HGSs cell also shows good cycling stability, the capacitance slightly increases from 190 to 220 F g 1 after 1000 cycles (Fig. 2d). In conclusion, we have demonstrated that 2D GO sheets were able to form HGSs through a simple one-step selfassemble hydrothermal process. The formation of the HGSs relies on the assembly of GO sheets promoted by H2SO4. HGSs exhibit a high capacitance of 207 F g 1, as well as good rate capability and cycling stability when used as electrode material for supercapacitors. The unique hollow structure, inherent biocompatibility and stability of carbon materials make the HGSs attractive in fields of biotechnology and electrochemistry. This study will provide a new understanding of the self-assemble behavior of GO sheets as 2D macromolecular building block and inspire novel designs of core/shell structures based on HGSs.
Acknowledgements The partial supports from the NSFC Grant Nos. 51021002 and 50701014, National Basic Science Research Program (2012CB933900), the Fundamental Research Funds for the Central Universities (HIT.BRETIII.201202) and the program
CARBON
5 6 (2 0 1 3) 3 8 3–39 1
for New Century Excellent Talents in University of China (NCET-08-0166) are gratefully acknowledged.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2012.12.075.
R E F E R E N C E S
[1] Xu YX, Shi GQ. Assembly of chemically modified graphene: methods and applications. J Mater Chem 2011;21:3311–23. [2] Xu YX, Sheng KX, Li C, Shi GQ. Self-assembled graphene hydrogel via a one-step hydrothermal process. ACS Nano 2010;4(7):4324–30. [3] Liu BY, Jia DC, Meng QC, Rao JC. A novel method for preparation of hollow carbon spheres under a gas pressure atmosphere. Carbon 2007;45:668–70.
391
[4] Guo P, Song HH, Chen XH. Hollow graphene oxide spheres self-assembled by W/O emulsion. J Mater Chem 2010;20:4867–74. [5] Vickery JL, Patil AJ, Mann S. Fabrication of graphene-polymer nanocomposites with higher-order three-dimensional architectures. Adv Mater 2009;21:2180–4. [6] Sohn K, Na YJ, Chang H, Roh KM, Jang HD, Huang J. Oil absorbing graphene capsules by capillary molding. Chem Commun 2012;48:5968–70. [7] Li D, Mu¨ller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of graphene nanosheets. Nat Nanotechnol 2008;3:101–5. [8] Sun XM, Li YD. Hollow carbonaceous capsules from glucose solution. J Colloid Interf Sci 2005;291:7–12. [9] Chen Y, Guo F, Jachak A, Kim SP, Datta D, Liu J, et al. Aerosol synthesis of cargo-filled graphene nanosacks. Nano Lett 2012;12:1996–2002. [10] Mao S, Wen Z, Kim H, Lu G, Hurley P, Chen J. A general approach to one-pot fabrication of crumpled graphene-based nanohybrids for energy applications. ACS Nano 2012;6:7505–13.
389
5 6 (2 0 1 3) 3 8 3–39 1
Hollow graphene spheres self-assembled from graphene oxide sheets by a one-step hydrothermal process Jianyun Cao a, Yaming Wang a,*, Ping Xiao b, Yingchun Chen b, Yu Zhou a, Jia-Hu Ouyang a, Dechang Jia a a b
Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin 150001, China Materials Science Centre, School of Materials, University of Manchester, Manchester M1 7HS, UK
A R T I C L E I N F O
A B S T R A C T
Article history:
We developed a one-step hydrothermal method to assemble graphene oxide (GO) sheets
Received 15 October 2012
into hollow graphene spheres (HGSs), using only a GO/H2SO4 aqueous suspension as the
Accepted 26 December 2012
starting material. Scanning electron microscope, focused ion beam scanning electron
Available online 3 January 2013
microscope and transmission electron microscope images show that the as-prepared HGSs vary from 1 to 3 lm in diameter and have a hollow interior structure. The as-prepared HGSs show a high capacitance of 207 F g 1, as well as good rate capability and cycling stability when used as electrode materials for supercapacitors. Ó 2013 Elsevier Ltd. All rights reserved.
As a two-dimensional (2D) nanomaterial prepared from natural graphite, graphene oxide (GO), could be thought as ‘‘soft’’ 2D macromolecules for self-assembly to achieve various novel structures [1,2]. Hollow carbon spheres have attracted great attention due to the wide applications including catalyst support, drug-delivery, electrode materials for energy storage devices and substrate for developing core/shell structure composites [3,4]. Thus, the recent attentions have been focused on the possibility of assembly of 2D GO sheets into three-dimensional hollow structures, such as hollow graphene oxide spheres [4], hollow graphene microspheres [5] and hollow graphene capsules [6]. Herein, we report a one-step self-assemble hydrothermal method (Fig. 1a) using a GO/H2SO4 aqueous suspension as the precursor to prepare hollow graphene spheres (HGSs), in which the GO sheets act as 2D macromolecules and H2SO4 as promoting agent. The as-prepared HGSs exhibit a high specific capacitance of 207 F g 1 as well as good rate capability and cycle stability when used as electrode material for supercapacitors. GO was prepared by the modified hummers’ method (see Supplementary material). The as-prepared monolayer GO sheets (0.727 nm in thickness) are several hundred nanometers in size as revealed by atomic force microscopy (AFM) images (Fig. 1b and Fig. S1). A certain amount of concentrated H2SO4 (98%) was added to the GO aqueous suspension and the final concentration of H2SO4 is 1 M, immediate agglomeration of GO sheets was observed (Fig. S2) after H2SO4 was added. This is because of the small zeta potential ( 7.8 mV) of GO
in acidic media caused by the protonation of the O and COO groups [7]. After being homogenized by stirring for several minutes, a total of 30 mL GO/H2SO4 aqueous suspension was transferred to a 40 mL Teflon-lined autoclave and heated at 160 °C for 10 h, through which the GO sheets were selfassembled to HGSs. Then the HGSs were collected by filtration, washed with distilled water for several times and dried at 80 °C in air for 10 h. The scanning electron microscope (SEM) image (Fig. 1c) shows that the as-prepared HGSs vary from 1 to 3 lm in diameter. The interior hollow structure of the HGSs was demonstrated both by a focused ion beam scanning electron microscope (FIB-SEM) and a transmission electron microscope (TEM). The inset of Fig. 1c shows a FIB-SEM image of split HGSs (also see Fig. S3), which clearly revealed the interior hollow structure of the HGSs. The TEM image of HGSs (inset of Fig. 1d) confirmed the hollow structure of the HGSs, and the corresponding high resolution TEM (HRTEM) image shows that the shell thickness is less than 2 nm with layered graphene structure. The TEM image of a broken hollow sphere (Fig. S4) also confirmed the sphere shell was assembled from the graphene oxide sheets. Fig. 1e shows the C1s spectrum of GO and HGSs, the C1s spectrum confirms the removal of most of C–OH, C@O and C(O)O groups after hydrothermal treatment. We also found that the other strong acids, such as H3PO4 and HCl, can also promote the formation of HGSs under hydrothermal conditions (Fig. S5), and the control experiment performed without adding of any acids shows that a graphene
* Corresponding author: Fax: +86 451 86413922. E-mail address: [email protected] (Y. Wang). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.12.075
CARBON
(a)
5 6 ( 2 0 1 3 ) 3 8 3 –3 9 1
(a) -1
Current (A g )
1 M H2SO4 o
160 C, 10 h Hydrothermal
(c)
(b)
6 4 2 0 -2 -4 -6
100 mV s-1
10 mV s-1
0.0
0.2 0.4 0.6 Potential (V)
5 nm
Capacitance (F g )
Relative Intensity
(d)
(e) GO
-C =O -C O O -
C -O sp 3 C
2
sp C
H G Ss 296 292 288 284 B inding Energy (eV )
280
Fig. 1 – Self-assembled HGSs. (a) Schematic illustration of one-step hydrothermal self-assembly of HGSs. Insert photographs: as-prepared GO/H2SO4 aqueous suspension (left) and black precipitate containing self-assembled HGSs after hydrothermal process (right). (b) AFM image of GO sheets. (c) SEM image of HGSs. The inset is a FIB-SEM image shows the interior hollow structure (scale bar: 500 nm). (d) HRTEM image of the HGSs shell. The inset is a TEM image of HGSs (scale bar: 1 lm). (e) C1s XPS spectrum of GO and HGSs.
hydrogel structure (Fig. S6) was formed as the previous study [2]. Since the polymerization and carbonization of glucose solution under hydrothermal condition could be acid-catalyzed [8], and this process is accompanied with formation of gaseous products (CO2, CO and H2), thus these bubbles serve as templates for construction of hollow carbonaceous spheres. GO sheets share similar functional groups (–OH and C@O) with glucose molecules, so we suspect that bubbles might be generated in the GO/H2SO4 aqueous suspension under hydrothermal conditions, and these bubbles might serve as templates for construction of HGSs. Driven by the minimization of interfacial energy, GO sheets may prefer to aggregate around the liquid/gas interface [9,10], and finally HGSs form. However, in the neutral GO suspension, although bubbles might also be generated, the GO sheets prefer to randomly disperse in water instead of aggregating around the liquid/ gas interface due to their strong hydrophilicity and electrostatic repulsion effect resulting from the high zeta potential ( 42.8 mV), and finally assemble to graphene hydrogel structure after the hydrothermal process [2]. The HGSs can find extensive potential applications in the fields of catalyst support, drug-delivery and energy storage devices. Here we provide an example of HGSs using as electrode material for supercapacitors. The electrochemical measurements were carried out in a two electrode cell and 6 M KOH aqueous solution was used as electrolyte. Fig. 2a shows the cyclic voltammetry (CV) curves of the HGSs cell at various
-1
200 150 100
-1
0.2A g
0.4 0.2
-1
0.5 A g
0.0
-1
1Ag
0
100 200 300 400 500 Time (s)
0
200 400 600 800 1000 Cycle number
(d) 250
-1
Capacitance (F g )
5 µm
0.6
-0.2
0.8
(c) 250 200 nm
(b) 0.8
50 mV s-1
Voltage (V)
390
0
1 2 3 4 5 Current density (A g-1)
200 150 100 50 0
Fig. 2 – Electrochemical characterization of HGSs. (a) CV curves of HGSs at various scan rates. (b) Galvanostatic charge and discharge curves of HGSs at different current densities. (c) Capacitance of HGSs at various discharge current densities. (d) Capacitance versus cycle number of HGSs at galvanostatic charge and discharge current density of 1 A g 1.
scan rates, the rectangular shape of the CV curves shows a capacitive behavior of the HGSs. The galvanostatic charge and discharge curves of the HGSs cells at various current densities are shown in Fig. 2b, the specific capacitance calculated from the discharge curves is 207 F g 1 at a discharge current density of 0.2 A g 1, and the capacitance slightly decreases to 163 F g 1 while the current density increases to 5 A g 1, indicating a good rate capability (Fig. 2c). The specific capacitance of the HGSs (193 F g 1 at 1 A g 1) is higher than that of the graphene hydrogel prepared by similar hydrothermal process (160 F g 1 at 1 A g 1) [2]. The HGSs cell also shows good cycling stability, the capacitance slightly increases from 190 to 220 F g 1 after 1000 cycles (Fig. 2d). In conclusion, we have demonstrated that 2D GO sheets were able to form HGSs through a simple one-step selfassemble hydrothermal process. The formation of the HGSs relies on the assembly of GO sheets promoted by H2SO4. HGSs exhibit a high capacitance of 207 F g 1, as well as good rate capability and cycling stability when used as electrode material for supercapacitors. The unique hollow structure, inherent biocompatibility and stability of carbon materials make the HGSs attractive in fields of biotechnology and electrochemistry. This study will provide a new understanding of the self-assemble behavior of GO sheets as 2D macromolecular building block and inspire novel designs of core/shell structures based on HGSs.
Acknowledgements The partial supports from the NSFC Grant Nos. 51021002 and 50701014, National Basic Science Research Program (2012CB933900), the Fundamental Research Funds for the Central Universities (HIT.BRETIII.201202) and the program
CARBON
5 6 (2 0 1 3) 3 8 3–39 1
for New Century Excellent Talents in University of China (NCET-08-0166) are gratefully acknowledged.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2012.12.075.
R E F E R E N C E S
[1] Xu YX, Shi GQ. Assembly of chemically modified graphene: methods and applications. J Mater Chem 2011;21:3311–23. [2] Xu YX, Sheng KX, Li C, Shi GQ. Self-assembled graphene hydrogel via a one-step hydrothermal process. ACS Nano 2010;4(7):4324–30. [3] Liu BY, Jia DC, Meng QC, Rao JC. A novel method for preparation of hollow carbon spheres under a gas pressure atmosphere. Carbon 2007;45:668–70.
391
[4] Guo P, Song HH, Chen XH. Hollow graphene oxide spheres self-assembled by W/O emulsion. J Mater Chem 2010;20:4867–74. [5] Vickery JL, Patil AJ, Mann S. Fabrication of graphene-polymer nanocomposites with higher-order three-dimensional architectures. Adv Mater 2009;21:2180–4. [6] Sohn K, Na YJ, Chang H, Roh KM, Jang HD, Huang J. Oil absorbing graphene capsules by capillary molding. Chem Commun 2012;48:5968–70. [7] Li D, Mu¨ller MB, Gilje S, Kaner RB, Wallace GG. Processable aqueous dispersions of graphene nanosheets. Nat Nanotechnol 2008;3:101–5. [8] Sun XM, Li YD. Hollow carbonaceous capsules from glucose solution. J Colloid Interf Sci 2005;291:7–12. [9] Chen Y, Guo F, Jachak A, Kim SP, Datta D, Liu J, et al. Aerosol synthesis of cargo-filled graphene nanosacks. Nano Lett 2012;12:1996–2002. [10] Mao S, Wen Z, Kim H, Lu G, Hurley P, Chen J. A general approach to one-pot fabrication of crumpled graphene-based nanohybrids for energy applications. ACS Nano 2012;6:7505–13.