Journal of Colloid and Interface Science 436 (2014) 41–46
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Preparation of colloidal graphene in quantity by electrochemical exfoliation Kunfeng Chen, Dongfeng Xue ⇑ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 27 June 2014 Accepted 27 August 2014 Available online 6 September 2014 Keywords: Colloidal graphene Electrochemical exfoliation Mass production Energy storage Printed electronics
a b s t r a c t We reported the preparation of colloidal graphene in quantity via the anodic exfoliation of graphite in (NH4)2SO4 aqueous solution. In the currently designed electrochemical exfoliation route, mass highquality graphene was produced within short reaction time, around 1 h. The proposed electrochemical and H2O can be intercalated into those graphite sheets, exfoliation mechanism showed that SO2 4 monolayer and few-layer graphene were obtained by the formation of gaseous SO2 and O2 within graphite sheets. Stability evaluation showed that our exfoliated colloidal graphene can be perfectly stabilized in DMF solvent more than 1 week. The colloidal graphene can be used to construct various simple and complex patterns by writing it on A4 paper, which can be applied to flexible printed electronic devices. Furthermore, colloidal graphene can show promising applications in the fabrication of binder- and additive-free electrodes for supercapacitors and lithium-ion batteries. Our present method shows huge potential for industrial-scale synthesis of high-quality graphene and further commercialization of graphene colloid for numerous advanced applications in flexible printed electronics and energy storage devices. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Due to its ultrathin, two-dimensional and sp2-bonded carbon nature and unprecedented properties, graphene has become the most studied nanomaterials [1,2], which can show potential and important applications in printed electronics, conductive coatings, composite fillers, solar cells, sensors, lithium-ion batteries and supercapacitors [3–5]. The large-scale preparation of colloidal form of graphene is an important requirement for next-generation printed electronics [6]. The solar cell, battery and sensor electrodes almost certainly are produced by solution-coating process and so will require large quantities of graphene in the form of colloids, liquid suspensions, inks, or dispersions [1,3]. However, all these applications require large-scale production of defect-free graphene in a processable form. The current routes for the preparation of graphene mainly include mechanically exfoliation, epitaxial growth, chemical vapor deposition (CVD), solvent- and/or surfactantassisted liquid-phase exfoliation of graphite, and the thermal/ chemical reduction of graphene oxide [7,8]. Among these methods, electrochemical exfoliation of graphite has attracted specific attention due to its easy, fast, and environmentally friendly nature to produce high-quality graphene [6,9,10]. The electrochemical exfoliation reaction mainly conducted in ionic liquids or acidic ⇑ Corresponding author. E-mail address: [email protected] (D. Xue). http://dx.doi.org/10.1016/j.jcis.2014.08.057 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
aqueous electrolyte [9,10]. However, the large quantities production of colloidal graphene in mild near neutral solution by electrochemical method is rarely reported [6]. Colloids represent a formation stage of crystallization of all kinds of materials; generally speaking, atoms initially form clusters in various kinds of solutions or melts, then during nucleation a colloid is formed which further promotes crystal growth [11,12]. The combination of strongly size- and shape-dependent physical properties and ease of fabrication and processing makes colloidal nanocrystals promising building blocks for materials with designed functions [13]. Recently, we have found that the in-situ formed colloid during the electrochemical reaction can display high electrochemical performance as energy storage devices [14– 17]. The as-formed colloids have very large surface area, thus can effectively utilize their active surfaces to improve electrochemical performance. Therefore, the colloidal form of graphene can also produce novel performances compared with other forms of graphene. In addition, colloidal graphene can be served as versatile platform to construct devices by the easy solution-route. However, graphene is prone to aggregate when reducing graphene oxide in water using reducing agent without additional reagents, where graphene colloid was produced by the transformation of graphene oxide colloid. The development of a straightforward process to prepare colloidal graphene from graphite materials is a critical point for commercial applications.
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K. Chen, D. Xue / Journal of Colloid and Interface Science 436 (2014) 41–46
Scheme 1. Schematic drawing shows the large-scale production of graphene sheets by electrochemistry using two graphite electrodes in aqueous (NH4)2SO4 solution. Left, experimental setup diagram; middle, exfoliation of the graphene sheets from the graphite anode; right, formation of colloidal graphene. The applied direct current (DC) bias voltage was constant 10 V. The electrochemical exfoliation only occurred at the anodic graphite, while the cathodic graphite kept intact. The electrochemical process was finished until the fully consumption of anodic graphite.
Scheme 2. Flow chart and the exfoliation mechanism of our designed electrochemical strategy toward high quality colloidal graphene. (a) Flow chart representation of the reaction processes of graphite anode in aqueous (NH4)2SO4 solution at DC bias voltage of 10 V. (b) Crystal structure of graphite, (c) the intercalation of SO2 4 and H2O within graphite layers, (d) the evolution of bubbles within the graphite layer due to the electrochemical reactions, (e and f) these gaseous species can exert large forces on the graphite layers, and separate weakly bonded graphite layers from one another to form graphene sheets. (g) The formation of colloidal graphene with dispersing graphene sheets in DMF solvent.
Herein, mass colloidal graphene was produced by electrochemical exfoliation of graphite in (NH4)2SO4 aqueous solution within short reaction time, about 1 h. The exfoliated graphene can be stable dispersed in DMF solvent to form colloidal solution. Furthermore, colloidal graphene can show potential applications in printed electronics, fabrication of binder- and additive-free electrodes for supercapacitors and lithium-ion batteries. Our present results are promising for industrial-scale synthesis of high-quality graphene and further commercialization of graphene for numerous advanced applications.
2. Experimental
(DC) bias voltage was set to 10 V and remained until the full consumption of anodic graphite. When 10 V bias voltage as applied to electrodes, the bulk graphite expanded quickly and was thoroughly consumed within 1 h. Colloidal graphene ink was prepared by dispersing graphene in N, N-dimethylformamide (DMF) with a high concentration at sonication conditions. 2.2. Material characterizations The morphology and structure of the samples were investigated by field emission scanning electron microscope (Hitachi S4800), transmission electron microscope (FEI Tecnai G2 F20) and X-ray diffraction (Bruker D8 Focus). Raman spectra were recorded with a Renishaw 2000.
2.1. Materials preparation 2.3. Preparation and electrochemical measurement of supercapacitor In a typical electrochemical synthesis of graphene, a conventional two-electrode system was used, which consisted of two graphite flakes as anode and cathode. The electrolyte was 40 mL aqueous solution containing 0.1 M (NH4)2SO4. The direct current
The as-prepared graphene ink was coated on nickel foil to form binder-free and additive-free working electrode. The electrochemical measurements were carried out using a classical three-electrode
K. Chen, D. Xue / Journal of Colloid and Interface Science 436 (2014) 41–46
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Fig. 1. Characterization of the exfoliated graphene and pristine graphite. (a) SEM images of pristine graphite flakes. (b) SEM image of the exfoliated graphene. (c and d) TEM image and SAED pattern of the exfoliated graphene. Monolayer graphene can be obtained as evidenced by its electron diffraction pattern.
cell configuration in 2 M KOH. The Pt wire was served as the counter electrode, and the saturated calomel electrode (SCE) was used as the reference electrode. The cyclic voltammograms (CV), and galvanostatic charge–discharge measurements were carried out by an electrochemical workstation (CHI 660D). Specific capacitance values were calculated from galvanostatic charge–discharge data according to the following equation:
SC ¼
I Dt mDE
ð1Þ
where I (A) is the current used for charge/discharge, Dt (s) is the time elapsed for the discharge cycle, m (g) is the mass of the active electrode material, and DE (V) is the voltage interval. 2.4. Preparation and electrochemical measurement of lithium-ion battery The as-prepared graphene ink was directly coated on copper foil using paintbrush. After drying, the copper foils were cut into disks, which were directly applied as the working electrode without using binders and conductive agents. Lithium metal was used as the counter and reference electrode, Celgard 2400 was used as separator, and the electrolyte was 1 M LiPF6 in ethylene carbonate/ dimethyl carbonate/diethyl carbonate (EC/DMC/DEC, 1:1:1 vol%). A galvanostatic cycling test of these assembled half-cells was conducted on a LAND CT2001A system in the voltage range of 0.01–3.0 V (vs. Li+/Li) at current density of 100 mA/g. 3. Results and discussion Scheme 1 shows the experimental setup and the procedure of electrochemical anodic exfoliation of graphite. The exfoliation process was performed in a two-electrode system using two graphite flakes as anode and cathode, and the electrolyte is 0.1 M (NH4)2SO4 aqueous solution. When the direct current voltage of 10 V was applied to two-electrode setup, vigorous bubbles were produced
at both two electrodes and anodic graphite began to dissociate into electrolyte (Scheme 1). The voltage was kept constant until completing the exfoliation process. The reaction mechanism of electrochemical exfoliation of graphite is shown in Scheme 2. Graphite has a layered, planar structure. In each layer, the sp2 carbon atoms are arranged in a honeycomb lattice and are only weakly bonded to the graphite sheets above and below. Graphene are essentially single-atomthick sheets of graphite, which can be pulled from a crystal of graphite with something as simple as adhesive tape [18]. Herein, electrochemical exfoliation of graphite to produce graphene sheets in aqueous solution was reported. Scheme 2a shows flow chart of the reaction processes of graphite anode in aqueous (NH4)2SO4 solution at the DC bias voltage of 10 V. The exfoliation mechanism of our electrochemical strategy toward high quality colloidal graphene is shown in Scheme 2b–f. Firstly, SO2 and H2O were 4 intercalated into graphite layers by electrochemistry [6]. Then, the reduction of SO2 4 anions and self-oxidation of water produced gaseous species such as SO2, O2, and others, which can be evidenced by the vigorous gas evolution at anode during the electrochemical process. These gaseous species can produce larger forces beyond the weakly bonded force between the graphite layers [19]. Finally, the weakly bonded graphite layers were exfoliated from one another to form monolayer graphene sheets. The graphene colloid can be formed by dispersing graphene sheets in DMF solvent, which can be used as ink for printed electronics and energy storage devices. The morphology of the exfoliated graphene sheets was investigated by SEM and TEM. Fig. 1a shows SEM image of pristine graphite, which includes many thick graphite flakes. After exfoliated in (NH4)2SO4 solution, the crumpled and thin sheets can be found, indicated the formation of graphene sheets (Fig. 1b). The formation of crumpled sheets is due to the thermodynamic stability of the 2D graphene resulting in microscopic crumpling via bending or buckling. Fig. 1c shows TEM image of the exfoliated graphene. A transparent sheet-like structure can be found and these crumpled
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Wavenumber (cm-1) Fig. 2. (a) XRD patterns and (b) Raman spectra of graphite flakes and the exfoliated graphene. The graphite exhibits a sharp peak centered at 26.6° and 54.7° corresponding to the (0 0 2) and (0 0 4) planes. The standard XRD pattern of graphite crystal with JCPDS No. 1-640 is also shown in graph. Two remarkable peaks in the Raman spectra are the D band (defects) and the G band (in-plane vibration of sp2 carbon atoms).
Fig. 3. Photographs show the potential applications of colloidal graphene in flexible printed devices. The graphene words ‘‘CIAC’’ and ‘‘J. Colloid Interface Sci.’’ were written on A4 paper using a Chinese maobi and graphene colloid ink. The assynthesized colloidal graphene can be well stored in DMF and water.
and loose graphene sheets were stacked. The selected area electron diffraction (SAED) was performed on the graphene sheets and the corresponding SAED pattern is shown in Fig. 1d. Several diffraction spots in a hexagonal pattern were identified, suggesting that the selected graphene sheet is single crystal and the obtained graphene products include monolayer graphene sheets [20].
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Time (s) Fig. 4. Supercapacitor performance of the exfoliated graphene. (a) CV curves of graphene electrode at different scan rates and (b) charge–discharge curves at different current densities. CV and charge–discharge curves of pure graphite are also shown in a and b for comparison. All data are collected in 2 M KOH electrolyte at room temperature.
The crystal structure of the pristine graphite and the exfoliated graphene were analyzed by conducting X-ray diffraction (XRD) studies (Fig. 2a). The pristine graphite flakes exhibit a sharp peak centered at 26.6° and 54.7° corresponding to the (0 0 2) and (0 0 4) planes of graphite crystal. The exfoliated graphene displays a broad (0 0 2) diffraction peak due to the corrugated structure of the graphene and the stacked graphene layers. XRD result proved the successful exfoliation of graphite to producing graphene, which is in good agreement with the reported value. Raman spectra of graphite flakes and the exploited graphene reflect the significant structural changes from graphite to graphene (Fig. 2b). Two remarkable peaks in the Raman spectrum are the D band (defects) and the G band (in-plane vibration of sp2 carbon atoms). The Raman spectrum of the graphite (Fig. 2b) displays a prominent G peak at 1600 cm1 corresponding to the first-order scattering of the E2g vibration mode [9]. The Raman spectrum of the exfoliated graphene shows that the G band is broadened and the peak of D band is present at 1370 cm1. The ratio of ID/IG corresponds to the number of defects in graphene. The intensity of G band is higher than that of D band for the exfoliated graphene, thus, the ratio of ID/IG is less than 1, which is much lower than that of chemically or thermally reduced graphene oxide (1.2–1.5) [21,22]. The Raman spectrum, XRD and SAED analyses clearly demonstrate the synthesis of high quality graphene by electrochemical exfoliation methods when compared to chemical methods. Our present electrochemical exfoliation process shows several advantages, such as simplicity, high productivity, economical viability, and short processing time when compared to previously reported techniques and methods [21,22].
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K. Chen, D. Xue / Journal of Colloid and Interface Science 436 (2014) 41–46
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Cycle number (n) Fig. 5. Lithium-ion battery performance of the exfoliated graphene. (a) The initial three charge–discharge curves and (b) cycling performance of the exfoliated graphene electrode for lithium-ion battery at a current density of 100 mA/g and the potential range from 0.01 to 3 V vs. Li+/Li.
In order to find potential applications in flexible printed electronics, colloidal graphene was firstly prepared and its printed application was further studied. The graphene colloid was obtained by dispersing graphene in DMF, water or ethanol (Fig. S1). The assynthesized colloidal graphene can be well stored in water and DMF for long-time. The long-term stability was examined by leaving the suspensions undisturbed for different times. For example, colloidal graphene in DMF was stable for more than 1 week, while graphene colloid in water was stable for 24 h. Fig. 3 shows the potential applications of colloidal graphene. The obtained colloidal graphene in DMF was served as ink (Fig. S2). Using a Chinese maobi, colloidal graphene can be written on A4 paper. Fig. 3a and b show the written graphene words ‘‘CIAC’’ and ‘‘J. Colloid Interface Sci.’’ on A4 paper. With this simple and easy-operating strategy, various simple and complex graphene patterns can be constructed on many substrates, such as paper, current collector for flexible electronic devices. The present results prove that our exfoliated colloidal graphene ink has promising application in flexible printed electronic devices. Large-scale production of graphene is another requirement for the commercial application of graphene. Among various methods for synthesizing graphene, electrochemical exfoliation is a promising route due to its advantages of cost-effective, simple, convenience, and without using complex equipment [20]. In our designed experiment, two graphite electrodes, one DC power source, and aqueous electrolyte were only needed to produce mass graphene within short reaction time. With 1 h reaction time, 0.2 g of crude graphite was fully exfoliated to produce graphene and the production rate of graphene was 0.2 g/h. In fact, 80% of literatures
45
had production rate of graphene below 0.04 g/h [1]. Therefore, our designed electrochemical exfoliation method can show huge potential in large-scale production of graphene. To further confirm the potential application of the exfoliated graphene, the performances of supercapacitors and lithium-ion batteries were evaluated. In this work, the used graphene electrodes were binder-free and additive-free. The exfoliated colloidal graphene ink was directly coated on Ni foam current collector to form supercapacitor electrode, while the graphene ink coated on copper foil was directly served as anode for lithium ion battery. The binder-free and additive-free electrode is an important requirement for high-performance flexible printed energy storage devices. Fig. 4 shows supercapacitor performance of the exfoliated graphene. The graphene electrode exhibits a typical double-layer capacitive behavior with quasi-rectangular CV curves and linear charge–discharge curves (Fig. 4a and b) [23]. The outline of CV curves at 50 and 100 mV/s, and the profile of charge–discharge curves at 1 and 3 A/g are similar, indicating the good capacitive behavior. The specific capacitance of the exfoliated graphene is 56.6 F/g at the current density of 1 A/g according to the discharge curves calculated from Eq. (1). The supercapacitor performance of the pure graphite is also shown in Fig. 4 for comparison. Pure graphite also shows typical double-layer capacitive behavior with quasi-rectangular CV curves and linear charge–discharge curves. However, the specific capacitance, 0.62 F/g at current density of 1 A/g, is very lower than the value of the exfoliated graphene. The results proved that the exploited graphene show significant enhanced specific capacitance. Furthermore, we studied the lithium-ion battery performance of the exfoliated graphene. Fig. 5 shows the charge–discharge curves and cycling performance of the exfoliated graphene electrode for lithium-ion batteries. The first discharge capacity is 342 mA h/g, while the 10th and 20th discharge capacities are 117.9 and 100.2 mA h/g. High Coulombic efficiency >91% can be obtained after 10th charge–discharge cycle. After 50 charge–discharge cycles, the discharge capacity is 80.1 mA h/g with Coulombic efficiency of 98.4%, indicating that the exfoliated graphene electrode shows better cycling stability. The present results prove that the exfoliated colloidal graphene can be served as effective electrode materials for the development of flexible energy storage devices, because that colloidal graphene can be used to directly fabricate binder-free and additive-free electrodes. 4. Conclusions In summary, we reported that mass high-quality colloidal graphene was produced by electrochemical exfoliation of graphite in an aqueous (NH4)2SO4 solution within short reaction time, about 1 h. Because SO2 4 and H2O can be intercalated into graphite sheets, monolayer and few-layer graphene were formed in the designed electrochemical exfoliation route. The production rate of graphene in present work was about 0.2 g/h, which is higher than those values (below 0.04 g/h) reported in 80% of literatures. The colloidal graphene can be used to construct various simple and complex patterns by writing the as-prepared graphene ink on A4 paper, which can be applied to flexible printed electronic devices. In addition, colloidal graphene can be directly coated on Ni foam and Cu current collector to fabricate binder-free and additive-free electrodes for supercapacitors and lithium-ion batteries. We showed that the electrochemical anodic exfoliation of graphite in aqueous solution is a promising route for industrial-scale synthesis of highquality graphene and the exfoliated colloidal graphene can show potential applications in printed electronics, binder- and additive-free electrodes for energy storage devices, solar cells and sensors.
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Acknowledgments Financial support from the National Natural Science Foundation of China (grant nos. 50872016, 20973033 and 51125009) and National Natural Science Foundation of China for Creative Research Group (grant no. 21221061), and Hundred Talents Program of Chinese Academy of Science are acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.08.057. References [1] K.R. Paton, E. Varrla, C. Backes, R.J. Smith, U. Khan, A. O’Neill1, C. Boland, et al., Nat. Mater. 13 (2014) 624. [2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [3] K.S. Novoselov, V.I. Fal’ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, Nature 490 (2012) 192. [4] F. Liu, S. Song, D. Xue, H. Zhang, Adv. Mater. 24 (2012) 1089. [5] F. Liu, D. Xue, Chem. Eur. J. 19 (2013) 10716. [6] K. Parvez, Z. Wu, R. Li, X. Liu, R. Graf, X. Feng, K. Müllen, J. Am. Chem. Soc. 136 (2014) 6083.
[7] C.J. Shih, A. Vijayaraghavan, R. Krishnan, R. Sharma, J.H. Han, M.H. Ham, Z. Jin, S.C. Lin, G.L.C. Paulus, N.F. Reuel, Q.H. Wang, D. Blankschtein, M.S. Strano, Nat. Nanotechnol. 6 (2011) 439. [8] P.W. Sutter, J.I. Flege, E.A. Sutter, Nat. Mater. 7 (2008) 406. [9] V.V. Singh, G. Gupta, A. Batra, A.K. Nigam, M. Boopathi, P.K. Gutch, B.K. Tripathi, A. Srivastava, M. Samuel, G.S. Agarwal, B. Singh, R. Vijayaraghavan, Adv. Funct. Mater. 22 (2012) 2352. [10] C.Y. Su, A.Y. Lu, Y.P. Xu, F.R. Chen, A.N. Khlobystov, L.J. Li, ACS Nano 5 (2011) 2332. [11] D. Nykypanchuk, M.M. Maye, D. van der Lelie, O. Gang, Nature 451 (2008) 549. [12] K. Chen, D. Xue, CrystEngComm 16 (2014) 4610. [13] Y. Yin, A.P. Alivisatos, Nature 437 (2005) 664. [14] K. Chen, D. Xue, Colloids Interface Sci. Commun. 1 (2014) 39. [15] K. Chen, D. Xue, J. Colloid Interface Sci. 424 (2014) 84. [16] K. Chen, D. Xue, J. Colloid Interface Sci. 416 (2014) 172. [17] K. Chen, D. Xue, J. Colloid Interface Sci. 430 (2014) 265. [18] K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V.V. Khotkevich, S.V. Morozov, A.K. Geim, Proc. Natl. Acad. Sci. USA 102 (2005) 10451. [19] F. Beck, J. Jiang, H. Krohn, J. Electroanal. Chem. 389 (1995) 161. [20] K. Parvez, R.J. Li, S.R. Puniredd, Y. Hernandez, F. Hinkel, S.H. Wang, X.L. Feng, K. Müllen, ACS Nano 7 (2013) 3598. [21] C. Mattevi, G. Eda, S. Agnoli, S. Miller, K.A. Mkhoyan, O. Celik, D. Mastrogiovanni, G. Granozzi, E. Garfunkel, M. Chhowalla, Adv. Funct. Mater. 19 (2009) 2577. [22] V. López, R.S. Sundaram, C.G. Navarro, D. Olea, M. Burghard, J.G. Herrero, F. Zamora, K. Kern, Adv. Mater. 21 (2009) 4683. [23] M.F. El-Kady, V. Strong, S. Dubin, R.B. Kaner, Science 335 (2012) 1326.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Preparation of colloidal graphene in quantity by electrochemical exfoliation Kunfeng Chen, Dongfeng Xue ⇑ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 27 June 2014 Accepted 27 August 2014 Available online 6 September 2014 Keywords: Colloidal graphene Electrochemical exfoliation Mass production Energy storage Printed electronics
a b s t r a c t We reported the preparation of colloidal graphene in quantity via the anodic exfoliation of graphite in (NH4)2SO4 aqueous solution. In the currently designed electrochemical exfoliation route, mass highquality graphene was produced within short reaction time, around 1 h. The proposed electrochemical and H2O can be intercalated into those graphite sheets, exfoliation mechanism showed that SO2 4 monolayer and few-layer graphene were obtained by the formation of gaseous SO2 and O2 within graphite sheets. Stability evaluation showed that our exfoliated colloidal graphene can be perfectly stabilized in DMF solvent more than 1 week. The colloidal graphene can be used to construct various simple and complex patterns by writing it on A4 paper, which can be applied to flexible printed electronic devices. Furthermore, colloidal graphene can show promising applications in the fabrication of binder- and additive-free electrodes for supercapacitors and lithium-ion batteries. Our present method shows huge potential for industrial-scale synthesis of high-quality graphene and further commercialization of graphene colloid for numerous advanced applications in flexible printed electronics and energy storage devices. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Due to its ultrathin, two-dimensional and sp2-bonded carbon nature and unprecedented properties, graphene has become the most studied nanomaterials [1,2], which can show potential and important applications in printed electronics, conductive coatings, composite fillers, solar cells, sensors, lithium-ion batteries and supercapacitors [3–5]. The large-scale preparation of colloidal form of graphene is an important requirement for next-generation printed electronics [6]. The solar cell, battery and sensor electrodes almost certainly are produced by solution-coating process and so will require large quantities of graphene in the form of colloids, liquid suspensions, inks, or dispersions [1,3]. However, all these applications require large-scale production of defect-free graphene in a processable form. The current routes for the preparation of graphene mainly include mechanically exfoliation, epitaxial growth, chemical vapor deposition (CVD), solvent- and/or surfactantassisted liquid-phase exfoliation of graphite, and the thermal/ chemical reduction of graphene oxide [7,8]. Among these methods, electrochemical exfoliation of graphite has attracted specific attention due to its easy, fast, and environmentally friendly nature to produce high-quality graphene [6,9,10]. The electrochemical exfoliation reaction mainly conducted in ionic liquids or acidic ⇑ Corresponding author. E-mail address: [email protected] (D. Xue). http://dx.doi.org/10.1016/j.jcis.2014.08.057 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
aqueous electrolyte [9,10]. However, the large quantities production of colloidal graphene in mild near neutral solution by electrochemical method is rarely reported [6]. Colloids represent a formation stage of crystallization of all kinds of materials; generally speaking, atoms initially form clusters in various kinds of solutions or melts, then during nucleation a colloid is formed which further promotes crystal growth [11,12]. The combination of strongly size- and shape-dependent physical properties and ease of fabrication and processing makes colloidal nanocrystals promising building blocks for materials with designed functions [13]. Recently, we have found that the in-situ formed colloid during the electrochemical reaction can display high electrochemical performance as energy storage devices [14– 17]. The as-formed colloids have very large surface area, thus can effectively utilize their active surfaces to improve electrochemical performance. Therefore, the colloidal form of graphene can also produce novel performances compared with other forms of graphene. In addition, colloidal graphene can be served as versatile platform to construct devices by the easy solution-route. However, graphene is prone to aggregate when reducing graphene oxide in water using reducing agent without additional reagents, where graphene colloid was produced by the transformation of graphene oxide colloid. The development of a straightforward process to prepare colloidal graphene from graphite materials is a critical point for commercial applications.
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K. Chen, D. Xue / Journal of Colloid and Interface Science 436 (2014) 41–46
Scheme 1. Schematic drawing shows the large-scale production of graphene sheets by electrochemistry using two graphite electrodes in aqueous (NH4)2SO4 solution. Left, experimental setup diagram; middle, exfoliation of the graphene sheets from the graphite anode; right, formation of colloidal graphene. The applied direct current (DC) bias voltage was constant 10 V. The electrochemical exfoliation only occurred at the anodic graphite, while the cathodic graphite kept intact. The electrochemical process was finished until the fully consumption of anodic graphite.
Scheme 2. Flow chart and the exfoliation mechanism of our designed electrochemical strategy toward high quality colloidal graphene. (a) Flow chart representation of the reaction processes of graphite anode in aqueous (NH4)2SO4 solution at DC bias voltage of 10 V. (b) Crystal structure of graphite, (c) the intercalation of SO2 4 and H2O within graphite layers, (d) the evolution of bubbles within the graphite layer due to the electrochemical reactions, (e and f) these gaseous species can exert large forces on the graphite layers, and separate weakly bonded graphite layers from one another to form graphene sheets. (g) The formation of colloidal graphene with dispersing graphene sheets in DMF solvent.
Herein, mass colloidal graphene was produced by electrochemical exfoliation of graphite in (NH4)2SO4 aqueous solution within short reaction time, about 1 h. The exfoliated graphene can be stable dispersed in DMF solvent to form colloidal solution. Furthermore, colloidal graphene can show potential applications in printed electronics, fabrication of binder- and additive-free electrodes for supercapacitors and lithium-ion batteries. Our present results are promising for industrial-scale synthesis of high-quality graphene and further commercialization of graphene for numerous advanced applications.
2. Experimental
(DC) bias voltage was set to 10 V and remained until the full consumption of anodic graphite. When 10 V bias voltage as applied to electrodes, the bulk graphite expanded quickly and was thoroughly consumed within 1 h. Colloidal graphene ink was prepared by dispersing graphene in N, N-dimethylformamide (DMF) with a high concentration at sonication conditions. 2.2. Material characterizations The morphology and structure of the samples were investigated by field emission scanning electron microscope (Hitachi S4800), transmission electron microscope (FEI Tecnai G2 F20) and X-ray diffraction (Bruker D8 Focus). Raman spectra were recorded with a Renishaw 2000.
2.1. Materials preparation 2.3. Preparation and electrochemical measurement of supercapacitor In a typical electrochemical synthesis of graphene, a conventional two-electrode system was used, which consisted of two graphite flakes as anode and cathode. The electrolyte was 40 mL aqueous solution containing 0.1 M (NH4)2SO4. The direct current
The as-prepared graphene ink was coated on nickel foil to form binder-free and additive-free working electrode. The electrochemical measurements were carried out using a classical three-electrode
K. Chen, D. Xue / Journal of Colloid and Interface Science 436 (2014) 41–46
a
b
c
d
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5 1/nm
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Fig. 1. Characterization of the exfoliated graphene and pristine graphite. (a) SEM images of pristine graphite flakes. (b) SEM image of the exfoliated graphene. (c and d) TEM image and SAED pattern of the exfoliated graphene. Monolayer graphene can be obtained as evidenced by its electron diffraction pattern.
cell configuration in 2 M KOH. The Pt wire was served as the counter electrode, and the saturated calomel electrode (SCE) was used as the reference electrode. The cyclic voltammograms (CV), and galvanostatic charge–discharge measurements were carried out by an electrochemical workstation (CHI 660D). Specific capacitance values were calculated from galvanostatic charge–discharge data according to the following equation:
SC ¼
I Dt mDE
ð1Þ
where I (A) is the current used for charge/discharge, Dt (s) is the time elapsed for the discharge cycle, m (g) is the mass of the active electrode material, and DE (V) is the voltage interval. 2.4. Preparation and electrochemical measurement of lithium-ion battery The as-prepared graphene ink was directly coated on copper foil using paintbrush. After drying, the copper foils were cut into disks, which were directly applied as the working electrode without using binders and conductive agents. Lithium metal was used as the counter and reference electrode, Celgard 2400 was used as separator, and the electrolyte was 1 M LiPF6 in ethylene carbonate/ dimethyl carbonate/diethyl carbonate (EC/DMC/DEC, 1:1:1 vol%). A galvanostatic cycling test of these assembled half-cells was conducted on a LAND CT2001A system in the voltage range of 0.01–3.0 V (vs. Li+/Li) at current density of 100 mA/g. 3. Results and discussion Scheme 1 shows the experimental setup and the procedure of electrochemical anodic exfoliation of graphite. The exfoliation process was performed in a two-electrode system using two graphite flakes as anode and cathode, and the electrolyte is 0.1 M (NH4)2SO4 aqueous solution. When the direct current voltage of 10 V was applied to two-electrode setup, vigorous bubbles were produced
at both two electrodes and anodic graphite began to dissociate into electrolyte (Scheme 1). The voltage was kept constant until completing the exfoliation process. The reaction mechanism of electrochemical exfoliation of graphite is shown in Scheme 2. Graphite has a layered, planar structure. In each layer, the sp2 carbon atoms are arranged in a honeycomb lattice and are only weakly bonded to the graphite sheets above and below. Graphene are essentially single-atomthick sheets of graphite, which can be pulled from a crystal of graphite with something as simple as adhesive tape [18]. Herein, electrochemical exfoliation of graphite to produce graphene sheets in aqueous solution was reported. Scheme 2a shows flow chart of the reaction processes of graphite anode in aqueous (NH4)2SO4 solution at the DC bias voltage of 10 V. The exfoliation mechanism of our electrochemical strategy toward high quality colloidal graphene is shown in Scheme 2b–f. Firstly, SO2 and H2O were 4 intercalated into graphite layers by electrochemistry [6]. Then, the reduction of SO2 4 anions and self-oxidation of water produced gaseous species such as SO2, O2, and others, which can be evidenced by the vigorous gas evolution at anode during the electrochemical process. These gaseous species can produce larger forces beyond the weakly bonded force between the graphite layers [19]. Finally, the weakly bonded graphite layers were exfoliated from one another to form monolayer graphene sheets. The graphene colloid can be formed by dispersing graphene sheets in DMF solvent, which can be used as ink for printed electronics and energy storage devices. The morphology of the exfoliated graphene sheets was investigated by SEM and TEM. Fig. 1a shows SEM image of pristine graphite, which includes many thick graphite flakes. After exfoliated in (NH4)2SO4 solution, the crumpled and thin sheets can be found, indicated the formation of graphene sheets (Fig. 1b). The formation of crumpled sheets is due to the thermodynamic stability of the 2D graphene resulting in microscopic crumpling via bending or buckling. Fig. 1c shows TEM image of the exfoliated graphene. A transparent sheet-like structure can be found and these crumpled
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a
a
4
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Wavenumber (cm-1) Fig. 2. (a) XRD patterns and (b) Raman spectra of graphite flakes and the exfoliated graphene. The graphite exhibits a sharp peak centered at 26.6° and 54.7° corresponding to the (0 0 2) and (0 0 4) planes. The standard XRD pattern of graphite crystal with JCPDS No. 1-640 is also shown in graph. Two remarkable peaks in the Raman spectra are the D band (defects) and the G band (in-plane vibration of sp2 carbon atoms).
Fig. 3. Photographs show the potential applications of colloidal graphene in flexible printed devices. The graphene words ‘‘CIAC’’ and ‘‘J. Colloid Interface Sci.’’ were written on A4 paper using a Chinese maobi and graphene colloid ink. The assynthesized colloidal graphene can be well stored in DMF and water.
and loose graphene sheets were stacked. The selected area electron diffraction (SAED) was performed on the graphene sheets and the corresponding SAED pattern is shown in Fig. 1d. Several diffraction spots in a hexagonal pattern were identified, suggesting that the selected graphene sheet is single crystal and the obtained graphene products include monolayer graphene sheets [20].
-1.0
0
20
40
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Time (s) Fig. 4. Supercapacitor performance of the exfoliated graphene. (a) CV curves of graphene electrode at different scan rates and (b) charge–discharge curves at different current densities. CV and charge–discharge curves of pure graphite are also shown in a and b for comparison. All data are collected in 2 M KOH electrolyte at room temperature.
The crystal structure of the pristine graphite and the exfoliated graphene were analyzed by conducting X-ray diffraction (XRD) studies (Fig. 2a). The pristine graphite flakes exhibit a sharp peak centered at 26.6° and 54.7° corresponding to the (0 0 2) and (0 0 4) planes of graphite crystal. The exfoliated graphene displays a broad (0 0 2) diffraction peak due to the corrugated structure of the graphene and the stacked graphene layers. XRD result proved the successful exfoliation of graphite to producing graphene, which is in good agreement with the reported value. Raman spectra of graphite flakes and the exploited graphene reflect the significant structural changes from graphite to graphene (Fig. 2b). Two remarkable peaks in the Raman spectrum are the D band (defects) and the G band (in-plane vibration of sp2 carbon atoms). The Raman spectrum of the graphite (Fig. 2b) displays a prominent G peak at 1600 cm1 corresponding to the first-order scattering of the E2g vibration mode [9]. The Raman spectrum of the exfoliated graphene shows that the G band is broadened and the peak of D band is present at 1370 cm1. The ratio of ID/IG corresponds to the number of defects in graphene. The intensity of G band is higher than that of D band for the exfoliated graphene, thus, the ratio of ID/IG is less than 1, which is much lower than that of chemically or thermally reduced graphene oxide (1.2–1.5) [21,22]. The Raman spectrum, XRD and SAED analyses clearly demonstrate the synthesis of high quality graphene by electrochemical exfoliation methods when compared to chemical methods. Our present electrochemical exfoliation process shows several advantages, such as simplicity, high productivity, economical viability, and short processing time when compared to previously reported techniques and methods [21,22].
a
3.0
Potential (V vs. Li+/Li)
K. Chen, D. Xue / Journal of Colloid and Interface Science 436 (2014) 41–46
2.5 1st 2nd 3rd
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b
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Capacity (mAh/g)
300
20 50 0
0
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0
Cycle number (n) Fig. 5. Lithium-ion battery performance of the exfoliated graphene. (a) The initial three charge–discharge curves and (b) cycling performance of the exfoliated graphene electrode for lithium-ion battery at a current density of 100 mA/g and the potential range from 0.01 to 3 V vs. Li+/Li.
In order to find potential applications in flexible printed electronics, colloidal graphene was firstly prepared and its printed application was further studied. The graphene colloid was obtained by dispersing graphene in DMF, water or ethanol (Fig. S1). The assynthesized colloidal graphene can be well stored in water and DMF for long-time. The long-term stability was examined by leaving the suspensions undisturbed for different times. For example, colloidal graphene in DMF was stable for more than 1 week, while graphene colloid in water was stable for 24 h. Fig. 3 shows the potential applications of colloidal graphene. The obtained colloidal graphene in DMF was served as ink (Fig. S2). Using a Chinese maobi, colloidal graphene can be written on A4 paper. Fig. 3a and b show the written graphene words ‘‘CIAC’’ and ‘‘J. Colloid Interface Sci.’’ on A4 paper. With this simple and easy-operating strategy, various simple and complex graphene patterns can be constructed on many substrates, such as paper, current collector for flexible electronic devices. The present results prove that our exfoliated colloidal graphene ink has promising application in flexible printed electronic devices. Large-scale production of graphene is another requirement for the commercial application of graphene. Among various methods for synthesizing graphene, electrochemical exfoliation is a promising route due to its advantages of cost-effective, simple, convenience, and without using complex equipment [20]. In our designed experiment, two graphite electrodes, one DC power source, and aqueous electrolyte were only needed to produce mass graphene within short reaction time. With 1 h reaction time, 0.2 g of crude graphite was fully exfoliated to produce graphene and the production rate of graphene was 0.2 g/h. In fact, 80% of literatures
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had production rate of graphene below 0.04 g/h [1]. Therefore, our designed electrochemical exfoliation method can show huge potential in large-scale production of graphene. To further confirm the potential application of the exfoliated graphene, the performances of supercapacitors and lithium-ion batteries were evaluated. In this work, the used graphene electrodes were binder-free and additive-free. The exfoliated colloidal graphene ink was directly coated on Ni foam current collector to form supercapacitor electrode, while the graphene ink coated on copper foil was directly served as anode for lithium ion battery. The binder-free and additive-free electrode is an important requirement for high-performance flexible printed energy storage devices. Fig. 4 shows supercapacitor performance of the exfoliated graphene. The graphene electrode exhibits a typical double-layer capacitive behavior with quasi-rectangular CV curves and linear charge–discharge curves (Fig. 4a and b) [23]. The outline of CV curves at 50 and 100 mV/s, and the profile of charge–discharge curves at 1 and 3 A/g are similar, indicating the good capacitive behavior. The specific capacitance of the exfoliated graphene is 56.6 F/g at the current density of 1 A/g according to the discharge curves calculated from Eq. (1). The supercapacitor performance of the pure graphite is also shown in Fig. 4 for comparison. Pure graphite also shows typical double-layer capacitive behavior with quasi-rectangular CV curves and linear charge–discharge curves. However, the specific capacitance, 0.62 F/g at current density of 1 A/g, is very lower than the value of the exfoliated graphene. The results proved that the exploited graphene show significant enhanced specific capacitance. Furthermore, we studied the lithium-ion battery performance of the exfoliated graphene. Fig. 5 shows the charge–discharge curves and cycling performance of the exfoliated graphene electrode for lithium-ion batteries. The first discharge capacity is 342 mA h/g, while the 10th and 20th discharge capacities are 117.9 and 100.2 mA h/g. High Coulombic efficiency >91% can be obtained after 10th charge–discharge cycle. After 50 charge–discharge cycles, the discharge capacity is 80.1 mA h/g with Coulombic efficiency of 98.4%, indicating that the exfoliated graphene electrode shows better cycling stability. The present results prove that the exfoliated colloidal graphene can be served as effective electrode materials for the development of flexible energy storage devices, because that colloidal graphene can be used to directly fabricate binder-free and additive-free electrodes. 4. Conclusions In summary, we reported that mass high-quality colloidal graphene was produced by electrochemical exfoliation of graphite in an aqueous (NH4)2SO4 solution within short reaction time, about 1 h. Because SO2 4 and H2O can be intercalated into graphite sheets, monolayer and few-layer graphene were formed in the designed electrochemical exfoliation route. The production rate of graphene in present work was about 0.2 g/h, which is higher than those values (below 0.04 g/h) reported in 80% of literatures. The colloidal graphene can be used to construct various simple and complex patterns by writing the as-prepared graphene ink on A4 paper, which can be applied to flexible printed electronic devices. In addition, colloidal graphene can be directly coated on Ni foam and Cu current collector to fabricate binder-free and additive-free electrodes for supercapacitors and lithium-ion batteries. We showed that the electrochemical anodic exfoliation of graphite in aqueous solution is a promising route for industrial-scale synthesis of highquality graphene and the exfoliated colloidal graphene can show potential applications in printed electronics, binder- and additive-free electrodes for energy storage devices, solar cells and sensors.
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Acknowledgments Financial support from the National Natural Science Foundation of China (grant nos. 50872016, 20973033 and 51125009) and National Natural Science Foundation of China for Creative Research Group (grant no. 21221061), and Hundred Talents Program of Chinese Academy of Science are acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.08.057. References [1] K.R. Paton, E. Varrla, C. Backes, R.J. Smith, U. Khan, A. O’Neill1, C. Boland, et al., Nat. Mater. 13 (2014) 624. [2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [3] K.S. Novoselov, V.I. Fal’ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, Nature 490 (2012) 192. [4] F. Liu, S. Song, D. Xue, H. Zhang, Adv. Mater. 24 (2012) 1089. [5] F. Liu, D. Xue, Chem. Eur. J. 19 (2013) 10716. [6] K. Parvez, Z. Wu, R. Li, X. Liu, R. Graf, X. Feng, K. Müllen, J. Am. Chem. Soc. 136 (2014) 6083.
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