Journal of Colloid and Interface Science 437 (2015) 304–310
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Free-standing porous Manganese dioxide/graphene composite films for high performance supercapacitors Wang-Huan Guo a,b,1, Teng-Jiao Liu b,1, Peng Jiang b,c,⇑, Zhan-Jun Zhang a,⇑ a
University of Chinese Academy of Sciences, Beijing 100049, China National Center for Nanoscience and Technology, Beijing 100190, China c CAS Key Laboratory of Standardization and Measurement for Nanotechnology, Beijing 100190, China b
a r t i c l e
i n f o
Article history: Received 23 June 2014 Accepted 28 August 2014 Available online 16 September 2014 Keywords: Graphene MnO2 Porous material Hybrid Symmetric supercapacitor
a b s t r a c t A simple hard template method and hydrothermal process have been employed to fabricate a self-standing hierarchical porous MnO2/graphene film. Thus-constructed electrode materials for binder-free supercapacitors exhibit a high specific capacitance of 266.3 F g1 at the density of 0.2 A g1. Moreover, the two-electrode device demonstrates an excellent rate capability and cycling stability with capacitance retention of 85.1% after 2000 charge–discharge cycles at a current density of 1 A g1. The porous nanostructured design can effectively improve the specific surface areas and account for the shorter relaxation time for the electrodes, resulting in a high electrochemical performance. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction The emerging supercapacitors with high power density, exceptional cycle life, high reversibility, good pulse charge–discharge characteristics and environmental friendliness (no heavy metals used) [1–3], are promising energy storage devices, which have potential applications in many potable systems and hybrid electric vehicles. The design of free-standing electrodes without any active and binder is becoming more and more important for high performance supercapacitors due to the avoidance of the ‘dead surface’ in traditional slurry-derived electrode and allowance for more efficient charge and mass exchange [4]. Because of the unique properties, such as high specific surface area up to 2675 m2 g1 [5], excellent chemical stability, superior electrical and mechanical properties [6], as well as feasibility for large-scale production, graphene has been regarded as a potential candidate for a kind of ideal free-standing supercapacitor electrode material. Much effort for special graphene structure materials such as graphene paper [7], planar graphene [8], graphene gel [9], has already been investigated. However, large interlayer van de Waals attraction and
⇑ Corresponding authors at: National Center for Nanoscience and Technology, Beijing 100190, China (P. Jiang). Fax: +86 010 62656765. E-mail addresses: [email protected] (P. Jiang), [email protected] (Z.-J. Zhang). 1 The authors contributed equally to this work. http://dx.doi.org/10.1016/j.jcis.2014.08.060 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
strong p–p interaction make graphene sheets easily aggregate or re-stack, leading to incomplete utilization of graphene surface. To prevent the stacking, various sandwich and three-dimensional (3D) porous graphene structures have been developed. For instance, Yu and co-workers [8] made graphene-based planar supercapacitors by a self-assembled process. Xie et al. [10,11] designed a three-dimensional graphene structure by CVD and template method. Although some progresses have been achieved, studies on three-dimensional free-standing porous graphene-based films directly used as electrodes are still few. Unfortunately, the capacitance for pure graphene material is relatively low because of its intrinsic charge storage mechanism. The way of adding pseudo-capacitance materials, which has higher capacitance due to reversible Faradaic redox reaction, is an alternative solution to combine the advantages of the two types of materials. Among the available electrode materials, MnO2 has proved to be an outstanding candidate for its low cost, environmental friendliness, natural abundance and high theoretical specific capacitance (1370 F g1) [12–14]. In present work, a new hard template-directed assembly for a three-dimensional porous graphene (3D-RGO) film loaded with MnO2 by a hydrothermal method has been explored. The facile way can produce controllable and hierarchical porous composite films by using polystyrene (PS) colloidal particles as sacrificial templates. The binder-free MnO2/graphene-based supercapacitor with MnO2 loaded on the inner surface of the hollow structure films demonstrates a high rate capability, mechanical flexibility and enhanced capacitance.
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2. Experimental part 2.1. Synthesis of 3D hierarchical porous MnO2/graphene composite films Synthesis of PS@PDA spheres: All the reagents were of analytical grade and used without further purification. Monodispersed polystyrene (PS) spheres with an average diameter of 300 nm were synthesized according to the procedure reported elsewhere [15]. Then, 100 mg as-prepared PS spheres were dispersed in 2 mg of dopamine (DA) per 1 mL of 10 mM Tris–HCl buffer solution (pH 8.5) [16,17]. The solution was vigorously stirred for 15 h at room temperature to generate PS@PDA composite particles [18]. Thus-prepared spheres were harvested by alternatively centrifugating and washing with the mixture of deionized water and ethanol several times, followed by drying at 60 °C. The PS@PDA spheres were re-dispersed in deionized water to obtain aqueous suspensions (10 mg mL1) for further application. Synthesis of free-standing hierarchical porous MnO2/graphene films: Graphene oxides (GO) were initially prepared from natural flaked graphite by the modified Hummers method [19]. An approximate 1.25 mg mL1 GO hydrosol was prepared by dispersing solid GO in water using an ultrasonication process. The GO hydrosol (10 mL) was mixed with PS@PDA sphere suspension (1 mL) and sonicated for 40 min to get a homogeneous colloidal suspension, which was then vacuum filtrated to realize the sandwich PS@PDA@GO film. The free-standing 3D-GO films were obtained by peeling them off from the millipore filter membrane, and then were annealed at 500 °C (heating rate 5 °C min1) in Ar gas (120 sccm) and located in a tubular furnace for 5 h so as to remove the PS sphere templates. The final 3D hierarchical porous MnO2/graphene composite films were obtained in a typical process as follows: KMnO4 (15 mL, 5 mM) was added into a 20 mL Teflon-lined stainless steel autoclave lines with the as-prepared films immersed into the reaction solution. The autoclave was sealed and maintained at 180 °C for 20 min. The free-standing porous MnO2/graphene films were obtained by washing with deionized water, followed by cooling the autoclave under water flow for 10 min. 2.2. Structural characterization SEM images were obtained by using a field emission scanning electron microscopy (Hitachi S4800). Transmission electron microscopy (TEM) observations were performed on a Tecnai G2 F20 U-TWIN instrument operated at 200 kV. The X-ray diffraction (XRD) data were collected using a Shimadzu X-ray diffractometer (XRD-6000) with Cu Ka radiation (k = 0.154178 nm). X-ray photoelectron spectroscopy (XPS) spectra were measured using an ESCALAB 250 electron spectrometer from Thermo Scientific Corporation. 2.3. Electrochemical measurements All electrochemical characterizations of the free-standing hierarchical porous MnO2/graphene films were performed in an
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electrochemical workstation (CHI 660D) using Swagelok-type cells with two symmetrical electrodes, a glass fiber as separator (Whatman Cat No.1823 047) and 1.0 M sodium sulfate aqueous solution as electrolyte at room temperature. The two-electrode test cell configuration is found to provide more reliable and practical data for evaluating the material performance for supercapacitors. Cyclic voltammograms were recorded between 0 and 0.8 V at various scan rates ranging from 2 to 300 mV s1. Galvanostatic charge–discharge tests were conducted between 0 and 0.8 V at different current densities from 0.2 to 40 A g1. The electrochemical impedance spectroscopy (EIS) measurements were performed over a frequency range from 105 to 102 Hz at the amplitude of the sinusoidal voltage of 5 mV. The specific capacitance (Cs) can be calculated from CV and galvanostatic charge–discharge curves according to the following formulas, respectively:
Rb
IðVÞdV a DV m m 2 I Dt C¼ DV m
C¼
ð1Þ ð2Þ
Rb where a IðVÞdV is the integrated area of CV curve in one cycle, t is the scan rate, I is the constant charge–discharge current, Dt is the discharging time, DV is the potential change during discharge process, and m is the mass of one piece of the whole film electrode. 3. Results and discussion The scheme in Fig. 1 illustrates the preparation process of porous MnO2/graphene composite film. The PS@PDA spheres were obtained by vigorously stirring the solution of PS and DA for 15 h at room temperature. The mixture of the as-prepared spheres and GO hydrosol was subsequently vacuum filtrated to realize the sandwich type assembly of PS@PDA@GO film. When PS was removed by annealing at 500 °C in Ar atmosphere, a bendable 3D-rGO skeleton was produced. MnO2 nanoparticles were grown on the inner surface and margin of 3D-rGO through hydrothermal process, thus yielding porous MnO2/graphene composite films. Fig. 2(a–e) demonstrate the typical SEM images of the PS@PDA, PS@PDA@GO, 3D-rGO, and porous MnO2/graphene. It can be seen that the PS@PDA spheres were uniform in size with an average diameter of 300 nm (see Fig. 2(a)), which is well agreed with the average size of holes in the 3D-rGO (see Fig. 2(c)). In Fig. 2(b), the PS@PDA spheres were observed to be coated by wrinkled GO sheets. This is probably originated from strong interactions such as hydrogen bonds and electrostatic interactions between the PS@PDA spheres and GO flakes. The coating process is very important for the preparation of the porous MnO2/graphene film. The shell thickness of the PS@PDA is determined by the coating time and the concentration of the DA solution. In this work, the coating time is 15 h and the concentration is 2 mg of DA per 1 mL of 10 mM Tris–HCl buffer solution (pH 8.5) [16]. Due to the PDA shell, the coating spheres were positively charged. GO were synthesized by a modified Hummers method and dispersed in water by ultrasonication. Thus-prepared GO suspension was negatively charged owing to the introduction
Fig. 1. Schematic illustration of the fabrication process for 3D porous MnO2/graphene composite film.
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Fig. 2. Typical SEM images of (a) PS@PDA spheres, (b) PS@PDA@GO, (c, d) cross-section of 3D-rGO, (e) cross-section of porous 3D MnO2/rGO, and (f) digital image of the composite film.
of functional groups such as hydroxyl (AOH) and carboxylic acids (ACOOH). Obviously, the components of PS@PDA and GO were oppositely charged and the electrostatic interactions between the two components led to the uniform distribution of PS@PDA spheres in GO films. The porous structure was well preserved (see Fig. 2(c) and (d)) after removal of the PS template, which could provide sufficient mechanical strength to be as an integral freestanding film. The pores were not out of shape possibly due to the high strength of PDA shells. Moreover, the coating PDA layers were gradually changed into N-doped carbon layers during the calcination, which synergistically supported the porous structure and could also serve as reductant for formation of MnO2 nanoparticles in the hydrothermal process (Eq. (3)). 4MnO4 þ 3C þ H2 O ! 4MnO2 þ CO2 3 þ 2HCO3
ð3Þ
Also, the N-doped carbon shells provided high surface for the deposition of the MnO2 nanocrystal and great mechanical properties. As seen from the SEM image in Fig. 2(e), the original smooth 3D-rGO surfaces became much rougher after growing the MnO2 nanoparticles. The film exhibited a hierarchical structure with interconnected holes, which could be expected to promote ion diffusion and enlarge the effective specific surface areas when used as electrodes. The digital photo in Fig. 2(f) presents the as-synthesized free-standing porous MnO2/graphene film with dark color. The film could be arbitrarily bended without damage, showing relatively good flexibility. The area density of the MnO2/graphene film is about 0.32 mg cm2, demonstrating the lightness of the obtained film [20]. The porous structure and composition of the MnO2/graphene film can be further characterized by a TEM. Fig. 3(a) presents a typical low-magnified TEM image of the film. Many interlaced or adjoined circles with sharp edge contrast prove the porous structure again. The average diameter (295 nm) of the circles reflects the average size of the PS spheres. This is in accord with the SEM data. Fig. 3(b) gives a typical high-magnified TEM image of a circle. A lot of small dark dots were found to distribute on the surface of the hollow spheres. The selected area electron diffraction (SAED)
(see Fig. 3(c)) detection demonstrates two obvious rings corresponding to {0 1 1 0} (0.216 nm) and {1 2 1 0} (0.126 nm) crystal planes for graphene, respectively [21]. Other small blurry discrete spots could originate from various crystal planes of MnO2 nanocrystals. Fig. 3(d) shows a typical high-resolution TEM image of the selected area in Fig. 3(b), clear well-defined lattice fringes were observed from the MnO2 nanocrystals. The measured d-spacing values of 0.242 and 0.201 nm well match those of (3 1 1) and (4 0 0) planes reported for single crystal a-MnO2 [22,23]. The crystalline structure of MnO2 in the composite was verified by thin film XRD measurement. As shown in Fig. 4(a), the XRD patterns unambiguously demonstrate three main peaks at 2h = 37.057°, 45.091°, 65.649°, corresponding to the (3 3 1), (4 0 0) and (4 4 0) planes of a-MnO2 (JCPDS Card No. 42-1169, space group: Fd3m(2 2 7)), respectively. It indicates that the a-MnO2 nanocrystals have been formed on surfaces of the carbon shells [22,24,25]. Furthermore, appearance of a broad diffraction peak at 22° implies that the graphene sheets are poorly ordered along the stacking direction [8,24]. To further investigate the chemical composition and chemical states of various elements in the porous MnO2/graphene film, XPS analysis was performed. The XPS signals of elements Mn, O, C, N and K could be seen in Fig. S1. Due to the low weight percentage of N in the composite film, the signal of N1s was very weak (see Fig. S2). The C1s signals (see Fig. S3) mainly resulted from CAC and C@C, indicating that GO were reduced to graphene after calcination and hydrothermal process [26]. The Mn2p spectrum in Fig. 4(b) exhibits multiple splitting. Two strong peaks centered at about 654.13 and 642.58 eV with an binding energy separation of 11.7 eV, can be assigned to Mn2p1/2 and Mn2p3/2 of Mn4+ in MnO2, respectively [12,27,28]. In addition, the much weaker peaks corresponding to Mn3+ and Mn5+ species can also be observed in the spectrum, suggesting that the predominant oxidation of element Mn is +4. The electrochemical performance of the free-standing composite films with large accessible surface areas and pore structures was evaluated by a two-electrode system with 1 M NaSO4 as electrolyte. Without any binder or conductive additive, two
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Fig. 3. (a, b) Typical TEM images of 3D porous MnO2/rGO under different magnifications; (c) the corresponding SAED pattern of the area in (b); (d) HRTEM image of the selected area in (b), the insert at the lower right corner is the typical HRTEM image of a single MnO2 nanocrystal.
Fig. 4. (a) XRD pattern of the 3D porous MnO2/rGO, (b) XPS spectra of the Mn2p region (‘‘Raw’’ represents the raw data, and ‘‘Sum’’ is the total fitted curve).
symmetrical composite film electrodes were tested in Swageloktype cells at room temperature. Fig. 5(a) illustrates the CV curves of both the porous graphene and porous MnO2/graphene at a scan rate of 20 mV s1 in the potential range from 0 to 0.8 V. The two curves exhibit approximately rectangle shape, indicating the efficient intralayer charge transfer and good capacitive behavior. It is worth noting that the CV curve of the porous MnO2/graphene becomes much wider than that of the porous graphene, suggesting capacitive increase with formation of the MnO2 nanocrystals on surface of the porous graphene. Fig. 5(b) demonstrates the comparison of galvanostatic charge–discharge at the same current density of 0.2 A g1. The charge–discharge time for the porous MnO2/ graphene film, as expected, is much longer than that for the porous
graphene. According to Eq. (2), the specific capacitance for the porous MnO2/graphene electrode material was calculated to be 266.3 F g1 at a current density of 0.2 A g1, which is almost 2.5 times of the pure porous graphene film (105.48 F g1). It is also higher than those reported on MnO2/graphene hybrid materials including freestanding three-dimensional graphene/MnO2 composite (130 F g1 at 2 mV s1) [29], graphene-wrapped honeycomb MnO2 nanospheres (210 F g1 at 0.5 A g1) [30], needle-like graphene-MnO2 composites (216 F g1 at 0.15 A g1) [31], and graphene/ MnO2 composite papers (256 F g1 at 0.5 A g1) [32]. The excellent specific capacitive ability of the porous MnO2/graphene hybrid device probably originated from the synergic effect of both components of porous graphene and MnO2 nanocrystals.
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Fig. 5. (a) CV curve comparisons of the porous graphene and the porous MnO2/rGO at a scan rate of 20 mV s1, (b) comparison between the charge–discharge curves of the porous graphene the porous MnO2/rGO at a current density of 0.2 A g1.
The typical CV curves for the MnO2/graphene hybrid electrodes at different scan rates from 1 to 300 mV s1 exhibit nearly rectangular and symmetric shapes, as shown in Fig. 6(a). For clarity, the insert CV curve at the lower left corner of Fig. 6(a) demonstrates an almost ideal rectangle at the potential scan rate of 5 mV s1.
Galvanostatic charge–discharge curves of the supercapacitor device at various current densities from 0.2 to 40 A g1 are further illustrated in Fig. 6(b). All the charge/discharge curves show liner sloops and approximately symmetric triangular shapes, suggesting good electrochemical reversibility and capacitive behavior again.
Fig. 6. Typical CV curves of the free-standing porous MnO2/rGO films at different scan rates, (b) galvanostatic charge–discharge curves at various current densities, (c) the calculated specific capacitance as a function of various current densities, (d) specific capacitance as a function of cycle number at 1 A g1, the insert shows the charge– discharge curves for last ten cycles.
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Owing to the porous structure and N-doped carbon layer, it allows for effective ion migration into active sites and stress relaxing, resulting in a high rate capacity of the electrodes. Variation of specific capacitance calculated from the charge–discharge curves as a function of current density for free-standing porous MnO2/graphene electrodes is shown in Fig. 6(c). The behavior is similar to that in the case of increasing scan rate (Fig. S4). The decrease of the specific capacitance is mainly attributed to limited accessible area for ion diffusion with the increasing current density. Maximum value of specific capacitance, calculated using Eq. (2), is about 266.3 F g1 at a current density of 0.2 A g1. Nearly 46% capacity retention could be observed at the current density of 40 A g1, indicating high-rate capability of the free-standing film electrodes. The 3D porous structure having a high specific surface area can effectively improve the double layer capacitance of the hybrids while the pseudo-capacitance of MnO2 in the composite film is also enhanced by the highly conductive 3D network formed by the rGO component. A higher current density limits the accessibility of ions inside every pore of the composite films, the specific capacitance of the nanostructure electrodes decreases with the increase of the current densities. To further confirm the ion-transport kinetics and capacitive behavior of the MnO2/graphene composite film, Electrochemical Impedance Spectroscopy test was performed at a frequency range from 105 to 102 Hz. The Nyquist plot has a small semicircle located at high frequency and a nearly vertical line at low frequency as shown in Fig. S5. According to the equivalent circuit [33,34], the charge-transfer resistance of the composite was calculated to be ca. 2 X, indicating fast charge–discharge characteristic. The porous structure of the 3D MnO2/graphene plays a critical role. In addition, the plot at the high frequency that is nearly parallel to the imaginary axis suggests the fast ion diffusion and ideal capacitive behavior of the devices. The result clearly illustrates that the electrolyte ions move easily into 3D network and charge transforms fast. However, the real axis intercept is approximately 15, which could present the equivalent series resistances of the both electrodes. The resistance between the crocodile clips and the device contributes much. Therefore, this threedimensional porous structure based on the interconnected rGO sheets, which provide continuous electron pathways, allows efficient ion diffusion and fast charge transfer. A long cycle life is another important parameter for evaluating the practical application of supercapacitors. Excellent cycling stability is crucial for real supercapacitors. The long-term cycle stability of the supercapacitor based on the free-standing porous MnO2/ graphene film was tested by repeating the galvanostatic charge– discharge at a current density of 1 A g1 for 2000 cycles, as shown in Fig. 6(d). It is evident from the figure that the capacitance retention is above 100% at the very beginning and kept constant in the first 250 cycles. This interesting phenomenon is indicative of the fact that there is a gradual activation process of the supercapacitor electrodes [35,36]. The decreases of the specific capacitance would be attributed to the mechanical expansion of MnO2 nanoparticles due to the continuous ion insertion/de-insertion process. After 2000 cycles, 85.1% of initial specific capacitance can be retained. Meanwhile, the charge–discharge curves of the last ten cycles still maintain almost symmetric. This shows high electrochemical stability of the porous MnO2/grapheme electrodes during the longterm cycling test.
4. Conclusions In summary, we have demonstrated a rational design and fabrication of three-dimensional porous binder-free MnO2/graphene films as supercapacitor electrodes. The porous 3D-rGO films were constructed by combining a simple hard template-directed
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ordered assembly approach and hydrothermal process. The porous structure was well obtained and the average size of the pores was in agreement with that of the template PS spheres. The hollow structures of the film by utilizing PS as templates largely enhanced the specific surface areas, which can provide more electroactive regions and short diffusion lengths. The coating layer of PDA can not only interact well with the wrapped graphene that prevents collapse of the porous structure during the charge/discharge process but also serve as a reductant for formation of MnO2 nanocrystal. Thus-prepared free-standing films, which were directly used as supercapacitor electrodes without any additives or binders, exhibited a high specific capacitance of 266.3 F g1 at the density of 0.2 A g1 and an excellent cycling performance with capacitance retention of 85.1% after 2000 charge–discharge cycles at a current density of 1 A g1. Nearly 46% capacity retention at the current density of 40 A g1 demonstrates good rate capability of the freestanding film electrodes. All of these features contribute to the synergic effect of the two components of 3D-rGO and MnO2 nanocrystals and the porous structure. Our work provides a rational and green synthetic route to obtain binder-free electrodes with porous graphene hybrids, which shows promising applications in future high-performance, low-cost electrode materials. Acknowledgment This work was financially supported by National Basic Research Program of China (No. 2012CB933402). 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.2014.08.060. References [1] Y.G. Patrice Simon, Nat. Mater. 7 (2008) 10. [2] M. Inagaki, H. Konno, O. Tanaike, J. Power Sources 195 (2010) 7880–7903. [3] A.L. Mohana Reddy, S.R. Gowda, M.M. Shaijumon, P.M. Ajayan, Adv. Mater. 24 (2012) 5045–5064. [4] C. Zhou, Y. Zhang, Y. Li, J. Liu, Nano Lett. 13 (2013) 2078–2085. [5] 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) 10451–10453. [6] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [7] W. Lv, Z. Xia, S. Wu, Y. Tao, F.-M. Jin, B. Li, H. Du, Z.-P. Zhu, Q.-H. Yang, F. Kang, J. Mater. Chem. 21 (2011) 3359. [8] L. Peng, X. Peng, B. Liu, C. Wu, Y. Xie, G. Yu, Nano Lett. 13 (2013) 2151–2157. [9] X. Yang, J. Zhu, L. Qiu, D. Li, Adv. Mater. 23 (2011) 2833–2838. [10] W.C. Yongmin He, Xiaodong Li, Zhenxing Zhang, Jiecai Fu, Changhui Zhao, Erqing Xie, ACS Nano 7 (2012) 174–182. [11] W. Chen, Y. He, X. Li, J. Zhou, Z. Zhang, C. Zhao, C. Gong, S. Li, X. Pan, E. Xie, Nanoscale 5 (2013) 11733–11741. [12] T.B. Mathieu, Chem. Mater. 16 (2004) 3184–3190. [13] X. Li, G. Wang, X. Wang, X. Li, J. Ji, J. Mater. Chem. A 1 (2013) 10103. [14] L. Bao, J. Zang, X. Li, Nano Lett. 11 (2011) 1215–1220. [15] J. Zhang, Z. Chen, Z. Wang, W. Zhang, N. Ming, Mater. Lett. 57 (2003) 4466– 4470. [16] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Science 318 (2007) 426– 430. [17] L. Zhang, J. Wu, Y. Wang, Y. Long, N. Zhao, J. Xu, J. Am. Chem. Soc. 134 (2012) 9879–9881. [18] K. Ai, Y. Liu, C. Ruan, L. Lu, G.M. Lu, Adv. Mater. 25 (2013) 998–1003. [19] J. William, S. Hummers, Richard E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [20] X.C. Chen, W. Wei, W. Lv, F.Y. Su, Y.B. He, B. Li, F. Kang, Q.H. Yang, Chem. Commun. (Camb.) 48 (2012) 5904–5906. [21] L.Z. Xuan, Nano Lett. 8 (2008) 323–327. [22] W.L. Jipeng Ni, Liangmiao Zhang, Baohua Yue, Xingfu Shang, Yong Lv, J. Phys. Chem. C 113 (2009) 54–60. [23] L. Wang, L. Chen, Y. Li, H. Ji, G. Yang, Powder Technol. 235 (2013) 76–81. [24] P. Lian, X. Zhu, S. Liang, Z. Li, W. Yang, H. Wang, Electrochim. Acta 55 (2010) 3909–3914. [25] L. Xing, C. Cui, C. Ma, X. Xue, Mater. Lett. 65 (2011) 2104–2106. [26] Y. Meng, K. Wang, Y. Zhang, Z. Wei, Adv. Mater. 25 (2013) 6985–6990. [27] H.-W.L. Sung-Wook Kim, Pandurangan Muralidharan, Dong-Hwa Seo, WonSub Yoon, Do Kyung Kim, Kisuk Kang, Nano Res. 4 (2011) 505–510. [28] S. Li, L. Qi, L. Lu, H. Wang, RSC Adv. 2 (2012) 3298.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Free-standing porous Manganese dioxide/graphene composite films for high performance supercapacitors Wang-Huan Guo a,b,1, Teng-Jiao Liu b,1, Peng Jiang b,c,⇑, Zhan-Jun Zhang a,⇑ a
University of Chinese Academy of Sciences, Beijing 100049, China National Center for Nanoscience and Technology, Beijing 100190, China c CAS Key Laboratory of Standardization and Measurement for Nanotechnology, Beijing 100190, China b
a r t i c l e
i n f o
Article history: Received 23 June 2014 Accepted 28 August 2014 Available online 16 September 2014 Keywords: Graphene MnO2 Porous material Hybrid Symmetric supercapacitor
a b s t r a c t A simple hard template method and hydrothermal process have been employed to fabricate a self-standing hierarchical porous MnO2/graphene film. Thus-constructed electrode materials for binder-free supercapacitors exhibit a high specific capacitance of 266.3 F g1 at the density of 0.2 A g1. Moreover, the two-electrode device demonstrates an excellent rate capability and cycling stability with capacitance retention of 85.1% after 2000 charge–discharge cycles at a current density of 1 A g1. The porous nanostructured design can effectively improve the specific surface areas and account for the shorter relaxation time for the electrodes, resulting in a high electrochemical performance. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction The emerging supercapacitors with high power density, exceptional cycle life, high reversibility, good pulse charge–discharge characteristics and environmental friendliness (no heavy metals used) [1–3], are promising energy storage devices, which have potential applications in many potable systems and hybrid electric vehicles. The design of free-standing electrodes without any active and binder is becoming more and more important for high performance supercapacitors due to the avoidance of the ‘dead surface’ in traditional slurry-derived electrode and allowance for more efficient charge and mass exchange [4]. Because of the unique properties, such as high specific surface area up to 2675 m2 g1 [5], excellent chemical stability, superior electrical and mechanical properties [6], as well as feasibility for large-scale production, graphene has been regarded as a potential candidate for a kind of ideal free-standing supercapacitor electrode material. Much effort for special graphene structure materials such as graphene paper [7], planar graphene [8], graphene gel [9], has already been investigated. However, large interlayer van de Waals attraction and
⇑ Corresponding authors at: National Center for Nanoscience and Technology, Beijing 100190, China (P. Jiang). Fax: +86 010 62656765. E-mail addresses: [email protected] (P. Jiang), [email protected] (Z.-J. Zhang). 1 The authors contributed equally to this work. http://dx.doi.org/10.1016/j.jcis.2014.08.060 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
strong p–p interaction make graphene sheets easily aggregate or re-stack, leading to incomplete utilization of graphene surface. To prevent the stacking, various sandwich and three-dimensional (3D) porous graphene structures have been developed. For instance, Yu and co-workers [8] made graphene-based planar supercapacitors by a self-assembled process. Xie et al. [10,11] designed a three-dimensional graphene structure by CVD and template method. Although some progresses have been achieved, studies on three-dimensional free-standing porous graphene-based films directly used as electrodes are still few. Unfortunately, the capacitance for pure graphene material is relatively low because of its intrinsic charge storage mechanism. The way of adding pseudo-capacitance materials, which has higher capacitance due to reversible Faradaic redox reaction, is an alternative solution to combine the advantages of the two types of materials. Among the available electrode materials, MnO2 has proved to be an outstanding candidate for its low cost, environmental friendliness, natural abundance and high theoretical specific capacitance (1370 F g1) [12–14]. In present work, a new hard template-directed assembly for a three-dimensional porous graphene (3D-RGO) film loaded with MnO2 by a hydrothermal method has been explored. The facile way can produce controllable and hierarchical porous composite films by using polystyrene (PS) colloidal particles as sacrificial templates. The binder-free MnO2/graphene-based supercapacitor with MnO2 loaded on the inner surface of the hollow structure films demonstrates a high rate capability, mechanical flexibility and enhanced capacitance.
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2. Experimental part 2.1. Synthesis of 3D hierarchical porous MnO2/graphene composite films Synthesis of PS@PDA spheres: All the reagents were of analytical grade and used without further purification. Monodispersed polystyrene (PS) spheres with an average diameter of 300 nm were synthesized according to the procedure reported elsewhere [15]. Then, 100 mg as-prepared PS spheres were dispersed in 2 mg of dopamine (DA) per 1 mL of 10 mM Tris–HCl buffer solution (pH 8.5) [16,17]. The solution was vigorously stirred for 15 h at room temperature to generate PS@PDA composite particles [18]. Thus-prepared spheres were harvested by alternatively centrifugating and washing with the mixture of deionized water and ethanol several times, followed by drying at 60 °C. The PS@PDA spheres were re-dispersed in deionized water to obtain aqueous suspensions (10 mg mL1) for further application. Synthesis of free-standing hierarchical porous MnO2/graphene films: Graphene oxides (GO) were initially prepared from natural flaked graphite by the modified Hummers method [19]. An approximate 1.25 mg mL1 GO hydrosol was prepared by dispersing solid GO in water using an ultrasonication process. The GO hydrosol (10 mL) was mixed with PS@PDA sphere suspension (1 mL) and sonicated for 40 min to get a homogeneous colloidal suspension, which was then vacuum filtrated to realize the sandwich PS@PDA@GO film. The free-standing 3D-GO films were obtained by peeling them off from the millipore filter membrane, and then were annealed at 500 °C (heating rate 5 °C min1) in Ar gas (120 sccm) and located in a tubular furnace for 5 h so as to remove the PS sphere templates. The final 3D hierarchical porous MnO2/graphene composite films were obtained in a typical process as follows: KMnO4 (15 mL, 5 mM) was added into a 20 mL Teflon-lined stainless steel autoclave lines with the as-prepared films immersed into the reaction solution. The autoclave was sealed and maintained at 180 °C for 20 min. The free-standing porous MnO2/graphene films were obtained by washing with deionized water, followed by cooling the autoclave under water flow for 10 min. 2.2. Structural characterization SEM images were obtained by using a field emission scanning electron microscopy (Hitachi S4800). Transmission electron microscopy (TEM) observations were performed on a Tecnai G2 F20 U-TWIN instrument operated at 200 kV. The X-ray diffraction (XRD) data were collected using a Shimadzu X-ray diffractometer (XRD-6000) with Cu Ka radiation (k = 0.154178 nm). X-ray photoelectron spectroscopy (XPS) spectra were measured using an ESCALAB 250 electron spectrometer from Thermo Scientific Corporation. 2.3. Electrochemical measurements All electrochemical characterizations of the free-standing hierarchical porous MnO2/graphene films were performed in an
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electrochemical workstation (CHI 660D) using Swagelok-type cells with two symmetrical electrodes, a glass fiber as separator (Whatman Cat No.1823 047) and 1.0 M sodium sulfate aqueous solution as electrolyte at room temperature. The two-electrode test cell configuration is found to provide more reliable and practical data for evaluating the material performance for supercapacitors. Cyclic voltammograms were recorded between 0 and 0.8 V at various scan rates ranging from 2 to 300 mV s1. Galvanostatic charge–discharge tests were conducted between 0 and 0.8 V at different current densities from 0.2 to 40 A g1. The electrochemical impedance spectroscopy (EIS) measurements were performed over a frequency range from 105 to 102 Hz at the amplitude of the sinusoidal voltage of 5 mV. The specific capacitance (Cs) can be calculated from CV and galvanostatic charge–discharge curves according to the following formulas, respectively:
Rb
IðVÞdV a DV m m 2 I Dt C¼ DV m
C¼
ð1Þ ð2Þ
Rb where a IðVÞdV is the integrated area of CV curve in one cycle, t is the scan rate, I is the constant charge–discharge current, Dt is the discharging time, DV is the potential change during discharge process, and m is the mass of one piece of the whole film electrode. 3. Results and discussion The scheme in Fig. 1 illustrates the preparation process of porous MnO2/graphene composite film. The PS@PDA spheres were obtained by vigorously stirring the solution of PS and DA for 15 h at room temperature. The mixture of the as-prepared spheres and GO hydrosol was subsequently vacuum filtrated to realize the sandwich type assembly of PS@PDA@GO film. When PS was removed by annealing at 500 °C in Ar atmosphere, a bendable 3D-rGO skeleton was produced. MnO2 nanoparticles were grown on the inner surface and margin of 3D-rGO through hydrothermal process, thus yielding porous MnO2/graphene composite films. Fig. 2(a–e) demonstrate the typical SEM images of the PS@PDA, PS@PDA@GO, 3D-rGO, and porous MnO2/graphene. It can be seen that the PS@PDA spheres were uniform in size with an average diameter of 300 nm (see Fig. 2(a)), which is well agreed with the average size of holes in the 3D-rGO (see Fig. 2(c)). In Fig. 2(b), the PS@PDA spheres were observed to be coated by wrinkled GO sheets. This is probably originated from strong interactions such as hydrogen bonds and electrostatic interactions between the PS@PDA spheres and GO flakes. The coating process is very important for the preparation of the porous MnO2/graphene film. The shell thickness of the PS@PDA is determined by the coating time and the concentration of the DA solution. In this work, the coating time is 15 h and the concentration is 2 mg of DA per 1 mL of 10 mM Tris–HCl buffer solution (pH 8.5) [16]. Due to the PDA shell, the coating spheres were positively charged. GO were synthesized by a modified Hummers method and dispersed in water by ultrasonication. Thus-prepared GO suspension was negatively charged owing to the introduction
Fig. 1. Schematic illustration of the fabrication process for 3D porous MnO2/graphene composite film.
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Fig. 2. Typical SEM images of (a) PS@PDA spheres, (b) PS@PDA@GO, (c, d) cross-section of 3D-rGO, (e) cross-section of porous 3D MnO2/rGO, and (f) digital image of the composite film.
of functional groups such as hydroxyl (AOH) and carboxylic acids (ACOOH). Obviously, the components of PS@PDA and GO were oppositely charged and the electrostatic interactions between the two components led to the uniform distribution of PS@PDA spheres in GO films. The porous structure was well preserved (see Fig. 2(c) and (d)) after removal of the PS template, which could provide sufficient mechanical strength to be as an integral freestanding film. The pores were not out of shape possibly due to the high strength of PDA shells. Moreover, the coating PDA layers were gradually changed into N-doped carbon layers during the calcination, which synergistically supported the porous structure and could also serve as reductant for formation of MnO2 nanoparticles in the hydrothermal process (Eq. (3)). 4MnO4 þ 3C þ H2 O ! 4MnO2 þ CO2 3 þ 2HCO3
ð3Þ
Also, the N-doped carbon shells provided high surface for the deposition of the MnO2 nanocrystal and great mechanical properties. As seen from the SEM image in Fig. 2(e), the original smooth 3D-rGO surfaces became much rougher after growing the MnO2 nanoparticles. The film exhibited a hierarchical structure with interconnected holes, which could be expected to promote ion diffusion and enlarge the effective specific surface areas when used as electrodes. The digital photo in Fig. 2(f) presents the as-synthesized free-standing porous MnO2/graphene film with dark color. The film could be arbitrarily bended without damage, showing relatively good flexibility. The area density of the MnO2/graphene film is about 0.32 mg cm2, demonstrating the lightness of the obtained film [20]. The porous structure and composition of the MnO2/graphene film can be further characterized by a TEM. Fig. 3(a) presents a typical low-magnified TEM image of the film. Many interlaced or adjoined circles with sharp edge contrast prove the porous structure again. The average diameter (295 nm) of the circles reflects the average size of the PS spheres. This is in accord with the SEM data. Fig. 3(b) gives a typical high-magnified TEM image of a circle. A lot of small dark dots were found to distribute on the surface of the hollow spheres. The selected area electron diffraction (SAED)
(see Fig. 3(c)) detection demonstrates two obvious rings corresponding to {0 1 1 0} (0.216 nm) and {1 2 1 0} (0.126 nm) crystal planes for graphene, respectively [21]. Other small blurry discrete spots could originate from various crystal planes of MnO2 nanocrystals. Fig. 3(d) shows a typical high-resolution TEM image of the selected area in Fig. 3(b), clear well-defined lattice fringes were observed from the MnO2 nanocrystals. The measured d-spacing values of 0.242 and 0.201 nm well match those of (3 1 1) and (4 0 0) planes reported for single crystal a-MnO2 [22,23]. The crystalline structure of MnO2 in the composite was verified by thin film XRD measurement. As shown in Fig. 4(a), the XRD patterns unambiguously demonstrate three main peaks at 2h = 37.057°, 45.091°, 65.649°, corresponding to the (3 3 1), (4 0 0) and (4 4 0) planes of a-MnO2 (JCPDS Card No. 42-1169, space group: Fd3m(2 2 7)), respectively. It indicates that the a-MnO2 nanocrystals have been formed on surfaces of the carbon shells [22,24,25]. Furthermore, appearance of a broad diffraction peak at 22° implies that the graphene sheets are poorly ordered along the stacking direction [8,24]. To further investigate the chemical composition and chemical states of various elements in the porous MnO2/graphene film, XPS analysis was performed. The XPS signals of elements Mn, O, C, N and K could be seen in Fig. S1. Due to the low weight percentage of N in the composite film, the signal of N1s was very weak (see Fig. S2). The C1s signals (see Fig. S3) mainly resulted from CAC and C@C, indicating that GO were reduced to graphene after calcination and hydrothermal process [26]. The Mn2p spectrum in Fig. 4(b) exhibits multiple splitting. Two strong peaks centered at about 654.13 and 642.58 eV with an binding energy separation of 11.7 eV, can be assigned to Mn2p1/2 and Mn2p3/2 of Mn4+ in MnO2, respectively [12,27,28]. In addition, the much weaker peaks corresponding to Mn3+ and Mn5+ species can also be observed in the spectrum, suggesting that the predominant oxidation of element Mn is +4. The electrochemical performance of the free-standing composite films with large accessible surface areas and pore structures was evaluated by a two-electrode system with 1 M NaSO4 as electrolyte. Without any binder or conductive additive, two
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Fig. 3. (a, b) Typical TEM images of 3D porous MnO2/rGO under different magnifications; (c) the corresponding SAED pattern of the area in (b); (d) HRTEM image of the selected area in (b), the insert at the lower right corner is the typical HRTEM image of a single MnO2 nanocrystal.
Fig. 4. (a) XRD pattern of the 3D porous MnO2/rGO, (b) XPS spectra of the Mn2p region (‘‘Raw’’ represents the raw data, and ‘‘Sum’’ is the total fitted curve).
symmetrical composite film electrodes were tested in Swageloktype cells at room temperature. Fig. 5(a) illustrates the CV curves of both the porous graphene and porous MnO2/graphene at a scan rate of 20 mV s1 in the potential range from 0 to 0.8 V. The two curves exhibit approximately rectangle shape, indicating the efficient intralayer charge transfer and good capacitive behavior. It is worth noting that the CV curve of the porous MnO2/graphene becomes much wider than that of the porous graphene, suggesting capacitive increase with formation of the MnO2 nanocrystals on surface of the porous graphene. Fig. 5(b) demonstrates the comparison of galvanostatic charge–discharge at the same current density of 0.2 A g1. The charge–discharge time for the porous MnO2/ graphene film, as expected, is much longer than that for the porous
graphene. According to Eq. (2), the specific capacitance for the porous MnO2/graphene electrode material was calculated to be 266.3 F g1 at a current density of 0.2 A g1, which is almost 2.5 times of the pure porous graphene film (105.48 F g1). It is also higher than those reported on MnO2/graphene hybrid materials including freestanding three-dimensional graphene/MnO2 composite (130 F g1 at 2 mV s1) [29], graphene-wrapped honeycomb MnO2 nanospheres (210 F g1 at 0.5 A g1) [30], needle-like graphene-MnO2 composites (216 F g1 at 0.15 A g1) [31], and graphene/ MnO2 composite papers (256 F g1 at 0.5 A g1) [32]. The excellent specific capacitive ability of the porous MnO2/graphene hybrid device probably originated from the synergic effect of both components of porous graphene and MnO2 nanocrystals.
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Fig. 5. (a) CV curve comparisons of the porous graphene and the porous MnO2/rGO at a scan rate of 20 mV s1, (b) comparison between the charge–discharge curves of the porous graphene the porous MnO2/rGO at a current density of 0.2 A g1.
The typical CV curves for the MnO2/graphene hybrid electrodes at different scan rates from 1 to 300 mV s1 exhibit nearly rectangular and symmetric shapes, as shown in Fig. 6(a). For clarity, the insert CV curve at the lower left corner of Fig. 6(a) demonstrates an almost ideal rectangle at the potential scan rate of 5 mV s1.
Galvanostatic charge–discharge curves of the supercapacitor device at various current densities from 0.2 to 40 A g1 are further illustrated in Fig. 6(b). All the charge/discharge curves show liner sloops and approximately symmetric triangular shapes, suggesting good electrochemical reversibility and capacitive behavior again.
Fig. 6. Typical CV curves of the free-standing porous MnO2/rGO films at different scan rates, (b) galvanostatic charge–discharge curves at various current densities, (c) the calculated specific capacitance as a function of various current densities, (d) specific capacitance as a function of cycle number at 1 A g1, the insert shows the charge– discharge curves for last ten cycles.
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Owing to the porous structure and N-doped carbon layer, it allows for effective ion migration into active sites and stress relaxing, resulting in a high rate capacity of the electrodes. Variation of specific capacitance calculated from the charge–discharge curves as a function of current density for free-standing porous MnO2/graphene electrodes is shown in Fig. 6(c). The behavior is similar to that in the case of increasing scan rate (Fig. S4). The decrease of the specific capacitance is mainly attributed to limited accessible area for ion diffusion with the increasing current density. Maximum value of specific capacitance, calculated using Eq. (2), is about 266.3 F g1 at a current density of 0.2 A g1. Nearly 46% capacity retention could be observed at the current density of 40 A g1, indicating high-rate capability of the free-standing film electrodes. The 3D porous structure having a high specific surface area can effectively improve the double layer capacitance of the hybrids while the pseudo-capacitance of MnO2 in the composite film is also enhanced by the highly conductive 3D network formed by the rGO component. A higher current density limits the accessibility of ions inside every pore of the composite films, the specific capacitance of the nanostructure electrodes decreases with the increase of the current densities. To further confirm the ion-transport kinetics and capacitive behavior of the MnO2/graphene composite film, Electrochemical Impedance Spectroscopy test was performed at a frequency range from 105 to 102 Hz. The Nyquist plot has a small semicircle located at high frequency and a nearly vertical line at low frequency as shown in Fig. S5. According to the equivalent circuit [33,34], the charge-transfer resistance of the composite was calculated to be ca. 2 X, indicating fast charge–discharge characteristic. The porous structure of the 3D MnO2/graphene plays a critical role. In addition, the plot at the high frequency that is nearly parallel to the imaginary axis suggests the fast ion diffusion and ideal capacitive behavior of the devices. The result clearly illustrates that the electrolyte ions move easily into 3D network and charge transforms fast. However, the real axis intercept is approximately 15, which could present the equivalent series resistances of the both electrodes. The resistance between the crocodile clips and the device contributes much. Therefore, this threedimensional porous structure based on the interconnected rGO sheets, which provide continuous electron pathways, allows efficient ion diffusion and fast charge transfer. A long cycle life is another important parameter for evaluating the practical application of supercapacitors. Excellent cycling stability is crucial for real supercapacitors. The long-term cycle stability of the supercapacitor based on the free-standing porous MnO2/ graphene film was tested by repeating the galvanostatic charge– discharge at a current density of 1 A g1 for 2000 cycles, as shown in Fig. 6(d). It is evident from the figure that the capacitance retention is above 100% at the very beginning and kept constant in the first 250 cycles. This interesting phenomenon is indicative of the fact that there is a gradual activation process of the supercapacitor electrodes [35,36]. The decreases of the specific capacitance would be attributed to the mechanical expansion of MnO2 nanoparticles due to the continuous ion insertion/de-insertion process. After 2000 cycles, 85.1% of initial specific capacitance can be retained. Meanwhile, the charge–discharge curves of the last ten cycles still maintain almost symmetric. This shows high electrochemical stability of the porous MnO2/grapheme electrodes during the longterm cycling test.
4. Conclusions In summary, we have demonstrated a rational design and fabrication of three-dimensional porous binder-free MnO2/graphene films as supercapacitor electrodes. The porous 3D-rGO films were constructed by combining a simple hard template-directed
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ordered assembly approach and hydrothermal process. The porous structure was well obtained and the average size of the pores was in agreement with that of the template PS spheres. The hollow structures of the film by utilizing PS as templates largely enhanced the specific surface areas, which can provide more electroactive regions and short diffusion lengths. The coating layer of PDA can not only interact well with the wrapped graphene that prevents collapse of the porous structure during the charge/discharge process but also serve as a reductant for formation of MnO2 nanocrystal. Thus-prepared free-standing films, which were directly used as supercapacitor electrodes without any additives or binders, exhibited a high specific capacitance of 266.3 F g1 at the density of 0.2 A g1 and an excellent cycling performance with capacitance retention of 85.1% after 2000 charge–discharge cycles at a current density of 1 A g1. Nearly 46% capacity retention at the current density of 40 A g1 demonstrates good rate capability of the freestanding film electrodes. All of these features contribute to the synergic effect of the two components of 3D-rGO and MnO2 nanocrystals and the porous structure. Our work provides a rational and green synthetic route to obtain binder-free electrodes with porous graphene hybrids, which shows promising applications in future high-performance, low-cost electrode materials. Acknowledgment This work was financially supported by National Basic Research Program of China (No. 2012CB933402). 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.2014.08.060. References [1] Y.G. Patrice Simon, Nat. Mater. 7 (2008) 10. [2] M. Inagaki, H. Konno, O. Tanaike, J. Power Sources 195 (2010) 7880–7903. [3] A.L. Mohana Reddy, S.R. Gowda, M.M. Shaijumon, P.M. Ajayan, Adv. Mater. 24 (2012) 5045–5064. [4] C. Zhou, Y. Zhang, Y. Li, J. Liu, Nano Lett. 13 (2013) 2078–2085. [5] 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) 10451–10453. [6] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [7] W. Lv, Z. Xia, S. Wu, Y. Tao, F.-M. Jin, B. Li, H. Du, Z.-P. Zhu, Q.-H. Yang, F. Kang, J. Mater. Chem. 21 (2011) 3359. [8] L. Peng, X. Peng, B. Liu, C. Wu, Y. Xie, G. Yu, Nano Lett. 13 (2013) 2151–2157. [9] X. Yang, J. Zhu, L. Qiu, D. Li, Adv. Mater. 23 (2011) 2833–2838. [10] W.C. Yongmin He, Xiaodong Li, Zhenxing Zhang, Jiecai Fu, Changhui Zhao, Erqing Xie, ACS Nano 7 (2012) 174–182. [11] W. Chen, Y. He, X. Li, J. Zhou, Z. Zhang, C. Zhao, C. Gong, S. Li, X. Pan, E. Xie, Nanoscale 5 (2013) 11733–11741. [12] T.B. Mathieu, Chem. Mater. 16 (2004) 3184–3190. [13] X. Li, G. Wang, X. Wang, X. Li, J. Ji, J. Mater. Chem. A 1 (2013) 10103. [14] L. Bao, J. Zang, X. Li, Nano Lett. 11 (2011) 1215–1220. [15] J. Zhang, Z. Chen, Z. Wang, W. Zhang, N. Ming, Mater. Lett. 57 (2003) 4466– 4470. [16] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Science 318 (2007) 426– 430. [17] L. Zhang, J. Wu, Y. Wang, Y. Long, N. Zhao, J. Xu, J. Am. Chem. Soc. 134 (2012) 9879–9881. [18] K. Ai, Y. Liu, C. Ruan, L. Lu, G.M. Lu, Adv. Mater. 25 (2013) 998–1003. [19] J. William, S. Hummers, Richard E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [20] X.C. Chen, W. Wei, W. Lv, F.Y. Su, Y.B. He, B. Li, F. Kang, Q.H. Yang, Chem. Commun. (Camb.) 48 (2012) 5904–5906. [21] L.Z. Xuan, Nano Lett. 8 (2008) 323–327. [22] W.L. Jipeng Ni, Liangmiao Zhang, Baohua Yue, Xingfu Shang, Yong Lv, J. Phys. Chem. C 113 (2009) 54–60. [23] L. Wang, L. Chen, Y. Li, H. Ji, G. Yang, Powder Technol. 235 (2013) 76–81. [24] P. Lian, X. Zhu, S. Liang, Z. Li, W. Yang, H. Wang, Electrochim. Acta 55 (2010) 3909–3914. [25] L. Xing, C. Cui, C. Ma, X. Xue, Mater. Lett. 65 (2011) 2104–2106. [26] Y. Meng, K. Wang, Y. Zhang, Z. Wei, Adv. Mater. 25 (2013) 6985–6990. [27] H.-W.L. Sung-Wook Kim, Pandurangan Muralidharan, Dong-Hwa Seo, WonSub Yoon, Do Kyung Kim, Kisuk Kang, Nano Res. 4 (2011) 505–510. [28] S. Li, L. Qi, L. Lu, H. Wang, RSC Adv. 2 (2012) 3298.
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