Nanoscale View Article Online
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
PAPER
View Journal | View Issue
Cite this: Nanoscale, 2014, 6, 4297
Scalable synthesis of silicon nanosheets from sand as an anode for Li-ion batteries† Won-Sik Kim, Yoon Hwa, Jung-Hoo Shin, Myung Yang, Hun-Joon Sohn and Seong-Hyeon Hong* The silicon nanostructure is a promising candidate for an anode of Li-ion batteries due to its high theoretical capacity. In this work, we have demonstrated the scalable synthesis of Si nanosheets from natural sand by magnesiothermic reduction, and suggested a new formation mechanism for Si nanosheets. In the suggested mechanism, an Mg2Si intermediate phase was formed at an early stage of the reduction process, which leads to the two-dimensional Si nanostructure. The synthesized Si nanosheets have a leaf-like sheet morphology ranging from several ten to several hundred nanometers, and show
Received 8th October 2013 Accepted 26th January 2014
comparable electrochemical properties to the commercial Si nanopowder as an anode for lithium ion
DOI: 10.1039/c3nr05354g
batteries. For the improved electrochemical performance, Si nanosheets are encapsulated with reduced graphene oxide (RGO), and the RGO-encapsulated Si nanosheet electrode exhibits high-reversible
www.rsc.org/nanoscale
capacity and excellent rate capability.
Introduction The demand for high-energy, high-power density, and high ratecapability lithium ion batteries (LIBs) has increased for use in hybrid electric vehicles (HEVs), back-up electricity storage units, and light-weight and portable electric devices.1 Carbon-based materials have been the most common anode materials in commercial LIBs, but their specic capacity is relatively low (372 mA h g1) with a poor rate capability.2–4 Silicon (Si) is a promising alternative, because it has the highest theoretical capacity (Li15Si4, 3579 mA h g1 at room temperature) and low operating voltage (0.1 V vs. Li/Li+).5–8 However, silicon-based electrodes undergo a large volume change during cycling, which causes pulverization and loss of electrical connectivity leading to poor cycle performance. To overcome this problem, various Si nanostructures have been fabricated including nanoparticles, nanowires, nanosheets, nanohollows, and nano-core–shells.9–13 These Si nanostructures show relatively excellent electrochemical properties, but the fabrication of nanostructured Si commonly requires complicated routes with toxic precursors or high-cost techniques such as CVD (chemical vapor deposition).9,10,14–16 The magnesiothermic reduction of SiO2 is an attractive and cost-effective method to obtain nanostructured Si.11,12,17–20 It requires a lower processing temperature and a shorter reaction time, compared to the other reduction processes.21–23 The magnesiothermic reduction of SiO2 generally Department of Materials Science and Engineering and Research Institute of Advanced Materials, Seoul National University, Seoul 151-744, Republic of Korea. E-mail: [email protected] † Electronic supplementary 10.1039/c3nr05354g
information
(ESI)
This journal is © The Royal Society of Chemistry 2014
available.
See
DOI:
yields 3-dimensional macroporous or mesoporous Si. Recently, free-standing ultrathin Si nanosheets were prepared by magnesiothermic reduction, but graphene oxide (GO) was used as a sacricial template, and the synthesis was a rather complicated and high-cost process.11 In this study, Si nanosheets are successfully prepared by magnesiothermic reduction using commercial sand as a precursor and a template. This is the rst attempt and low-cost scalable process for the preparation of Si nanostructures (2-dimensional silicon nanostructures) without any specic template. A new formation mechanism of Si nanosheets during magnesiothermic reduction is also proposed. In addition, Si nanosheets are encapsulated with reduced graphene oxide (RGO) to enhance the electrochemical properties. The RGO encapsulation is expected to increase the electrical conductivity and prevent the pulverization of Si nanosheets resulting from the large volume expansion during lithium insertion/extraction.
Results and discussion The synthesis procedure of RGO-encapsulated Si nanosheets is schematically presented in Fig. 1. Commercially available sand was crushed by planetary ball milling, and then mixed with magnesium (Mg) powder as a reducing agent. The magnesiothermic reduction was carried out at 700 C, which yielded the multi-layered nanosheets. The recovery of Si nanosheets was achieved by two-stage leaching using HCl and HF solutions. The magnesium-containing compounds (MgO and Mg2Si) were removed by HCl, and the unreacted SiO2 was etched out by HF. Finally, RGO was uniformly coated onto Si nanosheets by the APTES modied process.24
Nanoscale, 2014, 6, 4297–4302 | 4297
View Article Online
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
Nanoscale
Fig. 1 Schematic illustration of the synthesis of RGO-encapsulated Si nanosheets.
The commercial sand comprises irregularly shaped particles with a rough surface, and the particle size is 200 mm (see the ESI, Fig. S1a†). The particle fracture occurred during the planetary ball milling, and the particle size was signicantly reduced. The milled powder exhibits a fractured morphology with a wide size distribution (Fig. S1b†). Flat cleavage-like surfaces are discernible in the micron-sized particles. Without the ball milling process, similar Si nanosheets were obtained, but the amount of obtained Si nanosheets was small. Thus, we have employed the ball milling to crush the sand and produce a more at surface, which increases the production of Si nanosheets. Fig. 2a shows a representative scanning electron microscopy (SEM) image of as-synthesized Si. The obtained Si has a leaf-like sheet morphology, and the size ranges from several ten to several hundred nanometers. The silicon sheets appear to be exible and very thin although the exact thickness cannot be estimated from the SEM image. The specic surface area determined from the nitrogen absorption–desorption isotherm is 55.8 m2 g1, which is larger than that of commercial Si nanopowder (33.7 m2 g1) (Fig. S2†). The low-magnication transmission electron microscopy (TEM) image further
(a) FESEM, (b) low-magnification TEM, (c) selected area electron diffraction (SAED) pattern, (d) high-magnification TEM, and (e) highresolution TEM images of as-synthesized Si nanosheets.
Paper
conrms the 2-dimensional sheet morphology (Fig. 2b). The spots and rings in the selected area diffraction (SAED) pattern are indexed to be Si (ICDD #27–1402). Consequently, well-crystallized Si sheets are synthesized from the commercial sand using a facile magnesiothermic reduction reaction. The highmagnication TEM image indicates that the Si sheets are composed of several nanosheets (Fig. 2d). The lattice fringes are clearly seen in the high-resolution TEM image of the individual nanosheet revealing that the Si nanosheet is a single crystal (Fig. 2e). The determined interplanar spacing of 0.314 nm corresponds well with the (111) plane of the Si diamond structure. The magnesiothermic reduction of silica (SiO2) generally yields 3-dimensional macroporous or mesoporous silicon.17–20 Thus, the Si nanosheets obtained in this study appear to be formed with a different mechanisim or route from that of the conventional magnesiothermic reduction. The phase developments during magnesiothermic reduction and acid leaching were investigated by X-ray diffraction (XRD), and the patterns for milled sand, as-reduced, HCl-etched, and HF-etched specimens are shown in Fig. 3. The milled sand is a crystalline quartz and no phase change is observed aer planetary ball milling (Fig. 3a). The XRD pattern reveals that the asreduced specimen is composed of Si, MgO, and unreacted SiO2 (Fig. 3b). A stoichiometric weight ratio of SiO2 : Mg ¼ 1 : 0.8 (or 1 : 2 molar ratio) based on the following reaction SiO2 + 2Mg ¼ Si + 2MgO is used, but unreacted SiO2 is still present. The volatilization of gaseous Mg and/or the formation of intermediate phases (magnesium silicide, Mg2Si) have been attributed to the presence of unreacted SiO2 in the magnesiothermic reduction reaction.25 The peak broadening is observed in the diffraction patterns for Si and MgO, which can be attributed to the formation of nano-crystallites. MgO is completely dissolved in HCl solution (Fig. 3c), and unreacted SiO2 is removed by HF solution, which results in the crystalline Si (Fig. 3d). To nd out the formation mechanism of Si nanosheets during magnesiothermic reduction, the mixture of milled sand and magnesium was heat-treated at different temperatures, and the corresponding XRD patterns are presented in Fig. 4a. At
Fig. 2
4298 | Nanoscale, 2014, 6, 4297–4302
XRD patterns of (a) ball-milled, (b) as-reduced, (c) HCl-etched, and (d) HF-etched samples.
Fig. 3
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
Paper
Nanoscale
Fig. 4 (a) Ex situ XRD patterns of as-reduced sand at different temperatures. (b–d) TEM images of HCl-etched products (reduced at 700 C for 4 h). (e) Schematic illustration of the formation of Si nanosheets.
500 C, the as-reduced specimen was composed of Mg2Si, MgO, and unreacted SiO2. Surprisingly, Mg2Si was found, and Si was not detected. The peak for Si appeared at 550 C. Upon increasing the reduction temperature, the peak intensity for Si and MgO increased while the intensity for Mg2Si and SiO2 decreased. At 650 C, the Mg2Si phase completely disappeared, and no further phase change occurred at higher temperatures. The obtained results indicate that Mg2Si is formed as an intermediate phase at an early stage of the reduction process, and Si is produced by consuming Mg2Si and SiO2. Generally, the magnesiothermic reduction of SiO2 is expressed as follows: SiO2 + 2Mg / Si + 2MgO
(1)
For the complete reduction of SiO2, an Mg/SiO2 molar ratio of 2.0 is required. Recently, it was reported that Mg2Si is formed as an intermediate phase even below the stoichiometric ratio (1.5),25 and Mg2Si is further reduced into Si through the following reaction: Mg2Si + SiO2 / 2Si + 2MgO
(2)
Based on the previous report and the results obtained in this study, the following overall reactions are proposed for the magnesiothermic reduction of SiO2: 4Mg + SiO2 / Mg2Si + 2MgO
(3)
Mg2Si + SiO2 / 2Si + 2MgO
(4)
These two reactions are thermodynamically possible (Fig. S3†) and reaction (3) has more negative Gibbs free energy at the reduction temperatures. Thus, it is suggested that the magnesiothermic reduction of SiO2 can occur via an intermediate Mg2Si phase and the formation of Mg2Si might result in the nanosheet morphology of Si. The HCl-etched specimen
This journal is © The Royal Society of Chemistry 2014
(reduced at 700 C) was examined by TEM to reveal the microstructure of the reduction products (Fig. 4b). The low-magnication TEM image showed that HCl-etched powder had either a hollow or yolk/shell-like structure. The high-magnication image indicated that the yolk/shell structure consisted of three parts: a dense shell, a porous middle layer, and a dense core (Fig. 4c). The core was the unreacted SiO2, implying that the yolk/shell-like powder was the on-going reduction product. The outer shell was identied as crystalline Si with planar defects, and the middle layer was composed of 5 nm Si nanoparticles (Fig. 4d). The distinctive microstructure of Si suggests that two layers were formed in the different routes. Assuming the transient presence of Mg2Si during the magnesiothermic reduction, the following formation mechanism of Si nanosheets is proposed (Fig. 4e). Initially, Mg2Si is formed between Mg and SiO2 through reaction (3). The excess Si in Mg2Si is precipitated as a Si layer, while Mg2Si reduces SiO2 into Si through reaction (2) forming the Si/MgO nanocomposite. The unreated Mg and MgO (or Mg2Si) are dissolved with HCl, which results in the dense-Si/porous-Si/unreacted SiO2 core–shell structure. Finally, Si with a nanosheet morphology is obtained by removing the unreacted SiO2 and nanoparticle Si with HF solution. The precipitation of Si from Al–Mg–Si alloys with an excess of Si has been reported previously.26,27 The reduction of SiO2 by Mg2Si and the synthesis of Si with the nanosheet morphology are under investigation. For further evidence, as-reduced specimens at 550 and 600 C were examined by TEM. The crosssectional TEM specimens were prepared by ultra-microtome. The presence of Mg2Si, MgO, and Si was conrmed by SAED patterns, but the individual layers could not be distinguished (see the TEM images of microtomed samples, Fig. S4†). To enhance the electrochemical properties, Si nanosheets are encapsulated with RGO by the APTES modied process. The SEM micrograph shows that RGO-encapsulated Si maintains the sheet-like morphology (Fig. 5a). The individual nanosheets are well separated with increased thickness, indicating that the
Nanoscale, 2014, 6, 4297–4302 | 4299
View Article Online
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
Nanoscale
Paper
(a) FESEM, (b) low-magnification TEM, (c) selected area electron diffraction (SAED) pattern, (d) high-magnification TEM, and (e) highresolution TEM images of RGO-encapsulated Si nanosheets.
Fig. 5
Si nanosheets are thoroughly covered with RGO. The highmagnication TEM image further conrms the sheet morphology (Fig. 5b). The SAED pattern corresponds to that of Si (Fig. 5c), but the high-resolution TEM image clearly shows that the crystalline Si was well coated with the layered RGO of 5–10 nm thickness (Fig. 5d). In addition, the Raman spectrum of RGO-encapsulated Si nanosheets displays two broad peaks (D band at 1367 cm1 and G band at 1582 cm1) revealing the presence of RGO in the sheet-like morphology (see the ESI, Fig. S5†). The discharge/charge voltage proles of the Si nanosheet and RGO-encapsulated Si nanosheet electrodes are shown in Fig. 6a. The cycling experiments are conducted in the voltage range between 0.0 and 2.0 V (vs. Li/Li+) at a current density of 200 mA g1. The shape of the voltage proles is similar to that of a typical Si electrode.12,19,20,28 The initial discharge and charge capacities of the Si nanosheets are 3563 and 2431 mA h g1, respectively. The RGO-encapsulated Si nanosheets shows slightly higher capacities (3769 and 2625 mA h g1, respectively). The coulombic efficiency of the rst cycle was 70% for both electrodes. The observed values for RGO-encapsulated Si nanosheets are higher than those for the previously reported nano-sized Si/RGO composite.29 The lower capacities of the nano-sized Si/RGO composite can be attributed to the existence of SiOx due to a high reaction activity of 5 nm Si. As a reference, the cyclic performance of commercial Si nanopowder was performed (Fig. 6b) and the obtained results are similar to the previous studies.30 The fabricated Si nanosheet electrode exhibits the comparable reversible capacity to the commercial Si nanopowder (Fig. 6b), even though it was synthesized from sand. As expected, the RGO-encapsulated Si nanosheets showed the improved cycle retention. The cycle performance of the RGO-encapsulated Si nanosheet electrode at a high rate (500 mA g1) exhibited the similar tendency with slightly lower capacity (see the ESI, Fig. S6†). The average capacity of ve cells was 1762 mA h g1 aer 25 cycles and the standard deviation was 87 mA h g1. Thus, the electrochemical data were quite consistent and reproducible. Compared to the previously
4300 | Nanoscale, 2014, 6, 4297–4302
Fig. 6 (a) Voltage profiles of Si nanosheet and RGO-encapsulated Si nanosheet electrodes after 1, 2, 10, 30, and 50 cycles at 200 mA g1 between 0.0 and 2.0 V, (b) cyclability of commercial Si nanopowder, Si nanosheet, and RGO-encapsulated Si nanosheet electrodes, and (c) rate capability of RGO-encapsulated Si nanosheet electrode between 0.01 and 2.0 V.
reported graphene/nano-sized Si composites,31 the RGO encapsulation effect was more signicant, and the improved cycle retention in this study can be attributed to the better encapsulation of Si with RGO due to the nanosheet morphology. However, the capacity degradation was still observed in the RGO-encapsulated Si nanosheet electrode and further surface treatment is required to improve the cycle performance. The rate capability of the RGO-encapsulated Si nanosheet electrode is promising and it exhibited a capacity of 1113 mA h g1 even at a high current density of 3 A g1 (Fig. 6c).
Experimental procedures Synthesis of silicon nanosheets 1 g of commercial sand (Ficher Scientic) was crushed by planetary ball milling for 4 h. The crushed sand was washed with D.I. water, dried, and calcined at 600 C for 1 h to remove the organic impurities. Then, 0.4 g of calcined sand and 0.32 g
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
of magnesium (325 mesh, Stream) were hand-mixed. The mixture was heat-treated at 700 C for 4 h under a 5 vol.% H2/N2 atmosphere. The obtained brown products were washed with hydrochloric acid and hydrouoric acid solution (10 wt%).
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
Synthesis of graphene oxide (GO) GO was synthesized through Hummers' method by reacting commercially available graphite akes (Sigma Aldrich) in a mixture of H2SO4, NaNO3, and KMnO4.31 Aer completion of the reaction, H2O2 was added to the reaction vessel. The brown GO was obtained by ltering, washing with 1 M HCl and D.I. water twice, and drying. RGO encapsulation 0.1 g of prepared silicon nanosheets was dispersed in 100 mL of absolute ethanol, and 2 mL of APTES was added.24 Aer stirring for 4 h, the solution was washed and centrifuged. Then, 0.15 g of synthesized GO was dispersed in 80 mL of water by tipsonication, and 20 mL of silicon nanosheet ethanol solution was added to the prepared GO solution. Aer stirring for 4 h, the solution was washed, centrifuged, and dried. The obtained composite powder was reduced at 400 C for 6 h under a 5 vol.% H2/N2 atmosphere. Characterization The morphology of silicon nanosheets and RGO-encapsulated silicon nanosheets were examined using a eld emission scanning electron microscope (FESEM, JSM-6330F, JEOL) and a 300 kV transmission electron microscope (TEM, JEM-3000F, JEOL). The crystal structure and phase evolution were investigated using X-ray diffraction (XRD, D8-Advance, BRUKER) with Cu-Ka Radiation. The RGO-encapsulated silicon sheets were further characterized using a high-resolution Raman microscope (LabRAM HR UV/Vis/NIR, Horiba Scientic) with Raman shi from 100 to 4000 cm1. Electrochemical test The test electrodes consisted of an active powder material (70 wt.%), carbon black (Ketchen Black, 10 wt.%) as a conducting agent, and polyamide imide (PAI, 20 wt.%) dissolved in N-methyl pyrrolidinone (NMP) as a binder. The weight of the electrode was about 2.26–2.59 mg cm2. Each component was well mixed to form a slurry using a magnetic stirrer. The slurry was coated on a copper foil substrate, pressed, and dried at 200 C for 4 h under a vacuum. A coin-type electrochemical cell was used with lithium foil as the counter and reference electrodes and 1 M LiPF6 and 10% uoroethylene carbonate (FEC) in ethylene carbonate (EC)/diethylene carbonate (DEC) (3 : 7 (v/v), PANAX) as the electrolyte. The cell assembly and all of the electrochemical tests were carried out in an Ar-lled glove box. The cycling experiments were galvanostatically performed using a Maccor automater tester at a constant current density of 200 mA g1 (lithiation ¼ 200, 500, 1000, 2000, 3000, and 200 mA g1, delithiation ¼ 200 mA g1 in the rate capability test) for the This journal is © The Royal Society of Chemistry 2014
Nanoscale
active material within the voltage range between 0.0 V and 2.0 V (vs. Li/Li+).
Conclusions In summary, we facilely synthesized the Si nanosheets from natural sand by magnesiothermic reduction. The thin Si nanosheets were precipitated from an intermediate Mg2Si phase to make the composition balance of Mg2Si. The synthesized Si nanosheets were well crystallized and showed comparable electrochemical properties to commercial Si nanopowder as an anode for lithium rechargeable batteries, and further enhancement was achieved by RGO encapsulation. The RGOencapsulated Si nanosheets showed excellent rate capability. In this work, we have demonstrated the facile synthesis of an Si nanostructure from natural sand with improved electrochemical properties, and suggested a new formation mechanism for Si nanosheets.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (NRF-2012R1A2A4A01008226).
Notes and references 1 Y. Nishi, J. Power Sources, 2001, 100, 101. 2 J. R. Dahn, T. Zheng, Y. Liu and J. S. Xue, Science, 1995, 270, 590. 3 M. Winter, J. O. Besenhard, M. E. Spahr and P. Novak, Adv. Mater., 1998, 10, 725. 4 N. A. Kaskhedikar and J. Maier, Adv. Mater., 2009, 21, 2664. 5 M. N. Obrovac and L. Christensen, Electrochem. Solid-State Lett., 2004, 7, A93. 6 J. Li and J. R. Dahn, J. Electrochem. Soc., 2007, 154, A156. 7 C.-M. Park, J.-H. Kim, H. Kim and H.-J. Sohn, Chem. Soc. Rev., 2010, 39, 3115. 8 W.-S. Chang, C.-M. Park, J.-H. Kim, Y.-U. Kim, G. Jeong and H.-J. Sohn, Energy Environ. Sci., 2012, 5, 6895. 9 H. Kim, M. Seo, M.-H. Park and J. Cho, Angew. Chem., Int. Ed., 2010, 49, 2146. 10 C. K. Chan, H. Peng, G. Liu, K. Wrath, X. F. Zhang, R. A. Huggins and Y. Cui, Nat. Nanotechnol., 2008, 3, 31. 11 Z. Lu, J. Zhu, D. Sim, W. Zhou, W. Shi, H. H. Hng and Q. Yan, Chem. Mater., 2011, 23, 5293. 12 D. Chen, X. Mei, G. Ji, M. Lu, J. Xie, J. Lu and J. Y. Lee, Angew. Chem., Int. Ed., 2012, 51, 2409. 13 Y. Hwa, W.-S. Kim, S.-H. Hong and H.-J. Son, Electrochim. Acta, 2012, 71, 201. 14 J. D. Holmes, K. P. Johnston, R. C. Doty and B. A. Korgel, Science, 2000, 287, 1471. 15 K. Peng, Y. Yan, S. Gao and J. Zhu, Adv. Mater., 2002, 14, 1164. 16 H. Okamoto, Y. Sugiyama and H. Nakano, Chem. – Eur. J., 2011, 17, 9864.
Nanoscale, 2014, 6, 4297–4302 | 4301
View Article Online
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
Nanoscale
17 Z. Bao, M. R. Weatherspoon, S. Shian, Y. Cai, P. D. Graham, S. M. Allan, G. Ahmad, M. B. Dickerson, B. C. Church, Z. Kang, H. W. Abernathy, C. J. Summers, M. Liu and K. H. Sandhage, Nature, 2007, 446, 172. 18 Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P. A. V. Aken and J. Maier, Adv. Mater., 2010, 22, 2247. 19 J.-K. Yoo, J. Kim, Y. S. Jung and K. Kang, Adv. Mater., 2012, 24, 5452. 20 H. Jia, P. Gao, J. Yang, J. Wang, Y. Nuli and Z. Yang, Adv. Energy Mater., 2011, 1, 1036. 21 E. H. Myrhaug, J. K. Tuset and H. Tveit, Electrical Energy Management in the Ferroalloy Business. Proceedings: Tenth International Ferroalloys Congress, 2004, vol. 1, p. 726. 22 M. Nagamori, I. Malinsky and A. Claveau, Metall. Trans. B, 1986, 17, 503. 23 T. Nohira, K. Yasuda and Y. Ito, Nat. Mater., 2003, 2, 397.
4302 | Nanoscale, 2014, 6, 4297–4302
Paper
24 S. Yang, X. Feng, S. Ivanovici and K. M¨ ullen, Angew. Chem., Int. Ed., 2010, 49, 8408. 25 M. Barati, S. Sarder, A. McLean and R. Roy, J. Non-Cryst. Solids, 2011, 357, 18. 26 D. G. Eskin, V. Massardier and P. Merle, J. Mater. Sci., 1999, 34, 811. 27 D. P. Yu, Y. J. Xing, Q. L. Hang, H. F. Yan, J. Xu, Z. H. Xi and S. Q. Feng, Physica E, 2001, 9, 305. 28 Y. Hwa, W.-S. Kim, B.-C. Yu, H. Kim, S.-H. Hong and H.-J. Sohn, J. Mater. Chem. A, 2013, 1, 3733. 29 S.-L. Chou, J.-Z. Wanga, M. Choucair, H.-K. Liu, J. A. Stride and S.-X. Dou, Electrochem. Commun., 2010, 12, 303. 30 H. Xiang, K. Zhang, G. Ji, J. Yang Lee, C. Zou, X. Chen and J. Wu, Carbon, 2011, 49, 1781. 31 W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.
This journal is © The Royal Society of Chemistry 2014
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
PAPER
View Journal | View Issue
Cite this: Nanoscale, 2014, 6, 4297
Scalable synthesis of silicon nanosheets from sand as an anode for Li-ion batteries† Won-Sik Kim, Yoon Hwa, Jung-Hoo Shin, Myung Yang, Hun-Joon Sohn and Seong-Hyeon Hong* The silicon nanostructure is a promising candidate for an anode of Li-ion batteries due to its high theoretical capacity. In this work, we have demonstrated the scalable synthesis of Si nanosheets from natural sand by magnesiothermic reduction, and suggested a new formation mechanism for Si nanosheets. In the suggested mechanism, an Mg2Si intermediate phase was formed at an early stage of the reduction process, which leads to the two-dimensional Si nanostructure. The synthesized Si nanosheets have a leaf-like sheet morphology ranging from several ten to several hundred nanometers, and show
Received 8th October 2013 Accepted 26th January 2014
comparable electrochemical properties to the commercial Si nanopowder as an anode for lithium ion
DOI: 10.1039/c3nr05354g
batteries. For the improved electrochemical performance, Si nanosheets are encapsulated with reduced graphene oxide (RGO), and the RGO-encapsulated Si nanosheet electrode exhibits high-reversible
www.rsc.org/nanoscale
capacity and excellent rate capability.
Introduction The demand for high-energy, high-power density, and high ratecapability lithium ion batteries (LIBs) has increased for use in hybrid electric vehicles (HEVs), back-up electricity storage units, and light-weight and portable electric devices.1 Carbon-based materials have been the most common anode materials in commercial LIBs, but their specic capacity is relatively low (372 mA h g1) with a poor rate capability.2–4 Silicon (Si) is a promising alternative, because it has the highest theoretical capacity (Li15Si4, 3579 mA h g1 at room temperature) and low operating voltage (0.1 V vs. Li/Li+).5–8 However, silicon-based electrodes undergo a large volume change during cycling, which causes pulverization and loss of electrical connectivity leading to poor cycle performance. To overcome this problem, various Si nanostructures have been fabricated including nanoparticles, nanowires, nanosheets, nanohollows, and nano-core–shells.9–13 These Si nanostructures show relatively excellent electrochemical properties, but the fabrication of nanostructured Si commonly requires complicated routes with toxic precursors or high-cost techniques such as CVD (chemical vapor deposition).9,10,14–16 The magnesiothermic reduction of SiO2 is an attractive and cost-effective method to obtain nanostructured Si.11,12,17–20 It requires a lower processing temperature and a shorter reaction time, compared to the other reduction processes.21–23 The magnesiothermic reduction of SiO2 generally Department of Materials Science and Engineering and Research Institute of Advanced Materials, Seoul National University, Seoul 151-744, Republic of Korea. E-mail: [email protected] † Electronic supplementary 10.1039/c3nr05354g
information
(ESI)
This journal is © The Royal Society of Chemistry 2014
available.
See
DOI:
yields 3-dimensional macroporous or mesoporous Si. Recently, free-standing ultrathin Si nanosheets were prepared by magnesiothermic reduction, but graphene oxide (GO) was used as a sacricial template, and the synthesis was a rather complicated and high-cost process.11 In this study, Si nanosheets are successfully prepared by magnesiothermic reduction using commercial sand as a precursor and a template. This is the rst attempt and low-cost scalable process for the preparation of Si nanostructures (2-dimensional silicon nanostructures) without any specic template. A new formation mechanism of Si nanosheets during magnesiothermic reduction is also proposed. In addition, Si nanosheets are encapsulated with reduced graphene oxide (RGO) to enhance the electrochemical properties. The RGO encapsulation is expected to increase the electrical conductivity and prevent the pulverization of Si nanosheets resulting from the large volume expansion during lithium insertion/extraction.
Results and discussion The synthesis procedure of RGO-encapsulated Si nanosheets is schematically presented in Fig. 1. Commercially available sand was crushed by planetary ball milling, and then mixed with magnesium (Mg) powder as a reducing agent. The magnesiothermic reduction was carried out at 700 C, which yielded the multi-layered nanosheets. The recovery of Si nanosheets was achieved by two-stage leaching using HCl and HF solutions. The magnesium-containing compounds (MgO and Mg2Si) were removed by HCl, and the unreacted SiO2 was etched out by HF. Finally, RGO was uniformly coated onto Si nanosheets by the APTES modied process.24
Nanoscale, 2014, 6, 4297–4302 | 4297
View Article Online
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
Nanoscale
Fig. 1 Schematic illustration of the synthesis of RGO-encapsulated Si nanosheets.
The commercial sand comprises irregularly shaped particles with a rough surface, and the particle size is 200 mm (see the ESI, Fig. S1a†). The particle fracture occurred during the planetary ball milling, and the particle size was signicantly reduced. The milled powder exhibits a fractured morphology with a wide size distribution (Fig. S1b†). Flat cleavage-like surfaces are discernible in the micron-sized particles. Without the ball milling process, similar Si nanosheets were obtained, but the amount of obtained Si nanosheets was small. Thus, we have employed the ball milling to crush the sand and produce a more at surface, which increases the production of Si nanosheets. Fig. 2a shows a representative scanning electron microscopy (SEM) image of as-synthesized Si. The obtained Si has a leaf-like sheet morphology, and the size ranges from several ten to several hundred nanometers. The silicon sheets appear to be exible and very thin although the exact thickness cannot be estimated from the SEM image. The specic surface area determined from the nitrogen absorption–desorption isotherm is 55.8 m2 g1, which is larger than that of commercial Si nanopowder (33.7 m2 g1) (Fig. S2†). The low-magnication transmission electron microscopy (TEM) image further
(a) FESEM, (b) low-magnification TEM, (c) selected area electron diffraction (SAED) pattern, (d) high-magnification TEM, and (e) highresolution TEM images of as-synthesized Si nanosheets.
Paper
conrms the 2-dimensional sheet morphology (Fig. 2b). The spots and rings in the selected area diffraction (SAED) pattern are indexed to be Si (ICDD #27–1402). Consequently, well-crystallized Si sheets are synthesized from the commercial sand using a facile magnesiothermic reduction reaction. The highmagnication TEM image indicates that the Si sheets are composed of several nanosheets (Fig. 2d). The lattice fringes are clearly seen in the high-resolution TEM image of the individual nanosheet revealing that the Si nanosheet is a single crystal (Fig. 2e). The determined interplanar spacing of 0.314 nm corresponds well with the (111) plane of the Si diamond structure. The magnesiothermic reduction of silica (SiO2) generally yields 3-dimensional macroporous or mesoporous silicon.17–20 Thus, the Si nanosheets obtained in this study appear to be formed with a different mechanisim or route from that of the conventional magnesiothermic reduction. The phase developments during magnesiothermic reduction and acid leaching were investigated by X-ray diffraction (XRD), and the patterns for milled sand, as-reduced, HCl-etched, and HF-etched specimens are shown in Fig. 3. The milled sand is a crystalline quartz and no phase change is observed aer planetary ball milling (Fig. 3a). The XRD pattern reveals that the asreduced specimen is composed of Si, MgO, and unreacted SiO2 (Fig. 3b). A stoichiometric weight ratio of SiO2 : Mg ¼ 1 : 0.8 (or 1 : 2 molar ratio) based on the following reaction SiO2 + 2Mg ¼ Si + 2MgO is used, but unreacted SiO2 is still present. The volatilization of gaseous Mg and/or the formation of intermediate phases (magnesium silicide, Mg2Si) have been attributed to the presence of unreacted SiO2 in the magnesiothermic reduction reaction.25 The peak broadening is observed in the diffraction patterns for Si and MgO, which can be attributed to the formation of nano-crystallites. MgO is completely dissolved in HCl solution (Fig. 3c), and unreacted SiO2 is removed by HF solution, which results in the crystalline Si (Fig. 3d). To nd out the formation mechanism of Si nanosheets during magnesiothermic reduction, the mixture of milled sand and magnesium was heat-treated at different temperatures, and the corresponding XRD patterns are presented in Fig. 4a. At
Fig. 2
4298 | Nanoscale, 2014, 6, 4297–4302
XRD patterns of (a) ball-milled, (b) as-reduced, (c) HCl-etched, and (d) HF-etched samples.
Fig. 3
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
Paper
Nanoscale
Fig. 4 (a) Ex situ XRD patterns of as-reduced sand at different temperatures. (b–d) TEM images of HCl-etched products (reduced at 700 C for 4 h). (e) Schematic illustration of the formation of Si nanosheets.
500 C, the as-reduced specimen was composed of Mg2Si, MgO, and unreacted SiO2. Surprisingly, Mg2Si was found, and Si was not detected. The peak for Si appeared at 550 C. Upon increasing the reduction temperature, the peak intensity for Si and MgO increased while the intensity for Mg2Si and SiO2 decreased. At 650 C, the Mg2Si phase completely disappeared, and no further phase change occurred at higher temperatures. The obtained results indicate that Mg2Si is formed as an intermediate phase at an early stage of the reduction process, and Si is produced by consuming Mg2Si and SiO2. Generally, the magnesiothermic reduction of SiO2 is expressed as follows: SiO2 + 2Mg / Si + 2MgO
(1)
For the complete reduction of SiO2, an Mg/SiO2 molar ratio of 2.0 is required. Recently, it was reported that Mg2Si is formed as an intermediate phase even below the stoichiometric ratio (1.5),25 and Mg2Si is further reduced into Si through the following reaction: Mg2Si + SiO2 / 2Si + 2MgO
(2)
Based on the previous report and the results obtained in this study, the following overall reactions are proposed for the magnesiothermic reduction of SiO2: 4Mg + SiO2 / Mg2Si + 2MgO
(3)
Mg2Si + SiO2 / 2Si + 2MgO
(4)
These two reactions are thermodynamically possible (Fig. S3†) and reaction (3) has more negative Gibbs free energy at the reduction temperatures. Thus, it is suggested that the magnesiothermic reduction of SiO2 can occur via an intermediate Mg2Si phase and the formation of Mg2Si might result in the nanosheet morphology of Si. The HCl-etched specimen
This journal is © The Royal Society of Chemistry 2014
(reduced at 700 C) was examined by TEM to reveal the microstructure of the reduction products (Fig. 4b). The low-magnication TEM image showed that HCl-etched powder had either a hollow or yolk/shell-like structure. The high-magnication image indicated that the yolk/shell structure consisted of three parts: a dense shell, a porous middle layer, and a dense core (Fig. 4c). The core was the unreacted SiO2, implying that the yolk/shell-like powder was the on-going reduction product. The outer shell was identied as crystalline Si with planar defects, and the middle layer was composed of 5 nm Si nanoparticles (Fig. 4d). The distinctive microstructure of Si suggests that two layers were formed in the different routes. Assuming the transient presence of Mg2Si during the magnesiothermic reduction, the following formation mechanism of Si nanosheets is proposed (Fig. 4e). Initially, Mg2Si is formed between Mg and SiO2 through reaction (3). The excess Si in Mg2Si is precipitated as a Si layer, while Mg2Si reduces SiO2 into Si through reaction (2) forming the Si/MgO nanocomposite. The unreated Mg and MgO (or Mg2Si) are dissolved with HCl, which results in the dense-Si/porous-Si/unreacted SiO2 core–shell structure. Finally, Si with a nanosheet morphology is obtained by removing the unreacted SiO2 and nanoparticle Si with HF solution. The precipitation of Si from Al–Mg–Si alloys with an excess of Si has been reported previously.26,27 The reduction of SiO2 by Mg2Si and the synthesis of Si with the nanosheet morphology are under investigation. For further evidence, as-reduced specimens at 550 and 600 C were examined by TEM. The crosssectional TEM specimens were prepared by ultra-microtome. The presence of Mg2Si, MgO, and Si was conrmed by SAED patterns, but the individual layers could not be distinguished (see the TEM images of microtomed samples, Fig. S4†). To enhance the electrochemical properties, Si nanosheets are encapsulated with RGO by the APTES modied process. The SEM micrograph shows that RGO-encapsulated Si maintains the sheet-like morphology (Fig. 5a). The individual nanosheets are well separated with increased thickness, indicating that the
Nanoscale, 2014, 6, 4297–4302 | 4299
View Article Online
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
Nanoscale
Paper
(a) FESEM, (b) low-magnification TEM, (c) selected area electron diffraction (SAED) pattern, (d) high-magnification TEM, and (e) highresolution TEM images of RGO-encapsulated Si nanosheets.
Fig. 5
Si nanosheets are thoroughly covered with RGO. The highmagnication TEM image further conrms the sheet morphology (Fig. 5b). The SAED pattern corresponds to that of Si (Fig. 5c), but the high-resolution TEM image clearly shows that the crystalline Si was well coated with the layered RGO of 5–10 nm thickness (Fig. 5d). In addition, the Raman spectrum of RGO-encapsulated Si nanosheets displays two broad peaks (D band at 1367 cm1 and G band at 1582 cm1) revealing the presence of RGO in the sheet-like morphology (see the ESI, Fig. S5†). The discharge/charge voltage proles of the Si nanosheet and RGO-encapsulated Si nanosheet electrodes are shown in Fig. 6a. The cycling experiments are conducted in the voltage range between 0.0 and 2.0 V (vs. Li/Li+) at a current density of 200 mA g1. The shape of the voltage proles is similar to that of a typical Si electrode.12,19,20,28 The initial discharge and charge capacities of the Si nanosheets are 3563 and 2431 mA h g1, respectively. The RGO-encapsulated Si nanosheets shows slightly higher capacities (3769 and 2625 mA h g1, respectively). The coulombic efficiency of the rst cycle was 70% for both electrodes. The observed values for RGO-encapsulated Si nanosheets are higher than those for the previously reported nano-sized Si/RGO composite.29 The lower capacities of the nano-sized Si/RGO composite can be attributed to the existence of SiOx due to a high reaction activity of 5 nm Si. As a reference, the cyclic performance of commercial Si nanopowder was performed (Fig. 6b) and the obtained results are similar to the previous studies.30 The fabricated Si nanosheet electrode exhibits the comparable reversible capacity to the commercial Si nanopowder (Fig. 6b), even though it was synthesized from sand. As expected, the RGO-encapsulated Si nanosheets showed the improved cycle retention. The cycle performance of the RGO-encapsulated Si nanosheet electrode at a high rate (500 mA g1) exhibited the similar tendency with slightly lower capacity (see the ESI, Fig. S6†). The average capacity of ve cells was 1762 mA h g1 aer 25 cycles and the standard deviation was 87 mA h g1. Thus, the electrochemical data were quite consistent and reproducible. Compared to the previously
4300 | Nanoscale, 2014, 6, 4297–4302
Fig. 6 (a) Voltage profiles of Si nanosheet and RGO-encapsulated Si nanosheet electrodes after 1, 2, 10, 30, and 50 cycles at 200 mA g1 between 0.0 and 2.0 V, (b) cyclability of commercial Si nanopowder, Si nanosheet, and RGO-encapsulated Si nanosheet electrodes, and (c) rate capability of RGO-encapsulated Si nanosheet electrode between 0.01 and 2.0 V.
reported graphene/nano-sized Si composites,31 the RGO encapsulation effect was more signicant, and the improved cycle retention in this study can be attributed to the better encapsulation of Si with RGO due to the nanosheet morphology. However, the capacity degradation was still observed in the RGO-encapsulated Si nanosheet electrode and further surface treatment is required to improve the cycle performance. The rate capability of the RGO-encapsulated Si nanosheet electrode is promising and it exhibited a capacity of 1113 mA h g1 even at a high current density of 3 A g1 (Fig. 6c).
Experimental procedures Synthesis of silicon nanosheets 1 g of commercial sand (Ficher Scientic) was crushed by planetary ball milling for 4 h. The crushed sand was washed with D.I. water, dried, and calcined at 600 C for 1 h to remove the organic impurities. Then, 0.4 g of calcined sand and 0.32 g
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
of magnesium (325 mesh, Stream) were hand-mixed. The mixture was heat-treated at 700 C for 4 h under a 5 vol.% H2/N2 atmosphere. The obtained brown products were washed with hydrochloric acid and hydrouoric acid solution (10 wt%).
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
Synthesis of graphene oxide (GO) GO was synthesized through Hummers' method by reacting commercially available graphite akes (Sigma Aldrich) in a mixture of H2SO4, NaNO3, and KMnO4.31 Aer completion of the reaction, H2O2 was added to the reaction vessel. The brown GO was obtained by ltering, washing with 1 M HCl and D.I. water twice, and drying. RGO encapsulation 0.1 g of prepared silicon nanosheets was dispersed in 100 mL of absolute ethanol, and 2 mL of APTES was added.24 Aer stirring for 4 h, the solution was washed and centrifuged. Then, 0.15 g of synthesized GO was dispersed in 80 mL of water by tipsonication, and 20 mL of silicon nanosheet ethanol solution was added to the prepared GO solution. Aer stirring for 4 h, the solution was washed, centrifuged, and dried. The obtained composite powder was reduced at 400 C for 6 h under a 5 vol.% H2/N2 atmosphere. Characterization The morphology of silicon nanosheets and RGO-encapsulated silicon nanosheets were examined using a eld emission scanning electron microscope (FESEM, JSM-6330F, JEOL) and a 300 kV transmission electron microscope (TEM, JEM-3000F, JEOL). The crystal structure and phase evolution were investigated using X-ray diffraction (XRD, D8-Advance, BRUKER) with Cu-Ka Radiation. The RGO-encapsulated silicon sheets were further characterized using a high-resolution Raman microscope (LabRAM HR UV/Vis/NIR, Horiba Scientic) with Raman shi from 100 to 4000 cm1. Electrochemical test The test electrodes consisted of an active powder material (70 wt.%), carbon black (Ketchen Black, 10 wt.%) as a conducting agent, and polyamide imide (PAI, 20 wt.%) dissolved in N-methyl pyrrolidinone (NMP) as a binder. The weight of the electrode was about 2.26–2.59 mg cm2. Each component was well mixed to form a slurry using a magnetic stirrer. The slurry was coated on a copper foil substrate, pressed, and dried at 200 C for 4 h under a vacuum. A coin-type electrochemical cell was used with lithium foil as the counter and reference electrodes and 1 M LiPF6 and 10% uoroethylene carbonate (FEC) in ethylene carbonate (EC)/diethylene carbonate (DEC) (3 : 7 (v/v), PANAX) as the electrolyte. The cell assembly and all of the electrochemical tests were carried out in an Ar-lled glove box. The cycling experiments were galvanostatically performed using a Maccor automater tester at a constant current density of 200 mA g1 (lithiation ¼ 200, 500, 1000, 2000, 3000, and 200 mA g1, delithiation ¼ 200 mA g1 in the rate capability test) for the This journal is © The Royal Society of Chemistry 2014
Nanoscale
active material within the voltage range between 0.0 V and 2.0 V (vs. Li/Li+).
Conclusions In summary, we facilely synthesized the Si nanosheets from natural sand by magnesiothermic reduction. The thin Si nanosheets were precipitated from an intermediate Mg2Si phase to make the composition balance of Mg2Si. The synthesized Si nanosheets were well crystallized and showed comparable electrochemical properties to commercial Si nanopowder as an anode for lithium rechargeable batteries, and further enhancement was achieved by RGO encapsulation. The RGOencapsulated Si nanosheets showed excellent rate capability. In this work, we have demonstrated the facile synthesis of an Si nanostructure from natural sand with improved electrochemical properties, and suggested a new formation mechanism for Si nanosheets.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (NRF-2012R1A2A4A01008226).
Notes and references 1 Y. Nishi, J. Power Sources, 2001, 100, 101. 2 J. R. Dahn, T. Zheng, Y. Liu and J. S. Xue, Science, 1995, 270, 590. 3 M. Winter, J. O. Besenhard, M. E. Spahr and P. Novak, Adv. Mater., 1998, 10, 725. 4 N. A. Kaskhedikar and J. Maier, Adv. Mater., 2009, 21, 2664. 5 M. N. Obrovac and L. Christensen, Electrochem. Solid-State Lett., 2004, 7, A93. 6 J. Li and J. R. Dahn, J. Electrochem. Soc., 2007, 154, A156. 7 C.-M. Park, J.-H. Kim, H. Kim and H.-J. Sohn, Chem. Soc. Rev., 2010, 39, 3115. 8 W.-S. Chang, C.-M. Park, J.-H. Kim, Y.-U. Kim, G. Jeong and H.-J. Sohn, Energy Environ. Sci., 2012, 5, 6895. 9 H. Kim, M. Seo, M.-H. Park and J. Cho, Angew. Chem., Int. Ed., 2010, 49, 2146. 10 C. K. Chan, H. Peng, G. Liu, K. Wrath, X. F. Zhang, R. A. Huggins and Y. Cui, Nat. Nanotechnol., 2008, 3, 31. 11 Z. Lu, J. Zhu, D. Sim, W. Zhou, W. Shi, H. H. Hng and Q. Yan, Chem. Mater., 2011, 23, 5293. 12 D. Chen, X. Mei, G. Ji, M. Lu, J. Xie, J. Lu and J. Y. Lee, Angew. Chem., Int. Ed., 2012, 51, 2409. 13 Y. Hwa, W.-S. Kim, S.-H. Hong and H.-J. Son, Electrochim. Acta, 2012, 71, 201. 14 J. D. Holmes, K. P. Johnston, R. C. Doty and B. A. Korgel, Science, 2000, 287, 1471. 15 K. Peng, Y. Yan, S. Gao and J. Zhu, Adv. Mater., 2002, 14, 1164. 16 H. Okamoto, Y. Sugiyama and H. Nakano, Chem. – Eur. J., 2011, 17, 9864.
Nanoscale, 2014, 6, 4297–4302 | 4301
View Article Online
Published on 29 January 2014. Downloaded by National Chung Hsing University on 29/03/2014 09:46:31.
Nanoscale
17 Z. Bao, M. R. Weatherspoon, S. Shian, Y. Cai, P. D. Graham, S. M. Allan, G. Ahmad, M. B. Dickerson, B. C. Church, Z. Kang, H. W. Abernathy, C. J. Summers, M. Liu and K. H. Sandhage, Nature, 2007, 446, 172. 18 Y. Yu, L. Gu, C. Zhu, S. Tsukimoto, P. A. V. Aken and J. Maier, Adv. Mater., 2010, 22, 2247. 19 J.-K. Yoo, J. Kim, Y. S. Jung and K. Kang, Adv. Mater., 2012, 24, 5452. 20 H. Jia, P. Gao, J. Yang, J. Wang, Y. Nuli and Z. Yang, Adv. Energy Mater., 2011, 1, 1036. 21 E. H. Myrhaug, J. K. Tuset and H. Tveit, Electrical Energy Management in the Ferroalloy Business. Proceedings: Tenth International Ferroalloys Congress, 2004, vol. 1, p. 726. 22 M. Nagamori, I. Malinsky and A. Claveau, Metall. Trans. B, 1986, 17, 503. 23 T. Nohira, K. Yasuda and Y. Ito, Nat. Mater., 2003, 2, 397.
4302 | Nanoscale, 2014, 6, 4297–4302
Paper
24 S. Yang, X. Feng, S. Ivanovici and K. M¨ ullen, Angew. Chem., Int. Ed., 2010, 49, 8408. 25 M. Barati, S. Sarder, A. McLean and R. Roy, J. Non-Cryst. Solids, 2011, 357, 18. 26 D. G. Eskin, V. Massardier and P. Merle, J. Mater. Sci., 1999, 34, 811. 27 D. P. Yu, Y. J. Xing, Q. L. Hang, H. F. Yan, J. Xu, Z. H. Xi and S. Q. Feng, Physica E, 2001, 9, 305. 28 Y. Hwa, W.-S. Kim, B.-C. Yu, H. Kim, S.-H. Hong and H.-J. Sohn, J. Mater. Chem. A, 2013, 1, 3733. 29 S.-L. Chou, J.-Z. Wanga, M. Choucair, H.-K. Liu, J. A. Stride and S.-X. Dou, Electrochem. Commun., 2010, 12, 303. 30 H. Xiang, K. Zhang, G. Ji, J. Yang Lee, C. Zou, X. Chen and J. Wu, Carbon, 2011, 49, 1781. 31 W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.
This journal is © The Royal Society of Chemistry 2014
