Electrochimica Acta 69 (2012) 328–333
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
In situ synthesis of TiO2 –graphene nanosheets composites as anode materials for high-power lithium ion batteries Hua-Chao Tao, Li-Zhen Fan ∗ , Xiaoqin Yan, Xuanhui Qu School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 1 January 2012 Received in revised form 14 February 2012 Accepted 3 March 2012 Available online 15 March 2012 Keywords: Graphene Titanium dioxide Anode Lithium ion batteries Nanocomposites
a b s t r a c t TiO2 –graphene nanosheets (GNS) composites are prepared via an in situ chemical synthesis method, which enables a homogenous dispersion of TiO2 nanoparticles on the graphene nanosheets. The obtained TiO2 –GNS composites are anatase-GNS, while TiO2 is nanosized rutile without the incorporation of GNS. The resulting TiO2 –GNS composites are employed as anode materials for lithium-ion batteries, showing a high initial reversible capacity of 306 mAh g−1 as well as a good cycling stability. The TiO2 –GNS composites can deliver 60 mAh g−1 at a current rate as high as 5 A g−1 and demonstrate negligible fade even after 400 cycles. The superior electrochemical performances of the TiO2 –GNS composites can be attributed to their unique structures, which intimately combine the conductive graphene nanosheets network with uniformly dispersed TiO2 nanoparticles. The TiO2 –GNS composites could be promising candidate materials for high-power, low-cost, and environmentally friendly anodes for lithium ion batteries. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Rechargeable lithium-ion batteries are the most promising candidates for energy conversion and storage devices because of their high energy and power densities. Carbon-based materials are normally used as anode in commercial lithium-ion batteries . However, they have the disadvantage of low lithium intercalation voltage which is in close proximity to metallic lithium plating voltage. Compared with carbon materials, TiO2 -based materials show a higher lithium insertion–extraction potential of about 1.6 V versus Li/Li+ . In addition, TiO2 is abundant, low cost and environmentally benign and its lattice change is negligible during Li ion intercalation or de-intercalation, which is conducive to structure stability and long cycle life. The TiO2 polymorphs anatase (I41 /amd) , rutile (P42 /mnm) , brookite (Pbca) , and TiO2 [B] (bronze, C2/m)  have been studied in order to investigate their electrochemical behavior upon lithium insertion/extraction. Under standard conditions, rutile is the thermodynamically most stable structure of TiO2 , and is also the most common natural. Microsized rutile materials generally are reported to host only a negligible amount of Li ions, because the one-dimensional character of the diffusion limits the Li ions insertion over signiﬁcant three-dimensional volumes, leading to the poor performance .
∗ Corresponding author. Tel.: +86 10 62334311; fax: +86 10 62334311. E-mail address: [email protected]
(L.-Z. Fan). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.03.022
Nanosized rutile strongly increases both the capacity and the rate capability . Anatase is generally considered to be the most electroactive lithium ion insertion host. Nanosized anatase materials present increased capacity upon cycling compared with bulk micrometer sized anatase due to a better electrolyte/active materials interaction . Generally, the applications of TiO2 are limited due to the sluggish Li-ion diffusion, poor electron transport and increased resistance at the interface of electrode/electrolyte at high charge–discharge rates. In order to obtain high energy and power densities, it is very important to design and fabricate electrodes with enhanced Li ion transport and electronic conduction. To improve the lithium ion transport of the TiO2 -based materials, many efforts have been made to shorten the Li diffusion distance in the bulk materials, such as nanoparticles , nanotubes , nanowires , nanobelts  and nanoporous structures . In addition, many methods have been adopted to increase the electron transport in the TiO2 -based electrode materials, such as using conductive coating (carbon , Ag , and Sn ) and adding conductive additive nanophases (RuO2  and carbon nanotubes ). Graphene, a single layer of carbon atoms bonded together in a hexagonal lattice, has great advantages owing to its high speciﬁc surface area , excellent electronic conductivity , and good mechanical properties . Such merits suggest that graphene nanosheets may be an ideal conductive additive for metal oxides electrodes to improve their electrochemical performances. Graphene based nanocomposite electrode materials such as SnO2 , Si , Co3 O4 , Li4 Ti5 O12 , Fe3 O4 ,
H.-C. Tao et al. / Electrochimica Acta 69 (2012) 328–333
TiO2 [26–28] have been studied in recently years to enhance the storage capacity and cycle stability. In these composites, graphene nanosheets not only decrease the contact resistance of active particles and improve electrical conductivity of the electrode, but also prevent the aggregation of nanoparticles during cycling. TiO2 –graphene hybrid nanostructures and mesoporous anatase TiO2 –graphene composites synthesized by directly using graphene as raw materials exhibit an enhanced rate performance [26,27]. Different from these preparation methods, we employed here an in situ synthesis method using graphene oxide nanosheets as raw materials to ensure the uniform dispersion of TiO2 nanoparticles on the graphene nanosheets. Also, TiO2 @TiOx Ny /TN–graphene sheets hybrid nanocomposite synthesized by using NH3 in the annealing condition to reduce TiO2 @TiOx Ny /TN–graphene oxide nanocomposite show an enhanced capacity at high current densities . In this research, we present an in situ synthesis method to prepare TiO2 –graphene nanosheets composites using hydrazine as a reducing agent in the liquid phase to avoid the agglomeration of graphene. This in situ synthesis method could assure the uniform distribution of TiO2 on GNS surfaces. The TiO2 –graphene nanosheets composites exhibit not only high capacity, but also excellent rate capability.
2. Experimental Graphite oxide (GO) was prepared following the Hummers method  using graphite (Aldrich) as raw materials. GO (100 mg) was ultrasonicated for 2 h in the deionized water (200 mL) to obtain graphene oxide nanosheets (GONS). To prepare TiO2 –GNS composites, the predetermined amount of TiCl4 was added into GONS with vigorous stirring and heated at 60 ◦ C for 24 h in the water bath. Hereafter, the resultant was allowed to cool and the solid products (TiO2 –GONS) were centrifuged and washed with deionized water. TiO2 –GONS composites were sonicated for 1 h in deionized water and then hydrazine hydrate (3 mL) was added with vigorous stirring and heated at 100 ◦ C for 24 h in the oil bath. The resulted solid was TiO2 –GNS, and then it was washed and dried in a vacuum oven at 60 ◦ C. For comparison purpose, pure GNS was synthesized by directly dispersion of GONS in deionized water by sonication and then reduction to GNS by hydrazine hydrate. TiO2 powders without GNS were prepared from the hydrolysis of TiCl4 . X-ray diffraction (XRD) patterns were recorded on Rigaku/mac250 using Cu K␣ radiation. To quantify GNS in the composites, thermogravimetric analysis (TGA) was carried out in air from 25 ◦ C to 1000 ◦ C by using a Perkin-Elmer 2400II analyzer. The morphology and particle size of the materials were observed by transmission electron microscopy (TEM, JEOL JEM-200CX). C and Ti maps were collected using a ﬁeld emission scanning electron microscope (FE-SEM, JSM-6330) equipped with an energy-dispersive X-ray(EDX) analyzer (PHI5300 USA). Raman spectra were recorded using a 514.5 nm Ar-Ion laser on HR800 (Horiba Jobin Yvon). The lithium insertion/extraction properties were investigated at 25 ◦ C with CR2032 coin cell using pure Li foil as counter electrode. The working electrodes were prepared by mixing active materials, carbon black (Super-P), and poly (vinyl diﬂuride) (PVDF) at a weight ratio of 80:10:10 and pasted on pure copper foil. The electrolyte was consisted of a solution of 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethylmethyl carbonate (EMC) (1:1:1 by volume). The cells were assembled in an argonﬁlled glove box with the concentrations of moisture and oxygen below 1 ppm. The cells were charged and discharged galvanostatically at various rates in the ﬁxed voltage window from 0.8 to 3 V. Electrochemical impedance spectroscopy (EIS) analysis of the coin cells was performed from 0.1 Hz to100 kHz.
Scheme 1. The fabrication of TiO2 –GNS composites: (1) oxidation of graphite to graphite oxide; (2) the exfoliation of graphene oxide in water by ultrasonication and the hydrolysis of TiCl4 to form TiO2 –GONS; (3) reduction of TiO2 –GONS by hydrazine.
3. Results and discussion As schematically illustrated in Scheme 1, the procedures for preparation of TiO2 –GNS composites are carried out in three steps. Firstly, GO was synthesized from graphite by the Hummers method. Secondly, GONS–TiO2 composites were obtained after the exfoliation of graphite oxide by ultrasonication and the hydrolysis of TiCl4 (Eq. (1)) in water. Finally, TiO2 –GNS composites were synthesized by reducing TiO2 –GONS precursor using hydrazine. No additional subsequent heat treatment or reduction by NH3 in heated condition was employed as reported before in the process of graphene/TiO2 composite synthesis [26–28]. This process is simple and environmentally friendly. Two different TiO2 –GNS composites were prepared here from TiCl4 /GO weight ratios = 6 and 12. TiCl4 + GO + 2H2 O → TiO2 –GO + 4H+ + 4Cl−
Fig. 1a shows XRD patterns of TiO2 and TiO2 –GNS composites. Similar to the reduction of GONS to GNS by aqueous hydrazine, the TiO2 –GONS can also be reduced to TiO2 –GNS at 100 ◦ C. The peaks of the TiO2 –GNS composites can be indexed to main anatase (JCPDS NO: 01-089-4921) with a minor brookite phase (JCPDS NO: 003-0380). It is worth noting that the (1 0 1) major peak position of anatase is very close to the (1 1 1) peak of brookite. The estimated crystalline sizes of pure TiO2 and TiO2 in TiO2 –GNS composites are 13 nm and 7 nm by application of Scherrer formula respectively. Different from TiO2 –GNS composites, it is interesting that the rutile TiO2 (JCPDS NO: 01-076-1940) is obtained from the direct hydrolysis of TiCl4 without GO. The probable reason for the appearance of different crystallographic phases may be attributed to different pH values of the solution . The synthesis of TiO2 and TiO2 –GNS composites were also conﬁrmed by Raman spectra, as seen in Fig. 1b. In the spectrum of pristine graphite, the G band corresponds to an E2g mode of graphite and is related to the vibration of sp2 bonded carbon atoms. Compared with raw graphite, the ratio of the intensities (ID /IG ) (D band,
H.-C. Tao et al. / Electrochimica Acta 69 (2012) 328–333
Mass retained / %
graphite oxide (GO)
Intensity / a.u.
60 40 20 GNS
40 50 o 2θ /
Temperature / C Fig. 2. TGA curves of GNS, pure TiO2 , and TiO2 –GNS composite from TiCl4 /GO with weight ratio = 12, 6 under air atmosphere.
Intensity / a.u.
pure TiO2 graphite oxide graphite
800 1200 -1 Raman shift / cm
Fig. 1. XRD patterns (a), and Raman spectra (b) of graphite, graphite oxide, pure TiO2 (rutile) obtained from the hydrolysis of TiCl4 and TiO2 (anatase and brookite)GNS composites obtained from TiCl4 /GO with different weight ratios of 6 and 12. (*) Anatase TiO2 , () brookite TiO2 , () rutile TiO2 , () graphite oxide (GO), (•) graphite.
the breathing mode of -point phonons of A1g symmetry) for GO was markedly increased, indicating the formation of some sp3 carbon by functionalization. After the reduction, compared with raw graphite, the G band of TiO2 –GNS was broadened and ID /IG intensity ratio was increased. This phenomenon can be attributed to the signiﬁcant decrease of the size of the in-plane sp2 domain due to the oxidation and ultrasonic exfoliation, and partially disordered graphite crystal structure of graphene nanosheets . The Raman spectrum of as prepared pure TiO2 had four bands at 143, 248, 443, and 609 cm−1 , which are in consistency with the fundamental modes observed in the spectra of a rutile phase [32,33]. The peak at 248 cm−1 is assigned to a disordered-induced scattering mode of rutile. The TiO2 –GNS bands at 145, 395, 510 and 615 cm−1 correspond to those in the spectra of anatase phase . TGA (Fig. 2) of the GNS–TiO2 reveals that the weight content of GNS in the composites obtained from TiCl4 /GO weight ratio = 12 and 6 are 10 wt% and 20 wt% respectively. To characterize the morphology and structure of TiO2 –GNS composites, TEM and FE-SEM with EDX spectroscopy were carried out. It is clearly seen from Fig. 3a that a large quantity of particles with a size of about 5–10 nm are randomly and homogeneously distributed onto the GNS. The size of nanocrystalline is in
consistency with the size calculated from XRD patterns. The selected area electron diffraction (SAED) pattern (Fig. 3a inset) of these nanoparticles in the composites reveals the lattice spacings corresponding to the (1 0 1), (0 0 4) and (2 0 0) diffractions of the anatase phase. It is determined from TEM that the GNS is consisted of about 10 graphene layers (Fig. 3b). The uniform distribution of TiO2 on GNS could also be further conﬁrmed from FE-SEM image with corresponding C and Ti maps using the characteristic X-ray spectroscopy (Fig. 3c). Galvanostatic charge–discharge (lithium insertion–extraction) measurements were carried out to evaluate the electrochemical performance of TiO2 –GNS composites. For comparison, the control sample GNS and TiO2 were also tested under the same conditions. Fig. 4a illustrates the ﬁrst charge–discharge curves of GNS, TiO2 –GNS composites from TiCl4 /GO weight ratio = 6, 12 and pure TiO2 at a current density of 50 mA g−1 . It is remarkable to note a very high reversible capacity 306 mAh g−1 of TiO2 –GNS composites from TiCl4 /GO weight ratio = 6 in the range from 0.8 to 3 V, which is higher than those of TiO2 –GNS composites from TiCl4 /GO weight ratio = 12 (216 mAh g−1 ), pure TiO2 (162 mAh g−1 ) and GNS (103 mAh g−1 ). The ﬁrst discharge capacity of TiO2 –GNS composites from TiCl4 /GO weight ratio = 6 is also higher than those ever reported for nano-sized rutile, nano-sized anatase, TiO2 -B nanowires TiO2 nanotube array and TiO2 –graphene hybrid nanostructures [3–6]. Compared with pure TiO2 , the higher storage capacity and reversible capacity for composite are attributed to the uniformly dispersed TiO2 nanoparticles and GNS. The uniformly dispersed TiO2 nanoparticles can store more lithium ions on the surface region and the cavities and the edges provided by GNS can adsorb more lithium ions. The initial coulombic efﬁciencies of TiO2 –GNS are 81% (from TiCl4 /GO weight ratio = 6) and 76% (from TiCl4 /GO weight ratio = 12) respectively, which are also better than the values of pure TiO2 (47%) and GNS (30%), indicating a distinguished synergetic effect between GNS and TiO2 in the composites. Because electrolyte reduction reaction is negligible in this study due to its high operation voltage, therefore, the irreversible capacity in the ﬁrst cycle can be ascribed to the distortion of anatase/rutile framework, which results in the trapping of lithium ions and also the surface defects from GNS such as surface voids or vacancies, which act as trapping centers for lithium ions. Compared with pure TiO2 , the partial oxyhydrogen-containing surface functional groups from GNS in the TiO2 –GNS composites are prone to form stable chemical bonding solid electrolyte interface when the voltage is higher than 0.8 V , which usually occurs in the
H.-C. Tao et al. / Electrochimica Acta 69 (2012) 328–333
Fig. 3. (a) Low- and (b) high-magniﬁcation TEM images of TiO2 –GNS composite from TiCl4 /GO with a weight ratio of 6 showing the homogeneous distribution of the TiO2 nanoparticles on the GNS. The inset is the SAED pattern. Lattice frings are present indicating the crystalline characteristic; (c) FE-SEM image of TiO2 –GNS composite from TiCl4 /GO with weight ratio of 6 and the corresponding elemental C and Ti energy-dispersive X-ray spectroscopy (EDX) maps.
electrodes between Li ions and surface functional groups. Lithium ions bind to hydrogen-terminated edges of carbon fragment to form the organolithium molecule C2 H2 Li3 . The left oxygencontaining surface functional groups in the GNS may interact with the TiO2 nanoparticles, which improves the interface stability of the electrode and electrolyte. In the case of pure GNS, the large initial irreversible capacity and low coulombic efﬁciencies are due to the large surface area from a large amount of cavities and edges. It can also be seen from Fig. 3a that the charge–discharge plateau for the TiO2 –GNS composite from the TiCl4 /GO weight ratio = 6 is at approximately 1.8 V and it shows extended plateau and a larger charging–discharging capacity than TiO2 . In addition, a larger decrease in peak separation between anodic and cathodic peaks is observed for TiO2 –GNS compared to TiO2 , which indicates that the polarization is suppressed by the incorporation of GNS. The charge–discharge speciﬁc capacity of GNS, pure TiO2 and TiO2 –GNS composites from TiCl4 /GO weight ratio = 6, 12 at different current densities are depicted in Fig. 4b. The reversible capacity of the TiO2 –GNS composites from TiCl4 /GO weight ratio = 6 is as high as 197 mAh g−1 after 50 cycles at the current density of 50 mA g−1 , which is much higher than those of TiO2 –GNS composite from TiCl4 /GO weight ratio = 12 (148 mAh g−1 ), pure TiO2 (96 mAh g−1 ) and GNS (93 mAh g−1 ). At higher current density, the enhancement of reversible capacity for TiO2 –GNS composites from TiCl4 /GO weight ratio = 6 is more evident. For example, relatively high capacity of 60 mAh g−1 is obtained at 5 A g−1 (corresponding to charge–discharge time of 2 min). This value is in obvious contrast to that of GNS which has a capacity of 20 mAh g−1 at the same current density. The calculated pure TiO2 contribution of TiO2 –GNS composites from TiCl4 /GO weight ratio = 6 (80%
TiO2 ) is about 70 mAh g−1 at the current density of 5 A g−1 from Eq. (2). CT =
CA − CG × 0.2 0.8
where CT , CA , and CG are the speciﬁc capacity of calculated pure TiO2 , TiO2 –GNS composites, and pristine graphene, respectively. This value is in sharp contrast to that of pure TiO2 which decays to 6 mAh g−1 at the current density of 1 A g−1 . The reversibility is demonstrated by the fact that the capacity is regained when the current density is lowered to 50 mA g−1 . To further testify the enhanced electronic conductivity, EIS measurements on TiO2 –GNS composites and pure TiO2 were performed. The Nyquist plots of the TiO2 –GNS composites and TiO2 (Fig. 5) all show depressed semicircles at high frequencies. Obviously, the high frequency semicircle related to the resistivity of the cells decreased from 162 to 72 with the addition of 20% GNS. The highly conductive graphene sheets can facilitate electron transfer and thus decreasing resistance. Therefore, improved high charge rate performance and the depressed polarization could be attributed to the substantial decrease in charge-transfer resistance. In summary, the excellent electrochemical performances of TiO2 –GNS composites can be ascribed to their unique structures. Firstly, TiO2 nanoparticles are homogenously dispersed onto GNS, which prevents the agglomeration of TiO2 nanoparticles, and the porosities formed by lateral GNS and TiO2 nanoparticles can also facilitate ion conveyance; Secondly, GNS provides a large contact surface for dispersion of TiO2 nanoparticles and it can act as an excellent conductive agent to provide a high electrical conductivity for the electrode; Thirdly, the phases of the crystalline TiO2 change
H.-C. Tao et al. / Electrochimica Acta 69 (2012) 328–333
Voltage / V vs. Li/Li
TiCl4/GO=12 pure TiO2
0 50 100150200250300350400450 -1 Capacity / mAh g
350 Capacity / mAh g-1
250 200 150
pure TiO2 GNS
0.1 0.2 0.5 1
Acknowledgements This work was supported by NSF of China, Fundamental Research Funds for the Central Universities of China and State Key Laboratory of New Ceramic and Fine Processing (Tsinghua University).
0 50 100150200250300350400450 Cycle number
Fig. 4. (a) The ﬁrst charge–discharge curves cycled at a current density of 50 mA g−1 , and (b) cycle performances and rate capabilities of as-prepared TiO2 –GNS composite from TiCl4 /GO with weight ratio = 12 and 6, GNS, and pure TiO2 obtained from the hydrolysis of TiCl4 .
from rutile to anatase, which is conducive to the electrochemical performance. The capacity of anatase is larger than that of rutile because of the vigorous Lithium diffusion in anatase phase . Anatase is obtained in TiO2 –GNS composites, while only rutile phase exists for TiO2 without the incorporation of GNS.
400 pure TiO2
300 -Z" / ohm
We have demonstrated here convincingly TiO2 –GNS composites to be prospective electrode materials for lithium-ion batteries. TiO2 –GNS composites were successfully in situ synthesized by the reduction of TiO2 –GONS composites. This process enables a homogenous dispersion of TiO2 nanoparticles onto graphene nanosheets. The resulting TiO2 –GNS composites are anatase-GNS, while only rutile phase is obtained for pure TiO2 . The TiO2 –GNS composites exhibit a high reversible speciﬁc capacity, excellent cycling stability and superior rate performance in comparison with pure TiO2 . The complementary synergetic effect of the composites may be attributed to the prevented aggregation of TiO2 nanoparticles and increased electrical conductivity and mechanical stability of electrode materials in the presence of graphene nanosheets. In addition, the results demonstrate the importance of electronically conducting layers for enhancing the electrochemical properties of TiO2 -based electrode materials. Both the structure design and the synthesis method provide a wide insight for the synthesis of other GNS-based materials to improve the performance of electrodes for electrochemical energy storage and conversion.
250 200 150 100 50 0
50 100 150 200 250 300 350 400 Z' / ohm
Fig. 5. Nyquist plots of the electrodes of TiO2 –GNS composites and pure TiO2 .
 H. Li, Z.X. Wang, L.Q. Chen, X.J. Huang, Adv. Mater. 21 (2009) 4593.  A.R. Armstrong, G. Armstrong, G. Canales, R. Garcia, P.G. Bruce, Adv. Mater. 17 (2005) 862.  U. Lafont, D. Carta, G. Mountjoy, A.V. Chadwick, E.M. Kelder, J. Phys. Chem. C 114 (2010) 1372.  Y.S. Hu, L. Kienle, Y.G. Guo, J. Maier, Adv. Mater. 18 (2006) 1421.  M.A. Reddy, M.S. Kishore, V. Pralong, U.V. Varadaraju, B. Raveau, Electrochem. Solid-State Lett. 10 (2007) A29.  A.R. Armstrong, G. Armstrong, J. Canales, P.G. Bruce, Angew. Chem., Int. Ed. 43 (2004) 2286.  F. Gligor, S.W. de Leeuw, Solid State Ionics 177 (2006) 2741.  L.J. Hardwick, M. Holzapfel, P. Novak, L. Dupont, E. Baudrin, Electrochim. Acta 52 (2007) 5357.  G.F. Ortiz, I. Hanzu, T. Djenizian, P. Lavela, J.L. Tirdo, P. Knauth, Chem. Mater. 21 (2009) 63.  J.M. Li, W. Wang, H.H. Zhou, J.J. Li, D.S. Xu, Chem. Commun. 47 (2011) 3439.  W.Q. Han, X.L. Wang, Appl. Phys. Lett. 97 (2010) 243104.  J. Wang, Y.K. Zhou, Y.Y. Hu, R.O. Hayre, Z.P. Shao, J. Phys. Chem. C 115 (2011) 2529.  S.J. Park, Y.J. Kim, H. Lee, J. Power Sources 196 (2011) 5133.  M.M. Rahman, J.Z. Wang, D. Wexler, Y.Y. Zhang, X.J. Li, S.L. Chou, H.K. Liu, J. Solid State Electrochem. 14 (2010) 571.  H.S. Kim, S.H. Kang, Y.H. Chung, Y.E. Sung, Electrochem. Solid-State Lett. 13 (2010) A15.  Y.G. Guo, Y.S. Hu, W. Sigle, J. Maier, Adv. Mater. 19 (2007) 2087.  I. Moriguchi, R. Hidaka, H. Yamada, T. Kudo, H. Murakami, N. Nakashima, Adv. Mater. 18 (2006) 69.  M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Nano Lett. 8 (2008) 3498.  K.I. Bolotin, K.J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P. Kim, H.L. Stormer, Solid State Commun. 146 (2008) 351.  C. Lee, X. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385.  J. Yao, X.P. Shen, B. Wang, H. Liu, G.X. Wang, Electrochem. Commun. 11 (2009) 1849.  H.C. Tao, L.Z. Fan, Y.F. Mei, X.H. Qu, Electrochem. Commun. 13 (2011) 1332.  H. Kim, D.H. Seo, S.W. Kim, S.W. Kim, J. Kim, K. Kang, Carbon 49 (2011) 326.  N. Zhu, W. Liu, M.Q. Xue, Z. Xie, D. Zhao, M.N. Zhang, J. Chen, T.B. Cao, Electrochim. Acta 55 (2010) 5813.  H.L. Wang, L.F. Cui, Y. Yang, S. Casalongue, J.T. Robinson, Y.Y. Liang, Y. Cui, H.J. Dai, J. Am. Chem. Soc. 132 (2010) 13978.  D.H. Wang, D.W. Choi, J. Li, Z.G. Yang, Z.M. Nie, R. Kou, D.H. Hu, C.M. Wang, L.V. Saraf, J.G. Zhang, I.A. Aksay, J. Liu, ACS Nano 3 (2009) 907.  N. Li, G. Liu, C. Zhen, F. Li, L.L. Zhang, H.M. Cheng, Adv. Funct. Mater. 21 (2011) 1717.  C. Qiu, K.Y. Yan, S.H. Yang, L.M. Jin, H. Deng, W.S. Li, ACS Nano 4 (2010) 6515.  W. Hummers, R. Oﬂeman, J. Am. Chem. Soc. 80 (1958) 1339.
H.-C. Tao et al. / Electrochimica Acta 69 (2012) 328–333  N.M. Kinsinger, A. Wong, D.S. Li, F. Villalobos, D. Kisailus, Crystal Growth Design 10 (2010) 5254.  G.X. Wang, X.P. Shen, J. Yao, J. Park, Carbon 47 (2009) 2049.  M.S. Wainwright, N.R. Foster, Catal. Rev. – Sci. Eng. 19 (1979) 211.  M. Wu, W. Zhang, Z. Du, Y. Huang, Mod. Phys. Lett. B 13 (1999) 167.
 T. Ohsaka, F. Izumi, Y. Fujiki, Raman Spectrosc. 7 (1978) 321.  M.C. Tsai, J.C. Chang, H.S. Sheu, H.T. Chiu, C.Y. Lee, Chem. Mater. 21 (2009) 499.  T. Zheng, W.R. McKinnon, J.R. Dahn, J. Electrochem. Soc. 143 (1996) 2137.  D. Deng, M.G. Kim, J.Y. Lee, J. Cho, Energy Environ. Sci. 2 (2009) 818.