Graphene-Nanosheets Composites

Nov 11, 2010 - Yueming Li, Xiaojun Lv, Jin Lu, and Jinghong Li*. Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry and Chemic...
0 downloads 0 Views 4MB Size
Download PDF
21770

J. Phys. Chem. C 2010, 114, 21770–21774

Preparation of SnO2-Nanocrystal/Graphene-Nanosheets Composites and Their Lithium Storage Ability Yueming Li, Xiaojun Lv, Jin Lu, and Jinghong Li* Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology, Tsinghua UniVersity, Beijing 100084, P. R. China ReceiVed: June 1, 2010; ReVised Manuscript ReceiVed: October 22, 2010

To improve the performance of SnO2 as anode materials for lithium battery, a facile and efficient method to prepare the composites of SnO2-nanocrystal/graphene-nanosheets was developed on the basis of the reduction of graphene oxide (GO) by Sn2+ ion. Changing the ratio of Sn2+ and GO led to the morphology changes of SnO2/graphene-nanosheets composite. The performance as anode materials for lithium battery was studied in this report. The results showed that the electrochemical performance of composites was greatly enhanced, indicating that the composites might have a promising future as application in Li-ion battery. 1. Introduction Modern electronic devices such as cell phones, laptop computers, and electric vehicles require high-performance batteries to power them.1 Li-ion battery is one of the most suitable candidates to satisfy the requirements because of its high energy density, high voltage, and light weight.2 Within the battery, electrode materials are a determining factor for the battery performance. Although graphite performs well as anode for commercial Li-ion batteries, its theoretical capacity (372 mAh/g) is insufficient to satisfy the increasing demand for batteries with higher capacity. Thus, exploring novel anode materials for Li-ion batteries is of key importance. Many materials including metal oxide, metal sulfide, and nonmetal with large specific capacity have been studied to replace graphite.1,3,4 Among these materials, tin dioxide (SnO2)5-7 has attracted much attention because of its high theoretica1 capacity (782 mAh/g),8 low cost, and no toxicity. However, the major drawback affecting the practical applications of SnO2 is its poor cycling performance caused by volume expansion during the charge-discharge process.9,10 In order to tackle this problem, various methods have been tested.11-15 Results have shown that the cycle performance can be improved via compounding of carbon and SnO2.16-18 However, these methods are usually very complex; a facile and efficient method is needed to synthesis SnO2/carbon composites. Graphene, as the basic plane of graphite, because of its excellent conductivity and high specific surface area, is expected to have a wide range of applications.19 Composites based on graphene have shown improved mechanical strength, electronic conductivity, and electrochemical properties.20 Recently, it has been reported that SnO2/graphene-nanosheets composites showed greatly enhanced cycle performance compared to bare SnO2 nanoparticles.21-23 However, the SnO2/graphene-nanosheets composites processes were complicated, involving a multi-step approach or the use of additional chemicals such as urea or NaBH4. By considering the strong reductive capability of Sn2+, we herein designed a simple method by combining GO reduction and Sn2+ oxidation in one step by using just GO and SnCl2 as reagents to yield SnO2-nanocrystal/graphene-nanosheets composites. The electrochemical experiments showed that the cycle * E-mail: [email protected].

performance of composites was greatly enhanced and might pave a way to commercialize the composites electrode because of the simplicity. 2. Experimental section 2.1. Preparation of samples. GO was obtained by the Hummer method,24,25 followed by ultrasonication in water. SnO2-nanocrystal/graphene-nanosheets were prepared by chemical reduction of 100 mL GO (0.2 g/L, aqueous solution) with SnCl2 · 2H 2O. The amount of SnCl2 · 2H2O was varied (300, 600, and 1200 mg in order to control the ratio of graphene and SnO2.) The mixture was stirred for 24 h at room temperature, leading to a black suspension solution. The products were obtained by centrifugation, washed with distilled water, and dried at 100 °C. The samples were notated as SnO2/G-0.3, SnO2/ G-0.6, and SnO2/G-1.2, respectively. 2.2. Characterization. The powder X-ray diffraction (XRD) measurements of the samples were recorded on a Bruker D8Advance X-ray powder diffractometer by using a graphite monochromator with Cu KR radiation (λ ) 1.5406 Å). The data were collected between scattering angles (2θ) of 5-80° at a scanning rate of 8°/min. Fourier-transform IR (FT-IR) spectra were carried out through a Perkin-Elmer spectrophotometer operating in the infrared domain 400-4000 cm-1 by a KBr matrix. TEM specimens were prepared by drop-casting the asprepared sample dispersions onto carbon-coated TEM grids and dried in air. A JEOL JEM 2010 transition electronic microscopy was used for TEM analysis and HRTEM analysis. The XPS data were taken on an AXIS Ultra instrument from Kratos Analytical in the range of 0-1100 eV. Thermogravimetric analysis (TGA) was conducted on a TGAQ5000 IR instrument (TA Instruments Co., USA) from 100 to 800 °C at a speed of 10 °C/min under air. 2.3. Electrochemical Performance. To evaluate electrochemical performance, composite electrodes were constructed by mixing the active materials, conductive carbon black, and polyvinylidene fluoride (PVDF), in the weight ratio 80:15:5. The mixture was prepared as slurry in N-methyl pyrrolidinone and spread onto copper foil by using the doctor-blade technique. The electrode was dried under vacuum at 120 °C for 8 h. The cells were assembled inside an argon-filled glovebox by using a lithium-metal foil as the counter electrode and the reference

10.1021/jp1050047  2010 American Chemical Society Published on Web 11/11/2010

Preparation of SnO2-Nanocrystal/Graphene-Nanosheets Composites

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21771

Figure 1. XRD patterns of graphite oxide (a), SnO2/G-0.3 (b) SnO2/ G-0.6 (c), and SnO2/G-1.2 (d) .

electrode and microporous polypropylene as the separator. The electrolyte used was 1 M LiPF6 in a 1:1 weight ratio ethylene carbonate (EC):dimethyl carbonate (DMC) solvent. Assembled cells were allowed to soak overnight, and then, we began electrochemical testing on a Land battery testing unit (Wuhan, China). Galvanostatic charge and discharge of the assembled cells were performed at the current density of 200 mA/g between voltage limits of 0.01 and 1.5 V (versus Li) at room temperature. 3. Results and Discussion XRD data of the products are shown in Figure 1. The peak at 10.4° in curve a is characteristic for GO with an interlayer spacing of 0.85 nm, resulting to facile exfoliation because of weakened the van der Waals forces between layers of GO.26 The four dominant broadened peaks (110), (101), (211), and (301) (as shown in curves b, c, and d) are attributed to the SnO2 phase (JCPDS No. 41-1445), indicating the formation of tetragonal SnO2 nanocrystals. These diffraction peaks are considerably broadened, suggesting a small particle size of SnO2. The mean crystallite sizes for SnO2/G-0.3, SnO2/G-0.6, and SnO2/G-1.2 are 8.4, 7.8, and 8.4 nm, respectively, according to the Scherer equation, which is consistent with the broadened peaks. Transmission electronic microscopy (TEM) observation of GO shows a typical exfoliated nanostructure (Figure S1 in the Supporting Information). The TEM images of SnO2/G-0.3 (Figure 2a) clearly illustrate that SnO2 nanocrystals are distributed on the surface of the graphene nanosheets and that the diameters of the SnO2 grains are less than 10 nm (Figure 2b), which is consistent with calculations from XRD. The lattice fringe spacing d (110) is 0.34 nm, in good agreement with the predicted value by Bragg’s equation (0.337 nm). The selected area electron diffraction (SAED) pattern (inset of Figure 2a) shows four sharp diffraction rings corresponding to the (110), (101), (211), and (301) crystalline planes of the cassiterite SnO2, confirming the formation of polycrystalline SnO2 nanocrystal. The graphene nanosheets for SnO2/G-0.6 and SnO2/G-1.2 become smaller compared with that of SnO2/G-0.3 with increase of the Sn2+/GO ratio. For SnO2/G-0.3, the SnO2 was distributed sporadically on the surface of graphene nanosheets, shown in Figure 3a. And more SnO2 nanocrystals are observed on graphene nanosheets with an increased Sn2+/GO ratio (Figure 3b,c). It can be seen that the morphology evolution of SnO2/ graphene-nanosheets composite is related to the Sn2+/GO ratio. FTIR spectrum of GO (Figure 3d) shows the O-H deformation peak at 1401 cm-1, a weak band at 1735 cm-1 assigned to

Figure 2. TEM images of SnO2/G-0.3 (a,b), SnO2/G-0.6 (c,d), and SnO2/G-1.2 (e,f) with different magnifications. Insets: SAED patterns.

COOH, 1220 cm-1 assigned to C-OH stretching, 1052 cm-1 due to C-O stretching vibrations, and 827 cm-1 (weak) due to (OsCdO). The spectrum also depicts a peak at 1620 cm-1 which corresponds to the remaining sp2 character.27,28 The introduction of Sn2+ ion eliminated most of oxygen-containing groups such as COOH peaks (1735 cm-1), C-O stretching vibrations (1052 cm-1), O-H deformation peak (1401 cm-1), and OsCdO peak (827 cm-1). A strong peak at 606 cm-1 was assigned to Sn-O. The IR results confirmed the reduction of GO to graphene and the Sn2+ oxidiation to SnO2 nanocrystal, which indicated the formation of composites of graphene/SnO2. Raman spectroscopy is a powerful tool to characterize carbonaceous materials. As seen from Figure 4, the Raman spectrum of GO contains both G band (1591.3 cm-1, E2g phonon of C sp2 atoms) and D bands (1365.6 cm-1, κ-point phonons of A1g symmetry).29 The Raman spectra of the SnO2-nanocrystal/ graphene-nanosheets composites also contain both G and D bands. The D/G intensity ratios of these three samples (1.13, 1.25, and 1.34 for SnO2/G-0.3, SnO2/G-0.6, and SnO2/G-1.2, respectively) were larger than that of GO, and they increased with the increase of the Sn2+/GO ratio. This suggests a decrease in the average size of the sp2 domains upon reduction of the exfoliated GO,30 and it is consistent with the results observed from the TEM images. X-ray photoelectron spectroscopy (XPS) was employed to analyze graphene oxide and as-prepared composites. Figure S2 in the Supporting Information shows the XPS spectra of GO and SnO2/G-0.3 nanocomposites. The peaks of tin (Sn 3p, 3d, 4s, 4p, 4d) in the SnO2/G-0.3 composite spectra can be observed, which are attributed to SnO2.23 Compared to O 1s spectrum of GO, one obvious peak shift to 531.4 eV was observed; it is due to the existence of O2- species in the SnO2/G-0.3 composites. The Sn 3d5/2(487.3 eV) and Sn 3d3/2 (495.8 eV, Figure 6) further confirmed the formation of SnO2.31 The peak of C 1s is

21772

J. Phys. Chem. C, Vol. 114, No. 49, 2010

Li et al.

Figure 3. HRTEM image of SnO2/G-0.3 (a), SnO2/G-0.6 (b), and SnO2/G-1.2 (c); FT-IR spectra of graphite oxide (black line), sample SnO2/G-0.3 (red line), sample SnO2/G-0.6 (blue line), and sample SnO2/G-1.2 (green line) (d).

Figure 4. Raman spectra of graphite oxide (black line), sample SnO2/ G-0.3 (red line), sample SnO2/G-0.6 (blue line), and sample SnO2/G1.2 (green line).

attributed to graphene sheets. In comparison to the C 1s spectrum of the GO, that of SnO2/G-0.3 sample showed sharply decreased intensity for peak(s) corresponding to the epoxy/ether group (286.9 eV, Figure 5), supporting that these oxygencontaining groups have been removed by Sn2+ ion. This proved that the Sn2+ ions lead to the deoxygenation process, and this trend is the same for SnO2/G-0.6 and SnO2/G-1.2. In order to quantify the mass percentage of SnO2, the asprepared composites were analyzed by TGA. As shown in the TGA curves (Figure 6), SnO2 mass percent is about 63, 68, and 74% for SnO2/G-0.3, SnO2/G-0.6, and SnO2/G-1.2, respectively. The SnO2 mass fraction did not increase as quickly as the increase of Sn2+/GO ratio, suggesting the partial loss of SnCl2 during the reaction. To evaluate their abilities in Li storage, the samples were used as the anodes for Li-ion battery. The charge/discharge profiles of SnO2/graphene-nanosheets electrode at a current

Figure 5. XPS spectra of (A) O 1s of the GO and SnO2/G-0.3 composites, (B) Sn 3d doublet, (C) C 1s of the GO, and (D) C 1s of SnO2/G-0.3 composites.

Preparation of SnO2-Nanocrystal/Graphene-Nanosheets Composites

Figure 6. TGA curve of SnO2/G-0.3 (red line), SnO2/G-0.6 (blue line), and SnO2/G-1.2 (green line).

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21773 the reversible capacity, and SnO2/G-0.6 and SnO2/G-1.2 exhibited about 34.2 and 33.6% retention of the reversible capacity, respectively. The good retention of the reversible capacity for SnO2/G-0.3 demonstrated the excellent charge-discharge performance for a large current density. The difference among these three samples demonstrated that the cycle performance was related to the different compositions and different morphologies of the samples. It is well known that the main reason for a rapid fading of SnO2 electrode is that a large volume expansion of the SnO2 occurs during the charge-discharge cycle, leading to the pulverization of the electrode.32 The bare SnO2 nanoparticles have a poor cycle performance;21,22 the improved performance observed in our experiments should be attributed to the combination of SnO2 and graphene nanosheets. Graphene nanosheets themselves could store Li and act as electronic conductor. Furthermore, the graphene nanosheets in the composites can limit the volume expansion upon lithium insertion. As shown in TEM images, there is less SnO2 nanocrystals on the surface of graphene nanosheets with larger sizes (SnO2/G0.3) than on the surface of graphene nanaosheets with smaller sizes (SnO2/G-0.6 and SnO2/G-1.2), where the volume expansion of less SnO2 can be easily confined by the larger graphene nanosheets. On the other hand, the graphene nanosheets with large sizes can help to build a better conductive network which is favorable for electron transportation. 4. Conclusion In conclusion, a facile method to synthesize SnO2-nanocrystal/ graphene-nanosheets composites was developed. The synthesis strategy is based on the reduction of GO with Sn2+ ion that combines tin oxidation and GO reduction in one step, which provides a simple, low-cost, and effective way to prepare SnO2/ graphene-nanocomposites because no additional chemicals were needed. It is found that changing the ratio of Sn2+ and GO could lead to changes of the morphology. The electrochemical experiments show that a larger graphene sheet size and suitable SnO2 dispersion provide a better Li-storage performance. When considering the plentiful properties for both SnO2 and graphene, the composites might have potential applications in biosensor, gas sensor, and electrochemical analysis in the future. Furthermore, the methodology described in this report may provide a simple, economic, and effective strategy for the preparation of other graphene/metal-oxide composites. Acknowledgment. This work was supported by National Natural Science Foundation of China (No. 11079002) and National Basic Research Program of China (No. 2007CB310500 and No. 2011CB935700). The authors thank Dr. Xue-Mei Li at Nanjing University of Technology for her helpful discussions.

Figure 7. (A) Charge-discharge profiles and (B) cycle performance plot of SnO2/G-0.3 (red line), SnO2/G-0.6 (blue line), and SnO2/G-1.2 (green line) at the current density of 200 mA/g.

density of 200 mA/g and a voltage cutoff of 1.5/0.01 V versus Li/Li + are shown in Figure 7. All these three samples have a large and irreversible capacity, and the reversible capacities of the SnO2/graphene composites are 541.3, 402.7, and 222.4 mAh/ g, respectively. The irreversible initial capacity loss is due to the formation of amorphous Li2O matrix and intense surface reactions with the Li-Sn compounds and the electrolyte solution.32,33 After 35 cycles, the capacity of SnO2/G-0.3 remained at 377.3 mAh/g, which was about 69.7% retention of

Supporting Information Available: TEM image of GO, XPS spectrum of GO and SnO2/G-0.3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359. (2) Armand, M.; Tarascon, J. M. Nature 2008, 451, 652. (3) Chan, C. K.; Peng, H. L.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. Nature Nanotechnol. 2008, 3, 31. (4) Arico`, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Schalkwijk, V. W. Nat. Mater. 2005, 4, 366. (5) Cheng, B.; Russell, J. M.; Shi, W. S.; Zhang, L.; Samulski, E. T. J. Am. Chem. Soc. 2004, 126, 5972. (6) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. AdV. Funct. Mater. 2003, 13, 9.

21774

J. Phys. Chem. C, Vol. 114, No. 49, 2010

(7) Zhang, D. F.; Sun, L. D.; Jia, C. J.; Yan, Z. G.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2005, 127, 13492. (8) Winter, M.; Besenhard, J. O. 1999, 45, 31. (9) Larcher, D.; Beattie, S.; Morcrette, M.; Edstroem, K.; Jumas, J. C.; Tarascon, J. M. J. Mater. Chem. 2007, 17, 3759. (10) Yang, J.; Takeda, Y.; Imanishi, N.; Yamamoto, O. J. Electrochem. Soc. 1999, 146, 4009. (11) Guo, Z. P.; Du, G. D.; Nuli, Y. N.; Hassanb, M. F.; Liu, H. K. J. Mater. Chem. 2009, 19, 3253. (12) Wang, Y.; Su, F.; Lee, J. Y.; Zhao, X. S. Chem. Mater. 2006, 18, 1347. (13) Park, M. S.; Wang, G. X.; Kang, Y. M.; Wexler, D.; Dou, S. X.; Liu, H. K. Angew. Chem., Int. Ed. 2007, 46, 750. (14) Zhao, N.; Wang, G.; Huang, Y.; Wang, B.; Yao, B.; Wu, Y. Chem. Mater. 2008, 20, 2612. (15) Wang, C.; Zhou, Y.; Ge, M. Y.; Xu, X. B.; Zhang, Z. L.; Jiang, J. Z. J. Am. Chem. Soc. 2010, 132, 46. (16) Lou, X. W.; Li, C. M.; Archer, L. A. AdV. Mater. 2009, 21, 2536. (17) Yuan, L.; Konstantinov, K.; Wang, G. X.; Liu, H. K.; Dou, S. X. J. Power Sources 2005, 146, 180. (18) Wen, Z. H.; Wang, Q.; Zhang, Q.; Li, J. H. AdV. Funct. Mater. 2007, 17, 2772. (19) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666.

Li et al. (20) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282. (21) Paek, S. M.; Yoo, E. J.; Honma, I. Nano Lett. 2009, 9, 72. (22) Yao, J.; Shen, X. P.; Wang, B.; Liu, H. K.; Wang, G. X. Electrochem. Commun. 2009, 11, 1849. (23) Li, F. H.; Song, J. F.; Yang, H. F.; Gan, S. Y.; Zhang, Q. X.; Han, D. X.; Ivaska, A.; Niu, L. Nanotechnology 2009, 20, 455602. (24) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771. (25) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (26) Hontoria-Lucas, C.; Lopez-Peinado, A. J.; Lopez-Gonzalez, J. D. D.; Rojas-Cervantes, M. L.; Martin-Aranda, R. M. Carbon 1995, 33, 1585. (27) Bissessur, R.; Liu, P. K. Y.; White, W.; Scully, S. F. Langmuir 2006, 22, 1729. (28) Peckett, J.; Trens, P.; Gougeon, R.; Poppl, A.; Harris, R. Carbon 2000, 38, 345. (29) Ferrari, A. C.; Robertson, J. Phy. ReV. B 2000, 61, 14095. (30) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53, 1126. (31) Ahn, H. J.; Choi, H. C.; Park, K. W.; Kim, S. B.; Sun, Y. E. J. Phys. Chem. B 2004, 108, 9815. (32) Kim, M. G.; Cho, J. AdV. Funct. Mater. 2009, 19, 1497. (33) Deng, D.; Kim, M. G.; Lee, J. Y.; Cho, J. Energy EnViron. Sci. 2009, 2, 818.

JP1050047