DOI: 10.1002/chem.201406678
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& Electrochemistry
Synthesis of Mesoporous Wall-Structured TiO2 on Reduced Graphene Oxide Nanosheets with High Rate Performance for Lithium-Ion Batteries Mengmeng Zhen,[a] Meiqing Sun,[a] Guandao Gao,[a] Lu Liu,*[a] and Zhen Zhou*[b] development of mesoporous materials has attracted great attention, because of their wide use in various areas, such as electrodes, catalysts, and advanced energy storage materials. Mesoporous wall-structured materials are a new type of mesoporous material.[10–12] Many different kinds of mesoporous inorganic materials, such as TiO2, MnO2, Fe2O3, and Co3O4, with ordered pores and crystalline walls have been prepared by using various templates.[10–12] The novel mesoporous materials have shown better performance than general mesoporous materials, Yang et al.[13] synthesized aluminium-containing mesoporous benzene-silicas with crystal-like pore wall structure and it exhibits higher catalytic activity than the same material without the pore wall structure under identical reaction conditions. The mesoporous wall structure, combined with the virtues of the TiO2 materials, is one of the main reasons that mesoporous wall-structured TiO2 materials are one of the most promising anode materials.[14] However, the agglomeration and dissolution of TiO2 nanomaterials during charge/discharge cycles are still major obstacles for achieving superior rate capability.[15] To solve these problems, the addition of conductive phases, such as carbon, polymers, and metals, is always necessary. Compared with conventional conductive additives, graphene, a single-layer, twodimensional sheet of sp2-bonded carbon atoms, exhibits superior electrical conductivity, large surface area, structural flexibility, and chemical stability.[16, 17] Reduced graphene oxide (RGO) is a graphene derivative that is a preferred conductive agent for various applications,[17, 18] including LIBs.[19, 20] Li et al.[21] used a hydrothermal method to synthesize mesoporous TiO2 nanospheres/graphene composites, which exhibited a higher capacity of 200 mAh g 1 at 1 C after 100 cycles. Ding et al.[22] used solvothermal treatment to control the orientation of TiO2 nanosheets loaded on graphene and grew the highly active facet directly onto the TiO2 nanosheets/graphene composites, which exhibit a higher capacity 180 mAh g 1 at 1 C after 120 cycles. Together with relatively complicated experimental procedures, the practical application is not easily achieved, although the performance of TiO2/graphene composites can be significantly improved in these works.[23] Therefore, finding a simple and cost-effective way for preparing TiO2/graphene anode materials that possess satisfactory nanostructure and excellent electrochemical performance is extremely urgent. To the best of our knowledge, there are few reports on the synthesis of RGO sheets–supported anatase TiO2 with stable mesoporous wall structure, which has an advantage with respect to the electrochemical properties. In this work, we syn-
Abstract: Mesoporous wall-structured TiO2 on reduced graphene oxide (RGO) nanosheets were successfully fabricated through a simple hydrothermal process without any surfactants and annealed at 400 8C for 2 h under argon. The obtained mesoporous structured TiO2–RGO composites had a high surface area (99 0307 m2 g 1) and exhibited excellent electrochemical cycling (a reversible capacity of 260 mAh g 1 at 1.2 C and 180 mAh g 1 at 5 C after 400 cycles), demonstrating it to be a promising method for the development of high-performance Li-ion batteries.
Rechargeable lithium-ion batteries (LIBs) as energy storage devices have attracted tremendous attention in the scientific and industrial communities, due to their individual characteristics in the fields of high energy density, low self-discharge, light weight, and long cycling life.[1–4] Recently, LIBs have increasing requirement for excellent rate capability, higher capacity, stronger cyclic stability, and higher power density, especially for electrical/hybrid vehicle applications.[5] TiO2 and TiO2-based composite materials have been widely prepared and applied in lithium-ion batteries, owing to their enhanced capacity retention, small volume change, lower self-discharge rate, and high safety during lithium insertion/extraction.[6, 7] Looking for methods to further promote the electrode reaction rates, specific capacities and cycling stability of rechargeable LIBs based on TiO2 materials are a scientific problem that needs to be solved.[8] Nanosizing, carbon coating, and construction of a mesoporous architecture in the material are the major methods currently used to improve the rate performance.[8, 9] Recently, the [a] M. M. Zhen, M. Q. Sun, Prof. G. D. Gao, Prof. L. Liu Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering Nankai University, Tianjin 300071 (P.R. China) E-mail: [email protected] [b] Prof. Z. Zhou Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) Nankai University, Tianjin 300071 (P.R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201406678 and from the author. Chem. Eur. J. 2015, 21, 1 – 7
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Communication thesized mesoporous wall-structured TiO2 on reduced graphene oxide nanosheets by a simple hydrothermal reaction between NaOH and TiCl3, followed by the addition of graphene oxide (GO) solution in another hydrothermal process and subsequent heat treatment in argon. The current system avoids the use of templates and complicated experimental procedures. More importantly, the as-prepared mesoporous wall-structured TiO2 on reduced graphene oxide nanosheets had a high surface area and presented an excellent lithium storage performance, and ultralong cycle life. This work is favorable to explore advanced TiO2-based composites as anodes for LIBs with high power density. The as-prepared GO, pure TiO2, and TiO2–RGO composites were characterized by using X-ray diffraction (XRD) (Figure 1). The XRD pattern of as-prepared GO had a characteristic peak
Figure 2. SEM (A and B) and TEM (C, D, E and F) images of the TiO2–RGO composites.
ure 2 F) demonstrates that the crystalline lattice distances in the red is 0.352 nm, corresponding to the (101) plane of anatase TiO2, which is consistent with the XRD results. To further understand the structure of as-synthesized TiO2– RGO composites, we used the N2 adsorption-desorption measurement (Figure 3 and inset) for the composites. As seen in Figure 1. XRD patterns of GO, pure TiO2 and TiO2–RGO composites.
at ~ 108, which corresponds to the (001) plane of GO sheets.[19] The atomic force microscopy (AFM) image (Figure S1 in the Supporting Information) indicated the height of the graphene oxides to be 0.35–1 nm, which revealed that the graphene oxide sheets were either mono- or bi-layered. The XRD patterns of pure TiO2 and TiO2–RGO composites revealed that all the peaks can be well assigned to the pure anatase phase according to JCPDS 21-1272. No characteristic XRD peaks of GO and RGO were observed in the TiO2–RGO nanostructures (Figure 1). This result was attributed to the destruction of the regular stacking of GO upon anchoring the TiO2 nanosheets.[24] Scanning electron microscopy (SEM) images of the TiO2– RGO composites, presented in Figure 2, show the morphology and size. Figure 2 A shows that the TiO2 nanobulks were wrapped by RGO sheets. The TiO2–RGO composites consist of nanobulks with a width of 0.1–0.3 mm and a length of 0.3–1 mm. The high-magnification SEM image (Figure 2 B) shows that the TiO2–RGO had very coarse surfaces and they were formed from nanocrystals. The transmission electron microscopy (TEM) images were used to further observe the morphology and structure of TiO2–RGO composites. The low-resolution TEM image (Figure 2 C D) shows that TiO2 materials were very well distributed on the large surface of RGO nanosheets. This nanoarchitecture is beneficial to ensure all TiO2 materials participate in the electrochemical reaction and ensures fast electron transportation. Figure 2 D and E clearly showed that a single TiO2 nanobulk was highly porous and composed of interconnected nanocrystals. The high-resolution TEM (HRTEM) image (Fig&
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Figure 3. Nitrogen adsorption-desorption isotherms of the TiO2–RGO composites and the pore size distribution (inset).
Figure 3, the isotherm curves of TiO2–RGO composites present the typical adsorption hysteresis that belongs to type IV isotherm curves, indicating that the composites had a mesoporous structure[25] and a high surface area (99 0307 m2 g 1), which may mainly resulted from mesoporous TiO2 and RGO nanosheets. The pore-size distribution, obtained from the isotherm, indicated a number of pores less than 4 nm in the samples. These pores presumably arise from the spaces among the nanocrystals. More importantly, the mesoporous architectures and high surface area would be beneficial for the electrochemical performance of TiO2–RGO composites.[26] 2
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Communication The electrochemical cycle performances of the as-prepared TiO2–RGO composites as anode materials for LIBs were investigated by galvanostatic discharge/charge tests. For comparison, the electrochemical cyclic performances of the pure TiO2 electrodes, without adding GO and RGO, were also assembled and tested under the same conditions. Figure 4 shows the discharge/charge curves for the TiO2–RGO composites (Figure 4 A) samples and pure TiO2 (Figure 4 B) at a current density of 1.2 C
composites was only 9.5 wt % (Figure S2 in the Supporting Information). Figure S3 (see the Supporting Information) shows the reversible capacity of RGO nanosheets only kept at 190 mAh g 1 at 1.2 C after 100 cycles, and based on its low content in the TiO2–RGO composites, the contribution from RGO nanosheets was very limited, indicating the high reversible capacity of TiO2–RGO composites was not only caused by RGO nanosheets. The difference in the reversible capacity of the pure TiO2 and TiO2–RGO composites was even more remarkable with various current rates. The TiO2–RGO composites exhibited a superior rate performance, with 325 mAh g 1 at 0.6 C, 285 mAh g 1 at 1.2 C, 203 mAh g 1 at 5 C, and 177 mAh g 1 at 10 C (Figure 5 A). More importantly, no clear capacity loss happened when the rate was reduced to 1.2 C. Compared with TiO2– RGO composites, pure TiO2 exhibited a poor rate performance. When cycled at 0.6 C, 1.2 C, 5 C, 10 C, the reversible capacities were 184, 145, 86, and 65 mAh g 1, reFigure 4. Discharge/charge profiles of A) TiO2–RGO composites and B) pure TiO2, cycling performance of C) TiO2– spectively, indicating TiO2–RGO RGO composites and D) pure TiO2. composites also exhibited excellent performances at high (1 C = 167.5 mA g 1) in the potential window of 0.01– rates due to the favorable fea3 V. The potential plateaus around 1.75 V (Li + insertion) and 2.0 V (Li + extraction) for TiO2–RGO composites and pure TiO2 are clearly observed, and the difference of plateau voltage indicates that the TiO2/ RGO electrode had low polarization and good reaction kinetics. The redox potential plateaus of pure TiO2 rapidly became smaller and disappeared in the 100th cycle, due to the large size and intrinsically poor electronic conductivity. The initial discharge/ charge capacities of TiO2–RGO composites were 685/ 332 mAh g 1 and still kept at 240 mAh g 1 after 400 cycles at a current density of 1.2 C (Figure 4 C). More importantly, the reversible capacities of TiO2– Figure 5. A) Rate capabilities of pure TiO and TiO –RGO composites, B) Nqyuist plots for 2 2 RGO composites showed only minor capacity fading the EIS data of pure TiO2 and TiO2–RGO composites at different states. Inset: the correand exhibited a high specific capacity of 180 mAh g 1 sponding equivalent circuit mode of two samples. even at 5 C current after 400 cycles (Figure 4 C), indicating that the TiO2–RGO composites exhibit an extures of the superior hybrid nanostructures. cellent cycling stability and high reversible capacity. Compared Lithium-ion insertion/extraction properties of the TiO2–RGO with TiO2–RGO composites, pure TiO2 exhibited a lower discharge/charge capacity of 341/279 mAh g 1 at the initial cycle composites electrode were investigated using the CV technique. In the first five cycles, the samples exhibited a pair of (Figure 4 D) and inferior lithium storage performance (reversicathodic/anodic peaks at ~ 1.73 and 2.02 V, which can be asble capacity was 130 mAh g 1 at 1.2 C and 65 mAh g 1 at 5 C cribed to the Li-ion insertion and extraction in an anatase TiO2 after 100 cycles, see Figure 4 c). Thermogravimetry analysis (TGA) further confirmed that the carbon content in TiO2–RGO lattice, respectively. The cathodic/anodic peaks were in accordChem. Eur. J. 2015, 21, 1 – 7
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Communication ance with the discharge/charge profiles in Figure S4 in the Supporting Information. The upholding of the cathodic/anodic peak positions suggested that the reversibility of the TiO2 was well-maintained in the TiO2–RGO composites.[27] To gain insight into the remarkable rate performance of the TiO2–RGO composites, we performed electrochemical impedance spectroscopy (EIS) on the cells comprising pure TiO2 and TiO2–RGO composites as the working electrode. As shown in Figure 5 B, the Nyquist plots display an inclined line in the lowfrequency range and only one semicircle in the high-frequency range. The inclining line and high frequency semicircle correspond to the Li + diffusion process and charge-transfer impedance in the electrode/electrolyte interface, respectively.[28] Before cycling and after 400 cycles, the EIS profiles of TiO2– RGO composites exhibited a smaller semicircle in the high-frequency range and an inclined line with a larger slope than pure TiO2 and the corresponding equivalent circuit mode is shown in the inset (Figure 5). In this equivalent circuit, Rs represent electrolyte resistance and Rct represent charge-transfer resistance,[29] the two indices were calculated by the equivalent circuit in Table 1. The values of the Rs and Rct before cycling
xe QLixTiO2, in which x depends on the crystallography and microstructure of the materials. Because the poor electronic conductivity and Li + diffusivity, x is limited to 0.5 in the anatase phases of TiO2.[25, 32 ] Herein, as-prepared TiO2–RGO composites can reversibly store and release 0.7 Li + per TiO2, even at 5 C current after 400 cycles, which can be attributed to the following points. Firstly, the as-prepared TiO2–RGO composites had a mesoporous wall structure and high surface area. It is well-known that the high surface area and porous structure can accelerate the transport of lithium ions across the electrolyte/electrode interface and provide short path lengths with less resistance for both lithium ions and electron transport within the electrolyte. The high surface area and porous structure are beneficial for easy electrolyte immersion and diffusion and can offer a high electrolyte/electrode contact area.[26, 33–35] Especially, mesoporous wall-structured TiO2 showed excellent performance relative to nanoparticles and bulk TiO2 for Li + insertion/extraction because the mesoporous structure promote the electron transport along the long dimension and the two short dimensions ensure fast Li + insertion/extraction.[36, 37] Secondly, the composite structure of adding RGO nanosheets can minimize TiO2 agglomeration and RGO restacking, which helps maintain a good cycling stability and high surface areas during lithium insertion/extraction.[24, 38] Moreover, the highly conductive RGO matrix with large surface area is beneficial to the quality of the electrode-electrolyte contact.[16] More importantly, the cycle stabilities of as-prepared TiO2–RGO composites at high rates indicated the ultrafast diffusion of lithium ions in bulk, because of the mesoporous wall structure, short diffusion path length, high surface area, and stability of the graphene structure.[39] The synergistic effect of TiO2 nanobelts and RGO nanosheets leads to excellent lithium storage performance, and strong cycling stability of the TiO2–RGO composites. In summary, mesoporous wall-structured TiO2 on reduced graphene oxide nanosheets were successfully fabricated through a simple and steerable method. The as-prepared mesoporous wall-structured TiO2 on reduced graphene oxide nanosheets had a high surface area and outstanding electrochemical performance. Compared with pure TiO2 and other TiO2–graphene composites prepared through hydrothermal/solvothermal methods, the as-prepared TiO2–RGO composites exhibited excellent lithium storage performance, highly reversible capacity, and strong cycling stability. More importantly, the as-prepared TiO2–RGO composites still showed a higher reversible capacity (177 mA h g 1 at 10 C current). The simple and steerable synthesis, excellent electrochemical cycling, and the mesoporous wall structure could provide a new direction for developing higher-performance lithium storage electrodes.
Table 1. The values of the Rs and Rct calculated from EIS spectra (in Figure 5 B) Samples
Cycles
Rs [W]
Rct [W]
pure TiO2
0 400 0 400
5.64 5.23 3.84 3.24
350.3 183.2 94.97 25.21
TiO2–RGO
and after 400 cycles for TiO2–RGO composites were remarkably lower than those of the pure TiO2. Compared with pure TiO2, TiO2–RGO composites possessed a smaller Rct value, indicating that the synergistic effect of RGO nanosheets and nanostructured TiO2 can effectively improve the electron transport and Li + diffusivity, and can lead to higher discharge/charge capacities of TiO2–RGO composites than pure TiO2. These electrochemical performances of as-prepared TiO2– RGO composites were superior to other TiO2–graphene composites prepared by hydrothermal/solvothermal methods.[27, 30] For examples, Ren et al.[30] synthesized TiO2/carbon/RGO composites through hydrothermal reaction and the sample exhibited a relatively lower reversible capacity (158 mA h g 1 at 5 C current after 100 cycles); Zhang and co-workers[27] reported that anatase TiO2 nanoparticle/graphene displayed relatively lower capacity and weaker cycling stability (200 mAh g 1 at 0.6 C after 100 cycles). Transition-metal oxides, such as Fe2O3, NiO, and CoO3, have been extensively studied as anode materials for LIBs because of their higher theoretical capacities.[31] Compared with these high-performance materials, although they have higher theoretical capacities than TiO2, the as-prepared TiO2-RGO composite possessed excellent cycle stability and high-rate performance, due to its high surface area and mesoporous wall structure. Also notable was that the Li + insertion/extraction mechanism can be expressed as TiO2 + xLi + + &
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Experimental Section Preparation of GO nanosheets GO nanosheets were prepared through the oxidation of graphite according to the improved Hummers’ method. Briefly, graphite powder (2.0 g) and KMnO4 (6.0 g) were dispersed in sequence into cooled H2SO4 (100 mL, 98 %, 0 8C) while stirring vigorous. Then, the
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opment, NSFC (21273118), Tianjin Municipal Science and Technology Commission (11JCZDJC24800) and MOE Innovation Team (IRT13022) in China. Keywords: anodes · electrochemistry · Li-ion batteries · mesoporous wall · nanostructures [1] J. M. Tarascon, M. Armand, Nature 2001, 414, 359 – 367. [2] Y. C. Qiu, K. Y. Yan, S. H. Yang, L. M. Jin, H. Deng, W. S. Li, ACS Nano 2010, 4, 6515 – 6526. [3] J. Y. Liao, D. Higgins, G. Lui, V. Chabot, X. C. Xiao, Z. W. Chen, Nano Lett. 2013, 13, 5467 – 5473. [4] H. Xia, W. Xiong, C. K. Lim, Q. Yao, Y. Wang, J. Xie, Nano Res. 2014, 7, 1797 – 1808. [5] J. S. Chen, L. A. Archer, X. W. Lou, J. Mater. Chem. 2011, 21, 9912 – 9924. [6] Z. Ali, S. N. Cha, J. I. Sohn, I. Shakir, C. Z. Yan, J. M. Kim, D. J. Kang, J. Mater. Chem. 2012, 22, 17625 – 17629. [7] T. Song, H. Han, H. Choi, J. W. Lee, H. Park, S. Lee, W. I. Park, S. Kim, L. Liu, U. Paik, Nano Res. 2014, 7, 491 – 501. [8] J. Y. Shen, H. Wang, Y. Zhou, N. Q. Ye, L. J. Wang, CrystEngComm 2012, 14, 6215 – 6220. [9] M. J. Armstrong, C. O’Dwyer, W. J. Macklin, J. D. Holmes, Nano Res. 2014, 7, 1 – 62. [10] M. Kim, K. Sohn, J. Kim, T. Hyeon, Chem. Commun. 2003, 652 – 653. [11] F. Jiao, A. Harrison, J. C. Jumas, A. V. Chadwick, W. Kockelmann, P. G. Bruce, J. Am. Chem. Soc. 2006, 128, 5468 – 5474. [12] B. T. Yonemoto, G. S. Hutchings, F. Jiao, J. Am. Chem. Soc. 2014, 136, 8895 – 8898. [13] Q. H. Yang, J. Yang, Z. C. Feng, Y. Li, J. Mater. Chem. 2005, 15, 4268 – 4274. [14] H. Huang, J. W. Fang, Y. Xia, X. Y. Tao, Y. P. Gan, J. Du, W. J. Zhu, W. K. Zhang, J. Mater. Chem. A 2013, 1, 2495 – 2500. [15] H. Q. Cao, B. J. Li, J. X. Zhang, F. Lian, X. H. Kong, M. Z. Qu, J. Mater. Chem. 2012, 22, 9759 – 9766. [16] C. J. Chen, X. L. Hu, Y. Jiang, Z. Yang, P. Hu, Y. H. Huang, Chem. Eur. J. 2014, 20, 1383 – 1388. [17] D. C. Elias, R. R. Nair, T. M. G. Mohiuddin, S. V. Morozov, P. Blake, M. P. Halsall, A. C. Ferrari, D. W. Boukhvalov, M. I. Katsnelson, A. K. Geim, K. S. Novoselov, Science 2009, 323, 610 – 613. [18] L. Li, A. Kovalchuk, J. M. Tour, Nano Res. 2014, 7, 1319 – 1326. [19] Q. Tang, Z. Zhou, Z. F. Chen, Nanoscale 2013, 5, 4541 – 4583. [20] X. Zhao, M. J. Li, K. H. Chang, Y. M. Lin, Nano Res. 2014, 7, 1429 – 1438. [21] N. Li, G. Liu, C. Zhen, F. Li, L. L. Zhang, H. M. Cheng, Adv. Funct. Mater. 2011, 21, 1717 – 1722. [22] S. J. Ding, J. S. Chen, D. Luan, F. Y. C. Boey, S. Madhavi, X. W. Lou, Chem. Commun. 2011, 47, 5780 – 5782. [23] X. Xin, X. F. Zhou, J. H. Wu, X. Y. Yao, Z. P. Liu, ACS Nano 2012, 6, 11035 – 11043. [24] V. Etacheri, J. E. Yourey, B. M. Bartlett, ACS Nano 2014, 8, 1491 – 1499. [25] L. F. He, R. G. Ma, N. Du, J. G. Ren, T. L. Wong, Y. Y. Li, S. T. Lee, J. Mater. Chem. 2012, 22, 19061 – 19066. [26] Y. Xiao, C. W. Hu, M. H. Cao, Chem. Asian J. 2014, 9, 351 – 356. [27] J. X. Qiu, P. Zhang, M. Ling, S. Li, P. Liu, H. J. Zhao, S. Q. Zhang, ACS Appl. Mater. Interfaces 2012, 4, 3636 – 3642. [28] L. W. Su, Z. Zhou, M. M. Ren, Chem. Commun. 2010, 46, 2590 – 2592. [29] Y. Wang, H. Liu, K. Wang, H. Eiji, Y. Wang, H. Zhou, J. Mater. Chem. 2009, 19, 6789 – 6795. [30] Y. M. Ren, J. Zhang, Y. Y. Liu, H. B. Li, H. J. Wei, B. J. Li, X. Y. Wang, ACS Appl. Mater. Interfaces 2012, 4, 4776 – 4780. [31] S. Z. Huang, J. Jin, Y. Cai, Y. Li, H. Y. Tan, H. E. Wang, G. Van Tendeloo, B. L. Su, Nanoscale 2014, 6, 6819 – 6827. [32] M. Zukalov, M. Kalbˇ, L. Kavan, I. Exnar, M. Graetzel, Chem. Mater. 2005, 17, 1248 – 1255. [33] Z. Li, Z. W. Xu, X. H. Tan, H. L. Wang, C. M. B. Holt, T. Stephenson, B. C. Olsen, D. Mitlin, Energy Environ. Sci. 2013, 6, 871 – 878. [34] W. Jiao, N. Li, L. Z. Wang, L. Wen, F. Li, G. Liu, H. M. Cheng, Chem. Commun. 2013, 49, 3461 – 3463. [35] H. G. Jung, C. S. Yoon, J. Prakash, Y. K. Sun, J. Phys. Chem. C 2009, 113, 21258 – 21263.
Synthesis of TiO2-RGO composites In a typical synthesis, TiCl3 (4 mL) was added dropwise into NaOH solution (40 mL, 10 m) under vigorous stirring and then the mixture was stirred for a further 1 h. The mixture was transferred into a Teflon-lined autoclave (50 mL) and kept at 180 8C for 48 h. After it naturally cooled to room temperature, the resulting solid was centrifuged and washed with HCl solution (0.1 m) and distilled water until the pH value was about 7, and then dried at 90 8C for 10 h, resulting in the production of intermediate. The as-prepared intermediate (0.2 g) and GO (3 mL, 10 g L 1) were added into HCl solution (20 mL, 0.1 m), then the mixture was transferred into a Teflonlined autoclave (30 mL). The autoclave was kept at 180 8C for 6 h and left to naturally cool to room temperature. The obtained precipitate was isolated from the solution by centrifugation and subsequently washed with deionized water and then dried at 90 8C. The TiO2–RGO composites were prepared by annealing the obtained precipitate at 400 8C for 2 h under argon.
Material characterization The obtained samples were characterized by X-ray diffraction (XRD, Rigaku D/Max III diffractometer with Cu K radiation, l = 1.5418 ), scanning electron microscopy (SEM, Nova Nano SEM 230), transmission electron microscopy (TEM, Tecnai G2F20, FEI), high-resolution TEM (HRTEM, Tecnai G2F20, FEI), atomic force microscopy (AFM, MMAFM/STM, D3100M, Digital Ltd.), and thermogravimetry analysis (TGA, Rigaku PTC-10A TG-DTA analyzer).
Electrochemical measurements For electrochemical tests, the working electrodes were prepared with active materials, acetylene black (AB), and polytetrafluoroethylene (PTFE) at at weight ratio of 80:10:10. The average weight of the electrodes was approximately 2 mg. In the test cells, lithium metal was used as the counter and reference electrode. The electrolyte was LiPF6 (1 m), dissolved in a 1:1:1 mixture of ethylene carbonate (EC), ethylene methyl carbonate (EMC), and dimethyl carbonate (DMC). The cells were assembled in a glovebox filled with high-purity argon. The galvanostatic charge and discharge experiments were performed with a battery tester LAND-CT2001A in the voltage range of 0.001–3.0 V at room temperature. Cyclic voltammetry measurements were carried out at a scan rate of 0.01 mV s 1 between 0.01–3.0 V (vs. Li/Li + ). EIS was taken by using an IM6e electrochemical workstation at 25 8C with the frequency range from 10 kHz to 100 mHz and an AC signal of 5 mV in amplitude as the perturbation. The specific capacity was calculated according to the corresponding total weight of active materials in each electrode.
Acknowledgements This work was supported by NSFC (21271108), China–US Center for Environmental Remediation and Sustainable DevelChem. Eur. J. 2015, 21, 1 – 7
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Communication [36] J. M. Li, W. Wan, H. H. Zhou, J. J. Li, D. S. Xu, Chem. Commun. 2011, 47, 3439 – 3441. [37] P. G. Bruce, B. Scrosati, J. M. Tarascon, Angew. Chem. Int. Ed. 2008, 47, 2930 – 2946; Angew. Chem. 2008, 120, 2972 – 2989. [38] C. X. Peng, B. D. Chen, Y. Qin, S. H. Yang, C. Z. Li, Y. H. Zuo, S. Y. Liu, J. H. Yang, ACS Nano 2012, 6, 1074 – 1081.
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[39] B. C. Qiu, M. Y. Xing, J. L. Zhang, J. Am. Chem. Soc. 2014, 136, 5852 – 5855.
Received: December 31, 2014 Published online on && &&, 0000
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Mesoporous wall-structured TiO2 on reduced graphene oxide nanosheets was successfully fabricated through a simple and steerable two-step hydrothermal process without any surfactants. The obtained mesoporous structured TiO2–reduced graphene oxide composites have high surface areas and exhibit excellent electrochemical cycling (a reversible capacity of 260 mAh g 1 at 1.2 C and 180 mAh g 1 at 5 C after 400 cycles).
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M. M. Zhen, M. Q. Sun, G. D. Gao, L. Liu,* Z. Zhou* && – && Synthesis of Mesoporous WallStructured TiO2 on Reduced Graphene Oxide Nanosheets with High Rate Performance for Lithium-Ion Batteries
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Synthesis of Mesoporous Wall-Structured TiO2 on Reduced Graphene Oxide Nanosheets with High Rate Performance for Lithium-Ion Batteries Mengmeng Zhen,[a] Meiqing Sun,[a] Guandao Gao,[a] Lu Liu,*[a] and Zhen Zhou*[b] development of mesoporous materials has attracted great attention, because of their wide use in various areas, such as electrodes, catalysts, and advanced energy storage materials. Mesoporous wall-structured materials are a new type of mesoporous material.[10–12] Many different kinds of mesoporous inorganic materials, such as TiO2, MnO2, Fe2O3, and Co3O4, with ordered pores and crystalline walls have been prepared by using various templates.[10–12] The novel mesoporous materials have shown better performance than general mesoporous materials, Yang et al.[13] synthesized aluminium-containing mesoporous benzene-silicas with crystal-like pore wall structure and it exhibits higher catalytic activity than the same material without the pore wall structure under identical reaction conditions. The mesoporous wall structure, combined with the virtues of the TiO2 materials, is one of the main reasons that mesoporous wall-structured TiO2 materials are one of the most promising anode materials.[14] However, the agglomeration and dissolution of TiO2 nanomaterials during charge/discharge cycles are still major obstacles for achieving superior rate capability.[15] To solve these problems, the addition of conductive phases, such as carbon, polymers, and metals, is always necessary. Compared with conventional conductive additives, graphene, a single-layer, twodimensional sheet of sp2-bonded carbon atoms, exhibits superior electrical conductivity, large surface area, structural flexibility, and chemical stability.[16, 17] Reduced graphene oxide (RGO) is a graphene derivative that is a preferred conductive agent for various applications,[17, 18] including LIBs.[19, 20] Li et al.[21] used a hydrothermal method to synthesize mesoporous TiO2 nanospheres/graphene composites, which exhibited a higher capacity of 200 mAh g 1 at 1 C after 100 cycles. Ding et al.[22] used solvothermal treatment to control the orientation of TiO2 nanosheets loaded on graphene and grew the highly active facet directly onto the TiO2 nanosheets/graphene composites, which exhibit a higher capacity 180 mAh g 1 at 1 C after 120 cycles. Together with relatively complicated experimental procedures, the practical application is not easily achieved, although the performance of TiO2/graphene composites can be significantly improved in these works.[23] Therefore, finding a simple and cost-effective way for preparing TiO2/graphene anode materials that possess satisfactory nanostructure and excellent electrochemical performance is extremely urgent. To the best of our knowledge, there are few reports on the synthesis of RGO sheets–supported anatase TiO2 with stable mesoporous wall structure, which has an advantage with respect to the electrochemical properties. In this work, we syn-
Abstract: Mesoporous wall-structured TiO2 on reduced graphene oxide (RGO) nanosheets were successfully fabricated through a simple hydrothermal process without any surfactants and annealed at 400 8C for 2 h under argon. The obtained mesoporous structured TiO2–RGO composites had a high surface area (99 0307 m2 g 1) and exhibited excellent electrochemical cycling (a reversible capacity of 260 mAh g 1 at 1.2 C and 180 mAh g 1 at 5 C after 400 cycles), demonstrating it to be a promising method for the development of high-performance Li-ion batteries.
Rechargeable lithium-ion batteries (LIBs) as energy storage devices have attracted tremendous attention in the scientific and industrial communities, due to their individual characteristics in the fields of high energy density, low self-discharge, light weight, and long cycling life.[1–4] Recently, LIBs have increasing requirement for excellent rate capability, higher capacity, stronger cyclic stability, and higher power density, especially for electrical/hybrid vehicle applications.[5] TiO2 and TiO2-based composite materials have been widely prepared and applied in lithium-ion batteries, owing to their enhanced capacity retention, small volume change, lower self-discharge rate, and high safety during lithium insertion/extraction.[6, 7] Looking for methods to further promote the electrode reaction rates, specific capacities and cycling stability of rechargeable LIBs based on TiO2 materials are a scientific problem that needs to be solved.[8] Nanosizing, carbon coating, and construction of a mesoporous architecture in the material are the major methods currently used to improve the rate performance.[8, 9] Recently, the [a] M. M. Zhen, M. Q. Sun, Prof. G. D. Gao, Prof. L. Liu Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering Nankai University, Tianjin 300071 (P.R. China) E-mail: [email protected] [b] Prof. Z. Zhou Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin) Nankai University, Tianjin 300071 (P.R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201406678 and from the author. Chem. Eur. J. 2015, 21, 1 – 7
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Communication thesized mesoporous wall-structured TiO2 on reduced graphene oxide nanosheets by a simple hydrothermal reaction between NaOH and TiCl3, followed by the addition of graphene oxide (GO) solution in another hydrothermal process and subsequent heat treatment in argon. The current system avoids the use of templates and complicated experimental procedures. More importantly, the as-prepared mesoporous wall-structured TiO2 on reduced graphene oxide nanosheets had a high surface area and presented an excellent lithium storage performance, and ultralong cycle life. This work is favorable to explore advanced TiO2-based composites as anodes for LIBs with high power density. The as-prepared GO, pure TiO2, and TiO2–RGO composites were characterized by using X-ray diffraction (XRD) (Figure 1). The XRD pattern of as-prepared GO had a characteristic peak
Figure 2. SEM (A and B) and TEM (C, D, E and F) images of the TiO2–RGO composites.
ure 2 F) demonstrates that the crystalline lattice distances in the red is 0.352 nm, corresponding to the (101) plane of anatase TiO2, which is consistent with the XRD results. To further understand the structure of as-synthesized TiO2– RGO composites, we used the N2 adsorption-desorption measurement (Figure 3 and inset) for the composites. As seen in Figure 1. XRD patterns of GO, pure TiO2 and TiO2–RGO composites.
at ~ 108, which corresponds to the (001) plane of GO sheets.[19] The atomic force microscopy (AFM) image (Figure S1 in the Supporting Information) indicated the height of the graphene oxides to be 0.35–1 nm, which revealed that the graphene oxide sheets were either mono- or bi-layered. The XRD patterns of pure TiO2 and TiO2–RGO composites revealed that all the peaks can be well assigned to the pure anatase phase according to JCPDS 21-1272. No characteristic XRD peaks of GO and RGO were observed in the TiO2–RGO nanostructures (Figure 1). This result was attributed to the destruction of the regular stacking of GO upon anchoring the TiO2 nanosheets.[24] Scanning electron microscopy (SEM) images of the TiO2– RGO composites, presented in Figure 2, show the morphology and size. Figure 2 A shows that the TiO2 nanobulks were wrapped by RGO sheets. The TiO2–RGO composites consist of nanobulks with a width of 0.1–0.3 mm and a length of 0.3–1 mm. The high-magnification SEM image (Figure 2 B) shows that the TiO2–RGO had very coarse surfaces and they were formed from nanocrystals. The transmission electron microscopy (TEM) images were used to further observe the morphology and structure of TiO2–RGO composites. The low-resolution TEM image (Figure 2 C D) shows that TiO2 materials were very well distributed on the large surface of RGO nanosheets. This nanoarchitecture is beneficial to ensure all TiO2 materials participate in the electrochemical reaction and ensures fast electron transportation. Figure 2 D and E clearly showed that a single TiO2 nanobulk was highly porous and composed of interconnected nanocrystals. The high-resolution TEM (HRTEM) image (Fig&
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Figure 3. Nitrogen adsorption-desorption isotherms of the TiO2–RGO composites and the pore size distribution (inset).
Figure 3, the isotherm curves of TiO2–RGO composites present the typical adsorption hysteresis that belongs to type IV isotherm curves, indicating that the composites had a mesoporous structure[25] and a high surface area (99 0307 m2 g 1), which may mainly resulted from mesoporous TiO2 and RGO nanosheets. The pore-size distribution, obtained from the isotherm, indicated a number of pores less than 4 nm in the samples. These pores presumably arise from the spaces among the nanocrystals. More importantly, the mesoporous architectures and high surface area would be beneficial for the electrochemical performance of TiO2–RGO composites.[26] 2
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Communication The electrochemical cycle performances of the as-prepared TiO2–RGO composites as anode materials for LIBs were investigated by galvanostatic discharge/charge tests. For comparison, the electrochemical cyclic performances of the pure TiO2 electrodes, without adding GO and RGO, were also assembled and tested under the same conditions. Figure 4 shows the discharge/charge curves for the TiO2–RGO composites (Figure 4 A) samples and pure TiO2 (Figure 4 B) at a current density of 1.2 C
composites was only 9.5 wt % (Figure S2 in the Supporting Information). Figure S3 (see the Supporting Information) shows the reversible capacity of RGO nanosheets only kept at 190 mAh g 1 at 1.2 C after 100 cycles, and based on its low content in the TiO2–RGO composites, the contribution from RGO nanosheets was very limited, indicating the high reversible capacity of TiO2–RGO composites was not only caused by RGO nanosheets. The difference in the reversible capacity of the pure TiO2 and TiO2–RGO composites was even more remarkable with various current rates. The TiO2–RGO composites exhibited a superior rate performance, with 325 mAh g 1 at 0.6 C, 285 mAh g 1 at 1.2 C, 203 mAh g 1 at 5 C, and 177 mAh g 1 at 10 C (Figure 5 A). More importantly, no clear capacity loss happened when the rate was reduced to 1.2 C. Compared with TiO2– RGO composites, pure TiO2 exhibited a poor rate performance. When cycled at 0.6 C, 1.2 C, 5 C, 10 C, the reversible capacities were 184, 145, 86, and 65 mAh g 1, reFigure 4. Discharge/charge profiles of A) TiO2–RGO composites and B) pure TiO2, cycling performance of C) TiO2– spectively, indicating TiO2–RGO RGO composites and D) pure TiO2. composites also exhibited excellent performances at high (1 C = 167.5 mA g 1) in the potential window of 0.01– rates due to the favorable fea3 V. The potential plateaus around 1.75 V (Li + insertion) and 2.0 V (Li + extraction) for TiO2–RGO composites and pure TiO2 are clearly observed, and the difference of plateau voltage indicates that the TiO2/ RGO electrode had low polarization and good reaction kinetics. The redox potential plateaus of pure TiO2 rapidly became smaller and disappeared in the 100th cycle, due to the large size and intrinsically poor electronic conductivity. The initial discharge/ charge capacities of TiO2–RGO composites were 685/ 332 mAh g 1 and still kept at 240 mAh g 1 after 400 cycles at a current density of 1.2 C (Figure 4 C). More importantly, the reversible capacities of TiO2– Figure 5. A) Rate capabilities of pure TiO and TiO –RGO composites, B) Nqyuist plots for 2 2 RGO composites showed only minor capacity fading the EIS data of pure TiO2 and TiO2–RGO composites at different states. Inset: the correand exhibited a high specific capacity of 180 mAh g 1 sponding equivalent circuit mode of two samples. even at 5 C current after 400 cycles (Figure 4 C), indicating that the TiO2–RGO composites exhibit an extures of the superior hybrid nanostructures. cellent cycling stability and high reversible capacity. Compared Lithium-ion insertion/extraction properties of the TiO2–RGO with TiO2–RGO composites, pure TiO2 exhibited a lower discharge/charge capacity of 341/279 mAh g 1 at the initial cycle composites electrode were investigated using the CV technique. In the first five cycles, the samples exhibited a pair of (Figure 4 D) and inferior lithium storage performance (reversicathodic/anodic peaks at ~ 1.73 and 2.02 V, which can be asble capacity was 130 mAh g 1 at 1.2 C and 65 mAh g 1 at 5 C cribed to the Li-ion insertion and extraction in an anatase TiO2 after 100 cycles, see Figure 4 c). Thermogravimetry analysis (TGA) further confirmed that the carbon content in TiO2–RGO lattice, respectively. The cathodic/anodic peaks were in accordChem. Eur. J. 2015, 21, 1 – 7
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Communication ance with the discharge/charge profiles in Figure S4 in the Supporting Information. The upholding of the cathodic/anodic peak positions suggested that the reversibility of the TiO2 was well-maintained in the TiO2–RGO composites.[27] To gain insight into the remarkable rate performance of the TiO2–RGO composites, we performed electrochemical impedance spectroscopy (EIS) on the cells comprising pure TiO2 and TiO2–RGO composites as the working electrode. As shown in Figure 5 B, the Nyquist plots display an inclined line in the lowfrequency range and only one semicircle in the high-frequency range. The inclining line and high frequency semicircle correspond to the Li + diffusion process and charge-transfer impedance in the electrode/electrolyte interface, respectively.[28] Before cycling and after 400 cycles, the EIS profiles of TiO2– RGO composites exhibited a smaller semicircle in the high-frequency range and an inclined line with a larger slope than pure TiO2 and the corresponding equivalent circuit mode is shown in the inset (Figure 5). In this equivalent circuit, Rs represent electrolyte resistance and Rct represent charge-transfer resistance,[29] the two indices were calculated by the equivalent circuit in Table 1. The values of the Rs and Rct before cycling
xe QLixTiO2, in which x depends on the crystallography and microstructure of the materials. Because the poor electronic conductivity and Li + diffusivity, x is limited to 0.5 in the anatase phases of TiO2.[25, 32 ] Herein, as-prepared TiO2–RGO composites can reversibly store and release 0.7 Li + per TiO2, even at 5 C current after 400 cycles, which can be attributed to the following points. Firstly, the as-prepared TiO2–RGO composites had a mesoporous wall structure and high surface area. It is well-known that the high surface area and porous structure can accelerate the transport of lithium ions across the electrolyte/electrode interface and provide short path lengths with less resistance for both lithium ions and electron transport within the electrolyte. The high surface area and porous structure are beneficial for easy electrolyte immersion and diffusion and can offer a high electrolyte/electrode contact area.[26, 33–35] Especially, mesoporous wall-structured TiO2 showed excellent performance relative to nanoparticles and bulk TiO2 for Li + insertion/extraction because the mesoporous structure promote the electron transport along the long dimension and the two short dimensions ensure fast Li + insertion/extraction.[36, 37] Secondly, the composite structure of adding RGO nanosheets can minimize TiO2 agglomeration and RGO restacking, which helps maintain a good cycling stability and high surface areas during lithium insertion/extraction.[24, 38] Moreover, the highly conductive RGO matrix with large surface area is beneficial to the quality of the electrode-electrolyte contact.[16] More importantly, the cycle stabilities of as-prepared TiO2–RGO composites at high rates indicated the ultrafast diffusion of lithium ions in bulk, because of the mesoporous wall structure, short diffusion path length, high surface area, and stability of the graphene structure.[39] The synergistic effect of TiO2 nanobelts and RGO nanosheets leads to excellent lithium storage performance, and strong cycling stability of the TiO2–RGO composites. In summary, mesoporous wall-structured TiO2 on reduced graphene oxide nanosheets were successfully fabricated through a simple and steerable method. The as-prepared mesoporous wall-structured TiO2 on reduced graphene oxide nanosheets had a high surface area and outstanding electrochemical performance. Compared with pure TiO2 and other TiO2–graphene composites prepared through hydrothermal/solvothermal methods, the as-prepared TiO2–RGO composites exhibited excellent lithium storage performance, highly reversible capacity, and strong cycling stability. More importantly, the as-prepared TiO2–RGO composites still showed a higher reversible capacity (177 mA h g 1 at 10 C current). The simple and steerable synthesis, excellent electrochemical cycling, and the mesoporous wall structure could provide a new direction for developing higher-performance lithium storage electrodes.
Table 1. The values of the Rs and Rct calculated from EIS spectra (in Figure 5 B) Samples
Cycles
Rs [W]
Rct [W]
pure TiO2
0 400 0 400
5.64 5.23 3.84 3.24
350.3 183.2 94.97 25.21
TiO2–RGO
and after 400 cycles for TiO2–RGO composites were remarkably lower than those of the pure TiO2. Compared with pure TiO2, TiO2–RGO composites possessed a smaller Rct value, indicating that the synergistic effect of RGO nanosheets and nanostructured TiO2 can effectively improve the electron transport and Li + diffusivity, and can lead to higher discharge/charge capacities of TiO2–RGO composites than pure TiO2. These electrochemical performances of as-prepared TiO2– RGO composites were superior to other TiO2–graphene composites prepared by hydrothermal/solvothermal methods.[27, 30] For examples, Ren et al.[30] synthesized TiO2/carbon/RGO composites through hydrothermal reaction and the sample exhibited a relatively lower reversible capacity (158 mA h g 1 at 5 C current after 100 cycles); Zhang and co-workers[27] reported that anatase TiO2 nanoparticle/graphene displayed relatively lower capacity and weaker cycling stability (200 mAh g 1 at 0.6 C after 100 cycles). Transition-metal oxides, such as Fe2O3, NiO, and CoO3, have been extensively studied as anode materials for LIBs because of their higher theoretical capacities.[31] Compared with these high-performance materials, although they have higher theoretical capacities than TiO2, the as-prepared TiO2-RGO composite possessed excellent cycle stability and high-rate performance, due to its high surface area and mesoporous wall structure. Also notable was that the Li + insertion/extraction mechanism can be expressed as TiO2 + xLi + + &
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Experimental Section Preparation of GO nanosheets GO nanosheets were prepared through the oxidation of graphite according to the improved Hummers’ method. Briefly, graphite powder (2.0 g) and KMnO4 (6.0 g) were dispersed in sequence into cooled H2SO4 (100 mL, 98 %, 0 8C) while stirring vigorous. Then, the
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opment, NSFC (21273118), Tianjin Municipal Science and Technology Commission (11JCZDJC24800) and MOE Innovation Team (IRT13022) in China. Keywords: anodes · electrochemistry · Li-ion batteries · mesoporous wall · nanostructures [1] J. M. Tarascon, M. Armand, Nature 2001, 414, 359 – 367. [2] Y. C. Qiu, K. Y. Yan, S. H. Yang, L. M. Jin, H. Deng, W. S. Li, ACS Nano 2010, 4, 6515 – 6526. [3] J. Y. Liao, D. Higgins, G. Lui, V. Chabot, X. C. Xiao, Z. W. Chen, Nano Lett. 2013, 13, 5467 – 5473. [4] H. Xia, W. Xiong, C. K. Lim, Q. Yao, Y. Wang, J. Xie, Nano Res. 2014, 7, 1797 – 1808. [5] J. S. Chen, L. A. Archer, X. W. Lou, J. Mater. Chem. 2011, 21, 9912 – 9924. [6] Z. Ali, S. N. Cha, J. I. Sohn, I. Shakir, C. Z. Yan, J. M. Kim, D. J. Kang, J. Mater. Chem. 2012, 22, 17625 – 17629. [7] T. Song, H. Han, H. Choi, J. W. Lee, H. Park, S. Lee, W. I. Park, S. Kim, L. Liu, U. Paik, Nano Res. 2014, 7, 491 – 501. [8] J. Y. Shen, H. Wang, Y. Zhou, N. Q. Ye, L. J. Wang, CrystEngComm 2012, 14, 6215 – 6220. [9] M. J. Armstrong, C. O’Dwyer, W. J. Macklin, J. D. Holmes, Nano Res. 2014, 7, 1 – 62. [10] M. Kim, K. Sohn, J. Kim, T. Hyeon, Chem. Commun. 2003, 652 – 653. [11] F. Jiao, A. Harrison, J. C. Jumas, A. V. Chadwick, W. Kockelmann, P. G. Bruce, J. Am. Chem. Soc. 2006, 128, 5468 – 5474. [12] B. T. Yonemoto, G. S. Hutchings, F. Jiao, J. Am. Chem. Soc. 2014, 136, 8895 – 8898. [13] Q. H. Yang, J. Yang, Z. C. Feng, Y. Li, J. Mater. Chem. 2005, 15, 4268 – 4274. [14] H. Huang, J. W. Fang, Y. Xia, X. Y. Tao, Y. P. Gan, J. Du, W. J. Zhu, W. K. Zhang, J. Mater. Chem. A 2013, 1, 2495 – 2500. [15] H. Q. Cao, B. J. Li, J. X. Zhang, F. Lian, X. H. Kong, M. Z. Qu, J. Mater. Chem. 2012, 22, 9759 – 9766. [16] C. J. Chen, X. L. Hu, Y. Jiang, Z. Yang, P. Hu, Y. H. Huang, Chem. Eur. J. 2014, 20, 1383 – 1388. [17] D. C. Elias, R. R. Nair, T. M. G. Mohiuddin, S. V. Morozov, P. Blake, M. P. Halsall, A. C. Ferrari, D. W. Boukhvalov, M. I. Katsnelson, A. K. Geim, K. S. Novoselov, Science 2009, 323, 610 – 613. [18] L. Li, A. Kovalchuk, J. M. Tour, Nano Res. 2014, 7, 1319 – 1326. [19] Q. Tang, Z. Zhou, Z. F. Chen, Nanoscale 2013, 5, 4541 – 4583. [20] X. Zhao, M. J. Li, K. H. Chang, Y. M. Lin, Nano Res. 2014, 7, 1429 – 1438. [21] N. Li, G. Liu, C. Zhen, F. Li, L. L. Zhang, H. M. Cheng, Adv. Funct. Mater. 2011, 21, 1717 – 1722. [22] S. J. Ding, J. S. Chen, D. Luan, F. Y. C. Boey, S. Madhavi, X. W. Lou, Chem. Commun. 2011, 47, 5780 – 5782. [23] X. Xin, X. F. Zhou, J. H. Wu, X. Y. Yao, Z. P. Liu, ACS Nano 2012, 6, 11035 – 11043. [24] V. Etacheri, J. E. Yourey, B. M. Bartlett, ACS Nano 2014, 8, 1491 – 1499. [25] L. F. He, R. G. Ma, N. Du, J. G. Ren, T. L. Wong, Y. Y. Li, S. T. Lee, J. Mater. Chem. 2012, 22, 19061 – 19066. [26] Y. Xiao, C. W. Hu, M. H. Cao, Chem. Asian J. 2014, 9, 351 – 356. [27] J. X. Qiu, P. Zhang, M. Ling, S. Li, P. Liu, H. J. Zhao, S. Q. Zhang, ACS Appl. Mater. Interfaces 2012, 4, 3636 – 3642. [28] L. W. Su, Z. Zhou, M. M. Ren, Chem. Commun. 2010, 46, 2590 – 2592. [29] Y. Wang, H. Liu, K. Wang, H. Eiji, Y. Wang, H. Zhou, J. Mater. Chem. 2009, 19, 6789 – 6795. [30] Y. M. Ren, J. Zhang, Y. Y. Liu, H. B. Li, H. J. Wei, B. J. Li, X. Y. Wang, ACS Appl. Mater. Interfaces 2012, 4, 4776 – 4780. [31] S. Z. Huang, J. Jin, Y. Cai, Y. Li, H. Y. Tan, H. E. Wang, G. Van Tendeloo, B. L. Su, Nanoscale 2014, 6, 6819 – 6827. [32] M. Zukalov, M. Kalbˇ, L. Kavan, I. Exnar, M. Graetzel, Chem. Mater. 2005, 17, 1248 – 1255. [33] Z. Li, Z. W. Xu, X. H. Tan, H. L. Wang, C. M. B. Holt, T. Stephenson, B. C. Olsen, D. Mitlin, Energy Environ. Sci. 2013, 6, 871 – 878. [34] W. Jiao, N. Li, L. Z. Wang, L. Wen, F. Li, G. Liu, H. M. Cheng, Chem. Commun. 2013, 49, 3461 – 3463. [35] H. G. Jung, C. S. Yoon, J. Prakash, Y. K. Sun, J. Phys. Chem. C 2009, 113, 21258 – 21263.
Synthesis of TiO2-RGO composites In a typical synthesis, TiCl3 (4 mL) was added dropwise into NaOH solution (40 mL, 10 m) under vigorous stirring and then the mixture was stirred for a further 1 h. The mixture was transferred into a Teflon-lined autoclave (50 mL) and kept at 180 8C for 48 h. After it naturally cooled to room temperature, the resulting solid was centrifuged and washed with HCl solution (0.1 m) and distilled water until the pH value was about 7, and then dried at 90 8C for 10 h, resulting in the production of intermediate. The as-prepared intermediate (0.2 g) and GO (3 mL, 10 g L 1) were added into HCl solution (20 mL, 0.1 m), then the mixture was transferred into a Teflonlined autoclave (30 mL). The autoclave was kept at 180 8C for 6 h and left to naturally cool to room temperature. The obtained precipitate was isolated from the solution by centrifugation and subsequently washed with deionized water and then dried at 90 8C. The TiO2–RGO composites were prepared by annealing the obtained precipitate at 400 8C for 2 h under argon.
Material characterization The obtained samples were characterized by X-ray diffraction (XRD, Rigaku D/Max III diffractometer with Cu K radiation, l = 1.5418 ), scanning electron microscopy (SEM, Nova Nano SEM 230), transmission electron microscopy (TEM, Tecnai G2F20, FEI), high-resolution TEM (HRTEM, Tecnai G2F20, FEI), atomic force microscopy (AFM, MMAFM/STM, D3100M, Digital Ltd.), and thermogravimetry analysis (TGA, Rigaku PTC-10A TG-DTA analyzer).
Electrochemical measurements For electrochemical tests, the working electrodes were prepared with active materials, acetylene black (AB), and polytetrafluoroethylene (PTFE) at at weight ratio of 80:10:10. The average weight of the electrodes was approximately 2 mg. In the test cells, lithium metal was used as the counter and reference electrode. The electrolyte was LiPF6 (1 m), dissolved in a 1:1:1 mixture of ethylene carbonate (EC), ethylene methyl carbonate (EMC), and dimethyl carbonate (DMC). The cells were assembled in a glovebox filled with high-purity argon. The galvanostatic charge and discharge experiments were performed with a battery tester LAND-CT2001A in the voltage range of 0.001–3.0 V at room temperature. Cyclic voltammetry measurements were carried out at a scan rate of 0.01 mV s 1 between 0.01–3.0 V (vs. Li/Li + ). EIS was taken by using an IM6e electrochemical workstation at 25 8C with the frequency range from 10 kHz to 100 mHz and an AC signal of 5 mV in amplitude as the perturbation. The specific capacity was calculated according to the corresponding total weight of active materials in each electrode.
Acknowledgements This work was supported by NSFC (21271108), China–US Center for Environmental Remediation and Sustainable DevelChem. Eur. J. 2015, 21, 1 – 7
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Communication [36] J. M. Li, W. Wan, H. H. Zhou, J. J. Li, D. S. Xu, Chem. Commun. 2011, 47, 3439 – 3441. [37] P. G. Bruce, B. Scrosati, J. M. Tarascon, Angew. Chem. Int. Ed. 2008, 47, 2930 – 2946; Angew. Chem. 2008, 120, 2972 – 2989. [38] C. X. Peng, B. D. Chen, Y. Qin, S. H. Yang, C. Z. Li, Y. H. Zuo, S. Y. Liu, J. H. Yang, ACS Nano 2012, 6, 1074 – 1081.
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[39] B. C. Qiu, M. Y. Xing, J. L. Zhang, J. Am. Chem. Soc. 2014, 136, 5852 – 5855.
Received: December 31, 2014 Published online on && &&, 0000
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Mesoporous wall-structured TiO2 on reduced graphene oxide nanosheets was successfully fabricated through a simple and steerable two-step hydrothermal process without any surfactants. The obtained mesoporous structured TiO2–reduced graphene oxide composites have high surface areas and exhibit excellent electrochemical cycling (a reversible capacity of 260 mAh g 1 at 1.2 C and 180 mAh g 1 at 5 C after 400 cycles).
Chem. Eur. J. 2015, 21, 1 – 7
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M. M. Zhen, M. Q. Sun, G. D. Gao, L. Liu,* Z. Zhou* && – && Synthesis of Mesoporous WallStructured TiO2 on Reduced Graphene Oxide Nanosheets with High Rate Performance for Lithium-Ion Batteries
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