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Novel assembly and electrochemical properties of anatase TiO2-graphene aerogel 3D hybrids as lithium-ion battery anodes
Chemical Physics Letters 662 (2016) 214–220
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Novel assembly and electrochemical properties of anatase TiO2-graphene aerogel 3D hybrids as lithium-ion battery anodes Jingjie Zhang a, Yizhuo Zhou a, Guangping Zheng b, Qiuying Huang a,c, Xiucheng Zheng a,⇑, Pu Liu a, Jianmin Zhang a, Xinxin Guan a,⇑ a b c
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong, China Department of Chemical Engineering, Henan Polytechnic Institute, Nanyang 473009, China
a r t i c l e
i n f o
Article history: Received 28 July 2016 In final form 14 September 2016 Available online 15 September 2016 Keywords: TiO2-GA hybrids Hierarchical structure Electrochemical property Lithium ion battery
a b s t r a c t TiO2-graphene aerogel (TiO2-GA) 3D hybrids were directly assembled via a one-pot hydrothermal process followed by freeze-drying without using any structure-directing agent. The hybrids with a hierarchical structure exhibited large surface area (SBET = 283.6 m2 g 1) and high pore volume (Vp = 0.278 cm3 g 1), in which the ultradispersed TiO2 nanoparticles were in a single crystal phase of anatase. When used as the anodes for lithium ion battery, the TiO2-GA hybrids exhibited higher reversible capacity, more stable cycling performance and better rate-capability than TiO2 ascribed to the unique 3D nanoporous structure and the synergistic interaction of GA and TiO2. Ó 2016 Published by Elsevier B.V.
1. Introduction Titanium dioxide (TiO2) has been well investigated because it has the advantages of strong redox ability, relative non-toxicity, high efficiency good stability, low cost, solar cells/batteries, field emission and so on [1–3]. On the other hand, three-dimensional (3D) graphene aerogel (GA) with nano-porous structures is capable of facilitating ion and mass transport and is desirable for applications in electrode materials [4]. Hence, the hybrids containing TiO2 and GA are expected to be the anode material for lithium ion battery (LIB). The self-assembly technique is one of the most effective strategies in implementing the practical applications of nanomaterials which are used as nanoscale building blocks to construct bulk materials [5]. To date, although a few publications have been reported about the TiO2-graphene composites with 3D bulk forms such as hydrogels, aerogels or other macroscopic structures prepared by self-assembly methods [6–14], scarce works have been focused on the one-pot self-assembly of TiO2-GA hybrids from tetrabutyl titanate (TBT) without using structure-directing agent.
⇑ Corresponding authors. E-mail addresses: [email protected] (X. Zheng), [email protected] (X. Guan). http://dx.doi.org/10.1016/j.cplett.2016.09.044 0009-2614/Ó 2016 Published by Elsevier B.V.
Recently, we successfully prepared a series of TiO2-GA hybrids from TBT via a one-pot hydrothermal process followed by freeze-drying without using any structure-directing agent [15]. In the present paper, the 3D hierarchical TiO2-GA hybrids were characterized in details and then used as anode materials for LIB to investigate their electrochemical properties. 2. Experimental methods 2.1. Synthesis of TiO2-GA hybrids The detailed synthesis procedure for TiO2-GA hybrids was reported in Reference [15]. Firstly, 17.5 mL of absolute ethanol was added into 17.5 mL of GO solution (3.6 mg mL 1), which was prepared with a modified Hummers’ method [4], and sonicated for 1 h. Secondly, 0.63 g of TBT was added dropwise and the reactant was sonicated for another 1 h. Thirdly, the mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and kept at 180 °C for 24 h. Finally, the resulting TiO2-graphene hydrogels (TiO2-GH) were hydrothermally treated in ammonia solution (10 v/v%) at 120 °C for 3 h, following by a freeze-drying process. The resulting hybrids were denoted as TiO2-GA. For comparison, GA and TiO2 were prepared with the same procedure without using TBT and GO, respectively. The thermogravimetric analysis (TGA) curves shown in Fig. S1 display that the resulting hybrids exhibit a total weight loss of about 29% below 580 °C,
J. Zhang et al. / Chemical Physics Letters 662 (2016) 214–220
which is attributed to the evaporation of adsorbed water molecules and the combustion of GA. Therefore, the content of TiO2 in the hybrids is 71 wt%. 2.2. Materials characterizations The porous nature of the samples was investigated by nitrogen adsorption-desorption isotherms at 196 °C using a Micromeritics ASAP 2420 surface area and porosity analyser. The specific surface area was calculated from the nitrogen adsorption isotherms within the relative pressure range of 0.05–0.25 by the Brunauer-EmmettTeller (BET) method. The pore size distributions were measured from the adsorption branches. TGA was measured by a BJ HENVEN with a heating rate of 10 °C min 1 in the air to determine the chemical composition of TiO2-GA hybrids. The microscopic features of the TiO2-GA hybrids were examined using a Zeiss Ultra 55 scanning electron microscopy (SEM) and a JEM-2100 high-resolution transmission electron microscopy (HRTEM). Crystal structures of the synthesized samples were characterized by X-ray diffraction (XRD) on a Panalytical X’pertPro diffractometer operated at 40 kV and 40 mA using Cu Ka radiation (k = 0.154 nm). Surface chemistry study was conducted by X-ray photoelectron spectroscopy (XPS) using an RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Al Ka radiation (hm = 1486.6 eV). Binding energies were
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calibrated using the containment carbon (C 1s = 284.6 eV). Raman spectra recorded on a Renishaw RM-1000 spectrometer and Fourier-transform infrared (FT-IR) spectra recorded on a Thermo scientific Nicolet 380 Fourier transform spectrometer using a KBr pellet technique were used to characterize the samples.
2.2. Electrochemical measurements Electrochemical performances of the electrode materials for lithium ion batteries were tested in lithium cells (CR 2016 coin type cell). The working electrodes were prepared by mixing 80 wt% of active materials, 10 wt% of super P as the conductive material and 10 wt% of polyvinylidene fluoride as the binder in N-methylpyrrolidinone solvent to form a slurry, which was coated onto a copper foil and dried at 120 °C for 12 h in a vacuum oven. The cells were assembled in an argon-filled glove box using the working electrode, the lithium foil counter electrode and the Celgard 2400 microporous membrane separator. The organic electrolyte was 1 M LiPF6 in a mixture solution of ethylene carbonate, diethyl carbonate and ethyl methyl in a 1: 1: 1 vol ratio. Galvanostatic charge-discharge characteristics were tested on a LAND battery system between 0.01 and 3.00 V at different current densities. Cyclic voltammetry tests were performed on a
Fig. 1. XRD patterns (a), N2 adsorption-desorption isotherms (b), pore size distributions (insert of b), FT-IR spectra (c) and Raman spectra (d) of the resulting materials.
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Fig. 2. Photograph (a), SEM image (b), TEM image (c), HRTEM image (d) and the SAED patterns (insert of d) of the TiO2-GA hybrids.
CHI660C electrochemistry workstation at a scan rate of 0.2 mV s 1 and over a potential range of 0.01–3.00 V (vs. Li/Li+) at room temperature. Electrochemical impedance spectra (EIS) of the electrodes were measured on a CHI660C electrochemistry workstation. The frequency range was set from 100 mHz to 10 kHz and the potential amplitude was 5 mV.
3. Results and discussion As shown in Fig. 1a, the sharp diffraction peaks appeared in the XRD patterns for TiO2 and the TiO2-GA hybrids are perfectly consistent with those for anatase TiO2 [16–18]. The disappearance of the characteristic peak of (0 0 2) plane for GA at ca. 24.8o in the TiO2-GA hybrids could be explained by the low GA content and the overlapping or screening caused by the strong (1 0 1) peak for TiO2. On the other hand, the average crystal size decreases from 7.5 nm for the pure TiO2 to 5.7 nm for TiO2 in the TiO2-GA hybrids. This is mainly because the incorporation of GA promotes the nucleation of TiO2
nanoparticles. As shown in Fig. 1b, Compared to the step of N2 adsorption and desorption branches of TiO2 occurs at a relative pressure (P/P0) of about 0.55–1.00 and the weak capillary condensation for GA which occurs at P/P0 = 0.35–0.75, the isotherm of TiO2-GA is typically of IV type with an H1 hysteresis loop (P/P0 = 0.40–0.85), suggesting that it has a mesoporous structure [19]. In addition, both GA and TiO2-GA display a much narrower pore size distribution in comparison with that of TiO2 (inset of Fig. 1b). The adsorption average pore width of TiO2-GA (3.92 nm) is larger than that of GA (2.40 nm) due to the incorporation of TiO2 nanoparticles. Furthermore, the specific surface area of TiO2-GA hybrids (SBET = 283.6 m2 g 1) is also higher than that of TiO2 (SBET = 152.8 m2 g 1). The FT-IR spectra shown in Fig. 1c imply that the C=O and C–O stretching vibration bands at 1725 cm 1 and 1045 cm 1 for COOH groups almost disappear for GA and the TiO2-GA hybrids, indicating that they are reduced during the solvothermal process. For TiO2 and the TiO2-GA hybrids, the broad absorption below 1000 cm 1 is presumably ascribed to the combination of Ti–O–Ti and Ti–O–C vibration modes resulting
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Fig. 3. XPS survey spectrum (a), high-resolution Ti 2p spectrum (b), high-resolution C 1s spectrum (c) and high-resolution O 1s spectrum (d) of the TiO2-GA hybrids.
from the chemical interaction between TiO2 and graphene [20]. As shown in Fig. 1d, the TiO2-GA hybrids imply the characteristic Raman bands of anatase TiO2, such as B1g, (A1g + B1g) and Eg appeared at about 398, 515, and 639 cm 1 [21]. The intensity ratio of ID/IG for GA and the resulting hybrids keeps constant (1.20), suggesting that the incorporation of TiO2 has no obvious effect on the defects and disorders in the graphitized structures of GA. Remarkably, the hybrids display a black cylindrical shape (Fig. 2a), indicating that the TiO2-GA hybrids retain the 3D monolithic architecture of GA. Such 3D graphene-based aerogel embedded with nanoparticles may enhance the interfacial contacts and suppress the dissolution and agglomeration among the nanoparticles, facilitate ion and mass transport, thereby improves the photoelectrochemical activities and stability of the hybrids [4,22]. The SEM and TEM images also reveal that there are abundant well-defined and interconnected 3D porous network and the TiO2 with a particle size of around 5–7 nm are anchored uniformly on the surfaces of graphene nanosheets or wrapped by the graphene nanosheets in the TiO2-GA hybrids (Fig. 2b and c). The HRTEM image further reveal that the crystal lattice fringe of TiO2 is 0.35 nm (Fig. 2d), which is ascribed to the (1 0 1) plane of anatase TiO2 [23]. Meanwhile, the SAED ring patterns
further indicate the presence of TiO2 anatase phase (inset of Fig. 2d). The XPS survey scan for the TiO2-GA hybrids shown in Fig. 3a confirms the presence of the elements C, O, and Ti, which are also detected by energy dispersive X-ray (EDX) spectrum (Fig. S2). The small numbers of N element could result from the ammonia solution used for the pretreatment of TiO2-GA hybrids before freeze drying. Meanwhile, the peaks ascribed to Cu element are also shown in Fig. S2, which is come from the copper grids used in the TEM measurement. In the high-resolution Ti 2p spectrum shown in Fig. 3b, the doublet peaks of Ti 2p1/2 (B.E. 459.2 eV) and Ti 2p3/2 (B.E. 464.9 eV) elucidate the Ti4+ chemical state [18]. The O/Ti ratio is higher than that of the stoichiometry of TiO2 because of the additional oxygen atoms detected in the functional groups of GA, which may be found in the high-resolution C 1s and O 1s spectra (Fig. 3c and d). Meanwhile, the very low intensity of the peak for O–C=O groups also indicates that GO is efficiently reduced into graphene after the hydrothermal treatment. As shown in Fig. 4a and b, the cyclic voltammetry (CV) curves of TiO2 and TiO2-GA electrodes display a cathodic peak at ca. 1.62 V representing the reduction of Ti4+ by Li to Ti3+ and a anodic peak at 2.18 V representing the oxydic reaction from Ti3+ to Ti4+.
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Fig. 4. CV curves of TiO2 (a) and the TiO2-GA hybrids (b); the discharge-charge profiles of TiO2 (c) and the TiO2@GA hybrids (d).
Moreover, the CV curves for the TiO2-GA composite in the subsequent two cycles almost overlap, demonstrating its good reversibility. As shown in Fig. 4c and d, the voltage profiles of the first cycle exhibit a discharge voltage plateau at about 1.7 V and a charge plateau at about 2.1 V, which are the typical characteristics of TiO2 during the charging/discharging process [24]. In the following cycles, the TiO2-GA hybrids exhibit better performance in the electrochemical lithium storage than TiO2. After three discharge/charge cycles, TiO2-GA exhibits a reversible capacity of 256.7 mA h g 1, while the corresponding value for TiO2 is only 198.9 mA h g 1. After 100 cycles, shown in Fig. 5a, the discharge capacity of TiO2 electrode drastically decreases to 74.3 mA h g 1, which is obviously lower than that of the TiO2-GA hybrids (150.2 mA h g 1). The results demonstrate that the TiO2-GA electrode possesses a superior charge/discharge cycling stability due to the incorporation of GA when TiO2-GA is used as an active material for lithium ion storage. In addition, the fluctuation for the discharge capacity for the TiO2-GA electrode may be explained that the improved Li-ion diffusion kinetics by an activation and stabilization process during cycling and the unique configuration of aerogel [4]. The rate capabilities of TiO2 and the TiO2-GA hybrids shown in Fig. 5b reveal that the TiO2-GA hybrids show excellent cyclic capacity retention with specific capacities of 185.3, 124.7, 80.2, 51.9, and 36.1 mA h g 1 at current densities of 100, 200,
500, 1000 and 2000 mA g 1, respectively. Furthermore, the discharge capacity of TiO2-GA declines slowly with increasing current density as compared with that of TiO2. The Nyquist plots for TiO2 and TiO2-GA shown in Fig. 5c are similar in shape, and have a semi-circle in the high-frequency region. Obviously, the radius of semicircle in the Nyquist plots for TiO2-GA anode is much smaller than that of TiO2, suggesting that the electron transfer of TiO2-GA is better than that of TiO2. 4. Conclusions A one-pot hydrothermal method combined with freeze drying was developed to fabricate 3D TiO2-graphene aerogel (TiO2-GA) hybrids without using any structure-directing agent. The TiO2-GA hybrids were found to have high capacity, good reversibility, outstanding high-rate capacity and stability over cycling conditions for LIB applications. Acknowledgements The authors are grateful for the supports of the National Natural Science Foundation of China (Nos. U1304203 and J1210060), the Foundation of He’nan Educational Committee (Nos. 16A150046
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Fig. 5. Cycling performance (a), rate capability (b) and Nyquist plots profiles (c) of TiO2 and the TiO2@GA hybrids.
and 16B150002) and the Innovation Foundation of Zhengzhou University (Nos. 201610459062 and 2016xjxm251). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.09. 044. References [1] P. Zhang, T. Tachikawa, M. Fujitsuka, T. Majima, Efficient charge separation on 3D architectures of TiO2 mesocrystals packed with a chemically exfoliated MoS2 shell in synergetic hydrogen evolution, Chem. Commun. 51 (2015) 7187– 7190. [2] W.K. Wang, J.J. Chen, M. Gao, Y.X. Huang, X. Zhang, H.Q. Yu, Photocatalytic degradation of atrazine by boron-doped TiO2 with a tunable rutile/anatase ratio, Appl. Catal. B-Environ. 195 (2016) 69–76. [3] X.J. Li, M.Y. Li, J.C. Liang, X.F. Wang, K.F. Yu, Growth mechanism of hollow TiO2(B) nanocrystals as powerful application in lithium-ion batteries, J. Alloy. Compd. 681 (2016) 471–476. [4] J.K. Meng, Y. Cao, Y. Suo, Y.S. Liu, J.M. Zhang, X.C. Zheng, Facile fabrication of 3D SiO2@graphene aerogel composites as anode material for lithium ion batteries, Electrochim. Acta 176 (2015) 1001–1009. [5] S. Man, Self-assembly and transformation of hybrid nano-objects and nanostructures under equilibrium and non-equilibrium conditions, Nat. Mater. 8 (2009) 781–792. [6] W. Yan, F. He, S.L. Gai, P. Gao, Y.J. Chen, P.P. Yang, A novel 3D structured reduced graphene oxide/TiO2 composite: synthesis and photocatalytic performance, J. Mater. Chem. A 2 (2014) 3605–3612. [7] Y. Agrawal, G. Kedawat, P. Kumar, J. Dwivedi, V.N. Singh, R.K. Gupta, B.K. Gupta, High-performance stable field emission with ultralow turn on voltage from rGO conformal coated TiO2 nanotubes 3D arrays, Sci. Rep. 5 (2015) 11612.
[8] W.J. Han, C. Zang, Z.Y. Huang, H. Zhang, L. Ren, X. Qi, J.X. Zhong, Enhanced photocatalytic activities of three-dimensional graphene-based aerogel embedding TiO2 nanoparticles and loading MoS2 nanosheets as Co-catalyst, Int. J. Hydrogen Energy 39 (2014) 19502–19512. [9] Z.Y. Zhang, F. Xiao, Y.L. Guo, S. Wang, Y.Q. Liu, One-pot self-assembled threedimensional TiO2-graphene hydrogel with improved adsorption capacities and photocatalytic and electrochemical activities, ACS Appl. Mater. Interfaces 5 (2013) 2227–2233. [10] B.C. Qiu, M.Y. Xing, J.L. Zhang, Mesoporous TiO2 nanocrystals grown in situ on graphene aerogels for high photocatalysis and lithium-ion batteries, J. Am. Chem. Soc. 136 (2014) 5852–5855. [11] Y.C. Chen, H. Ishihara, W.J. Chen, N. DeMarco, A. Siordia, Y.S. Sun, O. Lin, C.W. Chu, V.C. Tung, Electrohydrodynamically assisted deposition of efficient perovskite photovoltaics, Adv. Mater. Interfaces 2 (2015) 1500292. [12] X. Jiang, X.L. Yang, Y.H. Zhu, H.L. Jiang, Y.F. Yao, P. Zhao, C.Z. Li, 3D nitrogendoped graphene foams embedded with ultrafine TiO2 nanoparticles for high performance lithium-ion batteries, J. Mater. Chem. A 2 (2014) 11124–11133. [13] F.H. Zhao, B.H. Dong, R.J. Gao, G. Su, W. Liu, L. Shi, C.H. Xia, L.X. Cao, A threedimensional graphene-TiO2 nanotube nanocomposite with exceptional photocatalytic activity for dye degradation, Appl. Surf. Sci. 351 (2015) 303– 308. [14] F. Li, J.Z. Jiang, X.J. Wang, F. Liu, J.Z. Wang, Y.W. Chen, S. Han, H.L. Lin, Assembly of TiO2/graphene with macroporous 3D network framework as an advanced anode material for Li-ion batteries, RSC Adv. 6 (2016) 3335–3340. [15] J.J. Zhang, Y.H. Wu, J.Y. Mei, G.P. Zheng, T.T. Yan, X.C. Zheng, P. Liu, X.X. Guan, Synergetic adsorption and photocatalytic degradation of pollutants over 3D TiO2-graphene aerogel composites synthesized via a facile one-pot route, Photochem. Photobiol. Sci. (2016), http://dx.doi.org/10.1039/c6 pp00133e. [16] G. Cheng, M.S. Akhtar, O.B. Yang, F.J. Stadler, Novel preparation of anatase TiO2@reduced graphene oxide hybrids for high-performance dye-sensitized solar cells, ACS Appl. Mater. Interfaces 5 (2013) 6635–6642. [17] K.R. Parmar, S. Basha, Z.V.P. Murthy, Application of graphene oxide as a hydrothermal catalyst support for synthesis of TiO2 whiskers, Chem. Commun. 50 (2014) 15010–15013. [18] K.M. Cho, K.H. Kim, H.O. Choi, H.T. Jung, A highly photoactive, visible-lightdriven graphene/2D mesoporous TiO2 photocatalyst, Green Chem. 17 (2015) 3972–3978.
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[19] B. Yu, X.S. Jiang, J. Yin, Silica/titania sandwich-like mesoporous nanosheets embedded with metal nanoparticles templated by hyperbranched poly(ether amine), HPEA, Nanoscale 5 (2013) 5489–5498. [20] A.J. Wang, W. Yu, Y. Fang, Y.L. Song, D. Jia, L.L. Long, M.P. Cifuentes, M.G. Humphrey, C. Zhang, Facile hydrothermal synthesis and optical limiting properties of TiO2-reduced graphene oxide nanocomposites, Carbon 89 (2015) 130–141. [21] T. Ohsaka, F. Izumi, Y. Fujiki, Raman spectrum of anatase, TiO2, J. Raman Spectrosc. 7 (1978) 321–324. [22] Z.S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng, K. Mȕllen, 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts
for the oxygen reduction reaction, J. Am. Chem. Soc. 134 (2012) 9082–9085. [23] W. Li, F. Wang, Y.P. Liu, J.X. Wang, J.P. Yang, L.J. Zhang, A.A. Elzatahry, General strategy to synthesize uniform mesoporous TiO2/graphene/mesoporous TiO2 sandwich-like nanosheets for highly reversible lithium storage, Nano Lett. 15 (2015) 2186–2193. [24] S. Jiang, R.W. Wang, M.J. Pang, H.B. Wang, S.J. Zeng, X.Z. Yue, L. Ni, Y.R. Yu, J.Y. Dai, S.L. Qiu, Z.T. Zhang, Assembling porous carbon-coated TiO2(B)/anatase nanosheets on reduced graphene oxide for high performance lithium-ion batteries, Electrochim. Acta 182 (2015) 406–415.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Novel assembly and electrochemical properties of anatase TiO2-graphene aerogel 3D hybrids as lithium-ion battery anodes Jingjie Zhang a, Yizhuo Zhou a, Guangping Zheng b, Qiuying Huang a,c, Xiucheng Zheng a,⇑, Pu Liu a, Jianmin Zhang a, Xinxin Guan a,⇑ a b c
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong, China Department of Chemical Engineering, Henan Polytechnic Institute, Nanyang 473009, China
a r t i c l e
i n f o
Article history: Received 28 July 2016 In final form 14 September 2016 Available online 15 September 2016 Keywords: TiO2-GA hybrids Hierarchical structure Electrochemical property Lithium ion battery
a b s t r a c t TiO2-graphene aerogel (TiO2-GA) 3D hybrids were directly assembled via a one-pot hydrothermal process followed by freeze-drying without using any structure-directing agent. The hybrids with a hierarchical structure exhibited large surface area (SBET = 283.6 m2 g 1) and high pore volume (Vp = 0.278 cm3 g 1), in which the ultradispersed TiO2 nanoparticles were in a single crystal phase of anatase. When used as the anodes for lithium ion battery, the TiO2-GA hybrids exhibited higher reversible capacity, more stable cycling performance and better rate-capability than TiO2 ascribed to the unique 3D nanoporous structure and the synergistic interaction of GA and TiO2. Ó 2016 Published by Elsevier B.V.
1. Introduction Titanium dioxide (TiO2) has been well investigated because it has the advantages of strong redox ability, relative non-toxicity, high efficiency good stability, low cost, solar cells/batteries, field emission and so on [1–3]. On the other hand, three-dimensional (3D) graphene aerogel (GA) with nano-porous structures is capable of facilitating ion and mass transport and is desirable for applications in electrode materials [4]. Hence, the hybrids containing TiO2 and GA are expected to be the anode material for lithium ion battery (LIB). The self-assembly technique is one of the most effective strategies in implementing the practical applications of nanomaterials which are used as nanoscale building blocks to construct bulk materials [5]. To date, although a few publications have been reported about the TiO2-graphene composites with 3D bulk forms such as hydrogels, aerogels or other macroscopic structures prepared by self-assembly methods [6–14], scarce works have been focused on the one-pot self-assembly of TiO2-GA hybrids from tetrabutyl titanate (TBT) without using structure-directing agent.
⇑ Corresponding authors. E-mail addresses: [email protected] (X. Zheng), [email protected] (X. Guan). http://dx.doi.org/10.1016/j.cplett.2016.09.044 0009-2614/Ó 2016 Published by Elsevier B.V.
Recently, we successfully prepared a series of TiO2-GA hybrids from TBT via a one-pot hydrothermal process followed by freeze-drying without using any structure-directing agent [15]. In the present paper, the 3D hierarchical TiO2-GA hybrids were characterized in details and then used as anode materials for LIB to investigate their electrochemical properties. 2. Experimental methods 2.1. Synthesis of TiO2-GA hybrids The detailed synthesis procedure for TiO2-GA hybrids was reported in Reference [15]. Firstly, 17.5 mL of absolute ethanol was added into 17.5 mL of GO solution (3.6 mg mL 1), which was prepared with a modified Hummers’ method [4], and sonicated for 1 h. Secondly, 0.63 g of TBT was added dropwise and the reactant was sonicated for another 1 h. Thirdly, the mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and kept at 180 °C for 24 h. Finally, the resulting TiO2-graphene hydrogels (TiO2-GH) were hydrothermally treated in ammonia solution (10 v/v%) at 120 °C for 3 h, following by a freeze-drying process. The resulting hybrids were denoted as TiO2-GA. For comparison, GA and TiO2 were prepared with the same procedure without using TBT and GO, respectively. The thermogravimetric analysis (TGA) curves shown in Fig. S1 display that the resulting hybrids exhibit a total weight loss of about 29% below 580 °C,
J. Zhang et al. / Chemical Physics Letters 662 (2016) 214–220
which is attributed to the evaporation of adsorbed water molecules and the combustion of GA. Therefore, the content of TiO2 in the hybrids is 71 wt%. 2.2. Materials characterizations The porous nature of the samples was investigated by nitrogen adsorption-desorption isotherms at 196 °C using a Micromeritics ASAP 2420 surface area and porosity analyser. The specific surface area was calculated from the nitrogen adsorption isotherms within the relative pressure range of 0.05–0.25 by the Brunauer-EmmettTeller (BET) method. The pore size distributions were measured from the adsorption branches. TGA was measured by a BJ HENVEN with a heating rate of 10 °C min 1 in the air to determine the chemical composition of TiO2-GA hybrids. The microscopic features of the TiO2-GA hybrids were examined using a Zeiss Ultra 55 scanning electron microscopy (SEM) and a JEM-2100 high-resolution transmission electron microscopy (HRTEM). Crystal structures of the synthesized samples were characterized by X-ray diffraction (XRD) on a Panalytical X’pertPro diffractometer operated at 40 kV and 40 mA using Cu Ka radiation (k = 0.154 nm). Surface chemistry study was conducted by X-ray photoelectron spectroscopy (XPS) using an RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Al Ka radiation (hm = 1486.6 eV). Binding energies were
215
calibrated using the containment carbon (C 1s = 284.6 eV). Raman spectra recorded on a Renishaw RM-1000 spectrometer and Fourier-transform infrared (FT-IR) spectra recorded on a Thermo scientific Nicolet 380 Fourier transform spectrometer using a KBr pellet technique were used to characterize the samples.
2.2. Electrochemical measurements Electrochemical performances of the electrode materials for lithium ion batteries were tested in lithium cells (CR 2016 coin type cell). The working electrodes were prepared by mixing 80 wt% of active materials, 10 wt% of super P as the conductive material and 10 wt% of polyvinylidene fluoride as the binder in N-methylpyrrolidinone solvent to form a slurry, which was coated onto a copper foil and dried at 120 °C for 12 h in a vacuum oven. The cells were assembled in an argon-filled glove box using the working electrode, the lithium foil counter electrode and the Celgard 2400 microporous membrane separator. The organic electrolyte was 1 M LiPF6 in a mixture solution of ethylene carbonate, diethyl carbonate and ethyl methyl in a 1: 1: 1 vol ratio. Galvanostatic charge-discharge characteristics were tested on a LAND battery system between 0.01 and 3.00 V at different current densities. Cyclic voltammetry tests were performed on a
Fig. 1. XRD patterns (a), N2 adsorption-desorption isotherms (b), pore size distributions (insert of b), FT-IR spectra (c) and Raman spectra (d) of the resulting materials.
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Fig. 2. Photograph (a), SEM image (b), TEM image (c), HRTEM image (d) and the SAED patterns (insert of d) of the TiO2-GA hybrids.
CHI660C electrochemistry workstation at a scan rate of 0.2 mV s 1 and over a potential range of 0.01–3.00 V (vs. Li/Li+) at room temperature. Electrochemical impedance spectra (EIS) of the electrodes were measured on a CHI660C electrochemistry workstation. The frequency range was set from 100 mHz to 10 kHz and the potential amplitude was 5 mV.
3. Results and discussion As shown in Fig. 1a, the sharp diffraction peaks appeared in the XRD patterns for TiO2 and the TiO2-GA hybrids are perfectly consistent with those for anatase TiO2 [16–18]. The disappearance of the characteristic peak of (0 0 2) plane for GA at ca. 24.8o in the TiO2-GA hybrids could be explained by the low GA content and the overlapping or screening caused by the strong (1 0 1) peak for TiO2. On the other hand, the average crystal size decreases from 7.5 nm for the pure TiO2 to 5.7 nm for TiO2 in the TiO2-GA hybrids. This is mainly because the incorporation of GA promotes the nucleation of TiO2
nanoparticles. As shown in Fig. 1b, Compared to the step of N2 adsorption and desorption branches of TiO2 occurs at a relative pressure (P/P0) of about 0.55–1.00 and the weak capillary condensation for GA which occurs at P/P0 = 0.35–0.75, the isotherm of TiO2-GA is typically of IV type with an H1 hysteresis loop (P/P0 = 0.40–0.85), suggesting that it has a mesoporous structure [19]. In addition, both GA and TiO2-GA display a much narrower pore size distribution in comparison with that of TiO2 (inset of Fig. 1b). The adsorption average pore width of TiO2-GA (3.92 nm) is larger than that of GA (2.40 nm) due to the incorporation of TiO2 nanoparticles. Furthermore, the specific surface area of TiO2-GA hybrids (SBET = 283.6 m2 g 1) is also higher than that of TiO2 (SBET = 152.8 m2 g 1). The FT-IR spectra shown in Fig. 1c imply that the C=O and C–O stretching vibration bands at 1725 cm 1 and 1045 cm 1 for COOH groups almost disappear for GA and the TiO2-GA hybrids, indicating that they are reduced during the solvothermal process. For TiO2 and the TiO2-GA hybrids, the broad absorption below 1000 cm 1 is presumably ascribed to the combination of Ti–O–Ti and Ti–O–C vibration modes resulting
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Fig. 3. XPS survey spectrum (a), high-resolution Ti 2p spectrum (b), high-resolution C 1s spectrum (c) and high-resolution O 1s spectrum (d) of the TiO2-GA hybrids.
from the chemical interaction between TiO2 and graphene [20]. As shown in Fig. 1d, the TiO2-GA hybrids imply the characteristic Raman bands of anatase TiO2, such as B1g, (A1g + B1g) and Eg appeared at about 398, 515, and 639 cm 1 [21]. The intensity ratio of ID/IG for GA and the resulting hybrids keeps constant (1.20), suggesting that the incorporation of TiO2 has no obvious effect on the defects and disorders in the graphitized structures of GA. Remarkably, the hybrids display a black cylindrical shape (Fig. 2a), indicating that the TiO2-GA hybrids retain the 3D monolithic architecture of GA. Such 3D graphene-based aerogel embedded with nanoparticles may enhance the interfacial contacts and suppress the dissolution and agglomeration among the nanoparticles, facilitate ion and mass transport, thereby improves the photoelectrochemical activities and stability of the hybrids [4,22]. The SEM and TEM images also reveal that there are abundant well-defined and interconnected 3D porous network and the TiO2 with a particle size of around 5–7 nm are anchored uniformly on the surfaces of graphene nanosheets or wrapped by the graphene nanosheets in the TiO2-GA hybrids (Fig. 2b and c). The HRTEM image further reveal that the crystal lattice fringe of TiO2 is 0.35 nm (Fig. 2d), which is ascribed to the (1 0 1) plane of anatase TiO2 [23]. Meanwhile, the SAED ring patterns
further indicate the presence of TiO2 anatase phase (inset of Fig. 2d). The XPS survey scan for the TiO2-GA hybrids shown in Fig. 3a confirms the presence of the elements C, O, and Ti, which are also detected by energy dispersive X-ray (EDX) spectrum (Fig. S2). The small numbers of N element could result from the ammonia solution used for the pretreatment of TiO2-GA hybrids before freeze drying. Meanwhile, the peaks ascribed to Cu element are also shown in Fig. S2, which is come from the copper grids used in the TEM measurement. In the high-resolution Ti 2p spectrum shown in Fig. 3b, the doublet peaks of Ti 2p1/2 (B.E. 459.2 eV) and Ti 2p3/2 (B.E. 464.9 eV) elucidate the Ti4+ chemical state [18]. The O/Ti ratio is higher than that of the stoichiometry of TiO2 because of the additional oxygen atoms detected in the functional groups of GA, which may be found in the high-resolution C 1s and O 1s spectra (Fig. 3c and d). Meanwhile, the very low intensity of the peak for O–C=O groups also indicates that GO is efficiently reduced into graphene after the hydrothermal treatment. As shown in Fig. 4a and b, the cyclic voltammetry (CV) curves of TiO2 and TiO2-GA electrodes display a cathodic peak at ca. 1.62 V representing the reduction of Ti4+ by Li to Ti3+ and a anodic peak at 2.18 V representing the oxydic reaction from Ti3+ to Ti4+.
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Fig. 4. CV curves of TiO2 (a) and the TiO2-GA hybrids (b); the discharge-charge profiles of TiO2 (c) and the TiO2@GA hybrids (d).
Moreover, the CV curves for the TiO2-GA composite in the subsequent two cycles almost overlap, demonstrating its good reversibility. As shown in Fig. 4c and d, the voltage profiles of the first cycle exhibit a discharge voltage plateau at about 1.7 V and a charge plateau at about 2.1 V, which are the typical characteristics of TiO2 during the charging/discharging process [24]. In the following cycles, the TiO2-GA hybrids exhibit better performance in the electrochemical lithium storage than TiO2. After three discharge/charge cycles, TiO2-GA exhibits a reversible capacity of 256.7 mA h g 1, while the corresponding value for TiO2 is only 198.9 mA h g 1. After 100 cycles, shown in Fig. 5a, the discharge capacity of TiO2 electrode drastically decreases to 74.3 mA h g 1, which is obviously lower than that of the TiO2-GA hybrids (150.2 mA h g 1). The results demonstrate that the TiO2-GA electrode possesses a superior charge/discharge cycling stability due to the incorporation of GA when TiO2-GA is used as an active material for lithium ion storage. In addition, the fluctuation for the discharge capacity for the TiO2-GA electrode may be explained that the improved Li-ion diffusion kinetics by an activation and stabilization process during cycling and the unique configuration of aerogel [4]. The rate capabilities of TiO2 and the TiO2-GA hybrids shown in Fig. 5b reveal that the TiO2-GA hybrids show excellent cyclic capacity retention with specific capacities of 185.3, 124.7, 80.2, 51.9, and 36.1 mA h g 1 at current densities of 100, 200,
500, 1000 and 2000 mA g 1, respectively. Furthermore, the discharge capacity of TiO2-GA declines slowly with increasing current density as compared with that of TiO2. The Nyquist plots for TiO2 and TiO2-GA shown in Fig. 5c are similar in shape, and have a semi-circle in the high-frequency region. Obviously, the radius of semicircle in the Nyquist plots for TiO2-GA anode is much smaller than that of TiO2, suggesting that the electron transfer of TiO2-GA is better than that of TiO2. 4. Conclusions A one-pot hydrothermal method combined with freeze drying was developed to fabricate 3D TiO2-graphene aerogel (TiO2-GA) hybrids without using any structure-directing agent. The TiO2-GA hybrids were found to have high capacity, good reversibility, outstanding high-rate capacity and stability over cycling conditions for LIB applications. Acknowledgements The authors are grateful for the supports of the National Natural Science Foundation of China (Nos. U1304203 and J1210060), the Foundation of He’nan Educational Committee (Nos. 16A150046
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Fig. 5. Cycling performance (a), rate capability (b) and Nyquist plots profiles (c) of TiO2 and the TiO2@GA hybrids.
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