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Preparation and electrochemical characterization of TiO2 nanowires as an electrode material for lithium-ion batteries
Electrochimica Acta 53 (2008) 7863–7868
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Preparation and electrochemical characterization of TiO2 nanowires as an electrode material for lithium-ion batteries Yunfei Wang a , Muying Wu b , W.F. Zhang a,∗ a b
Institute of Micro-system Physics and School of Physics & Electronics, Henan University, 85 Minglun Street, Kaifeng 475001, PR China Department of Electronic Engineering, Dongguan University of Technology, Dongguan 523808, PR China
a r t i c l e
i n f o
Article history: Received 1 March 2008 Received in revised form 28 May 2008 Accepted 29 May 2008 Available online 5 June 2008 Keywords: TiO2 nanowire Discharge–charge Electrochemistry Lithium-ion batteries
a b s t r a c t Anatase TiO2 nanowires containing minor TiO2 (B) phase were prepared by a hydrothermal chemical reaction followed by the post-heat treatment at 400 ◦ C. The phase structure and morphology were analyzed by X-ray diffraction, Raman scattering, transmission electron microscope, and field-emission scanning electron microscopy. The electrochemical properties were investigated by employing constant current discharge–charge test, cyclic voltammetry, and electrochemical impedance techniques. These nanowires exhibited high rate capacity of 280 mAh g−1 even after 40 cycles, and the coulombic efficiency was approximately 98%, indicating excellent cycling stability and reversibility. The electrochemical impedance spectra showed a stable kinetic process of the electrode reaction. These results indicated that the TiO2 nanowires have promising application for high energy density lithium-ion batteries. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The demand for rechargeable batteries is increasing in portable devices and power sources for automobile electrical systems. Lithium-ion batteries have been considered to be a good answer because of their high energy storage density, long cycle life, little memory effect, poisonous metals free and so on [1,2]. Most of the commercial lithium-ion batteries use graphite as an anode material. The graphite electrode, however, has several disadvantages such as its electrical disconnection, structural deformation, and initial loss of capacity [1,3], which dissatisfy the need of production. To avoid these drawbacks, transition metal oxides including WO3 , MoO3 , V2 O5 , and TiO2 have attracted much interest [2,4]. Especially, TiO2 is regarded as a promising active lithium intercalation material with low production cost and high capacity, low-voltage (below ca. 2.0 V vs. Li+ /Li) for lithium intercalation. In the structure of TiO2 , the TiO6 octahedra share vertices and edges to build up the three-dimensional framework, leaving favorable empty sites available for lithium insertion [5]. TiO2 crystallizes in various forms such as anatase, rutile, brookite, TiO2 (B), etc. [6–9]. Under standard conditions, rutile is the thermodynamically most stable structure of TiO2 , and is also the most common natural form. TiO2 (B) is a metastable monoclinic modification of titanium dioxide. Anatase is generally considered to be the most electroactive Li-insertion
∗ Corresponding author. Tel.: +86 378 3881 940; fax: +86 378 3880 659. E-mail addresses: [email protected], [email protected] (W.F. Zhang). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.05.068
host among these TiO2 polymorphs. However, rutile and TiO2 (B) for electrochemical Li-insertion also aroused increasing interests [9–12]. For the micrometer-sized rutile, only a small degree of Liinsertion (0.1–0.25 mol) can be observed, these values are typical for micrometer-sized rutile [10–12]. Li-insertion electrochemistry of microfibrous TiO2 (B) was recently reported and found to be basically different from that of anatase. The kinetics of lithium storage in anatase is controlled by solid-state diffusion of Li+ , whereas Liinsertion into TiO2 (B) is governed by a pseudocapacitive faradaic process [9]. Nanostructured materials as the anode of lithium-ion batteries stimulate great interest since they demonstrated excellent improvement in the electrochemical properties than that of the respective micrometer materials. Nanostructured TiO2 has been widely investigated as a key material in fundamental research and technological applications in the fields of optical devices, photovoltaic cells, photocatalysts, gas sensing, and electrochemical storage [13–16]. Particularly, much attention has been paid to nanostructured TiO2 for Li-insertion, because it is not only a lowvoltage insertion host for Li+ , but also a fast Li-insertion/extraction host. These characteristics render it a potential anode material for lithium-ion batteries [7,8,10]. Besides, nanoscale anatase, the electrochemical performance of nanoscale rutile was also investigated [10], which shows surprising advantages compared with micrometer-sized rutile and is able to reversibly accommodate Li up to Li0.5 TiO2 with excellent capacity retention and high rate capability on cycling. Recently, nanotubular TiO2 , nanostructured TiO2 nanorods, and TiO2 (B) nanowires have been studied for
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electrochemical performance as the anode material of lithium-ion batteries [17–20]. In this work, TiO2 nanowires were synthesized by a simple hydrothermal reaction with sodium hydroxide, ethanol and commercial P25. The preparation method is convenient, lowcost and easily adaptable to mass production. These TiO2 nanowires exhibit excellent electrochemical properties in comparison to nanotubular TiO2 and nanostructured TiO2 nanorods [17–20]. 2. Experimental 2.1. Synthesis of TiO2 nanowires Commercial P25 powders (Degussa) of 0.5 g were added into the mixture of absolute ethanol (30 mL) and aqueous NaOH solution (30 mL, 10 mol L−1 ). After stirring for 1 h, the resulting suspension was transferred into a Teflon-lined stainless steel autoclave (150 mL). The autoclave was maintained at 180 ◦ C for 24 h and then naturally cooled to room temperature. Subsequently the obtained precipitates were filtered, washed with HCl (0.1 mol L−1 ) solution and then with deionized water until the pH value approaching about 7.0. Finally, the precipitates were dried at 80 ◦ C and further annealed at 400 ◦ C for 2 h in air to obtain TiO2 nanowires. It is noted that TiO2 (B) and anatase TiO2 nanowires were synthesized by Yoshida et al. with a very similar method to that in this work [21]. They used commercial anatase TiO2 powder (Ishihara Sangyo Ltd.) and NaOH/H2 O solution as precursor to hydrothermally prepare Na-free titanate nanowires. These Na-free nanowires were transformed into TiO2 (B) nanowires at 300–500 ◦ C, and further transformed into anatase structure at 600–800 ◦ C with keeping 1D shape. While in the present work, the commercial P25 powders (Degussa), absolute ethanol and aqueous NaOH solution were used as the precursor, and the anatase nanowires containing minor TiO2 (B) phase can be obtained via the post-heat treatment of the protonated titanate nanowires at 400 ◦ C for 2 h. It was found that the ratio of ethanol to water was important for the formation of nanowires and a small amount of ethanol or its absence was not favorable for forming TiO2 nanowires. The presence of ethanol at a high concentration not only can cause the polarity of the solvent to change but also strongly affects zeta potential values of reactant particles, and increases solution viscosity. It is known that solvent plays a crucial role in determining the crystal morphology. Solvents with different physicochemical properties can influence the solubility, reactivity, and diffusion behavior of the reactants. Particularly, the polarity and coordinating ability of solvent can affect the morphology and the crystallization behavior of the finial products. The formation of nanowires may arise from the slow nucleation rate and the very fast growth rate [22].
agent, and 5 wt% polyvinylidene fluoride (PVDF) as a binder. In order to get slurry, several drops of N-methylpymolidinone (NMP) were added into the mixture of nanowires with graphite and PVDF. The prepared slurry was homogenized by stirring and then coated uniformly on 10 m thick copper foil substrates. The electrode with 1.11 mg of the active material (TiO2 nanowire powder) was dried in an oven at 80 ◦ C for 12 h and then cooled down to room temperature. Cells were assembled in a high purity argon filled glove box (H2 O 1 ppm, O2 1 ppm) using the TiO2 nanowires as the working electrode and lithium foil as the counter and reference electrodes. Celgard 2400 was used as the separator membrane and 1 mol L−1 LiPF6 and ethylene carbonate (EC)/dimethyl carbonate (DMC) at 1:1 (v/v) were used as the electrolyte. 2.4. Electrochemical measurements The assembled cells were aged for 12 h before testing. Using a LAND cell test (Land-CT 2001A) system, the galvanostatic charge/discharge test was performed at different current density, with cutoff voltage of 2.7–1.0 V vs. Li+ /Li. Cyclic voltammetry (CV) was recorded over a potential range of 2.6–0.8 V at a scan rate of 0.1 mV s−1 using a electrochemical workstation (CHI660B, Shanghai). The electrochemical impedance spectrum of the electrode was carried out with a small ac signal of 5 mV from 0.001 Hz to 100 kHz after 10 discharge–charge cycles on electrochemical workstation (CHI660B, Shanghai). The electrode was subjected to a galvanostatic charge injection followed by an open circuit equilibration for an hour before performing impedance tests, so that the equilibrium of the electrode can be ensured. The current density and specific capacity were calculated based on the mass of active materials in the electrode. All electrochemistry tests were carried out at ambient temperature (∼20 ◦ C). 3. Results and discussion 3.1. Characterization of samples The XRD pattern of the as-prepared TiO2 nanowires is given in Fig. 1. The main phase is anatase, and the coexisting residual phase can be assigned as TiO2 (B) (JCPDS 35-0088). The mixture of anatase-type TiO2 with a small quantity of TiO2 (B) is a typical product of hydrothermal treat for TiO2 particles using aqueous NaOH solution. The structure of the as-prepared samples is further ver-
2.2. Characterization of TiO2 nanowires X-ray diffraction (XRD) measurements were performed on a diffractometer (DX-2500, Fangyuan) with Cu K␣ radiation with ˚ The Raman spectrum characterization of the samples = 1.54145 A. was carried out on a laser Raman Spectrometer (RM1000, Renishaw) at an output power of 100 mW with 457.5 nm solid-state laser. Transmission electron microscopy (TEM, JEM-2010, JEOL) and field-emission scanning electron microscopy (FE-SEM, X-650) were combined to examine the morphology of the TiO2 nanowires. The thermogravimetric analysis (TGA) was performed on a DSC 6200 instrument in a N2 flow and at a heating rate of 10 ◦ C/min. 2.3. Preparation of the electrodes The working electrode was prepared by mixing 85 wt% TiO2 nanowires as an active material, 10 wt% graphite as a conducting
Fig. 1. XRD pattern of the as-prepared TiO2 nanowires.
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Fig. 2. Raman spectrum of the as-prepared TiO2 nanowires.
ified by Raman spectroscopy because it is more sensitive to the microstructure of materials than XRD. The result is shown in Fig. 2. The Raman modes at 144, 195, 396, 516, and 638 cm−1 , respectively, are in well agreement with the typical Raman features of the anatase phase. The additional five vibrational modes with weak intensity marked by arrows can be attributed to the TiO2 (B) phase [23,24], which is consistent with the above XRD result. Shown in Fig. 3(a) and (b) are the morphology images of the as-prepared sample by FE-SEM and TEM observations, which exhibit a high yield of nanowires with layered-structure. These nanowires are of diameter 40–80 nm and can extend up to 1500 nm in length. The dehydration process of the original protonated titanate nanowires was investigated by thermogravimetric analysis, and the TGA plot is depicted in Fig. 4. The protonated titanate nanowires show a weight loss of 12% after heating up to 500 ◦ C, depending on the temperature and time for drying or different washing times [25]. The steep weight loss is found below 300 ◦ C, which is usually attributed to the absorbed water and interlayer water [26]. The weight loss above 300 ◦ C is thought to be the dehydration of structural water. When the annealing temperature is 400 ◦ C, there is still 0.3% structural water within nanowires at 400 ◦ C. 3.2. Electrochemical characteristics of TiO2 nanowires The discharge and charge cycling performance of TiO2 nanowires at different current densities is shown in Fig. 5. It can be seen that the TiO2 nanowires exhibit large discharge and charge capacity and excellent cycling stability. The 10th discharge capacity can reach 327.0 mAh g−1 , which is close to the theoretical discharge capacity of TiO2 and much larger than that of ordinary anatase TiO2 in the form of Li0.5 TiO2 [23,24]. The reversible capacity diminishes rapidly in the initial several cycles, and in the following cycles, it becomes relatively stable and the efficiency reaches 98%. The irreversible capacities of the sample in the first several cycles are obviously larger than those of ordinary nanocrystalline titanate [27]. The binding water in the layered titanate nanowires could react irreversibly with lithium to form Li2 O on the surface and insider the layer, this is the major reason for the larger capacity loss. The discharging/charging tends to stabilization in the following cycles, because the binding water is consumed gradually during the first several cycles. It is noted that there is only slightly irreversible capacities after the 10th discharge and charge cycle. This may be explained by the interface-based nature of the irreversible reaction process between electrode and electrolyte.
Fig. 3. FE-SEM (a) and TEM (b) images of the as-prepared TiO2 nanowires.
It consumes lithium ions and forms a passivation interface (solid electrolyte interface—SEI). The SEI film is completely formed after several charge and discharge cycles. In addition, the film allowing a more controlled and sustainable lithium diffusion, which is well agreement with the high-frequency region of the ac impedance
Fig. 4. Thermogravimetric diagram of the original protonated titanate nanowires.
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Fig. 5. Cycling performance of the TiO2 nanowires at different current densities of 35, 70, 105, and 140 mA g−1 . The voltage window is 1.0–2.7 V. Fig. 7. Discharge–charge curves of the TiO2 nanowires at different current densities of 35, 70, 105, and 140 mA g−1 , respectively.
spectroscopy (shown in Fig. 8). There is another explanation that the irreversible capacities are attribute to lithium intercalation into irreversible sites and side reaction [18]. The charge capacity retention from the 10th to 40th cycle was 86.6% with an average capacity fade of 1.23% per cycle. After 40 cycles, a large rate capacity of 280 mAh g−1 and a high coulomb efficiency of about 98% still are maintained. The high reversibility is mainly determined by the intrinsic transport features and good electrical contact between the active material and the conductive additive [28]. Fig. 6 displays the cyclic voltammetry of the TiO2 nanowire electrode after the 10th discharge and charge cycle. The cyclic voltammograms (CVs) of the TiO2 nanowires (scan rate: 0.1 mV s−1 ) is displayed in Fig. 6. The CVs of the sample exhibits a pair of cathodic/anodic peaks at 1.70 and 2.05 V (formal potential 1.87 V), which are characteristic for lithium-ion intercalation/deintercalation reaction in anatase lattice. In addition to that, the voltammogram shows another two pairs of peaks with formal potentials of 1.52 and 1.59 V vs. Li+ /Li, respectively. This notation can be assigned to the reaction of lithium-ion intercalation/deintercalation reaction in TiO2 (B) phase lattice [29,30]. The redox peaks should be attributed to the oxidation and reduction of Ti3+ /Ti4+ along with the lithium insertion/extraction in the materials. It is also seen that the formal potentials (1.85, 1.52, and 1.59 V) in the voltammogram correspond with the discharge/discharge
potentials in Fig. 7. It can be observed that the ratio of anode and cathodic peak current, ipa /ipc , is nearly 1, demonstrating that Li+ is intercalated and deintercalated reversibly and this redox system remains in equilibrium throughout the potential scan. This indicates an improved kinetics for Li+ insertion by reducing the Li+ diffusion distance in TiO2 nanowires. The CVs curves show the peak interval of the TiO2 nanowire electrode are 0.30, 0.13, and 0.11 V, respectively, when the scan rate is 0.1 mV s−1 , which is much lower than the reported value (0.49 V) of the nanocrystalline TiO2 performed at the same scan rate. Because the peak separation is determined by the potential polarization of the material during the lithium-ion insertion or extraction, the lower peak potential interval indicates the lithium inserting reaction of TiO2 nanowires more easily [31,32]. Fig. 7 shows the galvanostatic charging and discharging carried out at different current densities. The discharge (or charge) curves descend (or rise) with a small potential plateau. Generally, lithium intercalation in TiO2 causes an increase in the fraction of Li-rich titanate and a decrease in the fraction of the Li-poor anatase fraction [33]. The two coexisting phases are responsible for the appearance of the potential plateau. Therefore, the discharge and charge potential plateaus are presented at about 1.75 and 1.95 V (vs. Li+ /Li) for lithium insertion and extraction, respectively, which is related to the fraction of the anatase phase. The formation of the solid solution lithium titanate within layer nanowires contributes to the slopping discharge and charge curves [23,24,29,34,35]. The overall cell reaction for the lithium insertion/deinsertion reaction in TiO2 electrode can be written as TiO2 + xLi+ + xe− ↔ Lix TiO2
Fig. 6. Cyclic voltammetry of the TiO2 nanowires at a scan rate of 0.1 mV s−1 (voltage range: 0.8–2.6 V).
(1)
The insertion coefficient x depends on the crystallography and microstructure of materials, being usually close to 0.5 in anatase [18]. In this work, the corresponding coefficient x is 0.84, which is much larger than ordinary anatase TiO2 [24,31,36]. This high insertion coefficient may be explained from the discharge curve of TiO2 nanowires. The discharge curve can be divided into three domains. The first domain characterized by a monotonous potential decrease corresponds to a solid solution insertion mechanism [37]. The second domain characterized by a plateau at about 1.70 V is a biphasic transition from the tetragonal TiO2 to orthorhombic Li0.5 TiO2 . In general, such a phase transition is associated with 3–4% increase of the lattice volume upon intercalation [38,39], causing local lattice strain during lithium insertion. Therefore, materials with welldeveloped plateau domain may undergo more lattice strain during
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are corresponding to the semicircle at medium frequency, and ZW (the Warburg impedance) is corresponding to a straight slopping line at low frequency [46,47]. It can be observed that the diameter of the semicircle at high frequency is larger than that of medium frequency, meaning the Li+ transfer is slower in SEI film than in double layer. In addition, the high- and middle-frequency impedance spectrum changes little under different potentials, meaning that the kinetic process of electrode reaction has become stable after 10 cycles, which is consistent with the stability of discharge–charge cycles shown in Fig. 5. 4. Conclusions
Fig. 8. Electrochemical impedance spectra of the TiO2 nanowires. ac impedance spectrum was measured on the frequency range from 0.001 Hz to 100 kHz with ac amplitude 5 mV (peak-to-peak).
lithiation process and therefore induce faster capacity fading during cycling [40]. This short plateau in the second domain indicates that the nanowires can accommodate more lithium and are expected to have higher cycling stability. The third domain beyond the plateau is a further Li-insertion into Li0.5 TiO2 as the voltage drops linearly. The extension of this domain means that Li inserting into TiO2 (B). The Li-storage capacity of TiO2 (B) is larger than that of anatase TiO2 [41]. This improved performance may be ascribed to a shorter diffusion length for both electron and Li+ , and a larger electrode/electrolyte contact area of TiO2 nanowires compared with traditional materials [23], which facilitate the lithium insertion and extraction. There are some extra site occupations of the nanowires for lithium insertion, related to the surface imperfection and the formation of solid solution lithium titanate on the surface. The capacity has only a little decrease when increasing current density, indicating a good power characteristic. This is probably also due to the feature of nanostructured electrodes [42]. At high discharge current density, high Li+ insertion-flux density and slow Li+ transport within normal electrode result in concentration polarization of Li+ . For nanowires, increasing the surface area of the electrode and decreasing the diffusion distance can lead to a decrease of concentration polarization at high discharge current, and consequently, the cycling capability can be improved [3]. On the other hand, the large specific surface area of nanowires material can also relieve the stress associated with the volume changes more easily in the process of lithium intercalation/extraction, increase the electrode–electrolyte contact area, and decrease the specific current density of the active material. In addition, the majority of capacity is obtained between 2.0 and 1.0 V, larger discharge–charge voltage than that of graphite, which can improve safety and stability for lithium-ion batteries. ac impedance spectroscopy is a powerful technique to determine the kinetic process of electrode reactions [43,44]. The Nyquist plots obtained at three potential of 1.62, 1.65, and 1.68 V were shown in Fig. 8, exhibiting two partially overlapped semicircles at the high-to-medium frequencies and a straight slopping line at low frequency [45]. The equivalent circuit for this cell system is depicted in the inset of Fig. 8. Where R´ (bulk resistance of the electrolyte, separator, and electrode) is obtained from the highfrequency intercept of the semicircle with the real axis, CSEI and RSEI (the capacitance and resistance of SEI, respectively) are corresponding to the semicircle at high frequency, Cd and Rct (the double layer capacitance and the charge transfer resistance, respectively)
TiO2 nanowires were prepared by a hydrothermal reaction followed by the post-heat treatment at 400 ◦ C in air. The XRD, Raman and TEM results showed that the products crystallized in anatase TiO2 containing minor TiO2 (B) phase, with 40–80 nm in diameters and 400–1500 nm lengths. These nanowires exhibited high rate capacity of 280 mAh g−1 even after 40 cycles. Furthermore, the cyclability was excellent at different current densities, resulting from the improved electrochemical kinetics for Li+ insertion in the case of nanowires. The coulombic efficiency approaching 98% and the good symmetry of the cyclic voltammogram reveal excellent reversibility. The electrochemical impedance spectroscopy exhibited a stable kinetics of Li+ intercalation into TiO2 nanowires. The TiO2 nanowires might be promising for high energy density lithium-ion batteries. Acknowledgements This work has been supported by National Natural Science Foundation of China (grant no. 60476001) and by the Project of Cultivating Innovative Talents for Colleges & Universities of Henan Province (no. 2002006). References [1] K.M. Abrham, Electrochim. Acta 38 (1993) 1233. [2] C. Natarajan, K. Setoguchi, G. Nogami, Electrochim. Acta 43 (1998) 3371. [3] Y.K. Zhou, L. Cao, F.B. Zhang, B.L. He, H.L. Li, J. Electrochem. Soc. 150 (9) (2003) A1246. [4] S.Y. Huang, L. Kavan, I. Exnar, M. Gratzel, J. Electrochem. Soc. 142 (1995) L142. [5] A. Kuhn, R. Amandi, F. Garc´ıa-Alvarado, J. Power Sources 92 (2001) 221. [6] J.F. Banfield, D.R. Veblen, Am. Miner. 77 (1992) 545. ¨ [7] L. Kavan, M. Gratzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, J. Am. Chem. Soc. 118 (1996) 6716. ¨ [8] I. Exnar, L. Kavan, S.Y. Huang, M. Gratzel, J. Power Sources 68 (1997) 720. ´ M. Kalbaˇ ´ c, L. Kavan, I. Exnar, M. Gratzel, ¨ [9] M. Zukalova, Chem. Mater. 17 (2005) 1248. [10] Y.S. Hu, L. Kienle, Y.G. Guo, J. Maier, Adv. Mater. 18 (2006) 1421. [11] W.J. Macklin, R.J. Neat, Solid State Ionics 53–56 (1992) 694. [12] L. Kavan, D. Fattakhova, P. Krtil, J. Electrochem. Soc. 146 (1999) 1375. ¨ [13] B. O’Regan, M. Gratzel, Nature 353 (1991) 737. [14] G.K. Boschloo, A. Goossens, J. Schoonman, J. Electrochem. Soc. 144 (1997) 1311. [15] Y. Li, D.S. Hwang, N.H. Lee, S.J. Kim, Chem. Phys. Lett. 404 (2005) 25. [16] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Science 297 (2002) 2243. [17] Y.K. Zhou, C. Lin, F.B. Zhang, B.L. He, H.L. Li, J. Electrochem. Soc. 150 (2003) A1246. [18] J.R. Li, Z.L. Tang, Z.T. Zhang, Electrochem. Solid-State Lett. 8 (2005) A316. [19] A.R. Armstrong, G. Armstrong, J. Canales, R. Garc´ıa, P.G. Bruce, Adv. Mater. 17 (2005) 862. [20] D.V. Bavykin, J.M. Friedrich, F.C. Walsh, Adv. Mater. 18 (2006) 2807. [21] R. Yoshida, Y. Suzuki, S. Yoshikawa, J. Solid State Chem. 178 (2005) 2179. [22] B.M. Wen, C.Y. Liu, Y. Liu, New J. Chem. 29 (2005) 969. [23] J. Tu, X.B. Zhao, G.S. Cao, J.P. Tu, T.J. Zhu, Mater. Lett. 60 (2006) 3251. [24] A.R. Armstrong, G. Armstrong, J. Canales, P.G. Bruce, Angew. Chem. Int. Ed. 43 (2004) 2286. [25] (a) N. Wang, H. Lin, J.B. Li, X.Z. Yang, B. Chi, C.F. Lin, J. Alloys Compd. 424 (2006) 311; (b) X.M. Sun, Y.D. Li, J. Chem. Eur. 9 (2003) 2229. [26] Y. Lan, X.P. Gao, H.Y. Zhu, Z.F. Zheng, T.Y. Yan, F. Wu, S.P. Ringer, D.Y. Song, Adv. Funct. Mater. 15 (2005) 1330.
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Preparation and electrochemical characterization of TiO2 nanowires as an electrode material for lithium-ion batteries Yunfei Wang a , Muying Wu b , W.F. Zhang a,∗ a b
Institute of Micro-system Physics and School of Physics & Electronics, Henan University, 85 Minglun Street, Kaifeng 475001, PR China Department of Electronic Engineering, Dongguan University of Technology, Dongguan 523808, PR China
a r t i c l e
i n f o
Article history: Received 1 March 2008 Received in revised form 28 May 2008 Accepted 29 May 2008 Available online 5 June 2008 Keywords: TiO2 nanowire Discharge–charge Electrochemistry Lithium-ion batteries
a b s t r a c t Anatase TiO2 nanowires containing minor TiO2 (B) phase were prepared by a hydrothermal chemical reaction followed by the post-heat treatment at 400 ◦ C. The phase structure and morphology were analyzed by X-ray diffraction, Raman scattering, transmission electron microscope, and field-emission scanning electron microscopy. The electrochemical properties were investigated by employing constant current discharge–charge test, cyclic voltammetry, and electrochemical impedance techniques. These nanowires exhibited high rate capacity of 280 mAh g−1 even after 40 cycles, and the coulombic efficiency was approximately 98%, indicating excellent cycling stability and reversibility. The electrochemical impedance spectra showed a stable kinetic process of the electrode reaction. These results indicated that the TiO2 nanowires have promising application for high energy density lithium-ion batteries. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction The demand for rechargeable batteries is increasing in portable devices and power sources for automobile electrical systems. Lithium-ion batteries have been considered to be a good answer because of their high energy storage density, long cycle life, little memory effect, poisonous metals free and so on [1,2]. Most of the commercial lithium-ion batteries use graphite as an anode material. The graphite electrode, however, has several disadvantages such as its electrical disconnection, structural deformation, and initial loss of capacity [1,3], which dissatisfy the need of production. To avoid these drawbacks, transition metal oxides including WO3 , MoO3 , V2 O5 , and TiO2 have attracted much interest [2,4]. Especially, TiO2 is regarded as a promising active lithium intercalation material with low production cost and high capacity, low-voltage (below ca. 2.0 V vs. Li+ /Li) for lithium intercalation. In the structure of TiO2 , the TiO6 octahedra share vertices and edges to build up the three-dimensional framework, leaving favorable empty sites available for lithium insertion [5]. TiO2 crystallizes in various forms such as anatase, rutile, brookite, TiO2 (B), etc. [6–9]. Under standard conditions, rutile is the thermodynamically most stable structure of TiO2 , and is also the most common natural form. TiO2 (B) is a metastable monoclinic modification of titanium dioxide. Anatase is generally considered to be the most electroactive Li-insertion
∗ Corresponding author. Tel.: +86 378 3881 940; fax: +86 378 3880 659. E-mail addresses: [email protected], [email protected] (W.F. Zhang). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.05.068
host among these TiO2 polymorphs. However, rutile and TiO2 (B) for electrochemical Li-insertion also aroused increasing interests [9–12]. For the micrometer-sized rutile, only a small degree of Liinsertion (0.1–0.25 mol) can be observed, these values are typical for micrometer-sized rutile [10–12]. Li-insertion electrochemistry of microfibrous TiO2 (B) was recently reported and found to be basically different from that of anatase. The kinetics of lithium storage in anatase is controlled by solid-state diffusion of Li+ , whereas Liinsertion into TiO2 (B) is governed by a pseudocapacitive faradaic process [9]. Nanostructured materials as the anode of lithium-ion batteries stimulate great interest since they demonstrated excellent improvement in the electrochemical properties than that of the respective micrometer materials. Nanostructured TiO2 has been widely investigated as a key material in fundamental research and technological applications in the fields of optical devices, photovoltaic cells, photocatalysts, gas sensing, and electrochemical storage [13–16]. Particularly, much attention has been paid to nanostructured TiO2 for Li-insertion, because it is not only a lowvoltage insertion host for Li+ , but also a fast Li-insertion/extraction host. These characteristics render it a potential anode material for lithium-ion batteries [7,8,10]. Besides, nanoscale anatase, the electrochemical performance of nanoscale rutile was also investigated [10], which shows surprising advantages compared with micrometer-sized rutile and is able to reversibly accommodate Li up to Li0.5 TiO2 with excellent capacity retention and high rate capability on cycling. Recently, nanotubular TiO2 , nanostructured TiO2 nanorods, and TiO2 (B) nanowires have been studied for
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electrochemical performance as the anode material of lithium-ion batteries [17–20]. In this work, TiO2 nanowires were synthesized by a simple hydrothermal reaction with sodium hydroxide, ethanol and commercial P25. The preparation method is convenient, lowcost and easily adaptable to mass production. These TiO2 nanowires exhibit excellent electrochemical properties in comparison to nanotubular TiO2 and nanostructured TiO2 nanorods [17–20]. 2. Experimental 2.1. Synthesis of TiO2 nanowires Commercial P25 powders (Degussa) of 0.5 g were added into the mixture of absolute ethanol (30 mL) and aqueous NaOH solution (30 mL, 10 mol L−1 ). After stirring for 1 h, the resulting suspension was transferred into a Teflon-lined stainless steel autoclave (150 mL). The autoclave was maintained at 180 ◦ C for 24 h and then naturally cooled to room temperature. Subsequently the obtained precipitates were filtered, washed with HCl (0.1 mol L−1 ) solution and then with deionized water until the pH value approaching about 7.0. Finally, the precipitates were dried at 80 ◦ C and further annealed at 400 ◦ C for 2 h in air to obtain TiO2 nanowires. It is noted that TiO2 (B) and anatase TiO2 nanowires were synthesized by Yoshida et al. with a very similar method to that in this work [21]. They used commercial anatase TiO2 powder (Ishihara Sangyo Ltd.) and NaOH/H2 O solution as precursor to hydrothermally prepare Na-free titanate nanowires. These Na-free nanowires were transformed into TiO2 (B) nanowires at 300–500 ◦ C, and further transformed into anatase structure at 600–800 ◦ C with keeping 1D shape. While in the present work, the commercial P25 powders (Degussa), absolute ethanol and aqueous NaOH solution were used as the precursor, and the anatase nanowires containing minor TiO2 (B) phase can be obtained via the post-heat treatment of the protonated titanate nanowires at 400 ◦ C for 2 h. It was found that the ratio of ethanol to water was important for the formation of nanowires and a small amount of ethanol or its absence was not favorable for forming TiO2 nanowires. The presence of ethanol at a high concentration not only can cause the polarity of the solvent to change but also strongly affects zeta potential values of reactant particles, and increases solution viscosity. It is known that solvent plays a crucial role in determining the crystal morphology. Solvents with different physicochemical properties can influence the solubility, reactivity, and diffusion behavior of the reactants. Particularly, the polarity and coordinating ability of solvent can affect the morphology and the crystallization behavior of the finial products. The formation of nanowires may arise from the slow nucleation rate and the very fast growth rate [22].
agent, and 5 wt% polyvinylidene fluoride (PVDF) as a binder. In order to get slurry, several drops of N-methylpymolidinone (NMP) were added into the mixture of nanowires with graphite and PVDF. The prepared slurry was homogenized by stirring and then coated uniformly on 10 m thick copper foil substrates. The electrode with 1.11 mg of the active material (TiO2 nanowire powder) was dried in an oven at 80 ◦ C for 12 h and then cooled down to room temperature. Cells were assembled in a high purity argon filled glove box (H2 O 1 ppm, O2 1 ppm) using the TiO2 nanowires as the working electrode and lithium foil as the counter and reference electrodes. Celgard 2400 was used as the separator membrane and 1 mol L−1 LiPF6 and ethylene carbonate (EC)/dimethyl carbonate (DMC) at 1:1 (v/v) were used as the electrolyte. 2.4. Electrochemical measurements The assembled cells were aged for 12 h before testing. Using a LAND cell test (Land-CT 2001A) system, the galvanostatic charge/discharge test was performed at different current density, with cutoff voltage of 2.7–1.0 V vs. Li+ /Li. Cyclic voltammetry (CV) was recorded over a potential range of 2.6–0.8 V at a scan rate of 0.1 mV s−1 using a electrochemical workstation (CHI660B, Shanghai). The electrochemical impedance spectrum of the electrode was carried out with a small ac signal of 5 mV from 0.001 Hz to 100 kHz after 10 discharge–charge cycles on electrochemical workstation (CHI660B, Shanghai). The electrode was subjected to a galvanostatic charge injection followed by an open circuit equilibration for an hour before performing impedance tests, so that the equilibrium of the electrode can be ensured. The current density and specific capacity were calculated based on the mass of active materials in the electrode. All electrochemistry tests were carried out at ambient temperature (∼20 ◦ C). 3. Results and discussion 3.1. Characterization of samples The XRD pattern of the as-prepared TiO2 nanowires is given in Fig. 1. The main phase is anatase, and the coexisting residual phase can be assigned as TiO2 (B) (JCPDS 35-0088). The mixture of anatase-type TiO2 with a small quantity of TiO2 (B) is a typical product of hydrothermal treat for TiO2 particles using aqueous NaOH solution. The structure of the as-prepared samples is further ver-
2.2. Characterization of TiO2 nanowires X-ray diffraction (XRD) measurements were performed on a diffractometer (DX-2500, Fangyuan) with Cu K␣ radiation with ˚ The Raman spectrum characterization of the samples = 1.54145 A. was carried out on a laser Raman Spectrometer (RM1000, Renishaw) at an output power of 100 mW with 457.5 nm solid-state laser. Transmission electron microscopy (TEM, JEM-2010, JEOL) and field-emission scanning electron microscopy (FE-SEM, X-650) were combined to examine the morphology of the TiO2 nanowires. The thermogravimetric analysis (TGA) was performed on a DSC 6200 instrument in a N2 flow and at a heating rate of 10 ◦ C/min. 2.3. Preparation of the electrodes The working electrode was prepared by mixing 85 wt% TiO2 nanowires as an active material, 10 wt% graphite as a conducting
Fig. 1. XRD pattern of the as-prepared TiO2 nanowires.
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Fig. 2. Raman spectrum of the as-prepared TiO2 nanowires.
ified by Raman spectroscopy because it is more sensitive to the microstructure of materials than XRD. The result is shown in Fig. 2. The Raman modes at 144, 195, 396, 516, and 638 cm−1 , respectively, are in well agreement with the typical Raman features of the anatase phase. The additional five vibrational modes with weak intensity marked by arrows can be attributed to the TiO2 (B) phase [23,24], which is consistent with the above XRD result. Shown in Fig. 3(a) and (b) are the morphology images of the as-prepared sample by FE-SEM and TEM observations, which exhibit a high yield of nanowires with layered-structure. These nanowires are of diameter 40–80 nm and can extend up to 1500 nm in length. The dehydration process of the original protonated titanate nanowires was investigated by thermogravimetric analysis, and the TGA plot is depicted in Fig. 4. The protonated titanate nanowires show a weight loss of 12% after heating up to 500 ◦ C, depending on the temperature and time for drying or different washing times [25]. The steep weight loss is found below 300 ◦ C, which is usually attributed to the absorbed water and interlayer water [26]. The weight loss above 300 ◦ C is thought to be the dehydration of structural water. When the annealing temperature is 400 ◦ C, there is still 0.3% structural water within nanowires at 400 ◦ C. 3.2. Electrochemical characteristics of TiO2 nanowires The discharge and charge cycling performance of TiO2 nanowires at different current densities is shown in Fig. 5. It can be seen that the TiO2 nanowires exhibit large discharge and charge capacity and excellent cycling stability. The 10th discharge capacity can reach 327.0 mAh g−1 , which is close to the theoretical discharge capacity of TiO2 and much larger than that of ordinary anatase TiO2 in the form of Li0.5 TiO2 [23,24]. The reversible capacity diminishes rapidly in the initial several cycles, and in the following cycles, it becomes relatively stable and the efficiency reaches 98%. The irreversible capacities of the sample in the first several cycles are obviously larger than those of ordinary nanocrystalline titanate [27]. The binding water in the layered titanate nanowires could react irreversibly with lithium to form Li2 O on the surface and insider the layer, this is the major reason for the larger capacity loss. The discharging/charging tends to stabilization in the following cycles, because the binding water is consumed gradually during the first several cycles. It is noted that there is only slightly irreversible capacities after the 10th discharge and charge cycle. This may be explained by the interface-based nature of the irreversible reaction process between electrode and electrolyte.
Fig. 3. FE-SEM (a) and TEM (b) images of the as-prepared TiO2 nanowires.
It consumes lithium ions and forms a passivation interface (solid electrolyte interface—SEI). The SEI film is completely formed after several charge and discharge cycles. In addition, the film allowing a more controlled and sustainable lithium diffusion, which is well agreement with the high-frequency region of the ac impedance
Fig. 4. Thermogravimetric diagram of the original protonated titanate nanowires.
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Fig. 5. Cycling performance of the TiO2 nanowires at different current densities of 35, 70, 105, and 140 mA g−1 . The voltage window is 1.0–2.7 V. Fig. 7. Discharge–charge curves of the TiO2 nanowires at different current densities of 35, 70, 105, and 140 mA g−1 , respectively.
spectroscopy (shown in Fig. 8). There is another explanation that the irreversible capacities are attribute to lithium intercalation into irreversible sites and side reaction [18]. The charge capacity retention from the 10th to 40th cycle was 86.6% with an average capacity fade of 1.23% per cycle. After 40 cycles, a large rate capacity of 280 mAh g−1 and a high coulomb efficiency of about 98% still are maintained. The high reversibility is mainly determined by the intrinsic transport features and good electrical contact between the active material and the conductive additive [28]. Fig. 6 displays the cyclic voltammetry of the TiO2 nanowire electrode after the 10th discharge and charge cycle. The cyclic voltammograms (CVs) of the TiO2 nanowires (scan rate: 0.1 mV s−1 ) is displayed in Fig. 6. The CVs of the sample exhibits a pair of cathodic/anodic peaks at 1.70 and 2.05 V (formal potential 1.87 V), which are characteristic for lithium-ion intercalation/deintercalation reaction in anatase lattice. In addition to that, the voltammogram shows another two pairs of peaks with formal potentials of 1.52 and 1.59 V vs. Li+ /Li, respectively. This notation can be assigned to the reaction of lithium-ion intercalation/deintercalation reaction in TiO2 (B) phase lattice [29,30]. The redox peaks should be attributed to the oxidation and reduction of Ti3+ /Ti4+ along with the lithium insertion/extraction in the materials. It is also seen that the formal potentials (1.85, 1.52, and 1.59 V) in the voltammogram correspond with the discharge/discharge
potentials in Fig. 7. It can be observed that the ratio of anode and cathodic peak current, ipa /ipc , is nearly 1, demonstrating that Li+ is intercalated and deintercalated reversibly and this redox system remains in equilibrium throughout the potential scan. This indicates an improved kinetics for Li+ insertion by reducing the Li+ diffusion distance in TiO2 nanowires. The CVs curves show the peak interval of the TiO2 nanowire electrode are 0.30, 0.13, and 0.11 V, respectively, when the scan rate is 0.1 mV s−1 , which is much lower than the reported value (0.49 V) of the nanocrystalline TiO2 performed at the same scan rate. Because the peak separation is determined by the potential polarization of the material during the lithium-ion insertion or extraction, the lower peak potential interval indicates the lithium inserting reaction of TiO2 nanowires more easily [31,32]. Fig. 7 shows the galvanostatic charging and discharging carried out at different current densities. The discharge (or charge) curves descend (or rise) with a small potential plateau. Generally, lithium intercalation in TiO2 causes an increase in the fraction of Li-rich titanate and a decrease in the fraction of the Li-poor anatase fraction [33]. The two coexisting phases are responsible for the appearance of the potential plateau. Therefore, the discharge and charge potential plateaus are presented at about 1.75 and 1.95 V (vs. Li+ /Li) for lithium insertion and extraction, respectively, which is related to the fraction of the anatase phase. The formation of the solid solution lithium titanate within layer nanowires contributes to the slopping discharge and charge curves [23,24,29,34,35]. The overall cell reaction for the lithium insertion/deinsertion reaction in TiO2 electrode can be written as TiO2 + xLi+ + xe− ↔ Lix TiO2
Fig. 6. Cyclic voltammetry of the TiO2 nanowires at a scan rate of 0.1 mV s−1 (voltage range: 0.8–2.6 V).
(1)
The insertion coefficient x depends on the crystallography and microstructure of materials, being usually close to 0.5 in anatase [18]. In this work, the corresponding coefficient x is 0.84, which is much larger than ordinary anatase TiO2 [24,31,36]. This high insertion coefficient may be explained from the discharge curve of TiO2 nanowires. The discharge curve can be divided into three domains. The first domain characterized by a monotonous potential decrease corresponds to a solid solution insertion mechanism [37]. The second domain characterized by a plateau at about 1.70 V is a biphasic transition from the tetragonal TiO2 to orthorhombic Li0.5 TiO2 . In general, such a phase transition is associated with 3–4% increase of the lattice volume upon intercalation [38,39], causing local lattice strain during lithium insertion. Therefore, materials with welldeveloped plateau domain may undergo more lattice strain during
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are corresponding to the semicircle at medium frequency, and ZW (the Warburg impedance) is corresponding to a straight slopping line at low frequency [46,47]. It can be observed that the diameter of the semicircle at high frequency is larger than that of medium frequency, meaning the Li+ transfer is slower in SEI film than in double layer. In addition, the high- and middle-frequency impedance spectrum changes little under different potentials, meaning that the kinetic process of electrode reaction has become stable after 10 cycles, which is consistent with the stability of discharge–charge cycles shown in Fig. 5. 4. Conclusions
Fig. 8. Electrochemical impedance spectra of the TiO2 nanowires. ac impedance spectrum was measured on the frequency range from 0.001 Hz to 100 kHz with ac amplitude 5 mV (peak-to-peak).
lithiation process and therefore induce faster capacity fading during cycling [40]. This short plateau in the second domain indicates that the nanowires can accommodate more lithium and are expected to have higher cycling stability. The third domain beyond the plateau is a further Li-insertion into Li0.5 TiO2 as the voltage drops linearly. The extension of this domain means that Li inserting into TiO2 (B). The Li-storage capacity of TiO2 (B) is larger than that of anatase TiO2 [41]. This improved performance may be ascribed to a shorter diffusion length for both electron and Li+ , and a larger electrode/electrolyte contact area of TiO2 nanowires compared with traditional materials [23], which facilitate the lithium insertion and extraction. There are some extra site occupations of the nanowires for lithium insertion, related to the surface imperfection and the formation of solid solution lithium titanate on the surface. The capacity has only a little decrease when increasing current density, indicating a good power characteristic. This is probably also due to the feature of nanostructured electrodes [42]. At high discharge current density, high Li+ insertion-flux density and slow Li+ transport within normal electrode result in concentration polarization of Li+ . For nanowires, increasing the surface area of the electrode and decreasing the diffusion distance can lead to a decrease of concentration polarization at high discharge current, and consequently, the cycling capability can be improved [3]. On the other hand, the large specific surface area of nanowires material can also relieve the stress associated with the volume changes more easily in the process of lithium intercalation/extraction, increase the electrode–electrolyte contact area, and decrease the specific current density of the active material. In addition, the majority of capacity is obtained between 2.0 and 1.0 V, larger discharge–charge voltage than that of graphite, which can improve safety and stability for lithium-ion batteries. ac impedance spectroscopy is a powerful technique to determine the kinetic process of electrode reactions [43,44]. The Nyquist plots obtained at three potential of 1.62, 1.65, and 1.68 V were shown in Fig. 8, exhibiting two partially overlapped semicircles at the high-to-medium frequencies and a straight slopping line at low frequency [45]. The equivalent circuit for this cell system is depicted in the inset of Fig. 8. Where R´ (bulk resistance of the electrolyte, separator, and electrode) is obtained from the highfrequency intercept of the semicircle with the real axis, CSEI and RSEI (the capacitance and resistance of SEI, respectively) are corresponding to the semicircle at high frequency, Cd and Rct (the double layer capacitance and the charge transfer resistance, respectively)
TiO2 nanowires were prepared by a hydrothermal reaction followed by the post-heat treatment at 400 ◦ C in air. The XRD, Raman and TEM results showed that the products crystallized in anatase TiO2 containing minor TiO2 (B) phase, with 40–80 nm in diameters and 400–1500 nm lengths. These nanowires exhibited high rate capacity of 280 mAh g−1 even after 40 cycles. Furthermore, the cyclability was excellent at different current densities, resulting from the improved electrochemical kinetics for Li+ insertion in the case of nanowires. The coulombic efficiency approaching 98% and the good symmetry of the cyclic voltammogram reveal excellent reversibility. The electrochemical impedance spectroscopy exhibited a stable kinetics of Li+ intercalation into TiO2 nanowires. The TiO2 nanowires might be promising for high energy density lithium-ion batteries. Acknowledgements This work has been supported by National Natural Science Foundation of China (grant no. 60476001) and by the Project of Cultivating Innovative Talents for Colleges & Universities of Henan Province (no. 2002006). References [1] K.M. Abrham, Electrochim. Acta 38 (1993) 1233. [2] C. Natarajan, K. Setoguchi, G. Nogami, Electrochim. Acta 43 (1998) 3371. [3] Y.K. Zhou, L. Cao, F.B. Zhang, B.L. He, H.L. Li, J. Electrochem. Soc. 150 (9) (2003) A1246. [4] S.Y. Huang, L. Kavan, I. Exnar, M. Gratzel, J. Electrochem. Soc. 142 (1995) L142. [5] A. Kuhn, R. Amandi, F. Garc´ıa-Alvarado, J. Power Sources 92 (2001) 221. [6] J.F. Banfield, D.R. Veblen, Am. Miner. 77 (1992) 545. ¨ [7] L. Kavan, M. Gratzel, S.E. Gilbert, C. Klemenz, H.J. Scheel, J. Am. Chem. Soc. 118 (1996) 6716. ¨ [8] I. Exnar, L. Kavan, S.Y. Huang, M. Gratzel, J. Power Sources 68 (1997) 720. ´ M. Kalbaˇ ´ c, L. Kavan, I. Exnar, M. Gratzel, ¨ [9] M. Zukalova, Chem. Mater. 17 (2005) 1248. [10] Y.S. Hu, L. Kienle, Y.G. Guo, J. Maier, Adv. Mater. 18 (2006) 1421. [11] W.J. Macklin, R.J. Neat, Solid State Ionics 53–56 (1992) 694. [12] L. Kavan, D. Fattakhova, P. Krtil, J. Electrochem. Soc. 146 (1999) 1375. ¨ [13] B. O’Regan, M. Gratzel, Nature 353 (1991) 737. [14] G.K. Boschloo, A. Goossens, J. Schoonman, J. Electrochem. Soc. 144 (1997) 1311. [15] Y. Li, D.S. Hwang, N.H. Lee, S.J. Kim, Chem. Phys. Lett. 404 (2005) 25. [16] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Science 297 (2002) 2243. [17] Y.K. Zhou, C. Lin, F.B. Zhang, B.L. He, H.L. Li, J. Electrochem. Soc. 150 (2003) A1246. [18] J.R. Li, Z.L. Tang, Z.T. Zhang, Electrochem. Solid-State Lett. 8 (2005) A316. [19] A.R. Armstrong, G. Armstrong, J. Canales, R. Garc´ıa, P.G. Bruce, Adv. Mater. 17 (2005) 862. [20] D.V. Bavykin, J.M. Friedrich, F.C. Walsh, Adv. Mater. 18 (2006) 2807. [21] R. Yoshida, Y. Suzuki, S. Yoshikawa, J. Solid State Chem. 178 (2005) 2179. [22] B.M. Wen, C.Y. Liu, Y. Liu, New J. Chem. 29 (2005) 969. [23] J. Tu, X.B. Zhao, G.S. Cao, J.P. Tu, T.J. Zhu, Mater. Lett. 60 (2006) 3251. [24] A.R. Armstrong, G. Armstrong, J. Canales, P.G. Bruce, Angew. Chem. Int. Ed. 43 (2004) 2286. [25] (a) N. Wang, H. Lin, J.B. Li, X.Z. Yang, B. Chi, C.F. Lin, J. Alloys Compd. 424 (2006) 311; (b) X.M. Sun, Y.D. Li, J. Chem. Eur. 9 (2003) 2229. [26] Y. Lan, X.P. Gao, H.Y. Zhu, Z.F. Zheng, T.Y. Yan, F. Wu, S.P. Ringer, D.Y. Song, Adv. Funct. Mater. 15 (2005) 1330.
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