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Hydrothermal synthesis and electrochemical properties of crystalline Zn2V2O7 nanorods
Materials Letters 107 (2013) 35–38
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hydrothermal synthesis and electrochemical properties of crystalline Zn2V2O7 nanorods Zhanjun Chen a, Wenda Huang b, Dongliang Lu c, Ruirui Zhao a, Hongyu Chen a,n a
School of Chemistry and Environment, South China Normal University, Guangzhou, Guangdong 510006, China Guangzhou Tinci Materials Technology Co., Ltd., Guangzhou, Guangdong 510006, China c Department of Electronics and Information, Foshan Polytechnic, Foshan 528137, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 7 March 2013 Accepted 25 May 2013 Available online 2 June 2013
Zn2V2O7 nanorods were successfully synthesized by a hydrothermal route for the application in the Lithium ion batteries. The effects of the reacting time on the Zn2V2O7 nanorods as well as the corresponding electrochemical properties were investigated. Results show that the obtained sample under the optimum condition possesses high crystallinity; the width and the thickness are in the range of 40–60 nm and 20–40 nm, respectively. Based on the observations, a possible growth mechanism of Zn2V2O7 nanorods could be described as a reacting–exfoliating–splitting process. Besides, the Zn2V2O7 nanorods used as anode materials in rechargeable lithium-ion battery can exhibit a highly reversible discharge/charge capacity and excellent rate property at a current density as high as 0.1 A g−1. This might be attributed to the intrinsic characteristics of the layered structure of Zn2V2O7 nanorods. & 2013 Elsevier B.V. All rights reserved.
Keywords: Zn2V2O7 Nanorods Hydrothermal method Lithium ion battery
1. Introduction During the past several decades, lithium-ion batteries (LIB) had become a commercial reality and been widely applied as a power supply for portable electronic devices. A traditional LIB is mainly constituted of two intercalation compounds as electrode materials, a lithiated metal oxide cathode and a graphite anode. Graphite had been widely used as anode material for LIB owing to the favorable low potential (vs. Li+/Li) and the good cycle performance. However, the specific capacity of graphite is relatively low (372 mAh g−1) and the rate property is unsatisfactory either, which are great drawbacks for electric and/or hybrid vehicle applications of LIB. Many efforts have been made in finding out alternative materials to replace graphite anode and a variety of materials such as oxides [1,2], nitrides [3], and metals [4,5] were investigated. Among these candidate compounds, the vanadium oxides and its derivatives attract more attention due to their layered structure and high specific capacity. According to the report published by Sides et al. [6] recently, the lithium ion diffusion of the one-dimensional (1D) nanomaterials is restricted to the radius direction and the diffusion distance is significantly shortened, thus the rate capability of the 1D nanomaterial-based electrode is superior to the electrode composed of bulk materials. One-dimensional nanomaterials such as
n
Corresponding author. Tel.: +86 20 39310183. E-mail address: [email protected] (H. Chen).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.05.112
TiO2–B nanowire [7], TiO2 nanotube [8], and β-AgVO3 nanorods [9] have been synthesized successfully and applied to LIB already, and the specific capacity and the rate capability of the batteries can be improved greatly.
Fig. 1. XRD patterns of the products obtained at 205 1C for different reaction times: (a) 12 h; (b) 24 h; (c) 36 h and (d) 48 h.
36
Z. Chen et al. / Materials Letters 107 (2013) 35–38
To the best of our knowledge, there are no reports on the synthesis of Zn2V2O7 nanorods. With this background, a simple hydrothermal route for the preparation of Zn2V2O7 nanorods was described and its electrochemical properties were further studied in this paper.
2. Experimental Sample synthesis: The synthesis of Zn2V2O7 nanorods was performed by a simple hydrothermal process. At first, 0.5 mmol
of commercial ZnSO4 powder and 1 mmol of commercial NH4VO3 power were dispersed into 40 ml H2O, and then was transferred into a 50 ml Teflon-lined autoclave in an oven at 205 1C for several hours. After the reaction, the products were obtained. Sample characterization: X-ray powder diffraction (XRD) patterns were recorded using a diffractometer (Co Kα, PANalytical, X’Pert, data were convert into Cu Kα). Scanning electron microscope (SEM) and transmission electron microscopy (TEM) were taken on a Philip-XL30 instrument and a JEOL 2010 instrument, respectively.
1µm
1µm
1µm
1µm
d022=0.312 nm
5 nm
200 nm
reacting
VO3-
Zn2+
splitting
exfoliating
layered structure
Zn2V2O7sheets
Zn2V2O7nanorods
Fig. 2. SEM images of the products synthesized at the temperature of 205 1C for different reaction times: (a) and (b) 24 h, (c) 36 h and (d) 48 h; (e)TEM and (f) HRTEM image of the product synthesized at the temperature of 205 1C for 48 h; (g) Schematic illustration of the formation process of Zn2V2O7 nanorods.
Z. Chen et al. / Materials Letters 107 (2013) 35–38
Electrochemical measurement: For the electrochemical measurement, 60 wt% active materials were mixed and grounded with 10 wt% polyvinylidene fluoride (PVDF) powder as a binder and 30 wt% acetylene back carbon (AB) powder as the conductive assistant materials. The mixture was spread and pressed on Cu foil circular flakes as the working electrode (WE), and dried at 120 1C for 24 h under the vacuum conditions. Metallic lithium foils were used as negative electrode. The electrolyte was 1 M LiPF6 in a 1/1/1 (volume ratio) mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC). The separator was UBE 3093 (Japan) micro-porous polypropylene membrane. The cells were assembled in a glove box filled with highly pure argon gas (O2 and H2O levels o1 ppm), and charge/discharge tests were performed in the voltage range of 0.05–3 V (Li+/Li) at different current densities on a Land automatic batteries tester (Land CT 2001A, Wuhan, China).
3. Results and discussion
Based on the XRD, SEM and TEM results mentioned above, a possible model for the formation of nanorods is described as below: (i) NH4VO3 is dissolved in H2O to form HVO3 solution; (ii) HVO3 reacted with ZnSO4 and H2O, then a layered structure product Zn3(OH)2V2O7.2 H2O is obtained; (iii) Zn3(OH)2V2O7.2∙H2O is gradually exfoliated and dehydrated to form Zn2V2O7 sheets under the hydrothermal conditions; and (iv) the Zn2V2O7 sheets are split in order to release strong stress and lower the total energy, and then the nanorods are formed. This process is summarized in a simple diagram, as shown in Fig. 2g. In fact, the synthesis of 1D nanomaterials using layered compounds as precursors has been widely reported [10–12]. The electrochemistry properties of the Zn2V2O7 nanorods have also been investigated. Fig. 3a shows the discharge–charge curves at a current density of 0.03 A g−1. As can be seen, it exhibits a large irreversible discharge capacity at the first cycle, which can be attributed to the structure transforming into amorphous at the first cycle [13]. However, then a high reversible discharge capacity can be obtained at the second and the third cycles. From Fig. 3b, it also can be seen that the capacity decreases rapidly at first and later it tends to flatten and eventually reaches a ceiling at a current density of 0.03 Ag-1. A reversible discharge capacity of 528 mAh g−1 is obtained after 30 cycles. Thus those results indicate that the Zn2V2O7 nanorods possess a high reversible discharge/charge capacity and good cycle performance. The rate property of the Zn2V2O7 nanorods is also discussed. As shown in Fig. 3b, the discharge capacity decreases with the increase of the current density. However, a reversible capacity of 497 mAh g−1 can be achieved even when the current density is 0.1 A g−1, which is much larger than that of commercial graphite anode. This excellent rate property might be attributed to the intrinsic characteristics of 1D nanostructure, because the nanorods about 40–60 nm in width can reduce the diffusion distance of lithium ions and electrons in the solid state greatly. On the other hand, the layered structure of Zn2V2O7 nanorods could provide a diffusion space and multiple electron transfer during lithium-ion intercalation/ deintercalation into/out of the electrodes, resulting in very good cycling stability and high reversible discharge/charge capacity.
4. Conclusions In conclusion, a simple synthetic route for preparing Zn2V2O7 nanorods has been demonstrated. The synthesized nanorods are highly crystalline and their thickness is found to be ca. 20–40 nm. Based on the observations, a possible model, reacting–exfoliating– splitting, is proposed for the formation of Zn2V2O7 nanorods. These nanorods used as the electrode materials in a rechargeable
3.5
1400
3
1200
Specific capacity / mAhg-1
Potential / V vs Li/Li+
Fig.1 shows the XRD patterns of the products synthesized at 205 1C for 12, 24, 36, and 48 h. As can be seen, a typical layered structure compound Zn3(OH)2V2O7.2∙H2O (JCPDS 50-0570) is obtained after 12 h from Fig. 1a. With the reaction time increasing to 24, 36 and 48 h, however, the diffraction reflections of all samples can be indexed to a layered structure Zn2V2O7 (JCPDS 70-1532). Moreover, the intensity of diffraction reflection is increasing while the reaction time is prolonging, which indicates the crystallinity of the products are also improved. The morphology and particle information of the samples synthesized at 205 1C for different hours are observed by SEM. As can be seen from Fig. 2(a,b), the products with a synthesis time of 24 h are composed of a large quantity of sheets (Fig. 2a) and some sheets trend to exfoliate (Fig. 2b). And then, a few single rodlike samples can be seen when the reaction time increases to 36 h from Fig. 2c. And moreover, we can see that a stacking rod-like sample is splitting to nanorods. Finally, nanorods are formed when the reaction time further increases to 48 h according to Fig. 2d. The information of the nanorods synthesized at 205 1C for 48 h is further confirmed by the TEM and HRTEM measurement. In Fig. 2e, the data clearly indicates that the width and the thickness of these nanorods are found to be about 40–60 nm and 20–40 nm, respectively. This result is consistent with the SEM observations. The high magnification image exhibits highly crystalline nanorods (Fig. 2f), where the lattice fringe corresponds to a d-spacing of 0.312 nm. This is in agreement with the d022 spacing in the XRD patterns of Zn2V2O7.
2.5 2 1.5 1 0.5
1 st
2 nd
3 th
37
Discharge
Charge
1000 800
0.03A/g 0.1A/g
600 400
0.5A/g
200 0
0 0
200
400
600
800
1000
Special capacity / mAhg -1
1200
1400
0
20
40
60
80
100
Cycle number
Fig. 3. Charge–discharge performance at the current density of 0.03 A g−1 (a) and cycle performance at different current densities (b) for the products synthesized at the temperature of 205 1C for 48 h.
38
Z. Chen et al. / Materials Letters 107 (2013) 35–38
lithium-ion battery show a much larger reversible capacity than that of commercial graphite anode and excellent rate property even at a current density as high as 0.1 A g−1. Such a highly favorable electrochemical performance might be attributed to the intrinsic characteristics of the layered structure of Zn2V2O7 nanorods.
Acknowledgments This work was financially supported by the Development and Reform Commission of Guangdong Province (No. 301-5) and the South China Normal University (2012kyjj120).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Zhang WM, Wu XL, Hu JS, Guo YG, Wan LJ. Adv Funct Mater 2008;18:3941–6. Wen Z, Wang Q, Zhang Q, Li J. Adv Funct Mater 2007;17:2772–8. Shodai T, Okada S, Tobishima S, Yamaki J. Solid State Ionics 1996;86–88:785–9. Kim H, Cho J. Chem Mater 2008;20:1679–81. Wang X, Wen ZY, Liu Y, Wu XW. Electrochim Acta 2009;54:4662–7. Sides CR, Li NC, Patrissi CJ, Scrosati B, Martin CR. MRS Bull 2002;27:604–7. Armstrong AR, Armstrong G, Canales J, Bruce PG. Angew Chem Int Ed 2004;43:2286–8. Wang K, Wei M, Morris MA, Zhou H, Holmes JD. Adv Mater 2007;19:3016–20. Liang S, Zhou J, Pan A, Li Y, Chen T, Tian Z, et al. Mater Lett 2012;74:176–9. Wei MD, Sugihara H, Honma I, Ichihara M, Zhou HS. Adv Mater 2005;17:2964–9. Song RQ, Xu AW, Deng B, Fang YP. J Phys Chem B 2005;109:22758–66. Chen ZJ, Gao SK, Jiang LL. Mater Chem Phys 2010;121:254–8. Delmas C, Cognac-Auradou H, Cocciantelli JM, Ménétrier M, Doumerc JP. Solid State Ionics 1994;69:257–64.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hydrothermal synthesis and electrochemical properties of crystalline Zn2V2O7 nanorods Zhanjun Chen a, Wenda Huang b, Dongliang Lu c, Ruirui Zhao a, Hongyu Chen a,n a
School of Chemistry and Environment, South China Normal University, Guangzhou, Guangdong 510006, China Guangzhou Tinci Materials Technology Co., Ltd., Guangzhou, Guangdong 510006, China c Department of Electronics and Information, Foshan Polytechnic, Foshan 528137, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 7 March 2013 Accepted 25 May 2013 Available online 2 June 2013
Zn2V2O7 nanorods were successfully synthesized by a hydrothermal route for the application in the Lithium ion batteries. The effects of the reacting time on the Zn2V2O7 nanorods as well as the corresponding electrochemical properties were investigated. Results show that the obtained sample under the optimum condition possesses high crystallinity; the width and the thickness are in the range of 40–60 nm and 20–40 nm, respectively. Based on the observations, a possible growth mechanism of Zn2V2O7 nanorods could be described as a reacting–exfoliating–splitting process. Besides, the Zn2V2O7 nanorods used as anode materials in rechargeable lithium-ion battery can exhibit a highly reversible discharge/charge capacity and excellent rate property at a current density as high as 0.1 A g−1. This might be attributed to the intrinsic characteristics of the layered structure of Zn2V2O7 nanorods. & 2013 Elsevier B.V. All rights reserved.
Keywords: Zn2V2O7 Nanorods Hydrothermal method Lithium ion battery
1. Introduction During the past several decades, lithium-ion batteries (LIB) had become a commercial reality and been widely applied as a power supply for portable electronic devices. A traditional LIB is mainly constituted of two intercalation compounds as electrode materials, a lithiated metal oxide cathode and a graphite anode. Graphite had been widely used as anode material for LIB owing to the favorable low potential (vs. Li+/Li) and the good cycle performance. However, the specific capacity of graphite is relatively low (372 mAh g−1) and the rate property is unsatisfactory either, which are great drawbacks for electric and/or hybrid vehicle applications of LIB. Many efforts have been made in finding out alternative materials to replace graphite anode and a variety of materials such as oxides [1,2], nitrides [3], and metals [4,5] were investigated. Among these candidate compounds, the vanadium oxides and its derivatives attract more attention due to their layered structure and high specific capacity. According to the report published by Sides et al. [6] recently, the lithium ion diffusion of the one-dimensional (1D) nanomaterials is restricted to the radius direction and the diffusion distance is significantly shortened, thus the rate capability of the 1D nanomaterial-based electrode is superior to the electrode composed of bulk materials. One-dimensional nanomaterials such as
n
Corresponding author. Tel.: +86 20 39310183. E-mail address: [email protected] (H. Chen).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.05.112
TiO2–B nanowire [7], TiO2 nanotube [8], and β-AgVO3 nanorods [9] have been synthesized successfully and applied to LIB already, and the specific capacity and the rate capability of the batteries can be improved greatly.
Fig. 1. XRD patterns of the products obtained at 205 1C for different reaction times: (a) 12 h; (b) 24 h; (c) 36 h and (d) 48 h.
36
Z. Chen et al. / Materials Letters 107 (2013) 35–38
To the best of our knowledge, there are no reports on the synthesis of Zn2V2O7 nanorods. With this background, a simple hydrothermal route for the preparation of Zn2V2O7 nanorods was described and its electrochemical properties were further studied in this paper.
2. Experimental Sample synthesis: The synthesis of Zn2V2O7 nanorods was performed by a simple hydrothermal process. At first, 0.5 mmol
of commercial ZnSO4 powder and 1 mmol of commercial NH4VO3 power were dispersed into 40 ml H2O, and then was transferred into a 50 ml Teflon-lined autoclave in an oven at 205 1C for several hours. After the reaction, the products were obtained. Sample characterization: X-ray powder diffraction (XRD) patterns were recorded using a diffractometer (Co Kα, PANalytical, X’Pert, data were convert into Cu Kα). Scanning electron microscope (SEM) and transmission electron microscopy (TEM) were taken on a Philip-XL30 instrument and a JEOL 2010 instrument, respectively.
1µm
1µm
1µm
1µm
d022=0.312 nm
5 nm
200 nm
reacting
VO3-
Zn2+
splitting
exfoliating
layered structure
Zn2V2O7sheets
Zn2V2O7nanorods
Fig. 2. SEM images of the products synthesized at the temperature of 205 1C for different reaction times: (a) and (b) 24 h, (c) 36 h and (d) 48 h; (e)TEM and (f) HRTEM image of the product synthesized at the temperature of 205 1C for 48 h; (g) Schematic illustration of the formation process of Zn2V2O7 nanorods.
Z. Chen et al. / Materials Letters 107 (2013) 35–38
Electrochemical measurement: For the electrochemical measurement, 60 wt% active materials were mixed and grounded with 10 wt% polyvinylidene fluoride (PVDF) powder as a binder and 30 wt% acetylene back carbon (AB) powder as the conductive assistant materials. The mixture was spread and pressed on Cu foil circular flakes as the working electrode (WE), and dried at 120 1C for 24 h under the vacuum conditions. Metallic lithium foils were used as negative electrode. The electrolyte was 1 M LiPF6 in a 1/1/1 (volume ratio) mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC). The separator was UBE 3093 (Japan) micro-porous polypropylene membrane. The cells were assembled in a glove box filled with highly pure argon gas (O2 and H2O levels o1 ppm), and charge/discharge tests were performed in the voltage range of 0.05–3 V (Li+/Li) at different current densities on a Land automatic batteries tester (Land CT 2001A, Wuhan, China).
3. Results and discussion
Based on the XRD, SEM and TEM results mentioned above, a possible model for the formation of nanorods is described as below: (i) NH4VO3 is dissolved in H2O to form HVO3 solution; (ii) HVO3 reacted with ZnSO4 and H2O, then a layered structure product Zn3(OH)2V2O7.2 H2O is obtained; (iii) Zn3(OH)2V2O7.2∙H2O is gradually exfoliated and dehydrated to form Zn2V2O7 sheets under the hydrothermal conditions; and (iv) the Zn2V2O7 sheets are split in order to release strong stress and lower the total energy, and then the nanorods are formed. This process is summarized in a simple diagram, as shown in Fig. 2g. In fact, the synthesis of 1D nanomaterials using layered compounds as precursors has been widely reported [10–12]. The electrochemistry properties of the Zn2V2O7 nanorods have also been investigated. Fig. 3a shows the discharge–charge curves at a current density of 0.03 A g−1. As can be seen, it exhibits a large irreversible discharge capacity at the first cycle, which can be attributed to the structure transforming into amorphous at the first cycle [13]. However, then a high reversible discharge capacity can be obtained at the second and the third cycles. From Fig. 3b, it also can be seen that the capacity decreases rapidly at first and later it tends to flatten and eventually reaches a ceiling at a current density of 0.03 Ag-1. A reversible discharge capacity of 528 mAh g−1 is obtained after 30 cycles. Thus those results indicate that the Zn2V2O7 nanorods possess a high reversible discharge/charge capacity and good cycle performance. The rate property of the Zn2V2O7 nanorods is also discussed. As shown in Fig. 3b, the discharge capacity decreases with the increase of the current density. However, a reversible capacity of 497 mAh g−1 can be achieved even when the current density is 0.1 A g−1, which is much larger than that of commercial graphite anode. This excellent rate property might be attributed to the intrinsic characteristics of 1D nanostructure, because the nanorods about 40–60 nm in width can reduce the diffusion distance of lithium ions and electrons in the solid state greatly. On the other hand, the layered structure of Zn2V2O7 nanorods could provide a diffusion space and multiple electron transfer during lithium-ion intercalation/ deintercalation into/out of the electrodes, resulting in very good cycling stability and high reversible discharge/charge capacity.
4. Conclusions In conclusion, a simple synthetic route for preparing Zn2V2O7 nanorods has been demonstrated. The synthesized nanorods are highly crystalline and their thickness is found to be ca. 20–40 nm. Based on the observations, a possible model, reacting–exfoliating– splitting, is proposed for the formation of Zn2V2O7 nanorods. These nanorods used as the electrode materials in a rechargeable
3.5
1400
3
1200
Specific capacity / mAhg-1
Potential / V vs Li/Li+
Fig.1 shows the XRD patterns of the products synthesized at 205 1C for 12, 24, 36, and 48 h. As can be seen, a typical layered structure compound Zn3(OH)2V2O7.2∙H2O (JCPDS 50-0570) is obtained after 12 h from Fig. 1a. With the reaction time increasing to 24, 36 and 48 h, however, the diffraction reflections of all samples can be indexed to a layered structure Zn2V2O7 (JCPDS 70-1532). Moreover, the intensity of diffraction reflection is increasing while the reaction time is prolonging, which indicates the crystallinity of the products are also improved. The morphology and particle information of the samples synthesized at 205 1C for different hours are observed by SEM. As can be seen from Fig. 2(a,b), the products with a synthesis time of 24 h are composed of a large quantity of sheets (Fig. 2a) and some sheets trend to exfoliate (Fig. 2b). And then, a few single rodlike samples can be seen when the reaction time increases to 36 h from Fig. 2c. And moreover, we can see that a stacking rod-like sample is splitting to nanorods. Finally, nanorods are formed when the reaction time further increases to 48 h according to Fig. 2d. The information of the nanorods synthesized at 205 1C for 48 h is further confirmed by the TEM and HRTEM measurement. In Fig. 2e, the data clearly indicates that the width and the thickness of these nanorods are found to be about 40–60 nm and 20–40 nm, respectively. This result is consistent with the SEM observations. The high magnification image exhibits highly crystalline nanorods (Fig. 2f), where the lattice fringe corresponds to a d-spacing of 0.312 nm. This is in agreement with the d022 spacing in the XRD patterns of Zn2V2O7.
2.5 2 1.5 1 0.5
1 st
2 nd
3 th
37
Discharge
Charge
1000 800
0.03A/g 0.1A/g
600 400
0.5A/g
200 0
0 0
200
400
600
800
1000
Special capacity / mAhg -1
1200
1400
0
20
40
60
80
100
Cycle number
Fig. 3. Charge–discharge performance at the current density of 0.03 A g−1 (a) and cycle performance at different current densities (b) for the products synthesized at the temperature of 205 1C for 48 h.
38
Z. Chen et al. / Materials Letters 107 (2013) 35–38
lithium-ion battery show a much larger reversible capacity than that of commercial graphite anode and excellent rate property even at a current density as high as 0.1 A g−1. Such a highly favorable electrochemical performance might be attributed to the intrinsic characteristics of the layered structure of Zn2V2O7 nanorods.
Acknowledgments This work was financially supported by the Development and Reform Commission of Guangdong Province (No. 301-5) and the South China Normal University (2012kyjj120).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Zhang WM, Wu XL, Hu JS, Guo YG, Wan LJ. Adv Funct Mater 2008;18:3941–6. Wen Z, Wang Q, Zhang Q, Li J. Adv Funct Mater 2007;17:2772–8. Shodai T, Okada S, Tobishima S, Yamaki J. Solid State Ionics 1996;86–88:785–9. Kim H, Cho J. Chem Mater 2008;20:1679–81. Wang X, Wen ZY, Liu Y, Wu XW. Electrochim Acta 2009;54:4662–7. Sides CR, Li NC, Patrissi CJ, Scrosati B, Martin CR. MRS Bull 2002;27:604–7. Armstrong AR, Armstrong G, Canales J, Bruce PG. Angew Chem Int Ed 2004;43:2286–8. Wang K, Wei M, Morris MA, Zhou H, Holmes JD. Adv Mater 2007;19:3016–20. Liang S, Zhou J, Pan A, Li Y, Chen T, Tian Z, et al. Mater Lett 2012;74:176–9. Wei MD, Sugihara H, Honma I, Ichihara M, Zhou HS. Adv Mater 2005;17:2964–9. Song RQ, Xu AW, Deng B, Fang YP. J Phys Chem B 2005;109:22758–66. Chen ZJ, Gao SK, Jiang LL. Mater Chem Phys 2010;121:254–8. Delmas C, Cognac-Auradou H, Cocciantelli JM, Ménétrier M, Doumerc JP. Solid State Ionics 1994;69:257–64.