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Ultrathin SnO nanosheets as anode materials for rechargeable lithium-ion batteries
Materials Letters 120 (2014) 200–203
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Ultrathin SnO nanosheets as anode materials for rechargeable lithium-ion batteries Haijiao Zhang a,b, Qingquan He a, Fengjun Wei a, Yingjie Tan a, Yong Jiang a, Guanghong Zheng c,nn, Guoji Ding a, Zheng Jiao a,n a
Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, PR China State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, PR China c School of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 November 2013 Accepted 10 January 2014 Available online 20 January 2014
SnO ultrathin nanosheets with a highly pure crystal phase were successfully prepared via a simple and effective hydrothermal process, using tin dichloride dihydrate (SnCl2 2H2O) as a stannous source in the presence of hexamethylenetetramine (HMT). SEM and TEM images showed that the obtained SnO products are uniform and ultrathin nanosheets. The electrochemical properties results indicated that the obtained SnO nanosheets exhibited excellent discharge capacity, the reversible capacity was 559 mAh g 1 after 20 cycles, and the SnO nanosheets retain high capacity even at high discharge current densities. The enhanced performance was attributed to the unique structure of ultrathin SnO nanosheets and highly pure crystal phase. & 2014 Elsevier B.V. All rights reserved.
Keywords: SnO Nanosheets Hexamethylenetetramine (HMT) Lithium-ion batteries
1. Introduction Graphite is the widely used commercial anode material for rechargeable lithium-ion batteries, but its theoretical specific capacity is only 372 mAh g 1, which cannot meet the increasing demand for high performance lithium-ion batteries. Great efforts have been devoted to develop different types of materials with high reversible capacity, long cycle life, and low cost [1,2]. Among the materials, SnO is regarded as one of the most promising candidate for anode materials owing to its high theoretically gravimetric lithium storage capacity and low potential of lithium ion intercalation [3]. Unfortunately, tin-based materials have a disadvantage, in that they undergo a significant volume expansion and contraction during Li þ insertion and extraction. This causes cracking and crumbling, resulting in ‘‘dead volume’’, which is electrically disconnected from the current collector, and results in subsequent degradation of electrode performance during cycling [4]. Therefore, the tailoring of nanostructure has become a critical process in developing electrode materials to alleviate the volume changes and mechanical stress. As we know, SnO nanocrystals are very difficult to be synthesized due to their easy transformation into SnO2 by oxidization. Although some successful approaches have been reported for the preparation of SnO crystals with
n
Corresponding author. Tel./fax: þ 86 21 66137803. Corresponding author. E-mail address: [email protected] (Z. Jiao).
nn
0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2014.01.044
various morphologies, such as nanowires [5,6], nanoribbons [7], nanosheets [8,9], and diskettes [10]. However, these structures are of large size, tedious synthetic procedures, and/or low battery capacity, which further hinder their large-scale industrial applications. Herein, we present a facile one-step method for mass production of the ultrathin SnO nanosheets with highly pure crystal phase by using SnCl2 2H2O and HMT as the precursors. The electrochemical properties of the products were studied. According to the experimental results, a possible Li þ insertion/extraction mechanism of the SnO anode materials was also discussed. 2. Experimental All chemicals were of analytical grade and used without further purification. Typically, 0.4513 g SnCl2 2H2O and 0.8411 g HMT were dissolved in 30 mL deionized water with vigorous stirring for 1 h to form a homogeneous solution, and then added 30 mL nbutanol dropwise into the above solution. Subsequently, the reaction mixture was transferred into a Teflon-lined stainless steel autoclave and kept at 160 1C for 24 h. Afterwards, the autoclave was cooled naturally down to room temperature. The precipitates were collected by centrifugation, washed several times with deionized water and ethanol, respectively, and dried at 80 1C for 12 h in vacuum. The products were characterized with an X-ray diffraction (XRD, RigaKu D/max-2550), a scanning electron microscope (SEM, JEOL JSM-6700F), a transmission electron microscope
H. Zhang et al. / Materials Letters 120 (2014) 200–203
(TEM, JEOL JEM-200CX) and a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2010F), energy dispersive X-ray spectroscopy (EDS, OXFORD INCA), FT-IR (Nicolet AVATAR 370), thermogravimetric (TG, NETZSCH STA 409 PC/4/H), and N2 sorption isotherms (BET, Quadrasorb SI). Electrochemical properties: For the electrochemical measurement, the active materials (SnO, 70 wt%) and carbon black conductive additives (Super-P, 20 wt%) were mixed and rolled with polytetrafluoroethylene (PTFE, 10 wt%) powder to form a film. The obtained film was pressed onto a copper mesh and dried under vacuum for 12 h at 80 1C. The electrolyte used was 1 M LiPF6 dissolved in dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylene carbonate (EC) (1:1:1 by weight). Cyclic voltammetry was performed on an AUTOLAB electrochemical workstation using the above-mentioned cell in the voltage range of 3–0.005 V (vs Li/Li þ ) at a sweep rate of 0.2 mV s 1. Discharge/charge testing was carried out on Land-CT2001A battery test system between 0.005 V and 3.0 V with a constant current density of 100 mA g 1. All the electrochemical tests were carried out at room temperature.
3. Results and discussion Fig. 1a and b shows the SEM and TEM images of the obtained SnO products, respectively. Clearly, the morphologies of SnO ultrathin nanosheets are disorderly accumulated on a large scale. Seen from Fig. 1c, an HRTEM image confirmed that the prepared SnO was highly crystalline with the lattice spacing of 0.299 nm, corresponding to (101) planes of romarchite SnO. The EDS results in Fig. 1d show the intense peaks of Sn and O, indicating that the composition of products are only Sn and O elements, and while the Cu and C signals are derived from the supporting TEM grid. Meanwhile, the EDS analysis gives an average Sn/O composition of 1:1 within the accuracy of the technique, in good accordance with the stoichiometry of SnO.
201
Fig. 2a illustrates the typical XRD pattern, and all the peaks can be well indexed to the tetragonal SnO structure (JCPDS No: 06–0395). Fig. 2b shows the FT-IR spectra of the SnO nanosheets. Obviously, the absorption band at 513 cm 1 can be attributed to stretching vibrations of Sn–O in the SnO nanosheets [6]. In the TG curves shown in (Fig. 2c), the loss of weight up to 100 1C was caused by the residual water molecule and the weight increased around 300 1C results from the oxidation of SnO to SnO2. The N2 sorption isotherm was shown in Fig. 2d, the prepared SnO products exhibited a hysteresis hoop at high relative pressure, suggesting the existence of mesopores in the material [11]. The obtained SnO has a BET surface area of 51.4 m2 g 1. The electrochemical reaction of SnO with lithium-ion is generally described as follows [12]: SnO þ2Li þ þ2e -Sn þLi0O þ
(1)
Li þe þelectrolyte-SEI (Li)
(2)
Snþ xLi þ þxe 2LixSn (0 r xr4.4)
(3)
Fig. 3a shows the cyclic voltammograms of SnO sample. In the first cyclic voltammogram, the cathodic peaks at the potential of 1.15 V and 0.74 V can be attributed to the decomposition of SnO to Sn and Li2O composite (Eq. (1)) and formation of solid electrolyte interphase (SEI, Eq. (2)), respectively. Moreover, these two processes are believed to be irreversible. A pair of cathodic and anodic peaks at 0.07 and 0.71 V can be ascribed to the reversible alloying and de-alloying reaction of LixSn (Eq. (3)). The results are similar to those reported by other groups [9]. The properties of the SnO nanosheets as an anode material for a rechargeable lithium-ion battery were studied using constant current discharge/charge measurements as shown in Fig. 3b. The first discharge capacity is very high, which is 1496 mAh g 1, and the observed capacity during the first charge is 1016 mAh g 1, corresponding to the coulombic efficiency which is 67.9%. From Fig. 3b, the 5th discharge and charge capacities are determined as 882 and 829.4 mAh g 1,
Fig. 1. SEM image (a), TEM image (b), HRTEM image (c), and EDS spectrum (d) of the SnO nanosheets.
202
H. Zhang et al. / Materials Letters 120 (2014) 200–203
Fig. 2. XRD pattern (a), FT-IR spectra (b), TG and DTG curves (c), and N2 adsorption–desorption isotherm (d) of SnO nanosheets, inset in (d) is the corresponding pore distribution curve.
Fig. 3. Representative cyclic voltammograms (a), galvanostatic curves for different cycles (b), cycling performance (c), and discharge capacity of SnO electrode at various discharge current densities (d).
respectively, giving a coulombic efficiency up to 94%. These performances are better than many previous reports [3,12]. The high reversibility of the underlying electrochemical reactions
over many discharge and charge cycles is shown in Fig. 3c, it can be easily found that the cell exhibits a high discharge capacity of 559 mAh g 1 after 20 cycles with the retention of 53% compared
H. Zhang et al. / Materials Letters 120 (2014) 200–203
203
4. Conclusions In summary, a simple and environmentally benign route was developed for the fabrication of the ultrathin SnO nanosheets. The results indicated that the as-synthesized SnO nanosheets exhibited excellent discharge capacity, which may be attributed to their unique nanostructures. This result indicated that the obtained SnO nanosheets could be potentially applied as good anodes for highcapacity rechargeable lithium-ion batteries.
Acknowledgments
Fig. 4. Schematic illustrations of Li
þ
insertion/extraction of nanosheets.
with the first reversible capacity, which is obviously higher than those of meshed SnO (320 mAh g 1) [12] and SnO nanoflowers (450 mA h g 1) [3]. In addition, the specific capacity is still maintained at 305.4 mA h g 1 even after 40 cycles. The cycling data at various discharge current densities are shown in Fig. 3d. The charge capacity at the current density of 200 mA g 1 is 785.4 mAh g 1; even at the current density as high as 500 mA g 1, it retains high capacity of 606.3 mAh g 1. Based on above experimental results, a possible discharge and charge mechanism for the SnO nanosheets is displayed in Fig. 4. For the first step, SnO is reduced to Sn and Li2O, and then further reduced to LixSn. The significant improvement of the electrochemical performance may be attributed to the unique structure of SnO nanosheets with a variety of favorable properties: the ultrathin nanosheets promise to provide large surface and fast transport channels for the conductive ions (e.g., Li þ ); the interspace of nanosheets disordered accumulation is expected to buffer well against the local volume change during the Li–Sn alloying and dealloying reactions; the short diffusion length for Li þ insertion due to the ultrathin SnO nanosheets is beneficial for keeping the structural stability as well as leading to a good cycling performance and high rate capability.
The work was supported by the Natural Science Foundation of China (61174011, 11275121, 21241002, 21371116), Specialized Research Fund for the Doctoral Program of Higher Education (20103108120021, 20113108110017), and State Key Laboratory of Pollution Control and Resource Reuse Foundation, Tongji University (PCRRF12003).
References [1] Lian PC, Zhu XF, Liang SZ, Li Z, Yang WS, Wang HH. Electrochim Acta 2011;56:4532–9. [2] Yin XM, Li CC, Zhang M, Hao QY, Liu S, Chen LB, et al. J Phys Chem C 2010;114:8084–8. [3] Ning JJ, Dai QQ, Jiang T, Men KK, Liu DH, Xiao NR, et al. Langmuir 2009;25:1818–21. [4] Guo ZP, Du GD, Nuli Y, Hassan MF, Liu HK. J Mater Chem 2009;19:3253–7. [5] Dai ZR, Gole JL, Stout JD, Wang ZL. J Phys Chem B 2002;106:1274–9. [6] Sakaushi K, Oaki Y, Uchiyama H, Hosono E, Zhou H, Imai H. Nanoscale 2010;2:2424–30. [7] Wang ZL, Pan ZW. Adv Mater 2002;14:1029–32. [8] Kumar B, Lee D-H, Kim S-H, Yang B, Maeng S, Kim S-W. J Phys Chem C 2010;114:11050–5. [9] Sakaushi K, Oaki Y, Uchiyama H, Hosono E, Zhou H, Imai H. Small 2010;6:776–81. [10] Dai ZR, Pan ZW, Wang ZL. J Am Chem Soc 2002;124:8673–80. [11] Sun P, Zhao W, Cao Y, Guan Y, Sun YF, Lu GY. Cryst Eng Commun 2011;13:3718–24. [12] Uchiyama H, Hosono E, Honma I, Zhou H, Imai H. Electrochem Commun 2008;10:52–5.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Ultrathin SnO nanosheets as anode materials for rechargeable lithium-ion batteries Haijiao Zhang a,b, Qingquan He a, Fengjun Wei a, Yingjie Tan a, Yong Jiang a, Guanghong Zheng c,nn, Guoji Ding a, Zheng Jiao a,n a
Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, PR China State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, PR China c School of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 November 2013 Accepted 10 January 2014 Available online 20 January 2014
SnO ultrathin nanosheets with a highly pure crystal phase were successfully prepared via a simple and effective hydrothermal process, using tin dichloride dihydrate (SnCl2 2H2O) as a stannous source in the presence of hexamethylenetetramine (HMT). SEM and TEM images showed that the obtained SnO products are uniform and ultrathin nanosheets. The electrochemical properties results indicated that the obtained SnO nanosheets exhibited excellent discharge capacity, the reversible capacity was 559 mAh g 1 after 20 cycles, and the SnO nanosheets retain high capacity even at high discharge current densities. The enhanced performance was attributed to the unique structure of ultrathin SnO nanosheets and highly pure crystal phase. & 2014 Elsevier B.V. All rights reserved.
Keywords: SnO Nanosheets Hexamethylenetetramine (HMT) Lithium-ion batteries
1. Introduction Graphite is the widely used commercial anode material for rechargeable lithium-ion batteries, but its theoretical specific capacity is only 372 mAh g 1, which cannot meet the increasing demand for high performance lithium-ion batteries. Great efforts have been devoted to develop different types of materials with high reversible capacity, long cycle life, and low cost [1,2]. Among the materials, SnO is regarded as one of the most promising candidate for anode materials owing to its high theoretically gravimetric lithium storage capacity and low potential of lithium ion intercalation [3]. Unfortunately, tin-based materials have a disadvantage, in that they undergo a significant volume expansion and contraction during Li þ insertion and extraction. This causes cracking and crumbling, resulting in ‘‘dead volume’’, which is electrically disconnected from the current collector, and results in subsequent degradation of electrode performance during cycling [4]. Therefore, the tailoring of nanostructure has become a critical process in developing electrode materials to alleviate the volume changes and mechanical stress. As we know, SnO nanocrystals are very difficult to be synthesized due to their easy transformation into SnO2 by oxidization. Although some successful approaches have been reported for the preparation of SnO crystals with
n
Corresponding author. Tel./fax: þ 86 21 66137803. Corresponding author. E-mail address: [email protected] (Z. Jiao).
nn
0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2014.01.044
various morphologies, such as nanowires [5,6], nanoribbons [7], nanosheets [8,9], and diskettes [10]. However, these structures are of large size, tedious synthetic procedures, and/or low battery capacity, which further hinder their large-scale industrial applications. Herein, we present a facile one-step method for mass production of the ultrathin SnO nanosheets with highly pure crystal phase by using SnCl2 2H2O and HMT as the precursors. The electrochemical properties of the products were studied. According to the experimental results, a possible Li þ insertion/extraction mechanism of the SnO anode materials was also discussed. 2. Experimental All chemicals were of analytical grade and used without further purification. Typically, 0.4513 g SnCl2 2H2O and 0.8411 g HMT were dissolved in 30 mL deionized water with vigorous stirring for 1 h to form a homogeneous solution, and then added 30 mL nbutanol dropwise into the above solution. Subsequently, the reaction mixture was transferred into a Teflon-lined stainless steel autoclave and kept at 160 1C for 24 h. Afterwards, the autoclave was cooled naturally down to room temperature. The precipitates were collected by centrifugation, washed several times with deionized water and ethanol, respectively, and dried at 80 1C for 12 h in vacuum. The products were characterized with an X-ray diffraction (XRD, RigaKu D/max-2550), a scanning electron microscope (SEM, JEOL JSM-6700F), a transmission electron microscope
H. Zhang et al. / Materials Letters 120 (2014) 200–203
(TEM, JEOL JEM-200CX) and a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2010F), energy dispersive X-ray spectroscopy (EDS, OXFORD INCA), FT-IR (Nicolet AVATAR 370), thermogravimetric (TG, NETZSCH STA 409 PC/4/H), and N2 sorption isotherms (BET, Quadrasorb SI). Electrochemical properties: For the electrochemical measurement, the active materials (SnO, 70 wt%) and carbon black conductive additives (Super-P, 20 wt%) were mixed and rolled with polytetrafluoroethylene (PTFE, 10 wt%) powder to form a film. The obtained film was pressed onto a copper mesh and dried under vacuum for 12 h at 80 1C. The electrolyte used was 1 M LiPF6 dissolved in dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylene carbonate (EC) (1:1:1 by weight). Cyclic voltammetry was performed on an AUTOLAB electrochemical workstation using the above-mentioned cell in the voltage range of 3–0.005 V (vs Li/Li þ ) at a sweep rate of 0.2 mV s 1. Discharge/charge testing was carried out on Land-CT2001A battery test system between 0.005 V and 3.0 V with a constant current density of 100 mA g 1. All the electrochemical tests were carried out at room temperature.
3. Results and discussion Fig. 1a and b shows the SEM and TEM images of the obtained SnO products, respectively. Clearly, the morphologies of SnO ultrathin nanosheets are disorderly accumulated on a large scale. Seen from Fig. 1c, an HRTEM image confirmed that the prepared SnO was highly crystalline with the lattice spacing of 0.299 nm, corresponding to (101) planes of romarchite SnO. The EDS results in Fig. 1d show the intense peaks of Sn and O, indicating that the composition of products are only Sn and O elements, and while the Cu and C signals are derived from the supporting TEM grid. Meanwhile, the EDS analysis gives an average Sn/O composition of 1:1 within the accuracy of the technique, in good accordance with the stoichiometry of SnO.
201
Fig. 2a illustrates the typical XRD pattern, and all the peaks can be well indexed to the tetragonal SnO structure (JCPDS No: 06–0395). Fig. 2b shows the FT-IR spectra of the SnO nanosheets. Obviously, the absorption band at 513 cm 1 can be attributed to stretching vibrations of Sn–O in the SnO nanosheets [6]. In the TG curves shown in (Fig. 2c), the loss of weight up to 100 1C was caused by the residual water molecule and the weight increased around 300 1C results from the oxidation of SnO to SnO2. The N2 sorption isotherm was shown in Fig. 2d, the prepared SnO products exhibited a hysteresis hoop at high relative pressure, suggesting the existence of mesopores in the material [11]. The obtained SnO has a BET surface area of 51.4 m2 g 1. The electrochemical reaction of SnO with lithium-ion is generally described as follows [12]: SnO þ2Li þ þ2e -Sn þLi0O þ
(1)
Li þe þelectrolyte-SEI (Li)
(2)
Snþ xLi þ þxe 2LixSn (0 r xr4.4)
(3)
Fig. 3a shows the cyclic voltammograms of SnO sample. In the first cyclic voltammogram, the cathodic peaks at the potential of 1.15 V and 0.74 V can be attributed to the decomposition of SnO to Sn and Li2O composite (Eq. (1)) and formation of solid electrolyte interphase (SEI, Eq. (2)), respectively. Moreover, these two processes are believed to be irreversible. A pair of cathodic and anodic peaks at 0.07 and 0.71 V can be ascribed to the reversible alloying and de-alloying reaction of LixSn (Eq. (3)). The results are similar to those reported by other groups [9]. The properties of the SnO nanosheets as an anode material for a rechargeable lithium-ion battery were studied using constant current discharge/charge measurements as shown in Fig. 3b. The first discharge capacity is very high, which is 1496 mAh g 1, and the observed capacity during the first charge is 1016 mAh g 1, corresponding to the coulombic efficiency which is 67.9%. From Fig. 3b, the 5th discharge and charge capacities are determined as 882 and 829.4 mAh g 1,
Fig. 1. SEM image (a), TEM image (b), HRTEM image (c), and EDS spectrum (d) of the SnO nanosheets.
202
H. Zhang et al. / Materials Letters 120 (2014) 200–203
Fig. 2. XRD pattern (a), FT-IR spectra (b), TG and DTG curves (c), and N2 adsorption–desorption isotherm (d) of SnO nanosheets, inset in (d) is the corresponding pore distribution curve.
Fig. 3. Representative cyclic voltammograms (a), galvanostatic curves for different cycles (b), cycling performance (c), and discharge capacity of SnO electrode at various discharge current densities (d).
respectively, giving a coulombic efficiency up to 94%. These performances are better than many previous reports [3,12]. The high reversibility of the underlying electrochemical reactions
over many discharge and charge cycles is shown in Fig. 3c, it can be easily found that the cell exhibits a high discharge capacity of 559 mAh g 1 after 20 cycles with the retention of 53% compared
H. Zhang et al. / Materials Letters 120 (2014) 200–203
203
4. Conclusions In summary, a simple and environmentally benign route was developed for the fabrication of the ultrathin SnO nanosheets. The results indicated that the as-synthesized SnO nanosheets exhibited excellent discharge capacity, which may be attributed to their unique nanostructures. This result indicated that the obtained SnO nanosheets could be potentially applied as good anodes for highcapacity rechargeable lithium-ion batteries.
Acknowledgments
Fig. 4. Schematic illustrations of Li
þ
insertion/extraction of nanosheets.
with the first reversible capacity, which is obviously higher than those of meshed SnO (320 mAh g 1) [12] and SnO nanoflowers (450 mA h g 1) [3]. In addition, the specific capacity is still maintained at 305.4 mA h g 1 even after 40 cycles. The cycling data at various discharge current densities are shown in Fig. 3d. The charge capacity at the current density of 200 mA g 1 is 785.4 mAh g 1; even at the current density as high as 500 mA g 1, it retains high capacity of 606.3 mAh g 1. Based on above experimental results, a possible discharge and charge mechanism for the SnO nanosheets is displayed in Fig. 4. For the first step, SnO is reduced to Sn and Li2O, and then further reduced to LixSn. The significant improvement of the electrochemical performance may be attributed to the unique structure of SnO nanosheets with a variety of favorable properties: the ultrathin nanosheets promise to provide large surface and fast transport channels for the conductive ions (e.g., Li þ ); the interspace of nanosheets disordered accumulation is expected to buffer well against the local volume change during the Li–Sn alloying and dealloying reactions; the short diffusion length for Li þ insertion due to the ultrathin SnO nanosheets is beneficial for keeping the structural stability as well as leading to a good cycling performance and high rate capability.
The work was supported by the Natural Science Foundation of China (61174011, 11275121, 21241002, 21371116), Specialized Research Fund for the Doctoral Program of Higher Education (20103108120021, 20113108110017), and State Key Laboratory of Pollution Control and Resource Reuse Foundation, Tongji University (PCRRF12003).
References [1] Lian PC, Zhu XF, Liang SZ, Li Z, Yang WS, Wang HH. Electrochim Acta 2011;56:4532–9. [2] Yin XM, Li CC, Zhang M, Hao QY, Liu S, Chen LB, et al. J Phys Chem C 2010;114:8084–8. [3] Ning JJ, Dai QQ, Jiang T, Men KK, Liu DH, Xiao NR, et al. Langmuir 2009;25:1818–21. [4] Guo ZP, Du GD, Nuli Y, Hassan MF, Liu HK. J Mater Chem 2009;19:3253–7. [5] Dai ZR, Gole JL, Stout JD, Wang ZL. J Phys Chem B 2002;106:1274–9. [6] Sakaushi K, Oaki Y, Uchiyama H, Hosono E, Zhou H, Imai H. Nanoscale 2010;2:2424–30. [7] Wang ZL, Pan ZW. Adv Mater 2002;14:1029–32. [8] Kumar B, Lee D-H, Kim S-H, Yang B, Maeng S, Kim S-W. J Phys Chem C 2010;114:11050–5. [9] Sakaushi K, Oaki Y, Uchiyama H, Hosono E, Zhou H, Imai H. Small 2010;6:776–81. [10] Dai ZR, Pan ZW, Wang ZL. J Am Chem Soc 2002;124:8673–80. [11] Sun P, Zhao W, Cao Y, Guan Y, Sun YF, Lu GY. Cryst Eng Commun 2011;13:3718–24. [12] Uchiyama H, Hosono E, Honma I, Zhou H, Imai H. Electrochem Commun 2008;10:52–5.