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Hydrothermal synthesis and characterization of Bi2O3 nanowires
Materials Letters 65 (2011) 1134–1136
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Hydrothermal synthesis and characterization of Bi2O3 nanowires Changle Wu a,⁎, Li Shen b, Qingli Huang a, Yong-Cai Zhang b a b
Testing Center of Yangzhou University, Yangzhou 225009, China College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China
a r t i c l e
i n f o
Article history: Received 6 November 2010 Accepted 13 January 2011 Available online 19 January 2011 Keywords: Nanoparticles Semiconductors Electron microscope Optical materials and properties
a b s t r a c t An alternative two-step method has been proposed for the synthesis of Bi2O3 nanowires with a diameter of about 40 nm from common and cost-effective Bi(NO3)3·5H2O, Na2SO4, and NaOH. That is, first, Bi2O(OH)SO4 nanowires were prepared through the precipitation reaction of Bi(NO3)3·5H2O and Na2SO4 in distilled water under the ambient condition and second, monoclinic phase Bi2O3 nanowires were prepared via the hydrothermal reaction of Bi2O(OH)SO4 and NaOH at 120 °C for 12 h. The resultant products were characterized by X-ray diffraction, field emission scanning electron microscope, and high resolution transmission electron microscopy. In addition, the photocatalytic studies indicated that the as-synthesized Bi2O3 nanowires were a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Bi2O3, an important semiconductor with a direct band gap of 2.8 eV [1], has been widely used in catalyst [2], fuel cells [3], and photocatalyst [4], etc. Recently, promising performance in catalyst and photocatalyst has been demonstrated in one-dimensional (1D) semiconductor nanostructures in the form of nanorods, nanowires, nanotubes or nanofibers [5,6], which triggers a wide range of subsequent research in searching for newer synthetic methods of 1D Bi2O3 nanostructures [4]. At present, there exist several kinds of methods to prepare Bi2O3 nanorods, nanowires, nanotubes or nanofibers [6–10], including calcinations treatment of bismuth nitrate composite [6], atomicpressure chemical vapor deposition (CVD) [7], metallorganic CVD [8], oxidative metal vapor-phase deposition [9], and slow oxidation of Bi nanowires at 750 °C [10], etc. Apparently, it is difficult for the existing methods to produce large-scale Bi2O3 nanorods or nanowires at low cost in an environmental-friendly way, because they require sophisticated equipments and high temperatures [6–10]. Hydrothermal route has been proved to be a very powerful method in synthesizing 1D nanostructures of inorganic functional materials [11]. But to the best of our knowledge, the reports on the hydrothermal preparation of Bi2O3 nanowires are scarce up to now. Herein, we report the synthesis of Bi2O3 nanowires by a simple hydrothermal route, as well as the characterization of the resultant products by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), and high resolution transmission electron microscopy (HRTEM). Further-
⁎ Corresponding author. Tel.: + 86 0514 87979022; fax: + 86 0514 87979244. E-mail address: [email protected] (C. Wu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.01.021
more, the photocatalytic activities of the as-synthesized Bi2O3 nanowires are also studied by degrading Rhodamine B in water under the visible light (λ N 420 nm) irradiation. 2. Experimental section All the chemical reagents used in this work, including Bi (NO3)3·5H2O, Na2SO4, and NaOH, are A. R. grade. Bi2O3 nanowires were prepared according to the following procedure. Firstly, 2.0 mmol of Bi(NO3)3·5H2O and 3.0 mmol of Na2SO4 were dissolved in 40 ml of distilled water and the solution was stirred at room temperature for 45 min. Then, 18.0 mmol NaOH dissolved in 40 ml of distilled water was added drop by drop into the above solution with stirring. The mixed solution was transferred into a Teflon-lined stainless steel autoclave of 100 ml capacity, sealed and heated at 120 °C for 12 h, then allowed to cool to room temperature naturally. The as-formed yellow precipitates were filtered, washed with deionized water and ethanol, and finally dried in air at 80 °C. XRD patterns were recorded on a German Bruker AXS D8 ADVANCE X-ray diffractometer. The morphological and structural analysis was carried out on Japan Hitachi S-4800 FESEM, and Holland F-30 HRTEM. In the photocatalytic experiment, Rhodamine B was used as a probe molecule to evaluate the photocatalytic reactivity of the assynthesized Bi2O3 nanowires. The experiments were carried out as follows: 20 mg of the samples was dispersed in 200 mL of 0.02 mmol/ L Rhodamine B solution in a 300 mL beaker. Prior to illumination, the suspensions were magnetically stirred in the dark for 2 h to ensure the establishment of absorption equilibrium of Rhodamine B on the sample surfaces. Subsequently, the suspension was irradiated under a 1000 W Xe lamp (equipped with a filter of λ N 420 nm), which was
#
15
(041) (140) (-312) (113)
(023)
Bi2O(OH)2SO4+Bi(OH)3
# # (b)
Bi2O(OH)2SO4
(a) 10
Bi2O3 (-222) (131) (-213) (122)
(-211) (-122) (200) (022) (-212) (031) (102) (130) (112)
(-102) (002) (111) (012)
(020)
(c)
(-111)
Intensity (a.u.)
(120)
C. Wu et al. / Materials Letters 65 (2011) 1134–1136
20
25
30
35
40
45
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Fig. 2 shows the typical FESEM and HRTEM images of the products prepared under different conditions. As can be seen from the image in Fig. 2(a) and (b), an abundance of nearly wire-like particles of pure Bi2O(OH)2SO4 and the mixture of Bi2O(OH)2SO4 and Bi(OH)3 were prepared at room temperature. After the hydrothermal reaction at 120 °C for 12 h, Bi2O3 nanowires with a diameter of about 40 nm were found in Fig. 2(c). In Fig. 2(d) and (e), the HRTEM image shows sharp lattice fringes with 0.325 nm spacings, corresponding to the (1 2 0) planes of monoclinic phase Bi2O3 crystals. Moreover, this sample displayed sharp lattice fringes with no lattice defects such as stacking faults, indicating good crystallinity. This was also consistent with the result of their XRD patterns. Since the source materials used in the present system were Bi (NO3)3·5H2O, Na2SO4, and NaOH, and the synthesis was conducted in two steps, the reactions involved in the formation of Bi2O3 can be described by the following equations:
50
2Theta/degree Fig. 1. XRD patterns of (a) Bi2O(OH)2SO4 and the products prepared at (b) room temperature and (c) 120 °C. The reflections marked by # belong to Bi(OH)3.
positioned about 10 cm away from the breaker. UV–vis adsorption spectra (Shimazu, UV2101) were recorded at different time intervals to monitor the process. For comparison, the Degussa P25 TiO2 was also tested under the same conditions. 3. Results and discussion The crystalline phase and purity of the as-prepared samples were determined by XRD, and the obtained results are shown in Fig. 1. The monoclinic structure Bi2O(OH)2SO4 was formed when Bi(NO3)3·5H2O and Na2SO4 were mixed in distilled water (Fig. 1(a)).When excess NaOH was added to the reaction system, the mixture of Bi2O(OH)2SO4 and Bi(OH)3 was obtained at room temperature (Fig. 1(b)), while pure monoclinic phase Bi2O3 (JCPDS file No. 41-1449) was obtained after the hydrothermal reaction at 120 °C for 12 h (Fig. 1(c)). The strong and sharp XRD peaks of Fig. 1(c) indicated that the as-prepared Bi2O3 crystals were highly crystalline.
2BiðNO3 Þ3 :5H2 O þ Na2 SO4 →Bi2 OðOHÞ2 SO4 þ 2NaNO3 þ 4HNO3 þ 7H2 O ð1Þ Bi2 OðOHÞ2 SO4 þ 2NaOH þ H2 O→2BiðOHÞ3 þ Na2 SO4
ð2Þ
2BiðOHÞ3 →Bi2 O3 þ 3H2 O:
ð3Þ
When Bi(NO3)3 and Na2SO4 were mixed in distilled water at room temperature, Bi2O(OH)2SO4 nanowires were formed, which may behave as a template in the following process of the reaction. By dropping NaOH solution into the reaction system, OH− ions reacted with Bi2O(OH)2SO4 (Kθsp = 1.8 × 10− 31) gradually to form Bi(OH)3 (Kθsp = 3.2 × 10− 40). When the Bi(OH)3 nanowires were subjected to hydrothermal treatment at 120 °C for 12 h, they would dehydrate and convert into Bi2O3 nanowires. This process is similar to the formation process of ZnO nanowires [11]. In order to investigate if the additive Na2SO4 played an important role in gaining Bi2O3 nanowires, control experiment was also conducted without adding Na2SO4. It can be seen from the FESEM image in Fig. 2(f) that only Bi2O3 micro rods were obtained, in the absence of Na2SO4. The above result clearly indicated that the
Fig. 2. FESEM images of (a) Bi2O(OH)2SO4 and the products prepared at (b) room temperature, and (c) 120 °C; (d)–(e) HRTEM images of the products prepared at 120 °C; and (f) FESEM image of Bi2O3 prepared at 120 °C without adding Na2SO4.
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C. Wu et al. / Materials Letters 65 (2011) 1134–1136
1.1 1.0
C/C0
0.9 0.8
P25 TiO2
0.7
Bi2O3
0.6
without catalyst
place at a much slower rate under the same conditions. For example, the decolorization ratio of Rhodamine B is nearly 60% over 0.02 g of Bi2O3 nanowires, which is higher than only 1.1% over 0.02 g of P25 TiO2, when irradiated by the visible light for 130 min. Furthermore, in the absence of any photocatalyst, the degradation of Rhodamine B hardly occurs when subjected to the visible light irradiation for 130 min.
0.5
4. Conclusion
0.4 0.3 0.2 0.1 0.0
0
10
20
30
40
50
60
70
80
90 100 110 120 130 140
Irradiation time (min)
In the absence of any surfactant and template, monoclinic phase Bi2O3 nanowires with a diameter of about 40 nm were successfully obtained by a simple hydrothermal process at 120 °C, and verified by XRD, FESEM, and HRTEM. The proposed method is simple, mild and cost-effective, which may be suitable for industrial production of Bi2O3 nanowires with promising applications such as visible-light photocatalyst.
Fig. 3. Photodegradation of Rhodamine B using the Bi2O3 photocatalyst.
References formation of Bi2O(OH)2SO4 intermediate due to the addition of Na2SO4 may limit the conversion rates of Bi(NO3)3 → Bi(OH)3 → Bi2O3, thus yielding Bi2O3 nanowires with similar morphology to the Bi2O(OH)2SO4 intermediate. The photocatalytic activities of the as-prepared Bi2O3 nanowires are shown in Fig. 3. C0 and C in Fig. 3 are the initial concentration after the equilibrium adsorption and the reaction concentration of the Rhodamine B, respectively. As seen in Fig. 3, Rhodamine B aqueous solution can be obviously decolorized by the Bi2O3 photocatalyst under visible irradiation. By contrast, when P25 TiO2 substitutes for Bi2O3 as the photocatalyst, the decolorizing of Rhodamine B takes
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Ma MG, Zhu JF, Sun RC, Zhu YJ. Mater Lett 2010;24:524–7. Huang TJ, Huang MC, Huang MS. Appl Catal A Gen 2009;354:127–31. Gong YH, Ji WJ, Zhang L, Xie B, Wang HQ. J Power Sources 2011;196:928–34. Sambandam A, Lee GJ, Chen PK, Fan CH, Wu JJ. Ind Eng Chem Res 2010;49: 9729–37. Xie X, Li Y, Liu ZQ, Masatake H, Shen WJ. Nature 2009;458:746–9. Wang C, Shao C, Wang LJ, Zhang L, Li XH, Liu YC. J Colloid Interface Sci 2009;333: 242–8. Shen XP, Wu SK, Zhao H. Phys E Amsterdam 2007;39:133–8. Kim HW, Lee JW, Shim SH. Sens Actuat B 2007;126:306–10. Kim HW, Na HG, Yang JC, Kim HS, Lee JH, Yoob KH. Electrochem Solid State Lett 2010;13:K67–71. Li L, Yang YW, Li GH, Zhang LD. Small 2006;2:548–53. Ye F, Peng Y, Chen GY, Deng B, Xu AW. J Phys Chem C 2009;113:10407–15.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Hydrothermal synthesis and characterization of Bi2O3 nanowires Changle Wu a,⁎, Li Shen b, Qingli Huang a, Yong-Cai Zhang b a b
Testing Center of Yangzhou University, Yangzhou 225009, China College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China
a r t i c l e
i n f o
Article history: Received 6 November 2010 Accepted 13 January 2011 Available online 19 January 2011 Keywords: Nanoparticles Semiconductors Electron microscope Optical materials and properties
a b s t r a c t An alternative two-step method has been proposed for the synthesis of Bi2O3 nanowires with a diameter of about 40 nm from common and cost-effective Bi(NO3)3·5H2O, Na2SO4, and NaOH. That is, first, Bi2O(OH)SO4 nanowires were prepared through the precipitation reaction of Bi(NO3)3·5H2O and Na2SO4 in distilled water under the ambient condition and second, monoclinic phase Bi2O3 nanowires were prepared via the hydrothermal reaction of Bi2O(OH)SO4 and NaOH at 120 °C for 12 h. The resultant products were characterized by X-ray diffraction, field emission scanning electron microscope, and high resolution transmission electron microscopy. In addition, the photocatalytic studies indicated that the as-synthesized Bi2O3 nanowires were a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Bi2O3, an important semiconductor with a direct band gap of 2.8 eV [1], has been widely used in catalyst [2], fuel cells [3], and photocatalyst [4], etc. Recently, promising performance in catalyst and photocatalyst has been demonstrated in one-dimensional (1D) semiconductor nanostructures in the form of nanorods, nanowires, nanotubes or nanofibers [5,6], which triggers a wide range of subsequent research in searching for newer synthetic methods of 1D Bi2O3 nanostructures [4]. At present, there exist several kinds of methods to prepare Bi2O3 nanorods, nanowires, nanotubes or nanofibers [6–10], including calcinations treatment of bismuth nitrate composite [6], atomicpressure chemical vapor deposition (CVD) [7], metallorganic CVD [8], oxidative metal vapor-phase deposition [9], and slow oxidation of Bi nanowires at 750 °C [10], etc. Apparently, it is difficult for the existing methods to produce large-scale Bi2O3 nanorods or nanowires at low cost in an environmental-friendly way, because they require sophisticated equipments and high temperatures [6–10]. Hydrothermal route has been proved to be a very powerful method in synthesizing 1D nanostructures of inorganic functional materials [11]. But to the best of our knowledge, the reports on the hydrothermal preparation of Bi2O3 nanowires are scarce up to now. Herein, we report the synthesis of Bi2O3 nanowires by a simple hydrothermal route, as well as the characterization of the resultant products by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), and high resolution transmission electron microscopy (HRTEM). Further-
⁎ Corresponding author. Tel.: + 86 0514 87979022; fax: + 86 0514 87979244. E-mail address: [email protected] (C. Wu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.01.021
more, the photocatalytic activities of the as-synthesized Bi2O3 nanowires are also studied by degrading Rhodamine B in water under the visible light (λ N 420 nm) irradiation. 2. Experimental section All the chemical reagents used in this work, including Bi (NO3)3·5H2O, Na2SO4, and NaOH, are A. R. grade. Bi2O3 nanowires were prepared according to the following procedure. Firstly, 2.0 mmol of Bi(NO3)3·5H2O and 3.0 mmol of Na2SO4 were dissolved in 40 ml of distilled water and the solution was stirred at room temperature for 45 min. Then, 18.0 mmol NaOH dissolved in 40 ml of distilled water was added drop by drop into the above solution with stirring. The mixed solution was transferred into a Teflon-lined stainless steel autoclave of 100 ml capacity, sealed and heated at 120 °C for 12 h, then allowed to cool to room temperature naturally. The as-formed yellow precipitates were filtered, washed with deionized water and ethanol, and finally dried in air at 80 °C. XRD patterns were recorded on a German Bruker AXS D8 ADVANCE X-ray diffractometer. The morphological and structural analysis was carried out on Japan Hitachi S-4800 FESEM, and Holland F-30 HRTEM. In the photocatalytic experiment, Rhodamine B was used as a probe molecule to evaluate the photocatalytic reactivity of the assynthesized Bi2O3 nanowires. The experiments were carried out as follows: 20 mg of the samples was dispersed in 200 mL of 0.02 mmol/ L Rhodamine B solution in a 300 mL beaker. Prior to illumination, the suspensions were magnetically stirred in the dark for 2 h to ensure the establishment of absorption equilibrium of Rhodamine B on the sample surfaces. Subsequently, the suspension was irradiated under a 1000 W Xe lamp (equipped with a filter of λ N 420 nm), which was
#
15
(041) (140) (-312) (113)
(023)
Bi2O(OH)2SO4+Bi(OH)3
# # (b)
Bi2O(OH)2SO4
(a) 10
Bi2O3 (-222) (131) (-213) (122)
(-211) (-122) (200) (022) (-212) (031) (102) (130) (112)
(-102) (002) (111) (012)
(020)
(c)
(-111)
Intensity (a.u.)
(120)
C. Wu et al. / Materials Letters 65 (2011) 1134–1136
20
25
30
35
40
45
1135
Fig. 2 shows the typical FESEM and HRTEM images of the products prepared under different conditions. As can be seen from the image in Fig. 2(a) and (b), an abundance of nearly wire-like particles of pure Bi2O(OH)2SO4 and the mixture of Bi2O(OH)2SO4 and Bi(OH)3 were prepared at room temperature. After the hydrothermal reaction at 120 °C for 12 h, Bi2O3 nanowires with a diameter of about 40 nm were found in Fig. 2(c). In Fig. 2(d) and (e), the HRTEM image shows sharp lattice fringes with 0.325 nm spacings, corresponding to the (1 2 0) planes of monoclinic phase Bi2O3 crystals. Moreover, this sample displayed sharp lattice fringes with no lattice defects such as stacking faults, indicating good crystallinity. This was also consistent with the result of their XRD patterns. Since the source materials used in the present system were Bi (NO3)3·5H2O, Na2SO4, and NaOH, and the synthesis was conducted in two steps, the reactions involved in the formation of Bi2O3 can be described by the following equations:
50
2Theta/degree Fig. 1. XRD patterns of (a) Bi2O(OH)2SO4 and the products prepared at (b) room temperature and (c) 120 °C. The reflections marked by # belong to Bi(OH)3.
positioned about 10 cm away from the breaker. UV–vis adsorption spectra (Shimazu, UV2101) were recorded at different time intervals to monitor the process. For comparison, the Degussa P25 TiO2 was also tested under the same conditions. 3. Results and discussion The crystalline phase and purity of the as-prepared samples were determined by XRD, and the obtained results are shown in Fig. 1. The monoclinic structure Bi2O(OH)2SO4 was formed when Bi(NO3)3·5H2O and Na2SO4 were mixed in distilled water (Fig. 1(a)).When excess NaOH was added to the reaction system, the mixture of Bi2O(OH)2SO4 and Bi(OH)3 was obtained at room temperature (Fig. 1(b)), while pure monoclinic phase Bi2O3 (JCPDS file No. 41-1449) was obtained after the hydrothermal reaction at 120 °C for 12 h (Fig. 1(c)). The strong and sharp XRD peaks of Fig. 1(c) indicated that the as-prepared Bi2O3 crystals were highly crystalline.
2BiðNO3 Þ3 :5H2 O þ Na2 SO4 →Bi2 OðOHÞ2 SO4 þ 2NaNO3 þ 4HNO3 þ 7H2 O ð1Þ Bi2 OðOHÞ2 SO4 þ 2NaOH þ H2 O→2BiðOHÞ3 þ Na2 SO4
ð2Þ
2BiðOHÞ3 →Bi2 O3 þ 3H2 O:
ð3Þ
When Bi(NO3)3 and Na2SO4 were mixed in distilled water at room temperature, Bi2O(OH)2SO4 nanowires were formed, which may behave as a template in the following process of the reaction. By dropping NaOH solution into the reaction system, OH− ions reacted with Bi2O(OH)2SO4 (Kθsp = 1.8 × 10− 31) gradually to form Bi(OH)3 (Kθsp = 3.2 × 10− 40). When the Bi(OH)3 nanowires were subjected to hydrothermal treatment at 120 °C for 12 h, they would dehydrate and convert into Bi2O3 nanowires. This process is similar to the formation process of ZnO nanowires [11]. In order to investigate if the additive Na2SO4 played an important role in gaining Bi2O3 nanowires, control experiment was also conducted without adding Na2SO4. It can be seen from the FESEM image in Fig. 2(f) that only Bi2O3 micro rods were obtained, in the absence of Na2SO4. The above result clearly indicated that the
Fig. 2. FESEM images of (a) Bi2O(OH)2SO4 and the products prepared at (b) room temperature, and (c) 120 °C; (d)–(e) HRTEM images of the products prepared at 120 °C; and (f) FESEM image of Bi2O3 prepared at 120 °C without adding Na2SO4.
1136
C. Wu et al. / Materials Letters 65 (2011) 1134–1136
1.1 1.0
C/C0
0.9 0.8
P25 TiO2
0.7
Bi2O3
0.6
without catalyst
place at a much slower rate under the same conditions. For example, the decolorization ratio of Rhodamine B is nearly 60% over 0.02 g of Bi2O3 nanowires, which is higher than only 1.1% over 0.02 g of P25 TiO2, when irradiated by the visible light for 130 min. Furthermore, in the absence of any photocatalyst, the degradation of Rhodamine B hardly occurs when subjected to the visible light irradiation for 130 min.
0.5
4. Conclusion
0.4 0.3 0.2 0.1 0.0
0
10
20
30
40
50
60
70
80
90 100 110 120 130 140
Irradiation time (min)
In the absence of any surfactant and template, monoclinic phase Bi2O3 nanowires with a diameter of about 40 nm were successfully obtained by a simple hydrothermal process at 120 °C, and verified by XRD, FESEM, and HRTEM. The proposed method is simple, mild and cost-effective, which may be suitable for industrial production of Bi2O3 nanowires with promising applications such as visible-light photocatalyst.
Fig. 3. Photodegradation of Rhodamine B using the Bi2O3 photocatalyst.
References formation of Bi2O(OH)2SO4 intermediate due to the addition of Na2SO4 may limit the conversion rates of Bi(NO3)3 → Bi(OH)3 → Bi2O3, thus yielding Bi2O3 nanowires with similar morphology to the Bi2O(OH)2SO4 intermediate. The photocatalytic activities of the as-prepared Bi2O3 nanowires are shown in Fig. 3. C0 and C in Fig. 3 are the initial concentration after the equilibrium adsorption and the reaction concentration of the Rhodamine B, respectively. As seen in Fig. 3, Rhodamine B aqueous solution can be obviously decolorized by the Bi2O3 photocatalyst under visible irradiation. By contrast, when P25 TiO2 substitutes for Bi2O3 as the photocatalyst, the decolorizing of Rhodamine B takes
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Ma MG, Zhu JF, Sun RC, Zhu YJ. Mater Lett 2010;24:524–7. Huang TJ, Huang MC, Huang MS. Appl Catal A Gen 2009;354:127–31. Gong YH, Ji WJ, Zhang L, Xie B, Wang HQ. J Power Sources 2011;196:928–34. Sambandam A, Lee GJ, Chen PK, Fan CH, Wu JJ. Ind Eng Chem Res 2010;49: 9729–37. Xie X, Li Y, Liu ZQ, Masatake H, Shen WJ. Nature 2009;458:746–9. Wang C, Shao C, Wang LJ, Zhang L, Li XH, Liu YC. J Colloid Interface Sci 2009;333: 242–8. Shen XP, Wu SK, Zhao H. Phys E Amsterdam 2007;39:133–8. Kim HW, Lee JW, Shim SH. Sens Actuat B 2007;126:306–10. Kim HW, Na HG, Yang JC, Kim HS, Lee JH, Yoob KH. Electrochem Solid State Lett 2010;13:K67–71. Li L, Yang YW, Li GH, Zhang LD. Small 2006;2:548–53. Ye F, Peng Y, Chen GY, Deng B, Xu AW. J Phys Chem C 2009;113:10407–15.