- Email: [email protected]
Synthesis of SnO2 single-layered hollow microspheres and flowerlike nanospheres through a facile template-free hydrothermal method
Materials Letters 95 (2013) 67–69
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of SnO2 single-layered hollow microspheres and flowerlike nanospheres through a facile template-free hydrothermal method Xiaohong Zhang n, Mingxing Huang, Yingjie Qiao Institute of Structured-Function Integration Materials, Harbin Engineering University, Harbin 150001, China
a r t i c l e i n f o
abstract
Article history: Received 29 October 2012 Accepted 29 December 2012 Available online 7 January 2013
SnO2 single-layered hollow microspheres and SnO2 flowerlike nanospheres have been fabricated by one step template-free hydrothermal reaction with the help of PEG400. The concentration of NaOH plays an important role in the formation of SnO2 nanostructure. When the molar ratio of SnCl4/NaOH is 1:6, the nanostructure of SnO2 turns out to be a single-layered hollow microsphere. And it will be a flowerlike nanosphere when molar ratio of SnCl4/NaOH is 1:8. These single-layered hollow microspheres with diameters of 1–5 mm have a very thin shell thickness of about 200 nm. These flowerlike nanospheres with diameters of 0.5–1 mm are composed of SnO2 submicron rods with diameters of 200–300 nm. A possible growth mechanism is proposed. & 2013 Elsevier B.V. All rights reserved.
Keywords: SnO2 Single-layered microspheres Flowerlike nanospheres Nanoparticles Crystal growth
1. Introduction As an important low-cost, n-type wide band gap (Eg ¼3.6 ev, at 300 K) semiconductor, SnO2 is wellknown for its potential applications in transparent conductive electrodes [1]; lithium rechargeable batteries [2], dye-sensitized solar cells [3], ultrasensitive gas sensors [4] and catalyst supports [5], due to its excellent physiochemical properties. Thus, tin dioxide has attracted great attention for a long time. Some studies have shown that many fundamental physical or chemical properties of semiconductor materials strongly depend on the size and morphology of the materials, which define their further applications. Thus, it is of great interest to synthesize SnO2 nanocrystals with desired structure and morphology [6–11]. Particularly, 3D structures have attracted considerable attention because of their improvable performance, such as large surface area, efficient catalytic activity, and structural stability. Several methods have recently been reported to prepare different kinds of SnO2 3D structures. Of all the methods, the hydrothermal method is an easy, common and effective way. It has been reported that SnO2 3D structures can be prepared with the help of the hydrothermal method, including hollow spheres [12,13], core–shell microspheres [14], flowerlike nanospheres [15], urchin-like hollow spheres [16] and so on. However, in order to obtain different 3D structures we usually need different hydrothermal systems and some relatively sophisticated procedures. To the best of our knowledge, there have been fewer reports in regard to preparation of more than one 3D structure with only one facile
n
Corresponding author. Tel./fax: þ 86 451 82568337. E-mail address: [email protected] (X. Zhang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.108
hydrothermal system. Herein, we report an efficient and simple approach for synthesis of single-layered hollow spheres and flowerlike nanospheres by the one-step template-free hydrothermal method with the help of PEG400. We found that NaOH plays an important role in the formation of SnO2 nanostructure.
2. Experimental All chemicals were analytical-grade reagents and used without any further purification. In a typical procedure, 3 mmol of SnCl4H2O was added into 35 ml of NaOH (18 mmol) solution under stirring. Then 10 ml PEG400 was added into above solution with vigorous stirring. Later 35 ml of absolute ethanol was added, and a white translucent suspended solution was obtained, which was then transferred into a Teflon-lined autoclave (100 ml), and held in an electric oven at 180 1C for 24 h. Finally the autoclave was allowed to cool to room temperature naturally. The white powder was harvested by filtration and washed with de-ionized water several times before drying in vacuum at 70 1C for 6 h. All the characterization experiments were performed at room temperature. The phase was identified by X-ray diffraction (XRD) ˚ radiation. using a Philips X’Pert Pro MPD with Cu Ka (l ¼1.5406 A) Field-emission scanning electron microscopy (FESEM) observations were carried out with a FEI Sirion 200 microscope.
3. Results and discussion Fig. 1 shows a typical XRD pattern of the as-synthesized SnO2 with different molar ratios of SnCl4/NaOH. All the diffraction peaks in Fig. 1 (S1–S3) were well indexed to the tetragonal
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X. Zhang et al. / Materials Letters 95 (2013) 67–69
˚ structure of SnO2 with the lattice constants of a ¼4.755 A, ˚ This is in good agreement with the literature values c¼3.199 A. (JCPDS 77-0452). No impurity peaks are observed, indicating the high purity of the final products. With a decrease of the molar ratio, from SnCl5H2O/NaOH ¼1:6 to SnCl5H2O/NaOH¼1:7, diffraction peaks were obviously sharpened. It is further noticed that X-ray diffraction peaks of the hollow spheres (S1) are much broader than those of the broken hollow sphere (S2) and the
Fig. 1. XRD pattern of samples prepared at different molar ratios of SnCl4/NaOH: S1, single-layered hollow microspheres (SnCl5H2O/NaOH¼ 1:6); S2, broken hollow microspheres (SnCl4/NaOH ¼ 1:7); and S3, flowerlike nanospheres (SnCl4/ NaOH¼ 1:8).
spherical flowerlike architectures (S2). This reveals that the SnO2 hollow spheres have comparatively smaller crystallite size. Fig. 2a shows a typical low-magnification SEM image of SnO2 hollow microshperes. These microspheres have diameters in the range of 1–5mm. Some of them aggregate together (Fig. 2b). In a high-magnification SEM image (Fig. 2c), there is a hole on the surface of the microsphere with a diameter about 200 nm that may have formed by the evaporation process of PEG during the hydrothermal and dry treatment. With increase in the dosage of NaOH, as shown in Fig. 2d, it is clear that the hollow spheres were broken into shells and pieces, and the shell thickness of SnO2 microspheres is about 200 nm. This indicates that there is a single-layered shell of hollow microsphere structure in as-synthesized SnO2 (S1). Fig. 3a presents a typical low-magnification SEM image of SnO2 flowerlike nanospheres. The as-prepared samples (S3) consist of a large quantity of flowerlike nanospheres with the diameters in the range of 0.5–1mm. These flowerlike nanospheres are well dispersed with good monodispersity. In a highmagnification SEM image (Fig. 3b), the flowerlike nanospheres are assembled from submicron rods with the diameters of 200– 300 nm. A possible growth mechanism for the formation of SnO2 single-layered hollow microspheres and flowerlike nanospheres is illustrated in Fig. 3c. Firstly, the overall reaction for the growth of SnO2 crystals may be expressed by the following equations: SnCl4 þ6NaOHaNa2Sn(OH)6 þ4NaCl
(1)
Na2Sn(OH)6-Sn O2 þ2NaOH þ2H2O
(2)
By dissolving SnCl45H2O in alkaline ethanol aqueous solution, a white translucent solution with an intermediate phase
Fig. 2. SEM images of single-layered hollow microspheres with MSnCl4:MNaOH ¼ 1:6 at (a) low magnification, (b) medium magnification and (c) high magnification. Images of broken single hollow spheres with shells and pieces (d) with MSnCl4:MNaOH ¼ 1:7.
X. Zhang et al. / Materials Letters 95 (2013) 67–69
69
Fig. 3. SEM images of flowerlike nanospheres with MSnCl4:MNaOH ¼ 1:8 at (a) low magnification, and (b) high magnification and (c) a possible model for the formation of SnO2 single-layered hollow microspheres and flowerlike nanospheres.
Na2Sn(OH)6 was prepared because of its low solubility in ethanol/ water solvent. In the subsequent hydrothermal reaction, the Na2Sn(OH)6 intermediate phase decomposed and changed into a large quantity of SnO2 nuclei. It is supposed that the formation of SnO2 hollow spheres is related to PEG400 which played a ‘‘template-like’’ role. PEG can absorb the SnO2 cell on its surface to form spheres and the SnO2 nanocrystals have a tendency to aggregate around the surface of PEG400 due to minimization of interfacial energy; then the SnO2 single-layered hollow microspheres are formed. There is a small hole on the surface of spheres that may be formed by the evaporation of PEG. The single-layered hollow microspheres will be broken into nanoparticles and shells when the dosage of NaOH is increased to 21 mmol. With continued increase of the dosage of NaOH to 24 mmol, the broken shells completely change into nanoparticles. And driven by the minimization of the total energy of the system, these SnO2 nanocrystals aggregated together to grow into SnO2 submicron rods via an oriented attachment process in reaction time, and thus the particles with irregular morphology gradually evolved into flowerlike nanospheres.
4. Conclusions To conclude, we have demonstrated a one-step template-free hydrothermal system to prepare two kinds of SnO2 3D structures, including the single-layered hollow microspheres and flowerlike nanospheres. Based on the experiment, the formation of SnO2 single-layered hollow microspheres and flowerlike nanospheres was investigated and discussed. Such a simple synthetic method without any templates or reactants can be used to obtain two different interesting 3D structures and will be economical and environment friendly.
Acknowledgments The project was sponsored by the Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, PR China.
References [1] Wang WW, Zhu YJ, Yang LX. ZnO–SnO2 hollow spheres and hierarchical nanosheets: hydrothermal preparation, formation mechanism, and photocatalytic properties. Adv Funct Mater 2007;17:59–64. [2] Idota Y, Kubota T, Matsufuji A, Maekawa Y, Miyasaka T. Tin-based amorphous oxide: a high-capacity lithium-ion-storage material. Science 1997;276:1395–7. [3] Ferrere S, Zaban A, Gregg BA. Dye sensitization of nanocrystalline tin oxide by perylene derivatives. J Phys Chem B 1997;101:4490–3. [4] Lou XW, Wang Y, Yuan CL, Lee JY, Archer LA. Template-free synthesis of SnO2 hollow nanostructures with high lithium storage capacity. Adv Mater 2006;18: 2325–9. [5] Chen Y, Zhu CL, Shi XL, Cao MS, Jin HB. The synthesis and selective gas sensing characteristics of SnO2/alpha-Fe2O3 hierarchical nanostructures. Nanotechnology 2008;19:205603. [6] Wang YL, Jiang XC, Xia YN. A solution-phase, precursor route to polycrystalline SnO2 nanowires that can be used for gas sensing under ambient conditions. J Am Chem Soc 2003;125:16176–7. [7] Zhang J, Wang SR, Wang YM, Zhu BL, Xia HJ, Guo XZ, et al. NO2 sensing performance of SnO2 hollow-sphere sensor. Sens Actuators B 2009;135:610–7. [8] Comini E, Faglia G, Sberveglieri G. Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts. Appl Phys Lett 2002;81:1869–71. [9] Jia Y, He LF, Guo Z, Chen X, Meng FL, Luo T, et al. Preparation of porous tin oxide nanotubes using carbon nanotubes as templates and their gas-sensing properties. J Phys Chem C 2009;113:9581–7. [10] Hyodo T, Sasahara K, Shimizu Y, Egashira M. Preparation of macroporous SnO2 films using PMMA microspheres and their sensing properties to NOx and H2. Sens Actuators B 2005;106:580–90. [11] Yin XM, Li CC, Zhang M, Hao QY, Liu S, Li QH, et al. SnO2 monolayer porous hollow spheres as a gas sensor. Nanotechnology 2009;20:455503. [12] Zheng C.Z., Zheng X.Z., Hong Z.S., Ding X.K., Wei M.D. Template-free synthesis of SnO2 nanostructural hollow spheres coverd by nanorods. Mater Lett 2011:65:1645-1647. [13] Yan WW, Fang M, Tan XL, Liu M, Liu PS, Hu XY, et al. Template-free fabrication of SnO2 hollow spheres with photoluminescence from Sni. Mater Lett 2010;64:2033–5. [14] Guo YQ, Li Y, Tan RQ, Song WJ. Synthesis of SnO2 hollow microspheres with core–shell structures through a facile template-free hydrothermal method. Mater Sci Eng B 2010;17:20–4. [15] Wang H, Liang QQ, Wang WJ, An YR, Li JH, Guo L. Preparation of flower-like SnO2 nanostructures and their applications in gas-sensing and lithium storage. Cryst Growth Des 2011;11:2942–7. [16] Liu SQ, Xie MJ, Li YX, Guo XF, Ji WJ, Ding WP, et al. Novel sea urchin-like hollow core–shell SnO2 superstructures: facile synthesis and excellent ethanol sensing performance. Sens Actuators B 2010;151:229–35.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of SnO2 single-layered hollow microspheres and flowerlike nanospheres through a facile template-free hydrothermal method Xiaohong Zhang n, Mingxing Huang, Yingjie Qiao Institute of Structured-Function Integration Materials, Harbin Engineering University, Harbin 150001, China
a r t i c l e i n f o
abstract
Article history: Received 29 October 2012 Accepted 29 December 2012 Available online 7 January 2013
SnO2 single-layered hollow microspheres and SnO2 flowerlike nanospheres have been fabricated by one step template-free hydrothermal reaction with the help of PEG400. The concentration of NaOH plays an important role in the formation of SnO2 nanostructure. When the molar ratio of SnCl4/NaOH is 1:6, the nanostructure of SnO2 turns out to be a single-layered hollow microsphere. And it will be a flowerlike nanosphere when molar ratio of SnCl4/NaOH is 1:8. These single-layered hollow microspheres with diameters of 1–5 mm have a very thin shell thickness of about 200 nm. These flowerlike nanospheres with diameters of 0.5–1 mm are composed of SnO2 submicron rods with diameters of 200–300 nm. A possible growth mechanism is proposed. & 2013 Elsevier B.V. All rights reserved.
Keywords: SnO2 Single-layered microspheres Flowerlike nanospheres Nanoparticles Crystal growth
1. Introduction As an important low-cost, n-type wide band gap (Eg ¼3.6 ev, at 300 K) semiconductor, SnO2 is wellknown for its potential applications in transparent conductive electrodes [1]; lithium rechargeable batteries [2], dye-sensitized solar cells [3], ultrasensitive gas sensors [4] and catalyst supports [5], due to its excellent physiochemical properties. Thus, tin dioxide has attracted great attention for a long time. Some studies have shown that many fundamental physical or chemical properties of semiconductor materials strongly depend on the size and morphology of the materials, which define their further applications. Thus, it is of great interest to synthesize SnO2 nanocrystals with desired structure and morphology [6–11]. Particularly, 3D structures have attracted considerable attention because of their improvable performance, such as large surface area, efficient catalytic activity, and structural stability. Several methods have recently been reported to prepare different kinds of SnO2 3D structures. Of all the methods, the hydrothermal method is an easy, common and effective way. It has been reported that SnO2 3D structures can be prepared with the help of the hydrothermal method, including hollow spheres [12,13], core–shell microspheres [14], flowerlike nanospheres [15], urchin-like hollow spheres [16] and so on. However, in order to obtain different 3D structures we usually need different hydrothermal systems and some relatively sophisticated procedures. To the best of our knowledge, there have been fewer reports in regard to preparation of more than one 3D structure with only one facile
n
Corresponding author. Tel./fax: þ 86 451 82568337. E-mail address: [email protected] (X. Zhang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.108
hydrothermal system. Herein, we report an efficient and simple approach for synthesis of single-layered hollow spheres and flowerlike nanospheres by the one-step template-free hydrothermal method with the help of PEG400. We found that NaOH plays an important role in the formation of SnO2 nanostructure.
2. Experimental All chemicals were analytical-grade reagents and used without any further purification. In a typical procedure, 3 mmol of SnCl4H2O was added into 35 ml of NaOH (18 mmol) solution under stirring. Then 10 ml PEG400 was added into above solution with vigorous stirring. Later 35 ml of absolute ethanol was added, and a white translucent suspended solution was obtained, which was then transferred into a Teflon-lined autoclave (100 ml), and held in an electric oven at 180 1C for 24 h. Finally the autoclave was allowed to cool to room temperature naturally. The white powder was harvested by filtration and washed with de-ionized water several times before drying in vacuum at 70 1C for 6 h. All the characterization experiments were performed at room temperature. The phase was identified by X-ray diffraction (XRD) ˚ radiation. using a Philips X’Pert Pro MPD with Cu Ka (l ¼1.5406 A) Field-emission scanning electron microscopy (FESEM) observations were carried out with a FEI Sirion 200 microscope.
3. Results and discussion Fig. 1 shows a typical XRD pattern of the as-synthesized SnO2 with different molar ratios of SnCl4/NaOH. All the diffraction peaks in Fig. 1 (S1–S3) were well indexed to the tetragonal
68
X. Zhang et al. / Materials Letters 95 (2013) 67–69
˚ structure of SnO2 with the lattice constants of a ¼4.755 A, ˚ This is in good agreement with the literature values c¼3.199 A. (JCPDS 77-0452). No impurity peaks are observed, indicating the high purity of the final products. With a decrease of the molar ratio, from SnCl5H2O/NaOH ¼1:6 to SnCl5H2O/NaOH¼1:7, diffraction peaks were obviously sharpened. It is further noticed that X-ray diffraction peaks of the hollow spheres (S1) are much broader than those of the broken hollow sphere (S2) and the
Fig. 1. XRD pattern of samples prepared at different molar ratios of SnCl4/NaOH: S1, single-layered hollow microspheres (SnCl5H2O/NaOH¼ 1:6); S2, broken hollow microspheres (SnCl4/NaOH ¼ 1:7); and S3, flowerlike nanospheres (SnCl4/ NaOH¼ 1:8).
spherical flowerlike architectures (S2). This reveals that the SnO2 hollow spheres have comparatively smaller crystallite size. Fig. 2a shows a typical low-magnification SEM image of SnO2 hollow microshperes. These microspheres have diameters in the range of 1–5mm. Some of them aggregate together (Fig. 2b). In a high-magnification SEM image (Fig. 2c), there is a hole on the surface of the microsphere with a diameter about 200 nm that may have formed by the evaporation process of PEG during the hydrothermal and dry treatment. With increase in the dosage of NaOH, as shown in Fig. 2d, it is clear that the hollow spheres were broken into shells and pieces, and the shell thickness of SnO2 microspheres is about 200 nm. This indicates that there is a single-layered shell of hollow microsphere structure in as-synthesized SnO2 (S1). Fig. 3a presents a typical low-magnification SEM image of SnO2 flowerlike nanospheres. The as-prepared samples (S3) consist of a large quantity of flowerlike nanospheres with the diameters in the range of 0.5–1mm. These flowerlike nanospheres are well dispersed with good monodispersity. In a highmagnification SEM image (Fig. 3b), the flowerlike nanospheres are assembled from submicron rods with the diameters of 200– 300 nm. A possible growth mechanism for the formation of SnO2 single-layered hollow microspheres and flowerlike nanospheres is illustrated in Fig. 3c. Firstly, the overall reaction for the growth of SnO2 crystals may be expressed by the following equations: SnCl4 þ6NaOHaNa2Sn(OH)6 þ4NaCl
(1)
Na2Sn(OH)6-Sn O2 þ2NaOH þ2H2O
(2)
By dissolving SnCl45H2O in alkaline ethanol aqueous solution, a white translucent solution with an intermediate phase
Fig. 2. SEM images of single-layered hollow microspheres with MSnCl4:MNaOH ¼ 1:6 at (a) low magnification, (b) medium magnification and (c) high magnification. Images of broken single hollow spheres with shells and pieces (d) with MSnCl4:MNaOH ¼ 1:7.
X. Zhang et al. / Materials Letters 95 (2013) 67–69
69
Fig. 3. SEM images of flowerlike nanospheres with MSnCl4:MNaOH ¼ 1:8 at (a) low magnification, and (b) high magnification and (c) a possible model for the formation of SnO2 single-layered hollow microspheres and flowerlike nanospheres.
Na2Sn(OH)6 was prepared because of its low solubility in ethanol/ water solvent. In the subsequent hydrothermal reaction, the Na2Sn(OH)6 intermediate phase decomposed and changed into a large quantity of SnO2 nuclei. It is supposed that the formation of SnO2 hollow spheres is related to PEG400 which played a ‘‘template-like’’ role. PEG can absorb the SnO2 cell on its surface to form spheres and the SnO2 nanocrystals have a tendency to aggregate around the surface of PEG400 due to minimization of interfacial energy; then the SnO2 single-layered hollow microspheres are formed. There is a small hole on the surface of spheres that may be formed by the evaporation of PEG. The single-layered hollow microspheres will be broken into nanoparticles and shells when the dosage of NaOH is increased to 21 mmol. With continued increase of the dosage of NaOH to 24 mmol, the broken shells completely change into nanoparticles. And driven by the minimization of the total energy of the system, these SnO2 nanocrystals aggregated together to grow into SnO2 submicron rods via an oriented attachment process in reaction time, and thus the particles with irregular morphology gradually evolved into flowerlike nanospheres.
4. Conclusions To conclude, we have demonstrated a one-step template-free hydrothermal system to prepare two kinds of SnO2 3D structures, including the single-layered hollow microspheres and flowerlike nanospheres. Based on the experiment, the formation of SnO2 single-layered hollow microspheres and flowerlike nanospheres was investigated and discussed. Such a simple synthetic method without any templates or reactants can be used to obtain two different interesting 3D structures and will be economical and environment friendly.
Acknowledgments The project was sponsored by the Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, PR China.
References [1] Wang WW, Zhu YJ, Yang LX. ZnO–SnO2 hollow spheres and hierarchical nanosheets: hydrothermal preparation, formation mechanism, and photocatalytic properties. Adv Funct Mater 2007;17:59–64. [2] Idota Y, Kubota T, Matsufuji A, Maekawa Y, Miyasaka T. Tin-based amorphous oxide: a high-capacity lithium-ion-storage material. Science 1997;276:1395–7. [3] Ferrere S, Zaban A, Gregg BA. Dye sensitization of nanocrystalline tin oxide by perylene derivatives. J Phys Chem B 1997;101:4490–3. [4] Lou XW, Wang Y, Yuan CL, Lee JY, Archer LA. Template-free synthesis of SnO2 hollow nanostructures with high lithium storage capacity. Adv Mater 2006;18: 2325–9. [5] Chen Y, Zhu CL, Shi XL, Cao MS, Jin HB. The synthesis and selective gas sensing characteristics of SnO2/alpha-Fe2O3 hierarchical nanostructures. Nanotechnology 2008;19:205603. [6] Wang YL, Jiang XC, Xia YN. A solution-phase, precursor route to polycrystalline SnO2 nanowires that can be used for gas sensing under ambient conditions. J Am Chem Soc 2003;125:16176–7. [7] Zhang J, Wang SR, Wang YM, Zhu BL, Xia HJ, Guo XZ, et al. NO2 sensing performance of SnO2 hollow-sphere sensor. Sens Actuators B 2009;135:610–7. [8] Comini E, Faglia G, Sberveglieri G. Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts. Appl Phys Lett 2002;81:1869–71. [9] Jia Y, He LF, Guo Z, Chen X, Meng FL, Luo T, et al. Preparation of porous tin oxide nanotubes using carbon nanotubes as templates and their gas-sensing properties. J Phys Chem C 2009;113:9581–7. [10] Hyodo T, Sasahara K, Shimizu Y, Egashira M. Preparation of macroporous SnO2 films using PMMA microspheres and their sensing properties to NOx and H2. Sens Actuators B 2005;106:580–90. [11] Yin XM, Li CC, Zhang M, Hao QY, Liu S, Li QH, et al. SnO2 monolayer porous hollow spheres as a gas sensor. Nanotechnology 2009;20:455503. [12] Zheng C.Z., Zheng X.Z., Hong Z.S., Ding X.K., Wei M.D. Template-free synthesis of SnO2 nanostructural hollow spheres coverd by nanorods. Mater Lett 2011:65:1645-1647. [13] Yan WW, Fang M, Tan XL, Liu M, Liu PS, Hu XY, et al. Template-free fabrication of SnO2 hollow spheres with photoluminescence from Sni. Mater Lett 2010;64:2033–5. [14] Guo YQ, Li Y, Tan RQ, Song WJ. Synthesis of SnO2 hollow microspheres with core–shell structures through a facile template-free hydrothermal method. Mater Sci Eng B 2010;17:20–4. [15] Wang H, Liang QQ, Wang WJ, An YR, Li JH, Guo L. Preparation of flower-like SnO2 nanostructures and their applications in gas-sensing and lithium storage. Cryst Growth Des 2011;11:2942–7. [16] Liu SQ, Xie MJ, Li YX, Guo XF, Ji WJ, Ding WP, et al. Novel sea urchin-like hollow core–shell SnO2 superstructures: facile synthesis and excellent ethanol sensing performance. Sens Actuators B 2010;151:229–35.