Template-free synthesis of SnO2 nanostructural hollow spheres covered by nanorods

Template-free synthesis of SnO2 nanostructural hollow spheres covered by nanorods

Materials Letters 65 (2011) 1645–1647 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 ...

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Materials Letters 65 (2011) 1645–1647

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

Template-free synthesis of SnO2 nanostructural hollow spheres covered by nanorods Chunlong Zheng, Xiangzhen Zheng, Zhensheng Hong, Xiaokun Ding, Mingdeng Wei ⁎ Institute of New Energy Technology and Nano-Materials, Fuzhou University, Fuzhou, Fujian 350002, China

a r t i c l e

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Article history: Received 30 October 2010 Accepted 18 February 2011 Available online 24 February 2011 Keywords: SnO2 Hollow sphere Nanostructures Crystal growth

a b s t r a c t In the present work, SnO2 nanostructural hollow spheres have been successfully synthesized in the absence of template by a simple synthetic route, and their surfaces were covered by nanorods. The synthesized nanostructural hollow spheres covered by nanorods were further characterized by XRD, SEM and TEM measurements. The diameter of SnO2 hollow spheres and the thickness of shells are found to be ca. 150–200 and 20–30 nm, respectively. The size of the nanorod is found to be ca. 5 nm, and the length up to tens of nanometers. Based on a series of experimental results, an oxidizing-aggregating-Ostwald ripening model has been proposed for the formation of SnO2 nanostructural hollow spheres. © 2011 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

In the past decade, a wide range of applications employing nanoscale SnO2 materials have been reported, including gas sensors [1,2], solar cells[3,4], catalysts [5,6], transistors [7,8], and lithium rechargeable batteries [9–11], due to their excellent physiochemical properties. To date, various nanostructures of SnO2, such as nanorods [12,13], nanowires [14], nanotubes [15,16], nanoboxes [17], nanodisks [18], and hollow structures [19–21], have been synthesized. Among them, hollow structures have attracted considerable attention because of their promising applications such as nanoscale chemical reactors and drug-delivery carriers [22,23]. So far, a series of strategies have been successfully developed in the synthesis of hollow nanostructures SnO2. Hard templates have been widely used in the synthesis of unique hollow nanostructures, for instance, SnO2 nanococoons [24] and SnO2 nanotubes[16]. However, the hard templating routes suffer from disadvantages related to high cost and tedious synthetic procedures, which may prevent them from being used in large-scale applications. More recently, a lot of template-free routes have been developed for generating hollow nanostructures. Lou et al. employed an inside–out Ostwald ripening mechanism and prepared SnO2 core-shell hollow nanostructures via a self-assembly route in the absence of template[20]. Du et al. demonstrated the formation of SnO2 nanotubes based on a nanoscale Kirkendall effect [15]. In the present work, we first report a template-free synthesis of SnO2 nanostructural hollow spheres, of which the shells are covered by nanorods. Furthermore, a possible mechanism for the formation of SnO2 nanostructural hollow spheres covered by nanorods is discussed.

2.1. Synthesis of SnO2 hollow spheres

⁎ Corresponding author: Fax: +86 591 837353180. E-mail address: [email protected] (M. Wei). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.061

In a typical synthesis, stannous sulfate (SnSO4, Aldrich, N99.0%) was dissolved in distilled water, and a solution with a concentration of 5 mM was obtained (pH ≈2.9). After stirring for several minutes, the solution was transferred to a 75 ml Teflon-lined autoclave and kept at 200 °C for 24 h. Finally, a white precipitate was collected by centrifugation, and then washed with distilled water for several times before it was dried at 70 °C for 6 h in the air. 2.2. Characterizations Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were taken on a Philip-XL30 instrument and a JEOL 2010 instrument, respectively. X-ray diffraction (XRD) pattern was recorded on a PANalytical X'Pert spectrometer using the Co Kα radiation (λ = 1.78897 Å), and the data would be converted to Cu Kα data. 3. Results and discussion Fig. 1 shows the XRD pattern of the product synthesized at 200 °C for 24 h. All the diffractions can be indexed to a tetragonal structural SnO2 with the parameters a = b = 4.738 Å and c = 3.187 Å (JCPDS 41– 1445), indicating that the synthesized product is of high purity. TEM images of the products synthesized at 200 °C for different times are depicted in Fig. 2. Fig. 2a shows the result for 12 h, only nanoparticles could be observed. The synthesized nanoparticles are highly crystalline and their size is found to be ca. ~10 nm. After the reaction time was increased to 16 h, solid spheres and core-shell hollow spheres were observed, as depicted in Fig. 2b and c. The

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2θ/deg Fig. 1. XRD pattern of SnO2 hollow nanostructures composed of nanorods.

formation of those solid spheres should be ascribed to the aggregation of nanoparticles for 12 h. While some solid spheres were subjected to an inside–out Ostwald ripening, core-shell spheres were formed. It is noticeable that the Ostwald ripening process was accompanied by the emergence of few short nanorods on the external surface of the coreshell spheres whereas the lateral nanostructures of solid spheres were just agglomerated nanoparticles. High-resolution TEM (HRTEM) image of a single nanorod on the peripheral surface of core-shell spheres is depicted in Fig. 2c (inset). The lattice fringe is found to be ca. 0.337 nm, which corresponds to d110-spacing in the XRD pattern of SnO2, indicating a preferential growth along [001] direction. When the reaction time was further increased to 24 h, the hollow spheres

with external surfaces covered by nanorods were observed, as shown in Fig. 2d. The diameter of hollow spheres and the thickness of shells are found to be ca. 150–200 and 20–30 nm, respectively. An HRTEM image of a single nanorod on the surface of shells is depicted in Fig. 2d (inset). The size of nanorod is found to be ca. 5 nm, and the length up to tens of nanometers. It is also found that the formed nanorods are highly crystalline and the lattice fringes are clearly visible. The distance between the adjacent lattice fringes is ca. 0.266 nm, corresponding to the d101-spacing in the XRD pattern of SnO2. On the other hand, all nanorods on the surface of shells grow along the c-axis, since the (001) planes have the highest surface energies, resulting in a preferential growth in [001] direction [2,24]. In the present work, a simple method was employed to synthesize SnO2 nanostructural hollow spheres using SnSO4 as a precursor. The following chemical reactions may take place in an aqueous solution, resulting in the formation of SnO2 phase. SnSO4 þ 1=2O2 þ H2 O→SnO2 þ H2 SO4

ð1Þ

In this reaction, SnSO4 reacted with oxygen in the solution under the hydrothermal conditions to yield SnO2 hollow nanostructures. To understand the formation progress of the unique nanostructured SnO2 hollow spheres, a series of experiments were taken to track the reaction process and the products were observed by SEM measurement. Fig. 3 is SEM images of the products synthesized over 200 °C for 24 h at pH values of 1.5 and 3.8, respectively. Fig. 3a clearly shows that the synthesized products are hollow spheres composed of nanoparticles. After the pH value was altered to 3.8, only rod-like particles were observed in the product, as shown in Fig. 3b. These results

Fig. 2. TEM images of the products synthesized at 200 °C for different times: (a) 12, (b, c) 16 and (d) 24 h (inset, HRTEM images).

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Fig. 3. SEM images of the products synthesized over 200 °C for 24 h at pH values of (a) 1.5 and (b) 3.8, respectively.

Scheme 1. A possible model for the formation of SnO2 hollow nanostructures composed of nanorods on the surface of shells.

indicate that pH value plays a key role in the formation of the SnO2 nanostructural hollow spheres covered by nanorods. On the basis of the experimental results mentioned above, a possible growth mechanism for the formation of SnO2 nanostructural hollow spheres is illustrated in scheme 1. Firstly, Sn2+ ions in aqueous solution are oxidized and tiny nanoparticles of SnO2 are formed. Secondly, tiny nanoparticles aggregate together to form solid spheres in order to reduce the surface energy. Thirdly, the hollow sphere structure is formed based on the localized Ostwald ripening process. In general, reducing the overall surface energy provides the driving force for Ostwald ripening within an ensemble of particles, whereby the initial particle population on the shells grow at the expense of the inner tiny particles that dissolve over time [25]. In addition, the (001) planes of SnO2 have the highest surface energy, a proper pH value provides the preferential growth along [001] direction to reduce the surface energy. Finally, the formation of hollow sphere structure is accompanied by the growth of the nanorods on the external shells. Therefore, an oxidizing-aggregating-Ostwald ripening model might be described for the formation of the SnO2 nanostructural hollow spheres covered by nanorods. 4. Conclusions In conclusion, we have demonstrated a simple synthetic route for preparing SnO2 hollow spheres covered by nanorods using SnSO4 as a precursor in the absence of templates. Based on the experimental results, an oxidizing-aggregating-Ostwald ripening model, has been proposed for the formation of SnO2 nanostructural hollow spheres covered by nanorods. Such a simple synthetic method, which involves no templates or reactants and requires no expensive and precise equipment, will offer great opportunities for the scale-up preparation of hollow nanostructures materials.

Acknowledgments This study was financially supported by Nos. 2007AA05Z438, 2007HZ005-1, 200803860004, 2008J0332 and 20173039(NSFC).

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