Self-construction of SnO2 cubes based on aggration of nanorods
Dan Qin, Peng Yan ⁎, Guangzhong Li, Juan Xing, Yukuan An
Physics Department, Binzhou Medical University, Yantai, 264003, China
Received 4 October 2007; accepted 8 December 2007Available online 3 January 2008
Abstract
The SnO2 cubes with the rutile structure have been successfully synthesized without using any catalyst. Their morphology and microstructurewere studied by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), and electedarea electron diffraction (SAED). It is revealed that the SnO2 nanocubes exhibit high crystalline quality. The size of the nanocubes ranges from100 nm to 300 nm. The side surfaces of nanocubes are {110} planes, while their cube axes are [001] direction. The growth mechanism of SnO2
Keywords: SnO2 nanocubes; Chemical vapor Deposition (CVD); Nanomaterials
1. Introduction
One-dimensional (1D) and quasi-one-dimensional (quasi-1D) nanostructures are attracting a great deal of attention dueto their unique properties and novel applications [1–3].Oxides are the basis of smart and functional materials thathave tunable properties and important technological applica-tions. As an important low-cost, n-type semiconductor withwide band gap (Eg=3.6 eV), 1D SnO2 nanostructures havecurrently attracted considerable attention for their promisingapplication in optical waveguides [4], solar cells [5] and gassensors [6]. Recently, 1D SnO2 nanostructures have beenprepared by different methods, such as hydrothermal process[7], thermal evaporation [8], template-directed synthesis [9],and solution method [10]. Compared with other methods, thechemical vapor deposition (CVD) approach is a better alter-native with the advantages of high yield, high purity and lowpollution.
Oriented attachment has attracted increasing interest inrecent years as a new means for fabrication and self-organizaionof nanocrystalline materals [11–16]. Recent examples include
formation of 1D nanorods from their respective 0D nanocrys-tallites [11–13]. The formed 1D nanostructures can also furtherself-attach through stacking by lateral “lattice fusion” togenerate either length-multiplied 1D nanostructures or 2Dcrystal sheets and walls [14–16].
Here we report another novel organizing route with anunderlying oriented-attachment mechanism in which geome-trical structures (cubes) can be built by the assembly route1D→3D. We demonstrate that the SnO2 nanocubes can beprepared by CVD method without using any catalyst.
2. Experiment
In our experiments, the SnO2 nanocubes were synthe-sized through a CVD method. The synthesis route isdepicted as follows: a horizontal ceramic tube was mountedinside a tube furnace. SnO/Sn powder with molar ratio of~4:1 was put into an alumina boat, and then loaded into thecentral region of the ceramic tube. The substrate was a Siwafer. The distance between the substrate and sourcematerials was about 8 cm. After the tube had been purgedwith high-purity Argon (Ar) for about 1 h, the Ar/O2 flowwas introduced into the system. The volume ratio of Ar/O2
was 50:1 and flow rate was 50 sccm. The temperature ofthe central region of the furnace was rapidly increased to
Fig. 3. TEM images of the nanocubes. (a) General morphology of two kinds of
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1000 °C in 10 min and then maintained at this temperaturefor 45 min. After the furnace was cooled to room tempe-rature, white wool-like products were collected from theSi wafer.
The morphology and microstructure of the as-preparedsamples were studied by field emission scanning electronmicroscopy (FE-SEM, Sirion 200), and high-resolution trans-mission electron microscopy (HRTEM, JEOL-2010, at 200 KV)equipped with an energy-dispersive X-ray spectroscopy (EDX)attachment. Before HRTEM imaging, the samples wereultrasonically dispersed in ethanol.
Fig. 2. SEM images of the SnO2 nanocubes. (a) overall product morphology (b) general sample morphology. (c) and (d) the high-magnification image of the typicalnanocubes.
nanocubes. (b) A typical nanocube with two caves in the top and bottom.
Fig. 5. (a) Dispersive nanorods founded in the products. (b) Well-developednanocubes.
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3. Results and discussion
The XRD pattern (Fig. 1) reveals the overall crystal structure andphase purity of the as-synthesized products. All of the diffraction peakscan be indexed to a tetragonal rutile structure of SnO2, which agreeswell with the reported values (a=4.738 and c=3.187 Å) from JCPDScard (41-1445). No characteristic peaks from impurities, such aselemental Sn or other tin oxides, can be detected.
Scanning electron microscopy (SEM) was employed to study themorphologies of the as-synthesized products. The general morphologyof the nanocubes is displayed in Fig. 2(a) and (b), which demonstratesthat the samples are in large quantity and their structures are uniform.Fig. 2(c) and (d) reveal that the nanocubes mainly have two kinds ofmorphologies. The size of the nanocubes is in the range of 100 nm–300 nm. Cube-like architectures have rough side-faces, which consistof four SnO2 nanorods with diameters of 60–80 nm. Fig. 2(d) showssome nanocubes have smooth side-faces with two caves in the top andbottom.
Transmission electron microscopy (TEM) can give us more detailedmicrostructural information on the complex SnO2 nanostructures. TEMimage of nanocubes and dispersive nanorods is presented in Fig. 3(a).The nanocubes are clearly shown in the image, with a high contrastbetween the joining part and the edge of the cube. The TEM image ofthe samples shows that the SnO2 cube-like architectures with roughside-faces are veritably formed by nanorods side by side. Fig. 3(b)shows the typical nanocube with smooth side faces and two caves in thetop and bottom.
Fig. 4 is a HRTEM image taken near the edge of a nanocube,together with an inset of the corresponding SAED pattern. Fig. 4(a)reveals that the resolved spacing of about 0.34 nm corresponds to thespacing of (110) lattice planes (d=0.335 nm) of SnO2. SAED patternindicates that the SnO2 nanocube is a single crystal with rutilestructure (a=0.473 nm and c=0.318 nm). Combining HRTEM andSAED, the axis direction of the SnO2 cube could be determined as[001]. The four peripheral surfaces of the SnO2 nanocubes are {110}planes, as schematically shown in Fig. 4(b). Further, the representa-tive energy-dispersive X-ray spectroscopy (EDX) analysis of thenanocubes indicates that the atomic ration of tin/oxygen isapproximately 1:2. Cu and C peaks in the spectrum arise from theCu grids with carbon film.
The VLS mechanism is supported usually by the observation of aspherical particle of tin at the tip of each nanostructure. In our case,there are no special metallic particles found at the growth fronts of thenanocubes. Accordingly, the growth of SnO2 nanocubes was thoughtmost likely to be dominated by the vapor-solid (VS) mechanism.
Fig. 4. (a) HRTEM image of the edge of the nanocube. The EDX spectrum and SAEDthe cube.
We suggest that the process for the formation of the nanocubes canbe divided into two steps. Firstly, SnO2 vapor was formed through thefollowing reaction [17,18]:
2Snþ O2Y2SnO ð1Þ
2SnOYSnO2 þ Sn ð2ÞThe melting of metallic tin took place during the experiment since
its melting point is as low as 231 °C. The liquid tin could provide an
pattern were inserted. (b) A schematic figure showing the growth orientation of
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energetically favored site for the absorption of oxygen, or react withoxygen to yield tin oxides. As the temperature further increases, pureliquid tin might be rapidly oxidized. It is generally believed that SnOforms at the initial stage of oxidation of tin, as shown in Eq. (1). As wehave known, SnO is metastable and will decompose to SnO2 and Sn.The reaction between tin and oxygen decreased the oxygen partialpressure in system and accelerated the decomposition rate of SnO [18],which could supply more SnO2 species. The generated SnO2 vapor wascarried by Ar atmosphere followed by nucleation and growth. Theseeds were formed in the deposition region through a VS process.Because SnO2 had an anisotropic structure, the growth direction waslargely confined to the b001N direction and the single crystalline seedstended to grow into the cylindrical shape. Oriented attachment involvesspontaneous self-organization of adjacent particles so that they share acommon crystallographic orientation [11]. When such rod-likestructures were close to each other, the chance of joining existed,that is, the cubes grew into each other. Dispersive nanorods were alsofounded in our product. A similar phenomenon has been observed inother reports [19]. Afterwards, the neighboring nanorods graduallyunited to form a smooth side-face by continuous deposition of gaseousSnO2 into the joining parts (Fig. 2(d)). At the same time, the condensedvapor species could diffuse across the surfaces of the nanorods andincorporated into the growth fronts, prolonging the cubes. Eventually,well-developed nanocubes (Fig. 5(b)) were obtained. With the increaseof growth time, the supply of vapor species was deficient and thefurther addition of SnO2 species would preferentially occur at thecircumferential edges of each cube because these sites had relativelyhigher free energies than other sites on the surface, leading to theformation of caves in the nanocubes [20]. In the growth process, thehigh degree super-saturation for the two-dimensional nucleation playsan important factor in the formation of such nanocubes.
4. Conclusion
In summary, single-crystal SnO2 nanocubes were synthe-sized by CVD method. TEM characterization shows that theSnO2 nanocubes are of a crystalline rutile structure. The side-faces of SnO2 nanocubes are {110} planes and the growthdirection is [001]. A reasonable mechanism is suggested toexplain the formation of the nanocubes. These nanostructures
represent an important example of spontaneous organization ofnanorods.
Acknowledgement
This work was supported by the Binzhou medical universityproject of Fundamental Research: Nanomaterials andNanostructure.
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