sno2 - fe2o3 hierarchical nanostructure hydrothermal preparation and formation mechanism

SnO 2 /a-Fe 2 O 3 hierarchical nanostructure: Hydrothermal preparation and formation mechanism Wei-Wei Wang * School of Materials Science and Engineering, Shandong University of Technology, Shandong 255049, PR China Received 31 January 2007; received in revised form 29 April 2007; accepted 19 September 2007 Available online 25 September 2007 Abstract In this paper, we reported a simple solution method to assemble SnO 2 nanorods hierarchically on the surface of a-Fe 2 O 3 nanosheets using Fe 3 O 4 nanosheets as precursor. The product was characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). Our experimental results show that the lattice mismatch at the interface of SnO 2 nanorods with a-Fe 2 O 3 nanosheets played an important role in determining the growth direction of SnO 2 nanorods. The interface prefers to take the least lattice mismatch and thus the preferential growth direction of SnO 2 nanorods was along [1 0 1] direction. The result may have important impact on the understanding of the nucleation growth process in a heterogeneous system. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructure; A. Oxides; B. Chemical synthesis; C. Electron microscopy 1. Introduction The uniqu e chemical and physical prope rties of nanocrystals are determined not only by the size of the particles but also by the particle shape [1]. The particle shapes are closel y related to the crystallographic surfaces that enclose the particles [2,3]. For example (0 1 0) plane of BiVO 4 has a larger atom density than that of the (0 0 1) and (1 0 0) planes. This is the important reason for the improved color properties of the BiVO 4 nanosheets as compared with the BiVO 4 nanoparticles [4]. Yamaguchi et al. [5] also found that the photocatalytic activity of ZnO was improved as the ratio of (1 0 À 1 0) plane to (0 0 0 2) plane increased. Great interests in nanostructured materials have been focused on their shapes and finding novel properties [6]. Among these shapes, the synthesis of organized structures based on the assembly of nanostructured building blocks has attracted much attention because the resulting hierarchical, multi- functional materials can be applied in various fields. Semiconductor oxides possess outstanding physical and chemical properties and are widely used in the fields of optics, electronics, magnetic storage and catalysis. In order to improve their physical and chemical properties, many efforts have been made in the controlled synthesis of oxide composites such as ZnO – SnO 2 [7,8],TiO 2 –SnO 2 [9,10] and WO 3 –TiO 2 [11,12]. It was found that SnO 2 /Fe 2 O 3 materials show higher catalytic activity and gas sensitivity than pure SnO 2 and Fe 2 O 3 [13–19]. SnO 2 /Fe 2 O 3 composites have been prepared by mechanical alloying of a-Fe 2 O 3 and www.elsevier.com/locate/matresbu Materials Research Bulletin 43 (2008) 2055–2059 * Tel.: +86 533 2782232; fax: +86 533 2781660. E-mail address: weiweiwangsd@yahoo.com.cn. 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.09.010 SnO 2 powder [20], polymerization between citric acid and ethylene glycol to form polymeric precursor at 180–200 8C [21], or by the co-precipitation and impregnation techniques [22]. In this paper, we reported a simple solution method to assemble SnO 2 nanorods hierarchically on the surface of a-Fe 2 O 3 nanosheets using Fe 3 O 4 nanosheets as precursor. 2. Experimental Iron(II) sulfate (FeSO 4 Á7H 2 O), ethylene glycol (EG), tin (IV) chloride (SnCl 4 Á5H 2 O) and sodium hydroxide (NaOH) were purchased and used without further purification. The precursor (Fe 3 O 4 nanosheets) was prepared according to reference [23]. 0.200 g of FeSO 4 Á7H 2 O was dissolved in a mixtu re of EG (2 mL) and deionized water (8 mL). Then 1 mL of NaOH (0.190 g) aqueous solutio n was added into the above solution at room temperature. The above suspension was refluxed at boiling point under microwave heating for 1 min and cooled down to room temperature. The products were separated by centrifugation, washed with absolute ethanol three times, and dried at 60 8C in a vacuum. The preparation of SnO 2 /Fe 2 O 3 hierarchical nanostructures: SnCl 4 Á5H 2 O (0.158 g) and the precursor Fe 3 O 4 nanosheets (0.01 g) were added into 30 mL of NaOH aqueous solution (0.15 M) to form uniform suspension. The starting mole ratio of Sn 4+ :Fe 3+ was about 3.5. The suspension was transferred into a 40-mLTeflon-lined stainless steel autoclave. The autoclave was maintained at 180 8C for 24 h without stirring and shaking. After cooled down to room temperature, the products were separated by centrifuga tion, washed with absolute ethanol three times, and dried at 60 8C in a vacuum (sample 1). For comparison, sample 2 was prepared by mixing powders of SnO 2 and a-Fe 2 O 3 in the mole ratio of about 7:1 (the mole ratio of Sn:Fe was about 3.5), which was similar to the starting mole ratio of sample 1. Sample 3 was prepared under the same experimental conditions as sample 1 but without adding Fe 3 O 4 nanosheets. X-ray powder diffraction (XRD) patterns were recorded using a Rigaku D/max-rB X-ray d iffractometer with high- intensity Cu Ka radiation (l = 1.54178 Å) and a graphite monochromator. The transmission electron microscopy (TEM) micrographs, selected-area electron diffraction (SAED) patterns, high-resolution transmission electron microscopy (HRTEM) micro graphs and energy dispersive spectroscopy (EDS) were taken with a Philip Tecnai20U transmission electron microscope with an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) micrograph was recorded on a FEI-Sirion200 field emission scanning electron microscope. The final products were also examined with an INCA energy dispersive spectrometer (EDS). 3. Results and discussion Fig. 1 shows the XRD patterns of samples 1 and 2. Sample 1 mainly consisted of tetragonal SnO 2 (space group: P4 2 / mnm, JCPDS file no. 41-1445, Fig. 1a). The measured lattice constants were a = b = 4.743 Å and c = 3.178 Å, which were similar to the reported data of tetragonal SnO 2 (JCPDS file no. 41-1445, a = b = 4.738 Å and c = 3.187 Å). Weak diffraction peaks from the hexagonal a-Fe 2 O 3 (JCPDS file no. 33-0664) were observed. No compounds of tin and iron W W. Wang / Materials Research Bulletin 43 (2008) 2055–20592056 Fig. 1. XRD patterns of (a) sample 1 and (b) sample 2. X stands for a-Fe 2 O 3 ; O stands for SnO 2 . were detected by XRD. It differed from Liu’s reports that SnFe 2 O 3 nanoparticles were prepared using FeCl 3 , SnCl 2 and NH 3 ÁH 2 O solution as starting materials [24]. In addition, no diffraction peaks of Fe 3 O 4 were detected by XRD, indicating Fe 3 O 4 was oxidized to form a-Fe 2 O 3 . For comparison, we mixed powders of SnO 2 and a-Fe 2 O 3 in the mole ratio of 7:1 (sample 2). It showed the similar diffraction patterns to that of sample 1(Fig. 1b). So the mole ratio of Sn to Fe of sample 1 was similar to that of sample 2, i.e. 3.5. In addition, the presence of Sn and Fe were also confirmed by EDS (figures not shown). The EDS analysis for sample 1 yielded an average atomic ratio of 7:2 for Sn/Fe. Fig. 2a shows TEM micrograph for sample 1. Sample 1 was in rod like morphology with diameters about 10 nm and lengths up to 20 nm. After carefully investigated, one can see that these nanorods were parallel to the surface of the nanosheet. This nanosheet was transparent to the electron beam, suggesting very thin nanosheet. The dimension of the nanosheet was up to 100 nm. The SAED pattern taken from the region including nanosheets and nanorods (inset of Fig. 2a) displays their single-crystalline structure. SEM micrograph of sample 1 also confirmed that the surfaces of these nanosheets were constructed by nanorods with diameters of about 10 nm (Fig. 2b). EDS analysis was employed to determine the composition of nanosheets and nanorods, respectively. Both nanosheets and nanorods contained Fe, Sn and O elements (Fig. 2c and d; copper came from the TEM copper grid of the sample holder). The EDS spectrum of the nanosheet (Fig. 2c; as indicated by the circle in Fig. 2a) shows much stronger Fe and O bands than that of nanorods (Fig. 2d; as indicated by the square in Fig. 2a). Fe element (shown in Fig. 2d) may come from nanosheets due to nanorods attached to the surface of nanosheets. According to the above analysis, we deduced that Fe 2 O 3 was in sheet-like morphology and SnO 2 was in rod shape. The structure of the nanocomposite was further characterized by HRTEM. Fig. 2e shows the HRTEM micrograph of nanosheets (as indicated by the circle in Fig. 2a) and its fast Fourier transform (FFT). The HRTEM micrograph of the nanosheet shows that the spacing between clear lattice fringes with an angle of 608 is 0.251 nm, which is consistent with the (1 1 À 2 0) and (1 À 2 1 0) planes of hexagonal a-Fe 2 O 3 (0.252 nm). The FFT is characterized by hexagonal symmetry (inset in Fig. 2e). Fig. 2f shows the HRTEM micrograph of nanorods (as indicated by the square in Fig. 2a) and its FFT. The interplanar spacing of $0.264 nm agrees well with the spacing between (1 0 1) plane of tetragonal SnO 2 (0.264 nm). This indicates that Sn O 2 nanorods had the preferential growth direction along the [1 0 1]. The lattice mismatch between SnO 2 nanorods and Fe 2 O 3 nanosheets is a very important factor in heterostructure growth and it is W W. Wang / Materials Research Bulletin 43 (2008) 2055–2059 2057 Fig. 2. (a) TEM micrograph; (b) SEM micrograph; (c) and (d) EDS spectra; (e) and (f) HRTEM micrographs of sample 1. (c) and (e) as indicated by the circle in (a); (d) and (f) as indicated by the square in (a); the inset in Fig. 1(a) is corresponding SAED pattern; the insets in (e) and (f) are corresponding fast Fourier transform (FFT). useful to observe the interface layer between nanorods and nanosheets. Unfortunately, we did not obtain a figure for a clear view of the interface layer under TEM observation. It may be due to the dimension of the nanosheet (up to 100 nm). The nanosheet is too thin to stand on the copper grid. According to the above TEM, SAED and HRTEM analysis, we deduced that the interface was composed of f101g SnO 2 and f110g aÀFe 2 O 3 . The comparison experiment was conduc ted in order to study the growth process of SnO 2 /a-Fe 2 O 3 hierarchical nanostructures. Single phase of SnO 2 (sample 3) was obtained by hydrothermally treating SnCl 4 Á5H 2 O and NaOH aqueous solution at 180 8C for 24 h. All the diffraction peaks can be indexed as tetragonal structure SnO 2 (space group: P4 2 /mnm, JCPDS file no: 41–1445, Fig. 3a). Without adding Fe 3 O 4 nanosheets, only SnO 2 irregular sheets were observed (as shown in Fig. 3b). The edges of the SnO 2 sheets should correspond to the (1 0 1) and (2 0 0) planes, which was supported by the SAED pattern (the inset of Fig. 3b). For sample 1, the preferential growth direction for SnO 2 was [1 0 1] and the presence of Fe 3 O 4 nanosheets limited the growth of SnO 2 along [2 0 0] direction. For SnO 2 nanorods growing along [1 0 1], the interface is composed of f101g SnO 2 and f110g aÀFe 2 O 3 . The crystallographic lattice structure at the interface is important in defining the structural characteristics of SnO 2 nanorods and a-Fe 2 O 3 nanosheets [25]. As we know, the interface prefers to take the least lattice mismatch. The lattice mismatch of f101g SnO 2 and f 110g aÀFe 2 O 3 is lower than that of the interface of f200g SnO 2 and f 110g aÀFe 2 O 3 . Thus, the crystalline structure of a-Fe 2 O 3 determined the growth of SnO 2 nanorods along [1 0 1] on the surface of a-Fe 2 O 3 nanosheets. According to the above analysis, the whole process can be simplified as the following stages: the precursor was single phase of cubic Fe 3 O 4 and in sheet -like morphol ogy [23].Fe 3 O 4 was easily oxidized to form a-Fe 2 O 3 and without shape changing under hydrothermal treating, consistent with the TEM and XRD results. After the addi tion of Sn 4+ , Sn(OH) 6 2À anions were formed in the strong basic solution. With the hydrothermal treating, SnO 2 nuclei were formed on the surfaces of nanosheets by lowering the activation energy of nucleation [26]. Dr iven by the decreased lattice mismatch, the SnO 2 nanoparticles took [1 0 1] as its preferential orientation. The SnO 2 /a-Fe 2 O 3 hierarchical nanostructures were formed. 4. Conclusions In summary, SnO 2 /a-Fe 2 O 3 hierarchical nanostructure has been prepared under hydrother mal treating of SnCl 4 and NaOH aqueous solution using Fe 3 O 4 nanosheets as precursor. Fe 3 O 4 was easily oxidized to form a-Fe 2 O 3 and without shape changing under hydrothermal treating. The presence of a-Fe 2 O 3 limited the growth of SnO 2 nanorods along [2 0 0] direction. The least interfacial lattice mismatch between SnO 2 and a-Fe 2 O 3 could lower the nucleation energy barrier and thus make it an important effect in determining the growth behavior of SnO 2 nanorods on the surface of a- Fe 2 O 3 nanosheets. The special SnO 2 /a-Fe 2 O 3 hierarchical str ucture may be a promising candidate with improved properties. Acknowledgement We thank Scientific Resear ch Foundation from Shandong University of Technology (406020) for the fund. W W. Wang / Materials Research Bulletin 43 (2008) 2055–20592058 Fig. 3. (a) XRD pattern and (b) TEM micrograph of sample 3, the inset is corresponding SAED pattern. References [1] Z.L. Wang, J. Phys. Chem. B 104 (2000) 1153. [2] A. Vittadini, A. Selloni, F.P. Rotzinger, M. Gratzel, Phys. Rev. Lett. 81 (1998) 2954. [3] K. Zhou, X. 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