sno2 - fe2o3 hierarchical nanostructure hydrothermal preparation and formation mechanism

SnO 2 /a-Fe 2 O 3 hierarchical nanostructure: Hydrothermalpreparation and formation mechanism School of Materials Science and Engineering, Shandong University of Technology, Shandong 255

SnO 2 /a-Fe 2 O 3 hierarchical nanostructure: Hydrothermal

preparation and formation mechanism

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 SnO2nanorods hierarchically on the surface ofa-Fe2O3

nanosheets using Fe3O4nanosheets 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 SnO2nanorods witha-Fe2O3nanosheets played

an important role in determining the growth direction of SnO2nanorods The interface prefers to take the least lattice mismatch and thus the preferential growth direction of SnO2nanorods 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 unique chemical and physical properties of nanocrystals are determined not only by the size of the particles but also by the particle shape[1] The particle shapes are closely related to the crystallographic surfaces that enclose the particles[2,3] For example (0 1 0) plane of BiVO4has 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 BiVO4nanosheets as compared with the BiVO4 nanoparticles[4] Yamaguchi et al.[5]also found that the photocatalytic activity of ZnO was improved as the ratio of (1 0 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–SnO2[7,8], TiO2–SnO2[9,10] and WO3–TiO2[11,12] It was found that SnO2/Fe2O3materials show higher catalytic activity and gas sensitivity than pure SnO2and Fe2O3[13–19] SnO2/Fe2O3composites have been prepared by mechanical alloying ofa-Fe2O3and

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SnO2powder[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 SnO2nanorods hierarchically on the surface ofa-Fe2O3nanosheets using Fe3O4nanosheets as precursor

2 Experimental

Iron(II) sulfate (FeSO47H2O), ethylene glycol (EG), tin (IV) chloride (SnCl45H2O) and sodium hydroxide (NaOH) were purchased and used without further purification

The precursor (Fe3O4nanosheets) was prepared according to reference[23] 0.200 g of FeSO47H2O was dissolved

in a mixture of EG (2 mL) and deionized water (8 mL) Then 1 mL of NaOH (0.190 g) aqueous solution 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 SnO2/Fe2O3 hierarchical nanostructures: SnCl45H2O (0.158 g) and the precursor Fe3O4

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 Sn4+:Fe3+was about 3.5 The suspension was transferred into a 40-mL Teflon-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 centrifugation, 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 SnO2anda-Fe2O3in 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 Fe3O4nanosheets

X-ray powder diffraction (XRD) patterns were recorded using a Rigaku D/max-rB X-ray diffractometer 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) micrographs 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 1shows the XRD patterns of samples 1 and 2 Sample 1 mainly consisted of tetragonal SnO2(space group: P42/ 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 SnO2(JCPDS file no 41-1445, a = b = 4.738 Å and c = 3.187 Å) Weak diffraction peaks from the hexagonala-Fe2O3(JCPDS file no 33-0664) were observed No compounds of tin and iron

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Fig 1 XRD patterns of (a) sample 1 and (b) sample 2 X stands for a-Fe O ; O stands for SnO

were detected by XRD It differed from Liu’s reports that SnFe2O3nanoparticles were prepared using FeCl3, SnCl2 and NH3H2O solution as starting materials[24] In addition, no diffraction peaks of Fe3O4were detected by XRD, indicating Fe3O4was oxidized to forma-Fe2O3 For comparison, we mixed powders of SnO2anda-Fe2O3in 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 inFig 2a) shows much stronger Fe and O bands than that of nanorods (Fig 2d; as indicated by the square inFig 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 Fe2O3was in sheet-like morphology and SnO2was 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 2 0) and (1  2 1 0) planes of hexagonal a-Fe2O3 (0.252 nm) The FFT is characterized by hexagonal symmetry (inset inFig 2e).Fig 2f shows the HRTEM micrograph of nanorods (as indicated by the square inFig 2a) and its FFT The interplanar spacing of0.264 nm agrees well with the spacing between (1 0 1) plane of tetragonal SnO2(0.264 nm) This indicates that SnO2nanorods had the preferential growth direction along the [1 0 1] The lattice mismatch between SnO2nanorods and Fe2O3nanosheets is a very important factor in heterostructure growth and it is

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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 off1 0 1gSnO

2andf1 1 0gaFe

2 O3.

The comparison experiment was conducted in order to study the growth process of SnO2/a-Fe2O3hierarchical nanostructures Single phase of SnO2(sample 3) was obtained by hydrothermally treating SnCl45H2O and NaOH aqueous solution at 180 8C for 24 h All the diffraction peaks can be indexed as tetragonal structure SnO2(space group: P42/mnm, JCPDS file no: 41–1445,Fig 3a) Without adding Fe3O4nanosheets, only SnO2 irregular sheets were observed (as shown inFig 3b) The edges of the SnO2sheets should correspond to the (1 0 1) and (2 0 0) planes, which was supported by the SAED pattern (the inset ofFig 3b) For sample 1, the preferential growth direction for SnO2was [1 0 1] and the presence of Fe3O4nanosheets limited the growth of SnO2along [2 0 0] direction For SnO2nanorods growing along [1 0 1], the interface is composed of f1 0 1gSnO2and f1 1 0gaFe2O3 The crystallographic lattice structure at the interface is important in defining the structural characteristics of SnO2 nanorods and a-Fe2O3

nanosheets [25] As we know, the interface prefers to take the least lattice mismatch The lattice mismatch of f1 0 1gSnO

2andf1 1 0gaFe

2 O3is lower than that of the interface off2 0 0gSnO

2andf1 1 0gaFe

2 O3 Thus, the crystalline structure ofa-Fe2O3determined the growth of SnO2nanorods along [1 0 1] on the surface ofa-Fe2O3nanosheets According to the above analysis, the whole process can be simplified as the following stages: the precursor was single phase of cubic Fe3O4and in sheet-like morphology [23] Fe3O4was easily oxidized to form a-Fe2O3and without shape changing under hydrothermal treating, consistent with the TEM and XRD results After the addition of

Sn4+, Sn(OH)6anions were formed in the strong basic solution With the hydrothermal treating, SnO2nuclei were formed on the surfaces of nanosheets by lowering the activation energy of nucleation[26] Driven by the decreased lattice mismatch, the SnO2nanoparticles took [1 0 1] as its preferential orientation The SnO2/a-Fe2O3hierarchical nanostructures were formed

4 Conclusions

In summary, SnO2/a-Fe2O3hierarchical nanostructure has been prepared under hydrothermal treating of SnCl4and NaOH aqueous solution using Fe3O4nanosheets as precursor Fe3O4was easily oxidized to forma-Fe2O3and without shape changing under hydrothermal treating The presence ofa-Fe2O3limited the growth of SnO2nanorods along [2 0 0] direction The least interfacial lattice mismatch between SnO2anda-Fe2O3could lower the nucleation energy barrier and thus make it an important effect in determining the growth behavior of SnO2nanorods on the surface of

a-Fe2O3nanosheets The special SnO2/a-Fe2O3hierarchical structure may be a promising candidate with improved properties

Acknowledgement

We thank Scientific Research Foundation from Shandong University of Technology (406020) for the fund

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Fig 3 (a) XRD pattern and (b) TEM micrograph of sample 3, the inset is corresponding SAED pattern.

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