SnO
2
/a-Fe
2
O
3
hierarchical nanostructure: Hydrothermal
preparation andformation 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.
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W W. Wang / Materials Research Bulletin 43 (2008) 2055–2059 2059
. SnO
2
/a-Fe
2
O
3
hierarchical nanostructure: Hydrothermal
preparation and formation mechanism
Wei-Wei Wang
*
School of Materials Science and Engineering, Shandong. oxides possess outstanding physical and chemical properties and are widely used in the fields of
optics, electronics, magnetic storage and catalysis. In order