A simple method to synthesize nanowires titanium dioxidefrom layered titanate particles National Institute of Advanced Industrial Science and Technology AIST, Energy Technology Research
Trang 1A simple method to synthesize nanowires titanium dioxide
from layered titanate particles
National Institute of Advanced Industrial Science and Technology (AIST), Energy Technology Research Institute,
AIST Tsukuba Central #5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan Received 11 September 2004; in final form 25 October 2004
Available online 11 November 2004
Abstract
Nanowires TiO2were successfully synthesized from layered titanate Na2Ti3O7particles by a simple soft chemical process Com-pared with other synthetic routes where some templates or reactants were introduced into reaction system, only raw material and dilute HCl were used in this simple method The diameters of nanowires are ca 20–100 nm and the length up to several hundred micrometers Formation of brookite TiO2phase in the nanowires was confirmed by XRD and TEM measurement Based on our experimental results, an exfoliating-splitting model was proposed for formation of nanowire structure
Ó 2004 Elsevier B.V All rights reserved
1 Introduction
Since carbon nanotubes were discovered by Ijima in
1991 [1], nanoscale materials have attracted immense
interest due to structure, electronic and optical
proper-ties and their potential applications in electronic-,
pho-tonic-device [2], gas sensor, solar cell, lithium battery
and fuel cell The growing Ônano-toolboxÕ contains
organic and inorganic components, which come in a
variety of shapes and dimensionalities, including
particles/quantum dots (zero-dimension, 0D),
nano-tubes/nanorods/nanobelt/nanoribbons/nanowires (one
dimension, 1D), and nanosheets/nanohorns (two
dimen-sion, 2D) Materials in 1D form include TiO2[3], MnO2
[4], V2O5 [5], ZnO [6], CdSe [7]and MoS2[8] Among
them, nanoscale TiO2is particularly interesting because
they have large surface area, leading to a higher
poten-tial of application in environment purification, gas
sen-sor, and photovoltaic cell[9]
Several methods have been reported to prepare 1D nanowires TiO2 Jiang et al [10]reported the synthesis
of nanowires TiO2using polyol-mediated process How-ever, the products are required to remove organic com-pound, resulting in increase the cost of product Using the hydrothermal process, nanowires TiO2were synthe-sized in high concentration KOH solution [11,12] As well known, alkali solution is a strong corrosiveness and very harmful to human health and environment Nanowires TiO2 were also formed in anodic alumina membranes by a sol–gel process [13–15] However, the diameters of formed nanowires were decided by through-hole membrane Xu et al [16] prepared nano-wires TiO2via annealing TiS2precursor with or without the presence of molten-salt flux in ammonia gas atmos-phere Among them, the templates or reactants were introduced into the reaction system This means a much more complicated process, and might bring about an in-crease of impurity concentration in the final product To our knowledge, however, synthesis of nanowires TiO2
using a soft chemical process has not been concerned
In this Letter, we investigated the preparation of 1D structural nanowire TiO2 from the layered material
0009-2614/$ - see front matter Ó 2004 Elsevier B.V All rights reserved.
doi:10.1016/j.cplett.2004.10.114
* Corresponding author Fax: +81 29 8616771.
E-mail address: wei-mingdeng@aist.go.jp (M Wei).
www.elsevier.com/locate/cplett Chemical Physics Letters 400 (2004) 231–234
Trang 2Na2Ti3O7particles using a soft chemical process, with
no existence of reactants or templates, only the raw
material and dilute HCl were used Further, a formation
mechanism of nanowires TiO2was proposed according
to the results of SEM, TEM and XRD
2 Experimental
2.1 Preparation of samples
The layered material Na2Ti3O7was synthesized from
Na2CO3 (Wako) and TiO2 (ST-01, Ishihara Sangyo
Kaisha LTD.) in the stoichiometrical ratio 1:3 The
pow-ders were mixed together and repeatedly ground in an
agate mortar, and calcined at 1000°C for 2 h in the air
Synthesis of nanowires TiO2 was performed by a
hydrothermal soft chemical process In a typical
synthe-sis, 0.15 g Na2Ti3O7was dispersed into a 15 ml 0.05–0.1
M HCl solution, then was transferred into a 30 ml
auto-clave, and kept at 140–170°C for 3–7 days The
as-prod-uct was filtered, washed with H2O, and finally dried at
60°C for 4 h
2.2 Characterization of samples
X-ray powder diffraction (XRD) patterns were
re-corded using a diffractometer (Mac Science) Scanning
electron microscope (SEM) and transmission electron
micrographs (TEM) were taken on a TOPCON DS
instrument and a JEOL instrument, respectively
3 Results and discussion
Fig 1 shows SEM images of the raw material Na
2-Ti3O7and the sample obtained at 170°C for 7 days It
is very different between the raw material Na2Ti3O7
and the sample Only particles were observed for the
raw material, as depicted in Fig 1a The SEM image
of the sample shown inFig 1b has revealed the presence
of numerous nanowires with typical lengths ranging
from several micrometers to several tens of micrometers
It noticed that some brush-like sheets attached to some
of the nanowires were observed (Fig 1b, circle A) In
addition, some particle aggregates and sheets with rolled
edge were also found (Fig 1b, circle B) These
nano-wires lie close to each other Further magnification
clearly shows that the diameter of these nanowires is
ca 20–100 nm (seeFig 1c)
The formation of numerous nanowires was further
confirmed by TEM measurement Fig 2 depicts TEM
images of the sample containing lots of nanowires
The low magnification image, as shown inFig 2a,
indi-cates that the size and length of nanowires are same as
those observed in SEM experiment One single nanowire
was selected, as shown inFig 2b It is clear that the end
of nanowire is cone-shaped Further magnification clearly shows the single-crystalline nature of sample (in-set in Fig 2c, left upper), and the lattice fringes corre-spond to a d-spacing of 0.22 nm
XRD patterns of the raw material Na2Ti3O7and the product containing lots of nanowire were depicted in
Fig 3 The former shows a typical profile of layered compound, and could be indexed to a layered titanate
Na2Ti3O7 [17] The latter is obviously different from the former TiO2as the main phase was observed besides the phase of H2Ti3O7(JCPDS 47-0561), and it could be indexed to a brookite TiO2with an orthorhombic struc-ture (JCPDS 72-1582) The d202-spacing is 0.22 nm This
is in agreement with the lattice fringe obtained from TEM image, indicating that the nanowires in product are TiO2 with brookite structure Formation of
H Ti O was contributed to ion-exchange reaction In
Fig 1 SEM images of (a) the raw material Na 2 Ti 3 O 7 together with the product containing lots of nanowires (b) low magnification and (c) high magnification.
232 M Wei et al / Chemical Physics Letters 400 (2004) 231–234
Trang 3the dilute HCl solution, Na+ in the raw material Na
2-Ti3O7was replaced by H+ to form H2Ti3O7phase
In this study, only the raw material Na2Ti3O7and
di-lute HCl were used, no any templates or reactants were
introduced into the reaction system Based on the
exper-imental results of XRD, SEM and TEM, an
exfoliating-splitting model of nanowires formation from layered
Na2Ti3O7 was proposed, as illustrated in Scheme 1 It
is well known that Na+ cations reside between
edge-shared [TiO6] octahedral layers in Na2Ti3O7[17] The
strong static interaction between the Na+ cations and
[TiO6] unit holds the layers together tightly Under
hydrothermal conditions, Na+ cations will be replaced
gradually with intercalated Hþ3O molecule into the
inter-layer space of [TiO6] sheets Because the size of HþO is
larger than that of Na+ cation, the interlayer distance was enlarged, resulted in weakened static interaction be-tween neighboring [TiO6] octahedral sheets As a result, the layered compound Na2Ti3O7particles were gradu-ally exfoliated In the meantime, Na+ in Na2Ti3O7was exchanged by H+ in the dilute HCl solution to form numerous H2Ti3O7 sheet-shaped products The nano-sheet does not has an inversion symmetry, i.e the layer
is asymmetric, an intrinsic tension exists which might gradually tends to roll from the edges of nanosheets
[18] In order to release the strong stress and lower the total energy, the nanosheets are splitted, resulting in for-mation of the nanowires This forfor-mation model can be used to explain the formation of brush-like nanosheets observed in SEM images There exist three procedures
Scheme 1 An exfoliating-splitting model of nanowires formation from the layered Na 2 Ti 3 O 7 particles by a soft chemical process.
Fig 2 TEM images of the sample containing lots of nanowires (a) low
magnification, (b, c) high magnification.
Fig 3 XRD patterns of (a) the raw material Na 2 Ti 3 O 7 and (b) product containing lots of nanowires.
M Wei et al / Chemical Physics Letters 400 (2004) 231–234 233
Trang 4in the reaction: (i) the exfoliation from layered Na2Ti3O7
particles to form nanosheets; (ii) the split of nanosheets
to form nanowires; (iii) the exfoliation accompanied by
splitting to form brush-like nanosheets Therefore, the
formation of brush-like nanosheets can be contributed
to fact that both exfoliating and splitting are incomplete
4 Conclusions
In this Letter, a simple method involving a soft
chem-ical process started from the layered compound Na
2-Ti3O7 particles has been used to synthesize
single-crystal TiO2 nanowires According to the comparative
experimental results, an exfoliating-splitting model was
proposed for formation of nanowire structure Based
on the simple method with no participation of catalysts
or templates and requiring no expensive and precise
equipment, will greatly reduce the production cost,
and thus offer great opportunity for scale-up
prepara-tion of 1D nanostructure materials
Acknowledgement
M.D.W acknowledges Mr M Ichihara of Institute
of Solid State Physics at Tokyo University for his
help-ful in the TEM experiments
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