Tech-niques X-ray diffraction XRD, scanning electron microscopy SEM, transmission electron microscopy TEM, high-resolution transmission electron microscopy HRTEM, Fourier transform infra
Trang 1Characterization of h-WO 3 nanorods synthesized by hydrothermal process
S Salmaouia, F Sediria,b,*, N Gharbia
a
Laboratoire de Chimie de la Matière Condensée, IPEIT, Université de Tunis, 2 rue Jawaher Lel Nehru 1008, B.P 229 Montfleury, Tunis, Tunisia
b
Faculté des Sciences de Tunis, Université Tunis-Elmanar, 2092 Elmanar, Tunis, Tunisia
a r t i c l e i n f o
Article history:
Received 10 November 2009
Accepted 15 February 2010
Available online 19 February 2010
Keywords:
Tungsten oxide
Aniline
Hydrothermal synthesis
Nanorods
a b s t r a c t
Hexagonal tungsten oxide nanorods have been synthesized by hydrothermal strategy using
Na2WO42H2O as tungsten source, aniline and sulfate sodium as structure-directing templates Tech-niques X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectros-copy (FTIR) and Raman spectrosspectros-copy have been used to characterize the structure, morphology and com-position of the nanorods The h-WO3nanorods are up to 5lm in length, and 50–70 nm in diameter
Ó 2010 Elsevier Ltd All rights reserved
1 Introduction
One-dimensional (1D) nanostructure materials, such as
nano-tubes, nanowires, nanorods and nanobelts have attracted
consider-able attention due to their remarkconsider-able physicochemical properties
and their great potential for nanodevices[1–6] The outstanding
structural versatility of the metal oxides compounds such as
tung-sten oxides, and their derivatives has been receiving significant
attention especially with respect to applications in electrochromic
or photochromic devices, secondary batteries, gas sensors and
photocatalysts [7–9] In these fields, recent studies showed that
WO3 nanostructured have superior sensitivity compared with
those of bulk materials[10,11] For such an application,
character-istics of the material such as size, shape and crystallographic
struc-ture are very important and strongly depend on the growth
method Until now various methods for the synthesis of
nanostruc-tured tungsten oxides with various morphologies have been
re-ported such as electrospinning[12], chemical vapour deposition
[13], pulsed laser irradiation [14], electro-deposition [15],
va-pour–solid growth [16], gas deposition [17], precipitation [18],
sol–gel[19]and hydrothermal methods[20–24]
Moreover, WO3can crystallize according to several structures
All the WO3 structures can be described as deformations of the
ReO3 cubic perfect model Such a perfect structure is composed
of a three-dimensional network of WO6octahedral linked by their
oxygen corner Among various crystal structures of WO3,
hexago-nal form is of great interest owing to its well-known tunnel
struc-ture in which WO6octahedrons share their corners with each other forming hexagonal tunnels along c-axis[25] Hexagonal tungsten oxide (h-WO3) has been widely investigated, especially as an inter-calation host for obtaining hexagonal tungsten bronzes MxWO3 (M = K+, Li+, etc.) and a promising material for negative electrodes
of rechargeable lithium batteries[26–28] Although many methods have been developed to elaborate nanostructured tungsten oxides, to the best of our knowledge, it
is the first time to report the optimization of reaction conditions
of the synthesis of hexagonal tungsten oxide nanorods using ani-line with sodium sulfate as structure-directing templates by hydrothermal self-assembling process
2 Experimental 2.1 Hydrothermal synthesis All of the chemical reagents were analytical grade They were purchased from across and used without further purification Na2
-WO42H2O was used as tungsten source The aniline and sodium sulfate together have been used as structure-directing template for the first time
Hexagonal WO3 nanorods were hydrothermally synthesised from a mixture of Na2WO42H2O, H2O, C6H5–NH2, HCl and Na2SO4
in the molar ratio 1:258:1:2.8:5 Reactants were introduced in this order and stirred a few minutes before introducing the solution in
a Teflon-lined steel autoclave and the temperature set at 180 °C for
3 days under autogenous pression The pH of the solution remains close to pH 1 during the whole synthesis The obtained powder was washed with acetone to remove organics residues and then dried at 80 °C
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd All rights reserved.
* Corresponding author Address: Faculté des Sciences de Tunis, Université
Tunis-Elmanar, 2092 Tunis-Elmanar, Tunis, Tunisia Tel.: +216 71872600; fax: +216 71871666.
E-mail addresses: faouzi.sediri@ipeit.rnu.tn (F Sediri), neji.gharbi@ipeit.rnu.tn
(N Gharbi).
Contents lists available atScienceDirect
Polyhedron
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / p o l y
Trang 2In order to control the morphology of h-WO3 a set of
experi-ments was performed by varying the molar ratio Na2SO4:W from
0.15:1 to 5:1
2.2 Characterization techniques
X-ray diffraction (XRD) patterns were obtained on a X‘Pert Pro
Panalytical diffractometer with Co Ka radiation (k = 1.78901 Å)
The XRD measurements were carried out by a step scanning
meth-od (2h range from 3° to 60°), the scanning rate is 0.03°/s and the
step time is 3 s
Scanning electron microscopy (SEM) images were obtained
with a Cambridge Instruments Stereoscan 120
Transmission electron microscopy (TEM) studies were recorded
on JEOL 100 CX II electron microscope operated at 200 KV One
droplet of the powder dispersed in CH3CH2OH was deposited onto
a carbon-coated copper grid and left to dry in air
Fourier transform infrared spectra (FTIR) were recorded with a
Nicolet 380 Spectrometer
Raman spectroscopy was performed using a Jobin Yvon T 64000
Spectrometer
3 Results and discussion
3.1 X-ray diffraction
By controlling the appropriate molar ratio of the starting
mate-rials (Na2SO4:W), the nanorods can be prepared conveniently with
high purity.Fig 1shows the XRD patterns of the as-obtained
prod-ucts at different molar ratio Na2SO4:W = (a) 5:1, (b) 2.5:1, (c)
1.25:1, (d) 0.62:1 (e) 0.30:1 and (f) 0.15:1, respectively All the
dif-fraction peaks can be perfectly indexed to hexagonal tungsten
oxide crystalline phase (h-WO3) with lattice constants of
a = 7.298 Å and c = 3.899 Å (JCPDS # 33-1387) The peak intensities
in the reported spectra were not exactly the same as the reference
spectrum because of differences in the molar ratio No peaks of any
other phases or impurities were observed from the XRD patterns,
indicating that h-WO3 crystalline phase with high purity could
be obtained using the present synthetic process
3.2 Scanning and transmission electronic microscopy
To understand the formation of the rod-like morphology, we conducted the experiment in the presence of different amount of
Na2SO4while keeping other conditions unchanged The morphol-ogy of the products was investigated by SEM and TEM images It
is found that the amount of Na2SO4has clear effects on the forma-tion of nanorods Comparative experiments have shown that when the molar ratio Na2SO4:W = 0.62:1, only irregular particles of nano-rods and nanowires were formed (Fig 2a) With increasing the mo-lar ratio to 1.25:1, nanorods were generated with an increase in the diameter (Fig 2b) When the molar ratio = 2.5:1 the h-WO3 rod-like nanostructures became predominant products (Fig 2c) SEM and TEM images, shown inFig 2d and e, indicate that with
a Na2SO4:W ratio of 5:1, a rod-like structure with diameters of about 50–70 nm and length of about 5lm were obtained Close observation shown inFig 2f revealed that these rod-shape prod-ucts are formed by the oriented attachment of large, numerous, highly aligned, and closely packed nanoneedles According to the literature[29], sodium sulfate tends to induce the formation of the 1D nanostructures of h-WO3 Thus, the size and the yield of the rod-like nanostructures increase with increasing the amount
of the salt As a result, the presence of an appropriate amount of
Na2SO4plays a key role in the formation of the h-WO3nanorods evolved from the oriented attachment of h-WO3nanoneedles Controlled experiments have shown that the presence of aniline without Na2SO4, only irregular particles coexist with a small frac-tion of nanoneedles of orthorhombic WO31/3H2O (JCPDS # 35-0270) (Figs 3a and 4a) were obtained Furthermore, when the preparations are made without aniline and sodium sulfate only regular plate-like of monoclinic WO3 (JCPDS # 72-1465) are formed (Figs 3band 4b) Our results show that the addition of so-dium sulfate without aniline leads to the formation of irregular nanoneedles of the h-WO3(Fig 5)
As reported in the literature, organic molecules were found to play an important role in the controlling the morphologies Thus,
it appears that aniline, due to its anisotropic character, is used as capping agent which induce and enhance the directing role of
Na2SO4in the formation of 1D h-WO3nanostructures[24,30,31]
In view of these results, we can conclude that aniline and so-dium sulfate play important roles on the types of structure, mor-phology and size of particles constituting the product
The details of the effect of sulfate on the formation of WO3 nanocrystal are not clear up to date However, it is known that the anisotropic growth of the particles can be explained by the spe-cific adsorption of ions to particular crystal surface, therefore, inhibiting the growth of these faces by lowering their surface en-ergy[32,33]
This study allowed us to determine the optimum conditions of synthesis of h-WO3nanoneedles Indeed, the observation by SEM (Fig 2d) of the obtained product with the molar ratio Na2SO4:Na2
-WO4= 5:1 shows that the synthesized material is made of homog-enous phase with uniform particles which display nanorods mor-phology sizing about 5lm in length TEM image (Fig 2e) shows that the as-synthesized product exhibits the same rod-like mor-phology These nanorods are straight and uniform in diameter in the range of 50–70 nm
The high-resolution HRTEM image inFig 6, clearly, reveals that the inter-plane distance is 0.383 nm This could be indexed as [001] of the h-WO3crystal, according to JCPDS # 33-1387 This re-sult is good agreement with X-ray diffraction
3.3 Infrared spectroscopy The structure information was further provided by FTIR spec-troscopy.Fig 7displays the FTIR spectrum of as-synthesized WO
Fig 1 XRD patterns of h-WO 3 synthesized at different molar ratio Na 2 SO 4 :W (a)
5:1, (b) 2.5:1, (c) 1.25:1, (d) 0.62:1 (e) 0.30:1 and (f) 0.15:1.
Trang 3nanorods The broad band located between 949 and 870 cm1
ob-served for the WO3is present in many tungsten oxide compounds
It is attributed to the stretching of short W@O bonds that are
also present in h-WO3, while the bands located at 817 and
728 cm1 are assigned to O–W–O stretching modes [34] The
vibrational bands centered at 659 and 590 cm1are attributed to
W–O–W stretching modes[35]
3.4 Raman spectroscopy
Raman spectroscopy was used to characterize this material
since this technique is suitable to obtain details of the WO
chem-ical structure (Fig 8) Well defined peaks centered at 236, 265, 328,
612, 702 and 807 cm1, can be observed According to the litera-ture [35,36], these bands can be assigned to the fundamental modes of crystalline h-WO3 The bands at 807 and 702 cm1are attributed to the symmetric and asymmetric vibrations of W6+–O bonds (O–W–O stretching modes), while the bands at 328, 265 and 236 cm1can be attributed to the W–O–W bending mode of the bridging oxygen The band at 460 cm1 can be attributed to the characteristic band of crystalline WO3[37–39] FTIR and Ra-man results confirm those obtained by X-ray diffraction
This study suggests to propose a mechanism for the formation processes of h-WO nanorods under hydrothermal conditions:
Fig 2 SEM images for the products prepared at different molar ratio Na 2 SO 4 :W (a) 0.62:1, (b) 1.25:1, (c) 2.5:1 and (d) 5:1 TEM images for the products prepared at molar ratio Na 2 SO 4 :W (e) and (f) 5:1.
Trang 4Fig 3 SEM images of the as-obtained materials: (a) with aniline and without
sodium sulfate (b) without sodium sulfate and aniline.
Fig 4 XRD patterns of the as-obtained materials: (a) without sodium sulfate (b)
Fig 5 SEM image of the as-obtained material in presence of Na 2 SO 4 only.
Fig 6 HRTEM image of the individual WO 3 nanorod.
Trang 5Na2WO4þ 2HCl þ nH2O ! H2WO4nH2O þ 2NaCl ð1Þ
H2WO4nH2O !Hydrothermal treatment
C 6 H 5 —NH 2 þNa 2 SO 4
WO3þ ðn þ 1ÞH2O ð2Þ
In this treatment, with sufficient energy provided by the
hydro-thermal system, the WO3nuclei were formed In the presence of
aniline and Na2SO4, these nuclei serve as seeds and following grow
along the c-axis direction of h-WO3unit cell due to the aniline and
sulfate preferentially absorb on the faces parallel to the c-axis of
the WO3, resulting in the formation of the nanorods More in-depth
studies are necessary to further understand their growth process,
which can provide important information for structure design
and morphology controlled synthesis of oxides
4 Conclusion
In summary, we have successfully synthesized h-WO3nanorods
through a hydrothermal process The h-WO3 nanorods are up to
somelm in length, and 50–70 nm in diameter The pure hexagonal
phase crystalline WO3nanostructure was confirmed by XRD, SEM,
TEM, HRTEM, IR and Raman analyses The formation of nanorods
greatly relies on the presence of aniline and sodium sulfate
Be-sides, a particular amount of sodium sulfate in the reaction
med-ium plays a critical role even though the presence of aniline
required for producing the morphology of rod-like structure of
WO3 This versatile method provides a straightforward and
effi-cient means of obtaining WO3nanostructure having unique
mor-phology Furthermore, this synthetic method is simple, mild, and
controllable, and it provides a novel method for direct solution
growth of highly oriented and hierarchical nanostructures Indeed,
we anticipate that this technique can be exploited for the
fabrica-tion of other nanomaterial oxides
Acknowledgements
We would like to acknowledge Prof M Ben Salem, Faculté des Sciences de Bizerte, for TEM studies and Prof M Oueslati, Faculté des Sciences de Tunis, for Raman experiments
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Fig 8 Raman spectrum of h-WO 3 nanorods.