Characterization of h-WO
3
nanorods synthesized by hydrothermal process
S. Salmaoui
a
, F. Sediri
a,b,
*
, N. Gharbi
a
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
article info
Article history:
Received 10 November 2009
Accepted 15 February 2010
Available online 19 February 2010
Keywords:
Tungsten oxide
Aniline
Hydrothermal synthesis
Nanorods
abstract
Hexagonal tungsten oxide nanorods have been synthesized by hydrothermal strategy using
Na
2
WO
4
Á2H

[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, WO
3
can crystallize according to several structures.
All the WO
3
structures can be described as deformations of the
ReO
3
cubic perfect model. Such a perfect structure is composed
of a three-dimensional network of WO
6
octahedral linked by their
oxygen corner. Among various crystal structures of WO
3
, hexago-
nal form is of great interest owing to its well-known tunnel struc-
ture in which WO
6
octahedrons share their corners with each other
forming hexagonal tunnels along c-axis [25]. Hexagonal tungsten
oxide (h-WO
3
) has been widely investigated, especially as an inter-
calation host for obtaining hexagonal tungsten bronzes MxWO
3
(M = K
+

Á2H
2
O, H
2
O, C
6
H
5
–NH
2
, HCl and Na
2
SO
4
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.
doi:10.1016/j.poly.2010.02.025
* Corresponding author. Address: Faculté des Sciences de Tunis, Université Tunis-
Elmanar, 2092 Elmanar, Tunis, Tunisia. Tel.: +216 71872600; fax: +216 71871666.
E-mail addresses: (F. Sediri),
(N. Gharbi).
Polyhedron 29 (2010) 1771–1775
Contents lists available at ScienceDirect
Polyhedron

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 (Na
2
SO
4
:W), the nanorods can be prepared conveniently with
high purity. Fig. 1 shows the XRD patterns of the as-obtained prod-
ucts 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, respectively. All the dif-
fraction peaks can be perfectly indexed to hexagonal tungsten
oxide crystalline phase (h-WO
3
) 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-WO
3
crystalline phase with high purity could
be obtained using the present synthetic process.
3.2. Scanning and transmission electronic microscopy

:W ratio of 5:1, a rod-like structure with diameters of
about 50–70 nm and length of about 5
l
m were obtained. Close
observation shown in Fig. 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-WO
3
. 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
Na
2
SO
4
plays a key role in the formation of the h-WO
3
nanorods
evolved from the oriented attachment of h-WO
3
nanoneedles.
Controlled experiments have shown that the presence of aniline
without Na
2
SO
4
, only irregular particles coexist with a small frac-
tion of nanoneedles of orthorhombic WO

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-WO
3
nanoneedles. Indeed, the observation by SEM
(Fig. 2d) of the obtained product with the molar ratio Na
2
SO
4
:Na
2
-
WO
4
= 5:1 shows that the synthesized material is made of homog-
enous phase with uniform particles which display nanorods mor-
phology sizing about 5
l
m 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 in Fig. 6, clearly, reveals that
the inter-plane distance is 0.383 nm. This could be indexed as
[001] of the h-WO
3

are 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
3
chem-
ical structure (Fig. 8). Well defined peaks centered at 236, 265, 328,
612, 702 and 807 cm
À1
, can be observed. According to the litera-
ture [35,36], these bands can be assigned to the fundamental
modes of crystalline h-WO
3
. The bands at 807 and 702 cm
À1
are
attributed to the symmetric and asymmetric vibrations of W
6+
–O
bonds (O–W–O stretching modes), while the bands at 328, 265
and 236 cm
À1
can be attributed to the W–O–W bending mode of
the bridging oxygen. The band at 460 cm
À1
can be attributed to
the characteristic band of crystalline WO
3
[37–39]. FTIR and Ra-

3
nanorods.
1774 S. Salmaoui et al. /Polyhedron 29 (2010) 1771–1775
Na
2
WO
4
þ 2HCl þ nH
2
O ! H
2
WO
4
ÁnH
2
O þ 2NaCl ð1Þ
H
2
WO
4
ÁnH
2
O
!
Hydrothermal treatment
C
6
H
5
—NH

In summary, we have successfully synthesized h-WO
3
nanorods
through a hydrothermal process. The h-WO
3
nanorods are up to
some
l
m in length, and 50–70 nm in diameter. The pure hexagonal
phase crystalline WO
3
nanostructure 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
WO
3
. This versatile method provides a straightforward and effi-
cient means of obtaining WO
3
nanostructure 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

(2004) 1415.
[18] O. Nimittrakoolchai, S. Supothina, J. Mater. Chem. Phys. 112 (2008) 270.
[19] A. Patra, K. Auddy, D. Ganguli, J. Livage, P.K. Biswas, Mater. Lett. 58 (2004)
1059.
[20] S.J. Yoo, Y.H. Jung, J.W. Lim, H.G. Choi, D.K. Kim, Y.E. Sung, Sol. Energy Mater.
Sol. Cells 92 (2008) 179.
[21] Z. Xiao, L. Zhang, Z. Wang, Q. Lu, X. Tian, H. Zeng, Mater. Lett. 61 (2007) 1718.
[22] X.C. Song, Y.F. Zheng, E. Yang, Y. Wang, Mater. Lett. 61 (2007) 3904.
[23] J.M. Wang, E. Khoo, P.S. Lee, J. Ma, J. Phys. Chem. C 112 (2008) 14306.
[24] Z.J. Gu, T.Y. Zhai, B.F. Gao, X.H. Sheng, Y.B. Wang, H.B. Fu, Y. Ma, J.N. Yao, J. Phys.
Chem. B 110 (2006) 23829.
[25] J.D. Guo, K.P. Reis, M.S. Whittingham, Solid State Ionics 53–56 (1992) 305.
[26] K.P. Reis, A. Ramanan, M.S. Whittingham, Chem. Mater. 2 (1990) 219.
[27] K.P. Reis, A. Ramanan, M.S. Whittingham, J. Solid State Chem. 96 (1992) 31.
[28] N. Kumagai, Y. Umetzu, K. Tanno, J.P.P. Ramos, Solid State Ionics 86–88 (1996)
1443.
[29] K. Huang, Q. Pan, F. Yang, S. Ni, X. Wei, D. He, J. Phys. D: Appl. Phys. 41 (2008)
155417.
[30] J.H. Ha, P. Muralidharan, D.K. Kim, J. Alloys Compd. 475 (2009) 446.
[31] F. Sediri, F. Toauti, N. Gharbi, Mater. Sci. Eng. B 129 (2006) 251.
[32] Z.D. Xiao, L.D. Zhang, X.K. Tian, X.S. Fang, Nanotechnology 16 (2005) 2647.
[33] K. Lee, W.S. Seo, J.T. Park, J. Am. Chem. Soc. 125 (2003) 3408.
[34] T. Nishide, F. Mizukami, Thin Solid Films 259 (1995) 212.
[35] M.F. Daniel, B. Desbat, J.C. Lassegues, B. Gerand, M. Figlarz, J. Solid State Chem.
67 (1987) 235.
[36] G.L. Frey, A. Rothschild, J. Sloan, R. Rosentsveig, R. Popovitz-Biro, R. Tenne, J.
Solid State Chem. 162 (2001) 300.
[37] R. Solarska, B.D. Alexander, J. Augustynski, J. Solid State Electrochem. 8 (2004)
748.
[38] I.M. Szilàgyi, J. Madaràsz, G. Pokol, P. Király, G. Tárkányi, S. Saukko, J. Mizsei, A.