Solid State Sciences 7 (2005) 67–72
www.elsevier.com/locate/ssscie
Facile large scale synthesis of WS
2
nanotubes from WO
3
nanorods
prepared by a hydrothermal route
Helen Annal Therese
a
, Jixue Li
b
,UteKolb
b
, Wolfgang Tremel
a,∗
a
Institut für Anorganische Chemie und Analytische Chemie der Johannes Gutenberg-Universität, Duesbergweg 10-14, 55099 Mainz, Germany
b
Institut für Physikalische Chemie, Welderweg 11, 55099 Mainz, Germany
Received 17 June 2004; received in revised form 27 July 2004; accepted 8 October 2004
Available online 13 December 2004
Abstract
Hexagonal WO
3
nanorods of5–50 nmin diameter and 150–250 nmin length have been synthesised in gram quantitiesby a low temperature
hydrothermal route using citric acid as a structural modifier and hexadecylamine as a templating agent. The ratio of [A]/[W] play an important
role on WO
3
nanorods formation. These WO
3

tum size. Among nanomaterials, nanowires and nanotubes
are good candidates for studying the phenomena such as
electrical resistivity, strength, magnetic and optical proper-
ties in one dimension. During the past few years, nanotubes
of various materials with graphite-like layered structures
has been synthesised successfully using techniques such as
arc discharge [2], laser ablation [3], electron beam irradi-
ation [4], sonochemical [5], hydrothermal reaction [6] and
*
Corresponding author.
E-mail address: [email protected] (W. Tremel).
iodine transport [7], etc. Among the various classes of non-
carbon nanotubes, transition metal chalcogenides of the gen-
eral formula MQ
2
(M = W,Mo,V,Nb,Ta,Zr;Q= S,
Se) are significant materials and reveal interesting electronic
and optical properties [8–10]. Inorganic fullerene like MoS
2
nanoparticles and MoS
2
nanotubes exhibit excellent lubri-
cating properties [11] and show high scope as tips for scan-
ning probemicroscopes[12]. Recent electrochemicalstudies
on MoS
2
nanotubes revealed that the nanotubes can store
relatively large amounts of gaseous hydrogen by electro-
chemical storage [13].MoS
2

under reduced pressure in the presence of water vapour [19,
20]. Walton and co-workers have synthesised WS
2
tubes
from W
18
O
49
rods produced by heating tungsten foil at high
voltages under Ar in the presence of SiO
2
[21].
Various strategies have been employed for the synthe-
sis of WO
3
nanorods as synthesising large quantities WO
3
nanostructures of the required size becomes a greater task.
WO
3
nanorods were produced by heating the tungsten fil-
ament in an inert atmosphere either in the presence of ox-
ides like SiO
2
[22],B
2
O
3
[23] or in the presence of water
vapour [20] or by IR irradiation of W in the presence of

WS
2
nanotubes.
2. Experimental
2.1. Synthesis of WO
3
nanorods
An aqueous solution containing a mixture of 1.32 g of
(NH
4
)
10
W
12
O
41
·7H
2
O and 2.10 g of citric acid was heated
around 120
◦
C under constant stirring for 4–5 h until a gel
was formed, which was allowed to stand overnight. 2.45 g of
hexadecyl amine dissolved in ethanol was added as an addi-
tive to the gel and stirred for 10 h. The resulting mixture was
transferred to a Teflon autoclavewith a stainless steel protec-
tive outer body and heated at 180
◦
C for 7 days. The product
obtained was washed with ethanol, cyclohexene, water and

The products obtained from hydrothermal reactions after
washing and drying were analysed by X-ray powder diffrac-
tion in θ/2θ reflection geometry using Siemens D8 powder
diffractometer equipped with a position sensitive detector.
Data was collected between 2θ = 5
◦
and 60
◦
(using Cu-Kα
radiation), at an operation potential of 40 kV and a current
of 40 mA. The morphology of the WO
3
nanorods and WS
2
nanotubeswas characterisedbyhigh-resolutiontransmission
electron microscopy (FEI Tecnai F30 ST operated at an ex-
traction voltage of 300 kV, equipped with an EDXA energy
dispersive X-ray spectrometer) and by selected area electron
diffraction techniques (SAED).
For transmission electron microscopic(TEM) studies, the
sample was prepared by crushing them mechanically with
a mortar and pestle followed by dispersing the powder ul-
trasonically in absolute ethanol and placing a drop of this
suspension on to a copper grid coated with a holey carbon
films.
3. Results and discussion
Representative TEM images of the tungsten oxide sam-
ples obtained from two different trials of hydrothermal reac-
tions are given in Fig. 1 (with m and w given in the caption
of Fig. 1). Fig. 1ashowsapartofaWO

WO
3
rods. The particle size of the nanorods calculated from
the XRD pattern using Scherrer’s formula varies between 50
and 185 nm.
Fig. 3 shows experimental and simulated HRTEM im-
ages and SAED diffraction patterns of a WO
3
nanorod. The
lattice parameters of 0.38 and 0.63 nm correspond to the
d-spacings of (001) and (100) of the WO
3
hexagonal cell.
H.A. Therese et al. / Solid State Sciences 7 (2005) 67–72 69
Fig. 1. TEM images of WO
3
nanorods synthesised by hydrothermal reactions at different mole percentages m =[W]/([A]+[W]+[HDA]) (%) and ratios
of w =[W]/[HAD] (%) ([W], [A] and [HAD] correspond to the number of moles of ammonium tungstate, citric acid and hexadecylamine). (a) Depicts the
bunch-like morphology of the WO
3
particle grown at m = 3.42 and w = 18.6, whereas (b) shows the morphology of WO
3
rods obtained at grown at m = 1.8
and w = 1.6. The WO
3
rods are shown at higher magnification in the inset. w values < 5andm values  1 resulted in the formation of nanorods.
Fig. 2. Powder X-ray diffraction pattern of WO
3
nanorods. All reflections
are indexed based on a hexagonal WO

tioned about the yield of such needles. In the present report,
it is important to mention that samples prepared for m  1
and w<5 contain exclusively hexagonal WO
3
nanorods.
Recently, nanotubes of VO
x
have been synthesised [32–34]
by low temperature sol–gel techniques. In both cases of VO
x
nanotube synthesis, hexadecyl amine used as a template gets
Fig. 3. Experimental HRTEM image of a WoO
3
nanorod along b axis (top);
the filtered image is shown as inset (a) the corresponding simulated image as
inset (b) together with experimental SAED pattern (bottom, left-hand side)
and dynamically calculated ED pattern for zone [010] (bottom, right-hand
side).
intercalated into the vanadium oxide structure, resulting in
larger d-spacings (∼ 3nm). From the lattice spacings of the
WO
3
rods reported here one could see that the hexadecyl
amine is not intercalated into the WO
3
nanorods. This also
helps us in availing this material as a precursor for WS
2
–NT
synthesis. Synthesis of WO

tion of WO
3
nanorods. One could speculate that at higher m
values, the 3 carboxylategroup of each citric acid could bind
to more than one WO
6
octahedron and helping in the olation
of WO
6
in a directed manner while hindering the oxolation
in all directions due to steric effects. This could lead to the
formation of shorter and thinner rods. On the other hand, at
lower m values, two or more oxygens of the WO
6
octahe-
dra could be contributed from a single citric acid molecule,
hence hindering both the olation and oxolation to a large
extent. During hydrothermal reaction at 180
◦
C, when citric
acid decomposes the hydrophobic hexadecyl amine template
could be helping in preserving the rod like structure of the
tungsten oxides. For w values > 5 the surface coverage of
the growing WO
3
nanorods is not sufficient to limit the par-
ticle growth and WO
3
bunches and rods are obtained. When
citric acid is fully replaced by hydrochloric acid condensa-

nanotube encapsulated
by a WS
2
mantle (b). The shorter nanotubes obtained when the nanorods were reduced for a duration of 10 min under H
2
S are shown in (c) and (d).
observed. The nanotube thicknesses range broadly from 20
to 200 nm and their length varies approximately from 1
to 8 µm. A large fraction of the nanotubes has open ends
(Fig. 4b). Studies on these nanotubes by HRTEM combined
with EDX analyses reveal the complete conversion of oxide
rods to sulphide tubes during the reduction process which
allows the synthesis of large amounts of multiwalled nan-
otubes (MWNTs). A HRTEM image of one such represen-
tative MWNT is shown in Fig. 4c. The interlayer spacing
of 0.65 nm between the tubular walls is consistent with the
(002) d-spacing of 2H–WS
2
lattice. The helicity of the nan-
otube (Fig. 4c) could be calculated as ∼ 10
◦
based on the
selected area diffraction(SAED) pattern (Fig. 4c) of the mul-
tiwalled WS
2
nanotube [35].
The synthesis of WS
2
nanotubes by reduction of WO
x

/N
2
and H
2
S
flow. During the course of the reaction this embryonic WS
2
layer starts growing inward as well as slowly converting the
oxide, which is continuously growing on the other end of
the particles by the condensation of WO
x
from the vapour
state. A similar mechanism is plausible in the present nano-
tube synthesis, where the role of the reducing H
2
/N
2
gas has
been replaced by the pretreatment of the oxide with Ar gas.
A TEM analysis of the oxiderodsafter the pretreatmentwith
72 H.A. Therese et al. / Solid State Sciences 7 (2005) 67–72
Ar shows the formation of an intermediate tungsten oxide
with many defects while the appearance of the rods was re-
tained. This could be related to a higher surface activity of
the oxide nanorods.
We have also observed many interesting structures such
as open multiwalled WS
2
tubes coated with a WS
2

MS
2
nanotubes. It was also possible to control the wall
thickness and the length of the nanotubes by selecting ap-
propriate reaction times for the reduction.
Acknowledgement
We are grateful to the Federal Ministry for Research and
Technology (BMBF) for the support of this research within
the program “Multifunctional Materials and Miniaturized
Devices” at the University of Mainz and the Deutsche
Forschungsgmeinschaft (DFG, SFB 625).
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