Tungsten oxide (WO3) nanorods array prepared using chemical vapor deposition techniques was studied. The influence of oxygen gas concentration on the nanoscale tungsten oxide structure was observed; it was responsible for the stoichiometric and morphology variation from nanoscale particle to nanorods array. Experimental results also indicated that the deposition temperature was highly related to the morphology; the chemical structure, however, was stable. The evolution of the crystalline structure and surface morphology was analyzed by scanning electron microscopy, Raman spectra and X-ray diffraction approaches. The stoichiometric variation was indicated by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy

A. Talledo

B. Q. Yang

C. Julien

C. Klinke

C.G. Granquist

F. Galléa

G. Micocci

H. X. Zhang

J. Livage

K. Nonaka

L. Chi

M. Ponzi

P. Poizot

P. X. Feng

S. Pal

X. P. Wang

Z. Lu

English

PubMed

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Tungsten Oxide Nanorods Array and Nanobundle Prepared by Using Chemical Vapor Deposition Technique

AbstractTungsten oxide (WO3) nanorods array prepared using chemical vapor deposition techniques was studied. The influence of oxygen gas concentration on the nanoscale tungsten oxide structure was observed; it was responsible for the stoichiometric and morphology variation from nanoscale particle to nanorods array. Experimental results also indicated that the deposition temperature was highly related to the morphology; the chemical structure, however, was stable. The evolution of the crystalline structure and surface morphology was analyzed by scanning electron microscopy, Raman spectra and X-ray diffraction approaches. The stoichiometric variation was indicated by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy.</p

Wang XP

Yang BQ

Zhang HX

Feng PX

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Nanoscale Research Letters

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NANO EXPRESS
Tungsten Oxide Nanorods Array and Nanobundle Prepared
by Using Chemical Vapor Deposition Technique
X. P. Wang Æ B. Q. Yang Æ H. X. Zhang Æ
P. X. Feng
Received: 16 May 2007 / Accepted: 15 June 2007 / Published online: 7 July 2007
 to the authors 2007
Abstract Tungsten oxide (WO3) nanorods array prepared
using chemical vapor deposition techniques was studied.
The influence of oxygen gas concentration on the nano-
scale tungsten oxide structure was observed; it was
responsible for the stoichiometric and morphology varia-
tion from nanoscale particle to nanorods array. Experi-
mental results also indicated that the deposition
temperature was highly related to the morphology; the
chemical structure, however, was stable. The evolution of
the crystalline structure and surface morphology was ana-
lyzed by scanning electron microscopy, Raman spectra and
X-ray diffraction approaches. The stoichiometric variation
was indicated by energy dispersive X-ray spectroscopy and
X-ray photoelectron spectroscopy.
Keywords Nanostructure  Tungsten oxide  Nanorod 
Nanobundle  CVD
Introduction
Nanostructured transition metal oxides are outstanding
candidates for a wide range of applications including lith-
ium-ion batteries, [1, 2] catalysts, [3] electrochromic
materials, [4, 5] and sensors. [6, 7] Nanostructured tungsten
oxide, as a typical transition metal oxide material, has been
researched frequently these years.
The nanostructured tungsten oxide material exhibited
many excellent properties because of their particular phase
structure and huge surface areas, which depend greatly on
the experimental parameters. In previous experiment of
chemical vapor deposition (CVD), it was realized that
several factors, such as filament temperature, electrical
current, gas flow and the composition of gas, would affect
the structure of the sample. The major factors could be the
substrate temperature and the chamber pressure [8].
Moreover, the effect of the reaction gas concentration on
the sample properties was also preliminarily studied [9].
Based on the previous achievements, the focus of the
present paper would be on two issues: to analyze the
influence of oxygen gas concentration (OGC) on the stoi-
chiometry phase, and to study an effect of substrate tem-
perature on the crystalline structure of tungsten oxide
nanorods array. All samples have been characterized by
using Raman spectra, scanning electron microscopy
(SEM); energy dispersive X-ray spectroscopy (EDS), X-
ray photoelectron spectroscopy (XPS) and X-ray diffrac-
tion (XRD) were also employed to characterize the sam-
ples.
Experimental Set Up
The nanostructured tungsten oxide materials were synthe-
sized using a CVD technique. The Molybdenum (Mo)
wafer was used as deposition substrate. Before placing the
substrates in the CVD chamber, the mirror-like surface of
the polished substrates were ultrasonically washed in a
methanol solution for 5 min, rinsed with acetone, and dried
with helium. After placing the substrate, the chamber was
pumped down to 2.0 · 10–5 Torr before feeding the gases.
Two kinds of gas mixture, 8.7% of CH4, 0.3% O2, and 91%
H2 and 8.3% of CH4, 0.7% of O2, and 91% H2 gases were
used. The flow rate of mixed gases was 5SCCM. The gas
pressure inside the deposition chamber was maintained at
X. P. Wang  B. Q. Yang  H. X. Zhang  P. X. Feng (&)
Department of Physics, University of Puerto Rico, P.O. Box
23343, San Juan, PR 00931, USA
e-mail: pfeng@cnnet.upr.edu
123
Nanoscale Res Lett (2007) 2:405–409
DOI 10.1007/s11671-007-9075-3
500 mTorr during the deposition. An AC power supply
with electric current of 10 A and voltage of 8 V was used
to heat the tungsten filament to temperature 2,400 C to
promote gas phase activation.
Results and Discussions
Figure 1 showed two SEM images of surfaces of the
samples prepared under OGC of (a) 0.3% and (b) 0.7% in
mixture gases at 400 C for 1 h of deposition. Differences
between the two surface structures were distinguishable.
Generally, low OGC resulted in yielding nanoscale WO
particles, which was shown in Fig. 1a. The particles uni-
formly distributed. The scale of the particles was similar,
which was 1 lm around. The film’s color looked like iv-
ory-white, and its surface appeared glossy. Figure 1b
showed the SEM image of the samples under high OGC.
The sample’s color was violet blue, which was the typical
color of tungsten trioxide. The sample’s surface was dim. It
could be easily observed that the tungsten oxide rods ar-
rayed very well. They were vertical to the substrate. The
diameter of the rod was 400 nm averagely. The number of
rods per unit area (Fig. 1b) was almost the same as the
number of the total particles on the top surface of the
sample (Fig. 1a). Therefore, it could be assumed that the
particles on the top layer in sample (a) could be the base of
the tungsten oxide rods shown in Fig. 1b. The particles on
the bottom layer covered the substrate surface tightly, and
then the top layer provided the seeds of the rods. If the
OGC were high enough as the precursor, the tungsten oxide
nanorods would keep yielding. If not, the nanorods would
not exist. As mentioned above, the gas pressures of both
experiments were kept nearly constant except slight vari-
ation of OGC from 0.3% to 0.7% in the gas mixture.
Interest is that so little variation of OGC resulted in com-
pletely different WO3 nanostructure.
Figure 2a, b showed the EDS of two tungsten oxide
samples (Fig. 1a, b). The element component quantitative
result was presented in Table 1. The EDS pattern in Fig. 2
indicated the sample not only consisted of tungsten and
oxygen, but also some carbon atoms. The EDS signal
(Fig. 2a) of oxygen was obviously weaker than that in
Fig. 2b. This was in good agreement with the experimental
conditions. From Table 1, it could be seen that variation of
the oxygen concentration in the mixture gases in the
chamber from 0.3% to 0.7% yielded oxygen component up
to nearly 7% and 51%, respectively, inside the tungsten
oxide samples. The ratio of atomic percentages between
tungsten and oxygen was almost 9 under low OGC. In this
case the phase of tungsten oxide supposed to be in sub-
stoichiometry state. It could also be observed that the
composition of carbon was remarkable which was even
4 times more than oxygen. On the other hand, under high
OGC, the tungsten content decreased down to 38.66%,
which was 20% lower than former one, whereas the com-
ponent of oxygen largely increased up to 50%. Accord-
ingly, the carbon component decreased rapidly, less than
20%.
Based on the data above, the stoichiometry variation can
be given. As seen under low OGC, the carbon content
inside the sample was higher than that of oxygen. There-
fore, two possible chemical states might coexist inside the
sample. One was tungsten oxide together with tungsten
carbide. Due to lack of oxide component, the stoichiometry
phase of tungsten oxide should be WO3–x, where x was
related to the stoichiometry phase of tungsten carbide. The
second possibility was that carbon atoms, tungsten atoms,
and tungsten oxide mixed but independently existed, which
mean there were no chemical bonds among them. This
expectation has been confirmed by XPS or XRD mea-
surements below.
When the OGC in the mixture gases was high, the ob-
tained oxygen content inside the sample was up to 50%.
The percentage of the carbon inside the sample descended
to 10%. Consequently, tungsten oxide dominated the
sample. This was verified by using XPS. Figure 3a showed
the tungsten peaks for the sample prepared under low
OGC. Four peaks were observed at 37.97 eV, 35.87 eV,
33.38 eV and 31.2 eV. The typical doublet W4f peaks
were clearly visible in the spectra, which were at 31.2 eV
and 33.38 eV. The existence of these two peaks strongly
Fig. 1 SEM images of the
samples prepared under (a) low
OGC, and (b) high OGC
406 Nanoscale Res Lett (2007) 2:405–409
123
proved the assumption of deposition of atomic tungsten.
The two upper binding energy peaks exhibited the presence
of oxygen modified W4f5/2 and W4f7/2 status. It was also
found that there was a shoulder at upper energy side of
each atomic tungsten peak. By looking up the database of
National Institute of Standard Technology (NIST), these
shoulders were related to WO2 and WOx. No specified
carbon modified tungsten peaks could be found in the XPS
profiles of the samples. Considering the assumption of
compound component mentioned above, it was concluded
that atomic carbon, atomic tungsten and sub-stoichiometry
tungsten oxide existed in this tungsten oxide particle-based
thin film.
The XPS profile of sample (b) prepared under high OGC
was shown in Fig. 3b. The oxygen modified tungsten fea-
tures remained unchanged, indicating the presence of
stoichiometry tungsten oxide. The peaks of atomic tungsten
vanished. Moreover, the shoulder peaks related to WO2 or
WOx also disappeared. This evidence strongly supported
the stoichiometry phase evolution of the tungsten oxide.
Figure 4 showed the Raman spectra of the tungsten
oxide samples related to Fig. 1a, b. Two broad but weak
bands marked with K1 and K2 located in 770 cm–1 and
870 cm–1 which was shown in Fig. 4a, were related to
tungsten oxide. Typical Raman peaks of crystalline tung-
sten oxide located at 700 cm–1 and 800 cm–1. The shift
resulted from the different chemical experimental condi-
tions. In fact, such weak humps revealed that the obtained
tungsten oxide particle-based film was in amorphous states.
Under high OGC, much more prominent Raman spectra
peaks at 701 cm–1 and 801 cm–1 were indicated in Fig. 4b,
which supported the existence of tungsten trioxide. A
conclusion from Raman spectra was revealed: the crystal-
line tungsten oxide has been yielded under high OGC.
Variation from the two weak humps in Fig. 4a to promi-
nent peaks in Fig. 4b indicated that the crystalline struc-
tural evolution followed the change of OGC.
The XRD patterns of the samples were showed in Fig. 5.
All four XRD peaks of the first sample shown in Fig. 5a
were from tungsten components. From XPS, we have also
known that the first sample included atomic tungsten
component, WO2 component, and WO3 component. These
results provided evidence again that the first sample was in
amorphous status. It was in good agreement with the data
obtained from Raman spectra. XRD pattern of the second
Fig. 2 EDS of the samples
(Fig. 1) for (a) low and (b) high
OGC
38 36 34 32 30
(a)
O
W
x
W
0
O
W
2
Ve79.73
.53
Ve78
.33
Ve83
Ve2.13
W
0
f4
W
2/ 7
f4
W
2/5
(
N
)
E
Binding Energy[eV]
38 36 34 32 30
f4
W
2/7
4
W
f
2 /5(b)
(
N
)
E
Binding Energy (eV)
Fig. 3 XPS plot of (a) low
OGC and (b) high OGC
Table 1 EDS elements component quantitative results
Low OGC High OGC
Wt% At% Wt% At%
W K 95.39 58.78 88.33 38.66
O K 0.95 6.72 10.07 50.63
C K 3.66 34.50 1.60 10.72
Nanoscale Res Lett (2007) 2:405–409 407
123
sample shown in Fig. 5b was complicated. It included
atomic tungsten, WO2 and WO3 XRD peaks. WO3 XRD
peaks dominated all spectral lines. The peaks marked as
002, 020 and 200, were related to monoclinic tungsten
trioxide. The peak of crystalline orientations of 020 was
much stronger than that of 200 and 002 orientations.
Similar phenomenon was observed at the 2h diffraction
angles between 47o and 51o, which were associated with
the orientations of 004, 040, and 400. Meanwhile, the peak
of 040 was the strongest one. This fact showed that the
polycrystalline tungsten trioxide of the sample was yielded,
and the orientation of 020 dominated the trend of growth.
In summary, the stoichiometry phase evolution of
tungsten oxide highly depended on the variation of OGC in
mixture gases during deposition. Following an increase of
OGC, the sub-stoichiometry tungsten oxide-WO2 and
WOx-would become stoichiometry tungsten oxide. It was
also found that atomic tungsten and carbon without any
chemical bond structure would be mixed inside the sample.
The variation of OGC also determined the structural evo-
lutions from amorphous to crystal. Low OGC caused
yielding amorphous tungsten oxide, whereas high OGC
resulted in producing polycrystalline WO3.
As a comparison, the effect of variation of substrate
temperature on the sample nanostructure and chemical
bond was also studied. Figure 6 shows SEM images of the
samples prepared at substrate temperature of (a) 800 C,
(b) 1,000 C and (c) 1,200 C. All other conditions such as
OGC in the mixture gas, gas pressure, gas flow rate, fila-
ment temperature and deposition duration were kept same
as the sample shown in Fig. 1b.
The morphologies of these three samples were promi-
nently different. The sample of tungsten oxide prepared at
800 C (Fig. 6a) was thin, sharp and short. Nanobundle
was generated in this sample. The diameter of single
nanorod was around 200 nm, and the length was 2 lm. The
nanobundle was so compact that it tended to yield to larger
nanorod. The sample yielded at 1,000C was shown in
Fig. 6b. The hump-like nanostructured tungsten oxide was
obtained. The diameter of the hump at this temperature was
500 nm approximately. It could be assumed that the
nanobundle in Fig. 6a gathered then became a hump. This
dynamic phenomenon was similar to the little drip gath-
ering to be a larger drip. Furthermore, the tungsten oxide
nanorod was clearly shown in Fig. 6c. The diameter of the
nanorod was more than 500 nm, and the length was longer
than 5 lm.
Based on these three samples, it was concluded that the
growth rate of tungsten oxide increased following the ris-
ing of temperature, resulting in that the sample diameter
was large and the length was long. Similar result was re-
ported by Chi et al. [10] and Pal et al. [11]. It was espe-
cially mentioned that the root of each nanobundle was
thinner than the main body [10]. So it could be inferred that
the thin nanobundle at lower temperature could be the base
of the sample prepared at higher temperature, which was
similar to the condition shown at former paragraph con-
cerning the different OGC.
600 900
10
20
30
(a)
K2
K1
In
te
ns
ity
 [a
.u
]
Wavelength (cm-1)
(b)Fig. 4 (a) Raman profile for
low OGC. (b) Raman profile for
high OGC
20 40 60 80
0
1
2
3
In
te
ns
ity
 [a
.u
.]
In
te
ns
ity
 [a
.u
.]
2 Theta (Degrees) 2 Theta (Degrees)
(a)
20 40 60 80
0
1
2
3
4
5
O
W
2
O
W
2
04
000
4
40
0
o
M
o
M
o
M
o
M00
2
02
0
20
0
(b)Fig. 5 (a) XRD pattern for low
OGC. (b) XRD pattern for high
OGC
408 Nanoscale Res Lett (2007) 2:405–409
123
Figure 7 showed Raman profiles for these three samples
(Fig. 6). Several Raman peaks were clearly visible, where
peaks (signals) marked with J and K in the Raman profile,
respectively. No shift existed for all these three spectra,
which showed the structural stability of the samples at high
temperature was very well. In general, the bands situated at
around 700 and 800 cm–1 could be assigned to W–O
stretching model, whereas the bands situated at around 130
and 270 cm–1 were associated to W–O bending modes of
monoclinic WO3 [12] Miyakawa has shown that the Raman
bands at 809 and 718 cm–1 were for monoclinic WO3 and
did not change as a function of temperature (<500 C), [13]
indicating the formation of a highly stable monoclinic
crystalline WO3. The present data indicated that the
stretching mode would not change even the temperature
higher than 500 C. However, the bending mode vanished
at 1,200 C. Therefore the bending mode was not as stable
as stretching mode and it depended on the substrate tem-
perature.
Conclusion
In conclusion, the variation of the properties of tungsten
oxide highly depended on the OGC of the gas mixture.
Slight rise of OGC from 0.3% to 0.7% in the mixture gas
during deposition resulted in large change of the oxygen
quantitative component in samples from 7% to 51%; the
morphologies of the samples varied from particle-based
film to nanorods array, and the chemical phase developed
from sub-stoichiometry phase to stoichiometry. The crys-
talline structure also altered to crystalline tungsten oxide
from amorphous structure. The sample prepared under high
substrate temperature (800 C–1,200 C) was also inves-
tigated. The diameter and length of the samples’ nano-
structure grew up by raising the substrate temperature. The
evolution of morphology was prominent, whereas the
structure was quite stable. Based on the evidences above, it
could be concluded that the properties of tungsten oxide
were highly sensitive to the experimental parameters dur-
ing deposition, especially the OGC in the gas mixture.
Acknowledgements This work has been supported by NSF-EP-
SCoR and DoD grants. We would like to thank Mr. William’s
assistance of Raman measurements, Mr. Ortiz and Ms. Hernandez for
SEM and EDS measurements, Mr. Esteban for XPS measurements
and Mr. Wu for XRD measurements.
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Fig. 6 SEM images for the
samples at substrate
temperatures of (a) 800 C (b)
1,000 C and (c) 1,200 C
0 400 800 1200
400
800
1200
1600
2000
2400
1200°C
1000°C
800°C
K2
K1J2J1
etnI
sn
yti
].u. a[
Wavelength (cm-1)
Fig. 7 Raman profiles for samples at different deposition tempera-
tures
Nanoscale Res Lett (2007) 2:405–409 409
123


Escobar-Alarco `n,

Handbook of Inorganic Electrochromic Materials.

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Tungsten Oxide Nanorods Array and Nanobundle Prepared by Using Chemical Vapor Deposition Technique

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