growth of monoclinic wo3 nanowire array for highly

Growth of monoclinic WO
3
nanowire array for highly
sensitive NO
2
detection
Baobao Cao,† Jiajun Chen,† Xiaojun Tang and Weilie Zhou
*
Received 23rd September 2008, Accepted 15th January 2009
First published as an Advance Article on the web 23rd February 2009
DOI: 10.1039/b816646c
A monoclinic tungsten trioxide nanowire array has been grown on silicon substrates using tungsten
powders as source materials by thermal evaporation under specific synthesis conditions (1000

C, 13–15
Torr, 200 sccm air flow). The morphology, chemical composition and crystal structure of the as-
prepared tungsten trioxide nanowires were characterized by scanning electron microscopy, energy
dispersive spectroscopy, X-ray diffraction, and transmission electron microscopy. The nanowires were
identified as monoclinic in structure, with diameters ranging from 40 to 100 nm and lengths up to 5 mm.
It was found that sufficient oxygen and air flow are the major factors to influence the nanowire array
growth. The nanowire array was employed directly for gas sensor fabrication using photolithography.
The gas sensing experiments revealed that the nanowire array sensors are highly sensitive to NO
2
(50 ppb), making the tungsten trioxide nanowire array a competitive candidate for highly sensitive gas
sensor fabrication.
Introduction
In the family of transitional metal oxides, tungsten oxides have
attracted great attention in the past few decades due to their
unique electrochromic, optochromic and gasochromic proper-
ties. Their applications have spread to various fields including
flat-panel displays, smart windows, gas sensors and field emit-

ters.
1–6
Since one-dimensional tungsten oxides exhibit large
surface to volume ratios compared to their bulk materials,
extensive efforts have been made for the synthesis of tungsten
oxide nanostructures and assembled ordered superstructures for
sensor applications.
7–19
Different from single nanowires, nano-
wire arrays have a higher surface to volume ratio and can be
potentially used to fabricate large area, highly sensitive, and
more stable sensors for practical applications. However, studies
on synthesis of tungsten oxide nanowire arrays are compara-
tively rare due to the high melting point of tungsten metal and
tungsten compounds. Several groups have performed studies on
nanowire array growth.
20–23
One of the interesting works was
demonstrated by Huang et al.,
23
in which they successfully
synthesized W
18
O
49
nanowire arrays directly on ITO (indium tin
oxide) coated glass substrate using tungsten trioxide powders by
a thermal evaporation method at a relatively low temperature
(1100


C).
Generally, among all the tungsten oxide nanostructures synthe-
sized for gas sensors, W
18
O
49
are mostly obtained and studied.
24,25
However, according to the very recent study by Sun et al.,
26
W
18
O
49
nanowires were completely transformed to monoclinic WO
3
at
above 500

C, along with the change of morphology from nano-
wires to nanoparticles. Since monoclinic WO
3
is quite stable up to
1000

C, sensors based on monoclinic WO
3
could be potentially
used in severe environments. Moreover, monoclinic WO
3

contains
more oxygen content than other WO
3 À x
phases, which might lead
to unique physical or chemical properties. In this paper, a conven-
tional thermal evaporation approach is applied to synthesize
a quasi-aligned, single crystal WO
3
nanowire array using tungsten
powder as the source material. In addition, the gas sensors based on
such WO
3
nanowire arrays demonstrate a highly sensitive capa-
bility to detect NO
2
concentrations down to the 50 ppb level,
demonstrating a very promising application in the field of low
concentration gas detection.
Experimental
In a typical synthesis, 2 g tungsten powder (12 mm, 99.9%
provided by Sigma–Aldrich) was loaded into the center of the
tube furnace, acting as a source material. Silicon substrate (1 cm
 3 cm) was cleaned by alcohol and deionized water in an
ultrasonic cleaner, then positioned at the low temperature zone,
20 cm downstream of the source material. After the quartz tube
was pumped down to a vacuum of 500 mTorr, the temperature of
the tube was increased from room temperature to 1000

Cat
a ramping rate of 50


C/min. During the whole heating process,
constant air flows of 20, 100 or 200 sccm, respectively, were
introduced for each experiment and pressure inside the tube was
kept at 13–15 Torr in order to explore an optimum synthesis
condition for nanowire array growth. After grown for 1 h, the
furnace was naturally cooled to room temperature by switching
off the heating power.
The as-prepared products on Si substrates were characterized
by Carl Zeiss 1530 VP field emission scanning electron micro-
scopy (FESEM), Philips X’pert-MPD X-ray diffraction (XRD),
JEOL 2010 transmission electron microscopy (TEM), and energy
dispersive spectroscopy (EDS) attached on the TEM. The
nanowire array gas sensor was prepared by a multiple-step
photolithography process
27
and the sensor was mounted on
a heating stage connected to the electrical feedthrough in
Advanced Materials Research Institute, University of New Orleans, New
Orleans, Louisiana, 70148, USA. E-mail: ; Fax: +1-504-
280-318; Tel: +1-540-280-1068
† Baobao Cao and Jiajun Chen contributed equally to this work.
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a testing cell for sensor measurements. The gas testing experi-
ments of the sensor under air-diluted NO
2

were conducted at
180

C using a Keithley 2400 source meter.
Results and discussion
A large area and quasi-aligned tungsten nanowire array was
successfully synthesized under a pressure of 13–15 Torr with an
air flow of 200 sccm at a temperature of 1000

C. Fig. 1 shows
typical FESEM images of an as-synthesized nanowire array
grown on Si substrate. Nanowires are distributed evenly with the
length of about 3–5 mm as shown in the top view of Fig. 1(a) and
cross-sectional view of Fig. 1(b), respectively. In fact, the nano-
wire array grows out from a thin layer of tungsten oxide nano-
particles formed prior to the nanowire growth shown in the insert
of the enlarged interface in Fig. 1(b).
The XRD pattern of tungsten oxide nanowires is shown in
Fig. 2. The diffraction peaks can be well indexed to a monoclinic
WO
3
phase with unit cell parameters as a ¼ 0.7297 nm, b ¼
0.7539 nm, c ¼ 0.7688 nm, b ¼ 90.91

(JCPDS 43-1035). In the
XRD pattern, the (002) diffraction peak is the strongest reflec-
tion, indicating the (002) is the preferential growth plane of the
nanowires. The strong peak marked by a star in the pattern is
from the Si substrate. It is apparent that the tungsten oxide
synthesized in our experiment is WO

3
phase, other than W
18
O
49
23
which was further confirmed by TEM investigation.
Fig. 3(a) presents a low magnification TEM image of the
nanowires with a diameter around 40–50 nm. Fig. 3(b) is the
[100] high resolution electron image (HREM) of the nanowire
from the rectangular area in Fig. 3(a), in which the lattice
spacings are measured to be 0.385 and 0.379 nm along two
orthogonal directions, corresponding to the (002) and (020)
planes of monoclinic WO
3
, respectively. The selected area elec-
tron diffraction pattern (SAED) proves the nanowire is single
crystalline with the growth plane parallel to (002), as shown in
the inset in Fig. 3(b). Furthermore, EDS analysis shows the
atomic ratio of W and O are close to 3 : 1. The C and Cu peaks
are from the copper grid in the EDS spectrum. From the XRD
and TEM analysis, it can be further concluded that the WO
3
nanowires are single monoclinic crystalline with growth plane
parallel to the (002) plane.
In our experiment, the whole synthesis process follows the
vapor–solid (VS) mechanism.
28
Generally, gas-phase supersatu-
ration in VS growth is a most determining factor for the

formation of different nanostructures, which is influenced by
source materials temperature, tube pressure, gas flow rate, etc.
29
Therefore, a series of experiments were performed to investigate
the optimum growth conditions. It is found that relatively low
pressure (13–15 Torr), moderate temperature (1000

C), and high
air flow rate (200 sccm) are able to produce a suitable gas-phase
supersaturation for nanowire array growth. During the heating,
the color of the source materials changed from dark gray to
orange-reddish, accompanied with an obvious volume increase.
Owing to the sufficient oxygen introduced from the air flow,
WO
3
species were directly carried down stream, forming WO
3
nanowire arrays on the Si substrate. No catalysts were observed
at the tips of the nanowires by SEM or TEM. It should be noted
that a layer of WO
3
nanoparticles, acting as a seed layer, was
formed prior to the nanowire array growth.
A gas sensor was prepared based on this WO
3
nanowire array
and the schematic diagram and corresponding SEM images of
the preparation process are shown in the Fig. 4. A layer of 8 mm
polymethyl methacrylate (PMMA) Resist 950 A4 (Microchem
Inc.) was first coated on the nanowire array by spin coating,

shown in Fig. 4(b). After the heat treatment on the PMMA resist,
the nanowire array was loaded to an oxygen plasma etching
machine. A 100 W input power resulted in an etch rate of 0.08
mm/min, and after 75 min etching, 2 microns of PMMA was left
Fig. 1 SEM images of a WO
3
nanowire array grown on a Si substrate
from a top view (a) and a cross-sectional view (b).
Fig. 2 XRD spectra of the nanowire array on Si substrate. The peak
marked by a star is (004) diffraction from the Si substrate.
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and the tips of the nanowires were exposed to the environment
(Fig. 4(c)). By using sputtering and a metal mask, a 100 nm gold
layer with 2 mm  2 mm area was coated onto the residue
PMMA and connected to the tips of the nanowires. After that,
the nanowire array was soaked into acetone, isopropyl alcohol,
and deionized water, successively, to remove the PMMA.
Finally, the sensor was dried by slow nitrogen flow. Silver paste
was used to make connections from the sensor testing system to
the bottom layer and the top gold electrode (Fig. 4(d)). By
applying a voltage on the top electrode, current flowing through
the nanowires could be provided and all of the nanowires were
exposed to the atmosphere.
The sensor was first stabilized at 180

C in pure air. IV curves
were obtained when air-diluted NO

2
with different concentra-
tions were introduced into the gas sensor testing cell, as shown in
Fig. 5(a). All the IV curves are linear, implying ohmic contacts
were formed between the deposited Au and WO
3
. The sensor can
detect NO
2
with a concentration down to 50 ppb. When the
sensor is exposed to 50 ppb NO
2
, the resistance of the sensor
increased to about 37% from 45.5 MOhm to 62.5 MOhm. Even at
a relatively low concentration, 5 ppm, the resistance change can
be as high as 3400% from 45.5 MOhm to 1.6 GOhm. The sensing
response curves to air-diluted NO
2
are plotted in Fig. 5(b). The
sensor showed immediate response to NO
2
, the typical response
times are less than 30 s; however, it took a relatively long time for
the sensor recovery, generally more than 800 s, and it increased as
the exposed concentration increased. The response curves for the
low concentrations are also plotted in the inset of Fig. 5(b), which
indicates that the response signal was much larger than the
Fig. 3 (a) A typical low magnification TEM image of tungsten oxide
nanowires. (b) [100] HREM image of a tungsten oxide nanowire denoted
by the white rectangle in (a). The inset shows the corresponding SAED

pattern. The white arrow in (a) indicates the growth direction of the
nanowire. (c) EDS spectra of the as-prepared nanowires reveal that
nanowires consist of W and O. The C and Cu elements are from the TEM
copper grids.
Fig. 4 Schematics and SEM images showing the procedures to prepare
a gas sensor based on WO
3
nanowire array. All the scale bars are 4 mm.
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background noise level, even at a concentration as low as 50 ppb
and can be easily identified. Therefore, the sensor presents high
sensitivity to detect NO
2
gases, which might be attributed to the
large surface area of the nanowire array and intrinsic properties
of the monoclinic WO
3
nanowires.
WO
3
is an n-type semiconductor. Absorbing oxidizing gas
molecules, such as O
2
,NO
2
, to its surface, can induce surface
states that trap electrons and result in depletion of carrier on the

material surface. For nanowires, forming a carrier depletion layer
will reduce the effective cross area for carrier transport and cause
reduced current in our constant voltage bias measurement.
Considering that the nanowires are resistors with parallel
connection, overall resistance increase in our device requires the
gas can cause resistance increase in most of the nanowires. Since
the response speed of this sensor is very fast, gas diffusion into the
nanowire array sensor must be very effective so that it can induce
a resistance increase in most nanowires in a short time. Though
the response trend is the same as other WO
3
based sensors,
30,31
the
unique parallel connection with large surface area provides a new
route to fabricate highly sensitive gas sensors.
Conclusions
In summary, a large area WO
3
nanowire array with diameters
ranging from 40 to 100 nm and lengths up to 5 mm was
synthesized on Si substrate using a convenient thermal evapo-
ration method at a relatively low temperature. The nanowire
grew on top of WO
3
nanoparticle seed layers, forming a quasi-
aligned nanowire array. The nanowires were single crystalline
with monoclinic structure. The nanowire array gas sensors were
also fabricated using a multiple step photolithography method.
Gas sensing tests revealed that the sensor based on the WO

3
nanowire array had the capability of detecting NO
2
concentra-
tions as low as 50 ppb, demonstrating a promising application in
the field of low concentration gas detection.
Acknowledgements
This work was supported by the DARPA Grant No.HR0011-07-
1-0032 and research grants from the Louisiana Board of Regents
Contract Nos. LEQSF(2007-12)-ENH-PKSFI-PRS-04 and
LEQSF(2008-11)-RD-B-10. We acknowledge the help from Dr
Daniela Caruntu for the XRD measurements.
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Fig. 5 (a) IV curves of gas sensors based on the WO
3
nanowire
array being exposed to air-diluted NO
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with different concentrations.
(b) Sensing response curves of the sensor.
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