# Ambient Pressure Synthesis of Nanostructured Tungsten Oxide Crystalline Films

Authors: H. X. Zhang; B. Q. Yang; P. X. Feng
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

We report the results of the ambient pressure synthesis of tungsten oxide nanowires and nanoparticles on AlN substrates using the hot filament CVD techniques. The morphologic surface, crystallographic structures, chemical compositions, and bond structures of the obtained samples have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and Raman scattering, respectively. Different morphologies were observed for different substrate temperatures, but otherwise identical growth conditions. The experimental measurements reveal the evolutions of the crystalline states and bond structures following the substrate temperatures. Besides, different substrate materials also affected the tungsten oxide nanostructures. Bundles of wire-type tungsten oxide nanowires with a length of up to 5 mm were obtained onAl2O3 substrate. Furthermore, the sensitive properties of the super long nanowires to the gas and different temperature were investigated. The dependence of the sensitivity of tungsten oxide nanowires to the methane as a function of the time was obtained. The sensitive properties of the tungsten oxide nanowires have almost linear relationship with the temperature.


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## Body

## 1. Introduction

Over the last decade, nanostructured tungsten oxide materials have
attracted much interest due to their potential for
catalyst [1], gas sensors [2, 3], and electrochromic material  applications [4].
Several studies have been conducted by using various
techniques, including the oxygen plasma processing [5], plasma
sputtering [6], chemical
solution [7, 8], sol-gel
techniques [9], electron beam
evaporation deposition [10],
electrochemical etching [11],
and the chemical vapor deposition techniques [12]. Most work concerned the synthesis of nanoparticles for catalytic applications based on chemical solution methods [4, 7–9].Using a simple hot filament CVD technique, we previously synthesized various
nanostructured tungsten oxide (WO3) films [13, 14]. The advantage of this technique was that both a.c. and d.c.
electrical power supply could be used for heating the substrates. Low electrical power was used, less than 80 W. The temperature of the
substrate was easily controlled by adjusting the distance between the substrate and the hot filament, and no extra
heater was  needed.In the present work, we address ambient
pressure syntheses of tungsten oxide nanoparticles and nanowires, and then the demonstration of their sensing
applications. The main advantages
of the ambient pressure deposition technique are that the temperature and density of plasma can be independently
controlled during deposition. Furthermore, the experimental set-up is easy and effective,
and the size of the sample is virtually unlimited. The effect of substrate temperature
on the nanostructures of tungsten oxidefilms has also been investigated. Preliminary
results of sensing material applications have been obtained.


## 2. Experimental Setup

The nanostructured tungsten oxide materials were synthesized using a simple hot
filament CVD technique. The details of the
process were described elsewhere in our previous publications [13, 14]. The tungsten filament acted as a precursor for tungsten oxide, and no catalyst or other tungsten-containing
compound precursor was used. Both AlN and Al2O3 ceramic substrates were used. Prior to the experiments, the substrates were ultrasonically washed in the methanol solution for 5 minutes, and dried
with helium.  After placing the substrate, the chamber was pumped down to 2.0×10−5 Torr and
then fed with the Ar gas (purity: 75%) to ambient pressure. During deposition, the gas inside the chamber was in a static state.
The distance between the hot filament and substrate remained unchanged. The
substrate temperature was
controlled by adjusting electrical current
on the hot filament, which was different from our previous experiments where the substrate temperature was controlled by simply changing the distance between the substrate and the hot filament.


## 3. Results and Discussions

### 3.1. Grows and Characterizations

Figure1
shows typical SEM images of the nanostructured tungsten oxide films deposited
on the AlN substrates at the substrate temperature
of (a) 400°C and (b) 1000°C, respectively. The experiments
were conducted at ambient pressure and the duration for each sample was 30
minutes. Different morphologies were observed for different substrate
temperatures, but otherwise identical growth conditions.SEM images of the films prepared at substrate temperature of (a) 400°C and (b) 1000°C.
(a)(b)At 400°C, nanowires with an average diameter of
about 200 nm and length of about 1 μm are uniformly distributed on the surface of sample (Figure 1(a)). Raising the substrate
temperature to 1000°C, the
submicrometer size, well-shaped particles appear (Figure 1(b)).From the naked eyes, the color of sample at 400°C appears green and dull. In contrast, following growth of substrate temperature to 1000°C, slightly shiny surface was obtained. It should be mentioned that with
further increase of substrate temperature, the color of obtained film would
become white, and the particles on the surface of the substrate look like
diamond particles, very shiny.In general, the growth of thin film may follow three types of models:
layer by layer growth of Franck and Van der Merwe (FM), initial layer growth
followed by island growth of Stranski and Krastanov (SK), and the island growth
of Volmer and Weber (VW). A determinant factor is the competition among the surface free energies of the overlayerEo,
the substrate Es, and the overlayer/substrate interface free energy
Ei. The common model is the case that the substrate free energies is low and Es<Eo+Ei,
where three dimensional island
growth (VW) occurs [14]. The epitaxial growth of tungsten
oxide was conducted, where the lattice constants differ from the AlN substrates, strains were introduced into
the layer, which prevents epitaxial growth of thicker layers without defects [14]. Since we have large misfit between the
film and the substrate, our case can
possibly be considered as a VW model. Furthermore, the misfit in lattices between films and substrates can be enhanced with the
increase of substrate temperature. Basically, the overlayer will grow with a
stacking fault after a first coherent layer.When the film was much
thicker than critical thickness, the initial smooth surface of the film will become the fractal-like
structure, and eventually the thin film was transformed to nanoparticles
shown in Figure 1.Energy dispersive X-ray spectroscopy
(EDX) measurements were also conducted to quantitatively analyze the chemical
composition of the tungsten oxide samples (shown in Figures1(a) and 1(b)). The oxygen and
tungsten atom concentrations are approximately 62.47 at% and 30.21 at%, respectively,
for the first sample (Figure 1(a)). For the second one (Figure 1(b)), the oxygen concentration drops down to 55.78 at%, whereas the relative tungsten
content increases up to 41.62 at%. The oxygen and tungsten concentration decreases from 2.07
to 1.34 from 400°C to 1000°C.Clearly, the concentration ratio O/W is high at low temperature
of deposition. With the increase of the substrate temperature, the
concentration ratio O/W decreases, probably resulting in a significant effect
on the crystalline structures and the phase of the present two samples. In fact, the following XRD data have also shown distinct properties related to relative oxygen
content inside the films.Figure2 shows XRD patterns of the two
tungsten oxide samples in Figures 1(a) and 1(b). WO2, WO3, and W peaks in XRD
spectra of the films have been identified based on the literatures [15, 16]. The present two samples exhibit mixed states including WO2, WO3,
and W. The polycrystalline
WO3 dominates the composition of the sample prepared at 400°C.
Evidence is that XRD peaks of WO3 with (002) and (200) orientations are much stronger than those of WO2 and W peaks. At 1000°C, WO2 signals remain nearly
unchanged but the W components become dominant with the vanishing of WO3 components.
The XRD profiles of the films provide the direct evidence that the states of tungsten oxide films
transfer from polycrystalline WO3 domination to polycrystalline W domination.Figure 2
XRD of the tungsten oxide films corresponding to the samples shown in
Figure1.Raman scattering spectra of the tungsten
oxide films were also obtained at room temperature by using a triple monochromator (ISA J-Y Model T64000)
with an excitation
wavelength of 514 nm (Ar+ ion laser). The samples were scanned
from 50 cm-1 to 1200 cm-1. Several peaks marked with J and k in the Raman spectra were identified,
respectively. In general, the bands situated at around 700 and 800 cm-1 can be assigned to W-O
stretching models, whereas the bands
situated at around 130 and 270 cm-1 are associated to W-O bending modes
of monoclinic WO3 [16, 17]. It was also
reported that J bands at 270 and 330 cm-1 can also be assigned to W-O bending modes
of monoclinic WO3 [18].  According to our case, J1, J2,
and J3 bands in Raman spectra of all our tungsten oxide samples
always coexist. Therefore, the present three bands have been
assigned to J-bands associated to W-O bending modes
of monoclinic WO3.Setting substrate temperature up to 1000°C, both
K bands and J bands were obviously broadened, and their signal-to-noise ratios
greatly reduced, suggesting changes of states of the samples. This is in
agreement with the data obtained from XRD above. A shoulder close to higher
wave number of WO3K bands is also clearly visible. According to
literature, this shoulder (Raman band) should be related to the contribution
from W component [16].Slightly blue shifts of k bands from 711 to 719 cm-1 for k1 and from 806
to 808 cm-1 for k2 following an increase of the substrate temperature from 400 to 1000°C
have been observed from Figure 3. Generally, the blue shifts of the Raman peaks probably
result from the change in crystal symmetry or the high stress of the particles. The variations
of content of WO2 and W inside unavoidably change crystal symmetry of WO3.
However, referring to the previous work [19], the stress due
to mismatch and the different thermal expansion between the substrate and
overlayer is qualitatively associated with increasing the shift and width of the band. The blue shifts of
the samples are also due to the high stress of particles
caused by the high temperature’s deposition process.Figure 3
Raman spectra of the films at 400°C and 1000°C.Besides, it was found that different substrate materials also affected
the tungsten oxide structures. Figure4 shows the super long tungsten oxide nanowires deposited on Al2O3 substrates
at ambient pressure with the substrate temperature 1000°C. Bundles of wire-type tungsten oxide nanostructures with a length of up to 5 mm are distributed on the surface of Al2O3 substrate
(Figure 4(a)). The average diameter of the nanowires is around 850 nm.SEM images and
Raman spectrum of the super long tungsten oxide wire-type nanostructures deposited on
Al2O3 substrates
at ambient pressure at 1000°C.
(a)(b)From SEM images in Figure4, it can be
observed that each wire
consists of good-shaped segments. Typical
WO3 Raman spectrum has been obtained. The bands situated at 717.8 and 806.5 cm-1 are assigned to W-O stretching models, whereas the
bands situated at 133.9, 272.1, and 327.3 cm-1 are associated to W-O bending modes
of monoclinic WO3. The synthesis of super long tungsten oxide nanowires are probably
related to the chemical reactions between the Al2O3 substrates and the tungsten
oxide materials.


### 3.2. The Sensitive Properties

Furthermore, the sensitive properties of the
super long nanowires were also investigated as an example. A
prototypic sensor was constructed, which consisted of a voltage-current-resistor
(V-I-R) electrical circuit. The tungsten oxide nanowires were serially connected to a precise resistor and a power supply. The variations of the voltage across the precise
resistor were detected by using WaveSurfer 452 oscilloscope LeCroy Inc. (NY, USA), from which the
variations of resistances or conductivities of tungsten oxide nanowires could be estimated. The measurements of the electrical conductivity were
conducted in order to understand the variations of the sensitive properties of the sensor at
different conditions.For traditional sensors, a disadvantage is that the sensitivity of the sensor mainly relies
on the operating temperature. The extra heater installed inevitably leads to the
complicated geometry of the prototypic
sensor [20, 21]. The following
measurements were conducted at room temperature, and the sensitivity of the prototypic sensor to
the gas was obtained.A spraying nozzle was used to eject liquid methane
as fine particles or gas to the surface of the nanowires. Flow
distribution was determined by gas (air) and liquid superficial velocities. The electrical conductivities of the sensor during the whole process were recorded by National
Instruments LabVIEW software.The dependence of the sensitivity of tungsten oxide nanowires as
a function of the time is shown in Figure5. The response time
(rise time) is about 90 ± 0.5 milliseconds, and recovery
time of the sensor is around 550 ± 0.5 milliseconds, which depends on the solvent, gas,
liquid superficial velocities from spraying nozzle, and spraying duration.Figure 5
Evolution of sensitivity of nanowires after spraying methane gas.After heavily spray of liquid methane, an obvious multipeak type of sensitive variation would be observed, which was possibly caused by piston vibration inside the
valve. Similar
phenomenon was observed in our previous supersonic atomic beam experiments [22]. In general, the width and density of the second peak in the pulsed
atomic beam profile are much smaller than the first one. Meanwhile, a weak spray would yield a
single peak of molecular pulse,
which consequently led to the weak response signal of the
prototypical sensor.Figure6 shows the dependence of the resistance
of tungsten oxide nanowires as a function of temperature. Clearly,
the resistance of nanowires reduces from 1.75 MΩ
to 1.49 MΩ with an increase of the temperature from 45°C to
80°C under atmosphere condition. Signal output of
the prototypical sensor has almost linear relationship with the temperature.Figure 6
Variation of resistance of nanowires following the
increase of temperature.


## 3.1. Grows and Characterizations

Figure1
shows typical SEM images of the nanostructured tungsten oxide films deposited
on the AlN substrates at the substrate temperature
of (a) 400°C and (b) 1000°C, respectively. The experiments
were conducted at ambient pressure and the duration for each sample was 30
minutes. Different morphologies were observed for different substrate
temperatures, but otherwise identical growth conditions.SEM images of the films prepared at substrate temperature of (a) 400°C and (b) 1000°C.
(a)(b)At 400°C, nanowires with an average diameter of
about 200 nm and length of about 1 μm are uniformly distributed on the surface of sample (Figure 1(a)). Raising the substrate
temperature to 1000°C, the
submicrometer size, well-shaped particles appear (Figure 1(b)).From the naked eyes, the color of sample at 400°C appears green and dull. In contrast, following growth of substrate temperature to 1000°C, slightly shiny surface was obtained. It should be mentioned that with
further increase of substrate temperature, the color of obtained film would
become white, and the particles on the surface of the substrate look like
diamond particles, very shiny.In general, the growth of thin film may follow three types of models:
layer by layer growth of Franck and Van der Merwe (FM), initial layer growth
followed by island growth of Stranski and Krastanov (SK), and the island growth
of Volmer and Weber (VW). A determinant factor is the competition among the surface free energies of the overlayerEo,
the substrate Es, and the overlayer/substrate interface free energy
Ei. The common model is the case that the substrate free energies is low and Es<Eo+Ei,
where three dimensional island
growth (VW) occurs [14]. The epitaxial growth of tungsten
oxide was conducted, where the lattice constants differ from the AlN substrates, strains were introduced into
the layer, which prevents epitaxial growth of thicker layers without defects [14]. Since we have large misfit between the
film and the substrate, our case can
possibly be considered as a VW model. Furthermore, the misfit in lattices between films and substrates can be enhanced with the
increase of substrate temperature. Basically, the overlayer will grow with a
stacking fault after a first coherent layer.When the film was much
thicker than critical thickness, the initial smooth surface of the film will become the fractal-like
structure, and eventually the thin film was transformed to nanoparticles
shown in Figure 1.Energy dispersive X-ray spectroscopy
(EDX) measurements were also conducted to quantitatively analyze the chemical
composition of the tungsten oxide samples (shown in Figures1(a) and 1(b)). The oxygen and
tungsten atom concentrations are approximately 62.47 at% and 30.21 at%, respectively,
for the first sample (Figure 1(a)). For the second one (Figure 1(b)), the oxygen concentration drops down to 55.78 at%, whereas the relative tungsten
content increases up to 41.62 at%. The oxygen and tungsten concentration decreases from 2.07
to 1.34 from 400°C to 1000°C.Clearly, the concentration ratio O/W is high at low temperature
of deposition. With the increase of the substrate temperature, the
concentration ratio O/W decreases, probably resulting in a significant effect
on the crystalline structures and the phase of the present two samples. In fact, the following XRD data have also shown distinct properties related to relative oxygen
content inside the films.Figure2 shows XRD patterns of the two
tungsten oxide samples in Figures 1(a) and 1(b). WO2, WO3, and W peaks in XRD
spectra of the films have been identified based on the literatures [15, 16]. The present two samples exhibit mixed states including WO2, WO3,
and W. The polycrystalline
WO3 dominates the composition of the sample prepared at 400°C.
Evidence is that XRD peaks of WO3 with (002) and (200) orientations are much stronger than those of WO2 and W peaks. At 1000°C, WO2 signals remain nearly
unchanged but the W components become dominant with the vanishing of WO3 components.
The XRD profiles of the films provide the direct evidence that the states of tungsten oxide films
transfer from polycrystalline WO3 domination to polycrystalline W domination.Figure 2
XRD of the tungsten oxide films corresponding to the samples shown in
Figure1.Raman scattering spectra of the tungsten
oxide films were also obtained at room temperature by using a triple monochromator (ISA J-Y Model T64000)
with an excitation
wavelength of 514 nm (Ar+ ion laser). The samples were scanned
from 50 cm-1 to 1200 cm-1. Several peaks marked with J and k in the Raman spectra were identified,
respectively. In general, the bands situated at around 700 and 800 cm-1 can be assigned to W-O
stretching models, whereas the bands
situated at around 130 and 270 cm-1 are associated to W-O bending modes
of monoclinic WO3 [16, 17]. It was also
reported that J bands at 270 and 330 cm-1 can also be assigned to W-O bending modes
of monoclinic WO3 [18].  According to our case, J1, J2,
and J3 bands in Raman spectra of all our tungsten oxide samples
always coexist. Therefore, the present three bands have been
assigned to J-bands associated to W-O bending modes
of monoclinic WO3.Setting substrate temperature up to 1000°C, both
K bands and J bands were obviously broadened, and their signal-to-noise ratios
greatly reduced, suggesting changes of states of the samples. This is in
agreement with the data obtained from XRD above. A shoulder close to higher
wave number of WO3K bands is also clearly visible. According to
literature, this shoulder (Raman band) should be related to the contribution
from W component [16].Slightly blue shifts of k bands from 711 to 719 cm-1 for k1 and from 806
to 808 cm-1 for k2 following an increase of the substrate temperature from 400 to 1000°C
have been observed from Figure 3. Generally, the blue shifts of the Raman peaks probably
result from the change in crystal symmetry or the high stress of the particles. The variations
of content of WO2 and W inside unavoidably change crystal symmetry of WO3.
However, referring to the previous work [19], the stress due
to mismatch and the different thermal expansion between the substrate and
overlayer is qualitatively associated with increasing the shift and width of the band. The blue shifts of
the samples are also due to the high stress of particles
caused by the high temperature’s deposition process.Figure 3
Raman spectra of the films at 400°C and 1000°C.Besides, it was found that different substrate materials also affected
the tungsten oxide structures. Figure4 shows the super long tungsten oxide nanowires deposited on Al2O3 substrates
at ambient pressure with the substrate temperature 1000°C. Bundles of wire-type tungsten oxide nanostructures with a length of up to 5 mm are distributed on the surface of Al2O3 substrate
(Figure 4(a)). The average diameter of the nanowires is around 850 nm.SEM images and
Raman spectrum of the super long tungsten oxide wire-type nanostructures deposited on
Al2O3 substrates
at ambient pressure at 1000°C.
(a)(b)From SEM images in Figure4, it can be
observed that each wire
consists of good-shaped segments. Typical
WO3 Raman spectrum has been obtained. The bands situated at 717.8 and 806.5 cm-1 are assigned to W-O stretching models, whereas the
bands situated at 133.9, 272.1, and 327.3 cm-1 are associated to W-O bending modes
of monoclinic WO3. The synthesis of super long tungsten oxide nanowires are probably
related to the chemical reactions between the Al2O3 substrates and the tungsten
oxide materials.


## 3.2. The Sensitive Properties

Furthermore, the sensitive properties of the
super long nanowires were also investigated as an example. A
prototypic sensor was constructed, which consisted of a voltage-current-resistor
(V-I-R) electrical circuit. The tungsten oxide nanowires were serially connected to a precise resistor and a power supply. The variations of the voltage across the precise
resistor were detected by using WaveSurfer 452 oscilloscope LeCroy Inc. (NY, USA), from which the
variations of resistances or conductivities of tungsten oxide nanowires could be estimated. The measurements of the electrical conductivity were
conducted in order to understand the variations of the sensitive properties of the sensor at
different conditions.For traditional sensors, a disadvantage is that the sensitivity of the sensor mainly relies
on the operating temperature. The extra heater installed inevitably leads to the
complicated geometry of the prototypic
sensor [20, 21]. The following
measurements were conducted at room temperature, and the sensitivity of the prototypic sensor to
the gas was obtained.A spraying nozzle was used to eject liquid methane
as fine particles or gas to the surface of the nanowires. Flow
distribution was determined by gas (air) and liquid superficial velocities. The electrical conductivities of the sensor during the whole process were recorded by National
Instruments LabVIEW software.The dependence of the sensitivity of tungsten oxide nanowires as
a function of the time is shown in Figure5. The response time
(rise time) is about 90 ± 0.5 milliseconds, and recovery
time of the sensor is around 550 ± 0.5 milliseconds, which depends on the solvent, gas,
liquid superficial velocities from spraying nozzle, and spraying duration.Figure 5
Evolution of sensitivity of nanowires after spraying methane gas.After heavily spray of liquid methane, an obvious multipeak type of sensitive variation would be observed, which was possibly caused by piston vibration inside the
valve. Similar
phenomenon was observed in our previous supersonic atomic beam experiments [22]. In general, the width and density of the second peak in the pulsed
atomic beam profile are much smaller than the first one. Meanwhile, a weak spray would yield a
single peak of molecular pulse,
which consequently led to the weak response signal of the
prototypical sensor.Figure6 shows the dependence of the resistance
of tungsten oxide nanowires as a function of temperature. Clearly,
the resistance of nanowires reduces from 1.75 MΩ
to 1.49 MΩ with an increase of the temperature from 45°C to
80°C under atmosphere condition. Signal output of
the prototypical sensor has almost linear relationship with the temperature.Figure 6
Variation of resistance of nanowires following the
increase of temperature.


## 4. Conclusions

Tungsten oxide nanoparticles and nanowires
were synthesized at ambient pressure on AlN substratesusing the hot filament CVD techniques. The amount of 400°C
of deposition yields WO3 film consisting of nanowires with an
average diameter and length of about 200 nm
and 1 μm,
respectively. Increasing substrate temperature results in submicrometer size
particles. The
profiles of XRD of the films provide evidence of the mixture state changes from
polycrystalline WO3 domination to polycrystalline W domination.
Typical EDX measurements of the
films indicated that the concentration
ratio O/W decreases from 2.07 to 1.34 from 400°C to 1000°C. Up to 5 mm super
long tungsten oxide nanowires with diameter of about 850 nm were obtained on Al2O3 substrate at 1000°C, but
otherwise identical growth condition.Experiments have proved that tungsten oxide nanowires can be
used for sensing applications. The resistance
of nanowires reduces from 1.75 MΩ to 1.49 MΩ with an increase
of the temperature from 45°C to 80°C under atmosphere condition. After spray of liquid methane, the rise time of the
signal is about 90 milliseconds,
and the recovery time for the sensor is around 550 milliseconds.



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*Source: 957935-2008-12-04.xml*