# Conductivity-Type Sensor Based on  CNT-WO3 Composite for

Authors: Takeshi Hashishin; Jun Tamaki
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

The CNTs with 20–50 nm in diameter were directly grown on Au microgap electrode by means of thermal CVD at700∘C for 60 minutes under EtOH-Ar-H2 atmosphere (6 kPa). The CNTs with entangled shape formed the network structure with contacting each other. In the CNTs-WO3 composite, WO3 grains with disk shape (50–200 nm) were independently trapped. The CNTs-WO3 composite sensor showed the fairly good sensor response (Ra/Rg = 3.8 at 200∘C). The sensor response was greatly improved with CNTs-WO3 composite, comparing with that of CNT sensor (Ra/Rg = 1.05). This phenomenon can be explained by formation of p-n junction, between CNT(p) and WO3(n), and thus improvement of NO2 adsorption. The sensor response was decreased with increasing the WO3 amount in CNTs-WO3 composite, suggesting the electronic conduction due to WO3 connection.


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## Body

## 1. Introduction

Conductivity-type gas sensors based on carbon nanotubes (CNTs) have received
considerable attention because of their intrinsic properties such as high-surface
area, size, hollow geometry, and chemical inertness [1–6]. To elucidate
the effects of gas adsorption on the electrical properties of CNTs for gas
sensing, it has been estimated that NO2 and O2 molecules
would yield considerable larger adsorption energies than H2O, NH3,
CH4, CO2, and so on [7, 8]. However, it has been reported
that sensor response (Ra/Rg), which is used for resistance decrease of sensing
materials by adsorption of gas molecule, was as low as 1.2, or 1.3 to 5 ppm NO2 [9–13]. It is
important to enhance the sensor response to NO2 for future
application of CNTs gas sensor. It was well known that WO3 is an
excellent sensing material for NOx detection [14].
The conductive-type sensors using WO3 have enhanced their sensor
response to NO2 by adopting thin film structure [15–17] and by doping
foreign oxides [18, 19]. Recently, we have developed the high sensitivity NO2 sensor by employing disk shape WO3 particles and Au interdigitated
microelectrode [20, 21]. These WO3 sensors can detect dilute NO2 less than 1 ppm with high sensitivity. Interestingly, the modification of CNT
with WO3 nanoparticles would be nanocomposite with p-n junction and
might give us new concept for effect of interaction between CNT and WO3 on NO2 detection.In this paper, we modified the
surface of CNT with the WO3 grains with 300 nm in diameter and 50 nm
in thickness to improve sensor response to NO2, and discuss the
interaction between CNT and WO3 when varied the additional amount of
WO3 to CNT.


## 2. Experimental

At first, the microgap
electrodes with various gap sizes were fabricated by means of MEMS techniques
[16]. The Au line with width of 20 μm, gap size of 5 μm,
and thickness of 0.3 μm was formed on SiO2 substrate by photolithography (lift off), as
shown in Figure 1. Second, growth catalyst for CNTs, Ni was deposited on Au
electrode with 5 μm gap,
in which 0.05 wt% Ni(CH3COO)2 aqueous solution was
dropped by using microinjector, and dried at room temperature for 30 minutes.
The Ni-deposited substrate was subsequently set on the electric furnace, and
CNTs were grown on the microgap at 700°C for 60 minutes from the
nickel growth catalyst under gas mixture of ethanol, argon, and hydrogen
(33/53/14 vol% = 6 kPa).Figure 1
Schematic diagram of Au electrode with the gap
of 5μm and line width of 20 μm on SiO2 substrate fabricated by
means of photolithography (lift off).The WO3 powder was prepared from (NH4)10W12O41·5H2O by
wet process. Aqueous solution of (NH4)10W12O41·5H2O was neutralized by dilute nitric acid solution. The precipitate obtained (H2WO4)
was thoroughly washed with deionized water, dried, and dispersed into deionized
water to be a suspension. The microdrop of suspension ranged from 0.1 to 7 wt%
H2WO4 was directly dropped on the surface of CNTs grown
between Au electrodes with microgap (5 μm) by using microinjection, dried, and
calcined at 400°C for 3 hours under inert gas of argon to prevent
oxidation of CNTs. The microstructure of the WO3 trapped on CNTs
microsensor was measured by high-resolution FE-SEM (S-4800, Hitachi Ltd.),  TEM (JEM-2010, Jeol Ltd.), and Raman spectroscopy (NRS-2100, JASCO Co. Ltd.).The CNTs-WO3 composite microsensor was set into a flow apparatus equipped with electric
furnace and the sensing properties to dilute NO2 (5 ppm) were
measured at room temperature to 200°C. The sensor
response (S = Ra/Rg) was defined as a ratio of resistance in air (Ra) to that in
NO2-containing atmosphere (Rg).


## 3. Results and Discussion

The CNTs with 20–50 nm in diameter
were grown at 700°C for 60 minutes
under gas mixture of ethanol, argon, and H2 (6 kPa) have entangled
shape, as shown in Figures  2(a), 2(b).
The microstructural analysis by means of Raman and TEM techniques indicated
that the ratio of G- to D-band was mostly closed to 1 and they had multiwalled
carbon layers. The entangled CNTs formed the network structure with contacting
each other (Figure 2(b)).(a) SEM image of CNT directly grown on Au
microgap; (b) magnified view of (a) indicates the entangled CNTs contacting
each other.
(a)(b)The SEM images of
CNT-WO3 composites with various WO3 amounts are shown in
Figure 3. In the CNT-WO3 composite with 0.1 wt% WO3, WO3 grains were independently trapped and CNTs were clearly observed (Figure 3(a)).
The grain size of WO3 trapped on CNTs was ranging from 50 to 200 nm
and the grains were almost disk-like or platelet. With increasing WO3 amount (Figures 3(b), 3(c)), WO3 grains were increased and CNTs
could not be visible, suggesting the formation of WO3 connection.SEM images of (a) CNT-0.1 wt% WO3 composite microsensor; (b)
CNT-1 wt% WO3 composite microsensor; and (c) 7 wt% WO3 composite microsensor.
(a)(b)(c)Figure4 shows the
response transients of CNT and CNT-WO3 microsensors to 5 ppm NO2 at 200°C. The resistances
of both CNT and CNT-WO3 microsensors were decreased upon exposure to
NO2, suggesting that the conduction occurs through p-type CNT in
both CNT and CNT-WO3 microsensors. The sensor response (Ra/Rg) of
CNT-0.1 wt% WO3 microsensor was as high as 3.8, while the CNT
microsensor showed almost no response (Ra/Rg = 1.05).Figure 4
Response transients of CNT and CNT-WO3 microsensors to 5 ppm NO2 at 200°C.Figure5 depicts the
sensor resistance and the sensor response of CNTs-WO3 composite
microsensors as a function of amount of WO3. The resistance was
steeply increased at 0.1 wt% WO3 addition. After the maximum at 0.1 wt%, the
resistance was gradually decreased with increasing WO3 amount. This behaviour can be explained by the formation of p-n junction at 0.1 wt%
and the WO3 connection higher than 1 wt%. The similar behaviour
was observed for the sensor response, which had the maximum at 0.1 wt%. At 0.1 wt%, the p-n junction was formed between CNT and WO3 grains to
generate the large depletion layer within CNT, inducing the large resistance of
CNTs-WO3 composite sensor. The highly depleted surface state of CNT
resulted in the increasing amount of NO2 adsorption on CNT and thus
high sensor response to NO2 of CNT-WO3 composite sensor.
When higher than 1 wt%, the conduction pass was formed via WO3 grains to decrease the sensor resistance. At more than 1 wt% WO3,
the WO3 grains begin to contact with each other to dominant n-type
conduction pass due to WO3. It is well known that WO3 is
excellent sensing material for NO2 detection. Although the WO3 sensor shows the response of resistance-increase, the CNT-WO3 (7 wt%)
composite sensor exhibited no response to 5 ppm NO2 (Ra/Rg = 1).
It is considered that the sensor response (Ra/Rg) would be decreased to less
than unity (resistance increase) at higher amount of WO3.  Finally, the reproducibility of sensor
response to 5 ppm NO2 was examined at 200°C for 0.1 and 1 wt% WO3-CNT composites. As the result, the sensor response of both
composites was, respectively, 3.6 and 1.4, which was closed to the plotted data
of Figure 5.Figure 5
The sensor resistance and the sensor response of CNTs-WO3 composite microsensors as  function of
amount of WO3.


## 4. Conclusion

Conductivity-type gas sensor based on carbon nanotubes (CNTs)-WO3 composite showed the fairly good sensor response (Ra/Rg) to dilute NO2,
comparing that the sensor fabricated from only CNT exhibited almost no
response. The large depletion layer due to p-n junction was formed on CNT,
inducing the enhancement of NO2 adsorption on the surface of CNT.



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*Source: 352854-2008-04-14.xml*