Conductivity-Type Sensor Based on CNT-WO 3 Composite for NO 2 Detection

The CNTs with 20–50 nm in diameter were directly grown on Au microgap electrode by means of thermal CVD at 700◦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.


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][2][3][4][5][6].To elucidate the effects of gas adsorption on the electrical properties of CNTs for gas sensing, it has been estimated that NO 2 and O 2 molecules would yield considerable larger adsorption energies than H 2 O, NH 3 , CH 4 , CO 2 , 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 NO 2 [9][10][11][12][13].It is important to enhance the sensor response to NO 2 for future application of CNTs gas sensor.It was well known that WO 3 is an excellent sensing material for NO x detection [14].The conductive-type sensors using WO 3 have enhanced their sensor response to NO 2 by adopting thin film structure [15][16][17] and by doping foreign oxides [18,19].Recently, we have developed the high sensitivity NO 2 sensor by employing disk shape WO 3 particles and Au interdigitated microelectrode [20,21].These WO 3 sensors can detect dilute NO 2 less than 1 ppm with high sensitivity.Interestingly, the modification of CNT with WO 3 nanoparticles would be nanocomposite with p-n junction and might give us new concept for effect of interaction between CNT and WO 3 on NO 2 detection.
In this paper, we modified the surface of CNT with the WO 3 grains with 300 nm in diameter and 50 nm in thickness to improve sensor response to NO 2 , and discuss the interaction between CNT and WO 3 when varied the additional amount of WO 3 to CNT.

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 SiO 2 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(CH 3 COO) 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).The CNTs-WO 3 composite microsensor was set into a flow apparatus equipped with electric furnace and the sensing properties to dilute NO 2 (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 NO 2 -containing atmosphere (Rg).

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 H 2 (6 kPa) have entangled shape, as shown in Figures 2(a The SEM images of CNT-WO 3 composites with various WO 3 amounts are shown in Figure 3.In the CNT-WO 3 com- Figure 4 shows the response transients of CNT and CNT-WO 3 microsensors to 5 ppm NO 2 at 200 • C. The resistances of both CNT and CNT-WO 3 microsensors were decreased upon exposure to NO 2 , suggesting that the conduction occurs through p-type CNT in both CNT and CNT-WO 3 microsensors.The sensor response (Ra/Rg) of CNT-0.1 wt% WO 3 microsensor was as high as 3.8, while the CNT microsensor showed almost no response (Ra/Rg = 1.05).
Figure 5 depicts the sensor resistance and the sensor response of CNTs-WO 3 composite microsensors as a function of amount of WO 3 .The resistance was steeply increased at 0.1 wt% WO 3 addition.After the maximum at 0.1 wt%, the resistance was gradually decreased with increasing WO 3 amount.This behaviour can be explained by the formation of p-n junction at 0.1 wt% and the WO 3 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 WO 3 grains to generate the large depletion layer within CNT, inducing the large resistance of CNTs-WO 3 composite sensor.The highly depleted surface state of CNT resulted in the increasing amount of NO 2 adsorption on CNT and T. Hashishin and J. Tamaki   Resistance in air (GΩ) Sensor response (Ra/Rg) Resistance in air thus high sensor response to NO 2 of CNT-WO 3 composite sensor.When higher than 1 wt%, the conduction pass was formed via WO 3 grains to decrease the sensor resistance.At more than 1 wt% WO 3 , the WO 3 grains begin to contact with each other to dominant n-type conduction pass due to WO 3 .It is well known that WO 3 is excellent sensing material for NO 2 detection.Although the WO 3 sensor shows the response of resistance-increase, the CNT-WO 3 (7 wt%) composite sensor exhibited no response to 5 ppm NO 2 (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 WO 3 .Finally, the reproducibility of sensor response to 5 ppm NO 2 was examined at 200 • C for 0.1 and 1 wt% WO 3 -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.

CONCLUSION
Conductivity-type gas sensor based on carbon nanotubes (CNTs)-WO 3 composite showed the fairly good sensor response (Ra/Rg) to dilute NO 2 , 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 NO 2 adsorption on the surface of CNT.

Figure 1 :
Figure 1: Schematic diagram of Au electrode with the gap of 5 μm and line width of 20 μm on SiO 2 substrate fabricated by means of photolithography (lift off).
), 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)).

Figure 2 :
Figure 2: (a) SEM image of CNT directly grown on Au microgap; (b) magnified view of (a) indicates the entangled CNTs contacting each other.

Figure 5 :
Figure 5: The sensor resistance and the sensor response of CNTs-WO 3 composite microsensors as function of amount of WO 3 .