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Journal of Nanomaterials
Volume 2008, Article ID 352854, 4 pages
http://dx.doi.org/10.1155/2008/352854
Research Article

Conductivity-Type Sensor Based on CNT- Composite for NO2 Detection

Department of Applied Chemistry, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan

Received 27 June 2007; Accepted 14 March 2008

Academic Editor: Y. Wang

Copyright © 2008 Takeshi Hashishin and Jun Tamaki. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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

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 [16]. To elucidate the effects of gas adsorption on the electrical properties of CNTs for gas sensing, it has been estimated that N and molecules would yield considerable larger adsorption energies than O, N , C , C , 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 N [913]. It is important to enhance the sensor response to N for future application of CNTs gas sensor. It was well known that W is an excellent sensing material for detection [14]. The conductive-type sensors using W have enhanced their sensor response to N by adopting thin film structure [1517] and by doping foreign oxides [18, 19]. Recently, we have developed the high sensitivity N sensor by employing disk shape W particles and Au interdigitated microelectrode [20, 21]. These W sensors can detect dilute N less than 1 ppm with high sensitivity. Interestingly, the modification of CNT with W nanoparticles would be nanocomposite with p-n junction and might give us new concept for effect of interaction between CNT and W on N detection.

In this paper, we modified the surface of CNT with the W grains with 300 nm in diameter and 50 nm in thickness to improve sensor response to N , and discuss the interaction between CNT and W when varied the additional amount of W 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  m, gap size of 5  m, and thickness of 0.3  m was formed on Si substrate by photolithography (lift off), as shown in Figure 1. Second, growth catalyst for CNTs, Ni was deposited on Au electrode with 5  m gap, in which 0.05 wt% Ni(C 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).

352854.fig.001
Figure 1: Schematic diagram of Au electrode with the gap of 5  m and line width of 20  m on Si substrate fabricated by means of photolithography (lift off).

The WO3 powder was prepared from (N )10 ·5 O by wet process. Aqueous solution of (N )10 ·5 O was neutralized by dilute nitric acid solution. The precipitate obtained ( W ) 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% W was directly dropped on the surface of CNTs grown between Au electrodes with microgap (5  m) 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 W 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 N (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 N -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 (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)).

fig2
Figure 2: (a) SEM image of CNT directly grown on Au microgap; (b) magnified view of (a) indicates the entangled CNTs contacting each other.

The SEM images of CNT-W composites with various W amounts are shown in Figure 3. In the CNT-W composite with 0.1 wt% W , W grains were independently trapped and CNTs were clearly observed (Figure 3(a)). The grain size of W trapped on CNTs was ranging from 50 to 200 nm and the grains were almost disk-like or platelet. With increasing W amount (Figures 3(b), 3(c)), W grains were increased and CNTs could not be visible, suggesting the formation of W connection.

fig3
Figure 3: SEM images of (a) CNT-0.1 wt% W composite microsensor; (b) CNT-1 wt% W composite microsensor; and (c) 7 wt% W composite microsensor.

Figure 4 shows the response transients of CNT and CNT-W microsensors to 5 ppm N at 200°C. The resistances of both CNT and CNT-W microsensors were decreased upon exposure to NO2, suggesting that the conduction occurs through p-type CNT in both CNT and CNT-W microsensors. The sensor response (Ra/Rg) of CNT-0.1 wt% W microsensor was as high as 3.8, while the CNT microsensor showed almost no response (Ra/Rg = 1.05).

352854.fig.004
Figure 4: Response transients of CNT and CNT-W microsensors to 5 ppm N at 200°C.

Figure 5 depicts the sensor resistance and the sensor response of CNTs-W composite microsensors as a function of amount of W . The resistance was steeply increased at 0.1 wt% W addition. After the maximum at 0.1 wt%, the resistance was gradually decreased with increasing W amount. This behaviour can be explained by the formation of p-n junction at 0.1 wt% and the W 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 W grains to generate the large depletion layer within CNT, inducing the large resistance of CNTs-W composite sensor. The highly depleted surface state of CNT resulted in the increasing amount of N adsorption on CNT and thus high sensor response to N of CNT-W composite sensor. When higher than 1 wt%, the conduction pass was formed via W grains to decrease the sensor resistance. At more than 1 wt% W , the W grains begin to contact with each other to dominant -type conduction pass due to W . It is well known that W is excellent sensing material for N detection. Although the W sensor shows the response of resistance-increase, the CNT-W (7 wt%) composite sensor exhibited no response to 5 ppm N (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 W . Finally, the reproducibility of sensor response to 5 ppm N was examined at 200°C for 0.1 and 1 wt% W -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.

352854.fig.005
Figure 5: The sensor resistance and the sensor response of CNTs-W composite microsensors as function of amount of W .

4. Conclusion

Conductivity-type gas sensor based on carbon nanotubes (CNTs)-W composite showed the fairly good sensor response (Ra/Rg) to dilute N , 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 N adsorption on the surface of CNT.

Acknowledgments

This work was partly supported by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (no. 17750136), from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, and the 21st Century COE Program, “Micro-Nano Science Integrated System”.

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