Metal oxides have often been utilized for commercial gassensor materials.[1] In spite of their high sensitivity, their high power consumption due to high resistance has been a serious drawback. Conducting polymers have also been tried, since they operate at room temperature and have good sensitivity and reproducibility. However, the effects of humidity and degradation by ultraviolet irradiation, in particular, have hindered further practical applications. [2] Recently, carbon nanotube (CNT)-based gas sensors have received a great deal of attention. Nanosized CNT-based gas sensors of the field-effect-transistor type have superb sensitivity at room temperature due to a drastic change in the electrical conductivity upon the adsorption of various gases. [3,4] Despite such advantages, however, their application is still limited by a long recovery time and a complex fabrication process. In particular, the single-walled carbon nanotubes (SWNTs) used for gas sensors must be semiconducting.[5] However, the presence of both metallic and semiconducting carbon nanotubes in conventional powder samples reduces the reproducibility and/or yield of the devices. The issue here is to introduce an easy fabrication process of gas sensors whilst still retaining high sensitivity. To meet this criterion, we fabricated a gas sensor from a nanocomposite by polymerizing pyrrole monomer with SWNTs. Polypyrrole (Ppy) was prepared by a simple and straightforward in situ chemical polymerization of pyrrole mixed with SWNTs, and the sensor electrodes were formed by spin-casting SWNT/Ppy onto pre-patterned electrodes. Ppy was uniformly coated on the wall of the SWNTs to increase the specific surface area. The measured resistivity was greatly reduced due to the presence of the conductive SWNT network, whereas the specific surface area was increased about threefold. The sensitivity of the gas sensor fabricated with the SWNT/Ppy nanocomposite towards NO 2 gas, as measured by a direct voltage divider at room temperature, was very high and similar to that of the fabricated SWNTs alone.[3] Figure 1 shows the typical field-emission scanning electron microscope (FESEM) images of the Ppy, SWNTs, and SWNT/ Ppy nanocomposite formed by a simple in situ chemical polymerization. Pure Ppy synthesized without SWNTs (Fig. 1a) shows a typical granular morphology. The granule size of the pure Ppy is about 0.2±0.3 lm. The purified SWNTs shown in Figure 1b are entangled and crosslinked with a typical bundle diameter of 20 nm. The pyrrole monomer becomes anchored to the carbon nanotube walls during polymerization, covering them completely, as shown in Figure 1c. The inset of this figure shows the shape of the electrodes formed on the substrate. The two electrodes are separated by 500 lm. The specific surface areas of Ppy, SWNTs, and SWNT/Ppy nanocomposite were 23, 100, 65 m 2 g ±1 , respectively. [6] The surface area of the COMMUNICATIONS
