Synthesis And Gas Sensitivity Of Aligned Carbon Nanotube Array Film

Carbon nanotube (CNT) is a kind of typical one-dimensional nanomaterial. Due to its special structure, CNTs have good absorption properties for gas. The gas sensor made from CNTs has better sensitivity, lower working temperature and smaller dimension. Vertically-aligned CNTs film displays excellent gas-sensitivity. However, there is still lack of a mature technique for fabrication of CNT based sensor devices. The key to the application of CNTs array as gas sensors is how to synthesize the CNTs with the desired structure and property. In this paper, CNTs array films were fabricated on the Al2O3 and Si substrates by a physical method and the direct chemical vapor deposition (CVD) process. The influences of the preparation methods, the technologic parameters of fabrication and the choice of substrates on the gas sensitivity of CNTs array were studied.CNTs were prepared through catalytic cracking of C2H4 in Ar flow. The metallic catalyst was obtained by the coprecipitation of Fe(NO3)3·9H2O and Mg(NO3)2·9H2O. Gas sensor was fabricated by coating the CNTs onto a Al2O3 substrate. Measurement showed that the gas sensor thus made exhibited poor sensitivity to gases, without detectable signals to NH3.Nitrogen-doped CNTs were prepared on Al2O3 and Si substrates by the double injection method developed in this lab. Ferrocene was used as the catalyst, and the mixture of ethanol and ethylenediamine as the carbon source and nitrogen source respectively. Scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) were used to study the morphologies and structures of the CNTs. The results showed that highly pure and oriented CNTs array films were obtained using the following synthesis conditions: synthesis temperature 950oC, growth time 60min, the injection rate of ethylenediamine 5.0ml/h, ferrocene concentration in ethanol 0.0125g/ml and the injection rate 10.0ml/h. The thickness of CNTs on the Al2O3 substrates was about 160μm, and 83μm on Si substrates.The gas sensitivity measurements indicated that the gas sensor fabricated under the above optimized conditions poccessed excellent sensitivities to Ar and NH3. The gas sensors fabricated by growing the CNTs on the Al2O3 substrate displayed a sensitivity of 69.44% to Arand 50% to NH3. Somewhat reduced but still noticeable sensitivities were obtained for the CNTs grown on Si substrates, which were 40.00% and 28.57% to Ar and NH3, respectively.The systematic studies of the synthesized structures of CNTs and their gas sensitivities found that better oriented and longer CNTs enhanced their sensitivities to gases. Moreover, the nitrogen doped CNTs of the bamboo-shaped structures furthure increased their sensitivity to NH3.