Effects of various annealing temperature on carbon nanotubes for N 2 detection
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Effects of various annealing temperature on carbon nanotubes for N2 detection Chien-Sheng Huang,*, Bohr-Ran Huang, Yung-Huang Jang, Chih-Fu Hsieh, Chia-Ching Wu, Min-Chao Chen, Kun-Lin Yang Department and Institute Electronic Engineering, National Yunlin University of Science and Technology, Taiwan, ROC Abstract The vertically aligned carbon nanotubes (CNTs) deposited by microwave plasma-enhanced chemical vapor deposition (MPCVD) were utilized as resistive gas sensors. The carbon nanotubes were annealed between 200 to 800℃ under N2 flow (500 sccm) for 15 minute, respectively. After that, the carbon nanotubes were exposed to an N2 filling and pumping environment. Upon exposure to N2 the electrical resistance of vertically aligned carbon nanotubes was found to increase. It was found that the N2 absorption of unannealed carbon nanotubes was reversible, whereas which of annealing ones was not. However, the sensitivity of the N2 absorption on carbon nanotubes was improved after annealing. From the Raman spectra, the ID/IG ratio of carbon nanotubes also decreased after annealing, indicating that more graphenes were formed by the annealing process. Furthermore, from X-ray photoelectron spectroscopy (XPS), it was observed that the ratio of the oxygen to carbon (O/C) signal intensity increased from 0.094 to 3.943 as the annealing temperature increased. As a consequence, it was suggested that the surface of carbon nanotubes was oxygenated and the absorption of N2 changed from physisorption to chemisorption. Keywords: CNTs; N2 ; gas sensor; annealing; microwave plasma-enhanced CVD; Raman; XPS
1. Introduction Since their discovery in 1991 [1], carbon nanotubes (CNTs) have been the most promising material due to their unique structure and properties. These unique properties of CNTs make them as field emission devices [2], tips for scanning probe microscopes [3], hydrogen storage [4], quantum wires [5], random access memory [6], and gas sensor [7-9]. Although traditional gas sensors made using semiconducting oxides are inexpensive, and safe and sensitive, these ones have been operated at high temperatures to enhance chemical reactivity between the material and the gas molecule. The major fault makes traditional gas sensors have a limit for future application. Then the discovery of carbon nanotubes in 1991 takes a whole new beginning of gas sensors. The high specific surface area, small size, hollow geometry, mechanical properties, good electrical, chemical inertness, excellent sensitivity and
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rapid response times of carbon nanotubes (CNTs) makes them become the most promising material for the applications of gas sensors. Electrical properties and dielectric properties are the common principles of the gas detection. Upon exposure of detecting gas the electrical resistance of carbon nanotubes was found to increase or decrease. When the sensors were exposed to reducing gaseous species such as ammonia, carbon monoxide, carbon dioxide, ethanol and nitrogen gas the electrical resistance of carbon nanotubes was foun
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