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Fabrication and Characterization of Hydrogen Gas sensors Based on Metal Oxides/Carbon Nanotubes Hybrid System

Ghasempour, Roghayeh | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 41788 (48)
  4. University: Sharif University of Technology
  5. Department: Institute for Nanoscience and Nanotechnology
  6. Advisor(s): Iraji zad, Azam; Taghavinia, Nima
  7. Abstract:
  8. We have studied binary structures contain; as grown thin film of carbon nanotubes (CNTs) , CNT rich polymeric composite, Pd doped WO3 thin film, and also hybrid thin film which was mixed CNTs and WO3 nanoparticles and ternary structure which was obtained by adding Pd to hybrid structure as a hydrogen gas sensor. The structure, morphology, chemical composition and quality of as–grown and functionalized CNTs and sensitive films were studied by SEM, TEM, XRD, XPS, DLS, FTIR and Raman spectroscopy methods. Variation of film’s electrical resistance after exposure gases is utilized as the principle of gas sensing. Multi-Walled Carbon Nanotube (MWCNTs) films were obtained by Thermal Chemical Vapor Deposition method (TCVD) and coated by palladium sputtering process. Hydrogen sensing behavior of the films was investigated at 50 oC. The conductance of Pd/MWCNTs films changes 5.6 orders in presence of 1% hydrogen. Also it presents an irreversible gas response to hydrogen in the first cycle of gas exposure due to chemically adsorption of the hydrogen gas. In addition, we studied gas sensing property of CNTs Polymethylmethacrylate (PMMA) and Polystyrene (PS) composite as the active element for acetone, methanol and ethanol vapour detection at room temperature. The experimental results showed that Pd increased the gas sensitivity and the samples depicted chemical selectivity and reversibility toward polar gases. Pd-doped WO3 film is one of binary structure that was used as hydrogen sensor too. We observed that Pd can modify the growth kinetic of WO3 nanoparticles by reducing the crystallite size and as a result can improve hydrogen sensitivity. Resistance-sensing measurements indicated sensitivity of about 24000 at room temperature in hydrogen concentration of 1.3% in air. In addition, we investigated the hydrogen sensing properties of The WO3/MWNTs hybrid films. Results showed that WO3 nanoparticles were nucleated on oxygenated functionalized MWNTs in hybrid suspension. After coating and annealing the films at 350 oC, the hybrid WO3/MWNTs films showed that the sensitivity of hybrid films is 9 and 54 times more than pure WO3 and MWNTs film respectively. Considering all sensing parameters, the best working temperature was decreased to 250 oC, and linear gas sensitivity as a function of hydrogen concentrations was observed at 300 oC. Also, the results were discussed based on surface increase as well as development of two types of schottky potential barrier height, one at the n-WO3/p-MWNTs hetero-junction and the other at WO3 grain boundaries. To improve the hydrogen sensing properties of hybrid films, Pd was added by sol-gel and elecroless methods (ternary structure). Results indicated that sol-gel method is suitable method. The sensitivity of Pd doped hybrid film increased 50.6 times rather than hybrid films. Also, it presented p-type semiconductor behavior to hydrogen due to increasing p-type semiconductor PdO
  9. Keywords:
  10. Hydrogen Gas Sensor ; Carbon Nanotubes ; Hybrid Gas Sensors ; Ethanol-Methanol-Acetone Gas Sensor

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