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Study of Physical Properties of Three Dimensional Graphene-Based Structures for Sensing Application

Mirmotallebi, Mona | 2019

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 53001 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Iraji zad, Azam; Jafari, Akbar
  7. Abstract:
  8. In this thesis, physical properties and gas sensing application of three-dimensional graphene-based structures are studied. The three-dimensional structures of reduced graphene oxide and relative hybrids were synthesized through reducing graphene oxide aqueous solution. The arrangement of two-dimensional graphene layers in three-dimensional architectures has some advantages such as high surface area, hence increasing available adsorption sites. These properties result in changing electrical parameters of the structure, such as electrical impedance and phase angle in the presence of different mediums and alteration of temperature and pressure. Various characterization techniques like atomic force microscopy, scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, Brauner-Emmett-Teller analysis (BET), scanning tunneling spectroscopy were employed to characterize structures’ morphology, chemical composition, crystalline structure, specific surface area and semiconducting properties of the samples. In this research, the variations in electrical resistance, impedance, phase angle and capacitance in various temperatures and under mechanical force were measured through direct and alternative current-voltage for different applications. Due to presence of adsorbed oxygen-functional groups on the reduced graphene oxide layers, heating the structures weakens the bonds between the plane and the adsorbed species; which modifies structure capacitance. Our measurements confirmed 7.2% (K-1) capacitance sensitivity to change in the temperature. Also, applying mechanical force on the three-dimensional structures resulted in change in its electrical resistance with a response of 22 Ω/kPa, a demonstration of the three-dimensional structure application as a pressure sensor. Due to potential application of structures with elevated surface area in gas sensing, the sensing parameters of three-dimensional graphene-based structures for sensing of toxic hydrogen sulfide gas with concentrations of 1-20 ppm was improved by hybridizing graphene with copper oxide. The optimized molar ratio of metal oxide to graphene and its sensing behavior in different environmental conditions were studied by DC measurement of electrical resistivity. The optimized structures showed 30% response to 5 ppm of H2S gas at room temperature. The proposed sensing mechanism was based on adsorbing target gas molecules on the sensor surface and electron donation to the sample, which has p-type semiconductive behaviour. Three-dimensional hybrid structure of graphene/molybdenum disulfide was synthesized for sensing of low concentrations (1-40 ppm) of ethanol gas, where the electrical impedance and phase angle changes in the presence of gas species were measured by applying alternative voltage to the sensor. Electrical impedance and phase angle response of the samples to 40 ppm of ethanol vapor was measured to be 2.7% and 40%, respectively. Thereupon, it was shown that according to the type of structure and polar molecules of ethanol gas, the alteration in electrical phase is a better candidate for response calculation in comparison with electrical resistivity. In addition, the sensors behavior in the presence of ethanol gas at various frequencies was studied with potentiostat workstation, which provided appropriate sensing mechanism on the basis of electron transport through the hybrid structure and between the structure and environment gas species, electrical double layer formation and diffusion through the porous structure
  9. Keywords:
  10. Graphene ; Physical Properties ; Sensing ; Three Dimensional Structural Models ; Chemical Reduction ; Three Dimentional Hybrid Structures

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