Electrophoretic deposition (EPD) of functional metal oxide nano/micro-structures for VOC sensor devices
Type of DegreePhD Dissertation
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In this dissertation, two nano/micro-structured functional metal oxides, ZnO and NiO, were synthesized by a simple hydrothermal and precipitation method. Synthesized particles were then electrophoretically deposited as a film and used as sensing materials for sensor applications. Based on the fundamental study of the electrophoretic deposition (EPD) process, this study explored an optimized process condition for improved deposition rate and properties of deposits, and this condition was compared with the empirical equations. The researcher further studied the selection of adequate solvent and additives to achieve a stable suspension by DC/AC EPD with aqueous suspension and a mixture of water and ethanol suspensions. Also, this study explored process conditions with flexible substrates, such as metallic or metal coated fiber, carbon fabric, and carbon papers, to improve deposition rate and properties of deposits, and these conditions were compared with the empirical equation. The effect of frequency on the kinetics of AC-EPD was investigated with ZnO in aqueous suspensions and ethanol suspensions. Different morphologies of ZnO and NiO, such as cubes, rods, and flakes, and hollow microspheres, were used to grow nano/micro-structured materials in order to fabricate highly sensitive and selective VOC sensor devices. Also, this study explored the synthesis of ZnO by the precipitation method at low temperatures with controlling morphology to apply the in-situ EPD process. Synthesized ZnO and NiO materials were characterized by FE-SEM, TEM, XRD, EDS, DSC, FT-IR, BET, zeta potential analyzer, impedance analyzer, and Keithley 2400 source meter to examine the surface morphology, crystalline phase, atomic composition, chemical bonding, and electrical resistance change. Gas sensing properties of nanostructured metal oxides were studied with different morphologies. Three different gases (ethanol, acetone, and ammonia) mixed with synthetic air were tested in a closed chamber by continuously flowing gases. ZnO and NiO films were electrophoretically deposited from synthesized particles on gas sensing properties. Geometric effects of the oxides on the gas sensing properties were investigated by inducing additives or surfactants. ZnO hollow microspheres were prepared by a simple precipitate method by inducing sodium citrate. Different morphologies of NiO were synthesized by a simple hydrothermal method using triethanolamine. Higher specific area of particles appeared to be a dominant factor for gas sensing applications. In addition to controlling the selectivity and sensitivity, silver nanoparticles as a metallic catalyst were decorated on ZnO hollow microspheres.