Doctor of Philosophy
Chemical and Biochemical Engineering
Paul A. Charpentier
Progress in the development of energy-efficient coatings has led to recent investigations of smart windows with the ability to control heat and light. Of particular interest for this dissertation is the fabrication of vanadium dioxide (VO2) thermochromic nanomaterials using an environmentally friendly approach with good performance for optical switching. Tungsten (W) doped VO2/GO (graphene oxide) hybrids were successfully prepared using a one-step hydrothermal method. At 2 at. % doping, VO2 nanotubes were successfully synthesized with tunable phase transition temperatures (Tc) achieved as low as 42oC. In addition, various experimental parameters including reaction temperature, reaction time, and W doping concentration were examined. Compared to the other reaction parameters, reaction time was found to have the most important influence on the vanadium morphology, phase purity and phase Tc.
In addition to nanotube structure, nanofiber shaped VO2 was synthesized via an environmental friendly sol-gel method with supercritical CO2. ScCO2 was found to help facilitate nanostructure formation and maintain high surface areas of the VO2 nanostructure. The vanadium products were characterized by XRD, XPS, SEM, EDX and DSC. At a reaction temperature of 40oC, microsphere structure was formed with a Tc around 80oC. Further increasing reaction temperature to 70oC led to nanobelt structure forming, which gave a Tc around 80oC. Reaction temperature was found to play a signification role in phase purity, morphology. The reaction pressure and volumetric ratio R showed only a minor effect on nanostructure.
The mechanism of formation of VO2(M) microspheres and nanofiber structure was further investigated by in-situ Fourier transmission infrared (ATF-FTIR) analysis. The results showed that pentavalent vanadium was much easier to synthesize than tetravalent vanadium with a lower reaction temperature. Nanofiber formation was enhanced by a higher volumetric ratio and higher reaction temperature. Significant band shifts in the 1350-1520 cm-1 region of the FTIR spectrum indicated a structural change from sphere to fiber formation. The FTIR results also indicated that the formation of nanofibers was favored by one-dimensional condensation, while microspheres were favored by three-dimensional condensation. .
Qin, Hanna, "Synthesis of VO2 Nanomaterials Using Green Solvents for Thermochromic Window Coatings" (2017). Electronic Thesis and Dissertation Repository. 5113.