Doctor of Philosophy
Chemical and Biochemical Engineering
Paul A. Charpentier
Graphene has been recognized as one of the most exciting carbon based materials of the present decade due to its unique electronic, mechanical and thermal properties. High surface area exfoliated graphene sheets with controllable surface functionality is an attractive two-dimensional surface for attaching different metals and semiconductors for improving the performance of catalysts, sensors, photoelectronic and energy conversion devices. Graphene is an ideal material which can be used for improving various metal oxide properties such as those of titania (TiO2). TiO2/graphene composites have shown excellent properties compared to bare TiO2 in various applications. In high quality graphene sheets, electrons can travel without scattering at room temperature making them potentially ideal electron transfer bridges, that can also act as an extended charge carrier network resulting in the potential for reduced electron-hole recombination rates when in direct contact with TiO2. The absorbance capability of TiO2 has also been improved when anchored onto highly porous graphene. By chemical modification of these networks, the surface properties can be adjusted for using the composites for tailored applications.
This research has focused on the synthesis and modification of TiO2 nanomaterials on the surface of graphene sheets via an acid modified sol-gel process in supercritical carbon dioxide (scCO2). Acetic acid was used as the polycondensation agent while titanium isopropoxide was used as the Ti alkoxide. The resultant materials were characterized by electron microscopy (SEM and TEM), N2 physisorption, FTIR, XRD, XPS, thermal analysis, Raman spectroscopy, UV-Vis and PL analysis. The results showed that functionalized graphene sheets containing carboxylate groups acted as templates for anchoring modified TiO2 nanoparticles and nanowires on the surface. First, sonication following by scCO2 washing was used to anchor commercial anatase TiO2 on the graphene. TiO2 nanowires were then synthesized by using a sol-gel method in the green solvent scCO2 using titanium alkoxides. Uniform TiO2 nanowires with diameters less than 40 nm were decorated on the surface of the graphene sheets. One-dimensional, precisely oriented TiO2 nanostructures are more effective than TiO2 nanoparticles as their percolation pathways are excellent for charge transfer. Fe doped TiO2 morphologies on the graphene sheets were also prepared in both organic solvents and scCO2 to extend these devices band gap into the visible region. TiO2 nanoparticles less than 5 nm were uniformly formed on the graphene sheets when ethanol was used as the solvent. Doped TiO2 nanoparticles and nanowires showed smaller crystal size, higher visible absorption, higher surface area and higher thermal stability compared to similar materials without graphene. However, Fe doped nanowires prepared in scCO2 showed higher surface area and photocatalytic activity than those prepared in organic solvents. ZrO2-TiO2 bimetallic nanomaterials were also synthesized on the graphene sheets in scCO2 with different morphologies including nanofibers and nanotubes depending on the initial concentrations of precursors. The synthesized materials were examined for both photocatalytic and photovoltaic performance, with both materials providing higher activity than bare TiO2 and corresponding materials without graphene.
Possible interactions between TiO2 and Fe doped TiO2 with graphene sheets and functionalized graphene sheets were studied by calculating adsorption energy values using the Vienna ab-initio Simulation Package (VASP) based on density functional theory (DFT). The results showed that both physical and chemical interactions are present and responsible for stable interaction between TiO2 and graphene sheets. The resulting modified TiO2 nanostructured materials on graphene sheets exhibited a higher visible adsorption, higher surface area, smaller crystallite size, and greater thermal stability, which are all desirable features for catalysts, support materials, semiconductors, and electrodes in dye-sensitized solar cells (DSSC). Nanomaterials prepared by these simple, scalable, environmentally friendly synthesis procedures are potentially attractive for commercial employment.
Farhangi, Nasrin, "Nano TiO2/Graphene Composites for Photovoltaic and Photocatalytic Materials" (2012). Electronic Thesis and Dissertation Repository. 919.