Ab initio Investigation on the Surface Chemistry of Functionalized Titania Membranes

OF DISSERTATION AB INITIO INVESTIGATION ON THE SURFACE CHEMISTRY OF FUNCTIONALIZED TITANIA MEMBRANES Titania (titanium dioxide) is a metal oxide which has recently been investigated as a photocatalyst, most commonly for use in hydrolysis, which absorbs mostly in the UV range. However, the range of absorption can be shifted to fall within the visible light range either by doping or by functionalizing the surface with atomic or molecular adsorbates. Over the course of this research, a series of Density Functional Theory (DFT) calculations are performed to ascertain the effects of these different methods on the photocatalytic performance of titania. While the effects of nitrogen doping and oxygen vacancies are well known, more recent studies point to hydrogen doping as a possible improvement to drive photocatalytic reactions, which will be investigated here. Surface chemistry with metal oxides is commonly affected by surface coverage by hydroxyl groups. The effects of different explicit coverages and implicit solvation were attained separately. All surfaces were found to be favorably solvated with varying coverage being most stable for each crystallographic surface. Various molecules were adsorbed to titania surfaces to determine their interactions in realistic environments. These include: polydopamine for thin film composite membranes, poly-acrylic acid (PAA) for its possible use in a one-step filtration-catalysis membrane, poly(3-hexylthiophene-2,5-diyl) (P3HT) for its use in organic solar cells, the flavonoid quercetin for therapeutic use, and aminopropylsilane as a means of carbon capture. Hydrogen doping was found to affect light absorption to a similar degree as does nitrogen doping for an equivalent molar concentration. With a more favorable energy of formation and the ability to reduce titania, hydrogen doping may be able to overtake nitrogen doping as a means to improve the visible light photoactivity of titania. PAA was found to alter its steric length when electrons were added or removed from the structure, in much the same way as it does when changing pH in solution, suggesting PAA can be used to create one-step filtration-catalysis membranes in conjunction with titania. Four different attachment geometries of PAA to the two main crystallographic surfaces were tested, most of which were favorable (-0.5 to -2 eV), suggesting a robust catalog of stable structure. Dopamine had two different adsorption geometries tested with four different proposed monomers for polydopamine. Monomers were found to absorb favorably to rutile with adsorption energies of around -1 eV and unfavorably to anatase at +1 eV. P3HT, used in dye synthesized solar cells, was found to adsorb unfavorably through the sulfur, suggesting the pi bonding investigated in previous research is the correct mechanism. Quercetin, a therapeutic flavonoid with functional groups similar to dopamine, attained a similar conformation, but may need a larger super to confirm the results. Finally, aminopropylsilane, used in the capture of carbon dioxide from the atmosphere, was the only one of the molecules tested here to prefer a monodentate adsorption mechanism over the bidentate bridging mechanism as was the case with the previous molecules. In addition to the work done with titania, work was done on ruthenium dioxide to see if the rules which had been found for titania also applied to other metal oxide. This included investigating the effects of hydroxyl coverage and atoms/molecule interactions with nanoparticle surfaces. One reaction of interest was that of a mechanism whereby atomic mercury from coal power plant emissions is passed through a ruthenium dioxide catalyst with hydrogen halides, producing mercury chloride or mercury bromide.