(274b) Tunable Porosity Gradients in Photocatalytic Titania Nanostructures for Air and Water Purification

Titanium dioxide nanoparticles have been used extensively a photocatalyst to breakdown organic compounds when exposed to ultraviolet light.  However, it is difficult to control the morphology and structure of a collection of nanoparticles.  Oblique angle deposition allows for the growth of catalytic nanostructures on solid surfaces and the ability to tailor the shape, structure, and density of the material as a function of deposition conditions.  The efficiency of porous TiO₂ films developed for this purpose is limited by light intensity losses due to absorption as well as the ability of the reactant to diffuse into the pores.  The interplay of light absorption and reactant diffusion in films grown by oblique angle deposition was investigated to find the optimal reaction conditions.  High surface area nanorods of titanium dioxide were grown on a transparent substrate to investigate their effectiveness as photocatalytic agents for the destruction of organic contaminants in air and water. Optical transmission measurements were made that allowed for an estimation of the porosity of the film (75%-78%). The photocatalytic degradation of an indigo carmine dye solution and a gas mixture containing isobutylene on the porous films was shown to depend on film thickness and annealing conditions.  The effectiveness of the film was assessed by observing the change in absorbance of the dye at 610nm over time and quantifying the film performance using a pseudo-first-order reaction rate model. Reaction rates increased as the film thickness increased from 600nm to 1000nm, but leveled out or decreased at thicknesses beyond 1500nm.  A transport/reaction model was used to show that there exists an optimal geometry that maximizes the overall reaction rate and that such a geometry can be simply produced using glancing angle deposition. The nanorod films were benchmarked against nanoparticle films and were shown to perform as well as 0.73g/L of 25nm diameter anatase nanoparticles having a surface area of 50m²/g.