(708c) Titania Nanotubes as Photocatalytic Material: Synthesis and Characterization

The objective of this work is to explore the effect of synthesis conditions on the structure, morphology, and photocatalytic activity of titania nanotubes. Titania nanotubes were synthesized in a clean room environment by anodic oxidation of thin films of silica-supported titanium, to create uniform arrays of nanotubes aligned perpendicularly to the silica substrate surface. Anodic oxidation of titanium films deposited on silica wafers resulted in the formation of amorphous titania nanotubes. Upon heating in air, the titania nanotubes crystallized in the rutile form of titania, while maintaining the nanotube morphology. The transformation into rutile was confirmed by Raman spectroscopy. Annealing of nanotubes in oxygen changed the oxygen to titanium ratio. Characterization of titania photocatalysts by high resolution transmission electron microscopy and scanning transmission electron microscopy showed nanotubes with 50 nm diameter, 150 nm length, and 15 nm wall thickness, but these dimensions could be adjusted by changing the anodic oxidation conditions and fluoride concentration in the synthesis electrolyte.

Titania is a well-known photocatalytic material, thanks to its wide band gap. The nanotube structure provides an increased surface to volume ratio, and permits the introduction of other catalytically active species to enhance the activity. The photocatalytic activity of the nanotubes was investigated and correlated with parameters such as nanotube wall thickness, oxygen stoichiometry, and degree of crystallization. An initial assessment of photocatalytic activity was carried out by immersing an array of titania nanotubes in a methylene blue solution. The degradation of methylene blue was measured spectrophotometrically while oxygen was bubbled into the solution and the sample was exposed to ultraviolet (UV) light from a 100W Hg lamp. In addition to these batch reactor experiments, flow reactor studies were conducted in a custom-built photocatalytic reactor allowing the exposure of the catalyst to UV light under elevated temperatures and pressures.