Photocatalytic properties of porous titania grown by oblique angle deposition

Abstract

High surface area nanorods of titanium dioxide were grown by oblique angle deposition 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%). Comparing transmission measurements with those from a dense anatase film showed that the penetration depth for the light into the nanorod film was 2.5 times that in a dense, anatase film. The photocatalytic degradation of indigo carmine dye 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 610 nm over time and quantifying the film performance using a pseudo-first-order reaction rate model. Reaction rates increased as the film thickness increased from 600 nm to 1000 nm, but leveled out or decreased at thicknesses beyond 1500 nm. 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 0.73 g/L of 25-nm-diameter anatase nanoparticles with surface area of 50 m2/g.

Surface area and porosity in obliquely grown photocatalytic titanium dioxide for air purification

“…A complete derivation of this can be found in previous work. 4,17,32 The reaction rate constant is lowest for the unannealed film and reaches its highest value after only 3 min of annealing whether in nitrogen or oxygen (not shown). Further annealing in nitrogen for a total of 10 min or 20 min lowers the rate slightly, possibly because surface diffusion lowers the surface area, but is counteracted by the growth of the anatase phase.…”

“…28 Upon annealing between 450 C and 600 C, the film composition increases in anatase content, 29 which is preferable for photocatalysis. 4,28,30 However, there is still a considerable amount of amorphous material present. The as-deposited films are also oxygen deficient, as a consequence of the evaporation process.…”

“…On a transparent substrate, the light that provides the energy required for photocatalysis can be delivered either from behind the film or in front of the film, and the porosity can be adjusted as a function of thickness to maximize both the access of gas from the front and the absorption of light from behind. 4 These films inherently act as anti-reflection coatings. 5,6 Backlighting the film with LEDs, for example, will allow the design of flat surfaces that can be incorporated into interior lighting systems to allow for both the enhanced transmission of visible light and air purification.…”

“…This is in contrast to optical transmission methods used in the liquid phase that require monitoring a change in the absorbance of a dye, which adds restrictions to the substrate material that can be used and the active agent that can be used. 4,14 A film in a device designed for gas phase processing would also not suffer from capillary forces acting on the nanorod film every time the reactant is changed leading to a potential loss of active surface. Most gas phase systems employ a flow reactor but in a small, exploratory system, a batch reactor will be shown the be just as effective a tool.…”

Annealing effect on the photoelectrochemical and photocatalytic performance of TiO2 nanorod arrays

“…25,26 It has been found that the as grown TiO 2 lms or columns by physical vapor deposition method under room temperature were typically amorphous, 27,28 and annealing at a temperature ranging from 300 to 700 C in air caused the formation of single anatase crystalline phase, 29 and also annealing at 400 and 500 C under O 2 ow varied the porosity and oxygen defect. 30 However, the photoelectrochemical and photocatalytic properties were not been discussed.…”