Paper TF1+SE-TuM3
Nanophotocatalysts Engineered by Glancing Angle Deposition Method

Tuesday, November 10, 2009, 8:40 am, Room B3

TiO₂ has long been used as an efficient and effective photocatalyst material, with applications in water purification, water splitting for hydrogen generation, clean windows, and many others. The photocatalytic efficiency of TiO₂ can be enhanced by increasing its surface area as well coupling it with another semiconductor which can create a charge separation effect. There are many methods to produce high surface area nano-sized TiO₂ such as sol-gel, hydrothermal, and ball-milling, but these techniques are governed by surface chemistry and random aggregation, and are difficult to control the overall size and morphology of the nanoparticles. These issues can be fixed by utilizing an oblique angle deposition (OAD) technique and glancing angle deposition (GLAD) technique, that can create ordered nanorod arrays with tunable height, separation, density and heterostructures. With these unique advantages, we systematically studied the photocatalytic rate of methylene blue versus the TiO₂ nanorod height, and found a scaling relationship that can be interpreted by a surface reaction model. We also created WO₃-TiO₂ two-layer thin film, tilted nanorods, and vertical nanorods by e-beam deposition, OAD, and GLAD. Two important factors played a role in the observed photocatalytic properties; the crystal phase of each material, and the interfacial area between TiO₂ and WO₃. The best sample was found to be the GLAD multi-layer nanorod array, which showed an enhancement up to 3 times over single layer TiO₂ GLAD nanorods. The GLAD structure had a higher interfacial area between TiO₂ and WO₃ than other samples. To maximize the interfacial area between the two materials, a dynamic shadowing growth (DSG) method was used to create a core-shell nanorod array. WO₃ nanorods were first grown on a bare substrate using GLAD to serve as the “core”. A TiO₂ “shell” was then deposited such that the entire WO₃ “core” nanorod was covered. The photocatalytic decay rate for these core-shell samples again showed further improvement over single layer TiO₂ thin films and multi-layer c-TiO₂/a-WO₃ films by 13 and 3 times respectively.

These results show that the GLAD based nanofabrication technique is a versatile tool to design new photocatalytic nanostructures. With more structural and material engineering, better photocatalyst structures can be engineered.