Titanium dioxide (TiO2) is one of the most attractive environment friendly cleaning materials, because of its high oxidation–reduction ability by photocatalysis under ultra violet (UV) radiation. After the commercialization of UV LEDs, it becomes imperative to investigate the photocatalytic activity of TiO2 under narrow band light source in order to use it for such as indoor air cleaner. However, its photocatalytic property is not utilized very efficiently because of its low optical absorbance. Here, we introduce TiO2 nanocolumn arrays/dielectric/mirror structure. This structure is expected to allow us to enhance the optical absorption of the TiO2 layer at a selected wavelength because of the optical interference. We also expect that the porous TiO2 columnar structure will increase the surface area and subsequent photocatalytic reactions. Thus, in this study, we aim to develop a highly efficient photocatalyst by using TiO2 nanocolumn arrays/SiO2/Al mirror structures.
For achieving strong interference, it is important to control the thicknesses of TiO2 and SiO2 layers. In order to optimize those thicknesses, we calculated the theoretical absorptance of the multilayered structure by using a transfer matrix method. The designed multilayered structures were then fabricated by using the dynamic oblique deposition technique. First, a smooth Al layer (150 nm) and a SiO2 layer (110—230 nm) were deposited on a glass substrate. On the SiO2 layer, TiO2 nanocolumn arrays (180 nm) were prepared. During the deposition, the deposition angle measured from the surface normal was fixed at 70°, and the substrate was rotated at 20 rpm. After the deposition, samples were annealed in air at 500°C for 1 hour. As a result, we successfully obtained the samples, which have high optical absorption peaks of more than 80% in the wavelength range of 300—340 nm.
The photocatalytic activity of the samples was evaluated by photocatalytic degradation of methylene blue in water using a LED light with a wavelength of 340 nm. As the optical absorption of the sample became the higher, the degradation rate became the higher. The maximum rate reached twice as high as that of the simple TiO2 nanocolumn arrays without dielectric and mirror layers. These results suggest that the TiO2 nanocolumn arrays/dielectric/mirror structure realizes the enhancement of the photocatalytic activity.