Effect of Film Thickness on the Photocatalytic Performance of TiO₂ Thin Films Deposited by Spin Coating

TiO2 thin films of varying thicknesses (similar to 50-430 nm) were fabricated by depositing sol-gel (prepared using titanium tetraisopropoxide and isopropanol) on soda-lime silica glass substrates by spin coating, followed by annealing at 450 degrees C for 2 h. Mineralogical analyses using glancing angle X-ray diffraction showed that the films were comprised of anatase, the peak intensities of which increased with increasing thickness. Ultra-high resolution field emission scanning electron microscopy revealed that the grain size was approximately constant at similar to 5 nm but that the agglomerate size increased from similar to 20 nm to similar to 30 nm with increasing film thickness. Optical analyses by UV-VIS spectrophotometry showed that the absorption edge exhibited a red shift with increasing film thickness while the optical indirect band gap decreased. Photocatalytic testing using methylene blue decomposition showed increasing photoactivity with increasing film thickness. Since the particle size was constant, the films were porous (thus allowing limited dye solution and UV radiation penetration), the agglomerate size increased, and the impurity concentrations were assumed to decrease at the film surface, these data allow the conclusion that the optical and photocatalytic properties of the films were altered principally through an increase in the charge carrier diffusion distance that was determined by the agglomerate size.