Effect of heat treatment temperature on properties of nanocrystalline, photoactive, titania, thin films on polymer and fused quartz

Successful implementation of flexible photocatalysts (TiO2 films on polymeric substrates) has been hindered by the low photoactivity of the resultant amorphous films and difficulties with adhesion. In this work, thin films (~570 nm) of nanocrystalline TiO2 were deposited on PET sheets at room temperature using a novel sol-gel process. The precursor solution (titanium tetra-isopropoxide and propanol) was subjected to hydrolysis, peptisation, and condensation reactions in dilute HNO3 solution, followed by refluxing at 80°C /15 h to increase the crystallinity of TiO2 precursor. Dip-coating was used to apply the films on different substrates (PET/fused quartz), followed by drying/annealing at different temperatures, viz., l50°C / 15 h on PET and 150°C / 5 h, 350°C / 5 h, and 500°C / 5 h on fused quartz. The mineralogy, particle size/morphology, interfacial characteristics, crystallite sizes, transparency, photoactivity, and adhesion of the films were determined using X-ray diffraction (XRD), scanning electron microscopy (SEM), focussed ion beam milling (FIB), high-resolution transmission electron microscopy (HRTEM), UV-Visible spectrophotometry, methylene blue decomposition, and pull-off tape testing, respectively. The films dried/annealed at l50°C were highly transparent and comprised principally of crystalline anatase particles (~20 nm size); moreover, the films were strongly adhered to the PET sheets and displayed excellent photoactivity with regard to methylene blue degradation under UV irradiation. The superior photoactivity of these films is attributed to: (a) increased film surface area, which increased the number of reaction sites, and (b) decreased contamination of the films at lower temperatures