Effect of the added polyethylene glycol molecular weight and calcination heating rate on the morphology of TiO2 films formed by sol-gel

Abstract

This research investigated the changing morphology of silicon (Si)-supported titanium dioxide (TiO2) thin films with different heating rates and molecular weights (MW) of the added polyethylene glycol (PEG). The TiO2 films were deposited on a Si wafer (100) by sol-gel spin coating with PEG (MW = 6,000 or 35,000 g·mol-1) as pore generating agents. Calcination at 450°C completely decomposed all the organic residues in the TiO2 sol, and the resultant films were in the anatase phase. The combustion nature of PEG was found to be the main factor controlling the film's morphology, where the exothermic heat of PEG combustion tended to be higher with increased heating rates and dependent on the type of PEG (extended and folded chain crystal). At heating rates of 10°C·min-1 or higher, the exothermic heat led to localized grain coalescence in the TiO2 films, which decreased the film porosity. However, this exothermic heat also simultaneously induced pore agglomeration. Hence, the average pore size of PEG-containing films were larger than in films without PEG. In contrast, the heating rate did not significantly affect the morphology films without PEG.