Preparation And Pore-forming Mechanism Of Superhydrophilic TiO₂Porous Thin Films

Superhydrophilic TiO2thin films have potential application in self-cleaning,antibacterium, antifogging and heat transfer, due to their unique advantages. However, it isdifficult for TiO2films to maintain their superhydrophilic properties when UV illuminationwas eliminated, which severely limits their practical applications. So it is of importanttheoretical and practical significance to prepare TiO2thin films with superhydrophilicitywithout UV illumination via pore structure regulation.Porous titanium dioxide (TiO2) films with superhydrophilicity were prepared by afacile method under template-free conditions. Different complexing agents (acetylacetone,ethanolamine, diethanolamine, triethanolamine) were used as inhibitors to regulate thehydrolysis and polycondensation rates of tetrabutyl titanate as well as pore-formingmaterials to control macropore size, together with the connectivity of pores and gelskeleton. Surface morphologies, structure of crystalline zone and surface chemicalcomposition of TiO2thin films were characterized by scanning electron microscope(SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and Ramanspectra. Wetting behaviors of the resulting films were characterized using contact anglemeasurements. The results show that TiO2films prepared with two complexing agentsexhibit superhydrophilic characteristic without ultraviolet irradiation comparing with flatones with single stabilizer.A series of TiO2surfaces with different pore sizes were fabricated byneutralization-driven phase separation in sol-gel processes under template-free conditions.In this study, TiO2films with well-defined macropores were obtained by applying differentalternative acid-base combinations (inorganic/organic acid and base as HCl, H2SO4, HNO3,acetic acid, Na2CO3, NaHCO3, KOH, K2CO3, NaOH, ethanediamine, triethylamine,formamide and butylamine) during synthesis, in which the size of pores could be tuned byregulating ratios of acid and base. Surface morphologies, crystal structures and wettingbehaviors of TiO2thin films were characterized by SEM, XRD, Raman spectra and contactangle measurements. The results showed that porous TiO2films prepared throughneutralization-driven phase separation exhibited promising superhydrophilic characteristic without ultraviolet irradiation comparing with flat ones.Formation mechanism of porous morphology as well as systematic control of porousstructure was discussed in details. The results showed that when pH reached to neutralcondition, which was closed to isoelectric point, polycondensation carried out quickly.With the reaction of debutanation polycondensation and dehydration polycondensation,the polarity of gel phase decreaseddue to the consumation of hydroxyl groups, whereasthe polarity of the solvent remained high. The phase separation was induced by thereduction of miscibility between the polar solvent and polycondensed inorganic species.When the sol system was acidic, compact and flat thin films could be obtained, withthe increasing of pH, porous structure appeared on the film surface, when pH reached toisoelectric point of TiO2, the larger and more homogeneous distribution pores could begained. As pH increased further, since sol-gel transition happened before phase separation,only large cracks appeared on thin film surface.