Domain size change of spinodal phase separation structure in the sol-gel derived TiO₂ thin film

TiO₂ thin films were prepared with a spinodal phase separation structure ranging from hundreds of nanometers to micrometers in size by the sol-gel dip-coating method from a titanium tetraisopropoxide solution containing polyoxyethylene (20) nonylphenyl ether (NPE-20). It was possible to change the titania domain size of the spinodal phase separation structure by selecting alcohol as the solvent because the polycondensation and the phase separation are severely influenced by its evaporation rate from the surface of the coated film. It was also observed that the titania domains became larger in size as the film thickness increased in the bottom area of the substrate when the dip-coating method was applied, while uniformly sized titania domains were possibly formed by a spin-coating method. Furthermore, the contrast between the TiO₂ phase and the air phase was not sharp when the titania domain size was smaller than several hundreds of nanometers. It was possible to prepare TiO₂-air interconnective films with the spinodal phase separation structure 200–300 nm in domain size by subsequent HF etching. In addition, combining spinodal phase separation structure (SPSS) TiO₂ thin film with different skeleton size has the potential to create TiO₂ thin film with high photo-electron conversion efficiency due to its particular structure, higher surface area, and lack of bottleneck for electron transfer.