Preparation and Properties of TiO₂ Films by Complexing Agent-Assisted Sol-Gel Method

Abstract

The TiO₂ films were prepared on glass or silicon wafer substrates by sol-gel method using glycols or their ether derivatives as ligands, directed toward control of optical properties. The effect of ligands on the indices and crystal phases of TiO₂ films was investigated. On the glass substrates, the refractive indices of TiO₂ films depended on the ligands used, although all the films were fired under the same conditions. The 2-(2-methoxyethoxy) ethanol (MEE) ligand gave the most high refractive indices among the ligands used. On the other hand, the 2-methyl-2, 4-pentanediol (MPO) ligand gave the most low refractive indices when the films were fired under 510°C. The change in refractive indices is discussed in connection with crystal phases of TiO₂ films measured by X-ray diffraction analysis. In the TiO₂ film prepared using MEE as the ligand, the anatase crystal appeared at low temperature (400°C). It gave high refractive indices. In the TiO₂ film prepared using MPO as the ligand, amorphous TiO₂ appeared under 510°C and gave low refractive indices. The ligands influence size of titanium oxide sols and TiO₂ crystal phases, and change the refractive indices of the films. In other words, the ligands can control the refractive indices of TiO₂ films on glass substrates. The crystal phases of TiO₂ films on silicon wafer also depended on the ligand used. The MPO and triethylene glycol monomethyl ether (TME) ligand accelated the transformation from anatase to rutile form. In contrast MEE ligand inhibited the change to rutile form even at 900°C. Thus, anatase or rutile in the TiO₂ film on the silicon wafer can be selectively formed by the selection of ligands used in sol-gel process.