Synthesis of ordered mesostructured TiO x thin films from tetrahydrofuran–water solutions

Abstract

The co-self-assembly of hybrid mesostructured TiO x -based films from water-deficient tetrahydrofuran (THF) solutions, including a titania precursor species and ethylene oxide structure-directing agents (SDA), is reported. A variety of TiO x mesostructures could be obtained, evident from small angle X-ray scattering and electron microscopy measurements, depending on the chemical composition and concentration of the SDA, and the concentration of water in the precursor solution. We find that the TiO x mesostructured phase evolution with increasing SDA concentration in the THF-based solution follows the cubic → hexagonal → lamellar sequence reported for mesostructured TiO x films deposited from the conventional aqueous/ethanol evaporation induced self-assembly (EISA) process. However, the unavailability of water molecules and the limited hydrolysis of the titania precursor due to the water-poor conditions in the THF-based solution, effectively reduce the concentration of hydrophilic species in the solution and their interactions with the SDA. As a result, directing a specific TiO x mesostructure from a THF-based precursor solution requires SDA concentrations lower than that known to direct the same TiO x mesostructure from the conventional, well studied aqueous/ethanol process. Furthermore, SDAs with very long ethylene oxide hydrophilic segments, which require high overall hydrophilic species concentration to promote self-assembly, are extremely sensitive to the concentration of water and hydrolyzed titania precursor concentrations in the solution, showing sharp mesophase transitions as a function of minute changes in water concentration.