Synthesis of ordered mesostructured TiOx thin films from tetrahydrofuran–water solutions

The co-self-assembly of hybrid mesostructured TiOx-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 TiOx 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 TiOx mesostructured phase evolution with increasing SDA concentration in the THF-based solution follows the cubic → hexagonal → lamellar sequence reported for mesostructured TiOx 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 TiOx mesostructure from a THF-based precursor solution requires SDA concentrations lower than that known to direct the same TiOx 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.

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► SDA-directed TiOx mesostructures from THF-precursor solutions.
► Increasing SDA concentration leads to cubic → hexagonal → lamellar phase evolution.
► Lower SDA concentration directs a specific phase from THF compared to water/ethanol