Formation of TiO2 Nanoparticles in Reverse Micelles and Their Deposition as Thin Films on Glass Substrates

Abstract

Low-water content reverse micelles have been formed in cyclohexane employing Triton X-100 or AOT (bis(2-ethylhexyl) sulfosuccinate sodium (salt) as the surfactant. Steady-state and time-resolved luminescence quenching studies, using Ru(bpy)32+ as the luminophore and Fe(CN)63- or MV2+ as the quencher, have shown that at low-water content, reverse micelles can not properly solubilize polar species but the surfactant molecules tend to reorganize themselves around the polar molecules forming structures that depend on the nature of the surfactant and the charge. When titanium isopropoxide is solubilized in such reverse micelles, it is slowly hydrolyzed and it polymerizes to give an −O−Ti−O−Ti− network. Gelling is faster in the case of AOT than in the case of Triton X-100-based solutions. Gelling, i.e., polymerization, is a complex process that mainly depends on the surfactant, which tends to organize itself around the polar TiO2 particles. Gels can be deposited by dip coating on glass slides as thin films of thickness of the order of 100 nm. When dipping at an early stage of gelation, films are transparent and optically uniform. Study with steady-state and time-resolved pyrene fluorescence reveals that AOT-based films consist of domains of lower dimensionality than Triton-based films. Indeed, when films are heated up to 450 °C giving pure inorganic (oxide) material, after burning the organic part, Triton-based TiO2 shows up as monodisperse spherical particles of a size of a few tens of nanometers. On the contrary, AOT-based TiO2 films consist of long particles with a high degree of orientation. It is obvious that with AOT reverse micelles one can obtain both hybrid organic/inorganic and pure inorganic mesoporous films of highly structured and oriented domains. It is believed that the difference between the materials created by the two surfactants mainly originates from the higher hydrolysis rates obtained in the AOT-based system.