Effects of Sintering Atmosphere on the Microstructure and Surface Properties of Symmetric TiO₂ Membranes

The effects of sintering atmosphere on the properties of symmetric TiO₂ membranes are studied with regard to sintering behavior, porosity, mean pore size, surface composition, and surface charge properties. The experimental results show that the symmetric TiO₂ membranes display better sintering activity in the air than in argon, and the mean pore diameters and porosities of the membrane sintered in argon are higher than those of the membrane sintered in the air at the same temperature. The surface compositions of the symmetric TiO₂ membrane sintered in the air and in argon at different temperatures, as studied by X-ray photoelectron spectroscopy, are discussed in terms of their chemical composition, with particular emphasis on the valence state of the titanium ions. The correlation between the valence state of the titanium ions at the surface and the surface charge properties is examined. It is found that the presence of Ti³⁺, introduced at the surface of the symmetric TiO₂ membranes by sintering in a lower partial pressure of oxygen, is related to a significant decrease in the isoelectric point. TiO₂ with Ti⁴⁺ at the interface has an isoelectric point of 5.1, but the non-stoichiometric TiO_2-x with Ti³⁺ at the interface has a lower isoelectric point of 3.6.