Evidence of nanopores in sol–gel based TiO2 and TiN ultrafiltration membranes

An essential criterion for applying TiO2 and TiN ultrafiltration (UF) membranes in the cross-flow filtration is the pore size distribution in the active filter layer. This layer is very thin (0.1–0.5 μm) compared to the entire cross-section of the membrane. Therefore the pores inside the layer cannot be determined by means of the classical method of N2-adsorption, but only by high resolution techniques which are demanding (as the pore sizes amount to only a few nanometers), and require intricate preparation methods. However, adsorption measurements are easy to perform in so-called unsupported layers which are manufactured by the crystallization of the gel in a glass dish. Using high resolution electron microscopy and image analysis methods we have directly evidenced (i) the pore and particle sizes and (ii) the very good agreement of the results of the adsorption measurements in unsupported layers with the real pore size distributions in the UF layer of the multilayer membranes. Converting the volume distribution of the spatial pore diameters from the N2-adsorption measurements into a number distribution of the chord lengths allows the comparison of both methods. In addition, scanning electron microscopy combined with energy dispersive X-ray microanalysis revealed the thicknesses of the UF layers of the multilayer membranes to be 130 nm for TiO2, and 550 nm for TiN. No sol particles had penetrated into the multilayer supports.