Properties of the TiO₂ membranes prepared by CVD of titanium tetraisopropoxide

Thin films of TiO₂ were deposited on the inner surface of porous Vycor glass tubes by atmospheric pressure chemical vapor deposition (APCVD) at temperatures ranging from 200 to 400°C using titanium isopropoxide as a precursor. Dense and hydrogen-permselective membranes were formed only in the temperature range between 230 and 300°C. Gas permeation through the dense membrane was governed by an activated diffusion mechanism and the H₂:N₂ permeation ratio was far beyond the range of Knudsen diffusion. At 200 and 400°C, however, highly porous films were formed and the permselectivity of the membrane was not improved at all, because gas permeation through the porous membrane was in the Knudsen diffusion regime. Selectivity (H₂:N₂ permeation ratio) profile showed a maximum value of 57 at a deposition temperature of 250°C. The oxygen added to the reaction environment appeared to have some effects on the stability and the permselectivity of the membrane formed. The permeation properties of the TiO₂ membranes were not so appreciable and the stability of the membrane was poor when compared with dense SiO₂ membranes. The characterization of the TiO₂ films by SEM and TEM reveals that phase change from amorphous to crystalline state occurs at a temperature about 300°C.