Plasma treatment of hydrophobic sub-layers to prepare uniform multi-layered films and high-performance gas separation membranes

Uniform and defect-free silica films were prepared by spin-coating silica sols on plasma-treated hydrophobic sub-layers. Three kinds of silica films were prepared using tetraethoxysilane (TEOS), bis(triethoxysilyl)ethane (BTESE) and bis(triethoxysily)octane (BTESO) via sol-gel method. First, hydrophobic sub-layers were pre-coated on silicon wafers with Me-SiO2 sols prepared from mixtures of methyltrimethoxysilane (MTMS) and TEOS. After firing at 400 °C, the films showed water contact angles of 120°. Then TEOS- and BTESE-derived sols were directly spin-coated on the Me-SiO2 films, resulting in separated and scattered coatings. A H2O/N2 plasma modification method was used to change the properties of the Me-SiO2 films from hydrophobicity to hydrophilicity without damaging either the surface morphology or the bulk chemistry. After the treatment, the TEOS- and BTESE-derived sols formed homogenous films. On the other hand, the Me-SiO2 films were fully coated with BTESO either with or without plasma treatment. This was probably due to both the polar (-OH) and non-polar (long -CH2) portions of the BTESO-derived sols. For gas separation applications, the corresponding BTESE membranes showed great improvement in gas selectivity after the plasma treatment of hydrophobic Me-SiO2 layers.