Formation process of precursor solution for preparation of transparent mesoporous silica thin film

Abstract

Viscosity, conductivity, pH and concentration of silicic acid in a solution of tetraethylorthosilicate (TEOS)-cetyltrimethylammonium bromide (CTAB)-water (H2O)-nitric acid (HNO3) system have been measured, and the formation process of a precursor solution for preparing transparent mesoporous silica thin films was postulated. For the precursor solution of TEOS/CTAB/H2O = 8/1/185 molar ratio, the relationship of viscosity vs. concentration of silica in a solution at aging times of 15 to 23 h showed good linearity with a slope which became steeper with time, while the viscosity of silica remained almost unchanged for aging times of 3 to 11 h. These results showed that the initial stage includes spherical polysilicate and the later stage a linear one. The concentration of silicic acid in precursor solutions showed the maximum at the aging time of about 1.6 h. The conductivity in the precursor solution of TEOS/CTAB/H2O = 8/1/185 molar ratio was measured, and it slowly decreased with slight fluctuation, exhibiting a the maximum value at the aging time of about 2.5 h. This type of conductivity change suggested that silicic acid or polysilicate interacts with CTAB. The pH of the precursor solution remained unchanged at around 3 throughout the measurement. From these results, the formation process of a precursor solution was assumed as follows; 1st step: cationization of +H3O-Si(OH) 3; 2nd step: complexation of silicic acid cation and CTAB micell; 3rd step: polymerization of silicic acid on CTAB micell; 4th step: separation of CTAB and polysilicate; 5th step: adsorption of CTAB on polysilicate; 6th step: complexation of CTAB and polysilicate; 7th step: formation of hexagonal structure.