X-ray diffraction of silica gels made by sol–gel method under different conditions

Abstract

Silica gels made from tetra–ethyl–orthosilicate (TEOS) under the basic condition and one made by two-step hydrolysis were studied by X-ray radial distribution function and correlation function analyses, and 29Si NMR. X-ray scattering intensity, the medium range feature of the radial distribution function (RDF) curve and correlation function curve of silica gels was different from those of melt-derived silica. No difference in X-ray data was noted among the gels. The X-ray RDF data of the silica gels prepared under the basic condition and by the two-step by hydrolysis method were explained by using a medium-range-order (MRO) structure model consisting of fourfold siloxane ring units instead of sixfold siloxane rings which prevail in the melt-derived silica glass. It was estimated that the primary particles of the acid-catalyzed silica gels built up by fourfold rings were around 1.0 nm in size, and those of the base-catalyzed silica gels were a little larger. 40–50% of silicon atoms in the primary particle in the acid-catalyzed gels, and 60–70% of silicon in the base-catalyzed gels were bonded to other particles through forming Si–O–Si bonds. The remaining silicon atoms were terminated by forming Si–OH bonds which were considered to stabilize the fourfold siloxane ring unit in the gels.