First Sharp Diffraction Peak of Silanol- and Water-Rich Amorphous Silica Synthesized by Stöber Method

Abstract

The structure of silanol- and water-rich amorphous silica is still not well understood. In this study, high-energy x-ray diffraction experiments were conducted on amorphous silica (called sol–gel silica in this study) synthesized by the Stöber method, which consists of the hydrolysis of tetramethoxysilane (TMOS) and subsequent dehydration and condensation processes for silicic acid. The first sharp diffraction peak (FSDP) of the sol–gel silica was compared with that of bulk silica glass, which contains almost no silanol or water. The as-synthesized sol–gel silica sample contains a lot of water owing to a large amount of Q³ units (silanol), such that the density of the sol–gel silica (true specific gravity measured using helium) is low, and the length of intermediate-range structure becomes small owing to the many cleavages of Si─O─Si bonds, resulting in the shift of FSDP to a higher k region. The density of the sol–gel silica increased with heat treatment at 200°C for 5 h under a pressure of 1.56 MPa, but the FSDP shifted to a lower k region and approached that of the bulk silica glass. This is because, as the amount of Q⁴ units increases and Si─O─Si condensation progresses, the density increases, but the length of the intermediate-range structure becomes greater than that of the as-synthesized sample. Furthermore, the height of the small-angle x-ray scattering peak of the sol–gel silica decreased upon heating and approached that of the bulk silica glass. Since it is well known that the FSDP of bulk silica glass shifts to a lower k region with decreasing density, we concluded that the density-dependent behavior of FSDP of the sol–gel silica is different from that of the bulk silica glass.