Obtaining sols, gels and mesoporous nanopowders of hydrothermal nanosilica

Abstract

Experiments were performed to obtain SiO2 sols, gels, and mesoporous nanopowders based on hydrothermal medium. To achieve this, polycondensation of orthosilicic acid (OSA) at certain pH and temperature, as well as ultrafiltration membrane concentration, sol–gel transition, and cryochemical vacuum sublimation were done. The dynamic light scattering, scanning and tunneling electron microscopy, low-temperature nitrogen adsorption, and others methods determined the physical and chemical characteristics of nanosized SiO2 samples. By choosing the pore size of the ultrafiltration membranes at the sol concentration stage, one can control the ratio of the SiO2 content to the total salt content and provide a zeta potential of nanoparticles sufficient for the stability of the sols. It was shown that by varying the temperature at the polycondensation stage from 20 to 90 °С at pH = 8.5–9.3, it is possible to control the final average diameter of SiO2 particles in the range from 5 to 100 nm, respectively. The specific surface area of particles are 50–500 m2/g, the average diameter of the mesoporous powders in the range of 2–15 nm; the fraction of the area (<4%) and volume (<0.25%) of micropores are low.