Preparation of silica sphere with porous structure in supercritical carbon dioxide.

Abstract Silica sphere with porous structure has been synthesized in supercritical carbon dioxide. The structure originates from a delicate CO 2 trapping phenomenon intended for void formation in the inorganic framework. Silicate polymerization and subsequent removal of CO 2 by depressurization leaves the porous architecture. The key factor to obtain stable porous spherical structure was CO 2 pressure. Different characterization techniques such as X-ray diffraction, scanning and transmission electron microscopy and N 2 adsorption–desorption isotherm were used to determine the framework structure, morphology and porosity of the material. Microscopic visualization of calcined material suggested that the spherical structure was consisted of macroporous windows of diameter ∼100 nm and the space between macropores presents a wormhole like mesoporous/microporous structure. The pore diameter of the mesoporous structure has been calculated as ∼3 nm. X-ray diffraction and N 2 adsorption isotherm analysis confirmed the presence of micropores and also the macropores. In addition, the resulting material possess high thermal and hydrothermal stability associated with fully SiO 4 cross-linking. The spherical structure with different types of porosity was successfully obtained without using any molding agent.