Direct Crystallization of Layered Silicates on the Surface of Amorphous Silica

We overview the entire surface coverage of amorphous silica with layered silicates (smectite-like) using the direct crystallization technique under hydrothermal conditions. Various shapes (e. g., microspheres, fibers, microcapsules) have been available as the silica substrates, which involve partial dissolution triggered by the hydrolysis of urea for supplying a source of the layered silicates. Other sources (e. g., Li⁺ , Mg²⁺ , Al³⁺ ) have been supplied from aqueous solutions, participating the heterogeneous nucleation of the layered silicates. Because of partial dissolution of the silica substrates, the original shape has been preserved after the hydrothermal reactions. Cation exchange reactions, usually observed for conventional smectite systems, occurred by replacing the interlayer cations in the silicates with metal (a rare-earth ion) and organic (a quaternary long-chain alkylammonium, methylene blue, and a tris-chelated complex) cations. We have discussed the mechanism of the crystallization and displayed some applications of the hybrids (e. g., chiral discrimination, filtering, and magnetic recovery).