Structure-Directing Effect on Silica Nanoparticle Growth in Sodium Silicate Solutions through Small-Angle X-ray Scattering

Understanding the effect of the microstructure of a sodium silicate solution on the growth behavior of silica nanoparticles is necessary for the preparation of functional silica. The structural evolution of silica aggregates in sodium silicate solutions was studied by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The sodium silicate solution mainly contained three types of particles: monomers with a radius of gyration (Rg) of <0.6 nm, SiO2 clusters formed by monomer polymerization, and large colloidal particles. Notably, primary particles with different structures in sodium silicate solutions exhibited a structure-directing effect for silica nanoparticles formation. Assembly growth occurs through the continuous addition of primary particles to the surface. For SiO2/Na2O < 4.2, the primary particles are ellipsoidal, and there are more hydroxyl groups grafted on both ends of the ellipsoid, so the condensation reaction is more likely to occur at both ends, eventually the ellipsoidal aggregates are formed. For SiO2/Na2O > 4.2, condensation reactions occur at equal rates in all directions, resulting in the formation of spheroid aggregates. Additionally, for SiO2/Na2O > 4.2, the primary particles maintain the fractal structure and are not easily destroyed during the carbonization reaction, so the aggregates formed by primary particles have relatively denser fractal structure than SiO2/Na2O < 4.2. Moreover, an understanding of the sodium silicate structure and different structural regulation mechanisms for silica nanoparticles synthesis provided an important theoretical foundation for fabricating high-performance silica.