The formation and growth mechanisms of silica thin film and spherical particles through the Stöber process

Silica nanoparticles and thin film on flat substrate were simultaneously prepared by the Stöber method. The growth kinetics of these precipitates and the fine structure of the film surface were investigated by atomic force microscopy. The thin film formed on a hydrophilic substrate was flat and uniform, with root-mean-square (RMS) roughness of ≈1 nm. The growth of the film was observed to be slower than that of the particles, indicating that the growth of these silica precipitates was limited by the diffusion of reactive silica species. On the other hand, the thin film formed on a hydrophobic substrate was not flat, with RMS roughness of ≈10 nm, and was characterized by convex microstructures of two distinct sizes on its surface. The appearance of these microstructures and the delay observed in film formation on the hydrophobic surface indicate that these precipitates, including spherical Stöber colloids, formed as aggregations of primary particles appearing in the precursor solution, and then grew by the adhesion of primary particles forming continuously in the solution.