Scalable and Stable Manufacture of Molecular-Sized Silica Nanoparticles by Evaporation-Induced Self-Assembly

Silica nanoparticles on the single-nanometer scale are promising building blocks for the construction of functional nanomaterials. Generally, silica nanoparticles are formed via controlled nucleation and growth in dilute aqueous solutions to avoid aggregation, which hampers their large-scale production and surface modification with organochlorosilanes. Herein, we report a simple and scalable process to prepare uniform and unprecedentedly small silica nanoparticles (∼2 nm in size) via the evaporation-induced self-assembly of silicate species and surfactants in a rotary evaporator. The as-synthesized reverse-type silica-surfactant mesostructured composite was disassembled in tetrahydrofuran to obtain a clear colloidal dispersion. Surface modification was achieved by direct silylation of the mesostructured composite with organochlorosilanes. Small-angle X-ray scattering analysis of the nanoparticle dispersion after surface trimethylsilylation indicated the absence of larger nanoparticles. The trimethylsilylated nanoparticles were assembled by solvent evaporation to form a transparent film. Nanoindentation tests revealed that this nanoparticle film exhibited plasticity and softness similar to organic polymers. Additionally, the thermal stability became significantly higher (10% weight loss at ∼500 °C in the air) when the surface was modified with dimethylsilyl (Me₂Si(OSi)₂) groups. These features of the silica nanoparticles prepared using our method have promising applications in emerging fields.