Poly(acrylic acid) Enabling the Synthesis of Highly Uniform Silica Nanoparticles of sub‐100 nm

Abstract Robust synthesis of small colloidal silica (SiO₂) nanoparticles with narrow size distributions (<5%) is challenging because the possible involvement of continuous nucleation and growth of SiO₂ in the widely used Stöber process usually results in silica nanoparticles with large sizes and broad size distributions. Promoting heterogeneous nucleation and growth of silica is promising to tackle the challenges by in‐situ forming transient colloidal seeds that provide nucleation surfaces for silica condensate. In this work, poly(acrylic acid) (PAA) is introduced to the Stöber synthesis solutions, which are usually composed of ethanol (containing a trace amount of water), tetraethyl orthosilicate (TEOS), and ammonia, to mediate the nucleation of TEOS hydrolysates and growth of SiO₂ nanoparticles. PAA reacts with ammonia to form soft PAA‐NH₃ complex colloids that serve as transient seeds to facilitate heterogeneous nucleation and growth of SiO₂ nanoparticles after adding TEOS to the synthesis solution. The number of PAA‐NH₃ complex colloidal particles, which strongly depends on the molar ratio of PAA and ammonia, determines the number of the resulting SiO₂ nanoparticles and thus the size of the SiO₂ nanoparticles correspondingly according to the supply of TEOS. Highly uniform SiO₂ nanoparticles with finely tuned diameters in the sub‐100 region have been successfully synthesized, and the synthesis protocol is feasible to be scaled up while maintaining the high quality of SiO₂ nanoparticles.