Hollow Silica Nanoparticles with Tunable Morphology and Porosity

Hollow silica nanoparticles (HSNPs) are promising nanomaterials for applications such as antireflection coatings, catalysis, and gas separation membranes. Their hollow cores and tunable porosity provide a unique combination of a low refractive index, a high surface area, and selective transport properties. However, precise control of the structure of HSNPs remains challenging because the formation pathways are highly sensitive to synthesis conditions. We systematically examine the influence of poly(acrylic acid) (PAA), tetraethyl orthosilicate (TEOS), and ammonia concentrations on particle morphology to gain insight into the mechanisms governing the formation of HSNPs. Distinct regimes of nanospheres, nanochains, and aggregates are identified, with the core size and shell thickness of nanospheres independently governed by the concentrations of PAA and TEOS. Increasing the PAA/TEOS ratio promotes the formation of nanochains with higher aspect ratios and increased porosity. Elevated ammonia concentrations induce a transition to yolk-shell nanochains by accelerating silica condensation and entrapping sub-10 nm particles within the hollow cores. Applying this mechanistic insight, we extend the approach to fabricate ceria-silica yolk-shell composites. These results highlight a versatile design principle for tailoring HSNP morphology and porosity, providing practical guidelines for scalable synthesis and application across diverse structures.