One-Pot Preparation of Hollow Silica Spheres by Using Thermosensitive Poly(N-isopropylacrylamide) as a Reversible Template

Hollow silica nanospheres with mesoporous shells were successfully fabricated with a new one-pot strategy by using a thermosensitive polymer, poly(N-isopropylacrylamide) (PNIPAm), as a reversible template without the need of further calcination or chemical etching. By simply regulating the solution temperature with respect to the lower critical Solution temperature (LCST) of PNIPAm, PNIPAm chains can reversibly form aggregates or dissolve in aqueous solution. The thermosensitive character makes PNIPAm chains behave as soft templates for the formation of core-shell silica nanospheres at elevated temperature (> LCST), and they will then diffuse out Of the cores at lower temperature (< LCST), leading to the formation of hollow silica nanospheres. The TEM, SEM, XRD, and N-2 adsorption-desorption results indicate that the shells of such hollow silica nanospheres also contain large quantities of irregular mesopores. This new strategy was also tested with another thermosensitive polymer, poly(vinyl methyl ether) (PVME). However, only solid silica nanospheres with a broad size distribution were obtained when PVME was used. We speculated on the possible formation mechanism of hollow silica nanospheres with PNIPAm templates. The effects of the initial concentration of PNIPAm, the molecular weight of PNIPAm, and the pretreatment of silica precursor on the morphology and size of the resultant hollow silica nanospheres were also investigated. The PNIPAm soft templates were confirmed to be recyclable.