Fabrication of hollow inorganic microspheres by chemically induced self-transformation

Two contrasting approaches, involving either polymer-mediated or fluoride-mediated self-transformation of amorphous solid particles, are described as general routes to the fabrication of hollow inorganic microspheres. Firstly, calcium carbonate and strontium tungstate hollow microspheres are fabricated in high yield using sodium poly (4-styrenesulfonate) as a stabilizing agent for the formation and subsequent transformation of amorphous primary particles. Transformation occurs with retention of the bulk morphology by localized Ostwald ripening, in which preferential dissolution of the particle interior is coupled to the deposition of a porous external shell of loosely packed nanocrystals. Secondly, the fabrication process is extended to relatively stable amorphous microspheres, such as TiO2 and SnO2, by increasing the surface reactivity of the solid precursor particles. For this, fluoride ions, in the form of NH4F and SnF2, are used to produce well-defined hollow spheroids of nanocrystalline TiO2 and SnO2, respectively. Our results suggest that the chemical self-transformation of precursor objects under morphologically invariant conditions could be of general applicability in the preparation of a wide range of nanoparticle-based hollow architectures for technological and biomedical applications.