Crystallization at Nanodroplet Interfaces in Emulsion Systems: A Soft-Template Strategy for Preparing Porous and Hollow Nanoparticles

A heterophase method to prepare hollow and/or porous crystalline
nanoparticles of metal oxides at room temperature is presented, taking
cerium(IV) oxide and γ-iron(III) oxide (i.e., maghemite) as
representative cases. The crystallization begins at the oil–water
interface in aqueous nanodroplets of the precursor in inverse (water-in-oil)
miniemulsion systems, and it may continue toward the inner part of
the droplets. A poly(styrene-b-acrylic acid) block
copolymer is used as a structuring agent because the ability of the
carboxylic groups to bind metal ions improves the inorganic shell
formation. A precipitating base is added from the continuous phase,
generating hydroxide species at the interface that begin the crystallization.
We analyze the effects of the synthetic parameters in terms of colloidal
stability and morphology of the resulting materials. In the case of
maghemite samples, the prepared dispersions of hollow particles present
a distinct magnetofluidic behavior.