Synthesis of double-shell hollow magnetic Au-loaded ellipsoids as highly active and recoverable nanoreactors

Herein, a novel combined strategy was developed for the preparation of double-shell hollow magnetic ultra-small Au-loaded ellipsoids (Fe@MO2–Au@H–SiO2) as powerful nanoreactors; these ellipsoids comprised double mesoporous shell structures, CeO2 or TiO2 inner active yolks, plenty of sub-3 nm Au nanoparticles (NPs), and magnetic Fe cores. The hierarchical yolk–shell architectures with ellipsoidal Fe2O3@MO2 (M: Ce or Ti)/mSiO2 as yolks/shells were fabricated first via a facile bottom-up assembly process based on sol–gel reactions. After this, encapsulation of numerous extremely stable Au NPs within the shell structures was accomplished via a two-stage reduction process based on the unique deposition–precipitation method mediated with Au(en)2Cl3 compounds; moreover, strong magnetism was integrated into the ellipsoids and inner voids were formed due to the transformation of Fe2O3 into smaller magnetic Fe. Note that ethylenediamine was used as a ligand to synthesize the stable gold precursors [Au(en)2]3+ that were chemically modified onto the double ellipsoidal shells under alkaline conditions. Due to their superior structural properties and enhanced composite synergy, the Fe@MO2–Au@H–SiO2 ellipsoids, especially Fe@CeO2–Au@SiO2, were shown as a highly efficient and recoverable nanocatalysts with outstanding activity and reusability in catalyzing the reduction of 4-nitrophenol to 4-aminophenol.