Seed Size-Dependent Formation of Fe₃O₄/MnO Hybrid Nanocrystals: Selective, Magnetically Recyclable Catalyst Systems

A family of Fe 3 O 4 /MnO hybrid nanocrystals differing in heterojunction structure and number of MnO domains were synthesized through the seed-mediated growth process of MnO at the surface of Fe 3 O 4 nanocrystal. In this process, the resulting heterostructures, including core/shell, dumbbell-like, and flowerlike structures, were found to be largely influenced by the size of initially injected Fe 3 O 4 seeds. The further transformation of Fe 3 O 4 /MnO heterodimer through a nanoscale etching process within silica nanosphere generated the nanoreactor framework composed of a superparamagnetic Fe 3 O 4 nanocrystal, a functionalized cavity with a catalytically active Mn 3 O 4 layer, and a porous silica shell. The newly developed nanoreactor successfully catalyzed the cyanosilylation reaction of aromatic aldehydes in a size selective manner and could be recovered magnetically and reused without loss of catalytic activity even after ten successive cycles.