Magnetic Rattle-Type Core–Shell Particles Containing Iron Compounds with Acid Tolerance by Dense Silica

Abstract

Magnetic rattle-type particles, comprising magnetite or metallic iron in nonporous dense hollow silica microspheres, were fabricated by using sol–gel reactions of alkylsilyl trichlorides around droplets of aqueous iron nitrate solution in a water-in-oil emulsion. After evaporation of water within the silica capsules to leave iron salts, calcination of the dried sample was conducted to transform into a hematite (α-Fe2O3) core and porous hollow silica shell by losing alkyl groups of polyalkylsiloxane. Hydrogen gas penetrated through the silica shell and reduced hematite to magnetite (Fe3O4) at 310 °C and metallic iron (α-Fe) at 450 and 500 °C. The reduction at 310 °C resulted in largest magnetization at 12 kOe among the present magnetic particles. The core magnetic compounds were enclosed by a dense silica shell, which was transformed from porous silica by annealing in nitrogen at 700 °C. Because the magnetic particles were encapsulated by the dense silica shell, the magnetism was shown even after immersion...