Colloids of superparamagnetic shell: synthesis and self-assembly into 3D colloidal crystals with anomalous optical properties

Abstract

Silica microspheres were coated with Fe3O4 and carbon by the decomposition of ferrocene in an acetone system containing hydrogen peroxide. The system was heated at the temperature of 100 °C for the first 24 h and then 180 °C for the next 24 h. The as-prepared superparamagnetic colloids of SiO2@Fe3O4@C were dispersed in acetone and efficiently self-assembled into centimetre-sized 3D magnetic colloidal crystals under the induction of a 0.2 T magnetic field. Both optical microscope image and reflection spectrum show a blue color of the colloidal crystals, demonstrating photonic crystal features. Because of the thinness of the shell and the large difference in reflective index between the core and shell, a new pseudogap whose reflection cannot be predicted by Bragg's law was generated.