Evaluation of Color and Structure of α-Fe₂O₃ Nanocapsules by Tuning of the SiO₂ Morphology

In this study, we synthesized a core/yolk–shell α-Fe₂O₃@SiO₂ structure by treatment of SiO₂ shell with mild basic solution. Spindle shaped α-Fe₂O₃ nanoparticles were initially prepared as core materials and subsequently coated with silica via sol–gel method. The yolk shell of α-Fe₂O₃ was produced by dissolution of silica shell from core–shell type hematite in NH₄OH solution. In addition, the correlation between the density and morphology of the silica coating layer and the chromaticity according to the NH₄OH concentration and the etching time was confirmed. We found that, after the conversion, the spindle shapes of the composite colloids were retained for both the hematite core and the silica shell, but the physical properties had a median value.