Preparation and Properties of Uniform Coated Inorganic Colloidal Particles

Uniformly coated particles were obtained when dispersions of colloidal ellipsoidal hematite in manganese(II) 2,4-pentanedionate (MP), manganese(II) methoxide (MMO), and manganese sulfate/urea (MSU) solutions were aged under optimum experimental conditions. The shell formed in the hematite-MP system was amorphous, while those prepared with the other two manganese compounds were identified as crystalline MnO/MnO2 and MnCO3, respectively. On calcination at 750°C, amorphous and MnCO3 coatings converted to Mn2O3, whereas MnO/MnO2 acquired only higher crystallinity without significant change in the morphological features of the particles. Electrophoretic measurements indicated that in all cases, shells significantly decreased the isoelectric point as compared to bare cores. The aging of the coating solutions of MP, MMO, and MSU in the absence of hematite resulted in uniform dispersed particles of Mn3O4/Mn2O3, MnO/MnO2, and MnCO3, respectively. On treating Mn3O4/Mn2O3 dispersions with a 30% solution of H2O2 for 72 h, partial oxidation to Mn2O3/MnO2 took place with simultaneous aggregation of the original particles to much larger spheres of narrow size distribution.