Synthesis and characterization of cerium substituted hematite by sol-gel method

Fine particles of cerium substituted hematite with different compositions were prepared by the sol-gel method from the aqueous solution of their corresponding metal salts. Mixed hydroxides of Fe³⁺ and Ce ⁴⁺ were obtained by reacting aqueous NH₃ with the aqueous solution of FeCl₃ and Ce(NH₃)₂(NO ₃)₆ at pH 2.5-3.0) and they were heat treated at different temperatures between 400 and 1200°C in air for 2 h to get the mixed oxide product. Thermal analysis and X-ray diffraction studies revealed the coexistence of Fe₂O₃ and CeO₂ phase separately up to 900°C, and covalent interaction between them above 1000°C, which was evidenced by the shifting of the XRD peaks. Unit cell parameters and the cell volumes (V) of the samples derived between 1000 and 1200°C were found to be in the range a = 5.045-5.048 ± 0.007 Å, c = 13.774-13.815 ± 0.041 Å and V = 303.608-304.874 ų respectively. It was observed that both the lattice parameters and the cell volumes were higher than that of the pure α-Fe₂O₃ (a = 5.036 Å, c = 13.749 Å, V = 301.976 ų) system. The expansion of lattice parameters further supported the incorporation of the large Ce ⁴⁺ ion into the α-Fe₂O₃ matrix. The lattice parameter increased with increasing Ce⁴⁺ ion loading. FESEM studies showed the growth of the particles with increasing temperature though it was suppressed due to the presence of Ce⁴⁺ ions as compared to the pure system.