Effect of Ce(IV) doping on formation of goethite and its transformation to hematite

Cerium-doped goethite (α-FeOOH) samples were prepared by aqueous precipitation route. During doping Ce(IV)/Fe(III) atomic ratio was varied in the range of ∼0.014 to ∼0.07. The characterization by XRD showed that irrespective of the amount of cerium doping, all the sample were well crystallized showing only goethite as the crystalline phase. The cell parameter a of goethite marginally decreased while b and c marginally increased as the Ce(IV)/Fe(III) atomic ratio increased till 0.035:1 and further increase in this ratio showed much more increase in b and c cell parameters. The lattice image of a typical sample with Ce(IV)/Fe(III) = 0.035:1, indicated (1 1 0) plane of goethite lattice and no plane growth corresponding to any phase of cerium. On heating the samples to 673 K, it was observed that goethite is completely transformed to nano-sized α-Fe2O3 while the formation of cerium oxide was only partial. Further calcination to 1073 K showed the formation of two distinct phases of CeO2 and α-Fe2O3. Lattice image of the same sample revealed that crystal growth of CeO2 occurs along (1 1 1) plane. CeO2 crystallites did not separate from α-Fe2O3. Nano-structures of CeO2-α-Fe2O3 were retained even after calcination at 1073 K.