Hydrothermal synthesis of α-Fe2O3 nanorings with the help of divalent metal cations, Mn2+, Cu2+, Zn2+ and Ni2+

The morphologies of α-Fe2O3 spindles and nanotubes was modified by the addition of divalent metal cations M2+ (M = Mn, Cu, Zn, Ni). Divalent metal cations gradually modify the spindle to pseudosphere and the nanotube to nanoring particle morphologies. At a higher concentration of added divalent cations the nanodisc morphology was obtained. The SEM/EDS and TEM/EDX analyses showed that precipitates contained Fe, O and P elements, but none of the precipitates contained divalent metal cations. The XRD patterns of each investigated sample fitted well to pure hematite (α-Fe2O3) and there were no other phases observed. The Mössbauer and XRD lines broadened with the addition of divalent metal cations. The mean crystallite sizes were calculated using the Scherrer equation from XRD line broadening of 104 and 110 lines of hematite. With the addition of a Mn2+ cation the mean crystallite size gradually increased in the a-axis direction. The relative high value of the mean crystallite size in the a-axis direction was found for nanorings modified by a Cu2+ cation. This high crystal distortion in the a-axis direction upon adding Cu2+ was explained by a strong Jahn–Teller effect of Cu2+ that has a tetragonally distorted coordination sphere in an octahedral coordination. Thus, quite opposite to a pure system, the modification by divalent metal cations induced the preferential growth of α-Fe2O3 crystals in the a-axis direction. It was suggested that divalent metal cations introduced the defects into the planes parallel to the basal plane (0 0 1) and thus changed the type of hydroxyl groups on hematite planes, which as a consequence switched the preferential growth of the α-Fe2O3 along the c-axis to the growth along the a-axis.