Effect of Ru3+ ions on the formation, structural, magnetic and optical properties of hematite (α-Fe2O3) nanorods

Nanosized hematite (alpha-Fe2O3) is a widely investigated material due to its favourable properties (chemically stable, environmentally safe, inexpensive) and very good performance in several advanced applications. Properties and performance of hematite nanoparticles can be adjusted and improved by modification of particle size and shape, as well as by substitution of Fe3+ ions in the crystal structure of hematite with other metal ions. Ru3+ ions are potentially suitable metal ions for the substitution of Fe3+ ions in hematite because of the same charge and similar ionic radii. In the present work, the effects of Ru3+ ions on the formation and properties of hematite nanorods prepared by combination of hydrothermal precipitation and calcination were investigated. The influence of the molar fraction of Ru3+ ions in the hydrothermal precipitation system on the formation and properties of iron oxide phases was studied. Single-phase goethite (alpha-FeOOH) nanorods containing incorporated Ru3+ ions were formed in the presence of low levels of Ru3+ ions (<3 mol%), while at higher levels (4 and 5 mol % Ru) hematite nanocylinders, consisting of self-assembled and fused nanoparticles, were obtained. The mechanism of the formation of these iron oxide nanostructures in the presence of Ru3+ ions was explained and compared with the effect of other metal cations reported in the literature. Ru-doped hematite nanorods were formed after calcination of Ru-doped goethite nanorods at 500 degrees C. A gradual elongation of hematite nanorods with increased Ru doping to highly elongated Ru-doped hematite nanoneedles at 2 mol% Ru was observed and explained. The influence of the Ru doping level on the magnetic properties of hematite nanorods was investigated using Mossbauer spectroscopy and magnetic measurements. Temperature of the transition between anti-ferromagnetic (AFM) and weakly ferromagnetic (WFM) spin ordering state (Morin transition) gradually rose with increasing Ru3+-for-Fe3+ substitution in hematite. Besides, magnetization of the WFM hematite gradually decreased with Ru doping which was attributed to the reduced canting of two almost antiparallel spin sublattices in this phase. The optical band gap in hematite nanorods was found to get gradually narrower with increased Ru doping due to the modified electronic structure.