Magnetic Properties And Electrochemical Performances Of Iron Oxides Nanostructures

Iron oxides nanostructures are attracting fast growing research interests, due to their wide range of important applications. The structure and morphology of iron oxides nanostructures have a critical influence over their properties. Two important kinds of iron oxides nanostructure have been chosen for this study. Hematite nanorings with different thickness and nanotubes are fabricated through a hydrothermal method, and magnetic properties are systematically invested by Physical Property Measurement System. The hematite nanorings have also been invested as anode materials for high performance li-ion batteries. The main results are as follow:(1) We have prepared single-crystalline hematite nanorings and nanotubes using a hydrothermal method. High-resolution transmission electron microscope and selected-area electron diffraction confirm that the axial directions of both nanorings and nanotubes are parallel to the crystalline c-axis. Magnetic measurements show that there exists a first-order Morin transition at about 210 K in the nanoring crystals while this transition disappears in nanotube crystals. The current results suggest that the Morin transition depends very strongly on the shape of nanostructures. This strong shape dependence of the Morin transition can be well explained by a negative and a positive surface anisotropy constant in the surface planes parallel and perpendicular to the crystalline c-axis, respectively.(2) Magnetic measurements up to 980 K have been performed on single-crystalline hematite(α-Fe2O3) nanostructures with different shapes. Magnetic measurements under a high vacuum(<9.5×10-6 Torr) up to 920 K were used to characterize thermal stability of the nanostructures. The onset temperature of the phase transformation of α-Fe2O3 into Fe3O4 and the transformed fraction were found to depend strongly on the shape of the nanostructure. The data demonstrate that the phase transformation mainly occurs at the(001) surfaces. The high thermal stability of the nanoring and nanotube samples allows us to measure their Néel temperatures. The Néel temperatures of the nanoring and nanotube samples were found to decrease with decreasing the mean wall-thickness of the nanoring/nanotube assembly. The data confirm the two-dimensional finite-size scaling law for the Néel temperature.(3) The α-Fe2O3@C nanorings have been successfully fabricated through a facile large-scale two-step method. Compared with bare α-Fe2O3 nanorings, the as-synthesized α-Fe2O3@C nanorings exhibit a much higher specific capacity with excellent charge/discharge rate capability. More remarkably, α-Fe2O3@C nanorings have very high stability for LIBs. Their specific capacity can be well maintained(815 mAhg-1) after cycling 160 times at a high rate of 1000 mAg-1. In view of their special nanostructures and excellent electrochemical performances, these α-Fe2O3@C nanorings might serve as very promising anode materials for high performance LIBs. The novel nanostructure in this work not only shows a promising avenue for designing high performance Fe2O3-based anode materials, but also can be used to fabricate other TMOs anode materials for next-generation LIBs.