Phase Transition In Nanosized Amorphous Iron Oxide And Assembly Of Shell/Core Nanowires

The enhancement of the structural stability of amorphous nanostructured materials isvery important for their practical applications since they have a lot of surfaces andinterfaces as well as the metastability nature of amorphous solids. Many research workshave been focused on the shell/core composite nanowires due to their novel physical andchemical properties. In present work, both the structural stability of amorphousnanostrucured iron oxide and the synthesis of Ni /γ-Fe₂O₃ shell/core nanowire have beeninvestigated by the employment of a x-ray diffractometer (XRD), a transmission electronmicroscopy (TEM), and differential scanning calorimetry (DSC) measurements, etc.Nanosized amorphous iron oxide powders were prepared by chemical method and theirsurface were processed by employing the NaOH solution. Compared with as-preparednanosized iron oxide powders, the crystallization temperature of surface-processedpowders was increased by a value of 49.3 oC, and the crystallization activation energyincreases from 81.5 kJ/mol to156.8 kJ/mol. The surface-modified amorphous Fe₂O₃powders show an entirely different crystallization behavior as compared with as-preparedamorphous powders. The enhanced structural stability is attributed to the increase of theamount of hydroxide groupings at the surfaces of amorphous powders, which lowers theirsurface energy and suppresses the crystal-surface nucleation.Amorphous iron oxide nanowires were prepared by chemical method using the porousanodic alumina template (PAAT). The amorphous iron oxide nanowires without PAAThave a crystallization temperature of about Tx=365.6 oC and a crystallization activationenergy of 88.3 kJ/mol, a little higher than that of the as-prepared nanosized amorphousiron oxides powders. However, the amorphous nanowires show a larger crystallizationactivation energy of about 115 kJ/mol at the initial stage of crystallization transition.The crystallization transition of amorphous iron oxide nanowires with the restriction ofamorphous PAAT was studied by using differential scanning calorimetry and in-situ x-raydiffraction. The amorphous iron oxide nanowires restricted in PAAT show acrystallization temperature of about Tx=850 oC, much higher than that Tx=365 oC ofamorphous iron oxide nanowires without PAAT. The enhancement structural stability isattributed to both the suppression of the heterogeneous nucleation of α-Fe2O3 crystals atthe interfaces of amorphous Fe2O3 / Al2O3 and the suppression of crystallization transitionof amorphous Fe2O3 nanowires due to a pressure induced by the thermal expansionbehavior of amorphous Fe2O3 nanowires during the heating.An assembly of Ni/γ-Fe2O3 shell/core nanowire arrays was prepared by employing theelectrodeposition technique combined with the chemical precipitation method using thesilanized PAAT. The thickness of Ni shell can be controlled by varying electrodepositionparameters, Ni shell is identified to have an amorphous nature and it coats apolycrystalline γ-Fe2O3 core which comes from the transform of crystal β-FeOOHannealed at a temperature of 300 oC.