Controllable synthesis of hollow α-Fe₂O₃ nanostructures, their growth mechanism, and the morphology-reserved conversion to magnetic Fe₃O₄/C nanocomposites

In this work we present a controllable synthesis of hollow nanostructures of α-Fe₂O₃ by a facile hydrothermal route using FeCl₃·6H₂O as the iron source and NH ₄H₂PO₄ as the shape-directing agent. We found that the morphology of α-Fe₂O₃ experienced a continuous change from thin nanospindles to flat nanodisks via finely tuning the phosphate ion concentration due to the selective absorption of phosphate ions onto the Fe₂O₃ nanostructures. At a given phosphate ion concentration, prolonging the reaction time led to the formation of hollow nanostructures (nanotubes, nanobeads, and nanorings) due to the dissolution and re-crystallization of Fe₂O₃. We also found that morphology-preserved conversion from α-Fe₂O₃ to magnetic Fe₃O₄/C can be achieved during the CVD reactions in C₂H₂. The Fe₃O₄/C nanocomposites with uniformly coated carbon layers exhibited morphology-dependent magnetic properties.