Tuning the composition of Zn-Fe-O nanotube arrays: from zinc ferrite ZnFe2O4 to hematite α-Fe2O3

Large-scale vertically aligned nanotube arrays with composition of zinc ferrite (ZnFe2O4), hematite (alpha-Fe2O3) and their composite (ZnFe2O4/a-Fe2O3) are fabricated by a simple sacrificial ZnO-nanowire-array templating method. Hydrolysis of Fe3+ ions on the surface of ZnO nanowires in the arrays leads to deposition of Fe(OH)(3) shell at the expense of ZnO. The controllable ratio between the newly deposited Fe(OH)(3) and the remaining ZnO determines the chemical composition of the final products after solid state reaction between the two at elevated temperatures. We have shown that the nanotube morphology can always be obtained while its chemical composition can be tuned from stoichiometric ZnFe2O4 to ZnFe2O4/alpha-Fe2O3 composite, and eventually to alpha-Fe2O3. By using ZnO nanowires as both sacrificing and reacting template, the present work suggests a feasible methodology for ternary nanotube array growth. The optical and magnetic properties of obtained nanotube arrays are studied. Both ZnFe2O4 and alpha-Fe2O3 nanotubes show strong visible light absorption. ZnFe2O4 nanotubes exhibit relatively large ferromagnetism at both 3 K and 300 K, while alpha-Fe2O3 nanotubes demonstrate a Morin transition around 235 K under small external field and a spin-flop transition under large external field.