Highly ordered transition metal ferrite nanotube arrays synthesized by template-assisted liquid phase deposition

Abstract

Highly ordered spinel ferrite M_xFe_3−xO₄ (M = Ni, Co, Zn) nanotube arrays were synthesized in anodic aluminium oxide (AAO) templates with a pore size of 200 nm by combining a liquid phase deposition (LPD) method with a template-assisted route. The morphology of the transition metal ferrite nanotubes was characterized by electron microscopy (FE-SEM; TEM, SAED and HRTEM) and atomic force microscopy (AFM), whereas their chemical composition was determined by inductive coupling plasma (ICP). The phase purity was studied by X-ray diffraction (XRD) and the magnetic properties of the nanotubes were measured by SQUID measurements. Unlike the deposition of thin film structures, nanotube arrays form within the pores of the AAO templates in a much shorter time due to the attractive interactions between the positively charged AAO and the negatively charged metal complex species formed in the treatment solution. The as-deposited nanotubes are amorphous in nature and can be converted into polycrystalline metal ferrites via a post-synthesis heat treatment which induce the dehydroxylation, crystallization and formation of the spinel structure. The resulting nanotubes are uniform with smooth surfaces and open ends and their wall thickness can be varied from 4 to 26 nm by increasing the deposition time from 1 to 4 h. Significant differences in the magnetic properties of the ferrite nanotubes have been observed and these differences seem to result from the chemical composition, the wall thickness and the annealing temperature of the spinel ferrite nanotubes.