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Keywords:

  • magnetic nanoparticles;
  • nanotubes;
  • porous materials;
  • thin films;
  • colloids

Abstract

This work presents a synthesis route for low-aspect-ratio nanotubes consisting of a layer of magnetite (Fe3O4) sandwiched between SiO2 layers. In this template-based strategy, self-ordered porous alumina membranes are combined with the atomic layer deposition of SiO2 and Fe2O3. An optimized electrochemical setup yields nanoporous Al2O3 membranes on 4-inch Al substrates, which serve as templates for the large-scale fabrication of nanotubes. A selective chemical etching step releases the magnetic tubes for suspension in a carrier fluid and permits recycling of the underlying aluminum foils for the fabrication of subsequent nanotube batches. The nanotubes consisting of an iron oxide layer protected by a silica shell are magnetically characterized in suspensions as well as in dried form on a substrate. High-resolution transmission electron imaging reveals a polycrystalline, magnetite spinel structure of iron oxide, with the proper stoichiometry proven by the presence of the Verwey transition. Furthermore, field-dependent viscosity measurements show an enhancement of the magnetoviscosity, thus demonstrating the technological potential of nanotube suspensions as a new class of ferrofluidic solutions. Owing to the tubular shape being closed at one end, these nanoparticles might additionally function as magnetic containers for targeted drug-delivery or as chemical nanoreactors.