M. Zacharias, H.J. Fan, K. Nielsch, D. Hesse, and M. Knez
Using MgO nanowires as a reactive template, we fabricated single-crystal MgAl2O4 spinel nanotubes through an interfacial solid-state reaction of MgO–Al2O3 core–shell nanowires. Single-crystal MgO nanowires are coated with a conformal thin layer of amorphous Al2O3 via atomic layer deposition. Subsequent annealing at 700 °C activates the interfacial reaction between MgO and Al2O3, transforming the alumina shell into a spinel shell. Finally, after etching away the remaining MgO core in ammonia sulfuric solution, MgAl2O4 spinel nanotubes are obtained. As a transition from conventional planar spinel layers via thin-film interface reactions, our result might open a window for the fabrication of a wide variety of MgO-based spinel one-dimensional nanostructures.
(a) A summary of typical approaches to generate nanotubes using nanowires or tubes as templates. (i) Eliminating by etching or dissolution the core of a pre-formed core-shell nanowire. (ii) Reaction of shell material or solvent with a tube to form complex compositions. (iii) Interfacial reaction of core-shell nanowires involving the Kirkendall effect. (b) Schematics of the fabrication process of MgAl2O4 spinel in this study. (i) The MgO nanowires are coated with a conformal layer of amorphous Al2O3 with a uniform thickness (10 nm) via atomic layer deposition, forming core-shell MgO-Al2O3 wires. (ii) The sample is annealed to activate interfacial solid-state reactions, growing a MgAl2O4 spinel shell. (iii) The excess MgO is dissolved, leading to single-crystal spinel MgAl2O4 nanotubes.
TEM image of core-shell MgO-Al2O3 nanowires obtained by atomic layer deposition of alumina around MgO nanowires. Note that the image contrast is not uniform for all wires due to the variation of the lattice orientation relative to the electron beam. Not shown in this image are some MgO wires with larger diameters of ≈100 nm
TEM images showing the typical morphology of MgAl2O4 spinel nanotubes obtained by selectively dissolving the MgO core. Part of MgO cores were not completely dissolved, as seen in (a). The double arrows in (b-c) indicate the square cross sections of tubes. The tubes in (c) are from thick MgO nanowires (diameter: ≈100 nm) which occupies a percentage less than 10% of the as-deposited structures.
For more details see:
H. J. Fan. et al., Nanotechnology 17(2006) 5157-5162.