The control of thickness on aluminum oxide nanotubes by Atomic Layer Deposition using carbon nanotubes as removable templates

•ALD technology combined with the template-based growth to fabricate Al2O3 nanotubes•CNTs as organic templates removed by oxidation at a slow heating rate; TGA study.•Easy method for preparing Al2O3 nanotubes with tunable thickness to 1 nm

Aluminum oxide (Al2O3) nanotubes with wall thicknesses from 2 to 20 nm were synthesized by atomic layer deposition (ALD) technique using multiwalled carbon nanotubes (MWCNTs) as templates. Al2O3 material was deposited on the MWCNT at 100 °C by a hot walls ALD reactor. Trimethyl-aluminum (TMA) and water were used as precursor and oxidant, respectively. Transmission Electron Microscopy (TEM) shows that the wall thickness of Al2O3 coatings is precisely controlled by adjusting the number of ALD-cycles of TMA/H2O. In order to remove the carbon template by oxidation, the Al2O3-coated MWCNT were heated under air flow from ambient temperature up to 800 °C. The correlation between thicknesses and thermal properties of the Al2O3-coated MWCNTs were studied in detail by thermogravimetric analysis (TGA). It was found that the heating rate is a critical step in the removal of the carbon template, being a slower rate mandatory to avoid the spontaneous combustion of carbon at the kindling point. We recommend heating rates of 3 °C/min or less; higher rates might produce nanotubes with significant proportions of voids. TEM confirmed the production of amorphous Al2O3 nanotubes that retained the cylindrical shape of the parent MWCNTs template with excellent control of wall thickness, except for the very thin films with thicknesses below 2 nm (< 10 ALD-cycles).

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