Atomic Layer Deposition of Silica on Carbon Nanotubes

Low-temperature ozone-assisted atomic layer deposition (ALD) of SiO 2 with four silane derivatives (3-aminopropyl)triethoxysilane (APTES), bis(diethylamino)silane (BDEAS), diphenylaminosilane (DPAS), and triethylsilane on carbon nanotubes (CNTs) leads to the one step formation of SiO 2 nanotubes. In the process, CNTs act as templates and are removed during the ongoing deposition. From transmission electron microscopy images, the formation of a void between the CNTs surface and the SiO 2 coating was observed, indicating an unexpected removal of carbon from the CNTs. This gap grows as the number of ALD cycles is increased, eventually leading to SiO 2 nanotubes almost free of carbon. ATR-IR and EELS spectra proved the SiO 2 formation. Depending on the CNTs templates used in this process, different morphologies of one-dimensional SiO 2 nanostructures are obtained, including simple nanotubes, hollow wall nanotubes, tube-in-tube structures, and SiO 2 nanowires. The application of this process on vertically aligned CNTs (VACNTs) templates allows the formation of a perfect SiO 2 replica of the VACNTs. From experiments with different oxygen and silicon precursors, it is proposed that peroxides and oxygen-based radicals, which can be formed from the reaction of surface Si–H species with ozone, are the main reactive species leading to the unexpected etching of carbon from the CNTs during silica ALD.