Understanding and Controlling Nucleation and Growth of TiO2 Deposited on Multiwalled Carbon Nanotubes by Atomic Layer Deposition

Controlled deposition of thin conformal oxide films on carbon nanotubes (CNTs) by atomic layer deposition (ALD) for applications in solar energy and photocatalysis is still challenging, as the early stages of nucleation and subsequent growth are not yet well understood. In this work, we employed ALD to grow TiO2 on multiwalled carbon nanotubes (MW-CNTs). The effects of deposition temperature (120-240 degrees C), number of ALD cycles (20-750), and surface pretreatment of the MW-CNTs with oxygen plasma on the morphology and crystallinity of the TiO2 were systematically studied using transmission electron microscopy (TEM). By tuning the deposition conditions, controllable nucleation and growth of TiO2 on CNTs was achieved. In particular, high-quality crystalline anatase conforming to the CNTs was obtained with an ALD growth temperature as low as 200 degrees C. Direct observation using aberration-corrected atomic-resolution TEM imaging at 120 keV revealed an island structure of crystalline TiOx at the very early stage of nucleation, followed by coalesced growth of crystalline anatase at this temperature. The study also paves the way to understand the interface between the two materials on an atomic level.