Morphology and crystallinity control of ultrathin TiO2 layers deposited on carbon nanotubes by temperature-step atomic layer deposition

Carbon nanotubes (CNTs) coated with titanium oxide (TiO2) have generated considerable interest over the last decade and become a promising nanomaterial for a wide range of energy applications. The efficient use of the outstanding electrical properties of this nanostructure relies heavily on the quality of the interface and the thickness and morphology of the TiO2 layer. However, complete surface coverage of the chemically inert CNTs and appropriate control of the morphology of the TiO2 layer have not been achieved so far. Here, we report a new strategy to obtain ultrathin TiO2 coatings deposited by "Temperature-step" Atomic Layer Deposition (TS-ALD) with complete surface coverage of non-functionalized multiwall carbon nanotubes (MWCNTs) and controlled morphology and crystallinity of the TiO2 film. This strategy consists of adjusting the temperature during the ALD deposition to obtain the desired morphology. Complete coverage of long non-functionalized MWCNTs with conformal anatase layers was obtained by using a low temperature of 60 degrees C during the nucleation stage followed by an increase to 220 degrees C during the growth stage. This resulted in a continuous and amorphous TiO2 layer, covered with a conformal anatase coating. Starting with the deposition at 220 degrees C and reducing to 60 degrees C resulted in sporadic crystal grains at the CNT/TiO2 interface covered with an amorphous TiO2 layer. The results were accomplished through an extensive study of nucleation and growth of titanium oxide films on MWCNTs, of which a detailed characterization is presented in this work.