CNTs’ array growth using the floating catalyst-CVD method over different substrates and varying hydrogen supply

High, homogenous and dense CNT arrays of high-aspect ratio tubes grown over sapphire substrate with floated CVD-catalyst. [Display omitted] •Substrate Crystallinity affects the CNTs properties.•CNTs carpet alignment changes from the horizontal to vertical during CCVD on sapphire.•Synergetic effect of substrate crystallinity and hydrogen on high aspect ratio and low walls number CNTs. In the present investigation, we point out the effect of the substrate crystallinity on the growth rate, efficiency, quality and the structure of synthesized aligned carbon nanotubes (CNTs). Three substrates are tested: amorphous alumina (Al2O3), silica (Si/SiO2), and crystalline alumina (sapphire). The growth is carried out using the floating catalyst-CVD process with the ferrocene-toluene solution as precursor. In addition, different concentrations of H2 in the gas supply are investigated. It is observed that the sapphire substrate provides a more homogenous forest of vertically oriented straight nanotubes with small diameter, despite initial horizontal alignment of the nanotubes. In the presence of H2, long arrays of high L/Φ aspect ratio nanotubes were obtained with a reduction of diameter and the number of walls. L/Φ aspect ratio in the case of 7% H2 is c.a. 3600 for sapphire substrate, 1500 and 2000 for alumina and silica amorphous substrate, respectively. When sapphire substrate is used, the increase of H2 to 15% provides the L/Φ aspect ratio of ∼14,000.