Catalyst-support interactions and their influence in water-assisted carbon nanotube carpet growth

We report results from characterization studies focused on a diverse selection of catalyst support materials in order to understand what makes a good catalyst support during carbon nanotube (CNT) carpet growth via water-assisted chemical vapor deposition. The growth and catalyst morphological changes occurring for thin Fe layers deposited on Al2O3, MgO, TiN, and ZrO2 are compared. The growth behaviors of the catalyst substrates were evidently different, with Al2O3/Fe supporting CNT carpet growth and showing the highest activity and longest lifetime. The TiN/Fe catalyst also supported CNT carpet growth, albeit with much lower activity, shorter lifetime, and lower CNT quality while MgO/Fe and ZrO2/Fe did not support CNT carpet growth under standard growth conditions. Studies using a combination of atomic force microscopy and X-ray photoelectron spectroscopy revealed a general correlation between the catalyst behavior (activity and lifetime) and the 3D evolution of the catalyst for active catalysts (Al2O3/Fe and TiN/Fe). Analysis of inactive catalysts under standard conditions (MgO/Fe and ZrO2/Fe) raise interesting questions related to additional chemical interactions between the substrate and catalyst that could influence nucleation and CNT growth. This work provides a step toward understanding the challenges that arise in engineering efficient CNT growth processes on a desired substrate.