Chemistry of Water-Assisted Carbon Nanotube Growth over Fe−Mo/MgO Catalyst

Introducing a small amount of water vapor in the catalytic chemical vapor deposition (CVD) growth of single- and double-walled carbon nanotubes (SWNTs and DWNTs) extends the catalyst lifetime and increases the nanotube yield. We study the mechanism of this water-assisted nanotube growth over a Fe−Mo/MgO catalyst, based on analysis of the effluent gas, in terms of chemistry of the water-induced oxidation and its effects on catalytic activity. Water vapor was found to etch away carbon precipitate covering the metal catalyst, based primarily on the chemical reaction C + H 2 O → CO + H 2, thus maintaining the catalytic activity. This oxidative etching was strongly dependent on the CVD temperature, and the balance between the etching and carbon precipitation was important for effective nanotube growth. With an optimized water concentration, the etching rate of the carbon precipitate was estimated to be ca. 1/1000 of the formation rate of carbon precipitate consisting mainly of SWNTs and DWNTs.