Monitoring the chemical vapor deposition growth of multiwalled carbon nanotubes by tapered element oscillating microbalance.
The growth of multiwalled carbon nanotubes (MWCNTs) produced by a catalytic chemical vapor deposition (CCVD) process has been monitored using a tapered element oscillating microbalance (TEOM) probe. This technique displays a high sensitivity (<1 microg). Growths in the TEOM microreactor are investigated with catalytic particles (Fe, Ni) dispersed on different supports. First, high surface area FeAl2O3 or Fe (Ni) exchanged on zeolite powders is used. Second, growths are performed on array of nickel dots or FeSi-nc particles dispersed on large holes patterned on Si(100) substrates. An accurate monitoring of the early stages of growth permits a precise evaluation of the growth rates and shows substantial differences between these samples which greatly differ by the surface area. On catalysts dispersed on Si(100) the mass uptake is linear throughout the process. On high surface area catalysts, however, a saturation of the mass uptake is indifferently observed. This saturation is explained either by diffusion limitation by the growing MWCNTs or by internal diffusion through the pores or external diffusion through the grains of the catalyst. The kinetic dependence with partial pressure of the incoming C2H6:H2 gas mixture is then explored on the FeAl2O3 catalyst. A linear dependence of the MWCNT growth an (P(C2H6)/P(H2))(1/2) is found. A simple model is then developed that accounts for this dependence only if an associative and competitive adsorption of ethane is the rate determining step of the overall process. These results thus bring insight to improve and control the CCVD growth kinetics of MWCNTs.