Growth of carbon nanotubes inside porous anodic alumina membranes: Simulation and experiment

Abstract

Porous anodic alumina (PAA) membranes represent a widely used and extensively studied template for production of carbon nanotubes (CNT). The PAA–CNT membranes possess a number of unique properties, such as controllable nanotube geometry, size– and chemically–based selectivity as well as high water permeability. In this work, we first propose a combination of gas phase and surface reaction models to quantitatively describe the growth of carbon nanotubes in PAA membranes in a commercial CVD reactor. A complimentary experimental study of CNT formation from ethanol precursor with argon as a carrier gas is performed. A new method for characterizing carbon nanotubes geometry by SEM and TEM image processing of membrane cross–sections is proposed. The simulations show that the carbon growth rate (in nm/min) averaged over the membrane remains constant during the deposition process until the pore diameter becomes relatively small, and rapidly falls to zero after that. The carbon nanotube thickness near the membrane surface is slightly higher than that in the membrane center. The carbon growth rate increases with synthesis temperature and pressure, while it decreases with the argon flow rate. The dependence of carbon growth rate on the ethanol/water flow rate reaches maximum at some intermediate value. These results are supported by the experimental data obtained from SEM/TEM image processing. It is found that the SEM data provide overestimated values of nanotube diameter and thickness in comparison with the TEM data. The obtained results provide new insights into the CNT growth kinetics in nanoporous media, and develop quantitative guidelines for synthesis of CNT–PAA membranes with precisely controlled nanopore geometry. It also validates the combined homogenous / heterogeneous reaction model by comparison with carbon deposition kinetics on a nanometer scale.