In Situ Monitoring the Thermal Dependence of the Growth of Carbon Nanotubes by Chemical Vapor Deposition Investigated by Tapered Element Oscillating Microbalance

摘要

Recently, it has been shown that the catalytic chemical vapor deposition (CCVD) synthesis at atmospheric pressure of multiwalled carbon nanotubes (MWCNTs) and carbon nanofibers can be very well monitored with a tapered element oscillating microbalance (TEOM) (V. Svrcek et al., J. Chem. Phys. 2006,124, 184705). In this paper, the temperature dependence of the MWCNTs growth by thermal CCVD is investigated. Iron nanoparticle catalysts are dispersed on porous alumina powders. It is shown by scanning electron microscopy that MWCNTs appear above 903 K. The mass increase obtained from decomposition of an ethane-hydrogen gas mixture, monitored by TEOM, occurs with a large initial transient rate v1 generally followed by a constant steady-state rate v2. Activation energy of around 100 kJ/mol is derived for the constant steady-state mass increase throughout the temperature range. A kinetic three-dimensional model based on finite differences is developed to account for these kinetic results. With only two basic assumptions, the calculations well agree with the experimental results. The first assumption supposes a variable competitive adsorption/desorption kinetics on the catalytic surface, and the other one assumes a transition of the iron catalyst from a solid to a liquidlike state at 973 K. It is thus inferred that the rate of the steady-state growth is controlled by the competitive adsorption of hydrocarbon with hydrogen on the catalytic surface, the first step of the process. By contrast, the transient step displays abrupt changes that are governed by a partial melting of the iron-based nanoparticles above 973 K. In the solid state below 973K, carbon diffusion is controlled by the surface diffusion. Above 973K, the carbon diffusion is enhanced by several orders of magnitude corresponding to "liquidlike" diffusion. At high temperature, the suppression of the transient state is accounted for by an enhanced hydrocarbon desorption. A nucleation step involving preliminary carbon saturation of the catalytic nanoparticle as well as carbon surface coverage by the nucleation precursor is observed at low temperatures. From the simulations, it is proposed that a carbyne chain circumventing the catalytic nanoparticle may provide the nucleation precursor. A partial or collective poisoning of the catalyst interferes at high temperatures with this general scheme.