Longer Nanotubes at Lower Temperatures:  The Influence of Effective Activation Energies on Carbon Nanotube Growth by Thermal Chemical Vapor Deposition

Abstract

Growth of carbon nanotubes (CNTs) by metal-catalyzed thermal chemical vapor deposition (CVD) upon flat silicon substrates is studied as a function of growth temperature. It is found that the CNT growth rate at a given temperature is constant for a certain amount of growth time, after which growth ceases; the product of the growth rate and the growth time gives the ultimate length of the CNTs. Both the growth rate and the growth time are found to depend on the CVD temperature, and this dependence is such that the ultimate CNT length increases as temperature decreases; that is, longer CNTs can be grown at lower temperatures than at higher temperatures. This surprising and counter-intuitive result reflects the interaction of competing factors affecting the CNT growth: the rate at which carbon is incorporated into growing CNTs versus the rate at which catalytic metal particles become inactive. Both of these rates are found to have an Arrhenius form of temperature dependence, with activation energies of 2.0 and 3.4 eV, respectively, when an Al 2 O 3 diffusion barrier layer is used. These energies are interpreted as "effective" activation barriers arising from activation energy contributions from multiple chemical processes. CNT bundles as long as one millimeter have been grown at a temperature of 600 °C.

Distinct termination morphologies for vertically aligned carbon nanotube forests

“…From the start of forest growth to the sudden termination point, an excellent fit is obtained for an exponentially decaying initial growth rate that is consistent with a proposed growth equation [7]: H (t) = βτ(1 − e −t/τ ),whereβ is the initial growth rate and τ is the characteristic time. However, the growth suddenly stops at a terminal height H t (indicated by a horizontal line) as previously reported from in situ observations [15,22]a n d inferred from ex situ data [11,19].…”

“…Millimeter tall forests of SWNTs and/or MWNTs (multi-walled carbon nanotubes) are now regularly synthesized [1,3,[5][6][7][8][9][10][11][12][13][14][15][16][17] and research has shifted focus from how nanotube growth is initiated to the increasingly important question of how nanotube growth terminates [15,18,19]. Several reports demonstrate that nanotube forests grow at an initially rapid rate which drops off either gradually [7,20,21] or suddenly [15,22,23].…”

Kinetic critical temperature and optimized chemical vapor deposition growth of carbon nanotubes

“…Interestingly, this energy is the same, within error, to the activation energy for defect diffusion, which has recently been determined to be $2.2 eV [26]. We speculate that defect diffusion to the growth front is a possible explanation for the growth termi- attributed the activation energy of s to catalyst deactivation via interaction with the catalyst support [25]. Elementary considerations provide some insight into these characteristic energies.…”

“…The lifetime s also shows a change in temperature dependence at T c % 870 C. Above T c ; 1=s shows an activation energy of 2:4 AE 0:2 eV, a similar order of magnitude for iron catalyst and ethylene source gas [25]. Below T c ; 1=s shows an activation energy of 1:2 AE 0:1 eV, the same value within error as b (as expected the activation energies of b are simply the sums of the energies in H f and 1=s).…”

Carbon Nanotube Mass Production: Principles and Processes

“…The possible rate-determining steps of CNT growth are hydrocarbon decomposition, [197,225,227] catalyst activity, [223,229] and diffusion of carbon into the catalyst particles. [198,224,230] Both the kinetic and the rate-determining step of CNT growth varies with different catalysts and growth parameters. [225,228,231] It is important to test the kinetics by experimental results and select the appropriate kinetic model for further scaling up.…”