Study of the catalyst evolution during annealing preceding the growth of carbon nanotubes by microwave plasma-enhanced chemical vapour deposition

Abstract

A two-step catalyst annealing process is developed in order to control the diameter of nickel catalyst particles for the growth of carbon nanotubes (CNTs) by microwave plasma-enhanced chemical vapour deposition (MW PECVD). Thermal annealing of a continuous nickel film in a hydrogen (H 2 ) environment in a first step is found to be insufficient for the formation of nanometre-size, high-density catalyst particles. In a second step, a H 2 MW plasma treatment decreases the catalyst diameter by a factor of two and increases the particle density by a factor of five. An x-ray photoelectron spectroscopy study of the catalyst after each step in the annealing process is presented. It is found that the catalyst particles interact with the substrate during thermal annealing, thereby forming a silicate, even if a buffer layer in between the catalyst and the substrate is intended to prevent silicate formation. The silicate formation and reduction is shown to be directly related to the CNT growth mechanism, determining whether the catalyst particles reside at the base or the tip of the growing CNTs. The catalyst particles are used for the growth of a high-density CNT coating by MW PECVD. CNTs are analysed with electron microscopy and Raman spectroscopy.

Current understanding of the growth of carbon nanotubes in catalytic chemical vapour deposition

“…Mattevi et al[183] observed that metallic Fe on Al 2 O 3 formed Fe(II) and Fe(III) interface states making the interaction of Fe with Al 2 O 3 much stronger than with SiO 2 in agreement with the lower electronegativity of Al compared to Si. They proposed that these interface states anchor Fe particles to the oxide surface and limit their coarsening.Finally, metal thin films on silica/alumina can also form surface silicates/aluminates when heat-treated[184][185][186][187][188][189]. In presence of oxygen, a mixed oxide interlayer of NiAl 2 O 4 can form by interdiffusion between Ni and Al 2 O 3…”

Multifunctional porous catalyst produced by mechanical alloying

“…Although supported catalysts, especially those produced from thin films, may be treated through annealing or other processes to control particle size [27], surface break-up preempts CNF formation in coarsegrained materials, and the CNF deposition process can be designed to prevent sintering in metallic powders [28]. Break-up has been studied in Ni [29,30], and it is generally attributed to carbon accumulation at crystalline defects and subsequent stress build-up and fracturing of grains from the surface.…”

Effect of Temperature Gradient Direction in the Catalyst Nanoparticle on CNTs Growth Mode

“…Similar investigation indicates that plasma treatment may also affect the growth mode by decreasing the bond between catalyst and substrate [33]. However, this does little to control or define the bond strength between catalysts and substrates, due to the fact that catalyst nanoparticles are in a liquid state during the high temperature CNT growth process [5,66].…”