Evolution of directly-spinnable carbon nanotube catalyst structure by recycling analysis

Carbon nanotubes (CNTs), particularly of the substrate-grown directly spinnable variety, have a vast number of potential applications. We have devised a recycling methodology to enable the evolution of catalyst morphology and activity and its effect on CNT growth quality and failure to be studied incrementally. Direct spinnability is particularly sensitive to many variables and so provides verification that each cycle is essentially identical. Two processes, utilising acetylene with and without hydrogen, are studied. Acetylene alone gives four cycles of spinnable CNTs before failing abruptly at the fifth at which point catalyst particle (and CNT) areal density increase while CNT diameter and length fall sharply. In contrast, addition of hydrogen doubles the initial growth rate but spinnability declines on the third cycle and fails on the fourth as catalyst (and CNT) areal density decreases sharply. CNT length also falls although diameter increases. Our observations support a proposed 'sinking plateau' model of catalyst behaviour where growth rate is driven by the essentially flat accumulation area or 'plateau plain' surrounding a local high spot (active catalyst particle). The growth rate remains stable until the plateau plain drops below the substrate surface through diffusion at the base, at which point it falls sharply. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.