Page 49 - Carbon Nanotube Fibres and Yarns

42 Carbon Nanotube Fibers and Yarns

of sulfur availability at lower carbon concentration. It was proposed that
availability of carbon and sulfur when the iron particles are still small
(~1 nm) could assist SWNT growth [21]. In another study [33], CS 2 was
used to effectively limit the size of the catalyst particle which decomposes
earlier at a temperature (~650°C) lower than thiophene (~800°C). It was
also reported that due to the existence of the iron particle, thiophene
might decompose at a temperature around 400°C [11].
Other groups consisting of 16 elements of Se and Te have also been used
as promotors for CNT growth [29]. Similar to sulfur, low Se concentrations
(0.0018 at%, or 2.1 wt% of selenophene) promotes predominantly armchair
SWNT, and high Se concentration causes an increase in the diameter and
wall number of the synthesized CNTs.
Under specific conditions, growth promoter could help control the chi-
rality of the synthesized CNTs. Sundaram et al. synthesized CNTs with
dominant metallic chirality [33] using CS 2 to introduce sulfur in the earlier
stage of ferrocene decomposition, which limits the catalyst size. In another
study, using promoter of S or Se, Aleman et al. achieved predominantly high
chiral angle metallic CNTs [44], which were independent of the carbon
precursor (toluene, butanol) or promoter. They proposed that this predom-
inance of armchair CNTs is an inherent feature of high-temperature CVD
growth of CNTs. Chirality control is very challenging, and more results are
required to understand the mechanism further.

3.2.2 Synthesis temperature

The synthesis temperature has a significant influence on the CNT structure
and property. High-temperature synthesis methods produce CNTs of better
quality. In this section, the effect of synthesis temperature (1200–1500°C)
on the CNTs are discussed, including the structure and quality of the syn-
thesized CNTs.
In the floating catalyst method, the synthesized CNTs are usually a
combination of single-walled carbon nanotubes (SWNTs) and MWNTs.
There is a transition from SWNT to MWNT when increasing synthesis
temperature. At higher temperature, there is an increase of CNT diame-
ter, indicated by more MWNTs (Fig. 3.3). The transition can be further
confirmed by the reduced radial breathing mode (RBM) signals in Raman
spectra. At higher temperature (>1300°C) the RBM signals drop signifi-
cantly (Fig. 3.4), which indicates a reduction of the SWNT percentage in
the sample. This transition could be related to the larger catalyst particles at
a higher temperature.