Nanotechnology’s Wonder Material: Synthesis of Carbon Nanotubes - Hierarchical Nanostructures for Energy Devices - page 39

d n 3 r 4 n g | 6

Figure 3.5 Gas purification effect on CNT growth kinetics. (A) Schematic describing
the action of deoxo gas purifiers. (B) Kinetic curves of CNT growth with
purified or unpurified gases (inset: enlarged plot of the purified gas
data). (C) Plot of catalyst lifetime as a function of water concentration
under different gas purification conditions. (D) Final height data corres-
ponding to the data in (C). (B, C, D) Reprinted with permission from
ref. 47. Copyright 2011, AIP Publishing LLC.

.

catalysts. 47,50,51 Other oxygen-containing molecules including alcohol car-
bon precursors exhibit similar effects.
Unfortunately, In et al. reported that the performance of a water promoter
differs when the water vapor is combined with other gas impurities. 47 The
lifetime and growth height data in Figure 3.5C and D show that the impurities
still influence nanotube growth in spite of a quantitatively dominant water
introduction. This result implies that the multiple oxygen-containing species
make the growth chemistry complicated by promoting or deactivating
the catalyst depending on their molecular forms and concentrations. 43,52
Moreover, their roles in the gas-phase reactions have not yet been explored
rigorously. Therefore, gas purification should be a required step to avoid this
complication and improve growth controllability and reproducibility.

3.2.2 Growth Mechanism
In this section, we describe the CNT growth mechanism in detail. In order to
understand this sophisticated physical and chemical process, we first out-
line the overall growth process and then go over the details of each growth
step. For brevity, we skip the catalyst pretreatment process and focus on the
transport and chemical conversion events of carbon from its gaseous state to