Nanotechnology’s Wonder Material: Synthesis of Carbon Nanotubes - Hierarchical Nanostructures for Energy Devices - page 34

Recent studies demonstrated that CNT growth by CVD is feasible even
from non-metallic catalysts such as semiconductors 34 and oxide nano-
particles. 35,36 Considering the compatibility requirement of CMOS (com-
plementary metal-oxide-semiconductor) fabrication, these semiconducting
or ceramic catalysts could be technologically important. However, the pro-
duction yields of these catalysts have not yet matched those of conventional
metallic catalysts. Precious metals such as gold, silver, palladium, and
platinum have been widely used in various catalysis processes using
hydrocarbon gases. However, these precious metals are either less active for
growth or only co-catalytic to other active catalysts. For instance, gold is a
well-known catalytic material for the growth of inorganic nanowires, but
without an additional activation treatment, 37 its application to CNT growth
was not successful. Ding et al.'s study gives a fundamental clue to this
elemental selection of catalysts. Based on their MD simulation results, 38 they
proposed that the ineciency of these metals is attributed to the weak bond
strength between a nanotube and its catalyst (Figure 3.3). Indeed, their
calculations on various metal elements are consistent with the empirical
preference: Fe, Co, Ni better than Pt, Pd.
Researchers have expended a large amount of effort on designing catalysts
for the selective growth of CNTs and assembling them on the substrate
surface. First, prepared catalyst nanoparticles can be directly deposited on a
substrate from solution-state dispersions. This wet process has an advantage
in that the areal density of particles can be easily controlled by adjusting the
concentration of the solution or coating speed. Besides, in this straight-
forward approach, researchers can control the particle size during the
particle preparation step.

d n 3 r 4 n g | 6

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Figure 3.3 Energy diagrams describing detachment of a SWNT from a catalyst
nanoparticle and the subsequent nanotube opening closure. The growth
stability of the iron catalyst is attributed to the stronger adhesion of the
CNT-iron (left) system. Relatively weak binding between CNT-gold
(right) energetically favors separation of a nanotube from its catalyst.
Reprinted with permission from ref. 38. Copyright 2008 American
Chemical Society.