Biology Reference
In-Depth Information
1993bytwopaperssubmittedindependently,onebyIijimaandIchi-
hashi [60], and the other by Bethune
et al.
[61]. CNTs can be grown
by using the arc-discharge method, the laser vaporization method,
and the chemical vapor deposition (CVD) [62]. Compared with
arc-discharge and laser methods, CVD is a simple and economic
technique for synthesizing CNTs at low temperature and ambient
pressure,atthecostofcrystallinity.Itisversatileinthatitharnesses
a variety of hydrocarbons in any state (solid, liquid, or gas), enables
the use of various substrates, and allows CNT growth in a variety
of forms, such as powder, thin or thick films, aligned or entangled,
straightorcoiled,orevenadesiredarchitectureofnanotubesatpre-
defined sites on a patterned substrate. It also offers better control
over growth parameters.
Carbon-based materials have found intensive use as adsorbents
because of their porous and highly developed internal surface areas
as well as their complex chemical structures. The porous struc-
ture and the chemical nature of the carbon surface are signifi-
cantly related to its crystalline constitution. The crystal structure
of graphite consists of parallel layers of condensed, regular hexag-
onal rings. The in-plane C---C distance is intermediate between the
Csp
3
---Csp
3
andtheCsp
2
===Csp
2
bondlengths.Grapheneisthehypo-
thetical infinite aromatic sheet of sp
2
-bonded carbon that is the 2-D
counterpart of naturally occurring 3-D graphite. It is found in the
π
-stacked hexagonal structure of graphite with an interlayer spac-
ing of 3.34 A, which is the van der Waals distance for sp
2
-bonded
carbon [63].
The pore structure and surface area of carbon-based materials
determine their physical characteristics, while the surface chemi-
calstructureaffectsinteractionswithpolarandnonpolarmolecules
due to the presence of chemically reactive functional groups. Active
sites—edges,dislocations,anddiscontinuities—determinethereac-
tivityofthecarbonsurface.Graphiticmaterialshaveatleasttwodis-
tinct types of surface sites, namely, the basal-plane and edge-plane
sites [64]. It is generally considered that the active sites for electro-
chemical reactions are associated with the edge-plane sites, while
the basal plane is mostly inactive. Heteroatoms (usually oxygen)
play an important role in the chemical nature of the carbon “active”
surface [57]. The adsorption process is thus strongly dependent on
