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Scheme 2 Diagrammatic representation of the electrochemical transformation of GNRs from
MWCNTs: a Pristine MWCNT; b MWCNT deposited on glassy carbon electrode after oxidation
to generate functional groups on edges under controlled potential so that it gets broken;
c electrochemical and d chemical reduction to graphene layers (adapted from Ref. [
99
] with
permission from American Chemical Society)
However, in order to completely realize these properties and applications, a
consistent, reliable and inexpensive method for preparing high-quality graphene
layers is crucial, as the existence of residual defects will heavily impact their
electronic properties, despite their expected insensitivity to impurity scattering.
Unfortunately, many of the existing methods of graphene preparation have several
major limitations. We recently reported a remarkable transformation of CNTs to
nanoribbons (Scheme
2
) composed of a few layers of graphene by a two-step
electrochemical approach using the oxidation of CNTs at controlled potential
(Fig.
6
), followed by reduction to form graphene nanoribbons (GNRs) having
smooth edges and fewer defects, as evidenced by multiple characterization tech-
niques, including Raman spectroscopy, atomic force microscopy and transmission
electron microscopy [
99
]. This type of 'electrochemical unzipping' of CNTs
(single-walled, multiwalled) provides unique advantages with respect to the ori-
entation of CNTs, facilitating possible the production of GNRs with controlled
widths and fewer defects for energy storage applications.
In principle, it should be possible to make use of the high degree of graphiti-
zation, electrical conductivity and corrosion resistance of CNTs to impart high
stability to ORR electrocatalysts. However, the ORR activities of carbon-nanotube-
based catalysts have been found to be low in acids. Nitrogen-doped multiwalled
CNTs or aligned carbon nanotube arrays have been made by feeding in nitrogen
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