The
mechanism of thermal defunctionalization of multiwalled carbon
nanotubes (CNTs) oxidized by nitric acid was studied. X-ray photoelectron
spectroscopy and thermal analysis under different heating rates combined
with mass spectrometry of evolved gases (TGA–MS) were used
to reveal the transformations on the CNT surface. Hydrogen–deuterium
exchange and mathematical handling of TGA–MS curves were carried
out to evaluate the impact of a small amount of residual oxygen on
CNT defunctionalization. Water, CO, CO2, and NO/CH2O mass curves recorded during TGA–MS study were curve
fitted. The resultant peaks were attributed to the different stages
of CNT defunctionalization. Deuterium exchanged CNTs allowed one to
reveal the mechanism of water release during heating. Kissinger’s
model was applied to estimate the activation energy of the decomposition
of different functional groups on the surface of CNTs.
Thermal defunctionalization of oxidized jellyfish-like few-layer graphene nanoflakes was studied under non-isothermal conditions by simultaneous thermal analysis. Activation energies for thermal decomposition of different oxygen functional groups were calculated by the Kissinger method and compared with those for oxidized carbon nanotubes. Oxygen content in graphene nanoflakes was found to significantly affect the decomposition activation energies of carboxylic and keto/hydroxy acids because of their acceptor properties and strong distortion of the graphene layers at the edges of the nanoflakes. The structure of the carbon material and the oxygen chemical state significantly influence the decomposition kinetics of thermally stable oxygen-containing groups. The activation energy for thermal decomposition of phenol groups (110-150 kJ mol) is close to that for graphene oxide reduction.
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