Biomedical Engineering Reference
In-Depth Information
C
through first principles molecular dynamics simulations;
in this case, the authors have demonstrated that the previous not
dissociative C
B
54
6
hydrogenation is unstable against the dissociative
hydrogen chemisorption and the reaction path has been calculated
accordingly; the dissociative chemisorption has been calculated to
occur within the
B
54
6
timescale [62].
Among the other doping species, silicon is particularly interesting
because industrial C
ps
synthesis is performed on Si substrates [74].
In this case, however, an intermediate state between physical and
chemical adsorption emerges, similarly to the Ni-doped fullerenes
case [89] where a gravimetric ratio of 6.8 wt.% has been calculated.
60
8.5.1.3
Carbon Nanotubes
Following some experimental evidences concerning hydrogen
chemisorption on CNTs, Lee and co-workers [63] have studied, by
DFT total energy calculation, different coverage configurations of
hydrogenated CNTs showing that the most stable one is characterized
by hydrogen atoms alternating outside and inside the tube (
zig-zag
configuration
). Later on, Bauschlicher and So [8] have demonstrated,
with accurate quantum chemistry models, that half-full hydrogen
coverage of the CNTs exterior sidewall is favored over the full
coverage case suggesting that hydrogen adsorption can affect the
stability of the CNT structure.
Doping of CNTs with metallic impurities can improve the
situation as for fullerenes and thus Yildirm and Ciraci [116] have
shown that Ti on SWCNTs increases the hydrogen binding energy up
to 0.83 eV. However, Ti decorated CNTs behave quite differently from
the fullerene case where H
molecules are simply physisorbed; this
is mainly due to the different coordination numbers of Ti atoms for
the two cases (2 and 6, respectively) that favor H
2
dissociation and
chemisorprion on Ti doped CNTs. After the first H
2
chemisorption
event, the Ti-decorated site can bind up to three other hydrogen
molecules that, unlike the first one, are simply physisorbed and
do not dissociate. However, it has also been demonstrated another
configuration involving four hydrogen molecules exists where only
physisorption occurs. In this case four Hydrogen molecules can
be simultaneously physisorbed, thanks to quite similar processes
as the ones described by the Dewar, Chatt, and Duncanson model.
High-temperature molecular dynamics simulations have shown that
H
2
molecules are desorbed above 800 K without breaking the C-Ti
bonds.
2
