Environmental Engineering Reference
In-Depth Information
The average binding energy is the highest at 50% coverage when the H atoms are adsorbed on the adjacent B and n atoms along
the tube axis, and the value is −53.93 kcal/mol, which is similar to half of the H-H binding energy. In addition, the band gap
(−4.29 eV) of the pristine (10, 0) nanotube was decreased up to −2.01 eV for the 50% hydrogen coverage. Zhang et al. [32]
performed dFT calculations to investigate the adsorption of atomic hydrogen H on the wrapping axis of nonpolar armchair
(5, 5) and chiral (8, 4) Bn nanotubes with a view toward understanding the chemisorption-induced polarization field in Bn
nanotubes. The adsorption of H along the zigzag B-n bonds that lie on the wrapping axis of the Bn nanotubes enhances the
macroscopic polarization field. depending on whether the B or n site near the edge of the nanotube is adsorbed with H, the
direction of the polarization field, as well as the work function of the tube ends, can be changed significantly. The relationships
between the chemical effect as well as the geometric distortion of the tube caused by H-chemisorptions and the induced polar-
ization were investigated, respectively. These results have implications for the application of Bn nanotubes as electron emitters.
Zhao and ding systematically studied [33] the effects of several gaseous adsorbates, including H
2
, on the electronic properties
of open edges of Bn nanotubes by using dFT calculations. The results indicated that H
2
molecules dissociate and chemisorb
on open Bn nanotube edges with large adsorption energy because the tube edge has either an open or capped structure and thus
has dangling bonds or pentagonal defects. The high reactivity of an open-ended Bn nanotube can be comparable with that of
its carbon counterpart, although the wall of the Bn nanotube is chemically more stable than a single-walled carbon nanotube's
wall. Moreover, it was noted that adsorption of the gas at the tips of open Bn nanotubes can modify their electronic properties.
A considerable amount of charge transferred for the adsorption of gaseous hydrogen on the open Bn nanotubes may account
for the changes of the electronic properties. Interestingly, the open (5, 5) Bn nanotube exhibits the properties of wide-band gap
materials when gas is adsorbed at top sites, while a smaller band gap is observed when this gas is adsorbed on seat sites. These
results might be helpful in the design of Bn nanotube-based nanomaterials such as field emitters or nanojunctions. Ab initio
local spin density approximation calculations were performed [34] to study the magnetic properties of hydrogenated Bn nano-
tubes (H-Bn). It was found that the adsorption of a single H atom on the external surface of a Bn nanotube can induce
spontaneous magnetization in the H-Bn nanotubes, whereas no magnetism was observed when two H atoms were adsorbed on
two neighboring n atoms or on two neighboring B and n atoms. However, spontaneous magnetization was also found in the
H-Bn nanotubes with two H atoms on two B atoms not next to each other. This may be experimentally accessible when the
coverage of H atoms adsorbed on the external surface of Bn nanotubes is low. defects produce spontaneous magnetization on
Bn nanotubes. When one or three H atoms are adsorbed on vacancy defects, the H-Bn nanotubes are nonmagnetic, while
magnetic H-Bn nanotubes can be obtained by two H atoms adsorbed on vacancy defects, which may be difficult to control
experimentally. The key issue for magnetism is the existence of unpaired electrons, which can be realized either by low coverage
of hydrogen atoms or by making defects on perfect Bn nanotubes. This indicates that it is possible to tune the magnetic prop-
erties of Bn nanotubes by hydrogenation or defects, thus providing a new synthetic route toward metal-free magnetic materials.
Further theoretical investigations of chemical hydrogen storage showed [35] that the dehydrogenation of chemisorbed hydrogen
atoms on Bn nanotubes could be triggered by appropriate reagents through simultaneous proton and hydride transfer. The
computed free energy of the activation barrier for the reduction of formaldehyde to methanol by chemisorbed hydrogen atoms
on a zigzag Bn nanotube was predicted to be 12.7 kcal/mol. The thermodynamic and kinetic feasibilities of H
2
dissociation on
some zigzag nanotubes including the Bn ones have been investigated [36] theoretically by calculating the dissociation and
activation energies. The tubes were determined to be inert toward H
2
dissociation, both thermodynamically and kinetically. The
reaction is endothermic by 5.8 kcal/mol, exhibiting high activation energy of 38.8 kcal/mol.
The structural and electronic characteristics of fully hydrogenated Bn nanotubes were determined [37] by quantum chemical
methods. single-walled nanotubes up to and over 10 nm in diameter were fully optimized by periodic calculations, made possible
by the utilization of line group symmetries. The preferred fully
exo
-hydrogenated Bn nanotubes have diameters below 1 nm.
Partial
endo
-hydrogenation was shown to stabilize large Bn nanotubes, producing energetically favored tubes with diameters of
3.5 nm. The calculated band gaps suggest that perhydrogenated boron nitride nanotubes could be insulators, the band gaps being
practically equal for zigzag and armchair tubes.
Adsorption of some chemical species, including H, H
2
, and nH
3
, on the sidewall of zigzag (8, 0) and armchair (5, 5) Bn
nanotubes was studied [38] using dFT. Particular attention was paid to searching for the most stable configuration of the
adsorbates and the surface reactivity at a perfect site and near a stone-Wales defect. reactivity near the stone-Wales defect
is generally higher than that at the perfect site because of the formation of frustrated B-B and n-n bonds and the local strain
caused by pentagonal and heptagonal pairs. The adsorption of nH
3
on the sidewall of Bn nanotubes can be described as
molecular chemisorption due to modest interaction between highest-occupied molecular orbital (HOMO) of nH
3
with lowest-
unoccupied molecular orbital (LUMO) of Bn. Adsorption of nH
3
can affect electronic properties of Bn nanotubes by raising
the Fermi level. As such, nH
3
can be viewed as an n-type impurity. As for H
2
, it can only be physisorbed on the sidewall of
Bn nanotubes through van der Waals interaction. The geometries, formation energies, electronic properties, and reactivities
of stone-Wales defects in single-walled (8, 0) Bn nanotubes were investigated [39] by means of gradient-corrected dFT
