Environmental Engineering Reference
In-Depth Information
attention paid to potential applications of boron nitrides in hydrogen gas sorption was insufficient. In a study by Weng et al.
[17], a novel Bn material, that is, porous microbelts, with the highest specific surface area up to 1488 m
2
/g, was obtained
through one-step template-free reaction of a boron acid-melamine precursor with ammonia. The obtained Bn phase was
partially disordered and belonged to an intermediate state between h-Bn and amorphous a-Bn phases. By changing the
synthesis temperatures, the textures of obtained porous microbelts are adjustable. H
2
sorption evaluations demonstrated that
the materials exhibit high and reversible H
2
uptake from 1.6 to 2.3 wt.% at 77 K and at a relatively low pressure of 1 MPa.
Zhao et al. performed [18] a first-principles theoretical investigation of the adsorption of hydrogen molecules between
bilayers of solid matrix layers—Bn sheets (BBn) and graphene/Bn heterobilayers (gBn)—with variable interlayer distance.
It was found that the H
2
adsorption energy has a minimum by expanding the interlayer spacing, along with further interlayer
expansion, arising from many H
2
binding states and electrostatic interaction induced by the polar nature of B-n bonds. To
determine whether successive addition of H
2
molecules is indeed possible using the minimal H
2
adsorption energy as the
reference state, the hydrogen storage capacity of BBn and gBn was simulated with different stacking types, and it was found
that the gBn with Bernal stacking is superior for reversible hydrogen storage. Up to eight molecules of H
2
can be adsorbed with
an average adsorption energy of approximately 0.20 eV/H
2
, corresponding to approximately 7.69 wt.% hydrogen uptake.
gradient-corrected dFT computations were performed [19] to probe the local chemical reactivity of stone-Wales defects
and edge sites in zigzag- and armchair-edge Bn nanoribbons with the CH
2
cycloaddition. Independent of the nanoribbon types
and the defect orientations, the reactions at stone-Wales defect sites were found to be more exothermic than those at the center
of perfect Bn nanoribbons. The intriguing electronic and magnetic properties of fully and partially hydrogenated Bn nanorib-
bons were investigated [20] by means of first-principles computations. They showed that independent of ribbon width, fully
hydrogenated armchair nanoribbons are nonmagnetic semiconductors, while the zigzag counterparts are magnetic and metallic.
The partially hydrogenated zigzag nanoribbons (using hydrogenated and pristine ones as building units) exhibit diverse
electronic and magnetic properties: they are nonmagnetic semiconductors when the percentage of hydrogenated nanoribbons
blocks is minor, while a semiconductor → semimetal → metal transition occurs, accompanied by a nonmagnetic → magnetic
transfer, when the hydrogenated part is dominant. Although the semimetallic property is not robust when the hydrogenation
ratio is large, this behavior is sustained for partially hydrogenated zigzag Bn nanoribbons with a smaller degree of hydrogena-
tion. Thus, controlling the hydrogenation ratio can precisely modulate the electronic and magnetic properties, which endows
Bn nanomaterials many potential applications in novel integrated functional nanodevices. A quantum mechanical description
was reported [21] based on the dFT of the structures and electronic properties of armchair Bn nanoribbons edge-terminated
with O atoms and OH groups. The O-edge termination was found to give a peroxide-like structure that is nonmagnetic and
semiconducting. The O-terminated Bn nanoribbon was stabilized by the reduction of the peroxide groups with H atoms leading
to a polyol-like structure. The two chains of hydrogen bonds created along the edges led to alternating five- and seven-mem-
bered rings and caused the ribbon to become nonplanar with rippled edges. Three configurations of different ripple periods and
amplitudes were found with energy differences up to 2 eV per unit cell but with virtually the same band gap of 4.2 eV.
Bhattacharya et al. performed [22] dFT calculations to explore the possibility of using a metal-functionalized hydrogenated
Bn sheet for the storage of molecular hydrogen. The chair BHnH conformer is ideally suited for the adsorption of metal
adatoms on the surface of the sheet. The Li metal, in particular, binds to the sheet with a binding energy of ~0.88 eV/Li atom
and becomes cationic, which thereby attracts hydrogen molecules. However, the interaction of the BHnH sheet and the absorbed
H
2
molecules with Li
+
is different from the conventionally known dewar coordination or Kubas-type interaction for hydrogen
storage. each Li
+
can adsorb up to four H
2
molecules, and the hydrogen binding energy is in the desired energy window for
effective storage of molecular hydrogen. The fully Li-functionalized BHnH sheet yields a reasonably high gravimetric density,
which is >7 wt.%. By dFT calculations, Chen et al. investigated [23] the adsorption of transition-metal (TM) atoms (TM = sc,
Ti, V, Cr, Mn, Fe, Co, and ni) on a carbon-doped h-Bn sheet and the corresponding cage B
12
n
12
. The carbon-doped Bn
nanostructures with dispersed sc could store up to five and six molecules of H
2
, respectively, with the average binding energy
of 0.3-0.4 eV, indicating the possibility of fabricating hydrogen storage media with high capacity. It was also demonstrated that
the geometrical effect is important for hydrogen storage, leading to a modulation of the charge distributions of
d
-levels, which
dominate the binding between H
2
and TM atoms. In a study by shahgaldi et al. [24], titanium-coated boron nitride nanofibers
were produced by the electrospinning method, and the effect of heat treatment on the nanofibers was studied. TiO
2
-coated Bn
nanofibers, with a diameter of 100 nm, were obtained after heat treatment and nitridation. The X-ray diffraction (Xrd) and
FT-Ir spectroscopy depicted hexagonal structures of Bn with sharp peaks related to titanium. The hydrogen uptake capacities
of the nanofibers were investigated by pressure composition isotherms in the range of 1-70 bars at room temperature. different
heat treatment temperatures resulted in different morphologies and specific surface areas for the nanofibers, which had direct
effects on the hydrogen absorption of the samples. Hydrogen absorption measurements revealed that coatings on Bn nanofilms
that were synthesized at a lower temperature during the nitridation process had the highest hydrogen absorption at room
temperature. This result can be explained by the presence of titanium and the specific morphology and higher surface area
