BN is a binary compound made of Group III and Group V elements in the periodic table. However, due to its specific structure and properties, BN is much closer to the C system compared to other Groups III- V compounds [1]. Zhi et al. [1] has reviewed the current literature on Boron Nitride nanotubes.
The obvious and most appealing difference between BNNTs and CNTs is their visible appearance: BNNTs are pure white (sometimes slightly yellowish due to N vacancies) while CNTs are totally black, as shown in Fig. 2. BN materials are isoelectronic with their all-carbon analogues but possess local dipole moments due to a difference in electronegativity of B and N atoms. The B-N bond contains a significant ionic component; this polarity can remarkably alter both molecular and solid-state electronic properties as well as optical properties of the system. The band gap of BNNTs has been reported to be between 5.0 and 6.0 eV independent to tube chirality, providing good electrical insulation, while CNTs can be a metal or a narrow band-gap semiconductor. This difference in electronic structure results in different luminescence emission: BNNTs have violet or ultraviolet luminescence under excitation by electrons or photons, while CNTs can emit infrared light and the wavelengths depend on their chiralities.
Fig. 2. Images of (a) CNTs and (b) BNNTs exhibiting totally different appearance
Both BNNTs and CNTs have superb mechanical properties: the Young's modulus of CNTs has been predicted to reach a TPa level. The BNNTs' Young's modulus is a bit lower, around 0.7-0.9 TPa, according to theoretical calculations. However, experimentally, both BNNTs and CNTs' Young's moduli vary in samples fabricated by different methods. Sometimes the BNNTs' data are even better than for CNTs due to better crystallization. In regard of thermal properties, CNTs were calculated to have astonishingly high thermal conductivity (6000W/mK). Although, for BNNTs theoretical calculations have given totally different predictions, but a recent experimental work has revealed that at a similar diameter, BNNTs thermal conductivity is comparable with that of CNTs. Besides that, BNNTs possess better thermal and oxidation stability than CNTs.
To sum up, on one hand, both tube types may be used in similar applications due to property similarities, for example, for the mechanical reinforcement or thermal conductivity improvement of matrix materials, etc. On the other hand, the differences are obvious: BNNTs are basically electrically insulating, whereas CNTs are conductive. This causes differences in their usages; for instance, BNNTs are suitable fillers for insulating materials, while CNTs are usually used to improve electrical conductivity of polymers.
As an important potential application for nanomaterials, the hydrogen storage of BNNTs has been intensively studied by theoretical calculations. Although, totally different results were obtained and it is difficult to make a well-established judgment on hydrogen storage capacity of BNNTs, but reports stated that hydrogen storage capacity of BNNT arrays is obviously much better than for CNTs, and can reach and/or exceed the commercial standard presented by the US Department of Energy. Moreover, a conclusion can be made that defects, doping, and/or deformation of BNNTs may remarkably improve their ability to absorb hydrogen.
Kankaala et al. [2] investigated the structural phase transition induced by hydrogen adsorption on a W(100) surface using Monte Carlo simulations. They showed that hydrogen chemisorption tends to switch displaced surface atoms and reconstruct the surface.
Yu et al. [3] studied the adsorption mechanism of hydrogen on the possible isomers of Boron-doped fullerenes using the local-spin-density approximation (LSDA) method. Their results indicated that a single hydrogen molecule could be strongly adsorbed on two isomers, C34BCð‘ŽH and C34BCð‘H, with binding energies of 0. 42 and 0. 47 eV, respectively, hence indicating the possibility of reversible hydrogen adsorption/desorption near room temperature.
Li et al. [4] studied the adsorption of hydrogen on a single-walled BN nanotube, incorporating a transition metal atom, Pt, into the BNNT using density functional theory (DFT). The Pt atom in the Pt-doped armchair (5, 5) BN nanotube protrudes to the exterior of the sidewall and favors attack from an approaching molecule, due to a smaller energy gap than that of the pristine BNNT. The binding energies of H2 with Pt-doped BNNTs are in the optimal range for hydrogen storage.
Fig. 1. Optimized geometries for Pt-doped BNNTs and H2-adsorbed Pt-doped BNNTs, for states SB (a B atom substituted by a Pt atom) and SN (a N atom substituted by a Pt atom) types.
Cabria et al. [5] performed density functional calculations of hydrogen adsorption on recently discovered boron nanotubes and boron sheets, considering both molecular physisorption and dissociative atomic chemisorption. The calculations predict physisorption as the leading adsorption mechanism of hydrogen at moderate temperatures and pressures, but the hydrogen adsorption capacity of these novel B materials is even smaller than that of CNTs.
Hayden and Lamont [6] showed interest in the dissociative hydrogen adsorption and its reaction with an oxygen overlayer on copper, using supersonic molecular beam technique.
Tew et al. [7] determined the particle size effect on the formation of palladium hydride and on surface hydrogen adsorption, at room temperature, using X-ray absorption spectroscopy and distinguished the proportion of bulk-dissolved and surface hydrogen. Their results indicated that the ratio of surface hydrogen versus that in the bulk increased with decreasing particle size, implying that smaller particles dissolved less hydrogen.
Ambridge and Carter [8] have investigated the effect of hydrogen adsorption on the conductivity of CdS thin-film. Having measured the changes in electrical conductivity and by plotting this rate of change against reciprocal of the filament's absolute temperature, the activation energy for the adsorption of atomic hydrogen on CdS has been deduced.
Kishi et al. [9] investigated the effect of hydrogen adsorption on the magnetic properties for Fe adatoms on Si(001) using first-principles calculations. Results indicated that the magnetic moment of the Fe adatom on H-terminated symmetric dimers surface is larger than that of the Fe adatom on the surface having buckling dimers.
Wang et al. [10] investigated the effect of surface hydrogen coverage on the electron field emission properties of diamond films using high-resolution electron energy loss spectroscopy. It was found that increase of surface hydrogen coverage could improve the field emission properties, due to the decrease of electron affinity of the diamond surface through hydrogen adsorption.
Nikfarjam and Kalantari
Marsili and Pulci [11] investigated the effect of hydrogen adsorption on the electronic band structure and electron affinity of diamond and graphene surfaces with the aid of ab initio method. Results show that the electron affinity is strongly reduced becoming negative for the hydrogenated diamond surfaces, and almost zero in graphane, that is graphene functionalized with hydrogen.
Venkataramanan et al. [12] investigated hydrogen storage on Nickel and Rhodium-doped hexagonal boron nitride (BN) sheet using the first principle method, and first, found the most stable site for Ni and Rh atoms on the hexagonal BN sheet (Fig. 3).
Fig. 3. The optimized and most stable geometric structure of BN sheets along with (a) the possible sites of metal doping. (b) the doped Ni atom. (c) the doped Rh atom.
Further results show that the first hydrogen molecule is absorbed dissociatively over Rh atom, and molecularly on Ni doped BN sheet, and both Ni and Rh atoms are capable to absorb up to three hydrogen molecules chemically (Fig. 4). The bonding between the metal atom and the hydrogen molecules is due to the hybridization of metal d orbital with the hydrogen s orbital.
Fig. 4. The optimized geometric structures for the hydrogen adsorbed on the metal doped BN sheet (a) one hydrogen molecule adsorbed over Ni doped BN sheet (b) one hydrogen molecule absorbed over Rh doped BN sheet (c) three hydrogen molecules adsorbed over Ni doped BN sheet (d) three hydrogen molecules adsorbed over Rh doped BN sheet.
In the quest for suitable materials for hydrogen storage, Meisner and Hu [13] have proposed and synthesized high surface area microporous carbon materials, used for cryogenic hydrogen storage. The hydrogen storage capacity of 4.2 wt% was obtained using excess hydrogen adsorption measurements at 77 K and up to 40 bar hydrogen pressure, for the direct one-step synthesis method with a specific surface area value of up to nearly 2000 m2/g, and an excess hydrogen adsorption capacity of nearly 5.8 wt% was achieved for the chemical activation-based synthesis method with a specific surface area value of nearly 3000 m2/g.
Son et al.
Muscat [14] investigated hydrogen adsorption on a Cu sample containing Ni impurities and concluded that the presence of a Ni impurity close to the hydrogen adsorption site results in a lowering of the hydrogen-substrate binding energy compared to that for hydrogen adsorbed on a pure Cu sample.
Ansón et al.[15] carried out measurements of the hydrogen adsorption of a mixed carbon material containing single-walled carbon nanotubes (SWNTs), using three different techniques. The H2 gas adsorption capacity (volumetric and gravimetric) was found to be very low, around 0.01 wt% at room temperature and pressure, increasing to 0.1 wt% at 20 bar. Electrochemical measurements show a slightly higher capacity (0.1-0.3 wt%) than volumetric and gravimetric data. Although very different values of the hydrogen storage capacity have been reported in the literature for nanostructured materials, but the authors believe that compatible results of hydrogen adsorption can be found if the following hold: (i) Samples are obtained by the same method and the same kind of physical or chemical treatment after production. (ii) Well calibrated and high-precision devices are used for measuring hydrogen adsorption.
Miwa, et al.[16] explored the electronic and structural properties of boron doped graphene sheets, and the chemisorption processes of hydrogen adatoms on the boron doped graphene sheets, by ab initio calculations. After determining the energetically most stable configuration for substituting boron atoms on graphene sheets, the authors considered the hydrogen adsorption process as a function of the boron concentration. The calculated binding energies indicate that the C-H bonds are strengthened near boron substituting sites and the formation (or not) of H clusters on graphene sheets can be tuned by the concentration of substituting boron atoms.
Fig. 5. Structural models of a boron doped graphene sheet for (a) a single substitutional boron atom (B1), and (b) two substitutional boron atoms per unit cell (B1-B2), corresponding to boron concentrations of ∼1.2 and 2.4%, respectively.
Wöll [17] reported investigations of hydrogen adsorption on metal oxide surfaces (TiO2, ZnO, Al2O3), by exposing clean metal oxide surfaces to either molecular or atomic hydrogen, using helium-atom scattering (HAS).
Eichler et al. [18] used ab initio calculations to study the adsorption of atomic hydrogen on Rh and Pd surfaces and reported detailed results for the adsorption energies, the stabilities of various adsorption geometries, and the adsorption- induced changes in the surface relaxations and in the work-functions. Findings suggest that the adsorption of a monolayer of hydrogen changes the inward relaxation of the top layer of the substrate into an outward relaxation, but this has only a very small influence on the adsorption energy and geometry.
Nie et al.[19] used first-principles calculations to investigate the adsorption, dissociation, and diffusion of hydrogen on the surface of Uranium and observed a weak molecular chemisorption for H2 approaching with its molecular axis parallel to the surface (Fig. 6).
Fig. 6. Horizontal and vertical adsorption configurations for H2: (a) top view of the HOR1 configuration, (b) top view of the HOR2 configuration, (c) side view of the VER configuration, (d) top view of the VER configuration. The small white spheres correspond to the hydrogen atoms and the dark ones represent the uranium atoms.
Liu et al. [20] employed a density functional theory (DFT) and grand canonical Monte Carlo simulations (GCMC) to study the physisorptions of molecular hydrogen in single-walled BC3 nanotubes and carbon nanotubes in order to provide useful information about the nature of hydrogen adsorption and physisorption energies of these two nanotubes. Results show that the hydrogen storage capacity of BC3 nanotubes is superior to that of carbon nanotubes.
Fig. 7. The adsorption sites of a H2 in BC3 (8,0) nanotube: (a) above B-C bond; (b) on top of a B atom; (c) the centre of a hexagon.
Watari et al. [21] studied hydrogen storage in a Pd cluster (Fig. 8) using density functional theory and found stable sites for hydrogen adsorption.
Fig. 8. Hydrogen adsorption at different sites in (a) the Pd13/H6 cluster and (b) the Pd13/H8 clusters.
Schmidt et al. [22] studied the effect of hydrogen adsorption on gadolinium (Gd) islands grown on tungsten (W) tips of atomic force microscopes. A local reduction of the work function and the presence of localized charges on hydrogen-covered areas lead to a variation of the contact potential difference between tip and surface areas, which are clean or hydrogen-covered, and helps identify clean regions. These results are also important, because hydrogen alters the magnetic properties locally.
Sato [23] made an attempt to interpret the FEM patterns of a hydrogen adsorbed tungsten tip and discussed its application to hydrogen adsorption on single-crystal planes.
Babenkova et al. [24] have reviewed and compared quantum-chemical and experimental studies of the mechanism of formation of metal-hydrogen bonds and the properties metals of the iron sub-group in the chemisorption of hydrogen
Jia et al. [25] has used first-principle density functional calculations to show that the exposure of ZnO nanowires surfaces to atomic hydrogen can result in drastic changes the electronic properties of insulating nanowires.
Purewal et al. [26] has measured the pore size distribution (PSD) and supercritical molecular hydrogen adsorption in activated carbon fibers and has shown that the surface area and the PSD both depend on the degree of activation to which the ACF has been exposed.
Egawa and McCash [27] have studied the reactivity of hydrogen on ultra-thin FCC iron films grown on Cu, and measured the heat of adsorption of hydrogen, which can be particularly important in catalytic activities.
Oliveira et al. [28] studied the role of hydrogen adsorption on carbon terminated β-SiC and showed that the presence of adsorbed hydrogen atoms affects the atomic equilibrium positions, as well as electronic properties, of the atoms of the clean structure and concluded that a possible metallization, as a result of hydrogen adsorption, is theoretically postulated.
Ren [29] studied hydrogen adsorption and diffusion on the surface of austenitic stainless steel, using atom probe field ion microscope (AP-FIM) and calculated the diffusion activation energy.
White and Woodruff [30] investigated the effect of hydrogen adsorption on the surface reconstruction of silicon.
Reddy et al. [31] reported the synthesis, characterization and hydrogen storage properties of different types of boron nitride nanostructures, and discussed the dependence of hydrogen storage capacity on the morphology of BN nanostructures
Venkataramanan et al. [32] investigated hydrogen adsorption on alkali atom doped B36N36 clusters using first-principles calculations. Adsorption of alkali atoms involves a charge transfer process, creating positively charged alkali atoms, polarizing the H2 molecules thereby, increasing their binding energy. The fully doped Li6B36N36 cluster has been found to hold up to 18 hydrogen molecules with the average binding energy of 0.146 eV, corresponding to a gravimetric density of hydrogen storage of 3.7 wt.%. Chemisorption on the Li6B36N36 has been found to be an exothermic reaction, in which 60 hydrogen atoms chemisorbed with an average chemisorption energy of ~2.13 eV. Thus, the maximum hydrogen storage capacity of Li doped BN fullerene is 8.9 wt.% in which 60 hydrogen atoms were chemisorbed and 12 hydrogen molecules were adsorbed in molecular form.
