Chemisorption of BH3 and BF3 on aluminum nitride nanocluster: quantum-chemical investigations
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In this study, two functionals (B3LYP and ωB97XD) were used for density functional theory (DFT) calculation of two major boron compounds (BH3 and BF3) adsorption on fullerene-like Al12N12 nanocluster. High values of adsorption energy, −268.6 (−244.7) for BF3 and −224.5 (−196.4) kJ/mol for BH3 were found using ωB97XD (B3LYP) functional, indicating strong chemisorption which is the result of Lewis acid–base interaction of adsorbent and adsorbates. The high negative values of ΔG (Gibbs free energy) and ΔH (enthalpy) confirm spontaneous exothermic adsorption process. Further studies were done by taking into account the charge analysis, FMO (frontier molecular orbitals), MEP (molecular electrostatic potential), density of states (DOS), and reactivity of resulted systems.
KeywordsFullerene-like cluster Al12N12 Nanocage Chemisorption Boron trifluoride Borane
Borane (trihydridoboron) and boron trifluoride are two inorganic compounds with chemical formulas of BH3 and BF3, respectively. They are drab gases that are famous as substantial reagents in reaction pathway [1, 2]. In addition, they are known as significant Lewis acids (because of the electron deficiency of boron atom in their compounds) and general construction blocks for further boron complexes. Studying the interface conception of different B–N bonds is vital for energy storage viewpoint. Group III-nitrogen compounds are well-recognized as storage energy owing to their lightweight and inherent high releasing energy .
Although BH3 itself has not normally used as a reactant; however, it is a potential intermediate in adsorption of di-borane, B2H6, that is broadly used as a source of boron . Similarly, boron halides, including BF3 and BCl3, have been used as substitute probe molecules owing to the 2p1 electronic structure of boron that provides a powerful Lewis acid when shared with a halogen atom [5, 6]. The boron atom of BF3 has an unfilled pz-like orbital in perpendicular to the molecular plane and has a propensity to receive electron pairs, and this property causes adsorption of boron compounds. For example in our recent study , we investigated the adsorption of some boron compounds on the surface of nitrogen-doped graphene. We found formation of new bond between nucleophilic atom (N) and electrophilic atom (B), whereas there was weak physisorption in the case of pristine graphene. There are various investigations on BF3 and BH3 in literature in this regard. For example, Xu et al.  used DFT to study the hydroboration of the Ge(1 0 0)−2 × 1 surface with BH3. Based on their result, the Ge(1 0 0) surface displays rather different surface reactivity to dissociative adsorption of BH3 in comparison with the C(1 0 0) and Si(1 0 0) surfaces. In another work, dissociative adsorption of BH3 on the Si(100) surface has been searched with nonlocal DFT by Konecny and Doren . They revealed that a Si–B bond is constructed through a nucleophilic attack on boron, leaving BH2 and H fragments bonded to the surface.
Abee and Cox  searched on BF3 as a probe molecule to interrogate the basicity of Cr2O3 (101̄2) surfaces.
After a successful development of CNT and fullerene C60, various spherical fullerene-like configurations composed of inorganic non-carbon materials have been stimulated a great deal of interest [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. Strout et al. revealed that the fullerene-like nano-cages X12Y12 are the most stable structures among all (XY)n (X = B, Al, Ga,… and Y = N, P, As, …) semiconductors . Al12N12, Al12P12, B12N12, and B12P12 are of great significance because of their high steadiness, large energy band gap, and outstanding chemical and physical properties. Among them, B12N12 nanoclusters were synthesized by Oku et al. in 2004 . Different applications of nanostructure materials have been studied in recent literature [11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]. Because of important roles of boron compounds in different areas of study, recently we rummaged on the chemisorption property of boron trichloride (BCl3) on the surface of Al12N12 nanocluster . Following successful potential of mentioned nanocluster as an ideal adsorbent, we persuaded to search on the adsorption of other boron compounds on this kind of nanocluster. As we found, there is no investigation in literature on the BH3 and BF3 adsorption on the Al12N12 nanocluster. In this paper, I have employed DFT method to investigate the adsorption of above-mentioned molecules on the surface of Al12N12 nanocluster. The attained results were analyzed by considering adsorption energies, the natural bond orbital (NBO) charge transfer, frontier molecular orbitals, density of states, and global indices of activities.
Results and discussion
The experimental bond length of B–H and B–F in free BH3 and BF3 molecules is reported 1.19 and 1.31 Å, respectively [5, 34], which are very close to the values of 1.194 and 1.317 Å calculated by this study. It is shown in Fig. 2 that after adsorption of BF3 molecule, fully dissociation in one of the B-F bonds is happened, whereas partial dissociation in one of the B–H bonds is resulted. In the case of BF3, one B…F bond is elongated to 2.42 Å (83.3% increase) and two other bonds are partially extended to 1.33 Å. So, because of the large change in one of the bonds in the molecule, we can conclude fully dissociation of BF3 upon adsorption on Al12N12 nanocluster. On the other hand, for BH3, the elongation is less pronounced. It is shown in Fig. 2 that one of the bond lengths of B…H reaches to 1.36 Å (13.3% increase) and two other reach to 1.20 Å upon adsorption which attribute to the partial dissociation of molecule.
The values of adsorption energy (Ead, kJ/mole) using two functionals (wB97XD and B3LYP), nearest equilibrium distance of analyte- surface (de, Å), mean of bond distances b@66, and b@64 (Å) at the area of interaction, NBO net charge transfer (QNBO, e), change in the enthalpy and Gibbs free energy (kJ/mol)
In order to examine the thermodynamic feasibility of BF3 and BH3 adsorption on the Al12N12 at ambient temperature and pressure (T = 298.14 K, and P = 1 atm), I have calculated free energies (ΔG) and enthalpy changes (ΔH) of each system using the results of vibrational frequency calculations,. The computed values of ΔH are −240.3 and −187.3 kJ/mol, and those of ΔG are −195.1 and −140.6 kJ/mol, respectively. However, the lower value of ΔG compared to that of ΔH is because of the entropic influence. The negative value of Gibbs free energy as well as enthalpy of each system confirms that adsorption of above-mentioned adsorbates is a spontaneous exothermic process.
The electronic properties of free Al12N12 and its complexes
The HOMO–LUMO energies of Al12N12 nanocluster are −6.472 and −2.541 eV, correspondingly. As shown in Fig. 5 and the data of Table 2, the adsorption of BF3 and BH3 causes some changes in the electronic structure of the Al12N12 nanocluster. While the energy gap of pristine Al12N12 nanocluster is 3.931 eV, upon BF3 and BH3 adsorption, the energy gaps are reduced to 3.618 and 3.869 eV, respectively.
For BF3 adsorption, the HOMO of resulted system increases while the LUMO decreases; however, the change in the LUMO is more pronounced than HOMO, resulting in a decrease in the energy gap of system. On the other hand, upon adsorption of BH3, the energy of HOMO approximately remains unchanged while the LUMO decreases, resulting in a little decrease in the energy gap of system.
The global indices of reactivity for pristine Al12N12 and its complexes are given in Table 2. They are pretty vital parameters owing to they exemplify the reactivity and steadiness of a system. Entertainingly, the hardness of nanocluster (1.965 eV) decreases on complexation with BF3 (1.809 eV) and BH3 (1.934 eV). One can see that the change in hardness is comparatively small for BH3 adsorption (Δŋ = −0.031 eV) whereas meaningfully large alteration is observed for BF3 adsorption (Δŋ = −0.156). Meanwhile, hardness attributes to the steadiness of a complex toward distortion in the attendance of electrical field, based on the result, the Al12N12 complexes are more susceptible to deformation under electrical field than pristine nanocluster. Softness inversely changes compared to hardness, hence, it is predictable that softness for Al12N12−BF3 and Al12N12−BH3 will increase, resulting increase in the reactivity of complexes.
BF3 and BH3 with a low-lying empty orbital are strong Lewis acids, so their capability to accept electrons along with their planar geometries suggests that nucleophilic attack by a nucleophilic atom is performed. The goal of this research is investigation on the adsorption properties of BF3 and BH3 molecules on Al12N12 nanocluster considering DFT method. I found fully dissociative adsorption for BF3 and partial dissociative adsorption for BH3 molecules. The computed values of ΔH are −240.3 and −187.3 kJ/mol, and those of ΔG are −195.1 and −140.6 kJ/mol, for BF3 and BH3 adsorption, respectively. The negative value of Gibbs free energy approves that adsorption of above-mentioned adsorbates is a spontaneous process. Results of frontier molecular orbitals confirm some important changes in the electronic structure of the nanocluster upon adsorption.
I highly acknowledge the financial support received from the Iran Nanotechnology Initiative Council, Iran.
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