Theoretical Investigations On Structure, Electronic Properties, And Gas Adsorption Of BN Nanotube With Vacancy Defects Or Doped Defects

In recent years, much research interest was focused on nanotubes, due to their unique structure, electronic properties, and potential application. Advanced theoretical-computational techniques combined with the power of computers provide an understanding of nanosized materials at the atomic scale with an unprecedented level of detail and accuracy, enabling nanosized material design and property prediction to realize. Boron nitride nanotube (BNNT) is an analogous to carbon nanotubes in many respects. Surprisingly, the amount of successful research work performed on BNNTs was negligibly lower compared to that on CNTs. We note that the reports on the doped BNNT mainly about metal adsorption on the NT sidewall, which could lead to detachment of metal atoms from the BNNT when approaching molecular is excessive. Few works have been reported on the substituted doped BNNT. However, the C-doped and Si-doped BNNT have been synthesized. It would be valuable to explore the structures, electronic properties, related mechanism, hydrogen storage, and small molecular adsorption of substituted doped BNNTs.Using density functional theory (DFT)-based first-principles calculation, such as Gaussian 03 and DMOL3 in Material Studio, we report a systematic study on the BNNTs with defects or substituted doped BNNT. According to natural bond orbital (NBO), frontier molecular orbital (FMO), band structure, and local density of states (LDOS) analyses, we explore their structure, electronic properties, hydrogen storage, and small molecular adsorption.At first, a systematic study of armchair BNNTs with defects has been carried out within DFT. All the seven defects we investigested show symmetrical charge distribution according to natural bond orbital analysis. Those atoms close to the defect point alleviate the charge redistribution brought by the removed or replaced atom. The analyses of spin density and LDOS reveal that the defects have localized electronic states and are reactive towards attacking reagents. The defects change the energy gap and the conductivity of the BNNTs. The substitution of boron with carbon enhances the field emission of the tubes. Doping or vacancy defect is a good way to functionalize nanotubes and create active center on nanotubes, thus broadening the applications of nanotubes in chemistry and material sciences.Secondly, substitution of all ten 3d transition metal (TM) atoms Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn on a [8, 0] zigzag single walled BNNT has been investigated with DFT based methods. The TM atoms protrude to the exterior of the sidewall, which may facilitate this site to react with an approaching molecular or atomic species. The doping of TM atoms induces certain impurity states within the band gap of the pristine BNNT, thereby reducing the band gap and increasing the conductivity. Generally, the TM-doped SB is the major substitution doped BNNT and the substitution is site selective when Nd≤4 (Cr). However, the other TM atom (from Mn to Zn) doped BNNT would be a mixture of SB and SN. Spontaneous magnetization occurs in those cases of TM with even d electrons.Thirdly, novel Pt doped armchair (5, 5) single-walled BNNTs have been studied within DFT. The Pt atom protrudes to the exterior of the sidewall and favors attack from approaching molecule. The smaller energy gap for the Pt-doped BNNTs implies that their conductivity is higher than that of the pristine BNNT. The DFT predictions suggest a strong affinity of the Pt atom in BNNT towards hydrogen molecules. The adsorption of H2 onto the Pt-doped BNNT is predicted to be different from that of a pristine BNNT and from that of Pt-doped CNT. The adsorption of the first H2 molecules on the Pt atom of the Pt-doped BNNT is exothermic. The binding energies of H2 with Pt-doped BNNTs are in the optimal range for hydrogen storage. The mechanism of H2 adsorption on Pt-doped BNNT is a combination of charge transfer and charge polarization.Up to two H2 can be partially dissociated with weak chemistry adsorption, which improves the hydrogen storage capacity.At last, we report a systematic study of several small moleculars adsorption, such as CO,CO2,O2,NH3,H2O,C2H2,C2H4, on Pt substituted doped BNNT. According to NBO, FMO, and LDOS analyses, we note that the small molecular adsorption would not destroy the stability of BNNT. The conductivity of the Pt-doped BNNT is decreased when one or more molecular is adsorbed. The adsorption energy, adsorption mechanism, and charge chansfer is different with each other. These results help us to obtain the insight on the gas sensor materials, nano-electronic, catalysts and novel organic compound application of doped BNNT.The enhanced reactivity of doping or defect BNNT reveals that these modifications are benifit to functionalize nanotubes, improve conductivity of the BNNT, hydrogen storage ability, and small molecular identification, thus broadening the application of nanotubes in chemistry and material sciences.