Theoretical Study On Electronic Structure Regulation And Gas Molecular Adsorption Properties Of Boron Nitride Nanotubes

Currently,human energy is mainly obtained by burning non-renewable fossil energy sources.Although natural gas is also a non-renewable energy source,compared to coal and oil,its combustion only produces carbon dioxide and water,and does not produce other harmful gases.Natural gas（NG）,whose main component is methane（CH₄）,is a transitional fuel to the low-carbon energy future because of its abundant reserves,high calorific value of combustion,and low CO₂ production.Current storage NG methods include liquefied natural gas（LNG）,compressed natural gas（CNG）,and adsorbed natural gas（ANG）.The main challenge for natural gas fuel applications is to develop safe,economical and efficient CH₄ molecular adsorbent materials.Boron nitride nanotubes（BNNT）can be an ideal material in the field of gas storage because of its suitable pore size and mechanical strength,ultra-high specific surface area,and lighter mass.Therefore,in this paper,the adsorption properties of gas small molecules with metal-modified,non-metal atom-doped BNNT structures and their mechanisms are investigated based on density flooding theory（DFT）and Giant Canonical Monte Carlo（GCMC）methods.BNNTs are classified into sawtooth,armchair,and helical types according to the curl pattern,and are classified according to each layer（m,n）,where m,n are integers,indicating that each layer of nanotubes contains the number of different kinds of atoms.When m>n,the nanotube configuration is sawtooth type;when m=n,the nanotube configuration is armchair type;when m<n,the nanotube configuration is spiral type.Since BNNT and carbon nanotubes（CNT）have similar structures and CNT has good adsorption ability to CH₄ molecules,two structures,BNNT（11,0）and BNNT（8,0）,were selected for the study.It was found that the adsorption performance of the system obtained by GCMC method for CH₄ can correspond well with the results obtained by DFT method.First,the predicted structures of CH₄ molecule adsorption using the GCMC method for BNNT（11,0）and BNNT（8,0）showed that the CH₄molecule concentration reached 5423.265 mol/m³ and 4048.633 mol/m³ at a temperature of233 K and a pressure of 60 bar,respectively,with adsorption amounts of 50.544 wt.%and51.188 wt.%.To verify the accuracy of the results,the adsorption performance of CH₄molecules was studied using the DFT method for BNNT（11,0）and BNNT（8,0）,and it was calculated that BNNT（11,0）was able to adsorb up to 33 CH₄ molecules,and its adsorption amount was able to reach 49.23 wt.%,when the average adsorption energy of CH₄ was-0.145e V.When the BNNT（8,0）substrate is used for adsorption of CH₄ molecules,it can adsorb up to 24 CH₄ molecules on its surface,and its adsorption capacity also reaches 49.23 wt.%,but its average CH₄ adsorption energy decreases to-0.134 e V.During adsorption,the first layer of CH₄ molecules produces weak polarization in the electric field generated by the substrate perpendicular to the direction of the substrate,which weakens the Coulombic repulsion between CH₄molecules.This weakened the Coulombic repulsion between CH₄ molecules and enabled the CH₄ molecules to adsorb more firmly onto the substrate.Since the CH₄ molecules in the second and third layers are far away from the substrate,the adsorption force between CH₄ molecules mainly comes from the interaction force between CH₄ molecules.Since both BNNT（11,0）and BNNT（8,0）structures failed to meet the US DOE standard for CH₄ storage capacity.Through a comprehensive comparison,BNNT（8,0）,which has a stronger symmetry,was selected for surface modification.First,using metal atoms directly modified to the substrate surface,it was found that due to the low surface chemical activity of BNNT（8,0）,the metal atom binding energy was much smaller than the cohesion energy,which would greatly increase the difficulty of experimental preparation of the metal atom modified structures.If B atoms were selected for doping,it was found that the B-atom-doped BNNT（8,0）structure had excellent adsorption properties for CH₄ molecules,but after the adsorption of CH₄ molecules onto the substrate,the CH₄ molecules would have undergone cleavage at the substitution site,which in turn could not be well desorbed.It was calculated that after B doping to the nanotube surface,the nanotube surface chemical activity was significantly improved,and the binding energy was much higher than the cohesion energy after the metal atoms were modified to its surface.So attempts were made to modify the B-doped BNNT（8,0）structure using different metal atoms.Finally,Li atom was chosen as the modification atom for nanotube surface modification because of its light mass and the significant improvement of CH₄ adsorption energy after modification.Considering the structural symmetry and stability,four B atoms were used for nanotube surface doping and four Li atoms were used for surface modification to finally obtain the 4Li-BBNNT（8,0）structure.Firstly,the energy storage performance of the structure was predicted using the GCMC method for CH₄ molecules,and the results showed that the adsorption amount of CH₄molecules could reach 57.96 wt.%at a temperature of 233 K with a pressure of 60 bar,and the concentration of CH₄ molecules in space was 5423.471 mol/m³.Using the DFT method,it was found that the optimal adsorption sites for CH₄ molecules were located in the vicinity of Li atoms nearby and there are only two optimal adsorption sites,and when the optimal adsorption sites are occupied,the adsorption energy of CH₄ molecules at other sites decreases significantly,which is due to the fact that these CH₄ molecules are far away from the doping and modification sites and have less charge transfer between them and the substrate,so when CH₄ molecules adsorb to positions far from the doping and modification sites,they can only rely on the induced polarization induced with the neighboring CH₄ molecules to stabilize adsorption to the substrate surface.The final storage capacity of 4Li-BBNNT for CH₄molecules can reach 57.10 wt.%and the concentration of CH₄ molecules in space is 5225.149mol/m³,when the average adsorption energy of CH₄ molecules is-0.146 e V.It can be seen that the BNNT（8,0）structure with B-doping by Li modification has more excellent storage performance for CH₄ molecules.To investigate the adsorption performance of BNNT on NO molecules,the BNNT（8,0）structure with higher symmetry was chosen to adsorb NO molecules.It was calculated that the outer surface of the unmodified BNNT（8,0）could not adsorb NO molecules,while the inner part could adsorb NO molecules through the confinement effect.To improve the adsorption capacity of BNNT（8,0）for NO molecules,two B atoms were used to replace the N atoms on the surface of BNNT（8,0）,resulting in four structures.The calculations show that NO can be stably adsorbed to the outer surface of the nanotube only when it is near the doping site,and the adsorption energy of NO is maximum only when the N atomic end is facing the substrate.The modification of the substrate using metal atoms is an effective way to improve the CH₄ adsorption performance of BNNT materials,which can not only significantly increase the adsorption energy of neighboring CH₄ molecules,but also increase the adsorption energy of the next-nearest CH₄ molecules by strongly polarizing the neighboring CH₄ molecules through increasing the negative electrical properties of the substrate,and thus increase the average adsorption energy of CH₄ molecules as a whole.Also,non-metal atom doping can increase the binding energy of metal atoms in the substrate as well as the adsorption energy of NO on the substrate surface.