Theoretical Study On The Decoration And Gas Sensitivity Of BN Nanotubes

Some nanotubes, such as carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs), and so on, are special cylinders-like material, having hollow, fibroid structures, with extremely larger surface area. They exhibit many effects such as surface effects, volume effects, quantum effects, these characters are quite different with those of bulk materials. They have attracted much attention in science and technology, for example thousands of papers on nanotubes were published every year, due to their extraordinary hardness, well elastic, good toughness, better thermal and electric conductive, optical and magnetic, chemical catalyse properties and many other physical or chemical properties. The nanotubes will have widely potential applications in constructing nano-scale electronic devices, monitor, hydrogen storage, high strength fibres, adsorption material and so on in the future.The BN nanotubes are another nanotubular material synthesized in laboratory after the discovery of carbon nanotubes, they are alike in their structures with hollow cylinders and hexagon network on the nanotube sidewall.The B-N atoms on the BN nanotubes form sp² bonds just like the structure of graphite.The electronic property of BN nanotubes is different from that of carbon nanotubes. The band gap and electronic properties of carbon nanotubes depend on their chiralitys and diameters sensitively. However, BN nanotubes have the uniform wide band gap of 5.5eV, always showing semi-conductive properties regardless of their chiralitys and diameters. This unique property of BN nanotubes together with other two characters of high temperature endurance and anti-oxidant ability makes BN nanotubes a better candidate for nano-electronic devices and other nano technology applications. But the BN nanotubes are ionic and inertial material, and don't easily react with other materials. We can increase the activity of BN nanotubes and induce their semiconductor properties by doping or adsorbing some atoms on sidewall of them.In this dissertation, Our work, which was performed by using DFT calculation based on the current progress in the research of BN nanotubes, can be divided into two parts The first part introduces the theoretical fundamentals that we used in our research work, and the second part includes my main work done during my master degree. The following gives a brief outline of the main contents of this dissertation.1. In chapter 1, we studied the electronic structure, the mechanical, electronical and magnetic property of BNNTs and their main applications. The highest occupied valance band and the lowest unoccupied conduction band are mainly localized to N and B atoms, respectively, because of the higher ionicity of BNNTs. This indicates that when exotic atoms decorate on the sidewall of BNNTs at different sites may result in different electronic properties. For example, the hydrogen-decorated BNNTs have the character of p-type semiconductor when an H atom is adsorbed on a B atom. However, they have the character of n-type semiconductor when an H atom is adsorbed on an N atom. This property of BNNTs can be used in designing nano-electronic devices.2. In chapter 2, we introduced the basic knowledge of density functional theory, pseudopotential theory and so on.3. In chapter 3, we have investigated the optimized configuration and electronic structure of Al-doped BN (8, 0) nanotubes with Al atom substituting the B or N atom on the sidewall of BN(8,0) nanotubes. It shows that the Al-doping on N site induces some impurity states in the band gap, thereby reduces the band gap, and then increases the conductivity of the pristine BN nanotubes. But we find that it doesn't evidently modify the electronic structures of BN nanotubes when Al atom substitute the B atom, they are still semiconductor with wide band gap (>3.3eV), a little smaller than that of BN nanotubes. When gases, such as H₂ and CO, are introduced to the pristine and doped BNNTs near the Al site, the activity of Al-doped BNNTs increase and gases can be more easily adsorbed on the surface of doped BN nanotube with larger binding energy. CO can be physically adsorbed or chemically adsorbed on the surface of doped BNNTs with some impurity states induced within the band gap, which may influence the conductivity of doped BNNTs. However, H₂ is adsorbed on the surface of BNNTs with poor effects to the property of pristine and doped tubes. Therefore, BNNTs can be used as gas sensitive materials to detect CO gas. This discovery has not reported before.4. In chapter 4, the interaction between Li atoms and BN (8, 0) nanotubes was investigated, including the adsorb energy and energy barrier. We analyze the ability of Li storage in BN nanotubes, and find that the disfigurement will effects the action between Li atom and BN nanotubs. A very high barrier was found when the Li atom come near to the sidewall of BN nanotubes at the distance smaller than 2.0 A, and prevents Li atoms from going through the sidewall of BNNTs easily. However, there is a low potential energy area, cylinder-like, with a radius of 1.0 A inside of the BN nanotubes. We also saw the oscillation of Li atoms inside BN nanotubes in the process of molecular dynamical simulation.5. In chapter 5, the adsorption of some small gas molecules, such as CH₄, CO₂, H₂, H₂O, N₂, NH₃, NO₂, O₂, F₂ and so on, on the sidewall of pristine and C-doped BN nanotubes (9, 0) were investigated. We found that some gas molecules can be physically or chemically adsorbed on the sidewall of BNNTs according to their chemical activity, and the abilities of gas adsorbtion can be increase for the C-doped BNNTs . Usually, CH₄, CO₂, H₂, H₂O, N₂ and NH₃ can't be chemically adsorbed on the BN nanotubes surface easily, and the electronic structures of BNNTs can't be modified obviously no matter which the adsorption of gas molecules on pristine or doped BNNTs. However, NO₂, O₂, F₂ can be easily adsorbed on the sidewall of BNNTS and induce some impurity states within the band gap, thus increasing the conductivity of the BNNTs. So the BN nanotubes may be used as a promising gas sensitive material to detect some gases.6. The results had been concluded in chapter 6.