First-Principles Study Of Chemisorption And Difffusion Of Small Molecules On Carbon Nanotubes

Carbon nanotubes (CNTs) exhibit unique electronic, mechanical, and chemical properties that make them attractive building blocks in many fields of nanotechnology. In particular, the large surface area and hollow geometry of carbon nanotubes make them easy to adsorb small molecules and sensitive to environmental exposure. Although great efforts have been made on these issues, there are still many concerns remaining elusive. In this thesis, the first-principles calculations are systematically performed to study the chemisorption behaviors of the molecular hydrogen, oxygen, and water on CNTs. The following advances have been achieved:1. Axial deformation and the net charge remarkably influence the H2 chemisorption on CNTs. The tensile and compressive axial deformation of the (8, 0) CNT can effectively influence the energy barrier for the direct chemisorption of H2. It is shown that, for the optimal chemisorption/desorption pathway, about a 7.05% compressive deformation of the CNT can lower the chemisorption barrier from 1.98 to 1.56 eV, and the desorption barrier from 1.64 to 1.20 eV. Besides, the chemisorption configurations would transform to an energetically more favorable configuration, through the diffusion of H on the CNT. However, the tensile or compressive axial deformation can not effectively modulate the energy barriers for the transitions between chemisorption configurations. The study on chemisorption of H2 on a series of charged CNTs reveals that the net charge can remarkably change the chemisorption barrier of H2. Generally, the negative charge can more remarkably lower the chemisorption barrier for the armchair CNTs, while the positive charge can more efficiently lower the barrier for the zigzag CNTs. Therefore, our results show that it is possible to improve the hydrogen storage capacity of carbon nanotube by applying the axial strain or injecting the charge to the carbon nanotubes.2. The kinetics properties of chemisorption, dissociation and diffusion of oxygen on the CNTs are strongly sensitive to the surface concavity-convexity. The first-principles calculations show that the chemisorption (cycloaddition) of a singlet O2 on the outer surface of the (8, 0) CNT has an energy barrier about 0.94eV, while the chemisorption barrier for the inner surface is much higher. On both the inner and outer surfaces of the CNT, the cycloadded O2 is likely to dissociate into two isolated O with an energy barrier of about 0.6~1.3 eV. Becausse of the different bonding strength on inner and outer surfaces and the presence of unique configuration with oxygen on the inner surface, the pathways of oxygen diffusion on the inner surface are completely different from that on the outer surface. And the dissociated O on the inner surface can diffuse easily with a small energy barrier of 0.33eV, while the diffusion on the outer surface has a relatively high energy barrier of 1.11 eV. It is also found that the radial deformations of CNT can remarkably enhance the chemisorption of O2 even on the inner surface. It is could be deduced that, when the CNTs are deformed by the alternating mechanical force, the protruding site can chemisorb O2 easily and the following concave surface can facilitate the diffusion. Therefore, the oxidation can be strongly enhanced.3. The water chemisorption is found to remarkably influence the electronic property of CNTs and water dissociative adsorption on the CNT may occur easily when an external electric field or doping atoms being introduced. The influence of water chemisorption on the electronic propertis of the (8, 0) CNT and the kinetics behavior of water chemisorption is studied by density founctional calcualtions. The results show that the water molecule dissociative adsorption on the (8, 0) CNT can obviously decrease the energy gap. Using the elastic band method, the chemisorption of water on the (8, 0) CNT is found to be very difficult, and the smallest energy barrier of the chemisorptions is 2.89 eV in our work, with the H and OH dissociated from water bonding at the opposite sites of carbon hexagon. However, the external electric field in tranverse direction or the substitution atom N on the CNT surface can obviously lower the energy barrier for water chemisorption. Therefore, the foreign atoms, such as the iron as the catalyst for growing CNTs, or the external electric field may lead the water can easy to chemisorb on the CNTs and further influence the electronic properties as the previously reported experiment.