Theoretical Calculations Of Carbon Nanotubes Adsorbed With Alkali Metals In The Seawater

Among the nanomaterials, the carbon nanotubes (CNTs) attract great attentions due to unique geometrical structure and physical and chemical properties. The adsorption property is of great important to the material modification and practical application of the carbon nanotubes. In this thesis, first-principles calculations and researches are performed on an armchair (5, 5) single-walled carbon nanotube or a zigzag (9, 0) single-walled carbon nanotube with alkali metal adatoms (lithium, sodium or cesium). Conclusions are as follows:1. Alkali metal adsorption on carbon nanotubes. The adsorption property is determined by the physical and chemical properties of carbon nanotubes and alkali metal adatoms. The adsorption energy of a Li/Na/Cs adatom on the (9, 0) carbon nanotube is lager than that of an alkali metal adatom on the (5, 5) carbon nanotube. The (5, 5)/(9, 0) carbon nanotube with a cesium adatom is most stable on structure because of the smallest electronegativity of cesium element. The alkali metal atom moves towards the vacancy defect and the structure becomes more stable after an carbon atom is removed from the tip of the (5, 5)/(9, 0) carbon nanotube. The nearer distance between the vacancy defect and the alkali metal atom is, the more stable structure and the lager adsorption energy of the adsorption system becomes.2. Alkali metal adsorption on hydrated carbon nanotubes. The structure of (5, 5) or (9, 0) carbon nanotubes adsorbed with a water molecule is stable, while the adsorption energy is quite small. For the complete (5, 5)/(9, 0) single-walled carbon nanotube, the cesium/sodium atom has the largest/smallest adsorption energy. However, For the defective (5, 5)/(9, 0) carbon nanotube, the lithium/sodium atom has the largest/smallest adsorption energy. This result indicate that complete nanotubes prefer to adsorb cesium atom, while the defective ones prefer to adsorb lithium atom. Therefore, carbon nanotubes can be used to perform the selective adsorption of alkali metals in the seawater. 3. Influence of alkali metal adsorption on the properties of carbon nanotubes. All the axial and radial work functions of (5, 5)/(9, 0) CNTs increase linearly with electronegativities of alkali metals. Therefore, the work functions of complete/defective CNTs can be distinctly modulated by the alkali-metal adsorption. This research is of great significance in field emission of CNTs. After the alkali-metal adsorption, the semiconducting properties of capped (5, 5) P-CNT are converted into metallic properties. Work functions of the sidewalls of the CNTs are slightly higher than the axial work functions. The variations in the work functions are mainly attributed to the changes in the Fermi levels, while charge transfer from an alkali-metal atom to a carbon atom influences the vacuum levels. The induced dipole moments decrease the vacuum levels, leading to small variations in the work functions.