Theoretical Studies On The Geometry And Electronic Structure Of The Calcium Modified Carbon Nanotubes

Carbon nanotubes (CNTs), as a kind of novel nanomaterial, have attracted wide interests in the field of material,chemical, physical and even biological sciences for their unique physical and chemical properties since they were found in 1991. The adsorption properties of carbon nanotubes lay the basis for their modification and practical application. The surface doping and metal atoms adsorption on the nanotubes become the commonly used ways to achieve the modification of nanotubes. In this thesis, first-principles calculations are performed to study the adsorption configurations and electronic structures of calcium atoms adsorbed on an armchair (5, 5) single-walled carbon nanotube and a zigzag (9,0) single-walled carbon nanotube. The effects of nanotubes type and calcium atoms coverage are also discussed. Finally the adsorption behavior of CO2 on a calcium atom decorated (5,5) carbon nanotube is deeply studied.The adsorption configurations and electronic structures of (5,5) single-walled carbon nanotubes are systematically studied when different number of Ca atoms (from 1 to 15) adsorbed on them. The results show that the optimal adsorption site lies in the hollow (H) site at the center of a hexagon, and the Ca—C covalent bond are formed. The adsorption tendency turned out to be along the axial when more Ca atoms adsorbed on the (5,5) single-walled carbon nanotubes. With the increase of Ca atoms, the average adsorption energy increases firstly and then decreases, and reaches the maximum value in the case of four Ca atoms adsorption. The Ca adsorption is accompanied with electrons (more than 0.5 e) transferred to the carbon naotubes, which leads a rise of Fermi level and increases their electronical conductivity. The 4s orbital of Ca atom mainly concentrates around the Fermi level, and forms adsorption bonds with the 2p orbital of C atom in the hexagon.Take the (9,0) zigzag single-walled carbon naotubes for an example, the effect of nanotube type for the Ca adsorption behavior is also discussed. Similar to the (5,5) carbon nanotubes, the optimal adsorption site remains above the center of a hexagon, the electron transfer and bond formation style are still unchanged. Even though the diameter of the two kind of nanotubes are very close, the Ca adsorption behavior are greatly different for their unique arrangement modes of six-membered rings. The binding ability of Ca adsorption on (9,0) nanotubes is stronger, and there is no evident change in the adsorption stability of different orientations. In addition, the saturated coverage of Ca adsorption on (9,0) nanotubes comes lower than that of (5,5) nanotubes.The adsorption performance of CO2 molecules on the Ca modified (5,5) carbon nanotubes is further discussed. The calculation results indicate that the single CO2 molecule can be bonded with Ca atom in two ways, namely η2(C,O) and η2(O,O), and the linear CO2 changed into V-like structure after adsorption. When more CO2 molecules adsorbed, only one CO2 molecule bonds in the manner of η2(O,O), which correspond to the strong interaction. The others are combined with Ca atom by means of η1(O), which is a relatively weak interaction. For their special binding style, the average adsorption energy decreases with the increased number of CO2 molecules. The single Ca atom modified (5,5) carbon nanotubes can adsorb up to five CO2 molecules. From the electronic structure point of view, the carbon nanotubes mainly act as a carrier for Ca atom.