A Density Functional Theory Study Of Metal Rh Adsorption On The Carbon Nanotube

Carbon nanotubes (CNTs) have attracted increasing attention from researchers since their discovery by Iijima. Because of their striking structural and electronic properties such as small diameters, high aspect ratios, strong mechanical properties, and high thermal and chemical stability, CNTs have rapidly become the worldwide research focus. Doped or adsorption on the carbon nanotubes is an important way to synthesize functionalized carbon materials. The Rh adsorption behaviors inside and outside of a series single wall carbon nanotubes (SWCNTs) and double wall carbon nanotubes (DWCNT) were systematically investigated using density functional theory (DFT).The Rh adsorption behaviors inside and outside of a series of (n,0) zigzag single-walled carbon nanotubes (SWCNTs) were systematically. According to the optimized configurations and adsorption energies, it is found that the most stable configurations are I-H1 and O-H2. The adsorption energies of Rh outside the SWCNTs are higher than the inside ones. With the decrease of SWCNT curvatures, the differences of the adsorption energies between inside and outside configurations reduce, which are almost close to the adsorption energy on the graphene sheet. Charge density indicates that the well-proportioned charge density of the graphene redistributes due to the curvature effects, which leads more charge to the outside SWCNT than the inside one. The charge differences between the inside and outside tube were decreased with the increase of the tube diameter, which is well consistent with the adsorption energy difference. Bader charge populations inside and outside of the SWCNTs also show the similar trend. According to the partial density of states (PDOS),5s electrons of Rh transfer to the 4d orbital, then 4d electrons transfer to the SWCNTs. Therefore, the Rh atom is positively charged, while the SWCNT is negatively charged.The curvature and helicity of single-wall carbon nanotubes (SWCNTs) are the important factors which influence the adsorption behaviors of metal inside and outside carbon tubes. However, it is difficult to investigate the separate effects of SWCNT helicity on the adsorption behaviors of metal. In the present work, the armchair (6,6), zigzag (10, 0), and chiral (8,4) tubes with similar curvature were selected, then the Rh adsorption behaviors inside and outside the tubes were systematically investigated using density functional theroy (DFT). Due to the different SWCNT helicities, the stable configurations of Rh on tubes are different. The neighbor carbons interacted with Rh are varied with the different tube helicities, therefore, the Rh adsorption energies for the similar configuration are also different. It indicates that the outside charge density of SWCNT is higher than the inside one. Different helicities lead to different charge density variation along the radial direction. Charge density difference (CDD) shows that the orbital orientation of Rh adatom and the electrons obtained and lost are slightly different due to the different helicities. The bandstructures indicate that the doping band appears near the Fermi energy level. The (6,6) tube with Rh adtom still exhibits metallicity. When Rh adsorbs inside the (10,0) tube, the nanotube transforms from the semiconductor into the metallic pipe. However, the band gap reduces when Rh adsorbs outside the tube. After the Rh adsorption, the (8,4) tube band gap reduces.The Rh adsorption behaviors inside and outside of a series of (n,0)@ (n+9,0) (n=7-15) double-walled carbon nanotubes (DWCNTs) were systematically investigated. According to the optimized configurations and adsorption energies, it is found that the most stable configurations are I-H1 and O-H2,O-H1. The adsorption energies of Rh outside the SWCNTs are higher than the inside ones. With the decrease of SWCNTs curvatures, the differences of the adsorption energies between inside and outside configurations reduce. The adsorption energies of inside of DWCNT are higher than those inside of SWCNT. Howerver, the adsorption energies outside of DWCNT are lower than outside of SWCNT. Bader charge analysis show that the bader charges inside is higher than that outside.The bandstructures indicate that the bandgap of DWCNTs reduce compared with SWCNTs. A local DWCNT model was built and tested to be reasonable. Based on the simplified model, the adsorption behaviors of metal atoms on multi-wall carbon nanotubes are possible.