Structure, Properties, And Hydrogen Storage Of Dual-Ti-Doped Single-Walled Carbon Nanotubes Within Density Functional Theory

Nanomaterials, especially carbon nanotubes, is one of the most active research areas in the current world. Although the discovery of carbon nanotubes is less than 20 years, the pace of research on nanotubes is increasing rapidly, the results are very significant and the rapid progress on the experiment is amazing. The nanotubes own unique structure, electronic and chemical properties. It is one of potential hydrogen storage materials. It is insensitive to some gas impurities and can be reused. Our work based on density functional theory with Gaussian03 and Gaussian09, the structures, electronic properties and hydrogen abilities of a fragment of (7, 7) single-walled carbon nanotube (SWCNT) doped with one or two Ti atoms have been investigated.Our work includes the following sections:Firstly, We show forteen different dual-Ti-doped SWCNT fragments and two clips of single-Ti-doped SWCNT for comparison. Using Gaussian03-based theoretical calculation. The results show that:1 We study relative stability and geometry of dual-Ti-doped SWCNTs and we know the doping with a C dimer replaced by a Ti dimer is most energetically feasible. the optimized structure of XIV have changed greatly. The relative stability of the dual-Ti-doped SWCNTs correlates with the relative positions of the two Ti atoms, shorter distance comes with higher stability. The triplet state of IX is the most stable conformation. 2. We study electronic structure and reactivity of dual-Ti-doped SWCNTs, we know relative doping sites of the two Ti atoms have significant effect on the electronic structure of the doped SWCNTs, thus bringing about different properties and chemical reactivity for the metal-doped SWCNTs. The HOMO and LUMO mainly localize around the two Ti atoms. Spin polarization occurs at the doping sites with net spin mainly localizes on the metal centers. Closer distance between the two Ti atoms causes relatively larger spin on Ti atoms in those dual-Ti-doped SWCNTs in triplet. Doing of Ti in SWCNT affects the conductivity of doped SWCNTs according to the change of HOMO-LUMO energy gap before and after doping.Secondly, we choose typical eight dual-Ti-doped SWCNT from the fourteen structures and one single-Ti-doped SWCNT for comparision. Using Gaussian09-based theoretical calculation for studying hydrogen absorption ablilities. The results show that: 1. We study the single H2 adsorption, the first H2 in VI and VIII dissociates and the two H atoms bond to the two Ti atoms. These two H atoms carry negative charges, there exit charge transfer between the two H atoms and the Ti-doped SWCNT. The first H2 still keeps molecular frame adsorbed on the Ti atom in the other structures. The hydrogen adsorption stability of the dual-Ti-doped SWCNTs correlates with the relative positions of the two Ti atoms. There exit different influence for different Ti-doped SWCNTs according to the change of HOMO-LUMO energy gap before and after hydrogen adsorption. The adsorption energy of the first H2 for V is endothermic, the others are exthothermal. The adsorption energy of VI and VIII is largest, and the adsorption energy of II and VII render the system in the optimal range to work as hydrogen storage madia. 2. We study multiple H2 adsorption, conformation I can adsorb the most hydrogen molecules(six hydrogen molecules), VIII can adsorb five H2, the substitutionally doped Ti atom can adsorb one H2, but the adsored Ti atom can adsorb four H2. Although the conformation VII only can stablely adsorb four H2, the Ti dimmer inserts to SWCNT with one Ti atom lying outsides of the tube while the other Ti atom lying inside of the tube, the hydrogen adsorption on the Ti atom which lying inside the tube is difficult to escape from the tube, it is benefit to improve the hydrogen storage capacity of nanotubes. The distance between two Ti atoms in conformation I are long enough that essentially the hydrogen adsorption behave as isolated single-doping Ti atom in SWCNT as that in IX. NBO charges show that the charges of H2 are quite different, with the successive introduction of hydrogen, it have great influence on previous one, but those isolated hydrogen molecule have no effect on others. From the molecular orbital (MO) analyses, we can know there exit more or less orbital overlap for these stable adsorption hydrogen molecules, there is no orbital overlap for these isolated adsorption hydrogen molecule. The average adsorption energy of II, VII and VIII render the system in the optimal range to work as hydrogen storage madia.The relative positions of the two Ti atoms play a decisive role in all the dual-Ti-doped SWCNTs. When the two Ti atoms are in a appropriate location, the dual-Ti-doped SWCNT can own an ideal hydrogen storage capacity. This kind of hydrogen storage material is worthy of study.