Interaction of a Titanium Atom with the Surface of Perfect and Defective Carbon Nanotubes

The dispersion of metal atoms across the surface of 1D and 2D carbon systems is the most accessible method for controlling their properties, which are attractive for various applications in electronics, energy, and catalysis. This work explores the features of the interaction between titanium atoms and the surface of carbon nanotubes using ab initio computer-simulation methods based on density functional theory. The investigation focuses on the peculiarities induced by the presence of various types of structural defects on these surfaces. To conduct the study, we select (7, 7) and (11, 0) nanotubes with similar diameters (≈1 nm) but with different types of conductivity: metallic and semiconductor type, respectively. Three types of defects are investigated: single vacancy, double vacancy, and topological defect. Two possible orientations of each type of defect relative to the axis of the tube are considered. The primarily used basis set is the atomic orbital basis (SIESTA package), and the plane-wave basis set (VASP package) is also employed in some test calculations. Computational experiments show that the binding energy of Ti atoms to a defect-free nanotube is always lower than that to defective nanotubes, regardless of the approximation used for the exchange-correlation functional (LDA or GGA). The binding energy values predicted in the LDA approximation are noticeably higher than those in the GGA approximation (up to ~15% for the (7, 7) tube and up to ~50% for the (11, 0) tube). Strongest binding occurs when titanium is adsorbed on a nanotube with a single vacancy; the resulting configuration can be considered as a defect where one carbon atom is replaced by a titanium atom.