Single walled CNTs (SWCNT) with chirality (8, 0) and saturated hydrogen atoms on each ends were constructed in this work. They have zigzag conformations with 128 carbon atoms in total. Seven repeat units were arrayed along the tube axis and the size was set at lengths of 17.84 Å and diameters of 3.19 Å. The hydroxyl group functionalized CNTs (CNTs-OH) were formed by chemisorbing 24 hydroxyl groups at the side walls of CNT and four hydroxyl groups were averagely distributed at two ends. The origin of the coordinate system was set the same as pristine CNTs and CNT-NH2 located at the center of the nanotube referred to our previous work.29 The ends of the CNTs were initially set at z = 8.92 Å and − 8.92 Å. Two different lithium ion densities were studied for lithium ions intercalation utilizing MD simulations.
For low lithium density study, eight Li ions were set at the upper and bottom ends of CNTs. Their coordinates were set at (0.85, 1.47, z0), (0.85, 1.47, z0), (5.20, 3.00, z0) and (5.20, 3.00, z0), where z0 = 13.0 or -13.0 Å. To exclude the influence of Li ion with CNTs’ interactions at the beginning of simulation, the distance between lithium and CNT end edge was set to be 4.1 Å. For high lithium ion density study, twenty lithium atoms were added into the system. Aside from previous eight lithium ions, additional twelve ions were set at (3.90, 2.25, z0), (3.90, 2.25, z0), (6.06, 3.50, z0), (6.06, 3.50, z0), (5.00, 0.00, z0), (0.00, 5.00, z0), where z0 = 13.0 or -13.0 Å. To make the study more specific, two confined hexagonal boxes with dimensions of 13.0×13.0×32.0 Å3 and 15.0×15.0 ×32.0 Å3 were adopted for the distance investigations of CNT-OH interacting with lithium ions.
VMD32 was utilized to create the initial structure of CNT-OH. The initial conformation with or without the addition of lithium ions were submitted for further calculations using Car–Parrinello MD simulations. In this work, Car–Parrinello MD31 was performed using the CPMD software package30. In all calculations, Perdew, Burke, and Ernzerhof (PBE) exchange–correlation functional33–34 was used. To assure energy convergence, a plane wave set with 80 Ry kinetic energy cutoff was adopted for valence electrons. The Troullier–Martins normconserving nonlocal pseudopotential35 was utilized to describe the valence electrons and ionic cores interactions in the Kleinman–Bylander form.36 The preconditioned conjugate-gradients (PCG) method and the L-BFGS method37 were utilized for wave function optimization and geometry calculation. In the MD calculations, the temperature was set to 300 K, the massive Nose–Hoover chain algorithm37–38 were utilized to achieve thermostatting. All the hydrogen atoms were substituted by deuterium atoms, and a time step of 4.0 a.u. (0.09676 fs) and a fictitious electronic mass of 500 a.u were used. At the beginning of MD simulations, the electronic wave functions were quenched to the Born–Oppenheimer surface. The kinetics study lasted 20 ps for CNT-OH system, and trajectories were collected every five steps. Periodic boundary conditions (PBC) are applied in all directions to simulate a bundle of CNTs, and only the Ʈ point of the Brillouin zone was considered for periodically repeated system calculation.