3.1. Optimized Structures
In this work, first the structures of CNT and DNTs containing one doped atom like sulfur, nitrogen and boron (named as SCNT, NCNT and BCNT respectively) in the zigzag (5,5) CNT with 58 carbon atoms were optimized. To simplify the calculations, both ends of the CNT and DNTs were saturated with hydrogen atoms (10 hydrogens at both ends, totally 68 atoms). Afterward, to obtain the most stable configurations of CO molecule on the surface of CNTs, one CO molecule was located at two different configurations. In the first position (perpendicular, named as d position) CO molecule was placed vertically versus the orientation of CNT, and at the second position (parallel, named as p position) CO molecule was parallel with the orientation of CNT. Regarding the multiplicity of models, the acronyms were used including the simple and S, N and B-doped carbon nanotubes with perpendicular CO molecule with CNT-d, SCNT-d, NCNT-d and BCNT-d respectively and the simple and S, N and B-doped carbon nanotubes with parallel CO molecule with CNT-p, SCNT-p, NCNT-p and BCNT-p respectively. The optimized structures of the most stable configurations of CO molecule on their surfaces were illustrated in Fig.1. As shown in this figure, both N and B atoms retain on the surface of NCNT and BCNT but the S atom was on the top of the surface of SCNT (at the distance of 0.23 Å). In addition, optimized parameters of all structures were extracted and the results are shown in Table 1.