These calculations were carried out based on the density functional theory by using a plane-wave base set and pseudopotential implemented in the Quantum Espresso package . The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) form was employed to compute the exchange-correlation energy-density functional. According to the Monkhorst-pack scheme, the Brillouin zone was sampled by 8 × 1 × 1 k-point mesh for structural optimization. An energy cut-off of 80 Ry was utilized for the plane wave expansion to solve the Kohn–Sham equations. In all calculations, atomic positions were optimized until the convergence threshold on total energy (a.u) and forces (a.u) were 10− 8 and 10− 7, respectively. Moreover, the convergence threshold (conv-thr) for selfconsistency was 10− 10. An armchair single-walled nanotube structure was considered with 30 zinc atoms and 30 oxygen atoms.
CO and CuO get close to ZnO in various positions with respect to the symmetry axis parallel and perpendicular to the plane in four states at top positions of zinc-oxygen atoms (Top or T1-Type and T2-Type), the zinc-oxygen bond bisector perpendicular (Bridge or B-Type) and the center of the hexagonal structure of ZnO nanotube (Hollow or H-Type) as shown in Figs. 1A and 1B. Therefore, the 16 possible configurations with CO close to ZnO nanotube were: a1T1, a2T1, a3T1, a4T1, a1B, a2B, a3B, a4B, a1T2, a2T2, a3T2, a4T2, a1H, a2H, a3H, and a4H. The 16 possible configurations with CuO close to ZnO nanotube were: b1T1, b2T1, b3T1, b4T1, b1B, b2B, b3B, b4B, b1T2, b2T2, b3T2, b4T2, b1H, b2H, b3H, and b4H. Next, the most optimal configuration was chosen.
CO gets close to CuO-ZnO nanotube in three states with the symmetry axis parallel and perpendicular to the plane in three states according to Fig. 1C. 3; possible configurations in this case were: C1T, C2T, and C3T, so the most optimal configuration could be investigated (Fig. 1C).
The binding energy (Ebind) of all possible configurations of adsorption of CO and CuO molecules on ZnO nanotube was calculated by:
Ebind = Etotal – (E(Zno nanotube) + Emolecule) (1)
where Etotal is the total energy of CuO adsorbed on ZnO nanotube, E(Zno nanotube) is the total energy of the ZnO nanotube, and Emolecule is the total energy of free CO or CuO molecules. Moreover, to find the binding energy, Ebind, between CO and CuO-ZnO nanotube structure (CO/CuO-ZnO nanotube), the following equation was used:
Ebind = Etotal – (E(CuO−Zno nanotube) + ECO) (2)
where Etotal is the total energy of the CuO-ZnO nanotube structure interacting with CO gas, ECuO−ZnO nanotube is the total energy of the CuO-ZnO nanotube structure, and ECO is the total energy of free CO molecules. The most stable configuration corresponds to a binding energy with the largest negative value.
Charge transfer is calculated from Mulliken charge analysis as follows:
Qt = Q(Adsorbed molecule) - Q(Isolateded molecule) (3)
In addition, the recovery time of (CO molecules from) ZnO and CuO-ZnO nanotube structures can be obtained as follows :
where k and T are the Boltzmann’s constant (8.62 × 10− 5 eV K− 1) and temperature, respectively; ϑ0 denotes the attempt frequency (ϑ0 = 1012s-1).