Density Functional Theory ( DFT ) Study of O 2 , N 2 Adsorptions on H-Capped ( 5 , 0 ) Single – Walled Carbon Nanotube ( CNT )

CNTs are one of the most significant achievements of nano-technology with important applications in the design of electronic nano-devices. Nano-tubes with small size, physical stability and sensitivity of their electric properties to adsorption of N2 and O2 make them ideal materials for use in gas sensors. In this investigation the (DFT) method is utilized to study the adsorption of oxygen, nitrogen molecules on the surface of (5,0) CNT. The electronic structure, NMR spectrum, dipole moment of nitrogen, oxygen and carbon nuclei's are thoroughly studied. The computational results indicate that rich adsorption patterns may result from the interaction of oxygen and nitrogen with the CNTs. Sometimes, C-O bounds are formed via breaking C-C bounds and sometimes a carbon atom in the nanotube is replaced with an oxygen atom. Sometimes oxygen and nitrogen molecular are attracted to a C-C bound. In summary, the optimized adsorption rates are calculated. The nitrogen molecules adsorb with a comparatively lower rate and almost never a chemical binding is formed with the CNT. Gaussian 98 software has been used to carry out quantum chemistry calculations. Keyword: Density functional theory (DFT), NMR spectrum, single–walled CNT, electronic structure, Gaussian 98 software.


Introduction
Carbon nanotubes have many fascinating properties 1,2 and numerous studies have been conducted on their synthesis 3,4 , treatments 5,6 and physical properties 7,8 .However, only a limited number of studies have been made on the adsorption of gases in carbon nanotubes; these include theoretical studies 9,10 , experimental work 11,12 Many researchers studied endohedral adsorption of nitrogen in single walled nanotubes of 10.2 Ǻ of diameters 13,14 .A large number of experimental studies have been carried out thus far on the adsorption of nitrogen 15,16 , oxygen 17,18 , carbon nanotubes single-walled.
Sensitivity of CNTs to oxygen and nitrogen exposure has been also demon-started by means of quantum mechanics calculations.Difficulties in CNTs production and spectroscopic characterization have made computational studies of these systems indispensable.Even though experimental techniques have advanced, theoretical calculations have preserved their significant role in predicting geometries and energy levels and interpreting spectroscopic data 19 .Pathways to oxygen macular chemisorptions on the (5,0) CNTs were probed by George E. Froudakis et al 20 and L.S. Cahill et al 21 , based on density functional theory, adsorption of O 2 molecule on the (8,0) CNT 22,23 , was studied.Binding energies, charge transfer and density of states corresponding to O 2 chemisorption and physisorption on (8, 0) SWCNT also was studied 24 .The effect of adsorbed oxygen molecule on the outer surface of a (10, 0) CNT was analyzed in more detailed by S. Peng et al 25 , which considered adsorption of O 2 molecule on graphite and (8,0) nano-tube, also the similar studies was performed 26,27 .Both local density approximation and gradient-corrected approximation (GCA) methods predict that O 2 molecular is physisorbed at perfect and defect sites of CNT, studies on physisorption and chemisorptions of O 2 on the outer surface of (4,2) CNT, performed by Liu et al 28 , indicating that (4,4) tube is less air-stable than (5,0) tube.
In this study, electronic structure properties of (5,0) SWCNT involved with an O 2 and N 2 molecules are investigated.Nuclear magnetic resonance NMR properties are calculated in order to obtain useful information about the nature of interactions in nano-tubes.Consequently, the bond strength of O 2 is greater than N 2 species adsorbed on CNTs.In our study, oxygen and Nitrogen electronic structure modification effects on NMR chemical shielding and dipole moment (5,0) of CNT, the influence of hydrogen doping (H-doping) on the electrostatic properties of the zigzag SWCNTs is studied by means of the CS tensor calculations at the sites of 13 C nuclei in representative model including H-doped of 7.1 nm long (5,0) zigzag SWCNTs 29 , (Table 2 and Figure 1).

Experimental
This study was performed in Payame Noor University, Sari center, Sari, Iran, as a research project over carbon nanotubes adsorption property in 2009 and 2010.

Computational details
In the present study, O 2 and N 2 molecules adsorption behaviors on the SWCNT is taken in to consideration.A (5, 0) CNT containing 40 carbon atoms with length of 7.1 Ǻ has chosen for the purpose.Due to the absence of periodic boundary conditions in molecular calculations, it is necessary to saturate the carbon dangling bonds with hydrogen atoms.All calculations were carried out at all-electron level using Gaussian 98 suite of programs. 30It has been established that DFT is able to accurately treat such systems due to incorporation of the exchange-correlation effects 31,32 .Geometry optimizations were performed using 6-311G * basis set with B3LYP functional 29,33 .We see that greater dipole moments are associated with higher absolute values of energy, this seems reasonable because greater dipole moments means larger shifts in the probability distribution of electrons and hence a larger change in energy levels.NMR chemical shielding calculations were carried out of theory using gauge independent atomic orbital's (GIAO) approach 34 .We have performed experiments on singlewall carbon nanotube SWCNT networks and compared with DFT calculations to identify the microscopic origin of the observed sensitivity of the network conductivity to physisorbed O 2 and N 2 .NMR chemical shielding calculations were carried out of theory using gauge independent atomic orbital's (GIAO) approach 34 .We have performed experiments on singlewall carbon nanotube SWCNT networks and compared with DFT calculations to identify the microscopic origin of the observed sensitivity of the network conductivity to physisorbed O 2 and N 2 .Previous DFT calculations of the transmission function for isolated pristine SWNTs have found physisorbed molecules have a little influence on their conductivity.However, by calculating we show that, physisorbed O 2 and N 2 affect the junction's conductance.This may be understood as an increase in tunneling probability due to hopping via molecular orbitals.We find the effect is substantially larger for O 2 than for N 2 SWNTs junctions, in agreement with experiment 35 .This may be understood as an increase in tunneling probability due to hopping via molecular orbitals.We find the effect is substantially larger for O 2 than for N 2 SWNTs junctions, in agreement with experiment 36 .Components of CS tensor are defined by following relation 37 : Where E is energy of the system and μ j and B i are components of magnetic moment and external magnetic field, respectively.CS tensor in the principal axes system (PAS) ( 33 > 22 > 11 ) is diagonal and thus, principal values for specification of shielding is defined for this coordinate system: Where σ ισο , Δς and η σ are isotropic, anisotropic and asymmetric parts of CS tensor, respectively, in which η σ in certain cases vanishes.

Results and Discussion
In this study, geometries and binding energies, 13 C NMR chemical shielding tensors, and total dipole moment of states of H-capped (5,0) SWCNT in terraced with O 2 and N 2 molecules, are taken in Tables(1&2).In the following sections, molecular geometries and binding energies, NMR chemical shielding resulted from O 2 and N 2 adsorptions are discussed, separately.

Molecular geometries and binding energies
The optimized geometries of calculated configurations of O 2 and N2 molecules adsorbed on (5, 0) SWCNT are schematically displayed in Figure 1.Geometrical parameters, binding energies and dipole moment are summarized in Table1.The nature of stationary points are confirmed by vibrational frequency calculations at the B3LYP/6-311G * level.For nitrogen and oxygen molecules we have considered distinct adsorption sites, marked as CNT (A, A 1, A 2 , A 3 and A 4 (Table1).CNT, CNT-O 2 and CNT-N 2 binding energies,  E ab , are calculated using: Where, E tot (CNT), E tot (O 2 ) and E tot (CNT+O 2 ), E tot (N2) and E tot (CNT+N 2 ) are the energies of the optimized tubes, that are adsorbate and tube-adsorb ate systems, respectively.Zigzag (5, 0) tube has two different C-C bonds ((C-C) 1 =1.401Ǻ and (C-C) 2 =1.465Ǻ) and thus offers two distinct adsorption sites.Such a structure has also been observed for other SWCNTs, using DFT-GCA method 12,13,38,39 .For the molecular O 2 -CNT system, O 2 is constrained to lie parallel to the outer surface of the tube.Two different types of adsorbed O 2 , N 2 molecules were identified (Figure 1. A, A 1 , A 2 , A 3 and A 4 ).The calculated binding energies were predicted to be -1111.617and -1110.745eV at the sites A 1 and A 2 and the sites A 3 and A 4 1.737 and 2.858 eV, respectively.Such adsorptions of O 2 molecule are known as cycloaddition which is very similar to those found for larger diameter tubes 40 .The nitrogen molecules adsorb with a comparatively lower rate and almost never formed a chemical binding with the carbon nanotube.Once such oxidation occurs and physisorbed product is formed, the geometry of (5,0) tube is considerably modified.However, this is not the case for the (4,2) tube as reported previously. 38The electron configuration of O 2 is KK ( 2s ) 2 ( 2s *) 2 ( 2pz ) 2 (  2px ) 2 ( 2py ) 2 ( 2px *) 1 (  2py *) 1 , The electron configuration of N 2 is KK (  2s ) 2 ( 2s *) 2 (  2px ) 2 (  2py ) 2 ( 2pz ) 2 , and the transferred electron is filled in the half-filled anti-bonding orbital of O 2 , hence weaken the O-O bond.The binding orbital of N 2 molecule is filled, so that the electron can't enter into this binding orbital.The O 2 -tube and N 2tube equilibrium distances for adsorption sites A 1 , A 2 , A 3 and A4 illuminate the interaction of O 2 and N 2 with CNT belongs to physisorption.The C-C distance increases from(C-C) 1 =1.401A˚ to (C-C) 1 =1.50 and (C-C) 2 =1.465A to (C-C) 2 =1.49A and hence C-O bind in A 2 site shows better adsorption, which is very similar to literature 24,40 .
It was found that O 2 binding energy decreases as the diameter of (n, 0) CNT increases.It is well known that the tendency for sp 2 -sp 3 rehybridization upon O 2 adsorption is strong for thin nanotubes, because highly bent sp 2 bonding of thin nanotubes is favored for the transition to sp 3 bonding.This is an essential reason for the binding performed studies, show that the values of energies of nitrogen molecules adsorption on armchair model with determined diameter and length have the differences about twice in grandeur.In addition, all these energies are positive which show the reaction is improbable.Based on these results we can conclude that the physical adsorption over the surface area of nanotube occurs very difficultly and so this is not a suitable case.We approach that, the adsorption carry out over open ends of nanotubes, based on performed calculations, has more advantages.
Based on these results we can conclude that the physical adsorption over the surface area of nanotable occurs very difficultly and so this is not a suitable case.We approach that, the adsorption carry out over open ends of nanotubes, based on performed calculations, has more advantages.
2.5764 2.858 a All calculated distances in A˚.All calculated binding energies in electron volt (eV).All calculated dipole moment in Debye.a Calculated ii ,  iso , ∆  values in ppm.
Tables 2 exhibit the calculated 13 C chemical shielding for CNTs.O 2 and N 2 adsorption on the CNT has a remarkable in flounce on 13 C NMR tensors which is in complete accordance with the facts mentioned above Previously, it has been indicated that for the H-capped CNTs, the calculated 13 C chemical shielding value sat the ends are smaller than in the tube's center if the carbon is directly bound to a hydrogen; otherwise it is larger 38 .It is also depicted that chemical shielding components converge in a way similar to that of the chemical shifts when increasing the tube length albeit not as smoothly as the isotropic shielding.On the other hand, the calculated 13 C chemical shielding values in the middle of The (5,0) CNT seem to approach values 149.1757and149.1663ppm (Table2).It may be noted that 13 C chemical shielding tensor a the carbon sites depends remarkably on the tube size and nature of frontier orbital's 38 .
DFT study of 13Cchemical shielding tensors on small-to-medium -diameter infinite SWCNTs revealed that chemical shielding decreases roughly inversely proportional to the tubes diameter 29 .The NMR chemical shielding of finite SWCNTs were found to converge very slowly, to the in finite limit, indicating that hydrogen capped tube fragments are not necessarily good models of infinite systems.For the hydrogen capped (9,0) tube case, all of the frontier orbital's have carbon p-s character, they are localized at each end of the tube 38 .As the length of the fragment increases, these orbital's do not yield a contribution to the electron density along the tube (except at the ends) and must therefore be regarded as artifacts due to treating the finite sized systems.
According to GIAO calculations performed after adsorption of O 2 13 C NMR parameters of those interacted carbon atoms are also modified.As understood by comparison of sites A 1 , A 2 , A 3 and A 4 , the carbon atoms included in O 2 adsorption become more shielded.
Among the two NMR principal components, intermediate shielding component,  22 , shows more change from nanotube to the O 2 -CNT system.The results are consistent with strong interaction between the tube and O 2 molecule.The discrepancy between the 13 C chemical shielding tensor for the site CNT, A 1 , A 2 , A 3 and A 4 systems must be attributed to the different nature of the frontier orbitals

Conclusion
According to DFT calculations, adsorption of O 2 molecules dramatically influences geometrical and electronic structure properties of (5,0) SWCNT.For the molecular oxygen adsorption, the binding energy at the A2 site is found to be1110.745eV, which is stable than A1 site.It is found that 13 C chemical shielding is appropriate parameter to investigate the nature of interactions in (5,0) SWCNT.The 13 C chemical shielding isotropy and anisotropy values vary remarkably from CNT to the CNT-O 2 and CNT-N 2 systems for two O 2 , N 2 adsorption sites.

Table 1 .
Calculated structural parameters and adsorption energies of N 2 and O 2 adsorbed on the (5, 0) SWCNT a .dipole moment