Adsorption Behavior of CO on Pristine and Doped B 12 P 12 Nanocage : A DFT Study

Adsorption of the pollutant gas CO on B12P12 nanocage surface is studied through density functional calculations. HOMO and LUMO energy levels ,binding energies and energy bond gaps of three possible configurations of CO on pristine B12P12 has been calculated by means of B3LYP and M062X functional with 6-31g+ basis set.The results showed that there is no or very slight adsorption of CO molecule by pristine B12P12.To overcome the fault, CO adsorption is investigated on Al and N doped B12P12 nanocage with the same method and basis set.The electronic and structural parameters like HOMO and LUMO energy levels and binding energies of possible configurations are calculated and showed that doped B12P12 with both Al and N atoms have increased about % 1.7 with CO molecule which also indicates more Van der Waals attraction between CO and Al and N doped B12P12 nanocage.


INTRODUCTION
Detection and control of gases are important issues in environmental studies and much experiments have been made on the development of materials which can be used as gas-sensitive and dangerous chemical removal devices 1,2 .Carbon monoxide (CO) is one of the gases which has been vastly studied because it is one of the most important pollutant gases in the environment.The main sources of this gas are automobiles and industrial process.CO can effect dangerously on the human body and the ecosystem of animals.In most studies CO gas has been detected in a range of 4-10 ppm.This gas has no color and no odor 3 .
Carbon nanotubes as known as CNTs can adsorb gases such as CO and NO molecules but the energy for adsorption of these two gases by CNTs are very weak 4,5 .Some quasi-onedimensional nanotubes and III-V compound semiconductors which are isoelectronic to CNTs such as BN, AlN, BP and AlP have been investigated [6][7][8][9][10] .These materials may be good substitutes for detecting pollutant gases such as CO and NO.Theoretical studies of several (XY)n cages (X=B, Al, …and Y=N, P, …) showed the fullerene-like cages X 12 Y 12 can be the most stable ones which demonstrate specific stability when n is 12 [11][12] .At first, BP nanocages with 4-and 6membered rings have been synthesized, but later nanocages of B 12 N 12 were synthesized by Oku et al. indicated that B 12 N 12 nanocages have 4-and 6membered rings of BN 13 .BP are polar materials because of the slight positive charges of boron (B) atoms and the slight negative charges of phosphorus (P) atoms.The BP is a semiconductor compound which can be used in high temperatures and hard conditions in optoelectronic and microelectronic devices because of their special properties and abilities 14 .
In the first step of this work, we investigate the interaction and the adsorption energy of CO molecule on pristine B 12 P 12 nanocage using two density functional theory (DFT) calculations.As the next step, we continue calculating the interaction of Al and N doped B 12 P 12 nanocage with CO molecule using same (DFT) calculations.Our porpose of this experiment is to examine the capability of B 12 P 12 nanocage and its doped variations for detecting CO molecule and the effects of Al and N atoms on B 12 P 12 /CO on electronic level.

Computional methods
By using DFT functional methods (B3LYP,M062x) with 6-31g* basis set, Geometry optimizations , energy calculations , and density of states (DOS) analysis of B 12 P 12 , CO , three different CO/B 12 P 12 complexes configurations , three doped B 12 P 12 nanocages and their complexes with CO were performed in GAMESS suite of the program 15,18 .Vibration frequencies were also calculated at the same level to confirm that all the stationary points correspond to true minima on the potential energy surface .In this study, the dopant atoms are Aluminum and Nitrogen which are chosen from the third and fifth groups of the periodic table of elements, respectively.B3LYP is a basis set which is mostly used for investigations and calculations of nano structure topics [19][20][21][22][23][24] but in this paper M062x is used as well 16 .

Adsorption of CO on pristine B 12 P 12
Formation carbon monoxide from the free atoms requires separations and a charge transfer from the atom C to O, but before reaching the equilibrium bond distance, this charge transfer reverses.Politzer et al. calculated the MEP of the carbon monoxide and showed that negative regions belonged to both C and O atoms.It means both carbon and oxygen atoms have neucleophilic characters 25 .Azizi et al. showed that the adsorption of CO molecule on CNTs via the oxygen head is stronger than that via its carbon head 26 .Figure 1a shows the MEP of the carbon monoxide.
In order to find the most stable COadsorbed configuration, several configurations were used for optimization including C head or O head located on top and parallel on B 12 P 12 nanocage.In this report, CO molecule is optimized by using two DFT functional methods (B3LYP,M062x). the results can be seen in table 1.
The MEP of B 12 P 12 is shown in figure 1b, it shows charge integration on top of P atoms on B 12 P 12 surface.By comparing figures 1(a) and 1(b), it can be concluded that the most stable configuration of CO and B 12 P 12 is due O atom of CO molecule and B atom of B 12 P 12 nanocage.Figures 2 and 3 show the optimized structure of B 12 P 12 nanocage by using B3LYP and M062x and also theirs DOS plot.According to these results, by using B3LYP and M062x methods, the optimized nanocage has HOMO-LUMO energy gap (Eg) of 3.70 eV and 5.54 eV, respectively (table2).It can be concluded that, with M062X as DFT functional HOMO-LUMO energy gap is increased.The figures also show slight changes of angles in optimized B 12 P 12 by changing from B3LYP to M062X.
Figure 4 shows three possible configuration of CO/B 12 P 12 complexes.In these configurations, CO molecule was located in 1.9 Å distance from B 12 P 12 nanocage .In the first step of this work, , these three configurations are optimized by using DFT functional method B3LYP.Then their DOS plots and HOMO-LUMO energy gap (Eg) are determined and compared.The optimized complexes and their DOS plots are shown in figure 5.
As it is shown in figure 5, in the three optimized configurations the distance between CO molecule and B 12 P 12 nanocage are more than 1.9 Å .In the configuration (I), the distance between O atom of CO and B atom of B 12 P 12 is 3.38Å.While in the configurations (II) and (III) the distance between C atom of CO and P atom of B 12 P 12 is 3.81 Å and 3.77 Å ,respectively.Figures 5 and 6 show that by using M062X, the calculated Van der Waals bonds between B 12 P 12 and CO in configurations (I) and (III) are slightly shorter than the calculated bonds using B3LYP.While the calculated bond in configuration (II) using M062X is longer than the calculated Van der Waals bond using B3LYP.

The effect of Al and N doping on B 12 P 12 nanocage and their adsorption
In this level, we decided to study the effect of structure variation on the adsorption of B 12 P 12 in comparison with its pristine form.To do such a doping, first one of the Boron atoms is replaced with an Aluminum atom which has more metallic characteristic.Second, one of the Phosphorous atoms is replaced with one Nitrogen atom and in third step, one of the Phosphorous atoms and one of the Boron atoms of pristine B 12 P 12 are replaced with one Nitrogen and one Aluminum atom, respectively.In this step all optimizations are done using M062X method and 6-31g + basis set.
At the first step, the effect of Al doping is investigated by the obtained results using M062X method and 6-31g + basis set, the distance between O of CO molecule and Al atom in B 12 P 12 is 2.51 Å which is shorter than the distance between B and O in the configuration (I) of CO/B 12 P 12 using same method and basic set.In the next step, the interaction of N doping in B 12 P 12 and CO molecule is calculated using same method and basic set.The results show that the distance between C of CO molecule and N atom in B 12 P 12 is 3.43 Å which is shorter than the bond length between C atom and P atom in the configuration (II) of CO/B 12 P 12 using same method and basic set.In the third step, the effect of Al and N doping together in pristine B 12 P 12 on the adsorption of CO is being studied.As the figure shows after optimization the CO molecule is placed with O head on top of Al atom of the nanocage.The distance between O atom of CO and Al in the complex is 2.44 Å which is shorter than the bond length in the two previous configurations and between B atom and O atom in the configuration (I) of CO/B 12 P 12 using same method and basic set.It can be cocluded that Al and N doped B 12 P 12 nanocage has more interaction with CO molecule than other studied configurations in this paper.

Fig. 8 :
Fig. 8: B 12 P 11 N and B 12 P 11 N /CO and their DOS plots (distances are in A° and angles are in degree)

Fig. 9 :
Fig. 9:AlB 11 P 11 N and AlB 11 P 11 N /CO and their DOS plots (distances are in A° and angles are in degree)

Table 4 : Total energy, binding energy, energy of HOMO and LUMO orbitals, Eg value and %Eg of Al and N doping B12P12 nanocages and their complexes with CO molecule
Figures 7,8 and 9 show the change of angles in pristine B 12 P 12 and its Al and N doped configurations.Slightly differences in angles of B 12 P 12 and AlB 11 P 12 can be seen but this differences in angles are increased in B 12 P 11 N and AlB 11 P 11 N.It can be the result of the effect of lone pair of doped N atom.The DOS plots of the Al-and N-doped B 12 P 12 and their complexes with CO are shown in figures 7,8 and 9.As it is shown in those figures, the HOMO-LUMO energy gap (Eg) of AlB 11 P 12 , B 12 P 11 N and AlB 11 P 11 N after optimization are 5.06eV, 5.50eV and 5.27ev, respectively.It indicates no considerable change in the electronic properties of the three nanocages comparing with pristine B 12 P 12 .The results in these figures also show that interactions of the three doped nano cages with CO molecule.According to comparison Eg value of AlB 11 P 12 with AlB 11 P 12 /CO , the calculated % Eg is 1.38% .It can conclude that there is some weak van der Waals interactions in the system.Also comparing HOMO-LUMO energy gap of B 12 P 11 N with B 12 P 11 N /CO show us no difference in Eg value of the two structure so the calculated band energy gap variation is 0%.It shows no significant interaction exists between CO and N doped B 12 P 12 .Finally, According to the DOS plots and HOMO-LUMO energy gaps of AlB 11 P 11 N with AlB 11 P 11 N / CO ,a band energy gap variation of 1.7% is calculated .The result again shows a weak van der Waals interactions exists in the system.CONCLUSION The behavior of CO molecule adsorbed on the external surface of B 12 P 12 nanocage cluster was studied using two DFT functional methods, B3LYP and M062X.Calculating the HOMO/LUMO energy gap and adsorption energy of B 12 P 12 and its three configuration with CO with two DFT functional methods showed no significant change in the HOMO-LUMO gap of the nanocage.Again the behavior of CO molecule was detected on Al and N doped B 12 P 12 nanocage.The results showed that adsorption of CO on Al and N doped B 12 P 12 , above the Aluminum atom of the surface as AlB 11 P 11 N/CO has the most stable state of adsorption and most increased in the HOMO-LUMO gap of the other Al and N doped B 12 P 12 configurations.The %Eg of AlB 11 P 11 N/CO is calculated %1.7 which less than %7-8.As the result, B 12 P 12 cannot be a potential efficient gas sensor for CO detection .