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DFT Study of CO<sub>2</sub> Adsorption on the Zn<sub>12</sub>O<sub>12</sub> Nano-cage
DFT Study of CO2 Adsorption on the Zn12O12 Nano-cage
Bulletin of the Korean Chemical Society. 2013. Dec, 34(12): 3722-3726
Copyright © 2013, Korea Chemical Society
  • Received : August 25, 2013
  • Accepted : September 22, 2013
  • Published : December 20, 2013
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Mohammad T. Baei

Abstract
Covalent functionalization of a Zn 12 O 12 nano-cage with CO 2 molecule in terms of energetic, geometry, and electronic properties was investigated by density functional theory method. For chemisorption configurations, the adsorption energy of CO 2 on the Zn 12 O 12 nano-cage for the first CO 2 was calculated −1.25 eV with a charge transfer of 1.00|e| from the nano-cage to the CO 2 molecule. The results show that CO 2 molecule was significantly detected by pristine Zn 12 O 12 nano-cage, therefore the nano-cage can be used as CO 2 storage. Also, more efficient binding could not be achieved by increasing the CO 2 concentration. For Physisorption configurations, HOMO–LUMO gap of the configurations has not changed, while slight changes have been observed in the chemisorption configurations.
Keywords
Introduction
Carbon dioxide (CO 2 ) is known as a greenhouse gas (GHG) and has an important contribution in global climate changes. 1 2 The main source of CO 2 emission worldwide comes from fossil fuel electric power plants. 3 Capture and sequestration of the CO 2 emitted from different sources is thus one of the most pressing issues in the environmental protection. Therefore, it is very important to develop a simple, rapid and reliable method for the capture and sequestration of CO 2 in many cases. Adsorption of CO 2 on zinc oxide (ZnO) surfaces has attracted considerable attention in the last decade. Adsorption of carbon dioxide on the ZnO surface has been studied by different groups. 4 5
Fink 4 has studied adsorption of CO 2 on the ZnO
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surface. Its results showed CO 2 dissociation at oxygen vacancy of
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surface. Also, Sergio et al . 5 have reported CO 2 adsorption on polar surfaces of ZnO. They showed a clear interaction between the CO 2 molecule and the surface.
Nanostructures due to their novel properties are intriguing in cluster protection, nano-ball bearings, nano-optical magnetic devices, catalysis, gas sensors, and biotechnology. 6 7 In recent years, there have been numerous studies of the adsorption of CO 2 on solid surfaces 4 5 ; while there are few studies about the adsorption of CO 2 on nanostructures surfaces. Therefore, further study of CO 2 adsorption on the nanoclusters is important task. ZnO nanoclusters have been widely investigated both theoretically and experimentally. 6 7 Recently, stability of fullerene-like cages of (XY) n nanostructures have been investigated and it has been suggested that the fullerene-like cage (XY) 12 is energetically the most stable cluster among different types of (XY) n structures. 8 9 Therefore, it can be concluded that the fullerene-like cage (ZnO) 12 is energetically the most stable cluster in this family and would thus be an ideal inorganic fullerene-like cage. The aim of this work is to investigate theoretically adsorption of CO 2 on Zn 12 O 12 nano-cage based on analyses of structure, energies, stability, electronic properties, etc . Our results are likely to be useful in functionalization of ZnO nanoclusters, construction of a CO 2 storage material, nano electronic devices, and other applications.
Computational Methods
Spin-unrestricted B3LYP/6-31G* level of theory has been largely used to describe the adsorption CO 2 molecule on surfaces of Zn 12 O 12 nano-cage, specifically the structural and electronic properties. For the Zn atoms, the standard LANL2DZ basis set 10 was used. Earlier studies indicated that the computations based on the B3LYP/6-31G* level of theory could yield reliable results in study of different nanostructures. 6 11 This method was used to calculate the adsorption energy ( Ead ) of CO 2 molecule on the surface of Zn 12 O 12 nano-cage as follows:
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Where E CO2 / ZnO is the total energy of an adsorbed CO 2 molecule on the pure Zn 12 O 12 nano-cage, E CO2 is referred to the energy of a single CO 2 molecule, and E ZnO is the energy of the pristine Zn 12 O 12 nano-cage. Negative or positive value for Ead is referred to exothermic or endothermic processes, respectively. All the calculations were carried out by using the GAMESS suite of programs. 12
Results and Discussion
Optimized Structure of Zn12O12. The pristine Zn 12 O 12 nano-cage was allowed to relax in the optimization at B3LYP/ LANL2DZ level of theory. Optimized structure of the Zn 12 O 12 nano-cage is formed from eight 6-membered (hexagon) rings and six 4-membered (tetragon) rings with Th symmetry. Optimized structure and geometrical parameters of the Zn 12 O 12 nano-cage is shown in Figure 1 . As is shown in Figure 1 , two types of Zn–O bonds are computed in Zn 12 O 12 nano-cage, one with the bond length of 1.91 Å which is shared between two hexagon rings, and the other which is shared between a tetragon and hexagon ring with length of 1.98 Å. The angles in 4-membered and 6-membered rings in Zn 12 O 12 nano-cage vary from 88.9 to 90.8 and from 116.2 to 123.6, respectively. The calculated energy gap ( Eg = ELUMO ˗ EHOMO ) of the Zn 12 O 12 nano-cage was calculated from the total densities of states (DOS) results. As is shown in Figure 1(b) , the Eg of nano-cage is 4.19 eV, indicating that the nano-cage is a semiconductor.
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Structure of optimized Zn12O12 nano-cage and its electronic density of states (DOS). Distances are in angstrom.
Adsorption of CO2 on the Zn12O12. In order to determine the minimum adsorption energy structure of adsorbed CO 2 on the Zn 12 O 12 nano-cage, various possible initial adsorption geometries including both the carbon and oxygen atoms of CO 2 close to hexagon and tetragon rings, oxygen atom close to Zn atom, two oxygen atoms locating top of the two Zn atoms of a hexagon or tetragon rings and one of the oxygen atoms above the center of 4-hexagon or tetragon rings. After careful structural optimizations without any constraints, reorientation of the molecule has been observed in some states, and finally it was found that only three kinds of the considered configurations are stable and are shown in Figure 2 .
As shown in Figure 2(a) , the C atom of CO 2 molecule is bonded to O atom of the nano-cage, so that the plane of CO 2 has bent due to the intramolecular steric repulsion. In configuration (a), length of the newly formed C-O bond is 1.36 Å. The adsorption of CO 2 shows an apparent local structural deformation on both the CO 2 and the Zn 12 O 12 nano-cage. In the configuration, O–C–O angle of CO 2 molecule is reduced from 180° to 128.6° and the bond length of C-O is increased from 1.17 Å in isolated CO 2 to 1.27 Å in the adsorbed state. In addition, the length of Zn–O bonds in adsorbed ring increased from 1.91 and 1.98 Å to 2.14 and 2.39 Å in the configuration. Further indication of the deformation degree in the geometry of CO 2 due to the adsorption process is given by the bond reorganization energy ( Ebr ). Ebr is as the calculated energy difference between the full relaxed CO 2 molecule and its adsorbed state, in which for each state is summarized in Table 1 . Ebr of CO 2 molecule for this configuration is 2.5 eV and the Ead is ˗1.25 eV, indicating a strong interaction and chemisorption process. Natural bond orbital (NBO) analysis shows a charge transfer of ˗1.00|e| from the nano-cage to the CO 2 molecule. In the configuration, the vacant π* orbital of C=O in the CO 2 molecule accepts the electrons from the Zn 12 O 12 nano-cage and CO 2 π-bond breaking due to electron backdonation from the Zn 12 O 12 to CO 2 and the CO 2 molecule undergoes the structural distortion to a bent structure. Therefore, the O-CO angle is reduced to 128.6°, and the broken C-O bond is significantly elongated to 1.27 Å.
In configuration (b) ( Fig. 2(b) ), one of the oxygen atoms of CO 2 molecule is close to a Zn atom of the Zn 12 O 12 nanocage by an interaction distance of 2.37 Å. The Ead and Ebr of CO 2 molecule for this configuration are ˗0.40 and 0.01 eV, respectively and a charge of 0.03|e| is transferred from the CO 2 molecule to the nano-cage. The results indicate that this interaction is weak and should be considered as a physisorption. Another CO 2 physisorption approach is shown in Figure 2(c) , in which the interaction distance between both of the oxygen atoms of CO 2 molecule and the Zn atoms of a tetragon ring of the nano-cage is about 2.80 Å. This configuration has an Ead ˗0.37eV and do not shows charge transfer to take place between the CO 2 and Zn 12 O 12 nano-cage. Also, Ebr of CO 2 molecule for this configuration is zero.
There are several hexagon and tetragon rings in structure of the Zn 12 O 12 nano-cage as potential adsorption site; therefore the possibility of the second adsorption is interesting for consideration. In this configuration ( Fig. 3(d) ), two CO 2 molecule is adsorbed on the Zn 12 O 12 nano-cage. The Ead and Ebr of CO 2 molecule for this process is about ˗1.03 and 2.44 eV per CO 2 molecule with a charge transfer of ˗0.98|e|, which are slightly lower than that of one CO 2 adsorption due to the steric repulsion between two CO 2 molecules. In the next step, three and four CO 2 molecules are adsorbed on the Zn 12 O 12 nano-cage ( Fig. 3(e) and (f) ). The Ead for these configurations are about ˗1.05 and ˗1.12 eV per CO 2 molecule for three and four molecules adsorption. Ebr of CO 2 molecule for these processes are 2.43 and 2.45 eV per CO 2 , respectively. In comparison with the one CO 2 adsorption model ( Fig. 2(a) ), the Ead and Ebr of CO 2 molecule due to the steric repulsion between the CO 2 molecules is reduced.
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Models for three optimized structure of CO2/Zn12O12 configurations and their density of state (DOS) plots. Distances are in angstrom.
Adsorption of CO2 on the Electronic Properties of Zn12O12 Nano-cage. Finally, to better understand the interaction between CO 2 with the Zn 12 O 12 nano-cage, the influence of CO 2 adsorption on the electronic properties of the nano-cage was studied. The difference in energy between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), Eg , was calculated from DOS plots. As shown in Table 1 , with comparison of DOS of the free ZnO nano-cage and the physisorption configurations ( Fig. 2(b) and (c) ), it is found that their Eg value have changed about 0.24-0.72% after the CO 2 adsorption. In the physisorption configurations the valence and conduction level energies are relatively the same as for the pristine Zn 12 O 12 valence and conduction level energies. Therefore, the results show that the CO 2 adsorption through these configurations has not sensible effects on the electronic properties of the nano-cage. For functionalization or chemisorption cases ( Fig. 2(a) and Fig. 3(d)-(f) ), it is revealed from DOS plots that their valence level energies in the cases are approximately similar to that of the Zn 12 O 12 , while the conduction level energies some shift downwards. As shown in Table 1 , upon the CO 2 adsorption on the Zn 12 O 12 nano-cage, the Eg value of the nano-cage are more changed compared to the physisorption cases, in other words, when number of CO 2 molecules increased from 0 to 3, band gap of the ZnO nano-cage has changed about (1.67-9.78%). However, when 4CO 2 molecules are adsorbed, the band gap has changed about 0.95% due to the steric repulsion between the CO 2 molecules. In fact, with increasing of CO 2 numbers, the Ead of CO 2 molecules is decreased (see Table 1 ) and increasing of CO 2 molecules has no sensible effects on the electronic properties of the nano-cage. Therefore, Change of Eg value in the configuration F (with 4CO 2 molecules) is reduced.
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Model for 2CO2, 3CO2, and 4CO2 chemisorbed- Zn12O12 configurations and their density of states (DOS) plots. Distances are in angstrom.
Calculated adsorption energy per molecule (Ead), HOMO energies (EHOMO), LUMO energies (ELUMO), HOMO–LUMO energy gap (Eg), Sum of NBO charge on the adsorbed CO2per molecule (QT), Fermi level energy (EFL), and bond reorganization energy (Ebr) per molecule. Energies are in eV
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aThe change of HOMO–LUMO gap of Zn12O12 nano-cage after CO2 adsorption. bQ is defined as the average of total natural bond orbital charges (NBO on the CO2 molecule). cEbr is calculated as the average of energy difference between the geometry of CO2 after adsorption on Zn12O12 nano-cage and the full relaxed molecule
In a molecule at 0 Kelvin, Fermi level lie approximately middle of the Eg . Table 1 indicates that the Fermi level energy ( EFL ) of the physisorption configurations is increased from ˗4.84 eV in the pristine Zn 12 O 12 nano-cage to ˗4.75 and ˗4.81 eV in the (b) and (c) configurations. This increasing of EFL with CO 2 adsorption leads to a decrement in the work function which is important in field emission applications. The work function is the minimum energy required for one electron to be removed from the Fermi level to the vacuum. The decrement in the work function shows that the field emission properties of the configurations are improved upon the CO 2 adsorption. While, the EFL of the chemisorption configurations is shifted down (see Table 1 ) which leads to an increment in the work function. The increment in the work function shows that the field emission properties of the configurations are impeded upon the CO 2 adsorption and have a disadvantageous effect on the field emission properties of Zn 12 O 12 nano-cage.
Conclusions
Physisorption and chemical functionalization of CO 2 molecule on the Zn 12 O 12 nano-cage were studied using density functional calculations. Binding energy corresponding to adsorption of CO 2 on the Zn 12 O 12 in the most stable configuration was calculated to be ˗1.25eV with a charge transfer of 1.00|e| from the nano-cage to the CO 2 molecule. On the basis of our calculations, it seems that attachment of the CO 2 molecule on the walls of the Zn 12 O 12 nano-cage induces some changes in electronic properties of the cluster and its Eg is slightly reduced after covalent functionalization process. The results show that pristine Zn 12 O 12 nano-cage can significantly detect CO 2 molecule. Also, more efficient binding could not be achieved by increasing the CO 2 concentration. The strong adsorption of the CO 2 on the Zn 12 O 12 nano-cage shows the potential application of the ZnO-based materials for CO 2 capture and storage.
Acknowledgements
The publication cost of this paper was supported by the Korean Chemical Society.
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