DFT Study of CO₂ Adsorption on the Zn₁₂O₁₂ Nano-cage

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.

Introduction

Carbon dioxide (CO2) is known as a greenhouse gas (GHG) and has an important contribution in global climate changes.12 The main source of CO2 emission worldwide comes from fossil fuel electric power plants.3 Capture and sequestration of the CO2 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 CO2 in many cases. Adsorption of CO2 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.45

Fink4 has studied adsorption of CO2 on the ZnO surface. Its results showed CO2 dissociation at oxygen vacancy of surface. Also, Sergio et al.5 have reported CO2 adsorption on polar surfaces of ZnO. They showed a clear interaction between the CO2 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.67 In recent years, there have been numerous studies of the adsorption of CO2 on solid surfaces45; while there are few studies about the adsorption of CO2 on nanostructures surfaces. Therefore, further study of CO2 adsorption on the nanoclusters is important task. ZnO nanoclusters have been widely investigated both theoretically and experimentally.67 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.89 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 CO2 on Zn12O12 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 CO2 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 CO2 molecule on surfaces of Zn12O12 nano-cage, specifically the structural and electronic properties. For the Zn atoms, the standard LANL2DZ basis set10 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.611 This method was used to calculate the adsorption energy (Ead) of CO2 molecule on the surface of Zn12O12 nano-cage as follows:

Where ECO2/ZnO is the total energy of an adsorbed CO2 molecule on the pure Zn12O12 nano-cage, ECO2 is referred to the energy of a single CO2 molecule, and EZnO is the energy of the pristine Zn12O12 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 Zn12O12 nano-cage was allowed to relax in the optimization at B3LYP/ LANL2DZ level of theory. Optimized structure of the Zn12O12 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 Zn12O12 nano-cage is shown in Figure 1. As is shown in Figure 1, two types of Zn–O bonds are computed in Zn12O12 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 Zn12O12 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 Zn12O12 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.

Figure 1.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 CO2 on the Zn12O12 nano-cage, various possible initial adsorption geometries including both the carbon and oxygen atoms of CO2 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 CO2 molecule is bonded to O atom of the nano-cage, so that the plane of CO2 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 CO2 shows an apparent local structural deformation on both the CO2 and the Zn12O12 nano-cage. In the configuration, O–C–O angle of CO2 molecule is reduced from 180° to 128.6° and the bond length of C-O is increased from 1.17 Å in isolated CO2 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 CO2 due to the adsorption process is given by the bond reorganization energy (Ebr). Ebr is as the calculated energy difference between the full relaxed CO2 molecule and its adsorbed state, in which for each state is summarized in Table 1. Ebr of CO2 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 CO2 molecule. In the configuration, the vacant π* orbital of C=O in the CO2 molecule accepts the electrons from the Zn12O12 nano-cage and CO2 π-bond breaking due to electron backdonation from the Zn12O12 to CO2 and the CO2 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 CO2 molecule is close to a Zn atom of the Zn12O12 nanocage by an interaction distance of 2.37 Å. The Ead and Ebr of CO2 molecule for this configuration are ˗0.40 and 0.01 eV, respectively and a charge of 0.03|e| is transferred from the CO2 molecule to the nano-cage. The results indicate that this interaction is weak and should be considered as a physisorption. Another CO2 physisorption approach is shown in Figure 2(c), in which the interaction distance between both of the oxygen atoms of CO2 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 CO2 and Zn12O12 nano-cage. Also, Ebr of CO2 molecule for this configuration is zero.

There are several hexagon and tetragon rings in structure of the Zn12O12 nano-cage as potential adsorption site; therefore the possibility of the second adsorption is interesting for consideration. In this configuration (Fig. 3(d)), two CO2 molecule is adsorbed on the Zn12O12 nano-cage. The Ead and Ebr of CO2 molecule for this process is about ˗1.03 and 2.44 eV per CO2 molecule with a charge transfer of ˗0.98|e|, which are slightly lower than that of one CO2 adsorption due to the steric repulsion between two CO2 molecules. In the next step, three and four CO2 molecules are adsorbed on the Zn12O12 nano-cage (Fig. 3(e) and (f)). The Ead for these configurations are about ˗1.05 and ˗1.12 eV per CO2 molecule for three and four molecules adsorption. Ebr of CO2 molecule for these processes are 2.43 and 2.45 eV per CO2, respectively. In comparison with the one CO2 adsorption model (Fig. 2(a)), the Ead and Ebr of CO2 molecule due to the steric repulsion between the CO2 molecules is reduced.

Figure 2.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 CO2 with the Zn12O12 nano-cage, the influence of CO2 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 CO2 adsorption. In the physisorption configurations the valence and conduction level energies are relatively the same as for the pristine Zn12O12 valence and conduction level energies. Therefore, the results show that the CO2 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 Zn12O12, while the conduction level energies some shift downwards. As shown in Table 1, upon the CO2 adsorption on the Zn12O12 nano-cage, the Eg value of the nano-cage are more changed compared to the physisorption cases, in other words, when number of CO2 molecules increased from 0 to 3, band gap of the ZnO nano-cage has changed about (1.67-9.78%). However, when 4CO2 molecules are adsorbed, the band gap has changed about 0.95% due to the steric repulsion between the CO2 molecules. In fact, with increasing of CO2 numbers, the Ead of CO2 molecules is decreased (see Table 1) and increasing of CO2 molecules has no sensible effects on the electronic properties of the nano-cage. Therefore, Change of Eg value in the configuration F (with 4CO2 molecules) is reduced.

Figure 3.Model for 2CO2, 3CO2, and 4CO2 chemisorbed- Zn12O12 configurations and their density of states (DOS) plots. Distances are in angstrom.

Table 1.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 Zn12O12 nano-cage to ˗4.75 and ˗4.81 eV in the (b) and (c) configurations. This increasing of EFL with CO2 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 CO2 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 CO2 adsorption and have a disadvantageous effect on the field emission properties of Zn12O12 nano-cage.

 

Conclusions

Physisorption and chemical functionalization of CO2 molecule on the Zn12O12 nano-cage were studied using density functional calculations. Binding energy corresponding to adsorption of CO2 on the Zn12O12 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 CO2 molecule. On the basis of our calculations, it seems that attachment of the CO2 molecule on the walls of the Zn12O12 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 Zn12O12 nano-cage can significantly detect CO2 molecule. Also, more efficient binding could not be achieved by increasing the CO2 concentration. The strong adsorption of the CO2 on the Zn12O12 nano-cage shows the potential application of the ZnO-based materials for CO2 capture and storage.