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Ambient carbon dioxide capture by different dimensional AlN nanostructures: A comparative DFT study
Superlattices and Microstructures 96 (2016) 164e173
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Ambient carbon dioxide capture by different dimensional AlN nanostructures: A comparative DFT study Mehdi D. Esrafili*, Roghaye Nurazar, Parisa Nematollahi Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, P.O. Box: 5513864596, Maragheh, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2016 Received in revised form 16 May 2016 Accepted 19 May 2016 Available online 20 May 2016
Strong binding of an isolated carbon dioxide molecule over three different aluminium nitride (AlN) nanostructures (nanocage, nanotube and nanosheet) is verified using density functional calculations. Equilibrium geometries, electronic properties, adsorption energies and thermodynamic stability of each adsorbed configuration are also identified. Optimized configurations are shown at least one corresponding physisorption and chemisorption of CO2 molecule over different AlN nanostructures. Also, the effect of chirality on the adsorption of CO2 molecule is studied over two different finite-sized zigzag (6,0) and armchair (4,4) AlN nanotubes. It is found that the electronic properties of the Al12N12 nanocage are more sensitive to the CO2 molecule than other AlN nanostructures. This indicates the significant potential of Al12N12 nanocage toward the CO2 adsorption, fixation and catalytic applications in contrast to other AlN nanostructures. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Carbon dioxide capture DFT Aluminum nitride Al12N12 AlN nanosheet
1. Introduction Nowadays, carbon-based fossil fuels supply about 80% of the world’s energy needs which are the major source of carbon dioxide (CO2) in the atmosphere. CO2 is a renewable, non-toxic and non-flammable gas, and its emission is one of the most important causes of greenhouse effect. Thus, its fixation and adsorption over different surfaces become one of the most exciting challenges for the scientific community [1,2]. To date, numerous studies have been performed to separate the CO2 molecule from fuel gas streams, such as cryogenic separation, membrane separation and adsorption processes, such as pressure and temperature swing adsorption [3e5]. The successful performance of the adsorption processes depends on materials which can selectively adsorb the CO2 molecule. In order to achieve this purpose, an ideal CO2 sequestration material with a large surface to volume ratio, strong adsorption between the framework and the CO2 molecule compared to other small gas molecules is needed. There are several different CO2 adsorbents which have been previously reported such as metalorganic frameworks [2], zeolitic imidazolate frameworks [6], carbon nanotubes (CNTs) [7] and boron nitride nanotubes (BNNTs) [8]. But, the problem is the weak interaction between CO2 and the atoms on those frameworks which is due to the weak van der Waals and electrostatic interactions [9,10]. Recently, it has been found that the covalent bonds can be formed between the carbon atom of CO2 and N atoms of azabenzene when extra electrons are present [11]. So, it can be expected that introduction of N atoms into the adsorbent has a significant effect on its surface activity. This might potentially lead to the formation of chemical bonds between C atom of CO2 and the N atom of the surface that is favorable for CO2 capture applications.
* Corresponding author. E-mail address: esrafi[email protected] (M.D. Esrafili). http://dx.doi.org/10.1016/j.spmi.2016.05.025 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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Recently, many theoretical and experimental investigations have been devoted to study the possible nanostructures such as nanosheets, nanotubes or fullerene-like nanocages which consist of other elements, rather than carbon, for their specific physical and chemical properties [12e14]. These studies have shown that the counterparts of group III-V as wide gap semiconductor nanostructures, result in an unexpected rich chemical and physical characteristics [15,16]. Therefore, they are considered as important candidates for the next generation materials in microelectronic and optical industry [17,18]. For instance, different boron nitride (BN) nanostructures have been extensively synthesized experimentally and predicted theoretically [19e21]. It is also interesting to know that if the heavier elements of group III and V are substituted with boron and nitrogen atoms of BN clusters, respectively, they become less strained [13]. Among these, aluminium nitride (AlN) nanostructures have been considered evidently due to their low electron affinity, excellent physical and chemical properties, and wide band gap [22,23] which make these graphite-liked layered clusters energetically stable. On the basis of ab initio calculations, Wu et al. [24] studied the geometric and energetic aspects of (AlN)n (n ¼ 2e41) clusters and showed that among these large family, the Al12N12 cluster with Th symmetry is the most favorable nanostructure and would be an ideal inorganic fullerene-like cage. Also, other theoretical studies predicted that the fullerene-like cage Al12N12 can be a magic cluster which has intrinsic special stability [25]. For example, Huang et al. [22] in a computational study predicted the hydrogen storage on Al12N12 cage. Wang et al. [26] reported the capability of Li-decorated (AlN)n (n ¼ 12, 24, 36) clusters for hydrogen storage by using density functional theory calculations. It is found that each Al atom is capable of binding one H2 molecule up to a gravimetric density of hydrogen storage of 4.7 wt% with an average binding energy of 0.189, 0.154, and 0.144 eV/H2 in the pristine (AlN)n (n ¼ 12, 24, 36) nanocages, respectively. Additionally, the hexagonal AlN nanotubes (AlNNTs) were successfully synthesized experimentally by Tondare et al. [27], using the DC arc plasma reactor for fabricating the vapors of Al in a reactive nitrogen atmosphere. Also, in another experimental work these nanotubes were fabricated differently by Wu et al. [28]. AlNNTs are semiconductors with low electron affinity, excellent physical and chemical properties with a large band gap [22,23]. The band gap which reported experimentally by Meyer et al. [29] was about 6.2 eV, while it differs with the change of calculation methods [30]. Recently, many investigations have been performed on the unique characteristics of these nanotubes. Machado et al. [31] theoretically compared the stability and electronic properties of zigzag and armchair single-walled AlNNTs under different conditions. They concluded that the tube response to the electric field applications can be significantly changed with the enhancement of its diameter. Baei et al. [32] studied the fluorination of zigzag AlNNTs with the fluorine atoms theoretically. They found that the fluorine atoms tend to be chemisorbed on Al atoms of the tube in the energy range of 3.79e4.25 eV. Kuang et al. [33] reported the atomic and molecular adsorption of hydrogen on zigzag and armchair AlNNTs via first-principles calculations. Their results indicated that each Al atom can bind to one H atom while their adsorption configurations are energetically favorable. However, although great results are obtained from AlN nanocages and nanotubes, but still graphene-like structures are the major topics in material science fields because of their fantastic quantum confinement and surface effects [34,35]. In our case, despite few studies have been done on AlN nanosheets (AlNNSs) but there are still lots of interest for these outstanding nanostructures [35] with their high surface to volume ratio which can be utilized for the adsorption of gas pollutants and facilitate the accommodation of gas molecules in large amounts. For example, Rastegar et al. [36] studied the adsorption of NO2 on the AlNNS in the presence of NH3 and they found that the AlNNS can detect NO2 molecules in the presence of NH3 molecule. These authors also reported [37] another interesting theoretical study about the selective detection of SO2 molecule in the presence of O3 molecules by AlN nanosheets. Jiao et al. [38] explored the adsorption of CO2 and N2 molecules on single-layer AlN nanostructure and depicted that this single-layer nanosheet can have a great potential for CO2 capture and storage. In another theoretical study [39] which has been done on the adsorption of CO2 and N2 molecules over single-walled AlNNTs, the same authors reported that N2 molecule can be only physisorbed on the nanotube suggesting the potential application of AlN based materials for CO2 fixation. Also, Beheshtian et al. [40] investigated the CO adsorption on BN, AlN, BP and AlP nanotubes with two different calculation methods. Their results suggested that AlNNTs are energetically the most favorable candidate for CO adsorption. Although some efforts have been made for tuning of these nanostructure properties, but, the interaction of AlN nanostructures with CO2 molecule in comparison with other gaseous molecules such as H2 [41], NH3 [42,43], N2 [38] and NO2 [36], have seldom been studied and make this topic to a largely unexplored areas. In light of this, the present study theoretically investigates the interaction of a single CO2 molecule with three different AlN nanostructures, including Al12N12 nanocage, AlNNT and AlNNS. Also, in order to explore the effects of the tube chirality on the CO2 adsorption over AlNNTs, finite-sized zigzag (6,0) and armchair (4,4) type single-walled AlNNTs are considered. Density functional theory (DFT) calculations are performed to elucidate the strength and nature of CO2 adsorption over AlN nanostructures in order to explore their potential application as gas sensors. Our results are likely to be useful for future studies related to different AlN nanostructures as chemical sensors for the construction of nanodevices. 2. Computational details The calculations were performed using the Gaussian 09 package [44]. The geometries of the monomers and complexes were fully optimized at the M06-2X/6-31G* level of theory. Then, corresponding frequency calculations were performed at the same level in order to identify whether the optimized complexes correspond to a true local minimum or not. It is noteworthy that the M06-2X density functional is known as a reliable method to study non-covalent interactions, so, we preferred to apply this method for our calculations [45,46]. In order to investigate the most activate structure toward CO2 adsorption, three different AlN nanostructures were chosen, i.e. Al12N12 nanocage, AlNNTs and AlNNS. Also, in order to study
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the chirality effects on the CO2 adsorption, two model AlNNT systems with the approximately same length (z12 Å) and diameter (z7 Å) were considered, namely finite-sized (6,0) and (4,4) AlN single-walled nanotubes (Fig. 1). In addition, a finite-sized monolayer sheet, having a total of 23 Al and 23 N atoms was considered for the pristine AlNNS. The unsaturated boundary effect of all the AlN nanostructures (except Al12N12 cluster) was avoided by adding hydrogen atoms terminated on their two ends. To better understand the nature of CO2 interaction with AlN nanostructures, the total and partial density of states of the pristine and adsorbed configurations were studied in detail. The adsorption energies (Ead) were calculated as the difference between the total energies of the complexes and the corresponding isolated monomers at the M06-2X/6-31G* level via the following equation:
Eads ¼ Eadsorbatenanostructre Eadsorbate Enanostructure
(1)
where the Eadsorbateenanostructre, Eadsorbate and Enanostructure are the total energies of the CO2 molecule adsorbed on a nanostructure, the isolated CO2 molecule and nanostructure, respectively. 3. Results and discussions 3.1. Geometry and electronic structure of Al12N12, AlNNTs and AlNNS First of all, the geometry and electronic structure of different AlN nanostructures are studied. Fig. 1 shows the optimized structures and total density of states (TDOS) plots of Al12N12, (6,0) and (4,4) AlNNTs, and AlNNS. The geometry of Al12N12 cage is found to be composed of six four-membered rings having Th symmetry. There are two nonequivalent AleN bonds: AleN bond in the four-membered ring has a length of 1.84 Å, while the AleN bond between the four-membered rings has a length of 1.78 Å. These are consistent with the results obtained in the previous theoretical studies [25]. The lengths of the AleN bonds in the (6,0) and (4,4) AlNNTs are about 1.80 Å, which are in good agreement with those of other theoretical studies [32,33]. It is also seen that the electronic structures of these nanotubes are almost independent of the tube chirality and diameter, due to the ionicity of AleN bonds. A partial structure of the optimized AlNNS is shown in Fig. 1. In the AlN hexagonal plane, each N atom covalently binds with the three adjacent Al atoms. Also, the sp2-hybridization of all orbitals in the AlNNS is due to the lone-electron pairs which are distributed over the N atoms of the nanosheet and the bonding combination of Al-pz and N-pz orbitals. Due to the significant electronegativity difference between Al and N atoms, the Al atoms are positively charged and hence, all of the AleN bonds have an ionic character. Therefore, all these AlN nanostructures can be considered as a non-metal catalyst with Lewis acidebase pairs [25,47]. Fig. 1 also compares the computed TDOS plots of the pristine Al12N12, AlNNTs and AlNNS. The TDOS of the Al12N12 exhibits that the energy gap (Eg) between the highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) is 6.31 eV. This value is in good agreement with that reported in previous studies [13], introducing it as a relatively wide gap semiconductor. As it is also clear from the corresponding TDOS plots, the Eg for zigzag AlNNT (6.49 eV) is about 0.10 eV smaller than that of armchair (6.59 eV), indicating that they are also semiconductor materials. The calculated TDOS plot in Fig. 1 indicates the Eg value of 7.38 eV for AlNNS. The experimental value for HOMO-LUMO gap for bulk AlN is about 6.2 eV [48]; the main reason for the difference between this Eg and our estimated value is most likely due to our used simplified model in this study. Comparison of Eg values obtained for AlNNTs and AlNNS also indicates that the Eg decreases with the increase of the nanosheet curvature, suggesting that it is easier for the electrons in AlNNT than those in AlNNS to be excited from the HOMO to LUMO. This can be also attributed to the different degree of contribution of electron pair of nitrogen atoms to form a delocalized conjugated system with their adjacent Al atoms. 3.2. CO2 capture over the Al12N12 nanocage According to Fig. 2, there are three stable configurations for the adsorption of CO2 over the Al12N12 cage, corresponding two chemisorptions (A and B complexes) and one physisorption (complex C). The stable optimized structures of possible adsorption configurations of CO2 over the Al12N12 surface and their corresponding bond lengths are shown in Fig. 2. Also, the corresponding Ead, Gibbs free energy change (DG298), enthalpy change (DH298) and charge transfer (qCT) for each complex are listed in Table 1. In the A complex, the calculated Ead is 49.54 kcal/mol, which is about 21 kcal/mol more negative than that of formic acid adsorption over Al12N12 nanocage [46]. In this configuration, two O atoms of the CO2 molecule are placed above Al atoms of the Al12N12 and CO2 is chemisorbed with its positive C atom by the negative N atom of the surface (CeN ¼ 1.36 Å) and average AleO binding distance of 1.92 Å (Fig. 2). The formation of this configuration is exothermic (DH298 ¼ 48.37 kcal/ mol), and the negative value of DG298 (36.22 kcal/mol) shows that the reaction can occur at room temperature. In addition, the strong interaction of CO2 with the Al12N12 cage is attributed to the significant charge transfer (qCT) from the surface to the gas molecule (0.61 e), which occupy the CO2-2p* orbital and subsequently lead to elongation of both CeO bonds (z1.28 Å) compared to the free state of CO2 molecule. This can be also explained with the electron density difference (EDD) map which is demonstrated in Fig. 2. It is clear that there is a great charge accumulation around the CeN and OeAl bonds which indicates the strong chemical binding in these regions. In another chemisorbed complex B, CO2 molecule is attached to the Al12N12 cluster via two bonds: CeN and OeAl, with the distinct binding distances of 1.46 and 1.81 Å, respectively (Fig. 2). In comparison with the complex A, the calculated Eads for this complex is decreased from 49.54 to 40.78 kcal/mol. It can be
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Fig. 1. Optimized structures (left) and TDOS plots (right) of different AlN nanostructures: (a) Al12N12, (b) (6,0) AlNNT, (c) (4,4) AlNNT and (d) AlNNS. All distances are in Å.
168 M.D. Esrafili et al. / Superlattices and Microstructures 96 (2016) 164e173 Fig. 2. Optimized structures, EDD maps and PDOS plots of CO2eAl12N12 complexes. In the electron density difference plots, charge depletion and accumulation are displayed in red and blue, respectively. In each PDOS plot, the vertical dashed line indicates the Fermi level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Calculated binding distances (R), adsorption energy (Eads), change of Gibbs free energy (DG298), change of enthalpy (DH298), amount of charge transfer and HOMO-LUMO energy gap of CO2 adsorption over different AlN nanostructures. Surface Al12N12 A B C AlNNT (6,0) D E AlNNT (4,4) D′ E′ AlNNS F G
RN-C (Å)
Eads (kcal/mol)
DG298 (kcal/mol)
DH298 (kcal/mol)
qCT (e)
1.36 1.46 2.75
49.54 40.78 14.43
36.22 28.46 4.13
48.37 39.68 13.55
0.61 0.50 0.09
1.39 2.88
41.60 11.62
29.80 2.46
38.90 9.94
0.58 0.09
1.40 2.77
35.96 10.65
23.16 1.41
31.88 6.73
0.58 0.07
1.42 2.90
32.19 8.89
22.20 5.53
27.17 4.12
0.58 0.05
Eg (eV) 6.31 6.28 6.13 6.31 6.49 6.46 6.45 6.59 6.62 6.61 7.38 7.30 7.40
understood from the negative values of DH298 and DG298 that the formation of complex B is exothermic and is thermodynamically feasible (Table 1). Also, from the Mullikan charge analysis, a net charge about 0.50 e is transferred from the nanocage to the 2p* orbital of CO2 molecule. In comparison with complex A, despite the smaller qCT value of complex B, the CeO binding distances are increased more significantly (1.32 Å). This can be attributed to the number of OeAl bonds which are formed in this complex. One can see from the EDD map in Fig. 2 that like complex A, there is also a strong binding between CO2 and the Al12N12 surface, because of the great charge accumulation between the C (O) atom of the CO2 molecule and N (Al) atoms of the surface. On the other hand, the last CO2eAl12N12 complex is related to the configuration in which CO2 molecule is physically adsorbed over the Al12N12 surface (complex C). In this complex, CO2 molecule is just placed above the nanocluster with a weak formed CeN bond (2.75 Å) and Eads of about 14.43 kcal/mol (Fig. 2). According to Table 1, despite the small amount of DH298 and DG298, the formation of complex C is exothermic and it is accessible at an ambient condition. Also, due to the weak interaction between CO2 and the Al12N12 surface, a small net charge about 0.09 e is transferred from the CO2 molecule to the cage which is also confirmed by the EDD analysis of this complex (Fig. 2), in which there is no any electron density accumulation between the gaseous molecule and the cluster surface. Finally, it can be understood from the projected density of states (PDOS) plots that upon the adsorption of CO2, the calculated HOMO-LUMO energy gap is reduced from 6.31 eV in the pristine Al12N12 to 6.28 and 6.13 eV for the complexes A and B, respectively, while it doesn’t considerably change in complex C (Table 1). This can be explained with the slightly movement of both conduction and valence levels to higher energies, so that Eg of the nanocage is slightly decreased for the complex B. Hence, it is concluded that the electronic properties of the Al12N12 are sensitive to the CO2 molecule, and therefore, this nanocage can be used as a potential sensor for CO2 detection. 3.3. CO2 capture over the AlNNTs Different adsorption configurations of CO2 over zigzag and armchair AlNNTs with their corresponding bond lengths are shown in Fig. 3. There are two adsorption configurations for zigzag (armchair): named D (D )׳and E (E)׳, in which D and D׳ complexes are chemisorbed while E and E ׳are physisorbed. Additionally, the calculated AleN binding distances (R) and the corresponding thermodynamic parameters for both nanotubes after the adsorption of CO2 are individually summarized in Table 1. As mentioned before, Al and N atoms of AlNNTs are positively and negatively charged, respectively. Also, a considerable charge density is transferred from the electropositive atom (Al) to the electronegative one (N). Thus, large values of Eads may be achieved due to the interaction of these electron rich sites (N atoms) of the AlNNTs with the electron poor atom of the CO2 gas (C atom). It is important to know that the adsorption configurations of CO2 over these nanotubes are exactly the same with that over Al12N12 cluster. In the formation process of complex D, the CO2 molecule is chemically adsorbed over the (6,0) zigzag AlNNT with the CeN distance of 1.39 Å. In this configuration, two O atoms of the gas molecule is also chemisorbed with Al atoms of the AlNNT while the average binding distance is about 1.91 Å. The calculated Eads of this configuration is 41.60 kcal/mol which proceeds via an exothermic reaction (Table 1). Due to the strong charge transfer of 0.58 e from the tube to the 2p* orbital of CO2 molecule, the CeO bond length is increased from 1.16 to 1.27 Å. This means that the reactivity of this molecule is increased after adsorption over the tube surface. Also, the EDD map of complex D shows a great interaction between CO2 molecule and the tube surface. Likewise, similar adsorption configuration is obtained in the formation of complex D׳, where the CO2 chemisorbed over the (4,4) armchair AlNNT with the corresponding CeN bond length and Eads of 1.40 Å and 35.96 kcal/mol, respectively (Fig. 3). Additionally, similar to complex D, the charge amount of 0.58 e is transferred from the surface to the CO2 molecule in this configuration. This sizable charge transfer is also clear from its EDD map as depicted in Fig. 3. It should be noted that in comparison with the average OeAl bond in complex D, here, the OeAl bond lengths are slightly increased to 1.93 and 1.98 Å. Analogous to complex D, the amount of charge analysis (see Table 1) and the CeO bond elongation can be explained with the obtained EDD map which implies that a chemisorption occurs between CO2 and tube surface. Comparing the results of these two chemisorption configurations (D
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Fig. 3. Optimized structures, EDD maps and PDOS plots of CO2-AlNNT complexes. In the electron density difference plots, charge depletion and accumulation are displayed in red and blue, respectively. In each PDOS plot, the vertical dashed line indicates the Fermi level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and D )׳indicates that the CO2 adsorption is slightly more stable over the (6,0) zigzag tube than (4,4) armchair one (Table 1). Another adsorption configuration (complex E) is found when the CO2 molecule is weakly adsorbed on the (6,0) AlNNT surface. Despite the large bond length of CeN (2.88 Å) and small Eads of 11.62 kcal/mol, the formation of this complex is exothermic (DH298 ¼ 9.94 kcal/mol) and thermodynamically favorable (DG298 ¼ 2.46 kcal/mol). The relatively small charge transfer (0.09 e) from the CO2 molecule to the AlNNT and the studied EDD isosurface confirm a weak CO2 adsorption over the zigzag nanotube surface. Like complex E, upon the physisorption of CO2 molecule over an armchair (4,4) AlNNT, the complex E ׳with the Eads of 10.65 kcal/mol and the CeN distance of about 2.77 Å is achieved exothermically with DH298 ¼ 6.73 kcal/mol and a relatively low value of DG298 ¼ 1.41 kcal/mol. Analogous with the configuration E, due to the weak adsorption of CO2 molecule with the surface, the amount of charge transferring is negligible (0.07 e). Comparing these two different tubular AlNNTs indicates that the CeN binding between CO2 molecule and the (6,0) AlNNT is stronger than that in (4,4) tube. Besides, the calculated Eads values for CO2 adsorption in the present study are more negative than that of CH4 molecule over (4,4) AlNNT and SiCNT [30]. The calculated PDOS plots of the (6,0) and (4,4) AlNNTs are shown in Fig. 3. It is clear from Table 1 that upon the adsorption of CO2, the Eg values of complexes D, E, D ׳and E ׳change by 0.02 (0.3%), 0.04 (0.6%), 0.03 (0.5%) and 0.02 eV (0.3%), respectively. Thus, it can be concluded that the electronic properties of these nanotubes are less sensitive to the CO2 molecule than Al12N12 nanocage.
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Fig. 4. Optimized structures, EDD maps and PDOS plots of CO2eAlNNS complexes. In the electron density difference plots, charge depletion and accumulation are displayed in red and blue, respectively. In each PDOS plot, the vertical dashed line indicates the Fermi level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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3.4. CO2 capture over the AlNNS In order to examine the sensitivity of the AlNNS toward the CO2, the gas molecule is placed at different adsorption sites with different positions (Fig. 4). Analogues to the results of the Al12N12 cluster and AlNNTs, the planar surface of the AlNNS is deformed upon the chemisorptions of CO2 molecule (complex F). According to the reported thermodynamic parameters which are listed in Table 1, it is clear that in contrast to other studied nanostructures, the Eads of this system is quite small (32.19 kcal/mol). This finding can be explained by the curvature effect, since the sidewall reactivity of these AlNNTs depends critically on their local curvature. These are also supported by charge transfer values from the surface to the CO2 molecule. It should be noted that the estimated Eads value in this work is more negative than that of reported by Jia et al. (20.47 kcal/mol) as obtained at the B3LYP/6-31G* [38]. This may be attributed to the failure of the B3LYP method to properly include dispersion forces, which is the main constituent part of the weak intermolecular interactions. In addition, the calculated Eads value of CO2 in this work is larger than those of SO2 molecule over the AlNNS [37]. Also, a charge of 0.58 e is transferred from the plane to the CO2 molecule and occupies its 2p* orbital which leads to an outstanding charge accumulation around the CeN bond (see EDD map in Fig. 4). Unlike the chemisorption configuration (complex F), the physisorbed complex G is formed in which two oxygen atoms are just located above two aluminum atoms of the sheet without any bond formation. Due to the small Eads of 8.89 kcal/mol and the large average OeAl (CeN) bond lengths of 2.90 Å, the physically adsorbed CO2 molecule does not have any considerable effect on the electronic structure of AlNNS. Another reason for this weak interaction can be attributed to the negligible charge of about 0.05 e which is transferred from the gas molecule to the plane. Additionally this is clear from the electron depletion region in EDD map where there is not any considerable electron accumulation between the CO2 molecule and the AlNNS. 4. Conclusion In summary, a comparative DFT study was performed to study the adsorption of CO2 molecule over three different AlN nanostructures surfaces, i.e. Al12N12, AlNNTs and AlNNS. In order to find the effect of chirality on the CO2 adsorption, two different AlNNTs were chosen: a finite-sized (6,0) zigzag and (4,4) armchair AlNNT, with the approximately same diameter. Totally, nine different configurations (named from A to G) were obtained from these four nanostructures. In all these calculations, CO2 shows strong chemical binding with all of the AlN nanostructures in contrast of its physisorption configurations. However, all of these different configurations were formed via an exothermic reaction and were thermodynamically feasible at ambient temperatures. The smallest Eads values were achieved for the physisorbed configurations and the relative magnitude order of the AlN nanostructure surfaces toward CO2 adsorption is as follows: Al12N12 > (6,0) AlNNT > (4,4) AlNNT > AlNNS. This trend can be attributed to the curvature effect of these AlN nanostructures and to the different contribution of electron pair of nitrogen atoms to form a delocalized conjugated system with the adjacent Al atoms. As a result, the large curvature of Al12N12 facilitates the sp3-hybridization of the site and thus helps in the binding of CO2 on the surface. These results also reveal that the small-diameter AlNNTs with large curvature would be beneficial for the capture of CO2. In addition, this indicates the significant potential of Al12N12 nanocage toward the CO2 adsorption, fixation and catalytic applications in contrast to other AlN nanostructures. It is also found that the electronic structures of the AlNNTs are almost independent of the tube chirality and diameter, due to the ionicity of AleN bonds. This comparative study presented here may provide a new understanding about the CO2 adsorption over three important AlN nanostructures. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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Ambient carbon dioxide capture by different dimensional AlN nanostructures: A comparative DFT study Mehdi D. Esrafili*, Roghaye Nurazar, Parisa Nematollahi Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, P.O. Box: 5513864596, Maragheh, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2016 Received in revised form 16 May 2016 Accepted 19 May 2016 Available online 20 May 2016
Strong binding of an isolated carbon dioxide molecule over three different aluminium nitride (AlN) nanostructures (nanocage, nanotube and nanosheet) is verified using density functional calculations. Equilibrium geometries, electronic properties, adsorption energies and thermodynamic stability of each adsorbed configuration are also identified. Optimized configurations are shown at least one corresponding physisorption and chemisorption of CO2 molecule over different AlN nanostructures. Also, the effect of chirality on the adsorption of CO2 molecule is studied over two different finite-sized zigzag (6,0) and armchair (4,4) AlN nanotubes. It is found that the electronic properties of the Al12N12 nanocage are more sensitive to the CO2 molecule than other AlN nanostructures. This indicates the significant potential of Al12N12 nanocage toward the CO2 adsorption, fixation and catalytic applications in contrast to other AlN nanostructures. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Carbon dioxide capture DFT Aluminum nitride Al12N12 AlN nanosheet
1. Introduction Nowadays, carbon-based fossil fuels supply about 80% of the world’s energy needs which are the major source of carbon dioxide (CO2) in the atmosphere. CO2 is a renewable, non-toxic and non-flammable gas, and its emission is one of the most important causes of greenhouse effect. Thus, its fixation and adsorption over different surfaces become one of the most exciting challenges for the scientific community [1,2]. To date, numerous studies have been performed to separate the CO2 molecule from fuel gas streams, such as cryogenic separation, membrane separation and adsorption processes, such as pressure and temperature swing adsorption [3e5]. The successful performance of the adsorption processes depends on materials which can selectively adsorb the CO2 molecule. In order to achieve this purpose, an ideal CO2 sequestration material with a large surface to volume ratio, strong adsorption between the framework and the CO2 molecule compared to other small gas molecules is needed. There are several different CO2 adsorbents which have been previously reported such as metalorganic frameworks [2], zeolitic imidazolate frameworks [6], carbon nanotubes (CNTs) [7] and boron nitride nanotubes (BNNTs) [8]. But, the problem is the weak interaction between CO2 and the atoms on those frameworks which is due to the weak van der Waals and electrostatic interactions [9,10]. Recently, it has been found that the covalent bonds can be formed between the carbon atom of CO2 and N atoms of azabenzene when extra electrons are present [11]. So, it can be expected that introduction of N atoms into the adsorbent has a significant effect on its surface activity. This might potentially lead to the formation of chemical bonds between C atom of CO2 and the N atom of the surface that is favorable for CO2 capture applications.
* Corresponding author. E-mail address: esrafi[email protected] (M.D. Esrafili). http://dx.doi.org/10.1016/j.spmi.2016.05.025 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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Recently, many theoretical and experimental investigations have been devoted to study the possible nanostructures such as nanosheets, nanotubes or fullerene-like nanocages which consist of other elements, rather than carbon, for their specific physical and chemical properties [12e14]. These studies have shown that the counterparts of group III-V as wide gap semiconductor nanostructures, result in an unexpected rich chemical and physical characteristics [15,16]. Therefore, they are considered as important candidates for the next generation materials in microelectronic and optical industry [17,18]. For instance, different boron nitride (BN) nanostructures have been extensively synthesized experimentally and predicted theoretically [19e21]. It is also interesting to know that if the heavier elements of group III and V are substituted with boron and nitrogen atoms of BN clusters, respectively, they become less strained [13]. Among these, aluminium nitride (AlN) nanostructures have been considered evidently due to their low electron affinity, excellent physical and chemical properties, and wide band gap [22,23] which make these graphite-liked layered clusters energetically stable. On the basis of ab initio calculations, Wu et al. [24] studied the geometric and energetic aspects of (AlN)n (n ¼ 2e41) clusters and showed that among these large family, the Al12N12 cluster with Th symmetry is the most favorable nanostructure and would be an ideal inorganic fullerene-like cage. Also, other theoretical studies predicted that the fullerene-like cage Al12N12 can be a magic cluster which has intrinsic special stability [25]. For example, Huang et al. [22] in a computational study predicted the hydrogen storage on Al12N12 cage. Wang et al. [26] reported the capability of Li-decorated (AlN)n (n ¼ 12, 24, 36) clusters for hydrogen storage by using density functional theory calculations. It is found that each Al atom is capable of binding one H2 molecule up to a gravimetric density of hydrogen storage of 4.7 wt% with an average binding energy of 0.189, 0.154, and 0.144 eV/H2 in the pristine (AlN)n (n ¼ 12, 24, 36) nanocages, respectively. Additionally, the hexagonal AlN nanotubes (AlNNTs) were successfully synthesized experimentally by Tondare et al. [27], using the DC arc plasma reactor for fabricating the vapors of Al in a reactive nitrogen atmosphere. Also, in another experimental work these nanotubes were fabricated differently by Wu et al. [28]. AlNNTs are semiconductors with low electron affinity, excellent physical and chemical properties with a large band gap [22,23]. The band gap which reported experimentally by Meyer et al. [29] was about 6.2 eV, while it differs with the change of calculation methods [30]. Recently, many investigations have been performed on the unique characteristics of these nanotubes. Machado et al. [31] theoretically compared the stability and electronic properties of zigzag and armchair single-walled AlNNTs under different conditions. They concluded that the tube response to the electric field applications can be significantly changed with the enhancement of its diameter. Baei et al. [32] studied the fluorination of zigzag AlNNTs with the fluorine atoms theoretically. They found that the fluorine atoms tend to be chemisorbed on Al atoms of the tube in the energy range of 3.79e4.25 eV. Kuang et al. [33] reported the atomic and molecular adsorption of hydrogen on zigzag and armchair AlNNTs via first-principles calculations. Their results indicated that each Al atom can bind to one H atom while their adsorption configurations are energetically favorable. However, although great results are obtained from AlN nanocages and nanotubes, but still graphene-like structures are the major topics in material science fields because of their fantastic quantum confinement and surface effects [34,35]. In our case, despite few studies have been done on AlN nanosheets (AlNNSs) but there are still lots of interest for these outstanding nanostructures [35] with their high surface to volume ratio which can be utilized for the adsorption of gas pollutants and facilitate the accommodation of gas molecules in large amounts. For example, Rastegar et al. [36] studied the adsorption of NO2 on the AlNNS in the presence of NH3 and they found that the AlNNS can detect NO2 molecules in the presence of NH3 molecule. These authors also reported [37] another interesting theoretical study about the selective detection of SO2 molecule in the presence of O3 molecules by AlN nanosheets. Jiao et al. [38] explored the adsorption of CO2 and N2 molecules on single-layer AlN nanostructure and depicted that this single-layer nanosheet can have a great potential for CO2 capture and storage. In another theoretical study [39] which has been done on the adsorption of CO2 and N2 molecules over single-walled AlNNTs, the same authors reported that N2 molecule can be only physisorbed on the nanotube suggesting the potential application of AlN based materials for CO2 fixation. Also, Beheshtian et al. [40] investigated the CO adsorption on BN, AlN, BP and AlP nanotubes with two different calculation methods. Their results suggested that AlNNTs are energetically the most favorable candidate for CO adsorption. Although some efforts have been made for tuning of these nanostructure properties, but, the interaction of AlN nanostructures with CO2 molecule in comparison with other gaseous molecules such as H2 [41], NH3 [42,43], N2 [38] and NO2 [36], have seldom been studied and make this topic to a largely unexplored areas. In light of this, the present study theoretically investigates the interaction of a single CO2 molecule with three different AlN nanostructures, including Al12N12 nanocage, AlNNT and AlNNS. Also, in order to explore the effects of the tube chirality on the CO2 adsorption over AlNNTs, finite-sized zigzag (6,0) and armchair (4,4) type single-walled AlNNTs are considered. Density functional theory (DFT) calculations are performed to elucidate the strength and nature of CO2 adsorption over AlN nanostructures in order to explore their potential application as gas sensors. Our results are likely to be useful for future studies related to different AlN nanostructures as chemical sensors for the construction of nanodevices. 2. Computational details The calculations were performed using the Gaussian 09 package [44]. The geometries of the monomers and complexes were fully optimized at the M06-2X/6-31G* level of theory. Then, corresponding frequency calculations were performed at the same level in order to identify whether the optimized complexes correspond to a true local minimum or not. It is noteworthy that the M06-2X density functional is known as a reliable method to study non-covalent interactions, so, we preferred to apply this method for our calculations [45,46]. In order to investigate the most activate structure toward CO2 adsorption, three different AlN nanostructures were chosen, i.e. Al12N12 nanocage, AlNNTs and AlNNS. Also, in order to study
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the chirality effects on the CO2 adsorption, two model AlNNT systems with the approximately same length (z12 Å) and diameter (z7 Å) were considered, namely finite-sized (6,0) and (4,4) AlN single-walled nanotubes (Fig. 1). In addition, a finite-sized monolayer sheet, having a total of 23 Al and 23 N atoms was considered for the pristine AlNNS. The unsaturated boundary effect of all the AlN nanostructures (except Al12N12 cluster) was avoided by adding hydrogen atoms terminated on their two ends. To better understand the nature of CO2 interaction with AlN nanostructures, the total and partial density of states of the pristine and adsorbed configurations were studied in detail. The adsorption energies (Ead) were calculated as the difference between the total energies of the complexes and the corresponding isolated monomers at the M06-2X/6-31G* level via the following equation:
Eads ¼ Eadsorbatenanostructre Eadsorbate Enanostructure
(1)
where the Eadsorbateenanostructre, Eadsorbate and Enanostructure are the total energies of the CO2 molecule adsorbed on a nanostructure, the isolated CO2 molecule and nanostructure, respectively. 3. Results and discussions 3.1. Geometry and electronic structure of Al12N12, AlNNTs and AlNNS First of all, the geometry and electronic structure of different AlN nanostructures are studied. Fig. 1 shows the optimized structures and total density of states (TDOS) plots of Al12N12, (6,0) and (4,4) AlNNTs, and AlNNS. The geometry of Al12N12 cage is found to be composed of six four-membered rings having Th symmetry. There are two nonequivalent AleN bonds: AleN bond in the four-membered ring has a length of 1.84 Å, while the AleN bond between the four-membered rings has a length of 1.78 Å. These are consistent with the results obtained in the previous theoretical studies [25]. The lengths of the AleN bonds in the (6,0) and (4,4) AlNNTs are about 1.80 Å, which are in good agreement with those of other theoretical studies [32,33]. It is also seen that the electronic structures of these nanotubes are almost independent of the tube chirality and diameter, due to the ionicity of AleN bonds. A partial structure of the optimized AlNNS is shown in Fig. 1. In the AlN hexagonal plane, each N atom covalently binds with the three adjacent Al atoms. Also, the sp2-hybridization of all orbitals in the AlNNS is due to the lone-electron pairs which are distributed over the N atoms of the nanosheet and the bonding combination of Al-pz and N-pz orbitals. Due to the significant electronegativity difference between Al and N atoms, the Al atoms are positively charged and hence, all of the AleN bonds have an ionic character. Therefore, all these AlN nanostructures can be considered as a non-metal catalyst with Lewis acidebase pairs [25,47]. Fig. 1 also compares the computed TDOS plots of the pristine Al12N12, AlNNTs and AlNNS. The TDOS of the Al12N12 exhibits that the energy gap (Eg) between the highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) is 6.31 eV. This value is in good agreement with that reported in previous studies [13], introducing it as a relatively wide gap semiconductor. As it is also clear from the corresponding TDOS plots, the Eg for zigzag AlNNT (6.49 eV) is about 0.10 eV smaller than that of armchair (6.59 eV), indicating that they are also semiconductor materials. The calculated TDOS plot in Fig. 1 indicates the Eg value of 7.38 eV for AlNNS. The experimental value for HOMO-LUMO gap for bulk AlN is about 6.2 eV [48]; the main reason for the difference between this Eg and our estimated value is most likely due to our used simplified model in this study. Comparison of Eg values obtained for AlNNTs and AlNNS also indicates that the Eg decreases with the increase of the nanosheet curvature, suggesting that it is easier for the electrons in AlNNT than those in AlNNS to be excited from the HOMO to LUMO. This can be also attributed to the different degree of contribution of electron pair of nitrogen atoms to form a delocalized conjugated system with their adjacent Al atoms. 3.2. CO2 capture over the Al12N12 nanocage According to Fig. 2, there are three stable configurations for the adsorption of CO2 over the Al12N12 cage, corresponding two chemisorptions (A and B complexes) and one physisorption (complex C). The stable optimized structures of possible adsorption configurations of CO2 over the Al12N12 surface and their corresponding bond lengths are shown in Fig. 2. Also, the corresponding Ead, Gibbs free energy change (DG298), enthalpy change (DH298) and charge transfer (qCT) for each complex are listed in Table 1. In the A complex, the calculated Ead is 49.54 kcal/mol, which is about 21 kcal/mol more negative than that of formic acid adsorption over Al12N12 nanocage [46]. In this configuration, two O atoms of the CO2 molecule are placed above Al atoms of the Al12N12 and CO2 is chemisorbed with its positive C atom by the negative N atom of the surface (CeN ¼ 1.36 Å) and average AleO binding distance of 1.92 Å (Fig. 2). The formation of this configuration is exothermic (DH298 ¼ 48.37 kcal/ mol), and the negative value of DG298 (36.22 kcal/mol) shows that the reaction can occur at room temperature. In addition, the strong interaction of CO2 with the Al12N12 cage is attributed to the significant charge transfer (qCT) from the surface to the gas molecule (0.61 e), which occupy the CO2-2p* orbital and subsequently lead to elongation of both CeO bonds (z1.28 Å) compared to the free state of CO2 molecule. This can be also explained with the electron density difference (EDD) map which is demonstrated in Fig. 2. It is clear that there is a great charge accumulation around the CeN and OeAl bonds which indicates the strong chemical binding in these regions. In another chemisorbed complex B, CO2 molecule is attached to the Al12N12 cluster via two bonds: CeN and OeAl, with the distinct binding distances of 1.46 and 1.81 Å, respectively (Fig. 2). In comparison with the complex A, the calculated Eads for this complex is decreased from 49.54 to 40.78 kcal/mol. It can be
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Fig. 1. Optimized structures (left) and TDOS plots (right) of different AlN nanostructures: (a) Al12N12, (b) (6,0) AlNNT, (c) (4,4) AlNNT and (d) AlNNS. All distances are in Å.
168 M.D. Esrafili et al. / Superlattices and Microstructures 96 (2016) 164e173 Fig. 2. Optimized structures, EDD maps and PDOS plots of CO2eAl12N12 complexes. In the electron density difference plots, charge depletion and accumulation are displayed in red and blue, respectively. In each PDOS plot, the vertical dashed line indicates the Fermi level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Calculated binding distances (R), adsorption energy (Eads), change of Gibbs free energy (DG298), change of enthalpy (DH298), amount of charge transfer and HOMO-LUMO energy gap of CO2 adsorption over different AlN nanostructures. Surface Al12N12 A B C AlNNT (6,0) D E AlNNT (4,4) D′ E′ AlNNS F G
RN-C (Å)
Eads (kcal/mol)
DG298 (kcal/mol)
DH298 (kcal/mol)
qCT (e)
1.36 1.46 2.75
49.54 40.78 14.43
36.22 28.46 4.13
48.37 39.68 13.55
0.61 0.50 0.09
1.39 2.88
41.60 11.62
29.80 2.46
38.90 9.94
0.58 0.09
1.40 2.77
35.96 10.65
23.16 1.41
31.88 6.73
0.58 0.07
1.42 2.90
32.19 8.89
22.20 5.53
27.17 4.12
0.58 0.05
Eg (eV) 6.31 6.28 6.13 6.31 6.49 6.46 6.45 6.59 6.62 6.61 7.38 7.30 7.40
understood from the negative values of DH298 and DG298 that the formation of complex B is exothermic and is thermodynamically feasible (Table 1). Also, from the Mullikan charge analysis, a net charge about 0.50 e is transferred from the nanocage to the 2p* orbital of CO2 molecule. In comparison with complex A, despite the smaller qCT value of complex B, the CeO binding distances are increased more significantly (1.32 Å). This can be attributed to the number of OeAl bonds which are formed in this complex. One can see from the EDD map in Fig. 2 that like complex A, there is also a strong binding between CO2 and the Al12N12 surface, because of the great charge accumulation between the C (O) atom of the CO2 molecule and N (Al) atoms of the surface. On the other hand, the last CO2eAl12N12 complex is related to the configuration in which CO2 molecule is physically adsorbed over the Al12N12 surface (complex C). In this complex, CO2 molecule is just placed above the nanocluster with a weak formed CeN bond (2.75 Å) and Eads of about 14.43 kcal/mol (Fig. 2). According to Table 1, despite the small amount of DH298 and DG298, the formation of complex C is exothermic and it is accessible at an ambient condition. Also, due to the weak interaction between CO2 and the Al12N12 surface, a small net charge about 0.09 e is transferred from the CO2 molecule to the cage which is also confirmed by the EDD analysis of this complex (Fig. 2), in which there is no any electron density accumulation between the gaseous molecule and the cluster surface. Finally, it can be understood from the projected density of states (PDOS) plots that upon the adsorption of CO2, the calculated HOMO-LUMO energy gap is reduced from 6.31 eV in the pristine Al12N12 to 6.28 and 6.13 eV for the complexes A and B, respectively, while it doesn’t considerably change in complex C (Table 1). This can be explained with the slightly movement of both conduction and valence levels to higher energies, so that Eg of the nanocage is slightly decreased for the complex B. Hence, it is concluded that the electronic properties of the Al12N12 are sensitive to the CO2 molecule, and therefore, this nanocage can be used as a potential sensor for CO2 detection. 3.3. CO2 capture over the AlNNTs Different adsorption configurations of CO2 over zigzag and armchair AlNNTs with their corresponding bond lengths are shown in Fig. 3. There are two adsorption configurations for zigzag (armchair): named D (D )׳and E (E)׳, in which D and D׳ complexes are chemisorbed while E and E ׳are physisorbed. Additionally, the calculated AleN binding distances (R) and the corresponding thermodynamic parameters for both nanotubes after the adsorption of CO2 are individually summarized in Table 1. As mentioned before, Al and N atoms of AlNNTs are positively and negatively charged, respectively. Also, a considerable charge density is transferred from the electropositive atom (Al) to the electronegative one (N). Thus, large values of Eads may be achieved due to the interaction of these electron rich sites (N atoms) of the AlNNTs with the electron poor atom of the CO2 gas (C atom). It is important to know that the adsorption configurations of CO2 over these nanotubes are exactly the same with that over Al12N12 cluster. In the formation process of complex D, the CO2 molecule is chemically adsorbed over the (6,0) zigzag AlNNT with the CeN distance of 1.39 Å. In this configuration, two O atoms of the gas molecule is also chemisorbed with Al atoms of the AlNNT while the average binding distance is about 1.91 Å. The calculated Eads of this configuration is 41.60 kcal/mol which proceeds via an exothermic reaction (Table 1). Due to the strong charge transfer of 0.58 e from the tube to the 2p* orbital of CO2 molecule, the CeO bond length is increased from 1.16 to 1.27 Å. This means that the reactivity of this molecule is increased after adsorption over the tube surface. Also, the EDD map of complex D shows a great interaction between CO2 molecule and the tube surface. Likewise, similar adsorption configuration is obtained in the formation of complex D׳, where the CO2 chemisorbed over the (4,4) armchair AlNNT with the corresponding CeN bond length and Eads of 1.40 Å and 35.96 kcal/mol, respectively (Fig. 3). Additionally, similar to complex D, the charge amount of 0.58 e is transferred from the surface to the CO2 molecule in this configuration. This sizable charge transfer is also clear from its EDD map as depicted in Fig. 3. It should be noted that in comparison with the average OeAl bond in complex D, here, the OeAl bond lengths are slightly increased to 1.93 and 1.98 Å. Analogous to complex D, the amount of charge analysis (see Table 1) and the CeO bond elongation can be explained with the obtained EDD map which implies that a chemisorption occurs between CO2 and tube surface. Comparing the results of these two chemisorption configurations (D
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Fig. 3. Optimized structures, EDD maps and PDOS plots of CO2-AlNNT complexes. In the electron density difference plots, charge depletion and accumulation are displayed in red and blue, respectively. In each PDOS plot, the vertical dashed line indicates the Fermi level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and D )׳indicates that the CO2 adsorption is slightly more stable over the (6,0) zigzag tube than (4,4) armchair one (Table 1). Another adsorption configuration (complex E) is found when the CO2 molecule is weakly adsorbed on the (6,0) AlNNT surface. Despite the large bond length of CeN (2.88 Å) and small Eads of 11.62 kcal/mol, the formation of this complex is exothermic (DH298 ¼ 9.94 kcal/mol) and thermodynamically favorable (DG298 ¼ 2.46 kcal/mol). The relatively small charge transfer (0.09 e) from the CO2 molecule to the AlNNT and the studied EDD isosurface confirm a weak CO2 adsorption over the zigzag nanotube surface. Like complex E, upon the physisorption of CO2 molecule over an armchair (4,4) AlNNT, the complex E ׳with the Eads of 10.65 kcal/mol and the CeN distance of about 2.77 Å is achieved exothermically with DH298 ¼ 6.73 kcal/mol and a relatively low value of DG298 ¼ 1.41 kcal/mol. Analogous with the configuration E, due to the weak adsorption of CO2 molecule with the surface, the amount of charge transferring is negligible (0.07 e). Comparing these two different tubular AlNNTs indicates that the CeN binding between CO2 molecule and the (6,0) AlNNT is stronger than that in (4,4) tube. Besides, the calculated Eads values for CO2 adsorption in the present study are more negative than that of CH4 molecule over (4,4) AlNNT and SiCNT [30]. The calculated PDOS plots of the (6,0) and (4,4) AlNNTs are shown in Fig. 3. It is clear from Table 1 that upon the adsorption of CO2, the Eg values of complexes D, E, D ׳and E ׳change by 0.02 (0.3%), 0.04 (0.6%), 0.03 (0.5%) and 0.02 eV (0.3%), respectively. Thus, it can be concluded that the electronic properties of these nanotubes are less sensitive to the CO2 molecule than Al12N12 nanocage.
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Fig. 4. Optimized structures, EDD maps and PDOS plots of CO2eAlNNS complexes. In the electron density difference plots, charge depletion and accumulation are displayed in red and blue, respectively. In each PDOS plot, the vertical dashed line indicates the Fermi level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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3.4. CO2 capture over the AlNNS In order to examine the sensitivity of the AlNNS toward the CO2, the gas molecule is placed at different adsorption sites with different positions (Fig. 4). Analogues to the results of the Al12N12 cluster and AlNNTs, the planar surface of the AlNNS is deformed upon the chemisorptions of CO2 molecule (complex F). According to the reported thermodynamic parameters which are listed in Table 1, it is clear that in contrast to other studied nanostructures, the Eads of this system is quite small (32.19 kcal/mol). This finding can be explained by the curvature effect, since the sidewall reactivity of these AlNNTs depends critically on their local curvature. These are also supported by charge transfer values from the surface to the CO2 molecule. It should be noted that the estimated Eads value in this work is more negative than that of reported by Jia et al. (20.47 kcal/mol) as obtained at the B3LYP/6-31G* [38]. This may be attributed to the failure of the B3LYP method to properly include dispersion forces, which is the main constituent part of the weak intermolecular interactions. In addition, the calculated Eads value of CO2 in this work is larger than those of SO2 molecule over the AlNNS [37]. Also, a charge of 0.58 e is transferred from the plane to the CO2 molecule and occupies its 2p* orbital which leads to an outstanding charge accumulation around the CeN bond (see EDD map in Fig. 4). Unlike the chemisorption configuration (complex F), the physisorbed complex G is formed in which two oxygen atoms are just located above two aluminum atoms of the sheet without any bond formation. Due to the small Eads of 8.89 kcal/mol and the large average OeAl (CeN) bond lengths of 2.90 Å, the physically adsorbed CO2 molecule does not have any considerable effect on the electronic structure of AlNNS. Another reason for this weak interaction can be attributed to the negligible charge of about 0.05 e which is transferred from the gas molecule to the plane. Additionally this is clear from the electron depletion region in EDD map where there is not any considerable electron accumulation between the CO2 molecule and the AlNNS. 4. Conclusion In summary, a comparative DFT study was performed to study the adsorption of CO2 molecule over three different AlN nanostructures surfaces, i.e. Al12N12, AlNNTs and AlNNS. In order to find the effect of chirality on the CO2 adsorption, two different AlNNTs were chosen: a finite-sized (6,0) zigzag and (4,4) armchair AlNNT, with the approximately same diameter. Totally, nine different configurations (named from A to G) were obtained from these four nanostructures. In all these calculations, CO2 shows strong chemical binding with all of the AlN nanostructures in contrast of its physisorption configurations. However, all of these different configurations were formed via an exothermic reaction and were thermodynamically feasible at ambient temperatures. The smallest Eads values were achieved for the physisorbed configurations and the relative magnitude order of the AlN nanostructure surfaces toward CO2 adsorption is as follows: Al12N12 > (6,0) AlNNT > (4,4) AlNNT > AlNNS. This trend can be attributed to the curvature effect of these AlN nanostructures and to the different contribution of electron pair of nitrogen atoms to form a delocalized conjugated system with the adjacent Al atoms. As a result, the large curvature of Al12N12 facilitates the sp3-hybridization of the site and thus helps in the binding of CO2 on the surface. These results also reveal that the small-diameter AlNNTs with large curvature would be beneficial for the capture of CO2. In addition, this indicates the significant potential of Al12N12 nanocage toward the CO2 adsorption, fixation and catalytic applications in contrast to other AlN nanostructures. It is also found that the electronic structures of the AlNNTs are almost independent of the tube chirality and diameter, due to the ionicity of AleN bonds. This comparative study presented here may provide a new understanding about the CO2 adsorption over three important AlN nanostructures. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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