J Mol Model (2014) 20:2071 DOI 10.1007/s00894-014-2071-5
ORIGINAL PAPER
DFT study of ozone dissociation on BC3 graphene with Stone–Wales defects Ali Ahmadi Peyghan & Morteza Moradi
Received: 8 July 2013 / Accepted: 4 November 2013 / Published online: 24 January 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Ozone (O3) adsorption on pristine Stone–Wales (SW) defective BC3 graphene-like sheets was investigated using density functional calculations. It was found that O3 is weakly adsorbed on the pristine sheet. Two types of SWdefective sheets were studied, SW-CC and SW-BC, in which a defect is formed by rotating a C–C or B–N bond, respectively. O3 molecules were found to be more reactive on SWBC defective sheets. It was predicted that O3 molecules are reduced to O2 molecules on SW-BC sheets, overcoming an energy barrier of 34.2 kcal/mol−1 at the B3LYP level of theory and 27.2 kcal/mol−1 at the BP98 level of theory. Therefore, SW-BC sheets could potentially be employed as a metal-free catalyst for O3 reduction. The HOMO–LUMO gap of a SWBC sheet decreases from 2.16 to 1.21 eV after O3 dissociation on its surface in the most stable state. Keywords Semiconductors . Nanostructures . Ab initio . Electronic properties
Introduction Ozone (O3) is toxic to the respiratory tissues and ocular mucosa following inhalation. As expected, the primary target of airborne ozone is the lung, and it has been shown to cause adverse effects on the eyes and the nervous system [1]. Accordingly, ozone has long been recognized as one of the most hazardous air pollutants, and one of the criteria of air pollutants. The off-gas leaving an ozone contact chamber A. A. Peyghan Central Tehran Branch, Islamic Azad University, Tehran, Iran M. Moradi (*) Department of Semiconductors, Materials and Energy Research Center, P.O. Box 31787–316, Karaj, Iran e-mail: [email protected]
may contain high levels of ozone. The emission of ozone must be controlled or it must be removed before discharging the effluent into the atmosphere. The most commonly used methods for removing ozone are thermal destruction, catalytic decomposition, and adsorption/decomposition on activated carbon. Many articles focusing on methods of catalytically decomposing ozone have been published [2, 3]. Nanostructures are intriguing from the perspectives of both scientific research and future applications in devices, such as in cluster protection, nano ball bearings, nano-optical magnetic devices, catalysis, gas sensors, and biotechnology [4–8]. Recently, spurred on by the creation of graphene in laboratories [9], two-dimensional graphene has aroused interest among theoretical and experimental researchers due to its characteristic electronic properties. It is well established that a pristine layer of graphene is a zero-gap semiconductor with a point-like Fermi surface and a linear dispersion at the Fermi level. A great amount of experimental and theoretical work has demonstrated that the existence of defects, such as Stone– Wales (SW) defects, vacancies, antisites, and pentagon–octagons in graphene and nanotubes, can drastically change their properties [10–12]. It is typically possible to tailor the desired band gap for such a system through the formation of nanoribbons by cutting a graphene sheet or attaching foreign atoms to the pristine layer [13]. These graphene sheets can be transformed into multi-dimensional carbon materials by self-assembly, and they can be used as (2D) building blocks for many carbon materials. Nevertheless, graphene usually shows various types of structural defects that arise during its growth. Boron atoms have been widely used as dopants in carbon nanostructures to build functional materials due to the similar atomic radii of C and B. Not surprisingly, a uniform BC3 sheet (h-BC3) has already been synthesized in the laboratory [14]. Its geometric structure was found to be almost identical to graphene, except that some carbon atoms are replaced with boron atoms, so six
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carbon atoms form a hexagon that is surrounded by six boron atoms [15, 16]. In addition, Pontes et al. found that boron atoms can substitute for carbon atoms in a graphene sheet without any activation barrier [17]. However, it should be stressed that the BC3 honeycomb sheet is a semiconductor with an indirect gap, while graphene is a semiconductor with a zero gap. Theoretical studies revealed that it is possible to tune the electronic properties of h-BC3 [18, 19]. Moreover, a study by Ding et al. [19] further indicated that hydrogen adsorption on h-BC3 results in the appearance of a semiconductor–semiconductor–metal transition. It has also been demonstrated theoretically that a BC3 nanotube may be a potential chemical sensor for CS2 molecules [20]. The adsorptive characteristics of graphene-like nanosheets in the gas phase have led to their use as gas sensors, storage for fuels, and for the decomposition of hazardous pollutants from gas streams [21–24]. In the work described in the present paper, we attempted to find, using a theoretical approach, the answers to the following questions: (1) could h-BC3 serve as a suitable surface for O3 dissociation, and if not, (2) what kind of strategy can be applied to improve the applicability of hBC3 to O3 dissociation?
Computational methods We selected a BC3 sheet consisting of 102 C and 34 B atoms and in which the end atoms were saturated with hydrogen atoms to reduce boundary effects. Geometry optimizations, energy calculations, and density of states (DOS) analysis were performed on the h-BC3 and different O3/h-BC3 complexes using the B3LYP functional with the 6–31G (d) basis set as implemented in the GAMESS software suite [25]. The GaussSum program was used to obtain DOS results [26]. B3LYP has proven to be a reliable and commonly used functional in studies of different nanostructures [27–31]. We defined adsorption energy in the usual way, as E ad ¼ EðO3 =h−BC3 Þ–Eðh−BC3 Þ–EðO3 Þ þ EBSSE ;
ð1Þ
where E(O3/h-BC3) corresponds to the energy of h-BC3 with O3 adsorbed on its surface, E(h-BC3) is the energy of the isolated sheet, E(O3) is the energy of a single O3 molecule, and EBSSE is the energy of the basis set superposition error. By definition, a negative value of Ead corresponds to exothermic adsorption. To investigate the changes in the electronic charge on the h-BC3, the net charge transfer (QT) between the O3 molecule and the sheet was calculated using Mulliken population analysis and defined as the difference between the charge on an O3 molecule adsorbed on h-BC3 and the charge on an isolated O3 molecule.
Results and discussion The typical topological defect found in nanostructures is the SW defect, which consists of two pairs of five-membered and seven-membered rings. The SW defect can be created by rotating a bond 90° and geometry of SW defected sheet (SW-h-BC3). In it, two types of bonds (B–C and C–C) can be identified, with lengths of 1.56 and 1.42 Å, respectively. Consequently, two types of SW defects are possible for each h-BC3, labeled type I and type II. The former is obtained by rotating a C–C bond in the sheet, while the latter is obtained by rotating a tilted B–C bond. The atomic configurations for the two types of SW defects in h-BC3 and the perfect configuration are shown in Fig. 1. The formation energy of an SW defect is defined as: Ef =ESW − Epristine, where ESW and Epristine are the energy of h-BC3 containing an SW defect and the energy of a perfect sheet, respectively. The formation energies obtained by Zhao et al. for BC3 nanotubes are lower than the values for the corresponding carbon nanotubes [32]. Unlike in a BN nanotube [33], an SW defect in a BC3 nanotube does not form unstable homoelemental B−B bonds, so its structure is more stable. For type I SW defects (SW-CC), Ef was calculated to be +62.3 kcal/mol−1, but Ef was found to be +84.4 kcal/ mol−1 for type II SW defects (SW-BC). These positive values indicate that both of the defect formation processes are endothermic, and they also suggest that the SW-CC process may be a more energetically favorable one than the SW-BC process. The main change from the pristine structure is that the bond belonging to the defect is compressed and stretched. In particular, the rotated C–C and B–C bond lengths decrease to 1.34 and 1.48 Å, respectively. As shown by the calculated DOSs and the values for the energy gap (Eg) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in Fig. 1 and Table 1, the pristine h-BC3 is found to be a semiconductor with an Eg of 2.65 eV. Figure 2 shows the electronic density differences for the defected structures shown in Figure 1. It should be noted that in SW-BC (Fig. 2a), electron accumulation is dispersed across different regions of the sheet. However, for SW-CC, the charge mainly accumulates in the defective region (Fig. 2b). In order to obtain stable configurations (local minima) of a single O3 molecule adsorbed on pristine h-BC3, various possible initial adsorption geometries, including single (terminal and central oxygen), double (O–O), and triple (O–O–O) bonded atoms close to B and C atoms were considered. For the sake of simplicity, only the three most stable complexes are shown in Fig. 3. For O3 adsorption on the pristine sheet, optimization does not cause noticeable change in geometry for both O3 and the sheet compared with the isolated molecule and the sheet. This means that the interaction between O3 and the pristine sheet is very weak, as demonstrated by the range of values for Ead (+0.1 to −0.9 kcal/mol−1) and the large interaction distance of 3.56–4.48 Å.
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Fig. 1a–c Partial structure of optimized a pristine, b SW-CC defective h-BC3, and c SW-BC defective h-BC3, and the corresponding density of states (DOS) plots. Bond lengths are in Å
Furthermore, the effect of an SW defect on the adsorption behavior of O3 was investigated. Different possible adsorption configurations, including those with the central or a terminal O atom of the O3 molecule close to the site of the defect in the sheet, were investigated. More detailed information on the simulation of different O3/SW-h-BC3 systems, including values of Ead, electronic properties, and the charge transfer for these configurations, is listed in Table 1. First, we focused on the adsorption of O3 on a defective SW-CC sheet. After geometry optimization, it was found that only one kind of configuration is stable (A). In this configuration (see Fig. 4), the O3 molecule was located on top of the B atom, with a terminal oxygen atom closest to the B. The bond length in this case was 1.53 Å and the corresponding calculated value of Ead was about −9.1 kcal/mol−1. Mulliken population analysis
showed a net charge transfer (of 0.295 e) from the sheet to the adsorbate, which can be explained by the high electronegativity of the O atoms in O3. In the next step, we investigated the adsorption of O3 on a defective SW-BC sheet. Band Cin Fig. 4 are the most stable and the second most stable configurations for O3 adsorption on an SW-BC sheet. In the second most stable configuration (B), a terminal oxygen atom of O3 was located on top of the boron atom of the SW-CC sheet, with a bond length of 1.51 Å (O–B). For the most stable configuration (Cin Fig. 4), two oxygen atoms of the molecule were located on top of C and B atoms of the sheet at the site of the defect, so two new bonds were formed: O– B and O–C, with lengths of 1.39 and 1.41 Å, respectively. The adsorption process in configuration B is obviously weaker (Ead = −13.3 kcal/mol −1 ) than configuration C (Ead =
Table 1 Adsorption energies (Ead in kcal/mol−1), HOMO energies (EHOMO), LUMO energies (ELUMO), and HOMO–LUMO energy gaps (Eg) of defective systems (in eV), as calculated at the B3LYP/6-31G(d) level EHOMO
ELUMO
Eg
– –
– –
−6.40 −6.11
−3.75 −3.96
2.65 2.15
– –
−16.3
−0.295 – −0.421 −0.621 −0.821
−6.17 −5.93 −5.81 −5.71 −5.96
−5.30 −3.77 −4.50 −3.76 −4.75
0.87 2.16 1.31 1.95 1.21
59.5 – 39.3 10.7 44.0
Eada
h-BC3 SW-CC
– –
(A) O3/SW-CC SW-BC (B) O3/SW-BC (C) O3/SW-BC (D) O3/SW-BC
−9.1 – −13.3 −67.9 −86.1
a
ΔEg (%)c
QT (|e|)b
Ead
Configuration
−18.2 −70.5 −91.3
Calculated at the B3LYP-D/6-31G(d) level of theory, rather than the B3LYP/6-31G(d) level of theory
b
QT is defined as the total Mulliken charge on the molecule
c
Change in the Eg of the sheet after O3 adsorption
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Fig. 2a–b Electronic density differences for a SW-BC and b SW-CC sheets. Blue and red regions correspond to positive and negative polarization, respectively
−67.9 kcal/mol−1). Furthermore, O3 adsorption induces structural deformations of both SW-BC and h-BC3. For example, in configuration C, the length of the B–C bond attached to O3 increases from 1.48 to 2.72 Å, which implies the scission of the B–C bond and the formation of an open structure on the side wall of the sheet. Such structural deformation can be attributed to the change from sp2 to sp3 hybridization on the adsorbed atoms. Also, it is necessary to mention that structural changes in the O3 molecule were observed, such as the increased O–O bond length of O3 (1.53 Å) as compared to that (1.20 Å )in isolated O3 and the reduced O–O–O bond angle (≈ 96.6° as compared to 117.9° in isolated O3). Next, the influence of the adsorption of O3 on the electronic properties of the SW-CC and SW-BC sheets was studied (configurations A, B, and C). After SW defect formation, both the conduction and the valence levels shift slightly towards the Fermi level, so the Eg of the sheet decreased from 2.65 eV in Fig. 3 Model for the stable adsorption of O3 on pristine hBC3 and the corresponding Ead. Bond lengths are in Å. Energies are in kcal/mol−1
pure h-BC3 to 2.15 and 2.16 eV in SW-CC and SW-BC, respectively. The DOS plots in Fig. 1b and c clearly show that the SW-defective h-BC3 is still a semiconductor. After O3 adsorption in configurations A and B, the conduction level shifts to lower energies compared to that in the nonadsorbed SW sheet, so the Eg decreases to 0.87 or 1.31 eV, respectively (Fig. 5a and b). It is well known that Eg (or the band gap in bulk materials) is a major influence on the electrical conductivity of a material, and there is a well-known relation between these parameters, as follows [34]: −E g σ∝exp ; ð2Þ 2kT where σ is the electrical conductivity and k is Boltzmann’s constant. According to the equation, increasing the value of Eg at a given temperature decreases the electrical conductivity.
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Fig. 4a–d Various configurations (A–D) for the adsorption of geometry-optimized O3 on an SWBC or SW-CC sheet. Bond lengths are in Å
The considerable change (about 59.5 or 39.3 %, respectively) demonstrates that the electronic properties of SW-CC and SW-BC sheets are highly sensitive to the configuration of Fig. 5a–d Plots showing the density of states for each O3/SWh-BC3 adsorption configuration (A–D) shown in Fig. 4
O3 adsorption if we consider configurations A and B (Table 1). However, the calculated DOS plot shows that adsorbing O3 via configuration C has no discernible effect on the electronic
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properties of the sheet, and the Eg of the SW-BC decreases only slightly (by about 0.21 eV) when adsorption occurs in this configuration (Fig. 5). The most interesting case, however, is the dissociative adsorption of the O3 molecule on the SW-BC sheet (configuration D). In this case, the O–O bond of the molecule was initially aligned vertically above and close to the B–C bond, and then full relaxation was performed (Fig. 4). Based on the NBO results and geometry analysis, the B–C bond of the sheet is broken after the adsorption process and the oxygen atom of the molecule interacts with the B–C bond, resulting in the formation of two new bonds. In this configuration, an O2 molecule escapes from the surface of the sheet, leaving an O atom attached to the B–C bond of the SW-BC sheet. For this functionalized h-BC3, we predict that local structural deformation occurs at the adsorption site, where the adsorbing pentagonal ring is destroyed and six-membered and eightmembered rings are formed. In this configuration, the C–O– B angle is about 101.3° and the newly formed O–C and O–B bond lengths are about 1.39 and 1.46 Å, respectively. This configuration is the most stable of all the adsorption configurations obtained, with an Ead of −86.1 kcal/mol−1. This suggests that the O3 molecule could be reduced to an O2 molecule on the SW-BC-h-BC3. Hence, defective h-BC3 may be a candidate metal-free catalyst for O3 reduction. We think that a type II SW defect, in which charge distribution is concentrated in the region of the defect, is suitable for O3 dissociation, whereas a type I SW defect cannot reduce the O3 to O2. Subsequently, the kinetic favorability of this energetically possible configuration was explored. As shown in Fig. 6, an energy barrier must be overcome to reach the final configuration. The geometry of the transition state (TS) structure is shown in Fig. 6. In this configuration, during the generation of the TS structure, the O–O bond becomes so weak that its length increases to 1.79 Å and new O–C and O–B bonds are formed with lengths of 1.53 and 1.67 Å, respectively. The activation energy for configuration D is about 34.2 kcal/ mol−1. It should be noted that an impurity peak appears at −4.75 eV in the DOS of the sheet after the dissociation of O3 over SW-h-BC3 (see D in Fig. 5), reducing the Eg from 2.16 to 1.21 eV. The appearance of this peak indicates that, after O3 adsorption, the electrical conductivity of the SW-defective BC3 sheet is increased. Finally, we investigated the effect of the DFT functional on the obtained results. The dispersion term makes a nonnegligible contribution to the total energy, so we repeated all of the energy calculations using the semiempirical dispersioncorrected functional B3LYP-D with the same (6-31G (d)) basis set. The results are summarized in Table 1, and they show that, in all cases, the Ead values obtained at the B3LYP-D level are somewhat more negative than those obtained at the B3LYP level. This may be due to the fact that B3LYP-D takes dispersion interactions into account. We also checked the results by
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Fig. 6 Geometry of the transition state structure (partial view) in the dissociation of O3 over the SW-BC sheet via configuration D. Energies are in eV
performing single point energy calculations using the popular BP86 functional with the 6-31G(d) basis set. The results listed in Table 2 show that the Ead values obtained with this functional are slightly more negative than those obtained using B3LYP. In terms of the electronic properties, the BP86 functional gives smaller Eg and larger ΔEg (i.e., the change in Eg after adsorption) values than B3LYP does, but the trend in ΔEg is the same. It was also predicted at the BP98 level of theory that the O3 molecule may be reduced to an O2 molecule on the SW-BC sheet, overcoming an energy barrier of 27.4 kcal/mol−1.
Conclusions The adsorption of an O3 molecule on pristine and SWdefective BC3 sheets was investigated using DFT calculations. The adsorption of O3 on an SW-h-BC3 sheet is very weak due to its small Ead, large binding distance, and small net charge transfer between the sheet and O3. Consequently, two types of SW defects are predicted for h-BC3, termed SW-CC and SWTable 2 HOMO energies (EHOMO), LUMO energies (ELUMO), and HOMO–LUMO energy gaps (Eg) of defective systems (in eV), as calculated at the BP86/6-31G(d) level of theory a
QT (|e|) EHOMO ELUMO Eg
ΔEg (%)
b
Configuration
Ead
h-BC3 SW-CC (A) O3/SW-CC SW-BC (B) O3/SW-BC
– – – – −12.1 −0.097 – – −15.2 −0.327
−6.04 −5.78 −5.83 −5.40 −5.27
−4.09 −4.37 −5.47 −4.14 −4.85
1.95 – 1.41 – 0.36 74.4 1.26 – 0.42 66.6
(C) O3/SW-BC (D) O3/SW-BC
−68.1 −0.555 −90.4 −0.751
−5.33 −5.35
−4.63 −5.04
0.87 30.9 0.31 75.4
a
QT is defined as the total Mulliken charge on the molecule
b
Change in the Eg of the sheet after O3 adsorption
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BC. SW-CC is energetically more stable than SW-BC. In contrast to its interactions with a pristine sheet, the O3 molecule shows strong interactions with a defective sheet. The significantly more negative Ead and greater charge transfer between O3 and the modified BC3 sheet are expected to induce significant changes in the electrical conductivity of the sheet. Interestingly, the results indicated that the O3 molecule could be reduced to O2 on SW-BC-h-BC3.
References 1. Felix EP, Filho JP, Garcia G, Cardoso AA (2011) Microchim J 99:530 2. Yu Q, Pan H, Zhao M, Liu Z, Wang J, Chen Y, Gong M (2009) J Hazard Mater 172:631 3. Li D, Zhu J, Ray MB, Ray AK (2011) Chem Eng Sci 66:4615 4. Chelmecka E, Pasterny K, Kupka T, Stobiński L (2012) J Mol Model 18:1463 5. Rajarajeswari M, Iyakutti K, Kawazoe Y (2012) J Mol Model 18:771 6. Chełmecka E, Pasterny K, Kupka T, Stobiński L (2012) J Mol Model 18:2241 7. Beheshtian J, Baei MT, Bagheri Z, Peyghan AA (2012) Microelectron J 43:452 8. Beheshtian J, Ahmadi Peyghan A, Bagheri Z (2012) Phys E 44:1963 9. Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, Grigorieva IV, Firsov AA (2004) Science 306:666 10. Ouyang T, Chen Y, Xie Y, Yang K, Zhong J (2010) Solid State Commun 150:2366 11. Shayeganfar F, Neek-Amal M (2012) Solid State Commun 152:1493 12. Ahmadi Peyghan A, Hadipour N, Bagheri Z (2013) J Phys Chem C 117:2427
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ORIGINAL PAPER
DFT study of ozone dissociation on BC3 graphene with Stone–Wales defects Ali Ahmadi Peyghan & Morteza Moradi
Received: 8 July 2013 / Accepted: 4 November 2013 / Published online: 24 January 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Ozone (O3) adsorption on pristine Stone–Wales (SW) defective BC3 graphene-like sheets was investigated using density functional calculations. It was found that O3 is weakly adsorbed on the pristine sheet. Two types of SWdefective sheets were studied, SW-CC and SW-BC, in which a defect is formed by rotating a C–C or B–N bond, respectively. O3 molecules were found to be more reactive on SWBC defective sheets. It was predicted that O3 molecules are reduced to O2 molecules on SW-BC sheets, overcoming an energy barrier of 34.2 kcal/mol−1 at the B3LYP level of theory and 27.2 kcal/mol−1 at the BP98 level of theory. Therefore, SW-BC sheets could potentially be employed as a metal-free catalyst for O3 reduction. The HOMO–LUMO gap of a SWBC sheet decreases from 2.16 to 1.21 eV after O3 dissociation on its surface in the most stable state. Keywords Semiconductors . Nanostructures . Ab initio . Electronic properties
Introduction Ozone (O3) is toxic to the respiratory tissues and ocular mucosa following inhalation. As expected, the primary target of airborne ozone is the lung, and it has been shown to cause adverse effects on the eyes and the nervous system [1]. Accordingly, ozone has long been recognized as one of the most hazardous air pollutants, and one of the criteria of air pollutants. The off-gas leaving an ozone contact chamber A. A. Peyghan Central Tehran Branch, Islamic Azad University, Tehran, Iran M. Moradi (*) Department of Semiconductors, Materials and Energy Research Center, P.O. Box 31787–316, Karaj, Iran e-mail: [email protected]
may contain high levels of ozone. The emission of ozone must be controlled or it must be removed before discharging the effluent into the atmosphere. The most commonly used methods for removing ozone are thermal destruction, catalytic decomposition, and adsorption/decomposition on activated carbon. Many articles focusing on methods of catalytically decomposing ozone have been published [2, 3]. Nanostructures are intriguing from the perspectives of both scientific research and future applications in devices, such as in cluster protection, nano ball bearings, nano-optical magnetic devices, catalysis, gas sensors, and biotechnology [4–8]. Recently, spurred on by the creation of graphene in laboratories [9], two-dimensional graphene has aroused interest among theoretical and experimental researchers due to its characteristic electronic properties. It is well established that a pristine layer of graphene is a zero-gap semiconductor with a point-like Fermi surface and a linear dispersion at the Fermi level. A great amount of experimental and theoretical work has demonstrated that the existence of defects, such as Stone– Wales (SW) defects, vacancies, antisites, and pentagon–octagons in graphene and nanotubes, can drastically change their properties [10–12]. It is typically possible to tailor the desired band gap for such a system through the formation of nanoribbons by cutting a graphene sheet or attaching foreign atoms to the pristine layer [13]. These graphene sheets can be transformed into multi-dimensional carbon materials by self-assembly, and they can be used as (2D) building blocks for many carbon materials. Nevertheless, graphene usually shows various types of structural defects that arise during its growth. Boron atoms have been widely used as dopants in carbon nanostructures to build functional materials due to the similar atomic radii of C and B. Not surprisingly, a uniform BC3 sheet (h-BC3) has already been synthesized in the laboratory [14]. Its geometric structure was found to be almost identical to graphene, except that some carbon atoms are replaced with boron atoms, so six
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carbon atoms form a hexagon that is surrounded by six boron atoms [15, 16]. In addition, Pontes et al. found that boron atoms can substitute for carbon atoms in a graphene sheet without any activation barrier [17]. However, it should be stressed that the BC3 honeycomb sheet is a semiconductor with an indirect gap, while graphene is a semiconductor with a zero gap. Theoretical studies revealed that it is possible to tune the electronic properties of h-BC3 [18, 19]. Moreover, a study by Ding et al. [19] further indicated that hydrogen adsorption on h-BC3 results in the appearance of a semiconductor–semiconductor–metal transition. It has also been demonstrated theoretically that a BC3 nanotube may be a potential chemical sensor for CS2 molecules [20]. The adsorptive characteristics of graphene-like nanosheets in the gas phase have led to their use as gas sensors, storage for fuels, and for the decomposition of hazardous pollutants from gas streams [21–24]. In the work described in the present paper, we attempted to find, using a theoretical approach, the answers to the following questions: (1) could h-BC3 serve as a suitable surface for O3 dissociation, and if not, (2) what kind of strategy can be applied to improve the applicability of hBC3 to O3 dissociation?
Computational methods We selected a BC3 sheet consisting of 102 C and 34 B atoms and in which the end atoms were saturated with hydrogen atoms to reduce boundary effects. Geometry optimizations, energy calculations, and density of states (DOS) analysis were performed on the h-BC3 and different O3/h-BC3 complexes using the B3LYP functional with the 6–31G (d) basis set as implemented in the GAMESS software suite [25]. The GaussSum program was used to obtain DOS results [26]. B3LYP has proven to be a reliable and commonly used functional in studies of different nanostructures [27–31]. We defined adsorption energy in the usual way, as E ad ¼ EðO3 =h−BC3 Þ–Eðh−BC3 Þ–EðO3 Þ þ EBSSE ;
ð1Þ
where E(O3/h-BC3) corresponds to the energy of h-BC3 with O3 adsorbed on its surface, E(h-BC3) is the energy of the isolated sheet, E(O3) is the energy of a single O3 molecule, and EBSSE is the energy of the basis set superposition error. By definition, a negative value of Ead corresponds to exothermic adsorption. To investigate the changes in the electronic charge on the h-BC3, the net charge transfer (QT) between the O3 molecule and the sheet was calculated using Mulliken population analysis and defined as the difference between the charge on an O3 molecule adsorbed on h-BC3 and the charge on an isolated O3 molecule.
Results and discussion The typical topological defect found in nanostructures is the SW defect, which consists of two pairs of five-membered and seven-membered rings. The SW defect can be created by rotating a bond 90° and geometry of SW defected sheet (SW-h-BC3). In it, two types of bonds (B–C and C–C) can be identified, with lengths of 1.56 and 1.42 Å, respectively. Consequently, two types of SW defects are possible for each h-BC3, labeled type I and type II. The former is obtained by rotating a C–C bond in the sheet, while the latter is obtained by rotating a tilted B–C bond. The atomic configurations for the two types of SW defects in h-BC3 and the perfect configuration are shown in Fig. 1. The formation energy of an SW defect is defined as: Ef =ESW − Epristine, where ESW and Epristine are the energy of h-BC3 containing an SW defect and the energy of a perfect sheet, respectively. The formation energies obtained by Zhao et al. for BC3 nanotubes are lower than the values for the corresponding carbon nanotubes [32]. Unlike in a BN nanotube [33], an SW defect in a BC3 nanotube does not form unstable homoelemental B−B bonds, so its structure is more stable. For type I SW defects (SW-CC), Ef was calculated to be +62.3 kcal/mol−1, but Ef was found to be +84.4 kcal/ mol−1 for type II SW defects (SW-BC). These positive values indicate that both of the defect formation processes are endothermic, and they also suggest that the SW-CC process may be a more energetically favorable one than the SW-BC process. The main change from the pristine structure is that the bond belonging to the defect is compressed and stretched. In particular, the rotated C–C and B–C bond lengths decrease to 1.34 and 1.48 Å, respectively. As shown by the calculated DOSs and the values for the energy gap (Eg) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in Fig. 1 and Table 1, the pristine h-BC3 is found to be a semiconductor with an Eg of 2.65 eV. Figure 2 shows the electronic density differences for the defected structures shown in Figure 1. It should be noted that in SW-BC (Fig. 2a), electron accumulation is dispersed across different regions of the sheet. However, for SW-CC, the charge mainly accumulates in the defective region (Fig. 2b). In order to obtain stable configurations (local minima) of a single O3 molecule adsorbed on pristine h-BC3, various possible initial adsorption geometries, including single (terminal and central oxygen), double (O–O), and triple (O–O–O) bonded atoms close to B and C atoms were considered. For the sake of simplicity, only the three most stable complexes are shown in Fig. 3. For O3 adsorption on the pristine sheet, optimization does not cause noticeable change in geometry for both O3 and the sheet compared with the isolated molecule and the sheet. This means that the interaction between O3 and the pristine sheet is very weak, as demonstrated by the range of values for Ead (+0.1 to −0.9 kcal/mol−1) and the large interaction distance of 3.56–4.48 Å.
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Fig. 1a–c Partial structure of optimized a pristine, b SW-CC defective h-BC3, and c SW-BC defective h-BC3, and the corresponding density of states (DOS) plots. Bond lengths are in Å
Furthermore, the effect of an SW defect on the adsorption behavior of O3 was investigated. Different possible adsorption configurations, including those with the central or a terminal O atom of the O3 molecule close to the site of the defect in the sheet, were investigated. More detailed information on the simulation of different O3/SW-h-BC3 systems, including values of Ead, electronic properties, and the charge transfer for these configurations, is listed in Table 1. First, we focused on the adsorption of O3 on a defective SW-CC sheet. After geometry optimization, it was found that only one kind of configuration is stable (A). In this configuration (see Fig. 4), the O3 molecule was located on top of the B atom, with a terminal oxygen atom closest to the B. The bond length in this case was 1.53 Å and the corresponding calculated value of Ead was about −9.1 kcal/mol−1. Mulliken population analysis
showed a net charge transfer (of 0.295 e) from the sheet to the adsorbate, which can be explained by the high electronegativity of the O atoms in O3. In the next step, we investigated the adsorption of O3 on a defective SW-BC sheet. Band Cin Fig. 4 are the most stable and the second most stable configurations for O3 adsorption on an SW-BC sheet. In the second most stable configuration (B), a terminal oxygen atom of O3 was located on top of the boron atom of the SW-CC sheet, with a bond length of 1.51 Å (O–B). For the most stable configuration (Cin Fig. 4), two oxygen atoms of the molecule were located on top of C and B atoms of the sheet at the site of the defect, so two new bonds were formed: O– B and O–C, with lengths of 1.39 and 1.41 Å, respectively. The adsorption process in configuration B is obviously weaker (Ead = −13.3 kcal/mol −1 ) than configuration C (Ead =
Table 1 Adsorption energies (Ead in kcal/mol−1), HOMO energies (EHOMO), LUMO energies (ELUMO), and HOMO–LUMO energy gaps (Eg) of defective systems (in eV), as calculated at the B3LYP/6-31G(d) level EHOMO
ELUMO
Eg
– –
– –
−6.40 −6.11
−3.75 −3.96
2.65 2.15
– –
−16.3
−0.295 – −0.421 −0.621 −0.821
−6.17 −5.93 −5.81 −5.71 −5.96
−5.30 −3.77 −4.50 −3.76 −4.75
0.87 2.16 1.31 1.95 1.21
59.5 – 39.3 10.7 44.0
Eada
h-BC3 SW-CC
– –
(A) O3/SW-CC SW-BC (B) O3/SW-BC (C) O3/SW-BC (D) O3/SW-BC
−9.1 – −13.3 −67.9 −86.1
a
ΔEg (%)c
QT (|e|)b
Ead
Configuration
−18.2 −70.5 −91.3
Calculated at the B3LYP-D/6-31G(d) level of theory, rather than the B3LYP/6-31G(d) level of theory
b
QT is defined as the total Mulliken charge on the molecule
c
Change in the Eg of the sheet after O3 adsorption
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Fig. 2a–b Electronic density differences for a SW-BC and b SW-CC sheets. Blue and red regions correspond to positive and negative polarization, respectively
−67.9 kcal/mol−1). Furthermore, O3 adsorption induces structural deformations of both SW-BC and h-BC3. For example, in configuration C, the length of the B–C bond attached to O3 increases from 1.48 to 2.72 Å, which implies the scission of the B–C bond and the formation of an open structure on the side wall of the sheet. Such structural deformation can be attributed to the change from sp2 to sp3 hybridization on the adsorbed atoms. Also, it is necessary to mention that structural changes in the O3 molecule were observed, such as the increased O–O bond length of O3 (1.53 Å) as compared to that (1.20 Å )in isolated O3 and the reduced O–O–O bond angle (≈ 96.6° as compared to 117.9° in isolated O3). Next, the influence of the adsorption of O3 on the electronic properties of the SW-CC and SW-BC sheets was studied (configurations A, B, and C). After SW defect formation, both the conduction and the valence levels shift slightly towards the Fermi level, so the Eg of the sheet decreased from 2.65 eV in Fig. 3 Model for the stable adsorption of O3 on pristine hBC3 and the corresponding Ead. Bond lengths are in Å. Energies are in kcal/mol−1
pure h-BC3 to 2.15 and 2.16 eV in SW-CC and SW-BC, respectively. The DOS plots in Fig. 1b and c clearly show that the SW-defective h-BC3 is still a semiconductor. After O3 adsorption in configurations A and B, the conduction level shifts to lower energies compared to that in the nonadsorbed SW sheet, so the Eg decreases to 0.87 or 1.31 eV, respectively (Fig. 5a and b). It is well known that Eg (or the band gap in bulk materials) is a major influence on the electrical conductivity of a material, and there is a well-known relation between these parameters, as follows [34]: −E g σ∝exp ; ð2Þ 2kT where σ is the electrical conductivity and k is Boltzmann’s constant. According to the equation, increasing the value of Eg at a given temperature decreases the electrical conductivity.
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Fig. 4a–d Various configurations (A–D) for the adsorption of geometry-optimized O3 on an SWBC or SW-CC sheet. Bond lengths are in Å
The considerable change (about 59.5 or 39.3 %, respectively) demonstrates that the electronic properties of SW-CC and SW-BC sheets are highly sensitive to the configuration of Fig. 5a–d Plots showing the density of states for each O3/SWh-BC3 adsorption configuration (A–D) shown in Fig. 4
O3 adsorption if we consider configurations A and B (Table 1). However, the calculated DOS plot shows that adsorbing O3 via configuration C has no discernible effect on the electronic
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properties of the sheet, and the Eg of the SW-BC decreases only slightly (by about 0.21 eV) when adsorption occurs in this configuration (Fig. 5). The most interesting case, however, is the dissociative adsorption of the O3 molecule on the SW-BC sheet (configuration D). In this case, the O–O bond of the molecule was initially aligned vertically above and close to the B–C bond, and then full relaxation was performed (Fig. 4). Based on the NBO results and geometry analysis, the B–C bond of the sheet is broken after the adsorption process and the oxygen atom of the molecule interacts with the B–C bond, resulting in the formation of two new bonds. In this configuration, an O2 molecule escapes from the surface of the sheet, leaving an O atom attached to the B–C bond of the SW-BC sheet. For this functionalized h-BC3, we predict that local structural deformation occurs at the adsorption site, where the adsorbing pentagonal ring is destroyed and six-membered and eightmembered rings are formed. In this configuration, the C–O– B angle is about 101.3° and the newly formed O–C and O–B bond lengths are about 1.39 and 1.46 Å, respectively. This configuration is the most stable of all the adsorption configurations obtained, with an Ead of −86.1 kcal/mol−1. This suggests that the O3 molecule could be reduced to an O2 molecule on the SW-BC-h-BC3. Hence, defective h-BC3 may be a candidate metal-free catalyst for O3 reduction. We think that a type II SW defect, in which charge distribution is concentrated in the region of the defect, is suitable for O3 dissociation, whereas a type I SW defect cannot reduce the O3 to O2. Subsequently, the kinetic favorability of this energetically possible configuration was explored. As shown in Fig. 6, an energy barrier must be overcome to reach the final configuration. The geometry of the transition state (TS) structure is shown in Fig. 6. In this configuration, during the generation of the TS structure, the O–O bond becomes so weak that its length increases to 1.79 Å and new O–C and O–B bonds are formed with lengths of 1.53 and 1.67 Å, respectively. The activation energy for configuration D is about 34.2 kcal/ mol−1. It should be noted that an impurity peak appears at −4.75 eV in the DOS of the sheet after the dissociation of O3 over SW-h-BC3 (see D in Fig. 5), reducing the Eg from 2.16 to 1.21 eV. The appearance of this peak indicates that, after O3 adsorption, the electrical conductivity of the SW-defective BC3 sheet is increased. Finally, we investigated the effect of the DFT functional on the obtained results. The dispersion term makes a nonnegligible contribution to the total energy, so we repeated all of the energy calculations using the semiempirical dispersioncorrected functional B3LYP-D with the same (6-31G (d)) basis set. The results are summarized in Table 1, and they show that, in all cases, the Ead values obtained at the B3LYP-D level are somewhat more negative than those obtained at the B3LYP level. This may be due to the fact that B3LYP-D takes dispersion interactions into account. We also checked the results by
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Fig. 6 Geometry of the transition state structure (partial view) in the dissociation of O3 over the SW-BC sheet via configuration D. Energies are in eV
performing single point energy calculations using the popular BP86 functional with the 6-31G(d) basis set. The results listed in Table 2 show that the Ead values obtained with this functional are slightly more negative than those obtained using B3LYP. In terms of the electronic properties, the BP86 functional gives smaller Eg and larger ΔEg (i.e., the change in Eg after adsorption) values than B3LYP does, but the trend in ΔEg is the same. It was also predicted at the BP98 level of theory that the O3 molecule may be reduced to an O2 molecule on the SW-BC sheet, overcoming an energy barrier of 27.4 kcal/mol−1.
Conclusions The adsorption of an O3 molecule on pristine and SWdefective BC3 sheets was investigated using DFT calculations. The adsorption of O3 on an SW-h-BC3 sheet is very weak due to its small Ead, large binding distance, and small net charge transfer between the sheet and O3. Consequently, two types of SW defects are predicted for h-BC3, termed SW-CC and SWTable 2 HOMO energies (EHOMO), LUMO energies (ELUMO), and HOMO–LUMO energy gaps (Eg) of defective systems (in eV), as calculated at the BP86/6-31G(d) level of theory a
QT (|e|) EHOMO ELUMO Eg
ΔEg (%)
b
Configuration
Ead
h-BC3 SW-CC (A) O3/SW-CC SW-BC (B) O3/SW-BC
– – – – −12.1 −0.097 – – −15.2 −0.327
−6.04 −5.78 −5.83 −5.40 −5.27
−4.09 −4.37 −5.47 −4.14 −4.85
1.95 – 1.41 – 0.36 74.4 1.26 – 0.42 66.6
(C) O3/SW-BC (D) O3/SW-BC
−68.1 −0.555 −90.4 −0.751
−5.33 −5.35
−4.63 −5.04
0.87 30.9 0.31 75.4
a
QT is defined as the total Mulliken charge on the molecule
b
Change in the Eg of the sheet after O3 adsorption
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BC. SW-CC is energetically more stable than SW-BC. In contrast to its interactions with a pristine sheet, the O3 molecule shows strong interactions with a defective sheet. The significantly more negative Ead and greater charge transfer between O3 and the modified BC3 sheet are expected to induce significant changes in the electrical conductivity of the sheet. Interestingly, the results indicated that the O3 molecule could be reduced to O2 on SW-BC-h-BC3.
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