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Oxygen adsorption on single layer graphyne: a DFT study† Baotao Kang, Hongguang Liu and Jin Yong Lee* Graphyne is a rising two-dimensional (2D) carbon allotrope with excellent electronic properties. In this paper, theoretical calculations were performed to study the corresponding electronic properties of the oxygenated graphyne. Atomic oxygen when bound to the carbon atom of graphyne forms a stable oxide, with a much larger binding energy compared to that on graphene. Owing to the oxygen adsorption, the
Received 31st July 2013, Accepted 22nd October 2013
a- and b-graphyne change from a zero-band-gap material to a semiconductor as indicated in the band
DOI: 10.1039/c3cp53237b
g-graphyne. These electronic properties are tunable by altering the oxygen coverage through changing
structure calculations. Moreover, spin splitting was observed from the band structure of the oxygenated the supercell size. Our results based on the first-principles calculations imply that oxygenation is a
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promising method to functionalize graphyne to achieve designated properties.
Introduction Besides the existing carbon allotropes (graphite, diamond and carbon black) in the natural world, other artificial allotropes, especially graphene, have received enormous scientific interest due to their fascinating physical, mechanical and electrical properties.1–6 Recently other carbon allotropes, e.g., graphyne, graphdiyne and derivates7–10 are actively envisioned. Although graphene is the only 2D carbon allotrope that can be synthesized so far,11 the 2D periodic graphyne systems, which share some of remarkable graphene properties such as Dirac cone,12 are receiving growing attention. It is encouraging to see a large-area graphdiyne sheet and corresponding nanotube have recently been experimentally obtained.13,14 Structurally, graphyne systems are built by inserting the acetylenic linkage (–CRC–) between two bonded carbons of graphene but graphdiyne is theoretically estimated to be less stable than graphyne by about 0.18 eV per atom.15 Despite genuine graphyne not yet being available, some finite building blocks or cutouts have already been synthesized.16,17 Continuing efforts are thus devoted to developing more extended structures. Whether attempts are worthwhile to produce graphyne relies on the properties such material should have. Graphyne, which was first suggested by Baughman et al. in 1987,18 was theoretically reported to have outstanding properties, including good chemical stability, large surface area, and excellent electronic conductivity.19–21 The unique electronic properties may allow
Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Korea. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp53237b
974 | Phys. Chem. Chem. Phys., 2014, 16, 974--980
graphyne to have some potential applications, which have been theoretically investigated. For instance, Li mobility in the electrodes is a crucial issue for the Li-ion battery. The insertion of an acetylenic linkage can reduce the Li diffusion barrier and intensify the Li storage capability.22 Another application of graphyne is to serve as the electrode for fuel cells. The oxygen reduction reaction (ORR) is the dominant process which will strongly affect the performance of fuel cells.23–25 Graphyne with many positively charged sites can improve the efficiency of ORR.26 Ca-decorated graphyne was reported to be a promising material for H2 storage with an estimated gravimetric density of 9.6 wt%.27 It is no wonder that graphene is considered, from the application viewpoint, to be even more promising than other nanostructured carbon allotropes. Nevertheless, the zero band gap of graphene restricts its performance; one example is the low on–off ratios of graphene-based FETs.28 Based on different combinations of –CRC– with the graphene network, a-, band g-graphyne (illustrated in Scheme 1) are assembled with different electronic properties.12 The a- and b-graphyne are zero-gap materials, whereas the g-graphyne turns out to be a semiconductor with a band gap of 0.471 eV. The zero (or small) band gap of graphyne is inevitably a major impediment for the potential applications as a semiconductor. Thus one of the thrust areas in the field of graphyne has been on techniques to open or enlarge the band gap without compromising on any of its other properties. Various techniques and methods have come up to functionalize graphene.29–31 Among them, surface decoration is proved to be an efficient approach. It was reported that hydrogen (H) chemisorption on the 5 5 1 supercell of graphene opens a band gap of 1.20 eV which is tunable by changing the supercell size.32 In addition, a magnetic moment
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Scheme 1
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The unit cell for each type of graphyne.
will be induced by hydrogenation. Both theoretical33 and experimental34 studies revealed that O-decorated graphene can open a finite band gap as well. It was theoretically estimated to be 3.39 eV when the O coverage is 50%, and it is much reduced as the coverage decreases. All of these suggest that the H- or O-decorated graphene can be used as semiconductor or spintronics materials. Such an approach would be transplanted to graphyne. Theoretical work has been performed on hydrogenated graphyne.35 It clarified H prefers to be chemically adsorbed on the sp-hybridized carbon. A band gap of 1.01 eV is opened and it is tunable with the hydrogen coverage. However, to our knowledge, the band structure change by oxygen adsorption on single layer graphyne has not yet been reported. Herein, density function theory (DFT) studies were performed on three graphyne models to foresee the electronic property change caused by oxygenation. Oxygen is found tightly adsorbed on graphyne with a much larger binding energy than that on graphene. It is further revealed that oxygen adsorption can open a band gap for a- and b-graphyne, and induce a magnetic moment for g-graphyne. At last, we also explored the O coverage associated with the supercell size which might shed light on the strategies to attain desirable electronic properties.
Computational method In this work, all calculations were performed by the Dmol3 package.36 The generalized gradient approximation (GGA) method proposed by Perdew and Wang (PW91)37 was used to deal with the exchange and correlation functionals. The OBS method was employed to treat the dispersion energy. The lattice constants of each graphyne model were first fully optimized with a 20 Å vacuum space. All-electron treatment was performed and the double numerical plus polarization (DNP) basis set was utilized. The convergence tolerance of energy is taken as 10 5 Hartree, and the maximum allowed force and displacement are 0.002 Hartree per Å and 0.005 Å, respectively. For each type of graphyne, 1 1 1, 2 2 1 and 4 4 1 supercells were investigated with Brillouin zone k-point meshes of 7 7 1, 4 4 1 and 2 2 1, respectively. The binding energy (Eb) was defined as: Eb = Etotal Egraphyne EO, where Etotal stands for the total energy of the oxygen-adsorbed graphyne, Egraphyne is the energy of the isolated graphyne and EO represents the energy of the isolated oxygen.
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Result and discussion To study the presented idea, first of all, the unit cells of three graphyne models were fully optimized with 2D symmetry by both GGA (PW91) and LDA (PWC)38 methods. As seen in Table 1 and the ESI† (Table S1), it is clear that the GGA and LDA functionals do not give a significant difference for the 2D graphyne systems. Hereafter, we will discuss based on the GGA (PW91) results. The lattice constants were calculated to be 6.961 Å, 9.472 Å and 6.881 Å for a-, b- and g-graphyne, respectively. Different lattice constants stem from the unique combination characteristic between the –CRC– and graphene. Our calculated lattice constants are in line with those in the previous studies.12,27,39 For graphene, all the carbon atoms are equivalent with sp2-hybridization. The insertion of –CRC– induces two types of carbons that co-exist in graphyne, C1 (sp-hybridized) and C2 (sp2-hybridized). Two types of carbon– carbon bonds, C1–C1 and C1–C2, appear in a-graphyne as exhibited in Scheme 1. Meanwhile, another type, C2–C2, rises in both b-graphyne and g-graphyne. Because of the insertion of –CRC–, all C1 atoms are positively charged with 0.096e, 0.100e, 0.093e, while the C2 atoms are all negatively charged with 0.287e, 0.199e, 0.093e for a-, b- and g-graphyne, respectively. The charge distribution of a-graphyne is different from the earlier result.26 The mismatch would originate from the periodic consideration here rather than a finite model. It is expected that the positively charged C1 atoms in graphyne can enhance oxygen adsorption. The corresponding band structure of each graphyne model was further calculated. As clearly shown in Fig. 1, similar to the case in graphene, the a-graphyne and b-graphyne have a zero band gap in the band structure. However, a direct band gap emerges at the M point of Brillouin zone in the band structure of g-graphyne. The estimated band gap of g-graphyne is 0.435 eV, which is comparable to the earlier reported value (0.471 eV, ref. 12). For graphene, there are only three possible adsorption sites, i.e., top site, hollow site and bridge site.32,40 The oxygen
Table 1 The optimized lattice constant (a) and carbon–carbon bond lengths of each graphyne model
a (Å)
RC1–C1 (Å) RC1–C2 (Å) RC2–C2 (Å) Band gap (eV)
a-Graphyne 6.961 1.231 b-Graphyne 9.472 1.232 g-Graphyne 6.881 1.223
1.394 1.387 1.405
1.456 1.423
0 0 0.435
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Fig. 1
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Band structure of a-, b-, and g-graphyne.
adsorbed at the bridge site of graphene to form an epoxide structure is the most favorable configuration, with a binding energy of 2.40 eV.41 The –CRC– insertion enlarges the 6-membered ring in graphene to an 18-membered ring in a-graphyne. Comparably, 12- and 18-membered (6- and 12-membered) rings arise in b-graphyne (g-graphyne). Therefore, many possible binding sites for oxygen adsorption appear in graphyne as a consequence of the –CRC– insertion. We tested those possible binding sites for each type of graphyne, which were demonstrated in the ESI† (Fig. S1) including the corresponding Eb. In the following discussion, only the most stable configuration of each oxygenated graphyne model is involved and demonstrated in Fig. 2. The optimum configuration is when an oxygen atom binds to the carbon lattice of a- and b-graphyne in the epoxide form, while for g-graphyne a carbonyl is formed. We rationalized the unique binding geometries by analyzing the frontier molecular orbitals of three types of graphyne. The corresponding highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of each graphyne model are displayed in Fig. 3. The HOMOs and LUMOs exhibit significant p electron cloud character for all graphynes, however, the shapes of the electron clouds are different for different types of graphyne. For a-graphyne, the electrons in the HOMO are localized at the –CRC– groups, and those in the LUMO are at the top of the –CRC– groups and C2 atoms. For b-graphyne, the electrons in the HOMO are localized at the top of the C1–C2–C1 link, and those in the LUMO lie at the top of the –CRC– and C2–C2 bond. For g-graphyne, the electrons in the HOMO are localized at the top of the –CRC– and benzene ring, and those in the LUMO are at the top of the C1–C2 bond. During the oxidation, the oxygen electrons move to the LUMO of
Fig. 2
Fig. 3 HOMO and LUMO of a-, b- and g-graphyne (different color stands for different spin orientation).
graphyne, then chemical bonds are formed between oxygen and graphyne. For a- and b-graphyne, the LUMO shows that the –CRC– is covered by p electron cloud. The O electrons jumping to the LUMO of graphyne breaks one p bond of –CRC– but C1 keeps its sp-like hybridization. Then, O is bonded with two C1s via p orbitals. The bond length of C1–O (RC1––O) is 1.423 Å/ 1.421 Å for a-/b-graphyne. Compared to pristine graphyne, the oxygen adsorption elongates the bond length of C1–C1 (RC1–C1) in a-/b-graphyne to 1.364 Å/1.368 Å, and decreases the bond length of C1–C2 (RC1–C2) to 1.358 Å/1.366 Å. For g-graphyne, the
The most stable structure of the oxygenated a-, b- and g-graphyne.
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p electron cloud in the LUMO is localized on the C1–C2 bond, not the –CRC– bond. Considering the positive charge, the O electrons prefer to localize at C1 driving C1 to be sp2-like. A double bond is formed between O and C1 with RC1–O of 1.250 Å.
Fig. 4 HOMO and LUMO of oxygenated a-, b- and g-graphyne.
Fig. 5
In the meantime, the RC1–C1 is enlarged from 1.223 Å to 1.417 Å and RC1–C2 is elongated from 1.405 Å to 1.521 Å. The calculated Eb of a-, b- and g-graphyne are 4.154 eV, 4.088 eV and 3.301 eV respectively, indicating the epoxide structure is indeed energetically more favorable. Recently, Galvao and coworkers investigated such oxidation on graphyne systems by molecular dynamics (MD) simulations.42 Their MD results are qualitatively consistent with our ab initio results. That is, the oxidation reactions are strongly site dependent (C1 atoms are preferential sites for chemical attacks) and the effectiveness of the oxidation is in the following order; a-graphyne 4 b-graphyne 4 g-graphyne. Even though the Eb of g-graphyne is smaller than that of the other two, it is still significantly much larger than that of graphene.41 The adsorbed oxygen localizes in the same plane of a- and b-graphyne. Nevertheless, for g-graphyne, oxygen extrudes the underlying C1 out of plane by 0.604 Å (hC1). To explain it, the HOMO and LUMO of oxygenated graphyne are plotted in Fig. 4. For the oxygenated a- and b-graphyne, the HOMO exhibits a diffuse p electron cloud at the adsorption site, which leads to the planar configurations. However, the oxygen adsorption breaks the nearby p bond in g-graphyne, resulting in a nonplanar configuration. The a-graphyne has been studied as a catalyst in the ORR process by a theoretical approach,26 where OOH+ or O2 was initially placed on top of the –CRC– bond while the hollow site was not taken into account. Our result suggests, other than molecular oxygen, atomic oxygen preferentially locates at
Band structures of the oxygenated a-, b- and g-graphyne with different supercell sizes.
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the hollow site of the carbon lattice, which might largely diminish the hollow area that blocks the species penetration through graphyne. The band structure of each oxygenated graphyne model is shown in Fig. 5. For a-graphyne, it is worthy to note that the oxidation leads to an up-shifted conduction band minimum (CBM), and the valence band maximum (VBM) still stays at the Fermi level. Thus, a band gap is opened for the degenerate states around the K point. The direct band gap was estimated to be 1.361 eV, implying a transition from a zero-gap material to a semiconductor. Similarly, a direct band gap of 0.816 eV is induced at the M point in the band structure of the oxygenated b-graphyne. The opened band gap of oxygenated a- and b-graphyne could originate from the charge transfer (CT) from graphyne to O. The O atom is negatively charged by 0.358 and 0.348, respectively. Considering the large binding energy and considerable band gap, oxygenated a- and b-graphyne could be potential candidates for semiconductors. On the contrary, the adsorption of oxygen slightly reduces the band gap of g-graphyne to 0.399 eV. In addition, spin spliting is observed in the oxygenated g-graphyne (in Fig. 5). To further confirm it, the spin density of the oxygenated g-graphyne is shown in Fig. 6. As can be seen, most of the unpaired electrons are localized at the O and the neighboring C1 atom. Spin splitting induces a magnetic moment of 1.850 mB in the oxygenated g-graphyne, which may be used as another class of spintronic material. The above results are based on the unit cell of an individual graphyne model. The O coverage associated with the supercell size would be another influencing factor that would be relevant to the specific electronic properties. The corresponding property changes with the ratio of the numbers of oxygen to carbon atoms (O/C) were analyzed. As shown in Fig. 7(a), the O coverage within the range has little influence on the Eb. The largest energy deviation is within 0.2 eV difference for a-graphyne. The band structures under different O coverage were studied as well (in Fig. 5). For oxygenated a-graphyne, the CBM moves towards the Fermi level while the VBM nearly remains unchanged when O/C decreases. As a result, a diminished band gap would be discerned (0.408 eV at 3.13% and 0.109 eV at 0.78% coverage as indicated in Fig. 7(b)). The decreasing tendency is also discerned for oxygenated b-graphyne. The band gap is almost negligible at 1.39% or 0.35% coverage. The reduced band gap may originate from the enhanced CT as the O/C decreases. The amount of transferred charge increases to 0.368/ 0.349 at 3.13/1.39% of O/C and to 0.369/ 0.352 at 0.78/0.35% of O/C for a-/b-graphyne, respectively. Comparably, the band gap of the oxygenated g-graphyne
Fig. 6
Spin density of the oxygenated g-graphyne.
978 | Phys. Chem. Chem. Phys., 2014, 16, 974--980
Fig. 7 Variation of the binding energy (a) and band gap (b) of the oxygenated graphyne as a function of the supercell size. The values in parenthesis are O/C corresponding to the supercell size.
seems to be insensitive to the O coverage. However, the total magnetic moment of the oxygenated g-graphyne decreases as a function of the supercell size (Fig. 8). The reduced magnetic moment may be induced by the enhanced CT as O/C decreases. The O is negatively charged by 0.301, 0.314 and 0.316 at O/C of 8.33%, 2.08% and 0.52%. It can be concluded that variation of the O coverage would be an efficient strategy to modify the electronic properties of the oxygenated graphyne.
Fig. 8 Variation of the magnetic moment of the oxygenated g-graphyne as a function of the supercell size. The values in parenthesis are O/C corresponding to the supercell size.
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Conclusions In summary, DFT calculations were performed on three types of graphyne model to foresee the electronic property changes caused by oxygenation. Due to the insertion of the –CRC– bond, all C1 atoms are positively charged, which can enhance the oxygen adsorption as confirmed by the large binding energy of 4.154 eV, 4.088 eV and 3.301 eV for a-, b- and g-graphyne. The oxygen binds with two C1 atoms in a- and b-graphyne, while it only binds with one C1 atom in the g-graphyne due to the different characteristics of the frontier molecular orbitals. For the oxygenated g-graphyne, a spin split in the band structure and a magnetic moment of 1.850 mB were induced. The oxygenation changes the a- and b-graphyne from a zero-gap material to a semiconductor with band gap of 1.36 eV and 0.82 eV, respectively, while it slightly decreases the band gap of g-graphyne from 0.435 eV to 0.399 eV. The supercell size effect which is relevant to the O coverage was also taken into account. The O coverage can tune the band gap but has a limited effect on the binding energy of our graphyne models. Moreover, the total magnetic moment of the oxygenated g-graphyne decreases to 1.814 mB and 1.713 mB at the O/C of 2.08% and 0.52%. Our results suggest that oxygen decoration would be a promising method to modify graphyne materials with desirable electronic properties, and the oxygenated graphyne could be a potential candidate for spintronic devices. Indeed, graphyne is still in its infancy and will face many challenges. Our study might serve as a modest spur to induce extensive efforts in this field.
Acknowledgements This work was supported by National Research Foundation (NRF) grants funded by the Korean government (MEST) (20070056343) and (2011-0015767). We acknowledge the financial support from the Ministry of Education, Science and Technology, subjected to the project EDISON (Education-research Integration through Simulation On the Net, Grant No.: 2012M3C1A6035359).
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Oxygen adsorption on single layer graphyne: a DFT study† Baotao Kang, Hongguang Liu and Jin Yong Lee* Graphyne is a rising two-dimensional (2D) carbon allotrope with excellent electronic properties. In this paper, theoretical calculations were performed to study the corresponding electronic properties of the oxygenated graphyne. Atomic oxygen when bound to the carbon atom of graphyne forms a stable oxide, with a much larger binding energy compared to that on graphene. Owing to the oxygen adsorption, the
Received 31st July 2013, Accepted 22nd October 2013
a- and b-graphyne change from a zero-band-gap material to a semiconductor as indicated in the band
DOI: 10.1039/c3cp53237b
g-graphyne. These electronic properties are tunable by altering the oxygen coverage through changing
structure calculations. Moreover, spin splitting was observed from the band structure of the oxygenated the supercell size. Our results based on the first-principles calculations imply that oxygenation is a
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promising method to functionalize graphyne to achieve designated properties.
Introduction Besides the existing carbon allotropes (graphite, diamond and carbon black) in the natural world, other artificial allotropes, especially graphene, have received enormous scientific interest due to their fascinating physical, mechanical and electrical properties.1–6 Recently other carbon allotropes, e.g., graphyne, graphdiyne and derivates7–10 are actively envisioned. Although graphene is the only 2D carbon allotrope that can be synthesized so far,11 the 2D periodic graphyne systems, which share some of remarkable graphene properties such as Dirac cone,12 are receiving growing attention. It is encouraging to see a large-area graphdiyne sheet and corresponding nanotube have recently been experimentally obtained.13,14 Structurally, graphyne systems are built by inserting the acetylenic linkage (–CRC–) between two bonded carbons of graphene but graphdiyne is theoretically estimated to be less stable than graphyne by about 0.18 eV per atom.15 Despite genuine graphyne not yet being available, some finite building blocks or cutouts have already been synthesized.16,17 Continuing efforts are thus devoted to developing more extended structures. Whether attempts are worthwhile to produce graphyne relies on the properties such material should have. Graphyne, which was first suggested by Baughman et al. in 1987,18 was theoretically reported to have outstanding properties, including good chemical stability, large surface area, and excellent electronic conductivity.19–21 The unique electronic properties may allow
Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Korea. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp53237b
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graphyne to have some potential applications, which have been theoretically investigated. For instance, Li mobility in the electrodes is a crucial issue for the Li-ion battery. The insertion of an acetylenic linkage can reduce the Li diffusion barrier and intensify the Li storage capability.22 Another application of graphyne is to serve as the electrode for fuel cells. The oxygen reduction reaction (ORR) is the dominant process which will strongly affect the performance of fuel cells.23–25 Graphyne with many positively charged sites can improve the efficiency of ORR.26 Ca-decorated graphyne was reported to be a promising material for H2 storage with an estimated gravimetric density of 9.6 wt%.27 It is no wonder that graphene is considered, from the application viewpoint, to be even more promising than other nanostructured carbon allotropes. Nevertheless, the zero band gap of graphene restricts its performance; one example is the low on–off ratios of graphene-based FETs.28 Based on different combinations of –CRC– with the graphene network, a-, band g-graphyne (illustrated in Scheme 1) are assembled with different electronic properties.12 The a- and b-graphyne are zero-gap materials, whereas the g-graphyne turns out to be a semiconductor with a band gap of 0.471 eV. The zero (or small) band gap of graphyne is inevitably a major impediment for the potential applications as a semiconductor. Thus one of the thrust areas in the field of graphyne has been on techniques to open or enlarge the band gap without compromising on any of its other properties. Various techniques and methods have come up to functionalize graphene.29–31 Among them, surface decoration is proved to be an efficient approach. It was reported that hydrogen (H) chemisorption on the 5 5 1 supercell of graphene opens a band gap of 1.20 eV which is tunable by changing the supercell size.32 In addition, a magnetic moment
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Scheme 1
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The unit cell for each type of graphyne.
will be induced by hydrogenation. Both theoretical33 and experimental34 studies revealed that O-decorated graphene can open a finite band gap as well. It was theoretically estimated to be 3.39 eV when the O coverage is 50%, and it is much reduced as the coverage decreases. All of these suggest that the H- or O-decorated graphene can be used as semiconductor or spintronics materials. Such an approach would be transplanted to graphyne. Theoretical work has been performed on hydrogenated graphyne.35 It clarified H prefers to be chemically adsorbed on the sp-hybridized carbon. A band gap of 1.01 eV is opened and it is tunable with the hydrogen coverage. However, to our knowledge, the band structure change by oxygen adsorption on single layer graphyne has not yet been reported. Herein, density function theory (DFT) studies were performed on three graphyne models to foresee the electronic property change caused by oxygenation. Oxygen is found tightly adsorbed on graphyne with a much larger binding energy than that on graphene. It is further revealed that oxygen adsorption can open a band gap for a- and b-graphyne, and induce a magnetic moment for g-graphyne. At last, we also explored the O coverage associated with the supercell size which might shed light on the strategies to attain desirable electronic properties.
Computational method In this work, all calculations were performed by the Dmol3 package.36 The generalized gradient approximation (GGA) method proposed by Perdew and Wang (PW91)37 was used to deal with the exchange and correlation functionals. The OBS method was employed to treat the dispersion energy. The lattice constants of each graphyne model were first fully optimized with a 20 Å vacuum space. All-electron treatment was performed and the double numerical plus polarization (DNP) basis set was utilized. The convergence tolerance of energy is taken as 10 5 Hartree, and the maximum allowed force and displacement are 0.002 Hartree per Å and 0.005 Å, respectively. For each type of graphyne, 1 1 1, 2 2 1 and 4 4 1 supercells were investigated with Brillouin zone k-point meshes of 7 7 1, 4 4 1 and 2 2 1, respectively. The binding energy (Eb) was defined as: Eb = Etotal Egraphyne EO, where Etotal stands for the total energy of the oxygen-adsorbed graphyne, Egraphyne is the energy of the isolated graphyne and EO represents the energy of the isolated oxygen.
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Result and discussion To study the presented idea, first of all, the unit cells of three graphyne models were fully optimized with 2D symmetry by both GGA (PW91) and LDA (PWC)38 methods. As seen in Table 1 and the ESI† (Table S1), it is clear that the GGA and LDA functionals do not give a significant difference for the 2D graphyne systems. Hereafter, we will discuss based on the GGA (PW91) results. The lattice constants were calculated to be 6.961 Å, 9.472 Å and 6.881 Å for a-, b- and g-graphyne, respectively. Different lattice constants stem from the unique combination characteristic between the –CRC– and graphene. Our calculated lattice constants are in line with those in the previous studies.12,27,39 For graphene, all the carbon atoms are equivalent with sp2-hybridization. The insertion of –CRC– induces two types of carbons that co-exist in graphyne, C1 (sp-hybridized) and C2 (sp2-hybridized). Two types of carbon– carbon bonds, C1–C1 and C1–C2, appear in a-graphyne as exhibited in Scheme 1. Meanwhile, another type, C2–C2, rises in both b-graphyne and g-graphyne. Because of the insertion of –CRC–, all C1 atoms are positively charged with 0.096e, 0.100e, 0.093e, while the C2 atoms are all negatively charged with 0.287e, 0.199e, 0.093e for a-, b- and g-graphyne, respectively. The charge distribution of a-graphyne is different from the earlier result.26 The mismatch would originate from the periodic consideration here rather than a finite model. It is expected that the positively charged C1 atoms in graphyne can enhance oxygen adsorption. The corresponding band structure of each graphyne model was further calculated. As clearly shown in Fig. 1, similar to the case in graphene, the a-graphyne and b-graphyne have a zero band gap in the band structure. However, a direct band gap emerges at the M point of Brillouin zone in the band structure of g-graphyne. The estimated band gap of g-graphyne is 0.435 eV, which is comparable to the earlier reported value (0.471 eV, ref. 12). For graphene, there are only three possible adsorption sites, i.e., top site, hollow site and bridge site.32,40 The oxygen
Table 1 The optimized lattice constant (a) and carbon–carbon bond lengths of each graphyne model
a (Å)
RC1–C1 (Å) RC1–C2 (Å) RC2–C2 (Å) Band gap (eV)
a-Graphyne 6.961 1.231 b-Graphyne 9.472 1.232 g-Graphyne 6.881 1.223
1.394 1.387 1.405
1.456 1.423
0 0 0.435
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Fig. 1
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Band structure of a-, b-, and g-graphyne.
adsorbed at the bridge site of graphene to form an epoxide structure is the most favorable configuration, with a binding energy of 2.40 eV.41 The –CRC– insertion enlarges the 6-membered ring in graphene to an 18-membered ring in a-graphyne. Comparably, 12- and 18-membered (6- and 12-membered) rings arise in b-graphyne (g-graphyne). Therefore, many possible binding sites for oxygen adsorption appear in graphyne as a consequence of the –CRC– insertion. We tested those possible binding sites for each type of graphyne, which were demonstrated in the ESI† (Fig. S1) including the corresponding Eb. In the following discussion, only the most stable configuration of each oxygenated graphyne model is involved and demonstrated in Fig. 2. The optimum configuration is when an oxygen atom binds to the carbon lattice of a- and b-graphyne in the epoxide form, while for g-graphyne a carbonyl is formed. We rationalized the unique binding geometries by analyzing the frontier molecular orbitals of three types of graphyne. The corresponding highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of each graphyne model are displayed in Fig. 3. The HOMOs and LUMOs exhibit significant p electron cloud character for all graphynes, however, the shapes of the electron clouds are different for different types of graphyne. For a-graphyne, the electrons in the HOMO are localized at the –CRC– groups, and those in the LUMO are at the top of the –CRC– groups and C2 atoms. For b-graphyne, the electrons in the HOMO are localized at the top of the C1–C2–C1 link, and those in the LUMO lie at the top of the –CRC– and C2–C2 bond. For g-graphyne, the electrons in the HOMO are localized at the top of the –CRC– and benzene ring, and those in the LUMO are at the top of the C1–C2 bond. During the oxidation, the oxygen electrons move to the LUMO of
Fig. 2
Fig. 3 HOMO and LUMO of a-, b- and g-graphyne (different color stands for different spin orientation).
graphyne, then chemical bonds are formed between oxygen and graphyne. For a- and b-graphyne, the LUMO shows that the –CRC– is covered by p electron cloud. The O electrons jumping to the LUMO of graphyne breaks one p bond of –CRC– but C1 keeps its sp-like hybridization. Then, O is bonded with two C1s via p orbitals. The bond length of C1–O (RC1––O) is 1.423 Å/ 1.421 Å for a-/b-graphyne. Compared to pristine graphyne, the oxygen adsorption elongates the bond length of C1–C1 (RC1–C1) in a-/b-graphyne to 1.364 Å/1.368 Å, and decreases the bond length of C1–C2 (RC1–C2) to 1.358 Å/1.366 Å. For g-graphyne, the
The most stable structure of the oxygenated a-, b- and g-graphyne.
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p electron cloud in the LUMO is localized on the C1–C2 bond, not the –CRC– bond. Considering the positive charge, the O electrons prefer to localize at C1 driving C1 to be sp2-like. A double bond is formed between O and C1 with RC1–O of 1.250 Å.
Fig. 4 HOMO and LUMO of oxygenated a-, b- and g-graphyne.
Fig. 5
In the meantime, the RC1–C1 is enlarged from 1.223 Å to 1.417 Å and RC1–C2 is elongated from 1.405 Å to 1.521 Å. The calculated Eb of a-, b- and g-graphyne are 4.154 eV, 4.088 eV and 3.301 eV respectively, indicating the epoxide structure is indeed energetically more favorable. Recently, Galvao and coworkers investigated such oxidation on graphyne systems by molecular dynamics (MD) simulations.42 Their MD results are qualitatively consistent with our ab initio results. That is, the oxidation reactions are strongly site dependent (C1 atoms are preferential sites for chemical attacks) and the effectiveness of the oxidation is in the following order; a-graphyne 4 b-graphyne 4 g-graphyne. Even though the Eb of g-graphyne is smaller than that of the other two, it is still significantly much larger than that of graphene.41 The adsorbed oxygen localizes in the same plane of a- and b-graphyne. Nevertheless, for g-graphyne, oxygen extrudes the underlying C1 out of plane by 0.604 Å (hC1). To explain it, the HOMO and LUMO of oxygenated graphyne are plotted in Fig. 4. For the oxygenated a- and b-graphyne, the HOMO exhibits a diffuse p electron cloud at the adsorption site, which leads to the planar configurations. However, the oxygen adsorption breaks the nearby p bond in g-graphyne, resulting in a nonplanar configuration. The a-graphyne has been studied as a catalyst in the ORR process by a theoretical approach,26 where OOH+ or O2 was initially placed on top of the –CRC– bond while the hollow site was not taken into account. Our result suggests, other than molecular oxygen, atomic oxygen preferentially locates at
Band structures of the oxygenated a-, b- and g-graphyne with different supercell sizes.
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the hollow site of the carbon lattice, which might largely diminish the hollow area that blocks the species penetration through graphyne. The band structure of each oxygenated graphyne model is shown in Fig. 5. For a-graphyne, it is worthy to note that the oxidation leads to an up-shifted conduction band minimum (CBM), and the valence band maximum (VBM) still stays at the Fermi level. Thus, a band gap is opened for the degenerate states around the K point. The direct band gap was estimated to be 1.361 eV, implying a transition from a zero-gap material to a semiconductor. Similarly, a direct band gap of 0.816 eV is induced at the M point in the band structure of the oxygenated b-graphyne. The opened band gap of oxygenated a- and b-graphyne could originate from the charge transfer (CT) from graphyne to O. The O atom is negatively charged by 0.358 and 0.348, respectively. Considering the large binding energy and considerable band gap, oxygenated a- and b-graphyne could be potential candidates for semiconductors. On the contrary, the adsorption of oxygen slightly reduces the band gap of g-graphyne to 0.399 eV. In addition, spin spliting is observed in the oxygenated g-graphyne (in Fig. 5). To further confirm it, the spin density of the oxygenated g-graphyne is shown in Fig. 6. As can be seen, most of the unpaired electrons are localized at the O and the neighboring C1 atom. Spin splitting induces a magnetic moment of 1.850 mB in the oxygenated g-graphyne, which may be used as another class of spintronic material. The above results are based on the unit cell of an individual graphyne model. The O coverage associated with the supercell size would be another influencing factor that would be relevant to the specific electronic properties. The corresponding property changes with the ratio of the numbers of oxygen to carbon atoms (O/C) were analyzed. As shown in Fig. 7(a), the O coverage within the range has little influence on the Eb. The largest energy deviation is within 0.2 eV difference for a-graphyne. The band structures under different O coverage were studied as well (in Fig. 5). For oxygenated a-graphyne, the CBM moves towards the Fermi level while the VBM nearly remains unchanged when O/C decreases. As a result, a diminished band gap would be discerned (0.408 eV at 3.13% and 0.109 eV at 0.78% coverage as indicated in Fig. 7(b)). The decreasing tendency is also discerned for oxygenated b-graphyne. The band gap is almost negligible at 1.39% or 0.35% coverage. The reduced band gap may originate from the enhanced CT as the O/C decreases. The amount of transferred charge increases to 0.368/ 0.349 at 3.13/1.39% of O/C and to 0.369/ 0.352 at 0.78/0.35% of O/C for a-/b-graphyne, respectively. Comparably, the band gap of the oxygenated g-graphyne
Fig. 6
Spin density of the oxygenated g-graphyne.
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Fig. 7 Variation of the binding energy (a) and band gap (b) of the oxygenated graphyne as a function of the supercell size. The values in parenthesis are O/C corresponding to the supercell size.
seems to be insensitive to the O coverage. However, the total magnetic moment of the oxygenated g-graphyne decreases as a function of the supercell size (Fig. 8). The reduced magnetic moment may be induced by the enhanced CT as O/C decreases. The O is negatively charged by 0.301, 0.314 and 0.316 at O/C of 8.33%, 2.08% and 0.52%. It can be concluded that variation of the O coverage would be an efficient strategy to modify the electronic properties of the oxygenated graphyne.
Fig. 8 Variation of the magnetic moment of the oxygenated g-graphyne as a function of the supercell size. The values in parenthesis are O/C corresponding to the supercell size.
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Conclusions In summary, DFT calculations were performed on three types of graphyne model to foresee the electronic property changes caused by oxygenation. Due to the insertion of the –CRC– bond, all C1 atoms are positively charged, which can enhance the oxygen adsorption as confirmed by the large binding energy of 4.154 eV, 4.088 eV and 3.301 eV for a-, b- and g-graphyne. The oxygen binds with two C1 atoms in a- and b-graphyne, while it only binds with one C1 atom in the g-graphyne due to the different characteristics of the frontier molecular orbitals. For the oxygenated g-graphyne, a spin split in the band structure and a magnetic moment of 1.850 mB were induced. The oxygenation changes the a- and b-graphyne from a zero-gap material to a semiconductor with band gap of 1.36 eV and 0.82 eV, respectively, while it slightly decreases the band gap of g-graphyne from 0.435 eV to 0.399 eV. The supercell size effect which is relevant to the O coverage was also taken into account. The O coverage can tune the band gap but has a limited effect on the binding energy of our graphyne models. Moreover, the total magnetic moment of the oxygenated g-graphyne decreases to 1.814 mB and 1.713 mB at the O/C of 2.08% and 0.52%. Our results suggest that oxygen decoration would be a promising method to modify graphyne materials with desirable electronic properties, and the oxygenated graphyne could be a potential candidate for spintronic devices. Indeed, graphyne is still in its infancy and will face many challenges. Our study might serve as a modest spur to induce extensive efforts in this field.
Acknowledgements This work was supported by National Research Foundation (NRF) grants funded by the Korean government (MEST) (20070056343) and (2011-0015767). We acknowledge the financial support from the Ministry of Education, Science and Technology, subjected to the project EDISON (Education-research Integration through Simulation On the Net, Grant No.: 2012M3C1A6035359).
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