Rules of boron-nitrogen doping in defect graphene sheets: a first-principles investigation of band-gap tuning and oxygen reduction reaction catalysis capabilities.

CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201402147

Rules of Boron–Nitrogen Doping in Defect Graphene Sheets: A First-Principles Investigation of Band-Gap Tuning and Oxygen Reduction Reaction Catalysis Capabilities Dipayan Sen,[a] Ranjit Thapa,[b] and Kalyan Kumar Chattopadhyay*[a] Introduction of defects and nitrogen doping are two of the most pursued methods to tailor the properties of graphene for better suitability to applications such as catalysis and energy conversion. Doping nitrogen atoms at defect sites of graphene and codoping them along with boron atoms can further increase the efficiency of such systems due to better stability of nitrogen at defect sites and stabilization provided by BN bonding. Systematic exploration of the possible doping/codoping configurations reflecting defect regions of graphene pres-

ents a prevalent doping site for nitrogen-rich BN clusters and they are also highly suitable for modulating (0.2–0.9 eV) the band gap of defect graphene. Such codoped systems perform significantly better than the platinum surface, undoped defect graphene, and the single nitrogen or boron atom doped defect graphene system for dioxygen adsorption. Significant stretching of the OO bond indicates a lowering of the bond breakage barrier, which is advantageous for applications in the oxygen reduction reaction.

1. Introduction As per the current notion, platinum-based catalyst materials fail to provide a promising outlook in the full commercialization of fuel cells[1] and dye-sensitized solar cells.[2] During the energy generation process in hydrocarbon-based fuel cells, adsorption of carbon monoxide (CO) on the active catalytic sites can lead to the inactivation of the platinum surfaces[3] in a composite electrode. Additionally, the inadequacy of the platinum reservoir on earth coupled with the high cost of such catalysts has initiated the hunt for alternative carbon-based, platinumfree electrocatalysts due to their cost efficiency, higher durability, improved electrocatalytic activity,[4] and their resistance to cross-over related issues and CO poisoning.[5] Among different carbon nanostructures, graphene is particularly suitable for designing a metal-free oxygen reduction reaction (ORR) electrocatalyst because its 2D nature yields a very high surface to volume ratio, which can vastly improve the catalytic performance parameters. Previous reports suggested for the ORR at the cathode of a low-temperature fuel cell that substitutional nitrogen atoms on carbon materials exhibited excellent catalytic performances.[6] Because nitrogen is more electrophilic than carbon, substitutional nitrogen atoms in carbon structures can accumulate higher amount of electrons, which are readily grabbed by dioxygen because it is even more elec[a] D. Sen, Prof. K. K. Chattopadhyay Thin Film and NanoScience Laboratory Department of Physics, Jadavpur University Kolkata 700032 (India) E-mail: [email protected] [b] R. Thapa SRM Research Institute, SRM University Kattankulathur, Chennai 603 203, Tamil Nadu (India)

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trophilic than nitrogen atoms. The net result manifests as much better binding of dioxygen around substitutional nitrogen sites.[6a] On the other hand, another approach to tailor the properties of graphene lies in the introduction of defects,[7] as in the case of low-dimensional systems, structural imperfections can severely distort electronic and other properties.[7] To date, the formation of several types of defects on graphene sheets have been theoretically studied and experimentally observed; this includes defects arising due to pure rearrangement [Stone–Wales (SW) defect][8] or reconstruction due to the removal of one,[9] two,[10] or multiple carbon atoms[11] from the graphenic lattice. Among them, a double vacancy (DV), produced by the coalescence of two single vacancies (SVs) or by removing two adjacent atoms, seems to be the most suitable for doping or adsorption because it is thermodynamically more favorable and practically immobile up to very high temperatures.[7] Recent studies regarding the formation of DVs in graphene have shown the possibility of different configurations, among which the DV(555-777) defect with three pentagons and three heptagons is energetically most stable.[7] Doping of heteroatoms in defect sites of graphene might open up further possible avenues for designing metal-free ORR electrocatalysts; however, very few works are reported in this regard.[12] Hou et al. presented a report on the stability of nitrogen doping on graphene with vacancies and SW defects by the means of first-principles calculations.[12b] They showed that, for nitrogen doping, in the case of a monovacancy, the most stable position is the pyridine-like configuration, whereas for SW defects and divacancies nitrogen prefers a site in the pentagonal ring. In another report,[6a] Feng et al. showed that subChemPhysChem 0000, 00, 1 – 9

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CHEMPHYSCHEM ARTICLES stitutional nitrogen clusters on pristine graphene were much better than single nitrogen-doped pristine graphene for adsorbing O2. They also noted that high formation energies of such clusters were a critical problem and showed that codoping with other atoms, such as boron, iron, or cobalt could stabilize substitutional nitrogen clusters on graphene efficiently without affecting the catalytic performance. However, to the best of the our knowledge, no prior investigations into the effects of BN codoping on defect sites of graphene, especially detailing the gradual formation of BN clusters at the DV (555777) defect sites of graphene, and its effect as a catalyst in the ORR have been reported. This could provide vital information on improving the heteroatomic doping capacity and catalytic activity of a defect graphene system. Herein, we present an exhaustive study on B, N, and BN codoping on the DV(555-777) defect graphene surface by using density functional theory (DFT). The stabilities of different doping configurations are tested against each other to reach the most probable doping configuration. The viability of controlled boron and nitrogen doping as a tool to manipulate the band gap of the DV(555-777) defect graphene system is also investigated by using electronic structure calculations. The catalytic properties of the most stable BN codoping configuration on DV(555-777) defect graphene for the cathode of the ORR are also explored and compared to that of the undoped DV(555-777) defect graphene system and the single boronand nitrogen-doped DV(555-777) defect graphene system.

Computational Methods Our first-principles calculations were performed by CASTEP code,[13] which implemented a supercell approach to DFT. The Perdew– Burke–Ernzerhof (PBE) functional[14] within the generalized gradient approximation (GGA) was used to deal with the exchange and correlation term. The Vanderbilt ultrasoft pseudopotential[15] was used to represent the carbon, boron, nitrogen, and oxygen atoms, and plane waves up to an energy cutoff of 500 eV was used in the calculation. Brillouin zone integrations were performed within the Monkhorst Pack scheme[16] with an approximate k point separation of 0.04 1. For geometrical optimization, the system was allowed to fully relax by using the BFGS (Broyden–Fletcher–Goldfarb– Shanno) scheme[17] until the total energy converged to less than 2 105 eVatom1, the maximum force converged to less than 0.05 eV 1 and the maximum displacement was 0.002 . All calculations were performed in a spin-unrestricted manner. The graphene surface was built by cleaving the geometrically optimized graphite (space group P63/mmc) structure. Thus, the obtained graphene unit cell consisted of two carbon atoms and had a lattice parameter of a = b = 2.439 . A vacuum slab of length 15 was used along the c axis to ward off spurious interactions with its own periodic image. The DV(555-777) defect graphene model was built by removing 2 carbon atoms from a 6 6 1 supercell of graphene containing 72 carbon atoms (so that the defect concentration was 2.777 %), rotating the bonds as required and then geometrically optimizing the resulting structure. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org 2. Results and Discussion 2.1. Stability To study the relative stability of the different doping configurations, their formation energies were computed by using Equation (1): E for ¼ E ddg E dg þ nv mC nB mB nN mN

ð1Þ

in which Efor is the formation energy of the doped structure; Eddg and Edg are the total ground state energies of the doped defect graphene and defect graphene, respectively; nv is the number of carbon atoms replaced due to the course of doping; mC is the chemical potential of the carbon atom (reference state: graphene); nB is the number of doped boron atoms; mB is the chemical potential of the boron atom (reference state: a-rhombohedral phase of boron); nN is the number of doped nitrogen atoms; and mN is the chemical potential of the nitrogen atom (reference state: N2 molecule). Low values of formation energy, as obtained from Equation (1), indicate better stability of the system under consideration.

2.1.1. Single Boron or Nitrogen Atom Doping To investigate the effect of BN codoping, as the first step, single boron and single nitrogen atom substitution were considered at mainly four types of symmetric sites of the DV(555777) defect graphene sheet, namely, at the defect center (C0); a common vertex of one pentagon and two heptagons (Cin); a common vertex of a pentagon, a hexagon, and a heptagon (Cmid); and a common vertex of one pentagon and two hexagons (Cout) (schematically shown in Figure 1). Our obtained results indicated, for single nitrogen atom doping, Cout sites (NCout) in a pyridine-like fashion with a dangling s state[12b] were most favorable and we calculated the formation energy for this configuration to be 0.480 eV. The negative value of the computed formation energy implies that the nitrogen dopants are attracted by the defect locations and the nitrogen dopant–defect interaction is exothermic in nature. These inferences are in full agreement with previous work.[12b] In the case

Figure 1. Schematic representation of the most symmetric doping sites on a the DV(555-777) graphene sheet. The graph shows the formation energies of single B atom and single N atom doping at different highly symmetric sites. Minima in the graphs represent the most favorable sites for doping.

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CHEMPHYSCHEM ARTICLES of nitrogen doping at Cmid, Cin, and C0 sites of the DV(555-777) defect graphene sheet, the formation energies were much higher: 0.071, 0.331, and 1.332 eV, respectively. Better stability of nitrogen doping at Cout sites can be attributed to higher aromaticity, according to Hckel’s rule.[18] For single boron atom doping on the DV(555-777) defect graphene sheet, the Cin site (BCin) is most favorable with a formation energy of 2.095 eV, whereas, for Cout, Cmid, and C0 sites, we found formation energies of 4.013, 2.913, and 2.232 eV, respectively. The site preference of boron dopants can be explained by the preference of hole doping (i.e. B doping) for lower aromaticity sites (C0 and Cin), in contrast to the case of electron doping (N doping). Other than the four types of symmetry sites mentioned above, single boron an nitrogen doping at common vertices of two hexagons and a heptagon (C2out site) were also considered, for which we obtained high formation energies of 2.843 and 0.518 eV for boron and nitrogen doping, respectively. The formation energies of single nitrogen and boron doping, with respect to different doping sites, are graphically shown in Figure 1 and optimized structures of NCout and BCin are shown in Figure 2 a and b, respectively. The formation energy (Efor)

Figure 2. Optimized structures of B, N, and BN codoped DV(555-777) defect graphene: a) most favorable configuration of single N doping: NCout, b) most favorable configuration of single B doping: BCin, c) 3NCoutBCin, d) 3NCinBC0, e) 3BCinNC0, and f) 3NCoutBC0. Black and white dot overlays on B and N atoms, respectively, are used for convenience of representation.

and computed Mulliken charge of dopants (QB, QN) for all doping configurations under consideration are listed in Table 1. Additionally, for single nitrogen and boron doping in normal hexagonal rings of DV(555-777) defect graphene, the formation energies were computed to be 0.205 and 3.405 eV, respectively; this indicates that defect sites on graphene are more suitable for doping than normal hexagonal rings.


www.chemphyschem.org 2.1.2. Boron and Nitrogen Codoping The interplay of several different factors, such as 1) site preference of nitrogen dopant (Cout site), 2) site preference of boron dopant (Cin site), 3) interaction between boron and nitrogen in the resultant codoped configuration (formation of the BN bond reduces the net formation energy, and thus, is more preferable from the stability point of view),[6a] and 4) the ratio of boron to nitrogen in the resultant codoped structure (lower is better due to the relatively higher formation energy of B doping in comparison to that of N doping), and most importantly the tradeoff between them, plays a pivotal role in determining the most stable configuration when boron and nitrogen atoms are codoped in a DV(555-777) defect graphene sheet. Depending on the abovementioned factors, to investigate the smallest case of BN codoping, 3N1B doping seems reasonable owing to a low ratio of boron to nitrogen. Following the site preference of the respective dopants rigidly, without giving BN bonding any priority, a doping configuration in which three nitrogen dopants are placed in Cout sites and one boron dopant in Cin is the most logical choice. As per our calculations, the formation energy of this configuration (3NCoutBCin ; shown in Figure 2 c) is 0.485 eV. However, if both the site preference of the dopants and their interactions are taken into account, another logical 3N1B codoping configuration, namely, 3NCinBC0, becomes relevant, in which both dopants are moved into a less preferred site (for B, Cin !C0 and for N Cout !Cin) in favor of interdopant interactions. This structure is shown in Figure 2 d and we calculated a formation energy of 0.112 eV for this configuration. The above data illustrates how the tradeoff between different factors plays a key role in determining the most favorable codoping configuration. For the 3NCinBC0 configuration, the interaction between dopants helps to further lower the formation energy, so that the stability of it supersedes that of the 3NCoutBCin configuration, which, from the point of view of site preference of individual dopants, is supposed to be the most stable structure. Moreover, if the position of the dopants of 3NCinBC0 is reversed (i.e. 3BCinNC0, as shown in Figure 2 e), we obtain a configuration in which boron dopants are doped into the most favorable locations site-preference wise, along with favorable interdopant bonding. However, we obtained a high formation energy of 4.167 eV for this configuration owing to a high boron to nitrogen ratio and the nitrogen dopant being doped to the least favorable C0 site. This configuration is of importance because the 3BCinNC0 core can further be utilized to design codoping configurations with a higher number of dopants and low boron to nitrogen ratio. Interestingly, among the different cases of 3N1B doping on the DV(555-777) defect graphene sheet, neither 3NCinBC0 nor 3NCoutBCin represent the most stable configuration. A thorough investigation of different possible 3NB doping configurations reveals that a hybrid of these two, a 3NCoutBC0 structure (as shown in Figure 2 f), is the most stable with a formation energy of 0.024 eV. The higher stability of 3NCoutBC0 than that of 3NCinBC0 can be explained by the site preference of nitrogen dopants superseding the effect of BN cohesion due to ChemPhysChem 0000, 00, 1 – 9

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ergetically feasible. Previous reports suggest, for more than Configuration Efor [eV] QB [e] QN [e] three nitrogen atoms doping DV(555-777) defect graphene, NCout 0.480 – 0.240 the C0 site becomes most pref0.071 – 0.260 NCmid 0.331 – 0.280 NCin erable after three nitrogen NC0 1.332 – 0.250 atoms are placed in three Cout 2 NC out 0.518 – 0.260 sites in a pyridine-like manBCout 4.013 0.650 – ner.[12b] Coupled with this site 2.913 0.540 – BCmid BCin 2.095 0.510 – preference of nitrogen dopants 2.232 0.490 – BC0 and the already demonstrated 2 2.843 0.550 – BC out preference for the formation of 3NCoutBCin 0.485 0.480 0.230 interdopant bonds, several con0.112 0.910 0.470 3NCinBC0 4.167 0.620 0.790 3BCinNC0 figurations of BN codoping on 0.024 0.510 0.240 3NCoutBC0 the DV(555-777) defect graph3B7N1 0.838 0.860 0.470 ene sheet for cases larger than 0.933 0.700 0.860 (C0) 3B7N2 3N1B were considered. The for0.450 0.230 (Cout) mation energy (Efor) and comput5.221 0.900 0.320 (C0) 3B7N3 ed Mulliken charge of dopants 0.490 (QB, QN) for all such doping con0.460 (Cout) figurations are presented in 3B10N 2.730 1.030 0.310 0.460 (outer of Cout) Table 1. Among different 3B7N 0.490 (Cmid) codoping configurations on the 0.470 (Cout) DV(555-777) defect graphene 0.820 (C0) 9B7N 13.975 0.830 (Cin) sheet, the energetically most fa0.680 (Cout) 0.810 (Cmid, bonded with 3B) 0.610 (C2out) 0.380 (Cmid, bonded with 1B) vorable one (3B7N1) is obtained 0.810 (Cin) 7B9N 5.857 0.840 (C0) by extending the previously 0.900 (Cmid, bonded with 3N) 0.420 (Cout) mentioned 3BCinNC0 configura0.540 (Cmid, bonded with 1N) 0.430 (C2out) tion by doping six more nitro[a] Efor is the formation energy, QB is the computed Mulliken charge of B atoms, and QN is the computed Mullikgen atoms, as shown in Figen charge of N atoms. ure 3 a. We obtained a formation energy of 0.838 eV in this case. In addition, Mulliken charge analysis shows that the 3B7N1 configuration has a high a small net number of BN bonds (only three). However, the relatively small difference in formation energy between these amount of charge (0.860 e B1 and 0.470 e N1) distributed two structures (0.024 eV for 3NCoutBC0 compared with 0.112 eV uniformly among the dopants, which indicates that this configfor 3NCinBC0) imply, for configurations with a higher number of uration holds great potential as a possible dioxygen adsorption BN bonds, that the situation is more than likely to be remedium. Also, the stability of this configuration is much better versed. On the other hand, a comparatively higher difference than the site-preference-wise more preferable 3B7N2 configuration (shown in Figure 3 b) which is constructed from the in formation energy between 3NCoutBC0 and 3NCoutBCin config3BCinNC0 structure with the outer three nitrogen dopants on urations (0.024 eV for 3NCoutBC0 compared with 0.485 eV for 3NCoutBCin) indicate, in the case of codoping, a preference of the Cout sites, so that four nitrogen atoms are doped in the sites for individual dopants (as shown in Figure 1) might devimost stable configuration (3NCout, 1NC0)[12b] and the other ate slightly because, in this case, for boron dopants, the C0 site three nitrogen atoms are bonded to boron dopants, which are also doped in their most preferred sites. We obtained a formahas clearly become more preferable than that of Cin sites. The tion energy of 0.933 eV for the 3B7N2 configuration and the formation energy (Efor) and computed Mulliken charges of dopants (QB, QN) for all codoping configurations mentioned above higher stability of 3B7N1 could be attributed to the formation are listed in Table 1. of a higher number of BN bonds (seven for 3B7N1 and six for 3B7N2). Also, for the 3B7N3 configuration (shown in Figure 3 c), even though the number of BN bonds remains the same as 2.1.3. Formation of Larger BN Clusters that of 3B7N2, as a result of the 3BCinNC0 core being broken A higher number of bonded boron and nitrogen atoms doped and boron dopants being moved to higher formation energy into adjacent sites can give rise to the formation of large BN Cout sites, a much higher formation energy of 5.221 eV was obclusters in the host material. In the following section, we extained. Interestingly, for the 3B10N configuration (as shown in plore various possible codoping configurations of ten or more Figure 3 d), which is constructed from the 3B7N3 configuration, dopants and whether the formation of such clusters of BN the formation energy decreased to 2.730 eV owing to the fordopants in defect area of DV(555-777) defect graphene are enmation of a higher number of BN bonds (9 for 3B10N and 6 Table 1. Parameters for B, N, and BN codoping on the DV(555-777) defect graphene system.[a]



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the electronic properties of graphene. Previous works reported a 0.3 eV calculated band gap for DV(5555-6-7777) defect graphene and no band gap for DV(555-777) defect graphene upon using a system with a defect concentration of 1 %.[21] Our obtained results indicate that the undoped DV(555-777) defect graphene sheet used in our calculation has a small band gap of 0.366 eV. Because the defect concentration is much higher for the current case (2.777 %), it can be inferred that, for a high DV(555-777) defect concentration, a small band gap Figure 3. Optimized structures of large BN clusters doped on DV(555-777) defect graphene: a) most favorable structure: 3B7N1, b) 3B7N2, c) 3B7N3, d) 3B10N, e) 9B7N, and f) 7B9N. Black and white dot overlays on B and N opening occurs, the value of atoms, respectively, are used for convenience of representation. which decreases with decreasing doping concentration and the corresponding band structure tends to that of pristine graphfor 3B7N3), even though the total number of dopants is higher ene for the limiting case. Doping this system with single boron in the 3B10N configuration (13 for 3B10N and 10 for 3B7N3). or nitrogen atoms in their respective most stable configuraThe above results conclusively indicate, for BN codoping on tions, that is, at Cin and Cout sites, induced a small modulation the DV(555-777) defect graphene sheet, that the formation of of the band gap and incorporated spin polarizations at the the BN cluster at the center of the defect site is the most faFermi level in the ground states. Higher BN codoped configuvorable configuration for doping. Keeping the BN bonding rations demonstrated much higher band gap modulations and, sequence coherent, the 3BCinNC0 core can even be further exalthough a few less stable codoped structures showed minor panded into the maximum of 16 dopants, 9B7N, structure (as spin polarizations at the Fermi level, the stable and intermedishown in Figure 3 e). However, the introduction of six more ately stable structures of the various codoping configurations, boron dopants into the system increases the formation energy in general, favored non-spin polarized ground states. The band of such a structure greatly to 13.975 eV, so the formation of gap of the DV(555-777) defect graphene system after doping this kind of structure is predicted to be highly improbable. Alwith a single boron atom, a single nitrogen atom, three nitroternately, by using the 3NCinBC0 core, another possible 16gen and one boron atoms, and three boron and seven nitrodopant configuration, namely, 7B9N, as shown in Figure 3 f, gen atoms at their most stable configurations, that is, at the can be designed, and owing to lower the boron to nitrogen Cin site, the Cout site, 3NCoutBC0, and 3B7N1, respectively, along ratio, the formation energy for this configuration is 5.857 eV. with some other relevant doping configurations, such as However, because the formation energies of both 9B7N and 3NCinBC0, 9B7N, and 7B9N, are shown graphically in Figure 4. 7B9N were much higher than that of the 3B7N1 configuration, based on the above analysis it can safely be inferred that, for We obtained a minimum band gap of about 0.252 eV (averall mentioned cases of BN codoping on the DV(555-777) defect graphene sheet, the formation of the BN cluster in the form of a 3B7N1 structure at defect sites is energetically most favorable. The high stability of BN structures on defect regions of graphene, as shown above, is analogous to that for grain boundary defects in graphene.[12a] Experimental measures for introducing DV defects in graphene have already been summarized in the literature.[7] These, coupled with various chemical vapor deposition techniques that are usually deployed to design BN-doped graphene systems,[19] might play a crucial role in practically realizing similar BN cluster substituted defect graphene systems. 2.2. Electronic Properties The introduction of boron or nitrogen dopants on pure[20] or defect[12b] graphene is a well-established method to modulate 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 4. Graphical representation of the band-gap tuning of the DV(555777) defect graphene sheet by varying the doping configuration. Double bars denote up and down spin channels, respectively.


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aged) for the BCin configuration and a maximum band gap of 0.912 eV for the 3B7N1 configuration; the band gaps of the NCout, 3NCinBC0, 3NCoutBC0, 9B7N, and 7B9N configurations were 0.400 (averaged), 0.559, 0.710, 0.544, and 0.478 eV, respectively. Our results demonstrate that B, N, and BN codoping can be utilized as an effective measure for band-gap tuning of the DV(555-777) defect graphene system in the range 0.2–0.9 eV. The calculated band structures (0 is set at the top of the valence band and the Fermi level is set at the middle of the band gap for ease of comparison) of the 6 6 1 supercell of the DV(555-777) defect graphene sheet and the 3B7N1 doping configuration are shown in Figure 5 a and b, respectively. It is

Figure 6. TDOS comparison of the 3B7N1 configuration, the DV(555-777) defect graphene sheet, and a pristine h-BN sheet.

phene can boost the stability of the system even further and may pave the way for designing better carbon-based, platinum-free electrocatalysts. To study the dioxygen adsorption capacity of BN codoped DV(555-777) defect graphene, the adsorption energy was computed from Equation (2): E a ¼ E total E host E dioxygen

Figure 5. Calculated band structures of a) the 6 6 1 supercell of the DV(555-777) defect graphene sheet and b) the 3B7N1 configuration. (0 is set at the top of the valence band and the Fermi level is set at the middle of the band gap for ease of comparison. The Fermi levels are shown by solid lines.)

well known that, in graphene, boron doping is analogous to hole doping and nitrogen doping is analogous to electron doping. In this system, the nitrogen dopant is higher than the boron dopants, so the net effect of doping manifests as a net upward shift of the Fermi level of the doped system by 0.283 eV; the band-gap opening in the doped system also increases by 0.546 eV in comparison with the undoped DV(555777) defect graphene system.[12b] This implies that greater control of modulation of the transport properties of the DV(555777) defect graphene system can be achieved by controlled B, N, or BN codoping. Calculated total density of states (TDOS) of the 3B7N1 configuration along with the same for DV(555777) defect graphene and a pristine h-BN (h = hexagonal) sheet are plotted in Figure 6 and, as shown, incorporation of dopants drastically affects the TDOS of the DV(555-777) defect graphene sheet.

ð2Þ

in which Ea is the adsorption energy of dioxygen, Etotal is the total energy of dioxygen adsorbed on the host system, Ehost is the total energy of the host system (i.e. the doped defect or defect graphene system), and Edioxygen is the total energy of an isolated dioxygen molecule. For dioxygen adsorption at the defect center of undoped DV(555-777) defect graphene [O2– DV(555-777)], as shown in Figure 7 a, we obtained an adsorption energy of 1.001 eV, which was not only much better than that for the pristine graphene system,[6a] but was also comparable to that of the Pt(1 1 1) surface;[6a] however, it was not better than the previously reported adsorption energy of 1.26 eV on a three substitutional nitrogen-atom-doped pris-

2.3. Dioxygen Adsorption and Its Role on the ORR Our results indicate that not only doping of individual boron and nitrogen atoms on DV(555-777) defect graphene provides improved stability relative to the same on pristine graphene, but also the codoping of BN clusters on defect sites of gra 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 7. Optimized structures of dioxygen adsorption: a) O2–DV(555-777), b) O2–NCout, c) O2–BCin, and d) O2–3B7N1


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tine graphene system.[6a] For isolated O2, we calculated the O O bond length to be 1.238 , which is in good agreement with previous studies.[22] However, after adsorption on the undoped DV(555-777) defect graphene, the OO bond length of dioxygen is found to stretch to 1.245 . For the most stable case of single nitrogen-atom-doped DV(555-777) defect graphene, that is, single nitrogen atom doping at the Cout site, we obtained an O2 adsorption energy of 1.312 eV, which was much better than the previously reported adsorption energy of 0.180 eV O2 on a single substitutional nitrogen-doped pristine graphene system,[6a] and even better than the same for a three substitutional nitrogen-atom-doped pristine graphene system.[6a] For the most stable configuration of a single boron-atom-doped DV(555-777) defect graphene system, we also obtained a comparable O2 adsorption energy of 1.358 eV. We obtained OO bond lengths of 1.250 and 1.330 at O2–NCout and O2–BCin, respectively; optimized structures of both are shown in Figure 7 b and c, respectively. For dioxygen adsorption on the 3B7N1 codoped configuration, as shown in Figure 7 d, we obtained an adsorption energy of 1.720 eV, which was much better than those for all systems under consideration, that is, the Pt (1 1 1) surface, the pristine graphene system, the nitrogen-doped pristine graphene system, and the NCout and BCin system. Accordingly, in this case, we found that the OO bond length stretched to a maximum value of 1.346 . The O2 adsorption energy (Ea); OO bond length (dOO); OO stretching after adsorption (DdOO); the average height of the adsorbed O2 molecule from the host (hO2); and the computed Mulliken charges of B (QB), N (QN), and dioxygen (QO) for dioxygen adsorption on 3B7N1; and all other cases under discussion are listed in Table 2. Mulliken charge analysis reveals, in the case of the 3B7N1 codoping configuration, that the highest amount of charge (0.280, 0.260 e) is transferred to the dioxygen adsorbate. This can be attributed to the behavior of the nitrogen atom as an electron dopant, due to which the surroundings become electron rich. As a consequence, more charge is readily grabbed by dioxygen through the least electronegative element in the system, that is, the boron dopant, because oxygen atoms are the most electrophilic in this system. A higher charge accumulation in the dioxygen molecule for 3B7N1 translates as even better binding of dioxygen with the adjacent boron atoms through the formation of stronger ionic bonds; hence the stability of the system increases. This is also supported by the atomic electron density difference diagram for O2 adsorption on the 3B7N1 configuration, as shown in Figure 8.

Figure 8. Atomic electron density difference diagram for O2 adsorption on the 3B7N1 configuration.

Also, as a result of strong BO interactions, the adjacent boron atom is lifted up by 0.405 from the graphene plane after O2 adsorption. Additionally, stretching of the OO bond in all adsorption configurations, and especially the greater stretching (8.723 %) in the O2–3B7N1 configuration, indicates that the OO bond becomes weaker after adsorption, as suggested by previous reports.[23] This OO bond weakening, that is, lowering of the bond breaking barrier, is convenient from the point of view of the ORR process.

3. Conclusion We performed an exhaustive study regarding B, N and BN codoping on the DV(555-777) defect graphene system by using density functional theory. The results obtained indicate that the formation of a cluster comprised of three boron atoms and seven nitrogen atoms at the center of the DV(555-777) defect (3B7N1) was most favorable with a formation energy of 0.838 eV. The incorporation of boron dopants improved the stability and charge distribution of nitrogen atoms. Varying the boron and nitrogen doping concentration and configuration was an effective measure to modulate the band gap of the doped DV(555-777) defect graphene system within the range of 0.2–0.9 eV. Our results also demonstrated, for 3B7N1 doping, even though the linear dispersion nature at the Dirac point was preserved, the band gap of the system increase by 0.546 eV in comparison to the undoped DV(555-777) defect graphene system.

Table 2. Parameters of O2 adsorption on the undoped and doped DV(555-777) defect graphene system.[a] Configuration

Ea [eV)]

dOO []

DdOO [%]

hO2 []

QB [e]

QN [e]

QO [e]

DV(555-777) NCout BCin 3B7N1

1.001 1.312 1.358 1.720

1.245 1.250 1.330 1.346

0.565 0.969 7.431 8.723

3.241 3.178 2.140 2.110

– – 0.570 0.910

– 0.230 – 0.830

0.050, 0.060, 0.220, 0.280,

0.050 0.060 0.240 0.260

[a] Ea is the adsorption energy, dOO is the bond length of dioxygen, DdOO is the amount of OO bond stretching occurring after adsorption, hO2 is the average height of the adsorbed O2 molecule from the undoped/doped DV(555-777) defect graphene sheet, QB is the computed Mulliken charge of B atoms, QN is the computed Mulliken charge of N atoms, and QO is the computed Mulliken charge of each oxygen atom of the dioxygen molecule.



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CHEMPHYSCHEM ARTICLES Our calculated results indicated that 3B7N1 was a good dioxygen adsorption medium with an adsorption energy of 1.720 eV, which was much lower than the same on the Pt(1 1 1) surface ( 1 eV), undoped DV(555-777) defect graphene system (1.001 eV), and single nitrogen-atom-doped (1.312 eV) and single boron-atom-doped (1.358 eV; at their most favorable sites) DV (555-777) defect graphene system. The electron-donating nature of the nitrogen dopant and the low electronegativity of the boron dopant governed the higher amount of charge transfer to the dioxygen molecule, the boron dopant behavior as an active adsorption site, and the higher stability of the dioxygen adsorbate in O2–3B7N1. Also, significant stretching of the OO bond ( 8.723 %), calculated for the O2–3B7N1 configuration, indicated lowering of the OO bond breaking barrier, which was highly advantageous for the ORR process.

Acknowledgements D.S. wishes to thank the West Bengal State Government for providing financial support during the execution of this work. R.T. wishes to thank the Science and Engineering Research Board (SERB) for financial support (grant no: SB/FTP/PS028/2013). We also wish to thank the University Grants Commission, the Government of India, for financial support under the ‘University with Potential for Excellence (UPE-II)’ scheme and also from the TEQIP programme. Keywords: ab initio calculations · boron · carbon · doping · graphene [1] a) X. Kong, Q. Chen, Z. Sun, ChemPhysChem 2013, 14, 514 – 519; b) N. Alonso-Vante, ChemPhysChem 2010, 11, 2732 – 2744. [2] a) M.-H. Yeh, L.-Y. Lin, L.-Y. Chang, Y.-A. Leu, W.-Y. Cheng, J.-J. Lin, K.-C. Ho, ChemPhysChem 2014, 15, 1175 – 1181; b) G. Wang, W. Xing, S. Zhuo, Electrochim. Acta 2013, 92, 269 – 275. [3] a) T. H. M. Housmans, J. M. Feliu, R. Gmez, M. T. M. Koper, ChemPhysChem 2005, 6, 1522 – 1529; b) T. C. M. Nepel, P. P. Lopes, V. A. Paganin, E. A. Ticianelli, Electrochim. Acta 2013, 88, 217 – 224; c) T. Diemant, J. Bansmann, H. Rauscher, ChemPhysChem 2010, 11, 1482 – 1490; d) M. K. Debe, Nature 2012, 486, 43 – 51.


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Received: March 19, 2014 Published online on && &&, 2014


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ARTICLES D. Sen, R. Thapa, K. K. Chattopadhyay* && – &&

Who needs to be replaced? Various boron and nitrogen doping/codoping schemes on double-vacancy defect graphene sheets are systematically ex-

plored by the means of first-principles calculations. The oxygen reduction reaction capabilities of such structures are investigated in detail (see picture).


Rules of Boron–Nitrogen Doping in Defect Graphene Sheets: A FirstPrinciples Investigation of Band-Gap Tuning and Oxygen Reduction Reaction Catalysis Capabilities


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Tuning the catalytic activity of graphene nanosheets for oxygen reduction reaction via size and thickness reduction.

Co-Doping of Activated Graphene for Synergistically Enhanced Electrocatalytic Oxygen Reduction Reaction.

Electrochemical doping of three-dimensional graphene networks used as efficient electrocatalysts for oxygen reduction reaction.

Molecular doping of graphene as metal-free electrocatalyst for oxygen reduction reaction.

Layered SiC sheets: a potential catalyst for oxygen reduction reaction.

Oxygen reduction reaction by electrochemically reduced graphene oxide.

Concurrent phosphorus doping and reduction of graphene oxide.

Tuning hydrogen storage in lithium-functionalized BC2N sheets by doping with boron and carbon.

Few-layered graphene-supported palladium as a highly efficient catalyst in oxygen reduction reaction.

Dramatic increase in the oxygen reduction reaction for platinum cathodes from tuning the solvent dielectric constant.

Evolution Reaction of CoMn2O4 Nanodots Supported on Graphene.

Synthesis and characterizations of Ni-NiO nanoparticles on PDDA-modified graphene for oxygen reduction reaction.

Nitrogen-doped mesoporous graphene as a synergistic electrocatalyst matrix for high-performance oxygen reduction reaction.

A density functional theory study on oxygen reduction reaction on nitrogen-doped graphene.

Investigation of Supported Pd-Based Electrocatalysts for the Oxygen Reduction Reaction: Performance, Durability and Methanol Tolerance.

Observation of active sites for oxygen reduction reaction on nitrogen-doped multilayer graphene.

Well-coupled graphene and Pd-based bimetallic nanocrystals nanocomposites for electrocatalytic oxygen reduction reaction.

ZIF-8 derived graphene-based nitrogen-doped porous carbon sheets as highly efficient and durable oxygen reduction electrocatalysts.

Graphene supported non-precious metal-macrocycle catalysts for oxygen reduction reaction in fuel cells.

Tuning the mechanical properties of vertical graphene sheets through atomic layer deposition.

Oxygen-activated growth and bandgap tunability of large single-crystal bilayer graphene.

Tuning the Magnetic Properties of Carbon by Nitrogen Doping of Its Graphene Domains.

Graphene-based transition metal oxide nanocomposites for the oxygen reduction reaction.

Defect Control and n-Doping of Encapsulated Graphene by Helium-Ion-Beam Irradiation.