Another Way of Looking at Reactivity Enhancement in Large-Area

Article pubs.acs.org/JPCC

Another Way of Looking at Reactivity Enhancement in Large-Area Graphene: The Role of Exchange Splitting from First-Principles Methods Mary Clare Sison Escaño,*,† Tien Quang Nguyen,‡ and Hideaki Kasai§,∥,⊥ †

Graduate School of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan § National Institute of Technology, Akashi, Hyogo 974-9501, Japan ∥ Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan ⊥ Institute of Industrial Science, The University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan ‡

S Supporting Information *

ABSTRACT: In spintronics field, the spin polarization of large-area graphene has been experimentally observed, yet its reaction to gaseous molecules remains elusive because of the lack of spin-resolved adsorption/mechanistic probe. In this work, we quantified the exchange splitting and magnetic stability of graphene on Ni(111) and found the relationship with its enhanced reactivity toward oxygen molecule in comparison to freestanding graphene and to nitrogen-doped graphene using spin density functional theory calculations with van der Waals corrections. The significant improvement in the reactivity is attributed to the formation of spin-down density of states at the Fermi level (LDOS at EF) because of the exchange splitting. Interestingly, the LDOS at EF property is maintained on graphene atoms not bound to oxygen. Also, the spin-polarization of graphene is found to be stable under oxygen adsorption. These findings pose a new deviation from the conventional ways of improving reactivity of large-area graphene such as straining, doping, and defects introduction. In the spintronics field, the spin-polarization of graphene deposited on Ni(111) was observed experimentally using spinpolarized metastable de-excitation spectroscopy (SPDMS).21 Although SPDMS can not yet provide the direct measurement of the exchange splitting in graphene (on Ni), it can however probe spin asymmetry, sufficient to confirm the spin-polarization and hence the presence of exchange splitting. In this work, we quantify the exchange splitting near the Fermi level, obtain its magnetic stability, and demonstrate how it can lead to a reactive carbon surface without geometric and chemical modifications, paving the way for surface reactivity only previously associated with transition metal surfaces. Using an oxygen molecule as our reactivity probe and implementing firstprinciples methods based on spin density functional theory (SDFT) with van der Waals correction (DFT-D2), the enhancement of reactivity of exchange-split graphene (ESG) as compared to that of freestanding graphene (FSG) and nitrogen-doped graphene (NDG) is demonstrated, and the corresponding mechanism is revealed. Oxygen has a wide

1. INTRODUCTION Among the different carbon structures, freestanding graphene is a simple 2D honeycomb structure that exhibits novel electronic and magnetic phenomena. In the field of nonprecious catalysts, graphene is widely considered because of its robustness attributed to the σ bond between the carbon atoms and its high conductivity due to the linear dispersion of π and π* bands near the Fermi level (EF) at K and K′ points.1,2 However, graphene remains unreactive to many molecules. Therefore, various geometric and chemical methods have been adopted to improve the reactivity of graphene such as straining,3−5 “cutting” to produce semi-infinite nanoribbons,6−8 doping (e.g., substitutional doping, charge doping, heteroatom doping, etc.),9−16 and defects introduction such as vacancies.17−19 In general, enhanced reactivity is observed and is attributed to the formation of reactive states localized within the strained C−C bonds, edge sites, dopants, or vacancies. Thus, characteristic surface reactivity found in transition metals where all surface atoms are reactive seemed farfetched on these carbon structures. Carbon “cage” structures have also been explored such as single-shell carbon-encapsulating transition metal nanoparticles, where enhanced catalytic activity is observed, but mechanisms remain elusive.20 © 2015 American Chemical Society

Received: September 30, 2015 Revised: October 22, 2015 Published: November 6, 2015 26636

DOI: 10.1021/acs.jpcc.5b09549 J. Phys. Chem. C 2015, 119, 26636−26642

Article

The Journal of Physical Chemistry C

∼12 Å are employed. This surface unit cell size gives an oxygen coverage of 0.25 ML, typical in oxidation reduction catalysis experiments (0.15−0.35 ML).40 The oxygen is placed at different configurations on the systems. Thus, a potential energy scan (PES) to obtain the most stable configuration is conducted. The PES is depicted and further explained in Supporting Information Figures 1 and 2. In the FSG and NDG, the O2 and the surface atoms are all relaxed. In the ESGs, all the C atoms of graphene and the topmost Ni layer are relaxed. The binding energy of O2 on the systems is referenced to the isolated gas-phase molecule and the surface. Lastly, the charge transfer upon the oxygen molecule adsorption is obtained using Bader’s charge analysis/algorithm, where the charge density partitioning is conducted at zero-flux surfaces, that is, the surfaces where the charge density is minimum perpendicular to it.42−44

implication in the oxidation−reduction reaction (ORR) in fuel cells9−14,22−24 and lithium−air batteries11,12 and in the oxidation of toxic gases in automotive catalysts, among others. In nonprecious catalysts, the O2 reaction process proceeds via a molecular precursor state.10,12,25 Thus, the study of oxygen molecule adsorption is a vital step.

2. COMPUTATIONAL METHODS The theoretical study is based on spin density functional theory (SDFT)26,27 with van der Waals corrections (DFT-D2),28 which is implemented in Vienna Ab Initio Simulation Package (VASP).29,30 The DFT-D2 method of Grimme for the introduction of van der Waals effects in the adsorption calculations is conducted by adding the dispersion term (Edisp) to the Kohn−Sham DFT energy (EKS−DFT): E DFT‐D2 = E KS − DFT + Edisp (1)

3. RESULTS AND DISCUSSIONS We found two stable structures of graphene on Ni (111) as shown in Figure 1 below. (For other configurations considered,

. The dispersion energy is a semiempirical dispersion correction given by Nat − 1

Edisp = −s6

Nat

∑ ∑ i=1

j=i+1

C6ij R ij6

fdmp (R ij)

(2)

Cij6

where Nat is the number of atoms in the system, is the dispersion coefficient for atom pair ij, s6 is a global scaling factor, Rij is the interatomic distance, and fdmp(Rij) is a damping function used to prevent near-singularities for small R. The exchange and correlation is described by the generalized gradient approximation in Perdew−Burke−Ernzerhof functional (GGA-PBE).31 The ion−electron interaction is treated within projector-augmented wave (PAW) method.32 The plane-wave cutoff energy is 550 eV. The Brillouin zone integration is sampled in 5 × 5 × 1 gamma-centered mesh of k points. An increase in k-point sampling to 15 × 15 × 1 leads to minute change of 0.0026 eV in binding energy of O2 on ESG. To obtain more accurate density of states, a 21 × 21 × 1 kpoints sampling is used. Optimization of atomic structures is conducted using the conjugate-gradient algorithm.33 The above calculations parameters give a Ni lattice parameter (3.515 Å), a graphene−Ni interfacial distance (2.092 Å), ESG’s C−C bond (a very minimal expansion of ∼1.20%), and O2 binding energy on FSG (0.094 eV) in excellent agreement with experiments: 3.52 Å,34 2.10 ± 0.10 Å,35 1.30%,36 and 0.100 eV,37 respectively. Moreover, the GGA-PBE excellently predicts the magnetic moment of bulk Ni, 0.60 μB (experiment: 0.60−0.62 μB),38,39 and the exchange splitting of Ni bulk states, 0.40 eV (experiment: ∼ 0.35 eV; other theoretical work: 0.39 eV).39 Other details of the calculation method are described below. A four-layer Ni(111) slab is used to model the substrate in ESGs. This slab has a surface relaxation of 0.010 Å in excellent agreement with that of experiment (0.007 ± 0.003 Å)40 and is thus sufficient to model a surface. In addition, use of such Ni slab successfully reproduced the experimental graphene−Ni interfacial distance in ESGs as mentioned above. Here, the graphene and top two layers of Ni are relaxed in a (1 × 1) surface unit cell. For the bare NDG, the experimentally attainable N-dopant coverage of 13 atom %41 is modeled; thus, a (2 × 2) surface unit cell is used with one substitutional N atom doping. This coverage is large enough for a reactive NDG without distorting the structure.41 For the adsorption calculations on all systems (FSG, ESGs, and NDG), a (2 × 2) surface unit cell and a vacuum layer of

Figure 1. Atomic structure for (a) exchange-split graphene at top fcc (ESG-TF), (b) exchange-split graphene at bridge-top (ESG-BT), (c) nitrogen-doped graphene (NDG), and (d) freestanding graphene (FSG). For ESGs, the Ni atoms at different layers are labeled (e.g., Ni1 for Ni at the first layer, and so on). For ESGs and FSG, C1 and C2 depict the carbon atoms in the (1 × 1) unit cell. The two C atoms are at near-top sites for ESG-BT, whereas one C atom is at the top site and the other is on a fcc-hollow site for ESG-TF. For the NDG, one nitrogen (per seven carbon atoms or 13 atom %) substitutional doping is used. The nitrogen is labeled, and C1 and C2 are identified as atoms where the density of states (DOS) are projected.

please refer to Supporting Information Table 1.) The ferromagnetic Ni substrate is in almost perfect conformation with graphene leaving the latter unstrained. Experiment via high-resolution X-ray photoelectron spectroscopy (HR-XPS) has detected the same two unstrained stable structures.45 We used the same labeling of the two structures as that in the experiment, namely, graphene on top-fcc and graphene on bridge-top configurations. Because this graphene is exchangesplit, the structures are depicted as ESG-TF in Figure 1a and as ESG-BT in Figure 1b. The ESG-TF is characterized by one C atom (C1) positioned on the top site of Ni surface and the 26637


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Figure 2. Density of states (s and p) projected on carbon atoms labeled previously in Figure 1 as C1 and C2 for: (a and b) exchange-split graphene at bridge-top (ESG-BT), (c and d) freestanding graphene (FSG), (e and f) exchange-split graphene at top-fcc (ESG-TF), and (g and h) nitrogendoped graphene (NDG). Spin-up (spin-down) states are shown in black (red) curves. The exchange splitting values (Δex) near the EF for ESGs are shown by dashed arrows, and the consequent local density of states at the Fermi level (LDOS at EF) for ESG-BT, ESG-TF, and NDG are depicted by solid arrows. Semimetallic property of FSG and spin-up electrons of ESG-BT are depicted in c and a, respectively.

Figure 3. d-states projected on the Ni substrate of (a) ESG-TF and (b) ESG-BT. The arrow on the spin-down component (red) shows clearly the hybridization with the LDOS at EF on graphene identified in Figure 2a for ESG-BT and Figure 2e for ESG-TF. The arrow for the spin-up component also depicts the hybridization with the spin-up sp states of graphene near the EF.

an exchange splitting (Δex) of ∼0.46 eV is noted near EF (Figure 2e); thus, spin-down LDOS at EF also arises. These peaks are more localized as compared to that of ESG-BT. However, no such significant peaks can be observed for the C2 atom of ESG-TF (Figure 2f). This indicates that three out of six carbon atoms within the benzene ring possess the LDOSEF property for spin-down electrons in ESG-TF. The exchange splitting for ESGs are obtained using the energies of the peaks identified in LDOS plots. For ESG-TF, the peaks are localized; thus, energies are easily determined. The spin-up LDOS peak is at ca. −0.42 eV, and the spin-down LDOS peak is at ∼0.04 eV, thus giving a Δex of ∼0.46 eV. For ESG-BT, the spin-up peak is at ca. −0.45 eV and the spin-down peak is at ∼0.05 eV. The Δex is ∼0.50 eV. Because the LDOS near the EF in the ESG-BT are more broadened than in the ESG-TF, the Δex value in the former is checked using a band structure plot (Supporting Information Figure 3). We note that the exchange splitting at Γ is 0.502 eV. The same Δex magnitude is noted in the M → Γ and Γ → K-band dispersions.

other (C2) on the fcc-hollow site, whereas the ESG-BT is characterized by two C atoms on the “near-top” sites. As mentioned previously, a (1 × 1) Ni surface unit cell is used in the optimization of ESGs; hence, two C atoms (C1 and C2) are enough to span the graphene sheet. For comparison, the atomic structures of NDG and FSG are also shown in Figure 1c,d. The NDG structure is obtained by typical substitutional doping of N.12,13 The same two C atoms (C1 and C2) are used to span the freestanding graphene as in the ESGs. The corresponding electronic structures for the ESGs are shown in Figure 2 using density of states projected (LDOS) on the C1 and C2 atoms. Here, we show the magnitude of the exchange splitting near the Fermi level. As shown in Figure 2a,b, an exchange splitting (Δex) of ∼0.50 eV is noted for both C1 and C2 atoms of ESG-BT. As a consequence, spin-down LDOSs at EF (shown by the arrows) arise for all the carbon atoms, whereas a semimetallic property is maintained for the spin-up electrons. Such LDOSs at EF do not exist on FSG as can be seen in Figure 2c,d, and only the semimetallic property can be observed in agreement with experiments.1 For ESG-TF, 26638


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The Journal of Physical Chemistry C We also include in Figure 2 the DOS projected on the C1 and C2 atoms of NDG (Figure 2g,h). LDOS at EF also forms on the C1 but not on the C2, which is almost similar to that of ESG-TF. A minute spin-polarization is observed. A closer look at the C atoms within the benzene ring reveals that C1 and the C atoms indicated by asterisk (*) in Figure 1 possess the LDOS at EF property. This finding confirms the N-doping-induced asymmetry in spin-density distribution in graphene structures such as pyridine and pyrrole.12 The exchange splitting on ESGs is due to the ferromagnetic Ni substrate. Figure 3 shows the LDOS plot for the Ni underlayer in ESG-TF (Figure 3a) and in ESG-BT (Figure 3b). For the spin-down component, the arrow depict the d states at the EF that hybridize with the LDOS at EF of graphene, identified previously in Figure 2a,e. Similar hybridization can be noted for the spin-up component. Thus, graphene exhibits exchange splitting characteristic of its ferromagnetic substrate. We expect this trend to be manifested when other ferromagnetic surfaces (e.g., Fe and Co) are used, provided the graphene is unstrained. Next, we show the oxygen molecule adsorption properties on the four systems presented above. An exhaustive PES incorporating the molecule’s orientation (vertical and parallel), rotation, and translation to obtain the most favorable adsorption configuration and energies on FG, NDG, ESGBT, and ESG-TF is conducted, and the binding energies are detailed in Supporting Information Tables 2−5. Only the most stable configurations are shown in Figure 4a−d, whereas the summary of the adsorption properties is given in Table 1. First, it is worthwhile to point out that the SDFT + DFT-D2 methods employed for adsorption calculations provide an O2

Table 1. O2 Adsorption Properties on FSG, NDG, ESG-TF, and ESG-BT at the Most Stable Configurationa catalyst

binding energy (eV)

O2−surface distance (Å)

O−O bond length (Å)

charge transferred to adsorbed O2 (e−)

FSG NDG ESG-TF ESG-BT

−0.094 −0.206 −0.240 −0.218

3.019 2.907, 2.928 2.777, 2.815 2.870

1.236 1.246 1.248 1.248

0.032 0.120 0.150 0.140

a

The binding energy is referenced to isolated O2 and the surface. The charge transferred to adsorbed O2 is referenced to the gas phase’s total valence electrons.

binding energy on FSG (−0.094 eV) in excellent agreement with experiment (0.100 eV).37 Here, we further provide other important adsorption properties. On the basis of Figure 4a, the molecule prefers a parallel orientation with its center-of-mass on the hollow site and the O−O bond spanning the bridge site (i.e., p-cmh-b configuration). In Table 1, the O2−surface distance is 3.019 Å, the longest among the four systems. The O−O bond length is maintained as compared to the gaseous state (1.235 Å). This is not stretched at all, which is due to the minute amount of charge transferred to O2 as shown in Table 1 (last column). This kind of inertness of graphene is the bottleneck of its supposed wide use in future energy devices. The O2 on NDG, however, is much more bound (−0.206 eV) as shown in Table 1. This confirms the wide interest given to N-doped graphene structures as candidates for nonprecious catalysts for various energy device applications.9 In contrast to that of FSG, one of the oxygen atoms (O1) is lower than other. This tilted configuration in NDG results to two O2−surface distances (column 3, Table 1). Furthermore, the O−O bond length is better stretched (1.246 Å), and the charge transferred to O2 is more significant. The adsorption configuration (p-cmh-t) is depicted in Figure 4b. Despite this reactivity enhancement in doped graphene, the ESGs are found to exhibit greater interaction with O2 as indicated by the binding energies on ESG-BT (−0.218 eV) and ESG-TF (−0.240 eV) in Table 1. The O2−surface distances for the ESGs are also little smaller than that of NDG (2.777 and 2.815 Å for ESG-TF and 2.870 Å for ESG-BT), indicating better interaction. The charge transferred to O2 is also more significant. Figure 4c,d depicts the most stable configuration on ESG-BT (p-oh-b) and on ESG-TF (p-cmh-t). The O2 on the former spans toward the bridge sites, whereas on the latter it spans toward the top sites. Thus, there is a similarity in the adsorption geometry of O2 between ESG-BT and FSG and between ESG-TF and NDG. What is the correlation of the exchange splitting with reactivity enhancement of unstrained, undoped infinite graphene? First, it is apparent from Figure 2c,d that the lack of LDOS near or at EF for the FSG makes it unreactive to gas molecules. In contrast, ESGs exhibit significant LDOS at EF because of the exchange splitting as depicted previously in Figure 2a,b,e,f. Thus, an increased charge transfer to adsorbed O2 and a longer O−O bond length is observed. We confirm this using the LDOS plots in Figure 5a−c, showing the hybridization of O2 sp states with the LDOS at EF of the ESGs. The hybridized (antibonding state + LDOS at EF) is depicted by the solid arrows in Figure 5a, leading to partial occupation of this state of O2. The DOS projected only on the C atoms within the benzene ring for ESG-TF and ESG-BT is shown in Figure 5b,c. This confirms the hybridization with LDOS at EF

Figure 4. O2 at the most stable configuration in (a) freestanding graphene (FSG), (b) nitrogen-doped graphene (NDG), (c) exchangesplit graphene at bridge-top (ESG-BT), and (d) exchange-split graphene at top-fcc (ESG-TF). Only the part showing the benzene ring is depicted. Two oxygen atoms are labeled as O1 and O2. A (2 × 2) surface unit cell is used; hence, a total of 8 carbon atoms are considered for adsorption on ESGs. The same corresponding unit cell size is used for NDG and FSG. C1 and C2 depict the same carbon atoms as in Figure 1. The most stable configuration is labeled for each system. The details of the potential energy scan are given in Supporting Information Tables 2−5 and Figures 1−2. 26639


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Figure 5. Density of states (s and p) projected on (a) O2 and carbon atoms of exchange-split graphene at top-fcc (ESG-TF), (b) carbon atoms within the benzene ring of ESG-TF, and (c) carbon atoms within the benzene ring of ESG-BT. The arrows show the hybridization of O2 states with the local density of states at the Fermi level (LDOS at EF) of the ESGs.

for both types of ESGs (arrows). Furthermore, the similarly adsorbed O2 configuration on ESG-BT compared to that of FSG (Figure 4a,b) can be associated with same electronic properties for all C atoms (as in FSG), rendering projection of O2 toward the bridge. Using the same line of thought, the O2 configuration on ESG-TF is also similar to that of the NDG because of similar electronic properties. We note that because of the unequal positions of the C atoms on Ni for ESG-TF, formation of LDOS at EF is observed on some C atoms just as in the case of NDG. The difference, however, is that LDOS at EF property can be observed on three equally spaced carbon atoms (symmetric) in ESG-TF as indicated by the triangle in Figure 4d, in contrast to that of NDG, where such electronic property is observed for only two carbon atoms within the benzene ring. Thus, the adsorption configuration and other properties are similar, except for a higher binding energy on the ESG-TF. For ESG-BT, the more delocalized LDOS at EF as compared to that of ESG-TF renders a weaker adsorption of O2 on the former; however, it is still higher than that of NDG. Next, we show the electronic structure of the carbon atoms on ESGs that are not involved in the O2 bonding. Figure 6 depicts the repeated surface unit cell to show the carbon atoms outside the benzene ring (e.g., C3 and C4) in ESGs as well as the corresponding LDOS plots. We note that the LDOS at EF property is maintained in both ESG-TF (Figure 6a) and ESG-BT (Figure 6b). This suggests that even in molecular O2 preadsorbed cases other carbon atoms still remain reactive. This poses a significant implication in catalysis where reactions can involve preadsorption or coadsorptions. Finally, the magnetic stability of the ESGs with the Ni substrate is obtained using the energy difference (ΔE) between the spin-polarized and non-spin-polarized ESG systems. Figure 7 depicts the schematic diagram of the evaluation of the magnetic stability. Here, the ΔE is found to be 0.82 and 0.79 eV for ESG-TF and ESG-BT, respectively. These energies are higher than the O2 molecular binding energies so that the extra energy associated with the adsorption of oxygen does not induce transition to a nonmagnetic state.

Figure 6. Repeated surface unit cell for (a) O2/ESG-TF and (b) O2/ ESG-BT. The carbon atoms not involved in the O2−graphene bond formation (that is, outside the benzene ring) are depicted as C3 and C4. The corresponding LDOS plots are shown on the right.

Figure 7. Schematic diagram for the magnetic stability of the ESGs obtained using total energies of spin-polarized (left) and non-spinpolarized ESGs (right). The energy difference (ΔE) for ESG-TF and ESG-BT are shown.

stability of ESGs and to demonstrate their enhanced reactivity toward an oxygen molecule in comparison to that of FSG and NDG. It is found that the graphene placed at top-fcc position on Ni(111) or ESG-TF gives the largest adsorption energy. This is followed by graphene placed at bridge-top position on Ni(111) or ESG-BT. Nitrogen-doped graphene (NDG)

4. CONCLUSIONS First-principles methods based on spin density functional theory (SDFT) with van der-Waals correction (DFT-D2) are conducted to quantify the exchange splitting and magnetic 26640


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provides a little lower adsorption energy, whereas free-standing graphene (FSG) binds O2 very weakly. The O−O bond length (O2−surface distance) increases (decreases) on the same order. The improvement of the reactivity of the exchange-split graphene is attributed to the presence of spin-down LDOS at EF, which is nonexistent in the free-standing graphene. This can facilitate better charge transfer to O2 molecule via more effective O2 sp states − LDOS at EF hybridization. LDOS at EF is also observed in NDG induced by the nitrogen substitutional doping; however, LDOS at EF arises on few carbon atoms around the dopant. Interestingly, a structure of ESG (ESG-BT) is found to exhibit LDOS at EF on all carbon atoms, rendering a reactive nonprecious carbon surface rather than reactive local sites. This mimics the surface reactivity of metal surfaces. Lastly, the LDOS at EF property is maintained on carbon atoms of ESGs that are not involved in the O2 bonding, retaining the enhanced reactivity even on preadsorbed cases. Thus, the exchange splitting reveals a novel pathway toward reactive large-area graphene without doping, straining, or defect introduction.

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ASSOCIATED CONTENT

S Supporting Information *

Figures are supplied for detailed description of configurations, potential energy scan and verifications whereas tables are provided for the binding energies etc. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09549. Figure captions and tables. (PDF) Supporting Information Figure 1: potential energy scan (rotation, translation) to obtain the configurations of O2 on freestanding graphene (FSG) and nitrogen-doped graphene (NDG). (PDF) Supporting Information Figure 2: potential energy scan (rotation, translation) to obtain the configurations of O2 considered for exchange-split graphene (ESGs). (PDF) Supproting Information Figure 3: Band structure plot for ESG-BT along high symmetry k-points (M, Γ, K). (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81776-27-9802. Fax: +81776-27-9742. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.C.S.E. extends gratitude to Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT)/Japan Science and Technology Agency (JST)/Tenure Track Program for Innovative Research and Japan Society for Promotion of Science (JSPS) Grant-in-aid for Young Scientist B - Grant Number 15K21028 for research funds. The calculations are done using the Kyoto University Supercomputer and the HighPerformance Computing Cluster Fukui (HPCCF) of Escaño Research Group, University of Fukui.

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REFERENCES

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