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Effect of Hydrogen Termination on Carbon K-Edge X-ray Absorption Spectra of Nanographene Zhufeng Hou,† Xianlong Wang,† Takashi Ikeda,‡ Shen-Feng Huang,z Kiyoyuki Terakura,*,z,† Mauro Boero,z,§ Masaharu Oshima,|| Masa-aki Kakimoto,† and Seizo Miyata† †

)

Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 S5-20, Ookayama,Tokyo 152-8552, Japan ‡ Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA), 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan z Research Center for Integrated Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan § Institut de Physique et Chimie des Materiaux de Strasbourg (IPCMS), UMR 7504 CNRS-University of Strasbourg, 23 rue du Loess, 67034 Strasbourg, France Department of Applied Chemistry, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 113-8656, Japan

bS Supporting Information ABSTRACT: Carbon K-edge X-ray absorption spectra of nanographene have been simulated by density functional theory calculations to obtain information on the edge termination by hydrogen. Such information is crucially important to understand and predict functions such as transport and catalysis. Our results show that different edge terminations significantly affect the binding energy of the 1s core-level of C atoms in the vicinity of edges because of the change in chemical bonding and the localized edge states. We find that a shoulder or a peak appears below the π* peak at relatively different positions with respect to the π* peak position in the theoretical spectra of zigzag graphene nanoribbons, depending on the ratio of monohydrogen- to dihydrogen-terminations. We also point out that the two additional features observed between the π* and σ* peaks of an ideal graphene originate from the σ* states of C-H bonding and C-H2 bonding at the edges.

1. INTRODUCTION Graphene, a single atomic layer of graphite consisting of sp2hybridized carbons, has attracted enormous attention because of its two-dimensional crystal lattice feature, its atomic thickness, and its unique electronic structures.1,2 These peculiarities have disclosed exciting opportunities for developing novel nanoelectronic devices.3 Furthermore, due to high surface area and rich edges, nanometer-sized graphene (nanographene hereafter) potentially has a wide range of fascinating applications such as biosensing,4,5 energy storage and conversion,4 and catalyst.6 In particular, recent intensive activities have revealed that carbon alloy catalysts (CAC), whose basic structural components are multilayered nanographene (nanographite), are strong candidates for the Pt-free cathode catalyst of a polymer-electrolyte fuel cell.7-20 Graphene edges can have unique electronic structures localized along edges depending on the details of the atomic structure21-24 and those edge states will play important roles in the functions mentioned above. Therefore, in order to tune these functions, precise information on the atomic structure of nanographene is indispensable. r 2011 American Chemical Society

X-ray absorption spectroscopy (XAS) is an element-specific technique, which involves the excitation of electrons from a core level to unoccupied states.25 Hence, XAS can provide the electronic, structural, and bonding information not only about nanographene but also about atoms belonging to functional groups at the surface or at the edge that are possibly introduced during the chemical treatment. Although the graphene samples were prepared by several groups with different methods and through different chemical treatments, some common features were found in the C K-edge XAS spectra. According to the available experimental results,26-33 we briefly summarized the main features in the C Kedge XAS spectra of graphene-related materials in Table 1. Roughly, there are five peaks in the near-edge region of the measured spectra. Here, these peaks are labeled as p1, p2, p3, p4, and p5, respectively, from lower to higher transition energies, for

Received: November 15, 2010 Revised: February 1, 2011 Published: March 04, 2011 5392

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special extended final state or to Stone-Wales (SW) defects. Moreover, the energy position of p1 observed in the experiments shows a certain variation; the energy separation between features p1 and p2 varies as 1.3,26 1.8,30 and 1.0 ( 0.1 eV.31 Nonetheless, the origin of such large variation is still unclear. To the best of our knowledge, this issue has not yet been addressed. For feature p3 at 287.526,35 or 287 eV,27 and feature p4 at 288 eV, their energy positions are very close to each other. Feature p3 at 287.5 eV observed in nanographite films26 and carbon thin films35 was ascribed to the C 1s-σ*(C-H) transition.26,35 Feature p3 at 287.0 eV observed in graphene sheets with acid treatment27 was attributed to the CdO bonds incorporated via OH groups in the carbon network. Similarly, the origin of feature p4 is still controversial (see Table 1), because the corresponding peak was observed in the C K-edge XAS spectra of the oxidized pristine graphite (and graphene)28,29,32 as well as a single-layered graphene.30 We will make some comments in Section 3.1 on the possible role of interlayer states suggested by Pacile et al.30 SW defects were also assigned to the origin of either p3 or p4 by a recent theoretical work.34 As described above, the assignment of peaks p1, p3, and p4 is still debated. Given this scenario, a systematic theoretical simulation of C K-edge XAS spectra of nanographene can in principle complement experiments and straightforwardly provide more convincing assignment. However, it is practically impossible to inspect all possible configurations including various functional groups. In the present work, we therefore focus our attention only on the effects of hydrogen termination of graphene edges in various ratios of mono- to dihydrogenation. A detailed theoretical study on the XAS of graphene and graphene clusters was recently done in ref 34. In this work, the effect of stacking of graphene layers was also studied. However, all of the edge carbons are monohydrogenated and edge effects were not so systematically analyzed.34 Effects of oxygen related functional groups will be the target of a forthcoming work. A recent theoretical study36 has shown that the stability of edge structures of graphene nanoribbons (GNRs) strongly depends on the hydrogen partial pressure. The zigzag edges with only monohydrogen termination have been the main target of theoretical studies due to the presence of spin-polarized π states localized on zigzag edges.23 However, such a termination is suggested to be stable only at extremely low hydrogen partial pressure. Under more standard conditions, stable structures are likely to be a mixture of mono- (-CH) and dihydrogen (-CH2)

some of which the assignment is still ambiguous as described in detail below. Peaks p2 and p5 exhibit strong intensity and their energy positions are the same as those of the main features (π* and σ* peaks, respectively) observed in graphite (see the upper curve of Figure 1(b)). Thus, it is commonly accepted that features p2 (∼285 eV) and p5 (∼292 eV) come from C 1s-π* and C 1s-σ* transitions, respectively. In contrast, no conclusive assignment is reached for other features. Particularly, the origin of feature p1, a shoulder below the feature p2 (i.e., π* peak), is still a matter of debate.26,30,31,34 Entani et al.26 considered that feature p1 originates from the zigzag edge states21-23 of nanographite. Their samples were grown by exposing a benzene gas to a clean Pt(111) substrate annealed up to high temperatures. Alternatively, Pacile et al.30 argued that the shoulder below the π* peak is ascribed to the splitting of π* bands in graphene and that it is irrelevant to zigzag edges. In their experiment, the samples were prepared by micromechanical cleavage of highly ordered pyrolytic graphite (HOPG) on a SiO2 substrate. They also claimed that a similar shoulder appears in a measurement for a large-sized (>10 μm) graphene sample, in which the contribution of edges and C-H species at the surface could be negligible. On the contrary, Joly et al.31 ascribed again the shoulder below the π* peak to the zigzag edge states of nanographene. Recent theoretical work34 on the XAS spectra of graphene clusters ascribed the shoulder to a

Figure 1. (a) Total density of states (TDOS) of single-layered graphene. The zero of energy is set at the Fermi level (EF). The inset shows the TDOS in the energy region of 1-2 eV above EF. (b) The simulated C K-edge XAS spectrum of single-layered graphene. The experimental spectrum of graphite taken from ref 46 is shown for comparison.

Table 1. Main Features in Measured C K-edge XAS Spectra of Graphene-Related Materials peak

energy (eV)

p1

283.8 283.7

p2

p3

assignment

ref

zigzag edge states the splitting of the π* band

Entani et al.26 Pacile et al.30

284.5 ( 0.1

edge state

Joly et al.31

285.1

π* (C-C)

Entani et al.26

285.5

Pacile et al.30

285.5

Joly et al.31

287

C-O states

Coleman et al.27

287.5

σ* (C-H)

Entani et al.26

An analogue of the interlayer state of graphite COOH and/or C-H contamination

Pacile et al.30 Jeong et al.28,29,32

σ* (C-C)

Entani et al.26

p4

288 288

p5

292.8 291.5

Pacile et al.30

291.85

Joly et al.31 5393

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Figure 2. (a) Atomic structure of graphene nanocluster C54H20. The symmetry-independent C atoms are labeled as C1, ..., C15. Atom colors are black for C and gray for H. (b) Total density of states (TDOS) of C54H20 and projected density of states (PDOS) for the p orbitals of representative C atoms in the graphene nanocluster. The zero of energy is set at the Fermi level. The rapid increase in the total DOS from 3 eV above the Fermi level is due to the appearance of s states of edge hydrogen atoms. (c) Simulated C K-edge XAS spectra of graphene nanocluster C54H20. Contributions from the representative C atoms in the cluster are also shown below the total spectrum. The isosurface plots of final-state orbitals evaluated in the presence of the C 1s core hole are also shown for the representative transitions. Vertical bars indicate computed line spectra before convolution with Gaussian functions.

terminations along armchair and zigzag edges. Different arrangements of these two types of hydrogen terminations strongly affect the electronic states including the core levels. It is generally believed that the core level is solely determined by the very local chemical environment. This may be true for the core level of unperturbed ground state. However, our study deals with the case of a 1s core hole associated with X-ray absorption, which is screened by 2s and 2p valence electrons. The screening by extended 2pz states is generally affected by the fairly extended environment. In this work, we first calculate the chemical shifts of 1s corelevel binding energy of C atom in the sp2 C-H, sp3 H-C-H, and the sp2 C-C bondings in nanographene and then perform the theoretical simulation of C K-edge XAS spectra within the density functional theory (DFT) framework. Our computed chemical shifts of C 1s levels are rationalized in terms of change of electronic states near the Fermi level due to the mixture of mono- and dihydrogen terminations. We also show that these

hydrogen termination effects are directly reflected in the C Kedge XAS spectra. The remainder of this work is organized as follows. In Section 2, we introduce the computational methods for electronic structures and XAS spectra. Our computed chemical shifts of C 1s levels and simulated C K-edge XAS spectra of graphene in different structures are presented in Section 3. Finally, we draw conclusions in Section 4.

2. METHOD AND COMPUTATIONAL DETAILS The total XAS spectrum at the C K-edge was obtained by summing up the corresponding spectra for the symmetryindependent C atoms with the relative abundance of every type of C atom. The calculation was performed using the CP2K code37 in the Gaussian and augmented plane wave (GAPW) all-electron formalism.38 We used the gradient correction after Perdew-Burke-Ernzerhof (PBE)39 for the exchange-correlation 5394

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The Journal of Physical Chemistry C functional. The 6-311G** basis set was used for both C and H atoms orbitals, whereas a cutoff of 280 Ry was used for the plane wave expansion. The theoretical spectra were obtained using the Slater transition potential (TP) method,40 in which a potential with a half-core hole (HCH) was created on the X-ray absorbing atom and hundreds of unoccupied orbitals under such constructed TP were computed. The transition energies were taken as differences between the corresponding Kohn-Sham energies. To determine the energy position of the spectrum more accurately, we resorted to ΔKohn-Sham (ΔKS) calculations as described in ref 40. All of the calculated spectra were calibrated so that the transition energy of the first spectral feature corresponding to the transition from the 1s level to the lowest unoccupied molecular orbital (LUMO) coincides with the one obtained from the ΔKS calculations, in which the total energy difference between the ground state and the excited state is calculated. A Gaussian function within the scheme described in ref 40 for the choice of full width at half-maximum (fwhm, i.e., σ) was used for convoluting the spectra. Namely, below the ionization threshold, the Gaussian width σ was fixed to 0.4 eV, while linearly increasing up to 8 eV over the following 20 eV. For the analysis of the C K-edge XAS spectra, we studied the following three categories of systems. In all cases, a supercell model is used, where the distance between neighboring planes in the direction perpendicular to the (nano)graphene plane is 12.0 Å. The Supporting Information gives supplementary description of the conditions in the actual numerical calculations. 2.1. Infinite Graphene. An infinite graphene was studied as a reference. Before the XAS calculation, the lattice constant a was optimized with the PWSCF code,41 which uses the plane-wave basis with the ultrasoft psudopotential method.42 The optimized value of a is 2.460 Å corresponding to the C-C bond length of 1.420 Å. The result is almost identical to the experimental data. The density of states (DOS) of unperturbed graphene is calculated also with PWSCF. For the structural optimization and the DOS calculation, a primitive cell with two atoms was used and a sufficiently large number of k points were used for the Brillouin zone integration. The XAS calculation performed with CP2K needs a large supercell in order to avoid the interference between the electrons excited to unoccupied states. The supercell size used in the present work is 49.12  51.05 Å2 within the basal plane, which contains 960 atoms. In this supercell, the lattice constant of graphene is the same as the one optimized with PWSCF. With such a large supercell, the only Γ point sampling is sufficient. 2.2. Graphene Nanocluster. A rectangular piece of graphene sheet shown in Figure 2(a) was used to analyze the overall features of the XAS spectra associated with monohydrogenated edges and corners of graphene nanoclusters. The chemical formula of this graphene nanocluster is C54H20. In the supercell model, the shortest distance in the basal plane between edge hydrogen atoms of two neighboring nanoclusters is 16.236 Å between zigzag edges and 18.089 Å between armchair edges. Both structural optimization and XAS calculation were done with CP2K with the Γ point sampling. All of the atoms in the supercell were allowed to relax until the atomic forces were below 0.01 eV/Å. We have confirmed that the optimized structure for graphene nanoclusters obtained by CP2K agrees quite well with that obtained by PWSCF. 2.3. Graphene Nanoribbons. For graphene nanoribbons (GNRs), we considered a set of their zigzag and armchair edges as representatives of different degrees of hydrogenation. The

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Figure 3. Schematic illustration with wireframe model for geometric structures of graphene nanoribbons (GNRs) with different degrees of hydrogenation at the edges: (a) zigzag GNRs (ZGNRs) z1m2n and (b) armchair GNRs (AGNRs) a1m2n. The m and n stand for the numbers of H-termination and H2-termination at edges, respectively. The periodic boundary of unit cell is indicated by arrows. The rows of carbon at edge, edge-1, and interior sites in GNRs are also indicated by arrows with text.

hydrogenated zigzag and armchair edges of GNRs are respectively denoted as z1m2n and a1m2n, where m and n stand for the number of -CH and -CH2 edge C in the primitive cell of GNRs, respectively. For example, z1120 means that all edge C atoms in the zigzag GNRs are monohydrogenated, while a1022 denotes the armchair GNRs with only dihydrogenated edge C. (See Figures s1 and s2 in the Supporting Information for some of the structures. The details of the combinations (m, n) and also the width of the ribbons are also given in the Supporting Information.) The lattice constant of GNRs along the edge was fixed to be the same as the corresponding one in the infinite graphene and the atomic positions in the perpendicular direction was optimized with PWSCF. The convergence threshold on forces for the optimization of atomic positions was set as 0.01 eV/Å. The primitive cell of a GNR was used for the structural optimization and the DOS calculation, while a supercell was used for the XAS calculation with CP2K. The supercell length of GNRs was chosen so that the number of edge C atoms (not including edge-1 C in Figure 3) along one ribbon edge varies from 12 to 15. (Figures s1 and s2 in Supporting Information also show the relation between the primitive cells and supercells for GNRs.)

3. RESULTS AND DISCUSSION 3.1. Infinite Graphene Sheet. Figure 1(a) shows the total density of states (DOS) of single-layered graphene, as obtained from the plane wave pseudopotential approach41,42 with a 60  60  1 grid of k-point sampling combined with a tetrahedron method.43 A pronounced peak is present at about 1.7 eV above the Fermi level (EF) and a small peak appears at 1.6 eV. These two peaks in the DOS are associated with the splitting of the π* bands.30 The energy separation between these two features is 0.1 eV 5395

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Table 2. the Chemical Shifts (ΔEb(1s), in eV) of 1s Core Level Binding Energy (Eb(1s), in eV) of the Symmetry-Independent C Atoms in the Graphene Nanocluster C54H20a zigzag edge

armchair edge

edge

a

edge-1

edge

edge-1

interior

site

C2

C4

C1

C3

C5

C6

C9

C7

C8

C13

ΔEb(1s)

-1.00

-0.72

0.02

0.02

-0.22

-0.71

-0.40

0.02

-0.13

0.00

ΔEb(1s) is given with respect to the Eb(1s) of interior C (i.e., C13). The label of C atoms is shown in Figure 2(a).

in our calculation, while a larger value of 0.8 eV was estimated by Pacile et al.30 for graphene with a different computational method,44 in which a small basis set without including the polarization functions was used. In refs 27, 34, and 45 no such small peak in the DOS of graphene was found and it should be noted that a broadening technique was used there to smooth the DOS curve. Due to the lifetime broadening and experimental resolution limit in the XAS spectrum, the effect of such small energy separation associated with the splitting of the π* band would be smeared out. In Figure 1(b) we show the simulated C K-edge XAS spectrum of single-layered graphene sheet. For comparison, the experimental C K-edge XAS spectrum46 of graphite is also presented. Only two main features are found in the theoretical spectrum, i.e., a sharp peak at 285.0 eV which is due to the π* resonance and an intense peak at 292.4 eV which is contributed by the σ* resonance. The positions of the π* and σ* peaks in our calculations agree well with the experimental results within less than 0.5 eV, although the structure at 291.7 eV in the experimental spectrum due to exciton cannot be reproduced by the present calculation. The simulated C K-edge XAS spectrum for a single-layered graphene shows no shoulder below the π* peak and no additional peaks between the π* and σ* resonances. The peak of the DOS at 1.6 eV is too small and too close to the main peak at 1.7 eV to reproduce the feature p1 seen in the measured XAS spectra for nanographene or nanographite. For the feature p4 at 288 eV in the experimental spectra, Pacile et al.30 ascribed it to an analogue of the interlayer state of graphite,47 i.e., the “image potential states” (1þ and 1-) described in ref 48. However, the main distributions of these 1þ and 1- image potential states are about 2.1 and 3.7 Å above the graphene layer, respectively,48 and hence the very tiny proportion of these states located at the atomic layer of graphene sheet will not be the main contribution to the feature p4. Nevertheless, the role of interlayer states is still a problem under debate. Some works claim that the clean graphene and graphite will not have any appreciable structures in their XAS spectra between π* and σ* peaks,28,29,31,34,46,49 while others claim that the structures around 288 to 289 eV are intrinsic originating from interlayer states.30,47,50 As the XAS spectra depend on the measurement condition and sample preparation, there may be more than one sources for each structure. Anyway, a detailed analysis of the role of interlayer states in the XAS spectrum is under way and a full discussion on this matter will be made in a future publication. 3.2. Graphene Nanocluster with Monohydrogen Termination. In order to see the effects of edges on XAS as a whole, we use the graphene nanocluster C54H20 shown in Figure 2(a). Figure 2(b) shows its total DOS and local DOSs at C atoms along the zigzag and armchair edges. As in other calculations,16 the zigzag edge C atoms have spin-polarized edge states close to the Fermi level (EF) and the lowest unoccupied state is located at 0.2 eV. The energy separation between this unoccupied edge state and the π* peak at 1.7 eV in Figure 1(a) is 1.5 eV. Therefore, the

assignment of p1 to the zigzag edge states seems to be reasonable if one considers only the initial-state model. However, the simulated C K-edge XAS spectrum of this graphene nanocluster depicted in Figure 2(c) shows that a small peak is located at 282.5 eV, about 2.5 eV lower than the π* peak (p2). By checking the final-state orbital in the presence of the C 1s core hole, we can assign this small peak to the zigzag edge state. The energy separation between these structures is too large compared with the experimental result (∼1.5 eV). The difference in the two energy separations mentioned above, one estimated from the DOS of unoccupied states and the other seen in the calculated XAS, comes from the difference in the 1s core level binding energy between the C atom in the interior of the graphene nanocluster and the one along the zigzag edge. The effect of core level shift has not been taken into account in the discussions so far on the feature p1. In Table 2, we present the chemical shifts ΔEb(1s) of 1s core level binding energies Eb(1s) of the symmetry-independent C atoms in the graphene nanocluster C54H20. The ΔEb(1s) of each symmetry-independent C atom is given with respect to Eb(1s) of the center C atom (i.e., C13 shown in Figure 2(a)). For monohydrogen-terminated edge C atoms at the zigzag (i.e., C2 and C4) and armchair edges (i.e., C5, C6, and C9) of graphene nanocluster, their 1s core level binding energies are smaller than that of C13 (see Table 2), indicating that their 1s core levels are shallower than that of C13 atom. Particularly for C2, C4, and C6 which have large local DOSs near EF coming from the zigzag edge states, their ΔEb(1s) is about -1.0 eV. (The zigzag edge states along the top side edge extend to C6 because it can be regarded as a part of the zigzag edge.) We can thus explain why the XAS peak originating from the zigzag edge states is too far from the π* peak to be assigned to the feature p1. The present analysis strongly suggests that the origin of the shoulder of the π* peak is not the zigzag edge states. In the present model of graphene nanocluster, all of the edge carbons are terminated by -CH. We will show in the following that different hydrogen termination is needed to reproduce the observed shoulder. Comparing ΔEb(1s) in Table 2 with the local DOSs in Figure 2(b), we notice an important trend in the chemical shifts of monohydrogen-terminated edge C atoms. Localized edge states near EF have the largest weight at C2 and smaller but yet appreciable weights at C4 and C6. Even C9 near the middle of the armchair edge has an appreciably negative ΔEb(1s) due to a small amount of localized states near the Fermi level coming from the tail of zigzag edge states along the down-side edge. There is no weight of edge states at C5. Larger weights of edge states make ΔEb(1s) more negative. This trend can be understood in terms of screening for the created core hole by 2s and 2p valence electrons. With larger local DOS near EF, the screening of the core hole will be more efficient to reduce the attractive potential due to the core hole. The core level, therefore, becomes 5396

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The Journal of Physical Chemistry C shallower. A more systematic analysis of 1s core level shift will be made in the following sections. 3.3. Graphene Nanoribbons: z1m2n and a1m2n. 3.3.1. Chemical Shifts of C 1s Levels. Figure 3 schematically shows the structures of GNRs with different degrees of edge hydrogenation. Considering that the edge hydrogenation can induce changes in the chemical bond of edge C differing from the sp2 hybridized C-C bond in the interior region of GNRs, we pay attention to the edge and edge-1 C atoms in GNRs. For the sake of simplicity, the di- and monohydrogenated edge C atoms are denoted as CH2 and CH, respectively. For the edge-1 C atoms in zigzag GNRs (ZGNRs), according to the edge hydrogenation of their two nearest neighbors we can classify them into three groups: CH2CCH2, CH2CCH, and CHCCH. For instance, CH2CCH means that one of its nearest neighbors in ZGNRs is dihydrogenated and the other is monohydrogenated. For edge-1 C atoms in the armchair GNRs (AGNRs), each has only one nearest neighbor at the edge site, and hence they can be classified into two groups: CCH2 and CCH. Figure 4 shows the chemical shifts ΔEb(1s) of edge and edge-1 C atoms in GNRs. Here the ΔEb(1s) is defined as the energy difference between the 1s binding energies Eb(1s) of the edge or edge-1 C atoms of interest and those of the sp2 hybridized interior C in GNRs. The positive (negative) value of ΔEb(1s) means that the corresponding 1s core level is deeper (shallower) than that of interior C. The ΔEb(1s) values reported in Figure 4 are obtained by creating a half core hole in the 1s core level and the range of variation is 3 eV. In contrast to them, the variation in the chemical shift of the unperturbed 1s core level is 1 order of magnitude smaller. For instance, in the absence of a core hole the range of variation of ΔEb(1s) in z1120, z1021, and a1121 is less than 0.24 eV. The difference between the two sets of core level shift comes from the difference in the screening efficiency against the presence of a half core hole. Detailed discussion about this issue will be given below. 3.3.1.1. Edge C: CH2. For the dihydrogenated C atoms along the zigzag edges of GNRs, the ΔEb(1s) is positive in all the cases considered here and the corresponding magnitude keeps a nearly constant value around 0.8 eV (see Figure 4(a)). This suggests that the 1s core level of dihydrogenated C is much deeper than that of the interior C in ZGNRs. This is also the case in AGNRs (see Figure 4(c)) although the chemical shift for a1022 is slightly larger. The local DOS of CH2 is shown in Figure 5(b) and Figure 6(b),(c) for GNRs with several different configurations. In all cases, CH2 has a fairly large local band gap of about 4 eV because of the sp3 bonding character of CH2. Therefore, the attractive potential due to a half core hole is only weakly screened, making the core level deeper. 3.3.1.2. Edge C: CH. For the monohydrogenated C atoms, ΔEb(1s) is negative in all the cases considered here and varies significantly from about -2.0 eV for a1121 to about -0.2 eV for z1122 and a1220 (see Figures 4(a),(c)). By comparing the variation in ΔEb(1s) among different sites of CH with the corresponding local DOS shown in Figures 5 and 6, we clearly see a general trend in which more negative ΔEb(1s) occurs for the site with larger local DOS near the Fermi level. The situation in ZGNRs will become clearer by categorizing CH into three groups (B1, B2, and B3) as shown in Figure 4(a). In B1 the two nearest neighbor edge carbons of the CH are also CH, in B2 one of the two is CH and the other CH2, and in B3 both the two are CH2. As is well-known, z1120 has zigzag-edge states at the Fermi level and such zigzag-edge states exist also at CH of z1m21 for m g 3.

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Figure 4. The chemical shift (ΔEb(1s), in eV) of 1s binding energy Eb(1s) of C at edge (a) and edge-1 (b) sites in zigzag GNRs, and in the armchair GNRs (c). The ΔEb(1s) is given with respect to the Eb(1s) of interior C in GNRs. The insets show the edge structures of GNRs.

The amplitude of these states is larger at the CH surrounded by more neighboring CH (the case of B1). Even if the zigzag-edge states exist, their amplitude at the CH next to CH2 is reduced significantly (the cases of B2 and B3). It has also been recently shown that a1121 has strongly localized armchair-edge states near EF (Figure 6 (c)).24,36 With large DOS near EF, the screening of the core hole is effective to reduce its attractive potential and the core level becomes shallower. The above arguments on the zigzag edge states also explain the tendency of ΔEb(1s) to increase by increasing the content of CH2 along the zigzag edge. However, for AGNR, the absence of an edge state for a1120 and the presence of that for a1121 produce 5397

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Figure 5. (a) Total density of states (TDOS) for ZGNRs z1m2n, and (b)-(h) projected density of states (PDOS) for the p orbitals of C atom at different position: (b) H2 edge-hydrogenated C (i.e., CH2); (c) H edge-hydrogenated C (i.e., CH) closest to CH2; (d) edge-1 C next to one CH2 and one CH (i.e., CH2CCH); (e) edge-1 C next to two CH (i.e., CHCCH); (f) edge-1 C next to two CH2 (i.e., CH2CCH2); (g) interior C; (h) three inequivalent H edgehydrogenated C1, C2, and C3 in z1521 and their distances to CH2 in the order of C1 < C2 < C3. The zero of energy is set at the Fermi level. The upward and downward arrows indicate the majority and minority spins, respectively.

the opposite tendency of ΔEb(1s) as a function of the CH2 content. 3.3.1.3. edge-1 C. For the edge-1 C in ZGNR, there is a tendency of ΔEb(1s) to decrease by increasing the CH2 content as seen in Figure 4, which is just opposite to the tendency for CH. The reason for this can be understood by the complementary relationship between the standard zigzag edge state (which can be referred to as Fujita state21) and the Klein state.51 The Fujita state appears when a certain length of edge carbons (more than three edge carbons) are monohydrogenated and it has a large amplitude along the edge CH with a very small amplitude at edge1 C. However, for a similar length of dihydrogenated edge carbons CH2, the Klein state appears and has a large amplitude at edge-1 C with a nearly vanishing amplitude at the edge CH2. As for AGNR, a localized edge state appears for a1121. Its amplitude is large at the edge CH, still significant at the edge-1 CCH2 and vanishingly small at the edge-1 CCH (Figure 6 (c)). The relation between local DOS near EF and ΔEb(1s) holds also here. Figure 7 summarizes the above arguments on the correlation between ΔEb(1s) and the local DOS near EF. Related discussion was made on the screening effect in the surface core level shift on Si and Ge(001) surfaces where the unoccupied surface states with localized wave functions play important roles.52 3.3.2. Simulated C K-Edge XAS Spectra. Now we turn to the discussion of the simulated C K-edge XAS spectra of GNRs. Figure 8(a) shows the total spectra of all considered cases of

ZGNRs. The contributions from specific C atoms at the edge and edge-1 sites are shown in Figure 8(b)-(f). In all cases, the total spectrum shows two intense peaks at about 285 and 291.5 eV which originate from the resonances of the π* and σ* states of sp2 hybridized C-C, respectively, as in the cases of infinite graphene sheet and graphene nanocluster (see Figures 1(b) and 2(c)). In this subsection, we will discuss the origins of additional features below the π* peak and between π* and σ* peaks. We checked the dependence of the simulated XAS spectra upon the ribbon width. As the width of the ribbon increases, the population of edge and edge-1 atoms decreases, and hence their contribution to the features characteristic to nanographene in the XAS spectra will be reduced. However, we found that the ribbon-width dependence of the positions of the main peaks in the respective XAS spectra of these edge and edge-1 C atoms is quite small. Therefore, the conclusion based on the following analyses would not be altered even for much wider GNRs. 3.3.2.1. Structures below the π* Peak. A small peak, or a shoulder, can be found below the π* peak in the total spectra. However, the positions of these additional features relative to the π* peak depend on the degree of edge hydrogenation, i.e., the ratio of CH2 to CH. For instance, in z1120, where each edge C is monohydrogenated, a small peak at 2.5 eV below the π* peak appears, arising from a transition from C 1s to the unoccupied edge state localized at CH. When the concentration of CH2 increases up to 33.3%, the position of this small peak shifts 5398

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Figure 6. Total density of states (TDOS) for AGNRs and projected density of states (PDOS) for the p orbitals of C atoms at the edge, edge-1, and interior sites, respectively: (a) a1220; (b) a1022; and (c) a1121. The zero of energy is set at the Fermi level. Here CH2 and CH stand for the H2 and H edgehydrogenated C, respectively. The edge-1 C next to CH (CH2) and the interior C are denoted as CCH (CCH2) and Cinterior, respectively. The upward and downward arrows indicate the majority and minority spins, respectively.

toward the position of the π* peak and becomes a shoulder at about 1.3 eV below the π* peak. In z1121, the concentration of CH2 is 50% and a shoulder can also be found at about 1.6 eV below the π* peak. However, its main contribution comes from the excitation of 1s core electrons of the edge-1 C. When the concentration of CH2 increases further to 66.7% ∼ 100%, the shoulder becomes a small peak again, which shifts downward to 2.0∼2.4 eV below the π* peak. This peak arises from the excitation of 1s to the unoccupied edge states strongly localized at the edge-1 C. Our results suggest that the degree of hydrogenation can significantly affect the position of additional features below the π* peak in the theoretical spectra of ZGNRs. In experimental studies, the assignment of a shoulder (i.e., p1) below the π* peak to the zigzag edge states is made by considering only the initial unoccupied conduction states and neglecting the effect of edge hydrogenation on the 1s core level. Our results suggest that this assignment is incorrect. Quite recently, an experimental work of point-by-point energy-loss near-edge fine structure (ELNES) measurement for graphene

edges was reported and well-defined peaks 2.5 to 3.0 eV deep below the π* peak was actually observed.53 The local structures responsible to these peaks look rather complicated and the relation between this experiment and the present work may not be so straightforward. Anyway, this experiment suggests that the localized states at edges can produce a peak in the absorption spectrum deep below the π* peak, supporting our results on the edge localized states. 3.3.2.2. Structures between π* and σ* Peaks. In the simulated XAS spectra of ZGNRs, additional features, i.e., small peaks, can also be found between the π* and σ* peaks. From the contribution of dihydrogenated C (see Figure 8(b)), we find a sharp peak at 288.0 eV for the ZGNRs with CH2 concentration from 16.7% to 50%. When the CH2 concentration in ZGNRs increases further up to 100%, the position of such a peak shifts slightly downward. From the local DOS of dihydrogenated C, we can infer that this sharp peak is due to the resonance of the σ* states of sp3 hybridized CH2. Monohydrogenated C (see Figure 8(c)), in addition to some features below 285 eV, produces a sharp 5399

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Figure 7. Schematic illustration for the chemical shift (ΔEb(1s), in eV) of 1s core level binding energy (Eb(1s)) of C with different atomic coordination and intensity of localized edge states in GNRs. The insets show the atomic coordinations of specific C atoms. The size of blue-transparent circles indicates the degree of localization of edge states at respective C atom.

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peak at 287 eV and its position is not affected by the degree of edge hydrogenation. From the local DOS of monohydrogenated C, we can ascribe this sharp peak to a transition from the 1s to σ* state of sp2 hybridized CH. Besides the π* peak at 285 eV and σ* peak at 291.5 eV, some of the above features can also be found in the simulated C K-edge XAS spectra of AGNRs, as shown in Figure 9. Two small peaks are found at 2.2 and 2.8 eV below the π* in the total spectra of a1121, in which the CH2 concentration is 50%, and they originate from the transition of 1s to the unoccupied edge states localized at edge-1 and edge C atoms. However, in the total spectra of a1220 and a1022, which correspond to CH2 concentrations of 0% and 100%, respectively, no notable features are found below the π* peak. This is because no edge states appear in these two cases of AGNRs.36 Furthermore, from the contributions of mono- and dihydrogenated C atoms, we find that sharp peaks are produced near 287 and 288 eV, originating from the resonances of the σ* states of sp2 hybridized CH and sp3 hybridized CH2, respectively.

Figure 8. (a) Simulated total spectra of C 1s XAS for ZGNRs z1m2n, Contributions from (b) the H2-terminated edge C (i.e., CH2), (c) the H-terminated edge C (i.e., CH) being closest to CH2, (d) CH with different distance to CH2 in z1521, (e) edge-1 C atom next to one CH2 and one CH (i.e., CH2 CH C ), and (f) edge-1 C atom next two CH2 (i.e., CH2CCH2). The insets show the edge structures of ZGNRs. The positions of C atoms with core hole are indicated by the symbol (circle). 5400

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Figure 9. (a) Simulated total spectra of C 1s XAS for AGNRs a1m2n, Contributions from (b) the edge and edge-1 C atoms in a1220 and a1022, and (c) the edge and edge-1 C atoms in a1121. The insets show the edge structures of AGNRs. The positions of C atoms with core hole are indicated by the symbols (triangle, circle, and square).

3.3.2.3. Possible Origins of the Observed Features in XAS. On the basis of the analyses presented so far, we summarize our explanation about the origins of the observed features in XAS under the condition that hydrogen is the only element for the edge C termination. First, we can state that the feature p1 below the π* peak cannot be ascribed to the zigzag edge states in contrast to the proposal made in refs 26 and 31. Although this assignment seems to be reasonable from the DOS for unoccupied states, the large upward shift (about 1 eV) of 1s core level at the edge CH makes the transition energy too small to be assigned to the shoulder of the π* peak. This is also the case for the Klein state of z1021, where the edge-1 C has the large amplitude of the edge localized states. Similarly, the edge state associated with a1121 will not be the origin for the shoulder below the π* peak. However, our results suggest that the possible origins of the feature p1 may include contributions from (i) the edge CH closest to CH2 (see Figure 8(c)), and (ii) the edge-1 C atoms with CH2 CH C configuration (see Figure 8(e)). The edge state amplitude is small or even no edge states appear at these C atoms in ZGNRs. Hence, the chemical shift for them is small and the contribution from the difference in their chemical bonding becomes dominant. Moreover, our results suggest that the variation in the energy separation between the feature p1 and the π* peak reported in experiments may be due to the degree of edge hydrogenation, i.e., the ratio of CH2 to CH. Second, as for the feature p3 at 287 or 287.5 eV reported in experiments, it arises from the resonance of the σ* states of sp2 hybridized CH, as proposed in ref 26. Finally, feature p4 at 288 eV may originate from the resonance of the σ* states of the sp3 hybridized CH2. 3.3.2.4. Comments on the Stability of Edge Terminations. Stability in equilibrium at low temperatures of various edge terminations by hydrogen was studied as a function of the hydrogen chemical potential.36 In this work, two qualitatively important predictions were made. One is that within a reasonable range of hydrogen partial pressure, stable edge terminations do not have localized edge states. As mentioned earlier, z1120, which has the well-known zigzag edge states is stabilized only at very low hydrogen pressure. The other is that the armchair edge is

more frequently realized than the zigzag edge. Only z1221 (z211 in the notation of ref 36) may exist in a narrow pressure range sandwiched by a1120 and a1021. However, in the actual sample preparation processes, kinematical aspects will also play important roles. For example, it is known that iron nanoparticles in a graphene sheet cut the graphene sheet to produce exclusively zigzag edges.54-56 Similarly, some oxidation processes of graphene and carbon nanotubes will also cut the sheet and produce zigzag edges.57-62 Therefore, we expect that the population of zigzag edges may be much larger than expected from the equilibrium phase diagram. Then, within the reasonable hydrogen partial pressure, z1321 and z1221 may have a greater chance to be realized.

4. CONCLUSIONS We have analyzed the effect of different hydrogen terminations of edge carbons on the C 1s XAS spectra of nanostructured graphenes using DFT electronic structure calculations. One of the targets of this analysis is to clarify the origin of the shoulder (feature p1) below the π* peak (feature p2) observed in experiments. A first conclusion that can be drawn is that the shoulder cannot be ascribed to the states of an infinite graphene sheet. Our results also indicate that the shoulder cannot be ascribed to the edge localized states near the Fermi level because they make the 1s core level shallower due to their efficient screening of the created core hole. The main contribution of p1 may come from the excitation of 1s core electrons at the monohydrogenated edge C atom and the C atom at edge-1 site both being next to the dihydrogenated edge C atom. The variation in energy separation between p1 and p2 in experiments may be caused by the different types of edge hydrogenation of nanographenes. For the features between the π* and σ* peaks, i. e., the p3 at 287.0 or 287.5 eV and p4 at 288 eV reported in experiments, they can be ascribed to the σ* states in the sp2 hybridized CH and sp3 hybridized CH2, respectively. Finally, we focused on the role of edge termination by hydrogen in the XAS spectrum in the present work. The analysis 5401

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’ ASSOCIATED CONTENT

bS

Supporting Information. (i) Detailed computational setup for the ab initio calculations of nanostructured graphene done by the PWSCF code and (ii) the atomic geometries of graphene nanoribbons (GNRs) used in the XAS calculation by the CP2K code and the DOS calculation by the PWSCF code. This material is available free of charge via the Internet at http:// pubs.acs.org

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was performed under Project 08003441-0 at the New Energy and Industrial Technology Development Organization (NEDO). The computation was performed using the supercomputing facilities in the Center for Information Science in JAIST and JAEA Supercomputer Facility. ’ REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (2) Neto, A. H. C.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. Rev. Mod. Phys. 2009, 81, 109. (3) Geim, A.; Novoselov, K. Nat. Mater. 2007, 6, 183. (4) Shang, N. G.; Papakonstantinou, P.; McMullan, M.; Chu, M.; Stamboulis, A.; Potenza, A.; Dhesi, S. S.; Marchetto, H. Adv. Funct. Mater. 2008, 18, 3506. (5) Shao, Y.; Wang, J.; Wu, H.; Liu, J.; Aksay, I.; Lin, Y. Electroanalysis 2010, 22, 1027. (6) Yu, D.; Nagelli, E.; Du, F.; Dai, L. J. Phys. Chem. Lett. 2010, 1, 2165. (7) Ozaki, J.; Tanifuji, S.; Kimura, N.; Furuichi, A.; Oya, A. Carbon 2006, 44, 1324. (8) Ozaki, J.; Anahara, T.; Kimura, N.; Oya, A. Carbon 2006, 44, 3358. (9) Ozaki, J.; Tanifuji, S.; Furuichi, A.; Yabutsuka, K. Electrochim. Acta 2010, 55, 1864. (10) Bashyam, R.; Zelenay, P. Nature 2006, 443, 63. (11) Matter, P. H.; Ozkan, U. S. Catal. Lett. 2006, 109, 115. (12) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Science 2009, 323, 760. (13) Ikeda, T.; Boero, M.; Huang, S.-F.; Terakura, K.; Oshima, M.; Ozaki, J. J. Phys. Chem. C 2008, 112, 14706. (14) Ikeda, T.; Boero, M.; Huang, S.-F.; Terakura, K.; Oshima, M.; Ozaki, J.; Miyata, S. J. Phys. Chem. C 2010, 114, 8933. (15) Niwa, H.; Horiba, K.; Harada, Y.; Oshima, M.; Ikeda, T.; Terakura, K.; Ozaki, J.; Miyata, S. J. Power Sources 2009, 187, 93. (16) Huang, S.-F.; Terakura, K.; Ozaki, T.; Ikeda, T.; Boero, M.; Oshima, M.; Ozaki, J.; Miyata, S. Phys. Rev. B 2009, 80, 235410. (17) Nabae, Y.; Moriya, S.; Matsubayashi, K.; Lyth, S. M.; Malon, M.; Wu, L.; Islam, N. M.; Koshigoe, Y.; Kuroki, S.; Kakimoto, M.; Miyata, S.; Ozaki, J. Carbon 2010, 48, 2613. (18) Shao, Y.; Sui, J.; Yin, G.; Gao, Y. Appl. Catal., B 2008, 79, 89, and references therein. (19) Biddinger, E. J.; Ozkan, U. S. J. Phys. Chem. C 2010, 114, 15306–15314.

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