Spectroscopic Fingerprints of Graphitic, Pyrrolic, Pyridinic and

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C: Physical Processes in Nanomaterials and Nanostructures

Spectroscopic Fingerprints of Graphitic, Pyrrolic, Pyridinic and Chemisorbed Nitrogen in N-Doped Graphene Petr Lazar, Radim Mach, and Michal Otyepka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02163 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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The Journal of Physical Chemistry

Spectroscopic Fingerprints of Graphitic, Pyrrolic, Pyridinic and Chemisorbed Nitrogen in N-doped Graphene Petr Lazar,a Radim Mach,a,b and Michal Otyepkaa,b* a

Regional Centre for Advanced Technologies and Materials, Faculty of Science, Palacký

University Olomouc, Šlechtitelů 27, 771 46 Olomouc, Czech Republic b

Department of Physical Chemistry, Faculty of Science, Palacký University Olomouc, 17.

listopadu 1192/12, 771 46 Olomouc, Czech Republic

ACS Paragon Plus Environment

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ABSTRACT: Doping and functionalization of graphene significantly modulate its properties and extend its application potential. Detailed and accurate chemical characterization of the final material is critical for understanding its properties and reliable design of new graphene derivatives. Spectroscopic methods commonly used for this purpose include Raman, Fourier Transform InfraRed (FTIR), and X-ray photoelectron spectroscopy (XPS). However, the correct interpretation of observed bands is sometimes hampered by ambiguities when assigning measured binding energies or IR/Raman peaks to specific atomic structures. N-doped graphene has many potential applications but can contain several different chemical forms of nitrogen whose relative abundance strongly affects the doped material’s properties. We present clear spectroscopic fingerprints of the various chemical forms of nitrogen that can occur in N-doped/functionalized graphene to facilitate the identification and quantification of the different forms of N present in experimentally prepared samples. The calculated XPS binding energies of the N 1s state for graphitic, pyrrolic, pyridinic, and chemisorbed nitrogen in N-doped graphene are 401.5, 399.7, 397.9, and 396.6 eV, respectively, and hydrogenation of pyridinic N shifts its peak to 400.5 eV.


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INTRODUCTION Since graphene’s discovery in 2004,1 many researchers have sought to controllably modify its properties to improve its functionality or enable new applications. Many different kinds of modification have been reported, with substitutional heteroatom doping and covalent functionalization being among the most prominent.2 Substitutional doping of graphene with atoms such as boron or nitrogen induces significant changes in its electronic structure and creates a band gap, which is crucial for electronics applications.3–6 Covalent functionalization of graphene, on the other hand, involves grafting organic groups onto the graphene surface.2,7–9 In this way, the unique properties of organic functional groups can be combined with those of graphene, such as its large surface area and conductivity. Covalently grafting organic molecules onto the graphene surface perturbs the symmetry and sp2 character of the graphene lattice, giving another way to widen and possibly control its band gap.10 Despite recent progress, probing the exact structural arrangement of grafted functional groups and their attachment to the graphene lattice is one of the most challenging aspects of graphene’s covalent chemistry. The placement of grafted functional groups can profoundly affect the properties of functionalized graphene. This is clearly illustrated by the example of nitrogen, which is one of the most widely used and studied dopant heteroatoms. Nitrogen is adjacent to carbon in the periodic table, so it has a similar atomic size but one more valence electron. Nitrogen doping can imprint active centers into graphene, yielding materials suitable for electrochemical (energy storage and sensors)11–13 and spintronic applications.7–11 However, nitrogen can adopt three distinct bonding configurations in the graphene lattice (pyridinic, pyrrolic, and graphitic), and can also exist in adsorbed forms such as chemisorbed N and NH 2 groups (Figure 1). Moreover, any combination of these forms could in principle coexist in any given sample.14 The different forms


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of nitrogen have markedly different properties. Graphitic nitrogen centers preserve the graphene lattice’s symmetry and electron conjugation. Upon doping by graphitic N, the Dirac point in the graphene band structure shifts below the Fermi level3,15 and the ferromagnetic spin-ordering may appear.16 Substitutional defects can also either preserve the Dirac cones or open a band gap, depending on the superlattice symmetry.3 Pyrrolic nitrogen centers occur in sp2-hybridized fivemembered rings. Pyridinic nitrogen centers have lone pairs localized in p x -like non-bonding orbitals that are not part of the aromatic system. Consequently, they are basic and can reversibly bind protons. DFT calculation revealed p-type doping for pyridinic and pyrrolic nitrogen in graphene, but the charge transfer per N atom is much smaller than in the case of graphitic N.17,18 The differing reactivity of these different forms of N is illustrated by the studies of Guo et al.,19,20 who found that an oxygen reduction reaction (ORR) catalyzed by graphite-based model catalysts with well-defined π conjugation and well-controlled doping of N species occurred exclusively at active sites featuring pyridinic N centers. More recently, Blonski et al. reported that N-doped graphene derivatives undergo a transition to a ferromagnetic state at 69 K when the level of nitrogen doping exceeds five percent.21 This magnetic effect was attributed to graphitic nitrogen; pyridinic and chemisorbed nitrogen contributed much less to the ferromagnetic ground state. Ito et al. reported that pyrrolic nitrogen even reduced the spin population and reduced magnetism in graphene.22 These results show that the type of N-doping can significantly affect the electronic, magnetic, and catalytic properties of N-doped graphene derivatives. In principle, this could be exploited to tune the properties of nitrogen-doped graphene derivatives by selectively introducing nitrogen centers with specific atomic configurations. However, nitrogen doping is usually achieved via rather harsh chemical procedures such as chemical vapor deposition of nitrogen-containing molecules,23 high temperature NH 3 treatment of GO24 or graphene,19 or solvothermal reactions of


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GO with N-containing molecules followed by high temperature annealing.25,26 It is very challenging to control doping under such conditions; typically, many different kinds of nitrogen center are introduced into the graphene lattice simultaneously. Individual chemical forms of nitrogen can be distinguished by microscopic and spectroscopic techniques. The surface of an Ndoped graphene sample can be analyzed directly by scanning probe microscopy, which can distinguish individual surface atoms or vibrational properties of adsorbed molecules.27–29 However, such techniques require specific operating conditions, which limits their practicality for routine analysis of chemically prepared N-doped graphenes. Experimental analyses of such materials therefore rely primarily on spectroscopic techniques, i.e., X-ray photoemission spectroscopy (XPS), infra-red (IR) spectroscopy, and Raman spectroscopy. The drawback of spectroscopic methods is that assigning measured binding energies or IR/Raman peaks to specific atomic structures is rarely straightforward, especially in chemically complex materials. Sparse previous studies focused mainly on spectroscopy of graphitic nitrogen,30 and the binding energy shifts for nitrogen-containing graphene in specific structural environments.31 Consequently, there is a clear need for reliable indicators that can be used to distinguish different nitrogen configurations.32 Here, we use quantum mechanical density functional theory (DFT) calculations to identify spectroscopic features associated with specific configurations of nitrogen in N-doped graphene. Specifically, we model a set of graphene derivatives bearing graphitic, pyridinic, pyrrolic, and chemisorbed nitrogen centers (all of which bond to the graphene lattice in different ways), simulate their IR and Raman spectra, and calculate their XPS binding energies. Our calculations suggest that XPS is the best spectroscopic method for distinguishing between these different forms of nitrogen. We also present N 1s binding energies for physisorbed N 2 and NO molecules, graphene functionalized with amine and nitrile groups, hydrogenated (quaternary)


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pyridinic nitrogen, and oxidized pyridinic nitrogen. Physisorbed N 2 is responsible for a broad N 1s peak at ~405 eV, while amine and nitrile groups generate peaks that could be mistakenly assigned to pyrrolic N. Hydrogenation of pyridinic N leads to an appearance of a peak at 400.5 eV, between the signals of graphitic and pyridinic N. IR and Raman spectroscopy are less suitable for distinguishing between different forms of nitrogen in N-doped graphene because the nitrogenspecific vibrations of these materials are coupled to new vibrations of carbon atoms originating from symmetry changes induced by doping. Nevertheless, our calculations reveal the peak at ~1340 cm–1 to be the IR fingerprint of graphitic nitrogen. This peak also appears in Raman spectra, but coincides with the well-known D band of graphene.

METHODS All calculations were performed using the projector-augmented wave method as implemented in the Vienna ab-initio simulation package (VASP).33,34 The energy cutoff for the plane-wave expansion was set to 400 eV. Total energies and forces were calculated using the generalized gradient approximation with the PBE parametrization. The graphene sheet was modeled by a 4×4 supercell (32 carbon atoms). For each initial orientation, a local energy minimum was found by force-relaxation. The periodically repeated sheets were separated by at least 16 Å of vacuum, and a 3×3×1 k-point grid was used to sample the Brillouin zone. The elements of the force-constant matrix for the calculation of vibrational modes were determined using density-functional perturbation theory. The IR intensity of each mode was calculated using the matrix of Born effective charges, which describes the change of atoms' polarizabilities with respect to an external electric field.35 Born effective charges were calculated using VASP. Each mode’s off-resonance Raman activity was calculated by evaluating the derivative of the polarizability (or macroscopic


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dielectric tensor) with respect to the pertinent mode coordinate using the Raman-sc package. The calculated line spectra were broadened using a Lorentzian resolution function with a width of 10 cm−1. When simulating photoemission, core-level binding energies can be obtained using either initial or final state methods. Initial state methods only consider the energy level of the core electron. Conversely, final state methods explicitly include a core hole in the calculation, and the system’s electronic structure is relaxed in its presence. We calculated binding energies using the final state method as the total difference in energy between a system with a core hole and the corresponding ground state. These calculations were performed using the FHI-aims code, which is an all-electron full-potential code that treats core electrons explicitly and at the same level as the valence electrons.36 The final state method was validated and calibrated by calculating the C 1s binding energy of pristine graphene, which was computed to be 284.3 eV using the tight basis set in the FHI-aims; the experimental reference value of the C 1s binding energy of graphite is 284.42 eV.37,38 Experimentally determined values for graphene depend on the substrate used and range from 283.97 eV for graphene on Pt(111) to 284.8 eV for graphene on SiC (see ref 39 and references therein). For N-doped graphene, the tight basis set of FHI-aims yielded systematically too low binding energies by 1 eV, worsening the agreement with experimental values, so the light basis was used in all calculations of the N 1s energy.

RESULTS AND DISCUSSION Infra-red Spectroscopy. Ideal pristine graphene has two equivalent atoms per unit cell, giving six normal modes for the zone center. In pristine defect-free graphene, the ring stretching


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vibrations are IR-inactive,40 because of the system’s symmetry, so graphene is completely IRsilent. This degeneracy should be lifted in the presence of defects; accordingly, several vibrational modes become IR-active in N-doped graphene (Figure 1).

Figure 1. Calculated IR spectra of graphene doped with graphitic, pyridinic, pyrrolic, and chemisorbed N (the corresponding structures are shown on the right). The spectrum calculated for graphene with a chemisorbed epoxy oxygen center (O bound at a bridge site between two carbon atoms) is plotted with a red dashed line in the lower panel for comparative purposes. The inset shows the full spectrum for the pyrrolic N system, including the high wavenumber stretch mode of the N-H bond at 3514 cm–1. Pyrrolic and pyridinic N reside in vacancy defects within graphene lattice, so their IR spectra are intermixed with IR activity due to the vacancy’s presence. Pyrrolic nitrogen could in principle be distinguished by the absorption peak at 3514 cm–1 due to the N-H stretch vibration (inset of Figure 1). The experimental wavenumber of the fundamental N-H stretch is 3530.8 cm–1 for


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isolated pyrrole.41,42 Chemisorbed N forms a bridge-like structure on the graphene surface, i.e., it is bonded between two adjacent C atoms, which become sp3 hybridized. The resulting vibrational modes are primarily due to the sp3 hybridization of these carbon atoms and the associated disruption of the graphene lattice’s symmetry. For comparative purposes, we also predicted the IR spectrum of the analogous system with an epoxy oxygen, which bonds in the same manner as chemisorbed N (forming a C-O-C bridge). Both spectra feature strong bands at ~1450 cm–1 and ~1100 cm–1. The identification of chemisorbed nitrogen is therefore hampered by its spectral similarity to epoxy oxygen, which is often present in graphene, especially in samples prepared by reducing graphene oxide. Graphitic N preserves the graphene sheet’s sp2 character because N has roughly the same atomic radius as C. The main feature of the IR spectrum of graphitic N is an intense peak at 1330 cm–1. Because of the periodic boundary conditions used in the calculation, the nitrogen defect creates a superlattice whose symmetry depends on the defect’s placement. A single N dopant embedded in a honeycomb-shaped superlattice creates a hexagonal superlattice with the point symmetry reduced to D 3h . When two N atoms are placed into opposite graphene sublattices, the point symmetry is lowered to C 2v . It should be noted that two substitutional N atoms can be positioned to form a honeycomb superlattice that preserves the D 6h point group symmetry of pristine graphene,3 but this high-symmetry superlattice is rather artificial. Figure 2 shows how varying the placement of graphitic N centers affects the IR spectrum of N-doped graphene.


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Figure 2. Calculated IR spectra of graphene doped with graphitic N: a) the displacement vectors of the two almost degenerate vibrational modes responsible for the peak at 1331 cm-1, b) IR spectra of N-doped graphene having various placements of the N centers (which affects the symmetry of the resulting superlattice). The main feature of the IR spectrum of graphitic N, clearly visible in all symmetry configurations, is high IR adsorption between 1315 cm–1 and 1350 cm–1. This adsorption peak is unique to graphitic nitrogen and thus distinguishes it from pyrrolic, pyridinic, and chemisorbed nitrogen. The vibrational mode associated with the peak is rather complex (Figure 2a); the motion of the atoms in the ring containing the N dopant resembles the B 2 symmetric waggling mode of pyridine, but the surrounding carbon atoms exhibit various stretching movements. A similar mode exists in the graphene lattice containing two N atoms, but it is shifted to a slightly higher wavenumber.


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The honeycomb and hexagonal superlattices may not accurately represent graphitic N in experimental samples, which are likely to contain randomly dispersed nitrogen atoms. Random alloys are an obstacle for periodic calculations, but as a model of a quasi-random distribution of graphitic N, we performed calculations using a 5×5 graphene supercell with three N defects positioned to generate a very low point symmetry (C 1h ). The resulting spectrum is shown in Figure 2. The peak at 1318 cm–1 remains visible, albeit less dominant, and would presumably be distinguishable in experimental spectra. The experimental evidence for this peak is complicated because there are many potentially overlapping vibrational modes in the 1100–1500 cm–1 region. A new sharp peak at 1385 cm–1, attributed to C-N vibration, was observed in the IR spectra of Ndoped graphene foams produced from freeze-dried graphene oxide.43 A sharp peak around 1350 cm–1 can also be distinguished in the spectrum of N-doped graphene prepared by pyrolysis of graphene oxide and urea.44 Additionally, strong peaks at 1250 and 1372 cm–1 were observed in the IR spectra of N-doped multiwalled carbon nanotubes.45 The authors assumed that nitrogen chemisorbed on the nanotubes’ walls, and therefore attributed these peaks to C-N and N-CH 3 vibrations, respectively. However, our calculations suggest the latter peak actually arises from graphitic N in the nanotubes; chemisorbed N seems unlikely based on its calculated spectrum. Interestingly, the calculated spectra contain no peaks due to skeletal ring stretching vibrations, which are commonly invoked to explain a peak at around 1600 cm–1 in experimental studies.46 Vibrational modes are typically assigned using spectroscopic tables based on the vibrations of small organic molecules.47 However, the IR activity of two- and three-dimensional periodic structures differs in many ways from that of small aromatic molecules. Ideal single-layer graphene has no IR-active modes due to its symmetry.48 Graphite crystals have two IR-active modes: E 1u at 1587 cm−1 and A 2u at 868 cm−1.48 Because of its frequency, it may be tempting to relate the E 1u


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mode to two characteristic bands in the 1600 cm−1 region of the IR spectra of phenyl compounds (in benzene, these displacements cause no change in dipole moment, so the mode is not IR-active). These bands correspond to C=C stretching vibrations, and can be regarded as a pair of water-like molecules undergoing symmetric stretching in phase with one-another. However, the E 1u mode in graphite has a different origin: it is associated with an anti-phase vibration of the two unit cell atoms in adjacent AB stacked planes. That is, it is a consequence of graphite’s three-dimensional structure.

Figure 3. Calculated IR spectra of graphite (lower panel), graphite doped with graphitic nitrogen in one plane (middle panel), and double-layer graphene doped with graphitic N (upper panel). The calculated IR spectrum of graphite correctly reproduces both the experimentally observed IR-active modes (Figure 3); the E 1u mode is at 1580 cm−1 and the A 2u mode is at 867 cm−1, in good agreement with the experimental frequencies.48 The E 1u mode is much more intense than the A 2u mode. We note that the calculated spectrum of pristine graphene (not shown) contains no IR-active modes, in accordance with the symmetry arguments outlined above. Doping graphite with graphitic nitrogen shifts the E 1u peak to a higher frequency (~1600 cm–1) and leads to the


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appearance of a new mode at 1300 cm–1 that is also observed in the spectrum of graphene doped with graphitic nitrogen (Figure 3). Raman Spectroscopy. The first-order optical spectrum of graphene has only one Raman active mode: the E 2g mode, at 1587 cm–1.48,49 This mode, which is often called the G band, is a doubly degenerate Raman-active optical vibration in which the carbon atoms move in the graphene plane. Because of the peculiar character of graphene’s electronic structure, the screening of the atomic vibrations associated with the Γ and K points of the Brillouin zone changes abruptly, leading to an anomalous softening of the phonon dispersion at those points that is known as a Kohn anomaly.50 The calculated Raman spectrum of pristine graphene (shown in the upper panel of Figure 4) features a single peak at 1561 cm–1, in accordance with the above symmetry arguments. The presence of graphitic nitrogen causes the appearance of another intense peak at 1339 cm–1. Note that a similar peak due to graphitic N also appears in the IR spectrum. This peak coincides with the so-called D band of graphene. The D band is usually described as being disorder-induced; it is attributed to structural defects and other disordered structures on the graphitic plane. It was once assigned to the breathing modes of sp2 atoms in rings based on a “molecular” conceptualization of carbon materials. However, this interpretation was challenged and debated for several decades;51 the current understanding is that the D peak in defective graphene is due to longitudinal optical phonons around the Dirac point, is active by double resonance which connects K and K’ points in the first Brillouin zone of graphene,52 and is strongly dispersive with laser excitation energy due to the Kohn anomaly at the Dirac point.50 Our calculations show that the D mode can also appear in the non-resonant Raman spectrum of doped graphene. It is plausible that double resonance might greatly enhance its Raman intensity. However, we did not investigate this possibility further


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because while second-order processes can be modelled using first-principles methods, the computational cost of doing so is tremendous, even for simple cubic solids such as silicon.53

Figure 4. Calculated off-resonance Raman spectra of N-doped graphene with graphitic, pyrrolic, pyridinic, and chemisorbed N. Pyrrolic and pyridinic nitrogen appear to be less readily detected by Raman scattering. Pyrrolic nitrogen centers give rise to several new peaks between 1500 and 1600 cm–1. These peaks are likely to convolute with the G band, making them very difficult to distinguish in Raman spectra. The spectrum of pyridinic N resembles that of pyrrolic N, with one notable exception: the G band almost disappears due to strong disruption of graphene lattice. The pyrrolic, pyridinic, and chemisorbed nitrogen systems also all exhibit a peak at ~1610 cm–1. The spectra of lower-quality graphene samples feature a peak at 1620 cm–1 known as the D’ band,54 which originates from an intra-valley double-resonant scattering process that activates phonons with a small q.51 However,


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because the predicted off-resonant spectrum of graphitic N lacks a D’ band, we assume that the 1610 cm–1 peak observed in our spectra has a different origin and its proximity to the D’ band is mere coincidence.

Figure 5. a) High-resolution N 1s XPS pattern of substitutionally N-doped graphene containing 5.1 at.% of nitrogen (see ref 21 for details of the GN 0.051 sample. Reprinted from ref 21, copyright 2017 American Chemical Society) with the original empirical peak assignments (above peaks) and the new assignments based on theoretical calculations (blue arrows, for details see Table 1). b) Structures of additional forms of nitrogen considered in the XPS calculations: a physisorbed N 2 molecule, an adsorbed nitrile group -C≡N (Gr-CN), and an adsorbed amino group -NH 2 (Gr-NH 2 ). XPS Spectroscopy. The calculated binding energies of the N 1s state differ strongly enough for XPS to be used to identify different forms of nitrogen (Table 1). Figure 5 compares calculated binding energies to an experimental spectrum, showing that the predicted and observed graphitic and pyridinic peaks are in excellent agreement. Our calculations also clearly show that a broad peak at ~405 eV, originally assigned to chemisorbed N/N 2 , originates from physisorbed molecular N 2 . Chemisorbed N would cause photoemission at 396.6 eV, which is not observed in the


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experimental spectrum. Reported binding energies for N substitutions (graphitic N) based on experimental studies span a wide range, from 400.2 eV to 401.8 eV.55 Such spreads are rather unusual and may have multiple causes, including inaccuracies in measurements and spectral deconvolution, the assignment of measured binding energies to incorrect atomic configurations, or hybridization between the valence orbitals of N and the atoms of an underlying substrate. To address the latter possibility, Usachov et al. performed elaborate measurements on N-doped graphene in which the substrate’s influence was eliminated by intercalation, yielding a binding energy of 401.3 eV for the substitutional configuration, in excellent agreement with our result.56 Our additional calculations also suggest that graphitic nitrogen centers shift the 1s binding energy of adjacent carbon centers from 284.3 eV to 286.3 eV. This is consistent with earlier reports that ascribed a carbon component centered at 285.9 eV to carbons bonded to nitrogen atoms in Ndoped graphene formed by NH 3 -treatment.21

Table 1. XPS binding energies of various forms of nitrogen in graphene, calculated as the energy difference between the final state (with a core-hole due to the excitation of a 1s electron) and the initial state. Type of N dopant

N 1s Binding energy (eV)

Graphitic N

401.5

Pyrrolic N

399.7

Pyridinic N

397.9

Pyridinic N-H

400.5

Pyridinic N-O

401.8

Chemisorbed N

396.6


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Physisorbed N 2

404.7

-NH 2

399.1

-C≡N

399.3

Figure 6. Pyridinic configuration of nitrogen in the vicinity of a vacancy; a) three N atoms surrounding a vacancy, b) four N atoms surrounding a divacancy. Pyridinic nitrogen is commonly associated with a peak around 398.3 eV,55,57 which is slightly higher than our calculated binding energy of 397.9 eV. A possible explanation of this discrepancy is that pyridinic nitrogen can be located either next to a vacancy, as in our calculation, or at the edge of the graphene lattice. The formation energy of a single pyridinic vacancy is very high,58 so pyridinic nitrogens are most likely to occur at the edges of graphene flake, which may shift their binding energies. In nitrogen-doped single-walled carbon nanotubes, which lack edge sites, a pronounced peak at 397.9 eV was observed,59 in excellent agreement with our calculation. Pyridinic N can be hydrogenated or oxidized, i.e., nitrogen can be terminated by a H or O atom, which increases the N 1s binding energy to 400.5 eV or 401.8 eV, respectively. The increase of the binding energy upon hydrogenation would lead to an appearance of a peak between signals of graphitic and pyridinic N. Späth et al.60 performed hydrogenation of N-doped graphene experimentally and observed that the pyridinic peak was decreased and the region between the graphitic and pyridinic peak was filled upon exposure to atomic hydrogen, in agreement with our theoretical result.


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Pyridinic configurations may also exist with three N atoms surrounding a vacancy, or possibly even four N around a divacancy.61 For the sake of completeness, we also considered these configurations (Figure 6). The binding energies for three pyridinic nitrogens surrounding a vacancy and four pyridinic nitrogens surrounding a divacancy are 398.3 eV and 398.2 eV, respectively, indicating that the N 1s binding energy is rather insensitive to the specific position of the pyridinic nitrogen center. Li et al. showed that additional information might be obtained from the N 1s near-edge X-ray absorption fine structure (NEXAFS) spectrum, because the NEXAFS spectra of graphene doped by pyridinic N were sensitive to the doping concentration of N in the π* region.62 The calculated binding energy of pyrrolic nitrogen was 399.7 eV. Liu et al. synthesized nitrogendoped single-walled carbon nanotubes on silicon and quartz substrates and observed a secondary peak at 399.8 eV, in addition to the peak from graphitic nitrogen at 401.8 eV.63 They attributed this secondary peak to pyridinic N, but our results suggest that it is more likely to have originated from pyrrolic nitrogen. It should be noted that the identification of pyrrolic nitrogen may be complicated by the presence of other nitrogenous species. For example, amine and cyano groups bonded to graphene have binding energies of 399.1 eV (Table 1) and 399.3 eV, respectively. It is therefore impossible to conclusively determine a sample’s atomic structure based on XPS alone when these groups are, or may be, present.

CONCLUSIONS The main feature of the IR spectrum of graphitic N is the intense peak at ~1340 cm–1, which can be considered a fingerprint of graphitic N because it is not present in the IR spectra of other nitrogen forms. The vibrational mode associated with this peak is rather complex; the motion of


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the atoms in the ring containing the N dopant resembles the B 2 symmetric waggling mode in pyridine, but the surrounding carbon atoms exhibit various stretching movements. Interestingly, there is no peak around 1600 cm–1 in the spectra of graphene containing graphitic N, which has been ascribed to skeletal ring stretching vibrations in experimental studies. Ideal single-layer graphene has no IR-active modes due to its symmetry, and doping with graphitic N is insufficient to make the ring stretching vibration IR-active. This peak should instead be attributed to the presence of few-layer graphene. Pyrrolic and pyridinic N centers reside in vacancy defects within the graphene lattice, so their IR spectra are intermixed with IR activity due to the vacancy. The calculated Raman spectrum of pristine graphene features a single peak at 1561 cm–1, in accordance with the symmetry of ideal pristine graphene. The presence of graphitic nitrogen causes the appearance of an intense peak at 1339 cm–1, which is attributed to the disorder-induced D band of graphene. This peak is also present in the IR spectrum of graphene doped with graphitic N. The IR spectra of pyrrolic, pyridinic, and chemisorbed nitrogen all feature a peak at ~1610 cm–1 that coincides with the D’ band appearing at 1620 cm–1 in the spectra of lower quality graphene samples.54 XPS seems to be the most powerful spectroscopic method for distinguishing between different kinds of N-doping in graphene because different forms of nitrogen have markedly different binding energies: the calculated XPS binding energies of the N 1s state for graphitic, pyrrolic, pyridinic, and chemisorbed nitrogen in N-doped graphene are 401.5, 399.7, 397.9, and 396.6 eV, respectively. Hydrogenation of pyridinic N would lead to an appearance of a peak at 400.5 eV, between the signals of graphitic and pyridinic N. Physisorbed N 2 molecules give rise to an additional peak at 404.7 eV, which clarifies some earlier experimental observations. Experimentally determined N 1s binding energies for graphitic N range from 400.2 eV to 401.8


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eV.55 This range is so wide that some of these measurements likely include significant experimental inaccuracies or represent mis-assigned binding energies for pyrrolic nitrogen. When graphitic nitrogen is present in the lattice, the C 1s state’s binding energy shifts to 286.3 eV for carbon atoms bonded to nitrogen. Our results show that precise XPS and IR measurements can identify most forms of nitrogen that may occur in graphene, but experimental data must be analyzed in conjunction with computational results, which can effectively support correct assignment of observed spectroscopic features to corresponding structural motives especially in new derivatives.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M.O.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Operational Programme Research, Development and Education – European Regional Development Fund via the Ministry of

Education,

Youth

and

Sports

of

the

Czech

Republic

(grant

no.

CZ.02.1.01/0.0/0.0/16_019/0000754), an internal grant from Palacký University Olomouc (IGA_PrF_2019_031), and funding via the European Union’s Horizon 2020 research and innovation program to M. O. under grant agreement no. 683024 (ERC-CoG).


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Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1)

Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.;

Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666–669. (2)

Georgakilas, V.; Otyepka, M.; Bourlinos, A. B.; Chandra, V.; Kim, N.; Kemp, K. C.;

Hobza, P.; Zboril, R.; Kim, K. S. Functionalization of Graphene: Covalent and Non-Covalent Approaches, Derivatives and Applications. Chem. Rev. 2012, 112, 6156–6214. (3)

Casolo, S.; Martinazzo, R.; Tantardini, G. F. Band Engineering in Graphene with

Superlattices of Substitutional Defects. J. Phys. Chem. C 2011, 115, 3250–3256. (4)

Kwon, O. S.; Park, S. J.; Hong, J.-Y.; Han, A. R.; Lee, J. S.; Lee, J. S.; Oh, J. H.; Jang, J.

Flexible FET-Type VEGF Aptasensor Based on Nitrogen-Doped Graphene Converted from Conducting Polymer. ACS Nano 2012, 6, 1486–1493. (5)

Lazar, P.; Zboril, R.; Pumera, M.; Otyepka, M. Chemical Nature of Boron and Nitrogen

Dopant Atoms in Graphene Strongly Influences Its Electronic Properties. Phys. Chem. Chem. Phys. 2014, 16, 14231–14235. (6)

Wang, X.; Li, X.; Zhang, L.; Yoon, Y.; Weber, P. K.; Wang, H.; Guo, J.; Dai, H. N-Doping

of Graphene Through Electrothermal Reactions with Ammonia. Science 2009, 324, 768–771. (7)

Sturala, J.; Luxa, J.; Pumera, M.; Sofer, Z. Chemistry of Graphene Derivatives: Synthesis,

Applications, and Perspectives. Chem. Eur. J. 2017, 24, 5992–6006.


21


(8)

Page 22 of 30

Chronopoulos, D. D.; Bakandritsos, A.; Lazar, P.; Pykal, M.; Cepe, K.; Zboril, R.; Otyepka,

M. High-yield Alkylation and Arylation of Graphene via Grignard Reaction with Fluorographene. Chem. Mater. 2017, 29, 926–930. (9)

Lazar, P.; Chua, C. K.; Hola, K.; Zboril, R.; Otyepka, M.; Pumera, M. Dichlorocarbene-

Functionalized Fluorographene: Synthesis and Reaction Mechanism. Small 2015, 11, 3790–3796. (10) Englert, J. M.; Dotzer, C.; Yang, G.; Schmid, M.; Papp, C.; Gottfried, J. M.; Steinrück, H.P.; Spiecker, E.; Hauke, F.; Hirsch, A. Covalent Bulk Functionalization of Graphene. Nat. Chem. 2011, 3, 279–286. (11) Wang, H.; Maiyalagan, T.; Wang, X. Review on Recent Progress in Nitrogen-Doped Graphene: Synthesis, Characterization, and Its Potential Applications. ACS Catal. 2012, 2, 781– 794. (12) Reddy, A. L. M.; Srivastava, A.; Gowda, S. R.; Gullapalli, H.; Dubey, M.; Ajayan, P. M. Synthesis of Nitrogen-Doped Graphene Films for Lithium Battery Application. ACS Nano 2010, 4, 6337–6342. (13) Chen, L.-F.; Zhang, X.-D.; Liang, H.-W.; Kong, M.; Guan, Q.-F.; Chen, P.; Wu, Z.-Y.; Yu, S.-H. Synthesis of Nitrogen-Doped Porous Carbon Nanofibers as an Efficient Electrode Material for Supercapacitors. ACS Nano 2012, 6, 7092–7102. (14) Tian, L. L.; Wei, X. Y.; Zhuang, Q. C.; Jiang, C. H.; Wu, C.; Ma, G. Y.; Zhao, X.; Zong, Z. M.; Sun, S. G. Bottom-up Synthesis of Nitrogen-Doped Graphene Sheets for Ultrafast Lithium Storage. Nanoscale 2014, 6, 6075–6083.


22

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15) Mukherjee, S.; Kaloni, T. P. Electronic Properties of Boron- and Nitrogen-Doped Graphene: A First Principles Study. J. Nanopart. Res. 2012, 14 (8). (16) Park, H.; Wadehra, A.; Wilkins, J. W.; Neto, A. H. C. Spin-polarized Electronic Current Induced by Sublattice Engineering of Graphene Sheets with Boron/Nitrogen. Phys. Rev. B 2013, 87, 085441. (17) Jalili, S.; Vaziri, R. Study of the Electronic Properties of Li-intercalated Nitrogen Doped Graphite. Mol. Phys. 2011, 109, 687–694. (18) Wang, C. D.; Zhou, Y. A.; He, L. F.; Ng, T. W.; Hong, G.; Wu, Q. H.; Gao, F.; Lee, C. S.; Zhang, W. J. In Situ Nitrogen-Doped Graphene Grown from Polydimethylsiloxane by Plasma Enhanced Chemical Vapor Deposition. Nanoscale 2013, 5, 600–605. (19) Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active Sites of Nitrogen-Doped Carbon Materials for Oxygen Reduction Reaction Clarified Using Model Catalysts. Science 2016, 351, 361–365. (20) Boukhvalov, D. W.; Son, Y. W. Oxygen Reduction Reactions on Pure and Nitrogen-Doped Graphene: A First-principles Modeling. Nanoscale 2012, 4, 417–420. (21) Błoński, P.; Tuček, J.; Sofer, Z.; Mazánek, V.; Petr, M.; Pumera, M.; Otyepka, M.; Zbořil, R. Doping with Graphitic Nitrogen Triggers Ferromagnetism in Graphene. J. Am. Chem. Soc. 2017, 139, 3171–3180. (22) Ito, Y.; Christodoulou, C.; Nardi, M. V.; Koch, N.; Kläui, M.; Sachdev, H.; Müllen, K. Tuning the Magnetic Properties of Carbon by Nitrogen Doping of Its Graphene Domains. J. Am. Chem. Soc. 2015, 137, 7678–7685.


23


Page 24 of 30

(23) Ito, Y.; Christodoulou, C.; Nardi, M. V.; Koch, N.; Sachdev, H.; Müllen, K. Chemical Vapor Deposition of N-Doped Graphene and Carbon Films: The Role of Precursors and Gas Phase. ACS Nano 2014, 8, 3337–3346. (24) Wang, L.; Sofer, Z.; Luxa, J.; Pumera, M. Nitrogen Doped Graphene: Influence of Precursors and Conditions of the Synthesis. J. Mater. Chem. C 2014, 2, 2887–2893. (25) Sui, Z.-Y.; Wang, C.; Yang, Q.-S.; Shu, K.; Liu, Y.-W.; Han, B.-H.; Wallace, G. G. A Highly Nitrogen-Doped Porous Graphene - an Anode Material for Lithium Ion Batteries. J. Mater. Chem. A 2015, 3, 18229–18237. (26) Chang, D. W.; Lee, E. K.; Park, E. Y.; Yu, H.; Choi, H.-J.; Jeon, I.-Y.; Sohn, G.-J.; Shin, D.; Park, N.; Oh, J. H.; et al. Nitrogen-Doped Graphene Nanoplatelets from Simple Solution Edgefunctionalization for N-type Field-effect Transistors. J. Am. Chem. Soc. 2013, 135, 8981–8988. (27) Sugimoto, Y.; Pou, P.; Abe, M.; Jelinek, P.; Pérez, R.; Morita, S.; Custance, Ó. Chemical Identification of Individual Surface Atoms by Atomic Force Microscopy. Nature 2007, 446, 64– 67. (28) Stipe, B. C.; Rezaei, M. A.; Ho, W. Single-Molecule Vibrational Spectroscopy and Microscopy. Science 1998, 280, 1732–1735. (29) Iwata, K.; Yamazaki, S.; Mutombo, P.; Hapala, P.; Ondracek, M.; Jelinek, P.; Sugimoto, Y. Chemical Structure Imaging of a Single Molecule by Atomic Force Microscopy at Room Temperature. Nat. Commun. 2015, 6, 7766.


24

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30) Kabir, S.; Artyushkova, K.; Serov, A.; Kiefer, B.; Atanassov, P. Binding Energy Shifts for Nitrogen-containing Graphene-based Electrocatalysts – Experiments and DFT Calculations. Surf. Interface Anal. 2016, 48, 293-300. (31) Artyushkova, K.; Kiefer, B.; Halevi, B.; Knop-Gericke, A.; Schlogl, R.; Atanassov, P. Density Functional Theory Calculations of XPS Binding Energy Shift for Nitrogen-containing graphene-like Structures. Chem. Commun. 2013, 49, 2539-2541. (32) Hellgren, N.; Haasch, R. T.; Schmidt, S.; Hultman, L.; Petrov, I. Interpretation of X-ray Photoelectron Spectra of Carbon-nitride Thin Films: New Insights from in Situ XPS. Carbon 2016, 108, 242-252. (33) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953–17979. (34) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758–1775. (35) Karhánek, D.; Bučko, T.; Hafner, J. A Density Functional Study of the Adsorption of Methane-thiol on the (111) Surfaces of the Ni-group Metals: II. Vibrational Spectroscopy. J. Phys.: Condens. Matter 2010, 22, 265006. (36) Blum, V.; Gehrke, R.; Hanke, F.; Havu, P.; Havu, V.; Ren, X.; Reuter, K.; Scheffler, M. Ab Initio Molecular Simulations with Numeric Atom-centered Orbitals. Comput. Phys. Commun. 2009, 180, 2175–2196. (37) Prince, K. C.; Ulrych, I.; Peloi, M.; Ressel, B.; Chab, V.; Crotti, C.; Comicioli, C. CoreLevel Photoemission from Graphite. Phys. Rev. B 2000, 62, 6866–6868.


25


Page 26 of 30

(38) Speranza, G.; Minati, L.; Anderle, M. The C1s Core Line in Irradiated Graphite. J. Appl. Phys. 2007, 102, 043504. (39) Susi, T.; Mowbray, D. J.; Ljungberg, M. P.; Ayala, P. Calculation of the Graphene C 1s Core Level Binding Energy. Phys. Rev. B 2015, 91, 081401(R). (40) Terrones, H.; Lv, R.; Terrones, M.; Dresselhaus, M. S. The Role of Defects and Doping in 2d Graphene Sheets and 1d Nanoribbons. Rep. Prog. Phys. 2012, 75, 062501. (41) Mellouki, A.; Georges, R.; Herman, M.; Snavely, D. L.; Leytner, S. Spectroscopic Investigation of Ground State Pyrrole (12C 4 H 5 N): The N–H Stretch. Chem. Phys. 1997, 220, 311– 322. (42) Dauster, I.; Rice, C. A.; Zielke, P.; Suhm, M. A. N–H···π Interactions in Pyrroles: Systematic Trends from the Vibrational Spectroscopy of Clusters. Phys. Chem. Chem. Phys. 2008, 10, 2827–2835. (43) Xue, Y.; Liu, J.; Chen, H.; Wang, R.; Li, D.; Qu, J.; Dai, L. Nitrogen-Doped Graphene Foams as Metal-Free Counter Electrodes in High-Performance Dye-Sensitized Solar Cells. Angew. Chem. Int. Ed. 2012, 51, 12124–12127. (44) Lin, Z.; Waller, G.; Liu, Y.; Liu, M.; Wong, C.-P. Facile Synthesis of Nitrogen-Doped Graphene Via Pyrolysis of Graphene Oxide and Urea, and Its Electrocatalytic Activity Toward the Oxygen-Reduction Reaction. Adv. Energy Mater. 2012, 2, 884–888. (45) Misra, A.; Tyagi, P. K.; Singh, M. K.; Misra, D. S. FTIR Studies of Nitrogen Doped Carbon Nanotubes. Diamond Relat. Mater. 2006, 15, 385–388.


26

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(46) Geng, D.; Yang, S.; Zhang, Y.; Yang, J.; Liu, J.; Li, R.; Sham, T.-K.; Sun, X.; Ye, S.; Knights, S. Nitrogen Doping Effects on the Structure of Graphene. Appl. Surf. Sci. 2011, 257, 9193–9198. (47) Mayo, D. W. Characteristic Frequencies of Aromatic Compounds (Group Frequencies of Arenes). In Course Notes on the Interpretation of Infrared and Raman Spectra; Mayo D. W., Miller F. A., and Hannah R. W., Eds.; John Wiley & Sons, Inc., Hoboken, New Jersey, 2004. (48) Kostic, R.; Miric, M.; Radic, T.; Radovic, M.; Gajic, R.; Popovic, Z. V. Optical Characterization of Graphene and Highly Oriented Pyrolytic Graphite. Acta Phys. Pol., A 2009, 116, 718–721. (49) Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Raman Spectroscopy in Graphene. Phys. Rep. 2009, 473, 51–87. (50) Piscanec, S.; Lazzeri, M.; Mauri, F.; Ferrari, A. C.; Robertson, J. Kohn Anomalies and Electron-Phonon Interactions in Graphite. Phys. Rev. Lett. 2004, 93, 185503. (51) Ferrari, A. C. Raman Spectroscopy of Graphene and Graphite: Disorder, Electron-Phonon Coupling, Doping and Nonadiabatic Effects. Solid State Commun. 2007, 143, 47–57. (52) Thomsen, C.; Reich, S. Double Resonant Raman Scattering in Graphite. Phys. Rev. Lett. 2000, 85, 5214–5217. (53) Gillet, Y.; Kontur, S.; Giantomassi, M.; Draxl, C.; Gonze, X. Ab Initio Approach to Second-Order Resonant Raman Scattering Including Exciton-Phonon Interaction. Sci. Rep. 2017, 7, 7344–7344.


27


Page 28 of 30

(54) Nemanich, R. J.; Solin, S. A. 1st-order and 2nd-order Raman-scattering from Finite-size Crystals of Graphite. Phys. Rev. B 1979, 20, 392–401. (55) Susi, T.; Pichler, T.; Ayala, P. X-Ray Photoelectron Spectroscopy of Graphitic Carbon Nanomaterials Doped with Heteroatoms. Beilstein J. Nanotechnol. 2015, 6, 177–192. (56) Usachov, D.; Vilkov, O.; Grüneis, A.; Haberer, D.; Fedorov, A.; Adamchuk, V. K.; Preobrajenski, A. B.; Dudin, P.; Barinov, A.; Oehzelt, M.; et al. Nitrogen-Doped Graphene: Efficient Growth, Structure, and Electronic Properties. Nano Lett. 2011, 11, 5401–5407. (57) Gammon, W. J.; Kraft, O.; Reilly, A. C.; Holloway, B. C. Experimental Comparison of N(1s) X-Ray Photoelectron Spectroscopy Binding Energies of Hard and Elastic Amorphous Carbon Nitride Films with Reference Organic Compounds. Carbon 2003, 41, 1917–1923. (58) Fujimoto, Y.; Saito, S. Formation, Stabilities, and Electronic Properties of Nitrogen Defects in Graphene. Phys. Rev. B 2011, 84, 245446. (59) Ibrahim, E. M. M.; Khavrus, V. O.; Leonhardt, A.; Hampel, S.; Oswald, S.; Rümmeli, M. H.; Büchner, B. Synthesis, Characterization, and Electrical Properties of Nitrogen-Doped SingleWalled Carbon Nanotubes with Different Nitrogen Content. Diamond Relat. Mater. 2010, 19, 1199–1206. (60) Spath, F.; Zhao, W.; Gleichweit, C.; Gotterbarm, K.; Bauer, U.; Hofert, O.; Steinruck, H. P.; Papp, C. Hydrogenation and Dehydrogenation of Nitrogen-Doped Graphene Investigated by X-ray Photoelectron Spectroscopy. Surf. Sci. 2015, 634, 89-94.


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(61) Rocha, A. R.; Rossi, M.; da Silva, A. J. R.; Fazzio, A. Realistic Calculations of Carbonbased Disordered Systems. J. Phys. D: Appl. Phys. 2010, 43, 374002. (62) Li, X.; Hua, W.; Guo, J.; Luo, Y., Electronic Structure of Nitrogen-Doped Graphene in the Ground and Core-Excited States from First-Principles Simulations. J. Phys. Chem. C 2015, 119, 16660-16666. (63) Liu, Y.; Jin, Z.; Wang, J.; Cui, R.; Sun, H.; Peng, F.; Wei, L.; Wang, Z.; Liang, X.; Peng, L. Nitrogen‐Doped Single‐Walled Carbon Nanotubes Grown on Substrates: Evidence for Framework Doping and Their Enhanced Properties. Adv. Funct. Mater. 2011, 21, 986–992.


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Spectroscopic Fingerprints of Graphitic, Pyrrolic, Pyridinic and

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