Tracking the Hydrogen Motion in Defective Graphene - The Journal

Laboratorio di Micro e Submicro Tecnologie Abilitanti dell'Emilia Romagna (MIST. ... (16, 17) High storage capacity of chemisorbed H atoms on graphene...
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Tracking the Hydrogen Motion in Defective Graphene Daniele Pontiroli,†,‡ Matteo Aramini,‡ Mattia Gaboardi,‡ Marcello Mazzani,‡ Samuele Sanna,§ Filippo Caracciolo,§ Pietro Carretta,§ Chiara Cavallari,‡,∥ Stephane Rols,∥ Roberta Tatti,⊥ Lucrezia Aversa,⊥ Roberto Verucchi,⊥ and Mauro Riccò*,‡ †

Laboratorio di Micro e Submicro Tecnologie Abilitanti dell’Emilia Romagna (MIST.E-R), via P. Gobetti 101, 40129 Bologna, Italy Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Parma, Via G. Usberti 7/a, 43124 Parma, Italy § Dipartimento di Fisica “A. Volta”, Università di Pavia, Via Bassi, 6, 27100 Pavia, Italy ∥ Institut Laue Langevin, BP156, 6, rue Jules Horowitz, 38042 Grenoble Cedex, France ⊥ Istituto dei Materiali per l’Elettronica e il Magnetismo, IMEM-CNR, Via alla Cascata 56/C, 38123 Povo (Tn), Italy ‡

S Supporting Information *

ABSTRACT: Bulk defective graphene produced by thermal exfoliation of graphite oxide was treated under H2 and investigated with X-ray photoemission spectroscopy, neutron spectroscopy, and solid state nuclear magnetic resonance. Graphene defects appear effective in dissociating H2 molecule and in promoting H covalent absorption on the carbon backbone. Measured generalized phonon density of states shows the presence of localized peaks ascribed to C−H bending modes already in pristine graphene, whose intensities enhance when samples are treated under H2 at 1273 K. However, 1H NMR evidences a thermally activated dynamics with a correlation time of a few microseconds assigned to a part of H atoms bound onto the graphene plane. These findings point toward a diffusive dynamics of the hydrogen chemically bound to graphene sheets, already active at room temperature.

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INTRODUCTION

It has been shown that the dissociation of H2 molecule can become energetically favorable by the presence of catalysts (metals) or even defects at the graphene plane.12 Also physisorption can be enhanced by the metal decoration of the graphene plane, through the Coulomb and/or interactions involving the partial hybridization between H2 and metal orbitals (Kubas interactions).13,14 Although the experimental efforts to decorate graphene with metal atoms failed so far, due to the tendency of transition metals to clusterize,15 it has been recently proved that the formation of small metallic clusters can still improve the hydrogen absorption in carbon nanostructures. For example, intecalation of C60 with alkali clusters (Li and Na) have shown the lowering of the hydrogenation temperature of the fullerene molecule.16,17 High storage capacity of chemisorbed H atoms on graphene relies on the so-called H-spillover effect. The latter process requires the ability of H atoms, fed from H2 molecules dissociated by metallic clusters,18 to jump onto the graphene substrate and diffuse over long distance at the surface to sustain the hydrogenation process. The saturation of the whole graphene surface with hydrogen (formation of graphane) should in principle allow to reach 7.7 wt % of stored hydrogen.

The interest on hydrogen interaction with graphene is broad. It embraces fundamental problems, such as the formation of molecular H2 in the interstellar media,1 or the onset of magnetism in carbon-based materials.2 The physicochemical understanding of the peculiar state of hydrogen at the graphene surface is also relevant for technological issues, such as graphene-based nanoelectronics and energy storage. In the recent past, massive production of graphene-based compounds, exploiting chemical methods starting from graphite,3,4 opened the route to new practical applications, such as in the field of hydrogen storage.5,6 In particular, thermal exfoliation of graphite oxide has attracted considerable attention since it is an easy, efficient, and low-cost method to produce single-layer graphene nanosheets with high rate, applicable also at the industrial level.7,8 Thanks to its large specific surface area (2630 m2/g), hydrogen adsorption is sizable in graphene, although at low temperatures, since the H2 binding energy on the carbon backbone is small (in the order of 0.08 eV).9 Beside physisorption, atomic hydrogen can also bind to graphene via a chemisorption process. While dissociative absorption of H2 on perfect graphene plane at normal conditions is unlikely since it requires to overcome a large energy barrier (3.3 eV),10 the absorption of H atoms is exothermic after overcoming a small barrier (0.2 eV).11 © 2014 American Chemical Society

Received: August 20, 2013 Revised: March 12, 2014 Published: March 17, 2014 7110

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of the neutron investigation. The thermal exfoliation and the subsequent handling and hydrogen treatment were performed under strict oxygen and moisture free conditions (i.e., vacuum or ≤1 ppm O2 and H2O Argon glovebox). The availability of macroscopic amount of graphene made possible the physical investigation to techniques requiring bulk amount of samples, such as NMR and INS, both of them being particularly sensitive to hydrogen. XPS investigation has been carried out on both TEGO and H-TEGO, using samples that were first dispersed in isopropanol, hence deposited onto a Cu cleaned surface and, after the solvent evaporation, collected on a polyethylene foil. The reliability of this procedure has been checked, and it did not introduce any significant contribution or artifact (see Supporting Information, Figure S1). The photoemission spectroscopies were performed using the Mg Kα photon at 1253.6 eV for XPS (total energy resolution of 0.86 eV), and the HeI photon at 21.21 eV for UPS (total energy resolution of 0.86 eV). All the core level binding energies (BE) were normalized to the Au 4f7/2 core level signal (at 84.0 eV), obtained from a sputtered gold surface. The core level analysis has been performed by Voigt line-shape deconvolution after the background subtraction of a Shirley function. The typical precision for each component’s energy position is ±0.05 eV. The uncertainty for the full width at half-maximum (fwhm) is less than ±5%, while for the area evaluation it is about ±5%. XPS analysis shows the presence of only carbon (C) and oxygen (O) in the samples, with relative atomic percentage for TEGO and H-TEGO of 97.1%/2.8% and 96.9%/3.1%, respectively. In both cases, the amount of oxygen appears very low, thus indicating that the thermal exfoliation process is effective in removing almost the totality of the oxidizing groups, which are present in the precursor graphite oxide. The C 1s core level in TEGO appears as a structured peak, with the presence of four specific components (see Figure 1, bottom curve). The main peak (68.9% of the whole C 1s area, see Table 1) is located at 284.53 eV, and it is associated to sp2 C. The asymmetric shape of this peak, together with the presence of the C plasmon peak at 290.36 eV, suggest that the majority of graphene sheets in TEGO are probably stacked to form few-layered structures, of 3 or more layers, as confirmed also by the UPS measurements (see Supporting Information, Figure S2) and by previous TEM investigation.6 The peak at 287.94 eV arises from carbon bound to oxygen species, typical of C−O groups (carbonyl and epoxide). A fourth contribution is located at 285.55 eV (21.3% of C 1s area) and originates from C in sp3 configuration, as it was shown in studies of weakly sputtered HOPG.27 This could be rationalized by the presence of defects in graphene sheets and between planes. The C 1s peak in H-TEGO (see Figure 1, top curve) shows essentially the same four components, plus a further peak at 285.19 eV, ascribable to carbon bound to hydrogen.28,29 This peak represents about 6.6% of the C 1s emission, that is the amount of C involved in C−H bonds, in good agreement with the estimation done with INS (see below). It is worth noting in this case that the growing of this peak is at expenses of the intensity of the C sp3 one (see Table 1), suggesting that hydrogenation should involve mainly the defective graphene regions, in which the dangling bonds of in plane sp3 carbons get saturated by the addition of hydrogen. In Figure 2 are shown the O 1s core levels for both TEGO (bottom curves) and H-TEGO (top curves) samples. The lower O 1s signal-to-noise (S/N) ratio in H-TEGO with

A large debate on the possibility of H diffusion on graphene and on its precise mechanism can be found in the recent literature. While scanning tunneling microscopy (STM) investigations of atomic deuterium attached on clean HOPG (0001) surface via plasma treatment indicates the absence of diffusion at room temperature,19,20 recent Raman and IR studies of different Pt-doped carbon systems report evidence of the H spillover on graphenic surfaces.21 Also, first-principle molecular dynamics (MD) simulations suggest that chemisorbed hydrogen diffusion onto the graphene plane is feasible and may even compete with desorption.22 Owing to the large C−H binding energy (≳1 eV), it appears unlikely at first sight that the H atoms jump from one C atom to its neighbor. However, the C−H binding energy and the barrier for H chemical diffusion are not necessarily correlated, and it was shown that strongly bound C−H states on graphene could on the contrary display relatively low mobility barriers.23 In particular, diffusion can be further enhanced by hole-doping the graphene plane using either substitutional atoms24 and/or attaching functional groups25 at its surface. Hence, the mobility capabilities of H on graphene depend strongly on its chemical state and it is therefore extremely sensitive to the graphene synthesis, manipulation, and history. At the moment, however, the literature still lacks from any direct evidence of H diffusion on graphene. In this article we performed a detailed investigation of the hydrogen interaction with defective chemically produced graphene, obtained by thermal expansion of graphite oxide. The analysis was carried out by means of X-ray and UV photoemission spectroscopy (XPS and UPS), inelastic neutron scattering (INS), and solid-state nuclear magnetic resonance (NMR). We showed that the defects in graphene are effective in dissociating hydrogen molecules, promoting the covalent absorption of atomic H. Despite the relatively large binding energy of the C−H covalent bond, we found that a significant fraction of atomic hydrogen are mobile already at ambient temperature, as probed by 1H NMR. We found a surprisingly low activation energy of about 30 meV and a correlation time of about 2 μs at room temperature, which cannot be explained as due to the presence of H 2 molecules or alkyl-group reorientations in the samples. Several hypotheses for the H dynamics chemisorbed on the graphene plane are presented and discussed.

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EXPERIMENTAL METHODS AND RESULTS Graphene samples under investigation were produced via thermal exfoliation of graphite oxide, thoroughly described elsewhere.26 Although this method provides compounds with a certain level of disorder, such as defects (i.e., carbon vacancies and edges) and corrugations of the planes, thermally expanded graphite oxide (TEGO) can be produced in large amount and the synthesis is in principle scalable at industrial level, thus being remarkable for large scale applications. Previous characterization of TEGO with transmission electron microscopy (TEM), scanning electron microscopy (SEM), and selected area electron diffraction (SAED) clearly demonstrated that the samples contain both single and few layers of sp2 carbon.6,26 Investigation with XPS and UPS was included in the present work, which allowed to complete the chemical/physical analysis. The hydrogenation was performed by annealing TEGO at 1273 K under H2 flux at ambient pressure for one hour in a quartz vial (H-TEGO). The same treatment was done on TEGO sample using deuterium (D-TEGO) for the purpose 7111

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Figure 2. XPS analysis of O 1s core levels from TEGO (bottom curves) and H-TEGO (top curves). Spectra are background subtracted and normalized in intensity. A single component is used to reproduce peak line shape for both samples. Figure 1. XPS analysis of C 1s core levels from TEGO (bottom curves) and H-TEGO (top curves). Spectra are background subtracted and normalized in intensity. Components used to reproduce peak line shape are shown.

respect to TEGO is only due to the lower amount of materials investigated, while for C 1s photoemission signal the significantly higher atomic percentage has enabled analysis with better S/N ratio. Peaks are characterized by the presence of a single main component, located at about the same BE of 233.30 eV (see Table 1). This is in good agreement with the presence of a single well-defined C−O feature in the C 1s spectra in both TEGO and H-TEGO, suggesting that the hydrogenation procedure does not increase the amount of C− O species nor introduce new and different C−O groups. The INS experiments on TEGO, D-TEGO, and H-TEGO samples have been performed at the Institut Laue Langevin, using the IN4C time-of-flight spectrometer.30 The spectra are shown in the form of the generalized phonon density of states (GDOS), as derived from a standard treatment of the data.31 Figure 3 shows the neutron GDOS on H-TEGO at 10 K, obtained by combining the spectra using the incident neutron wavelengths of 1.5, 1.1, and 0.74 Å (squares), in neutron energy loss. The three spectra were combined in a single spectrum after proper data treatment and normalization. The D-TEGO (diamonds), TEGO (circles), and H-TEGO (triangles) (from bottom to top) spectra were recorded using a 2.41 Å incident wavelength at 320 K, in neutron energy gain. All the spectra are normalized to the mass of carbon (∼250 mg) in the neutron

Figure 3. Inelastic neutron spectra obtained on IN4 at 320 K with incident wavelength of 2.41 Å on as-prepared TEGO (circles), DTEGO (diamonds), and H-TEGO (triangles). Squares: INS spectra obtained on H-TEGO at 10 K with incident neutron wavelength of 1.5, 1.1, and 0.74 Å. Arrows are guides to indicate the main features attributed to the H localized modes in the H-TEGO spectrum.

beam. The intensity enhancement observed from TEGO to HTEGO denotes an increased quantity of hydrogen. However, the signal decrease in the D-TEGO sample witnesses that hydrogen is already present in TEGO and that the D treatment is effective in promoting H/D exchange. From the neutron data it was possible to estimate the amount of hydrogen in TEGO before and after hydrogenation, by rescaling the elastic

Table 1. XPS Analysis of C 1s and O 1s Core Levels from TEGO and H-TEGO as Shown in Figures 1 and 2a C 1s C−C sp2 TEGO H-TEGO a

C−C plasmon

O 1s C−C sp3

C−O

C−H

eV

%

eV

%

eV

%

eV

%

284.53 284.53

68.9 73.2

290.36 290.36

2.3 2.7

287.94 287.94

7.5 8

285.55 285.54

21.3 9.5

eV

O−C %

eV

%

533.39 533.23

100 100

The single feature BE is shown in eV, and the percentage is the fraction of the component area over the whole core level area. 7112

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nanosecond time scale. In particular, these neutron investigations suggest the absence of physisorbed molecular H2 in HTEGO sample, as the latter would give rise to intense features around 14.7 meV at low temperature, where the para−ortho transition for molecular H2 is expected.33,34 Moreover, the absence of quasielastic signal in the spectra, together with the lack of any evident rotational band at low temperature, allowed us also to exclude the presence of possible methyl groups attached to graphene planes, whose reorientational motion is expected to be thermally activated at the investigated temperatures.35 In order to investigate the behavior of hydrogen in the HTEGO samples, we measured the temperature dependence of the 1H NMR line shape, which is highly sensitive to slow frequency diffusive motions.36 1H NMR spectra have been obtained from the Fourier transform of half of the echo signal generated after a π/2−τ−π/2 pulse sequence. The π/2 pulse length was kept between 2 and 5 μs, and the applied magnetic field μ0H ranged between 1.5 and 5.6 T. The 1H NMR spectrum profile at room temperature, shown in Figure 4,

scattered intensity to that of a reference vanadium sample of calibrated mass, i.e., knowing the number of V atoms (nV) illuminated by the incident neutron beam. A particular care was taken to keep the vanadium sample measurement in strictly the same configuration as the TEGO and H-TEGO samples. The elastic intensity (integrated area) of the vanadium standard (IV) and of the H-containing sample (IH) was evaluated by fitting these contributions to a Gaussian function, and their integrated area was evaluated in a common range of Q, avoiding some non-negligible coherent contributions from the carbon framework. We make the assumption that the Debye−Waller factor in both estimations is a second order correction to the quantity derived. The number of H atoms in the H-containing sample (nH) is derived from the relationship nH = (IH/IV )(σ V /σH)nV

(1)

with σV and σH being the V and H incoherent neutron scattering cross-section, respectively. We found that the H amount in TEGO is 0.17(2) wt %, while after hydrogenation it increases by a factor of 4, reaching 0.69(2) wt % in H-TEGO. The latter information appears in very good agreement with a similar estimation derived from NMR data (see Table 2) and confirms the effectiveness of the H treatment. Table 2. Mass Percentage of Hydrogen in TEGO and HTEGO Samples Evaluated by INS and NMR Dataa INS NMR

TEGO

H-TEGO

0.17(2) wt %

0.69(2) wt % 0.6(1) wt %

a

The H amount in H-TEGO is about four times that in TEGO, corresponding to roughly 1 H for every 12 carbon atoms.

The TEGO and H-TEGO spectra are dominated by intense bands located around 105 and 150 meV, which clearly enhance in the hydrogenated sample. The structure centered around 105 meV is composed of three peaks, respectively, located at 95 (shoulder), 104, and 106 meV. These structures are better seen in the low temperature spectrum, due to the improved resolution in this energy range and to the reduced thermal disorder. Such contributions, whose energy fall in the range of C−H bending modes, are compatible with the presence of atomic H covalently bound to the graphene plane. By contrast, no sharp feature and no important temperature evolution is observed in the low frequency region of the spectra. For all the samples, and at all temperatures, a linear increase of the GDOS is observed up to ∼20 meV, followed by a more or less flat region up to 40 meV. This [0, 40 meV] range mostly features out-of-plane modes of the graphene surface. The constant shape, but increased intensity of the GDOS in this range from D-TEGO to H-TEGO, reflects that the hydrogen atoms follow adiabatically the carbon framework motions for these modes.32 The H-TEGO GDOS in this range is observed to be temperature independent for T varying from 10 to 320 K. This indicates that the scattered intensity follows a Bose dependence with T and suggests that the signal originates from phonon excitations scattering. This observation excludes the possibility of low frequency contributions from quantum rotations and quasielastic-like signal, which would originate from relaxation in the picosecond time scale. Additional investigations, using spin echo neutron spectroscopy, allowed us to affirm that no relaxation in this system can be measured with a characteristic time from the picosecond up to the

Figure 4. 1H NMR spectrum profile at T = 295 K and μ0H = 1.5 T (circles). The fit (solid line) shows that two contributions are clearly distinguishable, one Gaussian (dotted line) and one Lorentzian shaped (dashed line).

evidences two main contributions. In fact, the best fit is obtained with a Gaussian (dotted) plus a Lorentzian (dashed) curve with the 3:1 weight ratio, with very different linewidths. The full width at half-maximum (fwhm) being Δv = 2(2 ln 2)1/2σ ≃ 30 kHz (where σ is the square-root of the Gaussian second moment), and Γ ≃ 3 kHz for the Gaussian and Lotentzian, respectively. This behavior reflects the presence of at least two different H atomic species in the sample. The multipanel Figure 5 shows the temperature evolution of the 1H NMR spectrum, measured at μ0H = 5.6 T in order to enhance the S/N ratio. The curve fit (solid lines) evidences the presence of two contributions over the whole temperature range 70 K < T < 360 K. The Gaussian component (dotted line) shows a T-independent width Δv0 ≃ 30 kHz, while the Lorentzian one (dashed line) displays a significant broadening upon cooling, approaching Γ0 ≃ 20 kHz below ∼120 K. These results were obtained by assuming that the amount of each H species does not vary with temperature, i.e., the fit of the Gaussian and Lorentzian weights is constrained to the 3:1 constant ratio, as it is reasonable to assume that different species of hydrogens do not interconvert. The broad temperature independent component is reasonably attributed to clusters of hydrogens covalently bonded to graphene planes, in which the homonuclear dipolar interaction 7113

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than the dynamics probed here by 1H NMR spectra. However, the nuclear dipole−dipole interaction may be modulated either by H vibrations or by the H motions throughout the graphene lattice. Since only the latter dynamic does reach a frequency range, which may affect the NMR line width, it is tempting to associate the motional narrowing of Γ(T) to the H nuclei diffusion. 36 When the time scale of the fluctuations, characterized by a correlation time τc, becomes short as compared to the inverse of the frequency distribution Δω0, the system is driven to the fast motion regime and the observed line shape is expected to narrow. From the value of the constant width of the Gaussian shape we can then evaluate that this condition, i.e., Δω0τc < 1, is roughly achieved when τc ≲ 10 μs. Specifically, from the behavior of Γ(T) displayed in Figure 6 (right axis scale), the crossover between the slow and the fast regimes is found to occur around 150 K. Above this temperature, the line shape narrows from the slow motion value of Δω0 = Γ0 ≃ 20 kHz to the fluctuating limit described in the fast motion regime. In this limit the correlation time can be evaluated36 as τc = 2πΓ/(2πΓ0)2 and its temperature evolution, τc(T), is displayed in Figure 6 (left axis scale). The plot shows that at high temperature the condition τc < 10 μs is well satisfied. The data fit to a thermally activated behavior τc = τ∞ eU/kBT (dotted line), which allows to estimate an activation energy barrier U = 29 ± 4 meV and τ∞ = 0.5 ± 0.1 μs. Since these correlation times are significantly longer than the inverse of 1H Larmor frequency, the hydrogen diffusive dynamics does not sizeably contribute to the 1H spin−lattice relaxation rate 1/ T1. In fact, 1/T1 shows only a moderate T dependence, which is possibly driven by the fluctuations of the paramagnetic centers (dangling bonds) localized around the hydrogen. It is important to point out that the slow time scale deduced by NMR is not compatible neither with the presence of molecular hydrogen, possibly physisorbed on graphene (this is expected to give rise to a Pake doublet, which was not observed down to 1.6 K), or with a localized motion of a small group like a methyl group (as evidenced in the case of ball milled graphite37) or an hydroxy group (which could originate from an incomplete desorption of functional groups during exfoliation). Much shorter time scales are involved in the dynamics of these groups’ motion and, in our case, also a INS and XPS signature of their presence is missing. In addition, our NMR results rule out the possibility that H could be attached to unsaturated carbons, in which the large hyperfine coupling with the paramagnetic electron would deeply affect the hydrogen line shape.

Figure 5. Temperature evolution of 1H NMR spectra profile measured at μ0H = 5.6 T (circles). The fit (solid line) shows that the large Gaussian contribution (dotted line) has a fixed width while the Lorentzian one (dashed line) is temperature dependent.

gives rise to the observed broadening. We analyzed in detail various types of locations, like zigzag edges, armchair edges, and graphane-like in-plane regions (see Supporting Information, Figure S3), and we found that the 30 kHz broadening observed is compatible either with saturated armchair edges (on the edge or on large vacancy regions) or with pure sp3 graphane. However, saturated and diluted zigzag edges and intermediate sp2/sp3 regions are expected to give narrower contributions, which are compatible with the narrower component of the 1H spectrum at low temperature. The temperature dependence of the Lorentzian width Γ(T) is reported in Figure 6. The observed narrowing of the line

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DISCUSSION The clear increase of H chemisorbed in H-TEGO samples, as derived from the INS measurements, witnesses the effectiveness of chemically produced graphene to dissociate the H 2 molecules and induce C−H bonds. This probably arises from the presence of paramagnetic defects, such as (unsaturated) carbon vacancies, sp2/sp3 mixed regions (as shown by XPS), in which sp3 carbons would bear a dangling bond, and edges. Recent ab initio calculations indicated that the dissociative absorption barrier of H2 on graphene is noticeably reduced in the presence of edges, and in particular, zigzag edges were predicted to induce barrierless dissociation of H2.38,39 In addition, large carbon vacancies at the graphene planes, created by the evaporation of chemical groups (i.e., hydroxide and epoxide) during the exfoliation process, are expected to behave similarly to edges.12

Figure 6. Temperature dependence of the correlation time of the diffusion process (squares). The dotted line is the fit to a thermally activated behavior, which yields an activation energy barrier of U = 29 ± 4 meV.

width upon increasing T indicates the onset of low-frequency dynamics, which modulates the local field at the 1H nuclei. The local field at the protons may originate either from the dipolar interaction with the other nuclei or from the hyperfine interaction with the electron spins. This latter interaction, however, is not expected to give rise to the observed Tdependence of Γ(T) since spin fluctuations are much faster 7114

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Notes

These paramagnetic regions can be either completely or incompletely saturated during the hydrogenation process. In the former case, the covalently bound hydrogen is stationary (its only dynamics is the vibrational one detected by INS) and the local high density of H spins gives rise to the large static Gaussian broadening found in the NMR spectrum. In the latter case, on the contrary, either on the edges or on the sp2/sp3 mixed regions, the density of hydrogens would be smaller and many carbons would bear a paramagnetic dangling bond. The higher dilution of H atoms would produce a narrower NMR line (see Supporting Information, Figure S3) and the local hyperfine field distribution generated by paramagnetic impurities can induce a Lorentzian line shape (rather than the expected Gaussian). This fits the features of the second narrow component observed in the 1H NMR spectrum. Moreover, in this case, a diffusive dynamics can take place, in which covalently bound H would move to the neighboring unsaturated carbons. These dynamics would involve not only the movement of the H atom but also a rearrangement of the whole graphene/graphane area or a bending of the zigzag edge. The involvement of a large number of atoms slows down the time scale of this diffusion (with respect to the one expected for a single H diffusion), which could fall into the range found in the NMR experiment. Although the different techniques involved in this study still do not allow us to formulate a precise dynamical model for the motion of hydrogen in TEGO, the two above-mentioned possibilities, namely, the H dynamics in defective sp2/sp3 mixed regions or on unsaturated zigzag edges, appear to us the most reasonable hypothesis for this motion and indeed the ones that are compatible with the several experimental results presented in the present work.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Institut Laue Langevin for provision of beamtime, and we acknowledge the SNF-HyCarbo project (grant No. CRSII2-130509) for the financial support. We thank Dr. Rocco Martinazzo for the useful discussions.

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(1) Bonfanti, M.; Martinazzo, R.; Tantardini, G. F.; Ponti, A. Physisorption and Diffusion of Hydrogen Atoms on Graphite from Correlated Calculations on the H−Coronene Model System. J. Phys. Chem. C 2007, 111, 5825−5829. (2) Yazyev, O. V.; Helm, L. Defect-Induced Magnetism in Graphene. Phys. Rev. B 2007, 75, 125408. (3) Park, S.; Ruoff, R. S. Chemical Methods for the Production of Graphenes. Nat. Nano. 2009, 4, 217−224. (4) Soldano, C.; Mahmood, A.; Dujardin, E. Production, Properties and Potential of Graphene. Carbon 2010, 48, 2127−2150. (5) Schlapbach, L.; Zuttel, A. Hydrogen-Storage Materials for Mobile Applications. Nature 2001, 414, 353−358. (6) Gaboardi, M.; Bliersbach, A.; Bertoni, G.; Aramini, M.; Vlahopoulou, G.; Pontiroli, D.; Mauron, P.; Magnani, G.; Salviati, G.; Zuttel, A.; Riccó, M. Decoration of Graphene with Nickel Nanoparticles: Study of the Interaction with Hydrogen. J. Mater. Chem. A 2014, 2, 1039−1046. (7) McAllister, M. J.; Li, J.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Single Sheet Functionalized Graphene by Oxidation and Thermal Expansion of Graphite. Chem. Mater. 2007, 19, 4396− 4404. (8) Mikhailov, S. Physics and Applications of Graphene: Experiments; InTech: Rijeka, Croatia, 2011. (9) Arellano, J. S.; Molina, L. M.; Rubio, A.; Alonso, J. A. Density Functional Study of Adsorption of Molecular Hydrogen on Graphene Layers. J. Chem. Phys. 2000, 112, 8114−8119. (10) Miura, Y.; Kasai, H.; Dino, W.; Nakanishi, H.; Sugimoto, T. First Principles Studies for the Dissociative Adsorption of H2 on Graphene. J. Appl. Phys. 2003, 93, 3395−3400. (11) Hornekær, L.; Rauls, E.; Xu, W.; Sljivancanin, Z.; Otero, R.; Stensgaard, I.; Lægsgaard, E.; Hammer, B.; Besenbacher, F. Clustering of Chemisorbed H(D) Atoms on the Graphite (0001) Surface due to Preferential Sticking. Phys. Rev. Lett. 2006, 97, 186102. (12) Allouche, A.; Ferro, Y. Dissociative Adsorption of Small Molecules at Vacancies on the Graphite (0001) Surface. Carbon 2006, 44, 3320−3327. (13) Lee, H.; Ihm, J.; Cohen, M. L.; Louie, S. G. Calcium-Decorated Graphene-Based Nanostructures for Hydrogen Storage. Nano Lett. 2010, 10, 793−798. (14) Zhou, W.; Zhou, J.; Chen, C. J.; Ouyang; Shi, S. First-Principles Study of High-Capacity Hydrogen Storage on Graphene with Li Atoms. J. Phys. Chem. Solids 2012, 73, 245−251. (15) Sun, Q.; Wang, Q.; Jena, P.; Kawazoe, Y. Clustering of Ti on a C60 Surface and Its Effect on Hydrogen Storage. J. Am. Chem. Soc. 2005, 127, 14582−14583. (16) Teprovich, J. A.; Wellons, M. S.; Lascola, R.; Hwang, S.; Ward, P. A.; Compton, R. N.; Zidan, R. Synthesis and Characterization of a Lithium-Doped Fullerane (Lix-C60-Hy) for Reversible Hydrogen Storage. Nano Lett. 2012, 12, 582−589. (17) Mauron, P.; Remhof, A.; Bliersbach, A.; Borgschulte, A.; Zuttel, A.; Sheptyakov, A.; Gaboardi, M.; Choucair, M.; Pontiroli, D.; Aramini, M.; et al. Reversible Hydrogen Absorption in Sodium Intercalated Fullerenes. Int. J. Hydrogen Energy 2012, 37, 14307−14314. (18) Singh, A. K.; Ribas, M. A.; Yakobson, B. I. H-Spillover Through the Catalyst Saturation: an ab Initio Thermodynamics Study. ACS Nano 2009, 3, 1657−1662.

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CONCLUSIONS The main result of this work is represented by the evidence of dynamics of hydrogen chemisorbed on the graphene plane, already at low temperature. INS, XPS, and NMR data have shown that chemically produced graphene is effective to dissociate H2 molecule and to support H motion with a surprisingly low energy barrier of about 30 meV. Although the extended investigation presented here still cannot allow us to formulate a precise diffusion model, we identified two possible hydrogen dynamics, which are compatible with the presented data. The first takes place within highly defective sp2/sp3 regions (present in pristine TEGO as highlighted by XPS), where the thermally activated rearrangement of these highly distorted areas induces the jump of the H atom to neighboring unsaturated carbons. The second could take place on unsaturated zigzag edges, in which H could jump to the neighboring edge site through an intermediate binding to the middle carbon, which would convert to sp3. This diffusion would be coupled to the torsional dynamics of the edges.

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ASSOCIATED CONTENT

S Supporting Information *

We provide details of XPS analysis and simulations of NMR spectrum for different distributions of hydrogen on the graphene plane. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(M.R.) E-mail: mauro.ricco@fis.unipr.it. Tel: +390521905217. Fax: +390521905223. 7115

dx.doi.org/10.1021/jp408339m | J. Phys. Chem. C 2014, 118, 7110−7116

The Journal of Physical Chemistry C

Article

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