Effective Pathway for Hydrogen Atom Adsorption on Graphene

Yoshio MIURA1;2, Hideaki KASAI1�, Wison Agerico DINO1, Hiroshi NAKANISHI1 and Tsuyoshi SUGIMOTO3

1Department of Applied Physics, Osaka University, Suita, Osaka 565-08712Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012

3Toyota Motor Corporation, Toyota-cho, Aichi 471-8572

(Received December 9, 2002)

We investigate and discuss the interaction of a hydrogen atom (H) with graphene based on the densityfunctional theory (DFT). Our calculation results show that reconstructions of carbon atoms play animportant role in the H adsorption on graphene. When constituent carbon atoms are held rigid,endothermic H adsorption is about 0.2 eV, and the activation barrier is 0.3 eV for H adsorption, due to thestrong �-bonding network of the hexagonal carbon. On the other hand, when carbon atoms are allowed torelax, the carbon atom directly below the H atom moves 0.33 �A upward towards the gas phase, and ansp3-like geometry is formed between the H and carbon atoms of graphene. This relaxation stabilizes thehydrogen–carbon interaction, and the exothermic hydrogen adsorption on the graphene has a bindingenergy of 0.67 eV. We also show that the effective pathway for H adsorption on graphene, which gives anactivation barrier for the H adsorption on graphene of 0.18 eV.

KEYWORDS: hydrogen, graphene, density functional theory, adsorption, relaxationDOI: 10.1143/JPSJ.72.995

In recent years, carbon-based nanomaterials, such ascarbon nanotubes (CNTs) and graphite nanofibers (GNFs),have attracted considerable attention because of theirsuggested suitability as materials for gas storage. Inparticular, the reportedly high hydrogen uptake of thesematerials makes them attractive for hydrogen storagedevices in fuel-cell-powered electric vehicles.1,2) Manyexperimental and theoretical studies have been carried outto understand the storage mechanisms, capacity and neces-sary structure for hydrogen storage of CNTs andGNFs.3–10,13–16,18–20) Dillon et al.3) measured the H2

adsorption capacity of single-walled carbon nanotubes(SWNTs), using temperature-programmed desorption(TPD) spectroscopy. They estimated an H2 storage capacity,for the SWNT, of 5 to 10wt% (wt% means the weight of H2

divided by the weight of SWNTs plus the H2 adsorbed onthe SWNTs). They concluded that SWNTs have a highstorage capacity for hydrogen, and the storage capacityincreases with increasing SWNT diameter. Chambers et al.4)

claimed that instead of SWNTs, certain GNFs can absorband retain 67wt% H2 at room temperature at a pressure of12MPa. However, up to now, this H2 absorption amount onthe GNFs has yet to be confirmed, either experimentally ortheoretically, by others. More recently, Liu et al.5) found thatSWNT storage can take place at room temperature,corresponding to a storage capacity of 4.2wt% undermoderate pressure, and 78.3% of the adsorbed H2 can bereleased under ambient pressure and at room temperature. Inthe same year, Chen and Yang6) reported that high H2 uptakevalues of 20 and 14wt% can be achieved for Li-doped andK-doped multiwalled carbon nanotubes (MWNTs) between400�C and room temperature under ambient pressure. Theyalso showed that the stored H2 could be released attemperatures higher than 400�C and the adsorption–de-sorption cycle can be repeated with minor loss in storagecapacity.

Darkrim and Levesque7) performed Monte Carlo simula-

tions based on model potentials. They discussed theinfluence of SWNT diameter on H2 storage, and found thatH2 adsorption on SWNTs decreases with increasing SWNTdiameter. Wang and Johnson8) and Simonyan et al.9) studiedhydrogen adsorption in neutral, positively, and negativelycharged SWNTs using model potentials for the H2–H2

interaction and the H2–SWNT interaction. They demon-strated that graphite nanofibers show a significantly betterperformance for hydrogen storage than SWNTs and a 0:1e/Ccharging of the nanotubes increases the hydrogen adsorptionup to 30%. Williams and Eklund10) simulated H2 physisorp-tion on SWNT ropes using the Monte Carlo method. Theyshowed that small-diameter ropes are preferable for H2

storage. These Monte Carlo simulations and classicalmolecular mechanics calculations mainly suggest that theH2 physisorption on graphene is crucial to explain theexperimental estimations for H2 storage in CNTs and GNFs.The importance of the H2 chemisorption on graphene wasalso suggested by some groups. Lee and Lee12) reportedcalculation results for the hydrogen storage behavior inSWNTs based on density functional theory (DFT) calcula-tions, and proposed that the adsorption of hydrogen inSWNTs is a chemisorption process. They predicted that thehydrogen storage capacity in (10,10) nanotubes can exceed14wt%. Furthermore, Liu et al.5) observed that, aftertreating carbon nanotubes with hydrogen gas under highpressures, there were residual H2 during the desorption cyclethat could be released only upon heating to temperaturesabove 400 K. They concluded that these residual H2 may berelated to chemical adsorption.

In this paper, we investigate the interaction of an H atomwith graphene (basal plane of a graphite sheet) by perform-ing DFT calculations, and discuss the effective H adsorptionpathway on graphene. The purpose of this study is to obtaina microscopic understanding of H interaction with graphene,which is essential for predicting the maximum storagecapacity and interpretation of hydrogen adsorption in CNTsand GNFs.

In this study, we performed total energy calculations

LETTERS

�E-mail: kasai@dyn.ap.eng.osaka-u.ac.jp

Journal of the Physical Society of Japan

Vol. 72, No. 5, May, 2003, pp. 995–997

#2003 The Physical Society of Japan

995

based on the DFT, using the plane waves and pseudopoten-tial code DACAPO.21) In our calculations, graphene isrepresented in a supercell by a slab of one graphene sheetand vacuum region 14.67 �A. The graphene sheet consists ofeight carbon atoms with a nearest neighbor distance of1.42 �A as shown in the inset of Fig. 1. Labels A to E in theinset of Fig. 1 indicate the positions of the H atom betweenthe (hexagonal center) hollow and bridge sites (A), bridgesites (B), top sites (C), between top and hollow sites (D), andhollow sites (E), respectively. The size of the supercell issufficiently large to avoid interactions between H atoms inneighboring supercells. The electron–ion interaction isdescribed by optimized ultra-soft pseudopotentials,22) andthe Kohn–Sham equations are solved using plane waves withkinetic energies up to 400 eV. The surface Brillouin zoneintegration is performed using the special point samplingtechnique of Monkhorst and Pack (with 4� 4� 1 samplingmeshes).23) For the exchange correlation energy, we adoptedthe generalized gradient approximation (GGA).24,25) Wedetermined that the numerical results converged with respectto the kinetic energy cutoff and the k-point set in eachcalculation.

In Fig. 1, we show the potential energy curves for Hinteraction with graphene as a function of the normaldistance of the H atom from graphene. In the calculation, theH atoms are positioned at highly symmetric sites (A–E) ofgraphene as shown in the inset of Fig. 1, and the carbonatoms are held rigid. V ¼ 0:0 eV in Fig. 1 corresponds to thecase when the H atom is in the gas phase, far from thegraphene sheet. From Fig. 1, we can see that the endother-mic H adsorption on graphene is more than 0.2 eV in thisrigid configuration. We can also see that the activationbarrier is �0:3 eV. Due to the strong �-bonding network ofthe pz orbital of the hexagonal carbon, repulsion between theH and carbon atoms dominates in the H interaction with therigid graphene. Furthermore, in the H interaction with thehollow site of graphene (dashed line in Fig. 1), nometastable adsorption occurs, and there is an activation

barrier corresponding to 4.0 eV when the H penetrates thecenter of the hexagonal carbon of graphene. From this result,we can say that H absorption and desorption through thehexagonal center of graphene hardly occur in the thermalenergy region. The most stable H adsorption site is near siteC (top site) of graphene. In this case, the hybridizationbetween the s orbital of the H atom and the 2pz orbital of thecarbon atom is large, and a metastable chemical bond isformed between the H and the carbon atom.

Next, we consider how the relaxation of the carbon atomsaffects the H interaction with graphene. The positions of theH atom (A–E) in Fig. 2 are the same as those of Fig. 1.When we allow the carbon atoms in graphene to relax, weobserve a change in both the adsorption energy and theactivation barrier, when compared with the results for a rigidconfiguration of carbon atoms. The most prominent changein potential energy can be seen for the case when the H atomis at sites C, i.e., on the top site of graphene. In this case, thecarbon atom under the hydrogen atom moves 0.33 �A upwardtowards the gas phase. This relaxation stabilizes thehydrogen–carbon interaction, and the exothermic hydrogenadsorption on graphene has a binding energy of 0.67 eV.This result indicates that the carbon atom relaxation is veryimportant for characterizing the hydrogen–graphene inter-action. The adsorbed hydrogen atom is located 1.5 �A abovethe graphene plane, and the H–C bond length is about1.17 �A. To show in detail the effects of the carbon atomrelaxation, we show the geometry of the H position atZ ¼ 1:4 �A above graphene in the inset of Fig. 2. Since thecarbon atom just below the hydrogen atom moves 0.33 �A

upward towards the gas phase, an sp3-like geometry isformed between the adsorbed H and the carbon atoms ofgraphene. This indicates that the �-bonding network formedby the hexagonal carbon atoms of graphene weakens, and anattractive interaction dominates between the adsorbed H andthe carbon atoms of graphene.

In the following, we discuss the effective adsorptionpathway of H on graphene. Figure 3 shows a two-dimen-sional cut through the potential energy surface [along a plane

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eV]

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B C D EM N

Fig. 1. Calculated potential energy curves for the interaction of an H atom

with graphene as a function of the normal distance of the H atom from the

graphene Z, with the carbon atoms held rigid. Labels A to E indicate the

positions of the H atom as shown in the inset—A: between (hexagonal

center) hollow and bridge sites, B: bridge sites, C: top site, D: between

top and hollow sites, E: hollow sites, of the upper graphite sheet,

respectively.

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Fig. 2. Calculated potential energy curves for the interaction of an H atom

with graphene as a function of the normal distance of the H atom from

graphene Z, when we allow the carbon atoms to relax. Labels A to E

indicate the positions of the H atom as shown in the inset of Fig. 1. Inset:

the geometry of the H atom at Z ¼ 1:4 �A and the reconstructed graphene.

996 J. Phys. Soc. Jpn., Vol. 72, No. 5, May, 2003 LETTERS Y. MIURA et al.

perpendicular to the graphene sheet, passing through the lineM � N, as shown in Fig. 1(inset)] as a function of the lateralposition of H on the graphene sheet and the normal distanceof H from the graphene sheet. The bold solid line in Fig. 3indicates the effective adsorption pathway of H on graphene.First, the incident H atom goes towards the hollow site ofgraphene, then moves parallel to the surface towards the topsite of graphene. The activation barrier for the H diffusion ongraphene corresponds to 0.1 eV. If the incident H atomcomes near the top site of graphene, the carbon atom underthe H atom moves upwards towards gas phase, and the Hatom is attracted to the carbon atom on graphene. In thisadsorption pathway, there is an activation barrier of 0.18 eV.

Summarizing, we investigate and discuss the adsorptionof an H atom on graphene based on the density functionaltheory (DFT). Our calculation results show that reconstruc-tions of carbon atoms play an important role in the Hadsorption on graphene. When constituent carbon atoms areheld rigid, endothermic H adsorption is about 0.2 eV, andthere is an activation barrier corresponding to 0.3 eV for Hadsorption, due to the strong �-bonding network of thehexagonal carbon. On the other hand, when carbon atomsare allowed to relax, the carbon atom below the H atommoves 0.33 �A upward towards the gas phase, and an sp3-likegeometry is formed between the H and carbon atoms ofgraphene. This relaxation stabilizes the hydrogen–carboninteraction, and the exothermic hydrogen adsorption on thegraphene has a binding energy of 0.67 eV. We also show theeffective adsorption pathway of H on graphene, where theincident H atom first goes towards the hollow site ofgraphene, then moves parallel to the surface towards the topsite of graphene, inducing the carbon atom on graphene torelax. The activation barrier of this reaction pathway is0.18 eV.

Acknowledgements

This work is partly supported by the Ministry ofEducation, Culture, Sports and Science and Technology ofJapan (MEXT) through their Grant-in-Aid for COE Re-search (10CE2004), Scientific Research (11640375,13650026) programs, MEXT Special Coordination Fundsfor Promoting Science and Technology (NanospintronicsDesign and Realization), and by the New Energy andIndustrial Technology Development Organisation (NEDO)through the Materials and Nanotechnology program, theJapan Science and Technology Corporation (JST) throughtheir Research and Development Applying Advanced Com-putational Science and Technology program, and the ToyotaMotor Corporation through their Cooperative Research inAdvanced Science and Technology program. Some of thecalculations presented here were carried out using thecomputer facilities of the Japan Science and TechnologyCorporation (JST), the Yukawa Institute Computer Facility(Kyoto University) and the Institute for Solid State Physics(ISSP) Supercomputer Center (University of Tokyo). Y.M.gratefully acknowledges a fellowship grant from the JapanScience and Technology Corporation (JST) through theirResearch and Development Applying Advanced Computa-tional Science and Technology program.

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Fig. 3. Calculated potential energy surface as a function of the parallel

position of the H atom on the graphene X and the normal distance of the

H from the graphene Z, when we allow the carbon atoms to relax. The

contour spacing is 0.1 eV. The bold solid line indicates the effective

adsorption path reaction way of H on graphene.

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