Adsorption of hydrogen on boron-doped graphene: A first-principles prediction
Y. G. Zhou, X. T. Zu, F. Gao, J. L. Nie, and H. Y. Xiao
Citation: J. Appl. Phys. 105, 014309 (2009); doi: 10.1063/1.3056380
View online: dx.doi/10.1063/1.3056380
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Adsorption of hydrogen on boron-doped graphene:
Afirst-principles prediction
Y.G.Zhou,1,a͒X.T.Zu,1,2F.Gao,3J.L.Nie,1and H.Y.Xiao1
1Department of Applied Physics,University of Electronic Science and Technology of China,
Chengdu610054,People’s Republic of China
劝学的翻译2International Center for Material Physics,Chine Academy of Sciences,Shenyang110015,
People’s Republic of China
3Pacific Northwest National Laboratory,MS K8-93,P.O.Box999,Richland,Washington99352,USA
͑Received18September2008;accepted14November2008;published online12January2009͒The doping effects of boron on the atomic adsorption of hydrogen on graphene have been investigated using density functional theory calculations.The hydrogen adsorption energies and electronic structures have been considered for pristine and B-doped graphene with the adsorption of hydrogen on top of carbon or boron atom.It is found that the B-doping forms an electron-deficient structure and decreas the hydrogen adsorption energy dramatically.For the adsorption of hydrogen on top of other sites,similar results have also been found.The results indicate that the hydrogen storage capacity is improved by the doping of B atom.©2009American Institute of Physics.͓DOI:10.1063/1.3056380͔
I.INTRODUCTION
Hydrogen͑H͒storage is one of the key technologies in
the application as the future energy carrier.However,many
traditional H storage alloys usually have the problems of low
gravimetric storage capacity,and thus are not able to satisfy
the requirements for practical H storage.Recently,carbon-
bad nanomaterials,such as carbon nanotubes͑CNTs͒and
graphite nanofibers,have attracted much attention as candi-
dates for high H storage capacity.1–11It is well known that
车队管理办法chemical doping is a practical and feasible way to alter and
adjust the binding configurations and the electronic proper-
ties of carbon-bad nanomaterials.B-or N-substitution in
CNT materials has been widely studied experimentally and
theoretically.3–10From the investigation of the doping effects
of B and N on H adsorption in single-walled carbon nano-tubes,Zhou et al.1concluded that B-doping forms an electron-deficient six-membered ring structure and decreas the H atomic adsorption energy.On the contrary,the N-doping forms an electron-rich six-membered ring structure and increas the H adsorption energy.Additionally,from the investigation of the lithium͑Li͒adsorption in BN-doped nanotubes,they found that the Li adsorption in͑8,0͒BN-doped nanotubes is unfavorable energetically.11Also,B-or N-doping in graphitic carbon materials has been widely stud-ied experimentally and theoretically.12–16
However,the in-vestigation of doping effects of B and N on H adsorption on graphene are still controversial.In order to fundamentally understand how to optimize graphene for H storage devices, we investigated the interaction of H atom with pristine and B-doped graphene and discusd the effects of H diffusion abilities.The purpo of this study is to obtain a microscopic understanding of H interaction with graphene.IIPUTATIONAL METHOD
All the calculations were performed using the discrete Fourier transform͑DFT͒method,as implemented in Vienna ab initio simulation package͑V ASP͒.17,18The graphene is rep-rented in a supercell by a slab of one graphene sheet and a vacuum region of14Å.Three specific sites are investigated, as shown in Fig.1͑a͒.The electron-ion interaction is de-scribed by optimized ultrasoft pudopotentials,19and the Kohn–Sham equations are solved using plane waves with kinetic energies up to450eV.The surface Brillouin zone is sampled using a7ϫ7ϫ1k-point grid for2ϫ2cell and6ϫ6ϫ1k-point grid for a3ϫ3cell.For the exchange corre-lation energy,we adopted the generalized gradient approximation.20–22All the calculations were performed in
a͒Author to whom correspondence should be addresd.Electronic mail: FIG.1.͑a͒The schematic view of three high symmetry sites of hydrogen chemisorbed on graphene and͓͑b͒and͑c͔͒the arrangements of H atoms on the top sites of pristine graphe
ne plane.͑b͒and͑c͒correspond to the cov-erages of1/8and1/18ML,respectively.͑d͒The calculated PEC for the interaction of H with graphene in1/8ML͑solid line͒and1/18ML͑dotted line͒.
JOURNAL OF APPLIED PHYSICS105,014309͑2009͒
0021-8979/2009/105͑1͒/014309/4/$23.00©2009American Institute of Physics
105,014309-1
spin manner.The atomic adsorption energy ͑E atomicadsorption ͒of H is defined as the difference between the total energy of H-host ͑E host-H ͒and the sum of the energies of the parated host ͑E host ͒with a H atom ͑E H ͒and is calculated as follows:
E atomicadsorption =E host-H −E host −E H .
͑1͒
紫罗兰原产地The total energy of H-host ͑E host-H ͒and the energy of host ͑E host ͒were calculated using the same supercell and calcula-tion parameters.By definition,after relaxation,E atomicadsorption Ͻ0in Eq.͑1͒corresponds to a stable and exo-thermic chemical absorption.If E atomicadsorption Ͼ0,it is endo-thermic,which corresponds to a local minimum.III.RESULTS AND DISCUSSION
A.Effect of H–H interaction on the binding of H to graphene
六年级语文第四单元作文As a starting point it is worth considering the size of the supercell for the adsorption of H.Two coverages correspond-ing to different parations between H in neighboring super-cells have been shown in Figs.1͑b ͒and 1͑c ͒.The hexagonal unit cell in Fig.1͑b ͒,corresponding to the coverage of 1/8ML,contains eight carbon atoms and one H atom,with the paration of 4.92Åbetween neighboring H atoms.The unit cell in Fig.1͑c ͒,corresponding to the coverage of 1/18ML,contains eighteen carbon atoms and one H atom,with the paration of 7.38Åbetween neighboring H atoms.The po-sitions of H atoms were fixed at different distances along a line perpendicular to the graphene layer toward the top of carbon atom.The calculated potential energy curves ͑PEC ͒are shown in Fig.1͑d ͒.From the similar PEC,we find that no repulsion exists when H are parated by 4.92Å,which in-dicates that the coverage of 1/8ML is sufficient for studying the H-graphene interaction.
B.PECs for H adsorption on a top site
Bad on a supercell of 2ϫ2,the interaction of H with pristine and B-doped graphene for the adsorption on a top site has been analyzed.In Fig.2͑a ͒,we show the PEC for H interaction with pristin
e and B-doped graphene as a function of the normal distance of the H atom from graphene.In pristine graphene H is adsorbed on the top of a C atom and in
B-doped graphene H is adsorbed on the top of B atom.In the calculations,the C and B atoms are held rigid.V =0.0eV in Fig.2͑a ͒corresponds to the H atom in the gas pha,far from the graphene sheet.From the potential energy curves for H interaction with pristine graphene ͑A ͒,it can be en that the endothermic H adsorption on graphene is more than 0.2eV .A metastable adsorption is found and the activation barrier is about 0.3eV .The results are well in agreement with the theoretical calculation of Miura et al.23When H atom is adsorbed on the top of B-doped graphene ͑B ͒,we clearly e a change in the adsorption energy comparing with the result of pristine graphene.The minimum adsorption en-ergy of Ϫ1.7eV is found when the exothermic H is located 1.30Åabove the B atom.This indicates that when the H atom is at parations less than stable binding distance the H atom is pushed away from the graphene surface toward the stable binding site.When the H atom is at parations slightly larger than the stable binding paration,H atom is pulled toward the graphene surface.The changed magnitude of the H position is dependent on the original.The results indicate that the doped B atom leads to the optimistic expec-tation of enhancing the capacity of H storage.
C.Adsorption energies and local geometric structures after full optimization
The adsorption of H on pristine and B-doped graphene after full optimization has also been considered,as shown in Fig.2͑b ͒.The H atomic adsorption energies are found to be about Ϫ0.73and Ϫ1.82eV ,corresponding to pristine and B-doped graphene,respectively.The results suggest that the B-doping reduces the adsorption energy about 1eV and increas the storage capacity.To gain some insight into the storage capacity of H,N-doped graphene has also been in-vestigated.The adsorption energy of H is found to be about Ϫ0.71eV ,which is similar to the ca of pristine graphene without doping,indicating that the storage capacity is slightly decread with the doping of N atom.The reason may be due to the fact that N-doping forms an electron-rich six-membered ring structure.The bond length and angle are listed in Table I for comparison.It is clear that B–C bonds in B-doped graphene are longer than C–C bonds in pristine graphene.For the doping of N atom,the bonds of N–C are found to be the same as C–C bonds in pristine graphene.The results clearly indicate that B-doping destabilizes the graphene structure slightly while for the N-doping,no desta-bilization has been found.This change is due to the different numbers of valence electrons in B,N,and C atoms.After H atom adsorbing on the top of the center atom in Fig.2͑b ͒,all the bonds around the center atom become longer.The in-crea in B–C,C–C,and N–C bond lengths is found to
be about 1.4%,4.5%,and 5.6%,respectively,and thus,the dop-ing of B and N affects the bond lengths upon H adsorption.Especially,the B-doping could counteract the structure insta-bility due to the atomic H adsorption to some degree,result-ing in the decrea in H atomic adsorption energies.The newly formed H–C,H–B,and H–N bonds are found to be about 1.12,1.25,and 1.04Å,respectively.The differences in the bond length of H–C,H–B,and H–N are due to the fact that the valence electrons in the center atoms cau
西班牙留学费用different
项的成语FIG. 2.͑a ͒The calculated PEC for the interaction of a H atom with graphene as a function of the normal distance of the H atom from the graphene,where the curve labeled A corresponds to the adsorption of H on top of carbon atoms of pristine graphene,and the curve labeled B corre-sponds to the adsorption of H on top boron atoms of B-doped graphene.͑b ͒The optimized local geometric structure for graphene before and after hy-drogen atom is adsorbed on the top of center atoms.
bond orders between center atoms and H.From Table I ,the changes in the bond length and angle clearly show the bind-ing transformation of the center C,B,and N atoms from sp 2hybridization to sp 3hybridization upon the H adsorption.D.Electronic structure
To shed light on the mechanism for B doping effect on H storage,the spin-up density of state ͑DOS ͒for different graphenes is shown in Fig.3.From Fig.3͑a ͒,we clearly e that the pristine graphene shows a typical miconductor band structure.After H atom adsorbing on the top of a car-bon atom,a donor level forms below the conduction band minimum in the band gap region in Fig.3͑b ͒.We also find that the peak at the Fermi level for the H-covered graphene is attributed to 2p states of nearby substrate C atoms.In the B-doped graphene,an acceptorlike level above the valence band maximum
appears,indicating that the B-doped graphene is electron deficient,as shown in Fig.3͑c ͒.When H atom is adsorbed on the top of the B atom,the electron-deficient structure is optimized to accept electrons,and the former acceptor level has been filled with electrons,as shown in Fig.3͑d ͒.A coordinationlike B–H bond has been formed,which indicates that the changes of electronic struc-ture in graphene due to the impurity doping and H atomic adsorption.To gain some insight into the electronic structure
of this chemisorption system,the electron density in the slices including the substrate C or B atoms for various graphenes have been investigated,and the corresponding contour plots are shown in Fig.4.The area with den red contour lines shows a high electron density region and the area with den green contour lines shows a lower electron density region.Comparing to the contour plots of Fig.4͑a ͒,we find that when the H atom is adsorbed on the top of the C atom the electron density decreas in the region between this C atom and the neighboring C atoms ͓Fig.4͑b ͔͒.From Fig.4͑c ͒,we find that the doping B atom has the similar ,decreasing electron density.Therefore,the exis-tence of B atom leads to an electron-deficient structure.When the H is adsorbed on the top of B atom in Fig.4͑d ͒,H transfers some electronic charges to the B atom,which de-creas the adsorption energy of H in B-doped graphene.The results indicate that the change in electron density in the graphene is due to the impurity doping,which is in agreement with our DOS result.
E.PEC for H adsorption on the other sites
First,we discuss the PEC of H adsorption on a hollow site,as shown in Fig.5͑a ͒.For the interaction of H with pristine graphene ͑A ͒,no metastable adsorption occurs.The activation barrier for H penetrating the center of the hexago-nal carbon is found to be about 3.7eV ,suggesting that H
TABLE I.The C–C ͑B or N ͒bond length and C–C ͑B or N ͒-C angle for various graphenes with and without hydrogen atomic adsorption after full geometric optimization.
d 1͑Å͒
d 2͑Å͒d 3͑Å͒a 1͑o ͒a 2͑o ͒a 3͑o ͒
Pristine grapheme 1.420 1.420 1.420120.0120.0120.0H-covered graphene 1.484 1.484 1.484114.9114.9114.9B-doped graphene
1.458 1.458 1.458120.0120.0120.0H-covered B-doped graphene 1.478 1.478 1.478117.8117.9117.9N-doped graphene
1.422 1.422 1.422120.0120.0120.0H-covered N-doped graphene
1.502
1.502
1.502
113.6
113.6
113.6
整理床铺FIG.3.The spin-up DOS in pristine and B-doped graphenes and the corre-sponding atomic hydrogen
adsorbed graphenes,where the solid line indi-cates the position of Fermi level.
FIG.4.͑Color online ͒Electron density distribution for atomic adsorption of a single H on the top of a C atom in pristine graphene ͑b ͒,on top of B atom in B-doped graphene ͑d ͒.͓͑a ͒and ͑c ͔͒The contour plots for the correspond-ing graphenes without H atom adsorption.
adsorption and desorption through the hexagonal center of pristine graphene hardly occur in the thermal energy region.The most prominent change in adsorption energy is the ca when the H atom is adsorbed on B-doped graphene ͑B ͒.In this ca,the activation barrier for H penetrating the center of the hexagonal carbon descends to about 1.0eV ,indicating that the capability of H penetrating to the graphite bulk is enhanced.We also clearly e that there is a minimum ad-sorption energy of Ϫ0.1eV ,which corresponds to the H atom located 0.88Åabove the B-doped graphene.For this site,the activation barrier is found to be about 0.3eV .Fi-nally,the absorption of H on a bri ͑bridge ͒site has also been considered,which is shown in Fig.5͑b ͒.For the adsorption of H on pristine graphene,a metastable site is formed with the adsorption energy of 0.3eV .For the adsorption of H on B-doped graphene,the minimum adsorption energy is found to about Ϫ1.9eV ,and this means that the doping of B atom decreas the adsorption energy of H.However,the activa-tion energy barrier for H penetrating the center of the hex-agonal carbon of B-doped graphene plane is found to be abo
ut 10.0eV ,while the energy barrier in pristine graphene is even much higher.The results clearly show that it is difficult for H to penetrate the hexagonal carbon of graphene through the bri sites.IV.CONCLUSION
The adsorption of an H atom on pristine and B-doped graphenes has been investigated and discusd bad on the
DFT calculations.For the adsorption of H on a top site,B-doping forms an electron-deficient structure and decreas the adsorption energy of equilibrium position about 1eV .This indicates that the H storage capacity is improved with the doping of B atom.For the adsorption of H on hollow site,B-doping decreas the activation barrier about 2.7eV for H penetrating the center of the hexagonal carbon,which en-hances the penetrating ability of H to the bulk graphite.For the adsorption of H on bri site,the adsorption energy also is found to be decread dramatically,but with high activation energy for penetrating.ACKNOWLEDGMENTS
审计人员
This study was supported financially by the NSAF Joint Foundation of China ͑Grant No.10376006͒and by the Si-chuan Young Scientists Foundation ͑Grant No.03ZQ026-059͒and by the Project-sponsored by SRF for ROCS,SEM.F.Gao was supported by the Division of Materials Sciences and Engineering,
Office of Basic Energy Sciences,U.S.De-partment of Energy under Contract No.DE-AC05-76RL01830.
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FIG.5.The calculated PEC for the interaction of a H atom with graphene as a function of the normal distance of the H atom from the graphene.͑a ͒The adsorption of H on hollow sites and ͑b ͒the adsorption of H on bri sites.
