石墨烯 储氢

Multifunctional Porous Graphene for Nanoelectronics and Hydrogen Storage:

New Properties Revealed by First Principle Calculations

Aijun Du,*,?Zhonghua Zhu,?and Sean C.Smith*,?

The Uni V ersity of Queensland,Australian Institute for Bioengineering and Nanotechnology,Centre for

Computational Molecular Science,QLD 4072,Brisbane,Australia,and The Uni V ersity of Queensland,School of

Chemical Engineering,QLD 4072,Australia

Received January 8,2010;E-mail:a.du@https://www.sodocs.net/doc/ac17781331.html,.au;s.smith@https://www.sodocs.net/doc/ac17781331.html,.au

The discovery of single layer graphite or graphene has opened up exciting opportunities for the design of novel electronic devices and interconnects 1due to its unique electronic properties.2Among them,the use of making a truly tiny transistor offers much hope for making faster,smaller electronics devices once silicon reaches its limits.3However,the lack of an obvious “band gap”is a formidable problem that hinders the practical application of graphene-based nanoelectronics.4In the area of clean energy,graphene doped with Li atoms has been predicted to be a promising candidate for hydrogen storage.5Li-doped graphene is expected to be superior to a Li-doped carbon nanotube because both sides might be readily utilized to ensure ef?cient hydrogen storage.5,6However,with a high coverage of Li atoms on graphene,the adsorption of hydrogen will be signi?cantly weakened by the strong electrostatic interaction between Li cations.5

Most recently,Bieri et al.have for the ?rst time succeeded in synthesizing a well-de?ned porous graphene.7,8With the insertion of holes of speci?c size and distribution into graphene sheets,the electronic structure is expected to be greatly modi?ed.Additionally,the separation of Li dopants on porous graphene is expected to be large (4.23?)compared to that on Li-doped graphene and hence the electrostatic interaction between Li cations should be lower,leading to enhanced adsorption of hydrogen molecules.Moreover,the hydrogen storage capacity may also be signi?cantly increased.5

In this communication,we report a series of calculations to explore (1)whether the creation of porous graphene may open a band gap and (2)whether hydrogen storage performance in Li-decorated porous graphene can be greatly improved compared to that in Li-doped graphene.Remarkably,we ?nd that porous graphene has a direct gap (3.2eV),which is similar to that of TiO 2and graphitic C 3N 4materials with high photocatalytic activities.9Importantly,the adsorption energy of hydrogen on Li-decorated porous graphene is 0.26eV on average (0K),which is much closer to the optimal adsorption enthalpy (-0.155eV)at ambient conditions.10Additionally,the adsorption energy of the ?rst hydrogen molecule is at least 50%larger than that in Li-doped graphene as reported by all the existing studies.5Our results predict porous graphene to be a promising optical and hydrogen storage material for nanoelectronics and clean energy application.

First,we will explore the electronic structure of porous graphene by using the plane-wave basis VASP code 11implementing the projector augmented wave method.12Figure 1a and 1b present the optimized geometry and band structure for porous graphene based on the local density approximation (LDA).13The lattice constant is calculated to be 7.45?,which is in good agreement with the experimental value.6The LDA calculations predict a direct gap of

2.34eV.It is well-known that LDA underestimates the band gap and hybrid functionals such as HSE0614are better able to predict experimentally measured band gaps with high accuracy.Figure 1c presents the calculated band structure for porous graphene using the HSE06functional.Clearly,valence bands have similar disper-sions for both the LDA and HSE06functional calculations,but the conduction band minimum computed with the hybrid functional is shifted to higher energy relative to the Fermi level.The band gap is increased to

3.2eV,i.e.in the range of UV light.This is comparable to TiO 2and graphitic C 3N 4materials,which have shown potential application in photocatalyzed splitting of water into hydrogen.9These results suggest that porous graphene may solve the obvious “band gap”problem of graphene for nanoelectronics applications and display a photocatalytic activity similar to those of TiO 2and graphitic C 3N 4materials.

We now turn to examine the adsorption of Li atoms on porous graphene.Similar to graphene,5we ?nd the hexagonal center is still the most energetically favorable adsorption site for a Li atom on porous graphene.However,the adsorption energy (-1.81eV)for a Li atom onto porous graphene (7.45?×7.45?)is much stronger than that (-0.86eV)on a (2×2)graphene (5.92?×5.92?).5It is however very close to that (-1.65eV)on a (3×3)graphene (7.38?×7.38?).Clearly,this effect may therefore be attributed to the natural separation of Li adsorption sites on porous graphene,which avoids strong electrostatic interactions between the Li cations.To examine (and discount)the likelihood of Li clustering,we computed the adsorption of a second Li atom onto porous graphene in a large (3×3)supercell (22.35?×22.35?)and found that it adsorbs preferentially on another hexagonal center,which can be understood by the fact that the cohesive energy of bulk Li (-1.795eV per Li atom)15is very close to the calculated Li atom adsorption energy.

Figure 2a presents the adsorption energies and optimized geometries for ?rst,second,thirdm and fourth hydrogen molecules around the adsorbed Li atom on porous graphene.In contrast with previous studies on Li-doped graphene,5the adsorption energy for the ?rst hydrogen molecule is calculated to be as high as -0.265

?Australian Institute for Bioengineering and Nanotechnology.?

School of Chemical Engineering.

Figure 1.(a)Optimized geometry for 2D porous graphene (dark atoms C and light atoms H).Calculated band structures are shown based on LDA (b)and HSE06(c)exchange correlation functional,respectively.The dotted line at zero indicates the Fermi

level.

Published on Web 02/15/2010

10.1021/ja100156d 2010American Chemical Society

28769

J.AM.CHEM.SOC.2010,132,

2876–2877

eV and the H-H bond length was elongated to0.80?.Interestingly, the adsorption energies for the second and third hydrogen molecules remain nearly unchanged(-0.27and-0.256eV,respectively). There is a sudden drop(-0.206eV)for the fourth hydrogen molecule due to the strong steric interaction between the adsorbed H2molecules.To explore the adsorption mechanism,three-dimensional charge density differences for Li-decorated porous graphene in the presence of one and three adsorbed hydrogen molecules are plotted in Figure2b and2c,respectively.Charge depletion and accumulation at both sides of the H2molecule clearly indicate a polarization mechanism.The above calculations are based

on LDA,since the van der Waals contribution in alkali doped carbon materials has been shown to be better accounted for by LDA.16Similar calculations based on the PW91functional17were performed for comparison.The adsorption energies are calculated to be-0.153,-0.125,and-0.095eV for?rst,second,and third hydrogen molecules on Li-decorated porous graphene,which are much higher than that reported in Li-decorated C60(-0.075eV).18 Lithium atoms can form a dense coverage on porous graphene with a Li-Li distance of4.23?and are expected to resist clustering due to electrostatic interaction between the cationic adsorbed Li atoms.The maximum number of absorbed Li atoms in one unit cell of porous graphene is two and four for one and both sides, respectively.We examine the case of three adsorbed hydrogen molecules around each Li atom since this ratio retains the higher adsorption energies as shown in Figure2a.Figure3a and3b present the optimized geometries for6and12hydrogen molecules adsorbed on2and4Li-decorated porous graphene,respectively.The average adsorption energies of H2molecules are calculated to be around 0.255and0.243eV in both cases,respectively.The H2storage capacities corresponding to Figure3a and3b are7.0and12wt%, respectively.These are much higher than the limit(6wt%)set for the feasible hydrogen storage capacity.Recently,unphysical overbinding by density functional calculations in the Ca-H2 system19has been revealed with the correct description of hybridization involving the d orbital on Ca being implicated.This is not expected to be problematic in the Li-H2system,however, due to the absence of d orbitals on the Li atom.Previous MP2 calculations have indeed also shown favorable adsorption of H2in Li-H2system.20

In summary,we offer a new prediction that porosity in graphene opens a direct band gap.The predicted band gap(3.2eV)is comparable to the case of TiO2and graphitic C3N4materials with potential applications in photocatalysis.9Most interestingly,the adsorption of hydrogen molecules on Li-decorated porous graphene is signi?cantly enhanced with up to a12wt%hydrogen storage capacity potentially feasible,in signi?cant contrast to the case of Li-doped graphene.In light of the recent experimental synthesis of porous graphene,7,8our results suggest multiple new potential applications for this fascinating material. Acknowledgment.This research was undertaken in the NCI National Facility in Australia,which is supported by the Australian Commonwealth Government.

Supporting Information Available:Computational details and a side view of Figure3.These materials are available free of charge via the Internet at https://www.sodocs.net/doc/ac17781331.html,.

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JA100156D

Figure2.(a)Adsorption energies and optimized geometries for?rst,

second,third,and fourth hydrogen molecules on Li-decorated porous

graphene.Green,white,blue,and yellow balls represent C,H,Li,and

adsorbed hydrogen molecules,respectively.(b)and(c)present plots of

charge density difference with an isovalue of0.01e/?3for Li-decorated

porous graphene in the presence of one and three hydrogen molecules.The

red and yellow iso-surface indicates space charge depletion and accumulation.

Figure3.Top views of the optimized geometries for(a)6and(b)12

hydrogen molecules adsorbed on two and four Li-decorated porous graphene.

Green,white,blue,and yellow balls represent C,H,Li,and the physisorbed

hydrogen molecules,respectively.

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