Role of sodium decoration on the methane storage properties of BC3 nanosheet

ORIGINAL RESEARCH

Role of sodium decoration on the methane storage propertiesof BC3 nanosheet

Morteza Moradi • Ali Ahmadi Peyghan

Received: 3 September 2013 / Accepted: 10 December 2013

� Springer Science+Business Media New York 2013

Abstract First-principles calculations including disper-

sion correction are carried out to investigate pristine and

Na-decorated graphene-like BC3 (h-BC3) for their appli-

cation as methane storage materials. Structural optimiza-

tion shows that the methane is physisorbed on the pristine

sheet via van der Waals forces with adsorption energy of

-2.7 kcal/mol. It was found that unlike the pristine

graphene, sodium decorated sheet can effectively interact

with the CH4 molecule, so that each metal atom bound on

sheet may adsorb up to four CH4. Furthermore, no bond

dissociation was observed for the adsorption of CH4 on Na-

decorated h-BC3, which means that decorated sheet can act

as a storage device for methane safety storage. The results

indicate that decoration of the Na atom on surface of sheet

induces significant changes in electronic properties of the

sheet and its Eg is unchanged after adsorption of CH4

molecules. Theoretical methane storage capacity of Na-

decorated BC3 nanosheet could approach 18.1 wt%.

Keywords BC3 nanosheet � DFT � Methane storage �Metal decoration

Introduction

It is well know that energy shortage is a major problem in

the world. However, fossil fuels, such as oil and natural gas

(NG), pollute the environment, contribute to global warn-

ing, and have the limited supply. In recent years, consid-

erable efforts have been devoted to develop new fuels

technologies to meet transportation energy demand and

reduce reliance on conventional petroleum-based gasoline

[1]. Among the possible fuels, NG (methane is the primary

constituent) is one of the most promising alternatives

because of its relatively clean properties and cheaper cost.

However, its energy density is only 0.038 MJ/L, 0.11 % of

that of gasoline. Thus, the mileage per unit volume of the

fuel tank is very low [2]. In order to make NG vehicles

competitive with other types of vehicle and successful in

the market, an adequate amount of CH4 must be stored in

the on board fuel tank. Due to the low critical temperature

(191 K), CH4 cannot be liquefied by only implementing

pressure at room temperature. Compressed natural gas

(CNG) is an alternative solution, but has some disadvan-

tages such as requirements of costly multi-stage compres-

sion to 20–25 MPa high pressures. Compared to CNG,

absorbed natural gas (ANG) is promising and efficient by

storing NG in porous media [3]. The materials used for

methane storage have been studied for quite a long time

(both experimentally and theoretically) and several criteria

have been set up for the porous (carbonaceous materials or

metal-organic framework systems) materials for such

storage. The DOE criteria are 180 cm3 (STP)/cm3 at 35 bar

for methane storage and the adsorption energy criteria are

*2.9 kcal/mol [4]. Higher adsorption energy is not advo-

cated as it could lead to other complicacies. Moreover, the

materials should have surface are of[2,000 m2/g and high

volume pores (\2 nm) [5].

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11224-013-0384-0) contains supplementarymaterial, which is available to authorized users.

M. Moradi (&)

Semiconductors Department, Materials and Energy Research

Center, P.O. Box 31787-316, Karaj, Iran

e-mail: [email protected]

A. A. Peyghan

Islamic Azad University, Central Tehran Branch, Tehran, Iran

123

Struct Chem

DOI 10.1007/s11224-013-0384-0

http://dx.doi.org/10.1007/s11224-013-0384-0

The discovery of carbon nanotube (CNT) [6] and graph-

ene [7] has aroused considerable interest in the properties of

these nanostructures [8–10]. One of the trends in these

investigations is the study of the adsorption properties of

nanostructures. A considerable number of works is dedicated

to the investigation of the adsorption of hydrogen [11],

methane [12], ethane [13], and other gases. For instance,

Mackie et al. [14] have studied adsorption and wetting

behavior of methane films on catalytic CNTs. Talapatra et al.

[15] have experimentally obtained adsorption isotherm of

Ne, CH4, and Xe on CNTs bundles. Bekyarova et al. [16]

measured adsorption of methane in single wall nanohorn

by experimental method, the volumetric capacity up to 160

(v/v). Tanaka et al. [17] studied the adsorption of methane on

isolated CNT by density functional theory (DFT) method,

and obtained gravimetric capacity of 198 g CH4/kg C at

room temperature. The results of a number of papers show

that even various improvements such as improvement in

structure arrangement [18], electrification [19], functionali-

zation [20], etc., do not result in significant improvement of

the adsorption properties of nanostructures, which are suit-

able for industrial use. In recent years, metal dispersed car-

bon nanostructures have been approved to be efficient for

hydrogen storage due to its light weight, diversity in struc-

tures, large surface area, and interesting hydrogen adsorption

properties [21, 22]. Most of the results suggest that the

adsorption energy of the hydrogen molecule on the decorated

carbon-based systems increases compared with their pristine

ones.

Boron atoms have been widely used as dopants in carbon

nanostructures for building functional materials owing to

their similar atomic radius. Not surprisingly, a uniform BC3

sheet has already been synthesized in the laboratory [23]. Its

geometric structure was found to be almost identical to

graphene, except that some carbon atoms are replaced by

boron atoms so that six carbon atoms form a hexagon sur-

rounded by six boron atoms [24]. In addition, Pontes et al.

[25] found that boron atoms can substitute carbon atoms in a

graphene sheet without any activation barrier. However, it

should be stressed that the BC3 honeycomb sheet (h-BC3) is a

semiconductor with an indirect gap, while graphene is a

semiconductor with a zero gap. Here, we are interested in

finding out if there is a potential possibility for pristine h-BC3

serving as a chemical storage for CH4 molecule, and if not,

can we find a method for improving the affinity of the sheet to

CH4? To these ends, we have studied the reactivity of CH4 to

the pure and Na-decorated h-BC3 by using DFT calculations.

Computational methods

We selected a BC3 sheet consisted of 102 C and 34 B

atoms, in which the end atoms have been saturated with

hydrogen atoms to reduced boundary effects. Structure

optimizations and natural bond orbitals (NBO) analyses

were performed using three parameter hybrid generalized

gradient approximation with the B3LYP functional aug-

mented with an empirical dispersion term (B3LYP-D) with

6-31G basis set including the d-polarization function

[denoted as 6-31G (d)] as implemented in the GAMESS

suite of program [26]. GaussSum program has been used to

obtain the density of states (DOS) results [27]. The B3LYP

is demonstrated to be a reliable and commonly used

functional in the study of different nanostructures [28–31].

We have defined adsorption energy in the usual way as:

Ead ¼ E CH4=h-BC3ð Þ�E h-BC3ð Þ�E CH4ð Þþ E BSSEð Þ; ð1Þ

where E (CH4/h-BC3) corresponds to the energy of the

h-BC3 in which CH4 has been adsorbed on the surface,

E (h-BC3) is the energy of the isolated sheet, E (CH4) is the

energy of a single CH4 molecule, and E (BSSE) is the

energy of the basis set superposition error. Hence,

according to Eq. (1), negative adsorption energy indicates

that the system is stable. In some cases, the adsorption

energy is positive which corresponds to the local minimum

where the adsorption of CH4 on the sheet is prevented by a

barrier.

Results and discussion

Pristine h-BC3

In Fig. 1, we have shown structure of the optimized h-BC3,

where two types of bonds namely B–C and C–C can be

identified, with corresponding lengths of 1.56, and 1.42 A,

respectively. In order to obtain the stable configurations

(local minima) of single CH4 adsorbed on the h-BC3,

various possible initial adsorption geometries are consid-

ered. However, only two local minima structures were

obtained upon the relaxation process (Fig. 2). More

detailed information from the simulation of the different

CH4/h-BC3 systems, including values of Ead and electronic

properties for these configurations is listed in Table 1. The

A configuration configuration shows a weak interaction

between CH4 molecule and the h-BC3. In this configuration,

CH4 molecule was adsorbed to boron site of sheet from its

one of hydrogen atoms, and the corresponding calculated

Ead value (Table 1) is about -1.3 kcal/mol. The low Ead of

CH4 on the sheet in this structure and large interaction dis-

tance between them ([4 A) reveal the physical nature of the

interaction. As shown in Fig. 2b, the most stable configu-

ration of the CH4/h-BC3 system (B) is that in which the three

hydrogen atom of CH4 is close to sheet with equilibrium

distance of 2.85 A and corresponding Ead of -2.7 kcal/mol.

Struct Chem

123

The obtained results indicate that CH4 molecule adsorbed on

the h-BC3 through weak van der Waals interaction, which

means that the process is physisorption. For this weak

adsorption, the charge transfer between sheet and CH4

molecule is very small (about 0.017 e). Additional data

about geometric features of the studied materials are avail-

able in the supplementary section (Table 1S). Similar to our

results, Qiu et al. [32] have shown that the Ead of CH4

adsorbed on the carbon sheet is very low within the DFT

including dispersion correction (DFT-D3).

To verify the effects of the adsorption of CH4 molecule on

the electronic properties of h-BC3, the DOS of the molecule-

sheet adsorption systems are calculated. As shown by the

calculated DOS and the energy gaps (Eg) between the highest

occupied molecular orbital (HOMO) and lowest unoccupied

molecular orbital (LUMO) in Fig. 1 and Table 1, the pure

sheet is found to be a semiconductor with Eg of 2.65 eV.

Weak interactions are evident in the DOS structures (Fig. 2a, b),

which show little change after the adsorption. For example,

in configuration A, both of conduction and valence levels

slightly move to lower energies, so that Eg of the sheet

slightly decreased from 2.65 to 2.63 eV due to charge

transfer between molecule and sheet. This change in elec-

tronic properties is negligible, indicating that the h-BC3 is

still a semiconductor. Thus, we conjecture that the electronic

properties of pure h-BC3 are insensitive to the CH4 molecule.

Na-decorated h-BC3

The above results indicate that the interaction between CH4

molecule and pure h-BC3 is very weak and that the CH4

molecule cannot strongly adsorb on pristine one. So, the

influence of decoration Na metal atom on the h-BC3 on the

adsorption of CH4 molecule is considered. Now, we con-

sider the adsorption of a single Na atom on the sheet. We

define the binding energy (Eb) using the expression:

Eb ¼E ðnNa=h � BC3Þ � E ðh � BC3Þ � nE ðNaÞ½ �

n;

ð2Þ

Fig. 1 Structure of optimized

h-BC3 sheet and its DOS

Fig. 2 Model for two stable adsorption of CH4 on the h-BC3 and their DOS

Struct Chem

123

where n is the number of the Na atom. E (nNa/h-BC3),

E (h-BC3), and E (Na) are the total energy of the h-BC3

with Na adsorption, the pristine h-BC3 sheet, and the free

Na atom, respectively. By this definition, the negative

value of Eb indicates that the adsorption is exothermic, and

hence stable. There are three kinds of possible sites, i.e., on

top, bridge, and hollow sites, which metal atom can locate.

In fact, according to the calculated results, the top and the

bridge sites are energetically unfavorable compared to the

hollow sites. The h-BC3 has two different kinds of hollow

sites, involving the hollow center of the C6 hexagon (H1)

and C4B2 hexagon (H2) as shown in Fig. 1. To ensure the

most preferred location of Na atom, we optimize the Na

added on different hollow center of the hexagonal rings of

the h-BC3 and compare the relative stability. Out of the two

H sites for metal adsorption, the H1 site is energetically

most preferred. These results are in agreement with Li

decoration in h-BC3 obtained by Yang and Ni [33]. The

calculated value of Eb of Na-decorated h-BC3 is

-44.3 kcal/mol. This implies that the clustering of Na

atoms on the h-BC3 is suppressed. In this case, the nearest

distances between the surfaces of the sheet and Na atom are

about 2.13 A. According to the NBO charge analysis of

single Na atom adsorbed on h-BC3, metal atom donates

electrons to the neighboring carbon atoms on the h-BC3,

and this charge transfer decreases for boron and carbon

atoms far away from the metal atom. This charge transfer

behavior leads to metal atoms in cationic form and renders

extensive heteropolar bonding between the metal atom and

the nearest carbon atoms. As a consequence, extra dipole

moments are formed, thus resulting in an increase in the

CH4 molecule uptake (Fig. 3).

We now turn to discussion of the adsorption of CH4

molecule on the Na-doped h-BC3. The geometries of CH4/

Na-decorated h-BC3 systems were fully optimized without

any restrictions. The Ead of CH4 in decorated sheet is

calculated using following equation:

Ead ¼E ðmCH4=nNa=h � BC3Þ � E ðnNa=h � BC3Þ � mE ðCH4Þ½ �

m;

ð3Þ

where m is the number of the CH4 molecules and E (mCH4/

nNa/h-BC3) is the total energy of the nNa/h-BC3 system

with mCH4 molecules adsorption. We first consider the

interaction between CH4 molecules and the 1Na-decorated

h-BC3 by attaching additional CH4 step by step. Figure 4

denotes side views of the equilibrium configuration for the

adsorption of one to four CH4 molecules. When the first

CH4 molecule is adsorbed on Na-decorated h-BC3, it

adsorbs on top of the Na atom with Ead of -5.6 kcal/mol.

Fig. 3 Structure of optimized a 1Na-decorated h-BC3, b 2Na-decorated h-BC3 complex and their DOS

Table 1 Adsorption energy (Ead, kcal/mol), HOMO energies

(EHOMO), LUMO energies (ELUMO), and HOMO–LUMO energy gap

of systems in eV

Configuration Ead EHOMO ELUMO Eg DEg (%)a

h-BC3 – -6.40 -3.75 2.65 –

(A) CH4/h-BC3 -1.3 -6.43 -3.80 2.63 -0.7

(B) CH4/h-BC3 -2.7 -6.40 -3.76 2.64 -0.3

a Change of Eg of sheet after CH4 adsorption

Struct Chem

123

Compared with the Na-decorated h-BC3 in the absence of

CH4 molecule, the adsorption of the CH4 molecule weak-

ens the interaction between Na and h-BC3. This Na atom

donates electrons to the CH4 molecule and these electrons

are forced to occupy the antibonding orbital of the CH4

molecule due to the Pauli exclusion principle. In addition,

the charge transfer from Na to h-BC3 decreases upon

adsorption of the CH4 molecule, which weakens the

interaction between Na and the sheet. The obtained data of

partial charges indicate that the electrostatic interaction is

one of the major factors contributed to the overall stabili-

ties of the adsorption process. We gradually increase the

number of CH4 molecule close to the Na atom, where the

system with CH4 molecules is fully optimized. For a single

Na atom adsorbed on the h-BC3, each Na atom could

adsorb four CH4 molecules (Table 2). In the case of mul-

tiple CH4 molecules adsorbed on decorated sheet, it is

found that the CH4 molecules present a symmetric con-

figuration near to Na atom. If we further increase the

number of CH4 molecule, the adsorption energy of the fifth

CH4 molecule is positive, which seems too weak. Hence

the fifth CH4 molecule attached to Na atom is thermody-

namically unstable at room temperature. It should be noted

that the H atoms of CH4 molecule is directly bonded to the

Na atom in which the distance between Na and CH4

molecule is in the range of 2.44–2.96 A. In Fig. 5, the

charge densities of typical CH4 adsorption on pristine and

Na-decorated h-BC3 are shown. For Na-decorated h-BC3,

the charges of the CH4 molecule and Na atom are over-

lapped seriously, indicating the formation of chemical

bonds. However, for pristine one, there is not any super-

position appearing, showing the decoupling between the

molecule and the h-BC3 surface. As a result, it seems CH4

molecule is strongly adsorbed on Na-decorated h-BC3.

Fig. 4 Structures of optimized 1Na-decorated mCH4/h-BC3

Fig. 5 Charge densities of CH4 adsorption on a pristine and b Na-

decorated h-BC3



of systems in eV


Na-decorated

h-BC3

– -4.37 -3.69 0.68 –

CH4/Na-decorated

h-BC3

-5.6 -4.36 -3.69 0.67 -1.4

2CH4/Na-decorated

h-BC3

-9.1 -4.33 -3.67 0.66 -2.9

3CH4/Na-decorated

h-BC3

-10.4 -4.31 -3.66 0.65 -4.4

4CH4/Na-decorated

h-BC3

-9.3 -4.33 -3.67 0.66 -2.9


Struct Chem

123

In the next step, we consider the double-sided adsorption

of Na atoms. Since the first Na atom prefers to be adsorbed

on H1 site, we add the second Na atom to two possible sites

(H1 and H2) on the opposite side of the h-BC3 layer. We

denote these possible adsorption sites of the two metal

atoms as H1–H1 and H1–H2, respectively. We note that

the average Eb of the configuration with two atoms occu-

pying the H1–H1 sites is -40.1 kcal/mol (Fig. 3b). The

average Eb of the two Na atoms adsorbed on H1–H2

-134.5 kcal/mol, which are unstable than that of H1–H1.

Therefore, with the distance between two Na atoms on both

side of h-BC3 being 4.18 A, the H1–H1 is the most stable

structure which is suitable for adsorption CH4 molecules.

Table 3 lists the electronic properties for the mCH4/2Na/h-

BC3 systems, and the corresponding Ead of CH4 molecules.

It is shown that when a single Na atom is adsorbed on the

one side of h-BC3, each Na atom can adsorb four CH4

molecules with the average Ead in the range of -5.6 to

-10.4 kcal/mol. Similarly each Na atom can adsorb four

CH4 molecules with an Ead of -4.3 to -18.8 kcal/mol

when Na atoms are adsorbed on both sides of sheet. For

two side adsorption, the distance between CH4 molecules

and the nearest Na atom is in the range of 2.48-3.02 A

(Fig. 6). The CH4 molecules are more crowded after four

CH4 molecules are adsorbed by each Na atom for two side

adsorption. Due to the repulsion between adsorbed CH4

molecules, there is no enough space to adsorb more CH4

molecules. In our calculations, if we further increase the

number of adsorbed CH4 molecules, after relaxation, the

fifth hydrogen molecule goes away from Na atom. This

indicates that the fifth CH4 molecule cannot be adsorbed.

It is known that the more metal dopants are adsorbed on

the storage material, the higher hydrogen storage capacity

can be obtained. So, to the finding of maximum storage

capacity of Na-decorated BC3 nanosheet, we have inves-

tigated more Na decoration in the studied sheet. As can be

seen in Fig. 1S (in the supplementary material), six Na can

be decorated in both side of BC3 sheet, exothermically.

Unlike to 2Na-decorated BC3 nanosheet in which four CH4

physisorbed near to each Na, after full relax optimization

3CH4 can be attached to each sodium of 6Na-decorated

BC3 nanosheet, exothermically (Fig. 2S, in the supple-

mentary material). Adsorption energy for 18CH4/6Na-

decorated BC3 nanosheet is -1.7 kcal/mol. In other word,

the methane storage capacity of Na-decorated BC3 nano-

sheet could approach 18.1 wt%. Yulong et al. [34] have

investigated methane storage in multi-walled CNT exper-

imentally. They found that an optimal 11.7 % of mass

storage capacity was achieved at room temperature. From

stand point of comparison, methane storage capacity of Na-

decorated BC3 sheet is two-times larger than CNT, indi-

cating graphene-like materials such as BC3 nanosheet can

interact with CH4 from its both sides, whereas only exterior

surface of CNT is potential place for methane uptake.

Next criteria for candidate materials for methane storage

related to their surface area and porosity. As we know

classic materials with macro-structure size have low

capacity for gas storage due to small surface area. Thus,

they should have high volume pores for methane trapping.

But in last decade, development of graphene-based mate-

rials for gas storage systems has investigated due to large

theoretical specific surface area (2,630 m2/g). Since, BC3

nanosheet has a similar geometry with graphene, its spe-

cific surface area is larger than 2,000 m2/g (minimum

accepted surface area). These unique features of BC3

nanosheet make it suitable for gas storage systems. For

example, recently Yang and Ni [35, 36] have focused on

hydrogen storage properties of metal-decorated BC3

nanosheet. They found that Li or Ca decorated can dra-

matically enhance storage capacity of this sheet. Although

there is no experimental report about volume pores of BC3

nanosheet, but its functionalization or decoration with

different alkali or alkaline earth metals can make it

promising candidate for methane storage.

Consequently, Eg were considered, since it is known to

be the index of stability (reactivity) and electrical con-

ductivity. Calculated DOS of 1Na- and 2Na-decorated

h-BC3 are shown in Fig. 3 (panel a and b, respectively),

indicating that after Na decoration conduction level almost

remains constant, while valence level moves to higher

energies so that its Eg value is dramatically reduced to 0.68

and 0.71 eV, respectively. It means that the system behaves

like a metal systems and the reactivity of the system were

increased. It should be noted that herein, the Eg also stands

for singly occupied molecular orbital (SOMO)/LUMO

energy gaps for the open shell systems. Tables 2 and 3

present the results of the HOMO, LUMO, and Eg for Na-

decorated h-BC3 before and after CH4 adsorption which

indicated that the Eg of systems were unchanged. The

contribution of CH4 is largely away from the Fermi level.



of systems in eV


2Na-decorated

h-BC3

– -4.37 -3.66 0.71 –

2CH4/2Na-decorated

h-BC3

-4.3 -4.31 -3.61 0.70 -1.4

4CH4/2Na-decorated

h-BC3

-8.9 -4.31 -3.61 0.70 -1.4

6CH4/2Na-decorated

h-BC3

-18.8 -4.26 -3.59 0.67 -5.6

8CH4/2Na-decorated

h-BC3

-11.4 -4.27 -3.58 0.69 -2.8


Struct Chem

123

These Eg behaviors suggest that the conductivity of deco-

rated sheet is insensitive to CH4 adsorption. In short,

chemical storage of CH4 upon decorated sheet can be

supposed as some kind of harmless modification. In other

words, the decorated h-BC3 with Na metal atom improves

ability of sheet for adsorption of CH4 molecule. Further-

more, no bond dissociation was observed for the adsorption

of CH4 on Na-decorated h-BC3, which means that deco-

rated sheet can act as a storage device for methane safety

storage.

Conclusion

The geometric structures and electronic properties of the

pristine and Na-decorated h-BC3 in the presence and

absence of multiple adsorbed CH4 molecules were

explored using DFT. It was found that the CH4 molecule is

physisorbed on the pristine sheet via van der Waals forces.

Also, we have investigated systemically how supporting

Na atom on h-BC3 can affect their capability for adsorption

of CH4 molecule onto sheet without any bond dissociation.

The Na-doped h-BC3 is demonstrated to be a good

candidate for safety adsorption of CH4 molecule. The sta-

ble structure of Na-decorated h-BC3 is that the Na atom is

put in C6 hexagon ring. We find that the Na atoms on the

BC3 sheet do not have the problem of metal clustering,

which is a major issue in metal dispersion. Our results

show that up to four CH4 molecules can be adsorbed on

each Na atom. The results also indicate that decoration of

the Na atoms on surface of h-BC3 induces some changes in

electronic properties of the sheet and its Eg is unchanged

after adsorption of CH4 molecules.

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Role of sodium decoration on the methane storage properties of BC3 nanosheet

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