Author's Accepted Manuscript

Alkali metal ions on a nanosized tube of BC2N:computational study

Ali Hashemi, Maziar Noei

PII: S1386-9477(13)00285-3DOI: http://dx.doi.org/10.1016/j.physe.2013.08.020Reference: PHYSE11359

To appear in: Physica E

Received date: 8 July 2013Revised date: 8 August 2013Accepted date: 19 August 2013

Cite this article as: Ali Hashemi, Maziar Noei, Alkali metal ions on a nanosizedtube of BC2N: computational study, Physica E, http://dx.doi.org/10.1016/j.physe.2013.08.020

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1

Alkali metal ions on a nanosized tube of BC2N: computational study

Ali Hashemi1, and Maziar Noei2

1Department of Chemistry, Gachsaran Branch, Islamic Azad University, Gachsaran, Iran

2 Department of Chemistry, Mahshahr Branch, Islamic Azad University, Mahshahr, Iran

*To whom correspondence should be addressed: E-mail: noeimaziar@gmail.com, Tel:

+989125759236

Abstract

Using density functional theory calculations, we have investigated the adsorption of Li+, Na+ and

K+ cations on the surface of a BC2N nanotube in terms of energetic, geometric and electronic

properties. The adsorption is site-selective, so that the cations prefer to be adsorbed atop a

hexagonal ring of the tube surface which involves more N atoms. The adsorption energy of Li+ (-

54.6 kcal/mol) is more negative than that of the others (about -41.5 kcal/mol for Na+ and -18.4

kcal/mol for K+). By increasing the number of adsorbed Li+ cation, the adsorption energy per a

cation is decreased and also the HOMO, LUMO and Fermi level are more shifted to lower

energies. The adsorption of the alkali cations may impede the electron emission from the BC2N

2

nanotube surface by increasing the work function due to the charge transfer from the tube to the

cation.

Keywords: DFT, Adsorption, Nanotubes, B3LYP, Cations

1. Introduction

In the last two decades great development in the area of small particles and nanostructured

materials have been achieved [1-3]. The adsorption of metal ions on the nanotubes has attracted a

growing interest due to its important role in many applications [4]. The addition of metal ions to

the nanostructures can also improve their electrical, conducting, and chemical properties. Now,

with the recent discovery of new crystalline forms of carbon, specifically carbon nanotubes

(CNTs) with dimensions of 1–100 nm, it appears that there may be a new paradigm in carbon-

based battery electrode materials. The increasing demand for alkali-metal-ion (AMI) batteries

with high energy storage densities, applicable in portable electronic devices, has spawned many

explorations of new AMI intercalation materials with superior performance both as cathode and

anode materials [5]. For example, Zhao et al. have simulated Li intercalation in CNT bundles and

demonstrated the possibility of high saturation density of about Li3C6 [6].

Purification or controlled synthesis of CNTs with selected helicity has not been achieved so

far which has made electronic devices manufacturing with CNTs difficult. Therefore, it is

interesting and important to further find or design other new low-dimensional tube-like

nanostructured materials suitable for miniaturization of electronic devices. For example, the BC3,

BC2N and B2C nanotubes were theoretically proposed [7, 8], among which the BC3 nanotubes as

well as composite Cx(BN)y nanotubes [9], including signature of BC2N, have been

experimentally realized. Considerable experimental efforts have been devoted to the synthesis of

3

Cx(BN)y nanotubes, and they have been successfully obtained by electrical pyrolysis, laser

ablation, hot-filament chemical vapor deposition , and the template route [10, 11]. The electronic

structure properties of BC2N nanotubes (BC2NNTs) have been theoretically studied by different

groups [12, 13]. Tuning the electronic structures of the BC2NNTs for specific applications is

important in building specific electronic and mechanical devices. Recently, theoretical

calculations on the pristine BxCyNz compound nanotubes have shown that these nanostructures

are very promising materials for energy-storage applications [14]. In this work, we report a

density functional theory study on the adsorption of selected alkali metal ions (AMIs, Li+, Na+

and K+) in the exterior surface of the BC2NNTs. Our results may help to direct experimental

developments of new nanostructured materials.

2 Computational methods

Geometry optimizations, natural bond orbital (NBO), and density of states (DOS) analyses

were performed on a (8, 0) zigzag BC2NNT (constructed of 36 B, 36 N, and 72 C atoms), and

different AMI/BC2NNT complexes at B3LYP/6-31G (d) level of theory as implemented in

GAMESS suite of program [15]. B3LYP is a popular functional which has been commonly used

for nanotube structures [16-20]. GaussSum program has been used to obtain the DOS results

[21]. The length and diameter of the optimized pure BC2NNT have been computed to be about

17.13 Å and 6.22 Å, respectively, which consisted of nine atomic layers along the tube axis.

In order to avoid boundary effects, atoms at the open ends of the tube have been saturated

with hydrogen atoms. With the optimized structures, the adsorption energy (Ead) of the AMI-

adsorbed nanotube is obtained using the following equation:

Ead = E (AMI-BC2NNT) – E (AMI) – E (BC2NNT) + EBSSE (1)

4

where E (AMI-BC2NNT) is the total electronic energy of the AMI-adsorbed BC2NNT complex,

and E (BC2NNT) and E (AMI) are the total electronic energies of the pristine BC2NNT and the

cation, respectively. EBSSE, the basis set superposition error (BSSE), has been corrected for all of

the interaction energies. By the definition, a negative value of Ead corresponds to an exothermic

functionalization. It should be noted that the electronic energies do not include zero point energy.

We think that it has no influence on the Ead in our work. The zero point energies can be ignored

when the energies of cations /BC2NNT complexes are compared to those of the isolated cations

and BC2NNT based on the same calculation method. The Fermi level (EFL) in the pristine tube

lies approximately in the middle of the HOMO-LUMO energy gap (Eg).

3 Results and discussion

In Fig. 1, we have shown a partial structure of the optimized BC2NNT, where four types of

bonds namely B-N, B-CI, N-CII and CI-CII can be identified, with corresponding lengths of 1.46,

1.52, 1.44, and 1.36 Å, respectively. As it can be seen, there are two types of carbon atoms in

BC2NNT; CI is the carbon atom which is bonded to two B atoms and another C atom, while CII is

the carbon atom which is bonded to two N atoms and another C atom. Buckling of B–N and C–C

bonds was found in the structures. After optimization, it has been found that the N atoms are

pushed inward while the B atoms are pushed outward of the nanotube surface. On the other hand,

in C–C bonds, the CI atoms are relaxed outward while the CII ones inward of the nanotube

surface. Buckling of atoms from perfect cylindrical model is according to the previous results for

BNNTs and is a solution to minimize the total and strain energies [22].

In order to find minimum adsorption configurations for AMI/BC2N complexes, the AMIs

were initially placed at different positions above the BC2NNT. For each adsorbate, six different

sites (B, CI, CII, N, H1, and H2) for outside adsorption (as shown in Fig. 1) have been selected to

examine the interaction between the tube and a single cation. Figure 2 displays side view of the

5

stable (local minimum) configurations of the adsorbed AMIs on the tube. From the results shown

in Table 1, it can be seen that the selected cations can be adsorbed on top of two different

hexagonal rings namely H1 (as C4BN) and H2 (as C2B2N2), so that in other configurations the

target AMI re-oriented to these stable sites. Our calculations show that all of the cations are

exothermically adsorbed on the outside of the BC2NNT (negative values for Ead). As shown in

Table 1, when cations are added to the sidewall of the BC2NNTs, the H2 site is more

energetically preferable than H1 site.

Relative magnitude order of the Ead for different AMIs is: Li+ > Na+ > K+. The computed

values of Ead for Li+, Na+, and K+ in the H2 site are about -54.6, -41.5, and -18.4 kcal/mol with

corresponding distances of 1.84, 2.22, and 2.77 Å, respectively. This observation may be

explained based on the Pearson’s hard-soft acid-base theory, which states that the soft acids react

strongly with soft bases and in contrary, the hard acids react strongly with the hard bases, when

all other factors being equal. The Li+ ion is a hard acid with smaller size, and is weakly

polarizable compared to Na+ and K+ ions. Therefore, it tends to interact more preferably with the

nitrogen atoms of the tube which are known as hard bases, in comparison with B and C atoms.

In order to consider the influence of AMI adsorption on the structural and electronic

properties of the BC2NNT, these compounds were analyzed in terms of two global reactivity

indices, namely, hardness (η) and electrophilicity (ω) as well as DOS calculations. The absolute

hardness has been quantified by Parr and Pearson as the second derivative of the electronic

energy of the system with respect to the number of electrons at a constant external potential [23]:

)r(2

2

NE

21

νδδη ⎟⎟

⎠

⎞⎜⎜⎝

⎛= (2)

The hardness evaluations were all based on the commonly used finite difference

approximation, leading to:

6

)AI( −=21η

(3)

where I and A represent the ionization potential and the electron affinity, respectively. According

to Koopmans’ theorem [24], Eq. 3 becomes:

( )2

LUMOHOMO EE −−=η (4)

where ELUMO and EHOMO are Kohn-Sham one-electron eigenvalues associated with the

LUMO and HOMO, respectively, obtained from DFT calculations. The chemical hardness

generally shows the resistance of a system against change in the electron number or the shape of

the electron cloud. Thus, it can be used as a global reactivity index to predict reactivity in the

context of charge-transfer processes. Any decrease in η after the adsorption of AMI in the

BC2NNT implies less stability, i.e. more reactivity of the system. The results in Table 1 show that

the Li+ adsorption in H1 site leads to lower hardness (1.01 eV) values than the pristine BC2NNT

(1.28 eV).

The electrophilicity index is a measure of electrophilic power of a molecule. The

electrophilicity index as defined by Parr is given by the expression (ημω2

2

= ) [25]. When two

species react with each other, one of them behaves as a nucleophile while the other one acts as an

electrophile. Higher electrophilicity index shows higher electrophilic nature of the molecule. In

all of the AMI-adsorbed BC2NNT configurations, the electrophilicity of the complex (16.2-20.3

eV) is drastically higher than that of the pristine model (6.6 eV). Therefore, AMI intercalation on

the BC2NNT can increase the electrophilicity of the nanotube by about 125-207%, which is

consistent with the nature of the cations.

7

The nature of the nanotube’s DOS near the Fermi level is critical for understanding the

electrical transport through these materials. Therefore, we have drawn DOS plots for the ion-

adsorbed tubes and compared them with that of the bare BC2NNT. For the bare BC2NNT, from

Fig. 1 and Table 1, it can be seen that it is a semi-conductor material with an Eg of about 2.57 eV.

Comparing DOSs of the free BC2NNT and those of the AMI-adsorbed systems (Fig. 3), it can be

concluded that the presence of the cations decrease the value of Eg from 2.57 eV to 2.32-2.02 eV

in the AMI-adsorbed tube (about 21% change). Table 1 shows that the conduction, valence, and

Fermi levels (EFL) of the tube simultaneously shift to lower energies upon the adsorption of AMI

cations. It indicates that intercalation of inherent semi-conductor BC2NNT with the AMIs will

result in new semiconductive material with enhanced conductivity.

With each H2 site in the BC2NNT being a potential intercalation site, the possibility for

multiple adsorptions should has been considered to evaluate the concentration of Li+ cation (as an

example) on the nanotube (Fig. 4). First, adsorption of two Li+ cations is examined. Two Li+ are

placed on two H2 sites as far as possible, ensuring the least steric repulsion between them. The

Ead of the two Li+ atop the two H2 sites is about -33.9 eV with a charge of 0.436 e in each cation

(table 2). Interestingly, when two Li+ are adsorbed on the tube simultaneously, the absolute value

of Ead per cation is much smaller than that of the case of single adsorption. Furthermore, the

adsorption of more Li+ (nLi+ = 3 to nLi+ = 4) have been considered. We can find from Fig. 5 that

the Ead per Li+ decreases gradually by increasing the number of the adsorbed cations (-17.6 and -

10.9 eV per Li+ atom for three- and four-Li+ adsorption, respectively). These findings can be

explained based on the electrophilicity index. As mentioned above, when more Li+ ions are

adsorbed on the surface of the BC2NNT, more and more increases are occurred in the

electrophilicity index. This observation clearly shows that the BC2NNT surface has lower

tendency for adsorption of the third and fourth electron-deficient Li+.

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In the following, we have studied the influence of the multiple Li+ adsorption on the

electronic properties of the nanotube. At high concentrations of Li+, both the conduction and

valence levels of the tube are significantly changed, thus EFL moves to lower energy, but the Eg

changes slightly between 0.14-0.61 eV. It can be found that all of the AMI adsorbed-BC2NNTs

are still semiconductors with a wide Eg close to that of the pristine BC2NNT. By increasing the

number of Li+ cations, the HOMO, LUMO, and Fermi levels are more shifted to lower energies.

The value of Eg of the tube shows an oscillation with the number of the adsorbed Li+. It was

found that the Eg of the tube with odd adsorbed Li is smaller than that with even number. We

think that it may be sue to symmetry broken in odd Li+ adsorbed systems.

Recently, great interest has been attracted in field emission properties of the nanostructured

materials [26]. As shown in Table 2, the EFL is decreased after multiple Li+ adsorption. For

instance, the EFL obviously decreased from -4.12 eV to -12.89 eV in 4Li+/BC2NNT complex.

However, these phenomena lead to an increment in the work function that is important in field

emission applications. The work function can be found using the standard procedure by

calculating the potential energy difference between the vacuum level and the Fermi level, which

is the minimum energy required for one electron to be removed from the Fermi level to the

vacuum. The increment in the work function indicates that the field emission properties of the

tube are impeded upon the AMI adsorption. Furthermore, this will raise the potential barrier of

the electron emission for the tube, and makes the field emission difficult. In summary, we believe

that the present work may help researchers to design of new BC2NNT-based materials with

different electronic properties than the pristine form.

9

4. Conclusions

Adsorption of three cations (Li+, Na+, and K+) on the surface of BC2NNTs has been investigated

by density functional theory. It has been found that the reactions are site-selective, and cations

prefer to be adsorbed atop the H2 ring of the tube surface (as B2C2N2). The Ead of Li+ (-54.6

kcal/mol) is more negative than that of the other cations (about -41.5 kcal/mol for Na+ and -18.4

kcal/mol for K+) which can be explained using Pearson’s hard-soft acid-base theory. The average

adsorption energy per cation is decreased by increasing the number of the adsorbed Li+ ions. It

has also been found that the adsorption of cations may impede the electron emission from the

BC2NNT surface by increasing the work function due to the charge transfer from the tube to the

cation.

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Figure captions

Fig. 1. Partial structure of optimized (8,0) BC2NNT and its density of states (DOS).

Fig. 2. Models for different AMI adsorption on (a) H1 and (b) H2 sites of BC2NNT. Distances

are in Å.

Fig. 3. Comparison between the density of states (DOS) plots for different AMI adsorption on the

BC2NNT, (a) H1 and (b) H2 site.

Fig. 4. Models for multiple AMI adsorption H2 site of BC2NNT, and corresponding density of

states (DOS) plots.

Fig. 5. Variation of adsorption energy (per Li+) as a function of the number of adsorbed Li+ on a

BC2NNT.

12

Table 1. Adsorption energy of AMI on BC2NNT (Ead, kcal/mol), Mulliken charge on the

adsorbed ion (QT), HOMO energies (EHOMO), LUMO energies (ELUMO), Fermi level (EFL),

HOMO-LUMO energy gap (Eg), global hardness (η) and electrophilicity index (ω) for the

pristine and AMI-adsorbed BC2NNT in eV.

* QT is defined as the total Mulliken charge on the AMI. **Change of Eg of BC2NNT upon adsorption of AMI.

ω η **ΔEg(%) Eg ELUMOEFL EHOMO*QT (e) Ead System

6.6 1.28- 2.57-2.84 -4.12 -5.41 - - BC2NNT

20.31.0121.4 2.02-5.40 -6.41 -7.42 0.393 -45.7 (H1) Li-BC2NNT

17.91.1212.8 2.24-5.22 -6.34 -7.46 0.377 -54.6 (H2) Li-BC2NNT

19.51.0220.2 2.05-5.30 -6.32 -7.35 0.538 -36.1 (H1) Na-BC2NNT

17.41.1312.0 2.26-5.14 -6.27 -7.40 0.531 -41.5 (H2) Na-BC2NNT

17.81.0716.7 2.14-5.11 -6.18 -7.25 0.787 -12.3 (H1) K-BC2NNT

16.21.169.7 2.32-4.98 -6.14 -7.30 0.743 -18.4 (H2) K-BC2NNT

13

Table 2. Adsorption energy of multiple Li+ on BC2NNT (Ead, kcal/mol), Mulliken charge on the

adsorbed ion (QT), HOMO energies (EHOMO), LUMO energies (ELUMO), Fermi level (EFL),

HOMO-LUMO energy gap (Eg), global hardness (η) and electrophilicity index (ω) for the

pristine and nLi+ / BC2NNT complexes in eV.

* QT is defined as the total Mulliken charge on the Li+. **Change of Eg of BC2NNT upon adsorption of nLi+.

ω η **ΔEg(%) Eg ELUMO EFL EHOMO*QT (e) Ead System

6.6 1.28- 2.57 -2.84 -4.12 -5.41 - - BC2NNT

17.91.1212.8 2.24 -5.22 -6.34 -7.46 0.377 -54.6 First

27.21.355.4 2.71 -7.24 -8.6 -9.95 0.436 -33.9 Second

55.20.9823.7 1.96 -9.42 -10.4 -11.380.525 -17.6 Third

58.11.4311.3 2.86 -11.46 -12.9 -14.320.654 -10.9 Fourth

14

Fig. 1.

15

Fig. 2.

16

Fig. 3.

17

Fig. 4.

18

Fig. 5.

Highlights

> Adsorption of Li+, Na+ and K+ cations on a BC2N nanotube is studied

> Adsorption energy of Li+ (-54.6 kcal/mol) is more negative than that of the others

> By increasing the number of Li+ cation, the adsorption energy is decreased

> Alkali cations impede the electron emission from the BC2N nanotube surface

> Alkali cations increase the work function of the tube

19

Graphical abstract

Using density functional theory calculations, we have investigated the adsorption of Li+, Na+ and

K+ cations on the surface of a BC2N nanotube.