Exohedral chemical functionalization of C48B6N6 with NH3: Binding energies and electronic structures of C48B6N6–(NH3)n=1−6

Superlattices and Microstructures 51 (2012) 290–299

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Exohedral chemical functionalization of C48B6N6 with NH3: Binding energies and electronic structures of C48B6N6–(NH3)n=16 Ehsan Zahedi ⇑, Ahmad Seif Chemistry Department, Shahrood Branch, Islamic Azad University, Shahrood, Iran Chemistry Department, Boroojerd Branch, Islamic Azad University, Boroojerd, Iran

a r t i c l e

i n f o

Article history: Received 18 September 2011 Received in revised form 24 November 2011 Accepted 30 November 2011 Available online 7 December 2011 Keywords: C48B6N6 Chemical functionalization DFT NBO DOS

a b s t r a c t Theoretical study of exohedral chemical functionalization of C48B6N6 with NH3 molecules has been investigated using DFT. It was found that NH3 molecule can be chemically adsorbed on boron sites of C48B6N6, with a charge transfer from NH3 to C48B6N6. Adsorption energy and the quantity of electron charge transfer from latest adsorbed ammonia to C48B6N6 decreased with increasing in the adsorbed NH3 molecules. Despite the strong adsorption energies, electronic properties of C48B6N6 is preserved after modification(s) with NH3 molecule(s) and chemical modification of C48B6N6 with NH3 molecules can be viewed as some kind of safe modification. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction C60 (most important member of the family of fullerenes) is one of the carbon allotropes with spherical form that was discovered in 1985 unexpectedly during vaporization of graphite by laser irradiation [1]. In this molecule, 60 carbon atoms (with sp2 hybridizations) are placed at the vertices of a truncated icosahedron (Ih) with 12 pentagons and 20 hexagons so that none of the pentagons are in contact with each other and this is the socalled isolated pentagon rule (IPR) [2]. Boron nitride (BN) is a binary compound made of Group III and Group V elements in the periodic table that is closely related to the carbon system. BN materials are the best candidates to replace carbon because they are isoelectronic with their all-carbon analogues [3]. The inclusion of dopants, such as nitrogen or boron, ⇑ Corresponding author at: Chemistry Department, Shahrood Branch, Islamic Azad University, Shahrood, Iran. Tel.: +98 912 273 3755. E-mail addresses: [email protected], [email protected] (E. Zahedi). 0749-6036/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2011.11.021

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in the C60 shell predicted experimentally and theoretically in the previous studies [4–24]. The B–N bond in heterofullerence is polar which can potentially change the electronic, optical, and magnetic properties of C60 by modifying the character of the frontier molecular orbitals [25] and make the resulting clusters candidates for interesting applications. Chemical functionalization of heterofullerenes through either covalent sidewall or noncovalent exohedral/endohedral functionalization offers an effective route to enhance physical and electronic properties of heterofullerenes and can expand potential applications of heterofullerenes in developing of electronic devices, gas sensors, biosensors industry, and filters. Theoretically, chemical functionalization of the heterofullerenes with several molecular functional groups has been studied. Recently, Zahedi et al. [26] have been investigated exohedral functionalization of C30B15N15 with NH3 using DFT. Authors claimed that ammonia does not modify the electronic structure of C30B15N15 and suggested that C30B15N15 heterofullerene is an ideal material for elimination or/and filtering of ammonia. Also, Zahedi [27] studied the influence of exohedral NH3-attaching on the nuclear magnetic shielding and electric field gradient tensors of N and B nuclei in the C30B15N15 for the first time. In the new published research, Zahedi and Seif [28] have been reported and compared electronic properties of C48B6N6 functionalized with only one NH3 and one NO2. The calculated results suggested that the C48B6N6 heterofullerene is a suitable sensor material for NO2 and is an ideal material for elimination and filtering of ammonia. In the present work, we report exohedral chemical functionalization of C48B6N6 with 1–6 molecules of NH3 using density functional calculations. This research is a theoretical study on changes of binding energies with increasing in NH3 adsorbed molecules and the number of the adsorbed (NH3)n=1–6 species is effective on the electronic structure of C48B6N6. This study can help in applications of C48B6N6 for sensing and/or filtering of NH3. 2. Computational details The starting geometry of C48B6N6 is generated from C60 by replacing six carbon atoms by nitrogen atoms and another six carbon atoms by boron atoms. All calculations were performed on a C48B6N6 and the C48B6N6–(NH3)n=1–6 complexes using Gaussian 03 [29] package of programs. It has been established that DFT is capable of accurately treating such systems due to incorporation of the exchangecorrelation effects [30–33]. Geometry optimizations and electronic analysis were performed using 6-31G basis set with B3LYP functional [34,35]. NBO (natural bond orbital analysis) calculation was performed with the use of the NBO program version 3.1 (link 607, Gaussian 03) [36] by the B3LYP method and 6-311G standard basis set in the optimized structures in order to evaluate the electron charge transfer.

3. Results and discussion Interaction of C48B6N6 heterofullerene with n single molecules (n = 1–6) is considered. To show the position of atoms on a C48B6N6 heterofullerene, we used the same labeling and numbering of C60 as the Schlegel diagram shows in Fig. 1 with adsorption site pointed out in bold. First, we studied the stable structure of pristine C48B6N6 where six B–N bonds with bond length of 1.44–1.45 Å and 12 B–C bonds with bond length of 1.53–1.54 Å, can be identified. The charge analysis using the NBO method indicates that all six B–N bonds are polarized with positive partial charges 0.83 d+ on B atom and negative partial charges 0.60 d on N atom, that is, the B–N bonds of C48B6N6 are partially ionic, which is similar to B–N bonds in BNNTs [37]. Thereafter, we scrutinized several different orientation of the one NH3 molecule, including the N and H atoms of NH3 molecule was nearest to the N atom, B atom, C atom, pentagon, and hexagon ring. The most stable configuration is such that the N atom of ammonia is nearest to the B atom with staggered conformation. To evaluate the interaction between a NH3 molecule and C48B6N6–(NH3)n1 (n = 1–6), we computed adsorption energies (Eads), according to following equation [33,38,39]:

Eads ¼ EC48 B6 N6 —ðNH3 Þn  EC48 B6 N6 —ðNH3 Þn1  ENH3

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Fig. 1. Schlegel showing the C60 core C-atoms, and the dopant atoms of nitrogen and boron.

Table 1 Calculated adsorption distance, charge transfer, total electronic energies, adsorption energies, and recovery times of NH3 on the C48B6N6 heterofullerene. Configuration

a

c d

QT (el)b

E (a.u.)

C48B6N6

2306.8128420

NH3

56.5479477

C48B6N6–(NH3)1 C48B6N6–(NH3)2 C48B6N6–(NH3)3 C48B6N6–(NH3)4 C48B6N6–(NH3)5 C48B6N6–(NH3)6 b

D (Å)a

1.67 1.67 1.68 1.69 1.69 1.70

0.36 0.35 0.34 0.33 0.33 0.32

2363.3935476 2419.9732532 2476.5485814 2533.1157972 2589.6820581 2646.2465009

Adsorption distance between the C48B6N6 and latest NH3 molecule. Charge transfer between C48B6N6 and latest NH3 molecule. The adsorption energy between C48B6N6 and latest NH3 molecule. Recovery time of latest ammonia in 298.15 K supposing that t0 = 1012 s1.

Eads (kJ/mol)c

s (s)d

86.00 83.38 71.88 50.59 48.08 43.30

1168 406 4 7.3  104 2.6  104 3.8  105

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where EC48 B6 N6 —ðNH3 Þn is the total energy of a n NH3 molecule adsorbed on the C48B6N6 surface and EC48 B6 N6 —ðNH3 Þn1 is the total energy of the n  1 NH3 molecule adsorbed on the C48B6N6 surface and ENH3 is the total energy of an individual ammonia molecule. Hence, the negative value of the Eads denotes an exothermic adsorption [40]. For the C48B6N6–NH3 complex in the most stable configuration at B1 site, the equilibrium C48B6N6–NH3 distance is 1.67 Å and the adsorption energy is 86.00 kJ/mol (Table 1). The magnitude of the adsorption energy supports the hypothesis that the adsorption of ammonia is chemical in nature. Due to transformation of lone pair of N atom of NH3 to the empty orbital of B1 atom, 0.36e¯ transferred from NH3 to C48B6N6, and local hybridization of the B1 atom changed from sp2 to sp3 orbital. Table 1 displays that adsorption distance, charge transfer, adsorption energy between C48B6N6 and latest NH3 are dependent to n, when other NH3 molecules adsorbed on the B2–B6 sites. The equilibrium distance between NH3 and boron atom of heterofullerene in the adsorption site is sensitive to n and adsorption distance increased from 1.67 to 1.70 Å with increasing in n from 1 to 6, respectively. As shown in Table 1, the computed charge transfer from NH3 to C48B6N6– (NH3)n1 is sensitive to the n. With the exception of adding the fifth NH3 molecule, the quantity of electron charge transfer from latest adsorbed ammonia decreased with increasing in the n number. If the C48B6N6 heterofullerene had hole carriers, NH3 would have reduced the hole concentration by giving up electrons to the C48B6N6 heterofullerene. Using transition state theory the recovery time can be described as: ðEads =K B T Þ s ¼ m1 0 e

DOS (1/eV)

where T is temperature, KB is Boltzmann constant, and t0 (supposing that t0 = 1012 s1) is the attempt frequency. Increasing the adsorption energy will prolong the recovery time in an exponential manner. From Table 1, it can be seen that adsorption energy and recovery time in 298.15 K decreased with increasing in n. Therefore, in C48B6N6–(NH3)n (n = 4–6) recovery times are short with quick response. In short, the sensitivity of the response to the gas molecule seems to be related to the amount of

C48B6N6

4

2

0 -20

-15

-10

-5

0

5

10

0

5

10

DOS (1/eV)

E (eV)

NH3

2

1

0 -20

-15

-10

-5 E (eV)

Fig. 2. Calculated total electronic density of states (TDOS) for the C48B6N6 heterofullerene and individual NH3. HOMO, LUMO, and Fermi level in each are respect to individual C48B6N6.

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4

DOS (1/eV)

DOS (1/eV)

6

n=1

2

0

n=2

4 2 0

-20

-15

-10

-5

0

5

10

-20

-15

-10

E (eV)

n=3

4 2 0

5

10

0

5

10

0

5

10

6 4 2 0

-20

-15

-10

-5

0

5

10

-20

-15

-10

E (eV)

10

12

n=5

8

-5 E (eV)

n=6

10 DOS (1/eV)

DOS (1/eV)

0

n=4

8 DOS (1/eV)

DOS (1/eV)

6

-5 E (eV)

6 4 2

8 6 4 2

0

0 -20

-15

-10

-5 E (eV)

0

5

10

-20

-15

-10

-5 E (eV)

Fig. 3. Calculated total electronic density of states (TDOS) for the C48B6N6–(NH3)n complexes.

electron charge transfer and the adsorption energy. This is also reflected in the stronger adsorption energy and shorter adsorption distance of systems with smaller n. Next, we studied the effect of chemical modification with NH3 molecules on the electronic properties of C48B6N6. For this purpose, we analyzed the density of state (DOS) for the C48B6N6–(NH3)n (n = 1– 6) complex and compared with C48B6N6. DOS analysis can provide basic information on the effects of the adsorption of adsorbed gas on the C48B6N6 heterofullerene’s electronic properties. Figs. 2 and 3 show the total electronic DOS for the most stable state of NH3, C48B6N6, and C48B6N6–(NH3)n (n = 1– 6) complexes in terms of Mulliken population analysis were calculated and created by convoluting the molecular orbital information with Gaussian curves of unit height and FWHM (full width at half maximum) of 0.3 eV. Total DOS (TDOS) of C48B6N6–(NH3)n (n = 1–6) complexes are almost exactly the superposition of the TDOS of C48B6N6. In particular, the TDOS of C48B6N6 and C48B6N6–(NH3)n (n = 1–6) between HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) states, near the Fermi level, are nearly unaffected upon the chemical modification with NH3 molecules. Partial density of state (PDOS) spectrum (Fig. 4) shows that the electronic states contributed from the NH3 molecules are far away from the Fermi level of C48B6N6–(NH3)n complexes. In fact, electronic properties of C48B6N6 are largely preserved after chemical modification with 1–6 NH3

295

6

n=1

4

PDOS (1/eV)

PDOS (1/eV)

E. Zahedi, A. Seif / Superlattices and Microstructures 51 (2012) 290–299

2

0

n=2

4 2 0

-20

-15

-10

-5

0

5

10

-20

-15

-10

E (eV) 6

n=3

PDOS (1/eV)

PDOS (1/eV)

6 4 2 0

0

5

10

0

5

10

0

5

10

n=4

4 2 0

-20

-15

-10

-5

0

5

10

-20

-15

-10

E (eV) 8

n=5

6

-5 E (eV)

n=6

8 PDOS (1/eV)

PDOS (1/eV)

-5 E (eV)

4 2 0

6 4 2 0

-20

-15

-10

-5

0

5

10

-20

-15

E (eV)

-10

-5 E (eV)

Fig. 4. Calculated partial electronic density of states (PDOSs) for C48B6N6 (black), and NH3 molecule(s) (pink). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)

Table 2 Calculated electronic parameters for NH3, C48B6N6 heterofullerene, and C48B6N6–(NH3)n complexes in 298.15 K.

a

Configuration

HOMO (eV)

LUMO (eV)

HLG (eV)

EF (eV)

Ncb (electron/m3)a

NH3 C48B6N6 C48B6N6–(NH3)1 C48B6N6–(NH3)2 C48B6N6–(NH3)3 C48B6N6–(NH3)4 C48B6N6–(NH3)5 C48B6N6–(NH3)6

6.87 5.61 5.03 4.52 4.02 3.50 3.04 2.70

2.14 3.08 2.56 2.12 1.59 1.16 0.73 0.30

– 2.53 2.47 2.40 2.43 2.34 2.31 2.40

– 4.35 3.80 3.32 2.81 2.33 1.89 1.50

– 1.03  104 3.31  104 1.29  105 7.21  104 4.15  106 7.45  106 1.29  105

Population of conduction electrons in 298.15 K.

molecules and can be viewed as some kind of safe modification. It is manifestly clear that in spite of the fact that the recovery times for C48B6N6–(NH3)n (n = 4–6) are short but for the reason that the chemical modification of C48B6N6 with NH3 molecules unaffected on the density of states near Fermi level, C48B6N6 heterofullerene is not a suitable sensor material for NH3 and is an ideal material for

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Fig. 5. Overlap population DOS curves between the B atom of C48B6N6 (in the adsorption sites) and N atom of gas molecule. Bonding, antibonding, and nonbonding B–N interactions are easily observable.

elimination and filtering of ammonia. The additional results for electronic parameters of C48B6N6 and C48B6N6–(NH3)n complexes are listed in Table 2. When NH3 molecules attached to the C48B6N6, with increasing in the NH3 molecules, HOMO, LUMO, and Fermi level shifts towards higher energy. As shown in Table 2, the band gap of heterofullerene decreased upon adsorption of NH3 molecules. With the exception of C48B6N6–(NH3)n (n = 3, 6), the complex band gap decreased by addition of NH3 molecules. The population of conduction electrons is given by [41]:

Ncb ¼ AT 3=2 ebandgap=2K B T

and A ¼

25=2 ðme pK B Þ3=2 3

h

3

4:83  1021 electrons=m K3=2

where me is electron mass and h is Planck constant. The population of conduction electrons for C48B6N6 and C48B6N6–(NH3)n complexes are tabulated in Table 2. It is quit clear that, with the exception of C48B6N6–(NH3)n (n = 3, 6), with increasing in the ammonia attached, the larger fraction of the

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n=1, B1–N E= – 13.0 eV

n=4, B4–N E= – 12.8 eV

n=2, B2–N E= – 12.7 eV

n=5, B5–N E= – 12.1 eV

n=3, B3–N E= – 13.2 eV

n=6, B6–N E= – 11.9 eV

Fig. 6. Visualized wave function molecular orbitals of C48B6N6–(NH3)n complexes in the bonding states of latest adsorbed NH3.

electrons can bridge the HLG (HOMO–LUMO gap) and participate in electrical conduction. The most important application of the DOS plots is to demonstrate MO compositions and their contributions to chemical bonding through the OPDOS plots which are also referred in the literature as crystal orbital overlap population (COOP) diagrams. The OPDOS is similar to DOS because it results from multiplying DOS by the overlap population. OPDOS show the bonding, antibonding, and nonbonding nature of the interaction of the two orbitals, atoms or groups. A positive value of the OPDOS indicates a bonding interaction (because of the positive overlap population), negative value means that there is an antibonding interaction (due to negative overlap population) and zero value indicates nonbonding interactions [41]. To analyze the interaction between B atom in heterofullerene and N atom in NH3, we have plotted OPDOS spectrum of each attachment in Fig. 5. It is easily seen that the positive OPDOS of B(1–6)–N are located in the frontier HOMO orbitals and in lower occupied orbitals, where are far a way from Fermi level. To get better insight into the interaction of NH3 with C48B6N6, we visualized wave function molecular orbitals of C48B6N6–(NH3)n complexes in the bonding states Fig. 6) of B atom of C48B6N6 and N atom of NH3. The interaction between the lone pair of N atom with C48B6N6 is easily observable. It should be noted that the interaction of NH3 and C48B6N6 is not limited in the prediction of OPDOS spectrum and can occur in the occupied orbitals close to HOMO level, even where the OPDOS is negative. The interaction between the p electrons of C48B6N6 and the valence electrons of NH3 molecules is the main reason for this phenomenon, while OPDOS spectrums predict the interaction between B atom of the adsorption site and N atom of NH3. For example, this interaction for C48B6N6–(NH3) complexes is shown in Fig. 7.

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E= –8.7 eV

Fig. 7. Visualized wave function molecular orbitals of C48B6N6–NH3 complexes. The interaction between the p electrons of C48B6N6 and the valence electrons of NH3 molecule is easily observable.

4. Conclusions Density functional theory calculations with B3LYP method has been used to investigate the chemical modification of C48B6N6 with 1–6 NH3 molecules. Interaction between C48B6N6 and NH3 molecules are considered in terms of the adsorption energy, charge transfer, adsorption distance, density of states (DOS, PDOS, OPDOS), and molecular orbitals. The current results clearly indicate that NH3 molecule can be chemically adsorbed on boron sites of C48B6N6 along with charge transfer from NH3 to C48B6N6. The adsorption energy, charge transfer, and adsorption distance between C48B6N6 and NH3 molecules are sensitive to the number of adsorbed NH3 molecules. The DOS, partial DOS, and overlap population DOS show that after modification of C48B6N6 with NH3 molecules electronic properties of C48B6N6 are largely preserved and chemical adsorptions are due to overlap of atomic orbitals of B atom with N (in the adsorption sites) below the Fermi level, wherein there are bonding states. Acknowledgements The financial support of this work by the Iranian Nanotechnology Initiative is gratefully acknowledged. E. Zahedi expresses his gratitude to the Islamic Azad University of Shahrood. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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