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Adsorption of pyrrole on Al12N12, Al12P12, B12N12, and B12P12 fullerene-like nano-cages; a first principles study
Vacuum 131 (2016) 135e141
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Adsorption of pyrrole on Al12N12, Al12P12, B12N12, and B12P12 fullerene-like nano-cages; a first principles study Ali Shokuhi Rad a, *, Khurshid Ayub b a b
Department of Chemical Engineering, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran COMSATS Institute of Information Technology, University Road, Tobe Camp, Abbottabad, 22060, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 May 2016 Received in revised form 15 June 2016 Accepted 16 June 2016 Available online 17 June 2016
Adsorption of pyrrole on the surfaces of four X12Y12 semiconductors (Al12N12, Al12P12, B12N12, and B12P12) is studied through density functional theory (DFT) calculations at B3LYP/6-31G(d,p) level of theory. The highest interaction energy is calculated for the adsorption of pyrrole on the surface of Al12N12 nano-cage. The adsorption energies of pyrrole on Al12N12, Al12P12, B12N12, and B12P12 are 64.6, 42.6, 12.0, 9.2 kJ mol1, respectively. Pyrrole acts as an electron donor and adsorbs at the electrophilic site of nanocage. Charge transfer to aluminum nano-cages is higher than to boron nano-cages. Changes in electronic properties such as band gap, Fermi level, and densities of states are also analyzed in order to better understand the sensing abilities of nano-cages for pyrrole molecule. Band gaps of aluminum nano-cages (Al12N12 and Al12P12) are unaffected by adsorption of pyrrole because of comparable effect on HOMOs and LUMOs. On the other hand, band gaps of boron nano-cages are significantly reduced on adsorption of pyrrole. Boron nano-cages are better sensor for pyrrole molecule despite their lower binding energies. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Pyrrole Al12N12 Al12P12 B12N12 B12P12 Nano-cages
1. Introduction An important challenge in current semiconductor technology is to understand important parameters that influence the interaction of organic molecules with different semiconductors [1,2]. A few reports exist in the literature where adsorption of small organic molecules such as cyclopentene, benzene and pyrrole is demonstrated on the surface of semiconductor materials [3e5]. Pyrrole (C4H4NH) is colorless, volatile, nitrogen containing heterocyclic compound (see Fig. 1). Pyrrole is an aromatic compound and it exhibits chemical reactivity very similar to benzene. For example, pyrrole resists to hydrogenation and Diels-Alder reactions. However, the electron density in pyrrole is greater than that of benzene; therefore, pyrrole is categorized as p-electron excessive aromatic molecule [6]. Pyrrole has noxious impact on metal catalysts utilized in petrochemical industry therefore removal of pyrrole from crude petroleum is very important. A number of studies demonstrate the adsorption of pyrrole on different surface [7e9]. For example, Qiao et al. [7] studied the adsorption and thermal reaction of pyrrole on
* Corresponding author. E-mail address: [email protected] (A. Shokuhi Rad). http://dx.doi.org/10.1016/j.vacuum.2016.06.012 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
Si(1 0 0) through X-ray and ultra-violet photoelectron spectroscopy. Bruhn et al. [8] studied the adsorption mechanism of pyrrole on Asrich GaAs(001)-c(4 4) surface. Noei et al. [9] examined the electrical sensitivity of boron nitride nanotube toward pyrrole through DFT calculations. Recently, nanostructure semiconductors have gained significant interest from the scientific community because of their distinct physical and chemical properties [10e12]. Particularly, group IIIeV semiconductors have been used in fast microelectronic devices and light-emitting diodes [13e15]. Group IIIeV sheets such as AlN sheets have been studied experimentally on Ag(111) support [16], and also theoretically for defects [17]. Moreover, hexagonal BN sheets are also studied as sensor [18]. Group IIIeV semiconductors have also been used as adsorbent and sensor for different analytes. Design of these new solid-state adsorbents/sensors is based on the high electrophilicity of IIIeV semiconductors. Electronic structure of these semiconductors changes on interaction with analytes. The small size of solid-state chemical adsorbents, easy synthesis, low cost and their reproducibility make them fascinating candidates for sensor applications [19]. Molecular simulation of different XnYn semiconductors (X ¼ B, Al, … and Y ¼ N, P, …) showed that the fullerene-like X12Y12 cages are the most stable ones [20,21]. Al12N12, Al12P12, B12N12 and B12P12 are four important fullerene-like nano-cages because of their
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that B3LYP/6-31G(d,p) is an optimal level of theory to deliver reliable and accurate results at minimum cost for a variety of systems [34e39]. Moreover, the theoretical studies on adsorption and sensor properties of nano-cages are exclusively studied at B3LYP/631G(d,p). Therefore, the results obtained here can be compared directly with the literature. Geometries of pyrrole-X12Y12 complexes are optimized without an symmetry constraints at B3LYP/631G(d,p) level of theory. The default grid for optimization in G09 is fine. The maximum force and RMS forces are 3 104 and 4.5 104 (Hartrees/Bohr and Hartrees/Radian) whereas maximum displacement and RMS displacement are 1.8 103 and 1.2 103. The binding energy (Eb) of pyrrole on nano-cages is defined as:
Eb ¼ EðX12 Y12 pyrroleÞ EðX12 Y12 Þ EðpyrroleÞ
(1)
where E(X12Y12-pyrrole) is the total energy of pyrrole-X12Y12 complex, and E(X12Y12) and E(pyrrole) are the total energies of the free X12Y12 and free pyrrole molecule, respectively. The charge transfer is calculated through Mullikan and Natural Bonding Orbitals (NBO) schemes. The changes in electronic structures of nanocages are evaluated through Frontier Molecular Orbitals (FMO), B and gap (Eg), Fermi levels and density of states. Electronic properties are also calculated at above mentioned level of theory (B3LYP/6-31G(d,p)). 3. Results and discussion Fig. 1. The Scheme of pyrrole molecule along with its DOSs and HOMO-LUMO distribution.
particular stability, large band (HOMO-LUMO) gap, and excellent physical and chemical properties. Accordingly, these nano-cages have been overwhelmingly investigated as sensors [22e24]. Adsorption properties of a variety of analytes on the surfaces of Al12N12, Al12P12, B12N12, and B12P12 [25e28] have been demonstrated in the literature. For example, we explored the adsorption property of guanine molecule on the surface of the above mentioned nano-cages using DFT calculations [25]. Although adsorption of guanine on Al12N12 has the highest adsorption energy; however, B12N12 and B12P12 show more changes in electronic property upon adsorption of guanine. Moreover, adsorption of nickel on the surface of Al12P12 [26] and Al12N12 [27] are also studied. The nickel atom was shown to adsorb on Al12P12 and Al12N12 in four distinct adsorption sties (four distinct geometries). Beheshtian et al. studied the interaction of CO on B12N12 nano-cage [28]. Soltani et al. [29] studied the interaction of phenol with diverse nano-cages through DFT calculations. In the present study, we theoretically investigate the potential of four fullerene-like X12Y12 nano-cages (X ¼ Al, B, and Y ¼ N, P) as adsorbent for pyrrole. Based on our knowledge, the adsorption of pyrrole molecule has not been studied on the surface of abovementioned nano-cages. The adsorption of pyrrole on the surface of nano-cages is studied through geometric, energetic and electronic analysis. For this purpose, binding energies, charge transfer, distribution of HOMO and LUMO, band gaps and densities of states are analyzed for free X12Y12 and X12Y12-P complexes.
2. Computational methods All calculations were performed at B3LYP/6-31G(d,p) level of DFT, as implemented in Gaussian 09 suite of program [33]. The B3LYP is a reliable method for the study of geometric and electronic properties of nano-cages [27e32]. It has been previously shown
The relaxed structure of pure Al12N12, Al12P12, B12N12 and B12P12 semiconductors are well discussed in our recent paper [25] therefore, the discussion in this manuscript is restricted to changes observed on complexation with pyrrole. The pyrrole molecule is placed on X12Y12 nano-cages in two possible orientations on the semiconductor; (a) pyrrole interacts with the nano-cage through nitrogen atom(N-side), (b) p stacking where aromatic ring of pyrrole interacts with the nano-cage. Both geometries are allowed to relax during optimization in order to find the most stable structure. For all systems, relaxed structuresin side and top views are given in Fig. 2. The binding energies of pyrrole (Table 1) adsorption on Al12N12, Al12P12, B12N12 and B12P12are 64.6, 42.6, 12.0, 9.2 kJmol-1, respectively. The X12Y12 ——— Pyrrole distances in Al12N12-P, Al12P12-P, B12N12-P and B12P12-P are 2.13 Å, 2.20 Å, 2.39 Å and 3.56 Å, respectively. The high bind energies of pyrrole on the surface of aluminum nano-cages suggest chemisorption of pyrrole. On the other hand, low binding energies of pyrrole on the surface of boron nano-cages suggest physisorption. Strong adsorption of pyrrole on aluminum nano-cages relates to the higher charge density on aluminum compared to boron (vide infra, charge analysis). The binding energy is inversely proportional to X12Y12 ——— Pyrrole distance. The binding energies data suggest that aluminum containing nano-cages are good adsorbent for pyrrole. Calculated binding distances for adsorption of pyrrole on the surface of Al12N12 and Al12P12 are very close to the value of 1.95 Å and 2.01 Å for adsorption of guanine on the same surfaces [25]. The binding energies of pyrrole on nitrogen containing nanocages are higher than their phosphorus counterparts which can be attributed to higher charge density on group III atom, and hydrogen bonding interaction in nitrogen nano-cages. Al and B atoms in nitrogen containing semiconductors are more electron deficient than those in phosphorus nano-cages. The higher charge density on Al and B in nitrogen containing semiconductor render them more susceptible to nucleophilic attack of pyrrole therefore, these complexes are categorized as p-type semiconductors. The results of charge analysis show transfer of charge from pyrrole to semiconductor (see Table 1). The nitrogen atom in
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Fig. 2. Relaxed structures of Al12N12-pyrrole (a), Al12P12-pyrrole (b), B12N12-pyrrole (c), and B12P12-pyrrole (d) in side and top view (to simplicity only some parts of semiconductors are shown in top views).
Table 1 Some parameters of relaxed structure: the nearest intera-molecular distance (dn), transferred charge of adsorbed Pyrrole based on Mulliken (QM) and NBO (QNBO), and binding energy (Eb) of X12Y12-pyrrole complex. System
dn (Å)
QM (e)
QNBO (e)
Eb kJmole1
Al12N12-P Al12P12-P B12N12-P B12P12-P
2.13 2.20 2.39 3.56
0.234 0.244 0.009 0.076
0.117 0.132 0.004 0.047
64.6 42.6 12.0 9.2
pyrrole is electron-rich whereas Al or B atoms of semiconductor are electron deficient; therefore, the adsorption of pyrrole on these surfaces is a consequence of charge transfer. In order to realize the change in the electronic structure of the nano-cages on adsorption of pyrrole, the net charge transfer from pyrrole to nano-cage is calculated for all nano-cages using natural bond orbital (NBO) and Mullikan charge schemes (see Table 1).
Both NBO and Mulliken charge analyses revealed more charge transfer for Al-containing nano-cages compared to boron containing nano-cages. The net charge transfer to Al12N12, Al12P12, B12N12, and B12P12 are þ0.117, þ0.132, þ0.004 and þ0.047 e (based on NBO) and þ0.234, þ0.244, þ0.009, and þ0.076 e (based on Mulliken), respectively. Two trends in charge transfer can be realized; (1) the charge transfer is higher for aluminum nano-cages (2) the charge transfer increases when nitrogen atom of the nano-cage is replaced with phosphorus atom. The pronounced charge transfer to aluminum may be attributed to strong electrophilic nature of aluminum than boron. Complexation of Al12N12 and Al12P12 with pyrrole causes higher charge transfer which results in higher polarity of resulted complex compared to those for B12N12 and B12P12. The higher charge transfer in phosphorus nano-cages may be relates to the longer BeP and AleP bonds compared BeN and AleN bonds. The nitrogen atom of the nano-cage better interacts with boron or aluminum atom, and makes the latter less electrophilic in
Fig. 3. The dipole moment of (left to right): free Pyrrole, Al12N12-Pyrrole, Al12P12-Pyrrole, B12N12-Pyrrole, and B12P12-Pyrrole.
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Table 2 Orbital parameters: HOMO energies (EHOMO), LUMO energies (ELUMO), energy of Fermi level (EFL), HOMO-LUMO energy gap (Eg), dipole moment (mD) of pyrrole and all free and complexed forms of X12Y12 at B3LYP/6-31G(d,p) level of theory. System
EHOMO (ev)
EFL (ev)
ELUMO (ev)
Eg (ev)
mD (Debye)
Pyrrole Al12N12 Al12N12-P Al12P12 Al12P12-P B12N12 B12N12-P B12P12 B12P12-P
5.50 6.47 6.03 6.75 6.32 7.71 5.47 6.83 5.86
2.07 4.50 4.08 5.05 4.65 4.28 3.24 6.54 4.43
1.35 2.54 2.13 3.36 2.99 0.86 1.02 3.13 3.00
6.85 3.93 3.90 3.39 3.33 6.85 4.45 3.70 2.86
1.90 0.0 5.63 0.0 6.87 0.0 2.20 0.0 2.02
nature. On the other hand, weak bonding between phosphorus and aluminum or boron causes the latter more electrophilic. To further validate our notion, we have calculated the amounts and directions of the dipole moment (Fig. 3 and Table 2). The calculated dipole moments of all free X12Y12 nano-cages are fundamentally zero (Table 2) whereas the dipole moment of pyrrole is 1.90 Debye. Considerable changes in the dipole moment of
different semiconductors are observed upon adsorption of pyrrole (Table 2). For each system, the dipole moment increases considerably upon complexation. The calculated dipole moments of Al12N12-P, Al12P12-P, B12N12-P, and B12P12-P complexes are 5.63, 6.87, 2.20 and 2.02 Debye, respectively. The trend of dipole moments of pyrrole nano-cage complexes is in accordance with the charge analyses. The highest change in dipole moment is observed when pyrrole complexes with Al12P12 followed by complexation with Al12N12nano-cage. Moreover, the low dipole moments for Bcontaining nano-cages result from insignificant charge transfer upon adsorption of pyrrole. The dipole moment depends on the quantity of charges, as well as their separation. Less intense charges on boron nano-cages are responsible for their low dipole moments. Moreover, it can be seen in Fig. 3 that the vectors of dipole moment for Al12N12-pyrrole and Al12P12-pyrrole originate from the semiconductors and point toward the pyrrole part whereas reverse and angular directions are found for B12N12-P and B12P12-P, respectively. Frontier molecular orbitals (HOMO and LUMO) are analyzed in order to gain deeper understanding of adsorption process. Fig. 1 shows the distributions of HOMO and LUMO of pyrrole along with the density of states (DOS). It can be seen from Fig. 1 that the HOMO of pyrrole is located on the unsaturated carbon atoms while
Fig. 4. HOMO and LUMO distributions along with DOSs for free Al12N12 (left) and Al12N12-pyrrole complex (right).
Fig. 5. HOMO and LUMO distributions along with DOSs for free Al12P12 (left) and Al12P12-pyrrole complex (right).
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Fig. 6. HOMO and LUMO distributions along with DOSs for free B12N12 (left) and B12N12-pyrrole complex (right).
the LUMO is spread on all atoms. The large band gap (6.85 eV) demonstrates high thermal stability of free pyrrole. HOMO and LUMO distributions as well as DOSs for free and X12Y12-P complexes are given in Figs. 4e7. The energies of HOMO (and LUMO) for free Al12N12, Al12P12, B12N12, and B12P12 are 6.47 (2.54), 6.75 (3.36), 7.71 (0.86), and 6.83 (3.13) eV with band gaps of 3.93, 3.39, 6.85, and 3.70 eV, respectively (see Table 2). The Fermi level of Al12N12, Al12P12, B12N12 and B12P12 are 4.50, 5.05, 4.28, and 6.54 eV. After adsorption of pyrrole, the energies of HOMO (and LUMO) was changed to 6.03 (2.13), 6.32 (2.99), 5.47 (1.02), and 5.86 (3.00) Ev and the band gaps (Eg) are 3.90, 3.33, 4.45, and 2.86 eV, respectively. The Fermi levels changed to 4.08, 4.65, 3.24, and 4.43 eV in Al12N12-P, Al12P12-P, B12N12-P and B12P12-P, respectively. Except for B12N12, the HOMO in all free semiconductors is mostly situated on nitrogen and phosphorus atoms, and the LUMO has density uniformly spread on all atoms. Nevertheless, the HOMO in B12N12 resides on nitrogen atoms as
well as BeN bonds whereas the LUMO is centered mostly on B atoms. Quite significant changes in the frontier molecular orbitals are observed on complexation with pyrrole. Figs. 4 and 5 show that upon adsorption of pyrrole, HOMO and LUMO of Al12N12eP and Al12P12-P are still situated on the nano-cage part but their distributions are changed considerably compared to free semiconductors. These changes in distribution of both HOMO and LUMO associate with shifting of their energy level which leads to changes in Eg. However, the changes in the Eg of these two semiconductors upon complexation are not significant (0.8% for Al12N12 and 1.8% for Al12P12) since the amounts of positive shift for both HOMO and LUMO are approximately the same as can be found by comparing their DOSs in Figs. 4 and 5. Interesting re-distribution of HOMOs and LUMOs is observed for B12N12-P and B12P12eP complexes. The LUMOs of B12N12-pyrrole and B12P12-pyrrole reside completely on nano-cage (Figs. 6 and 7)
Fig. 7. HOMO and LUMO distributions along with DOSs for free B12P12 (left) and B12P12-pyrrole complex (right).
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whereas their HOMOs are located entirely on the pyrrole moiety. This new distribution causes significant shifts in the energies of HOMOs and LUMOs. Quite contrary to Al-containing semiconductors, the shifts in energies are not equal for both HOMO and LUMO. As a result, the changes in the Eg of B-containing nano-cages are much higher than those for Al-containing nano-cages (35.0% for B12N12 and 22.7% for B12P12). These results suggest that B12N12 and B12P12 are efficient sensors for pyrrole despite their lower binding energies due to much pronounced change in their electronic structure (Eg) (Figs. 6 and 7). These observations clearly imply that the Al12P12 and Al12N12 are not good sensors for pyrrole. The highest sensitivity towards pyrrole is expected for B12N12 nanocage. The decrease in the band gap of systems corresponds to increase in their conductivity through the following equation:
s∝exp Eg 2KT
(2)
where s defined as the electric conductivity and k defined as the Boltzmann constant [40]. It can be concluded from the changes in electronic structure that boron containing semi-conductors are good sensor for pyrrole molecule despite their lower binding energy. 4. Conclusion Adsorption of pyrrole on the surfaces of four X12Y12 semiconductors (Al12N12, Al12P12, B12N12, and B12P12) is studied through density functional theory (DFT) calculations at B3LYP/6-31G(d,p) level of theory. The highest interaction energy is calculated for the adsorption of pyrrole on the surface of Al12N12nano-cage. The adsorption energies of pyrrole on Al12N12,Al12P12, B12N12, and B12P12 are 64.6, 42.6, 12.0, 9.2 kJ mol1, respectively. Pyrrole acts as electron donor and adsorbs at the electrophilic site of nanocage. Charge transfer to aluminum nano-cages is higher than to boron nano-cages. Changes in electronic properties such as band gap, Fermi level, and densities of states are also analyzed in order to better understand the sensing abilities of nano-cages for pyrrole molecule. Band gaps of aluminum nano-cages (Al12N12 and Al12P12) are unaffected by adsorption of pyrrole because of comparable effect on HOMOs and LUMOs. The changes in Eg for Al12N12 and Al12P12 on complexation with pyrrole are 0.8% and 1.8%, respectively. On the other hand, band gaps of boron nano-cages are significantly reduced on adsorption of pyrrole. The changes in Eg for B12N12 and B12P12 on complexation with pyrrole are 35% and 22.7%, respectively. We find higher change in the Eg of B-containing semiconductors compared with Al-containing semiconductors. Boron nano-cages are better sensor for pyrrole molecule despite their lower binding energies. All these nano-clusters are found to be p-type semiconductor owing to direction of charge transfer which is from pyrrole to semiconductor. Acknowledgments A. S. R highly acknowledge financial support from Iran Nanotechnology Initiative Council, Iran. K. A. acknowledge the financial and technical support from Higher Education Commission of Pakistan (Grant Nos. 1899, 2469 and 2981) and COMSATS Institute of Information Technology. References [1] R.J. Hamers, Formation and characterization of organic monolayers on semiconductor surfaces, Annu. Rev. Anal. Chem. 1 (2008) 707e736. [2] F. Tao, S.L. Bernasek, G.-Q. Xu, Electronic and structural factors in modification and functionalization of clean and passivated semiconductor surfaces with
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Adsorption of pyrrole on Al12N12, Al12P12, B12N12, and B12P12 fullerene-like nano-cages; a first principles study Ali Shokuhi Rad a, *, Khurshid Ayub b a b
Department of Chemical Engineering, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran COMSATS Institute of Information Technology, University Road, Tobe Camp, Abbottabad, 22060, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 May 2016 Received in revised form 15 June 2016 Accepted 16 June 2016 Available online 17 June 2016
Adsorption of pyrrole on the surfaces of four X12Y12 semiconductors (Al12N12, Al12P12, B12N12, and B12P12) is studied through density functional theory (DFT) calculations at B3LYP/6-31G(d,p) level of theory. The highest interaction energy is calculated for the adsorption of pyrrole on the surface of Al12N12 nano-cage. The adsorption energies of pyrrole on Al12N12, Al12P12, B12N12, and B12P12 are 64.6, 42.6, 12.0, 9.2 kJ mol1, respectively. Pyrrole acts as an electron donor and adsorbs at the electrophilic site of nanocage. Charge transfer to aluminum nano-cages is higher than to boron nano-cages. Changes in electronic properties such as band gap, Fermi level, and densities of states are also analyzed in order to better understand the sensing abilities of nano-cages for pyrrole molecule. Band gaps of aluminum nano-cages (Al12N12 and Al12P12) are unaffected by adsorption of pyrrole because of comparable effect on HOMOs and LUMOs. On the other hand, band gaps of boron nano-cages are significantly reduced on adsorption of pyrrole. Boron nano-cages are better sensor for pyrrole molecule despite their lower binding energies. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Pyrrole Al12N12 Al12P12 B12N12 B12P12 Nano-cages
1. Introduction An important challenge in current semiconductor technology is to understand important parameters that influence the interaction of organic molecules with different semiconductors [1,2]. A few reports exist in the literature where adsorption of small organic molecules such as cyclopentene, benzene and pyrrole is demonstrated on the surface of semiconductor materials [3e5]. Pyrrole (C4H4NH) is colorless, volatile, nitrogen containing heterocyclic compound (see Fig. 1). Pyrrole is an aromatic compound and it exhibits chemical reactivity very similar to benzene. For example, pyrrole resists to hydrogenation and Diels-Alder reactions. However, the electron density in pyrrole is greater than that of benzene; therefore, pyrrole is categorized as p-electron excessive aromatic molecule [6]. Pyrrole has noxious impact on metal catalysts utilized in petrochemical industry therefore removal of pyrrole from crude petroleum is very important. A number of studies demonstrate the adsorption of pyrrole on different surface [7e9]. For example, Qiao et al. [7] studied the adsorption and thermal reaction of pyrrole on
* Corresponding author. E-mail address: [email protected] (A. Shokuhi Rad). http://dx.doi.org/10.1016/j.vacuum.2016.06.012 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
Si(1 0 0) through X-ray and ultra-violet photoelectron spectroscopy. Bruhn et al. [8] studied the adsorption mechanism of pyrrole on Asrich GaAs(001)-c(4 4) surface. Noei et al. [9] examined the electrical sensitivity of boron nitride nanotube toward pyrrole through DFT calculations. Recently, nanostructure semiconductors have gained significant interest from the scientific community because of their distinct physical and chemical properties [10e12]. Particularly, group IIIeV semiconductors have been used in fast microelectronic devices and light-emitting diodes [13e15]. Group IIIeV sheets such as AlN sheets have been studied experimentally on Ag(111) support [16], and also theoretically for defects [17]. Moreover, hexagonal BN sheets are also studied as sensor [18]. Group IIIeV semiconductors have also been used as adsorbent and sensor for different analytes. Design of these new solid-state adsorbents/sensors is based on the high electrophilicity of IIIeV semiconductors. Electronic structure of these semiconductors changes on interaction with analytes. The small size of solid-state chemical adsorbents, easy synthesis, low cost and their reproducibility make them fascinating candidates for sensor applications [19]. Molecular simulation of different XnYn semiconductors (X ¼ B, Al, … and Y ¼ N, P, …) showed that the fullerene-like X12Y12 cages are the most stable ones [20,21]. Al12N12, Al12P12, B12N12 and B12P12 are four important fullerene-like nano-cages because of their
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that B3LYP/6-31G(d,p) is an optimal level of theory to deliver reliable and accurate results at minimum cost for a variety of systems [34e39]. Moreover, the theoretical studies on adsorption and sensor properties of nano-cages are exclusively studied at B3LYP/631G(d,p). Therefore, the results obtained here can be compared directly with the literature. Geometries of pyrrole-X12Y12 complexes are optimized without an symmetry constraints at B3LYP/631G(d,p) level of theory. The default grid for optimization in G09 is fine. The maximum force and RMS forces are 3 104 and 4.5 104 (Hartrees/Bohr and Hartrees/Radian) whereas maximum displacement and RMS displacement are 1.8 103 and 1.2 103. The binding energy (Eb) of pyrrole on nano-cages is defined as:
Eb ¼ EðX12 Y12 pyrroleÞ EðX12 Y12 Þ EðpyrroleÞ
(1)
where E(X12Y12-pyrrole) is the total energy of pyrrole-X12Y12 complex, and E(X12Y12) and E(pyrrole) are the total energies of the free X12Y12 and free pyrrole molecule, respectively. The charge transfer is calculated through Mullikan and Natural Bonding Orbitals (NBO) schemes. The changes in electronic structures of nanocages are evaluated through Frontier Molecular Orbitals (FMO), B and gap (Eg), Fermi levels and density of states. Electronic properties are also calculated at above mentioned level of theory (B3LYP/6-31G(d,p)). 3. Results and discussion Fig. 1. The Scheme of pyrrole molecule along with its DOSs and HOMO-LUMO distribution.
particular stability, large band (HOMO-LUMO) gap, and excellent physical and chemical properties. Accordingly, these nano-cages have been overwhelmingly investigated as sensors [22e24]. Adsorption properties of a variety of analytes on the surfaces of Al12N12, Al12P12, B12N12, and B12P12 [25e28] have been demonstrated in the literature. For example, we explored the adsorption property of guanine molecule on the surface of the above mentioned nano-cages using DFT calculations [25]. Although adsorption of guanine on Al12N12 has the highest adsorption energy; however, B12N12 and B12P12 show more changes in electronic property upon adsorption of guanine. Moreover, adsorption of nickel on the surface of Al12P12 [26] and Al12N12 [27] are also studied. The nickel atom was shown to adsorb on Al12P12 and Al12N12 in four distinct adsorption sties (four distinct geometries). Beheshtian et al. studied the interaction of CO on B12N12 nano-cage [28]. Soltani et al. [29] studied the interaction of phenol with diverse nano-cages through DFT calculations. In the present study, we theoretically investigate the potential of four fullerene-like X12Y12 nano-cages (X ¼ Al, B, and Y ¼ N, P) as adsorbent for pyrrole. Based on our knowledge, the adsorption of pyrrole molecule has not been studied on the surface of abovementioned nano-cages. The adsorption of pyrrole on the surface of nano-cages is studied through geometric, energetic and electronic analysis. For this purpose, binding energies, charge transfer, distribution of HOMO and LUMO, band gaps and densities of states are analyzed for free X12Y12 and X12Y12-P complexes.
2. Computational methods All calculations were performed at B3LYP/6-31G(d,p) level of DFT, as implemented in Gaussian 09 suite of program [33]. The B3LYP is a reliable method for the study of geometric and electronic properties of nano-cages [27e32]. It has been previously shown
The relaxed structure of pure Al12N12, Al12P12, B12N12 and B12P12 semiconductors are well discussed in our recent paper [25] therefore, the discussion in this manuscript is restricted to changes observed on complexation with pyrrole. The pyrrole molecule is placed on X12Y12 nano-cages in two possible orientations on the semiconductor; (a) pyrrole interacts with the nano-cage through nitrogen atom(N-side), (b) p stacking where aromatic ring of pyrrole interacts with the nano-cage. Both geometries are allowed to relax during optimization in order to find the most stable structure. For all systems, relaxed structuresin side and top views are given in Fig. 2. The binding energies of pyrrole (Table 1) adsorption on Al12N12, Al12P12, B12N12 and B12P12are 64.6, 42.6, 12.0, 9.2 kJmol-1, respectively. The X12Y12 ——— Pyrrole distances in Al12N12-P, Al12P12-P, B12N12-P and B12P12-P are 2.13 Å, 2.20 Å, 2.39 Å and 3.56 Å, respectively. The high bind energies of pyrrole on the surface of aluminum nano-cages suggest chemisorption of pyrrole. On the other hand, low binding energies of pyrrole on the surface of boron nano-cages suggest physisorption. Strong adsorption of pyrrole on aluminum nano-cages relates to the higher charge density on aluminum compared to boron (vide infra, charge analysis). The binding energy is inversely proportional to X12Y12 ——— Pyrrole distance. The binding energies data suggest that aluminum containing nano-cages are good adsorbent for pyrrole. Calculated binding distances for adsorption of pyrrole on the surface of Al12N12 and Al12P12 are very close to the value of 1.95 Å and 2.01 Å for adsorption of guanine on the same surfaces [25]. The binding energies of pyrrole on nitrogen containing nanocages are higher than their phosphorus counterparts which can be attributed to higher charge density on group III atom, and hydrogen bonding interaction in nitrogen nano-cages. Al and B atoms in nitrogen containing semiconductors are more electron deficient than those in phosphorus nano-cages. The higher charge density on Al and B in nitrogen containing semiconductor render them more susceptible to nucleophilic attack of pyrrole therefore, these complexes are categorized as p-type semiconductors. The results of charge analysis show transfer of charge from pyrrole to semiconductor (see Table 1). The nitrogen atom in
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Fig. 2. Relaxed structures of Al12N12-pyrrole (a), Al12P12-pyrrole (b), B12N12-pyrrole (c), and B12P12-pyrrole (d) in side and top view (to simplicity only some parts of semiconductors are shown in top views).
Table 1 Some parameters of relaxed structure: the nearest intera-molecular distance (dn), transferred charge of adsorbed Pyrrole based on Mulliken (QM) and NBO (QNBO), and binding energy (Eb) of X12Y12-pyrrole complex. System
dn (Å)
QM (e)
QNBO (e)
Eb kJmole1
Al12N12-P Al12P12-P B12N12-P B12P12-P
2.13 2.20 2.39 3.56
0.234 0.244 0.009 0.076
0.117 0.132 0.004 0.047
64.6 42.6 12.0 9.2
pyrrole is electron-rich whereas Al or B atoms of semiconductor are electron deficient; therefore, the adsorption of pyrrole on these surfaces is a consequence of charge transfer. In order to realize the change in the electronic structure of the nano-cages on adsorption of pyrrole, the net charge transfer from pyrrole to nano-cage is calculated for all nano-cages using natural bond orbital (NBO) and Mullikan charge schemes (see Table 1).
Both NBO and Mulliken charge analyses revealed more charge transfer for Al-containing nano-cages compared to boron containing nano-cages. The net charge transfer to Al12N12, Al12P12, B12N12, and B12P12 are þ0.117, þ0.132, þ0.004 and þ0.047 e (based on NBO) and þ0.234, þ0.244, þ0.009, and þ0.076 e (based on Mulliken), respectively. Two trends in charge transfer can be realized; (1) the charge transfer is higher for aluminum nano-cages (2) the charge transfer increases when nitrogen atom of the nano-cage is replaced with phosphorus atom. The pronounced charge transfer to aluminum may be attributed to strong electrophilic nature of aluminum than boron. Complexation of Al12N12 and Al12P12 with pyrrole causes higher charge transfer which results in higher polarity of resulted complex compared to those for B12N12 and B12P12. The higher charge transfer in phosphorus nano-cages may be relates to the longer BeP and AleP bonds compared BeN and AleN bonds. The nitrogen atom of the nano-cage better interacts with boron or aluminum atom, and makes the latter less electrophilic in
Fig. 3. The dipole moment of (left to right): free Pyrrole, Al12N12-Pyrrole, Al12P12-Pyrrole, B12N12-Pyrrole, and B12P12-Pyrrole.
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Table 2 Orbital parameters: HOMO energies (EHOMO), LUMO energies (ELUMO), energy of Fermi level (EFL), HOMO-LUMO energy gap (Eg), dipole moment (mD) of pyrrole and all free and complexed forms of X12Y12 at B3LYP/6-31G(d,p) level of theory. System
EHOMO (ev)
EFL (ev)
ELUMO (ev)
Eg (ev)
mD (Debye)
Pyrrole Al12N12 Al12N12-P Al12P12 Al12P12-P B12N12 B12N12-P B12P12 B12P12-P
5.50 6.47 6.03 6.75 6.32 7.71 5.47 6.83 5.86
2.07 4.50 4.08 5.05 4.65 4.28 3.24 6.54 4.43
1.35 2.54 2.13 3.36 2.99 0.86 1.02 3.13 3.00
6.85 3.93 3.90 3.39 3.33 6.85 4.45 3.70 2.86
1.90 0.0 5.63 0.0 6.87 0.0 2.20 0.0 2.02
nature. On the other hand, weak bonding between phosphorus and aluminum or boron causes the latter more electrophilic. To further validate our notion, we have calculated the amounts and directions of the dipole moment (Fig. 3 and Table 2). The calculated dipole moments of all free X12Y12 nano-cages are fundamentally zero (Table 2) whereas the dipole moment of pyrrole is 1.90 Debye. Considerable changes in the dipole moment of
different semiconductors are observed upon adsorption of pyrrole (Table 2). For each system, the dipole moment increases considerably upon complexation. The calculated dipole moments of Al12N12-P, Al12P12-P, B12N12-P, and B12P12-P complexes are 5.63, 6.87, 2.20 and 2.02 Debye, respectively. The trend of dipole moments of pyrrole nano-cage complexes is in accordance with the charge analyses. The highest change in dipole moment is observed when pyrrole complexes with Al12P12 followed by complexation with Al12N12nano-cage. Moreover, the low dipole moments for Bcontaining nano-cages result from insignificant charge transfer upon adsorption of pyrrole. The dipole moment depends on the quantity of charges, as well as their separation. Less intense charges on boron nano-cages are responsible for their low dipole moments. Moreover, it can be seen in Fig. 3 that the vectors of dipole moment for Al12N12-pyrrole and Al12P12-pyrrole originate from the semiconductors and point toward the pyrrole part whereas reverse and angular directions are found for B12N12-P and B12P12-P, respectively. Frontier molecular orbitals (HOMO and LUMO) are analyzed in order to gain deeper understanding of adsorption process. Fig. 1 shows the distributions of HOMO and LUMO of pyrrole along with the density of states (DOS). It can be seen from Fig. 1 that the HOMO of pyrrole is located on the unsaturated carbon atoms while
Fig. 4. HOMO and LUMO distributions along with DOSs for free Al12N12 (left) and Al12N12-pyrrole complex (right).
Fig. 5. HOMO and LUMO distributions along with DOSs for free Al12P12 (left) and Al12P12-pyrrole complex (right).
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Fig. 6. HOMO and LUMO distributions along with DOSs for free B12N12 (left) and B12N12-pyrrole complex (right).
the LUMO is spread on all atoms. The large band gap (6.85 eV) demonstrates high thermal stability of free pyrrole. HOMO and LUMO distributions as well as DOSs for free and X12Y12-P complexes are given in Figs. 4e7. The energies of HOMO (and LUMO) for free Al12N12, Al12P12, B12N12, and B12P12 are 6.47 (2.54), 6.75 (3.36), 7.71 (0.86), and 6.83 (3.13) eV with band gaps of 3.93, 3.39, 6.85, and 3.70 eV, respectively (see Table 2). The Fermi level of Al12N12, Al12P12, B12N12 and B12P12 are 4.50, 5.05, 4.28, and 6.54 eV. After adsorption of pyrrole, the energies of HOMO (and LUMO) was changed to 6.03 (2.13), 6.32 (2.99), 5.47 (1.02), and 5.86 (3.00) Ev and the band gaps (Eg) are 3.90, 3.33, 4.45, and 2.86 eV, respectively. The Fermi levels changed to 4.08, 4.65, 3.24, and 4.43 eV in Al12N12-P, Al12P12-P, B12N12-P and B12P12-P, respectively. Except for B12N12, the HOMO in all free semiconductors is mostly situated on nitrogen and phosphorus atoms, and the LUMO has density uniformly spread on all atoms. Nevertheless, the HOMO in B12N12 resides on nitrogen atoms as
well as BeN bonds whereas the LUMO is centered mostly on B atoms. Quite significant changes in the frontier molecular orbitals are observed on complexation with pyrrole. Figs. 4 and 5 show that upon adsorption of pyrrole, HOMO and LUMO of Al12N12eP and Al12P12-P are still situated on the nano-cage part but their distributions are changed considerably compared to free semiconductors. These changes in distribution of both HOMO and LUMO associate with shifting of their energy level which leads to changes in Eg. However, the changes in the Eg of these two semiconductors upon complexation are not significant (0.8% for Al12N12 and 1.8% for Al12P12) since the amounts of positive shift for both HOMO and LUMO are approximately the same as can be found by comparing their DOSs in Figs. 4 and 5. Interesting re-distribution of HOMOs and LUMOs is observed for B12N12-P and B12P12eP complexes. The LUMOs of B12N12-pyrrole and B12P12-pyrrole reside completely on nano-cage (Figs. 6 and 7)
Fig. 7. HOMO and LUMO distributions along with DOSs for free B12P12 (left) and B12P12-pyrrole complex (right).
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whereas their HOMOs are located entirely on the pyrrole moiety. This new distribution causes significant shifts in the energies of HOMOs and LUMOs. Quite contrary to Al-containing semiconductors, the shifts in energies are not equal for both HOMO and LUMO. As a result, the changes in the Eg of B-containing nano-cages are much higher than those for Al-containing nano-cages (35.0% for B12N12 and 22.7% for B12P12). These results suggest that B12N12 and B12P12 are efficient sensors for pyrrole despite their lower binding energies due to much pronounced change in their electronic structure (Eg) (Figs. 6 and 7). These observations clearly imply that the Al12P12 and Al12N12 are not good sensors for pyrrole. The highest sensitivity towards pyrrole is expected for B12N12 nanocage. The decrease in the band gap of systems corresponds to increase in their conductivity through the following equation:
s∝exp Eg 2KT
(2)
where s defined as the electric conductivity and k defined as the Boltzmann constant [40]. It can be concluded from the changes in electronic structure that boron containing semi-conductors are good sensor for pyrrole molecule despite their lower binding energy. 4. Conclusion Adsorption of pyrrole on the surfaces of four X12Y12 semiconductors (Al12N12, Al12P12, B12N12, and B12P12) is studied through density functional theory (DFT) calculations at B3LYP/6-31G(d,p) level of theory. The highest interaction energy is calculated for the adsorption of pyrrole on the surface of Al12N12nano-cage. The adsorption energies of pyrrole on Al12N12,Al12P12, B12N12, and B12P12 are 64.6, 42.6, 12.0, 9.2 kJ mol1, respectively. Pyrrole acts as electron donor and adsorbs at the electrophilic site of nanocage. Charge transfer to aluminum nano-cages is higher than to boron nano-cages. Changes in electronic properties such as band gap, Fermi level, and densities of states are also analyzed in order to better understand the sensing abilities of nano-cages for pyrrole molecule. Band gaps of aluminum nano-cages (Al12N12 and Al12P12) are unaffected by adsorption of pyrrole because of comparable effect on HOMOs and LUMOs. The changes in Eg for Al12N12 and Al12P12 on complexation with pyrrole are 0.8% and 1.8%, respectively. On the other hand, band gaps of boron nano-cages are significantly reduced on adsorption of pyrrole. The changes in Eg for B12N12 and B12P12 on complexation with pyrrole are 35% and 22.7%, respectively. We find higher change in the Eg of B-containing semiconductors compared with Al-containing semiconductors. Boron nano-cages are better sensor for pyrrole molecule despite their lower binding energies. All these nano-clusters are found to be p-type semiconductor owing to direction of charge transfer which is from pyrrole to semiconductor. Acknowledgments A. S. R highly acknowledge financial support from Iran Nanotechnology Initiative Council, Iran. K. A. acknowledge the financial and technical support from Higher Education Commission of Pakistan (Grant Nos. 1899, 2469 and 2981) and COMSATS Institute of Information Technology. References [1] R.J. Hamers, Formation and characterization of organic monolayers on semiconductor surfaces, Annu. Rev. Anal. Chem. 1 (2008) 707e736. [2] F. Tao, S.L. Bernasek, G.-Q. Xu, Electronic and structural factors in modification and functionalization of clean and passivated semiconductor surfaces with
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