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How can nickel decoration affect H2 adsorption on B12P12 nano-heterostructures?
Journal of Molecular Liquids 255 (2018) 168–175
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
How can nickel decoration affect H2 adsorption on B12P12 nano-heterostructures? Ali Shokuhi Rad a,⁎, Khurshid Ayub b a b
Department of Chemical Engineering, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran Department of Chemistry, COMSATS Institute of Information Technology, University Road, Tobe Camp, 22060 Abbottabad, Pakistan
a r t i c l e
i n f o
Article history: Received 27 October 2017 Received in revised form 24 January 2018 Accepted 25 January 2018 Available online xxxx Keywords: Nano-heterostructure B12P12 Ni decoration Inorganic material Hydrogen adsorption
a b s t r a c t Density functional theory calculations have been performed to study structural, electronic and hydrogen adsorption properties of Ni doped B12P12 fullerene-like clusters. Four geometries of Ni doped B12P12 nanoheterostructures were optimized and the potential of each geometry was investigated towards hydrogen adsorption. Natural bond orbital (NBO), molecular electrostatic potential (MEP), dipole moment, frontier orbitals, and density of states (DOS) analyses are performed to observe the changes in the electronic structure of B12P12 upon decoration by Ni followed by H2 adsorption. Ni doping significantly enhances the adsorption property of B12P12 nano-heterostructure for H2 molecule. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Interaction of hydrogen with metals is fundamental to several chemical reactions and industries. For example, hydrogenation reaction on the surface of nickel nano-particles involves gas-solid interaction of sub-surface hydrogen with nickel [1–4]. Moreover, the interaction of H2 with a variety of metals including Pd [5], Ru [6], Fe [7], Pt [8], is already reported in the literature. Therefore, profound studies of hydrogen interaction with transition metals are very important for understanding interaction mechanism of hydrogen on transition metals for different applications. Recently, fullerene-like group III–V semi-conductor nanoheterostructures (XnYn (X = B, Al, Ga, and Y = N, P, As)) have gained large attention due to their outstanding chemical and physical properties [9–12]. The X12Y12 nano-cluster is energetically the most stable cluster among different sized (XY)n structures [13,14]. Boron phosphide (BP) is an attractive member of XnYn family which has unique physical and electrochemical properties. The BP can exist in planer, tubular or spherical shapes, quite analogous to graphene, carbon nano-tubes and fullerenes, respectively. All shapes of its family are eminent as vast band-gap semiconductor material having many advanced properties. B12P12 is a spherical fullerene like member of BP family, and one of the best candidates to build devices in the violet area. Beheshtian et al. [15] studied the stability, geometry and electronic structure of Al12N12, Al12P12, B12N12, and B12P12 nano-heterostructures ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (A.S. Rad).
https://doi.org/10.1016/j.molliq.2018.01.149 0167-7322/© 2018 Elsevier B.V. All rights reserved.
using DFT calculations. A few studies have appeared in the recent literature where B12P12 and other analogues have been used as sensors for different analytes. In our recent report [16], we studied the adsorption property of guanine molecule on the surface of four nano-cages using DFT calculations. We found that despite the fact that Al12N12 has the highest adsorption energy, B12N12 and B12P12 show more changes in electronic property upon adsorption of guanine. Beheshtian et al. [17] investigated the interaction of a hydrogen atom with B12P12 nano-cluster through DFT methods. Their results showed that the HOMO-LUMO energy gap of B12P12 cluster is dramatically reduced to one-half of its original value upon H adsorption on the B atom. Moreover, the adsorption of ammonia on above four mentioned clusters was investigated through DFT methods by Peyghan and Soleymanabadi [18]. They found that ammonia prefers to be adsorbed on B or Al atom of the clusters. More recently, dopants have been introduced in nanostructures to enhance the electronic and adsorption properties [19–33]. For example, Baei et al. [34] demonstrated that silicon-doping transforms the B12N12 cluster to an n- or p-type semiconductor (depend on the location of doped Si). We have studied nickel doping of Al12P12 cluster [35], and observed four possible sites for decoration of nickel atom. Zhang et al. [36] investigated the feasibility of bare and Ni decorated Al12N12 cages for hydrogen storage. They reported significant enhancement in hydrogen adsorption capacity of this cluster upon decoration of nickel. In this paper, we explore all possible sites of nickel decoration on B12P12 cluster. Next, the possibility of free and Ni-decorated B12P12 cluster for H2 adsorption is scrutinized by DFT methods. We expect quite significant changes in the electronic properties of pure B12P12 cluster
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by nickel doping. To the best of our knowledge, this is the first systematic study of the adsorption of H2 molecules on the surface of Ni-decorated B12P12. The disparities of structural and electronic properties of these nano-clusters are compared with each other. The results can be particularly important for promoting the use of inorganic clusters in the future nano-scale electronic devices for H2 adsorbent/sensor applications. 2. Computational methods We used B3LYP method of DFT with 6–31 G(d,p) basis set. The B3LYP is a quite reliable hybrid DF method for the study of X12Y12 nanostructures [15–18]. Geometric, energetic and electronic properties (including NBO charge analyses, MEP, DOSs analyses, frontier molecular orbital analyses) of B12P12 and Ni-B12P12 nano-cluster are calculated to study the sensing of these clusters for hydrogen molecule (B12P12, NiB12P12, B12P12-H2, and B12P12-Ni-H2). A number of possible orientations were considered for decoration of Ni on B12P12 nano-cluster but all input geometries resulted in optimization of four distinct structures; P1, P2, P3, and P4 (vide infra). The adsorption energy of Ni on BP in all positions is calculated by: Ead ¼ ENi−BP −ðEBP þ ENi Þ
ð1Þ
where ENi-BP is the total (electronic) energy of B12P12 interacting with Ni whereas EBP and ENi are the total energy of isolated B12P12 and Ni, respectively. The interaction energy of H2 with isolated BP and Ni-BP is calculated by: E intð BPÞ ¼ EH2−BP −ðEBP þ EH2 Þ
ð2Þ
E int ðNi−BPÞ ¼ EH2−Ni−BP −ðENi−BP þ EH2 Þ
ð3Þ
where Eint(BP) and Eint (Ni-BP) correspond to interaction energy of H2 with BP and Ni-BP, respectively. EH2-BP and EH2-Ni-BP are total electronic energies of BP and Ni-BP interacting with the H2, respectively, whereas EH2 is the total energy of an isolated H2. All calculations were performed using G09 suite of program [37]. 3. Results and discussion The BP nano-heterostructure has four and six membered rings connected to one another. The B\\P bond lengths differ slightly among each other depending whether the bond is between two six membered rings, or between a six and a four membered ring. The B\\P bond shared between two six membered rings is 1.91 Å whereas the bond shared
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between a six and a four membered ring is 1.93 Å in length (see Fig. 1). For nickel decoration on the surface of BP nano-heterostructure, one finds several positions for interaction; on an atom (P or B), on a bond, and in the center of a ring. When nickel is adsorbed on a bond or in the center of a ring, there are two possibilities for each category. The nickel atom can be placed on a bond shared between two six membered ring, or on a bond shared between four and six membered ring. Similarly, the nickel atom can be placed on a four membered ring or a six membered ring. Although six different possibilities exist for nickel decoration on the surface of nano-heterostructure, we could get only four different optimized structures because some of the initial input geometries converted to other geometries during optimization. The four optimized geometries are denoted as P1, P2, P3 and P4. The nickel atom is incorporated into a bond shared between four and six membered rings (denoted as M@b64) in P1 whereas the nickel atom is incorporated into a bond shared between two six membered rings in P2 (M@ b66). P3 and P4 geometries contain nickel atom above four (M@r4) and six membered rings (M@r6), respectively. In general, nickel adsorption causes significant distortion in the geometries of BP nano-heterostructure (see Fig. 2). The nickel atom is inserted into a four membered ring in P1 geometry (vide supra) and this causes the B\\P bond to elongate to 2.66 Å compared to 1.93 Å in pure BP nano-heterostructure. Despite this distortion, the interaction energy for adsorption of nickel on BP nano-heterostructure in P1 geometry is very high (−481.1 kJ mol−1). The high interaction energy is indicative of chemisorption. The coordination of some atoms of BP nano-heterostructure combined with release of strain from four to five membered ring probably accounts for this high interaction energy. A similar distortion by incoming nickel atom is seen in P2 geometry. The B\\P bond is elongated to 2.76 Å compared to 1.91 Å in bare BP nanoheterostructure. The interaction energy of nickel with BP nanoheterostructure in P2 geometry is relatively less (−445 kJ mol−1) compared to that in P1 geometry. The least interaction energy is calculated for P3 geometry (−425 kJ mol −1 ). The nickel atom interacts with boron and two phosphorus atoms from the nano-heterostructure without distorting the geometry of BP nano-heterostructure in P3. The BP bond is slightly elongated to 2.05 Å compared to 1.93 Å in pure BP nanoheterostructure. The interaction of nickel with BP nano-heterostructure in P3 is unsymmetrical where only three atoms (out of four atoms of four membered ring) interact with nickel (see Fig. 2). The interaction energy is considerably enhanced for P4 geometry; however, the distortion caused by nickel adsorption is also highest for P4. The interaction energy is −580.7 kJ mol−1. The geometry of P4 is considerably different than P3; the nickel atom in the former is oriented at the center of the ring (in the plane of the ring) but above the plane of the ring in the
Fig. 1. The relaxed structure of BP at 6–31 G(d,p)/B3LYP level of theory.
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Fig. 2. The relaxed structure of Ni-BP for each position; to simplicity for each position some part of relaxed structures in top view is shown too. Some inter-molecular distances of BP's atoms are shown upon Ni adsorption (black ones) followed by H2 adsorption (blue ones) (vide infra). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
latter. This difference is probably due to cavity size for four and six membered ring. The nickel atom finds enough space to sit almost in the plane of the six membered ring. The P4 structure is more stabilized by enhanced interactions from increased number of coordinating atoms. This placement of nickel in the center of six membered ring pushes away the other atoms of the ring. The B\\P bond lengths are increased to 3.01, 2.60 and 2.72 Å compared to 1.91 and 1.93 Å in BP nano-heterostructure. Then, the interactions of these geometries (P1-P4) for H2 are analyzed. Pure BP nano-heterostructure has very weak interaction with H2 molecule. The interaction energy is −0.7 kJ mol−1, and the H2
molecule is located from the nano-heterostructure at a distance of 4.50 Å. The interaction of H2 molecule is significantly increased for metal decorated BP nano-heterostructures. The interaction energies range from −107.8 kJ mol−1 to +8.9 kJ mol−1. The highest interaction energy for hydrogen adsorption is seen for P3 geometry (−107.8 kJ mol−1) followed by −99 kJ mol−1 for P2, −85.8 kJ mol−1 for P1 and +8.9 kJ mol−1 for P4. Interestingly, the interaction energies for hydrogen adsorption are inversely proportional to the adsorption energies of nickel itself (see Fig. 4). The inverse relationship may be rationalized by the fact that higher stability of nickel decorated nanoheterostructure resist towards hydrogen adsorption. The P4 geometry
Fig. 3. The relaxed structure of adsorbed H2 on: pristine BP (a), Ni(P1)BP (b), Ni(P2)BP (c), Ni(P3)BP (d), and Ni(P4)BP (e).
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Fig. 4. The inverse relationship of Ni adsorption and H2 adsorption (the unit of energies: kJ/mol).
is the most stable geometry for Ni adsorbed system, and it resists towards hydrogen adsorption to the extent that interaction is endothermic. The resistance towards hydrogen adsorption can be demonstrated based on geometric changes caused by hydrogen adsorption. Nickel adsorption on BP nano-heterostructure distorts the geometries of BP (vide supra), and B\\P bond lengths are elongated. However, the hydrogen adsorption reverts those changes to some extent. For example, the hydrogen adsorption in P1 causes the B\\P bond length to change from 2.66 to 2.39 (more close to the 1.93 for bare BP nano-heterostructure). Similar changes are also observed for P2 where B\\P bond length is decreased to 2.66 from 2.76. The behavior of change in bond lengths for P3 and P 4 is of mixed type where some bond lengths are increased and some are decreased (see Fig. 1 for details). The strength of interaction of nickel with hydrogen can be realized from the H\\H bond lengths as well as Ni\\H\\H bond lengths. For H2 adsorption on bare BP nanoheterostructure, the H\\H bond length is 0.74 Å (Fig. 3) which is comparable to the calculated 0.743 Å for free H2 molecule [38]. The H\\H bond lengths for H2 complexes are 0.83 Å, 0.83 Å, 0.88 Å and 0.77 Å, respectively. These values suggest that the interaction of H2 is strongest for P3 and lowest for P4, respectively, which is consistent with the analysis from interaction energies. The M-H2 bond lengths are 1.57 Å, 1.58 Å, 1.47 Å and 1.84 Å for P1, P2, P3 and P4, respectively. The weak interaction of H2 in P4 is reflected from relatively large bond (1.84 Å) whereas the strong bond in P3 is evident from short bond length (1.47 Å). The interaction of H2 with Ni-BP in P2 geometry is slightly greater than that of P1 geometry. The increase in bond length of H\\H on interaction with Ni-BP nano-heterostructures is indicative of partial bond dissociation. Next, we analyzed the charge transfer and changes in dipole moments on nickel decoration as well as hydrogen adsorption. The BP
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nano-heterostructure has almost zero dipole moment which can be expected for a nearly symmetrical molecule. Decoration of nickel brings certain significant changes in the dipole moment. The dipole moments of nickel decorated nano-heterostructures ranges from 0.8 to 4.0 D (see Table 1). The highest dipole moment is calculated for P2 geometry followed by P3, P1 and P4, respectively. A close look on the data reveals that certain trends are observed in dipole moments of Ni-BP nanoheterostructures but after classifying these nano-heterostructures based on the type of interaction. P1 can be compared with P2 because both involve insertion of nickel in a bond. Similarly, P3 can be compared with P4 (both involve interaction above a ring). In each category, the dipole moment increases with increase in the distance of metal from the nano-heterostructure (or inversely with the interaction energy). This inverse relationship with the interaction energies can be explained based on the distance of nickel from the nano-heterostructure. The large distance in P2, although results in lower binding energies, but causes larger dipole moment due to large vector. The dipole moment of P2 (3.99 D) is considerably higher than 2.08 D for P1. Similarly, the dipole moment of P3 (3.33 D) is much higher than 0.81D for P4. The orientation of vector in P4 is away from the metal center, and it is in marked contradiction with P1-P3 geometries where the vector points towards metal atom. The dipole moments change on interaction with H2. Dipole moments decrease for P2 and P3, but increase for P1 and P4 geometries. The increase in dipole moment for P4 is very nominal. Next, the changes in NBO charges are analyzed to rationalize for the dipole moments changes on nickel decoration and hydrogen adsorption. The NBO charges on nickel atom in P1-P4 geometries are positive. The QNBO on Ni are 0.54, 0.582, 0.562 and 0.279 for P1, P2, P3 and P4 geometries. The amount of NBO charge demonstrates the sequence for dipole moments of P1-P4 geometries; the highest dipole moment is observed for P2 which is consistent with the highest positive charge (0.582) based on NBO. Similarly, the next highest positive charge is observed for P3 which again matches with the dipole moment of P3 (3.33D). The QNBO on metal for all geometries decrease on interaction with H2; however, the change varies among different geometries. The highest change in QNBO charge is observed for P4 (from 0.279 to 0.082). But this change in QNBO also brings reversal in the direction of dipole moment. The dipole moment of P4 points away from nickel atom but in P4-H2, the dipole moment is oriented towards nickel atom. For P1-P3, the directions of dipole moments stay the same (towards the metal atom). Among P1-P3, the change in QNBO is higher for P2 followed by P3 and P1. P3 and P2 have strong interaction with H2 molecule (vide supra) which would suggest that their interaction with BP nanoheterostructure would be much reduced after H2 molecule is attached on the surface. This would probably reduce the dipole moment of the H2-comlexed BP nano-heterostructures. Alternatively, it may be
Fig. 5. The MEP of different systems. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 6. DOSs along with HOMO-LUMO distribution for different systems. The value of the isosurfaces is 0.02 e/Å3.
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Table 1 The nearest distance of Ni to BP (for Ni adsorption) and the distance of H2 to free BP and Ni-decorated BP at different positions. System
d Ni-BP (A°)
dH2-Ni (A°)
QNBO on H2 (e)
QNBO on Ni (e)
μD (Debye)
Eadc(kJ mol−1)
Ni BP BP-H2 Ni-BP (P1) Ni-BP(P1)-H2 Ni-BP (P2) Ni-BP(P2)-H2 Ni-BP (P3) Ni-BP(P3)-H2 Ni-BP (P4) Ni-BP(P4)-H2
– – – 1.71 1.82 1.77 1.78 1.81 1.83 1.87b 1.88b
– 4.50a – 1.57 – 1.58 – 1.47 – 1.84
– – 0.00 – −0.001 – 0.024 – −0.028 – 0.113
0.00 – – 0.540 0.469 0.582 0.446 0.562 0.456 0.279 0.082
0.00 0.00 0.01 2.08 3.39 3.99 3.80 3.33 2.72 0.81 0.82
– – −0.7 −481.1 −85.8 −445.0 −99.0 −428.0 −107.8 −580.7 +8.9
a b c
For BP-H2 means the nearest distance of H2 to BP. Ni is situated at the center of six-membered ring. The calculated energy upon Ni adsorption (for H2-free systems) and upon H2 adsorption for all H2 included systems.
attributed to the generation of slightly negative hydrogen atoms (in H2P3) which reduces the amount of positive charge at transition metal terminus of the nano-heterostructure. The increase in the dipole moment after hydrogen adsorption in P1 geometry of Ni-BP is attributed to change in the direction of dipole moment vector. The dipole moment vector of Ni-BP (P1) originates from the center and points to the center of six membered ring. However, the dipole moment vector direction is changed in H2-Ni-BP (P1) and it points towards the metal atom. The change in the direction of the dipole moment vector is probably causing an increase in the dipole moment of the hydrogen adduct. These inferences are also consistent with the electrostatic potential analysis (MEP shown in Fig. 5). The bare BP nano-heterostructure has symmetrical potential due to symmetrical structure. Boron atoms have positive potential (blue in color) and phosphorus atoms have negative potential (yellow color). Adsorption of H2 molecule in bare BP nanoheterostructure does not cause any observable change in the electrostatic potential of pure BP nano-heterostructure. For P4, the metal end of the nano-heterostructure has positive potential which changes to negative potential after adsorption of H2. For P1-P3, the nickel atom has positive potential whereas the atoms in the nano-heterostructure have almost neutral potential. The negative potential on phosphorus and positive potential on boron in bare BP nano-heterostructure is reduced in nickel decorated nano-heterostructure. Upon adsorption of hydrogen molecule, the negative potential is regenerated on the nano-heterostructure. This change in potential corroborate nicely with drop in dipole moment for Ni-BP (P1)-H2 compared to Ni-BP (P1). The MEP analysis also reveals that upon hydrogen adsorption, the position of positive potential also shifts in close proximity of nickel. In Ni-BP (P1) and Ni-BP (P2), the region of positive potential is directly on top of Ni which shifts to sides after hydrogen adsorption. The results from energetic analysis reveal that P4 geometry is not suitable for interaction with H2 molecule (endothermic interaction); therefore, the subsequent discussion is only limited to P1-P3. Electronic energy level and their densities are analyzed to unveil the effect of nickel decoration and H2 adsorption on BP nano-heterostructures and Ni-BP nano-heterostructures, respectively. The orbital parameters such as energies of HOMO and LUMO, EFL (energy of Fermi level), Eg (band gap) are given in Table 2 whereas the HOMO and LUMO distributions along with densities of states are shown in Fig. 6. The BP nanoheterostructure is a semi-conductor with a band gap of 3.70 eV whereas Ni is a metal with a calculated band gap of 0.22 eV. The energies of HOMO and LUMO of BP nano-heterostructure are −6.83 eV and −3.13 eV, respectively, and the energy of Fermi level is −4.98 eV. Fermi level (EFL) in a molecule (at T = 0 K) is approximately located at the mid of the HOMO-LUMO energy gap (Eg). Decoration of BP nano-heterostructure brings significant changes in the energies of HOMO and LUMO. For P1 and P3 geometries, the effect of Ni adsorption is similar; the energies of HOMOs and LUMOs are increased, although the effect is more
pronounced for P3 compared to P1. The energies of HOMO and LUMO for P1 are −5.70 eV and −3.02 eV whereas the corresponding values for P3 are −5.59 eV and 2.91 eV. Despite these differences, Eg for both geometries is 2.68 eV. For P2 geometry, the effect of Ni decoration on the energy of HOMO and LUMO is quite different. The energy of HOMO is increased (compared to bare BP nano-heterostructure) to −6.0 eV whereas the energy of LUMO is decreased (compared to bare BP nanoheterostructure) to −3.4 eV. We have analyzed the shapes of frontier orbitals to account for this difference. In general, the HOMO is shifted to nickel metal upon adsorption. When a metal atom binds with several electronegative atoms (as in BP cluster) then the lone pairs of electrons from these electronegative atoms (phosphorus) cause the metal atom to be electron rich. However, the metal atom, due to its electropositive nature, cannot hold these electrons therefore, they are spread out as diffuse excess electrons which increase the energy level of newly formed HOMO [39–42]. The location of densities of HOMO on metal center in all geometries is supportive to our notion. The density of LUMO for P2 geometry is quite different than those for P1 and P3. For P1 and P3, the density on LUMO is uniformly distributed over the entire skeleton whereas the density of LUMO in P2 geometry is mainly located on the metal center. From the above analysis, it can be generalized that if HOMO is shifted on the metal center, its energy will rise however, the energy of LUMO will decrease if shifted on metal center. Significant changes in HOMO and LUMO are observed for Ni-BP nano-heterostructures upon adsorption of hydrogen. The interaction of H2 with metal causes decrease in interaction of metal with the nano-heterostructure (vide supra) and the effect is quite obvious from HOMO analysis (the HOMO is shifted back to the nano-heterostructure). The decrease in coordination to the metal center decreases the push of outer d electron to become more diffuse like, therefore, HOMO is no longer residing solely on Ni. The HOMO in H2-Ni-BP complexes has some density on the nano-heterostructure part but yet more density on the
Table 2 The orbital parameters: HOMO energies (EHOMO), LUMO energies (ELUMO), energy of Fermi level (EFL), HOMO-LUMO energy gap (Eg) for different systems. System
EHOMO (ev)
E FL (ev)
E LUMO (ev)
Eg (ev)
Ni BP BP-H2 Ni-BP (P1) Ni-BP(P1)-H2 Ni-BP (P2) Ni-BP(P2)-H2 Ni-BP (P3) Ni-BP(P3)-H2
−5.82 −6.83 −6.83 −5.70 −6.06 −6.00 −6.26 −5.59 −6.07
−5.71 −4.98 −4.98 −4.36 −4.50 −4.70 −4.64 −4.25 −4.52
−5.60 −3.13 −3.13 −3.02 −2.95 −3.40 −3.02 −2.91 −2.97
0.22 3.70 3.70 2.68 3.11 2.60 3.24 2.68 3.10
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metal atom. The LUMO for all complexes has density uniformly distributed on the entire complex. The energies of HOMO are also expected to decrease again on adsorption of H2 because the interaction of metal with the nanoheterostructure decreases. Indeed, this is the case. The energies of HOMOs are −6.06 eV, −6.26 eV and −6.07 eV for H2-P1, H2-P2 and H2-P3, respectively, which are quite lower than −5.70 eV, −6.00 and −5.59 eV for P1, P2 and P3, respectively. The effect of H2 adsorption on LUMO is of mixed type. The energies of LUMO for H2-P1 and H2-P2 increase (compared to P1 and P2) but the energy of LUMO decreases for H2-P3 (see Table 2 for details). Next, the densities of states are analyzed to further explore the effect of Ni chemisorption on the electronic structure of BP nanoheterostructure, and the effect of hydrogen adsorption on the electronic properties of Ni-BP nano-heterostructures. The densities of states for pure, Ni doped BP nano-heterostructures are compared with their hydrogen adsorbed complexes in Fig. 6. By comparison, it is observed that doping of a nickel atom causes significant shift of the occupied orbitals of BP nano-heterostructure to high energy levels and the newly formed HOMOs lie between the original HOMO and LUMO of BP nanoheterostructure. The densities of the states change on chemisorption of nickel on BP nano-heterostructure. More pronounced changes are observed in the occupied orbitals compared to virtual orbitals. For P1 geometry, the intensities of peaks in the region of HOMO and LUMO decrease, and this effect is more pronounced in P1 compared to P2. After hydrogen adsorption the density of states graph starts restoring (as before nickel decoration) back similar to bare BP nanoheterostructure. Peaks shifting as well as increase in the intensities are also observed. The effect of H2 adsorption on reversion of density of states is more pronounced for P1compared to P2, which is very consistent with the effect on band gap. The change in band gap leads to changes in conductivity. Relationship between conductivity and Eg can be given by the following equation [43]: σ α exp −Eg =kT
ð4Þ
where it can be seen clearly that a small decrease in Eg leads to significantly higher electrical conductivities. From the results above, one can easily conclude that bare BP nano-heterostructure is not a good sensor for hydrogen molecule but adsorption of nickel on BP nanoheterostructure significantly enhances its ability towards hydrogen adsorption. 4. Conclusions In summary, we investigated all possible sites of nickel adsorption on B12P12 nano-cluster. All of these Ni-adsorbed sites have been studied further to find the best position for adsorption of H2 molecule. We found four optimized geometries for Ni decoration denoted as P1, P2, P3 and P4 while their order in binding energy is P4 N P1 N P2 N P3. Important changes in the electronic structure of pure B12P12 cluster are observed for decoration of nickel followed by H2 adsorption. We analyzed the changes in NBO charges to rationalize dipole moments changes during nickel decoration and hydrogen adsorption. The amount of NBO charge correlates nicely with the trends of dipole moments of P1-P4 geometries; the highest dipole moment is observed for P2 which has the highest positive charge. Finally the electronic energy level and their densities are analyzed to reveal the effect of nickel decoration and H2 adsorption on B12P12 and Ni-BP nano-heterostructures, respectively. Acknowledgments The corresponding author acknowledges financial support from Iran Nanotechnology Initiative Council, Iran.
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Ayub, Adsorption of thiophene on the surfaces of X12Y12 (X = Al, B, and Y = N,P) nanoclusters; A DFT study, J. Mol. Liq. 238 (2017) 303–309. [14] K. Ayub, Binding affinity and permeation of X12Y12 nanoclusters for helium and neo, J. Mol. Liq. 244 (2017) 124–134. [15] J. Beheshtian, Z. Bagheri, M. Kamfiroozi, A. Ahmadi, A comparative study on the B12 N12, Al12 N12, B12 P12 and Al12 P12 fullerene-like cages, J. Mol. Model. 18 (2012) 2653–2658. [16] A.S. Rad, K. Ayub, A comparative density functional Theory study of guanine chemisorption on Al12N12, Al12P12, B12N12, and B12P12 nano-cages, J. Alloys Compd. 672 (2016) 161–169. [17] J. Beheshtian, M. Kamfiroozi, Z. Bagheri, A. Ahmadi, Theoretical study of hydrogen adsorption on the B12P12 fullerene-like nanocluster, Comput. Mater. Sci. 54 (2012) 115–118. [18] A.A. Peyghan, H. Soleymanabadi, Computational study on ammonia adsorption on the X 12 Y 12 nano-clusters (X = B, Al and Y = N, P), Curr. Sci. 108 (2015) 1910–1914. [19] A.S. Rad, E. Abedini, Chemisorption of NO on Pt-decorated graphene as modified nanostructure media: a first principles study, Appl. Surf. Sci. 360 (2016) 1041–1046. [20] A.S. Rad, Adsorption of C2H2 and C2H4 on Pt-decorated graphene nanostructure: abinitio study, Synth. Met. 211 (2016) 115–120. [21] A.S. Rad, Y.M. Jouibary, V.P. Foukolaei, E. Binaeian, Study on the structure and electronic property of adsorbed guanine on aluminum doped graphene: first principles calculations, Curr. Appl. Phys. 16 (2016) 527–533. [22] A.S. Rad, S.S. Shabestari, S. Mohseni, S.A. Aghouzi, Study on the adsorption properties of O3, SO2, and SO3 on B-doped graphene using DFT calculations, J. Solid State Chem. 237 (2016) 204–210. [23] A.S. Rad, V.P. Foukolaei, Density functional study of Al-doped graphene nanostructure towards adsorption of CO, CO2 and H2O, Synth. Met. 210 (2015) 171–178. [24] A.S. Rad, Al-doped graphene as a new nanostructure adsorbent for some halomethane compounds: DFT calculations, Surf. Sci. 645 (2016) 6–12. [25] A.S. Rad, O.R. Kashani, Adsorption of acetyl halide molecules on the surface of pristine and Al-doped graphene: ab initio study, Appl. Surf. Sci. 355 (2015) 233–241. [26] A.S. Rad, Al-doped graphene as sensitive nanostructure sensor for some ether molecules: ab-initio study of adsorption, Synth. Met. 209 (2015) 419–425. [27] A.S. Rad, First principles study of Al-doped graphene as nanostructure adsorbent for NO2 and N2O: DFT calculations, Appl. Surf. Sci. 357 (2015) 1217–1224. [28] A.S. Rad, K. Ayub, Enhancement in hydrogen molecule adsorption on B12N12 nanocluster by decoration of nickel, Int. J. Hydrog. Energy 41 (2016) 22182–22191. [29] A.S. Rad, K. Ayub, Coordination of nickel atoms with Al12X12 (X = N, P) nanocages enhances H2 adsorption: a surface study by DFT, Vacuum 133 (2016) 70–80. [30] A.S. Rad, K. Ayub, Adsorption properties of acetylene and ethylene molecules onto pristine and nickel-decorated Al12N12 nanoclusters, Mater. Chem. 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A.S. Rad, K. Ayub / Journal of Molecular Liquids 255 (2018) 168–175 [36] Y. Zhang, X. Zheng, S. Zhang, S. Huang, P. Wang, H. Tian, Bare and Ni decorated Al12N12 cage for hydrogen storage: A first-principles study, Int. J. Hydrog. Energy 37 (2012) 12411–12419. [37] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, G09, Revision D.01(Wallingford CT) 2009. [38] C.G. Van de Walle, Hydrogen as a cause of doping in zinc oxide, Phys. Rev. Lett. 85 (2000) 1012–1015. [39] K. Ayub, Are phosphide nano-cages better than nitride nano-cages? A kinetic, thermodynamic and non-linear optical properties study of alkali metal encapsulated X12Y12 nano-cages, J. Mater. Chem. C 4 (2016) 10919–10934.
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Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
How can nickel decoration affect H2 adsorption on B12P12 nano-heterostructures? Ali Shokuhi Rad a,⁎, Khurshid Ayub b a b
Department of Chemical Engineering, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran Department of Chemistry, COMSATS Institute of Information Technology, University Road, Tobe Camp, 22060 Abbottabad, Pakistan
a r t i c l e
i n f o
Article history: Received 27 October 2017 Received in revised form 24 January 2018 Accepted 25 January 2018 Available online xxxx Keywords: Nano-heterostructure B12P12 Ni decoration Inorganic material Hydrogen adsorption
a b s t r a c t Density functional theory calculations have been performed to study structural, electronic and hydrogen adsorption properties of Ni doped B12P12 fullerene-like clusters. Four geometries of Ni doped B12P12 nanoheterostructures were optimized and the potential of each geometry was investigated towards hydrogen adsorption. Natural bond orbital (NBO), molecular electrostatic potential (MEP), dipole moment, frontier orbitals, and density of states (DOS) analyses are performed to observe the changes in the electronic structure of B12P12 upon decoration by Ni followed by H2 adsorption. Ni doping significantly enhances the adsorption property of B12P12 nano-heterostructure for H2 molecule. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Interaction of hydrogen with metals is fundamental to several chemical reactions and industries. For example, hydrogenation reaction on the surface of nickel nano-particles involves gas-solid interaction of sub-surface hydrogen with nickel [1–4]. Moreover, the interaction of H2 with a variety of metals including Pd [5], Ru [6], Fe [7], Pt [8], is already reported in the literature. Therefore, profound studies of hydrogen interaction with transition metals are very important for understanding interaction mechanism of hydrogen on transition metals for different applications. Recently, fullerene-like group III–V semi-conductor nanoheterostructures (XnYn (X = B, Al, Ga, and Y = N, P, As)) have gained large attention due to their outstanding chemical and physical properties [9–12]. The X12Y12 nano-cluster is energetically the most stable cluster among different sized (XY)n structures [13,14]. Boron phosphide (BP) is an attractive member of XnYn family which has unique physical and electrochemical properties. The BP can exist in planer, tubular or spherical shapes, quite analogous to graphene, carbon nano-tubes and fullerenes, respectively. All shapes of its family are eminent as vast band-gap semiconductor material having many advanced properties. B12P12 is a spherical fullerene like member of BP family, and one of the best candidates to build devices in the violet area. Beheshtian et al. [15] studied the stability, geometry and electronic structure of Al12N12, Al12P12, B12N12, and B12P12 nano-heterostructures ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (A.S. Rad).
https://doi.org/10.1016/j.molliq.2018.01.149 0167-7322/© 2018 Elsevier B.V. All rights reserved.
using DFT calculations. A few studies have appeared in the recent literature where B12P12 and other analogues have been used as sensors for different analytes. In our recent report [16], we studied the adsorption property of guanine molecule on the surface of four nano-cages using DFT calculations. We found that despite the fact that Al12N12 has the highest adsorption energy, B12N12 and B12P12 show more changes in electronic property upon adsorption of guanine. Beheshtian et al. [17] investigated the interaction of a hydrogen atom with B12P12 nano-cluster through DFT methods. Their results showed that the HOMO-LUMO energy gap of B12P12 cluster is dramatically reduced to one-half of its original value upon H adsorption on the B atom. Moreover, the adsorption of ammonia on above four mentioned clusters was investigated through DFT methods by Peyghan and Soleymanabadi [18]. They found that ammonia prefers to be adsorbed on B or Al atom of the clusters. More recently, dopants have been introduced in nanostructures to enhance the electronic and adsorption properties [19–33]. For example, Baei et al. [34] demonstrated that silicon-doping transforms the B12N12 cluster to an n- or p-type semiconductor (depend on the location of doped Si). We have studied nickel doping of Al12P12 cluster [35], and observed four possible sites for decoration of nickel atom. Zhang et al. [36] investigated the feasibility of bare and Ni decorated Al12N12 cages for hydrogen storage. They reported significant enhancement in hydrogen adsorption capacity of this cluster upon decoration of nickel. In this paper, we explore all possible sites of nickel decoration on B12P12 cluster. Next, the possibility of free and Ni-decorated B12P12 cluster for H2 adsorption is scrutinized by DFT methods. We expect quite significant changes in the electronic properties of pure B12P12 cluster
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by nickel doping. To the best of our knowledge, this is the first systematic study of the adsorption of H2 molecules on the surface of Ni-decorated B12P12. The disparities of structural and electronic properties of these nano-clusters are compared with each other. The results can be particularly important for promoting the use of inorganic clusters in the future nano-scale electronic devices for H2 adsorbent/sensor applications. 2. Computational methods We used B3LYP method of DFT with 6–31 G(d,p) basis set. The B3LYP is a quite reliable hybrid DF method for the study of X12Y12 nanostructures [15–18]. Geometric, energetic and electronic properties (including NBO charge analyses, MEP, DOSs analyses, frontier molecular orbital analyses) of B12P12 and Ni-B12P12 nano-cluster are calculated to study the sensing of these clusters for hydrogen molecule (B12P12, NiB12P12, B12P12-H2, and B12P12-Ni-H2). A number of possible orientations were considered for decoration of Ni on B12P12 nano-cluster but all input geometries resulted in optimization of four distinct structures; P1, P2, P3, and P4 (vide infra). The adsorption energy of Ni on BP in all positions is calculated by: Ead ¼ ENi−BP −ðEBP þ ENi Þ
ð1Þ
where ENi-BP is the total (electronic) energy of B12P12 interacting with Ni whereas EBP and ENi are the total energy of isolated B12P12 and Ni, respectively. The interaction energy of H2 with isolated BP and Ni-BP is calculated by: E intð BPÞ ¼ EH2−BP −ðEBP þ EH2 Þ
ð2Þ
E int ðNi−BPÞ ¼ EH2−Ni−BP −ðENi−BP þ EH2 Þ
ð3Þ
where Eint(BP) and Eint (Ni-BP) correspond to interaction energy of H2 with BP and Ni-BP, respectively. EH2-BP and EH2-Ni-BP are total electronic energies of BP and Ni-BP interacting with the H2, respectively, whereas EH2 is the total energy of an isolated H2. All calculations were performed using G09 suite of program [37]. 3. Results and discussion The BP nano-heterostructure has four and six membered rings connected to one another. The B\\P bond lengths differ slightly among each other depending whether the bond is between two six membered rings, or between a six and a four membered ring. The B\\P bond shared between two six membered rings is 1.91 Å whereas the bond shared
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between a six and a four membered ring is 1.93 Å in length (see Fig. 1). For nickel decoration on the surface of BP nano-heterostructure, one finds several positions for interaction; on an atom (P or B), on a bond, and in the center of a ring. When nickel is adsorbed on a bond or in the center of a ring, there are two possibilities for each category. The nickel atom can be placed on a bond shared between two six membered ring, or on a bond shared between four and six membered ring. Similarly, the nickel atom can be placed on a four membered ring or a six membered ring. Although six different possibilities exist for nickel decoration on the surface of nano-heterostructure, we could get only four different optimized structures because some of the initial input geometries converted to other geometries during optimization. The four optimized geometries are denoted as P1, P2, P3 and P4. The nickel atom is incorporated into a bond shared between four and six membered rings (denoted as M@b64) in P1 whereas the nickel atom is incorporated into a bond shared between two six membered rings in P2 (M@ b66). P3 and P4 geometries contain nickel atom above four (M@r4) and six membered rings (M@r6), respectively. In general, nickel adsorption causes significant distortion in the geometries of BP nano-heterostructure (see Fig. 2). The nickel atom is inserted into a four membered ring in P1 geometry (vide supra) and this causes the B\\P bond to elongate to 2.66 Å compared to 1.93 Å in pure BP nano-heterostructure. Despite this distortion, the interaction energy for adsorption of nickel on BP nano-heterostructure in P1 geometry is very high (−481.1 kJ mol−1). The high interaction energy is indicative of chemisorption. The coordination of some atoms of BP nano-heterostructure combined with release of strain from four to five membered ring probably accounts for this high interaction energy. A similar distortion by incoming nickel atom is seen in P2 geometry. The B\\P bond is elongated to 2.76 Å compared to 1.91 Å in bare BP nanoheterostructure. The interaction energy of nickel with BP nanoheterostructure in P2 geometry is relatively less (−445 kJ mol−1) compared to that in P1 geometry. The least interaction energy is calculated for P3 geometry (−425 kJ mol −1 ). The nickel atom interacts with boron and two phosphorus atoms from the nano-heterostructure without distorting the geometry of BP nano-heterostructure in P3. The BP bond is slightly elongated to 2.05 Å compared to 1.93 Å in pure BP nanoheterostructure. The interaction of nickel with BP nano-heterostructure in P3 is unsymmetrical where only three atoms (out of four atoms of four membered ring) interact with nickel (see Fig. 2). The interaction energy is considerably enhanced for P4 geometry; however, the distortion caused by nickel adsorption is also highest for P4. The interaction energy is −580.7 kJ mol−1. The geometry of P4 is considerably different than P3; the nickel atom in the former is oriented at the center of the ring (in the plane of the ring) but above the plane of the ring in the
Fig. 1. The relaxed structure of BP at 6–31 G(d,p)/B3LYP level of theory.
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Fig. 2. The relaxed structure of Ni-BP for each position; to simplicity for each position some part of relaxed structures in top view is shown too. Some inter-molecular distances of BP's atoms are shown upon Ni adsorption (black ones) followed by H2 adsorption (blue ones) (vide infra). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
latter. This difference is probably due to cavity size for four and six membered ring. The nickel atom finds enough space to sit almost in the plane of the six membered ring. The P4 structure is more stabilized by enhanced interactions from increased number of coordinating atoms. This placement of nickel in the center of six membered ring pushes away the other atoms of the ring. The B\\P bond lengths are increased to 3.01, 2.60 and 2.72 Å compared to 1.91 and 1.93 Å in BP nano-heterostructure. Then, the interactions of these geometries (P1-P4) for H2 are analyzed. Pure BP nano-heterostructure has very weak interaction with H2 molecule. The interaction energy is −0.7 kJ mol−1, and the H2
molecule is located from the nano-heterostructure at a distance of 4.50 Å. The interaction of H2 molecule is significantly increased for metal decorated BP nano-heterostructures. The interaction energies range from −107.8 kJ mol−1 to +8.9 kJ mol−1. The highest interaction energy for hydrogen adsorption is seen for P3 geometry (−107.8 kJ mol−1) followed by −99 kJ mol−1 for P2, −85.8 kJ mol−1 for P1 and +8.9 kJ mol−1 for P4. Interestingly, the interaction energies for hydrogen adsorption are inversely proportional to the adsorption energies of nickel itself (see Fig. 4). The inverse relationship may be rationalized by the fact that higher stability of nickel decorated nanoheterostructure resist towards hydrogen adsorption. The P4 geometry
Fig. 3. The relaxed structure of adsorbed H2 on: pristine BP (a), Ni(P1)BP (b), Ni(P2)BP (c), Ni(P3)BP (d), and Ni(P4)BP (e).
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Fig. 4. The inverse relationship of Ni adsorption and H2 adsorption (the unit of energies: kJ/mol).
is the most stable geometry for Ni adsorbed system, and it resists towards hydrogen adsorption to the extent that interaction is endothermic. The resistance towards hydrogen adsorption can be demonstrated based on geometric changes caused by hydrogen adsorption. Nickel adsorption on BP nano-heterostructure distorts the geometries of BP (vide supra), and B\\P bond lengths are elongated. However, the hydrogen adsorption reverts those changes to some extent. For example, the hydrogen adsorption in P1 causes the B\\P bond length to change from 2.66 to 2.39 (more close to the 1.93 for bare BP nano-heterostructure). Similar changes are also observed for P2 where B\\P bond length is decreased to 2.66 from 2.76. The behavior of change in bond lengths for P3 and P 4 is of mixed type where some bond lengths are increased and some are decreased (see Fig. 1 for details). The strength of interaction of nickel with hydrogen can be realized from the H\\H bond lengths as well as Ni\\H\\H bond lengths. For H2 adsorption on bare BP nanoheterostructure, the H\\H bond length is 0.74 Å (Fig. 3) which is comparable to the calculated 0.743 Å for free H2 molecule [38]. The H\\H bond lengths for H2 complexes are 0.83 Å, 0.83 Å, 0.88 Å and 0.77 Å, respectively. These values suggest that the interaction of H2 is strongest for P3 and lowest for P4, respectively, which is consistent with the analysis from interaction energies. The M-H2 bond lengths are 1.57 Å, 1.58 Å, 1.47 Å and 1.84 Å for P1, P2, P3 and P4, respectively. The weak interaction of H2 in P4 is reflected from relatively large bond (1.84 Å) whereas the strong bond in P3 is evident from short bond length (1.47 Å). The interaction of H2 with Ni-BP in P2 geometry is slightly greater than that of P1 geometry. The increase in bond length of H\\H on interaction with Ni-BP nano-heterostructures is indicative of partial bond dissociation. Next, we analyzed the charge transfer and changes in dipole moments on nickel decoration as well as hydrogen adsorption. The BP
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nano-heterostructure has almost zero dipole moment which can be expected for a nearly symmetrical molecule. Decoration of nickel brings certain significant changes in the dipole moment. The dipole moments of nickel decorated nano-heterostructures ranges from 0.8 to 4.0 D (see Table 1). The highest dipole moment is calculated for P2 geometry followed by P3, P1 and P4, respectively. A close look on the data reveals that certain trends are observed in dipole moments of Ni-BP nanoheterostructures but after classifying these nano-heterostructures based on the type of interaction. P1 can be compared with P2 because both involve insertion of nickel in a bond. Similarly, P3 can be compared with P4 (both involve interaction above a ring). In each category, the dipole moment increases with increase in the distance of metal from the nano-heterostructure (or inversely with the interaction energy). This inverse relationship with the interaction energies can be explained based on the distance of nickel from the nano-heterostructure. The large distance in P2, although results in lower binding energies, but causes larger dipole moment due to large vector. The dipole moment of P2 (3.99 D) is considerably higher than 2.08 D for P1. Similarly, the dipole moment of P3 (3.33 D) is much higher than 0.81D for P4. The orientation of vector in P4 is away from the metal center, and it is in marked contradiction with P1-P3 geometries where the vector points towards metal atom. The dipole moments change on interaction with H2. Dipole moments decrease for P2 and P3, but increase for P1 and P4 geometries. The increase in dipole moment for P4 is very nominal. Next, the changes in NBO charges are analyzed to rationalize for the dipole moments changes on nickel decoration and hydrogen adsorption. The NBO charges on nickel atom in P1-P4 geometries are positive. The QNBO on Ni are 0.54, 0.582, 0.562 and 0.279 for P1, P2, P3 and P4 geometries. The amount of NBO charge demonstrates the sequence for dipole moments of P1-P4 geometries; the highest dipole moment is observed for P2 which is consistent with the highest positive charge (0.582) based on NBO. Similarly, the next highest positive charge is observed for P3 which again matches with the dipole moment of P3 (3.33D). The QNBO on metal for all geometries decrease on interaction with H2; however, the change varies among different geometries. The highest change in QNBO charge is observed for P4 (from 0.279 to 0.082). But this change in QNBO also brings reversal in the direction of dipole moment. The dipole moment of P4 points away from nickel atom but in P4-H2, the dipole moment is oriented towards nickel atom. For P1-P3, the directions of dipole moments stay the same (towards the metal atom). Among P1-P3, the change in QNBO is higher for P2 followed by P3 and P1. P3 and P2 have strong interaction with H2 molecule (vide supra) which would suggest that their interaction with BP nanoheterostructure would be much reduced after H2 molecule is attached on the surface. This would probably reduce the dipole moment of the H2-comlexed BP nano-heterostructures. Alternatively, it may be
Fig. 5. The MEP of different systems. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 6. DOSs along with HOMO-LUMO distribution for different systems. The value of the isosurfaces is 0.02 e/Å3.
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Table 1 The nearest distance of Ni to BP (for Ni adsorption) and the distance of H2 to free BP and Ni-decorated BP at different positions. System
d Ni-BP (A°)
dH2-Ni (A°)
QNBO on H2 (e)
QNBO on Ni (e)
μD (Debye)
Eadc(kJ mol−1)
Ni BP BP-H2 Ni-BP (P1) Ni-BP(P1)-H2 Ni-BP (P2) Ni-BP(P2)-H2 Ni-BP (P3) Ni-BP(P3)-H2 Ni-BP (P4) Ni-BP(P4)-H2
– – – 1.71 1.82 1.77 1.78 1.81 1.83 1.87b 1.88b
– 4.50a – 1.57 – 1.58 – 1.47 – 1.84
– – 0.00 – −0.001 – 0.024 – −0.028 – 0.113
0.00 – – 0.540 0.469 0.582 0.446 0.562 0.456 0.279 0.082
0.00 0.00 0.01 2.08 3.39 3.99 3.80 3.33 2.72 0.81 0.82
– – −0.7 −481.1 −85.8 −445.0 −99.0 −428.0 −107.8 −580.7 +8.9
a b c
For BP-H2 means the nearest distance of H2 to BP. Ni is situated at the center of six-membered ring. The calculated energy upon Ni adsorption (for H2-free systems) and upon H2 adsorption for all H2 included systems.
attributed to the generation of slightly negative hydrogen atoms (in H2P3) which reduces the amount of positive charge at transition metal terminus of the nano-heterostructure. The increase in the dipole moment after hydrogen adsorption in P1 geometry of Ni-BP is attributed to change in the direction of dipole moment vector. The dipole moment vector of Ni-BP (P1) originates from the center and points to the center of six membered ring. However, the dipole moment vector direction is changed in H2-Ni-BP (P1) and it points towards the metal atom. The change in the direction of the dipole moment vector is probably causing an increase in the dipole moment of the hydrogen adduct. These inferences are also consistent with the electrostatic potential analysis (MEP shown in Fig. 5). The bare BP nano-heterostructure has symmetrical potential due to symmetrical structure. Boron atoms have positive potential (blue in color) and phosphorus atoms have negative potential (yellow color). Adsorption of H2 molecule in bare BP nanoheterostructure does not cause any observable change in the electrostatic potential of pure BP nano-heterostructure. For P4, the metal end of the nano-heterostructure has positive potential which changes to negative potential after adsorption of H2. For P1-P3, the nickel atom has positive potential whereas the atoms in the nano-heterostructure have almost neutral potential. The negative potential on phosphorus and positive potential on boron in bare BP nano-heterostructure is reduced in nickel decorated nano-heterostructure. Upon adsorption of hydrogen molecule, the negative potential is regenerated on the nano-heterostructure. This change in potential corroborate nicely with drop in dipole moment for Ni-BP (P1)-H2 compared to Ni-BP (P1). The MEP analysis also reveals that upon hydrogen adsorption, the position of positive potential also shifts in close proximity of nickel. In Ni-BP (P1) and Ni-BP (P2), the region of positive potential is directly on top of Ni which shifts to sides after hydrogen adsorption. The results from energetic analysis reveal that P4 geometry is not suitable for interaction with H2 molecule (endothermic interaction); therefore, the subsequent discussion is only limited to P1-P3. Electronic energy level and their densities are analyzed to unveil the effect of nickel decoration and H2 adsorption on BP nano-heterostructures and Ni-BP nano-heterostructures, respectively. The orbital parameters such as energies of HOMO and LUMO, EFL (energy of Fermi level), Eg (band gap) are given in Table 2 whereas the HOMO and LUMO distributions along with densities of states are shown in Fig. 6. The BP nanoheterostructure is a semi-conductor with a band gap of 3.70 eV whereas Ni is a metal with a calculated band gap of 0.22 eV. The energies of HOMO and LUMO of BP nano-heterostructure are −6.83 eV and −3.13 eV, respectively, and the energy of Fermi level is −4.98 eV. Fermi level (EFL) in a molecule (at T = 0 K) is approximately located at the mid of the HOMO-LUMO energy gap (Eg). Decoration of BP nano-heterostructure brings significant changes in the energies of HOMO and LUMO. For P1 and P3 geometries, the effect of Ni adsorption is similar; the energies of HOMOs and LUMOs are increased, although the effect is more
pronounced for P3 compared to P1. The energies of HOMO and LUMO for P1 are −5.70 eV and −3.02 eV whereas the corresponding values for P3 are −5.59 eV and 2.91 eV. Despite these differences, Eg for both geometries is 2.68 eV. For P2 geometry, the effect of Ni decoration on the energy of HOMO and LUMO is quite different. The energy of HOMO is increased (compared to bare BP nano-heterostructure) to −6.0 eV whereas the energy of LUMO is decreased (compared to bare BP nanoheterostructure) to −3.4 eV. We have analyzed the shapes of frontier orbitals to account for this difference. In general, the HOMO is shifted to nickel metal upon adsorption. When a metal atom binds with several electronegative atoms (as in BP cluster) then the lone pairs of electrons from these electronegative atoms (phosphorus) cause the metal atom to be electron rich. However, the metal atom, due to its electropositive nature, cannot hold these electrons therefore, they are spread out as diffuse excess electrons which increase the energy level of newly formed HOMO [39–42]. The location of densities of HOMO on metal center in all geometries is supportive to our notion. The density of LUMO for P2 geometry is quite different than those for P1 and P3. For P1 and P3, the density on LUMO is uniformly distributed over the entire skeleton whereas the density of LUMO in P2 geometry is mainly located on the metal center. From the above analysis, it can be generalized that if HOMO is shifted on the metal center, its energy will rise however, the energy of LUMO will decrease if shifted on metal center. Significant changes in HOMO and LUMO are observed for Ni-BP nano-heterostructures upon adsorption of hydrogen. The interaction of H2 with metal causes decrease in interaction of metal with the nano-heterostructure (vide supra) and the effect is quite obvious from HOMO analysis (the HOMO is shifted back to the nano-heterostructure). The decrease in coordination to the metal center decreases the push of outer d electron to become more diffuse like, therefore, HOMO is no longer residing solely on Ni. The HOMO in H2-Ni-BP complexes has some density on the nano-heterostructure part but yet more density on the
Table 2 The orbital parameters: HOMO energies (EHOMO), LUMO energies (ELUMO), energy of Fermi level (EFL), HOMO-LUMO energy gap (Eg) for different systems. System
EHOMO (ev)
E FL (ev)
E LUMO (ev)
Eg (ev)
Ni BP BP-H2 Ni-BP (P1) Ni-BP(P1)-H2 Ni-BP (P2) Ni-BP(P2)-H2 Ni-BP (P3) Ni-BP(P3)-H2
−5.82 −6.83 −6.83 −5.70 −6.06 −6.00 −6.26 −5.59 −6.07
−5.71 −4.98 −4.98 −4.36 −4.50 −4.70 −4.64 −4.25 −4.52
−5.60 −3.13 −3.13 −3.02 −2.95 −3.40 −3.02 −2.91 −2.97
0.22 3.70 3.70 2.68 3.11 2.60 3.24 2.68 3.10
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metal atom. The LUMO for all complexes has density uniformly distributed on the entire complex. The energies of HOMO are also expected to decrease again on adsorption of H2 because the interaction of metal with the nanoheterostructure decreases. Indeed, this is the case. The energies of HOMOs are −6.06 eV, −6.26 eV and −6.07 eV for H2-P1, H2-P2 and H2-P3, respectively, which are quite lower than −5.70 eV, −6.00 and −5.59 eV for P1, P2 and P3, respectively. The effect of H2 adsorption on LUMO is of mixed type. The energies of LUMO for H2-P1 and H2-P2 increase (compared to P1 and P2) but the energy of LUMO decreases for H2-P3 (see Table 2 for details). Next, the densities of states are analyzed to further explore the effect of Ni chemisorption on the electronic structure of BP nanoheterostructure, and the effect of hydrogen adsorption on the electronic properties of Ni-BP nano-heterostructures. The densities of states for pure, Ni doped BP nano-heterostructures are compared with their hydrogen adsorbed complexes in Fig. 6. By comparison, it is observed that doping of a nickel atom causes significant shift of the occupied orbitals of BP nano-heterostructure to high energy levels and the newly formed HOMOs lie between the original HOMO and LUMO of BP nanoheterostructure. The densities of the states change on chemisorption of nickel on BP nano-heterostructure. More pronounced changes are observed in the occupied orbitals compared to virtual orbitals. For P1 geometry, the intensities of peaks in the region of HOMO and LUMO decrease, and this effect is more pronounced in P1 compared to P2. After hydrogen adsorption the density of states graph starts restoring (as before nickel decoration) back similar to bare BP nanoheterostructure. Peaks shifting as well as increase in the intensities are also observed. The effect of H2 adsorption on reversion of density of states is more pronounced for P1compared to P2, which is very consistent with the effect on band gap. The change in band gap leads to changes in conductivity. Relationship between conductivity and Eg can be given by the following equation [43]: σ α exp −Eg =kT
ð4Þ
where it can be seen clearly that a small decrease in Eg leads to significantly higher electrical conductivities. From the results above, one can easily conclude that bare BP nano-heterostructure is not a good sensor for hydrogen molecule but adsorption of nickel on BP nanoheterostructure significantly enhances its ability towards hydrogen adsorption. 4. Conclusions In summary, we investigated all possible sites of nickel adsorption on B12P12 nano-cluster. All of these Ni-adsorbed sites have been studied further to find the best position for adsorption of H2 molecule. We found four optimized geometries for Ni decoration denoted as P1, P2, P3 and P4 while their order in binding energy is P4 N P1 N P2 N P3. Important changes in the electronic structure of pure B12P12 cluster are observed for decoration of nickel followed by H2 adsorption. We analyzed the changes in NBO charges to rationalize dipole moments changes during nickel decoration and hydrogen adsorption. The amount of NBO charge correlates nicely with the trends of dipole moments of P1-P4 geometries; the highest dipole moment is observed for P2 which has the highest positive charge. Finally the electronic energy level and their densities are analyzed to reveal the effect of nickel decoration and H2 adsorption on B12P12 and Ni-BP nano-heterostructures, respectively. Acknowledgments The corresponding author acknowledges financial support from Iran Nanotechnology Initiative Council, Iran.
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