# Zinc-Doped Boron Phosphide Nanocluster as Efficient Sensor for SO2

Authors: Shahid Hussain; Shahzad Ali Shahid Chatha; Abdullah Ijaz Hussain; Riaz Hussain; Muhammad Yasir Mehboob; Shabbir Muhammad; Zaheer Ahmad; Khurshid Ayub
Journal: Journal of Chemistry
 (2020)

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## Abstract

Adsorption of SO2 on pure B12P12 and Zn-doped B12P12 is investigated through density functional theory methods. Zn adsorption on BP delivers four optimized geometries: B-Top, P-top, b64, and ring-enlarged geometry with adsorption energies of −57.12 kJ/mol, −14.50 kJ/mol, −22.94 kJ/mol, and −14.83 kJ/mol, respectively. The adsorption energy of SO2 on pristine boron phosphide is −14.92 kJ/mol. Interaction of SO2 with Zn-doped boron phosphide gives four different geometries with adsorption energies of −69.76 kJ/mol, −9.82 kJ/mol, −104.92 kJ/mol, and −41.87 kJ/mol. Geometric parameters such as dipole moment, QNBO, frontier molecular orbital analysis, PDOS, and global indices of reactivity are performed to visualize the changes in electronic properties of B12P12 after Zn and SO2 adsorption.


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## Body

## 1. Introduction

Air pollution is a major concern in recent years, which has a severe toxicant impact on our environment and human health. It is generally a result of releasing poisonous substances such as noxious gases and very fine particles including carbon monoxide (CO) and sulphur dioxide (SO2) [1–3]. Sulphur dioxide is an industrial effluent and contributes to air contamination. Burning of petroleum products is also a main source of sulphur dioxide which leads to the number of breathing problems and lung and vascular diseases [4]. Lefohn and Tingey [5] studied the impacts of SO2 and O3 impact on vegetation. They analyzed air quality data to determine the co-occurrence frequency for pollutant pairs.Due to reactive nature and significant presence of ozone and sulphur dioxide, their monitoring is highly essential. Therefore, development of new ozone (O3) and sulphur dioxide (SO2) sensors have gained significant importance in scientific community. In these days, nanostructure semiconductors are of great interest due to their unique physical and chemical properties [6–8].Recently, (XY)n nanostructures such as nanotubes, nanocages, and nanowires are of great scientific interest due to their unique electronic properties [9–13]. Fullerene-like cages (XY)n when n = 12, 16, 24, and 28 are mainly interesting and attracting due to their unique properties. In recent theoretical studies on different (XY)n clusters, fullerene-like cages (XY)12 were shown as most stable structures [14, 15]. Fullerene structures of group III–V are being considered of great interest due to their significant applications in imaging material (LED’s), environmental processes, and for magnetic recording [14–17].Due to remarkable chemical and physical properties, Al12N12, Al12P12, B12P12, and B12N12 are appealing to the scientific community [18–22]. Study on B12N12 nanocage has shown that because of exceptional structural properties, B12N12 has its applications in adsorption and electrical devices [23, 24]. Al12P12 is also an important nanocluster of several theoretical studies for various applications [25, 26]. B12P12 is a refractory semiconductor substance with unique properties. Many reports are available in the literature for adsorption and catalytic applications of B12P12 nanocages [27–31].Several studies on different nanocages in the literature show that adsorption of metal significantly enhances the adsorption and catalytic properties [24, 32, 33]. In this regard, there are several theoretical reports describing the improved catalytic and adsorption abilities for metal-doped AlN and GaN nanocages [34–36]. Zhang et al. [37] studied the possibility of Ni-adsorbed AlN nanocage for hydrogen (H2) storage. They found that in exposed AlN, each Al atom adsorbs one hydrogen (H2) molecule, whereas in the Ni-adsorbed nanocage (AlN), the Ni atom is able to absorb up to three H2 molecules. Pan et al. [38] studied the effect of Si-doping on AlN films. In another study, Cui et al. [39] concluded that Cr atom has enough tendencies to separate to form Cr-N-Cr-bonded embed clusters. Despite these advances, the potential of Zn group metals in the adsorption and catalytic process is not well documented. We became interested in studying the potential of Zn-doped boron phosphide nanocages for the adsorption of SO2.In this paper, we search all potential sites of zinc-decorated B12P12 nanocage. Then, the possibility of bare and Zn decoration on B12P12 nanocage for SO2 adsorption is investigated by DFT methods. We observe some considerable changes in the electronic properties of B12P12 nanocage by zinc (Zn) doping. To our knowledge, such theoretical studies have not been reported so far in literature where adsorption of sulphur dioxide (SO2) on the surface of Zn-decorated B12P12 is studied. We report the result on adsorption through binding energy values, the net charge transfer, and HOMO-LUMO distribution on all possible forms of B12P12 nanocages.


## 2. Computational Methods

All calculations in this study are performed at B3LYP/6-31G (d, p) level of theory by using Gaussian 09 [40]. Geometry optimization, adsorption energies, dipole moment, charge transfer (QNBO), the MEP (molecular electrostatic potential), frontier molecular orbitals analysis (HOMO-LUMO distribution), and PDOS (Partial density of states) are calculated to study the interaction mechanism. B3LYP/6-31G (d, p) is a reliable level of theory which is frequently used for nanoclusters [6, 23]. Many different possible orientations of zinc on BP nanocage (M@b66, M@b64, M@R4, M@R6, M@Btop, and M@Ptop) are considered for optimization, but all abovementioned input geometries converged into four optimized structures which were named as M1 (ring large, in which metal atom is inserted in the ring), M2 (M@Btop), M3 (M@B64), and M4 (M@Ptop).Equation (1) is used to calculate the interaction or adsorption energy of Zn on the BP nanocage:(1)Ead=EZn−BP−EBP+EZn,where EZn-BP is the energy of Zn-doped BP nanocage, whereas EBP and EZn are the energies of pure BP nanocage and Zn metal, respectively. Equations (2) and (3) are used to calculate the interaction or adsorption energy of SO2 with pure BP nanocage and zinc-decorated BP nanocage.(2)EintBP=ESO2−BP−EBP+ESO2,(3)EintZn−BP=ESO2−Zn−BP−EZn−BP+ESO2.Here,Eint (BP) and Eint (Zn-BP) represent the interaction/adsorption energy of sulphur dioxide with BP nanocage and Zn-decorated BP, respectively. ESO2−BP and ESO2−Zn−BP represent the total electronic energies of SO2-adsorbed BP nanocage and SO2-adsorbed Zn-BP nanocage. ESO2 stands for the total energy of the single SO2.Parr et al. [41] in 1999 studied the chemical potential (μ), and he expressed it by the following equation:(4)μ=−I+A2,where I represents ionization potential which is –EHOMO/eV and A stands for electron affinity which is –ELUMO/eV.Moreover, the electronic properties such as softness (S), hardness (η), and electrophilicity (ω) can be determined by using Koopmans’ theorem [42]:(5)η=I−A2,S=12η,ω=μ22η.Partial density of states (PDOS) for all systems is generated by using MultiWFN software [43].


## 3. Results and Discussion

Optimized structure of boron phosphide at B3LYP/6-31G (d, p) level of theory is shown in Figure1. B12P12 consists of tetragonal and hexagonal rings which are interconnected with each other. Two types of bonds are present in B12P12: (i) b66 (bond shared between two hexagonal rings) and (ii) b64 (bond shared between one hexagonal ring and tetragonal ring). The B-P bond length in the case of b66 is 1.91 Å and in the case of b64 is 1.93 Å.Figure 1
Optimized structure of B12P12 at B3LYP/6-31G (d, p) basis set.


### 3.1. Adsorption Energies and Bond Lengths

First, we decorated B12P12 nanocage with Zn metal. Decoration of Zn on B12P12 was done at different positions, i.e., Zn@b66 (Zn is placed on the bond shared between two hexagonal rings), Zn@b64 (Zn is placed on the bond shared between one hexagonal and one tetragonal ring), Zn@Btop (Zn is placed on the top site of Boron atom), Zn@Ptop (Zn is placed on the top site of Phosphide atom), Zn@R6 (Zn is placed on the hexagonal ring), and Zn@R4 (Zn is placed on the tetragonal ring). With six different input geometries, only four distinct geometries could be optimized. Some of the initial input geometries converged to others during optimization. Among four distinct optimized geometries, one geometry is Zn@b66 which obviously looks like a ring enlarged structure (in which Zn is inserted into hexagonal ring). The second geometry is Zn@Btop (in which Zn is adsorbed on the top site of boron atom), while third geometry is Zn@b64 (in which Zn is adsorbed on the bond shared between one hexagonal and tetragonal ring). Similarly, the fourth geometry is Zn@Ptop (in which Zn is adsorbed on the top of Phosphorus atom). The ring enlarged geometry, Zn@b66 is named as M1, whereas Zn@Btop, Zn@b64, and Zn@Ptop geometries are termed as M2, M3, and M4, respectively. Zn decoration on B12P12 (in M1 geometry) causes distortion in the B12P12 geometry, and B-P bond lengths at the side of zinc adsorption increase to 3.98 Å as compared to 1.91 Å in bare B12P12. The adsorption energy value for M1 geometry is very low about (−14.83 kJ/mol). Low adsorption energy for M1 reflects physisorption of zinc on the cage. The low interaction energy is attributed to the distortion caused by Zn insertion into the nanocage. No such distortion of the B12P12 nanocage is caused by Zn in M2 geometry which results in higher adsorption energy (−57.12 kJ/mol). B-P bond length slightly elongated to 1.98 Å as compared to 1.93 Å (Figure 2). In M3 and M4 geometries, B-P bond lengths are increased slightly to 1.94 Å, as compared to 1.91 Å in bare nanocage. This change is very small because Zn in both geometries does not cause any significant geometric changes (as Zn is adsorbed on bond shared between hexagonal and tetragonal ring and on the top of phosphorus atom). Adsorption energy values of Zn are −22.94 kJ/mol and −14.50 kJ/mol in M3 and M4, respectively.Figure 2
Optimized structures of Zn-doped B12P12 nanocages: (a) M1, (b) M2, (c) M3, and (d) M4.
(a)
(b)
(c)
(d)Next, adsorption of SO2 on bare and Zn-decorated B12P12 was analyzed. Here, we got one geometry in which SO2 is adsorbed on bare B12P12 (Figure 3). The value of interaction energy (−14.92 kJ/mol) shows that SO2 is favorably adsorbed on bare B12P12, but through physisorption. SO2 interacts with boron phosphide nanocage through O-B bond where oxygen of SO2 interacts with boron of the nanocage. When SO2 is adsorbed on M1, M2, M3, and M4 geometry, four geometries named as N1, N2, N3, and N4 are obtained (Figure 3). The values of adsorption energies in N1, N2, N3, and N4 are −69.76 KJ/mol, −9.82 KJ/mol, −104.92 KJ/mol, and −14.87 kJ/mol, respectively (Table 1). The distances of SO2 from Zn in N1, N2, N3, and N4 are 1.94 Å, 2.86 Å, 2.01 Å, and 2.38 Å, respectively. Among these geometries, N1–N3 has oxygen of SO2 coordinated to Zn atom, whereas N4 is characterized by the Zn-S bond. The high interaction energy of SO2 adsorption on Zn in N3 is attributed to chelation of SO2 with Zn. Interestingly, the values of adsorption energies of SO2 adsorption on bare and Zn-doped B12P12 shows inverse relation with Zn-decorated B12P12 except N2 geometry. This relation shows that Zn-doped B12P12 are more stable and resists SO2 adsorption and for N2 geometry reversal is true.Figure 3
(a) SO2-adsorbed B12P12 and SO2-adsorbed Zn-doped B12P12 systems: (b) N1, (c) N2, (d) N3, and (e) N4.
(a)
(b)
(c)
(d)
(e)Table 1
The closest distance of SO2 to BP, Zn metal to pure BP, QNBO (Charge) on metal and gas, dipole moment, and adsorption energies of different systems.
Systems
d
Zn-BP (Å)
d
SO
2
−
Zn (Å)
Q
NBO on SO2 (eV)
Q
NBO on Zn (eV)
μ
D (D)
E
ad
b (KJ/mol)
Zn
—
—
—
0.00
0.00
—
BP
—
—
—
—
0.00
—
BP-SO2
—
−0.217
—
3.08
−14.92
Zn-BP (M1)
2.10
—
1.004
2.42
−14.83
Zn-BP-SO2 (N1)
—
1.94a
−0.163
—
1.70
−69.76
Zn-BP (M2)
2.15
—
—
0.524
3.24
−57.12
Zn-BP-SO2 (N2)
—
2.86a
−0.093
—
2.25
−9.82
Zn-BP (M3)
2.15
—
—
0.937
2.26
−22.94
Zn-BP-SO2 (N3)
—
2.01a
−0.152
—
0.53
−104.09
Zn-BP (M4)
3.00
—
—
0.081
0.61
−14.50
Zn-BP-SO2 (N4)
—
2.38a
−0.147
—
5.26
−41.87
aMetal nearest to SO2 (gas) atom towards Zn metal. bThe calculated (adsorption) energy, which is calculated by using equations (1), (2), and (3).


### 3.2. Dipole Moment

Pure B12P12 has equal number of boron and phosphorus atoms and show zero dipole moment due to its centrosymmetric nature. Upon decoration of Zn on B12P12 nanocage (M1, M2, M3, and M4), change in dipole moment is observed. Values of dipole moment for M1, M2, M3, and M4 geometries are 2.42 D, 3.24 D, 2.26 D, and 0.61 D. The highest value is counted for M2 geometry. This increase in dipole moment is due to large distance of Zn from BP cage. The vector is pointed away from B12P12 nanocage toward Zn in all geometries (Zn-decorated BP) which is the reason of change in dipole moment after decoration of Zn on BP. Significant change in dipole moment is observed when SO2 is adsorbed on bare B12P12 (3.08 D). On interaction of SO2 with N1, N2, N3, and N4 geometries, the dipole moment values decrease (except N4) compared to M1–M4. Values of dipole moment for N1, N2, N3, and N4 are 1.70 D, 2.25 D, 0.53 D, and 5.26 D. This decrease in dipole moment is due to decrease in distance of SO2 from Zn and higher value of interaction energies except geometry N2 where reversal is true. Dipole moment vector is pointed away from nanocage, and it is toward SO2 in all SO2-adsorbed Zn-doped BP geometries (N1–N4).


### 3.3.QNBO

Next,QNBO analysis is performed to correlate the charge transfer with dipole moment. The charges of Zn in M1–M4 geometries are 1.004, 0.524, 0.937, and 0.081, respectively. Close analysis reveal lack of any correlation between QNBO and dipole moment for Zn-doped B12P12. This lack of correlation in QNBO and dipole moment suggested that intensity of charge is not only the factor in deciding the dipole moment rather separation between charges also matters a lot, besides the geometry of the Zn-doped complex itself. So, dipole moment is depending on intensity of charges as well as displacement between Zn and nanocage. QNBO charges on SO2 in SO2-adsorbed Zn-doped BP is slightly negative which reflects that SO2 is taking charge from the Zn-doped geometries. QNBO charges on N1, N2, N3, and N4 are −0.163, −0.093, −0.152, and −0.147, respectively. This negative charge is due to generation of negative SO2 atoms (which is attached with Zn transition metal). The trend between dipole moment and QNBO is true (consistent) for N1, N2, N3, and N4 (Table 1).


### 3.4. MEP Analysis

Next, molecular electrostatic potential (MEP) analysis is performed to unveil the change in the properties after decoration of Zn metal and SO2 adsorption on Zn-doped BP as shown in Figure 4. This analysis also gives relationship between chemical reactivity and charge distribution. Yellow color shows electron-rich area (negative charge), blue color shows electron-deficient area (positive charge), and green color shows mean potential area (neutral charge). Pure B12P12 has equal number of boron and phosphorus atoms, so it shows no charge distribution, or in other words, it is neutral as shown in Figure 4. When SO2 is adsorbed on BP, the yellow area is shifted on SO2 which suggested that SO2 end is an electron-rich end. But metal end shows electropositive charge (blue in color) in Zn-doped complex, whereas the cage shows uniform charge distribution. Similarly, when SO2 is adsorbed on Zn-decorated B12P12, then charge transfer towards SO2 occurs on SO2; yellow color shows negative charge end, blue color on Zn metal shows electropositive nature of transition metal, and cage shows equal charge distribution this trend is consistent with QNBO.Figure 4
The molecular electrostatic potential map of different systems: (a) B12P12 nanocage, (b) SO2-adsorbed B12P12, (c) M1, (d) M2, (e) M3, (f) M4, (g) N1, (h) N2, (i) N3, and (j) N4.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)


### 3.5. Electronic Properties

It is seen that decoration of Zn on B12P12 and subsequent SO2 adsorption on Zn-doped B12P12 brings some changes in the electronic properties of pure B12P12 and Zn-B12P12. Zn decoration on BP causes narrowing of HOMO-LUMO gap which is very vital for conductivity (useful for sensing materials). The following equation shows relationship between HOMO-LUMO energy gap and conductivity [44]:(6)σαexp−EgKT.Here, “K” represents Boltzmann’s constant, and σ represents conductivity. HOMO energies, LUMO energies, Fermi level, and HOMO-LUMO gap are calculated and given in Table 2. Fermi level makes define as the midpoint of HOMO-LUMO gap in molecule at zero Kelvin temperature [45].Table 2
The orbital parameters, HOMO-LUMO energies, fermi level, and HOMO-LUMO energy gap of different systems.
System
E
HOMO (ev)
E
FL (ev)
E
LUMO (ev)
E
g (ev)
Zn
−6.10
−3.32
−0.55
5.55
BP
−6.83
−4.98
−3.13
3.70
BP-SO2
−6.21
−5.21
−4.20
2.01
Zn-BP (M1)
−6.33
−4.66
−2.99
3.34
Zn-BP-SO2 (N1)
−5.90
−5.32
−4.75
1.15
Zn-BP (M2)
−6.25
−4.71
−3.16
3.09
Zn-BP-SO2 (N2)
−6.35
−5.10
−3.84
2.51
Zn-BP (M3)
−5.92
−4.45
−2.99
2.93
Zn-BP-SO2 (N3)
−5.47
−4.82
−4.18
1.29
Zn-BP (M4)
−5.57
−4.37
−3.17
2.40
Zn-BP-SO2 (N4)
−6.43
−5.19
−3.94
2.49Pure B12P12 is a semiconductor with HOMO-LUMO gap of 3.70 eV. Energies of HOMO and LUMO are −6.83 eV and −3.13 eV, respectively, with the Fermi level of −4.98 eV. Upon decoration of Zn (transition metal) on B12P12 nanocage, the energies of both HOMO and LUMO increase which causes narrowing of the HOMO-LUMO gap. M1 has HOMO-LUMO gap of 3.34 eV with the Fermi level of −4.66 eV. Similarly, M2 has HOMO-LUMO gap of 3.09 eV with the Fermi level of −4.71 eV. HOMO-LUMO gaps of M3 and M4 geometries are 2.93 eV and 2.40 eV with the Fermi levels of −4.45 eV and −4.37 eV, respectively. When SO2 is adsorbed on bare and Zn-doped B12P12, then HOMO-LUMO gaps become narrower as compared to Zn-doped B12P12. SO2 adsorption on bare B12P12 causes narrowing the HOMO-LUMO to 2.01 eV as compared to 3.70 eV (in pure B12P12). Adsorption of SO2 on Zn-doped B12P12 increase the conductivity of B12P12 as it causes narrowing of HOMO-LUMO gap. HOMO-LUMO gap values for N1, N2, N3, and N4 are 1.15 eV, 2.51 eV, 1.29 eV, and 2.49 eV with the Fermi level of −5.32 eV, −5.10 eV, −4.82 eV, and −5.19 eV, respectively (Table 2). Energies of HOMOs for N1, N2, N3, and N4 are −5.90 eV, −6.35 eV, −5.47 eV, and −6.43 eV, respectively. Similarly, LUMO energies for N1, N2, N3, and N4 are −4.75 eV, −3.84 eV, −4.18 eV, and −3.94 eV, respectively.Moreover, frontier molecular orbital shapes also reveal significant information regarding the reactivities. When B12P12 is decorated with transition metal like Zn, then HOMO is shifted to Zn atom. In general, when a metal such as Zn binds with many electronegative atoms present in B12P12 (phosphorus), then electrons from phosphorus causes the outer electron of Zn to be pushed out and more diffuse in nature. This results in increase in the energies of electrons present on Zn. Therefore, HOMO resides on Zn. If we see the frontier molecular orbital shapes of Zn-doped B12P12, then HOMO in all geometries is located on metal which support our notion [46]. LUMO density is also shifted on metal center after adsorption of Zn on B12P12. Therefore, we can say that if LUMO energy decreases, then it shifted to metal center, and if HOMO energy increases, then it shifted to metal center. When we adsorbed SO2 on bare and Zn-doped B12P12, then HOMO energies increases and HOMO density is shifted on SO2 and on zinc metal in all SO2-adsorbed Zn-doped B12P12 geometries. Similarly, LUMO energies decrease in all SO2-adsorbed Zn-doped B12P12 geometries which causes shifting of LUMO to metal as well as SO2 as shown in Figure 5.Figure 5
Side views of HOMO and LUMO of different systems.


### 3.6. Partial Densities of States

Next, partial densities of states analysis is performed to visualize the change in electronic behavior of B12P12 upon adsorption of Zn as well as SO2 adsorption on bare and Zn-doped B12P12. When Zn is adsorbed on B12P12 (M1, M2, M3, and M4), the HOMO has high density on cage and less density on Zn metal. And, LUMO has equal density on cage and metal. Similarly, when SO2 is adsorbed on B12P12, then HOMO has high density on cage and less density on SO2 and LUMO has equal densities on cage and SO2. However, we SO2 is adsorbed on Zn-doped B12P12 (N1, N2, N3, and N4) then, HOMO has high density on cage and Zn metal and has less density on SO2. But LUMO has high density on SO2 and has less density on B12P12-Zn as shown in Figure 6.Figure 6
Different systems with TDOS and PDOS graph. The isosurfaces is 0.02e/Å3. (a) B12P12 nanocage. (b) B12P12–SO2. (c) M1. (d) M2. (e) M3. (f) M4. (g) N1. (h) N2. (i) N3. (j) N4.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)


### 3.7. Global Indices of Reactivity

Changes in electronic properties of B12P12 after adsorption of Zn and SO2 are analyzed in terms of global indices of reactivity (Table 3). Different parameters such as ionization potential (I), electron affinity (A), chemical potential (μ), chemical hardness (η), softness (s), and electrophilicity (ω) are discussed in Table 3. Ionization potential is the negative of energy of HOMO and electron affinity is negative of the energy of LUMO, according to Koopman’s theorem. Pristine B12P12 has ionization potential of 6.83 eV with an electron affinity of 3.13 eV. When Zn is placed on pure B12P12, then ionization potential decreases to 6.33 eV (M1), 6.25 eV (M2), 5.92 eV (M3), and 5.57 eV (M4), and electron affinity show mixed behavior to 2.99 eV (M1), 3.16 eV (M2), 2.99 eV (M3), and 3.17 eV (M4) as shown in Table 3. When we adsorbed SO2 on Zn-doped B12P12, then increase in ionization potential and decrease in electron affinity as compared to pure B12P12 is seen. Electrophilicity tells the reactivity of a molecule or compound. Adsorption of Zn on pure B12P12 increases electrophilicity. But adsorption of SO2 on pure and Zn-doped B12P12 highly increases the electrophilicity as compared to Zn-doped B12P12. Chemical hardness and softness is directly related to HOMO-LUMO of a compound. A compound is said to be hard in nature if it has large HOMO-LUMO gap and vice versa. Adsorption of Zn on pure B12P12 makes pure B12P12 harder in nature. And adsorption of SO2 on pure and Zn-doped B12P12 also have same effect except N1 and N3 where reversible is true.Table 3
The ionization potential (I), electron affinity (A), chemical hardness (η), chemical potential (μ), softness (S), and electrophilicity (ω) for different systems.
Systems
I
A
η
μ
S
ω
Zn
6.10
0.55
2.78
−3.33
0.18
1.99
BP
6.83
3.13
1.06
−4.66
0.47
6.79
BP-SO2
6.21
4.20
1.01
−5.21
0.50
13.46
Zn-BP (M1)
6.33
2.99
1.67
−4.66
0.30
6.51
Zn-BP-SO2 (N1)
5.90
4.75
0.58
−5.33
0.87
24.52
Zn-BP (M2)
6.25
3.16
1.55
−4.71
0.32
7.16
Zn-BP-SO2 (N2)
6.35
3.84
1.26
−5.10
0.40
10.32
Zn-BP (M3)
5.92
2.99
1.47
−4.46
0.34
6.77
Zn-BP-SO2 (N3)
5.47
4.18
0.65
−4.83
0.78
17.95
Zn-BP (M4)
5.57
3.17
1.20
−4.73
0.42
7.96
Zn-BP-SO2 (N4)
6.43
3.94
1.25
−5.19
0.40
10.78


## 3.1. Adsorption Energies and Bond Lengths

First, we decorated B12P12 nanocage with Zn metal. Decoration of Zn on B12P12 was done at different positions, i.e., Zn@b66 (Zn is placed on the bond shared between two hexagonal rings), Zn@b64 (Zn is placed on the bond shared between one hexagonal and one tetragonal ring), Zn@Btop (Zn is placed on the top site of Boron atom), Zn@Ptop (Zn is placed on the top site of Phosphide atom), Zn@R6 (Zn is placed on the hexagonal ring), and Zn@R4 (Zn is placed on the tetragonal ring). With six different input geometries, only four distinct geometries could be optimized. Some of the initial input geometries converged to others during optimization. Among four distinct optimized geometries, one geometry is Zn@b66 which obviously looks like a ring enlarged structure (in which Zn is inserted into hexagonal ring). The second geometry is Zn@Btop (in which Zn is adsorbed on the top site of boron atom), while third geometry is Zn@b64 (in which Zn is adsorbed on the bond shared between one hexagonal and tetragonal ring). Similarly, the fourth geometry is Zn@Ptop (in which Zn is adsorbed on the top of Phosphorus atom). The ring enlarged geometry, Zn@b66 is named as M1, whereas Zn@Btop, Zn@b64, and Zn@Ptop geometries are termed as M2, M3, and M4, respectively. Zn decoration on B12P12 (in M1 geometry) causes distortion in the B12P12 geometry, and B-P bond lengths at the side of zinc adsorption increase to 3.98 Å as compared to 1.91 Å in bare B12P12. The adsorption energy value for M1 geometry is very low about (−14.83 kJ/mol). Low adsorption energy for M1 reflects physisorption of zinc on the cage. The low interaction energy is attributed to the distortion caused by Zn insertion into the nanocage. No such distortion of the B12P12 nanocage is caused by Zn in M2 geometry which results in higher adsorption energy (−57.12 kJ/mol). B-P bond length slightly elongated to 1.98 Å as compared to 1.93 Å (Figure 2). In M3 and M4 geometries, B-P bond lengths are increased slightly to 1.94 Å, as compared to 1.91 Å in bare nanocage. This change is very small because Zn in both geometries does not cause any significant geometric changes (as Zn is adsorbed on bond shared between hexagonal and tetragonal ring and on the top of phosphorus atom). Adsorption energy values of Zn are −22.94 kJ/mol and −14.50 kJ/mol in M3 and M4, respectively.Figure 2
Optimized structures of Zn-doped B12P12 nanocages: (a) M1, (b) M2, (c) M3, and (d) M4.
(a)
(b)
(c)
(d)Next, adsorption of SO2 on bare and Zn-decorated B12P12 was analyzed. Here, we got one geometry in which SO2 is adsorbed on bare B12P12 (Figure 3). The value of interaction energy (−14.92 kJ/mol) shows that SO2 is favorably adsorbed on bare B12P12, but through physisorption. SO2 interacts with boron phosphide nanocage through O-B bond where oxygen of SO2 interacts with boron of the nanocage. When SO2 is adsorbed on M1, M2, M3, and M4 geometry, four geometries named as N1, N2, N3, and N4 are obtained (Figure 3). The values of adsorption energies in N1, N2, N3, and N4 are −69.76 KJ/mol, −9.82 KJ/mol, −104.92 KJ/mol, and −14.87 kJ/mol, respectively (Table 1). The distances of SO2 from Zn in N1, N2, N3, and N4 are 1.94 Å, 2.86 Å, 2.01 Å, and 2.38 Å, respectively. Among these geometries, N1–N3 has oxygen of SO2 coordinated to Zn atom, whereas N4 is characterized by the Zn-S bond. The high interaction energy of SO2 adsorption on Zn in N3 is attributed to chelation of SO2 with Zn. Interestingly, the values of adsorption energies of SO2 adsorption on bare and Zn-doped B12P12 shows inverse relation with Zn-decorated B12P12 except N2 geometry. This relation shows that Zn-doped B12P12 are more stable and resists SO2 adsorption and for N2 geometry reversal is true.Figure 3
(a) SO2-adsorbed B12P12 and SO2-adsorbed Zn-doped B12P12 systems: (b) N1, (c) N2, (d) N3, and (e) N4.
(a)
(b)
(c)
(d)
(e)Table 1
The closest distance of SO2 to BP, Zn metal to pure BP, QNBO (Charge) on metal and gas, dipole moment, and adsorption energies of different systems.
Systems
d
Zn-BP (Å)
d
SO
2
−
Zn (Å)
Q
NBO on SO2 (eV)
Q
NBO on Zn (eV)
μ
D (D)
E
ad
b (KJ/mol)
Zn
—
—
—
0.00
0.00
—
BP
—
—
—
—
0.00
—
BP-SO2
—
−0.217
—
3.08
−14.92
Zn-BP (M1)
2.10
—
1.004
2.42
−14.83
Zn-BP-SO2 (N1)
—
1.94a
−0.163
—
1.70
−69.76
Zn-BP (M2)
2.15
—
—
0.524
3.24
−57.12
Zn-BP-SO2 (N2)
—
2.86a
−0.093
—
2.25
−9.82
Zn-BP (M3)
2.15
—
—
0.937
2.26
−22.94
Zn-BP-SO2 (N3)
—
2.01a
−0.152
—
0.53
−104.09
Zn-BP (M4)
3.00
—
—
0.081
0.61
−14.50
Zn-BP-SO2 (N4)
—
2.38a
−0.147
—
5.26
−41.87
aMetal nearest to SO2 (gas) atom towards Zn metal. bThe calculated (adsorption) energy, which is calculated by using equations (1), (2), and (3).


## 3.2. Dipole Moment

Pure B12P12 has equal number of boron and phosphorus atoms and show zero dipole moment due to its centrosymmetric nature. Upon decoration of Zn on B12P12 nanocage (M1, M2, M3, and M4), change in dipole moment is observed. Values of dipole moment for M1, M2, M3, and M4 geometries are 2.42 D, 3.24 D, 2.26 D, and 0.61 D. The highest value is counted for M2 geometry. This increase in dipole moment is due to large distance of Zn from BP cage. The vector is pointed away from B12P12 nanocage toward Zn in all geometries (Zn-decorated BP) which is the reason of change in dipole moment after decoration of Zn on BP. Significant change in dipole moment is observed when SO2 is adsorbed on bare B12P12 (3.08 D). On interaction of SO2 with N1, N2, N3, and N4 geometries, the dipole moment values decrease (except N4) compared to M1–M4. Values of dipole moment for N1, N2, N3, and N4 are 1.70 D, 2.25 D, 0.53 D, and 5.26 D. This decrease in dipole moment is due to decrease in distance of SO2 from Zn and higher value of interaction energies except geometry N2 where reversal is true. Dipole moment vector is pointed away from nanocage, and it is toward SO2 in all SO2-adsorbed Zn-doped BP geometries (N1–N4).


## 3.3.QNBO

Next,QNBO analysis is performed to correlate the charge transfer with dipole moment. The charges of Zn in M1–M4 geometries are 1.004, 0.524, 0.937, and 0.081, respectively. Close analysis reveal lack of any correlation between QNBO and dipole moment for Zn-doped B12P12. This lack of correlation in QNBO and dipole moment suggested that intensity of charge is not only the factor in deciding the dipole moment rather separation between charges also matters a lot, besides the geometry of the Zn-doped complex itself. So, dipole moment is depending on intensity of charges as well as displacement between Zn and nanocage. QNBO charges on SO2 in SO2-adsorbed Zn-doped BP is slightly negative which reflects that SO2 is taking charge from the Zn-doped geometries. QNBO charges on N1, N2, N3, and N4 are −0.163, −0.093, −0.152, and −0.147, respectively. This negative charge is due to generation of negative SO2 atoms (which is attached with Zn transition metal). The trend between dipole moment and QNBO is true (consistent) for N1, N2, N3, and N4 (Table 1).


## 3.4. MEP Analysis

Next, molecular electrostatic potential (MEP) analysis is performed to unveil the change in the properties after decoration of Zn metal and SO2 adsorption on Zn-doped BP as shown in Figure 4. This analysis also gives relationship between chemical reactivity and charge distribution. Yellow color shows electron-rich area (negative charge), blue color shows electron-deficient area (positive charge), and green color shows mean potential area (neutral charge). Pure B12P12 has equal number of boron and phosphorus atoms, so it shows no charge distribution, or in other words, it is neutral as shown in Figure 4. When SO2 is adsorbed on BP, the yellow area is shifted on SO2 which suggested that SO2 end is an electron-rich end. But metal end shows electropositive charge (blue in color) in Zn-doped complex, whereas the cage shows uniform charge distribution. Similarly, when SO2 is adsorbed on Zn-decorated B12P12, then charge transfer towards SO2 occurs on SO2; yellow color shows negative charge end, blue color on Zn metal shows electropositive nature of transition metal, and cage shows equal charge distribution this trend is consistent with QNBO.Figure 4
The molecular electrostatic potential map of different systems: (a) B12P12 nanocage, (b) SO2-adsorbed B12P12, (c) M1, (d) M2, (e) M3, (f) M4, (g) N1, (h) N2, (i) N3, and (j) N4.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)


## 3.5. Electronic Properties

It is seen that decoration of Zn on B12P12 and subsequent SO2 adsorption on Zn-doped B12P12 brings some changes in the electronic properties of pure B12P12 and Zn-B12P12. Zn decoration on BP causes narrowing of HOMO-LUMO gap which is very vital for conductivity (useful for sensing materials). The following equation shows relationship between HOMO-LUMO energy gap and conductivity [44]:(6)σαexp−EgKT.Here, “K” represents Boltzmann’s constant, and σ represents conductivity. HOMO energies, LUMO energies, Fermi level, and HOMO-LUMO gap are calculated and given in Table 2. Fermi level makes define as the midpoint of HOMO-LUMO gap in molecule at zero Kelvin temperature [45].Table 2
The orbital parameters, HOMO-LUMO energies, fermi level, and HOMO-LUMO energy gap of different systems.
System
E
HOMO (ev)
E
FL (ev)
E
LUMO (ev)
E
g (ev)
Zn
−6.10
−3.32
−0.55
5.55
BP
−6.83
−4.98
−3.13
3.70
BP-SO2
−6.21
−5.21
−4.20
2.01
Zn-BP (M1)
−6.33
−4.66
−2.99
3.34
Zn-BP-SO2 (N1)
−5.90
−5.32
−4.75
1.15
Zn-BP (M2)
−6.25
−4.71
−3.16
3.09
Zn-BP-SO2 (N2)
−6.35
−5.10
−3.84
2.51
Zn-BP (M3)
−5.92
−4.45
−2.99
2.93
Zn-BP-SO2 (N3)
−5.47
−4.82
−4.18
1.29
Zn-BP (M4)
−5.57
−4.37
−3.17
2.40
Zn-BP-SO2 (N4)
−6.43
−5.19
−3.94
2.49Pure B12P12 is a semiconductor with HOMO-LUMO gap of 3.70 eV. Energies of HOMO and LUMO are −6.83 eV and −3.13 eV, respectively, with the Fermi level of −4.98 eV. Upon decoration of Zn (transition metal) on B12P12 nanocage, the energies of both HOMO and LUMO increase which causes narrowing of the HOMO-LUMO gap. M1 has HOMO-LUMO gap of 3.34 eV with the Fermi level of −4.66 eV. Similarly, M2 has HOMO-LUMO gap of 3.09 eV with the Fermi level of −4.71 eV. HOMO-LUMO gaps of M3 and M4 geometries are 2.93 eV and 2.40 eV with the Fermi levels of −4.45 eV and −4.37 eV, respectively. When SO2 is adsorbed on bare and Zn-doped B12P12, then HOMO-LUMO gaps become narrower as compared to Zn-doped B12P12. SO2 adsorption on bare B12P12 causes narrowing the HOMO-LUMO to 2.01 eV as compared to 3.70 eV (in pure B12P12). Adsorption of SO2 on Zn-doped B12P12 increase the conductivity of B12P12 as it causes narrowing of HOMO-LUMO gap. HOMO-LUMO gap values for N1, N2, N3, and N4 are 1.15 eV, 2.51 eV, 1.29 eV, and 2.49 eV with the Fermi level of −5.32 eV, −5.10 eV, −4.82 eV, and −5.19 eV, respectively (Table 2). Energies of HOMOs for N1, N2, N3, and N4 are −5.90 eV, −6.35 eV, −5.47 eV, and −6.43 eV, respectively. Similarly, LUMO energies for N1, N2, N3, and N4 are −4.75 eV, −3.84 eV, −4.18 eV, and −3.94 eV, respectively.Moreover, frontier molecular orbital shapes also reveal significant information regarding the reactivities. When B12P12 is decorated with transition metal like Zn, then HOMO is shifted to Zn atom. In general, when a metal such as Zn binds with many electronegative atoms present in B12P12 (phosphorus), then electrons from phosphorus causes the outer electron of Zn to be pushed out and more diffuse in nature. This results in increase in the energies of electrons present on Zn. Therefore, HOMO resides on Zn. If we see the frontier molecular orbital shapes of Zn-doped B12P12, then HOMO in all geometries is located on metal which support our notion [46]. LUMO density is also shifted on metal center after adsorption of Zn on B12P12. Therefore, we can say that if LUMO energy decreases, then it shifted to metal center, and if HOMO energy increases, then it shifted to metal center. When we adsorbed SO2 on bare and Zn-doped B12P12, then HOMO energies increases and HOMO density is shifted on SO2 and on zinc metal in all SO2-adsorbed Zn-doped B12P12 geometries. Similarly, LUMO energies decrease in all SO2-adsorbed Zn-doped B12P12 geometries which causes shifting of LUMO to metal as well as SO2 as shown in Figure 5.Figure 5
Side views of HOMO and LUMO of different systems.


## 3.6. Partial Densities of States

Next, partial densities of states analysis is performed to visualize the change in electronic behavior of B12P12 upon adsorption of Zn as well as SO2 adsorption on bare and Zn-doped B12P12. When Zn is adsorbed on B12P12 (M1, M2, M3, and M4), the HOMO has high density on cage and less density on Zn metal. And, LUMO has equal density on cage and metal. Similarly, when SO2 is adsorbed on B12P12, then HOMO has high density on cage and less density on SO2 and LUMO has equal densities on cage and SO2. However, we SO2 is adsorbed on Zn-doped B12P12 (N1, N2, N3, and N4) then, HOMO has high density on cage and Zn metal and has less density on SO2. But LUMO has high density on SO2 and has less density on B12P12-Zn as shown in Figure 6.Figure 6
Different systems with TDOS and PDOS graph. The isosurfaces is 0.02e/Å3. (a) B12P12 nanocage. (b) B12P12–SO2. (c) M1. (d) M2. (e) M3. (f) M4. (g) N1. (h) N2. (i) N3. (j) N4.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)


## 3.7. Global Indices of Reactivity

Changes in electronic properties of B12P12 after adsorption of Zn and SO2 are analyzed in terms of global indices of reactivity (Table 3). Different parameters such as ionization potential (I), electron affinity (A), chemical potential (μ), chemical hardness (η), softness (s), and electrophilicity (ω) are discussed in Table 3. Ionization potential is the negative of energy of HOMO and electron affinity is negative of the energy of LUMO, according to Koopman’s theorem. Pristine B12P12 has ionization potential of 6.83 eV with an electron affinity of 3.13 eV. When Zn is placed on pure B12P12, then ionization potential decreases to 6.33 eV (M1), 6.25 eV (M2), 5.92 eV (M3), and 5.57 eV (M4), and electron affinity show mixed behavior to 2.99 eV (M1), 3.16 eV (M2), 2.99 eV (M3), and 3.17 eV (M4) as shown in Table 3. When we adsorbed SO2 on Zn-doped B12P12, then increase in ionization potential and decrease in electron affinity as compared to pure B12P12 is seen. Electrophilicity tells the reactivity of a molecule or compound. Adsorption of Zn on pure B12P12 increases electrophilicity. But adsorption of SO2 on pure and Zn-doped B12P12 highly increases the electrophilicity as compared to Zn-doped B12P12. Chemical hardness and softness is directly related to HOMO-LUMO of a compound. A compound is said to be hard in nature if it has large HOMO-LUMO gap and vice versa. Adsorption of Zn on pure B12P12 makes pure B12P12 harder in nature. And adsorption of SO2 on pure and Zn-doped B12P12 also have same effect except N1 and N3 where reversible is true.Table 3
The ionization potential (I), electron affinity (A), chemical hardness (η), chemical potential (μ), softness (S), and electrophilicity (ω) for different systems.
Systems
I
A
η
μ
S
ω
Zn
6.10
0.55
2.78
−3.33
0.18
1.99
BP
6.83
3.13
1.06
−4.66
0.47
6.79
BP-SO2
6.21
4.20
1.01
−5.21
0.50
13.46
Zn-BP (M1)
6.33
2.99
1.67
−4.66
0.30
6.51
Zn-BP-SO2 (N1)
5.90
4.75
0.58
−5.33
0.87
24.52
Zn-BP (M2)
6.25
3.16
1.55
−4.71
0.32
7.16
Zn-BP-SO2 (N2)
6.35
3.84
1.26
−5.10
0.40
10.32
Zn-BP (M3)
5.92
2.99
1.47
−4.46
0.34
6.77
Zn-BP-SO2 (N3)
5.47
4.18
0.65
−4.83
0.78
17.95
Zn-BP (M4)
5.57
3.17
1.20
−4.73
0.42
7.96
Zn-BP-SO2 (N4)
6.43
3.94
1.25
−5.19
0.40
10.78


## 4. Conclusion

We investigate the adsorption of Zn on B12P12. We find that Zn is strongly adsorbed on B12P12. Moreover, when we adsorbed SO2 on Zn-doped B12P12, SO2 favorably adsorbed on Zn-doped B12P12. The decreasing order of binding energy for SO2-adsorbed Zn-doped B12P12 is N3 > N1 > N4 > N2. Charge separation occurs when Zn is adsorbed on B12P12. But small charge separation occurs when SO2 is adsorbed on Zn-doped B12P12. When Zn is adsorbed on B12P12, it causes destabilization of HOMO and stabilization of LUMO which causes of narrowing of HOMO-LUMO gap. Similar effect is seen in the case of SO2 adsorption on Zn-doped B12P12. And SO2 adsorption on Zn-doped B12P12 increases electrophilicity of the B12P12. At the end, we also performed PDOS to see the change in electronic properties of B12P12 after Zn and SO2 adsorption.



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*Source: 2629596-2020-02-23.xml*