A Theoretical Framework of Zinc-Decorated Inorganic Mg12O12 Nanoclusters for Efficient COCl2 Adsorption: A Step Forward toward the Development of COCl2 Sensing Materials.

Gas sensors are widely explored due to their remarkable detection efficiency for pollutants. Phosgene is a toxic gas and its high concentration in the environment causes some serious health problems like swollen throat, a change in voice, late response of nervous systems, and many more. Therefore, the development of sensors for quick monitoring of COCl2 in the environment is the need of the time. In this aspect, we have explored the adsorption behavior of late transition metal-decorated Mg12O12 nanoclusters for COCl2. Density functional theory at the B3LYP/6-31G(d,p) level is used for optimization, frontier molecular orbital analysis, dipole moment, natural bonding orbitals, bond lengths, adsorption energies, and global reactivity descriptor analysis. Decoration of Zn on pure Mg12O12 delivered two geometries named as Y1 and Y2 with adsorption energy values of -388.91 and -403.11 kJ/mol, respectively. Adsorption of COCl2 on pure Mg12O12 also delivered two geometries (X1 and X2) with different orientations of COCl2. The computed adsorption energy values of X1 and X2 are -44.92 and -71.32 kJ/mol. However, adsorption of COCl2 on Zn-decorated Mg12O12 offered two geometries named as Z1 and Z2 with adsorption energy values of -455.22 and -419.04 kJ/mol, respectively. These adsorption energy values suggested that Zn decoration significantly enhances the adsorption capability of COCl2 gas. Further, the narrow band gap and large dipole moment values of COCl2-adsorbed Zn-decorated Mg12O12 nanoclusters suggested that designed systems are efficient candidates for COCl2 adsorption. Global reactivity indices unveil the great natural stability and least reactivity of designed systems. Results of all analyses suggested that Zn-decorated Mg12O12 nanoclusters are efficient aspirants for the development of high-performance COCl2 sensing materials.

Introduction

Group ΙΙ–VΙ matericlass="Chemical">als such as class="Chemical">pan class="Chemical">magnesium oxide received valuable attention in nanoscience due to a large number of applications like sensors,[1] ceramics,[2] energy storage,[3] nonlinear optics,[4] and catalysis.[5] Magnesium oxide clusters are quite stable in nature, which was proved by experimental and theoretical techniques.[6−8] Further, the stable nature of magnesium oxide clusters (MgO) with n = 6, 9, 12, and 15 was proved by spectroscopic techniques conducted by Saunders.[9] Similarly, a time-of-flight-based laser ionization technique had been used by Ziemann and Castleman for exploring the stable nature of small magnesium oxide clusters.[7] Magnesium oxide clusters, especially in nanocage form, offers extra stability due to low energy and minimum chemical reactivity. In nanocage forms of magnesium oxides, Mg12O12 is more stable as compared to other nanocage forms. In a Mg12O12 nanocluster, the energy is quite low with a large band gap, which offers extra stability with extraordinary electronic and optical properties. A Mg–O bond is ionic in nature, which allows marvelous electronic properties.

Adsorpclass="Chemical">tion of difclass="Chemical">pan class="Chemical">ferent analytes on the surface of the Mg12O12 nanocluster is already reported in the literature. Acetylene was adsorbed vertically and horizontally on the surface of the Mg12O12 nanocluster by Nicholas and his co-workers.[10] Acetylene was chemisorbed on the MgO surface by bonding with a Mg atom. Their report revealed that the resultant systems were quite stable and electronically efficient and showed good agreement with spectroscopic findings. Vertical adsorption of nitrogen mono-oxide on Mg12O12 and Zn12O12 was studied by Noei and Peyghan.[11] They examined different interactions like Mg–O and Mg–N. Their study unveiled the electronic response of Mg12O12 and Zn12O12 with and without NO. In addition, Solimannejad and his co-authors studied the adsorption of different anions and cations like F–, Cl–, Li+, and Na+ on the exterior surface of Mg12O12 in gas and water (solvent) phases. They studied that anions and cations showed strong adsorption on the top of a Mg12O12 nanocage.[12]

class="Chemical">Metal declass="Chemical">pan class="Chemical">coration on the surface of an X12Y12 nanocluster is an effective approach for enhancing the optical, nonlinear, electronic, and adsorption properties of the X12Y12 nanocluster. In the literature, many reports are present in which decoration of a metal on X12Y12 was done for enhancing the adsorption, nonlinear, and electronic properties of resultant nanoclusters.[13,14] Ni decoration on the surface of B12N12 was studied by Ayub and Rad for SO2, O3, and H2 adsorption.[15,16] They studied SO2 and O3 molecule adsorption on a B12N12 nanocluster in the presence and absence of Ni metal. They also examined H2 molecule adsorption (vertical) on a Ni-decorated B12N12 nanocluster and suggested that these adsorptions were excellent and designed systems were good sensors for the mentioned gases. Similarly, in another report, Ayub and Rad studied the adsorption of H2 on a Ni-decorated Al12N12 nanocluster.[17] They studied vertical and horizontal adsorption of a hydrogen molecule on an Al12N12 nanocluster with the aid of DFT. They used different analyses like DOS, NBO, FMOs, and MEP to examine the effect of Ni metal decoration on the nanocage for efficient adsorption of H2. Further, the nonlinear optical properties of alkali metal-doped B12P12 and Al12N12 nanoclusters were studied, which suggested that alkali metals, being rich in electron density, significantly improved the nonlinear optical properties.[18]

Many reports in the literature are present in which declass="Chemical">coraclass="Chemical">pan class="Chemical">tion of metals on the surface of Mg12O12 was done like Shakarzedah et al. who studied the enhancement in electronic, optical, and nonlinear optical properties of Mg12O12 nanoclusters by alkali metal doping.[19] A detailed DFT study was performed by Kakemam and Peyghan in which they used Si- and carbon-doped Mg12O12 nanoclusters for unveiling the geometric, electronic, solvation, energetic, and field emission properties of MgO nanocages.[20] Magnetic properties of Mg12O12 were also explored through decoration of Mn, Fe, Co, and Ni metals on Mg12O12.[21] The most stable structures were determined with full geometry optimization near the minimum of the binding energy curves of all the examined paths inside and outside of the Mg12O12 nanocage. The results reveal that for all stable structures, the Ni atom has a larger binding energy than the other TM atoms. Similarly, Shamlouei et al. studied the Mg12O12 doped with a transition metal for the change in the structure–property relationship and optical and electronic properties of resultant nanoclusters.[22]

In the vclass="Chemical">aluable literature, difclass="Chemical">pan class="Chemical">ferent reports are present in which the Mg12O12 nanocluster was efficiently utilized for different analyte adsorptions. Sulfur mustard (a chemical warfare), Tabun molecule, hydrogen molecule, alkali metal, 6-thioguanine (anticancer drug), hydroxyurea (anticancer drug), HCN, and CICN (poisonous gases) adsorption on the Mg12O12 nanocluster was successfully studied in the literature.[19,23−28] These reports suggested that Mg12O12 nanoclusters are effective nanoclusters for adsorption of gas and other analytes. Moreover, optical and magnetic properties of transition metal-encapsulated, Sc- and Al-doped Mg12O12 nanoclusters are also studied in the literature, which suggested that Mg12O12 is a good candidate for high nonlinear and electronic properties.[29−31] They studied Mg12O12 and the various Al-doped Mg12O12 nanoclusters by DFT and TD-DFT calculations with M06 and B97D functionals. They found that energy doping by MgAl is energetically more considerable than OAl. Moreover, doping of Al atoms leads to changes in adsorbent energy gap owing to the rise of new states in the gap region of the Mg12O12 nanocluster.[29] Similarly, the majority of TM atoms are encapsulated in the Mg12O12 cage except for Sc, Ti, Zr, and Hf atoms. The structural stability of Mg12O12 clusters was higher than that of TM@Mg12O12 clusters by average binding energies. Furthermore, Sc, Co, Zr, Rh, Hf, W, and Pt atoms were more suitable for the Mg12O12 cage than their neighbors.[31]

Zinc is a late tranclass="Chemical">siclass="Chemical">pan class="Chemical">tion metal with a filled d orbital; thus, it has an electron-rich nature. Further, the attachment and de-attachment of Zn is easy as compared to earlier transition metals (Z = 21–29). In addition, Zn decoration on various nanoclusters was also examined in the literature, which suggested that Zn is an efficient metal with an electron-rich nature and better electronic properties when considered with nanoclusters.[32] So, we become interested to study the Zn-decorated Mg12O12 nanoclusters for COCl2 adsorption. Phosgene gas is actually an acid chloride that is formed from carbonic acid. COCl2 is colorless gas, which is used in many industrial processes like pesticides and isocyanates. Besides these advantages, many disadvantages are associated with phosgene gas as it is a chemical weapon that was used in World War Ι. In addition, exposure to phosgene causes many health problems like a change in voice before aging, bad throat condition, and abnormal pulmonary edema.[33] Due to these reasons, its concentration is strictly monitored in the atmosphere to eliminate the public health risks. In the literature, different nanoclusters like B12N12 and nanosheets like AlN were used for efficient adsorption of phosgene gas.[34,35] Motivated from these reports, we also decide to explore the electronic, optoelectronic, and adsorption properties of Mg12O12 upon Zn decoration and COCl2 adsorption.

Herein, we have performed a detailed denclass="Chemical">sity funcclass="Chemical">pan class="Chemical">tional theory study for Zn decoration and COCl2 adsorption on pure and Zn-decorated Mg12O12 nanoclusters. By designing such systems, we made an efficient attempt to explore the electronic properties of Mg12O12. Different parameters like frontier molecular orbitals, dipole moment, natural bonding orbitals, global reactivity indices, adsorption energies, bond lengths, and partial density of states are obtained for evaluating the electronic behavior of Mg12O12 nanoclusters upon Zn decoration and COCl2 adsorption. Results of all analyses suggested that designed systems are quite stable in nature with low reactivity. Thus, we report an efficient system for COCl2 adsorption.

Results and Discussion

class="Chemical">Magnesium oxide nanocages are electricclass="Chemical">pan class="Chemical">ally neutral in nature as the number of electropositive and electronegative atoms is equal. Among magnesium oxide nanocages, the Mg12O12 nanocage is more stable due to its large band gap and great charge separation. The optimized geometry of the Mg12O12 nanocage at the B3LYP/6-31G(d,p) basis set of density functional theory is shown in Figure . Two types of bonds are noted in the Mg12O12 nanocage, i.e., b (bond common between two hexagonal rings) and b (bond common between tetragonal and hexagonal rings). The bond lengths of b and b are 1.77 and 1.80 Å, respectively, as shown in Figure .

Figure 1

Optimized geometry of Mg12O12 at the B3LYP/6-31G(d,p) level of theory.

Opclass="Chemical">timized geometry of class="Chemical">pan class="Chemical">Mg12O12 at the B3LYP/6-31G(d,p) level of theory.

Interaction Energy along with Bond Lengths

A class="Chemical">Mg12O12 nanocage class="Chemical">pan class="Chemical">consists of eight hexagonal and six tetragonal rings. There are a number of places on Mg12O12 for doping of metals. In the current study, zinc (late transition metal) is initially installed on b (Zn on the bond common between two hexagonal rings), b (Zn on the bond common between hexagonal and tetragonal rings), O (Zn on the top position of oxygen), Mg (Zn on the top position of magnesium), r (Zn above the hexagonal ring), and r (Zn above the tetragonal ring). All initial inputs are converging into two types of geometries, i.e., O and r. These geometries are named as Y1 and Y2, respectively, as shown in Figure . The DFT-based computed adsorption energy values of Y1 and Y2 are −388.91 and −403.11 kJ/mol, respectively. The calculated bond distances of zinc from the magnesium oxide nanocage in Y1 and Y2 are 2.05 and 2.02 Å, respectively. The small distance in Y2 geometry causes strong adsorption of Zn on the Mg12O12 nanocluster.

Figure 2

Optimized geometries of Zn-decorated Mg12O12 (Y1 and Y2) at the B3LYP/6-31G(d,p) level of DFT.

Opclass="Chemical">timized geometries of class="Chemical">pan class="Chemical">Zn-decorated Mg12O12 (Y1 and Y2) at the B3LYP/6-31G(d,p) level of DFT.

Further, adsorpclass="Chemical">tion of class="Chemical">pan class="Chemical">phosgene on a pristine Mg12O12 nanocage is also studied. Different orientations of COCl2 on the Mg12O12 nanocluster were analyzed and the most stable geometries with different orientations of COCl2 were selected. These geometries were named as X1 (Cl-Mg12O12) and X2 (O-Mg12O12), which disclosed adsorption energy values of −44.92 and −71.32 kJ/mol, respectively, as shown in Figure . The distances of COCl2 from the Mg12O12 nanocage are 2.85 and 2.19 Å in X1 and X2, respectively. Again, the small distance of COCl2 from Mg12O12 offers efficient adsorption of COCl2 on the nanocluster in X2 geometry.

Next, class="Chemical">COCl2 was adsorbed on class="Chemical">pan class="Chemical">Zn-decorated Mg12O12 (Y1 and Y2), and two geometries named as Z1 (COCl2-Y1) and Z2 (COCl2-Y2) were obtained as shown in Figure . The adsorption energy values of Z1 and Z2 are −455.22 and −419.04 kJ/mol, respectively (Table ). The adsorption energy values suggested that Z1 is more efficient (for COCl2 adsorption) as compared to Z2, which is might be due to the small distance of COCl2 from the Zn-Mg12O12 nanocage. The large distance of COCl2 from the Zn-Mg12O12 nanocage offers a low adsorption aptitude. But both geometries (Z1 and Z2) disclosed better adsorption energy as compared to COCl2-Mg12O12 nanocages (X1 and X2), which is probably due to decoration of a late transition metal on the Mg12O12 nanocage. Furthermore, these Zn-decorated Mg12O12 nanoclusters offer more efficient adsorption of COCl2 with large adsorption energy values as compared to previously reported Cu-decorated B12N12 (Ead = −16.954 kJ/mol)[14] and B12P12 nanoclusters (Ead = −119.031 kJ/mol) for COCl2 adsorption.[36] Similarly, Padash et al. investigated the adsorption of COCl2 on Al12N12, Al12P12, B12N12, and B12P12 with adsorption energy values of −78.732, −26.244, −26.243, and −26.242 kJ/mol, respectively.[37] Mella and Cortés-Arriagada reported adsorption of COCl2 on Cr-decorated graphene sheets with an adsorption energy value of −93.951 kJ/mol.[38] Further, COCl2 adsorption on boron nitride nanocones disclosed an adsorption energy value of −69.452 kJ/mol.[34] Adsorption of COCl2 on aluminum nitride nanotubes and boron nitride nanoflasks disclosed adsorption energy values of −24.520 and −12.670 kJ/mol,[39,40] respectively, as shown in Table . All previous reports showed that adsorption of COCl2 on Zn-decorated Mg12O12 nanocages (present report) is quite good, which suggested that our designed systems are more efficient for COCl2 adsorption. From the above discussion, it is evidently suggested that Zn-decorated Mg12O12 nanoclusters are efficient aspirants for COCl2 adsorption.

Figure 3

Optimized geometries of COCl2-Mg12O12 (X1 and X2) and COCl2-Zn-Mg12O12 (Z1 and Z2) at the B3LYP/6-31G(d,p) level of DFT.

Table 1

Bond Distances, QNBO, and Adsorption Energies of All Studied Systems at the B3LYP/6-31G(d,p) Level of DFT

system	dM-Cagea	dCOCl2-Metalb	QNBO on metal	QNBO on COCl2	Eadc
Mg12O12
COCl2-Mg12O12 (X1)		2.85		0.053	–44.92
COCl2-Mg12O12 (X2)		2.99		0.075	–71.32
Zn-Mg12O12 (Y1)	2.05		0.553		–388.91
COCl2-Zn-Mg12O12 (Z1)	2.19	3.63		1.031	–455.22
Zn-Mg12O12 (Y2)	2.02		0.322		–403.11
COCl2-Zn-Mg12O12 (Z2)	2.09	3.87		1.001	–419.04

Distance of metal from the Mg12O12 cluster in Å.

Distance of NO2 from a metal in Å.

Adsorption energy in kJ/mol.

Table 2

Adsorption Energy Values (kJ/mol) of Some Previously Reported Systems

systems	Ead in kJ/mol	reference
COCl2-Cu-B12N12	–16.954	Hussain et al. (2020)[14]
COCl2-Cu-B12P12	–119.031	Younas et al. (2021)[36]
COCl2-Al12P12 and COCl2-Al12N12	–26.244 and −78.732	Padash et al. (2019)[37]
COCl2-B12P12 and COCl2-B12N12	–26.242 and −26.243	Padash et al. (2019)[37]
COCl2-Cr-graphene	–93.951	Mella and Cortés-Arriagada (2019)[38]
COCl2-boron nitride nanocones	–69.452	Vesally et al. (2018)[34]
COCl2-aluminum nitride nanotubes	–24.520	Shahabi and Raissi (2016)[39]
COCl2-boron nitride nanoflasks	–12.670	Moladoust et al. (2019)[40]

Opclass="Chemical">timized geometries of class="Chemical">pan class="Chemical">COCl2-Mg12O12 (X1 and X2) and COCl2-Zn-Mg12O12 (Z1 and Z2) at the B3LYP/6-31G(d,p) level of DFT.

Distance of pan class="Chemical">metal from the class="Chemical">pan class="Chemical">Mg12O12 cluster in Å.

Distance of pan class="Chemical">NO2 from a class="Chemical">pan class="Chemical">metal in Å.

Adsorppan class="Chemical">tion energy in kJ/mol.

Dipole Moment

Dipole moment is a key parameter for understanding the charge separaclass="Chemical">tion within a system.[41−44] Addiclass="Chemical">pan class="Chemical">tionally, dipole moment also measures the change in the electronic behavior of Mg12O12 upon zinc decoration and COCl2 adsorption. Dipole moment also describes the charge moment within a system, i.e., from one end to another end. For this purpose, a dipole moment vector is commonly used. The DFT-based computed dipole moment values of all designed systems are represented in Figure . From the figure, it is clearly indicated that the pure Mg12O12 nanocage expressed zero dipole moment, which is due to the centrosymmetric nature of the Mg12O12 nanocage. An equal number of electropositive and electronegative atoms cancel the resultant dipole moment. Zn-doped Mg12O12 showed dipole moment values of 3.10 D (Y1) and 2.88 D (Y2). A large dipole moment value is noted in Y1, which is probably due to the large distance of Zn from the nanocage, and the reverse is true for Y2. However, adsorption of COCl2 on pure Mg12O12 brings a significant change in dipole moment values, i.e., μ = 2.44 D (X1) and μ = 3.91 D (X2), which suggested that adsorption of COCl2 disturbs the electronic behavior of the Mg12O12 nanocage, which offers efficient adsorption properties. Further, adsorption of COCl2 on the Zn-decorated Mg12O12 nanocage (Z1 and Z2) brings a significant change in dipole moment. The DFT-based computed dipole moments in Z1 and Z2 are 7.71 and 8.66 D, respectively. A large dipole moment value is noted in Z2 geometry, which is probably due to the large distance of COCl2 from the Zn-Mg12O12 nanocage, which allows high charge separation. From the above discussion, it is noted that COCl2 adsorption on Zn-decorated Mg12O12 nanocages efficiently enhances dipole moment values. So, designed systems are fine candidates for the development of highly efficient COCl2 sensing materials.

Figure 4

Dipole moment of all studied systems at the B3LYP/6-31G(d,p) level of DFT.

Dipole moment of pan class="Chemical">all studied systems at the B3LYclass="Chemical">pan class="Chemical">P/6-31G(d,p) level of DFT.

Natural Bonding Orbital Analysis

Next, naturclass="Chemical">al bonding orbitclass="Chemical">pan class="Chemical">al (QNBO) analysis has been performed to determine the charges on metal and phosgene gas and also to correlate the dipole moment with QNBO. For QNBO analysis, the B3LYP/6-31G(d,p) level of DFT has been used. The results of QNBO on metal and COCl2 are tabulated in Table . From Table , it is observed that QNBO values on Zn in Zn-Mg12O12 nanocages are 0.553 (Y1) and 0.322 e (Y2). The high NBO charge on zinc in Y1 geometry might be due to the high dipole moment value, which created a high NBO charge. The QNBO values on the COCl2 analyte in X1, X2, Z1, and Z2 are 2.85, 2.99, 3.63, and 3.87 e, respectively. These values of NBO charges show good coherence with calculated dipole moment values, which are also in the trend Z2 > Z1 > X2 > X1. The highest NBO charge is noted in Z2 geometry, which is probably due to the large distance and high value of dipole moment. The second highest value of QNBO is noted in the Z1 nanocage, which is also due to the large value of dipole moment. So, a good correlation is seen between QNBO and dipole moment in all studied systems. To sum up, zinc decoration on Mg12O12 significantly enhances the QNBO on COCl2, which suggested that designed systems are good candidates for efficient COCl2 adsorption.

Frontier Molecular Orbital Analysis

The highest occlass="Chemical">cuclass="Chemical">pied moleclass="Chemical">pan class="Chemical">cular orbital (HOMO) and lowest unoccupied molecular orbitals are generally known as frontier molecular orbitals (FMOs), and their position in a molecule is vital for measuring the resultant HOMO–LUMO energy gap. The HOMO–LUMO energy gap also determines the sensing ability and charge transfer within a system. The band gap (HOMO–LUMO energy gap) of a system can be calculated by utilizing the following equation.[45−48]

The class="Chemical">Fermi level is defined as the midclass="Chemical">point of the HOMO–class="Chemical">pan class="Chemical">LUMO energy gap at a temperature equal to 0 K. The Fermi level, H–L energy gap, and energies of HOMO and LUMO are tabulated in Table . Mg12O12 is a semiconductor with a large band gap value (Eg = 4.87 eV). However, the adsorption of COCl2 on the pure Mg12O12 nanocage significantly reduces the band gap. The computed values of the band gap in X1 and X2 are 4.21 and 3.37 eV along with Fermi level values of −2.11 and −1.74 eV, respectively. In both geometries, adsorption of phosgene on pristine Mg12O12 causes narrowing of the band gap. In X1 and X2 geometries, the destabilization of HOMO and stabilization of LUMO cause the formation of a new Fermi level at the midpoint of newly assigned HOMO and LUMO. The decoration of Zn on the pristine Mg12O12 nanocluster causes more destabilization of HOMO (EHOMO = −4.95 and −5.37 eV in Y1 and Y2) and stabilization of LUMO (ELUMO = −1.78 and −1.68 eV in Y1 and Y2), which in turn cause narrowing of the HOMO–LUMO energy gap (Eg = 3.17 and 3.69 eV in Y1 and Y2). The computed Fermi levels in Y1 and Y2 are located at −1.58 and −1.85 eV, respectively. Both Zn-decorated Mg12O12 geometries disclosed a narrow band gap as compared to pure Mg12O12, which might be due to decoration of an electron-rich metal, which significantly causes shifting of charge from HOMO to LUMO. Also, at the end, adsorption of COCl2 on Zn-decorated Mg12O12 also causes a major change in the electronic behavior of the Mg12O12 nanocluster. The frontier molecular orbital, i.e., HOMO and LUMO, energy values of Z1 and Z2 are −4.89 and −5.33 eV and −1.91 and −1.76 eV, respectively. In both geometries Z1 and Z2, destabilization of HOMO and stabilization of LUMO are observed as compared to pure and COCl2-Mg12O12 (X1 and X2) nanoclusters. Destabilization of HOMO and stabilization of LUMO significantly reduce the gap between two molecular orbitals, which in turn enhances charge transfer from HOMO to LUMO. So, from the above discussion, it is evidently concluded that Zn decoration significantly favors the COCl2 adsorption, which narrows HOMO–LUMO energy gap values.

Table 3

Energies of HOMO and LUMO and Fermi Level along with the Band Gap of All Studied Systems at the B3LYP/6-31G(d,p) Basis Set of DFT

systems	EHOMO (eV)	ELUMO (eV)	EFL (eV)	Eg (eV)
Mg12O12	–6.56	–1.69	–4.13	4.87
COCl2-Mg12O12 (X1)	–6.43	–2.22	–2.11	4.21
COCl2-Mg12O12 (X2)	–6.27	–2.79	–1.74	3.47
Zn-Mg12O12 (Y1)	–4.95	–1.78	–1.58	3.17
COCl2-Zn-Mg12O12 (Z1)	–4.89	–1.91	–1.49	2.98
Zn-Mg12O12 (Y2)	–5.37	–1.68	–1.85	3.69
COCl2-Zn-Mg12O12 (Z2)	–5.33	–1.76	–1.78	3.56

Fronclass="Chemical">tier moleclass="Chemical">pan class="Chemical">cular orbital distribution delivers some useful information.[49,50] The distribution pattern of frontier molecular orbitals in a studied system is estimated at the B3LYP/6-31G(d,p) level of theory. The DFT-based computed frontier molecular orbital distribution pattern is expressed in Figure . From Figure , it is noted that HOMO and LUMO are equally distributed in the Mg12O12 nanocluster. However, adsorption of COCl2 on the pure Mg12O12 nanocluster (X1 and X2) causes a major change in the distribution pattern of HOMO and LUMO. HOMO is distributed in the Mg12O12 nanocage, while the LUMO density is distributed on COCl2 gas, which suggested that the charge is shifted from Mg12O12 to COCl2 on excitation. Zn decoration on the Mg12O12 nanocluster (Y1 and Y2) shows a similar pattern of HOMO and LUMO distribution as HOMO is majorly present on Zn. The presence of HOMO on Zn suggested that the metal is electron-rich and could not hold the excess of electron density. Thus, it spread the extra electrons, which shifted to the nanocage in the form of LUMO. Adsorption of COCl2 on Zn-decorated Mg12O12 nanoclusters (Z1 and Z2) causes distribution of HOMO majorly on Zn and the Mg12O12 nanocluster, while LUMO is present on COCl2 gas.

Figure 5

Distribution pattern of HOMO and LUMO in all studied systems.

Distribuclass="Chemical">tion class="Chemical">pattern of HOMO and class="Chemical">pan class="Chemical">LUMO in all studied systems.

Partial Density of States Analysis

class="Chemical">Parclass="Chemical">pan class="Chemical">tial density of states (PDOS) analysis is performed to unveil the distribution pattern of HOMO and LUMO in a studied system.[50,51] The B3LYP/6-31G(d,p) level of DFT is used for evaluating the partial density of states (Figure ). From the PDOS graph, it is evidently seen that HOMO and LUMO are equally distributed in the Mg12O12 nanocage. Meanwhile, in X1 and X2, HOMO is present only on the Mg12O12 nanocluster and LUMO is present majorly on COCl2, which suggested that adsorption of COCl2 on the Mg12O12 nanocluster efficiently moves the charge density from HOMO to LUMO. In the Zn-decorated Mg12O12 nanocluster, the HOMO is distributed on Zn and Mg12O12 with a higher amount on the metal, which shows that the metal is electron-rich. The LUMO of Y1 and Y2 geometries is distributed majorly on Zn and the nanocage with more amount on the nanocage, which shows that an excess of electrons from the metal end goes to the nanocage. However, in the COCl2-adsorbed Zn-decorated Mg12O12 nanocluster (Z1), the HOMO density is majorly present on the Zn-Mg12O12 end and no HOMO density is present on COCl2, while LUMO is also present on Zn-Mg12O12 with a small amount on COCl2, which suggested that the large distance of COCl2 from Zn-Mg12O12 causes small shifting of charge density. In Z2, the HOMO density is majorly present on the Zn-Mg12O12 end and LUMO is present on COCl2 with a small amount on the Zn-Mg12O12 end. From the above discussion, it is evidently concluded that our Zn decoration is an efficient approach for COCl2 adsorption.

Figure 6

Partial density of states plots of all studied systems at the B3LYP/6-31G(d,p) level of DFT.

class="Chemical">Parclass="Chemical">pan class="Chemical">tial density of states plots of all studied systems at the B3LYP/6-31G(d,p) level of DFT.

Global Reactivity Descriptor

Globclass="Chemical">al reacclass="Chemical">pan class="Chemical">tivity descriptors like electronegativity, electrophilic index, ionization potential, electron affinity, global hardness, global softness, and chemical potential are calculated at the B3LYP/6-31G(d,p) level of DFT. Global reactivity descriptors are useful for demonstrating the natural stability and chemical reactivity. The calculated global reactivity descriptors are tabulated in Table . The ionization potential is considered as an inverse of HOMO and electron affinity is considered as an inverse of LUMO. The computed ionization potential values of Mg12O12, X1, X2, Y1, Y2, Z1, and Z2 are 6.560, 6.430, 6.270, 4.950, 5.370, 4.890, and 5.330 eV with electron affinity values of 1.690, 2.220, 2.790, 1.780, 1.680, 1.910, and 1.760 eV, respectively. The highest ionization potential is noted in the Mg12O12 nanocluster followed by X1, which is due to low values of HOMO of respective systems. Similarly, the highest electron affinity is seen in X2 geometry followed by X1. The electronegativity and chemical potential values of studied systems are 4.125, 4.325, 4.530, 3.365, 3.525, 3.400, and 3.545 for Mg12O12, X1, X2, Y1, Y2, Z1, and Z2, respectively. The highest electronegativity is noted in pure and COCl2-adsorbed Mg12O12 nanoclusters. Fine values of electronegativity are noted in COCl2-adsorbed Zn-decorated Mg12O12 (Z1 and Z2) nanoclusters, which suggested that designed systems are quite stable in nature. Generally, the global softness is associated with the natural stability and least reactivity of a system. Low values of global softness suggested that resultant systems are quite stable in nature with the least reactivity. The computed global softness values of Mg12O12, X1, X2, Y1, Y2, Z1, and Z2 are 0.205, 0.238, 0.287, 0.315, 0.271, 0.336, and 0.280 eV–1, respectively. All studied systems disclosed low values of global softness and large values of chemical hardness, which suggested that designed systems are quite stable in nature with minimum reactivity. Electrophilic indices of all studied systems are also calculated at the B3LYP/6-31G(d,p) level of DFT. The computed value of the electrophilic index of Mg12O12 is 3.494 eV. The decoration of Zn on Mg12O12 significantly enhances the electrophilic index (ω = 3.572 eV for Y1). Adsorption of COCl2 on zinc-decorated Mg12O12 nanoclusters significantly enhances the electrophilic index, i.e., ω = 3.879 and 3.520 eV, for Z1 and Z2. These values suggested that Zn decoration on Mg12O12 significantly enhances the electrophilic index. The above discussion cleared that Zn decoration on Mg12O12 potentially enhances the adsorption capability of resultant systems for COCl2 with fine values of the electrophilic index. Similarly, large global hardness and small global softness values suggested that designed systems are quite stable in nature with minimum reactivity.

Table 4

IP (Ionization Potential in eV), EA (Electron Affinity in eV), X (Electronegativity), μ (Chemical Potential in eV), η (Global Hardness in eV), S (Global Softness in eV–1), and ω (Global Electrophilic Index in eV) of Studied Systems

systems	IP	EA	X	μ	η	S	ω
Mg12O12	6.560	1.690	4.125	–4.125	2.435	0.205	3.494
COCl2-Mg12O12 (X1)	6.430	2.220	4.325	–4.325	2.105	0.238	4.443
COCl2-Mg12O12 (X2)	6.270	2.790	4.530	–4.530	1.740	0.287	5.897
Zn-Mg12O12 (Y1)	4.950	1.780	3.365	–3.365	1.585	0.315	3.572
COCl2-Zn-Mg12O12 (Z1)	4.890	1.910	3.400	–3.400	1.490	0.336	3.879
Zn-Mg12O12 (Y2)	5.370	1.680	3.525	–3.525	1.845	0.271	3.367
COCl2-Zn-Mg12O12 (Z2)	5.330	1.760	3.545	–3.545	1.785	0.280	3.520

Conclusions

In summary, advance quantum chemicclass="Chemical">al aclass="Chemical">pclass="Chemical">proaches were used for exclass="Chemical">ploring the oclass="Chemical">pan class="Chemical">ptoelectronic and electronic response of Mg12O12 nanoclusters in the presence and absence of Zn followed by COCl2 adsorption. Density functional theory at the B3LYP/6-31G(d,p) level of theory was employed for all calculations. Zinc decoration on Mg12O12 offered two geometries (Y1 and Y2) with adsorption energy values of −388.91 and −403.11 kJ/mol, respectively. Adsorption of COCl2 on pure Mg12O12 also delivered two geometries (X1 and X2) with different orientations of COCl2. The computed adsorption energy values of X1 and X2 are −44.92 and −71.32 kJ/mol. However, adsorption of COCl2 on Zn-decorated Mg12O12 offered two geometries named as Z1 and Z2 with adsorption energy values of −455.22 and −419.04 kJ/mol, respectively. These adsorption energy values suggested that Zn decoration significantly enhances the adsorption capability of COCl2 gas. Adsorption of COCl2 also causes narrowing of the HOMO–LUMO energy gap (Eg = 2.98–3.56 eV) as compared to pure Mg12O12 (4.87 eV) and COCl2-Mg12O12 (3.47–4.21 eV) nanoclusters, which allows maximum charge shifting. The large dipole moment and high QNBO of COCl2-adsorbed Zn-decorated Mg12O12 nanoclusters suggested that designed systems are efficient candidates for COCl2 sensing. Moreover, our reported systems show better adsorption energy and band gap values as compared to systems that were reported in the literature. Last, small values of global softness and large electrophilic index along with fine chemical potential values suggested that designed systems are excellent aspirants for the development of COCl2 sensing materials.

Computational Methodology

class="Chemical">All cclass="Chemical">pan class="Chemical">alculations of the present study were performed by using Gaussian 09, whereas GaussView 6.0 suit of program was used for visualization. Initially optimization of all studied systems was done at the B3LYP/6-31G(d,p) level of density functional theory. B3LYP/6-31G(d,p) is a famous and frequently used level of DFT for nanocluster-like systems.[4−16,36] It is also used to unveil the natural stable nature of magical X12Y12 nanoclusters. Different positions of metals on nanocages like r, r, b, b, Mg, and O were investigated, but all these inputs were converged to Otop and r6. Due to this reason, we used these two geometries for further analysis.[13,32,36,51] After optimization, PDOS, frontier molecular orbital analysis, natural bonding orbital analysis, dipole moment, and global reactivity index analysis were also performed at the B3LYP/6-31G(d,p) level of DFT.[51−58]

The interacclass="Chemical">tion energies of class="Chemical">pan class="Chemical">Zn with Mg12O12 (Y1 and Y2) were calculated by utilizing eq . Similarly, interaction/adsorption energies of COCl2 adsorbed with Mg12O12 nanoclusters (X1 and X2) and COCl2-adsorbed Zn-Mg12O12 nanoclusters (Z1 and Z2) were calculated through eqs and 4, respectively.

Difclass="Chemical">ferent equaclass="Chemical">pan class="Chemical">tions were utilized for calculating different global reactivity indices such as electrophilic index (ω in eV), global softness (S in eV–1), electronegativity (X in eV), global hardness (η in eV), and chemical potential (μ in eV) that were calculated by using the following equations:[59−66]