ZnO Nanocrystal-Based Chloroform Detection: Density Functional Theory (DFT) Study

: We investigated the detection of chloroform (CHCl 3 ) using ZnO nanoclusters via density functional theory calculations. The e ﬀ ects of various concentrations of CHCl 3 , as well as the deposition of O atoms, on the adsorption over ZnO nanoclusters were analyzed via geometric optimizations. The calculated di ﬀ erence between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for ZnO was 4.02 eV. The most stable adsorption characteristics were investigated with respect to the adsorption energy, frontier orbitals, elemental positions, and charge transfer. The results revealed that ZnO nanoclusters with a speciﬁc geometry and composition are promising candidates for chloroform-sensing applications.


Introduction
The rapid development of various important industries, such as automobiles, pharmaceuticals, textiles, food, and agriculture, has substantially contributed to environmental pollution [1]. The release of various toxic and harmful gases and chemicals from such industries has significantly disturbed the ecosystem, and poses a great threat not only to humans, but to all living beings [2,3]. Among the various toxic gases, chloroform, which is also known as tri-chloromethane or methyl-tri-chloride, is considered to be one of the most toxic gases, and evaporates quickly when exposed to air [4]. It is widely used by chemical companies and in paper mills. Chloroform lasts for a long time in the environment, and its breakdown products, such as phosgene and hydrogen chloride, are as toxic or even more toxic [5]. The exposure of humans to chloroform severely affects the central nervous system, kidneys, liver, etc. Long-term exposure may result in vomiting, nausea, dizziness, convulsions, depression, respiratory failure, coma, and even sudden death [6,7]. It is important to efficiently detect the release of chloroform because of its serious health hazards. Thus, various methods have been reported for detecting chloroform, which involve optical sensors, colorimetric sensors, fluorescent   The DOS of the Zn 12 O 12 nanocage was calculated, as shown in Figure 1. A geometric optimization was performed for the chloroform molecule (CHCl 3 ). It is a tetrahedral molecule, as shown in Figure 1. The calculated structural properties of CHCl 3 indicated that bond lengths R C-H and R C-Cl were 1.09 and 1.79 Å, respectively, and angles A H-C-Cl and A Cl-C-Cl were 107.5 • and 11.4 • , respectively. The energy gap (E g ) between the HOMO and LUMO was calculated to be 7.27 eV. The DOS for CHCl 3 was calculated, and is presented in Figure 1.

CHCl 3 Interaction with the Zn 12 O 12 Nanocage
The geometric optimizations for four probable orientations of CHCl 3 on the surface of the Zn 12 O 12 nanocage were investigated. Figure 2 shows the four orientations where the CHCl 3 molecule may interact via its H head or Cl head, and may be absorbed over the O site or Zn site of the Zn 12 O 12 nanocage. The adsorption energy was calculated using Equation (1). The electronic properties of the CHCl 3 adsorption modes are presented in Table 1. For the first adsorption mode (a), the CHCl 3 molecule was weakly chemically adsorbed, and for the other modes (b, c, and d), the CHCl 3 molecule was physically adsorbed. The boundary value between the physical and chemical adsorption was considered to be 0.21 eV [50,51]. In mode (a), owing to the chemical interaction, the Fermi level (E FL ) for the cluster was reduced by 0.17 eV, and the dipole moment (D) was increased to 3.05 Debye. There was no noticeable change in the HOMO-LUMO energy gap. In all of the adsorption modes, it was found that the HOMO-LUMO energy gaps of the CHCl 3 /ZnO complexes were in the range of 4.00-4.03 eV. Consequently, the adsorption of CHCl 3 on the ZnO nanocage had no significant effect on the HOMO-LUMO energy gap.     Additionally, to investigate the effect of the CHCl 3 concentration on the adsorption over the Zn 12 O 12 nanocage, we performed geometric optimizations for n CHCl 3 molecules (n = 1, 2, 3, and 4) adsorbed simultaneously over the Zn 12 O 12 nanocage to form (CHCl 3 ) n /ZnO complexes. All of the CHCl 3 molecules had an orientation in which the H head of the CHCl 3 molecule was directed toward an O site of the Zn 12 O 12 nanocage, which is the most energetic stable orientation, as presented in Figure 2. The adsorption energies (E ads ) were calculated using Equation (1), and are presented in Table 2. The optimized structures of (CHCl 3 ) n /ZnO and their DOSs are shown in Figure 3. As indicated by Table 2, after the second molecule was adsorbed, the adsorption energy (E ads ) increased as n-the number of adsorbed CHCl 3 molecules-increased. Additionally, as the number of adsorbed CHCl 3 molecules increased, the Fermi level decreased. Furthermore, although there were no significant changes in the average acquired charge (Q CHCl 3 ) on the CHCl 3 molecules, the dipole moment was sensitive to the number of adsorbed CHCl 3 molecules. The HOMO-LUMO energy gap (E g ), compared with that of the pristine Zn 12 O 12 nanocage (4.02 eV), was not affected by the number of adsorbed CHCl 3 molecules.

O Atom Interaction with the Zn12O12 Nanocage
To improve the sensitivity of Zn12O12 to the CHCl3 molecules, an O atom was deposited onto the cluster. To investigate the ability of the Zn12O12 nanocage to adsorb an O atom, the O atom was added at three different sites, namely: an O site, a Zn site, and the middle of the ZnO bond. Then, a full geometric optimization was performed for the O/Zn12O12 complexes. With the optimization, there are only two possible O/Zn12O12 complexes, as shown in Figure 4. The Eads were calculated using Equation (2). As shown in Table 3, the Eads values of the O atom on the Zn12O12 nanocage were -1.98 and -1.62 eV for complexes (a) and (b), respectively. This indicated that a chemical bond was formed between the O atom and the Zn12O12 cluster. Additionally, the NBO analysis indicated that the O atom gained negative charges (QO) of -0.71|e| and -0.61|e| for complexes (a) and (b), respectively.

O Atom Interaction with the Zn 12 O 12 Nanocage
To improve the sensitivity of Zn 12 Figure 4. The E ads were calculated using Equation (2). As shown in Table 3, the E ads values of the O atom on the Zn 12 O 12 nanocage were −1.98 and −1.62 eV for complexes (a) and (b), respectively. This indicated that a chemical bond was formed between the O atom and the Zn 12 O 12 cluster. Additionally, the NBO analysis indicated that the O atom gained negative charges (Q O ) of −0.71|e| and −0.61|e| for complexes (a) and (b), respectively. O/Zn12O12 for complexes (a) and (b) were reduced (to 3.53 and 3.78 eV, respectively) compared with that of the pristine Zn12O12 nanocage (4.02 eV; Table 1). Furthermore, for O/Zn12O12 complexes (a) and (b), increases of 0.24 and 0.12 eV, respectively, were observed for the Fermi level (EFL), and the dipole moment increased to 2.03 and 0.72, respectively. This indicated that the deposited O atom significantly affected the electronic properties of the Zn12O12 nanocage, and consequently may have affected its ability to adsorb CHCl3 molecules.

CHCl3 Interaction with O Atoms Deposited on the Zn12O12 Nanocage
The CHCl3 molecule could interact via its H head or Cl head, and the O atom could be deposited on the Zn or O sites of the nanocage; thus, there were four possible geometric structures for the CHCl3/O/Zn12O12 complexes. Consequently, we performed geometric optimization for the four aforementioned CHCl3/O/Zn12O12 complexes. During the optimization process, we found only three stable CHCl3/O/Zn12O12 complexes, as shown in Figure 5. The properties of the interaction among the CHCl3 molecule, deposited O atom, and Zn12O12 nanocage are presented in Table 4.   This strong interaction is attributed to the charge transfer from the Zn 12 O 12 nanocage to the adsorbed O atom. As indicated by the DOS in Figure 4, the HOMO-LUMO energy gaps (E g ) of O/Zn 12 O 12 for complexes (a) and (b) were reduced (to 3.53 and 3.78 eV, respectively) compared with that of the pristine Zn 12 O 12 nanocage (4.02 eV; Table 1). Furthermore, for O/Zn 12 O 12 complexes (a) and (b), increases of 0.24 and 0.12 eV, respectively, were observed for the Fermi level (E FL ), and the dipole moment increased to 2.03 and 0.72, respectively. This indicated that the deposited O atom significantly affected the electronic properties of the Zn 12 O 12 nanocage, and consequently may have affected its ability to adsorb CHCl 3 molecules.

CHCl 3 Interaction with O Atoms Deposited on the Zn 12 O 12 Nanocage
The CHCl 3 molecule could interact via its H head or Cl head, and the O atom could be deposited on the Zn or O sites of the nanocage; thus, there were four possible geometric structures for the CHCl 3 /O/Zn 12 O 12 complexes. Consequently, we performed geometric optimization for the four aforementioned CHCl 3 /O/Zn 12 O 12 complexes. During the optimization process, we found only three stable CHCl 3 /O/Zn 12 O 12 complexes, as shown in Figure 5. The properties of the interaction among the CHCl 3 molecule, deposited O atom, and Zn 12 O 12 nanocage are presented in Table 4.  The adsorption energies (Eads) for the complexes ranged from -0.92 to -2.44 eV. These values indicate a chemical interaction, which may have been due to a charge transfer. This can be explained by the NBO analysis, which revealed that in complexes (a) and (b), the deposited O atom gained negative charges of -0.63|e| and -0.62|e|, respectively. These charges were mainly transferred from   The adsorption energies (E ads ) for the complexes ranged from −0.92 to −2.44 eV. These values indicate a chemical interaction, which may have been due to a charge transfer. This can be explained by the NBO analysis, which revealed that in complexes (a) and (b), the deposited O atom gained negative charges of −0.63|e| and −0.62|e|, respectively. These charges were mainly transferred from the Zn 12 O 12 nanocage, which gained positive charges of 0.56|e| and 0.59|e|, respectively. Additionally, there was a small charge from the CHCl 3 molecule, which gained positive charges of 0.07|e| and 0.03|e|, respectively. However, in complex (c), the charge was transferred from the CHCl 3 molecule, which gained a positive charge of 0.86|e|, to both the Zn 12    The interaction energies are presented in Table 5. The adsorption energy remained relatively constant (approximately -0.96 eV) for the first three CHCl3 interacting molecules, and decreased for the fourth CHCl3 molecule (to -0.86 eV). Furthermore, as the number of adsorbed CHCl3 molecules increased, the average acquired positive charges on CHCl3 (  The interaction energies are presented in Table 5. The adsorption energy remained relatively constant (approximately −0.96 eV) for the first three CHCl 3 interacting molecules, and decreased for the fourth CHCl 3 molecule (to −0.86 eV). Furthermore, as the number of adsorbed CHCl 3 molecules increased, the average acquired positive charges on CHCl 3 (Q CHCl 3 ) decreased, and the negativity of the average charges on the deposited O atom (Q O ) decreased, while the negativity of the charges on the Zn 12 O 12 nanocage increased. Additionally, with the increasing number of adsorbed CHCl 3 molecules, the Fermi level (E FL ) increased and the HOMO-LUMO energy gap (E g ) decreased, compared with the pristine Zn 12 O 12 nanocages. The dipole moment of (CHCl 3 ) n /O/Zn 12 O 12 was sensitive to the number of CHCl 3 molecules. It has been observed that during the adsorption process, the change in the HOMO-LUMO energy gap (E g ) is related to the sensitivity of the sorbent for the adsorbate. However, the reduction of E g of the cluster significantly affects the electrical conductivity, as indicated by the following equation [52]: where σ represents the electrical conductivity, K represents Boltzmann's constant, and T represents the temperature. According to Equation (5) and the E g values in Tables 1 and 2, the adsorption of the CHCl 3 molecule in the gas phase did not lead to significant changes in the E g of the Zn 12 O 12 nanocage. According to Tables 4 and 5, the CHCl 3 molecule adsorption over the oxygenated ZnO significantly reduced the E g values. The energy difference between the nucleophile HOMO and electrophile LUMO is one of the important factors for HOMO-LUMO interactions. As previously mentioned, the chemical bonding between CHCl 3 and the oxygenated ZnO cluster in the CHCl 3 /O/Zn 12 O 12 complexes is due to the charge-transfer mechanism. It can be explained as the contribution from the HOMO of the O/Zn 12 O 12 cluster to the vacant LUMO of the CHCl 3 molecule. Figure 7 shows the surfaces of the frontier molecular orbitals (FMOs; HOMO/LUMO) for CHCl 3 Figure 8 shows the energy diagrams of the FMOs (HOMO/LUMO) for CHCl3, Zn12O12, O/Zn12O12, and CHCl3/O/Zn12O12. Our FMO studies revealed that the deposited O atom increased the HOMO of the ZnO cluster from -6.81 to -6.32 eV. Consequently, the energy gap between the HOMO of ZnO and the LUMO of CHCl3 decreased, making the charge transfer from the O/Zn12O12 cluster to the CHCl3 easier than that from the pristine Zn12O12 cluster. Thus, the O/Zn12O12 cluster is more sensitive to the CHCl3 molecule than the pristine Zn12O12 cluster.    Our FMO studies revealed that the deposited O atom increased the HOMO of the ZnO cluster from -6.81 to -6.32 eV. Consequently, the energy gap between the HOMO of ZnO and the LUMO of CHCl3 decreased, making the charge transfer from the O/Zn12O12 cluster to the CHCl3 easier than that from the pristine Zn12O12 cluster. Thus, the O/Zn12O12 cluster is more sensitive to the CHCl3 molecule than the pristine Zn12O12 cluster.