3.1. Pure ZnONS
We displayed the geometry of optimized ZnONS in Fig. 1. Zn–O is predicted in the structure of the sheet with lengths of 1.95 Å. From DOS plot in Fig. 1, it is seen that the Eg between the LUMO and HOMO of the pure ZnONS is about 3.81 eV, indicating a semiconducting property. In order to find the most stable complex of XH3/ZnONS, we considered various primary adsorption geometries. Figures 2 and 3 show the four most stable states. In complex A and C, the molecules is adsorbed almost in parallel with the ZnONS surface. In configuration A and C, the XH3 approaches to the one X and one H atoms through the Zn and O atoms, and in configuration B and D, the XH3 interacts with one X atom through the Zn atom on the sheet.
Configuration B and D indicate a poor interaction between the ZnONS surface and XH3 (Figs. 2 and 3) and the Ead are − 4.5 and − 7.6 kcal/mol, respectively. They indicate the physical nature of these interaction, which are stronger than the configuration A and C, respectively. It should be noted that the weak interaction in complexes A and C compared to B and D is the higher strain and steric effect when the X and H atoms simultaneously react with the surface of the sheet. By the adsorption of XH3, there is a slight change in both conduction and valence levels, which causes a decrease in Eg of the complex from 3.81 eV in the pure ZnONS to 3.52, 3.49, 3.41 and 3.33 eV for A, B, C and D complexes, respectively. Thus, the adsorption of XH3 on the pure ZnONS is a physisorption process. The electrical conductivity of the nanosheet can change because of the change of Eg based on the equation below:
(3)
Here, k designates the Boltzmann constant and σ is the electrical conductivity and A designates a constant (electrons/m3K3/2)[47]. At a constant temperature, as the amount of Eg gets lower, the electrical conductivity becomes higher. So, the electrical conductivity of ZnONS increases significantly following a decrease in Eg, which is caused by the adsorption process. Therefore, this change of Eg is very small and negligible which cannot generate a proper electronic noise to detect the presence of XH3 gases. Thus, pure ZnONS cannot be a proper sensor for XH3 detection.
3.2. XH3 adsorption on the SW ZnONS
The typical topological defect in nanostructures is the SW defect which consists of seven-membered and two pairs of five-membered rings. The SW defects are made by rotating the bond 90◦. SW is obtained by rotating a Zn-O bond on the pure ZnONS. The atomic configuration for the SW defect in ZnONS is shown in Fig. 4. From DOS plot in Fig. 4, it is seen that the Eg between the LUMO and HOMO of the SW ZnONS is about 3.24 eV, indicating a semiconducting property.
The interaction of a SW ZnONS with an XH3 molecules will be investigated. Here, the adsorption of XH3 is inspected on the SW ZnONS. For this purpose, the XH3 molecules are located in above SW sites on the SW ZnONS surface such as the over the center of a five, six and seven-membered ring, top of the bond bridge and on the O or Zn atom. Totally, the XH3 are located in parallel with the surface of SW ZnONS or perpendicular to it. Two local minima is predicted for each XH3 after the optimization process (Figs. 5 and 6). In configuration I, the PH3 interacts by its one P and one H atoms with one Zn and O atoms of the defected site SW ZnONS. Moreover, the corresponding interaction distances between the Zn and O atoms of defected sheet and the P and H atoms of the complex I is about 2.98 and 3.11 Å, respectively, and an Ead is about − 12.1 kcal/mol (Table 2). In configuration II (Fig. 5), the PH3 interacts by its one P atom with one Zn atom of the defected site SW ZnONS with distance 2.77 Å, and an Ead is about − 13.4 kcal/mol (Table 2). According Eq. 3, the adsorption of PH3 on the SW ZnONS is a physisorption process and SW ZnONS cannot detect PH3 gas. Therefore SW ZnONS isn’t a sensor for PH3 gas.
Table 1
Adsorption energy (Ead, kcal/mol) for PH3 and AsH3 adsorption on the pristine ZnO nanosheet (Figs. 2 and 3). Energy of HOMO, LUMO, and HOMO-LUMO energy gap (Eg) in eV. The ∆Eg indicates the change of Eg after the adsorption process. Work function (Φ) for Pristine ZnO nanosheet
Structure
|
Ead
|
EHOMO
|
EF
|
ELUMO
|
Eg
|
ΔEg(%)
|
Φ
|
%∆Φ
|
ZnONS
|
-
|
-6.36
|
-4.46
|
-2.55
|
3.81
|
-
|
4.46
|
-
|
A
|
-3.7
|
-6.04
|
-4.28
|
-2.52
|
3.52
|
-7.6
|
4.28
|
-3.9
|
B
|
-4.5
|
-5.98
|
-4.24
|
-2.49
|
3.49
|
-8.4
|
4.24
|
-5.0
|
C
|
-6.7
|
-5.89
|
-4.19
|
-2.48
|
3.41
|
-10.5
|
4.19
|
-6.2
|
D
|
-7.6
|
-5.80
|
-4.14
|
-2.47
|
3.33
|
-12.6
|
4.14
|
-7.3
|
Table 2
Adsorption energy (Ead, kcal/mol) for PH3 and AsH3 adsorption on the Stone-Wales ZnO nanosheet (SW ZnONS). Energy of HOMO, LUMO, and HOMO-LUMO energy gap (Eg) in eV. The ∆Eg indicates the change of Eg after the adsorption process. Work function (Φ) for SW ZnONS (Figs. 5 and 6).
Structure
|
Ead
|
EHOMO
|
EF
|
ELUMO
|
Eg
|
ΔEg(%)
|
Φ
|
%∆Φ
|
SW ZnONS
|
-
|
-5.76
|
-4.14
|
-2.51
|
3.25
|
-
|
4.14
|
-
|
I
|
-12.1
|
-5.32
|
-3.91
|
-2.49
|
2.83
|
-12.9
|
3.91
|
-5.6
|
II
|
-13.4
|
-5.24
|
-3.86
|
-2.47
|
2.77
|
-14.8
|
3.86
|
-6.9
|
III
|
-18.5
|
-4.97
|
-3.71
|
-2.45
|
2.52
|
-22.5
|
3.71
|
-10.4
|
IV
|
-23.3
|
-4.81
|
-3.63
|
-2.44
|
2.37
|
-27.1
|
3.63
|
-12.4
|
In configuration III, the AsH3 approaches to one Zn and one O atoms with an interacting distance of 2.82 Å for Zn…As and 2.64 Å for H…O. In configuration IV, the AsH3 interacts with one Zn atom through one of its As atom which the Zn…As distance is about 2.57 Å. For configuration III, Ead is -18.5 kcal/mol and it is -23.3 kcal/mol for configuration IV (Table 2 and Fig. 6). It seems that because As atom is larger than P atom, it was able to react better with the SW ZnONS surface. Therefore, the results display that SW defected significantly strengthens the adsorption of AsH3 on SW ZnONS. The adsorption of XH3 in the SW ZnONS is more favorable compare to the pure ZnONS, which is due to the fact that the localization of the HOMO is mainly on the defected site in the SW ZnONS (Fig. 7).
As shown in Fig. 8 (configuration IV), the partial DOS plot illustrates that a new occupied orbital appears on the SW ZnONS electron forbidden area (Eg) at -2.81 eV due to the presence of AsH3. The HOMO profile shown in Fig. 8 also confirms that the HOMO of complex shifts on the AsH3 with changing the HOMO energy. The Eg of SW ZnONS decreases significantly, following the adsorption of AsH3 and the sheet becomes more conductive. Numerically, its Eg in complex IV decreased from 3.25 to 2.37 eV (by approximately − 27.1%). According to Eq. 3, the SW ZnONS sensitivity toward the AsH3 molecule increase. Thus, it was found that SW ZnONS can selectively detect AsH3 molecule.
To further evaluate the sensitivity of the surfaces, the changes in the work function (Φ) were investigated before and following the adsorption process. Φ of a semiconductor is the least amount of work needed for the extraction of an electron from the Fermi level. The re-examination of the gas-induced Φ by the suspended amplitude effect modifiers has been accepted for many years as the basis for realizing sensor operating systems [48]. Theoretically, in vacuum, the released electron current density is defined as follows:
(4)
Where, T is the temperature in Kelvin, A is the Richardson constant in A/m2 and Φ is the work function. Φ was computed as follows:
Φ = Einf – EF (5)
Where EF is the energy of Fermi level and Einf is the electrostatic potential, which is supposed to be equal to 0 at infinity. Φ of the sheet was subtracted from that of the complexes and obtained Φ changes (∆Φ). Φ for pristine ZnONS was about 4.46 eV and changed very slightly after adsorbing the XH3 molecules, which can be ignored. But when AsH3 is adsorbed onto SW ZnONS (configuration IV), Φ is significantly reduced from 4.14 to 3.63 eV. According to Eq. 4, the emitted current density and Φ are exponentially related to each other. Therefore, it can be said that after the adsorption of AsH3, by decreasing the Φ, the current density of the emitted electron increases dramatically. Accordingly, we think that SW defected in ZnO is a hopeful way to increase the sensitivity of ZnONS toward AsH3 which pristine ZnONS did not.
3.3. Recovery time
An important factor which should be taking into account in the development of sensors is the strength of interactions. A higher recovery time is obtained if Ead gets more negative:
(6)
Here, T is the temperature, ν0 is the attempt frequency, and k shows the Boltzmann's constant which is ~ 2.0 ⋅ 10− 3 kcal/mol.K. At 298 K and the ultra-violet light (υ ~ 1016 s− 1), τ for the desorption of AsH3 from the SW ZnONS surface is computed to be 9.5 s, which indicates the nanosheet has a short τ as a sensor for the detection of AsH3. For the sake of comparison, τ value have been provided for different chemical agents, inclufing SO2/Al-doped h-BN [49], adrucil/Si-doped phagraphene [50], metronidazole/B36 borophene [51], phosgene/BN nanocones-180 [52], and cathinone/B12N12 [53], which is 27.6, 0.02, 1.53, 0.48, and 0.54 s, respectively.