3.1. Structure and Adsorption Characteristics
For intrinsic g−ZnO, the lattice constant, the Zn-O bond length, and the bond angle of Zn-O-Zn are a = 3.289 Å, 1.899 Å, and 120°, respectively. These results aligned with the previously reported [
41]. To obtain the most stable adsorption conformation, four adsorption sites were considered, as shown in
Figure 1a. To research the intrinsic g−ZnO adsorption system, the DOS of the intrinsic g−ZnO was calculated and displayed in
Figure 1b. The DOS shows that spin up and down components of the intrinsic g−ZnO are symmetric, indicating that the intrinsic g−ZnO is non-magnetic. Moreover, the valence band maxima of intrinsic g−ZnO are mainly determined by the O atom, while the conduction band minima are determined primarily by the Zn atom.
For the defective g−ZnO, two defect types (Zinc vacancy and Oxygen vacancy) were considered, as depicted in
Figure 1c,e, respectively. For the Zinc vacancy g−ZnO (V
Zn/g−ZnO), O atoms around the V
Zn are all far away from the vacancy canter. Moreover, the Zn-O bond length around the V
Zn decreases to 1.832 Å, and the bond angle of Zn-O-Zn increases to 134.093°. The reason for this phenomenon is that the charge of the adjacent atom is transferred to the vacancy after the Zn atom is removed, and the V
Zn becomes a negative electric center, which has a positive Coulomb repulsion potential [
42]. Huang et al. [
43] calculated that the change of Zn-O-Zn bond angle around the vacancy of V
Zn/g−ZnO increases to 133.79°, and our results are similar. However, for the Oxygen vacancy g−ZnO (V
O/g−ZnO), the three Zn atoms around the V
O are all near the vacancy center. The O-Zn bond length around V
O increases to 1.946 Å, and the bond angle of O-Zn-O decreases to 107.336°, consistent with previous reports [
43,
44]. The reason is that the charge density around the vacancy changes after the Zn atom is removed, and the V
O becomes a positive electric center with a negative Coulomb attraction potential [
45].
To probe into the gas adsorbed on the defective g−ZnO system, the DOS of V
Zn/g−ZnO was calculated and displayed in
Figure 1d. It can be observed that the spin up and down is asymmetric. The spin-down impurity levels appear at 0.099 eV and 0.311 eV above the Fermi level, indicating that the introduction of V
Zn induces the production of magnetic. The V
Zn/g−ZnO have magnetic moment of 1.996
μB. In addition, the DOS shows that the impurity levels are mainly devoted by the O atom. The DOS of V
O/g−ZnO were calculated and displayed in
Figure 1f. It can be observed that no magnetic properties are generated after the introduction of V
O. The DOS indicates that valence band maxima is mainly contributed by the O atoms, and conduction band minima is mainly by the Zn atoms.
The most stable structure of gas (CO, NH
3, NO, and NO
2) adsorbed on intrinsic g−ZnO system is shown in
Figure 2a–e. Overall, CO, NH
3, and NO molecules are tilted concerning the intrinsic g−ZnO plane, and the C-atom, N-atom, and N-atom near the intrinsic g−ZnO aircraft, respectively. In contrast, the NO
2 molecule, with the O atom close to the intrinsic g−ZnO aircraft, is parallel to the intrinsic g−ZnO plane. In addition, the adsorption sites for CO, NH
3, NO, and NO
2 molecules are A
C, A
C, A
Zn, and A
O, respectively. The most stable configurations of gas adsorbed on the V
Zn/g−ZnO system and the V
O/g−ZnO system are shown in
Figure 2f–h and 2i–l, respectively. For the gas adsorbed on V
Zn/g−ZnO systems, gas molecules are tilted to V
Zn/g−ZnO, and nearly embedded in V
Zn/g−ZnO. For CO molecules, the C atom is closer to the V
Zn/g−ZnO, while the H atom is for NH
3 molecules. For NO and NO
2 molecules, the N atom is closer to the V
Zn/g−ZnO plane. For the gas adsorbed on V
O/g−ZnO systems, all gas molecules are inclined to V
O/g−ZnO. The difference is that the N atoms of the NH
3 is closer to the V
O/g−ZnO. And, for the NO
2 molecule, the O atom is closer to the V
O/g−ZnO plane.
To explore the sensitivity of the subject material to gas molecules and the type of adsorption, the
Ead and adsorption height was shown in
Figure 3 and
Figure 4, respectively. Overall, the
Ead are all negative, indicating that the adsorption process of each system is exothermic and stable. Moreover, the absolute values of adsorption energy of V
Zn/g−ZnO adsorbed by CO, NH
3, NO, and NO
2 systems are higher than other adsorption systems. It shows that V
Zn/g−ZnO is suitable for detecting four gas molecules [
46]. In addition, the intrinsic g−ZnO showed the best detection capability for NH
3 gas molecules. In comparison, due to the introduction of defects, the V
Zn/g−ZnO adsorption systems showed the most significant improvement in detecting NO and NO
2 gas. And V
O/g−ZnO adsorption systems showed the most remarkable enhancement in detecting NO
2 gas molecules.
The calculated adsorption height is defined as the closest atomic spacing between the gas and the g−ZnO or defective g−ZnO [
5,
47]. For the gas adsorbed on intrinsic g−ZnO system, the adsorption heights of the intrinsic g−ZnO adsorbed by CO gas (CO@ g−ZnO) system are 2.500 Å, more significant than the Zn-C bond length (2.010 Å [
48]). The adsorption heights of the adsorption of intrinsic g−ZnO by NO gas (NO@ g−ZnO) system are 2.336 Å, larger than the Zn-O (1.950 Å [
49]) or O-O (1.410 Å [
50]) bond length. The adsorption heights of the adsorption of intrinsic g−ZnO by NO
2 gas (NO
2@ g−ZnO) system are 2.380 Å, bigger than the Zn-O bond length. The smaller
Ead and larger adsorption height indicate that these are physical adsorption. On the contrary, the adsorption heights of the NH
3 gas adsorbed on the intrinsic g−ZnO (NH
3@ g−ZnO) system are 2.181 Å, which is less than the Zn-N bond length (2.22~2.25 Å [
51,
52]), and its larger
Ead proves to be chemisorption. For the gas adsorption V
Zn/g−ZnO systems, the adsorption heights of the V
Zn/g−ZnO systems for CO, NH
3, NO, and NO
2 adsorption are 1.219 Å, 0.686 Å, 0.910 Å, and 0.858 Å, respectively. The adsorption height of the V
Zn/g−ZnO adsorbed by CO (CO@ V
Zn/g−ZnO) system is greater than the O-C bond length (1.136 Å [
53]), and its smaller adsorption energy demonstrates physical adsorption. The NH
3 adsorbed on V
Zn/g−ZnO (NH
3@ V
Zn/g−ZnO) system, the NO adsorbed on V
Zn/g−ZnO (NO@ V
Zn/g−ZnO) system, and NO
2 adsorbed on V
Zn/g−ZnO (NO
2@ V
Zn/g−ZnO) system have smaller adsorption heights and larger adsorption energy indicating that they are chemisorbed. The smaller adsorption height, the stronger the interaction between layers [
46]. For the gas adsorption V
O/g−ZnO systems, the adsorption height of the V
O/g−ZnO adsorbed by CO (CO@ V
O/g−ZnO) system is 2.537 Å, which is higher than the Zn-C bond length and have smaller adsorption energy. Thus, it is physical adsorption. For the NH
3 adsorbed on V
O/g−ZnO (NH
3@ V
O/g−ZnO) system, NO adsorbed on V
O/g−ZnO (NO@ V
O/g−ZnO) system, and NO
2 adsorbed on V
O/g−ZnO (NO
2@ V
O/g−ZnO) system, the adsorption heights are 2.190 Å, 1.812 Å, and 1.941 Å, respectively, which are less than the Zn-N, Zn-N, and Zn-O bond lengths, respectively, and are chemisorption.
3.2. Electronic Characteristics
To explore the interaction mechanism of the adsorption process between the gas molecules and the host material, the CDD was calculated for each system.
Figure 5a–d illustrates the CDD of the intrinsic g−ZnO adsorption systems. For the CO@ g−ZnO system, the electrons lost by the C atom in the CO gas and the Zn atom below are mainly captured by the CO molecule and the O atom in g−ZnO. However, for the NH
3@ g−ZnO system, the electron is mainly distributed between the NH
3 molecule and the g−ZnO layer, which may be used to form chemosynthetic bonds consistent with the chemisorption in the previous section. For the NO@ g−ZnO system, the electrons lost by the Zn atoms below the NO molecule are mainly distributed around the NO molecule. For NO
2@ g−ZnO system, it is similar to the NO system, but the difference is that NO
2 molecules capture more electrons. To obtain precise charge transfer amounts, Bader charges were calculated as listed in
Table 1a. What can be found is that CO, NO, and NO
2 molecules act as electron acceptors, receiving 0.007e, 0.083e, and 0.252e from the intrinsic g−ZnO, respectively. Since the polarity of NO
2 molecules is higher than that of CO and NO molecules, the charge transfer of the NO
2@ g−ZnO system is significantly higher than that of the CO@ g−ZnO and NO@ g−ZnO systems. However, NH
3 molecules act as electron donors with 0.111e charge transfer to the intrinsic g−ZnO.
The CDD notations for the gas molecule adsorption V
Zn/g−ZnO systems and the gas molecule adsorption V
O/g−ZnO systems are shown in
Figure 5e–h and
Figure 5i–l, respectively. The Bader charge calculation is displayed in
Table 1b,c, respectively. For the CO adsorption defective g−ZnO system, the charge transfers amounts for the CO@ V
Zn/g−ZnO system and CO@ V
O/g−ZnO system are 0.019e and 0.017e, respectively, and both are transferred from the defective g−ZnO to the CO gas. Compared with the CO@ g−ZnO system, the charge transfer amounts are increased, and the corresponding regions of CO-gaining electrons are larger. For the NH
3 adsorbed defective g−ZnO systems, NH
3 still acts as an electron donor, transferring 0.110e and 0.101e to the V
Zn/g−ZnO and V
O/g−ZnO layers, respectively, with no significant change in charge transfer compared to the CO@ g−ZnO system. However, for the NO adsorption defective g−ZnO system, in the NO@ V
Zn/g−ZnO system, NO was converted from an electron acceptor to an electron donor and provided 0.110e to the V
Zn/g−ZnO layer. In the NO@ V
O/g−ZnO system, NO receives 0.312e from the V
O/g−ZnO layer as an electron acceptor, and the charge transfer is significantly increased compared to the NO@ g−ZnO system. And the charge density around NO is also increased considerably. In the NO
2@ V
Zn/g−ZnO system. 0.017e is transferred from the V
Zn/g−ZnO to the NO
2 gas. The charge density around NO
2 decreased substantially compared to the NO
2@ g−ZnO system. In contrast, in the NO
2@ V
O/g−ZnO system, NO
2 acts as an electron acceptor and receives 0.626e from the V
O/g−ZnO layer, and the amount of charge transfer and the charge density around NO
2 increases dramatically.
The band structure was calculated to research the effect of defects further. The band structure of the intrinsic g−ZnO is shown in
Figure 6a for comparative study. The intrinsic g−ZnO has a direct bandgap (1.651 eV) at the Γ point. Hu et al. [
54] used a similar algorithm to calculate the bandgap of the intrinsic g−ZnO is 1.670 eV, which is consistent. The band structure of the intrinsic g−ZnO adsorption systems is displayed in
Figure 6b–e. It can be observed that the band structure of the CO@ g−ZnO system and NH
3@ g−ZnO system have not significantly changed compared to that of the intrinsic g−ZnO, remaining non-magnetic direct bandgap with the value of 1.683 eV and 1.645 eV, respectively. Similar results were obtained by Zhou et al. [
55] in a study of WS
2 adsorption by CO and NH
3 gases. For the NO@ g−ZnO system and NO
2@ g−ZnO system, a splitting of the spin up and down bands can be noted, indicating that the adsorption of NO and NO
2 induces magnetic properties. For the NO@ g−ZnO system, NO turns into a magnetic direct bandgap semiconductor with a value of 1.717 eV, while the CBM of the NO
2@ g−ZnO system crosses the Fermi level and exhibits metallic behavior. Besides, in the NO@ g−ZnO system, spin-up impurity energy bands appear near −0.237 eV and 0.320 eV on both sides of the Fermi energy level and spin-down impurity energy bands appear at 0.676 eV above the Fermi energy level, presumably introduced by NO. Moreover, in the NO
2@ g−ZnO system, a spin-down impurity energy level appears near the Fermi energy level, which may be provided by NO
2. Similarly, in NO and NO
2 adsorption of transition metal-doped MoS
2 has been reported by Salih et al. [
56]. A similar statement was made in the study of gas adsorption of WS
2 by Zhou et al. [
55]
The band structure of V
Zn/g−ZnO is displayed in
Figure 6f as a comparison and exhibits magnetic semiconductor properties at K with a direct bandgap of 0.058 eV. The band structure of the V
Zn/g−ZnO adsorption system is shown in
Figure 6g–j. It can be observed that for the CO@ V
Zn/g−ZnO system, there is no significant change compared to that of V
Zn/g−ZnO, which is still a magnetic direct bandgap semiconductor at K, and the value is 0.089 eV. However, for the NH
3@ V
Zn/g−ZnO system, the band structure shows spin up and down bands crossing the Fermi level, indicating that the NH
3@ V
Zn/g−ZnO system exhibits magnetic metallic behavior. For the NO@ V
Zn/g−ZnO system and NO
2@ V
Zn/g−ZnO system, the band structures exhibit semi-metallic properties, with the spin-down band structure crossing the Fermi energy level to exhibit metallic behavior. In contrast, the spin-up band structure maintains the semiconductor properties. The spin-up bandgap values are 1.882 eV and 1.828 eV, respectively.
The band structure of V
O/g−ZnO is displayed in
Figure 6k as a comparison, and shows that V
O/g−ZnO have a non-magnetic direct bandgap (2.208 eV) at Γ. The band structure of the V
O/g−ZnO adsorption system is shown in
Figure 6l–o. From the figure, it can be understood that the CO and NH
3 molecules have little effect on V
O/g−ZnO. These show that the non-magnetic direct semiconductor properties are still maintained, with values of 2.140 eV and 2.151eV, respectively in the CO@ V
O/g−ZnO system and NH
3@ V
O/g−ZnO system. However, for the NO@ V
O/g−ZnO system and NO
2@ V
O/g−ZnO system, the band structure shows spin up and down splitting, indicating the appearance of magnetic behavior. Due to the adsorption of NO and NO
2, the V
O/g−ZnO transforms into a magnetic semiconductor. The bandgap of the NO@ V
O/g−ZnO system and the NO
2@ V
O/g−ZnO system are 0.808 eV and 0.233 eV, respectively.
3.3. Magnetism
To reveal the band structure composition and the origin of the magnetism, the DOS and the spin density are researched. The DOS of gas adsorbed on intrinsic g−ZnO are displayed in
Figure 7. What can be observed is that the states contributed by CO and NH
3 gas molecules have less effect on the intrinsic g−ZnO. However, NO and NO
2 gas molecules have a more significant impact on the DOS of intrinsic g−ZnO. The DOS of the NO@ g−ZnO system shows that the DOS is asymmetric in the upper and lower Brillouin zone. The spin-up impurity levels on both sides of the Fermi level, and those above the Fermi level are mainly contributed by NO gas. It indicates that the adsorption of NO introduces magnetic properties.
The spin density, as shown in
Figure 8a, confirms that the magnetic moment is mainly from NO gas molecules, and the magnetic moment is 1
μB. The Brillouin zone above and below the DOS of the NO
2@ g−ZnO system is also asymmetric. The spin-down impurity level near the Fermi level is mainly contributed by NO
2, and it crosses the Fermi level. And the spin-up is also. This suggests that the adsorption of NO
2 not only introduces magnetic properties but makes the NO
2@ g−ZnO system exhibit metal properties. The spin density shown in
Figure 8b shows that the magnetic of the NO
2@ g−ZnO system comes from the NO
2 gas molecule, but a small part of the magnetic moment comes from the O atom below the NO
2 gas. The magnetic moment is 0.858
μB. Consistent with the band structure results.
The DOS of the V
Zn and V
O g−ZnO adsorption systems are shown in
Figure 9 and
Figure 10, respectively. What can be observed is that the DOS of the upper and lower Brillouin zone of the V
Zn/g−ZnO adsorption systems are asymmetric due to the magnetic properties of V
Zn/g−ZnO, indicating that all adsorption systems possess magnetic properties. Besides, it can be found that CO gas molecules do not contribute to the forbidden band of V
Zn/g−ZnO, and the CO@ V
Zn/g−ZnO system is still a narrow bandgap magnetic semiconductor. Although the NH
3 gas molecules hardly contribute to the total DOS, the NH
3 causes the spin up and down DOS to cross the Fermi level, making the NH
3@ V
Zn/g−ZnO system exhibit metallic behavior. In the NO@ V
Zn/g−ZnO system, NO makes the spin-down DOS cross the Fermi level, making it exhibit semi-metallic properties. The NO
2@ V
Zn/g−ZnO system is similar to the NO@ V
Zn/g−ZnO system, except that the NO
2 gas molecules do not induce the production of impurity energy levels in the V
Zn/g−ZnO forbidden band. Not surprisingly, the magnetic of the CO@, NH
3@, NO@, and NO
2@ V
Zn/g−ZnO systems are mainly contributed by the uncoordinated O atoms around V
Zn, and the gas molecules do not contribute to the magnetic moments in
Figure 8c–f. The magnitudes of the magnetic moments are 1.967
μB, 1.191
μB, 1
μB, and 1
μB.
For the gas adsorption V
O/g−ZnO system, the CO and NH
3 gas molecules have no contribution within the forbidden band of V
O/g−ZnO, and the NH
3 gas molecules have almost no contribution to the total DOS. Therefore, no significant change in DOS compared to that of V
O/g−ZnO. The DOS of the NO@ V
O/g−ZnO system and the NO
2@ V
O/g−ZnO system are asymmetric in the upper and lower Brillouin zone, suggesting that the adsorption of NO and NO
2 gases induces magnetism. In the NO@ V
O/g−ZnO system, the VBM and CBM are mainly contributed by NO gas molecules. However, in the NO
2@ V
O/g−ZnO system, the NO
2 contribution to the in-band DOS of V
O/g−ZnO is not apparent. Correspondingly, the magnetic moments of the NO@ V
O/g−ZnO system, as shown in
Figure 8g, mainly originate from NO gas molecules, while the magnetic moments of the NO
2@ V
O/g−ZnO system mainly come from the Zn atoms near V
O as displayed in
Figure 8h. The magnetic moments of the NO@ V
O/g−ZnO system and NO
2@ V
O/g−ZnO system are 1
μB and 1
μB, respectively.