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Article

Effects of Vacancy Defects and the Adsorption of Toxic Gas Molecules on Electronic, Magnetic, and Adsorptive Properties of g−ZnO: A First-Principles Study

by
Yang Shen
1,*,
Zhihao Yuan
1,
Zhen Cui
2,*,
Deming Ma
1,
Pei Yuan
1,
Kunqi Yang
1,
Yanbo Dong
1,
Fangping Wang
1 and
Enling Li
1,*
1
School of Science, Xi’an University of Technology, Xi’an 710054, China
2
School of Automation and Information Engineering, Xi’an University of Technology, Xi’an 710048, China
*
Authors to whom correspondence should be addressed.
Chemosensors 2023, 11(1), 38; https://doi.org/10.3390/chemosensors11010038
Submission received: 25 November 2022 / Revised: 23 December 2022 / Accepted: 28 December 2022 / Published: 2 January 2023
(This article belongs to the Special Issue Dedicated to Professor Giorgio Sberveglieri on the Occasion of His 75th Birthday for His Outstanding Contributions to the Field of Chemical Sensors)
Browse Figures
Versions Notes

Abstract

:
Using first principles based on density functional theory (DFT), the CO, NH3, NO, and NO2 gas adsorbed on intrinsic Graphite-like ZnO (g−ZnO) and vacancy-deficient g−ZnO were systematically studied. For intrinsic g−ZnO, the adsorption energy of NH3, NO, and NO2 adsorption defective g−ZnO systems increased significantly due to the introduction of Zn vacancy (VZn). Especially, for NH3, NO, and NO2 adsorbed Zn-vacancy g−ZnO (VZn/g−ZnO) systems increased to 1.366 eV, 2.540 eV and 2.532 eV, respectively. In addition, with the introduction of vacancies, the adsorption height of the gases adsorbed on VZn/g−ZnO system is significantly reduced, especially the adsorption height of the NH3 adsorbed on VZn/g−ZnO system is reduced to 0.686 Å. It is worth mentioning that the introduction of O-vacancy (VO) significantly enhances the charge transfer between NO or NO2 and VO/g−ZnO. This suggest that the defective g−ZnO is more suitable for detecting NH3, NO and NO2 gas. It is interesting to note that the adsorption of NO and NO2 gases gives rise to magnetic moments of 1 μB and 0.858 μB for g−ZnO, and 1 μB and 1 μB for VO/g−ZnO. In addition, VZn induced 1.996 μB magnetic moments for intrinsic g−ZnO, and the CO, NH3, NO and NO2 change the magnetic of VZn/g−ZnO. The adsorption of NO2 causes the intrinsic g−ZnO to exhibit metallic properties, while the adsorption of NH3 gas molecules causes VZn/g−ZnO also to show metallic properties. The adsorption of NO and NO2 causes VZn/g−ZnO to display semi-metallic properties. These results facilitate the enrichment of defect detection means and the design of gas detection devices.

1. Introduction

Zinc Oxide (ZnO) has various dimensional anamorphs, such as zero-dimensional nanocrystals [1], one-dimensional nanowires [2], and two-dimensional (2D) nanofilms [3,4], which has a direct bandgap with the value of 3.37 eV [5,6]. In particular, 2D ZnO possesses high chemical or thermal stability [7,8,9], which is essential in the fields of gas detector devices [10,11], high-efficiency UV laser emitter devices at room temperature [12,13], etc. Therefore, 2D ZnO is rapidly becoming a recent research hotspot [14].
After Claeyssens et al. [15] revealed that graphene-like ZnO (g−ZnO) has a stable structure through theoretical calculation, Tusche et al. [8] experimentally verified that ZnO with a fibrillated ZnO structure could be converted to g−ZnO with a stable structure when the thickness is small enough (A few atoms). Sahoo et al. [16] synthesized planar 2D ZnO by hydrothermal method and revealed the formation of a 2D honeycomb lattice and the aggregated structure of layered ZnO. Altuntasoglu et al. [17] successfully prepared ZnO nanosheets by delamination of layered ZnO films. Qin et al. [18] prepared LaCoO3 modified ZnO nanosheet materials with great improvement in gas sensitivity. Shen et al. [19] recently prepared ZnO and carbon materials composite aerogels for enhancing the photocatalytic properties of ZnO. Shishiyanu et al. [20] prepared Sn-doped ZnO films, and they found good selectivity for NO2 gas.
In addition, theoretical research on g−ZnO modification is also developing speedily. Cui et al. [21] covered that the formation of MoSSe/ZnO heterojunction structure changes the optical absorption properties of g−ZnO. Wang et al. [22] found that MoS2/ZnO vdW heterostructure has application potential in photovoltaic and photocatalytic devices. Shen et al. [23] found that molecular doping (organic molecules) could achieve effective p-type doping. Guo et al. [24] found that the adjustable magnetic and electronic properties of monolayer ZnO can be achieved by nonmetallic doping. Theoretical studies to improve the gas detection performance of g−ZnO have also been widely followed by scholars. Meng et al. [5] investigated graphene (MoS2) as a heterogeneous layer material stacked with g−ZnO to enhance interaction with NH3 gas, and found that g−ZnO and its homogeneous and heterogeneous bilayer structures could be a candidate for gas-sensitive materials. However, there have been relatively few studies on g−ZnO and vacancy defective g−ZnO modifications for adsorption and detection of toxic gases [25].
Here, the gas molecules (CO, NH3, NO, and NO2) adsorbed on intrinsic g−ZnO and vacancy g−ZnO were systematically researched. The influence of defects on the interaction between g−ZnO and gas molecules and on the electronic properties is revealed by the adsorption energy, adsorption height, charge density difference (CDD), band structure, the density of states (DOS) and spin density (magnetic system) of the system, and the origin of the magnetism is also demonstrated. Our results provide theoretical guidance for the design of gas detection devices.

2. Computation Methods and Models

Using the Vienna ab initio simulation package (VASP) [26] to research the adsorption behavior of gas (CO, NH3, NO, and NO2) on intrinsic and defective g−ZnO. Electron exchange and correlation are achieved by the Perdew-Burke-Ernzernhof (PBE) function [27], while electron-ion interactions use the projection-enhanced wave method [28]. The DFT-3 method describes weak dispersion forces between layers [29,30]. The truncation energy of the plane wave is set to 500 eV [31]. The Brillouin zone is described by 3 × 3 × 1 K-points centered on Γ point. The vacuum layer of 20 Å is set to avoid interactions between periodic structures [32,33]. Additionally, the data export and processing are realized by the VASPKIT code [34].
In addition, the adsorption energy is calculated as follows [35,36]:
E ad = E total − E I/D-g-ZnO − E G
where Ead is the adsorption energy, Etotal and EG represent the energy of the adsorption system and gas, respectively. EI/D-g−ZnO represents the energy of intrinsic g−ZnO or defective g−ZnO. CDD can describe the distribution of charge transfer between gas and intrinsic or vacancy g−ZnO, and is calculated by [37]:
Δ ρ = ρ total − ρ I/D-g-ZnO − ρ G
where ρtotal and ρG represent the charge density (CD) of the adsorbed system and gas, respectively. ρI/D-g−ZnO represents the CD of intrinsic g−ZnO or defective g−ZnO. The spin density is calculated as follows [38,39]:
ρ = ρ spin-up − ρ spin-down
where ρspin-up and ρspin-down represent the spin up and down of the magnetic systems, respectively.
The 4 × 4 × 1 supercells are studied both intrinsic g−ZnO and defective g−ZnO. The difference is that defective g−ZnO has a Zinc vacancy (VZn) or Oxygen vacancy (VO). All atoms are entirely relaxed until the Hellmann-Feynman force is less than 10−2 eV Å−1, and the total energy change is below 10−5 eV [40].

3. Results and Discussion

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 (VZn/g−ZnO), O atoms around the VZn are all far away from the vacancy canter. Moreover, the Zn-O bond length around the VZn 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 VZn 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 VZn/g−ZnO increases to 133.79°, and our results are similar. However, for the Oxygen vacancy g−ZnO (VO/g−ZnO), the three Zn atoms around the VO are all near the vacancy center. The O-Zn bond length around VO 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 VO 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 VZn/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 VZn induces the production of magnetic. The VZn/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 VO/g−ZnO were calculated and displayed in Figure 1f. It can be observed that no magnetic properties are generated after the introduction of VO. 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, NH3, NO, and NO2) adsorbed on intrinsic g−ZnO system is shown in Figure 2a–e. Overall, CO, NH3, 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 NO2 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, NH3, NO, and NO2 molecules are AC, AC, AZn, and AO, respectively. The most stable configurations of gas adsorbed on the VZn/g−ZnO system and the VO/g−ZnO system are shown in Figure 2f–h and 2i–l, respectively. For the gas adsorbed on VZn/g−ZnO systems, gas molecules are tilted to VZn/g−ZnO, and nearly embedded in VZn/g−ZnO. For CO molecules, the C atom is closer to the VZn/g−ZnO, while the H atom is for NH3 molecules. For NO and NO2 molecules, the N atom is closer to the VZn/g−ZnO plane. For the gas adsorbed on VO/g−ZnO systems, all gas molecules are inclined to VO/g−ZnO. The difference is that the N atoms of the NH3 is closer to the VO/g−ZnO. And, for the NO2 molecule, the O atom is closer to the VO/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 VZn/g−ZnO adsorbed by CO, NH3, NO, and NO2 systems are higher than other adsorption systems. It shows that VZn/g−ZnO is suitable for detecting four gas molecules [46]. In addition, the intrinsic g−ZnO showed the best detection capability for NH3 gas molecules. In comparison, due to the introduction of defects, the VZn/g−ZnO adsorption systems showed the most significant improvement in detecting NO and NO2 gas. And VO/g−ZnO adsorption systems showed the most remarkable enhancement in detecting NO2 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 NO2 gas (NO2@ 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 NH3 gas adsorbed on the intrinsic g−ZnO (NH3@ 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 VZn/g−ZnO systems, the adsorption heights of the VZn/g−ZnO systems for CO, NH3, NO, and NO2 adsorption are 1.219 Å, 0.686 Å, 0.910 Å, and 0.858 Å, respectively. The adsorption height of the VZn/g−ZnO adsorbed by CO (CO@ VZn/g−ZnO) system is greater than the O-C bond length (1.136 Å [53]), and its smaller adsorption energy demonstrates physical adsorption. The NH3 adsorbed on VZn/g−ZnO (NH3@ VZn/g−ZnO) system, the NO adsorbed on VZn/g−ZnO (NO@ VZn/g−ZnO) system, and NO2 adsorbed on VZn/g−ZnO (NO2@ VZn/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 VO/g−ZnO systems, the adsorption height of the VO/g−ZnO adsorbed by CO (CO@ VO/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 NH3 adsorbed on VO/g−ZnO (NH3@ VO/g−ZnO) system, NO adsorbed on VO/g−ZnO (NO@ VO/g−ZnO) system, and NO2 adsorbed on VO/g−ZnO (NO2@ VO/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 NH3@ g−ZnO system, the electron is mainly distributed between the NH3 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 NO2@ g−ZnO system, it is similar to the NO system, but the difference is that NO2 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 NO2 molecules act as electron acceptors, receiving 0.007e, 0.083e, and 0.252e from the intrinsic g−ZnO, respectively. Since the polarity of NO2 molecules is higher than that of CO and NO molecules, the charge transfer of the NO2@ g−ZnO system is significantly higher than that of the CO@ g−ZnO and NO@ g−ZnO systems. However, NH3 molecules act as electron donors with 0.111e charge transfer to the intrinsic g−ZnO.
The CDD notations for the gas molecule adsorption VZn/g−ZnO systems and the gas molecule adsorption VO/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@ VZn/g−ZnO system and CO@ VO/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 NH3 adsorbed defective g−ZnO systems, NH3 still acts as an electron donor, transferring 0.110e and 0.101e to the VZn/g−ZnO and VO/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@ VZn/g−ZnO system, NO was converted from an electron acceptor to an electron donor and provided 0.110e to the VZn/g−ZnO layer. In the NO@ VO/g−ZnO system, NO receives 0.312e from the VO/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 NO2@ VZn/g−ZnO system. 0.017e is transferred from the VZn/g−ZnO to the NO2 gas. The charge density around NO2 decreased substantially compared to the NO2@ g−ZnO system. In contrast, in the NO2@ VO/g−ZnO system, NO2 acts as an electron acceptor and receives 0.626e from the VO/g−ZnO layer, and the amount of charge transfer and the charge density around NO2 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 NH3@ 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 WS2 adsorption by CO and NH3 gases. For the NO@ g−ZnO system and NO2@ g−ZnO system, a splitting of the spin up and down bands can be noted, indicating that the adsorption of NO and NO2 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 NO2@ 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 NO2@ g−ZnO system, a spin-down impurity energy level appears near the Fermi energy level, which may be provided by NO2. Similarly, in NO and NO2 adsorption of transition metal-doped MoS2 has been reported by Salih et al. [56]. A similar statement was made in the study of gas adsorption of WS2 by Zhou et al. [55]
The band structure of VZn/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 VZn/g−ZnO adsorption system is shown in Figure 6g–j. It can be observed that for the CO@ VZn/g−ZnO system, there is no significant change compared to that of VZn/g−ZnO, which is still a magnetic direct bandgap semiconductor at K, and the value is 0.089 eV. However, for the NH3@ VZn/g−ZnO system, the band structure shows spin up and down bands crossing the Fermi level, indicating that the NH3@ VZn/g−ZnO system exhibits magnetic metallic behavior. For the NO@ VZn/g−ZnO system and NO2@ VZn/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 VO/g−ZnO is displayed in Figure 6k as a comparison, and shows that VO/g−ZnO have a non-magnetic direct bandgap (2.208 eV) at Γ. The band structure of the VO/g−ZnO adsorption system is shown in Figure 6l–o. From the figure, it can be understood that the CO and NH3 molecules have little effect on VO/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@ VO/g−ZnO system and NH3@ VO/g−ZnO system. However, for the NO@ VO/g−ZnO system and NO2@ VO/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 NO2, the VO/g−ZnO transforms into a magnetic semiconductor. The bandgap of the NO@ VO/g−ZnO system and the NO2@ VO/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 NH3 gas molecules have less effect on the intrinsic g−ZnO. However, NO and NO2 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 NO2@ g−ZnO system is also asymmetric. The spin-down impurity level near the Fermi level is mainly contributed by NO2, and it crosses the Fermi level. And the spin-up is also. This suggests that the adsorption of NO2 not only introduces magnetic properties but makes the NO2@ g−ZnO system exhibit metal properties. The spin density shown in Figure 8b shows that the magnetic of the NO2@ g−ZnO system comes from the NO2 gas molecule, but a small part of the magnetic moment comes from the O atom below the NO2 gas. The magnetic moment is 0.858 μB. Consistent with the band structure results.
The DOS of the VZn and VO 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 VZn/g−ZnO adsorption systems are asymmetric due to the magnetic properties of VZn/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 VZn/g−ZnO, and the CO@ VZn/g−ZnO system is still a narrow bandgap magnetic semiconductor. Although the NH3 gas molecules hardly contribute to the total DOS, the NH3 causes the spin up and down DOS to cross the Fermi level, making the NH3@ VZn/g−ZnO system exhibit metallic behavior. In the NO@ VZn/g−ZnO system, NO makes the spin-down DOS cross the Fermi level, making it exhibit semi-metallic properties. The NO2@ VZn/g−ZnO system is similar to the NO@ VZn/g−ZnO system, except that the NO2 gas molecules do not induce the production of impurity energy levels in the VZn/g−ZnO forbidden band. Not surprisingly, the magnetic of the CO@, NH3@, NO@, and NO2@ VZn/g−ZnO systems are mainly contributed by the uncoordinated O atoms around VZn, 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 VO/g−ZnO system, the CO and NH3 gas molecules have no contribution within the forbidden band of VO/g−ZnO, and the NH3 gas molecules have almost no contribution to the total DOS. Therefore, no significant change in DOS compared to that of VO/g−ZnO. The DOS of the NO@ VO/g−ZnO system and the NO2@ VO/g−ZnO system are asymmetric in the upper and lower Brillouin zone, suggesting that the adsorption of NO and NO2 gases induces magnetism. In the NO@ VO/g−ZnO system, the VBM and CBM are mainly contributed by NO gas molecules. However, in the NO2@ VO/g−ZnO system, the NO2 contribution to the in-band DOS of VO/g−ZnO is not apparent. Correspondingly, the magnetic moments of the NO@ VO/g−ZnO system, as shown in Figure 8g, mainly originate from NO gas molecules, while the magnetic moments of the NO2@ VO/g−ZnO system mainly come from the Zn atoms near VO as displayed in Figure 8h. The magnetic moments of the NO@ VO/g−ZnO system and NO2@ VO/g−ZnO system are 1 μB and 1 μB, respectively.

4. Conclusions

In summary, the adsorption of CO, NH3, NO, and NO2 gas molecules on intrinsic g−ZnO and vacancy g−ZnO were systematically studied by the first principle based on DFT. the Ead, adsorption height, CDD, band structure, DOS, and spin density of the magnetic systems for each adsorption system were considered. The effect of vacancies on the interaction between g−ZnO and gas, and on the electronic properties were investigated. In addition, the source of magnetic generation is revealed. The results show that the adsorption energy is promoted, and the adsorption height is reduced due to the introduction of defects. Compared with the NH3@, NO@, and NO2@ g−ZnO systems, the Ead of the NH3@, NO@, and NO2@ VZn/g−ZnO systems increased to 1.366 eV, 2.540 eV, and 2.532 eV, respectively. The adsorption height was significantly reduced to 0.686 Å for the NH3@ VZn/g−ZnO system. It is worth mentioning that the NH3, NO, and NO2 gas molecules adsorbed on the defective g−ZnO system are converted to chemical adsorption due to the larger adsorption energy and smaller adsorption height. The CDD indicate that the introduction of VO significantly enhances the charge transfer between NO and NO2 and VO/g−ZnO. The band structure, DOS, and spin density results show that the introduction of VZn splits the spin-up band and spin-down band structures, producing a magnetic moment value of 1.996 μB for g−ZnO. NO and NO2 gases can induce the magnetic properties of intrinsic g−ZnO (1 μB and 0.858 μB) and VO/g−ZnO (1 μB and 1 μB), while the adsorption of gases can degrade the magnetic properties of VZn/g−ZnO itself (1.967 μB, 1.191 μB, 1 μB, and 1 μB). Notably, the adsorption of NO2 causes the intrinsic g−ZnO to show metallic properties, while the adsorption of NH3 gas molecules causes VZn/g−ZnO to also exhibit metallic properties. The adsorption of NO and NO2 causes VZn/g−ZnO to exhibit semi-metallic properties. These are beneficial to enrich the detection means of defects and the design of gas detection devices [57].

Author Contributions

Conceptualization, Y.S., Z.C. and E.L.; methodology, D.M.; software, Z.C.; validation, Y.S., Z.C., D.M., P.Y., K.Y., Y.D., F.W. and E.L.; formal analysis, Z.Y.; investigation, Z.Y., P.Y., K.Y., Y.D. and F.W.; resources, Z.C. and E.L.; data curation, Z.Y.; writing—original draft preparation, Y.S. and Z.Y.; writing—review and editing, Y.S.; visualization, Z.Y.; supervision, Y.S., Z.C. and E.L.; project administration, E.L.; funding acquisition, Y.S. and Z.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Natural Science Basic Research Program of Shaanxi (Program No. 2022JM-176), Scientific Research Program Funded by Shaanxi Provincial Education Department (Program No. 21JK0789), Xi’an Science and Technology Project (Program No. 22GXFW0080), the Opening Project of Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology (Program No. ammt2020A-6), College Students’ Innovative Entrepreneurial Training Plan Program (Program No. 202210700008 and Program No. 202210700105), the National Natural Science Foundation of China (No. 12104362) and China Postdoctoral Science Foundation (Program No. 2020M683684XB).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) The crystal structure and (b) DOS of intrinsic g−ZnO. AZn is above the Zn atom. (c) The crystal structure and (d) the DOS of Zinc vacancy g−ZnO (VZn/g−ZnO), and (e) The crystal structure and (f) the DOS of Oxygen vacancy g−ZnO (VO/g−ZnO). AO is upper the O atom, AM is upper the bond of Zn-O, and AC is upper the center of the hexagonal structure. VZn is upper the Zn vacancy, and VO is above the O vacancy. The pink spheres represent Zn atoms, and the grey spheres represent O atoms. The Fermi level is shifted to 0 eV.
Figure 1. (a) The crystal structure and (b) DOS of intrinsic g−ZnO. AZn is above the Zn atom. (c) The crystal structure and (d) the DOS of Zinc vacancy g−ZnO (VZn/g−ZnO), and (e) The crystal structure and (f) the DOS of Oxygen vacancy g−ZnO (VO/g−ZnO). AO is upper the O atom, AM is upper the bond of Zn-O, and AC is upper the center of the hexagonal structure. VZn is upper the Zn vacancy, and VO is above the O vacancy. The pink spheres represent Zn atoms, and the grey spheres represent O atoms. The Fermi level is shifted to 0 eV.
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Figure 2. The stable structures of (a) CO, (b) NH3, (c) NO, or (d) NO2 adsorbed on intrinsic g−ZnO systems, (e–h) VZn/g−ZnO systems, and (i–l) VO/g−ZnO systems. The pink, grey, yellow, orange, and blue balls represent Zn, O, C, H, and N atoms, respectively.
Figure 2. The stable structures of (a) CO, (b) NH3, (c) NO, or (d) NO2 adsorbed on intrinsic g−ZnO systems, (e–h) VZn/g−ZnO systems, and (i–l) VO/g−ZnO systems. The pink, grey, yellow, orange, and blue balls represent Zn, O, C, H, and N atoms, respectively.
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Figure 3. The absorption energy of gas molecules on intrinsic g−ZnO, VZn/g−ZnO, and VO/g−ZnO systems.
Figure 3. The absorption energy of gas molecules on intrinsic g−ZnO, VZn/g−ZnO, and VO/g−ZnO systems.
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Figure 4. The absorption height of gas molecules on intrinsic g−ZnO, VZn/g−ZnO, and VO/g−ZnO systems.
Figure 4. The absorption height of gas molecules on intrinsic g−ZnO, VZn/g−ZnO, and VO/g−ZnO systems.
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Figure 5. The isosurface of the CDD with a value of 0.001 e Å−3 for the (a) CO, (b) NH3, (c) NO, or (d) NO2 adsorbed on intrinsic g−ZnO systems, (e–h) for the VZn/g−ZnO adsorption systems, and (i–l) for the VO/g−ZnO adsorption systems. The white area and peachy red area represent the depletion and accumulation of electrons, respectively. The pink, grey, yellow, orange, and blue balls are the Zn, O, C, H, and N atoms, respectively.
Figure 5. The isosurface of the CDD with a value of 0.001 e Å−3 for the (a) CO, (b) NH3, (c) NO, or (d) NO2 adsorbed on intrinsic g−ZnO systems, (e–h) for the VZn/g−ZnO adsorption systems, and (i–l) for the VO/g−ZnO adsorption systems. The white area and peachy red area represent the depletion and accumulation of electrons, respectively. The pink, grey, yellow, orange, and blue balls are the Zn, O, C, H, and N atoms, respectively.
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Figure 6. The band structures of (a) intrinsic g−ZnO, and (b) CO, (c) NH3, (d) NO, or (e) NO2 adsorbed on intrinsic g−ZnO systems. The band structures of (f) VZn/g−ZnO, and (g) CO, (h) NH3, (i) NO, or (j) NO2 adsorbed on VZn/g−ZnO systems. The band structures of (k) VO/g−ZnO, and (l) CO, (m) NH3, (n) NO, or (o) NO2 adsorbed on VO/g−ZnO systems. The dark blue and pink lines denote spin up and down, respectively. The Fermi level is set to 0 eV.
Figure 6. The band structures of (a) intrinsic g−ZnO, and (b) CO, (c) NH3, (d) NO, or (e) NO2 adsorbed on intrinsic g−ZnO systems. The band structures of (f) VZn/g−ZnO, and (g) CO, (h) NH3, (i) NO, or (j) NO2 adsorbed on VZn/g−ZnO systems. The band structures of (k) VO/g−ZnO, and (l) CO, (m) NH3, (n) NO, or (o) NO2 adsorbed on VO/g−ZnO systems. The dark blue and pink lines denote spin up and down, respectively. The Fermi level is set to 0 eV.
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Figure 7. The DOS of (a) CO, (b) NH3, (c) NO, or (d) NO2 adsorbed on intrinsic g−ZnO systems. The Fermi level is shifted to 0 eV and respected by a black dash line.
Figure 7. The DOS of (a) CO, (b) NH3, (c) NO, or (d) NO2 adsorbed on intrinsic g−ZnO systems. The Fermi level is shifted to 0 eV and respected by a black dash line.
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Figure 8. The spin density of (a) NO, or (b) NO2 molecules adsorbed on intrinsic g−ZnO, of (g,h) VO/g−ZnO adsorption systems, and of (c) CO, (d) NH3, (e) NO, or (f) NO2 molecules adsorbed on VZn/g−ZnO systems. The peachy red area denotes the spin-up, whereas the white area represents the spin-down. The pink, grey, yellow, orange, and blue balls are the Zn, O, C, H, and N atoms, respectively.
Figure 8. The spin density of (a) NO, or (b) NO2 molecules adsorbed on intrinsic g−ZnO, of (g,h) VO/g−ZnO adsorption systems, and of (c) CO, (d) NH3, (e) NO, or (f) NO2 molecules adsorbed on VZn/g−ZnO systems. The peachy red area denotes the spin-up, whereas the white area represents the spin-down. The pink, grey, yellow, orange, and blue balls are the Zn, O, C, H, and N atoms, respectively.
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Figure 9. DOS of (a) CO, (b) NH3, (c) NO, or (d) NO2 adsorbed on VZn/g−ZnO systems.
Figure 9. DOS of (a) CO, (b) NH3, (c) NO, or (d) NO2 adsorbed on VZn/g−ZnO systems.
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Figure 10. DOS of (a) CO, (b) NH3, (c) NO, or (d) NO2 adsorbed on VO/g−ZnO systems.
Figure 10. DOS of (a) CO, (b) NH3, (c) NO, or (d) NO2 adsorbed on VO/g−ZnO systems.
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Table
Table 1. The magnetic moments (Mtotal), bandgap (Eg), and charge transfer (ΔQ) for the stable configurations of gas adsorbed on intrinsic g−ZnO, VZn/g−ZnO, and VO/g−ZnO.
Table 1. The magnetic moments (Mtotal), bandgap (Eg), and charge transfer (ΔQ) for the stable configurations of gas adsorbed on intrinsic g−ZnO, VZn/g−ZnO, and VO/g−ZnO.
Adsorption System TypeConfigurationMtotal (μB)Eg (eV)ΔQ (e)
IntrinsicCO01.6830.007
NH301.645−0.111
NO11.7170.083
NO20.85800.252
Zn-vacancyCO1.9670.0890.019
NH31.1910−0.110
NO11.882−0.110
NO211.8280.017
O-vacancyCO02.1400.012
NH302.151−0.101
NO10.8080.312
NO210.2330.626
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Shen, Y.; Yuan, Z.; Cui, Z.; Ma, D.; Yuan, P.; Yang, K.; Dong, Y.; Wang, F.; Li, E. Effects of Vacancy Defects and the Adsorption of Toxic Gas Molecules on Electronic, Magnetic, and Adsorptive Properties of g−ZnO: A First-Principles Study. Chemosensors 2023, 11, 38. https://doi.org/10.3390/chemosensors11010038

AMA Style

Shen Y, Yuan Z, Cui Z, Ma D, Yuan P, Yang K, Dong Y, Wang F, Li E. Effects of Vacancy Defects and the Adsorption of Toxic Gas Molecules on Electronic, Magnetic, and Adsorptive Properties of g−ZnO: A First-Principles Study. Chemosensors. 2023; 11(1):38. https://doi.org/10.3390/chemosensors11010038

Chicago/Turabian Style

Shen, Yang, Zhihao Yuan, Zhen Cui, Deming Ma, Pei Yuan, Kunqi Yang, Yanbo Dong, Fangping Wang, and Enling Li. 2023. "Effects of Vacancy Defects and the Adsorption of Toxic Gas Molecules on Electronic, Magnetic, and Adsorptive Properties of g−ZnO: A First-Principles Study" Chemosensors 11, no. 1: 38. https://doi.org/10.3390/chemosensors11010038

APA Style

Shen, Y., Yuan, Z., Cui, Z., Ma, D., Yuan, P., Yang, K., Dong, Y., Wang, F., & Li, E. (2023). Effects of Vacancy Defects and the Adsorption of Toxic Gas Molecules on Electronic, Magnetic, and Adsorptive Properties of g−ZnO: A First-Principles Study. Chemosensors, 11(1), 38. https://doi.org/10.3390/chemosensors11010038

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