Nitrogen Dioxide Gas Sensor Based on Ag-Doped Graphene: A First-Principle Study

: High-performance tracking trace amounts of NO 2 with gas sensors could be helpful in protecting human health since high levels of NO 2 may increase the risk of developing acute exacerbation of chronic obstructive pulmonary disease. Among various gas sensors, Graphene-based sensors have attracted broad attention due to their sensitivity, particularly with the addition of noble metals (e.g., Ag). Nevertheless, the internal mechanism of improving the gas sensing behavior through doping Ag is still unclear. Herein, the impact of Ag doping on the sensing properties of Graphene-based sensors is systematically analyzed via ﬁrst principles. Based on the density-functional theory (DFT), the adsorption behavior of speciﬁc gases (NO 2 , NH 3 , H 2 O, CO 2 , CH 4 , and C 2 H 6 ) on Ag-doped Graphene (Ag–Gr) is calculated and compared. It is found that NO 2 shows the strongest interaction and largest Mulliken charge transfer to Ag–Gr among these studied gases, which may directly result in the highest sensitivity toward NO 2 for the Ag–Gr-based gas sensor. on an show that the adsorption energy and Mulliken charge of NO 2 adsorbed on Ag–Gr are both the among all tested NO 2 adsorption, the DOS changes obviously, and the total/difference charge densities illustrate a strong interaction of NO 2 and Ag–Gr. Therefore, Ag–Gr-based sensors exhibit desirable sensitivity toward NO 2 . As for NH 3 and the theoretical analysis demonstrates that the interaction of these two gases with Ag–Gr is relatively weaker than


Introduction
High-performance detecting of toxic gas molecules, such as NO 2 and NH 3 , is crucial for industry application and real-time monitoring of environmental pollutants [1,2], as it is widely agreed that NO 2 can produce acid rain and have adverse effects on plant growth [3,4]. Moreover, long-term exposure to NO 2 can result in human respiratory diseases, such as bronchitis, emphysema, and asthma [5,6]. Consequently, extensive attention and research have been focusing on the development of NO 2 sensors that are based on diverse sensing materials. Over the past decade, various kinds of relevant sensitive materials were announced, which were mainly focused on metal oxide semiconductors [7], transitionmetal dichalcogenides [8][9][10], carbon-based materials [11], and ionic liquids [12][13][14]. For instance, Jang et al. reported 2D NbS 2 nanosheets for selective sensing NO 2 , and studied the sensing mechanisms through (density-functional theory) DFT calculations [8]. In addition, Myung et al. prepared SnO 2 /ZnO heterostructure films for detecting NO 2 , which presented high response performance and quick response/recovery speed [7]. Note that among the aforementioned sensing materials, graphene exhibits many attractive properties such as facile structural control, diversified synthetic methods, and exceptionally large surface areas; graphene in particular, with its physically/chemically stable behavior is competent to endure harsh environments (for example, strong alkali/acidic conditions) [15,16]. Thus, it is believed that graphene would be a good candidate for developing high-performance NO 2 sensors.
To date, pristine and derivatives of graphene have been intensively studied, while the pristine graphene-based sensors suffered the problem of low response signals, long recovery time and poor selectivity toward NO 2 , which restricted its sensing application [11,[17][18][19]. For example, Leenaerts et al. investigated the adsorption properties of NO 2 on pristine graphene via DFT calculation, the adsorption energy is only 0.067 eV and charge transfer is −0.099 e, indicating that the interaction between NO 2 and pristine graphene was weak [20,21]. On the contrary, noble metal nanoparticles (NMPs) doping has proven to be a facile and effective method of improving the response performance of graphene for gas sensing [22]. Typical examples are demonstrated by Zhang et al. who used Au to modify graphene, which showed more sensitivity to H 2 S and SOF 2 than pristine graphene [23]. They also carried out DFT calculations to confirm that graphene modified with Au could enhance the electrochemical reactivity of pristine graphene and activate the adsorbed gas molecules [24]. In 2019, Guha et al. reported that the response of Pt decorated and reduced graphene oxide (rGO) toward 400 ppm HCHO is 4.5 times that of bare rGO [25]. The DFT-based first-principle calculation also illustrated that Pt promoted the chemisorption of HCHO on Pt-rGO, which contributed to improving the sensing performance. In contrast to Pt-rGO, HCHO is only physically adsorbed on bare rGO. In 2020, Zeng et al. reported the simulation based on the first-principle study and demonstrated that Pddoped graphene showed a more outstanding SO 2 adsorption performance than intrinsic graphene [26]. Compared with these frequently reported NMPs dopants (e.g., Au, Pt, Pd) [23,26,27], Ag has more potential due to its nontoxicity, antibacterial properties, high electrical/thermal conductivity, and cost-effectiveness. In our previous work [28], we have experimentally demonstrated that Ag nanoparticles modified reduced graphene oxide materials display outstanding sensing performance toward NO 2 , including excellent sensitivity, fast response/recovery speed, low limit of detection (LOD), and good selectivity. Table 1 summarizes the comparison of the NO 2 sensing performance of different types of graphene-based materials in recent experimental works. Nevertheless, the internal mechanism of the improved gas sensing behavior through doping Ag is still unclear. In view of the fact that DFT has a wide range of applications in physics and chemistry, we especially studied the properties of molecules and condensed matter, such as determined the interactions of molecules in Deep Eutectic Solvents (DES) [29][30][31][32]. Herein, we theoretically illuminate the adsorption properties of NO 2 gas on Ag-Gr via the DFT. In order to reduce the simulation complexity, the graphene is doped by a single Ag atom. There are many DFT studies recently reported on metal-doped graphene, but the selectivity studies are limited [20]. Considering this circumstance, the adsorption performances for NH 3 , H 2 O, CO 2 , and some volatile organic compounds (CH 4 , C 2 H 6 ) are also calculated to evaluate and clarify the selectivity of Ag-Gr. We anticipate that the initiatory theoretical study can provide helpful information and instructive direction on developing high-performance NO 2 sensors using noble metal-doped graphene. a Response = (R g − R a )/R a × 100%, where R g is the resistance in NO 2 ; R a is the resistance in background gas.

Materials and Methods
First principle calculation is implemented in the DMol3 package [43][44][45][46]. Generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) is used. Grimme custom method for DFT-D is chosen for better describing Van der Waals functionalization [47]. DFT Semi-core Pseudopots is used for core treatment, and basis sets DNP with 4.4 files. Monkhorst-pack grid parameters are set as 9 × 9 × 1 for calculating electronic properties. The model contains a 4 × 4 graphene supercell doped with an Ag atom in the middle by replacing a carbon atom. The vacuum with 2 nm spacing in the z-axis direction is adopted for minimizing the interaction of adjacent layers of graphene. Gas molecules are added more than the distance of 0.36 nm above the Ag atom. All configurations are optimized until the energy tolerance is less than 2 × 10 −5 Ha. Besides, NO 2 is a paramagnetic molecule [48,49], spin polarization is carried out in the adsorption calculation of NO 2 on Ag-Gr.
The adsorption energy (E ad ) of gas molecules on Ag-Gr is defined as: where E Ag/G + gas is the total energy after gas adsorbed on the Ag-Gr, E Ag/G is the isolated optimized energy of Ag-Gr, and E gas is the isolated optimized energy of the gas molecule.

Results
For the purpose of investigating the adsorption effect of gas molecules on single Ag-doped graphene, some typical gas molecules are tested. The adsorption distance (d), adsorption energy (E ad ), and Mulliken charge (Q) are calculated and the values are presented in Table 2. Finally, the interaction between gas molecules and Ag-doped graphene will be analyzed in detail. After finishing the Geometry Optimization task, the optimal stable adsorption configurations of all molecules are shown in Figure 1. For NO 2 , the N atom in NO 2 is far from Ag-Gr. The adsorption distance is defined as the nearest distance of atoms between gas molecules and Ag-Gr. Thus, the adsorption distance in NO 2 /Ag-Gr configuration is 0.2224 nm, which is less than the Van der Waals radii (O = 0.155 nm, Ag = 0.177 nm). The adsorption energy is −2.209 eV, which is the largest value among all gas molecules, indicating strong adsorption properties. Meanwhile, the Mulliken transfer charge is −0.45 e, suggesting that the charges transfer from Ag-Gr to NO 2 , where NO 2 acts as an electron acceptor. The Mulliken charge value is also the largest compared with other gases, meaning the super electron capture capability of NO 2 . Ag-Gr is one of the p-type semiconductor materials. When NO 2 molecules as the electron acceptor are adsorbed on Ag-Gr, the resistance of Ag-Gr will dramatically decrease. The total density of states (DOS) is calculated to further study the interaction between NO 2 and Ag-Gr. As presented in Figure 2a, the DOS changes obviously after NO 2 adsorbed on Ag-Gr, especially, near both sides of the Fermi level. Thus, both the largest adsorption energy and Mulliken transfer charge with the addition of dramatic DOS change illuminate stronger interaction between NO 2 and Ag-Gr than others, which is beneficial for gas sensitivity.
x FOR PEER REVIEW 4 of 10 molecules and Ag-Gr. Thus, the adsorption distance in NO2/Ag-Gr configuration is 0.2224 nm, which is less than the Van der Waals radii (O = 0.155 nm, Ag = 0.177 nm). The adsorption energy is −2.209 eV, which is the largest value among all gas molecules, indicating strong adsorption properties. Meanwhile, the Mulliken transfer charge is −0.45 e, suggesting that the charges transfer from Ag-Gr to NO2, where NO2 acts as an electron acceptor. The Mulliken charge value is also the largest compared with other gases, meaning the super electron capture capability of NO2. Ag-Gr is one of the p-type semiconductor materials. When NO2 molecules as the electron acceptor are adsorbed on Ag-Gr, the resistance of Ag-Gr will dramatically decrease. The total density of states (DOS) is calculated to further study the interaction between NO2 and Ag-Gr. As presented in Figure 2a, the DOS changes obviously after NO2 adsorbed on Ag-Gr, especially, near both sides of the Fermi level. Thus, both the largest adsorption energy and Mulliken transfer charge with the addition of dramatic DOS change illuminate stronger interaction between NO2 and Ag-Gr than others, which is beneficial for gas sensitivity. Many recent studies on NO2 sensors based on graphene or graphene derivatives have reported that the sensors have a response to NH3 [50]. Thus, the adsorption performance of NH3 on Ag-Gr should be investigated. After calculation, the adsorption distance of NH3 on Ag-Gr is 0.2297 nm (Figure 1b), also less than Van der Waals radii (N = 0.16 nm, Ag = 0.177 nm). The adsorption energy is −1.115 eV and the Mulliken charge is 0.136 e, which indicates that obvious interaction takes place in the adsorption process, and the charge transfers from NH3 to Ag-Gr. Contrary to NO2, NH3 plays the donor role. When exposed to the NH3 atmosphere, the resistance of p-type Ag-Gr-based gas sensors will increase. The different change direction of resistance caused by NH3 and NO2 can be used for discriminating the two gas molecules. Besides, the DOS near the Fermi level has a small change (Figure 2b). Hence, NH3 on Ag-Gr can slightly change the conductivity of Ag-Gr-based sensor.
Humidity interference in the sensing process often influences the accuracy of the gas sensors. The cross-response of humidity should be suppressed. Therefore, the adsorption properties of water on Ag-Gr are studied. The adsorption distance of H2O on Ag-Gr is 0.2296 nm, less than Van der Waals radii (O = 0.155 nm, Ag = 0.177 nm). The adsorption energy (−0.930 eV) and Mulliken charge (0.122 e) show that the charge transfers from H2O transfer charge of C2H6 is only 0.050 eV, indicating the weak sensitivity of Ag-Gr toward C2H6. For CH4, both adsorption energy and Mulliken transfer charge (0.031 e) are very small, corresponding to the poor response toward CH4 for Ag-Gr. Actually, the gas sensor based on Ag nanoparticles modified reduced graphene oxide has been experimentally demonstrated to have a poor response toward VOCs in our previous work. This characteristic allows Ag-Gr-based NO2 sensors to have good selectivity.  Many recent studies on NO 2 sensors based on graphene or graphene derivatives have reported that the sensors have a response to NH 3 [50]. Thus, the adsorption performance of NH 3 on Ag-Gr should be investigated. After calculation, the adsorption distance of NH 3 on Ag-Gr is 0.2297 nm (Figure 1b), also less than Van der Waals radii (N = 0.16 nm, Ag = 0.177 nm). The adsorption energy is −1.115 eV and the Mulliken charge is 0.136 e, which indicates that obvious interaction takes place in the adsorption process, and the charge transfers from NH 3 to Ag-Gr. Contrary to NO 2 , NH 3 plays the donor role. When exposed to the NH 3 atmosphere, the resistance of p-type Ag-Gr-based gas sensors will increase. The different change direction of resistance caused by NH 3 and NO 2 can be used for discriminating the two gas molecules. Besides, the DOS near the Fermi level has a small change (Figure 2b). Hence, NH 3 on Ag-Gr can slightly change the conductivity of Ag-Gr-based sensor.
Humidity interference in the sensing process often influences the accuracy of the gas sensors. The cross-response of humidity should be suppressed. Therefore, the adsorption properties of water on Ag-Gr are studied. The adsorption distance of H 2 O on Ag-Gr is 0.2296 nm, less than Van der Waals radii (O = 0.155 nm, Ag = 0.177 nm). The adsorption energy (−0.930 eV) and Mulliken charge (0.122 e) show that the charge transfers from H 2 O (donor role) to Ag-Gr. In Figure 2c, after adsorption, the contribution from H 2 O electronic states to the total DOS of Ag-Gr is localized from −9.0 eV to −3.7 eV in valence bands and from 0.4 eV to 5 eV in conduction bands. Although H 2 O can interact with Ag-Gr, the interaction could be weakened by some suitable methods, such as heating through a microheater, which is demonstrated as an effective method in our previous work [28].
In the case of CO 2 , the adsorption distance on Ag-Gr is 0.2626 nm. Figure 2d shows that the contribution of CO 2 electronic states to total DOS is far away from the Fermi level. The adsorption energy is −0.360 eV, and the Mulliken transfer charge is only 0.018 e. The weak charge transfer indicates that the sensor based on Ag-Gr is not sensitive to CO 2 .
For organic volatile compounds (VOCs), such as CH 4 and C 2 H 6 , the adsorption distances on Ag-Gr are 0.2218 nm and 0.2395 nm, respectively. From Figure 2e-f, the contributions of CH 4 and C 2 H 6 electronic levels are both far away from the Fermi level. The adsorption energy of the two gas is −0.335 eV (CH 4 ) and −0.514 eV (C 2 H 6 ). The Mulliken transfer charge of C 2 H 6 is only 0.050 eV, indicating the weak sensitivity of Ag-Gr toward C 2 H 6 . For CH 4 , both adsorption energy and Mulliken transfer charge (0.031 e) are very small, corresponding to the poor response toward CH 4 for Ag-Gr. Actually, the gas sensor based on Ag nanoparticles modified reduced graphene oxide has been experimentally demonstrated to have a poor response toward VOCs in our previous work. This characteristic allows Ag-Gr-based NO 2 sensors to have good selectivity.
The total charge densities are calculated for further investigating the interaction of gas molecules and Ag-Gr. As shown in Figure 3a, a distinct electron orbital overlap between Ag-Gr and NO 2 molecules is observed. The strong overlap is consistent with the intense interaction once the Ag-Gr-based gas sensor is exposed to the NO 2 polluted air. Additionally, the difference charge densities are also performed. Figure 4 shows the difference charge densities of gas molecules' adsorption on Ag-Gr. The colors blue and red are represented as losing electrons and obtaining electrons, respectively. As seen from Figure 4a, the conspicuous red regions surround O atoms of NO 2 , and at the same moment, the obvious blue region surrounds the Ag atom. The electrons enrichment areas around O atoms nearly contact with the core electrons area around the Ag atom. In this case, strong interaction occurs between Ag and O atoms. Consequently, adsorbed NO 2 will significantly change the electrical conductivity of Ag-Gr, resulting in outstanding sensitivity. Compared with NO 2 , the electron orbital overlap between both NH 3 and H 2 O with Ag-Gr are smaller (Figure 3b-c), indicating a low sensitivity for Ag-Gr-based gas sensors. From difference charge densities plots in Figure 4b-c, we can see that the blue regions are obvious, and small red regions appear around both NH 3 and H 2 O molecules. This is due to the donor properties of these two types of molecules. As for CO 2 and VOCs, shown in Figure 3d-f, it is noted that no electron orbital overlap exists between the other three gases (CO 2 , CH 4 , and C 2 H 6 ) with Ag-Gr, demonstrating that the adsorption of CO 2 , CH 4 , and C 2 H 6 on Ag-Gr layers are physisorption, resulting in the facts of very few charges transferring between gas molecules and Ag-Gr. This could be one of the main reasons that an Ag-Grbased gas sensor is not sensitive to VOCs. From the six inserts of deformation electron density isosurfaces in Figure 3, we can also obtain consistent results, because the electron accumulation (pink) and depletion (yellow) regions between NO 2 and Ag are more obvious than those between other gas molecules and Ag.

Conclusions and Discussions
First-principle calculations are implemented to theoretically study the gas adsorption properties on an Ag-Gr surface. The results show that the adsorption energy and Mulliken transfer charge of NO2 adsorbed on Ag-Gr are both the largest among all tested gases. After NO2 adsorption, the DOS changes obviously, and the total/difference charge densities illustrate a strong interaction of NO2 and Ag-Gr. Therefore, Ag-Gr-based sensors exhibit desirable sensitivity toward NO2. As for NH3 and humidity, the theoretical analysis demonstrates that the interaction of these two gases with Ag-Gr is relatively weaker than that of NO2. In addition to responding to NO2, some experimental reports also show that

Conclusions and Discussions
First-principle calculations are implemented to theoretically study the gas adsorption properties on an Ag-Gr surface. The results show that the adsorption energy and Mulliken transfer charge of NO 2 adsorbed on Ag-Gr are both the largest among all tested gases. After NO 2 adsorption, the DOS changes obviously, and the total/difference charge densities illustrate a strong interaction of NO 2 and Ag-Gr. Therefore, Ag-Gr-based sensors exhibit desirable sensitivity toward NO 2 . As for NH 3 and humidity, the theoretical analysis demonstrates that the interaction of these two gases with Ag-Gr is relatively weaker than that of NO 2 . In addition to responding to NO 2 , some experimental reports also show that graphene and its derivatives have a weak response to ammonia and water [37,[51][52][53][54]. The potential interference of NH 3 and humidity can be suppressed by heating or UV illuminating the sensor. As for CO 2 and VOCs (CH 4 and C 2 H 6 ), the small Mulliken transfer charge and no electron orbital overlap between Ag-Gr and gas molecules indicate the physisorption properties and poor sensitivity, corresponding to the good selectivity to NO 2 of Ag-Gr-based sensors. Conclusively, although there will be a minor deviation in our theoretical analysis, we still anticipate that these results will be helpful in providing instructive direction on designing future high-performance sensing materials.