## 3 Results
### Adsorption of CO, NO2, and SO2 on Ag-WS2 Monolayer  
Adsorption studies involving CO, NO2, and SO2 molecules on Ag-WS2 were conducted using the Ag atom as the active adsorption site. Various adsorption directions were considered for different molecules. For diatomic molecules, such as CO, the adsorption direction options included C downward and O downward. For symmetrical triatomic molecules (NO2 and SO2), two directions were examined: central atom downward and edge atom downward. Following complete geometric optimization, four adsorption configurations with the minimum total energy were identified. These adsorption parameters are presented in Table 1, and the corresponding configurations are illustrated in Figure 3. For NO2 adsorption, only the O downward configuration with higher adsorption energy was analyzed in greater detail. Concerning CO adsorption on the Ag-WS2 monolayer, the CO molecule was observed to be nearly perpendicular to the surface, with C facing downward, as shown in Figure 3a. The calculated adsorption energy was approximately \(-\)0.73 eV. The distance between the Ag and C atoms measured 2.13 A, significantly smaller than the  
Figure 2: Electronic properties and stability of Ag-WS2: (**a**) CDD of Ag-WS2 (the isosurface is 0.01 e/Å\({}^{3}\), and the pink and yellow represent the electron accumulation and depletion region, respectively); (**b1,b2**) band structure of pristine WS2 monolayer and Ag-doped WS2 monolayer; (**c1,c2**) DOS of Ag-WS2, and atomic orbitals of Ag and one of the adjacent W atoms; (**d1,d2**) thermal stability of Ag-WS2 monolayer at 500 K.  
sum of van der Waals radii of the Ag and C atoms (1.72 A and 1.70 A, respectively) [50]. This phenomenon suggests the potential formation of a newly formed Ag-C chemical bond after CO adsorption. Hirshfeld analysis indicated that CO loses 0.08 e and gains a positive charge during the adsorption process, implying that CO functions as an electron donor. Upon NO\({}_{2}\) adsorption, two configurations were identified. The O downward configuration exhibits a calculated adsorption energy of \(-\)1.40 eV, while the N downward configuration displayed an adsorption energy of \(-\)1.11 eV. In comparison, NO\({}_{2}\) demonstrated a preference for adsorption on the Ag site, with O facing downward, as depicted in Figure 3b. This study focused solely on adsorption configurations with higher adsorption energies for NO\({}_{2}\). The distance between Ag and O was measured at 2.34 A, also smaller than the sum of van der Waals radii of Ag and O atoms (1.72 A and 1.52 A, respectively) [50], indicating potential chemical interactions between NO\({}_{2}\) and the Ag atom. During this process, NO\({}_{2}\) accepted 0.40 electrons from the Ag-WS\({}_{2}\) monolayer, resulting in a negatively charged identity. Regarding SO\({}_{2}\) adsorption, one O atom within the SO\({}_{2}\) molecule displayed an affinity for proximity to the Ag atom. This configuration exhibited an adsorption energy of \(-\)0.69 eV and an adsorption distance (Ag-O) of 2.30 A, which is smaller than the sum of van der Waals radii of Ag and O atom. Similar to NO\({}_{2}\), SO\({}_{2}\) acted as an electron acceptor, involving an electron transfer of 0.20 e.  
To investigate the electron density contribution, the CDD maps of the three molecules on Ag-WS\({}_{2}\) are depicted in Figure 4. In the case of CO adsorption, electron accumulation manifests below and around the C atom within CO, while electron depletion is observed just above the Ag atom and around the O atom within CO. Notably, the electron movement within CO aligns with the direction of electron transfer. Upon NO\({}_{2}\) adsorption, an electron accumulation region emerges above the N atom. Prominently, a distinct accumulation region surrounds the NO\({}_{2}\) molecule, indicating its role as an electron acceptor. This same pattern is observed for SO\({}_{2}\) adsorption, where conspicuous electron accumulation regions are evident around S and O atoms.