## 3 Results
### Dimethoate Interaction with IG and GO Surfaces
#### 3.2.2 Adsorption of DMT on Defect Graphene Surfaces  
After adsorption on pristine graphene, the investigation is expanded to four defect graphene model surfaces, in a 3 \(\times\) 3 graphene supercell--Stone-Walles (SW) defect, N-defect, oxydefect, and monovacancy (MV) defect (Figure 6). As mentioned in the previous section, these surface models are designed to represent the most representative surface features expected on the graphene surface and predict possible causes of altered reactivity of the investigated surfaces compared to pristine graphene. It should be emphasized that the structure of real (particularly oxygen-containing) surface groups may be considerably different from the oxydefect model (hydroxyl groups, carboxyl groups, etc.). However, it is expected that the model accurately describes at least the trends in the adsorption reactivity when oxygen is introduced. Similarly, the monovacancy defect is present on both IG and GO in a small amount (for example, on unsaturated edges). Still, it exhibits significant theoretical importance, since its increased reactivity might trigger considerable chemical  
Figure 5: Total electronic density of states (DOS) of pristine graphene, compared to the graphene with adsorbed DMT, for (**a**) 6 \(\times\) 6 supercell and (**b**) 3 \(\times\) 3 supercell. Fermi level is taken as energy zero. Grey surface—isolated DMT, red line—pristine graphene, blue line—graphene with adsorbed dimethoate. changes in the system [48]. Optimized adsorption geometries of DMT on investigated surfaces, together with \(E_{ads}\) and charge transfer, are given in Table 4.  
As observed from the obtained results, adsorbate behavior and reactivity on defect model surfaces--SW, oxydefect, and N-defect--is similar to the one on pristine graphene, at least because of adsorption energetics and surface-to-adsorbate distance. Adsorption energy values are close to pristine graphene in 3 \(\times\) 3 cell. Additionally, the formation of a particular chemical bond between the surface oxygen and DMT has not been detected. There was no charge transfer exceeding 0.012 e, and the PDOS structure changes (Figure S1, Supplementary material) are not likely to originate from chemical changes. However, in the case of the MV defect, geometry optimization results in dimethoate dissociation, pointing to the substantially different MV defect reactivity compared to other investigated surfaces (Table 4). The dissociation represents a chemical change, where DMT oxygen or sulfur atoms (depending on the input geometry) are incorporated into the vacant site of the graphene structure. This process resulted in the overall stabilization of the system by 2.35 eV in case of O-binding, and 2.43 eV in case of S-binding geometry (Equation (1), Table 4). As observed, the dissociation process is more complex than simple non-dissociative chemisorption, electron transfer was not calculated for the case of MV defect, while \"adsorption energy\" rather refers to \"stabilization energy\", denoted by \"\(\approx\)\" sign in Table 4. Figure 7 represents the DOS structure of graphene with the MV defect, before and after adsorption of DMT. Widening of DOS peaks is obvious, particularly for the peaks from the Fermi level to about \(-\)2.5 eV. Particularly, the appearance of a DOS minimum at the Fermi level represents the evidence of system stabilization upon dimethoate adsorption on MV-defect surface. Based on the overall results of DFT calculations, a conclusion can be drawn that the intrinsic affinity of graphene towards adsorption of dimethoate is not influenced by the introduction of any among the investigated surface defects, except the MV defect, which induces dimethoate dissociation. Figure 6: Investigated defect model surfaces (3 \(\times\) 3 graphene supercell): (**a**) monovacancy (MV) defect, (**b**) oxydefect, (**c**) SW defect, and (**d**) N-defect (carbon = black, oxygen = red, nitrogen = grey).