# Natural Stones with a Self-Cleaning Surface via Self-Assembled Monolayers
## 3 Results
### Impact of PDA Layers on Water/Oil Wettability of the Stone Surfaces  
In order to investigate the impact of the PDA layers on the liquid wettability of the stone surfaces, dopamine Tris-HCl solution with concentrations of 0.75, 1.5, 2, 5, and 8 mg/mL were adopted in the polymerization, while the concentration of Al2O3@FPA was fixed at 1.0 mg/mL. Static, advancing, and receding water and peanut-oil CAs on the treated stones are shown in Figure 2.  
As shown in Figure 2a,c, the static and advancing water CAs hardly show differences and are in the range of 165-170\({}^{\circ}\) for the treated marble and Hedishi for all tested dopamine concentrations. On the other hand, as shown in Figure 2e, for the treated Qingshi surface, when the dopamine concentration is 1.5 mg/mL, its water CAs are smaller and remain unchanged when the concentration is larger than 2 mg/mL. Thus, the treated stones are superhydrophobic, which is necessary for self-cleaning surface. As shown in Figure 2b,d,f, the advancing CAs of peanut oil on the three treated stones are around 150\({}^{\circ}\), and the static counterparts are slightly lower, reaching approximately 145\({}^{\circ}\). This experimental result suggests that the treated surface is not only superhydrophobic but also superoleophobic. Figure 2: Static, advancing, and receding contact angles of water (represented as SC, AC, and RC, respectively) (**a**,**c**,**e**) and peanut oil (**b**,**d**,**f**) on marble, Hedishi, and Qingshi as a function concentration of dopamine polymerized on the stones as the adhesive layer, on which the concentration of Al2O3@FPA used to render the surface hydrophobic had a concentration 1.0 mg/mL, respectively. Inserted in the figures are the hysteric angles (HAs), that is, the difference between the advancing and receding contact angles. As shown in the inserts in Figure 2, the water HAs, or the difference between the advancing and receding contact angles, are in the range of 20-25\({}^{\circ}\), while the peanut-oil HAs are in the range of 10-20\({}^{\circ}\). Figure 3 shows SEM images of a PDA layer polymerized on a silicon wafer surface with a dopamine of 2 mg/mL for the polymerization. Spherical aggregations with a diameter of about 200 nm are observed on a continuous and dense PDA film with a thickness of \(\sim\)1 \(\upmu\)m. Hence, the reason for the slight variation of the water and oil CAs with the concentration of dopamine comes from the smaller roughness of PDA surfaces compared with the intrinsic micro-roughness of the stone surfaces and the micro- and nano-roughness formed by Al\({}_{2}\)O\({}_{3}\)@FPA particles. Since 2 mg/mL dopamine is enough for PDA layer formation and for self-cleaning surfaces construction, in the following experiments, the concentration of dopamine was fixed at 2 mg/mL. SEM images of the surfaces and cross-sections of three blank stones are shown in Figure 4. On the polished stone surfaces, nonuniform interstices and holes are distinct. The width of the interstices and the diameter of the holes are in the range of several hundred nanometers, and the length of the interstices is mainly several micrometers (Figure 4a-c). From the cross-sections of the three stones shown in Figure 4d-f, we see that the depth of these interstices and holes ranges from nanometers to micrometers. SEM images of the surfaces and cross-sections of three stones after being treated with PDA and Al\({}_{2}\)O\({}_{3}\)@FPA are shown in Figure 5. The stone surfaces present hierarchical structures, including micro- and nanoscale roughness. As shown in Figure 5d-f, the PDA and Al\({}_{2}\)O\({}_{3}\)@FPA particles were permeated into the interstices and holes of the stones beneath the surfaces, and protective layers were formed on the inner surfaces of the interstices and holes, as well. Figure 4: SEM images of the surface and cross section of a blank marble (**a**,**d**), Hedishi (**b**,**e**), and Qingshi (**c**,**f**), respectively.