Figure 1. (a) Schematic view of the preparation of 3D MF@HPB-PPyn-OA by facile one-step self-assembly of H3PMo12O40·nH2O (POM), pyrrole (Py) and oleic acid (OA) on melamine foam (MF); (b) On the surface coating of MF@HPB-PPyn-OA, the hydrophobic regions (left) is mainly occupied by OA molecules, and the hydrophilic regions (right) is mainly composed of HPBs.
ethanol solution containing H3PMo12O40·nH2O (POM), pyrrole (Py) and oleic acid (OA) is poured on commercially available the melamine foam (MF) that serves as a carrier for self-assembly. In this process, POM is reduced to HPB by triggering the oxidative polymerization of Py to PPy. Then, HPB and OA (acting as electron donors) undergo respectively electrostatic self-assembly with PPy (serving as electron acceptor) on the surface of MF to form a series of hydrophilic-hydrophobic composite interfacial materials (denoted as MF@HPB-PPyn-OA, n refers to the amount of Py added, n = 5 µL, 10 µL, 20 µL). Influenced by the electrostatic repulsion of HPB and OA, the hydrophilic-hydrophobic units formed by them present distinct spatial separation characteristics on the MF@HPB-PPyn-OA surface. As shown in Figure 1b, on the surface region without HPB, the PPy surface is modified by OA, forming a hydrophobic region (left). In contrast, the region containing HPBs exhibits affinity to water owing to the absence of OA modification (right). This spatially separated feature implies that the hydrophilic regions of the HPB might form nano water channels for water transport. On one hand, the exposed hydrophilic regions of HPBs enhance the coupling-activation, transport and convection of bulk water under the action of hydrogen bonding or electrostatic forces, thus enhancing the evaporation efficiency of water. On the other hand, the hydrophobic regions endow the 3D porous MF@HPB-PPyn-OA surface with a powerful anti-salt fouling property. Moreover, salt is also prevented from crystallizing in the hydrophilic regions due to the advantages of the nanoscale dimension of hydrophilic regions and the strong salt-water convection effect between the water channels (It is something like “pits” formed by HPBs with a high salt concentration) and the original microporous channels of MF (with a low salt concentration). The surfacial hydrophilic-hydrophobic dual feature of the porous MF@HPB-PPyn-OA is further modulated by adjusting the loading different amount of HPBs. Among them, MF@HPB-PPy10-OA stands out in terms of its ability in photothermal water evaporation and anti-salt fouling properties, thus, it is selected as an example for elucidation in detail.
SEM and TEM show the morphology of MF@HPB-PPy10-OA. As shown in Figure 2a and Figure S1, the HPB, PPy, and OA are clearly self-assembled on the smooth skeleton surface of MF (Figure S1a and Figure S1b), forming a HPB-PPy10-OA self-assembled coating. Meanwhile, the resulting MF@HPB-PPy10-OA preserves the original 3D porous structure of MF. TEM further reveals that HPB-PPy10-OA is stacked by irregular lamellar structures (Figure S2b and Figure S2c). In the HR-TEM image (Figure 2b), we observe that the HPB clusters are uniformly dispersed in the amorphous PPy lamellar structures. The uniformity distribution of C, N, O, Mo and P elemental components and the completeness of HPB-PPy10-OA coating coverage are evidenced by TEM-EDX and SEM-EDX mapping analysis (Figure 2c), respectively. Notably, the cross-sectional SEM images (Figure S1e, Figure S1h and Figure S1k) display that the self-assembled layers of MF@HPB-PPy5-OA, MF@HPB-PPy10-OA and MF@HPB-PPy20-OA became progressively thicker with increasing the proportion of Py substrate in the reaction system. At the same time, the loading of HPB is gradually diminished (Figure S3), implying that the hydrophilic-hydrophobic properties of the surface are controllable. The corresponding water contact angle measurements verify this conclusion (Figure S1f, Figure S1i and Figure S1l). A control sample MF@PPy10-OA without HPB is firstly prepared (see Synthesis 2.1 section for details), and the water contact angle is about 132.0 ± 3o, exhibiting a strong hydrophobicity as demonstrated in Figure 2d. When the hydrophilic HPB is introduced, the water contact angle on the surface of the material gradually decreases with the increase of HPB loading, and the corresponding hydrophilicity is