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