Results and Discussion
According to Ref. [28],
Zn2(bim)4 can be ex situ obtained
through hydrothermal transformation of 3D zeolitic imidazolate framework
ZIF-7, and the layered Zn2(bim)4 can
then be exfoliated through a soft-physical process. Later, there have
been many trials to in situ grow layered
Zn2(bim)4 nanosheet on different
substrates. For example, Zhang et al. reported a direct growth technique
by converting ZnO to Zn2(bim)4 membrane
with the assistance of ammonia on a porous hollow fiber
substrate[30]. Recently, the GVD method was
reported to in situ grow a ZIF-8 membrane on PVDF fibers by Li et al.[31] Inspired by this, we apply the GVD method for
the first time to in situ grow a 2D MOF layer on the zinc
surface, as shown in Figure 2a.
Firstly, Zn-based sol was prepared by dissolving zinc acetate dihydrate
in ethylene glycol monomethyl ether and ethanolamine. The as-prepared
zinc-based sol was then dip-coated on the Zn surface and heat treated to
remove the solvent, resulting in the zinc-based gel that was tightly
attached to the Zinc surface. As shown in Figure 2a, the Zn-based gel
layer serves as the Zn source and oriented growth sites for 2D MOF
nanosheets. After interacting with the benzimidazole vapor, the
Zn2(bim)4 MOF layer in lamellated
structure would in situ grow on the zinc surface. It is shown
that the Zn2(bim)4 nanosheets grow on
the Zn surface with an oriented direction and stack layer by layer (Fig.
2b and e). The thickness of the MOF layer is around 1 µm, and C and Zn
are the main elements of the Zn2(bim)4MOF (Figure 2b-d, and Figure 2e-g). The top view of the
Zn@Zn2(bim)4 shows that the oriented 2D
MOF nanosheet can form a continuous membrane with almost no slit, which
ensures that the zinc ions transport through the pores of MOFs instead
of directly passing from the slits. In addition, as discussed in recent
work, continuous MOF membranes can avoid the growth of zinc dendrites
along the grain boundaries of crystalline
MOFs[26]. The structure of the 2D nanosheet is
characterized by TEM images, as shown in Figure 2h, with Zn, C, and N
elements being detected (Figure S1, supporting information). Since the
samples for TEM characterization are prepared by scraping the MOF layer
from Zn@Zn2(bim)4 samples, more than one
layer of the nanosheet is observed in the TEM image. In addition, the
scarce N element not detected from the SEM image can be examined from
the TEM characterization. Moreover, the surface of
Zn@Zn2(bim)4 turns gray as compared to
the metallic luster of bare zinc, as shown in Figure 2i. The contact
angle test shows that after growing the MOF layer, the surface becomes
more hydrophobic, as witnessed by the increase in the contact angle from
93.7° on bare zinc and to 122.4° on
Zn@Zn2(bim)4 (Figure 2j and k). The
hydrophobic surface is conducive to avoiding the direct contact of water
molecules with the Zn surface, thus suppressing water-induced corrosion.
The X-ray diffraction (XRD) pattern of the crystal structure of
Zn2(bim)4 is determined according to
Cambridge Crystallographic Data Centre (CCDC), no. 675375, as depicted
in Figure 2l. The peak at 9° corresponds to the (002) plane of
Zn2(bim)4 nanosheet, suggesting that the
membrane is highly oriented. In contrast to previous methods that usedex situ fabricated MOF particles with binders to coat the Zn
surface, the in situ formed MOF nanosheet layer grows into a
continuous membrane. In addition, compared with other in situ grown MOF
particles, the MOF nanosheet with square shapes stacks layer by layer
and forms a tightly connected layer.