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.