Syntheses And Photoelectrochemical Properties Of ZnO@ZIFs Nanomaterials

MOFs are a kind of crystalline materials with structures based on classical coordination bonds between metal cations (such as Zn2+) and electron donors (such as carboxylates or amines). The self-assembly of these components, typically in solution, creates rigid pores that do not collapse upon removal of solvent or other "guest" molecules occupying the pores. The presence of both inorganic and organic components enables both the pore size and chemical environment to betailored to achieve specific properties. Compared with pure MOFs, the heterostructures integrating MOFs with other functional materials show great advantages due-to their synergism effect. ZnO is one of the important functional materials with semiconducting properties,especially for the applications in photoelectrochemistry. ZnO@MOFs heterostructures should possess potential application in photoelectro-chemical sensors with high selective response towards molecules of different sizes.Thereinto, we proposed a self-template strategy to fabricate a series of ZnO@MOF core-shell heterostructures. In this synthetic strategy, ZnO nanorods and their arrays not only act as the template, but also provide Zn2+ions for the formation of Zn-MOFs.On the basis of this, the photoelectrochemical response of as-synthesized ZnO@MOF core-shell heterostructures was systematically explored. The main research results are summarized as follows:(1)By the ZnO-based self-template strategy, we successfully prepared four kinds of ZnO@MOF heterostructures with a core-shell structure, including ZnO@Zn(Im)2, ZnO@Zn(EIm)2, ZnO@ZIF-7, and ZnO@ZIF-71, using H2O or N, N-dimethyl formamide as reaction medium. These MOFs grown on ZnO nanorods were proved to possess different pore sizes. We detailedly investigated the influence of solvent ratio and reaction temperature on the morphology of as-prepared ZnO@MOF heterostructures.(2) Through tracking the thickness of MOFs shell and the diameter of ZnO nanorods through time-dependent reactions, two growth mechanisms were found among the as-prepared ZnO@MOF heterostructures, which is closely associated with the sizes of the pores and the ligands in MOFs. For two cases of Zn(Im)2and ZIF-7, the thickness of MOFs shell increased as reaction time, whereas the diameter of ZnO nanorods significantly decreased. According to this feature, it can be deduced that in the synthetic process, the ligands can pass the pores of the initially formed MOFs shell,due to their large pore sizes, and coordinate with Zn atoms dissolved from ZnO nanorods. Different from the Zn(Im)2case, the increase of the shell thickness originated from the"gate opening" or "breathing effect" of ZIF-7.By contrast, for Zn(EIm)2and ZIF-71,the thickness of the MOFs shell and diameter of the ZnO core were unchanged once the ZnO nanorods were completely covered by the compact MOF shell.This means that it is very difficult for the ligands to pass through the pores of the MOFs shell from the reaction medium.(3)We systematically tested photoelectrochemical responses of as-prepared ZnO@MOFs heterostructures in the presence of hole scavengers with different molecule sizes, including methanol, isopropanol, tert butyl alcohol, and phenylcarbinol.Our experimental results indicated that ZnO@Zn(EIm)2and ZnO@ZIF-71expectedly showed distinct photoelectrochemical response stewards these hole scavengers, depending on their molecule size relative to the pore sizes of MOFs. In addition, ZnO@ZIF-7showed positively enhanced photoelectrochemical response for all hole scavengers due to the"gate opening" or"breathing effect" of ZIF-7.