| Reasonably building precursors which contain pre-transition metals(Mo,W)and post-transition metals(Co,Ni,Fe)with specific morphology is the key to obtain bi-transition metal carbides(nitrides).Polyoxometalates(POMs)are inorganic nanoclusters composed of precursor atoms such as Mo(W).Polyoxometalates possess a stable structure,close to the nanometer size,and are suitable raw materials for the synthesis of transition metal carbides(nitrides).In the paper,we assumes that if the POMs are limited into the nano-scale space containing the post-transition metals Co(Ni,Fe),the pre-transition metals and post-transition metals will have close contact,which will provide opportunities for obtaining the bi-transition metal carbides(nitrides).Metal organic frameworks(MOFs)are functional materials rich in pores formed by coordination of metal ions and organic ligands.MOFs can be used as effective catalysis and adsorption materials.Because they contains both metal components and carbon sources,MOFs can be used to synthesize catalytic and energy storage materials such as transition metal oxides,phosphides,and porous carbons.MOFs have abundant pores close to nanometer scale,and polyoxometalates clusters are nanometer sized,so MOFs and polyoxometalates clusters can effectively combine to obtain materials containing pre-transition metals Mo(W)and post-transition metals Co(Ni,Fe),which provides a new chance for the synthesis of bimetallic carbides(nitrides).Severial studies have indicated that POMs/MOFs(POMOFs)can be obtained by combining POMs(PMo12)and MOFs by means of simultaneous self-assembly(self-assembly of polyoxometalates clusters,post-transition metal ions and ligand in the solution).In the POMOF-based route,the introduction of POMs makes the MOFs synthesis system with more difficult influential factors,which limits the morphology control and compositional modulation of the obtained POMs/MOFs materials.All of these factors are not conducive to obtaining suitable precursors for the preparation of homogenous,tunable bi-transition metal carbides(nitrides).Therefore,it is necessary to develop a strategy for effectively combining POMs with different structures and MOFs with uniform size and morphology.In fact,the abundant pores of MOFs can provide suitable spaces for adsorbed guest molecules.Therefore,using a step-by-step route to assemble polyoxometalates with pre-prepared MOFs is a feasible way to achieve the combination of MOFs and POMs.Compared with the POMOF route(simultaneous self-assembly route),the MOFs in the step-by-step route are pre-synthesized which can achieve a good control of the morphology and size of the MOFs.At the same time,this kind of step-by-step assembly route broadens the types of selectable MOFs and POMs,and provides the possibility for wide-scale control of the structure and composition of materials.By adjusting the matching of MOFs structure(pore and cage size)and POMs size,electrical properties(matching principle),the two are assembled,and the post-transition metals in the MOFs and the pre-transition metals in the POMs are contacted in the nanoscale space.It is beneficial to form bimetallic carbides(nitrides)during the heat treatment.The question which should be concerned is whether the POMs/MOFs obtained by this assembly method can be used as precursors for preparation of hollow structures(self-sacrificing templates).Recently,we used PMo12(phosphomolybdic acid hydrate,approximately 1 nm in size)as Mo source,and pre-synthesized ZIF-67(cage size is approximately 1.16 nm,which matches the size of polyoxometalates clusters)as a cobalt source conducted a preliminary exploration.The results show that designing a reasonable route can make the two effectively assembled.By controlling the nitriding conditions,the conversion of the precursor to hollow bimetallic nitrides can be achieved,demonstrating the applicability of the proposed post assembly method for the assembly of POMs and MOFs and for the synthesis of hollow bi-transition metal nitrides. |
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