Modification Of Structure And Regulation Of Ferroelectricity Of Transition Metal Based MOFs

Ferroelectric metal-organic frameworks(MOFs),as a classification of organic-inorganic hybrid-based materials,have garnered tremendous interest in recent years depending on the component versatility(such as metal ions,ligands and guest molecules)as well as the synthetic flexibility(such as temperature,solvent and template).However,there still has the challenges of the scarce quantity,unsatisfactory ferroelectricity and limited methods,hindering their further development.In order to solve the problems of ferroelectric MOFs,three MOFs are selected as pristine which crystallized in the non-centrosymmetric polar space group.A series ferroelectric MOFs with controllable ferroelectricity are obtained by means of metal ions doping,modification of substituent on ligands and guest ions exchange,respectively.The main research contents as follows:1.Based on the parent Ni-MOF with the polar space group Pba2,a series metal doped MOFs were prepared by Mg²⁺doped in situ using solvothermal method.ICP,EDS and XPS were adopted to confirm the doped into the lattice nodes and the uniform distribution of Mg²⁺cations in crystal.Doped MOFs maintain the framework from the parent Ni-MOF.Due to the large electronegativity difference between Ni²⁺and Mg²⁺,doped MOFs exhibit obvious lattice distortion and significant dipole moment,resulting in large polarization.Therefore,the doped MOFs can exhibit ferroelectricity at room temperature,and the ferroelectric performance gradually increased with increasing of Mg²⁺content.2.A 2D layered cationic metal-organic framework was synthesized,denote as Zn-MOF,crystallizing in polar space group P2₁ and containing guest NO₃^-anions.And then,Zn-MOF was placed into three solutions which contained NO₂^-,Br^-and I^-,respectively,to obtain Zn-MOF-X(X=NO₂^-,Br^-and I^-)by means of guest ions exchange.XPS was used to verify the presences of exchanges of guest ions in the whole frameworks.The structural phase transition and ferroelectric performance of the samples were also tested.The results showed that the ferroelectricity change greatly from Zn-MOF to Zn-MOF-X(X=NO₂^-,Br^-and I^-),and rendered as Zn-MOF-NO₂^-<Zn-MOF-Br^-<Zn-MOF-I^-.The variations of ferroelectricity might be caused by the change of positive and negative charge gravities in Zn-MOF-X(X=NO₂^-,Br^-and I^-).3.In order to introduce F atoms on ligands partially in pristine Cu-MOF,which crystallizes in polar space group Pca2₁ and possesses ferroelectricity,a series of Cu-MOF-Fs were obtained in situ via the mixture of isonicotinic acids and 3-fluoroisonicotinic acids using solvothermal method.FT-IR and XPS have been employed to confirm the introduction of F atoms in crystal structure.The frameworks of Cu-MOF-Fs are consistent with the parent Cu-MOF,and the introduction of F atoms lead to a large polarization in the whole crystal structure.Therefore,Cu-MOF-Fs exhibited stronger ferroelectricity at room temperature,and the ferroelectricity were further improved with increasing of the content of F atoms.