Effect Of Metal Ions Doping On Ferroelectric Properties Of MOFs

MOFs ferroelectrics as a crucial branch of molecular ferroelectrics,exhibit competitive prospect because of their designable structure and adjustable properties.However,the weak ferroelectric performance of MOFs ferroelectrics hinders their further development and application.Post–synthetic modification,as a valid technique for assembling new MOFs or improving the original properties of MOFs,has attracted great attention of researchers.Post–synthetic modification includes metal ions doping,organic linker modification and guest molecules modulation.Thereinto,metal ions doping exhibits unique superiority in modulating ferroelectric properties of MOFs owing to various metal ions and lattice distortion.Based on the above background,this thesis aims to enhance or modulate ferroelectric properties of MOFs through constructing different metal ions doped MOFs according to metal ions doping methodology,study the effect of metal ions doping on the crystal structure and ferroelectric performance of MOFs,and analyze the mechanism of metal ions doping improving ferroelectric properties of MOFs.The main research contents are as follows:1.On the basis of literature,Co–Gallate crystallized in polar point group was synthesized,which exhibited the weak ferroelectric performance.Subsequently,a series of M–doped Co–Gallate（M=Mg,Mn,Ni）were constructed according to the principle of similar ion radii,analogous coordination configuration and different electronegativity.M–doped Co–Gallate exhibit enhanced ferroelectric performance,which is attributed to the lattice distortion triggered by doping metal ions,and further improve polarity of the whole structure.Compared to parent Co–Gallate,the remanent polarization is enhanced by twice for Mg–doped Co–Gallate,six times for Mn–doped Co–Gallate and four times for Ni–doped Co–Gallate.By comparing the effect of different metal ions doping on ferroelectric properties,it is found that the difference of ionic radius（△r）between Co²⁺ions and M²⁺metal ions is the key factor influencing the polarization of M–doped Co–Gallate.2.Considering the higher polarity of high valence metal ions,it will be beneficial to increase the polarity of the whole structure and further enhance the ferroelectric properties.Therefore,an Eu–MOF belonged to polar point group was synthesized according to the reported work.Then,a couple of Ln–doped Eu–MOF（Ln=Ce,Sm,Ho）were assembled through doping Ln³⁺metal ions into crystal structure of Eu–MOF.The larger difference on electronegativity and ionic radius between Ho³⁺ions and Eu³⁺ions will together influence the crystal structure of Ho–doped Eu–MOF,cause lattice deformation,lead to the variation of the position of the center of positive and negative charges,trigger the change of dipole moment,further alter the polarity of the overall structure.P–E hysteresis loops indicate that the remanent polarization increases from0.008μC/cm² to 0.471μC/cm² by 57 times when the doping content of Ho³⁺ions increase to 50%.3.The above works mainly research the influence of single metal ions doping on the ferroelectric properties of MOFs.Then,mixed metal ions co-doping on ferroelectric properties of MOFs will be explored on the basis of single metal ion doping.Thus,two isostructural MOF–802（Zr）and MOF–802（Hf）crystalized in polar point group were obtained according to the reported literature,and MOF–802（Hf）displays the better ferroelectric performance than MOF–802（Zr）,but that still is very weak.Then,Zr–doped MOF–802（Hf）and Ce–doped MOF–802（Hf）were built with MOF–802（Hf）as the parent structure through metal ions doping.P–E hysteresis loops prove Zr–doped MOF–802（Hf）exhibit enhanced ferroelectric properties at the high voltage,meanwhile Ce–doped MOF–802（Hf）show improved ferroelectric performances at the low voltage.Subsequently,Ce/Zr co–doped MOF–802（Hf）were successfully constructed by through metal ion co–doping,displaying the better ferroelectric performance than the single metal ions doped MOF–802（Hf）with the synergy of Ce⁴⁺ions and Zr⁴⁺ions,which is the first example on modulating ferroelectric behaviors of MOFs through metal ion co–doping.And this work would serve as an excellent guide for subsequent research.The enhanced ferroelectric of M–doped MOF–802（Hf）（M=Ce,Zr）is attributed to the changed dipole moment and polarization of crystal structure.