Crystal Structure And Negative Thermal Expansion In Organic-inorganic Hybrid Materials

Organic-inorganic hybrid materials have attracted considerable attention.Benefitting from the blending of two components in a single phase,organic-inorganic hybrid materials can offer combined or enhanced properties of individual components,which makes them possess outstanding performance in photovoltatics,catalysis,and drug releases.Compared with organic polymers,organic-inorganic hybrid materials are highly crystallized,and their crystal structures are predictable and designable.In addition,compared with traditional inorganic materials,organic-inorganic hybrid materials possess the advantage of easy-decoration,tunable structure etc.The incorporation of organic components can enhance the flexibility of the framework,which makes it an ideal platform for the tailoring of thermal expansion behaviors.What’s more,compared with the inorganic negative thermal expansion(NTE)materials,such as ZrW2O8,ScF3,and invar alloy,organic-inorganic hybrid materials possess higher compatibility with inorganic and polymer materials,which makes them ideal candidates as composites.However,most of the reported organic-inorganic hybrid materials with NTE are on the cyanogen bridged Prussian blue analogues and some terephthalate bridged metal organic frameworks(MOFs).What’s more,most of these NTE materials are isotropic,which largely limits their using and can hardly satify the demands of various kinds of applications.In this thesis,we focused on some rigid ligand(such as squarate,2-Ethylimidazole,1,3-Benzenedicarboxylic acid and 1,4-Benzenedicarboxylic acid)bridged organic-inorganic hybrid materials.The crystal structure,thermal expansion behavior,and mechanism of NTE of these materials were investigated.Two squarate bridged compounds BaC4O4 and PbC4O4 with pillar-layered architectures were synthesized.High resolution synchrotron X-ray diffraction measurements showed that the two compounds exhibited zero thermal expansion(ZTE)along the organic-inorganic hybrid direction(c axis).Further synchrotron X-ray pair distribution function and Raman spectra studies revealed that the rotation and geometric deformation of the Ba2O2 or Pb2O2 plane resulted in the NTE in the inorganic layer,while the organic pillar showed positive thermal expansion(PTE).So the cooperation of NTE in the inorganic layer and PTE in the organic pillar resulted in the observed ZTE behavior along c axis.Two helical compounds M(eim)2(M=Zn,Cd;eim=2-Ethylimidazole)with 2-Ethylimidazole as bridging ligand were synthesized.Variable temperature single crystal and powder X-ray diffraction measurements showed that these two compounds exhibited large NTE along c axis with the coefficients αc=-21×10-6 K-1 for Zn(eim)2 and αc=-55×10-6 K-1 for Cd(eim)2.Further structural analysis revealed that the uniaxial NTE was caused by the spring like deformation.In addition,the compound Cd(eim)2 showed remarkable thermal quenching.Upon heating,the intensity of the emission nearly linearly decreased.The combination of the linearly decrease of the emission intensity and the large negative thermal expansion coefficient makes it a candidate for luminescence thermometers or other thermoresponsive devices.Variable temperature synchrotron X-ray diffraction measurements revealed that Zn8(SiO4)(m-BDC)12(m-BDC=isophthalic acid)showed NTE along c axis and PTE in the ab plane.What’s more,the compound showed volumetric near-ZTE over the entire temperature range.Further structural analysis revealed that the spring-like deformation resulted in the anisotropic thermal expansion.At the same time,the π…π interaction resulted in the contraction of the helical chain Zn8(SiO4)-(m-BDC)-Zn8(SiO4),the synergistic effect of spring like deformation and π…πinteraction resulted in the overall volumetric near-ZTE.The thermal expansion behavior of terephthalate bridged MIL-68(In)was investigated.High resolution synchrotron X-ray diffraction measurements showed that,MIL-68(In)exhibited NTE along its all crystallographic axes.Further structural analysis revealed that,the terephthalate ligand showed strong transverse vibration perpendicular to the linkage In-BDC-In(BDC = terephthalate),which resulted in the NTE in the ab plane.Variable temperature single crystal x-ray diffraction and pair distribution function analyses showed that the NTE along the c axis was caused by the rotation of the InO6 octahedrons.The thermal expansion behavior of squarate linked compound Co(C4O4)·2H2O was investigated,which showed isotropic low thermal expansion.SEM and TEM images showed that the products synthesized by the ultrasonic-standing approach possessed"dice-like"morphology.A series of controlled experiments revealed that the products underwent a dissolution-regrowth prossess which finally resulted in the observed morphology.After calcination in air,the Co(C4O4)·2H2O can be converted to Co3O4 nanoparticles with the"dice-like"morphology well retained.The drived Co3O4 materials showed high electrocatalytic performance toward OER.