Functional Polyoxometalate Nanocomposites Construction Based On Multi-level Driving Forces

Polyoxometalates(POMs)are a group of mono-disperse,nanoscale metal oxide clusters with well-defined structures and tunable electronic and surface properties.The surface of POMs is mainly composed of oxo,hydroxyl,and/or water ligands,providing access to the formation of hydrogen bonding.The anionic feature of POMs facilitates the study of their electrostatic interaction with cationic molecules.On the other hand,owning excellent redox properties and wide band gaps,POMs can also act as good electron reservoirs that are capable of capturing and storing electrons from QDs.POMs are attracting more and more attention due to their excellent properties and have emerged at the forefront of materials chemistry and other fields,such as biomedical,sensing,electrochemistry,optics and catalysis.The stability energy of the non-covalent bond cluster is usually in the range of 2-200 k J/mol,while the binding energy of the covalent bond complex is about 300-950 k J/mol.The melting point,boiling point,solubility,viscosity,hardness and other physical properties of the composite materials constructed by non-covalent bonds are all affected.The chemical properties of covalent bond composites vary greatly.Accurate prediction of composite functionality is very important for the application and design of composite materials.It is one of the most challenging tasks to properly regulate molecular interactions and energy matching to achieve functional prediction of composite materials.Therefore,it is of great significance to construct POM-organic and POM-inorganic materials based on multi-level composite driving force,to study their synergistic properties and to predict their potential applications.The specific contents of this paper are as follows:(1)Hydrogen bonding directed co-assembly of polyoxometalates and polymers to core-shell nanoparticles.A general strategy has been developed here to co-assemble polyoxometalates and polymers into core-shell hybrid nanoparticles via hydrogen bonding interaction.Due to the hydrogen bonds between the pyridine groups of poly(4-vinyl pyridine)(P4VP)and the hydrogen bonding donor groups on the POM surface,P4VP is stabilized by the POMs and dispersed as discrete hybrid core-shell nanoparticles in aqueous solution.For these thermodynamically stable nanoparticles,the P4VP cores are covered with hexagonal close-packed POMs.The morphologies of the obtained NP complexes are studied by electron microscopy and scattering techniques while the non-covalent interactions between the POMs and polymers are probed by vibration spectroscopy and scattering techniques.The size of the core-shell particles is controlled by the electrostatic repulsive interaction among the POMs.The introduction of extra salts screens the repulsive force among the POMs,thus increasing the size of the core-shell structures.In the first part,we proved that POMs of various topologies can be enriched on the surface of the polymer.(2)U₆₀(Li₄₄K₁₆[UO₂(O₂)(OH)]₆₀,U₆₀)and polymers are directionally enriched on the polymer surface by hydrogen bonding.In this part,we use several different polymers and U₆₀for self-assembly.The morphology and non-covalent interaction between the core-shell nanocomplexes are studied by electron microscopy and vibration spectroscopy.Under the synergistic action of hydrogen bonding and nanometer phase separation,block copolymers and U₆₀ formed different structures of core-shell nanocomposites.At low concentration of U₆₀,spherical nanocomposites with frozen structure can be formed,at high concentration of U₆₀,thermodynamically stable worm-like structure and micelles can be formed.We used block copolymers to absorb U₆₀,and after one month,about 96%of U₆₀ could be removed from the solution.(3)Uranyl peroxide cluster nanocomposites for solid-state electrolytes.Uranyl peroxide cluster nanocomposites are synthesized from the ionic interaction between anionic uranyl peroxide cluster(U₆₀)and cationic surfactants.As a porous nano-capsule,the U₆₀ clusters can uptake?44 Li⁺ions,serving as Li⁺storage and transportation centers.The nanocomposites show enriched micro-phase separated structures ranging from lamellar to hexagonal columnar structures depending on the geometries of surfactants and the U₆₀ enriched phases serve as nano-scaled channels for Li⁺ion transportation.Temperature dependence of Li⁺conductivity in the nanocomposites exhibits a crossover from obeying a Vogel-Fulcher-Tammann equation to an Arrhenius equation at 295 K,suggesting different Li⁺conducting mechanism across the critical temperature(T_c).This crossover reflects the change in the transport mechanism of the ions which are diffusing through a segmental motion above T_c,and through a hopping mechanism below T_c.The design of the nanocomposites offered approaches to decouple the ion transportation and dynamics of surfactant chains and therefore pave new ways to fabricate solid state electrolytes with both high ion conductivity and mechanical strength.(4)Nanocomposites of Preyssler-type POM for super-structures and single-ion conductivities.A simple inorganic polyoxometalates–surfactants system provides a convenient and potential approach for developing SEPs with solid-state electrolyte.The conductivity of the complex can reach up to 10^-6 S cm^-1,and Ag⁺can be rapidly transported in a two-dimensional layered structure.The conductivity of SEPs-Ag exhibits a clear crossover from a VFT equation at T>T_c to an Arrhenius equation at T<T_c,suggesting different Ag⁺conducting mechanism across the critical temperature(T_c).Temperature dependence of?_?and?_?in SEPs-Ag system above T_c,?_?follows Arrhenius's laws and?_?follows VFT laws,indicating that a decoupling of ion dynamics from structural dynamics.The transference number of SEPs-Ag is found to equal t _Ag+=0.89,comparable to single-ionic polymer electrolytes.(5)The co-assembly of polyoxometalates and quantum dots for hybrid core-shell nanoparticles.A co-assembly protocol of quantum dots(QDs)-polyoxometalates(POMs)hybrids is developed using Keggin-type POMs as surface capping ligands.Phase transfer strategy is used to prepare POM/QDs nanocomposites.These newly developed hybrid materials have been explored for their optical properties and aggregation behavior in solutions.UV/Vis and FT-IR are used to reveal the optical properties of POM/QDs.Small-angle X-ray scattering and transmission electron microscopy are applied to characterize the co-assembled core-shell structures of POM/QDs complexes under different reaction conditions.A general trend to be noted is that the average photo-luminescence lifetime of QDs decreases after phase transfer.The lifetime attenuation is caused by the surface property change of newly obtained POM/QDs complexes,which is resulted from the changed band edge recombination and shallow trap assisted recombination.