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Synthesis and Application of Metal-organic Frameworks M3[Co(CN)6]2·nH2O Nanoparticles

Author: HuLin
Tutor: ChenQianWang
School: University of Science and Technology of China
Course: Inorganic Chemistry
Keywords: metal-organic framework Prussian blue analogues nanomaterials solvothermal synthesis lithium ion battery heavy metal ions dye wastewater
CLC: TB383
Type: PhD thesis
Year: 2012
Downloads: 345
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Though Prussian blue analogues (PBA) were discovered in the17th century, gas adsorption properties of these materials were explored very recently. However, in these studies the main focus is the properties of bulk PBA, few research focuses on the properties of PBA and other metal-organic framework at the nanoscale. It was found that the shape and size are important factors to fine-tune the properties of the material. Therefore, it is a strong desire to develop a simple method to prepare the PBA nanomaterials with enhanced or new properties.The objective of this dissertation is to explore new simple avenue for synthesis the nanoparticles of PBA with molecular formula M3Ⅱ[Co(CN)6]2·nH2O, and investigate their gas storage properties. Moreover, a new facile strategy has been designed to fabricate metal oxides and composite metal oxides, which involved a morphology conserved and pyrolysis-induced transformation of Prussian Blue Analogues (PBA) in air. Owing to the release of born gases in the process of decomposition, the products with porous structure were effectively obtained. The applications of these porous nanomaterials in lithium-ion batteries and waste water were also investigated.1. M3Ⅱ[Co(CN)6]2·nH2O (M=Mn, Co, Cd, Zn, Fe) nanoparticles have been successfully synthesized at room temperature using K3[Co(CN)6] and metal salt as starting materials. Surfactants (polyethylene pyrrole and dodecylbenzenesulfonic acid sodium) and solvents (ethanol and water) were employed to adjust the growth habit of nanoparticles, and nanoparticles with various morphologies were fabricated, including nanocubes, truncated nanocubes, octahedrons, spheres and so on. Full nitrogen sorption showed that the surface area of nanoparticle were lower than that of bulk materials, which is resulted from a large number of residual surfactants appears in the porous framework of M3Ⅱ[Co(CN)6]2-nH2O. The adsorbed CO2wt%of Mn3[Co(CN)6]2porous at room temperature and670mm Hg pressure was10.9%, higher than Mn3[Co(CN)6]2bulk materials (≈10%). The adsorbed H2wt%of Cd3[Co(CN)6]2nanocubes and octahedrons at77K and1bar pressure were1.3%and1.05%, while the adsorbed CO2wt%were53cm3g-1and38.5cm3g-1, respectively. The adsorbed CO2wt%of Co3[Co(CN)6]2was8.7%. Although the surface area of M3n[Co(CN)6]2bulk materials is little higher than that of nanoparticles, better gas adsorption (CO2, H2) performance was shown by using nanoparticles. This result proves that the surface area is not the only factor that influences gas adsorption, and the materials at the nanoscale are more favorable to adsorption applications. Indeed, the porous framework, or in another word, the internal surface, is not easy to be changed. However, downsizing MOFs to the nanometer regime will lead to more possibilities of enhancing their gas storage capacity for the following reasons at the same time. As we all know, chemical and physical phenomena are usually strongly affected when material becomes nanometer-sized. With decreased particle size, the proportion of the atoms lies on the surface will increase as a result. Furthermore, compared with the bulk material, the nano-sized PCPs possess higher surface energy. The binding energy between the surface atoms is higher than that of the internal atomic. Surface atom, lacking adjacent atom, display unsaturated properties and are easy to combine with other atoms, which will improve their surface adsorption as well. Moreover, Mn[Co(CN)e]2·nH2O nanoparticles display excellent adsorption properties for heavy metal ions in water solution, and the adsorption efficiency was94%. Mn3[Co(CN)6]2·nH2O shows paramagnetic properties at room temperature, and a magnet can be used to separate precipitates from the treated solutions.2. Co3O4nanocages with porous shell and foam-like MnxCo3-xO4porous nanocubes have been successfully synthesized, which involved a pyrolysis-induced transformation of M3Ⅱ[Co(CN)6]2·nH2O (M=Co, Mn) in air at400℃. Co3O4nanocages possess high surface area (66m2/g), porous shell, small size (60nm) and a small amount carbon, therefore can overcome the disadvantage (capacity fade fast) of traditional Co3O4as the lithium ion battery anode materials, displaying high capacity and better cycle stability. The capacities up to1465mA h g-1are attained after50cycles at a current density of300mA g-1. This result breaks the highest record capacity of Co3O4anode materials. Foam-like MnxCo3-xO4porous nanocubes also possess high surface area (129m2/g) and small size (200nm), which are different from the spinel materials with small surface area and large size obtained by traditional high temperature firing methods. When evaluated as electrode materials for lithium-ion, the foam-like MnxCo3-xO4porous nanocubes display high specific discharge capacity and excellent rate capability. The capacity of733mAh g-1could be maintained after30cycles at a relatively high current density of200mA g-1. We deduce that the high lithium storage capacity and good cyclic stability of Co3O4nanocages and MnxCo3-xO4porous nanocubes might be due to the large surface areas which are more convenient for the intercalation of Li+ions into the active materials and accelerating their diffusion velocity. Moreover, the large surface areas can enlarge the electrolyte/materials contact area, shorten the Li+-ion diffusion length in the materials and reduce the resistance of electrolyte. On the other hand, the porous structure may endure the volume expansion/contraction during the Li+-ion insertion/extraction processes, enhance the cycle stability.3. ZnO/Co3O4nanocomposites and FexCo3-xO4porous spheres have been successfully synthesized, which involved a pyrolysis-induced transformation of M3Ⅱ[Co(CN)6]2·nH2O (M=Zn, Fe) in air. ZnO/Co3O4porous nanocomposites display ferromagneticlike behavior at room temperature, and the coercive field is230Oe. It is shown that the magnetism seems to be related to the following two factors. One is Co3O4surface atomic electron spin-track coupling change the particle internal magnetic-order. Another is cobalt atoms dope into ZnO semiconductor to form dilute magnetic semiconductor. The specific reason needs to be studied further. FexCo3-xO4porous spheres possess high surface area (81m2/g). When served as the adsorbent for Congo red in water, the as-prepared FexCo3-xO4porous spheres exhibit a high adsorption capacity for the dye removal with adsorption efficiency of88.24%, suggesting their potential use in water treatment. Moreover, the product can be easily separated from water system duo to its ferromagnetism at room temperature.4. Mn2O3and CoMn2O4hierarchical microspheres self-assembled with porous nanosheets have been obtained by a two-step method. First, a solvothermal treatment is employed to prepare the Mn-EG (EG=ethylene glycol) and Mn-Co-EG precursor by coordination of-OH with the metal ions, and then Mn2O3microspheres and CoMn2O4microspheres were obtained by annealing the precursor powder in air at600℃for3h. Owing to the release of born gases in the process of decomposition, the products possess porous structure. When evaluated as electrode materials for lithium-ion, the products display high specific discharge capacity. After45cycles, the capacity of Mn2O3microspheres can be retained at750mAh g-1at a current density of50mA g-1, while the capacity of Mn2O3microspheres can be retained at900mAh g-1at a current density of100mA g-1after65cycles. Moreover, the excellent rate capability demonstrate the product have a great potential as a high-rate anode material in lithium ion batteries. The superior electrochemical performances of Mn2O3and CoMn2O4hierarchical microspheres can be attributed to the following two factors:(1) The nanometer-sized subunits (porous nanosheets) not only allow the reversible formation/dissolution of polymeric gel-like film at the surface of the active materials but also make the conversion reaction more feasible, both of which can contribute to the high specific capacities.(2) The hierarchical structure and porosity in the surfaces can buffer the large volume change of anodes based on the conversion reaction during the repeated Li+insertion/extraction, thus alleviating the pulverization problem and enhancing the cycling performance.

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