Controllable Synthesis And Properties Of Biomorphic LDH Based Composites

Layered double hydroxides (LDHs), as an important class of anionic clay materials, have received great attention due to their versatile applications in pollution control, bioseparations, optoelectronic materials. However, the positively charged hydroxide sheets and intercalated anions are orderly stacked based on the strong electrostatic interaction and hydrogen bonding, resulting in the highly ordered structures and extremely limited interlayer space of LDHs materials. The potential applications of which highly dependent on surface properties are limited due to their very low specific surface area and small interlayer distance.Hierarchically structured LDHs, which possess dual or multiple morphologies and structures, are attracting significant attention owing to their high surface areas and special surface properties. In this work, focusing on clarifying the relationships between interface structures and properties of LDHs, the multifunction LDHs based composites with tunable micro/nanostructures and chemical compositions are designed by combining the biological template method and in situ growth technique for applications in pollution control, bioseparations, optoelectronic materials. The physical and chemical properties of synthesized materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and N2 adsorption-desorption isotherm. All the research contents and results are as follows:(1) Hierarchically structured Mg-Al LDHS/Al2O3 composites were fabricated from waste paper fibers using a two-step method. In the first step the microscaled Al2O3 fibers were prepared by template-directed synthesis employing paper fibers as templates and in the second step the nanoscaled LDHs platelets were fabricated into hierarchical architectures by in situ growth on Al2O3 fibers surface. The XRD results confirmed the amorphous nature of Al2O3 fibers and relatively crystalline nature of LDHs. BET analysis showed that the surface area was increased from 76.66 m2/g (Al2O3 fibers) to 165.0 m2/g (composites) by the by the growth of LDHs platelets on the surfaces of Al2O3 fibers. The Langmuir isotherm model was found to agree well with the equilibrium data, while the pseudo-second order model provided the highest correlation of the kinetic data for fluoride adsorption. As compared to bare LDHs particles and Al2O3 fibers, the Mg-Al LDHs/Al2O3 composites show a high fluoride adsorption capacity, and the maximum adsorption capacity can reach up to 58.7 mg/g. The results indicated that the as-fabricated Mg-Al LDHs/Al2O3 composites have potential application in pollution control.(2) The hierarchically porous Ni-Al LDHs/Al2O3 composites were fabricated by combining the biological template method and hydrothermal method, which involves direct growth of nanoscaled LDHs platelets on the surfaces of Al2O3 fibers. The structure and morphology of composites can be tuned by adjusting the content of hexamethylenetetramine and hydrothermal temperatures. Furthermore, the calcined Ni-Al LDHs/Al2O3 composites were applied for BSA separation. It is found that the calcined composites show a high binding capacity for BSA, where the appropriate adsorption capacity is achieved at neutral pH. The results also indicate that the macroporous structures have significant effects on BSA adsorption. The anions with negative charge are easily adsorbed on the surface of positively charged LDHs nanosheets via electrostatic interactions. Hence, the desorption process of BSA may be controlled by the addition of high negative charge anions. Since the BSA adsorbed composites show near complete (over 95%) desorption in a high negative charge density of salt solution, suggesting that the paper-based Ni-Al LDHs/Al2O composites are practically usable for bioseparation.(3) The multi-scale structures and multi-components LDHs based composites were fabricated using metal cations of Zn-Al LDHs as metallic precursors. The Zn-Al LDHs/Al2O3 composites can be obtained by controlling the hydrothermal time using Al2O3 fibers as aluminum source. In order to optimize the chemical compositions of Zn-Al LDHs/ZnO/Al2O3 composites, the multi-component LDHs based composites can be observed by controlling the hydrolysis of the zinc salts and crystal growth of LDHs. The experimental results show that the longer reaction times are favorable for the crystal growth of LDHs, and the higher hydrothermal temperatures are favorable for the formation of ZnO. The low infrared emissivity values were obtained in biomorphic Zn-Al composites, indicating that the microstructure of composites may play an important role in infrared emissivity control.These systemic theoretical studies reveal the intrinsic relationships among the microstructures, components and physical and chemical properties (adsorption, bioseparation and optical properties) of LDHs based composites. This would pave a new way to design other LDHs based for applications in research and industrial fields. It can provide useful guidance for the development of composite materials with high specific surface area and excellent surface activities.