Study On The Oriented Growth And Properties Of Layered Double Hydroxide Films

Layered double hydroxides (LDHs) are a calss of anionic lamellar functional materials. They have a wide variety of applications including as additives in polymers, as precursors to magnetic materials, in biology and medicine, in catalysis and environmental remediation. Recently, ways have been reported to organize LDH microcrystals into large uniform films, which have widened the range of applications of LDH as catalysts, metallic anti-corrosion coatings, and in optical, electric, magnetic devices.In this dissertation, we successfully achieve to fabricate LDH films with different orientations (i.e., the c-axis of the LDH crystallites parallel and perpendicular to the substrate and three dimentional LDH composite membranes with network structure) through in-situ growth methods. To widen the application area of the LDH films, aluminum, glass, poly (vinyl alcohol) (PVA) and eggshell membrane (ESM) were chosen as substrates which represent metal, inorganic material, polymer and biomaterial four typical textures respectively. The effect of the different texture on the morphology of the LDH films was also investigated. The growth mechanisms of the LDH films with different orientations were discussed and compared. The properties of the resulting films were explored and tuned by varying the preparation conditions. The details are shown below:1. For the LDH films with c-axis parallel to the substrate, we obtained ZnAl-LDH films and MgAl-LDH films on aluminum substrate, and MgAl-LDH films on glass substrate. The films were characterized in detail and the properties were investigated respectively.(1) We obtained ZnAl-NO3--LDH films with c-axis parallel to the aluminum substrate by using one-step in-situ growth method. The films were continous and densely packed on the substrate with strong adhesion. The anti-corrosion ability of the films on the aluminum was investigated by polarization curve tests, electrochemical impedance spectroscopy (EIS) tests and immersion tests, and it results that the film could provide long-term protection to the substrate. The densely oriented LDH films could serve as passive layers with high charge transfer resistance contributing to the anticorrosion ability. In addition, the NO3--LDH could exchange the corrosive ions, such as Cl-, into the interlayer, and the ions would be immobilized by the strong interaction between the host layer and the interlayer anions.(2) MgAl-CO32--LDH films with c-axis perpendicular to the aluminum substrates were fabricated by using urea hydrolysis method. The hexagonal platelet-like LDH crystallites are interlaced with each other forming a network of vacant pockets which results in a low density of the film. After being calcined at 500℃, the LDH films were transferred to MgAl-MMO films which could retain the morphology of the precursor films. Dielectric tests showed that the low density of the MMO film results in a low k-value. Furthermore, we successfully controlled the microstructure of the MMO films by varying the crystallization conditions (metal ion concentration, and crystallization time) of the corresponding precursor LDH films. Relatively low k-values in the range 2.0-4.7 were achieved.(3) MgAl-CO32--LDH films with c-axis perpendicular to the glass substrates were fabricated by using urea hydrolysis method. After being calcined at 500℃, the LDH films were transferred to MgAl-MMO films. Bactericidal experiments with colon bacillus and S. aureus were carried out using the as-synthesized MgO-Al2O3 composite film, and both the bactericidal efficiency could reach 99.9% at high concentration gradient. It seems that there are rich Fs+ colour centers on the surface of the highly dispersive nano MgO particles, and highly active superoxide ions O2-could be produced on these colour centers, which can react with the peptide linkages in the cell wall of bactrials and thus destroy them.2. For the LDH films with c-axis perpendicular to the substrate, we obtained this aimed oriented MgAl-LDH films on substrate through in-situ growth method by using PVA as structure-direct agent. We propose that the hydroxyl groups in PVA interact with the Mg/Al-bound hydroxyl groups on the surface of the LDH crystallites via hydrogen bonding which finally orients the metal hydroxide sheet of the LDH crystallites with their c-axis perpendicular to the substrate.3. For the three-dimensional LDH films with complicated structure, we fabricated a LDH/ESM composite membrane by using ESM as the substrate as well as template. SEM images showed that the LDH crystallites could grow on the protein fiber of the ESM, and the final composite membrane retained the network structure of the ESM. Further investigate showed that the inorganic-bio composite membrane could be used as highly efficient adsorbent for hexavalent chromium Cr(VI) removal even when the pH value of the Cr(VI) solution was not further adjusted by addition of acid, which is of practical significance in a wastewater treatment plant. The adsorption isotherm showed a good fit with the Langmuir isotherm model.