Oriented Growth Of Layered Double Hydroxides Films And Their Wettability

Layered double hydroxides(LDHs),also known as hydrotalcite-like materials or anionic clays,are an important class of lamellar functional materials.They have a wide range of applications in many important industry fields,e.g.adsorption,catalysis,separation,biology and medicine. In recent years,layered double hydroxides films or membranes have aroused great interest because of their potential novel applications as optical, electrical and magnetic devices.Therefore,considerable research efforts have been directed to the fabrication,characterization,and application of LDHs films.In this dissertation,we have demonstrated in situ crystallization technology for preparation of LDHs films by using porous anodic alumina/aluminums(PAO/Al)as both substrate and sole source of aluminum. Compared with the LDHs films prepared by the method reported in the previous literature,the obtained LDHs film via in situ crystallization technology have better adherence and mechanical stability.Moreover, considering the unique lamellar structure,variability of chemical composition and ion-exchange capability as salient characteristics of LDHs allow us design and preparation of a class of LDHs and organic guest intercalated LDHs films.In recent years,there has been considerable interest in the development of the materials whose surface properties can be modulated dynamically. Super-hydrophobic surface have recently attracted a great deal of interest because of their potential practical applications in microfluidic devices as well as in developing new waterproof coatings,stain-resistant finishes,and self-cleaning materials.Super-hydrophobic surface are conventionally devised and fabricated by combining appropriate surface roughness with low-surface-energy materials.With this in mind,we take full advantage of the as-prepared oriented LDHs films,the surface of which has peculiar micro-/nano-structure resulting from the vertically aligned hexagonal LDHs nano-sheet to design super-hydrophobic surface.The as-prepared film has shown excellent super-hydrophobicity after simple modification with inexpensive long-chain fatty acid salt.The investigation will make LDHs material posses superior performance and wide application.This thesis was focused on the investigation of the fabrication, characterizations and wettability of the as-prepared LDHs films and the super-hydrophobic LDHs films after surface modification using long-chain fatty acid salt.In this work,the main results are as follows:Firstly,oriented NiAl-CO₃^2--LDHs films with curved hexagonal microstructure have been prepared by the in situ crystallization in a closed hydrothermal system without use of any adscititious aluminium source and shape-directed surfactant on porous anodic alumina / aluminium(PAO/Al) substrate.The crystal orientation and morphology of the polycrystalline films obtained were investigated by powder X-ray diffraction(XRD), IR-spectroscopy,scanning electron microscope(SEM),atomic force microscopy(AFM),and energy dispersive X-ray(EDX)spectrometry.The lack of any(00l)reflection for the monolayer film reveals an extremely well oriented assembly of LDHs microcrystals in the film and the orientation is preserved along the film thickness.The morphology observed by scanning electron microscopy shows that the surface micro/nano-structure of the resulting oriented and curved hexagonal microstructures of LDHs films can be flexibly adjusted over a wide range by controlling the crystallization temperature and time.After simple surface modification,films re-treated under certain condition have found to exhibit excellent super-hydrophobicity.Based on the unique LDHs super-molecular structure and the ion-exchange capability as salient characteristics of LDHs, lauric-intercalated ZnAl layered double hydroxide(ZnAl-La-LDH)hybrid films have been prepared for the first time on porous anodic alumina/aluminum(PAO/Al)substrates by one step anion-exchange with ZnAl-NO₃^--LDH precursor films under mild conditions.XRD results reveal that La anions have been successfully intercalated into the ZnAl-LDH interlayer region and accommodated at a slanting angle of ca.60°in the interlayer region,forming the tail-to-tail bilayer with hydrophilic headgroups oriented towards metal hydroxide layers.The morphology observed by SEM shows that a large number of hemispherical protrusions,which consist of LDH plate-like microcrystals,randomly locate on the surface of the obtained hybrid films.The contact angle measurement results showed that the ZnAl-La-LDHs hybrid films not only have excellent hydrophobic properties, but also strong adhesive to water.The maximum adhesive force estimated by a high-sensitivity microelectromechanical balance on the ZnAl-La-LDH hybrid film is ca.85μN.The excellent sticky super-hydrophobic property of hybrid films can be attributed to their randomly distributed hemispherical protrusions microstructure whose voids are of 40～80μm.This presumption can be proved by the characterization of dynamic CA measurements via Wilhelmy plate method.Moreover,the formation mechanism of hemispherical protrusions have been investigated and proposed,and the mechanism model proved to be suitable through the confirmation of design experiment.Secondly,two type of different metal-beating hydrotalcite-like MAl-LDH(M=Ni,Zn)films have been successfully prepared via the in situ hydrothermal crystallization technology.XRD and FT-IR characterization reveal that NiAl-LDH film and ZnAl-LDH film possess different orientation and interlayer anions,though they are synthesis at same experimental conditions.The evolution and growth mechanism of NiAl-LDH and ZnAl-LDH films have been comparative investigated.The "homogeneous nucleation" and "heterogeneous nucleation" mechanism model were proposed to explain NiAl-LDH and ZnAl-LDH films growth process, respectively.The different growth mechanism can satisfactorily explain the different orientation and types of interlayer ions for different metal bearing MAl-LDH(M=Zn or Ni)films.Finally,mixed metal oxide(MMO)films containing Ni²⁺and Al³⁺have been fabricated by a simple process involving calcinations of layered double hydroxide(LDH)film precursors at 300-600℃for 4 h in air.SEM reveals that the resulting NiAl-MMO thin films maintain the original nest-like morphology of the precursor films without any deformation of the microstructure during the thermal decomposition process.After surface treatment with an aqueous solution of sodium laurate,the wettability of the NiAl-LDH and NiAl-MMO thin film surfaces changes from hydrophilic to hydrophobic.Moreover,whereas the NiAl-LDH film exhibits a low adhesion for water(Cassie-Baxter behavior)after surface modification,the NiAl-MMO films exhibit a high adhesion for water(Wenzel behavior).A possible explanation for this phenomenon has been suggested.