Studies On The Fabrication And Application Of Oriented Films Derived From Layered Double Hydroxides

In this paper we are trying to find a simple,low cost and environmentally friendly method for preparing a large class of nanostructured functional films. Based on this conception we began with low-cost inorganic materials of Layered Double Hydroxides(LDHs)and have prepared a series of high quality LDHs,Mixed Metal Oxide(MMO)and spinel films.The main results are as follows:1.Uniform small LDHs nanoparicles was prepared by a method involving separate nucleation and aging steps(SNAS),in which rapid mixing and nucleation in a colloid mill is followed by a separate aging process. Transparent self-supporting and(00l)-oriented LDH films with large dimensions up to centimeters was obtained by a simple evaporation method of LDHs suspensions,which favors not only face-to-face type interactions but also edge-to-edge type interaction between individual LDH platelets. Compared with those produced by other methods reported in the literature,the resulting films are superior in that whilst they are sufficiently thick and mechanically robust to be self supporting,they retain a high degree of optical transparency. Europium complexes of ethylenediaminetetraacetate(EDTA)and nitrilotriacetate(NTA),[Eu(EDTA)(H₂O)₃]^-and[Eu(NTA)₂(H₂O)]^3-,which have similar geometries but different charge densities,have been incorporated in ZnAl-LDHs.The thermal stability,reorientation behavior and luminescence properties are significantly influenced by their charge density.The (00l)-oriented Eu-containing films show interesting polarized luminescence.2.Large(～1 cm²)transparent highly(111)-oriented self-supporting Ni-Al oxide films of uniform small nanoparticles have been prepared using (00l)-oriented Ni₂Al(OH)₆(NO₃)·2.1 H₂O LDH as single precursors based on oriented interaction and topotactic transformation of LDHs.The NiAl-LDH was first convert to Al-doped NiO phase upon calcinations.Further heat-treatment gives formation of NiAl₂O₄ spinel phase and the film converts to composite NiO/NiAl₂O₄ films.The resulting self-supporting Ni-Al oxide films have a unique set of properties including crystalline orientation,large continuity,optical transparency,crack-free form,narrow mesoporosity and high thermal stability,showing their great potential for application as catalysts or catalyst supports,in sensors,and as ultrafiltration membranes in harsh environments.The Ni-Al oxide films formed by heating(00l)-oriented NiAl-LDH film have a preferred(111)orientation,which can be rationalized in terms of a topotactic transformation of the close-packed oxygen layers:(00l) NiAl-LDH→(111)Al-doped NiO→(111)NiO +(111)NiAl₂O₄.Oxide nucleation occurs on the(00l)plane of LDHs where the packing of cations is the densest,giving the lowest nucleation energy,resulting in the formation of the(111)NiO +(111)NiAl₂O₄ film via the intermediate (111)-oriented Al-doped NiO film.The mesoporous structures of the Ni-Al oxide films were investigated the possibility of tailoring by varying the synthesis parameters such as the calcination temperature,the temperature ramping rate and the particle size of the precursor NiAl-LDH.In all these cases the as-prepared Ni-Al oxide films have typeâ…£isotherms with pronounced H2 type hysteresis loops and a narrow unimodal pore size distribution.An increase in both pore size and broadening of pore size distribution were observed by elevating calcination temperature,decreasing the temperature ramping rate and increasing particle size of the precursor NiAl-LDH.It was found that the mesoporosity of Ni-Al oxide films was most sensitive to the calcination temperature and both the pore size and pore size distribution show a steep monotonic increase with increasing temperature.3.Large oriented macroporous NiAl₂O₄ spinel films were prepared involving a very simple three-step process which is based on a combination of oriented assembly,topotactic transformation,and selective leaching of NiO and,unlike all previous reports in the literature,the high degree of orientation is achieved without the requirement for a template,structure-directing agent or single crystal substrate.It should also be emphasized that the size of the pores and the spinel particles can be easily and independently tailored by tuning the ratio of M²⁺to M³⁺(higher amounts of M²⁺lead to more metal oxide MO being formed and larger pores)and the heating temperature(higher temperatures lead to both larger particles and larger pores after the oxide particles are removed).The NiAl₂O₄ nanoparticles along(111)plane increases from 22.7 to 32.7nm,when the temperature increases from 950 to 1100. Correspondingly,an decreasing of BET surface area,an increase in both pore size and broadening of pore size distribution were observed by elevating calcination temperature.In summary,we have developed a novel method for fabricating of a large class of highly homogeneous large continuous and oriented nanostructured films.The method employs LDHs as single inorganic precursors without the necessity for any templates,structure-directing agents or single-crystalline substrates and is simple,economic,and amenable to scale-up.The method is very versatile because a wide range of LDH precursors are readily available, and a variety of functional LDHs,MMO amd spinel films for specific applications can be obtained by changing the composition of the LDH precursor.For example,we have used the same route to form a macroporous nickel ferrite(NiFe₂O₄)film with a high degree of magnetic anisotropy due to its highly orientated structure.The nanostructures and porosity of the resulting oxide films can be controlled by changing the calcination temperature.This method can be readily extended to a wide range of other functional bi- or multi-metallic oxide systems tailored for specific applications by changing the metal composition of the LDH precursors.We believe that this method,based on oriented interaction and topotactic transformation of LDHs,represents an important advance in fundamental studies of LDHs and the resulting mesoporous mixed metal oxide films have a unique set of properties which suggest promising applications in the fields of catalysis,catalyst supports,separation membranes, sensors,optical devices and magnetic media.