关于LDHs的专业英语课程论文.docx

1、关于LDHs的专业英语课程论文Research Development on Structure and morphology synthesis of layered double hydroxidesAbstract: Layered Double Hydroxides (LDHs) or hydrotalcite-like compounds (HTLc) are a class of anionic clays materials whose structure is based on brucite (Mg(OH)2)-like layers in which some of the

2、 divalent cations have been replaced by trivalent metallic cations giving positively-charged sheets and hydrated interlayer domains containing anionic species. This characteristic results in much more difficulty in controlling its structure and morphology than single metal hydroxides. This review pr

3、ovides an overview of the LDH synthesis methods, and the recent advancement that has been achieved in the morphology synthesis of layered double hydroxides.Keywords: Morphology control; Layered double hydroxides; Synthesis; Template1. IntroductionLayered double hydroxides (LDHs) are lamellar mixed h

4、ydroxides containing positively charged main layers and undergoing anion exchange chemistry 1. The expected hexagonal plate- or flake-like crystallites with diameters from nanometers to microns were grown by various conventional preparation methods such as co-precipitation 2, urea hydrolysis 3, sepa

5、rate nucleation 4 and aging, ion-exchange 5, metal oxide precursor 6 method, reflecting the rigidity and hardness of LDHs host layer, which badly limited the application of layered double hydroxides in catalysis, adsorption and other domains. In recent years, however, The morphology and size control

6、 of inorganic solids has achieved great development in the material science. A number of imaginative routes to synthesize inorganic materials with unusual morphology have been recently reported 7. many studies have been devoted to investigating the parameters on the Structure and morphology of LDHs.

7、 Fortunately, with the development of bio-mineralization 8, confined growth of nano-particles 9, the inorganic/organic self-assembly 10 and template chemistry 11, Template chemistry has been developed to tailor the material formation of minerals12, porous solids 13, polymers 14, etc. Some templates

8、that consist of bio-macromolecules 15, such as vesicles 16, micelles 17, micro-emulsions 18, organo-gels 19, and conformational defined peptides 20, polymers 21, are used to direct the formation of bio-minerals and have led to materials with specific size, morphology and properties. Several new meth

9、ods have been used to control the morphology synthesis of LDHs.2. Composition and structure of LDHsLDHs are a class of two-dimensional nano-structured anionic clays. The structure of LDHs can be described as a cadmium iodide-type layered hydroxide (e.g., brucite, Mg(OH)2), where a fraction of the di

10、valent cations coordinated octahedrally by hydroxyl groups have been isomorphously replaced by trivalent cations, giving positively charged sheets. The net positive charge is compensated by anions in the interlayer region between the brucite-like sheets. Some hydrogen-bonded water molecules may occu

11、py the free space in this interlayer region. The structure of LDHs and a typical octahedral unit are shown in Fig. 1 22. The basal spacing (c) is the total thickness of the brucite-like sheet and the interlayer region. The octahedral units of M2+ or M3+ (six fold coordinated to OH-) share edges to f

12、orm infinite sheets. These sheets are stacked on top of each other and are held together by hydrogen bonding. LDHs can be represented by the general formula M1-x2+ Mx 3+(OH)2x+(An-)x/n -mH2O, where M2+ and M3+ are divalent and trivalent cations, respectively; the value of x is equal to the molar rat

13、io of M3+/( M2+ M3+), whereas A is the interlayer anion of valence n. The identities of M2+, M3+, x, and An- may vary over a wide range, thus giving rise to a large class of isostructural materials with varied physicochemical properties 23. The parent material of these anionic clays is the naturally

14、 occurring mineral hydrotalcite which has the formula Mg6Al2(OH)16CO3 - 4H2O.Different stacking arrangements of the brucite-like sheets have been observed. The brucite-like sheets can be stacked either with two layers per unit cell in hexagonal symmetry (manasseite) or with three layers per unit cel

15、l in rhombohedral symmetry (hydrotalcite), or in less symmetrical arrangements. In general, LDHs of rhombohedral symmetry have mainly been found. M2+ (Mg2+, Fe2+, Co2+, etc.) and M3+ (Al3+, Cr3+, Fe3+, etc.) ions that have ionic radius similar to that of Mg2+ can be accommodated in the holes of the

16、close-packed configuration of OH groups in the brucite-like sheets to form LDHs 24. In addition, LDHs containing more than two species of the second cation have been synthesized. Another class of LDHs, which contains monovalent and trivalentmatrix cations such as LiAl2(OH)6+A-mH2O, has also been pre

17、pared. The charge density and the anion exchange capacity of the LDHs may be controlled by varying the M2+/M3+ ratio. The most common anion found in the naturally occurring LDHs is carbonate. In practice, however, a broad spectrum of charge-balancing anions may be incorporated, namely halides, oxy-a

18、nions, silicates, poly-oxometalate anions, complex anions, and organic anions 25.Fig.1 - Schematic representation of the LDH structure.3. Conventional preparation methods of LDHsLDHs can be regarded as a class of materials that are simple to synthesize in the laboratory, although not always as pure

19、phases. In general, there are several approaches to prepare LDHs. Recently, a thorough review has been prepared by He et al 26 on the preparation of LDHs.The simplest and most commonly used method is coprecipitation 2. In this method, aqueous solutions of M2+ and M3+ containing the anion that is to

20、be incorporated into the LDHs are used as precursors, of which Mg and Al are the most frequently used metal precursors. In order to ensure simultaneous precipitation of two or more cations, it is necessary to carry out the synthesis under conditions of super-saturation. There are generally two types

21、 of co-precipitation conditions, namely co-precipitation at low super-saturation and co-precipitation at high super-saturation. Co-precipitation at high super-saturation generally gives rise to less crystalline materials compared to those with low super-saturation, due to the formation of a large nu

22、mber of crystallization nuclei. After precipitation at low and high super-saturation, a thermal treatment process is performed to increase the yields and crystallinity of the materials. This is followed by an aging process conducted for a period ranging from a few hours to several days. In order to

23、ensure the purity of the synthesized LDHs, the use of the de-carbonated ultra-pure water and the application of vigorous stirring in combination with nitrogen purging in the synthesis process are necessary.A synthesis method of LDHs involving separate nucleation and aging steps has been proposed by

24、Zhao et a 3. The key features of this method are a very rapid mixing and nucleation process in a colloid mill, followed by a separate aging process. As compared to the conventional coprecipitation process, this method results in a slightly higher degree of crystallinity of the LDH materials, smaller

25、 crystallites with a higher aspect ratio, and a narrower distribution of crystallite sizes, owing to the extreme forces to which the nucleation mixture is subjected in the colloid mill.A number of studies have reported the synthesis of LDHs using the urea hydrolysis method 4. Urea has a few unique p

26、roperties such as its weak brnsted base characteristic, high solubility in water, and its hydrolysis rate that can be easily controlled, making it an attractive agent to precipitate several metal ions as hydroxides or as insoluble salts when in the presence of a suitable anion. The optimum condition

27、s to prepare the LDHs with a good crystal quality in a relatively short time using this method have been suggested to involve dissolving solid urea in a 0.5M solution of the chosen metal chlorides to give a urea/metal ion molar ratio of 3.3. The compounds prepared using this method display homogeneo

28、us sizes and platelet-like primary particles with well-defined hexagonal shapes, which may be very interesting from the viewpoint of nanotechnology since LDHs offer nano-size two-dimensional spaces for the creation of functional materials.LDHs can be also prepared by the ion exchange method 5. This

29、method is useful when the coprecipitation method is inapplicable, e.g. when the divalent or trivalent metal cations or the anions involved are unstable in the alkaline solution, or when the direct reaction between the metal ions and the guest anions is more favorable. In this method, the guests are

30、exchanged with the anions present in the interlayer regions of the LDHs to produce specific anion-pillared LDHs.Another common method to produce LDHs is rehydration/ reconstruction using the structural memory effect 27. This method involves calcination of LDHs to remove the interlayer water, interla

31、yer anions, and the hydroxyl groups, resulting in mixed metal oxides. It is interesting to note that the calcined LDHs are able to regenerate the layered structure when they are exposed to water and anions. In addition, the anions included need not be the same species originally present in the inter

32、layer of the uncalcined LDHs, and therefore this is an important method to synthesize LDHs with desired inorganic or organic anions to fulfill specific application requirements.The hydrothermal method 28 is usually used when organic guest species with low affinity for LDHs are required to be intercalated into the interlayers, and when the ion exchange and coprecipitation methods are not applicable. This method has been shown to be effective because only the desired organic anions can occupy the interlayer space under the hydrothermal condition since insoluble magnesium and alumin