Layered double hydroxide (LDH) films have been widely investigated in the last few years because of their promising applications in areas such as catalysis, anti-corrosion coatings for metals, and as components in optical, electrical, and magnetic devices. In this Feature Article we review recent work, from our own laboratory and elsewhere, on the synthesis, properties and applications of functional LDH films, and also offer some perspectives for the design of future multifunctional LDH films.

The costs of metal corrosion amount to several percent of the GDP of an industrialized country. [1] In the case of aluminum, chromate-based coatings [2] provide highly effective corrosion protection, but environmental regulations are increasingly restricting their use. Anodization [3] increases the thickness of the oxide layer, but it retains its porous nature.[4] Layered materials such as anionic clays (e.g., layered double hydroxides ) [5,6] and cationic clays (e.g., montmorillonite) [7] have been widely investigated as additives in organic anticorrosion coatings or as polymer-clay nanocomposite corrosion-resistant coatings. Zeolites [8,9] have also been explored as corrosion-resistant coating materials. Hydrophobic self-assembled monolayers (SAMs) [10] of surfactant molecules on the surface have recently been proposed as corrosion inhibitors but suffer from the drawbacks that the layers have limited stability and molecule-sized defects which allow water to reach the underlying surface. These problems should be mitigated if the surfactant could be incorporated in an inorganic host matrix, a thin film of which has been previously strongly bonded to the aluminum surface.Layered double hydroxides (LDHs) are one such potential inorganic host. They can be expressed by the generalwhere the cations M 2+ and M 3+ occupy the octahedral holes in a brucite-like layer and the anion A nÀ is located in the hydrated interlayer galleries.[11] The ability to vary the composition over a wide range allows materials with a wide variety of properties to be prepared. We recently showed [12] that an NiAl-LDH-CO 3 2Àfilm can be formed directly on porous anodic alumina/ aluminum (PAO/Al) substrates; since PAO/Al is the only source of Al 3+ , the thin film grows directly on the substrate and thus exhibits good adhesion and mechanical stability. [13] Treatment with sodium laurate (n-dodecanoate) results in surface-bonded laurate films showing superhydrophobicity with water contact angles (CA) greater than 1608. Here we show that intercalation of laurate anions by ion exchange with ZnAl-LDH-NO 3 À film precursors on a PAO/Al substrate leads to a hierarchical micro/nanostructured superhydrophobic film which provides a very effective corrosion-resistant coating for the underlying aluminum.The aluminum substrate was first coated with a layer of porous anodic alumina by conventional anodization and subsequently treated with an alkaline solution of zinc nitrate in the presence of an excess of nitrate anions. In addition to the peaks of the PAO/Al substrate, the XRD pattern of the film shows two low-angle reflections at 8.874 and 4.462 (Figure 1 a), which can be assigned to the [003] and [006] reflections of an LDH phase with a basal spacing of 0.887 nm, consistent with the literature for ZnAl-LDH-NO 3 À .[14] The presence of NO 3 À in the interlayer galleries of the LDH film was confirmed by the characteristic peak at 1384 cm À1 in the FTIR spectrum. It is well known that LDHs in their usual powder form readily exchange NO 3 À ions for oth...

Thermal decomposition of layered double hydroxides (LDHs) is a way of fabricating mixed metal oxide (MMO) nanocomposite materials composed of metal oxide and spinel phases. A detailed understanding of the mechanism of the transformation of the LDH precursor to the MMO should allow the properties of the resulting MMO nanocomposites to be tailored to specific applications. Here we report a systematic investigation of the structure, composition, and morphology evolution from ZnAl-LDHs to ZnO-based MMO nanocomposites composed of ZnO and ZnAl 2 O 4 on calcination at different temperatures. The nucleation and oriented growth of ZnO crystallites and the formation of ZnAl 2 O 4 were monitored by high resolution transmission electron microscopy (HRTEM) combined with selected-area electron diffraction (SAED), in situ X-ray diffraction (XRD), solid-state 27 Al magic-angle spinning nuclear magnetic resonance ( 27 Al MAS NMR), and thermogravimetric and differential thermal analyses (TG-DTA). The layered structure of the LDH precursor was maintained as the temperature was increased from room temperature to 180 °C. Upon further heating from 200 to 400 °C, ZnO nuclei doped with Al 3þ were first formed as an amorphous phase and then underwent oriented growth along the AE1010ae direction. The high aspect ratio of the LDH platelets is responsible for the oriented growth of the resulting ZnO crystallites. On further increasing the calcination temperature, Zn 2þ ions were continuously released from the amorphous phase resulting in the formation of crystalline ZnO nanoparticles doped with Al 3þ , which are homogeneously dispersed throughout the amorphous phase. When the calcination temperature reached 500 °C, Al 3þ ions were released from the ZnO-like structure resulting in the formation of ZnAl 2 O 4 spinel and the crystallinity of the spinel increased gradually with increasing temperature. Sintering of ZnO and ZnAl 2 O 4 , with concomitant loss of the platelet-like morphology, occurred below 800 °C. UV-visible spectroscopy showed that the ZnO/ZnAl 2 O 4 nanocomposite prepared by calcination of the ZnAl-LDH precursor at 800 °C has superior UV-blocking properties to both commercial ZnO and a physical mixture of ZnO and ZnAl 2 O 4 .

Layered double hydroxides (LDHs), also known as hydrotalcite-like materials or anionic clays, are an important class of layered materials. Various studies show that LDHs have a wide range of applications in industry, e.g., catalyst precursors, ion exchangers, adsorbents for environmental contaminants, and substrates for the immobilization of biological material. [1][2][3][4] However, for the purpose of developing novel innovative applications of LDHs as materials for chemical sensors, [5,6] clay-modified electrodes, [7,8] corrosion-resistant coatings, [9,10] membrane catalysis, or components in optical and magnetic devices, intensive studies have been conducted aimed at organizing LDH microcrystals into large uniformly aligned 2D arrays or films. Several methods have been employed to fabricate LDH films on different substrates thus far. For example, LDH microcrystals have been deposited on indium-doped SnO 2 coated glass, platinum disks, and gold electrode surfaces from colloidal suspensions in order to prepare LDH films for electrode modification by deposition [11,12] and Langmuir-Blodgett methods. [8] Most of the films obtained, however, were not oriented or uniformly aligned as it is hard to control the LDH crystallite orientation using these methods. Recently, new techniques have been reported for the fabrication of oriented LDH films. Pinnavaia and co-workers found that colloidal suspensions of LDHs obtained through hydrolysis of LDH/methoxide were able to form transparent and smooth films, [13] in which the LDH microcrystals were extremely well oriented. By employing ultrasonification, Jung and co-workers obtained a monolayer of LDH films with a high packing density and a preferred orientation with the c-axis perpendicular to the substrate surface (ab-face parallel). [14,15] However, this route did not allow control of the orientation of the LDH microcrystals with respect to the substrate plane because of the intrinsic propensity of the microcrystals to align in an orientation that leads to maximum faceto-face contact between the crystals and the substrate. In spite of the progress made during the last decade in research on LDH films and their crystal orientation, there has been no synthetic method for directly growing uniformly aligned LDH polycrystalline films from a substrate. Growing thin films directly from a substrate considerably improves the adherence and the mechanical stability of the resulting thin film, compared to colloidal-deposition techniques (for example, spin-coating, dip-coating, and screen-printing).[16] Therefore, the exploration of new approaches to fabricate oriented LDH films on substrates is of significant importance. Among the existing synthetic methods to fabricate inorganic films, hydrothermal synthesis shows high flexibility in terms of control of the structure and morphology of the resulting inorganic materials. It is also a well-known pathway for fabricating inorganic films with the desired micro-or nanostructure and controlled crystal orientation. Our group has recently repor...

A zinc-aluminum layered double hydroxide (ZnAl-LDH)/alumina bilayer film has been fabricated on an aluminum substrate by a one-step hydrothermal crystallization method. The LDH film was uniform and compact. XRD patterns and SEM images showed that the LDH film was highly oriented with the c-axis of the crystallites parallel to the substrate surface. The alumina layer existing between the LDH film and the substrate was formed prior to the LDH during the crystallization process. Polarization measurements showed that the bilayer film exhibited a low corrosion current density value of 10(-8) A/cm(2), which means that the LDH/alumina bilayer film can effectively protect aluminum from corrosion. Electrochemical impedance spectroscopy (EIS) showed that the impedance of the bilayer was 16 MOmega, meaning that the film served as a passive layer with a high charge transfer resistance. The adhesion between the film and the substrate was very strong which enhances its potential for practical application.