Layered double hydroxides (LDHs) are a class of two-dimensional anionic clays. The structure of these layered materials is based on positively charged brucite-like layers in which divalent metal cations occupy octahedrally coordinated positions in the layered structure. Some portion of divalent metal cations are isomorphously replaced by trivalent metal cations to generate net positive charge, which are compensated with exchangeable anions located in the interlayer region. LDHs can be represented by the general formula of [M_1−x²⁺M_x³⁺(OH)₂]^x+(Aⁿ⁻)_x/n·mH₂O, where M²⁺ and M³⁺ are divalent and trivalent cations, respectively; the value of x is equal to the molar ratio of M³⁺ /(M²⁺ + M³⁺), where A is the interlayer anion of valence n.

Scheme of layered double hydroxides physical appearance and morphology

The cation in the brucite layer, the anion in the interlayer region and the ratio between divalent and trivalent cations can vary to give a class of isostructural materials with variable physicochemical properties. These materials variation make them useful for a wide range of applications like polymer additive, catalyst and its support, adsorbent, drug delivery, slow-release fertilizer, and thin films.

Synthetic efforts have been done to design structural and morphological features of well-defined particles with improved materials’ performance. The syntheses under mild conditions are another issue for green chemistry concepts. Due to the high anion exchange capacity of LDHs, functional organic anions or molecules can be further intercalated in the interlayer space of LDHs, which results organic-inorganic hybrids with unique function to which host or guest alone does not have access.

Layered double hydroxides can be synthesized by different methods including homogeneous precipitation from aqueous solution or urea hydrolysis under hydrothermal conditions. It has been shown successfully, that the composition and morphology of LDHs can be controlled by the adjustment of the synthesis conditions like temperature and concentration in order to obtain well-defined particles with narrow size-distribution. Moreover, with a newly developed pH adjustment method using ion- exchange resin the controlled synthesis of finite particles has been reported. Furthermore, the defined intercalation of organic guest molecules in the interlayer space of LDHs has been achieved in order to prepare organic-inorganic hybrids with novel material properties. For the first time, the direct correlation between nanostructure and particle morphology during intercalation has been visualized which confirmed the topochemical reaction of the intercalation.

Scheme of a brucite-like LDH structure	Scheme of layered double hydroxides synthesis