Layered Double Hydroxides (LDHs)

Abstract

Hydrotalcite, the first natural mineral belonging to the family of layered materials, was discovered in Sweden by Hochstetter in 1842, but it was not until 1930 that the first study on its synthesis, solubility, stability, and structure was carried out by Feitknecht.Since then, the family of layered materials has become wider and wider, and also been variously named over time.Layered Double Hydroxides (LDHs), Layered Hydroxicarbonates, Hidrotalcite-like Materials, Anionic Clays, etc., are just some of the many examples, although none of them are sufficiently exhaustive and reflect the current situation.Regardless of the denomination, these materials are not as naturally occurring as cationic clays are, nonetheless they are easy to prepare and are low-cost.What made and makes these materials extremely interesting is the fact that the nature of the layer cations can be varied within a wide selection, and the nature of the interlayer anion can be chosen, almost at will, between organic and inorganic anions, polymetalates, simple anionic coordination compounds, etc.Like the cationic clays, they can be pillared and, even more importantly, the interlayer anions can be easily exchanged.This property increases the possible applications and opens new routes to the synthesis of derivatives.Furthermore, unlike cationic clays, they are able to recover the lamellar structure after undergoing thermal decomposition.This property can also be used as a synthesis technique.Since the pioneering work of Feitknecht, LDHs have been synthesized by direct and indirect methods, such as coprecipitation, hydrothermal growth, sol-gel synthesis, soft chemistry, electrochemical synthesis, anion exchange, and those in which LDHs are used as precursors.The opportunities offered by these properties are extremely ample, and it is precisely for this reason that the applications of LDHs are constantly growing.The main areas of interest range from renewable energy production to water purification and remediation, including functionalized materials for piezoelectric nanogenerators and gas sensing.Great attention is paid to biomedical applications and to the synthesis of hybrid smart nanocomposites, which involve expanding sectors such as drug-delivery, food packaging and safety.Within this Special Issue, eight articles are collected, and are divided between synthesis techniques [1-3], applications [4-6], and review works [7,8].The first work, related to the synthesis of Mg-Al, Ca-Al, Zn-Al and Cu-Al LDHs, is the one carried out by Barnard and Labuschagne [1].The authors, in order to propose a green synthesis technique, implement the wet mechanochemical method through the use of a Netzsch LME 1 horizontal bead mill, designed "ad hoc" for wet grinding applications.In this way they are able to eliminate the production of salt-rich effluent and the control of pH.Furthermore, an aging phase allows a better conversion of raw materials into LDH structures, as well as a morphological improvement of the structures.Another notable point is that the selected mill can be easily scaled up for the production of large quantities of LDH products.In addition, in the work proposed by Gevers and Labuschagné [2], the authors take care to adopt an environmentally friendly synthesis.In particular, they present the results obtained through the hydrothermal synthesis of hydrocalumite (HC) and Al(OH) 3 in water, examining the parameters