Al ^III - and Fe ^III -based layered double hydroxides: a review of their characterization techniques and formation mechanisms by co-precipitation with the variable pH method

This study provides a review on the synthesis, the techniques of characterization, and the formation mechanisms of AlIII- and FeIII-based layered double hydroxides (LDHs) by co-precipitation with the variable pH method. Based on this literature review, the use of many complementary characterization techniques – from in situ/operando to ex situ – is required to obtain the most detailed information on solid and liquid phases during the synthesis. With the current knowledge, the accepted formation mechanism can be summarized in four main consecutive steps with increasing pH: (i) the precipitation of MIII (oxyhydr)oxides at low pH; (ii) the adsorption of M2+ on the MIII (oxyhydr)oxide surface – with possible associated redox reactions for redox-sensitive elements, transformation of MIII (oxyhydr)oxide or nucleation of MII (oxyhydr)oxide –; (iii) the formation of primitive LDHs, with cationic substitution and anion intercalation, and the growth of LDH particles by dissolution-reprecipitation; and (iv) the transformation of LDH phases associated with the formation of secondary phases or impurities. However, the precise description of the successive phases formed during the synthesis depends on several parameters, i.e., the nature of salt precursors and divalent cations, or the composition and rate of addition of base solution, which does not allow for providing a universal detailed formation mechanism of LDHs. Further experimental and computational studies for a large variety of MII/MIII LDHs are still required in the quest to control the synthesis of LDHs with their required properties of interest for their specific applications.