Formation pathways of MII/MIII layered double hydroxides: A review

A major issue of layered double hydroxide (LDH) materials now used in a huge amount of applications is to tailor the composition and morphological properties through a comprehensive knowledge of their formation pathways. Various cationic combinations and preparation methods have been investigated; however, the specificity of LDH materials existing for each couple of M ^II and M ^III cations complicates any implementation of a general formation mechanism. This review focuses on relevant comprehensive studies of the synthesis steps of several families of di-cationic LDH through complementary physico-chemical analyses of both the species in solution and the precipitated solid phases leading to convincing proposals of synthesis pathways. Coprecipitation of a cationic salts solution in alkaline media was the most commonly used preparation method. That of Mg/Al LDH corresponding to hydrotalcite mineral was the most widely studied. Several experimental approaches allowed separating nucleation and growth and gave unique insights on the nucleation step. It revealed the fast nucleation rate, the heterogeneous composition and the different growth rate of the Mg/Al seeds. The formation of Mg/Al, Zn/Al and M ^II/Fe ^III LDH (M ^II = Fe, Mg, Ni, Co) by coprecipitation obeyed to multistep processes. The reactivity between the M ^III(OH) ₃ species precipitated in the first step and the M ^II species in solution led in the second precipitation step to Mg/Al and Zn/Al LDH by dissolution/reprecipitation and to M ^II/Fe ^III LDH (M ^II = Fe, Mg, Ni) by surface reaction with in situ nucleation and growth. Dissolution/reprecipitation also accounted for the formation of Co/Fe LDH from Co and Fe salts or using hexaamminecobalt(III) trichloride (Co-Amm ₆ ) as reactant with complex intermediate reactions induced by the different redox reactions of the Fe ^II/Fe ^III and Co ^II/Co ^III couples of cations. A one-step process prevailed for the formation by coprecipitation of M ^II/Cr LDH (M ^II = Zn ^II, Cu ^II ) occurring by heterocondensation of hexaaquo Zn or Cu species with deprotonated chromium monomers. Other synthesis routes more adapted to the control of size and morphology of the particles or to peculiar cationic species like urea hydrolysis and epoxide methods, mixed oxide and salt-oxide reactions and condensation between aqueous cationic complexes are also detailed and compared to the results obtained by coprecipitation, particularly for Mg/Al, Zn/Al systems.