Synthesis of NiGa Layered Double Hydroxides. A Combined Extended X-ray Absorption Fine Structure, Small-Angle X-ray Scattering, and Transmission Electron Microscopy Study. 1. Hydrolysis in the Pure Ni2+ System

Takovites are nickel-based layered double hydroxides (LDHs) with a general formula that can be written as Ni 1 - x Al x (OH) 2, A z - x / z, y H 2 O, where A is a compensating interlayer anion. As in some other LDH samples, the positive charge of the layer can be adjusted upon synthesis and various anions can be exchanged in the interlayer region. It is then important to understand the synthesis pathway of these materials. We then undertook a study on the hydrolytic behavior of pure Ni salts and mixtures of Ni and Ga salts. This paper focuses only on the hydrolysis of Ni 2+ ions carried out by base addition. The samples will be defined by their hydrolysis ratio R = [OH - ]/[Ni 2+ ]. For all R values, colloids are observed in the final suspensions. Each hydrolyzed sample was studied by Ni K-edge extended X-ray absorption fine structure (EXAFS) to obtain information on the local structure of the colloids. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) are also used to obtain concomitant information at larger scale. EXAFS results reveal that upon Ni 2+ hydrolysis by NaOH, the number of nickel neighbors increases with R, atomic distances corresponding only to edge-sharing nickel octahedra. These results could then be interpreted as revealing the precipitation of increasing amounts of Ni(OH) 2, with increasing R values. However, SAXS curves display a significant evolution with R . Modeling shows the presence of rod-shaped colloids for low R values. These rodlike particles progressively disappear in favor of disklike platelets, that are the only species present in suspension for R = 2.0. Such an evolution is confirmed by TEM analysis.