Study on the formation process for Co²⁺-Ni²⁺-Fe³⁺-CO₃^2- LDHs prepared by trisodium citrate complexing agent-assisted homogeneous precipitation method

A new complexing agent-assisted homogeneous precipitation technology was successfully developed to synthesize Co²⁺–Ni²⁺–Fe³⁺– LDHs materials with high crystallinity and well-defined hexagonal shapes. An experiment to trace changes of phase composition, morphological features of crystallinity, molar ratio of Co²⁺/Ni²⁺/Fe³⁺, and the pH changes in the reaction system was designed to investigate the formation process of Co²⁺–Ni²⁺–Fe³⁺– LDHs materials. The formation process was proposed on the basis of the experimental results. In the case of no TSC added, Fe³⁺ ions in the reaction system precipitated to Fe(OH)₃ at low pH value, and  the Fe(OH)₃ is hard to convert to Co²⁺-Ni²⁺-Fe³⁺-- LDHs materials of highly crystalline and regular shapes with free Co²⁺ and Ni²⁺. In addition, β-Ni(OH)₂ as an impurity phase is easily formed in company with the formation of Co²⁺–Ni²⁺–Fe³⁺– LDHs. When TSC is added and optimized amount was 0.6–1.0 mmol L⁻¹, a parts of Fe³⁺ ions complex with TSC form the [Fe(C₆H₄O₇)₂]⁵⁻ ligand ions and reduce the amount production of Fe(OH)₃, causing pH deposition range of Fe³⁺ rise. Moreover, the formed [Fe(C₆H₅O₇)₂]⁵⁻ ligand ions changed the direct conversion process of Fe(OH)₃ to Co²⁺–Ni²⁺–Fe³⁺– LDHs, which reacted with free Co²⁺, Ni²⁺ ions and formed Co²⁺–Ni²⁺–Fe³⁺– LDHs materials with high crystallinity and well-defined hexagonal shapes in the higher pH value of the reaction system. While TSC optimized amount has more than 1.5 mmol L⁻¹, the excess TSC also complex some of Ni²⁺ ions except for Fe³⁺ in the solution and also induce pH deposition range of Ni²⁺ raise, which is not favorable to formation of Co²⁺–Ni²⁺–Fe³⁺– LDHs material because Ni²⁺ still existed in the complex form even in the higher precipitation pH value of the reaction system.