Study on the formation process for Co2+-Ni2+-Fe3+-CO32- 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.