# Fine Characterization of Co/Fe-Based Materials: Insights into the Influence of Cation Ratios Between 2/2 and 10/2 on Obtaining Layered Double Hydroxides

## TL;DR

This paper studies how varying cobalt and iron ratios affects the formation of layered double hydroxides, revealing insights into their structure and properties.

## Contribution

The study provides a detailed characterization of Co/Fe LDHs using multiple techniques, revealing how cation ratios and pH influence phase formation.

## Key findings

- Well-crystallized LDHs were synthesized except for the 2/2 ratio at pH 11.5, which produced byproducts.
- A pH of 8.0 yielded quasi-hexagonal LDH particles with high purity and consistent composition.
- Rietveld refinement confirmed LDH phases in the range of 95–98% for most compositions.

## Abstract

Co/Fe layered double hydroxides (LDHs) are among the most promising materials for advanced industrial and energy applications. Controlling the synthesis conditions of LDH materials is thus crucial to precisely tailoring cation composition and distribution, thereby regulating surface charge, ion sorption, and electron transfer required for optimal chemical and electrochemical performance. Therefore, characterizing Co/Fe precipitates (chemical composition, purity, morphology, and crystallinity) is also required to further exploit their controlled properties. Thus, solids with Co/Fe cation ratios between 2/2 and 10/2 were synthesized under an air atmosphere, at pH 8 or 11.5. For the first time, multiscale physicochemical techniques (FTIR, TEM-EELS, SEM, AAS, TGA, CHN elemental analysis, and XRD complemented by Rietveld refinement) were used to provide a fully documented characterization of the structure, texture, purity, chemical composition, and thermal properties of Co/Fe LDH-based materials. The combined interpretation of data from these complementary techniques enabled the precise identification and chemical characterization of the mineralogical phases formed. Both acid–base and redox reactions govern the overall CoII/FeIII LDH formation process. Well-crystallized LDHs were synthesized, except for the 2/2 ratio at pH 11.5, which led to the formation of α-Co(OH)2, γ-Fe2O3, and Co3O4 byproducts. A pH value of 8.0 provides valuable LDH materials made of quasi-hexagonal particles with diagonal lengths between 200 and 500 nm. Rietveld refining showed the presence of LDH phases in the range of 95–98%. Multiple local chemical analyses using EDX on chosen particles demonstrated pure 4/2 and 6/2 LDHs. For the 2/2 ratio, the cumulative mass fraction of two LDH-type products consistently reached 97%, distributed between Co/Fe 1.5/2 (71%) and Co/Fe 4/2 (29%). For the 10/2 ratio, only partial Co precipitation was observed, forming 95% Co/Fe LDH phases distributed between Co/Fe 10/2 (72%) and 7/2 (28%).