Comprehension of the Route for the Synthesis of Co/Fe LDHs via the Method of Coprecipitation with Varying pH

Co/Fe-based layered double hydroxides (LDHs) are among the most promising materials for electrochemical applications, particularly in the development of energy storage devices, such as electrochemical capacitors. They have also been demonstrated to function as energy conversion catalysts in photoelectrochemical applications for CO(2) conversion into valuable chemicals. Understanding the formation mechanisms of such compounds is therefore of prime interest for further controlling the chemical composition, structure, morphology, and/or reactivity of synthesized materials. In this study, a combination of X-ray diffraction, vibrational and absorption spectroscopies, as well as physical and chemical analyses were used to provide deep insight into the coprecipitation formation mechanisms of Co/Fe-based LDHs under high supersaturation conditions. This procedure consists of adding an alkaline aqueous solution (2.80 M NaOH and 0.78 M Na(2)CO(3)) into a cationic solution (0.15 M Co(II) and 0.05 M Fe(III)) and varying the pH until the desired pH value is reached. Beginning at pH 2, pH increases induce precipitation of Fe(III) as ferrihydrite, which is the pristine reactional intermediate. From pH > 2, Co(II) sorption on ferrihydrite promotes a redox reaction between Fe(III) of ferrihydrite and the sorbed Co(II). The crystallinity of the poorly crystalized ferrihydrite progressively decreases with increasing pH. The combination of such a phenomenon with the hydrolysis of both the sorbed Co(III) and free Co(II) generates pristine hydroxylated Fe(II)/Co(III) LDHs at pH 7. Above pH 7, free Co(II) hydrolysis proceeds, which is responsible for the local dissolution of pristine LDHs and their reprecipitation and then 3D organization into Co(II)(4)Fe(II)(2)Co(III)(2) LDHs. The progressive incorporation of Co(II) into the LDH structure is accountable for two phenomena: decreased coulombic attraction between the positive surface-charge sites and the interlayer anions and, concomitantly, the relative redox potential evolution of the redox species, such as when Fe(II) is re-oxidized to Fe(III), while Co(III) is re-reduced to Co(II), returning to a Co(II)(6)Fe(III)(2) LDH. The nature of the interlamellar species (OH(−), HCO(3)(−), CO(3)(2−) and NO(3)(−)) depends on their mobility and the speciation of anions in response to changing pH.