Topochemical Synthesis of Co−Fe Layered Double Hydroxides at Varied Fe/Co Ratios: Unique Intercalation of Triiodide and Its Profound Effect

Abstract

Co−Fe layered double hydroxides at different Fe/Co ratios were synthesized from brucite-like Co²⁺_1−xFe²⁺_x(OH)₂ (0 ≤ x ≤ 1/3) via oxidative intercalation reaction using an excess amount of iodine as the oxidizing agent. A new redoxable species: triiodide (I₃⁻), promoted the formation of single-phase Co−Fe LDHs. The results point to a general principle that LDHs with a characteristic ratio of total trivalent and divalent cations (M³⁺/M²⁺) at 1/2 may be the most stable in the oxidative intercalation procedure. At low Fe content, e.g., starting from Co²⁺_1−xFe²⁺_x(OH)₂ (x < 1/3), partial oxidation of Co²⁺ to Co³⁺ takes place to reach the M³⁺/M²⁺ threshold of 1/2 in as-transformed Co²⁺_2/3−(Co³⁺_1/3−x−Fe³⁺_x) LDHs. Also discovered was the cointercalation of triiodide and iodide into the interlayer gallery of as-transformed LDH phase, which profoundly impacted the relative intensity ratio of basal Bragg peaks as a consequence of the significant X-ray scattering power of triiodide. In combination with XRD simulation, the LDH structure model was constructed by considering both the host layer composition/charge and the arrangement of interlayer triiodide/iodide. The work provides a clear understanding of the thermodynamic and kinetic factors associated with the oxidative intercalation reaction and is helpful in elucidating the formation of LDH structure in general.