Interlayer interaction in Ca–Fe layered double hydroxides intercalated with nitrate and chloride species

Abstract

Ca–Fe layered double hydroxide (LDH) intercalated with chloride and nitrate ions has been synthesized with varying CaII:FeIII molar ratios of the initial solution. Phase pure LDH is observed with CaII:FeIII molar ratio of 2:1 and a mixture of LDH and Ca(OH)2 is formed for CaII:FeIII molar ratios higher than 2:1. Vibrational spectroscopies (Raman and IR) were used successfully to understand the interaction between the cationic and anionic sheets. The Raman bands positions at lower frequencies (150–600 cm−1) are intimately correlated to the nature of the divalent and trivalent ions but also to the nature of the anions. Indeed, a shift of ∼9 cm−1 is observed for the Raman double bands situated in the 300–400 cm−1 region when comparing Raman spectra of CaFe-LDH containing either nitrate or chloride ions. Two types of nitrate environments are observed namely free (non-hydrogen bonded) nitrate and nitrate hydrogen bonded to the interlayer water or to the ‘brucite-like’ hydroxyl surface. Multiple types of water structure are observed and would result from different hydrogen bond structures. Water bending modes are identified at 1645 cm−1 greater than the one observed for LDH intercalated with chloride anions (1618 cm−1), indicating that the water is strongly hydrogen bonded to the nitrate anions.