Simple and rapid synthesis of Ni–Fe layered double hydroxide by a new mechanochemical method

This paper presents a new mechanochemical method for quickly and simply synthesizing a crystalline Ni(II)–Fe(II)–Fe(III) layered double hydroxide (LDH) with intercalated chloride ions. An acid nickel chloride solution is subjected to ball milling at room temperature for 3 hours using a tumbling mill with steel balls. No reagent as the iron source is needed, nor any alkali to precipitate the metal hydroxides and adjust the pH. Furthermore, no additional heating of the solution, for purposes such as aging, is required. The sample obtained was characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), laser particle size analysis, thermogravimetric/differential thermal analysis (TG/DTA), ion chromatography (IC), and inductively coupled plasma (ICP) spectroscopy. The analyses showed that the sample had high crystallinity and purity, and was almost identical to Ni–Fe–Cl LDHs synthesized by conventional methods. The LDH crystals were shaped like platelets with a median size of 25 nm. A possible mechanism for the LDH formation is that iron ions are gradually released from the steel balls during milling, generating electrons which produce hydroxide ions via decomposition of water molecules. This controls the pH automatically, and nickel ions are incorporated into green rust one formed from released iron ions and newly formed hydroxide ions, resulting in the formation of a Ni–Fe LDH.