Insight on the synthesis of LDH with urea method: morphology and intercalated anion control

Layered double hydroxides (LDH) are a class of layered solids applied in many application fields. The study on synthetic methods able to control the interlayer composition and morphology of LDH is an open issue. The urea method, that exploits the thermal decomposition of urea, is known for yielding highly crystalline LDH in carbonate form. This form is highly stable and, to replace carbonate ions with more easily exchangeable anions, a second step is required. In this work, we modified the urea method to obtain MgAl and ZnAl LDH in chloride or nitrate form through a one-step synthesis. The effect of the urea/(Al+M(II)) molar ratio (R), reaction time and metal salt concentrations were deeply investigated. We found that LDH in chloride and nitrate forms can be prepared from solutions of metal salts not exceeding 1M by adjusting R and maintaining the reaction time at 48 hours. The morphology of these products was found to depend on the R value and on the metal salts used in the synthesis. High R value and nitrate salts favoured the formation of sand-rose crystals, while chloride salts induced the formation of plate-like crystals. The crystal growth mechanism and the parameters influencing the morphology are discussed with reference to ZnAl LDH by monitoring the synthesis over time.