Thermally induced crystal phase transitions in Zn–Al layered double hydroxides: zinc and aluminium pathways at various pH values

Due to their lamellar nano-sheet structure, layered double hydroxides (LDH) are known as materials suitable for carrying certain active, functional molecules which can be easily intercalated. The properties of LDHs depend strongly on the synthesis parameters, especially on the pH value of the reaction medium. To gain a thorough understanding of how pH impacts the synthesis outcomes, a range of ZnAl-LDH suspensions were created through a modified method of low supersaturation co-precipitation at different pH levels spanning from 7 to 10. Almost all synthesis routes of LDHs, both direct and indirect, employ some kind of thermal treatment, either during the synthesis or after it, aiming to improve the crystallinity of the products. Accordingly, comprehensive thermogravimetric and dynamic scanning calorimetry assessments were carried out on synthesised samples. For those produced at the lowest and highest pH levels, an analysis of emitted gases was conducted. Extensive microstructure examinations via X-ray powder diffraction (XRD) was performed both before and after thermal treatment. Through XRD analysis, complemented by Fourier transform infrared spectroscopy measurements, the crystal phases within the synthesised ZnAl-LDHs were identified, and the microstructure results were compared to delineate the pathways of thermally induced zinc and aluminium crystal phase transitions relative to the pH of the reaction medium.