Control of Micro- and Nanostructures of Layered Double Hydroxides by Hydrothermal Treatment

Layered double hydroxides (LDHs) are mixed M(II) and M(III) hydroxides with positively charged lamellar brucite layers and interlayered anions. LDHs have attracted significant attention due to their anion- and cation-exchange ability, adsorption capacity, and potential applications in drug delivery and catalysis. The properties of LDHs directly correlate with their structures. In this study, we synthesized nickel-aluminum LDHs by the sol–gel method. We investigated the changes in their crystal structure/morphology introduced by hydrothermal treatments that varied from 0 to 48 h. Based on measured data using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and synchrotron-based small-angle X-ray scattering, we demonstrated that the duration of the hydrothermal treatment can effectively control the micro- and nanostructures of Ni–Al LDHs. We found that the hydrothermal treatment leads to a two-step change in the structure of the LDHs. In the first step, crystallization occurred during the first 24 h of hydrothermal treatment, increasing the interlayer width (from 2.28 to 2.31 nm) and the crystallite size (from 14.2 to 50.6 nm). In the second step, the predominant phenomena were stacking and planar expansion after the crystallization process approached equilibrium. Thus, the layered structure already developed in the first step was further stacked, and the hexagonal platelet characteristic of LDH expanded in a planar direction. Our study suggests that the duration of the hydrothermal treatment is a crucial factor in the structural evolution of LDHs and can be used to control the structure of LDHs for different applications.