A unique growth mechanism of donut-shaped Mg-Al layered double hydroxides crystals revealed by AFM and STEM-EDX

Donut-like crystals of Mg-Al layered double hydroxides (LDH) are synthesized using a hydrothermal method with microwave heating. This morphology provides enlargement of the specific surface area of the {h k 0} faces, needed for adsorption application. The growth mechanism for donut-shaped crystals is proposed on the basis of AFM and STEM-EDX images. The nucleation of Mg-Al LDH starts on the amorphous surface of spherical MgO particles, which have a much lower solubility compared to the aluminum trihydroxides at the synthesis pH (∼11). The outgrowing nucleus provides re-entrant corners on both sides. These are preferential sites for the addition of new growth units, allowing lateral growth of the LDH crystal encircling the MgO particles. The dissolving MgO provides the supersaturation needed for growth and once it is depleted, a donut-like structure remains.