Influence of Atmospheric Conditions on the Structural Evolution upon Heating of the ZnAl-Layered Double Hydroxide: A Comprehensive In Situ Transmission Electron Microscopy Study.

Abstract

Layered double hydroxides (LDHs) are versatile materials with diverse applications, including catalysis, water separation, adsorption, biomedicine, and fire retardancy. This study investigates the thermal evolution of ZnAl LDH ([Zn²⁺₂Al³⁺(OH)₆]⁺ [CO₃^2-_0.5]^-) using in situ scanning nanobeam electron diffraction, high-resolution transmission electron microscopy, and energy-dispersive X-ray analysis. The structural and morphological changes are examined during heating in both ambient air and vacuum. The pristine ZnAl LDH exhibited edge dislocations and irregular interlayer distances, leading to significant diffuse scattering in the electron diffraction patterns. Both upon heating in ambient air conditions and in vacuum, the characteristic hexagonal LDH morphology transformed into a porous nanostructure. The exact crystal structure transitions differed as a function of the environment. Heating under vacuum conditions first led to a ZnAl₂O₄ phase with a spinel-type structure, and subsequently to ZnO particles embedded within an Al₂O₃ matrix. When heated in a closed cell filled with ambient air, the LDH initially transitioned to an Al-doped ZnO-type structure, followed by conversion to a ZnAl₂O₄ phase. This study is the first gas in situ TEM LDH study and shows the recrystallization behavior of the LDH into the different phases upon heating and uncovers the nanoscale distribution of these phases within the particles, forming ZnAl₂O₄, ZnO, and Al₂O₃ composites.