Structural transformation of layered double hydroxides: an in situ TEM analysis

Abstract

A comprehensive nanoscale understanding of layered double hydroxide (LDH) thermal evolution is critical for their current and future applications as catalysts, flame retardants and oxygen evolution performers. In this report, we applied in situ transmission electron microscopy (TEM) to extensively characterise the thermal progressions of nickel-iron containing (Ni-Fe) LDH nanomaterials. The combinative approach of TEM and selected area electron diffraction (SAED) yielded both a morphological and crystallographic understanding of such processes. As the Ni-Fe LDH nanomaterials are heated in situ, an amorphization occurred at 250 °C, followed by a transition to a heterogeneous structure of NiO particles embedded throughout a NiFe₂O₄ matrix at 850 °C, confirmed by high-resolution TEM and scanning TEM. Further electron microscopy characterisation methodologies of energy-filtered TEM were utilised to directly observe these mechanistic behaviours in real time, showing an evolution and nucleation to an array of spherical NiO nanoparticles on the platelet surfaces. The versatility of this characterisation approach was verified by the analogous behaviours of Ni-Fe LDH materials heated ex situ as well as parallel in situ TEM and SAED comparisons to that of an akin magnesium-aluminium containing (Mg-Al) LDH structure. The in situ TEM work hereby discussed allows for a state-of-the-art understanding of the Ni-Fe material thermal evolution. This is an important first, which reveals pivotal information, especially when considering LDH applications as catalysts and flame retardants.

Layered double hydroxides: in situ TEM uncovers thermal evolution pathways

In situ transmission electron microscopy unveils the thermal evolution of Ni-Fe and Mg-Al containing layered double hydroxides (LDH). A team led by Valeria Nicolosi at Trinity College Dublin combined transmission electron microscopy with selected area electron diffraction to provide in-depth morphological and crystallographic understanding of the thermal processes occurring when LDH nanomaterials are heated in situ. In Ni-Fe LDH, after an initial amorphisation stage occurring at 250 °C, a transition to NiO particles arranged within a NiFe₂O₄ trevorite matrix was detected. Larger NiO particles tended to form on the central sections of the platelets, with smaller NiO crystallites arranged on the platelet edge regions. Conversely, Mg-Al containing LDH evolved into Al₂O₃ and MgAl₂O₄ structures, with the development of a porous matrix as opposed to the generation of spherical particles.