Combination of (100), (110) and (111) facets in MgO crystals shapes from dry to wet environment

Abstract

At the onset of dissolution in water, cubic MgO smoke crystals present (110) cuts of the edges of the cubes. Next, (111) facets progressively dominate the shape of the crystallites, which finally transform into truncated octahedra. The morphology of the crystallites that are derived from surface energies computed within the density functional theory (DFT), only involve (100) and (111) facets. We explain the unexpected (110) cuts via a "constrained" Wulff equilibrium shape that arises from a slower kinetics of formation of (111) facets than (110) ones. Experiment and theory fully agree on the hierarchy of hydroxylated surface energies: Gamma(111) < Gamma(100) < Gamma(110), both supporting the partial dissociation of water on MgO(100). Finally, from low to high P(H2O) (high to low T), DFT-based calculations predict a switch from Wulff shapes involving dry (100) facets, in which the (100)/(111) area ratio decreases upon increasing P(H2O), to shapes involving hydroxylated (100) surfaces, in which the above ratio increases with P(H2O).

Influence of natural adsorbates of magnesium oxide on its reactivity in basic catalysis

“…27,40,41 The proportion of (110) planes decreases when the dehydroxylation process goes further, consistently with the unstability of those planes without help of surface hydroxyls.…”

Morphology of MgO smoke crystallites upon etching in wet environment

“…In this context, it has been previously shown that MgO smoke crystallites offer a rich system [7][8][9]. The more stable orientation of the magnesium oxide in vacuum or neutral gases is by far (1 0 0), with a surface energy which is half that of MgO(11 0) [10].…”

“…It is only in a second stage of the dissolution process that (111) truncations appear at the corners of the cubes which then progressively transform into octahedra [7,8]. The unexpected formation of (11 0) facets has been assigned to kinetic effects on the basis of the examination of the growth rates of the (11 0) and (111) cuts with the support of a density functional approach [9]. The present work aims at analyzing the formation of these facets by means of transmission electron microscopy (TEM) in various wet environments.…”

Defects in Metal Oxide Nanoparticle Powders

“…Although, some fingerprints related to defects are apparently independent from the gross morphology of the substrate particle, this does not apply for all spectroscopically accessible defects. Ensembles of vapor phase grown cubic MgO particles have been the subject of numerous investigations and perhaps belong to one of the best understood ceramic nanomaterials [14,[143][144][145]. In previous work, a variety of spectroscopic techniques [78,79,92,137,[146][147][148] as well as first-principles theoretical calculations [66,[149][150][151] have been used to investigate the electronic properties of MgO nanocubes formed by the controlled combustion of magnesium vapor.…”

“…9.2d, however, covers only 50 of them. There is a growing number of publications-predominantly on colloidal particle systems-which underline the potential of high resolution transmission electron microscopy (HR-TEM) for the determination of the geometrical shape of crystalline polyhedra and for the estimation of relative and absolute surface areas of different crystal facets [13][14][15][16]. Technically used powders of functional metal oxides (e.g.…”