Hydration behavior of MgO single crystals and thin films

Abstract

MgO single crystals and thin films were intentionally hydrated to determine the critical factors affecting the hydration behavior. The degree of hydration was affected by the crystallographic orientation in the initial stages. The (111) plane showed a higher tendency to hydrate than (100). The shape of the hydration clusters also differed according to the orientation of MgO single crystals. After long-term hydration, the density and grain size appeared to influence the hydration along with the orientation. On low-density thin films, Mg atoms are easily supplied to the surface, which induces large hydration clusters. As the grain boundary area increased, the number of nucleation sites for the formation of hydration clusters increased, which increases the number of clusters. Hydration also occurred in the inner part of thin films. The density of thin films is the most important property in this case because it governs the diffusion of Mg atoms, water, and OH through the thin films.

The surface chemistry of nanocrystalline MgO catalysts for FAME production: An in situ XPS study of H2O, CH3OH and CH3OAc adsorption

“…The total surface oxygen content of NanoMgO remained largely unaffected by dosing even for water exposure up to 1x10 6 L ( temperature. [59][60][61][62][63][64][65][66][67][68] In contrast, the surface oxygen concentration of NanoMgO-700 increased monotonically with water exposure, equating to a 2 % rise following 1x10 6 L. This corroborates the hypothesis that NanoMgO-700 is the more reactive surface reflecting its higher proportion of (111) facets and low coordination defects. [27,28] Further evidence for changes in surface oxygen species upon H2O adsorption was apparent from the corresponding high resolution O 1s spectra.…”

“…Indeed, any observed adsorption over this facet appears due to the presence of low-coordinate defects on the oxide surface, as evidenced by studies in which defects are deliberately introduced through ion bombardment. [60,67] Efforts to study adsorption over the less stable/more electron-rich (111) surface [60,62,66,68,70] have proven problematic due to surface reconstruction resulting in a highly facetted termination. [71,72] In spite of this difficulty, adsorption studies over the highly stepped (111) surface which contains a high proportion of corner sites, [72] have provided a useful contrast to the comparatively atomically smooth (100) terraces.…”

“…Dissociative adsorption of water, small acids, aldehydes and alcohols is more prevalent over reconstructed (111) surfaces than (100) counterparts. [59], [60,62,66,70] 2. Experimental…”

Influence of crystallographic orientation on hydration of MgO single crystals

“…This is because the protonation of the MgO (1 1 1) creates a surface with the same structure as the Mg(OH) 2 (0 0 0 1) cleavage plane, and this may stabilize MgO (1 1 1) surface [14]. MgO has a higher tendency to hydrate when the atoms on the plane have a coordination number of 3 and a lower tendency to hydrate for a coordination number of 5 [13].…”

“…Delplancke-Ogletree et al studied the effect of annealing and exposure time on brucite growth on (1 0 0) single crystals [12]. Lee et al investigated the effects of moisture on single crystals of different orientations of MgO that are grown via thin film deposition [13]. But no study has yet been done on the degree of chemical attack of moisture on MgO bulk single crystals as function of crystallographic orientation.…”