Thin water films and particle morphology evolution in nanocrystalline MgO

Abstract

A key question in the field of ceramics and catalysis is how and to what extent residual water in the reactive environment of a metal oxide particle powder affects particle coarsening and morphology. With X‐ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), we investigated annealing‐induced morphology changes on powders of MgO nanocubes in different gaseous H2O environments. The use of such a model system for particle powders enabled us to describe how adsorbed water that originates from short exposure to air determines the evolution of MgO grain size, morphology, and microstructure. While cubic nanoparticles with a predominant abundance of (100) surface planes retain their shape after annealing to T = 1173 K under continuous pumping with a base pressure of water p(H2O) = 10−5 mbar, higher water partial pressures promote mass transport on the surfaces and across interfaces of such particle systems. This leads to substantial growth and intergrowth of particles and simultaneously favors the formation of step edges and shallow protrusions on terraces. The mass transfer is promoted by thin films of water providing a two‐dimensional solvent for Mg2+ ion hydration. In addition, we obtained direct evidence for hydroxylation‐induced stabilization of (110) faces and step edges of the grain surfaces.

Water dissociation on the low-coordinated sites of MgO nanopowders

“…The previous findings indicate that a fine analysis of the reactivity of defective MgO surfaces and nanopowders with respect to water vapor imperatively requires ultra-high vacuum (UHV) conditions. Exposure to water vapor at higher pressure than 10 -5 mbar can indeed induce profound restructuring of the MgO surfaces [6,[12][13][14] and may cause mass transport [15]. Further, the marginal fraction of the surface occupied by defects suggests to study their hydroxylation on powders by infrared spectroscopy in transmission, in such way that the high specific surface area of the samples should compensate for the scarcity of the sites under study.…”

Impurity Segregation and Nanoparticle Reorganization of Indium Doped MgO Cubes

“…The characteristic cubic habit of MgO nanoparticles in anhydrous environments is due to the low surface energies of low index surfaces of alkaline‐earth metal oxides. The surface energy for a bare and dehydroxylated MgO(100) surface has been calculated to be 0.93 J/m 2 , whereas the corresponding values for MgO(110) and MgO(111) with 2.25 and 2.21 J/m 2 , respectively, are substantially higher ,. (The presence of water, however, changes the relative ordering of the surfaces, since the adsorption energies of water to the high index planes are much stronger.…”

“…(The presence of water, however, changes the relative ordering of the surfaces, since the adsorption energies of water to the high index planes are much stronger. ), Hence, after gas phase synthesis and/ or thermal processing in a dry environment undoped and dehydroxylated MgO particles adopt a cubic shape. Application of processing cycles which alternate between continuous pumping with a base pressure below 10 −5 mbar and thermal treatment in pure oxygen guarantee such experimental conditions (Figure S3, Supporting Information).…”

“…Application of processing cycles which alternate between continuous pumping with a base pressure below 10 −5 mbar and thermal treatment in pure oxygen guarantee such experimental conditions (Figure S3, Supporting Information). In comparison to corresponding materials, which were grown in liquids, mass transfer and sintering is strongly suppressed ,…”

“…This work represents a first study on the nanocrystalline and particle based In−Mg−O system and focuses on the stability of the In 3+ ions within the MgO host lattice. Comparison of structure and composition of the composite nanoparticles, either as‐synthesized or after thermal processing, reveal trends, that are key for the later exploitation of their functional properties . The choice of two processing temperatures, T=873 K and 1173 K, is based on previous observations: oxygen treatment at 873 K was found to be effective for complete carbon contaminant elimination, whereas vacuum annealing at 1173 K leads to dehydroxylated MgO nanoparticle surfaces .…”