Thin water films and particle morphology evolution in nanocrystalline MgO

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.

Hydroxylation Induced Alignment of Metal Oxide Nanocubes

Water vapor is ubiquitous under ambient conditions and may alter the shape of nanoparticles. How to utilize water adsorption for nanomaterial functionality and structure formation, however, is a yet unexplored field. Herein, we report the use of water vapor to induce the self-organization of MgO nanocubes into regularly staggered one-dimensional structures. This transformation evolves via an initial alignment of the MgO cubes, the formation of intermediate elongated Mg(OH)₂ structures, and their reconversion into MgO cubes arranged in staggered structures. Ab initio DFT modelling identifies surface-energy changes associated with the cube surface hydration and hydroxylation to promote the uncommon staggered stacked assembly of the cubes. This first observation of metal oxide nanoparticle self-organization occurring outside a bulk solution may pave novel routes for inducing texture in ceramics and represents a great test-bed for new surface-science concepts.

Hydration of magnesia cubes: a helium ion microscopy study

Physisorbed water originating from exposure to the ambient can have a strong impact on the structure and chemistry of oxide nanomaterials. The effect can be particularly pronounced when these oxides are in physical contact with a solid substrate such as the ones used for immobilization to perform electron or ion microscopy imaging. We used helium ion microscopy (HIM) and investigated morphological changes of vapor-phase-grown MgO cubes after vacuum annealing and pressing into foils of soft and high purity indium. The indium foils were either used as obtained or, for reference, subjected to vacuum drying. After four days of storage in the vacuum chamber of the microscope and at a base pressure of p < 10(-7) mbar, we observed on these cubic particles the attack of residual physisorbed water molecules from the indium substrate. As a result, thin magnesium hydroxide layers spontaneously grew, giving rise to characteristic volume expansion effects, which depended on the size of the particles. Rounding of the originally sharp cube edges leads to a significant loss of the morphological definition specific to the MgO cubes. Comparison of different regions within one sample before and after exposure to liquid water reveals different transformation processes, such as the formation of Mg(OH)2 shells that act as diffusion barriers for MgO dissolution or the evolution of brucite nanosheets organized in characteristic flower-like microstructures. The findings underline the significant metastability of nanomaterials under both ambient and high-vacuum conditions and show the dramatic effect of ubiquitous water films during storage and characterization of oxide nanomaterials.