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.
Highlights
Water adsorption and subsequent surface reactions can be a key factor for the functionalization and performance of oxide nanomaterials
The two H2O desorption features at T = 303 K and 573 K are attributed to desorption of weakly bound surface water at T = 303 K and to the decomposition of Mg(OH)[2] into MgO and gas phase water at T = 573 K, respectively.[51]. Two desorption features of lower intensity at T = 773 K and 1023 K are attributed to the elimination of chemisorbed surface hydroxyls
Starting with MgO nanocubes, as particulate model system for surface and interface studies, we have experimentally investigated their morphology evolution in different gaseous H2O containing environments
Summary
Water adsorption and subsequent surface reactions can be a key factor for the functionalization and performance of oxide nanomaterials. Depending on the water concentration in the surrounding continuous phase, the film thickness of the adsorption layer covering the nanostructures can be in the range between that of a vacuum/solid and of a bulk liquid/solid interface This transition is associated with a significant increase in the level of complexity with regard to the physico-chemical description of a materials system.[1,2,3,4] At a relative humidity typical for ambient conditions, solid surfaces are covered with water molecules up to a thickness of a few nanometers.[5,6] The properties of related water layers depend on the surface properties of the substrate[6] and are typically very different from those of macroscopically thick films. In such a form, water does contribute to the conversion of oxide surface layers into hydroxides,[7] it can act as a two-dimensional solvent, which drives the alignment of oxide particles[8,9] and enables the spontaneous structural and microstructural transformation of particle systems under ambient conditions.[10,11,12]
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