The hydration reaction of MgO to amorphous Mg(OH)₂ is a model hydration reaction and is important to diverse research fields, ranging from catalysis to Earth Sciences. Although the bulk thermodynamics and surface energies of these phases are well studied,[1,2] real time and real space analysis of the reaction at ambient pressure is lacking. In this study, the hydration of MgO nanocrystals is studied at the single particle level, both in real space and in diffraction space using in-situ Transmission Electron Microscopy (TEM) at near-ambient pressure and temperature. Upon exposure to water vapor and the electron beam, the MgO nanocrystals react with H₂O and convert to amorphous Mg(OH)₂.

Real-time recordings of the hydration reaction reveal that the reaction starts at the MgO nanocrystal surface and proceeds inwards at a constant rate while the Mg(OH)₂ shell expands outwards. The growth rate is found to be constant throughout the reaction. Furthermore, as the applied dose rate is increased, the growth rate increases accordingly. Possible mechanisms for the beam-promoted transformation are discussed, including the role of defect formation and migration at the interior and at the surface of the MgO nanocrystals, H₂O diffusion towards the MgO surface, and the possible influence of beam-generated H₂O dissociation products. Assemblies of converting MgO/Mg(OH)₂ nanocrystals exhibited a reorganization of the assembly framework due to the solid volume increase (~100%) of each individual nanocrystal.

References

[1] de Leeuw, N.H., Watson, G.W., and Parker, S.C., J. Phys. Chem., 1995, 99 (47), 17 219-17 225

[2] Geysermans, P., Finocchi, F., Goniakowski, J., Hacquart, R., und Jupille, J., Phys. Chem. Chem. Phys., 2009, 11 (13), 2228-2233

Figures:

In-situ hydration of MgO NCs during exposure to air with 100% relative humidity (RH) at 23 °C and a dose rate of 324 e nm-2 s-1. A: snapshots of the cluster are shown at the start, halfway and at the end of the reaction (scalebar is 500 nm). Detailed view of regions I, II and III are shown in B-D (scalebars are 50 nm). E: the intensity profile along the lines drawn in B-D evolving in time. Colors represent intensity gray values (0-255). The darker core of the particles shrinks whereas the particle as a whole grows.

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