MgO nanocube hydroxylation and carbonation by nanometric water films

Hydrophilic nanominerals exposed to air moisture host thin water films that are key drivers of reactions of interest in terrestrial and atmospheric settings. Water films can trigger irreversible mineralogical transformations, and control chemical fluxes across networks of aggregated nanomaterials. Using X-ray diffraction, vibrational spectroscopy, electron microscopy, and (micro)gravimetry we tracked water film-driven transformations of periclase (MgO) nanocubes to brucite (Mg(OH)2), as well as to amorphous magnesium carbonate (AMC) in the presence of moist CO2. We show that 3-4 monolayer-thick water films first triggered the nucleation-limited growth of brucite and AMC, and that water film populations continuously grew on newly-formed nanoparticles. Small (8 nm-wide) nanocubes were completely converted to brucite under this growth regime, while growth on larger (32 nm-wide) nanocubes transitioned to a diffusion-limited regime when (~1.3 nm-thick) brucite nanocoatings began hampering the flux of reactive species to growth fronts. In contrast, AMC growth was limited to the nucleation-limited regime as nanocoatings hindered the transport of reactive species from the MgO core to growth fronts. By resolving nanocoating growth on a model reactive hydrophilic mineral, this work provides new insight into the study of water film-driven nanomineral transformations that are important to geosciences.