Factors Governing MgO(111) Faceting in the Thermal Decomposition of Oxide Precursors

The preparation of metal oxide particles, such as MgO, NiO, and ZnO, exposing polar facets via the decomposition of suitable precursors in air, molten salts, ionic liquids, or other media has been reported; however, the main driving force(s) and factors that determine their morphology have largely remained elusive. For instance, the adsorption of ions within a molten medium on a growing oxide surface has been proposed to stabilize MgO{111} facets by the so-called molten salt route (MSR). In this article, we examine the thermal decomposition of MgO precursors to assess the influence of precursor decomposition pathways, including the physical state of the reaction intermediates and the possibility of dissolution–recrystallization processes in the formation of octahedral MgO particles. We found that solid-to-solid conversions and recrystallizations in molten nitrates or chlorides are usually incapable of producing well-defined MgO{111} facets, indicating that ion adsorption on MgO may not be the main morphological driving force. Here, we show for Mg(NO3)2·6H2O, the most suited MgO(111) precursor, that its decomposition trajectory is crucial in determining the exposure of {111} facets. The decomposition trajectory may be regulated by using controlled heating profiles, regulated atmospheres, or by incorporating particular salts in the reaction mixture. Our findings indicate that, for neat Mg(NO3)2·6H2O decomposition in air, MgO{111} facets are promoted via magnesium nitrate intermediates in the molten state and in the presence of residual water. These conditions can be achieved by heating the hexahydrate precursor at high rates. On the contrary, decomposition of anhydrous Mg(NO3)2 results in ill-defined MgO morphologies. In a molten NaNO3:KNO3 medium, the formation of liquid eutectic mixtures facilitate H2O retention and ionic mobility, from which well-defined octahedral MgO crystals form, thereby emphasizing the crucial role of water in Mg(II)-nitrate systems. In a NaCl:KCl ionic medium, precursor decomposition occurs via a K3NaMgCl6 intermediate, which melts before converting to the oxide. MgO(111) forms under local melting of the intermediate (before NaCl:KCl melts) and when the H2O content is negligible. In summary, the formation of polar MgO(111) particles is facilitated in molten salt media when MgO is generated via a liquid-to-solid reaction (with intermediates in the molten state). The presence of residual water and ions impact MgO(111) crystallization in ways that still remain elusive, but are not necessarily governed by adsorption–stabilization processes.