(163i) Molten Salt Synthesis of MgO and NiO Exposing Polar and High Index Facets

Metal oxides exposing high-index facets have potential impact in absorption and catalytic applications owing to the presence of unusual ionic surface arrangements exposing under-coordinated ions and increased surface polarities. Few examples have been reported and the crystal formation mechanism is still scarcely understood. In this talk, we will discuss the formation of metal oxide particles consisting of rock-salt cubic structured MgO and NiO exposing a variety of crystal facets synthesized by molten salt routes (MSRs). 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, which may be regulated by using controlled heating profiles, or by incorporating particular salts in the reaction mixture. We show that the formation of polar MgO(111) particles occurs via locally molten intermediate phases by liquid-to-solid reactions. For equivalent NiO syntheses, we will discuss our results showing that the morphology is sensitive to the combination of alkali cations and anions, which result in the formation of (311), (100), (111), and other high-index facets. We posit that the crystallization mechanism is governed by the formation of growth units consisting of Ni(II) complexes in (local) molten states whose coordination numbers are determined by the used anion (associated to the coordination sphere) and the alkali cation (associated with the outer sphere). Our results illustrate the vast potential of the MSR for developing scalable metal oxide synthesis methods for the production of high surface energy materials. Moreover, we will discuss the growth mechanism(s) of these metal oxide crystal facets and stability tests revealing that NiO(311) facets can withstand prolonged periods of catalysis and steaming.