Manufacturing nanoceramics is challenging owing to the instability of the grain size resulting from the high driving force toward growth associated with the interfaces. Nanometric ceramics of some oxides have exceptional mechanical and optical properties, eg, magnesium aluminate spinel (MgAl 2 O 4 ). The production of these fully conformed ceramics requires a precursor powder, which generally contains sintering-promoting additives. Li salts are typically used as sintering promoters for MgAl 2 O 4 , but the interface stability associated with the segregation of the additive is poorly understood. In this study, MgAl 2 O 4 samples containing 0-2.86 mol% Li ions were synthesized via a simultaneous-precipitation method in an ethylic medium and subsequently calcined at 800°C in air. The nanopowders exhibited only the MgAl 2 O 4 phase, and the crystallite size was determined by the Li 2 O concentration. The crystallite size was changed via the chemical modification of the interfaces by the segregation of Li ions. The solubility in the bulk material was very low at the fabrication temperature, and small amounts of Li ions saturated the bulk material and segregated to the grain boundaries (GBs), significantly stabilizing the grain-grain interface compared with the surface. The resulting powder was then aggregated further owing to the initial stage of sintering. The surface excess obtained via the selective lixiviation method confirmed that the segregation to the GBs was greater than that to the surface. Energetics calculations confirmed these results, indicating a high enthalpy of segregation at the GBs (ΔH segr GB = −52.0 kJ∕mol) compared with that at the surfaces (ΔH segr s = −31.5 kJ/mol). The enthalpy of segregation together with theinterface excess allowed us to estimate the reduction in the interface energy with Li + segregation of 0.8% to the surface and 11.2% to the GBs. The Li + segregation to the surfaces started by Al 3+ substitution, and for powders with ≥1.8 mol% Li ions, Mg +2 was preferentially substituted at the surfaces.

Titanium dioxide nanoparticles are of particular interest in photocatalysis and artificial photosynthesis studies. Their properties are dependent on parameters such as the crystal polymorph, for example, anatase/rutile phases, the specific surface area, and the capacity to adsorb CO2 and H2O gas molecules. A cause-effect relationship exists between those parameters, the surface thermochemistry, and the performance in application. However, the lack of systematic thermodynamic data has hindered a more comprehensive understanding to enable control. This work investigates the relationship between the surface thermochemistry of alkaline earth metals doped TiO2 and the associated micro and nanostructural features. TiO2 nanoparticles doped with Mg2+, Ca2+, Sr2+, or Ba2+ showed spontaneous segregation of dopants to the nanoparticles’ interfaces, leading to incremental surface energy reduction as directly measured by microcalorimetry of adsorption. The phenomenon was a direct function of ionic radius and led to the improved thermal stability of the nanoparticles against coarsening, stabilizing the anatase polymorph, and increasing the specific surface area.

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