Conversion of MgO nanocrystal surfaces into ceramic interfaces: Exsolution of BaO as photoluminescent interface probes.

Ion exsolution can be instrumental to engineer intergranular regions inside ceramic microstructures. BaO admixtures that were trapped inside nanometer-sized MgO grains during gas phase synthesis undergo annealing-induced exsolution to generate photoluminescent surface and interface structures. During their segregation from the bulk into the grain interfaces, the BaO admixtures impact grain coarsening and powder densification, effects that were compared for the first time using an integrated characterization approach. For the characterization of the different stages the materials adopt between powder synthesis and compact annealing, spectroscopy measurements (UV-Vis diffuse reflectance, cathodo- and photoluminescence [PL]) were complemented by an in-depth structure characterization (density measurements, X-ray diffraction [XRD], and electron microscopy). Depending on the Ba2+ concentration, isolated impurity ions either become part of low-coordinated surface structures of the MgO grains where they give rise to a characteristic bright PL emission profile around λ = 500 nm, or they aggregate to form nanocrystalline BaO segregates at the inner pore surfaces to produce an emission feature centered at λ = 460 nm. Both types of PL emission sites exhibit O2 gas adsorption-dependent PL emission properties that are reversible with respect to its pressure. The here-reported distribution of BaO segregates between the intergranular region and the free pore surfaces inside the MgO-based compacts underlines that solid-based exsolution strategies are well suited to stabilize nanometer-sized segregates of metal oxides that otherwise would coalesce and grow in size beyond the nanoscale.

Oxygen Radicals Entrapped between MgO Nanocrystals: Formation, Spectroscopic Fingerprints, and Reactivity toward Water.

Compaction of dehydroxylated MgO nanocrystal powders produces adsorbed oxygen radicals with characteristic UV-vis spectroscopic fingerprints. Identical absorption bands arise upon UV excitation in an oxygen atmosphere but in the absence of uniaxial pressure. Photophysical calculations on MgO gas-phase clusters reveal that the observed optical transitions at 4.4 and 3.0 eV are consistent with adsorbed superoxide (O2·-) and ozonide (O3·-) species, respectively. The presence of these oxygen radicals is corroborated by electron paramagnetic resonance spectroscopy. Upon reaction with interfacial water, oxygen radicals convert into diamagnetic products with no absorptions in the UV-vis range. Since superoxide O2·- and ozonide anions O3·- play a key role in a variety of processes in heterogeneous catalysis, sensing, or as transient species in cold sintering, their UV-vis spectroscopic detection will enable in situ monitoring of transient oxygen radicals inside metal oxide powders.

Segregation Engineering in MgO Nanoparticle-Derived Ceramics: The Impact of Calcium and Barium Admixtures on the Microstructure and Light Emission Properties.

Nanostructured segregates of alkaline earth oxides exhibit bright photoluminescence emission and great potential as components of earth-abundant inorganic phosphors. We evaluated segregation engineering of Ca2+- and Ba2+-admixtures in sintered MgO nanocube-derived compacts. Compaction and sintering transform the nanoparticle agglomerates into ceramics with residual porosities of Φ = 24-28%. Size mismatch drives admixture segregation into the intergranular region, where they form thin metal oxide films and inclusions decorating grain boundaries and pores. An important trend in the median grain size evolution of the sintered bodies with dCa(10 at. %) = 90 nm < dBa(1 at. %) = 160 nm < dMgO = 250 nm ∼ dCa(1 at. %) = 280 nm < dBa(10 at. %) = 870 nm is rationalized by segregation and interface energies, barriers for ion diffusion, admixture concentration, and the increasing surface basicity of the grains during processing. We outline the potential of admixtures on interface engineering in MgO nanocrystal-derived ceramics and demonstrate that in the sintered compacts, the photoluminescence emission originating from the grain surfaces is retained. Interior parts of the ceramic, which are accessible to molecules from the gas phase, contribute with oxygen partial pressure-dependent intensities to light emission.

Rubbing Powders: Direct Spectroscopic Observation of Triboinduced Oxygen Radical Formation in MgO Nanocube Ensembles.

Powder compaction-induced surface chemistry in metal oxide nanocrystal ensembles is important for very diverse fields such as triboelectrics, tribocatalysts, surface abrasion, and cold sintering of ceramics. Using a range of spectroscopic techniques, we show that MgO nanocube powder compaction with uniaxial pressures that can be achieved by gentle manual rubbing or pressing (p ≥ 5 MPa) excites energetic electron-hole pairs and generates oxygen radicals at interfacial defect structures. While the identification of paramagnetic O- radicals and their adsorption complexes with O2 point to the emergence of hole centers, triboemitted electrons become scavenged by molecular oxygen to convert into adsorbed superoxide anions O2 - as measured by electron paramagnetic resonance (EPR). By means of complementary UV-photoexcitation experiments, we found that photon energies in the range between 3 and 6 eV produce essentially the same EPR spectroscopic fingerprints and optical absorption features. To provide insights into this effect, we performed density functional theory calculations to explore the energetics of charge separation involving the ionization of low-coordinated anions and surface-adsorbed O2 - radicals at points of contact. For all selected configurations, charge transfer is not spontaneous but requires an additional driving force. We propose that a plausible mechanism for oxygen radical formation is the generation of significant surface potential differences at points of contact under loading as a result of the highly inhomogeneous elastic deformations coupled with the flexoelectric effect.