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 O
2
point to the emergence of hole centers,
triboemitted electrons become scavenged by molecular oxygen to convert
into adsorbed superoxide anions O
2
–
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 O
2
–
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.
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 Ca
2+
- and Ba
2+
-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
d
Ca(10 at. %)
= 90 nm <
d
Ba(1 at. %)
= 160 nm <
d
MgO
=
250 nm
∼
d
Ca(1 at. %)
= 280
nm <
d
Ba(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.
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.
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