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

Abstract

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 (O 2 ·– ) and ozonide (O 3 ·– ) 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 O 2 ·– and ozonide anions O 3 ·– 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.