Identification and Distribution of Surface Ions in Low Coordination of CaO Powders with Photoluminescence Spectroscopy

Oxide ions in low coordination (O-LC(2-)) Play a major role in adsorption and catalysis properties of CaO powders. They also give specific photoluminescence features that could be used to characterize the distribution of these ions at the molecular level, but up to now, the photoluminescence of defective CaO powders was assigned incompletely. In this study, photoluminescence spectra of defective CaO powders prepared in various ways are recorded at 77 K with unprecedented resolution. A new feature is thus evidenced with an excitation at 320 nm and an emission at 490 nm. Combining infrared and photoluminescence spectroscopies, we show that this feature does not originate from remaining hydroxyl groups, but it completes the set of photoluminescence fingerprints of O-LC(2-) ions on CaO. A revisited assignment of this set is proposed, consistent with the dependence of transition energy on coordination numbers. An application example shows how the photoluminescence spectroscopy can be used as an original tool to compare surface morphologies of CaO samples at the molecular level and under in situ conditions.