Visible Photoluminescence from Photoinduced Molecular Species in Nanometer-Sized Oxides: Crystalline Al₂O₃ and Amorphous SiO₂ Nanoparticles

Photoluminescence (PL) characteristics of two structurally and compositionally different oxide nanoparticles, i.e., crystalline γ-alumina and amorphous silica nanoparticles, are investigated. Time-resolved PL measurements have shown that both of these nanoparticles exhibit blue PL emissions characterized by fast (several tens of nanoseconds) and slow (a few seconds) decay constants. The fast PL component is observed over a wide temperature range up to ∼400 K, whereas the slow PL component is observed only at temperatures below ∼200 K. The close similarity between the PL characteristics of γ-alumina and amorphous silica nanoparticles indicates that common molecular-like species are responsible for the observed PL emissions. From the detailed comparison of the present experimental observations with the reported data, it can be concluded that the fast and slow PL components result from O₂ molecules and OH radicals, respectively, both of which are photoinduced transient molecular species generated at the surface of these oxide nanoparticles.