Oxidation of Zn nanoparticles probed by online optical spectroscopy during nanosecond pulsed laser ablation of a Zn plate in H2O

Abstract

We report online UV-Visible absorption and photoluminescence measurements carried out during and after pulsed laser ablation of a zinc plate in water, which clarify the events leading to the generation of ZnO nanoparticles. A transient Zn/ZnO core-shell structure is revealed by the coexistence of the resonance absorption peak around 5.0 eV due to Zn surface plasmon resonance and the edge at 3.5 eV of ZnO. The growth kinetics of ZnO, selectively probed by the exciton luminescence at 3.3 eV, begins only after a ∼30 s delay from the onset of laser ablation. We also detect the luminescence at 2.3 eV of ZnO oxygen vacancies, yet rising with an even longer delay (∼100 s). These results show that the oxidation of Zn nanoparticles mainly occurs out of the ablation plume region and proceeds as a sequence of two stages: the earliest oxidation is only superficial and forms a defect-free ZnO shell around a Zn core, whereas core oxidation is driven by comparatively slower diffusion of water through the ZnO shell and leaves behind a defective ZnO material rich of oxygen vacancies.