Changing interfaces: Photoluminescent ZnO nanoparticle powders in different aqueous environments

We transformed vapor phase grown ZnO nanoparticle powders into aqueous ZnO nanoparticle dispersions and studied the impact of associated microstructure and interface property changes on their spectroscopic properties. With photoluminescence (PL) spectroscopy, we probed oxygen interstitials $O_i^{2-}$ in the near surface region and tracked their specific PL emission response at hv_EM = 2.1 eV during the controlled conversion of the solid-vacuum into the solid-liquid interface. While oxygen adsorption via the gas phase does affect the intensity of the PL emission bands, the O₂ contact with ZnO nanoparticles across the solid-liquid interface does not. Moreover, we found that the near band edge emission feature at hv_EM = 3.2 eV gains relative intensity with regard to the PL emission features in the visible light region. Searching for potential PL indicators that are specific to early stages of particle dissolution, we addressed for aqueous ZnO nanoparticle dispersions the effect of formic acid adsorption. In the absence of related spectroscopic features, we were able to consistently track ZnO nanoparticle dissolution and the concomitant formation of sol- vated Zinc formate species by means of PL and FT-IR spectroscopy, dynamic light scattering, and zeta potential measurements. For a more consistent and robust assessment of nanoparticle properties in different continuous phases, we discuss characterization challenges and potential pitfalls that arise upon replacing the solid-gas with the solid-liquid interface.