Activation and cold sintering of commercial ZnO powders: Compaction-induced oxide reduction

Defect formation and changes in electronic reducibility occur during cold sintering of ZnO powder compacts. For this reason, materials design must consider the material history and processing-related changes in the electronic structure. We performed spectroscopic investigations (UV-Vis-NIR diffuse reflectance, photoluminescence, Raman, electron paramagnetic resonance) on commercially available ZnO powders that were cold-sintered in H₂O or in aqueous acetic acid as a transient phase and find significant lattice oxygen depletion in the latter case. As a result, shallow donor states emerge in UV-Vis-NIR diffuse reflectance and electron paramagnetic resonance via an isotropic resonance signal at g = 1.96. The use of NH₄Cl, instead of aqueous and/or acidic liquids as an activator to promote cold sintering, leads to identical spectroscopic fingerprints that are indicative of stoichiometry changes in ZnO. The spectroscopic results underline that cold sintering-induced protonation of the metal oxide lattice favors the formation of defects and electronic donor states.