Journal of the European Ceramic Society / Activation and cold sintering of commercial ZnO powders: Compaction-induced oxide reduction

Abstract

Defect formation and changes in electronic reducibility occur during cold sintering of ZnO powder compacts. Forthis reason, materials design must consider the material history and processing-related changes in the electronicstructure. We performed spectroscopic investigations (UV-Vis-NIR diffuse reflectance, photoluminescence,Raman, electron paramagnetic resonance) on commercially available ZnO powders that were cold-sintered inH2O 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 resonancevia an isotropic resonance signal at g = 1.96. The use of NH4Cl, instead of aqueous and/or acidic liquids as anactivator to promote cold sintering, leads to identical spectroscopic fingerprints that are indicative of stoichi-ometry changes in ZnO. The spectroscopic results underline that cold sintering-induced protonation of the metaloxide lattice favors the formation of defects and electronic donor states