Surfactantless Electrophoretic Deposition of Nonaqueous CuO Nanoparticle Dispersions by Tailoring Their Surface...

Electrophoretic deposition (EPD) is valued for its simplicity, cost-effectiveness, and ability to assemble nanoparticles (NPs) into micrometer-thick functional coatings, membranes, or interconnects at room temperature. In this work, we present a surfactant-free strategy to optimize the colloidal stability and electrophoretic mobility of CuO NPs in nonaqueous solutions by tuning their surface defectiveness and resulting space-charge region in the dry state. To this end, various commercial and in-house synthesized CuO nanopowders (with unknown synthesis, storage, and atmospheric aging histories) are subjected to mild, organic-free surface treatments, such as washing and subsequent annealing in O2 or O3. These optimized treatments yield "pristine" CuO surfaces with reduced defect densities─primarily monovacancies and vacancy clusters, as confirmed by combining X-ray photoelectron and positron annihilation lifetime spectroscopies─which in turn produce narrower and sharper surface space-charge regions, corresponding to higher effective surface potentials. The resulting ζ-potential of treated CuO NPs dispersed in ethanol exceeds > |±100| mV, enabling excellent colloidal stability and high electrophoretic mobility, even for very high NP loadings. A relatively high and stable steady-state EPD current is observed during EPD, which is postulated to originate from capacitive discharging of the electrophoretically deposited CuO NPs at the electrode surface. As such, coherent, uniform, porous CuO coatings with micrometer-scale thickness are obtained, entirely free of organic contaminants often detrimental for surface reactivity and material performance. These findings enable the development of efficient, up-scalable, and environmental-friendly EPD manufacturing routes of organic-free NP-based oxide coatings and membranes, independent of the nanopowder type and aging history.