Tuning the Surface State and Zeta Potential of Aged CuO Nanoparticles by Additive‐Free Surface Treatments as Guided by XPS

Broad application of nanoparticles, for e.g., catalysis, antimicrobial coatings, medicine, electronics, batteries, and nanojoining technologies, is hindered by their limited shelf-life due to ambient aging. This study traces the changes in the surface chemistry of nanoparticles during atmospheric aging by monitoring the changes in the C 1s, O 1s, Cu 2p core-level spectra using XPS. The atmospheric aging of nanoparticles proceeds via hydroxylation and the adsorption of organic-O surface species, which introduces defects in the subsurface region. The position and shape of the main Cu 2p_3/2 peak and its satellite provide a sensitive fingerprint of the nanoparticle surface state, while the O 1s spectra reveal the formation of subsurface O defect species. Air annealing at 300°C and room-temperature ozonation provide additive-free, environmentally friendly surface treatments that can convert arbitrarily aged surface back to well-defined (pristine) CuO nanoparticle surface states. Thus, reproducible dispersion characteristics of CuO nanoparticles in ethanol can be obtained independently of the aging history. These findings help to extend the shelf-life of nanoparticles and enable reproducible processing routes for CuO nanopowders to be established for advancing nanotechnologies such as nanoporous oxide membranes, catalysts, functional coatings, and batteries.