Formation of copper(II) oxide nanoparticles by ESI microdroplets deposition.

The utilization of electrospray ionization (ESI) source from a mass spectrometer to generate charged microdroplets represents a cutting-edge strategy for inducing accelerated chemical transformations in confined volumes. These microdroplets act as extremely dynamic microreactors, defined by instantaneous solvent evaporation, intense interfacial electric fields, and sudden pH variations. Such conditions allow for reaction rates up to six orders of magnitude higher than those of the same processes conducted in bulk phase. Recently, copper(II) oxide nanoparticles (CuONPs) have been adopted as significant materials for catalysis, sensing, antimicrobial coatings, and energy storage; consequently, identifying alternative synthetic pathways is essential to promote the widespread application of these materials. This study evaluates the feasibility of synthesizing CuO nanoparticles through the deposition of ESI microdroplets from aqueous copper(II) acetate solutions under mild conditions, avoiding the strongly alkaline environments typical of traditional wet-chemical methods. Slightly acidic Cu(AcO)₂ aqueous solutions were nebulized via a Z-spray ESI source to produce charged droplets directed onto solid substrates, where their coalescence originated reactive thin films. The evolution of clusters and ionic species was monitored via ESI mass spectrometry, while the obtained deposits were analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The effects of deposition time and desolvation conditions were examined to optimize particle production. ESI-MS analyses highlighted the formation of copper-acetate clusters containing CuO units already during the microdroplets flight, suggesting that oxidative mechanisms begin within the confined environment of the droplet. CID analyses show a consistent loss of CuO centers, further confirming the proposed mass assignments. Prolonged deposition times generated solid films from which the CuO was selectively isolated following the removal of soluble precursors. The application of a negative voltage to the ESI source proved decisive in favoring oxidation, whereas positive or zero potentials facilitated the formation of metallic copper.