Gradual oxidation and enhanced NIR-I photothermal properties of Cu/CuO nanostructures prepared via DC arc discharge in liquid

Copper-based nanostructures offer cost-effective alternatives to noble metal photothermal agents due to their strong near-infrared absorption and high thermal conductivity. In this work, Cu/CuO nanostructures were synthesized via an eco-friendly DC arc discharge in water without chemical precursors or surfactants. Optical emission spectroscopy identified excited copper- and oxygen-containing species, while Wien’s displacement law applied to the continuum-emission maxima yielded approximate effective temperatures above 8300 K, indicating the highly energetic conditions associated with nanoparticle formation. The as-synthesized colloids were aged in water and characterized at 1, 3, 7, 15, and 30 days to explore structural and optical evolution. XRD analysis with RIR-based semi-quantitative phase analysis indicated a gradual phase transformation from Cu to Cu 2 O and ultimately CuO, accompanied by an increase in the estimated CuO content from approximately 21% to 88%, while the transient Cu 2 O phase became undetectable after 7 days. FE-SEM observations further revealed the formation of anisotropic CuO nanorods during prolonged aging. XPS analysis of the representative 1-day and 7-day samples further confirmed the surface oxidation process, showing an increase in the surface Cu(II) fraction from 85% to 96% and in the lattice-oxygen contribution from 26% to 46%. UV-visible spectra indicated a redshift and bandgap reduction from 2.19 to 2.04 eV. Under 808 nm laser irradiation, the sample aged for 7 days achieved the highest photothermal conversion efficiency of 55%, associated with a favorable balance between Cu/CuO phase composition, optical absorption, morphology, and energy dissipation pathways. MTT assay results showed approximately 85% viability of L929 fibroblast cells at a concentration of 10 μg mL −1 , suggesting preliminary cytocompatibility under the tested conditions. This study highlights DC arc discharge in liquid as a clean and scalable approach for producing Cu/CuO nanostructures with promising photothermal performance and preliminary cytocompatibility, warranting further biological and photothermal treatment investigations to evaluate their potential applicability in photothermal therapy.