Flow-Rate-Controlled Agglomeration and Ambient Oxide Phase Characteristics of Spark-Ablated Copper Nanoparticles

Spark ablation is an attractive gas-phase route for producing ligand-free metal nanoparticles, yet control over agglomeration and phase evolution during deposition remains challenging. The influence of carrier gas flow rate and deposition time on the agglomerate morphology, size distribution, and oxidation behavior of copper nanoparticles deposited onto silicon and glass substrates is investigated. Copper nanoparticles were generated by spark discharge and transported by nitrogen (N₂) carrier gas under flow rates ranging from 0.5 to 3.5 L min^-1 prior to deposition onto silicon substrates for 2 h and 5 h. Scanning electron microscopy reveals a transition from compact, isotropic agglomerates at low flow rates to elongated, chain-like structures at higher flow rates, indicating a shift from coalescence- and restructuring-favored conditions toward increasingly kinetically constrained aggregation. Transmission electron microscopy reveals predominantly spherical nanoparticles with a lognormal size distribution centered at approximately 10 nm, consistent with gas-phase nucleation and coalescence. Structural and spectroscopic analyses suggest CuO as the dominant oxide phase with minor Cu₂O contributions, with Raman spectroscopy providing the primary evidence due to limitations in high-resolution TEM analysis from substrate effects and peak overlap. These findings show that carrier-gas flow rate systematically influences agglomerate structure under the investigated conditions, while oxidation may occur both during aerosol transport and following ambient exposure; the present measurements do not allow the relative contributions of these processes to be quantitatively separated.