Influence of Argon Flow Rate on Plasma Parameters, Phase Evolution, and Optical Band Gap Tuning of Iron Oxide Nanoparticles Synthesized by Atmospheric Plasma Jet

Abstract

A systematic investigation into the effect of argon gas flow rate on the plasma-assisted synthesis of iron oxide nanoparticles (NPs) is presented. Optical emission spectroscopy (OES) revealed that increasing the flow rate from 0.5 to 1.5 L/min led to a rise in electron temperature from 0.369 to 0.480 eV and an increase in electron density from 12.162 × 10¹⁷ to 13.851 × 10¹⁷ cm^–3. X-ray diffraction (XRD) analysis confirmed the coexistence of magnetite (Fe₃O₄) and hematite (Fe₂O₃) phases. Crystallite sizes ranged from 7.5–14.0 nm at 0.5 L/min and increased to 8.9–17.1 nm at 1.5 L/min. Lower flow rates promoted the formation of smaller, more uniform grains, while higher flow rates led to increased agglomeration and more defined diffraction peaks. UV-Vis absorption spectroscopy showed distinct absorbance in the ultraviolet region for samples synthesized at 0.5 L/min, whereas those produced at 1.5 L/min exhibited extended absorption tails into the visible spectrum. Tauc plot analysis indicated that both direct and indirect optical band gaps decreased with increasing flow rate, from 4.22 eV (direct) and 2.82 eV (indirect) at 0.5 L/min to 3.85 and 1.94 eV at 1.5 L/min, consistent with reduced quantum confinement due to larger particle sizes. These findings highlight the critical role of gas flow rate in modulating plasma characteristics, structural evolution, and optical behavior of iron oxide NPs synthesized via plasma jet.