Morphological Evolution of Gold Nanoparticles Synthesized via Solution Plasma Sputtering: Effect of Sodium Chloride Concentration and Storage Time

Abstract

Morphological control of gold nanoparticles (AuNPs) is vital for tuning their optical, chemical, physical, and catalytic properties for desired specific applications. Despite numerous efforts dedicated in recent years, controlling the morphology of AuNPs synthesized via solution plasma sputtering (SPS) remains a challenge, requiring further investigation. Herein, AuNPs were synthesized via SPS in sodium chloride (NaCl) solutions of various concentrations (0.5, 1, 2, and 5 mM). Evolutions of localized surface plasmon resonance and morphology of AuNPs over a 30 day storage period were thoroughly examined using ultraviolet-visible spectroscopy and field-emission transmission electron microscopy. The formation and growth mechanisms of AuNPs during and after synthesis provide more insights into this nascent field. At the initial storage time, AuNPs with chain-like structures were observed at all the NaCl concentrations. Their size tended to increase from 6 to 10 nm with a broader size distribution with increasing NaCl concentration. At a prolonged storage time of 15 days, AuNPs became smaller and more isolated at all NaCl concentrations owing to the oxidative etching effect. After 30 days of storage, the number of Au ions dissolved from etching slowly reduced and grew on the AuNPs, affording increased particle sizes. The interplay between etching and growth processes proceeded until reaching equilibrium. AuNPs exhibited good colloidal stability at low NaCl concentrations (i.e., 0.5, 1, and 2 mM); however, coalescence and sedimentation occurred at a high NaCl concentration of 5 mM owing to a considerable reduction in the electrical double-layer thickness. These results demonstrated that the NaCl concentration and storage time crucially affect the morphological evolution and stability of AuNPs synthesized via SPS.