Studying citric acid-mediated synthesis of gold nanoparticles in ionic liquids by in situ liquid phase STEM: A reproducible approach

Understanding the nucleation and growth mechanisms of gold nanoparticles (Au-NPs) remains a major challenge in nanoscience, limiting the rational design of tailored nanomaterials. Ionic liquids (ILs) have recently emerged as promising media for nanoparticle synthesis due to their low vapor pressure, high thermal stability, and unique ability to act simultaneously as solvent, stabilizer, and reaction environment. Here, we present a reproducible synthesis strategy for Au-NPs based on citric acid reduction in 1-butyl-3-methylimidazolium chloride ([BMIm]Cl) and tetrabutylammonium chloride ([TBA]Cl) ionic liquids, optimized for in situ variable temperature liquid-phase scanning transmission electron microscopy (VT LP-STEM) studies. By adapting hot-injection protocols and controlling reaction parameters, gradual reduction kinetics were achieved, allowing real-time observation of nanoparticle nucleation and growth dynamics. In situ LP-STEM revealed distinct differences between the two ILs: [BMIm]Cl supported dynamic rearrangement, surface diffusion, and coalescence processes at elevated temperatures, while [TBA]Cl exhibited restricted nanoparticle mobility and growth. Moreover, the formation of periodic nanostructuring of the IL matrix was observed. Our approach establishes a reproducible model system for detailed investigations of nanoparticle formation in ILs.