Successful Synthesis of Gold Nanoparticles through Ultrasonic Spray Pyrolysis from a Gold(III) Nitrate Precursor and Their Interaction with a High Electron Beam

Herein, we report for the first time the successful preparation of a gold(III) nitrate [Au(NO 3 ) 3 ] water‐based precursor for use in a bottom‐up ultrasonic spray pyrolysis (USP) process. Due to its limited solubility in water, the precursor was prepared under reflux conditions with nitric acid (HNO 3 ) as the solvent and ammonium hydroxide (NH 4 OH) as a neutralizer. This precursor enabled the USP synthesis of gold nanoparticles (AuNPs) and the in situ formation of low concentrations of NO 2 − and NO 3 − ions, which were caught directly in deionized water in a collection system. These ions were proven to act as stabilizers for the AuNPs. Investigations showed that the AuNPs were monodispersed and spherically shaped with a size distribution over three groups: the first contained 5.3 % AuNPs with diameters (2  r ) <15 nm, the second contained 82.5 % AuNPs with 2  r between 15 and 200 nm, and the third contained 12.2 % AuNPs with 2  r >200 nm. UV/Vis spectroscopy revealed the maximum absorbance band of the AuNPs at λ =528 nm. Additionally, scanning transmission electron microscopy (STEM) observations of the smallest AuNPs (2  r <5 nm) revealed atomically resolved coalescence phenomena induced by interaction with the electron beam. Four stages of the particle‐growth process were distinguished: 1) movement and rotation of the AuNPs; 2) necking mechanism; 3) orientated attachment at matching facets; 4) reshaping of the AuNPs by surface diffusion. This provided important insight into the formation/synthesis process of the AuNPs.