Dynamic process of evaporation of Ni–Ce nitrate precursor droplet in flame-assisted spray pyrolysis by molecular dynamics simulation

Abstract

To achieve structural control of multi-component nanoparticles using flame-assisted spray pyrolysis (FASP), elucidation of particle formation mechanisms under flame conditions is required. In this study, molecular dynamics simulations are performed to investigate the evaporation of Ni–Ce nitrate aqueous nanodroplets under high-temperature nitrogen environments of 1200 K and more, focusing on solute-aggregation processes during evaporation and resulting metal-ions distributions. At an ambient temperature of 1200 K and a Ni : Ce = 50 : 50%, Ce ions coordinate with nitrate ions and initiate cluster aggregation before Ni ions do, forming a single cluster, while Ni ions disperse into multiple clusters. In addition, the proportion of Ce ions in the total metal ions increases in the inner region of the droplet and decreases near the gas–liquid interface as evaporation progresses. As the Ni ratio decreases from Ni : Ce = 100 : 0 to 25 : 75%, ratio of Ni concentration near the gas–liquid interface to that within the droplet interior gradually increases. Moreover, when temperature is varied to 1200, 2000, and 3000 K for Ni : Ce = 50 : 50%, the corresponding ratio shows a maximum at 2000 K. These results demonstrate that, in evaporation of binary precursor droplet, differences in the onset timing of solute aggregation, which may be attributed to variations in ionic charge and hydration stability, play a key role in determining both the final cluster morphology and the spatial distribution of components within the droplet. Finally, these results are discussed in relation to the experimental trend that Ni segregates near the surface of Ni/CeO₂ particles in FASP.