Stabilizer-Concentration Effects on the Size of Gold Submicrometer-Sized Spherical Particles Prepared Using Laser-Induced Agglomeration and Melting of Colloidal Nanoparticles

Pulsed laser melting in liquids (LMLs), a convenient method of submicrometer-sized spherical particle (SMP) preparation, induces nanoparticle (NP) melting and fusion using laser irradiation at a moderate fluence for colloidal NPs. Our earlier study, conducted to produce gold SMPs (AuSMPs) from gold NPs (AuNPs) stabilized by sodium carbonate, demonstrated that the AuSMP size increased concomitantly with decreasing stabilizer concentration, although it has been suggested that laser fluence fundamentally determines the SMP size. This study elucidates an explanation of this phenomenon. Results obtained through experimentation showed the same relation when LML was conducted using AuNPs stabilized by sodium citrate. The narrower size distribution (65 ± 4 nm) of AuNPs used in this study than that of AuNPs used in earlier studies enabled us to ascertain that AuNPs smaller than the source AuNPs (approximately 10 nm diameter) are generated during laser irradiation via AuNP evaporation that occurs simultaneously with the AuNP agglomeration. A theoretical calculation predicts that the temperature increased by laser heating of AuNP depends strongly on the AuNP size, suggesting that efficiencies of stabilizer removal and agglomeration of the 10 nm AuNPs are lower than those of the source 60 nm AuNPs. For that reason, 10 nm AuNPs are likely to remain at high stabilizer concentration, leading to the formation of smaller AuSMPs because of insufficient growth. We also confirmed that such a mechanism (AuNP-size-dependent agglomeration efficiency) is applicable to improve the growth efficiency of AuSMPs on LML using AuNPs prepared by laser ablation in liquids with a wide size distribution. When smaller AuNPs were removed from the colloids by centrifugation, the amount of AuNPs remaining after LML decreased and larger AuSMPs were formed.