Femtosecond laser-induced photochemical synthesis of gold nanoparticles in nitrate solution

Femtosecond laser pulses are capable of ionizing water molecules through multiphoton absorption. The generated hydrated electron and hydrogen peroxide are responsible for the nucleation and growth of nanoparticles during a photochemical synthesis. The photochemical reduction kinetics follow an autocatalytic rate law, which is determined by nucleation rate k₁ and growth rate k₂. In this work, we investigated the effect of adding nitrate as electron scavenger on the laser-based photochemical synthesis of gold nanoparticle. The experiment results showed enhancements of both k₁ and k₂ with increasing nitrate concentration, indicating the formation of a large number of nuclei followed by fast growth process. TEM images and size distribution analysis confirmed the improved growth of gold nanoparticles. In the absence of scavenger, the gold nanoparticles produced were dominated by small particles with sizes <7 nm. Adding small amount of nitrate increased the mass fraction of larger particles (∼7–15 nm), thus tuning the average size of gold nanoparticles from 6.52 nm to 7.49 nm. Exceedingly high concentration of nitrate caused the formation of even larger particles, which connected to each other. While salt effects might interfere with both nucleation and growth process, generation of reducing species from electron scavenging reaction significantly contributed to the counterintuitive increase of nucleation rate. This study demonstrated the potential use of nitrate as a modifying agent for morphological control of gold nanoparticles.