Methodological Improvement in Pulsed Laser-Induced Size Reduction of Aqueous Colloidal Gold Nanoparticles by Applying High Pressure

By applying high pressures well above the critical pressure of water (22.1 MPa), we prepared gold nanoparticles (Au NPs) using a nanosecond pulsed laser-induced size-reduction technique. The Au NPs thus obtained exhibited a narrow size distribution and size-selectivity dependent on the applied laser energy density (fluence). This is significant because previous attempts under ambient pressure failed to achieve such size-selective generation. Spherical Au NPs of diameters 46 and 33 nm, with a standard deviation of only 2–3 nm, were obtained at the expense of original faceted 58 nm Au NPs. We ascribed our results to the formation of a supercritical water layer surrounding the liquid droplet NP transformed by laser heating. The supercritical layer originates from heat transfer from the particle, leading to water temperatures above the critical point of 647 K. The supercritical water layer acts as an effective heat sink for photothermal layer-by-layer size reduction to release small fragments, leaving behind a core particle. The supercritical water can retard evaporation of atoms and clusters from the droplet, thus controlling the evaporation rate. In contrast, at an ambient pressure of 0.1 MPa, the size reduction takes place inside the bubble that can form through the explosive evaporation of superheated water. In this case, particles of various sizes were produced, accompanied by connected structures. This is because thermal insulation is achieved inside the bubble, where the size reduction takes place with insufficient cooling. This study clearly showed nanosecond laser-induced size reduction of Au NPs, better than that achieved previously, by introducing an additional parameter, pressure. This study provides a refined method for size control in laser ablation NP generation. We expect that a rich chemistry can proceed in this transient supercritical water layer surrounding the NP droplets as a reaction medium.