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Download fileMethodological Improvement in Pulsed Laser-Induced Size Reduction of Aqueous Colloidal Gold Nanoparticles by Applying High Pressure
journal contribution
posted on 2016-02-21, 18:29 authored by Daniel Werner, Tomoyuki Ueki, Shuichi HashimotoBy 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.