<div>
<div>
<div>
<p>We report the synthesis of silica-gold nanoparticles (silica-Au NPs) using a
one-step femtosecond-reactive laser ablation in liquid (fs-RLAL) technique by
focusing femtosecond laser pulses onto a silicon wafer immersed in an aqueous
KAuCl4 solution. Characterization of the silica-Au NPs revealed two populations of Au NPs: (i) larger, isolated Au NPs with diameter 7.3±2.1 nm, and (ii)
smaller Au NPs (3.4±0.8 nm) embedded in an amorphous silica matrix, along
with new species of silicon observed from XPS analysis. The catalytic activity
of the silica-Au NPs towards the reduction of para-nitrophenol by NaBH4 is
significantly higher compared to the control Au NPs synthesized in the absence
of a silicon wafer and other Au NPs recently reported in literature. The formation of the two populations of silica-Au NPs is ascribed to reaction dynamics
occurring on two distinct timescales. First, the dense electron plasma formed
within tens of femtoseconds of the laser pulse initiates reduction of the [AuCl4]–
complex, leading to the formation of larger isolated Au NPs. Second, silicon
species ejected from the wafer surface hundreds of picoseconds or later after the
initial laser pulse reduce the remaining [AuCl4 ]– and encapsulate the growing
clusters, forming ultrasmall Au NPs embedded in the silica matrix. The morphologies of the silica-Au NPs generated from fs-RLAL are distinct from those
reported in recent RLAL experiments with nanosecond lasers, reflecting distinct
mechanisms occurring on the different pulse duration timescales.
</p>
</div>
</div>
</div>
One-step femtosecond laser ablation synthesis of sub-3 nm gold nanoparticles stabilized by silica