One-Step Femtosecond Laser Ablation Synthesis of Sub-5 Nm Gold Nanoparticles Embedded in Silica

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.