One-step femtosecond laser ablation synthesis of sub-3 nm gold nanoparticles stabilized by 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 7.0 +/- 2.0 nm, and (ii) smaller Au NPs (1.9 +/- 0.7 nm) stabilized by an amorphous silica matrix, along with new species of silicon observed from XPS analysis. The silica-Au NPs were catalytically active towards the model reaction of para-nitrophenol reduction by NaBH4. 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 stabilized by 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.