Gold Nanoparticle Synthesis Using Spatially and Temporally Shaped Femtosecond Laser Pulses: Post-Irradiation Auto-Reduction of Aqueous [AuCl₄]⁻

Abstract

Simultaneous spatiotemporal focusing (SSTF) of femtosecond laser radiation is used to produce gold nanoparticles from aqueous [AuCl 4 ] − solutions. Multiphoton ionization and dissociation of water produces electrons and hydrogen atoms for the reduction of [AuCl 4 ] − to Au(0) during irradiation with the temporally chirped (36 ps) pulse and produces hydrogen peroxide (H 2 O 2 ) as a long-lived reducing agent which persists after irradiation is terminated. Aqueous H 2 O 2 is found to reduce [AuCl 4 ] − , remaining in solution after the laser irradiation is terminated, leading to growth and transformation of the existing Au(0) species. The highly efficient postirradiation reduction of [AuCl 4 ] − to Au(0) by H 2 O 2 is ascribed to reactions occurring on gold nanoparticle surfaces. In the absence of added surfactant, the negatively charged gold particles formed during irradiation are a complex mixture of irregularly shaped and spherical morphologies that are only metastable as aqueous dispersions. These particles become transformed into more perfectly shaped gold crystals, as the remaining [AuCl 4 ] − is reduced in the postirradiation period. The addition of polyethylene glycol (PEG 45 ) accelerates the rate of the [AuCl 4 ] − reduction during laser irradiation and directs the exclusive formation of spherical nanoparticles. Varying the concentration of PEG 45 tunes the diameter and size distribution of the Au nanoparticles formed by laser processing from 3.9 ± 0.7 to 11 ± 2.4 nm.

Elucidating Strong Field Photochemical Reduction Mechanisms of Aqueous [AuCl4]−: Kinetics of Multiphoton Photolysis and Radical-Mediated Reduction

“…To validate the need for a linear term in the model to correctly describe the reduction kinetics at an initial solution pH ≤ 5.4, we conducted a series of experiments at an initial solution pH of 3.4 in which laser irradiation was terminated before all of the [AuCl 4 ] − was consumed. Under these conditions, [AuCl 4 ] − remaining in solution after the laser is turned off is consumed by reaction with the existing AuNP and H 2 O 2 formed during irradiation, 30 whereas no further nucleation is expected in the absence of laser irradiation. 32 Reduction of [AuCl 4 ] − by H 2 O 2 in the presence of AuNPs is Figure 5A to Eqs 2 and 3 eq 2 eq 3 pH ] − concentration (after termination of laser irradiation) to eq 4 is therefore expected to converge to the "true" value of k 2 in Table 1 Table 1 for eqs 2 and 3 are shown.…”

Applications of Shaped Femtosecond near-IR Laser Irradiation in the Generation of Metal Nanoparticles

“…SSTF was investigated as a means to achieve high intensity fs laser irradiation of aqueous [AuCl 4 ] -for the production of AuNPs [10]. Laser irradiation of surfactant-free [AuCl 4 ] -solution resulted in formation of broad distributions of the gold nanoparticles (24.0 ± 12.7 nm).…”

Resolving the source of blue luminescence from alkyl-capped silicon nanoparticles synthesized by laser pulse ablation

“…Luminescent alkyl-capped Si NPs were synthesized by single-step laser ablation of a Si (111) wafer target using simultaneously spatially and temporally focused (SSTF) 28,29 femtosecond pulses generated using a homebuilt apparatus (details in ESI †). In brief, 35 fs, 100 µJ laser pulses centred at 790 nm generated by a titanium-sapphire-based chirped-pulse amplifier laser were first shaped using SSTF and then focused onto the surface of a piranha cleaned Si (111) wafer immersed in neat 1-octene solvent through a quartz cuvette.…”