Free electrons and HO formed in an optical breakdown plasma are found to directly control the kinetics of [AuCl] reduction to form Au nanoparticles (AuNPs) during femtosecond laser-assisted synthesis of AuNPs. The formation rates of both free electrons and HO strongly depend on the energy and duration of the 800 nm laser pulses over the ranges of 10-2400 μJ and 30-1500 fs. By monitoring the conversion of [AuCl] to AuNPs using in situ UV-vis spectroscopy during laser irradiation, the first- and second-order rate constants in the autocatalytic rate law, k and k, were extracted and compared to the computed free electron densities and experimentally measured HO formation rates. For laser pulse energies of 600 μJ and lower at all pulse durations, the first-order rate constant, k, was found to be directly proportional to the theoretically calculated plasma volume, in which the electron density exceeds the threshold value of 1.8 × 10 cm. The second-order rate constant, k, was found to correlate with the measured HO formation rate at all pulse energies and durations, resulting in the empirical relationship k ≈ HO. We have demonstrated that the relative composition of free electrons and HO in the optical breakdown plasma may be controlled by changing the pulse energy and duration, which may make it possible to tune the size and dispersity of AuNPs and other metal nanoparticle products synthesized with femtosecond laser-based methods.
Irradiation of aqueous [AuCl4]− with 532 nm nanosecond (ns) laser pulses produces monodisperse (PDI = 0.04) 5-nm Au nanoparticles (AuNPs) without any additives or capping agents via a plasmon-enhanced photothermal autocatalytic mechanism. Compared with 800 nm femtosecond (fs) laser pulses, the AuNP growth kinetics under ns laser irradiation follow the same autocatalytic rate law, but with a significantly lower sensitivity to laser pulse energy. The results are explained using a simple model for simulating heat transfer in liquid water and at the interface with AuNPs. While the extent of water superheating with the ns laser is smaller compared to the fs laser, its significantly longer duration can provide sufficient energy to dissociate a small fraction of the [AuCl4]− present, resulting in the formation of AuNPs by coalescence of the resulting Au atoms. Irradiation of initially formed AuNPs at 532 nm results in plasmon-enhanced superheating of water, which greatly accelerates the rate of thermal dissociation of [AuCl4]− and accounts for the observed autocatalytic kinetics. The plasmon-enhanced heating under ns laser irradiation fragments the AuNPs and results in nearly uniform 5-nm particles, while the lack of particles’ heating under fs laser irradiation results in the growth of the particles as large as 40 nm.
Photochemical reduction of aqueous
Ag+ and [AuCl4]− into alloy
Au–Ag nanoparticles
(Au–Ag NPs) with intense laser pulses is a green synthesis
approach that requires no toxic chemical reducing agents or stabilizers;
however size control without capping agents still remains a challenge.
Hydrated electrons produced in the laser plasma can reduce both [AuCl4]− and Ag+ to form NPs, but hydroxyl
radicals (OH·) in the plasma inhibit Ag NP formation by promoting
the back-oxidation of Ag0 into Ag+. In this
work, femtosecond laser reduction is used to synthesize Au–Ag
NPs with controlled compositions by adding the OH· scavenger
isopropyl alcohol (IPA) to precursor solutions containing KAuCl4 and AgClO4. With sufficient IPA concentration,
varying the precursor ratio enabled control over the Au–Ag
NP composition and produced alloy NPs with average sizes less than
10 nm and homogeneous molar compositions of Au and Ag. By investigating
the kinetics of Ag+ and [AuCl4]− coreduction, we find that the reduction of [AuCl4]− into Au–Ag NPs occurs before most of the Ag+ is incorporated, giving us insight into the mechanism of
Au–Ag NP formation.
Laser photoreduction of metal ions onto graphene oxide (GO) is a facile, environmentally friendly method to produce functional metal-GO nanocomposites for a variety of applications. This work compares Au-GO nanocomposites...
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