Surface Chemistry of Gold Nanoparticles Produced by Laser Ablation in Pure and Saline Water.

Pulsed laser ablation in liquid (PLAL) is a powerful method for producing nanoparticle colloids with a long-term stability despite the absence of stabilizing organic agents. The colloid stability involves different reactivities and chemical equilibria with complex ionic-specific effects at the nanoparticle/solvent interface which must be strongly influenced by their chemical composition. In this work, the surface composition of PLAL-produced gold nanoparticles in alkaline and saline (NaBr) water is investigated by X-ray photoelectron spectroscopy on free-flying nanoparticles, exempt from any substrate or radiation damage artifact. The Au 4f photoelectron spectra with a depth profiling investigation are used to evaluate the degree of nanoparticle surface oxidation. In alkaline water, the results preclude any surface oxidation contrary to the case of nanoparticles produced in NaBr solution. In addition, the analysis of Br 3d core-level photoelectron spectra agrees with a clear signature of Br on the nanoparticle surface, which is confirmed by a specific valence band feature. This experimental study is supported by DFT calculations, evaluating the energy balance of halide adsorption on different configurations of gold surfaces including oxidation or adsorbed salts.

Assessing the Surface Oxidation State of Free-Standing Gold Nanoparticles Produced by Laser Ablation.

The surface chemistry of gold nanoparticles produced by the pulsed laser ablation in liquids method is investigated by X-ray photoelectron spectroscopy (XPS). The presence of surface oxide expected on these systems is investigated using synchrotron radiation in conditions close to their original state in solvent but free from substrate or solvent effects which could affect the interpretation of spectroscopic observations. For that purpose we performed the experiment on a controlled free-standing nanoparticle beam produced by combination of an atomizer and an aerodynamic lens system. These results are compared with those obtained by the standard situation of deposited nanoparticles on silicon substrate. An accurate analysis based on Bayesian statistics concludes that the existence of oxide in the free-standing conditions cannot be solely confirmed by the recorded core-level 4f spectra. If present, our data indicate an upper limit of 2.15 ± 0.68% of oxide. However, a higher credence to the hypothesis of its existence is brought by the structureless valence profile of the free-standing beam. Moreover, the cross-comparison with the deposited nanoparticles case clearly evidences an important misleading substrate effect. Experiment with free-standing nanoparticles is then demonstrated to be the right way to further investigate oxidation states on Au nanoparticles.

Doping nanoparticles using pulsed laser ablation in a liquid containing the doping agent.

While doping of semiconductors or oxides is crucial for numerous technological applications, its control remains difficult especially when the material is reduced down to the nanometric scale. In this paper, we show that pulsed laser ablation of an undoped solid target in an aqueous solution containing activator ions offers a new way to synthesise doped-nanoparticles. The doping efficiency is evaluated for laser ablation of an undoped Gd2O3 target in aqueous solutions of EuCl3 with molar concentration from 10-5 mol L-1 to 10-3 mol L-1. Thanks to luminescence experiments, we show that the europium ions penetrate the core of the synthesised monoclinic Gd2O3 nanoparticles. We also show that the concentration of the activators in the nanoparticles is proportional to the initial concentration in europium ions in the aqueous solution, and a doping of about 1% ([Eu]/[Gd] atomic ratio) is reached. On the one hand, this work could open new ways for the synthesis of doped nanomaterials. On the other hand, it also raises the question of undesired penetration of impurities in laser-generated nanoparticles in liquids.