Ion-Specific Effects on Laser Ablation of Silver in Aqueous Electrolyte Solutions

Abstract

Silver nanoparticle hydrosol formation by laser ablation (LA) of a Ag target immersed in pure water and in aqueous electrolyte solutions (HCl, NaCl, NaOH, AgNO3, and/or Na2S2O3) of various concentrations has been followed by surface plasmon extinction (SPE) spectral measurements and, for selected samples, by transmission electron microscopy (TEM), scanning electron microscopy (SEM) imaging, and energy dispersive X-ray (EDX) analysis. The laser ablation process accompanied by the Ag nanoparticle fragmentation (NF) has been performed with nanosecond laser pulses, by employing either a continuous or an intermittent irradiation regime. SPE spectra have been recorded either after each irradiation step of the stepwise procedure or at the end of the continuous one and, additionally, during a subsequent aging of Ag hydrosols. The presence of HCl, NaCl, and/or NaOH during LA/NF has led to the stabilization of the resulting Ag nanoparticles, while the presence of AgNO3 and Na2S2O3 has shown a destabilizing effect. The alternations of light and dark periods in LA/NF process, along with the presence of electrolytes (having different affinities toward Ag nanoparticle surfaces and toward Ag+ ions as well) can be considered as a new physicochemical parameter affecting the outcome of LA/NF. Moreover, the light/dark alternations are useful for separating the effects of ions on Ag nanoparticle hydrosols directly under irradiation from those occurring during further aging. Ag hydrosols prepared by LA/NF in 1 × 10-3 M NaCl and aged for 1−2 days have been shown to be excellent substrates for surface-enhanced Raman scattering (SERS) measurements owing to the presence of compact aggregates with numerous interstices between nanoparticles and of reduced Ag(0) adsorption sites on chloride-modified Ag nanoparticle surfaces.