Particle size was investigated by measuring the hydrodynamic diameter

To clarify the effect of electrolytes on colloidal stability, zeta potential (ζ) measurements as function of ionic strength were performed. Results are shown in Fig. 3. Palladium nanoparticles fabricated in pure water (80 μg/mL) are just slightly negatively charged (ζ close to zero) which results in weak colloidal stability. The addition of salts strongly increases the charge of colloidal palladium nanoparticles ( Fig. 3). Interestingly, literature shows that adding stabilizing ligands or surfactants like SDS enhances colloidal PR619 of palladium nanoparticles only marginally, but obviously, dissolved salts are quite efficient to stabilize colloidal nanoparticles [20]. In this study, as shown in Fig. 3, a minute amount of salt (>0.001 mM) is sufficient to cause a significantly negative zeta potential, where colloidal stability is ensured (ζ < −30 mV). With increasing ionic strength (0.001–10 mM) the absolute value of zeta potential is further increased (?−50 mV), caused by higher surface charge density of palladium nanoparticles. According to prior investigations, the surface of nanoparticles produced by PLAL contains oxidized surface atoms, which was proven by X-ray photoelectron spectroscopy (XPS) [30], [31] and [38]. XPS spectra of palladium nanoparticles produced by an Nd:YAG-laser were measured by Mortazavi et al., where three kinds of Pd species were found on the surface [19]. A main peak was related to Pd0 and two smaller peaks resulted from Pd2+- and Pd4+-species. In this study (Section 3.3, Fig. 9a), the oxidation state on the surface of palladium nanoparticles was measured to be +II and +IV, additionally to metallic palladium, which is in good agreement the literature. The positive charge of the nanoparticle surface is overcompensated by covering the surface by a Stern layer of negative ions [33] and [38]. It has been reported that the nature of the anions determines its effectiveness of the charge delivery, in particular its number of hydration and polarizability showing that the adsorbed anions from the salt are more dominant compared to the oxidative charge delivery [30]. This ion adsorption at the nanoparticle/water interface results in a negative zeta potential in aqueous solution. Due to electrostatic repulsion, nanoparticle agglomeration is prevented and colloidal stability can be achieved.