Preparation of small size palladium nanoparticles by picosecond laser ablation and control of metal concentration in the colloid

We assessed a method for the preparation of small, highly stable and unprotected Pd nanoparticles by picosecond laser ablation in 2-propanol. The nanoparticles can be extracted from 2-propanol by centrifugation and redispersed in water, where a strongly negative ζ-potential assures long term stability. The proposed procedure permits reduction of particle size down to 1.6 nm and optimization of the Pd(0):Pd(II) ratio which, in the best cases, was of the order of 6:1. The increase of this ratio with ablation times has been correlated to the high temperature conversion of PdO to metallic Pd by a simple theoretical model. A study of the relationship between colloid absorption at 400 nm and Pd concentration permitted the role of PdO in the determination of the UV–vis spectra to be clarified and the limits of the Mie theory for the evaluation of colloid concentration to be established. The absorption at 400 nm can be used as a fast method to estimate the Pd content in the colloids, provided that a calibration of the ablation process is preliminarily performed.