EXAFS studies of palladium nanoparticles: Size control and hydrogenation

We synthesized several samples of thiol-stabilized Pd nanoparticles by controlling their size with the palladium/thiol ratio from 5:1 to 1:3. Size dependence on the Pd/thiol ratio was verified by comparing the Pd-S and Pd-Pd coordination numbers. The average size of free-standing nanoparticles decreased as the thiol concentration increased and stabilized at the 1:2 value of palladium/thiol ratio. In addition to the free-standing nanoparticles, we analyzed their thin films by depositing them as monolayers on Kapton film using Langmuir-Blodgett technique. We obtained that the structure of the nanoparticles was preserved, within the accuracy of the EXAFS measurement, after preparing Langmuir films. We also studied the effect of hydrogenation on the both type of the nanoparticles (the free standing ones and those in the monolayers). In both cases, exposure to 4 atm H₂ pressure resulted in the elongation of the average Pd-Pd distance. The larger change (0.06±0.03 Å, relative to the non-hydrogenated films) was observed in the smaller clusters (with 1:1 Pd thiol ratio), and the smaller change (0.03±0.01 Å) - in the larger clusters (with 3:1 ratio). Such bond length expansion is attributed to the defects caused by H₂ adsorption.