Synthesis of ultrasmall platinum nanoparticles and structural relaxation.

We report the synthesis of ligand-protected, ultrasmall Pt nanoparticles of ∼1 nm size via a one-phase wet chemical method. Using matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS), we determined the mass of the nanoparticles to be ∼8 kDa. Characterization of the Pt nanoparticles was further carried out by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), optical absorption spectroscopy, and X-ray photoelectron spectroscopy (XPS). Interestingly, we observed a large structural relaxation in the 8kDa nanoparticles (i.e. lattice parameter elongation by +10%) compared to bulk platinum. XPS analysis revealed a positive shift of Pt 4f core level energy by approximately +1 eV compared with bulk Pt, indicating charge transfer from Pt to S atom of the thiolate ligand on the particle. Compared to bulk Pt, the 5d band of Pt nanoparticles is narrower and shifts to higher binding energy. Overall, the ∼1 nm ultrasmall Pt nanoparticles exhibit quite distinct differences in electronic and structural properties compared to their larger counterparts and bulk Pt.