Effect of Size on the Electrochemical Stability of Pt Nanoparticles Deposited on Gold Substrate

Pulsed laser deposition was used to prepare Pt nanoparticles of various sizes ranging from 1.8 to 6.0 nm. These nanoparticles were deposited oil highly oriented pyrolytic graphite (HOPG) and gold substrates. For Pt deposited oil HOPG and Au, the size of the nanoparticles was established by scanning tunneling microscopy (STM) and calculated from the electrochemically active surface area (EASA) obtained through measurements of the hydrogen underpotential deposited charges, Q(Hupd). The diameters determined from these two sets of measurements agreed with each other to within 60%. X-ray photoelectron spectroscopy (XPS) Was used to assess the size of the nanoparticles before and after an electrochemical treatment that involved potential cycling in 0.5 M H2SO4. The Upper potential limit was progressively increased from 1.15 to 1.40 V in steps of 0.05 V, and the EASA Was Continuously monitored. The EASA decreased with cycle number and with increasing Upper potential limit to 1.35 V vs RHE. This effect was found to be larger for the smaller Pt nanoparticles (50% decrease For phi = 1.8 nm) than for the larger ones (20% decrease for phi = 3.0 nm). It was found by XPS that the diameter of the smaller Pt nanoparticles increased from phi = 1.8 nm to phi = 6.5 nm as a result of the electrochemical treatment, whereas the diameter of the larger nanoparticles (phi = 3.0 nm) remained constant. In this potential range, the corrosion of the gold Substrate is minimal, and this observation can be explained by an increase of the equilibrium Soluble Pt concentration with decreasing Pt nanoparticle diameter.