X-ray Photoelectron Spectroscopic Studies of Pd Nanoparticles Deposited onto Highly Oriented Pyrolytic Graphite: Interfacial Interaction, Spectral Asymmetry, and Size Determination

Pd nanoparticles, evaporated onto highly oriented pyrolytic graphite (HOPG) surfaces, were studied using in situ X-ray photoelectron spectroscopy (XPS). The evolutions of both core level and valence band spectra were followed as a function of Pd deposition. Both Pd3d 5/2 and Pd3p 3/2 core level spectra were separated into three symmetric peaks, which are attributed, in order of increasing binding energy, to surface, bulk, and surface oxide components. The intensity ratios among these components account fully for the asymmetry changes of the spectra with NP size. On analyzing the relationships among the C1s, Pd3d 5/2, and Pd3p 3/2 spectra, we identified the interaction at the Pd/HOPG interface as back-bonding. By using the Pd3p 3/2:Pd3d 5/2 intensity ratio, we estimated the size of the NPs as a function of deposition. Good agreement between the XPS and atomic force microscopic size determinations was found, suggesting XPS could be a reliable, and probably more accessible, alternative.