Determination of the Size of Supported Pd Nanoparticles by X-ray Photoelectron Spectroscopy. Comparison with X-ray Diffraction, Transmission Electron Microscopy, and H2 Chemisorption Methods

Supported palladium nanoparticles with different diameters were synthesized by the water-in-oil microemulsion method using TiO2 as support. The materials were characterized by different physicochemical methods such as X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and H-2 chemisorption. The results confirmed that the microemulsion method permits well-dispersed palladium nanoparticles to be obtained. The size of the nanoparticles was estimated by XPS intensity ratios using models proposed by Davis and by Kerkhof and Moulijn and compared with XRD, TEM, and H-2 chemisorption analysis. Good accordance of the two models was found for very small Pd particles (smaller than 3 nm). The Kerkhof Moulijn model seemed to be very sensitive to the small variation in the particle size distribution. The Davis model seemed to be more adequate to determine the size of small and biggest particles as compared with the Kerkhof Moulijn model. A good accordance between TEM results and the Davis model was found. The results obtained using the Davis model permitted also understanding of the differences observed between XRD and TEM studies. XPS analysis could be a good and probably more accessible alternative to determine rapidly and with high accuracy nanosize particles of materials, in particular when others physicochemical techniques are not accessible or have a limited resolution.