PtRu Alloy Nanoparticles I. Physicochemical Characterizations of Structures Formed as a Function of the Type of Deposition and Their Evolutions on Annealing

We have physicochemically characterized the formation of PtRu nanoparticles, deposited by evaporation onto highly oriented pyrolytic graphite, using in situ X-ray photoelectron spectroscopy, ex situ high-angle annular dark-field/scanning transmission electron microscopy, electron energy loss spectroscopy, and time-of-flight secondary ion mass spectrometry. We used three different orders of metal deposition: Pt evaporated onto Ru, Ru evaporated onto Pt and both metals evaporated simultaneously, and then followed the evolutions of the alloys as a function of annealing temperature. The C 1s, O 1s, Ru 3d, and Pt 4f core level spectra were employed to describe the alloying interactions between the metals. For all deposition methods, Ru diffuses to the NP surface through the Pt, and not the reverse. Each of the preparation methods produces surface and volume structures that differ from those of the others, even after prolonged annealing at temperatures over 700 degrees C, indicating the source of confusion in the literature concerning the physicochemical characterization of PtRu nanoparticles.