The surface characteristics of metallic nanoparticles (NPs), especially the atomic percent and distribution of each component, is critical to their catalytic and electrocatalytic behavior. Therefore, efforts are underway to design NP catalysts with full control of their composition, both in the bulk and at the surface. In our work, Pt-Ru and Pt-Ir NPs, either in alloy or core@shell forms, are of great interest, due to their suitability for a wide range of electrocatalytic and sensing applications, such as for the oxidation of formic acid, alcohols, and ammonia, for oxygen evolution and reduction in regenerative fuel cells, in the reduction of hydrogen peroxide, and as an electron mediating matrix in glucose biosensors ^1–8.

In the present work, Pt_xIr_y alloys, Ir_core@Pt_shell, Ru_core@Pt_shell, and Ru_core@Pt-Ir_shell NPs were synthesized using the simple and controllable polyol method ⁹. In the case of Pt_xIr_y alloy NPs, two sequential heating steps were employed to minimize the possibility of surface enrichment of Pt or Ir during NP formation. In the case of Ir_core@Pt_shell and Ru_core@Pt_shell NPs, the Pt_shell, with a coverage of between 0.1 and 2 monolayers (MLs), was controllably deposited on the surface of the Ir_core and Ru_core NPs, while for the Ru_core@Pt-Ir_shell NPs, one ML of the Pt_xIr_y alloy shell, containing different Pt:Ir ratios, was deposited on the Ru_core. Wavelength dispersive X-ray spectroscopy (WDS) and energy dispersive X-ray spectroscopy, coupled with high-resolution transmission electron microscopy (EDS/HRTEM) and powder X-ray diffraction (PXRD), were then used to determine the bulk composition and homogeneity of the NPs.

To determine the outer surface composition of these NPs, the underpotential deposition/stripping of Cu and oxalic acid oxidation have been used previously, e.g., for Pt-Ru NPs ^10–12. However, the effect of NP size, Pt-Ru interactions at the surface, and the degree of Ru oxidation on the catalytic activity have not been determined. In other prior work ¹³, CO stripping was used to establish the stability of PtRu NPs by tracking the changes in the surface composition. In the present work, several electrochemical fingerprinting methods were developed (underpotential deposition/removal of H atoms, CO stripping, and surface oxide reduction) to determine the precise coverage and thickness (fraction of MLs) of the Pt_shell on the Ir_core@Pt_shell and Ru_core@Pt_shell, NPs and the Pt:Ir ratio at the surface of the Pt_xIr_y alloy and Ru_core/Pt-Ir_shell NPs, as well as the real surface area of the exposed metals.

Figure 1 ^1,14,15 shows the CO stripping voltammetry of the NPs under study here as an example of how the peak potential and splitting correlate with the NP surface composition. A comparison will be given between the surface areas and compositions obtained by each of the electrochemical methods used here, as well as with what can be inferred from TEM imaging methods.

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