(Invited) Electrochemical Methods for Surface Composition Determination of Alloy and Core/Shell Nanoparticles
Authors: El Sawy Ehab N , Hoang Annie , Slaby Jachym , Birss Viola 
Published: February 2020

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, PtxIry alloys, Ircore@Ptshell, Rucore@Ptshell, and Rucore@Pt-Irshell NPs were synthesized using the simple and controllable polyol method 9. In the case of PtxIry 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 Ircore@Ptshell and Rucore@Ptshell NPs, the Ptshell, with a coverage of between 0.1 and 2 monolayers (MLs), was controllably deposited on the surface of the Ircore and Rucore NPs, while for the Rucore@Pt-Irshell NPs, one ML of the PtxIry alloy shell, containing different Pt:Ir ratios, was deposited on the Rucore. 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 Ptshell on the Ircore@Ptshell and Rucore@Ptshell, NPs and the Pt:Ir ratio at the surface of the PtxIry alloy and Rucore/Pt-Irshell 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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Figure 1