Characterization of Pt-rare-earth nanoparticles by STEM tomography and EDX

Abstract text (incl. figure legends and references)

The search for sustainable energy sources for transport has intensified recently because of the climate crisis. In addition to batteries for electric motors, fuel cells are gaining increasing interest. Platinum-Rare Earth Metal (Pt-RE) alloy nanoparticles (NP) are promising candidates for use in proton‑exchange membrane fuel cells (PEMFCs), which are a sustainable alternative to combustion engines in automobiles and buses (1). In such PEMFCs, Pt-RE nanoparticles on carbon black can be used as a catalyst for the oxygen reduction reaction and the oxidization of H₂ (Fig. 1) (1). They show long-term stability and high catalytic activity, which would be optimized by a strained Pt surface layer over the alloy core, providing optimal reactant binding energies and preventing dealloying (2). Recently, a newly developed chemical synthesis route to prepare Pt-RE nanoalloys with tunable alloy phases and particle sizes has been published (3). However, preliminary results on Pt₂Gd have shown that after acid leaching to remove byproducts, nanoporous structures with yet unclear 3D distribution of elements are obtained (2, 3).

Here, we present an analysis of the elemental distribution and 3D shape of Pt2Gd nanoalloys. The electron-dispersive X-ray (EDX) map in Figure 2b shows the segregation of Pt and Gd in the particle. The map has been acquired using a Super-X detector at an FEI Osiris S/TEM at 200 kV and a total dose of 3E+6 e^-/Ų from the particle depicted in Figure 2a. Noise in the elemental maps has been reduced by principal component analysis using the hyperspy toolbox (4). A comparison between the HAADF-STEM image and the elemental distribution indicates that darker areas in the STEM image contain more Gd, and appear darker due to the lower atomic number of Gd.

HAADF-STEM tilt series for electron tomography of Pt₂Gd NPs have been acquired using a probe‑corrected Titan³ 80-300 equipped with a Fischione photomultiplier STEM detector and operated at 300kV. Figure 2c shows an orthoslice of the volume reconstructed using a workflow based on the ASTRA toolbox (5), indicating a porous structure with a denser core. A comparison with Figure 2b however suggests that the darker areas observed in Figures 2a and 2c do not correspond to voids but rather locations in the sample where the Gd might have been segregated.

The investigation of a larger number of particles, ideally in conjunction with EDX tomography is necessary to further clarify this observation beyond doubt. EDX tomography, however, is challenging due to the beam sensitivity of the material.

Fig. 1: Scheme of a proton-exchange membrane fuel cell. Pt-RE nanoparticles on carbon black are used as the yellow-marked anode (left) and cathode (right).

Fig. 2: a) HAADF-STEM image b) Overlay elemental map of Pt (red) and Gd (blue) from the same particle as in (a). c) Orthosclice from STEM Tomography from another nanoparticle.

References

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