1. Introduction

Different Pt-Pd nanoalloys were prepared from organic precursors in solution. Their nucleation and growth in the liquid was in situ studied in a graphene oxide liquid cell by the direct reduction in the electron beam [1-3].

Then, the morphological evolution of the nanoalloys under gas pressure was in situ studied in an environmental sample holder [4] by standard TEM.

2.Pt@Pd core shell NPs

Increasing amounts of Pt could be deposited on Pd nanocubes cubes by sequential reduction, with a resulting concave shape as seen in fig. 1.

Fig.2 is a set of images during the Pt growth around the same Pd cube, during 20 mn. The operating conditions (magnification 800 K, electron density 3. 10⁵ A/m²) corresponds to an increasing electron dose.  During this time, the variation of liquid quantity in the drop was not visible. From the set of images, it seems that the Pt layer has an homogeneous thickness during the growth, so that the final concave shape results from other steps in the preparation process.

Then, the morphology of Pt@Pd nanocubes was in situ observed during oxido reduction cycles in a few mbar of pure O₂ and H₂.

Fig. 3 clearly shows that the Pd@Pt nanocubes have concave shapes in pure H₂. In pure O_2, they are much rounded at the corners and the (110) facets are extended compared to the same samples in H₂, as the anisotropy ratio between the surface free energies of  (001) and (110) faces,  increases from 0.7 in H₂ to 0.8 in O₂.

3. Properties

A maximal reactivity in gas has been found for an equivalent thickness of 0.4 atomic Pt layers on the Pd nanocubes. For this thickness the core-shell particles are more active than pure Pd cubes or similar Pt cubes reported in literature. This behavior is explained by a decrease of the adsorption energy of molecules, due a compressive strain, induced by the misfit between the two metal bulk lattices and by a ligand effect due to the modification of the electronic structure of Pt atoms in contact with Pd atoms. A similar qualitative evolution as a function of thickness of the Pt layer was already observed in electrocatalysis and was also explained by the decrease of the strength of adsorbed species.

References

[1]  Yuk M. et al.,Science , 336,  61  (2012)

[2]  De Clercq A. et al., J. Phys Chem. Letters, 5,  2126-2130  (2014)

[3]  Alloyeau D. et al. Nanoletters (2015)

[4]  Giorgio S. et al., Ultramicroscopy 106 -6, 503 (2006)  

Figures:

1. Pd nanocubes covered with 0.5 atomic layer of Pt, with a concave shape

2. Evolution with time of a Pd nanocube in a 3mM Pt(acac)2 solution in a liquid cell under electron beam irradiation (3. 105 A/m2 at a magnification of 800K).

3. ETEM observations of two different series (right and left) of Pd@Pt0.4eqL under (a-b) 1 mbar H2 with limited effect on the shape and (c-f) under 1 mbar O2 with a visible rounding of the sharp corners and extension of the (110) facets.

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