1. Introduction

Au-Rh nanoalloys were prepared from colloidal solution [1]. The growth mechanism was studied in situ by TEM in a graphene liquid cell [2-3]. Then, in an environmental sample holder [4],the influence of oxygen or hydrogen adsorption on the structure of AuRh/TiO₂ catalysts was observed at a pressure of a few mbar and at room temperature.

2. Growth mechanism

The colloidal solution was encapsulated in a graphene oxide liquid cell and directly observed in a standard electron microscope. The particle growth was initiated under the electron beam.

The same area was observed during about 80 s and images were recorded every 5 s. The average particle size increases until 4 nm. The density number plotted in fig. 1, indicates a maximal density after 25 s, then a drop after 50 s, corresponding to a growth mechanism by direct adsorption of the monomers. This process is followed by coalescence of the  particles, as directly seen in fig. 2.

3. Segregation in hydrogen

Au-Rh nanoparticles with an homogeneous structure, and supported on rutile TiO₂ nanorods powders, were observed during oxidation- reduction cycles in an environmental sample holder. In the largest NPs, core- shell formation was clearly observed during hydrogen adsorption, as seen in figure 3. On the other hand, the observation of Au-Rh NPs annealed ex situ in H₂ at 400 °C, also shows the core- shell contrast which was not visible before H₂ treatment.

The strong interaction between hydrogen and Rh is certainly responsible for the surface segregation of Rh.

Acknowledgments:

We thank the ANR DINAMIC  -11-BS10-009 for financial support and Région PACA for a grant given for the PhD thesis of A. De Clercq.

References

1. Konuspayeva et al., PCCP 17 (2015) 28122
2. J.M. Yuk et al., Science, 336 (2012) 61
3. De Clercq, et al., The Journal of Physical Chemistry Letters, 5 (2014) 2126-2130
4. S. Giorgio et al., Ultramicroscopy. 106 (2006) 503     

Figures:

1. Density number of particles in the same volume of liquid, during the growth of Au-Rh NPs.

2. Direct growth of Au- Rh NPs in a graphene oxide liquid cell under electron irradiation. The colored squares represent similar areas observed respectively at 10 s, 20 s, 40 s, 50 s, 60s. The coalescence starts at the 4th image.

3. In situ observation in an environmental sample holder of the Au@Rh core- shell formation due to hydrogen adsorption.

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