In the form of alloyed or segregated structures, bimetallic nanoparticles (NPs) exhibit remarkable catalytic (activity, selectivity, stability) and optical (localized surface plasmon resonance, LSPR) properties, both depending on the morphology and the chemical configuration that they adopt. Moreover, the structure of these nanoparticles may evolve under specific environmental conditions (pressure, temperature, presence of reactants). It follows that structural and optical characterization techniques must be implemented in a controlled environment for probing these restructuring phenomena.

Here, we present an environmental TEM (ETEM) study on Ag-In nanoparticles. These particles exhibit a reversible shift of their LSPR, after exposition to oxidizing (air, 25°C) and reducing (H₂+N₂, 500°C) atmospheres [1]. The particles were synthesized by Low Energy Cluster Beam Deposition (at PLYRA in Lyon), which allows an independent control of their size and composition. The microscope used was a FEI Titan ETEM operating at 300 kV, with a Cs corrector of the objective lens. HRTEM images and movies of the bimetallic nanoparticles were recorded at up to 500°C and under 10 mbar H₂ [2].

The starting chemical configuration of the particles is a core-shell structure, with an Ag or Ag-In alloyed core of 4-5 nm and an In₂O₃ amorphous shell with a thickness of 1-2 nm. The nanoparticles were exposed in the ETEM to successive (H₂ pressure; temperature) couples, from (1 mbar H₂; 25°C) to (10 mbar H₂; 500°C). These particles were monitored both at the local (single nanoparticle tracking) and global (observations on large assemblies, under low magnification) scales. The structural changes observed at the atomic scale range from the almost complete extraction of the Ag-rich core from the In₂O₃ shell, leading to “Janus” nanoparticles, to the complete reduction of the indium oxide shell. In₂O₃ reductions which occur at higher temperature than the melting point of indium (156°C) induce the competition between two behaviours: i) the melting or evaporation of reduced indium, and thus the decreasing thickness of the shell until its complete disappearance, ii) the diffusion from the shell to the core of reduced indium atoms, leading to a core growth (figure 1). This last point may be closely related to the evolution of the shell thickness during reduction. From these results, we constructed a (pressure, temperature) diagram highlighting the relationship between temperature and H₂ pressure in the reduction activation. These results provide new insights in both physical and chemical processes involved during reduction of oxidized metallic nanoparticles at the atomic scale.

The authors thank the technical staff of the CLYM facility for access to the Titan microscope, and the PLYRA cluster facility for the cluster synthesis. ARC Energie (Academic Research Community), Rhônes-Alpes regional council is acknowledged for thesis scholarship.

[1] E.Cottancin, C.Langlois, J.Lermé, M.Broyer, M.A Lebeault, M.Pellarin, Phys. Chem. Chem. Phys. 2014, 16, 5763

[2] J.R. Jinschek, Chem. Commun. 2014, 50, 2696

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EMC Abstracts - https://emc-proceedings.com/abstract/in-situ-environmental-hrtem-study-of-the-restructuration-under-reducing-atmosphere-of-small-oxidized-silver-indium-nanoparticles/