ID-12-P-2438 In-situ TEM reduction of core-shell Ag@In₂O₃ nanoparticles

Under gaseous operating conditions, the structure of bimetallic catalysts is highly dependent on the pressure, temperature, exposure time and composition of the surrounding atmosphere. Their catalytic properties are therefore prone to large variations during a catalytic experiment, due to the evolution of the morphology, chemical arrangement, and eventually oxidation reduction, of the two metals. For the same reasons their optical properties, particularly the frequency of the surface plasmon resonance (SPR), are very sensible to these environmental parameters. It follows that structural and optical characterization techniques must be implemented in an environment as close to the catalytic conditions as possible.
We present an environmental TEM study on Ag@In2O3 nanoparticles. These particles exhibit a reversible shift of their SPR, after air exposition and after hydrogen reduction at 300°C [1]. The synthesis was made using a physical route at the Low Energy Cluster Beam Deposition source of PLYRA in Lyon (France). With this kind of source, the size and the composition of the particles can be monitored independently and the obtained nanoparticles are very clean, with no chemical ligand around the particles.
The microscope used is a TITAN FEI Environmental Transmission Electron Microscope (ETEM) operating at 300 kV, with a Cs corrector of the objective lens; this instrument is installed at CLYM (www.clym.fr). The sample was mounted on a GATAN TEM heating holder with Inconel oven; HRTEM images and movies of the bimetallic nanoparticles were recorded up to 500°C and 10 mbar [2].
The starting chemical configuration of the particles is a core-shell structure, with silver cores of ~4 nm and In2O3 amorphous shell with a thickness between 1 and 2 nm. These nanoparticles have been exposed in the ETEM to successive (H2 pressure; temperature) couples, from (1 mbar H2 ; 25°C) to (10 mbar H2 ; 500°C). The structural changes observed at the atomic scale range from the formation of Janus Ag-crystalline In2O3 nanoparticles to the complete reduction of indium oxide, and the incorporation of indium into the remaining silver nanoparticles to form an InAg solid solution. Results are followed by a discussion about the precautions to be taken in the interpretation of such results and the complementary experiments needed to conclude about the effective role of each experimental parameter.
[1] Plasmon spectroscopy of small indium–silver clusters monitoring the indium shell oxidation, E. Cottancin, C. Langlois, J. Lerme, M. Broyer, M-A. Lebeault and M. Pellarin, Phys.Chem.Chem.Phys. (2014) 16 5763-5773
[2] Advances in the environmental transmission electron microscope (ETEM) for nanoscale in situ studies of gas–solid interactions , J.R. Jinschek, Chem. Commun. (2014) 50 2696-2706