Molecular dynamics simulations of initial Pd and PdO nanocluster growth in a magnetron gas aggregation source

Abstract

Molecular dynamics simulations are carried out for describing growth of Pd and PdO nanoclusters using the ReaxFF force field. The resulting nanocluster structures are successfully compared to those of nanoclusters experimentally grown in a gas aggregation source. The PdO structure is quasi-crystalline as revealed by HRTEM analysis for experimental PdO nanoclusters. The role of the nanocluster temperature in the MD simulated growth is highlighted.

The role of oxygen on the growth of palladium clusters synthesized by gas aggregation source

“…Furthermore, the two‐body mechanism or atom attachment can also contribute to the cluster growth and to the final size. The formation of a Pd‐O bond releases more energy than a Pd‐Pd bond therefore, the formation of PdO clusters takes longer to reach a thermal equilibrium, slowing the clustering process that can explain the size decrease at high OFR . The relative size dispersion increases when increasing the OFR from 0 to 1 sccm as also observed in Figure c and is much higher at the OFR value of 5 sccm.…”

“…Moreover, the local gas heating could modify the aggregation zone and contribute to the size distribution broadening. A recent work describing the cluster growth process by molecular dynamic (MD) simulations confirms that an increase of the temperature is observed during the first steps of formation of the Pd clusters. MD simulations also showed the same behavior during the initial steps of the formation of PdO clusters but a longer period of time was needed to reach the thermal equilibrium due to the formation of Pd‐O bonds.…”

“…The physicochemical phenomena that occur to the gas phase near the cathode and to the target surface could be related with these two specific OFR values. The formation of large clusters could be explained by a local increase of temperature, which has been recently observed in a GAS chamber and also, by plasma inhomogeneities (such as the electron temperature, electron density, and plasma voltage) that appear as a result of the cluster formation and which could trap some clusters in the aggregation zone allowing the formation of larger ones (as for the Ti‐O system) …”

“…In these previous studies, the growth mechanism and the cluster size were related to the main experimental parameters. Other studies have shown the effect of reactive environments, for example, on Pd subnanometric clusters exposed to H 2 and O 2 or Pd surfaces exposed to O 2 . However, an open question, still remains: how the presence of a reactive gas in the GAS chamber could affect the nanostructure and the chemical composition of Pd clusters during their growth.…”

Pt3MeAu (Me = Ni, Cu) Fuel Cell Nanocatalyst Growth, Shapes, and Efficiency: A Molecular Dynamics Simulation Approach

“…A neutral gas buffer is added for describing the energy dissipation during collisions. This is mimicking free nanoparticle growth in the context of a gas aggregation source (GAS) where atoms are sputtered in such a gas at a pressure high enough for allowing clustering in the flight [29][30][31][32][33].…”