Investigation of Iron Nanoparticle Oxidation under External Electrostatic Fields Using Reactive Molecular Dynamics

Abstract

A reactive molecular dynamics (MD) study of iron nanoparticle oxidation under externally applied electrostatic fields is performed, with a focus on the role of charge transfers and electrically induced polarization. Two charge equilibration methods that either enable or shield long-range charge transfers and two ReaxFF parameter sets that predict different bonded and nonbonded interactions are used in the evaluation. The field-induced polarization modeled in MD simulations is in good agreement with analytical solutions and density functional theory (DFT) computations. Results show that oriented external electric fields affect the transition states of reactions involved in oxygen adsorption on the nanoparticle surface and oxygen diffusion inside the iron lattice. The oxygen diffusion inside the nanoparticle can be either aided or hindered by the external electric field depending on the direction of motion of oxygen compared to the direction of the external electric field. Simulations also demonstrate that long-range charge transfers increase the reactivity of the system compared to shielded interactions. Regarding ambient conditions, a density increase can accelerate the kinetics of the system, while temperature variations have a smaller effect on the oxidation rate for the systems investigated here. The external electrostatic field affects the kinetic energy, collision frequency, and reactivity of the system. When charge transfers are limited to close interactions, the system's kinetics is accelerated only under strong external electric fields. On the contrary, if long-range charge transfers are enabled, an increase in the oxidation rate is observed for weak electric fields, whereas for stronger electric fields the oxidation rate decreases due to a more coherent movement of the charged particles under the Lorentz forces. In addition, external electric fields affect the chemical composition of iron and iron oxide nanoparticles. The diffusion of the negatively charged absorbed oxygen atoms toward one side of the nanoparticle causes an anisotropic thickness of the oxidation layer. The influence of the external electric field on the system's kinetics depends on the choice of the ReaxFF parameter set, highlighting the importance of accurate training of the force field with a focus on charge transfers and their distribution under external fields. These findings provide insight into the fundamental effects of charge transfers and field-induced polarization on the oxidation process of a nanoparticle in the presence of an external electrostatic field and offer a comprehensive evaluation of the capability of the MD-ReaxFF framework to predict the underlying physical mechanisms at the atomistic scale.

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