Inducing Porosity on Hollow Nanoparticles by Hypervelocity Impacts

Abstract

The large surface-to-volume ratio of hollow palladium nanoparticles (hNPs) offers room to improve their hydrogen storage capacity as well as their catalytic activity. However, a less explored possibility is to use, in addition, the internal cavity. Here we explore, through classical molecular dynamics, the possibility of boring channels across the hNP wall by collision with solid Pd nanoprojectiles at high velocities, as well as their resilience to maintain their spherical geometry. We choose a stable hNP with an inner diameter of 13 nm and an outer diameter of 15 nm. The projectiles are Pd NPs of 1.5, 2.4, and 3.0 nm, respectively. We consider collision speeds between 3 and 15 km/s, with an impact parameter between 0 to 7 nm. Four different regimes, as a function of kinetic energy and impact parameter of the projectile, are found. For low speeds, the projectile is not able to penetrate the target and only creates surface craters. For a narrow range of intermediate speeds, the projectile enters the target, but the hNP shell is able to self-heal, either totally or partially. For large speeds, the projectile penetrates the target without altering its spherical hollow geometry, but for even larger speeds, the hNP collapses into a solid structure. The specific threshold speed for each regime depends on the mass and speed of the projectile. In all noncollapsing cases, the results show a linear relationship between projectile kinetic energy and crater or perforation size. We also studied its behavior when the hNP suffers successive collisions, finding that it keeps its hollow shape but forms faceted structures, such as nanoframes or hollow cuboctahedron nanoparticles. All of our results suggest that Pd hNPs, with adequate combinations of external radius and thickness are very robust, can withstand hypervelocity impacts and that channels can be opened to allow molecules to reach the internal cavity.