Tunability of Interactions between the Core and Shell in Rattle-Type Particles Studied with Liquid-Cell Electron Microscopy

Abstract

Yolk–shell or rattle-type particles consist of a core particle that is free to move inside a thin shell. A stable core with a fully accessible surface is of interest in fields such as catalysis and sensing. However, the stability of a charged nanoparticle core within the cavity of a charged thin shell remains largely unexplored. Liquid-cell (scanning) transmission electron microscopy is an ideal technique to probe the core–shell interactions at nanometer spatial resolution. Here, we show by means of calculations and experiments that these interactions are highly tunable. We found that in dilute solutions adding a monovalent salt led to stronger confinement of the core to the middle of the geometry. In deionized water, the Debye length κ –1 becomes comparable to the shell radius R shell , leading to a less steep electric potential gradient and a reduced core–shell interaction, which can be detrimental to the stability of nanorattles. For a salt concentration range of 0.5–250 mM, the repulsion was relatively long-ranged due to the concave geometry of the shell. At salt concentrations of 100 and 250 mM, the core was found to move almost exclusively near the shell wall, which can be due to hydrodynamics, a secondary minimum in the interaction potential, or a combination of both. The possibility of imaging nanoparticles inside shells at high spatial resolution with liquid-cell electron microscopy makes rattle particles a powerful experimental model system to learn about nanoparticle interactions. Additionally, our results highlight the possibilities for manipulating the interactions between core and shell that could be used in future applications.

Frequency-controlled electrophoretic mobility of a particle within a porous, hollow shell

“…The salt concentration within the liquid-cell geometry was changed by flowing in a given aqueous LiCl solution for at least 15 min before any electric field experiments were performed. It was confirmed that the new salt concentration was present within the shell by observation of how close the particle could approach the shell [42].…”

“…In contrast, a free particle with a radius of 170 nm moves on average 50 nm in one dimension in 2 ms via Brownian motion, which is similar to the distance over which the particle is pushed back by the effects of the electric field. This is an upper bound to the diffusion, as the diffusion of a particle within a shell is reduced compared to that of a free particle [42,72,73]. We conclude that the effect of the electric field is strong enough to indeed limit diffusive motion of the core particles in the direction parallel to the electric field.…”

“…6 shows the projected area within the shell that was explored by the core particle at various frequencies of the applied electric field for ionic strengths of 0.200, 2.00, and 25.0 mM (ja particle % 8, 26, and 88). These concentrations were chosen as the equilibrium EDLs for these concentrations either confine the core particles to the middle of the shell (for 0.200 mM) or allow the core particles to approach the inner shell wall extremely closely (25.0 mM) [42]. When turning on the AC electric field, we observed similar motion for the core particles for all LiCl concentrations.…”

“…Here, we clarify how the positioning of a diffusive core particle within a porous shell is modified by applying an external driving AC electric field, using both liquid-phase electron microscopy (LP-EM) experiments and finite-element calculations. Recent liquid-phase electron microscopy results have shown that Brownian motion [40,41] and interactions of particles can be observed at sufficiently low electron dose rates, for which the effects of the electrons could be neglected [42]. This technique yields nanometer resolution [43] and we applied it here to gain information at the single particle level while applying an external AC electric field.…”

Carbon Nanofiber Growth Rates on NiCu Catalysts: Quantitative Coupling of Macroscopic and Nanoscale In Situ Studies

“…First, the conditions for which we were able to image the catalyst had to be investigated, as the electron beam has the potential to influence results drastically, especially for in situ experiments. ,− Figure S1 shows several in situ TEM experiments at 600 °C at 1 bar for various gas feed compositions. When hydrogen was not present, CNF growth was observed only in areas that were not irradiated by the electron beam.…”