Diffusion through the shells of yolk-shell and core-shell nanostructures in the liquid phase.

Recent advances in synthetic materials sciences such as the incorporation of self-assembly, sol–gel, and layer-by-layer deposition chemistry have afforded the development of many interesting and potentially useful nanostructures. Among those, nanoarchitectures in which a core nanoparticle, often a transition metal, is encapsulated by an outer layer of a second material, typically a porous oxide, have gained much popularity recently. The shells in these so-called core–shell nanostructures can offer a number of functionalities, including protection of the core from the outside environment, help in maintaining its compositional and structural integrity, prevention of the core from aggregating or sintering into larger particles, selective percolation of molecules in and out of the interior of the shell, increases in solubility and/or biocompatibility, and addition of new physical or chemical properties. More sophisticated nanostructures can also be synthesized with a void space, in a yolk–shell or rattle-type nanoarchitecture, to create individualized nanoreactors around the core nanoparticles. The new properties of these nanostructures have already been exploited in sensing, 8] drug delivery and biomedical imaging, catalysis and electrocatalysis (including fuel cells), and the development of batteries and energy storage devices. Although many of the applications of the core–shell and yolk–shell nanostructures require the diffusion of molecules through the outer shell to the core nanoparticle, often the active phase, surprisingly little work has been reported on the direct characterization of mass transport across such shells. Only a couple of examples have been reported in the gas phase, where IR absorption spectroscopy has been used to confirm adsorption of carbon monoxide on the core element of Pt@CoO and Au@TiO2 [18] yolk–shell samples. Herein we present what we believe is the first study on the accessibility of the core nanoparticles in these types of systems in liquid phase, an environment that is experimentally more difficult to probe. We have used both carbon monoxide and cinchonidine in the liquid solvent (carbon tetrachloride) as probes to assess the possible availability of the inner surfaces of Pt@void@TiO2 yolk–shell and Pt@SiO2 core–shell nanostructures. We first turn our attention to the Pt@void@TiO2 yolk– shell nanostructure, a transmission electron microscopy (TEM) image of which is provided in Figure 1 c. Its performance was evaluated in contrast with appropriate reference materials, namely, void@TiO2 hollow shells (Figure 1a), a void@TiO2@SiO2 hollow double-shell nanostructure (Figure 1b), and a regular 1 wt % Pt/TiO2 catalyst prepared by