Mass Transport across the Porous Oxide Shells of Core–Shell and Yolk–Shell Nanostructures in Liquid Phase

Abstract

The mass transport of a number of molecules dissolved in a liquid solvent through the shells of Pt@SiO2 core–shell and Pt@Void@TiO2 yolk–shell nanostructures was characterized in situ by infrared absorption spectroscopy. Our samples, which were used here to represent the core–shell and yolk–shell nanostructures that have become so popular in recent times, were determined to exhibit a dual distribution of pore sizes, with a majority of micropores with diameters of less than 1 nm and a second group of mesopores with windows approximately 4 nm in diameter. The uptake of carbon monoxide from CCl4 or ethanol solutions onto the surface of the metal of these nanostructures was characterized first. It was determined that adsorption is possible with both samples, and that saturation on the metal surface precedes saturation on the oxide shell. This latter observation suggests that the access of the CO molecules (and all other molecules studied here) to the inside of the yolk–shell structures may be controlled by the mass transport of the liquid through the porous network of the shells, not by the diffusion of the dissolved gas alone. Additional studies were carried out with a family of cinchona alkaloid derivatives and with a porphyrin, covering a range of molecular sizes of approximately 5–20 Å in diameter. Adsorption of all those molecules on the Pt surfaces of both nanoarchitectures was deemed possible by a combination of tests, including the detection of changes in peak frequencies or peak intensities and measurements of the reversibility of the adsorption. Our results indicate that although the sol–gel synthetic method used to prepare the shells usually produces solids with microporous structures, a secondary mesoporous network that also develops in these nanostructures can provide a direct path to the metal nanocores, affording the mass transport of large molecules in liquid solutions in and out of the inside volumes. We also show that the attoliter-size volumes encased by typical shells in yolk–shell nanostructures are sufficient to afford adsorption displacement processes.