Nanoparticles have attracted a great deal of attention in fields such as catalysis, magnetics, and optics, due to their extraordinary properties. In particular, the exploration of new frontiers in catalysis has been intimately related to developments of well-defined nanomaterials. Over the past 20 years, an explosion of interest in nanomaterials has greatly promoted fundamental understanding of catalysis. Effects of particle size, crystal plane orientations, and surface defects on catalytic performances have all been investigated on single component catalysts, partially enabled by the realization of nanomaterials with well-defined morphologies.
Recently, interest has shifted toward bimetallic catalyst systems, due to their potential to enhance catalytic activity or even create bi-functional surfaces capable of propagating technically challenging chemical reactions. For bimetallic catalysts, outstanding catalytic performance has been reported for various reactions, including alcohol oxidation and oxygen reduction. These successes have triggered intense interest in the preparation of bimetallic catalysts with controlled morphologies and sizes. Among the as-obtained bimetallic nanoparticles, those with core-shell structures are especially interesting. The core-shell nanoparticles have shown superior catalytic activity and/or selectivity in many reactions, which could be partially attributed to high index facets on the surface. Another benefit of the core-shell architecture is a reduced catalyst cost by minimizing the usage of the expensive active component.
Transition metals are often categorized as being noble and non-noble, in general the morphology and size control of noble-metal nanoparticles are relatively well understood. Despite the diverse spectrum of synthesized noble-metal nanoparticles, synthesis procedures are often similar, with most incorporating the use of capping agents, for instance poly (vinyl pyrrolidone) PVP. Mechanistically it is understood that the capping agents control the morphology and size of the nanoparticles by binding with the noble metals through carbonyl or amino groups.
In contrast to the case for noble-metal nanoparticles, non-noble metal nanoparticles are notorious for the difficulties associated with controlling the morphology and size. The binding energies of the capping agents on transition metal surfaces are different (lower in most cases) from that on noble metal surfaces. The disparity can be related to differences in the d-band structure, since the d-orbitals of non-noble metals are filled with fewer electrons than that of noble metals. Filled d-orbital are required to interact with the electrophilic group, carbonyl in PVP, and the electron deficient d-orbitals of non-noble metals thus has lower adsorption energy to PVP. In the literature, to maintain tight control of the morphology and size, two steps are needed to prepare core-shell structures, first forming the core of the nanoparticles and then coating the core with the other component as the shell.
Though two step can synthesis procedures produce core-shell particles with narrow size distribution, it results in a mono-metallic surface rather than presenting a solid solution of two metals on the surface. The mono-metallic shell is good for reactions with single-active-site mechanism, but not for those with double-active-site mechanisms. Furthermore, the two-step synthesis is prohibiting large scale production due to the complicated procedure, and delicate centrifuging/re-dispersion.