Concepts for the Stabilization of Metal Nanoparticles in Ionic Liquids

Nanoparticles assemblies of hundreds to thousands of atoms and a size in the range of 1-50 nm can be considered at first approximation as a state of matter intermediate between single atoms or molecules and bulk bodies. While the properties of atoms and molecules can be described via quantum mechanics, the properties of bulk bodies are described by solidstate physics. Using quantum chemical models for describing nanoparticles is demanding because of the huge number of strongly interacting atoms, which have to be taken into account. On the other hand, methods of solid state physics cannot always be applied as nanoparticles often demonstrate size dependent quantum effects,[1] e.g., in surface plasmon resonance.[2, 3] Magnetic,[4] thermodynamic,[5] catalytic [6] and other properties of nanoparticles can also depend on their size. Due to their small size, nanoparticles have a very high specific surface area and dispersion, the latter being defined as the ratio of the number of surface atoms to the total number of atoms in the particle. Platinum particles with 2 nm diameter, e.g., have a surface area of 140 m2/g and a dispersion of 50 %. Note that also the relative concentration of atoms located at corners, edges, and faces is strongly size dependent (Fig. 1).[7] These surface atoms are not equivalent to each other and frequently play different roles in catalysis. In the following, we focus on metal nanoparticles,[8-10] as one can find good reviews about metal oxide [11-16] and semiconductor [17-23] nanoparticles elsewhere. One of the most attractive features of unsupported metal nanoparticles is the possibility to apply physicochemical methods to investigate them and to catalyze chemical reactions by exactly the same material. Thus, unsupported metal nanoparticles are frequently used as a model system for conventional heterogeneous catalysts as many physicochemical methods cannot be applied directly to conventional catalysts due to their heterogeneous nature, interfering support effects, and low transparency for electromagnetic radiation. In other words, nanoparticles can serve as models for ideal surfaces and are good candidates to fill the so-called pressure material gap.[24, 25] It is of great interest to study nanoparticles in ionic liquids, primarily for applications in electrochemistry and catalysis, as they display advantages in comparison to polymer stabilized nanoparticles. These advantages lie, e.g., in the stabilization mechanism, the relative ease of adjusting the properties of the nanoparticle-ionic liquid system, as well as aspects associated with green chemistry. The applicability of nanoparticle suspensions