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44627-G10
Ionic Liquids: Versatile Solvent Systems for Nanoparticle Synthesis
The study of nanoscale materials has led to an explosion of research in the last few years due to the new properties that emerge as a function of reduced size as well as ultra high surface area. These studies have led to various new ideas for technologies based on these size-dependent properties. New technology is predicated, however, on the ability to develop new synthetic methods for high quality materials. Solution-phase synthesis has emerged as an elegant and rational method that can yield a high level of control over size, shape, surface properties, and connectivity.
Great progress has been made in the development of solution phase synthesis of nanoparticles, but there are many materials that have yet to be exploited for nanoscale applications because they pose significant synthetic challenges. This is due to the fact that many materials contain elements in unusual oxidation states. For example, CrO2 is a ferromagnetic solid that has found application as a recording medium for data storage. Interest in this material has been renewed because of its half-metallic ferromagnetic nature and exhibited magnetoresistance. The complete spin polarization in this compound makes it a desirable target for use as electrodes in spin-dependent tunneling devices. Typical synthetic methods involve high pressure, which is a hinderance to the use of organic ligands for control over size and shape. The goal of this project is to develop an ambient pressure solution-phase synthesis of crystalline CrO2 nanoparticles with control over the size, shape and surface chemistry.
We require a solvent with the ability to withstand oxidation or reduction over a large potential range. Ionic liquids, solvent systems that have not yet been well explored for nanoparticle synthesis, can have extremely large electrochemical windows. Ionic liquids are salts that have melting points below 100 °C, and normally consist of large nitrogen- or phosphorous-containing cations and inorganic anions. Ionic liquids have flexibility in the species they can dissolve, for example inorganic and organic, cationic and anionic species can be solubilized. The typical high-boiling point organic solvents used in nanoparticle synthesis are generally more limited in what they can dissolve. Coupling this versatility with the ability to exploit the large electrochemical window (up to 6 V).1 of these solvents should enable the synthesis of materials with metals in a host of exotic oxidation states.
While Cr(IV) is generally unstable, we have shown that the reaction of CrO3 and Cr(NO3)3 at ambient pressures in an aqueous solution yields CrO2. The drawback of that approach is that the reaction temperature required is 300 °C, leading to solvent evaporation. Ionic liquids would allow for elevated temperatures and would also be able to dissolve the required precursors as well as a wide variety of ligands which are known to stablize Cr(IV) species and which may be used to control the size and shape of the particles. In this case the electrochemical window afforded by the ionic liquids will be the important factor.
While the use of ionic liquids as solvents for electrodeposition1 is fairly well-developed, the use of these solvents in inorganic synthesis is still largely unexplored. In the last year we have explored the solubility of a wide range of Cr-precursors in various phosphonium containing ionic liquids. We observe that the 2+, 3+, and 6+ oxidation states of Cr can be isolated and characterized via UV-Vis spectroscopy and electrochemistry. Currently we are pursuing electrochemical methods to identify conditions for the stabilization of Cr4+. We have recently obtained positive results that hint at a possible redox process occurring between the potentials typically observed for the oxidation of Cr3+ to Cr6+. Further characterization is underway. We are also exploring anion exchange of our ionic liquid. Preliminary data suggests that Cl- is too strongly coordinating with the chromium species in solution compared with other possible anions frequently used for ionic liquids.
1 Endres, F. et al. Z. Phys. Chem. 2004, 218, 255.
