Ionic Liquids for Reactive Nano- materials

The limited reversibility of some electrode reactions might require consideration of consumable (cheap) ionic liquids in the anode compartment for technical applications and commercial electroplating. For example, the electrochemical oxidation of oxalate delivers carbon dioxide, hydride could be oxidized to hydrogen, halides to the halogen or trihalide salt in the case of iodide ionic liquids and so on. Since ionic liquids can readily form biphasic systems an alternative may be to have the anodic reaction in an immiscible solvent. In that case a common ion would be needed that can be transferred from one phase to the other. 

The electrodeposition of reactive elements like Al, Si, Ge, Ta and a few others is possible. As discussed in Chapter 4.4 the successful electrodeposition of Ti, Mg, Mo and many others in relevant layer thicknesses has not yet been described, though attempts have been made in some cases. Apart from the availability of suitable precursors there is at least one other issue to consider ionic liquids can be reactive. It was found that magnesium and its alloys can form passivating films in ionic liquids with the bis(trifluoromethylsulfonyl)amide (Tf2N) anion, especially in the presence of water. It was found by two of our groups (Endres, MacFarlane) that, under certain circumstances, the Tf2N ion is subject to cathodic 

A further important aspect is how to handle reactive elements It was found in the Clausthal group that nanocrystalline aluminum and nanoscale silicon made in 1-butyl-l-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide react even with the comparably low level of oxygen ( 1 ppm) in an inert gas glove box. Under air the deposit can be oxidized on the time scale of a few days. Maybe in situ passivation methods will have to be developed. One could think about deposition of a reactive element in an ionic liquid, washing off the ionic liquid, followed by passivation in a different liquid. 

Usually there is a lot of effort required to make nanomaterials by electrochemical means. In aqueous solutions the electrodeposition of nanocrystalline metals requires pulsed electrodeposition and the addition of additives whose reaction mechanism hitherto has only been partly understood (see Chapter 8). A further shortcoming is that usually a compact bulk material is obtained instead of isolated particles. The chemical synthesis of metal or metal oxide nanoparticles in aqueous or organic solutions by colloidal chemistry, for example, also requires additives and often the desired product is only obtained under quite limited chemical conditions. Changing one parameter can lead to a different product. 

In ionic liquids the situation seems to be totally different. It was surprising to us that the electrodeposition of metals and semiconductors in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide delivers nanocrystalline deposits with grain sizes varying from 10 to 200 nm for the different materials, like Si, Al, Cu, Ag and In, investigated to date. It was quite surprising in the case of Al deposition that temperature did not play a tremendous role. Between 25 and 125 °C we always got nanocrystalline Al with similar grain sizes. Similar results were obtained if the deposition was performed in tri-hexyl- tetradecylphos-phonium bis (trifluoromethylsulfonyl) amide. Maybe liquids with saturated nonaromatic cations deliver preferentially nanomaterials this is an aspect which, in our opinion, deserves further fundamental studies. 

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