Air- and Water-stable Ionic Liquids

Analysis of haloaluminate ionic liquids is much more limited than that of other ionic liquids. The most important analytical technique is surely NMR spectroscopy. The determination of residual water is difficult because of the instability of these materials. Hence it is crucial to work accurately to achieve the best results. 

To summarize the most important points for the synthesis of pure and colorless ionic liquids it is recommended to  

The importance of these first generation ionic liquids for metal deposition is summarized in Chapter 4.1. 

Physical Data of Haloaluminate-based Ionic Liquids 

A selection of physical data of selected haloaluminate-based ionic liquids is given in Table 2.1. 

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Endres, E and El Abedin, S.Z., Air and water stable ionic liquids in physical chemistry, Phys. Chem. Chem. Phys., 8,2101, 2006. 

Endres, R, El Abedin, S.Z., Borissenko, N., Probing lithium and alumina impurities in air- and water stable ionic liquids by cyclic voltammetry and in situ scanning tunneling microscopy, Zeits. Phys. Chem.—Int. ]. Res. Phys. Chem. Chem. Phys., 220, 1377-1394, 2006. 

In the 1990s John Wilkes and coworkers introduced air- and water-stable ionic liquids (see Chapter 2.2) which have attractive electrochemical windows (up to 3 V vs. NHE) and extremely low vapor pressures. Furthermore, they are free from any aluminum species per se. Nevertheless, it took a while until the first electrodeposition experiments were published. The main reason might have been that purity was a concern in the beginning, making reproducible results a challenge. Water and halide were prominent impurities interfering with the dissolved metal salts and/or the deposits. Today about 300 different ionic liquids with different qualities are commercially available from several companies. Section 4.2 summarizes the state-of-the-art of electrodeposition in air- and water-stable ionic liquids. These liquids are for example well suited to the electrodeposition of reactive elements such as Ge, Si, Ta, Nb, Li and others. 

Electrodeposition of Metals in Air- and Water-stable Ionic Liquids... 

Chloroaluminate ionic liquids are regarded as the first generation of ionic liquids. However, their hygroscopic nature has delayed progress in many applications since they must be prepared and handled under an inert gas atmosphere. Thus, the synthesis of air- and water-stable ionic liquids, which are considered as the second and third generations of ionic liquids, has attracted further interest in the use of ionic liquids in various fields. Unlike the chloroaluminate ionic liquids, these ionic liquids can be prepared and safely stored outside an inert atmosphere. Generally, they are water insensitive. However, water-containing [BMIM]PF< can... 

The electrodeposition of silver from chloroaluminate ionic liquids has been studied by several authors [45-47], Katayama et al. [48] reported that the room-temperature ionic liquid l-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4) is applicable as an alternative electroplating bath for silver. The ionic liquid [EMIM]BF4 is superior to the chloroaluminate systems since the electrodeposition of silver can be performed without contamination of aluminum. Electrodeposition of silver in the ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) and l-butyl-3-methylimidazoliumhexafluorophosphate was also reported [49], Recently we showed that isolated silver nanoparticles can be deposited on the surface of the ionic liquid Tbutyl-3-methylimidazolium trifluoromethylsulfonate ([BMIMJTfO) by electrochemical reduction with free electrons from low-temperature plasma [50] (see Chapter 10). This unusual reaction represents a novel electrochemical process, leading to the reproducible growth of nanoscale materials. In our experience silver is quite easy to deposit in many air- and water-stable ionic liquids. 

In this section we will show that air- and water-stable ionic liquids can be used for the electrodeposition of highly reactive elements which cannot be obtained from aqueous solutions, such as aluminum, magnesium and lithium, and also refractory metals such as tantalum and titanium. Although these liquids are no longer air-and water-stable when AICI3, TaFs, TiCU and others are dissolved, quite interesting results can be obtained in these liquids. 

In this chapter we have briefly discussed the high potential of air- and water-stable ionic liquids as electrolytes for metal deposition. Their extraordinary physical properties, superior to those of water or organic solvents, and their stability, open the door to the electrodeposition of many metals. Some advantages of air- and water-stable ionic liquids in electrodeposition are that they are quite easy to purify and handle and in most cases they do not decompose under environmental conditions. They can have pretty wide electrochemical windows of up to 6 V, and hence they... 

Over the past two decades, ionic liquids (ILs) have attracted considerable interest as media for a wide range of applications. For electrochemical applications they exhibit several advantages over the conventional molecular solvents and high temperature molten salts they show good electrical conductivity, wide electrochemical windows of up to 6 V, low vapor pressure, non-flammability in most cases, and thermal windows of 300-400 °C (see Chapter 4). Moreover, ionic liquids are, in most cases, aprotic so that the complications associated with hydrogen evolution that occur in aqueous baths are eliminated. Thus ILs are suitable for the electrodeposition of metals and alloys, especially those that are difficult to prepare in an aqueous bath. Several reviews on the electrodeposition of metals and alloys in ILs have already been published [1-4], A selection of published examples of the electrodeposition of alloys from ionic liquids is listed in Table 5.1 [5-40]. Ionic liquids can be classified into water/air sensitive and water/air stable ones (see Chapter 3). Historically, the water-sensitive chloroaluminate first generation ILs are the most intensively studied. However, in future the focus will rather be on air- and water-stable ionic liquids due to their variety and the less strict conditions under which... 

For the electrodeposition of metals or alloys from air- and water-stable ionic liquids, it is necessary first to dissolve the corresponding metal ions in the ionic liquid. Such a dissolution process is made possible by introducing excess amounts of halide ions (such as Cl ) to form soluble metal-halide complex anions. Alternatively, the metal is dectrochemically oxidized in the ionic liquid to form the soluble salt such as Sn(Tf2N) in the trimethyl-n-hexylammonium [bis(trifluoromethyl)sulfonyl]amide ([TMHAj TfiN ) ionic liquid. 

The electrodeposition of Zn-Mn was investigated at 80 °C in the hydrophobic tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)amide ([TBMA]+Tf2N ) [46] ionic liquid containing Zn(II) and Mn(II) species that were introduced into the ionic liquid by anodic dissolution of the respective metal electrodes. Cyclic voltam-mograms indicated that the reduction of Zn(II) occurs at a potential less negative than that of the Mn(II). Due to some kinetic limitations, which is a common phenomenon in air- and water-stable ionic liquids, incomplete oxidation of Mn electrodeposits was observed in this system. The current efficiency of Mn electrodeposition in this ionic liquid approaches 100%, which is a great improvement compared to the results obtained in aqueous solution (20-70%). Electrodeposition of Zn-Mn alloy coatings has never been carried out in chloroaluminate ionic liquid because of the unavoidable codeposition of Mn and Al. 

In this chapter some results on the electrodeposition of alloys from ionic liquids are summarized. Many fundamental studies have been performed in chloroaluminate first generation ionic liquids but the number of studies employing air- and water-stable ionic liquids rather than the chloroaluminates is increasing. Currently, new ionic liquids with better electrochemical properties are being developed. For example, Abbott et al. [47] have prepared a series of ionic liquids by mixing commercially available low-cost choline chloride and MCI2 (M = Zn, Sn) or urea and demonstrated that these ILs are good media for electrodeposition for pure metals (see Chapter 4.3). It can be expected that in the near future, the electrodeposition of alloys from ILs may become available for industrial applications. Furthermore, due to their variety, their wide electrochemical and thermal windows air- and water-stable ionic liquids have unprecedented prospects for electrodeposition. 

Currently silicon is still one of the most important semiconductors as it is the basis of any computer chip. It exhibits an indirect band gap of 1.1 eV at room temperature in the microcrystalline phase. Similar to Ge, silicon nanoparticles show a size-dependent photoluminescence. It was reported by Katayama el al. that a thin Si layer can be electrodeposited in l-ethyl-3-methylimidazolium hexafluorosilicate at 90 °C [44], However, upon exposure to air the deposit reacted completely to SiC>2, which makes it difficult to decide whether the deposit was semiconducting or not. Recently, we showed for the first time that silicon can be well electrodeposited from SiCU in the air and water stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([BMPJTfiN) [45, 46]. This ionic liquid can be... 

This appears to be contradictory to the smoother poly(pyrrole) films that are formed in air- and water-stable ionic liquids [46, 51]. 

Endres et al. [82] have demonstrated the suitability of an air- and water-stable ionic liquid for the electropolymerization of benzene. This synthesis is normally restricted to media such as concentrated sulfuric acid, liquid SO2 or liquid HF as the solution must be completely anhydrous. The ionic liquid used, l-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, can be dried to below 3 ppm water, and this ionic liquid is also exceptionally stable, particularly in the anodic regime. Using this ionic liquid, poly(para-phenylene) was successfully deposited onto platinum as a coherent, electroactive film. Electrochemical quartz crystal microbalance techniques were also used to study the deposition and redox behavior of the polymer from this ionic liquid (Section 7.4.1) [83]. 

Apart from our in situ STM studies in ionic liquids, there are a few papers dealing with this subject, especially in chloroaluminate ionic liquids, although air- and water-stable ionic liquids are now commercially available (see for example Refs. [1-3]). As chloroaluminate ionic liquids are extremely hygroscopic, in situ STM... 

In this chapter we present a few selected results on the nanoscale electrodeposition of some important metals and semiconductors, namely, Al, Ta and Si, in air- and water-stable ionic liquids. Here we focus on the investigation of the electrode/electrolyte interface during electrodeposition with the in situ scanning tunneling microscope and we would like to draw attention to the fascinating... 

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