Ionic liquids metal deposition from

Recently, there has been considerable interest in developing molten salts that are less air and moisture sensitive. Melts such as l-methyl-3-butylimidazolium hexa-fluorophosphate [211], l-ethyl-3-methylimidazolium trifluoromethanesulfonate [212], and l-ethyl-3-methylimidazolium tetrafluoroborate [213] are reported to be hydro-phobic and stable under environmental conditions. In some cases, metal deposition from these electrolytes has been explored [214]. They possess a wide potential window and sufficient ionic conductivity to be considered for many electrochemical applications. Of course if one wishes to take advantage of their potential air stability, one loses the opportunity to work with the alkali and reactive metals. Further, since these ionic liquids are neutral and lack the adjustable Lewis acidity common to the chloroaluminates, the solubility of transition metal salts into these electrolytes may be limited. On a positive note, these electrolytes are significantly different from the chloroaluminates in that the supporting electrolyte is not intended to be electroactive. 

What is dear from this introduction is that the journey into the area of metal deposition from ionic liquids has tantalizing benefits. It is also dear that we have only just begun to scratch the surface of this topic. Our models for the physical properties of these novel fluids are only in an early state of devdopment and considerably more work is required to understand issues such as mass transport, spedation and double layer structure. Nudeation and growth mechanisms in ionic liquids will be considerably more complex than in their aqueous counterparts but the potential to adjust mass transport, composition and spedation independently for numerous metal ions opens the opportunity to deposit new metals, alloys and composite materials which have hitherto been outside the grasp of electroplaters. 

Deposition of copper metal Since Cu(II) is the preferred oxidation state of copper, Cu2+ salts are more stable and more available, hence, in a technical application it would be favorable to use them as starting material. We tried to reduce Cu(CF3S03)2 dissolved in [EMIM][TfO], [BMP][TfO] and [BMIM][TfO] with an argon plasma (gas pressure 100 Pa) as well as with a nitrogen plasma (100 Pa), respectively. Additional experiments with Cu(CF3SC>3)2 dissolved in [EMIM][TfO] and Ar/H2 plasmas were carried out, with the distance between the hollow cathode in the gas phase and the surface of the ionic liquid metal salt solution being 3, 45 and 100 mm. Moreover, for the 3 mm distance several experiments with different gas pressures from 50 to 500 Pa were carried out. 

The main contaminants in an ionic liquid will be introduced from the synthesis, absorbed from the atmosphere or produced as breakdown products through electrolysis (see above). The main contaminants for eutectic-based ionic liquids will be from the components. These will be simple amines (often trimethylamine is present which gives the liquid a fishy smell) or alkyl halides. These do not interfere significantly with the electrochemical response of the liquids due to the buffer behavior of the liquids. The contaminants can be effectively removed by recrystallization of the components used to make the ionic liquids. For ionic liquids with discrete anions the major contaminants tend to be simple anions, such as Li+, K+ and Cl-, present from the metathesis technique used. These can give significant difficulties for the deposition of reactive metals such as Al, W and Ti as is demonstrated below with the in situ scanning tunnelling microscope. 

Table 6.5 Some examples of metals deposited from ionic liquids. ...

Arimoto, S., Kageyama, H., Torimoto, T. and Kuwabata, S. (2008) Development of in situ scanning electron microscope system for real time observation of metal deposition from ionic liquid. Electrochem. Commun, 10,1901-1904. 

In the case of Type 1 and 11 eutectics the potential window is limited at high potentials by chlorine gas evolution and at low potentials by the metal ion reduction with metal deposition from the melt. Type I eutectics have been prepared using Zn, Sn, Fe, Al, Ge and Cu chlorides. Their reduction potential is shifted towards more electronegative values as the metal halide is closer by Lewis acid. Because the reduction potential is associated with Lewis acidity, the corresponding proportions of metal and quaternary ammonium salts affect the potential window. Type 11 eutectics have been developed in order to extend the range of metals able to be electrodeposited from ionic liquids and Cr electrodeposition with good characteristics has been reported (Abbott et al., 2004 Abbott et al., 2004 Benaben Sottil, 2006). Hydration water plays a significant role on the stability and fluidity of choline chloride based ionic liquids. In this case water behaviour is different compared to the case of aqueous electrolytes and the potential window is limited rather by the metallic species... 

Not only cationic, but also anionic, species can be retained without addition of specially designed ligands. The anionic active [FFPt(SnCl3)4] complex has been isolated from the [NEt4][SnCl3] solvent after hydrogenation of ethylene [27]. The PtCl2 precursor used in this reaction is stabilized by the ionic salt (liquid at the reaction temperature) since no metal deposition occurs at 160 °C and 100 bar. The catalytic solution can be used repeatedly without apparent loss of catalytic activity. 

It was quite recently reported that La can be electrodeposited from chloroaluminate ionic liquids [25]. Whereas only AlLa alloys can be obtained from the pure liquid, the addition of excess LiCl and small quantities of thionyl chloride (SOCI2) to a LaCl3-sat-urated melt allows the deposition of elemental La, but the electrodissolution seems to be somewhat Idnetically hindered. This result could perhaps be interesting for coating purposes, as elemental La can normally only be deposited in high-temperature molten salts, which require much more difficult experimental or technical conditions. Furthermore, La and Ce electrodeposition would be important, as their oxides have interesting catalytic activity as, for instance, oxidation catalysts. A controlled deposition of thin metal layers followed by selective oxidation could perhaps produce cat-alytically active thin layers interesting for fuel cells or waste gas treatment. 

Tellurium and cadmium Electrodeposition of Te has been reported [33] in basic chloroaluminates the element is formed from the [TeCl ] complex in one four-electron reduction step, furthermore, metallic Te can be reduced to Te species. Electrodeposition of the element on glassy carbon involves three-dimensional nucleation. A systematic study of the electrodeposition in different ionic liquids would be of interest because - as with InSb - a defined codeposition with cadmium could produce the direct semiconductor CdTe. Although this semiconductor can be deposited from aqueous solutions in a layer-by-layer process [34], variation of the temperature over a wide range would be interesting since the grain sizes and the kinetics of the reaction would be influenced. 

The German public funded project NEMESIS focuses on the design and development of microreactors for the synthesis of ionic liquids at pilot scale [52], Scientific objectives are to increase the yield of the corresponding ionic liquid as well as to decrease reaction time from hours up to days currently. Ionic liquids, a new innovative class of materials, are synthesized using microreaction technology. Possible application fields are their use as electrolytes for the elaborate deposition of metals. A concept for regeneration of the electrolyte is also considered. 

Cation Cationic structure and size will affect the viscosity and conductivity of the liquid and hence will control mass transport of metal ions to the electrode surface. They will also be adsorbed at the electrode surface at the deposition potential and hence the structure of the double layer is dominated by cations. Some studies have shown that changing the cationic component of the ionic liquid changes the structure of deposits from microcrystalline to nanocrystalline [27]. While these changes are undeniable more studies need to be carried out to confirm that it is a double layer effect. If this is in fact the case then the potential exists to use the cationic component in the liquid as a built-in brightener. 

While the structure at the electrode/ionic liquid interface is uncertain it is clear that in the absence of neutral molecules the concentration of anions and cations at the interface will be potential dependent. The main difference between aqueous solutions and ionic liquids is the size of the ions. The ionic radii of most metal ions are in the range 1-2 A whereas for most ions of an ionic liquid they are more typically 3-5 A. This means that in an ionic liquid the electrode will be coated with a layer of ions at least 6-7 A thick. To dissolve in an ionic liquid most metal species are anionic and hence the concentration of metal ions close to the electrode surface will be potential dependent. The more negative the applied potential the smaller the concentration of anions. This means that reactive metals such as Al, Ta, Ti and W will be difficult to deposit as the effective concentration of metal might be too low to nucleate. It is proposed, as one explanation, that this is the reason that aluminum cannot be electrodeposited from Lewis basic chloroaluminate ionic liquids. More reactive metals such as lithium can however be deposited using ionic liquids because they are cationic and therefore... 

Eutectic mixtures have been used extensively for applications of molten salts to reduce the operating temperature and this is where the significant area of ionic liquids developed from i.e. the quest to find aluminum-based salt mixtures. While the development of aluminum-containing ionic liquids is technologically very important for the field of metal deposition it is clear that there are many other issues that also need to be addressed and hence methods need to be developed to incorporate a wide range of other metals into ionic liquid formulations. 

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. 

Habboush and Osteryoung were the first to describe the electrodeposition of a Group V metal from AlClj/1-butyl-pyridinium chloride-based ionic liquids. As antimony sources they used SbCh or Sb-rods, dissolved by anodic dissolution [16]. For the composition AlCl j BuPyCl (0.8 1) a deposition of Sb was observed at —0.885 V... 

In this chapter we will concentrate on the deposition of metals from eutectic-based ionic liquids. These have been developed since the end of the 1990s, primarily by our group and that of Sun. Figure 4.10 shows just some of the metals that... 

Fig. 4.10 A range of metal and metal alloy coatings deposited elec-trolytically from type II (Cr) and type III (Ni, Cu Zn Sn, Ag) choline chloride-based ionic liquids.

We were quite optimistic in the beginning as the second reduction process corresponds to the formation of a black deposit which was potentially the first electrochemical route to make thick tantalum layers. After having washed off all ionic liquid from the sample we were already a bit sceptical as the deposit was quite brittle and did not look metallic. The SEM pictures and the EDX analysis supported our scepticism and the elemental analysis showed an elemental Ta/Cl ratio of about 1/2. Thus, overall we have deposited a low oxidation state tantalum choride. Despite the initial disappointment we were still eager to obtain the metal and found some old literature from Cotton [122], in which he described subvalent clusters of molybdenum, tungsten and tantalum halides. In the case of tantalum the well-defined Ta6Cli22+ complex was described with an average oxidation number of 2.33 and thus with a Ta/Cl molar ratio very close to 1/2. Such clusters are depicted in Figure 4.15. 

The motivation of this chapter was to show that despite the enormous prospects of ionic liquids in electrodeposition some troublesome aspects have to be expected. Apart from the purity and price of ionic liquids the optimum temperature for any process has to be found. Furthermore, suitable additives for electrodeposition will have to be developed and cation/anion effects that can strongly alter the morphology of deposits have to be expected. Finally, the electrochemical window alone is not the only factor that needs to be considered for the deposition of reactive metals. Suitable precursors will have to be tailor-made and it is our personal opinion that the electrodeposition of metals like Mg, Ti, Ta and Mo may not be possible from metal halides but rather metal bis(trifluoromethylsulfonyl)amide salts and other ones may be more suitable. 

Mo are single phase, supersaturated solid solutions having an fee structure very similar to that of pure Al. Broad reflection indicative of an amorphous phase appears in deposits containing more than 6.5 atom% Mo. As the Mo content of the deposits is increased, the amount of fee phase in the alloy decreases whereas that of the amorphous phase increases. When the Mo content is more than 10 atom%, the deposits are completely amorphous. As the Mo atom has a smaller lattice volume than Al, the lattice parameter for the deposits decreases with increasing Mo content. Potentiodynamic anodic polarization experiments in deaerated aqueous NaCl revealed that increasing the Mo content for the Al-Mo alloy increases the pitting potential. It appears that the Al-Mo deposits show better corrosion resistance than most other aluminum-transition metal alloys prepared from chloroaluminate ionic liquids. 

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