Plasma Electrochemical Metal Deposition in Ionic Liquids

Plasma Electrochemical Metal Deposition in Ionic Liquids 

Deposition of silver metal In order to exemplify the plasma electrochemical deposition (PECD) technique in ionic liquids we first deposited silver 

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. 

Subsequent investigation of the obtained deposit with EDX revealed that it indeed consisted mainly of carbon and the residues of decomposed ionic liquid. Only a small amount of copper was found so the question remains as to whether this is copper metal or merely enclosed Cu+ or Cu2+. Hence, at this point we conclude that copper deposition from Cu(II) salts does not easily result in Cu(0) deposition. 

Deposition of platinum metal In the case of platinum no solid product was found. The ionic liquid darkened more and faster the smaller the distance between the surface of the ionic liquid [EMIM][TfO] containing tetrabutylammonium hexachloro-platinate ([n-Bu4N]2[PtCl6]) and the Ar/H2-plasma (3 1, overall pressure 100 Pa) was chosen. So far no other ionic liquid has been tested. The rate constant for the reduction of the tetrabutylammonium ion with a hydrated electron is only 1.4 x 106 LmoH1 s 1, hence the main rival pathway for reduction of platinum(IV) is the reduction of the imidazolium ion of the ionic liquid. As in the case of copper, a suitable platinum salt - maybe made by electro-oxidation of metallic platinum in a suitable ionic liquid - has to be found. 

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Plasma Electrochemical Metal Deposition in Ionic Liquids 275... 

A completely different approach might be the use of radio frequency plasma instead of a DC plasma. The ignition and sustainment of the plasma is decoupled from the application of voltages to the electrodes that are now used only for electrochemical reactions. Another method which has been proven to be quite successful is the application of an U-shaped tube in order to avoid an IR-drop over the ionic liquid (see Figure 10.2). Unfortunately, this set-up led to a large size distribution of the obtained particles but it showed that RF plasma could further improve the stability of the ionic liquids during the metal deposition process. 

Fig. 10.8 Schematic experimental set-up for the deposition of metal nanoparticles by plasma electrochemical reduction of a metal salt dissolved in an ionic liquid at room temperature.

Ionic liquids are interesting media for the electrodeposition of metals, alloys, and senoiconductors, despite the fact that they are more expensive than aqueous baths. Due to their large electrochemical windows and their good thermal and chemical stability, they are used nowadays to electrodeposit metals that can also be obtained from aqueous solution, such as Cr, Ag, Ni, Cu, but also to electrodeposit metals that cannot be obtained from aqueous baths. This paper deals with the electrodeposition of Si and Ta in the ionic liquid butyl-methyl-pyrrolidinium bis(trifluoro)sulfonyl imide ([BMP][TFSI]). Compared to other deposition techniques, such as chemical vapor deposition or plasma vapor deposition, electrodeposition is a relatively simple technique and in many cases significantly cheaper. Moreover, materials of different shapes and dimensions can be obtained relatively easy by electrodeposition. Thus, for industrial applications, it would be highly interesting to obtain reactive metals and semiconductors by electrodeposition. 

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