Electrodeposition from Ionic Liquids

Abstract

Electrodeposition is an interesting and often used technique to deposit thin films of metals onto conducting substrates. During electrodepositio n, film properties such as morphology or composition can be adjusted by controlling process parameters such as bath composition, temperature and applied potential or current density. The electrodeposition of metals f rom ionic liquids has become a heavily studied topic in the last decade but still lots of challenges remain. This thesis investigates three main topics:1. The use of liquid metal salts to increase the solubility of metals in ionic liquids, hereby improving the mass transport2. Th e application of high vacuum as an environment for electrodeposition 3. Influence of rate constants in chloroaluminatesFurthermore, the f irst electrochemical oscillator in ionic liquids was discovered by seren dipity. The techniques, used in this PhD thesis, consist of Cyclic Volta mmetry (CV), Chronoamperometry, Scanning Electron Microscopy (SEM), Tran smission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), Quart z Crystal Microbalance (QCM), Optical Spectroscopy, Raman Spectroscopy a nd Finite Element Modeling. The liquid metal salts tetrakis(acetonitrile) copper(I) bis(trifluoromet hylsulfonyl)imide ([Cu(MeCN)4][Tf2N]) and tetrakis(benzonitrile) copper( I) bis(trifluoromethylsulfonyl)imide ([Cu(PhCN)4][Tf2N]), bis(tetrakis(a cetonitrile) silver(I)) tris((trifluoromethylsulfonyl)imide) silver(I) ( [Ag(MeCN)4]2[Ag(Tf2N)3]) and bis(1-ethylimidazole) silver(I) bis(trifluo romethylsulfonyl)imide ([Ag(EtIm)2][Tf2N]) were electrochemically charac terized: for electrodeposition purposes, low melting salts with a cation ic complex are preferred over anionic complexes. The MeCN complexes are the easiest to be reduced and current densities up to 25 A dm−2 in unstirred solutions could be achieved. The resulting copper or silver d eposits, made at high current densities, have a smooth appearance, did n ot show cracks and were free from entrapped organic compounds. Lower cur rent densities result in nodular deposits. The use of thiourea or 1H-ben zotriazole as additive resulted in smooth morphologies at 1 A dm−2 . It is believed that excellent mass transport in this system is due to the high metal concentration and to the favorable electrostatic interact ions between the cationic electroactive species and the cathode. This ma kes the use of liquid metal salts with electrochemically active cationic complexes interesting for electrochemical applications where mass trans port is important. The fact that comparable results are obtained for two metals, indicates that the complexes with a metal center, surrounded by acetonitrile, lead to ionic liquids that can be used for high current d ensity electroplating. A second interesting property of liquid metal sal ts is that high overpotentials can be applied during electrodeposition. This leads to a small critical size for nucleation, which is advantageou s during the plating of copper on barrier materials such as tantalum. By using [Cu(MeCN)2][Tf2N] as medium for copper electrodeposition on a tantalum substrate, a nucleation density of 8e14 m−2 is reached. The resulting deposits contain very few pinholes and have thicknesses o f 19 nm. Electrochemical vacuum deposition is a technique which exploits the low vapor pressure of ionic liquids. In contrast with aqueous solutions or o rganic solvents, ionic liquids can be placed in a high vacuum environmen t, without noticeable loss of ionic liquid, to strongly decrease the amo unt of oxygen gas and water. Possible applications are the deposition of copper on tantalum, but results indicate that even at the low pressure of a high vacuum environment, the oxidation of tantalum cannot be avoide d. A finite element model of the electrodeposition of aluminium from chloro aluminate ionic liquids was made to study the influence of the rate cons tants kb and kf on the electrodeposition rate of aluminium. These rate c onstants determine the equilibrium between Al2Cl7−, AlCl4−, and Cl−. It was found that thermodynamics and kinetics change in o pposite directions for variations in kb or kf and these opposing effects lead to a minimum in the calculated current density as a function of kb . A comparison of the calculated data and the experimental limiting curr ent densities indicates that the value of kb lies between 1e−7.5 a nd 1e−6.7 m3 mol−1 s−1. Electrochemical oscillations are a phenomenon in which a constant applie d potential leads to an oscillating current or that a constant applied c urrent gives rise to an oscillating potential. A former type of electroc hemical oscillator was found in a mixture of monovalent with divalent co pper ions in 1-ethyl-3-methylimidazolium chloride. The oscillator is an N-NDR-type because the low frequency-end of the impedance spectrum has n egative real impedances and the steep decrease in current in the potenti al-current curve. The presence of chloride is also a nece