(Invited) Ultrasound Application and Multi-Step Reactions in Electrodeposition of Refractory Metals
Authors: Seidl Lukas , Asen Ludwig , Yesilbas Göktug , Fischer Pauline , Kühn Fritz , Schneider Oliver 
Published: February 2020

This contribution discusses the application of ultrasound in ionic liquids (ILs) and its effect on refractory metal deposition as well as the particular electrochemistry of NbCl5 in TFSI based ILs. While ILs have enabled the electrodeposition of materials not accessible from aqueous solutions [1, 2], the transfer to technological applications is still hampered by high costs, stringent requirements on humidity and oxygen levels, and aspects of integration into industrial scale processes. Further, there are metals that are difficult to deposit, particularly the refractory metals: The deposition of tantalum [3] and niobium [4, 5] has been accomplished, but not yet in the form of thick, crack-free layers with good adhesion and low impurity level. Titanium was only deposited in ultrathin layers [6, 7]. Research into fundamental properties of ILs and especially the interfacial properties has shed light on some causes for these difficulties, like the presence of strong cation-anion multilayers at the interface and partial reduction of the metal precursors, especially halides, resulting in subhalide precipitation [3, 8]. The application of ultrasound leads to strongly improved mass transport and thus can counteract the accumulation of halide ions and the depletion of metal precursor close to the interface. It also might disturb the interfacial IL layers by cavitation, facilitating thereby refractory metal deposition. In aqueous solutions, the application of ultrasound can significantly enhance the deposition rate and influence the morphology and grain size of deposited films [9]. ILs with their negligible vapor pressure represent a very different environment. Nevertheless, ultrasound has been applied in ILs [10-12], cavitation was reported [13, 14] and is also associated with locally extreme temperatures [13]. This induces the risk of electrolyte decomposition. Therefore a systematic study was carried out in order to study electrochemical reactions and electrolyte decomposition in the presence of ultrasound. Extended application of ultrasound induced partial IL decomposition and formation of solid precipitates. Changes in background current and conductivity were observed and the electrolytes characterized by different spectroscopic techniques. The enhancement of electrochemical reactions in ILs by ultrasound application was demonstrated for model reactions and by application to Ta deposition from BMP TFSI. Especially the application of short ultrasound pulses at elevated temperatures increased electrochemical reaction rates. The reduction of Ta and Nb precursors requires the transfer of five electrons for each ion. The exact speciation in the IL (complexation) and thus the counter ion of the metal precursor influence the redox potentials of these individual reduction steps. (Apparent) multi-electron transfer reactions may facilitate the deposition, sequential one or two-electron transfer steps increase the risk to form sparingly soluble intermediates and can lead to corrosion of as-deposited layers [15]. The reduction of NbCl5 from BMP and OMP TFSI ILs [15] is characterized by five separate distinct reduction and corresponding oxidation steps. This system was studied in depth using the electrochemical quartz crystal microbalance technique, among others, and revealed strong changes in the electrolyte properties close to the interface.
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