Sonoelectrodeposition : Microstructural Studies in Aqueous and Ionic Liquid Electrolytes

Abstract

Rare are the industrial examples of ultrasonic enhancement of the plating step itself. However, it has been demonstrated at laboratory scale that many advantages may be gained by use of ultrasound. Several authors such as Walker et al. (1973) [1] or Touyeras et al. [2] reported the beneficial effect of ultrasound in metal deposition and plating. This is particularly competitive as it may result in a reduction of chemical additives use, or even in their complete suppression. They also report that plating in an ultrasonic field may produce electrodeposits with an increased hardness and brightness, better adhesion to substrate, a finer grain and a reduced porosity and internal stress. Most of these properties are in direct relationship with the coating microstructure, which becomes therefore a major issue. Silver coating microstructures examined by XRD measurements reveals an important sensitivity of the crystalline preferential orientation to both current density and ultrasonic irradiation. Two main categories are identified by XRD: one poorly structured and the other following the [110] orientation, corresponding to low and high current densities. It is interesting to note that, while changing from still to mechanically stirred conditions, the value of the current density threshold moves from 2.5mA/cm² to 5mA/cm². When ultrasound is used (575 kHz or 20 kHz), this coating microstructure modification threshold occurs at higher current density values when coatings are produced under sonication, while agitation is kept at the same level (equivalent velocity). In both cases, the shift is about 15 mA/cm² [3]. It appears then as an interesting challenges to control the cavitation activity and especially the respective contribution from ultrasonic streaming vs. bubble collapse at the electrode vicinity [4]. This is possible by the use of the double reactor Besançon cell, filled with ionic liquids [5]. As ionic liquids present very low volatilities, their vaporization is reduced, and cavitation bubbles only depend on the presence of gases. Then, cavitation activity may be trigged by reactor atmosphere control. Applying severe depression within an irradiated ionic liquid medium contributes to removing dissolved gases, thus quenching progressively cavitation activity [6]. Electrolytes were prepared by dissolution of metallic salts composed by Tf2n as anion, and Copper or silver as cation in BuMIMTf2n, and deposits have been elaborated under various pressures (88, 55 and 26 kPa) to allow a progressive variation of the cavitation activity. All electrodepositions have been performed at a given potential, under the very same ultrasonic transmitted power. The charge was adapted to get the same thickness, and XRD measurements allow the comparison of crystalline preferential orientations. [1] C.T. Walker, R. Walker, Effect of ultrasonic agitation on some properties of electrodeposits, Electrodepos. Surf. Treat. 1 (1973) [2] F. Touyeras, J.Y. Hihn, X. Bourgoin, B. Jacques, L. Hallez, V. Branger, Effects of ultrasonic irradiation on the properties of coatings obtained by electroless plating and electro plating, Ultrason. Sonochem. 12 (2005) [3] A. Nevers, L. Hallez, F. Touyeras, J.-Y. Hihn, Effect of ultrasound on silver electrodeposition: Crystalline structure modification, Ultrason. Sonochem. 40 (2018) [4] J.-Y. Hihn, M.-L. Doche, A. Mandroyan, L. Hallez, B.G. Pollet, Respective contribution of cavitation and convective flow to local stirring in sonoreactors, Ultrason. Sonochem. 18 (2011) [5] C. Costa, J.-Y. Hihn, M. Rebetez, M.-L. Doche, I. Bisel, P. Moisy, Transport-limited current and microsonoreactor characterization at 3 low frequencies in the presence of water, acetonitrile and imidazolium-based ionic liquids, Phys. Chem. Chem. Phys. PCCP. 10 (2008) [6] B. Naidji, L. Hallez, A. EtTaouil, M. Rebetez, J.-Y. Hihn, Influence of pressure on ultrasonic cavitation activity in room temperature ionic liquids: An electrochemical study, Ultrason. Sonochem. 54 (2019) Figure 1