27Al NMR spectroscopy studies on speciation of Al complex ions in AlCl3+LiAlH4 solutions in tetrahydrofuran for electroplating of Al

Electrochemical kinetic and mechanistic studies on electrocrystallization of Al from non-aqueous solution baths have been described elsewhere. Electroplating takes place from mixed electrolytes of AlCl3 + LiAlH4 in e.g. tetrahydrofuran (thf) as solvent but not from the individual solutes, AlCl3 or LiAlH4 by themselves. In such mixed solute solutions, a variety of hydrido-chloro complexes arise dependent on the ratio of the above two solute components, variously ligated also by the thf. By means of Al-27 NMR Studies the speciation of Al complexes in such plating baths of various compositions has been examined. The various complexation equilibria are discussed as well as time effects which arise in the composition and plating efficacy of the baths due to auto-decomposition processes. The reactivity of the aluminate species is described and discussed on the basis of the MMR results. The various complexes can also be related to the compositions of intermediates involved in the sequential steps of Al electrodeposition requiring an overall three-electron process. (C) 1998 Elsevier Science S.A. All rights reserved.