Electroless Displacement Deposition of Nanocrystalline Al on Mg Surface from AlCl3-Emic Room Temperature Ionic Liquid

Abstract

Introduction Ionic liquids have received considerable attentions recently for their wide use in electrochemistry, synthesis, separation, energy-related applications and so on [1]. As alternative solvents and electrolytes for a number of electrochemical processes, ionic liquids show great advantages comparing with aqueous solution. A range of metal coatings, including water-sensitive metals like Al, Li, Mg, etc, were electrodeposited from ionic liquids [2]. However, the electroless deposition of metals from ionic liquids was rarely reported, except for the deposition of Ag on Cu from a choline chloride-based ionic liquid [3]. Here we report the electroless displacement deposition of Al on Mg from AlCl3-1-ethyl-3-methyl-imidazolium chloride (AlCl3-EMIC) room temperature ionic liquid, especially that nanocrystalline Al can be obtained. Experimental 99.95% Mg substrates used in this study were typically 10×25×1 mm, which were mechanically polished with silicon carbide paper and ultrasonically cleaned with acetone prior to use. An acidic ionic liquid with AlCl3 to EMIC molar ratio of 2:1 was used as the electrolyte. All the experiments referred to AlCl3-EMIC ionic liquid were performed in an argon-filled glove-box at room temperature, in which the oxygen content and moisture were maintained below 1 ppm. To remove the oxide layer on surface, the Mg substrates were firstly electrolytic etched in the 2:1 AlCl3-EMIC ionic liquid at 20 mA/cm2 for 10 min, according to ref. [4-5]. The electroless displacement deposition was carried out by just placing the electrolytic etched Mg plates in the ionic liquid for 20 min or by pulling the etched Mg out of ionic liquid and then placing in the glove-box with the residual ionic liquid on surface for 10 min. The samples after electroless displacement deposition were characterized with X-ray diffraction (XRD) and scanning electron microscope (SEM). Results and discussion XRD pattern of the Mg substrate after deposition is shown in Fig. 1a. The diffraction peaks corresponding to pure Al were clearly detected, which suggests that Al was successfully electroless deposited on Mg surface. The low intensity of Al peaks was due to the low thickness of deposited Al coating. Fig. 1b and Fig. 1c show the SEM micrograph of the deposited Al on Mg. For the sample placed in the ionic liquid (Fig. 1b), the Al crystals can be hardly observed by SEM under the magnification of 50000, while cracks of Al coating can be clearly observed. The deposited Al crystallites should be in the nanometer regime. The cracks of coating might be caused by the high stress in the nanocrystalline Al coating. In the case of placing in the glove-box (Fig. 1c), Al crystals can be clearly observed, which is ca. 200~300 nm. No crack of the coating was observed. Conclusion Al coating was successfully deposited on Mg surface through electroless displacement deposition from AlCl3-EMIC room temperature ionic liquid after an electrolytic etching pretreatment. The results showed that the deposited Al coating was nanocrystalline. Acknowledgment Financial support from National Natural Science Foundation of China (No. 51271166) is gratefully acknowledged.