Electrochemical QCM studies of aluminum deposition and alloy formation on Au(111) in ambient temperature molten salts/benzene mixtures

Certain aspects of the electrochemical deposition of Al on Au electrodes fr om ambient-temperature acidic AlCl3/1-ethyl-3-methyl imidazolium chloride ( EMIC) melts have been examined in situ using a quartz crystal microbalance (QCM). These experiments were performed using benzene as a cosolvent to dec rease the very high viscosity of the neat molten salt. A linear correlation was found between the change in the frequency of the QCM and the amount of electrodeposited Al for amounts of Al ranging from 0.05 to 0.27 mug/cm(2). This observation suggests that within this mass range the films are relati vely smooth and rigidly bound to the underlying substrate. For lower masses , the plot was also linear, except that the slope was significantly smaller than that expected based on the weight of Al, a phenomenon attributed to m ass compensation. Large deviations from linearity were observed when the am ount of deposited Al increased beyond 0.27 mug/cm(2). This behavior is cons istent with the growth of discrete Al clusters leading to increased surface roughness and to liquid entrapment within the cluster array, which contrib utes to the frequency changes measured by the QCM. Long times were required for the mass of the Au electrode to return to its original value following deposition and stripping, which is consistent with the formation of bulk A l/Au alloys as has been earlier proposed. (C) 2001 The Electrochemical Soci ety.