Electrodeposition of Zn and Au–Zn alloys at low temperature

Dimitar Borissov, Aparna Pareek, Frank Uwe Renner and Michael Rohwerder, Physical Chemistry Chemical Physics 2010, 12, 2059–2062. (Communication)

Electrodeposition of Zn from 60–40 mol% ZnCl₂–1-butyl-3-methylimidazolium chloride (BMIC) ionic liquid on Au substrates has been investigated. For the first time, initial stages of Zn electrocrystallization from BMIC has been studied by in situ X-ray diffraction (XRD) employing synchrotron radiation, which showed an initial epitaxial deposition of Zn and hexagonal Au_1.2Zn_8.8 phases on Au(111) single crystal substrates. In the later stages of electrodeposition, phase analysis showed a formation of several Zn–Au intermetallics, namely AuZn, AuZn₃, and Au_1.2Zn_8.8, along with the Zn phase.

The electrocrystallization of Zn and its alloys has attracted significant attention because of their important role in several technological applications, such as corrosion-resistant coatings in the automotive industry,1 energy storage devices and hydrogen production by water electrolysis.3 However, the electroplating of zinc and its alloys from aqueous electrolytes is usually accompanied by the simultaneous discharge of hydrogen ions and hydrogen embrittlement of the substrate and often leads to low current efficiencies. To overcome the problems of hydrogen evolution, highly conducting aprotic media such as room temperature molten salts can be employed. A wide range of room temperature molten salts have been developed, which are commonly known today as ‘ionic liquids’ (ILs). Ionic liquids possess some unique properties, e.g. low melting points, extremely low vapour pressures, good thermal stability and electrical conductivity. They also have wide electrochemical windows as compared to aqueous electrolytes, which enables the electrodeposition of semiconductors 4 and metals with large negative reduction potentials such as Al and Ti_5,6 as well as noble metals such as Ag.7 Due to these favourable characteristics, ILs are of great technological interest as new electrolytes for electroplating and electropolishing. For more details on the physical and chemical properties of ILs, we refer the interested reader to the recent reviews of Endres,8 Abbott 9 and Compton. Zinc electrodeposition in chloroaluminate ILs on different substrates in the underpotential (UPD) and overpotential deposition (OPD) regions was investigated. Current transients on glassy carbon substrates showed a very good agreement with the theoretical transients for the limiting case of progressive three-dimensional nucleation with hemispherical diffusion controlled growth of the nuclei.11 The initial stages of zinc electrocrystallization on Au(111) in the UPD have been studied by in situ scanning tunnelling microscopy (STM) in acidic AlCl₃–dialkyl-imidazolium ionic liquid, which showed a layer-by-layer growth mode and Zn–Au surface alloying. However, chloroaluminate ILs require elaborate handling, as they are very sensitive to air and moisture. Electrocrystallization of zinc and its alloys has also been extensively studied from the Lewis acid melt system (liquid at 40 1C) ZnCl₂–1-ethyl-3-methylimidazolium chloride (EMIC), which is less sensitive in comparison to chloroaluminate ILs to air and moisture. It was reported that melts containing less than 33 mol% ZnCl₂ are ‘basic’ and probably contain complexed anionic chlorozincate species. Huang and Sun have reported the electrodeposition of Zn–Pt, Zn–Fe, Zn–Sn, Zn–Ni alloys in the Lewis acidic ILs. Recently, electrochemical alloying and dealloying of a Au substrate in a ZnCl₂–EMIC ionic liquid has been performed to fabricate functional nanoporous gold. However, the mechanism of bulk alloy formation and the exact phase composition of the Zn–Au alloys have not been reported yet. In this communication, we report on the phase composition and characterization of the Zn–Au alloy by electrochemical deposition of zinc on gold from an ionic liquid comprising of 60.0–40.0 mol% ZnCl₂ and 1-butyl-3-methylimidazolium chloride (BMIC). Furthermore, the mechanism of alloy formation was investigated by using in situ X-ray diffraction using synchrotron radiation and single-crystal substrates.