The electrodeposition of Ag and Al on Ñame annealed Au(111) Ðlms from an acidic aluminium chlorideÈ1-methyl-3-butylimidazolium chloride room temperature molten salt has been investigated by electrochemical scanning tunneling microscopy, cyclic voltammetry and potential step experiments. The cyclic voltammogram of Ag on Au(111) is characterized by adsorption controlled Ag underpotential deposition (upd) and di †usion controlled Ag overpotential deposition (opd). Starting from the anodic limit, bulk oxidation of Au is observed to start near ]1.25 V vs. Ag/Ag`reference electrode (RE). In the upd range, two dimensional Ag islands form which merge in a coherent Ag monolayer near 0.05 V vs. RE. With further reduction of the potential, a second monolayer grows. The corresponding chronoamperometric measurements show exponential behaviour with time constants of the order 10 s~1 consistent with a Langmuir adsorption model. In the opd range, a di †usion controlled layer by layer growth of Ag clusters occurs, the bulk Ag`di †usion coefficient being (1.4 ^0.2) ] 10~6 cm2 s~1. Alloying of Ag with codeposited Al from the electrolyte has to be considered. For the Al electrodeposition on Au(111), strong indications for alloying have been observed starting at a potential of ]0.95 V vs. Al/Al3`RE. Below ]0.55 V, the formation of two dimensional Al islands is seen followed by a three dimensional growth whereby a strong tendency for alloying has to be considered.
The electrocrystallization of Co and Co-Al alloys has been investigated for the first time from a chloroaluminate ionic liquid, AlCl3/[BMIm] + Cl -, on Au(111) by in situ electrochemical scanning tunneling microscopy (STM) and spectroscopy (STS). In contrast to Ni deposition (Zell, C. A.; Freyland, W. Proc. Electrochem. Soc. 2002, 19, 660), the formation of a coherent Co monolayer is not observed in the underpotential range. Instead, 2D phase formation sets in at slightly cathodic potentials characterized by monatomically high Co islands with a radius of 2-3 nm and a narrow size distribution. Their growth behavior has been studied as a function of time and overpotential η in the range -0.05 e η/V e -0.17 versus Co/Co(II). It can be described by a classical nucleation model yielding a critical nucleus of 1-2 atoms. At potentials below -0.2 V, a fast 3D growth of Co clusters is manifest. Although Co and Au are immiscible in the bulk phase at room temperature, the STM images give clear evidence of surface alloying for 2D and 3D cluster deposits. Co-deposition of Co and Al has been studied in the potential range -0.3 e η/V e -0.7. It is found that the grain size of the CoxAl1-x clusters decreases with increasing Al content. Particularly remarkable is the result that the composition of CoxAl1-x can be controlled in situ with nanometer resolution on a time scale of milliseconds, the typical time for recording the tunneling spectra being ∼200 µs. This is demonstrated by the variation of the effective tunneling barrier φ as a function of η as determined from STS measurements. It quantitatively agrees with independent composition determinations by conventional electrochemical methods. This correlation has not been reported before.
Interfacial phase transitions like wetting and prewetting transitions are of considerable interest in physics and chemistry of condensed matter since they represent phase transitions in reduced dimensionality. Besides this interfacial properties are of profound practical and technological interest. Most systems studied experimentally in this respect are characterized by Van der Waals intermolecular interactions. However, in the last few years it was shown that Coulomb liquids like liquid alloys or metal molten salt solutions exhibit interfacial phase transitions similar to those known in Van der Waals systems.
In order to get more insight into these phenomena the fluid‐vapor interface of two different alloy systems have been studied using ellipsometry. Gallium‐bismuth is a binary alloy with large positive deviations from Raoult's law, exhibiting a distinct miscibility gap. Approaching liquid‐liquid coexistence a Bi‐rich film completely wets the fluid‐vapor interface. As can be estimated from the ellipsometric results the film thickness jumps at the monotectic temperature to a value of about 50 Å. In contrast, gallium‐germanium shows continuous miscibility and deviates much less from ideal mixing. As the Ge concentration in liquid Ga increases along the solid‐liquid coexistence curve the optical properties at the surface also vary continuously, which can be modelled within a simple effective medium approach.
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