Underpotential co-deposition

Several binary alloys of technological importance are known to form by way of an underpotential co-deposition mechanism. The abnormal composition-potential relationship observed in Cu-Zn alloys deposited from cyanide-based electrolytes, one of the most widely used commercial alloy plating processes, is attributed to the underpotential co-deposition of Zn [64]. The UPD of Zn is also known to occur on Co and Fe and has been included in treatments focusing on the anomalous co-deposition of Co-Zn [65] and Ni-Zn alloys [66-68]. Alloys of Cu-Cd have been shown to incorporate Cd at underpotentials when deposited from ethylene diamine solution [69-71]. 

Fig. 5. Potential regime for underpotential co-deposition process of alloy AB (UPCD). The UPCD regime is indicated relative to OPCD potential range and equilibrium potentials of elemental A and B.

Here, Wg and Wq2 represent temperature and composition independent constants called Margules parameters. The substitution of Eq. (11) into Eq. (7) yields the equilibrium potential of the less noble component in the alloy. If this expression is subtracted from the equilibrium potential of the elemental phase defined by Eq. (1), the relation between the underpotentially co-deposited alloy composition and corresponding value of underpotential can be obtained. The example of this approach is shown in Fig. 6 where the composition of UPCD CoPt and FePt is measured as a function of deposition underpotential. The solid lines in the plot indicate the fit of the asymmetric regular solution model. It is important to note that Eq. (11) combined with Eqs. (7) and (1) suggest that composition of UPCD AB alloys (CoPt and FePt) is not dependent on A and B (Co and Fe) deposition kinetics (concentrations in the solutions). 

The catalytic properties of a Pt/Sn combination were observed on different kinds of electrode materials alloys [90], electro co-deposits of Pt and Sn [89, 90], underpotential deposited tin [42] or a mixture of tin oxide and platinum deposited on glass [95], All different materials present a marked influence on methanol electrooxidation. 

The electrodeposition of alloys at potentials positive of the reversible potential of the less noble species has been observed in several binary alloy systems. This shift in the deposition potential of the less noble species has been attributed to the decrease in free energy accompanying the formation of solid solutions and/or intermetallic compounds [61, 62], Co-deposition of this type is often called underpotential alloy deposition to distinguish it from the classical phenomenon of underpotential deposition (UPD) of monolayers onto metal surfaces [63],... 

An empirical treatment developed by Kolb et al. [81, 82] relating UPD behavior to the difference in work function between the substrate and depositing species has been used to explain anomalous co-deposition behavior observed in Ni-Fe and Ni-Zn alloys [83]. Although the relationship appears to hold for pure underpotential deposition limited to a monolayer, it does not satisfactorily predict bulk alloy behavior. For example, based on work function data alone, one would expect Zn-Al and Sb-Al alloys to be formed by underpotential alloy deposition. Recent reports in the literature, however, indicate that alloying in these systems does not occur [46, 84]. 

The direct electrodeposition of GaAs from ionic liquids has been studied mainly by two groups. Wicelinski et al. [66] used an acidic chloroaluminate melt at 35-40 °C to co-deposit Ga and As. However, it was reported that Al underpotential deposition occurs on Ga. Carpenter et al. employed an ionic liquid that was based on GaQs to which AsQs was added [18,67]. Unfortunately, in these studies the quality of the deposits was not convincing and both pure arsenic and gallium could be found in the deposits. Nevertheless, these studies have to be regarded as the first steps in the electrodeposition of Ga-based semiconductors. Furthermore, thermal annealing could improve the quality of the deposits. 

Copper electrodeposition on Au(111) Copper is an interesting metal and has been widely investigated in electrodeposition studies from aqueous solutions. There are numerous publications in the literature on this topic. Furthermore, technical processes to produce Cu interconnects on microchips have been established in aqueous solutions. In general, the quality of the deposits is strongly influenced by the bath composition. On the nanometer scale, one finds different superstmctures in the underpotential deposition regime if different counter-ions are used in the solutions. A co-adsorption between the metal atoms and the anions has been reported. In the underpotential regime, before the bulk deposition begins, one Cu mono-layer forms on Au(lll) [66]. 

Rajeshwar and co-workers performed photocatalytic underpotential deposition of Cd and Pb onto the surface of Se-modified Ti02 particles to prepare CdSe/Ti02 and PbSe/Ti02 composites [97, 98]. The Se-modified Ti02 particles were prepared themselves by UV illumination of titania particles in a Se(fV)-containing aqueous solution. The photocatalytic UPD of Cd and Pb on the bare Ti02 surface was found... 

Hoshi N, Bae IT, Scherson DA. 2000. In situ infrared reflection absorption spectroscopic studies of coadsorption of CO with underpotential-deposited lead on Pt(lll) in an aqueous acidic solution. J Phys Chem B 104 6049-6052. 

Melroy and co-workers88 recently reported on the EXAFS spectrum of Pb underpotentially deposited on silver (111). In this case, no Pb/Ag scattering was observed and this was ascribed to the large Debye-Waller factor for the lead as well as to the presence of an incommensurate layer. However, data analysis as well as comparison of the edge region of spectra for the underpotentially deposited lead, lead foil, lead acetate, and lead oxide indicated the presence of oxygen from either water or acetate (from electrolyte) as a backscatterer. 

Due to the experimental difficulties involved, there have been only three reports of XSW measurements at electrochemical interfaces. Materlik and co-workers have studied the underpotential deposition of thallium on single-crystal copper electrodes under both ex situU9 and in situ120 conditions. In addition, they report results from studies in the absence and presence of small amounts of oxygen. 

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