Underpotential deposition

Underpotential deposition consists of the formation of metal submonolayers at potentials more positive than the reversible Nernst potential, before bulk deposition can occur [2, 47-51]. This technique is also called adatom deposition [52] or adsorption of metal atoms [53]. 

When M is deposited in a submonolayer (M ml) on the metal M, according to the reaction. 

The energetic aspects of underpotential deposition can be investigated by a slow (i.e., a few millivolts per second) potential scan starting at a potential so high that no adsorption takes place. As the potential is lowered, one or more current peaks axe observed, which are caused by the adsorption of the metal ions (see Fig. 4.9). According to the usual convention, the adsorption current is negative (i.e., cathodic). Different peaks may correspond to different adsorption sites, or to different structures of the adsorbate layer. If the potential is scanned further past the equilibrium potential foo, the usual bulk deposition is observed. 

Cu2+ on Au(lll) shown in Fig. 4.9. The charge under the first peak corresponds to the deposition of about 2/3 of a monolayer. The second peak near 0.03 V corresponds to about 1/3 of a monolayer. Note that the corresponding desorption peak is shifted toward a higher potential (near 0.07 V), possibly because the desorption is very slow.1 

The difference between the potential of the current peak for the desorption and the bulk deposition potential is known as the underpotential shift /Upd. For simple systems the value of /upd is independent of the concentration of ions in the bulk of the solution, since the Gibbs energies of adsorption and deposition shift both according to the Nernst equation. Deviations from this behavior may indicate coadsorption of other ions. 

Kolb [4] observed an interesting correlation for adsorption on polycrystalline substrates A plot of /upd (in volts) versus the difference A4 = 4 sub — 4 ad (in electron volts) between the work functions of the substrate and the adsorbate yields a straight line with a slope of about 1/2 (see Fig. 4.10). Often there are several upd potentials, and in these cases the highest value corresponding to the strongest substrate- 

1Recent studies have shown that the upd of Cu on Au(lll) is not as simple as it seems, but is complicated by the coadsorption of SO - or HSO ions. 

It was described above how the metal ion is adsorbed on its own metal. Metal ions, however, can also be adsorbed on other metal substrates. This deposition is observed before bulk deposition and sometimes it looks as if a metal deposition occurs against the rules of thermodynamics. Therefore, this phenomenon is called underpotential deposition (UPD). Budevski, Staikov, and Lorenz gave a comprehensive treatment of UPD. In the following only a short description of the typical problems in this research field and its influence on surface treatment and phase formation will be described. 

Unlike anions that specifically adsorb at electrodes, cations normally do not lose their solvation shell due to their smaller size and are electrostatically adsorbed at electrodes at potentials negative to the pzc. However, depending on the affinity with the foreign substrate, cations can be reduced to a lower oxidation state or even discharged completely to the corresponding metal atom at the sub-monolayer or monolayer level at potentials positive to the equilibrium Nernst potential for bulk deposition. This deposition of metal atoms on foreign metal electrodes at potential positive to that predicted by the Nernst equation for bulk deposition has been called underpotential deposition and has been extensively investigated in recent years. Detailed discussion of the 

The existence of a difference in electrode potential for the underpotential deposition (upd) at a given coverage, 0upd, equilibrium conditions with respect to the bulk deposition potential implies that there is a difference in the chemical potential between the upd layer and the corresponding bulk metal [14]. 

Underpotential deposition occurs because the upd metal has a stronger interaction with the foreign substrate than with the corresponding bulk metal and such potential difference is a measure of the binding energy of the upd layer on the foreign substrate. 

Kolb [109] found a correlation for a large number of upd systems of the underpotential at 0upd = 0.2 from voltametric curves and the differences between work functions of the substrate and the corresponding metal 

Trasatti [112], on the other hand, has correlated the underpotential value for the starting of the upd phenomena at 6 — 0 with A J  

We have seen earlier that metal M will be deposited on the cathode from the solution 

Metal UPD at the SAM/substrate interface is of interest for several reasons. Firstly, from an application point of view as the intercalation of another metal alters the thiol-substrate bond and, thus, the stability of a SAM that can be exploited to generate heterogeneous and patterned SAMs, a point we will return to later. Secondly, the intercalation and alteration of the thiol-substrate bond changes the morphology of a 

The change in stability by UPD was first reported by Jennings and Laibinis [199, 

STM and CVs are applied. This is exacerbated by the fact that the extent to which UPD features are suppressed in the CVs depends sensitively on the quality of the SAM. For such a pronounced quenching a good film quality is required, that is, a low defect density is required. To achieve this reproducibly is quite critical as has been pointed out in the literature [40, 183, 204—206]. Therefore, it is no surprise that substantial variations in the blocking properties refiected in the CVs have been observed [39, 183, 203, 207-209]. 

To elucidate the mechanism of metal UPD, a number of studies on Cu and Ag UPD have been performed over the years comprising phenomenological, CV-based studies and microscopic investigations with STM both ex situ and in situ [38, 39, 

210-214]. While some features, such as the formation of UPD islands, were commonly reported for various systems (different thiols and metals, that is, Ag and Cu) differing interpretations were given with respect to the details such as formation, extension or height, possibly due to the sometimes difficult interpretation of data that, furthermore, can vary with the details of the system and the experimental conditions applied. Some of the issues could be resolved in a recent study on high-quality aromatic SAMs where the UPD process could be extremely slowed down to allow time-resolved in-situ studies [43]. 

One can expect that the exact mechanism depends on the type of SAM. Furthermore, the various pathways for UPD might not only depend on the particular SAM system but also on the potential applied, since a SAM can undergo potential induced structural transitions as observed for ethane thiol (C2H5SH), for example [61, 210], 

Another common feature of metal UPD on SAM-modified electrodes is the pronounced suppression of nucleation that for bare Au occurs at steps unless the step density is very low [218]. Even though nucleation still occurs mostly at steps for 

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Chen C-FI, Washburn N and Gewirth A A 1993 In situ atomic force microscope study of Pb underpotential deposition on Au(111) Structural properties of the catalytically active phase J.Phys. Chem. 97 9754-60... 

Carnal D, Oden P I, Muller U, Schmidt E and Siegenthaler FI 1995 In situ STM investigation of T1 and Pb underpotential deposition on chemically polished Ag(111) electrodes E/ecfroc/r/m. Acta 40 1223-35... 

Li J and Abruna FI D 1997 Coadsorption of sulphate/bisulphate anions with Fig cations during Fig underpotential deposition on Au (111) An In situ x-ray diffraction study J. Phys. Chem. B 101 244-52... 

Blum L, Abruna FI D, White J, Gordon J G, Borges G L, Samant M G and Melroy 1986 Study of underpotentially deposited copper on gold by fluorescence detected surface EXAFS J. Chem. Phys. 85 6732-8... 

Huckaby D A and Blum L 1991 A model for sequential first-order phase transitions occuring in the underpotential deposition of metals J. Eiectroanai. Chem. 315 255-61... 

Zinc and tin The electrodeposition of Zn [52] has been investigated in acidic chloroaluminate liquids on gold, platinum, tungsten, and glassy carbon. On glassy carbon only three-dimensional bulk deposition was observed, due to the metal s underpotential deposition behavior. At higher overvoltages, codeposition with A1... 

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]. 

Hydrogen adsorption from solution Oxygen adsorption from solution Underpotential deposition of metals Adsorption of probe molecules from solution ... 

Based on the experimental conditions the gold electrode is most likely covered with underpotential deposited (upd) silver. Consequently the value of iip c should be compared with the corresponding value for a silver electrode. 

Gregory WB, Norton ML, Stickney JL (1990) Thin-layer electrochemical studies of the underpotential deposition of cadmium and tellurium on polycrystalline Au, Pt and Cu electrodes. J Electroanal Chem 293 85-101... 

Generally, the experimental results on electrodeposition of CdS in acidic solutions of thiosulfate have implied that CdS growth does not involve underpotential deposition of the less noble element (Cd), as would be required by the theoretical treatments of compound semiconductor electrodeposition. Hence, a fundamental difference exists between CdS and the other two cadmium chalcogenides, CdSe and CdTe, for which the UPD model has been fairly successful. Besides, in the present case, colloidal sulfur is generated in the bulk of solution, giving rise to homogeneous precipitation of CdS in the vessel, so that it is quite difficult to obtain a film with an ordered structure. The same is true for the common chemical bath CdS deposition methods. 

It appears that a way to efficient electrochemical growth of CdTe and other chalcogenides on silicon crystals is the utilization of light-assisted processes. Works in this direction will be discussed in a subsequent section regarding underpotential deposition studies. 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

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... 

Colletti LP, Teklay D, Stickney JL (1994) Thin-layer electrochemical studies of the oxidative underpotential deposition of sulfur and its application to the electrochemical atomic layer epitaxy deposition of CdS. J Electroanal Chem 369 145-152... 

Alois GD, CavaUini M, Innocent M, Foresti ML, Pezzatini G, GuideUi R (1997) In situ STM and electrochemical investigation of sulfur oxidative underpotential deposition on Ag(lll). J Phys Chem B 101 4774 780... 

Herrero E, Buller LJ, Abruna HD (2001) Underpotential deposition at single crystal surfaces of Au, Pt, Ag and other materials. Chem Rev 101 1897-1830... 

Michaelis R, Zei MS, Zhai RS, Kolb DM (1992) The effect of halides on the structure of copper underpotential-deposited onto Pt(lll) a low-energy electron diffraction and X-ray photoelectron spectroscopy study. J Electroanal Chem 339 299-310... 

Alanyahopu M, (Jakal H, Oztiirk AE, Demir U (2001) Electrochemical studies of the effects of pH and the surface stracture of gold substrates on the underpotential deposition of sulfur. J Phys Chem B 105 10588-10593... 

Santos MC, Machado SAS (2004) Microgravimetric, rotating ring-disc and voltammetric studies of the underpotential deposition of selenium on polycrystalline platinum electrodes. J Electroanal Chem 567 203-210... 

Santos MC, Machado SAS (2005) A voltammetric and nanogravimetric study of Te underpotential deposition on Pt in perchloric acid medium. Electrochim Acta 50 2289-2295... 

Osipovich NP, Streltsov EA, Susha AS (2000) Bismuth underpotential deposition on tellurium. Electrochem Commun 2 822-826... 

Ivanov DK, Osipovich NP, Poznyak SK, Streltsov EA (2003) Electrochemical preparation of lead-doped amorphous Se films and underpotential deposition of lead onto these films. Surf Sci 532-535 1092-1097... 

Streltsov EA, Poznyak SK, Osipovich NP (2002) Photoinduced and dark underpotential deposition of lead on selenium. 1 Electroanal Chem 518 103-114... 

The additivity principle was well obeyed on adding the voltammograms of the two redox couples involved even though the initially reduced platinum surface had become covered by a small number of underpotential-deposited mercury monolayers. With an initially anodized platinum disk the catalytic rates were much smaller, although the decrease was less if the Hg(I) solution had been added to the reaction vessel before the Ce(lV) solution. The reason was partial reduction by Hg(l) of the ox-ide/hydroxide layer, so partly converting the surface to the reduced state on which catalysis was greater. 

Fedchenfeld, H. and Weaver, M.J. (1989) Binding of alkynes to silver, gold, and underpotential deposited silver electrodes as deduced by surface-enhanced Raman spectroscopy. The Journal of Physical Chemistry, 93, 4276—4282. 

These primary electrochemical steps may take place at values of potential below the eqnilibrinm potential of the basic reaction. Thns, in a solntion not yet satnrated with dissolved hydrogen, hydrogen molecnles can form even at potentials more positive than the eqnilibrinm potential of the hydrogen electrode at 1 atm of hydrogen pressnre. Becanse of their energy of chemical interaction with the snbstrate, metal adatoms can be prodnced cathodically even at potentials more positive than the eqnilibrinm potential of a given metal-electrolyte system. This process is called the underpotential deposition of metals. 

Underpotential Deposition of Metal Atoms Because of the energy of interaction between a foreign substrate and the adsorbed metal atoms formed by discharge, cathodic discharge of a limited amount of metal ions producing adatoms is possible at potentials more positive than the equilibrium potential of the particular system, and also more positive than the potential of steady metal deposition. 

Kokkinidis, K., Underpotential deposition and electrocatalysis, J. Electroanal. Chem., 201, 217 (1986). 

An alternative type of tip-induced nanostructuring has recently been proposed. In this method, a single-crystal surface covered by an underpotential-deposited mono-layer is scanned at a close tip-substrate distance in a certain surface area. This appears to lead to the incorporation of UPD atoms into the substrate lattice, yielding a localized alloy. This procedure works for Cu clusters on Pt(l 11), Pt(lOO), Au(l 11), and for some other systems, but a model for this type of nanostructuring has not been available until now. (Xiao et al., 2003). 

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