Underpotential deposition thallium

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. 

As an example [13] we consider the underpotential deposition of thallium on silver (Fig. 15.13). At potentials above the onset of the upd of thallium the SHG signal decreases, at first slowly, then more rapidly. The adsorption of thallium causes a strong rise in a(o ), because the region in which the electronic density decays to zero becomes more extended with an angle of incidence of 45° this shows up as a drastic increase in the signal. A similar behavior is seen in other systems, and often even fractions of a monolayer can be detected. 

Figure 15.13 Cyclic voltammogram (top) and SHG signal for the underpotential deposition of thallium on silver The letters in the voltammogram denote various adsorption (A) and desorption (D) peaks. Reprinted with permission from Ref. 13.

The underpotential deposition of T1 on Cu(l 11) has been examined by Shannon et al. [133] under static conditions, and Richmond and Robinson in the time domain [117]. A large mismatch between the copper and the thallium lattice constants makes it an interesting case for comparison with the Tl/Ag(lll) SH study described above [122] where the lattice constants are more closely matched and the overlayer has been shown to form in an ordered manner. From strictly geometrical considerations, the silver lattice is able to accommodate a thallium overlayer without buckling. On the copper substrate, a close-packed thallium overlayer must be either buckled or rotated with respect to the substrate lattice. 

Fig. 5.21. Transient SH response of a Cu(l 11) electrode during thallium underpotential deposition. Incident wavelength = 1064 nm. (a) Isotropic SH signal, /PjP(f). The solid line is a single exponential fit to the entire transient with time constant, t= 10.7 0.3 msec, (b) Anisotropic SH signal, /p s(f). The intensity scale is 2X more sensitive than in (a). From Ref. 117.

Several studies have appeared which examine the SH response from bulk single crystals of gold under potential control. The first study reported was that of Koos [134] in which both the native and underpotential deposition of thallium was studied. This was later examined in more detail for a series of different metals on Au(lll) [155]. Fig. 5.23 shows the SH intensity from Au(lll) in HC104 as a function of the azimuthal angle 0. The SH response using a 1064 nm incident beam was collected for p-polarized input and p- and s-polarized output (Fig. 5.23 a and c) and s-polarized input and p-polarized output (Fig. 5.23 b). The responses are consistent... 

T vo different approaches have been made to increase the maximum deposition rate which is reached under a given set of deposition conditions. The first approach involves addition of a depolarizer, such as lead or thallium ions [162-164]. These ions are known to adsorb strongly on gold and to bring about underpotential deposition (UPD) during electrodeposition of gold [165]. The depolarization effect amounts to as much as 0.2 V, as reported by Matsuoka et al. [164] and illustrated in Fig. 34. These authors show that addition of such depolarizers to a borohydride bath increases the deposition rate by a factor of as much as 8 to 10 (Fig. 35). 

Investigated systems include various metal deposits, like underpotentially deposited silver on Au(l 11) [69], thallium on a Pt(l 11) surface as a function of solution pH and bisulfate coadsorption [70] and other upd-systems [36, 59]. Evidence of dealloying of Cu3Au(lll) has been reported [71]. Near-neighbor distances between atoms in upd-monolayers of various transition metals deposited on Ag(lll) and Au(lll) surfaces have been measured as a function of electrode potential [72]. Typical results of a study of thallium-upd showing the changing Tl-Tl distance as a function of electrode potential are shown in Fig. 6.9. 

Mao, B.W., Tian, Z.Q. and Fleischmann, M. (1992) Voltammetric studies of underpotential deposition of thallium from thallium(l) film confined to silver electrodes. Electrochimica Acta, 37, 1767. 

Leung, L.W.H. and Weaver, M.J. (1987) Extending the metal interface generality of surface-enhanced Raman-spectroscopy - underpotential deposited layers of mercury, thallium, and lead on gold electrodes. Journal ofElectroanalytical Chemistry, 217, 367-384. 

The first in-situ X-ray diffraction (XRD) investigations of phase transitions of adsorbed monolayers and multilayers [6] and reconstmction of a metal surface [7,8] were also reported by Fleischmann and Mao. The phase transitions were reported for the underpotential deposition (upd) and overpotential deposition (opd) of thallium onto a roughened silver electrode surfaces (similar to those used in surface-enhanced Raman spectroscopy (SERS) using the reflection mode of collection), and for upd of lead onto gold and silver... 

In addition to these direct NP oxidation detection experiments, the underpotential deposition (UPD) of metal ions from solution onto metal NPs during collisions between the NPs and an inert electrode was also reported (see Figure 8.18). Reactions for UPD of thallium and bulk electrodeposition of cadmium onto Ag NPs were used for detection, which formed bimetallic core-shell NPs (denoted Ag Tl and Ag Cd, respectively). For the case of thallium, it was shown that up to a... 

Zhou, Y.-G. Rees, N. V. Compton, R. G. 2011. NP-Electrode collision processes The underpotential deposition of thallium on silver NPs in aqueous solution. ChemPhysChem 12 2085-2087. 

Robinson and Richmond [117] also found that the time constant for adsorption of the first thallium overlayer depends on the final potential, E, of the potential step. They investigated the effect of underpotential and overpotential steps on the best fit values of z for exponential fits to /p p((). The under- and overpotentials, AE, are defined with respect to the maximum of the current peak, Ep, in the CVs for the first monolayer deposition. Fig. 5.22 displays the results for seven different potential steps. The time required to form the deposit increases at anodic potentials (A <0) closest to Ep, indicating that more time is required to deposit successively larger coverages. The form of the data suggests that z would reach a maximum near... 

Fleischmann, M. and Tian, Z.Q. (1987) The effects of the underpotential and overpotential deposition of lead and thallium on silver on the Raman spectra of adsorbates. Journal of... 

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