The Effect of Halide Adsorption on Pb Underpotential Deposition on Au
Authors: Vasiljevic Natasa , Szczepanska Alicja 
Published: February 2024

In many systems with a large lattice mismatch, the formation of surface alloys during deposition has been reported and theoretically predicted.1 Pb monolayer deposition on Au is one such system that has been studied extensively in the past in the UHV and electrochemical environment as the underpotential deposition.2-4

Pb UPD on Au(111) in perchlorate solution has been a model UPD system studied by many surface techniques.2-6 In particular, the in-situ surface stress and the in-situ STM studies5,6 have demonstrated changes associated with the surface alloying at the partial Pb coverage. Our recent electrochemical study showed the effect of extended polarisation and potential cycling on the surface alloying in this system.7 This talk will present further work on this system by examining the effect of Cl- and Br- adsorption on Pb UPD and surface alloying.
The formation of the UPD monolayer in the presence of halides is often accompanied by competitive adsorption, changes in the kinetics and energetics of deposition, and the co-adsorption that could stabilise and create new stages/phases of growth. In contrast to many other model UPD systems, the effect of halide adsorption in this system has not been explored much. To our knowledge, there are only a few studies reported so far. The in situ X-ray studies of Pb UPD on Au in the presence of Cl- and Br- by Wang et al. 8 suggested the formation of a series of ordered structures referred to as metal-halide hybrid structures. However, their model analysis did not include surface-alloying as a possible origin of the observed ordered structures.
The effect of Cl- and Br- adsorption on Pb UPD was studied by gradually increasing the concentrations of ions (from 5μM-10mM) in the perchlorate solution. The results as illustrated in the Figure below show that both Cl- and Br - compete with Pb UPD causing some of the characteristic peaks to shift towards more negative potentials. The cyclic voltammetry in Cl- reveals the similar stages of the Pb UPD formation as those observed in perchlorate. The most significant differences have been observed in Br- solutions. For bromide concentration above 50 μM pronounced surface reconstruction-type peaks were observed suggesting ordered structures that could include both Br and Au. The difference in the electrochemical behaviour between the perchlorate and halide containing solutions are also visible in the chronoamlerometric current transients in the UPD region. Besides the fundamental interest in understanding halide effects on this UPD system, the interest was in the surface alloy (Pt-Au) formation during polarisation experiments. The comparison with the results of surface alloying in the perchlorate solutions will be presented and discussed. References

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