X-ray Scattering Study of Tl Adlayers on the Au(111) Electrode in Alkaline Solutions: Metal Monolayer, OH- Coadsorption, and Oxide Formation

Abstract

Underpotential deposition (UPD) refers to theelectrodeposition of metals at potentials positive of the Nernst potential. It occurs when the adatom-substrate bonding is stronger than the adatomadatom bonding in the bulk phase.' On single crystal electrode surfaces, the UPD processes often exhibit voltammetry features which are correlated with the formation of well-ordered adlayer structures. It is now possible to determine the structure of these UPD adlayers by in situ surface X-ray scattering (SXS), scanning tunneling microscopy (STM), and atomic force microscopy (AFM) techniques (see e.g. refs 2-9). An important finding of all of these studies is that the nearest-neighbor separation of the deposited metal layer-at potentials close to the bulk deposition potential-is always very close to the bulk interatomic spacings. Theseclose-packed structures aresimilar to the structures formed by vapor deposition in vacuum. In contrast, for UPD adlayers formed at more positive potentials, the nearest-neighbor separations are much larger than the bulk value^.^-^ These open structures often exhibit pronounced electrocatalytic effects1° and may be technologically relevant. In this paper we report the first in situ structural study of UPD adlayers in alkaline solution. The results reveal the effects of OH- adsorption on the structures and chemical states of three out of four T1 adlayer structures at the Au(ll1) electrode. For instance, over a range of potentials OH- is coadsorbed with the T1, and together they form an incommensurate monolayer where the T1-TI separation is much larger than the bulk value. The existence of OH- coadsorption, in the present study, has also been confirmed by the measured electrosorption valence and X-ray specular reflectivity. The phase transition between this coadsorbed phase and the close-packed T1 monolayer phase is discontinuous. In the voltammetry curve, the corresponding current spikes support the notion of a first-order phase transition. Furthermore, the transition potential corresponds to the potential where the oxygen reduction switches from a two-electron to a