X-RAY-SCATTERING STUDY OF TL-ADLAYERS ON THE AU(111) ELECTRODE IN ALKALINE-SOLUTIONS - METAL MONOLAYER, OH- COADSORPTION, AND OXIDE FORMATION

The structure of electrodeposited Tl adlayers on Au(111) in 0.1 M NaOH has been investigated using surface X-ray scattering. Incommensurate, hexagonal Tl structures exist over a wide potential region positive of bulk deposition. In the potential region between -0.66 and -0.38 V (versus Ag/AgCl), the Tl monolayer is rotated from the substrate axes by 5.2 degrees-6.2 degrees and is closely packed with a Tl-Tl separation ranging from 3.34 to 3.43 Angstrom. Between -0.36 and -0.18 V, an incommensurate, aligned-hexagonal phase is observed in which the Tl-Tl spacing increases with increasing potential from 3.88 to 4.10 Angstrom. The first-order transformation between these two phases at -0.37 V involves the coadsorption of OH-. For the aligned-hexagonal phase the electrosorption valence is larger than unity, which indicates that partially-charged hydroxyl species are coadsorbed. The coadsorption of OH- facilitates the existence of this unusual low-coverage, incommensurate Tl phase. At more positive potentials, the hexagonal adlayer lattice continuously rotates away from the substrate axes. At E > -0.4 V the adlayer appears disordered, and a two Tl atom thick Tl2O adlayer forms. The data illustrate the power of the surface X-ray scattering techniques, coupled with electrochemical measurements, in studying the complex behavior of electrodeposited adlayers.