SCANNING-TUNNELING-MICROSCOPY OF ELECTRODEPOSITED THALLIUM MONOLAYERS ON AU(111) IN ALKALINE-SOLUTION

The structure of electrodeposited Tl adlayers on Au(111) in 0.1 M NaOH has been investigated using in situ scanning tunneling microscopy (STM). Incommensurate rotated hexagonal Tl adlayers exist over a wide potential region between the bulk deposition and -0.38 V. The Tl monolayer is closely packed with a Tl-Tl spacing of 3.4 +/- 0.2 A and is rotated from the substrate axis by 6 degrees +/- 1 degrees. Its coverage is 0.74. Between -0.36 and -0.18 V, an incommensurate aligned hexagonal phase is observed in which the Tl-Tl spacing is 3.8 +/- 0.2 Angstrom and the coverage is 0.56. The first-order transformation between these two phases at - 0.37 V involves the coadsorption of OH- which facilitates the existence of this unusual low coverage incommensurate Tl phase. At more positive potentials, a new hexagonal adlayer is observed without atomic resolution. The adlayer at E > - 0.04 V is disordered and appears to be formed during the oxidation of the Tl adsorbate with additional Tl adsorption from solution. There is an excellent agreement between the STM results and surface X-ray scattering results obtained earlier.