NATURE OF BR ADLAYERS ON PT(111) SINGLE-CRYSTAL SURFACES - VOLTAMMETRIC, CHARGE DISPLACEMENT, AND EX-SITU STM EXPERIMENTS

Strongly adsorbed bromine monolayers are formed when Pt(111) electrode s are exposed to aqueous bromide solutions at open circuit or to bromi ne gas atmospheres. These adlayers are stable in air as well as in bro mide-free acidic electrolyte (at sufficiently high potentials). The br omine coverage for these adlayers (evaluated from charge displacement experiments during potentiostatic CO adsorption) is very similar for t hese two dosing procedures and close to that existing at the Pt(111)/a queous bromide electrolyte interface in a relatively wide potential ra nge positive to the voltammetric peaks. This surface coverage is aroun d 0.46 Br/Pt. These irreversibly adsorbed Pt(111)-Br adlayers are stab le enough for protecting the surface against contamination for prolong ed periods of time, providing a surface protection procedure simpler t han the widely used I-CO replacement method. The structure of the brom ine adlayer can be studied ex situ by using STM, due to the immobile c haracter of the adlattice. The existence of a densely, close-packed he xagonal adlayer of bromine atoms has been evidenced. This adlayer give s rise to different coincidence structures in the coverage range near saturation. Some of these structures can be tentatively described as ( 3 x 3(root 3)/2) rect and ((3 0)(12-5)). The surface coverages calcula ted from the different structures imaged by STM range from 0.44 to 0.5 0 Br/Pt, which are in agreement with those obtained by charge displace ment and Auger data. This variation in coverage and structure indicate s that near the saturation a compression of the Br adlayer on Pt exist s, with a continuous variation of the interatomic distances in the adl ayer, in the vicinity of the van der Waals diameter of bromine.