The dependence of the potential of zero charge (pzc) for Pt(111) surfaces in acidic aqueous solution having increasing densities of ordered monoatomic steps in the (111)−(110) and (111)−(100) zones is evaluated from CO “charge-displacement” measurements, with the objective of elucidating the influence of the electrochemical double layer on the large step-induced changes in surface potential known for the clean uncharged surfaces in ultrahigh vacuum (UHV). This experimental strategy, which involves evaluating the charge flowing at controlled potentials upon “quenching” the aqueous double layer with chemisorbed CO, yields pzc values referring to zero “total” electronic charge, . The values in both 0.1 M HClO4 and 0.5 M H2SO4 electrolytes decrease noticeably (by ca. 0.15 V) upon increasing the (110) step density, N, whereas smaller effects are found for (100) steps. The location of the values within the so-called “hydrogen” region, however, complicates interpretation of the −N dependences due to the presence of faradaic charge associated with potential-dependent hydrogen adsorption. Procedures are outlined by which this influence upon can be removed, yielding approximate estimates of pzc values referring instead to zero “free” electronic charge, , as a function of step density. The analysis followed involves extrapolating charge−potential data from higher potentials where hydrogen adsorption is essentially absent, or evaluating instead “potentials of constant (nonzero) free charge” in this “double-layer” region, achievable most readily with the data in 0.1 M HClO4. The resulting −N plots exhibit substantial negative slopes for dilute (110) stepped surfaces (N ≤ 107 cm-1), Δ values approaching ca. −0.7 V being obtained, although the dependence changes sign close to the densely stepped (110) limit. Significantly, the −N profile obtained for (110) steps is comparable to the corresponding work function (ΦM−N) behavior for the Pt(111)−(110) surfaces in UHV. This indicates that the remarkably (ca. 1 eV) lower “local” ΦM values for Pt steps relative to (110) terrace regions known to be present in the latter environment are not attenuated (or otherwise altered) greatly by the presence of the aqueous double layer. Interpretation is given in terms of current understanding of interfacial solvation effects on , and some electrochemical implications are pointed out.

Strongly adsorbed bromine monolayers are formed when Pt(111) electrodes are exposed to aqueous bromide solutions at open circuit or to bromine gas atmospheres. These adlayers are stable in air as well as in bromide-free acidic electrolyte (at sufficiently high potentials). The bromine coverage for these adlayers (evaluated from charge displacement experiments during potentiostatic CO adsorption) is very similar for these two dosing procedures and close to that existing at the Pt(111)/aqueous bromide electrolyte interface in a relatively wide potential range positive to the voltammetric peaks. This surface coverage is around 0.46 Br/Pt. These irreversibly adsorbed Pt(111)−Br adlayers are stable enough for protecting the surface against contamination for prolonged periods of time, providing a surface protection procedure simpler than the widely used I-CO replacement method. The structure of the bromine adlayer can be studied ex situ by using STM, due to the immobile character of the adlattice. The existence of a densely, close-packed hexagonal adlayer of bromine atoms has been evidenced. This adlayer gives rise to different coincidence structures in the coverage range near saturation. Some of these structures can be tentatively described as (3 × 3(√3)/2) rect and ( ). The surface coverages calculated from the different structures imaged by STM range from 0.44 to 0.50 Br/Pt, which are in agreement with those obtained by charge displacement and Auger data. This variation in coverage and structure indicates that near the saturation a compression of the Br adlayer on Pt exists, with a continuous variation of the interatomic distances in the adlayer, in the vicinity of the van der Waals diameter of bromine.