Electrode surface coordination chemistry: ligand (-SH/I) substitutions at polycrystalline platinum

Ligand substitution (adsorbate displacement) reactions between iodine and the -SH functional group in 2,5-dihydroxythiophenol (DHT), previously investigated on Au, have been studied at a smooth polycrystalline platinum electrode in 1 M H2SO4. Experiments involved pretreatment of the Pt surface with a full monolayer of DHT (or iodine) followed by exposure to a dilute aqueous solution of KI (or DHT). Thin-layer electrochemistry and X-ray photoelectron spectroscopy were then used to characterize the DHT and/or I that remained on the surface. The results were as follows: (i) exposure of the I-coated surface to DHT led to the displacement of 40% of the original I and the incorporation of 70% of a monolayer of DHT; exposure of the DHT-coated Pt to KI yielded a layer that retained at least 90% of the original DHT and the coadsorption of no more than 10% of a monolayer of iodine. This observation indicates that, on Pt, the -SH functional group is selectively surface-coordinated over I; the converse occurs on Au where I is preferentially chemisorbed over DHT; (ii) The presence of coadsorbed I disrupts the substrate-mediated DHT-DHT interactions, as indicated by the sharpening of the reversible redox peaks of the pendant diphenolic moiety, on going from pure DHT to mixed I-DHT adlayers. On Au, the redox peaks were equally sharp whether or not iodine was present on the surface; (iii) The apparent molecular cross-section of DHT in the mixed I-DHT layer (0.20 nm(2)) was less than that in the pure DHT layer (0.31 nm(2)). This result, along with the coadsorbed-I-induced diminution of the redox peak widths, suggest that, as on Au, the coadsorbed I and DHT species are distributed homogeneously, not heterogeneously as distinct domains of pure I and pure DHT. (C) 1998 Elsevier Science B.V.