ELSEVIER Journal of ElectroanalyticaiChemistry428 (1997) !47-153 JOt,,iRNALOF Anion adsorption from sulfuric acid solutions on Pt(111) single crystal electrodes l A.M. Funtikov 2, U. Stimming *, R. Vogel lnstitut fiir Energieverfahrenstechnik (WV), Forschungszentrum J~lich (KFA), D-52425 Jiilich, Germany Received 14 November 1996 Abstract Anion adsorption from sulfuric acid solutions on a Pt(l I 1) electrode was investigated with in-situ STM and cyclic voltammetry. An ordered (~ 12 ) adlattice is observed in the potential range between 0.5 and 0.7 V- This adlattice is attributed to specifically adsorbed anions and intzrpreted as a coadsorbate of sulfate ardons and water, Similarities as well as differences to anion adsorption on Rh(11 l) and Au(l ! l) are discussed. © 1997 Elsevier Science S.A. Keywords: Scanning tunnelling microscopy; Anion adsorption; Sulphate; Pt(l 11)electrode 1. Introduction The specific adsorption of anions on electrode surfaces has been investigated intensively in recent years [1 ]. Con- ventional electrochemical methods and UHV-based ex situ techniques as well as newly developed surface-sensitive in situ techniques such as vibrational spectroscopy, X-ray surface diffraction (XRSD) or scanning probe microscopy were applied for this purpose in numerous studies. Impor- tant questions regarding the effect of adsorbate structures on electrochemical reactivity are related to the adsorption of sulfate and hydrogensulfate anions since sulfuric acid solutions are widely used for electrochemical studies. Strikingly similar structures have been observed with scan- ning tunneling microscopy (STM) for potential-dependent adlayers in sulfuric acid solutions on Au(lll) [2,3], on Pt(lll) [4] and on Rh(ll 1) [5] electrodes as well as on epitaxial monolayers of Cu on Pt(l 11) [6]. For all these cases the observed structure consists of close-packed rows of maxima with distances of V~" times the substrate inter- atomic spacing. These rows enclose an angle of 30 ° with a * Corresponding author. ' Dedicated to Professor Roger Parsons on the occasion of his 70th birthday. 2 Permanentaddress: A.N. FrumkinInstitute of Electrochemistry, Rus- sian Academy of Sciences, Leninski prospect 31, 117071 Moscow, Russia. 0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. HI S0022-0728(96)05051-6 close-packed row of the substrate lattice. The distance between maxima in neighboring rows equals ~ times the substrate interatomic spacing and the respective vector is 19.1 ° off from a close-packed row of the substrate lattice. (2 1)adlattice. The structure can thus be described as a "i 2 A primitive adlattice is sometimes observed, but often an additional feature is located close to the center of the unit cells. This additional feature shows a rather variable ap- pearance and may be absent, hardly visible or have the same corrugation as the comer positions. The actual con- trast seems to depend on the tip condition and on the orientation of the adlattice relative to the tip movement during imaging, The common approach for the interpretation of high- resolution STM images of adsorbates is to attribute ob- served maxima to species, which are contact-adsorbed on the substrate surface. It is well-known that this may in- volve an oversimplification of the imaging process, which is still little understood, but the validity of this empirical approach is in many cases supported by complementary techniques. Based on this empirical approach, the STM images can be regarded as real-space representations of the inner Helmholtz layer of contact-adsorbed species. The similar- ity of published STM images indicates that similar interfa- cial structures may be formed at the (111) faces of Au, Pt and Rh. On the other hand, marked differences between the respective structures were derived from recent ex situ