Spontaneously Formed Sulfur Adlayers on Gold in Electrolyte Solutions: Adsor or Gold Sulfide? P. G. Lustemberg, † C. Vericat,* ,‡ G. A. Benitez, ‡ M. E. Vela, ‡ N.Tognalli, † A. Fainstein, † M. L. Martiarena, † and R.C.Salvarezza ‡ Centro Ato´mico Bariloche, CNEA,Instituto Balseiro, UNC and CONICET, Bustillo 9500, 8400 Bariloche, RN, Argentina, and Instituto de InVestigaciones Fisicoquímicas Teo´ricas y Aplicadas (INIFTA), UniVersidad Nacional de La Plata, CONICET, Sucursal 4 Casilla de Correo 16, 1900 La Plata, Argentina ReceiVed: NoVember 26, 2007 High coverage S phases (surface coverage g0.33), spontaneously formed by immersion of Au(1 2 S aqueous solutions at room temperature, have been studied by scanning tunneling microscopy ( photoelectron spectroscopy (XPS), surface enhanced Raman spectroscopy (SERS), electrochem functional theory (DFT) calculations. XPS data show no evidence of a AuS phase, as no oxidized gold is detected. Voltammetric data are also inconsistent with the formation of a AuS phase with 0.5 st In situ and ex situ SERS measurements of S-covered nanostructured gold substrates demonstra the surface species present at the gold surface consist of a mixture of chemisorbed S and polysulfid already proposed based on in situ STM images. A DFT surface model thatis energetically feasible and reproduces well the experimental STM images is presented. The proposed model involves only a small rearrangement of the upper Au layer and coexistence of monomeric and polymeric S. Therefore, the high coverage S phase should be described as a mixture of monomeric and polymeric chemisorbed s than as an extended 2D AuS phase. Introduction Sulfur on metals at the sub-monolayer and monolayer levels hasattracted considerable attention in the fields of surface science, catalysis, and nanotechnology. The study of S-metal interactions is of great interest because S is a poison for some heterogeneous reactions involving metallic catalysts in techno- logicalprocesses of greateconomical importance. 1–3 Sulfur layers can be formed on metal surfaces as an undesired result of the adsorption and/or reaction of different compounds, such as SO 2 , disulfides, alkanethiols, thiosulfates, thiocyanates, and sulfides. 4,5 Sulfur adlayers are also of importance concerning the formation of semiconductor films (such as those of CdS or ZnS) by electrochemical atomic layer epitaxy (ECALE). 6 The S on gold system is of interest because gold is the preferred substrate for the preparation of well-ordered self- assembled alkanethiol monolayers (SAMs). 7 Sulfurcan be regardedastheshortestalkanethiolate chain,that is, an alkanethiolate with a number of C atoms equal to 0. 8 Therefore, the study of S adlayers on metals could be a way of discerning between the substrate-molecule and molecule-molecule in- teractions in SAMs on metals, which could help to elucidate the role of the weak intermolecular interactions for the formation of the assembled structures. Moreover, it has recently been reported that gold nanoparticles supported on oxides exhibitunusual catalytic properties in reactions such as CO oxidation. 9 Also, even ifS is usually considered as a poison, recent density functional theory (DFT) data have shown that preadsorbed S on Au(111) could act as an activator for water dissociation. 10 Sulfur monolayers can be easily formed on Au (and also on Ag) 11,12 by simple immersion in sulfide-containing solutions (S 2- , SH - , or SH 2 species) or by sample exposure to gaseous S 2 or SO 2 . 1,11–13 In all cases, S adsorbs on Au(111) forming a covalent bond and different structures, depending on the surface coverage. The initialstep ofS adsorption on Au(111) involvesS chemisorption at step edges. 12,13 In fact, in situ scanning tunneling microscopy (STM) images taken in aqueous 0.1 M NaOH after complete S desorption from the Au terraces sh that S atoms remain adsorbed at step edges. 12 Different phases can be found by increasing the S coverage, as described in literature. In gas phase, a diluted 5 × 5 S surface structure been observed by STM and low energy electron diffraction (LEED). 14 When more S atoms are added, a 3 × 3 R30 ° lattice is formed on the Au(111) terraces, both in solution (at controlled potential) and in the gas phase. 12–14 The estimated interatomic distance measured from STM images is d ) 0.5 nm, and the coverage is θ ) 1/3, as in the case of alkanethio 7,8 For such coverages, DFT calculations have indicated that the mostfavorable sites of the Au surface for S atom adsorption are indeed the hollow fcc sites. 1,15 Addition of more S atoms to the 3 × 3 R30 ° structure leads to the slow formation of denser domains of polymeric S, such as trimers, tetramers octomers, as revealed by STM imaging. 12,16 Rectangles with dimensions 0.62 ( 0.03 × 0.58 ( 0.03 nm and interatomic distances d ≈ 0.3 nm have been assigned to octomeric sur species. The 0.3 nm average distance (as compared to the nm distances typical of bulk polysulfides) has been taken a clear indication of the role of the Au substrate in S adsorpt 12,16 A model consisting of four atoms at hollow positions and fo atoms at bridge positions has been proposed. 12 However, other configurations could be possible; for similar rectangular st tures of Se 8 species on Au(111), six hollow and two atop sites have also been suggested. 17 In electrolyte solutions, the rect- * Corresponding author: C. Vericat; Fax: +54-221-4254642; Phone: +54- 221-4257430; Email: cvericat@inifta.unlp.edu.ar † Centro Atómico Bariloche. ‡ INIFTA. J. Phys.Chem. C 2008, 112, 11394–11402 11394 10.1021/jp8029055 CCC: $40.75  2008 American Chemical Society Published on Web 07/02/2008