Spontaneously Formed Sulfur Adlayers on Gold in Electrolyte

Typical tunneling currents, bias voltages, and scan rates were 10−30 nA, ..... (21) At potentials in the −0.9 to −0.5 V range, there is a slow t...
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J. Phys. Chem. C 2008, 112, 11394–11402

Spontaneously Formed Sulfur Adlayers on Gold in Electrolyte Solutions: Adsorbed Sulfur 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(111) in Na2S aqueous solutions at room temperature, have been studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), surface enhanced Raman spectroscopy (SERS), electrochemistry, and density 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 stoichiometry. In situ and ex situ SERS measurements of S-covered nanostructured gold substrates demonstrate that the surface species present at the gold surface consist of a mixture of chemisorbed S and polysulfide species, as already proposed based on in situ STM images. A DFT surface model that is 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 sulfur rather than as an extended 2D AuS phase. Introduction Sulfur on metals at the sub-monolayer and monolayer levels has attracted 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 technological processes of great economical 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 SO2, 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 selfassembled alkanethiol monolayers (SAMs).7 Sulfur can be regarded as the shortest alkanethiolate 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 interactions 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 exhibit unusual catalytic properties in reactions such as CO oxidation.9 Also, even if S 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 * Corresponding author: C. Vericat; Fax: +54-221-4254642; Phone: +54221-4257430; Email: [email protected] † Centro Atómico Bariloche. ‡ INIFTA.

easily formed on Au (and also on Ag)11,12 by simple immersion in sulfide-containing solutions (S2-, SH-, or SH2 species) or by sample exposure to gaseous S2 or SO2.1,11–13 In all cases, S adsorbs on Au(111) forming a covalent bond and different structures, depending on the surface coverage. The initial step of S adsorption on Au(111) involves S 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 show that S atoms remain adsorbed at step edges.12 Different phases can be found by increasing the S coverage, as described in the literature. In gas phase, a diluted 5 × 5 S surface structure has 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 alkanethiols.7,8 For such coverages, DFT calculations have indicated that the most favorable 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, and 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 surface species. The 0.3 nm average distance (as compared to the 0.22 nm distances typical of bulk polysulfides) has been taken as a clear indication of the role of the Au substrate in S adsorption.12,16 A model consisting of four atoms at hollow positions and four atoms at bridge positions has been proposed.12 However, other configurations could be possible; for similar rectangular structures of Se8 species on Au(111), six hollow and two atop sites have also been suggested.17 In electrolyte solutions, the rect-

10.1021/jp8029055 CCC: $40.75  2008 American Chemical Society Published on Web 07/02/2008

Spontaneously Formed Sulfur Adlayers on Gold angular structures are formed at applied potentials slightly more negative than the reversible potential corresponding to the formation of bulk S.12 Therefore, they have been considered as precursors of bulk sulfur (S in multilayers), which is formed at more positive potentials. In situ surface enhanced Raman spectroscopy (SERS) measurements of gold in sulfide solutions at relatively high positive potentials have shown a S-S stretching band, indicating the presence of adsorbed sulfur or polysulfide species.18 On the theoretical side, some ab initio Hartree-Fock19 and DFT calculations of the S on Au(111) system have been performed,1,14,20 which deal mainly with the energetics of systems for different geometries, charge density, coverage dependency, and phase transitions between the low and high coverage regime, respectively. However, the presence of chemisorbed polymeric S species in the rectangular configuration has recently been questioned.13 In fact, some papers on SO2 adsorption on Au(111) from the gas phase have suggested that the rectangles are not chemisorbed S, but a AuS phase formed by a corrosion process that is accompanied by a strong reconstruction of the Au surface, leading to vacancy islands.13,21 In these works the AuS stoichiometry has been established from a 0.5 monolayer (ML) S coverage estimated from Auger electron spectroscopy (AES) data and a 50% gold island coverage area from STM imaging, after sample annealing to 450 K. Theoretical analysis has suggested that the outer surface layer consists of S atoms bonded to Au1+, Au2+, and Au3+ species. According to the authors, the exposure to sulfur-containing species would dramatically change the chemistry of Au(111), giving a remarkably robust sulfide adlayer with rich coordination chemistry that shows longrange order after annealing.21 Even more, they have suggested that the rectangular structures observed for Au(111) in contact with aqueous sodium sulfide alkaline solutions are also AuS rather than adsorbed polymeric sulfur. Supporting this claim, a recent paper has reported on the surface structural transformations of the 3 × 3 R30° subjected to anodic potential increases in sulfide-free NaOH solutions, that is, at a fixed S coverage.22 In that paper the authors observed by ECSTM the reversible formation of a rhombic phase of S rings at approximately +0.4 V (vs SHE), which they identified with the rectangular S structures already described.12,16 They assigned this phase to a AuxS compound based on pit formation and the height of the rhombic phase domains, although it is well-known that STM images at atomic level are a convolution between topographic and electronic contributions. It is also interesting to note that the imaged rings are formed at potentials about 0.5 V more positive than those reported in refs 12 and 16 for the formation of the rectangular surface structures. Moreover, an atomistic model21 has been developed, which supports the existence of the AuS surface phase proposed in ref 13 by building an incommensurate isolated AuS layer that is then placed on a gold substrate. The model proposed a AuS layer on top of a six-layered Au(111) slab in that unit cell, even if in the paper it was not clearly explained if the model was energetically possible. More recently, an improved model for the ordered incommensurate gold sulfide phase was presented that better reproduces the experimental STM images shown in ref 13 and allows an interpretation of the unusual stoichiometry of the AuS layer.23 Although no conclusive evidence has been presented on the existence of the AuS (i.e., the nature of the rectangular species and the proposed stoichiometry is based only on STM imaging

J. Phys. Chem. C, Vol. 112, No. 30, 2008 11395 and on the AES-calculated coverage13), this is an interesting point that deserves further investigation. In particular, the precise structural and chemical description of the S/Au interface is important because it plays a key role not only in catalysis as mentioned above (either as a poison1 or as an activator9) but also in many fields of nanotechnology, such as molecular electronics.24 Note also that sulfur-induced Au(111) surface reconstruction is a hot topic in the field of thiol self-assembly, where many recent works report contradictory experimental and theoretical data.25 Therefore, elucidation of the S/Au interface is important for the understanding of the complex chemistry of sulfur and sulfur-containing molecules in two-dimensional (2D) systems. In this work we have studied sulfur adlayers spontaneously formed on Au(111) in sulfide-containing aqueous solutions by STM, X-ray photoelectron spectroscopy (XPS), and electrochemical techniques. We have found that, although the S coverage is close to 0.5, as reported in ref 21, the rectangular surface structures cannot be associated with a AuS phase. This conclusion is based on the fact that (a) XPS data do not show any clear evidence of an oxidized gold involving Au1+, Au2+, and/or Au3+, and (b) electrochemical data are inconsistent with the formation of a AuS monolayer. More importantly, for nanostructured gold samples SERS data show an intense S-S stretching band that has been assigned to polysulfide species in the potential region of stability of the rectangular species that should not be present in the case of the AuS phase. We have also performed theoretical calculations using DFT for S/Au(111) adsorption on a 3 × 23 supercell allowing the relaxation of all S atoms and of the Au atoms in the two topmost layers, which lead to arrays of rectangular structures very similar to those experimentally observed by STM. Our data show that only one out of twelve Au atoms from the first layer is lifted, thus precluding the formation of the 2D AuS phase, in agreement with our experimental data. Although some other theoretical models could be considered, we wanted to show that it is possible to propose an energetically feasible model where monomeric and polymeric S species coexist on Au(111) and that this is compatible with STM, SERS, XPS, and electrochemical data. Experimental and Theoretical Methods Experimental Details. Evaporated Au films on glass with (111) preferred orientation (AF 45 Berliner Glass KG, Germany) were used as substrates. After annealing for 3 min with a hydrogen flame, these Au substrates exhibit atomically smooth (111) terraces separated by monatomic steps in height, as revealed by STM.26 Nanostructured gold electrodes for SERS and XPS were prepared with the procedure described in ref 27. The Au foils, 99.99% pure, were prepared in a conventional three-electrode cell containing 0.5 M H2SO4 with a saturated calomel electrode reference (SCE; all potentials in the text are referred to this reference) and a high area platinum foil counter electrode. For the electrochemical roughening, the potential was held at 2.4 V (vs SCE) for 10 min (to form a thick hydrous Au oxide layer), and then a potential sweep was applied from 2.4 to -0.5 V at a rate of 0.02 V s-1 (to electroreduce the thick oxide). As already reported, these surfaces are highly reactive, a fact that should facilitate the mixing between gold and sulfide species. The same Au foils were used in different series of measurements to check for repeatability. Prior to each roughening procedure, the foils were mechanically polished, rinsed several times with MilliQ water, chemically etched during 3 min in aqua regia, and again carefully rinsed with MilliQ water.

11396 J. Phys. Chem. C, Vol. 112, No. 30, 2008 The S layers were formed on the Au(111) substrates by immersion of the clean substrates in deaerated aqueous 3 × 10-3 M sodium sulfide (Na2S) and 0.1 M NaOH solution for 10 min at room temperature.12 Some samples were then characterized by STM in the same electrolyte solution at open circuit potential (ocp). Other samples were removed from the electrolyte, rinsed with water, dried with N2, and either imaged in air by STM or immediately placed in the UHV chamber for XPS analysis. In the case of the nanostructured gold samples used in SERS, XPS, and electrochemical measurements, the S layers were formed by immersion of the freshly prepared substrates in nitrogen-saturated aqueous 3 × 10-3 M sodium sulfide (Na2S) and 0.1 M NaOH solution for 40 min at room temperature.12 We have used an electrochemically formed gold oxide monolayer on Au(111) to test our ability to detect the presence of oxidized Au species. Briefly, a clean planar gold electrode was immersed in a 0.5 M H2SO4 aqueous solution, and the potential was swept between -0.1 and 1.6 V, while the current was recorded.28 The electrode was then held for 1 min at 1.55 V and removed “in vivo” from the cell, rinsed, dried, and quickly transferred to ultra high vacuum (UHV) to be analyzed by XPS. In blank experiments we tested that, after a 1 min polarization, the charge related to gold oxide electroreduction measured by applying to the sample a potential sweep from 1.55 to -0.1 V was 440 µC per cm2, that is, the amount of charge corresponding to a gold oxide monolayer.27 In fact, an ideal Au(111) plane contains 1.39 × 1015 atoms cm-2 and, therefore, the charge required to form an oxide monolayer in a process involving two electrons per gold site is 440 µC cm-2 for this plane.27 We have also tested that the gold oxide monolayer survives in air during the time required for sample transfer into the UHV chamber (approximately 10 min). STM imaging was done in the constant current mode, either in air or in liquid at ocp, with a Nanoscope IIIa microscope from Veeco Instruments (Santa Barbara, CA). Commercial Pt-Ir tips were used, which were insulated with Apiezon wax for STM imaging in the Na2S electrolyte. Typical tunneling currents, bias voltages, and scan rates were 10-30 nA, 10-200 mV, and 15-30 Hz, respectively, for both STM in air and in liquid at ocp. X-ray photoelectron spectroscopy measurements were made with a Mg KR source (1253.6 eV) from XR50, Specs GmbH, and a hemispherical electron energy analyzer from PHOIBOS 100, Specs GmbH. Spectra were acquired with 10 eV pass energy, and a Shirley-type background was subtracted from each region. A two-point calibration of the energy scale was performed using gold cleaned by sputtering (Au 4f7/2, binding energy ) 84.00 eV) and copper (Cu 2p3/2, binding energy ) 933.67 eV) samples. C 1s at 285 eV was used as a charging reference. The spectra were fitted with the XPSPEAK 4.0 software package. A Shirley-type background was substracted from each spectrum. The curves in the S 2p (Au 4f) regions have two components split by 1.19 eV (3.65 eV) with a branching ratio of 0.5 (0.75) that accounts for the spin-orbit splitting. Raman experiments were performed using a Jobin-Yvon T64000 triple spectrometer operating in subtractive mode and equipped with a liquid-N2-cooled charge-coupled device. The excitation was done with the red 647.1 nm line of an Ar-Kr ion laser with 10 mW power concentrated on a 7 mm long and 100 µm wide line focus. The spectra were collected in the 170-530 cm-1 spectral window where S-metal and S-S stretching vibrations are expected to appear,18,29 both on dry

Lustemberg et al. S-modified Au substrates and in an electrochemical cell with the electrode immersed in a deoxygenated aqueous 0.1 M NaOH solution. The latter were initially acquired under ocp conditions and then at -0.75 V (vs SCE reference electrode), for which partial S desorption is anticipated. Blank experiments in 0.1 M NaOH solution of the Au rough substrates prior to the S assembly were also performed. First-Principle Density Functional Calculations. We have carried out DFT calculations within the slab-supercell approach30 by using the ab initio total energy and molecular dynamics program VASP (Vienna ab initio simulation program).30–33 The one-electron Khon-Sham orbitals are developed using a plane-wave basis set, whereas electron-ion interactions are described through the ultrasoft pseudopotentials (USSPs)34,35 obtained by Kresse and Hafner.36 Exchange and correlation (XC) is described within the generalized gradient approximation (GGA) introduced by Perdew and Wang (PW91),37 which performs well for the global energetics of several reactions involving sulfur-containing molecules on Au(111). The sampling of the Brillouin zone is carried out according to the MonkhorstPack method.38 Electron smearing was introduced following the Methfessel-Paxton technique39 with σ ) 0.2 eV, and all the energies are extrapolated to 0 K. The employed cut-off energy is 350 eV, and the calculations are spin restricted except otherwise stated. The theoretical lattice constant obtained for the Au bulk (using a 9 × 9 × 9 k-point mesh) is acalc ) 0.4185 nm, in good agreement with the experimental value, aexp ) 0.4078 nm. We used a three-layer slab to represent the Au(111) surface, using a 3 × 3 × 1 k-point mesh. A vacuum layer 1.4 nm in thickness was placed on top of the slab to ensure negligible interactions between periodic images normal to the surface. No significant relaxation of the interlayer distance with respect to the Au bulk value is observed. It is well-known that Au(111) presents a 22 × 3 reconstructed structure. Nevertheless, because a 0.05 ML sulfur coverage is enough to partially lift the herringbone reconstruction, we assume that this does not play an important role in the intermediate and high coverage S adsorption here considered. The adsorption energy (∆Ead) of sulfur is calculated by eq 1.

∆Ead ) -{(E(S) ⁄ Au(111)) - E(Au(111)) nE(S(atoms in cell))} (1) Spin polarization is used for calculating S in the gas phase, but it is not included in those calculations involving S/Au(111).1 The STM images presented in this work were calculated within the Tersoff-Hamann approach40 using the decomposed charge density obtained with VASP. The energy range of the states that contribute to the tunneling current in the images presented in this work are EFermi ( eVbias, with Vbias in the 10-200 mV range. Results and Discussion Experimental Results. Typical STM images taken after immersion of the Au(111) substrate in the sodium sulfide alkaline aqueous solution are shown in Figure 1. The image in Figure 1a was obtained at ocp in the electrolyte solution, and that in Figure 1b was taken in air after removal of the Au substrate from the sulfide-containing solution. These images show that, irrespective of the environment in which the images were taken (electrolyte or air) and the applied bias voltage (Vbias ) 10-200 mV), the Au(111) terraces are covered by the typical rectangular surface structures already described in the literature as a dense and disordered phase.12,16 In some cases a detailed inspection of the images also shows the presence of monomeric

Spontaneously Formed Sulfur Adlayers on Gold

J. Phys. Chem. C, Vol. 112, No. 30, 2008 11397

Figure 1. High-resolution STM images showing complex rectangular S structures on Au(111). (a) 7.5 × 7.5 nm2 in liquid STM image in 3 × 10-3 M Na2S and 0.1 M NaOH solution at ocp; (b) 7.5 × 7.5 nm2 STM image in air. (c) 3.0 × 3.0 nm2 STM image showing a detail of the rectangular S structures. (d) 24 × 24 nm2 STM image in air showing pits and mounds. The rectangular S structures can be observed.

species. In fact, previous in situ STM studies12 clearly indicate that the rectangular dense phase coexists with monomeric species, leading to a complex mixture on the Au(111) surface. In Figure 1c the rectangular surface structures are shown in more detail, and individual atoms can be clearly resolved. The STM contrast and interatomic distances of the individual atoms in the rectangles change slightly from one atom to another without any regular trend, in contrast to what should be expected for an alternate array of S-Au atoms. Note that, in those papers where the dense phases have been assigned to AuS, no resolution of atoms in the rectangular structures has been obtained.13,22 A larger size in air STM image (Figure 1d) shows the strong reconstruction of the Au surface upon S adsorption, that is, the typical pit and mound topography already described in ref 13 can be observed. It has been found that the pits are monatomic or diatomic in depth, in agreement with previous observations. As in the case of thiol adsorption on Au(111), several different explanations for pit formation have been proposed. In any case, this pit and mound structure is consistent with the adsorbateinduced reconstruction and faceting41 already described for S adsorption on metal surfaces. This point, however, which deserves further investigation, will not be further addressed in the present study. XPS data for the Au(111) samples covered by the dense phases shown in Figure 1 are shown in Figure 2a-b. The broad S 2p signal (Figure 2a) is similar to that described for S on Au by different authors.1,42,43 Two components, C1 and C2, with 2p3/2 binding energies (BE) at 161.1 and 162.1 eV, respectively, can be fitted into the spectra (Figure 2a). In a previous paper, the main component C2 (64% of the area in the spectrum in Figure 2a) was assigned to polymeric S species43 by correlating in situ STM information about the kinetics of the 3 × 3 R30° lattice S polymeric S transformation with the XPS spectrum. This interpretation agrees with that of a previous study42 for S adsorption on Au. On the other hand, C1 (the remaining 36% of the area) was related to monomeric S, that is, chemisorbed S in a 3 × 3 R30° lattice. Occasionally,

Figure 2. High-resolution XPS spectra: (a) S 2p region for a S on Au(111) sample. Components C1 (36%, red) and C2 (64%, blue) are indicated. The S coverage is 0.5. (b) Au 4f region for Au(111) covered by S. (c) Au 4f region for clean, sulfur-free Au(111) (d) Experimental Au 4f spectrum for Au(111) (black), and calculated Au 4f spectra for Au2S monolayer (blue) and Au2S3 monolayer (red). (e) S 2p region for S-covered nanostructured Au. (f) Au 4f region for S-covered nanostructured Au.

another small component (C3) at 163-164 eV has been observed,43 which has been assigned to species weakly bounded to the gold surface (S multilayers). This interpretation of XPS data recorded for S adsorption from electrolyte solutions agrees with conclusions from a previous study for S adsorption on Au from the gas phase.1 A detailed analysis of the oxidation states of Au is crucial to elucidate the chemical composition of the adlayer. The Au 4f signal for S-covered and clean Au(111) substrates are shown in Figure 2, panels b and c, respectively. In both cases the respective 4f7/2 peaks have a BE ) 84.0 eV and a full width at half-maximum (fwhm) of 1 eV. This indicates that only one component is present in our S- covered samples, that is, there is no evidence of oxidized Au from the XPS spectra. From the S 2p (Figure 2a) and the Au 4f (Figure 2b) peaks, the S/Au signal is consistent with a S surface coverage of 0.5, in agreement with the AES data reported in ref 13. Note that, as already mentioned,43 the S adlayer prepared in solution consists of a mixture of monomeric (C1, θ ) 1/3) and polymeric (C2, θ ≈ 2/3) S. Taking this into account and considering the percentage of each S 2p component (36% and 64%), we arrive at a calculated surface coverage of 0.5, in agreement with the measured S coverage. It could be argued that the oxidized gold observed in bulk gold sulfide44 cannot be detected in the samples due to the 2D

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Figure 3. (a) Cyclic voltammogram of Au(111) in 0.5 M H2SO4. The gold oxide formation (I) and reduction (II) peaks can be observed. The blue arrow shows the potential at which the scan was stopped to remove the gold electrode from the solution under electrochemical control. (b) High-resolution XPS Au 4f spectrum for the oxidized sample. Two components (blue and red) are necessary for a good fitting. (c) High resolution XPS O 1s spectrum for (i) clean gold (ii) oxidized gold samples. A new component at 529.5 eV is clearly present in trace ii.

nature of the AuS phase. To check if our XPS instrument is able to detect a 2D layer of oxidized gold, we have electrochemically prepared a gold oxide monolayer by holding the potential for 1 min at 1.55 V (Figure 3a) and removing the electrode in vivo from the cell (see Experimental Section). This electrochemically formed gold oxide monolayer had been first assigned in the literature to a Au(II) oxide,45 although nowadays the formation of Au(III) oxide species is proposed.46,47 The calculated gold oxide reduction charge for a AuO monolayer (considering 2 electrons) or a Au2O3 monolayer (considering three electrons) is the same (440 µC cm-2), so that we cannot infer the stoichoimetry from electrochemistry. In Figure 3b the Au 4f XPS spectrum of the oxidized gold sample shows that, in addition to the usual component at 84.0 eV corresponding to Au0, there is a second component (in red) at higher binding energy (85.6 eV), which is a clear evidence of oxidized Au species. In fact, it has been recently reported that the Au 4f peak for the electrochemically formed gold oxide monolayer can be fitted with two components, at approximately 84 and 86 eV, the latter is assigned to Au(III) oxide species,

Lustemberg et al. possibly Au2O3.47 The presence of oxidized gold was also confirmed by analyzing the O 1s signal for clean gold and the oxidized gold sample (Figure 3c, i and ii). It is evident that a new component appears at 529.5 eV in the case of the oxidized sample that corresponds to oxygen bonded to Au atoms.47 However, it could be possible that, despite our efforts to prepare an oxide monolayer, a gold oxide multilayer would be formed that would give an intense Au 4f signal. Therefore, we have estimated the thickness of the gold oxide layer considering a Au2O3 stoichoimetry on top of the Au surface. From the area ratio of the two Au 4f components (84.0 and 85.6 eV) and using the density of Au2O348 and Au(111), a layer thickness of 0.34 nm was obtained, a figure that is consistent with a gold oxide monolayer on top of the Au surface. To test for possible size effects that could originate the observed Au 4f shift (for example, if gold oxide nanoparticles were present), we have made blank experiments using different thiol-covered surfaces: 3 nm diameter Au nanoparticles; nanostructured gold formed by nanostructures 10-20 nm in size, as revealed by STM;49 and the (111) substrates used in this work. In all cases the Au 4f peak can be fitted with a single component at 84.0 eV with a fwhm of 1 eV, indicating that we cannot attribute the BE shift observed in the gold oxide to any effect other than a change in oxidation state. For thiol-capped gold nanoparticles with diameters