Solvent Organization above Metal Surfaces: Ordering of DMSO on Au

Solvent Organization above Metal Surfaces: Ordering of DMSO on Au. Siv K. Si, and Andrew A. ... Publication Date (Web): June 23, 2000. Copyright © 20...
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J. Phys. Chem. B 2000, 104, 10775-10782

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Solvent Organization above Metal Surfaces: Ordering of DMSO on Au† Siv K. Si and Andrew A. Gewirth* Department of Chemistry, and Frederick Seitz Materials Research Laboratory, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801 ReceiVed: February 7, 2000; In Final Form: May 8, 2000

In situ scanning tunneling microscopy (STM) is utilized to image the structures formed by neat dimethyl sulfoxide (DMSO) on Au surfaces at room temperature. The STM reveals that DMSO forms ordered arrays of chains on Au(111) lattices, but is substantively disordered on the other low Miller index faces of Au. The chain structures, which are insensitive to added water, non-oxidizing electrolytes, and DMSO preparation, are interpreted in terms of a head-to-tail association of DMSO with itself and the interchain repulsions also observed are interpreted in terms of dipolar misfit between the chains. Addition of nitric acid to the neat DMSO liquid results in the rapid formation of an entirely different structure resulting from the creation of the hydroxy sulfonium nitrate. While the neat DMSO structure evinces a low and nearly charge independent interfacial capacity, the capacity with added nitric acid is an order of magnitude higher and exhibits considerable structure over the potential range studied. The results from neat DMSO in ambient are compared with previous results obtained in the ultrahigh vacuum environment.

I. Introduction There is considerable interest in the structure of solvents on and near surfaces for many reasons. Many electrochemical phenomena have their origin in the structure of the solid-liquid interface. The presence of a surface leads to solvent behavior different from that of the bulk. Most importantly, there is a growing appreciation that solvent ordering both laterally and normal to the plane plays an important role in informing properties of the electrochemical double layer. Indeed, Grahame invoked an “ice-like” layer in some of his discussion of the application of the Gouy-Chapman model to the Hg/solution interface.1 Before more general questions concerning the structure of the solvent-electrolyte complex can be addressed, it is important to understand the way in which simple solvents interact with metal surfaces on their own. There has been a long-standing interest in understanding the way in which water adsorbs on metals,2-4 but relatively little effort has been expended for other solvents. The interaction of solvents with conductive, typically metallic, surfaces forms the basis for much of electrochemistry. This interaction, when combined with an appropriate electrolyte, leads to the formation of the electrochemical double layer. Adsorption of solvents onto electrode surfaces in electrolyte solutions has long been investigated to elucidate the nature of the solid-liquid interface.5 In this paper, we report the results of an in situ scanning tunneling microscopy (STM) study of dimethyl sulfoxide [(CH3)2SdO; DMSO] adsorbed on Au single-crystal surfaces from neat and mixed solutions under ambient conditions. Like water, DMSO is an important solvent in electrochemistry and its properties at electrode surfaces have long been studied.6-9 DMSO has been the focus of intense study by X-ray and neutron scattering methods because of its self-associative properties in neat solution,10-15 mixed with water,16,17 and mixed with other solvents.18,19 * To whom correspondence should be addressed. Tel: 217-333-8329. Fax: 217-333-2685. E-mail: [email protected]. † Part of the special issue “Thomas Spiro Festschrift”.

There has been an intense effort associated with vibrational spectroscopic characterization of neat DMSO. Early studies of the self-association constant of DMSO by Figueroa et al. were made on the basis of the variation of IR intensities of bands at 1060 and 1000 cm-1.20 Horrocks and Cotton,21 Forel and Tranquille,14 Tranquille et al.,22 and Geiseler and Hanschmann23 have made detailed studies of the vibrational spectra of DMSO and the corresponding normal coordinate analysis. In particular, Forel and Tranquille14 reported infrared and Raman spectra of neat DMSO and proposed the existence of dimer and higher polymers in the liquid phase. Fini and Mirone24 presented the isotropic and anisotropic Raman spectrum of liquid DMSO in the SO stretching vibration region which are explained by the presence of molecular clusters. Sastry and Singh25 reported dilution studies of DMSO in carbon tetrachloride and water. They assign the parallel-polarized Raman spectra in the SO stretching vibration region to monomer DMSO molecules, cyclic dimer, and linear polymer DMSO aggregates. Aqueous solutions of DMSO show an additional peak due to DMSO hydrogen-bonded with water. Finally, Gill et al.26 also found indications for monomer, dimer, and linear associated species of DMSO in solution. The DMSO molecule finds use as an antiinflammatory agent, as a cryoprotectant, and as a free-radical scavenger in cancer treatments.27,28 The structure of the water-DMSO interface has recently been probed because of the importance of the molecule in atmospheric chemistry.16,29 Most recently, Shen and Pemberton examined the structure of the DMSO/Ag interface with a LiBr supporting electrolyte.30,31 From surface-enhanced Raman scattering (SERS) and differential capacity measurements, these authors inferred that DMSO formed an ordered, head-to-tail array on the Ag surface at potentials positive of the potential of zero charge (pzc), but disordered around Li+ and water at negative potentials. UHV-based techniques have been used to examine the behavior of DMSO on Pt(111),32-34 Au(100),35-37 and Au(111).37 On Pt(111), DMSO exhibits a (2 × 2) LEED pattern in the lowest coverage regime and the molecule is thought to associate with this surface primarily through the S atom. On

10.1021/jp000487y CCC: $19.00 © 2000 American Chemical Society Published on Web 06/23/2000

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Au(100) DMSO exhibits two correlated monolayer-related desorption peaks which suggests that it has two different low coverage phases on this surface. On the basis of analysis of X-ray photoelectron spectroscopic (XPS) data, DMSO is thought to associate with gold via interactions from the lone pairs on both the S and O atoms. We recently reported that DMSO forms different structures on the Au(111) and Au(100) faces in the low-temperature UHV environment with the initial sites of DMSO growth on the terraces for Au(100) but at step edges for Au(111).37 This behavior has its origins in the differing charge density present on the terraces of the different faces. For the Au(100) face, the reconstructed surface contains a high density of electropositive sites at the reconstruction sites while these electropositive sites are not available on the Au(111) terraces. Hence, the DMSO molecules nucleate at step edges on Au(111) because the steps are the only appropriately electropositive features on the Au(111) surface. One question raised by studies of solvents in the UHV environment is their relevance to the actual behavior of the solvent in a condensed phase. Answers to this question require structure determinations in both environments. Most solvents are too weakly adsorbed and too fluxional to allow imaging by probe microscopic methods and many times lack the heavy atoms usually required for convenient X-ray scattering analysis. In this paper, we report the results of in situ STM studies of DMSO adsorbed on Au surfaces from neat DMSO solution. The structures we see reflect interactions between DMSO molecules as well as those between DMSO and the Au surface. II. Experimental Section The STM images were obtained with a Nanoscope III from Digital Instruments (Santa Barbara, CA) which was calibrated by imaging a highly ordered pyrolytic graphite (HOPG) surface in air. A homemade Kel-F cell held the solutions during the STM measurements which were obtained at room temperature (22 ( 1 °C in our laboratory). The STM tip was a mechanically cut Pt/Ir wire (Digital Instruments) coated with polyethylene to minimize the faradaic background current. STM images were obtained in the height mode and are unfiltered unless noted otherwise. The Au(111) surface was obtained as Au films deposited on borosilicate glass (Metallhandel Schro¨er GmbH, Lienen, Germany) and flame annealed before use.38 Au(100) and (110) single crystals (Monocrystals, Inc.) were annealed for three minutes in a hydrogen flame prior to use and quenched in ultrapure water. The orientation of these crystals was confirmed with Laue´ backscattering. DMSO (99.9% Certified ACS) and nitric acid (69.95% Certified ACS Plus) were from Fisher and used as received except where noted. Some solutions were made with ultrapure water (Milli-Q UV plus, Millipore Inc., 18.2 MΩ cm). Capacitance-potential curves were measured by using a two channel lock-in amplifier (Stanford Research System) coupled with a potentiostat in a range of potentials within the doublelayer region, as determined from a separate experiment. The working electrode was a Au(111) single-crystal mounted in a hanging drop orientation in the electrochemical cell. The counter electrode was a Au foil. The reference electrode was a Ag/ AgCl cell joined to the main cell by a Luggin capillary with the liquid junction made via a frit. The modulation signal was 5 mV in amplitude at 10 Hz. III. Results 3.1. Au(111)/DMSO. Figure 1a shows a STM image of the bare, as prepared, Au(111) surface in air. The surface is

Figure 1. STM images of DMSO on Au(111) (a) Bare Au(111) surface imaged in air. Vbias ) 96 mV, Itip ) 2.6 nA. (b) In situ STM image of DMSO on Au(111) following immersion in neat DMSO for 7 h. Vbias ) 54 mV, Itip ) 2.5 nA.

heterogeneous with several steps and terraces. Zooming in on one of the flat terraces, the surface (figure not shown) exhibits the hexagonal structure and interatomic spacing of 0.29 ( 0.02 nm associated with the bare Au(111) lattice. The reconstructed Au(111) surface has also been imaged in ambient from these samples. Addition of 0.2-0.3 mL of neat DMSO to the Au(111) surface maintained in ambient conditions initially yielded images little different from those obtained in air as shown in Figure 1a. Atomic resolution could still be obtained from the Au(111) surface. The step edges are sharper, clearer, and more distinct in DMSO than in air. If the surface was maintained covered with DMSO for several hours, however, an entirely different set of images was obtained. Figure 1b shows the Au(111) surface imaged in situ some 7 h after immersion into DMSO. The figure shows a series of ordered island structures/patches surrounded by regions of disorder. The islands evince a parallel stripe pattern covering the Au(111) surface. The island patches are oriented 60° or 120° with respect to each other, which reflects the symmetry of the Au(111) substrate. Over the step edges (Figure 2a) the stripes align perpendicular to the Au(111) step edges. A close-up look at the patches reveals three different kinds of structures. The first and most common structure with the patches is shown in Figure 2b. This image shows a pattern consisting of

Solvent Organization above Metal Surfaces

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Figure 2. (a) 50 nm × 50 nm in situ STM image of DMSO on Au(111) showing the relationship of the DMSO chains to the Au Vbias ) 200 mV, Itip ) 2.7 nA. (b) 15 nm × 15 nm in situ STM image obtained from neat DMSO showing the dominant chain on Au(111). Vbias ) 54 mV, Itip ) 2.5 nA (c) 25 nm × 25 nm in situ STM image showing a variation of the chain motif Vbias Itip ) 2.4 nA (d) 15 nm × 15 nm in situ STM image showing a square structure occasionally observed on Au(111) Vbias ) 57 2.4 nA.

alternating bright stripe rows and dark stripe spaces. The bright parallel stripes are evenly spaced 1.45 ( 0.05 nm apart, the same as for the dark parallel spaces. The distance from one bright row through the dark space to another bright row is ca. 1.0 nm. Inside the bright stripe region are a rhombically arranged series of spots. The spot-spot distance is 0.48 ( 0.02 nm and the spots form a rhombus with an angle of 55° and 125° as drawn in the figure. The upper right part of the figure shows a second domain with the same structure but oriented 120° from the first. Figure 2c shows another structure slightly different from the dominant stripe structure in Figure 2b. There are still alternating bright and dark spaces. The bright spots also form a rhombus and have the same spacing as the bright row spots in Figure

step edges. motif seen ) 96 mV, mV, Itip )

2b. The main difference between Figure 2b and c is the appearance of spots in the dark spaces in Figure 2c. The spots in the dark rows are evenly spaced 0.48 ( 0.02 nm apart along the row. Figure 2d shows the third type of structure observed within the patches. The image reveals a square pattern with a 0.48 ( 0.02 nm distance between protrusions which are displaced at an angle close to 90°. This square pattern is observed occasionally in all measurements performed but the stripe structure is the dominant motif observed. Lattice formation times were uniformly about 6 h. The lattice formed with or without prior immersion of the STM tip into the DMSO in the STM cell. We saw no changes in the structure of the patches whether we used DMSO as received or DMSO

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Figure 3. STM image obtained from neat DMSO on Au(100) Vbias ) 404 mV, Itip ) 2.4 nA.

that was further prepared by vacuum distillation on a Schlenk line. We also saw identical structures in measurements performed with a 1:1 (v/v) DMSO:water mixture. However, in this case the ordered stripes took much longer (>12 h) to form. Finally, we added 10 mM CsClO4 to the DMSO to examine the effect of this salt on the DMSO/Au interaction and found that identical structures were formed with about the same time required for their formation. Once the ordered patches form, they are stable for more than 20 h (the duration of the experiment). 3.2. Other Surfaces. Figure 3 shows an in situ STM image of a Au(100) surface covered with DMSO. The image shows what appear to be pairs of spots, one of which is marked with an arrow, exhibiting weak order on the Au(100) surface. The rows formed by these pairs of spots are roughly coincident with the 〈011〉 direction of the underlying Au(100) lattice. However, no long range ordered structure is observed. The spots are of varying sizes and the spacing from one spot to the next is close to 1.0 nm. The image suggests that the DMSO molecules try to order up but are weakly bound to the surface. Measurements on Au(110) (figure not shown) have also been undertaken. The resulting images evince no long range order and reveal spots which are even more random and disordered than they are on the Au(100) surface. We also attempted measurements on highly oriented pyrolytic graphite (HOPG) and saw no evidence of ordered structures save for the HOPG lattice itself on this surface. 3.3. Au(111)/DMSO with Added 8 M Nitric Acid. To perturb the DMSO structures shown in Figure 2, we examined structures formed on Au(111) following addition of 8 M HNO3 to the neat DMSO in the STM cell. Figure 4a and b show STM images obtained when about 0.1 mL of 8 M nitric acid is added to the neat Au(111)/DMSO interface. The images obtained are completely different from those without added HNO3. Many lamella-like/cigar-like shaped structures appear in the images. The formation of the lamella may look random but a closer inspection reveals that many of the lamella are oriented 60° or 120° with respect to each other. The length of the lamella vary from a long of ≈150 nm to a more typical length of ≈50 nm or shorter. The width varies anywhere from 4 to 10 nm. A closeup look at the lamella reveals a well-ordered structure (Figure 4c). The rows in the lamella are perpendicular to the lamella long axis. The parallel rows in the lamella are spaced 0.51 ( 0.02 nm apart and spots along the row are space 0.35 ( 0.02

Figure 4. (a) 250 nm × 250 nm (b) 100 nm × 100 nm and (c) 17 nm × 17 nm in situ STM images obtained from neat DMSO with added nitric acid Vbias ) 44 mV, Itip ) 2.7 nA.

nm apart. The lamella like/cigar-like shaped structure form instantaneously once the 8 M nitric acid is added to the Au(111)/DMSO sample. The lamella structures disappear after about 2 h.

Solvent Organization above Metal Surfaces

Figure 5. Capacitance-potential curve for the DMSO/Au(111) interface with (a) 10 mM CsClO4 supporting electrolyte and (b) with 30 mL of DMSO diliuted with 12 mL of 8 M HNO3.

Measurements made following addition of other acids including HClO4 and H2SO4 revealed only the structures formed in neat DMSO. 3.4. Differential Capacitance Measurements. Figure 5 shows the differential capacity curves obtained in the doublelayer region for Au(111) in DMSO with 10 mM CsClO4 (Figure 5a) and with 8 M nitric acid (Figure 5b). The two systems display rather different behaviors. The C(E) curve for 10 mM CsClO4 shows a minimum, which correspond to the potential of zero charge (pzc). The differential capacitance is also very small for this particular system. The C(E) curve for the 8 M HNO3 shows the opposite behavior. It has a broad and asymmetric maximal capacitance peak instead of minimum. The actual value of the capacity is well over an order of magnitude greater than that seen for CsClO4. IV. Discussion The images presented above show that DMSO can form an ordered overlayer over time on the Au(111) surface. However, this property appears unique to Au(111) as results from other substrates did not reveal any significant ordering. On Au(100) there were vestiges of what could be the beginning of an ordered array, but the images revealed substantially less order than on Au(111). On Au(110) and on the atomically flat surface of HOPG, STM images indicate that DMSO does not adsorb to form an ordered layer. In what follows we present our understanding of the origin of the structures found on the Au-

J. Phys. Chem. B, Vol. 104, No. 46, 2000 10779 (111) surface and the reason for the preference for ordering only on Au(111). We also describe the origin of the new structure obtained with addition of HNO3 to the DMSO and conclude with a discussion of the differential capacity. 4.1. Structure of the DMSO Adlayer. One of the most interesting features of the images presented in Figure 1 is that the DMSO molecule appears to form paired rows or chains on the Au(111) surface. The chains reflect the underlying symmetry of the Au surface in that they extend along the x3 direction of the unit cell. This speaks to what must be a significant interaction between the DMSO molecule and the Au(111) surface. We look to the properties of DMSO to understand the paired row structure. The relatively high boiling point (189 °C), large heat of vaporization (57 kJ/mol at 189 °C),39 results of vibrational spectroscopic studies,20,40 cryoscopic measurements,41 and other data of DMSO indicate that the molecules are highly associated both in bulk solution and in other environments.29 It has been suggested that the association is likely to be in a form of chainlike polymeric structure oriented by dipole-dipole interactions.27,42 Recent results of molecular dynamics computer simulation concluded that dipole-dipole interactions contribute significantly to the formation of local order in liquid DMSO, but the DMSO molecules associate in a head-to-tail fashion, rather than in a chain.17,43 Dielectric measurements by Kaatz et al.44 using the dipole orientation correlation factor obtained from the static permittivity reflects some kind of antiparallel ordering of dipole molecules. On the basis of X-ray and neutron diffraction experiments, Bertagnolli et al.,11 proposed a geometrical model of the DMSO cluster. In their model the cluster consisted of chain of DMSO molecules with parallel dipole moments, while the neighboring DMSO molecules from the adjacent chains were oriented with antiparallel dipole moments. Calculations examining the structure of solvents with dipole moments suggest that in the absence of other interactions the antiparallel orientation of solvent molecules dipoles with the dipoles confined to the interfacial plane is lowest in energy relative to other possible ensemble orientations.45 On the basis of this physical insight and the distances and orientations obtained in the STM measurements we propose in Figure 6 a structural model for neat DMSO on the Au(111) surface. From the STM angle and distance measurements, DMSO adsorbs on the Au(111) surface in a commensurate fashion along the x3 direction. The rows are arranged in groups of two DMSO molecules together and in our model the DMSO molecules are oriented with their dipole moments arranged in an antiparallel fashion due to dipolar coupling. Two DMSO molecules are required for each chain because each spot is of the correct size to accommodate only one DMSO molecule. This insight is consistent with that provided by Kaatze et al. who concluded from dielectric measurements that the molecular dipoles were ordered in an antiparallel fashion.44 The DMSO structure on the Au(111) surface features collinear dark rows in addition to the chains which show up as bright spots on the surface. The dark rows have almost the same width as the bright rows, but of course do not feature any internal structure. There are several possible origins of these dark rows. First, the dark rows could be regions where there are no molecules, the spaces between the bright rows. Second, the dark spaces could be regions where there were molecules, but these molecules were disordered or highly fluxional so that they could not be imaged by STM. Third, the dark spaces could be regions where molecules actually did associate with the surface, but due to some electronic effect are not visible to STM. Finally,

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Figure 6. Proposed structural models for DMSO on Au(111) (a) Stripe pattern with a (x3 × 2x7)R19.1° unit cell. (b) Square structure with a (x3×x15)R26.6° unit cell.

the dark spaces could be filled with another molecular species, not visible to STM. Insight into the correct choice of these four possibilities comes from a detailed examination of the images shown in Figure 2. In particular, the image in Figure 2b shows that the missing space is the correct size to be filled by an additional DMSO molecule. The DMSO molecules visible to STM clearly interact with the Au(111) surface to yield a structure which is commensurate with the Au lattice. The missing rows likely result from repulsion between the chains. The origin of this repulsion is not steric in origin because there is sufficient space for an additional DMSO molecule. The DMSO could also associate with the Au(111) lattice in the same manner as the DMSO in the chains in the empty spaces, so incomensurativity between the DMSO lattice and the Au surface is also not the origin of the missing molecules. We propose that the origin of the interchain repulsion is dipolar misfit. The chains arise from DMSO molecules coupled in an antiparallel fashion. In order for the chains to be close packed, the rows of DMSO molecules must be oriented in a consecutive antiparallel fashion, up, down, up, down, etc. If the chains are arranged such that two rows with DMSO molecules oriented such that their dipoles point in the same direction are close to each other, then the energy required to bring the rows close together is too high and the rows repel each other. When there is no dipolar misfit we get the occasional square structure shown in Figure 2d and schematically illustrated

Si and Gewirth in Figure 6b. Figure 2c is an immediate structure between the rows/spaces structure and square structure. Faint spots appear in the dark spaces in Figure 2c. The images here are different from the now well-known (x3 × x7) structure of water coadsorbed with sulfate which has been observed on a number of metallic surfaces by in situ STM46-49 in that there are no subsidiary spots that could be easily associated with another adsorbate. However, the experience of many in situ measurements involving coadsorption, especially during the underpotential deposition process, is that coadsorbates are not always easily imaged.50 The image shown in Figure 2d apparently arises as a consequence of filling in the row structure to form a full monolayer of DMSO on the Au(111) surface. The rows are still oriented along the x3 direction of the Au(111) surface. In this case, the dipolar mismatch hypothesized as the origin of the rows shown in Figure 2b must be removed and the molecules are now allowed to close pack on the surface. The slightly different structure of the full monolayer relative to the row structure may arise from these differences in packing forces between the molecules. Any discussion of DMSO structure must include a discussion of the possible role of water, since DMSO is very hygroscopic and it is virtually impossible to guarantee that all of the water has been removed from the DMSO. The measurements here were performed in ambient, and we anticipate that even if the DMSO was free of water at the outset of the experiment, substantial water was incorporated from the atmosphere after several hours of exposure. Solutions made incorporating water yielded structures identical with those obtained from neat solutions. However, the time scale of assembly was considerably slower when water was deliberately added. This suggests that the delay in assembly might be related to the time required to remove water from the interface. Preliminary interrogation of the neat DMSO/Au(111) interface with polarization modulation FTIR spectroscopy did not reveal any of the bands expected from water. Shen and Pemberton suggested that water was important only at potentials negative of the pzc.30,31 This suggests that water is not important in the structures we see. DMSO is thought to partition at the surface of the air/DMSOwater interface, as well.29 4.2. Substrate Effects. The ordered arrays described and analyzed above were found on the Au(111) surface but not on Au(100) or Au(110). DMSO is not the only molecule to order up only on a hexagonally symmetric plane. For example, studies of water adsorption on metal surfaces in UHV reveal that adsorbed water forms an ordered (x3×x3)R30° superlattice of hydrogen bonded clusters known as the “bilayer” on hexagonally symmetric fcc(111) and hcp(0001) planes. No ordered structure is formed on the more open fcc(100) and fcc(110) planes.2 The ordered DMSO seen on the Au(111) surface probably arises as a consequence of the better fit of this molecule with the hexagonal lattice which helps to promote chain formation along the x3 direction. The melting point of DMSO is 18.55 °C, which is only three or four degrees colder than the temperature at which these measurements were performed. The interaction with the Au(111) substrate can then be thought of as possibly assisting in the crystallization of the molecule. The interaction with the other faces is weaker. There is a large body of work on the adsorption behavior of long-chain alkanes and alkyl derivatives on the basal plane of graphite.51,52 The long-chain alkanes and derivatives adsorb from nonpolar solutions to form what are suggested to be closepacked monolayers on graphite. This work is similar in that

Solvent Organization above Metal Surfaces there is exchange from the solution above into an ordered layer near or on the surface. The packing of the alkanes reflects interactions between the alkanes and their association with the surface. The structures reported to date are always close packed in some way reflecting the dominant role of van der Waals contacts in structure determination. The alkane structures never evince the missing-row pattern described for DMSO on Au(111) which suggests that a different packing mechanism is available for DMSO relative to the alkanes. There also has been relatively little work examining alkane structure as a function of different substrates or preparation conditions. In this work, it is clear that the substrate has a strong influence on the molecular order of adsorbed layers. 4.3 Comparison with UHV Results. It is interesting to compare the structures formed by neat DMSO in ambient with those seen in UHV in a previous study.37 In UHV we found that DMSO exhibited order only at step edges for typical monolayer coverages of the molecule on Au(111). There was no order evident on the terraces, even for very low coverages. Alternatively, we found that DMSO did order on the terraces of the Au(100) surface, a result we attributed to the greater corrugation height of the reconstructed Au(100) surface relative to Au(111) leading to more electropositive sites on the Au(100) terrace. The results obtained in ambient appear at odds with the UHV results, because in ambient order was only seen on Au(111) and this order was on the terraces not at the step edges. There are two major differences in conditions between the UHV measurements and those reported here. First, the concentration of DMSO above the surface is considerably greater in the ambient measurements relative to the UHV environment. Second, the ambient measurements were performed with the DMSO at a temperature near 300 K, while the UHV measurements were carried out at 100 K or lower. The lower temperature in UHV will certainly restrict the mobility of DMSO molecules on the surface, which may hinder their ability to order. It is possible with appropriate annealing in UHV that DMSO could be made to order into the more associated structures found in ambient. The increased concentration under ambient conditions likely plays a more important role in forming the structures we observed here. That areas of disorder are interspersed with areas of order suggests that there is a critical nucleation event involved in chain formation. This nucleation event, and the maintenance of the chain structure, will likely be enhanced with a higher concentration of DMSO available. It may also be the case that the exchange between bulk and surface is a necessary condition for these ambient structures to form. Such conditions are not available in UHV. Finally, we found in UHV on Au(100) that increasing the exposure of the surface to DMSO led to the formation of lamellae with an unresolved local structure. This effect observed on increasing the concentration in UHV may mimic the ambient situation on Au(100). 4.4. Changes on the Addition of Nitric Acid. The addition of nitric acid changes the structure of the Au(111)/DMSO surface species in two ways. First, we observed the formation of a series of cigar-shaped structures. Second, the row structure so evident in the neat DMSO case was disrupted and a rectangular array 3.5 by 5 Å is formed. The addition of nitric acid changes the chemical composition of the species in solution and on the surface. Sulfoxides are weak bases and exist mainly as sulfonium salts in the presence of strong acids, such as nitric acid. DMSO reacts immediately with any excess nitric acid to form the hydroxy sulfonium nitrate.53 The failure of nitric acid oxidation to proceed beyond the sulfoxide stage, even in the

J. Phys. Chem. B, Vol. 104, No. 46, 2000 10781 presence of a large excess of oxidizing agent, is probably due to the formation of a sulfonium salt which is expected to be resistant to oxidation.53-56 The STM images in Figure 3 show how the sulfonium salt disrupts the dipole-dipole coupling of DMSO leading to a linear structure. While the internal resolution into the lamella formed is modest, the images suggest that the sulfonium salt structure is considerably different from that presented by DMSO. The different stability exhibited by the sulfonium salt (it disappears within a few hours of its formation) suggests that unlike the neat DMSO structure the salt structure is sensitive to water which may not partition away from the interface with the same facility as in the case of neat DMSO. 4.5. Differential Capacity. Our C(E) curve for the Au(111) electrode in DMSO with 10 mM CsClO4 shows a rather low differential capacitance throughout the double-layer region compared to the DMSO with nitric acid system. The CsClO4 curves are qualitatively similar in both the low magnitude and near charge independence to those reported previously using LiClO4 as the supporting electrolyte.7,9 The Pt/DMSO interface also had a similarly low and nearly charge independent capacity.57 The low and almost field-independent differential capacity at the Au(111) electrode in DMSO suggests that the electric field across the interface may not be able to reorient the solvent molecules.58 Such a situation would occur in the case of strong metal-solvent interactions. Strong interaction between Au and DMSO may also responsible for the influence of the crystallographic orientation of single-crystal Au electrodes on pzc values in DMSO solutions of LiClO4. The pzc is most negative for Au(111), more positive for Au(100), Au(110) and most positive for Au(210),9 contrary to the behavior in aqueous solution. It was concluded9 that the interactions between Au and DMSO are stronger and more dependent on the atomic structure of the Au surfaces than those between Au and water. Our observation of order at the DMSO/Au(111) interface is consistent with this understanding. Finally, we note that the very low capacity we observe is consistent with an interface where water is absent. Addition of nitric acid increases the capacity for several reasons. First, addition of water to the system increases the dielectric constant. The capacity we observed with nitric acid is similar to that reported for Au in strictly aqueous systems.59 Second, the nitric acid alters the strong association of DMSO with the Au surface and leads to the formation of what appears to be a salt at the interface. Without the strong molecular association, the capacity is no longer nearly charge independent, but exhibits considerable structure. 5. Conclusions We have shown that DMSO can form an ordered layer near the Au(111) surface under ambient conditions from neat solution. The structure of this layer reflects the properties of the DMSO molecule in that the DMSO appears to form chains along which there is a head-to-tail order and between which is an antiparallel organization. Our observations are consistent with inferences made using X-ray scattering methods for bulk solution, but speak as well to the unique role of the Au(111) lattice in ordering this solvent where a good match between this lattice and the DMSO molecule facilitates condensation. Our observations at the Au(111)/DMSO interface are consistent with those inferred for bulk solution and emphasize the associative properties of DMSO. With appropriate choice of electrolyte,30 it may now be possible, by means of in situ STM, to examine the interplay of solvent, anion, and cation at the solid/liquid interface

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