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A Scanning Tunneling Microscopy Study of the Interaction of H2S with a Au(111) Surface: Characterization of Corrosion and Monolayer Structures Igor Touzov and Christopher B. Gorman* Box 8204, Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695 Received March 19, 1997. In Final Form: June 4, 1997X Hydrogen sulfide was observed by scanning tunneling microscopy to easily corrode a Au(111) surface, as suggested by an increase in surface roughness upon H2S exposure to the surface. However, this process could be tempered by moderating the concentration of H2S in the vapor above the gold. When corrosion was minimized, corroded regions were observed to exist predominantly at step edges. At a sufficiently low concentration of H2S, linear triple-row structures were observed. These are not the same as those observed when aqueous sulfide was reacted with a gold surface. A model that accounts for all the features of the adlayer structure is presented that involves a packing density of hydrosulfide groups of 28.5 Å2/ molecule, which is lower than that found in a x3 x x3 R30 overlayer (21.5 Å2/molecule) found for long-chain alkanethiolate self-assembled monolayers on Au(111).
Introduction The interaction of sulfur-containing species with metal surfaces continues to be a subject of interest, as it is pertinent in a variety of frequently studied systems. In many cases, atomically well-ordered sulfide and hydrosulfide monolayers have been discerned on metal surfaces. These could be formed from the interaction of gaseous hydrogen sulfide or sulfide ions in solution with the metal surface. Metals in which such monolayers have been found and characterized by scanning probe microscopies include rhodium,1 rhenium,2 copper,3 nickel,4,5 silver,6,7 and gold.8-10 The last metal is of particular interest. Hydrogen sulfide sensors based upon the interaction of H2S with gold have been described,11-13 and some work to understand this interaction at the atomic level has been pursued, although not with gaseous H2S.8-10 In addition, a hydrosulfide monolayer is a potentially useful system in understanding gold/sulfur interactions in self-assembled monolayers (SAMs) of n-alkanethiolates on gold. Recently, much has been learned at the molecular length scale about such * Author to which correspondence should be addressed. Telephone number: (919)-515-4252 (voice). Fax: (919)-515-8920. Email:
[email protected]. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Wong, K. C.; Liu, W.; Saidy, M.; Mitchell, K. A. R. Surf. Sci. 1996, 345, 101-109. (2) Ogletree, D. F.; Hwang, R. Q.; Zeglinski, D. M.; Vazquez-de-Parga, A. L.; Somorjai, G. A.; Salmeron, M. J. Vac. Sci. Technol. B 1991, 9, 886-890. (3) Ruan, L.; Stensgaard, I.; Besenbacher, F.; Laegsgaard, E. Ultramicroscopy 1992, 42-44, 498-504. (4) Huntley, D. R. Surf. Sci. 1990, 240, 24-36. (5) Ruan, L.; Stensgaard, I.; Laegsgaard, E.; Besenbacher, F. Surf. Sci. 1993, 296, 275-282. (6) Heinz, R.; Rabe, J. P. Langmuir 1995, 11, 506-511. (7) Hatchett, D. W.; Gao, X.; Catron, S. W.; White, H. S. J. Phys. Chem. 1996, 100, 331-338. (8) Gao, X.; Zhang, Y.; Weaver, M. J. J. Phys. Chem. 1992, 96, 41564159. (9) McCarley, R. L.; Kim, Y.-T.; Bard, A. J. J. Phys. Chem. 1993, 97, 211-215. (10) Leavitt, A. J.; Beebe, T. P. Surf. Sci. 1994, 314, 23-33. (11) Fruhberger, B.; Grunze, M.; Dwyer, D. J. J. Phys. Chem. 1994, 98, 609-616. (12) Yoo, K. S.; Sorensen, L. W.; Glaunsinger, W. S. J. Vac. Sci. Technol. A 1994, 12, 192-198. (13) In this example, a gold-doped tungsten trioxide film was employed as the gas sensing element: Galipeau, J. D.; Falconer, R. S.; Vetelino, J. F.; Caron, J. J.; Wittman, E. L.; Schweyer, M. G.; Andle, J. C. Sens. Actuators B 1995, 24, 49-53.
S0743-7463(97)00295-3 CCC: $14.00
SAMs, including the range and stability of molecular packing arrangements of alkanethiolates on Au(111), the effect the formation of the monolayer has upon the arrangement of gold at the Au/SAM interface, and the effect the alkane chain length has upon the SAM structure.14-26 A hydrosulfide monolayer is a SAM in which the n-alkane tail is absent. Similarities and differences between its structure and those of SAMs containing n-alkane tails could suggest the relative influence that the gold/sulfur interaction has in determining the structure of quasi-crystalline SAMs with tail groups. In this case, the gold/sulfur interaction and tailgroup packing may have different roles in determining the SAM structure. Molecular adlayers produced from the interaction between H2S and Au(111) have not previously been observed, thus prompting this study. In this paper, scanning tunneling microscopy (STM) experiments are described that illustrate the effects of treating a Au(111) surface with H2S delivered through the gas phase. It is shown that both roughened (presumably corroded) Au(111) surfaces and monolayer structures can be observed in this mannersthe relative proportion of each depends on the H2S dosage. The structure of the monolayer is particularly interesting, as it differs from those of both the sulfide monolayers observed previously (14) Takami, T.; Delamarche, E.; Michel, B.; Gerber, C.; Wolf, H.; Ringsdorf, H. Langmuir 1995, 11, 3876-3881. (15) Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 28692871. (16) Delamarche, E.; Michel, B.; Kang, H.; Gerber, C. Langmuir 1994, 10, 4103-4108. (17) Sprik, M.; Delamarche, E.; Michel, B.; Ro¨thlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116-4130. (18) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071-6082. (19) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611-614. (20) Scho¨nenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259-3271. (21) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826-6834. (22) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853-2856. (23) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383-3386. (24) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 1096610970. (25) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148. (26) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257-13267.
© 1997 American Chemical Society
STM Study of H2S Interaction with Au(111)
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on gold and monolayer structures obtained by dosing H2S on other metal surfaces.1,2,6-10 Experimental Section Gold that had been evaporated onto heated mica was purchased from Molecular Imaging. For this work, hydrogen sulfide was vapor-deposited onto a Au(111) surface that had been peeled from the mica surface. This method was previously found to give a hexagonal decanethiolate SAM on Au(111) with a high degree of perfection over a large (up to hundreds of nanometers) length scale.27 In the case of the surfaces reported here, samples had typical terrace widths of 10-50 nm. To produce a fresh, clean gold surface, gold that had been evaporated onto heated mica was taken, and the metal was peeled off the mica. This was accomplished by slightly but repeatedly bending the sample until the gold de-adhered. The surface originally found at the gold/ mica interface was used (a so-called “template-stripped” surface).28-30 This procedure differs slightly from those previously reported in that the use of solvents in the de-adhesion process was avoided. The details of the procedure by which each sample was prepared are reported below and discussed in light of each of the results obtained. STM measurements were performed using a Digital Instruments Nanoscope III MultiMode microscope in low-current STM mode. Tips were chemically etched tungsten, as described in the documentation of the Nanoscope III manual or cut Pt/Ir from Digital Instruments. STM measurements were carried out in a dry atmosphere of prepurified helium. Each phenomenon reported here was observed on a minimum of three separate occasions using a different sample and tip. The STM was calibrated by imaging a decanethiolate self-assembled monolayer on Au(111) before and after imaging these samples. In this monolayer, intermolecular distances were found to be 5.01 ( 0.05 Å and were clearly distinguishable from the different intermolecular distances reported for adlayer structures here.
Results and Discussion Corrosion. Formation of a monolayer of a substance delivered from the gas phase has the advantage that this procedure can be performed under clean, well-defined, controlled conditions. Gas phase deposition conditions also, however, can result in an insufficient flux of molecules to the surface to ensure complete monolayer coverage. This feature can have advantages in that surface structures resulting from submonolayer coverage can be prepared and studied.25 However, the intention of this work was to prepare a monolayer produced from the interaction of H2S and Au(111) that had complete coverage of a thiol-containing species on the surface. Thus, it was initially sought to prepare a sample under conditions in which a high concentration of H2S was presented to the bare gold surface. To create these conditions, the gold sample (ca. 1 cm2) was placed into a 20 mL syringe. In a fumehood, at ambient temperature, this syringe was filled, emptied, and refilled with pure H2S gas, creating an atmosphere after the second fill consisting of virtually 100% H2S in the syringe.31 After a few seconds, the syringe was opened in a hood and the sample was removed and mounted atop the STM scanner without further delay. All STM was performed under a helium atmosphere. STM (27) Touzov, I.; Gorman, C. B. J. Phys. Chem., in press. (28) Parker, J. L.; Christenson, H. K. J. Chem. Phys. 1988, 88, 80138014. (29) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 3946. (30) Wagner, P.; Hegner, M.; Gu¨ntherodt, H.-J.; Semenza, G. Langmuir 1995, 11, 3867-3875. (31) In this case, some oxygen and/or water may have been present. For surfaces in which adsorbed oxygen is present, one cannot rule out the possibility that this species is involved in the chemistry (e.g., H2Sg + Oad f H2Og + Sad). In later experiments, the helium purge employed should minimize the presence of oxygen and water, avoiding this process. This is an important process on a Ni(110) surface that is more oxophilic than Au(111). (See: Ruan, L.; Stensgaard, I.; Laegsgaard, E.; Besenbacher, F. Surf. Sci. 1993, 296, 275-282. Huntley, D. R. Surf. Sci. 1990, 240, 24-36). We thank a reviewer for pointing out this possibility.
Figure 1. STM image of a Au(111) surface exposed to a ca. 100% H2S atmosphere. Setpoint current ) 7.9 pA, setpoint voltage ) 1.0 V, height mode data.
of this sample (Figure 1) revealed a rough and pitted surface. Such a surface would be expected for substantial reaction of the H2S with several layers of gold atoms at the surface. It is suggested that this image is of gold sulfide salts. In addition to the possibility that the observed pits were due to simple removal or rearrangement of material on the surface, it is possible that the pits observed were the result of imaging less conductive gold sulfide crystallites. No images with atomic resolution were obtained under these conditions. To avoid this severe roughening/corrosion of the surface by H2S, a second set of experiments were performed to reduce the H2S concentration in the vapor. Here, the bare gold sample was mounted in the STM and a cap was placed over the microscope and purged with helium gas for 45 min and sealed. This cap had a ca. 2 L volume. Under these conditions, the bare gold surface could be imaged reproduciblysthere was no visible contamination on the surface. A volume of 20 mL of H2S was injected into this cap, creating an atmosphere of ca. 1% H2S. Again, STM imaging was performed a few minutes after injection of the gas (Figure 2) and also revealed a largely corroded surface, but one in which some apparently molecular scale structures were observable (as indicated by the arrows in Figure 2). Since these molecular scale structures were found adjacent to corroded regions, it was concluded that the molecular density of this adlayer represented a full monolayer coverage. At a minimum, the density of molecules in the adlayer was not limited by insufficient mass transport of H2S to the surface. However, it was also clear that, under these conditions, this structure was a minimal component on the surface. Corrosion processes predominated. The step density observed in Figure 2 was similar to that observed on bare gold. In experiments to further reduce the concentration of H2S in the vapor, the same setup was used as in the second experiment, but only 1 mL of H2S was injected into the system, creating an atmosphere of ca. 0.05% H2S in helium. STM imaging a few minutes after injection of the gas (Figure 3) revealed a structure that still contained some corrosion but also contained a substantial fraction of what appeared to be monolayer structures. Specifically, corrosion was found on small atomically flat gold terraces and at step edges of larger terraces. This observation suggests a mechanism for gold corrosion by H2S that at
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Figure 2. STM image of a Au(111) surface exposed to a ca. 1% H2S atmosphere. Setpoint current ) 2.0 pA, setpoint voltage ) 1.0 V, height mode data. Arrows indicate some of the regions containing ordered adlayer structures.
Figure 3. STM image of a Au(111) surface exposed to a ca. 0.05% H2S atmosphere. Setpoint current ) 5.4 pA, setpoint voltage ) 1.0 V, height mode data.
least initially involves corrosion at step edges. This type of corrosion process can be complicated and involves several mechanistic aspects. Under these experimental conditions, it is not possible to be more definitive about its nature. It is similar in appearance although not necessarily in mechanism to that observed on a Cu(111) surface.3 Monolayer Structures. In addition to corrosion observed in Figure 3, molecular adlayer structures were observed in regions removed from step edges. In this image, the adlayer takes the form of stripes that have a slightly varying appearance on different atomically flat steps of gold. Figure 4 shows a closer view of this structure. Here, different contrasts were observed in the two domains. These different contrasts are more obvious in the three images shown in Figure 5. These three images
Touzov and Gorman
Figure 4. STM image of two domains of an ordered adlayer structure on Au(111). Setpoint current ) 6.3 pA, setpoint voltage ) 1.0 V, height mode data
were obtained as consecutive scans of the same region of the surface at different setpoint currents. Figure 5A (obtained at a setpoint current of 6.9 pA) shows striped structure in which each stripe was composed of rows of three molecules. Notably, in this structure, the positions of the molecules in one of the three rows were observed with substantially better resolution than the positions of the molecules in the other two rows. Figure 5B (obtained at a setpoint current of 2.7 pA) shows an apparent double-layer structure, and Figure 5C (obtained at a setpoint current of 1.3 pA) shows a structure in which only a single row was observed. Empirically, as the current was decreased, the number of rows of molecules resolvable by the STM in the adlayer decreased. All of these adlayers, however, have the same unit cell parameters (see below). Initially, it was speculated that the density of molecules in the adlayer structure might be decreasing with continued scanning. However, upon raising the setpoint current after obtaining the image shown in Figure 5C, the additional molecules in the adlayer were once again observed (e.g., an image indistinguishable from that shown in Figure 5A was obtained). This observation indicates that this contrast difference is at least partially determined by the tunneling conditions and is not due to changes in the adlayer structure. In this case, the contrast difference was not just a function of the tunneling parameters. Differing row structures were observed within a single image (Figure 4) in which the tunneling parameters remained unchanged over the course of acquiring the image. A correlation of the image’s features with tunneling parameters is not always meaningful.32 This is so because the image obtained is governed by interactions between the tip and sample that are more complicated than can be described just by the tunneling parameters. The tip/ sample junction is also influenced by any contaminants on the tip and sample, by the specific shape of the tip and also by the direction of travel of a nonsymmetrically shaped or covered tip with respect to the surface. These factors can change during the course of imaging. However, given that these factors are generally undeterminable, this series of images is presented to indicate the range of possible (32) Hoshino, A.; Isoda, S.; Kurata, H.; Kobayashi, T. J. Appl. Phys. 1994, 76, 4113-4120.
STM Study of H2S Interaction with Au(111)
A
B
C
Figure 5. STM images of the same region of the surface containing the striped adlayer at various setpoint currents: (A) 6.9 pA; (B) 2.7 pA; (C) 1.3 pA. Setpoint voltage of all images ) 1.0 V, height mode data. By varying the setpoint current, each different image could be subsequently obtained from any other.
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structures observable by STM under conditions that are relatively standard and as well-defined as possible. Based on this argument, all of the linear structures obtained in Figures 3-5 are assigned to the same adlayer structure. These molecular adlayers are attributed to hydrosulfide moieties on the surface (e.g., -SHads). This assigment is made on the basis of the X-ray photoelectron spectroscopy data reported by Leavitt and Beebe in which H2S was dosed onto a Au(111) surface and the species on the surface was characterized as a function of substrate temperature.10 These authors concluded that hydrosulfide was the predominant species on the surface between 165 and 520 K. This study did not involve atomic-scale characterization of adlayer structure. These data are most relevant to the assigment made here, as the conditions employed in that work for preparation of the adlayer structure are most similar to those used here. The conditions for both substrate preparation and adlayer formation have a dramatic effect on the structure of the monolayer that is produced. This point has been nicely illustrated for organothiolate SAMs in a recent review by Finklea.33 There is evidence for this in the case of monolayers formed from the interaction of H2S with Au(111) as well. An early LEED study by Kostelitz et al.34 found a complicated pattern for the adlayer. However, the sample was heated (to 300 °C) to obtain this pattern. Both Weaver et al.8 and Bard et al.9 characterized by STM the interaction of sulfide and/or hydrosulfide deposited from aqueous solution onto Au(111), and very different results were obtained in these studies. Specifically, both of these papers reported ringtype structures that involved either dissolution of the gold by the aqueous species or polysulfur ring structures adsorbed onto the gold. The species that interacted with the surface in these studies are arguably quite different from gaseous H2S. Aqueous solutions of H2S resulted only in gold corrosion9sa systematic study of adlayer structure as a function of H2S concentration such as that performed here was not reported. In any event, linear chain structures were not observed under conditions reported here. In addition, Bard et al.9 noted that both of these studies were performed using nanoampere setpoint currents in which tip/surface interactions may have played a role in inducing these structures. In the work reported here, much smaller (picoampere) currents were employed, resulting in substantially reduced tip/surface interactions. The linear chain structures observed here initially appeared to resemble those reported for short chain alkanethiolate SAMs on Au(111).22-25 A direct correlation in structure would have been believablesit is reasonable that hydrosulfide and, for example, butanethiolate would form similar structures on the gold surface. However, although short chain alkanethiolate SAMs have been observed to form a number of row structures, all of these contained a x3 periodicity along the chain (e.g., px3 structures). Measurement of the periodicity along the linear direction observed here revealed an intermolecular spacing of 5.7 Å rather than the 5.0 Å (e.g., x3) periodicity observed in short chain alkanethiolate SAMs. This difference in periodicity was confirmed by imaging a decanethiolate SAM on Au(111) before and after acquiring these images and verifying its 5.0 Å periodicity. This periodicity was observed in all of the linear structures imaged (e.g., Figures 3-5). Thus, the structures observed here are not the same as those observed in short chain alkanethiolate SAMs. Moreover, it is notable that a hexagonal (e.g., x3 × x3 R30) structure also was not (33) Finklea, H. O. In Electroanalytical Chemistry: A Series of Advances; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp 109-335. (34) Kostelitz, M.; Domange, J. L.; Oudar, J. Surf. Sci. 1973, 34, 431-449.
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continue to completion25 or in which thermal annealing (typically to temperatures greater than 100 °C) caused the desorption of some of the molecules from the surface.20 This is not the case here. Conclusions
Figure 6. A model of the adlayer structure that is consistent with all of the features observed in the atomic-resolution STM images.
observed. Beebe10 had originally suggested this structure for a hydrosulfide monolayer based on the basis of calculations by Sellers.35 Figure 6 presents a proposed structure for the hydrosulfide adlayers observed here. This model accounts for both the triple row structure (the three rows of hydrosulfide are nonequivalent in this structure) and the 5.7 Å spacing along the chains that is observed. Overall, this packing can be described by a rectangular unit cell with parameters a ) 5.7 Å, b ) 15.0 Å, and γ ) 90 °. This structure represents a packing motif that has a lower density than that of a x3 × x3 R30 overlayer (28.5 Å2/ molecule versus 21.5 Å2/molecule). However, it does not involve any vacant rows, consistent with the image shown in Figure 5A. Linear structures in which rows of molecules are missing are not uncommon to observe in alkanethiolate SAMs, particularly those composed of short chains in which the formation of the monolayer was not allowed to (35) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389-9401.
A study of the interaction of hydrogen sulfide and gold as the result of gas phase dosing of H2S onto an Au(111) surface has revealed both corrosion and monolayer structures. This type of sample preparation is relevant to the interaction in hydrogen sulfide sensors and offers some indications that gold/sulfur interactions are not the sole phenomena in determining the structure of either long chain or short chain organothiolate SAMs on Au(111). By moderating the concentration of H2S in the vapor above the gold, the relative surface area dominated by corrosion processes (e.g., reaction of more than the top layer or two of gold atoms with the H2S) and by the presence of monolayer structures can be semiquantitated. When corrosion is minimized, corroded regions were observed to exist predominantly at step edges. At a sufficiently low concentration of H2S, linear triple-row structures were observed. These are not the same as those observed when aqueous sulfide-containing species were reacted with a gold surface. A model that accounts for all the features of the adlayer structure was constructed that involves a lower packing density of hydrosulfide groups of 28.5 Å2/molecule, which is lower than that found in a x3 × x3 R30 overlayer (21.5 Å2/molecule). Tail group interactions in organothiolate SAMs are thus concluded to be a factor in maximizing the packing density of the SAM at the surface. Acknowledgment. This work was supported in part by NC State Start-Up Funds, by the U.S. Army Research Office under grant number 34422MS-YIP and by the Air Force Office of Scientific Research MURI Program in Nanoscale Chemistry. We thank the reviewers for helpful comments and references. LA9702953
