Chemical Imaging of Terrace-Based Active Sites on Gold - Langmuir

Feb 11, 2004 - Thomas M. Owens, Bonnie J. Ludwig, Daniel R. Fosnacht, Jeffrey M. Bartolin, Natalie Homann, Norman J. Wells, Bradford G. Orr, and Mark ...
0 downloads 0 Views 1MB Size
Download PDF
2250

Langmuir 2004, 20, 2250-2256

Chemical Imaging of Terrace-Based Active Sites on Gold Kevin S. Schneider,† Kenneth T. Nicholson,† Bradford G. Orr,*,‡,§ and Mark M. Banaszak Holl*,†,§ Chemistry Department, University of Michigan, Ann Arbor, Michigan 48109-1055, and Physics Department and the Applied Physics Program, University of Michigan, Ann Arbor, Michigan 48109-1120 Received December 6, 2003 Scanning tunneling microscopy data of a mixed monolayer comprised of a 40:60 ratio of H8Si8O12 and C6H13-H7Si8O12 clusters on gold are presented. The images display a composite monolayer surface with well-defined domain regions of the individual components. Holes present at face-centered cubic (fcc) sites of the starting Au/H7Si8O12 adsorbate layer indicate the location of active sites for impinging C6H13H7Si8O12 clusters. Adsorption of a C6H13-H7Si8O12 cluster likely yields a mobile hydrogen atom available to recombine with and desorb an adjacent H8Si8O12 cluster. Hydrogen atom diffusion along substrate [121] directions is the proposed pattern formation mechanism of the mixed monolayer. Imaging of the spherosiloxane cluster domains identifies a novel terrace-based active site located in the fcc regions of the Au(111) 23 × x3 surface reconstruction.

Introduction Gold has long been described as a noble metal and largely considered unreactive relative to most elements.1 Contributing to its chemically inert reputation are the high dissociation energy and low chemical adsorption energy of molecular hydrogen on gold.2 Indeed, in reviews published in the 1980s, gold is exceptional among the metals for its lack of utility with respect to heterogeneous catalysis.3-5 However, the popular adage coined by George Bernard Shaw stating “the golden rule is that there are no golden rules”6 holds especially true considering that a number of significant results delineating catalytic gold chemistry have been reported in the last two decades. In particular, gold has demonstrated catalytic reactivity with respect to combustion of CO and a variety of hydrocarbons in addition to several hydrogenation reactions.7-15 These reports ascribe gold particle size, gold/support interactions, and gold particle edge effects as critical to the observed catalytic chemistry. Ambient-temperature Si-H bond activation by gold was first observed in 1999.16 In this instance, H8Si8O12 and †

Chemistry Department, University of Michigan. Physics Department, University of Michigan. § The Applied Physics Program, University of Michigan. ‡

(1) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Oxford, 1997. (2) Hammer, B.; Nørskov, J. K. Nature 1995, 376, 238-240. (3) Wachs, I. E. Gold Bull. 1983, 16, 98. (4) Schwank, J. Gold Bull. 1983, 16, 103. (5) Schwank, J. Gold Bull. 1985, 18, 2. (6) Shaw, G. B. Man and Superman: A Comedy and a Philosophy; The University Press: Cambridge, MA, 1903. (7) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 2, 405-408. (8) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301-309. (9) Haruta, M. Catal. Today 1997, 36, 153-166. (10) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 16471650. (11) Ba¨r, T.; de Bocarme´, T. V.; Nieuwenhuys, B. E.; Kruse, N. Catal. Lett. 2001, 74, 127-131. (12) Liu, Z.-P.; Hu, P.; Alavi, A. J. Am. Chem. Soc. 2002, 124, 1477014779. (13) Mohr, C.; Hofmeister, H.; Radnik, J.; Claus, P. J. Am. Chem. Soc. 2003, 125, 1905-1911. (14) Mills, G.; Gordon, M. S.; Metiu, H. J. Chem. Phys. 2003, 118, 4198-4205. (15) Molina, L. M.; Hammer, B. Phys. Rev. Lett. 2003, 90, 206102.

H10Si10O15 spherosiloxane clusters were observed to form chemisorbed layers on the Au(111) surface.17,18 X-ray photoelectron spectroscopy (XPS) and reflection absorption infrared spectroscopy (RAIRS) studies indicated a precursor kinetic model for spherosiloxane cluster chemisorption. Spectroscopic and chemical evidence suggest that approximately 10% of the clusters desorb upon evacuation of excess cluster pressure from the ultrahigh vacuum (UHV) reaction chamber. Cluster desorption forms holes in the remaining chemisorbed layer, which act as reactive sites for subsequent H8Si8O12/H8Si8O12, H8Si8O12/D8Si8O12, H8Si8O12/C6H13-H7Si8O12,17,18 and H8Si8O12/C8H17SiH319 molecular exchange and displacement reactions. Spectroscopic data indicate that silsesquioxane18 and alkylsilane19 molecules actively displace H8Si8O12 up to ∼60% upon exposure to the Au/H7Si8O12 adsorbate layer. In the case of the monosubstituted C6H13-H7Si8O12 clusters, XPS data display a 15% increase in O(1s) core-level peak area indicating that, in addition to undergoing displacement reactions with H8Si8O12, C6H13-H7Si8O12 clusters also “fill in” the available adsorption sites (i.e., the adsorbate layer holes) initially present on the Au/H7Si8O12 surface.18,20 Molecular structures for H8Si8O12 and C6H13-H7Si8O12, the two clusters utilized in this paper, are provided in Figure 1. Schematic structures of H8Si8O12 and C6H13H7Si8O12 chemisorbed to Au(111) are provided as a part of Scheme 1. Submonolayer exposures to ambient atmosphere poison the Au(111) surface to H8Si8O12 chemisorption reactions even though spectroscopic characterization indicates that the substrate is “clean”. This suggests that active sites present on the Au(111) surface may also play a key role in the chemical reactivity with spherosiloxane clusters. Furthermore, these active sites are critical to subsequent cluster exchange reactions. Apparently, am(16) Nicholson, K. T.; Zhang, K. Z.; Banaszak Holl, M. M. J. Am. Chem. Soc. 1999, 121, 3232-3233. (17) Nicholson, K. T.; Zhang, K. Z.; Banaszak Holl, M. M.; McFeely, F. R.; Pernisz, U. C. Langmuir 2000, 16, 8396-8403. (18) Nicholson, K. T.; Zhang, K. Z.; Banaszak Holl, M. M.; McFeely, F. R.; Calzaferri, G.; Pernisz, U. C. Langmuir 2001, 17, 7879-7885. (19) Owens, T. M.; Orr, B. G.; Banaszak Holl, M. M. Unpublished results. (20) Schneider, K. S.; Nicholson, K. T.; Fosnacht, D. R.; Orr, B. G.; Banaszak Holl, M. M. Langmuir 2002, 18, 8116-8122.

10.1021/la0363027 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/11/2004

Terrace-Based Active Sites on Gold

Langmuir, Vol. 20, No. 6, 2004 2251

Scheme 1. H8Si8O12 and C6H13-H7Si8O12 Chemisorbed to Au(111)a

a (A and B) Chemisorption of H8Si8O12 to Au(111). This is a formal oxidative-addition reaction. (C) Formal reductive-elimination of H2 from the Au(111) surface. Reductive-elimination of H8Si8O12 also can occur. (D) Chemisorption of C6H13-H7Si8O12 to Au(111). (E) Reductive-elimination of H8Si8O12. The alkyl chain associated with the H7Si8O12 cluster is not shown attached to a specific vertex because multiple confirmations are possible for the chemisorbed clusters (see ref 18).

Figure 1. Molecular structures of (A) H8Si8O12 and (B) C6H13H7Si8O12 rendered as ball-and-stick models.

bient atmosphere exposure quenches these reactive sites necessary for the initial chemisorption reactions. As advocated in the recent catalytic literature, substrate terrace edges are probable sources of active sites on the Au(111) surface.12-14,21 Density functional theory (DFT) calculations indicate the highest occupied molecular orbitals of gold clusters possess substantial electron density at cluster edges, thereby allowing the greatest molecular orbital overlap with impinging adsorbates.14 Conversely, DFT calculations indicate that a flat gold terrace is the least reactive, or completely unreactive, site for catalytic or molecular chemisorption reactions.12,14 Thus far, active sites on a flat Au(111) terrace have not been experimentally observed or identified. (21) Mills, G.; Gordon, M. S.; Metiu, H. Chem. Phys. Lett. 2002, 359, 493-499.

Scanning tunneling microscopy (STM) data of the Au/ H7Si8O12 surface reveal a discontinuous adsorbate layer containing a complex pattern of holes and channels encompassing approximately 15% of the image areas.20 STM data offer additional surprising results: (1) the Au(111) 23 × x3 surface reconstruction is preserved following formation of the chemisorbed layer, (2) the H8Si8O12 clusters are preferentially bound to bridge and hexagonally close-packed substrate sites, and (3) the Au/ H7Si8O12 adsorbate layer holes are predominantly situated at face-centered cubic (fcc) sites of the underlying Au(111) 23 × x3 surface. The adsorbate layer holes serve as active sites for subsequent molecular exchange reactions and the preferential adsorption behavior of H8Si8O12.20 However, in sharp contrast to recent reports investigating active sites on Au nanoparticles, the Au/H7Si8O12 adsorbate layer holes are primarily located on flat portions of the substrate terraces and not the terrace edges. Because both the H8Si8O12 and the C6H13-H7Si8O12 cluster chemisorptions to gold have been independently characterized by XPS and RAIRS,16-18 these silsesquioxane clusters can serve as contrast agents for the visual identification of the active-site regions on Au(111) 23 × x3 by STM. Use of the nondesorbing C6H13-H7Si8O12 cluster is critical because it provides a STM image that is distinctly different than that obtained for H8Si8O12. Thus, the differing chemical properties of the clusters with respect to the surface reactivity and exchange allow the terrace regions involved in the cluster/cluster exchange reactions to be imaged. This paper addresses the role of Au(111) 23 × x3 terrace-based active sites in H8Si8O12 chemisorption and subsequent molecular exchange reactions. Novel STM data of the H8Si8O12/C6H13-H7Si8O12 mixed monolayer are presented and reveal a composite monolayer surface in which distinct domain regions of H8Si8O12 and C6H13H7Si8O12 are easily identified. Quantitative analysis of the data reveals that the mixed monolayer is comprised of 40% H8Si8O12 and 60% C6H13-H7Si8O12, in excellent agreement with previous spectroscopic characterization.16-18 Qualitative analysis of the STM data indicates that the fcc Au/H7Si8O12 adsorbate layer holes serve as

2252

Langmuir, Vol. 20, No. 6, 2004

Schneider et al.

reactive sites for subsequent molecular displacement reactions and are the likely active sites for the initial H8Si8O12 chemisorption reactions. Experimental Section H8Si8O12 and C6H13-H7Si8O1224,25 clusters were prepared by literature procedures. Cluster purity was checked using 1H NMR, IR, and GC-MS. After loading the individual cluster samples into separate glass or steel UHV-compatible sample containers, they were further purified by sublimation via gentle heating in a water bath at ∼323 K and prolonged (>24 h) exposure to pumping in UHV conditions. Clean Au(111) samples for STM imaging experiments were prepared by annealing a prefabricated sample of Au deposited on mica (Molecular Imaging) in UHV (base pressure 5 × 10-11 Torr). Sample annealing was accomplished by resistively heating to ∼673-773 K a ∼3 mm × 5 mm × 0.3 mm piece of Si(100) cut from a larger wafer (Virginia Semiconductor, p-type, B-doped, resistivity ∼0.003 Ω‚cm) situated directly underneath the mica. In a separate chamber (base pressure 3 × 10-10 Torr), gold (Cerac, Inc., 99.999% vacuum deposition grade, placed inside a coiled tungsten filament) was deposited on the surface at an approximate rate of 1 Å/s (film thickness determined by atomic force microscopy), and the sample was further annealed for at least 1 h. This sample preparation method effectively yields clean Au substrates (i.e., not exposed to air) with terraces ranging in size from a few hundred to several thousand angstroms with numerous screw dislocations. Prior to cluster adsorption, Au samples were imaged in UHV (base pressure 5 × 10-11 Torr) by STM to gauge overall sample cleanliness and confirm the presence of the Au(111) 23 × x3 surface reconstruction. Filled state, constant current images were obtained at room temperature using an RHK Technology, Inc., UHV300 Series variabletemperature STM at a typical sample bias (VS) of -1.0 V and tunneling current (IT) of 0.5-1.5 nA and a 512 × 512 pixel resolution. Low or high pass filtering was employed to attenuate extraneous IT noise. Gold samples for RAIRS experiments were prepared by first evaporating either Cr or Ti for use as an adhesion layer onto an oxidized Si(100) 2 × 1 sample, followed by at least 1000 Å of Au. Immediately prior to use, additional gold was evaporated onto the surface in UHV and the sample purity was assessed by XPS. Only trace amounts of carbon impurities could be detected. RAIRS experiments were performed with a Bio-Rad FTS-40 Fourier transform infrared spectrometer. Complete details of the RAIRS experimental apparatus have been described elsewhere.26 H8Si8O12 clusters were introduced into the UHV chamber via a leak valve at a dosing pressure of ∼2 × 10-7 Torr. C6H13H7Si8O12 clusters were introduced into the UHV chamber via a separate leak valve at a dosing pressure of ∼1 × 10-8 Torr. As a result of the decreased volatility of C6H13-H7Si8O12 (relative to H8Si8O12), the material was heated to ∼323 K in a water bath to obtain an adequate vapor pressure. All chemical reactions were performed at room temperature in UHV. Exposure of hydrogen atoms to the Au/H7Si8O12 adsorbate layer was accomplished by introducing H2 (∼1 × 10-5 Torr) to a heated tungsten filament (∼1673 K) situated ∼8 cm from the sample surface for 5 min. A control experiment utilizing a heated tungsten filament in the absence of H2 produced no observable change in the RAIRS spectrum of the adsorbate layer. 22,23

Results and Discussion Exposure of clean Au(111) 23 × x3 to 486 L (L ) langmuir, 1 × 10-6 Torr‚s) of H8Si8O12 forms a discontinuous adsorbate layer containing a complex pattern of (22) Agaskar, P. A. Inorg. Chem. 1991, 30, 2707-2708. (23) Agaskar, P. A.; Klemperer, W. G. Inorg. Chim. Acta 1995, 229, 355-364. (24) Calzaferri, G.; Imhof, R.; Tornroos, K. W. J. Chem. Soc., Dalton Trans. 1994, 3123-3128. (25) Marcolli, C.; Calzaferri, G. Appl. Organomet. Chem. 1999, 13, 213-226. (26) Greeley, J. N.; Meeuwenberg, L. M.; Banaszak Holl, M. M. J. Am. Chem. Soc. 1998, 120, 7776-7782.

Figure 2. 2000 Å × 2000 Å UHV STM image of the Au/H7Si8O12 adsorbate layer formed from exposure of Au(111) 23 × x3 to 486 L of H8Si8O12 in UHV. The white scale bar in the bottom right corner signifies 200 Å. The inset displays an expanded region of the adsorbate layer hole features and the accompanying cross-sectional profile. The figure has been plane- and slopesubtracted to minimize the vertical step-height differences. High pass filtering was employed to attenuate extraneous noise in IT.

adsorbate layer holes and channels ∼0.7 Å in depth (Figure 2).20 Previous quantitative analyses of the adsorbate layer features confirmed that the Au(111) 23 × x3 surface reconstruction is preserved following chemisorption of H8Si8O12 clusters and the adsorbate layer holes (encompassing ∼15% of the image area) are predominantly situated at fcc regions of the reconstructed surface.20 The fcc adsorbate layer holes indicate the location of active sites for subsequent molecular exchange reactions that provide available binding sites for impinging clusters. Exposure of the Au/H7Si8O12 adsorbate layer to C6H13H7Si8O12 in UHV dramatically alters the surface topography. A large-scale (2200 Å × 2200 Å) STM image of the H8Si8O12/C6H13-H7Si8O12 mixed monolayer formed from exposure of clean Au(111) 23 × x3 to 360 L of H8Si8O12 followed by 190 L of C6H13-H7Si8O12 displays substrate terraces decorated with cylindrical surface protrusions (Figure 3A). A STM image with increased resolution (650 Å × 650 Å) reveals a crosshatching pattern comprised of numerous rhombic and trapezoidal surface protrusions roughly 30-50 in length and width and 0.4 Å in apparent height relative to intersecting and surrounding depressed regions (Figure 3B). Cross-sectional analyses of the surface protrusions reveal a number of similarities to the previously characterized Au/H7Si8O12 adsorbate layer.20 Each surface protrusion contains two elevated stripes (∼0.1-0.2 Å) oriented parallel to one another and separated by roughly 18 Å, with pairs of stripes separated by approximately 40 Å. In addition, a majority of the surface protrusions are aligned in parallel and frequently reorient in unison at (120° angles (denoted by the dashed lines in Figure 3B). These geometric features coincide with the dimensions previously observed in STM images of the Au/H7Si8O12 adsorbate layer in which the underlying Au(111) 23 × x3 surface reconstruction is preserved.20 The large degree of qualitative and quantitative agreement between the surface features in Figure 3B and those of the Au/H7Si8O12 adsorbate layer allows

Terrace-Based Active Sites on Gold

Langmuir, Vol. 20, No. 6, 2004 2253

Figure 3. (A) 2200 Å × 2200 Å and (B) 650 Å × 650 Å UHV STM images of the mixed monolayer formed from exposure of clean Au(111) 23 × x3 to 360 L of H8Si8O12 followed by 190 L of C6H13-H7Si8O12 in UHV. The dashed lines in part B follow the orientation of aligned surface features. The white scale bar in the bottom right corner of each image signifies 100 Å.

assignment of these regions to areas with appreciable H8Si8O12 cluster coverage. Frequently, these regions are bisected at 120° angles by the darkly contrasted 0.4-Å depressed regions to form rhombic and trapezoidal shapes. Direct comparison of the H8Si8O12/C6H13-H7Si8O12 mixed monolayer to STM images of its individual components is further instructive. A high-resolution (350 Å × 350 Å) STM image of clean Au(111) clearly displays the characteristic herringbone pattern intrinsic to the 23 × x3 reconstructed surface (Figure 4A).27-29 Formation of the chemisorbed Au/H7Si8O12 adsorbate layer preserves the Au(111) 23 × x3 surface reconstruction and, consequently, the adsorbate layer displays identical lateral and vertical dimensions associated with the underlying substrate (Figure 4B). However, evacuation of excess H8Si8O12 clusters from the UHV reaction chamber results in ∼15% cluster desorption and formation of ∼0.7-Å-deep adsorbate layer holes predominantly located at fcc substrate positions (denoted by pointing fingers in Figure 4B).20 The adsorbate layer holes vary in size and shape, though they frequently occur in fcc regions opposite one another and spaced 30-50 Å apart. The monolayer comprised solely of C6H13-H7Si8O12 clusters displays a complex pattern of ∼7-Å-wide parallel and spiraling rows with a variable (5-10 Å) interrow spacing and vertical corrugation of ∼0.3 Å (Figure 4C). Unlike the Au/H7Si8O12 adsorbate layer, evacuation of excess C6H13-H7Si8O12 cluster pressure does not cause measurable cluster desorption, suggesting that the image in Figure 4C represents a full-coverage monolayer (lacking the holes observed in the discontinuous Au/H7Si8O12 adsorbate layer).17,18 Spectroscopic characterization of the C6H13-H8Si8O12 monolayer indicates that it is comprised of intact, tightly packed clusters attached via single vertexes with the hexyl chains oriented out of the surface plane (not parallel to the surface).17,18 The C6H13-H7Si8O12 monolayer features in Figure 4C are not well understood and, therefore, preclude definitive structural assignment. A possible interpretation assigns the bright features to aligned chemisorbed C6H13-H7Si8O12 molecules. It is not (27) Van Hove, M. A.; Koestner, R. J.; Stair, P. C.; Bibe´rian, J. P.; Kresmodel, L. L.; Bartosˇ, I.; Somorjai, G. A. Surf. Sci. 1981, 103, 189217. (28) Harten, U.; Lahee, A. M.; Toennies, J. P.; Wo¨ll, C. Phys. Rev. Lett. 1985, 54, 2619-2622. (29) Wo¨ll, C.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. B 1989, 39, 7988-7991.

clear how or to what extent the alkyl chains impart a contrast contribution in the STM images, although they may be responsible for the variable interrow spacing observed in Figure 4C. As a result of the seven available reactive sites per cluster (the Si-H bonds at the cluster vertexes), a number of cluster bonding and rotomer conformations are possible. Variations in cluster bonding inherently orient the hexyl chain in a different conformation and, thus, may account for the variable 5-10-Å interrow spacings evident in Figure 4C. A localized domain of similarly bonded clusters would yield a different interrow spacing than an adjacent localized domain of differently oriented clusters. As previously explained, a STM image of the H8Si8O12/ C6H13-H7Si8O12 mixed monolayer contains numerous surface protrusions with vertical and lateral dimensions equivalent to those observed in images of the Au/H7Si8O12 adsorbate layer (Figure 4D). These regions are attributed to areas of appreciable H8Si8O12 cluster coverage (a representative region is enclosed by the dashed oval in Figure 4D). Resolution of the surrounding depressed regions (∼0.4 Å in depth relative to the surface protrusions) is also apparent in Figure 4D. These regions display numerous parallel and curving features reminiscent of those observed for the C6H13-H7Si8O12 monolayer (a representative region is indicated in the dashed box). The ∼7-Å-wide features also exhibit variable (5-10 Å) interrow spacings. The orientation and dimensions of these features correspond to those observed in images of the C6H13H7Si8O12 monolayer (Figure 4C). In particular, the features denoted in the dashed box very closely resemble the features in the same-sized box in Figure 4C. The large degree of qualitative and quantitative agreement between the features in the depressed regions in Figure 4D to those of the C6H13-H7Si8O12 monolayer allows assignment of these regions to areas with appreciable C6H13-H7Si8O12 cluster coverage. Hence, STM images of the H8Si8O12/ C6H13-H7Si8O12 mixed monolayer are characterized by numerous 30-50 Å2 areas of H8Si8O12 cluster-covered regions 0.4 Å high relative to intersecting and surrounding depressed regions of C6H13-H7Si8O12. Assignment of specific domain regions in the H8Si8O12/C6H13-H7Si8O12 mixed monolayer permits facile quantification of the relative amounts of monolayer features. A 650 Å × 650 Å STM image of the H8Si8O12/C6H13H7Si8O12 mixed monolayer surface is presented in Figure 5A. Figure 5B displays a dual-tone image of Figure 5A

2254

Langmuir, Vol. 20, No. 6, 2004

Schneider et al.

Figure 4. UHV STM images of various surfaces. All images are 350 Å × 350 Å in area with the white scale bar in the bottom right corner of each image signifying 50 Å. (A) Clean reconstructed Au(111) 23 × x3. (B) Discontinuous adsorbate layer formed from exposure of clean Au(111) 23 × x3 to 162 L of H8Si8O12 in UHV. Pointing fingers denote ∼0.7-Å adsorbate layer holes. (C) Monolayer formed from exposure of clean Au(111) 23 × x3 to 9 L of C6H13-H7Si8O12 in UHV. (D) Mixed monolayer formed from exposure of clean Au(111) 23 × x3 to 360 L of H8Si8O12 followed by 190 L of C6H13-H7Si8O12 in UHV. The circled region denotes features similar to those observed in part B, whereas the boxed region denotes features similar to those observed in part C.

with white and black regions corresponding to regions of H8Si8O12 and C6H13-H7Si8O12 cluster coverage, respectively. A normalized height histogram plotting Figure 5B in the form of pixel shade frequency versus apparent height is presented in Figure 5C. Integration of the height histogram yields surface coverage values of the corresponding features in Figure 5B and indicates that the H8Si8O12/C6H13-H7Si8O12 mixed monolayer is comprised of 40% H8Si8O12 clusters and 60% C6H13-H7Si8O12 clusters. These values are consistent with previous spectroscopic characterization, which estimated that ∼60% of the mixed monolayer is comprised of C6H13-H7Si8O12 clusters.18 Thus far, quantitative details of the H8Si8O12/C6H13H7Si8O12 mixed monolayer have been presented; however, potential reasons regarding the size and shape of the monolayer features have not been addressed. Repeating unit cells of the reconstructed Au(111) 23 × x3 surface are aligned with substrate [121] directions to form the characteristic herringbone pattern (Figure 4A).27-29 Formation of the discontinuous Au/H7Si8O12 adsorbate layer preserves the underlying surface reconstruction, and the fcc adsorbate layer holes are consequently aligned with substrate [121] directions. Close inspection of the Au/ H7Si8O12 adsorbate layer reveals a number of holes with 30-50 Å spacing (Figure 4B). Chemisorption of an impinging C6H13-H7Si8O12 cluster via oxidative addition of a cluster Si-H bond at an active site located in an

absorbate layer hole yields a mobile hydrogen atom available to recombine with and reductively eliminate an adjacent H8Si8O12 cluster. Desorption of a H8Si8O12 cluster exposes a new reactive site for subsequent C6H13-H7Si8O12 cluster adsorption, and the process is repeated. The likelihood of hydrogen atoms to induce cluster desorption has been tested by exposing the Au/H7Si8O12 adsorbate layer to an impinging flux of H• radicals. As judged by the dramatic decrease in both the δ(Si-H), υas(Si-O-Si), and υ(Si-H) bands respectively located at 881, 1181, and 2281 cm-1 in the RAIRS spectrum (Figure 6) and the substantial decrease in the O(1s) core level in the XPS spectrum (data not presented), at least 50% of the H8Si8O12 clusters have desorbed from the surface following exposure to hydrogen radicals (Figure 6). Hydrogen atom diffusion paths are postulated to occur along substrate [121] directions, influenced by the threefold rotational symmetry of the Au(111) surface (Figure 7A). As a result, a mobile hydrogen atom likely diffuses in a Au/H7Si8O12 adsorbate layer hole in one of three possible directions: (1) along (parallel with) the fcc adsorbate layer hole in which it originated or along the paths rotated (2) -60° and (3) +60° relative to the position of the adsorbate layer hole. In the case of 1, C6H13H7Si8O12 clusters would chemisorb in the direction of the initial fcc adsorbate layer hole and, hence, run parallel to the H8Si8O12 covered regions (Figure 7D). In the cases of

Terrace-Based Active Sites on Gold

Figure 5. (A) 650 Å × 650 Å UHV STM image of the H8Si8O12/ C6H13-H7Si8O12 mixed monolayer formed from exposure of clean Au(111) 23 × x3 to 360 L of H8Si8O12 followed by 190 L of C6H13-H7Si8O12 in UHV. (B) The same image in part A displayed in dual tone. The white scale bar in the bottom right corner of each image signifies 100 Å. (C) A normalized height histogram of the dual-tone image in part B. The histogram plots image pixel frequency versus pixel height. Integration of the area under the curve yields the areal extent of each domain. The estimated error for this histogram, derived from varying the contrast of the dual tone image until distortions were apparent in comparison to panel A, is (2%.

Figure 6. (A) RAIRS spectrum of the Au/H7Si8O12 adsorbate layer. (B) The chemisorbed adsorbate layer in part A following exposure to an impinging flux of H• radicals. (C) Difference spectrum of A - B clearly displaying loss of spectral features. Transmission values have been offset for clarity.

2 and 3, chemisorbed C6H13-H7Si8O12 clusters would intersect adjacent H8Si8O12-covered regions at 120° angles to form rhombic features (Figure 7E,F). In the instance where hydrogen atoms diffuse in both directions relative to the starting position of the adsorbate layer holes, trapezoidal features would result (Figure 7G-I). The separation of fcc Au/H7Si8O12 adsorbate layer holes (frequently 30-50 Å) would, therefore, determine the 3050 Å size of the resulting H8Si8O12-covered regions in the H8Si8O12/C6H13-H7Si8O12 mixed monolayer. As evident in Figure 7, these potential mechanisms for hydrogen diffusion and cluster adsorption successfully generate the

Langmuir, Vol. 20, No. 6, 2004 2255

rhombic and trapezoidal features observed in Figures 3B, 4D, and 5A. Close inspection of the H8Si8O12/C6H13-H7Si8O12 mixed monolayer also reveals disordered domain regions lacking definite geometric shape and areas corresponding to complete cluster exchange extending beyond substrate fcc locations. The presence of these regions implies hydrogen radicals generated as a result of cluster chemisorption may be limited to localized diffusion radii centered at the points of origin (i.e., in the Au/H7Si8O12 adsorbate holes). A hydrogen atom unable to recombine with a H8Si8O12 cluster within this limited radius likely combines with another mobile hydrogen atom and desorbs as H2.30,31 This hypothesis is further supported by the absence of any observable υ(Au-H) expected at ∼2305 cm-1 in the RAIRS spectrum had gold hydride been formed.32 This reasoning may explain why the exchange process is limited to ∼60% and a monolayer comprised solely of monosubstituted clusters is not realized upon exposure to the Au/ H7Si8O12 adsorbate layer. Consideration of the size and area of the disordered domain regions permits estimation of a 40-50 Å hydrogen atom diffusion radius from an adsorbate layer hole at room temperature. Finally, the preferential adsorption behavior of H8Si8O12 clusters on Au(111) 23 × x3 displays a number of similarities to reports investigating the role of active sites in Au catalysis.7-15 Recently, indium was discovered to preferentially adsorb on the flat outer faces of gold nanoparticles while the edges remain predominantly uncovered.13 The uncovered edges were later identified as active sites for subsequent hydrogenation reactions. In a similar fashion, H8Si8O12 clusters preferentially adsorb on the flat terraces of Au substrates while the active sites remain uncovered. However, in sharp contrast to Au nanoparticles,12-14 terrace-based active sites have been identified on Au(111) 23 × x3. Frequently, catalytic properties of supported gold particles display a marked size effect with particles on the order of ∼3.5 nm, exhibiting maximum reactivity.9,10 Interestingly, Figures 2 and 4B also reveal nanometer-scale active sites on Au(111) 23 × x3 terraces. Conclusions STM data of the H8Si8O12/C6H13-H7Si8O12 mixed monolayer have been presented. The STM images reveal a composite monolayer surface with distinct domain regions of H8Si8O12 and C6H13-H7Si8O12 cluster coverage. Quantitative analysis of the data reveals that the mixed monolayer is comprised of 40% H8Si8O12 and 60% C6H13H7Si8O12, in excellent agreement with previous spectroscopic characterization.16-18 Holes present at fcc sites of the initial Au/H7Si8O12 adsorbate layer serve as active sites for subsequent molecular exchange reactions and mixed monolayer formation. C6H13-H7Si8O12 cluster adsorption yields mobile hydrogen atoms that recombine with and desorb adjacent H8Si8O12 clusters. Hydrogen atom diffusion along substrate [121] directions is the proposed pattern formation mechanism of the mixed H8Si8O12/C6H13-H7Si8O12 monolayer. The mixed cluster monolayer provides a novel chemical imaging agent for the gold surface. Examination of the adsorption pattern following treatment with each type of silsesquioxane cluster allows inferences to be drawn regarding the presence of chemically active sites on the gold terraces. Identification of terrace-based active sites (30) Stobin´ski, L.; Dus´, R. Vacuum 1994, 45, 299-301. (31) Stobin´ski, L. Appl. Surf. Sci. 1996, 103, 503-508. (32) Herzberg, G. Molecular Spectra and Molecular Structure, 2nd ed.; Van Nostrand Reinhold Company: New York, 1950; p 506.

2256

Langmuir, Vol. 20, No. 6, 2004

Schneider et al.

Figure 7. Models of potential pattern generation mechanisms for the H8Si8O12/C6H13-H7Si8O12 mixed monolayer. The models are for schematic purposes only and are not meant to infer or imply a specific cluster packing geometry. (A) Clean Au(111) starting substrate. (B) Cluster monolayer in the presence of excess H8Si8O12 cluster pressure in UHV. (C) Evacuation of excess H8Si8O12 cluster pressure from the UHV reaction chamber produces adsorbate layer holes. (D) Pattern formation as a result of hydrogen atom diffusion along the direction of the initial adsorbate layer holes. Hydrogen atom diffusion along the [121] direction rotated (E) -60° and (F) +60° relative to the initial adsorbate layer hole direction produces rhombic features. (G-I) Hydrogen atom diffusion along both [121] directions relative to the initial adsorbate layer hole produces trapezoidal features.

is surprising in light of the generally inert behavior of gold and recent reports relating the active sites to terraces edges and quantum size effects.7-15 Acknowledgment. RHK Technology, Inc., and the National Science Foundation (DMR-0093641) are grate-

fully acknowledged for support of this work. Prof. G. Calzaferri is gratefully acknowledged for supplying the sample of C6H13-H7Si8O12 used to synthesize the mixed cluster monolayers. LA0363027