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Atomic Roughness of an Intrinsically Chiral Surface Orientation of an fcc Metal: Cu{531} Marian L. Clegg, Stephen M. Driver,* Maria Blanco-Rey, and David A. King Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom ReceiVed: December 16, 2009; ReVised Manuscript ReceiVed: February 1, 2010
Intrinsically chiral metal surfaces would potentially offer a route toward enantioselective heterogeneous catalysis. A bulk-terminated {531} surface of an fcc metal has an intrinsically chiral unit mesh: it thus appears to be an attractive candidate for fundamental studies of the interaction of such an intrinsically chiral metal surface with chiral adsorbates. Using atomically resolved scanning tunnelling microscopy at 77 K, we show that the real Cu{531} surface departs strongly from ideal bulk termination, exhibiting a high degree of atomic-scale roughness. We examine the impact of the roughness on the low-energy electron diffraction pattern, discuss the origins and nature of the roughness in the context of thermal roughening, and comment upon the implications of the roughness for adsorption of chiral molecules. Introduction A key motivation for studying metal surfaces at the atomic scale is their role in heterogeneous catalysis of chemical reactions. Steps, kinks, and other structural defects are thought to be crucial to the catalytic activity of the small metal particles typically used in real industrial catalysts. A recent development has been an interest in the set of high Miller index surfaces that are obtained by cutting a metal crystal at an orientation that does not preserve any of the mirror symmetry of the bulk structure and which must, thus, in some sense be intrinsically chiral.1-5 It is believed that these surfaces may offer potential in enantioselective heterogeneous catalysis for the pharmaceutical industry or in enantio-discriminating sensors. This is based on the idea that the chiral arrangement of substrate atoms in the unit mesh may offer a more natural bonding configuration to one enantiomer of a chiral adsorbate than to the other. Enantiomeric effects with chiral adsorbates have been reported experimentally on such surfaces,2,3,6-13 and systems for classifying their chirality have been proposed.1-3,14,15 To provide a basis for understanding the interaction of chiral and pro-chiral molecules with chiral surfaces, it is clearly important to have an accurate picture of the initial state of the substrate prior to adsorption. The {531} orientation of fcc metal surfaces has seemed a particularly attractive candidate for fundamental study. The ideal {531} surface, shown schematically in Figure 1, has the smallest unit mesh of the chiral fcc surfaces. It is, thus, the most tractable from the point of view of structural analysis using quantitative low-energy electron diffraction (LEED) and density functional theory (DFT), while nevertheless presenting a variety of possible adsorption sites. It is a rather open surface, with one Cu atom in each of the topmost four atomic layers exposed within the unit mesh. Note that it can be thought of as a kinked-stepped surface vicinal to {111} with extremely narrow {111} terraces (the terrace width is 4.4 Å). Our focus is on the {531} surface of Cu. Experimentally, our key finding is that this surface departs strongly from ideal bulk termination. Atomic-scale roughness has previously been invoked to explain a curious phenomenon in LEED patterns obtained from Pt{531}, that of extinctions of * To whom correspondence should be addressed: E-mail: smd37@ cam.ac.uk.
Figure 1. Schematic of the Cu{531}-S surface. Narrow {111} terraces run down into the surface to the right of kinked steps (marked). A {531} unit mesh is defined by the green trapezoid. The basis vectors a1 and a2 refer to an alternative unit mesh used in the LEED analysis. Scale: |a2| ) 4.43 Å.
diffracted beams over unusually large energy ranges.16,17 Here, we demonstrate using STM that Cu{531} is rough at the atomic scale and examine whether extinctions similarly occur in the LEED patterns. We discuss the origins of the roughness, comparing and contrasting with recently published STM data for the thermal roughening of a structurally related surface, Cu{643},18 and we discuss its implications for the chiral nature of the surface. Experimental Methods The experiments were performed in ultrahigh vacuum (UHV) on the Cu{531}-S surface, its S-chirality1,6 and orientation being established by X-ray Laue diffraction corroborated by detailed analysis of the STM data. Laue diffraction also revealed that the crystal was in fact miscut by 1.5° from {531} toward {110}; the consequences of this are discussed below. The surface was
10.1021/jp9118869 2010 American Chemical Society Published on Web 02/17/2010
Atomic Roughnes of an fcc Metal
Figure 2. STM image (2000 × 2000 Å2, 5 Å greyscale range) of Cu{531} surface, showing rough, undulating morphology.
prepared in UHV by cycles of sputtering (Ar+, 1.5 kV, 10 µA sample drain current, 30 min) and annealing (900-980 K), until sharp LEED patterns that would normally be indicative of good surface ordering were obtained. For quantitative analysis, we recorded a LEED “movie” at 300 K, from which I(V) curves were extracted. The STM images were recorded at 77 K using an Omicron low-temperature STM, operated in constant-current (topographic) mode using electrochemically etched W tips. All images are shown as-recorded after subtraction of a planar background. Results and Discussion Figure 2 shows a typical large-area STM image of the Cu{531} surface. At this magnification on a typical low-index surface, one would expect to see planar terraces separated by steps. Here, we see gentle undulations, which we ascribe to the mechanical polishing stage of the initial sample preparation. From the 1.5° miscut of our crystal and the nominal 0.61 Å layer spacing of Cu{531}, we might anticipate seeing {531} terraces of average width 23 Å separated by steps running horizontally across the image. Higher magnification images, such as that shown in Figure 3, reveal that this is not the case. Terraces of {531} orientation can be seen, in which the STM resolves the Cu atoms in the outermost layer, such that four adjacent “bumps” in a trapezoidal configuration define the corners of a single {531} mesh. The terraces are very small in extent, with no more than a handful of {531} unit meshes exposed before a step up or down to another {531} terrace is encountered, and are typically defected, containing vacancies and adatoms, often arranged in lines or small groups. The surface is evidently rough at the atomic scale. All visible atoms occupy lattice sites, with disorder being manifest in the presence or absence of atoms in these sites rather than in any occupation of nonlattice positions. The surface can thus be thought of as a type of lattice gas, which is frozen at 77 K (no evidence of self-diffusion was seen in successive frames recorded on the same area). Despite an extensive survey, we found no regions of the surface showing more extended {531} terraces, even at the tops of the long-range undulations where one might anticipate that the terraces will be wider than on the more steeply
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Figure 3. Atomically resolved STM image (200 × 200 Å2, 5 Å) of clean Cu{531} (tunnelling parameters: -10 mV, 1.0 nA; surface temperature: 78 K). An annotated version of this image with the key features labeled is available in Supporting Information.
sloping parts. The steps between successive {531} planes are highly irregular. The general trend in their direction varies as the STM tip is moved from place to place on the surface, sampling different parts of the long-range undulations. Thus the average slope in a given image (steepness and direction) typically differs from the global miscut. For this reason, we conclude that the 1.5° miscut does not impact significantly on the surface properties: local variations due to polishing marks are more significant. In view of the atomic roughness of this surface, it is perhaps surprising that sharp LEED patterns are obtained from it. In a quantitative LEED study of Pt{531}, it was found that many of the diffracted beams exhibited negligible intensity over unusually large energy ranges.16,17 None of the (1 × 1)-periodic structural models tried could account for this: in the multiple scattering analysis, peaks were always present in the calculated I(V) curves in these regions. Instead, it was proposed that the extinctions are a consequence of roughness. A quasi-kinematical scattering treatment was used in the analysis, based on the idea that a random distribution of adatom and vacancy meshes imposed (in fixed ratios) on a planar {531} surface can be used to derive “modulation functions” by which the calculated I(V) curves are multiplied prior to comparison with the experimental I(V) curves. Minima in the modulation functions represent destructive interference that causes the extinctions of particular beams over particular energy ranges. A good match with experiment was reported.16 With the aim of further establishing the association between surface roughness and extinctions in the LEED beams, we recorded LEED data for the rough Cu{531} surface. We calculated I(V) curves using structural parameters from a very recently published quantitative LEED and DFT study of Cu{531},19 and calculated modulation functions using the method of ref 16 and a similar three-layer model with 25% coverages of adatom and vacancy meshes. The results, however, are somewhat inconclusive. A comparison of experimental and calculated I(V) curves (Figure 4) does show some indications of extinction regions in which the calculated curve has peaks where the experimental curve is flat, and the modulation function
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Figure 4. Sample experimental (solid red) and calculated (dashed green) LEED I(V) curves for Cu{531}, with modulation functions calculated for 0.61 Å (dotted pink) and 0.55 Å (dot-dashed blue) layer spacings.
has a corresponding minimum. However, the calculated peaks in these regions are relatively weak (prior to multiplication by the modulation function) and are sensitive to the exact parameter values chosen.20 The modulation functions are likewise sensitive to these values: for example, changing the layer spacing from 0.61 Å (the bulk-terminated value) to 0.55 Å (a relaxed value from ref 19) shifts the maxima and minima (by up to 50 eV at higher energies, as shown in Figure 4), with a consequent impact on the extent to which they account for possible extinctions. Thus, while some features of our LEED data may be consistent with extinctions arising from the roughness of the Cu{531} surface revealed by the STM, we cannot conclusively establish such a link. A simple rationale for the roughness of fcc {531} surfaces is provided by a nearest-neighbor (nn) bond-counting argument (a similar argument applies to the open {111} surfaces of bcc metals).21 In a low-index fcc surface, there is an energy penalty in creating an adatom-vacancy pair, because the coordination of an adatom is lower than that of an atom in a terrace. By contrast, in the {531} surface, six nn bonds are broken in creating a vacancy, and six are made in placing an adatom in a lattice site. The reduction in surface enthalpy that drives lowindex surfaces to form extended terraces is thus absent in {531}. The same argument applies to any chiral fcc surface, all of which have kinked steps. This picture of surface bonding is of course simplistic, but one can anticipate that, in a more sophisticated treatment, very similar surface free energies would be obtained
Clegg et al. for the ideal and rough {531} surfaces. The absence of a strong enthalpic driving force toward extended {531} planes is presumably the reason why the undulations due to the mechanical polishing, seen in Figure 2, are preserved even after UHV annealing. It has been suggested (e.g., ref.19) that the propensity to being rough might be less for Cu{531} than for Pt{531}. This is based on Pt and Cu being d-band and sp metals, respectively: because d-band bonding is shorter-ranged and more directional than sp-band bonding, one might argue that nn bondcounting is more valid for Pt than for Cu. For Cu, the influence of third-nearest neighbors, for which an imbalance in the numbers of bonds made and broken disfavors adatom/vacancy pair creation, is thus proportionately greater.22 From the high degree of roughness that we observe for Cu{531}, we infer that any difference in the degree of roughness between Cu{531} and Pt{531} is only marginal. Thermal roughening of surfaces, the spontaneous formation of steps on a flat surface or kinks on a stepped surface occurring above a critical temperature, the “roughening transition temperature”, is a well-established phenomenon (see, for example, the review by Kern23 and references therein). Of particular relevance to Cu{531}, thermal fluctuations specifically of kinked steps on fcc surfaces have been studied from both a theoretical24-26 and an experimental18,27,28 point of view. Kinetic Monte Carlo simulations of kinked-stepped fcc surfaces vicinal to {111} predict step meandering to form extended close-packed step facets between successive {111} terraces. The simulated morphology of a thermally roughened Pt{643} surface25 is in excellent agreement with recent STM images of thermally roughened Cu{643}.18 However, the authors of refs 24 and 25 stress that because the simulations are restricted to “periphery diffusion” (i.e., atoms cannot detach from steps) and neglect step-step interactions, they are not applicable in the limit of very narrow {111} terraces, such as for an ideal {531} surface. Correspondingly, the nature of the roughness seen in our STM images of Cu{531} is qualitatively different from that seen for Cu{643}. To relate the two pictures, we first note that adatoms and vacancies relative to the {531} plane are equivalent to meanders in the kinked steps separating adjacent {111} terraces; the corresponding argument holds true for {643}. Within the small {531} terraces, the kinked steps between successive {111} planes are regular, as shown schematically in 1, that is, these regions are not thermally roughened. We infer that repulsive interactions between the very closely spaced kinked steps evidently oppose step meandering, locally reducing the degree of disorder. The steps that separate adjacent {531} terraces can be seen as correlated meanders in the kinked steps that separate {111} terraces, the correlations again pointing to interactions between the kinked steps. Whereas small {531} terraces do occur on the Cu{531} crystal surface, no identifiable regions of ordered {643} can be seen in the STM images of Cu{643}.18 Evidently, the (average) width of the {111} terraces on {643} is sufficiently large that step-step repulsion does not stabilize the kinked steps to define regular arrays of {643} unit meshes. What are the implications of the atomic roughness of the Cu{531} surface in terms of its chirality? Perhaps the key point is that the initial state of the clean surface is not simply that of a perfectly ordered {531} structure. An incoming molecule is presented not only with intact {531} meshes, but also with a rich variety of defects, and it is not clear a priori which of these will dominate the adsorbate/substrate interaction. Moreover, one must consider the likelihood of adsorbate-induced restructuring of the substrate. A chiral adsorbate may cause metal atoms to reconfigure into a favorable bonding geometry, and this may
Atomic Roughnes of an fcc Metal drive reconstruction or faceting, or give rise to a disordered arrangement of locally modified sites. It has been shown that adsorbing glycine, alanine or lysine on Cu{100} causes step bunching to form small facets of {3,1,17} orientation.29-34 Such adsorbate-induced restructuring is more likely on high-index surfaces than on low-index surfaces, because of the ease of creating and repositioning adatoms and vacancies. Notwithstanding the roughness of the clean Cu{531} and {643} surfaces, clear indications of enantiomeric effects have been reported for alanine adsorbing on Cu{531},12,13 and for propylene oxide, 2-bromobutane, and 3-methylcyclohexanone on Cu{531} and Cu{643}.2,3,7-11 Evidently, in these cases at least, the adsorbate does respond to the overall chirality of the surface, but one cannot assume that the locally chiral bonding configuration around the adsorbate necessarily relates to the ideal {531} or {643} unit mesh. The details are likely to be subtle and would need to be determined on a case-by-case basis. Conclusions We have shown using STM that clean Cu{531} surfaces, prepared using standard methods and imaged at 77 K, deviate strongly from ideal bulk termination, showing instead a high degree of atomic roughness. Although the corresponding LEED patterns show some indications of extinctions of diffracted beams, we have not been able to establish a clear correlation between these and the roughness in the way previously proposed for Pt{531}. It is possible to account for the roughness using a simple nearest-neighbor bond-counting argument, indicating that the creation of adatoms and vacancies is a near-thermoneutral process: there is no strong enthalpic driving force for extended {531} terraces to form. In terms of thermal roughening, the behavior of the {531} surface is qualitatively different from that established for stepped-kinked fcc surfaces having wider terraces, such as Cu{643}; we infer that repulsive step-step interactions stabilize the small areas of perfect {531} ordering that occur. The roughness of the clean surface, coupled with the likelihood of adsorbate-induced restructuring, together imply that one should not think solely in terms of bonding to ideal {531} meshes in predicting the interaction with chiral or prochiral adsorbates. Indeed, it is likely that such high-index surfaces offer ready scope to chiral adsorbates to form “tailormade” bonding sites through local restructuring. Acknowledgment. We thank EPSRC for funding, P. J. Feibelman for stimulating discussions, and the authors of ref 19 for a preprint of their paper. M.B.-R. thanks the EC for a Marie Curie Intra-European Fellowship. Supporting Information Available: Annotated version of Figure 3, with key features labeled. Details of the LEED and modulation function calculations. Full set of figures showing experimental and calculated LEED I(V) curves and modulation functions. This material is available free of charge via the Internet at http://pubs.acs.org.
J. Phys. Chem. C, Vol. 114, No. 9, 2010 4117 References and Notes (1) McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Langmuir 1996, 12, 2483. (2) Attard, G. A.; Ahmadi, A.; Feliu, J.; Rodes, A.; Herrero, E.; Blais, S.; Jerkiewicz, G. J. Phys. Chem. B 1999, 103, 1381. (3) Ahmadi, A.; Attard, G.; Feliu, J.; Rodes, A. Langmuir 1999, 15, 2420. (4) Sholl, D. S. Langmuir 1998, 14, 862. (5) Sholl, D. S.; Gellman, A. J. AIChE J. 2009, 55, 2484. (6) Attard, G. A. J. Phys. Chem. B 2001, 105, 3158. (7) Horvath, J. D.; Gellman, A. J. J. Am. Chem. Soc. 2001, 123, 7953. (8) Horvath, J. D.; Gellman, A. J. J. Am. Chem. Soc. 2002, 124, 2384. (9) Horvath, J. D.; Koritnik, A.; Kamakoti, P.; Sholl, D. S.; Gellman, A. J. J. Am. Chem. Soc. 2004, 126, 14988. (10) Rampulla, D. M.; Francis, A. J.; Knight, K. S.; Gellman, A. J. J. Phys. Chem. B 2006, 110, 10411. (11) Huang, Y.; Gellman, A. J. Catal. Lett. 2008, 125, 177. (12) Scott, N. R. Ph.D. Thesis, University of Cambridge, Cambridge, 2007. (13) Gladys, M. J.; Stevens, A. V.; Scott, N. R.; Jones, G.; Batchelor, D.; Held, G. J. Phys. Chem. C 2007, 111, 8331. (14) Pratt, S. J.; Jenkins, S. J.; King, D. A. Surf. Sci. 2005, 585, L159. (15) Jenkins, S. J.; Pratt, S. J. Surf. Sci. Rep. 2007, 62, 373. (16) Puisto, S. R.; Held, G.; King, D. A. Phys. ReV. Lett. 2005, 95, 036102. (17) Puisto, S. R.; Held, G.; Ranea, V.; Jenkins, S. J.; Mola, E. E.; King, D. A. J. Phys. Chem. B 2005, 109, 22456. (18) Baber, A. E.; Gellman, A. J.; Sholl, D. S.; Sykes, E. C. H. J. Phys. Chem. C 2008, 112, 11086. (19) Jones, G.; Gladys, M. J.; Ottal, J.; Jenkins, S. J.; Held, G. Phys. ReV. B 2009, 79, 165420. (20) The inhomogeneity of the rough surface implies that the relaxations in the layer spacings will not be uniform across the surface: one can expect that the values applicable to a unit mesh within a perfect {531} terrace will differ from those applicable to a unit mesh next to a step or other defect. We believe that the LEED values reported in ref 19 must represent some kind of average of a distribution of real values. (21) Field ion microscope images of W tips prepared by field evaporation reveal a high degree of crystalline order, including {111} terraces, whereas thermally annealed W tips are disordered over most of their area, exhibiting no identifiable {111} terraces.35 For a thermally annealed W{111} surface, King and Wells invoked the existence of substantial disorder in the form of adatoms and/or vacancies to account for measured adsorption kinetics of nitrogen.36 (22) Conversely, one might argue that Pt, because it has a higher cohesive energy than Cu (5.84 eV cf. 3.49 eV37), should be less susceptible to being rough. (23) Kern, K. In The Chemical Physics of Solid Surfaces; King, D. A., Woodruff, D. P., Eds.; Elsevier: New York, 1994; Vol. 7. (24) Sholl, D. S.; Asthagiri, A.; Power, T. D. J. Phys. Chem. B 2001, 105, 4771. (25) Power, T. D.; Asthagiri, A.; Sholl, D. S. Langmuir 2002, 18, 3737. (26) Asthagiri, A.; Feibelman, P. J.; Sholl, D. S. Top. Catal. 2002, 18, 193. (27) Zhao, X.; Perry, S. S. J. Mol. Catal. A 2004, 216, 257. (28) Giesen, M.; Dieluweit, S. J. Mol. Catal. A 2004, 216, 263. (29) Zhao, X.; Gai, Z.; Zhao, R. G.; Yang, W. S.; Sakurai, T. Surf. Sci. 1999, 424. (30) Zhao, X.; Zhao, R. G.; Yang, W. S. Surf. Sci. 1999, 442. (31) Zhao, X.; Wang, H.; Zhao, R. G.; Yang, W. S. Mater. Sci. Eng., C 2001, 16, 41. (32) Zhao, X. J. Am. Chem. Soc. 2000, 122, 12584. (33) Rankin, R. B.; Sholl, D. S. J. Chem. Phys. 2006, 124, 074703. (34) Rankin, R. B.; Sholl, D. S. Langmuir 2006, 22, 8096. (35) Holscher, A. A. Ph.D. Thesis, University of Leiden, Leiden, The Netherlands, 1967. (36) King, D. A.; Wells, M. G. Surf. Sci. 1972, 29, 454. (37) Kittel, C. Introduction to Solid State Physics; Wiley: New York, 2005.
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