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Organic Adsorbate Induced Surface Reconstruction: p-Aminobenzoic Acid on Cu{110} Q. Chen,* D. J. Frankel, and N. V. Richardson School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, United Kingdom Received June 11, 2001. In Final Form: September 17, 2001 Large-scale facets, induced by annealing p-aminobenzoate on a Cu{110} surface, are observed in STM. The bunching of {110} steps aligned along the 〈112〉 directions with a separation of 12.75 Å forms the {11 13 1} facets. STM images show that there are two rows of molecules on each {110} terrace of the facet. The faceting mechanism and the relationship to the adsorbate structure are discussed. This large-scale modification of the surface morphology confirms that the adsorption of organic molecules on metal surfaces may cause a significant amount of mass transport in the process of two-dimensional adsorbate crystallization.
Introduction The chemical modification of heterogeneous catalysts plays a vital role in the chemical and pharmaceutical industries. It has been proposed that organic modifiers could influence the bare catalytic metal particles in several ways: by either forming templates which define the selectivity1 and/or reconstructing the substrate and varying the reactivity.2 Recently, there have been several examples of organic adsorbates which not only form wellordered commensurate superstructures on metal surfaces but can also modify the morphology of an atomically flat surface into highly faceted domains. For example, on Cu{001} surfaces, the {3 1 17} facet is found with the adsorption of glycine,3 alanine,4 and lysine,5 while on the Cu{110} surface, the 〈11 13 1〉 facet is found with the adsorption of formic acid,6 acetic acid,7 and benzoic acid.8 Significantly, all these molecules contain a carboxylate, which is the focus of bonding to the metal surfaces. Here, we present an example of large-scale facets formed by the adsorption of p-aminobenzoic acid on Cu{110}. Our previous study9 has shown that on Cu{110} surfaces, as a function of annealing temperature, p-aminobenzoic acid forms (3 × 4)g, (25 -24), (16 -52), and (14 -32) periodicities (R, β, γ, and δ phases) in which all the molecules are flat-lying. TPD and HREELS suggest a dehydrogenation process between the ortho-CH and the amino group. STM images reveal the formation of dimers, as a result of the dehydrogenation by annealing at 464 K. During this dehydrogenation-induced phase transition process, the molecules can also strongly interact with the Cu atoms on the step edges in such a way that the 3D * Corresponding author. Fax: (+44) 1334-467285. E-mail: qc@ st-andrews.ac.uk. (1) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature, 2000, 404, 376. (2) Jensen, F.; Besenbacher, F.; Læsgaard, E.; Stensgaard, I. Phys. Rev. B 1990, 41, 10233. (3) Zhao, X. Y.; Gai, Z.; Zhao, R. G.; Yang, W. S.; Sakurai, T. Surf. Sci. 1999, 424, L347-L351. (4) Zhao, X. Y.; Zhao, R. G.; Yang, W. S. Surf. Sci. 1999, 442, L995L1000. (5) Zhao, X. Y.; Zhao, R. G.; Yang, W. S. Langmuir 2000, 16, 98129818. (6) Leibsle, F. M.; Haq, S.; Frederick, B. G.; Bowker, M.; Richardson, N. V. Surf. Sci. 1995, 343, L1175-1181. (7) Haq, S.; Leibsle, F. M. Surf. Sci. 1996, 355, L345-L349. (8) Frederick, B. G.; Leibsle, F. M.; Haq, S.; Richardson, N. V. Surf. Rev. Lett. 1996, 3, 1523-1546. (9) Chen, Q.; Frankel, D. J.; Richardson, N. V. J. Chem. Phys., revised.
structure of the step edges is determined by the intermolecular interaction. High-resolution STM images reveal the details of the molecular species on the facets. The relative position of individual molecules indicates the underlying faceted substrate structure, which enables us to determine the orientation of the facets accurately. Comparison with the facets formed with other similar molecules suggests the driving force for the facet formation. Experimental Section The experiments were carried out in an UHV system equipped with LEED/AES (Omicron), a quadrupole mass spectrometer (Hiden), and vt-STM (Omicron). The Cu{110} crystal was cleaned by standard Ar+ bombardment (typically, 500 eV, 20 µA cm-2 ) and annealing (850 K) procedures until a clean surface was obtained, characterized by sharp (1 × 1) LEED patterns and large flat terraces in STM. The tunneling tip is made by dc electrochemically etching the polycrystalline W wire (diameter 0.38 mm) in a saturated KOH solution. p-Aminobenzoic acid was dosed onto the surface by vacuum deposition. The finely divided material was pumped overnight on a gas line, attached to the chamber via a gate valve. The doser consists of a glass tube with heating wire and thermocouple sensor, so the dosing temperature is well controlled and the reproducibility is ensured. Since the pumping speed for this chemical is very slow, overheating of the chemicals during dosing must be avoided and it is necessary to isolate the sample once the overlayer is formed. Dosing was carried out at room temperature at a pressure of ca. 1 × 10-9 mbar. Phase transitions and faceting were monitored in situ with LEED optics. All STM images were taken at room temperature in a constant current mode.
Results and Discussion Molecular-induced faceting implies large-scale, threedimensional mass transport of the substrate atoms across terraces and steps. Here, we demonstrate details of the size and orientation of the facets thermally induced, in the presence of adsorbate, p-aminobenzoic acid, adjacent to the {110} terraces of copper. The STM image in Figure 1 shows a large region (1000 nm × 1000 nm) formed by annealing at 540 K. All step edges are aligned along the well-defined directions, which are formed with small triangular features. These features, typical adsorbateinduced facets, have never been found on a clean Cu(110) surface. An estimate of the facet area suggests as much as 18% of the total surface, while all the steps are faceted, that is, the surface is completely faceted. Of course, the
10.1021/la0108712 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/20/2001
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Figure 1. Large-scale STM image (1000 nm × 1000 nm, bias ) 0.05 V, tunneling current ) 1.0 nA) showing that all the step edges are faceted.
size of the molecular adsorbate induced facet is limited by the initial clean substrate morphology. The appearance of so much faceting indicates that large-scale mass transport is involved in the annealing and adsorbate ordering process. Strong, attractive interadsorbate and adsorbate/substrate interactions with relatively weak interactions between substrate atoms favor the formation
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of large-scale facets. The crystalline orientation of microfacets resulting from the surface mass transport is an attempt to achieve the lowest surface energy in the presence of the adsorbate molecules and as such can be anticipated to depend closely on the adsorbate 2D structure. Figure 2 shows an enlarged faceted area with molecular resolution, in which the triangular step edges (Figure 1) are clearly resolved. Here, we want to emphasize the difficulties in studying highly faceted surfaces by STM. When the tunneling tip is close to the bottom of the facets, the side of the tip can also contribute to the tunneling current if the tip is not fine enough, which may blur the image. An extremely fine tip was achieved, in this case, by a slow etching process with a dc current as low as 10 mA. From Figure 2, it is clear that each step in this facet is initiated and terminated by 〈112〉 oriented steps on the {110} terraces. Also, two adjacent, reflection correlated facets meet at vectors tilting slightly in/out of the (11 h 0) plane. There are two types of STM features on the facet. Each has a dimension close to the “footprint” of the flatlying molecule; however, one is brighter than the other. We assign each feature to a flat-lying molecule consistent with our temperature-dependent EELS measurement.9 Both molecules are aligned with their longer axis aligned along the [100] direction. The upper third of the image also shows large but also stepped {110} terraces with an ordered (14 -32) structure formed by the dehydrogenated p-aminobenzoate dimers.9 Here, the two-dimensionally ordered structures are described in matrix
Figure 2. High-resolution STM image (50 nm × 50 nm, bias ) -1.14 V, tunneling current ) 6.1 nA) showing the molecular resolution of the faceted structure. The upper {110} terraces have the ordered (14 -32) structure.
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Figure 4. Model of the clean fcc {11 13 1} facet without adsorbate. Figure 3. 3D view of the relative positions of the {11 13 1} facets and their included vectors. i notation: (ab cd )(j) for the real space structures on the facecentered cubic (fcc) {110} surface. The translation vectors of the substrate unit cell, i and j, together with the outward surface normal, form a right-handed coordinate system with i and j directed along [11 h 0] and [001], respectively. These terraces have an elongated shape with a dimension of ∼30 Å × ∼120 Å, in which the longer dimension is aligned along the [11h 0] azimuth. Here, the step edge is defined by the molecular unit and terraces consist of an integer number of molecular units with the periodicity maintained across the step edge in the (1, 2) and (4, 3) directions. By use of this known overlayer structure, the drift of the piezo scanner can be accurately calibrated,10 which is essential for determining the orientation of the facets. The details of the (14 -32) structure have been discussed in an earlier paper.9 Looking at the correlation between the ordered (14 -32) structure and the facet at the boundary of the two, it is found that each row of the bright feature along the 〈112〉 directions arises from a single atomic step on the terrace. Thus, the height of each row on the facet is equal to the space between steps of Cu{110} surface, 1.275 Å, and the tilting angle of the vector on the facets into the (11 h 0) plane can be precisely measured as 3.36°. Therefore, the vectors at which two adjacent facets coalesce can be defined as the [1 1 24] and, equivalently, [1 h1 h 24] vectors, shown in Figure 3. All adjacent steps are merged together at these two vectors. Meanwhile, each facet contains either the [1h 1 2] or [1 1 h 2] vectors. Both the 〈1 1 24〉 and 〈1 1 2〉 vectors have been indicated in Figure 3 with exaggerated tilting angles. Combined with the [1 h 1 2] and [1 1 h 2] vectors, which are also included in the facets since they are the direction of the step edges, the crystalline index of these facets can be determined as (11 13 1), (11 13 1h ), (11 13 1h ), and (13 11 1). Figure 3 shows the details of the relationships between the different vectors and the facets on a
(10) Barrett, S. D. Image SXM, Σ5 ed.; IRC Surface Science: Liverpool, U.K., 1994. Available from the Internet by anonymous ftp from ssci.liv.ac.uk.
Figure 5. (A) Small area (200 Å × 150 Å) of the facet. (B) Further enlarged area (28 Å × 28 Å) with labeled vectors between adjacent molecules. The image has been flattened with the Image SXM software (ref 10).
three-dimensional frame. Of course, each of these facets is chiral, while the reflection correlation between pairs results in a racemic mixture of facets. The {11 13 1} facet consists of narrow {110} terraces. Each terrace has six Cu atoms along the 〈001〉 direction with an effective width of 12.75 Å ()5 × 2.55 Å), while along the [1 h 1 2] direction the {110} terrace has an unlimited dimension. Therefore, at the apex of the facet along [1 1 24], the step size is 21.7 Å ()6 × 3.61 Å). In
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Figure 6. The suggested overlayer structure on the {11 13 1} facets with substrate structure.
Figure 4, a constructed {11 13 1} facet model is shown to demonstrate the details of the arrangement of Cu atoms on this facet. Each {110} terrace coalesces with its neighbor {110} terraces along one of the 〈1 1 2〉 directions, while adjacent facets merge at the [1 1 24] direction. The well-ordered overlayer structure on the facet and indeed the drive to form this facet reflect the registration of the adsorbate on the substrate. By measurement of the angle and length of each unit cell vector from the calibrated image, shown in Figure 5a,b, the periodicity of the superstructure on the facet can be determined. The shorter vector can be easily identified as [1 1 h 2] with a doubled substrate lattice constant, which is equivalent to the (2, 2 h ) vector on the (110) terrace, labeled as a. The second vector, b, is aligned along 63° with respect to the [11h 0] azimuth with a length of 14 Å. This vector crosses one atomic step. The projection of this vector onto a (110) plane can be denoted as (2.5, 3.5) in matrix notation, or (-1.25 1.25 3.5) in bulk notation. In matrix notation, referred to 2 the {110} plane, the unit cell is (-2 2.5 3.5). This is similar to the matrix notation we used before. The half integral index is due to the crossing of a single step and the nature of the fcc crystal. Thus, the vector is formed with 2.5 times the Cu lattice along the [11 h 0] direction, 3.5 times the Cu lattice along the [001] direction, and one Cu lattice along the [110] direction (surface normal). Hence, the vector b can also be referred to as (2.5, 3.5, 1) with the 1 indicating the crossing of an atomic step. The vector between the adjacent two molecules could either cross one atomic step c or be on a single {110} terrace, d. The c vector is aligned at 79° with respect to the [11 h 0] azimuth with a length of 9.0 Å. The projection of this vector on the (110) terrace is
very close to (0.5, 2.5), which has an angle of 82° with respect to the [11h 0] direction with a length of 9.2 Å. If all the molecules are adsorbed on the same adsorption site, for example, the short bridge site for the carboxylate group, the vector (0.5, 2.5) must cross a single atomic step, which gives rise to the half order unit cell vector. Thus, the vector c should be described as [0.5, 2.5, 1]. The second vector, labeled as d, between the adjacent molecules can be determined as the (2, 1) vector, which does not cross an atomic step and can be described as [2, 1, 0]. This vector is aligned at 35° with respect to the [11 h 0] azimuth with a length of 6.2 Å. Because only the vectors b and c cross a single atomic step, judging from the tilting direction of the facet, it can be easily suggested that the less bright feature lies at the top of a step with the brighter feature at the bottom of the step. On the Cu(110) surface, the {11 13 1} facet is also found with the adsorption of benzoic acid.8 Although there is no bunching of {110} terraces to form the {11 13 1} facet, the adsorption of formic acid6 and acetic acid7 also modifies the step edges of the substrate to align along the [1 h 12] and [11 h 2] directions, which are part of the {11 13 1} facet. The formation of facets is due to the strong attractive chemical interaction between the substrate Cu atoms and the functional groups of the adsorbates. Annealing of the surface to a temperature above 400 K (in this case, the sample is annealed at 540 K) is necessary to allow sufficient movement of the substrate Cu atoms. However, for both formate and acetate molecules, the surface species is not sufficiently thermally stable which would limit the scale of the facet. What is interesting here is that a unique facet is formed on the {110} surface with the adsorption
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of molecules containing carboxylic acid. Thus, it strongly suggests that the other functional group, in this case, the amino group, has no effect on determining which crystalline facets are formed. On the other hand, the determination of the particular facets is independent of the molecular geometry; for example, when the {11 13 1} facet is formed both benzoic acid8 and p-aminobenzoic acid, here, are flat-lying on the surface, while formate11-13 and acetate7,14 are perpendicular to the surface. If the facets for all these molecules are formed with the same mechanism, one would expect that an ionic bond between the Cu atom and the carboxylate group is responsible, which would not depend on whether the molecule is upright or flat-lying to the surface. On the other hand, the amino group attached to the phenyl ring in the p-aminobenzoate may have little effect to influence the faceting. From the STM image in Figure 5, it is clear that all the molecules are aligned with their longer axis along the 〈001〉 azimuth. Thus, all the carboxylate groups are aligned along the 〈110〉 azimuth. Assuming that the carboxylate group is bound on the short bridge site,15-19 we propose a real space model, in Figure 6, for the faceted {11 13 1} plane. The relative position of individual molecules has been measured based on the image in Figure 5. The vectors a, b, c, and d, corresponding to (2, 2, 0), (2.5, 3.5, 1), (0.5, 2.5, 1), and (2, 1, 0), are also marked. As we have discussed before, vectors a and d are on a single (110) terrace, while b and c cross one atomic step. In this model, to position the molecule on the lower terrace of the edge, a Cu atom on the upper terrace has to be removed, which forms a reconstructed 〈1 1 2〉 vector with alternatively missing Cu atoms. There are two types of flat-lying molecules; one appears to be a brighter feature in the STM. The difference (11) Haq, S.; Leibsle, F. M. Surf. Sci. 1997, 375, 81-90. (12) Bowker, M.; Rowbotham, E.; Leibsle, F. M.; Haq, S. Surf. Sci. 1996, 349, 97-110. (13) Bowker, M.; Poulston, S.; Bennett, R. A.; Stone, P.; Jones, A. H.; Haq, S.; Hollins, P. J. Mol. Catal. A: Chem. 1998, 131, 185-197. (14) Hasselstrom, J.; Karis, O.; Weinelt, M.; Wassdahl, N.; Nilsson, A.; Nyberg, M.; Pettersson, L. G. M.; Samant, M. G.; Stohr, J. Surf. Sci. 1998, 407, 221-236. (15) Woodruff, D. P.; McConville, C. F.; Kilcoyne, A. L. D.; Lindner, T.; Somers, J.; Surman, M.; Paolucci, G.; Bradshaw, A. M. Surf. Sci. 1988, 201, 228. (16) Leibsle, F. M. Surf. Sci. 1994, 311, 45-52. (17) Caputi, L. S.; Chiarello, G.; Lancellotti, M. G.; Rizzi, G. A.; Sambi, M.; Granozzi, G. Surf. Sci. Lett. 1993, 291, 756. (18) Frederick, B. G.; Chen, Q.; Leibsle, F. M.; Dhesi, S.; Lee, M. B.; Kitching, K. J.; Richardson, N. V. Surf. Sci. 1997, 394, 1. (19) Baumann, P.; Bonzel, H. P.; Pirug, G.; Werner, J. Chem. Phys. Lett. 1996, 260, 215.
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in the corrugation reflects the difference in the adsorption environment. One of the molecules (labeled as the brighter feature A) is adsorbed at the top of the step, while the other one (labeled as the less bright feature B) is adsorbed at the bottom of the step. Along the 〈001〉 direction, the A type molecules are assumed to have their carboxylate group bonding toward the 〈112〉 edge of the (110) terrace at which they sit. The alignment of the carboxylate group along the 〈112〉 direction with a 2× periodicity resembles part of the c(2 × 2) structure. We suggest that in this geometry, the substrate-mediated dipole interactions between the adjacent carboxylate groups become strongly attractive. Thus, the high-temperature annealing of the surface with the adsorbate containing the carboxylate group can routinely form well-defined step edges along the 〈112〉 direction.6-8 Large-scale calculations are required for further investigation of the intermolecular interaction and the mechanism of the faceting. For the B type molecule, either the carboxylate or the amino group could bond toward the (lower) step edge. Here, in Figure 6, we assume that the carboxylate group is preferentially bonded to the step edge. The structure formed with a pair of molecules on the step edge is very similar to that of the Cucarboxylate complex20,21 with a syn-syn bidentate bond holding the four Cu atoms. We suggest that this structure is responsible for the formation of the 〈1 1 24〉 vector of the {11 13 1} facet, in other words, the bunching of the steps, which is characteristic for molecules containing carboxylate groups. Summary Large-scale facets induced by the annealing of the Cu(110) surface with p-aminobenzoic acid are studied with high-resolution STM images which reveal the details of the molecular arrangement on the facets. The measurements on the calibrated STM images suggest that the facets contain both the 〈1 1 2〉 and 〈1 1 24〉 vectors which defines a {11 13 1} facet. Reflection related facets coalesce along the [1 1 24] or [1 h1 h 24] vectors. On the basis of the measurement of the molecular features, a model and the molecular mechanism for the formation of the facets are proposed. LA0108712 (20) Mounts, R. D.; Ogura, T.; Fernando, Q. Inorg. Chem. 1974, 13, 802. (21) Drew, M. G. B.; Edwards, D. A.; Richards, R. J. Chem. Soc., Dalton Trans. 1977, 299-303.
