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STM, FTIR and quantum chemical calculation studies of acetate structures on Cu(110 ) surfaces
Surface Science 522 (2003) 34–46 www.elsevier.com/locate/susc
STM, FTIR and quantum chemical calculation studies of acetate structures on Cu(1 1 0) surfaces S.M. York a, S. Haq b, K.V. Kilway c, J.M. Phillips a, F.M. Leibsle
a,*
a
c
Department of Physics, University of Missouri at Kansas City, 5100 Rockhill Road, Kansas City, MO 64110, USA b Surface Science Research Centre, University of Liverpool, Liverpool L69 3BX, UK Department of Chemistry, University of Missouri at Kansas City, 5100 Rockhill Road, Kansas City, MO 64110, USA Received 24 April 2002; accepted for publication 16 August 2002
Abstract We have exposed clean and oxygen-precovered Cu(1 1 0) surfaces to acetic acid to create adsorbed acetate. Our studies show the formation of a variety of ordered acetate structures depending on the initial oxygen exposure. We have observed the restructuring of step edges and the formation of ordered arrays of up–down steps. Using a purposeful coadsorption strategy, we have obtained images that assist us in developing models for the acetate structures. Fast Fourier transform infrared spectra have been obtained and Hartree–Fock calculations are included for comparison. Ó 2002 Published by Elsevier Science B.V. Keywords: Scanning tunneling microscopy; Vibrations of adsorbed molecules; Chemisorption; Density functional calculations; Copper
1. Introduction
formation of adsorbed formate [4] and acetate [2], namely
The interaction of carboxylates with Cu(1 1 0) surfaces has long been studied using both experimental and computational methods. Carboxylic acids such as formic [1], acetic [2] and benzoic acids [3] deprotanate on clean Cu(1 1 0) surfaces at room temperature according to overall reaction stoichiometries summarized by
2RCOOHðgÞ þ OðadsÞ ! 2RCOOðadsÞ þ H2 OðgÞ
2RCOOHðgÞ ! 2RCOOðadsÞ þ H2ðgÞ It has been shown that the presence of preadsorbed oxygen can open up a new pathway for the
*
Corresponding author. Tel.: +1-816-235-2502; fax: +1-816235-5221. E-mail address: [email protected] (F.M. Leibsle).
Much work has been performed in attempting to determine the essential chemistry [1–4], molecular adsorption sites [5–7], orientation [5–8], and vibrational [9–11] and electronic spectra [12–14] for these systems, as well as to examine computationally [15–17] these issues. Only recently, has it been discovered that these molecules can form a large variety of ordered structures on this surface [10,14,18–20]. Much of this work was performed via STM experiments which also showed that these molecules could also cause significant restructuring of step edges [10,18–20]. The initial STM work for the acetate/Cu(1 1 0) system [10] was performed on clean Cu(1 1 0)
0039-6028/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 2 2 4 8 - 3
S.M. York et al. / Surface Science 522 (2003) 34–46
surfaces exposed to acetic acid. Here, we reexamine this system and also explore the effects of preadsorbed oxygen on the formation of acetate structures. In particular, we report the discovery of a number of new acetate structures and gain insight into the development of molecular order. Through purposeful coadsorption studies [21], we are able to develop models for the observed structures. Fast Fourier transform infrared (FTIR) spectra and the results of quantum chemical calculations are also included for comparison.
2. Experimental details These experiments were performed in an ultrahigh vacuum chamber with a base pressure of 1 1010 Torr. The STM used in this study was a commercial Omicron Vakuumphysik STM. The chamber also contained facilities for argonion bombardment, low energy electron diffraction (LEED) and a residual gas analyzer. The samples were mounted on Ta baseplates using Ta clips. Sample temperatures were monitored by means of a chromel–alumel thermocouple mounted on the sample manipulator approximately 2 cm away from the sample. The Cu(1 1 0) surfaces were prepared by repeated cycles of argonion bombardment followed by annealing to 720 K until a sharp (1 1) LEED pattern was observed. In order to facilitate cooling to room temperature, a portion of the manipulator in thermal contact with the sample plate was brought in contact with a water filled copper tube. The STM images obtained in this study were analyzed and prepared for publication using the image processing program Image SXM [22]. The chamber was also equipped with several precision leak valves (Varian). One leak valve was reserved for oxygen dosing while another was reserved for the acetic acid dosing. The exposures were measured in Langmuirs (1 L ¼ 1 106 Torr s). 99.7% glacial acetic acid (Alfa Aesar) was used in this study. The acetic acid was purified by several freeze-pump-thaw cycles prior to admission to the vacuum chamber. Acetic acid exposures were performed with the sample facing the leak valve.
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3. Results and discussion 3.1. Acetate structures on clean and oxygen-precovered surfaces When clean Cu(1 1 0) and oxygen-precovered surfaces are exposed to acetic acid, an adsorbed acetate species is formed [2]. Subsequent photoelectron diffraction experiments attempted to determine the acetate adsorption site [6]. More recently, combined FTIR, STM and LEED studies showed that on the clean Cu(1 1 0) surface acetate formed a complex structure consisting of two reflectionally-equivalent domains whose periodicities could described in matrix notation as be 1 1 1 1 and structures [10]. STM 4 4 4 4 images showed rows running in the h1 1 2i directions and a model was proposed which consisted of 1/8 ML of acetate molecules [10]. A virtually concurrent XPS and LEED study [14] resulted in the observation of the same LEED pattern as observed in Ref. [10], but analysis of the XPS spectra suggested a much higher acetate coverage of 3/8 ML. In both these studies, it was also shown that the acetate LEED patterns were susceptible to electron-beam induced damage. Here, we reinvestigate the behavior of acetate on the clean Cu(1 1 0) surface and explore the effect of preadsorbed oxygen on the formation of ordered acetate structures similar to work performed on Cu(1 1 0)(2 1)O surfaces exposed to formic acid. In the previous STM study [10], clean Cu(1 1 0) surfaces were exposed to saturation (10 L) doses of acetic acid. We wished to explore whether other structures existed for lower exposures of acetic acid and also the effect of preadsorbed oxygen. We discovered that we could reproduce the results of the earlier study in our vacuum chamber and saturate the clean surface with acetic acid exposures as low as 3 L. For acetic acid exposures below 3 L, we discovered only islands of the previously ob1 1 1 1 served and structures. No 4 4 4 4 other lower coverage ordered structures were observed. To study the influence of preadsorbed oxygen on the formation of acetate structures, Cu(1 1 0)
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surfaces were treated with a variety of different oxygen exposures prior to exposure to acetic acid. A practical difficulty in performing these experiments is that the oxygen pressure used to determine the exposure is not measured at the surface of the sample but rather at the position of the vacuum chamberÕs ion gauge and thus the actual exposure may be significantly different due to the chambers geometry and pumping speeds. In light of this, all oxygen exposures were performed with the sample in the same position and at room temperature. It should be noted that the actual oxygen coverage does not vary linearly with exposure as the sticking
coefficient will decrease with increasing oxygen coverage. An additional complication in this system is that once either acetic acid or formic acid is admitted into an ion-pumped chamber, a residual pressure of these gases will persist until the chamber is baked. These residual gases will react to remove oxygen from the Cu(1 1 0) surfaces. Fig. 1 shows images of Cu(1 1 0)-(2 1)O surfaces for a variety of oxygen exposures used in this study. These images should serve as a reasonable guide to the actual oxygen coverages. Fig. 2(a) shows a typical area of a surface like that shown in Fig. 1(a) after an acetic acid expo-
2 STM images of Cu(1 1 0) surfaces exposured to oxygen (a) 1.0 L, (b) 1.75 L and (c) 2.5 L. These images Fig. 1. Three 200 200 A show approximate oxygen coverages of 0.15, 0.24 and 0.30 ML, respectively.
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2 image of a surface like that shown in Fig. 1(a) following a 0.1 L acetic acid exposure. This image shows fewer Fig. 2. (a) 540 540 A 2 andsmaller areas of (2 1)O structures and the formation of acetate islands. (b) 500 500 A image of a surface completely covered 1 1 1 1 2 high resolution images showing both reflectionally-equivalent doand acetate structures. (c,d) 80 40 A by 4 4 4 4 mains. Inequivalent rows in the acetate structures are designated A and B.
sure of 0.1 L, in comparison to Fig. 1(a), one sees a reduction in the lengths of the (2 1)O islands on the of islands of the surface and the formation 1 1 1 1 and structures. We also 4 4 4 4 note the reorientation of the step edges adjacent to the acetate rows to run also along the h1 1 2i direction. Fig. 2(b) shows an area of a Cu(1 1 0) surface completely covered by these acetate structures. We note also the single bright line of features running across the terrace in the h1 1 2i direction. This surface was generated by exposing the clean Cu(1 1 0) surface to 1.0 L of oxygen followed by an 0.5 L exposure of acetic acid. This was a general trend observed in our experiments that we could saturate the surfaces with much lower exposures of acetic acid when oxygen was initially present on the surface. Fig. 2(c) and (d) show high
resolution images of the two reflectionally-equivalent acetate structures and show groups consisting of three rows, one bright (A) and two faint (B). below Features in the B rows are imaged 0.3–0.4 A those feature found in the A rows. These images were obtained from an image of a larger area which contained both domains. To avoid possible effects from an asymmetric tip, we believe that the most reliable images are those in which the STM tip images two reflectionally- or rotationallyequivalent domains in an identical manner within the same image. In the previous STM study of the system [10], only a single bright row was observed, this lead to the proposal of a model consisting of a 1/8 ML acetate coverage. The XPS and LEED study of Baumann et al. [14] suggested a 3/8 ML acetate coverage for this structure. We have taken much more data than the original STM study, with
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lower resolution images showing only one bright row for this structure and higher resolutions images like that shown in Fig. 2(c) and (d) showing three rows. If we associate all the features observed in these images with acetate molecules, we would obtain a coverage of 3/8 ML consistent with XPS measurements. For higher initial coverages of oxygen followed by exposures of 0.5 L of acetic acid, the resultant surfaces begin to differ significantly from the image shown in Fig. 2(b). Fig. 3(a) shows an image of a surface that had been exposed to 1.2 L of oxygen prior to acetic acid exposure. This image shows a much smaller number of the row structures and an increased number of bright lines (C) running in the h1 1 2i direction. These bright rows blend into adjacent terraces without an discernible change in height. Line scans across the bright lines (C) and rows denoted by A show a difference in tip retraction consistent with the interlayer spacing of ). In light of this, we refer Cu for this surface (1.3 A to these structures as up–down steps. The high resolution image shown in the inset shows what appear to be a group of three acetate rows like that observed in Fig. 2 running between the C rows. If one would wish to use matrix notation to describe the periodic structures like those shown in the inset in Fig. describe these 3(a), one would 1 1 1 1 structures as and domains. 5 5 5 5 If we attribute all the observed protrusions to acetate molecules, we would obtain a local coverage of 0.4 ML. As the initial oxygen coverage is increased, the acetate structures show the C rows occasionally doubling and the spacing between them increasing as shown in Fig. 3(b). As noted for the images shown in Fig. 3(a), the tip retraction between the C and A rows is consistent with monatomic step heights for Cu(1 1 0) surfaces. The high resolution images included in Fig. 3(b) as an inset shows the bright lines consisting of single or double C rows of features, and either groups of acetate rows like that observed in Figs. 2 and 3(a) running between the C rows or groups of four rows in a BAAB sequence. Where one sees pairs of C rows or A rows, one can associate a local c(2 2) periodicity with these structures as denoted in the figure.
Fig. 3(c) shows an image of a Cu(1 1 0) surface like that shown in Fig. 1(b) after a 0.5 L acetic acid exposure. One sees again up–down step structures and a greater degree of alignment between step edges similar to what has been observed for benzoate on Cu(1 1 0) surfaces. Compared to Fig. 3(b), the up–down step structures show a reduction in long-range order and the edges of these structures appear more ragged, indicative of adsorbate diffusion. The high resolution image in the inset show c(2 2) periodicities on both the upper and lower portions of the structures. For initial oxygen coverages in excess of 0.25 ML, surfaces after acetic acid exposure begin to look significantly different (Fig. 3(d)), one see no up–down step structures and occasional long narrow dark islands running in the h0 0 1i direction, step edges appear ragged but in general run along h1 1 2i directions. A high resolution image of the dark islands shows a c(6 2) periodicity with the internal structure virtually identical to images of c(6 2)O structures [23,24]. We believe that the acetate has compressed whatever oxygen that remained on the surface into c(6 2) structures. Similar behavior has been observed for the formate/O/Cu(1 1 0) system [19]. Fig. 3(e) shows an image of a typical area of a Cu(1 1 0) surfaces that was initially completely covered by (2 1)O structures prior to a 0.1 L acetic acid exposure. One sees a substantial amount of c(6 2) structures, some regions with a banded appearance and elsewhere featureless terraces, step edges which are not shown tend to run once again with edges parallel to the h1 1 0i directions. Similar banded regions have been observed in an STM study of Cu(1 1 0)-c(6 2)O surfaces exposed to formic acid [25]. There it was speculated that the banded regions consisted of formate adsorbed on c(6 2)O structures that had relaxed into a (6 1) structure. We feel that while such structures may be plausible, other explanations may exist as well. We note that unlike c(6 2)O islands, these banded regions are elongated in the h1 1 0i direction. The presence of oxygen in several chemically different environments, i.e. within acetate molecules, c(6 2) structures and probably within the banded regions will certainly complicate XPS spectra for this system [26].
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Fig. 3. STM images of oxygen-precovered Cu(1 1 0) surfaces exposed to 0.5 L acetic acid, for additional details see text. (a) 300 300 2 image with 60 30 A 2 inset showing acetate structures (oxygen exposure 1.2 L). (b) 300 300 A 2 image with 60 30 A 2 inset A 2 image with 60 30 A 2 inset showing acetate structures (oxygen showing acetate structures (oxygen exposure 1.4 L). (c) 300 300 A 2 image showing acetate and c(6 2)O structures with 60 30 A 2 inset showing c(6 2)O structures exposure 1.75 L). (d) 300 300 A 2 image showing split c(2 2) acetate (1), c(6 2)O (2) and banded regions (3) (oxygen (oxygen exposure 2.5 L). (e) 300 300 A 2 image of a c(2 2) acetate region with antiphase domains, the accompanying inset shows a correexposure 5 L). (f) 200 200 A sponding FFT.
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Fig. 3(f) shows a high resolution image of the structure occurring on the terraces of Fig. 3(d) and (e), one sees bands with internal c(2 2) periodicities. LEED patterns obtained from such surfaces show a c(2 2) pattern with splitting of the 1/2 order spots. A fast fourier transform of the image shown in Fig. 3(f) also shows this splitting. The splitting is indicative of ordered antiphase domain boundaries [27], which can also be observed through careful examination of the image. We also note, in this discussion of surface structure, the structures found at step edges. In particular in Figs. 2 and 3(a) and (b), when one finds acetate rows parallel to step edge, the A rows are found at the top of the step, while B rows are found at the base. This can also be observed in up– down step regions. The frequency of these B rows decreases with increasing oxygen precoverage. It also appears that the step edges become less stable (Fig. 3(c)). When the surface is fully converted to a c(2 2) structure, no B rows are found, nor is there any longer a preference for step edges to run along h1 1 2i directions. This suggests that the B rows play a role is stabilizing the step edges.
areas of both reflectionally-equivalent domains. One sees that the small domain eventually disappears. Fig. 4(c) shows a boundary between two reflectionally-equivalent domains, one sees that the ends of the rows oscillate back and forth. Our data (Fig. 4(b) and (c)) demonstrates the mobility of the domain walls between reflectionally-equivalent adsorbate regions. As is the case in the low temperature studies of physisorbed systems, i.e. Kr/ graphite monolayers, the domain walls in our chemisorbed system ‘‘breathe’’ also. These domain walls have a finite energy and the system can be observed to become more orderly. In Fig. 4(b) one can see the one domain expand at the expense of another. At constant temperature, one would expect the energy of the wall system to drop as the structural entropy of the adsorbate decreases [29]. Fig. 4(d) shows the movement of step edges on surfaces like that shown in Fig. 3(c). It should be noted that movement of step edges as shown in this figure clearly involves both the movement of Cu atoms as well as acetate molecules.
3.2. Evolution of surface structure
To aid in developing models for the acetate structures observed in this study, we have performed purposeful coadsorption studies. This methodology involves creating islands of a relatively inert reference structure on the surface and then surrounding these islands with acetate structures [21]. To introduce this methodology and gain insight into the orbitals that participate in the tunneling process Fig. 5(a) shows coexisting (2 3)N islands which serve as a reference structure and c(2 2) formate structures. The features labeled ‘‘B’’ in the (2 3)N islands occur over two-fold hollow sites. Comparison of the spatial relationships between these features and those protrusions in the c(2 2) formate structure show that the formate-related protrusions are centered over short-bridge sites. This is in agreement with PED and SEXAFS results for this system [5]. Line scans above show the formate molecules appearing 0.5 A the (2 3)N structures. Fig. 5(b) also shows the results of a similar experiment in which coexisting (2 3)N and acetate structures are formed. Comparisons of the spatial relationships as done
Acetate on the Cu(1 1 0) surface produces some very interesting surface structures. As seen in Fig. 2, it causes the reorientation of step edges to run along the h1 12i directions and forms extremely 1 1 1 1 well-ordered and domains. 4 4 4 4 We have taken data that give us some insight into how both processes occur. Fig. 4(a) shows consecutive images of an area of a Cu(1 1 0) surface containing (2 1)O islands and acetate rows. One sees that the development of the acetate structures is still continuing with the acetate islands growing and in one instance an acetate island appears to grow back into the adjacent terrace and either displacing the Cu atoms there or possibly incorporating them into the acetate structure. It has recently been shown in the case of formate on Cu(1 1 0) surfaces, that the presence of formate can enhance the movement of the step edges [28]. Fig. 4(b) shows three terraces covered by acetate structures, the middle terrace contains unequal
3.3. Purposeful coadsorption results
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2 images showing the growth of acetate islands. One Fig. 4. STM images showing the evolution of acetate structures. (a) 200 200 A 2 images showing the disappearance of a reflectionally-equivalent domain. island appears to grow back into a step edge. (b) 300 300 A 2 images showing the back and forth movement of a boundary between two reflectionally-equivalent domains. (d) (c) 160 160 A 2 images showing movement of molecules and presumeably substrate atoms along step edges for surfaces like that of Fig. 200 200 A 2(c). In each set of images, the times shown are relative to the first image of each set.
for formate, show that the c(2 2) acetate features are also positioned over short-bridge sites. Line
scans shows that these acetate protrusions are above the (2 3)N structures. It is imaged 1.2 A
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2 image showing coexisting (2 3)N and c(2 2) formate reFig. 5. Results of purposeful coadsorption experiments. (a) 70 70 A 2 image showing coexisting (2 3)N and acetate regions. (c) Diagram showing spatial relationships between gions. (b) 70 70 A features found in the STM images, both formate and acetate protrusions occur over short-bridge sites.
interesting to examine the difference in the line scans for the two systems, a much greater tip retraction occurs as the tip moves from the (2 3)N regions to the acetate regions than is the case for formate. These observations are consistent if electrons are tunneling from the C–H groups of the formate molecules and the CH3 groups of the acetate molecules. While tip changes can often render molecules invisible to the STM, a general idea of how molecular species are imaged can aid greatly in interpreting STM images, in particular, those from systems in which more that one molecular species may coexist on a surface. 3.4. Structural models Once an adsorption site for one of the acetaterelated features is determined, then high resolution scans of the individual acetate structures can be analyzed quantitatively. Fig. 6(a) shows the results of such an analysis of high resolution images like
those shown in Fig. 2(c) and (d). With the features found in the A rows positioned over short-bridge sites, measures of the relative positions of the features in the B rows indicate that these features are not centered over high symmetry sites. Their relative positions are indicated in Fig. 6(a). One cannot infer however that the positions shown in this figure are those of the actual adsorption sites in that this determination cannot take into account molecular tilt or possible rearrangement of substrate atoms. Based on these measurements, Fig. 1 1 6(b) shows and a model for the 4 4 1 1 structures. This model has an acetate 4 4 coverage of 0.375 ML consistent with XPS measurements [14]. Fig. 6(b) shows a model for the up–down step structures shown in Fig. 3(a) and (b). Specifically, 1 1 it shows a model of a domain like that 5 5 shown in the inset of Fig. 3(a). The A and B rows
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Fig. of acetatestructures 6. Models found on Cu(1 1 0) surfaces. (a) shows quantitative positions of B row features, (b) models of 1 1 1 1 structures, (c) structures, (d) c(2 2) structures with periodic antiphase domain boundaries. 4 4 5 5
are positioned as in Fig. 6(a) and (b), C rows are presumed to be due to acetate molecules adsorbed on short-bridge site on top of pairs of added Cu atoms. This model has an acetate coverage of 0.4 ML. Additional structures like those shown in Fig. 3(b) can modeled by adding additional A or C rows. Fig. 6(d) shows a model for the split c(2 2) structure with antiphase domain boundaries occurring every eight spacings in the h0 0 1i direction The acetate molecule are again positioned over short-bridge sites on the underlying substrate.
3.5. Low energy electron induced restructuring in both previous studies of It has been shown 1 1 1 1 and acetate structures that 4 4 4 4 these structures were sensitive to bombardment of low energy electrons with the LEED patterns disappearing quickly [10,14]. While we have not explored this exhaustively, we have observed that all the structures observed in this study were susceptible to damage from low energy electrons. The split c(2 2) pattern appeared to be the most
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2 STM images of a surface like that shown Fig. 7. 500 500 A in Fig. 2(b) after erradiation by 47 eV electrons. This image shows structures similar to the up–down step regions of Fig. 2 inset 3(b) separated by disordered regions, the 60 30 A shows molecular resolution and again is similar to the structures shown in Fig. 3(b).
stable. Fig. 7 shows an image of an area of a surface like that shown in Fig. 2(c) after bombardment with 47 eV electrons. This image shows a mixture of up–down step and disordered regions. High resolution images of the up–down step regions, like that shown in the inset in Fig. 7, show structures identical to those in Fig. 3(b). 3.6. FTIR and quantum chemical calculations FTIR spectra were obtained for two of the surface structures observed in this study. Fig. 8(a) shows a spectrum obtained from a clean Cu(1 1 0) surface exposed to acetic acid. The LEED pattern obtained afterward from was this surface consis 1 1 1 1 tent with coexisting and 4 4 4 4 domains. Fig. 8(b) shows a spectrum from a Cu(1 1 0)-(2 1)O surface exposed to acetic acid. The LEED pattern from this surface showed a split c(2 2) pattern, consistent with a surface like that shown in Fig. 3(e) and (f). The spectra from both surfaces were very similar and show vibrational bands largely consistent with a sym-
Fig. 8. FTIR spectra for (a) a clean Cu(1 1 0) surface exposed to acetic acid and (b) a Cu(1 1 0)-(2 1)O surface exposed to acetic acid.
metrically bound upright acetate species with characteristic ms (OCO) (1434 cm1 ), ds (CH3 ) (1335 cm1 ), and ms (CH3 ) (2932 cm1 ) d(OCO) (678 cm1 ) frequencies [30]. We note that a band at 1391 cm1 exists in Fig. 8(a). To gain insight into the possible origins of this band, we have performed quantum chemical calculations for this system. We used restricted Hartree–Fock (RHF) and density functional (DFT) optimizations. The calculations were carried out using the GAUSSIAN98 program [31]. We used an all electron model, i.e. we did not use any core potential approximations. The calculations were a self-consistent-field approach with linear combinations of gaussian type orbitals with a generalized gradient approximation, SCF–LCGTO–GGA. We calculated separately using two different wave functions (6-311G(d) and 6-31G(d)). The first basis set has five shells, an s-shell with six primitives, three sp-
S.M. York et al. / Surface Science 522 (2003) 34–46 Table 1 Experimental and calculated vibrational frequencies for acetate on Cu(1 1 0) surfaces Mode
ms (CH3 ) ms (OCO) da (CH3 ) ds (CH3 ) ds (OCO)
Experiment cm1
RHF 6-31G(d) cm1
6-311G(d) cm1
6-311G(d) cm1
2932 1434 1391 1335 678
2872 1459 1433 1352 645
2850 1446 1423 1338 645
3036 1434 1488 1382 675
B3LYP
shells with three primitives. The next two shells have one primitive each. There is also an uncontracted d-shell. The second wave function we used has one less shell. The results using this two basis sets are given in Table 1. The DFT calculations were using the B3LYP key word which employ methods given by Becke [32] and Lee et al. [33]. We approximated the copper surface with a ten atom cluster (Fig. 9). In our cluster calculations, the copper atoms are constrained to remain in their bulk solid positions. We found the optimized structure for the cluster complex and then computed the vibrational frequency spectra of our model. The short-bridge adsorption site agrees with experiments. Our primary goal computationally was to compare the FTIR data given in Fig. 8 to the cluster model calculations. This cluster model has forty four modes. All but the following five either have very low relative intensities or they do not have a significant dipole moment perpendicular to the copper surface. The resulting frequencies using RHF
Fig. 9. Diagram of the acetate–copper cluster used to calculate vibrational frequencies.
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have been scaled with the usual factor of 0.8929 and the DFT results are scaled using 0.989 [34] and are displayed along with the FTIR results in Table 1. The calculated vibrational frequencies are all well within 5% of those observed in the FTIR experiments. These calculations suggest that the band observed in Fig. 8(a) at 1391 cm1 is due to the da (CH3 ) mode. This vibrational mode results in a dipole moment that is only partially perpendicular to the surface and hence the associated band may be more intense for a tilted acetate species as suggested in the discussion of Fig. 6(a). In regard to the small size of the Cu cluster used in this model, we note that calculational predictions of adsorption sites, adsorbate structure, orbital configuration [35], and vibrational frequencies are consistent with experiment. The finite size of the substrate model does not effect these results significantly, since the properties we have studied are highly local in character. This is not the case when the band structure of the substrate figures prominently in the measurements, i.e. heats of adsorption.
4. Conclusions We have explored the effect of preadsorbed oxygen on the formation of ordered acetate structures on Cu(1 1 0) surfaces. On clean and 0.15 ML O precovered surfaces, acetic acid exposure 1 1 1 1 results in domains of and 4 4 4 4 structures with no other lower coverage acetate structures observed. For oxygen coverages between 0.15 and 0.24, acetic acid exposure results in arrays of up–down step structures. Above an oxygen coverage of 1/4 ML, c(2 2) acetate structures are formed. STM images and LEED patterns are consistent with ordered arrays of antiphase domain boundaries. Oxygen not consumed in reactions with acetic acid appears to form c(6 2) structures. Models are proposed for the acetate structuresobserved here and suggest that for the 1 1 1 1 and up–down step and 4 4 4 4 structures that acetate occupies two inequivalent adsorption sites. The STM images obtained in this
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study yield insight into the development of molecular order on this surface. The images show the movement of step edges and the rapid removal of domain boundaries. FTIR spectra were obtained from clean Cu(1 1 0) and Cu(1 1 0)-(2 1)O surfaces exposed to acetic acid. These spectra are consistent with adsorbed acetate. The computational results compare quite well to the experimental data. Acknowledgements We would like to thank A. Holder and T. Keith for helpful discussions and S. Silva for technical assistance during the early phase of these experiments. References [1] D.H.S. Ying, R.J. Madix, J. Catal. 61 (1980) 48. [2] M. Bowker, R.J. Madix, Appl. Surf. Sci. 8 (1981) 299. [3] B.G. Frederick, T.S. Jones, P.D.A. Pudney, N.V. Richardson, J. Electron Spectrosc. Relat. Phenom. 64 (1993) 115. [4] F.C. Henn, J.A. Rodriguez, C.T. Campbell, Surf. Sci. 236 (1990) 282. [5] D.P. Woodruff, C.F. McConville, A.L.D. Kilcoyne, Th. Lindner, J. Somers, M. Surman, G. Paolucci, A.M. Bradshaw, Surf. Sci. 201 (1988) 228. [6] K.-U. Weiss, R. Dippel, K.-M. Schindler, P. Gardner, V. Fritzsche, A.M. Bradshaw, A.L.D. Kilcoyne, D.P. Woodruff, Phys. Rev. Lett. 69 (1992) 3196. [7] M. Pascal, C.L.A. Lamont, M. Kittel, J.T. Hoeft, R. Terborg, M. Polcik, J.H. Kang, R. Toomes, D.P. Woodruff, Surf. Sci. 492 (2001) 285. [8] P. Hofmann, D. Menzel, Surf. Sci. 191 (1987) 353. [9] B.E. Hayden, K. Prince, D.P. Woodruff, A.M. Bradshaw, Surf. Sci. 131 (1983) 589. [10] S. Haq, F.M. Leibsle, Surf. Sci. 355 (1996) L345. [11] P.D.A. Pudney, B.G. Frederick, N.V. Richardson, Surf. Sci. 309 (1994) 46. [12] J. Hasselstr€ om, O. Karis, M. Weinelt, N. Wassdahl, A. Nilsson, M. Nyberg, L.G.M. Pettersson, M.G. Samant, J. St€ ohr, Surf. Sci. 407 (1998) 221. [13] O. Karis, J. Hasselstr€ om, N. Wassdahl, M. Weinelt, A. Nilsson, M. Nyberg, L.G.M. Pettersson, J. St€ ohr, M.G. Samant, J. Chem. Phys. 112 (2000) 8146.
[14] P. Baumann, H.P. Bonzel, G. Pirug, J. Werner, Chem. Phys. Lett. 260 (1996) 215. [15] A. Wander, B.W. Holland, Surf. Sci. 199 (1988) L403. [16] J.R.B. Gomes, J.A.N.F. Gomes, Surf. Sci. 432 (1999) 279. [17] H. Zhenming, R.J. Boyd, J. Chem. Phys. 112 (2000) 9562. [18] F.M. Leibsle, S. Haq, B.G. Frederick, M. Bowker, N.V. Richardson, Surf. Sci. Lett. 343 (1995) L1175. [19] S. Haq, F.M. Leibsle, Surf. Sci. 375 (1997) 81. [20] B.G. Frederick, F.M. Leibsle, S. Haq, N.V. Richardson, Surf. Rev. Lett. 3 (1996) 1523. [21] F.M. Leibsle, Surf. Sci. 311 (1994) 45. [22] Image SXM is available by anonymous ftp from ssci.liv. as.uk. [23] D. Coulman, J. Wintterlin, J.V. Barth, G. Ertl, R.J. Behm, Surf. Sci. 240 (1990) 151. [24] R. Feidenhansl, F. Grey, M. Nielsen, F. Besenbacher, F. Jensen, E. Lægsgaard, I. Stensgaard, K.W. Jacobsen, J.K. Nørskov, R.L. Johnson, Phys. Rev. Lett. 65 (1990) 2027. [25] P. Stone, S. Poulston, R.A. Bennett, N.J. Price, M. Bowker, Surf. Sci. 418 (1998) 71. [26] P. Baumann, G. Pirug, D. Reuter, H.P. Bonzel, Surf. Sci. 335 (1995) 186. [27] M.A. Van Hove, W.H. Weingberg, C.-M. Chan, LowEnergy Electron Diffraction, Springer Series in Surface Science, vol. 8, Springer-Verlag, Berlin, 1986, p. 85. [28] S.J. Harrington, K.V. Kilway, F.M. Leibsle, D.-M. Zhu, J.M. Phillips, J. Chem. Phys., submitted for publication. [29] F.F. Abraham, Adv. Phys. 35 (1986) 1. [30] K. Ito, H.J. Bernstein, Can. J. Chem. 34 (1956) 170. [31] Gaussian98, Revision A.4, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dappich. J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G. Lui, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. AlLaham, C.Y. Peng, A. Nanayakkara, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon E.S. Replogle, J.A. Pople, Gaussian Inc., Pittsburgh, PA, 1998. [32] A.D. Becke, Phys. Rev. A 38 (1988) 3098; A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [33] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [34] J.B. Foresman, A. Frisch, Exploring Chemistry with Electronic Structure Methods, second ed., Gaussian Inc., Pittsburgh, PA. [35] J.M. Phillips, F.M. Leibsle, unpublished.
STM, FTIR and quantum chemical calculation studies of acetate structures on Cu(1 1 0) surfaces S.M. York a, S. Haq b, K.V. Kilway c, J.M. Phillips a, F.M. Leibsle
a,*
a
c
Department of Physics, University of Missouri at Kansas City, 5100 Rockhill Road, Kansas City, MO 64110, USA b Surface Science Research Centre, University of Liverpool, Liverpool L69 3BX, UK Department of Chemistry, University of Missouri at Kansas City, 5100 Rockhill Road, Kansas City, MO 64110, USA Received 24 April 2002; accepted for publication 16 August 2002
Abstract We have exposed clean and oxygen-precovered Cu(1 1 0) surfaces to acetic acid to create adsorbed acetate. Our studies show the formation of a variety of ordered acetate structures depending on the initial oxygen exposure. We have observed the restructuring of step edges and the formation of ordered arrays of up–down steps. Using a purposeful coadsorption strategy, we have obtained images that assist us in developing models for the acetate structures. Fast Fourier transform infrared spectra have been obtained and Hartree–Fock calculations are included for comparison. Ó 2002 Published by Elsevier Science B.V. Keywords: Scanning tunneling microscopy; Vibrations of adsorbed molecules; Chemisorption; Density functional calculations; Copper
1. Introduction
formation of adsorbed formate [4] and acetate [2], namely
The interaction of carboxylates with Cu(1 1 0) surfaces has long been studied using both experimental and computational methods. Carboxylic acids such as formic [1], acetic [2] and benzoic acids [3] deprotanate on clean Cu(1 1 0) surfaces at room temperature according to overall reaction stoichiometries summarized by
2RCOOHðgÞ þ OðadsÞ ! 2RCOOðadsÞ þ H2 OðgÞ
2RCOOHðgÞ ! 2RCOOðadsÞ þ H2ðgÞ It has been shown that the presence of preadsorbed oxygen can open up a new pathway for the
*
Corresponding author. Tel.: +1-816-235-2502; fax: +1-816235-5221. E-mail address: [email protected] (F.M. Leibsle).
Much work has been performed in attempting to determine the essential chemistry [1–4], molecular adsorption sites [5–7], orientation [5–8], and vibrational [9–11] and electronic spectra [12–14] for these systems, as well as to examine computationally [15–17] these issues. Only recently, has it been discovered that these molecules can form a large variety of ordered structures on this surface [10,14,18–20]. Much of this work was performed via STM experiments which also showed that these molecules could also cause significant restructuring of step edges [10,18–20]. The initial STM work for the acetate/Cu(1 1 0) system [10] was performed on clean Cu(1 1 0)
0039-6028/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 2 2 4 8 - 3
S.M. York et al. / Surface Science 522 (2003) 34–46
surfaces exposed to acetic acid. Here, we reexamine this system and also explore the effects of preadsorbed oxygen on the formation of acetate structures. In particular, we report the discovery of a number of new acetate structures and gain insight into the development of molecular order. Through purposeful coadsorption studies [21], we are able to develop models for the observed structures. Fast Fourier transform infrared (FTIR) spectra and the results of quantum chemical calculations are also included for comparison.
2. Experimental details These experiments were performed in an ultrahigh vacuum chamber with a base pressure of 1 1010 Torr. The STM used in this study was a commercial Omicron Vakuumphysik STM. The chamber also contained facilities for argonion bombardment, low energy electron diffraction (LEED) and a residual gas analyzer. The samples were mounted on Ta baseplates using Ta clips. Sample temperatures were monitored by means of a chromel–alumel thermocouple mounted on the sample manipulator approximately 2 cm away from the sample. The Cu(1 1 0) surfaces were prepared by repeated cycles of argonion bombardment followed by annealing to 720 K until a sharp (1 1) LEED pattern was observed. In order to facilitate cooling to room temperature, a portion of the manipulator in thermal contact with the sample plate was brought in contact with a water filled copper tube. The STM images obtained in this study were analyzed and prepared for publication using the image processing program Image SXM [22]. The chamber was also equipped with several precision leak valves (Varian). One leak valve was reserved for oxygen dosing while another was reserved for the acetic acid dosing. The exposures were measured in Langmuirs (1 L ¼ 1 106 Torr s). 99.7% glacial acetic acid (Alfa Aesar) was used in this study. The acetic acid was purified by several freeze-pump-thaw cycles prior to admission to the vacuum chamber. Acetic acid exposures were performed with the sample facing the leak valve.
35
3. Results and discussion 3.1. Acetate structures on clean and oxygen-precovered surfaces When clean Cu(1 1 0) and oxygen-precovered surfaces are exposed to acetic acid, an adsorbed acetate species is formed [2]. Subsequent photoelectron diffraction experiments attempted to determine the acetate adsorption site [6]. More recently, combined FTIR, STM and LEED studies showed that on the clean Cu(1 1 0) surface acetate formed a complex structure consisting of two reflectionally-equivalent domains whose periodicities could described in matrix notation as be 1 1 1 1 and structures [10]. STM 4 4 4 4 images showed rows running in the h1 1 2i directions and a model was proposed which consisted of 1/8 ML of acetate molecules [10]. A virtually concurrent XPS and LEED study [14] resulted in the observation of the same LEED pattern as observed in Ref. [10], but analysis of the XPS spectra suggested a much higher acetate coverage of 3/8 ML. In both these studies, it was also shown that the acetate LEED patterns were susceptible to electron-beam induced damage. Here, we reinvestigate the behavior of acetate on the clean Cu(1 1 0) surface and explore the effect of preadsorbed oxygen on the formation of ordered acetate structures similar to work performed on Cu(1 1 0)(2 1)O surfaces exposed to formic acid. In the previous STM study [10], clean Cu(1 1 0) surfaces were exposed to saturation (10 L) doses of acetic acid. We wished to explore whether other structures existed for lower exposures of acetic acid and also the effect of preadsorbed oxygen. We discovered that we could reproduce the results of the earlier study in our vacuum chamber and saturate the clean surface with acetic acid exposures as low as 3 L. For acetic acid exposures below 3 L, we discovered only islands of the previously ob1 1 1 1 served and structures. No 4 4 4 4 other lower coverage ordered structures were observed. To study the influence of preadsorbed oxygen on the formation of acetate structures, Cu(1 1 0)
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surfaces were treated with a variety of different oxygen exposures prior to exposure to acetic acid. A practical difficulty in performing these experiments is that the oxygen pressure used to determine the exposure is not measured at the surface of the sample but rather at the position of the vacuum chamberÕs ion gauge and thus the actual exposure may be significantly different due to the chambers geometry and pumping speeds. In light of this, all oxygen exposures were performed with the sample in the same position and at room temperature. It should be noted that the actual oxygen coverage does not vary linearly with exposure as the sticking
coefficient will decrease with increasing oxygen coverage. An additional complication in this system is that once either acetic acid or formic acid is admitted into an ion-pumped chamber, a residual pressure of these gases will persist until the chamber is baked. These residual gases will react to remove oxygen from the Cu(1 1 0) surfaces. Fig. 1 shows images of Cu(1 1 0)-(2 1)O surfaces for a variety of oxygen exposures used in this study. These images should serve as a reasonable guide to the actual oxygen coverages. Fig. 2(a) shows a typical area of a surface like that shown in Fig. 1(a) after an acetic acid expo-
2 STM images of Cu(1 1 0) surfaces exposured to oxygen (a) 1.0 L, (b) 1.75 L and (c) 2.5 L. These images Fig. 1. Three 200 200 A show approximate oxygen coverages of 0.15, 0.24 and 0.30 ML, respectively.
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2 image of a surface like that shown in Fig. 1(a) following a 0.1 L acetic acid exposure. This image shows fewer Fig. 2. (a) 540 540 A 2 andsmaller areas of (2 1)O structures and the formation of acetate islands. (b) 500 500 A image of a surface completely covered 1 1 1 1 2 high resolution images showing both reflectionally-equivalent doand acetate structures. (c,d) 80 40 A by 4 4 4 4 mains. Inequivalent rows in the acetate structures are designated A and B.
sure of 0.1 L, in comparison to Fig. 1(a), one sees a reduction in the lengths of the (2 1)O islands on the of islands of the surface and the formation 1 1 1 1 and structures. We also 4 4 4 4 note the reorientation of the step edges adjacent to the acetate rows to run also along the h1 1 2i direction. Fig. 2(b) shows an area of a Cu(1 1 0) surface completely covered by these acetate structures. We note also the single bright line of features running across the terrace in the h1 1 2i direction. This surface was generated by exposing the clean Cu(1 1 0) surface to 1.0 L of oxygen followed by an 0.5 L exposure of acetic acid. This was a general trend observed in our experiments that we could saturate the surfaces with much lower exposures of acetic acid when oxygen was initially present on the surface. Fig. 2(c) and (d) show high
resolution images of the two reflectionally-equivalent acetate structures and show groups consisting of three rows, one bright (A) and two faint (B). below Features in the B rows are imaged 0.3–0.4 A those feature found in the A rows. These images were obtained from an image of a larger area which contained both domains. To avoid possible effects from an asymmetric tip, we believe that the most reliable images are those in which the STM tip images two reflectionally- or rotationallyequivalent domains in an identical manner within the same image. In the previous STM study of the system [10], only a single bright row was observed, this lead to the proposal of a model consisting of a 1/8 ML acetate coverage. The XPS and LEED study of Baumann et al. [14] suggested a 3/8 ML acetate coverage for this structure. We have taken much more data than the original STM study, with
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lower resolution images showing only one bright row for this structure and higher resolutions images like that shown in Fig. 2(c) and (d) showing three rows. If we associate all the features observed in these images with acetate molecules, we would obtain a coverage of 3/8 ML consistent with XPS measurements. For higher initial coverages of oxygen followed by exposures of 0.5 L of acetic acid, the resultant surfaces begin to differ significantly from the image shown in Fig. 2(b). Fig. 3(a) shows an image of a surface that had been exposed to 1.2 L of oxygen prior to acetic acid exposure. This image shows a much smaller number of the row structures and an increased number of bright lines (C) running in the h1 1 2i direction. These bright rows blend into adjacent terraces without an discernible change in height. Line scans across the bright lines (C) and rows denoted by A show a difference in tip retraction consistent with the interlayer spacing of ). In light of this, we refer Cu for this surface (1.3 A to these structures as up–down steps. The high resolution image shown in the inset shows what appear to be a group of three acetate rows like that observed in Fig. 2 running between the C rows. If one would wish to use matrix notation to describe the periodic structures like those shown in the inset in Fig. describe these 3(a), one would 1 1 1 1 structures as and domains. 5 5 5 5 If we attribute all the observed protrusions to acetate molecules, we would obtain a local coverage of 0.4 ML. As the initial oxygen coverage is increased, the acetate structures show the C rows occasionally doubling and the spacing between them increasing as shown in Fig. 3(b). As noted for the images shown in Fig. 3(a), the tip retraction between the C and A rows is consistent with monatomic step heights for Cu(1 1 0) surfaces. The high resolution images included in Fig. 3(b) as an inset shows the bright lines consisting of single or double C rows of features, and either groups of acetate rows like that observed in Figs. 2 and 3(a) running between the C rows or groups of four rows in a BAAB sequence. Where one sees pairs of C rows or A rows, one can associate a local c(2 2) periodicity with these structures as denoted in the figure.
Fig. 3(c) shows an image of a Cu(1 1 0) surface like that shown in Fig. 1(b) after a 0.5 L acetic acid exposure. One sees again up–down step structures and a greater degree of alignment between step edges similar to what has been observed for benzoate on Cu(1 1 0) surfaces. Compared to Fig. 3(b), the up–down step structures show a reduction in long-range order and the edges of these structures appear more ragged, indicative of adsorbate diffusion. The high resolution image in the inset show c(2 2) periodicities on both the upper and lower portions of the structures. For initial oxygen coverages in excess of 0.25 ML, surfaces after acetic acid exposure begin to look significantly different (Fig. 3(d)), one see no up–down step structures and occasional long narrow dark islands running in the h0 0 1i direction, step edges appear ragged but in general run along h1 1 2i directions. A high resolution image of the dark islands shows a c(6 2) periodicity with the internal structure virtually identical to images of c(6 2)O structures [23,24]. We believe that the acetate has compressed whatever oxygen that remained on the surface into c(6 2) structures. Similar behavior has been observed for the formate/O/Cu(1 1 0) system [19]. Fig. 3(e) shows an image of a typical area of a Cu(1 1 0) surfaces that was initially completely covered by (2 1)O structures prior to a 0.1 L acetic acid exposure. One sees a substantial amount of c(6 2) structures, some regions with a banded appearance and elsewhere featureless terraces, step edges which are not shown tend to run once again with edges parallel to the h1 1 0i directions. Similar banded regions have been observed in an STM study of Cu(1 1 0)-c(6 2)O surfaces exposed to formic acid [25]. There it was speculated that the banded regions consisted of formate adsorbed on c(6 2)O structures that had relaxed into a (6 1) structure. We feel that while such structures may be plausible, other explanations may exist as well. We note that unlike c(6 2)O islands, these banded regions are elongated in the h1 1 0i direction. The presence of oxygen in several chemically different environments, i.e. within acetate molecules, c(6 2) structures and probably within the banded regions will certainly complicate XPS spectra for this system [26].
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Fig. 3. STM images of oxygen-precovered Cu(1 1 0) surfaces exposed to 0.5 L acetic acid, for additional details see text. (a) 300 300 2 image with 60 30 A 2 inset showing acetate structures (oxygen exposure 1.2 L). (b) 300 300 A 2 image with 60 30 A 2 inset A 2 image with 60 30 A 2 inset showing acetate structures (oxygen showing acetate structures (oxygen exposure 1.4 L). (c) 300 300 A 2 image showing acetate and c(6 2)O structures with 60 30 A 2 inset showing c(6 2)O structures exposure 1.75 L). (d) 300 300 A 2 image showing split c(2 2) acetate (1), c(6 2)O (2) and banded regions (3) (oxygen (oxygen exposure 2.5 L). (e) 300 300 A 2 image of a c(2 2) acetate region with antiphase domains, the accompanying inset shows a correexposure 5 L). (f) 200 200 A sponding FFT.
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Fig. 3(f) shows a high resolution image of the structure occurring on the terraces of Fig. 3(d) and (e), one sees bands with internal c(2 2) periodicities. LEED patterns obtained from such surfaces show a c(2 2) pattern with splitting of the 1/2 order spots. A fast fourier transform of the image shown in Fig. 3(f) also shows this splitting. The splitting is indicative of ordered antiphase domain boundaries [27], which can also be observed through careful examination of the image. We also note, in this discussion of surface structure, the structures found at step edges. In particular in Figs. 2 and 3(a) and (b), when one finds acetate rows parallel to step edge, the A rows are found at the top of the step, while B rows are found at the base. This can also be observed in up– down step regions. The frequency of these B rows decreases with increasing oxygen precoverage. It also appears that the step edges become less stable (Fig. 3(c)). When the surface is fully converted to a c(2 2) structure, no B rows are found, nor is there any longer a preference for step edges to run along h1 1 2i directions. This suggests that the B rows play a role is stabilizing the step edges.
areas of both reflectionally-equivalent domains. One sees that the small domain eventually disappears. Fig. 4(c) shows a boundary between two reflectionally-equivalent domains, one sees that the ends of the rows oscillate back and forth. Our data (Fig. 4(b) and (c)) demonstrates the mobility of the domain walls between reflectionally-equivalent adsorbate regions. As is the case in the low temperature studies of physisorbed systems, i.e. Kr/ graphite monolayers, the domain walls in our chemisorbed system ‘‘breathe’’ also. These domain walls have a finite energy and the system can be observed to become more orderly. In Fig. 4(b) one can see the one domain expand at the expense of another. At constant temperature, one would expect the energy of the wall system to drop as the structural entropy of the adsorbate decreases [29]. Fig. 4(d) shows the movement of step edges on surfaces like that shown in Fig. 3(c). It should be noted that movement of step edges as shown in this figure clearly involves both the movement of Cu atoms as well as acetate molecules.
3.2. Evolution of surface structure
To aid in developing models for the acetate structures observed in this study, we have performed purposeful coadsorption studies. This methodology involves creating islands of a relatively inert reference structure on the surface and then surrounding these islands with acetate structures [21]. To introduce this methodology and gain insight into the orbitals that participate in the tunneling process Fig. 5(a) shows coexisting (2 3)N islands which serve as a reference structure and c(2 2) formate structures. The features labeled ‘‘B’’ in the (2 3)N islands occur over two-fold hollow sites. Comparison of the spatial relationships between these features and those protrusions in the c(2 2) formate structure show that the formate-related protrusions are centered over short-bridge sites. This is in agreement with PED and SEXAFS results for this system [5]. Line scans above show the formate molecules appearing 0.5 A the (2 3)N structures. Fig. 5(b) also shows the results of a similar experiment in which coexisting (2 3)N and acetate structures are formed. Comparisons of the spatial relationships as done
Acetate on the Cu(1 1 0) surface produces some very interesting surface structures. As seen in Fig. 2, it causes the reorientation of step edges to run along the h1 12i directions and forms extremely 1 1 1 1 well-ordered and domains. 4 4 4 4 We have taken data that give us some insight into how both processes occur. Fig. 4(a) shows consecutive images of an area of a Cu(1 1 0) surface containing (2 1)O islands and acetate rows. One sees that the development of the acetate structures is still continuing with the acetate islands growing and in one instance an acetate island appears to grow back into the adjacent terrace and either displacing the Cu atoms there or possibly incorporating them into the acetate structure. It has recently been shown in the case of formate on Cu(1 1 0) surfaces, that the presence of formate can enhance the movement of the step edges [28]. Fig. 4(b) shows three terraces covered by acetate structures, the middle terrace contains unequal
3.3. Purposeful coadsorption results
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2 images showing the growth of acetate islands. One Fig. 4. STM images showing the evolution of acetate structures. (a) 200 200 A 2 images showing the disappearance of a reflectionally-equivalent domain. island appears to grow back into a step edge. (b) 300 300 A 2 images showing the back and forth movement of a boundary between two reflectionally-equivalent domains. (d) (c) 160 160 A 2 images showing movement of molecules and presumeably substrate atoms along step edges for surfaces like that of Fig. 200 200 A 2(c). In each set of images, the times shown are relative to the first image of each set.
for formate, show that the c(2 2) acetate features are also positioned over short-bridge sites. Line
scans shows that these acetate protrusions are above the (2 3)N structures. It is imaged 1.2 A
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2 image showing coexisting (2 3)N and c(2 2) formate reFig. 5. Results of purposeful coadsorption experiments. (a) 70 70 A 2 image showing coexisting (2 3)N and acetate regions. (c) Diagram showing spatial relationships between gions. (b) 70 70 A features found in the STM images, both formate and acetate protrusions occur over short-bridge sites.
interesting to examine the difference in the line scans for the two systems, a much greater tip retraction occurs as the tip moves from the (2 3)N regions to the acetate regions than is the case for formate. These observations are consistent if electrons are tunneling from the C–H groups of the formate molecules and the CH3 groups of the acetate molecules. While tip changes can often render molecules invisible to the STM, a general idea of how molecular species are imaged can aid greatly in interpreting STM images, in particular, those from systems in which more that one molecular species may coexist on a surface. 3.4. Structural models Once an adsorption site for one of the acetaterelated features is determined, then high resolution scans of the individual acetate structures can be analyzed quantitatively. Fig. 6(a) shows the results of such an analysis of high resolution images like
those shown in Fig. 2(c) and (d). With the features found in the A rows positioned over short-bridge sites, measures of the relative positions of the features in the B rows indicate that these features are not centered over high symmetry sites. Their relative positions are indicated in Fig. 6(a). One cannot infer however that the positions shown in this figure are those of the actual adsorption sites in that this determination cannot take into account molecular tilt or possible rearrangement of substrate atoms. Based on these measurements, Fig. 1 1 6(b) shows and a model for the 4 4 1 1 structures. This model has an acetate 4 4 coverage of 0.375 ML consistent with XPS measurements [14]. Fig. 6(b) shows a model for the up–down step structures shown in Fig. 3(a) and (b). Specifically, 1 1 it shows a model of a domain like that 5 5 shown in the inset of Fig. 3(a). The A and B rows
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Fig. of acetatestructures 6. Models found on Cu(1 1 0) surfaces. (a) shows quantitative positions of B row features, (b) models of 1 1 1 1 structures, (c) structures, (d) c(2 2) structures with periodic antiphase domain boundaries. 4 4 5 5
are positioned as in Fig. 6(a) and (b), C rows are presumed to be due to acetate molecules adsorbed on short-bridge site on top of pairs of added Cu atoms. This model has an acetate coverage of 0.4 ML. Additional structures like those shown in Fig. 3(b) can modeled by adding additional A or C rows. Fig. 6(d) shows a model for the split c(2 2) structure with antiphase domain boundaries occurring every eight spacings in the h0 0 1i direction The acetate molecule are again positioned over short-bridge sites on the underlying substrate.
3.5. Low energy electron induced restructuring in both previous studies of It has been shown 1 1 1 1 and acetate structures that 4 4 4 4 these structures were sensitive to bombardment of low energy electrons with the LEED patterns disappearing quickly [10,14]. While we have not explored this exhaustively, we have observed that all the structures observed in this study were susceptible to damage from low energy electrons. The split c(2 2) pattern appeared to be the most
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2 STM images of a surface like that shown Fig. 7. 500 500 A in Fig. 2(b) after erradiation by 47 eV electrons. This image shows structures similar to the up–down step regions of Fig. 2 inset 3(b) separated by disordered regions, the 60 30 A shows molecular resolution and again is similar to the structures shown in Fig. 3(b).
stable. Fig. 7 shows an image of an area of a surface like that shown in Fig. 2(c) after bombardment with 47 eV electrons. This image shows a mixture of up–down step and disordered regions. High resolution images of the up–down step regions, like that shown in the inset in Fig. 7, show structures identical to those in Fig. 3(b). 3.6. FTIR and quantum chemical calculations FTIR spectra were obtained for two of the surface structures observed in this study. Fig. 8(a) shows a spectrum obtained from a clean Cu(1 1 0) surface exposed to acetic acid. The LEED pattern obtained afterward from was this surface consis 1 1 1 1 tent with coexisting and 4 4 4 4 domains. Fig. 8(b) shows a spectrum from a Cu(1 1 0)-(2 1)O surface exposed to acetic acid. The LEED pattern from this surface showed a split c(2 2) pattern, consistent with a surface like that shown in Fig. 3(e) and (f). The spectra from both surfaces were very similar and show vibrational bands largely consistent with a sym-
Fig. 8. FTIR spectra for (a) a clean Cu(1 1 0) surface exposed to acetic acid and (b) a Cu(1 1 0)-(2 1)O surface exposed to acetic acid.
metrically bound upright acetate species with characteristic ms (OCO) (1434 cm1 ), ds (CH3 ) (1335 cm1 ), and ms (CH3 ) (2932 cm1 ) d(OCO) (678 cm1 ) frequencies [30]. We note that a band at 1391 cm1 exists in Fig. 8(a). To gain insight into the possible origins of this band, we have performed quantum chemical calculations for this system. We used restricted Hartree–Fock (RHF) and density functional (DFT) optimizations. The calculations were carried out using the GAUSSIAN98 program [31]. We used an all electron model, i.e. we did not use any core potential approximations. The calculations were a self-consistent-field approach with linear combinations of gaussian type orbitals with a generalized gradient approximation, SCF–LCGTO–GGA. We calculated separately using two different wave functions (6-311G(d) and 6-31G(d)). The first basis set has five shells, an s-shell with six primitives, three sp-
S.M. York et al. / Surface Science 522 (2003) 34–46 Table 1 Experimental and calculated vibrational frequencies for acetate on Cu(1 1 0) surfaces Mode
ms (CH3 ) ms (OCO) da (CH3 ) ds (CH3 ) ds (OCO)
Experiment cm1
RHF 6-31G(d) cm1
6-311G(d) cm1
6-311G(d) cm1
2932 1434 1391 1335 678
2872 1459 1433 1352 645
2850 1446 1423 1338 645
3036 1434 1488 1382 675
B3LYP
shells with three primitives. The next two shells have one primitive each. There is also an uncontracted d-shell. The second wave function we used has one less shell. The results using this two basis sets are given in Table 1. The DFT calculations were using the B3LYP key word which employ methods given by Becke [32] and Lee et al. [33]. We approximated the copper surface with a ten atom cluster (Fig. 9). In our cluster calculations, the copper atoms are constrained to remain in their bulk solid positions. We found the optimized structure for the cluster complex and then computed the vibrational frequency spectra of our model. The short-bridge adsorption site agrees with experiments. Our primary goal computationally was to compare the FTIR data given in Fig. 8 to the cluster model calculations. This cluster model has forty four modes. All but the following five either have very low relative intensities or they do not have a significant dipole moment perpendicular to the copper surface. The resulting frequencies using RHF
Fig. 9. Diagram of the acetate–copper cluster used to calculate vibrational frequencies.
45
have been scaled with the usual factor of 0.8929 and the DFT results are scaled using 0.989 [34] and are displayed along with the FTIR results in Table 1. The calculated vibrational frequencies are all well within 5% of those observed in the FTIR experiments. These calculations suggest that the band observed in Fig. 8(a) at 1391 cm1 is due to the da (CH3 ) mode. This vibrational mode results in a dipole moment that is only partially perpendicular to the surface and hence the associated band may be more intense for a tilted acetate species as suggested in the discussion of Fig. 6(a). In regard to the small size of the Cu cluster used in this model, we note that calculational predictions of adsorption sites, adsorbate structure, orbital configuration [35], and vibrational frequencies are consistent with experiment. The finite size of the substrate model does not effect these results significantly, since the properties we have studied are highly local in character. This is not the case when the band structure of the substrate figures prominently in the measurements, i.e. heats of adsorption.
4. Conclusions We have explored the effect of preadsorbed oxygen on the formation of ordered acetate structures on Cu(1 1 0) surfaces. On clean and 0.15 ML O precovered surfaces, acetic acid exposure 1 1 1 1 results in domains of and 4 4 4 4 structures with no other lower coverage acetate structures observed. For oxygen coverages between 0.15 and 0.24, acetic acid exposure results in arrays of up–down step structures. Above an oxygen coverage of 1/4 ML, c(2 2) acetate structures are formed. STM images and LEED patterns are consistent with ordered arrays of antiphase domain boundaries. Oxygen not consumed in reactions with acetic acid appears to form c(6 2) structures. Models are proposed for the acetate structuresobserved here and suggest that for the 1 1 1 1 and up–down step and 4 4 4 4 structures that acetate occupies two inequivalent adsorption sites. The STM images obtained in this
46
S.M. York et al. / Surface Science 522 (2003) 34–46
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