J. Phys. Chem. C 2007, 111, 10503-10507
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Vanadium Oxide Overlayers on Vicinal Rh(15 15 13): The Influence of Surface Steps J. Schoiswohl, S. Surnev, and F. P. Netzer* Institute of Physics, Surface and Interface Physics, Karl-Franzens UniVersity Graz, A-8010 Graz, Austria ReceiVed: March 12, 2007; In Final Form: May 3, 2007
The growth of vanadium oxide monolayer films on a vicinal Rh(111) surface has been examined by scanning tunneling microscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy to determine the role of substrate steps on the oxide overlayer structures. While similar local V-O building blocks as on the flat Rh(111) surface are found on the stepped surface as a function of the chemical potential of oxygen, the effects of the interfacial chemistry of step atoms and of the strain relief at step edges lead to novel oxide structures that are not observed on the extended (111) surface. Specifically, the high chemical affinity of step atoms toward oxygen induces a reorientation of pyramidal VO5 units at high chemical potentials of oxygen generating a unique oblique superstructure near the step edges. For more reducing conditions a (2 × 2) surfaceV2O3 phase is formed, which is stabilized by the strain relief at step edges and is therefore not observed on the flat Rh(111) surface.
Introduction Vicinal metal surfaces provide natural templates for the growth of low-dimensional nanostructured materials using the selective decoration of their periodic step array.1 The intrinsic modification of the surface electronic structure by the step structure and periodicity is an important parameter that influences the growth of a different material on vicinal surfaces.2 In the ideal case, vicinal surfaces consist of a perfect staircase with the homogeneous step spacing determined by the miscut angle to the respective low-index parent surface. In practice, many different factors combine to make real vicinal surfaces much more complex than expected, and step bunching, the meandering of steps, and instabilities of the surface morphology toward adsorption are rather the common features than the exception. Apart from electronic effects, the steps also provide a geometrical perturbation of the surface. The latter may play an active role in the relaxation of adlayer strain. Both electronic and elastic effects may conspire to generate adlayers on vicinal surfaces that are different in structure and reactivity from those grown on the respective low-index surfaces.3 In the present paper, we examine the influence of the surface step structure of a vicinal metal surface, viz. Rh(15 15 13), on the growth of an ultrathin oxide overlayer (a so-called oxide nanolayer). The oxide material considered is vanadium oxide, which has been shown to form a range of highly ordered overlayer structures on Rh(111) surfaces. The growth and structure of vanadium oxides on Rh(111) have been studied in detail by a combination of experimental and theoretical methods in previous work.4,5 The vanadium oxide/Rh(111) surface phase diagram is characterized by a pronounced polymorphism, which means that many different oxide phases exist as a function of oxide coverage, stoichiometry, temperature, and chemical potential of oxygen. Here, we are interested in the influence of the surface steps of the Rh substrate on the atomic structure and morphology of V-oxide overlayers, and we will concentrate on interfacial effects, thus restricting the discussion to the first monolayers of V oxide. * Corresponding author. E-mail:
[email protected].
The pertinent questions concern the influence of the modified electronic structure of the Rh step atoms and of the geometrical confinement provided by the terrace width on the oxide overlayer growth. In other words, we investigate the role of the interfacial step chemistry and the possibility of overlayer strain relaxation at steps in determining the structure of an oxide nanolayer. And we will show that both effects may become important parameters. The vicinal Rh(15 15 13) surface exposes (111) terraces of 15 atoms wide, which are separated by monatomic steps with (111)-type microfacets (see the model of Figure 1a). In the ideal case of the stepped surface, ∼7% of the surface atoms correspond to step sites. Scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) have been used to monitor surface morphology, structure, and the oxidation state of the oxide overlayers, respectively. We find that the V-oxide overlayer structures are related in terms of their V-O building blocks to those that grow on the flat Rh(111) surface, but that significant structural modifications are introduced on the vicinal surface by the steps. These effects are interpreted in terms of stepinduced interfacial chemistry and overlayer stress relaxation. Experimental Methods The STM experiments have been performed in a customdesigned STM system as described previously.4 XPS spectra have been measured at beamline I311 in the Swedish synchrotron radiation laboratory MAX-lab, Lund.3 LEED was used to reproduce and to compare the different surface preparations in the two systems. The preparation of a well-ordered Rh(15 15 13) surface with a regular step array is difficult; on the one hand, the interaction between steps across the wide terraces, which provides the basis for the regular step arrangement, is weak. On the other hand, step bunching and faceting is observed under the influence of an oxygen atmosphere at elevated temperature; the latter has to be applied during surface cleaning and during the preparation of oxide overlayers if a reactive physical vapor deposition technique is used. However, the step bunching and faceting reconstructions can be lifted by a high-
10.1021/jp071984i CCC: $37.00 © 2007 American Chemical Society Published on Web 06/23/2007
10504 J. Phys. Chem. C, Vol. 111, No. 28, 2007
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Figure 1. The Rh(15 15 13) surface: (a) hard sphere model, the insert illustrates the step facet orientation; (b) STM image (540 × 540 Å2; sample bias U ) +2 V; tunneling current I ) 0.3 nA); (c) autocorrelation image of the STM image shown in (b); (d) line scan across the autocorrelation image along the white line, revealing the average terrace size.
temperature-annealing step in vacuum. Therefore, after the initial cleaning of the crystal including cycles of Ar sputtering, oxygen treatment, and annealing the Rh(15 15 13) surface has not been exposed to oxygen at an elevated temperature anymore. It has been gently sputtered with 0.5 keV Ar ions and then has been annealed to 1000 °C in ultrahigh vacuum (UHV) to obtain a clean starting surface for the various oxide overlayer preparation steps. The resulting morphology of the Rh(15 15 13) surface is shown in the STM image of Figure 1b. The step arrangement is not perfect; the step edges display some frizziness and the terraces some variation of widths. However, the autocorrelation image of the STM image given in Figure 1c and the line scan across the autocorrelation image (Figure 1d) reveals a mean terrace width of ∼33 Å, which is the expected theoretical value. The vanadium oxide overlayers have been prepared by deposition of submonolayer quantities of vanadium metal on to Rh(15 15 13) at room temperature followed by heating in 2 × 10-7 mbar O2 at 250 °C. The vanadium monolayer (ML) coverage is referred to as the number of Rh atoms on the flat Rh(111) surface (1.59 × 1015 atoms/cm2). It appears that the V-covered Rh(15 15 13) surface is more stable against step bunching and faceting in the subsequent oxidation step than the clean surface and this post-oxidation procedure has therefore been adopted for the generation of the oxide overlayers. Results and Discussion Figure 2 displays STM images of the Rh(15 15 13) surface after deposition of 0.2 ML V at room temperature (RT), subsequent annealing in 2 × 10-7 mbar O2 at 250 °C for 3 min, and cooling down to RT in oxygen. The large scale image of Figure 2a shows that the step structure of the surface has been preserved and that the step edges have been straightened
out after the V-oxide deposition; the step edges also display brighter contrast features in the STM. At higher magnification (Figure 2b) V-oxide structures can be identified that decorate the Rh step edges. The V oxide is predominantly arranged in two-dimensional islands that follow the step direction of the substrate ([110]) and extend over the entire terrace widths. Some oxide clusters close to step edges are also observed. The V-oxide structures are difficult to image with high resolution at this submonolayer coverage stage, but Figure 2c is an attempt. An ordered structure network can be inferred from the image in Figure 2c with an oblique (almost square) unit mesh of dimension ∼3.6 Å (as indicated on the image), which is rotated by 19.1° with respect to the substrate azimuthal directions; the smaller angle enclosed by the overlayer unit mesh is 81.8°. The LEED picture of Figure 2d confirms the oblique oxide overlayer unit cell: the pattern consists of (2 × 2) spots due to the (2 × 1) chemisorbed oxygen adlayer on the Rh terraces and of reflections due to an oblique unit cell (indicated on the picture). The latter are elongated as highlighted by the elipses and only one rotational domain is detected, indicating that the observed domain is determined by the step orientation of the Rh(15 15 13) surface. The ∼3.6 Å oxide unit cell length is x7/2 times the lattice constant of the Rh(111) surface unit cell (x7/2 × aRh ) 3.56 Å); the V-oxide structure corresponds to a superstructure with 1 1.5 a matrix of . On the flat Rh(111) surface a (x7 × 1 -1.5 x7)R19.1° vanadium oxide monolayer structure has been found previously under similar preparation conditions.4 This latter structure has been analyzed with the help of a combination of various experimental methods, including high-resolution STM, XPS and vibrational high-resolution electron energy loss
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Vanadium Oxide Overlayers on Vicinal Rh(15 15 13)
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10505
Figure 2. (a-c) STM images of the Rh(15 15 13) surface after deposition of 0.2 ML V and post-oxidation at 250 °C. (a) 1000 × 1000 Å2, U ) +2 V, I ) 0.1 nA. (b) 250 × 200 Å2, U ) +2 V, I ) 0.1 nA; the rectangles highlight V-oxide structures decorating the step edges. (c) 30 × 20 Å2, U ) +0.5 V, I ) 0.1 nA; a likely unit cell is indicated. (d) LEED pattern of the surface shown in (a); the oblique reciprocal unit cell of the V-oxide overlayer is indicated (electron primary energy 40 eV).
spectroscopy, and ab initio density functional theory (DFT).4 The stable structure model derived from the DFT calculations is reproduced in Figure 3a. The structure consists of slightly distorted tetragonal pyramidal building blocks O4-VdO, which contain the V atom in the center, four oxygen atoms in the basal plane and a double-bonded oxygen atom at the apex (the vanadyl group). These building blocks are linked together via the oxygen atoms in the basal plane, which are shared by two pyramids each. As indicated in Figure 3a, this leads to a (x7 × x7)R19.1° unit cell (dashed white line) with an overall stoichiometry of V3O9, which breaks down to VO3. Note that this stoichiometry is not in conflict with the formal maximum oxidation state of +5 of V atoms, because the basal oxygen atoms are attached to the substrate, and charge transfer from the Rh metal to the oxide overlayer can occur. The x7 structure is the stable V-oxide monolayer phase on Rh(111) at higher chemical potentials of oxygen.4 The V 2p3/2 core level binding energy of the oblique structure on Rh(15 15 13) is close to the one of the (x7 × x7)R19.1° phase on Rh(111): 515.0 eV versus 515.2 eV, respectively. This suggests that the two structures are closely related. Further support for this conjecture is obtained from Figure 4, which is an STM image from a Rh(15 15 13) surface covered entirely by the first V-oxide monolayer (prepared by post-oxidation of 0.6 ML V at 400 °C). This STM image shows a surface region with a rather irregular step distribution where two different oxide structures can be resolved. On the wider terraces, areas of the (x7 x x7) structure can be recognized (encircled on the image), whereas on the smaller terraces and close to the step edges regions of the oblique structure are detected (indicated by arrows). The x7 structure and the oblique structure can thus coexist at the surface and a structural similarity is therefore suggested.
It is tempting to apply the O4-VdO building block scheme of the x7 structure to derive the oblique structure. As shown in Figure 3b, the oblique unit cell can be constructed in a most natural way by linking the VO5 pyramids in such a way that they are oriented with one side of the basal plane parallel to the step edges. We note that the structure model of Figure 3b reproduces the experimentally observed structural parameters and puts the central V atoms of the pyramids in the bridge positions of the Rh substrate, which has been shown by DFT to be the most stable situation.4 The driving force for this reorientation of VO5 units on the stepped Rh surface may be ascribed to the high affinity of the Rh step edge atoms toward oxygen. As shown by Gustafson et al.,6 the adsorption energy of oxygen at Rh step sites is considerably enhanced as compared to the Rh terrace sites. The arrangement in the oblique structure allows the oxygen atoms of the VO5 basal plane to be located at the step sites, which leads to an increase of the adlayer-substrate bonding and to the shifting of the energy balance in favor of the oblique structure. The fact that the oblique structure is only observed in the vicinity of step edges or on narrow terraces indicates that it is intrinsically slightly less stable than the x7 structure and requires the step edges to acquire additional interfacial stabilization. As on the flat Rh(111) surface, the highly oxidized V-oxide monolayer structures can be transformed into more reduced V-oxide phases by annealing in UHV or by chemical reduction using H2 exposure.5 The STM images of Figure 5 have been recorded from a Rh(15 15 13) surface, which has been covered by a monolayer of the VO3-type oxide (oblique and x7 structure) and has then been reduced by exposure to 2 × 10-7 mbar H2, 3 min at 250 °C. The latter surface shows a wellordered oxide overlayer (Figure 5a), which displays two different
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Figure 3. (a) DFT-derived structure model of the (x7 x x7)R19.1° V-oxide monolayer structure on Rh(111) (ref 4); the (x7 x x7) unit cell (dashed white) and the VO5 building blocks (solid black) are indicated. Rh, light gray spheres; oxygen, red spheres; V, green spheres. (b) Analogous structure model of the oblique V-oxide layer (unit cell dashed white) on Rh(15 15 13), using the same VO5 building blocks (black lines); the decoration of a Rh step structure by the basal oxygen atoms of the VO5 building blocks is only schematic.
Figure 4. STM image of the stepped Rh surface covered by a full V-oxide monolayer (0.6 ML V post-oxidized at 400 °C) (250 × 250 Å2, U ) +1 V, I ) 0.1 nA). Circles denote areas of the (x7 x x7)R19.1° structure, the arrows point toward regions of the oblique structure.
commensurate geometries: a (9 × 9) superstructure on terraces wider than 30 Å (see Figure 5b), and a (2 × 2) superstructure on the narrower terraces (Figure 5c). The oxide structure thus depends on the terrace width. The (9 × 9) structure is well known and has also been observed on the flat Rh(111) surface,5 but a (2 × 2) structure has not been detected on Rh(111). Previous combined experimental and theoretical analysis has
Schoiswohl et al.
Figure 5. STM images of the Rh(15 15 13) surface covered with a V2O3-type overlayer (formed by reduction of the VO3 phase). (a) 500 × 500 Å2, U ) +2 V, I ) 0.1 nA. (b) Magnified view of the (9 × 9) V-oxide superstructure on the highlighted wide terrace area, the (9 × 9) unit cell is indicated (45 × 45 Å2, U ) +2 V, I ) 0.1 nA). (c) Magnified view of the (2 × 2) V-oxide superstructure on the highlighted narrow terraces, the (2 × 2) unit cell is indicated (75 × 75 Å2, U ) +1 V, I ) 0.1 nA).
determined the atomic structure of the (9 × 9) V-oxide phase: the structure is formed by flat hexagonal V6O12 building units, which are linked together in a complicated way to yield a (9 × 9) superstructure with a unit cell content of V36O54, thus an overall V2O3 stoichiometry (ref 5 and Figure 6). The (9 × 9) unit cell is too large to be accommodated on terraces
