Planar Vanadium Oxide Clusters: Two-Dimensional ... - CMS

VOLUME 92, N UMBER 20

PHYSICA L R EVIEW LET T ERS

week ending 21 MAY 2004

Planar Vanadium Oxide Clusters: Two-Dimensional Evaporation and Diffusion on Rh(111) J. Schoiswohl,1 G. Kresse,2 S. Surnev,1 M. Sock,1 M. G. Ramsey,1 and F. P. Netzer1,* 1

Institut fu¨ r Experimentalphysik, Karl-Franzens-Universita¨t Graz, A-8010 Graz, Austria 2 Institut fu¨ r Materialphysik, Universita¨t Wien, A-1090 Wien, Austria (Received 19 January 2004; published 21 May 2004)

The formation of novel vanadium oxide cluster molecules by oxidative two-dimensional evaporation from vanadium oxide nanostructures is reported on a Rh(111) metal surface. The structure and stability of the planar V6 O12 clusters and the physical origin of their 2D evaporation process have been elucidated by high-resolution scanning tunneling microscopy (STM) and ab initio density functional theory calculations. The surface diffusion of the clusters has been followed in elevated-temperature STM experiments, and the diffusion parameters have been extracted, indicating diffusion by hopping of the entire surface stabilized cluster units. DOI: 10.1103/PhysRevLett.92.206103

Clusters can be regarded as building blocks for the design of nanostructured materials, and much research effort is applied because of their potential use in different areas of the emerging nanotechnologies. Fundamental issues of understanding materials properties, e.g., how does structure and reactivity of a chemical compound change when passing from molecular units via nanosized clusters to the bulk solid, can be addressed in gas phase cluster studies. The evolution of bulk-type structural properties of a material with size is of prime interest; however, the characterization of cluster structures in the gas phase remains a challenge for the experimentalist. Metal oxide clusters in the gas phase have attracted extensive research activities recently because of the possibilities of addressing basic scientific questions of matter, on the one hand, and because of their relevance in technological applications, on the other hand. The latter include the use of fine powder materials, with their particular mechanical and magnetic properties [1], or their outstanding chemical reactivity in heterogeneous catalysis related systems [2– 4]. Many important oxidation catalysts contain vanadium oxides in highly dispersed form on various supports [5], and vanadium oxide clusters are good model systems to study and mimic the active sites in such catalysts. Gas phase vanadium oxide clusters have therefore been investigated recently in ionic and neutral forms, both experimentally [6 –8] and theoretically by density functional theory (DFT) [9,10]. Polynuclear vanadium oxide clusters have been predicted to form three-dimensional polyhedral cage structures [10], but the experimental determination of the geometry of free clusters is difficult and information on their structure has been obtained only by indirect means [7]. However, the size of the clusters has been recognized as an important parameter for their geometrical structure. Here, we show that the ‘‘boundary conditions’’ of clusters, that is, their environment and contact with interfaces, can be most important for the structure and stability of small oxide clusters. 206103-1

0031-9007=04=92(20)=206103(4)$22.50

PACS numbers: 68.47.Gh, 68.37.Ef, 68.55.–a, 71.15.Mb

In this Letter we report the formation and the diffusion properties of novel planar V6 O12 clusters on a Rh(111) metal surface. We have determined their atomic structure experimentally by high-resolution scanning tunneling microscopy (STM) measurements and theoretically by ab initio DFT calculations. The ‘‘cluster sources’’ in the present experiments were ultrathin ordered vanadium oxide nanostructures on a Rh(111) surface, from which the production of clusters could be followed during an oxidative two-dimensional evaporation process. The planar vanadium oxide clusters created in this way are in the form of hexagonal starlike features, which are remarkably stable and are able to diffuse as intact ‘‘molecules’’ across the surface. They are stabilized by the oxide-metal interface and constitute an as yet unrecognized kind of cluster material. The experiments have been performed in a variabletemperature STM system, which has been described previously [11]. Figure 1(a) shows a large-scale constant-current topographic STM image of the vanadium oxide cluster source, which is in the form of welldefined island structures partially covering the Rh(111) surface. The vanadium oxide islands have been prepared by evaporation of 0:25 monolayer of vanadium metal on to the oxygen-precovered Rh1112 1-O surface at room temperature, followed by a short annealing in ultrahigh vacuum at 250 C. The vanadium oxide islands constitute a two-dimensional mixed-valent p oxide monolayer phase with a rectangular 5 3 3-rect surface structure on the hexagonal (111) surface. The rectangular surface unit cell of this structure is indicated on the STM image of Fig. 1(b), which displays the details of this structure with higher magnification. p The stoichiometry and structure of the complex 5 3 3-rect phase has been resolved with the help of DFT calculations [12 –16]. The corresponding structure model is presented in Fig. 1(c), which reveals a V13 O21 unit cell, which may be regarded also in terms of 6V2 O3 1VO3 building units. Note that the formal VO3 unit in this latter  2004 The American Physical Society

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FIG. 1 (color online). (a) Constant current topographic STM image p of nanostructured islands of the vanadium oxide 5 2; 3 3-rect monolayer phase on Rh(111) (1500 1500 A sample bias U 2 V; tunneling pcurrent I 0:1 nA). (b) Magnified STM view of the 5 3 3-rect structure; the 2 ; U 1 V; I rectangular unit cell is indicated (55 55 A p 0:1 nA). (c) DFT derived model of the 5 3 3-rect structure. The V13 O21 unit cell is indicated by the square, and a (see text). (d) Simulated STM —O4 V—O unit is encircled p image of the 5 3 3-rect structure, considering tunneling into empty states between 0 and 1 eV.

—O unit, description corresponds to the central —O4 V— which is indicated in the figure. This is not in conflict with the maximal oxidation state of 5 of theVatoms, because the peripheral four bridging O atoms are shared with the Rh substrate. The simulated STM image, calculated with the structure model of Fig. 1(c) and the experimentally employed bias conditions of Fig. 1(b) [17,18], is shown in Fig. 1(d). The experimental image and the simulation agree in every contrast detail— see, e.g., the bright protrusion in the center due to the V— —O group or the dark holes at the corners and along the sides of the unit cell—


thus giving credence to the proposed structure model. Additional support for this model has been obtained from high-resolution electron energy loss spectra (not shown), where the characteristic phonon excitations of —O group have the bridging oxygen atoms and the pV— been observed. Note that the 5 3 3-rect phase is a stable structure in the V-oxide=Rh111 phase diagram, although it exists only in the single layer limit, where it is stabilized by the Rh(111) interface. oxidizing conditions, the vanadium oxide 5 pUnder 3 3-rect islands are observed to decompose and eject oxide clusters from their boundaries onto the free Rh surface, acting thus as a source for vanadium oxide surface clusters. The STM image of Fig. 2(a) demonstrates this two-dimensional evaporation process, which was induced by 5 108 mbar O2 in the gas phase at 130 C surface temperature: starlike, planar hexagonal clusters are observed, which move away from the oxide island boundaries. These unique ‘‘oxide molecules’’ are shown in more detail in Fig. 2(b). The stars have a preferred orientation with respect to the Rh(111) substrate [see Fig. 2(b)], their apparent and a diameter d 7 A height in the STM is relatively independent of the applied The DFT calsample bias and measured to 1:2 0:1 A. culations indicate that the V-oxide cluster stars are V6 O12 units, and their relaxed structure model is displayed in ˚ Fig. 2(c). Accordingly, the 6 V atoms are located 2.26 A above the fcc and hcp threefold hollow sites of the Rh(111) surface, six bridging O atoms are in approxi˚ above the Rh plane, and mately on-top positions 3.16 A ˚ above the Rh the peripheral O atoms are on-top 2.17 A ˚, plane. The bridging V—O bond distance is 1.79 A whereas the V—O distance of the peripheral O atoms is ˚ . These V—O bond lengths are within the typical 1.69 A range of V—O distances in vanadium oxide bulk structures [19]. The simulated STM image shown in the inset of Fig. 2(b) reflects exactly the experimental shape. Similar good experiment-theory agreement has been obtained for other tunneling conditions. The STM simulations clearly indicate the strong hybridization between V and O states. In the simulations, the apparent corrugation with respect to the surface is 1.6 , i.e., slightly larger than

p FIG. 2 (color online). (a) STM image of the 5 3 3-rect vanadium oxide phase showing the 2D evaporation of the planar 2 ; U 1 V; I starlike vanadium oxide clusters under oxidizing conditions (pO2 5 108 mbar, 130 C) (200 200 A 2 ; U 0:5 V; I 0:1 nA). The inset shows 0:1 nA). (b) High-resolution STM image of the V6 O12 starlike clusters (63 63 A the STM simulation based on the model structure of (c). (c) Relaxed DFT model geometry of V6 O12 clusters on Rh(111).

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in the experiment, but still significantly smaller than the ˚. real geometric corrugation of 3.16 A The structure and the chemical nature of the V6 O12 stars are determined by their boundary to the metal surface. Vyboishchikov and Sauer [10] have studied free clusters of V4 O10 and V6 O15 by DFT and found a tetrahedron and a trigonal prism as the most stable structures, respectively. This observed planar hexagonal geometry of the V6 O12 clusters is stabilized by 15.7 eVas a result of the bonding interaction to the Rh(111) substrate. The latter value has been calculated by lifting the stars off the surface and relaxing the freestanding stars in the vacuum. In the computations, it is also possible to oxidize the cluster by attaching double bonded oxygen atoms to V atoms. In this manner a V6 O15 or V6 O18 cluster can be created. These clusters are found to be unstable in the phase diagram, as oxygen would prefer to stay bonded directly to the Rh substrate. Nevertheless, the supported starlike V6 O15 cluster is 3.5 eV more stable than the unsupported trigonal prismatic V6 O15 cluster suggested by Vyboishchikov and Sauer [10], which is a clear indication that the substrate plays a decisive role and is capable of modifying the shape and morphology of small nanoparticles. p The ejection of V6 O12 clusters from the 5 3 3-rect phase on the Rh surface may be understood by considering the thermodynamic phase stability diagram of V oxides on Rh(111) as calculated by DFT. The diagram of Fig. 3(a) shows the relevant part of the phase stability

FIG. 3 (color online). (a) Section of the thermodynamic DFT phase stability diagram of vanadium oxide on Rh(111) in equilibrium with an O particle reservoir controlling the chemical potential O . The chemical potential can be related to the oxygen partial pressure through O T; p O T; p0 1=2kB T lnp=p0 [20]. The energy zero O 0 eV corresponds to oxygen molecules condensating on the surface. Lines are drawn at the nominal V coverage cV of each phase, if the corresponding phase is stable at a specific chemical potential O . Stars were considered at various coverages but found to be stable only at low coverages corresponding to a 7 7 supercell and cV 0:122. The structure ‘‘stars chem. O’’ was modeled in this supercell by adding between five and nine chemisorbed O atoms. (b) Schematic drawing illustrating a possible route for the formation of V6 O12 stars from the 5 p 3 3-rect structure.

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diagram, displaying the regions of stability of the 5 p 3 3-rect structure, of the V6 O12 stars, and of the oxygen-covered and bare Rh(111) surface as a function of the chemical potential of oxygen O and the vanadium coverage cV . At a reducing chemical potential of, e.g., 2:8 eV and a V coverage cV between 0 and 0.45 ML, only the clean surface and the rectangular structure are stable (this is indicated by the lower dot and the two arrows). Increasing O from, e.g., 2:8 to 2:2 eV, one goes from the coexistence of clean Rh and the rectangular p structure to a position where clean Rh, 5 3 3-rect, and the stars coexist. That is, applying oxidizing conditions leads to the stars becoming thermodynamically favored at the expense of the rectangular structurep(upper dot); thus, the stars evaporate from the 5 3 3-rect islands onto the Rh surface. Evaporation should occur once oxygen dissociation sets in on the Rh(111) surface. Figure 3(b) gives a pictorial p view of how the stars could evolve from the 5 3 3-rect structure. The overall reaction can be described formally by the chemical equation 2V13 O21 5O2 4V6 O12 V2 O4 ; thus, the evaporation process corresponds to an oxidation reaction of the mixed-valent V13 O21 oxide to VO2 -type oxide phases. Under reducing conditions, the reverse process can also be observed. We note parenthetically that under certain conditions fast moving rodlike features have been seen in the STM images, which are presumably associated with the V2 O4 products, but it is also conceivable that three V2 O4 clusters agglomerate to form another V6 O12 star. Having found the means of production of V6 O12 clusters on the Rh(111) surface, we have studied their dynamic behavior and have followed the surface diffusion of the stars at the molecular level. From STM movies at a number of temperatures (110 –140 C) we have determined the diffusion length versus time for low coverages of stars: Figs. 4(a) – 4(c) show STM images taken at 110 C [21] —the arrows indicate the displacement direction of the stars during the time elapsed between consecutive images. Note that in Fig. 4(c) a star has moved under the STM tip during the scanning process (encircled). Assuming 2D isotropic diffusion of isolated particles executing a random walk, the tracer diffusion coefficient D can be derived from a plot of the mean squared displacement hjrt r0j2 i versus time [22,23] according to hjrt r0j2 i 4Dt. Figure 4(e) shows this plot for T 110 C, where each data point has been averaged over 100 independent observations. An Arrhenius plot of D versus 1=T [Fig. 4(d)] following the equation D D0 expED =kb T yields the activation energy of diffusion, which is evaluated to be ED 1:30 0:07 eV, with a preexponential factor D0 0:43 cm2 s1 . This value of 206103-3




materials via the self-assembly route of molecular cluster building blocks. This work has been supported by the Austrian Science Fund.

FIG. 4 (color online). (a) –(c) STM images of V6 O12 stars on Rh(111), recorded consecutively after the indicated time at 2 ; U 1 V; I 0:1 nA). The arrows in110 C (85 85 A dicate the hopping directions. (d) Arrhenius plot of the tracer diffusion coefficient D versus 1=T. (e) Plot of the mean square displacement versus time at T 110 C, from which D has been evaluated.

the preexponential factor indicates a diffusion mechanism involving the entire clusters [24], which is supported by the experimental observation that the clusters seem to diffuse always as a whole. The height of the diffusion barrier has also been estimated theoretically by DFT calculations. Shifting a star from its favored position with theVatoms above hollow sites to an adjacent position above bridging sites costs 1.7 eV. This is of similar magnitude as the experimentally obtained value of the diffusion barrier and suggests that the configuration over bridging sites provides the saddle point of the diffusion process. In summary, the creation of a novel type of vanadium oxide cluster material has been observed, by scanning tunneling microscopy, to occur on a Rh(111) surface as the result of the oxidation reaction of a vanadium surface oxide phase. The structure, the stoichiometry, and the physical nature of the formation process of planar V6 O12 cluster molecules have been elucidated by highresolution STM measurements in combination with ab initio DFT calculations. The surface diffusion of the V-oxide clusters has been followed in elevatedtemperature STM experiments, and the diffusion parameters have been determined quantitatively, indicating that the oxide clusters hop as entire units. The results of this study demonstrate that the boundary conditions imposed by a solid surface can create new cluster forms, which are not stable in the gas phase. This has interesting implications for the fabrication of nanostructured oxide

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*Electronic address: [email protected] [1] A. E. Berkowitz, Magnetic Properties of Fine Particles (North-Holland, Amsterdam, 1992). [2] G. Busca et al., Appl. Catal. 18, 1 (1998). [3] I. Yamanaka et al., J. Mol. Catal. A 133, 251 (1998). [4] X. Gao and I. E. Wachs, J. Phys. Chem. B 104, 1261 (2000). [5] H. H. Kung, Transition Metal Oxides, Surface Chemistry and Catalysis (Elsevier, Amsterdam, 1989). [6] M. Foltin et al., J. Chem. Phys. 111, 9577 (1999). [7] R. C. Bell et al., J. Chem. Phys. 114, 798 (2001). [8] K. R. Asmis et al., Phys. Chem. Chem. Phys. 4, 1101 (2002). [9] M. Calatayud et al., J. Phys. Chem. A 105, 9760 (2001). [10] S. F. Vyboishchikov and J. Sauer, J. Phys. Chem. A 105, 8588 (2001). [11] S. Surnev et al., Phys. Rev. B 61, 13 945 (2000). [12] The simulations were performed using Vienna ab initio simulation package (VASP) [13] applying density functional theory in the generalized gradient approximation [14], the projector augmented wave method [15,16], and a plane wave cutoff of 250 eV. The slabs were modeled using four Rh layers, and a Brillouin sampling corresponding to 8 8 k points in the primitive surface cell. [13] G. Kresse and J. Furthmu¨ller, Comput. Mater. Sci. 6, 15 (1996). [14] Y. Wang and J. P. Perdew, Phys. Rev. B 44, 13 298 (1991). [15] P. E. Blo¨ chl, Phys. Rev. B 50, 17 953 (1994). [16] G. Kresse and D. Joubert, Phys. Rev. B 59, 1758 (1999). [17] STM simulations are based on the Tersoff-Hamann approach [18]. The charge isosurfaces were evaluated at a ˚ above the value that the brightest spots are located 4 A core of the topmost atom in the oxide. [18] J. Tersoff and D. R. Hamann, Phys. Rev. B 31, 805 (1985). [19] S. Surnev et al., Prog. Surf. Sci. 73, 117 (2003). [20] K. Reuter and M. Scheffler, Phys. Rev. B 65, 035406 (2002). [21] The STM images were taken with a high tunneling resistance (10 G") in order to minimize the influence of the scanning tip on the diffusion of stars. A typical STM image (512 512 pixel) has been obtained in 42 sec. However, variations in the interaction time between the tip and the clusters did not give differences in the derived values for D. Thus, we conclude that tip effects can be excluded. [22] G. L. Kellogg, Surf. Sci. Rep. 21, 1 (1994). [23] R. Gomer, Rep. Prog. Phys. 53, 917 (1990). [24] S. C. Wang et al., Phys. Rev. Lett. 81, 4923 (1998).

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