J. Phys. Chem. B 2006, 110, 15359-15367
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Ultrathin Wagon-Wheel-like TiOx Phases on Pt(111): A Combined Low-Energy Electron Diffraction and Scanning Tunneling Microscopy Investigation Francesco Sedona, Stefano Agnoli, and Gaetano Granozzi* Dipartimento di Scienze Chimiche and Unita` di Ricerca INFM-CNR and INSTM, UniVersita` di PadoVa, Via Marzolo, I-35131 PadoVa, Italy ReceiVed: April 7, 2006; In Final Form: June 9, 2006
Ultrathin ordered titanium oxide films on a Pt(111) surface have been prepared by reactive deposition and characterized by low-energy electron diffraction and scanning tunneling microscopy (STM). According to the postdeposition annealing condition, three different phases have been prepared which show a wagonwheel-like (hereafter ww) morphological pattern. Two of them can be prepared as single phases (w- and w′-TiOx) and one (wint-TiOx) as a mixed phase which always coexists with at least one of the other two phases. All of them are formed by a Ti-O bilayer, where the Ti atoms are located at the interface with the substrate, but they show a rather distinct STM ww pattern. The experimental STM contrast has been discussed on the basis of a Moire´-like model, i.e., as deriving from a modulation of the Ti occupancy of the different substrate sites (i.e., hollow, bridge and on-top sites). The major part of the STM data can be easily interpreted on the basis of this simplified model.
1. Introduction Titania films on a metal surface (here after indicated as titania/ metal) represents a very attractive topic1 for many applicative fields, e.g., photocatalysis2 and gas sensors.3 Among the different possible metals, platinum is one of the most intriguing: the large interest is justified by the promotion effect of Pt in photocatalysis and because the titania/Pt system represents a classical example of the well-known strong metal support interaction (SMSI) effect.4 Actually, the SMSI allows tailoring of the selectivity of a catalyst, and this has caused a widespread interest in the scientific community in order to understand the reasons for this behavior.5 Besides studies on real catalysts (metal particles on an oxide substrate), also the inVerse model catalysts have been investigated, where an oxide overlayer decorates a metal single-crystal surface.6 By this method it is possible to study the SMSI effect in a more ideal and ordered system, where the modern surface science techniques are easily applicable and provide very detailed information. Very recently, we have reported the results of a detailed study on the preparation and structural characterization of ultrathin TiOx films grown on a Pt(111) surface.7 This topic revealed a rich source of new and potentially interesting materials: we have shown that several surface-stabilized nanophases can been prepared in the monolayer regime. They have been obtained as almost single phases by reactive evaporation of Ti in the presence of oxygen (pO2 ) 10-4 Pa) and after a patient optimization of the preparation procedures (i.e., varying the deposition rate, coverage, annealing temperature and time, and oxygen partial pressure). The TiOx phases have been characterized by means of X-ray photoelectron spectroscopy (XPS), lowenergy electron diffraction (LEED), scanning tunneling microscopy (STM), and photoelectron diffraction (XPD). * To whom correspondence may be addressed. E-mail:
[email protected]. Phone: +39-0498275158. Fax: +39-0498275161.
In the present paper we will focus on a subset of them which we call wagon-wheel-like (hereafter ww) phases because of their quite distinctive STM pattern. The ww definition has been introduced for the first time by Zhang et al.8,9 to describe the STM images of Cr/Pt(111) (reported in Figure 1d), where the“hub”and the“spokes”of the ww can be easily identified. However, overlayers with a ww-type structure represent a general phenomenon, and they have been observed on many different hexagonal substrates. In section 3 we will propose a stringent and geometric definition of a ww structure, which is not based only on the experimental STM pattern, and we will briefly review the models adopted in the literature to describe this kind of system. We report here LEED and STM data of three different ww TiOx/Pt(111) phases. According to previous XPS and XPD data,7 these ww phases are formed by a Ti-O bilayer, where the Ti atoms are located at the interface with the substrate and the oxygen atoms most probably occupy hollow positions of the Ti layer. Accordingly, the stoichiometry of this nanophase is close to TiO, in agreement with the reported XPS results.7 In the following discussion we make an attempt to interpret the experimental STM contrast on the basis of a Moire´-like model, i.e., as deriving from a modulation of the Ti occupancy of the different substrate sites (i.e., hollow, bridge, and on-top sites). In the absence of a quantum mechanical determination of the STM contrast, we have no way to corroborate the topographic or electronic nature of such a contrast. However, the actual assignment of each STM feature to a specific atom (i.e., bright features at positive bias associated with Ti localized empty states or alternatively to oxygen atoms) is unnecessary within the adopted interpretative framework. Actually, reversing the actual assignment would only determine a change in the overlayersubstrate registry. Nevertheless, it is our opinion that the hypothesis which relates the bright features at positive bias to Ti-localized empty states is more justified on the basis of the prevalent literature interpretation of the contrast in STM images
10.1021/jp062180q CCC: $33.50 © 2006 American Chemical Society Published on Web 07/15/2006
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Sedona et al. recipes adopted were discussed in detail elsewhere.7 After the reactive evaporation of Ti onto clean freshly sputtered and annealed Pt(111) single crystals, two ww-like phases, namely, w-TiOx and the w′-TiOx, can be obtained as single phases (as judged from the LEED patterns) by changing the conditions of the postannealing, as described in reactions 1 and 2 reported in the following 823 K, pO ) 10-5 Pa 2
Pt(111) + Ti + O2 98 w-TiOx/Pt(111) (1) 723 K, pO < 10-8 Pa 2
Pt(111) + Ti 98 w′-TiOx/Pt(111)
(2)
These reactions show that the w-TiOx phase is obtained under oxidizing conditions, while the w′-TiOx one under reducing conditions. Between these two phases, there is a intermediate ww phase: the wint-TiOx that always coexists with at least one of the other two phases. As outlined in reactions 3 and 4, it is possible to transform the w-TiOx into the w′-TiOx and vice versa by a reducing or oxidative heat treatment. In the same reactions 3 and 4 reported below it is also shown as the wint-TiOx phase is an intermediate step in this reversible transformation
Reductive transformation 4 min@723 K, pO < 10-8 Pa 2
w-TiOx 98 w-TiOx 10 min @ 723 K, pO2 < 10-8 Pa wint-TiOx 98 w′-TiOx (3) w′-TiOx Oxidative transformation 7 min @ 823 K, pO < 10-5 Pa 2
Figure 1. Different wagon-wheel-like phases on different systems from literature: (a) TiOx on Pd(111) clusters on TiO2(110);17 (b) VOx on Pd(111) (0.1 V, 1.0 nA);18 (c) FeO on Pt(111) (0.2 V, 0.5 nA);19,20 (d) Cr on Pt(111) (-0.5 V, 0.6 nA);8,9 (e) Ce on Pt(111) (-0.08 V, 1.67 nA);21,22 (f) VOx on Rh(111) (2 V, 0.1 nA);23,24 (g) Al2O3 on Ni3Al(111) (3.2 V, 0.12 nA).25,26
of TiOx surfaces,1 and for this reason it will be adopted in the following. Obviously, without the support from theoretical simulations, such an interpretation is to be considered a tentative one. Finally, we would like to outline that apart from the scientific challenges, the study of such long-range ordered systems can find an interesting application in the field of nanotechnology. Actually, it has been recently demonstrated that some ww reconstructions (i.e., alumina/Ni3Al(111)10,11 and FeO/Pt(111)12) can be used as templates for self-organization of metal atoms and/or clusters into nanostructured arrays. 2. Experimental Section The Pt(111) surface is prepared by several Ar+ sputtering and annealing cycles at 870 K until the C 1s signal is below the detection limit of XPS. The subsequent inspection with LEED shows a clear (1 × 1) diffraction pattern. The deposition of titania is performed in an oxygen background (pO2 ) 10-4 Pa) by electron-beam evaporation from a Ti wire. The actual
w′-TiOx 98 w-TiOx 13 min @ 823 K, pO2 < 10-5 Pa wint-TiOx 98 w-TiOx (4) w′-TiOx The LEED and STM characterization was done at room temperature (RT) in an Omicron VT-STM system operating at a base pressure of 5 × 10-9 Pa. The system is equipped with four-grid LEED optics. Pt-Ir tips were used in all the experiments. Tunneling voltages are given with respect to the sample. The tunneling parameters are reported in the corresponding captions of the reported STM images. The scanner was calibrated in the z-direction with respect to the step edge of the clean Pt(111) surface. For the lateral calibration a (2 × 1) reconstructed Pt(110) surface has been used. 3. Results and Discussion Before entering into the description of the our results, we want first to put forward a general definition of a ww structure and briefly review the state of the art of the model adopted for these systems. We propose the following definition: A ww structure is a hexagonal commensurate superstructure with symmetry p6 which grows on a p6m substrate, and where the unit cell Vector of the commensurate superstructure is rotated by a θ angle with respect the principal direction of the substrate. Using the matrix notation and adopting an angle of 120° (and not 60°) between the two unit-cell vectors, it is possible to define
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TABLE 1: Summary of the Different Wagon-Wheel-like Structures Reported in Literature and in This Work (last three rows)a substrate unit cell dimensions (Å)
(x39 × x39)R16.1°
17.17
2.75
[7,2]//[6,2]
3.25
3
(x63 × x63)R19.1°
21.83
2.75
b
b
b
(x91 × x91)R5.21°
26.42
2.77
[10,1]//[9,1]
3.1
0.6
(x39 × x39)R16.1°
17.3
2.77
b
b
b
(x28 × x28)R19.1°
14.66
2.77
[6,2]//[6,1]
2.63
10.16
(7 × 7)R21.79°
18.83
2.69
[8,3]//[7,4]
3.1
3.5
(x67 × x67)R47.8°
41.6
5.076
b
b
b
(x43 × x43)R7.6°
18.16
2.77
[7,1]//[6,1]
3.26
1.36
15 5 -5 10
(x175 × x175)R19.1°
36.64
2.77
[15,5]//[13,5]
3.23
3.3
8 3 -3 5
(7 × 7)R21.79°
19.39
2.77
[8,3]//[7,4]
3.18
3.5
matrix notation
system TiOx on Pd(111) clusters on TiO2 (110)17 VOx on Pd(111)18 FeO on Pt(111)19,20 Cr on Pt(111)8,9 Ce on Pt(111)21,22 VOx on Rh(111)23,24 Al2O3 on Ni3Al(111)25,26 w-TiOx on Pt(111) (this study) wint-TiOx on Pt(111) (this study) w′-TiOx on Pt(111) (this study) a
supercell dimensions (Å)
[ [ [ [ [ [ [ [ [ [
7 2 -2 5 9 3 -3 6 10 1 -1 9 7 2 -2 5 6 2 -2 4 8 3 -3 5 9 2 -2 7 7 1 -1 6
] ] ] ] ] ] ] ] ] ]
Wood’s notation
substrate// overlayer alignment
overlayer unit cell dimensions (Å)
overlayer rotation (deg) with respect to substrate
The relevant geometric data of the commensurate superstructures with respect the different substrates have been reported. b Not reported.
the p6 ww structure with respect to the p6m substrate as a
[
x y -y x - y
]
(with x > y > 0)
If we adopt this definition, different systems can be encompassed and easily assembled in two different groups. The former is formed by adlayers having weak van der Waals interactions with the hexagonal substrate, as in the case of MoSe2/MoS2,13 C6Br6/MoS2,14 C6Br6/HOPG,14 graphite/Pt(111),15 and S/Al(111).16 The latter group of ww structures, studied more recently, is formed (as in our case) by oxide or metallic alloy films grown on different hexagonal substrates: TiOx/Pd(111)/TiO2(110),17 VOx/Pd(111),18 FeO/Pt(111),19,20 Cr/Pt(111),8,9 Ce/Pt(111), 21,22 VOx/Rh(111),23,24 and Al2O3/Ni3Al(111).25,26 Table 1 summarizes the different characteristics of the literature ww phases for the second group and for the phases described in this paper. In the first three columns we report the geometric peculiarities of the superstructures: the matrix notation, the Wood’s notation (where the angle between the unit cell and the principal direction of the substrate is directly visible), and the dimensions of the commensurate unit cell. In the fourth column we report the unit cell dimensions of the p6m substrate, and in the following columns the experimental data of the overlayer derived from the analysis of the STM are summarized. Some relevant literature STM images found for these different ww phases are also reported in Figure 1: it is evident that not all of them show a well developed STM ww motif, but this depends on the quality of the experimental STM images. There are two principal models adopted in the literature for describing the ww phases. The first one, proposed in the work of Zhang et al.,8,9 explains the peculiar STM contrast as due to
a local variation of the surface chemical composition, i.e., the presence of a surface alloy between the Pt substrate and the Cr atoms. The second model, adopted in the cases of VOx/Rh(111),24 FeO/Pt(111),20 and TiOx/Pd(111)/TiO2 (110)17(see Table 1), assumes that the STM contrast is due to a Moire´-like pattern obtained by superimposing two different long-range commensurate hexagonal meshes, rotated with respect to each other by a few degrees. In the following we will briefly report some more details of the two models. Alloy Model. In the Cr/Pt(111) system,9 the authors found a large contrast between the hub, the spokes, and the internal triangles of the ww. However, the STM images do not show atomic resolution (Figure 1d). As reported in Figure 2 (left), this model associates the dark triangles (at negative bias, while at positive bias it becomes bright) to a triangular cluster of 10 Cr atoms. On the contrary, the bright spokes are attributed to Pt atom lines which can be interpreted as dislocation lines due to Cr-induced reconstruction of the topmost layer of the Pt(111) substrate. In the right side of Figure 2 the gray-and-black atoms show the close packed face-centered cubic (fcc) and hexagonal close packed (hcp) stacking position of the reconstructed Pt layer. The light-gray atoms occupy bridge sites with respect to the Pt layer underneath and form a dislocation line which separates the hcp and fcc triangles. Referring to Table 1, this alloy model cannot be applied to the other systems for two main reasons: (a) As evidenced by the line a-b in Figure 2, the Cr atoms of the triangle and the topmost layer Pt atoms on the spoke lines are not in registry, whereas in the atomically resolved STM images reported in Figure 1 there is no evidence for any dislocation line, and the atoms form an ordered hexagonal mesh.
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Figure 2. Model of the Cr-induced Pt(111) reconstruction (from refs 8 and 9). The gray-and-black atoms show the close-packed fcc and hcp stacking positions of the reconstructed Pt layer, respectively. The light-gray atoms occupy bridge position and form a dislocation line. Small circles represent the Pt(111) layer underneath. On the left, toplayer Cr atoms are shown as big circles.
Figure 3. (a) Experimental LEED pattern of the w-TiOx phase. (b) Real space superstructure with respect to the substrate: a1 and a2 denote the lattice vectors of the coincidence unit cell rotated by 7.6° with respect to the substrate whereas the d1 and d2 denote the lattice vectors of the substrate..
(b) In the alloy model the Cr atoms are on the hollow positions with respect to the Pt atoms, i.e., there is no rotation between the two unit cells of the overlayer and of the substrate. On the contrary, as reported in Table 1, the substrate and overlayer unit cells for the other ww systems are not aligned. Moire` -like Model. This model has been proposed for the VOx/Rh(111),24 FeO/Pt(111),20 and TiOx/Pd(111)/TiO217 systems (see Table 1). It simply implies a superimposition of the hexagonal mesh of the overlayer above the substrate one, looking for long range coincidences. One first simplified approach is that the STM contrast is topographical, i.e., simply dictated by the atom height. Accordingly, the atoms sitting in on-top and bridge positions will appear brighter than the atoms sitting on hollow positions. However, it is expected that this topographic interpretation cannot explain the fine details of the experimental patterns, and electronic effects should be included to accurately simulate the STM contrast. 3.1. The w-TiOx Phase. The w-TiOx phase is characterized by a very sharp experimental LEED pattern (reported in Figure 3a) which can be assigned to a hexagonal coincidence lattice, described by
[ ] 7 1 -1 6
(or alternatively as (x43 × x43)R7.6° in Wood’s notation).
Sedona et al. This phase has been already reported by Boffa et al.27 Simple geometric simulations of the diffraction spots provide the observed LEED pattern when a real space net (Figure 3b) is considered where the w-TiOx phase has a hexagonal unit cell with a unit vector of 18.2 Å rotated by 7.6° with respect to the 〈11h0〉 direction of the Pt substrate. The w-TiOx phase gives rise to a flat and continuous film which wets the entire Pt surface. Figure 4 reports two large scale STM images of the w-TiOx phase which show two different regions marked on the figures with I and II. Region I is found at the center of big terraces and shows a hexagonal arrangement of black holes (Figure 4 b) in perfect agreement with the cell dimensions determined from the LEED pattern. Figure 4a shows, on the contrary, that the regions II are present near the step edges and are characterized by an arrangement of straight dark troughs that grow quasi-parallel to the step edge. Here the film appears very defective and it is hard to determine the unit cell univocally. These trough domains develop only near the step edge and therefore it is not possible to find large areas with this motif: this can explain the lack of extra spots on the observed LEED patterns. Figure 5 reports an atomically resolved STM image for region I obtained at a positive bias. The overall pattern of the hexagonal dark spots, separated by 18.2 Å from each other, and surrounded by six bright rhombs, looks like a ww motif. The dark spot in the center represents the hub and the six bright rhombs the spokes of the ww. The line profile reported in the same figure gives an estimate of the depression of the dark hub. The bumps on this film form a hexagonal mesh with a mean spacing of 3.3 ( 0.1 Å. As already pointed out in the Introduction, we will assume that bright features at positive bias are to be associated with Ti localized empty states. Since we have demonstrated by XPS and XPD measurements that the w-TiOx phase is formed by a Pt-Ti-O stacking, we suggest that in an STM experiment we are imaging the Ti atoms lying at the interface with the substrate. We do not have any indication about the mesh of the oxygen atoms, but as a first approach, we suggest that the oxygen atoms lie on the hollow positions generated by the Ti atoms. It is interesting to note how the contrast changes as a function of the applied bias: In Figure 6 we report three different highresolution STM images collected at different bias values. At a bias higher than 1 V the hub appears simply as a dark spot (Figure 6a), while at positive bias below ∼1 V a little bright bump appears in the center of the dark hub (Figure 6b). For a negative bias (Figure 6c), even if the image resolution is substantially worsened, a broader and elongated bump at the center of the hub becomes very evident. Figure 7a shows how the commensurate (x43 × x43)R7.6° superstructure can be obtained by superimposing to the hexagonal Pt(111) mesh (periodicity 2.77 Å) a hexagonal oxide overlayer mesh with an atomic spacing of 3.26 Å and rotated by 1.36° with respect to the 〈11h0〉 direction of the Pt substrate. This corresponds to a situation where the [7,1] lattice vector of the Pt mesh is parallel to the [6,1] lattice vector of the overlayer. As a first approach, we can try to understand the experimental contrast of the STM image (Figure 7b) assuming this simple Moire´-like geometrical model and that the contrast is topographic, i.e., simply dictated by the atom height. In Figure 7a we have used different colors for the atoms on different positions with respect to the Pt substrate: black Ti atoms lie at on-top (coincidence) sites, dark blue on quasi-on-top sites, yellow on quasi-bridge sites, and light blue on the quasi-hollow sites.
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Figure 4. (a) Large scale (770 × 700 Å2, V ) 1.1 V, I ) 0.6 nA) and (b) (380 × 380 Å2; V ) 1.2 V, I ) 1.6 nA) STM constant-current images of the w-TiOx phase. Both images show the presence of two domains: a large domain in the middle of the terrace (I) with hexagonal unit cell dimensions consistent with those determined from the LEED pattern, and a second domain (II) near the step edge characterized by the presence of troughs.
images. Finally, the coincidence sites (black in the figure) appear in Figure 7b as black holes (the hub of the ww). In this case the contrast does not seem to be determined by the anticipated topographic height. Actually, such black holes could be associated either to electronic effects, due to the coincidence, or to some Ti vacancies. As a whole the nature of the topographic contrast is not clear-cut in this w-TiOx phase and this could be an important point to be clarified by theoretical simulations of the STM maps. On the contrary, the contrast of the other two ww phases reported in the following seems to be rather well described on the basis of a purely topographic contrast. 3.2. The w′-TiOx Phase. The w′-TiOx LEED pattern reported in Figure 8a indicates a commensurate superstructure
[ ] 8 3 -3 5
Figure 5. High-resolution (75 × 75 Å2; V ) 1.3 V, I ) 1.9 nA) STM constant-current image of the w-TiOx phase and a line profile along the displayed direction.
This topographic model reproduces only some of the experimental contrast shown in the high-resolution STM image reported in Figure 7b. Actually, the higher atoms in a bridge position can be associated to the brighter rhombs in Figure 7b, but the light blue atoms on quasi-hollow and the dark blue on quasi-on-top positions show the same experimental contrast. However, to discuss this point, it is useful to remember that the quasi-on-top position is very close to a quasi-hollow position: if we displace (by only 0.2 Å) the quasi-on-top atom far from the dark hub, the atom assumes a quasi-hollow position. This small displacement is difficult to evaluate from the STM
(or (7 × 7)R21.8° in the Wood’s notation). The real space drawing reported in Figure 8b shows that the w′-TiOx phase presents a hexagonal unit cell with a unit vector of 19.4 Å, rotated by 21° with respect to the 〈11h0〉 direction of the Pt substrate. Similarly to the w-TiOx phase, also the w′-TiOx phase forms a flat and continuous film which wets the entire Pt surface, as evident from the large scale STM image reported in Figure 9a. In this image the motif of the unit cell is very complex and shows a low contrast. The periodicity is best recognized in the autocorrelation representation of the STM image, as displayed in Figure 9b. According to this plot, the observed STM image is perfectly compatible with the (7 × 7)R21.8 superstructure observed in the LEED pattern. A similar (7 × 7)R21.8° phase has been reported by Schoiswohl et al. for the reduced VOx/Rh(111) system.23,24 In this case, using a DFT approach, Schoiswohl et al. have derived a model starting from a VO(111) lattice. The ww phase is composed of a V-O bilayer, where V atoms are located at the Rh interface and the phase is oxygen terminated. The DFT simulations were successful in explaining the experimental dimensions of the unit cell overlayer, but the agreement of the simulated STM images with the experiment was defined by the authors themselves only modest.
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Figure 6. Bias dependence of the STM images of the w-TiOx phase: (a) 50 × 50 Å2, V ) 1.3 V, I ) 1.9 nA; (b) 54 × 54 Å2, V ) 0.9 V, I ) 1.6 nA; (c) 54 × 54 Å2, V ) -1.0 V, I ) 1.6 nA.
Figure 7. (a) Schematic drawing of the Moire´-like coincidence between the Pt(111) lattice and the w-TiOx superlattice (Ti atom color code: yellow ) quasi-bridge, light blue ) quasi-hollow, dark blue ) quasi-top, black ) coincidence or Ti vacancy, see text). (b) Corresponding atomically resolved STM image.
Figure 8. (a) Experimental LEED pattern of w′-TiOx phase. (b) Real space superstructure with respect to the substrate: a1 and a2 denote the lattice vectors of the coincidence unit cell rotated by 21.8° with respect to the substrate.
Figure 10b shows an atomically resolved STM image of the w′-TiOx phase. Another ww motif, different from the w-TiOx, is present and the lattice vectors nicely match the LEED cell dimensions. The hub of the ww is now created by a bright spot which is hexagonally surrounded by six other bright protrusions, and the six bright lozenges represent the spokes of the ww. Figure 10a displays how, in a fashion similar to what was done for the w-TiOx phase, one can construct the Moire´-like structure of the w′-TiOx: one must take an oxide lattice parameter of 3.18 Å, rotated by 3.5° with respect to the 〈11h0〉 direction of the Pt substrate, to create the observed 19.4 Å periodicity of the (7 × 7)R21.8° superstructure. In such a way, the [8,3] lattice vector of the Pt mesh is parallel to the [7,4]
Figure 9. (a) Large scale (310 × 310 Å2, V ) 1.0 V, I ) 1.35 nA) STM constant-current image of the w′-TiOx phase. (b) Autocorrelation diagram of the STM image.
lattice vector of the overlayer. Also, in Figure 10a we have indicated the atoms on different positions with respect to the Pt substrate with different colors (the color code is the same as that of the previous example). In this case the geometric model can suggest a simple satisfactory explanation of the contrast: the on-top, quasi-on-top, and quasi-bridge positions appear as bright and the quasi-hollow positions appear as darker, in agreement with the different topographic height. As already seen in the large-scale STM image (Figure 9a), this phase is more defective than the w-TiOx one and it is hard to find extended domains with a good contrast as the one reported in Figure 10b. 3.3. The wint-TiOx Phase. As mentioned above, the wint-TiOx phase appears during the transformation of w-TiOx into w′-TiOx or vice versa; therefore it always coexists with at least one of
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Figure 10. (a) Schematic drawing of the Moire´-like coincidence between the Pt(111) lattice and the w′-TiOx superlattice (Ti atom color code: yellow ) quasi-bridge, light blue ) quasi hollow, dark blue ) quasi-top, black ) coincidence). (b) Corresponding atomically resolved STM image.
the other two ww phases. Actually, during the reductive or the oxidative transformations, the LEED pattern shows a superimposed pattern of the w and w′ phases and a high background. However, no extra spot that could be associated to a new ordered structure is visible. Nevertheless, in the corresponding STM large scale image reported in Figure 11a, a new ww structure is visible. The geometric motif of the wint-TiOx phase is really complicated. In Figure 11b the positions of the different ww structures are outlined by asterisks and straight lines have been added to define the mesh formed by this motif. The hexagonal unit cell vector is very large, about 36.5 ( 0.2 Å, and it is rotated by about 19° with respect to the 〈11h0〉 direction of the Pt substrate. It is important to note that the right side of Figure 11b presents a well ordered mesh of ww structures, while on the left side they (marked by dotted lines) are not in registry with the mesh, but lie on the middle of the traced unit cells. It is easy to note that the dimensions of the wint-TiOx unit cell are about doubled with respect to the w- and w′-TiOx phases. Probably the LEED pattern of the wint-TiOx phase is not visible because, in the reciprocal space, the LEED spots are so close that they are obscured by the spots of the w and w′ phases. Moreover, the higher background in the LEED pattern could be due to the large wint-TiOx unit cell. In Figure 11c we report a high-resolution STM image of the wint-TiOx phase where the atoms and the complex ww motif are both well visible. A line profile is also reported in the same figure. In line with the previous discussion of the w and w′ phases, we try to explain the STM contrast of wint with a Moire´-like model. After several attempts, we have found that the wint-TiOx phase can be described as a
[
15 5 -5 10
]
(or alternatively as (x175 × x175)R19.11° in Wood’s notation) hexagonal coincidence lattice. As reported in Figure 12a, this superstructure can be obtained by superimposing to the Pt(111) mesh a hexagonal oxide overlayer mesh with an atom spacing of 3.23 Å and rotated by 3.3° with respect to the 〈11h0〉 direction of the Pt substrate. This leads to a situation where the [15,5] lattice vector of the Pt mesh is parallel to the [13,5] lattice vector of the overlayer (see Table 1). In Figure 12 we have outlined the different positions of the Ti atoms with respect to the Pt
Figure 11. (a) Large-scale (300 × 290 Å2, V ) 0.23 V, I ) 0.33 nA) STM constant-current image of the wint-TiOx phase. (b) The wagonwheel structures are evidenced by an asterisk and the straight lines trace the regular mesh formed by this motif. (c) High-resolution (160 × 160 Å2; V ) 0.2 V, I ) 1.02 nA) STM constant-current image of the wint-TiOx phase and line profile along the displayed direction.
atoms: we have connected the quasi-on-top atoms with a yellow line, the quasi-bridge atoms with a black line, and we have covered with blue geometric figures the quasi-hollow atoms. Finally, the coincidences are marked with black points. It is rather surprising that this simple model can provide a rather
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Figure 12. (a) Schematic drawing of the Moire´-like coincidence between the Pt(111) lattice and the wint-TiOx superlattice. (b) Corresponding atomically resolved STM image. Ti atom color codes: yellow lines connect quasi-on-top Ti, black lines connect quasi-bridge Ti, blue geometric figures cover quasi-hollow Ti, black points represent coincidence points.
Figure 13. (a) STM constant-current image of a boundary between w-TiOx and wint-TiOx phase (176 × 145 Å2 V ) 1 V, I ) 1.4 nA). (b) We report the Fourier transformed images of the two different images and the straight lines evidence the rotation of the two atomic meshes.
satisfactory agreement with the experimental contrast of the STM image. Taking into account this Moire´-like model, we can also explain the two different registries already evidenced in Figure 11b. Actually, Figure 12b makes evident that the on-top position marked by a black atom is very similar to the quasi-on-top site marked with a yellow rhomb. From the figure it is also easy to recognize that this quasi-on-top rhomb lies midway between two black marked centers of the ww structures. Therefore with a very small translation of the overlayer mesh, one of the rhomb atoms can take a perfect on-top position, thereby becoming the center of another ww structure. 4. Conclusions As reported in the Introduction, the ww structure is a phenomenon characteristic of some transition metal oxides or alloys grown on hexagonal metal substrates. This would suggest the presence of a general driVing force whose nature is still unclear, particularly with respect to its thermodynamic (i.e., most stable structures) or kinetic (i.e., local minima) origin. To clarify this point a quantum mechanical approach would be necessary. In the present paper, to set more experimental insights of relevance to discuss this general phenomenon, we have tested
the Moire´-like model and, as illustrated in Figures 7, 10, and 12, this approach seems to explain the main features of the STM images. On the other hand, some questions are still open. First, the origin of the dark hub in the w-TiOx phase: actually, it is not yet clear if it is related to a Ti vacancy or to an electronic effect. Second, we do not know the actual role of the electronic effects on the STM contrast. However, the body of the discussed results allows us to draw some considerations. In Table 1 we have summarized the geometric characteristics of the three different ww phases reported in this work. Looking at the last two columns of Table 1, it is easy to see how the hexagonal mesh of the overlayer changes after a reducing annealing. Actually, starting from the w-TiOx phase (unit cell 3.26 Å and θ ) 1.36°), after an ultrahigh vacuum (UHV) annealing the hexagonal mesh shrinks and rotates with respect to the Pt(111) substrate, obtaining the wintTiOx phase (unit cell 3.23 Å and θ ) 3.3°), and finally the w′-TiOx phase (unit cell 3.18 Å and θ ) 3.5°). Figure 13a displays an atomic resolved STM image of a boundary between the w-TiOx and the wint-TiOx phases. In Figure 13b, a Fourier Transform filter has been used to isolate the positions of single atoms in the two different phases: in this way we can detect
Ultrathin Titanium Oxide Films directly the rotation of ca. 2° of the two hexagonal meshes with respect to each other. Therefore, another open question is the origin of the driving force for this shrinking and rotation. Probably, the shrinkage of the overlayer lattice is a direct consequence of the UHV annealing and the rotation with respect to the substrate allows a decrease in the interface energy, but only theoretical simulation could definitely demonstrate this hypothesis. Another factor that we should consider is the possible influence of the reconstruction of the substrate. Indeed, it has been reported that the Pt(111) surface, under severe annealing conditions or in the presence of supersaturated Pt vapors, can reconstruct by forming a complex network.28,29 In this way the system increases the surface layer density. Some of these reconstruction networks have an STM pattern similar to the ww structures. This intriguing hypothesis is the same starting point adopted for the alloy model proposed for the Cr/Pt(111) system.8,9 However, one of the most important differences between the ww overlayer structures and the Pt(111) substrate reconstructions is the size of the respective unit cell: actually, the substrate reconstruction is characterized by a very large unit cell, much larger than our ww structures. Whereas the substrate reconstruction implies an increase of the atomic surface layer density of ca. 4-5%, a reconstruction with the size of the ww phases should imply an increased surface density of ca. 31%. On the other hand, to our best knowledge, substrate reconstructions have been reported for Pt(111),28,29 Ir(111),30 and Au(111),31 while the ww phases reported in Table 1 concern Pt(111), Pd(111), and Rh(111) substrates. However, the hypothesis that the ww phases could be induced by the substrate can represent an intriguing field of work. Indeed, if one looks at Table 1, it is easy to note that the different systems showing this reconstruction are more similar for the type of substrate rather than for the type of overlayer. At this stage, we cannot exclude the idea of the substrate-induced reconstruction, but at the same time we do not have any reliable model that can support this hypothesis. Finally we would like to mention that, even if the simple Moire´-like model seems to explain a wealth of experimental data, the hypothesis of a PtTiO alloy model could not be definitively ruled out. Only a complete quantum mechanical calculation could provide the final answer. Acknowledgment. This work has been funded by European Community through the STRP project with the acronym NanoChemSens (Nanostructures for Chemical Sensors) within the SIXTH FRAMEWORK PROGRAMME (Contract No. STRP 505895-1), by the Italian Ministry of Instruction, University and Research (MIUR) through the fund “Programs of
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