Scanning Probe Microscopy Study of Ba Overlayers on TiO2(110)

the TiO2 surface is almost completely covered, a c(6 × 2) overlayer is formed. Taller rows with 1 × 5 periodicity are formed at Ba coverages above a...
0 downloads 0 Views 752KB Size
Download PDF
J. Phys. Chem. C 2007, 111, 9221-9226

9221

Scanning Probe Microscopy Study of Ba Overlayers on TiO2(110) C. L. Pang,*,†,§ A. Sasahara,†,‡ and H. Onishi† Department of Chemistry, Faculty of Science, Kobe UniVersity, Kobe 657-8501, Japan, and Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan ReceiVed: January 9, 2007; In Final Form: March 28, 2007

Scanning tunneling microscopy, noncontact atomic force microscopy, and low-energy electron diffraction have been used to examine the structure of Ba overlayers on TiO2(110)1 × 1 formed by metal vapor deposition. At low coverages, rows of width ∼10 Å develop in the [11h0] and [001] azimuths. At higher coverages where the TiO2 surface is almost completely covered, a c(6 × 2) overlayer is formed. Taller rows with 1 × 5 periodicity are formed at Ba coverages above a single layer.

1. Introduction Alkali and alkaline earth metal and metal oxide additives are known to modify the reactivity of a variety of substrates, including metal oxides.1-12 Understanding the role they play in technologies such as catalysis and gas sensing is a key challenge. In particular, barium oxide has been the focus of much recent research due to its use as the storage component in de-NOx catalysts.2-4 As alumina is usually used as the support in these catalysts,2 the system is somewhat difficult to study using many surface science methods. This is because electronbased techniques, including scanning tunneling microscopy (STM), are difficult or impossible to apply to insulators such as alumina. Although noncontact atomic force microscopy (NC-AFM) can be applied to insulators in principle, atomic-scale images on such substrates are still rather rare, presumably due to strong charging effects.13,14 One method with which to circumvent the insulating problem is to employ “reverse catalysts” whereby the active metal is used as a support for the growth of an oxide film. This method was recently used to allow STM studies of BaO supported on Pt(111).3,4 Another common method to overcome the conductivity issue is to replace the alumina support with a “model” oxide, such as titania which is amenable to investigation by common surface science techniques such as STM.15 Here, we use rutile TiO2(110) as a model support for the growth of Ba. This surface of TiO2 is characterized by rows of 5-fold coordinated Ti which run in the [001] direction and alternate with rows of bridging-O rows. A model of this surface is shown in Figure 1 together with STM and NC-AFM images. Both images consist of bright rows which alternate with dark rows and run in the [001] direction. However, in the STM image, the bright rows originate from the Ti rows, whereas the bright rows in the NC-AFM image arise from the bridging-O rows. This means that the NC-AFM images provide complementary information to the STM and both are used throughout this study. * To whom correspondence should be addressed. E-mail: chi.pang@ ucl.ac.uk. Tel: +44 (0) 207 679 9910. † Kobe University. ‡ Japan Science and Technology Agency. § Current address: London Centre for Nanotechnology and Department of Chemistry, University College London, London WC1H 0AJ, United Kingdom.

Figure 1. (a) Ball model of TiO2(110), including an O-vacancy and an H-adatom. Large gray balls depict lattice oxygen with atoms nearer the surface shaded lighter. Small black spheres indicate Ti atoms. The O-vacancy is circled and the H-adatom is represented with a small gray sphere. (b) (85 Å)2 STM image of the (110) surface recorded with a silicon tip at +1.1 V, 1.0 nA. The bright rows correspond to 5-fold Ti sites and the bright spots between these bright rows are type-A defects which have been assigned to both oxygen vacancies and H-atoms.17,18 Some of these type-A defects are indicated with black squares. Much brighter species (one of which is circled) are likely to be TiOx particles. (c) (85 Å)2 NC-AFM image of the (110) surface recorded with ∆f ) -45 Hz. The bright rows correspond to the bridging-O rows. The depressions on these rows have been shown to derive from both oxygen vacancies and H-atoms.24,25 Some of these type-A defects are marked with white squares. The STM image was taken from the surface before deposition of the Ba clusters and rows, whereas the NC-AFM image is a representative image recorded on another occasion.

At relatively high Ba coverages, our low-energy electron diffraction (LEED), STM, and NC-AFM results show that an ordered c(6 × 2) overlayer forms in a similar way to Ca on

10.1021/jp0701789 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007

9222 J. Phys. Chem. C, Vol. 111, No. 26, 2007 TiO2(110).8-10 Photoemission spectroscopy measurements8,11 for submonolayer coverages of both Ba and Ca on TiO2(110) suggest that the alkali earth metal component is in the +2 oxidation state, although recent calculations dispute this assignment in the case of Ba/TiO2(110).16 At lower coverages, scanning probe microscopy (SPM) images exhibit rows which run along the [001] and [11h0] directions of the TiO2(110) substrate, again reminiscent of Ca on TiO2(110).8,10 Multiple layers of Ba lead to taller added rows which run in the [001] direction with a 1 × 5 periodicity. 2. Experimental Methods The experiments were performed using a JSPM-4500A (JEOL) microscope operated at room temperature and housed in an ultrahigh vacuum (UHV) chamber with a base pressure of ∼2 × 10-10 mbar. NC-AFM images were recorded in the constant frequency shift (∆f) mode using silicon cantilevers (Mikromasch) with resonant frequencies between ∼300-340 kHz, peak-to-peak amplitudes of ∼70 Å, and force constants of ∼14 Nm-1. NC-AFM images were optimized by applying a compensating bias to minimize the contact potential difference (CPD) between the Si tip and the sample. In some cases a fixed compensating bias (∼+0.5-1.5 V) was used, as determined from CPD curves taken at individual positions.13,14 In other cases, the CPD was minimized at each point of the NC-AFM scan by operating in the Kelvin probe force microscopy mode.6 STM images were recorded using the same Si tips as the NC-AFM measurements without oscillation and in the constant current mode with tunneling into sample states. All images are smoothed using Image SXM. Line profiles are taken from the original unsmoothed images with no further smoothing of the graphs. LEED patterns were taken from optics attached (BDL600, OCI) to the vacuum chamber. The TiO2(110) surfaces (Shinkosha) were prepared with cycles of Ar-ion bombardment (2 keV) and annealing to ∼1100 K. The crystal was heated by passing a current through a Si wafer pressed against the back of the sample. Ba was vapor-deposited from a Ba Getter source (SAES) heated to ∼1100 K. One monolayer (ML) is defined as the coverage of Ba required to cover the entire TiO2(110) surface with the c(6 × 2) overlayer. Other coverages are determined by assuming a linear dosing rate with time. To form the high coverage rows, Ba was sequentially evaporated onto the low coverage Ba rows, whereas the c(6 × 2) overlayer was formed on a different, clean TiO2(110) crystal. 3. Results 3.1. Low Coverage: Row Structure. Ba was evaporated onto the surface of TiO2(110) and annealed in UHV to ∼1000 K for ∼10 s. Figure 2 shows the resulting STM images and line profiles at ∼0.5 ML Ba coverage. The image in Figure 2b shows the substrate 1 × 1 rows together with small clusters which sometimes form into added rows running in the [001] and [11h0] directions. These rows and clusters cover about ∼40% of the TiO2 surface area. As the surface area covered by the likely TiOx clusters observed in Figure 1b is only ∼5% of the surface, almost all of the rows and clusters appear only after deposition of Ba. Furthermore, the rows and clusters which follow deposition have an internal structure not seen in the likely TiOx clusters. We therefore conclude that they are Ba related and will be referred to as Ba rows and Ba clusters. The rows have widths of ∼10 Å and apparent heights of ∼2 Å which are almost identical to the widths and heights of the Ca rows reported previously on TiO2(110).8,10 In the present

Pang et al. study, the length of the Ba rows in the [001] and the [11h0] directions extend up to ∼70 and ∼60 Å, respectively. The STM images show that the Ba rows which run in the [001] direction align on top of the bright substrate rows. As these bright substrate rows in STM correspond to 5-fold coordinated Ti rows (when defects can be seen between the bright rows17,18), the images indicate that the Ba rows are aligned over the Ti rows, as observed for the Ca rows on TiO2(110).8,10 Between the Ba clusters and rows, bright spots with heights of ∼0.8 Å can be seen on the bright rows. These spots are likely to originate from individually adsorbed Ba atoms which have not formed Ba clusters. The positions of the bright spots on the bright rows suggest that the Ba atoms adsorb on the 5-fold coordinated Ti atoms which is the same adsorption site identified for Na atoms adsorbed on TiO2(110).5,6 It should be noted, however, that recent theoretical calculations favor 3-fold hollow sites between the bridging-O atoms and the in-plane O atoms, for adsorption of both Ba and Na atoms.16,19 At present, the origin of this discrepancy is not known. Figure 3 shows NC-AFM images and line profiles of the same substrate. Added rows are imaged with apparent heights of ∼1.5 Å, similar to that measured with STM, and we assign these to the same Ba rows imaged in STM. As these Ba rows are aligned on top of the dark substrate rows in the NC-AFM images, we can deduce that the NC-AFM shows the bridging-O rows bright in these images. Thus, the bright spots which can be seen between the bright substrate rows in the NC-AFM images are the same as those already assigned to individual Ba atoms. Pit-like structures can be observed both in Figures 2 and 3, as indicated with blue ellipses and highlighted by the line profile in Figure 2c. The edges of these pits are decorated with the bright spots which we have attributed to Ba atoms. 3.2. Monolayer Coverage. Figure 4, panels a and b, shows STM images of the Ba/TiO2(110) surface following evaporation of ∼1 ML Ba and annealing in UHV at ∼1000 K for ∼3 min. A disordered, particulate film can be observed with a number of holes in the film. These holes have depths of ∼3-4 Å and widths of ∼10-60 Å, representing ∼10% of the surface area. Annealing this disordered film for another ∼3 min in UHV reduces the density of holes in the surface, so that they account for only ∼3% of the surface, as shown in the STM images of Figure 4, panels c and d. In the high-resolution STM image in Figure 4d, some atom-sized bright spots on the surface can be observed. A LEED pattern (not shown) of this surface shows faint additional spots on the surface indicating that the film has some long-range order. Annealing this film for ∼30 min at ∼900 K in 1 × 10-6 mbar O2 followed by annealing in UHV at ∼1000 K for ∼4 min orders the overlayer further, as revealed by SPM images and LEED. The STM images in Figure 5, panels a and b, show that an overlayer with a c(6 × 2) honeycomb structure almost completely covers the TiO2(110) substrate. A schematic representation of part of the honeycomb network is superimposed on the STM images in Figure 5. NC-AFM images reveal the same structure, as shown in Figure 5, panels c and d. Both STM and NC-AFM images are displayed together with a schematic depiction of the overlayer in Figure 6. In the [001] direction, the bright features are separated alternately by two substrate TiO2(110) primitive unit cells (2 × 2.96 Å) then four TiO2(110) primitive unit cells (4 × 2.96 Å). The line profiles in Figure 6d highlight this periodicity. The same configuration of bright features is seen in adjacent [001] direction rows, but they are shifted three unit cells along the [001] direction with respect to each other. Equivalent [001]

Study of Ba Overlayers on TiO2(110)

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9223

Figure 3. (a) (250 Å)2 NC-AFM image of ∼0.5 ML Ba on TiO2(110) recorded with ∆f ) -70 Hz. The red arrowheads indicate some spots attributed to individual Ba atoms on the dark Ti rows, and the blue ellipse is drawn around a pit-like feature. The four solid black lines are drawn over bright O rows. These are to guide the eye. The two dashed lines are drawn through the spine of two Ba [001] direction rows to show their alignment. (b) Line profile taken from the red line indicated in part a.

Figure 2. STM results for ∼0.5 ML Ba on TiO2(110) recorded at +1.0 V, 1.0 nA. (a) (500 Å)2 image and (b) (250 Å)2 image. The five short solid red lines are drawn over bright Ti rows. These are to guide the eye. The dashed red line drawn through the spine of one of the [001] direction Ba rows shows its alignment. The yellow lines indicate two of the Ba rows. Red arrowheads indicate some spots attributed to individual Ba atoms on the Ti rows and the blue ellipses are drawn round pit-like features. (c) Line profiles taken from the red and green lines in part b. The green profile is drawn across one of the pit-like structures. (d) (100 Å)2 image. The blue squares are drawn over some point defects. The red arrowheads indicate some spots attributed to individual Ba atoms on the Ti rows and the blue ellipse is drawn around a pit-like feature.

direction rows are separated by two primitive TiO2(110) unit cells (2 × 6.5 Å) in the [11h0] direction. The images of this Ba c(6 × 2) overlayer seem to be equivalent to the Ca c(6 × 2) overlayer observed for Ca on TiO2(110).10 Figure 7 shows a LEED pattern of the overlayer. Some of the spots are rather faint, but the c(6 × 2) unit cell is observable together with the substrate TiO2(110) 1 × 1 unit cell. The only difference between the Ca and Ba c(6 × 2) overlayers seems to be that the ordering of the Ba overlayer is much lower. This is reflected both in SPM images, which show the presence of anti-phase domain boundaries, and the poorer quality LEED pattern. The domain boundaries can be seen on the surface with an approximate parallelogram shape, as indicated in Figure 5c. However, it is not clear whether this disorder is intrinsic to the Ba film or whether the optimum conditions for growth of the film remain to be found. For instance, the role of oxygen in the film forming process is not yet known. 3.4. High Coverage: Row Structure. Dosing to higher coverages (∼1.5 ML) and annealing in UHV to ∼1000 K for 3 min generates rows which run in the [001] direction. As shown in the STM images in Figure 8, panels a and b, and the NCAFM images in Figure 8, panels d and e, the rows are much longer than the low coverage Ba rows, extending up to ∼1000 Å. These rows also have greater heights (∼6 Å) and widths (∼20 Å) than the low coverage rows. Most of these rows also have a 1 × 5 (32.5 Å) periodicity although this varies. The rows also appear predominantly near step edges so that in the center of terraces there are regions with no rows observable.

9224 J. Phys. Chem. C, Vol. 111, No. 26, 2007

Pang et al.

Figure 4. STM images of ∼1 ML Ba films recorded at +1.5 V and 1.0 nA. (a) (2000 Å)2 image of the disordered film with 3 min annealing. (b) (500 Å)2 image of the same surface as in part a. (c) (2000 Å)2 STM image of monolayer ordered Ba film with a further 3 min annealing. (d) (360 Å)2 image of the same surface as in part c. In part d, some atomic-scale spots are indicated with black arrowheads.

Figure 6. c(6 × 2) overlayer. (a) (70 Å)2 topographic NC-AFM image taken in the Kelvin probe force microscopy mode with ∆f ) -45 Hz. (b) (150 Å)2 STM image recorded at +2.0 V and 1.0 nA. (c) Schematic representation of the c(6 × 2) overlayer. The rectangular mesh represents the TiO2(110) primitive surface unit cells. (d) Line profiles taken from the green and red lines as indicated in parts a and b, respectively. An equivalent line (in red) is also indicated in the schematic diagram in part c. In part a, the yellow rectangle indicates the c(6 × 2) unit cell, and in part b, the blue diamond indicates a primitive c(6 × 2) unit cell. These unit cells are placed in the corresponding places in the schematic diagram of the overlayer in part c.

Figure 5. SPM images of ∼1 ML c(6 × 2) ordered Ba film. (a) (500 Å)2 STM image (+2.0 V, 1.0 nA). (b) (150 Å)2 STM image of the same surface (+2.0 V, 1.0 nA). (c) (200 Å)2 topographic NC-AFM image of the same c(6 × 2) ordered Ba film recorded in the Kelvin probe force microscopy mode taken with ∆f ) -45 Hz. The red parallelogram is drawn over domain boundaries. (d) (100 Å)2 topographic NC-AFM image taken in the Kelvin probe force microscopy mode with ∆f ) -45 Hz. In parts a-d, blue hexagons highlight the honeycomb network. In parts b and d, the blue circles further indicate the discrete spots which form the honeycomb network.

4. Discussion It is clear that, up to monolayer coverages, the Ba overlayers behave almost identically to the Ca overlayers previously reported.8-10 Thus, any model which describes the Ca c(6 × 2) overlayer would also explain our current Ba c(6 × 2) overlayer,

and we discuss the two together. First, we should note that, in accordance with ref 10, a definitive model cannot be constructed based on the available data. Instead, a selection of candidate models were proposed by Bikondoa et al.10 which could be tested with a combination of further experiments, such as surface X-ray diffraction (SXRD), and theoretical calculations. We note that, as Ba scatters X-rays more strongly than Ca, the Ba overlayer structure should be easier to solve with SXRD. Although most of the models proposed in ref 10 simply place Ca atoms in high symmetry adsorption sites, one of the suggestions is more severe, involving substitution of 5-fold coordinated Ti atoms. Such a model, or a model like it, seems likely as the overlayer seems to require some restructuring of the surface rather than simply adsorbing onto the surface. Neither the Ca nor the Ba c(6 × 2) overlayers form without annealing the earth metal/TiO2 system, and in the case of the Ca/TiO2(110) system, densely packed rows of Ca give rise to the c(6 × 2) LEED pattern.8 As the substrate 1 × 1 rows can

Study of Ba Overlayers on TiO2(110)

Figure 7. LEED pattern of the c(6 × 2) overlayer recorded at ∼40 eV. The black rectangle indicates the TiO2(110) 1 × 1 unit cell and the white, crossed rectangle represents the c(6 × 2) unit cell. The LEED photograph has been smoothed to enhance the visibility of the spots.

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9225 suggests that the Ba c(6 × 2) overlayer does not form simply by adsorption onto the TiO2(110) surface. In Figures 2 and 3, we observed Ba atoms positioned on the Ti rows. Many of these Ba atoms lie on either side of a pit-like structure. Such pit-like structures are not normally observed on TiO2(110) [many examples of typical TiO2(110) surfaces can be found in the literature, for example in ref 15 and references therein] and may be indicative of the surface being consumed by the Ba. A similar conclusion was drawn by Kubo and Nozoye for Ba grown on TiO2(001) where rows were observed predominantly at step edges and around pits.12 It was speculated that the Ti and O from the substrate was used to grow BaTi-O rows, thus creating pits in the TiO2 surface. At the anneal temperatures used for the Ba/TiO2 systems both in our current study and Kubo and Nozoye’s work (∼1000-1300 K), there is ample evidence for mass-flow in TiO2 single crystals.20-23 The Ba/TiO2(001) system may also be related to the high coverage Ba rows which we observe. The high coverage Ba rows in the present study have lengths of up to 1000 Å and periodicities of ∼32 Å which are comparable dimensions to the Ba rows on TiO2(001) which have periodicities and lengths of ∼70 and ∼500 Å, respectively.12 Furthermore, the Ba rows on the TiO2(001) surface appear at step edges in a similar way to the high coverage Ba rows here. Ca rows were also observed with a 1 × 4 periodicity (26 Å) under certain conditions and may also have a similar origin to these Ba rows.9 High-temperature STM experiments of the Ba/Ca/TiO2 systems would shed further light on the formation mechanism for both the overlayers and the low and high coverage Ba rows. Such experiments could also determine whether the formation of the pits is correlated to the formation of the Ba/Ca rows. 5. Conclusions In summary, at monolayer and submonolayer coverages, Ba overlayers on TiO2(110) behave almost identically to Ca on TiO2(110) at similar coverages. In both cases, submonolayer coverages lead to TiO2(110) surfaces decorated with rows which run in the [11h0] and [001] directions. At near monolayer coverages, a distinctive c(6 × 2) overlayer with a honeycomb motif forms. At higher coverages, the Ba forms disordered, particulate 1 × 5 rows. Acknowledgment. C.L.P. is grateful to JSPS (Japan) for the award of a Research Fellowship. References and Notes

Figure 8. SPM results for ∼1.5 ML Ba on TiO2(110). (a) (2000 Å)2 STM image recorded at +1.5 V and 1.6 nA. (b) (1000 Å)2 STM image recorded at +1.5 V and 1.6 nA. (c) Line profile taken from the line indicated in part b highlighting the 1 × 5 periodicity of the rows. (d) (2000 Å)2 NC-AFM image recorded with ∆f ) -30 Hz. (e) (1000 Å)2 NC-AFM image taken with ∆f ) -30 Hz.

be seen in the gaps between these Ca rows, the LEED pattern cannot originate from some c(6 × 2) layer beneath the rows. This implies that the Ca rows contain the same structure as the c(6 × 2) films. As the Ba rows in Figure 2 and 3 have a greater height than the spots identified as individual Ba atoms, this

(1) Diehl, R. D.; McGrath, R. Surf. Sci. Rep. 1996, 23, 43. (2) Shinjoh, H.; Tanabe, T.; Subukawa, H.; Sugiura, M. Top. Catal. 2001, 16-17, 95. (3) Stone, P.; Ishii, M.; Bowker, M. Surf. Sci. 2003, 537, 179. (4) Bowker, M.; Stone, P.; Smith, R.; Fourre, E.; Ishii, M.; de Leeuw, N. H. Surf. Sci. 2006, 600, 1973. (5) Onishi, H.; Iwasawa, Y. Catal. Lett. 1996, 38, 89. (6) Sasahara, A.; Uetsuka, H.; Onishi, H. Jpn. J. Appl. Phys. 2004, 43, 4647. (7) Pang, C. L.; Muryn, C. A.; Woodhead, A. P.; Raza, H.; Haycock, S. A.; Dhanak, V. R.; Thornton, G. Surf. Sci. 2005, 583, L147. (8) Zhang, L. P.; Li, M.; Diebold, U. Surf. Sci. 1998, 412/413, 242. (9) No¨renberg, H.; Harding, J. H. Surf. Sci. 2001, 473, 151. (10) Bikondoa, O.; Pang, C. L.; Muryn, C. A.; Daniels, B. G.; Ferrero, S.; Michelangeli, E.; Thornton, G. J. Phys. Chem. B 2004, 108, 16768. (11) Li, Z.; Jørgensen, J. H.; Møller, P. J.; Sambi, M.; Granozzi, G. Appl. Surf. Sci. 1999, 142, 135. (12) Kubo, T.; Nozoye, H. Nano Lett. 2002, 2, 1173. (13) Pang, C. L.; Ashworth, T. V.; Raza, H.; Haycock, S. A.; Thornton, G. Nanotechnology 2004, 15, 862. (14) Barth, C.; Claeys, C.; Henry, C. R. ReV. Sci. Instr. 2005, 76, 083907. (15) Diebold, U. Surf. Sci. Rep. 2003, 48, 53.

9226 J. Phys. Chem. C, Vol. 111, No. 26, 2007 (16) San Miguel, M. A.; Oviedo, J.; Sans, J. F. J. Phys. Chem. B 2006, 110, 19552. (17) Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G. Nat. Mater. 2006, 5, 189. (18) Wendt, S.; Matthiesen, J.; Schaub, R.; Vestergaard, E. K.; Laegsgaard, E.; Besenbacher, F.; Hammer, B. Phys. ReV. Lett. 2006, 96, 066107. (19) Albaret, T.; Finocchi, F.; Noguera, C.; De Vita, A. Phys. ReV. B 2001, 65, 035402. (20) Onishi, H.; Iwasawa, Y. Phys. ReV. Lett. 1996, 76, 791.

Pang et al. (21) Bennett, R. A.; Stone, P.; Price, N. J.; Bowker, M. Phys. ReV. Lett. 1999, 82, 3831. (22) Henderson, M. A. Surf. Sci. 1999, 419, 174. (23) McCarty, K. F.; Bartelt, N. C. Surf. Sci. 2003, 527, L203. (24) Pang, C. L.; Sasahara, A.; Onishi, H.; Chen, Q.; Thornton, G. Phys. ReV. B 2006, 74, 073411. (25) Lauritsen, J. V.; Foster, A. S.; Olesen, G. H.; Christensen, M. C.; Ku¨hnle, A.; Helveg, S.; Rostrup-Nielsen, J. R.; Clausen, B. S.; Reichling, M.; Besenbacher, F. Nanotechnology 2006, 17, 3436.