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Combinatorial Scanning Tunneling Microscopy Study of Cr Deposited on Anatase TiO2(001) Surface T. Ohsawa,† Y. Yamamoto,†,‡ M. Sumiya,§ Y. Matsumoto,| and H. Koinuma*,†,⊥,# Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, Research Center, Asahi Glass Co. Ltd., 1150 Hazawa, Kanagawa-ku, Yokohama 221-8755, Japan, Department of Electrical and Electronic Engineering, Shizuoka University, Hamamatsu 432-8561, Japan, Frontier Collaborative Research Center Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, NIMS-COMET, 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan, and CREST-JST, Japan Received May 9, 2003. In Final Form: February 5, 2004 We have investigated Cr deposited on anatase TiO2(001) surfaces using a combinatorial approach. Anatase TiO2(001) thin films covered with Cr metals were prepared on a Nb-doped SrTiO3(001) substrate by combinatorial laser molecular beam epitaxy and characterized by in situ low-energy electron diffraction and ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). STM images of a clean TiO2(001) surface showed that one-dimensional structures along the [100] and [010] directions were formed, resulting in a (1 × 4) surface reconstruction. When Cr atoms were deposited on the anatase TiO2 surface and the films were annealed at 900 K in UHV, the one-dimensional structures became branched and showed short-range disorder. The observed surface structural disordering will be discussed in terms of the lattice strain induced by the Cr atoms diffused into the bulk.
Introduction Metal-on-oxide heterogeneous systems are used in many technological applications, such as oxide-supported transition metal catalysts, gas sensors, and oxide-based electronics using metal electrodes. Isolated nanoparticles play an important role in heterogeneous catalysts functioning as active sites or modifying the chemical state of an oxide surface and/or an interface between a metal and an oxide. It is well-known, for example, that a considerable enhancement of its catalytic activity is observed when a small amount of transition metals are introduced on and/ or into titanium dioxide (TiO2).1 Titanium dioxide, which has three kinds of crystal structures, rutile, anatase, and brookite, has been extensively investigated for optical applications and also for elucidating photocatalytic processes such as water decomposition,2 from the viewpoint of surface science. To understand the mechanism of catalytic enhancement of TiO2 supported transition metal catalysts, it is highly desirable to employ well-defined oxide single crystals of TiO2. In fact, there are many studies on the behavior of transition metals on rutile single crystals,3-5 because rutile is the most thermodynamically stable, well-defined, and * To whom correspondence may be addressed. Telephone: +8145-924-5314. Facsimile: +81-45-924-5377. E-mail: koinuma1@ oxide.msl.titech.ac.jp. † Materials and Structures Laboratory, Tokyo Institute of Technology. ‡ Research Center, Asahi Glass Co., Ltd. § Department of Electrical and Electronic Engineering, Shizuoka University. | Frontier Collaborative Research Center Laboratory, Tokyo Institute of Technology. ⊥ NIMS-COMET. # CREST-JST. (1) Matsumoto, Y.; Shimizu, T.; Toyoda, A.; Sato, E. J. Phys. Chem. 1982, 86, 3581. (2) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (3) Diebold, U. Surf. Sci. Rep. 2002, 293, 1. (4) Pesty, F.; Steinriick, H. P.; Madey, T. E. Surf. Sci. 1995, 339, 83. (5) Tanner, R. E.; Goldfarb, I.; Castell, M. R.; Briggs, G. A. D. Surf. Sci. 2001, 486, 167.
commercially available crystal phase. In contrast, even though anatase is more attractive as a photocatalyst, there are few reports on anatase surfaces.6-14 This is because single crystals of anatase are difficult to obtain by bulk processing due to its thermodynamically unstable nature. Recently, some groups have reported the fabrication of epitaxial anatase TiO2(001) films, using appropriate substrates, such as SrTiO3(001) or LaAlO3(001).15-19 The epitaxial anatase thin films make it possible to examine the characteristics of transition metals on single crystalline anatase surfaces. In fact, we have extensively and parametrically within a rather short period investigated the dynamics of various transition metals deposited on the anatase surface using a combinatorial approach,20 as described later. In this Letter, we focus on the surface structure of Cr atoms deposited on the anatase TiO2(001) surface, as well as the clean surface investigated by using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). (6) Herman, G. S.; Sievers, M. R.; Gao, Y. Phys. Rev. Lett. 2000, 84, 3354. (7) Liang, Y.; Gan, S.; Chambers, S. A.; Altman, E. I. Phys. Rev. B 2001, 63, 235402. (8) Lazzeri, M.; Selloni, A. Phys. Rev. Lett. 2001, 87, 266105. (9) Herman, G. S.; Gao, Y. Thin Solid Films. 2001, 397, 157. (10) Hengerer, R.; Bolliger, B.; Erbudak, M.; Gratzel, M. Surf. Sci. 2000, 460, 162. (11) Sato, S. Shokubai (Catalyst) 1989, 31, 469. (12) Diebold, U. Surf. Sci. Rep. 2002, 293, 1. (13) Tanner, R. E.; Liang, Y.; Altman, E. I. Surf. Sci. 2002, 506, 251. (14) Gan, S.; El-azab, A.; Liang, Y. Surf. Sci. 2001, 479, L369. (15) Matsumoto, Y.; Murakami, M.; Jin, Z. W.; Nakayama, A.; Yamaguchi, T.; Ohmori, T.; Suzuki, E.; Nomura, S.; Kawasaki, M.; Koinuma, H. Proc. SPIE 2000, 3941, 19. (16) Murakami, M.; Matsumoto, Y.; Nakajima, K.; Makino, T.; Segawa, Y.; Chikyow, T.; Ahmet, P.; Kawasaki, M.; Koinuma, H. Appl. Phys. Lett. 2001, 78, 2664. (17) Herman, G. S.; Sievers, M. R.; Gao, Y. Phys. Rev. Lett. 2000, 84, 3354. (18) Liang, Y.; Gan, S.; Chambers, S. A.; Altman, E. I. Phys. Rev. B 2001, 63, 235402. (19) Herman, G. S.; Gao, Y. Thin Solid Films 2001, 397, 157. (20) Ohsawa, T.; Matsumoto, Y.; Koinuma, H. Appl. Surf. Sci. 2004, 223, 84.
10.1021/la034794h CCC: $27.50 © 2004 American Chemical Society Published on Web 03/09/2004
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Figure 2. STM images (50 nm × 50 nm) and LEED patterns of the (1 × 4) anatase surface on SrTiO3(001) after (a) cooling in UHV: VB ) 1.5 V, It ) 0.30 nA, (b) cooling in 10-5 Torr of oxygen atmosphere: VB ) 1.5 V, It ) 0.21 nA.
Figure 1. Schematic diagram of the combinatorial laser MBE system. Film thickness gradients can be obtained by using a linear shadow mask.
Experimental Section Thin film fabrication was carried out by combinatorial laser molecular beam epitaxy (CLMBE) as illustrated in Figure 1.21 In the deposition chamber, a focused pulsed laser beam (20 ns, 1-10 Hz) was impinged with an energy density of ∼1 J/cm2 through a quartz window onto a sintered target. Three targets can be installed in the deposition chamber and each of them can be ablated alternately. Films were depositedeither under ultrahigh vacuum (UHV) conditions (∼10-10 Torr base pressure) or in an O2 gas atmosphere. A linearly moving mask made it possible to deposit several films and nanostructures in a separate area on a single substrate under different conditions (the amount of deposited atoms, etc.) in one experimental cycle. The main advantage of fabricating an integrated sample in a combinatorial fashion is the reduction of the sample preparation time. To fabricate the anatase thin films, a ceramic TiO2 target (99.99% purity) was placed in the vacuum chamber and ablated with a KrF excimer laser (λ ) 248 nm) (Lambda Physik, COMPEX102-S) with a typical laser fluence of 1.5 J/cm2. The oxygen partial pressure was 10-6-10-5 Torr, and the substrate temperature was 900-950 K during deposition. The typical deposition rate was 0.002 nm/pulse. After a TiO2 film was deposited, the substrate was cooled to room temperature and a pure chromium target (99.9%) was ablated to grow a metal layer on the TiO2 surface with moving the mask to obtain a film thickness gradient. The 0.05 mol % Nb-doped SrTiO3(001) substrates used in this study were etched with a NH4F-HF (BHF) solution and annealed at 900 K in 10-5 Torr of oxygen to obtain 4 Å steps and atomically flat terraces.22 The STM used in this study was a commercial Rasterscope-5000 STM from the DME Corporation. All STM images presented here were recorded in the constant-current mode at room temperature using a Pt-Ir tip.
Results and Discussion After an anatase film was deposited, the anatase TiO2(001) surface underwent surface reconstruction, resulting in a clear (1 × 4) structure. Figure 2a is an STM image of the anatase film surface on a B-site-terminated SrTiO3(001) substrate. One-dimensional structures grown along the [100] and [010] directions were observed. Two types of steps exist on this surface, with the step edges either parallel or perpendicular to the atomic rows at upper terraces. The distance between neighboring rows was (21) Koinuma, H.; Kawasaki, M.; Itoh, T.; Ohtomo, A.; Murakami, M.; Jin, Z. W.; Matsumoto, Y. Physica C 2000, 335, 245. (22) Kawasaki, M.; Takahashi, K.; Maeda, T.; Tsuchiya, R.; Shinohara, M.; Ishiyama, O.; Yonezawa, T.; Yoshimoto, M.; Koinuma, H. Science 1994, 266, 1540.
approximately 15-16 Å, equal to four lattice spacings, giving a (1 × 4) structure, which agrees with previous reports.7 There have been proposed several structural models of (1 × 4) surface reconstruction: the “microfacets” model,6 the “added-and-missing row” (AMR) model,7 recently there is growing consensus favoring the “addmolecule” model (ADM).8 In addition, the observed step height is about 0.2 nm, corresponding to the minimum TiO2 unit, which can satisfy the charge neutrality. This value is consistent with the periodicity of the reflection high energy electron diffraction (RHEED) intensity oscillations during the growth of anatase films as reported in our previous paper.16 It is noteworthy that the number of bright spots that can be seen in Figure 2a was strongly dependent on the cooling process after depositing the films. When the anatase sample was cooled to room temperature in UHV after deposition, many bright spots were observed. In contrast, the anatase sample had almost no bright spots on the surface after cooling in an oxygen atmosphere condition (1 × 10-5 Torr O2), as shown in Figure 2b. Thus, although the possibility of impurities cannot be ruled out, the observed bright spots seem to be oxygen vacancies or reduced TiOx clusters. Cr atoms were deposited at room temperature in UHV by moving the shadow mask to control the amount of deposited Cr atoms ablated with 0, 20, 50, or 70 pulses. However, the (1 × 4) LEED pattern still remained visible, albeit with a higher background than before Cr deposition, as well as STM image. Therefore, the coverage of Cr atoms in this experiment is inferred to be much less than 1 monolayer (ML). In fact, the Cr composition was estimated to be approximately 7 atomic % by ex situ X-ray photoelectron spectroscopy (XPS) analysis. The Cr-deposited sample was slowly heated at a rate of 10 K/min in UHV and kept at 900 K for 30 min. Then, the sample was naturally cooled to room temperature in about 30 min. A significant change of the LEED pattern started at 700 K with the increase of the Cr atom coverage. The (1 × 4) spots gradually became elongated, finally turning into streaks along the [100] direction at 900 K, as shown in the LEED patterns of parts a and b of Figure 3. In addition, the reciprocal lattice spacing distance (∆G) between the (0, 1) and (0, 3/4) spots in the LEED patterns obtained after annealing Cr deposited anatase surfaces in UHV is plotted against the number of laser pulses in Figure 3c. ∆G increased monotonically by up to 20% with the increase of Cr atom coverage on the surface. The observed LEED patterns suggested the shrinking of the lattice spacing between neighboring rows (13%) and disordering along the rows. Figure 4 shows a series of STM images (25 nm × 25 nm) of four regions corresponding to the sample surface in Figure 3c. Figure 4a is a typical high-resolution
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Figure 3. Cr atoms were ablated with 70 pulses, which was corresponding to the Cr content of approximately 7 atomic % on the anatase surface at room temperature in UHV. The LEED patterns (a) before annealing and (b) after annealing at 900 K in UHV. (c) The change of the reciprocal space distance between (0, 1) and (0, 3/4) spots in the LEED patterns obtained after annealing Cr deposited anatase surfaces in UHV against the number of laser pulses.
Figure 4. High-resolution STM images (25 nm × 25 nm) of four regions corresponding to the sample surface in Figure 3c: (a) VB ) 2.9 V, It ) 0.23 nA; (b) VB ) 2.9 V, It ) 0.25 nA; (c, d) VB ) 2.9 V, It ) 0.30 nA. A Fourier transformed image of Figure 4d (inset) also showed the same reciprocal space pattern as the observed LEED pattern in Figure 3b.
STM image of the clean anatase TiO2(001) surface as shown in Figure 2. With an increase of deposited Cr coverage, one-dimensional structures started to branch at random into small pieces, resulting in disorder along the rows, consistent with the observed streaky (1 × 4) spots in the LEED patterns. In fact, a Fourier transformed image of Figure 4d (inset) also showed the same reciprocal space pattern as the observed LEED pattern in Figure 3b. As far as we investigated the behavior of all the transition metals from Ti to Cu on the anatase,20 only the case of Cr deposition was found to induce this significant surface structural change. It should be noted that the ex situ XPS analysis showed no Cr atom remained on the Cr-deposited anatase surface after the 900 K annealing.
Therefore, the Cr atoms should be diffused into the anatase film because the Cr metal and the related CrOx compounds are considered not to re-evaporate from the surface at the present annealing temperature of 900 K. In fact, the secondary ion mass spectroscopy (SIMS) measurement revealed that a sizable amount of Cr atoms were contained in the anatase film. On the basis of these experimental facts, we could deduce a possible scenario to explain this significant surface structural change of the Cr-deposited anatase surface. According to the ADM model,8 the (1 × 4) reconstruction is rationalized in terms of the relief of the large surface tensile stress present on the unreconstructed surface. The diffused Cr atoms in the interstitial and/or substituted sites in the anatase lattice should cause an ineligible lattice strain. Consequently, the surface stress might change as well, giving a different reconstruction of the anatase surface. In general, it has been considered that the reconstruction on the metal alloy surface is very sensitive to the surface composition rather than the bulk composition. In contrast, the reconstruction on the oxide surface could be relatively more sensitive to the bulk composition due to the less flexibility of the oxide lattice than the metal alloy. Conclusions The Cr-covered anatase TiO2(001) surface was found to exhibit a significant surface reconstruction after annealing at high temperatures in UHV. The Cr atom diffused into the bulk is responsible for the observed Cr-induced surface reconstruction on the anatase. This new anatase surface is also expected to change its electronic state and to improve its catalytic activities. Acknowledgment. The authors express their thanks for financial support from Japan Science and Technology, Science and Technology Agency Japan, Japan Society for the Promotion of Science. Y.M. also acknowledges the Asahi Glass Foundation for the financial support of this work. LA034794H
