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Observation of p(1×1)-type rows on TiO2(110)-p(1×2) surface by scanning tunneling microscope (STM)
Solid State Communications 129 (2004) 15–18 www.elsevier.com/locate/ssc
Observation of p(1 £ 1)-type rows on TiO2(110)-p(1 £ 2) surface by scanning tunneling microscope (STM) E. Asari*, R. Souda National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 14 March 2002; received in revised form 12 February 2003; accepted 28 August 2003 by D. E. Van Dyck
Abstract The TiO2(110) surfaces were observed by a Scanning Tunneling Microscope (STM). We found two types of bright p(1 £ 1)type rows on the p(1 £ 2) surface. One p(1 £ 1)-type formed independently and corresponds to the bridging oxygen rows. The second p(1 £ 1)-type appeared in a bright grouping, forming narrow rows, and corresponds to the five-fold titanium rows. The above results suggest the following two conclusions. First, the density of state (DOS) on the bridging oxygen rows becomes higher than that on the five-fold titanium atom rows when a bridging oxygen row exists independently on the p(1 £ 2) surface. Second, the bright rows on a TiO2(110)-p(1 £ 1) surface correspond to the five-fold titanium atom rows. The results further show the validity of DOS calculations on the TiO2(110)-p(1 £ 1) surface by Diebold et al. [Phys. Rev. Lett. 77 (1996) 1322]. The difference of width for Ti2O3 unit rows on the p(1 £ 2) and p(1 £ 3) surfaces in STM images are also discussed. q 2003 Elsevier Ltd. All rights reserved. PACS: 68.35.B; 68.37.E; 68.47.G Keywords: A. Surface and interfaces; C. Scanning tunneling microscopy
1. Introduction Studies of the TiO2 surface have received much attention after the discovery of light-induced decomposition of water [1]. Yet despite this attention, the photochemical reaction process on the surface in atomic scale is still a matter of controversy. This is because the precise structures on the surfaces could not be determine by a Scanning tunneling microscope (STM) or diffraction techniques. ‘Rutile’-type TiO2 is the most popular titanium dioxide because it is easy to obtain and prepare. The surface of rutile TiO2(110) has the highest thermodynamic stability, while other surfaces facet easily. In spite of this stability, experimental [2 – 12] and theoretical [13 – 16] studies, demonstrating the p(1 £ 1) surface with bridging oxygen rows [see Fig. 1(a)], and longer periodic structures [p(1 £ 2) * Corresponding author. Address: Yobito laboratory for environment and energy, 62-51 Yobito, Abashiri, Hokkaido 0992421, Japan. Fax: þ 81-152-48-2665. E-mail address: [email protected] (E. Asari). 0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2003.08.047
and p(1 £ 3)] at higher annealing temperature [8,17 –24] have been reported. At the initial stage of the TiO2(110) study, the bright rows in the ST M image on the TiO2(110)p(1 £ 1) surface were believed to be the bridging oxygen rows located on the top most layer. However, Diebold et al. presented, by the theoretical calculations, that the DOS on the five-fold titanium atom rows as higher than that of the bridging oxygen rows by theoretical calculations [23]. This indicates that the bright rows on the TiO2(110)-p(1 £ 1) surface in the STM image correspond to the five-fold titanium rows. Tanner et al. successfully observed the bridging oxygen rows and five-fold titanium rows simultaneously on the TiO2(110)-p(1 £ 1) surface by STM with a very small tip-surface distance condition [3]. This revealed the DOS on the five-fold titanium row to be close to that on the bridging oxygen rows. There are, however, few experimental studies, which support the results of Diebold et al. In this paper, we study two types of local narrow rows (p(1 £ 1)-type rows) on the TiO2(110)-p(1 £ 2) surface in order to discuss the attribution of bright rows on the TiO2(110)-p(1 £ 1) surface by STM.
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E. Asari, R. Souda / Solid State Communications 129 (2004) 15–18
Fig. 1. Ball models for TiO2(110), (a) p(1 £ 1) surface with bridging oxygen rows, (b) p(1 £ 2) surface with added Ti2O3 unit rows with cross link, (c) p(1 £ 3) surface with added Ti2O3 unit rows with bridging oxygen rows. Top and plane views are shown. Small circles: Ti; large circle: O.
2. Experimental The experiments were performed in a scanning tunnelling microscope (JEOL-JSTM4500XT) with a base pressure of 2 £ 1028 Pa. A tungsten tip was used to apply positive bias voltages to the sample. A mechanically polished TiO2(110) surface (Nakazumi crystal labo co.) was used for the experiments. The substrate was heated from behind by electron bombardment. Segregation of Ca impurity by the heating was below the sensitivity of ion scattering spectroscopy. The temperature of the surfaces was monitored by pyrometer. The color of TiO2(110) sample turned from the transparent to the marine blue after annealing. The color indicates the reduced surface.
previously showed that the TiO2(110)-p(1 £ 2) surface obtained by sputtering/annealing also has the Ti2O3 unit rows [10 – 12,25]. Ashino et al. also reported this structure on the TiO2(110)-p(1 £ 2) surface by STM and atomic force microscopy (AFM) experiments [26]. Though Fig. 3 is basically the p(1 £ 2) surface, some narrow rows exist for [001] direction at the phase boundary of the p(1 £ 2) array of the Ti2O3 rows with 3 spacing (see framed area in Fig. 3). Judging from the relative position of wider Ti2O3 unit rows and the independent narrow rows, it is obvious that the narrow rows correspond to the position of bridging oxygen rows; not to the position of five-fold titanium row, as depicted in Fig. 1(c) (termination of oxygen atoms O(6) at the narrow row is obviously stabler than that of four-fold
3. Results and discussion Fig. 2 is a STM image of a TiO2-p(1 £ 3) surface that was obtained by annealing at 830 8C for 2 h. In our previous work and experiments, we studied the TiO2(110)-p(1 £ 3) surface by impact collision ion scattering spectroscopy (ICISS) experiments [see Fig. 1(c)] [10] and found that the surface has Ti2O3 unit rows for [001]-direction and bridging ˚) oxygen rows between them. The broad rows (width of 16 A in Fig. 2 may correspond to the Ti2O3 unit rows. Though, the p(1 £ 3) surface has the bridging oxygen rows between Ti2O3 unit rows as shown in Fig. 1(c), it is difficult to recognize the oxygen rows in Fig. 2. It suggests that the DOS of the bridging oxygen rows on the p(1 £ 3) surface is lower than that of the Ti2O3 unit rows. Fig. 3 shows a TiO2(110)-p(1 £ 2) surface that was obtained by 2 keV Arþ sputtering and annealing at 1050 8C for 1.5 h. The surface showed a clear p(1 £ 2) LEED pattern (datum not shown). It is recognized that there are some defects, steps and cross links for ½110-direction on the ˚ . We surface. The interval of the p(1 £ 2) rows is 13 A
Fig. 2. STM image of TiO2(110)-p(1 £ 3) surface prepared by only annealing of new sample at 830 8C for 2 h (sample bias þ2.0 V, ˚ 2. tunneling current 0.07 nA), 315 £ 315 A
E. Asari, R. Souda / Solid State Communications 129 (2004) 15–18
˚ 2 STM image of TiO2(110), which contains Fig. 3. A 350 £ 350 A mainly the p(1 £ 2) phase, prepared by 2 keV Arþ ion sputtering and annealing of at 1050 8C for 1.5 h (sample bias þ 2.0 V, tunneling current 0.07 nA). The inset shows the magnified area of stage B0 . A height profile of the surface across the stages B–A –B – B0 (grey line in the image) is also shown. The scanning direction ˚. from the ½110-direction is about 20 A
titanium atoms Ti(5)) [10,26]. Though the stoichiometric structure of Ti2O3 – O –Ti2O3 in Fig. 2 is as same as that on the framed area in Fig. 3, we can not recognize narrow rows between Ti2O3 unit rows in Fig. 2. Furthermore, the width of ˚ ) is wider than that of the the p(1 £ 3) rows in Fig. 2 (16 A ˚ p(1 £ 2) rows in Fig. 3 (10 A). Above results suggest that the Ti2O3 unit rows on the p(1 £ 3) surface is wider than that on the p(1 £ 2) surface. Due to the ICISS experiments on the TiO2(110) surfaces, it became obvious that the width between O(4) and O(40 ) atoms on the p(1 £ 3) surface [see ˚ wider than that on the p(1 £ 2) surface Fig. 1(c)] is 0.3 A ˚ is not [see Fig. 1(b)] [10]. The difference of 0.3 A ˚ . The comparable to the difference of 6 ( ¼ 16 2 10) A wave functions, however, extend exponentially, small approach of the two-fold O(4) atom for the five-fold Ti(2) atom may largely enhance the electron hybridization between the rest of a O(4) bond and that of a Ti(2) bond in rutile. The increase of hybridization increases the electron
17
density or DOS around Ti(2) [or Ti(20 )] and Ti2O3 regions in Fig. 1(c). As a result, it may change the width of rows in the STM image. Considering the atomic radius of Ti(2) and Ti(20 ) atoms, the distance Ti(2) – Ti2O3 – Ti(20 ) in Fig. 1(c) ˚. should be about 16 A At the edge of stage B, we can recognize a group of p(1 £ 1)-type rows. The contrast between p(1 £ 1)-type rows and p(1 £ 2) rows on the stage B is largely discrepant, viz, the p(1 £ 1)-type rows exist at a higher layer (stage B0 ). A height profile of the surface across the stages B– A –B – B0 is also depicted in the Fig. 3. As the scanning angle from the ½110 direction is ,208 (see the grey line in Fig. 3), we must compensate the scanning distance with a factor of 0.94 ( ¼ cos208) to evaluate the certain intervals of the p(1 £ 1)type rows. But the factor 0.94 coincides 1 ^ 0.1 within 10% uncertainty of the STM measurements. Accordingly, the ˚ interval of the p(1 £ 1)-type rows is determined to be 6.5 A in average from the profile. The height of the stage A from ˚ (half length of 6.5 A ˚ ), suggests that the the stage B, 3.3 A stage A exists next layer on the stage B. The height of the ˚ , from the stage B does not reflect the reality stage B0 , ,2 A because the surface structure (or DOS) between the stage B [p(1 £ 2)] and the stage B0 [p(1 £ 1)] is largely discrepant. Comparing with the relative position of the p(1 £ 1)-type rows on the stage B0 and the p(1 £ 2) rows on the stage B (inset in Fig. 3), we can recognize that the bright p(1 £ 1)type rows on the stage B0 correspond to the five-fold titanium rows, because the dark lines between bright p(1 £ 1)-type rows, corresponding to bridging oxygen rows, always situate on the center of the Ti2O3 rows on stage B. The long narrow rows on the stage A (indicated by arrows in Fig. 3) obviously correspond to the bridging oxygen. Since the distance via p(1 £ 1)-type row –Ti2O3˚ , it corresponds well with p(1 £ 1)-type row in Fig. 3 is 20 A ˚ ) in the distance via O(6) – Ti2O3 –O(60 ) (9.75 £ 2 ¼ 19.5 A Fig. 1(c). The above results show that the DOS of the fivefold titanium rows, which appeared in a group on the p(1 £ 2) surface, is higher than that of the bridging oxygen rows. However, DOS of the independent bridging oxygen rows, which appeared at the p(1 £ 2) phase boundary, is higher than that of the five-fold titanium row. Although, it is difficult to assign directly the bright rows on the TiO2(110)p(1 £ 1) surface by STM, the result that the p(1 £ 1) type rows appeared in a group on the p(1 £ 2) surface, and that they correspond to the five-fold titanium rows all suggests that the bright rows on the TiO2(110)-p(1 £ 1) surface correspond to the five-fold titanium rows as well. This result therefore, coincides with the calculated results of DOS on the TiO2(110)-p(1 £ 1) surface by Diebold et al. [23]. As the mixed phase was observed within a local restricted area on the surface (one area at a time), frequency that just such a mixed phase appeared was very low. Ergo it seems to be very difficult to control the parameters needed to make the ideal mixed phase. The phase must have appeared by means of a random combination of the proper temperature and reduction rate in a local surface. This in
18
E. Asari, R. Souda / Solid State Communications 129 (2004) 15–18
turn must have been caused by chance, because the above parameters might not be perfectly uniform on the surface. In summary, TiO2(110) surfaces were observed by STM. We found that the p(1 £ 1)-type rows that appeared in a group on the p(1 £ 2) surface correspond to the five-fold titanium rows. On the other hand, a p(1 £ 1)-type row that appeared independently at the phase boundary of the p(1 £ 2) surface correspond to the bridging oxygen row. These results suggest that the bright rows in the STM image on the TiO2(110)-p(1 £ 1) surface correspond to the fivefold titanium rows, thereby, validating the DOS calculations results by Diebold et al.
References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] V.E. Henrich, R.L. Kurtz, Phys. Rev. B 23 (1981) 6280. [3] R.E. Tanner, M.R. Castell, G.A.D. Briggs, Surf. Sci. 412 (1998) 672. [4] G. Charlton, P.B. Howes, C.L. Nicklin, P. Steadman, J.S.G. Taylor, C.A. Muryn, S.P. Harte, J. Mercer, R. McGrath, D. Norman, T.S. Turner, G. Thornton, Phys. Rev. Lett. 78 (1997) 495. [5] B. Hird, R.A. Armstrong, Surf. Sci. 385 (1997) L1023. [6] B. Hird, R.A. Armstrong, Surf. Sci. 420 (1999) L131.
[7] D. Novak, E. Garfunkel, T. Gustafsson, Phys. Rev. B 50 (1994) 5000. [8] P.W. Murray, N.G. Condon, G. Thornton, Phys. Rev. B 51 (1995) 10989. [9] E. Asari, T. Suzuki, H. Kawanowa, J. Ahn, W. Hayami, T. Aizawa, R. Souda, Phys. Rev. B 61 (2000) 5679. [10] E. Asari, R. Souda, Phys. Rev. B 60 (1999) 10719. [11] E. Asari, W. Hayami, R. Souda, Appl. Surf. Sci. 167 (2000) 169. [12] E. Asari, R. Souda, Appl. Surf. Sci. 193 (2002) 70. [13] P. Reinhardt, B.A. Heß, Phys. Rev. B 50 (1994) 12015. [14] D. Vogtenhuber, R. Podloucky, A. Neckel, Phys. Rev. B 49 (1994) 2099. [15] S.P. Bates, G. Kresse, M.J. Gillan, Surf. Sci. 385 (1997) 386. [16] M. Ramamoorthy, D. Vanderbilt, R.D. King-Smith, Phys. Rev. B 49 (1994) 16721. [17] P.J. Møler, M.C. Wu, Surf. Sci. 224 (1989) 265. [18] H. Onishi, Y. Iwasawa, Surf. Sci. Lett. 313 (1994) L783. [19] M. Sander, T. Engel, Surf. Sci. Lett. 302 (1994) L263. [20] A. Szabo, T. Engel, Surf. Sci. 329 (1995) 241. [21] Q. Guo, I. Cocks, E.M. Williams, Phys. Rev. Lett. 77 (1996) 3851. [22] I.D. Gocks, Q. Guo, E.M. Williams, Surf. Sci. 390 (1997) 119. [23] U. Diebold, J.F. Anderson, K.O. Ng, D. Vanderbilt, Phys. Rev. Lett. 77 (1996) 1322. [24] K.O. Ng, D. Vanderbilt, Phys. Rev. B 56 (1997) 10544. [25] E. Asari, R. Souda, Vaccum 68 (2002) 123. [26] M. Ashino, et al., Phys. Rev. B 61 (2000) 13955.
Observation of p(1 £ 1)-type rows on TiO2(110)-p(1 £ 2) surface by scanning tunneling microscope (STM) E. Asari*, R. Souda National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 14 March 2002; received in revised form 12 February 2003; accepted 28 August 2003 by D. E. Van Dyck
Abstract The TiO2(110) surfaces were observed by a Scanning Tunneling Microscope (STM). We found two types of bright p(1 £ 1)type rows on the p(1 £ 2) surface. One p(1 £ 1)-type formed independently and corresponds to the bridging oxygen rows. The second p(1 £ 1)-type appeared in a bright grouping, forming narrow rows, and corresponds to the five-fold titanium rows. The above results suggest the following two conclusions. First, the density of state (DOS) on the bridging oxygen rows becomes higher than that on the five-fold titanium atom rows when a bridging oxygen row exists independently on the p(1 £ 2) surface. Second, the bright rows on a TiO2(110)-p(1 £ 1) surface correspond to the five-fold titanium atom rows. The results further show the validity of DOS calculations on the TiO2(110)-p(1 £ 1) surface by Diebold et al. [Phys. Rev. Lett. 77 (1996) 1322]. The difference of width for Ti2O3 unit rows on the p(1 £ 2) and p(1 £ 3) surfaces in STM images are also discussed. q 2003 Elsevier Ltd. All rights reserved. PACS: 68.35.B; 68.37.E; 68.47.G Keywords: A. Surface and interfaces; C. Scanning tunneling microscopy
1. Introduction Studies of the TiO2 surface have received much attention after the discovery of light-induced decomposition of water [1]. Yet despite this attention, the photochemical reaction process on the surface in atomic scale is still a matter of controversy. This is because the precise structures on the surfaces could not be determine by a Scanning tunneling microscope (STM) or diffraction techniques. ‘Rutile’-type TiO2 is the most popular titanium dioxide because it is easy to obtain and prepare. The surface of rutile TiO2(110) has the highest thermodynamic stability, while other surfaces facet easily. In spite of this stability, experimental [2 – 12] and theoretical [13 – 16] studies, demonstrating the p(1 £ 1) surface with bridging oxygen rows [see Fig. 1(a)], and longer periodic structures [p(1 £ 2) * Corresponding author. Address: Yobito laboratory for environment and energy, 62-51 Yobito, Abashiri, Hokkaido 0992421, Japan. Fax: þ 81-152-48-2665. E-mail address: [email protected] (E. Asari). 0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2003.08.047
and p(1 £ 3)] at higher annealing temperature [8,17 –24] have been reported. At the initial stage of the TiO2(110) study, the bright rows in the ST M image on the TiO2(110)p(1 £ 1) surface were believed to be the bridging oxygen rows located on the top most layer. However, Diebold et al. presented, by the theoretical calculations, that the DOS on the five-fold titanium atom rows as higher than that of the bridging oxygen rows by theoretical calculations [23]. This indicates that the bright rows on the TiO2(110)-p(1 £ 1) surface in the STM image correspond to the five-fold titanium rows. Tanner et al. successfully observed the bridging oxygen rows and five-fold titanium rows simultaneously on the TiO2(110)-p(1 £ 1) surface by STM with a very small tip-surface distance condition [3]. This revealed the DOS on the five-fold titanium row to be close to that on the bridging oxygen rows. There are, however, few experimental studies, which support the results of Diebold et al. In this paper, we study two types of local narrow rows (p(1 £ 1)-type rows) on the TiO2(110)-p(1 £ 2) surface in order to discuss the attribution of bright rows on the TiO2(110)-p(1 £ 1) surface by STM.
16
E. Asari, R. Souda / Solid State Communications 129 (2004) 15–18
Fig. 1. Ball models for TiO2(110), (a) p(1 £ 1) surface with bridging oxygen rows, (b) p(1 £ 2) surface with added Ti2O3 unit rows with cross link, (c) p(1 £ 3) surface with added Ti2O3 unit rows with bridging oxygen rows. Top and plane views are shown. Small circles: Ti; large circle: O.
2. Experimental The experiments were performed in a scanning tunnelling microscope (JEOL-JSTM4500XT) with a base pressure of 2 £ 1028 Pa. A tungsten tip was used to apply positive bias voltages to the sample. A mechanically polished TiO2(110) surface (Nakazumi crystal labo co.) was used for the experiments. The substrate was heated from behind by electron bombardment. Segregation of Ca impurity by the heating was below the sensitivity of ion scattering spectroscopy. The temperature of the surfaces was monitored by pyrometer. The color of TiO2(110) sample turned from the transparent to the marine blue after annealing. The color indicates the reduced surface.
previously showed that the TiO2(110)-p(1 £ 2) surface obtained by sputtering/annealing also has the Ti2O3 unit rows [10 – 12,25]. Ashino et al. also reported this structure on the TiO2(110)-p(1 £ 2) surface by STM and atomic force microscopy (AFM) experiments [26]. Though Fig. 3 is basically the p(1 £ 2) surface, some narrow rows exist for [001] direction at the phase boundary of the p(1 £ 2) array of the Ti2O3 rows with 3 spacing (see framed area in Fig. 3). Judging from the relative position of wider Ti2O3 unit rows and the independent narrow rows, it is obvious that the narrow rows correspond to the position of bridging oxygen rows; not to the position of five-fold titanium row, as depicted in Fig. 1(c) (termination of oxygen atoms O(6) at the narrow row is obviously stabler than that of four-fold
3. Results and discussion Fig. 2 is a STM image of a TiO2-p(1 £ 3) surface that was obtained by annealing at 830 8C for 2 h. In our previous work and experiments, we studied the TiO2(110)-p(1 £ 3) surface by impact collision ion scattering spectroscopy (ICISS) experiments [see Fig. 1(c)] [10] and found that the surface has Ti2O3 unit rows for [001]-direction and bridging ˚) oxygen rows between them. The broad rows (width of 16 A in Fig. 2 may correspond to the Ti2O3 unit rows. Though, the p(1 £ 3) surface has the bridging oxygen rows between Ti2O3 unit rows as shown in Fig. 1(c), it is difficult to recognize the oxygen rows in Fig. 2. It suggests that the DOS of the bridging oxygen rows on the p(1 £ 3) surface is lower than that of the Ti2O3 unit rows. Fig. 3 shows a TiO2(110)-p(1 £ 2) surface that was obtained by 2 keV Arþ sputtering and annealing at 1050 8C for 1.5 h. The surface showed a clear p(1 £ 2) LEED pattern (datum not shown). It is recognized that there are some defects, steps and cross links for ½110-direction on the ˚ . We surface. The interval of the p(1 £ 2) rows is 13 A
Fig. 2. STM image of TiO2(110)-p(1 £ 3) surface prepared by only annealing of new sample at 830 8C for 2 h (sample bias þ2.0 V, ˚ 2. tunneling current 0.07 nA), 315 £ 315 A
E. Asari, R. Souda / Solid State Communications 129 (2004) 15–18
˚ 2 STM image of TiO2(110), which contains Fig. 3. A 350 £ 350 A mainly the p(1 £ 2) phase, prepared by 2 keV Arþ ion sputtering and annealing of at 1050 8C for 1.5 h (sample bias þ 2.0 V, tunneling current 0.07 nA). The inset shows the magnified area of stage B0 . A height profile of the surface across the stages B–A –B – B0 (grey line in the image) is also shown. The scanning direction ˚. from the ½110-direction is about 20 A
titanium atoms Ti(5)) [10,26]. Though the stoichiometric structure of Ti2O3 – O –Ti2O3 in Fig. 2 is as same as that on the framed area in Fig. 3, we can not recognize narrow rows between Ti2O3 unit rows in Fig. 2. Furthermore, the width of ˚ ) is wider than that of the the p(1 £ 3) rows in Fig. 2 (16 A ˚ p(1 £ 2) rows in Fig. 3 (10 A). Above results suggest that the Ti2O3 unit rows on the p(1 £ 3) surface is wider than that on the p(1 £ 2) surface. Due to the ICISS experiments on the TiO2(110) surfaces, it became obvious that the width between O(4) and O(40 ) atoms on the p(1 £ 3) surface [see ˚ wider than that on the p(1 £ 2) surface Fig. 1(c)] is 0.3 A ˚ is not [see Fig. 1(b)] [10]. The difference of 0.3 A ˚ . The comparable to the difference of 6 ( ¼ 16 2 10) A wave functions, however, extend exponentially, small approach of the two-fold O(4) atom for the five-fold Ti(2) atom may largely enhance the electron hybridization between the rest of a O(4) bond and that of a Ti(2) bond in rutile. The increase of hybridization increases the electron
17
density or DOS around Ti(2) [or Ti(20 )] and Ti2O3 regions in Fig. 1(c). As a result, it may change the width of rows in the STM image. Considering the atomic radius of Ti(2) and Ti(20 ) atoms, the distance Ti(2) – Ti2O3 – Ti(20 ) in Fig. 1(c) ˚. should be about 16 A At the edge of stage B, we can recognize a group of p(1 £ 1)-type rows. The contrast between p(1 £ 1)-type rows and p(1 £ 2) rows on the stage B is largely discrepant, viz, the p(1 £ 1)-type rows exist at a higher layer (stage B0 ). A height profile of the surface across the stages B– A –B – B0 is also depicted in the Fig. 3. As the scanning angle from the ½110 direction is ,208 (see the grey line in Fig. 3), we must compensate the scanning distance with a factor of 0.94 ( ¼ cos208) to evaluate the certain intervals of the p(1 £ 1)type rows. But the factor 0.94 coincides 1 ^ 0.1 within 10% uncertainty of the STM measurements. Accordingly, the ˚ interval of the p(1 £ 1)-type rows is determined to be 6.5 A in average from the profile. The height of the stage A from ˚ (half length of 6.5 A ˚ ), suggests that the the stage B, 3.3 A stage A exists next layer on the stage B. The height of the ˚ , from the stage B does not reflect the reality stage B0 , ,2 A because the surface structure (or DOS) between the stage B [p(1 £ 2)] and the stage B0 [p(1 £ 1)] is largely discrepant. Comparing with the relative position of the p(1 £ 1)-type rows on the stage B0 and the p(1 £ 2) rows on the stage B (inset in Fig. 3), we can recognize that the bright p(1 £ 1)type rows on the stage B0 correspond to the five-fold titanium rows, because the dark lines between bright p(1 £ 1)-type rows, corresponding to bridging oxygen rows, always situate on the center of the Ti2O3 rows on stage B. The long narrow rows on the stage A (indicated by arrows in Fig. 3) obviously correspond to the bridging oxygen. Since the distance via p(1 £ 1)-type row –Ti2O3˚ , it corresponds well with p(1 £ 1)-type row in Fig. 3 is 20 A ˚ ) in the distance via O(6) – Ti2O3 –O(60 ) (9.75 £ 2 ¼ 19.5 A Fig. 1(c). The above results show that the DOS of the fivefold titanium rows, which appeared in a group on the p(1 £ 2) surface, is higher than that of the bridging oxygen rows. However, DOS of the independent bridging oxygen rows, which appeared at the p(1 £ 2) phase boundary, is higher than that of the five-fold titanium row. Although, it is difficult to assign directly the bright rows on the TiO2(110)p(1 £ 1) surface by STM, the result that the p(1 £ 1) type rows appeared in a group on the p(1 £ 2) surface, and that they correspond to the five-fold titanium rows all suggests that the bright rows on the TiO2(110)-p(1 £ 1) surface correspond to the five-fold titanium rows as well. This result therefore, coincides with the calculated results of DOS on the TiO2(110)-p(1 £ 1) surface by Diebold et al. [23]. As the mixed phase was observed within a local restricted area on the surface (one area at a time), frequency that just such a mixed phase appeared was very low. Ergo it seems to be very difficult to control the parameters needed to make the ideal mixed phase. The phase must have appeared by means of a random combination of the proper temperature and reduction rate in a local surface. This in
18
E. Asari, R. Souda / Solid State Communications 129 (2004) 15–18
turn must have been caused by chance, because the above parameters might not be perfectly uniform on the surface. In summary, TiO2(110) surfaces were observed by STM. We found that the p(1 £ 1)-type rows that appeared in a group on the p(1 £ 2) surface correspond to the five-fold titanium rows. On the other hand, a p(1 £ 1)-type row that appeared independently at the phase boundary of the p(1 £ 2) surface correspond to the bridging oxygen row. These results suggest that the bright rows in the STM image on the TiO2(110)-p(1 £ 1) surface correspond to the fivefold titanium rows, thereby, validating the DOS calculations results by Diebold et al.
References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] V.E. Henrich, R.L. Kurtz, Phys. Rev. B 23 (1981) 6280. [3] R.E. Tanner, M.R. Castell, G.A.D. Briggs, Surf. Sci. 412 (1998) 672. [4] G. Charlton, P.B. Howes, C.L. Nicklin, P. Steadman, J.S.G. Taylor, C.A. Muryn, S.P. Harte, J. Mercer, R. McGrath, D. Norman, T.S. Turner, G. Thornton, Phys. Rev. Lett. 78 (1997) 495. [5] B. Hird, R.A. Armstrong, Surf. Sci. 385 (1997) L1023. [6] B. Hird, R.A. Armstrong, Surf. Sci. 420 (1999) L131.
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