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Unraveling the Diffusion of Bulk Ti Interstitials in Rutile TiO2(110) by Monitoring Their Reaction with O Adatoms Zhen Zhang,† Junseok Lee,† John T. Yates, Jr.,*,† Ralf Bechstein,‡ Estephania Lira,‡ Jonas Ø. Hansen,‡ Stefan Wendt,‡ and Flemming Besenbacher‡ Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22904 and Interdisciplinary Nanoscience Center (iNANO), Department of Physics and Astronomy, UniVersity of Aarhus, DK-8000 Aarhus C, Denmark ReceiVed: October 13, 2009; ReVised Manuscript ReceiVed: December 3, 2009
The diffusion of interstitial Ti3+ species (Tii3+) in rutile TiO2(110) from the bulk to the surface has been studied utilizing two experimental techniques. Electron-stimulated desorption of O+ ions was employed to kinetically monitor the reaction between oxygen adatoms with Tii3+ species at temperatures between 360 and 400 K. Scanning tunneling microscopy was also used to measure the Tii3+ diffusion rate. Both methods yield a rate constant kTii3+ ) 5 × 10-4 s-1 at 393 K. The activation energy as measured by the rate dependence on temperature is ∼1.0 eV. Titania (TiO2) has wide applications in the fields of heterogeneous catalysis, photocatalysis, photovoltaic cells, and gas sensors.1-4 To better understand the relationship between atomic and electronic structure and reactivity, the rutile TiO2(110)(1 × 1) surface (Figure 1a) has been extensively studied as a model oxide surface.1-3,5,6 Surface as well as bulk defects have been postulated as active sites for the chemical and photochemical reactivity of the TiO2 surface.1,2,7,8 Since reduced TiO2 crystals contain interstitial Tii3+ ions,8-15 the diffusion of these ions to the TiO2 surface may well be involved in a variety of surface processes. Noteworthy is the SMSI (strong metal-support interaction) effect, where TiOx layers, produced by Tii3+ diffusion to the surface, cover the surface of metal particles deposited on the TiO2 surface, strongly affecting the rates of surface reaction on the particles.16-19 Therefore, accurate measurements of the rate and activation energy for Tii3+ bulk diffusion are crucial for an improved understanding of surface processes driven by Tii3+ interstitial ions in the TiO2 bulk. Because of a lack of sensitivity to buried species the study of interstitial species in the bulk and near surface region is a challenge to conventional surface science methods. Recently, however, it has been found from electron-stimulated desorption (ESD) studies that oxygen adatoms (Ot) on the TiO2(110)(1 × 1) surface (Figure 1a), produced by dissociation of O2 and chemisorption on the 5-fold-coordinated Ti (Ti5c) sites,6,20-22 have a very high ionic cross section for O+ production.23 The high sensitivity of ESD to the Ot adatoms therefore provides an excellent tool to measure the kinetics of the reaction between Ot adatoms and out-diffusing Tii3+ interstitial species. In this letter we study the kinetics of interstitial Tii3+ species diffusion through rutile TiO2(110) crystals using both ESD and high-resolution scanning tunneling microscopy (STM). Monitoring the reaction between chemisorbed oxygen atoms with Tii3+ species at temperatures between 360 and 400 K by means of ESD allows us to deduce the rate and activation energy for the diffusion process. Excellent agreement is found for the Tii3+ * To whom correspondence should be addressed. E-mail: johnt@ virginia.edu. † University of Virginia. ‡ University of Aarhus.
diffusion rate constant at 393 K, which is ∼5 × 10-4 s-1 as extracted independently from ESD and STM studies. In addition, an energy barrier of ∼1 eV was estimated for the diffusion of Tii3+ interstitials toward the surface. The ESD and STM experiments were performed in two separate ultra-high-vacuum chambers, both of which had base pressures in the low 10-11 mbar range. The depletion of Ot adatoms by reaction with Tii3+ interstitial species was measured using a time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD) apparatus to detect O+ ions from ESD.23 In the ESD experiments the Ot adatoms were isotopically labeled to accurately measure only the chemisorbed 18Ot surface species by TOF separation of 18O+ from 16O+. The STM studies were conducted using a home-built, temperature-variable Århus STM.24 The TiO2(110)-(1 × 1) crystals were cleaned by cycles of Ar+ ion sputtering at room temperature and vacuum annealing at 900-950 K. From the STM studies we estimated the Obr vacancy coverage of 26-42 × 1012 cm-2, which corresponds to 5-8% ML with 1 ML (monolayer) being the density of the (1 × 1) units, 5.2 × 1014 cm-2. Adsorption of O2 was reproducibly carried out using calibrated capillary array dosers.25 More details about the equipment utilized and sample preparation can be found in previous literature.8,21,23,26 Figure 1b-d depicts STM images acquired on clean and O2exposed TiO2(110) surfaces. The empty-state STM images of the TiO2(110) surface are dominated by electronic effects,1 i.e., the bright rows correspond to the Ti troughs, whereas geometrically protruding Obr atoms appear dark. Accordingly, the faint protrusions in the dark Obr rows obtained on the clean TiO2(110) surface (Figure 1b) correspond to single Obr vacancies.1,3,6,21 Exposure of O2 to the clean TiO2(110) surface at ∼120 K leads to stabilization of both O2 molecules20,27,28 and Ot adatoms8,21 on the surface. The Ot adatoms appear as bright spots in the Ti troughs (Figure 1c) and are the result of O2 dissociation in the Obr vacancies. However, in addition to the dissociation channel associated with the Obr vacancies, a second nonvacancy-related O2 dissociation channel also exists.8 As shown in Figure 1d this second O2 dissociation channel is evident from STM measurements on TiO2(110) crystals that were exposed to O2 at ∼120 K and subsequently heated to ∼266
10.1021/jp910358w 2010 American Chemical Society Published on Web 01/25/2010
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Figure 1. (a) Ball-and-stick model of the TiO2(110)-(1 × 1) surface with some of its known point defects. Large gray balls represent O atoms and medium-sized red balls 6-fold-coordinated Ti (Ti6c) and 5-fold-coordinated surface Ti atoms (Ti5c), respectively. The bridgebonded O species (Obr), single O vacancies (Obr vac.), and the on-topbonded O species (Ot) are indicated. The zoom-in STM images were acquired on (b) a clean vacuum-annealed TiO2(110) surface with a number of Obr vacancies and (c) a TiO2(110) surface with Ot adatoms obtained after O2 exposure at 120 K. (d) High-resolution STM image obtained after 6 L O2 [1 L (Langmuir) ) 1.33 × 10-6 mbar s] exposure at 120 K to a clean TiO2(110) crystal followed by heating up to 266 K. A tunneling voltage of +1.25 V and tunneling current of ∼0.1 nA were used throughout. Symbols indicate Obr vacancies (squares), Ot adatoms (circles), and adjacent Ot adatoms (ellipses).
K. After this preparation paired new features have appeared in the Ti troughs which we interpret as adjacent Ot adatoms resulting from O2 molecules that dissociated upon heating of the crystal. The results presented in Figure 1 are thus additional evidence for the second O2 dissociation channel and further show that this channel is relevant even at temperatures below room temperature. While STM is an ideal tool to unravel insights at the atomic scale, the technique of choice for a quantitative analysis (such as monitoring of the Ot adatom coverage at various temperatures) is ESD, because the Ot adatoms have a very high ionic cross section for O+ production.23 In fact, at 210 eV electron energy the ionic cross section is ∼3 × 10-18 cm-2 and thus ∼600 times higher than the ionic cross section for O+ production from O lattice atoms at the surface of the TiO2(110) crystal.23 Of particular interest is the temperature range between 360 and 400 K, because out-diffusion of Tii3+ from the bulk to the surface and their reaction with Ot adatoms sets in at these temperatures.8-10 In addition, previous ESD measurements have shown that molecular O2 is lost in this temperature range,28 and STM measurements have revealed increased coverage of Ot adatoms after heating to 393 K.8 After adsorption of 18O2 at 85 K followed by heating to various temperatures, we use the 18O+ yield to kinetically track the depletion of Ot adatoms as a
Zhang et al.
Figure 2. 18O+ yield decay from 18O2-exposed TiO2(110) surfaces upon ESD using electrons with 210 eV and a flux of 9.4 × 1010 electrons cm-2 s-1. A pulsed electron gun operating at 40 kHz with 75 ns pulse width was used for the TOF-ESDIAD.23,26 All measurements were conducted by keeping the sample at the indicated temperature (360-400 K) after 18O2 exposure at 85 K of (a) 2.5 × 1012 and (b) 38.4 × 1012 cm-2 molecules. The data (squares) are well fitted using a first-order exponential decay equation (lines). We find that there is no influence from ESD of 18O species since a reduction of the electron beam flux by 80% produces no change in the measured Ot consumption kinetics in this experiment.
function of crystal temperature. Selected kinetics measurements for the loss of Ot adatoms by reaction with Tii3+ diffusing from the bulk to the surface are shown in Figure 2. Figure 2a shows the 18O+ yield decays corresponding to an 18O2 exposure of 2.5 × 1012 cm-2 at 85 K and heating at the given temperatures, and a similar set of decay measurements is shown in Figure 2b for a higher 18O2 exposure (38.4 × 1012 cm-2 at 85 K). Three additional sets of kinetic measurements were made for 18O2 exposures between 2.5 × 1012 and 38.4 × 1012 cm-2. According to our current and previous20,28 temperature-programmed desorption measurements, no desorption of molecular O2 occurs for these O2 exposures, so that the O+ yield is directly related to the Ot coverage. For each Ot coverage the 18O+ yield decay was measured at 360, 370, 380, 390 and 400 K. The data points in Figure 2 are well fitted by first-order kinetics in the coverage of Ot, that is, the equation y ) y0 + ae- kOt can be used for the fits, where y is the O+ signal, y0 the background O+ signal, kO the rate constant for depletion of the Ot species, t the time, and a a variable that is proportional to the initial Ot coverage. Arrhenius plots of the rate constant for Ot consumption for all initial coverage of Ot adatoms are depicted in Figure 3. The activation energy for Ot depletion ranges between 0.93 ( 0.06 and 1.02 ( 0.08 eV over the Ot coverage range studied, i.e., within experimental error the activation energy, Ea, is constant ∼1 eV. This activation energy agrees well with the calculated value of ∼1.2 eV reported by Wendt et al.8
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Figure 5. Schematic diagrams showing the Tii3+ depletion layer in the near-surface region and the activated bulk diffusion of Tii3+ interstitials toward the surface where they react with Ot adatoms.
Figure 3. Diffusion data (Arrhenius plots) extracted from the 18O+ yield decays obtained in the ESD experiments for five different O2 exposures at 85 K, where kO is the rate constant for depletion of the Ot adatoms. In the inset kO is plotted versus the 18O2 exposure for the decays measured at 380 and 390 K. The red data point in the plot has been extracted from the STM data discussed below.
Figure 4. STM images acquired on TiO2(110) crystals (a) after exposure to 5 L O2 at 120 K and subsequently annealed at 393 K for 120 s and (b) followed by further annealing at 593 K for 120 s. Some of the TiOx islands on the terraces with an apparent height of ∼0.22 nm are indicated by circles.
The observation of first-order kinetics in the high Ot coverage range29 indicates that the rate of Ot depletion is proportional to the coverage of Ot and that a single kinetic process for Ot depletion is being observed between 360 and 400 K. In this picture, the rapid reaction of Ot adatoms with Tii3+ interstitials takes place instantly as the Tii3+ interstitials reach the surface. Upon heating in the 360-400 K temperature range, diffusion of Tii3+ interstitials occurs from the bulk to the surface which leads to the formation of TiOx islands on the terraces (cf. Figures 4 and 5). It is essentially the rate of this Tii3+ interstitial diffusion process that controls the decay rate of the O+ ESD ion signal from the Ot adatoms. The fortuitous high sensitivity of the ESD method to Ot adatoms allows us to measure the small flux of Tii3+ species to the surface at these rather low temperatures. Since clean reduced TiO2(110) surfaces exhibit very low ionic cross sections compared to the Ot adatoms on TiO2(110)(1 × 1)23 we anticipate that the O+ ESD yield from the TiOx islands is low as well. Therefore, the ESD measurements of Ot depletion are unlikely to be confounded by O+ ESD yields from the TiOx islands formed. To test the proposed model for Ot depletion through the formation of TiOx islands we performed a series of STM measurements (cf. Figure 4). In the STM studies, clean reduced
TiO2(110) crystals were exposed to O2 at 120 K and first annealed at 393 K for 120 s. This first anneal ensures that all molecularly adsorbed oxygen dissociates. After inspection by STM the crystals were further annealed at an even higher temperature. In the presented example the second anneal was performed at 593 K for 120 s (Figure 4b). Ot adatoms and small TiOx islands (designated by circles) are present on the terraces after both anneals. When comparing with the situation obtained after the first anneal, the coverage of small TiOx islands has markedly increased, whereas the Ot coverage simultaneously has decreased after the second anneal at 593 K. Thus, Tii3+ interstitials have indeed reacted with Ot adatoms on the surface. A closer look at the STM images reveals that at least three different types of TiOx islands exist on the terraces, all of which are assumed to be stoichiometric (Ti:O ) 1:2). The latter can be concluded based on valence band spectroscopic data which show no sign of reduction at annealing temperatures where the TiOx islands are present.8 From a quantitative analysis of the STM images obtained30 we found that, on aVerage, each TiOx island contains ∼3 O atoms and ∼1.5 Ti atoms. (This would be true, for example, for a mixture of 3 TiO2 and 4 Ti2O4 islands.) Thus, if we count the TiOx islands in the STM images obtained after annealing at 393 K30 we can extract the Ot depletion rate constant kO at 393 K, which was found to be (1.1 ( 0.4) × 10-3 s-1. This value is also plotted in Figure 3 (red) for a direct comparison with the depletion rate constants found by ESD. From the Ot depletion rate constants given in Figure 3 the flux of Tii3+ interstitials from the bulk to the surface can be estimated: For an O2 exposure of 38.4 × 1012 cm-2 in the ESD experiments the initial coverage of Ot adatoms is ∼19.2 × 1012 cm-2, assuming that the sticking coefficient at 85 K is ∼0.5.20 From the line in Figure 3 corresponding to this Ot coverage we extract at 393 K a rate constant kO for the depletion of Ot adatoms of ∼1.1 × 10-3 s-1. Note that this value for the rate constant agrees very well with the value extracted from the STM experiments [(1.1 ( 0.4) × 10-3 s-1]. We assume that the various islands have a stoichiometry of TiO2. On this basis the rate constant of Ot depletion is two times the rate constant of Tii3+ out-diffusion. Thus, at 393 K and high Ot coverage (g10 × 1012 cm-2) the Tii3+ interstitial diffusion rate is estimated to be kTii3+ ≈ 5 × 10-4 s-1 from both measurement techniques. A schematic diagram for the diffusion of Tii3+ interstitials to the TiO2(110) surface is shown in Figure 5. Here, interstitial Tii3+ species exist with a concentration gradient in the nearsurface region due to their reaction with adsorbed surface oxygen species. An interstitial depletion layer is thus formed in the near surface region upon annealing. The reaction of Ot adatoms with out-diffusing Tii3+ bulk species is controlled by an activation energy barrier of ∼1 eV for interstitial diffusion through the bulk to the surface. It is likely that this barrier is associated with the exchange of Tii3+ interstitial ions with Ti4+ lattice ions,
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as recently suggested based on density functional theory calculations.8,31 In conclusion, from the interplay of ESD and STM measurements we deduced important qualitative and quantitative understanding of the diffusion of Tii3+ interstitials in reduced rutile TiO2(110) crystals. By monitoring the reaction of Tii3+ interstitials with oxygen adatoms on the surface and from an analysis of STM results, we found excellent agreement for the Tii3+ diffusion rate constant which is ∼5 × 10-4 s-1 at 393 K. In addition, an energy barrier of ∼1 eV was estimated from the temperature dependence of the diffusion of Tii3+ interstitials toward the surface, which is in good agreement with an activation barrier of ∼1.2 eV derived by theoretical means, as reported recently.8 In the presented analysis of the ESD data we argue that the activation barrier found from the decay of oxygen adatoms on the surface is controlled by the diffusion of the Tii3+ interstitials toward the surface and that the rate of Tii3+ interstitial out-diffusion can thus be measured with high precision. Acknowledgment. We acknowledge with thanks the support of this work by the Defense Threat Reduction Agency (DTRA) under contract HDTRA-07-C0085 and the Danish Ministry of Science, Technology, and Innovation through the iNANO center and the Danish Research Councils. Note Added after ASAP Publication. This paper was published on the Web on January 25, 2010, with errors to the text. The corrected version reposted on February 1, 2010. References and Notes (1) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (2) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735. (3) Thompson, T. L.; Yates, J. T., Jr. Chem. ReV. 2006, 106, 4428. (4) Fujishima, A.; Zhang, X.; Tryk, D. A. Surf. Sci. Rep. 2008, 63, 515. (5) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994. (6) Pang, C. L.; Lindsay, R.; Thornton, G. Chem. Soc. ReV. 2008, 37, 2328. (7) Lu, G. Q.; Linsebigler, A. L.; Yates, J. T., Jr. J. Phys. Chem. 1994, 98, 11733.
Zhang et al. (8) Wendt, S.; Sprunger, P. T.; Lira, E.; Madsen, G. K. H.; Li, Z.; Hansen, J. Ø.; Matthiesen, J.; Blekinge-Rasmussen, A.; Lægsgaard, E.; Hammer, B.; Besenbacher, F. Science 2008, 320, 1755. (9) Henderson, M. A. Surf. Sci. 1999, 419, 174. (10) Li, M.; Hebenstreit, W.; Diebold, U. Phys. ReV. B 2000, 61, 4926. (11) Li, M.; Hebenstreit, W.; Diebold, U.; Tyryshkin, A. M.; Bowman, M. K.; Dunham, G. G.; Henderson, M. A. J. Phys. Chem. B 2000, 104, 4944. (12) Li, M.; Hebenstreit, W.; Gross, L.; Diebold, U.; Henderson, M. A.; Jennison, D. R.; Schultz, P. A.; Sears, M. P. Surf. Sci. 1999, 437, 173. (13) Smith, R. D.; Bennett, R. A.; Bowker, M. Phys. ReV. B 2002, 66, 035409. (14) Park, K. T.; Pan, M.; Meunier, V.; Plummer, E. W. Phys. ReV. B 2007, 75, 245415. (15) McCarty, K. F. Surf. Sci. 2003, 543, 185. (16) Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. Soc. 1978, 100, 170. (17) Dulub, O.; Hebenstreit, W.; Diebold, U. Phys. ReV. Lett. 2000, 84, 3646. (18) Bennett, R. A.; Pang, C. L.; Perkins, N.; Smith, R. D.; Morrall, P.; Kvon, R. I.; Bowker, M. J. Phys. Chem. B 2002, 106, 4688. (19) Fu, Q.; Wagner, T. Surf. Sci. Rep. 2007, 62, 431. (20) Henderson, M. A.; Epling, W. S.; Perkins, C. L.; Peden, C. H. F.; Diebold, U. J. Phys. Chem. B 1999, 103, 5328. (21) Wendt, S.; Schaub, R.; Matthiesen, J.; Vestergaard, E. K.; Walhlstro¨m, E.; Rasmussen, M. D.; Thostrup, P.; Molina, L. M.; Lægsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Surf. Sci. 2005, 598, 226. (22) Du, Y.; Dohna´lek, Z.; Lyubinetsky, I. J. Phys. Chem. C 2008, 112, 2649. (23) Lee, J.; Zhang, Z.; Yates, J. T., Jr. Phys. ReV. B 2009, 79, 081408(R). (24) Lauritsen, J. V.; Besenbacher, F. AdV. Catal. 2006, 50, 97. (25) Yates, J. T., Jr. Experimental InnoVations in Surface Science: A Guide to Practical Laboratory Methods and Instruments; Springer: New York, 1998. (26) Ahner, J.; Mocuta, D.; Yates, J. T., Jr. J. Vac. Sci. Technol. A 1999, 17, 2333. (27) Thompson, T. L.; Yates, J. T., Jr. Top. Catal. 2005, 35, 197. (28) Kimmel, G. A.; Petrik, N. Phys. ReV. Lett. 2008, 100, 196102. (29) As the Ot coverage approaches zero, the rate constant for Ot depletion increases and reaches a limiting value (see inset in Figure 3). The rate constant decreases monotonically as the initial O2 exposure at 85 K increases. We postulate that this decrease of the rate constant is due to increasing Tii3+ depletion in the near surface region for increasing O2 exposure before starting the kinetic experiments at temperatures g 360 K. (30) The quantitative analysis of the STM data corresponding to the images depicted in Figure 4a and 4b is based on total areas of ∼2000 nm2 in each experiment. From the data file corresponding to the STM image depicted in Figure 4a we estimated an Ot coverage of 44.4 × 1012 cm-2. In a control experiment we reached similar results both for the O content of the TiOx islands as well as for the Ot depletion rate constant at 393 K. ¨ g˘u¨t, S.; Zapol, P.; Browning, N. D. Phys. ReV. B 2007, (31) Iddir, H.; O 75, 073203.
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