Langmuir 2001, 17, 7339-7343
7339
Resonant Photoemission and X-ray Absorption Study of the Electronic Structure of the TiO2-Al2O3 Interface M. Sa´nchez-Agudo,† L. Soriano,*,† C. Quiro´s,† M. Abbate,‡ L. Roca,§ J. Avila,§ and J. M. Sanz† Departamento de Fı´sica Aplicada, Instituto de Ciencia de Materiales Nicola´ s Cabrera, Universidad Auto´ noma de Madrid, Cantoblanco, E-28049 Madrid, Spain, Departamento de Fisica, Universidade Federal do Parana, Caixa Postal 19091, 81531-990 Curitiba PR, Brazil, Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco E-28049 Madrid, Spain, and LURE, Universite´ Paris-Sud, Baˆ timent 209d, F-91405 Orsay, France Received June 11, 2001. In Final Form: August 15, 2001 The electronic structure of the TiO2-Al2O3 interface has been investigated using X-ray absorption and resonant photoemission spectroscopies. These two characterization techniques give a complete picture of the electronic structure of the interface. TiO2 deposits have been grown by thermal evaporation of Ti in an oxygen atmosphere at room temperature. The experimental absorption and photoemission spectra have been measured as a function of the deposition time. In particular, the spectra of a submonolayer of TiO2 grown on the Al2O3 substrate allowed us to study the electronic structure at the interface. The Ti 2p absorption spectra for the TiO2 submonolayer show a significant decrease of the crystal field for the Ti atoms at the interface, which is attributed to a reduction in the covalence of the bonding of the TiO2 overlayer. On the other hand, the corresponding resonant photoemission spectra through the 2pf3d absorption edge show resonance only from Ti states which are distributed at the lower binding energy side of the valence band spectra measured of TiO2. This is also an indication of a reduction of covalence. The results are compared to theoretical cluster model calculations with reduced interactions which support the interpretation of the experimental spectra. The results are also discussed in terms of the degree of covalence of the substrate.
1. Introduction The increasing importance of coatings and multilayered structures in technological applications has promoted many studies of oxide/oxide interfaces in the past few years. During the last two decades, several interfaces have been investigated, mainly in the field of semiconductors, by using surface sensitive techniques such as Auger electron spectroscopy (AES) and X-ray photoemission spectroscopy (XPS). However, the study of such interfaces formed by oxide materials using these techniques is not always an easy task. Several noncontrolled effects occurring during the interface formation (like charging, overlayer-support interaction, size effects, etc.) can affect the spectra and mislead their interpretation. We present here a combined X-ray absorption (XAS) and resonant photoemission (RPES) study of the electronic structure of the TiO2-Al2O3 interface. The absorption and photoemission spectra of a submonolayer of TiO2 grown on Al2O3 give a complete view of the bonding between these oxides at the interface. The experimental spectra are interpreted in terms of a reduction of the covalence of the Ti atoms at the interface in good agreement with cluster model calculations. The review work by Vurens et al. and references therein1 could be considered as the starting point of the research on oxide/oxide interfaces. Since then, other related works * Corresponding author. Phone: +34 91 397 4192. Fax: +34 91397 3969. E-mail:
[email protected]. † Departamento de Fı´sica Aplicada, Instituto de Ciencia de Materiales Nicola´s Cabrera, Universidad Auto´noma de Madrid. ‡ Departamento de Fisica, Universidade Federal do Parana. § Instituto de Ciencia de Materiales de Madrid, CSIC, and LURE, Universite´ Paris-Sud. (1) Vurens, G. H.; Salmero´n, M.; Somorjai, G. A. Prog. Surf. Sci. 1990, 32, 333.
have appeared in the literature.2-5 Very recently, a new review on oxide/oxide interfaces studied by electron spectroscopies has been published.6 In this review, it is shown that most of the recent studies on interfaces have been performed by XPS and AES whereas studies using XAS and RPES are very scarce. These spectroscopies are very useful in the analysis of the chemical bonding of oxides with an important covalent character and provide, in a first approximation, direct information on both occupied and nonoccupied electronic states of the interface. XAS is an unoccupied electronic state spectroscopy7 which has been mainly applied to the study of the electronic structure of bulk compounds, like oxides, nitrides, and carbides.8 The work by Himpsel et al.9 opened new perspectives for XAS as a new tool in the analysis of the electronic structure of surfaces and interfaces. However, as far as we know, only two works published by the present authors10,11 have applied XAS to the study of interfaces. Resonant photoemission spectroscopy (RPES) has been widely used also in the analysis of the electronic structure (2) Lad, R. J. Surf. Rev. Lett. 1995, 2, 109. (3) Henrich, V. E. Prog. Surf. Sci. 1995, 50, 77. (4) Henrich, V. E.; Cox, P. A. In The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994; Chapter 7. (5) Noguera, C. In Physics and Chemistry at Oxide Surfaces; Cambridge University Press: Cambridge, 1996; Chapter 5. (6) Gonza´lez-Elipe, A. R.; Yubero, F. In Handbook of Surfaces and Interfaces of Materials; Academic Press (to be published). (7) Fuggle, J. C.; Inglesfield, J. In Unoccupied Electronic States; Springer: Berlin, 1992. (8) Chen, J. G. Surf. Sci. Rep. 1997, 30, 1. (9) Himpsel, F. J.; Karlsson, U. O.; McLean, A. B.; Terminello, L. J.; De Groot, F. M. F.; Abbate, M.; Fuggle, J. C.; Yarmoff, J. A.; Thole, B. T.; Sawatzky, G. A. Phys. Rev. B 1991, 43, 6899. (10) Soriano, L.; Fuentes, G. G.; Quiro´s, C.; Trigo, J. F.; Sanz, J. M.; Bressler, P. R.; Gonza´lez-Elipe, A. R. Langmuir 2000, 16, 7066. (11) Sa´nchez-Agudo, M.; Soriano, L.; Quiro´s, C.; Avila, J.; Sanz, J. M. Surf. Sci. 2001, 482-485, 470.
10.1021/la010868i CCC: $20.00 © 2001 American Chemical Society Published on Web 10/12/2001
7340
Langmuir, Vol. 17, No. 23, 2001
Sa´ nchez-Agudo et al.
of bulk compounds. In particular, RPES has been used to separate the metal contribution to the valence band in covalent compounds. More details can be found in the review by Davis.12 TiO2 has been previously characterized by resonant photoemission at the 3pf3d edge13-15 and at the 2pf3d edge.16 As observed by Prince et al.,16 resonance effects at the L2,3 edge give much higher intensity than those at the M2,3 edge. This property gives support to our study, for which we need to detect a weak resonance signal from the TiO2 submonolayer. In this work, we first present and discuss the experimental Ti 2p XAS spectra of TiO2 grown on Al2O3 as a function of the coverage. Then, we analyze the valence band photoemission spectra of a submonolayer of TiO2 on Al2O3 as a function of the photon energy through the Ti 2pf3d absorption edge. Finally, cluster model calculations are used in the interpretation of the absorption and photoemission spectra. 2. Experimental Details Titanium dioxide has been grown by thermal evaporation of pure titanium in an oxygen atmosphere. The base pressure in the experimental chamber was around 1 × 10-10 Torr. Oxygen was introduced by a tube directed toward the sample in order to increase the oxygen pressure in the surroundings of the sample. The total pressure as measured far from the sample was 5 × 10-6 Torr. The evaporation rate, as estimated from the Ti 2p XAS intensities, was 0.5 monolayer (ML) per minute.11 The Al2O3 substrate was prepared by thermal oxidation of a pure aluminum foil at 350 °C for 30 min. After 188 min evaporation time, the sample was annealed at 300 °C for 30 min in the same oxygen atmosphere. This procedure gave a 200 Å thick TiO2 thin film whose Ti 2p XAS spectrum agrees with other spectra for TiO2.17 The XAS and RPES spectra were measured as a function of the evaporation time in the SU8 beam-line of the SuperAco storage ring at LURE. This beam-line is equipped with an ondulator and a plane grating-spherical mirror monochromator. The estimated resolution for the XAS spectra is better than 100 meV at the Ti 2p edge. The XAS spectra were taken in the total electron yield detection mode. The spectra were corrected from beam intensity losses with the photon flux current measured at the entrance of the chamber. The energy scale was calibrated according to the known position of the first peak of the Ti 2p EELS spectrum.17 The valence band photoemission spectra were measured with an angle resolved electron analyzer from VSW. The slits were opened in order to obtain a higher signal for photoemission measurements.
3. Results and Discussion Ti 2p XAS Spectra. In Figure 1, we present the Ti 2p XAS spectra of the TiO2 deposits as a function of the equivalent coverage. In general, the Ti 2p XAS spectra for TiO2 represent the atomic multiplets for Ti4+ ions affected by an octahedral crystal field (10Dq ) 1.8 eV).18 The topmost spectrum of the series, labeled as thin film, corresponds to a 20 nm thick thin film obtained as described in the experimental section. This spectrum agrees well with other spectra published for TiO2.17,18 The spectrum shows five well-defined peaks labeled as A-E as well as two small structures at the pre-edge. Peaks D (12) Davis, L. C. J. Appl. Phys. 1986, 59, R25. (13) Zhang, Z.; Jeng, S. P.; Henrich, V. E. Phys. Rev. B 1991, 43, 12004. (14) Heise, R.; Courths, R.; Witzel, S. Solid State Commun. 1992, 84, 599. (15) Nerlov, J.; Ge, Q.; Møller, P. J. Surf. Sci. 1996, 348, 28. (16) Prince, K. C.; Dhanak, V. R.; Finetti, P.; Walsh, J. F.; Davis, R.; Muryn, C. A.; Dhariwal, H. S.; Thornton, G.; van der Laan, G. Phys. Rev. B 1997, 55, 9520. (17) Brydson, R.; Sauer, H.; Engel, W.; Thomas, J. M.; Zeitler, E.; Kosugi, N.; Kuroda, H. J. Phys.: Condens. Matter 1989, 1, 797. (18) de Groot, F. M. F.; Fuggle, J. C.; Thole, B. T.; Sawatzky, G. A. Phys. Rev. B 1990, 41, 928.
Figure 1. Ti 2p X-ray absorption spectra as a function of the equivalent TiO2 coverage. For the thin film preparation, see the experimental section.
and E come from the Ti 2p1/2 states separated by spinorbit interaction from the Ti 2p3/2 states which generate the A, B, and C peaks. On the other hand, peaks A and D correspond to transitions to t2g states whereas peaks B, C, and E correspond to transitions to eg states. Also, the eg states are split into the B and C peaks due to distortion of the octahedron formed by the ligands in TiO2.18 More details on the interpretation of this spectrum can be found elsewhere.17,18 In contrast, the spectra corresponding to coverages equal to or lower than 1 monolayer, labeled as 0.5 and 1 in Figure 1, differ significantly from that of the thin film. They only show two main peaks and two weaker structures. This clearly indicates that the Al2O3 substrate is strongly affecting the electronic structure of the Ti atoms at the interface. In this case, XAS is a powerful tool for chemical analysis due to the strong dependence of the Ti 2p XAS spectra on the oxidation state as illustrated in refs 19 and 21. Therefore, these spectra cannot be assigned to Ti3+ or Ti2+ species, as correspond to transitions in d1 and d2 symmetries,20 since they would be much broader. They also cannot be explained in terms of metallic Ti since they would be broader and shifted in energy.21 These spectra have already been explained in terms of a reduction of the crystal field of the Ti4+ ions at the TiO2-Al2O3 interface11 (1.0 eV) with respect to bulk TiO2 (1.8 eV). In this picture, the Ti atoms at the interface are bonded to oxygen atoms forming Ti-O-Al cross-linking bonds. The relatively larger stability of the Al-O bond decreases the covalent contribution to the Ti-O bond. In fact, the loss of intensity observed in the XAS spectra is consistent with a decrease of the degree of covalence of the bonding.7,22 Similar effects have also been observed in the TiO2-SiO2 system.10 (19) Lusvardi, V. S.; Barteau, M. A.; Chen, J. G.; Eng, J., Jr.; Teplyakov, A. V. Surf. Sci. 1998, 397, 237. (20) de Groot, F. M. F.; Fuggle, J. C.; Thole, B. T.; Sawatzky, G. A. Phys. Rev. B 1990, 42, 5459. (21) Soriano, L.; Abate, M.; de Groot, F. M. F.; Alders, D.; Fuggle, J. C.; Hofmann, S.; Petersen, H.; Braun, W. Surf. Interface Anal. 1993, 20, 21. (22) Pedio, M.; Fuggle, J. C.; Somers, J.; Umbach, E.; Hasse, J.; Lindner, Th.; Ho¨fer, U.; Grioni, M.; de Groot, F. M. F.; Hillert, B.; Becker, L.; Robinson, A. Phys. Rev. B 1989, 40, 7924.
Electronic Structure of the TiO2-Al2O3 Interface
Langmuir, Vol. 17, No. 23, 2001 7341
Figure 3. Valence band resonant photoemission spectra of 0.5 ML of TiO2 grown on Al2O3 as a function of the photon energy. Figure 2. Valence band resonant photoemission spectra of the TiO2 thin film as a function of the photon energy. For an explanation of structures A-E, see the text.
Therefore, the unoccupied electronic states of the Ti atoms at the interface, as shown by the Ti 2p XAS spectra, are strongly affected by the effect of the support. A similar effect should be observed in the occupied valence band electronic states. For this reason, we have also studied the resonant photoemission spectra through the Ti 2pf3d absorption edge. Valence Band Resonant Photoemission Spectra. The valence band photoemission spectra as a function of the photon energy for the TiO2 thin film and for 0.5 ML of TiO2 grown on Al2O3 are shown in Figures 2 and 3, respectively. The spectra corresponding to the TiO2 thin film (Figure 2) show important changes in shape and intensity with photon energy. First of all, we have to note that the series of spectra for our thin film completely agree with the spectra reported by Prince et al.16 for a TiO2 single crystal. The main structures observed in the spectra have been labeled from A to D. Structures A and B are centered at 5.7 and 8.0 eV, respectively, and correspond to the valence band states of TiO2 formed mainly by the hybridization of the Ti 3d and O 2p states giving rise to the σ and π bands. The structure labeled as C peaks at 1.3 eV. This structure has been previously identified as due to surface defects produced by oxygen vacancies giving rise to Ti3+ species.16,23 On the other hand, the structure labeled as D moves to higher binding energies as the photon energy increases and has a constant kinetic energy. We assign this peak as an LM4,5M4,5 Auger peak for titanium in agreement with previous works.16 The spectra of the TiO2 submonolayer grown on Al2O3 (Figure 3) show also some changes throughout the series. In particular, the spectra around 459 eV photon energy are clearly enhanced in intensity in the region of peak B. In this case (i.e.,
