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Crystal-Field Effects at the TiO2-SiO2 Interface As Observed by X-ray Absorption Spectroscopy L. Soriano,* G. G. Fuentes, C. Quiro´s, J. F. Trigo, and J. M. Sanz Instituto de Ciencia de Materiales Nicola´ s Cabrera, Departamento Fı´sica Aplicada, Universidad Auto´ noma de Madrid, Cantoblanco, E-28049 Madrid, Spain
P. R. Bressler BESSY, Albert Einstein Strasse 15, D-12489 Berlin, Germany
A. R. Gonza´lez-Elipe Instituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla), Avda. Ame´ rico Vespucio s/n, Isla de la Cartuja, E-41092 Sevilla, Spain Received March 6, 2000 The electronic structure of the TiO2-SiO2 interface has been investigated using X-ray absorption spectroscopy (XAS). TiO2 overlayers have been grown on two substrates, amorphous SiO2 and highly oriented pyrolytic graphite (for comparison), by evaporation of Ti in an oxygen atmosphere at room temperature. The evaporation rate was low enough to allow a detailed study of the early stages of growth, i.e., the submonolayer regime (θ < 1). The Ti 2p XAS spectra of the TiO2 overlayers have been measured for different coverages. The spectra corresponding to the submonolayer regime show an unusual shape not reported up to now. A comparison with existing atomic multiplet calculations indicates a significant decrease of the crystal field (≈0.7 eV) as compared with bulk TiO2 (≈1.7 eV) due to strong electronic interactions at the interface. The presence of the SiO2 substrate and the covalent character of the Si-O bonds lowers the crystal field of the TiO2 overlayer at the interface. In contrast, the graphite substrate inhibits the total oxidation of the overlayer forming Ti2O3. XAS has proved to be a useful experimental tool for the study of the electronic structure of interfaces.
1. Introduction The main aim of this work is to study the electronic structure of the TiO2-SiO2 interface by X-ray absorption spectroscopy (XAS).The characterization of this interface by synchrotron photoemission spectroscopy (PES) and extended X-ray absorption fine structure (EXAFS) has been published very recently in this journal.1 In the previous work it was concluded that TiO2 spreads on the surface of SiO2, forming Si-O-Ti-O cross-linking bonds at the interface prior to thickening. The results indicated a strong interfacial electronic interaction between both oxides, which suggested the need for a further characterization using other techniques. Here we present a detailed analysis of the fine structure of the Ti 2p XAS spectra through the interface formation. We show that the fine structure of the XAS edges is a useful probe for the structural environment of atoms and their chemical state during the interface formation. In fact, we present here unique Ti 2p XAS spectra for TiO2-SiO2 in the submonolayer regime. The spectra reflect a significant lowering of the crystal field for the first TiO2 monolayer due to the influence of the silica substrate. These results are consistent with the formation of Si-O-Ti-O crosslinking bonds at the interface. The study of oxide-oxide interfaces is becoming a hot topic because of the interfaces’ role in relevant technological processes. The electronic interactions at oxide* Corresponding author. Fax, 34 91397 3969; e-mail, l.soriano@ uam.es. (1) Espino´s, J. P.; Lassaleta, G.; Caballero, A.; Ferna´ndez, A.; Gonza´lez-Elipe, A. R.; Stampfl, A.; Morant, C.; Sanz, J. M. Langmuir 1998, 14, 4908.
oxide interfaces determine many of the interfaces’ properties, and correspondingly, their applications. However, their study, both experimental and theoretical, is scarce in the literature, when compared, for instance, with metal-semiconductor or oxide-semiconductor systems. Several reviews dealing with metal-oxide and oxideoxide interfaces have been reported.2-6 In particular, the TiO2-SiO2 interface has been studied in connection with optical multilayers of these materials used for modulation of the refractive index in optical coatings.7 Other characterization studies of this interface include X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES).8,9 XAS has been widely used in the study of the electronic structure of bulk compounds. A review on applications of XAS to the study of the electronic structure of transition metal oxides, nitrides, carbides, sulfides, and other interstitial compounds has been recently published.10 XAS has also been nicely applied to the study of surfaces and interfaces.11 By controlling the experimental conditions (2) Lad, R. J. Surf. Rev. Lett. 1995, 2, 109. (3) Vurens, G. H.; Salmero´n, M.; Somorjai, G. A. Prog. Surf. Sci. 1990, 32, 333. (4) Henrich, V. E. Prog. Surf. Sci. 1995, 50, 77. (5) Henrich, V. E.; Cox, P. A. In The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994; Chapter 7. (6) Noguera, C. In Physics and Chemistry at Oxide Surfaces; Cambridge University Press: Cambridge, 1996; Chapter 5. (7) Yu-Zhang, K.; Boisjolly, G.; Rivory, J.; Kilian, L.; Colliex, C. Thin Solid Films 1994, 253, 299. (8) Lassaleta, G.; Ferna´ndez, A.; Espino´s, J. P.; Gonza´lez-Elipe, A. R. J. Phys. Chem. 1995, 99, 1484. (9) Mejı´as, J. A.; Jime´nez, V. M.; Lassaleta, G.; Ferna´ndez, A.; Espino´s, J. P.; Gonza´lez-Elipe, A. R. J. Phys. Chem. 1996, 100, 16255. (10) Chen, J. G. Surf. Sci. Rep. 1997, 30, 1.
10.1021/la000330x CCC: $19.00 © 2000 American Chemical Society Published on Web 07/29/2000
Crystal-Field Effects at TiO2-SiO2 Interface
of the measurements, i.e., taking advantage of the linear polarization of the incident light, collecting the convenient Auger electrons (partial yield), or even collecting photodesorbed positive ions (ion-yield), the surface sensitivity of XAS can be enhanced.11 In this work, we have investigated the growth of TiO2 overlayers on amorphous SiO2 substrate by measuring the Ti 2p XAS spectra in the total electron yield detection mode for different TiO2 coverages. The site-selective character of XAS allows us to study the electronic structure and chemical environment of the Ti atoms deposited on the SiO2 substrate. Special attention has been paid to the early stages of growth for coverages below one monolayer. In general, the L2,3 XAS spectra of the light transition-metal compounds can be regarded as atomic multiplets projected in a crystal field according to the corresponding symmetry.12,13 These spectra are very sensitive to the symmetry of the initial state, i.e., chemical state, local coordination, crystal field, etc. These properties support the use of XAS in this study. We first present and discuss the Ti 2p XAS spectra for different TiO2 coverages on highly oriented pyrolytic graphite (HOPG) substrate for comparison. Then, the spectra of the TiO2 overlayers grown on the SiO2 substrate are discussed. A quantitative analysis of the XAS intensities has been performed in order to estimate the thickness of the deposited overlayers. Finally, the experimental spectra have been compared with atomic multiplet calculations to support their interpretation. 2. Experimental Section The experiments were performed in a UHV chamber located at the VLS-PGM beam line of the BESSY-I storage ring in Berlin. XAS measurements were taken using a Varied Line SpacingPlane Grating Monochromator (VLS-PGM).14 The experimental resolution of this monochromator is estimated as better than 100 meV at the Ti 2p edge (455 eV). The spectra were acquired in the total-electron-yield detection mode and then normalized to the incident I0 current as measured from a gold grid located at the entrance of the chamber. The position of the sample and the voltage of the detector were maintained constant during the experiments. This allowed us to perform a quantitative analysis of the XAS intensities. The absolute energy scale was calibrated according to the known position of the first peak in the Ti 2p spectra in rutile (457.8 eV).15 The silica substrate consisted of a 200Å thick film of amorphous SiO2 grown by dry oxidation of a Si (111) wafer. An HOPG substrate from Advanced Ceramic Corporation, previously cleaved in air, was located close to the SiO2 substrate. Both substrates (about 1 × 1 cm2 each) were separated from the evaporation source by about 20 cm, thus the amount of deposited material on both substrates was comparable. TiO2 was grown by reactive Ti evaporation in an oxygen atmosphere at room temperature. Ti was evaporated from a Tungsten filament surrounded by a Ti wire (0.25 mm thick, 99.99% purity from Goodfellow) working at 11 Amps. This configuration provided a very low deposition rate (see below) to study the early stages of growth. Oxygen was introduced through a tube directed toward the substrates in order to maintain a higher pressure in their surroundings. The base pressure during the experiments was 1 × 10-10 Torr, whereas the partial oxygen pressure during evaporation was 5 × 10-7 Torr, as measured in the UHV chamber. An amorphous anodic TiO2 sample has also been used as reference. (11) 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. (12) De Groot, F. M. F.; Fuggle, J. C.; Thole, B. T.; Sawatzky, G. A. Phys. Rev. B 1990, 41, 928. (13) De Groot, F. M. F.; Fuggle, J. C.; Thole, B. T.; Sawatzky, G. A. Phys. Rev. B 1990, 42, 5459. (14) BESSY User’s Handbook. BESSY: Berlin, 1998. (15) Brydson, R.; Sauer, H.; Engel, W.; Thomas, J. M.; Zeitler, E.; Kosugi, N.; Kuroda, H. J. Phys.: Condens. Matter 1989, 1, 797.
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Figure 1. Ti 2p XAS spectra of TiO2 on HOPG as a function of the deposition time.
3. Results and Discussion Ti 2p XAS Spectra as a Function of Deposition Time. Figure 1 shows the XAS spectra for TiO2 on HOPG as a function of the deposition time. For comparison, the spectrum of the reference anodic TiO2 sample has been included. The spectrum of the reference TiO2 sample shows four well-defined peaks and a shoulder at threshold, in good agreement with data previously reported for bulk TiO2,16,17 although in this case the peaks appear broader, as expected for amorphous materials. The spectra corresponding to the TiO2 overlayers on HOPG show the same four peaks as the reference TiO2 sample throughout the whole series, except the spectrum labeled as 10 min, which is significantly different from the other spectra of the series. It shows only two broad structures and appears slightly shifted in energy. Both the shape and the energy position of this spectrum are completely consistent with the experimental spectrum of Ti2O3 reported by Lusvardi et al.18 and the multiplet calculation for Ti3+ as performed by de Groot et al.13 At higher coverages (i.e., evaporation time g15 min), the spectra show four well-defined peaks indicating the formation of TiO2, as deduced from their comparison with the spectrum of the reference sample. The spectra show an extra shoulder at threshold which is caused by the presence of small amounts of Ti3+ species. The Ti 2p XAS spectra for TiO2 deposited on SiO2 as a function of the evaporation time are shown in Figure 2. For very large deposition time (i.e. g210 min), the spectra show exactly the same features as the reference spectrum for TiO2. Interestingly, the spectra corresponding to low coverages (i.e., evaporation time e30 min) show significant differences with respect to the reference spectrum of TiO2. In fact, as far as we know, no experimental Ti 2p XAS spectra like those presented here for low coverages have been previously reported for Ti4+. These spectra show only two asymmetric narrow peaks instead of the four-peaked structure of the reference sample. The possibility that (16) Soriano, L.; Abbate, M.; Vogel, J.; Fuggle, J. C.; Ferna´ndez, A.; Gonza´lez-Elipe, A. R.; Sacchi, M.; Sanz, J. M. Surf. Sci. 1993, 290, 427. (17) Soriano, L.; Abbate, M.; Ferna´ndez, A.; Gonza´lez-Elipe, A. R.; Sanz, J. M. Surf. Interface Anal. 1997, 25, 804. (18) Lusvardi, V. S.; Barteau, M. A.; Chen, J. G.; Eng, J., Jr.; Teplyakov, A. V. Surf. Sci. 1998, 397, 237.
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Figure 2. Ti 2p XAS spectra of TiO2 on SiO2 as a function of the deposition time.
these spectra correspond to other Ti species, i.e., Ti3+ and Ti2+, is rejected because their spectra are much broader than those of TiO2, as expected for 2p f 3d transitions in d1 and d2 configurations, respectively.13,18 Its interpretation in terms of metallic Ti can also be discarded because the corresponding spectrum is much broader and shifted in energy as compared with the spectra of the oxides.19 Thus, as we will clearly see below, these unusual spectra are assigned to Ti4+ ions affected by the substrate. According to the interpretation of the XAS spectra presented here, we can conclude that for large coverages, amorphous TiO2 overlayers are formed on both substrates. However, the results for low coverages differ depending on the substrate. In the case of TiO2 on HOPG, the Ti 2p XAS spectra indicate the formation of Ti2O3 at the interface, followed by the formation of TiO2. The formation of the initial Ti2O3 instead of TiO2 suggests that the presence of the metallic graphite substrate inhibits the total oxidation of the Ti atoms. On the other hand, in the case of TiO2 on SiO2, the Ti atoms at the interface become fully oxidized to Ti4+, although they are strongly influenced by the SiO2 substrate. An important point to understand the origin of these unusual spectra is to estimate the equivalent thickness of deposited TiO2 for each spectrum. Quantitative Analysis of the XAS Intensities. We have performed a quantitative analysis of the Ti 2p XAS intensities. Despite the good properties of this tecnhique, up to now XAS has not been used to investigate a growth mode. The main reason is that very little is known about the probing depth of this technique in the total-electron yield detection mode. Only the work by Abbate et al.20 gives an experimental approach of the electron mean free path in the energy region of interest in this work. To perform this analysis we have used a common method in the XPS quantitative analysis of very thin overlayers.21 This method assumes an exponential dependence of the intensity on the thickness of the overlayer, i.e., in the (19) Soriano, L.; Abbate, M.; De Groot, F. M. F.; Alders, D.; Fuggle, J. C.; Hofmann, S.; Petersen, H.; Braun, W. Surf. Interface Anal. 1993, 20, 21. (20) Abbate, M.; Goedkoop, J. B.; De Groot, F. M. F.; Grioni, M.; Fuggle, J. C.; Hofmann, S.; Petersen, H.; Sacchi, M. Surf. Interface Anal. 1992, 18, 65. (21) Seah, M. P. In Practical Surface Analysis Vol. 1, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons: Chichester, 1990.
Soriano et al.
Figure 3. Quantitative analysis of the XAS intensities according to the overlayer growth mode: (2) TiO2-SiO2 and (9) TiO2-HOPG. For explanation, see text.
form I ) I∞(1-ed/λ), where I∞ stands for the XAS intensity of a bulk TiO2 sample, d is the overlayer thickness, and λ accounts for the mean free path of the electrons coming out from the sample. According to ref 20, we have estimated λ ≈ 40 Å at the Ti 2p energy (450 eV). In Figure 3 the experimental XAS intensities I, measured as the total area of the Ti 2p XAS spectra, in the form ln(1-I/I∞) ) d/λ, are shown as a function of the deposition time. Assuming a constant deposition rate, the overlayer thickness can be calculated from the slope of the straight line fitted to the experimental data, as shown in Figure 3. This procedure gives values of ≈0.09 Å/min for the evaporation rate and 34 min for the growth of the first equivalent monolayer. We would like to point out that, assuming an error of 20% for the λ value, this estimation would give values for the evaporation time of the first equivalent monolayer that would not vary significantly our final conclusions. According to these results, we can easily estimate that, for instance, the deposits after 30 min evaporation correspond to approximately 1 ML, whereas the last spectrum of the series (300 min) corresponds to about 10 ML. However, the main point given by the quantitative approach is that all the unusual Ti 2p spectra (i.e., evaporation time e30 min) correspond to coverages below the completion of the first monolayer. It appears to be clear now that the presence of the SiO2 substrate is strongly affecting the TiO2 submonolayer, as reflected in the unusual Ti 2p spectra. Similar results were obtained in the CaF2/Si(111) system in which the Ca 2p spectra of this interface has been characterized.11 The spectra for intermediate coverages (i.e., evaporation time 30-90 min) can be reproduced by the adequate linear combination of those of the first monolayer and the bulk. In other words, they have two contributions: coming from the buried first monolayer and from the outer layers (reference TiO2 spectrum). Comparison with Atomic Multiplets. To understand the effects that the SiO2 substrate produces on the Ti 2p XAS spectra, we have attempted a comparison of the experimental spectra with theory. As mentioned above, the Ti 2p XAS spectrum of TiO2 has been interpreted as atomic 2p63d0 f 2p53d1 multiplets projected in an octahedral crystal field.12 In Figure 4 we show the Ti4+ multiplet calculations for octahedral crystal fields ranging from 0 to 4.7 eV, taken from ref 12. For spherical symmetry (10Dq ) 0), the atomic multiplet consists of two main
Crystal-Field Effects at TiO2-SiO2 Interface
Figure 4. Atomic multiplet calculations for Ti4+ in Oh symmetry as a function of the crystal field strength (10Dq) taken from ref 12.
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of the second peak,12,15 as is reflected by the asymmetry of this peak in the experimental spectrum. On the other hand, in Figure 5b we have compared the spectrum corresponding to 10 min of evaporation (i.e., 0.3 ML) with the multiplet calculation for 10Dq ) 0.7 eV. Although the experimental spectrum is slightly broader, the agreement can be considered good. Therefore, from the above comparison we infer that the crystal field for the TiO2 at the interface with SiO2 is significantly lower than for bulk TiO2. This effect might be explained by the formation of Si-O-Ti-O bonds, as suggested in a previous work.1 In this picture, the Ti atoms at the interface would be octahedrally coordinated Ti4+, although those oxygen atoms of the TiO6 cluster which are also bonded to Si atoms would not contribute to the crystal field to the same extent as the oxygen atoms octahedrally coordinated in bulk TiO2 (see inset in Figure 5). However, in this case, no unambiguous information on the local coordination of the Ti atoms can be obtained from the XAS spectra. Multiplet calculations have also been performed in cubic symmetry (8-fold) with negative crystal field values (not shown here).12 For low crystal field values, the spectra are similar to those obtained in octahedral (6-fold) symmetry. In fact, for low crystal field strength, the spherical symmetry is weakly affected by the crystal field independently of its symmetry. Therefore, other symmetries, i.e., cubic or even tetrahedral, cannot be discarded. However, two experimental evidences give an approach to the local symmetry of the Ti atoms. First, the effect of the lowering of the crystal field, as observed in the Ti 2p XAS edges, is so strong that it should be produced by more than one oxygen atom in the TiO6 cluster. Second, EXAFS data of the same interface are consistent with octahedral symmetry although strongly distorted with two nearest neighbor distances.1 Therefore, we suggest an octahedral symmetry for the Ti atoms at the TiO2-SiO2 interface as depicted in the inset of Figure 5b. 5. Conclusions
Figure 5. Ti 2p XAS spectra (dots) for (a) TiO2 and (b) 0.3 ML of TiO2-SiO2 compared with multiplet calculation in Oh symmetry (solid line) for (a) 10Dq ) 1.8 eV and (b) 10Dq ) 0.7 eV.
peaks and a weaker one at threshold. As the crystal field increases, these structures split progressively into four main peaks and two weaker structures at threshold. The relative energies and intensities of the four peaks continuously change through the series. Based on the dependence of these parameters on the crystal field strength, the use of the Ti 2p XAS spectra to determine the crystal field in a Ti compound12 has been suggested. We have compared the multiplet calculation with our experimental data in Figure 5. The calculated spectra have been shifted conveniently to get a good fitting of the experimental data. As can be seen in Figure 5a, the spectrum of bulk TiO2 agrees with that calculated with 10Dq )1.8 eV, also in good agreement with other data reported in the literature.12 In the case of TiO2, the distortion of the octahedron surrounding the Ti cations (neglected in these calculations) gives rise to a splitting
XAS has given evidence of a strong influence of the SiO2 substrate on the crystal field of the Ti4+ ions accommodating at the interface. We have presented unique Ti 2p XAS spectra corresponding to a TiO2 submonolayer on the SiO2 substrate. A detailed analysis of the Ti 2p XAS spectra indicates that the crystal field of the Ti atoms at the interface is much weaker (0.7 eV) than in bulk TiO2 (1.8 eV). This interfacial electronic interaction would be consistent with the formation of SiO-Ti-O cross-linking oxygen bonds previously identified by PES. The interaction also would explain the reported changes observed in the electronic structure of very thin layers of TiO2 on SiO2, as well as the good wetting properties of this first overlayer of TiO2 on SiO2. On the other hand, when the SiO2 is changed by a metallic substrate such as graphite, the total oxidation of the Ti atoms at the interface is inhibited, leading to the formation of Ti2O3 at the TiO2-HOPG interface. Acknowledgment. This work has been financially supported by the DGICYT of Spain, contract number PB96-0061, and by the EU under the TMR program, contract number ERBFMGE-CT95-0031 at BESSY. We also thank the staff of BESSY for technical support. LA000330X
