X-ray photoelectron diffraction study of an anatase thin film: TiO2(001)

Surface Science 447 (2000) 201–211 www.elsevier.nl/locate/susc

X-ray photoelectron diffraction study of an anatase thin film: TiO (001) 2 G.S. Herman a, *, Y. Gao a, T.T. Tran a, J. Osterwalder b a Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-93, Richland, WA 99352, USA b Physik-Institut, Universita¨t Zu¨rich, Winterthurerstrasse 190, CH-8057 Zu¨rich, Switzerland Received 12 August 1999; accepted for publication 16 November 1999

Abstract An anatase thin film, TiO (001), has been grown by metal–organic chemical vapor deposition on a SrTiO (001) 2 3 substrate. The electronic and structural properties of the anatase thin film were investigated by X-ray photoelectron spectroscopy, low energy electron diffraction (LEED), and X-ray photoelectron diffraction ( XPD). Comparisons of the core-level binding energies for O 1s and Ti 2p photoelectrons was made between anatase and rutile, and were 3/2 found to be identical for both polymorphs. Structurally, the surface gave a (1×1) LEED pattern with a high background, indicating regions of good long-range order. Comparison between experimental XPD data and singlescattering cluster calculations indicate that there is good agreement with an anatase crystal termination, and poor agreement with a rutile crystal termination. Furthermore, good agreement was found at low takeoff angles for experiment and calculations suggesting that the thin film has good short-range order at the surface. This study indicates that anatase is stable and does not undergo a transformation to the rutile polymorph when exposed to atmosphere after film growth or after heating to low temperatures in ultrahigh vacuum. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Anatase; Polycrystalline thin films; Titanium; Titanium oxide; X-ray photoelectron spectroscopy

1. Introduction Titanium dioxide ( TiO ) has been extensively 2 studied because of its photocatalytic [1,2], catalytic [3], electronic [4], and optical properties [5]. TiO has three common polymorphs: rutile (tetra2 gonal ), anatase (tetragonal ), and brookite (orthorhombic). The commercial availability of highquality bulk samples has made rutile (the most stable polymorph) the subject of numerous surface * Corresponding author. Tel.: +1-509-376-5220; fax: +1-509-376-5106. E-mail address: [email protected] (G.S. Herman)

characterization and chemical reaction studies [6– 17]. However, anatase has been shown to have a higher photocatalytic activity than rutile for several reactions [18,19]. These activities may be related to differences in either the surface geometric and/or electronic structure of these two polymorphs. Because of the limited availability of anatase single crystals, significantly less work has been reported compared with rutile. Natural anatase mineral samples are available and have been investigated with traditional surface science methods. For example, anatase minerals, contaminated with iron, have been studied with high-resolution electron energy loss spectroscopy

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(HREELS) and low energy electron diffraction (LEED) [20]. LEED results from sputtered and annealed samples indicated that the (001) surface was unreconstructed with a (1×1) pattern and the (100) surface was disordered with a diffuse pattern. The surface phonon modes detected with HREELS indicated that anatase was very similar to rutile in that the high-energy loss peak (~95 meV ) was a composite with both A and E character, while 2u u the low-energy loss peak (~46 meV ) was predominantly E in character. u In order to avoid impurities that are often found in mineral samples, growth methods resulting in high-purity anatase single crystals [21] and thin films [22–24] of dimensions sufficiently large for surface characterization have been developed. Thin film anatase samples have been grown on SrTiO (001) substrates by molecular beam epitaxy 3 (MBE ) and metal–organic chemical vapor deposition (MOCVD). SrTiO (001) was chosen as a 3 substrate because it has a better lattice match to ˚, anatase than rutile (strontium titanate 3.905 A ˚ , and rutile 4.594 A ˚ ). In situ reflecanatase 3.785 A tion high-energy electron diffraction (RHEED) results have indicated that the anatase thin films are epitaxial with a lattice constant a approximately equal to that of bulk anatase [22,23]. Most investigations of anatase single crystals to date have examined the electronic, optical, and magnetic properties [25–32]. Core-level X-ray photoelectron spectroscopy ( XPS) indicated that the binding energies for the Ti 2p and O 1s photoelectrons are in the same range for anatase [26 ] as for rutile [11]. Furthermore, anatase and rutile were found to have the same valence band position below the Fermi energy, the same valence band width, and an absence of intrinsic surface states in the bulk band gap [26 ]. In general, the reflection spectra from both anatase and rutile can be divided into three regions based on their similar spectral features: 3–7, 7–12, and >12 eV [31]. However, anatase and rutile differ in several respects. For example, the location of the fundamental adsorption edges of anatase and rutile are at 3.2 and 3.0 eV, respectively [27]. In addition, band gap excitation of anatase and rutile results in photoluminescence from distinctly different self-trapped and free excitonic states, respectively [25]. No

surface structural work has been performed on single crystals or thin films of anatase to date, other than the observation of LEED and RHEED patterns. In the present work we have characterized an anatase sample grown by MOCVD with XPS, LEED, and X-ray photoelectron diffraction ( XPD). LEED and XPD provide measures of the long-range and short-range order of the thin films, respectively. Furthermore, XPD permits the degree of order for specific elemental components in the thin films (i.e. oxygen and titanium) to be determined. XPS results on anatase are compared directly with those obtained from rutile.

2. Experimental 2.1. Sample preparation The anatase sample, TiO (001), was grown by 2 MOCVD on a polished SrTiO (001) substrate 3 [24]. Samples grown by this technique were bulk characterized by ex situ X-ray diffraction ( XRD) and high-energy ion scattering (HEIS), and were determined to be of high quality with no XRD peaks related to rutile, and a low minimum yield along the [001] channeling direction with HEIS. The thickness of the film used in this study was ˚ . The rutile sample in this estimated to be ~500 A study was a TiO (110) single crystal that was 2 sputtered and annealed to remove impurities and a sharp (1×1) LEED pattern was obtained. 2.2. Sample characterization The experiments described here were performed in a custom VG ESCALAB 220 photoelectron spectrometer with a computer-controlled two-axis goniometer described elsewhere [33]. The sample was mounted on a molybdenum sample holder and transferred into the chamber via a load lock. The sample was then degassed at 100°C for 5 min in vacuum. Nonmonochromated Mg Ka radiation was used for both XPS and XPD, and all data were taken with the sample at 300 K. The XPS data were taken at normal emission (h=0°) for both anatase and rutile. The pass energy of the

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analyzer was set to 20 eV for the O 1s and Ti 2p scans. The binding energies reported in the present work are relative to the C 1s core level set at 285.0 eV, measured simultaneously on the lowlevel carbon contamination present on both surfaces (see Section 3), to account for charging and/or band bending at the surface. For the anatase sample, Ar ion sputtering was avoided to prevent any changes in the oxidation state or surface structure. The experimental XPD data of either the Ti 2p or O 1s photopeak intensities are presented in a linear grayscale for over 4150 emission angles above the surface. The emission angles are given in a stereographic projection where the center of the plot is normal emission (h=0°) and the edges are grazing emission (h=78°). In order to account for the low emission intensities at grazing angles, the individual azimuthal scans have been normalized by a procedure described previously [34]. For these measurements the angular acceptance of the analyzer was set to ±1.5°. Single scattering cluster (SSC ) calculations were performed to model the experimental XPD data. The calculations include the correct spherical wave nature of the final-state photoelectron, and the proper angular momentum final states involved as a result of dipole transitions from the initial angular momentum states to the final angular momentum states (e.g. p to s and d for Ti emission, and s to p for O emission) [35]. The partial wave phase

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shifts and electron attenuation length were obtained using the program FEFF [36 ]. Over 18 partial wave phase shifts were used for each atomic species for the kinetic energies of the Ti 2p and O 1s photoelectrons, and the electron attenuation ˚ . This attenuation length is length used was 9 A somewhat shorter than what would be expected for bulk TiO , but is consistent with a number of 2 prior studies that indicated that such reduced attenuation lengths gave better agreement between experiment and SSC calculations [37]. The inner potential used in the calculations was 15 eV [9]. Both oxygen- and titanium-terminated clusters were used for calculations of the anatase crystal structure. The results from the two different clusters were nearly identical and only those for the oxygen-terminated surface will be presented. The anatase and rutile clusters used for the calculations both had over 245 atoms. All inequivalent emitters in each layer were used for these simulations. The theoretical data were normalized by the same procedure used for the experimental data [34]. The unit cells for rutile and anatase are given in Fig. 1a and b, respectively. The large dark circles represent oxygen, while the small light circles represent titanium. Both anatase and rutile are tetragonal with the titanium cation coordinated to six oxygen anions located at the corners of slightly distorted octahedra, and each oxygen anion coordinated to three titanium cations. For both polymorphs, two TiMO bonds are slightly longer than

Fig. 1. Unit cells for (a) anatase and (b) rutile where the large dark circles represent oxygen and the small light circles represent titanium.

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Fig. 2. Two possible terminations for an anatase-TiO (001) surface (the coordination of the surface ions are indicated ): (a) the 2 oxygen-terminated surface, and (b) the titanium-terminated surface.

the other four, and some of the OMTiMO bond angles are distorted from 90°. The major difference between the two polymorphs is in how the octahedra are connected, where the structures can be described as chains of TiO octahedra having 6 common edges [38]. Rutile and anatase share two and four edges, respectively. Depending on where the anatase crystal is sliced, the [001] surface can be either oxygen or titanium terminated, as shown in Fig. 2a and b, respectively. For the oxygen-terminated surface the top layer oxygen ions are two-fold coordinated and the second layer titanium ions are fivefold coordinated. This termination results in an autocompensated surface, similar to rutile (110), with the anion dangling bonds filled and the cation dangling bonds empty [39]. For the titaniumterminated surface the top layer titanium ions are threefold coordinated and the second layer oxygen ions are fully threefold coordinated. This termination is not autocompensated and would likely be unstable. Although the (001) surface is essentially twofold symmetric, the experimental and theoretical XPD data show fourfold symmetry. The fourfold symmetry can be explained by the formation of steps smaller than the unit cell distance, in the z-direction. These steps will result in identical surfaces being exposed (e.g. oxygen terminated ), but with a 90° rotation.

3. Results and discussion 3.1. X-ray photoelectron spectroscopy Ti 2p, and O 1s XPS spectra from a rutile single crystal and an anatase thin film are shown in Figs. 3 and 4, respectively. The upper spectra in Figs. 3 and 4 are for a rutile TiO (110) sample 2 that was sputtered and annealed. This preparation resulted in a sharp (1×1) LEED pattern. The lower spectra in Figs. 3 and 4 are for an anatase TiO (001) thin film that was heated to 100° C for 2 5 min in vacuum. Carbon, potassium, and argon impurities were present in the rutile sample. The carbon coverage was determined to be 0.05 ML based on a non-attenuating overlayer model for the sputtered and annealed sample, and the potassium and argon impurities were determined to have concentrations of less than 0.03% and 0.3%, respectively [40]. Carbon, silicon, and aluminum impurities were present in the anatase sample. The silicon and aluminum impurities were likely due to contamination of the precursor lines in the MOCVD system from prior growths of silicon dioxide and alumina. The carbon coverage was determined to be 0.6 ML based on a non-attenuating overlayer model, and the silicon and aluminum impurities were determined to have concentrations of less than 0.4% and 1.3%, respectively [40]. The

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Fig. 3. Ti 2p XP spectra from rutile single crystal (upper) and anatase thin film ( lower).

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O 1s/Ti 2p ratios for rutile and anatase were found to be nearly identical after similar sample preparations. The binding energies for the Ti 2p and the 3/2 O 1s were 458.95±0.1 and 530.30±0.1 eV, respectively. We found that anatase and rutile had identical binding energies for both components, and these values are in good agreement with prior XPS spectra obtained from rutile [11] and anatase [26 ]. The binding energy of the Ti 2p component is 3/2 consistent with a surface region that has titanium primarily in the Ti4+ oxidation state [11]. The full widths at half-maximum (FWHM ) of the Ti 2p and O 1s peaks were 1.15 and 1.35 eV, 3/2 respectively, for both the anatase and rutile. These values are slightly higher than those reported in a prior study on anatase using monochromated Al Ka radiation [26 ]. Furthermore, the strong Ti 2p shake-up satellite structure has very similar binding energies for rutile and anatase as shown in Fig. 3. The position of the satellite peak to the main Ti 2p peak has been shown to be very sensitive to the ground state chemical properties of the emitting titanium atom [41,42]. Our results indicate that the main shake-up satellite for the Ti 2p peak is shifted to 13.46 and 13.54 eV 3/2 higher binding energy for rutile and anatase, respectively. This difference in the satellite position (+0.08 eV ), between rutile and anatase, is significantly less than that of the difference in the bandgap (+0.2 eV ). It has been proposed that the Ti 2p satellites are due to the creation of exitons on the surrounding ligand oxygen sites [43]. Thus it would not necessarily be expected that the difference in the satellite position and bandgap to be equivalent for rutile and anatase. 3.2. Low energy electron diffraction

Fig. 4. O 1s XP spectra from rutile single crystal (upper) and anatase thin film ( lower).

The LEED patterns obtained from the anatase thin film are shown in Fig. 5. The patterns were obtained for beam energies of 84 and 177 eV. There is a high background, but reasonably sharp spots in a square lattice. The pattern is consistent with a (1×1) structure. The high background is likely due to the high concentration of carbon left on the surface as determined by XPS. Prior studies in our laboratory have indicated that these anatase

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Fig. 5. LEED patterns from anatase thin film for 84 eV (upper) and 177 eV ( lower) beam energies.

thin films undergo a transition to a two-domain (1×4) reconstruction when they are sputtered and annealed in UHV or oxygen background pressures up to 1×10−7 Torr [44].

3.3. X-ray photoelectron diffraction Fig. 6a and b shows the stereographic projections of the experimental XPD data sets for O 1s

Fig. 6. Experimental XPD patterns obtained from the anatase thin film for (a) O 1s and (b) Ti 2p emission, and calculated XPD 3/2 patterns obtained using the SSC model for an anatase crystal for (c) O 1s and (d ) Ti 2p emission, and a rutile crystal for (e) O 1s 3/2 and (f ) Ti 2p emission. 3/2

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and Ti 2p emission, respectively, collected for 3/2 nearly the full 2p solid angle over the TiO (001) 2 anatase surface. The projections consist of forty 360° azimuthal scans that were collected for polar angles between h=0 and 78°, with Dh=2°. The data have been four-fold averaged to improve statistical accuracy. However, all the features were apparent in the raw data. The polar angle and azimuthal angle scales are indicated on the figures, as well as the grayscale values obtained by calculating the normalized anisotropy values x(h, w)=[I(h, w)−I (h)]/I (h), where I (h) is a o o o diffraction-free, smoothly varying polar intensity function [34]. Both the O 1s and Ti 2p XPD 3/2 data exhibit similarities, due in part to the crystal structure of anatase [45]. For example, along the [100] azimuth, the expected polar angles for forward scattering for both oxygen and titanium emission are h=77.7, 62.6, 38.5 and 0°, for neighbors within twice the nearest neighbor distance ˚ ). Along the [100] azimuth, oxygen emis(1.965 A sion should have one extra forward scattering peak at h=50.6°. For the [110] azimuth, the forward scattering directions for both oxygen and titanium emission are at h=43.8 and 0°, again for neighbors within twice the nearest neighbor distance ˚ ). These predicted forward scattering (1.965 A directions are in good agreement with the maxima observed in the experimental XPD data for both oxygen and titanium emission. However, some of these features do not stand out as prominent forward scattering peaks in these intricate diffraction patterns from a large unit cell, and in several cases there are shifts on the order of several degrees. Similar shifts have been observed in other oxide materials (MgO) and were determined not to be primarily structure related [46,47]. In the case of anatase, the observed shifts from the ideal near-neighbor forward scattering directions may be understood, in part, by the large number of forward scattering directions that are close to the forward scattering directions mentioned above, if the constraint of only nearest neighbor scattering is removed. For example, for neighbor distances ˚ , there are eight forward scattering direcof <7 A tions for oxygen emission along the [100] azimuth for the anatase structure. Single scattering cluster (SSC ) calculations for

the unreconstructed TiO (001) anatase surface are 2 shown in Fig. 6c and d for O 1s and Ti 2p 3/2 emission, respectively. The calculations are shown in the same format as the experimental data of Fig. 7a and b. A comparison of SSC calculations for different surface terminations (titanium versus oxygen) did not show significant differences except for grazing emission angles (h>78°). We therefore only show results for the oxygen-terminated surface, which has been predicted theoretically to be the low energy termination, and is the autocompensated termination [48,49]. A comparison of the experimental data and SSC calculations along the [100] and [110] azimuths indicate reasonable agreement for both oxygen and titanium emission. For example, the experimental and theoretical results show very good agreement in the width, shape, and position of the major features along the [110] azimuth for both oxygen and titanium emission. For O 1s emission along the [100] azimuth, the major difference is a broadening of the forward scattering feature at h#40° out of the plane formed by the [100] and [001] directions. Furthermore, there is a broadening of the feature at normal emission. The overestimation of peak widths for forward scattering peaks using SSC calculations have been observed previously, and can be corrected by using a full multiple scattering approach [50]. For Ti 2p emission along the [100] azimuth has excellent agreement. The position and relative intensities of the diffraction patterns are very well reproduced by the SSC calculations. Overall the agreement between experiment and theory is better for titanium emission compared with oxygen emission. Although there is a much larger lattice mismatch for rutile on strontium titanate than for anatase, it is necessary to rule out the formation of strained rutile at the films surface. SSC calculations for an unreconstructed TiO (001) rutile surface are shown 2 in Fig. 6e and f for O 1s and Ti 2p emission, 3/2 respectively. Four domains were used in the calculations to take into account the two possible orientations that can grow on the substrate ([100]:[100] and [100]:[010]) and the two different growth directions ([001] and [001: ]). The calculations are shown in the same format as the experimental data of Fig. 6a and b. For these calculations we used the

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Fig. 7. Azimuthal plot of the experimental data and anatase SSC calculations from Fig. 6 at polar angles of (a) 40° and (b) 68°.

bulk dimensions of the unit cell. The inclusion of a unit cell that is compressed in the surface plane would change the location of the observed forward scattering peaks on the order of a few degrees, but would not change the structure of the diffraction data. A comparison of the experimental data and SSC calculations along the [100] and [110] azimuths indicate extremely poor agreement for both oxygen and titanium emission. Overall the agreement between experiment and SSC calculations is much better for anatase than for rutile. These results indicate that these films are in fact anatase and not rutile at the surface, in agreement with the characterization of the bulk phase of the film by XRD [24]. For a more quantitative comparison of experiment to the SSC calculations for the anatase structure, azimuthal scans were extracted from the stereographic projections shown in Fig. 6. Experimental and calculated azimuthal scans are given in Fig. 6a and b for O and Ti emission at h=40 and 68°, respectively. The experimental anisotropies are reduced by roughly a factor of 2 from the calculations, which has been observed for other systems as well and is not fully understood [37,51]. Nevertheless, the peak positions and shapes are well reproduced with a lack of certain fine structure in the SSC curves which is mainly due to the neglect of multiple scattering and not

due to inadequacies in the structural model. The agreement shown in Fig. 7a, for a more bulk sensitive emission angle (h=40°), is rather good for both titanium and oxygen emission. For Ti emission all four of the major peaks are reproduced in position and relative intensity by the SSC calculations. The only discrepancy is the lack of the minor peaks at w=20 and 70°. Furthermore, for O emission three broad peaks are observed in both experiment and the SSC calculations at identical azimuthal angles. The agreement shown in Fig. 7b, for a more surface sensitive emission angle (h= 68°), is also quite good for both oxygen and titanium emission. For Ti emission the position of all the peaks are well reproduced when compared with the SSC calculations, other than a lack of the small peaks at w=12 and 78°. For O emission there is very good agreement between experiment and SSC calculations in terms of peak position. Much interest has been devoted to the coexistence/transformation of anatase and rutile materials [52–54]. For example, standard polycrystalline titanium dioxide catalysts (Degussa, P25) are known to contain both anatase and rutile phases in an approximately 80 to 20% ratio [52,53]. Early work suggested that the anatase particles contained in these catalysts were covered with a layer of rutile [52]. The authors speculated that

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the enhanced photoactivity of anatase is directly related to an increased efficiency of electron–hole separation related to the surface space-charge layer and the space-charge layer between the anatase and rutile thin film [52]. However, later work indicated that anatase and rutile coexist as individual single crystal particles, and no amorphous particles or thin films on particles occurs either before or after a photocatalytic reaction [53]. A recent investigation on anatase thin films indicated that an anatase-to-rutile transition can occur for temperatures of <400°C depending on the precursor used for the crystal growth [54]. Because of the good fit of our experimental XPD data to SSC calculations it appears that an anatase-to-rutile phase transition does not occur at the anatase surface under the conditions we investigated.

Acknowledgements The authors gratefully acknowledge assistance of S. Thevuthasan of Pacific Northwest National Laboratory (PNNL) in performing the SSC calculations, and T. Greber, F. Baumberger, and M. Muntwiler of the Universita¨t Zu¨rich in performing the experiments. This research was supported in part by the Swiss National Science Foundation, the U.S. Department of Energy (DOE) Environmental Management Science Program, and a PNNL Laboratory Directed Research and Development program. PNNL is operated for the U.S. DOE by Battelle Memorial Institute under Contract Number DE-AC06-76RLO 1830. The research described in this paper was performed at the Universita¨t Zu¨rich and at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research located at PNNL.

4. Conclusions Anatase thin films grown by MOCVD were characterized by XPS, LEED, and XPD. The XPS results indicate that the O 1s and Ti 2p core3/2 level binding energies are identical for anatase and rutile. Slight differences in relative binding energy were observed for the main shake-up satellite features for the Ti 2p XPS spectra from anatase and rutile. These satellite features are related to the coordination of the titanium cation and indicate that differences in the rutile and anatase structures can influence the relative binding energy of these features. A (1×1) LEED pattern with a high background was obtained for the anatase sample after heating to 100°C in vacuum. Experimental XPD results were compared with SSC calculations for anatase and rutile structures. Only calculations for the anatase structure gave good agreement, indicating that the structure of the thin film is fully consistent with an anatase bulk-like structure. We conclude that high quality single-crystalline anatase thin films can be grown on SrTiO (001) substrates. Future studies will 3 address the exact structural and chemical properties of these surfaces.

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