An X-ray photoelectron diffraction study of a TiO2(001) anatase single crystal mineral

PERGAMON

Solid State Communications 119 (2001) 101±104

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An X-ray photoelectron diffraction study of a TiO2(001) anatase single crystal mineral G. Silversmit*, H. Poelman, L. Fiermans, R. De Gryse Department of Solid State Sciences, Surface Division, Gent University, Krijgslaan 281, S1, B-9000 Gent, Belgium Received 3 November 2000; received in revised form 19 January 2001; accepted 2 May 2001 by D.E. Van Dyck

Abstract An anatase single crystal mineral is examined with low energy electron diffraction, X-ray photoelectron spectroscopy and X-ray photoelectron diffraction. Different gasses (He or Ar) are used to clean the sample and their in¯uence on the azimuthal XPD curves is checked. Azimuthal scans for the polar angles 15, 28 and 518 off surface are recorded. Experimental data are compared with single scattering cluster calculations on two different surface models. A comparison with literature results on thin anatase ®lms is made. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 61.14.Qp; 79.60.2i Keywords: A. Surfaces and interfaces; C. Crystal structure and symmetry; E. Photoelectron spectroscopies

1. Introduction We present, in the framework of a research project on a V2O5\TiO2 (anatase) model catalyst, an X-ray photoelectron diffraction (XPD) study of an anatase single crystal mineral. The V2O5\TiO2 (anatase) system is a catalyst for the selective oxidation of ortho-xylene to phthalic anhydride [1±4]. Supported V2O5 layers are known to have a higher activity and a better selectivity than unsupported layers [2], and in this respect anatase has drawn quite some attention. In our laboratory a model catalyst is developed by depositing a vanadium oxide layer on a TiO2(001) anatase single crystal mineral support. This simpli®ed system can be studied with diffraction techniques like LEED and XPD. In order to determine the surface and interface structure of the model catalyst knowledge of the supports surface termination is essential. Therefore XPD measurements are performed on single crystal anatase and a surface model for the TiO2(001) anatase surface is presented. Titanium dioxide (TiO2) crystallises in three polymorphs: rutile (tetragonal), anatase (tetragonal) and brookite (orthorhombic). Although less common than rutile, anatase is being

examined more and more to reveal its electronic [8,9], optical [9±11], magnetic [12] and vibrational properties [13]. Unlike rutile, anatase is dif®cult to synthesize. Thin anatase ®lms can be grown by metal±organic chemical vapor deposition [5] and small anatase single crystals have been obtained by chemical transport reactions [6,7]. However, the latter remain too small to assure easy handling and measuring. Therefore, in this study a natural mineral anatase single crystal sample was used. The anatase(001) surface was examined by means of LEED, XPS and XPD. XPD gives information about short-range order and structure. It is based on the increase of the intensity of the emitted photoelectron wave along the internuclear emitter±scatterer axis, due to the attractive potential of the scatterer [14,15]. Comparison with theoretical simulations then allows identifying the local environment of the emitting ions. Ion bombardment is applied to clean the sample. In order to check if the gas type of this pre-treatment has any in¯uence on the XPD pro®les two different gasses, He and Ar, are used for ion bombardment. 2. Experimental

* Corresponding author. Tel.: 132-9-264-4371; fax: 132-9-2644996. E-mail address: [email protected] (G. Silversmit).

Blackish anatase single crystal minerals are used for the experiments. The color is due to contamination present in

0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00224-1

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Fig. 1. (a) The unit cell of anatase, two polar angles examined with XPD are indicated; two surface terminations used for the SSC calculations. (b) Model 1. (c) Model 2.

the sample. Mineral anatase can be found in glacier valleys such as Binntal (Switzerland) or Hardangervidda (Norway). The crystals are sawn and Laue-oriented to the (001) surface. The 4 £ 4 mm 2 samples were polished with Al2O3 powder and etched in a boiling solution of ammonium sulfate in sulfuric acid and in boiling diluted chloric acid. After an ultrasonic treatment in acetone, the sample was introduced in the preparation chamber of a loss spectrometer (SEDRA, ISA Riber, France) equipped with LEED. The sample was cleaned by cycles of Ar 1 or He 1 bombardment (3±5 £ 10 21 Pa, 500 eV, 60 min, 1 mA) followed by annealing in O2 atmosphere (2 £ 10 21 Pa, 30 min, 300±3508C), until LEED spots were observed. The XPS and XPD measurements were performed in UHV in a Perkin Elmer Phi ESCA 5500 system with a monochromated 450 Watt Al Ka source, enabling polar and azimuthal variation with an angular resolution of 18. The base pressure of the system was below 1 £ 10 27 Pa, experiments were recorded with 200 W source power and an angular acceptance of 28. Azimuthal scans of the Ti 2p and O 1s photopeak over 3208 with a 28 step were measured for the polar angles (u ) 15, 28 and 518 off surface at room temperature. From the lattice structure of anatase intense forward scattering is expected for 27.6 and 51.58, see Fig. 1. The polar angle of 158 was chosen for its higher surface sensitivity. Binding energies reported are relative to the C 1s core level taken at 284.6 eV. Theoretical azimuthal scans were calculated with an SSC program 1. The clusters used contain over 420 atoms and the inner potential was set to 10 eV [16]. The kinetic energies for the Ti 2p and the O 1s photolines were taken at 1023 and 952 eV, respectively. Above a kinetic energy of a few hundred eV multiple scattering and backscattering become negligible, so that a single scattering approach is indeed allowed as a ®rst order approximation [14,15]. Anatase has a tetragonal structure with a D14 4h 2 I41 =amd 1 SSC program kindly put at our disposal by J. Osterwalder, UniversitaÈt ZuÈrich, Switzerland.

symmetry, the lattice parameters at room temperature are Ê and c ˆ 9.51 A Ê . The building-stone is a a ˆ b ˆ 3.78 A slightly distorted TiO6 octahedron, with the oxygens at the corners. Each octahedron is in contact with eight neighbors, four sharing an edge and four a corner. The Ti 41 cations are coordinated to six O 22 anions and the oxygen atoms are coordinated to three titanium atoms. Fig. 1a shows the unit cell of anatase. In calculations two possible surface terminations were considered, both oxygen terminated, see Fig. 1. The second model (Fig. 1c) is obtained from the ®rst (Fig. 1b) by removing the top oxygen atoms along the c axis. For the ®rst model, the third layer titanium ions is sixfold coordinated and for the second model the second layer titanium ions is ®vefold coordinated.

3. Results and discussion 3.1. X-ray photoelectron spectroscopy After the LEED measurements, the sample was transferred to the ESCA system through air. No additional cleaning was performed in the vacuum system in order not to disturb the surface layer. The binding energies for the Ti 2p3/2, Ti 2p1/2 and the O 1s core levels are 458.6, 464.3 and 529.9 eV, respectively. The details of the curve ®tting of the C 1s, Ti 2p and O 1s spectra are given in Table 1. The O 1s spectrum has two components, one at 529.9 eV attributed to Table 1 Details of the curve ®tting of the C 1s, Ti 2p and O 1s spectra. Binding energies (BE) are taken relative to the C 1s core level

C 1s Ti 2p3/2 Ti 2p1/2 O 1s

BE (eV)

FWHM (eV)

284.6 458.6 464.3 529.9 531.3

1.60 1.05 1.92 1.15 1.64

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Fig. 2. Azimuthal XPD scans, u ˆ 158, compared with the SSC calculations for the two surface models from Fig. 1.

the core level and one at 531.3 eV attributed to hydroxyl groups [8]. Potassium (0.5%), silicon (1.3%) and sodium (0.3%) contaminations are found in the XPS spectra, as well as a high carbon concentration (17%) due to the transport through air. Argon impurities are present when the sample is cleaned with an Ar 1 bombardment (0.5%). 3.2. X-ray photoelectron diffraction Azimuthal XPD scans of the C 1s photoline, recorded at the three polar angles, show no diffraction peaks. The carbon impurities are not sitting in well-de®ned adsorption sites with local order. So, the carbon surface contamination has no in¯uence on the structure of the Ti 2p and O 1s XPD signals. No signi®cant structural difference was found in the azimuthal scans for the two cleaning methods used (Ar 1 or He 1 bombardment). For comparison with the SSC calculations different measurements (three to four) on the same sample were added to enhance signal to noise ratio. The data were corrected for a smoothly varying instrumental background, due to changes in the X-ray ¯ux intensity and a possible slight misalignment of the sample orientation and the goniometer axis. In Fig. 2 we compared the experimental azimuthal scans

for the polar angle 158 off surface with the SSC calculations for the two surface models from Fig. 1. The agreement is slightly better for the second model. Peak positions are the same for the two models, but the form of the peak structure of the O 1s scan around the azimuthal angles 45, 135 and 2258 is better for model 2. R-factors were calculated as R ˆ P …Imodel 2 Iexp †2 on the normalised intensities for the two surface models and experiment. Model 2 indeed yields lower R factors for the Ti 2p and O 1s pro®les, as shown in Fig. 2. Moreover, model 2 is an autocompensated surface. The ®vefold coordinated Ti 41 from the second atom layer miss one O 22 in their coordination, so they are unsaturated. The O 22 anions from the ®rst atom layer are two-fold coordinated and are missing one Ti 41. Hence the charge on the unsaturated Ti 41 is 12/3 and 22/3 on the unsaturated O 22. Model 1 has only unsaturated oxygens and no compensating titaniums, because they have a full sixfold coordination. Hence model 2 is the more stable one, consistent with our experiment. A similar result was found on thin anatase(001) ®lms [5]. This surface model is used for simulating the azimuthal scans at the other polar angles. Figs. 3 and 4 show the azimuthal scans of the O 1s and Ti 2p photopeaks for the two polar angles 28 and 518 off surface. The SSC simulations and anisotropies …x ˆ …IMAX 2 IMIN †=IMAX † for the two polar angles are also presented. There is a good agreement between the

Fig. 3. Experimental and SSC calculated XPD scans, u ˆ 288.

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Fig. 4. Experimental and SSC calculated XPD scans, u ˆ 518.

experiment and the SSC calculations for the peak positions and shapes. The experimental anisotropies are two to three times lower than the theoretical ones, in agreement with the literature [5]. Although the anatase(001) surface has a twofold symmetry, the experimental and theoretical azimuthal scans display a more fourfold symmetry for the higher polar angles. This is due to the fact that the anatase structure is almost fourfold symmetric. The Ti±O bonding axis along the a and b directions deviate by ^128 from the (001) plane and cause the reduction to twofold symmetry. The higher the off surface, the lesser the in¯uence this deviation has upon the XPD pro®les. In combination with the experimental resolution the measurements hence show a more fourfold symmetry. We can, for completeness, compare our measurements at the polar angle 518 off surface with the XPD research on thin anatase ®lms with high purity reported in Ref. [5]. Therein Herman et al. present an azimuthal XPD scan for the polar angle 508 off surface. Taking into account the difference in experimental resolution, both measurements agree very well in shape and position for the Ti 2p and O 1s emission. From this correspondence we can conclude that at these higher polar angles off surface the impurities present in our anatase single crystal mineral do not in¯uence the azimuthal XPD scan.

4. Conclusions Anatase single crystal minerals were investigated with XPS, LEED and XPD. No difference is observed in the azimuthal XPD scans when different gases, He or Ar, are used for ion bombardment cleaning of the sample. The experimental azimuthal XPD pro®les are compared with the SSC calculations on two surface models. An oxygen terminated surface with ®vefold coordinated titanium atoms in the second atom layer agrees best with the experiment. A comparison with literature results on thin anatase ®lms shows that the mineral contamination does not struc-

turally interfere in the Ti 2p or O 1s azimuthal XPD patterns of the single crystal mineral anatase. Acknowledgements The authors are grateful to Ind. Ing. G. De Doncker for performing the XPS measurements. This text presents the results of the Belgian Program on Interuniversity Poles of Attraction initiated by the Belgian State, Prime Minister's Of®ce, Science Policy Programming. The scienti®c responsibility is assumed by its authors. References [1] G.C. Bond, J. Catal. 116 (1989) 531. [2] J. Haber, A. Kozlowska, R. Kozlowski, J. Catal. 102 (1986) 52. [3] EUROCAT, Catal. Today 20 (1994) 1±184. [4] Appl. Catal. A 157 (1997) 3±408. [5] G.S. Herman, Y. Gao, T.T. Tran, J. Osterwalder, Surf. Sci. 447 (2000) 201. [6] H. Berger, H. Tang, F. LeÂvy, J. Cryst. Growth 130 (1993) 108. [7] F. Izumi, K. Kodama, A. Ono, J. Cryst. Growth 47 (1979) 139. [8] R. SanjineÂs, H. Tang, H. Berger, F. Gozzo, G. Margaritondo, F. LeÂvy, J. Appl. Phys. 76 (1994) 2945. [9] S.D. Mo, W.Y. Ching, Phys. Rev. B 51 (1995) 13 023. [10] H. Tang, H. Berger, P.E. Schmid, F. LeÂvy, Solid State Communications 92 (1994) 267. [11] H. Tang, H. Berger, P.E. Schmid, F. LeÂvy, Solid State Communications 87 (1993) 847. [12] O. Chauvet, L. Forro, Solid State Communications 93 (1995) 667. [13] G. Durinck, H. Poelman, P. Clauws, L. Fiermans, J. Vennik, G. Dalmai, Solid State Communications 80 (1991) 579. [14] G. Grenet, Y. Jugnet, S. Holmberg, H.C. Poon, T.M. Duc, Surf. Interf. Anal. 14 (1989) 367. [15] W.F. Egelhoff Jr., Crit. Rev. Solid State Mater. Sci. 16 (1990) 213. [16] M. Sambi, G. Sangiovanni, G. Granozzi, Phys. Rev. B 55 (1997) 7850.