14154
J. Phys. Chem. C 2008, 112, 14154–14157
Low Energy Electron Diffraction Study of TiO2(110)(2 × 1)-[HCOO]R. Lindsay,*,† S. Tomic´,‡ A. Wander,‡ M. Garcı´a-Me´ndez,§ and G. Thornton| Corrosion and Protection Centre, School of Materials, The UniVersity of Manchester, PO Box 88, Manchester, M60 1QD, U.K., STFC, Daresbury Laboratory, Daresbury, Warrington WA4 4AD, U.K., Facultad de Ciencias Fı´sico-Matema´ticas de la UANL. Manuel L. Barraga´n S/N, Edificio de Posgrado, Ciudad UniVersitaria, San Nicola´s de los Garza, NL 66450, Me´xico, and London Centre for Nanotechnology and Chemistry Department, UniVersity College London, 20 Gordon Street, London WC1H 0AJ, U.K. ReceiVed: May 7, 2008; ReVised Manuscript ReceiVed: July 15, 2008
The structure of an ordered (2 × 1) overlayer of [HCOO]- on rutile TiO2(110)(1 × 1) has been elucidated using quantitative low energy electron diffraction. Both the location of adsorbate atoms, and substrate relaxation are determined. In agreement with previous work, it is concluded that the formate moiety binds to the surface through both of its oxygens to two adjacent 5-fold surface titanium atoms, so that its molecular plane is aligned with the [001] azimuth, i.e., it lies parallel to the bridging oxygen rows. The local adsorption geometry is in excellent quantitative agreement with that derived in a recent photoelectron diffraction study (Sayago, D. I.; et al. J. Phys. Chem. B 2004, 108, 14316). Introduction Given the importance of the carboxyl (-COOH) containing molecules in a variety of TiO2-based applications (e.g., Gra¨tzeltype solar cells), it is hardly surprising that there is a large archive of fundamental studies on the interaction of such species with single crystal TiO2 surfaces (see, for example, refs 1-14). To date, a large proportion of this effort has focused on adsorption of the simplest sCOOH containing species, namely formic acid (HCOOH), on the prototypical rutile TiO2(110)(1 × 1) surface. A number of techniques have been used to investigate the electronic structure and crystallography of this model system (1-11). Here we are concerned with elucidating the relaxation of the TiO2(110) surface following formic acid adsorption, which has not been determined previously, as well as the position of the adsorbate atoms. Exposure of TiO2(110)(1 × 1) to formic acid at room temperature gives rise to an ordered (2 × 1) overlayer at saturation, which occurs at a coverage of ∼0.5 monolayer (ML) (where 1 ML corresponds to one adsorbate molecule per exposed titanium).1-3,11 It is well established 1-11 that this adsorbed overlayer consists of formate moieties ([HCOO]-), formed through acidic hydrogen cleavage, i.e.,
HCOOH(g) f [HCOO]-(ads) + H+(ads) The geometry of this surface formate has already been the subject of a number of experimental studies, ranging from largely qualitative scanning probe work (e.g., ref 3) to more quantitative structure determinations.1,7,8,11 Of the latter type, the most complete is a study by Sayago et al.,1 in which both bonding site and adsorbate orientation are quantitatively determined through chemical-state specific scanned-energy mode photoelectron diffraction (PhD) from C 1s and O 1s core levels. It is concluded that formate binds to the surface through both * To whom correspondence should be addressed. Tel: +44 161 306 4824. Fax: +44 161 306 4865. E-mail:
[email protected]. † The University of Manchester. ‡ Daresbury Laboratory. § Ciudad Universitaria. | University College London.
Figure 1. Ball and stick model of the adsorption geometry of [HCOO]on TiO2(110)(1 × 1). Also indicated is the location of the acidic hydrogen (H+), resulting from dissociative adsorption, determined from PhD data,1 i.e., bonded to a bridging oxygen, forming a surface hydroxyl. The numerical labeling of atoms is employed in Table 1 for identification purposes. Symmetry pair atoms are denoted by * (e.g., 1*).
of its oxygens to two adjacent 5-fold surface titanium atoms, so that its molecular plane is aligned with the [001] azimuth, i.e. it lies parallel to the rows of bridging oxygens. A schematic diagram of the local adsorption geometry is shown in Figure 1. On the basis of previous work,4,8,11 a second formate configuration, rotated azimuthally by 90° and bonded to a bridging oxygen vacancy was also considered in the PhD study. However, there was no evidence for a significant surface concentration of this species. Interestingly, the location of the proton (H+(ads)) resulting from HCOOH dissociation was also determined from the O 1s PhD data. It was found to be attached to a bridging oxygen, forming a surface hydroxyl (OH), as depicted in Figure 1. The goals of this study are to test Sayago et al.’s optimized structure1 and augment it through elucidation of substrate relaxation, using quantitative low energy electron diffraction (LEED-IV).15 Given that quantitative surface structure determination is not, as yet, completely routine such application of complementary techniques is considered essential. Furthermore,
10.1021/jp804016d CCC: $40.75 2008 American Chemical Society Published on Web 08/19/2008
Diffraction Study of TiO2(110)(2 × 1)-[HCOO]-
J. Phys. Chem. C, Vol. 112, No. 36, 2008 14155
TABLE 1: Substrate Atomic Displacements Away from the Bulk-Terminated Structure of TiO2(110)(2 × 1)-[HCOO]-, Resulting from Analysis of the LEED-IV dataa displacement (Å) atom
LEED-IV
PhD1
LEED-IV (1 × 1)16
Ti (1) [110] Ti (1) [001] Ti (2) [110] Ti (3) [110] Ti (4) [110] Ti (5) [110] Ti (6) [110] Ti (6) [001] O (1) [110] O (1) [001] O (2) [110] O (2) [11j0] O (3) [110] O (3) [11j0] O (4) [110] O (4) [001] O (5) [110] O (5) [001] O (6) [110] O (6) [11j0] O(7) [110] O (7) [11j0] O (8) [110] O (8) [001] O (9) [110] O (9) [001]
-0.10 ( 0.03 -0.01 ( 0.13 -0.10 ( 0.04 0.05 ( 0.04 -0.02 ( 0.12 0.00 ( 0.08 -0.03 ( 0.05 0.05 ( 0.24 0.02 ( 0.04 0.02 ( 0.19 0.13 ( 0.09 -0.04 ( 0.18 0.05 ( 0.13 -0.22 ( 0.25 -0.02 ( 0.06 -0.12 ( 0.25 -0.02 ( 0.07 -0.04 ( 0.22 0.04 ( 0.15 0.14 ( 0.40 0.01 ( 0.22 -0.01 ( 0.32 -0.03 ( 0.22 0.10 ( 0.42 -0.06 ( 0.14 -0.09 ( 0.52
-0.07 ( 0.08 -0.03 ( 0.15 NC NC 0.02 ( 0.30 NC NC NC NC -0.04 ( 0.30 0.02 ( 0.22 -
-0.19 ( 0.03 SF 0.25 ( 0.03 0.25 ( 0.03 -0.09 ( 0.07 -0.09 ( 0.07 0.14 ( 0.05 SF 0.10 ( 0.05 SF 0.27 ( 0.08 -0.17 ( 0.15 0.27 ( 0.08 -0.17 ( 0.15 0.06 ( 0.10 SF 0.00 ( 0.08 SF 0.06 ( 0.12 -0.07 ( 0.18 0.06 ( 0.12 -0.07 ( 0.18 0.00 ( 0.17 SF 0.01 ( 0.13 SF
a Also listed are values obtained from earlier PhD measurements,1 and the atomic displacements derived from a LEED-IV study of clean TiO2(110)1 × 1.16 Figure 1 provides a key to the identity of the atoms. A negative value indicates that the atom moves towards the bulk for a displacement perpendicular to the surface plane, and in the [11j0] and [001] directions for lateral displacements. The table entry NC indicates that a quantitative comparison is inappropriate, due to different symmetry considerations in the LEED-IV and PhD1 studies. ‘-‘ denotes that no optimization of the coordinates of the atoms was undertaken. SF signifies that the displacement is symmetry forbidden on the clean (1 × 1) surface.
the LEED-IV data allow us to address the possibility, suggested by the PhD results, that the symmetry of the (2 × 1) overlayer may be lowered (loss of [11j0] mirror plane), due to the presence of a proton on alternate bridging oxygens. Experimental Methods LEED-IV measurements from TiO2(110)(2 × 1)-[HCOO]were performed in an ultra high vacuum (UHV) chamber (base pressure ∼1 × 10-10 mbar), equipped with facilities for sample cleaning, dosing, and characterization. A commercial lowcurrent (nA regime) LEED optics (MCP-LEED, OMICRON), which is fitted with a channel plate for image intensification, was employed for data acquisition. Such a system was utilized, as previously it had been observed that use of a standard LEED optics resulted in rapid electron-induced degradation of the ordered (2 × 1) overlayer.1,7 The TiO2(110) sample was prepared in situ by cycles of Ar+ bombardment and annealing at ∼1000 K until the surface was well-ordered and clean, as determined by LEED and Auger electron spectroscopy, respectively. LEED-IV data were recorded from the clean surface, and compared with those already published16 to ensure the integrity of the surface preparation. We note that throughout the experiment the TiO2(110) sample remained translucent green/blue, indicating a relatively low level of bulk reduction.2
Figure 2. Comparison of experimental and theoretical LEED-IV data for the best-fit structure. The value of RP for this fit is 0.17.
A saturated (2 × 1) overlayer of [HCOO]- was achieved through exposure of the substrate at room temperature to 10 L (1 langmuir ) 1.32 × 10-6 mbar s) of formic acid vapor by simply back filling the UHV chamber up to a partial pressure of 5 × 10-8 mbar. Prior to admitting the formic acid to the chamber, it was thoroughly degassed via repeated freeze-thaw cycles. Vapor purity was confirmed with an in situ mass spectrometer. Acquisition of the LEED-IV data, which was undertaken with the sample at ∼140 K, involved recording a series of LEED patterns as a function of the incident electron beam energy (EP) at 2 eV intervals over the range 50-400 eV. A CCD camera, interfaced to a computer, was employed for image capture. These measurements were conducted with the incident electron beam normal to the surface, as determined by comparing nominally symmetry-equivalent diffraction beams. Results For the structure determination, so-called IV-curves (i.e., plots of diffracted beam intensity (I) versus EP)15,17 were generated from the data set of LEED patterns. At each EP, the total intensity (IT) of individual diffracted beams was extracted from the LEED pattern, using a software package, by summing the pixel intensity within a square encompassing a single diffracted beam. A background (IB), which was estimated from the intensity at the edges of the square, was then subtracted to obtain I, i.e., I ) IT - IB. Following this procedure, IV-curves were compiled for eight nonequivalent integral order beams (i.e., (10), (01), (11), (02), (03), (04), (12), and (13)), and three nonequivalent fractional order beams (i.e., (1/2 0), (1/2 1), and (1/2 2)). The total energy range of these data is 1717 eV. Determination of the surface geometry from the IV-curves involved the usual trial-and-error methodology of generating simulated IV-curves for model structures, and then iteratively optimizing the geometry to find the best fit between experiment and theory as measured using the Pendry reliability factor (RP).18 The Barbieri/Van Hove Automated Tensor LEED code19 and the DL_LEED package20 were employed for the simulations. Phase shifts, which are required to accurately describe the electron scattering, were calculated following a self-consistent procedure outlined in ref.16 This approach entails no adjustable parameters, and so guarantees reliable phase shifts. Figure 2 displays the experimental IV-curves, together with theoretical simulations for the fully optimized structure. As evidenced by an RP of 0.17, there is excellent agreement between experiment and theory. Furthermore, the formate adsorption site is in accord with that determined in the PhD study,1 i.e., it binds through both of its oxygen atoms to two neighboring 5-fold titanium atoms, so that its molecular plane is aligned along the [001] azimuth. We note that the second, azimuthally rotated
14156 J. Phys. Chem. C, Vol. 112, No. 36, 2008
Lindsay et al. Discussion
Figure 3. Schematic diagrams of the local formate adsorption geometry, indicating various best-fit structural parameters derived from PhD (top)1 and LEED-IV (bottom).
Figure 4. Schematic plan view of TiO2(110)(2 × 1)-[HCOO]-, illustrating loss of [11j0] mirror plane due to presence of hydrogen on alternate bridging oxygen atoms.
formate species considered in the PhD study1 was not included in our analysis since these species are not long-range ordered, being associated with oxygen vacancies, and so are not expected to be apparent in the LEED-IV experiment. Figure 3 depicts the local formate adsorption geometry, indicating various bestfit structural parameters. Best-fit displacements of substrate atoms away from bulk termination are listed in Table 1. The error bars were calculated using standard methodology.18 Interestingly, the RP obtained here is lower than that reported for clean TiO2(110)(1 × 1).16 An explanation for this difference could be that vibrational modes of the clean surface, which are not well described by approximations in the simulation codes, are quenched upon adsorption of formate. For this optimized structure, it was assumed that the symmetry of the TiO2(110)1 × 1 substrate was maintained following adsorption of the (2 × 1)-[HCOO]- overlayer. The validity of this constraint was explored since the PhD results1 suggest that the [11j0] mirror plane may be removed due to presence of H on alternate bridging oxygens (surface OH’s), as illustrated in Figure 4. Simulations employing this lower surface symmetry resulted in a reduction in RP from 0.17 to 0.14, suggesting that this structure should be preferred over the one presented above. However, on the basis that the number of fitted parameters is much greater for this lower symmetry structure (i.e., 55 such parameters compared to only 30 for the original high symmetry structure), we conclude that this small improvement (0.03) in RP is not substantive.
The local adsorption geometry of (2 × 1) formate on TiO2(110)(1 × 1) derived from the PhD data1 is shown in Figure 3 along with that determined by LEED-IV. Comparison of the two sets of structural parameters demonstrates that these two independent structure determinations result in essentially identical local adsorption geometries. The greatest disparity between the two studies is in the separation of the two formate oxygens (d(O-O)), but the difference (∆) is still within experimental uncertainty (∆ ) 0.11 ( 0.24 Å). As displayed in Table 1, the substrate atom displacements determined by the two experiments also exhibit an excellent level of agreement. Furthermore, these experimental results are consistent with ab initio theoretical calculations of the adsorption geometry.4,6,9,10 Table 1 also lists the atomic displacements of the clean TiO2(110)(1 × 1) surface, as determined in a previous LEEDIV study.16 Comparing these values with those obtained for TiO2(110)(2 × 1)-[HCOO]- indicates that adsorption of formate induces significant modifications to the substrate structure. In general, following adsorption the atomic displacements away from bulk termination are lower in magnitude than on the clean surface. This is as expected since formate adsorption increases the coordination of the surface 5-fold Ti atoms to bulk-like 6-fold (as illustrated in Figure 1). Finally, we want to comment on the very small improvement in RP (0.03) following removal of the [11j0] mirror plane due to the presence of hydrogen on alternate bridging oxygen atoms. We note that this result does not strictly exclude the lower symmetry solution, rather it indicates that the higher and lower symmetry structures cannot be differentiated from the LEEDIV data. From the PhD analysis,1 the local structural changes around a bridging oxygen bonded to a hydrogen are marginal relative to a neighboring naked bridging oxygen. Therefore, although an ordered array of OH’s may exist, the induced structural perturbations may be too small to significantly modify the experimental IV-curves. Given the weak electron scattering of hydrogen atoms coupled with their relatively low surface density in this system, the mere presence of H, without displacement of others atoms, is expected to have little impact on the IV-curves. Test simulations with hydrogen atoms inserted atop bridging oxygen atoms confirmed this preconception. Furthermore, such hydroxyls may not display long-range order, and so although detectable by PhD, which only relies on local order,21 may be invisible in the LEED-IV experiment. Conclusions In summary, we have performed a LEED-IV study of TiO2(110)(2 × 1)-[HCOO]-, determining both adsorbate location and substrate relaxation with considerable precision. The structural solution is in excellent quantitative agreement with a recent PhD study of the same system.1 During the structure optimization, reduction in the symmetry of (2 × 1) unit cell was considered (i.e., loss of mirror plane due to presence of H on alternate bridging oxygens, as suggested by PhD results), but no substantial evidence for symmetry breaking emerged. Acknowledgment. The EPSRC (UK) are recognised for financially supporting this work through a Standard Research Grant (GR/N27774/01), and a UKCP Consortium Grant (GR/ N02337/01). We also thank D. P. Woodruff for his useful comments. References and Notes (1) Sayago, D. I.; Polcik, M.; Lindsay, R.; Toomes, R. L.; Hoeft, J. T.; Kittel, M.; Woodruff, D. P. J. Phys. Chem. B 2004, 108, 14316.
Diffraction Study of TiO2(110)(2 × 1)-[HCOO](2) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (3) Onishi, H.; Iwasawa, Y. Chem. Phys. Lett. 1994, 226, 111. (4) Wang, L.-Q.; Ferris, K. F.; Shultz, A. N.; Baer, D. R.; Engelhard, M. H. Surf. Sci. 1997, 380, 352. (5) Henderson, M. A. J. Phys. Chem. B 1997, 101, 221. (6) Ahdjoudj, J.; Minot, C. Catal. Lett. 1997, 46, 83. (7) Thevuthasan, S.; Herman, G. S.; Kim, Y. J.; Chambers, S. A.; Peden, C. H. F.; Wang, Z.; Ynzunza, R. X.; Tober, E. D.; Morais, J.; Fadley, C. F. Surf. Sci. 1998, 401, 261. (8) Hayden, B. E.; King, A.; Newton, M. A. J. Phys. Chem. B 1999, 103, 203. (9) Ka¨ckell, P.; Terakura, K. Appl. Surf. Sci. 2000, 166, 370. (10) Ka¨ckell, P.; Terakura, K. Surf. Sci. 2000, 461, 191. (11) Gutie´rrez-Sosa, A.; Martı´nez-Escolano, P.; Raza, H.; Lindsay, R.; Wincott, P. L.; Thornton, G. Surf. Sci. 2001, 471, 163. (12) Schnadt, J.; Schiessling, J.; O’Shea, J. N.; Gray, S. M.; Patthey, L.; Johansson, M. K.-J.; Shi, M.; Krempasky´, J.; Åhlund, J.; Karlsson, P. G.; Persson, P.; Mårtensoon, N.; Bru¨hwiler, P. A. Surf. Sci. 2003, 540, 39.
J. Phys. Chem. C, Vol. 112, No. 36, 2008 14157 (13) Thomas, A. G.; Flavell, W. R.; Chatwin, C.; Rayner, S.; Tsoutsou, D.; Kumarasinghe, A. R.; Brete, D.; Johal, T. K.; Patel, S.; Purton, J. Surf. Sci. 2005, 592, 159. (14) Sasahara, A.; Pang, C. L.; Onisihi, H. J. Phys. Chem B 2006, 110, 4751. (15) Pendry J. B. Low Energy Electron Diffraction; Academic Press: New York, 1974. (16) Lindsay, R.; Wander, A.; Ernst, A.; Montanari, B.; Thornton, G.; Harrison, N. M. Phys. ReV. Lett. 2005, 94, 246102. (17) Van Hove M. A.; Tong S. Y. Surface Crystallography by Low Energy Electron Diffraction: Theory, Computation and Structural Results; Springer: Heidelberg, 1979. (18) Pendry, J. B. J. Phys. C 1980, 13, 937. (19) Barbieri A.; Van Hove M. A. private communication; http://www. ap.cityu.edu.hk/personal-website/Van-Hove.htm. (20) Wander, A. Comput. Phys. Commun. 2001, 137, 4. (21) Woodruff, D. P.; Bradshaw, A. M. Rep. Prog. Phys. 1994, 57, 1029.
JP804016D
