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Article pubs.acs.org/JPCC Quantitative Structure of an Acetate Dye Molecule Analogue at the TiO2−Acetic Acid Interface Hadeel Hussain,†,‡,◆ Xavier Torrelles,§ Gregory Cabailh,∥ Parasmani Rajput,‡,¶ Robert Lindsay,⊥ Oier Bikondoa,#,∇ Marcus Tillotson,† Ricardo Grau-Crespo,○ Jörg Zegenhagen,‡,□ and Geoff Thornton*,† † London Centre for Nanotechnology and Department of Chemistry, University College London, 20 Gordon Street, London WC1H OAJ, U.K. ‡ ESRF, The European Synchrotron. 71, Avenue des Martyrs, CS40220, F-38043 Grenoble cedex 09, France § Institut de Ciència de Materials de Barcelona (CSIC), Campus UAB, 08193 Bellaterra, Spain ∥ Sorbonne Universités, UPMC Univ Paris 06, CNRS-UMR 7588, Institut des NanoSciences de Paris, F-75005 Paris, France ⊥ Corrosion and Protection Centre, School of Materials, The University of Manchester, Sackville Street, Manchester M13 9PL, U.K. # Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, U.K. ∇ XMaS, the U.K.-CRG Beamline, ESRF, The European Synchrotron, 71, Avenue des Martyrs, CS40220, F-38043 Grenoble cedex 09, France ○ Department of Chemistry, University of Reading, Reading RG6 6AD, U.K. * S Supporting Information ABSTRACT: The positions of atoms in and around acetate molecules at the rutile TiO2(110) interface with 0.1 M acetic acid have been determined with a precision of ±0.05 Å. Acetate is used as a surrogate for the carboxylate groups typically employed to anchor monocarboxylate dye molecules to TiO2 in dye-sensitized solar cells (DSSC). Structural analysis reveals small domains of ordered (2 × 1) acetate molecules, with substrate atoms closer to their bulk terminated positions compared to the clean UHV surface. Acetate is found in a bidentate bridge position, binding through both oxygen atoms to two 5-fold titanium atoms such that the molecular plane is along the [001] azimuth. Density functional theory calculations provide adsorption geometries in excellent agreement with experiment. The availability of these structural data will improve the accuracy of charge transport models for DSSC. ■ INTRODUCTION The interaction of carboxylic acids with TiO2 is important in a It is well established that exposing TiO2(110) to CH3COOH vapor at room temperature results in an ordered (2 × 1) number of applications. For instance, dyes in dye-sensitized overlayer at saturation coverage, i.e., 0.5 monolayers (ML).4−6 solar cells (DSSC) are typically anchored to TiO2 via one or The overlayer consists of acetate molecules ([CH3COO]−) more carboxylate groups.1,2 In this work, we use acetate as a formed via deprotonation, which bind to the surface through surrogate for a dye molecule with a single functionality. both oxygen atoms to two adjacent 5-fold surface titanium However, structural studies have thus far focused on exposure atoms, so that the molecular plane is aligned with the [001] of TiO2 to carboxylic acids at/near ultra high vacuum (UHV) azimuth. The cleaved H+ is thought to adsorb on neighboring conditions with a large proportion focused on formic acid. bridging oxygen atoms.4−6 A ball and stick model of the Recently, we used UHV scanning tunneling microscopy (STM) adsorption geometry is shown in Figure 1. Density functional to study the adsorption sites of carboxylates formed at aqueous theory (DFT) calculations have confirmed that the bidentate interfaces, finding that the same sites are occupied as those adsorption mode is thermodynamically preferred over mono- identified in UHV adsorption.3 Here we move even closer to dentate adsorption and that the p(2 × 1) pattern is more stable measurements in a technologically relevant environment, examining in a quantitative fashion the in situ structure of the than the c(2 × 2) pattern with the same concentration.7 Our TiO2(110) interface formed upon immersion in 0.1 M acetic SXRD results from the CH 3 COOH (aq) interface with acid using surface X-ray diffraction (SXRD). This concentration is chosen to match that used in an infrared spectroscopy study Received: January 7, 2016 of the TiO2 acetic acid interface, the results of which point to Revised: March 17, 2016 bidentate bonding of acetate.3 Published: March 18, 2016 © 2016 American Chemical Society 7586 DOI: 10.1021/acs.jpcc.6b00186 J. Phys. Chem. C 2016, 120, 7586−7590 The Journal of Physical Chemistry C Article SXRD measurements, the mylar thin film cell was in thin-layer- geometry, i.e., only a thin layer of CH3COOH(l) (<1 μm) was in contact with the sample surface.12 It should be noted that fractional order rods (FORs) were also investigated but no measurable intensity was found. Determination of the surface structure involved the use of a least-squares fitting procedure implemented within a modified version of ROD13 in which simulated CTRs are generated. By optimizing the geometry of the structure, a best-fit model between theory and experiment can be found as measured by optimized χ214 and R-factor.15 A χ2 value close to 1 and an R- Figure 1. Ball and stick model of the TiO2(110)(2 × 1)− [CH3COO]− surface. In the current study, the adsorbed acetate was value close to 0.10 is considered an excellent fit between the found to be in a bidentate bridge location with the cleaved H+ thought experimentally observed and the theoretically calculated to adsorb on neighboring bridging oxygen atoms. Large blue, small structure factors. We note that given their low X-ray scattering red, small black, and small pink spheres are oxygen, titanium, carbon, cross-section, H atoms were ignored in the fitting procedure. and hydrogen, respectively. The labeling identifies the atom positions Trial computer simulations for acetate adsorbed on in Tables 1, S1, S2, and S3. TiO2(110) (at the vacuum interface) were performed using the density functional theory (DFT) code VASP.16 Geometries TiO2(110) show that carboxylate forms small overlayer and total energies were obtained using the generalized gradient domains of (2 × 1) symmetry. approximation (GGA) with the exchange-correlation functional ■ EXPERIMENTAL DETAILS SXRD measurements were carried out using the six-circle of Perdew, Burke, and Ernzerhof (PBE).17 A Hubbard-type correction to the Ti 3d orbitals was applied, with a value of Ueff = 3 eV.18 No van der Waals corrections were applied as their diffractometer end station on ID32 at the European contribution to the strong chemisorption of carboxylic acids at Synchrotron Radiation Facility. The TiO2(110) (Pi-Kem) this surface can be expected to be small.7 The TiO2(110) sample was prepared in UHV using the Sample Character- surface was simulated using a periodic stack of quasi-two- ization Laboratory at ID32 (base pressure ≈ 1 × 10−10 mbar).8 dimensional slabs, where each slab is separated from the This involved cycles of Ar+ sputtering at room temperature and neighbor by a vacuum gap of 15 Å. We used slabs with four O− 1000 K annealing until a sharp (1 × 1) low energy electron Ti−O trilayers, with the bottom two layers fixed to equilibrium diffraction pattern was obtained and row-resolution imaging in bulk positions. A discussion about the adequacy of this model STM was achieved.9,10 Auger electron spectroscopy was used to to simulate the adsorption of molecules at this surface can be seen in ref 19. ■ confirm a lack of contamination. After preparation the sample was transferred under UHV to a small portable ion-pumped UHV chamber (“baby chamber”) RESULTS AND DISCUSSION with a base pressure in the 10−9 mbar range. This chamber SXRD analysis of UHV-prepared TiO2(110)(1 × 1) considers a incorporates a dome shaped beryllium window to allow surface slab consisting of 2.5 unit cells (six TiO atomic layers) unrestricted transmission of the incident and reflected X-ray in the [110] direction that involved a total of 49 parameters: 34 beams. Subsequently, this baby chamber was mounted on the atomic displacements, 12 Debye−Waller (DW) factors, a scale ID32 six-circle diffractometer for SXRD measurements, with factor, a roughness parameter, and a surface fraction parameter. the sample surface in the horizontal plane. Measurements were The best-fit model produced a χ2 and R-value of 1.5 and 0.14, performed with the sample at room temperature using a respectively. These values represent a high degree of agreement monochromatic focused beam with energy of 17.7 keV (λ = 0.7 between experiment and simulated patterns. From the Å), defined by slits to a size of (H × V: 200 μm × 20 μm). The comparison of the atomic displacements from this study and angle of incidence of the X-ray beam with respect to the surface that of a previous SXRD study20 on TiO2(110), listed in Table was kept constant at 0.3° for all measurements with 2 × 2 mm2 S1, it is clear that the two models are essentially in quantitative slits in front of a point detector. The experimental data were agreement with each other. This evidences the high measured using rocking scans in which scattered intensity is reproducibility of the preparation procedure as described in measured, while the sample is rotated about its normal. These the experimental section. All DW factors adopted reasonable intensities were then integrated, and geometrical correction values for both Ti and O atoms where the highest value factors11 were applied to evaluate the structure factors, which, corresponded to the topmost surface layer (BTi = 0.5 ± 0.1, BO when represented versus perpendicular momentum transfer, are = 0.9 ± 0.1) progressively decreasing with depth to their bulk known as crystal truncation rods (CTRs). The TiO2 (110) value (BTi = 0.3 ± 0.1, BO = 0.2 ± 0.1). The β roughness surface unit cell was described by lattice vectors (a1, a2, a3) parameter21 adopted a value (β = 0.07) consistent with the very parallel to the [11̅0], [001], and [110] directions, respectively, flat surface seen in STM images recorded prior to SXRD where a1 = a3 = a√2 and a2 = c (a = 4.593 Å and c = 2.958 Å). measurements. The surface fraction parameter confirmed that A large data set, comprising 1284 nonequivalent reflections, the entire surface adopted the geometry described by the best- was recorded for the UHV-prepared surface. Immediately fit model. afterward, the baby chamber was vented with N2 (99.998% The optimized TiO2(110)(2 × 1)−[CH3COO]− structure purity) within a glovebag where the sample was transferred to a obtained following analysis of the SXRD data produced a χ2 of mylar thin film cell,12 which was then mounted on the ID32 six- 1.05 and an R-value of 0.15. The total number of parameters circle diffractometer. Next, a 0.1 M aqueous solution of acetic used are the same as that for the UHV-prepared surface with an acid was injected into the mylar thin film cell, and a further additional nine parameters associated with the acetate 1479 nonequivalent reflections were measured. For these molecule; six parameters to determine orientation and site 7587 DOI: 10.1021/acs.jpcc.6b00186 J. Phys. Chem. C 2016, 120, 7586−7590 The Journal of Physical Chemistry C Article Figure 2. Experimentally observed (black error bars) and calculated CTRs for the TiO2(110)(2 × 1)−[CH3COO]− surface model (red line) and after removing the acetate molecule (blue line). It is clear that the addition of the acetate molecule to the model improves the overall goodness-of-fit. Table 1. Comparison of Molecular Acetic Acid (CH3COOH) Bond Distances and Angles26−28 with That of Acetate ([CH3COO]−) Resulting from SXRD and DFT-PBE in This Worka bond distance (Å)/angle (deg) atoms SXRD [CH3COO]− DFT-PBE [CH3COO]− [CH3COOH]26 [CH3COOH]27 [CH3COOH]28 C(1)−C(2) 1.54 ± 0.03 1.51 1.49 1.52 1.52 C(2)−O(11) 1.31 ± 0.04 1.28 1.31 1.36 1.38 C(2)−O(12) 1.32 ± 0.04 1.28 1.23 1.21 1.23 Ti(2)−O(11) 2.13 ± 0.03 2.11 Ti(2*)−O(12) 2.06 ± 0.03 2.08 ∠C(1)C(2)O(11) 117.7 ± 0.9 117.7 110.6 111.2 ∠C(1)C(2)O(12) 116.9 ± 0.8 117.0 126.6 125.9 a Atom labels are given in Figures 1 and S1. position, two DW factors (O and C atoms), and an occupancy averaged, and indeed the nominal asymmetry is almost within parameter. Excellent agreement between the best-fit model and the error bars (see Table 1). The expected orientational experiment is achieved, as can be seen from the comparison of disorder of the hydroxyl groups was confirmed by DFT the experimental and best fit simulated data in Figure 2. This calculations in a 2 × 2 supercell, where the results indicate that displays experimental CTRs (black error bars), with their best- all different orientational configurations are within 20 meV in fit theoretical simulations for the surface after exposure to 0.1 energy. M CH3COOH(aq) (red line). It also shows the effect of The apparent volume of the molecule has also been removing the acetate molecule from the model and investigated25 by simulating a uniform expansion/compression reoptimizing the structural and nonstructural parameters (i.e., of the molecule. The best fit is consistent with a small DW factors) (blue line). This increases the χ2 and R-factor to molecular expansion (2.9 ± 0.6%); however, this is considered values of 1.56 and 0.19, respectively. It is clearly evident that to be insignificant because of the lack of sensitivity of the χ2 and the presence of the acetate molecule significantly improves the R-factor to this change in volume (Table S3 provides optimized overall goodness-of-fit. The adsorbed acetate was found to be in positions of atoms in the acetate moiety). a bidentate bridge location, i.e., binding through both oxygen Table 1 lists the internal bond distances and angles of atoms to two 5-fold titanium atoms such that its molecular acetate/acetic acid emerging from this study and previous plane is along the [001] azimuth (Figure 1), in excellent literature values. There is excellent agreement with both agreement with the literature for acetate and other simple previous experimental26,27 and theoretical28 work regarding the carboxylates bound to this substrate at the vacuum inter- intramolecular bond distances. Discrepancies in bond angles are face.22−24 The resulting bond distances are Ti(2)−O(11) = due to previous experimental work being on molecular acetic 2.13 ± 0.03 Å, Ti(2*)−O(12) = 2.06 ± 0.03 Å, which compare acid rather than acetate. well with bond distances from the formate/TiO2(110) When comparing displacements of substrate atoms between system.23,24 the UHV-prepared TiO2(110)(1 × 1) and the TiO2(110)(2 × In contrast to refs23, 24., here we allow the adsorbed 1)−[CH3COO]− surfaces it is clear that adsorption of acetate carboxylate to be asymmetric in order to compare with DFT reduces the clean surface relaxations (see Tables S1 and S2 for calculations. The results of the latter indicate an asymmetric atomic displacements and atomic coordinates, respectively). carboxylate, which appears to arise from the orientational This almost certainly arises from an increase in substrate ordering of the hydroxyl groups. In the experiment, the surface atom coordination number and has been seen before for hydroxyls will likely be disordered and this asymmetry will be carboxylate/TiO2(110) at the vacuum interface23 and other 7588 DOI: 10.1021/acs.jpcc.6b00186 J. Phys. Chem. C 2016, 120, 7586−7590 The Journal of Physical Chemistry C Article metal oxide surfaces exposed to liquid water.29 This is also through the U.K.’s HPC Materials Chemistry Consortium, reflected in the DW parameters, which return to close to their which is funded by EPSRC (EP/L000202). bulk value. Good agreement of atomic displacements can also be seen between SXRD and DFT-PBE calculations from this study (see Supporting Information for details). As regards the β ■ REFERENCES (1) Thomas, A. G.; Syres, K. L. Adsorption of Organic Molecules on roughness parameter, this now takes a value of 0.2, increasing Rutile TiO2 and Anatase TiO2 Single Crystal Surfaces. Chem. Soc. Rev. from 0.07. 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