The Role of Hydrogen on the Adsorption Behavior of Carboxylic Acid

Article pubs.acs.org/JPCC

The Role of Hydrogen on the Adsorption Behavior of Carboxylic Acid on TiO2 Surfaces Wolfgang Heckel, Beatrix A. M. Elsner, Christian Schulz, and Stefan Müller* Institute of Advanced Ceramics, Hamburg University of Technology, 21073 Hamburg, Germany S Supporting Information *

ABSTRACT: In this work, we present binding energies of acetic acid on the (110), (100), and (011) surfaces of rutile TiO2 calculated with the two density functional theory (DFT) exchange-correlation functionals PBE and PBEsol. It is shown that the binding energies can be influenced, in this case slightly reduced for all three surfaces, via preadsorption of hydrogen. Additionally, we tested the performance of the densityfunctional based tight-binding (DFTB) method applied to these adsorbate systems. Analysis of the electronic density of states shows that DFTB provides qualitatively comparable results to DFT calculations as long as the Fermi energy level remains within the band gap.

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INTRODUCTION Natural nanostructured materials can exhibit exceptional mechanical, optical, or electrical properties, and especially the hierarchical structure of organic−inorganic composites, such as enamel or nacre, can serve as an example for modern materials design.1 The macroscopic properties of such materials are strongly governed by the interfaces between their organic and inorganic compounds. To produce similar hierarchically structured materials, it is therefore crucial to understand and influence those interfaces at all levels, beginning at the atomic scale. As a prototype for a hard−soft matter junction, we investigate the TiO2 rutile surfaces functionalized by carboxyl groups using electronic structure theory. As a technically important ceramic material with versatile applications, TiO2 has been subject of numerous studies.2−5 Especially on the two most stable rutile facets, the (110)6−19 and the (011),20,21 adsorption and photocatalytic reactions of a wide range of molecules have been investigated. In this context, molecules with a linking carboxyl functional group play an important role, as they have the ability to form bidentate and thus potentially strong bonds (e.g., amino acids for biocompatibility; (110): ref 22−39, (011): ref 40−44). Because a detailed, in theory45−47 and experiment,48,49 consistent picture exists for carboxylic acids on the (110) surface, this system can be used as a reference for examination of carboxylic acids on other TiO2 facets as well as for validating the reliability of novel methods. For more than two decades, density functional theory (DFT)50,51 has offered a supremely powerful and accurate method for atomic characterization of interfaces, which has perpetually been improved by introduction of new exchangecorrelation functionals. Despite the continuously increasing computing power, the DFT-treatable system sizes are limited, which especially poses an obstacle to the simulation of surface © 2014 American Chemical Society

defects, adsorption in hydrous atmosphere, or adsorption processes via molecular dynamics. This can be counteracted by applying semiempirical methods such as density-functionalbased tight-binding (DFTB), which manages far larger system sizes at comparable consumption of resources and accuracy.52 Simultaneously, it must be ensured that such a method is suitable for the particular task, i.e., that physically meaningful results are obtained with the required accuracy. Therefore, the applicability of DFTB, even though deemed a generally transferable method, needs to be carefully tested for each novel system. In this work, the following three topics are elucidated by means of the interface TiO2−acetic acid (as a representative for carboxylic acids in general): (1) Performance of DFT vs DFTB: Regarding the energetics and the electronic structure, we compare the performance of DFTB with two pure ab initio generalized gradient approximation (GGA) functionals, namely PBE and PBEsol. Thus, the opportunity arises to benchmark the scope of applicability for these three potentials in the context of adsorption. (2) Adsorption energies for acetic acid on TiO2: We present adsorption energies for acetic acid on the three stable surfaces of the TiO2 rutile structure: the well-known (110) surface, the least stable (100) surface, and the recently intensively studied (011) surface. In vacuum, the (011) surface forms a 2 × 1 superstructure. However, it has not yet been resolved whether the bulk-terminated or the reconstructed surface is more stable in the presence of an adsorbate layer. In Comparison of Adsorption Modes, we calculate formation enthalpies to show Received: January 16, 2014 Revised: April 16, 2014 Published: April 21, 2014 10771

dx.doi.org/10.1021/jp500507a | J. Phys. Chem. C 2014, 118, 10771−10779

The Journal of Physical Chemistry C

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Table 1. Lattice Parameters of Bulk TiO2 Rutile experiment69 PBE a in Å c in Å c/a a

PBEsol a

4.652 (4.634) 2.970 (2.963)a 0.6384 (0.6394)a

DFTB b

4.603 2.946 0.6400

4.672 (4.611) 2.997 (2.970)b 0.6415 (0.6412)b

15 K

295 K

4.587 2.954 0.6440

4.593 2.959 0.6442

Reference 70. bReference 13.

Table 2. TiO2 Rutile Surface Energies Esurf in meV/Å2 PBE a

(110) (011) 2 × 1 (011) (100) a

25 (26) 27 58 37 (52)b b

PBEsol

DFTB

LDA71

PW9172

B3LYP73

PBE074

37 46 70 49

71 71 84 75 (74)c

56

31

30

34

87

64 43

GGA75 26 56

42

52

c

Reference 74. Reference 76. Reference 68.

that it is sufficient to consider only the unreconstructed 1 × 1 surface. (3) Analysis of adsorption modes: We analyze various adsorption modes (acetate, acetic acid, acetic acid plus hydrogen) by means of the electronic density of states regarding the adjustability of the binding energy of such adsorbate systems. Here, the capability of the above-mentioned potentials to describe these modes will play an essential role. The following section contains computational details on the used methods DFT and DFTB. In Lattice Parameters and Surface Energy, the substrate TiO2 and its surfaces are briefly introduced. In Comparison of Adsorption Modes, it is shown that the energetically favorable adsorption configuration on the examined surfaces is the bidentate one, on which we will focus in subsequent sections. In Bidentate Binding Energies for TiO2 Rutile (110), (100), and (011), the binding energies of the considered adsorption modes on the three surfaces are presented, which are then analyzed in Discussion by means of the electronic structure. The performance of the different potentials is discussed in Performance of the Three Potentials. Finally, the results of this study are summarized.

asymmetry.62,63 Brillouin zone sampling using a 7 × 5 × 1 Monkhorst−Pack64 mesh for the laterally small (100) surface cell and 5 × 5 × 1 meshes for all other cells has been found sufficient. The supercells of the presented calculations are based on the DFT-optimized bulk rutile lattice with its tetragonal parameters (space group P42/mnm) listed in Table 1. For all PBE surface and adsorption calculations, the fully relaxed PBE bulk geometry is used to keep the two innermost layers of each slab fixed in their bulklike positions. This applies analogously for the PBEsol functional. DFTB. Background information on the semiempirical method of density-functional-based tight-binding can be taken, for example, from refs 65 and 66. For the DFTB calculations, we applied the program DFTB+52 with its second-order self-consitent charge approach of the Kohn−Sham total energy67 and the Slater−Koster potential files provided in the parameter sets mio67 and tiorg.68 Analogously to the DFT calculations, we constructed periodic slabs for DFTB calculations: 10 O−Ti−O trilayers, the two innermost layers fixed in their fully relaxed bulklike geometry, at least 110 Å vacuum, and 1 × 2 supercells for (110) and (100) surfaces and 1 × 1 for the (011) surface. Relaxation of surface and adsorbate atoms occurs via the conjugate gradient algorithm until the forces on each atom are less than 5 meV/Å. The Brillouin zone integration is accomplished by an 8 × 8 × 1 supercell folding with the Γ-point shifted to the center of the cell.

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COMPUTATIONAL DETAILS DFT. We performed density functional theory calculations using the Vienna Ab initio Simulation Package53−56 with its implemented PAW pseudopotentials57,58 and a plane wave basis with an energy cutoff of 520 eV. For the exchangecorrelation (xc) functional, two types of GGA functionals, namely PBE59,60 and PBEsol,61 have been used. PBEsol has been developed as a modification of the widely used PBE functional to improve lattice constants and surface energies. According to experimental data,48 the lateral dimensions of the periodic slabs have been chosen as small as possible but as large as necessary to adsorb a single acetic acid molecule in its bidentate mode without touching its neighbors. This results in 1 × 2 supercells for the (110) and (100) surfaces and in a 1 × 1 primitive surface cell for the (011) surface. Irrespective of the adsorbate and surface orientation, surface slabs contained at least 14 Å of vacuum in the direction normal to the surface and were constructed out of 10 O−Ti−O trilayers. The two innermost trilayers were fixed in their bulk positions, and all other atomic positions were relaxed using a conjugate gradient algorithm until the forces on the unconstrained atoms were less than 0.01 eV/Å. Molecules were adsorbed only on one side of the slab, applying dipole correction to account for the slab

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RESULTS Lattice Parameters and Surface Energy. To study the adsorption of carboxylic acid on TiO2, we initially investigate relevant properties of the substrate. Lattice parameters and surface energies for all surfaces and potentials are listed in Tables 1 and 2 together with their literature values. The lattice parameters of the fully relaxed bulk cells confirm known trends from other studies: DFTB (cf., refs 13, 68) and PBE (cf., e.g. ref 70) overestimate bond lengths systematically. This manifests in the lattice constants of DFTB and PBE in comparison to the experimental value of a = 4.587 Å. The lattice constant in [001] direction, c, behaves analogously to this, leading to an aspect-ratio c/a that deviates from the experimental value less than 0.9%. As expected, the PBEsol lattice constants a = 4.603 Å and c = 2.946 Å very precisely agree with the experimental values from 10772



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Figure 1. Clean, bulk-terminated TiO2 rutile surfaces. Under-coordinated surface atoms are denoted by Ti5c and O2c according to their coordination. The (011) surface normal is labeled as n(011). Color code: Ti = light blue, O = red, some TiO6 octahedrons are highlighted gray. All presented structures are visualized using VESTA.79

Figure 2. Bidentate adsorption of acetic acid on TiO2 rutile surfaces. Color code: Ti = light blue, O = red, C = brown, H = bright.

undercoordinated surface titanium atoms (see Figure 2a). Other molecules with a linking carboxylate group behave equivalently on the (110) surface, e.g., the amino acids glycine33,34 and cysteine35 as well as acetic acid,23,24 all of them having their molecular plane perpendicular to the surface. According to diverse theoretical studies,36,37,45,47 formic acids can also attach to the (110) surface in a monodentate manner. Here the molecule binds to one 5-fold coordinated surface Ti5c atom, while H dissociates to a bridging oxygen with hydrogen bonding to the oxygen of the molecules (Figure 3). Although

ref 69 as PBEsol has been designed to compute lattice constants closer to the experimental values. Table 2 also contains surface energies for all three potentials. Due to the usage of periodic slabs, the surface energy Esurf is determined as follows: Esurf =

1 O ‐ Ti ‐ O O ‐ Ti ‐ O (Eslab − Nslab E bulk ) 2A

(1)

Therein, Eslab is the total energy of the fully relaxed surface the energy of a O−Ti−O unit in the fully relaxed slab, EO−Ti−O bulk bulk structure, NO−Ti−O the number of O−Ti−O units in the slab surface slab, and A the surface area of the surface slab. The factor 1/2 arises from having two surfaces per surface slab. Typically, surface energies are underestimated by semilocal xc-functionals such as PBE, while they are increased by PBEsol.77,78 This behavior is reported here, too. Equally, the DFTB surface energies lie even higher, as expected according to, for example, ref 68. However, more important than the agreement of total energies is that the hierarchy of surface energies is consistent within all potentials, namely E(110) < surf (011)2 × 1 (100) (011) Esurf < Esurf < Esurf . The sole exception to this constitutes the DFTB values for the (110) and the (011)2 × 1 surfaces, being both equal to 71 meV/Å2. As seen from Table 2, our calculated values, especially the energetic hierarchy of the TiO2 rutile surfaces, completely conform to the results reported in numerous former studies. Comparison of Adsorption Modes. As many former studies state, the dissociative, bidentate adsorption mode of carboxylic acids plays an energetically significant role. For instance, experimental studies from the groups of Thornton49 and Woodruff48 (and references therein) show that formic acid, adsorbed on TiO2(110), forms an almost perfectly ordered 2 × 1 overlayer at saturation coverage. The acid dissociates into a hydrogen, sitting on top of a bridging oxygen (labeled as O2c in Figure 1a), and the main moiety (acetate), which binds symmetrically with both oxygen atoms to two neighbored

Figure 3. Monodentate adsorption of acetic acid on TiO2 (110) rutile surface. Color code: Ti = light blue, O = red, C = brown, H = bright.

this configuration would result in a 1 × 1 overlayer with a binding energy per area about 13% higher than that for the bidentate mode (cf., ref 47 or 45), this is not observed in experiment. In consequence of these results, we can exclude theoretically possible denser coverages. So, we investigate bidentate and monodentate adsorption in a 2 × 1 overlayer on the (110) surface. Binding energies are calculated as the difference between the total energies of the educts, Eadsorbate,gas and Esubstrate, and the total energy of the product, Eslab, E B = Eadsorbate,gas + Esubstrate − Eslab 10773

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meV), while on the unreconstructed (011) surface, bidentate adsorption is realized with a binding energy of 1.71 eV. The stability of a structure σ can be measured by its formation enthalpy ΔHf(σ). The mono- and bidentate formation enthalpies are given by

Results and literature values are listed in Table 3. In accordance with the literature, we find with all three potentials that the bidentate adsorption mode is energetically favorable.36,37,45 Table 3. Comparison of Mono- and Bidentate Binding Energies for TiO2(110) in eV adsorption mode

monodentate

bidentate

acetic acid PBE PBEsol DFTB

0.83 1.17 1.45

1.12 1.47 2.05

PW91 (formic acid)a PW91 (acetic acid)b PW91 (glycine)b PW91 (glycine)c PBE (glycine)c PBE0 (glycine)c PW91 (proline)c PBE (proline)c PBE0 (proline)c

1.39 1.24 1.56 1.17 0.97 1.15 0.93 0.86 1.10

1.94 1.37 1.67 1.57 1.36 1.72 1.42 1.33 1.66

experiments TDS (glycine)d TPXPS (proline)e

ΔHf,ads(σi) = Esurf (σi) − Esurf (σ0) −

1 E B(σi) Am

(3)

where Am corresponds to the surface area per adsorbed molecule. The index i = 0, 1 denotes the two surfaces, σ0 the 2 × 1 reconstructed surface and σ1 the bulk-terminated (011) surface. We used the binding energies reported by Muir et al.44 and obtained the formation enthalpies ΔHf,monodentate(σ0) = −15 meV/Å2 and ΔHf,bidentate(σ1) = −36 meV/Å2. We note that bidentate adsorption on the bulk-terminated surface is more favorable than monodentate adsorption on the 2 × 1 reconstructed one, even if, as claimed in ref 42, the coverage in the latter case could exceed 0.5 molecules per Ti5c atom. Whether the energetically favorable state is realized experimentally, is a question of activation barriers and cannot be clarified in this study. So, in the following, we restrict ourselves to the bidentate adsorption mode of acetic acid on the three bulk-terminated surfaces (110), (100), and (011). Bidentate Binding Energies for TiO2 Rutile (110), (100), and (011). Diverse studies report that the hydroxylation of the TiO 2 (110) and (011) 2 × 1 surfaces are stable,6−9,21,25and thus the adsorption behavior for additive molecules can change fundamentally in the presence of hydrogen. Accordingly, for example, O2 can be bound by the hydroxylated TiO2 surface, while it would not adsorb on a clean, stoichiometric surface.10 In this section, we will investigate the influence of hydrogen on the adsorption behavior of acetic acid on clean and hydroxylated TiO2. For this, we considered acetic acid on clean and hydroxylated TiO2, denoted as (CH3COOH/TiO2) and (CH3COOH/TiO2+H), respectively, as well as acetate on these surfaces: (CH3COO/ TiO2) and (CH3COO/TiO2+H). The binding energies for the surfaces (110), (100), and (011) as well as for the three potentials PBE, PBEsol, and DFTB are listed in Table 4. Additionally, the results for hydrogen on TiO2 and acetate on TiO2 under formation of H2 gas are given. In Figure 2 is shown the bidentate adsorption of (CH3COOH/TiO2) for all three surfaces. Detailed geometrical data for all structures are given in Supporting Information. The here presented adsorption energies of hydrogen on TiO2 originate from a coverage of 0.5 per surface Ti5c atom and are related to one free hydrogen molecule, i.e.,

1.2 1.08

a

Reference 45. bReference 37. cReference 36. dThermal desorption spectroscopy, ref 30. eTemperature-programmed X-ray photoelectron studies, ref 32.

Considering the clear trend of the (110) surface, we assume that also on the other surfaces the bridging adsorption mode is favorable as long as they provide reactive, undercoordinated Ti atoms in a bonding distance suitable for carboxylic acids. Such conditions are given on the (100) and the bulk-terminated (011) surfaces, contrary to the 2 × 1 reconstructed (011) surface. It is known that in vacuum the 2 × 1 reconstructed (011) surface is more stable than the unreconstructed one. However, it has not yet been clarified whether this is also true when the surface is covered by an adsorbate layer. Aschauer and Selloni computationally predicted that the 2 × 1 superstructure vanishes in the case of the adsorption of H2O.20 There is also experimental evidence that the adsorption of carboxylic acid might influence the superstructure in a similar manner. For example, this is indicated by the TPD desorption energies40,42 of 1.1−1.4 eV, which do not agree with the low adsorption energies for the (011) 2 × 1 surface, as calculated by Muir et al.44 According to this study, acetic acid binds only monodentally and weakly to the (011) 2 × 1 surface (38

Table 4. Binding Energies of Acetic Acid, Acetate, and Acetic Acid + H on TiO2 Surfaces in eV PBE acetic acid on TiO2 acetic acid on TiO2+H acetate on TiO2 acetate on TiO2+H acetate on TiO2, H2 gas H on TiO2

PBEsol

DFTB

(110)

(100)

(011)

(110)

(100)

(011)

(110)

(100)

(011)

1.12 1.07 1.56 3.64 −1.13 0.17

1.05 0.97 1.27 3.62 −1.41 0.12

1.54 1.43 1.92 4.05 −0.77 0.18

1.47 1.42 1.88 4.04 −0.97 0.29

1.43 1.41 1.61 4.07 −1.24 0.21

1.85 1.80 2.19 4.43 −0.66 0.27

2.05 1.92 1.95 5.11 −1.34 0.23

1.64 1.90 1.45 5.05 −1.84 −0.11

1.86 1.98 1.88 5.48 −1.41 −0.33

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Figure 4. Density of states (DOS) of acetate, acetic acid, acetic acid + H, and H on TiO2, calculated with PBE (a) and DFTB (b). The energy axis is shifted to the valence band maximum EVBM. For analysis, see Discussion. Color code: red = total DOS of the slab, green = partial DOS of the adsorbate (including the additional H in the case of (CH3COOH/TiO2+H)), blue = partial DOS of the substrate, black = total bulk DOS. For further clarification, Figure 5 shows a zoom-in of the orange framed region. 10775


The Journal of Physical Chemistry C ⎛ H ⎞ ⎞ ⎛ ⎛ H ⎞ 1 E B⎜ ⎟ − E⎜ ⎟ = E(H 2) + E⎜ ⎟ 2 ⎝ TiO2 ⎠ ⎝ TiO2 ⎠ ⎝ TiO2 ⎠

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Generally, the binding energies calculated with PBE lie systematically about 25% below the PBEsol values. These in turn are about 10% lower than the DFTB values, as expected from the surface energies. According to both DFT potentials, the adsorption of both acetic acid and acetate on the reactive (011) surface releases significantly more (∼0.4 eV) energy than on the other two surfaces, which differ less than ∼0.1 eV (in favor of the (110) surface). This hierarchy is not fully reproduced by DFTB. In the case of prehydroxylation only, the strongest binding is predicted for the (011) surface. If clean TiO2 is considered, DFTB predicts the strongest binding for the (110) surface. For all three surfaces, the binding energies for CH3COOH can be modified by preadsorption of hydrogen. For both DFT potentials, prehydroxylation results in a slight bond weakening. For PBE, this amounts to 50 to 110 meV, while for PBEsol, the weakening effect lies between 20 and 50 meV. In contrast, with DFTB this can only be observed for the (110) surface (130 meV). On the other two surfaces, prehydroxylation stabilizes the bond. The bond energy is increased by 120 and 260 meV for (100) and (011), respectively. The DFT calculations confirm the stability of the hydroxylated state, H/TiO2, for all surfaces, whereas in DFTB this reaction only appears exothermic for the (110) surface. The endothermic hydroxylation of the (100) and (011) surfaces correlates with the above-mentioned DFTB bond stabilization for (CH3COOH/TiO2+H). A detailed discussion of the adsorption of the neutral radical acetate will follow in The Energy Hierarchy EB(CH3COO/ TiO2+H) > EB(CH3COO/TiO2) > EB(CH3COOH/TiO2) and Adsorption of the Neutral Radical CH3COO.

Figure 5. DOS and partial DOS near the valence band maximum in the case of acetate on TiO2; zoom-in of the orange frame of Figure 4a. During the adsorption, electrons which primarily occupy states within the red shaded area are depleted into lower, empty states, originating from the adsorbate (green shaded area). This process releases the excess energy ΔE, which raises the binding energy.

slightly more than Egap + ΔE ≈ 2.5 eV as compared to (CH3COOH/TiO2). The two opposing effects (lowering of adsorbate states and depletion of states) also occur in DFTB calculations. However, the balance of the two effects is shifted by DFTB with respect to DFT, annihilating the energy hierarchy of (CH3COO/TiO2) and (CH3COOH/TiO2) (cf., Figure 4b). The binding energy differences of (CH3COO/TiO2+H) and (CH3COOH/TiO2) (≳3 eV) are due to the larger DFTB band gaps of about 2.5 eV, and the energy hierarchy of these adsorption modes is preserved. Adsorption of the Neutral Radical CH3COO. The high binding energies of (CH3COO/TiO2) and (CH3COO/ TiO2+H) are merely of theoretical avail because, as shown in the previous section, they are caused by empty states of the adsorbate CH3COO, which are located below the Fermi level of the substrate. Because this neutral molecule fragment does not exist among ordinary experimental conditions, the hydrogen atom cannot be neglected. There are three possible reaction paths for the dissociated hydrogen: formation of hydrogen gas, adsorption on the substrate, or reaction with a third substance. The adsorption of acetate on TiO2 upon generation of hydrogen gas is endothermic (see Table 4) and so is not among the likely reactions. A third substance involved in the reaction would have to bind the H atom during the dissociative adsorption of CH3COOH on TiO2 without influencing the substrate surface or the radical CH3COO. Additionally, it would have to outmatch the already considerable binding energies of CH3COOH on TiO2. Thus, the existence of such a substance is questionable. Consequently, the hydrogen atom dissociated from the carboxylic acid always coadsorbs on a TiO2 surface solely due to energetic reasons. Adjustability of Binding Energy (by hydrogen preadsorption). Coadsorption of appropriate molecules can influence adsorbate binding energies. H is particularly suitable for preadsorption on TiO2, on the one hand because it is geometrically small and so does not hinder the desired adsorption process. On the other hand, its electron occupies states at the current band minimum of the substrate and serves as an energy storage in the case that a subsequently adsorbed molecule creates free states located energetically below this energy storage. These states then get occupied while raising the heat of adsorption. Analogously, the adsorption of O2 on TiO2(+H)10 works because of the filling of the 2π orbital lying within the TiO2 band gap.

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DISCUSSION The Energy Hierarchy E B (CH 3 COO/TiO 2 +H) > EB(CH3COO/TiO2) > EB(CH3COOH/TiO2). The choice of reference energies can significantly influence the resulting binding energies. Therefore, in this section we will use the electronic density of states to discuss the energetic hierarchy of the three adsorption modes (CH 3 COO/TiO 2 +H), (CH3COO/TiO2), and (CH3COOH/TiO2). The two DFT potentials yield qualitatively equivalent results, and an exemplary plot of the density of states (PBE (110)) is shown in Figure 4a. For (CH3COOH/TiO2), the states of the adsorbate (green in Figure 4a) appear deeper than for (CH3COO/TiO2), which suggests a tougher bonding in the case of hydroxylation. However, the adsorbate CH3COO contributes empty states which lie below the Fermi energy of clean TiO2 and cause electrons from the highest occupied valence band to be depleted into these empty states. In case of the PBE (110) surface, this excess energy ΔE (see Figure 5) amounts to about 0.62 eV and overcompensates the energy gain caused by deeper adsorbate and adsorbate induced states of (CH3COOH/TiO2). This explains the amount of binding energy difference of ∼0.44 eV. The effect of depleting states also occurs for (CH3COO/ TiO2)+H. Here the lowest current band state, occupied by the electron of the preadsorbed hydrogen atom, is depleted into the empty states of the adsorbate. This results in an energy gain of 10776



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CONCLUSION In the present work, we have studied the adsorption behavior of acetic acid on the TiO2 rutile surfaces (110), (100), and (011) using DFT (PBE and PBEsol) and a tight-binding approach (DFTB). It has been shown that the results of both DFT potentials differ only quantitatively and that PBE reproduces the experimental binding energies more closely. Regarding both functionals, the electronic structure behaves qualitatively identically for the various adsorption modes studied. On all three surfaces, the dissociative bidentate adsorption, as already established for the (110) surface, is realized. The bond strength between substrate and adsorbate, and so the stability of this interface, can be modified by coadsorption of additive atoms or molecules. Our calculations show that the binding energy of the adsorbate acetic acid can be slightly lowered by preadsorption of hydrogen.

Carboxylic acids offer no free states below the current band minimum. That means, after the adsorption of CH3COOH on TiO2+H, the deepest current band states will remain occupied. Subsequently, the binding energy gain caused by depleting this energy storage is omitted, and only minor changes of binding energy, as compared to CH3COOH on bare TiO2, may be expected. Indeed, the binding energies of CH3COOH on TiO2+H are lower than those for CH3COOH on TiO2 (apart from aforesaid exceptions with DFTB, cf., Bidentate Binding Energies for TiO2 Rutile (110), (100), and (011) and Table 4). Noticeably, the adsorbate states of (CH3COOH/TiO2+H) appear slightly deeper than those of (CH3COOH/TiO2) and further reduce the reactivity of the overall system. This means that mainly the bonding of the preadsorbed hydrogen atom must cause the reduction of the binding energy by minimizing its degree of freedom when interacting with the carboxylic acid. Nevertheless, it may be reemphasized that the adsorption of H in addition to CH3COOH on TiO2 is exothermic. Thus, it is possible to coadsorb hydrogen and acetic acid on TiO2. Performance of the Three Potentials. DFT: PBE vs PBEsol. From the studies in refs 77 and 78, and references therein, no semilocal DFT xc-functional is capable of predicting precisely all three: the lattice parameter, the surface energies, and binding energies. Typically, surface energies are underestimated, while binding energies lie above the experimental values. If a potential, such as PBEsol, corrects the surface energies to more realistic, higher values, the binding energies will be overestimated even more. Consequently, all of our PBEsol binding energies are roughly 0.4 eV higher than corresponding PBE values, which accidentally reproduce the experimental binding energies quite well (cf., Comparison of Adsorption Modes and Table 3). However, the observed adsorption modes, with radicals, additive hydrogen atoms, or without them, show qualitatively the same physical effects, irrespective of the functional. DFTB vs DFT. Comparing DFTB to DFT, we find features typical for the DFTB method. This comprises high surface energies (even higher than PBEsol), larger band gaps than with DFT (here ∼1 eV larger), and compressed bands (see Figure 4) as a consequence of the DFTB minimal basis set. Unfortunately, the high surface energies also result in excessive binding energies. In the literature,68,80−85 there are numerous examples for the successful application of DFTB to adsorbate systems with the Fermi level remaining within the band gap after adsorption. Acetic acid on the TiO2 surfaces (110), (100), and (011) may be put in this row, although the binding energy differences with respect to DFT are considerable ((CH3COOH/TiO2): E(110) − E(100) = 0.07/0.41 eV (PBE/ B B (110) (011) − EB = −0.42/0.19 eV). Considering the DFTB), EB possibility to extend system sizes due to savings in computational resources and computing time, this might be a payable price. In contrast to DFT data, the DFTB binding energies show no consistent trend if either the lowest current band states are occupied or the highest valence band states are empty, as might be the case if electrons come from additional hydrogen atoms, or acetate contributes deep, empty states, respectively. Especially, the adsorption of hydrogen on TiO2 (100) and (011) appears erroneously endothermic. Evidently, the standard procedure for determining binding energies cannot be applied to DFTB calculations whenever the Fermi level is not located in the band gap.

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ASSOCIATED CONTENT

S Supporting Information *

Geometrical data for acetic acid on TiO2 surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 (0)40 428783137. Fax: +49 (0)40 428782647. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the German Research Foundation (DFG) via SFB 986 “M3”, project A4. REFERENCES

(1) Ortiz, C.; Boyce, M. C. Bioinspired Structural Materials. Science 2008, 319, 1053−1054. (2) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53−229. (3) Lun Pang, C.; Lindsay, R.; Thornton, G. Chemical Reactions on Rutile TiO2(110). Chem. Soc. Rev. 2008, 37, 2328−2353. (4) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515−582. (5) Henderson, M. A. A Surface Science Perspective on TiO2 Photocatalysis. Surf. Sci. Rep. 2011, 66, 185−297. (6) Leconte, J.; Markovits, A.; Skalli, M. K.; Minot, C.; Belmajdoub, A. Periodic ab Initio Study of the Hydrogenated Rutile TiO2(110) Surface. Surf. Sci. 2002, 497, 194−204. (7) Schaub, R.; Thostrup, P.; Lopez, N.; Lægsgaard, E.; Stensgaard, I.; Nørskov, J. K.; Besenbacher, F. Oxygen Vacancies as Active Sites for Water Dissociation on Rutile TiO2(110). Phys. Rev. Lett. 2001, 87, 266104. (8) Wendt, S.; Schaub, R.; Matthiesen, J.; Vestergaard, E. K.; Wahlström, E.; Rasmussen, M. D.; Thostrup, P.; Molina, L. M.; Lægsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Oxygen Vacancies on TiO2(110) and their Interaction with H2O and O2: A Combined High-Resolution STM and DFT Study. Surf. Sci. 2005, 598, 226−245. (9) Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G. Direct Visualization of Defect-Mediated Dissociation of Water on TiO2(110). Nat. Mater. 2006, 5, 189−192. (10) Liu, L. M.; McAllister, B.; Ye, H. Q.; Hu, P. Identifying an O2 Supply Pathway in CO Oxidation on Au/TiO2(110): A Density Functional Theory Study on the Intrinsic Role of Water. J. Am. Chem. Soc. 2006, 128, 4017−4022. 10777



Article

Proline on Rutile TiO2(110) Surfaces. Surf. Sci. 2008, 602, 2029− 2038. (33) Qiu, T.; Barteau, M. A. STM Study of Glycine on TiO2(110) Single Crystal Surfaces. J. Colloid Interface Sci. 2006, 303, 229−235. (34) Lerotholi, T. J.; Kröger, E. A.; Knight, M. J.; Unterberger, W.; Hogan, K.; Jackson, D. C.; Lamont, C. L. A.; Woodruff, D. P. Adsorption Structure of Glycine on TiO2(110): A Photoelectron Diffraction Determination. Surf. Sci. 2009, 603, 2305−2311. (35) Ataman, E.; Isvoranu, C.; Knudsen, J.; Schulte, K.; Andersen, J. N.; Schnadt, J. Adsorption of L-Cysteine on Rutile TiO2(110). Surf. Sci. 2011, 605, 179−186. (36) Tonner, R. Adsorption of Proline and Glycine on the TiO2(110) Surface: A Density Functional Theory Study. ChemPhysChem 2010, 11, 1053−1061. (37) Ojamäe, L.; Aulin, C.; Pedersen, H.; Käll, P.-O. IR and Quantum-Chemical Studies of Carboxylic Acid and Glycine Adsorption on Rutile TiO2 Nanoparticles. J. Colloid Interface Sci. 2006, 296, 71−78. (38) Monti, S.; van Duin, A. C. T.; Kim, S.-Y.; Barone, V. Exploration of the Conformational and Reactive Dynamics of Glycine and Diglycine on TiO2: Computational Investigations in the Gas Phase and in Solution. J. Phys. Chem. C 2012, 116, 5141−5150. (39) Li, C.; Monti, S.; Carravetta, V. Journey toward the Surface: How Glycine Adsorbs on Titania in Water Solution. J. Phys. Chem. C 2012, 116, 18318−18326. (40) Quah, E. L.; Wilson, J. N.; Idriss, H. Photoreaction of the Rutile TiO2(011) Single-Crystal Surface: Reaction with Acetic Acid. Langmuir 2010, 26, 6411−6417. (41) Wilson, J. N.; Dowler, R. M.; Idriss, H. Adsorption and Reaction of Glycine on the Rutile TiO2(011) Single Crystal Surface. Surf. Sci. 2011, 605, 206−213. (42) Tao, J.; Luttrell, T.; Bylsma, J.; Batzill, M. Adsorption of Acetic Acid on Rutile TiO2(110) vs (011)-2 × 1 Surfaces. J. Phys. Chem. C 2011, 115, 3434−3442. (43) McGill, P. R.; Idriss, H. DFT Study of Carboxylic Acids Modes of Adsorption on Rutile TiO2(011) Surfaces. Surf. Sci. 2008, 602, 3688−3695. (44) Muir, J. M. R.; Idriss, H. Study of the Interaction of Formic Acid, Formaldehyde and Formamide with the Bulk Terminated (1 × 1) and Reconstructed (2 × 1) Surfaces of Rutile TiO2(011). Surf. Sci. 2013, 607, 187−196. (45) Bates, S. P.; Kresse, G.; Gillan, M. J. The Adsorption and Dissociation of ROH molecules on TiO2(110). Surf. Sci. 1998, 409, 336−349. (46) Morikawa, Y.; Takahashi, I.; Aizawa, M.; Namai, Y.; Sasaki, T.; Iwasawa, Y. First-Principles Theoretical Study and Scanning Tunneling Microscopic Observation of De-hydration Process of Formic Acid on a TiO2(110) Surface. J. Phys. Chem. B 2004, 108, 14446−14451. (47) Uemura, Y.; Taniike, T.; Tada, M.; Morikawa, Y.; Iwasawa, Y. Switchover of Reaction Mechanism for the Catalytic Decomposition of HCOOH on a TiO2(110) Surface. J. Phys. Chem. C 2007, 111, 16379− 16386. (48) Sayago, D. I.; Polcik, M.; Lindsay, R.; Toomes, R. L.; Hoeft, J. T.; Kittel, M.; Woodruff, D. P. Structure Determination of Formic Acid Reaction Products on TiO2(110). J. Phys. Chem. B 2004, 108, 14316−14323. (49) Lindsay, R.; Tomić, S.; Wander, A.; García-Méndez, M.; Thornton, G. Low Energy Electron Diffraction Study of TiO2(110)(2 × 1)-[HCOO]−. J. Phys. Chem. C 2008, 112, 14154−14157. (50) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864−B871. (51) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133− A1138. (52) Aradi, B.; Hourahine, B.; Frauenheim, T. DFTB+, a Sparse Matrix-Based Implementation of the DFTB Method. J. Phys. Chem. A 2007, 111, 5678−5684. (53) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. 1993, 47, 558−561.

(11) Raghunath, P.; Lin, M. C. Adsorption Configurations and Decomposition Pathways of Boric Acid on TiO2 Rutile (110) Surface: A Computational Study. J. Phys. Chem. C 2009, 113, 3751−3762. (12) Raghunath, P.; Lin, M. C. A Computational Study on the Adsorption Configurations and Reactions of Phosphorous Acid on TiO2 Anatase (101) and Rutile (110) Surfaces. J. Phys. Chem. C 2009, 113, 8394−8406. (13) Luschtinetz, R.; Frenzel, J.; Milek, T.; Seifert, G. Adsorption of Phosphonic Acid at the TiO2 Anatase (101) and Rutile (110) Surfaces. J. Phys. Chem. C 2009, 113, 5730−5740. (14) Kieu, L.; Boyd, P.; Idriss, H. Trends within the Adsorption Energy of Alcohols over Rutile TiO2(110) and (011) Clusters. J. Mol. Catal. A: Chem. 2002, 188, 153−161. (15) Chang, J.-G.; Wang, J.; Lin, M. C. Adsorption Configurations and Energetics of BClx (x = 0 − 3) on TiO2 Anatase (101) and Rutile (110) Surfaces. J. Phys. Chem. A 2007, 111, 6746−6754. (16) Ashima, H.; Chun, W.-J.; Asakura, K. Room-TemperatureAdsorption Behavior of Acetic Anhydride on a TiO2(110) Surface. Surf. Sci. 2007, 601, 1822−1830. (17) Sorescu, D. C.; Lee, J.; Al-Saidi, W. A.; Jordan, K. D. Coadsorption Properties of CO2 and H2O on TiO2 Rutile (110): A Dispersion-Corrected DFT Study. J. Chem. Phys. 2012, 137, 074704. (18) Scheiber, P.; Riss, A.; Schmid, M.; Varga, P.; Diebold, U. Observation and Destruction of an Elusive Adsorbate with STM: O2/ TiO2(110). Phys. Rev. Lett. 2010, 105, 216101. (19) Li, S.-C.; Chu, L.-N.; Gong, X.-Q.; Diebold, U. Hydrogen Bonding Controls the Dynamics of Catechol Adsorbed on a TiO2(110). Surf. Sci. 2010, 328, 882−884. (20) Aschauer, U.; Selloni, A. Structure of the Rutile TiO2(011) Surface in an Aqueous Environment. Phys. Rev. Lett. 2011, 106, 166102. (21) Tao, J.; Cuan, Q.; Gong, X.-Q.; Batzill, M. Diffusion and Reaction of Hydrogen on Rutile TiO2(011)-2 × 1: The Role of Surface Structure. J. Phys. Chem. C 2012, 116, 20438−20446. (22) Hayden, B. E.; King, A.; Newton, M. A. Fourier Transform Refiection−Absorption IR Spectroscopy Study of Formate Adsorption on TiO2(110). J. Phys. Chem. B 1999, 103, 203−208. (23) Guo, Q.; Cocks, I.; Williams, E. M. The Orientation of Acetate on a TiO2(110) Surface. J. Chem. Phys. 1997, 106, 2924−2931. (24) Gutiérrez-Sosa, A.; Martínez-Escolano, P.; Raza, H.; Lindsay, R.; Wincott, P. L.; Thornton, G. Orientation of Carboxylates on TiO2(110). Surf. Sci. 2001, 471, 163−169. (25) Käckell, P.; Terakura, K. First-Principle Analysis of the Dissociative Adsorption of Formic Acid on Rutile TiO2(110). Appl. Surf. Sci. 2000, 166, 370−375. (26) Foster, A. S.; Nieminen, R. M. Adsorption of Acetic and Trifluoroacetic Acid on the TiO2(110) Surface. J. Chem. Phys. 2004, 121, 9039−9042. (27) Ferris, K. F.; Wang, L.-Q. Electronic Structure Calculations of Small Molecule Adsorbates on (110) and (100) TiO2. J. Vac. Sci. Technol., A 1998, 16, 956−960. (28) Lyubinetsky, I.; Deskins, N. A.; Du, Y.; Vestergaard, E. K.; Kim, D. J.; Dupuis, M. Adsorption States and Mobility of Trimethylacetic Acid Molecules on Reduced TiO2(110) Surface. Phys. Chem. Chem. Phys. 2010, 12, 5986−5992. (29) Roddick-Lanzilotta, A. D.; McQuillan, A. An in situ Infrared Spectroscopic Study of Glutamic Acid and of Aspartic Acid Adsorbed on TiO2: Implications for the Biocompatibility of Titanium. J. Colloid Interface Sci. 2000, 227, 48−54. (30) Lausmaa, J.; Lö fgren, P.; Kasemo, B. Adsorption and Coadsorption of Water and Glycine on TiO2. J. Biomed. Mater. Res. 1999, 44, 227−242. (31) Thomas, A. G.; Flavell, W. R.; Chatwin, C. P.; Kumarasinghe, A. R.; Rayner, S. M.; Kirkham, P. F.; Tsoutsou, D.; Johal, T. K.; Patel, S. Adsorption of Phenylalanine on Single Crystal Rutile TiO2(110) Surface. Surf. Sci. 2007, 601, 3828−3832. (32) Fleming, G. J.; Adib, K.; Rodriguez, J. A.; Barteau, M. A.; White, J. M.; Idriss, H. The Adsorption and Reactions of the Amino Acid 10778



Article

(54) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B: Condens. Matter Mater. 1994, 49, 14251−14269. (55) Kresse, G.; Furthmüller, J. Effcient Iterative Schemes for ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. 1996, 54, 11169−11186. (56) Kresse, G.; Furthmüller, J. Effciency of ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (57) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. 1994, 50, 17953−17979. (58) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. 1999, 59, 1758−1775. (59) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (60) Perdew, J. P.; Burke, K.; Ernzerhof, M. Errata: Generalized Gradient Approximation Made Simple [Phys. Rev. Lett. 77, 3865 (1996)]. Phys. Rev. Lett. 1997, 78, 1396−1396. (61) Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E.; Constantin, L. A.; Zhou, X.; Burke, K. e. Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces. Phys. Rev. Lett. 2008, 100, 136406. (62) Makov, G.; Payne, M. C. Periodic Boundary Conditions in ab Initio Calculations. Phys. Rev. B: Condens. Matter Mater. 1995, 51, 4014−4022. (63) Neugebauer, J.; Scheffer, M. Adsorbate-Substrate and Adsorbate-Adsorbate Interactions of Na and K Adlayers on Al(111). Phys. Rev. B: Condens. Matter Mater. 1992, 46, 16067−16080. (64) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B: Condens. Matter Mater. 1976, 13, 5188− 5192. (65) Frauenheim, T.; Seifert, G.; Elstner, M.; Niehaus, T.; Köhler, C.; Amkreutz, M.; Sternberg, M.; Hajnal, Z.; Di Carlo, A.; Suhai, S. Atomistic Simulations of Complex Materials: Ground-State and Excited-State Properties. J. Phys.: Condens. Matter 2002, 14, 3015. (66) Koskinen, P.; Mäkinen, V. Density-Functional Tight-Binding for Beginners. Comput. Mater. Sci. 2009, 47, 237−253. (67) Elstner, M.; Porezag, D.; Jungnickel, G.; Elsner, J.; Haugk, M.; Frauenheim, T.; Suhai, S.; Seifert, G. Self-Consistent-Charge DensityFunctional Tight-Binding Method for Simulations of Complex Materials Properties. Phys. Rev. B: Condens. Matter Mater. 1998, 58, 7260−7268. (68) Dolgonos, G.; Aradi, B.; Moreira, N. H.; Frauenheim, T. An Improved Self-Consistent-Charge Density-Functional Tight-Binding (SCC-DFTB) Set of Parameters for Simulation of Bulk and Molecular Systems Involving Titanium. J. Chem. Theory Comput. 2010, 6, 266− 278. (69) Burdett, J. K.; Hughbanks, T.; Miller, G. J.; Richardson, J. W.; Smith, J. V. Structural−Electronic Relationships in Inorganic Solids: Powder Neutron Diraction Studies of the Rutile and Anatase Polymorphs of Titanium Dioxide at 15 and 295 K. J. Am. Chem. Soc. 1987, 109, 3639−3646. (70) Lazzeri, M.; Vittadini, A.; Selloni, A. Structure and Energetics of Stoichiometric TiO2 Anatase Surfaces. Phys. Rev. B: Condens. Matter Mater. 2001, 63, 155409. (71) Ramamoorthy, M.; Vanderbilt, D.; King-Smith, R. D. FirstPrinciples Calculations of the Energetics of Stoichiometric TiO2 Surfaces. Phys. Rev. B: Condens. Matter Mater. 1994, 49, 16721−16727. (72) Perron, H.; Domain, C.; Roques, J.; Drot, R.; Simoni, E.; Catalette, H. Optimisation of Accurate Rutile TiO2 (110), (100), (101) and (001) Surface Models from Periodic DFT Calculations. Theor. Chem. Acc. 2007, 117, 565−574. (73) Hallil, A.; Amzallag, E.; Landron, S.; Tétot, R. Properties of Rutile TiO2 Surfaces from a Tight-Binding Variable-Charge Model. Comparison with ab Initio Calculations. Surf. Sci. 2011, 605, 738−745. (74) Labat, F.; Baranek, P.; Adamo, C. Structural and Electronic Properties of Selected Rutile and Anatase TiO2 Surfaces: An ab Initio Investigation. J. Chem. Theory Comput. 2008, 4, 341−352.

(75) Gong, X.-Q.; Khorshidi, N.; Stierle, A.; Vonk, V.; Ellinger, C.; Dosch, H.; Cheng, H.; Selloni, A.; He, Y.; a Dulub, O.; Diebold, U. The 2 × 1 Reconstruction of the Rutile TiO2(011) Surface: A Combined Density Functional Theory, X-Ray Diffraction, and Scanning Tunneling Microscopy Study. Surf. Sci. 2009, 603, 138−144. (76) Muscat, J.; Harrison, N. M.; Thornton, G. Effects of Exchange, Correlation, and Numerical Approximations on the Computed Properties of the Rutile TiO2 (100) Surface. Phys. Rev. B: Condens. Matter Mater. 1999, 59, 2320−2326. (77) Stroppa, A.; Kresse, G. The Shortcomings of Semi-Local and Hybrid Functionals: What We Can Learn from Surface Science Studies. New J. Phys. 2008, 10, 063020. (78) Schimka, L.; Harl, J.; Stroppa, A.; Grüneis, A.; Marsman, M.; Mittendorfer, F.; Kresse, G. Accurate Surface and Adsorption Energies from Many-Body Perturbation Theory. Nat. Mater. 2010, 9, 741−744. (79) Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272−1276. (80) Fan, W.; Tan, D.; Deng, W. Theoretical Investigation of Triphenylamine Dye/Titanium Dioxide Interface for Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2011, 13, 16159−16167. (81) Negre, C. F. A.; Fuertes, V. C.; Oviedo, M. B.; Oliva, F. Y.; Sánchez, C. G. Quantum Dynamics of Light-Induced Charge Injection in a Model Dye−Nanoparticle Complex. J. Phys. Chem. C 2012, 116, 14748−14753. (82) Ducasse, L.; Castet, F.; Méreau, R.; Nénon, S.; Idé, J.; Toupance, T.; Olivier, C. Structure and Absorption Properties of the C212 Dye Chemisorbed onto the TiO2 (101) Anatase Surface. Chem. Phys. Lett. 2013, 556, 151−157. (83) Kaledin, A. L.; Lian, T.; Hill, C. L.; Musaev, D. G. Structural Modification of TiO2 Surfaces in Bulk Water and Binding Motifs of a Functionalized C60 on TiO2 Anatase and Rutile Surfaces in Vacuo and in Water: Molecular Dynamics Studies. J. Phys. Chem. C 2012, 116, 20937−20948. (84) Hellström, M.; Jorner, K.; Bryngelsson, M.; Huber, S. E.; Kullgren, J.; Frauenheim, T.; Broqvist, P. An SCC-DFTB Repulsive Potential for Various ZnO Polymorphs and the ZnO−Water System. J. Phys. Chem. C 2013, 117, 17004−17015. (85) Saha, S.; Sarkar, S.; Pal, S.; Sarkar, P. Tuning the Energy Levels of ZnO/ZnS Core/Shell Nanowires To Design an Effcient NanowireBased Dye-Sensitized Solar Cell. J. Phys. Chem. C 2013, 117, 15890− 15900.

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