Adsorption of Phosphonic Acid at the TiO2 Anatase (101) - American

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J. Phys. Chem. C 2009, 113, 5730–5740

Adsorption of Phosphonic Acid at the TiO2 Anatase (101) and Rutile (110) Surfaces Regina Luschtinetz,*,† Johannes Frenzel,‡ Theodor Milek,† and Gotthard Seifert† Physikalische Chemie, Technische UniVersita¨t Dresden, Dresden, Germany, and Department of Chemistry, UniVersity of Calgary, Calgary, Canada ReceiVed: December 15, 2008; ReVised Manuscript ReceiVed: February 3, 2009

The adsorption of phosphonic acid on the TiO2 anatase (101) and rutile (110) surfaces have been investigated by means of efficient density-functional-based tight-binding calculations. We studied the geometries and adsorption energies of several adsorption models to achieve clarification of the discrepancy in the experimental finding of a preferred binding state. In this paper we show that there are several adsorption structures likely to be present on the specific TiO2 surfaces. Those structures have exclusively a bidentate configuration. They have similar adsorption energies but different geometries. For the monodentate complexes, we find a strong trend of the adsorption geometry relaxing toward the bidentate coordination. Also, they have significantly smaller adsorption energies. Furthermore, we extensively demonstrate the reliability of the SCC-DFTB method for this chemical system, which opens the way for studies of adsorption on more complex titania materials. Introduction The modification and functionalization of TiO2 surfaces by covalent attachment of functional organic molecules is of great interest in view of their applications in biomaterials,1-5 catalysis,6-8 photoelectrochemistry,7,9-14 corrosion protection,15 or optoelectronic devices.16 Depending on the nature of the TiO2 surface and the field of potential application, different families of functional organic molecules have been investigated in recent years. The most common anchoring molecules in the coupling of organic components to metal oxides are organosilanes or carboxylic acids.17-23 The latter are especially applied in dye-sensitized solar cells to anchor the dyes on nanocrystalline TiO2 electrodes.9,10,13,24,25 The following are a few examples of advantages and problems of using this functional unit as an anchoring group: Carboxylic acids are highly efficient in terms of good interfacial electronic coupling.26 However, they slowly desorb from the semiconductor surface in the presence of water.24,27,28 The problem of the long-term stability might be also ascribed to the photocatalytic activity of anatase to oxidize carboxylic acids in the presence of water and ultraviolet rays.29 Furthermore, bifunctional long-chain carboxylic acids tend to form undesirable looping structures.17,30,31 Phosphonic acid as an anchoring group offers a promising alternative and has attracted growing interest due to its high affinity toward metal oxide surfaces.32-39 Indeed, experimental studies have indicated that strong and stable P-O-Ti bonds are formed during adsorption of phosphonic acid on TiO2 surfaces.38,40-46 It has been also shown that phosphonic acid binds stronger to TiO2 than carboxylic acid.42,47 Therefore it gives better long-term stability of solar cells.11,48-50 To model the adsorption of carboxylic acids on TiO2 surfaces, the bonding of formic acid on both rutile (110) and anatase (101) surfaces has been intensively studied both experimentally51-62 and theoretically.47,62-69 A dissociated bridging bidentate geometry is the most stable adsorption structure of formic acid * E-mail: [email protected]. † Technische Universita¨t Dresden. ‡ University of Calgary.

on rutile (110).51-53,61,64-66,68,69 Concerning the adsorption of formic acid on anatase (101), a molecular monodentate or a bridging bidentate geometry is observed depending on the experimental conditions or the theoretical model.47,54,55,63,70 By contrast, the exact binding state (mono-, bi-, or tridentate) of the phosphonic acid coupling group to the TiO2 surface is not easy to assess experimentally and is still much debated.38,40-46,71 On the basis of FTIR spectroscopy, the tridentate binding is proposed to be the most likely adsorption structure of organophosphonic acids on TiO2 due to the absence of the P-OH and PdO absorption bands.41 However, this result has been challenged, since the absorption band of TiO2 occurs in the same region.38,42,71 Also, the absorption bands of the different P-O stretching modes greatly overlap and depend on the degree of hydrogen bonding or metal binding.38 Indeed, it has been found experimentally that hydrogen bond interactions are involved in the assembly and surface attachment of phosphonic acids on TiO2.46,71 On the basis of NMR spectroscopy, a mixture of bi- and tridentate-bound phosphonic acids is proposed to be present on TiO2. Besides signals ascribed to tridentate-bound phosphonate species, some spectroscopic evidence has been found for incomplete deprotonation of the phosphonic acid pointing toward a bidentate configuration.38,41,44,45,71 The presence of biand tridentate-linked phosphonic acids on TiO2 has been also found recently by X-ray diffraction characterization.72 Nilsing et al. have studied the adsorption of phosphonic acid on the surface of anatase (101) and rutile (110) using periodic boundary conditions within hybrid ab initio Hartree-Fock density-functional theory calculations.47,68 They have considered adsorption models including nondissociated and single dissociated phosphonic acid groups. According to their results, the phosphonic acid binds to the anatase (101) and rutile (110) surfaces in foremost a monodentate binding mode. In this paper we investigate the geometry and energetics of the adsorption of phosphonic acid (PA, HPO(OH)2) on titanium dioxide surfaces on the basis of a highly efficient quantum mechanical approach. We study the properties of various mono-, bi-, and tridentate adsorption models of phosphonic acid on anatase (101) and rutile (110) surfaces using a self-consistent

10.1021/jp8110343 CCC: $40.75  2009 American Chemical Society Published on Web 03/18/2009

Adsorption of Phosphonic Acid on TiO2 Surfaces

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charge density-functional-based tight-binding method (SCCDFTB). For the purpose of comparison, we closely align our study to the work of Nilsing et al.47,68 but extend our investigation to some more adsorption complexes especially those involving fully dissociated phosphonic acid molecules (HPO32-). Besides the clarification of the discrepancy in the experimental finding of a preferred binding state, one objective of this work was to test the reliability of the SCC-DFTB method to describe the adsorption of phosphonic acid on TiO-based surfaces using rather small test systems. Since the efficiency of the SCC-DFTB method allows models with some thousands of atoms, this paper will open the way for studies of adsorption on more complex titania materials.73 Our results will also extend the experimental and theoretical understanding of these materials in view of potential technological applications.

Figure 1. Unit cells of (a) rutile and (b) anatase bulk structures. Gray and red spheres are Ti and O atoms, respectively.

Computational Details DFTB Method. All calculations have been performed using a self-consistent charge density-functional-based tight-binding (SCC-DFTB) method.74-76 This method has been applied successfully to calculate properties of large molecules and clusters as well as periodic solids with high efficiency and good agreement with results obtained by ab initio calculations on the basis of density-functional theory (DFT) at much lower computational costs.77 In the following we give only a brief overview of the SCCDFTB method. For more details, see refs 74-76. The SCC-DFTB method is based on the density-functional theory of Hohenberg and Kohn in the formulation of Kohn and Sham.78,79 The single-particle Kohn-Sham eigenfunctions φi are expanded in a set of localized atom-centered basis functions ηµ. These functions are determined by self-consistent densityfunctional calculations on the isolated atoms R including an additional confining harmonic potential in the atomic Kohn-Sham equations. The effective one-electron potential in the Kohn-Sham Hamilton matrix is approximated as a superposition of the atomic potentials of the corresponding neutral atoms. Only oneand two-center integrals are calculated to set up the Hamiltonian matrix, but all matrix elements are calculated exactly within the Kohn-Sham basis, and none is determined through fitting to experimental results. The Hamilton and overlap matrices 0 ˆ [VR + Vβ]|ην〉 Hµν ) 〈ηµ |H

(1)

Sµν ) 〈ηµ |ην〉

(2)

are calculated for all combinations of orbitals ηµ and ην on atoms R and β. The total energy of the system within the SCC-DFTB approximation is written as

Etot )

∑ cµi ciνHµν0 + 21 ∑ γRβ∆qR∆qβ + 21 ∑ URβ iµν

Rβ

(3)

Rβ

where γRβ represents an effective damped Coulomb interaction between the atomic charges ∆qR and ∆qβ. URβ is a repulsive two-body potential between the atoms R and β. It is determined by comparison to the total energy from parameter-free densityfunctional calculations using suitable reference systems.

From the energy functional (eq 3), a generalized eigenvalue problem is derived, which has to be solved iteratively for the coefficients cµi , because the Hamilton matrix elements depend on the cµi via the atomic charges:

1 0 Hµν ) Hµν + Sµν 2

∑ ∆qδ(γRδ + γβδ)

(4)

δ

Practical Realization of the DFTB Method. For our calculations we employed the SCC-DFTB method as implemented in the dftb+ program.80 We used a minimal atomic valence basis for all atoms including 3d functions for titanium and phosphorus. The electrons in the core levels were treated within a frozen-core approximation. For the atomic calculations the local-density approximation (LDA)81 has been applied. For the contraction radii r0 of the atomic potential, we used the double covalent radius of the atoms. This is 5.0, 3.8, 2.7, and 1.3 au for the atoms Ti, P, O, and H, respectively. The Hubbard U (atomic hardness) parameters were calculated following ref 75: UTi ) 0.3545 au, UP )0.2894 au, UO ) 0.4954 au, and UH ) 0.4195 au. The short-range repulsive pair-potentials URβ in eq 3 have been fitted to results from DFT calculations. For UTiO we used the unit cells of rutile and anatase representations as reference structures and fitted them to reproduce atomic positions, lattice parameters, and relative energies. The repulsive potentails UTiH, UTiP, and UTiTi were fitted using small molecules as reference systems, TiH4, Ti-P, and bulk Ti, respectively. For all DFT reference calculations, we used the PBE parametrization for the exchange correlation potential.82 The bulk structures were calculated using the program Siesta83 applying norm-conserving Trouiller-Martins pseudopotentials. The molecular structures were calculated with the deMon code84 using a DZVP basis set and a gen-A2* auxiliary-basis set. For the P-O-H interactions, bond lengths and angles of a set of phosphonic acids are compared to DFT reference calculations, which are reported elsewhere.85 Further details on the accuracy of the DFTB parameters for the TiO2 and PA system are given in the section where we present our results. We note that this new set of SCC-DFTB parameters86 is compatible with previous ones generated for aluminum oxide, aluminum hydroxide, and aluminum silicon hydroxide.87,88 Modeling of the Structures. We optimized the atomic positions for all structures and additionally the lattice parameters in the case of the bulk structures using the conjugate-gradient

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Figure 3. TiO2 anatase (101) surface model: Optimized atomic positions in one unit cell of the slab model with a ) 7.618 76 Å in [010], b ) 10.453 45 Å in [1j01] and ≈9 Å in [101] directions. Gray and red spheres are Ti and O atoms, respectively, with different individual atomic species. Along the [010] direction O-Ti-O double chains are connected by rows of twofold-coordinated bridging O atoms (O-2c). Half fivefold- (Ti-5c) and half sixfold-coordinated (Ti-6c) Ti atoms are present, as well as threefold-coordinated O atoms (O-3c).

Eads ) Etot[TiO2] + 4*Etot[PA] - Etot[PA@TiO2] (5) Figure 2. TiO2 rutile (110) surface model: Optimized atomic positions in one unit cell of the slab model with a ) 5.9390 Å in [001], b ) 13.0421 Å in [11j0] and ≈16 Å in [110] directions. Gray and red spheres are Ti and O atoms, respectively, with different individual atomic species. Along the [001] direction, rows of sixfold-coordinated Ti atoms (Ti-6c) alternate with fivefold-coordinated Ti atoms (Ti-5c) with one dangling bond perpendicular to the surface. On the surfaces rows of threefold-coordinated O atoms (O-3c) connect the chains of Ti-6c and Ti-5c atoms, and the twofold-coordinated (O-2c) bridging O atoms connect surface Ti-6c atoms above the surface plane.

algorithm until the root-mean-square of the residual force was below 10- 4 hartree/bohr. Additionally, we used the simulated annealing technique to identify the stability of the local minima of the adsorption complex structures. Each of the corresponding molecular dynamics simulations used a velocity verlet algorithm with a total of 3000 time steps at a 0.25 fs interval. The NVT ensemble was used applying the Anderson thermostat.89 The temperature profile included a linear heating from 0 to 600 K within 500 time steps, a 2000 time step sampling at 600 K, and a final exponential cooling to 10 K. Furthermore, periodic boundary conditions and the Γ-point approximation were used for the bulk, surface slabs, and adsorption model structures. The symmetric and periodic slabs were modeled based on the optimized bulk structures, separated by 40 Å perpendicular to the surface to avoid self-interaction of the slabs. To minimize artificial dipoles, the adsorbate was added on both sides of the slabs. In detail, the rutile (110) surface model contains 40 TiO2 formula units in a 2 × 2 orthogonal surface slab in [001] and [11j0] directions and 5 layers of Ti2O4 units in the direction perpendicular to the surface; cf. Figure 2. The anatase (101) surface model contains 24 TiO2 formula units in a 2 × 1 slab in [010] and [1j01] directions and 18 layers of different atomic coordinate values along the direction perpendicular to the surface; cf. Figure 3. The adsorption energy is calculated from the difference in energy of the total energies of the adsorption complex, PA@TiO2, and the clean anatase or rutile surface slab, TiO2, plus four times the total energy of a phosphonic acid molecule in the gas phase, PA:

A positive value thus indicates stable adsorption. Results Rutile and Anatase Bulk and Surface Structures. Rutile and anatase are the most important crystallographic structures of titanium dioxide under ambient conditions.90 Rutile is the thermodynamically most stable phase. Its (110) surface has the lowest surface energy90,91 and is the most studied single-crystal TiO2 surface.92 In case of anatase, the (101) face is the thermodynamically most stable surface.90,93 The final atomic positions and lattice parameters of the optimized bulk structures, cf. Figure 1, are all within 1.5% for rutile and 2.5% for anatase compared to experimental values; cf. Table 1. These results are within or in some cases even better than those results obtained by other investigations using full ab initio Hartree-Fock (HF) or density-functional theory (DFT) methods.94,95 The calculated band gaps are 3.14 and 3.22 eV for rutile and anatase, respectively. They are in very good agreement with the experimental values of 3.0 eV for rutile96 and 3.2 eV for anatase,97 and they are larger than the respective band gaps usually obtained by DFT methods95,98-101 The rutile (110) surface remains stable in the full-geometry optimization; cf. Figure 2. In Table 2 we have listed the displacements of the atomic positions in the first two Ti2O4 layers with respect to the bulk system. The notation of the atoms follows that in Figure 2. The main displacements occur in the first Ti2O4 layer perpendicular to the surface, while those in the second Ti2O4 layer are much smaller. In agreement with the experiment,105 the Ti-6c(1) atoms relax outward considerably by 0.27 Å. The undercoordinated Ti-5c(2) atoms move inward by 0.11 Å and the neighboring O-3c(4,5) atoms outward by 0.22 Å. Only the bridging O-2c(3) atoms were calculated to relax outward by 0.10 Å, while they are found to relax inward in experiment by 0.27 Å.105 However, also other theoretical methods result in too small or almost no displacement for these O atoms (-0.02 Å to -0.16 Å).91,106,107 The atomic displacements of our anatase (101) surface model have been compared with other theoretical investigations, as



TABLE 1: Structural Parameters of Bulk Rutile and Anatase: Comparison of SCC-DFTB (This Work) with Other Experimental and Theoretical Resultsa method

a

c

dax

deq

exp.96 this work LDA94 PBE94 BLYP94 HF95 B3LYP95 PBE095

4.587 4.611(+0.5%) 4.546(-0.9%) 4.634(+1.0%) 4.679(+2.0%) 4.575(-0.3%) 4.639(+1.1%) 4.591(+0.1%)

rutile 2.954 2.970(+0.5%) 2.925(-1.0%) 2.963(+0.3%) 2.985(+1.0%) 2.987(+1.1%) 2.974(+0.7%) 2.955(+0.0%)

1.976 1.967(-0.5%) 1.952(-1.2%) 1.999(+1.2%) 2.021(+2.3%) 1.985(+0.5%) 2.013(+1.9%) 1.989(+0.7%)

1.946 1.969(+1.2%) 1.947(+0.1%) 1.954(+0.4%) 1.940(-0.3%)

exp.96 this work LDA94 PBE94 BLYP94 HF95 B3LYP95 PBE095

3.782 3.809(+0.7%) 3.735(-1.2%) 3.786(+0.1%) 3.828(+1.2%) 3.771(-0.3%) 3.783(+0.0%) 3.758(-0.6%)

anatase 9.502 9.735(+2.5%) 9.534(+0.3%) 9.737(+2.5%) 9.781(+2.9%) 9.688(+2.0%) 9.805(+3.2%) 9.704(+2.1%)

1.979 1.995(+0.8%) 1.973(-0.3%) 2.002(+1.2%) 2.014(+1.8%) 1.976(-0.2%) 2.000(+1.1%) 1.980(+0.1%)

1.932 1.955(+1.2%) 1.937(+0.3%) 1.945(+0.7%) 1.931(-0.1%)

a The latter have been obtained by ab initio HF calculations and by DFT calculations. Within DFT the different exchange-correlation functionals LDA,81,102 PBE,82 and BLYP103,104 have been used. The results of two hybrid HF/DFT functionals, B3LYP and PBE0, have been also included. The difference between calculated and experimental determined values are given as a percentage in parentheses. The lengths are given in angstrom.

TABLE 2: TiO2 Rutile (110) Surface Model: Calculated Displacements (in Å), Normal to the Surface, Using SCC-DFTB (This Work) in Comparison to Experiment (exp.) and Other Theoretical Modelsa this work

exp.105

FP-LAPW106

LCAO106

PW-PP-LDA91

PW-GGA107

0.27 -0.11 0.10 0.22 0.07 0.12 -0.05 0.01 0.04 0.01

0.12 ( 0.05 -0.16 ( 0.05 -0.27 ( 0.08 0.05 ( 0.05 0.03 ( 0.08 0.07 ( 0.04 -0.09 ( 0.04 0.00 ( 0.08 0.02 ( 0.06 -0.09 ( 0.08

0.08 -0.23 -0.16 0.09 -0.09 0.07 -0.13 -0.05 -0.04 -0.04

0.23 -0.17 -0.02 0.03 0.02 0.14 -0.10 0.00 0.03 -0.01

0.13 -0.17 -0.06 0.12 -0.07 0.06 -0.08 0.02 -0.03 -0.01

0.23 -0.11 -0.02 0.18 0.03 0.12 -0.06 0.03 0.00 0.03

Ti-6c(1) Ti-5c(2) O-2c(3) O-3c(4) O-3c(5) Ti-6c(6) Ti-6c(7) O-3c(8) O-3c(9) O-3c(10)

a FP-LAPW (full-potential linear augmented plane wave), LCAO (linear combination of atomic orbitals), and PW-PP (plane-wave pseudopotential), LDA (local density approximation), and GGA (generalized gradient approximation). The notation of the atoms follows that in Figure 2.

TABLE 3: TiO2 Anatase (101) Surface Model: Calculated Displacements (in Å), Normal to the Surface, Using SCC-DFTB (This Work) and Other Theoretical Modelsa O-2c(1) Ti-5c(2) O-3c(3) O-3c(4) Ti-6c(5) O-3c(6) O-3c(7) Ti-6c(8) O-3c(9)

this work

LDA94

GGA93

EIP108

GGA109

-0.04 -0.17 0.20 0.03 0.13 -0.15 0.07 -0.10 -0.06

-0.02 -0.18 0.15 0.06 0.20 -0.07 0.04 -0.14 -0.04

-0.06 -0.17 0.21 0.11 -

-0.08 -0.02 0.15 0.03 -

0.06 -0.12 0.28 -

a The notation of the atoms follows that in Figure 3. Empirical Interatomic Potentials (EIP), Plane-Wave Pseudopotentials Together with the Local Density Approximation (LDA) or the Generalized Gradient Approximation (GGA).

depicted in Figure 3 and summarized in Table 3. The notation of the atoms follows that in Figure 3. Upon relaxation the O-3c(3) atoms move outward by 0.20 Å, whereas the O-2c(1) atoms and the Ti-5c(2) atoms move inward by 0.04 and 0.17 Å, respectively. The Ti-6c(5) atoms relax outward by 0.13 Å. Thus the surface exhibits a slightly buckled geometry and

reproduces previous calculations using other theoretical methods.93,94,108,109 Adsorption of Water. To test further the reliability of our surface models, we have investigated the molecular and dissociative adsorption of water on the surfaces of rutile (110) and anatase (101). In the case of molecular adsorption, the oxygen of the nondissociated H2O molecule is coordinated by its O atom to a Ti-5c surface atom. In case of dissociative adsorption, the OH- group of the dissociated H2O molecule is attached by its O atom to a Ti-5c atom and the proton is bound to a bridging O-2c atom on the surface. In our model there were two H2O molecules on the top and on the bottom side of the surface slab corresponding to 0.5 monolayer coverage. The optimized structures of the molecular and dissociative adsorption complexes of water on the surfaces of rutile (110) and anatase (101) are shown in Figure 4. The calculated adsorption energies of molecularly adsorbed water are 70.5 and 83.4 kJ/mol for anatase (101) and rutile (110), respectively. The calculated adsorption energy for dissociative adsorption of water is 18.2 kJ/mol for anatase (101) and 48.5 kJ/mol for rutile (110).


Luschtinetz et al.

Figure 4. Optimized structures of molecular (a, b) and dissociative (c, d) adsorbed water on rutile (110). Optimized structures of molecular (e, f) and dissociative (g, h) adsorbed water on anatase (101). The bond lengths are given in angstrom. Dotted lines indicate hydrogen bonds. Gray, red, and white spheres are Ti, O, and H atoms, respectively.

TABLE 4: Calculated Adsorption Energies (kJ/mol) for Molecular (mol) and Dissociative (diss) Adsorption of Water on the Surface of Anatase (101) and Rutile (110) in Comparison to Experiment and Other Theoretical Results Using DFT-Based Methods method

mol

diss

this work theor.116 theor.119 theor.121 exp.111-113 exp.113

rutile (110) 83.4 95.5 82.1 77.1 71-80 59-101

48.5 87.8 58.9 -

this work theor.93 theor.120 theor.121 exp.110

anatase (101) 70.5 71.4 68.5 59.9 48-68

18.2 22.2 36.7 -

These results are in good agreement with the experimental ranges and with those results obtained by other theoretical investigations using DFT-based methods; cf. Table 4. Moreover, on both surfaces molecular adsorption of water is clearly favored in comparison to dissociative adsorption, which has been also found by experiment90,110-115 and other theoretical studies.93,116-120 Phosphonic Acid Adsorption. The chemically active sites on the surfaces of anatase (101) and rutile (110) are the unsaturated and pairwise arranged acidic cation (Ti-5c atoms) and basic anion sites (O-2c atoms).122,123 In our adsorption models there is one PA molecule per two acidic-basic pairs of Ti-5c and O-2c. This corresponds to grafting densities of 2.51 and 2.58 molecules/nm2 for anatase (101) and rutile (110), respectively, which are in agreement to thosedeterminedexperimentally(2.5-4.8molecules/nm2).22,40,41,45,124 There are two possibilities to classify PA molecules adsorbed on the TiO2 surface. The first one counts the number of covalent bonds between the adsorbate and the metal atoms of the substrate. This can be either monodentate, bidentate, or tridentate. The second possibility describes the chemistry of the adsorbate in the adsorption process. This can be molecular (nondissociative) and/or dissociative. In the molecular process

the phosphonic acid is adsorbed on the surface as complete, e.g., nondissociated molecule. This is only possible for the monodentate adsorption, where the double-bound phosphoryl oxygen is coordinated to a surface Ti atom. In case of dissociative adsorption, the phosphonic acid dissociates into a phosphonic acid anion (H2PO-3 , HPO23 -) and one or two surfacebound protons. The phosphonic acid anion is adsorbed on the surface by bonding of the one, two, or three available adsorbate O atoms to surface Ti atoms. Thus, in the dissociative process all three binding states are possible. Monodentate Adsorption. For molecular monodentate adsorption on the anatase (101) surface, three different, but chemically similar, complexes can be built as starting structures.47 These are denoted as M1-A, M2-A, and M3-A; cf. Figure 5a-f. Adsorbing PA on the rutile (110) surface in a molecular monodentate manner offers two different possible structures, M1-R and M2-R; cf. Figure 6a-d. In all cases the double-bound phosphoryl oxygen of the PA is coordinated to a surface Ti-5c atom. The OH groups of the PA are free to form hydrogen bonds to two surface O-2c surface atoms. We illustrate our optimized structures of the monodentate adsorption complexes in Figures 5g-l and Figure 6e-h. The geometrical details and calculated adsorption energies are summarized in Tables 5, 6 and 7, respectively. Contrary to the results of Nilsing et al.,47,68 where all complexes were stable, only structure M1-A remains its initial geometry during our optimization and simulated annealing. The final structure of the adsorbed PA in this complex is very similar to that of the symmetric resonance-stabilized HPO32 -, which has a P-O bond length of 1.57 Å. This feature is also present in the optimized structure of complex M2-R. However, the structure of M2-R changes to a bidentate adsorption state. In the optimized complexes M2-A, M3-A, and M1-R, the PA remains bound to the substrate, but with significantly changed structure compared to the starting models. For M2-A and M1-R, the adsorption geometries change from the initial molecular monodentate into stable dissociative bidentate ones. Also, the phosphoryl oxygen (PdO) is not involved in the surface bonding and remains unaffected in their final structures.



Figure 5. Monodentate adsorption of HPO(OH)2 on the anatase (101) surface: Starting structures (a-f) and geometry-optimized structures (g-l). In the adsorption complexes M1-A and M2-A, the PA is coordinated to three and in M3-A to two different pairs of Ti-5c and O-2c. Dotted lines indicate hydrogen bonds. The Ti, O, P, and H atoms are represented by gray, red, yellow, and white spheres, respectively.

Figure 6. Monodentate adsorption of HPO(OH)2 on the rutile (110) surface: Starting structures (a-d) and geometry-optimized structures (e-h). In (a-d) the adsorption complex M1-R and M2-R differ in the two phosphonic OH groups forming hydrogen bonds to two surface O-2c atoms, which belong to the same row in the case of M1-R or to different rows in the case of M2-R. Dotted lines indicate hydrogen bonds. The Ti, O, P, and H atoms are represented by gray, red, yellow, and white spheres, respectively.

In the optimized structures of M1-A and M3-A, one or two H atoms of the adsorbate are each located between two O atoms, one belonging to the surface and one to the acid. The O-H bond distances to the surface and to the acid are similar (1.2-1.3 Å), but 20-30% longer than in the free PA (0.98-1.00 Å).

The stability of the optimized adsorption complexes has been tested using the simulated annealing technique. Whereas the optimized structures M1-A, M2-A, and M1-R are stable, the complexes M3-A and M2-R change to the dissociative bidendate structures B2-A and B2-R; cf. Figure 7k, l and Figure 8g, h.


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TABLE 5: Calculated Bond Distances (Å) of the Free HPO(OH)2 Molecule (PA) and of Different Adsorption Complexes of HPO(OH)2 at the Surface of Anatase (101) anatase labels

PA85

M1-A

M2-A

M3-A

B1-A

B2-A

B3-A

B4-A

P-H3 P-O1 P-O2 P-O3 O1-H1 O2-H2 O3-H1 O3-H2 O1-Ti O2-Ti O3-Ti H1-O H2-O

1.42 1.68 1.68 1.47 0.98 0.98 -

1.41 1.59 1.58 1.56 1.22 1.24 1.94 1.30 1.29

1.42 1.89 1.52 1.47 0.98 2.31 2.26 1.98 0.99

1.43 1.71 1.55 1.52 0.98 1.27 2.22 1.27

1.42 1.71 1.55 1.52 0.99 1.97 2.23 1.99 0.99

1.42 1.74 1.53 1.51 0.99 1.98 2.24 0.99

1.43 1.62 1.62 1.47 3.44 3.54 1.89 1.89 0.99 1.00

1.42 1.62 1.62 1.48 2.12 1.89 1.89 0.99 1.00

TABLE 6: Calculated Bond Distances (Å) of the Free HPO(OH)2 Molecule (PA) and of Different Adsorption Complexes of HPO(OH)2 at the Surface of Rutile (110) rutile 85

labels

PA

P-H3 P-O1 P-O2 P-O3 O1-H1 O2-H2 O3-H1 O3-H2 O1-Ti O2-Ti O3-Ti H1-O H2-O

1.42 1.68 1.68 1.47 0.98 0.98 -

M1-R

M2-R

B1-R

B2-R

1.42 1.86 1.52 1.47 0.98 2.79 2.27 2.04 1.00

1.42 1.55 1.64 1.57 1.30 1.31 2.27 1.96 1.25 1.26

1.41 1.72 1.54 1.52 1.00 2.00 2.24 1.83 1.00

1.42 1.62 1.62 1.48 2.24 2.24 1.90 1.90 1.00 1.00

TABLE 7: Adsorption Energies (kJ/mol) of the Optimized Different Adsorption Complexes of HPO(OH)2 at the Surface of Anatase (101) and Rutile (110) labels M1 M2 M3 B1 B2 B3 B4

anatase (101)

rutile (110)

189.372 257.117 127.656 260.519 260.028 272.946 276.827

266.452 227.013 263.330 285.557 -

Bidentate Adsorption. In the dissociative bidentate adsorption complexes on the anatase(101) and rutile (110) surfaces, two adsorbate oxygens link the adsorbate to the surface. In total four chemically similar bidentate structures can be constructed in the case of anatase (101), and we denote them B1-A, B2-A, B3-A, and B4-A; cf. Figure 7a-h. In Figure 8a-d we depict the two possible dissociative, bidentate adsorption complexes of PA on rutile (110), B1-R and B2-R. It is characteristic for all these bidentate structures that one or both of the OH groups of the PA are dissociated. While two O atoms of the PA bind to two different Ti-5c atoms, the protons bind to surface O-2c atoms. The bidentate complexes retained their initial adsorption features during optimization and also simulated annealing treatment. The final optimized structures are depicted in Figures 7i-p and 8e-h. The calculated bond distances and adsorption energies are summarized in Tables 5, 6, and 7. Only the initial hydrogen bonds of the structures B2-A and B3-A are not stable.

Tridentate Adsorption. A tridentate adsorption on clean TiO2 could only be realized via a fully deprotonated PA (HPO23 ) with its three O atoms binding to three different surface Ti-5c atoms. However, we find no stable product structure of this type of adsorption for the clean anatase (101) surface nor the clean rutile (110) surface. On both surfaces the rows of Ti5c atoms are separated by rows of projecting O-2c atoms. Thus the distance to a third coordinatively unsaturated Ti is too long and sterically hindered. The same result has been reported by Nilsing et al. recently.47,68 Discussion and Conclusions The structure M1-A is the only stable molecular monodentate adsorption complex having an adsorption energy of 189 kJ/mol. However, the optimized bidentate adsorption structures and the structures M2-A, M1-R, and M2-R are 40-100 kJ/mol more stable due to the formation of a second P-O-Ti surface bond. M3-A and M2-R change into the dissociative bidentate adsorption complexes B2-A and B2-R during simulated annealing treatment. Thus, we conclude that bidentate adsorption on the surface of TiO2 is more stable than monodentate adsorption. In our investigation the overall most stable adsorption complexes are the bidentate structures B4-A and B2-R on the surface of anatase (101) and rutile (110), respectively, having adsorption energies of 277 and 286 kJ/mol. In both optimized structures the adsorbed phosphonic acid is fully dissociated. The PdO bond is not involved in the surface bonding but forms hydrogen bonds to surface OH groups. For B4-A the gain in energy due to this hydrogen bonding is only 4 kJ/mol compared to the adsorption complex B3-A, whereas the gain in energy due to dissociation of the second P-OH groups is four times larger (16 kJ/mol) compared to model B1-A. The stabilization due to dissociation in the case of rutile is 26 kJ/mol, if one compares the adsorption energy of B1-R and B2-R. Thus, we conclude that dissociation of the phosphonic acid is somewhat more important for the surface bonding than the formation of hydrogen bonds. Also, it is energetically favored that the doublebound phosphoryl oxygen does not coordinate to a surface Ti5c atom and remains unaffected. One might argue that the energetic barrier for the OH dissociation and the proton transfer to the surface O-2c atoms have not been taken into account in our investigation. However, in many cases the dissociation of the first P-OH group, the P-O-Ti bond formation of the dissociated O atom to the surface, the transfer of the proton to surface O-2c atoms, and the formation of surface OH groups happen spontaneously during optimization of the monodentate adsorption complexes



Figure 7. Bidentate adsorption of HPO(OH)2 on the anatase (101) surface: Starting structures (a-h) and geometry-optimized structures (i-p). In the structures B1-A and B2-A, the double-bond phosphoryl O atom and one dissociated O atom of the PA bind to two Ti-5c atoms, while in B3-A and B4-A the two dissociated O atoms are bound to the surface. The proton(s) H bind to surface O-2c atom(s), and the nondissociated OH groups in (a-h) have hydrogen bonds to the same (B1-A and B4-A) or to a different (B2-A and B3-A) pair of Ti-5c and O-2c surface atoms. Dotted lines indicate hydrogen bonds. The Ti, O, P, and H atoms are represented by gray, red, yellow, and white spheres, respectively.

resulting in dissociative bidentate structures. The dissociation of the second P-OH group is not barrier-free, since it does not take place spontaneously during relaxation of, e.g., B1-A and B1-R on anatase (101) and, respectively, rutile(110), but the total gain in energy is less than 30 kJ/mol as discussed above. All bidentate adsorption complexes, in which at least one P-OH group is dissociated, have similar adsorption energies of 257-286 kJ/mol on both rutile (110) and anatase (101). Also, the geometries of the bidentate structures present on the surface of rutile (110) are comparable to the corresponding adsorption structures on anatase (101), e.g., M2-A and M1-R, B1-A and B1-R, and B4-A and B2-R. Hence, from our present results we conclude that bidentate adsorption is the most stable binding state, and all bidentate adsorption complexes are likely to be present on the surface of TiO2. This is contrary to results of Nilsing et al.,47,68 who have calculated a molecular monodentate adsorption to be the most stable structure on anatase (101) and rutile (110). Although we have closely aligned our study to their investigation47,68 with respect to the possible adsorption configurations, we have

extended our investigation to adsorption complexes involving fully dissociated phosphonic acid molecules (HPO32 -), which we found are the most stable ones. Furthermore, we have used larger surface models, and we have added the adsorbate on both sides of the slabs to minimize artificial dipoles. Another striking difference is the surface coverage, which was 0.25 in the study of Nilsing et al. and 0.5 in our present work. (The coverage is here defined as the ratio between the number of adsorbed PA molecules and the number of surface 5c-Ti atoms.) Despite the theoretical method and applied basis set, the surface coverage and especially the slab thickness might have influence on the preference of either monodendate (molecular) or bidentate (dissociative) adsorption in theoretical investigations as has been found for the adsorption of water and formic acid on TiO2 surfaces.68,69,117,119 However, our results agree better with the experimental finding of the presence of bidentate-linked PA on TiO2.38,41,45,71 In agreement with our results, there is also some experimental evidence for the importance of hydrogen bond interactions in the surface attachment.46,71


Luschtinetz et al.

Figure 8. Bidentate adsorption of HPO(OH)2 on the rutile (110) surface: Starting structures (a-d) and geometry-optimized structures (e-h). In structure B1-R the double-bond phosphoryl O atom plus one dissociated O atom of the PA bind to two Ti-5c atoms, while in model B2-R the binding is done by the two dissociated O atoms. The proton(s) H bind to surface O-2c atom(s), which in model B2-R coordinate the phosphoryl O of the PA. Dotted lines indicate hydrogen bonds. The Ti, O, P, and H atoms are represented by gray, red, yellow, and white spheres, respectively.

But, on the basis of NMR spectroscopy, tridentate-bound phosphonic acids are proposed to be present on TiO2 surfaces, too.38,41,44,45,71 In contrast to this, we have found, in agreement with high-level hybrid Hartree-Fock DFT calculations,47,68 that the tridentate adsorption complexes are not stable on both clean anatase (101) and rutile (110) surfaces due to geometrical mismatch. But, most of the investigated stable mono- and bidentate adsorption complexes show three bonds as the remaining O atom or OH group forms hydrogen bonds to the surface. In addition, there is a stable monodentate adsorption complex M1-A (see Figure 5 g, h), in which the phosphonic acid is adsorbed corresponding to a symmetric resonancestabilized HPO32 - group, as it is usually proposed for the tridentate bonding. Furthermore, we note that on natural TiO2 surfaces there are always O-vacancy defects present in the rows of bridging twofold coordinated oxygen atoms.90 Under ambient conditions water molecules dissociate at these sites into an OH group, which fills the vacancy, and an H atom, which forms an OH group by binding to another bridging O-2c site.92,115 These surface OH groups might react with the OH groups of the PA via an acid-basic-condensation mechanism similar to those reported for PA on hydroxylated Al2O3 surfaces.85 This extends the possibilities for mono-, bi-, and tridentate adsorption structures enormously, and, thus, even tridentate adsorption might be stable. The adsorption of PA on defective and/or hydroxylated TiO2 surfaces will be the scope of future work. From our present results, we conclude that the discrepancy in the experimental finding of the preferred binding state is due to the fact that there are several chemically and structurally different but energetically similar bidentate adsorption complexes, which are equally likely to be present on TiO2 surfaces. Summary A large set of adsorption complexes for phosphonic acid on clean TiO2 anatase(101) and rutile (110) surfaces have been studied using the SCC-DFTB method. Particularly, we extended the recent theoretical work of Nilsing et al.47,68 to some

adsorption structures including fully dissociated phosphonic acid molecules (HPO32 -). Whereas the tridentate adsorption complexes of the phosphonic acid are not stable on anatase(101) and rutile (110), mono- and bidentate complexes resulted in stable configurations with adsorption energies exceeding 125 kJ/mol. For monodentate complexes we found a strong trend of the adsorption geometry relaxing toward the bidentate configuration. According to our calculations, the strongest adsorption complexes on both clean rutile (110) and anatase (101) surfaces have bidentate configurations with adsorption energies of 277 and 286 kJ/mol, respectively. But, there are several further bidentate adsorption complexes having similar adsorption energies but different geometries. In our present study, we extensively demonstrate the reliability of the SCC-DFTB method for the titania bulk and surface structures as well as for the adsorption model structures. We have found good agreement of many points with experimental and theoretical results. But, we have also found some differences, for which we cannot fully exclude methodological reasons. In our present study, we did optimizations of adsorbed PA in vacuum and at 0 K temperature using rather small test systems to demonstrate the reliability of the SCC-DFTB method for this chemical system. However, in experiment the adsorption of (organo)phosphonic acids takes place in aqueous solutions or other polar solvents at various temperatures (T * 0).38,40-46,72 The presence of water and/or other solvents might considerably influence the activation barriers for elementary steps of the adsorption reaction as well as the selectivity of the preferred adsorption structure. Thus, for a detailed, comprehensive, and realistic description of the adsorption of PA or similar on TiObased surfaces, the usage of small surface models is insufficient and much more complex systems are necessary. The efficiency of the SCC-DFTB method allows the calculation of such complex and extended structures with some thousands of atoms. Thus, this paper opens the way for studies of adsorption on more complex titania materials, e.g., including surface protonation and hydroxylation, solvent effects, larger

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Adsorption of Phosphonic Acid at the TiO2 Anatase (101) - American

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