and Rutile (110) - American Chemical Society

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J. Phys. Chem. C 2009, 113, 8394–8406

A Computational Study on the Adsorption Configurations and Reactions of Phosphorous Acid on TiO2 Anatase (101) and Rutile (110) Surfaces P. Raghunath† and M. C. Lin*,†,‡ Center for Interdisciplinary Molecular Science, Institute of Molecular Science, National Chiao Tung UniVersity, Hsinchu 300, Taiwan, and Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322 ReceiVed: January 26, 2009; ReVised Manuscript ReceiVed: March 18, 2009

We report the result of a density functional theory study on the adsorption and decomposition pathways of phosphorous acid (H3PO3) on TiO2 anatase (101) and rutile (110) surfaces. The most stable adsorption structure for H3PO3 and its isomer, HP(O)(OH)2, is a monodentate adsorption mode for the anatase surface with calculated adsorption energies 23.5 and 38.5 kcal/mol and a bidentate adsorption mode for rutile surface with 26.7 and 36.6 kcal/mol. The mechanisms for the surface reactions of these species have been explicitly elucidated with the computed potential energy surfaces. The barriers for the stepwise H3PO3 H-migration to two nearby bridged O atoms reactions on anatase leads to Ti-OP(OH)O-Ti(a) + 2H-Ob(a) with 7.9 and 6.8 kcal/mol barriers. Even lower activation barriers (1.3 and 2.9 kcal/mol) have been obtained on the rutile (110) surface for the same bond breaking modes. The intermediate Ti-OP(OH)O-Ti(a) thus formed on both surfaces can further decompose via two distinct pathways through H-migration to the P atom and H2O elimination to produce Ti-OP(H)(O)O-Ti(a) and Ti-OPO-Ti(a), respectively. In addition, we have calculated the adsorption and reactions of the dimer of H3PO3 on both surfaces. The most noticeable difference occurs in the energy levels of the H3PO3 reactions on the anatase and rutile surfaces, with the rutile being more reactive than the anatase surface. The predicted adsorption energies show that Ti-OP(OH)O-Ti(a) with two hydrogen atoms on bridged surface oxygen atoms is 47.1 kcal/mol for anatase and 42.4 kcal/mol for rutile; both are low when compared with the Ti-OB(OH)O-Ti(a) on the same surfaces, 140.1 and 134.6 kcal/mol, respectively. Our density of states analysis shows that OB(OH)O has a larger overlap with the TiO2 surface than OP(OH)O has, favoring the former’s charge transfer efficiency. 1. Introduction The titanium dioxide (TiO2) nanoparticle film is a polycrystalline material with different forms. Anatase (101) and rutile (110) surfaces have lower energies with similar characteristics1 for a wide range of applications, such as photophysics of TiO2 sensitized by dyes,2,3 polymers,4,5 and semiconductors,6-8 and have been widely studied particularly for photocatalysis and solar energy conversion by the photovoltaic effect. Dyesensitized TiO2 solar cells have been extensively investigated worldwide since Gra¨tzel’s group reported a high solar conversion efficiency of around 10%.2,3 Among the semiconductor sensitizers, for example, the quantum dots (QDs) of InP, InAs, PbS, Cds, and CdSe have been well-examined6-8 and showed evidence of electron transfer from quantum dots into the TiO2 nanoparticles. Recent experiments show that QD sensitized solar cells can be fabricated by anchoring the QDs of CdTe and CdSe through different linking molecules to TiO2 surfaces.9 Interaction of anchoring groups (COO- and -PO32-) with different surfaces has been a subject of several experimental and theoretical studies.10 For both dye- and QD-sensitized metal oxide solar cells, the anchoring group between the sensitizer and the metal oxide film plays a critical role. Recently, Wang11 has experimentally observed the effects of surface modifiers, such as boric acid and the phosphorous acid, on the photocurrent generated by InN/TiO2 nanoparticle films. Boric acid treatment of TiO2 nanoparticle films before InN * Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected]. † National Chiao Tung University. ‡ Emory University.

deposition by PECVD (plasma enhanced chemical vapor deposition) showed an enhancement in power conversion efficiency, whereas phosphorous acid treatment resulted in a distinctly different negative effect on the photocurrent conversion efficiency. To understand these effects we have evaluated the stability of boric acid and its dissociation products on TiO2 surfaces in an earlier work.12 On a related study, Chang et al.13 have recently investigated the adsorption and reactions of boron trichloride and its fragments (BClx) on TiO2 anatase (101) and rutile (110) surfaces by first-principles calculations. The objective of the present study lies in elucidating the reaction mechanism of the monomer and dimer of phosphorous acid on both TiO2 anatase (101) and rutile (110) surfaces by periodic density functional theory to identify which key stable species may potentially serve as a linkage between the semiconductor nanoparticles such as InN, CdTe, or CdSe and TiO2 for practical applications.9,11 This type of model calculations allows us to construct more complete potential energy surfaces for simulation of surface reactions and to predict the potential effect of H atom on the adsorbates. This article is organized as follows. The computational methods are described in detail in section 2, while the calculated results, including the optimized adsorbate structures, adsorption energies, potential energy surfaces, hydrogen coadsorption effect, Bader atomic charges, DOS, and the comparison of key points of with those of boric acid on TiO2 surfaces,12 are discussed in section 3. Finally, in section 4, the most important results are summarized. The calculated geometries and adsorption energies are expected to

10.1021/jp900747p CCC: $40.75  2009 American Chemical Society Published on Web 04/22/2009

Reactions of H3PO3 on TiO2 Anatase and Rutile Surfaces

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Figure 1. Perspective view of the TiO2 (a) anatase (101) and (b) rutile (110) surface slab models used in the present study. The dashed lines indicate the fixed atoms in the slab calculations. (c) Optimized structures of two isomers of H3PO3 calculated with PW91.

be helpful for the likely identification of the new anchoring groups for fabrication of solar cells. 2. Computational Details All calculations were made with the plane-wave-based Vienna ab initio simulation package (VASP) code, using ultrasoft Vanderbilt pseudopotentials (US-PP).13-17 The generalized gradient approximation (GGA)18-20 used for the total energy calculations was that of the spin-polarized Perdew-Wang 1991 (PW91) formulation. For the periodic condition, the electronic orbitals are represented by a plane-wave basis set. The planewave expansion includes all plane waves with their kinetic energies smaller than the chosen cutoff energy, pk2/2m < Ecut ) 500 eV, which ensures the convergence with respect to the basisset.TheBrillouinzonewassampledwiththeMonkhorst-Pack grid k-points.21 The climbing-image nudged-elastic band (CINEB) method was applied to locate the transition states for dissociative adsorption processes.22,23 All transition state structures were characterized by calculating the vibrational frequencies. In this study a slab model is adopted to simulate the interaction between both surfaces of TiO2 and H3PO3. The super cell was modeled as a periodically repeated slab consisting of 24 and 32 [TiO2] units of anatase and rutile phases, respectively, as shown in Figure 1. All these approximations were extensively tested in our previous study of boric acid on TiO2.12 A vacuum space of a sufficient separation (∼17 Å) was imposed for the surface reaction as well as to ensure no interaction with the lowest layer of the upper slab, in the direction perpendicular to the Z-axis of the super cell. The lowest layer of the super cell is fixed in the calculation to prevent surface deformation. Gas phase atoms and molecules were simulated in a cubic box 20 Å on each side, large enough to ensure negligible interactions between neighboring cells. Spin-polarized calculations are carried out throughout the system. The adsorption energy, Eads, was calculated as Eads ) Eslab + Emolecule - E(slab+molecule), where Eslab represents the energy of the clean slab, Emolecule is the energy of the adsorbate in the gas phase, and E(slab+molecule) is the total

energy of the slab with adsorbate. In the case of coadsorption of an adsorbate with H, we used Eads ) Eslab/H + Emolecule E(slab/H+molecule) to estimate the adsorption energy of the adsorbate, here Eslab/H represents the energy of the slab covered with H in the coadsorption configuration, and E(slab/H+molecule) is the total energy of the slab with the adsorbate and H coadsobed on it. Atomic charges of the optimized structures were computed by using the Bader method with a program by Henkelman et al.24 For the Bader charge analyses, the adsorbate structures are optimized by using the projector augmented wave (PAW) potential. With this program we can calculate the atomic charges of the optimized structures to examine the charge transfer between adsorbates and the substrate. 3. Results and Discussion 3.1. Verification. To test the reliability of our calculations, the methods and parameters employed in the current work were first examined by optimizing the bulk TiO2 in the anatase and rutile phases by characterizing their lattice constants, a and c. The structure of anatase (101) terraces exhibit a sawtooth-like surface, while the rutile structure is composed of the (110) oriented plane surface, as shown in Figures 1, parts a and b. for the anatase a model size 3 × 1 × 2 super cell was used for our simulations with Ti24O48 and a surface area of 11.355 Å × 10.24 Å, separated perpendicularly by a 17.8 Å vacuum space. Similarly, for the rutile (110) surface, a model size 4 × 1 × 4 super cell was used for our simulations, with Ti32O64 and a surface area of 11.832 Å × 6.496 Å, separated perpendicularly by a 17 Å vacuum space. Brillouin zone integrals were performed over regular Monkhorst-Pack grids with 2 × 2 × 1 and 2 × 4 × 1 k-points for the anatase (101) and rutile (110) surfaces, respectively. In a previous study, we have investigated different layers of the rutile (110) surface and concluded that a four-layer Ti slab is a reasonable model.12b Previous theoretical works have tested the reliability of these surface models by computing the adsorption/dissociation energies of H2O and HCOOH on both the anatase and rutile

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surfaces.12 The adsorption energy of water on TiO2 (101) was obtained as 19.2 kcal/mol, which is close to the experimental range of 11.5-17.3 kcal/mol,25 and the adsorption energy of water on TiO2 (110) was obtained as 18.9 kcal/mol, which is close to the previous results of 19.1-20.9 kcal/mol.26 3.2. Potential Energy Surfaces and Reaction Mechanism. 3.2.1. Reactions of H3PO3 Adsorbates on Anatase. The structures and bond lengths of the different isomers of gas phase molecules and fragments of H3PO3 were calculated by the PW91 method with VASP. We have characterized the optimized structures and geometrical parameters of the two tautomeric forms of the gaseous phosphorous acid i.e., P(OH)3 and HP(O)(OH)2, as shown in Figure 1c. The energy of the P(OH)3 is 9.8 kcal/mol higher than that of HP(O)(OH)2, which is in agreement with a previous study that predicted 9.2 kcal/mol by the MP2/6-31G(d,p) method.27 The predicted structures are also in close agreement with those obtained from a standard calculation by using the Gaussian 03 code. Adsorption of H3PO3. The adsorption characteristics of phosphorous acid on the four-layer TiO2 slab were mapped out by the DFT computation. We calculated several different bonding possibilities for an adsorbate that binds with the TiO2 surface. Coordination may be either monodentate or bidentate, depending on the number of oxygens used by the molecule (anionic group) to link with the surface Ti5c acid sites. Figure 2 shows the most stable adsorption configurations of H3PO3 and its fragments on the TiO2 anatase (101) surface, and the associated bond lengths and adsorption energies are listed in Table 1. We first determined the adsorption configurations of two tautomeric forms of phosphorous acid on TiO2 surfaces. The most stable adsorption structure for P(OH)3 on the surface is a molecular monodentate configuration with one hydroxyl oxygen attached to a surface Ti5c atom and the two other hydroxyl groups to form hydrogen bonds with two bridge surface oxygen (Ob) atoms denoted by Ti-O(H)P(OH)2(a) in Figure 2a. In this structure adsorption energy is 23.5 kcal/mol and the Ti5c-O2 bond length is 2.212 Å and the O3H-Ob bond length is 1.654 Å, as given in Table 1. The next lowest energy conformation is a bidentate configuration, where two adsorbate hydroxyl oxygens bind to two surface Ti5c atoms with an adsorption energy of 17.1 kcal/mol. Ti-O(H)P(OH)O(H)-Ti(a) bond lengths are slightly increased. The two Ti5c-O bond lengths are 2.571 and 2.429 Å, respectively (see Figure 2b). Similarly, the two most stable adsorption structures for B(OH)3 on the anatase (101) surface were also formed to have molecular monodentate and bidentate configurations, with adsorption energies of 17.2 and 13.5 kcal/mol, respectively.12 The most stable adsorption configuration of HP(O)(OH)2 on the surface Ti5c site is the unique monodentate binding of the >PdO group with an adsorption energy of 38.5 kcal/mol shown in Figure 2c. In this configuration, the two hydroxyl hydrogen atoms of HP(O)(OH)2 point toward a neighboring surface Ob atom. The distance of the Ti5c-O2 is 2.015 Å and those of the two hydroxyl hydrogens and Ob are 1.519 and 1.535 Å. The analogous binding configuration involving H3BO3 was found. The dissociative reactions of P(OH)3 and HP(O)(OH)2 on the surface potential energy surfaces are given in the following section. Adsorption and Dissociation of H3PO3 on the Anatase Surface. The computed potential energy surfaces of dissociative adsorption reactions of phosphorous acid on anatase (101) involving P(OH)3 and HP(O)(OH)2 are shown in Figures 3 and 4, respectively. Optimized structures of H3PO3 isomers and their

Raghunath and Lin

Figure 2. Calculated optimized configurations of phosphorous acid and its fragments on the anatase (101) surface. Their bond lengths are given in Table 1.

fragments on the anatase (101) surface are depicted in Figure 2 and selected bond lengths are listed in Table 1 as discussed above. The optimized structures including those of transition states and the potential energy surface of the bidentate conformation reaction are given in the Supporting Information in Figures S1 and S2, respectively. The results indicate that monodentate and bidentate bridge binding modes are energetically quite favorable, with a rather small difference in adsorption energies. In the first basic reaction, we investigate the P(OH)3 adsorption reaction on the surface, giving Ti-O(H)P(OH)2(a). In this reaction one of the OH groups’ lone-pair oxygen electrons can molecularly adsorb on an unsaturated surface Ti5c atom with the adsorption energy of 23.5 kcal/mol. Initially, the adsorbate can rotate molecularly on the surface requiring 6.5 kcal/mol. We found that the rotated adsorbate Ti-O(H)P(OH)2(a) can then undergo cleavage of O-H bonds which results in the formation of a covalently bonded Ti-O(H)P(OH)O-Ti(a) and H-Ob(a)



Figure 3. Calculated possible potential energy diagram for the reactions of the most stable monodentate configuration of P(OH)3 (g) on the TiO2 (101) surface. Their geometries are given in Figure 2.

TABLE 1: Optimized Bond Lengths (Å) and Adsorption Energies (kcal/mol) for Isomers of P(OH)3 and Its Fragments on the TiO2 (101) Surface anatase Ti-O(H)P(OH)2(a) Ti-O(H)P(OH)O(H)-Ti(a) Ti-OP(H)(OH)2(a) Ti-OP(H)(OH)O(H)-Ti(a) Ti-O(H)P(OH)2(a)rot Ti-O(H)P(OH)O-Ti(a)a Ti-OP(OH)O-Ti(a)b Ti-OB(OH)O-Ti(a) Ti-OB(OH)O-Ti(a)b Ti-OP(H)(OH)O-Ti(a) Ti-OP(H)(OH)O-Ti(a)-1a Ti-OP(H)(OH)O-Ti(a)-2a Ti-OPO-Ti(a) Ti-OPO-Ti(a)a H-Ob(a) 2H-Ob(a)

figure

Ti-O2

O2-P1

P1-O3

P1-O4

2a 2b 2c 2d 2e 2f 2g S4a S4b 2l 2h 2k 2m 2j 2n 2o

2.212 2.571 2.015 2.094 2.324 2.346 1.893 1.896 1.846 2.024 2.014 2.059 2.152 2.068 0.970g 0.975g

1.797 1.663 1.509 1.504 1.755 1.762 1.641 1.370 1.384 1.529 1.548 1.522 1.515 1.545

1.606 1.654 1.559 1.542 1.646 1.689 1.729 1.371 1.375 1.589 1.561 1.597

1.591 1.692 1.559 1.601 1.595 1.553 1.595 1.384 1.377 1.528 1.532 1.524 1.521 1.524

O4-Ti

P1-H

2.429 2.906 3.520 1.977 1.927 1.964 1.902 2.032 2.026 2.058 2.145 2.113

1.395 1.396

1.399 1.397 1.399

O3H-Ob

O4H-Ob

Ob-Ti6c

Eads

1.654 2.391c 1.519 1.431 2.176c 2.178c 2.656e

1.605 2.295 1.535 1.884 1.666 1.692d 1.586d

2.553f 2.255 1.499 2.860

2.584e

1.904 1.919 1.929 1.946 1.911 2.081 2.075 1.866 2.118 1.910 1.944 2.142 1.893 2.017 2.113 2.132

23.5 17.1 38.5 29.7 17.0 52.5 47.1 30.312a 140.112a 37.4 90.6 86.2 15.2 64.4 56.2 109.1

2.529e 2.592f 2.585e

a The adsorption is on the surface with one H(a) adsorbed on the anatase TiO2 (101) surface. b The adsorption is on the surface with two H(a) adsorbed on the anatase TiO2 (101) surface. c Bond length of O2H-Ob. d Bond length of O3-HOb. e Bond length of O2-HOb. f Bond length of O4-HOb. g Bond length of H-Ob.

(see Figure 2f) via TS1, with a reaction barrier of 7.9 kcal/mol. The exothermicity of the process is predicted to be 23.4 kcal/ mol, converting the molecule from the monodentate to the bidentate conformation. The transition state (TS1) of this process takes place with the formation of a Ti5c-O4 strong bond with a bond length of 1.977 Å and that of the Ob-H bond is 0.999 Å. The transition vector is dominated by the motion of the H atom in the dissociated OH group, which is 1.401 Å from the oxygen and 1.099 Å from the Ob. The transition state geometries are given in Figure S3 in the Supporting Information. The second dehydrogenation step that follows the O-H bond cleavage process in the Ti-OP(OH)O(H)-Ti(a) intermediate can also isomerize through a 6.8 kcal/mol barrier at TS2 with H-migration from the -O(H)-Ti moiety to the closest Ob to

produce Ti-OP(OH)O-Ti(a) + 2H-Ob(a), having an exothermicity of 26.9 kcal/mol. According to the predicted coordination configuration, the Ti-OP(OH)O-Ti(a) species can be bonded to the metal center via a bidentate bridging configuration (as illustrated in Figure 2g) with Ti-O bond lengths of 1.893 and 1.927 Å. At TS2 the O-H bond breaks and forms a H-Ob bond with bond lengths of 1.221 and 1.210 Å, respectively. We predicted the desorption energy for the OP(OH)O moieties on the anatase surface with two coadsorbed H-atoms on the bridged surface to be 47.1 kcal/mol. After the proton migration reactions, Ti-OP(OH)O-Ti(a) + 2H-Ob(a) can further be restructured in two ways. In the first pathway, Ti-OP(H)(OH)O-Ti(a)-1 + H-Ob(a) is formed by simultaneously undergoing hydrogen migrations from the

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Figure 4. Calculated possible potential energy diagram for the reactions of mono- and bidentate configurations of HP(O)(OH)2 (g) on the TiO2 (101) surface. Their geometries are given in Figure 2.

O-H group to the P atom and from Ob-H to O3. This process requires a high energy barrier (41.6 kcal/mol) via TS3 with their overall exotherimicity computed to be 47.2 kcal/mol (see Figures 2h and 3). At TS3, the bond lengths of O3-H and H-P1 are 1.343 and 1.512 Å, respectively (see Figure S3 in the Supporting Information). The desorption of the OP(H)(OH)O radical from Ti-OP(H)(OH)O-Ti(a)-1 with one coadsorbed H-Ob(a) requires 90.6 kcal/mol of energy. In the second pathway, the adsorbates Ti-OP(OH)O-Ti(a) + 2H-Ob(a) can produce the Ti-OP(H2O)O-Ti(a) complex with 4.4 kcal/mol endothermicity through the interaction of the hydrogen atom in the bridged oxygen migrating to the OH group in the adsorbate without an activation barrier (see Figure 2i). Finally, H2O elimination from the surface molecular complex requires 11.2 kcal/mol of energy and the final product OPO (g) + H2O (g) + H-Ob(a) is 53.1 kcal/mol above the initial reactants. We obtained similar results for the reaction mechanism of the bidentate configuration of phosphorous acid on the anatase (101) surface, which are given in Figure S2 in the Supporting Information. HP(O)(OH)2 Reaction. A dissociative adsorption pathway of HP(O)(OH)2 on the anatase involves both monodentate and bidentate adsorbates shown in Figure 4. The adsorption of HP(O)(OH)2 (g) on the surface giving rise to monodentate [Ti-OP(H)(OH)2(a)] and bidentate [Ti-OP(H)(OH)O(H)-Ti(a)] adsorbate was exothermic by 38.5 and 29.7 kcal/mol, respectively (see Figure 2, panels c and d). From the resulting stable intermediate, breaking of the O-H bond of an OH group occurs by a H-migration to a neighboring bridging O atom with a 12.6 kcal/mol barrier at TS4, which forms Ti-OP(H)(OH)O-Ti(a)-2 + H-Ob(a) (with the hydrogen adsorption on a different row of bridged oxygen; see Figure 2k), having bond lengths of 2.059 and 2.058 Å for the two Ti-O bonds. In the bidentate case, one of the O-H bonds binding with the surface can undergo H-migrationtoaneighboringOb toformTi-OP(H)(OH)O-Ti(a)-1 + H-Ob(a) via TS5 (5.7 kcal/mol) 36.5 kcal/mol below the initial reactants (see Figure 2h). The desorption of the final product, Ti-OP(H)(OH)O-Ti(a)-1 and -2 f OP(H)(OH)O (g), requires 86.2 and 89.6 kcal/mol, respectively, with one H-atom remaining on the surface.

Adsorption and Dissociation of P(OH)3 Dimer on the Anatase Surface. We further studied two P(OH)3 molecules adsorbed side by side on the surface, which, like the monodentate conformation, is more stable (shown as 2(Ti-O(H)P(OH)2(a)) in Figure 5a), with an adsorption energy of 38.6 kcal/mol. The detailed potential energy surface reaction is given in Figure 5 and geometrical parameters and adsorption energies are given in Table 3. Their detailed geometries are given in Figure S5 in the Supporting Information. The adsorbate 2(Ti-O(H)P(OH)2(a)) through H-atom migration from one of the OH groups of a P(OH)3 can react with one of the OH groups of another molecule intermolecularly to eliminate H2O, producing the -O-P-O-P-O- bridged bidentate and water complex, Ti-O(H)P(OH)OP(OH)(H2O)O(H)-Ti(a) (see Figure 5b). The product formed with an intrinsic energy barrier of 41.8 kcal/ mol (TS6) lies 31.5 kcal/mol below the initial reactants. From this H2O complex, H2O (g) can be eliminated from the surface, requiring an energy of 10 kcal/mol, to form the bridged Ti-O(H)P(OH)OP(OH)O(H)-Ti(a) (see Figure 5c). H-migration from one of the surface bonding O-H groups in the O(H)P(OH)OP(OH)O(H) adsorbate to a neighboring bridging O atom occurs with a 4.1 kcal/mol barrier at TS7. This indicates the ease of forming Ti-OP(OH)OP(OH)O(H)-Ti(a) + H-Ob(a) with an exothermicity of 6.4 kcal/mol (see Figure 5d). In the transition state, the breaking O-H bond and the forming H-Ob bond are 1.206 and 1.215 Å, respectively. Following a similar H-migration process, the second surface bonding OH group in Ti-OP(OH)OP(OH)O(H)-Ti(a) can undergo an isomerization reaction via TS8, producing the bidentate bridging Ti-OP(OH)OP(OH)O-Ti(a) and 2H-Ob(a) with an 8.8 kcal/ mol activation barrier. The exothermicity of the process is predicted to be 29.6 kcal/mol when compared with the initial reactants. 3.2.2. Reactions of H3PO3 Adsorbates on Rutile. We studied the H3PO3 adsorption and dissociation reactions on rutile (110) surface also and calculated all possible conformations of the two tautomeric forms of phosphorous acid adsorption on the surface; their geometries are given in Figure 6. The related bond lengths and adsorption energies are also listed in Table 2. The



Figure 5. Calculated possible potential energy diagram for the reactions of dimer P(OH)3 (g) on the TiO2 (101) surface. Their geometries are given in Figure S5 in the Supporting Information.

TABLE 2: Optimized Bond Lengths (Å) and Adsorption Energies (kcal/mol) for Isomers of P(OH)3 and Its Fragments on the TiO2 (110) Surface rutile Ti-O(H)P(OH)O(H)-Ti(a) Ti-O(H)P(OH)2(a) Ti-OP(H)(OH)O(H)-Ti(a) Ti-OP(H)(OH)2(a) Ti-OP(OH)O(H)-Ti(a) Ti-OP(OH)O(H)-Ti(a)a Ti-OP(OH)O-Ti(a) Ti-OP(OH)O-Ti(a)b Ti-OB(OH)O-Ti(a) Ti-OB(OH)O-Ti(a)b Ti-OP(H)(O)O-Ti(a) Ti-OP(H)(O)O-Ti(a)b Ti-OPO-Ti(a) Ti-OPO-Ti(a)a Ti-OP(H)(OH)O-Ti(a) Ti-OP(H)(OH)O-Ti(a)a H-Ob(a) 2H-Ob(a)

figure 6a 6b 6c 6d 6k 6e 6l 6f S4c S4d 6m 6g 6n 6i 6o 6j 6p 6q

Ti-O2 2.273 2.274 2.033 2.071 2.185 1.923 2.570 1.896 1.833 1.880 1.913 1.935 2.045 2.098 2.009 2.019 0.973g 0.972g

O2-P1 1.681 1.765 1.519 1.489 1.506 1.613 1.459 1.619 1.381 1.380 1.574 1.568 1.533 1.534 1.530 1.528

P1-O3 1.646 1.612 1.559 1.578 1.625 1.655 1.571 1.718 1.365 1.372 1.501 1.512 1.586 1.585

P1-O4 1.675 1.617 1.575 1.577 1.618 1.720 1.459 1.616 1.387 1.387 1.572 1.565 1.534 1.528 1.527 1.534

O4-Ti

O4H-Ob

Ob-Ti6c

Eads

2.216d

1.769 2.620 1.584 2.653 2.880 1.983

2.226e

2.261e

2.173d

2.363f

1.404 1.400

1.702e

1.668e

1.396 1.398

2.863d 2.938 1.668

2.627f

1.873 1.888 1.888 1.866 1.848 2.040 1.853 2.065 1.844 2.073 1.841 2.056 1.845 2.049 1.844 2.049 2.028 2.061

26.7 18.9 36.6 29.1 20.3 51.2 13.9 42.4 53.812b 134.612b 54.1 150.9 29.8 61.7 52.7 90.2 65.9 125.9

P1-H

2.304 3.159 2.805 2.256 2.359 1.886 1.841 1.878 1.912 1.945 2.050 2.069 2.007 2.040

1.393 1.394

O3H-Ob 1.744 2.214 1.541 2.539

c

a The adsorption is on the surface with one H(a) adsorbed on the rutile TiO2(110) surface. b The adsorption is on the surface with two H(a) adsorbed on the rutile TiO2(110) surface. c Bond length of the O2H-Ob. d Bond length of the O2-HOb. e Bond length of the O3-HOb. f Bond length of the O4-HOb. g Bond length of the H-Ob.

most stable adsorption structure for P(OH)3 on the surface is a bidentate configuration (Ti-O(H)P(OH)O(H)-Ti(a)) with an adsorption energy of 26.7 kcal/mol (see Figure 6a). Here the two adsorbate hydroxyl oxygens bind to two surface Ti5c atoms forming two Ti5c-O with bond lengths of 2.273 and 2.304 Å, respectively. Figure 6b shows the second stable monodentate conformation of the P(OH)3 molecule on the surface Ti-O(H)P(OH)2(a) with an adsorption energy 18.9 kcal/mol. The adsorption and dissociation reactions of most stable bidentate configurations of P(OH)3 on the rutile surface are shown in Figure 7. The isomerization of Ti-O(H)P(OH)O(H)Ti(a) to Ti-OP(OH)O(H)-Ti(a) + H-Ob(a), in which an H migrates via TS9 to a neighboring Ob, requires only 1.3 kcal/ mol of energy; the reaction is exothermic by 5.3 kcal/mol (see Figure 6e). Following a similar H-migration process, the second hydroxyl hydrogen group in the Ti-OP(OH)O(H)-Ti(a) also migrates to the neighboring surface Ob atom via TS10 to

produce Ti-OP(OH)O-Ti(a) + 2H-Ob(a) with a 2.9 kcal/mol barrier. The exothermicity of the process is predicted to be 40.6 kcal/mol when compared with the initial reactants. The lengths of the two Ti5c-O bonds are slightly different, 1.896 and 1.886 Å. The desorption energy of OP(OH)O in the presence of 2H-Ob(a) coadsorbates was found to be 42.4 kcal/mol. The intermediate Ti-OP(OH)O-Ti(a) + 2H-Ob(a) (see Figure 6f) can further decompose by two distinct pathways: (1) H-migration to the P atom and (2) H2O elimination. The first pathway involves the migration of the H atom from the O-H bond in the -OP(OH)O- group to the P atom, to produce the more stable Ti-OP(H)(O)O-Ti(a) + 2H-Ob(a) adsorbates (Figure 6g). This reaction has a 39.4 kcal/mol energy barrier via TS11, where the distances of O-H and H-P are 1.364 and 1.545 Å, respectively. The second reaction process involving one of the HO(a) adsorbates and the P-OH group requires a 7.1 kcal/mol activation barrier at TS12 in order to eliminate

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TABLE 3: Optimized Bond Lengths (Å) and Adsorption Energies (kcal/mol) for Isomers of P(OH)3 Dimer and Its Fragments on the TiO2 Surface structure anatase 2(Ti-O(H)P(OH)2(a) Ti-O(H)P(OH)OP(OH)O(H)-Ti(a) Ti-OP(OH)OP(OH)O(H)-Ti(a)a Ti-OP(OH)OP(OH)O-Ti(a)b rutile 2(Ti-O(H)P(OH)2(a) Ti-O(H)P(OH)OP(OH)O(H)-Ti(a) Ti-OP(OH)OP(OH)O(H)-Ti(a)a Ti-OP(OH)OP(OH)O-Ti(a)b

TiO2

O2P1

P1O3

P1O4

2.380 2.407 1.862 1.842

1.745 1.686 1.611 1.629

1.591 1.611 1.625 1.627

1.646

2.205 2.278 1.869 1.864

1.794 1.711 1.658 1.669

1.594 1.597 1.601 1.607

1.610

P1O7

TiO6

O6P5

P5O7

P5O8

O3HOb

O8HOb

Ob Ti6c

Eads

1.665 1.686 1.663

2.212 2.352 2.312 1.845

1.799 1.704 1.731 1.633

1.623 1.669 1.636 1.676

1.596 1.599 1.604 1.621

1.565 2.953 2.727c 2.721c

1.691 2.728 2.248 2.751d

1.915 1.867 2.000 1.999

38.6 23.4 55.5 93.5

1.667 1.692 1.675

2.209 2.261 2.219 1.866

1.757 1.713 1.738 1.664

1.626 1.663 1.645 1.671

1.617 1.599 1.596 1.609

1.718 1.830f 1.970c 2.003c

1.880e 1.825g 1.933g 1.983d

1.876 1.882 2.085 2.035

45.8 36.0 55.6 85.2

a The adsorption is on the surface with one H(a) adsorbed on the TiO2 surface. b The adsorption is on the surface with two H(a) adsorbed on the same sides on the TiO2 surface. c Bond length of the O2-HOb. d Bond length of the O6-HOb. e Bond length of the O4H-O7. f Bond length of the O2H-Ob. g Bond length of the O6H-Ob.

H2O from Ti-OP(OH)O-Ti(a) + 2H-Ob(a), producing Ti-OP(H2O)O-Ti(a) + H-Ob(a) with a 6.6 kcal/mol endothermicity. The elimination of H2O from the surface to produce Ti-OPO-Ti(a) + H-Ob(a) requires 15.4 kcal/mol of energy and the desorption of the OPO radical requires 24.5 kcal/mol. Finally, in Figure 7 the desorption of OPH(O)O (g) and OPO (g) from Ti-OPH(O)O-Ti(a) and Ti-OPO-Ti(a) with 2H-Ob(a) coadsorbates requires as much as 151 and 61.7 kcal/ mol, respectively. HP(O)(OH)2 Reaction. The proposed possible subsequent pathways for HP(O)(OH)2 reactions on the rutile (110) surface are schematically depicted in Figure 8. The geometrical structures are shown in Figure 6, and selected bond distances and adsorption energies are listed in Table 2. HP(O)(OH)2, which also can molecularly adsorb on the clean surface that has bidentate and monodentate Ti-OP(H)(OH)O(H)-Ti(a) and Ti-OP(H)(OH)2(a), has an adsorption energy of 36.6 and 29.1 kcal/mol as shown in Figure 6, panels c and d, respectively. A monodentate configuration that can easily rotate to bidentate was found to be the most stable molecular adsorption structure. In this configuration the double-bonded oxygen in the adsorbate bonds to the surface Ti5c atom and both hydroxyl hydrogen atoms point toward two neighboring surface Ob atoms. The distance between the O-Ti5c is 2.028 Å and that for the two H-Ob is 1.559 and 1.617 Å. Ti-OP(H)(OH)O(H)-Ti(a) can dissociate into Ti-OP(H)(OH)O-Ti(a) + H-Ob(a), by breaking one of the O-H groups in the adsorbate, with the breaking H atom migrating to a nearby Ob via TS13 with 1.5 kcal/mol of energy (see Figure 6j). This intermediate product lies 46.9 kcal/mol below the initial reactants (TiO2 rutile + HP(O)(OH)2). As shown in Figure 8, to desorb OP(H)(OH)O (g) from the Ti-OP(H)(OH)O-Ti(a) + H-Ob(a) requires 90.2 kcal/mol of endothermicity. The additional migration of H from the remaining O-H group in the Ti-OP(H)(OH)O-Ti(a) to a neighboring Ob yielding Ti-OP(H)(O)O-Ti(a) + 2H-Ob(a) via TS14 occurs with a negligible barrier of 0.6 kcal/mol (see Figure 6g). Similar to the anatase case, the desorption of the OP(H)(O)O moiety from Ti-OP(H)(O)O-Ti(a) requires a very large energy of 150.9 kcal/mol (see Figure 8). Adsorption and Dissociation of P(OH)3 Dimer on the Rutile Surface. Similar to dimer adsorption and dissociation reactions of P(OH)3 molecules on the anatase surface, we calculated the same processes for the rutile surface, as illustrated in Figure 9. Their brief geometries and bond lengths are given in Figure S6 in the Supporting Information and Table 3, respectively. The most stable adsorption configuration with two coadsorbed monodentates is 2(Ti-O(H)P(OH)2(a)) with 45.8 kcal/mol as

shown in Figure 9a. Like the anatase surface, the adsorbate 2(Ti-O(H)P(OH)2(a)) can undergo H2O elimination to form the Ti-O(H)P(OH)OP(OH)(H2O)O(H)-Ti(a) complex via TS15 with a 39.6 kcal/mol activation barrier and with 38.5 kcal/mol of exothermicity on the rutile surface. The energy required to detach H2O from Ti-O(H)P(OH)OP(OH)O(H)-Ti(a) is 3.7 kcal/mol (see Figure 9c). The intermediate can further decompose successively to a final product Ti-OP(OH)OP(OH)O-Ti(a) + 2H-Ob(a) with 38.4 kcal/mol of exothermicity. In this consecutive decomposition process, H from one of the bonding O-H groups migrates to a nearby Ob with a 2.8 kcal/mol of energy barrier at TS16 and a 4.9 kcal/mol barrier at TS17 for the H-migration from the second OH group. 3.2.3. Hydrogen Effect on Adsorbate Structures and Adsorption Energies for Both Surfaces. In our previous publications12 we demonstrated how the H-atom adsorption on the neighboring Ob surface affects the binding energies of H2O, HCOOH, and B(OH)3 and their fragments. In this section, we summarize the hydrogen-adsorbate effect on the decomposed fragments of H3PO3 on TiO2 anatase and rutile surfaces cited above. The optimized structures are shown in Figures 2 and 6, and related bond lengths and adsorption energies are listed in Tables 1 and 2. On the rutile surface, the calculated adsorption energy of Ti-OP(OH)O-Ti(a) is 13.9 kcal/mol (see Figure 6l) whereas the same adsorbate on the anatase surface is unstable. On the other hand Figures 2g and 6f show that the coadsorption of H on a neighboring bridged oxygen in the Ti-OP(OH)O-Ti(a) has an adsorption energy of 47.1 kcal/mol for the anatase and 42.4 kcal/mol for the rutile surface. For Ti-OP(H)(OH)O-Ti(a) on anatase (see Figure 2l), the adsorption energy is 37.4 kcal/ mol with bond lengths between O and Ti atoms, Ti-O2 and Ti-O4, of 2.024 and 2.032 Å, respectively. Similarly, the adsorption energies for the same molecule with one hydrogen on different sides of the bridge oxygen, Ti-OP(H)(OH)O-Ti(a) + H-Ob(a) (see Figure 2h,k), are 90.6 and 86.2 kcal/mol. The bond lengths between the phosphate O atoms and Ti atoms of the surface, i.e., Ti-O2 and Ti-O4, are 2.014 and 2.026 Å, respectively (Figure 2h). The bond angle ∠O2P1O4 of the phosphate group decreases slightly from 116.2° to 114.1°. Accordingly, in the case of rutile Ti-OP(H)(O)O-Ti(a) (see Figure 6m) the bond lengths of the O atoms with the two adsorbed Ti5c atoms, Ti-O2 and Ti-O4, are 1.913 and 1.912 Å, respectively, and the bond angle is 106.9° with an adsorption energy of 54.1 kcal/mol. The geometry of the same adsorbate Ti-OP(H)(O)O-Ti(a) is stabilized by the coadsorption of a hydrogen on a neighboring Ob atom, with an adsorption energy of 150.9 kcal/mol (Figure 6g). There is hydrogen bonding


Figure 6. Calculated optimized configurations of phosphorous acid and its fragments on the rutile (110) surface. Their bond lengths are given in Table 2.

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8401 between the H-Ob(a) and adsorbate OP(H)(O)O on the TiO2 and their bond lengths are 1.668 Å for H-Ob(a) and 1.703 Å for O3-HOb. The effect of hydrogen bonding on the adsorbate can play a significant role in adsorption energy.28 The adsorption of H on the bridging O atom leads to an increase of the Ob-Ti6c bond length from 1.841 Å to 2.056 Å, and this bond length is in good agreement with a previous finding.29 Woodruff et al.29a-d in their experimental and theoretical studies showed the coexistence of formate, HCOO, and hydroxyl, OH, surface species. The hydroxyl species was formed by H attachment to the surface Ob atom and have a Ti-O bond length of 2.02 ( 0.05 Å, which is significantly longer than that of the bridge oxygen atoms on a clean TiO2 (110) surface. Both panels m and j of Figure 2 (panels n and i of Figure 6) show the considerable energy difference in the adsorbate structures of OPO on the anatase (rutile) surface without and with coadsorbing hydrogen atom on the bridged oxygen having adsorption energies of 15.2 and 64.4 kcal/mol (29.8 and 61.7 kcal/mol for rutile), respectively. The adsorption energy of the H-Ob(a) for anatase is 56.2 and 65.9 kcal/mol for rutile. In the case of dimer adsorption, the most stable structure is the monodentate configuration 2(Ti-O(H)P(OH)2(a)), with an adsorption energy of 38.6 kcal/mol for anatase and 45.8 kcal/ mol for rutile. Figures S5c and S6c in the Supporting Information shows the adsorbate Ti-O(H)P(OH)OP(OH)O(H)-Ti(a) with adsorption energies of 23.4 and 36 kcal/mol, respectively. The adsorbate OP(OH)OP(OH)O is stabilized by the coadsorption of two hydrogen atoms onto nearby Ob atoms, with adsorption energies of 93.5 and 85.2 kcal/mol on anatase and rutile surfaces, respectively (Figures S5e and S6e in the Supporting Information). The same adsorbate OP(OH)OP(OH)O on both clean anatase and rutile surface is unstable. Bader Atomic Charges. In this section we analyzed the Bader charges on the adsorbates of OP(OH)O and OP(H)(O)O on anatase and rutile surfaces, and the H-coadsorbate on bridging oxygen as shown in Figures 10 and 11. As shown in panels b, c, and d of Figure 10, we compare the adsorbate structures of OP(H)(OH)O and the coadsorbed hydrogen atom on nearby Ob atoms. It is found that the H-coadsorption increases negligibly the charge of the OP(H)(OH)O adsorbate that is 0.2 e and 0.1 e for different configurations, where e is the magnitude of the charge on the electron. In the case of rutile, the H coadsorption increases the charge of the OP(OH)O [OP(H)(O)O] adsorbate by 0.91 e [0.33 e]. There is a simultaneous increase of the charge on the O atoms with the two Ti5c atoms of adsorption sites from -1.19 e and -1.20 e to -1.27 e and -1.25 e as shown in Figure 11, panels c and d, respectively. Previously we observed that, in the case of boric acid, the hydrogen coadsorption increased the charge of the OB(OH)O adsorbate by 0.28 e and 0.19 e on anatase and rutile, respectively.12 The changes in the estimated Bader charges are consistent with the increase in the binding energies of both OP(OH)O and OP(H)(OH)O. From the above results it can be seen that the presence of hydrogen atoms on bridged oxygens can significantly increase the adsorption energies of radical species. Comparison with B(OH)3 and H3PO3 Reactions on Both TiO2 Surfaces. As referred to above, we have recently studied the reactions of boric acid on TiO2 surfaces by first-principles calculations.12 Here, we compare the adsorption energies and reaction mechanisms of the lowest energy pathways of B(OH)3 and H3PO3 on TiO2 surfaces. On the clean anatase (101) and rutile (110) surface, B(OH)3 can molecularly adsorb on the surface with 17.2 and 20.1 kcal/mol adsorption energies, respectively. In the case of P(OH)3 and HP(O)(OH)2, adsorption

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Figure 7. Calculated possible potential energy diagram for the reactions of bidentate configuration of P(OH)3 (g) on the TiO2 (110) surface. Their geometries are given in Figure 6.

Figure 8. Calculated possible potential energy diagram for the reactions of mono- and bidentate configurations of HP(O)(OH)2 (g) on the TiO2 (110) surface. Their geometries are given in Figure 6.

energies are somewhat larger, 23.5 and 38.6 kcal/mol for anatase and 26.7 and 36.4 kcal/mol for rutile. On both surfaces, P(OH)3 [B(OH)3] molecule can dissociate by successive H-migrations yielding a bidentate configuration, Ti-OP(OH)O-Ti(a) [Ti-OB(OH)O-Ti(a)] with barriers 7.9 and 6.8 [8.4 and 15.2]

kcal/mol for anatase and 1.3 and 2.9 [0.5 and 8.2] kcal/mol for rutile (see Figure S4 in the Supporting Information). The overall exothermicities in anatase and rutile are 26.9 and 40.6 [10.8 and 23.7] kcal/mol, respectively. We observed a significant difference in the adsorption energies for Ti-OP(OH)O-Ti(a)



Figure 9. Calculated possible potential energy diagram for the reactions of dimer P(OH)3 (g) on the TiO2 (110) surface. Their geometries are given in Figure S6 in the Supporting Information.

Figure 10. Bader atomic charges (e) on some important adsorbate molecules on the anatase (101) surface.

with two hydrogen atoms on bridged surface oxygen atoms, 47.1 kcal/mol for anatase and 42.4 kcal/mol for rutile, which are low compared with the Ti-OB(OH)O-Ti(a) values on the same surfaces, 140.1 and 134.6 kcal/mol, respectively.12 The gas phase structure of OB(OH)O (bond length of O-B ) 1.371 Å and B-OH ) 1.353 Å) does not change much on the surface whereas that of the OP(OH)O gas phase molecule (O-P ) 1.452 Å and P-OH ) 1.599 Å) is very much different from that on the surface, where OB(OH)O has a planar triangle structure with B in the sp2 hybrid orbital while the P atom of OP(OH)O is with the sp3 hybrid and the tetrahedral structure.

Figure 11. Bader atomic charges (e) on some important adsorbate molecules on the rutile (110) surface.

To further understand the OX(OH)O [X ) B, P] adsorption modes on both anatase and rutile surfaces, we performed projected density of states (PDOS) analyses shown in Figures 12 and 13. In the bottom panel of Figures 12 and 13 we have the DOS of the isolated TiO2 slab (Figure 1) with and without

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Figure 12. Total density of states and its projected density of states for OX(OH)O [X ) B, P] adsorption on the TiO2 anatase (101) surface.

Figure 13. Total density of states and its projected density of states for OX(OH)O [X ) B, P] adsorption on the TiO2 rutile (110) surface.

2 hydrogens on the bridging oxygen (Figures 2o and 6q) and the isolated gas phase species of two adsorbates. Here we observe that among the two DOS curves, the conduction band edge of two hydrogens on bridged surface oxygen atoms moves by about 1.50 and 0.70 eV toward the Fermi level in anatase

and rutile surfaces, respectively. Figure 12b shows that the calculated PDOS of O orbitals of OB(OH)O has a large overlap with that of the Ti5c atom of the anatase surface and hydrogen bonding with the bridged OH group to stabilize the structure and to favor the charge transfer. The PDOS of the OB(OH)O

Reactions of H3PO3 on TiO2 Anatase and Rutile Surfaces molecule is pushed well below the valence band edge. This DOS shows that the valence and conduction bands have 1.5 eV shifting when compared with the pure 2H-Ob-TiO2, resulting in a band gap narrowing of about 0.3 eV. In the case of calculated PDOS of OP(OH)O on anatase, in Figure 12c, we notice that there is less overlap between the electronic states of 2H-Ob-TiO2 and OP(OH)O. OP(OH)O shows the formation of the impurity energy level in between the band gap of TiO2 and results in a lone distribution around -0.5 to -1.0 eV, compared with OB(OH)O. The total density of states shows the conduction and valence band edge shifting by 0.82 and 0.72 eV when compared with 2H-Ob-TiO2. A similar trend of DOS curves was observed in a study on both adsorbates on the rutile surface as shown in Figure 13. In summary, the OB(OH)O on the TiO2 surface is calculated to be a more favorable chemical link for highly efficient photoelectric conversion compared to the OP(OH)O group due to the stronger binding and better electronic overlap with the surface, consistent with our experimental finding.11 For the OB(OH)O adsorbate on clean surfaces the adsorption energies are 30.3 kcal/mol for anatase and 53.8 kcal/mol for rutile. In the phosphorus reaction case, the Ti-OP(OH)O-Ti(a) intermediate can further isomerize via H-migration from one of the OH groups in the adsorbate to the P atom yielding Ti-OP(H)(OH)O-Ti(a) exothermically by 20.3 kcal/mol on the anatase and 18.1 kcal/mol on the rutile surface. Comparing the adsorption energies of all the adsorbates on both surfaces, the rutile surface appears to be more active than the anatase surface. 4. Conclusions The adsorption and dissociation of the two tautomeric forms of phosphorous acid, P(OH)3 and HP(O)(OH)2 on anatase (101) and rutile (110) surfaces, have been explored by first-principles calculations and the relevant stationary points of the potential energy surfaces have been characterized. For the anatase surface, we found that P(OH)3 and HP(O)(OH)2 adsorption on a Ti5c site at the monodentate configuration is favored, although the bidentate configuration is favored on a rutile surface. In fact, the energy barrier for phosphorous acid dissociation processes on the rutile surface is significantly lower than the corresponding value for the anatase surface, but it is still lower energy than that for the B(OH)3 dissociation reactions. Initial adsorbents P(OH)3 can overcome the barriers for H atom migration to a neighboring bridged oxygen to produce Ti-OP(OH)O-Ti(a) + 2H-Ob(a), with an estimated exothermicity of 23.4 kcal/ mol for anatase and 40.6 kcal/mol for rutile. Another product is Ti-OP(H)(OH)O-Ti(a) + H-Ob(a) from HP(O)(OH)2 on the anatase surface, with a high energy barrier of 12.6 kcal/mol and an exothermicity of 33.1 kcal/mol. Similarly, the same adsorbate further dissociates on the rutile surface to produce Ti-OP(H)(O)(O)-Ti(a) and is predicted to be exothermic by 48.7 kcal/mol. The final dissociative products of phosphoric acid adsorptions, Ti-OP(H)(OH)O-Ti and Ti-OP(H)(O)O-Ti, bond very strongly with TiO2 surface with 90.6 and 151.0 kcal/ mol adsorption energies, respectively, when H atoms are coadsorbed on their nearby Ob atoms. In addition, we find that two P(OH)3 molecules undergo adsorption and dissociative reactions on the anatase and the rutile surface, attaching two Ti5c atoms with an adsorption energy of 38.6 and 45.8 kcal/ mol, respectively. Most of the H3PO3 reactions on the TiO2 surface are exothermic. According to DOS analysis, OB(OH)O on the TiO2 surface has a better electronic interaction compared to that of OP(OH). On the basis of the above results the

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8405 OB(OH)O linking group is useful for interface linking between the semiconductor quantum dots such as InN or CdSe on TiO2 nanoparticles in the fabrication of solar cells. Acknowledgment. . The authors thank the Institute of Nuclear Energy Research (INER), Taiwan, for the funding of this project. M.C.L. acknowledges the support from the Taiwan Semiconductor Manufacturing Company for the TSMC Distinguished Professorship and for the National Science Council of Taiwan for the Distinguished Visiting Professorship at National Chiao Tung University in Hsinchu, Taiwan. We are very much indebted to Taiwan’s National Center for High-Performance Computing for the extensive CPUs needed in this work. R.P. would also like to acknowledge partial support from the ATU Plan of MOE, Taiwan, and also Dr. J. H. Wang for useful discussions. Supporting Information Available: Table of optimized bond lengths and adsorption energies for P(OH)3 and its fragments on the TiO2 (101) surface and figures giving optimized structures of possible adsorbed and dissociatively adsorbed bidentate conformations of P(OH)3 on the anatase (101) surface (Figure S1), potential energy surface of the surface reactions starting with the bidentate conformation of P(OH)3 on the TiO2 (101) surface (Figure S2), optimized structures of possible transition states of H3PO3 reactions on anatase (101) and rutile (110) surfaces (Figure S3), optimized structures of some important dissociatively adsorbed bidentate conformations of B(OH)3 on anatase (101) and rutile (110) surfaces (Figure S4), calculated optimized configurations of dimer phosphorous acid and its fragments on the anatase (101) surface (Figure S5), and calculated optimized configurations of dimer phosphorous acid and its fragments on the rutile (110) surface (Figure S6). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Herman, G. S.; Dohnalek, Z.; Ruzycki, N.; Diebold, U. J. Phys. Chem. B 2003, 107, 2788. (2) O’Regan, B.; Gratzel, M. Nature (London) 1991, 353, 737. (3) (a) Gratzel, M. Nature (London) 2001, 414, 338. (b) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (4) Huisman, C. L.; Goossens, A.; Schoonman, J. Chem. Mater. 2003, 15, 4617. (5) Li, D.; Gu, C.; Guo, C.; Hu, C. Chem. Phys. Lett. 2004, 385, 55. (6) (a) Blackburn, J. L.; Selmarten, D. C.; Nozik, A. J. J. Phys. Chem. B 2003, 107, 14154. (b) Yu, P.; Zhu, K.; Norman, A. G.; Ferrere, S.; Frank, A. J.; Nozik, A. J. J. Phys. Chem. B 2006, 110, 25451. (7) (a) Robel, I.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2007, 129, 4136. (b) Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2008, 130, 4007. (8) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (9) (a) Lee, J.-H.; Kim, D. Y.; Yoo, J.-S.; Bang, J.; Kim, S.; Park, S.-M. Bull. Korean Chem. Soc. 2007, 28, 953. (b) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385. (c) Mann, J. R.; Watson, D. F. Langmuir 2007, 23, 10924. (10) (a) Zabri, H.; Gillaizeau, I.; Bignozzi, C. A.; Caramori, S.; Charlot, M.-F.; Cano- Boquera, J.; Odobel, F. Inorg. Chem. 2003, 42, 6655. (b) Pechy, P.; Rotzinger, F. P.; Nazeeruddin, M. K.; Kohle, O.; Zakeeruddin, S. M.; Humphrybaker, R.; Gratzel, M. J. Chem. Soc., Chem.Commun. 1995, 65. (c) Zakeeruddin, S. M.; Nazeeruddin, M. K.; Pechy, P.; Rotzinger, F. P.; HumphryBaker, R.; Kalyanasundaram, K.; Gratzel, M.; Shklover, V.; Haibach, T. Inorg. Chem. 1997, 36, 5937. (d) Gillaizeau-Gauthier, I.; Odobel, F.; Alebbi, M.; Argazzi, R.; Costa, E.; Bignozzi, C. A.; Qu, P.; Meyer, G. J. Inorg. Chem. 2001, 40, 6073. (e) Bae, E.; Choi, W.; Park, J.; Shin, H. S.; Kim, S. B.; Lee, J. S. J. Phys. Chem. B 2004, 108, 14093. (f) Nilsing, M.; Lunell, S.; Persson, P.; Ojamaee, L. Surf. Sci. 2005, 582, 49. (g) She, C.; Guo, J.; Irle, S.; Morokuma, K.; Mohler, D. L.; Zabri, H.; Odobel, F.; Youm, K.-T.; Liu, F.; Hupp, J. T.; Lian, T. J. Phys. Chem. A 2007, 111, 6832.

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