Modeling Ruthenium-Dye-Sensitized TiO2 Surfaces Exposing the

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Modeling Ruthenium Dye Sensitized TiO Surfaces Exposing the (001) or (101) Faces: A First Principles Investigation Filippo De Angelis, Giuseppe Vitillaro, L Kavan, Mohammad Khaja Nazeeruddin, and Michael Grätzel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp306186y • Publication Date (Web): 09 Aug 2012 Downloaded from http://pubs.acs.org on August 10, 2012

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The Journal of Physical Chemistry

Modeling Ruthenium Dye Sensitized TiO2 Surfaces Exposing the (001) or (101) Faces: A First Principles Investigation Filippo De Angelis,1,* Giuseppe Vitillaro,1 Ladislav Kavan,2,* Mohammad. K. Nazeeruddin,3 Michael Grätzel3 1

Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di

Scienze e Tecnologie Molecolari, Via Elce di Sotto 8, I-06123, Perugia, Italy. 2

J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech

Republic, Dolejškova 3, CZ-18223 Prague 8, Czech Republic 3

Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss

Federal Institute of Technology, CH-1015 Lausanne, Switzerland KEYWORDS : Dye-sensitized solar cells; Density Functional Theory; anatase TiO2 surfaces; dye adsorption mode; recombination kinetics;

ACS Paragon Plus Environment

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ABSTRACT

We present a first-principles computational investigation on the adsorption mode and electronic structure of the highly efficient heteroleptic Ruthenium dye C101, [NaRu(4,4′-bis(5hexylthiophene-2-yl)-2,2′-bipyridine)(4-carboxylic acid-4′-carboxylate-2,2′-bipyridine)(NCS)2], on anatase TiO2 models exposing the (001) and (101) surfaces. The electronic structure of the TiO2 models shows a conduction band energy upshift for the (001)-surface model ranging between 50 and 110 meV compared to the (101)-TiO2 model, in agreement with earlier interfacial impedance and recent spectro-electrochemical data. TDDFT excited state calculations provided the same optical band gap, within 0.01 eV, for the (001)- and (101) models.

Two dominant adsorption modes for C101 dye adsorption on the (001) and (101) surfaces were found, which differ by the binding of the dye carboxylic groups to the TiO2 surfaces (bridged bidentate vs. monodentate), leading to sizably different tilting of the anchoring bipyridine plane with respect to the TiO2 surface. The different adsorption mode leads to a smaller dye coverage on the (001) surface, as experimentally found, due to partial contact of the thiophene and alkyl bipyridine substituents with the TiO2 surface. For the energetically favored adsorption modes, we calculate a larger average spatial separation, by 1.3 Å, between the dye-based HOMO and the semiconductor surface in (001) and (101) TiO2 models. In terms of simple non-adiabatic electron transfer considerations, our model predicts a retardation of the charge recombination kinetics, in agreement with the experimental observations.


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1. Introduction Dye-sensitized solar cell (DSC) represents a particularly promising approach to the direct conversion of light into electrical energy at low cost and with high efficiency.1,2,3,4 In these devices, a dye sensitizer absorbs the solar radiation and transfers the photoexcited electron to a wide band-gap semiconductor electrode consisting of a mesoporous oxide layer composed of ca. 10-nanometer-sized particles, usually TiO2. The concomitant charge hole which is created on the dye molecule is transferred to an electrolyte solution or to a solid substrate functioning as hole conductor. In the first case, the dye is regenerated by electron donation from the electrolyte solution, containing suitable redox couple. The iodide/triiodide (I-/I3-) redox couple has been demonstrated to be one of the most efficient redox mediators in DSC to date,5 although it is now rivaled by metal complexes based on the Co(II)/Co(III)6,7,8,9, Cu(I)/Cu(II)10 or Fe(II)/Fe(III)11 redox couples. The three ingredients of a DSC, namely the dye, the semiconductor oxide and the redox mediator or hole conductor, can be individually or simultaneously optimized in search of higher efficiencies. Out of these three DSC components, the chemical nature and structure of the dye is by far the subject which has been more vastly investigated, with a general target of increasing the extinction coefficient and shifting the optical

absorption towards the near-IR region, thus

enhancing the overlap between the solar emission and the dye absorption spectrum and eventually achieving higher DSC’s photocurrents. Various Ru(II) polypyridyl complexes have primarily been employed as dye sensitizers.12,13The remarkable performance of the [cis-(dithiocyanato)-Ru-bis(2,2’-bipyridine4,4’-dicarboxylate)] complex (coded N3)14 and of its doubly protonated analogue (coded N719)15, had a central role in significantly advancing the DSCs technology, with solar to


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electric power efficiencies exceeding 11%.16 In the last years, substantial synthetic efforts have been devoted to Ru(II) complexes with improved characteristics compared to the prototype N719 dye. Thus, a flourishing family of heteroleptic and cyclometalated Ru(II) dyes have been designed and synthesized to provide higher molar extinction coefficient or peculiar supramolecular interactions compared to N719, thus enhancing the overall stability and/or efficiency of the DSCs.17-22 One of the most successful heteroleptic Ru(II) sensitizers is the C101 dye, [Na Ru (4,4′bis(5-hexylthiophene-2-yl) -2,2′-bipyridine) (4-carboxylic acid-4′-carboxylate-2,2′-bipyridine) (NCS)2], which was introduced in 2008 by Grätzel and coworkers.23 Thanks to its high molar extinction coefficient (17500 M-1 cm-1 at a wavelength of 547 nm) and to a corresponding improvement of charge-collection efficiency, it allowed demonstration of 11 % efficient solar cell. Such efficiency is still among the best values which were ever reported for the traditional (Imediated) DSCs, i.e. before the advent of Co-mediated DSCs.24 From the semiconductor side, mesoscopic TiO2 (anatase) is a unique photoanode material for DSC.25-27 Usual anatase crystals adopt mostly bipyramidal morphology, Scheme 1. The tetragonal bipyramid is terminated by the thermodynamically most stable (101) face, having the lowest surface energy of 0.44 J/m2.28,29 We can also find truncated bipyramidal crystals or rhombic-shaped crystals exposing the (010) face.30 These structures are reported less frequently,31,32 although the (010) face is predicted to be of similar stability as the ubiquitous (101) face.28,31 In real crystals, however, the second most abundant face is (001) with surface energy of 0.90 J/m2, that is about double of that of the (101) face.28 It contains exclusively fivecoordinated Ti-atoms which can be stabilized by fluorine as discovered by Yang et al.29 The


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optimization of the synthesis provided nanosheet materials enriched up to 90% with the (001) face.33,34 The anisotropy of electrochemical properties of anatase in the (101) or (001)-orientations is most straightforwardly investigated using large single-crystal electrodes,35,36 but the dye-sensitization was studied only on the (101) face of the single crystal electrode.35 The works on single-crystal electrodes concluded that the (001) face had more negative flatband potential (in electrochemical scale) and was more active for Li-insertion than the (101) face.36 Similarly, the rod-like crystals terminated by (001) face

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were more active for Li-insertion.37 The conclusions about flatband

potential shift38 and improved Li-insertion39 on (001)-oriented single crystal was later reproduced on polycrystalline electrodes, too. We notice, however, that different claim concerning the conduction band energy shift of (101) vs. (001) surfaces was communicated by Pan et al.40 based on combined XPS and UV-vis spectroscopy data. To the best of our knowledge - there is no computational study dealing with the interaction of Ru(II)-dyes with anatase (101) or (001) faces, while the electronic structure of anatase (101), (001) and (100) faces and their interaction with interaction of nitrogen-containing heterocycles was reported by Kusama et al.41 The anchoring of the prototypical N3 and N719 dyes has been carefully investigated in the past.42 Interestingly, Wu et al. reported that dye sensitized solar cell using N719 and (001) enriched anatase TiO2 had higher open circuit voltage and efficiency of conversion, compared to solar cell with ordinary (101) terminated anatase.43 Very recently, Laskova et al.

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presented an experimental study of the interaction of the top

performing C101 dye23 with polycrystalline TiO2 anatase

in (101) and (001) orientations.

According to this work, the C101/anatase (001) exhibited the following differences compared to the C101/anatase (101): (i) enhancement of the DSC voltage, (ii) smaller surface concentration


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of the dye and (iii) about six-times slower recombination kinetics, traced by transient absorption spectroscopy of the back electron transfer in pure solvent. The first conclusion was supported by the found negative shift of flatband potential of the (001) face, which is, however, still a subject of debate;36,38,40,44 the remaining two conclusions are just empirical findings at the moment. Hence, the motivation for a computational modeling of the system was to acquire further knowledge about the C101/titania interaction, which would address these observations.

2. Models and Methods Based on previous work by Persson et al.,45 we have set up a (101)-terminated (TiO2)82 anatase cluster of  2 nm side, 46 which we checked to show a comparable Density of States to that of the corresponding periodic surface model.47 To model the (001) anatase TiO2 surface, here we also set up a large cluster but due to the different topology of the (001) compared to the (101) surface we employed a surface-saturated model obtained by cutting a periodic (001)-surface slab of desired size to obtain a non-stoichiometric (TinO2n-x) cluster, i.e. with x missing oxygen atoms, to which we added 2x OH- groups which compensate the global charge and partly saturate tetracoordinated Ti atoms. The results is a (001)-cluster with global (TiO2)143(H2O)12 stoichiometry, see Figure 1. The two clusters expose roughly the same active surface area, but differ in the number of surface layers because of the saturation scheme adopted for the (001)-model. We optimized the geometries of the bare TiO2 models and of the corresponding dyeadsorbed structures in various configurations using a DZ basis set and a dispersion corrected D3PBE functional,48,49 as implemented in the ADF code.50 No simplifications in the dye structure were made. We considered the dye to alternatively carry two or one proton, which were allowed to stay on the dye or reach the surface during the unconstrained geometry optimizations. On the optimized geometries we performed single point energy evaluations and Time Dependent DFT


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(TDDFT) excited state calculations using the B3LYP functional51 and a 3-21G* basis set, including solvation effects by the C-PCM model,52 as implemented in the Gaussian 09 program package.53

3. Results and Discussion 3.1. Electronic structure of (101) and (001)-TiO2 surfaces We start our discussion by evaluating the electronic and optical properties for the (001)-TiO2 model against our previously reported (101)-TiO2 model. For the (001) and (101) models we calculate the two lowest TDDFT excitation energies at 3.53-3.60 and 3.49-3.52 eV, respectively, with the second excitation being optically active in (001), while in the (101) model the first excitation energy is optically active. Thus, considering the lowest optically allowed transition, the same optical band gap, within 0.01 eV, is calculated for the (001) and (101) models. The difference between the two data sets is definitely too small to be further commented, lying well within the accuracy of the employed models and methodology. We note, however, that our calculations overshoot the experimental optical band gap (3.2-3.3 eV) by 0.2-0.3 eV and that Liu et al.

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and Pan et al.40 reported a small blue-shift, within 0.1 eV, of the optical absorption

edge for decreasing (001)-TiO2 content in (101)/(001) samples. The electronic structure of the (101)- and (001)-TiO2 models are analyzed in terms of Density of States (DOS) in Figure 2. As it can be noticed, our results show a conduction band energy upshift for the (001)-surface model ranging between 50 and 110 meV. This translates into a corresponding negative shift, in the electrochemical scale, of the flatband potential for the (001) face of anatase. Our results are consistent with the theoretical work by Kusama et al., who found a sizable negative conduction band shift for (001)- compared to (101)-TiO2 surfaces.41


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Furthermore our data are in agreement with earlier report by Hengerer et al.55 who found a 60 mV downshift of the flatband potential for (101)-TiO2, using interfacial impedance (MottSchottky plots) as well as other electrochemical measurements on single crystal electrodes. Similarly, Kawakita et al.38 found comparable flatband downshift from Mott-Schottky plots for (001)-vs. (101)-oriented nanocrystals and Laskova et al.44 confirmed recently this trend by spectroelectrochemical methods. Despite the different models employed here, we believe the different conduction band energetics to be meaningful, since this data is referred to the entire conduction band structure, not only to a single energy level difference.

3.2 C101 dye anchoring geometry and surface coverage We calculated the adsorption mode of the C101 dye, carrying one and two protons, on the TiO2 clusters exposing the (001) and (101) surfaces. To screen possible dye adsorption modes, we carried out a series of 4-5 preliminary structural optimizations on a simplified dye model adsorbed on both (001) and (101) surfaces, in which the C101 5-hexylthiophene-2-yl bipyridine substituents were replaced by methyl groups, by varying the position of the protons and the binding mode (monodentate vs. bidentate) of the two carboxylic groups. We found two more stable adsorption modes, on both the (001) and (101) surfaces, which we label as A and B in Figure 3 and 4. Structures A and B differ in the binding of the dye carboxylic groups to the TiO2 surfaces: while A has a dissociative bridged bidentate and a dissociative monodentate adsorption mode of the two anchoring carboxylic groups on both the (001) and (101) surfaces, structure B has both carboxylic groups dissociatively adsorbed on TiO2 in a monodentate fashion on the (101) surface, while on the (001) surface one of the two monodentate carboxylic groups is undissociated, being stabilized by hydrogen-bonding to a nearby surface oxygen. The different


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adsorption modes give rise to the anchoring bipyridine group to lay almost perpendicular to the TiO2 surface plane in structures B, irrespective of the surface, while in A the anchoring bipyridine ligand shows a variable angle between 70 and 45  in the (001) and (101) TiO2 surfaces, respectively. While adsorption modes A and B show a similar binding motif on the (001) and (101) surfaces, their relative energies change substantially on the two surfaces. On the (001) surface, C101 carrying two protons shows adsorption mode A to be more stable than B by 15.5 kcal/mol, while on the (101) surface this difference reduces to 3.4 kcal/mol. Furthermore, moving to the monoprotonated dyes, on the (001) surface the A-B difference increases to 17.2 kcal/mol, while on the (101) surface the B adsorption mode is more stable by 5.6 kcal/mol. Thus, both for the dye carrying one and two protons, our calculations strongly indicate that the A adsorption mode is favored on the (001) surface, mainly because of a better structural matching between the two dye carboxylic groups and the pattern of five-coordinated Ti surface atoms. Furthermore, our data suggest that a smaller dye coverage should be found for the (001)-adsorbed dyes, due to the larger surface occupation of the energetically favored structure A, which is characterized by a partial contact of the thiophene and alkyl bipyridine substituents with the TiO2 surface. In this respect, our model study agrees with the experimental measurement by Laskova at al.42 who found the C101 surface coverage between 0.4 to 0.5 molecules/nm2 for the (001) face and 0.7 to 0.8 molecules/nm2 for the (101) face. Similarly, a reduced surface concentration of N719 dye on the (001) face was found by Yu et al. compared to the (101) surface.54 3.3. Electronic structure of dye-sensitized TiO2: Implications for charge recombination We investigated the HOMO spatial distribution in the A and B adsorption modes, which in a single particle approximation corresponds to the charge hole localized on the oxidized dye after


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electron injection. The C101 HOMOs are of mixed Ru-NCS character, as typical for this class of dyes,23 see Figure 5 and 6 for the dye adsorbed on (001)- and (101)-TiO2 surfaces. We calculate HOMO values of -5.41  0.02 eV for the four doubly protonated C101@TiO2 models considered here. The C101 dye oxidation potential in solution was measured at 0.90 V vs. SHE,23 i.e. -5.34 eV vs. vacuum. Thus our calculated HOMO energy values are in excellent agreement with available experimental data. Based on the calculated adsorption modes and electronic structure for the fully interacting C101 dye-sensitized (101)- and (001)-TiO2 models, we now wish to model the recombination process between TiO2-injected electrons and the oxidized dye. Following non-adiabatic electrontransfer theory57 the rate of electron transfer between an electron into TiO2 (donor) and the oxidized dye (acceptor) is given by:

where HAB2 is the electronic coupling between the donor and acceptor states, G0 is the reaction free energy, and  is the reorganization energy. The term HAB2 corresponds to electron tunneling through a potential barrier and therefore can be represented in terms of an exponential function of the spatial separation r between the donor and the acceptor:

where  is a function of the barrier height of the intervening media. For charge recombination in dye sensitized electrodes in absence of the electrolyte solution, r effectively corresponds to the spatial separation of the charge hole residing on the oxidized dye (approximated here by the


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neutral dye HOMO) from the electrode surface, while G0 corresponds to the difference in free energy between the dye oxidation potential and the electrode Fermi level. Previous work on Ru(II)-dye sensitized TiO2 by Clifford et al. has shown that the recombination dynamics were relatively insensitive to variations in the G0 parameter,58 a fact that was interpreted as the recombination reaction being nearly optimal in terms of Marcus parameters, i.e. with G0 almost equal to . A strong correlation was instead observed between the recombination dynamics and the estimated spatial separation r.58 Considering the very complex systems investigated here and the fact that we are looking at the same dye in the same environment on different TiO2 surfaces, here we will limit our attention to the analysis of distance dependence. This seems to be a reasonable assumption to trace some qualitative conclusions considering that, as discussed above, we do not find appreciable variations in the dye oxidation potential for the various investigated models and that the reorganization energy is not expected to appreciably vary by changing the semiconductor structure. The only variation resides in the different conduction band energetics for (101)- vs. (001)-TiO2. Thus, assuming G0 and  to be roughly constant, we can try to explain the observed differences in recombination kinetics between (101)- and (001)-TiO2 by investigating the varying distance dependent electronic coupling. Previous work by some of us applied similar arguments to qualitatively gauge the different injection/recombination kinetics in organic dyes with different anchoring groups,59 while a recent theoretical work by Maggio et al. has carefully evaluated the relevant electron transfer parameters, including the electronic coupling, affecting the recombination dynamics for a series of organic dyes.60


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We calculated the average distance distribution of the dye-based HOMO from the TiO2 surface in the four investigated cases, i.e. adsorption modes A and B on the (001) and (101) surfaces. This quantity is obtained by numerical integration of the dye-based HOMO distance from the average TiO2 surface plane. Most notably, structure A adsorbed onto the (001) surface shows an average HOMO spatial separation from the surface of 8.5 Å, while the corresponding value for structure B is 7.2 Å. On the (101) surface, structures A and B show closer values, 7.5 and 7.3 Å, respectively. As a matter of fact, while structures B are very similar on the (001) and (101) surfaces, hence the similar HOMO distribution relative to the TiO2 surface, structure A is more tilted with respect to the surface plane on the (101) surface, thus a shorter HOMO spatial separation from the semiconductor is calculated. As discussed above, our results show that structure A is strongly favored for both the diprotonated and mono-protonated C101 dye on the (001) surfaces. Considering that on average the HOMO spatial distribution is located at a larger distance from the surface in this structure than in the other investigated structures, our model suggests that a slower recombination between injected electrons and the oxidized dye might take place for the C101 dye adsorbed onto the (001) surface. Although based on a simple qualitative kinetic model, this conclusion is consistent with the transient absorbance study of the C101 oxidized dye measured after electron injection into TiO2 conduction band in the absence of redox mediator.44 Under these conditions, the back electron transfer followed a first-order kinetics, with rate constants of 1200 s−1 for the (001) nanosheets and 7700 s−1 for the (101) nanoparticles, respectively.44 The observed factor of 6 slowdown of the recombination rate was intuitively ascribed to a 1.5 Å increase of the distance between the Ru(III) center of the oxidized dye and the closest Ti(IV) site on the surface,44 which


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nicely compares with the results of our computational model. We notice that also Wu et al. reported on effectively retarded charge recombination process on the (001) surface.43

4. Conclusions We have presented a first-principles computational investigation on the adsorption and electronic structure of the highly efficient heteroleptic Ruthenium dye C101, [NaRu(4,4′-bis(5hexylthiophene-2-yl)-2,2′-bipyridine)(4-carboxylic acid-4′-carboxylate-2,2′-bipyridine)(NCS)2], on TiO2 models exposing the (001) and (101) surfaces. To model the TiO2 (001) anatase surface, we have set up a large cluster of (TiO2)143(H2O)12 stoichiometry and compared its electronic and optical properties with our previously reported (101)-terminated (TiO2)82 cluster. The electronic structure of the (101)- and (001)-TiO2 models have been analyzed in terms of Density of States, showing a conduction band energy upshift for the (001)-surface model ranging between 50 and 110 meV compared to the (101)-TiO2 model. This translates into a corresponding negative shift of the flatband potential in the electrochemical scale (referenced to SHE) for the (001) face of anatase, in agreement with previous theoretical calculations and with earlier interfacial impedance and recent spectroelectrochemical data. For the (001) and (101) TiO2 models we calculate essentially the same TDDFT optical band gap, within 0.01 eV. Although the difference between the two data sets is well within the accuracy of our computational approach, we note that optical absorption blueshifts within 0.1 eV were reported for (101)-terminated TiO2 surfaces. We found two dominant adsorption modes of the C101 dye on the (001) and (101) surfaces. The two structures differ in the binding of the dye carboxylic groups to the TiO2 surfaces (bridged bidentate vs. monodentate), leading to sizably different tilting of the anchoring


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bipyridine plane with respect to the TiO2 surface: for the most stable structure, the tilting angle is 70 on the (001) surface, which reduces to 45  on the (101) surface. The different adsorption mode implies a smaller surface dye coverage on the (001) surface, as experimentally found, due to partial contact of the thiophene and alkyl bipyridine substituents with the TiO2 surface. By taking the neutral dye-adsorbed HOMO as an approximation to the charge hole which is generated after electron injection, we calculate rather different average spatial separations between the HOMO and the semiconductor surface in (001) and (101) TiO2 models. As a matter of fact, a longer average spatial separation of 1.3 Å is found for the C101 dye HOMO to the (001)-TiO2 surface compared to the (101) case. In terms of simple non-adiabatic electron transfer arguments, our model predicts a retardation of the charge recombination kinetics on the (001) surface, in agreement with the experimental observations. In conclusion, our first-principles modeling has allowed us to unravel the atomistic features of the complex dye/semiconductor interface lying at the heart of dye-sensitized solar cells based on different TiO2 facets, allowing for further optimization of this important class of photovoltaic devices.

Acknowledgement: We thank FP7-NMP-2009 Project SANS (contract No. NMP-246124) for financial support of this work.LK thanks the Academy of Sciences of the Czech Republic (contracts IAA 400400804 and KAN 200100801) for financial support. FDA thanks Fondazione Istituto Italiano di Tecnologia, Platform Computation, Project SEED 2009 “HELYOS” for a grant.


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Scheme and Figure Captions: Scheme 1. Model TiO2crystal, showing the 101 and 001 surfaces. a) side view, b) top view. Figure 1. Optimized geometrical structures of the (001)- and (101)-anatase TiO2 models. Figure 2. Calculated density of states in the TiO2 conduction band for (001), green, and (101), red,TiO2 models. The inset shows a zoom in the conduction band edge energy range. Figure 3.Calculated C101 adsorption modes A and B on the (001)-TiO2 surface for the doubly protonated dye. Figure 4.Calculated C101 adsorption modes A and B on the (101)-TiO2 surface for the doubly protonated dye. Figure 5. Isodensity plot of the HOMO for C101 in adsorption modes A and B on the (001)TiO2 surface. Also reported are the average HOMO spatial separations from the TiO2 surface plane. Figure 6. Isodensity plot of the HOMO for C101 in adsorption modes A and B on the (101)TiO2 surface. Also reported are the average HOMO spatial separations from the TiO2 surface plane.


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Scheme 1


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Figure 1


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Figure 2

Figure 3


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Figure 4

Figure 5


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Figure 6


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For Table of Contents use:

Modeling Ruthenium Dye Sensitized TiO2 Surfaces Exposing the (001) or (101) Faces: A First Principles Investigation Filippo De Angelis, Giuseppe Vitillaro, Ladislav Kavan, Mohammad. K. Nazeeruddin, Michael Grätzel


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Modeling Ruthenium-Dye-Sensitized TiO2 Surfaces Exposing the

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