Structural Modification of TiO2 Surfaces in Bulk Water and Binding

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Structural Modification of TiO Surfaces in Bulk Water and Binding Motifs of a Functionalized C on TiO Anatase and Rutile Surfaces in Vacuo and in Water: Molecular Dynamics Studies 60

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Alexey Leonid Kaledin, Tianquan Lian, Craig Livingston Hill, and Djamaladdin G Musaev J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp307211h • Publication Date (Web): 06 Sep 2012 Downloaded from http://pubs.acs.org on September 9, 2012

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The Journal of Physical Chemistry

Structural Modification of TiO2 Surfaces in Bulk Water and Binding Motifs of a Functionalized C60 on TiO2 Anatase and Rutile Surfaces in Vacuo and in water: Molecular Dynamics Studies Alexey Kaledin,a) Tianquan Lian,b) Craig L. Hill,b) Djamaladdin G. Musaeva), * (a) Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, GA 30322; (b) Department of Chemistry, Emory University, Atlanta, GA 30322 Abstract: The nature of several TiO2 surfaces in liquid water, as well as the adsorption of a functionalized C60, L*C60, (where L is a carboxylic acid) on TiO2 anatase and rutile low index surfaces in vacuo and in liquid water have been studied at the self-consistent charge density functional tight-binding (SCC-DFTB) level of theory. It is shown that the SCCDFTB method provides very good agreement with the high-level DFT data. The typical binding motif of L*C60@TiO2 is found to be the formation of a strong HC1C(O2H)O1Ti5C/O1-Ti4C bond with a distance of 2.0-2.1 Å and a weaker HC1CO1O2-H…O2C/O2…H-O2C hydrogen-bond. In some cases, a terminal OH of the linking group coordinates with a Ti-O-Ti bridging oxygen and looses the H to the surface. The adsorption energies in vacuum range between 21 and 82 kcal/mol depending on the surface. Density of states of these species reveal the presence of peaks below the surface conduction band upon ligand adsorption, which is due to the low-lying LUMOs of the L*C60. Electron transfer from the surface to the ligand is thus possible via the initial UV photoexcitation of the surface followed by non-radiative relaxation of the excited electron to the LUMO of the ligand. This pattern was observed for all six surfaces considered in the present work. Solvation of C60@TiO2 in liquid water does not change the qualitative character of surface-ligand binding. In all cases the ligand remains bound to the surface in the presence of water. The interaction of water molecules with the surface shows various patterns depending on the surface index. Anatase (101) and (100) surfaces favor non-dissociative water adsorption, while anatase (001), rutile (001), (110) and (101) surfaces show dissociative water adsorption which results in OH/OH2-terminated TiO2 surfaces. The latter finding is in agreement with several previous DFT studies reported by others.

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1. Introduction Semiconductors with large band gaps, such as TiO2 and others, have been extensively used for conversion of UV light into chemical energy.1,2,3,4,5 Of these, TiO2 is the most frequently used semiconductor due its superior efficiency, thermal stability, low cost and non-toxic nature.6,7,8,9 It exists in nature in three possible phases, anatase, rutile and brookite. Although the rutile phase is the most thermodynamically stable one, anatase exhibits more suitable optical properties, such as a large band gap, for UV light harvesting.10 Recent experiments have shown, however, that catalytic efficiency of TiO2 can be further improved by introducing various dopants, creating impurities and adding various types of ligands to the surface.11,12,13,14,15,16,17,18,19,20,21 To yield more efficient photocatalytic activity of TiO2, conjugated electron scavenging materials, such as fullerenes, have been proposed as potent agents for more efficient electron-hole separation.22,23,24,25,26 In particular, C60, which has a large electron affinity, is shown to be capable of promoting a fast electron-hole separation by capturing the photo-excited electron and preventing its recombination with the hole. 27,28,29 To pursue this further, several experiments investigated the role of linkage group L between the surface (in some cases the quantum dot30) and C60.31 In the present work, we continue to investigate the role of linkage groups in fullerene binding on TiO2 surfaces. We consider two TiO2 phases, anatase and rutile, and three surface cuts for each phase, namely, anatase (101), (100), (001) and rutile (001), (110), and (101). There are several experimental and computational studies focusing on these surfaces that provide a valuable background for more detailed investigations. In particular, Park et al. studied interactions of C60(OH)12 with anatase (101), experimentally and theoretically,31 using a cluster model at DFT and configuration interaction (CI) levels of theory for the latter. In that work, the effect of hydration of anatase (101) was argued to be a main factor in facilitating surface-to-ligand charge transfer. Fungo and co-workers performed photovoltaic measurements of a PorphirineC60 system attached to TiO2 crystals via Porphirine-TiO2 interactions.32 More recently, Song et al.30 reported on electron transfer from a quantum dot (QD) to a C60 functionalized with two linking groups used to enhance QD-ligand binding. The abovementioned experiments are typically carried out in the presence of liquid water. Therefore,

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it is essential to elucidate the nature of the TiO2 surfaces, as well as the structure of a functionalized C60, L*C60, on TiO2 surfaces, i.e. L*C60@TiO2, on the liquid water. There are several density functional studies reported in the literature that examine water/TiO2 interaction in detail, both the binding motifs and dynamical and spectroscopic properties.10,33,34,35 However, no detailed studies of functionalized C60 interaction with TiO2 surfaces in vacuum or in water have been reported, to our knowledge. In the present paper, we address these issues at the self-consistent charge density functional tightbinding (SCC-DFTB) level of theory, which we will refer to simply as DFTB for the rest of the paper. The tight-binding theories36 have been a major cornerstone in solid-state physics, and are related to the extended Hückel and semiempirical molecular orbital theories. Briefly, the DFTB approach: (1) is based on a 2nd-order expansion of the Kohn-Sham total energy with respect to density fluctuations about a reference density ρ0. The introduced relaxation of the charge density has greatly decreased the dependence of the results on the initial density (ρ0) and increased the transferability of parameters, and (2) explicitly treats only the valence electrons represented in a minimal basis of atomic-like orbitals that makes it significantly faster than the regular DFT approach. Recent theoretical advancements 37,38,39,40,41 have made it routine to study single molecules, clusters, and especially periodic systems, involving hundreds of atoms. Since the electronic integrals are not computed but kept in memory on a radial grid and interpolated as needed, it makes DFTB significantly faster than the regular DFT approach. Although the extent of available parameters for transition metals is currently still rather limited in DFTB, a few important elemental interactions, including Ti-O, Ti-C and Ti-H and bulk TiO2 have been parameterized.39,42 These and other reported parameters, necessary for describing TiO2 with a functionalized C60 in vacuo and in water, make it possible to carry out large scale simulations of interaction of TiO2 surfaces with functionalized chromophores both in vacuo and in water. We note that several low index TiO2 anatase and rutile surfaces have been investigated in detail using DFTB.42 The reported band structure calculations revealed good agreement with experimental measurements of band gaps. For calculations of molecular systems, which included H, C and O atoms, adsorbed on several TiO2 surfaces,

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binding and atomization energies close to their DFT analogues were reported.42 In addition, proper performance of the method against results of full DFT calculations for water splitting on anatase (001) was demonstrated. A few exploratory bulk calculations reported in the section below also agree very well with experiment and previous theoretical calculations. Similarly, extensive DFTB calculations of a single C60 have been reported previously.43 Its structural parameters were shown to agree very well with B3LYP/cc-pVTZ results. However, there are no extensive studies on nature of interaction of TiO2 surfaces with bulk water and functionalized chromophores including C60. The rest of the paper is organized as follows. Section 2 describes the computational methods; section 3 details gas phase structures of the functionalized C60 groups, surface adsorbed C60, and surface-water interaction dynamics with and without C60. In section 4 we summarize the findings and make conclusions. 2. Computational details The electronic structure calculations reported in this work were done using the density functional tight binding theory, as implemented in the DFTB+ suite of codes, version 1.1.44 Long-range dispersion forces, which cannot be described by DFTB, were added via a Lennard-Jones term for all pairs of nuclei; the atomic parameters for the latter were taken from the UFF force field.45 Electronic overlap parameters were taken from two different databases: the Ti-A (where A=Ti, O, C, H) pairs are from the tiorg-0-1 set,Error! Bookmark not defined. while all non-metal involving pairs are from the mio-0-1 set.39 Most calculations were done as spin-unpolarized, i. e., as singlet electronic states. In several instances, such as for calculation of ionized surfaces and excitation energies, we performed spin-polarized calculations with corresponding spin coupling constants.46,47 To validate the methods and K-point sampling schemes we performed several exploratory calculations. It was found that 4-4-4 grids produced generally well-converged densities of states, along with a few other properties for bulk anatase and rutile. Specifically, the band gap for anatase was found to be 3.22 eV, very close to the measured value48 of ~3.2 eV. Similarly, rutile bulk calculations revealed a band gap of 3.18 eV, consistent with experimental measurements that show a smaller band gap than for anatase,49 i.e. ~3.0 eV.

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The anatase (101) surface was modeled by a (5 x 2) (TiO2)120 cell for L*C60 adsorption studies. We also used a smaller area (101) cell, namely (3 x 1) (TiO2)36, for simulations with liquid water, following the earlier work of Sumita et al.35 The monoclinic cell parameters used in the calculations are 18.925 x 20.478 x 45.42 and 11.355 x 10.239 x 45.42 Å respectively and α=111.7 º. The anatase (001) surface was modeled by a (4 x 4) tetragonal (TiO2)64 cell for interaction with L*C60, and by a (3 x 3) (TiO2)36 cell for water simulations, similar to the earlier work35. The respective cell dimensions are 15.14 x 15.14 x 38.056 and 11.355 x 11.355 x 28.542 Å. For anatase (100) we used a (3 x 3) tetragonal (TiO2)36 9.514 x 11.355 x 34.065 Å cell for water simulations and a larger (4 x 4) (TiO2)96 19.028 x 15.14 x 34.065 Å cell for simulations with an adsorbed L*C60. We also considered three rutile surfaces. The (001) surface was modeled by a (3 x 3) tetragonal (TiO2)54 cell for both L*C60 adsorption and water simulations. The cell dimensions were slightly different for the two cases, namely, 13.781 x 13.781 x 35.497 and 13.781 x 13.781 x 26.622 Å, respectively. Rutile (101) was modeled by a (3 x 3) (TiO2)36 10.9274 x 13.7811 x 35.4972 Å cell (α=57.2º), and rutile (110) by a (TiO2)70 11.8324 x 12.9929 x 41.3433 Å (α=55.0º) cell. A reduced dimensionality normal mode analysis was performed for a few structures of interest by generating and diagonalizing an 3M x 3M block of the full 3Nx3N Hessian matrix, with N being the total number of atoms and M a local sub-set of atoms. Since analytic second derivatives are presently not available in DFTB+ code,44 the Hessian was constructed by numerical differentiation of the gradient. We adjusted the Cartesian differentiation step Δ from 1e-6 to 1e-3 Å to monitor the convergence of frequencies for a few representative structures and found converged values at 1e-4 Å. Thus, all normal mode analyses reported below were performed with Δ=1e-4 Å. When constructing mass scaled Hessians, we used pure isotope masses, i.e. H(1.000), C(12.000), O(16.000) and Ti(48.000). For calculation of infrared (IR) absorption cross sections we used the double harmonic approximation for the matrix elements of the type, where ψi is the vibrational wavefunction (approximated by a harmonic one), and µ is the nuclear dipole moment (approximated by a 1st order Taylor series). Using the point charge model for

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the dipole, i. e., µ=Σaraqa, where the sum runs over relevant atomic sites “a”, the IR cross section at frequency ω is Σ (Σaua qama-1/2)2/(2ω), where ua is the normal mode α

α

α

eigenvector’s Cartesian α component at atom “a”, and ma are atomic masses (in units of electron mass). The corresponding normal mode reduced masses are evaluated by massunscaling the ua vectors, leading to (Σa|ua|2/ma)-1. Finally, for studies of thermal equilibrium and the effects of water on the surfaces, molecular dynamics simulations were performed as quasi NVT trajectories at 300 Kelvin. In this work we are interested primarily in structural motifs, binding patterns and binding energies, and since we do not report IR and power spectra we can obtain reliable data using a relatively large time integration step. Preliminary explorations of trajectories showed that a Δt=1 fs time integration step was a safe choice in this case. We used the Berendsen thermostat50 with the time response parameter τ=100 fs. All trajectories were propagated using the velocity Verlet algorithm. SCC convergence was monitored for all systems at every time step and no unconverged points were encountered. 3. Results and discussion 3.1) Structure of functionalized C60, L*C60 in gas-phase, where L = CHCO2H In general, C60 and a pendant carboxylate acid CHCO2H can bind in several different ways, the four most relevant ones are summarized in Figure 1 and Table 1.

C6 C6 O1

C1 C

C6

C5 O2

H

I(C6-C5)

H

C1

O2

C

C6' O1

C6 O1 H

I(C6-C6')

C

C5 C1

O2

II(C6-C5)

C6 O1 H

C6' C1

C

C6' 5 C

O2

II(C6-C6')

Figure 1. The four most relevant isomers and the used atomic notations of the carboxylate acid CHCO2H functionalized C60, L*C60, reported in this paper.

--- Table 1 ---

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As the calculations show, energetically the most favorable interaction mode of C60CHCO2H is interaction of the C1(H) atom of CHCO2H with a C6-C5 bond (~1.45 Å) of C60 shared by its connected C6 and C5 rings, called as I(C6-C5). As a result of this interaction the C6-C5 bond of C60 elongates to 2.23 Å, and two, C6-C1 and C5-C1, bonds form with a 1.50 Å bond distance. The HOOC-C1H bond of CHCO2H elongates from 1.42 Å in the free molecule to 1.54 Å in the bound group. The calculated C60-CHCO2H binding energy of this conformation is 74.2 kcal/mol. Interaction of the C1 atom of CHCO2H with a symmetric C6-C6’ bond of two connected six-membered rings of C60, called as I(C6-C6’), is energetically 8.6 kcal/mol less favorable than for I(C6-C5). The relatively weak interaction of the carboxylate acid in I(C6-C6’) is manifested in less elongation of the C6-C6’ bond distance, from 1.41 to 1.60 Å. Vibrational analysis of the infrared active modes reveals (see Table 2) typical bond stretching vibrations, most notably the carbonyl C=O1 stretch at 1745 (in I(C6-C6’)) and 1734 cm-1 (in I(C6-C5)). Table 2 includes several other important modes of vibration. --- Table 2 --In the other two reported low energy conformations of C1HCO1(O2H)-C60, called as II(C6-C6’) and II(C6-C5), the C1HCO1(O2H) bonded to C-C bonds of C60 via its carbonyl oxygen (C=O1) and C1 atoms. Attachment of carboxylic acid to the symmetric C6-C6’ bond of two connected six-membered rings, i.e. II(C6-C6’), is found to be more favorable than II(C6-C5): the calculated C60-CHCO2H binding energy of these isomers is 74.0 and 57.3 kcal/mol, respectively. The calculated C6-O1/C6/5-C1 bonds between C60 and CHCO2H fragments are 1.51/1.52 and 1.50/1.52 Å for II(C6-C5) and II(C6-C6’), respectively. 3.2) Adsorption of a functionalized C60, L*C60, on TiO2 in vacuo, where L = CHCO2H After examining the properties of the lowest energy isomers of the functionalized C60, L*C60, below we address its interaction with TiO2 surfaces, i.e. L*C60@TiO2, both in vacuo and in liquid water. Here, we consider interaction of two lowest energy isomers of L*C60, i.e. I(C6-C6’) and I(C6-C5), with two TiO2 phases, anatase and rutile, using three surface cuts for each phase, namely, anatase (101), (100), (001) and rutile (001), (110), and (101).

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a) anatase (101) Figure 2 summarizes the binding pattern of the two energetically lowest conformers, I(C6-C6’) and I(C6-C5), of functionalized fullerenes L*C60 on the anatase 2.21 (1.45)

C6 1.51

C6

C5

1.60 (1.41)

0.11 1.51

1.11 C1 1.53 0.09 H 0.68 0.13 1.25 1.31 C 2 0.47 O1 0.59 O 1.72

C1 1.54 0.07 H 1.12 0.66 C 1.25 1.31 O2 1.72 0.43 O1 0.57 0.35 H 0.99 O2C 0.49 (0.51)

1.92 0.98 (0.90) Ti5C

C6'

1.51

1.52

0.36 H 1.00 O2C 0.49 (0.51)

1.99 0.94 (0.90) Ti5c

a(101) 2.22 (1.45)

C6 1.50

C5 C6

1.50

1.61 (1.45) C6' 1.50

1.50 0.08

0.11 C1 1.54 H 1.12 0.64 C 1.24 1.33 O2 0.56 1.85 0.48 O1 2.0

0.98 (1.00) Ti5C

0.35

1.11 C1 1.53 0.09 H 0.14 0.65 C 1.24 1.33 O2 0.48 O1 0.56 1.87 H 2.0 0.35 0.98

H

0.98 O2C 0.47 (0.45)

0.98 (1.00) Ti5C

O2C 0.47 (0.45)

a(100)

C6

2.23 (1.45)

C5

1.48 (1.45)

1.51 1.50 1.41 (1.41)C

C6

C

1.52

2.18

2.87 1.12 C1 1.54 0.07 H 0.10 0.65 O1C C 1.24 0.48 (0.54) 1.34 2 0.42 O1 0.52 O 2.36 0.33 H 0.98 1.92 O1C 0.90 (0.90) Ti5C 0.55 (0.51)

1.61 (1.41) C6' 1.49 (1.45) C

1.51

2.61

1.11 C1 1.52 0.09 H 0.14 0.67 1.33 C 1.24 1 0.42 O

0.52 O2

1.91 0.89 (0.90) Ti5C

a(001)

O1C 0.49 (0.54)

2.35 H 0.32 0.98 O1C 0.55 (0.54)

Figure 2. The calculated structures of I(C6-‐C6')@TiO2 and I(C6-‐C5)@TiO2 and their schematic presentations for TiO2 a(101), a(100), and a(001) surfaces. The black numbers are bond distances (in Å), and blue/red numbers are negative/positive atomic charges (in e). The values in parentheses are for the free L*C60 and clean surfaces.

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(101) surface, called below as a(101) surface. L*C60 binding motif to a(101) surface is similar in both cases, i.e. terminal (carbonyl) O1 of L*C60 coordinates to a 5-center Ti (referred as Ti5C from here on) forming a single metal-oxygen bond. Simultaneously, the terminal O2H of L*C60 loses the H-atom to the bridging surface O (i.e. O2C). Population analysis for individual atoms shows about 0.35-0.35 |e| positive charge on the H-atom. At the same time, the charges of the Ti5C and O2C centers changes from +0.98 to +0.90 |e| and from -0.49 to -0.51 |e|, respectively. The calculated O1-Ti5C bond distances of L*C60@TiO2 a101 are 1.92 and 1.99 Å for the I(C6-C5), and I(C6-C6’) conformers of L*C60, respectively. These values are consistent with a single bond character of the O1-Ti5C bond. The I(C6-C5)@TiO2 a101 is found to be ~1.5 kcal/mol lower in energy than I(C6-C6’)@TiO2 a101 conformer (see Table 3). --- Table 3 Inspection of vibrational frequencies in the ester region, roughly between 1100 and 4000 cm-1, shows that there are significant shifts in the CO frequencies compared to those prior to the L*C60 adsorption on a(101) surface: the calculated C-O2 frequencies increased to 1224 cm-1 (from 884 cm-1) and 1278 cm-1 (from 919 cm-1) in I(C6C6’)@TiO2 and I(C6-C5)@TiO2 a101, respectively (see Table 4). The frequency of the C-O1 bond, which has double bond character with a 1.20 Å bond distance, reduces to 1518 (from 1745 cm-1) and 1543 cm-1 (from 1734 cm-1) in I(C6-C6’)@TiO2 and I(C6C5)@TiO2 a101, respectively. The O2C-H surface mode (3262 cm-1) appears significantly lower than a typical O2-H vibration (for example, 3638 and 3668 cm-1 in free I(C6-C6’), and I(C6-C5) conformers, respectively) and resembles the symmetric stretch in H3O+, both in frequency and the low IR activity.51 --- Table 4 --The above-presented vibrational changes upon going from free L*C60 to its adsorption state L*C60@TiO2 a(101) are consistent with the changes in geometry of linkage L-group. Indeed, upon L*C60 adsorption on the a(101) surface its C-O1 bond elongates from 1.20/1.21 Å to 1.31/1.31 Å for I(C6-C5)/I(C6-C6’) conformers. In contrast, the C-O2 bond distance shortens from from 1.39/1.38 Å to 1.25/1.25 Å upon L*C60 adsorption on a(101) surface.

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Analysis of the density of states (DOS), seen in Figure 3a, shows that the band gap of TiO2 anatase (101) shrinks from 3.19 eV to ~1.5 eV with either L*C60 isomer adsorbed on the surface. (We note that here and below that the value of the band gap was extracted by gradually increasing the Gaussian exponent used for smoothing the stick spectrum, until the lineshape at band edges converged.) The decrease of the band gap is due to the LUMO orbital of L*C60, which is located lower in energy than the conduction band of the surface (see caption of Figure 3 for more details). They are visible on the DOS curve as a pair of slight peaks roughly between -8 and -6 eV. Other regions where L*C60 bands are visible are the broad band at [-24,-16] eV, being mainly the C-C σ and π bonding orbitals, and a peak around -3 eV. Since the upper edge of the valence band of the a(101) surface appears to be unaffected by the interaction with the linkage group, the implications for photochemical experiments of L*C60@TiO2 a(101) are expected to be similar

to

the

ones

reported

for

TiO2 6

5

6

6’

anatase

(101)

surface.31,32

I(C -C )@TiO2 I(C -C )@TiO2 TiO2

(a)

Density of states

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-15

-10 -5 Energy (eV)

(d)

(b)

(e)

(c)

(f)

0

-15

-10 -5 Energy (eV)

0

Figure 3. The density of states of a clean TiO2 surface (black line) and with each of the two ligands adsorbed (red and green lines). MO energies (HOMO1, LUMO1-LUMO24) of the

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respective gas phase ligands are shown as red/green vertical sticks. The zero of energy for a particular surface is defined as the lowest electronic energy level of a singly ionized clean surface, cf. Table 3. The highest level of the valence band and the lowest level of the conduction band of the clean surface are: (a)-9.55 and -6.36 eV for a(101), (b) -9.75 and -6.72 eV for a(100), (c) 8.19 and -5.54 eV for a(001), (d) -8.16 and -4.94 eV for r(001), (e) -8.30 and -6.10 eV for r(110) and (f) -8.98 and -5.89 eV for r(101).

Thus, a photoelectron excited to the conduction band of TiO2 will relax into the LUMO orbitals of L*C60, by virtue of strong surface-ligand interactions, leaving the surface positively charged. To investigate this further, we performed additional calculations for a triplet electronic state by using a spin-polarized Hamiltonian and the corresponding spin coupling constants. The calculations predict a 3.27 eV vertical excitation energy for a clean TiO2 anatase (101) and a 1.60 eV vertical excitation energy for I(C6-C6’)@TiO2 a(101). These excitation energies nearly exactly map the pattern seen in the DOS curves on the [-8,-6] eV segment and are consistent with surface-toligand excitation scheme. b) anatase (100) Figure 2 also summarizes the binding pattern of the two functionalized fullerenes on the anatase (100) surface (below called as a(100)). The structure appears very similar to the a(101) surface with main differences in the O2-Ti5C and O2…H bonds, which are 0.08 and 0.13 Å longer, respectively. The longer bonds reflect the weaker binding of L*C60 to the a(100) surface compare to the a(101) surface. Indeed, the calculated energy of the reaction L*C60@TiO2 a(100) → L*C60 + a(100) is is -39.0 and -34.6 kcal/mol kcal/mol for I(C6-C6’) and I(C6-C5) conformers, respectively (see Table 3). Characterization of the most important vibrational modes is given in Table 5. There are clearly identifiable as C=O2, C1-H and surface O2C-H stretches. The surface bound C-O1 has a large component of oxygen motion (see large reduced mass) towards the surface and is thus ~100 cm-1 to the red of its a(101) counterpart. --- Table 5 ---

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The density of states of L*C60@TiO2 a(100) is shown in Figure 3b, and as can be seen, it closely mimics the one of L*C60@TiO2 a(101). The band gap of the clean a(100) surface is 3.03 eV, slightly narrower than a(101). The low lying LUMO orbitals of L*C60 ligands bring down the gap to ~1.6 eV; their presence can be discerned by two small bumps in the DOS on the [-8,-7] eV segment. c) anatase (001) Similar to the a(100) and a(101) surfaces, the a(001) surface is terminated by O2C oxygens which bridge two Ti atoms, however, both Ti atoms are 5-coordinated (see Figure 2). Placing either L*C60 on the surface and relaxing the geometry readily coordinates the terminal oxygen (O1) of the linkage group to a Ti5C site, forming a 1.92 Å O1-Ti5C bond. This results in cleavage of the adjacent O2C-Ti5C bond and formation of O1C center protruding from the surface. A nearby O2C (bridging two other Ti atoms) draws away the H-atom from the O2H group of L*C60 while simultaneously losing one of its two O2C-Ti bonds. The final converged structure for either isomer has an O1C and an O1CH pointing slightly upwards, and one of the former Ti5C centers becomes Ti4C. This formation leads to a highly stable structure, 83.7 kcal/mol below dissociation for I(C6C6’) conformer, and 75.5 kcal/mol below dissociation for I(C6-C5) conformer. Vibrational analysis shows a well-defined single bond C-O1 (1149/1128 cm-1) and double bond C=O2 (1616/1617 cm-1) stretches, for I(C6-C6’)@TiO2/I(C6-C5)@TiO2 respectively, both are infrared active (Table 6). Breaking of Ti-O surface bonds upon interaction with an L-ligand makes it possible to identify two infrared active Ti-O surface stretches, appearing immediately to the red of the C=O1 stretch. The surface O1C-H stretch, which is the second most IR active after C=O2, is also the fastest O1C-H mode among all surfaces considered, appearing at 3730/3717 cm-1. --- Table 6 --Analysis of the DOS shows that the clean a(001) surface has a relatively narrow band gap of 2.65 eV, smaller than four other surfaces (Figure 3c). With adsorption of a ligand, the unoccupied energy levels, due to ligand’s LUMO orbitals, appear toward the middle of the band gap as broad, faint bumps near -7 eV. A more precise inspection locates a much smaller band gap of ~1.28 eV.

12


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d) rutile (001)

2.22 (1.45)

C6 1.50

C5

1.50

1.12 C1 1.54 0.09 H 0.11 0.67 C 1.25 1.30 O2 1.73 1 0.46 O 0.57 0.33H 0.99 2.00 O2C 0.86 (0.80) Ti4C 0.49 (0.50)

C6

1.61 (1.41) C6'

1.51

1.51

1.11 C1 1.54 0.1 H 0.14 0.68 C 1.25 1.30 O2 1.73 0.46 O1 0.57 0.33 H 0.99 O2C 0.49 (0.50)

2.00 0.86 (0.80) Ti4C

r(001) C6

2.22 (1.45)

C6

C5

1.51

1.52

1.51

0.09

1.12 C1 1.55 0.07 H 0.11 0.66 C 1.26 1.31 O2 1 0.58 1.93 0.46 O 2.02

1.60 (1.41)

C6'

1.51

1.11 C1 1.54 H 0.15 0.67 C 1.25 1.31 2

0.57 O 1.95

0.46 O1

0.33 H

2.01

0.33 H

0.98 1.09 (1.00) O2C Ti5C 0.48 (0.45)

Ti5C 1.08 (1.00)

0.98 O2C 0.48 (0.45)

r(110)

C6

2.21 (1.45)

1.51

0.07

C5

C6

1.51

1.52

1.60 (1.41)

C6'

1.51

0.09 1.11 C1 1.53 0.66 H 0.13 C 1.31 1.25 O2 1 1.28 0.58 0.55 O H 1.13 0.38 2.89 O2C 1.04 (0.89) 0.52 (0.48) Ti5C

1.12 C1 1.55 0.65 H 0.11 C 1.31 1.25 O2 1.3 1 0.59 0.55 O 0.38 H 1.12 2.89 O2C 1.04 (1.00) Ti5C 0.52 (0.45)

r(101)

Figure 4. The calculated structures of I(C6-C6')@TiO2 and I(C6-C5)@TiO2 and their schematic presentations for

TiO2 r(001),

r(110), and r(101) surfaces. The black numbers are bond

distances (in Å), and blue/red numbers are negative/positive atomic charges (in e). The values in parentheses are for the free C60 and clean surfaces.

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Figures 4 provide an overview of the I(C6-C5), and I(C6-C6’) fullerenes adsorbed on the rutile (001) surface, called below as r(001) surface. This surface of TiO2 is terminated by rows of Ti4C and O2C centers with rows of Ti6C and O3C centers lying below. This allows for an unimpeded O1-Ti4C bond formation and a facile H-transfer to the nearest O2C site, an oxygen atom attached to the same Ti4C center. Additional calculations show that Htransfer to an O2C belonging to an adjacent row is less favorable by at least 6 kcal/mol, and so these structures will not be considered further in this work. The binding pattern of L*C60 on r(001) is very similar to that on a(101), namely, a well defined O1-Ti4C bond (2.00 Å) coupled with an H atom transfer to a nearest O2C site resulting in a short O2---H hydrogen bonding. Adsorption energies (Table 3) of -56.0 and -51.9 kcal/mol for I(C6-C6’)@TiO2 r(001) and I(C6-C5)@TiO2 r(001), respectively, are somewhat smaller than those on a(101), a difference which is reflected in a slightly longer O1-Ti bond. Among the cleanly identifiable IR active vibrational modes are the C-O1 bond stretch at ~1306 cm-1 (the 1745 cm-1 C=O1 mode in the free molecule) and the pure carbonyl C=O2 stretch at 1523 cm-1 (see Table 7). Both of these C-O type vibrations are slightly faster than their a(101) equivalents. The surface bound O2C-H stretch has a 3411 cm-1 frequency, slower than a typical OH stretching frequency and consistent with a three center O-center. --- Table 7 --The density of states is plotted in Figure 3d, including the clean surface, and the (C6-C5)@TiO2 r(001) and I(C6-C6’)@TiO2 r(001) surfaces. One can notice that the valence and conduction bands have shifted up by ~0.5 eV, compared with the corresponding anatase surfaces. The band gap of the clean r(001) surface was found to be 3.22 eV, only slightly larger than the band gap of the UV active a(101). However, the I(C6-C5)@TiO2 r(001) and I(C6-C6’)@TiO2 r(001) surfaces have much narrower band gaps, by ~1 eV, due to the aforementioned upward shift of the surface band and the fact that the LUMO orbitals of L*C60 remain unchanged. The two ligand LUMO peaks in the DOS are visible between -8 and -6 eV and seen to have less overlap with the conduction band of the surface than in the above discussed cases of anatase surfaces. Qualitatively, this suggests less efficient photoelectron transfer from surface to the ligand.

14


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e) rutile (110) Similar to a(101), the rows of Ti5C alternate with rows of Ti6C, however they lie in the same plane. This creates a sizeable gap between the O2C ridges and facilitates O1-Ti5C binding and a closer approach to the surface by the bulky C60. As the H-atom migrates to an O2C site, the O1-Ti5C interaction pulls the Ti atom up, the direction perpendicular to the surface, by ~0.2 Å (Figure 4). There are a few other relatively minor changes to the surface resulting from the interaction. The vibrational spectrum is qualitatively similar to r(001) (see Table 8). The IR active modes are localized on the surface-bound C-O1 and the carbonyl C=O2 stretches. The surface O2C-H stretching frequency is found at a higher frequency than the other surfaces described so far, just below 3600 cm-1. This frequency is close to the harmonic OH stretch in water and indicates a stronger H-surface binding. --- Table 8 --This surface has the narrowest band gap among the ones considered in this work, ~2.2 eV. The conduction band begins just below -6 eV, significantly overlapping with the LUMOs of the ligand. The resulting band gap of the surface with ligands is ~1.2 eV (see Figure 3e). f) rutile (101) Rutile (101) face, called below as r(101), is terminated by O2C that bridge Ti5C centers with alternating 1.8 and 1.9 Å O-Ti bond distances. The plane of Ti5C lies ~0.75 Å below the plane of the terminating O2C which contributes to impeding a close approach of the linking group’s oxygen to a Ti center (see Figure 4). The resulting O1-Ti5C distance is ~2.89 Å, however, the O2-H-O2C bonding structure appears unlike a typical H-bond and similar in nature to a Zundel cation. The energy of adsorption is comparable to that of r(001) at -51.0 kcal/mol (-45.8 kcal/mol for the I(C6-C5) isomer) (see Table 3). The presence of a shared H-atom complicates vibrational assignment in the most active IR region, where there are several closely spaced floppy H-modes, labeled H(flp)_A/B/C/D, of similar character and intensity (see Table 9). Of these four floppy H modes, H(flp)_B at 1262 cm-1 has the biggest overlap with H-transfer. The mode

15



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H(flp)_D has a significant overlap with the carbonyl C=O2 stretch and a C1-C6/5 stretch, and it could not be clearly identified in the I(C6-C5) isomer. The mode which can be best described as the carbonyl C=O2 mode, at 1519 cm-1, is strongly coupled with a C1-C6/5 stretch and a C1-H rock, which explains its small intensity. --- Table 9 --The density of states shows similar characteristics to the one of r(001), a broad band gap, ~3.1 eV, with well defined L*C60 “bumps” near the middle as can be seen in Figure 3f. 3.3) Structure and properties of H2O@TiO2 and L*C60@TiO2 systems in liquid water. Effects of liquid water on the nature of TiO2 surface and ligand absorption (in this case, L*C60) were considered by filling the “vacuum gap” between periodic surface layers with water molecules. We followed the work of Sumita et al. on TiO2 anatase simulations in bulk water35 to make a reasonable choice on the number of water molecules inserted into the empty space. All calculations involved pure water simulations on the surface followed by a simulation with each of the two, I(C6-C5) and I(C6-C6’), conformers of L*C60. For solvating L*C60 on TiO2 in water we proceed by superimposing an equilibrated water “cell” from the previous pure water calculation onto the identical cell with an L*C60 optimized geometry and removing water molecules overlapping with the ligand. In other words, we assume that the ligand is first adsorbed on the surface in vacuo before water is added. In this work, we did not consider the possibility of adsorbing a ligand on top of one or more layers of water already adsorbed on a surface. All 18 trajectories were propagated for a total of at least 8 ps, which included an initial equilibration stage. Visual inspection of structures and analysis of the paircorrelation spectra showed that the equilibrium was reached within 1-1.5 ps for each trajectory, thus the findings reported below are representative of 6.5-7 ps equilibrium simulations. a) anatase (101)

16


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At first, we equilibrated the water/TiO2 system without a ligand, using 68 water molecules. Figure 5 shows (a) representative snapshot along the equilibrated trajectory,

3 2 1 3 0 23 (a)

a(101) 1 2 3 01 23 0 214

(a)

01 32 0 24 0 a(100)1 2 0 4 0

3

Ti5C-Ow O2C-H Ti5C-H Ti6C-Ow Ti6C-H

2

Ti5C-O O2C-H !"""""""""""#"""""""""""$"""""""""""%"""""""""""&""""""""""'" Ti5C-H Ti5C-O -O (")"*" Ti 6C -H O 2C-H (b) Ti6C Ti5C-H Ti6C-O -H TiTi5C -O 6C -H OO2C -H 3C Ti5C-H Ti5C-Ow Ti6C-O O2C-H Ti6C -H Ti5C-H Ti5C-O Ti6C-Ow O2C-O Ti6C-H Ti5C-H O3C-H Ti5C-O2C Ti5C-O O2C-H !"!!!!!!!!!!#!!!!!!!!!!!$!!!!!!!!!!!%!!!!!!!!!!!&!!!!!!!!!!'! Ti5C-H Ti5C-O(!)!*! Ti6C-O O2C-O (b) Ti6C -H Ti5C-H O3C-H Ti5C-O2C

Ti5C-Ow O2C-O w Ti5C-H Ti5C-O2C

2 0 (a)

a(001)

1 3 0 2 3 21 3 01 23 0 12 4 01 32 0 24 0 1 2 0 4 0 2 0

!"""""""""""#"""""""""""$"""""""""""%"""""""""""&""""""""""'" (")"*" (b)

Figure 5. Results of simulations at 300K of the TiO2 anatase (101), (100) and (001) surfaces interacting with a bulk water: (a) representative snapshot along the equilibrated trajectory, where only the terminal surface layer and the first two water solvation layers are depicted, and (b) a summary of pair-correlation for the most important distances, where only the atoms from the terminal surface layer were used for statistics.

17


Ti4C-O O2C-H Ti4C-H Ti6C-O Ti6C-H O3C-H

Ti4C-O O2C-H Ti4C-H Ti -O Ti5C -O 6C-H O Ti2C -H 6C-H Ti5C O3C-H Ti6C-O -H TiTi4C -O 6C -H OO2C -H 3C Ti4C-H Ti5C-O Ti6C-O O2C-H Ti6C -H Ti -H O3C5C-H Ti6C-O Ti6C-H O3C-H Ti5C-O O2C-H Ti5C-H Ti6C-O Ti6C-H O3C-H


and (b) a summary of pair-correlation for the most important distances. In line with the previous BLYP study, we observe non-dissociative adsorption of water on a(101) surface with a ~2.1 Å H2O-Ti5C bond distance (see Figure 5). During the course of the simulation there are, on average, two well-defined H2O-Ti5C bonds, out of the possible six, given the size of the cell. There is an easily noticeable H-bond formation between the water molecules and surface oxygens: the calculated HOH…O2C bond distances are within 1.8-2.2 Å. This broad feature reflects two ways a water molecule approaches to single O2C site: vertical coordination (~1.8 Å) and side coordination (~2.1 Å). The side coordination occurs 6

6’

A: I(C -C )@TiO2 6 5 B: I(C -C )@TiO2

Å

4

4

(a) Ti5C-O Ti5C-O 2 O A-H 2 O B-H

3

1 1

4

(d)

Ti4C-O Ti4C-O 2 O A-H 2 O B-H

A B

3

2 (b)

4

Å

Ti5C-O A 1 Ti5C-O B 2 O A-H 2 O B-H

3

(e)

1

A B

1

Ti5C-O A 2 Ti5C-O A 2 O A-H

2 3.0

(c)

Ti5C-O Ti5C-O 2 O A-H 2 O B-H

4 3

1 1

1

3

2 5

1

2

1

Å

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

A

Ti5C-O B 2 Ti5C-O B 2 O B-H

2

(f)

O A-H 1 Ti5C-O A 2 Ti5C-O A 1 Ti5C-O B 2 Ti5C-O B 2 O B-H

B

2.5

2

2.0 1

2

3 4 time (ps)

5

6

1

2

3 4 time (ps)

5

6

Figure 6. Time profile of various Ti5C-O and O-H bonds for L*C60 absorbed on TiO2 in water at 300K, (a) a(101), (b) a(100), (c) a(001), (d) r(001), (e) r(110) and (f) r(101). Bond values are presented as running averages over a 1ps time window, therefore, short-time fluctuations are not seen.

18


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simultaneously with the H2O…Ti2C coordination, which shows up as a sharp peak at 2.9 Å in the Ti5C-O spectrum. This peak is absent in the BLYP simulations of Sumita et al.35 The hydrogen approach to the rows of Ti5C/Ti6C is long range, peaking at 2.9-3.1 Å. Unlike the earlier work on a(101),10,35 the present simulations show only non-dissociative water adsorption events: i.e. no evidence of O-H bond breaking and subsequent H adsorption on the surface. Placing L*C60 into a cell, filled by water, requires removal of extra water molecules. After all adjustments, the cell contains 59 water molecules for either, I(C6-C5) and I(C6-C6’), ligand. The dynamics shows no significant differences from the simulations with pure water on the surface. The I(C6-C5) and I(C6-C6’) ligands seem to act as a spectator and not interfere with water-surface interactions. The behavior of the ligands in the presence of water is inspected in Figure 6a where we plot two key distances, associated with the binding Ti5C-O1 interaction and the O2-H bond. As seen from this figure, after the 1ps mark the average of the metal-oxygen bond distance stays firmly at ~2 Å and the bond does not rupture. At the same time the O2-H bond appears well defined for the I(C6-C6’) ligand and remains unbroken during the course of the simulation. For the I(C6-C5) ligand, the CO2H group rotates away from the H adsorption site, which is reflected by a ~4 Å distance, and forms a H-bond with an adsorbed water molecule. Thus, the interaction motif of the I(C6-C5) ligand with the a(101) surface is different in water and “vacuum” conditions. In water, the ligand basically interacts with the a(101) surface via a single O1-Ti5C bond without any ligand-to-surface H-transfer. This could indicate the weaker I(C6-C5)C60 - a(101) interaction in water compared to in “vacuum”. b) anatase (100) A simulation with 56 water molecules was carried out for the a(100) surface, with results very similar to the one obtained for a(101) (see Figure 5). Water adsorption to the surface is non-dissociative and is dominated by H-bonding with the O2C sites, which is clearly reflected in the O2C-H pair-distribution. In Figure 5 one can see three water molecules bridging the gap between two adjacent O2C rows by forming an H-bond with

19



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the O2C atoms. The coordination pattern of water to Ti5C of the a(100) surface closely parallels that of a(101). The shortest bond formation occurs at about 2.1 Å followed by secondary coordination at 2.9 Å. The latter appears to be caused by a rather strong O3C-H coordination (dashed red line in Figure 5), which follows closely the O2C-H distribution pattern for distances exceeding 2.4 Å. As in a(101), for simulations with a ligand we overlapped bulk water on the relaxed surface-ligand system and removed extra water molecules. A short equilibration step of 1 ps was followed by a production run of about 7 ps. Figure 6b illustrates the trajectory by showing the most important bond lengths. The presence of water does not seem to impact the Ti5C-O1 bond as the fluctuations in the Ti5C-O1 distance are very small, less than 0.1 Å. The effect on the H-bond is more drastic, as can be seen in the rapid increase of the O2…H distance to the 3-3.5 Å range. Essentially, the H-atom is not coordinated to the O2 atom of the ligand in bulk water. These findings are similar for both I(C6-C5) and I(C6-C6’) conformers. In comparison with L*C60@TiO2 a(101) where only the I(C6-C5) isomer is bound to the surface by two bonds, in the L*C60@TiO2 a(100) both I(C6-C5) and I(C6-C6’) conformers are bound to the surface by only with one Ti5CO1 bond. c) anatase (001) For the same cell size as a(100), 56 water molecules were placed in the empty space between surface layers. The resulting trajectory showed an immediate onset of a strong Ti5C-OH2 interaction and subsequent breaking of O2C-Ti5C bonds, as captured in Figure 5. At equilibrium, the surface becomes partially OH terminated, i.e. H-OH bond of coordinated water molecule is broken. This pattern was reported in previous theoretical studies.10,35 The pair-correlation spectrum marks the position of a migrated H-atom onto an O2C site sharply at 1 Å. Hydrogen bonds of the type HOH…O2CH can also be seen forming at ~1.7 Å. There are two types of Ti-O bonds that can be discerned. At ~1.8 Å the short Ti5C-O2C surface bond (which is dominant) overlaps with Ti5C-OH bonds, which come from both types of O-atoms (surface and water). Closely situated is the second type, at 2.1 Å, which is a water molecule coordinated to Ti5C, similar to that on a(101) and a(100) surfaces. There is a corresponding Ti5C-H peak seen at 2.5 Å.

20


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Overlapping the I(C6-C5) and I(C6-C6’) ligands with a water cell removes 17 water molecules. The ensuing dynamics shows the ligand bound to the surface via a Ti5CO1 bond, which fluctuates around 2 Å. The O2…H hydrogen-bonding, however, is quickly broken in the case of I(C6-C6’) isomer (see Figure 6c). Around the 1 ps mark the O2CH bond starts to flip over toward the next row of O2C centers and by the 2 ps mark completely loses its interaction with O2. The O2…H hydrogen-bond dynamics with I(C6C5) is similar until about 0.7 ps when a water molecule coordinates to a Ti5C center [Ti5C-(OH)-Ti5C-] and the O2C-H bond is free to move higher above the surface and closer to O2, at about the 1ps mark. Eventually the adsorbed water molecule loses one of its Hatoms to the O2CH group, creating an HwO2CH water molecule. This is essentially a short time “window” into O-exchange dynamics where an O2C atom is bound to leave the surface as a water molecule and an Ow takes its place as a Ti5C bridge. Unlike the other anatase surfaces considered, a(001) interacts with water by breaking the HO-H and Ti5C -O2C bonds and forming an O2C-H terminating layer. The ligands are adsorbed with a strong Ti5C-OH bond and an O…H bond, which exists in vacuum but is broken by water-ligand interactions in liquid water. d) rutile (001) To study rutile surfaces somewhat larger periodic cells are needed. Here, we placed 77 water molecules in the empty space between r(001) surface layers. Since the clean r(001) surface is terminated by coordinately unsaturated Ti4C sites, the dynamics quickly leads to the Ti-atoms reacting with water molecules to form a dense “layer” of OH and OH2 above the surface (see Figure 7). Essentially, Ti4C is converted into Ti6C’ in the presence of water (the prime being used to differentiate from the inner surface layer of Ti6C atoms). Pair-correlations reflect this with the double peak at 2 Å. The splitting of the peak is the shorter Ti6C’-OH bond superimposed with the slightly longer Ti6C’-OH2 bond. Interestingly, the latter bond is easily broken by interactions with the H-bonded water molecules from the second solvation layer and reappears at 2.9 Å as an equally strong peak. Thus, there is an equilibrium balance between 5- and 6-coordinated Ti-atoms at surface-water interface. The water molecules adsorbed on Ti4C are observed to readily donate H to the bridging O2C creating a H-transfer balance between surface adsorbed OH

21



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Page 22 of 33

and O2CH. This also leads to instances where two water molecules are attached to one Ti4C center. Fifteen water molecules were displaced upon overlap with L*C60 ligand. The dynamics is rather uneventful, with a sturdy Ti4C-O1 bond (average at just over 2 Å) and a recurring O2…H hydrogen bond. The I(C6-C6’) ligand shows a spike in O2…H distance at 2 ps when the O2CH flips to point down toward a row of O3C centers (1-3 ps phase) before returning in its original position (see Figure 6d). Although the dynamics is rather short, there is evidence to suggest that water does not affect the binding pattern of either ligand.

3 2 1 3 0 23 3

12

01 23 0 124

01 3 2 0 24 0 1 2 0 4 0 2

3

Ti5C-O O2C-H Ti5C-H Ti6C-O Ti6C-H Ti5C-O O2C-H Ti5C-H -O TiTi -O 6C5C -H TiO -H 2C 6C Ti5C-H Ti6C-O Ti5C-O Ti6C-H O2C-H O3C-H Ti5C-H Ti -O Ti6C5C -O 2C TiO -H-H 6C Ti5C-H Ti6C-O Ti6C-H O3C-H

Ti5C-O O2C-H Ti5C-H Ti6C-O Ti6C-H O3C-H

2 1 3 0 23 (a)

r(001) 12 3

Ti5C-O O2C-O Ti5C-H Ti5C-O2C Ti5C-O O2C(a) -O Ti5C-H Ti5C-O2C

Ti5C-O O2C-O Ti5C-H Ti5C-O2C

0

01 3 2 0 24 0 r(110) 1 2 0 4 0

Ti5C-O w O2C-H Ti5C-H O2C-Ow O-H w

2 0

(a)

01 23 0 124

Ti4C-Ow O2C-H Ti4C-H Ti6C-Ow Ti6C-H O3C-H Ti4C-O O2C-H Ti -H !"""""""""""#""""""""""$"""""""""""%""""""""""""&""""""""""'" 4C (")"*" TiTi -O-O 5C 6C -H TiO -H (b) 6C2C OTi -H-H 3C5C Ti6C-O Ti4C-O Ti -H O2C-H6C O3C-H Ti4C-H Ti -O w Ti6C5C -O 2C TiO -H-H 6C Ti -H O3C5C -H Ti5C-O Ti6C-O w O2C-H Ti6C-H Ti5C-H O3C-H O2C-O Ti5C-O O-H O2C-H !"""""""""""#"""""""""""$"""""""""""%"""""""""""&""""""""""'" Ti5C-H (")"*" Ti5C-O Ti6C-O (b) O2C-H Ti6C-H Ti5C-H O3C-H O2C-O O-H

r(101)

22

!"""""""""""#"""""""""""$"""""""""""%"""""""""""&""""""""""'" (")"*" (b)


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Figure 7. Results of simulations at 300K of the TiO2 rutile (001), (110) and (101) surfaces interacting with bulk water: (a) representative snapshot along the equilibrated trajectory, where only the terminal surface layer and the first two water solvation layers are depicted, and (b) a summary of pair-correlation for the most important distances, where only the atoms from the terminal surface layer were used for statistics

e) rutile (110) The simulations for r(110) with water were done with 78 water molecules. Similar to r(001), the exposed Ti5C centers avail themselves to strong coordination with water, as seen in Figure 7. Analysis of pair-correlations and a visual inspection of the trajectory show that there are three binding signatures of Ti5C with water, the short Ti5COH bond distance at 1.9 Å, the medium Ti5C-OH2 bond distance at 2.1 Å and the long Ti5C…OH2 bond distance at 2.9 Å. The short and medium bonds are the result of an adsorbed water molecule losing one of the H’s to the nearby O2C, a bridge between two Ti6C centers. Hydrogen exchange dynamics between the two O’s, which is clearly captured in Figure 8 and indicated by the peaks at 1 and 1.8 Å in the O2C-H paircorrelation curve, results in alteration of the Ti5C-O distance between 1.9 and 2.1 Å. The long Ti5C-O distance near 2.9 Å appears to be due to solvent fluctuations that cause a water molecule that is adsorbed on a Ti5C to temporarily dissociate before re-adsorption. This particular feature has been described in the previous cases as well. Seventeen water molecules were removed upon solvation of the L*C60 ligand in the water filled shell. Molecular dynamics reveals an almost immediate formation of a second Ti5C-O bond formed on the same row of Ti5C centers. These two bonds (distinguished by labels “I” and “II” in Figure 7e.) with distance of ~2.2 Å on average, are dynamically identical and are observed for both ligands. This is the main difference between this surface and r(001) in liquid water conditions. The O2…HO2C bond, that is clearly present in the vacuum structures, is promptly broken in the early stages of the simulation, as a result of O2CH flipping between rows of Ti5C. However for the I(C6-C5) ligand the H-bond can still be identified after 2 ps hovering around 2.5 Å while in the I(C6-C6) ligand remains completely broken up to 7 ps. f) rutile (101)

23



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Page 24 of 33

The simulations for r(101) with water were done with 83 water molecules. In this case, the Ti5C-O pair-correlation spectrum shows less fine structure than the other rutile surfaces considered (see Figure 7). A single large peak centered at 2 Å and a weak feature at 2.9 Å suggests a stronger binding pattern of water oxygen to the Ti5C sites. This is reinforced by the O2C-H pair-correlation curve which parallels the O-H (water-water) curve up to ~2.2 Å and does not have a well defined peak near 1.7-1.8 Å, unlike r(001) and r(110). Essentially, this implies that the H-transfer equilibrium between water molecules and O2C sites is shifted towards Ti-OH / O2C-H, as can be seen in Figure 9a. The surface is predominantly OH terminated. Insertion of L*C60 ligand into the cell leads to the removal of 16 water molecules. The dynamics, represented in Figure 6f, shows a very stable ligand-surface interaction similar to the r(110) case, as indicated by the formation of a second Ti5C-O bond that is longer than the original by ~0.2 Å (the two bonds are discerned by labels “I” and “II” in the figure), formed soon after initial equilibration around the 1.5 ps mark. In the case of I(C6-C6’) the Ti5C-O_II bond lives for about 1.5 ps before breaking while the one involving the I(C5-C6) ligand exhibits a prolonged lifetime, at least up to ~ 6ps. The slight downturn in the I(C6-C6’) Ti5C-O_II curve observed just before the 6 ps mark suggests a possible re-formation of the bond within the next 1-2 ps. These characteristics make the adsorption of the ligands on r(101) similar to r(110) and clearly different from r(001). 4. Conclusions 1. Comparison of presented DFTB results for surface/water interaction with the available calculations at higher levels of theory is generally very favorable. We reproduced most of the key signatures of solvation dynamics of a(101) and a(001) reported by others35 at the BLYP level of theory. 2. Several

low

energy

isomers

of

carboxylate

acid

functionalized

C60

HC1C(O2H)O1-C60, L*C60, were found. Of these, the energetically favorable I(C6-C6’) and I(C6-C5) isomers were considered for extensive studies. 3. All six surfaces considered, except r101, bind L*C60 via one HC1C(O2H)O1-Ti bond. In all cases there is a H-bond involved, either via an HC1CO1O2-H…O2C

24


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(non-H-transfer

structure)

or

HC1CO1O2…H-O2C

(H-transfer

structure)

framework. In all cases except one, the surface remains intact upon ligand adsorption. 4. Electronic density of states reveals that the occupied orbitals of the ligands lie entirely within the valence band of the surfaces, while the first few LUMOs appear inside the surface band gap. This significantly reduces the band gap of the modified L*C60@TiO2 surfaces. However, the clean surface energy levels are only slightly perturbed by interaction with L*C60. Therefore, photoinduced electron transfer is expected to occur via excitation of a surface electron followed by a non-radiative transition to the LUMO orbitals of L*C60 overlapping with the lower edge of the conduction band. 5. In liquid water at 300K, two surfaces, a(101) and a(100), show non-dissociative adsorption of water, and four others, a(001), r(001), r(110) and r(101), show predominantly dissociative adsorption. In the latter case, the surfaces become OH terminated at both Ti and O surface sites. Under these conditions, i.e. liquid water at 300K, the L*C60-TiO2 surface interactions are weakened. Here we note that precise evaluation of binding energies in explicit solvent requires calculation of solvent free energy, a computationally extremely demanding task, which we leave for consideration in future works. Supporting Information Available: Cartesian coordinates of CHCO2H, I(C6-C6’), I(C6-C5) and L*C60@TiO2 systems reported in this paper. This information is available free of charge via the Internet at http://pubs.acs.org. Acknowledgment: We acknowledge U.S. Department of Energy, Office of Basic Energy Sciences, Solar Photochemistry Program (DE-FG02-07ER-15906) for support. Computer resources at the NERSC supercomputer are acknowledged. The authors gratefully acknowledge NSF MRI-R2 grant (CHE-0958205) and the use of the resources of the Cherry Emerson Center for Scientific Computation.

25



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For example, our DFTB calculations of H3O+ give 3529 cm-1 for the symmetric stretch

normal mode frequency, which has a very small IR intensity.

28


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Table 1. Bond distances (in Å) and binding energies (in kcal/mol) for the four important L*C60 structures (L=CHCO2H). I(C6-C5) 2.23 1.50 1.50 1.54 1.12

I(C6-C6’) 1.60 1.51 1.51 1.53 1.11

r(C6-C5)a r(C1-C6) r(C1-C5/C6’) r(C-C1) r(C1-H) 1 r(C6-O ) r(C-O1) 1.20 1.21 2 r(C-O ) 1.39 1.38 -74.2 -65.6 ΔEb a 6 5 6’ C , C and C are carbons of C60 (see Figure 1) b energy relative to L + C60

II(C6-C5) 1.61 1.52

II(C6-C6’) 1.60 1.52

1.36 1.09 1.51 1.39 1.359 -57.3

1.36 1.09 1.50 1.37 1.35 -74.0

Table 2. Harmonic vibrational frequencies (in cm-1), IR absorption cross sections (in a02×1000), and reduced masses (in amu) of the surface binding I(C6-C5) and I(C6-C6’) structures. I(C6-C6’) IR ω 2 O -H (wag) 415 17.6 O1CO2 (bend) 593 0.3 2 a C-O (str) 884 0.3 2 O -H(rock) 1197 6.3 C=O1 (str) 1745 1.6 1 C -H (str) 2927 0.2 O2-H (str) 3638 1.9 a “str” denotes a stretching mode

mass 1.3 6.2 5.4 1.4 11.0 1.1 1.1

29

Parameters

ω 562 613 919 1256 1734 2825 3668


I(C6-C5) IR 12.7 3.6 0.3 2.1 2.1 0.1 2.1

mass 1.9 4.2 7.0 1.5 10.6 1.1 1.1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

Table 3. Electronic energies of structures adsorbed on surfacesa relative to their respective dissociation limits (as ΔE) and to each other (in kcal/mol), and ionization potential (IP)b and band gap in eV. 6

a101 -65.7 -58.7 -1.5

6’

a100 -39.0 -34.6 -4.1

a001 -83.7 -75.5 -0.3

r001 -56.0 -51.9 -4.4

r110 -82.5 -75.0 -1.0

ΔE [I(C -C )@TiO2] ΔE [I(C6-C5)@TiO2] E [I(C6-C5)@TiO2] – E [I(C6C6’)@TiO2] IP (clean surface) 5.08 5.21 4.33 4.24 4.39 band gap (clean surface) 3.19 3.03 2.65 3.22 2.2 band gap [I(C6-C6’)@TiO2] 1.54 1.58 1.33 1.03 1.22 band gap [I(C6-C5)@TiO2] 1.49 1.56 1.27 0.97 1.16 a the surface index notation is shortened, e. g., a101 is anatase (101), see text b evaluated at the same geometry as the neutral surface

r101 -51.0 -45.8 -3.3 4.81 3.09 1.46 1.41

Table 4. Several stretch-type normal modes in the Amide IR band of I(C6-C6’)@a(101) and I(C6-C5)@a(101). Presented are frequencies (in cm-1), IR cross sections (in a02×1000) and reduced masses (in amu). Parameters 1

C-O C=O2 C1-H O2C-H

ω 1224 1518 2916 3262

I(C6-C6’)@a(101) IR 0.6 2.1 0.2 2.6

mass 5.9 8.9 1.1 1.1

ω 1278 1543 2839 3358

I(C6-C5)@a(101) IR mass 1.2 12.1 2.0 9.5 0.1 1.1 2.5 1.1

Table 5. Several stretch-type (str) and floppy (flp) shared hydrogen normal modes in the Amide IR band of the I(C6-C6’)@a(100) and I(C6-C5)@a(100) systems. Presented are frequencies (in cm-1), IR cross sections (in a02×1000) and reduced masses (in amu). Parameters 1

C-O C=O2 C1-H O2C-H

[I(C6-C6’)@a(100) IR mass ω 1135 2.8 13.0 1546 1.5 11.3 2904 0.2 1.1 3528 2.3 1.1

30

ω 1142 1541 2807 3517


[I(C6-C5)@a(100) IR 2.2 1.8 0.1 2.3

mass 10.6 12.4 1.1 1.1

Page 31 of 33

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Table 6. Several stretch-type normal modes in the Amide IR band of the I(C6C6’)@a(001) and I(C6-C5)@a(001) systems. Presented are frequencies (in cm-1), IR cross sections (in a02×1000) and reduced masses (in amu). Parameters C-O1 Ti5C-O1CH Ti5C=O1C C=O2 C1-H O1C-H

ω 1149 1433 1547 1616 2908 3730

I(C6-C6’)@a(001) IR 1.5 1.1 0.8 2.2 0.2 1.9

mass 12.7 15.6 19.3 13.7 1.1 1.1

ω 1128 1575 1606 1617 2795 3717

I(C6-C5)@a(001) IR 1.3 0.6 1.0 2.1 0.1 2.0

mass 12.2 15.6 18.6 14.1 1.1 1.1

Table 7. Several stretch-type normal modes in the Amide IR band of the I(C6C6’)@r(001) and I(C6-C5)@r(001) systems. Presented are frequencies (in cm-1), IR cross sections (in a02×1000) and reduced masses (in amu). Parameters 2

O ...H C-O1 C=O2 C1-H O2C-H

ω 424 1306 1523 2925 3411

I(C6-C6’)@r(001) IR mass

Structural Modification of TiO2 Surfaces in Bulk Water and Binding

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