Exploring Rutile (110) and Anatase (101) TiO2 Water Interfaces by

Article pubs.acs.org/JPCC

Exploring Rutile (110) and Anatase (101) TiO2 Water Interfaces by Reactive Force-Field Simulations Zdenek Futera*,† and Niall J. English* School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland S Supporting Information *

ABSTRACT: We have investigated static/structural as well as dynamical properties of anatase (101) and rutile (110) TiO2 interfaces with liquid bulk water by reactive force fields (ReaxFF). Layered, well-organized structure of water in the interface region was clearly observed within 6.5 Å of the surfaces. The first-hydration layer molecules adsorbed to unsaturated surface Ti atoms undergo spontaneous dissociation leading, rather controversially, to full coverage of O2c/Ob by H+ and partial coverage of Ti5c by OH−. Expected large variations of intrinsic electric field on the interfaces, and drop of electrostatic potential, were detected. Interfacial water was found to be heavily confined with a self-diffusion constant of 2 orders of magnitude lower than 2.28 × 10−9 m2/s measured in the bulk water region. Moreover, the rotational movement of adsorbed water molecules was found to be considerably hindered as well. On the other hand, the calculated hydrogen-bond lifetime on the interface was shorter than in bulk water for both surface types. Finally, the IR spectra obtained from collective-water-dipole variations in the interfacial region revealed stronger effects on stretching vibrations on anatase (101) than on rutile (110); however, description of liquid-water bond-stretching vibrations generally suffers from lack of accuracy in the applied reactive potential.

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INTRODUCTION Titanium dioxide (TiO2, known popularly as titania) is a widely studied semiconductor which became famous for its photocatalytic activity reported for the first time by Fujishima and Honda in 1972.1 Since then, this material has found many important applications in industry. It has been used, for instance, for hydrogen-production catalysis, energy conversion in solar cells or in lithium-ion batteries, for degradation of harmful organic pollutants, or as a component of paints.2−6 From the three main polymorphs of titania occurring naturally (rutile, anatase, and brookite) anatase is the most active one, and used abundantly in nanomaterials, while rutile is the most thermodynamically stable.7,8 From a technological perspective, rutile (110) and anatase (101) surface facets are the most important,9,10 and their properties and behavior have been studied extensively in recent decades. Yet, our understanding of structure−function relationships and behavior of TiO2/water interfaces, although it is crucial for fine-tuning of catalytic applications, is still rather limited and research in this area is very active. From accurate inelastic-scattering experiments, it is known that the interfacial layer of water adsorbed on a TiO2 surface is confined heavily and its dynamical and vibrational features exhibit behavior more redolent of ice than of liquid water.11−14 This observed behavior was later demonstrated by classical molecular dynamics (MD) simulations.15−22 The water on TiO2 interface was shown to have layered structure maintained by intermolecular hydrogen bonding.23−25 The structure of the © 2017 American Chemical Society

electric double layer (EDL) on rutile (110) interface was measured by standing X-ray methods26 and later investigated by classical MD simulations.27,28 Recently, and interestingly, the depletion layer in the TiO2-part of the aqueous interface was studied by Sang et al.29 Density functional theory (DFT) at various levels of exchange-correlation treatment has been employed to investigate not only structural properties but also electrochemical changes occurring on the TiO2 surface wetted by water and other solvents.30 Different regimes of water adsorption (molecular versus dissociative) were observed on different facets of titania,31−34 indicating complex structure−properties relationships. The dissociative adsorption, important for water splitting in photocatalytic applications, leads to protonation/ hydroxylation of TiO2 surfaces, which was observed in various experiments such as STM, potentiometry, or spectrometry.35−40 The adsorption depends naturally on temperature and coverage of the surface, which affect the arrangement and stability of interfacial hydrogen-bond network.41−45 Although the above-mentioned DFT studies have undoubtedly large consequences on understanding of TiO2 reactivity and electronic structure, such studies are limited in system size and length of MD simulations by high computational demands of first-principal computations. On the other hand, classical Received: December 20, 2016 Revised: February 28, 2017 Published: March 16, 2017 6701

DOI: 10.1021/acs.jpcc.6b12803 J. Phys. Chem. C 2017, 121, 6701−6711

Article

The Journal of Physical Chemistry C

located at the bottom part of the ridges. However, these latter two atom types are less important for interaction with water because their chemical valences are fully saturated by bonding within the TiO2 structure. The thermodynamically more stable rutile (110) is formed by plane of O3c, Ti5c, and Ti6c atoms above which rows of bridging, 2-coordinates oxygen atoms (Ob) are exposed. As in the anatase case, the Ob and Ti5c atoms constitute natural reactive sites on this surface. The potential energy of the whole system was described by the ReaxFF reactive force field47,52 with parameters optimized for TiO2/water systems.47 In general, the system energy can be written as

force-field approaches based on empirical potentials, which have proved to be a very good tool for investigating dynamical properties of solid/liquid interfaces, cannot describe dissociative changes on the surfaces. Therefore, we employ in the present study a reactive force-field approach (ReaxFF) developed by van Duin et al.,46 which has been designed to describe chemical changes empirically, in such a way as to achieve an apposite compromise between the accuracy of firstprinciples approaches and speed of nondissociative force-field (e.g., pairwise) simulations. The ReaxFF parameters for TiO2/ water interaction, as fitted by Kim et al.,47 can reproduce relative stability of rutile, brookite and anatase and they were shown to satisfactorily describe water dissociation on various titania surfaces although some controversies in level of dissociation were reported.48,49 Here, we investigate the dynamical properties of water in the interfacial region of anatase (101) and rutile (110) surfaces in contact with extended liquid water region by MD-simulation techniques. Recently, we have shown by classical (i.e., nondissociative force-field-potential-based) MD simulations that the adsorbed water molecules are affected by relatively strong intrinsic electric-field variations near the surface.50 In the present study, we demonstrate the ability of ReaxFF methods to describe static (including structural) as well as dynamical properties of water in contact with TiO2, and we discuss how these properties are affected by water molecules’ dissociative adsorption.

E = E b + Eoc + Euc + E lp + Eval + Epen + Etors + Econj + Evdw + Ecoul

where the individual terms stand for bonding energy (Eb), overcoordination penalty energy (Eoc), under-coordination penalty energy (Euc), lone-pair energy (Elp), valence-angle energy (Eval) and associated penalty energy (Epen), torsionangle energy (Etors), conjugation energy (Econj), nonbonded van der Waals interaction energy (E vdw), and electrostatic interaction energy (Ecoul). All but nonbonding terms depend on the bond order parameter, which is determined dynamically on the basis of interatomic distances in the system. Therefore, chemical changes, for instance, expected water dissociation on the TiO2 surfaces, can be handled by ReaxFF simulations. Further, in contrast to classical force-field potentials, the atomic charges are not kept constant during the simulation but they are reassigned by charge-equilibration methods53,54 and thus they reflect chemical changes in the system. This approach thus allows for charge transfer to take place in the system and polarization of water molecules, which play an important rôle at solid/liquid interfaces. The simulations were performed by the efficient parallel software package LAMMPS.55,56 Molecular dynamics (MD) runs were performed in the canonical NVT ensemble at a temperature of 300 K. The velocity Verlet scheme57 was employed to integrate equations of motion with a time step of 0.2 fs for 500 ps. Constant temperature of the whole system was imposed by a Nosé− Hoover chain thermostat58,59 with a relaxation time of 10 fs using five thermostat chains. Long-range electrostatic interactions were evaluated under periodic boundary conditions (PBC) using the charge-equilibration scheme53,54 with a 10 Å real-space cutoff and a relative precision of 10−6. The system was equilibrated carefully before the production run using initial structures from our previous classical-MD study.50 Structural properties of adsorbed water were analyzed by calculating water Ow−Hw bond lengths, Hw−Ow−Hw valence angles and individual-water dipoles as mean values ⟨x⟩ = ∫ xP(x)dx of corresponding normalized histograms P(x) collected from the simulations. Similarly, histograms of hydrogen-bond lengths, angles and average numbers of hydrogen bonds were computed. The hydrogen bonds were selected on the basis of geometry criteria for arrangement of donor (D), acceptor (A), and the hydrogen (H): the maximum allowed A−D distance was 3.5 Å and the A-D-H angle could not exceed 35°. The dynamics of the hydrogen bonds was described by the Luzar−Chandler model60 used for prediction of average hydrogen-bond lifetime. The self-diffusion constant of water was obtained using Einstein relation as a limit of meansquare displacement:

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COMPUTATIONAL DETAILS Here, we investigate properties of the two industrially most important titania surfaces, namely anatase (101) and rutile (110). Models of the surfaces were created by cutting the bulk anatase (I41/amd space group, tetragonal lattice, a = 3.785 Å, c = 9.514 Å) and rutile (P42/mnm space group, tetragonal lattice, a = 4.593 Å, c = 2.959 Å).51 Surface slabs of 224 TiO2 units were placed into orthorhombic supercells of dimensions specified in Table 1. The z dimensions of the supercells were Table 1. Side Lengths of the Orthorhombic Simulation Supercells a, b, and c, Thickness of the TiO2 Slab ΔZTiO2 Measured from Bottommost to Topmost Atom (ZTi), and Width of the Water Interface Layer ZWat Measured from ZTi [Å]a anatase (101) rutile (110)

a

b

c

ΔZTiO2

ZWat

26.495 23.624

40.956 45.381

69.873 69.450

11.7 9.9

6.3 6.6

(1)

a

Each supercell contains 224 TiO2 units and 2000 water molecules. Lower surface of the TiO2 slab is located at the bottom of the cell (z = 0).

chosen to accommodate 2000 water molecules of bulk density 1 g/cm3 in each system. The resulting structures are shown in Figure 1 where interface and bulk water regions are highlighted. For detailed structures of the two surfaces and atom-type labeling, see Figure 2. The characteristic terrace-like structure is typical for anatase (101), with surface ridges formed by 5coordinated titanium atoms (Ti5c) and 2-coordinated oxygens (O2c). These atoms are exposed to the interface and they can interact more directly with water. Further, 3-coordinated oxygen atoms can be found on the surface interconnecting rows of Ti5c, and, finally, 6-coordinated titanium (Ti6c) is 6702


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Figure 1. Supercell models of TiO2/water interfaces: (a) anatase (101) and (b) rutile (110). The bulk-water region is highlighted by orange color, and interface water in green.

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RESULTS AND DISCUSSION At first, we calculated density profile of water along the direction perpendicular to the TiO2 surface (z direction in laboratory system axes) to identify and measure the width of the interfacial region. The layered structure of water can be seen clearly in Figure 3 where the water-oxygen (Ow) profile

Figure 2. Structure of TiO2 (a) anatase (101) and (b) rutile (110) facets with labeled surface atoms. Titanium atoms are shown in brown, with oxygen atoms in red.

D=

⟨(r(t0 + t ) − r(t0))⟩ 1 lim t →∞ 6 t

(2)

Normalized autocorrelation functions C(t) = ⟨ai(t0)·ai(t0 + t)⟩/ ⟨ai(t0)·ai(t0)⟩ of internal water-molecule axes a1 = μ/|μ|, a2 = h/|h| and a3 = a1 × a2 (where μ is a dipole-moment and h is Hw-Hw vector) were computed on the basis of Wiener− Khintchine relations employing fast Fourier transform (FFT) algorithm: ⟨f (t0) f (t0 + t )⟩ =

1 2π

∫ ∫ f (t′)e−iωt′ dt′

2

eiωt dω

Figure 3. Density profile of atomic elements in the interfacial region of anatase (101) and rutile (110). The distance is measured from average position of the topmost Ti-atom layer on the surface. The vertical dashed green lines mark the boundary of the interface region.

(3)

Finally, the IR spectrum was computed as a power spectrum of the system-collective-dipole autocorrelation function I(ω) ∝ ω 2

∫ ⟨M(t0) M(t0 + t )⟩e−iωt dt

exhibits well-defined peaks corresponding to adsorbed-water layers on the surface. In a similar manner to our previous studies,50,61 the reference point of the density distribution was chosen as the mean position of the topmost Ti atom layer on the corresponding surface. The interface region boundary, indicated by the dashed green line in Figure 3, was then set as the position of last significant minimum in the profile before it becomes flat in the bulk-water region. The width of the

(4)

where M(t) can be written as sum of individual water dipoles M(t) = Σiμ(t). 6703


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Figure 4. Dissociative adsorption of water on (a) anatase (101) and (b) rutile (110) surface. The water molecule first adsorb molecularly (left-side panels) and consequently split to OH− that adsorbs on Ti5c and H+ adsorbing on O2c/Ob (right-side panels). Titanium atoms are shown in brown, TiO2 oxygen atoms in red, water oxygen in orange, and water hydrogen in green color.

hydration layer water hydrogen-bonded to the surface, while the peak between 4 and 6.3 Å delineates the second hydration layer. An analogous molecular arrangement was observed on rutile (110) surfaces. As in the previous case, the first hydration layer is formed by water coordinated by Ow to surface Ti5c and water hydrogen-bonded by Hw to bridging Ob atoms. In contrast to anatase (101), the water molecules hydrogen bonded simultaneously to neighboring Ob atoms were not found in the first hydration shell of rutile (110), and this region is characterized by double-peak pattern in density profile when the classical potential is used. For reactive force fields, splitting of the Ti5c-coordinated water was observed when one of the water protons is transferred to the nearest bridging Ob, as shown in the lower panel of Figure 4. The adsorbed OH− on Ti5c gives rise to a well-localized first peak in the water-oxygen profile positioned at 2 Å in Figure 3. The rest of the first hydration layer spans out to 3.9 Å, while the broad peak between 3.9 and 6.6 Å corresponds to the limits of the second hydration layer. The observed dissociation mechanism and adsorption of OH−/H+ to the surface is in agreement with previous ReaxFF studies of titania surfaces47−49 although it differs in surface coverage as we discuss below. To glean a better insight into surface water interactions, we calculated the radial distribution function (RDF) shown in Figure 5 for anatase (101) and in Figure 6 for rutile (110). As expected, O3c and Ti6c do not interact directly with the first hydration layer. The contact distance of these atoms with water is 2.5 Å and longer. The H+ released from dissociated water is obviously residing on O2c, which is indicated by the O2c−Hw peak located at 1.0 Å. The second peak at 1.8 Å corresponds to

interface was found ca. 6.5 Å on both studied surfaces (see Table 1 for details), which is slightly larger than we observed in the classical-potential calculations.50 The difference is of course caused mainly by the dissociative adsorption of water that is spontaneously occurring in the reactive-force-field simulations. Further, the position of TiO2 atoms was kept fixed in ref 50 and the width of interface region could therefore be partially narrowed by lack of surface vibrations. The water density converges to 1 g/cm3 value beyond the interface boundary and remains constant in the bulk regions. Water Adsorption and Dissociation. In the strictly molecular-adsorption regime, three types of adsorbed water molecules can be recognized on anatase (101) facet. First is the water interacting by its oxygen Ow with the unsaturated surface Ti5c cation. Adsorption of this water type is further stabilized by its hydrogen bonding to nearest surface O2c oxygen. The second type is water hydrogen-bonding to two neighboring O2c by its Hw atoms. Finally, the last type of water is hydrogenbonded by only one of its Hw to surface O2c while its second hydrogen can interact with other water molecules in the interface region. These three water types form the first hydration layer of anatase (101), which has a characteristic triple-peak pattern in its density profile when a classical potential is used. 50 However, for reactive force-fields, dissociation of the water attached to Ti5c was observed when the proton participating in H-bonding to O2c is transferred to this atom and negatively charged hydroxyl (OH−) remains coordinated to Ti5c. This process is illustrated in upper panel of Figure 4. The adsorbed hydroxyl gives rise to sharp Ow peak in density profile located 1.82 Å above Ti5c layer. The following band broad peak between 2 and 4 Å corresponds to first6704


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pointing outward of the surface with average angle of 27°. Interestingly, the coordination number of Ti5c is 0.634; thus, on average, 71 Ti5c atoms out of 112 available on both slab-end surfaces are covered by OH−. The rest of hydroxyl groups produced by dissociative adsorption of water are in the first hydration layer, interacting with surface O2c and the nearest water molecules by hydrogen bonding. In contrast to anatase, the weak interaction of O3c with Hw can be seen in the RDF obtained on the rutile (110) interface. The first small peak positioned at 1.0 Å integrates to a coordination number of 0.016 corresponding to coverage of 3.6 out of 224 O3c atoms by H+, on average. The second peak at 1.65 Å indicates a hydrogen-bonding interaction between O3c and the first hydration layer. The coordination number of this peak is 0.139: therefore, 31 out of 224 O3c participate in hydrogen bonding, on average. However, the most dominant interaction on the rutile (110) interface is the binding of H+ to bridging oxygen Ob and OH− to Ti5c. As in the anatase case, Ob atoms are fully covered by H+, while only 53.7 out of 112 Ti5c on average interact with adsorbed OH−, as deduced from a coordination number of 0.48 of the corresponding peak in the Ti5c-Ow RDF. Although released hydroxyl groups were observed in the first hydration layer, no free hydronium (H3O+) cations were found in the water. Structural Properties of Individual Water Molecules. To explore changes in structural properties of individual water molecules located near the titania surface with respect of those in bulk region, we collected values of Ow−Hw bond lengths, Hw−Ow−Hw angles and dipole moments to histograms, and calculated their mean values. The histograms are shown in Figure S1 (Supporting Information) and their mean values are summarized in Table 2. Obviously, the bond lengths are

Figure 5. Radial pair distribution function (RDF) detecting interaction between surface atoms of anatase (101) and water.

Table 2. Mean Values of Water-Molecule Ow−Hw Bond Length, Hw−Ow−Hw Angle, and Size of Dipole Moment in Bulk Water and Interface Regions of Anatase (101) and Rutile (110) bond length [Å] angle [deg] dipole [D]

bulk water

anatase (101)

rutile (110)

0.985 104.612 2.120

0.995 104.874 2.179

0.991 104.860 2.175

elongated slightly in the interface region, by 1% on anatase (101) and by 0.6% on rutile (110), and the same effect can be observed on the valence angles which increase by 0.3% on the anatase (101) surface and by 0.2% on the rutile-(110) one. In contrast to these tiny changes in geometry, the magnitude of the water dipole moments is considerably larger in the interfacial region: ca. 2.18 D on both studied surfaces while 2.12 D was found in bulk water. This almost 3% increase of dipole is supported by changes in atomic charges near the surfaces. Although the average charge of oxygen atoms remains close to those in bulk water, the charge of the hydrogen atoms is considerably higher in water molecules near the TiO2 surfaces. Electric Field Variations and Potential Drop on the Interface. As we discussed in our previous study,50 substantial changes of intrinsic electric field are characteristic of the interfacial region of TiO2 in contact with liquid water. To investigate these changes, we employed Poisson’s equation which provides the relationship between charge density ρ( r ⃗) in the system and electrostatic potential function φ( r ⃗). As we are

Figure 6. Radial pair distribution function (RDF) detecting interaction between surface atoms of rutile (110) and water.

a hydrogen-bonding interaction of molecular water/hydroxyl groups from the first hydration layer with the surface oxygen. The integral of the first peak gives rise to a coordination number of 1, corresponding to full coverage of O2c by H+ protons. The OH− attached to Ti5c is manifested itself as the sharp peak in the Ti5c−Ow RDF at 1.9 Å, together with a peak in the Ti5c−Hw RDF at 2.35 Å. The hydroxyl is therefore 6705


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The Journal of Physical Chemistry C interested only in changes proceeding along the direction perpendicular to the TiO2 surface, we can employ the onedimensional version of Poisson’s equation, ρ (z ) d2φ =− 2 εrε0 dz

(5)

The permittivity of the environment is written as product of vacuum permittivity, ε0, and relative permittivity of the environment, εr, in the above equation. Although the permittivity of water can change near and at the interface, we neglected these variations and used a uniform bulk-water value of 78.36 in all calculations presented here. The local charge of the system volume element dV containing discrete charges Qi is

∑

ρ ( r ⃗ ) dV =

Q iδ( r ⃗ − ri ⃗)

(6)

i ∈ dV

Therefore, the charge-profile function along z direction can be obtained as 0

ρ (z ) =

∫b ∫a

0

ρ( r ⃗)dx dy =

1 dz

∑ Qi i ∈ dz

(7)

The calculated profiles in the interface regions, averaged over the MD trajectories, are shown in Figure 7 for anatase (101) Figure 8. Calculated profile of charge distribution ρ(z), electric field profile E(z), and electrostatic potentials profile φ(z) on rutile (110) water interface. The distance is measured from average position of the topmost Ti-atom layer on the surface. The vertical dashed green lines mark the boundary of the interface region.

cannot (nor should not) be interpreted as real transfer of electrons/holes, as observed in DFT studies, for instance. Rather, the observed changes should be regarded as enhanced polarizations, and the presented profiles act mainly as qualitatively illustrative of the broad character of charge changes and interactions at the studied interfaces. To obtain the intrinsic-electric-field profile, we integrated the linear Poisson equation z 1 ρ(z′) dz′ E (z ) = εrε0 zbw (8)

∫

The integration was done numerically by Runge−Kutta method of fourth order from the middle of the bulk-water region (zbw) to the TiO2 surface with the boundary condition E(zbw) = 0. The calculated field profiles are shown in middle panels of Figures 7 and 8. The field variations of anatase (101) and rutile (110) and their magnitudes, changing from −2 to 6 V/Å near the surface, are qualitatively similar to our previous classicalpotential results where the intrinsic field was evaluated directly by Ewald sum50 and consistent with recent study of Sang et al.29 This extraordinary strong field (note that intrinsic field of magnitude ca. 2 V/Å acting on bulk-water molecules was found by classical as well as DFT calculations50,64) is an indicator of strong adsorption interactions on the titania interfaces and charge changes occurring in this region. As the water molecules interact with the electric field by their dipole moments, it is interesting to explore also the collective water-dipole profile in the interfacial region of titania. This curve is shown in Figure 9 where, for comparison, the intrinsic-field profile is plotted as well. It is evident that the collective dipole aligns to the field in more distant interface regions, ca. 3−4 Å above the surface and beyond, where the second hydration layer is located.

Figure 7. Calculated profile of charge distribution ρ(z), electric field profile E(z) and electrostatic potentials profile φ(z) on anatase (101) water interface. The distance is measured from the average position of the topmost Ti-atom layer on the surface. The vertical dashed green lines mark the boundary of the interface region.

and in Figure 8 for rutile (110). Obviously, the average charge is zero in the bulk region and variations can be seen only near the surface where the adsorbed water is confined and relatively well organized. Note that the charge equilibrium (QEq) method used in ReaxFF to assign atomic point charges may lead to artificial charge transfer at the solid/liquid interface. Such charge transfer is an inherent artifact of the method, and 6706


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(Supporting Information). The interfacial values are lower by a further order of magnitude vis-à-vis (3−4) × 10−10 m2/s which we found on these interfaces using classical force fields.50,61 The stronger confinement in the reactive force field is supported by charge changes near the interface which enhance the dipole moment of the adsorbed water molecules, as we discussed above. Further, in contrast to previous simulations, the titania surface position was not constrained here, and thus vibrations of the surface TiO2 atoms might affect the interfacial hydrogen-bond dynamics. The dynamics of water rotational movement was explored by calculating autocorrelation functions of internal water-molecule axes, i.e., a1 = μ/|μ|, a2 = h/|h|, and a3 = a1 × a2 (where μ is a dipole-moment and h is Hw−Hw vector), using Fouriertransform methods (cf. eq 3). The calculated functions are shown in Figure S4 (Supporting Information). The bulk-water relaxation time of a1 and a2 motion is about 20 ps, while the relaxation of a3 is somewhat faster (at ca. 15 ps, as we observed in classical-potential simulations61). Rotational dynamics of the interfacial water molecules are hindered considerably, as one would expect; however, the relaxation times are visibly longer than in the classical-potential case. This is probably an effect of dissociative adsorption in the ReaxFF simulations, causing different charge distributions on the surfaces. Rotational motion can be also restrained by hydrogen bonding, which has a slightly different character in ReaxFF than in the SPC/E water model, as we discuss below. The hydrogen bonds were identified on the basis of LuzarChandler geometry criteria60 and interaction between water molecules (Ow···Hw bonds) and between water molecules and surface oxygen atoms (O2c/Ob···Hw bonds) were distinguished on the interfaces. The hydrogen-bond mean values, their angles, average number of bonds per water oxygen Ow and mean lifetime are collected in Table 3, while the corresponding

Figure 9. Comparison of electric field profile E(z) with water dipole distribution in the interface region of antase (101) and rutile (110).

Distributions of dipole z-component in the interface region are shown in Figure S2 (Supporting Information). Water molecules in this region are not adsorbed directly to the surface, and their rotational movement is not so hindered: thus, they can minimize their energy by aligning the dipole with the field. However, nondissociated water molecules in the first hydration layer are confined by surface adsorption, and the collective dipole in this region is practically opposed to the interfacial intrinsic field. This observation is in qualitative agreement with the previous classical-potential study.50 Finally, we calculated the profile of the electrostatic potential by second integration of eq 5 from the middle-bulk region to the surface z z 1 φ (z ) = − E(z′) dz′ = − ρ(z′)(z − z′) dz′ εrε0 zbw z bw

∫

Table 3. Mean Value of Hydrogen Bond A−H Length, Mean A−D−H Angle, Mean Number of H-Bonds per Water Oxygen Ow and Mean H-Bond Lifetime Fitted by Luzar− Chandler model60a

∫

(9)

with the boundary condition φ(zbw) = 0. The potential curves on anatase (101) and rutile (110) interfaces are shown in lower panels of Figures 7 and 8. The potential drop across the interface was found to be 0.24 V on anatase (101) and 0.45 V on rutile (110). Note that the potential drop on anatase is smaller than 0.62 V which was found by Sang et al.29 in COMPASS potential with SPC/E water model. Not surprising the quantitative analyses are potential dependent, however, qualitative comparison of electric field and potential variations on the interface is consistent. Dynamical Properties of Interfacial Water. Next, we investigated the dynamical properties of the interfaces. To do so, first, we calculated the self-diffusion constant of water from the limiting slope of the mean square displacement according to eq 2. In the bulk-water region, the calculated diffusion constant is 2.28 × 10−9 m2/s, which is in excellent agreement with the experimental value of 2.3 × 10−9 m2/s.62,63 As wellknown from experimental measurements11−14 and from classical MD simulation,15−22 water adsorbed on the interface is confined significantly by strong interactions with the surface, and the diffusivity is hindered by 1−2 orders of magnitude. Here, we obtained a self-diffusion constant of 6.48 × 10−11 m2/ s on anatase (101) and 4.67 × 10−11 m2/s on rutile (110) − note that the MSD curves are provided in Figure S3

anatase (101) length [Å] angle [deg] number lifetime [ps]

rutile (110)

bulk water

wat/wat

wat/surf

wat/wat

wat/surf

1.954 13.541 3.028 5.205

1.884 13.477 2.114 5.289

1.884 14.441 1.337 2.400

1.898 13.644 2.093 4.570

2.183 18.756 0.510 0.804

a

H-bond interaction between water molecules (wat/wat) and between water and TiO2 surface (wat/surf) is distinguished on the interfaces.

distributions are shown in Figure S5 (Supporting Information). Water−water hydrogen bonds are slightly shorter near the TiO2 surfaces than 1.95 Å found in bulk, which indicates stronger intermolecular interactions in the interface regions. Rather surprisingly, hydrogen bonds between the first hydration layer and surface O2c/Ob atoms are of the same length on anatase (101), or even longer on rutile (110). The surface hydrogen bonds are therefore not stronger than the bonds within the first and second hydration layer. This finding is rather controversial, and it might indicate failure of ReaxFF potential to describe correctly the hydrogen-bonding interactions of water with the titania surface. The increase of A−D− H angle on the surface relative to bulk water reflects the structure of the TiO2 surfaces. Similarly, the number of H6707


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∼3000 cm−1, while the broad band near ∼3500 cm−1 mimics the experimental curve. Regarding the interfacial-water spectra, the most affected part is apparently the stretching band where the new broad peak between 2300 and 2850 cm−1 was detected on anatase (101). Analyzing the spectrum directional component, we found out that this peak is caused by stretching vibrations in x−y plane, i.e., the plane of the TiO2 surface. The x component is more pronounced than the y one, reflecting the ridged structure of this surface−rows of O2c atoms are oriented along the x direction. No such substantial changes can be seen on interfacial spectra of rutile (110) where the stretching band is only blue-shifted to 2850 cm−1 from its peak position of 3000 cm−1 in the bulk water. For comparison, we calculated the IR spectra also by Fourier-transforming the velocity autocorrelation function of hydrogen atoms, shown in Figure S6 (Supporting Information), where the stretching band exhibit significant dispersion to symmetric/antisymmetric modes. Nevertheless, the interfacial effects are, of course, the same as in Figure 10. In contrast to classical-potential results,50 the effects on the librational bands are rather minor on both studied surfaces, which is related to different absorption regimes in these studies. The strong directional effects observed in ref 50 were caused by translational/rotational movements of molecularly adsorbed water molecules in the first hydration layer, which were considerably affected by the surface structure. However, most of the first hydration layer is dissociated in ReaxFF and the librational peak is dominated by molecular water at the interface, which interacts with the surfaces only by hydrogen bonding (or, potentially, not via any hydrogen bonding at all). Therefore, the librational bands found on the interfaces resemble closely those of bulk water. Although there is undoubtedly some room for improvement of spectral accuracy, overall the ReaxFF potential performed reasonably well for description of dynamical properties of water in contact with TiO2 surfaces.

bonds is lower on the interface than in bulk, as we already observed in classical-potential simulations.50,61 The average hydrogen-bond lifetime was fitted on the basis of Luzar−Chandler model,60 which takes into account hydrogen-bond making/breaking rate, as well as diffusivity. The average lifetime of 5.2 ps that we found in the bulk-water region is considerably longer than 2.7 ps which we obtained previously with classical SPC/E water model.50 The water structure is more organized in ReaxFF (the average number of hydrogen bonds per Ow in the bulk is 3.0, compared to 2.3 found in SPC/E), and the hydrogen bonds persist for longer time. Interestingly, the lifetime of surface hydrogen bonds is considerably shorter, at 2.4 ps on anatase (101) and 0.8 ps on rutile (110), which contrasts with the classical model where a lifetime around 6−8 ps was measured at the interface. The discrepancy arises, of course, from different absorption régimes. In the classical model, the strongly adsorbed water molecules attached to Ti5c surface cations are not allowed to split, and their hydrogen bonds to the nearest O2c/Ob are very stable. On the other hand, dissociation occurs in ReaxFF as we described above and, moreover, the TiO2 atoms were allowed to move freely in contrast to the previous study. Water confinement on the TiO2 surfaces, that is restriction of the movement in the near interface region, affects the pattern of IR spectra. Here, we calculated the spectra by Fouriertransforming the collective-dipole propagation sampled during the MD simulations (cf. eq 4). The spectra are plotted in Figure 10, where the calculated as well as experimental bulk-water IR

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CONCLUSIONS We have investigated anatase (101) and rutile (110) TiO2water interface by reactive force-field (ReaxFF) techniques. We observed spontaneous dissociative adsorption of water on both studied surfaces when the first-hydration-shell water molecules attached to 5-coordinated titanium atom (Ti5c) split to form hydroxyl and hydrogen proton. While the hydroxyl group remains mostly on Ti5c, the proton moves to nearest 2coordinated surface oxygen (O2c/Ob). In contrast to previous studies involving only 1−3 water layers on the TiO2 surface, we found full coverage of surfaces by O2c/Ob by H+ and partial coverage of Ti5c by OH− (63% on anatase (101) and 48% on rutile (110) interface) in our simulations when the titania was in contact with the bulk water of room-temperature density. A typical layered water structure in the interfacial region, of width ca. 6.5 Å, was clearly observed. The intrinsic electric field, obtained by Poisson’s equation, changes rapidly in this region, varying from −2 to 6 V/Å on both studied surfaces. The electrostatic-potential drop across the interface layer was found to be 0.24 V on anatase (101) and 0.45 V on rutile (110). The well-known strong confinement of adsorbed water at the interfaces was confirmed by self-diffusion constant calculation. While the diffusion constant of 2.28 × 10−9 m2/s was measured in bulk water region, which is in excellent agreement with experimental data, the diffusivity at the interface is 2 orders of magnitude slower. Autocorrelation functions of internal water-

Figure 10. IR spectrum of water calculated as a power spectrum of system-collective-dipole autocorrelation function. Calculated bulk water spectrum (green) is compared with experimental curve measured in our laboratory. Spectra of nondissociated water molecules on the anatase (101) interface (red) and on the rutile (110) interface (blue) are shown in the lower panel.

spectra are also shown for comparison. First, one can notice that in contrast to classical water potentials50,61 the bulk-water spectrum is rather poorly described using ReaxFF. Although the reactive potential predicts all three characteristic bands of the spectra, the bending band located at 1950 cm−1 is noticeably red-shifted with respect to the experimental position of ∼1700 cm−1. On the other hand, the stretching band is significantly suppressed and blue-shifted with peak maximum located at 6708


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(3) Henderson, M. A. A Surface Science Perspective on TiO2 Photocatalysis. Surf. Sci. Rep. 2011, 66, 185−297. (4) Bourikas, K.; Kordulis, C.; Lycourghiotis, A. Titanium Dioxide (Anatase and Rutile): Surface Chemistry, Liquid-Solid Interface Chemistry, and Scientific Synthesis of Supported Catalyst. Chem. Rev. 2014, 114, 9754−9823. (5) O’Regan, B.; Gratzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (6) Berger, T.; Monllor-Satoca, D.; Jankulovska, M.; Lana-Villarreal, T.; Gomez, R. The Electrochemistry of Nanostructured Titanium Dioxide Electrodes. ChemPhysChem 2012, 13, 2824−2875. (7) Zhang, H.; Banfield, J. F. Thermodynamical Analysis of Phase Stability of Nanocrystalline Titania. J. Mater. Chem. 1998, 8 (9), 2073−2076. (8) Ranade, M. R.; Navrotsky, A.; Zhang, H. Z.; Banfield, J. F.; Elder, S. H.; Zaban, A.; Borse, P. H.; Kulkarni, S. K.; Doran, G. S.; Whitfield, H. J. Energetics of Nanocrystalline TiO2. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 6476−6481. (9) Park, N.-G.; van de Lagemaat, J.; Frank, A. J. Comparison of DyeSensitized Rutile- and Anatase-Based TiO2 Solar Cells. J. Phys. Chem. B 2000, 104, 8989−8994. (10) Pang, C. L.; Lindsay, R.; Thornton, G. Structure of Clean and Adsorbate-Covered Single-Crystal Rutile TiO2 Surfaces. Chem. Rev. 2013, 113, 3887−3948. (11) Levchenko, A. A.; Kolesnikov, A. I.; Ross, N. L.; Woodfield, B. F.; Li, G.; Navrotsky, A.; Boerio-Goates, J. Dynamics of Water Confined on a TiO2 (Anatase) Surface. J. Phys. Chem. A 2007, 111 (49), 12584−12588. (12) Spencer, E. R.; Levchenko, A. A.; Ross, N. L.; Kolesnikov, A. I.; Boerio-Goates, J.; Woodfield, B. F.; Navrotsky, A.; Li, G. Inelastic Neutron Scattering Study of Confined Surface Water on Rutile Nanoparticles. J. Phys. Chem. A 2009, 113 (12), 2796−2800. (13) Mamontov, E.; Wesolowski, D. J.; Vlcek, L.; Cummings, P. T.; Rosenqvist, J.; Wang, W.; Cole, D. R. Dynamics of Hydration Water on Rutile Studied by Backscattering Neutron Spectroscopy and Molecular Dynamics Simulation. J. Phys. Chem. C 2008, 112, 12334− 12341. (14) Mamontov, E.; Vlcek, L.; Wesolowski, D. J.; Cummings, P. T.; Rosenqvist, J.; Wang, W.; Cole, D. R.; Anovitz, L. M.; Gasparovic, G. Suppression of the Dynamic Transition in Surface Water at Low Hydration Levels. Phys. Rev. E 2009, 79, 051504. (15) Predota, M.; Bandura, A. V.; Cummings, P. T.; Kubicki, J. D.; Wesolowski, D. J.; Chialvo, A. A.; Machesky, M. L. Electric Double Layer at the Rutile (110) Surface. 1. Structure of Surfaces and Interfacial Water from Molecular Dynamics by Use of Ab Initio Potentials. J. Phys. Chem. B 2004, 108, 12049−12060. (16) Zhao, Z.; Li, Z.; Zou, Z. Structure and Properties of Water on the Anatase TiO2(101) Surface: From Single-Molecule Adsorption to Interface Formation. J. Phys. Chem. C 2012, 116, 11054−11061. (17) Nakamura, H.; Ohto, T.; Nagata, Y. Polarizable Site Charge Model at Liquid/Solid Interfaces for Describing Surface Polarity: Application to Structure and Molecular Dynamics of Water/Rutile TiO2(110) Interface. J. Chem. Theory Comput. 2013, 9, 1193−1201. (18) Kavathekar, R. S.; Dev, P.; English, N. J.; MacElroy, J. M. D. Molecular Dynamics Study of Water in Contact with the TiO2 Rutile110, 100, 101, 001 and Anatase-101, 001 Surface. Mol. Phys. 2011, 109 (13), 1649−1656. (19) Kavathekar, R. S.; English, N. J.; MacElroy, J. M. D. Study of Translational, Librational and Intra-molecular Motion of Adsorbed Liquid Monolayers at Various TiO2 Interfaces. Mol. Phys. 2011, 109 (22), 2645−2654. (20) Kavathekar, R. S.; English, N. J.; MacElroy, J. M. D. Spatial Distribution of Adsorbed Water Layers at the TiO2 Rutile and Anatase Interfaces. Chem. Phys. Lett. 2012, 554, 102−106. (21) English, N. J. Dynamical Properties of Physically Adsorbed Water Molecules at the TiO2 Rutile-(110) Surface. Chem. Phys. Lett. 2013, 583, 125−130.

molecule axes revealed that not only is translational motion, but also rotational movement, of water molecules is considerably hindered on the surfaces. On the other hand, interfacial hydrogen bonds are not significantly stronger compared to those found in the bulk water and their average lifetime, obtained by the Luzar−Chandler model, is relatively short−less than 1 ps. These results, in contrast with our previous studies,50,61 indicate the likely failure of the applied potential to describe correctly the hydrogen-bonding interaction of water with the titania surfaces. Although this is beyond the scope of the current study, careful comparison of simulation data with available CTR and QENS experimental data65,66 is needed to explore these discrepancies in more detail. In contrast to classical-potential water models, the IR spectrum of water obtained in ReaxFF is less accurate, exhibiting a considerable shift of bending as well as stretching band. Although the stretching band is clearly affected by surface interaction on anatase (101), rather minor changes were observed in interfacial IR spectrum of rutile (110). Despite some lack of accuracy in spectral description, the presented findings prove that ReaxFF can be used successfully for investigation of dynamical properties of TiO2/water interfaces, which are very hard to obtain by first-principles approaches.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12803. Distribution of water-molecule Ow−Hw bond length, Hw−Ow−Hw angle and size of the dipole moment; distributions of water−dipole orientation on the interfaces; mean square displacement curves; autocorrelation functions of internal water−molecule axes; distributions of hydrogen bond lengths, angles and number in bulk water and on the interfaces; IR spectra of bulk as well as interfacial water obtained from velocity autocorrelation function(PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(N.J.E.) E-mail: [email protected]. Tel: +353-1-7161646. Fax: +353-1-7161177. *(Z.F.) E-mail: [email protected]. Fax: +353-1-7161177. ORCID

Niall J. English: 0000-0002-8460-3540 Present Address †

Z.F.: Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Science Foundation Ireland for funding under Grant SFI 15/ERC-I3142, as well as Eduardo Bringa for technical advice and guidance in respect of reactive force fields.

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REFERENCES

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Exploring Rutile (110) and Anatase (101) TiO2 Water Interfaces by

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