Molecular Dynamics Simulations on the Interface between Titanium

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J. Phys. Chem. C 2009, 113, 10189–10197

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Molecular Dynamics Simulations on the Interface between Titanium Dioxide and Water Droplets: A New Model for the Contact Angle B. Ohler and W. Langel* Institut fu¨r Biochemie, UniVersita¨t Greifswald, 17487 Greifswald, Germany ReceiVed: December 19, 2008; ReVised Manuscript ReceiVed: March 30, 2009

The contact angle of H2O on stoichiometric and hydroxylated rutile (100) surfaces was studied by molecular dynamics simulations. On stoichiometric slabs, TIP3P water forms a highly ordered spreading film containing approximately two monolayers. On top of them, droplets with contact angles of 32-34° were formed. Hydroxylation induced complete spreading, whereas reduction of the partial charges of TiO2 mimicked a hydrophobic nonwetting surface. We postulate that the interface between ordered layers and droplets has an energy similar to that between ice and water. The Young equation is applied to this water/water interface rather than to the contact between H2O and TiO2. This approach reproduces the macroscopic contact angle and is consistent with models for radiation-induced hydrophilicity. UV-induced hydrophilicity, which is wellknown for several types of TiO2 films, was observed experimentally for oxide layers on titanium metal. The contact angles of water on different titanium samples exposed to air significantly depended on the surface pretreatment scattering in a range of 30-70°. In all cases, a significant light-induced decrease was observed, the values being in the range of 10-30° immediately after irradiation. 1. Introduction The high corrosion resistance of titanium metal is due to an impermeable oxide film, which spontaneously forms on air exposure. Titanium surfaces are characterized by the contact angle of water droplets,1 and treatments such as sandblasting alter the chemical composition and tend to reduce the contact angle, which is typically 40-60° on TiO2 single crystals2 or sputtered films on glass3 exposed to air. Titanium dioxide powders4 and thin films such as generated by sol-gel processes,5 chemical vapor deposition,3,6 or sputtering7 have been intensively studied as potential photocatalysts.8 After UV irradiation, a significant reduction of the contact angle to 10° or less, in some cases even spreading, was found.5,9-12 This is addressed as lightinduced hydrophilicity or superhydrophilicity. Recently, even anodically oxidized titanium dioxide has shown this effect,13 which has potential applications for self-cleaning coatings.14 Hydrophilicity of TiO2 films is reversible, and the contact angle increases after irradiation, attaining its initial value within several minutes or a few hours.15-17 Only radiation with wavelengths below 390 nm was found to be active.18 This was ascribed to surface charging,19 since light with quantum energies below the band gap of 3.2 eV in the TiO2 semiconductor8 cannot induce charge separation. Several experiments, however, give evidence that photoninduced electron transfer from the valence to the conduction band is not the only factor contributing to the reduction of the contact angles, which has also been observed after treatment by plasma6,20 or γ-rays2 and even after heating of nanocrystalline anatase films.3 Dissociative adsorption of water molecules in vacancies after UV-irradiation of (100) rutile resulted in additional hydroxylation.21 A necessary condition seems to be the presence of oxygen on the sample. According to refs 12, 22, and 23, the number of OH groups increases with surface charging, which affects the interface tension to water. * Corresponding author. Phone: +49 (0)3834 86 4423. Fax: +49 (0)3834 86 4475. E-mail: [email protected].

Radiation-induced superhydrophilicity was also attributed to the evaporation of hydrophobic hydrocarbon layers,24,25 which are generally found on titanium oxides exposed to air.26-28 According to ref 29, not only hydrocarbons, but also ultrathin water layers with 4.5-9.5 Å thickness are present. Their evaporation during irradiation and regeneration in a wet environment correlated with the variation of the contact angle. Two IR studies were interpreted in terms of ordered water layers on the TiO2 surface. Uosaki et.al.30 found an increase in signals for both icelike and bulk water adsorption after UV irradiation, and the enhanced adsorption was confirmed by QCM. Takeuchi et.al.31 proposed ordered water structures in droplets on TiO2, but these results were obtained by deconvoluting a broad combination band, and the ratio of the amounts of ordered and disordered water is probably subject to some uncertainty. Classical molecular dynamics simulations revealed that water close to a titanium dioxide surface has a high orientational order extending about 7 Å into the liquid.32 Water molecules at a distance of 10 Å or more from the surface are randomly oriented. This agrees with density profiles of water as a function of surface charge as calculated by a self-consistent approach.33 Recently, the orientation distributions of H2O molecules on hydrophobic surfaces were studied in detail by molecular dynamics.34 Contact angles on silica and platinum were directly calculated starting from orthorhombic water samples and simulating the transformation to lens-shaped droplets within less than 5 ns. Their images were evaluated in analogy to sessile drop experiments.35,36 A continuous transition from hydrophilic to hydrophobic surfaces was obtained by scaling down all partial charges of the atoms in the solid surface of silica.37 Force fields with few parameters as employed in these studies are, in general, only an approximation, resulting in significant divergence from the experiment for many relevant water properties. Both the threecenter TIP3P and the four-center TIP4P force fields underestimate the surface tension of water, yielding 50 and 56 mN/m at 300 K, respectively.38

10.1021/jp811257x CCC: $40.75  2009 American Chemical Society Published on Web 05/18/2009

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In this paper, we propose a new interpretation for the water/ titanium dioxide contact angle, providing an interface between a spreading ordered surface layer and the disordered droplet, and apply the Young equation to this system. Analysis of the interface structure from molecular dynamics simulations is shown to support this approach. Further on, measurements of the contact angle of water on passivated titanium after UV irradiation that complement extensive earlier work on various more specifically prepared samples are presented. 2. Experimental and Theoretical Approach Molecular Dynamics. Each simulation cell for molecular dynamics contained a rigid rutile (100) slab with five TiO2 layers. It is made up of vertical sticks containing 5 Ti and 10 O atoms, respectively, with a distance of 11 Å between the centers of top and bottom atoms. The slab fills the full cross section in the x-y direction,39 resulting in an extended contiguous surface under periodic boundary conditions. The surface area of each of these elements is determined by the lattice constants of rutile (100), being 4.594 × 2.959 Å2 ) 13.5 Å2. Systems with cross sections of 147.0 × 148.0 Å2 and 220.5 × 195.3 Å2 corresponding to 32 × 50 and 48 × 66 surface unit cells were set up. Heights of the simulation cells were 120-200 Å, allowing for spacing of 109-189 Å between the top of the slab and the bottom of its adjacent mirror image in the z direction. This vacuum was partially filled by 3700-9500 water molecules in tetragonal blocks of different sizes with initial shapes similar to configurations used in ref 35. In some runs, an additional layer of water was put on the surface. Water molecules were constrained to be rigid using the SHAKE option and were in general described by the TIP3P model.40 In two runs, the influence of the force field on the contact angle was checked, starting from the same initial configuration as in a TIP3P run but using the TIP4P force field.41 The Ti and O charges in the slab were +1.15 and -0.575e,42 respectively, unless otherwise stated. In ref 32, it was shown that protonation of 15% of the bridging oxygen atoms and hydroxylation of 30% of the 5-fold coordinated Ti atoms roughly correspond to a completely clean surface in contact with a physiological solution with pH ) 7.4. These surfaces with different degrees of protonation and hydroxylation had a net charge that was compensated by counterions in the solution. In contrast to these simulations, our cells are only partially filled with water, and the ratios of TiO2 surface areas to water volumes are very high. The compensation of a significant net surface charge by counterions in the droplet would have resulted in unrealistic ionic strengths, or nearly all ions would have been adsorbed, resulting in an unrealistic charge distribution. Highly polar but still neutral substrate surfaces were generated by adding OH- and H+ ions to Ti and O atoms. No charge transfer to the titanium dioxide was considered, the charges of the resulting singly and doubly coordinated surface hydroxyl groups being -e and +e, respectively. The condition at pH ) 7.4 is approximated by 15% or 30% hydroxylation, whereas a hydroxylation of 50% provides a very high concentration of highly polar adsorption sites for facilitating the interpretation of their influence on the local water structure. Dry (100) surfaces will, in general, be much less hydroxylated than those in contact with solutions, and we use the stoichiometric non-hydroxylated slab as the moderately polar standard model. The systems were subjected to energy minimization and NVT (constant number of atoms, volume of the cell and temperature) molecular dynamics runs scanning 5 ns at 300 K. All atoms of

Figure 1. Snapshots of molecular dynamics simulations for water on rutile (simulaton times 5 ns): (a) Stoichiometric surface with standard charges for Ti and O (1.15 and -0.575e): On top of the slab, a droplet is formed with a contact angle of 33°, and directly on the surface, a spread water layer is clearly seen. On the bottom of the slab, a higher amount of water is adsorbed, which, due to periodic boundary conditions, forms a contiguous layer. (b) On the stoichiometric surface with reduced charges (0.115 and -0.0575e), a droplet with a contact angle of about 100° is stable, and wetting is suppressed. Between droplet and surface, a gap is seen.

the stoichiometric and partially hydroxylated slabs were fixed. The curvatures at both edges of the droplet were fitted by one circle (Figure 1), and tangents were calculated to this circle rather than to the droplet itself.43 This is mathematically equivalent to the common tangents method, but removes some arbitrariness, since the edges of the droplet are better fitted by a circle than by tangents only. Moreover, the method yields identical values for both edges. For molecular graphics, the program VMD has been used.44 Measurement of Contact Angles. Titanium foils (Advent, Oxford; 99.6% purity, 0.5 and 1 mm thickness) were passivated by exposure to air only. Three commercial titanium samples with different surface preparations (polishing with organic materials, shot-peening with glass, and sandblasting with alumina) have been characterized by XPS.45 All of them showed carbon signal intensities similar to the corresponding Ti peaks. The C 1s peak from the shot-peened sample was only 30% lower than that from the polished one, indicating that atmospheric hydrocarbons significantly contribute to the carbon contamination. Sandblasting resulted in a significant aluminum signal and in a reduction of the Ti and C peaks by 30% with respect to those from the shot-peened sample. Silicon intensity on the latter sample was close to noise level. All samples were exposed to a 200 W XBO mercury highpressure arc lamp (Oriel 6137) with a UV-transmitting condenser lens for 1 h. From manufacturer’s data, we estimate the power to be 700 mW/cm2 and about 180 mW/cm2 in the vis and UV ranges, respectively. In some experiments, the spectrum was

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TABLE 1: Contact Angles on Titanium Dioxide Samples Ti sample passivated by air exposure shot peened (SiO2) shot peened (SiO2) sandblasted (alumina) polished polished

wavelength range UV/vis UV/vis UV UV UV/vis UV

dark angle/°

angle after irradiation/°

recovery time/min

62 39 48 28 51 72

17 16 20 11 26 33

16 35 8 20 9 52

limited to the range around 250 nm by a UV filter, the intensity being about 3 mW/cm2. Contact angles have been measured by the sessile drop method. They were evaluated using a graphical method analogous to that for the simulations. 3. Results and Discussion Experimental Contact Angles on Ti Foils. The contact angles of water on titanium surfaces oxidized by exposure to air were θ ) 30-70° (Table 1). The lowest angle was found for a sandblasted sample, in agreement with ref 1. Immediately after light irradiation by the full lamp spectrum and by UV, only the contact angles were significantly reduced. After the end of the irradiation, the contact angle, θ, increased again gradually, attaining its initial value θ0 (Table 1). This was described by a saturation function θ ) θ0 · [1 - exp (-(t/τ))] with time constants τ between 8 and 56 min. The effect was only qualitatively reproducible, the minimum and maximum angle and the relaxation time varying beyond the precision of the contact angle measurements of (2°. Simulation of Contact Angles by Molecular Dynamics. During all simulations, the initial configurations rapidly evolved and attained stable configurations within 1 ns. These consisted of droplets or layers spread on the surface depending on its charge and on the quantity of water in the cell. Using standard charges for Ti and O,32 contact angles of 32-34° are found in four runs on stoichiometric non-hydroxylated surfaces. A droplet is observed only for small amounts of water on the surface. In one case, the TiO2 slab was covered with a larger amount of water on the bottom than on top, and a droplet was seen only on top (Figure 1). In general, the amount of water had to be less than that of a homogeneous coverage of about n ) 2.5 monolayers, whereas above 3.0 layers, spreading was observed. This is an artifact of the small simulation system: On a periodic microscopic surface, the size and water content of the droplet is limited by the cross section, A, of the simulation cell and by the contact angle (Figure 2). If the droplet exceeds a certain size, it interacts with its images in the adjacent cells, and a smooth plane water layer is energetically more favorable. The volume, V, of a droplet with spherical curvature and a maximum diameter, d, of its surface contact and the corresponding number of layers for homogeneous coverage, n, is given by

V)

(

(

))

π 3 3 cos θ cos2 θ d · 1· 112 2 3 V 3 cos θ cos2 θ π d ⇒n) ≈ · · 1· 1A · h 12 h 2 3

(

(

))

where A · h is the volume of one layer completely covering the nearly quadratic cell. The height, h, of a close-packed layer of spherical water molecules with r ) 1.71 Å is given by h ) 3j · r ) 3.0 Å. The droplet volume corresponds to only n ) 0.5-1 equivalent monolayers for contact angles of 30-40°, h

Figure 2. Only small droplets are stable in a periodic cell (top). Larger droplets overlap with their mirror images, forming a contiguous layer (bottom).

) 3 Å, and d ) 150-200 Å. The spherical shape is an approximation; more precisely, the droplet will even have a slightly lower volume. The limit of 2.5 equivalent layers, as observed in the calculation, may be decomposed into 1-2 spread layers on the surface (below) and a droplet having an amount of water corresponding to about one layer. Density Distributions and Average Orientations. The density of water molecules was evaluated with a resolution of ∆z ) 0.1 Å as a function of the height, z, of their O atoms above the bridging oxygen atoms on the surface. Here, we discuss density profiles for stoichiometric surfaces with standard partial charges (Figure 3a), with reduced ones mimicking very low polarity (Figure 3b) and for partially hydroxylated systems (Figure 3c). Our respective density profiles are compared with three earlier calculations on hydroxylated,32 charged,33 and nonpolar34 surfaces. All profiles contained distinct peaks, which we assign to two water layers on the titanium dioxide, in which the molecules have significant preferential orientations (Figure 3a). On the stoichiometric surface, one intense peak with a maximum density value of about 4 g/cm3 at z ) 1.4 Å marks the first water layer. The absolute densities are identical for two very different amounts of water, indicating full surface coverage. Inspection of the simulated configurations indicated that the H2O molecules interact with both surface titanium and bridging oxygen atoms. Consequently, the bottom water layer is spreading and commensurate (Figure 4a). The surface area of 13.5 Å2 of each unit cell with one bridging oxygen atom is close to the effective molecular area of H2O, being AH2O ) 12.5 Å2.46 On top of the first layer, the density of water molecules drops nearly to zero at z ) 2.2 Å. This dip is much more pronounced than in the data of ref 33. A weak feature consisting of two maxima with a separation of about 1 Å is assigned to a second water layer. It is shifted by ∆z ) 2 Å with respect to the first layer, which is less than the spacing h ) 3.0 Å between closepacked layers. The density profiles have similar shapes for two different degrees of coverage: droplet formation (2, 3) and spreading (4). For the contiguous water film, the density has a constant value of about 0.95 g/cm3, close to bulk density in the range of 5-8 Å, where the cell cross section is completely filled (cf. Figure

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Figure 3. Plots of the water density, F, averaged over the cell cross section as a function of the height above the oxygen bridging atoms on the surface (F0: standard water density of 1 molecule/30 Å3). Positions were taken from molecular dynamics trajectories between 4 and 5 ns of simulation time. Arrows indicate significant features (see text). (a) H2O on the stoichiometric surface with standard charges (from bottom to top): low coverage (2.3 layers), TIP4P (trace 1), and TIP3P (2) force fields, respectively; low (1.5 layers, (3)) and high (3.0 layers, (4)) water coverages, as shown in Figure 1a. The second and third traces indicate the high reproducibility of the density distributions as obtained from different starting configurations. (b) The lower trace (5) corresponds to the “hydrophobic” surface with reduced charges and a low coverage (1.1 layers). For comparison, trace 3 from part a is redrawn on the same scale as trace 5. It is clearly seen that only a few molecules approach the hydrophobic surface. The peak for a spread water layer is missing in trace 5, the first peak corresponding to the second layer on the normally charged TiO2 (3). (c) As traces 3 and 4 in part a, but with 30% hydroxylation. Splitting of the peak for the first layer and shift of the second layer to higher z values is clearly seen.

2). For the droplet that only partially covers the surface (Figure 3a), the density is only 0.2 g/cm3 and decreases to 0.1 g/cm3. We compare the density profiles on the stoichiometric surfaces with those of Figures 3 and 4 in ref 33, in which qualitatively similar density distributions on negative and positive surfaces were reported with more pronounced structure at higher charges. These data show a strong peak, indicating a commensurate layer on the close-packed surface. A feature consisting of two peaks shifted by 1.8 Å with respect to the first maximum is assigned to a second layer of oriented water molecules. These two layers, which are reproduced in our molecular dynamics calculations on the stoichiometric surface, are ascribed to an “icelike” structure. On surfaces with 15%, 30%, and 50% of the TiO2 sites covered by hydroxyl groups, we observed spreading rather than well-defined droplets. The thickness of the water layer reflected inhomogeneous charge distributions in the surface. Islands of predominantly positive or negative charge are significantly less covered than border lines between them. We assume that the polar water molecules are adsorbed mainly there (Figure 5), since adjacent positive and negative hydroxyl groups generate highly polar adsorption sites in between. Consequently, a system with a regular chess board-type distribution of positive and negative OH groups did not show significant fluctuations in the water distribution.

Ohler and Langel

Figure 4. Snapshots of the H2O structure on the rutile surface from molecular dynamics simulations: (a) Stoichiometric surface with standard charges: The first water layer is commensurate, and the molecules have a uniform orientation (sites i). The water oxygen faces surface Ti (magenta), whereas the hydrogens form H bonds to bridging oxygen atoms (on top of the planes inclined to the right). (b) Randomly hydroxylated surface: 30% of the surface Ti and of the bridging O atoms are carrying a singly coordinated hydroxyl group and a proton, respectively. No ordered structure of the first layer can be recognized. Physisorbed water molecules in site ii form hydrogen bonds to positive protonated bridging atoms and to negative singly coordinated OH groups. In site iii, they form hydrogen bonds between their hydrogen atoms and non-protonated negative bridging oxygen, as in site i. Due to the interaction with the surface protons, the water molecules are on top of the bridging oxygen in site iii, whereas in site i, on the stoichiometric surface, the molecules were inclined and, thus, closer to the surface. (c) 50% of the surface is hydroxylated, and protons and hydroxyl groups form a chessboard pattern. The lowest water molecules are more regularly arranged than on the randomly distributed OH groups, but seem to have different orientations on hydroxylated and protonated sites. The molecules in the second layer seem to be orientationally disordered. Sites ii and iii can clearly be distinguished here.

The density and orientation profiles on hydroxylated surfaces are substantially different from those on stoichiometric ones. The first peak close to the surfaces splits into two components with a separation of only 1.2 Å and maximum densities of 2.0-2.5 g/cm3 (Figure 3c). We assign both peaks to one bottom layer with two preferential oxygen positions. A similar situation is known from ice Ih, where the oxygen atoms in each (0001)

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Figure 5. (top): Snapshot of the structure of water on the borderline between positive and negative hydroxyl groups. The surface is covered by singly (left) and doubly coordinated hydroxyl groups (right), and two periodic images on both sides of the simulation cell are shown in part. The water molecules are attached mainly to the boundary lines between the positive and negative zones of the surface. A small number of molecules are spread out on the cell, being oriented according to the local surface charge.(bottom): Orthographic view of the center part of the cell around the borderline between positive (left) and negative surface (right). The preferential orientation of the water molecules is clearly seen, indicating the high polarity of the surface. The water molecules are arranged in well-defined rows parallel to the surface.

plane are equally distributed over two z positions with a separation of ∆z ) 0.92 Å.47 Figure 4 shows three different surfaces, which are stoichiometric (a), and hydroxylated to 30% (b) and 50% (c), respectively. Many of the preferred H2O adsorption sites (i) on the stoichiometric surface (Figure 4a) are blocked by the additional hydroxyl groups in Figure 4b and c, but between the positive and negative OH groups, new favorable H2O sites (ii) exist with z positions similar to those O atoms in site i. On the remaining unprotonated bridging atoms, water molecules are adsorbed, forming hydrogen bonds similar to those in site i, but being shifted from the surface by about 1 Å (iii). Molecules in sites ii and iii contribute to the split peak of the first adsorption layer in the density distributions (Figure 3c).

The calculations for hydroxylated surfaces at different pH values32 have shown that the peak close to the surface is pronounced at low pH value where the water molecules are adsorbed due to the interaction between their negative O and the positive proton on a bridging atom (site ii). The second peak has enhanced intensities at high pH values, where water molecules interact via their positive end with unprotonated bridging atoms on the surface (site iii). On top of the doubly split first layer (Figure 3c), a feature consisting of two peaks with a splitting of ∆z ) 1 Å is found at z ) 4.4 Å. It is shifted by about ∆z ) 2 Å with respect to the second peak of the first layer and can be assigned to a second layer on top of adsorbate molecules in site iii, in analogy to the second layer on top of molecules in site i on the stoichiometric

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Figure 6. Average cosine of the angle between molecular dipole and z axis for the samples. The numbering of the traces is as in Figure 3. Arrows indicate significant features (see text). (a) On the stoichiometric surfaces with standard charges (1-4) a very reproducible pattern is observed, which does not siginificantly depend on the coverage (3 vs 4) but is less pronounced on TIP4P runs (1). On the hydrophobic surface (5), the average orientation is parallel to the surface. (b) Hydroxylated surfaces (3′) and (4′) show a reversal of the average orientation between the two peaks of the first layer. Features at larger z values are less reproducible.

surfaces. A second water layer around z ) 3.5 Å on top of molecules in sites ii is not seen on the hydroxylated surfaces, but no sharp dip in the density is observed here between the first and second layers at 2 and 4.4 Å, and the feature indicating the latter one is poorly reproducible for different hydroxylation states and coverage. This is seen from Figure 3, where at low coverage, only a single flat maximum remains at this z position, and from Figure 4 in ref 32, where the corresponding feature is strongly pH-dependent. The average cosine values of the angle between dipole direction and the z-axis (cf. ref 32) are plotted in Figure 6 as a function of z. The orientational functions of H2O on the stoichiometric surface for small coverage with droplet formation and for larger coverage with a spreading layer are identical. They are best described by a damped oscillation with an initial amplitude of 0.3 and a well-defined period of 2.36 Å, roughly coinciding with 1/3 of the spacing, c, between (0001) layers in the ice Ih lattice. We assume that the surface introduces a perturbation into the uniform distribution of water dipole orientations, which relaxes within about 10 Å. Using the TIP4P force field for water resulted in a nearly identical density distribution but a significantly different orientation function with less pronounced maxima (Figure 6). The TIP4P model provides a shift of the negative partial charge from the position of the oxygen atom toward the center of the positive charge of the hydrogen atoms and a slightly reduced dipole moment (2.18 vs 2.35 D). Under similar conditions, this force field shows smaller orientational polarization than TIP3P water and, consequently, yields a significantly smaller relative dielectric constant (53 vs 7348). Both dipole moments are significantly smaller than the experimental value for liquid H2O (2.95 D49). It is thus possible that the orientational ordering of the molecules near the surface is underestimated by both force fields.

Ohler and Langel On the hydroxylated surfaces, we analyzed in more detail the range of the first layer, in which the orientation changes its sign, switching from +0.3 to -0.4 between the two density maxima (Figure 6). Molecules in the peak close to the surface (site ii) are oriented with the oxygen pointing to it, and those in the second peak, with hydrogen down (site iii). This reversal of average orientations between the two halves of the first layer is also found in an ice-like structure, in which the molecules in the upper and lower part of an (0001) plane have opposite orientations. On top of the first layer, the average orientation shows only small deviations from zero. In one run, the charges of Ti and O in TiO2 were reduced to 10% of the standard values. On this surface with very low polarity, a droplet with a contact angle larger than 90° is formed (Figure 1). The first layer consisted of a broad feature whose maximum at z ) 3 Å is close to the position of the second layer on the stoichiometric surface. On a side view of the system (Figure 1b), indeed, no spread layer but a gap between droplet and TiO2 is seen. A single broad peak that is shifted by ∆z ) 2.9 Å with respect to the center of the first layer indicates a second one. This value is close to the height h ) 3.0 Å of closepacked layers. The orientation cosine shows a weak oscillation with amplitude of about 0.05 in the z range of the first layer and fluctuates around 0 above it. These density profiles (Figure 3b) and orientation distributions (Figure 6a) are very different from those on the more polar surfaces but closely resemble recent molecular dynamics simulations of water on nonpolar diamond surfaces34 (Figures 10 and 15 of the reference). Their analysis revealed a highly ordered molecular structure of the first layer with a strong preferential orientation of the dipole parallel to the surface. On top of the two layers described so far, no pronounced peaks are found in the density and orientation functions, and an additional third peak on charged surfaces, seen in ref 33, is not observed here. We assign this z range to bulk water. Description of the Interfaces by the Young Equation. The Young equation for the TiO2/bulk water interface reads (σTiO2 - σTiO2/H2O)/(σH2O) ) (∆σ)/(σH2O) ) cos Θ. No experimental data on the surface tensions of titanium dioxides σTiO2 are known to us. Calculations50-53 scan a wide range of different surface orientations and states of relaxation, but all results are in the range of 0.4 to 1.0 J/m2 which is about 1 order of magnitude higher than the surface tension of liquid water, σH2O ≈ 0.073 J/m2.54 For a nonspreading water layer with cos Θ < 1, the difference, ∆σ, between σTiO2 and the interface tension between TiO2 and water, σTiO2/H2O, must be smaller than σH2O. For Θ ) 60°, it follows that ∆σ ≈ 0.5σH2O ) 0.037 J/m2 , σTiO2, and consequently, σTiO2/H2O ≈ σTiO2 must hold within a few percent. On the other hand, generating the H2O/TiO2 interface obviously affords less energy than extending the surface of bare TiO2, since H2O is a polar molecule, which finds energetically favorable adsorption sites. Thus the interface tension σTiO2/H2O should be significantly lower than σTiO2 ) 0.5 J/m2, making it very unlikely that the interface between dry TiO2 and water accounts for droplet formation. We, therefore, propose a new model for the titanium dioxide/ water interface (Figure 7) consisting of three stages: (1) A chemisorbed layer of water and hydrocarbons on pure titanium dioxide determines its charge and electrostatic potential. (2) A thin film of physisorbed water is presumably present on any air-exposed titanium dioxide unless it is strongly contaminated and hydrophobic (cf ref 29). In the simulation, such a layer in direct contact with this titanium dioxide surface attains a structure that is very different from that of bulk liquid

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Figure 7. Schematic drawing of the model of water/water interface as presented in the text. We distinguish between a layer of oriented water molecules, which experiences a significant influence on its structure from the surface, and bulk water at distances above about 7 Å from the surface. The interface between these two water structures determines the observed contact angle.

water. The Young equation for the interface between TiO2/ oriented H2O layer predicts spreading for any value of the interface tension, σTiO2/oriented, between TiO2 and the oriented H2O layer (Figure 7) in the range below 0.4 J/m2:

σTiO2 - σTiO2|orderedH2O σH2O

≈

0.5 J/m2 - σTiO2|orderedH2O

)

0.07 J/m2

cos Θ > 1 (3) Because the structure of this water layer is significantly different from that of the bulk liquid, it is feasible to assume an interface with a significant energy σorderedH2O/H2O, which determines the macroscopic contact angle in our model. The surface tension of the ordered layer should be between σH2O and the surface tension of ice, σice ) 0.11 J/m2,55 and is given a tentative value of σorderedH2O ) 0.08 J/m2. The Young equation for this oriented layer/droplet interface (Figure 7) reads, for example, for Θ ) 60°,

cos Θ )

σorderedH2O - σorderedH2O/H2O σH2O

)

0.08 J/m2 - 0.044 J/m2 ) 0.5 0.073 J/m2 This affords an interface energy of 0.044 J/m2, which is comparable to that between liquid water and ice, σice/water ) 0.03 J/m2.56,57 An upper limit for σorderedH2O|H2O is given by the interface energy σD-D between two water layers in contact with each other with unrelaxed dipole orientations, which is the interaction energy ED-D of two oppositely oriented water molecules divided by the effective surface area of one water molecule,

σorderedH2O/H2O e

ED-D 1 ) · AH2O 4πε0 r

µ2

H2O-H2O

3

·

1 ) AH2O 0.25 J/m2

with µ ) 2.95 D in solution and the hard sphere diameter of H2O, rH2O-H2O ) 3.4 Å. This crude estimate shows that the dipole-dipole interaction alone between the adsorbed water layer and the bulk droplet may account for an interface tension that is largely sufficient for explaining the observed contact angles. By variation of σorderedH2O and σorderedH2O/H2O in respective ranges of 0.07-0.11 and 0-0.05 J/m2, contact angles from spreading up to 75° may be reproduced. Then a droplet is formed

if the surface tension of the layer is at least slightly lower than that of ice, and the water-water interface tension is close to that of water on ice. The simulated contact angle of 30-35° is lower than most experimental data. It depends on the force field applied and, especially, on the partial charges for the Ti and O surface atoms. A direct comparison of our calculated value with experimental data is difficult for two reasons: (1) a macroscopic droplet averages over different local structures and charges of the surface, and (2) titanium dioxide exposed to air is always covered by some carbon-containing species. This may generate additional electronic gap states and increase the hydrophobicity. Traces of carbon in anatase enhanced the superhydrophilicity, and on carbon-doped layers, the contact angle decreased even in visible light.58,59 This was ascribed to additional electronic states in the band gap. In our simulation, electronic states of the titanium dioxide are not considered, but a hydrophobic surface will result in a high contact angle. This is mimicked by reducing the charges of the Ti and O atoms, which had a strong effect and even suppressed wetting. The slab with low partial charges has properties similar to a strongly carbon-contaminated hydrophobic surface. Presumably, the interface between the ordered water film on the surface and the disordered molecules in the droplets is localized on top of the second water layer. Hydroxylation results in a highly inhomogeneous first layer on top of which no ordered second one can form. In turn, no welldefined interface with significant energy is present between a second layer and the disordered water phase, and no droplet is observed. In earlier simulations of water on platinum,36 a single spread layer of water molecules was found whose structure largely depended on the packing of the upper metal substrate. Only a vague explanation was given for the influence of the ordered water layer on the droplet formation, and no contact angle was determined. In addition to that, no density profiles that could reveal the second layer were evaluated. Our model is compatible with several factors contributing to the reduction of the contact angle under UV-irradiation: (1) A possible effect of the UV irradiation is the dissociation of water on the surface and the formation of hydroxyl groups. We have shown that significant hydroxylation reduces the contact angle. The observed spreading is due to charge separation between positive and negative OH groups, the hydroxyl groups generating high polarity and affecting the orientation of the water film in direct contact with the surface (Figures 5 and 6). Our calculations might overestimate this effect, since no relaxation of the TiO2 and, especially, of the hydroxyl groups was possible. Thus, we do not make a guess on the minimum coverage of hydroxyl groups inducing spreading. The experimentally observed removal and formation of a very thin water film during and after irradiation29 may be correlated to the radiation-induced dissociation of water and formation of hydroxyl groups, which must be reversible to explain the observed recovery of the contact angle. It seems unlikely that evaporation of physisorbed water can explain hydrophilicity, since this is in disagreement with QCM measurements, indicating an increase in the adsorbed water layer during UV irradiation.30 In addition to that, any measurement of the contact angle affords that liquid water is in contact with TiO2 and an adsorbed film will be formed immediately. (2) Local charges on the surface are induced by UV irradiation, which generates polar sites. We could not simulate net charges of the surface, but adjacent positive and negative

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sites can be considered as a model for local charges with a comparable effect on the structure of physisorbed water. (3) Evaporation of carbon will obviously reduce hydrophobicity. On a hydrophobic surface, the molecules are strongly oriented,34 and a well-defined interface between adsorbed film and droplet is possible. After evaporation of the film, this water orientation in the first layer will be perturbed, and the interface energy to disordered water may be reduced. Returning to the Young equation for the ordered/disordered interface of water, a reduction of the contact angle means that either the surface tension of the ordered layer σorderedH2O increases, approximating the value of ice, or that the interface tension σorderedH2O/H2O between ordered and disordered decreases and finally disappears. Arguments given above favor the second possibility. A radiation-induced decrease in σorderedH2O/H2O from 0.044 J/m2 to less than 0.01 J/m2 reproduces a reduction of the contact angle to 0° (spreading). 4. Conclusion The macroscopic contact angle of water on titanium dioxide is not determined by the direct contact of bulk liquid with the oxide surface. They are separated by a thin water film with an ordered, probably icelike structure, and the contact between this spread film and the droplet determines the macroscopic contact angle. We apply the Young equation for the first time to this interface, combining experimental and theoretical data for the respective interface tensions. The TIP3P force field for water and validated partial charges for TiO2 allow the macroscopic contact angle to be reproduced reasonably well in molecular dynamics simulations, even though standard force fields are based on few parameters and are known to describe the large variety of water properties with limited precision. It is not clear whether discrepancies between our calculations and experiment are due to the partial charges or a scaling problem going from microscopic to macroscopic rough systems. By the simulations, the structure of the water layer on the TiO2 surface was reproduced. UV irradiation may induce processes such as charging of the surface, reversible hydroxylation, and evaporation of hydrocarbon layers, which are adsorbed under atmospheric conditions. These surface modifications have influence on the structure of the ordered water film. It was shown here that the contact angle is very sensitive to the order in the adsorbed layers, and its variation is a likely mechanism for radiation-induced hydrophilicity. The moderate reproducibility of the contact angles in the experiment might be due to exposure of the sample to ambient atmospheres with varying contents of water and hydrocarbons. Several modifications of TiO2 show photoinduced hydrophilicity. We have reproduced this effect even on samples passivated by air exposure only. Our values for contact angles before and after irradiation and the recovery time constant for the titanium foils were in a range similar to that on oxide films prepared by other methods. Acknowledgment. Funding of this work by the Deutsche Forschungsgemeinschaft and useful discussions with Prof. C. Wo¨ll, Universita¨t Bochum, and Dr. S. Ko¨ppen, Universita¨t Bremen, are gratefully acknowledged. The XPS data were recorded by Y. Gao, Universita¨t Bochum, and some of the titanium samples were provided by OHST GmbH, Rathenow, Germany. We further acknowledge assistance of K. Lifson during contact angle measurements.

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