Enhancing Hydrophilicity of Anatase TiO2 Surfaces by Deposition of

Article pubs.acs.org/JPCC

Enhancing Hydrophilicity of Anatase TiO2 Surfaces by Deposition of Alkaline Earths: The Case of Ca Michel Posternak,*,† Simon Berner,‡ Alfonso Baldereschi,†,§ and Bernard Delley∥ †

Institute of Theoretical Physics, Swiss Federal Institute of Technology, SB, PH H2 482, Station 3, CH-1015 Lausanne, Switzerland Surfaces Research, Institut Straumann AG, Peter Merian-Weg 12, CH-4052 Basel, Switzerland § Physics Department, University of Trieste, Strada Costiera 11, I-34151 Trieste, Italy ∥ Paul Scherrer Institut, WHGA/123, CH-5232 Villigen PSI, Switzerland ‡

ABSTRACT: We show with DFT calculations within the COSMO framework that the wetting energy of clean or hydroxylated anatase TiO2 surfaces is strongly enhanced by Ca deposition at low coverage. Two mechanisms contribute to this behavior: the ionic character of Ca adsorption and the inhomogeneity of the transferred charge density, leading together to a strong polarization of the embedding medium. In turn, this polarization is responsible for a large wetting energy and the predicted hydrophilicity. By increasing coverage, Ca adsorption becomes more and more neutral and the transferred charge density is progressively more homogeneous: the effects of the two mechanisms are gradually reduced, leading eventually to surface hydrophobicity at full coverage. In order to support this theory, investigations of the wetting properties of TiO2 slabs in the presence of methane are presented.

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INTRODUCTION Titanium is one of the most used and documented biomaterials for dental and orthopedic applications, since it is a relatively inert and corrosion-resistant metal. The excellent biocompatibility of titanium implants is mainly due to the specific properties of the TiO2 passivation layer which is present on the surface. Apatite nucleation is initiated by adsorption of Ca2+ ions, followed at a later stage by incorporation of phosphate groups and formation of an amorphous calcium phosphate phase. Several studies1−7 have shown that the bone apposition process is favored by the presence of hydroxyl groups on the oxide film. This hydroxylation occurs naturally when the TiO2 surface is exposed to water contained in air, thus forming acidic and basic OH groups. The net charge of this surface enables therefore various ionic interactions. Macroscopically, this modification induced by hydroxylation is characterized by a high surface wettability, corresponding to contact angles close to zero. Hydrophilicity is largely dependent on surface energy and influences the degree of contact with the physiologic environment. Experiments suggest that the surface charge of the hydrophilic surface may selectively attract proteins. The increased wettability thus enhances the interaction between the implant surface and the biological environment. Inorganic molecules, such as calcium and phosphate ions, are readily adsorbed from the blood onto the hydroxylated TiO2 surface. However, both clean and hydroxylated TiO2 surfaces exposed to air become contaminated within minutes to hours by hydrocarbons and carbonates, lowering their surface energies and causing water contact angles to increase substantially, eventually making the surfaces hydrophobic. © 2013 American Chemical Society

For the above reasons, the wettability of implant surfaces influences the osseointegration process, i.e., integration of an implant in bone. Hydrophilic surfaces have a positive influence on the early bone response and lead thus to faster osseointegration compared to their hydrophobic counterparts.2,8,9 The commercially available SLActive implants (Institut Straumann AG, Basel, Switzerland) have a superhydrophilic surface due to surface treatments and special storage conditions. The SLActive surface is obtained by sand blasting and acid etching, followed by rinsing in pure water and storing the implant in aqueous NaCl solution. The aqueous solution protects the surface from contamination with carbonates and organic components which are present naturally in the atmosphere. The resulting implant surface is highly hydroxylated and has superhydrophilic properties. Several preclinical and clinical studies have shown that SLActive implants outperform their hydrophobic counterparts, i.e., SLA implants, in terms of early bone response of the osseointegration process.2,7,10 A proper understanding of the microscopic mechanisms, which are responsible for these beneficial properties, is therefore highly desirable. Another approach for enhancing implant hydrophilicity, and therefore cellular adhesion, is based on structural modifications of the surfaces.11 In particular, we demonstrated12 for the paradigmatic case of anatase TiO2 (the most stable phase when Received: July 10, 2013 Revised: October 30, 2013 Published: November 27, 2013 26013

dx.doi.org/10.1021/jp406817k | J. Phys. Chem. C 2013, 117, 26013−26020

The Journal of Physical Chemistry C

Article

particle size is small enough) that hydroxyl formation is substantially enhanced by a nanostructured state of the oxide surface involving the occurrence of undercoordinated Ti−O pairs located on ridge edges (not present on ideal surfaces) and displaying a particularly strong chemical activity. We consider here a different strategy for enhancing the hydrophilicity of TiO2 implant surfaces, which is based on changing the surface chemistry rather than the surface topography. Indeed, surface chemistry and surface free energy influence hydrophilicity and how proteins adsorb to the implant. Recent works13−16 have shown that addition of calcium ions to the implant surface changes the surface free energy and creates active sites with positive charge which are favorable for binding extracellular proteins as well as phosphate ions. However, a microscopic description of the relevant processes is still lacking. Incidentally, it is noteworthy to mention the possibility of changing the wettability of a surface by application of an external electric field: when an electric field is applied between a solid and a liquid, the charges and dipoles redistribute, lowering the surface energy at the interface and inducing/increasing hydrophilicity. This phenomenon, called electrowetting,17,18 is however of marginal interest for the design of dental implants, where hydrophilicity is required not only during implant preparation but also during their storage and installation into bone. In this work, we use first-principles methods for studying deposition of calcium atoms on a paradigmatic clean and hydroxylated anatase (001) surface. We show that at low coverage, Ca adsorption increases strongly the wetting energy and therefore surface hydrophilicity, so that even when contaminated by methane (a simple representative of hydrocarbon contaminants) the surfaces remain hydrophilic. We also explain why this beneficial effect is substantially reduced by increasing the Ca coverage. These properties should be of great interest for optimization of hydrophilic implants.

Figure 1. Side view of the relaxed periodic slab modeling the clean TiO2 anatase (001) surface. Large red spheres represent O atoms, while small blue spheres represent Ti atoms. Large (small) gray spheres indicate O (Ti) atoms which are kept frozen at their equilibrium bulk positions.

DFT quantum mechanical program using localized numerical orbitals as basis sets. The scheme can be considered as a generalized LCAO-type approach and works for both finite size and periodic systems. The equilibrium geometry of all studied slabs with relevant adsorbates is obtained by performing a series of structural optimizations. These optimizations are performed using a standard energy minimization procedure, which can be characterized as a quasi-Newtonian minimization of energy as a function of atomic positions. We use our optimized theoretical bulk lattice parameters of anatase, i.e., a = 3.817 Å, c = 9.715 Å, and u = 0.2065, for describing the frozen, central portion of the slab. In order to incorporate solvation effects, which are necessary for calculating wetting energies, we rely on the conductor-like screening model (COSMO), which is a continuum solvation model where the solute is placed inside a cavity embedded into a perfectly conducting medium representing the solvent. In general, for a solvent with arbitrary dielectric constant ε, the charge distribution of the solute polarizes the solvent medium and a screening charge σ(s) is generated on the cavity surface (the solvent-accessible surface, abbreviated SAS)

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METHODS AND COMPUTATIONAL DETAILS All simulations reported in this work have been performed for the anatase (001) surface. Indeed, though anatase crystals are dominated by the thermodynamically more stable {101} facets, the (001) surface is of great interest because of its high reactivity, potentially appropriate for many applications. Furthermore, it has been shown recently19 that it is possible to reverse the natural relative surface stability and synthesize TiO2 anatase single crystals with a high percentage of {001} facets. In the following, we will simulate adsorption of H2O, CH4, and Ca atoms on the TiO2 (001) surface with a slab model. The TiO2 slab consists of three layers, and the atoms of the central layer are kept frozen at the equilibrium bulk positions. Figure 1 displays the slab used for modeling the clean TiO2 anatase (001) surface. Regarding Ca deposition, we consider uniform and periodical atoms distributions at thermodynamical equilibrium, in agreement with experimental results in the literature for Ca and alkali metals,20−23 which do not show any evidence of cluster formation. We are therefore not interested here in this latter possibility, which would require the use of huge unit cells. We note the sensitivity of the total energy to Ca equilibrium positions, even in the case of a rigid substrate. This effect is enhanced if substrate reconstruction is allowed, as is the case in this work. All computations have been done using the all-electron DMol3 code,24,25 with the Perdew−Burke−Ernzerhof (PBE) functional26 for evaluating exchange and correlation. DMol3 is a

σ(s) =

ε−1 ∂ (V 0 + Vσ ) ·n 4π ∂n

(1)

0

where V (r) is the electrostatic potential due to the nuclei and the electron density ρ(r) of the solute and includes also the polarization of the electron density induced by the presence of the solvent. The latter contribution is taken into account through the self-consistent evaluation of ρ(r) V 0(r) =

∑ i

zi − |r − R i|

∫V |rρ−(r′r)′| d3r′

(2)

where zi and Ri are the values and positions of the solute nuclear charges. Vσ(r) is the reaction potential generated by the polarization of the dielectric medium 26014


The Journal of Physical Chemistry C Vσ(r) =

∫SAS |rσ−(s)s| d2s

Article

Table 1. Energetics and Wetting Properties of a TiO2 Anatase (001) Surface with Relevant Adsorbatesa

(3)

Resolution of eqs 1−3 with respect to σ(s) requires therefore an iterative procedure. COSMO greatly simplifies this procedure: in this case, the surface charge of the screening conducting medium (ε = ∞) can be obtained noniteratively from the boundary condition that in a conductor the total potential on the cavity surface is zero V 0(s) +

∫SAS |sσ−(s′s)′| d2s′ = 0

adsorbate + coverage Θ substrate TiO2

(4)

provided that ρ(r) has no tail extending outside the SAS. Given the exponential behavior of the charge density tail, the above condition is generally not exactly fulfilled and the total potential on the conducting cavity surface should really be a nonvanishing constant. Finally, σ(s) in eq 4 is computed numerically using a discretization of the continuous surface charge distribution in the integral. The distribution of the screening charges contains information about the hydrophilicity of the systems under consideration. At the COSMO level, the total wetting (solvation) energy Ewet is the difference between the total energy (including dielectric energy) of the system (slab + adsorbate) in the cavity surrounded by the conducting medium and the total energy of the same system in vacuum (gas phase). tot It is expressed as Ewet = Etot COS − Evac. In the present work, the DMol3−COSMO method27−29 is applied with periodic boundary conditions. In order to relate hydrophilicity properties to wetting energies and contact angles,30 we make use of Young’s law31 γSV = γSL + γLV cos(θ)

EXtotn/slab,

Etot slab,

COSMO

Δγ (mN/ m)

θ (deg)

−1.793 −1.497 −2.995 −2.869 −2.544 −2.218 −0.589 −0.329 −0.180

−1.459 −1.172 −3.855 −3.122 −2.610 −2.155 −0.475 −0.230 −0.101

−85 −80 −80 −323 −226 −143 −20 −59 −46 −19

∼0 ∼0 ∼0 ∼0 ∼0 ∼0 ∼74 ∼36 ∼51 ∼75

Wetting energies (Δγ) and contact angles (θ) are given for the clean surface as well as for the surface with dissociated H2O molecules at 1/4 ML and 1/2 ML coverage, with Ca atoms at 1/4 ML, 1/2 ML, 3/4 ML, and 1 ML coverage and with methane CH4 molecules at 1/4 ML, 1/2 ML, and 1 ML. For all these cases, adsorption energies per molecule (Eads) in the gas phase and in the COSMO environment are also displayed.

The highest possible coverage for purely dissociated H2O molecules on the (001) surface is 1/2 ML, in agreement with ref 32, where authors show that at coverages larger than 1/2 ML a mixed state, involving both dissociated and molecularly adsorbed molecules, is present. From Table 1, we note the high adsorption energies for Ca at any coverage value. The observed trend in adsorption energies is consistent with theoretical results33 for Ca deposition on the rutile (110) surface. At 1/4 ML coverage, wetting energy is the highest (−323 mN/m) and decreases to −20 mN/m at full coverage, leading to a nonzero contact angle. Up to 3/4 ML coverage, the wetting energies are substantially larger than those calculated for both the clean and the hydroxylated TiO2 surfaces. All surfaces with adsorbed CH4 molecules have low wetting energies, corresponding to nonvanishing contact angles. The physical mechanisms which determine the above trends are now examined. We discuss first in detail the mechanisms which are responsible for the strongly enhanced hydrophilicity of a clean TiO2 surface when Ca atoms are adsorbed at low coverage (1/4 ML). This system is indeed simpler than the hydroxylated surface case and is therefore paradigmatic. As we will see later, the behavior of more complex surfaces, e.g., hydroxylated TiO2 surfaces, is very similar. In all cases, two concomitant mechanisms must be taken into account. The first mechanism is related to the ionic bonding of Ca atoms to the TiO2 slab, a situation also observed during adsorption of alkali elements.21−23 At low coverage, Ca adsorption is mostly ionic, as can be seen in Figure 2 for the 1/4 ML case, where partial densities of states (PDOS) are displayed for a Ca atom in the adsorbate layer and for a Ti atom in the central layer. Figure 2b shows that this ionic adsorption gives rise to the occupation of states of the upper d band of the Ti atoms. The insert allows comparison with the case of a clean TiO2 slab. In addition, it is observed that Ca 4s electrons are transferred not only to the outermost Ti cations but also to those which occupy deeper sites into the solid, reducing therefore the positive charge of the Ti ions. At higher Ca and in particular for the full monolayer case considered here, the degree of ionicity of the Ca−TiO2 bond decreases strongly and

(5)

(6)

Using the experimental value of the water surface tension, i.e., γLV = 72.8 mN/m, and Ewet as defined above we can calculate the contact angle θ. Finally, we remind that the adsorption energy per molecule of a given species X on the slab is defined as tot E Xadsn = (E Xtotn /slab − Eslab − nE Xtot)/n

1/4 ML 1/2 ML 1/4 ML 1/2 ML 3/4 ML 1 ML 1/4 ML 1/2 ML 1 ML

gas

a

where θ is the contact angle, γ is the surface tension (or surface free energy), and the subscripts SV, SL, and LV refer to the solid−vapor, solid−liquid, and liquid−vapor interfaces, respectively. A supplementary condition is that if γSV − γSL > γLV then cos(θ) = 1. Therefore, at the COSMO level, the wetting energy Δγ per planar surface area Ac is Δγ = γSL − γSV = Ewet /Ac = −γLV cos(θ )

clean H2Odis H2Odis Ca Ca Ca Ca CH4 CH4 CH4

Eads (eV/mol)

(7)

Etot X

where and are the total energies of the relaxed slab with adsorbate X, the relaxed clean slab, and of species X, respectively; n is the number of species X per cell.

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RESULTS AND DISCUSSION We start by studying the wetting properties of some adsorbates which are relevant to our work. Table 1 shows adsorption energies Eads both in the gas phase and within the COSMO framework, wetting energies Δγ, and contact angles θ for H2O molecules, Ca atoms, and CH4 molecules adsorbed on a clean anatase (001) surface at several coverage values Θ. Calculations have also been performed for n-pentane. The results are quite similar to those obtained for methane, and therefore, only the latter are reported here, given the particular simplicity of CH4. 26015



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of the Ca charge qat from +1.64e to +0.63e for 1/4 and 1 ML coverage, respectively. The corresponding values for the Hirshfeld analysis, which is known to systematically underestimate charge values, are +0.81e and +0.28e, respectively. The charge transfer reduction is due to the increase of Coulomb repulsion between ionic adsorbates: by increasing Ca coverage, the complete ionization process Ca + 2Ti4+ →Ca2+ + 2Ti3+ becomes energetically less favorable and Ca 4s electrons tend to remain bound to their atomic core. In Figure 4 we display the charge density σ of the Ca adlayers as a function of the coverage Θ, calculated as

Figure 2. Partial densities of states for a TiO2 slab with Ca atoms adsorbed at 1/4 ML coverage. (a) PDOS of a Ca atom in the surface layer; (b) PDOS of a Ti atom in the central layer. Insert displays the PDOS of the same Ti atom in the central layer but in the case of a clean slab. Blue, green, and gray correspond to s, p, and d character, respectively.

Figure 4. Charge density σ in the Ca adlayers as a function of coverage Θ. Green dots correspond to the calculated values, while the red curve is simply a trendline to guide the eyes.

σ(Θ) = Θqat̅ (Θ)

the transfer of Ca s electrons to Ti d states is greatly reduced. Figure 3 shows indeed the presence of a partially filled Ca 4s orbital (plus some 4p admixture), the charge transfer to Ti atoms in the central layer of the slab remaining essentially unchanged. Mulliken population analysis indicates a reduction

(8)

where qa̅ t(Θ) is the average value of the charge of nonequivalent Ca adatoms for a given coverage Θ. Irregularities of the calculated values of the charge density (green dots) are likely due to the distinct reconstructions of the underlying TiO2 surface for different values of Ca coverage. The red curve is a best fit to the first-principles values. The most important feature to notice is that while the ionization charge qat of the Ca adatoms decreases for increasing coverage the charge density σ of the Ca adlayers increases and reaches saturation at about full coverage, Θ = 1. The second mechanism is related to the electrostatic properties of the slabs and in particular to the space extension of the electric field that they produce in vacuum (gas-phase adsorption) or in the embedding medium (COSMO environment). It is well known that surface electric dipole layers consisting of uniformly charged layers do not produce any total electric field in the regions of space outside the layers. If the charge distribution is inhomogeneous, the electric field is nonvanishing close to the outer charged layers but decreases rapidly as a function of distance from them. Both the intensity and the range in vacuum of the Fourier components VG∥0(z) of the electrostatic potential increase for growing charge inhomogeneity of the charged layers.34 In the above quantity, {G∥} is the set of two-dimensional reciprocal−lattice vectors characterizing the translational symmetry of the surface. In fact, the following exponential asymptotic behavior is valid: VG∥0(z) → BG∥e−G∥z, where BG∥ is a constant. Let us apply these considerations to understand the electrostatic potential produced in space by the TiO2 slabs with adsorbed Ca atoms at different coverages. As shown in Figure 4, the charge density σ increases sublinearly for

Figure 3. Partial densities of states for a TiO2 slab with Ca atoms adsorbed at 1 ML coverage. (a) PDOS of a Ca atom in the surface layer; (b) PDOS of a Ti atom in the central layer. Same color code as in Figure 2 26016



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increasing coverage and saturates at high values of Θ. This charge distribution, however, becomes more and more homogeneous and therefore more alike the model system of uniformly charged layers. Combining the two effects, one obtains a reduction of both the total electric field strength and the extension outside the slab, which in turn results in a reduction of the conducting medium polarization within the COSMO framework. For the case of TiO2 slabs with 1/4 ML and 1 ML Ca coverage, the combination of the two mechanisms produces the results of Figure 5, which displays,

Figure 5. Contour plots of the potential energy (positive probe charge) in a plane parallel to the slab surface at a distance of 5 au above the Ca layer. TiO2 anatase (001) surface in the gas phase with Ca adsorbed at 1/4 ML (a) and 1 ML (b) coverage. Consecutive contours differ by 0.01 Ha.

Figure 6. Potential energies (positive probe charge) along vertical lines through Ca (red) and O (blue, green, purple) representative atoms on the surface of a TiO2 anatase (001) slab in the gas phase, with Ca adsorbed at 1/4 ML (a) and 1 ML (b) coverage. Long tic mark on the horizontal axes indicates z location of the outermost atoms of the slabs. Distances are measured from the slab center. Large (small) gray spheres indicate the other equivalent O (Ca) atoms, while small black spheres represent Ti atoms.

for both Ca coverages, contour plots of the electrostatic potential energy V0(r) (with positive probe charge) for the gasphase case in a horizontal plane located 5 au above the Ca layers. It is evident that potential inhomogeneity is higher for Θ = 1/4 ML than for Θ = 1 ML. Figure 6 shows, for the two above coverages, the electrostatic potential energy in the gas phase along vertical lines through the outermost Ca and O atoms of the slab. This figure displays the larger potential inhomogeneity for Θ = 1/4 ML, close to the slab surface, and also for the same coverage the slower decrease of potentials as a function of distance. The larger inhomogeneity observed for Θ = 1/4 ML is the fingerprint of the strong lateral inhomogeneity of the corresponding charge density, which in turn is responsible for the enhanced polarization of the embedding fluid medium. Incidentally, the slower decrease of the electric field as a function of distance is particularly manifest along the lines through the Ca atoms: in fact, the z component of the field is less than 10−5 Ha/e·Bohr at distances larger than 22.0 and 11.7 au from the surface for Θ = 1/4 ML and Θ = 1 ML, respectively. Note that at 5 au from the surface (the distance considered in Figure 5) the electric field above Ca atoms is on the order of 0.03 Ha/e·Bohr, corresponding to 0.15 × 106 V/cm. This high value, which is still strongly increasing when going closer to the surface, should be compared, e.g., with the maximum electric field Emax ≈ 0.23 × 105 V/cm before dielectric breakdown, as evaluated in the case of an electrowetting study35 of some insulating fluids. Within the COSMO framework, reduction of the electrostatic potential produces a decrease of the polarization charge σ(s) present on the SAS, as given by eq 4. Indeed, Figure 7a shows a strong negative polarization charge above the positively charged Ca adatom for the case of 1/4 ML coverage, while there is practically no polarization charge present at full Ca coverage (Figure 7b). As the wetting energy can be approximated (see, e.g., ref 36) by Ewet ≈ (1/2)∫ SASσ(s) V0(s)d2s these data explain the large decrease of Ewet reported

Figure 7. COSMO charges on the SAS for the TiO2 anatase (001) slab with adsorbed Ca atoms at 1/4 ML (a) and 1 ML (b) coverage. Large red spheres represent O atoms, while small blue and large green spheres represent Ti and Ca atoms, respectively. Same cell orientation as in Figure 1. Color code for COSMO charges: blue −2 e/nm2, red +2 e/nm2, and white 0.

in Table 1 from −323 to −20 mN/m and the related occurrence of hydrophobicity. In Table 2 we report our results for the adsorption of Ca and CH4 but on a TiO2 surface which has been previously hydroxylated (1/4 ML and 1/2 ML of dissociated H2O molecules). Trends in wetting energies and contact angles are 26017



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Table 2. Energetics and Wetting Properties of a Relaxed TiO2 Anatase (001) Surface with Preadsorbed (dissociated) H2O Molecules at 1/4 ML and 1/2 ML Coverage, Subsequent Adsorption of Ca Atoms at 1/4 ML, 1/2 ML, 3/4 ML, and 1 ML Coverage, and Methane Molecules at 1/4 ML, 1/2 ML, and 1 ML Coveragea adsorbate + coverage Θ substrate TiO2/H2Odis 1/4ML

TiO2/H2Odis 1/2ML

a

Eads (eV/mol) gas

clean Ca Ca Ca Ca CH4 CH4 CH4 clean Ca Ca Ca Ca CH4 CH4 CH4

COSMO

1/4 ML 1/2 ML 3/4 ML 1 ML 1/4 ML 1/2 ML 1 ML

−3.236 −2.489 −2.198 −1.900 −0.105 −0.069 −0.080

−3.710 −2.721 −2.200 −1.885 −0.006 0.002 −0.011

1/4 ML 1/2 ML 3/4 ML 1 ML 1/4 ML 1/2 ML 1 ML

−3.636 −2.152 −2.209 −1.685 −0.151 −0.077 −0.075

−3.800 −2.328 −2.187 −1.619 −0.025 0.002 −0.014

Δγ (mN/m)

θ (deg)

−80 −211 −210 −84 −68 −58 −52 −26 −80 −126 −179 −64 −12 −50 −47 −34

∼0 ∼0 ∼0 ∼0 ∼21 ∼37 ∼45 ∼69 ∼0 ∼0 ∼0 ∼28 ∼81 ∼46 ∼50 ∼62

Same entries and notations as in Table 1.

Table 3. Energetics and Wetting Properties of a Relaxed TiO2 Anatase (001) Surface with Preadsorbed Ca Atoms at 1/4 ML, 1/ 2 ML, and 1 ML Coverage, and Subsequent Adsorption of Methane Molecules at 1/4 ML, 1/2 ML, and 1 ML Coveragea adsorbate + coverage Θ

Eads (eV/mol) gas

TiO2/Ca1/4ML

TiO2/Ca1/2ML

TiO2/Ca1ML

a

clean CH4 CH4 CH4 clean CH4 CH4 CH4 clean CH4 CH4 CH4

COSMO

1/4 ML 1/2 ML 1 ML

−0.187 −0.186 −0.153

0.108 0.100 0.111

1/4 ML 1/2 ML 1 ML

−0.057 −0.060 −0.055

0.057 0.079 0.080

1/4 ML 1/2 ML 1 ML

−0.028 −0.031 −0.033

−0.111 −0.082 −0.060

Δγ (mN/m)

θ (deg)

−323 −247 −176 −54 −226 −201 −161 −99 −20 −48 −59 −72

∼0 ∼0 ∼0 ∼43 ∼0 ∼0 ∼0 ∼0 ∼74 ∼49 ∼36 ∼10


Table 4. Energetics and Wetting Properties of a Relaxed TiO2 Anatase (001) Surface with Preadsorbed (dissociated) H2O Mmolecules at 1/2 ML and Ca Atoms at 1/4 ML and 1/2 ML, and Subsequent Adsorption of Methane CH4 at 1/4 ML, 1/2 ML, and 1 ML Coveragea adsorbate + coverage Θ

Eads (eV/mol)

substrate TiO2/H2Odis 1/2ML/Ca1/4ML

TiO2/H2Odis 1/2ML/Ca1/2ML

a

gas clean CH4 CH4 CH4 clean CH4 CH4 CH4

COSMO

1/4 ML 1/2 ML 1 ML

−0.145 −0.133 −0.092

−0.081 −0.023 −0.040

1/4 ML 1/2 ML 1 ML

−0.310 −0.183 −0.081

−0.359 −0.204 0.044

Δγ (mN/m)

θ (deg)

−126 −114 −76 −90 −179 −198 −201 −63

∼0 ∼0 ∼0 ∼0 ∼0 ∼0 ∼0 ∼31


3/4 ML the TiO2/H2Odis 1/2 ML surface has nonzero contact angles. COSMO charges on the SAS for the hydroxylated TiO2 systems in the presence of Ca atoms are very similar to those

essentially the same as those for the clean TiO2 surface of Table 1, except that the wetting energy values decrease somewhat with the increase of H2O coverage, so that at a Ca coverage of 26018


The Journal of Physical Chemistry C shown in Figure 7 and are not displayed. We note that the discontinuities of the energy values in the tables, as a function of coverage, are due to strong modifications of the TiO2 surface topography (reconstructions, etc.) produced by adsorption of Ca and/or H2O. Preadsorption of Ca, either on the clean TiO2 surface or on the hydroxylated one, leads to a noticeable change with respect to subsequent adsorption of methane. As indicated by the results reported in Tables 3 and 4 and except the case of the full Ca monolayer preadsorption, surfaces remain essentially hydrophilic after subsequent methane adsorption. This behavior is due to the initially high wetting energies of the TiO2 systems with low Ca coverage, as explained earlier. Finally, we briefly considered the adsorption of a Na atom on the fully hydroxylated TiO2 surface and its exchange reaction with a Ca atom. Such a reaction might be indeed relevant to processes involving preadsorbed Na atoms on the surface, Ca ions, and subsequent apatite formation. Disregarding features related to the energy barrier between reactant and product, we are led to the following exothermic reaction

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REFERENCES

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(9)

which corresponds to ΔE = −1.091 eV. Within the COSMO framework, we have ΔE = −1.048 eV.

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CONCLUSIONS The influence of Ca adsorption on the wetting properties of the clean and hydroxylated anatase TiO2 (001) surfaces was investigated in the presence of methane contaminant. We found a strong enhancement at low Ca coverage of the wetting energy and therefore of the hydrophilicity. This is due to the nearly complete ionization of adsorbed Ca atoms, with charge transfer to Ti sites and to the long range of the electrostatic potential in vacuum, related to the strong lateral inhomogeneity of the charge density which is transferred in the ionic bond. These two features contribute to an increase of the polarization charge of the embedding fluid medium, enhancing therefore the surface wetting energy well beyond its value in the absence of Ca. Nevertheless, at full monolayer coverage, adsorption is considerably less ionic and the charge density inhomogeneity is strongly reduced. Effects of the two processes turn out therefore to be less relevant, leading to a lowering of the wetting energy and eventually to surface hydrophobicity. Considering the important role that wettability plays in the osseointegration process, the results presented here might be relevant to the design of new hydrophilic surfaces as well as to surface pretreatments of titanium-based implants for improved osseointegration. Finally, we insist that conclusions of this work are not limited to Ca adsorption but apply to all alkalis (in particular, to sodium) and alkaline earths as long as their electronegativity is not too high. Calcium, however, is a precursor of apatite and seems to be an excellent candidate.

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ACKNOWLEDGMENTS

This project was supported in part by a grant from Institut Straumann AG, Basel, Switzerland.

dis TiO2 /H 2O1/2ML /Na1/4ML + 2Ca(g) dis → TiO2 /H 2O1/2ML /Ca1/4ML + 2Na(g)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: michel.posternak@epfl.ch. Notes

The authors declare no competing financial interest. 26019



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Enhancing Hydrophilicity of Anatase TiO2 Surfaces by Deposition of

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