Fluorination of the Hydroxylated α-Al2O3 (0001) and Its Implications

Article pubs.acs.org/JPCC

Fluorination of the Hydroxylated α‑Al2O3 (0001) and Its Implications for Water Adsorption: A Theoretical Study Jonas Wirth,† Julia Schacht,‡,§ Peter Saalfrank,† and Beate Paulus*,‡ †

,Institute of Chemistry, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany Institute of Chemistry and Biochemistry, Freie Universität Berlin, Takustrasse 3, 14195 Berlin, Germany § Victoria University of Wellington, P.O. Box 600, 6140 Wellington, New Zealand ‡

S Supporting Information *

ABSTRACT: Fluorination of the hydroxylated α-Al2O3 (0001) surface is studied using periodic density functional theory calculations. On the basis of a hypothetical reaction substituting surface hydroxyl groups with fluorine atoms, we find surface fluorination to be strongly exergonic but kinetically hindered. Fluorinated surface areas turn out to be rather hydrophobic as compared to hydroxylated areas, suggesting fluorination as a potential route for tuning oxide surface properties such as hydrophilicity.

1. INTRODUCTION Metal oxides as major constituents of earth’s crust play an important role not only in the environment but also in many technical applications, ranging from their use as ceramics to catalysts.1−4 Therefore, a deep understanding of the surface chemistry of these compounds is highly desirable, especially when it comes to the versatile phenomena governed by oxide/ water interaction, such as crystal growth, weathering, and dissolution. In this context, a well-established model system that has been studied extensively during the last decades is corundum, α-Al2O3, and its paradigmatic clean (0001) surface.5−8 For a long time, however, precise information on microscopic structures and reactions in water thin films on this substrate, which pose the setting of any chemical reaction under environmental conditions, has been, and in some parts still is, surprisingly scarce.2,9 Only recently have some major obstacles regarding the preparation of a well-defined surface termination as well as the experimental characterization of surface structures been overcome by a combination of molecular beam source (MBS) dosing and vibrational sum frequency (VSF) spectroscopy, allowing for direct comparison of experimental findings with first-principles calculations for an idealized surface model and eventually leading to a consistent description of water dissociation on α-Al2O3 (0001).10 Starting from the aluminum terminated α-Al2O3 (0001) surface, which is the thermodynamically most stable one under ultrahigh vacuum (UHV) conditions,8 at higher water partial pressures hydroxylation takes place, giving rise to complicated structures and a surface chemistry rather different from the UHV situation.9 While the precise mechanism of transformation between the Al- and OH-terminated surface is yet unknown, recently both end members of this process were © XXXX American Chemical Society

studied in terms of characteristic surface phonons that may serve as spectroscopic probes for alumina surface chemistry in future experiments.11 While the OH-terminated α-Al2O3 (0001) surface is rather hydrophilic due to its ability to form hydrogen bonds with an adsorbed water thin film,12 a surface modification that leads to a more hydrophobic behavior would open interesting perspectives for future studies and applications. In this respect, surface fluorination has proven successful in various fields from electrochemistry to weatherproof coatings,13,14 making this a promising approach also for passivation of hydroxylated α-Al2O3 (0001). Here, we study the OH-termination of this paradigmatic surface modified via substitution of surface hydoxyl groups with different amounts of fluorine, introducing a surface chemistry rather different from the fully hydroxylated case. To understand the thermodynamics of this process, in the first part of this Article (section 3.1), after introducing the methodology used, we analyze the energetics of the, currently still hypothetical, substitution reaction according to eq 1 (with ★ denoting the surface of the substrate): ★(OH)n + nHF → ★Fn + nH 2O

(1)

Studying the kinetics of the reaction itself gives a first estimate of the reaction probability. In the second part, a comprehensive account on water adsorption on both fully and partly fluorinated surfaces (section 3.2) follows; a final section summarizes our major conclusions and proposes possible follow-up studies. Received: November 9, 2015 Revised: April 4, 2016

A

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2. MODELS AND METHODS 2.1. Computational Details. All calculations presented in this theoretical study were performed within the framework of Kohn−Sham density functional theory (DFT)15 as implemented in the Vienna ab initio Simulation Package (VASP)16−18 using the PBE exchange-correlation functional19 and a projector-augmented wave basis.18,20 A 400 eV kinetic energy cutoff was used for the plane-wave basis, and a Γ-point centered (3 × 3 × 1) Monkhorst−Pack grid was found to be sufficiently accurate for the evaluation of total energies (up to a few tens of meV) and vibrational frequencies. The criterion for self-consistent field convergence was set to a total energy difference of less than 10−4 eV between iterations. Ionic relaxation was stopped for forces acting on ions dropping below 0.01 eV/Å, both in structure optimizations and in the Nudged Elastic Band (NEB) steps for the transition state search. Surface structures are modeled as periodic slabs with vacuum layers of approximately 25 Å in the direction of the surface normal. Because our slab models exhibit a finite dipole component perpendicular to the surface (due to broken inversion symmetry), dipole corrections as implemented in VASP were applied to all calculations to improve total energy convergence with respect to the size of the vacuum gap. Dispersion interaction was accounted for in a semiempirical fashion using Grimme’s D2 correction scheme21 with a maximum distance of 20 Å for atom-pair interactions to avoid spurious forces between adjacent slabs. 2.2. Structures. The slab model used in this work largely follows our approach previously published in refs 10 and 22 where further details can be found. First, starting from the optimized α-Al2O3 bulk structure with lattice parameters a = b = 4.83 Å and c = 13.09 Å of the hexagonal unit cell (overestimating experimental values23 by some GGA-typical 1%), a slab consisting of six aluminum and three oxygen layers, that is, one-half of the bulk unit cell, was chosen to represent the Al terminated surface. In a next step, the fully hydroxylated surface was created by substitution of the uppermost Al layer with three hydrogen atoms per unit cell, according to the highly schematic equation: ★O3Al + 3H 2O → ★(OH)3 + “Al(OH)3 ”

Figure 1. Periodic slab model of the fully hydroxylated α-Al2O3 (0001) surface used in this work; aluminum, oxygen, and hydrogen atoms are indicated as gray, red, and white balls, respectively. (a) Unit cell (side view); atoms below the dashed line are kept fixed at their bulk positions during relaxation and vibrational analysis. (b) (2 × 2) supercell (top view), featuring 12 surface hydroxyl groups.

cumbersome and effects are likely to cancel out in free energy differences to a large extent. While not the focus of this work, the kinetics of the OH/F substitution reaction (eq 2) was also studied, for a “direct mechanism” and using transition state theory. A summary of the most important results on kinetics will be given at the end of section 3.1, while more details will be provided in the Supporting Information. For kinetics, we follow our procedure already established for the Al-terminated (0001) surface,22 using the NEB method25 as implemented in the modified VASP version of Jónsson and co-workers. This features an improved band tangent estimate26 as well as a climbing image (CI) scheme27 for finding the transition state along the Minimum Energy Path (MEP) of the reaction. The NEB procedure used in this study includes successive linear interpolation and optimization steps as described in ref 22, but ending up with 15 images (plus reactant and product image) along the path. The transition state was then located using the CI scheme and characterized by vibrational analysis yielding a single imaginary frequency and a vibrational mode resembling the molecular motion between neighboring images along the path. Reaction rates as a function of temperature were estimated using the Eyring transition state theory28 expression:

(2)

with ★ denoting the further substrate and “Al(OH)3” as a placeholder for the various Al species existing in aqueous solution (see ref 11 for a detailed account on α-Al2O3 (0001) surface hydroxylation). The unit cell of the resulting slab model is shown in Figure 1a. For optimization as well as for vibrational analysis, all atoms below the dashed line were kept fixed at their bulk positions. Here, we use the corresponding (2 × 2) supercell structure of Figure 1b as a basis for our study of surface fluorination from 1 to 12 out of 12 surface hydroxyls. 2.3. Thermodynamics. Reaction (free) energies were calculated as ΔE = E(product) − E(reactant) and ΔG = G(product) − G(reactant), respectively, with E denoting the sum of electronic energy and nuclear repulsion. Enthalpic, H(T), and entropic, S(T), finite-temperature contributions to the free energies, G(T) = E + H(T) − TS(T), were calculated using standard procedures as outlined, for example, in ref 24. For surface structures, only vibrational contributions were considered because translation and rotation of adsorbate species also appear as vibrations in this case. Surface phonons and molecular vibrations were determined as normal modes at the Γ-point only, because accounting for phonon dispersion is

k(T ) =

kBT −ΔG⧧(T )/(kBT ) e h

(3)

with Boltzmann constant kB, Planck constant h, and activation free energy ΔG⧧ = G⧧ − G(reactant) as the difference between the free energy of reactant and transition state.

3. RESULTS AND DISCUSSION 3.1. Surface Fluorination. In a first step, substitution of surface OH by fluorine according to eq 1 was studied in increments of 1/12 using the supercell of Figure 1b). Within this model, in principle, the rather large number of 12

K=

⎛ 12 ⎞ ⎟ = 4095 i ⎠

∑ ⎜⎝ i=1

(4)

fluorination patterns can be realized, whereas many of these turn out to be equivalent due to the rotational and translational symmetry of the substrate (see below). To get a rough idea of the variation in the substitution energy per fluorine atom, ΔES, as a function of the degree of fluorination, first a small selection of substitution patterns as B

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Figure 2. Selected substitution patterns (blue: fluorine atoms) for fluorination grades of 1/12 (a), 2/12 (b), 3/12 (c), 4/12 (d), 8/12 (e), and 12/12 (f), respectively; the corresponding substitution energies are given in Table 1.

given in Figure 2 was studied. For each of these structures, an optimization was carried out (as well as for the gas-phase molecules HF and H2O), followed by a vibrational analysis giving access to the corresponding free energies (cf., section 2.3). Please note that the relative orientation of the remaining, rather floppy, OH groups was not scanned because no strong energetic effect is expected from this degree of freedom.11 From the resulting values for ΔES and ΔGS given in Table 1, it Table 1. Substitution Energies ΔES and Free Energies ΔGS (T = 300 K, p = 1 bar) According to Equation 1 as a Function of Fluorination Grade Θa pattern

Θ

ΔES/eV

ΔGS/eV

a b c d e f

1/12 2/12 3/12 4/12 8/12 12/12

−0.72 −0.64 −0.54 −0.65 −0.51 −0.42

−0.77 −0.70 −0.60 −0.71 −0.57 −0.49

Figure 3. F/OH substitution energies per fluorine atom on the hydroxylated α-Al2O3 (0001) surface. Values are given as an average for all symmetrically inequivalent substitution patterns (numbers given in brackets) realizable in a (2 × 2) supercell model for a given fluorination grade Θ; the blue area illustrates the maximum range of values.

associated spread of values, now showing a clear picture of fluorination energetics: (1) The before-mentioned decrease of atomic substitution energies with increasing fluorination grade is clearly visible (and even monotonous) for the average values, suggesting unfavorable interaction between surface fluorine atoms. (2) The spread in substitution energies between different patterns for a given fluorination grade is larger at smaller to intermediate values of Θ, supporting the notion of unfavorable “interfluorine” interaction, which can be best maximized or avoided in this range. For example, the dip at Θ = 4/12 ≈ 0.33 corresponds to the perfect distribution of fluorine atoms inside the supercell shown in Figure 2d, and the maximum value for Θ = 3/12 = 0.25 corresponds to the “island-like” fluorination of Figure 2c. After illuminating the F/OH substitution reaction from the thermodynamical point of view, taking a look at the kinetics of the process is useful to gain the overall picture. Here, we only summarize results that are described in more detail in the Supporting Information. Starting from the fully hydroxylated surface, substitution of a single hydroxyl group with fluorine was studied by means of the NEB method. The overall process can be thought of as comprising four individual steps: (1) dissociation of the HF molecule from an adsorbed state, (2) incorporation of the fluorine atom/ion into the surface layer

Values are given per fluorine atom for selected patterns a−f in Figure 2. a

is clearly visible that substitution of F for OH is energetically favorable up to full fluorination of the surface with even larger magnitudes for substitution free energies. Also, a slight trend showing a decrease in the magnitude of substitution (free) energies with increasing fluorination grade seems to be identifiable. To be comprehensive on the latter point, however, a systematic study of different substitution patterns for each fluorination grade is necessary. Therefore, in a next step, all combinations according to eq 4 were scanned for symmetrically inequivalent patterns by numerically comparing all possible patterns with each other as well as with their rotated and translated forms, resulting in a greatly reduced batch size (see bracketed values in Figure 3). Altogether, 366 patterns were considered. For each unique pattern, again, a structure optimization was carried out without scanning the relative orientation of the remaining surface hydroxyls. The resulting average substitution energies per fluorine atom for a given fluorination grade are plotted in Figure 3, along with the C

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Figure 4. (a) Partly fluorinated α-Al2O3 (0001) slab model with quadrants A, B, C, and D representing fluorination grades of 0, 1/3, 2/3, and 1, respectively. (b) Potential energy surface for water adsorption, obtained by relaxation of the water molecule for given lateral fractional coordinates xa and xb of the water oxygen atom on a (10 × 10) grid.

the optimization to find the energetically most favorable adsorption geometry for a given combination of fractional coordinates xa and xb. The resulting PES is depicted in Figure 4b, showing the water adsorption energy, calculated according to

while pushing away the OH oxygen, (3) formation of the water molecule, and (4) relaxation of the water molecule into an adsorbed state (see Figure S1). The whole reaction is exothermic by approximately 0.16 eV, differing from the overall substitution energy given in Table 1 due to the fact that only adsorbed states of H2O and HF are considered here. With respect to potential future experiments, the major question is, of course, if fluorination of the hydroxylated surfaces in a HF atmosphere can be expected. In the Supporting Information, it is also found that the “direct” reaction path studied here has an activation free energy of ΔG⧧ = 1.65 eV at T = 300 K; however, the reaction rate according to eq 3 amounts to a mere 1.4 × 10−15 s−1, effectively prohibiting the whole process at moderate temperatures despite its pronounced thermodynamic gradient. One possible workaround might be simply heating the sample, for example, to 800 K, yielding a fluorination rate of some 3.2 × 103 s−1, but by doing so one would have to accept the risk of significantly changing the surface structure at the same time.11 However, more complicated substitution mechanisms, possibly involving multiple adsorbate molecules and/or surface defects giving rise to lower reaction barriers, cannot be ruled out at this point, which is why a definite answer can only be given by experiment. 3.2. Water Adsorption. In the following, the consequences of α-Al2O3 (0001) surface fluorination for water adsorption are discussed. In a first step, adsorption of a single water molecule is studied to assess the hydrophilicity/hydrophobicity of the surface as a function of fluorination grade. We then investigate collaborative effects by introducing a second water molecule, allowing for tentative predictions of water layer growth and stability. 3.2.1. Single-Molecule Adsorption. Because performing a comprehensive potential energy surface (PES) scan of water adsorption for many different fluorination grades and patterns (cf., section 3.1) would be prohibitively costly in terms of computation time, here, we use the slab model shown in Figure 4a, featuring four quadrants A−D with fluorination grades 0 (quadrant A), 1/3 (B), 2/3 (C), and 1 (D), respectively, as a basis. A relaxed PES scan for this model provides us with an overview of favorable/unfavorable adsorption sites in environments with different ratios of surface hydroxyls and fluorine atoms. The surface was scanned on a (10 × 10) grid of lateral fixed fractional coordinates xa and xb (each on the range [0,1]) for the water oxygen atom, relaxing all remaining degrees of freedom of both the water molecule and the surface. At each point, different initial water orientations, both with oxygen and with hydrogen atoms pointing toward the surface, were used for

Eads = E★H2O − (E★ + E H2O)

(5)

as a function of lateral coordinates xa and xb. The rather complex pattern exhibits strong variations between the different quadrants of the surface model, showing a clear sign of increasing hydrophobicity with increasing fluorination grade from quadrant A to D. The energetically most favorable adsorption sites are located in the vicinity of at least two (quadrant B) or three (quadrant A) surface hydroxyl groups allowing for the formation of hydrogen bonds, which can be seen for example in Figure 5a showing the structure of the

Figure 5. Energetic minima for water adsorption on a partly fluorinated α-Al2O3 (0001) surface: (a) global minimum, fully hydroxylated case (quadrant A, Figure 4a); and (b) local minimum, fully fluorinated case (quadrant D).

global adsorption minimum. As compared to this, adsorption in completely fluorinated quadrant D is destabilized by more than 0.3 eV, while quadrant C with only one surface OH left shows intermediate adsorption energies. The most favorable structure for adsorption in the fully fluorinated case is given in Figure 5b, also showing a hydrogen bond-like orientation of the water hydrogen toward surface fluorine atoms, which is, however, less pronounced than in the case of surface hydroxyl groups as can be seen from the corresponding adsorption energies. The adsorption energy of one water molecule on the clean αAl2O3 (0001) surface is reported to be −1.03 eV both with the PBE-functional and plane wave basis29 and with BLYP functional and Gaussian-type basis.30 As compared to our adsorption energies of one water molecule on hydroxylated, partly fluorinated, and fully fluorinated areas of the α-Al2O3 (0001) surface in the range from −0.65 to −0.3 eV, the D

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Figure 6. Potential energy surfaces for adsorption of a second water molecule, with the first one sitting (a) at the global adsorption minimum position (cf., Figure 5a) or (b) at the local adsorption minimum position (each marked with an “X”) on top of the fluorine atoms in quadrant D (cf., Figure 5b). PESs were obtained by relaxation of the water molecule (as well as surface degrees of freedom) for given lateral coordinates xa and xb of the water oxygen atom on a (10 × 10) grid, while also keeping xa and xb coordinates of the first water molecule fixed.

adsorption minimum. Thus, the overall picture remains that of water adsorbing preferrably via hydrogen bonding to surface hydroxyl groups in the first place, whereas fluorinated parts of the surface are comparatively hydrophobic.

adsorption is up to a factor of 3 weaker. On the other hand, if one would regard the adsorption of water on a fluorideterminated (0112) AlF3,31 also there the adsorption is stronger (between −1.06 and −1.60 eV depending on the coverage) due to accessible undercoordinated Al-sites. 3.2.2. Collaborative Effects. While fluorination is clearly reducing the hydrophilicity of the surface, the question remaining is how the growth of a water layer might proceed. Therefore, in a next step, in analogy to the previous subsection, further PES scans were performed for adsorption of a second water molecule on the above model surface already accommodating one. Lateral fractional coordinates of both water oxygen atoms were kept fixed during optimizations, and adsorption energies of the second water molecule are defined as Eads = E★(H2O)2 − (E★H2O + E H2O)

4. CONCLUSIONS AND OUTLOOK Starting from the fully hydroxylated α-Al2O3 (0001) surface, which is the most stable one under “environmental” conditions, we study surface fluorination via a hypothetical substitution reaction with hydrogen fluoride by means of periodic slab density functional theory calculations. From the thermodynamical point of view, we find the reaction to be strongly exergonic up to complete fluorination of the surface but with a decreasing free energy gain per fluorine atom. Closer inspection of numerous fluorination patterns reveals that this is due to fluorination of neighboring surface positions being less favorable as compared to more distributed configurations. Studying the fluorination reaction by means of the nudged elastic band method, we find the whole process to follow a complicated mechanism of concerted HF dissociation, Al− (OH) bond breaking, and H2O formation, giving rise to a single, rather high energetic barrier. Therefore, the whole process is found to be effectively prohibited at moderate temperatures; collaborative effects featuring additional water molecules and/or surface defects might, however, assist in the process. Water adsorption was studied using a partly fluorinated model surface, revealing clear signs of preferred adsorption via hydrogen bonding to surface hydroxyl groups and/or other water molecules; in comparison, fluorinated surface parts were found to be more hydrophobic. Moreover, a preference of water/surface over water/water interaction suggests that water layer growth occurs in a rather distributed manner. However, a more systematical study of higher water coverages will be necessary to be conclusive on this point. Overall, fluorination was found to be a potentially useful strategy for modifying α-Al2O3 (0001) surface properties such as its hydrophilicity, which might be even tunable via the fluorination grade. Experimental implementation now seems to be a promising next step. On the theory side, however, further studies should focus on water thin film structures as a function of fluorination grade and eventually take the next step toward predicting/understanding macroscopic properties, such as the wetting behavior, which could be assessed, for example, by means of force field-based molecular dynamics simulations.32

(6)

Two extreme cases were studied, given by the optimized adsorption structures of Figure 5a and b, to assess the balance between adsorbate/surface and adsorbate/adsorbate interaction potentially governing water layer growth and structures. In case of one water molecule occupying the global adsorbate minimum of Figure 5a, the resulting PES (Figure 6a) clearly shows a preference of water/surface over water/water interaction. This becomes evident from the adsorption minima in quadrants B and C and can be easily understood by the number of “unsaturated” surface OH groups available for hydrogen bonding, which is basically zero in quadrant A due to the resident first water molecule. This preference of water/ surface interaction suggests a rather statistical, diffuse water layer growth with water molecules first occupying surface OH groups, favorably “pockets” of three, before water/water interaction would take over. A slightly different situation is visible in Figure 6b for the (less favorable) case of a first water molecule sitting on top of a fully fluorinated part of the surface (cf., Figure 5b). Here, instead of preferrably binding to unsaturated surface OH groups, adsorption energies of more than 0.8 eV are found in the vicinity of the first water molecule, suggesting an increased influence of water/water interaction. In fact, corresponding structures (not shown) exhibit hydrogen bonds between both water molecules, also allowing for stabilization of the first water molecule and thereby reducing the “penalty” for adsorption on top of fluorine atoms. However, the cumulative adsorption energies for both water molecules in this and similar configurations were found to be in the same range as in the above case with one water molecule occupying the global E

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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.5b10975. Kinetics of the substitution reaction of a hydroxyl group with a fluorine (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft for supporting this project through Collaborative Research Center 1109: Understanding of Metal Oxide/Water Systems at the Molecular Scale: Structural Evolution, Interfaces and Dissolution (projects B01 and C03). Additionally, we thank the HLRN (NorthGerman Supercomputing Alliance) as well as the ZEDAT at the Free University Berlin for computational time. J.S. also wants to acknowledge the help of Dr. Krista Steenbergen with the use of VASP.

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REFERENCES

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