Letter pubs.acs.org/JPCL
Catechol and HCl Adsorption on TiO2(110) in Vacuum and at the Water−TiO2 Interface Henrik H. Kristoffersen, Joan-Emma Shea, and Horia Metiu* Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106-9510, United States S Supporting Information *
ABSTRACT: Coadsorbed water is often unavoidable in electrochemistry and low-temperature catalysis. In addition, water influences the adsorption of biomolecules on surfaces. We use ab initio DFT molecular dynamics and ground-state calculations to study the adsorption of HCl and catechol on the rutile TiO2(110) surface and at a water−rutile interface. We find that a coadsorbed water film reduces the adsorption energy of both catechol and HCl significantly because water molecules must be displaced from the surface before catechol or HCl can adsorb. The adsorption energy of catechol (or HCl) at the water−rutile interface can be estimated as the adsorption energy in vacuum minus the energy to remove two water molecules (respectively, one water molecule) from the rutile surface in vacuum and place them in liquid water. This estimate predicts the effect of a surface water film on adsorption without the need of molecular dynamics.
R
energies of the systems studied here. In particular, we have compared the energies of a water film on TiO2(110) plus gasphase catechol, a water film on TiO2(110) with solvated catechol, and a water film on TiO2(110) with catechol adsorbed on the solid surface. The same type of studies have been performed for HCl. The main finding is that the adsorption energies of both catechol and HCl are significantly reduced, when the water film is present. This is due to the energy cost of displacing chemisorbed water molecules from the surface to allow catechol or HCl to adsorb. We find that the adsorption energy of catechol (or HCl) at the water−rutile interface is well approximated by the adsorption energy of catechol (or HCl) on rutile in vacuum minus the energy to move two water molecules (respectively, one water molecule) from the rutile surface in vacuum and place them in liquid water. The rutile TiO2(110) surface is modeled by a c(4 × 2) surface cell with four distinct bridging O atoms (Obr) and four distinct 5-fold coordinated Ti trough atoms (5c-Ti). The surface slab consists of four stoichiometric TiO2 layers. A single atom on the backside of the slab is fixed to ensure that the slab stays in the same place in the molecular dynamics simulations. With the added water film, a vacuum region of at least 12 Å exists between the top of the water film and the bottom of the TiO2 slab. Ground-state density functional theory (DFT) and ab initio molecular dynamics (MD) calculations are done with the VASP program.16−19 Exchange-correlation effects are
utile TiO2 is an important material with a wide range of applications from catalysis to biomedical implants. The adsorption of molecules on rutile TiO2 surfaces has been extensively studied, primarily at low temperatures and under ultra-high vacuum conditions.1−5 Adsorption of molecules at water−rutile interfaces has been less studied, even though water films form on TiO2 surfaces even at low humidities.6 Molecular species adsorbed at water−rutile interfaces are difficult to probe experimentally because the water film blocks signals from the surface. Computational studies provide a means of studying adsorption without the complications that arise in experiments. Ground-state density functional theory calculations have found that a single water layer reduces the adsorption strength of HCOOH and HCOONa on anatase TiO2(101).7 Ab initio molecular dynamics simulations have been used to study H+ and OH− adsorption from water,8,9 the solvation of a hole situated at the water−TiO2 interface,10,11 and solvent effects on the light absorption spectrum of an adsorbed dye molecule;12 however, we are not aware of any study that investigated the connection between adsorption of molecules at the water− TiO2 interface and adsorption of the same molecules in vacuum. We report on the adsorption of HCl and catechol on rutile TiO2(110) in contact with water and in vacuum. We chose HCl as a representative of an acid electrolyte. Catechol was chosen because it is used in photochemical studies4 and because it is an important moiety in protein surface adhesion.13 TiO2 is of interest for catalysis and electrochemical applications,1,14 and for protein adhesion to the oxide layer of titanium medical implants.15 We use ab initio, Nosé−Hoover, constant temperature molecular dynamics, which allows us to compute internal © XXXX American Chemical Society
Received: May 8, 2015 Accepted: June 2, 2015
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⟨ΔE⟩t is calculated as the difference in internal energy of the H2O(l)/TiO2(110) system with an adsorbed or solvated molecule (⟨E mol+system ⟩ t) minus the internal energy of H2O(l)/TiO2(110) (⟨Esystem⟩t) minus the internal energy of the molecule in gas phase (⟨Emol⟩t). 3/2kBT is added to the internal energy of the gas-phase molecule because the centerof-mass motion is not included in the MD simulation. Before considering the influence of a water film, we calculate the vacuum adsorption energies for water, catechol, and HCl (Figure 1). The O atom of water binds to a 5c-Ti of the surface (Figure 1a).26 The energy of the dissociative adsorption, where a H atom binds to the neighboring Obr and OH binds to the 5cTi, is close to the energy of the molecular configuration (−1.11 and −1.08 eV, respectively). The catechol molecule adsorbs in a tilted configuration with both hydroxyl groups bonded to 5cTi.27,28 We find that catechol binds most strongly (−2.25 eV) when both hydroxyl groups are dissociated and the H atoms are transferred to Obr (Figure 1b). The single-dissociated and nondissociated catechol bind less strongly with adsorption energies of −2.19 and −2.04 eV, respectively. Catechol adsorption calculated with the optB88-vdW functional is found to be −1 eV more stable than for calculations that use the PBE functional.28 Inclusion of van der Waals interactions stabilizes adsorbed catechol on anatase TiO2(101)29 and on SiO2(111),30 by a comparable amount. Finally, HCl adsorbs dissociatively (−1.36 eV) with Cl on a 5c-Ti and H on the neighboring Obr (Figure 1c). A water film containing 35 water molecules is added to the TiO2(110) computational cell to form H2O(l)/TiO2(110) (Figure 2a). Placing one molecule in this amount of water results in a molar concentration of 1.6 mol/L. The H2O(l)/ TiO2(110) system has been simulated by two distinct MD
approximated by the optB88-vdW functional20,21 to include van der Waals interactions. The optB88-vdW functional gives accurate structural properties of bulk liquid water22 and is important for water−catechol interactions. Indeed, MD simulations with PBE could not describe solvated catechol. Instead, a void was formed in the water film around the catechol molecule, and placing it in water increased the internal energy. This is contrary to the fact that catechol is very soluble in water. A DFT+U correction of 3.5 eV is applied to Ti d states. This U value is a compromise between the values recommended in the literature.23,24 A plane-wave basis with 350 eV energy cutoff is employed. The MD calculations are performed at the Γ-point to reduce the computational cost. For ground-state DFT calculations, we have used both Γ-point and 2 × 2 k-points. These results are shown in Figure 1. The use of Γ-point leads to
Figure 1. Ground-state adsorption energies with Γ-point (2 × 2 kpoints) for (a) dissociated H2O, (b) double-dissociated catechol, and (c) dissociated HCl. These are the configurations found to be most stable. Black diamond in panel a marks the surface cell used throughout this study.
an error in adsorption energies of ≤0.06 eV. We did not use spin-polarized DFT because the molecules considered here have closed-shell electronic structure. Spin-polarized test calculations support this choice by giving the same energies and electronic structures. MD simulations are based on DFT energies and forces. A Nosé thermostat25 keeps the temperature around 350 K and the time propagation uses 1 fs time steps. The internal energy of a system is calculated as the timeaverage kinetic plus potential energy (eq 1). ⟨E⟩t =
1 t − t0
∫t
t
E(t ′) dt ′ 0
(1)
We discard the results obtained in the first few picoseconds (t0) to allow the system to equilibrate with the thermostat. The internal energy is obtained by using t − t0 = 14 ps. In this study, adsorption energies of catechol, HCl, or water to TiO2(110) in vacuum are calculated from ground-state DFT energies. The adsorption energy of a molecule is defined as the energy of an adsorbed molecule on the surface, minus the energy of the clean surface, minus the energy of the molecule in the gas phase. In the presence of a water film, adsorption and solvation energies are obtained from MD simulations by calculating the change in internal energy (⟨ΔE⟩t) (eq 2).
Figure 2. (a) Picture of the H2O(l)/TiO2(110) system. Atoms from two unit cells are shown. (b) Water density as a function of distance to the 5c-Ti in the surface. (c) Accumulated average internal energy as a function of simulation time. Both runs for the H2O(l)/TiO2(110) system are shown in panels b and c.
⟨ΔE⟩t = ⟨Emol + system⟩t − ⟨Esystem⟩t − (⟨Emol⟩t + 3/2kBT) (2) 2278
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move the water molecules from gas phase to liquid (2 × −0.4 eV). We use the experimental water condensation enthalpy at 100 °C, which is −0.4 eV per molecule [NIST web page: kinetics.nist.gov/janaf/]. This gives an estimated reduction in adsorption energy of +1.4 eV, which is very close to the values obtained with the MD simulations (+1.4 and +1.6 eV). The catechol molecule is moved away from the surface and into the water film to calculate the solvation energy. Catechol has a pKa of 9.12 at 30 °C32 and is therefore not dissociated when dissolved in neutral water. We start the MD simulation with catechol in a nondissociated configuration and observe no dissociation during the 14 ps run. The internal energy of solvation is found to be −0.6 eV compared with gas-phase catechol (Figure 3c). This seems reasonable, considering the high solubility of catechol in water (31.2 g catechol per 100 g water at 20 °C32). The high solubility of catechol in water combined with the reduction in adsorption energy, due to the presence of the water film, means that the energy difference between adsorbed and solvated catechol is almost zero (∼−0.1 eV). The adsorption of HCl on the TiO2(110) surface covered by a water film was studied by placing the H atom on a surface Obr and the Cl atom on a 5c-Ti site (Figure 4a). This displaces one
simulations starting from different initial positions and velocities. Throughout both simulations, four water molecules bind to the four 5c-Ti in the surface. In the 14 ps timespan of the MD simulations, these water molecules do not dissociate and are not exchanged by other water molecules. Figure 2b shows the water density as a function of distance to the surface, for both MD simulations. The water molecules bound to 5c-Ti produce a peak at 2.2 Å. In addition, there is a clear second layer water peak (4 Å), suggesting that these molecules are stabilized by the surface or by the water molecules in the first layer. Michaelides and coworkers31 found a similar water structure, but the second peak in that work is less pronounced. This difference may be due to our use of a van der Waals functional rather than PBE. Ideally, the two MD simulations of the H2O(l)/TiO2(110) system should result in the same time-averaged internal energy (⟨Esystem⟩t). However, we find that they differ by 0.2 eV (Figure 2c). This difference gives a rough estimate of the statistical errors in our MD simulations. Because we need the energy of H2O(l)/TiO2(110) as a reference, we use the average of the two values. A catechol molecule is inserted into the H2O(l)/TiO2(110) system in a bidentate configuration, which displaces two water molecules from the 5c-Ti in the surface and into the water film. The double-dissociated (Figure 3a), single-dissociated (Figure
Figure 3. Final configuration in the MD sampling and adsorption/ solvation energy for (a) adsorbed and double-dissociated catechol, (b) adsorbed and single-dissociated catechol, and (c) solvated catechol. Atoms from two unit cells are shown.
3b), and nondissociated catechol configuration have been examined. During the MD simulation of the nondissociated adsorbed catechol, one of the hydroxyl groups dissociates. This indicates that the nondissociated configuration is not stable even for a short time. The double-dissociated catechol binds with −0.6 eV (Figure 3a), while the single-dissociated catechol binds with −0.8 eV (Figure 3b). When we compare with the vacuum calculations we find that the presence of the water film reduces the double-dissociated catechol adsorption energy by +1.6 eV and the single-dissociated catechol adsorption energy by +1.4 eV. The preferred binding configuration changes from double-dissociated in vacuum to single-dissociated in liquid water. As previously mentioned, the reduction in the catechol adsorption energy due to the water film can be estimated without the need for computationally expensive MD simulations. When catechol is adsorbed, two water molecules are displaced from the surface and added to the liquid water film. The simplest estimate for the associated energy cost is two times the vacuum water desorption energy to move the water molecules from the surface to the gas phase (2 × +1.1 eV) (Figure 1a), plus two times the water condensation energy to
Figure 4. Final configuration in the MD sampling and adsorption/ solvation energy for (a) adsorbed HCl, (b) partly solvated HCl, (c) solvated HCl, and (d) solvated HCl, where water forms an ordered structure. Atoms from two unit cells are shown. (e) Density profile of panel d as a function of distance to the 5c-Ti in the surface.
water molecule from the surface and into the liquid water film. The internal energy of this system was sampled for 25 ps and compared with the internal energy of the H2O(l)/TiO2(110), plus the internal energy of HCl in the gas phase (eq 2). The adsorption energy is found to be −0.6 eV. The water film has therefore reduced the adsorption energy by 0.8 eV compared with adsorption in vacuum. Again, the reduction in adsorption energy is well-approximated by the energy cost of desorbing one water molecule from the surface in vacuum (+1.1 eV) and condensing the water molecule into liquid water (−0.4 eV), 2279
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The Journal of Physical Chemistry Letters which gives 0.7 eV. The water film increases the Bader charge of the adsorbed Cl atom, which is −0.68 ± 0.02e when the water film is present and −0.57e in vacuum (Table 1). The negative charge is donated from the adsorbed H atom, which has a Bader charge of +0.67 ± 0.02e when the water film is present and +0.66e in vacuum.
and increase the free energy of the system. To further probe the stability of the ordered water film, we continued the MD simulation after removing HCl. After 5 ps, the ordered structure disappeared. The ordered water film is therefore not stable by itself, but it is stabilized by solvated HCl. Because adsorbed HCl is found to be less stable than solvated and partly solvated HCl, only a small fraction of Cl− ions are likely to displace water from the 5c-Ti and adsorb directly on the surface; however, the energy differences between these possibilities are within the errors of the MD simulations. To summarize, we have studied catechol and HCl adsorption on the TiO2(110) surface to quantify the influence of a liquid water film on adsorption energies. The presence of water reduces the adsorption energy of both molecules, because chemisorbed water must be displaced from the surface before catechol or HCl can adsorb. Our calculations show that one can estimate the adsorption energy of gas-phase catechol and HCl adsorbing on the water−rutile interface from the adsorption energies in vacuum and the experimental water condensation energy. If such an approximation is general, it makes it possible to estimate the adsorption energy at a liquid−solid interface by calculating the adsorption energies of the molecule and of the solvent in vacuum and using the experimental solvent condensation energy.
Table 1. Chlorine Bader Charge for Four Systems Containing HCla adsorbed, vacuum (Figure 1c)
adsorbed (Figure 4a)
partly solvated (Figure 4b)
solvated (Figure 4c)
−0.57e
−0.68 ± 0.02e
−0.73 ± 0.02e
−0.74 ± 0.01e
a
Each MD trajectory is represented by 15 configurations separated by 1 ps and the Bader charge is given with ± one standard deviation.
Given the energy cost of displacing water from the rutile surface, we have considered a partly solvated configuration, where the proton remains bound to Obr, while the Cl− ion is in the water film (Figure 4b). In this simulation, water is not displaced from the surface. The Cl− ion is initially placed close to the proton to keep the electrostatic cost small. This leads to an energy stabilization of −0.7 eV compared with gas-phase HCl. In the 14 ps simulation, the surface-bound proton diffuses from one Obr atom to the next. The diffusion happens by proton exchange with the water film, so the surface bound proton is not the same in the initial and final parts of the MD simulation (Movie in Supporting Information). Having the Cl− ion in the water film increases the amount of negative charge on Cl to −0.73 ± 0.02e compared with −0.68 ± 0.02e when Cl was at the water-covered surface (Table 1). The water film is therefore better at stabilizing the charge on the Cl− ion than the water-covered TiO2 surface. The solvation of HCl is considered by moving both H and Cl into the water film (Figure 4c). HCl remains dissociated into Cl− and H3O+, and the solvation energy is −0.7 eV (compared with gas phase HCl). This is in good agreement with the experimental HCl enthalpy of solution at low concentration and 25 °C, which is −2.0 kJ/g or −0.8 eV per molecule.33 The Bader charge on Cl in water (−0.74 ± 0.01e) is very similar to the previous case with Cl in water but H on the rutile surface (−0.73 ± 0.02e, Table 1). The proton moves around in the water film by proton exchange with the water molecules;34 the movement of the Cl− ion is more limited (Movie in Supporting Information). We performed an additional MD simulation of solvated HCl that resulted in a more stable HCl solvation energy (−1.6 eV) (Figure 4d). The associated water density profile is plotted in Figure 4e, and it shows that the water film is more ordered. A third layer peak (6 Å) and a fourth layer peak (9 Å) are visible in the plot of water density as a function of distance to the surface. This is significantly different from the density profiles of H2O(l)/TiO2(110) (Figure 2b) and the other MD simulations performed in this study (Supporting Information). Cl−−water pair correlation functions further show that the ordered structure allows for more pronounced second and third solvation shells around the Cl− ion (Supporting Information). The entropy cost of forming a more ordered water film is not readily accessible from constant temperature MD simulations, even though they sample a canonical ensemble. It is therefore unclear how much the two solvated HCl simulations differ in free energy because increased order will decrease the entropy
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ASSOCIATED CONTENT
S Supporting Information *
Movies showing the molecular dynamics simulations of adsorbed/solvated catechol and HCl. Plots of (i) density as a function of distance to the 5c-Ti in the surface for adsorbed/ solvated catechol and HCl, (ii) convergence of adsorption/ solvation energies as a function of time, and (iii) Cl−−water pair correlation functions for both simulations of solvated HCl. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b00958.
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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 Financial support was provided by the Department of Energy, Office of Science, Office of Basic Energy Sciences DE-FG0389ER14048, the Air Force Office of Scientific Research FA9550-12-1-0147, and the National Science Foundation (NSF) Grant MCB-1158577. We acknowledge support from the Center for Scientific Computing at the California NanoSystems Institute and the UCSB Materials Research Laboratory (an NSF MRSEC, DMR-1121053) funded in part by NSF CNS-0960316 and Hewlett-Packard. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract DE-AC02-06CH11357.
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