Water on Titanium Dioxide Surface: A Revisiting by Reactive

Nov 25, 2014 - The behavior of surface water, especially the adsorption and dissociation characteristics, is a key to understanding and promoting ...
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Water on Titanium Dioxide Surface: A Revisiting by Reactive Molecular Dynamics Simulations Liangliang Huang,*,† Keith E. Gubbins,‡ Licheng Li,§ and Xiaohua Lu∥ †

School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States § College of Chemical Engineering, Nanjing Forest University, Nanjing, Jiangsu 210037, China ∥ State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing, Jiangsu 210009, China

Langmuir 2014.30:14832-14840. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/31/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: The behavior of surface water, especially the adsorption and dissociation characteristics, is a key to understanding and promoting photocatalytic and biomedical applications of titanium dioxide materials. Using molecular dynamics simulations with the ReaxFF force field, we study the interactions between water and five different TiO2 surfaces that are of interest to both experiments and theoretical calculations. The results show that TiO2 surfaces demonstrate different reactivities for water dissociation [rutile (011) > TiO2−B (100) > anatase (001) > rutile (110)], and there is no water dissociation observed on the TiO2−B (001) surface. The simulations also reveal that the water dissociation and the TiO2 surface chemistry change, and the new surface Ti−OH and O− H functional groups affect the orientation of other near-surface water molecules. On the reactive surface, such as the rutile (110) surface, water dissociated and formed new Ti−OH and O−H bonds on the surface. Those functional groups enhanced the hydrogen bond networking with the near-surface water molecules and their configurations. On the nonreactive TiO2−B (001) surface where no molecular or dissociative water adsorption is observed, near-surface water can also form hydrogen bonds with surface oxygen atoms of TiO2, but their distance to the surface is longer than that on the rutile (011) surface.

I. INTRODUCTION

have different surface structures, surface free energies, surface reactivities, and other physical and chemical properties.9−11 Understanding the structure−property relationship and water behavior on different TiO2 surfaces is therefore highly desirable for many aqueous TiO2 applications.12 The H2O/TiO2 systems have been studied theoretically in the past two decades, from ab initio quantum mechanics calculations13 to force-field-based molecular dynamics simulations.14 Sun and coworkers reviewed the theoretical studies of H2O interactions on rutile and anatase surfaces.13 For the rutile (110) surface, the behavior of water depends on the water coverage: molecular adsorption at a higher coverage (e.g., 1 ML) and dissociative or molecular adsorption at a lower coverage (e.g., 1/8 ML).15For the rutile (100) surface, both molecular and dissociative adsorptions are possible.10 The rutile

1

Since the report of Fujishima and Honda in 1972, titanium dioxide (TiO2) has become one of the most widely studied semiconductors and has found applications in photocatalysis to degrade organic pollutants,2 in solar cells to produce hydrogen energy,3 in lithium ion batteries4 to improve the energy density and recharge rate, and in biomaterials and implants to enhance the biocompatibility.5 Nearly all of those applications involve an aqueous condition and thus require a fundamental understanding of the interactions between water and titanium dioxide surfaces. TiO2 crystallizers mainly as rutile, anatase, and brookite in nature, but other forms have been also reported through thermal or oxidation treatment of titanium-containing salts. For example, a monoclinic crystal known as TiO2−B is synthesized by the thermal hydrolysis of potassium tetratitanate (K2TiO4).6 The oxidations of potassium titanate (K0.25TiO2) and lithium titanate (Li0.5TiO2) produce new tetragonal and orthorhombic structures, known as TiO2−H7 and TiO2−R,8 respectively. Different TiO2 crystal morphologies and facets © 2014 American Chemical Society

Received: September 18, 2014 Revised: November 14, 2014 Published: November 25, 2014 14832

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studied TiO2 surfaces, the order of surface reactivity for the H2O/TiO2 interaction, and how water dissociation and the resulting TiO2 surface chemistry change affect the behavior of near-surface water. We then summarize our conclusions and discuss some future directions on this topic.

(011) surface can reconstruct the structure and accommodate molecular adsorption on the defect-free surface and dissociative adsorption on surface oxygen vacancies.16 The rutile (001) surface is very reactive because of the exposure of unsaturated Ti and O sites and thus favors the dissociative adsorption of water. As for anatase surfaces, molecular adsorption is generally favored on the anatase (101) surface, and dissociative adsorption is more stable on anatase (001) and (100) surfaces. However, it is worth noting that the anatase (001) surface exhibits a structural reconstruction under UHV conditions, thus not favoring water dissociation on its surface,17 and that a mixed molecular and dissociative adsorptions of water has been reported on the anatase (100) surface at a higher water coverage.18 Despite the understanding of water on TiO2 surfaces in the dilute limit, the following fundamental concerns remain unclear: (a) Do the TiO2 structure and surface properties depend on the water coverage? What happens if the TiO2 surface is in contact with a larger amount of bulk water? (b) Water molecules are known to dissociate or molecularly adsorb on TiO2 surfaces. How do water dissociation and the resulting TiO2 surface property changes affect the behavior of bulk water on TiO2 surfaces? State-of-the-art ab initio quantum mechanics methods, which calculate electron behavior and describe chemisorptions and reactions, generally have access to spatial and temporal scales on the order of tens of atoms and picoseconds, respectively. Classical molecular dynamics (MD) simulations are able to describe systems with hundreds of thousands of atoms. However, classical MD simulations use empirical force fields to describe atomic interactions and thus are not capable of describing chemisorptions or reactions where the behavior of electrons is important. ReaxFF is a firstprinciples-based bond-order-dependent reactive force field that provides an accurate description of bond breaking and bond forming.19 Being developed by fitting ab initio quantum mechanics results, ReaxFF is suitable for describing a reactive process by allowing bond formation and bond breaking under realistic conditions. It has been used to investigate the energetics for various reaction intermediates as well as for reactants and products.20 Within the large-scale atomic/ molecular massively parallel simulator (LAMMPS) package, MD calculations with a ReaxFF reactive force field are capable of simulating systems larger than 106 atoms on nanosecond time scales.21,22 In this article, we use a recently developed ReaxFF reactive force field23 and perform reactive molecular dynamics (RxMD) simulations to study the behavior of water on five different TiO2 surfaces, namely, rutile (110), rutile (011), anatase (001), TiO2−B (100), and TiO2−B (001). We choose those surfaces because the property of water has been reported to depend on either the surface water coverage (rutile (110) surface) or the surface structural reconstruction (rutile (011) and anatase (001) surfaces). The choice of the two TiO2−B surfaces is from recent efforts in applying the TiO2−B surface reactivity to NH3 dissociation24 and hydrodesulfurization25 applications. TiO2−B is considered to be a promising candidate in lithium ion battery research,26,27 where a fundamental understanding of the H2O/ TiO2−B interaction is also critical. The article is organized as follows. The Methods section briefly introduces the ReaxFF reactive force field applied to the TiO2/H2O systems and the RxMD simulation setup. We proceed to discuss in the Results and Discussion section the molecular and dissociation adsorptions of water on the five

II. METHODS A. ReaxFF Reactive Force Field for TiO2/H2O Systems. Originally developed by van Duin et al. in 2001,19 the ReaxFF reactive force field is one of the most widely used bond-orderdependent reactive force fields that enable a simultaneous description of bond breaking and bond forming for a variety of elements and many complex systems. ReaxFF uses a bondorder/bond-energy relationship and updates the bond-order information directly from instantaneous interatomic distances along the simulation trajectory. The structure, the connectivity (e.g., bonds, angles, and torsions), and the interaction energies depend on the instantaneous bond-order information. ReaxFF calculates both nonbonded van der Waals interactions and Coulomb interactions using the charge equilibration (QEq) method to calculate the charge and atomic polarization properties. Detailed descriptions of the ReaxFF force field method are provided in references by van Duin et al.19 and Chenoweth et al.28 Recent advances in the ReaxFF force field development has been reviewed.20 The implementation of the ReaxFF method in simulation software packages was discussed in the supplemental material of our previous paper.29 Kim and coworkers developed the ReaxFF reactive force field to calculate chemical reactions for the Ti/O/H systems.23 The force field parameters were fitted to a set of quantum mechanical calculations of TiO2/H2O systems and thus were able to predict the structures, energies, and equation of state for bulk TiO2 materials and calculate the reaction intermediates and reaction products for other Ti/O/H systems. The ReaxFF reactive force field was later extended by Monti and coworkers to Ti/O/N/H systems to study glycine adsorption on the rutile (110) surface. Both gas-phase glycine and glycine/water solution on rutile (110) have been reported.30 Kim and coworkers applied the Ti/O/N/H ReaxFF reactive force field to study the interactions between TiO2 nanoparticles and a mixture of water, methanol, formic acid, and Na+ and Cl− ions.31 The force field was also applied by Raju and coworkers in the study of water adsorption and dissociation on anatase (101), (100), (112), and (001) and rutile (110) at 300.0 K.14 They studied different water coverage on TiO2 surfaces and concluded that water has an ordered structure close to that of the TiO2 surfaces. The aggregation of anatase nanocrystals in vacuum and humid environments was also reported by RxMD simulations with this ReaxFF reactive force field.32 The hydrogen bond network between water and anatase was found to assist the nanocrystal aggregation under humidity. The ReaxFF reactive force field in this work23 has been tested on TiO2−B (100) and TiO2−B (001) surfaces. The calculated structural properties agreed well with ab initio density functional theory (DFT) results.33 B. Reactive Molecular Dynamics Simulation. The RxMD simulations are performed with the LAMMPS software package34 with the ReaxFF force field implemented as an external library. The isothermal−isobaric ensemble is applied where the number of molecules (N), the pressure (P), and the temperature (T) are fixed. The thermostatting (T = 300.0 K) and barostatting (P = 1 atm) are controlled by the Nosé− Hoover method with damping constants of 100.0 and 1000.0 fs, 14833

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Figure 1. (Left) x−z side view of the snapshot of the initial H2O/TiO2 simulation box of 48.71 Å (x) × 44.89 Å (y) × 40.0 Å (z, the vacuum height). Periodic boundary conditions have been applied along the x, y, and z directions. The number of water molecules is around 2900, and the calculation is performed at a fixed temperature and pressure: T = 300.0 K, P = 1 atm. (Right) x−y Top view of the five studied TiO2 surfaces, namely, rutile (110), rutile (011), anatase (001), TiO2−B (100), and TiO2−B (001). The color codes are Ti, gray; O, red; and H, white. Only the top-layer atoms are shown for clarity.

Figure 2. Partial final configurations of the five studied TiO2/H2O systems. The results are divided into three cases: on the rutile (011) surface, water exists in the dissociated form; on rutile (110), TiO2−B (100), and anatase (001), water can both molecularly adsorb or dissociate on the surfaces; water shows a very low molecular adsorption on the TiO2−B (001) surface. The color code is red, oxygen atoms from TiO2; blue, oxygen atoms from H2O; dark gray, Ti atoms; and white, H atoms. The bond lengths for water adsorption and dissociation are shown: black, new Ti−O bonds; red, new O−H bonds; blue, Ti−O distance for adsorbed water molecules. Bulk water molecules are not shown for clarity. The side view of TiO2−B (001)/H2O is displayed and the top views are used for the other four systems.

The top view in Figure 1 shows that the five studied surfaces have different topologies and different densities of unsaturated titanium and oxygen atoms. The rutile (110) surface has four different sites accessible to water molecules: two-coordinate or three-coordinate O ions and five-coordinate or six-coordinate titanium cations, noted as O2c, O3c, Ti5c, and Ti6c, respectively. For the rutile (011) surface, Ti5c and O2c sites are exposed to water molecules. The anatase (001) surface is also composed of Ti5c and O2c sites. The TiO2−B (100) surface has Ti5c, Ti6c, O2c, and O3c sites, and TiO2−B (001) exposes Ti5c, O3c, and O4c sites on the surface. As discussed by Liu and coworkers,35 TiO2−B (100) has a higher density of unsaturated oxygen sites on the surface than does the TiO2−B (001) surface. Generally speaking, the TiO2 surface reactivity is related to the unsaturated titanium and oxygen sites. On rutile (011), anatase (001), and TiO2−B (100) surfaces, the exposed oxygen sites are exclusively linked to two surface titanium atoms, and the ratio of surface atoms, O/Ti, is 2. For the other two surfaces, rutile (110) and TiO2−B (001), the surface oxygen sites are

respectively. The initial velocities are assigned according to the Boltzmann distribution. The dynamics of Newton’s equation are iterated using a 0.25 fs time step. A bond-order cutoff of 20% of the original bond length is used to identify the connectivity and molecular species and to monitor the evolution of TiO2/H2O systems as a function of simulation time. As shown in Figure 1, the cleaved clean titanium dioxide (TiO2) surface is modeled as a four-layer TiO2 slab model. The size of the TiO2 surface is fixed to be 48.71 Å (x) × 44.89 Å (y). The vacuum above the TiO2 surface is 40.0 Å and is randomly filled by about 2900 water molecules. The number of water molecules is estimated by using the liquid water density under ambient conditions. Periodic boundary periodic conditions are applied along the x, y, and z directions. This implies that the water molecules are essentially confined in a TiO2 slit pore with a pore width of 40.0 Å. We did not vary the water layer thickness in this work. 14834

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XRD40 and STM41 experiments have suggested that the rutile (011) surface is not stable because of high reactivity and can undergo a spontaneous surface reconstruction through the socalled microfaceting structural model.41 However, those experiments were carried out under vacuum conditions. We did not observe the surface reconstruction for the rutile (011) surface or the anatase (001) surface. This is probably because in our calculation setup the surface is in contact with a large number of water molecules. The interaction with water molecules helps to reduce the TiO2 surface energy and prevents them from the structure reconstruction. We did not study TiO2 structures in the absence of water in this work, but the (1 × 4) surface reconstruction of anatase (001) was reported by Raju and coworkers using the same ReaxFF reactive force field.14 Both molecular adsorption and dissociation of H2O were observed on the rutile (110) surface. New Ti−O bonds resulting from the dissociation are from 2.1 to 2.2 Å, larger those on the rutile (011) surface. The new O−H bonds are around 1.0 Å, similar to the ones on the rutile (011) surface. The adsorbed water molecule is about 2.3 Å from the unsaturated Ti atom. It is interesting to note that whether water can dissociate on the rutile (110) surface remains a point of controversy. As discussed in detail by Sun et al.,13 most experiments tend to conclude that low water coverage does not dissociate on the rutile (110) surface; however, ab initio DFT calculations suggest that a mixed molecular adsorption/ dissociation of water on the rutile (110) surface is favorable.15,42 Since ab initio DFT calculations are performed at 0 K, our RxMD simulations provide new evidence that under ambient conditions both the molecular and dissociative adsorption of water are favorable on the rutile (110) surface. TiO2−B surface structures and surface activity have been studied by ab initio DFT methods.33 The TiO2−B (001) surface was reported to be more stable than the TiO2−B (100) surface.43 H2O35 and NH324 were found to dissociate on the TiO2−B (100) surface but not on the TiO2−B (001) surface. Our results in Figure 2 agree with those previous calculations. Water can adsorb or dissociate on the TiO2−B (100) surface. The new Ti−O bonds are between 1.8 to 2.0 Å, relatively shorter than the corresponding Ti−O bonds on other TiO2 surfaces. On the TiO2−B (001) surface (the side view shown in Figure 2), however, only the molecular adsorption of water was observed. The results also demonstrate that the anatase (001) surface hosts both molecular and dissociative adsorptions of water, which agrees with ab initio DFT calculations from Gong and coworkers38 and recent RxMD calculations by Raju et al.14 B. Order of Surface Reactivity for Water Dissociation on TiO2. During the synthesis and crystal growth process, highly reactive TiO2 surfaces go through spontaneous surface reconstruction and react with water molecules. The reactive surfaces diminish rapidly, and the final product is generally dominated by the thermodynamically stable surfaces, not the most reactive ones. Engineering the shapes to achieve desirable morphologies has been actively pursued. Liu and coworkers reviewed the crystal facet engineering of the semiconductor photocatalyst, in particular, TiO2 crystals.44 Li and coworkers reported enhanced photocatalytic properties of anatase TiO2 nanofibers for phenol degradation by synthesizing the nanofiber surface with a higher density of the reactive anatase (001) surface.45 Inspired by ab initio DFT calculations of different anatase surface energies upon adsorption,46 Lu and coworkers

bonded to three surface titanium sites, and the ratio of surface atoms, O/Ti, is 1.5. More details about the TiO2 surface topologies are discussed in recent review papers.13,36,37 For each RxMD simulation, the simulation box (water and the TiO2 surface) is relaxed to optimize the structures, followed by a 10 ns calculation to further equilibrate the system. After that, the data is collected for 100 ps for analysis. There is no restriction on the structures. Both TiO2 structures and water molecules are allowed to relax and move during the simulation. It is worth pointing out that we did not relax the cleaved clean surface structures before exposing them to water. This is because we want to keep all surface reactive sites available for interactions with water molecules. As briefly reviewed in the previous section, the reliability of the ReaxFF reactive force field for TiO2 surface and cluster models has been tested during its development and in other applications.14,23,30−32 In this work, we put TiO2 surfaces in direct contact with a larger number of water molecules. The distribution of water molecules in the simulation box and the water coverage on different TiO2 surfaces will be determined automatically after the 10 ns relaxation process, unlike ab initio DFT calculations where the number of water molecules for a desired surface water coverage is precalculated and is manually placed close to the surface.

III. RESULTS AND DISCUSSION A. Molecular Adsorption and Dissociation of Water on TiO2 Surfaces. The behavior of water molecules near the TiO2 surface is affected by two factors: one is the surface chemistry, in which the unsaturated surface Ti atoms provide sites for water adsorption or dissociation and the unsaturated oxygen atoms form hydrogen bonds with the water molecules; the other is the surface geometry, in which different surfaces have unique configurations of the surface atoms, as shown in Figure 1. For instance, the large Ti−O−Ti bond angle at the rutile (011) surface implies that the 2p states on the oxygen atoms are destabilized and will be more reactive for water interactions.38 Figure 2 shows a collection of configurations of the five studied TiO2/H2O systems. They are the final configurations after a 10 ns RxMD simulation, where both TiO2 structures and water molecules have been fully relaxed and their interaction is at equilibrium. Only a small portion of the top-layer TiO2 surface is shown in Figure 2, and most water molecules are not shown for clarity. The oxygen atoms from TiO2 are colored red, and those from water are blue. Ti and H atoms are colored dark gray and white, respectively. Water is found to dissociate on four surfaces: rutile (011), rutile (110), TiO2−B (100) and anatase (001). For the TiO2−B (001) surface, however, we observed only the molecular adsorption of water. The same conclusion was also obtained from the ab initio DFT calculation.35 This is probably due to the low surface energy of TiO2−B (001) and the topology characteristics that on the TiO2−B (001) surface some oxygen atoms are bonded to three titanium sites whereas on other reactive TiO2 surfaces, oxygen atoms are bonded only to two titanium sites. On rutile (011) surface, we observed water dissociation: the dissociated OH groups formed new bonds with the unsaturated surface Ti atoms and the dissociated H atoms bonded with the neighboring undercoordinated surface oxygen atoms. The new Ti−O bonds are from 1.9 to 2.1 Å, as shown in black, and the new O−H bond length is about 1.0 Å, as shown in red in Figure 2. Recent results of ab initio DFT calculations39 and 14835

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high fraction of TiO2−B (100) or anatase (001) surfaces.48 It is also worth noting that a longer trajectory (250 ps) has been analyzed for the surface hydroxyl group density evolution. It shows no essential difference from the results in Figure 3. This is further confirmation that the system has been fully relaxed after the 10 ns relaxation, and the 100 ps analysis can well represent the properties discussed in Figure 3. As displayed in Figure 4, free radicals were traced for the 100 ps trajectory on the least-reactive rutile (110) surface: OH−,

developed a new solution synthesis method to crystallize anatase TiO2 with an unusually large fraction of reactive anatase (001) facets.47 The theoretical understanding of the TiO2 surface reactivity order can provide useful information for experiments and is the key to fine tuning the TiO 2 photocatalyst and its physicochemical properties to achieve high selectivity and desirable reactivity for various applications. To compare the reactivity of the five surfaces studied in this work, we monitor the density of hydroxyl groups on the five studied TiO2 surfaces. As shown in Figure 3, Ti−OH is the

Figure 3. Surface hydroxyl group density evolution: Ti−OH is the number of titanium atoms that are bonded with the OH groups from water dissociation. Ti(tot) is the total number of surface Ti atoms. The ratio of Ti−OH/Ti(tot) is monitored as a function of simulation time. More water molecules will dissociate on a more reactive TiO2 surface, which leads to a larger ratio of Ti−OH/Ti(tot).

Figure 4. Number of free radials on the rutile (110) surface: OH−, H+, (H2O)w·(OwH)−, (H2O)w·(OH)−, (OwH)−, and (H2O)·(OH)−. The subscript “w” demonstrates that the radical or the atom is directly from water: OwH− is the radical from water dissociation; (H2O)w is one molecule from the bulk water. The oxygen atoms of OH− and (H2O)·(OH)− are from the TiO2 surface, implying that multiple reactions are involved to produce those radials. The lines representing H+, (H2O)w·(OH)−, and (H2O)·(OH)− radials overlap at zero.

number of titanium atoms that are bonded with the OH groups from water dissociation. Ti(tot) is the total number of surface Ti atoms. The ratio of Ti−OH/Ti(tot) is monitored as a function of simulation time for 100 ps. The larger value of Ti− OH/Ti(tot) represents a more reactive TiO2 surface for water dissociation. The number and percentage of molecular adsorbed water molecules are not analyzed. This is because the strength of molecular adsorption depends greatly on the temperature (in this work, T = 300.0 K). Also, our main focus in this work is to sort out the surface reactivity order for water dissociation under ambient conditions and how the water dissociation and the resulting TiO2 surface property changes affect the behavior of bulk water near the TiO2 surface. It is important to study in future work how temperature and pressure will affect the TiO2 surface chemistry, the ratio between adsorption and dissociation of water molecules, and the stability and transferability of surface functional groups (OH and H) due to water dissociation. The results in Figure 3 illustrate that under ambient conditions the TiO2 surface reactivity for water dissociation follows this order: rutile (011) > TiO2−B (100) > anatase (001) > rutile (110) > TiO2−B (001). No water dissociation was observed on the TiO2−B (001) surface. The inert to water dissociation may find important applications for the TiO2−B (001) surface where waterproofing and structural stability are the dominating concerns. The rutile (011) surface is most reactive for water dissociation. We did not observe the structure reconstruction for the rutile (011) surface. This is probably due to the periodic boundary condition applied for the surface models. Figure 3 also shows that TiO2−B (100) and anatase (001) surfaces have similar surface reactivities for water dissociation. This provides a theoretical support for the experimental efforts in synthesizing TiO2 materials with a

H + , (H 2 O) w · (O w H) − , (H 2 O) w · (OH) − , (O w H) − , and (H2O)·(OH)−. The subscript “w” indicates that the radical or atom comes directly from water. For example, (OwH)− is a radical from water dissociation but OH− is a radical formed by an oxygen of TiO2 and a hydrogen atom of water, which requires multiple reactions to form this radical. Similarly, (H2O)w is a water molecule from the bulk, but H2O is the reaction product when an oxygen atom of TiO2 reacts with the OH− radical. It is obvious from Figure 4 that radicals generated from water dissociation and the water/TiO2 interaction can react back with the TiO2 surface: OH radicals are bonded with the unsaturated Ti sites, and the H radicals (protons) are bonded with the unsaturated O sites. Figure 4 also demonstrates that after the 10 ns optimization there are very few free radicals: there are fewer than two free radicals for (H2O)w·(OwH)−, OH−, and (OwH)− during the 100 ps analysis, and the number of H +, (H2O)w·(OH) −, and (H2O)·(OH)− radicals is zero. C. Surface Reactivity and Near-Surface Water. Before we discuss the properties of near-surface water, it is worth pointing out that the four-layer TiO2 surface model used in this work maintains the balance between the computational cost and the accuracy. Harris and Quong tested the slab thickness effect on the water interaction with the rutile (110) surface.49 Despite the slight adsorption energy difference with the slab thickness, the ab initio DFT results showed that no significant difference was observed. Both molecular adsorption and dissociation were reported when the slab changed from three to five layers. In the review paper, Sun and coworkers have summarized other factors, for example, the supercell size and 14836

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whether water molecules should be present on both sides of the slab13 and considered a four-layer TiO2 surface model to be a reasonable choice. The use of a four-layer TiO2 surface model has also been applied in recent RxMD simulations.14,23,31 We provide brief discussions of the four-layer TiO2 surface model and TiO2 bulk structures in the Supporting Information. The molecular perspective of interfacial water is one of the most exciting fields in computational chemistry and physics. The experimental understanding of interfacial water is very challenging because of the limited access to the dynamic disorder and entropic effect on interfacial water and the insufficient analysis of the interfacial water property change as a function of the contacting water layer thickness. Accurate and predictive theoretical models can help us to understand the fundamentals, including how interfacial structure and chemistry respond to molecular-level multilayer water adsorption and how interfacial properties will be altered by the dissociative adsorption of water and the resulting surface functionalization, the formation of a surface hydrogen bond network, and the surface hydrogen transfer. In general, interfacial water adsorption can be explained by the bilayer model in which the water molecules in direct contact (first layer) interact strongly with the surface, and water molecules in the second layer are connected through the hydrogen bond network and do not interact appreciably with the surface. It was not until the first experimental structural characterization of water on an atomically flat Ru(001) surface by Held and Menzel50 and the ab initio DFT calculation of Feibelman51 that it was known that partially dissociated interfacial water containing both OH radicals and water are significantly more stable than the traditional interfacial bilayer water model. With the development of surface analysis techniques, various interfacial water structures have been identified on different metal surfaces, from isolated water clusters,52,53 quasi-1D water chains,54,55 2D overlayers with mixture patterns of pentagons, heptagons and pyramids,56 and 3D crystalline pillars.57 One of the key concepts that has been accepted is that the contact layer of water and their OH group configurations facilitate the behavior of second and other upper layers of water. As discussed in the review paper by Carrasco and coworkers,58 the discussion of interfacial water has been focused on the contact layer between water and metal surfaces. There is an increasing interest in constructing a fundamental image of (a) multilayer water adsorption on metals and extending the understanding to metal oxides, polymers, and other surface and interfaces and (b) how the interfacial water properties are affected by the presence of bulk liquid and the change in pressure and temperature. We analyzed the water density distribution along the z direction of the simulation box, as shown in Figure 5. The density distribution profile z(ρ), with respect to the ambient liquid water density ρ0, was calculated along the z direction from surface oxygen atoms of TiO2 (origin) up to 1.0 nm away from the surface. Small shells (Δz = 0.1 Å) were used, shown as red dotted lines in the insets of Figure 5. On the reactive rutile (011) surface, there are three distinct peaks at 1.05, 1.69, and 3.18 Å, respectively. The increased local water density implies that the behavior of those water molecules is different from that of bulk water. The 1.05 Å peak is attributed to water dissociation on the rutile (011) surface. As illustrated in the inset of Figure 5a, 1.05 Å is the bond length of O−Hw, where O is the surface oxygen atom of the surface and Hw is the proton from water dissociation. The density distribution profile on

Figure 5. Density distribution of water molecules along the z direction: (a) rutile (011) with three distinct peaks and (b) TiO2−B (001) with two distinct peaks. As indicated by the inset, the origin was set to be at the oxygen atoms of the TiO2 surface. The peaks indicate an increased local density of water.

TiO2−B (001) revels a different image. As shown in Figure 5b, there are only two distinct peaks: one is 1.83 Å from the surface and the other is at 3.65 Å. This agrees with the previous discussion that no water dissociation is observed on the TiO2− B (001) surface. To understand the properties of water molecules at those peak shells, we analyzed their orientation in Figure 6. The dipole orientation angle, θ, is defined to be the angle formed by the water dipole moment and the negation z direction, shown in the inset of Figure 6. On the reactive rutile (011) surface, the layer 1 water molecules are dissociated on the surface, and dissociation products OH and H form new bonds with the surface Ti and oxygen atoms. Layer 2 water molecules interact with the new surface functional groups, Ti−(OH)w and O−Hw, to form a hydrogen bond network. This explains the preferred dipole orientation distribution and the peaks (red line in Figure 6a). The ordered orientation of the layer 2 water molecules can further affect the behavior of the layer 3 water molecules. This is demonstrated by the dipole orientation distribution (black line in Figure 6a). Water molecules that are further away from the surface do not have preferred dipole orientations and thus are not shown. In contrast, as shown in Figure 6b, the water molecules on TiO2−B (001) do not show a preferred dipole orientation. This is probably because water molecules do not dissociate on the TiO2−B (001) surface, thus there is no such hydrogen bond 14837

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of an enhanced hydrogen bond network, the near-surface water layer is 0.64 Å from the rutile (011) surface, which is the distance between the first peak (1.05 Å) and the second peak (1.69 Å) in Figure 5a. On the TiO2−B (001) surface, nearsurface water (peak one in Figure 5b) can also form hydrogen bonds with surface oxygen atoms of TiO2, but their distance to the surface is 1.83 Å, which is longer than that on the rutile (011) surface.

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CONCLUSIONS Titanium dioxide is considered to be a good candidate in solving environmental and energy challenges. The performance of TiO2 materials in environmental and energy applications strongly depends on complex material factors such as the surface geometry, surface chemistry, surface reactivity, and hydrostatic and thermal stability. The motivation here in this work is to understand theoretically the surface reactivity of TiO2 materials toward water dissociation. By using the ReaxFF reactive force field and RxMD simulations, we study the interactions between water and five different TiO2 surfaces, namely, anatase (001), rutile (011), rutile (110), TiO2−B (100), and TiO2−B (001). The calculation results show that the five studied TiO2 surfaces show different reactivities for water dissociation on four surfaces in the order of rutile (011) > TiO2−B (100) > anatase (001) > rutile (110), but there is no water dissociation observed on the TiO2−B (001) surface. Further analysis of the near-surface water suggests that the water dissociation and the TiO2 surface chemistry change, i.e., the new surface Ti−OH and O−H functional groups, can affect the orientation of near-surface water molecules. On the rutile (011) surface, representing the reactive TiO2 surfaces, the first layer of water molecules dissociates on the surface and the resulting new functional groups enhance the interaction with near-surface waters and bring the second layer of water closer to the surface. On the nonreactive TiO2−B (001) surface, where no molecular or dissociative water adsorption is observed, the near-surface water can also form a hydrogen bond network with the surface oxygen atoms of TiO2, but their (first-layer water molecules) distance to the surface is larger than it is on reactive TiO2 surfaces. The work reported here adds new fundamental understanding on the reactivity order of different TiO2 surfaces and also provides general theoretical insight into the near-surface water behavior on metal oxide surfaces. For future work, it will be important to understand quantitatively how the property of near-surface water changes on different metal and metal oxide surfaces and how this change in the near surface will affect the interaction between surface and adsorbents from the aqueous system. More efforts are needed from both ab initio quantum mechanics calculations and ReaxFF force field development for a fundamental understanding of complicated TiO2 systems, including TiO2 photocatalytic processes and TiO2 biomedical applications where the targeted adsorption and controllable binding of biomolecules are key.

Figure 6. Dipole orientation angle distribution analysis of near-surface water molecules: (a) rutile (011), (b) TiO2−B (001), and (c) cartoon illustration of the hydrogen bond network on reactive TiO2 surfaces. On the reactive rutile (011) surface, layer 1 corresponds to the dissociated water molecules. Layers 2 and 3 are near-surface water molecules whose configurations are more affected by the enhanced hydrogen bond network with the surface. On the nonreactive TiO2−B (001) surface, no water dissociation is observed.

network between near-surface water molecules and surface functional groups. It is important to point out that in this section we use rutile (011) to represent the reactive surfaces to compare with the inset TiO2−B (001) surface. The water layers and their dipole orientation distributions on the anatase (001), TiO2−B (100), and rutile (110) surfaces are similar to the one on the rutile (011) surface and are not shown. For the studied systems in this work, two types of water molecules and their behavior are influenced by the existence of the surface: the first-layer water molecules (layer 1) that are closest to the surface get dissociated or molecularly adsorbed on the rutile (011), rutile (110), TiO2−B (100), and anatase (001) surfaces; the nearsurface molecules whose properties are affected by the layer 1 water molecules. The dissociation of the first-layer water molecules changes the TiO2 surface chemistry by forming new functional groups Ti-(OH)w and O−Hw, where w implies that the atoms come from water molecules. The new surface functional groups further interact with near-surface water molecules to form hydrogen bond networks, thus affecting their orientation as illustrated in Figure 6c. In addition, because

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ASSOCIATED CONTENT

S Supporting Information *

Left and right configurations of H2O/rutile (110), H2O/TiO2− B (100), H2O/TiO2−B (001), and H2O/anatase (001). This material is available free of charge via the Internet at http:// pubs.acs.org. 14838

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS It is a pleasure to thank Dr. Luzheng Zhang for helpful discussions on titanium dioxide surface modifications. L.H acknowledges the support by startup funds from the University of Oklahoma and the State Key Laboratory of MaterialsOriented Chemical Engineering. K.E.G. acknowledges the U.S. National Science Foundation (NSF) for support through grants CBET-1133066 and CHE-1012780. X.L acknowledges support from the National Natural Science Foundation of China (21136004) and the 973 project (2013CB733501).

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