Molecular Behavior of Water on Titanium Dioxide Nanotubes: A

Article pubs.acs.org/jced

Molecular Behavior of Water on Titanium Dioxide Nanotubes: A Molecular Dynamics Simulation Study Wei Cao,† Linghong Lu,*,† Liangliang Huang,‡ Yihui Dong,† and Xiaohua Lu*,† †

State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing, Jiangsu 210009, China School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States

‡

S Supporting Information *

ABSTRACT: Understanding molecular behavior of water on titanium dioxide nanotubes (TiNTs) is of great interest in order to enable potential application of TiNTs in dye-sensitized solar cells, photocatalysis, and biomedical coatings. Using molecular dynamics simulations, we study the static and dynamic properties of water on TiNT with a diameter of ∼1.0 nm. The TiNT modified by a carbon nanotube (CNT) inside is built to investigate the effect of surface chemistry changes on the sorption and diffusion of water. The results show that the water molecules outside TiNT conform with the two-layer model for water on a planar surface. The difference is that the first water layer is further from Ti5c as an effect of surface curvature, indicating the easier water desorption on the surface. This layer disappears for water inside TiNT, leaving a water layer with the hydrogen atoms pointing to the O2c to form hydrogen bonds. The simulations also reveal that the orderly structure is destroyed in the carbon modified nanotubes. Diffusion of water contact with the outer TiNT surface is found to be slower than that of water inside tube. While the diffusion is highly improved of water confined in the tube covered by carbon, which is larger than that of water on the outer surface.

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INTRODUCTION Since Iijima first reported on the formation of carbon nanotubes (CNTs) in 1991, a range of transition metal-oxides was reported to form nanotubes (NTs).1−4 Among these transition metal-oxides, TiO2 is one of the most extensively studied materials in the past two decades, due to the combination of properties of bulk TiO2 and the nanotubular character.5 The synthesis of TiO2 nanotubes (TiNTs) is mainly achieved by hydrothermal methods, template-assisted methods, anodization approaches, and electrospinning process.6 The hydrothermal prepared TiNTs are open-ended, with the inner diameter ranging from 2 to 20 nm and a wall thickness in the range of atomic level.7 Aligned TiNT arrays are obtained via electrochemical anodization of titanium.5 By electrospinning method, TiNTs can also be formed with continuous and uniform structures.8 These kinds of TiO2 with tubular structures have extended the applications of classic TiO2, such as in dye-sensitized solar cells, photocatalysis, and biomedical coatings filed.9−11 Most of these applications for TiNTs involve an aqueous environment, and therefore understanding the TiO2−H2O interaction is essential. The TiO2−H2O system has been widely studied theoretically.12 The main TiO2 crystal morphologies are rutile, anatase, and brookite in nature. Recent studies have focused on the water behavior on rutile (110) and anatase (101) surface, the dominated surfaces of most TiO2 materials.13−15 In a previous study, we performed molecular dynamics (MD) simulations with the ReaxFF force field and concluded that different TiO2 © XXXX American Chemical Society

surface demonstrates different interaction between TiO2 and water.16 For example, the reactivity for water dissociation on the anatase (001) surface was larger than that on the rutile (110) surface and smaller than that on the rutile (011) surface. The diffusion of water adsorbed on the TiO2 surface above 1 monolayer coverage was also investigated by MD simulations.17 The results indicated that the residence time of water on the TiO2 surface was considerably longer than that on the graphite surface. When confined in the one-dimensional NT formed by rolling the two-dimensional surface, water behavior would change drastically. Take CNTs for instance; ultrafast mass transport for water in the tube was commonly reported.18,19 The hydrogen bonds of water molecules in CNTs were demonstrated to be tighter than bulk water.20 Compared with the planar surface, a water molecule inside the NT interacts with more surface atoms, while a water molecule outside the NT interacts with less surface atoms. Regarding TiNTs, the question how TiO2−H2O interaction changes remains for further discussion. To investigate the TiNT−H2O interaction, the structural properties should be verified first. Meng and co-workers applied density functional theory (DFT) to study the geometric and Special Issue: Proceedings of PPEPPD 2016 Received: June 30, 2016 Accepted: October 24, 2016

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DOI: 10.1021/acs.jced.6b00551 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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electronic properties of TiNT.21 Bandura et al.,22 Hossain et al.,23 and Szieberth et al.24 also studied the formation and structure of TiNTs, using DFT methods. Hart and co-workers conducted atomistic simulation techniques and found that NTs made by chiral wrapping of (101) sheets of anatase were relatively stable compared with other faces of anatase.25 Only a few theoretical studies have been focused on fluids adsorbed on the TiNT surface. Hydrogen was reported to chemisorb on the O atoms of anatase TiNT surfaces, while they physisorb on the Ti atoms.26 Formic acid adsorbed on TiNTs was found to form stronger hydrogen bond with the NT surface than the planar surface, obtained from a DFT study.27 For water on the TiNT surface, the results from Liu and Tan showed that the reactivity for water dissociation on NT was higher than that found on planar surface.28 Despite a few theoretical works concerning the TiNT−H2O system have been published, the study of a large amount water behavior, i.e., structure and dynamic properties, on TiNT surface is still lacking. Our previous work calculated the properties of water in TiO2 nanoslits with varying coverages of carbon.29 The results showed that changes to the surface chemistry of TiO2 can affect the hydrogen bond and diffusion of water on the surface. Here we extended this work to focus on the TiNTs. In this work, MD simulation techniques were utilized to study the structure and diffusion of water on TiNTs. The carbon modified TiNT model was also investigated. Water molecules inside and outside tube were both considered. To the best of our knowledge, the present study is first to investigate the behavior of water in TiNTs using MD simulations. In the calculations, the interaction between water and anatase (101) TiNTs of diameter ∼1 nm was examined. This paper is organized as follows. The Methods section briefly introduces the potential models of simulation systems and the setup. We proceed to discuss in the Results and Discussion section the structure, hydrogen bonds network, and diffusion coefficients of water molecules near NT surfaces. We also investigated the properties of water near a carbon modified TiNT to study the effect of surface chemistry changes.

Figure 1. (a) Atomic structure of TiNT viewed along the axial direction. (b) Side view of TiNT. Panels c and d are similar to a and b but for M-TiNT (TiNT with CNT inside). The color codes are Ti, silver; O, red; and C, gray. The fragments of NTs are shown for clarity.

In this work we choose the 1 nm TiNT as a host material model and compare the behaviors of water in TiNTs and CNTs. O atoms in the TiNT are either 2-fold coordinated (O2c) or 3-fold coordinated (O3c), while Ti atoms are 5-fold coordinated (Ti5c).22 Due to the employed NT models, the coordinated atoms such as 6-fold Ti and 3-fold O become undercoordinated 5-fold coordinated Ti and 2-fold O ions in the NT model. In order to figure out the water behavior on carbon modified TiNT (called M-TiNT below) surfaces, a combination of TiNT (0, 5) and CNT (8, 8) was constructed, as seen in Figure 1c and d. TiNT (0, 5) with an inner diameter of 1.418 nm is covered inside tube by a CNT (8, 8) with a diameter of 1.088 nm, similar to the work conducted by Wei and co-workers that the carbon (graphite) pattern was placed above the TiO2 surface.29 Water near a CNT (8, 8) is also discussed in this work for comparison. The lengths of all of the three NT models are all set to ∼4 nm. The TiNT and CNT both have a total of 480 atoms. M-TiNT has a total of 1082 atoms, including 480 carbon atoms. The extended simple point charge (SPC/E) model31 was used for water, with its internal degrees of freedom restrained through the SHAKE algorithm.32 The model was considered to be a better one in predicting some of the properties of water relevant for this study at room temperature.33 The partial charge of oxygen in water (Ow) was −0.848 e, and carbon atoms of CNTs were treated as neutral Lennard−Jones (LJ) interaction sites. The nonbonded interactions between water and CNTs were described by a union of LJ 12−6 potential and a Coloumbic potential, defined as

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COMPUTATIONAL METHODS Simulation Models. Three kinds of NTs in this study were modeled: TiNT, M-TiNT, and CNT. TiNT was formed by rolling the molecular monolayer TiO2 into a cylinder, the same way as for the formation of CNT by curling the graphene sheet. Anatase TiO2 (101) sheet was built first from bulk crystal. We focused on anatase TiO2 because it was produced experimentally by the hydrothermal method or sol−gel procedure.30 Then an anatase (101) planar sheet was wrapped along the [010] direction, giving rise to (0, m) zigzag tubes. The details of how the NT models were constructed and optimized have been described previously by Meng et al.21 A TiNT (0, 4) with an inner diameter of 1.088 nm and a wall thickness of 0.245 nm was shown in Figure 1a and b, though TiNTs synthesized by the hydrothermal method usually have the inner diameters from 2 to 20 nm. Experimental efforts are being made to produce smaller TiNTs in our group and others. The key reason for choosing a TiNT with diameter about 1 nm is that carbon nanotubes (CNTs) with diameters around 1 nm show significant confinement effects on the water inside CNTs. The behavior of water in larger CNTs (diameter >1 nm) is similar to that of bulk water. Although smaller CNTs provide stronger confine effects, water forms single files in those smaller CNTs, which might not be ideal for diffusion and separation purposes.

⎡⎛ ⎞12 ⎛ ⎞6 ⎤ qiqje 2 σij σij ⎥ ⎢ Uij = 4εij⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥ + r ⎝ rij ⎠ ⎦ 4πε0rij ⎣⎝ ij ⎠

(1)

where rij, εij, σij, qi, and qj are the separation, LJ well depth, LJ size, and partial charges, respectively; ε0 represents the dielectric constant of vacuum. For the TiNT, Matsui and Akaogi (MA) parameters were used.34 The site−site interaction between Ti and O ions was descripted by the function of Buckingham potential, defined as B



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⎛ r ⎞ C ij ij Uij = Aij exp⎜⎜ − ⎟⎟ − 6 ρ r ij ⎝ ij ⎠

The structure and dynamic properties of water were obtained by analyzing the last 1.0 ns data.

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(2)

RESULTS AND DISCUSSION Water Adsorption in NTs. Water can pass into NTs with either hydrophilic (i.e., aluminosilicate NTs38) or hydrophobic (i.e., CNTs39) surfaces. In our calculations, the equilibrium water numbers pass into the tubes are 77, 45, and 46 for TiNT, M-TiNT, and CNT, respectively. The larger amount of water in TiNTs comes from the stronger interaction between TiO2 and water molecules. Regarding M-TiNTs, the water density in tube is similar to that in CNTs, less than that in TiNTs. This shows that the interaction between water and TiNT decreases as an effect of surface modification by carbon. The free energy profiles of water molecules along the axial of NT models are calculated to understand the water permeation, as shown in Figure 2. Free energy is calculated by −kBT ln(ρ(z)/ρ0), where

where Aij, ρij, and Cij are the parameters. The partial charge of O and Ti atoms are +2.196 e and −1.098 e, respectively. The TiO2−H2O interaction was modeled in the same way as in our previous paper.29 NT models were treated as rigid units, and the interactions between atoms of either TiNT or CNT were not calculated in this study. The reasons we adopted the potential developed by Matsui and Akaogi and used a rigid TiNT model are as follows. We mainly study on understanding the behavior of water confined in nanopores. Our focus is neither the stability of the host TiNT material, nor water dissociation on reactive adsorbents. On the other hand, the Matsui and Akaogi potential has been tested and applied in various systems involving TiO2, such as the water−TiO2 interface for both hydroxylated (dissociative) and nonhydroxylated (associative) surfaces.12,17,29,35 Table 1 shows the potential parameters of NTs, the water model, and crossinteraction. Table 1. Force Field Parameters of Atom−Atom Interactionsa Buckingham potential Ti−Ti Ti−O(TiO2) O(TiO2)−O(TiO2) Ti−Ow Lennard−Jones (12,6) O(TiO2)−O(water) Ow−Ow *−Hw C(CNT)−C(CNT) C(CNT)−Ow

Aij (kcal·mol−1)

ρij (Å)

717647.4 0.154 391049.1 0.194 271716.3 0.234 28593.02 0.265 εij (kcal·mol−1) 0.15539 0.15539 0 0.07 0.104294

Cij (kcal·mol−1·Å6) 121.0676 290.3317 696.8883 148.0000 σij (Å) 3.166 3.166 0 3.55 3.358

The interactions between water and TiO2 derive from Bandura and Kubicki.35.

Figure 2. Free energy of water inside NTs along z-direction (axial direction of NTs). z = 0 represents the center of NTs. The free energy barrier is calculated according to the free energy difference between highest free energy of water inside NT and lowest free energy of water outside NT.

Simulation Details. MD simulation methods were performed with the LAMMPS software package.36 Appropriate water molecules were inserted to a cubic box with the size X, Y = 6.0 nm and Z = 12.0 nm, ensuring that the water density was 1.0 g·cm−3. NT models were fixed in the center of the box and occupied spaces less than 1/3 of the simulation box. Here the Ti and O atoms in TiNT models were fixed. The periodic boundary conditions were imposed along the x, y, and z directions. The particle-mesh Ewald method was employed to calculate the electrostatic interactions.37 The cutoff distances for short-range van der Waals interactions and Coulombic interactions were both taken to be 1.0 nm. The configurations were subjected to energy minimization before MD simulations were performed. The initial velocities of water molecules were assigned according to the Boltzmann distribution. The dynamics of Newton’s equation were iterated by a 1.0 fs time step, using the leap-frog algorithm. The isothermal−isobaric ensemble was used where the number of molecules (N), the pressure (P), and the temperature (T) were fixed. The thermostatting (T = 300.0 K) and barostatting (P = 100 bar) were controlled by the Nose−Hoover method with damping constants of 0.1 and 1.0 ps, respectively. The high pressure was set to prevent nanobubbles inside NTs. MD simulations were run for 5 ns. The trajectory of atoms was collected every 1.0 ps.

kB is Boltzmann’s constant, T is temperature, and ρ is density of water molecules occupied along the axial direction of NTs. In Figure 2 we can see that the free energy of water inside TiNTs is smaller than the other two. The free energy barrier was then calculated according to free energy difference between the highest free energy of water inside NT and lowest free energy of water outside NT. The highest free energies near the tube mouth of the TiNT, M-TiNT, and CNT are 0.48, 0.42, and 0.61, respectively. The lowest energies near the tube mouth of the TiNT, M-TiNT, and CNT are −0.31, −0.18 and −0.19, respectively. Thus, the free energy barriers for the TiNT, MTiNT, and CNT are 0.79, 0.6, and 0.8 kcal·mol−1, respectively. The energy barrier decreases when TiNT is modified by carbon, which favors water “entering” the NT. It is also worth noting that, though the TiNT−water interaction is larger than the CNT−water pair, the likelihood of water passing in TiNTs is almost the same as that for CNTs. Structure. In general, the behavior of water molecules near the TiO2 surface is mainly influenced by the surface geometry and chemistry. As seen in Figure 1, the anatase (101) NT surface is strongly corrugated, with ridges of 2-fold coordinated O atoms (O2c). Recent researchers reported the bilayer of water molecules on the anatase (101) planar surface.40 The oxygen

a

C



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atoms (Ow) in the first water molecule layer have been found to directly bind to the 5-fold coordinated Ti atoms (Ti5c). The hydrogen atoms (Hw) in the second water molecule layer form hydrogen bonds with the bridge oxygen atoms (O2c). The water molecules further from the surface show a higher mobility and a more disordered arrangement. To understand the structure of water on TiNTs, the radial density profiles of water molecules were studied, as shown in Figure 3. The

molecules outside M-TiNTs are nearly unchanged compared with those outside the TiNT model. The distribution profiles for the molecular dipole orientation of water in NTs and the two layers on outer NT surfaces (Figure 4) are calculated to further discuss the structure of water. Here we define an angle between the dipole moment and the radial direction (point from the tube center to the tube wall), and the distributions of this angle are calculated. A schematic image of the calculation was shown in Figure 4g. Snapshots from the MD simulations are also shown in this figure. It is obvious to find the ordered orientation of water near NTs. For water inside TiNT the angle is approximate to 90°. It means that the direction of the line between the two Hw atoms in a water molecule is parallel to the radial direction. We can see directly from Figure 4d that the −OH bond point to the bridge O2c. The water dipole vectors outside TiNT are mainly oriented in two directions. The Ow of a water molecule in the first water layer locates right on the surface Ti5c atoms, with the direction of the line between the two Hw atoms perpendicular to the radial direction. Meanwhile, the Hw from a water molecule in the second layer points to the O2c atoms on the outer surface, with the angle larger than those of water molecules inside tube. The separated angle distribution for the two water layers was shown in Figure 4h. From the comparison of the angle distributions of water inside and outside tube, we can find that the suggested first layer on TiO2 disappears when water adsorbed on the inside wall of TiNTs. In other words, it is hard for water molecules to contact with the Ti5c atoms. Regarding water near M-TiNTs, the angle distributions (Figure 4b) are similar to those near TiNTs. As illustrated in Figure 4, water molecules in the M-TiNT do not have a preferred dipole orientation, which is different from that of the CNT case. This is probably due to the smooth CNT wall and the weaker interaction between water and CNTs. Compared with the TiNT systems, water confined in CNTs can more easily adjust their orientation, which is illustrated by the preferred dipole orientation shown in Figure 4c. For the MTiNT, after the inner surface modification, the accessible volume is smaller than that of pristine TiNTs, which results in a different dipole orientation. The distances between the water molecules and the atoms of TiO2 are studied from the radial distribution functions (RDFs). The RDFs of water−water and water−surface pairs are calculated, as shown in Figure S1 in the Supporting Information. The closest distances between different atoms (the location of the first peak of a RDF) are presented in Table 2 and Table 3. Table 2 shows the distances of Ti5c−Ow pair and O2c−Hw pairs for water near TiNTs. In a recent DFT study, the Ti5c−Ow distances for a water molecule adsorbed in its molecular form on the inner and outer surface were found to be 2.27 and 2.3 Å, respectively.28 When compared to the data for water on a planar anatase (101) surface, the length of the Ti5c−Ow pair increases as an effect of surface curvature.42 However, when a large amount of water molecules are adsorbed on the TiNT surface, the local structure would change as an effect of the increases of water−water interactions. In our MD calculations, the distance between the surface Ti5c and Ow of the first water layer outside TiNT is 2.8 Å (Table 2), which is larger than the value (2.3 Å) from the DFT calculation. The difference is more than 15% of the equilibrium Ti−O bond length, which indicates a flexible probably also reactive force field is necessary to most accurately describe water/TiNT interactions. More DFT calculations are undergoing. We also

Figure 3. Radial density profile of water−oxygen atoms. z = 0 represents the center of NTs. The dashed lines show the borders of NT models: for TiNT the dashed lines represent the location of bridge O2c circle on the inner and outer surface; for M-TiNT the dashed lines represent the location of the C atoms in the CNT and bridge O2c circle on the inner and outer surface, respectively; for CNT the dashed line represents the location of the C atoms.

density profile ρ(r) with respect to the ambient liquid water density ρ0 was calculated along the radial direction, which originates from the tube center. From Figure 3, two distinct layers of water are observed, both inside and outside TiNTs. On the outer wall, the distances between Ti5c and the two water layers are ∼2.69 Å and ∼3.66 Å, respectively. From a recent MD simulation with the ReaxFF force field, the calculated two peaks on the planar surface were at ∼2.65 Å and ∼4 Å, respectively.41 It is worth noting that we do not consider the dissociated water layer on the TiO2 surface. By comparison, the first water layer on the outer TiNT surface is comparable to this result, indicating the water molecules bound to the surface Ti5c. Meanwhile, the water molecules in the second layer, which are suggested to form hydrogen bonds with the bridge O2c on TiNT, are closer to the Ti5c atoms as an effect of surface curvature. From Figure 3, we also find that the peaks for water inside tube are at ∼1.22 Å and ∼3.39 Å. The corresponding distances between the water layers and the Ti5c atoms are calculated to be ∼5.57 Å and ∼3.4 Å, respectively. We do not observe the peak at ∼2.69 Å, which is shown for water outside TiNT. This indicates that water molecules inside TiNT cannot form the water molecule layer in contact with Ti5c atoms. When the TiNT is modified by the CNT, the number of water layers changes from two to one as a result of a weak water−CNT interaction. The distribution is demonstrated to be the same as water in the CNT. We also find that the distance between the water molecules inside tubes and the surface increases after the TiNT being modified by carbon. The density profiles of water D



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Figure 4. (a−c) Distribution of the angle of the water molecules’ dipole vectors along the radial direction of NT (point from the tube center to the tube wall). (d−f) Snapshots of the equilibrium structure of water−NT system (along the axial direction). The water molecules inside tube and in the two water layers outside tube are calculated separately. The color codes are Ti, silver; O, red; H, white and C, gray. (g) A schematic image of the molecular dipole orientation calculation. (h) Distribution of the angle along the radial direction for the two water layers outside TiNT.

Table 2. First Peaks of the Atomic Pairs (Ti5c−Ow and O2c− Hw) Calculated from the Radial Distribution Functions (RDFs)a Ti5c−Ow (Å)

O2c−Hw (Å)

tube models

inside

outside

inside

outside

TiNT TiNT (DFT) planar TiO2

4.2 2.27

2.8 2.3

1.8 2.27

1.7 1.91

2.2

Table 3. First Peaks of the Atomic Pairs (Ow−Ow, Ow−Hw, and Hw−Hw) Calculated from the Radial Distribution Functions (RDFs)a Ow−Ow (Å) tube models TiNT/M−TiNT/ CNT liquid water

1.78

Ow−Hw (Å)

inside

outside

inside

2.8

2.8

1.8

2.875

Hw−Hw (Å)

outside inside 1.8

1.85

outside

2.5

2.4 2.45

a

a

plan to develop reactive force field under the framework of ReaxFF to simultaneously study the dynamics of confined water and the stability of TiNTs. Meanwhile, the minimum distance between the Ow of water inside tube and the Ti5c is 4.2 Å. This is found to be equal to the distance between the Ti5c and the Ow of the second water layer outside tube. From Table 2 we also see the similar O2c−Hw distance of water on the inner and outer TiNT surface. Meanwhile, this value is almost the same as the data of water on a planar TiO2 surface. The results suggest

that the surface curvature does not affect the equilibrium location of water molecules on the bridge oxygen atoms. The distances of water−water pair are discussed in Table 3. The shorter distance between water molecules confined in nanoporous materials were reported recently.43 An ice-like structure of water on anatase (101) surface was also found by Zhao and co-workers using the MD simulation methods.40 Here we calculated the RDFs of the atom pairs including Ow− Ow, Ow−Hw, and Hw−Hw. The results for water near TiNT (both the inner and outer surfaces) are quite consistent with

The water molecules inside NTs and the two layers outside NTs are calculated. The results of liquid water are for comparison.

The results of water molecules adsorbed on TiNT (DFT) and planar anatase (101) surface (first-principles MD) are for comparison.28,42.

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both the ⟨nHB⟩ and tR of water on the inner TiNT surface is larger than those on the outside surface. This indicates the stronger water−water interaction of water inside the tube. The hydrogen bonds between water molecules and the TiNT surface are also discussed. When we compare the results of the water layers near TiNTs with a similar O2c−Hw distance, it is appeared that water inside tubes can form more and stronger hydrogen bonds with the bridge oxygen atoms (O2c). Moreover, the water−surface interaction is suggested to be stronger than the water−water interaction by comparing the data in the two adjacent columns in Table 4. We also find the flexible hydrogen bonds formed between the first and second layer of water on the outer TiNT surface. Table 5 shows the ⟨nHB⟩ and tR of water near the surface of three NT models: TiNT, M-TiNT, and CNT. The water molecules inside and the two water layers outside NTs are calculated. For water inside tubes, the ⟨nHB⟩ follows this order: TiNT < M-TiNT < CNT, while the tR follows the opposite order: TiNT > M-TiNT > CNT. Particularly, the ⟨nHB⟩ of water inside TiNT is less than that of bulk water (2.46). The average hydrogen bond numbers of water increases when the inner TiNT surface is modified by carbon. This is because of the less dangling −OH bonds of water molecules in M-TiNT (seen from Figure 4). Moreover, the ⟨nHB⟩ value is slightly larger than the bulk counterpart. The modification also prevents the formation of hydrogen bonds between water and the NT surface. The results imply that the mobility would be enhanced for water confined in a CNT covered TiNT. For water outside tubes (only the first two water layers), the less and weaker hydrogen bonds are observed compared to water inside. The hydrogen bond networks can further influence the dynamic properties of water near the NT surfaces. Dynamic Properties. The residence time and diffusion coefficients of water molecule are discussed in this part to investigate the dynamic properties of water on TiNT. The calculations of residence time and diffusion coefficients are based on the methods described in our previous work.29 The residence time (τR) is related to the strength of hydration.46 It is shown in Table 6 that the hydration of water close to the

the data of liquid water. In other words, the phase transformation of water is not observed, though the orderly structure water molecules are shown. Moreover, the liquid phase of water confined in the tube is unchanged after modified by carbon. The water−water distances are the same for the water molecules near all of the three NT models. Hydrogen Bond Network. The hydrogen bond networks of water are discussed, as shown in Table 4 and Table 5. The Table 4. Average Number of Hydrogen Bonds (⟨nHB⟩) and Relaxation Time (tR) of Water Adsorbed on TiNTa inside tube location

water layer close to wall

(water layer close to wall)−surface

⟨nHB⟩ tR/ps

1.18 38.15

0.91 91.81 outside tube

location

1st water layer

1st water layer-surface

2nd water layer

2nd water layer−surface

1st layer− 2nd layer

⟨nHB⟩ tR/ps

0.33 2.43

0.13 5.57

0.77 6.73

0.75 60.99

0.55 3.62

a

The water layer close to the inner surface and the two layers outside NTs are calculated. The hydrogen bonds formed between water and bridge oxygen (O2c) are also shown. The “−surface” means the hydrogen bond between water and the nanotube surface.

Table 5. Average Number of Hydrogen Bonds (⟨nHB⟩) and Relaxation Time (tR) of Water near TiNT, M-TiNT, and CNTa inside tube tube models TiNT M-TiNT CNT

outside tube

location

water

water−surface

water

water-surface

⟨nHB⟩ tR/ps ⟨nHB⟩ tR/ps ⟨nHB⟩ tR/ps

2.16 47.82 2.51 25.57 2.75 25.05

0.7 92.47 0.0 0.0 0.0 0.0

1.12 6.28 1.29 9.08 2.34 5.89

0.47 62.87 0.44 80.93 0.0 0.0

a

The water molecules inside NTs and the two layers outside NTs are calculated. The “−surface” means the hydrogen bond between water and the nanotube surface.

Table 6. Residence Time (τR) of Water near TiNTs, Including Both the Inside and the Outside Surfaces inside tube

average number of hydrogen bonds (⟨nHB⟩) is calculated first. Hydrogen bonding is defined by applying the geometric criteria where the acceptor donor (Ow−Ow) distance is less than 0.35 nm and the angle (OHw−Ow) is less than 30°. The O2c atoms in TiNTs are considered as acceptors when we calculate the hydrogen bonds between water molecules and the pore wall. The hydrogen bond dynamics are then computed, which can be characterized by the hydrogen bond autocorrelation function (CHB(t)).44 The autocorrelation function CHB(t) is defined by

C HB(t ) =

h(0)h(t ) h

outside tube

TiNT

close to center

close to wall

1st layer

2nd layer

τR/ps

104.97

375.21

92.82

88.69

TiNT surface is stronger than that of water in other layers. In order to find the difference between water inside and outside TiNT, we compute the τR of water on both sides. The data shows that water inside tube prefers to stay on the inner surface. Regarding the molecules outside, the τR of water in the first layer is slightly larger than that in the second layer. This indicates the stronger interaction between the water molecules in the first water layer on the outer TiNT and surface. Figure 5 shows the diffusion coefficients of water molecules near NT surface. The diffusion coefficients presented in this figure have been calculated locally for separate radial layer. The diffusion coefficients of water along the tube axis were calculated in this work. For the TiNT, the diffusivity of water confined in the tube is less than that of water outside. This is because of the long-lasting hydrogen bonds of confined water molecules and between water and the surface. The diffusion

(3)

where h(t) equals 1 when a hydrogen bond is present at time t, and 0 otherwise. The CHB(t) of water can be found in Figure S2 in the Supporting Information. The relaxation time (tR) is then obtained by the integration of CHB(t), indicating the stability of the hydrogen bonds. The ⟨nHB⟩ of bulk water is 2.46.45 Water molecules near TiNT are studied, including the water layer close to the inner surface and the two layers on the outer TiNT surface (seen from Figure 2). From Table 4 we can see that F



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to be slower than that of water inside tube. Meanwhile, the diffusion is highly improved of water confined in the nanotube covered by carbon, which is larger than that of water on the outer M-TiNT surface. The work reported here adds new fundamental understanding of a large amount of water behavior on the TiNT surfaces. For future work, it will be important to understand the water dissociation and then the structure and dynamics of water on the TiNT.

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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.jced.6b00551. Figure S1 shows the radial distribution functions of different pairs. Figure S2 shows the hydrogen bond autocorrelation functions (PDF)

Figure 5. Diffusion coefficients (Ds) of water near the TiNT, M-TiNT, and CNT as a function of the distance from the center of the NTs, according to the density profiles shown in Figure 3. The red dashed line shows the borders of the inner NT. The error bars on molecular simulation calculations are also included. The diffusion coefficient of bulk water (2.49 × 10−9 m2·s−1) is also shown.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

coefficients of water in the first and second layer on the outer surface of TiNT are almost the same. The mobility of water increases when the distance of water−surface grows. To study the carbon modification effect of water mobility, we calculated the diffusion coefficients of water in M-TiNT. From Figure 5 we can find that the diffusion coefficients of water molecules in M-TiNT is highly improved (Ds = 2.1 × 10−9 m2· s−1). It is worth noting the small gap between the water diffusivity in M-TiNT and CNT, though the water−M-TiNT interaction is stronger than the water−CNT interaction. This indicates that the diffusion of water in NTs is mainly influenced by the surface which contacts directly with the water molecules. Water diffuses slower on the outer wall of M-TiNT than that of TiNT, resulting from the more stable hydrogen bonds discussed in Table 5.

Funding

This work has been supported by the National Basic Research Program of China under Grant No. 2015CB655301; the National Natural Science Foundation of China Grants under Grant Nos. 21176113, 91334202, and 21490584; the Jiangsu Natural Science Foundation under Grant BK20130062; and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS We have studied the molecular behavior of water on titanium dioxide nanotubes using MD simulations for the first time. A large water coverage near the anatase (101) nanotube with a diameter of ∼1 nm is studied. The TiNT modified by a carbon nanotube (CNT) inside is built to investigate the effect of surface chemistry changes on the sorption and diffusion of water. The results show that water can pass into TiNTs and form a two-layered structure. The resistance for water passing decreases when the TiNT is modified by carbon, following a decrease of water density. The structure and dynamic properties of water on the TiNT are studied. The structure of water is demonstrated by the radial density profiles, dipole orientation distributions, and radial distribution functions. Simulation results show that the water molecules outside the TiNT conform with the two-layer model for water on a planar surface. The difference is that the first water layer is further from Ti5c as an effect of surface curvature, indicating the easier desorption on the surface. This layer disappears for water inside the TiNT, leaving a water layer with the hydrogen atoms pointing to the O2c to form the hydrogen bonds. The simulations also reveal that the orderly structure and the long-lasting hydrogen bonds structure formed between water and TiO2 are destroyed in the carbon modified TiNTs. The diffusion of water contact with the outer TiNT surface is found G



Article

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Molecular Behavior of Water on Titanium Dioxide Nanotubes: A

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