Water Adsorption on Clean and Defective Anatase TiO2 (001

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Water Adsorption on Clean and Defective Anatase TiO (001) Nanotube Surfaces: A Surface Science Approach Stephane Kenmoe, Oleg Lisovski, Sergei Piskunov, Dmitry Bocharov, Yuri F. Zhukovskii, and Eckhard Spohr J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b11697 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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The Journal of Physical Chemistry

Water Adsorption on Clean and Defective Anatase TiO2 (001) Nanotube Surfaces: A Surface Science Approach Stephane Kenmoe,⇤,† Oleg Lisovski,‡ Sergei Piskunov,‡ Dmitry Bocharov,‡ Yuri F. Zhukovskii,‡ and Eckhard Spohr†,¶ †Department of Theoretical Chemistry, University of Duisburg-Essen, Universitätsstr. 2, D-45141 Essen, Germany ‡Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga LV-1063, Latvia ¶Center of Computational Science and Simulation, University of Duisburg-Essen, D-45141 Essen, Germany E-mail: [email protected]

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Abstract We use ab initio molecular dynamics simulations to study the adsorption of thin water films of 1ML and 2ML coverage on anatase TiO2 (001) nanotubes. The nanotubes are modeled as 2D slabs which consist of partially constrained and partially relaxed structural motifs from nanotubes. The e↵ect of anion doping on adsorption is investigated by substituting O atoms by N and S impurities on the nanotube slab surface. Due to strain induced curvature effects, water adsorbs molecularly on defect-free surfaces via weak bonds on Ti sites and H bonds to surface oxygens. While introduction of an S atom weakens the interaction of the surface with water, which adsorbs molecularly, the presence of an N impurity renders the surface more reactive to water with proton transfer from the water film and formation of an NH group at the N site. At 2ML coverage, a surface assisted proton transfer takes place in the water film, resulting in the formation of an OH group and an NH+ 2 cationic site on the surface.

Introduction Photocatalytic water splitting is considered as a clean and environmentally responsible way of satisfying the global energy demands. For this purpose, the ability of di↵erent photocatalysts to drive the water splitting reaction under sunlight illumination has been studied. Among these, TiO2 is one of the most-studied materials due to many beneficial properties such as chemical and optical stabil-

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ity, non-toxicity, low cost and abundance. 1 The higher efficiency of anatase among TiO2 polymorphs in photocatalytic applications 2 and the presence of water in operando conditions have motivated a considerable amount of studies on the anatase TiO2 /water interface for a better understanding of the elementary processes and for the improvement of its photocatalytic properties. 3 Crystalline anatase TiO2 grows naturally as a truncated octahedron exposing mainly the (101) and (001) facets, with the (101) facet covering more than 90 % of the surface. 4,5 The interaction of water with the predominant (101) and the less exposed (001) facets have been addressed both theoretically and experimentally. 3,6,7 The studies agree on the fact that the (001) facet shows a higher chemical reactivity than the (101) facet. The reason is that unlike the (101) facet, all surface species are undercoordinated on the (001) facet, confering it a higher surface energy, as shown in the Wul↵ construction for anatase TiO2 . 5 While on the (101) facet, molecular adsorption at the 5-fold coordinated Ti sites is favored from submonolayer coverage to bulk water, dissociative adsorption is prominent on the (001) facet. 8,9 Using a (2x2) supercell, Vittadini et al. 8 showed that water fully dissociates on the (001) surface at submonolayer coverage, while at monolayer coverage only one half of the water molecules adsorb dissociatively at the 5-fold coordinated Ti sites (Ti5c), with the OH radicals forming H bonds with the remaining water molecules located in an upper layer. A later study by Sumita et al. 9 showed that the adsorption mode in the contact water layer depends on the size of the supercell, as two consecutive Ti5c sites in the direction of

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[100] are required for dissociative adsorption to occur. Using a (3x3) supercell to model the interface of TiO2 (001) surface with bulk water in this later study, they found that intact water molecules and OH groups are present in the contact layer, forming strong and weak H bonds to molecules in a relatively mobile upper layer. In accordance with previous studies on water adsorption on TiO2 surfaces, this led to the conclusion that at low temperatures the second or third layer in the water films could already be considered as similar to the free water surface to the vacuum in a thicker film. Though e↵orts in characterizing the interface between water and TiO2 (001) 2D surfaces have hoisted the understanding of interfacial water as well as the properties of the TiO2 films 3 to a creditable level, very little is known on these properties for substrates of lower dimensionality. One of such are three monolayer thick TiO2 (001) nanotubes which are more stable than the corresponding 2D films, due to negative strain energy. 10 Also, nanostructuring TiO2 , e.g., by the formation of TiO2 nanotubes (NTs) with large surface area, can enhance the photocatalytic activity. In this respect, TiO2 (001) nanotubes were shown to be a promising candidate for photocatalytic water splitting as their visible-light-driven photocatalytic response can be tuned by N and S codoping. 11 Band gap engineering using anion impurities is known to be a practical way to tailor the optical response of nanotubes. 12 Particularly, N and S substitutional impurities increase the visible-light-driven photocatalytic response of TiO2 (001) nanotube, aligning suitably its bands relative to the water redox potential. 11 In reality, these nanotubes operate in an aqueous environment,

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hence the importance of simulating their surface in contact with water and together with its dynamic behavior. Yet information about the water/TiO2 NT interface is scarce, because of the difficulty to characterize this interface both at the theoretical and at the experimental levels. From the modeling point of view, the main issue is the size of the structures, since high photocatalytic performance can be expected, e.g., from a (001) TiO2 (36,0) nanotube co-doped with S and N, which then contain 648 atoms in a ring-like nanotube with diameter of 5 nm 11 and length of 0.7 nm, requiring a concomitant large number of water molecules. High-level quantum chemistry for this model is hardly achievable in aqueous medium, since the computational cost would be very high. Recently, a slab-based surface science approach, in which partially relaxed and partially constrained structural motifs from water-covered cylindrical NTs in a 2D periodic geometry have been proposed as an alternative to circumvent this difficulty. 13 Such a scheme, named the CFVS (constrained fixed volume slab) model in this recent work, can reduce the size of the system under study and allow for analysis of dynamics and excitations at the interface, while keeping characteristic nanotube properties. Though the efficiency of this approach is restricted to the description of water adsorption on the outer side of nanotubes with large diameters and relatively thick walls, it is nevertheless suitable for our purpose, since in reality most of the surface chemistry and dynamics take place on the outer part of the nanotube. Casarin et al. 14 proposed a 1D model in which only a fragment of the nanotube is included and which enables the study of water interactions with both sides of the nanotube. Such

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an approach is similar to the curved constrained fixed volume slab (CCFVS) model discussed in our recent study. 13 This model retains the original nanotube strain due to curvature and showed a good performance in reproducing the adsorption energy of the original tubular structure on both sides. However, it fails in describing the electronic structure properties, in particular the densities of state, which exhibit a strong mismatch due to unphysical properties near the kink sites and the ensueing changes due to varying distances from the kink. Experimentally, it is difficult to synthesize large high-quality anatase single crystals with a high percentage of (001) facets. Most of the synthesis methods are time-consuming and expensive. Recently, considerable progress in preparing anatase TiO2 with exposed (001) facets has been achieved by Lu et al. 15 who used hydrofluoric acid as a morphology-controlling agent to synthesize TiO2 single crystals with 47 % of (001) facets. From thereon, many experimental studies have followed, in which single-crystalline anatase TiO2 nanotubes, mainly with the exposed (001) plane were grown using di↵erent hydrothermal routes. Such nanotubes have shown enlarged photocatalytically active surface, enhanced conversion efficiency and improved electronic transport properties. 16,17 Photoinduced superhydrophilicity of anatase TiO2 nanotube surfaces 18–20 have motivated the study of environmental e↵ects on wetting properties. Using contact angle measurements and Fourier transform infrared (FTIR) spectroscopy, Shin and coworkers 21 showed that freshly anodized and annealed anatase TiO2 nanotubes have a hydrophilic character. Studying the e↵ect of contaminants upon

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air exposure, tube diameter, UV illumination and organic modifications of nanotube surfaces have enabled a better control of factors governing changes in wettability. Although a suitable combination of these factors allow, in principle, tuning the nanotube to acquire a desired wettability, ranging from superhydrophobic to superhydrophilic, 18–21 clear insight into the detailed atomistic interactions occurring at the interface is still missing. To shed some light on these open issues, studying the adsorption of water on nanotubes in vacuum is a primordial step towards understanding the complex mechanisms behind the wetting or photocatalytic water splitting processes. In the present paper, we address the structural properties, surface chemistry and dynamics of TiO2 (001) nanotubes covered with thin water films. Using a 2D slab representation of the nanotube which takes into account the strain and partial curvature of the nanotube, ab initio molecular dynamics calculations are performed to study the distribution of water molecules and the strength of their hydrogen bond network at the interface of the nanotube with water. As the presence of surface OH groups is crucial in most photocatalytic reactions, the impact of N and S dopants on the degree of dissociation of water molecules is investigated. The e↵ect of water coverage on these properties is investigated by increasing the thickness of the water film up to 2ML.

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Computational details The anatase TiO2 (001) nanotube was modeled by a slab of three O-Ti-O units separated by a vacuum region of 18.1 ˚ A. The atoms in the bottom O-Ti-O unit were kept fixed at their initial configurations in the nanotube, while the remaining ones were allowed to relax. The Ti and O atoms in the bottom layer were passivated with pseudo hydrogens. The importance of surface passivation with pseudo hydrogen on the structure and energetics of water-covered thin TiO2 films has previously been demonstrated by Kowalski et al. 22 An orthorhombic supercell with a (3x3) periodicity in the lateral directions x and y and dimensions 10.5 ˚ A ⇥ 12.6 ˚ A ⇥ 25.3 ˚ A was used to study the interaction of water films with the slab. (see Figure S1 in the Supporting Information for a graphical representation of the structure). This 2D slab model is used to simulate the interaction of a (36,0) nanotube of 5 nm diameter with a wall thickness of 6.5 ˚ A, corresponding to the slab thickness, surrounded by thin water films. It was recently shown that using this slab representation to simulate the adsorption of a water monolayer on the outer side of a TiO2 (001) nanotube with large radius retains the main structural and energetic features of the nanotube. 13 As a reference, we also studied the planar 2D interface. For this, a bigger supercell with dimensions 11.353 ˚ A ⇥ 11.353 ˚ A ⇥ 28.544 ˚ A was used as in a previous study by Sumita et al. 9 Due to the asymmetry imposed by the one-sided adsorption of water and the di↵erent terminations in the slab’s outermost layer and innermost layer, where the atoms are frozen, a dipole correction was introduced to cancel

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the electric field gradient in the vacuum. The CP2K/Quickstep package 23 was used to perform Born-Oppenheimer molecular dynamics simulations and static geometry optimizations. For the latter the BFGS minimizer was employed. The total energies were calculated at the

point only because of the

large size of the supercells. The Generalized Gradient Approximation (GGA) within its PBE formulation 24 was used to account for exchange correlation e↵ects. It is known that no GGA functional can simultaneously describe the structure, dynamics, and thermodynamics of liquid water. 25 Hopes in a fully satisfactory description may be achievable by dispersion-inclusive exchange-correlation functionals. 25–27 Therefore the PBE-D3 dispersion interaction correction as proposed by Grimme 28 was used here. Though this functional lacks structural softening due to its overbinding character, 25 its efficiency to describe the state of dissociation of water at TiO2 interfaces has been demonstrated. 29 The 3s, 3p, 4s, 3d electrons of Ti atoms and the 2s, 2p electrons of the O atoms were considered as valence electrons and the Goedecker-Teter-Hutter (GTH) pseudopotentials were used to treat the core electrons. The double-⇣ basis functions with one set of polarization functions (DZVP) were used as basis sets with a plane wave cuto↵ of 400 Ry. A Nosé-Hoover thermostat with a target temperature of 300 K and a time constant of 1 ps was used to impose NVT conditions to the system. A time step of 0.5 fs was used in Born-Oppenheimer molecular dynamics for a total simulation time of 20 ps after equilibration. The impact of dopants on the structure, reactivity and dynamics of the water films was investigated by replacing surface

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oxygens by one N, one S, or one N and one S atom simultaneously in the outermost layer of the surface. The N+S configurations were chosen by replacing two surface oxygen atoms 5.6 ˚ A apart along the diagonal direction of the 3⇥3 surface cell. This places the two dopants at the largest possible distance on the surfaces. The procedure corresponds to surface dopant concentrations of 1/18 and 1/9, respectively. To keep the whole system neutral, an extra H atom was added in the water film in case of N or N+S doping.

Results and discussions We start our investigation by studying the structure of the interface. Figure 1 shows the final snapshots after equilibrium trajectories lasting for 20 ps each. The images are (from left to right) for the interface of water in contact with the clean, the N-doped, the S-doped, and the N+S-codoped nanotube surfaces at coverages of 1ML (top) and 2ML (bottom). Water molecules bind on top of the five-fold coordinated Ti surface sites (Ti5c) and form H bonds to the two-fold coordinated O surface sites (O2c). On defect-free surfaces, water adsorbs molecularly. The situation is similar on Sdoped surfaces, but with the water molecules layer being buckled due to the larger size of the S atom. On N doped surfaces, an NH group is present on the N site at 1ML. At 2ML, a water molecule in the contact layer dissociates and transfers a proton via the Grothuss mechanism 30 to the neighboring NH group. This leads to the formation of an NH+ 2 cation at the N site and an OH group at the adjacent Ti5c site. On codoped surfaces, the NH group is also

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present at the N site. However, no further protonation is observed for this system. On all surfaces, we observe that there is a critical coverage of water molecules. Only 1/3 of the Ti5c atoms directly interact with water molecules. This observation was also pointed out by Sumita et al., 9 where full coverage of Ti5c with a monolayer of water could not be achieved. In the water film, molecules form H bonds to the surface and among each other. At 1ML, a disordered buckled water layer is formed; increasing the water coverage to 2ML leads to the formation of a multilayer structure.

pristine

N-doped

S-doped

N+S-codoped

1 ML

2 ML

Figure 1: Snapshots of equilibrium trajectories for the pristine, Ndoped, S-doped, and N+S-codoped nanotube surfaces covered with a single (top row) or two monolayers (bottom row). The density distribution of water molecules along the vertical direction is also investigated. We calculate the number density profiles of water oxygen and hydrogen atoms along the (001) direction, and the positions of Ti5c atoms is taken as reference (Figure 2). All Ti5c density profiles are centered around the 0 ˚ A value at monolayer (1ML) coverage. At 2ML water content, all peaks become broader. On the pristine surface and the N doped surface, the Ti5c density maximum splits into two peaks about 0.4 ˚ A apart. On the N+S codoped surface only a shoulder is visible at the larger dis11



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tance. These features of the Ti5c density are characteristic for the outward relaxation of only some Ti atoms in the presence of water. Note that the pristine surface also shows a small shoulder at about 0.2 ˚ A. The extent of these relaxation phenomena is a consequence of the interaction of the Ti atoms with water oxygen atoms. The O2c density profiles show single peaks localized around 0.6

0.7 ˚ A.

Thus the surface O2c atoms undergo very litte relaxation upon adsorption of water on the surface, independently of coverage. This originates from the weaker interaction of O2c sites with water via H bonds relative to the stronger interactions of Ti with water oxygen. Unlike on the two-dimensional flat surface, where the water oxygen profile shows distinct peaks, 9 the water oxygen distribution on the nanotube surfaces in Fig. 2 are broadened. They consist of double peaks centered at 2.3 ˚ A and 2.9 ˚ A for 1ML. The small separation between the two peaks indicates the presence of a buckled water layer at monolayer coverage. In fact, the maximum at 2.9 ˚ A is too far from the surface to be due to direct Ti-water interactions. Rather it appears to be due to water-water interactions. The first peak of the water oxygen density at 2ML coverage is located almost at the same position as the one at 1ML. The second peak, however is shifted to about 3.6 ˚ A; it is significantly broadened and extends to large coordinates, which shows the tendency of water molecules to form multilayers. The H atom profile supports the formation of the buckled monolayer feature at 1ML. It shows a main peak with its maximum between the two oxygen maxima; there is a shoulder on each side of the main hydrogen peak. These shoulders have di↵erent relative in-

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tensities for the di↵erent surfaces. The main hydrogen maximum at about 2.5-2.7 ˚ A is located about 2 ˚ A above the position of the O2c maxima and represents hydorgen atoms that form inter-water hydrogen bonds within the buckled (bilayer) of water. The shoulders at around 2.1 ˚ A indicate the formation of strong hydrogen bonds to the O2c surface atoms. The corresponding shoulders around 3.5 ˚ A represent the dangling OH bonds of the water layer pointing away from the surface. At 2ML, the hydrogen profile is less regular. The atom densities range to about 6 ˚ A and indicate the beginning formation of a multilayer film. n0s0 ⇢(z) / Å

0.20

n1s0 undoped

3

O H Ti

0.15

⇢(z) / Å

0.20

N doped

3

O H Ti N

0.15

0.10

0.10

0.05

0.05 n0s1

n1s1

0.15

6 z/Å S doped 0.20 O H Ti 0.15 S

0.10

0.10

0.05

0.05

0

1

0.20

0

1

2 3 ⇢(z) / Å 3

2

3

4

4

5

5

6

z/Å

0

1

2 3 ⇢(z) / Å 3

4

5

6 z/Å N+S doped O H Ti N S

0

1

2

3

4

5

6

z/Å

Figure 2: Number density profiles for: the defect-free (top left), N doped (top right), S doped (bottom left) and N+S codoped (bottom right) nanotube surfaces. The zero is defined to be located 4.9 ˚ A above the (rigid) bottom Ti layer; it corresponds approximately to the position of the top Ti layer. Lines with symbols are for 1ML water coverage, lines without symbol for 2ML coverage. Note that below about 1.1 ˚ A, the red oxygen curves represent O2c oxygen atoms, above 1.8 ˚ A they represent water oxygen atoms. Similar features are observed in the case of pure N-doping (Figure 2, top right), except for the presence of an N peak at 0.7 0.9 ˚ A. The N peak is shifted outward by about 0.2 ˚ A, indicating that the N impurity interacts more strongly with the aqueous phase than 13



the regular O2c sites. Furthermore, the small-coordinate tail of the water hydrogen density profile extends almost towards the position of the O2c and N density maxima. This shows that NH and OH bonds are present in the contact layers (see Fig. 1). The position of the N peak shifts even further outward from the surface when the water coverage increases to 2ML. This enhanced interaction is confirmed by the formation of an NH+ 2 entity. In the presence of the S co-dopant, no NH+ 2 is formed and the N profile remains at positions characteristic for a surface NH group. In fact, S doping makes the Ti5c sites in general less reactive. As can be inferred from Figure 2, S doping leads only to a single, somewhat broader density maximum where the characteristic splitting, or the pronounced shoulder, which are present in the sulphur-free case, is absent. Due to less efficient sp hybridization compared to O and N atoms, the S atom density relaxes more strongly (by about 0.8 ˚ A) outward than the O2c and N densities. Consequently, the sulfur position at around 1.5 ˚ A moves close to the onset of the water oxygen density peak at about 2 ˚ A, which leads to less ordered water multilayer, as illustrated in the broader Ow and Hw profiles. To gain deeper insight into the observed structural properties, characteristic radial distribution functions (RDFs) were calculated. On defect-free surfaces, where molecular adsorption has been observed on Ti5c sites, the peak positions in the Ti5c-Ow RDFs are located at 2.1 ˚ A and 2.2 ˚ A, respectively, at 1ML and 2ML (Figure 3, top). This shows that water binds coordinatively to the Ti5c sites. In the O2c-Hw RDFs (Figure 3, bottom), the first peak is centered around 1.9 ˚ A, which indicates the presence of strong H bonds

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between the water molecules and the O2c surface sites. Thus, on defect-free surfaces, water binds molecularly via coordinative bonds to the Ti sites and via hydrogen bonds to the oxygen sites. gT i5cO (r)

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gTi5c

pristine

O

S doped

6 4 2 gHWO c (r) 1 4

gO2c

2

3

4

H

5 pristine

r/Å

S doped

3 2 1

1

2

3

4

5

r/Å

Figure 3: Radial distribution functions gTi O (r) between surface Ti and water oxygen atoms (top) and gO H between and surface oxygen atoms and water hydrogen atoms on defect-free (red) and S-doped (blue) nanotube surfaces. Lines with symbols are for 1ML coverage, full and dashed lines for 2ML water coverage. It is known that the state of dissociation of water on TiO2 strongly depends on the supercell size and slab thickness. 3 For the anatase (001) surface modeled with a 4 TiO2 layered slab, partial dissociation was found at the interface with bulk water by Sumita et al. 9 They found no further change in the structure and adsorption mode upon increasing the slab thickness to 5 layers. Thus, for comparison, we also studied the interface of the 4 TiO2 layered slab surface with a single monolayer of water, using the same 15



supercell model as in Ref. 9 and the same settings as described in section 2. We also observed partial water dissociation in this case (see Figure 4 top left), and the interface looks similar to the contact layer at the interface with bulk water. 9 Our nanotube slab model consists of 3 TiO2 layers. Thus, to provide insight into the e↵ect of nanostructuring and curvature on the adsorption, we also simulated the adsorption of a monolayer of water on a 3 TiO2 layered slab surface and still observed partial dissociation (Figure 4 top right). We observe no qualitative change in the surface chemistry of the contact layer when decreasing the thickness from 5 to 3 TiO2 layers or when decreasing the water coverage from a ’bulk’ film to the monolayer. Thus, we can conclude that neither nanostructuring (which here corresponds to curvature e↵ects and the reduced (3 layer) slab thickness) nor solvation (water coverage) alters the interface chemistry. A similar observation also applies at low coverage. These conditions we investigated by simulating the adsorption of a single water molecule in a (2x2) supercell using a 3 TiO2 layer slab surface (Figure 4 bottom right). As observed earlier in Ref. 8 using a 4 layered slab, the water molecule spontaneously dissociates and transfers one proton to the titania surface. Similarly, the mode of water adsorption on the nanotube surface was also investigated from single molecule to monolayer coverage. At submonolayer coverage, static geometry optimizations where performed. The starting configurations were generated by systematically constructing low energy structures, considering molecular adsorption of intact water molecules, as well as dissociation into an OH group bound at Ti sites and a hydrogen adsorbed at a sur-

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3 layered-slab

4 layered-slab

1ML

1/4 ML

Figure 4: Characteristic snapshots of water at 1ML and 1/4ML coverage, adsorbed on a 4-layered and a 3-layered anatase TiO2 (001) 2D slab surface, respectively. face oxygen site. Figure 5 shows the most stable adsorption configurations at 1/4 ML and 1/2 ML for molecular and dissociative adsorption. In the regime of isolated molecules (1/4 ML) and at 1/2 ML coverage corresponding to 1 and 2 water molecules in a (2x2) supercell, respectively, molecular adsorption is prefered. The metastable structures can be found in the Supporting Material. To understand the factors favoring molecular adsorption over dissociative adsorption, we further analyzed contributions to the binding energies of the most stable structures (Table 1), following the decomposition scheme described in a recent study. 31 The corresponding values of these contributions for the 2D flat surface are also reported for comparison. For dissociated water molecules with O–H distances larger than 1.6 ˚ A, spin polarization was taken into

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1/4 ML

1/2 ML

-0.70 eV

-0.68 eV

-0.29 eV

-0.35 eV

M

D

Figure 5: Most stable adsorption configurations on the nanotube slab surface for molecular (M) dissociative (D) adsorption at submonolayer coverages of 1/4ML and 1/2ML account in the evaluation of the water relaxation energies. Due to the overbinding character of PBE-D3, the bond dissociation energy of 5.99 eV slightly overestimates the experimental value and that of high-level calculations (5.46 eV and 5.29 eV, respectively). The large distance between the dissociated hydrogen atom and the OHgroup (3.3 ˚ A) results in very high water relaxation energies (5.99 eV). From Table 1 it can be seen that at 1/4ML, the energy corresponding to adsorbing and dissociating a relaxed gas-phase water molecule on the frozen TiO2 surface (neglecting water–water interactions) can be obtained by combining the interaction energy of water with the surface E(TiO2 /H2 O) and the relaxation energy of water E(H2 O). The resulting value for the isolated dissociated water molecule (-2.37) eV emphasizes the high strength of this ad18


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sorbate–surface interaction via one Ti5c-OH bond and one O2c-H bond. The corresponding value for undissociated water, which is adsorbed via one Ti5c-O bond and two weak H-bonds to the surface, is -0.72 eV. The very strong interaction with the surface in case of dissociation requires large modifications in the substrate geometry reflected by the high relaxation energy E(TiO2 ) = 2.07 eV, compared to only 0.01 eV for molecular adsorption. Thus, the gain in water–surface interaction energy due to dissociation (-1.65 eV) is less than the increase in relaxation energies required to form the dissociated structure (2.06 eV); therefore, the dissociation of an isolated water molecule is not favourable on the TiO2 (001) nanotube surface. Meanwhile, on the flat surface where spontaneous dissociation of single molecules is observed (Figure 4 bottom left), less unfavorable adsorption-induced substrate relaxations (1.48 eV vs. 2.07 eV) occur. The substrate relaxations due to dissociation on the nanotube correspond to a 0.18 ˚ A extension of the Ti-Ti distances along the nanotube axis and a 0.14 ˚ A contraction of the same distances along the corresponding direction for dissociative adsorption on the flat surface. Similar trends are observed at 1/2ML. For thin water films of 1ML coverage on the nanotube surface modeled by a (3x3) supercell (see above), the large number of possible structures and the complexity of the H-bonding networks preclude a rational construction of all possible arrangements for water. Therefore, snapshots of MD runs were taken at regular time intervals (after each 1ps) and subjected to geometry optimization. We found that molecular adsorption still prevails, as no dissociation is observed. The absence of dissociative adsorption on the nanotube

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surface holds for our slab model, as well as for the original nanotubes with (0,36) chirality. Based on these observations, it can be concluded that molecular adsorption on the nanotube is most likely a curvature e↵ect due to strain-induced changes in the electronic structure of the slab, which leads to weaker interactions which move the equilibrium position of the molecules slightly away from the nanotube surface relative to the unstrained 2D TiO2 /water interface. The binding energies of equilibrium configurations at 1ML coverage are found to be in the range of -0.7 eV to -0.8 eV, showing that only weakly bound intact water molecules are present on the nanotube surface. These findings are consistent with recent NMR measurements, 32 which also concluded that on water covered TiO2 (001) nanotubes only intact water molecules are present. Table 1: Binding energy Ebin decomposition of most stable adsorption configurations on the nanotube slab surface for molecular (M) and dissociative (D) adsorption at submonolayer coverage (1/4ML and 1/2ML) into four contributions: (i) surface/water interaction energy E(TiO2 /H2 O), (ii) water/water interaction energy E(H2 O/H2 O), (iii) surface relaxation energy E(TiO2 ), and (iv) water relaxation energy E(H2 O). All energies are in eV. Supercell Coverage (ML) 1/4 1/4 n (H2 O) 1 1 Dissociation M D E(TiO2 /H2 O) -0.85 -8.36 E(H2 O/H2 O) -0.01 -0.01 E(TiO2 ) 0.01 2.07 E(H2 O) 0.13 5.99 a Ebin -0.70 -0.29

a

(2x2) 1/4b 1/2 1/2 1 2 2 D M D -8.61 -0.69 -8.04 -0.01 -0.06 -0.08 1.48 0.005 1.78 5.92 0.065 5.99 -1.22 -0.68 -0.35

Binding energy per molecule: at 1/2ML the water relaxation energies are

20


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identical, and the average is reported; b

flat unstrained 2D surface

Doping with nitrogen promotes proton transfer to the surface. After introducing an N dopant on the surface (replacing one O2c atom) and adding an extra H atom inside the water film, proton transfer takes place from the hydronium ion to the surface N atom after 0.5 ps. This is obvious from the N-Hw RDF (Figure 6 top left) where the first maximum is centered at the characteristic NH bond distance of 1 ˚ A. The running integration numbers n(r) of the RDF in Figure 6 top right with their value of ⇡1 or larger at r=1.5 ˚ A show that this process in all cases is complete and irreversible. At 2ML water coverage and in the absence of sulfur, a second proton transfer from one of the water molecules takes place to form an ˚ NH+ 2 group (the N–H running integral is 2 at 1.5 A in this case) and an OH ion in the first water layer, which binds strongly to a Ti5c surface atom as illustrated by the peak at 1.9 ˚ A in Figure 6 bottom left. These observations are consistent with FTIR spectra profiles of N-doped TiO2 nanocatalysts prepared by the solvothermal approach, which were reported in Ref. 33 There, the authors observed a peak at around 1630 cm 1 , which they assigned to the bending vibrations of O—H and N—H. In addition, a broad band at 3400 cm-1 corresponding to surface hydroxyl groups and absorbed water molecule was observed. Figure 7 shows the larger exclusion volume around the S atom, as can be seen from a comparison with gO2c

21

Ow

in Fig. 6 (bottom



gN

gN

8

H (r)

gN N doped

HW

8

N+S doped

6

6

4

4

2

nN

HW (r)

HW

N doped N+S doped

2 gO2c

gT i5cO (r) 1 8

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gTi5c

2

3

4

Ow

5 N doped

r/Å

1 4

N+S doped

6

3

4

2

2

1

1

2

3

4

5

r/Å

gO2c

2

3

Hw (r)

4

Hw

5 N doped

r/Å

N+S doped

1

2

3

4

5

r/Å

Figure 6: Selected radial distribution functions for N doped and N+S codoped nanotubes at 1ML and 2ML water coverage. right). Water in the vicinity of the S site interacts only weakly with the sulfur atoms and thus with the surface, as can be seen from the shift of the first maximum from the O-H RDF 1.8-1.9 ˚ A to the S-H RDF at 2.2-2.3 ˚ A. Consistent with the larger distance the height of the S-H maximum is smaller than the O-H and the N-H maxima in Fig. 3 and 6 on the defect-free and N doped surfaces. Thus, S doping weakens hydrogen bonding from water to the surface. This shows that weakening the water-surface interaction due to the presence of S dopant is not a local e↵ect and could explain why in the case of N and S co-doping the surface-assisted proton transfer observed on the N-doped surface at 2ML does not take place, and no OH groups are present. The weaker interactions between water and the S doped TiO2 can be regarded as an indication for low reactivity compared to N doped TiO2 . Such a low reactivity of S-doped anatase (001) surfaces was also reported in an experiment carried out by Gang et al., 34 where it is seen that even under visible light irradiation, these surfaces contain only a small concentration

22


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of OH groups compared to thegN doped (r) (001) surfaces. S Hw

4

gS

S doped

HW

N+S doped

3 2 1 gS 1 8

nS

2

Hw (r)

3

4

5

r/Å

HW

6

S doped N+S doped

4 2

1

2

3

4

5

r/Å

Figure 7: Radial distribution functions (top) and running integrals (bottom) for S doped and N+S codoped nanotube surfaces. The results reported above also allow a qualitative understanding of the curvature-induced superhydrophilicity of anatase TiO2 nanotubes, as reported in many studies. 18–20 Balur et al. 18 showed that the formation of nanotubular TiO2 layers from the originally flat hydrophilic TiO2 surface significantly changes the surface wetting properties, which become super-hydrophilic. Sumita et al. 9 showed that the flat TiO2 (001) surface interacts with water only via the formation of surface OH

groups and a few molecularly

adsorbed water molecules on Ti sites, while many of the undercoordinated surface atoms do not interact with water. However, our investigations showed that water forms strong H bonds to these undercordinated surface O sites in addition to being molecularly ad23



sorbed on Ti sites as shown in Fig. 3 (bottom) and Fig. 6 (bottom right). Even in the case of S doping, where the interaction with the surface is weakened, H bonds to the surface are still formed in the vicinity of S site. This agrees with the complete (super-hydrophilic) spreading of water on the nanotube surface as observed in. 18

Conclusion Our molecular dynamics simulations show that on the defect-free anatase TiO2 (001) nanotube surface, water adsorbs molecularly via weak interactions with the Ti sites and hydrogen bonds to surface oxygens. This binding mode is a consequence of strain-induced curvature e↵ects, as our investigations have shown that neither the nanostructuring by slab thickness reduction nor solvation of thin planar TiO2 (001) films alters the surface chemistry of the partially dissociated contact layer. The water molecules form a relatively strong hydrogen bond network both for a buckled monolayer and for a multilayer-like (2ML) film. While doping with sulfur, at the atom fraction (1/18) studied, weakens the interactions between the surface and water, nitrogen doping renders the surface more reactive to water, with proton transfer to the surface and formation of an NH group at the N site taking place in every studied case. At 2ML coverage, even a second surface-assisted proton transfer takes place within the water film, resulting in the formation of an OH group and an NH+ 2 ion on the surface. This e↵ect is present only in the absence of sulfur co-doping, due to the generally weaker water-nanotube interactions. The consequences of this behavior

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for the density of excited states will have to be investigated in a forthcoming publication.

Acknowledgements This study was supported by the ERA.Net RUS Plus project No. 237 Watersplit. The authors gratefully acknowledge computing time granted by the Center for Computational Sciences and Simulation (CCSS) of the Universität of Duisburg-Essen and provided on the supercomputer magnitUDE (DFG grants INST 20876/209-1 FUGG, INST 20876/243-1 FUGG) at the Zentrum f¨ ur Informationsund Mediendienste (ZIM). ES is also grateful for support by the Cluster of Excellence RESOLV (EXC1069) funded by the Deutsche Forschungsgemeinschaft.

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Water Adsorption on Clean and Defective Anatase TiO2 (001

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