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Spatial distribution of adsorbed water layers at the TiO2 rutile and anatase interfaces
Chemical Physics Letters 554 (2012) 102–106
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Spatial distribution of adsorbed water layers at the TiO2 rutile and anatase interfaces Ritwik S. Kavathekar a, Niall J. English a,b,⇑, J.M.D. MacElroy a,b a b
The SFI Strategic Research Cluster in Solar Energy Conversion, School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland Centre for Synthesis and Chemical Biology University College Dublin, Belfield, Dublin 4, Ireland
a r t i c l e
i n f o
Article history: Received 23 May 2012 In final form 1 October 2012 Available online 17 October 2012
a b s t r a c t The spatial distribution functions of adsorbed water layers at hydrated rutile-(1 1 0) and anatase-(1 0 1) surfaces have been studied at 300 K, using equilibrium classical molecular dynamics. It was found that there is evidence of some hydrogen-bonding with water molecules outside the adsorbed layer for anatase-(1 0 1), but less for rutile-(1 1 0) – on average, about 1.6 per water molecule in anatase-(1 0 1), in contrast to 0.9 for rutile-(1 1 0). This is due to adsorbed water molecules being confined more firmly in the area between bridging oxygen atoms at the rutile-1 1 0 surface, while the more open-like nature of anatase-1 0 1 allows for greater contact with molecules outside the first layer. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Since Fujishima and Honda discovered that TiO2 could split water under visible-light irradiation to produce hydrogen and oxygen gas [1], the study of the properties of aqueous solutions in contact with TiO2 interfaces has undergone a significant increase in activity. There are many potential applications for renewable energy using photo-electrochemical water splitting and in dyesensitised solar cells, not only for TiO2, but also for a range of other semiconductor-water interfaces; often, these will exploit supportmetal-support-interaction (SMSI) modification of photo-catalytic properties [2]. Although TiO2 is one of the most studied metal oxides in the literature, there is a rather limited understanding of interfacial water molecules’ behaviour and reactivity at interfaces, despite recent progress [3,4], including via theoretical and molecular simulation methods [5]. Such TiO2-water interfaces provide a rich environment for investigation of confinement of water molecules’ motion; this is particularly the case where hydrogen-bonded molecules play an important role in stabilising solutes via solvent interactions and in forming ‘cages’ [3,4]. For instance, experimental vibrational spectra of adsorbed water on both rutile rods and anatase powder have been reported via inelastic neutron scattering (INS) measurements, concluding that confined adsorbed water molecules exhibit vibrational and dynamical features closer to less mobile ice compared to their behaviour in the liquid state [6,7]. Molecular dynamics (MD) has been useful to some extent in characterising the dynamical and vibrational behaviour of adsorbed water molecules on rutile-(1 1 0) and anatase-(1 0 1) surfaces [8,9]; ⇑ Corresponding author at: School of Chemical and Bioprocess Engineering, Centre for Synthesis and Chemical Biology University College Dublin, Belfield, Dublin 4, Ireland. Fax: +353 1 716 1177. E-mail address: [email protected] (N.J. English). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.004
indeed, ab initio MD (AIMD), especially, has offered key insights into the librational motion of higher-frequency modes of water adsorbed to titania in very interesting recent studies [9,10]. However, the somewhat restrictive computational limits on the duration of AIMD simulation makes it difficult to ascertain lower-frequency translational modes for a desired level of accuracy. Recently, we have carried out longer classical MD [13] simulations to study, at least qualitatively, intramolecular strain in confined, adsorbed water molecules at various titania surfaces, in addition to orientations of the water dipoles with respect to the surface normal [11], as well as to reproduce vibrational spectra results of confined layers at the surfaces in reasonable agreement with INS spectra [12]. The rutile-(1 1 0)-water interface, in particular, has been studied in much detail in recent years via MD [14–19]. Zhang et al. have investigated ion adsorption [14], while Predota et al. have probed the electric double layer structure in the vicinity of rutile-(1 1 0) surfaces [15–17], and have studied the nature of the water layer structure [15], ionic adsorption [16], and viscosity and diffusivity of the water layers [17]. Machesky et al. have studied surface protonation effects [18]. In all of these experimental and simulation studies of relatively confined water layers, the hydrogen bonding properties with the bridging oxygen atoms at the titania surfaces, as well as with other water molecules, has been remarked as an influential factor in leading to this behaviour distinct from bulk liquid water. This has distinct ramifications on the structural features of these adsorbed water layers, and this study seeks to probe the threedimensional spatial distribution of the water layer to study its anisotropic nature. Although pair distribution functions (PDF), like the radial distribution function (RDF), have enjoyed popular use to represent nearest-neighbour information, their spatial analogue, the spatial distribution (SDF), has been relatively less conspicuous. The structural information of angular and radial distribution of molecules
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represented in the SDF, g(r, h, u) allows for the study of anisotropy of these density distributions not afforded by PDFs or RDFs. The use of SDFs were first reported by Svishchev and Kusalik [19] for liquid water in the form of pair-density isosurfaces maps and histograms of angular dependence of h, the angle made by the water molecule’s dipole vector (the local-frame z-axis [19]) and the separation vector between two water molecules’ oxygen atoms, and u, the angle made by the local-frame x-axis (H–H axis) projection of the water separation vector onto the local-frame x–y plane (with the y-axis being orthogonal to the dipole and H–H axes, cf. Ref. [19]). Such analysis of distributions has been performed for various species, namely for water molecules surrounding nucleic acid base pairs [20], RNA molecules [21], for cholesterol molecules and around proteins [22], but predominantly evaluated for liquid and amorphous low- and high-pressure ice and water [23–25]. We report here the three-dimensional isosurface SDF between the water molecules of the adsorbed layer and also with those outside this layer, at TiO2 rutile-(1 1 0) and anatase-(1 0 1) surfaces along with liquid water. The method for plotting SDFs as given by Svishchev and Kusalik [19] was followed to generate the 3D isosurfaces and g(r, h, u) histograms.
2. Simulation methodology We have performed a 100 ps NVT equilibrium classical molecular dynamics (CMD) simulation [13] for rutile-(1 1 0) and anatase-(1 0 1) TiO2 surfaces. The 3-D Ewald method was used for long-range non-bonded interactions with a precision of 10 5. The Ewald 3DC [26] method is a better approximation to the plain Ewald method for slab geometries, and will result in faster simulation and more accurate convergence of long-range Coulombic interactions; however, the 3-D Ewald technique used in this work would not be much different, although it is less computationally efficient. A Nosé–Hoover NVT ensemble at 300 K using VelocityVerlet integration with a 0.33 fs timestep was used. Bulk liquid water was equilibrated for about 200 ps via the Anderson–Hoover NPT ensemble before performing a 100 ps NVT run at 300 K with a relatively mild thermostat coupling period of 0.5 ps, with the appropriate number of water molecules added to achieve the appropriate bulk-like density of ca. 1 g/cm3 in the region between the titania surface and its periodic image. The Matsui-Akaogi (MA) [27] potential model was used for the titania surfaces and a flexible-SPC (FwSPC) [28] model for the water molecules were used. The cross parameters for Ti–Ow were obtained by using the same Buckingham potential and O–Ow LJ potential (see Table 1) [12] as used by Bandura and Kubicki, These have been previously used to
show qualitative comparisons between experimental and reported vibrational density of states and hydrogen bonding on similar surfaces [29]. A recent report on refining the TiO2–water interaction parameters, along the same lines as Bandura and Kubicki, but with explicit interaction with the water hydrogen atoms and TiO2 slab, has been reported by Alimohammadi et al. [30]. The MA potential consists of Buckingham interactions (see Table 1) which enables one to apply the potential to a stoichiometric slab model. All slabs used in the simulation were unconstrained. The details of the system sizes and simulation box dimensions are specified in Table 2. Similar simulations were also carried out for bulk liquid water. All surfaces were cut from a bulk rutile with lattice vectors a0 = b0 = 4.593 Å, c0 = 2.959 Å (P42/MNM) and a bulk anatase with lattice vectors a0 = b0 = 3.776 Å and c0 = 9.486 Å (I41/AMD). The details of the construction and topography of modeled surfaces are explained more fully elsewhere [11]. Rutile-(1 1 0) and anatase(1 0 1) is the most stable and studied titania surfaces, for which considerable experimental data on water layer arrangements are available. The rutile-(1 1 0) surface was cut for charge compensation to yield a non-polar and dipole-free surface [11] (cf. Fig. 1(a)). This leads to under-saturation of surface Ti atoms in its coordination. Rutile-(1 1 0) has a natural slanting angle with respect to the [0 0 1]-direction (or the laboratory z-direction, i.e., the direction of heterogeneity is orthogonal to the surface in the x–y plane, cf. Fig. 1(a)), which was corrected by aligning the entire slab orthogonal to the z-axis, so as to facilitate an orthorhombic periodic box. Anatase is the most photoactive polymorph of TiO2 and is more stable at nanoscale dimensions than rutile [31], and hence is an important surface for this study; the anatase-(1 0 1) surface is also tilted at an angle and was aligned vis-à-vis the z-axis (cf. Fig. 1(b)). We follow the method given in Ref. 19 to generate SDF plots and histograms, but a more general and complete method is also given by Bergmen et al. [32] for volumetric imaging. The 3D isosurface plots were prepared as Gaussian cube files [33]. The contour surfaces represent SDF values above a threshold such that 30% of the molecules are included in the SDF density plots [23], representing the main orientation in the shell corresponding to the selected distance (r) in the RDF. The O–O SDFs are plotted at distances guided by extrema in the corresponding RDF of (a) between water molecules present in the adsorbed layers [11,12], (b) between the water molecules in the first layer and rest of the water molecules, and (c) between the water molecules in the adsorbed layer and those outside it. These RDF used to guide the sampling distances for SDF plots are also reported (vide infra) as shown in Fig. 2. The SDFs were calculated in distance ranges of ± 0.05 Å about each nominal O–O separation to ensure good statistics. To assess for
Table 1 Force-field parameters. Buckingham potential for TiO2 and water oxygen: Aij exp( rij/qij) i–j Aij (kcal mol 1) Ti–O 391049.1 Ti–Ti 717647.4 O–O 271716.3 Ti–Ow 28593.0 Lennard–Jones potential for water: eij[(rij/rij)12 (rij/rij)6] i–j eij (kcal mol 1) Ow–Ow 0.1554
Cij/rij6
qij (Å)
Cij (kcal mol 290.331 121.067 696.888 148.000
0.194 0.154 0.234 0.265
rij (Å) 3.165492
2
Harmonic potential for water: k/2 (rij r0) i–j kij (kcal mol Ow–Hw 1059.162
1
Å
2
)
Harmonic angle bending potential for water: k/2 (h–h0) i–j–k h0 deg H–O–H 113.24 Atomic charges: q(Ti) = 2.196 e, q(O) =
1.098 e, q(Ow) =
r0ij (Å) 1.012 k (kcal mol 75.900
1
rad
2
)
0.82 e, q(Hw) = 0.41 e; Ow, Hw = water oxygen and hydrogen atoms
1
Å6)
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Table 2 Details of geometries used. Phase (surface), X, Y, Z (Å)
System size
Rutile (1 1 0) 26.26, 45.47, 69.490 Anatase (1 0 1) 71.46, 26.43, 72.680
(TiO2)630 (H2O)2000 (TiO2)1176 (H2O)3162
Figure 1. Layout of various titania surfaces, as discussed in the text; the laboratory z-direction is vertical. Here, Ob denotes a bridging oxygen, O3c a three-coordinated surface oxygen, Ti5c a penta-coordinated surface Ti atom (a lone coordinatively unsaturated site), while Ti6c stands for a hexa-coordinated Ti atom. (a) rutile-1 1 0, and (b) anatase-1 0 1.
the presence of hydrogen bonds, the geometric criteria of Luzar and Chandler were used [34].
3. Results and discussion We present the structural analysis of O–O SDF of liquid water and water adsorbed at rutile-(1 1 0) and anatase-(1 0 1) interfaces. For a given O–O separation, it is clear that in liquid water (Fig. 3), the probability of a nearby water’s oxygen being present is substantially larger than for the absorbed layer at either of the
titania interfaces (cf. Figs. 4 and 5). The number of (hydrogenbonded) nearest-neighbours is higher in liquid water (ca. 3.8, on average), with tetrahedral ordering observed at 2.7 Å O–O separation in the four tetrahedral-like ‘lobes’. The difference between anatase-(1 0 1) and rutile-(1 1 0) is rather striking. There is some evidence of hydrogen-bonding with water molecules outside the adsorbed layer for anatase-(1 0 1) (cf. Fig. 4(c) at 2.7 Å), but less for rutile-(1 1 0). This was confirmed by calculating the average number of hydrogen bonds formed with water molecules outside the adsorbed layer (as well as with bridging oxygen atoms in titania and with any other water molecules in the adsorbed layers), and these data are provided in Table 3. It can be seen that there are almost two hydrogen bonds formed with water molecules outside the adsorbed layer in anatase-(1 0 1), while there is almost one per adsorbed water in rutile-(1 1 0) – half that seen for anatase(1 0 1). This is seen in a somewhat less direct manner in the RDFs of Fig. 2, where there is a greater peak height at an O–O separation of around 2.8 Å with water molecules outside the adsorbed layer for anatase-1 0 1 (ca. 4) compared to around 2.8 for rutile-1 1 0; however, this observation is not as strong as the direct measure of the number of hydrogen bonds with water molecules outside the adsorbed layer, as it relates strictly to water O–O contacts. This is due to adsorbed water molecules being ‘perched’ more firmly in the area between bridging oxygen atoms in rutile-1 1 0 (cf. Fig. 1(a)), as opposed to the more ‘terraced’- or ‘corrugated’-like nature of the anatase-1 0 1 surface (cf. Fig. 1(b)), which is more open and allows for greater hydrogen bonding contact of adsorbed molecules with those outside the first layer. In contrast, rutile-1 1 0 does display two distinct nearest-neighbour water-water contacts at around 2.8–3.0 Å O–O separation (cf. Fig. 5(a)), but comparatively less with those outside the layer (cf. Fig. 5(b), and also the RDF on the left of Fig. 2 for peak heights of about 8.7 and 2.8 for contact with other adsorbed-layer water molecules and those outside this layer): this arises due to ‘stacking’ of absorbed water molecules laterally, in between the bridging oxygen atoms of titania, and is anisotropic, due to dipolar orientation that is essentially normal vis-à-vis the surface [11] between ‘ridges’ of bridging oxygen atoms (in contrast to largely parallel for anatase-(1 0 1) [11]) – the distance between nearest-neighbour Ti5c atoms is about 2.96 Å. It was found that the two nearestneighbour water-water contacts at around 2.8–3.0 Å O–O separation in rutile-1 1 0 within the adsorbed layer were not hydrogen-bonded (cf. Table 3, as can also be seen from the hydrogen atoms not pointing to the associated oxygen atoms in the
Figure 2. O–O RDFs of rutile-1 1 0 (on the left of the panel as ‘R110’) and anatase-1 0 1 (on the right-hand side as ‘A101’). ‘Layer 1 only’ denotes water-water contacts within the adsorbed layer only, while ‘Layer 1 and other’ denotes water-water distances between those of the adsorbed layer and other molecules outside this layer (up to 15 Å from the surface). ‘All Ow atoms’ means water-water contacts between those in the adsorbed layer and all other water molecules up to 15 Å from the surface including other water molecules in the adsorbed layer.
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Figure 3. O–O SDFs of bulk liquid-like water molecules at distances of 2.7, 3.2, 3.7, 4.44 Å.
Figure 5. O–O SDFs of reference layer-water molecules with other sets of water molecules at the rutile-(1 1 0) interface at 2.79 and 2.99 Å separation. SDFs are shown with other adsorbed-layer molecules on the left of the panel, and with only non-layer molecules on the right.
Table 3 Numbers of hydrogen bonds of various types for adsorbed layer of water molecules.
Figure 4. O–O SDFs of reference layer-water molecules with other sets of water molecules at the anatase-(1 0 1) interface at 2.7, 3.0, 3.3, 3.7, 4.6 Å separations. The vertical column shows SDFs with (A) other adsorbed-layer molecules, (B) only nonlayer molecules, and (C) all other molecules (both absorbed-layer and non-layer).
neighbours). There was also scant evidence of very limited and transient hydrogen bonding between nearest-neighbour water molecules in the adsorbed layer in anatase-(1 0 1) (cf. Table 3) – this arises due to the more open structure of anatase-(1 0 1) allowing transient water-water contacts due to lateral movements back and forth along the surface, unlike the ‘perched’ nature of rutile(1 1 0), even though the O–O distance range is greater at 3–4.5 Å than the corresponding nearest neighbor distances of 2.6–3.4 Å. These structural studies of anisotropic water orientations also help to rationalise earlier observations of less mobility in the water layer at rutile-1 1 0 interfaces vis-à-vis anatase-1 0 1 (and other titania interfaces) [12]. The presence of the bridging oxygen ‘ridges’ in rutile-1 1 0 impede diffusive motion of this layer along the surface. This is less prevalent in the anatase-1 0 1 surface, where the SDFs show more similarity to bulk-like liquid conditions. Of course, the importance of hydrogen bonds of water in the adsorbed layer with bridging oxygen atoms is important for both surfaces [5,9– 11,29] (the kinetics of which have been studied in Ref. 29), so this conclusion is necessarily tentative. There is a slightly larger extent
Surface
Bridging oxygens
Adsorbed-layer waters
Waters outside adsorbed layer
Rutile (1 1 0) Anatase (1 0 1)
0.11 ± 0.01 0.18 ± 0.02
0 0.05 ± 0.01
0.91 ± 0.05 1.59 ± 0.08
of hydrogen bonding to bridging oxygen atoms for anatase-(1 0 1) at an average of 0.18 relative to rutile-(1 1 0) at 0.11, which is explained by the more parallel arrangement of water molecules to the surface in anatase-(1 0 1) (as characterised by the dipole vector [11]). It was found in Ref. 29 that the kinetics of these hydrogen bonds were made up of frequent breakage and reformation events, with some particular hydrogen bonds stabilised to long times, with others being more transient on sub-picosecond scales. 4. Conclusions Equilibrium MD simulations have been performed to investigate structural anisotropy of adsorbed water layers at rutile-1 1 0 and anatase-1 0 1 interfaces. There is evidence of hydrogenbonding with water molecules outside the adsorbed layer for anatase-(1 0 1), but less for rutile-(1 1 0). This is due to adsorbed water molecules being confined more firmly in the area between bridging oxygen atoms at the rutile-1 1 0 surface. The more open-like nature
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of the anatase-1 0 1 surface, which allows for greater contact of adsorbed molecules with those outside the first layer, serves to increase the extent of hydrogen bonding with water molecules outside the adsorbed layer. This may serve to explain differing observations of mobility in the water layers, i.e., the lower level of mobility for rutile-1 1 0 interfaces, although this is a tentative conclusion owing to the nature and kinetics of hydrogen bonds with bridging oxygen atoms [29]. Also, at 298 K, there may well be some extent of chemical adsorption of water present on both surfaces which will alter the details of the water-water structural anisotropy in comparison to physical adsorption probed by classical MD. Indeed, there has been some extensive debate in the recent literature on the extent of chemical adsorption of water on rutile1 1 0 surfaces, if any [35–37]. However, notwithstanding the possibility of some degree of chemical adsorption of water, this study still offers a qualitative insight into water-water structural anisotropy. The use of ‘reactive’ potentials [30] and ab initio MD to probe these aspects in the future is suggested, owing to their ability to capture the rich physico-chemical nature of the surfaces more accurately. Acknowledgements We acknowledge useful discussions with John Tse and Damian Mooney. This material is based upon works supported by Science Foundation Ireland (SFI) under Grant No. [07/SRC/B1160]. We thank SFI and the Irish Centre for High-End Computing for the provision of high-performance computing facilities. The authors acknowledge the support of SSE Renewables. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] G.L. Haller, D.E. Resasco, Adv. Catal. 36 (1989) 173.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
U. Diebold, Surf. Sci. Rep. 48 (2003) 53. M.A. Henderson, Surf. Sci. Rep. 46 (2002) 1. C. Sun et al., J. Mater. Chem. 20 (2010) 10319. A.A. Levchenko, A.I. Kolesnikov, N.L. Ross, J. Boerio-Goates, B.F. Woodfield, G. Li, A.J. Navrotsky, Phys. Chem. A 111 (2007) 12584. E.C. Spencer et al., Phys. Chem. A 113 (2009) 2796. G. Mattioli, F. Filippone, R. Caminiti, A.A.J. Bonapasta, Phys. Chem. C 112 (2008) 13579. N. Kumar et al., Phys. Chem. C 113 (2009) 13732. D. Russo et al., Am. Chem. Soc. 133 (2011) 4882. R. Kavathekar, P. Dev, N.J. English, J.M.D. MacElroy, Mol. Phys. 109 (2011) 1649. K. Kavathekar, N.J. English, J.M.D. MacElroy, Mol. Phys. 109 (2011) 2645. M.P. Allen, Tildesley, D.J. Computer simulation of liquids; Oxford, 1987. Z. Zhang et al., Langmuir 20 (2004) 4954. M. Predota, A.V. Bandura, P.T. Cummings, J.D. Kubicki, D.J. Wesolowski, A.A. Chialvo, M.L. Machesky, J. Phys. Chem. B 108 (2004) 12049. M. Predota, J. Phys. Chem. B 108 (2004) 12061. M. Predota, J. Phys. Chem. C 111 (2007) 3071. M.L. Machesky, M. Predota, D.J. Wesolowski, L. Vlcek, P.T. Cummings, J. Rosenqvist, M.K. Ridley, J.D. Kubicki, A.V. Bandura, N. Kumar, J.O. Sofo, Langmuir 24 (2008) 12331. I.M. Svishchev, P.G. Kusalik, J. Chem. Phys. 99 (1993) 3049. B. Schneider et al., Biophys. J. 65 (6) (1993) 2291. K. Kulin´ska et al., Comput. Chem. 24 (3–4) (2000) 451. G. Khelashvili et al., Func. Bioinfo. 76 (2) (2009) 403. A.K. Soper, M.A. Ricci, Phys. Rev. Lett. 84 (13) (2000) 2881. J.L. Finney et al., Phys. Rev. Lett. 89 (20) (2002) 205503. S. Klotz et al., Phys. Rev. Lett. 89 (28) (2002) 285502. I.-C. Yeh, M.L. Berkowitz, J. Chem. Phys. 111 (1999) 3155. M. Matsui, M. Akaogi, Molecul. Sim. 6 (4–6) (1991) 239. Y. Wu, H.L. Tepper, G.A. Voth, J. Chem. Phys. 124 (2) (2006) 024503. N.J. English, R.S. Kavathekar, J.M.D. MacElroy, Molec. Phys. in press, http:// www.tandfonline.com/doi/abs/10.1080/00268976.2012.683888. M. Alimohammadi, K.A. Fichthorn, J. Chem. Phys. C 115 (2011) 24206. A.S. Barnard, L.A. Curtiss, Nano Lett. 5 (2005) 1261. D.L. Bergman, Graph Model. 15 (5) (1997) 301. Gaussian cube file format. Available from:. A. Luzar, D. Chandler, J. Chem. Phys. 98 (1993) 8160. L.-M. Liu, C. Zhang, G. Thornton, A. Michaelides, Phys. Rev. B 82 (2010) 161415. D.J. Wesolowski et al., Phys. Rev. B 85 (2012) 167401. L.-M. Liu, C. Zhang, G. Thornton, A. Michaelides, Phys. Rev. B 85 (2012) 167402.
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Spatial distribution of adsorbed water layers at the TiO2 rutile and anatase interfaces Ritwik S. Kavathekar a, Niall J. English a,b,⇑, J.M.D. MacElroy a,b a b
The SFI Strategic Research Cluster in Solar Energy Conversion, School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland Centre for Synthesis and Chemical Biology University College Dublin, Belfield, Dublin 4, Ireland
a r t i c l e
i n f o
Article history: Received 23 May 2012 In final form 1 October 2012 Available online 17 October 2012
a b s t r a c t The spatial distribution functions of adsorbed water layers at hydrated rutile-(1 1 0) and anatase-(1 0 1) surfaces have been studied at 300 K, using equilibrium classical molecular dynamics. It was found that there is evidence of some hydrogen-bonding with water molecules outside the adsorbed layer for anatase-(1 0 1), but less for rutile-(1 1 0) – on average, about 1.6 per water molecule in anatase-(1 0 1), in contrast to 0.9 for rutile-(1 1 0). This is due to adsorbed water molecules being confined more firmly in the area between bridging oxygen atoms at the rutile-1 1 0 surface, while the more open-like nature of anatase-1 0 1 allows for greater contact with molecules outside the first layer. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Since Fujishima and Honda discovered that TiO2 could split water under visible-light irradiation to produce hydrogen and oxygen gas [1], the study of the properties of aqueous solutions in contact with TiO2 interfaces has undergone a significant increase in activity. There are many potential applications for renewable energy using photo-electrochemical water splitting and in dyesensitised solar cells, not only for TiO2, but also for a range of other semiconductor-water interfaces; often, these will exploit supportmetal-support-interaction (SMSI) modification of photo-catalytic properties [2]. Although TiO2 is one of the most studied metal oxides in the literature, there is a rather limited understanding of interfacial water molecules’ behaviour and reactivity at interfaces, despite recent progress [3,4], including via theoretical and molecular simulation methods [5]. Such TiO2-water interfaces provide a rich environment for investigation of confinement of water molecules’ motion; this is particularly the case where hydrogen-bonded molecules play an important role in stabilising solutes via solvent interactions and in forming ‘cages’ [3,4]. For instance, experimental vibrational spectra of adsorbed water on both rutile rods and anatase powder have been reported via inelastic neutron scattering (INS) measurements, concluding that confined adsorbed water molecules exhibit vibrational and dynamical features closer to less mobile ice compared to their behaviour in the liquid state [6,7]. Molecular dynamics (MD) has been useful to some extent in characterising the dynamical and vibrational behaviour of adsorbed water molecules on rutile-(1 1 0) and anatase-(1 0 1) surfaces [8,9]; ⇑ Corresponding author at: School of Chemical and Bioprocess Engineering, Centre for Synthesis and Chemical Biology University College Dublin, Belfield, Dublin 4, Ireland. Fax: +353 1 716 1177. E-mail address: [email protected] (N.J. English). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.004
indeed, ab initio MD (AIMD), especially, has offered key insights into the librational motion of higher-frequency modes of water adsorbed to titania in very interesting recent studies [9,10]. However, the somewhat restrictive computational limits on the duration of AIMD simulation makes it difficult to ascertain lower-frequency translational modes for a desired level of accuracy. Recently, we have carried out longer classical MD [13] simulations to study, at least qualitatively, intramolecular strain in confined, adsorbed water molecules at various titania surfaces, in addition to orientations of the water dipoles with respect to the surface normal [11], as well as to reproduce vibrational spectra results of confined layers at the surfaces in reasonable agreement with INS spectra [12]. The rutile-(1 1 0)-water interface, in particular, has been studied in much detail in recent years via MD [14–19]. Zhang et al. have investigated ion adsorption [14], while Predota et al. have probed the electric double layer structure in the vicinity of rutile-(1 1 0) surfaces [15–17], and have studied the nature of the water layer structure [15], ionic adsorption [16], and viscosity and diffusivity of the water layers [17]. Machesky et al. have studied surface protonation effects [18]. In all of these experimental and simulation studies of relatively confined water layers, the hydrogen bonding properties with the bridging oxygen atoms at the titania surfaces, as well as with other water molecules, has been remarked as an influential factor in leading to this behaviour distinct from bulk liquid water. This has distinct ramifications on the structural features of these adsorbed water layers, and this study seeks to probe the threedimensional spatial distribution of the water layer to study its anisotropic nature. Although pair distribution functions (PDF), like the radial distribution function (RDF), have enjoyed popular use to represent nearest-neighbour information, their spatial analogue, the spatial distribution (SDF), has been relatively less conspicuous. The structural information of angular and radial distribution of molecules
103
R.S. Kavathekar et al. / Chemical Physics Letters 554 (2012) 102–106
represented in the SDF, g(r, h, u) allows for the study of anisotropy of these density distributions not afforded by PDFs or RDFs. The use of SDFs were first reported by Svishchev and Kusalik [19] for liquid water in the form of pair-density isosurfaces maps and histograms of angular dependence of h, the angle made by the water molecule’s dipole vector (the local-frame z-axis [19]) and the separation vector between two water molecules’ oxygen atoms, and u, the angle made by the local-frame x-axis (H–H axis) projection of the water separation vector onto the local-frame x–y plane (with the y-axis being orthogonal to the dipole and H–H axes, cf. Ref. [19]). Such analysis of distributions has been performed for various species, namely for water molecules surrounding nucleic acid base pairs [20], RNA molecules [21], for cholesterol molecules and around proteins [22], but predominantly evaluated for liquid and amorphous low- and high-pressure ice and water [23–25]. We report here the three-dimensional isosurface SDF between the water molecules of the adsorbed layer and also with those outside this layer, at TiO2 rutile-(1 1 0) and anatase-(1 0 1) surfaces along with liquid water. The method for plotting SDFs as given by Svishchev and Kusalik [19] was followed to generate the 3D isosurfaces and g(r, h, u) histograms.
2. Simulation methodology We have performed a 100 ps NVT equilibrium classical molecular dynamics (CMD) simulation [13] for rutile-(1 1 0) and anatase-(1 0 1) TiO2 surfaces. The 3-D Ewald method was used for long-range non-bonded interactions with a precision of 10 5. The Ewald 3DC [26] method is a better approximation to the plain Ewald method for slab geometries, and will result in faster simulation and more accurate convergence of long-range Coulombic interactions; however, the 3-D Ewald technique used in this work would not be much different, although it is less computationally efficient. A Nosé–Hoover NVT ensemble at 300 K using VelocityVerlet integration with a 0.33 fs timestep was used. Bulk liquid water was equilibrated for about 200 ps via the Anderson–Hoover NPT ensemble before performing a 100 ps NVT run at 300 K with a relatively mild thermostat coupling period of 0.5 ps, with the appropriate number of water molecules added to achieve the appropriate bulk-like density of ca. 1 g/cm3 in the region between the titania surface and its periodic image. The Matsui-Akaogi (MA) [27] potential model was used for the titania surfaces and a flexible-SPC (FwSPC) [28] model for the water molecules were used. The cross parameters for Ti–Ow were obtained by using the same Buckingham potential and O–Ow LJ potential (see Table 1) [12] as used by Bandura and Kubicki, These have been previously used to
show qualitative comparisons between experimental and reported vibrational density of states and hydrogen bonding on similar surfaces [29]. A recent report on refining the TiO2–water interaction parameters, along the same lines as Bandura and Kubicki, but with explicit interaction with the water hydrogen atoms and TiO2 slab, has been reported by Alimohammadi et al. [30]. The MA potential consists of Buckingham interactions (see Table 1) which enables one to apply the potential to a stoichiometric slab model. All slabs used in the simulation were unconstrained. The details of the system sizes and simulation box dimensions are specified in Table 2. Similar simulations were also carried out for bulk liquid water. All surfaces were cut from a bulk rutile with lattice vectors a0 = b0 = 4.593 Å, c0 = 2.959 Å (P42/MNM) and a bulk anatase with lattice vectors a0 = b0 = 3.776 Å and c0 = 9.486 Å (I41/AMD). The details of the construction and topography of modeled surfaces are explained more fully elsewhere [11]. Rutile-(1 1 0) and anatase(1 0 1) is the most stable and studied titania surfaces, for which considerable experimental data on water layer arrangements are available. The rutile-(1 1 0) surface was cut for charge compensation to yield a non-polar and dipole-free surface [11] (cf. Fig. 1(a)). This leads to under-saturation of surface Ti atoms in its coordination. Rutile-(1 1 0) has a natural slanting angle with respect to the [0 0 1]-direction (or the laboratory z-direction, i.e., the direction of heterogeneity is orthogonal to the surface in the x–y plane, cf. Fig. 1(a)), which was corrected by aligning the entire slab orthogonal to the z-axis, so as to facilitate an orthorhombic periodic box. Anatase is the most photoactive polymorph of TiO2 and is more stable at nanoscale dimensions than rutile [31], and hence is an important surface for this study; the anatase-(1 0 1) surface is also tilted at an angle and was aligned vis-à-vis the z-axis (cf. Fig. 1(b)). We follow the method given in Ref. 19 to generate SDF plots and histograms, but a more general and complete method is also given by Bergmen et al. [32] for volumetric imaging. The 3D isosurface plots were prepared as Gaussian cube files [33]. The contour surfaces represent SDF values above a threshold such that 30% of the molecules are included in the SDF density plots [23], representing the main orientation in the shell corresponding to the selected distance (r) in the RDF. The O–O SDFs are plotted at distances guided by extrema in the corresponding RDF of (a) between water molecules present in the adsorbed layers [11,12], (b) between the water molecules in the first layer and rest of the water molecules, and (c) between the water molecules in the adsorbed layer and those outside it. These RDF used to guide the sampling distances for SDF plots are also reported (vide infra) as shown in Fig. 2. The SDFs were calculated in distance ranges of ± 0.05 Å about each nominal O–O separation to ensure good statistics. To assess for
Table 1 Force-field parameters. Buckingham potential for TiO2 and water oxygen: Aij exp( rij/qij) i–j Aij (kcal mol 1) Ti–O 391049.1 Ti–Ti 717647.4 O–O 271716.3 Ti–Ow 28593.0 Lennard–Jones potential for water: eij[(rij/rij)12 (rij/rij)6] i–j eij (kcal mol 1) Ow–Ow 0.1554
Cij/rij6
qij (Å)
Cij (kcal mol 290.331 121.067 696.888 148.000
0.194 0.154 0.234 0.265
rij (Å) 3.165492
2
Harmonic potential for water: k/2 (rij r0) i–j kij (kcal mol Ow–Hw 1059.162
1
Å
2
)
Harmonic angle bending potential for water: k/2 (h–h0) i–j–k h0 deg H–O–H 113.24 Atomic charges: q(Ti) = 2.196 e, q(O) =
1.098 e, q(Ow) =
r0ij (Å) 1.012 k (kcal mol 75.900
1
rad
2
)
0.82 e, q(Hw) = 0.41 e; Ow, Hw = water oxygen and hydrogen atoms
1
Å6)
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Table 2 Details of geometries used. Phase (surface), X, Y, Z (Å)
System size
Rutile (1 1 0) 26.26, 45.47, 69.490 Anatase (1 0 1) 71.46, 26.43, 72.680
(TiO2)630 (H2O)2000 (TiO2)1176 (H2O)3162
Figure 1. Layout of various titania surfaces, as discussed in the text; the laboratory z-direction is vertical. Here, Ob denotes a bridging oxygen, O3c a three-coordinated surface oxygen, Ti5c a penta-coordinated surface Ti atom (a lone coordinatively unsaturated site), while Ti6c stands for a hexa-coordinated Ti atom. (a) rutile-1 1 0, and (b) anatase-1 0 1.
the presence of hydrogen bonds, the geometric criteria of Luzar and Chandler were used [34].
3. Results and discussion We present the structural analysis of O–O SDF of liquid water and water adsorbed at rutile-(1 1 0) and anatase-(1 0 1) interfaces. For a given O–O separation, it is clear that in liquid water (Fig. 3), the probability of a nearby water’s oxygen being present is substantially larger than for the absorbed layer at either of the
titania interfaces (cf. Figs. 4 and 5). The number of (hydrogenbonded) nearest-neighbours is higher in liquid water (ca. 3.8, on average), with tetrahedral ordering observed at 2.7 Å O–O separation in the four tetrahedral-like ‘lobes’. The difference between anatase-(1 0 1) and rutile-(1 1 0) is rather striking. There is some evidence of hydrogen-bonding with water molecules outside the adsorbed layer for anatase-(1 0 1) (cf. Fig. 4(c) at 2.7 Å), but less for rutile-(1 1 0). This was confirmed by calculating the average number of hydrogen bonds formed with water molecules outside the adsorbed layer (as well as with bridging oxygen atoms in titania and with any other water molecules in the adsorbed layers), and these data are provided in Table 3. It can be seen that there are almost two hydrogen bonds formed with water molecules outside the adsorbed layer in anatase-(1 0 1), while there is almost one per adsorbed water in rutile-(1 1 0) – half that seen for anatase(1 0 1). This is seen in a somewhat less direct manner in the RDFs of Fig. 2, where there is a greater peak height at an O–O separation of around 2.8 Å with water molecules outside the adsorbed layer for anatase-1 0 1 (ca. 4) compared to around 2.8 for rutile-1 1 0; however, this observation is not as strong as the direct measure of the number of hydrogen bonds with water molecules outside the adsorbed layer, as it relates strictly to water O–O contacts. This is due to adsorbed water molecules being ‘perched’ more firmly in the area between bridging oxygen atoms in rutile-1 1 0 (cf. Fig. 1(a)), as opposed to the more ‘terraced’- or ‘corrugated’-like nature of the anatase-1 0 1 surface (cf. Fig. 1(b)), which is more open and allows for greater hydrogen bonding contact of adsorbed molecules with those outside the first layer. In contrast, rutile-1 1 0 does display two distinct nearest-neighbour water-water contacts at around 2.8–3.0 Å O–O separation (cf. Fig. 5(a)), but comparatively less with those outside the layer (cf. Fig. 5(b), and also the RDF on the left of Fig. 2 for peak heights of about 8.7 and 2.8 for contact with other adsorbed-layer water molecules and those outside this layer): this arises due to ‘stacking’ of absorbed water molecules laterally, in between the bridging oxygen atoms of titania, and is anisotropic, due to dipolar orientation that is essentially normal vis-à-vis the surface [11] between ‘ridges’ of bridging oxygen atoms (in contrast to largely parallel for anatase-(1 0 1) [11]) – the distance between nearest-neighbour Ti5c atoms is about 2.96 Å. It was found that the two nearestneighbour water-water contacts at around 2.8–3.0 Å O–O separation in rutile-1 1 0 within the adsorbed layer were not hydrogen-bonded (cf. Table 3, as can also be seen from the hydrogen atoms not pointing to the associated oxygen atoms in the
Figure 2. O–O RDFs of rutile-1 1 0 (on the left of the panel as ‘R110’) and anatase-1 0 1 (on the right-hand side as ‘A101’). ‘Layer 1 only’ denotes water-water contacts within the adsorbed layer only, while ‘Layer 1 and other’ denotes water-water distances between those of the adsorbed layer and other molecules outside this layer (up to 15 Å from the surface). ‘All Ow atoms’ means water-water contacts between those in the adsorbed layer and all other water molecules up to 15 Å from the surface including other water molecules in the adsorbed layer.
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Figure 3. O–O SDFs of bulk liquid-like water molecules at distances of 2.7, 3.2, 3.7, 4.44 Å.
Figure 5. O–O SDFs of reference layer-water molecules with other sets of water molecules at the rutile-(1 1 0) interface at 2.79 and 2.99 Å separation. SDFs are shown with other adsorbed-layer molecules on the left of the panel, and with only non-layer molecules on the right.
Table 3 Numbers of hydrogen bonds of various types for adsorbed layer of water molecules.
Figure 4. O–O SDFs of reference layer-water molecules with other sets of water molecules at the anatase-(1 0 1) interface at 2.7, 3.0, 3.3, 3.7, 4.6 Å separations. The vertical column shows SDFs with (A) other adsorbed-layer molecules, (B) only nonlayer molecules, and (C) all other molecules (both absorbed-layer and non-layer).
neighbours). There was also scant evidence of very limited and transient hydrogen bonding between nearest-neighbour water molecules in the adsorbed layer in anatase-(1 0 1) (cf. Table 3) – this arises due to the more open structure of anatase-(1 0 1) allowing transient water-water contacts due to lateral movements back and forth along the surface, unlike the ‘perched’ nature of rutile(1 1 0), even though the O–O distance range is greater at 3–4.5 Å than the corresponding nearest neighbor distances of 2.6–3.4 Å. These structural studies of anisotropic water orientations also help to rationalise earlier observations of less mobility in the water layer at rutile-1 1 0 interfaces vis-à-vis anatase-1 0 1 (and other titania interfaces) [12]. The presence of the bridging oxygen ‘ridges’ in rutile-1 1 0 impede diffusive motion of this layer along the surface. This is less prevalent in the anatase-1 0 1 surface, where the SDFs show more similarity to bulk-like liquid conditions. Of course, the importance of hydrogen bonds of water in the adsorbed layer with bridging oxygen atoms is important for both surfaces [5,9– 11,29] (the kinetics of which have been studied in Ref. 29), so this conclusion is necessarily tentative. There is a slightly larger extent
Surface
Bridging oxygens
Adsorbed-layer waters
Waters outside adsorbed layer
Rutile (1 1 0) Anatase (1 0 1)
0.11 ± 0.01 0.18 ± 0.02
0 0.05 ± 0.01
0.91 ± 0.05 1.59 ± 0.08
of hydrogen bonding to bridging oxygen atoms for anatase-(1 0 1) at an average of 0.18 relative to rutile-(1 1 0) at 0.11, which is explained by the more parallel arrangement of water molecules to the surface in anatase-(1 0 1) (as characterised by the dipole vector [11]). It was found in Ref. 29 that the kinetics of these hydrogen bonds were made up of frequent breakage and reformation events, with some particular hydrogen bonds stabilised to long times, with others being more transient on sub-picosecond scales. 4. Conclusions Equilibrium MD simulations have been performed to investigate structural anisotropy of adsorbed water layers at rutile-1 1 0 and anatase-1 0 1 interfaces. There is evidence of hydrogenbonding with water molecules outside the adsorbed layer for anatase-(1 0 1), but less for rutile-(1 1 0). This is due to adsorbed water molecules being confined more firmly in the area between bridging oxygen atoms at the rutile-1 1 0 surface. The more open-like nature
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of the anatase-1 0 1 surface, which allows for greater contact of adsorbed molecules with those outside the first layer, serves to increase the extent of hydrogen bonding with water molecules outside the adsorbed layer. This may serve to explain differing observations of mobility in the water layers, i.e., the lower level of mobility for rutile-1 1 0 interfaces, although this is a tentative conclusion owing to the nature and kinetics of hydrogen bonds with bridging oxygen atoms [29]. Also, at 298 K, there may well be some extent of chemical adsorption of water present on both surfaces which will alter the details of the water-water structural anisotropy in comparison to physical adsorption probed by classical MD. Indeed, there has been some extensive debate in the recent literature on the extent of chemical adsorption of water on rutile1 1 0 surfaces, if any [35–37]. However, notwithstanding the possibility of some degree of chemical adsorption of water, this study still offers a qualitative insight into water-water structural anisotropy. The use of ‘reactive’ potentials [30] and ab initio MD to probe these aspects in the future is suggested, owing to their ability to capture the rich physico-chemical nature of the surfaces more accurately. Acknowledgements We acknowledge useful discussions with John Tse and Damian Mooney. This material is based upon works supported by Science Foundation Ireland (SFI) under Grant No. [07/SRC/B1160]. We thank SFI and the Irish Centre for High-End Computing for the provision of high-performance computing facilities. The authors acknowledge the support of SSE Renewables. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] G.L. Haller, D.E. Resasco, Adv. Catal. 36 (1989) 173.
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