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Interaction of Water with the Fluorine-Covered Anatase TiO2 (001) Surface Chenghua Sun,*,†,‡ Annabella Selloni,‡ Aijun Du,† and Sean C. Smith*,† †
Centre for Computational Molecular Science, Australia Institute for Bioengineering and Nanotechnology, The University of Queensland, Australia ‡ Department of Chemistry, Princeton University, Princeton, New Jersey 08542, United States ABSTRACT: The interaction of water with the fluorine-covered (001) surface of anatase titanium dioxide (TiO2) has been studied within the framework of density functional theory (DFT). Our results show that water dissociation is unfavorable due to repulsive interactions between surface fluorine and oxygen. We also found that the reaction of hydrofluoric acid with a surface hydroxyl group to form a surface TiF bond is exothermic, while the removal of fluorine from the surface needs additional energy of about half an eV. Therefore, water molecules are predicted to remain intact at the interface with the F-terminated anatase (001).
1. INTRODUCTION In nature, TiO2 crystals have three different structures: rutile, anatase, and brookite, all formed by TiO6 octahedra connected by shared edges and/or corners.1 Among these three, anatase TiO2 shows the highest reactivity in photocatalytic applications.2 This has motivated strong interest and intensive studies of the controlled synthesis of anatase TiO2 crystals in recent years.316 Due to the relative stabilities of low-index surfaces,17,18 anatase TiO2 crystals primarily expose (101) and (001) planes as the majority and minority surfaces, respectively. Recently, however, anatase crystals with high ratios of (001) surfaces have been successfully obtained by Yang et al., using hydrofluoric acid as the controlling agent.11,12 Increasing the concentration of fluorine (F) species, anatase TiO2 crystals with a fraction of (001) surfaces up to 90% or even more13 have been synthesized. These crystals show promise in applications such as solar cells,19 lithium batteries,2022 and photocatalysis.2330 The success of the synthesis of large (001) facets has been attributed to the fact that the fluorine atoms can strongly bond to the unsaturated titanium atoms and thus stabilize the surface (see Figure 1).11 This is essential for the synthesis, but potentially fluorine atoms may change the surface chemistry of TiO2 crystals and thus affect their performance for specific applications. In particular, water can spontaneously dissociate on the clean (001) surface,31,32 but when the surface Ti atoms are terminated by fluorine, this capacity may be lost. It is generally believed that water dissociation is driven by two interactions: one is the strong bonding between the water oxygen (labeled as Ow) and the unsaturated surface Ti atoms (five-coordinated titanium, Ti5c, on the (001) surface), and the other is the hydrogen bonds (HBs) r 2011 American Chemical Society
between the water hydrogen and the two-coordinated surface oxygen atoms (O2c).1,3335 When the surface is fully or partially covered by fluorine, however, Ti5c may be saturated, and Ti5c-Ow bonding is not available any more. Moreover, the repulsive interaction between F and O, due to their strong electronegativities, can influence the water adsorption. To elucidate the role of all the above effects, we have investigated the adsorption/dissociation of water on the fluorine-covered (001) surface (indicated as F-(001) below) using first-principle density functional calculations. We identified a stable configuration with water dissociation from total energy calculations, but calculated energy barriers and molecular dynamics simulations at T = 330 K show that water dissociation is unlikely on F-covered Anatase TiO2(001). The reaction of water with surface fluorine has also been investigated. As already mentioned, typically the F-(001) surface is obtained using hydrofluoric acid to stabilize the surface via TiF bonding.11,12 The merit of a fluorine-based agent is that F-(001) can be easily cleaned by heat treatment, typically at 400600 °C,11 to generate a fluorine-free (001) surface. For both the synthesis and the fluorine removal, the reaction, HF(aq) + TiOH(surf) T TiF(surf) + H2O, plays an essential role. Therefore, we also carried out calculations of the energetics associated with the dissociation and the formation of HF on the (001) surface. The knowledge gained from the present study should be useful for understanding the growth mechanism of F-(001) and the afterReceived: May 25, 2011 Revised: July 12, 2011 Published: July 28, 2011 17092
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The Journal of Physical Chemistry C synthesis removal of fluorine, as well as the chemistry of the F-terminated (001) surface.
2. COMPUTATIONAL METHOD We modeled the F-terminated (001) surfaces by 2 2 periodically repeated slabs consisting of 12 TiO2 layers (4 titanium layers), as shown in Figure 1. The vacuum space between . During the geometric neighboring slabs was larger than 15 Å optimization, the bottom titanium layer (4 TiO2 units) was fixed to represent the bulk structure based on the experimental data of the TiO2 crystal. To investigate the dependence on water coverage, four coverages were considered, namely, 1/4 monolayer (ML), 1 ML, 2 ML, and 3 ML. To study water adsorption, both molecular
Figure 1. Slab models (side views) for: (a) clean (001), and (b) F-terminated (001) surfaces. Titanium, oxygen, fluorine, and hydrogen atoms are represented as gray, red, cyan, and white spheres. Ballsand-sticks are used for the first layer, and sticks only are used for the other layers.
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and dissociated starting geometries were optimized, followed by a constrained relaxation when the maximum atomic force was less than 0.01 eV/Å. All calculations have been carried out using the spin-polarized density functional theory (DFT) within the generalized-gradient approximation (GGA),36 with the exchange-correlation functional of PerdewBurkeErnzerhof (PBE),37 as implemented in the Vienna ab initio simulation package (VASP).3840 Electronic states were expanded in plane waves with a kinetic energy cutoff of 380 eV. Reciprocal space sampling was restricted to the Γ point. In the geometry optimizations, all structures have been relaxed to an energy convergence of 104 eV (corresponding to a force convergence of 102 eV/Å). To check the reliability of the employed computational settings, we first studied the adsorption of water on the clean (001) surface. At a coverage of θ = 1/4 ML, the dissociative adsorption is predicted, with an adsorption energy of 1.25 eV, which agrees well with the result (1.25 eV) in ref 41 (PBE-functional) but is smaller than that (1.59 eV) in ref 32, where the PW91-functional was used. First-principle molecular dynamics (FPMD) simulations were employed to investigate the behavior of water multilayers on F-(001), with time step Δt = 1 fs and an overall time scale of 6.8 ps. For each MD step, the total energy and the force were converged to the above setted criteria. The FPMD simulation was performed using VASP as well. The transition state (TS) for water dissociation was searched based on a constrained minimization technique,4246 in which all of the degrees of freedon of the system are relaxed except the reaction coordinate (HOH bond for water dissociation). Such a technique has been widely used in the studies of surface reactions.4246 Specifically, the TS is identified when its total energy is the maximum among a series of images generated from initial state (IS) to final state (FS). Generally, barrier heights calculated by PBE functionals are underestimated, typically by 0.10.3 eV. However, the underestimation does not change the main conclusions of this work.
3. RESULTS AND DISCUSSION Experimentally two-coordinated oxygen (O2c) atoms in TiO2 samples obtained by wet synthetic methods may be terminated by hydrogen, and previous theoretical studies show that H-termination can significantly improve the surface stabilities.47,48 Consequently, tests with O2c terminated with hydrogen have been carried out. It was found that water dissociation is not favored on F-covered TiO2(001), which is consistent with the conclusion we made below with O2c exposed to water layers. No matter O2c is saturated or not, water prefers to bond with two fluorine atoms (rather than O2c atoms) and form two HF HBs as shown in Section 3.1. Therefore, the TiO2 surface is only covered by fluorine atoms in our calculations. This is reasonable because FTi bonds are much stronger (569 kJ/mol49) than
Figure 2. Adsorption and dissociation of single water on F-(001). (a) Molecular adsorption, (b) transition state, (c) dissociative adsorption, and (d) energy profile. Relevant atomic distances are given in Å. 17093
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The Journal of Physical Chemistry C typical OH bonds (300497 kJ/mol49) in terms of bond energies, consistent with the experimental observation that fluorine cannot be easily removed by water washing. 3.1. Adsorption and Dissociation of Single Water on F-(001). Figure 2a shows the adsorption geometry of single water on F-(001) with a coverage of 1/4 ML, initially starting with intact water. Different from clean (001),32,41 no water dissociation is found over F-(001). Instead, molecularly adsorbed water is stabilized by two FH HBs due to the strong electronegativity of the surface fluorine. The calculated adsorption energy is 0.31 eV, characteristic of physiadsorption. To investigate the possibility of water dissociation, one water hydrogen was shifted to one fluorine center step by step. From a series of images, the TS was identified as shown in Figure 2b. Here, TiFH bonding is present, leading to the formation of HF as a local stable state (see Figure 2c). However, the LS and TS are 1.37 and 1.65 eV above the IS, suggesting that the dissociation can hardly occur at room temperature. 3.2. Water Monolayer and Multilayers on F-(001). To investigate how waterwater interactions affect the results of Section 3.1, additional water molecules, forming one, two, and three monolayers, were included . Starting from the optimized structures shown in Figure 2a, we generated the initial geometries for one, two, and three water monolayer(s) on F-(001) by adding 3, 7 (3 + 4), and 11 (3 + 4 + 4) water molecules, respectively, in ad hoc “physically reasonable” positions. These geometries were then relaxed keeping the bottom TiO2 layer fixed. The resulting structures are shown in Figure 3ac. From these geometries, it
Figure 3. Optimized geometries of water layers on the F-(001) surface. (a) 1 ML, (b) 2 ML, and (c) 3 ML. Water and F-(001) are shown as sticks and ball-sticks, respectively.
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appears that molecularly adsorbed water at the water/F-(001) interface is stabilized by FH and OH hydrogen bonds, together with Ow-H HBs. During the optimization, no water dissociation is observed, as in the case of an isolated molecule in Figure 2a. 3.3. Formation and Dissociation of HF Molecule on F-(001). As mentioned in the introduction, the (forward) reaction Ti OH þ HF f Ti F þ H2 O
ð1aÞ
has a key role in the F-assisted growth of the (001) surface. Similarly, the after-synthesis removal of fluorine can be described by the backward reaction Ti F þ H2 O f Ti OH þ HF
ð1bÞ
Figure 4 shows the optimized structure for the left-hand side of eq 1a (or, equivalently, the right-hand side of eq 1b) with various coverages of coadsorbed water on F-(001). As shown in Figure 4a, three of four surface Ti5c centers are terminated by fluorine and only one terminated by OwH, together with one HF molecule around the OwH. This structure represents the fluorine-covered (001) in the HF solution. To simplify the surface reactions, only one TiOH group and one HF molecule are considered, while in Figure 4bd, more water molecules are introduced to represent the solution environment. Energy calculations show that reaction 1a is exothermic, with reaction energies of 0.35 eV, 0.43 eV, 0.56 eV, and 0.45 eV for θ = 1/4 ML, 1 ML, 2 ML, and 3 ML, respectively. This indicates that HF can easily exchange with surface OwH groups even when 75% of the Ti5c atoms are already terminated by fluorine. This is due to the strength of the TiF bond, which is believed to provide
Figure 5. Temperature profile for MD simulations at T = 330 K (indicated by the red line).
Figure 4. Optimized structure for the left side of reaction 1a. The water coverage is (a) 1/4 ML, (b) 1 ML, (c) 2 ML, and (d) 3 ML. 17094
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Figure 6. Snapshots along the MD trajectory.
the key stabilization for the growth of (001).11 The backward (1b) reaction is thus endothermic, indicating that additional energy of around half an eV is needed to remove fluorine from F-(001). This is consistent with our experimental work showing that heat treatment at 400600 °C is needed to remove fluorine from the surface.11 3.4. MD Simulation of Water Molecules on F-(001). The above investigations of water adsorption and dissociation were all performed at 0 K. To include the effect of thermal fluctuations and generate a more realistic interface structure, first-principle molecule dynamics has been employed to simulate the waterF(001) interface at 330 K. In our case 24 water molecules are introduced to fill the vacuum space on F-(001), followed by full optimization of the lattice of c-axis and atomic coordinates until the atomic forces fall into the convergence range (see Section 2). The simulation temperature increases slightly from 0 to 330 K, and the data are collected for analysis when the temperature stably fluctuates around 330 K, as shown in Figure 5. The MD simulation started from the initial state (Figure 6a, t = 0 ps) directly obtained from the optimization at 0 K, which shows an ice-like structure (Figure 6bd). After about 0.9 ps, the temperature steadily fluctuates around 330 K (as indicated by the red line in Figure 5), and snapshots at various stages are collected as shown in Figure 6bd. We did not observe water dissociation or the leaving of fluorine to form HF at the waterTiO2 interface, which is consistent with the results of the T = 0 K calculations. We can therefore conclude that water adsorbed at the interface with F-(001) is most likely intact, due to the large barrier (1.65 eV, see Figure 2d) for water dissociation.
4. CONCLUSION In this work, the adsorption and dissociation of water on the fully F-covered (001) surface have been investigated using DFT calculations. It is found that: (i) water dissociation is unlikely on F-(001) due to a large energy barrier (1.65 eV using PBE functional); (ii) HF can exchange with surface hydroxyl groups readily, leading to strong TiF bonding, and thus making the removal of fluorine endothermic (by ∼0.350.56 eV); and (iii) FPMD simulations at T = 330 K show that the water/F-(001) interface is characterized by intact water molecules and dominated by FH HBs, without water dissociation and the formation of HF molecules. The present results not only clarify the structure of the interface between water and F-(001) but also
provide a preliminary explanation of F-assisted growth of TiO2 (001) and the after-synthesis removal of fluorine.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (Sean C. Smith);
[email protected] (Chenghua Sun).
’ ACKNOWLEDGMENT This work is inspired by collaborations within the Australian Research Council Centre of Excellence for Functional Nanomaterials and financially supported by The University of Queensland (Research Excellence for C.S.), Australian Research Council, and Queensland Government (through Queensland Smart Future Fellowship for C.S.). A.S. acknowledges support from DoEBES, Chemical Sciences, Geosciences and Biosciences Division, Contract No. DE-FG02-05ER15702. We also appreciate the generous grants of CPU time from the University of Queensland, Princeton University, and the Australian National Computational Infrastructure Facility. ’ REFERENCES (1) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (2) Kavan, N. L.; Gr€atzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. J. Am. Chem. Soc. 1996, 118, 6716. (3) Wang, C. C.; Ying, J. Y. Chem. Mater. 1999, 11, 3113. (4) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. (5) Yin, H. B.; Wada, Y.; Kitamura, T.; Kambe, S.; Murasawa, S.; Mori, H.; Sakata, T.; Yanagida, S. J. Mater. Chem. 2001, 11, 1694. (6) Wu, M. M.; Lin, G.; Chen, D. H.; Wang, G. G.; He, D.; Feng, S. H.; Xu, R. R. Chem. Mater. 2002, 14, 1974. (7) Li, G. S.; Li, L. P.; Boerio-Goates, J.; Woodfield, B. F. J. Am. Chem. Soc. 2005, 127, 8659. (8) Nian, J. N.; Teng, H. S. J. Phys. Chem. B 2006, 110, 4193. (9) Ding, Z.; Hu, X. J.; Yue, P. L.; Lu, G. Q.; Greenfield, P. F. Catal. Today 2001, 68, 173. (10) Sugimoto, T.; Zhou, X. P.; Muramatsu, A. J. Colloid Interface Sci. 2003, 259, 53. (11) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Nature 2008, 453, 638. (12) Yang, H. G.; Liu, G.; Qiao, S. Z.; Sun, C. H.; Jin, Y. G.; Smith, S. C.; Zou, J.; Cheng, H. M.; Lu, G. Q. J. Am. Chem. Soc. 2009, 131, 4078. 17095
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