15862
J. Phys. Chem. C 2009, 113, 15862–15867
Dissociation of Water on Anatase TiO2 Nanoparticles: the Role of Undercoordinated Ti Atoms at Edges Michel Posternak,*,† Alfonso Baldereschi,*,† and Bernard Delley*,‡ Institute of Theoretical Physics, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland, and Paul Scherrer Institut, WHGA/123, CH-5232 Villigen PSI, Switzerland ReceiVed: April 7, 2009; ReVised Manuscript ReceiVed: July 16, 2009
We show with DFT calculations that hydroxyl formation on anatase TiO2 from dissociation of water molecules is strongly enhanced by a nanostructured state of the oxide, involving the occurrence of complexes consisting of undercoordinated 4-fold Ti-2-fold O pairs. Our study of ridges delimited by anatase TiO2 (101) surfaces of different orientations shows that hydroxyl formation is significantly increased by the high acidity/basicity combination of these Ti4c-O2c pairs located on the ridges and by a stabilization effect associated with chemical bonds of the hydrogen atoms. This is at variance with the cases of the ideal (101) surface and of a ridge with 5-fold coordinated Ti atoms, where water adsorption is molecular. We demonstrate that these pairs of undercoordinated atoms on ridge edges have a particularly strong reactivity, and we propose that they play a major role in the observed high chemical activity of TiO2 nanosystems. Introduction Titanium is a well-known biomaterial, since it is a relatively inert and corrosion-resistant metal. It is its thin surface oxide layer (usually TiO2) which is mainly responsible for its excellent tissue compatibility, related to the potential adsorption of Ca2 + ions on the TiO2 coating. Several studies1-5 have shown that the latter process requires the presence of hydroxyl groups on the oxide film. Optimal binding of OH groups on the ceramic layer is therefore the object of active research. However, microscopic mechanisms relevant to an efficient OH deposition are not yet fully understood. Recently, it has been shown that titania-based nanomaterials can be used to synthesize implants with similar surface roughness to that of natural tissues.6-9 Such materials can indeed simulate dimensions of constituent bone components since they possess particle or grain sizes less than 100 nm. Furthermore, the above studies have allowed the correlation of the adhesion and functions of bone cells on the implants with the nanoscale surface features of their TiO2 layer. For instance, three times the amount of calcium-mineral deposition was observed when osteoblasts were cultured for up to 28 days on nanophase, compared to conventional titania.9 These works give evidence that the increase in surface area of the nanoparticles is not the only factor contributing to the observed enhanced osteoblast adhesion and that changes in the chemical activity of titania in relation with its nanophase structure should also play a role. Particularly relevant among the TiO2 polymorphs is the anatase phase, since it exhibits higher chemical reactivity in several situations.10 Moreover, experimental and thermodynamic facts11 suggest that anatase may be more stable than rutile when crystals are only a few nanometers in size (∼14 nm) and that anatase nanocrystals start forming from the very beginning of TiO2 growth. Their equilibrium crystal shape is largely dominated by the very stable (101) surface12 (this latter point being * To whom correspondence should be addressed. E-mail:
[email protected] (M.P.);
[email protected] (A.B.);
[email protected] (B.D.). † Swiss Federal Institute of Technology. ‡ Paul Scherrer Institut.
however questioned in a recent work13). The origin of the higher chemical activity of anatase is still under debate, as there are indications that its (101) surface is not very reactive. For instance,wateradsorptiononanatase(101)ispurelymolecular.12,14,15 Here we study how boundaries of nanostructures can give rise to different organizations/coordinations of the surface atoms compared to those found in the bulk or at surfaces and how they affect water adsorption and hydroxyl formation. We demonstrate in this work that complexes consisting of undercoordinated Ti-O pairs, likely to be present on nanoparticle ridges, are particularly reactive, and we propose that they play a major role in the observed high chemical activity of TiO2 nanosystems. Using first-principles calculations within density functional theory (DFT), we study the hydroxylation mechanism and the reactivity of undercoordinated Ti-O pairs on a paradigmatic simulation model, consisting of edges between two anatase TiO2 (101) surfaces. Several calculations of water adsorbed on the ideal (101) surface exist in the literature.12,14,15 We repeated these computations for reasons of consistency with our new results. We note that there are no other theoretical works dealing with surface defects of this type on anatase, except some recent first-principles calculations of monatomic-height step edges16,17 observed on the (101) surface.18 This latter problem is, however, distinct from the one under study in our work. Methods and Computational Details At the present time, details of the atomic structure of TiO2 nanoparticles, as well as informations on the Ti coordination in these systems, are largely unknown. However, recent experimental studies performed on the anatase (101) surface have confirmed the existence of monatomic-height step edges.16,18 Furthermore, it has been shown that the formation energy is minimum for the step edge of D type and that the formation of corresponding steps of C type is less favorable (see ref 16 for the above notations). While coordination of Ti atoms is 6-fold in bulk anatase TiO2, and both 6- and 5-fold on the (101) surface, 4-fold coordinated Ti sites may exist only on “ridges” or “corners”, as, e.g., in clusters. For the step edges of interest
10.1021/jp9032113 CCC: $40.75 2009 American Chemical Society Published on Web 08/17/2009
Water Adsorption on Anatase Nanoparticles here, Ti coordination is 5-fold for the D type, and 4-fold for the C type edges. We stress that the above conclusions regarding energetics concern the case of monatomic-height step edges and that experimental, as well as theoretical, data are lacking for finite-height steps or ridges. In the present work, we intend to relate Ti coordination to the chemical activity and, in particular, to the adsorption energy of water. For this purpose, we construct ideal models of ridges/edges displaying 5- and 4-fold Ti coordination, using the experimental results for D and C steps edges as a guideline. We simulate these undercoordinated sites with a slab model, displaying a periodic roof-shaped section, whose (infinite) top ridges are along the [111j] and [1j11] crystallographic directions, for D and C ridge terminations, respectively. The two corresponding roof sides, which are perpendicular to each other, are (101)-type surfaces. Our choice is mainly motivated by the similarity of all chosen ridges, which differ essentially by the coordination of atoms at edges. The ridge of D type consists of a zigzag line of Ti5C atoms interspaced with O2C atoms, whose (lateral) periodical replicas are separated by 16.194 Å in our model. Distance between two Ti5C representatives in a given line is 5.557 Å. Two possible terminations of the C ridge (C I and C II, using the notation of ref 16) are consistent with stoichiometry. Both types of ridge consist of lines of Ti4C atoms surrounded by O2C atoms, whose periodical replicas are separated by 14.155 Å. Distance between two consecutive Ti4C atoms in a given line is 5.557 Å. We checked that all the above distances are large enough for minimizing interactions between successive replicas. This periodical approach allows to isolate effects solely due to the presence of the line of undercoordinated Ti-O sites, which would not be the case with the ridges of a finite cluster, where size effects are non-negligible and not well under control, or with a monatomic-height step edge.16,17 For all our models, we have “frozen” the atoms of the bottom layer of the slab at their equilibrium bulk positions, and adsorption is allowed to take place on the upper side only. Finally, the repeated slabs are separated vertically by a vacuum of ∼100 Å. We show in Figure 1 a top view of the ideal unrelaxed (101) surface, as a reference for atoms positions in this geometry, together with views of the roof sides (unrelaxed structures) in the perpendicular direction, for the D, C I, and C II terminations. These three latter pictures display the typical pattern of the anatase (101) surface, ending in the structure of the corresponding ridge edges. We show in Figure 2 a 3D view of our slabs with C I and C II terminations after atomic relaxation has been performed. We note that because these simulation systems consist of intersecting (101) surfaces, they display both Ti4C and Ti5C sites and should therefore allow the performing of calculations for such sites on the same footing. We used this property only for the purpose of a test (see below). Indeed, for consistency reasons with previous works, we have performed calculations for the (101) surface using the same cell geometry as in ref 14 but with a vacuum separation of 100 Å. All computations reported here have been done using the all-electron DMol3 code,19,20 with the Perdew-Burke-Ernzerhof (PBE) functional21 for evaluating exchange and correlation. DMol3 is a DFT quantum mechanical program, using localized numerical orbitals as basis sets. The scheme can be considered as a generalized LCAO-type approach and works for both finite size and periodic systems. Equilibrium geometry of slabs with molecularly/dissociatively adsorbed water molecules is obtained by performing a series of structural optimizations, updating positions of atoms which are free to relax, until atomic displacements and changes of total energy are sufficiently small.
J. Phys. Chem. C, Vol. 113, No. 36, 2009 15863
Figure 1. Projections of the unrelaxed simulation models. (a) Top view of the ideal (101) surface; (b, c, and d) View in a direction perpendicular to one of the two ridge surfaces for the D, C I, and C II terminations, respectively. These figures display the pattern of the (101) surface, together with the corresponding ridge edges. Large red spheres represent O atoms, while small blue, green, and purple spheres represent Ti6C, Ti5C, and Ti4C atoms, respectively.
We use our optimized theoretical bulk lattice parameters, i.e., a ) 3.817 Å, c ) 9.715 Å, and u ) 0.2065 for describing the lower portion of the structure containing atoms which are kept frozen. All structure plots in this work have been generated with the code XCrySDen.22 The adsorption energy per water molecule, needed in our calculations, is defined as
hyd dehyd Eads ) (Erelax - Erelax - nadsEwater)/nads
(1)
hyd dehyd where Erelax , Erelax , and Ewater are the energies of the relaxed hydrated system, the relaxed dehydrated system, and the water molecule, respectively, and nads stands for the number of water molecules per cell. Vibrational analysis is performed in the harmonic approximation, displacing only the atoms of the adsorbate and keeping all other atoms at their relaxed, equilibrium positions. Test calculations have shown that stretch frequencies of the adsorbate (which have the largest energies) are quite insensitive to this approximation (
