Surface Structure and Reactivity of Anatase TiO2 Crystals with

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Surface Structure and Reactivity of Anatase TiO2 Crystals with Dominant {001} Facets Sencer Selçuk and Annabella Selloni* Department of Chemistry, Princeton University, Frick Laboratory, Princeton, New Jersey 08544, United States S Supporting Information *

ABSTRACT: Hydrofluoric acid (HF)-assisted hydrothermal/solvothermal methods are widely used to synthesize anatase TiO2 single crystals with a high percentage of {001} facets, which are generally considered to be highly reactive. We have used Density Functional Theory calculations and first principles molecular dynamics simulations to investigate the structure of these facets, which is not yet well understood. Our results suggest that (001) surfaces exhibit the bulk-terminated structure when in contact with concentrated HF solutions. However, (1 × 4)-reconstructed surfaces, as observed in UHV, become always more stable at the typical temperatures, 400−600 °C, used to clean the as-prepared crystals in experiments. Since the (1 × 4)-reconstructed surfaces are only weakly reactive, our results predict that synthetic anatase crystals with dominant {001} facets should not exhibit enhanced photocatalytic activity, consistent with recent experimental observations.

1. INTRODUCTION Increasing the efficiency of photocatalytic materials is a major scientific and technological challenge.1−4 Among the available methods, intense research efforts have been recently focused on tailoring the crystal shape to expose more reactive facets.5−14 One of the most widely used photocatalytic materials is anatase TiO2, for which the equilibrium crystal shape is a truncated square bipyramid15−17 exposing majority {101} and minority {001} facets. Prompted by computational results suggesting that the minority (001) surface could play an important role in the reactivity of anatase,18,19 Yang et al.6 first showed that anatase single crystals with large amounts of (001) surface can be prepared by solvothermal synthesis using hydrofluoric acid (HF) as the controlling agent. (The samples prepared by this procedure will be called F-anatase in the following.) Their work was followed by many others,8−11 and crystals with up to almost 100% (001) surfaces have been synthesized.11 The structure of the resulting (001) surface is still unclear however. In particular, while anatase TiO2(001) is known to exhibit a 1 × 4 reconstruction under vacuum conditions,20 it is not yet established whether the (001) surface of F-anatase samples is also reconstructed or has instead the structure of the bulk terminated surface.21 Here, we use Density Functional Theory (DFT) calculations and first principles molecular dynamics (FPMD)22 simulations to investigate the structure and energetics of H2O and HF adsorption on the 1 × 1 bulk-terminated and 1 × 4 reconstructed anatase (001) surfaces, and determine the corresponding surface free energy diagrams in wide temperature and pressure ranges. We consider extended surface models, which are appropriate for crystals of μm size (as, e.g., in ref 6.), but are less well justified for crystals of nanometric dimensions (as, e.g., in ref 12.). We also assume anatase crystals © 2013 American Chemical Society

without defects and/or added metal impurities, as often are present in real catalytic materials,12,14 since these do not affect the equilibrium structure of extended surfaces.

2. METHODS AND MODELS Our DFT calculations are based on the Generalized Gradient Approximation (GGA) of Perdew−Burke−Ernzerhof (PBE),23 and the plane-wave-pseudopotential scheme as implemented in the Quantum Espresso package.24 Bulk-terminated slabs (Figure 1a) were generated by cleaving bulk anatase and

Figure 1. Side view of (a) bulk-terminated and (b) 1 × 4 reconstructed anatase (001) slabs. The bottom trilayer (shaded area) is fixed at bulk positions in the DFT optimizations. Red and gray spheres represent oxygen and Ti atoms, respectively.

inserting a vacuum layer (∼12 Å) between the two resulting surfaces. To describe the reconstructed surface, we used the “ad-molecule” model of ref 25. (Figure 1b), which agrees well with the available experimental information and is widely accepted. This model is characterized by added TiO2 rows forming “ridges” which run parallel to the [010] direction and Received: February 28, 2013 Published: March 6, 2013 6358

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Figure 2. Free energy diagram of the anatase (001) surface in the presence of water vapor (center), and selected water adsorption structures: (a) 0.25 ML, and (b) 1 ML water on the 1 × 1 surface; (c) 0.20 ML, and (d) 0.80 ML water on the 1 × 4 surface. Surface formation energies are plotted as a function of the water chemical potential (bottom) or water vapor pressure at different temperatures (top). The shaded area highlights the region of chemical potential corresponding to liquid water (298−500 K).

Figure 3. Free energy diagram of anatase (001) surface interacting with gaseous HF (center), and selected HF adsorption structures: (a) 0.25 ML, and (b) 1 ML HF on the 1 × 1 surface; (c) 0.20 ML, and (d) 0.80 ML HF on the 1 × 4 surface. Surface formation energies are plotted as a function of the HF chemical potential (bottom) or HF pressure at different temperatures (top). The shaded area highlights the region of chemical potential corresponding to HF aqueous solutions (2%-40%).

with cleavage of surface Ti−O bonds. At higher coverage, our results differ from those of previous studies, which were based on smaller unit cells.18,19 Here we find a mixed dissociatedmolecular adsorption structure where only 25% of the water molecules is dissociated, while the others form hydrogen bonds with the surface and between themselves (Figure 2b). This leads to a decrease of the adsorption energy per molecule which is reduced to 0.98 eV at full monolayer coverage. From Figure 2b, we can remark an interesting similarity between the structure of the fully hydrated bulk terminated surface and that of the clean reconstructed surface (Figure 1b). Calculations of water adsorption on the 1 × 4 reconstructed surface show that the latter is significantly less reactive than the 1 × 1 surface, consistent with the relative stabilities of the two surfaces.25,28 The site of highest reactivity for the 1 × 4 surface is the ridge, where water adsorbs dissociatively with an energy of 1.54 eV at low coverage. In the terrace region, water is physisorbed or not adsorbed at all (Figure 2d). The water adsorption energy is 0.63 eV/molecule at full coverage. The computed water adsorption structures and energies described above were used to construct the surface free energy diagram as a function of the water chemical potential (μH2O), or

expose 4-fold coordinated Ti atoms (Ti4c). In-between ridges, “terraces” expose 5-fold coordinated Ti (Ti5c) and 2-fold coordinated O (O2c) atoms, and have the structure of the unreconstructed surface with a lateral compression along [100]. For consistency, (2 × 4) surface supercells (twice the size of the primitive cell of the (1 × 4)-reconstructed surface) were used for all calculations. The approach of ref 26 was used to construct the surface free energy diagrams, with gas phase thermodynamic data taken from JANAF tables.27 Additional details on the methods and models used for the calculations are given in the Supporting Information (SI).

3. RESULTS AND DISCUSSION Anatase (001) in Humid Environment. A large number of structures were optimized in order to accurately determine the most stable configurations for water adsorbed on the bulk terminated and 1 × 4 reconstructed (001) surfaces at different coverages up to one monolayer (ML). In agreement with previous studies,18,19 we find that water readily dissociates on the 1 × 1 surface at low coverage. This adsorption is strongly exothermic, by 2.33 (2.38) eV/H2O in the case of one (two) adsorbed water molecule(s) per 2 × 4 supercell, and occurs 6359

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Figure 4. Snapshots from FPMD simulations of the bulk terminated (a,b) and reconstructed (c,d) anatase (001) surface in 32% HF aqueous solution (16 HF + 34 H2O molecules).

concentrated aqueous HF solutions, as used for the synthesis of the F-anatase crystals.9 More importantly, Figure 3 shows that the clean 1 × 4 surface is the stable structure at low μHF/ pressure, in particular at pressures below ∼0.01 atm at 900 K. Since, experimentally, heat treatment at 400−600 °C is required to clean the F-anatase samples,6 our calculations predict that the surface obtained after heat treatment is 1 × 4 reconstructed. Interaction of Anatase (001) with Gaseous HF−Water Mixtures and with a Liquid HF−Water Solution. We investigated the adsorption of mixed HF−water monolayers with varying amounts of the two molecules on the bulkterminated and reconstructed surfaces. In these mixtures HF is always found to be adsorbed dissociatively, whereas water forms H-bonds with the surface (SI, Figure S2). The results of these calculations were combined with those for the adsorption of a single species to build the surface phase diagram shown in Figure S3 of the SI. They predict that the reconstructed surface is always preferred over the bulk-terminated one in the presence of a gaseous HF mixture. The above static calculations still provide a very simplified description of the HF aqueous solution environment used for the synthesis of F-anatase. To further investigate the relative stabilities of the two surfaces in such an environment, we performed FPMD simulations at 350 K for these surfaces in contact with a 32% HF aqueous solution. The simulations were started using the most stable geometries established by total energy calculations, and, after 2 ps equilibration, were continued for additional 10 ps. The total potential energy evolutions during the simulations and their corresponding probability distributions are shown in SI, Figure S4 (see also Figure S5). Within the error bars of our simulations, we find no significant difference between the total energies of two surfaces in HF solution. However, analysis of the surface structures during the simulations provides some interesting clues, see Figure 4. While the unreconstructed surface is very stable, the ridges of the 1 × 4 reconstructed surface show important structural fluctuations in HF solution. A complete disruption of the ridges and transformation to the unreconstructed surface cannot be observed since the number of particles is fixed in our simulations. However these results suggest that the reconstructed surface is likely to be unstable in HF aqueous solution.

water pressure at different temperatures, see Figure 2. It is immediately evident from this diagram that the 1 × 4 surface is more stable than the 1 × 1 one in the whole range of water chemical potentials considered. In particular, the fully hydrated 1 × 4 surface is predicted to be the most stable (001) surface configuration at room temperature and a pressure of 0.023 atm, the partial pressure of water vapor in air. Moreover, a partially hydrated 1 × 4 surface is stable in most of the shaded region of the diagram, which corresponds to the range of μH2O values of liquid water. Even though the diagram refers to the surface in contact with water vapor, these results may be considered as an indication that the (001) surface in contact with liquid water is also 1 × 4 reconstructed. Anatase (001) in the Presence of HF(g). Calculations similar to those of water adsorption were performed to study the interaction of the bulk terminated and 1 × 4 reconstructed (001) surfaces with gaseous HF. On the bulk-terminated surface, the structure of adsorbed HF is similar to that of water at low coverage; that is, the adsorption is dissociative and is accompanied by cleavage of a surface Ti−O bond (Figure 3a). The adsorption energy is 2.98 eV per molecule at 0.25 ML, about 25% larger than that of water at the same coverage. In contrast to water, HF adsorption is dissociative also at high coverage, but in this case all Ti−O bonds remain intact (see Figure 3b). The adsorption energy decreases with increasing coverage, down to the value 1.22 eV per molecule at full coverage. On the 1 × 4 surface, HF adsorbs dissociatively on the Ti atoms of the ridge; the computed adsorption energy is 2.14 eV (1.54 eV/HF) in the case of a single (two) HF molecule(s) per cell. At higher coverage, terrace sites are occupied by a mixture of dissociatively adsorbed and H-bonded HF molecules. The adsorption energy is 0.81 eV/HF at full coverage (Figure 3c). Figure 3 shows the free energy diagram of the anatase (001) surface interacting with gaseous HF. In comparison to water (Figure 2), HF adsorption stabilizes the bulk-terminated surface more than the reconstructed one, due to a less favorable F bonding configuration on the latter, where the adsorbed fluorines cause significant distortions of the ridges. In addition, at variance with the bulk-terminated surface, some repulsion between the adsorbed fluorines and the surface oxygens appears to be present on the reconstructed surface. This changes the relative energies of the two surfaces making the 1 × 1 surface essentially degenerate with the reconstructed one in a small range of HF chemical potential (μHF) values characteristic of 6360

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4. CONCLUSIONS We have studied the (001) surface structure of anatase crystals prepared by HF-assisted hydrothermal synthesis using first principles calculations and extended surface models, which are well justified for crystals of micrometer size. Our results indicate that the {001} facets of these crystals exhibit a 1 × 1 bulk-terminated structure in concentrated HF solutions. However, (1 × 4) reconstructed surfaces should form after cleaning the samples at 400−600 °C to remove the adsorbed fluorine. Since the 1 × 4 reconstructed surface is only weakly reactive, our results predict that synthetic anatase samples with a large percentage of {001} facets should not exhibit enhanced photocatalytic activity, consistent with recent experimental observations.12,29,30

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information about the methodology including convergence criteria, structural data, snapshots from the MD simulation, and the two-component phase diagram. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by DoE-BES, Division of Chemical Sciences, Geosciences and Biosciences under Award DE-FG0212ER16286. We used resources of the Center for Functional Nanomaterials, Brookhaven National Laboratory. We also acknowledge use of the TIGRESS high performance computer center at Princeton University.

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REFERENCES

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