Fluorite TiO2(111) Surface Phase for Enhanced Visible-Light Solar

Aug 14, 2014 - ABSTRACT: The wide band gap of titanium dioxide (TiO2) limits its photoactivity only in the ultraviolet-light region and greatly blocks...
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Fluorite TiO2(111) Surface Phase for Enhanced Visible-Light Solar Energy Conversion Mang Niu, Daojian Cheng,* and Dapeng Cao* State Key Laboratory of Organic−Inorganic Composites, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *

ABSTRACT: The wide band gap of titanium dioxide (TiO2) limits its photoactivity only in the ultraviolet-light region and greatly blocks application of TiO2 in solar energy. Finding a pure TiO2 phase with a band gap around 2.0 eV is a very important issue for solar energy applications. We use the first-principles calculations to predict a fluorite TiO2(111) surface phase formed on the reconstructed high-energy rutile TiO2(011) surface. The band gap of the fluorite TiO2(111) surface phase is about 2.1 eV. We propose that engineering the high-energy surfaces of common TiO2 to obtain the fluorite TiO2(111) surface phase at room conditions is a promising method for the preparation of pure TiO2 materials with visible-light activity.

1. INTRODUCTION Titanium dioxide (TiO2) is a wide band gap semiconductor with a variety of applications from solar cells to photocatalysts by harvesting solar light.1−6 However, the large band gap of common TiO2 polymorphs (3.0 eV for rutile and 3.2 eV for anatase) limits their photoactivity to the ultraviolet (UV) light region (only 5% of the solar spectrum) and blocks application of common TiO2 in solar energy. As is well-known, the optimal band gap of the oxide should be around 2.0 eV, which can lead to efficient visible-light absorption. In past decades, great efforts have been devoted to narrow the band gap of TiO2 to make it suitable for sunlight absorption by composites7−9 and heteroatom doping.10−17 Although the doping method can improve the visible-light absorption of TiO2 to some extent, its photoactivity is still limited by the doping-induced charge carrier trapping and recombination18,19 and the low stability against photocorrosion.20,21 More recently, a new two-dimensional phase of TiO2 was prepared with a band gap of only ∼2.1 eV,22 making it possible to develop the dopant-free pure TiO2 phase for the visible-light absorption. However, the formation mechanism of this kind of new two-dimensional phase of TiO2 is still unclear. Besides the natural phases (rutile, anatase, and brookite) of TiO2, many high-pressure phases including the baddeleyite,23 columbite,24 orthorhombic,25 cotunnite,26 pyrite,27 and fluorite27 have been found in experiments. Although the band gap of the high-pressure TiO2 phase could match the visible-light absorption,28,29 the high-pressure TiO2 phases are not stable at atmospheric pressure and are thus not useful for photocatalysis or solar cells. In this work, we predict that the high-pressure phases of TiO2 can be stabilized as surface phases on the natural phases of TiO2 under room conditions by using firstprinciples calculations. First, we calculated the electronic properties of the existing high-pressure TiO2 phases by using hybrid density functional and found that the fluorite TiO2 has © 2014 American Chemical Society

desirable band gap of 2.178 eV. Then, we proposed that the fluorite TiO2(111) surface phase is likely to be formed on the high-energy rutile TiO2(011) surface, which can perfectly explain the formation mechanism of the new two-dimensional phase of TiO2.22

2. CALCULATION METHODS Our density functional theory (DFT) calculations were performed using the Vienna Ab-initio Simulation Package (VASP)30,31 with the frozen-core all-electron projector-augment-wave (PAW)32,33 method. The Perdew−Burke−Ernzerhof (PBE)34 of generalized gradient approximation (GGA) was adopted to describe the exchange and correlation potentials. To search for a high-pressure TiO2 phase that has a desirable band gap, the structural and electronic properties of baddeleyite, columbite, orthorhombic, cotunnite, pyrite, and fluorite TiO2 were calculated. The baddeleyite, columbite, cotunnite, pyrite, and fluorite TiO2 were modeled by 2 × 2 × 1 supercells, and the orthorhombic TiO2 was modeled by a 1 × 2 × 1 supercell, respectively. The special k-point meshes were generated using the Monkhorst−Pack35 scheme with 5 × 5 × 9 meshes for baddeleyite, columbite, pyrite, and fluorite TiO2 supercells and 5 × 7 × 7 meshes for orthorhombic and cotunnite TiO2 supercells, respectively. The cutoff energy of the plane-wave basis set was set to 500 eV for all systems. Extensive tests were carried out to ensure the convergences with respect to the cutoff energy and k-point meshes. The geometry optimizations were performed until the forces on each ion were reduced below 0.01 eV/Å, and the resulting structures were then used to calculate the electronic structures. Received: May 16, 2014 Revised: June 30, 2014 Published: August 14, 2014 20107

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Table 1. Calculated Structural Parameters of Baddeleyite-Type, Columbite-Type, Orthorhombic-Type, Cotunnite-Type, PyriteType, and Fluorite-Type TiO2, Compared with Available Experimental Dataa cell parameters (Å) space group

Z

this work

experiment

Wykoff parameters

ref

baddeleyite

P21/c (14)

4

23

Pbcn (60)

4

Ti (0.000, 0.177, 0.250)

24

orthorhombic

Pbca (61)

8

cotunnite

Pnma (62)

4

pyrite

Pa3̅ (205)

4

a = 4.64 b = 4.76 c = 4.81 β = 99.2° a = 4.55 b = 5.46 c = 4.92 a = 9.052 b = 4.836 c = 4.617 a = 5.163 b = 2.989 c = 5.966 a = 4.516

Ti (0.277, 0.056, 0.215) O (0.064, 0.324, 0.351) O (0.447, 0.759, 0.464)

columbite

a = 4.840 b = 4.918 c = 5.084 β = 99.483° a = 4.586 b = 5.577 c = 4.933 a = 9.426 b = 4.988 c = 4.834 a = 5.240 b = 3.163 c = 6.297 a = 4.902

fluorite

Fm3m (225)

4

a = 4.836

a = 4.516

phase

a

O (0.271, 0.381, 0.419) Ti (0.885, 0.042, 0.250) O (0.789, 0.376, 0.138) O (0.975, 0.737, 0.496) Ti (0.253, 0.250, 0.105) O (0.363, 0.250, 0.418) O (0.013, 0.750, 0.347) Ti (0.000, 0.000, 0.000) O (0.341, 0.341, 0.341) Ti (0.000, 0.000, 0.000) O (0.250, 0.250, 0.250)

25b

26b

27 27

Z is the number of TiO2 units in the choice of unit cell. bCalculated from molar volume.

Figure 1. Band structures of baddeleyite, columbite, orthorhombic, cotunnite, pyrite, and fluorite TiO2 along high-symmetry lines of the Brillouin zone, calculated with the HSE06 functional. The band gaps are illustrated with blue lines with two arrows, and the values are also displayed in blue numbers.

agreement with the experimental value of 3.0 eV. It suggests that the HSE06 hybrid functional with 20% mixing of HF exchange can predict the accurate electronic structures of TiO2 systems. In the common phase of rutile, anatase, and brookite TiO2, each Ti atom is octahedrally surrounded by six nearest O atoms. Alongside the rutile structure, there is also the columbite (α-PbO2) structure, where the Ti atoms in columbite TiO2 are also surrounded by six O atoms but now in a highly distorted octahedral coordination (see Figure S1 in the Supporting Information). For baddeleyite (ZrO2) and orthorhombic structures, the coordination of Ti increases to seven so that

It is well-known that the standard DFT functional fails to give the accurate electronic structures and especially band gap values of TiO2, due to the self-interaction errors inherent to such functionals. To overcome these shortcomings of the standard DFT functional, we used the Heyd−Scuseria− Ernzerhof (HSE06)36,37 hybrid functional to calculate the electronic structures. To test the reliability of HSE06 hybrid functional, bulk rutile TiO2 was chosen as an example for the energy band calculations. The HSE06 calculated band gap value of rutile TiO2 is 2.929 eV, with a mixing of 20% HF exchange in the short-range of exchange contribution, which is in good 20108

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each Ti atom has one more O neighbor. In the fluorite (CaF2) structure, Ti atoms occupy the (0, 0, 0) position defining a facecentered cubic (fcc) sublattice, while O atoms occupy the ± (0.25, 0.25, 0.25) positions. In this structure, each Ti atom is eight-coordinated with O atoms. The pyrite (FeS2) structure is a slight distortion of the fluorite structure, with Ti atoms still defining an fcc sublattice, while O atoms located at ± (0.34, 0.34, 0.34). In pyrite TiO2, each Ti atom is rhombohedrally surrounded by eight O atoms, with six O atoms from an inner shell and two O atoms displayed in farther positions. For the cotunnite (PbCl2) TiO2, each Ti atom is nine-coordinated to O atoms (see Figure S1 in the Supporting Information). The higher oxygen coordinated phases have been attributed to the harder high-pressure phase of TiO2, so the cotunnite TiO2 is the hardest formation oxide among the existing TiO 2 polymorphs.26 Table 1 summarizes the equilibrium 0 GPa structural details of baddeleyite-type, columbite-type, orthorhombic-type, cotunnite-type, pyrite-type, and fluorite-type TiO2.

the UV light, with a wavelength less than 420 nm, due to its large band gap of 3.0 eV. In contrast, the light absorption of fluorite TiO2 is extended to the visible region, reaching 550 nm, which is in good agreement with the band gap prediction. Our DFT calculation results indicate the high-pressure phase of fluorite TiO2 has potential applications as a light absorber in solar energy conversion. Previous report indicates that the high-pressure cubic phase of TiO2 (fluorite or pyrite) can be stable at ambient temperatures with pressures above 9 GPa.27 However, considering the applications of TiO2 materials for solar energy conversion, the synthesis of metastable fluorite TiO2 at room conditions has great advantages. Although the high-pressure phase of TiO2 is not stable at room conditions, we expect that the high-pressure phases can be stabilized as thin films or surface phases on the surface of the common natural phases.38 Since the static electric fields of Ti atoms in the highly symmetric [TiO8] cubes can tune the band gap of TiO2 into visible-light activity, engineering the surface structures of common TiO2 phase to form a fluorite TiO2 surface with [TiO8] cubes becomes a possible approach to avoid the high pressure required by the synthesis of bulk fluorite TiO2. Notably, it has been reported that the reconstructed rutile TiO2(011) surfaces under the oxidizing condition was found to be visible-light active without any doping.22 Moreover, the new pure TiO2 surface phase formed on rutile TiO2(011) surfaces has a band gap of 2.1 eV, with a quasi-hexagonal structure. In fact, the band gap energy of this new TiO2 surface is very close to that of fluorite TiO2 (2.178 eV). Moreover, the cubic fluorite TiO2(111) plane has a similar hexagonal structure. To verify this deduction, the structures and properties of the fluorite TiO2(111) surface were investigated. The most stable rutile TiO2(110) surface, the rutile TiO2(011) surface, and the cubic pyrite TiO2(111) surface were also studied for comparison (see Figure S4 in the Supporting Information). In four-layer slab models of rutile TiO2(110) and pyrite TiO2(111), each layer is composed of an O−Ti2O2−O trilayer (three atomic layers). For rutile TiO2(011) and fluorite TiO2(111) slabs, the repeated unit layer is composed of an O−Ti−O trilayer. Although both the pyrite TiO2(111) and fluorite TiO2(111) are cubic (111) planes, their surface structures are quite different. Because the fluorite TiO2 has a high symmetry of the Fm3m space group, the top view of the fluorite TiO2(111) surface exhibits a perfect hexagonal structure. However, the top view of the pyrite TiO2(111) surface is complex due to the lower symmetry of pyrite TiO2 than that of fluorite TiO2. The scanning tunneling microscopy (STM) image simulations of fluorite TiO2(111) and pyrite TiO2(111) surfaces were performed based on the relaxed 4√3 × 4 rectangular four-layer slab models, with the bottom two layers fixed in the bulk positions. The sample bias and tunneling height were set to +1.0 V and 1.60 Å, respectively. As shown in Figure 2a, the surface structure of fluorite TiO2(111) in the STM image is hexagonal, which is consistent with the experimental result of the reconstructed rutile TiO2(011) surface (see Figure 2b).22 On the other hand, the STM image of the pyrite TiO2(111) shows a nested hexagonal structure, with a big bright hexagonal structure that contains a small dim hexagonal structure (see Figure 2c). Therefore, the visible-light response of the new TiO2 surface phase can be explained by the formation of a fluorite TiO2(111) surface structure. To further understand the formation mechanism of the fluorite TiO2(111) structure on the rutile TiO2(011) surface,

3. RESULTS AND DISCUSSION To explore a high-pressure phase of TiO2 with narrower band gap than rutile and anatase TiO2, we calculated energy band structures of these six high-pressure TiO2 phases by using the HSE06 hybrid functional, and these are displayed in Figure 1. It is found that the columbite, cotunnite, and pyrite TiO2 phases are indirect band gap semiconductors with the band gap values of 3.693, 2.846, and 2.688 eV, respectively, while the baddeleyite, orthorhombic, and fluorite TiO2 have a direct band gap of 3.324, 2.846, and 2.178 eV, respectively. Among these high-pressure phases of TiO2, fluorite TiO2 has the narrowest band gap of 2.178 eV, which is about 0.8 eV smaller than that of rutile TiO2. In general, the chemical environments of Ti atoms determine the electronic structure of TiO2. In fluorite TiO2, the coordination of the Ti atoms forms a perfect [TiO8] cube with the Oh (m3m) symmetry. According to crystal field theory, the interactions between the Ti-3d electrons and their static electric fields, generated by the eight surrounding O anions in the [TiO8] cube, lead to the splitting of five degenerate Ti-3d states into triply degenerate t2g and doubly degenerate eg states.29 The t2g orbitals of dxy, dyz, and dxz point toward the edges of the cube, while the eg orbitals of dx2−y2 and dz2 point toward the cube faces. As a result, the distances from t2g and eg orbitals to the O anions are a/2 and √2a/2 (a is the lattice constant of fluorite TiO2), respectively. The shorter distance from t2g orbitals to the O anions leads to a stronger repulsion between the electrons in the Ti-3d t2g orbitals and the surrounding O atoms, compared to a spherical field. Therefore, the Ti-3d t2g orbitals will be increased in energy, while the Ti-3d eg orbitals will be lowered in energy and hence stabilized. The lowering of Ti-3d eg states is supported by the calculated band structure (see Figure 1) and density of states (DOSs) (see Figure S2 in the Supporting Information) for fluorite TiO2. It is noticed that the Ti-3d derived conduction state in the highly symmetric [TiO8] cube is likely split in two sections, and the lowering of one of the two substates causes a significant band gap reduction for fluorite TiO2. The optical absorption property of fluorite TiO2 has been evaluated through the frequency-dependent imaginary part of the dielectric tensor ε2(ω) (see Figure S3 in the Supporting Information). Here we also adopted the HSE06 functional to calculate ε2(ω). It is found that the rutile TiO2 mainly absorb 20109

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rutile TiO2(110), rutile TiO2(011), and fluorite TiO2(111) are stable surfaces. Since the surface energy of fluorite TiO2(111) is only slightly greater than that of the most stable rutile TiO2(110) surface, the fluorite TiO2(111) surfaces should exist in the real material. The high-energy rutile TiO2(011) surface is likely to be reconstructed to lower its surface free energy by reducing the number of dangling bonds. Because the fluorite TiO2(111) has a lower surface energy than the rutile TiO2(011) surface and both of them have a similar O−Ti−O unit layer (see Figure S4 in the Supporting Information), the formation of fluorite TiO2(111) surface phase on the reconstructed rutile TiO2(011) substrate is very possible. It is noted that a reversible rutile-to-fluorite phase transition in the transition metal oxide of CeO2 has been proved.39 Therefore, engineering the high-energy surfaces of commonest TiO2 to stabilize the high-pressure phase of the fluorite TiO2(111) surface is a promising method for the preparation of pure TiO2 materials with visible-light activity.

Figure 2. Simulated STM image of a 4√3 × 4 rectangular four-layer slab for (a) fluorite TiO2(111) surface, experimental STM image of (b) new TiO2 phase on reconstructed rutile TiO2(011) surface (reproduced from ref 22), and simulated STM image of a 4√3 × 4 rectangular four-layer slab for (c) pyrite TiO2(111) surfaces. The sample bias and tunneling height were set to +1.0 V and 1.60 Å, respectively. The quasi-hexagonal symmetries of the fluorite TiO2(111) and pyrite TiO2(111) surfaces are displayed with colored hexagons.

4. CONCLUSIONS In summary, we have used hybrid density functional calculations to investigate the electronic structures of the existing high-pressure phases of TiO2, including the baddeleyite, columbite, orthorhombic, cotunnite, pyrite, and fluorite types. Among these structures, the Ti-3d derived conduction state splitting of the highly symmetric [TiO8] cubes in fluorite TiO2 causes a significant band gap reduction. As a result, the fluorite TiO2 is found to be visible-light-active with a reduced band gap of 2.178 eV, which may be used in solar energy systems to improve the visible-light absorption. Furthermore, based on the analysis of the simulated STM images and the calculated surface energies, a fluorite TiO2(111) surface phase on the reconstructed high-energy rutile TiO2(011) surface may be the experimentally reported new two-dimensional phase of TiO2.22 It is expected that engineering the high-energy surfaces of common TiO2 to stabilize the high-pressure phase of fluorite TiO2(111) surface would avoid the high pressure required by the synthesis of bulk fluorite TiO2, which is a promising method for the preparation of pure TiO2 materials with visiblelight activity without any heteroatom doping.

the surface energies of rutile TiO2(011), fluorite TiO2(111), rutile TiO2(110), and pyrite TiO2(111) surfaces were calculated, and the results are displayed in Figure 3. For rutile

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ASSOCIATED CONTENT

S Supporting Information *

Figure 3. Dependence of calculated surface energy on slab thickness for rutile TiO2(011)S, fluorite TiO2(111), rutile TiO2(110), and pyrite TiO2(111) surfaces. All the atoms were allowed to relax during the geometry optimizations. Vacuum width is 10 Å in all cases.

The bulk structures, DOSs, optical absorption properties, and surface structures of different TiO2 polymorphs. This material is available free of charge via the Internet at http://pubs.acs.org.

TiO2(110) and rutile TiO2(011) surfaces, the surface energies exhibit an oscillatory behavior. This is caused by the different symmetry of slabs with an odd or even number of layers. The slabs with an odd number of layers have a symmetry plane at the center of the slab, which inhibits relaxations and increases the surface energy. The opposite happens for slabs with an even number of layers, and the combination of these effects delays the convergence of the surface energy with increasing slab thickness. The fluorite TiO2(111) surface energy is almost constant because in this structure the interaction between layers is very small. For the pyrite TiO2(111) surface, the surface energy shows a divergent trend and decreased to negative values, indicating that the pyrite TiO2(111) surface is not stable. The calculated surface energies in Figure 3 confirm that

Corresponding Authors

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AUTHOR INFORMATION

*E-mail: [email protected]. Phone: +8610-64427616. *E-mail: [email protected]. Phone: +8610-64443254. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National NSF of China (91334203, 21106003, 21121064), National 863 Program (2013AA031901), Beijing Novel Program (Z12111000250000), Outstanding Talents Plans and “Chemical Grid Project” of BUCT, and the Beijing Computing Center (BCC). 20110

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