ARTICLE pubs.acs.org/JPCC
Hexagonal TiO2 for Photoelectrochemical Applications Y. H. Lu,*,†,‡ B. Xu,§ A. H. Zhang,† M. Yang,† and Y. P. Feng† †
Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China § College of Physics and Communication Electronics, Jiangxi Normal University, Nanchang, Jiangxi 330022, People’s Republic of China ‡
ABSTRACT: Band gap narrowing is crucial for applications of TiO2, especially for photoelectrochemical water splitting. In this article, a novel hexagonal TiO2 phase is predicted by means of the first-principles calculations with the hybrid density functional. The crystal structure of this hexagonal phase of TiO2 consists of metastable TiO2 layers (space group P63/MMC), similar to that of graphite. The calculated electronic structure reveals that bulk hexagonal TiO2 is a semiconductor with a much smaller band gap of 1.7 eV compared to the rutile and anatase phases, while that of a single layer of TiO2 is 2.1 eV, which are in excellent agreement with available experimental results. This hexagonal phase of TiO2 would be a good candidate for photocatalyst in hydrogen production using sun light which is considered to be the ultimate clean fuel.
’ INTRODUCTION The photocatalytic properties of titanium dioxide (TiO2) have attracted much attention since the discovery of water splitting on TiO2 almost 40 years ago.1 As a prime photocatalyst for hydrogen production through photoelectrochemical (PEC) water splitting as well as other applications such as water or air purification and dye-sensitized solar cells, TiO2 has many advantages including strong catalytic activity, high chemical stability, and a long lifetime of photon-generated carriers.1,2 Furthermore, TiO2 is safe, abundant, and inexpensive.3 However, the efficiency of solar-driven photocatalytic processes involving TiO2 is limited by the large band gap of TiO2 in its commonly available phases: rutile and anatase. In the past decades, various experimental and theoretical studies have been carried out in attempt to reduce the band gap of TiO2 to increase its absorption of visible light. A popular approach is to dope TiO2 with metal or nonmetal impurities to generate donor or acceptor states in the band gap.4 Although activity in the visible light range has been reported for doped-TiO2,2c,5 this increase in light absorption does not necessarily translate into higher photocatalytic activity because charge traps are introduced that act as recombination centers for the photoexcited charge carriers.4a It was also proposed that passivated codoping by both metal or nonmetal impurities can reduce the carrier recombination centers and narrow the band gap of TiO2.6 But this requires heavy doping which is difficult due to the solubility limit. Strain could be an alternative way to tune the band gap of TiO2.7 However, large strain is needed to produce a significant change in the band gap and it is not feasible for practical applications. Clearly, a pure TiO2 phase with a band gap in the visible light energy range would be ideal for such applications. In this article, by means of first-principles calculations, we propose a new phase of TiO2 (hex-TiO2) that has intriguing r 2011 American Chemical Society
physical properties. The new crystal structure is hexagonal with the space group P63/MMC, similar to that of MoS2 and graphite. Results of our calculations show that the layered hex-TiO2 structure is metastable and a single hex-TiO2 layer could be easily formed, similar to graphene, due to weak interaction between the hex-TiO2 layers. Most importantly, calculated electronic structures reveal that hex-TiO2 is a semiconductor with a band gap of 1.74 eV, much narrower than those of all existing TiO2 phases. The band gap of few layer hex-TiO2 increases due to quantum confinement effect and that of a single hex-TiO2 layer reaches 2.1 eV, which is sufficient to absorb a main part of sun light. The calculated energy gap is consistent with observation in a recent scanning tunneling spectroscopy (STS) experiment.8 The narrow energy gap makes this hexagonal phase of TiO2 a good candidate for photocatalyst in hydrogen production using sun light which is considered the ultimate clean fuel.
’ METHOD First-principles calculations based on the density-functional theory (DFT),9 as implemented in the Vienna ab initio simulation package (VASP),10 were carried out using the projector augmented wave method.11 The generalized gradient approximation (GGA) developed by Perdew and Wang12 was adopted for the exchange-correlation potential. The plane-wave cutoff energy was taken to be 500 eV and k-point sampling based on the Monkhorst Pack scheme13 was performed to ensure that the total energy is converged within 0.01 eV/TiO2 unit. All structures were optimized until the remanent Hellmann Feynman Received: June 9, 2011 Revised: July 29, 2011 Published: July 29, 2011 18042
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force on each ion is less than 0.01 eV/Å. Due to underestimation of band gap by GGA,14 the hybrid density functional (HSE06)15 was used to estimate the band gap. Band gaps of rutile and anatase phases calculated using the same approach (3.0 and 3.2 eV, respectively) are in excellent agreement with experimental values.
’ RESULTS AND DISCUSSION Similar to the structure of MoS2, each Ti center in hex-TiO2 occupies a trigonal prismatic coordination sphere and is bound to six oxide ligands, as shown in Figure 1a. Each oxygen center is pyramidal and is connected to three Ti centers. In this way, the trigonal prisms are interconnected to form a layered structure, wherein Ti atoms are sandwiched between layers of oxygen atoms. In this new phase of TiO2, each unit cell contains two TiO2 units with four O atoms and two Ti atoms. The calculated lattice constant, equilibrium density, bond length, cohesive energy, energy gap between the valence band maximum (VBM), and the conduction band minimum (CBM), bulk modulus of hex-TiO2 are summarized in Table 1, along with those of rutile and anatase phases for comparison. All data presented here were obtained using the same approach and can thus be meaningfully compared. It is noted that the coordination numbers of Ti atom and O atom in hex-TiO2 are the same as those in rutile and anatase phases and the bond lengths
Figure 1. (a) Schematic depiction of the structure of hexagonal TiO2. Gray and red balls represent Ti and O atoms, respectively. (b) The total energy per TiO2 unit as a function of volume per TiO2 unit for hexagonal, anatase, and rutile phases, obtained by GGA functional. The curve of hexagonal phase around minimum point is shown in the inset. (c) Simulated XRD patterns of hexagonal, anatase, and rutile phases. The X-ray wavelength we use is 1.54059 Å.
in different structures are very close. There are, however, two different types of bond angles, 68° in the triangular prism and 91.7° between two pyramids. The former is much less than that in the rutile phase (90°), implying that the O Ti O unit in hex-TiO2 is strained which may lead to an increase in the total energy. Indeed, our calculations confirm that the cohesive energy of hex-TiO2 is 19.61 eV per TiO2 unit at the GGA level, which is about 1.7 eV smaller than those of the two common phases (rutile and anatase). The lower cohesive energy suggests that hex-TiO2 might be less stable thermodynamically compared to the common phases at zero pressure. The equilibrium density (F) of hex-TiO2 is 3.28 g/cm3 which is much smaller than those of rutile and anatase phases due to a large space between hex-TiO2 layers. The calculated bulk modulus of hex-TiO2 is also much smaller compared to those of rutile and anatase TiO2, indicating weak interaction between hex-TiO2 layers. Figure 1b shows the total energy per TiO2 unit as a function of unit cell volume of hex-TiO2, as well as those of the rutile and anatase phases. The total energy of the hexagonal phase has a single minimum around 40 Å3/TiO2 unit, indicating that the structure can exist. However, the total energy changes very slowly when the volume is increased beyond the equilibrium value. As the volume change is mainly due to increase in interlayer spacing, the results suggest a weak interaction between the TiO2 layers and easy formation of a single hexagonal TiO2 layer, similar to graphite.16 It is also noted that the minimum in total energy of the hexagonal phase is about 1.7 eV higher per TiO2 unit than the other two common phases, suggesting that hex-TiO2 is a metastable phase. To provide information for direct experimental verification, we simulated the X-ray diffraction (XRD) spectrum with a wavelength of 1.54059 Å for hex-TiO2 and the two common phases and present the results in Figure 1c. Distinguishable from the anatase and rutile phases where the first strong peak appears at 2θ ∼ 26°, the first strong peak of hex-TiO2 appears at 2θ = 15.79°. Several small peaks appear at large diffraction angles, similar to the diffraction spectrum of MoS2. These features may be useful for experimental identification of the hexagonal phase of TiO2. The most interesting and important property of hex-TiO2 is its smaller band gap compared to all existing phases of TiO2. The calculated band gap of bulk hex-TiO2 is 1.74 eV at the HSE06 level. As a comparison, the band gaps of the rutile and anatase phases calculated using the same approach are 3.0 and 3.2 eV, respctively, which are in excellent agreement with experimental values. A filled band with a bandwidth of 1.0 eV appears at 0.7 eV above the valence band, significantly increasing the VBM of hexTiO2. As shown in Figure 2b, this band is mainly composed of a O 2p orbital with a little Ti d character as a result of hybridization with the empty Ti 3d conduction states. The real space distribution of the wave function corresponding to this band shown in Figure 2d also exhibits pz character localized on O atoms. This
Table 1. The Space Group, Calculated Lattice Constant (a and c), Equilibrium Density (F), Bond Length (d), Cohesive Energy (Ecoh), Energy Gap (Eg), and Bulk Modulus (B) at Zero Pressure of Rutile, Anatase, and Hexagonal Phases of TiO2, Respectivleya
a
structure
space group
rutile
P42/MNM
a (Å)
c (Å)
F (g/cm3)
d (Å)
Ecoh (eV/TiO2 unit)
1.96, 2.01
21.28
4.65
2.97
4.14
(4.59)
(2.95)
(4.27)
anatase
I41/AMD
3.81 (3.78)
9.70 (9.52)
3.79 (3.89)
hexagonal
P63/MMC
2.89
11.17
3.28
Eg (eV) 3.0
B (GPa) 198
(3.0)
(211) 165 (179)
1.95, 2.01
21.38
3.2 (3.2)
2.02
19.61
1.7
42
Experimental values18 are shown in parentheses. All values are obtained by GGA functional except for Eg, which is obtained by HSE06 functional. 18043
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single minimum around 7.25 Å2/TiO2 unit, indicating that the structure can exist. We also calculated the electronic structure of a single hex-TiO2 layer and compared it with that of bulk hex-TiO2 in Figure 3. It is clear that the profile of the narrow filled band above the valence band in bulk hex-TiO2 remains in the single hex-TiO2 layer. However, the gap between this band and the conduction band is larger than that of the bulk, which can be attributed to quantum size effect in low dimensional system.17 The calculated band gap of a single hex-TiO2 layer is 2.1 eV, in excellent agreement with experimental observation,8 where a band gap of 2.1 eV was reported for hexagonal TiO2 film grown on a rutile TiO2(011) surface. This suggests that the new hexagonal phase predicted here could be grown on a specific surface by careful fabrication. It is noted that the in-plane surface unit cell (about 5.4 Å 5.4 Å) measured from the scanning tunneling microscope (STM) image is about twice the lattice constant of bulk hex-TiO2 (see Table 1) that we predicted here, which may be attributed to P(2 2) reconstruction induced by substrate with a cubic crystalline structure.8 Figure 2. (a) Total density of states of hexagonal, anatase, and rutile phases. (b) PDOS of hexagonal phase projected on oxygen p and titanium d and p orbitals. The Fermi level is set to zero. (a.u. = arbitrary units.) (c) Isosurface of wave function of the lowest unoccupied band. (d) Isosurface of wave function of the highest occupied band. All results are obtained by HSE06 functional.
’ CONCLUSIONS In summary, a novel hexagonal TiO2 phase, similar to MoS2 and graphite, is predicted by means of first-principle calculations. In this new phase, the Ti atoms are 6-fold coordinated with O atoms, same as that in the rutile or anatase phase. The band gap of this hexagonal TiO2 is only 1.74 eV, much narrower than that of all existing TiO2 phases. In addition, our calculation predicts easy formation of a single layer of hex-TiO2. Such two-dimensional structures with a large surface area are very important for effective chemical reaction. The band gap of a single hex-TiO2 layer is 2.1 eV, consistent with the experimental measurement. This hexagonal TiO2 phase can be an excellent visible light absorber and good candidate for PEC applications in the future. ’ AUTHOR INFORMATION Corresponding Author
Figure 3. (a) Total density of states of bulk hex-TiO2 (black) and single layer hex-TiO2 (red), obtained by HSE06 functional. The Fermi level is set to zero. (a.u.= arbitrary units.) (b) Total energy per TiO2 unit within one single hex-TiO2 layer as a function of area per TiO2 unit of the layer.
band is directly related to the hexagonal and layered structure of hex-TiO2. Compared to the px and py orbitals of O atom, which are strongly hybridized with Ti atoms, the pz orbital of O atom at the apex of the pyramid hybridizes with Ti atoms weakly and its tail extends into the space between hex-TiO2 layers. As a result, the energy of these nearly nonhybridized pz orbitals is higher than that of px or py, and they form a band above the usual valence band. This explains the p character of the VBM of hex-TiO2 observed in experiment and the upward shift of O 2p peak in energy after formation of this new phase.8 The lowest unoccupied band is composed of the d orbitals of Ti atoms with some character of O 2p orbital (see Figure 2c), very similar to the conduction band of the rutile and anatase phases, which is also in good agreement with experimental observation.8 As the interaction between layers of hex-TiO2 is weak, a single hex-TiO2 layer may be easily fabricated experimentally, as in the case of graphene. Figure 3b shows the total energy per TiO2 unit within one single hex-TiO2 layer as a function of area per TiO2 unit of the layer. The total energy of the hex-TiO2 layer has a
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[email protected].
’ ACKNOWLEDGMENT This work is supported by a National Research Foundation (Singapore) Competitive Research Program (Grant No. NRFG-CRP 2007-05) and the National Natural Science Foundation of China (Grant No. 11004171). Calculations were carried out at the Shanghai Supercomputer Center. ’ REFERENCES (1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238 (5358), 37–38. (2) (a) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95 (1), 69–96. (b) Gratzel, M. Photoelectrochemical Cells. Nature 2001, 414 (6861), 338–344. (c) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2 2. Science 2002, 297 (5590), 2243–2245. (3) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Photocatalysis on TiO2 Surfaces—Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95 (3), 735–758. (4) (a) Choi, W. Y.; Termin, A.; Hoffmann, M. R. The Role of Metal-Ion Dopants in Quantum-Sized TiO2—Correlation between Photoreactivity and Charge-Carrier Recombination Dynamics. J. Phys. 18044
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