J. Phys. Chem. C 2010, 114, 6035–6038
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High Photocatalytic Activity of Rutile TiO2 Induced by Iodine Doping Jingfu He, Qinghua Liu, Zhihu Sun, Wensheng Yan, Guobin Zhang, Zeming Qi, Pengshou Xu, Ziyu Wu, and Shiqiang Wei* National Synchrotron Radiation Laboratory, UniVersity of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China ReceiVed: NoVember 27, 2009; ReVised Manuscript ReceiVed: January 29, 2010
To improve the energy conversion of solar irradiation, a photocatalyst with high reactivity under visible light is required. Using density functional theory, the structural and electronic properties of iodine cation-doped rutile TiO2 are studied. The total energy calculations show that iodine substituting for titanium sites in TiO2 matrix is energetically favorable. The electronic structure calculations reveal that iodine doping induces a delocalized band consisting of I 5s states and O 2p states at the top of the valence band of TiO2. Due to this delocalized state, the band gap is markedly narrowed by about 0.4 eV, the optical absorption is extended to the visible light region, and the excited electron-hole pairs are expected to have better mobility. Moreover, the conduction band edge is raised above the reduction level of H2/H2O by I-doping, which enables the achievement of high photocatalytic efficiency of I-doped rutile TiO2. I. Introduction Titanium dioxide (TiO2) with its exceptional properties such as nontoxicity, low cost, and long-term stability has attracted worldwide attention as one of the most promising photocatalysts for hydrogen production, water or air purification, and dyesensitized solar cells.1–4 However, TiO2 only shows relatively high reactivity under ultraviolet (UV) light, whose energy exceeds the large band gap of TiO2, and the photoexcited carriers mobility is relatively low.5 To extend its optical absorption edge to visible light and enhance the energy conversion efficiency, intense research activity has been devoted to metal- and nonmetal-doped titanium dioxide.6–18 Unfortunately, metal dopants suffer from a thermal instability,14 and the formation of strongly localized d states within the band gap results in an increase of carrier-recombination centers. Nonmetal atoms doping such as N, C, and S is expected to be an effective method to improve energy conversion efficiency of the metastable polymorph of TiO2, anatase,11–13 whereas the improvement of visible light absorption on the most stable rutile form is relatively poor.15–18 Recently, some experimental investigations of I-doped rutile TiO2 have been reported,19–21 but the effect of I doping on the photoactivity is still under debate. Liu et al. observed that the I doping increased the photoactivity of TiO2 under both visible light and UV light while pure TiO2 showed no photocatalytic activity under visible light.19 Tojo et al. reported that the absorption edge of I-doped rutile TiO2 was shifted to 600 nm with an enhancement of photocatalytic activity under both visible light and UV light.20 On the contrary, Hong et al. found that I-doped rutile TiO2 only had a slight red-shift of absorption edge and showed no photocatalytic activity under visible light irradiation.21 To reduce the amount of the demanding experimental trials, there is a general need to perform theoretical studies that can provide guidelines for the rational doping and interpret the experimental observations. The theoretical methods have the advantages that they are low in cost but are promising * To whom correspondence should be addressed. E-mail: sqwei@ ustc.edu.cn.
to yield insights into the underlying photocatalytic mechanism of the doped TiO2. Indeed theoretical studies with first-principles calculations have been performed16,22 to clarify the origin of the observed red shift or blue shift of the optical absorption edge and the photocatalytic activity mediated by nonmetal elements (such as N and F) doping, focusing on the analysis of the localization of the impurity bands and their influence on the band structure. Whereas for the I-doped rutile TiO2, the basic questions such as what determines the red shift of the optical absorption edge and whether the impurity states induced by I doping retain the transfer ability of the photocarriers of TiO2 are unclear. To address these questions and discover the nature of photocatalytic activity of rutile TiO2 modified by I doping, it is necessary to investigate the energetics, electronic structure, and band edge modifications of this doped system in the theoretical framework. In this work, density functional theory (DFT) calculations were conducted to explore the atomic occupation of I dopant and the electronic structures of I-doped rutile TiO2. The goal is to gain insight into the modification of iodine impurities on the band structure of rutile TiO2, so as to give some hints on improving the photocatalytic activity of I-doped TiO2. Our calculation reveals the origin of red-shift absorption of I-doped rutile TiO2 and provides an extensive comprehension for the photocatalytic activity of I-doped rutile TiO2 under both visible light and UV light. II. Computational Details The first-principles DFT calculations were performed by the Vienna ab initio simulation package23,24 based on the planewave method. Our calculations employed a generalized gradient approximation, the Perdew-Wang 91 exchange-correlation functional,25 and the ultrasoft pseudopotential.26 The basis set cutoff was 400 eV. The k space integration was done with a 5 × 5 × 5 k-mesh.27 The convergence threshold for self-consistent iteration was set at 10-6 eV, and the lattice parameters and all the atomic positions were fully optimized until all components of the residual forces were smaller than 0.025 eV/Å. The optimized lattice parameters of rutile TiO2 are a ) 4.614 Å
10.1021/jp911267m 2010 American Chemical Society Published on Web 03/15/2010
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He et al.
TABLE 1: Optimized Structural Parameters of I-Cation-Doped Rutile undoped (exptl) undoped (calcd) I-doped
a (Å)
b (Å)
c (Å)
I-O(4)
I-O(2)
4.594 4.614 4.640
4.594 4.614 4.640
2.959 2.976 2.991
1.960 2.066
1.986 2.058
TABLE 2: Calculated Formation Energies for I-Doped Rutile TiO2 formation energy (eV) type
subI-O
subI-Ti
Ti-rich O-rich
4.37 8.98
10.16 0.92
and c ) 2.976 Å (Table 1), in good agreement with experimental results,28 indicating that our methodology is reasonable. The density of states (DOS) for each optimized I-doped TiO2 structure was calculated by using the tetrahedron method with Blo¨chl corrections. III. Results and Discussion First of all, we investigate whether the doped I impurity tend to occupy the site of O (anion doped) or Ti (cation doped) in rutile TiO2 from the energetics point of view. For this purpose, we calculated the formation energies Ef of the dopant defects according to the following formulas:
Ef ) Etot(I-doped) - [Etot(pure) + µI - µO] Ef ) Etot(I-doped) - [Etot(pure) + µI - µTi] Here Etot(I-doped) is the total energy of the supercell with the I impurity and Etot(pure) is the total energy of the ideal cell. µI is the chemical potential of I impurity; µTi and µO are the chemical potentials of Ti and O, respectively. In practice, the formation energy depends on the growth condition, which can be Ti-rich, O-rich, or anything in between.29 Correspondingly, we simulated the energetics calculation under different circumstances and the obtained results are listed in Table 2. The formation energies for I substituting O in rutile TiO2 under O-rich and Ti-rich conditions are 8.98 and 4.37 eV, respectively. In contrast, the smallest formation energy of 0.92 eV is required for I substituting Ti under O-rich condition. Thus, the I substituting O defects are difficult to form because of their large formation energy under both conditions. Therefore, we simulated the I cation-doped TiO2 using the 48-atom 2 × 2 × 2 repetition of rutile by replacing one Ti atom with one I atom (corresponding to the doping concentration about 2.1 atom %). The supercell
Figure 1. The iodine cation-doped rutile 2 × 2 × 2 supercell (48atom). The red spheres represent O atoms, the gray spheres represent Ti atoms, and the yellow sphere represents the iodine atom.
Figure 2. Band structure plots for (a) pure and (b) I cation-doped rutile TiO2.
system is shown in Figure 1. In our calculations, the lattice parameters and all the atomic positions were fully optimized until all components of the residual forces were smaller than 0.025 eV/Å. The iodine atom with a larger size causes obvious near neighbor displacements compared with the average crystallographic structure of TiO2. The I-O bond lengths in iodine cation-doped rutile TiO2 (see Figure 1) are 2.066 and 2.058 Å (the bond lengths of undoped rutile are 1.960 and 1.986 Å). The lattice parameters of iodine cation-doped TiO2 are a ) 4.640 Å (4.614 Å for undoped rutile) and c ) 2.991 Å (2.976 Å for undoped rutile), which are slightly greater than that of pure TiO2. This is reasonable since the I5+ radius (0.95 Å) is greater than that of Ti4+ (0.61 Å), which leads to the extension of the bond lengths and the expansion of the lattice. To clarify the effects of I doping on the electronic structure of rutile TiO2, the band structures of pure rutile and I cationdoped rutile were calculated, as shown in Figure 2. The calculated band gap of pure rutile TiO2 is 1.82 eV (Figure 2a), which is consistent with the reported result.16 The band gap underestimation compared with the experimental value (3.0 eV for rutile) is due to the well-known shortcoming in DFT.16 The band structures of I-doped rutile TiO2 are shown in Figure 2b. It is found that the I cation dopant introduces some impurity states at the top of the valence band. The impurity states overlap sufficiently with the valence band states of TiO2, extending the valence band into the band gap by about 0.6 eV. Furthermore, the conduction band minimum (CBM) is raised by about 0.2 eV and several states are mixed with the conduction band edge of rutile TiO2. Thus, the corrected band gap is markedly narrowed to 2.6 eV. The excitation from the impurity band to the conduction band raises the absorption edge from 410 to 480 nm, absorbing one-fourth of the visible light. This is consistent with a recent experimental result that, for I-doped rutile TiO2, the band gap is shifted to the visible light region with a stronger absorption in the wavelength range from 400 to 600 nm.19–21 This suggests that iodine cation doping can change the band gap width and increase the visible light response of rutile TiO2. To investigate the origin of the band structure modification in the I-doped rutile TiO2 system, the total density of states (TDOS) and projected density of states (PDOS) of the undoped and I-doped models were calculated and the results are illustrated in Figure 3. From panels a and c of Figure 3, it can be seen that for pure TiO2, the valence band consists mainly of O 2p states, whereas the Ti 3d states mainly contribute to the
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Figure 3. (a) Total density of states (TDOS) and (c) projected density of states (PDOS) for a 48-atom rutile supercell. (b) TDOS and (d) PDOS for a 48-atom iodine cation-doped rutile supercell. The energy is measured from the top of the valence band of pure rutile TiO2.
conduction band. A comparison of panels c and d of Figure 3 shows that the I 5p states contribute to the conduction band and slightly raise the position of the CBM (about 0.2 eV) while the I 5s states are located at the top of the valence band. This indicates that the iodine dopant at the Ti site exists as an I5+ cation, consistent with the experiment.19–21 The 5s states of the iodine dopant span a range from -0.4 to 0.6 eV, which indicates that the 5s states fully overlap with the valence band, extending the valence band into the band gap by 0.6 eV. Thus, the band gap of I cation-doped rutile has a marked band gap narrowing of about 0.4 eV, which is consistent with the above band structure results. As shown in Figure 3c, the PDOSs of iodine’s first neighboring O atoms show a similar character with that of I 5s states at the position of valence band edge, indicating the strong hybridization of I and its first neighboring O atoms. The full overlap of impurity band and valence band can reduce the number of charge carrier traps and suppress the recombination of the photoexcited carriers. According to the experimental and theoretic results, doping of nonmetal elements such as N,16 C,30 and S31 always induces an isolated band above the valence band maximum (VBM). For a heavy doping semiconductor, the isolated states introduced by the dopants would form a large number of charge recombination centers and greatly restrain the photocurrent. Hence, although the visible light response of TiO2 is increased, the UV-light photocatalytic activity of N-,32 C-,33 and S-doped34 TiO2 is lower than that of undoped TiO2. Notably, the iodine doping in rutile TiO2 may avoid this problem due to the fully hybridized states, thus responsible for the enhancement of photocatalytic activity of I-doped rutile TiO2 under both visible light and UV light.19,20 The full hybridizations of I 5s and O 2p are related to the special structural property of rutile phase among the TiO2 polymorphs. It is known that the rutile and anatase TiO2 are with the cell volume of 10.4 and 11.3 Å3 per formula unit, respectively. Thus, the rutile TiO2 is more compact than the anatase phase, inducing stronger interactions among the lattice atoms of rutile.16 As a consequence, the overlap between the O 2p orbitals is stronger, which leads to a great Pauli repulsion of O 2p states. This makes the O 2p bands wider for the rutile TiO2 (6.0 eV35) than for the anatase TiO2 (4.7 eV36) phase. For the same reason, the I 5s and O 2p hybridization is stronger in rutile TiO2 compared with that of anatase TiO2. As a result, the strong interaction in rutile reduces the total energy and makes the I 5s states more delocalized. This finally results in the full overlap of impurity bands with the valence bands in rutile TiO2 (as shown in Figure 3), in contrast with the case in anatase.
This picture is consistent with the experimental observation reported by Liu et al.,37 in turn confirming our hypothesis given above. In the photocatalytic reaction process, the absorbed photon first excites an electron from an occupied state to the conduction band and creates electron-hole pairs. And then, the excited electron-hole pairs should be transferred to the surface or electrode in their lifetime. Ultimately, the electrons and holes at the surface split water to form hydrogen and oxygen gas.38 Since every photon absorbed by the photocatalyst can only transfer 1.23 eV to chemical energy,39 the limiting efficiency is proportional with the number of absorbed photons. Murphy et al. predicted a maximum efficiency of 16.8% for a hypothetically ideal material with a band gap of 2.03 eV.39 As discussed above, the I doping successfully narrowed the band gap from 3.0 to 2.6 eV and the limiting efficiency is increased from 2% to 6%. To approach the limiting efficiency, two requirements should be met: suitable band edge positions and efficient transportation of photocarriers. The CBM of pure rutile TiO2 is below the hydrogen production level by 0.1-0.2 eV and thus restrains the course of reducing water to hydrogen gas.5,40 Fortunately, the I doping can raise the CBM above the hydrogen production level as shown in Figure 3, overcoming the shortcoming of pure rutile. This results from the ease of losing the outermost orbital electrons of I which strengthens the coulomb interaction and raises the whole conduction band. Also, since the impurity band in I-doped rutile TiO2 fully overlaps with the valence band of TiO2, the photoexcited holes are delocalized so that it is favorable for them to be transferred to the surface in their lifetime. Accordingly, the solar energy conversion efficiency is expected to increase obviously compared to that of pure rutile TiO2. It is worth noting that, in the process of surface water splitting, not only the electronic structure but also some other characters such as surface properties and electrode structure can affect the energy conversion efficiency. For instance, the catalytic efficiency is also influenced by other factors, such as the crystallinity, surface area, and mesoporous structure of the material suggested by Liu et al.19 Still, this principle for the band-edge modification might be applicable to the future design of photocatalysts. IV. Conclusion In summary, using first-principles calculations, we investigate the electronic and optical properties of iodine cation-doped rutile TiO2. The calculated results reveal that iodine dopant induces
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its 5s states in band gap and contributes its 5p states to the conduction bands, which is consistent with experimental XPS results. An impurity band that consists of I 5s states and O 2p states is localized at the top of the valence band of TiO2 and fully overlaps the valence band. The full overlap of impurity level and valence band is benefited from the strong electronic delocalization caused by the high density of the rutile structure. Thus, the band gap is markedly narrowed, leading to a red shift of the absorption edge to 480 nm. The full overlap of I 5s states and valence band delocalizes the excited electron-hole pairs and favors the transfer of photocarrier. Besides, the I doping can raise the conduction band edge of rutile TiO2 above the hydrogen production level to ensure the hydrogen evolving, helpful for I-doped rutile TiO2 to approach the limiting efficiency. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 10725522, 10635060, 10605024, and 20621061), and Knowledge Innovative Program of The Chinese Academy of Sciences (KJCX2YW-N40). References and Notes (1) Fujishima, A.; Honda, K. Nature (London) 1972, 238, 37. (2) Grazel, M. Nature (London) 2001, 414, 338. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnmann, D. W. Chem. ReV. 1995, 95, 69. (4) Lewis, N. S. Science 2007, 315, 798. (5) Kavan, L.; Gra1tzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. J. Am. Chem. Soc. 1996, 118, 6716. (6) Anpo, M.; Takeuchi, M. J. Catal. 2003, 216, 505. (7) Klosek, S.; Raftery, D. J. Phys. Chem. B 2001, 105, 2815. (8) Zhao, W.; Chen, C. C.; Li, X. Z.; Zhao, J. C.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 2002, 106, 5022. (9) Kato, H.; Kudo, A. J. Phys. Chem. B 2002, 106, 5029. (10) Cong, Y.; Zhang, J. L.; Chen, F.; Anpo, M. J. Phys. Chem. C 2007, 111, 6976. (11) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (12) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Science 2002, 297, 2243. (13) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. Appl. Phys. Lett. 2002, 81, 454. (14) Choi, W. J. Phys. Chem. 1994, 98, 13669.
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