5658
J. Phys. Chem. C 2009, 113, 5658–5663
First-Principles Characterization of Bi-based Photocatalysts: Bi12TiO20, Bi2Ti2O7, and Bi4Ti3O12 Wei Wei, Ying Dai,* and Baibiao Huang School of Physics, State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, People’s Republic of China ReceiVed: NoVember 25, 2008; ReVised Manuscript ReceiVed: January 27, 2009
The geometric, electronic, and optical properties of three Bi-based structures (Bi12TiO20, Bi2Ti2O7, and Bi4Ti3O12) with and without C and N doping as possible photocatalytic material were investigated systemically by means of the first-principles DFT calculations within the GGA scheme to explore the origin of different band gaps and high photocatalytic activity under visible light observed in experiment. Our calculated results illuminate that BTO structures show an indirect band gap characteristic and the actual band gaps of BTO structures should be wider than 3.0 eV except for Bi4Ti3O12. C and N elements can be very easily introduced into the BTO lattices during the sample preparation process according to the very small defect formation energies, and the unintentional C or N doping may be responseble for the observed different band gaps and the high photocatalytic activity under visible light. Accordingly, it can be predicted that nonmetal element doping of BTO structures may improve photocatalytic activity under visible light and can be an excellent candidate for TiO2. 1. Introduction Although bismuth is seldom used as a dopant in TiO2 materials, Bi-based mixed oxides such as BiVO4,1 CaBi2O4,2 BaBiO3,3 Bi24AlO3,4 Bi2WO6,5 and Bi2MoO66,7 have been widely investigated as visible light-driven photocatalysts due to their unique crystal structure and high activity. Bismuth titanate is a kind of wide band gap semiconductor with several crystal phases, such as refractive sillenite phase Bi12TiO20, dielectric pyrochlore phase Bi2Ti2O7, ferroelectric perovskite phase Bi4Ti3O12, and so on. Due to their ferroelectric, high dielectric constant and excellent electrooptical and photoelectric properties, the polymorphs have been studied extensively. In particular, photocatalytic applications of Bi12TiO20, Bi2Ti2O7, and Bi4Ti3O12 have been attracting more and more attention (hereafter, we denote the three bismuth titanate phases as BTO uniformly) and a number of works on the preparation and photocatalytic activity of the BTO samples have been reported.8-16 Yao8 and Thanabodeekij9 et al. reported respectively that Bi12TiO20 manifested high photocatalytic activity to photodegrade the organic pollution and has a band gap of about 2.4 eV. A photocatalytic property study10 of Ba-doped Bi12TiO20 implied that Ba can significantly increase the photocatalytic activity of the Bi-based system. Bi12TiO20 prepared by Xu et al.11,12 with an evaluated band gap of about 2.75 eV displayed high photocatalytic activity for the degradation of methyl orange and phenol in water under ultraviolet light (300 nm < λ < 400 nm) and visible light (λ > 400 nm) irradiation. They also found the photocatalytic activity increased as the Bi12TiO20 sample supported on nickel ferrite increased (Bi12TiO20/SiO2/NiFe2O4). Zhou et al.13 investigated the photocatalytic activity of Bi12TiO20 prepared by a simple solid-state reaction between Bi2O3 and TiO2 powders for the decomposition of methanol under visible light irradiation. The hybridization between Bi 6s and O 2p states, which may increase the mobility of the photogenerated carriers and eliminate the * To whom correspondence should be addressed. E-mail: daiy60@ sdu.edu.cn.
recombination center, was assumed to be responsible for the high photocatalytic activity of Bi12TiO20. The band gap of Bi12TiO20 was suggested to be about 2.78 eV. Kudo14 and Yao15 et al. found excellent photocatalytic properties of Bi2Ti2O7 for water splitting and photodecolorization of methyl orange. The estimated band gap of Bi2Ti2O7 was about 2.95 eV.15 Besides Bi12TiO20 and Bi2Ti2O7, the photocatalytic properties of Bi4Ti3O12 have also received much attention. Yao et al.16 assumed that the improved photocatalytic activity of Bi4Ti3O12 to the methyl orange as a model organic compound resulted from both the bond angle of Ti-O-Ti and the intraelectric field formed between the (Bi2O2)2+ layer and the (Bi2Ti3O10)2- layer, which motivates the separation of the photogenerated electron-hole pairs. The band gap of Bi4Ti3O12 was evaluated to be about 3.08 eV. The measured band gaps of the three BTO structures are smaller than 3.0 eV (except that in ref 16). However, other researchers pointed out that the BTO structures can only absorb photons that have a wavelength below 400 nm,17-20 indicating the band gap should be wider than 3 eV. Consequently, the band gaps of the BTO structures are controversial and it is important and necessary to understand the relationship between the electronic structures and the photocatalytic properties of the BTO structures definitely. However, seldom theoretical work has been performed on the BTO structures and the origin of its photocatalytic activity under visible light is ambiguous. In the present work, we explore the geometric, electronic, and optical properties of the BTO structures with and without C or N doping systematically by means of the first-principles DFT calculations to probe the possible reason for the various observed band gaps and high photocatalytic activities. From the results it can be proposed that the BTO structures can be candidates for photocatalysts. 2. Computational Details The first-principles calculations were performed by means of the density functional theory (DFT) method based on the
10.1021/jp810344e CCC: $40.75 2009 American Chemical Society Published on Web 03/18/2009
Characterization of Bi-based Photocatalysts
J. Phys. Chem. C, Vol. 113, No. 14, 2009 5659
Figure 1. Bulk structures of (a) Bi12TiO20, (b) Bi2Ti2O7, and (c) Bi4Ti3O12. The purple spheres represent Bi atoms, the gray spheres represent Ti atoms, and the red spheres represent O atoms.
plane-wave pseudopotential21 approach using the CASTEP package.22 The exchange-correlation potential was described by generalized gradient approximation (GGA)23 with the Perdew-Burke-Ernzerhof (PBE)24 scheme. The interaction between valence electrons and ion core was substituted by ultrasoft pseudopotential.25 Electronic wave functions were expanded in terms of a discrete plane wave basis set. The k-space integrations were done with the Monkhorst-Pack26 grid with 3 × 3 × 3 k-points in the Brillouin zone of the crystal to obtain the accurate density of the electronic states while the kinetic energy cutoff for wave function expansion was 340 eV. Geometry optimization was done before single point energy calculation and the self-consistent convergence accuracy was set at 5 × 10-5 eV/atom. The convergence criterion for the maximal force between atoms was 0.1 eV/Å, the maximum displacement was 5 × 10-3 nm, and the stress was 0.2 Gpa, respectively. To check the reliability of our results, we also performed a test calculation with higher plane-wave cutoff energy and more k-points. Compared with the cutoff energy of
340 eV and 3×3×3 k-points, negligible changes were obtained for both structural and electronic structures and the difference between the total energies is less than 0.02%. So the calculations in the present work are available. All of the electronic structures were calculated on the corresponding optimized geometries. 3. Optimized BTO Structures The three BTO structures are modeled by the 66-atom supercell for Bi12TiO20, the 88-atom supercell for Bi2Ti2O7, and the 76-atom supercell for Bi4Ti3O12, shown in Figure 1. The optimized Ti-O and Bi-O bond lengths of BTO structures are summarized in Table 1. Bi12TiO20 (Figure 1a) with cubic symmetry belongs to a family of sillenite compounds with metstable cubic γ-Bi2O3, being a well-known group of noncentrosymmetric crystals. It has the general formula Bi12MO20 where M represents a tetravalent ion or a combination of ions, which gives an average charge of 4+. Ti atoms bond to the four adjacent O atoms and form a TiO4 tetrahedral unit while the Ti
5660 J. Phys. Chem. C, Vol. 113, No. 14, 2009 TABLE 1: Optimized Bond Lengths (Å) in Bi12TiO20, Bi2Ti2O7, and Bi4Ti3O12
Wei et al. TABLE 2: The Experimental Band Gaps and Calculated Band Gaps of the BTO Structures
bond lengths Ti-O Bi12TiO20 Bi2Ti2O7 Bi4Ti3O12
1.845 2.009 1.919, 1.975
band gap (eV) Bi-O
exptl
2.027, 2.179, 2.194 2.256 2.257
Bi12TiO20 Bi2Ti2O7 Bi4Ti3O12
a
b
theoretical c
d
2.4, 2.75, 2.78, 3.2 2.95,e 2.9f 3.08,g 3.2h
3.28 (2.61) 2.89 (2.46) 1.24 (0.50)
a Reference 10. b reference 12. c reference 13. d reference 29. reference 15. f reference 14. g reference 16. h reference 30. The theoretical values are the calculated direct band gaps of the BTO structures at Γ point and those in parentheses are the indirect band gaps.
e
cation occupies the tetrahedral interstice. A Bi-O polyhedron network connects to the geometrical regular TiO4 tetrahedral. The optimal Ti-O bond length is 1.845 Å and the three kinds of optimized Bi-O bond lengths are 2.027 (I), 2.179 (II), and 2.194 (III) Å, respectively. Considering the crystal structure of Bi12TiO20, the Bi-O polyhedron in Bi12TiO20 may serve as active electron donor sites, which enhance the electron transformation and eliminate the recombination of the photogenerated electron-hole pairs.8 Bi2Ti2O7 (Figure 1b), also with cubic symmetry, belongs to a family of A2B2O7 compounds with A23+B24+O7 pyrochlore structure comprised of Ti-O octahedral and Bi-O tetrahedral units. The optimized Ti-O bond length is 2.009 Å and the Bi-O bond length is 2.256 Å. Bi4Ti3O12 (Figure 1c), with orthorhombic symmetry, belongs to a family of aurivillius compounds consisting of (Bi2O2)2+ layers and perovskite-type (Bi2Ti3O10)2- units with TiO6 octahedral layers, which is assumed to motivate the separation of photogenerated electron-hole pairs and then improve the photocatalytic activity of the catalyst. The two optimized kinds of Ti-O bond lengths are 1.919 Å (I) and 1.975 Å (II) and the optimal Bi-O bond length is 2.257 Å. On the basis of the bond lengths of Ti-O and B-O in the three BTO structures (see Table 1), it is clear that the interaction between Ti and O is stronger than that between Bi and O. A study on luminescent properties of tantalates and niobates has shown that the closer the bond angle of M-O-M is to 180°, the more the excitation energy is delocalized.27,28 This means that the bond length as well as the bond angle of M-O-M in MO6 octahedrons are the two important factors affecting the photocatalytic and photophysical properties of such semiconductors. Our calculated bond angles of Ti-O-Ti in Bi2Ti2O7 and Bi4Ti3O12 are close to the ideal 180°, so it might facilitate the mobility of photogenerated electron-hole pairs and eliminate the recombination of the photogenerated electron-hole pairs and finally improve the photocatalytic activity. In summary, compared with the TiO2 structure,34 the introduction of the Bi element surely increases the photocatalytic activity of the BTO structures.
reported in previous work.30 Furthermore, our previous studies indicate that the calculated band gaps for anatase and rutile TiO2 are 2.10 and 1.89 eV, which are underestimated compared with the experimental values of 3.2 and 3.0 eV,31-34 respectively. The underestimation mainly results from the well-known shortcoming of exchange-correction function in describing excited states in the DFT calculation. This indicates that, compared with TiO2, the actual band gaps of BTO structures should be wider than 3.0 eV according to our calculated results except for Bi4Ti3O12. Therefore, the photocatalytic activity of the BTO structures under visible light observed in experiment should result from other factors as discussed in section 5. To explore the details of the electronic properties, the total density of states (TDOS) and the projected density of states (PDOS) of the three BTO structures are shown in Figure 3. It can be seen that there is no spin polarization for all the BTO models and the electronic characters of O and Ti atoms are similar to those of TiO2,31,32 namely, O 2p states and Ti 3d states contribute mainly to the valence band and the conduction band, respectively. For the three BTO structures, Bi 6s states contribute mainly to the valence band and the Bi 6p states to the conduction band. In particular, the conduction band of the Bi12TiO20 structure is dominantly composed of the Bi 6p states rather than Ti 3d states and the Bi 6p states responsible for the bottom of the conduction band. Moreover, since Bi4Ti3O12 represents the lowest symmetry among the three structures, some degenerate states are eliminated and thus introduce some localized Bi 6p, 6s and O 2p related gap states. Integrating the Bi 6s and 6p states in the valence band and the conduction band, it can be seen that the interaction between Bi and O atoms should favor the generation and separation of the photoexcited electron-hole pairs and thus enhance the photocatalytic activity of the BTO structures.
4. Electronic Structures of BTO Band structures of the three BTO structures are shown in Figure 2, and the calculated and experimental band gaps are listed in Table 2. Figure 2 displays that all three BTO structures show an indirect band gap characteristic. For the Bi12TiO20 (Figure 2a) and Bi2Ti2O7 (Figure 2b) structures, the direct band gaps at the Γ point manifest at 3.28 and 2.89 eV and indirect band gap at 2.61 and 2.46 eV, respectively. For Bi4Ti3O12 (Figure 2c), both the valence band and the conduction band indicate exquisite undulation, the direct band gap at the Γ point and the indirect band gap are 1.24 and 0.50 eV, respectively. It is noticeable that the calculated band gaps of Bi4Ti3O12 are largely different from experimental values. This results from the crystal phase of the aurivillius structure depending seriously on temperature and experimental condition.29 In the present work, we chose one of the various Bi4Ti3O12 structures reported in ref 29 and our theoretical results are consistent with those
5. C- and N-Doped BTO Structures It is known that C and N atoms may be easily introduced into the TiO2 samples.35-37 Considering the resemblance of the electronic character between the BTO and TiO2 structures, one might speculate if the unintentional introduction of C and N during the preparation process is responsible for the photocatalytic activity of the BTO samples under visible light. If so, Cand N-doping may be an efficient approach to enhance the photocatalytic activity of the BTO structures under visible light, which is similar to the C- and N-doped TiO2 structures.35-37 We modeled the C- and N-doped BTO structures by substituting C or N for O atoms in the three BTO structures: 3.03 atom % C (N)-doped Bi12TiO20, 2.27 atom % C (N)-doped Bi2Ti2O7, and 2.63 atom % C (N)-doped Bi4Ti3O12, respectively, to examine the effects of C- and N-doping on the electronic structure and optical properties of the BTO structures.
Characterization of Bi-based Photocatalysts
J. Phys. Chem. C, Vol. 113, No. 14, 2009 5661
Figure 2. Band structures of (a) Bi12TiO20, (b) Bi2Ti2O7, and (c) Bi4Ti3O12. The red dashed lines represent the top of the valence band.
Figure 3. TDOS and PDOS of (A) Bi12TiO20, (B) Bi2Ti2O7, and (C) Bi4Ti3O12. The red dashed lines represent the top of the valence band.
TABLE 3: Calculated Defect Formation Energies for Cand N-Doped BTO Structuresa defect formation energy subC-O subN-O
Bi12TiO20
Bi2Ti2O7
Bi4Ti3O12
7.41 0.73
-4.56 -9.60
-0.19 -4.96
a subC-O represents the substitutional C for O-doped structures and subN-O represents the substitutional N for O-doped structures, respectively.
5.1. Formation Energy. To determine the energies required for substituting C or N for O atoms in the BTO structures, we calculated the defect formation energy Ef according to the following equation:34
EXf ) EX-doped - Epure - nµX + nµO
(1)
where EX-doped and Epure are the total energies of X (i.e., C or N)-doped and undoped BTO structures, respectively). µX and µO are the chemical potential of X and O atoms, respectively; and n is the number of the substitutional C and N atoms. The calculated results are summarized in Table 3. It is noticed that the substitutional doping is energetically more favorable while the EfX value becomes smaller. From the EfX values listed in Table 3, it can be seen that N is easier to dope into the BTO structures than C. In addition, Bi12TiO20 is the energetically hardest to dope and Bi2Ti2O7 is the energetically most favorable
to dope by C and N elements among the three BTO structures. Moreover, due to the relatively larger defect formation energy, the C-doped Bi12TiO20 structure is harder to realize than the others. Overall, C and N ions can be introduced into the BTO lattices very easily due to the small EfX values, even negative for the Bi2Ti2O7 and Bi4Ti3O12 structures, which indicate that it is very possible to incorporate C and N elements into the BTO structures during the sample preparation process. 5.2. Electronic Structure. To understand the effects of Cand N-doping on the electronic structure, the band structures of C- and N-doped BTO structures are calculated and summarized in Figures 4 and 5, respectively. For a further investigation, the PDOS of C 2p and N 2p states are calculated and plotted in Figure 6, panels A and B, respectively. For C-doped Bi12TiO20 (Figures 4(a) and 6A(a)), some C 2p related states appear above the valence O 2p states leading to overlapping between the C 2p states and the valence band. Moreover, some spin-polarized C 2p isolated levels appear within the band gap. Thus the valence band is expanded obviously compared with that of undoped Bi12TiO20 (Figure 2a), which results in a narrowed band gap between the top of the valence band and the bottom of the conduction band of about 2.33 eV. Subsequently, the narrowed band gap as well as the gap states may result in the absorption of visible light. For the C-doped Bi2Ti2O7 structure (Figures 4(b) and 6A(b)), C 2p related levels mostly contribute to the gap levels and the most upper C 2p gap level is localized at about 1.11 eV below the bottom of the conduction band. The electronic structures of the C-doped Bi4Ti3O12 (Figures 4(c) and 6A(c)) manifest that both the valence band
5662 J. Phys. Chem. C, Vol. 113, No. 14, 2009
Wei et al.
Figure 4. Band structures of (a) C-doped Bi12TiO20, (b) C-doped Bi2Ti2O7, and (c) C-doped Bi4Ti3O12. The red dashed lines represent the top of the valence band of the corresponding undoped BTO structures.
Figure 5. Band structures of (a) N-doped Bi12TiO20, (b) N-doped Bi2Ti2O7, and (c) N-doped Bi4Ti3O12. The red dashed lines represent the top of the valence band of the corresponding undoped BTO structures.
Figure 6. PDOS of (A) C and (B) N atoms in doped (a) Bi12TiO20, (b) Bi2Ti2O7, and (c) Bi4Ti3O12. The red dashed lines represent the top of the valence band of the corresponding undoped BTO structures.
and the conduction band undergo an extensive modification. Some C 2p states overlap with the upper valence band and the band gap at the Γ point to be about 0.95 eV. For the N-doped BTO structures (Figures 5 and 6B), similar results to those of C-doped BTO structures are obtained. Comparing the electronic character of C 2p with N 2p, it can be seen that the C 2p states mainly contribute to gap states while the N 2p states mainly contribute to the valence band of the doped BTO structures. Finally, according to the results mentioned above it can be
summarized as follows: (I) The valence bands of the BTO structures are expanded due to the hybridization between C 2p or N 2p and valence O 2p states. (II) Gap levels consisting of C 2p or N 2p states are introduced within the band gap or overlap with the band edge of the BTO structures leading to band gap narrowing, which results in the photocatalytic absorption and activity under visible light, thus it can be responsible for the different band gaps observed in the experiment. (III) Similar to the C- and N-doped TiO2, C- and N-doping can be
Characterization of Bi-based Photocatalysts an effective approach to enhance the photocatalytic activity under visible light of the BTO structures. 6. Concluding Remarks We carried out first-principles calculations systematically on the BTO structures by means of the plane-wave DFT method within the GGA scheme. For the undoped BTO structures, the electronic characters of O and Ti are similar to that of TiO2 structures. Bi 6s states mainly contribute to the valence band while Bi 6p states mainly contribute to the conduction band. The actual band gaps of BTO structures should be wider than 3.0 eV except for the Bi4Ti3O12. Because of the small defect formation energies, C and N elements can be very easily incorporated into the BTO structures during the sample preparation process. The results illuminate that some C 2p and N 2p states are introduced above and mix with the valence band, which results in the narrowed band gap and the photocatalytic activity under visible light. Introducing the C or N atom into the BTO crystal is an efficient method to enhance the photocatalytic activity of the BTO structures under visible light. Consequently, BTO doped with C or N elements act as a kind of promising photocatalyst and needs further study. Acknowledgment. This work is supported by the National Basic Research Program of China (973 program, Grant No. 2007CB613302), National Natural Science Foundation of China under Grant No. 10774091, Natural Science Foundation of Shandong Province under Grant No. Y2007A18, and the Specialized Research Fund for the Doctoral Program of Higher Education (20060422023). References and Notes (1) Kudo, A.; Omori, K.; Kato, H. J. Am. Chem. Soc. 1999, 121, 11459. (2) Tang, J.; Zou, Z.; Ye, J. Angew. Chem., Int. Ed. 2004, 43, 4463. (3) Tang, J.; Zou, Z.; Ye, J. J. Phys. Chem. C 2007, 111, 12779. (4) Yao, W. F.; Xu, X. H.; Zhou, J. T.; Yang, X. N.; Zhang, Y.; Shang, S. X.; Wang, H.; Huang, B. B. J. Mol. Catal. A 2004, 212, 323. (5) Fu, H.; Pan, C.; Yao, W.; Zhu, Y. J. Phys. Chem. B 2005, 109, 22432. (6) Shimodaira, Y.; Kato, H.; Kobayashi, H.; Kudo, A. J. Phys. Chem. B 2006, 110, 17790. (7) Bi, J.; Wu, L.; Li, J.; Li, Z.; Wang, X.; Fu, X. Acta Mater. 2007, 55, 4699. (8) Yao, W. F.; Wang, H.; Xu, X. H.; Cheng, X. F.; Huang, J.; Shang, S. X.; Yang, X. N.; Wang, M. Appl. Catal., A 2003, 243, 185.
J. Phys. Chem. C, Vol. 113, No. 14, 2009 5663 (9) Thanabodeekij, N.; Gulari, E.; Wongkasemjit, S. Powder Technol. 2005, 160, 203. (10) Yao, W. F.; Wang, H.; Xu, X. H.; Zhang, Y.; Yang, X. N.; Shang, S. X.; Liu, Y. H.; Zhou, J. T.; Wang, M. J. Mol. Catal. A 2003, 202, 305. (11) Xu, S. H.; Shangguan, W. F.; Yuan, J.; Shi, J. W.; Chen, M. X. Sci. Technol. AdV. Mater. 2007, 8, 40. (12) Xu, S. H.; Shangguan, W. F.; Yuan, J.; Shi, J. W.; Chen, M. X. Mater. Sci. Eng. B 2007, 137, 108. (13) Zhou, J. K.; Zou, Z. G.; Ray, A. K.; Zhao, X. S. Ind. Eng. Chem. Res. 2007, 46, 745. (14) Kudo, A.; Hijii, S. Chem. Lett. 1999, 28, 1103. (15) Yao, W. F.; Wang, H.; Xu, X. H.; Zhou, J. T.; Yang, X. N.; Zhang, Y.; Shang, S. X. Appl. Catal., A 2004, 259, 29. (16) Yao, W. F.; Xu, X. H.; Wang, H. J; Zhou, J. T.; Yang, X. N.; Zhang, Y.; Shang, S. X.; Huang, B. B. Appl. Catal., B 2004, 52, 109. (17) Rao, A. V. P.; Robin, A. I.; Komarneni, S. Mater. Lett. 1996, 28, 469. (18) Skorikov, V. M.; Zakharov, I. S.; Volkov, V. V.; Spirin, E. A. Inorg. Mater. 2002, 38, 172. (19) Gu, H. S.; Bao, D. H.; Wang, S. M.; Gao, D. F.; Kuang, A. X.; Li, X. J. Thin Solid Films 1996, 283, 81. (20) Vasconcelos, F.; Pimenta, M. A.; Sombra, A. S. B. J. Mater. Sci. 2001, 36, 587. (21) Milman, V.; Winkler, B.; White, J. A.; Pickard, C. J.; Payne, M. C.; Akhmatskaya, E. V.; Nobes, R. H Int. J. Quantum Chem. 2000, 77, 895. (22) Segall, M. D.; Lindan, P. L. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. J. Phys.: Condens. Matter 2002, 14, 2717. (23) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (24) Perdew, J. P.; Bruke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (25) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (26) Monkorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (27) Blasse, G. J. Solid State Chem. 1988, 72, 72. (28) Srivastsva, A. M.; Ackerman, J. F. J. Solid State Chem. 1977, 134, 187. (29) Pintilie, L.; Pintilie, I.; Alexe, M. J. Eur. Ceram. Soc. 1999, 19, 1473. (30) Skorikov, V. M.; Zakharov, I. S.; Volkov, V. V.; Spirin, E. A.; Umrikhin, V. V. Inorg. Mater. 2001, 37, 1348. (31) Yang, K. S.; Dai, Y.; Huang, B. B. J. Phys. Chem. C 2007, 111, 18985. (32) Yang, K. S.; Dai, Y.; Huang, B. B. J. Phys. Chem. C 2007, 111, 12086. (33) Yang, K. S.; Dai, Y.; Huang, B. B. Phys. ReV. B 2007, 76, 195201. (34) Yang, K. S.; Dai, Y.; Huang, B. B.; Whangbo, M. H. Chem. Mater. 2008, 20, 6528. (35) Li, Y.; Hwang, D.-S.; Lee, N. H.; Kim, S.-J. Chem. Phys. Lett. 2005, 404, 25. (36) Morikawa, T.; Asahi, R.; Ohwaki, T.; Aoki, K.; Taga, Y. Jpn. J. Appl. Phys. 2001, 40, L561. (37) Diwald, O.; Thompson, T. L.; Zubkov, T.; Goralski, E. G.; Walck, S. D.; Yates, J. T., Jr. J. Phys. Chem. B 2004, 108, 6004.
JP810344E
