Band structures of TiO₂ doped with N, C and B

Band structures of TiO 2 doped with N, C and B first-author corresp ‡Corresponding Author ‡Corresponding Author †E-mail:Songcl@zju.edu.cn †E-mail:Songcl@zju.edu.cn Songcl@zju.edu.cn Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China This study on the band structures and charge densities of nitrogen (N)-, carbon (C)- and boron (B)-doped titanium dioxide (TiO 2 ) by first-principles simulation with the CASTEP code (Segall et al., 2002 2002 ) showed that the three 2p bands of impurity atom are located above the valence-band maximum and below the Ti 3d bands, and that along with the decreasing of impurity atomic number, the fluctuations become more intensive. We cannot observe obvious band-gap narrowing in our result. Therefore, the cause of absorption in visible light might be the isolated impurity atom 2p states in band-gap rather than the band-gap narrowing. CASTEP code Titanium dioxide Band structure Charge density INTRODUCTION 1 Titanium dioxide (TiO 2 ) is known as a useful photosensitive material, such as for photoanodes (Byrne et al., 1998 1998 ) and photocatalysts (Yoneyama and Torimoto, 2000 2000 ). However, only the ultraviolet part of the solar irradiation (amounting to ~4% of the incoming solar energy on the earth’s surface) could be absorbed by TiO 2 due to its high intrinsic band gap (3.2 eV for anatase and 3.0 eV for rutile). For designing highly efficient and low cost photocatalytic systems, many approaches, such as doping with various transition metal cations (Zhu et al., 2000 2000 ), have been introduced to extend the optical absorption of TiO 2 -based systems to the visible-light region. Though that way could more or less extend the optical absorption, the thermal instability and increase of carrier recombination centers greatly limit the performances of transition metal dopants (Choi et al., 1994 1994 ). On the other hand, anionic nonmetal dopants, such as nitrogen (N) (Asahi et al., 2001 2001 ; Irie et al., 2003 2003 ), carbon (C) (Khan et al., 2002 2002 ) and boron (B) (Moon et al., 2000 2000 ) atoms, have also been investigated widely aim at extending photocatalytic activity into the visible-light region. Asahi et al.( 2001 2001 ) reported that N-doped TiO 2 showed dramatic improvement over undoped TiO 2 in its optical absorption and photocatalytic activity for visible light. Based on their analysis of the density of states, they concluded that the substitution doping of N for O in the anatase TiO 2 crystal would yield a band-gap narrowing driven by mixing of N 2p states with O 2p states. However, recent experimental data seemed contradict their theoretical. Irie et al.( 2003 2003 ) suggested that the visible-light response in N-doped TiO 2 might be due to N 2p states isolated above the valence-band maximum of TiO 2 . Similarly, the red-shift in C-doped TiO 2 had been observed by Choi et al.( 2004 2004 ), and Moon et al.( 2000 2000 ) reported the absorption band shifted towards longer wavelengths in B/TiO 2 . In this article, we calculated the band structures and charge densities of N-, C- and B-doped anatase TiO 2 by first-principles simulation with the CASTEP code, which is a state of the art quantum mechanics based program designed specifically for solid state materials science (Segall et al., 2002 2002 ). Based on the results, we explained the photocatalytic activity of three kinds of anionic nonmetal doping in visible-light region. METHODS 1 Our first-principles quantum mechanics calculations were performed using the local-density approximation (LDA) with local functional CA-PZ (Ceperley and Alder, 1980 1980 ; Perdew and Zunger, 1981 1981 ) in energy calculations and the generalized gradient approximation (GGA) with gradient corrected functional PBE (Perdew et al., 1996 1996 ) in geometry optimizations. The pseudopotential representation is in the reciprocal space, because it has several advantages both in efficiency and veracity. K-point mesh is set to 5×5×2. The N (C, B)-doped TiO 2 system’s calculations are carried on by using the super-cell geometry with four unit cells of anatase TiO 2 , where one O atom is replaced by one N, C or B atom. Thus the x in TiO 2− x N x , TiO 2− x C x and TiO 2− x B x is 0.0625 (Fig. 1 1 ). The basic structure of the undoped anatase TiO 2 crystal is picked from the Materials Studio Structures Library, which shows good agreement with the experiment and theory. The structural parameters such as the bond lengths and bond angles (Fig. 1 1 ) are given in Table 1 1 together with some previous calculations and experiment (Asahi et al., 2000 2000 ; Lee et al., 2005 2005 ; Burdett et al., 1987 1987 ). Brillouin zone for tetragonal lattice which is used in the calculations is shown in Fig. 2 2 . This work Lee’s work FLAPW Experiment a (Å) 3.776 3.845 3.692 3.782 c (Å) 9.486 9.537 9.471 9.502 c / a 2.512 2.481 2.566 2.512 d eq (Å) 1.930 1.960 1.893 1.932 d aq (Å) 1.973 2.002 1.948 1.979 θ (degree) 156.1 157.6 154.4 156.3 RESULTS AND DISCUSSION 1 The band structure of undoped anatase TiO 2 calculated by our method is shown in Fig. 3a 3a . The energy zero represents the valence-band maximum. The minimum band gap at the G point is 2.47 eV, less than the experimental value ( E g =3.2 eV), and is due to the well-known underestimate of the LDA (Perdew, 1983 1983 ). Before calculating the band structures of other doped TiO 2 , we optimize the crystal geometry with GGA method. The changes of the crystal after geometry optimization are shown in Table 2 2 . We found that the doped atom displacement increases with decreasing atomic number. This phenomenon could be explained by the incorporation of the doped atoms. The larger the atom is, the more is the strain on the neighboring Ti atoms. Δ x (Å) Δ y (Å) Δ z (Å) N-doped TiO 2 0 0 0.03 C-doped TiO 2 0 0 0.12 B-doped TiO 2 0 0 0.23 Next, we use LDA method in the N (C, B)-doped TiO 2 band structure calculations, and the results are given in the Figs. 3b~3d 3b~3d . The energy zero point has the same meaning with Fig. 3a 3a . The energy gaps between the lowest Ti 3d band and the highest O 2p band are denoted in each figure, and values are also listed in Table 3 3 for comparison. Moreover, we draw the charge density of each crystal in (1/2, 0, 0) plane. They are shown in Fig. 4 4 . The plots are drawn in (1/2, 0, 0) crystal plane. Contour spacings are 50 e/bohr 3 TiO 2 N-doped TiO 2 C-doped TiO 2 B-doped TiO 2 E g (eV) 2.47 2.33 2.44 2.85 Fig. 3 3 shows that the energy gap of N-doped TiO 2 ( E g =2.33 eV) is a little bit smaller than that of undoped TiO 2 ( E g =2.47 eV), and that three N 2p bands are located around the Fermi level and above the valence-band maximum of TiO 2 . Comparing Fig. 4a 4a with Fig. 4b 4b , we found that the doped N atom has very weak influence on the TiO 2 energy structure. This finding negated previous explanation that N atom doping would narrow the band gap, because it can only be produced by the strong mixing of the N 2p states with O 2p states. Therefore, we could draw the conclusion that the isolated N 2p bands should be the cause of the photocatalytic activity for visible-light. This result is consistent with the work of Di Valentin et al.( 2004 2004 ). Due to the relatively lower electronegativity, C and B atom can intensively affect TiO 2 charge distribution. This result is shown in Fig. 4c 4c and Fig. 4d 4d . However, in Fig. 3b 3b and Fig. 3c 3c , we cannot observe the band-gap narrowing either. C-doped TiO 2 E g is 2.44 eV while the B-doped TiO 2 E g is 2.85 eV, even larger than that of TiO 2 . We contribute this phenomenon to the low or no mixing of the C or B atom 2p bands with O 2p bands, because the matching of p bands decreases along with the difference in electronegativity. On the other hand, we found that the impurity atom’s three 2p bands were still in the band gap and along with the decreasing of impurity atomic number, the fluctuations become more intensive. It can greatly reduce the energy for electron transition between the O 2p states and Ti 3d states, and make the transition easier. 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The chemical states and properties of doped TiO2 film photocatalyst prepared using the Sol-Gel method with TiCl4 as a precursor We thank Dr. Xu Gang for reading our manuscript and Prof. Jiang Jianzhong for providing us software support. 10.1631/jzus.2006.B0299 Project (No. 2004AA32G040) supported by the Hi-Tech Research and Development Program (863) of China 2005-08-10 2005-10-19 2006-03-13 2022-01-05 Copyright © 2006, Journal of Zhejiang University Science 2006 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 04 07