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Magnetic and optical properties of N-doped, Fe-doped and (N, Fe)-codoped anatase TiO2
Accepted Manuscript Magnetic and optical properties of N-doped, Fe-doped and (N, Fe)-codoped anatase TiO2 Lin Chen, Hongping Li, Kun Zhang, Linting Yin, Hua Tang, Xiaojuan Liu, Jian Meng, Changsheng Li PII: DOI: Reference:
S0009-2614(14)00479-5 http://dx.doi.org/10.1016/j.cplett.2014.05.097 CPLETT 32242
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
26 March 2014 26 May 2014
Please cite this article as: L. Chen, H. Li, K. Zhang, L. Yin, H. Tang, X. Liu, J. Meng, C. Li, Magnetic and optical properties of N-doped, Fe-doped and (N, Fe)-codoped anatase TiO2, Chemical Physics Letters (2014), doi: http:// dx.doi.org/10.1016/j.cplett.2014.05.097
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Magnetic and optical properties of N-doped, Fe-doped and (N, Fe)-codoped anatase TiO2 Lin Chen,a Hongping Li,a,b,* Kun Zhang,a Linting Yin,a Hua Tang,a Xiaojuan Liu,c Jian Meng,c Changsheng Li,a,b,* a
School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, P. R. China b Institute for Advanced Materials, Jiangsu University, Zhenjiang, 212013, P. R. China c State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China * Corresponding author. E-mail address: [email protected] [email protected] Tel: +86-511-88783268
Abstract Electronic structure, magnetic and optical properties of N-doped, Fe-doped and N, Fe-codoped anatase TiO2 have been investigated by using first-principle calculations. Both N-doped and Fe-doped anatase TiO2 exhibit ferromagnetic semiconductors, while N, Fe-codoped one manifests metallic nature with antiferromagnetic ground state. The optical absorption spectrum for all the doped models extends the optical absoption to the visible-light region. Especially, N, Fe-codoped anatase TiO2 presents the highest absorption coefficient during the visible-light energy range (1.63-3.1 eV). Furthermore, the ultraviolet absorption is also enhanced, particularly for N-doped model.
Keywords: Magnetic properties; Optical properties; N, Fe-codoping; Anatase TiO2
1
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1. Introduction Since Fujishima and Honda [1] reported the monocrysral TiO2 can analyse water into hydrogen and oxygen under the ultraviolet (UV) light, titania has been extensively studied as a promising material for photochemical applications due to its excellent properties, i.e., chemical stability, non-toxicity, low cost, etc.. [2,3] However, the applications of TiO2 are rather limited owing to several reasons listed below: (i) a wide band gap (3.23 eV for anatase), which makes it inactive in the visible spectral region leading to a low utilization of sunlight; (ii) high recombination rate of photoexcited electron–hole pairs; and (iii) low efficiency of charge transfer to the surface. To overcome the existing limitations and shift the photocatalytic effect of titanium dioxide from the characteristic UV to visible range of the spectrum, great efforts have been made to improve the electronic property of TiO2 by means of the metal ion/nonmental ion doping, codoping with two or more foreign ions [4-6]. Over the past decade, many studies have been conducted to investigate the doping effect of TiO2 theoretically and experimentally [7,8] since Sato made a breakthrough that N-doped TiO2 showed visible-light photocatalytic activity [9]. Doping with small amounts (＜10 atom%) of non-metal impurities into TiO2 is proved to be an effective strategy to narrow band gap and increases photocatalytic activity [10,11]. However, their incorporations concurrently lead to poor photoresponse because the partially occupied impurity bands can act as killers for photogenerated carriers making the photo-excited current or photocatalytic efficiency decreased [12]. It has been recognized that donor-acceptor codoping at Ti and O sites can reduce the recombination centers which can effectively improve the charge carriers’ migration efficiency and enhance the photocatalytic activity [13,14]. Transition-metal ions were widely applied as dopants (doped at Ti site) into TiO2 due to their d electron configuration. As Jiaguo Yu’s work [15], it was reported that the photocatalytic activity of Fe-doped titanate nanotubes is 2 times higher than P25 under visible light illumination. The most acceptable explanation for the improvement of photocatalytic properties of TiO2 by doping with Fe3+ is that Fe3+ cations can act as 2
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shallow traps to decrease the recombination rate. Therefore, the synergistic effects of codoping with nonmetal and transition-metal ions into the TiO2 deserve more attention, which might eventually help to design high-efficient photocatalytic materials. Up to now, there are several papers related to N-doped, Fe-doped and N, Fe-codoped anatase TiO2 by experimental or theoretical method [4,16,17], which give explanations or predictions about the significantly increasing photocatalytic activity under visible-light irradiation. Meanwhile, reserchers have found magnetism after N-doped and Fe-doped anatase TiO2 [18,19], which is extremely useful as to photocatalytic materials. However, few researches study the magnetic properties confirming the magnetic ground states after N-doped, Fe-doped and N, Fe-codoped anatase TiO2. Here, the electronic structure, magnetic ground states and absorption properties of (N, Fe)-codoped TiO2 have been investigated by using the density functional theory (DFT) to reveal the microscopic mechanism for band gap narrowing and the origin of the enhanced photocatalytic activity. For comparison, the corresponding calculation and theoretical analysis were also conducted for pure, N-doped and Fe-doped anatase TiO2.
2. Computational methods First-principles plane-wave pseudopotential calculations based on the density functional theory [20] is performed applying the Cambridge Sequential Total Energy Package Code (CASTEP) [21]. The geometry optimization is carried out by using the Broyden-Fletcher-Goldfrab-Shanno
(BFGS)
method
before
their
properties’
calculations. The plane wave cut-off energy is chosen to be 380 eV for all the calculations.
The
generalized
Perdew-Burke-Ernzerhof
(PBE)
gradient scheme
approximation [22]
is
(GGA)
employed
within to
the
describe
exchange-correlation potential. Moreover, to properly describe the strong electron correlation in the 3d transition metal oxide, the GGA plus on site repulsion U method (GGA+U) is employed. In this paper, a series of U values for Ti 3d have been tested to better represent the experimental results. Our results show that the band gap for 3
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pure TiO2 is 3.05 eV when the U is set as 7.5 eV, which is in good agreement with experimental result 3.23 eV as well as previously theoretical results [23,24]. Hence, we mainly discuss the GGA+U results with U = 7.5 eV in this paper. Self-consistent was achieved once the total energy is converged to less than 1.0 ×10-5 eV and the maximum force being within 3.0 ×10-2 eV/Å, respectively. Anatase TiO2 belongs to body-centered tetragonal crystal structure with I41/amd space group, and there are four Ti atoms and eight O atoms in each unit cell. For N-doped anatase TiO2, 2 × 1 × 1 supercell configuration is considered and the Monkhorst-Pack k-mesh is set as 5×9×4. In regard to Fe-doped and N, Fe-codoped anatase TiO2, 2×2×1 supercell with four primitive unit cells is constructed and the Monkhorst-Pack k-mesh of 5×5×4 is adopted. All of the three configurations are based on the previous reports [25,26]. All the atoms are relaxed to their equilibrium position before the electronic, magnetic and optical properties calculated. As for the calculations of optical properties, interactions of photons in suitable frequency with the orbital electrons can shift the electron from the valence band to conduction band, during which these transitions will give spectra caused by the photon absorption or emission. The optical properties include the dielectric constant, absorption coefficient, reflectivity, refractive index and loss function are interrelated with each other. The dielectric function is defined as εω ε 1
where ε and are the real and the imaginary parts of dielectric function,
respectively. Generally, the real ε and imaginary parts of the dielectric
functions are associated with the electronic polarizability and electronic absorption of the material, respectively. The imaginary part can be calculated by [27]
| | 2 3
here ω is the angular frequency of light, the subscripts l and j demonstrates the conduction and valence bands, respectively, e and m represent the charge and the mass 4
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of electron, respectively and is the momentum matrix element with the wave
vector k. In addition, the real part ε of the dielectric function can be obtained
from the imaginary part with the Kramers-Kroning relations. Then the other optical properties, such as absorption coefficient (αω), reflectivity (R(ω)), refractivity index (n(ω)), and energy-loss (L(ω)) can be derived from ε and [28]: αω
√2 #$
'ω (
,ω
) * 1
) * 1
#$
%
⁄
3
( 4 ⁄
%
-√2 5
/ω ⁄0 1 6 3. Results and discussion To understand the origin of spontaneous spin-polarization in doped systems, the models of one-N-doped, one-Fe-doped and one-N, one-Fe-codoped anatase have been adopted and the total spin density of states (TDOS) have been calculated as well as the partial spin density of states (PDOS). In the case of N-doped system, one O atom (O(1)) is substituted by one N atom in 2×1×1 TiO2 supercell as showed in Figure 1(a) and this amounts to a doping concentration of 4.17 at.%. As to the Fe-doped anatase TiO2 system, one Ti atom (Ti(1)) is replaced by one Fe atom in 2×2×1 TiO2 supercell as shown in Figure 1(b) and the corresponding doping concentration is 2.08 at.%. For N, Fe-codoped anatase 2×2×1 TiO2 supercell, one Ti atom (Ti(1)) is replaced by one Fe atom and one O atom(O(1)) is concurrently substituted by one N atom in 2×2×1 TiO2 supercell (shown in Figure 1(b)) with a doping concentration of 4.17 at.%. Figure 2 shows the total and partial spin density of states of different configurations. For N-monodoped TiO2, it is clear from Figure 2(b) that the width of the conduction band becomes wider and the conduction band bottom has an obvious 5
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downward shift, which results in a band gap narrowing of about 0.18 eV compared with the pure TiO2. From Figure 2(b’) it can be observed the isolated N 2p PDOS peak are formed within the band gap resulting in stronger interactions between the interbands from the N 2p impurity states to the Ti 3d states. These results could provide a good explanation for the red-shift phenomenon in the recent experiments [29]. The appearance of the impurity N band results in asymmetrical distributions of the N 2p PDOS between the majority-spin and the minority-spin channels near Fermi level indicating the magnetism in N-doped anatase TiO2 is induced. The magnetic moment for this doped system is 0.96 34 /supercell. For Fe-monodoped TiO2, the remarkable feature of TDOS (Figure 2(c)) is that the Fermi level shifts upward into the band gap area compared with the undoped TiO2. Besides, the band gap is narrowed to 2.25 eV, which can partly explain why N-doped TiO2 has a better photocatalytic property compared with pure TiO2 [30,31]. It is also noticed that the states of the valence band and the conduction band exhibit a tendency toward weak spin polarization mainly due to the polarization of the Fe 3d electrons (Figure 2(c’)). The calculated magnetic moment of the supercell is 3.98 34 , which is consistent with the result of V. N. Krasil’nikov’s work [30]. For N, Fe-codoped TiO2, both the majority-spin and the minority-spin electrons are across the Fermi level, revealing N, Fe-codoped anatase TiO2 obviously shows stable metallic nature. Codoping with N and Fe would be expected to modify the conduction and valence band edges simultaneously because of different p and d orbital energies provided by N and Fe, respectively. The calculated magnetic moments of the codoped supercell are 4.92 34
(3.82 34 /Fe and 0.82 34 /N). There is a strong hybridization between the Fe 3d states and the N 2p states around the Fermi level, inducing the decrease of the N atom’s magnetic moment as well as the Fe atom’s magnetic moment. To investigate the magnetic coupling interaction between two spin-polarized atoms (N-N, Fe-Fe and N-Fe), we adopt three different doped configurations for each doped configuration, in which two atoms are replaced. In the case of N-doped system, three different structures of two-N-atom doped 2×1×1 TiO2 supercell marked as 6
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Ti8N2O14 are considered (Figure 1(a)). It is convenient to define the N-doped structures in terms of the four oxygen positions 1-4 in Figure 1(a). The three configurations are obtained by substituting two N atoms for the two O positions at O(1, 2), O(1, 3) and O(1, 4), respectively. For simplicity, the corresponding three configurations are labeled as ⅰ,ⅱ and ⅲ. In the Fe-doped anatase TiO2 system (Ti14Fe2O32), two Ti atoms are replaced by two Fe atoms and three different doped configurations are examined in 2×2×1 supercell, which are shown as the Ti(1, 2), Ti(1, 3) and Ti(1, 4) in Figure 1(b), respectively. Correspondingly, the above three configurations are designated as ⅳ, ⅴand ⅵ. For N, Fe-codoped anatase, we design three different configurations for the structure Ti15FeNO31: the Ti (1) atom is substituted by one Fe atom; meanwhile, the O(1), O(2) and O(3) atoms are replaced by one N atom (shown in Figure 1(b)), respectively. The three configurations are also labeled as ⅶ , ⅷ and ⅸ . In addition, both the ferromagnetic (FM) and the antiferromagnetic (AFM) couplings between the two dopants have been considered for each configuration. Table 1 depicts the calculated total energies of Ti8N2O14, Ti14Fe2O32 and Ti15FeNO31 for both FM and AFM states. For the system of Ti8N2O14, it is clear that the total energy of the FM state is apparently lower than that of the AFM state for all of the three configurations. Therefore, the N dopants in anatase TiO2 can induce FM coupling effects, which is consistent with both of the recent experimental and theoretical facts that there exists stable ferromagnetic after N doped into transition-metal oxides [32-36]. Moreover, configuration ⅲ has the lowest FM state energy among the three configurations, indicating it is the most stable structure for Ti8N2O14. As to Ti14Fe2O32, the total energy in the FM state is lower than that in the AFM state for configuration ⅳ and ⅴ, but opposite in configuration ⅵ. However, it can be seen that configuration ⅴ is the most stable doping structure. Therefore FM phase is the stable magnetic ground state for Ti14Fe2O32, which is in reasonable 7
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concordance with previous experimental and theoretical results about magnetic coupling of transition mental doped in TiO2 [37-39]. In the case of Ti15FeNO31, the energy difference between AFM and FM state in ⅸ configuration is almost negligible, meaning there exists a competition between FM state and AFM state. However, the lowest energy exists in ⅶ configuration, revealing its AFM magnetic ground state for Ti15FeNO31. The optical properties of N-doped, Fe-doped and N, Fe-codoped anatase TiO2 are shown in Figure 3 and Figure 4 on the basis of the absorption coefficient, dielectric function, reflectivity, refractivity index and energy-loss spectrum. In the present research, components of the optical properties corresponding to the polarization vectors perpendicular to the c axis (E⊥c) have been considered. As our calculated band gap for pure anatase TiO2 is 0.18 eV lower than the experimental band gap value (3.23eV), hence we added a scissor approximation of 0.18 eV to calculate the absorption edge to make them compatible with experimental results. From the absorption coefficient spectrum in Figure 3, it is crystal clear to see that pure anatase TiO2 only shows absorption in UV light region due to the intrinsic band gap nature. However, along with the stronger UV light absorption, improved visible light absorption can be found for all doped systems making the absorption edges shifted to the visible-light region. In Figure 3(a), the inset is the enlarged optical absorption spectrum with the photon energy ranging from 0 eV to 4.0 eV, which is just the slightly enlarged range of visible-light energy (1.63-3.1 eV). It can be seen from the inset that there exists an intersection point of the absorption spectrum as to the two doped systems (N-doped and Fe-doped ones) and the horizontal ordinate of the intersection point is 1.97 eV. Furthermore, it is worth noting that N, Fe-codoped TiO2 presents the highest absorption coefficient from the photon energy of 1.63 to 3.1 eV (i.e., the visible-light energy). Additionally, the Fe-doped TiO2 has the better optical absorption property from 1.63 to 1.97 eV than N-doped one but converse result can be seen from 1.97 to 3.1 eV, which makes it more convenient to choose the most efficient photoelectrochemical application as to the three doped systems. Meanwhile, the UV 8
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absorption is also enhanced for the three doped models, especially for N-doped TiO2 model, which could also be applied in short-wavelength optoelectronic devices, such as UV detector and UV light-emitting diodes (LEDs). Figure 3(b) shows the imaginary part of the dielectric function, which is the pandect of the optical properties for the four different configurations of the TiO2 systems. Compared with the imaginary part of the dielectric function of pure TiO2, the visible range has been expanded obviously due to the intrinsic plasma frequencies for the three doped configurations. The photon-excited carrier will be trapped by the impure state and then transferred into the conduction band making the transition energy becoming smaller, which improves the utilization of the solar light. Figure 4 depicts the reflectivity (R(ω)), refractivity index (n(ω)), and energy-loss (L(ω)) spectrum of the four models of the TiO2 systems in the photon energy range of 0~12 eV. Reflectivity of the three doped models are enhanced compared with that of pure TiO2 especially in the photon energy of range of 0-2 eV, which shows Fe-doped and N, Fe-codoped models have the higher reflectivity. The photon-energy range (0-4 eV) is characterized by small reflectivity but appreciable refractivity for the four different TiO2 models. At the photon-energy range of 8-12 eV, the reflectivity and refractivity show an opposite trend. The energy-loss function describes the energy loss of the fast moving electrons traversing the material medium. The inset in energy-loss functions is the enlarged figure for the four peaks of the energy-loss functions as to the four different TiO2 models. The peaks in the loss function spectrum describe the trailing edges in the reflectivity of the corresponding model [40]. The strong peaks in the loss functions spectra describe the strong and abrupt variation in the reflectivity value of the corresponding doped models. Different doped models have loss function peaks at different values indicating the decreasing tendency in the reflectivity in the corresponding region. Taking N, Fe-codoped TiO2 model as an example, the reflectivity reduces abruptly at about 10.34 eV, which corresponds to the peak of the energy-loss spectrum.
4. Conclusions 9
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In summary, first-principle calculations have been performed to study the electronic structure, magnetic ground states and optical properties of N-doped, Fe-doped as well as N, Fe-codoped anatase TiO2. Our results reveal that both N-doped and Fe-doped anatase TiO2 exhibit stable FM semiconductors, while N, Fe-codoped one manifests AFM metallic nature. The optical absorption spectrum shows that N-doping, Fe-doping and N, Fe-codoping anatase TiO2 all extend the optical absoption to the visible-light region. Most importantly, N, Fe-codoped prsents the highest absorption coefficient during the visible-light energy (1.63-3.1 eV). Our calculations provide electronic structure evidence in choosing the most efficient photoelectrochemical application. In addition, the UV absorption is also enhanced for the three doped models especially N-doped TiO2, which makes them to be potential candidates for short-wavelength optoelectronic devices.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grants No. 51275213, 21301075, 51372244 and 51302112), Specialized Research Fund for the Doctoral Program of Higher Education (Grants No. 20133227120003), Open Project of State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (Grants No. RERU2014006), Jiangsu Industry-University-Research Jointinnovation Foundation (Grants No. BY2013065-05, BY2013065-06) and Project Funded by Priority Academic Program Development of Jiangsu Higher Education Institutions.
References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] A.M. Volodin, Catal. Today 58 (2000) 103. [3] U. Diebold, Surf. Sci. Rep. 48 (2003) 53. [4] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [5] S. Girish Kumar, L. Gomathi Devi, J. Phys. Chem. A 115 (2011) 13211. [6] X. Dan, K.L. Yao, G.Y. Gao, L. Yang, J. Magn. Magn. Mater. 335 (2013) 118. 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
[7] C.D. Valentin, G. Pacchioni, A. Selloni, Phys. Rev. B 70 (2004) 085116. [8] B. Chi, L. Zhao, T. Jin, J. Phys. Chem. C 111 (2007) 6189. [9] S. Sato, Chem. Phys. Lett. 123 (1986) 126. [10] X.X. Yang, C.D. Cao, L. Erickson, K. Hohn, R. Maghirang, K. Klabunde, Appl. Catal. B : Environ. 91 (2009) 657. [11] M. Li, J. Zhang, Y. Zhang, Chem. Phys. Lett. 527 (2012) 63. [12] D.W. Jing, Y.J. Zhang, L.J. Guo, Chem. Phys. Lett. 415 (2005) 74. [13] Y.Q. Gai, J.B. Li, S.S. Li, J.B. Xia, S.H. Wei, Phys. Rev. Lett. 102 (2009) 036402.
[14] W.G. Zhu,
X.F. Qiu, V. Lancu, X.Q. Chen, H. Pan, W. Wang, N.M.
Dimitrijevic, T.
Rajh, H.M.
Meyer,
III, M.
Parans
Paranthaman, G.M.
Stocks, H.H. Weitering, B.H. Gu, G. Eres, Z.Y. Zhang, Phys. Rev. Lett. 103 (2009) 226401. [15] J.G. Yu, Q.J. Xiang, M.H. Zhou, Appl. Catal. B : Environ. 90 (2009) 595. [16] E. Thimsen, S. Biswas, C.S. Lo, P. Biswas, J. Phys. Chem. C 113 (2009) 2014. [17] W.X. Liu, J. Ma, X.G. Qu, W.B. Cao, Res. Chem. Intermed. 35 (2009) 321. [18] Q.Y. Wen, H.W. Zhang, Q.H. Yang, Y.L. Jing, P.X. Zhang, International Magnetics Conference, Taipei 47 (2011) 3987. [19]
Y.J.
Kim, S.
Thevuthasan, T.
Droubay, A.S.
Lea, C.M.
Wang, V.
Shutthanandan, S.A. Chambers, R.P. Sears, B. Taylor B. Sinkovic, Appl. Phys. Lett. 84 (2004) 3531. [20] W. Kohn, L.J Sham, Phys. Rev. A 140 (1965) 1133. [21] M.C. Payne, M.P. Teter, D.C. Allen, T.A. Arias, J.D. Joannopoulos, Rev. Mod. Phys. 64 (1992) 1045. [22] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [23] R. Long, N.J. English, Chem. Mater. 22 (2010) 1616. [24] R. Long, N.J. English, Chem. Phys. Lett. 478 (2009) 175. [25] L.P. Wen, B.S. Liu, X.J. Zhao, K. Nakata, T. Murakami, A. Fujishima, Int. J. Photoenergy 2012 (2012) 368750. [26] K.S. Yang, Y. Dai, B.B. Huang, J. Phys. Chem. C 111 (2007) 12086. 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
[27] H.I. Zhang, J. Callaway, Phys. Rev. 181 (1969) 1163. [28] S. Saha, T.P. Sinha, A. Mookerjee, Phys. Rev. B 62 (2000) 8828. [29] M. Batzill, E.H. Morales, U. Diebold, Phys. Rev. Lett. 96 (2006) 026103. [30] V.N. Krasil’nikov, V.P. Zhukov, L.A. Perelyaeva, I.V. Baklanova, I.R. Shein, Phys. Solid State 55 (2013) 1903. [31] Z. Ambrus, N. Balazs, T. Alapi, G. Wittmann, P. Sipos, A. Dombi, K. Mogyorosi, Appl. Catal. B 81 (2008) 27. [32] C.F. Yu, T.J. Lin, S.J. Sun, H. Chou, J. Phys. D: Appl. Phys. 40 (2007) 6497. [33] L. Shen, R.Q. Wu, H. Pan, G.W. Peng, M. Yang, Z.D. Sha, Y.P. Feng, Phys. Rev. B 78 (2008) 073306. [34] I.S. Elfimov, A. Rusydi, S.I. Csiszar, Z. Hu, H.H. Hsieh, H.J. Lin, C.T. Chen, R. Liang, G.A. Sawatzky, Phys. Rev. Lett. 98 (2007) 137202. [35] B. Gu, N. Bulut, T. Ziman, S. Maekawa, Phys. Rev. B 79 (2009) 024407. [36] M.M. Cruz, R.C. da Silva, N. Franco, M. Godinho, J. Phys.: Condens. Matter 21 (2009) 206002. [37] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Science 291 (2001) 854. [38] N.H. Hong, J. Sakai, W. Prellier, A. Hassini, A. Ruyter, F. Gervais, Phys. Rev. B 70 (2004) 195204. [39] X.S. Du, Q.X. Li, H.B. Su, J.L. Yang, Phys. Rev. B 74 (2006) 233201. [40] J. Sun, H.T. Wang, J.L. He, Y.J. Tian, Phys. Rev. B 71 (2005) 125132.
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Figure captions
Figure 1. (a) 25151 TiO2 supercell for N-doped configuration; (b) 25251 TiO2 supercell for Fe-doped and N, Fe-codoped configurations. Red and gray balls represent O and Ti atoms, respectively. The atoms involved in being substituted are marked by the element names.
Figure 2. Calculated TDOS and PDOS of pure, N-doping, Fe-doping and N, Fe-codoping anatase TiO2 are shown at the left and right side, respectively. Vertical dash line indicates Fermi level. Curves above and below the horizontal axis refer to the majority-spin and minority-spin DOS, respectively.
Figure 3. (a) the absorption coefficient spectrum (α(105cm-1)); (b) the imaginary part of the dielectric function ( (ω)) for pure, N-doping, Fe-doping and N, Fe-codoping anatase TiO2. The inset in figure (a) denotes the enlarged optical absorption spectrum with the photon energy ranging from 0 to 4 eV (the range of visible-light energy).
Figure 4. (a) reflectivity (R); (b) refractivity index (n); (c) energy-loss functions (L) of anatase TiO2 systems with four different structures. The inset in energy-loss functions is the enlarged figure for the four peaks of the energy-loss functions as to the four different TiO2 models. 13
Table 1 The calculated total energies and stable magnetic ground states for various configurations of Ti8N2O14, Ti14Fe2O32 and Ti15FeNO31. EFM and EAFM represent the relative energy in the FM and AFM states (the reference energy of each configuration marked with asterisk at the upper right). ΔE is the value of EFM minus EAFM for each configuration. In the column of the Ground state, FM/AFM state is the final determinate magnetic state for corresponding doped system.
Ti8N2O14
Ti14Fe2O32
Ti15FeNO31
Configuration
EFM(eV)
EAFM(eV)
ΔE(eV)
ⅰ
0*
7.5443
-7.5443
ⅱ
-0.7738
6.9147
-7.6885
ⅲ
-1.0405
6.7940
-7.8345
ⅳ
0*
0.2937
-0.2937
ⅴ
-3.7117
-3.5751
-0.1366
ⅵ
1.0677
0.5884
0.4793
ⅶ
0*
-0.6616
0.6616
ⅷ
0.1105
0.2943
-0.1838
ⅸ
0.6317
0.6305
0.0012
Ground state
FM
FM
AFM
Figure 1
Figure 2
Figure 3
Figure 4
Highlights
N-doped and Fe-doped anatase TiO2 exhibit ferromagnetic semiconductors.
N, Fe-codoped anatase TiO2 manifests antiferromagnetic metallic nature.
The optical absorption spectrum extends to the visible-light region.
The ultraviolet absorption is enhanced for all doped models.
S0009-2614(14)00479-5 http://dx.doi.org/10.1016/j.cplett.2014.05.097 CPLETT 32242
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
26 March 2014 26 May 2014
Please cite this article as: L. Chen, H. Li, K. Zhang, L. Yin, H. Tang, X. Liu, J. Meng, C. Li, Magnetic and optical properties of N-doped, Fe-doped and (N, Fe)-codoped anatase TiO2, Chemical Physics Letters (2014), doi: http:// dx.doi.org/10.1016/j.cplett.2014.05.097
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Magnetic and optical properties of N-doped, Fe-doped and (N, Fe)-codoped anatase TiO2 Lin Chen,a Hongping Li,a,b,* Kun Zhang,a Linting Yin,a Hua Tang,a Xiaojuan Liu,c Jian Meng,c Changsheng Li,a,b,* a
School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, P. R. China b Institute for Advanced Materials, Jiangsu University, Zhenjiang, 212013, P. R. China c State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China * Corresponding author. E-mail address: [email protected] [email protected] Tel: +86-511-88783268
Abstract Electronic structure, magnetic and optical properties of N-doped, Fe-doped and N, Fe-codoped anatase TiO2 have been investigated by using first-principle calculations. Both N-doped and Fe-doped anatase TiO2 exhibit ferromagnetic semiconductors, while N, Fe-codoped one manifests metallic nature with antiferromagnetic ground state. The optical absorption spectrum for all the doped models extends the optical absoption to the visible-light region. Especially, N, Fe-codoped anatase TiO2 presents the highest absorption coefficient during the visible-light energy range (1.63-3.1 eV). Furthermore, the ultraviolet absorption is also enhanced, particularly for N-doped model.
Keywords: Magnetic properties; Optical properties; N, Fe-codoping; Anatase TiO2
1
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1. Introduction Since Fujishima and Honda [1] reported the monocrysral TiO2 can analyse water into hydrogen and oxygen under the ultraviolet (UV) light, titania has been extensively studied as a promising material for photochemical applications due to its excellent properties, i.e., chemical stability, non-toxicity, low cost, etc.. [2,3] However, the applications of TiO2 are rather limited owing to several reasons listed below: (i) a wide band gap (3.23 eV for anatase), which makes it inactive in the visible spectral region leading to a low utilization of sunlight; (ii) high recombination rate of photoexcited electron–hole pairs; and (iii) low efficiency of charge transfer to the surface. To overcome the existing limitations and shift the photocatalytic effect of titanium dioxide from the characteristic UV to visible range of the spectrum, great efforts have been made to improve the electronic property of TiO2 by means of the metal ion/nonmental ion doping, codoping with two or more foreign ions [4-6]. Over the past decade, many studies have been conducted to investigate the doping effect of TiO2 theoretically and experimentally [7,8] since Sato made a breakthrough that N-doped TiO2 showed visible-light photocatalytic activity [9]. Doping with small amounts (＜10 atom%) of non-metal impurities into TiO2 is proved to be an effective strategy to narrow band gap and increases photocatalytic activity [10,11]. However, their incorporations concurrently lead to poor photoresponse because the partially occupied impurity bands can act as killers for photogenerated carriers making the photo-excited current or photocatalytic efficiency decreased [12]. It has been recognized that donor-acceptor codoping at Ti and O sites can reduce the recombination centers which can effectively improve the charge carriers’ migration efficiency and enhance the photocatalytic activity [13,14]. Transition-metal ions were widely applied as dopants (doped at Ti site) into TiO2 due to their d electron configuration. As Jiaguo Yu’s work [15], it was reported that the photocatalytic activity of Fe-doped titanate nanotubes is 2 times higher than P25 under visible light illumination. The most acceptable explanation for the improvement of photocatalytic properties of TiO2 by doping with Fe3+ is that Fe3+ cations can act as 2
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shallow traps to decrease the recombination rate. Therefore, the synergistic effects of codoping with nonmetal and transition-metal ions into the TiO2 deserve more attention, which might eventually help to design high-efficient photocatalytic materials. Up to now, there are several papers related to N-doped, Fe-doped and N, Fe-codoped anatase TiO2 by experimental or theoretical method [4,16,17], which give explanations or predictions about the significantly increasing photocatalytic activity under visible-light irradiation. Meanwhile, reserchers have found magnetism after N-doped and Fe-doped anatase TiO2 [18,19], which is extremely useful as to photocatalytic materials. However, few researches study the magnetic properties confirming the magnetic ground states after N-doped, Fe-doped and N, Fe-codoped anatase TiO2. Here, the electronic structure, magnetic ground states and absorption properties of (N, Fe)-codoped TiO2 have been investigated by using the density functional theory (DFT) to reveal the microscopic mechanism for band gap narrowing and the origin of the enhanced photocatalytic activity. For comparison, the corresponding calculation and theoretical analysis were also conducted for pure, N-doped and Fe-doped anatase TiO2.
2. Computational methods First-principles plane-wave pseudopotential calculations based on the density functional theory [20] is performed applying the Cambridge Sequential Total Energy Package Code (CASTEP) [21]. The geometry optimization is carried out by using the Broyden-Fletcher-Goldfrab-Shanno
(BFGS)
method
before
their
properties’
calculations. The plane wave cut-off energy is chosen to be 380 eV for all the calculations.
The
generalized
Perdew-Burke-Ernzerhof
(PBE)
gradient scheme
approximation [22]
is
(GGA)
employed
within to
the
describe
exchange-correlation potential. Moreover, to properly describe the strong electron correlation in the 3d transition metal oxide, the GGA plus on site repulsion U method (GGA+U) is employed. In this paper, a series of U values for Ti 3d have been tested to better represent the experimental results. Our results show that the band gap for 3
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pure TiO2 is 3.05 eV when the U is set as 7.5 eV, which is in good agreement with experimental result 3.23 eV as well as previously theoretical results [23,24]. Hence, we mainly discuss the GGA+U results with U = 7.5 eV in this paper. Self-consistent was achieved once the total energy is converged to less than 1.0 ×10-5 eV and the maximum force being within 3.0 ×10-2 eV/Å, respectively. Anatase TiO2 belongs to body-centered tetragonal crystal structure with I41/amd space group, and there are four Ti atoms and eight O atoms in each unit cell. For N-doped anatase TiO2, 2 × 1 × 1 supercell configuration is considered and the Monkhorst-Pack k-mesh is set as 5×9×4. In regard to Fe-doped and N, Fe-codoped anatase TiO2, 2×2×1 supercell with four primitive unit cells is constructed and the Monkhorst-Pack k-mesh of 5×5×4 is adopted. All of the three configurations are based on the previous reports [25,26]. All the atoms are relaxed to their equilibrium position before the electronic, magnetic and optical properties calculated. As for the calculations of optical properties, interactions of photons in suitable frequency with the orbital electrons can shift the electron from the valence band to conduction band, during which these transitions will give spectra caused by the photon absorption or emission. The optical properties include the dielectric constant, absorption coefficient, reflectivity, refractive index and loss function are interrelated with each other. The dielectric function is defined as εω ε 1
where ε and are the real and the imaginary parts of dielectric function,
respectively. Generally, the real ε and imaginary parts of the dielectric
functions are associated with the electronic polarizability and electronic absorption of the material, respectively. The imaginary part can be calculated by [27]
| | 2 3
here ω is the angular frequency of light, the subscripts l and j demonstrates the conduction and valence bands, respectively, e and m represent the charge and the mass 4
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of electron, respectively and is the momentum matrix element with the wave
vector k. In addition, the real part ε of the dielectric function can be obtained
from the imaginary part with the Kramers-Kroning relations. Then the other optical properties, such as absorption coefficient (αω), reflectivity (R(ω)), refractivity index (n(ω)), and energy-loss (L(ω)) can be derived from ε and [28]: αω
√2 #$
'ω (
,ω
) * 1
) * 1
#$
%
⁄
3
( 4 ⁄
%
-√2 5
/ω ⁄0 1 6 3. Results and discussion To understand the origin of spontaneous spin-polarization in doped systems, the models of one-N-doped, one-Fe-doped and one-N, one-Fe-codoped anatase have been adopted and the total spin density of states (TDOS) have been calculated as well as the partial spin density of states (PDOS). In the case of N-doped system, one O atom (O(1)) is substituted by one N atom in 2×1×1 TiO2 supercell as showed in Figure 1(a) and this amounts to a doping concentration of 4.17 at.%. As to the Fe-doped anatase TiO2 system, one Ti atom (Ti(1)) is replaced by one Fe atom in 2×2×1 TiO2 supercell as shown in Figure 1(b) and the corresponding doping concentration is 2.08 at.%. For N, Fe-codoped anatase 2×2×1 TiO2 supercell, one Ti atom (Ti(1)) is replaced by one Fe atom and one O atom(O(1)) is concurrently substituted by one N atom in 2×2×1 TiO2 supercell (shown in Figure 1(b)) with a doping concentration of 4.17 at.%. Figure 2 shows the total and partial spin density of states of different configurations. For N-monodoped TiO2, it is clear from Figure 2(b) that the width of the conduction band becomes wider and the conduction band bottom has an obvious 5
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downward shift, which results in a band gap narrowing of about 0.18 eV compared with the pure TiO2. From Figure 2(b’) it can be observed the isolated N 2p PDOS peak are formed within the band gap resulting in stronger interactions between the interbands from the N 2p impurity states to the Ti 3d states. These results could provide a good explanation for the red-shift phenomenon in the recent experiments [29]. The appearance of the impurity N band results in asymmetrical distributions of the N 2p PDOS between the majority-spin and the minority-spin channels near Fermi level indicating the magnetism in N-doped anatase TiO2 is induced. The magnetic moment for this doped system is 0.96 34 /supercell. For Fe-monodoped TiO2, the remarkable feature of TDOS (Figure 2(c)) is that the Fermi level shifts upward into the band gap area compared with the undoped TiO2. Besides, the band gap is narrowed to 2.25 eV, which can partly explain why N-doped TiO2 has a better photocatalytic property compared with pure TiO2 [30,31]. It is also noticed that the states of the valence band and the conduction band exhibit a tendency toward weak spin polarization mainly due to the polarization of the Fe 3d electrons (Figure 2(c’)). The calculated magnetic moment of the supercell is 3.98 34 , which is consistent with the result of V. N. Krasil’nikov’s work [30]. For N, Fe-codoped TiO2, both the majority-spin and the minority-spin electrons are across the Fermi level, revealing N, Fe-codoped anatase TiO2 obviously shows stable metallic nature. Codoping with N and Fe would be expected to modify the conduction and valence band edges simultaneously because of different p and d orbital energies provided by N and Fe, respectively. The calculated magnetic moments of the codoped supercell are 4.92 34
(3.82 34 /Fe and 0.82 34 /N). There is a strong hybridization between the Fe 3d states and the N 2p states around the Fermi level, inducing the decrease of the N atom’s magnetic moment as well as the Fe atom’s magnetic moment. To investigate the magnetic coupling interaction between two spin-polarized atoms (N-N, Fe-Fe and N-Fe), we adopt three different doped configurations for each doped configuration, in which two atoms are replaced. In the case of N-doped system, three different structures of two-N-atom doped 2×1×1 TiO2 supercell marked as 6
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Ti8N2O14 are considered (Figure 1(a)). It is convenient to define the N-doped structures in terms of the four oxygen positions 1-4 in Figure 1(a). The three configurations are obtained by substituting two N atoms for the two O positions at O(1, 2), O(1, 3) and O(1, 4), respectively. For simplicity, the corresponding three configurations are labeled as ⅰ,ⅱ and ⅲ. In the Fe-doped anatase TiO2 system (Ti14Fe2O32), two Ti atoms are replaced by two Fe atoms and three different doped configurations are examined in 2×2×1 supercell, which are shown as the Ti(1, 2), Ti(1, 3) and Ti(1, 4) in Figure 1(b), respectively. Correspondingly, the above three configurations are designated as ⅳ, ⅴand ⅵ. For N, Fe-codoped anatase, we design three different configurations for the structure Ti15FeNO31: the Ti (1) atom is substituted by one Fe atom; meanwhile, the O(1), O(2) and O(3) atoms are replaced by one N atom (shown in Figure 1(b)), respectively. The three configurations are also labeled as ⅶ , ⅷ and ⅸ . In addition, both the ferromagnetic (FM) and the antiferromagnetic (AFM) couplings between the two dopants have been considered for each configuration. Table 1 depicts the calculated total energies of Ti8N2O14, Ti14Fe2O32 and Ti15FeNO31 for both FM and AFM states. For the system of Ti8N2O14, it is clear that the total energy of the FM state is apparently lower than that of the AFM state for all of the three configurations. Therefore, the N dopants in anatase TiO2 can induce FM coupling effects, which is consistent with both of the recent experimental and theoretical facts that there exists stable ferromagnetic after N doped into transition-metal oxides [32-36]. Moreover, configuration ⅲ has the lowest FM state energy among the three configurations, indicating it is the most stable structure for Ti8N2O14. As to Ti14Fe2O32, the total energy in the FM state is lower than that in the AFM state for configuration ⅳ and ⅴ, but opposite in configuration ⅵ. However, it can be seen that configuration ⅴ is the most stable doping structure. Therefore FM phase is the stable magnetic ground state for Ti14Fe2O32, which is in reasonable 7
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concordance with previous experimental and theoretical results about magnetic coupling of transition mental doped in TiO2 [37-39]. In the case of Ti15FeNO31, the energy difference between AFM and FM state in ⅸ configuration is almost negligible, meaning there exists a competition between FM state and AFM state. However, the lowest energy exists in ⅶ configuration, revealing its AFM magnetic ground state for Ti15FeNO31. The optical properties of N-doped, Fe-doped and N, Fe-codoped anatase TiO2 are shown in Figure 3 and Figure 4 on the basis of the absorption coefficient, dielectric function, reflectivity, refractivity index and energy-loss spectrum. In the present research, components of the optical properties corresponding to the polarization vectors perpendicular to the c axis (E⊥c) have been considered. As our calculated band gap for pure anatase TiO2 is 0.18 eV lower than the experimental band gap value (3.23eV), hence we added a scissor approximation of 0.18 eV to calculate the absorption edge to make them compatible with experimental results. From the absorption coefficient spectrum in Figure 3, it is crystal clear to see that pure anatase TiO2 only shows absorption in UV light region due to the intrinsic band gap nature. However, along with the stronger UV light absorption, improved visible light absorption can be found for all doped systems making the absorption edges shifted to the visible-light region. In Figure 3(a), the inset is the enlarged optical absorption spectrum with the photon energy ranging from 0 eV to 4.0 eV, which is just the slightly enlarged range of visible-light energy (1.63-3.1 eV). It can be seen from the inset that there exists an intersection point of the absorption spectrum as to the two doped systems (N-doped and Fe-doped ones) and the horizontal ordinate of the intersection point is 1.97 eV. Furthermore, it is worth noting that N, Fe-codoped TiO2 presents the highest absorption coefficient from the photon energy of 1.63 to 3.1 eV (i.e., the visible-light energy). Additionally, the Fe-doped TiO2 has the better optical absorption property from 1.63 to 1.97 eV than N-doped one but converse result can be seen from 1.97 to 3.1 eV, which makes it more convenient to choose the most efficient photoelectrochemical application as to the three doped systems. Meanwhile, the UV 8
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absorption is also enhanced for the three doped models, especially for N-doped TiO2 model, which could also be applied in short-wavelength optoelectronic devices, such as UV detector and UV light-emitting diodes (LEDs). Figure 3(b) shows the imaginary part of the dielectric function, which is the pandect of the optical properties for the four different configurations of the TiO2 systems. Compared with the imaginary part of the dielectric function of pure TiO2, the visible range has been expanded obviously due to the intrinsic plasma frequencies for the three doped configurations. The photon-excited carrier will be trapped by the impure state and then transferred into the conduction band making the transition energy becoming smaller, which improves the utilization of the solar light. Figure 4 depicts the reflectivity (R(ω)), refractivity index (n(ω)), and energy-loss (L(ω)) spectrum of the four models of the TiO2 systems in the photon energy range of 0~12 eV. Reflectivity of the three doped models are enhanced compared with that of pure TiO2 especially in the photon energy of range of 0-2 eV, which shows Fe-doped and N, Fe-codoped models have the higher reflectivity. The photon-energy range (0-4 eV) is characterized by small reflectivity but appreciable refractivity for the four different TiO2 models. At the photon-energy range of 8-12 eV, the reflectivity and refractivity show an opposite trend. The energy-loss function describes the energy loss of the fast moving electrons traversing the material medium. The inset in energy-loss functions is the enlarged figure for the four peaks of the energy-loss functions as to the four different TiO2 models. The peaks in the loss function spectrum describe the trailing edges in the reflectivity of the corresponding model [40]. The strong peaks in the loss functions spectra describe the strong and abrupt variation in the reflectivity value of the corresponding doped models. Different doped models have loss function peaks at different values indicating the decreasing tendency in the reflectivity in the corresponding region. Taking N, Fe-codoped TiO2 model as an example, the reflectivity reduces abruptly at about 10.34 eV, which corresponds to the peak of the energy-loss spectrum.
4. Conclusions 9
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In summary, first-principle calculations have been performed to study the electronic structure, magnetic ground states and optical properties of N-doped, Fe-doped as well as N, Fe-codoped anatase TiO2. Our results reveal that both N-doped and Fe-doped anatase TiO2 exhibit stable FM semiconductors, while N, Fe-codoped one manifests AFM metallic nature. The optical absorption spectrum shows that N-doping, Fe-doping and N, Fe-codoping anatase TiO2 all extend the optical absoption to the visible-light region. Most importantly, N, Fe-codoped prsents the highest absorption coefficient during the visible-light energy (1.63-3.1 eV). Our calculations provide electronic structure evidence in choosing the most efficient photoelectrochemical application. In addition, the UV absorption is also enhanced for the three doped models especially N-doped TiO2, which makes them to be potential candidates for short-wavelength optoelectronic devices.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grants No. 51275213, 21301075, 51372244 and 51302112), Specialized Research Fund for the Doctoral Program of Higher Education (Grants No. 20133227120003), Open Project of State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (Grants No. RERU2014006), Jiangsu Industry-University-Research Jointinnovation Foundation (Grants No. BY2013065-05, BY2013065-06) and Project Funded by Priority Academic Program Development of Jiangsu Higher Education Institutions.
References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] A.M. Volodin, Catal. Today 58 (2000) 103. [3] U. Diebold, Surf. Sci. Rep. 48 (2003) 53. [4] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [5] S. Girish Kumar, L. Gomathi Devi, J. Phys. Chem. A 115 (2011) 13211. [6] X. Dan, K.L. Yao, G.Y. Gao, L. Yang, J. Magn. Magn. Mater. 335 (2013) 118. 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
[7] C.D. Valentin, G. Pacchioni, A. Selloni, Phys. Rev. B 70 (2004) 085116. [8] B. Chi, L. Zhao, T. Jin, J. Phys. Chem. C 111 (2007) 6189. [9] S. Sato, Chem. Phys. Lett. 123 (1986) 126. [10] X.X. Yang, C.D. Cao, L. Erickson, K. Hohn, R. Maghirang, K. Klabunde, Appl. Catal. B : Environ. 91 (2009) 657. [11] M. Li, J. Zhang, Y. Zhang, Chem. Phys. Lett. 527 (2012) 63. [12] D.W. Jing, Y.J. Zhang, L.J. Guo, Chem. Phys. Lett. 415 (2005) 74. [13] Y.Q. Gai, J.B. Li, S.S. Li, J.B. Xia, S.H. Wei, Phys. Rev. Lett. 102 (2009) 036402.
[14] W.G. Zhu,
X.F. Qiu, V. Lancu, X.Q. Chen, H. Pan, W. Wang, N.M.
Dimitrijevic, T.
Rajh, H.M.
Meyer,
III, M.
Parans
Paranthaman, G.M.
Stocks, H.H. Weitering, B.H. Gu, G. Eres, Z.Y. Zhang, Phys. Rev. Lett. 103 (2009) 226401. [15] J.G. Yu, Q.J. Xiang, M.H. Zhou, Appl. Catal. B : Environ. 90 (2009) 595. [16] E. Thimsen, S. Biswas, C.S. Lo, P. Biswas, J. Phys. Chem. C 113 (2009) 2014. [17] W.X. Liu, J. Ma, X.G. Qu, W.B. Cao, Res. Chem. Intermed. 35 (2009) 321. [18] Q.Y. Wen, H.W. Zhang, Q.H. Yang, Y.L. Jing, P.X. Zhang, International Magnetics Conference, Taipei 47 (2011) 3987. [19]
Y.J.
Kim, S.
Thevuthasan, T.
Droubay, A.S.
Lea, C.M.
Wang, V.
Shutthanandan, S.A. Chambers, R.P. Sears, B. Taylor B. Sinkovic, Appl. Phys. Lett. 84 (2004) 3531. [20] W. Kohn, L.J Sham, Phys. Rev. A 140 (1965) 1133. [21] M.C. Payne, M.P. Teter, D.C. Allen, T.A. Arias, J.D. Joannopoulos, Rev. Mod. Phys. 64 (1992) 1045. [22] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [23] R. Long, N.J. English, Chem. Mater. 22 (2010) 1616. [24] R. Long, N.J. English, Chem. Phys. Lett. 478 (2009) 175. [25] L.P. Wen, B.S. Liu, X.J. Zhao, K. Nakata, T. Murakami, A. Fujishima, Int. J. Photoenergy 2012 (2012) 368750. [26] K.S. Yang, Y. Dai, B.B. Huang, J. Phys. Chem. C 111 (2007) 12086. 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
[27] H.I. Zhang, J. Callaway, Phys. Rev. 181 (1969) 1163. [28] S. Saha, T.P. Sinha, A. Mookerjee, Phys. Rev. B 62 (2000) 8828. [29] M. Batzill, E.H. Morales, U. Diebold, Phys. Rev. Lett. 96 (2006) 026103. [30] V.N. Krasil’nikov, V.P. Zhukov, L.A. Perelyaeva, I.V. Baklanova, I.R. Shein, Phys. Solid State 55 (2013) 1903. [31] Z. Ambrus, N. Balazs, T. Alapi, G. Wittmann, P. Sipos, A. Dombi, K. Mogyorosi, Appl. Catal. B 81 (2008) 27. [32] C.F. Yu, T.J. Lin, S.J. Sun, H. Chou, J. Phys. D: Appl. Phys. 40 (2007) 6497. [33] L. Shen, R.Q. Wu, H. Pan, G.W. Peng, M. Yang, Z.D. Sha, Y.P. Feng, Phys. Rev. B 78 (2008) 073306. [34] I.S. Elfimov, A. Rusydi, S.I. Csiszar, Z. Hu, H.H. Hsieh, H.J. Lin, C.T. Chen, R. Liang, G.A. Sawatzky, Phys. Rev. Lett. 98 (2007) 137202. [35] B. Gu, N. Bulut, T. Ziman, S. Maekawa, Phys. Rev. B 79 (2009) 024407. [36] M.M. Cruz, R.C. da Silva, N. Franco, M. Godinho, J. Phys.: Condens. Matter 21 (2009) 206002. [37] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Science 291 (2001) 854. [38] N.H. Hong, J. Sakai, W. Prellier, A. Hassini, A. Ruyter, F. Gervais, Phys. Rev. B 70 (2004) 195204. [39] X.S. Du, Q.X. Li, H.B. Su, J.L. Yang, Phys. Rev. B 74 (2006) 233201. [40] J. Sun, H.T. Wang, J.L. He, Y.J. Tian, Phys. Rev. B 71 (2005) 125132.
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Figure captions
Figure 1. (a) 25151 TiO2 supercell for N-doped configuration; (b) 25251 TiO2 supercell for Fe-doped and N, Fe-codoped configurations. Red and gray balls represent O and Ti atoms, respectively. The atoms involved in being substituted are marked by the element names.
Figure 2. Calculated TDOS and PDOS of pure, N-doping, Fe-doping and N, Fe-codoping anatase TiO2 are shown at the left and right side, respectively. Vertical dash line indicates Fermi level. Curves above and below the horizontal axis refer to the majority-spin and minority-spin DOS, respectively.
Figure 3. (a) the absorption coefficient spectrum (α(105cm-1)); (b) the imaginary part of the dielectric function ( (ω)) for pure, N-doping, Fe-doping and N, Fe-codoping anatase TiO2. The inset in figure (a) denotes the enlarged optical absorption spectrum with the photon energy ranging from 0 to 4 eV (the range of visible-light energy).
Figure 4. (a) reflectivity (R); (b) refractivity index (n); (c) energy-loss functions (L) of anatase TiO2 systems with four different structures. The inset in energy-loss functions is the enlarged figure for the four peaks of the energy-loss functions as to the four different TiO2 models. 13
Table 1 The calculated total energies and stable magnetic ground states for various configurations of Ti8N2O14, Ti14Fe2O32 and Ti15FeNO31. EFM and EAFM represent the relative energy in the FM and AFM states (the reference energy of each configuration marked with asterisk at the upper right). ΔE is the value of EFM minus EAFM for each configuration. In the column of the Ground state, FM/AFM state is the final determinate magnetic state for corresponding doped system.
Ti8N2O14
Ti14Fe2O32
Ti15FeNO31
Configuration
EFM(eV)
EAFM(eV)
ΔE(eV)
ⅰ
0*
7.5443
-7.5443
ⅱ
-0.7738
6.9147
-7.6885
ⅲ
-1.0405
6.7940
-7.8345
ⅳ
0*
0.2937
-0.2937
ⅴ
-3.7117
-3.5751
-0.1366
ⅵ
1.0677
0.5884
0.4793
ⅶ
0*
-0.6616
0.6616
ⅷ
0.1105
0.2943
-0.1838
ⅸ
0.6317
0.6305
0.0012
Ground state
FM
FM
AFM
Figure 1
Figure 2
Figure 3
Figure 4
Highlights
N-doped and Fe-doped anatase TiO2 exhibit ferromagnetic semiconductors.
N, Fe-codoped anatase TiO2 manifests antiferromagnetic metallic nature.
The optical absorption spectrum extends to the visible-light region.
The ultraviolet absorption is enhanced for all doped models.