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Tailoring the photocatalytic activity of WO3 by Nb-F codoping from first-principles calculations
Chinese Journal of Physics xxx (xxxx) xxx–xxx
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Chinese Journal of Physics journal homepage: www.elsevier.com/locate/cjph
Tailoring the photocatalytic activity of WO3 by Nb-F codoping from first-principles calculations ⁎
Xu Yinga, , Zhou Yinga, Nie Guo Zhenga, Zou Daifenga, Ao Zhi Minb a
School of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan 411201, China Institute of Environmental Health and Pollution Control, School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, China b
ABS TRA CT
In this letter, the electronic structure properties of Nb, F monodoping and Nb-F codoping are explored by first-principles calculations. Our results show that Nb-F codoping can reduce the band gap notably. The band edge analysis indicates that both conduction band maximum (CBM) and valence band minimum (VBM) move to higher energies, which is desirable for water splitting. The formation energy and pair binding energy calculation shows that this anion-cation codoping is easy to realize in both O-rich and O-poor conditions. The calculated optical absorption spectra indicate that the visible light absorption can be significantly improved by Nb-F codoping in WO3. Therefore, Nb-F co-doped WO3 is predicted to be a promising visible light photocatalyst for water splitting.
1. Introduction Since Fujishima and Honda reported water photocatalysis on a TiO2 electrode in 1972 [1], TiO2 has been extensively studied as a promising material for photocatalytic and photochemical applications. However, its band gap, about 3.2 eV [2], is too wide, only ultraviolet light can be absorbed and only about 3% − 5% solar energy are utilized. The low photocatalytic efficiency remains a main challenge hindering the practical applications of TiO2. Tungsten trioxide (WO3) is another promising oxide for use in photoelectrochemical water-splitting systems due to its photosensitivity [3–5], stability against photocorrosion [6,7], and good electron transport properties [8]. Meanwhile, its smaller band gap, about 2.8 eV, smaller than that of other semiconductors (e.g., TiO2) makes it appropriate for absorption of visible solar light. However, the gap of WO3 is still too wide to realize a sufficient absorption of the solar spectrum. For efficient photocatalysis, as we known, the band edge positions must match the water-splitting potential. However, the conduction band minimum (CBM) of bulk WO3 is still too low for hydrogen production, about 0.4 eV below the hydrogen redox potential [9,10].To overcome this limitation, several attempts in experiments have been tried with doping different metal atoms into WO3, such as Mg, Mo, Ti, Ta, etc [11–17]. In contrast with TiO2, works on band structure tuning by DFT prediction on WO3 are relatively less. Huda et al reported N, N2 doping, metallic atoms (Mo, Ta, Hf) doping and Re-N and Ta-F codoping [18]. Interstitial N2 was found to be relatively more favorable. For codoping cases, Ta-F or Re-N is easily formed but it did not shift the conduction band as required. Wang et al. studied the effect of different metallic atoms doping and codoping [19]. They find Hf + O vacancy results in a shift of both VBM and CBM to higher energies and a reduction of the band gap. Therefore, doping and codoping are possible ways to tailor the electronic structure as well as placing the band edges in the desired position. However, doping with donors or acceptors can also bring partially occupied impurity states acting as recombination centers [20]. This problem can be overcome by passivated co-doping with donors and acceptors [21]. In this work, we choose Nb-F codopants to improve the photoactivity of WO3. Nb-F co-doping is isoelectronic with the undoped WO3. Thus, defect formation due to charge
⁎
Corresponding author. E-mail address: [email protected] (Y. Xu).
https://doi.org/10.1016/j.cjph.2018.07.003 Received 1 June 2018; Received in revised form 23 June 2018; Accepted 6 July 2018 0577-9073/ © 2018 The Physical Society of the Republic of China (Taiwan). Published by Elsevier B.V. All rights reserved.
Please cite this article as: Xu, y., Chinese Journal of Physics (2018), https://doi.org/10.1016/j.cjph.2018.07.003
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Fig. 1. The structure of WO3. The red atoms are oxygen and blue are tungsten. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
mismatch can be avoided. The Nb was selected because of its higher 4d orbital energy than W-5d orbital energy and not localized d states within the band gap. The other interest of choosing F and Nb is that the ionic radius for F-(1.33 Å) and Nb5+(0.64 Å) are close to that of O2-(1.40 Å), W6+(0.60 Å), Nb and F can be expected to be incorporated into WO3 lattice easily. 2. Computation method The first-principles calculations were carried out using Vienna ab initio simulation package (VASP) based on density functional theory (DFT) [22,23]. The local density approximation (LDA) with the projected augmented wave (PAW) method as were used [24,25]. For the k-point sampling of the Brillouin-zone integrals in the total energy calculations, the Monkhorst-Pack scheme and Gaussian broadening of 0.05 eV were used [26]. A 2 × 2 × 2 k-point grid for structure optimization and 8 × 8 × 8 grid for density of states plot were used. All plane waves with a cutoff energy of 450 eV were used in the basis function. The valence electrons configurations for F, O, W, Nb atoms were 2s22p5, 2s22p4, 5p66s25d4, 4d45s1, respectively. Monoclinic structure WO3 with space group P21/n is the most common and stable phase of tungsten oxide. The unit cell of WO3 consists of 8 W atoms and 24 O atoms. Eight oxygen atoms form corner-sharing octahedrons in a slightly distorted cubic arrangement as shown in Fig. 1. The experimental lattice constants are 7.306 Å, 7.540 Å, 7.692 Å and β = 90.9° [27]. The relaxed lattice constants are a = 7.381 Å, b = 7.472 Å, c = 7.633 Å and β = 90.42°, which are well consistent with the LDA calculation result in ref [18]. After the optimization of geometry of undoped WO3, the lattice constants were kept fixed throughout the following calculations. The atom 2
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Fig. 2. Partial density of states (PDOS) of pure WO3 (a), F-doped WO3 (b), Nb-doped WO3 and Nb-F codoped WO3. The VBM is set at 0 eV.
positions were relaxed until the force is less than 0.01 eV/Å.
3. Results and discussion We start from undoped WO3. The calculated partial density of states (PDOS) and band structure of pure WO3 are shown in Figs. 1a and 2a, which are consistent with previous calculation results [18]. The valence band of WO3 mainly consists of O-2p bands, while its conduction band has predominantly W-5d character. The calculated direct band gap was 1.31 eV, which is much smaller than the experimental band gap of 2.7 eV. This underestimation of band gap comes from the LDA used in our calculation. Because we are mostly concerned with the relative energy changes after doping, this computational underestimation will not significantly affect our results. To find the effect of individual dopant elements, we studied F-doped WO3 and Nb doped WO3, respectively. The optimized structures for these systems show that the substitution of either F or Nb dose not reduces significant structural distortion to WO3 lattice. This is due to the close ionic radius for O2-(1.40 Å), F-(1.33 Å), Nb5+(0.64 Å), W6+(0.60 Å). The relaxed Nb-O bond length and F-W bond length are slightly larger than that of W-O bond length. The PDOS and band structure are shown in Figs. 2b and 3b. The top of valance band for F-doped WO3 are composed mainly by O2p states, since F-2p states have lower energy than O-2p states. F-doped WO3 behaves as n-type semiconductor as F contains one more electron than that of O atom. There are partially occupied states at the Fermi level. The F-2p states are hybridized with neighboring W-5d states and very small part of hybridized with neighboring O-2p states. There are no localized states in N-doped WO3, which indicates that F-doping produces delocalized states. The band structure still has indirect character with VBM at B and CMB at Γ. The band gap is narrowed to be 1.09 eV, which is in favor of visible light absorption. To obtain efficient photoactivity, the VBM and CBM must be located in suitable positions with 3
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Fig. 3. Band structure of pure WO3 (a), F-doped WO3 (b), Nb-doped WO3 (c) and Nb-F codoped WO3 (d).
respect to the water redox level, hence we calculate the band edge positions respect to the undoped sysytem. The positions of CBM and VBM are determined according to the shifts of the CBM and the VBM with reference to the pristine system. The band-edge shifts are calculated with respect to the O-s core level far away from the impurity N atom. For F-doped WO3, the band edges of VBM shifts up slightly and CBM shifts down slightly, resulting the narrowing of band gap. Although the band gap of F-doped WO3 is reduced, the CBM shifts to lower energy, which is detrimental for hydrogen production. Therefore, the excess electron environment is created due to the presence of one more electron in F than that O. This leads to occupied impurity states near the CBM, which results CBM is lowered with respect of that of non-doped WO3. To improve the photocatalytic activity of F-doped WO3, another dopant element is required, which can compensate the electron deficiency. For Nb-doped WO3, the PDOS and band structure are shown in Figs. 2c and 3c. Since Nb has one less electron compared with W, the system shows p-type character and the Fermi level runs through the valence band. The bottom of conduction band is mainly made up by W-5d and the top of valence band is mainly of O-2p states, similar to that of undoped WO3. Nb-4d states have higher d-orbital than W-5d states. Nb atom has one less electron and slightly larger ionic radius compared with W atom. The band structure is also indirect. Both CBM and VBM shift to higher energy, and the band gap is narrowed to be 1.15 eV. However, the empty states at valence band are undesirable for photocatalytic activity as they can trap the charge carriers and enhance the electron − hole recombination process. For Nb-F co-doped WO3, the PDOS and band structure are shown in Figs. 2d and 3d. The excess electrons with F doping saturate the holes produced by Nb doping. Therefore there are no partial occupied states at the Fermi level. Nb-4d states appear at both valence band and conduction band. There are obviously hybridization between Nb-4d and F-2p states. The valence band is contributed by O-2p states, and the CBM is contributed by W-5d states. The system has an indirect band gap of 1.06 eV with VBM at B and CBM at Г. Compared with pure WO3, the band gap is reduced by 20%. The band edge analysis shows that the VBM of the codoped system rises up by 0.3 eV, and the CBM rises up by 0.17 eV. As a result, the band gap is narrowed by 0.13 eV, which will improve the absorption of visible region. The decrease in the band gaps can be explained mainly by the electronegativity difference between the Nb and W atoms or the O and F atoms. The valence bands of the co-doped structures consists mainly p-orbitals of O or F. The conduction bands consists mainly d-orbitals of W or Nb. As we known, the electronegativity of F is stronger than that of O, hence the energy level of F is higher than that of O. The electronegativity of Nb is stronger than that of W, and the energy level of Nb is higher than that of W. Thus the band gaps of WO3 decrease as Nb-F co-doping. 4
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Table 1 Calculated formation energy of the F, Nb and F-Nb dopants under O-rich or O-poor condition. Dopants
Ef in eV(O-rich )
Ef in eV(O-poor)
F Nb F-Nb
−0.27 −6.07 −8.34
−4.04 1.55 −4.48
Therefore, the combined effect of the Nb-F co-doping thus reduces the band gap of WO3 but also shifts CBM to higher energies, which are beneficial to obtain a good photocatalyst for water splitting. To find the optimal growth conditions in experiments, we calculate the formation energies of Nb substituting at W site (NbW), F substituting at O site (FO) and F-Nb codoping at W and O site (NbWFO). The formation energy is defined as:
EXf = EX = Et [WO3] + ni μi
(1)
where EXf is the total energy obtained from a supercell calculation with impurity X(X = Nb, F or Nb-F). Et[WO3] is the total energy of the perfect crystal. ni denotes the number of the atoms of type i and μi is the chemical potential of the corresponding atom. The chemical potentials for the atoms are related with the experimental growth conditions. For oxygen, we assume the bounds imposed by the formation of WO3. The formation energy of WO3 is given by the thermal equation, μW + 3μO = μWO3 . In our calculations, the upper limit of μO in oxygen rich condition is determined by the binding energy of O2 molecule, while μWis simulated by 1 μW = μWO3 − 3μO . In the oxygen poor condition, μW is determined from bulk W and μO is calculated from μO = 3 (μWO3 − μW ) . μF is bulk under oxygen poor determined from molecule F2 and μNb is determined by μ Nb = μ NbO − μO under O rich condition and μNb = μNb condition. The calculated formation energies for these doped systems are summarized in Table 1. The negative value of the formation energy indicates F substituting O site is easy in both O-poor and O-rich conditions. The formation energy of Nb substituting W site is − 6.07 eV, indicating that substitution of Nb at W site is easy under O-rich condition. The formation energy of F-Nb co-doping is much less than those of Nb and F monodoping. The predicted lower formation energy of Nb-F codoping suggests that this kind of cation-anion codoping could be easily realized. To compare the relative stability of doped systems, we further calculated the defect binding energies (Eb) for co-doped systems using the following equation:
Eb = E (D) + E (A) − E (DA) − nE (WO3)
(2)
where E(D), E(A), and E(DA) are the total energies of a supercell with donor D, acceptor A, and donor–acceptor DA, respectively. E (WO3) is the total energy of the primitive cell, and n is the number of primitive cells in the supercell containing dopants. The calculated binding energies of the near co-doped configurations are all more than 1.99 eV. This relatively large positive value means that the co-doped configurations are more favorable as compared with individual doping. This relative large defect pair binding energy originates from the charge transfer from donor to acceptor. To better understand its photovoltaic properties, the optical properties are also calculated. The optical properties are determined by the frequency dependent complex dielectric function ɛ(ω) = ɛ1 (ω) + iɛ2 (ω) . The absorption spectra were calculated according to the equation: α (ω) = 2[ ɛ12 (ω) + ɛ 22 (ω) − ɛ1 (ω)] , where α(ω) is the absorption coefficient. Fig. 4 shows the absorption coefficient of visible light for Nb-F codoped WO3 has a significant redshift of the absorption edge by 113 nm compared with that of undoped WO3.
Fig. 4. The optical absorption spectra for the undoped and (Nb-F)-codoped WO3. 5
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And the absorption of visible light is enhanced greatly. These conclusions are consistent with the results of band gap calculations that Nb-F codoping can reduce the band gap of WO3, which benefits photovoltaic conversion efficiency. 4. Conclusion In this letter, we explore the electronic structure properties of Nb, F monodoping and Nb-F codoping by First-principles calculations. Our results show that Nb-F codoping can decrease the band gap significantly. The band edge analysis indicates that the both CBM and VBM move to higher energies, which is desirable for water splitting. The decrease in the band gaps can be explained mainly by the electronegativity difference between the Nb and W atoms or the O and F atoms. The formation energy and pair binding energy calculation shows that this anion-cation codoping is easy to realize in both O-rich and O-poor condition. The calculated optical absorption spectra indicate that the visible light absorption can be significantly improved by Nb-F codoping in WO3. Our results predict that Nb-F co-doped WO3 is a promising visible light photocatalyst for water spitting. Acknowledgements We acknowledge National Supercomputing Center in Shenzhen for providing the computational resources and VASP5.4. This work was supported by Research Foundation of Education Bureau of Hunan Province, China (Grant No. 15B083, 17B090), the Natural Science Foundation of Hunan Province, China (Grant No. 2016JJ2059). ZA acknowledges the financial supports from “100 talents” program of Guangdong University of Technology, “1000 plan” for young professionals program of Chinese Government. Reference [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]
A. Fujishima, K. Honda, Nature 37 (1972) 238. A. Amtout, R. Leonelli, Phys. Rev. B 51 (1995) 6842. S.K. Deb, Sol. Energy. Mater. Sol. Cells 92 (2008) 245. S.K. Deb, Appl. Opt. 3 (1969) 192. S.K. Deb, Philos. Mater 27 (1973) 801. M.A. Butler, R.D. Nasby, R.K. Quinn, Solid State Commun. 19 (1976) 1011. D.E. Scaife, Sol. Energy 25 (1980) 41. J.M. Berek, J.J. Sienko, Solid State Chem. 2 (1970) 109. A.J. Nozik, Annu. Rev. Phys. Chem. 29 (1978) 189. G.R. Bamwenda, H. Arakawa, Appl. Catal. A 210 (2001) 181. D.W. Hwang, J. Kim, T.J. Park, J.S. Lee, Catal. Lett. 80 (2002) 53. A Hameed, MA Gondal, ZH Yamani, Catal. Commun. 5 (2004) 715. M Radecka, P Sobas, M Wierzbicka, M Rekas, Physica B 364 (2005) 85. X.F. Cheng, W.H. Leng, D.P. Liu, J.Q. Zhang, C.N. Cao, Chemosphere 68 (2007) 1976–1984. H. Liu, T. Peng, D. Ke, Z. Peng, C. Yan, Mater. Chem. Phys 104 (2007) 377. B Yang, V Luca, Chem. Commun. (2008) 4454–4456. A Enesca, A Duta, Schoonman, J. Phys. Status Solidi A 205 (2008) 2038. MN Huda, Y Yan, CY Moon, SH Wei, MM Al-Jassim, Phys. Rev. B 77 (2008) 195102. G Wang F, D Valentin C, G Pacchioni, J. Phys. Chem. C 116 (2012) 8901. Y. Gai, J. Li, S.S. Li, J.B. Xia, S.H. Wei, Design of narrow-gap TiO2: a passivated codoping approach for enhanced photoelectrochemical activity, Phys. Rev. Lett. 102 (2009) 036402. M.N. Huda, Y. Yan, S.H. Wei, M.M. Al-Jassim, Electronic structure of ZnO:GaN compounds: asymmetric bandgap engineering, Phys. Rev. B 78 (2008) 195204. G Kresse, J Hafner, Phys. Rev. B 47 (1993) 558. G Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169. J.P. Perdew, A. Zunger, Phys. Rev. B 23 (1981) 5048. P.E. Blöchl, Phys. Rev. B 50 (1994) 17953. H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. B.O. Loopstra, H.M. Rietveld, Acta Crystallogr. Sect. B B25 (1969) 1420.
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Chinese Journal of Physics journal homepage: www.elsevier.com/locate/cjph
Tailoring the photocatalytic activity of WO3 by Nb-F codoping from first-principles calculations ⁎
Xu Yinga, , Zhou Yinga, Nie Guo Zhenga, Zou Daifenga, Ao Zhi Minb a
School of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan 411201, China Institute of Environmental Health and Pollution Control, School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, China b
ABS TRA CT
In this letter, the electronic structure properties of Nb, F monodoping and Nb-F codoping are explored by first-principles calculations. Our results show that Nb-F codoping can reduce the band gap notably. The band edge analysis indicates that both conduction band maximum (CBM) and valence band minimum (VBM) move to higher energies, which is desirable for water splitting. The formation energy and pair binding energy calculation shows that this anion-cation codoping is easy to realize in both O-rich and O-poor conditions. The calculated optical absorption spectra indicate that the visible light absorption can be significantly improved by Nb-F codoping in WO3. Therefore, Nb-F co-doped WO3 is predicted to be a promising visible light photocatalyst for water splitting.
1. Introduction Since Fujishima and Honda reported water photocatalysis on a TiO2 electrode in 1972 [1], TiO2 has been extensively studied as a promising material for photocatalytic and photochemical applications. However, its band gap, about 3.2 eV [2], is too wide, only ultraviolet light can be absorbed and only about 3% − 5% solar energy are utilized. The low photocatalytic efficiency remains a main challenge hindering the practical applications of TiO2. Tungsten trioxide (WO3) is another promising oxide for use in photoelectrochemical water-splitting systems due to its photosensitivity [3–5], stability against photocorrosion [6,7], and good electron transport properties [8]. Meanwhile, its smaller band gap, about 2.8 eV, smaller than that of other semiconductors (e.g., TiO2) makes it appropriate for absorption of visible solar light. However, the gap of WO3 is still too wide to realize a sufficient absorption of the solar spectrum. For efficient photocatalysis, as we known, the band edge positions must match the water-splitting potential. However, the conduction band minimum (CBM) of bulk WO3 is still too low for hydrogen production, about 0.4 eV below the hydrogen redox potential [9,10].To overcome this limitation, several attempts in experiments have been tried with doping different metal atoms into WO3, such as Mg, Mo, Ti, Ta, etc [11–17]. In contrast with TiO2, works on band structure tuning by DFT prediction on WO3 are relatively less. Huda et al reported N, N2 doping, metallic atoms (Mo, Ta, Hf) doping and Re-N and Ta-F codoping [18]. Interstitial N2 was found to be relatively more favorable. For codoping cases, Ta-F or Re-N is easily formed but it did not shift the conduction band as required. Wang et al. studied the effect of different metallic atoms doping and codoping [19]. They find Hf + O vacancy results in a shift of both VBM and CBM to higher energies and a reduction of the band gap. Therefore, doping and codoping are possible ways to tailor the electronic structure as well as placing the band edges in the desired position. However, doping with donors or acceptors can also bring partially occupied impurity states acting as recombination centers [20]. This problem can be overcome by passivated co-doping with donors and acceptors [21]. In this work, we choose Nb-F codopants to improve the photoactivity of WO3. Nb-F co-doping is isoelectronic with the undoped WO3. Thus, defect formation due to charge
⁎
Corresponding author. E-mail address: [email protected] (Y. Xu).
https://doi.org/10.1016/j.cjph.2018.07.003 Received 1 June 2018; Received in revised form 23 June 2018; Accepted 6 July 2018 0577-9073/ © 2018 The Physical Society of the Republic of China (Taiwan). Published by Elsevier B.V. All rights reserved.
Please cite this article as: Xu, y., Chinese Journal of Physics (2018), https://doi.org/10.1016/j.cjph.2018.07.003
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Fig. 1. The structure of WO3. The red atoms are oxygen and blue are tungsten. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
mismatch can be avoided. The Nb was selected because of its higher 4d orbital energy than W-5d orbital energy and not localized d states within the band gap. The other interest of choosing F and Nb is that the ionic radius for F-(1.33 Å) and Nb5+(0.64 Å) are close to that of O2-(1.40 Å), W6+(0.60 Å), Nb and F can be expected to be incorporated into WO3 lattice easily. 2. Computation method The first-principles calculations were carried out using Vienna ab initio simulation package (VASP) based on density functional theory (DFT) [22,23]. The local density approximation (LDA) with the projected augmented wave (PAW) method as were used [24,25]. For the k-point sampling of the Brillouin-zone integrals in the total energy calculations, the Monkhorst-Pack scheme and Gaussian broadening of 0.05 eV were used [26]. A 2 × 2 × 2 k-point grid for structure optimization and 8 × 8 × 8 grid for density of states plot were used. All plane waves with a cutoff energy of 450 eV were used in the basis function. The valence electrons configurations for F, O, W, Nb atoms were 2s22p5, 2s22p4, 5p66s25d4, 4d45s1, respectively. Monoclinic structure WO3 with space group P21/n is the most common and stable phase of tungsten oxide. The unit cell of WO3 consists of 8 W atoms and 24 O atoms. Eight oxygen atoms form corner-sharing octahedrons in a slightly distorted cubic arrangement as shown in Fig. 1. The experimental lattice constants are 7.306 Å, 7.540 Å, 7.692 Å and β = 90.9° [27]. The relaxed lattice constants are a = 7.381 Å, b = 7.472 Å, c = 7.633 Å and β = 90.42°, which are well consistent with the LDA calculation result in ref [18]. After the optimization of geometry of undoped WO3, the lattice constants were kept fixed throughout the following calculations. The atom 2
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Fig. 2. Partial density of states (PDOS) of pure WO3 (a), F-doped WO3 (b), Nb-doped WO3 and Nb-F codoped WO3. The VBM is set at 0 eV.
positions were relaxed until the force is less than 0.01 eV/Å.
3. Results and discussion We start from undoped WO3. The calculated partial density of states (PDOS) and band structure of pure WO3 are shown in Figs. 1a and 2a, which are consistent with previous calculation results [18]. The valence band of WO3 mainly consists of O-2p bands, while its conduction band has predominantly W-5d character. The calculated direct band gap was 1.31 eV, which is much smaller than the experimental band gap of 2.7 eV. This underestimation of band gap comes from the LDA used in our calculation. Because we are mostly concerned with the relative energy changes after doping, this computational underestimation will not significantly affect our results. To find the effect of individual dopant elements, we studied F-doped WO3 and Nb doped WO3, respectively. The optimized structures for these systems show that the substitution of either F or Nb dose not reduces significant structural distortion to WO3 lattice. This is due to the close ionic radius for O2-(1.40 Å), F-(1.33 Å), Nb5+(0.64 Å), W6+(0.60 Å). The relaxed Nb-O bond length and F-W bond length are slightly larger than that of W-O bond length. The PDOS and band structure are shown in Figs. 2b and 3b. The top of valance band for F-doped WO3 are composed mainly by O2p states, since F-2p states have lower energy than O-2p states. F-doped WO3 behaves as n-type semiconductor as F contains one more electron than that of O atom. There are partially occupied states at the Fermi level. The F-2p states are hybridized with neighboring W-5d states and very small part of hybridized with neighboring O-2p states. There are no localized states in N-doped WO3, which indicates that F-doping produces delocalized states. The band structure still has indirect character with VBM at B and CMB at Γ. The band gap is narrowed to be 1.09 eV, which is in favor of visible light absorption. To obtain efficient photoactivity, the VBM and CBM must be located in suitable positions with 3
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Fig. 3. Band structure of pure WO3 (a), F-doped WO3 (b), Nb-doped WO3 (c) and Nb-F codoped WO3 (d).
respect to the water redox level, hence we calculate the band edge positions respect to the undoped sysytem. The positions of CBM and VBM are determined according to the shifts of the CBM and the VBM with reference to the pristine system. The band-edge shifts are calculated with respect to the O-s core level far away from the impurity N atom. For F-doped WO3, the band edges of VBM shifts up slightly and CBM shifts down slightly, resulting the narrowing of band gap. Although the band gap of F-doped WO3 is reduced, the CBM shifts to lower energy, which is detrimental for hydrogen production. Therefore, the excess electron environment is created due to the presence of one more electron in F than that O. This leads to occupied impurity states near the CBM, which results CBM is lowered with respect of that of non-doped WO3. To improve the photocatalytic activity of F-doped WO3, another dopant element is required, which can compensate the electron deficiency. For Nb-doped WO3, the PDOS and band structure are shown in Figs. 2c and 3c. Since Nb has one less electron compared with W, the system shows p-type character and the Fermi level runs through the valence band. The bottom of conduction band is mainly made up by W-5d and the top of valence band is mainly of O-2p states, similar to that of undoped WO3. Nb-4d states have higher d-orbital than W-5d states. Nb atom has one less electron and slightly larger ionic radius compared with W atom. The band structure is also indirect. Both CBM and VBM shift to higher energy, and the band gap is narrowed to be 1.15 eV. However, the empty states at valence band are undesirable for photocatalytic activity as they can trap the charge carriers and enhance the electron − hole recombination process. For Nb-F co-doped WO3, the PDOS and band structure are shown in Figs. 2d and 3d. The excess electrons with F doping saturate the holes produced by Nb doping. Therefore there are no partial occupied states at the Fermi level. Nb-4d states appear at both valence band and conduction band. There are obviously hybridization between Nb-4d and F-2p states. The valence band is contributed by O-2p states, and the CBM is contributed by W-5d states. The system has an indirect band gap of 1.06 eV with VBM at B and CBM at Г. Compared with pure WO3, the band gap is reduced by 20%. The band edge analysis shows that the VBM of the codoped system rises up by 0.3 eV, and the CBM rises up by 0.17 eV. As a result, the band gap is narrowed by 0.13 eV, which will improve the absorption of visible region. The decrease in the band gaps can be explained mainly by the electronegativity difference between the Nb and W atoms or the O and F atoms. The valence bands of the co-doped structures consists mainly p-orbitals of O or F. The conduction bands consists mainly d-orbitals of W or Nb. As we known, the electronegativity of F is stronger than that of O, hence the energy level of F is higher than that of O. The electronegativity of Nb is stronger than that of W, and the energy level of Nb is higher than that of W. Thus the band gaps of WO3 decrease as Nb-F co-doping. 4
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Table 1 Calculated formation energy of the F, Nb and F-Nb dopants under O-rich or O-poor condition. Dopants
Ef in eV(O-rich )
Ef in eV(O-poor)
F Nb F-Nb
−0.27 −6.07 −8.34
−4.04 1.55 −4.48
Therefore, the combined effect of the Nb-F co-doping thus reduces the band gap of WO3 but also shifts CBM to higher energies, which are beneficial to obtain a good photocatalyst for water splitting. To find the optimal growth conditions in experiments, we calculate the formation energies of Nb substituting at W site (NbW), F substituting at O site (FO) and F-Nb codoping at W and O site (NbWFO). The formation energy is defined as:
EXf = EX = Et [WO3] + ni μi
(1)
where EXf is the total energy obtained from a supercell calculation with impurity X(X = Nb, F or Nb-F). Et[WO3] is the total energy of the perfect crystal. ni denotes the number of the atoms of type i and μi is the chemical potential of the corresponding atom. The chemical potentials for the atoms are related with the experimental growth conditions. For oxygen, we assume the bounds imposed by the formation of WO3. The formation energy of WO3 is given by the thermal equation, μW + 3μO = μWO3 . In our calculations, the upper limit of μO in oxygen rich condition is determined by the binding energy of O2 molecule, while μWis simulated by 1 μW = μWO3 − 3μO . In the oxygen poor condition, μW is determined from bulk W and μO is calculated from μO = 3 (μWO3 − μW ) . μF is bulk under oxygen poor determined from molecule F2 and μNb is determined by μ Nb = μ NbO − μO under O rich condition and μNb = μNb condition. The calculated formation energies for these doped systems are summarized in Table 1. The negative value of the formation energy indicates F substituting O site is easy in both O-poor and O-rich conditions. The formation energy of Nb substituting W site is − 6.07 eV, indicating that substitution of Nb at W site is easy under O-rich condition. The formation energy of F-Nb co-doping is much less than those of Nb and F monodoping. The predicted lower formation energy of Nb-F codoping suggests that this kind of cation-anion codoping could be easily realized. To compare the relative stability of doped systems, we further calculated the defect binding energies (Eb) for co-doped systems using the following equation:
Eb = E (D) + E (A) − E (DA) − nE (WO3)
(2)
where E(D), E(A), and E(DA) are the total energies of a supercell with donor D, acceptor A, and donor–acceptor DA, respectively. E (WO3) is the total energy of the primitive cell, and n is the number of primitive cells in the supercell containing dopants. The calculated binding energies of the near co-doped configurations are all more than 1.99 eV. This relatively large positive value means that the co-doped configurations are more favorable as compared with individual doping. This relative large defect pair binding energy originates from the charge transfer from donor to acceptor. To better understand its photovoltaic properties, the optical properties are also calculated. The optical properties are determined by the frequency dependent complex dielectric function ɛ(ω) = ɛ1 (ω) + iɛ2 (ω) . The absorption spectra were calculated according to the equation: α (ω) = 2[ ɛ12 (ω) + ɛ 22 (ω) − ɛ1 (ω)] , where α(ω) is the absorption coefficient. Fig. 4 shows the absorption coefficient of visible light for Nb-F codoped WO3 has a significant redshift of the absorption edge by 113 nm compared with that of undoped WO3.
Fig. 4. The optical absorption spectra for the undoped and (Nb-F)-codoped WO3. 5
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And the absorption of visible light is enhanced greatly. These conclusions are consistent with the results of band gap calculations that Nb-F codoping can reduce the band gap of WO3, which benefits photovoltaic conversion efficiency. 4. Conclusion In this letter, we explore the electronic structure properties of Nb, F monodoping and Nb-F codoping by First-principles calculations. Our results show that Nb-F codoping can decrease the band gap significantly. The band edge analysis indicates that the both CBM and VBM move to higher energies, which is desirable for water splitting. The decrease in the band gaps can be explained mainly by the electronegativity difference between the Nb and W atoms or the O and F atoms. The formation energy and pair binding energy calculation shows that this anion-cation codoping is easy to realize in both O-rich and O-poor condition. The calculated optical absorption spectra indicate that the visible light absorption can be significantly improved by Nb-F codoping in WO3. Our results predict that Nb-F co-doped WO3 is a promising visible light photocatalyst for water spitting. Acknowledgements We acknowledge National Supercomputing Center in Shenzhen for providing the computational resources and VASP5.4. This work was supported by Research Foundation of Education Bureau of Hunan Province, China (Grant No. 15B083, 17B090), the Natural Science Foundation of Hunan Province, China (Grant No. 2016JJ2059). ZA acknowledges the financial supports from “100 talents” program of Guangdong University of Technology, “1000 plan” for young professionals program of Chinese Government. Reference [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]
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