First-principles investigation of structural and electronic properties of pure bismuth tungstate

71 HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2018-0072 Natural Sciences 2018, Volume 63, Issue 11, pp. 71-76 This paper is available online at FIRST-PRINCIPLES INVESTIGATION OF STRUCTURAL AND ELECTRONIC PROPERTIES OF PURE BISMUTH TUNGSTATE Tran Phan Thuy Linh 1 , Pham Van Hai 1 , Nguyen Dang Phu 1 , Duong Quoc Van 1 , Nguyen Thi Thao 1 and Tran Thien Lan 2 1 Faculty of Physics, Hanoi National University of Education 2 Vietnam-Japan University Abstract. We investigated the crystal structure and electronic properties of Bismuth tungstate Bi2WO6 by first-principles calculation based on the Density Functional Theory. The calculated electronic density of states confirmed that Bi2WO6 possess the semiconducting properties with the band gap of 2.198 eV. The contribution of each atomic orbital to electronic densities of states was evaluated from the partial densities of states. By elucidating the curvature of valence and conduction band along G-Z, we calculated the effective mass of photogenerated holes and electrons of 1.393me and 0.774me, respectively. Keywords: First-principles calculation, Bismuth tungstate, photocatalyst, band gap, effective mass. 1. Introduction Together with the rapidly development in economics, there raises the severe problem in environmental pollution, especially water pollution. Therefore, a number of scientific researchers have been established to solve the problem. Photocatalytic semiconductor materials have been widely studied due to their remarkable properties for pollution remediation and hydrogen production from water splitting using solar energy [1-6]. Presently, TiO2-based photocatalysts is mostly studied and efficient photocatalyst due to their high reactivity, good chemical stability, environmental friendly, and low cost [6-10]. However, the disadvantages of TiO2 is intrinsic band gap (rutile 3.05eV, anatase 3.26eV), TiO2 is able to be excited only the ultraviolet or near- ultraviolet radiation which occupies only about 3% to 4% of the solar spectrum which limits his application into practice [11]. Therefore, it is essential to develop novel visible-light-induced photocatalyst with high efficiency under normal solar light condition. Bismuth tungstate Bi2WO6 has been recently found to possess excellent photocatalytic performance under visible-light irradiation when used to decompose water dyes and indoor pollutants [12]. Bi2WO6, the simplest and a typical Aurivillius oxide family from Bi2An-1BnO3n+3 (A=Ca, Sr, Ba, Pb, Bi, Na, K and B=Ti, Nb, Ta, Mo, W, Fe) (when n=1) with layered structure, has excellent intrinsic physical and chemical properties [13-15]. Hence, presently, Bi2WO6-based photocatalysts have been widely studied [16-28]. And thus, it is significant to understand the precise crystal structure and electronic properties of pure Bi2WO6. Received November 5, 2018. Revised November 16, 2018. Accepted November 23, 2018. Contact Tran Phan Thuy Linh, e-mail address: linhtpt@hnue.edu.vn Tran Phan Thuy Linh, Pham Van Hai, Nguyen Dang Phu, Duong Quoc Van, Nguyen Thi Thao and Tran Thien Lan 72 In this paper, we elucidate the optimized crystal structure of pure Bi2WO6. Then, we analyze the electronics properties by considering the total density of states (TDOS) and band structure. The partial densities of states (PDOS) are figured out to determine the contribution of each atom to the TDOS. Finally, from the curvature of the conduction band minimum (CBM) and valence band maximum (VBM), the effective masses of photogenerated carriers are calculated. Besides, throughout this work, we use Visualization for Electronic and Structure Analysis (VESTA) to view the structure of our system. 2. Content 2.1. Computational method Our study performs the Density Functional Theory (DFT) [29, 30] + U calculations that is implemented in software package CASTEP [31]. The plane-wave basis set is employed for the valence electron wave function with cut-off energy of 580 eV. Reference configurations of valence electrons were 6s 2 6p 3 for Bi, 5d 4 6s 2 for W and 2s 2 2p 4 for O. For the exchange- correlation energy, the generalized gradient approximation (GGA) was employed within the Perdew-Burke-Ernzerhof (PBE) functional [32, 33]. The supercell 2 × 1 × 1 was constructed by repetition of the unit cell of Bi2WO6. This supercell was composed of 72 atoms: 16 Bi atoms, 8 W atoms and 48 O atoms. The Brillouin zone was sampled using 2 × 4 × 7. Monkhorst-Pack k-point grids [34] which showed total energy convergence within 1 meV per atom. Structural relaxation was terminated when the maximum Hellman–Feynman forces acting on each atom in the unit cell dropped to 0.001 eV/Å. To obtain accurate total energies and electronic densities of states, static calculations were performed after geometry relaxations with the denser k points sampling of 9 × 9 × 9. A Hubbard U parameter, which is applied to the 5d states of the W atoms to increase their localization, is introduced to improve accuracy in the standard DFT calculation. The value for U is set to 6.2 eV [35], which is a relevant value to describe strongly correlated electrons of the W atoms. 2.2. Results and discussions 2.2.1. Structure parameters The optimized crystal structure of pure 2 × 1 × 1 supercell of Bi2WO6 is shown in Figure 1. It is shown that Bi2WO6 belongs to the orthorhombic system, space group Pca21, that is in agreement with other theoretical and experimental results [35]. According to the space symmetry, atomic positions in Bi2WO6 can be grouped in Bi(1), Bi(2), W, O(1), O(2), O(3), O(4), O(5) and O(6) each of which consists of four equivalent positions (Figure 1). The structure consists of two types of layers: the WO6 octahedral layers and the Bi–O–Bi layers. The WO6 octahedral are connected to each other by corner-sharing O atom. It is found that this corner-sharing structure contributed to the visible-light response and the photocatalytic performance because it makes excitation energy and photogenerated electrons and hole pairs begin to migrate easily [36, 37]. From Table 1, our DFT calculation indicates the equilibrium lattice parameters for pure Bi2WO6 are similar to other studies [38] with the differences about 0.08% to 1%, within the standard error of DFT calculations (a few percent). First-principles investigation of structural and electronic properties of pure bismuth tungstate 73 Figure 1. Crystal structure of supercell Bi2WO6 viewed along c axis Gray octahedra indicate WO6 substructures. A black solid box presents a boundary of the super- cell 2 × 1 × 1. Red, green and blue arrows indicate a, b and c axes, respectively Table 1 Structure parameters of optimized 2 × 1 × 1 supercell Bi2WO6 lattice constant/Å angle/deg. a b c  β γ This work Other a Exp. b 10.877 10.963 10.868 16.480 16.556 16.425 5.441 5.497 5.456 89.999 90.000 90.000 90.000 90.000 90.000 90.000 90.000 90.000 a Ref. [35], b Ref. [12] 2.2.2. Electronic properties We next analyze the electronic properties of this material. The photocatalytic properties strongly depend on the electronic structures of semiconductors. To increase the efficiency of the photocatalytic reaction in the region of the visible light, a reasonable band gap of the system is required for the maximum conversion of visible light [39]. The TDOS and band structure of pure Bi2WO6 are presented in Figure 2. The Fermi energy is set at an energy of 0 eV, corresponding to the valence-band top. Since it is found that the spin-polarized calculations always converge to nonmagnetic solutions, only the DOS of spin-up electrons are shown. The pure Bi2WO6 performs electronically semiconducting properties with a band gap of 2.198 eV, which is in consistent with previous DFT calculations [38]. This is quite acceptable for photocatalytic materials to be active in visible light spectrum. However, for a better performance of this photocatalyst, the band gap may be reduced by some method such as doping, which will be our next study. In order to investigate the contribution of each atom, we analyze the partial density of states (PDOS) projected onto valence orbitals at each atomic sites and summed over all sites for the same element (Figure 3). In the portion from -10 to -8 eV (Fermi energy), there are major contributions from orbitals d of Bi, minor contributions from p orbitals of O, and much smaller contributions from the d orbitals of W. The upper energy band, from -8 to 0 eV, is dominated by the contribution from the p orbital of O, followed by contribution from d orbital of W and p orbital of Bi. Both valence and conduction bands are comprised by the hybridization of Bi 6p, O 2p and W 5d orbitals. The strong hybridization of O 2p and W 5d is observed and may be attributed to the W-O covalent bonds. The spreading of the states from -8 to 0 eV orbital p of Bi may facility for generated electron to the CBM. Tran Phan Thuy Linh, Pham Van Hai, Nguyen Dang Phu, Duong Quoc Van, Nguyen Thi Thao and Tran Thien Lan 74 Figure 2. The band structure (a) and TDOS (b) of optimized pure Bi2WO6 Figure 3. Partial densities of states of Bi2WO6 projected onto valance orbitals of each atomic species and summed over the magnetic quantum numbers and all sites for the same element First-principles investigation of structural and electronic properties of pure bismuth tungstate 75 2.2.3 Effective mass The mobility of photogenerated electrons and holes play an important role in the separating process and can be characterized by their effective mass that is defined by [40]: 2 * 2 2/ m d E dk   (1) where m * is the effective mass and E is the energy of wave vector k, ħ is the reduced Plank’s constant. The positive and negative signs correspond to electrons and holes, respectively. From eqn. (1), it can be seen that the effective mass is inversely proportional to the curvature of the band, i.e., the stronger dispersive band is, the smaller effective mass is attained and vice versa. The carrier with small effective mass possess a high mobility which assists to increase the probability of photogenerated electrons or holes transferring to the surface, and hence will increase the efficiency of a photocatalyst. The obtained effective masses of VBM and CBM along G-Z are 1.393me and 0.774me, respectively. This indicates that the photogenerated holes have quite high effective mass. And that for photogenerated electrons is small, but still heavy to mobile easily. Consequently, it is significant to lower the effective mass by, let say, making some defect in pure Bi2WO6. 3. Conclusions In summary, our DFT calculations indicate that Bi2WO6 exhibits the electronically semiconducting properties. The band-gap nature is direct. The valence bands are mainly composed of Bi 6p, O 2p and W 5d orbitals. The W-O covalency is observable due to the strong hybridization of O 2p and W 5d orbitals. 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