Effects of Mo/W codoping on the visible-light photocatalytic activity of monoclinic BiVO4 within the GGA + U framework

Jihua Zhangab, Mingsen Deng*ac, Fengzhu Renb, Yu Wua and Yuanxu Wang*ab
aGuizhou Provincial Key Laboratory of Computational Nano-Material Science, Guizhou Education University, 115 Gaoxin Road, Guiyang, 550018, China. E-mail: deng@gznc.edu.cn; wangyx@henu.edu.cn
bInstitute for Computational Materials Science, School of Physics and Electronics, Henan University, Kaifeng 475004, China
cGuizhou Synergetic Innovation Center of Scientific Big Data for Advanced Manufacturing Technology, Guizhou Education University, Guiyang, 550018, China

Received 29th October 2015 , Accepted 22nd January 2016

First published on 26th January 2016


Abstract

The formation energy, electronic properties, and photocatalytic activity of Mo, W mono-doped and Mo/W codoped BiVO4 were investigated using density functional theory plus U calculations (DFT + U). The calculated formation energies show that both Mo and W atoms prefer to substitute V atoms under the oxygen-rich condition, in agreement with previous experimental results. Mo or W atom doping on the V site can form continuum states above the conduction band edge of BiVO4, which is advantageous for the photochemical catalysis response. Moreover, we found that the W doped BiVO4 has a smaller band gap than the Mo doped one, and the effect of Mo and W doping on the electronic structure of BiVO4 is different. Mo/W/Mo and W/Mo/W co-doped BiVO4 have smaller formation energies and smaller band gaps than the other doping case, which may enhance the optical absorption. Thus, Mo/W/Mo and W/Mo/W co-doped BiVO4 is particularly suitable for visible-light photocatalysis.


1. Introduction

The photocatalytic activity of semiconductors is one of the key factors that limit the quantum efficiency of photocatalysis and must be significantly enhanced to accelerate the photoreaction under light, in particular for visible light.1,2 Due to the suitable band gap energy and the chemical stability, BiVO4 is considered an attractive photocatalyst and has been extensively studied.3–8 Several different phases of BiVO4 have been observed, such as tetragonal zircon (tz-), tetragonal scheelite (ts-), and monoclinic scheelite (ms-).9 Among these phases, under visible-light irradiation, ms-BiVO4 has been found to exhibit the highest photocatalytic activity.10–12 The valence bands (VB) of BiVO4 are composed of hybridized Bi 6s and O 2p orbitals. The hybridization of the Bi 6s and O 2p levels leads to a large dispersion of the VB, which favors the mobility of photogenerated holes and is beneficial to the oxidation reaction.13,14 In addition, the effective masses of the electron and the hole in BiVO4 were predicted to be much lighter than those in other semiconductors (e.g., TiO2 and In2O3).15,16

However, the typical efficiency of pure BiVO4 for water oxidation is not impressive, due to excessive electron–hole recombination and poor water oxidation kinetics.17–19 These deficiencies thus greatly limit the practical applications of BiVO4. Many attempts have been made to enhance the photocatalytic and photoelectrochemical (PEC) activity of BiVO4 by controlling the morphology,20,21 forming composite structures or heterojunctions,22–24 doping or composition tuning,25–27 and coupling with oxygen evolution catalysts (OECs).28 Among these methods, ion doping is a simple approach and one of the most effective methods.29 Using a modified metal–organic decomposition method, Parmar et al. prepared BiVO4 doped with various metal ions and observed that only Mo6+ or W6+ doping enhanced the water photo-oxidation activity.30 For water oxidation or organic compound degradation, Mo-doped BiVO4 was remarkably enhanced.31 Recently, Abdi et al.32 demonstrated that the carrier-separation efficiencies of W-doped BiVO4 photoanode can achieve to 80%. More importantly, instead of single element-doping, co-doping with W and Mo was found to further improve the photoelectrochemical (PEC) performance of BiVO4.33,34 The carrier density of the Mo/W co-doped BiVO4 sample was demonstrated to be twice that of the W-doped sample.

For doped BiVO4, the impurities can be at either substitutional Bi or V sites.8,35,36 However, the existing forms of Mo and W atoms in doped BiVO4 are often debated. It is expected that the mechanism for Mo impurity incorporation could be different from that for W. Some of the incorporated impurities may contribute to catalytic activity, whereas others may simply affect the crystal growth or alter the formation of defects and subsequently the doping properties. To achieve the optimal performance of Mo/W co-doped BiVO4, it is fundamentally important to identify the forms of the defects that are potential shallow donors or are harmful to the PEC response. It is also essential to determine the growth conditions for the formation of desirable defects and the suppression of harmful defects.

Herein, we systematically investigated the geometry structures, formation energies, and electronic properties of the Mo, W mono-doped and Mo/W codoped BiVO4. The result of our investigation enables us to determine the most stable model for Mo/W co-doped BiVO4. The origin of visible-light absorption and photoactive enhancement for the Mo/W co-doped BiVO4 were revealed by exploring the effects of the changes in band gaps, distributions of the impurity states, and energies of the band edges.

2. Models and methods

2.1. Computational details

All geometric optimizations and electronic structure calculations were performed using spin-polarized density functional theory (DFT), as implemented in the Vienna Ab initio Simulation Package (VASP).37,38 The project-augmented wave method for core valence interactions and the generalized gradient approximation (GGA) of the Perdew–Burke–Ernzerhof (PBE) form for the exchange–correlation function were used.39–42 For selected systems, we also used DFT (GGA) + U within Dudarev's approach.43 We applied the U = 2.7 eV (ref. 44) for the V 3d states in BiVO4. For the Mo/W co-doped BiVO4 cases, U = 2.3 eV for Mo 4d and U = 2.1 eV for W 5d were chosen, according to ref. 44. Note that 4d- and 5d-valence orbitals are generally less spatially localized than 3d-valence orbitals, resulting in smaller U values. Although the properties of doped BiVO4 can be affected by the choice of U, these DFT + U calculations should be appropriate to draw reasonable conclusions. In fact, the hybrid exchange functional (HSE06)45 has previously been shown to be better at accurately predicting the structure and band gap of BiVO4 compared to the standard DFT functional.46–48 However, the HSE06 calculations for doped BiVO4 are restricted by our computational resources. From Section 3, we will see that the calculated band gap of pure BiVO4 using the GGA + U method is 2.3 eV, which is in good agreement with the experimental value of 2.5 eV.49 Therefore, instead of HSE06, the GGA + U method is used in the current calculations. The Kohn–Sham one-electron states were expanded in a plane wave basis set up to 500 eV. For pure BiVO4 and Mo/W co-doped BiVO4, the Monkhorst–Pack k-point mesh of 5 × 3 × 7 and 3 × 3 × 3 was used to perform geometry optimizations, and 10 × 6 × 7 and 6 × 6 × 6 k-point mesh was used for the electronic structure calculations,50 respectively. At the end of the structural optimizations process, the residual Hellmann–Feynman forces on each ion became less than 0.03 eV Å−1. The criterion for the total energy is set as 1 × 10−5 eV. The density of states (DOS) was calculated using the tetrahedron method with Blöchl corrections. Moreover, the accuracy of the calculations was tested by increasing the cutoff energy and the number of k points, and negligible changes in the energy and geometry structure were observed. After finishing the geometry optimization, the band structure and projected density of state (PDOS) of the pure, Mo, W mono-doped, and Mo/W co-doped BiVO4 were calculated.

2.2. Doped configuration

Both I2/b and C2/c space groups are commonly used to describe the monoclinic scheelite structure of BiVO4.8 The space group, I2/b, with which the monoclinic scheelite structure of BiVO4 was originally reported is a non-standard space group.51 It can be converted to a standard space group, C2/c, which is used in some recent studies of BiVO4.15,23 Here we choose I2/b space groups, this because which has the advantage of easily showing its structural relationship to the tetragonal scheelite structure.8 The monoclinic BiVO4 structure was determined through careful volume optimization and atomic position relaxation with a primitive unit cell (consisting of two BiVO4 units). By optimizing the pure ms-BiVO4 structure, we obtained the following lattice parameters: a = 5.1507 Å, b = 5.0958 Å, c = 11.6067 Å, and γ = 90.2416° (space group I2/b). These lattice parameters are in good agreement with experimental values:51 a = 5.1935 Å, b = 5.0898 Å, c = 11.6972 Å, and γ = 90.3871°. These results indicate that our calculation methods can give reasonably good values. For doping structures, as shown in Fig. 1, the 2 × 2 × 1 supercell (containing 16 bismuth or vanadium atoms and 64 oxygen atoms) of monoclinic BiVO4 was simultaneously doped with one Mo atom and one W atom. The impurity atoms were introduced into the supercell with the modes of MoBi (Mo atom substituting for the lattice Bi atom), MoV (Mo atom substituting for the lattice V atom), WBi (W atom substituting for the lattice Bi atom), and WV (W atom substituting for the lattice V atom), resulting in four different modes of Mo/W co-doped monoclinic BiVO4 models (MoVWV–BiVO4, MoVWBi–BiVO4, MoBiWBi–BiVO4, and MoBiWV–BiVO4). To further determine the stable mono-doped configurations, we constructed 16 possible mono-doped systems for MoBi, MoV, WBi, and WV and calculated their total energies. It is found that MoBi at (0.75, 0.625, 0.8665), MoV at (0.75, 0.375, 0.63), WBi at (0.75, 0.875, 0.1335), and WV at (0.5, 0.875, 0.87) positions have lower total energy than other positions. For MoBiWV–BiVO4 and MoBiWBi–BiVO4, we fixed MoBi at the (0.75, 0.625, 0.8665) position, constructed 15 and 16 possible co-doped systems for WBi, and WV, respectively, and then calculated their total energies. It is found that WBi at (0.75, 0.875, 0.1335) and WV at (0.5, 0.875, 0.87) (seen in Fig. 2(j)) have lower total energy than at the other positions. Similarly, for MoVWV–BiVO4 and MoVWBi–BiVO4, we fixed MoV at the position of (0.75, 0.375, 0.63) and then constructed 16 and 15 possible co-doped systems for WBi, and WV, respectively. The calculated total energies demonstrate that WBi at (0, 0.125, 0.6335) and WV at (0.25, 0.375, 0.63) (seen in Fig. 2(k)) have lower total energy than at the other positions. In the following, we will only focus on the configuration with the lowest energy for mono-doped or co-doped systems. Furthermore, to compare the electronic properties of Mo/W co-doped BiVO4 with those of mono-doped BiVO4, the supercell models of MoBi–BiVO4, MoV–BiVO4, WBi–BiVO4 and WV–BiVO4 were also calculated.
image file: c5ra22659g-f1.tif
Fig. 1 The 2 × 2 × 1 supercell of BiVO4, which contains 16 bismuth or vanadium atoms and 64 oxygen atoms. The light blue, green, and red spheres represent Bi, V, and O atoms, respectively.

image file: c5ra22659g-f2.tif
Fig. 2 Band structures and projected density of state (PDOS) for pure, Mo, W monodoped and Mo/W codoped BiVO4: (a) pure, (b) WV, (c) MoBi, (d) MoV, (e) WVMoBi (f) WVMoV. (g)–(k) are the band decomposed charge density within the energy range of −0.5 to 0 eV (isosurface values 0.01 e Å−3) for (b) WV, (c) MoBi, (d) MoV, (e) WVMoBi, and (f) WVMoV. The dashed lines denote the Fermi level at 0 eV. The black lines represent the PDOS of O 2p, blue for V 3d, red for the quadruplicate of Mo 4d and magenta for the quadruplicate of W 5d. The light blue, green, red, purple and black spheres represent Bi, V, O, Mo and W atoms, respectively. Only (Mo or W)–O bands and (Mo or W)–V bands are shown. The Mo 4d and W 5d states are multiplied by 4 times to show their distribution clearly.

Experimentally, the doping amounts of Mo and W are often not equal.34 To explore its possible mechanism, we increased the Mo or W doping concentration further and considered Mo/W/Mo (atomic number ratio of Mo and W is 2[thin space (1/6-em)]:[thin space (1/6-em)]1) and W/Mo/W (atomic number ratio of Mo and W is 1[thin space (1/6-em)]:[thin space (1/6-em)]2) co-doped BiVO4, respectively. Moreover, the Mo, W dopant concentrations (mole ratio) in our calculations can be achieved 3.125%. This mole ratio could be comparable to those in experiments, which are 6% (ref. 34) or 8% (ref. 33) respectively. From Section 3, we will see that when Mo doped on the Bi lattice site, it might be harmful for PEC efficiency. In this case, we only focus on Mo or W doped on the V lattice site. We fixed MoV at the position of (0.75, 0.375, 0.63) and WV at the position of (0.25, 0.375, 0.63) and then constructed 14 and 14 possible co-doped systems for MoV, and WV, respectively. The calculated total energies show that MoV at the position of (0.5, 0.375, 0.87) (seen in Fig. 3(c)) and WV at the position of (0.5, 0.375, 0.87) (seen in Fig. 3(d)) have lower total energy than the other positions. In the following, we will only focus on the configurations with the lowest energy for 1[thin space (1/6-em)]:[thin space (1/6-em)]2 BiVO4 co-doped systems.


image file: c5ra22659g-f3.tif
Fig. 3 (a) and (b) Band structures, projected density of state (PDOS) plots, and (c) and (d) the band decomposed charge density within the energy range of −0.5 to 0 eV (isosurface values 0.01 e Å−3) for Mo/W/Mo–BiVO4 and W/Mo/W–BiVO4. The labeling of the atoms is the same as in Fig. 2. Only (Mo or W)–O bands and (Mo or W)–V bands are shown. The Mo 4d and W 5d states are multiplied by 4 times to show their distribution clearly.

2.3. Formation energies

To compare the relative feasibilities of the doping modes above, the formation energies (Eform) for Mo/W co-doped as well as for mono-doped BiVO4 were calculated; the formation energy is defined by the following expression:
 
Eform = EdopedEpurep × μMoq × μW + x × μBi + y × μV, (1)
where Edoped is the total energy of the Mo, W mono-doped or Mo/W co-doped BiVO4 supercell and Epure is the total energy of the pure BiVO4 supercell.52,53 μMo, μW, μBi and μV are the chemical potentials of Mo, W, Bi, and V atoms, respectively. The coefficients p and q (equal to 0, 1, or 2) represent the numbers of Mo and W, respectively, and x and y (equal to 0, 1, or 2) represent the numbers of Bi and V atoms, respectively. Note that Eform is not fixed but depends on the growth conditions. By adjusting the O2 pressure, the growth conditions can be changed from O-rich to O-poor. The relationships between oxygen and the chemical potentials of Mo, W, Bi, and V atoms are as follows:
 
μMo + 3μO = μ(MoO3), (2)
 
μW + 3μO = μ(WO3), (3)
 
2μBi + 3μO = μ(Bi2O3), (4)
 
μV + 2μO = μ(VO2). (5)

Under O-rich growth conditions, μO is determined by the ground-state energy of the O2 molecule (μO = μO(O2)/2). Thus, the chemical potentials of Mo, W, Bi, and V atoms can be obtained from eqn (2)–(5), respectively. While under extreme reducing conditions, μV is determined by the ground-state energy of bulk V (μV = μbulk/n, n is the number of V atom in the bulk V). The value of μBi determined by the ground-state energy of bulk Bi (μBi = μbulk/n, n is the number of Bi atom in the bulk Bi) does not change.29 According to eqn (5), μO is calculated. Then, μMo and μW can be obtained from eqn (2) and (3), respectively. Considering that the calculated μO in O-rich growth conditions is larger by 3 eV than that in O-poor growth conditions, μO is transformed into the form of μO = 1/2 × μO(O2)+ μO, where μO = −3 eV corresponds to O-poor growth conditions and μO = 0 eV corresponds to O-rich growth conditions (condensation oxygen). According to eqn (1)–(5), Eform can thus be calculated from the following expression:

 
Eform = EdopedEpurep × μ(MoO3) − q × μ(WO3) + (x/2) × μ(Bi2O3) + y × μ(VO2) + (3p/2 + 3q/2 − 3x/2 − y) × μ(O2) + (3p + 3q − 3x/2 − 2y) × μO (6)

Table 1 lists the formation energies of doped supercells under the O-poor and O-rich growth conditions. Under the oxygen-poor condition, the doping processes of Mo, W mono-doped and Mo/W co-doped BiVO4 with positive Eform become non-spontaneous reactions, implying that the oxygen-poor condition inhibits the doping process of Mo, W mono-doped and Mo/W co-doped BiVO4. In particular, under oxygen-rich growth conditions, MoVWV–BiVO4 is the most stable system due to its lowest formation energy (−0.79 eV), and the next most stable ones are MoV–BiVO4 (−0.65 eV) and WV–BiVO4 (−0.15 eV). These results confirm that the Mo and W atoms prefer to substitute V atom under the oxygen-rich condition, consistent with the experimental results.54 This result also indicates that the chosen parameters in the calculations are quite reasonable.

Table 1 Formation energies (eV) for Mo, W monodoped and Mo/W co-doped BiVO4
Doped BiVO4 O-Poor O-Rich
WBi 6.89 2.39
WV 2.84 −0.15
MoBi 5.31 0.81
MoV 2.34 −0.65
WVMoBi 8.03 0.53
WVMoV 5.20 −0.79
WBiMoV 9.11 1.61
WBiMoBi 11.75 2.75
Mo/W/Mo 7.61 −1.38
W/Mo/W 8.14 −0.86


It is intriguing to see in Table 1 that under oxygen-rich growth conditions, the Mo/W/Mo–BiVO4 is the most stable system due to its lowest formation energy (−1.38 eV), indicating that Mo/W/Mo co-doping could be easier to obtain experimentally than the other combinations (e.g., MoV–BiVO4, MoVWV–BiVO4, and WV–BiVO4). The formation energy of W/Mo/W–BiVO4 is smaller than that of WVMoV (−0.79 eV) and smaller than that of WV (−0.15 eV), which results from the better ion size matching of Mo and V.

3. Results and discussion

The Eform of MoBi–BiVO4 and MoBiWV–BiVO4 is 0.81 eV and 0.53 eV, respectively, which is close to that of MoV–BiVO4 (−0.65 eV) and WV–BiVO4 (−0.15 eV). Therefore, to compare Mo or W atom doping on V site BiVO4, the electronic properties of the MoBi–BiVO4 and MoBiWV–BiVO4 are also calculated. To clarify the origination of enhanced visible-light photocatalytic activity of Mo or W doped BiVO4, the band structures, the projected density of states (PDOS), and the band decomposed charge densities for the pure-BiVO4, WV–BiVO4, MoBi–BiVO4, MoV–BiVO4, MoBiWV–BiVO4, and MoVWV–BiVO4 were calculated using the DFT + U method and were plotted in Fig. 2(a)–(k). Comparing Fig. 2(a)–(f) and S1–S4 (Fig. S1–S4 in ESI material which geometrical structure is the same as Fig. 2), we found that the band gap increases with DFT + U method, while the shape of the electronic band structure remains nearly unchanged with respect to the normal DFT calculations.

As shown in Fig. 2(a), the pure BiVO4 is an indirect band gap semiconductor, which is consistent with the experimental result.49 The band gap of BiVO4 increases to 2.3 eV from DFT + U calculations, which is close to the experimental result (2.5 eV) and is consistent with the previous theoretical study.33 This result indicates that the chosen U value is sufficiently large for V 3d; in the literature, the commonly applied U values for V 3d are in the range of 2 to 4 eV.44,55 The conduction bands of pure BiVO4 are mainly composed of O 2p and V 3d states, while the valence bands are composed of O 2p states. It is noted that band gaps from the DFT + U method are not directly comparable to experiment. However, we mainly focus on the change of the band gap after Mo, W mono-doped and Mo/W co-doping, which can be well described by DFT + U method in present work.

In the cases of WV–BiVO4, MoBi–BiVO4, MoV–BiVO4, MoBiWV–BiVO4, and MoVWV–BiVO4 (see Fig. 2(b)–(f)), in comparison with pure BiVO4, one can clearly see that (1) for MoV and/or WV, the Mo 4d state tends to be localized in the bottom of the conduction bands, which leads to the extending of the conduction bands [see Fig. 2(b) and (f)], while the W 5d state goes deeply into the conduction bands, which strongly affects the V 3d state [see Fig. 2(c) and (f)]; (2) all of these doping are potential donors for realizing good n-type conductivities; and (3) doping with Mo, W leads to narrowing of the band gap, and W doped system shows a smaller gap than Mo doped one.

For the WV–BiVO4 (seen in Fig. 2(b)), there is no isolated state in the band gap, and the W 5d impurity states appear in the same energy region with V 3d states, implying a hybridization between the W 5d and V 3d states. There is one main peak of the V 3d spin-down state near EF (−0.05 V). The corresponding band decomposed charge density isosurface (see Fig. 2(g)) reveals that the charge density spreads over four V atoms. The conduction bands are mainly composed of V 3d states, while the valence bands mainly consisted of O 2p states. Thus, the valence bands extends towards the conduction bands and the eigenvalue gap (Eg), and the distance between O 2p states and V 3d states around EF (see Fig. 2), is 2.23 eV, which is close to that of pure BiVO4 (2.3 eV). Therefore, W doping weakly changes the band gap of BiVO4, which is in agreement with the experimental result.33

As shown in Fig. 2(c) and (e), for MoBi–BiVO4 and MoBiWV–BiVO4, Mo doping on the Bi lattice site will induce Mo 4d impurity states located in the band gap. These impurity states can easily trap the carriers and lead to the reduction in carrier mobility and conversion efficiency,56 which is a harmful for the application of MoBi–BiVO4 and MoBiWV–BiVO4 in the photoelectrochemical conversion of solar energy. Thus, although MoBi–BiVO4 and MoBiWV–BiVO4 systems have a significant reduction in the photo transition energy, they are not so suitable for enhancing the photocatalytic activity in the visible light region. In practice, to increase the PEC efficiency, we should control the growth conditions to avoid the formation of MoBi–BiVO4 and MoBiWV–BiVO4. For MoBi–BiVO4, the value of Eg is 2.27 eV, which is close to that of pure BiVO4 (2.3 eV). This is consistent with the previous results.33,57 While for MoBiWV–BiVO4, the value of Eg is decreased to 2.15 eV. This result indicates that the influence of W doping on Eg is stronger than that of Mo doping. The corresponding band decomposed charge density isosurface (see Fig. 2(h) and (j)) reveals that the charge density spreads over the Mo/O atoms and V atoms.

As shown in Fig. 2(d), a significant perturbation occurs at the conduction band minimum (CBM) and the Fermi level is above the conduction band. In addition, the conduction band edge is still determined by the V 3d state. The Eg is decreased to 2.23 eV. The corresponding band decomposed charge density isosurface (see Fig. 2(i)) reveals that the charge density spreads over the Mo atom. As shown in Fig. 2(f), the band edge shifts of the MoVWV–BiVO4 exhibit the same chemical trends as those observed in both the MoV–BiVO4 and WV–BiVO4. Simultaneously, the value of Eg is decreased to 2.08 eV. The band decomposed charge density isosurface (see Fig. 2(k)) reveals that the charge density spreads over the Mo atom and five V atoms.

To further investigate the effect of doping on the band gap, the Mo/W/Mo–BiVO4 and W/Mo/W–BiVO4 is considered. Fig. 3(a)–(d) shows the band structures, PDOS plots, and the band decomposed charge density within the energy range of −0.5 to 0 eV (isosurface values 0.01 e Å−3) for Mo/W/Mo–BiVO4 and W/Mo/W–BiVO4. As shown in Fig. 3, with increasing Mo or W doping, the impurity state continuously moves toward the valence bands by approximately 0.2 eV. The corresponding band decomposed charge density isosurface for Mo/W/Mo–BiVO4 indicates that the charge density spreads over the two Mo atoms and the seven V atoms, while for W/Mo/W–BiVO4, the impurity state comes from different V atoms. In this case, continuum states above the CB edge are formed rather than isolated states, and thus leads to a real band gap narrowing and consequently a redshift of the optical absorption edge,58 which is favorable for enhancing the lifetimes of photoexcited carriers.56,59 This enhancement is similar to the phenomenon in Nb/C/Nb co-doped TiO2 (ref. 56) and N/H-codoped TiO2.58 Compared with MoVWV–BiVO4, the values of Eg of Mo/W/Mo–BiVO4 and W/Mo/W–BiVO4 are reduced to 1.80 eV and 1.78 eV, respectively, which can enhance the absorptions of visible light.

Characteristic charge redistribution behavior can be obtained by calculating the charge density difference of the Mo, W mono-doped and Mo/W co-doped BiVO4 before and after the charge transfer. Fig. 4 shows the calculated charge density difference of WVMoBi, WVMoV, Mo/W/Mo, and W/Mo/W–BiVO4, co-doped BiVO4. As observed from Fig. 4(a)–(d), the charge redistribution is dominantly restricted on Mo/W and O. The amount of the charge transfer of MoBi is less than WV and MoV, which is consistent with Mo, W mono-doped BiVO4 (Fig. S5 in the ESI material). In the case of WV and MoV, substantial charge accumulates in the region between the WV/MoV and O atoms. This demonstrates that the bonding between the Mo/W and O atoms is characterized by covalent behavior.


image file: c5ra22659g-f4.tif
Fig. 4 Charge density difference isosurfaces of (a) WVMoBi (b) WVMoV (c) Mo/W/Mo and (d) W/Mo/W co-doped BiVO4. The cyan region represents charge depletion, and the yellow region represents charge accumulation. The isosurface value is 0.03 e Å−3. The labeling of the atoms is the same as in Fig. 2. Only (Mo or W)–O bands and (Mo or W)–V bands are shown.

The conduction band and valence band potentials of a semiconductor affect its photocatalytic activity.1 Based on the Mulliken electronegativity theory,60 the conduction band potentials at the point of zero charge of a semiconductor could be predicted by

 
ECB = χEc − 0.5Eg, (7)

ECB, Eg and χ are the conduction band potential, band-gap energy and absolute electronegativity of a semiconductor, respectively. Ec is the energy of the free electron in the hydrogen scale (approximately 4.5 eV).61 The valence band potential could be calculated from EVB = ECBEg. The χ value for BiVO4 is 6.04 eV.62 The Mulliken electronegativity of V, Mo, and W are 3.6, 3.9,63 and 4.4 eV.64 This indicates that Mo and/or W doping very weakly changes the χ value of the doped system. Thus, the ECB of doped systems are almost equal to that of the pure which is consistent with the previous computational results.65 It means that the driving force required for reduction process almost unchanged. Correspondingly, for the smaller band gap, EVB of doped systems are smaller than that of pure, and thus oxidation processes is lowered in these system.

4. Conclusions

We have carefully examined the formation energy, electronic property, and photocatalytic activity of the Mo, W mono-doped and Mo/W codoped BiVO4 using DFT + U calculations. Much important structure information has been obtained, which will provide useful guidelines for the growth of crystals. We found that under oxygen-rich conditions, the Mo and W atoms prefer to substitute V atom, but the doping of Mo gives a more stable structure. The electronic structure of the mono-doped is very different, resulting in very different optical absorption behavior. The Mo/W/Mo and W/Mo/W co-doping cases are found to enhance optical absorption due to their reduced band gap compared with the undoped case. The Mo/W/Mo and W/Mo/W co-doped BiVO4 can be considered as good candidates for visible-light photocatalysis in practical applications.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 11547011, 51371076, 21203037, and 11264005), the Program for Innovative Research Team in the University of Henan Province (no. 13IRTSTHN017), the Construction Project for Guizhou Provincial Key Laboratories (no. ZJ[2013]4009), the Natural Science Foundation of Guizhou Province (no. QKH-J[2010]2144, J[2011]2097), and the Guizhou Provincial High-Performance Computing Center of Condensed Materials and Molecular Simulation, the Program for Innovative Research Team of Guizhou Province (No. QKTD[2014]4021). J. H. Z. acknowledges the scientific research fund of GPED (no. 2114118006zx, QJTD[2013]16) and the GZNC startup package (no. 13BS027). M. S. D acknowledges the support by the Excellent Youth Scientific and Technological Talents of Guizhou Province (no. QKH-RZ[2013]01).

References

  1. A. L. Linsebigler, G. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735–758 CrossRef CAS.
  2. R. Li, J. Hu, M. Deng, H. Wang, X. Wang, Y. Hu, H. L. Jiang, J. Jiang, Q. Zhang, Y. Xie and Y. Xiong, Adv. Mater., 2014, 26, 4783–4788 CrossRef CAS PubMed.
  3. A. Kudo, K. Omori and H. Kato, J. Am. Chem. Soc., 1999, 121, 11459–11467 CrossRef CAS.
  4. A. Kudo, K. Omori and H. Kato, J. Am. Chem. Soc., 1999, 121, 11459–11467 CrossRef CAS.
  5. A. Kudo, K. Ueda, H. Kato and I. Mikami, Catal. Lett., 1998, 53, 229–230 CrossRef CAS.
  6. N. Aiga, Q. Jia, K. Watanabe, A. Kudo, T. Sugimoto and Y. Matsumoto, J. Phys. Chem. C, 2013, 117, 9881–9886 CAS.
  7. H. W. Jeong, T. H. Jeon, J. S. Jang, W. Choi and H. Park, J. Phys. Chem. C, 2013, 117, 9104–9112 CAS.
  8. Y. Park, K. J. McDonald and K. S. Choi, Chem. Soc. Rev., 2013, 42, 2321–2337 RSC.
  9. J. D. Bierlein and A. W. Sleight, Solid State Commun., 1975, 16, 69–70 CrossRef CAS.
  10. S. Tokunaga, H. Kato and A. Kudo, Chem. Mater., 2001, 13, 4624–4628 CrossRef CAS.
  11. J. Yu and A. Kudo, Adv. Funct. Mater., 2006, 16, 2163–2169 CrossRef CAS.
  12. H. Yoon, M. G. Mali, J. Y. Choi, M. W. Kim, S. K. Choi, H. Park, S. S. Al-Deyab, M. T. Swihart, A. L. Yarin and S. S. Yoon, Langmuir, 2015, 31, 3727–3737 CrossRef CAS PubMed.
  13. M. Oshikiri, M. Boero, J. Ye, Z. Zou and G. Kido, J. Chem. Phys., 2002, 117, 7313–7318 CrossRef CAS.
  14. L. Zhang, D. Chen and X. Jiao, J. Phys. Chem. B, 2006, 110, 2668–2673 CrossRef CAS PubMed.
  15. Z. Zhao, Z. Li and Z. Zou, Phys. Chem. Chem. Phys., 2011, 13, 4746–4753 RSC.
  16. A. Walsh, Y. Yan, M. N. Huda, M. M. Al-Jassim and S.-H. Wei, Chem. Mater., 2009, 21, 547–551 CrossRef CAS.
  17. F. F. Abdi, T. J. Savenije, M. M. May, B. Dam and R. van de Krol, J. Phys. Chem. Lett., 2013, 4, 2752–2757 CrossRef CAS.
  18. J. Ravensbergen, F. F. Abdi, J. H. van Santen, R. N. Frese, B. Dam, R. van de Krol and J. T. M. Kennis, J. Phys. Chem. C, 2014, 118, 27793–27800 CAS.
  19. Y. Ma, S. R. Pendlebury, A. Reynal, F. Le Formal and J. R. Durrant, Chem. Sci., 2014, 5, 2964 RSC.
  20. W. Luo, Z. Wang, L. Wan, Z. Li, T. Yu and Z. Zou, J. Phys. D: Appl. Phys., 2010, 43, 405402 CrossRef.
  21. G. Xi and J. Ye, Chem. Commun., 2010, 46, 1893–1895 RSC.
  22. J. Su, L. Guo, N. Bao and C. A. Grimes, Nano Lett., 2011, 11, 1928–1933 CrossRef CAS PubMed.
  23. J. Zhang, F. Ren, M. Deng and Y. Wang, Phys. Chem. Chem. Phys., 2015, 17, 10218–10226 RSC.
  24. Y. Yang, J. Wang, J. Zhao, B. A. Nail, X. Yuan, Y. Guo and F. E. Osterloh, ACS Appl. Mater. Interfaces, 2015, 7, 5959–5964 CAS.
  25. L. Chen, F. M. Toma, J. K. Cooper, A. Lyon, Y. Lin, I. D. Sharp and J. W. Ager, ChemSusChem, 2015, 8, 1066–1071 CrossRef CAS PubMed.
  26. M. Wang, H. Zheng, J. Liu, D. Dong, Y. Che and C. Yang, Mater. Sci. Semicond. Process., 2015, 30, 307–313 CrossRef CAS.
  27. D. K. Zhong, S. Choi and D. R. Gamelin, J. Am. Chem. Soc., 2011, 133, 18370–18377 CrossRef CAS PubMed.
  28. T. H. Jeon, W. Choi and H. Park, Phys. Chem. Chem. Phys., 2011, 13, 21392–21401 RSC.
  29. Z. Zhao, W. Luo, Z. Li and Z. Zou, Phys. Lett. A, 2010, 374, 4919–4927 CrossRef CAS.
  30. K. P. Parmar, H. J. Kang, A. Bist, P. Dua, J. S. Jang and J. S. Lee, ChemSusChem, 2012, 5, 1926–1934 CrossRef CAS PubMed.
  31. W. Yao, H. Iwai and J. Ye, Dalton Trans., 2008, 1426–1430,  10.1039/b713338c.
  32. F. F. Abdi, L. Han, A. H. M. Smets, M. Zeman, B. Dam and R. van de Krol, Nat. Commun., 2013, 4, 2195 Search PubMed.
  33. H. S. Park, K. E. Kweon, H. Ye, E. Paek, G. S. Hwang and A. J. Bard, J. Phys. Chem. C, 2011, 115, 17870–17879 CAS.
  34. S. P. Berglund, A. J. Rettie, S. Hoang and C. B. Mullins, Phys. Chem. Chem. Phys., 2012, 14, 7065–7075 RSC.
  35. B. Zhou, X. Zhao, H. Liu, J. Qu and C. P. Huang, Sep. Purif. Technol., 2011, 77, 275–282 CrossRef CAS.
  36. W.-J. Yin, S.-H. Wei, M. M. Al-Jassim, J. Turner and Y. Yan, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 83, 155102 CrossRef.
  37. G. Kresse and J. Furthmüller, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 11169–11186 CrossRef CAS.
  38. G. Kresse and J. Furthmüller, Comput. Mater. Sci., 1996, 6, 15–50 CrossRef CAS.
  39. G. Kresse and D. Joubert, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 59, 1758–1775 CrossRef CAS.
  40. P. E. Blöchl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 49, 16223–16233 CrossRef.
  41. J. P. Perdew and Y. Wang, Phys. Rev. B: Condens. Matter Mater. Phys., 1992, 45, 13244–13249 CrossRef.
  42. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865–3868 CrossRef CAS PubMed.
  43. S. L. Dudarev, S. Y. Savrasov, C. J. Humphreys and A. P. Sutton, Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 57, 1505–1509 CrossRef CAS.
  44. I. V. Solovyev and P. H. Dederichs, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 16861–16871 CrossRef CAS.
  45. J. Heyd, G. E. Scuseria and M. Ernzerhof, J. Chem. Phys., 2006, 124, 219906 CrossRef.
  46. S. K. Pilli, T. G. Deutsch, T. E. Furtak, J. A. Turner, L. D. Brown and A. M. Herring, Phys. Chem. Chem. Phys., 2012, 14, 7032–7039 RSC.
  47. N. Wadnerkar and N. J. English, Comput. Mater. Sci., 2013, 74, 33–39 CrossRef CAS.
  48. K. E. Kweon and G. S. Hwang, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 87, 205202 CrossRef.
  49. J. K. Cooper, S. Gul, F. M. Toma, L. Chen, Y.-S. Liu, J. Guo, J. W. Ager, J. Yano and I. D. Sharp, J. Phys. Chem. C, 2015, 119, 2969–2974 CAS.
  50. H. J. Monkhorst and J. D. Pack, Phys. Rev. B: Condens. Matter Mater. Phys., 1976, 13, 5188–5192 CrossRef.
  51. A. W. Sleight, H. y. Chen, A. Ferretti and D. E. Cox, Mater. Res. Bull., 1979, 14, 1571–1581 CrossRef CAS.
  52. R. Long and N. J. English, J. Phys. Chem. C, 2009, 113, 8373–8377 CAS.
  53. P. Zhou, J. Yu and Y. Wang, Appl. Catal., B, 2013, 142, 45–53 CrossRef.
  54. W. Luo, J. Wang, X. Zhao, Z. Zhao, Z. Li and Z. Zou, Phys. Chem. Chem. Phys., 2013, 15, 1006–1013 RSC.
  55. B. N. Cox, M. A. Coulthard and P. Lloyd, J. Phys. F: Met. Phys., 1974, 4, 807–820 CrossRef CAS PubMed.
  56. X. Ma, Y. Wu, Y. Lu, J. Xu, Y. Wang and Y. Zhu, J. Phys. Chem. C, 2011, 115, 16963–16969 CAS.
  57. K. Ding, B. Chen, Z. Fang, Y. Zhang and Z. Chen, Phys. Chem. Chem. Phys., 2014, 16, 13465–13476 RSC.
  58. L. Mi, P. Xu, H. Shen, P.-N. Wang and W. Shen, Appl. Phys. Lett., 2007, 90, 171909 CrossRef.
  59. M. Li, J. Zhang and Y. Zhang, Chem. Phys. Lett., 2012, 527, 63–66 CrossRef CAS.
  60. M. A. Butler, J. Electrochem. Soc., 1978, 125, 228 CrossRef CAS.
  61. S. R. Morrison, Electrochemistry at Semiconductor and Oxidized Metal Electrodes, Plenum Press, New York, NY, USA, 1980 Search PubMed.
  62. M. L. Guan, D. K. Ma, S. W. Hu, Y. J. Chen and S. M. Huang, Inorg. Chem., 2011, 50, 800–805 CrossRef CAS PubMed.
  63. M. V. Putz, N. Russo and E. Sicilia, Theor. Chem. Acc., 2005, 114, 38–45 CrossRef CAS.
  64. E. A. Boudreaux, J. Phys. Chem. A, 2011, 115, 1713–1720 CrossRef CAS PubMed.
  65. K. Lai, Y. Zhu, J. Lu, Y. Dai and B. Huang, Solid State Sci., 2013, 24, 79–84 CrossRef CAS.

Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22659g

This journal is © The Royal Society of Chemistry 2016
Click here to see how this site uses Cookies. View our privacy policy here.