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Double-Hole-Mediated Codoping on KNbO3 for Visible Light Photocatalysis Guangzhao Wang, Yuhong Huang, Anlong Kuang, Hongkuan Yuan, Yang Li, and Hong Chen* School of Physical Science and Technology, and Key Laboratory of Luminescent and Real-Time Analytical Chemistry, Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, People’s Republic of China

ABSTRACT: In this theoretical study, the double-hole-mediated codoping strategy has been adopted to improve the photocatalytic activity of cubic KNbO3 as compared with the corresponding individual doping. The strong double-hole-mediated dopant−dopant coupling significantly reduces the effective bandgaps for the anionic−anionic (N−N, P−P, N−P, C−S) codoped systems with removing the appearing acceptor states above the Fermi level. No dopant−O coupling occurs in the cationic− anionic (V−C, Ti−P, Ti−N, Zr−P, Zr−N, Sc−S, Y−S) codoped systems. The V−C and Ti−P codoping could lead to narrowed bandgaps without unfilled localized states appearing above the Fermi level. N, Ti, Zr, Sc, Y monodoping and Ti−N, Zr−P, Zr−N, Sc−S, Y−S codoping introduce unoccupied impurity states between the valence band maximum and conduction band minimum, which makes them unfavorable for photocatalysis as these impurity states may serve as electron−hole recombination centers. For P−P, N−P, and C−S codoped systems, the intermediate states are higher or close to the hydrogen evolution potential, which is thermodynamically unfavorable for production of both oxygen and hydrogen. Producing hydrogen only, the N−N and C−S codoped KNbO3 materials will be good choices for Z-scheme photocatalysis. V, S, and V−C codoped KNbO3 may be promising visible light photocatalysts for water splitting, as they have suitable effective bandgaps without the introduction of unoccupied impurity states above the Fermi level, and they also own proper band edge positions with respect to the water redox level. The calculated optical absorption curves also indicate that C, V, and S monodoping and N−N, V−C, and Ti−P codoping can effectively enhance the visible light absorption.

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INTRODUCTION Development of semiconductors which can store hydrogen, produce hydrogen from water, and destroy environmental pollutants has been a promising approach to solve energy and environment problems.1−9 The ideal photocatalyst should have proper bandgap and band edge positions; i.e., the bandgap should be around 2 eV with the valence band maximum (VBM) lower than the oxygen evolution potential (OEP) and the conduction band minimum (CBM) higher than the hydrogen evolution potential (HEP).10 Recently, cubic KNbO3 has been found to be an excellent photocatalyst for production of hydrogen and degradation of pollution.11−13 However, its large bandgap of 3.14 eV14 limits photocatalytic activity only to the range of ultraviolet light accounting for only 4% of incoming solar energy. It is significant to narrow the bandgap of KNbO3 so that the visible light accounting for 43% of incoming solar energy could be utilized. Introducing metal15−17 or nonmetal elementals18−22 into the crystal lattice has been demonstrated to be one of the most effective approaches to adjust the band structure so as to improve © XXXX American Chemical Society

the visible photocatalytic ability of wide bandgap semiconductors. Recently, Shen et al.23 theoretically predicted that replacing the K atom in KNbO3 crystals with a Cu atom leads to the shifting of the peak of absorption spectra due to the high peak generated by Cu 3d states around the Fermi level. Later on, Lau et al.24 have successfully synthesized N doped KNbO3 nanocube by a simple hydrothermal method. Their study indicates that N doping decreases the bandgap of a KNbO3 nanocube from 3.13 to 2.76 eV, and the visible light photocatalytic activity of an N doped KNbO3 nanocube in degradation of organic pollutants and water splitting is higher than that of a pure KNbO3 nanocube. Usually, monodoping can not significantly enhance the visible light photocatalytic efficiency. This may be attributed to the fact that some major issues which are associated with the monodoping approach will affect the photocatalytic performance. First, the desirable monodopant may have only limited solubility.25,26 The second aspect, monodoping, especially when Received: June 3, 2016

A

DOI: 10.1021/acs.inorgchem.6b01306 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(N−N, P−P, N−P, C−S) and cation−anion (V−C, Ti−P, Ti−N, Zr−P, Zr−N, Sc−S, and Y−S) double-hole-mediated codoped KNbO3 to find if there exists any hole-mediated coupling which could improve the visible light photocatalytic activity.

the dopant and host elements have different valences, is usually accompanied by the spontaneous formation of compensating vacancies, which will trap efficient charge carriers.27,28 Another major obstacle is that the introduced unoccupied localized states between CBM and VBM may reduce the photocatalytic efficiency through boosting electron−hole recombination.29 To overcome these problems, charge compensated codoping, which can not only improve the solubility by the Coulomb interaction between the codoping pairs but also avoid the compensating vacancy defects, has been demonstrated to be a promising strategy.30−38 Our previous study39 has reported the photocatalytic performance of Cr−N and Mo−N codoped KNbO3 as compared to the results of Mo, Cr, and N monodoped KNbO3. The Mo or Cr doping will obviously promote the N doping. Furthermore, the Mo−N codoped KNbO3 is a promising visible light photocatalyst with suitable bandgap and band edge positions with respect to the water redox level, while the appearance of unoccupied localized states between VBM and CBM boosting the recombination of electron−hole pairs limits the photocatalyst efficiency of Cr−N codoped KNbO3. Recently, Ghosh et al.40 have reported that W−N codoping could improve the visible light photocatalytic activity of KNbO3. In recent years, Yin et al.41 have proposed a novel strategy of double-holemediated dopants to narrow the bandgap of TiO2 without producing partially filled states above the Fermi level. The considerable bandgap reduction is due to the fact that the twohole-mediated dopant−dopant coupling leads to the fully filled bands above the valence band of TiO2. To ensure the coupling occurs, there must exist two holes for per dopant complex. Besides, the distance between the two O sites must be small, and there exists enough empty room for the movement of the dopants. Then, the double-hole-mediated coupling mechanism has been applied to design N−N, P−P, N−P, and C−S doublehole-mediated codoped ZrO2, BiNbO4, BaTaO4, SrTiO3, NaTaO3, Li2Ti2O7, and Sr2Nb2O7.42−48 However, there are only a few studies focusing on the double-hole-mediated coupling in metal−nonmetal codoped semiconductors. For instance, Niu et al.49 have reported the metal-assisted S−O coupling could enhance the photocatalytic performance of anatase TiO2, and Wang et al.50 have indicated metal-assisted P−O coupling could improve the photocatalytic activity of anatase TiO2. There is no study on the photocatalytic performance of double-hole-mediated codoped KNbO3; it is of great significance to investigate whether the anion−anion coupling and metalassisted anion−O coupling will occur in the cubic KNbO3 which has higher symmetry than the anatase TiO2. In this work, we have investigated the geometry structures, defect binding energies, defect formation energies, electronic structure, and optical properties of anionic monodoped, cationic monodoped, and doublehole-mediated codoped KNbO3. The CBM of KNbO3 is good enough as compared to the hydrogen reduction potential, and the VBM of KNbO3 is deep enough in contrast to the water oxidation potential. So moving up the VBM is a good choice to modify the band structure of KNbO3 for effective utilization of visible light. We choose the anion elements (N, P, C, S), which have higher p orbital energies than that of oxygen, to dope the O site of KNbO3 so as to shift upward the VBM. Besides, S, N, P, and C give us opportunities to study the effect on the band structure of KNbO3 caused by an isoelectronic, single hole, and double-hole dopant. The cation elements (V, Ti, Zr, Sc, Y) have been chosen to form double-hole-mediated codoped KNbO3 with N, P, C, and S. Especially, V and Ti doped KNbO3 have been grown and studied.24,51 We mainly investigate the anion−anion

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COMPUTATIONAL METHODS

All of the density functional theory (DFT) calculations using the projected augmented wave (PAW) method52,53 have been performed by using the Vienna ab initio simulation package (VASP).54 The valence states of K (3s23p64s1), Nb (4p65s14d4), O (2s22p4), N (2s22p3), P (3s23p3), C (2s22p2), S (3s23p4), V (3p63d44s1), Ti (3p63d34s1), Zr (4s24p65s24d2), Sc (3s23p64s23d1), and Y (4s24p65s24d1) are used to construct the PAW potentials. The geometry optimization has been performed by using the Perdew−Burke−Ernzerhof (PBE)55 functional for exchange correlation contribution within a generalized gradient approximation (GGA).56 In addition, the energy cutoff of 400 eV, a Monkhorst−Pack57 k-point mesh of 6 × 6 × 6, the energy convergence criterion of 10−6 eV, and the force convergence criterion of 0.02 eV/Å have been found to be sufficient for structural relaxation. We have adopted the HSE06 (Heyd−Scuserid−Ernzerhof)58,59 functional with the mixing parameter of 30% and k-point mesh of 3 × 3 × 3 to calculate the electronic structure and optical properties. The optimized lattice parameters for pure cubic KNbO3 are a = b = c = 4.013 Å, in excellent agreement with the experiment results of a = b = c = 4.022 Å.60 Besides, the obtained bandgap for pure KNbO3 is 3.01 eV, which is a little smaller than the experimental value of 3.14 eV.60 A 2 × 2 × 2 supercell of cubic KNbO3 (Figure 1) consisting of 12 K, 12 Nb,

Figure 1. 2 ×2 × 2 supercell structure for cubic KNbO3. and 24 O atoms has been chosen to model the doped systems. The anionic monodoped system is formed by substituting one of anionic elements (N, P, C, S) for one O atom in the crystal lattice of the pure system, while the cationic monodoped system is modeled by replacing one O atom with one of cationic elements (V, Ti, Zr, Sc, Y). Thus, the doping concentration for anionic or cationic monodoped KNbO3 is 2.5 at. %. The anionic−anionic codoped KNbO3 system is built by using one anionic dopant to substitute the O atom (A site in Figure 1) in the center of the supercell with another anionic dopant replacing one O atom at a B site (Figure 1). Similarly, the cationic−anionic codoped KNbO3 system is constructed by using one anionic dopant to replace the O atom (A site in Figure 1) in the center of the supercell with one cationic dopant to substitute for the Nb atom at a C site (Figure 1). The corresponding doping concentration for anionic−anionic or B


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Inorganic Chemistry

To find suitable growth conditions for doped systems, we also calculate the defect formation energy (Ef(D)) for a D defect system in the neutral charge state by using the definition63,64

cationic−anionic codoped KNbO3 is 5.0 at. %. Recently, the Rh doped SrTiO361 with dopant concentration of 10 at. % and the Bi−Yb codoped KNbO362 with the codopant concentration of 12 at. % have been successfully synthesized, so we believe the monodoped or codoped KNbO3 with the amount of dopants up to 5 at. % will be easily realized. We have also doped with two N atoms into a 3 × 3 × 3 KNbO3 supercell to examine whether the 2 × 2 × 2 supercell is big enough to avoid the self-interaction of N dopants. The obtained distance between these two N dopants in a 3 × 3 × 3 supercell of 1.378 Å is almost equal to that of 1.373 Å in a 2 × 2 × 2 supercell, which proves the 2 × 2 × 2 supercell is big enough to avoid the self-interaction of impurity. On the other hand, it was also confirmed by many previous studies41−50 that, for the codoped system, the double-hole-mediated codoping with two adjacent dopants has the lowest energy. To explore whether it is energetically favorable to form codoped systems in the neutral charge state, we calculate the defect binding energy (Eb) for X−Y codoped system using the following relationship:

E b = E(X) + E(Y) − E(pure) − E(X + Y)

Ef (D) = E(D) − E(pure) −

∑ niμi

(2)

i

where E(D) and E(pure), respectively, denote the total energies of the D defect and pure system. ni is the number of atoms added to (ni > 0) or removed from (ni < 0) the pure system to form the doped system; μi is the chemical potential of the corresponding atom, relying on the synthetic environment. When there exists equilibrium between KNbO3 and the reservoirs of Nb, K, O atoms, the chemical potential of the bulk KNbO3 (μKNbO3(bulk)), which is calculated from the total energy of the KNbO3 primitive cell, must be constrained by

μ Nb + μK + 3μO = μKNbO (bulk)

(3)

3

The chemical potentials of the host elements satisfy the following relationships:

(1)

Here, E(pure), E(X), E(Y), and E(X + Y) are the total energies of pure, X monodoped, Y monodoped, and X−Y codoped KNbO3, respectively. The positive value of defect binding energy means the codoped system is more favorable as compared with the corresponding monodoped system.

min μ Nb ≤ μ Nb ≤ μ Nb(bulk)

(4a)

μKmin ≤ μK ≤ μK(bulk)

(4b)

μOmin ≤ μO ≤ μO(gas)

(4c)

That is to say, the chemical potentials of Nb, K, and O are up to those of bulk Nb (μNb(bulk)), bulk K (μK(bulk)), and the O2 molecule (μO(gas)). For spontaneous formation of bulk KNbO3, the minima of μNb, μK, and μO satisfy

μKmin = E(K nNbnO3n) − E(K n − 1NbnO3n)

(5a)

min μ Nb = E(K nNbnO3n) − E(K nNbn − 1O3n)

(5b)

μOmin = E(K nNbnO3n) − E(K nNbnO3n − 1)

(5c)

where E(K n Nb n O 3n ), E(K n−1 Nb n O 3n ), E(K n Nb n−1 O 3n ), and E(KnNbnO3n−1) are, respectively, the total energies of pure, K defect, Nb defect, and O defect KNbO3 systems with n primitive cells. To facilitate discussion, we set ΔμO = μO − μO(gas), and ΔμNb = μNb − μNb(bulk). For dopants, we suppose bulk Sc, bulk Y, bulk Ti, bulk Zr, bulk V, bulk P2O5, gas SO2, gas CO2, and gas N2 act as Sc, Y, Ti, Zr, V, P, S, C, and N reservoirs, respectively. Following the above constraints,

Figure 2. DOS and PDOS for pure KNbO3. The vertical black dashed line indicates the Fermi level.

Figure 3. DOS and PDOS for (a) N, (b) P, (c) C, and (d) S monodoped KNbO3. The vertical black dashed lines indicate the Fermi levels. C


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Inorganic Chemistry

Figure 4. DOS and PDOS for (a) V, (b) Ti, (c) Zr, (d) Sc, and (e) Y monodoped KNbO3. The vertical black dashed lines indicate the Fermi levels. Here, ε1 and ε2 are the real and imaginary parts of the dielectric tensor, respectively. ε2 is given by71

the physically allowed region of chemical potentials and the defect formation energies of doped systems can be determined. Because there is no existence of absolute energy reference for different periodic systems, we use the preferred method proposed by Van de Walle et al.63,64 to obtain common energy reference levels by aligning the electrostatic potentials of the atoms far away from the doping centers between pure and defect systems. The same strategy has been adopted in a previous study on bandgap narrowing in doped La2Ti2O765,66 and our previous work on the photocatalytic activity of charge compensated codoped NaNbO3 and KNbO3.33,39 For KNbO3, the CBM position can be determined by the following formula67

ECBM = (χK χNb χO3 )1/5 − 0.5Eg + EO

(6a)

E VBM = ECBM + Eg

(6b)

ε2(ℏω) =

2ω

ε12(ω) + ε22(ω) − ε1(ω)

∑ |⟨ψkc|u·r|ψkv⟩|2 δ(Ekc − Ekv − ℏω) k ,v ,c

(8)

where Ω, v, c, ω, u, ψvk, and ψck are, respectively, the unit-cell volume, valence bands, conduction bands, photon frequencies, the vector defining the polarization of the incident electric field, and the occupied and unoccupied wave functions at point k in reciprocal space, while ε1 can be evaluated from ε2 by the Kramer−Kronig relationship:72

ε1(ω) = 1 +

2 p π

∫0

∞

ε2(ω′)ω′ d ω′ ω′ 2 − ω2

(9)

Here, p is the principal value of the integral.

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where ECBM and EVBM are, respectively, the potentials of CBM and VBM for the KNbO3 system; χO, χK, χNb denote the absolute electronegativity of O, K, and Nb atoms, respectively. Eg is the bandgap of experimental value, and EO, which denotes the scale factor relating the reference electrode redox level to the absolute vacuum scale, is −4.5 eV for the normal hydrogen electrode. Besides, χO, χK, and χNb are, respectively, 7.54, 2.42, and 4 eV.68 When the CBM and VBM potential levels are obtained, the CBM and VBM positions for doped systems are determined according to the CBM and VBM shifting with respect to those of the pure system. Next, we calculate the frequency dependent dielectric function to discuss the shift in the absorption spectrum by using the HSE06 functional. The absorption coefficient can be estimated by the following equation:69,70

I(ω) =

2e 2π Ωε0

RESULTS AND DISCUSSION The density of states (DOS) and projected density of states (PDOS) for pure KNbO3 are displayed in Figure 2. The obtained bandgap of 3.01 eV is slightly smaller than the experimental value of 3.14 eV.60 As we mainly focus on the effects of doping on the narrowing of the bandgap, the absolute value of bandgap is not a significant concern, and our obtained bandgap is big enough to estimate the bandgap reduction caused by doping. The valence and conduction band edges are mainly composed of occupied O 2p states and unoccupied Nb 4d states, respectively. However, no K electronic states appear around the CBM and VBM, which indicates that K only builds up the structure skeleton for KNbO3, but does not affect the electronic structure around the Fermi level. For the case of anionic monodoping, cationic monodoping,

(7) D


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Inorganic Chemistry

state and CBM as an effective bandgap, which has also been adopted in many previous theoretical studies.35,42,44,45,47,48 For N monodoped KNbO3, as a N dopant has one less valence electron than an O atom, the N doped KNbO3 is deficient by one electron. The DOS and PDOS are displayed in Figure 3a; the VBM has slightly moved up by 0.05 eV, and the CBM remains almost unchanged as compared with that of a pure system. Some occupied and unoccupied N 2p states mixing with O 2p states appear between VBM and CBM; this is due to the N 2p orbital energy being higher than the O 2p orbital energy.73 After doping, the effective bandgap reduces to 1.11 eV, which is beneficial to absorb visible light. However, the partially occupied impurity states around the Fermi level may act as an electron−hole recombination center, which will affect the photocatalytic performance. For P monodoped KNbO3, substituting one O atom by one P dopant will also act as a single acceptor, because the valence electron number for P is one less than that of O. The DOS and PDOS for a P doped system are shown in Figure 3b; some occupied localized states, which mainly originate from the mixing of P 3p and O 2p states, appear below the Fermi level. Besides, some unoccupied localized states mainly contributed by P 3p and Nb 4d states overlap with the conduction band edge. This should be attributed to the fact that the P 3p orbital energy is much higher than the O 2p orbital energy.73 The effective bandgap is only 0.40 eV, which is too narrow to make use of visible light. The DOS and PDOS of C monodoped KNbO3 are presented in Figure 3c; because the C 2p orbital energy is 2.3 eV higher than the O 2p orbital energy,73 and C has two fewer electrons than O, the C 2p states overlap with Nb 4d states. The VBM mainly consisting of C 2p, O 2p, and Nb 4d states shifts upward by 1.42 eV, while the CBM is dominated by contributions from Nb 4d states which decreased by 0.11 eV. It is mentioned that there is no appearance of unoccupied localized states above the Fermi level. Furthermore, the effective bandgap is narrowed to 1.48 eV, which makes it a promising visible light photocatalyst. For S doped KNbO3, the number of valence electrons for S is equal to that of O, but the S 3p orbital energy is 2.2 eV higher than the O 2p orbital energy.73 When one S atom substitutes for one O atom, S 3p states will interact with Nb 4d states. As seen from the DOS and PDOS depicted in Figure 3d,

Figure 5. Side view of the optimized structures of (a) pure, (b) N−N, (c) P−P, (d) N−P, (e) C−S, (f) V−C, (g) Ti−P, (h) Ti−N, (i) Zr−P, (j) Zr−N, (k) Sc−S, and (l) Y−S codoped KNbO3.

and double-hole-mediated codoping, the total number of electrons may either increase or decrease as compared to that of a pure system, so spin-polarization calculations have been adopted to treat these monodoped systems. It is mentioned that, usually, the bandgap of semiconductor is the energy difference between the CBM and VBM. However, for the doped semiconductors, there are impurity states appearing between the valence band maximum and the conduction band minimum; we define the energy difference between an impurity localized

Figure 6. DOS and PDOS for (a) N−N, (b) P−P, (c) N−P, and (d) C−S codoped KNbO3. The vertical black dashed lines indicate the Fermi levels. E


Article

Inorganic Chemistry

Figure 7. DOS and PDOS for (a) V−C, (b) Ti−P, (c) Ti−N, (d) Zr−P, (e) Zr−N, (f) Sc−S, and (g) Y−S codoped KNbO3. The vertical black dashed lines indicate the Fermi levels.

composed of Nb 4d states. The effective bandgaps for Ti and Zr monodoped KNbO3 are, respectively, 2.35 and 2.72 eV. The difference between Ti and Zr doped KNbO3 should be attributed to the different d orbital energy. Though the bandgap reductions are helpful to enhance the visible light absorption, the partially unoccupied states appearing above the Fermi level will serve as electron−hole recombination centers, which will suppress the photocatalytic activity. Sc or Y has two fewer valence electrons than Nb, so Sc and Y doped KNbO3 are both p-type semiconductors. The DOS and PDOS for Sc and Y monodoped KNbO3 are presented in Figure 4d,e; there exist impurity states between the VBM and CBM for both Sc and Y monodoped systems. These unoccupied impurity states reduce the effective bandgap and make the photoexcited electron require lower energy to be transferred to the conduction band. However, the empty states above the Fermi level are unfavorable for photocatalytic activity as they may trap photoexcited charge carriers and promote the electron−hole recombination. So, Sc or Y monodoping is not a good choice to improve the photocatalytic performance of KNbO3. In the case of N−N codoped KNbO3, the doped system has two net holes. As displayed in Figure 5a,b, the N−N distance is

there appear occupied localized states contributed by S 3p states and O 2p states above the valence band, while the CBM consisting of Nb states decreases by 0.06 eV. The effective bandgap is 2.10 eV without unoccupied localized states appearing above the Fermi level, which is helpful for absorbing visible light. Next, let us consider the case of cationic monodoping. For the case of V doped KNbO3, because V and Nb lie in the same column of the periodic table, and V is isoelectronic to the Nb atom, it may be expected that the V monodoping will not significantly change the electronic properties. We have displayed the DOS and PDOS of V doped KNbO3 in Figure 4a; the VBM mainly dominated by O 2p states moves up by 0.16 eV, and the CBM contributed by both Nb 4d and V 3d states decreases by 0.60 eV. The reason is that the V 3d orbital energy is lower than the Nb 4d orbital energy. The effective bandgap is narrowed to 2.25 eV, and the bandgap reduction is good enough for the visible light photocatalysis. For the case of Ti and Zr monodoping, as a Ti or Zr atom has one less valence electron than the Nb atom, Ti or Zr monodoped KNbO3 is a p-type semiconductor. The DOS and PDOS for Ti and Zr doped KNbO3 are plotted in Figure 4b,c; the valence band edges for them are both contributed by O 2p states, and the conduction band edges are both F


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Inorganic Chemistry Table 1. Calculated Defect Binding Energies (Eb’s) of Codoped KNbO3, the Minimum Defect Formation Energies (Ef) for Doped KNbO3 with Respect to Pure KNbO3, and the Corresponding Chemical Potentials of Host Elementsa system N P C S N−N P−P N−P C−S V Ti Zr Sc Y V−C Ti−P Ti−N Zr−P Zr−N Sc−S Y−S a

Eb

1.88 5.33 4.00 2.93

1.13 0.25 2.28 0.16 1.60 1.47 1.09

ΔμO

ΔμNb

Ef

−4.64 −4.64 −4.64 −4.64 −4.64 −4.64 −4.64 −4.64 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 −13.91 −13.91 −13.91 −13.91 −13.91 −13.91 −13.91 −13.91 −13.91 −13.91 −13.91 −13.91

1.14 12.01 9.61 4.67 0.38 18.69 9.14 11.36 −11.81 −12.86 −13.83 −11.75 −11.13 1.32 1.51 −7.34 1.23 −8.22 −3.92 −2.92

the DOS and PDOS have been plotted in Figure 6b. The VBM mainly contributed by O 2p states decreases by 0.05 eV, while the CBM composed of Nb 4d states moves upward by 0.31 eV. As each P atom has one unpaired 3p electron, when these two neighboring P atoms move toward each other, these two partially filled P 3p orbitals couple with each other, resulting in a bonding and an antibonding state. The coupling between these two P 3p orbitals introduces three fully filled sub-bands above the VBM, and the antibonding state lies in the conduction band, so no unfilled states appear above the Fermi level. Besides, because the P 3p orbital energy is higher than that of the O 2p orbital, the P−P codoping gives a bandgap of 0.76 eV. The N−P codoping adds two holes to the KNbO3 system, where one comes from the P atom and the other one from the N atom. The DOS and PDOS for N−P codoped KNbO3 are shown in Figure 6c; the VBM composed of O 2p states moves up by 0.11 eV, whereas the CBM owning Nb 4d characters goes up by 0.24 eV. As either the N or P atom has one unpaired electron, when the N and P atoms move close to each other, the unpaired N 2p electron and P 3p electron interact with each other and lead to a bonding state and an antibonding state. The bonding state sits close to the valence band, and the antibonding state lies in the conduction band, so no unfilled localized states are between the VBM and CBM. Moreover, the fully filled impurity states contributed by the mixing of N 2p, P 3p, and O 2p states appear above the VBM. The effective bandgap of the N−P codoped system is reduced to 0.91 eV. The N−P distance is 1.676 Å (Figure 5d), which is much smaller than the O−O distance of 2.845 Å for the pure KNbO3 system. This also reflects strong double-hole-mediated coupling between N 2p and P 3p orbitals. We also investigate the C−S codoped KNbO3. Many other studies43−48 have shown that C−S codoping will obviously reduce the bandgap by the double-holemediated coupling. The distance between the C and S atom is 1.734 Å, much shorter than the O−O distance of 2.845 Å. This also proves the strong coupling between C and S atoms. The DOS and PDOS are plotted in Figure 6d; the VBM composed of O 2p states is lifted up by 0.07 eV, whereas the CBM contributed by Nb 4d states goes up by 0.19 eV. When the C atom and S atom move toward each other, they may also couple with each other and form a bonding state and an antibonding state. However, the case for the C−S codoped system is different from those of N−N, P−P, and N−P codoped systems; the two holes are contributed by the C atom. As the bonding state sits close to the valence band and the antibonding state goes into conduction band, all the localized states between VBM and CBM are occupied. The occupied impurity states above the VBM are dominantly contributed by the mixing of C 2p and S 3p states, and the effective bandgap for C−S codoped KNbO3 is 0.85 eV, which is very small as compared with that for the pure KNbO3 system. Next, we discuss the effect on the electronic structure of KNbO3 caused by the cation−anion double-hole-mediated codoping: here, we consider V−C, Ti−P, Ti−N, Zr−P, Zr−N, Sc−S, and Y−S codoped systems, which all have two net holes. As shown in Figure 5f−l, the distances of C−O in the V−C codoped system, P−O in the Ti−P codoped system, N−O in the Ti−N codoped system, P−O in the Zr−P codoped system, N−O in the Zr−N codoped system, S−O in the Sc−S codoped system, and S−O in the Y−S codoped system are, respectively, 2.702, 2.794, 2.886, 2.821, 2.988, 2.936, and 2.956 Å; these distances are close to the distance of 2.845 Å between the two adjacent O atoms in pure KNbO3. This suggests that there is no appearance of coupling between doped anions and O atoms, which is quite different from the case of Ga−N,41 Sc−P, In−P,34

All the units are in eV.

Figure 8. Physically accessible region of the potentials in ΔO and ΔμNb planes for pure KNbO3.

1.373 Å, which is much smaller than the two O atom distance of 2.845 Å for the pure KNbO3 system. This indicates the strong coupling between the two N atoms. The DOS and PDOS for N−N codoped KNbO3 are presented in Figure 6a; the VBM consisting of O 2p states does not shift, and the CBM contributed by Nb 4d states shifts upward by 0.16 eV. Because each N atom has one unpaired electron and the half-filled state of each N atom is energetically slightly higher than the two filled states, when these two adjacent N atoms move close to each other, these two unpaired p electrons will hybridize with each other and form a bonding state and an antibonding state. The occupied bonding state is located close to the valence band, and the unoccupied antibonding state lies in the conduction band. Therefore, there is no appearance of unfilled localized states above the Fermi level, and the effective bandgap is reduced to 1.32 eV. No acceptor states appear above the Fermi level, so the recombination losses will be less as compared to the N monodoped system. The N−N codoped system is a good candidate for visible light photocatalysis. The P−P codoped KNbO3 is very similar to the N−N codoped system, as it also has two net holes. There exists a weak coupling between the two P atoms, as the P−P distance is 2.048 Å (Figure 5c). To understand the effect of the P−P double-hole-mediated codoping, G


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Inorganic Chemistry

Figure 9. Physically accessible region of the chemical potentials in ΔμO and ΔμNb plane for (a) N, (b) P (c) C, (d) S, (e) V, (f) Ti, (g) Zr, (h) Sc, and (i) Y monodoped KNbO3. The color of the map corresponds to the value of defect formation energy, and all the units are eV.

Mg−S, 2Al−S, Ca−S, and 2Ga−S49 codoped anatase TiO2, where doped metal atoms assist the coupling of the incorporated anions with the neighboring O atoms. The reason for no coupling occurring may be that when the metal atom substitutes the Nb atom and the anion substitutes the O atom adjacent to the metal dopant in the KNbO3, the four O sites adjacent to the anion dopant are all equivalent, and the interaction between the metal atom and the adjacent four O atoms are equal; i.e., the high symmetry prevents the occurrence of coupling. The DOS and PDOS for V−C, Ti−P, Ti−N, Zr−P, Zr−N, Sc−S, and Y−S codoped systems are presented in Figure 7a−g; the effective bandgaps for all these cation−anion codoped systems have been obviously reduced, but unoccupied dopant states appear between the VBM and CBM in Ti−N, Zr−P, Zr−N, Sc−S, Y−S codoped systems. These unoccupied impurity states may promote electron−hole recombination and reduce the photogenerated current. The DOS and the PDOS of V−C codoped KNbO3 in Figure 7a show that the VBM mainly contributed by O 2p and C 2p states shifts upward by 0.80 eV, whereas the CBM mainly composed of Nb 4d states and V 3d states moves downward by 0.21 eV. The effective bandgap is 2.00 eV, and no partially unoccupied states are above the Fermi level. The C 2p orbital energy is higher than the O 2p orbital energy,73 and the V 3d orbital energy is lower than the Nb 4d orbital energy,26 which causes the VBM to shift up and the CBM to move down, so the effective bandgap has been reduced. Besides, the C 2p states and V 3d states have been incorporated into the valence and conduction bands, so no impurity unfilled states

appear above the Fermi level. For the Ti−P codoped KNbO3, the DOS and PDOS are illustrated in Figure 7b. The VBM is mainly composed of O 2p and P 3p states, and the CBM is dominated by contributions from the Nb 4d state. The VBM moves up by 1.14 eV, and the CBM slightly shifts up by only 0.01 eV. The effective bandgap is reduced to 1.88 eV without unoccupied impurity states above the Fermi level, which is beneficial for photocatalysis. Since the P 3p orbital energy is higher than the O 2p orbital energy73 and the Ti 3d orbital energy is lower than the Nb 4d orbital energy,26 which causes the VBM to shift up and the CBM to shift down, the effective bandgap is narrowed. Moreover, the P 3p and Ti 3d states have been incorporated into the valence and conduction bands, so no unoccupied states appear above the Fermi level. As displayed in Table 1, the calculated defect binding energies for the codoped systems we have considered are all positive values, which indicates that both anionic−anionic and cationic− anionic codoping are energetically favorable as compared with the corresponding individual monodoping in KNbO3. The reason should be attributed to the strong double-hole-mediated coupling for the N−N, P−P, N−P, and C−S codoped systems and the strong coulomb interaction between dopants and hosts for V−C, Ti−P, Ti−N, Zr−P, Zr−N, Sc−S, and Sc−Y codoped systems. We further discuss the defect formation energies of the monodoped and codoped KNbO3 under O-rich or O-poor conditions to evaluate the stability of doped KNbO3 and to find a suitable growth environment. Figure 8 gives the physically accessible region of the chemical potentials for pure KNbO3 in H


Article

Inorganic Chemistry

Figure 10. Physically accessible region of the chemical potentials in ΔμO and ΔμNb plane for (a) N−N, (b) P−P (c) N−P, (d) C−S, (e) V−C, (f) Ti−P, (g) Ti−N, (h) Zr−P, (i) Zr−N, (j) Sc−S, and (k) Y−S codoped KNbO3. The color of the map corresponds to the value of defect formation energy, and all the units are eV.

the ΔμO and ΔμNb. In addition, the calculated defect formation energies dependent on ΔμO and ΔμNb for monodoped and codoped systems are, respectively, presented in Figures 9 and 10; the minima defect formation energies are given in Table 1. The defect formation energies for N, P, C, S monodoped systems decrease with the decline of the chemical potential of the O atom; i.e., they are energetically favorable for formation under O-poor conditions. The is because the O vacancies, which are necessary for the occupation of anionic dopants, are easy to form under O-poor conditions. Similarly, the defect formation energies for V, Ti, Zr, Sc, and Y monodoped systems increase with the increase in chemical potentials of the Nb atom, and they are easy to form under Nb-poor conditions. The N−N, P−P, N−P, and C−S codoped KNbO3 are energetically favorable under O-poor and Nb-rich conditions, while the V−C, Ti−P, Ti−N, Zr−P, Zr−N, Sc−S, and Y−S codoped systems are energetically favorable under O-rich and Nb-poor conditions. Besides, the defect formation

energies for cationic−anionic codoped systems are lower than the corresponding anionic monodoped systems. This means anionic doping could be obviously promoted by cationic doping. However, a suitable bandgap does not necessarily mean there is enhancement of the visible light photocatalytic activity. The valence and conduction band edge potentials also play an important role in determining the thermodynamic possibility of the photo-oxidation and photoreduction reactions. This usually requires that the VBM and CBM positions must straddle the water redox level. On the basis of the VBM and CBM positions, some materials may be good for only hydrogen reduction reaction, and some are good for only water oxidation. We could employ two different photocatalysts for the hydrogen evolution and oxygen evolution, which is called the Z-scheme photocatalysis system.74 Here, the band alignments for pure and doped KNbO3 with respect to the water redox level are displayed in Figure 11. The obtained CBM and VBM positions for cubic I


Article

Inorganic Chemistry

Figure 11. Calculated VBM and CBM positions of pure and C, S, V monodoped, and N−N, P−P, N−P, C−S, V−C, Ti−P codoped KNbO3 with respect to the water redox level. The horizontal dotted lines indicate the energy levels of redox potentials of H+/H2 (0 eV vs NHE) and O2/H2O (1.23 eV).

KNbO3 are, respectively, −0.78 and 2.36 eV. For doped systems, we only consider the C, S, V monodoping and N−N, P−P, N−P, C−S, V−C, Ti−P codoping, as these dopings lead to a suitable effective bandgap for visible light photocatalysis without the appearance of unoccupied impurity states. It is possible for pure KNbO3 to split water into hydrogen and oxygen, as both the hydrogen evolution potential and oxygen evolution potential lie between VBM and CBM. It is also possible for S, V monodoped and V−C codoped systems to produce oxygen and hydrogen. For the N−N codoped system, the intermediate states are above the water oxidation potential level, which is thermodynamically unfavorable for oxygen production. The VBM of Ti−P codoped KNbO3 is close to the VBM of oxygen evolution potential, which is not good for oxygen production. However, N−N and Ti−P codoped KNbO3 may be considered for Z-scheme photocatalysis. For P−P, N−P, and C−S codoped systems, the intermediate states are higher than or close to the hydrogen evolution potential, which is thermodynamically unfavorable for production of both oxygen and hydrogen. Finally, we calculate the frequency dependent dielectric function to discuss the shift in the absorption spectrum. Here we only calculate the optical spectra of pure; C, S monodoped; and N−N, V−C, Ti−P codoped KNbO3, as our discussion earlier indicates that only C, V, S monodoped and N−N, V−C, Ti−P codoped systems have suitable bandgaps and band edge positions. The optical absorption spectra of pure; C, V, S monodoped; and N−N, V−C, Ti−P codoped systems are displayed in Figure 12; these reveal that pure KNbO3 can only

absorb ultraviolet light. The C, V, S monodoping and N−N, V−C, Ti−P codoping give reasonable bandgap reductions, which significantly shift the optical absorption toward the visible light region.

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CONCLUSIONS Hybrid density functional calculations have been applied to explore whether double-hole-mediated codoping will improve the photocatalytic activity of KNbO3 as compared to the corresponding individual doping. The codoped KNbO3 samples we considered are energetically favorable as compared to the corresponding individual samples, due to the fact that the strong double-hole-mediated dopant−dopant coupling occurs in anionic−anionic codoped systems and there is strong Coulomb interaction between dopants and hosts in cationic−anionic codoped systems. The anionic monodoped and anionic−anionic codoped KNbO3 are energetically favorable under O-poor and Nb-rich growth conditions, and the cationic monodoped and cationic−anionic coopded systems are energetically favorable under O-rich and Nb-poor growth conditions. Double-holemediated dopant−dopant coupling appears in the N−N, P−P, N−P, and C−S codoped systems, which will significantly reduce the effective bandgaps. No metal-assisted coupling occurs in the V−C, Ti−P, Ti−N, Zr−P, Zr−N, Sc−S, and Y−S codoped systems, as the O atoms around the anion dopant are equivalent. For Ti−N, Zr−P, Zr−N, Sc−S, and Y−S codoped systems, the effective bandgaps are also obviously narrowed, but the appearance of unoccupied localized states between the VBM and CBM serving for electron−hole recombination suppresses the photocatalytic activity. V−C and Zr−P codoping could reduce the bandgaps without the appearance of impurity located states above the Fermi level, as the C 2p, P 3p, and V 3d states have been incorporated into valence and conduction bands. For N, Ti, Zr, Sc, and Y monodoped systems, unoccupied localized states appear above the Fermi level, which limits the photocatalytic performance. The P, C, S, and V monodoping could reduce the bandgap without introducing unoccupied localized states, but the bandgap of the P doped system is too small and is unfavorable for visible light photocatalysis. The intermediate states of P−P, N−P, and C−S codoped systems are higher or close to the hydrogen evolution potential, which makes them thermodynamically unfavorable for production of both oxygen and hydrogen. S, V monodoped and V−C codoped systems are good

Figure 12. Calculated absorption curves for the pure; C, S, V monodoped; and N−N, V−C, Ti−P codoped KNbO3. J


Article

Inorganic Chemistry

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choices for visible light photocatalysis as they have appropriate bandgaps with respect to the water redox levels. Producing hydrogen only, C monodoped and Ti−P, N−N codoped KNbO3 are suitable for Z-scheme photocatalysis. The calculated optical absorption curves of C, V, S monodoping and N−N, V−C, Ti−P codoping significantly improve the visible light absorption. We believe that our findings will encourage experimentalists to design highly efficient KNbO3 based visible light photocatalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-023-68367040. Fax: +86-023-68367040. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Grants 11175146 and 10904125, the Natural Science Foundation of Chongqing under Grants CSTC2011BA6004 and CSTC-2008BB4253, and the Fundamental Research Funds for the Central Universities under Grants XDJK2012C038, XDJK2014D044, and XDJK2015C045.

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