Understanding Photoelectrochemical Properties of B–N Codoped

Article pubs.acs.org/JPCC

Understanding Photoelectrochemical Properties of B−N Codoped Anatase TiO2 for Solar Energy Conversion Mang Niu, Daojian Cheng,* and Dapeng Cao* Division of Molecular and Materials Simulation, State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: Understanding the band gap narrowing of anatase TiO2 induced by B−N codoping is attractive and significant for their potential applications in renewable energy by converting sunlight to electricity or fuels. In this work, we use hybrid density functional calculations to investigate the electronic structures of B−N codoped TiO2 and further explore the mechanism of band gap narrowing of anatase TiO2 induced by B−N codoping. It is found the band gap narrowing of anatase TiO2 induced by the B-assisted N−O coupling effect (i.e., the substitution of Ti by B and the substitution of O by N, marked as (B[sub], N) codoping) is more effective than the compensation effect between the interstitial B donor and the substitutional N acceptors on O site (marked as (B[int], 3N) codoping). Results indicate that the (B[sub], N) codoped anatase TiO2 is an intrinsic semiconductor with a band gap of 1.762 eV, exhibiting a figure-of-merit for photoelectrochemical (PEC) catalysis in the visible light region. By considering the formation energy, we suggest adopting the strong O-rich environment to synthesize the (B[sub], N) codoped anatase TiO2. Actually, the B-assisted N−O coupling effect could significantly improve the visible light PEC performance of anatase TiO2. It is expected that this work can provide valuable information for design of new TiO2-based photocatalysts.

1. INTRODUCTION Semiconductor-based photocatalysts (for example, anatase TiO2) have attracted increasing attention owing to their potential applications in renewable energy by converting sunlight to electricity or fuels.1−4 For example, the photoelectrochemical (PEC) water splitting on TiO2 displays the great advantage of hydrogen production by harvesting solar energy.5−7 However, the activity of TiO2 for PEC water splitting is very low under solar light irradiation because its wide band gap (3.2 eV for anatase TiO2) only absorbs ultraviolet light (about 5% of solar spectrum) for photocatalytic activation. To achieve high performance of PEC water splitting under visible light irradiation, the desirable photocatalyst should have an appropriate band gap (1.6−2.2 eV) and correct band-edge positions that matched with the redox potentials of water.5−7 In the past decade, great efforts have been devoted to the band gap engineering of TiO2. The most common approach to improve the visible-light photocatalytic performance of TiO2 is to dope TiO2 with foreign elements.8−14 The dopants are including nonmetals (C, N, F, S, etc.),8−10 transition metals (V, Mn, Fe, etc.),11,12 and rare earths (La, Ce, Er, etc.).13,14 However, the experimental studies show that the PEC efficiency of these monodoped systems is very low because of the formation of recombination centers and the strongly localized states of monodoping.15−17 To overcome the shortcomings of monodoped TiO2, two most important © 2013 American Chemical Society

theoretical concepts have been proposed. One is the passivated codoping approach with the coincorporation of donor− acceptor pairs,6 and the other one is the double-hole-mediated coupling of dopants. 18 It is believed that the charge compensation effect in donor−acceptor codoped TiO26,19−21 and the coupling effect in certain two-acceptor codoped TiO218,22 can not only reduce the band gap of TiO2 effectively but also prevent the recombination of photogenerated carriers. Recently, several experimental studies on the synthesis of B− N codoped anatase TiO2 through various preparation methods have been published.23−27 It is reported that the B−N codoped anatase TiO2 exhibits high activity of dye degradation23−26 or PEC water splitting27 under visible light irradiation. However, the origin of the visible-light responsive B−N codoped TiO2 is still under debate. For instance, on the basis of standard density functional theory (DFT)28 calculation, Liu et al.27 proposed that the enhanced photocatalytic performance of the red B−N codoped anatase TiO2 derives from the charge compensation between the interstitial B atoms (donors) and the substitutional N atoms (acceptors) on O sites, while Uddin et al.26 suggested that the production of unoccupied midgap states below the conduction band minimum (CBM) of TiO2 induced by B and N codoping enhanced the photocatalytic activity of TiO2. Received: April 18, 2013 Revised: July 14, 2013 Published: July 16, 2013 15911

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Unfortunately, the standard DFT often fails to predict the correct band gaps of transition metal oxides like TiO229−31 due to the self-interaction inherent error in such functional (Liu et al.27 calculated band gap of pure anatase TiO2 is only 2.12 eV, which is much smaller than their experimental value of 3.22 eV). Therefore, the more accurate calculation method is necessary to further understand the photoresponse mechanisms of the B−N codoped anatase TiO2. One effective approach for correcting the self-interaction error is the hybrid density functional, which is well suitable to describe the electronic structures and dopant levels of pure or doped TiO 2 systems.22,32 It is recognized that the calculation of electronic structures is helpful to understand the mechanism of enhanced the PEC performance of photocatalysts and is also very useful to further design new photocatalysts.6,18 In order to illustrate how the coincorporation of B and N change the electronic structures of the host anatase TiO2 and to further explain the origin of the enhanced photocatalytic activity of B−N codoped anatase TiO2, here we performed a systematical study on the B−N codoped anatase TiO2 systems by using the hybrid density functional theory.

Figure 1. Inner part of the optimized supercell geometries for (a) pure anatase TiO2, (b) interstitial B monodoped TiO2, (c) (B[int], 3N) codoped TiO2, and (d) (B[sub], N) codoped TiO2. HSE E XC =

2. CALCULATION METHODS In this study, the first-principles calculations were performed by using the projector augmented wave (PAW)33,34 pseudopotentials in the Vienna Ab-initio Simulation Package (VASP).35,36 The Perdew−Burke−Ernzerhof (PBE)37 parametrization of generalized gradient approximation (GGA) was adopted to describe the exchange and correlation potentials. The cutoff energy of plane-wave basis was set to 500 eV. The standard density functional theory calculation (GGA-PBE) was performed for the geometry optimization with a Monkhorst− Pack38 k-point mesh of 12 × 12 × 5 for the primitive cell of anatase TiO2 and 7 × 7 × 5 for the 2 × 2 × 1 anatase supercell. Both the cell parameters and atomic positions were optimized until the force on each ion was smaller than 0.01 eV/Å. The 2 × 2 × 1 anatase supercell contains 32 O atoms and 16 Ti atoms. Such a supercell model has been successfully used to theoretically investigate various elements codoped TiO2.7,18 The doped TiO2 systems were constructed by inducing dopants to the inner part (shown in Figure 1a) of the 2 × 2 × 1 anatase supercell. As is shown in Figure 1b, the interstitial B doping is constructed by placing one B atom at the center of four O atoms (O1, O2, O3, and O4 atoms), and the substitutional N doping is modeled by replacing the O atoms with N atoms (see Figure 1c). Hybrid functionals can provide a more accurate band gap for TiO2 systems but need much more CPU time especially for the case of using a plane wave basis set (VASP).32 The geometry optimization using hybrid functional is very heavy and could not be performed here. It is known that DFT-GGA gives good structures of oxides compared to experimental data. Therefore, the electronic structures were calculated based on the GGAPBE optimized geometries by using the Heyd−Scuseria− Ernzerhof (HSE06)39,40 hybrid functional. In HSE06 hybrid functional, the exchange contribution is divided into short- and long-ranged parts, and the short-ranged part of PBE exchange is weighted by 25% Hartree−Fock (HF) exchange. The expression for the exchange-correlation in HSE06 is given by

1 HF,SR 3 EX (μ) + E XPBE,SR (μ) + E XPBE,LR (μ) 4 4

+ ECPBE

(1)

where SR and LR refer to the short- and long-ranged parts of the exchange interaction, μ is the parameter that defines the range-separation of Coulomb kernel, and μ = 0.2 Å−1 in this work. To reduce the HSE06 computational costs,32,40 density of states (DOSs) and band structures were calculated by using a reduced Monkhorst−Pack k-point mesh of 5 × 5 × 5 and 3 × 3 × 3, respectively. The convergences with respect to the cutoff energy and the k-point mesh have been checked to be within 0.001 eV.

3. RESULTS AND DISCUSSION 3.1. Pure Anatase TiO2. The optimized lattice parameters of primitive anatase TiO2 are a = b = 3.821 Å and c = 9.683 Å (see Table 1), which are in good agreement with experimental values of a = b = 3.782 Å and c = 9.502 Å.41 The calculated band structure of pure anatase TiO2 is displayed in Figure 2. As is shown in Figure 2, the CBM of pure anatase TiO2 is located at Gamma (G) points and the valence band maximum (VBM) is along M → G. The HSE06 calculation results indicated that the anatase TiO2 is an indirect band gap semiconductor with Table 1. Formation Energies Eform (eV) and Lattice Constants (Å) of Different Kinds of TiO2 formation energy Eform (Ti-rich)

structure pure B doped N doped theoretical [ref 50] (B[int], 3N) codoped (B[sub], N) codoped 15912

Eform (O-rich)

0.108 0.801 0.69 −4.757

0.108 5.237 5.69 8.551

4.525

8.406

lattice constants a

b

c

3.821 3.831 3.818

3.821 3.831 3.837

9.683 9.795 9.688

3.795

3.798

9.937

3.784

3.811

9.749

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Figure 2. HSE06 calculated band structures of pure anatase TiO2, (B[int], 3N) codoped, and (d) (B[sub], N) codoped anatase TiO2 along high symmetry lines of Brillouin zone. The energy zero is taken as the Fermi level and displayed with a red dashed line.

the band gap of 3.26 eV, which is consistent with the experimental result of 3.2 eV41 and the previous HSE06 calculated value of 3.30 eV.32 Apparently, the HSE06 functional reveals a significant improvement for electronic structure calculation of the TiO2 system compared to standard DFT calculation results. 3.2. Interstitial B Monodoped and Substitutional N Monodoped Anatase TiO2. A few B−N codoped anatase TiO2 models jointed with the corresponding experimental investigations25−27 have been proposed to supporting the experimental results. However, most of these models were constructed without a theoretical basis. Considering the processes of experiments, we believed that the B−N codoped TiO2 model from Liu et al. is rational, but the standard DFT used by them often fails to predict the correct band gaps of TiO2.27 Therefore, in order to illustrate how the coincorporation of interstitial B and substitutional N affects the electronic structures of the host anatase TiO2, we first used the HSE06 hybrid functional calculations to investigate the interstitial B monodoped and substitutional N monodoped anatase TiO2. The electronic structures of the host anatase TiO2 were also calculated for comparison. The HSE06 calculated total density of states (TDOSs) and projected density of states (PDOSs) for pure TiO2, interstitial B monodoped TiO2, and substitutional N monodoped TiO2 are displayed in Figure 3. As can be seen from the TDOSs and PDOSs of pure TiO2, the valence band (VB) is dominated by O-2p states, including some hybridization with the Ti-3d orbitals. On the other hand, the conduction band (CB) consists mainly of Ti-3d states. These results are consistent with the previous theoretical studies.6,21,42 The optimized inner part of the supercell geometry for interstitial B monodoped anatase TiO2 is shown in Figure 1b. It is found that the doped interstitial B atom attracts the surrounding four O atoms, forming the tetragonal BO4 unit.27,43 As a result, the Ti−O bonds nearby the B atom were weakened, which facilitates the substitution of O in BO4 unit by N.27 As seen from the TDOSs and PDOSs of interstitial B monodoped anatase TiO2 in Figure 3, the Fermi level locates at the bottom of CB, which is a typical n-type doping. Moreover, the incorporation of interstitial B in anatase TiO2 does not induce impurity level within the band gap but splits the Ti-3d states nearby the Fermi level. This splitting comes from the reduction of Ti4+ to Ti3+ by the interstitial B doping: the excess electrons of interstitial B donor are delocalized over several Ti atoms.43 As B has three valence electrons (2s22p1), the incorporation of interstitial B in TiO2 acts as a triple donor. It is known that Ti3+ states are localized about 0.8 eV below the

Figure 3. HSE06 calculated TDOSs (left side) and the corresponding PDOSs (right) of pure and interstitial B monodoped, substitutional N monodoped, (B[int], 3N) codoped, and (B[sub], N) codoped anatase TiO2. The O-2p states in the PDOSs plot of (B[sub], N) codoped anatase TiO2 were inverted for clarity. The Fermi level of all these systems is displayed with a red dashed line.

CB of the reduced TiO2.44,45 The localized Ti3+ states are usually reproduced by the pure hybrid-functional calculations (hybrid-functional electronic structures with hybrid-functional optimized geometry) and the hybrid-functional electronic structures with DFT optimized geometry often give partially localized pictures of Ti3+ states.46 However, our HSE06 calculation result with PBE optimized geometry is very similar to the DOSs of interstitial B doped TiO2 calculated by pure hybrid-functionals (B3LYP), which provide the partially localized Ti3+ states.43 This partially localized Ti3+ states result from the absence of a pronounced polaronic distortion of the lattice around Ti3+ ion.46 It should be noticed that the Ti3+ ions 15913

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codoped anatase TiO2, one hole is provided by the substitutional B on Ti site (B acts as a single acceptor) and the other hole is provided by the substitutional N on O site (Ti1 and O2 atoms were replaced by one B and one N atom, respectively; see Figure 1d). It is noticed that this (B[sub], N) codoping arrangement is structurally helpful for the strong coupling of dopants, which is in good agreement with the previous theoretical works.18,49 The optimized inner part of the supercell geometry for (B[sub], N) codoped anatase TiO2 is shown in Figure 1d. It is found that the incorporated N forms the N−O bond (bond length of 1.345 Å) with the nearest-neighboring O1 atom. However, in substitutional N monodoped TiO2, the distance between N (O2 atom) and O1 is 2.782 Å, which is close to the O2−O1 (see Figure 1a) distance of 2.786 Å in pure anatase TiO2, suggesting the absence of N−O bond at this case. As shown in Figure 1d, the doped B atom forms Ti−O−B bond and the doped N atom forms Ti−O−N and Ti−N−O bonds, which are consistent with the XPS studies that the incorporated B forms Ti−O−B or Ti−B−O or O−Ti−B bonds and the incorporated N forms O−Ti−N or Ti−O−N or Ti−N−O linkages.26 It indicates that this B-assisted N−O coupling is very similar to the acceptor metals assisted S−O coupling that has been reported in our previous study.22 As shown in the band structure of (B[sub], N) codoped anatase TiO2 in Figure 2, the (B[sub], N) codoped anatase TiO2 is still a semiconductor that has a narrower band gap than the pure anatase TiO2. In addition, there are two fully occupied energy levels appear inside the band gap of TiO2. The new band gap after (B[sub], N) codoping is determined by the positions of these fully occupied energy levels and the CBM of TiO2. The obtained band gap of (B[sub], N) codoped anatase TiO2 is 1.762 eV, which is about 1.5 eV smaller than that of pure anatase TiO2. It suggests that the band gap narrowing induced by (B[sub], N) codoping is much more effective compared with the case induced by (B[int], 3N) codoping. Since the fully occupied energy levels play an important role in the band gap reduction of TiO2, understanding the origin of these fully occupied energy levels is necessary. Therefore, we calculated the TDOSs and PDOSs of (B[sub], N) codoped anatase TiO2. For clarity, the other partial states were not shown and the O-2p states of the bonding O atom were inverted in the PDOSs of (B[sub], N) codoped anatase TiO2. As seen from the TDOSs and PDOSs of (B[sub], N) codoped anatase TiO2 in Figure 3, the two fully occupied states just below the Fermi level of (B[sub], N) codoped anatase TiO2 are comprised of the N-2p orbital and the 2p orbital of its neighboring O atom (in N−O bond). The B-assisted N−O coupling through (B[sub], N) codoping induces two fully occupied states in the band gap of the anatase TiO2, which not only reduces the band gap of TiO2 effectively but also prevents the recombination of photogenerated electron−hole pairs. Therefore, our calculation results indicate that the enhanced photocatalytic performance of B−N codoped anatase TiO2 in visible light region derives from the B-assisted N−O coupling in (B[sub], N) codoped anatase TiO2 rather than the charge compensation between B donor and N acceptor in (B[int], 3N) codoped TiO2. The mechanism of B-assisted N−O coupling in (B[sub], N) codoped anatase TiO2 was also proposed and shown in Figure 4. The incorporation of B on Ti site induces a hole that can capture one electron. Because the electronegativity of N is weaker than that of O, an electron in N-2p orbital is captured

exist in the n-type TiO2 systems (such as reduced or interstitial B doped TiO2). For the p-type and intrinsic TiO2 systems, our strategy of calculating the electronic properties with HSE06 hybrid functionals on the GGA-PBE optimized structures is still reasonable.32,47 As shown in the TDOSs and PDOSs of substitutional N monodoped anatase TiO2 in Figure 3, when the N was used to substitute for O (O2 atom was replaced by one N atom; see Figure 1a), the partially occupied N-2p states appear above VBM of anatase TiO2, which is in good agreement with previous theoretical results.42,48 Although these partially occupied N-2p impurity states lead to the enhanced visible light absorption in N-doped TiO2, the N anion act as a recombination center and these localized N-2p states also results in the reduction of carrier mobility. Generally, the position of acceptor level in TiO2 is determined by the anion’s p orbital energies.6 Because the neutral N-2p orbital energy is 2.0 eV higher than the O-2p orbital energy, the acceptor levels induced by N is higher than O-2p states in VB of TiO2 (about 0.6 eV above the VBM). The incorporation of substitutional N on O site in TiO2 acts as a single acceptor because N has one less valence electron than O. 3.3. (B [int] , 3N) Codoped Anatase TiO 2 . In the compensation effect, the electrons induced by the donor passivate the same amount of holes induced by the acceptor, making the codoped system keep semiconductor character and preventing the formation of recombination centers.6 Therefore, the dopant combination was selected to be one interstitial B with three substitutional N and the system was labeled as (B[int], 3N) codoped TiO2. Figure 1c shows the relaxed inner part of the supercell structure for (B[int], 3N) codoped TiO2. It is found that the structure is similar to the structure of interstitial B monodoped TiO2 whereas three O atoms in BO4 unit are replaced by N atoms. The HSE06 calculated band structure of (B[int], 3N) codoped TiO2 is displayed in Figure 2. As expected, the Fermi level of (B[int], 3N) codoped TiO2 locates just above the VBM, indicating a typical semiconductor character. However, the calculated band gap of (B[int], 3N) codoped TiO2 is 2.996 eV, which is only 0.264 eV smaller than that of HSE06 calculated pure anatase TiO2 (3.26 eV). This indicates that the band gap narrowing induced by interstitial B donor and substitutional N acceptors is very limited. To further understand this result, the TDOSs and PDOSs of (B[int], 3N) codoped TiO2 were calculated. As is shown in Figure 3, the N2p states are fully occupied because of the charge transfer from B donor to N acceptors (compensation effect). The band gap of the (B[int], 3N) codoped TiO2 is determined by the position of N-2p states (act as the VBM) and the CBM of TiO2. Because the N-2p state in (B[int], 3N) codoped TiO2 is not as higher as that in substitutional N monodoped system, the band gap narrowing induced by (B[int], 3N) codoping is very limited even under high N doping concentration [9.375% in (B[int], 3N) codoped case]. Therefore, there may be other mechanisms responsible for the effective band gap narrowing in the B−N codoped anatase TiO2. 3.4. (B[sub], N) Codoped Anatase TiO2. By analyzing the possible B−N dopant combinations, we found that the combination that substitutional B on Ti site with substitutional N on O site in TiO2 [this dopant combination is labeled as (B[sub], N)] matches with the criteria of dopant coupling: (i) the provision of net two holes18 and (ii) the presence of at least one nonmetal dopant with higher p orbital than the O-2p orbital (N) in each dopant combination.22 For the (B[sub], N) 15914

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anatase TiO2. It is found that the area between N and O cores is covered by relatively high density of electrons, suggesting that the N−O bond has strong covalency. As expected, the N−O bond has a considerably greater covalency character than the Ti−O ones. Because the electronegativity of N is weaker than that of O, the charge density of O is higher than that of N. The strong covalent coupling between the N atom and its neighboring O atom in (B[sub], N) codoped anatase TiO2 is the main reason for its band gap narrowing. 3.5. Formation Energy. To evaluate the relative difficulty of the incorporation of different dopants into the anatase TiO2 lattice, we calculated the formation energies of interstitial B doped, substitutional N doped, (B[int], 3N) codoped, and (B[sub], N) codoped anatase TiO2 by the following formulas:

Figure 4. Schematic plot of the bonding mechanism for the incorporated N atom and its neighboring O atom in (B[sub], N) codoped anatase TiO2.

Eform(Ti16O32 B) = E(doped) − E(pure) − μB

by the hole. The electron transfer of N makes its 2pz orbital empty, which facilitates the formation of covalent N−O bond. During the N−O bond formation, the 2pz orbitals of N and O form the bonding state σ and the antibonding state σ*. Simultaneously, their 2px and 2py orbitals form the bonding state π and the antibonding state π*. As shown in the TDOSs and PDOSs of (B[sub], N) codoped anatase TiO2 in Figure 3, the fully occupied σ and π* states locate below the VB and in the band gap of TiO2, respectively. Furthermore, the fully occupied π state and the unoccupied σ* state are dispersed in the VB and CB of TiO2, respectively. Because the position of antibonding state π* is higher than that of N-2p state, the Bassisted N−O coupling in (B[sub], N) codoped anatase TiO2 leads to an effective band gap reduction. To investigate the properties of N−O bond, we calculated the charge density of (B[sub], N) codoped anatase TiO2. The charge density of pure anatase TiO2 was also calculated for comparison. As shown in the charge density profile of pure anatase TiO2 in Figure 5a, the Ti−O bond contains large proportion of ionic bonds since there is very few charge left on Ti4+ (about 0.05 to 0.1 e/bohr3). Figure 5b shows the charge density profiles of the N−O bond in (B[sub], N) codoped

(2)

Eform(Ti16O31N) = E(doped) − E(pure) − μ N + μO (3)

Eform(Ti16O29N3B) = E(doped) − E(pure) − 3μ N + μO − μB

(4)

Eform(Ti15BO31N) = E(doped) − E(pure) − μ N + μO − μB + μTi

(5)

where E(doped) is the total energy of TiO2 with dopants and E(pure) is the total energies of pure TiO2. μB is the chemical potential of B, which is determined by the energy of rhombohedral bulk boron [12 B atoms within the primitive unit cell, μB = (μB12)/12]. μN and μO represent the calculated chemical potential of free molecule N2 and O2 [μN = (μN2)/2 and μN = (μN2)/2], respectively. μTi is the chemical potential of Ti, which is calculated by the energy of hexagonal bulk titanium [2 Ti atoms within the primitive unit cell, μTi = (μTi2)/2]. The growth of an engineered TiO2 is a kinetic process at either Tirich, O-rich, or the one between the two conditions. Under Tirich conditions, μTi is still the chemical potential of Ti while the μO is determined by μTi + 2μO = μ(TiO2 )

(6)

Under O-rich conditions, μO is still the chemical potential of O while the μTi is calculated by formula 6. The calculated formation energies are listed in Table 1 (the optimized lattice constants of doped TiO2 systems are also summarized in Table 1). As seen in this table, the formation energy of interstitial B doped TiO2 is 0.108 eV under both Ti-rich and O-rich conditions. This relatively low formation energy indicates that the incorporation of interstitial B in TiO2 is relatively easy because the interstitial B doping is an n-type doping. The obtained formation energy of substitutional N doped TiO2 is 0.801 and 5.237 eV under Ti-rich and O-rich conditions, respectively, which are in good agreement with the previous theoretical values of 0.69 and 5.69 eV, respectively.50 Under the Ti-rich growth condition, the (B[int], 3N) codoped TiO2 is more energetically favorable in comparison with the other doped TiO2 system due to the smallest formation energy of −4.757 eV. It is found that the Ti-rich growth condition is helpful to the incorporation of p-type (substitutional B and N) dopants in TiO2. However, in the synthesis process of doped TiO 2 , the O-rich growth condition is closest to the

Figure 5. Part of total charge density map for (a) pure anatase TiO2 and (b) the N−O bond in (B[sub], N) codoped anatase TiO2. The values of the contours in the plots are from 0.001 to 1.0 e/bohr3 with an increment of 0.05 e/bohr3. 15915

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experimental environment.27,50 Under the O-rich growth conditions, the formation energy of (B[sub], N) codoped TiO2 is 8.406 eV, which is only 0.145 eV smaller than that of (B[int], 3N) codoped TiO2 (8.551 eV). Therefore, the growth of B−N codoped anatase TiO2 may contain both (B[sub], N) codoped TiO2 and (B[int], 3N) codoped TiO2. To obtain the (B[sub], N) codoped anatase TiO2, we suggest that the synthesis of B−N codoped TiO2 should be performed at strong O-rich environment, which would narrow the band gap of anatase TiO2 effectively.

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4. CONCLUSIONS In summary, we have used hybrid density functional calculations to investigate the electronic structures of (B[int], 3N) and (B[sub], N) codoped anatase TiO2 and further explore the mechanism of band gap narrowing of anatase TiO2 induced by B−N codoping. It is found that the band gap reduction induced by B-assisted N−O coupling in (B[sub], N) codoped anatase TiO2 is much effective than that induced by charge compensation between B donors and N acceptors in (B[int], 3N) codoped anatase TiO2. In addition, our calculation results indicate that the experimentally observed enhancement of PEC water splitting performance for the red B−N codoped anatase TiO2 derives form the covalent coupling between the N and its neighboring O in (B[sub], N) codoped anatase TiO2. By considering the formation energy, we found that the strong Orich environment is helpful for the growth of (B[sub], N) codoped anatase TiO2. Therefore, we believe that the dopant coupling effect is very effective to improve the visible light photocatalytic performance of TiO2 and the criteria for the dopant coupling are very important to design new TiO2-based photocatalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], [email protected]; Fax +8610-64427616; Tel +8610-64443254. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National NSF of China (21106003, 21274011, 21121064), National 863 Program (2013AA031901), Beijing Novel Program (Z12111000250000), Outstanding Talents Plans and “Chemical Grid Project” of BUCT, and Supercomputing Center of Chinese Academy of Sciences (SCCAS).

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REFERENCES

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dx.doi.org/10.1021/jp4038792 | J. Phys. Chem. C 2013, 117, 15911−15917

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