Visible-light-driven photocatalysts: (La/Bi+N)-codoped ...

JOURNAL OF APPLIED PHYSICS 109, 063103 (2011)

Visible-light-driven photocatalysts: (La/Bi 1 N)-codoped NaNbO3 by first principles Guodong Liu, Shulin Ji, Liangliang Yin, Guoping Xu, Guangtao Fei, and Changhui Yea) Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Technology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China

(Received 18 August 2010; accepted 14 January 2011; published online 17 March 2011) To improve the photocatalytic activity of NaNbO3 for water splitting, the bandgap and the band edges of NaNbO3 should be tailored to match the visible part of the solar spectrum and hydrogen and oxygen redox potentials. By analyzing the band structures of La/Bi-doped and (La/Bi þ N)-codoped NaNbO3, we found that the pseudointermediate band (PIB) was formed in the bandgap in all the doped systems because of the orbital splitting of the Nb 4d induced by the dramatically enlarged O-Nb-O angles. The PIB could make the wide bandgap semiconductors absorb visible-light photons as long as it was degenerate or partially degenerate. Considering that the appropriate band edges and absorption properties, we believe that (La/Bi þ N)-codoped NaNbO3 materials are promising photocatalysts for hydrogen production through water splitting under visible-light irradiation without C 2011 American Institute of Physics. [doi:10.1063/1.3554697] other modifications. V I. INTRODUCTION

NaNbO3 is one of the ternary metal oxides that are promising as photoanodes with efficient ultraviolet-lightdriven photocatalytic performance owing to the large bandgap of 3.49 eV (Ref. 1) and high photochemical stability.2 The optical absorption property of the photocatalyst governs the photocatalytic reaction rate and efficiency; therefore, increasing the spectral response (decreasing the width of the bandgap) is a good way to improve its efficiency. Currently, the effort has been devoted to tune the bandgap energy and the bandedge positions via metal or nonmetal doping.3 There have been few papers on the experimental or theoretical investigations of the doped-NaNbO3 systems, except that the N-doped NaNbO3 was studied,4 where the enhanced photocatalytic efficiency was attributed to the narrowing of the bandgap by mixing N 2p and O 2p states. As a similar ternary metal oxide, the doped tantalates have been studied by many researchers by focusing on lanthanum doping,5–7 however, the electronic structure of the doped systems have not been investigated in depth. In order to clarify the role of La atom and provide some design rules for doped ternary metal oxideas visible-light-driven photocatalysts, we calculated the electronic structures and optical properties of doped NaNbO3 using density functional theory. The selection rules of dopants are formulated as follows. Firstly, we chose the La atom as being the impurity atom to change the conduction bandedge. Because the localization degree of d electron states is 3d > 4d > 5d,8 and only the impurity atom which has 5d electrons could broaden the Nb 4d states and further reduce the bandgap energy. Secondly, we have studied the Bi-doped NaNbO3 to see whether Bi 6s states could hybrid with O 2p states to elevate the valenceband maximum when Bi atom doped at A position in perovskite structure. The usage of Bi dopant is substantiated by the findings from several groups, for example, Kudo’s a)

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group9 and Wei’s group10 have proposed that an upward dispersion of the valence band in BiVO4 structure was caused by the coupling between Bi 6s lone pair electrons and O 2p states. However, it was reported by Li et al. that if Bi atom was located at the B position in the perovskite structure, the Bi 6s states would not hybrid with O 2p electrons in Na(BixTa1x)O3 (x ¼ 0–0.10) solid-solution.11 Thirdly, we use (La/Bi þ N)-codoping approach to redshift the NaNbO3 optical absorption edge and shift its band edges for hydrogen production. The monodoping could induce defect bands as recombination centers,12 whereas the unwanted recombinations are impeded by codoping approach.13 Besides, the N atom doped in oxide semiconductors could effectively shift the valence band more negatively.14–16 In the present study, we investigated the electronic and optical absorption properties of La and Bi atoms-doped NaNbO3 systems by substituting La and Bi for Na atom, and (La/Bi þ N)-codoped systems, ellucidated the underlying mechanism for the appearance of PIB in all systems and visible-light absorption peak in the codoped systems, and brought out the design rules for visible-light-driven ternary metal oxide photocatalysts. According to our calculation results, new band in the bandgap emerged in the doped systems, which was totally separated from the valence band and partially separated from the conduction band bottom, and completely composed of unoccupied states. We named the new band as pseudointermediate band (PIB), by analogy to the concept of intermediate band (IB).17,18 However, so far the IB materials have been limited in only several kinds of high mismatch alloys (HMA). We believed that this kind of PIB materials will represent a new family of materials that could split water under visible light irradiation with enhanced efficiency. II. COMPUTATIONAL METHOD

NaNbO3 has a cubic perovskite structure (space group P21ma) where the NbO6 octahedra connect with each other 109, 063103-1

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by one O atom, forming a three-dimensional net structure. We used 2 1 1 supercell containing 8 Na atoms, 8 Nb atoms and 24 O atoms to model bulk NaNbO3. The doped concentration was 12.5% (the codoped system must simultaneously contain one La or Bi atom and three N atoms to get this doped concentration), and all the doped atoms were as far apart as possible from one another. Our calculations were based on the pseudopotential plane-wave method as implemented in the CASTEP (Ref. 19) density-functional software package, which is widely used for investigating electronic structure and optical properties of a wide variety of doped systems.20–24 The geometry optimization was carried out using the generalized gradient approximation (GGA) proposed by Perdew, Burke, and Ernzerhof (PBE)25 combined with Vanderbilt ultrasoft pseudopotentials.26 The electronic structures were calculated by introducing effective Hubbard U (Ref. 27) to treat the d electrons with Ueff ¼ 2.5 eV for Nb 4d electrons and Ueff ¼ 0 for La 5d electrons,28,29 the actual spacing ˚ of k-point sampling in the Brillouin zone was set as 0.025/A for both two processes. The optical absorption spectra were calculated with the norm-conserving pseudopotentials.30 It is known that the KS equations always underestimate the forbidden bandwidth, therefore the scissors operator31 was used for adjusting the width of bandgap (we assumed that the calculated bandgap was at the same underestimation level for all the systems) in order to get the reasonable absorption spectra that include the direct optical transitions of valence to conduction band (ignoring the indirect transitions). The ˚ . The plane-wave basis k-point sampling was set as 0.020/A set was truncated at a kinetic energy of 450 eV and self-consistent iteration was carried out with a total energy convergence tolerance of less than 1.0e-6 eV/atom for all the computations.

Considering that both the La 4f (Ref. 32) and the Bi 5d states10 have no contribution to the density of states close to the Fermi level EF, we could take the electron configuration of La and Bi atom as 5s25p65d16s2 and 6s26p3, respectively. III. RESULTS A. Pure NaNbO3

The parameters of optimized NaNbO3 structure and the bandgap were listed in Table I. The calculated lattice parameters agreed well with the experimental results33 and the O-Nb-O angles of NbO6 octahedra had a large difference in the direction parallel and perpendicular to the y axis of NaNbO3 structure. The band structure and electronic density of states (DOS) for NaNbO3 were shown in Fig. 1, where the direct bandgap was calculated as 1.71 eV. The O 2p states constituted the valence bandedge and the conduction bands were contributed by Nb 4d and O 2p states, therefore changing the Nb-O interaction would vary the bandgap width and bandedge components. B. La-doped and (La 1 N)-codoped NaNbO3

When La atoms substituted Na atoms, the optimized lattice parameters changed slightly because the ionic radii of La3þ and Naþ are similar. While the O-Nb-O angles of NbO6 octahedra were obviously enlarged to over 173 (listed in Table I), and the angle disparities in two different directions (parallel and perpendicular to the y axis) became much smaller than that of pure NaNbO3, implying that the NbO6 octahedra were elongated. Such a distinct angle variations would change the Nb-O interactions and the electronic structures. As shown in Fig. 2(a), the degenerate PIB was present in the band structure. The PIB was completely separated

˚ , O-Nb-O angle/ (the Nb atom is at nearest the impurity atom, bold fonts represent the angles of perpendicular to the TABLE I. The lattice constant a/A y axis) and the bandgap width Eg/eV in theory (at the same underestimation level) and experiment of the pure and doped NaNbO3 systems. NaNbO3 (211) pure

N doped

La doped

LaþN codoped

Bi doped BiþN codoped

a

Ref. 1.

˚ a/A

O-Nb-O angle/

Eg/eV (calculated)

Eg/eV (experimentally)

a ¼ 11.30 b ¼ 7.78 c ¼ 5.58 a ¼ 11.28 b ¼ 7.87 c ¼ 5.62 a ¼ 11.21 b ¼ 7.92 c ¼ 5.58 a ¼ 11.36 b ¼ 7.87 c ¼ 5.54

155.7 (parallel to the y axis) 164.0 (perpendicular to the y axis)

1.71

3.49a

O-Nb-N:163.3 167.9

1.28

2.61

173.7177.7 174.9~177.2

0.75

1.53

O-Nb-N:178.5 167.2172.6 167.8~171.9

0.68

1.39

177.2177.7 174.3~177.6

1.06

2.16

O-Nb-N:177.9 166.1170.9 163.6~172.7

0.94

1.92

a ¼ 11.22 b ¼ 7.88 c ¼ 5.54 a ¼ 11.37 b ¼ 7.81 c ¼ 5.63

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FIG. 1. (Color online) The calculated spin polarized band structures (a) (solid and dot lines represent spin up and down states, respectively) and the projected density of states (b) of NaNbO3 using GGAþU method.

from the valence band and partially separated from the conduction band. It became indistinguishable from conduction band bottom at the Q point in the reciprocal space, resulting in the decrease of the calculated bandgap to 0.75 eV. As can be seen from the projected DOS in Fig. 3(a), the valence band was formed by hybridization of O 2p and La 5d, 5p, 6s states, consequently, O 2p states in valence band narrowed and moved far away from the Fermi level; La 5d states that are more delocalized than 4d states8,34 were also coupled with the Nb 4d states in the conduction band. Therefore the Nb 4d states were expanded, and approached more to the Fermi level than that in pure NaNbO3, which is essential for the PIB forming discussed in detail in Sec. IV. Because the O 2p states of La-doped NaNbO3 were far away from the Fermi level from the analysis above, the valence bandedge was more positive and not suitable for the oxidation of water any more. Thanks to the fact that N atom doped in oxide semiconductors could effectively shift the valence band more negatively, the (La þ N)-codoped system were calculated with the same method. The optimized structural parameters and the calculated bandgap were also listed in Table I, where the O-Nb-O angles were both enlarged to 167.2–172.6 in two different directions and had a smaller disparity than those of pure NaNbO3. The band structure of (La þ N)-codoped system in Fig. 2(b) revealed a direct bandgap with the decrease of the calculated bandgap to 0.68

FIG. 2. (Color online) The calculated spin polarized band structures (similar to Fig. 1) of (a) La-doped; (b) (La þ N)-codoped in NaNbO3 systems using GGAþU method.

FIG. 3. (Color online) The projected density of states for (a) La-doped; (b) (La þ N)-codoped NaNbO3 systems using GGAþU method. (The blue solid, red dot, and black dash lines represent the outermost shell d, p and s electron states, respectively).

eV. The degenerate PIB became totally nondegenerate and approached to the conduction bandedge more than that of La-doped system, because the elevated valence bandedge (more negative) by N doping repelled the PIB to the conduction bandedge and made the degenerate states split into two nondegenerate states. Comparing the projected DOS in Fig. 3(b) to that in Fig. 3(a), the only difference was that the valence bandedge was forced to move toward high levels because of the N atom doping. It is well-known that the IB materials could extend the optical absorption to visible-light region;35 therefore, it is interesting to know whether these PIB materials do the same as that of IB materials. We calculated the optical absorption spectra of pure, N-, La-doped, and (La þ N)-codoped NaNbO3 as shown in Fig. 4. The optical absorption edge of pure NaNbO3 was at about 3.5 eV, lying in the UV light region, agreeing with the experimental result.1 When the N atom was doped in NaNbO3, the optical absorption edge shifted slightly to the lower energy, while the absorption peak did not shift at all. Many literatures36,37 had reported the similar findings. When La atom was doped in NaNbO3, however, the optical absorption edge evidently moved to the lower energy due to the decreased bandgap, and a new moderate-intensity optical absorption peak appeared at around 1.6–2.1 eV in the visible-light region. The optical absorption edge of (La þ N)-codoped system was the lowest in energy in the four systems because it had the narrowest bandgap (Table I). Although there was PIB in (La þ N)-codoped structure, it had no such new absorption peak as that of La-doped one. The underlying mechanism for the appearance or disappearance of the visible-light optical absorption peak will be explained in Sec. IV. The ratio of the integrated intensity of the absorption in the visible-light region (1.5–3 eV) for pure, La-doped, and (La þ N)-codoped systems is 1: 31.2: 30.2, which quantitatively verify the advantage of the visible-light responsivity of the doped systems.

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FIG. 4. (Color online) The calculated absorption spectra for pure, N-doped, La-doped, and (La þ N)-codoped NaNbO3 systems using GGAþU method combined with norm-conserving pseudopotential.

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FIG. 6. (Color online) The same as Fig. 3, but for the Bi-doped and (Bi þ N)-codoped in NaNbO3.

C. Bi-doped and (Bi 1 N)-codoped NaNbO3

The optimized parameters and bandgap width of Bi-doped and (Bi þ N)-codoped NaNbO3 were also listed in Table I. It is surprising that the geometry optimized structure of Bi-doped and (Bi þ N)-codoped NaNbO3 had similar changes to those of La-doped and (La þ N)-codoped systems relative to pure NaNbO3, although Bi and La atom possesses totally different chemical properties and outermost electrons. The band structures of Bi-doped and (Bi þ N)-codoped NaNbO3 as shown in Figs. 5(a) and 5(b) were similar to those of La-doped and (La þ N)-codoped systems as shown in Figs. 2(a) and 2(b), respectively, except for the bandgap width. From the projected DOS shown in Figs. 6(a) and 6(b), we can see that the valence bandedge was constituted by the hybridization of O 2p and Bi 6s states in both systems. The valence bandedge was higher than that of pure NaNbO3 in Bi-doped system, which demonstrated that as an impurity atom, Bi 6s states did elevate the valence bandedge.9,10 Iden-

tically, the Nb 4d states also extended to the Fermi level in both systems and the N atom doping elevated the valence bandedge by comparing Fig. 6(a) with Fig. 6(b). Similar electronic structures give similar optical absorption properties. Figure 7 showed the absorption spectra of pure, N-, Bi-doped, and (Bi þ N)-codoped systems, with the same variation tendency as that of La-doped and (La þ N)codoped systems as shown in Fig. 4. The ratio of integrated intensity for the absorption in the visible-light region (1.5–3 eV) for pure, Bi-doped, and (Bi þ N)-codoped systems is 1: 40.7: 25.2. The results indicate that the Bi-doped and the (Bi þ N)-codoped systems were also candidates for water splitting under visible-light irradiation.



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was induced by the change of O-Nb interaction by enlarged O-Nb-O angles in doped systems. For La and Bi doping, they will donate two electrons to the conduction bands, and made the Nb d states more broad than that in NaNbO3, resulting in the splitting of the conduction band. The results indicate that two of the Nb states dropped-forming the so-called PIB which was degenerate or partially degenerate. When La and Bi were co-doped with N, the elevated valence bandedge caused by doped N atom repelled the PIB to the conduction bandedge, and made the degenerate states splitting into two nondegenerate states as shown in Figs. 2(b) and 5(b). If the formed PIB was degenerate or partially degenerate, it could capture the excited electrons from valance band and caused a moderate-intensity absorption peak in the visible-light region as the case for the La- and Bi-doped systems, whereas there was no visible-light absorption peak if the PIB was totally nondegenerate as in the (La/Bi þ N)-codoped systems. B. Relative positions of band edges FIG. 8. (Color online) The Band structures (similar to Fig. 1) of (a) La-dopedþVNa and (b) the nongeometry optimized La-doped NaNbO3; The calculated absorption spectra for (c) La-dopedþVNa and (d) þ2 charges of La-doped comparing with pure, La-doped and (La þ N)-codoped NaNbO3.

IV. DISCUSSION A. The reason for existence of the PIB and a visiblelight absorption peak

La- and Bi-doped NaNbO3 materials were both the high valence doping and may induce other defects to balance the excess charges for stabilizing the systems. To elucidate whether the visible-light absorption was induced by the PIB or other defects, we studied the La-dopedþVNa (one La atom with one Na vacancy) system and carried out other related calculations. From the band structure of La-dopedþVNa NaNbO3 we can see that the PIB was partially degenerate and the calculated bandgap was increased to 1.21 eV as shown in Fig. 8(a). Figure 8(c) showed the absorption spectrum of La-dopedþVNa NaNbO3 which had a small peak at about 2 eV. Comparing with those of La-doped and (La þ N)-codoped systems, we can concluded that the visible-light absorption peak was induced by the degenerate or partially degenerate PIB (electron transitions between the valence band and the PIB), instead of other defects. We have also calculated La-doped NaNbO3 system with þ2 excess charges and nongeometry optimized configuration, respectively. We found that the absorption spectrum of þ2 charges system was rather similar to that in charge balanced (La þ N)-codoped system as shown in Fig. 8(d), and there was a degenerate PIB in the bottom of the conduction band (the band structure of þ2 charges system was not shown here), which indicated that the visible-light absorption peak had no relevance to the charge imbalance effect. In addition, there was no PIB in the band structure of the nongeometry optimized La-doped NaNbO3 as shown in Fig. 8(b). By comparing the band structure of the nongeometry optimized with that of the geometry optimized La-doped NaNbO3 in Fig. 2(a), we deduced that the formation of PIB

The band positions relative to redox electrodepotential of water splitting are as significant as the width of bandgap. In order to investigate the relative band position, the conduction band bottoms (ECB) were calculated empirically according to formula (1) proposed by M. A. Butler and R. G. Pearson,38–40 1 ECB ¼ vAaBbCc Eg þ E0 ; 2

(1)

where Egis bandgap, E0 a scale factor relating the reference electrode redox level to the absolute vacuum scale (E0 ¼ 4.5 eV for normal hydrogen electrode), and vAaBbCc the absolute electronegativity of compound AaBbCc. The derived relative band positions were shown in Fig. 9, where the compact blue lines signified the PIB jointing with conduction band bottom. Considering the width of bandgap shown in Table I and the relative band edges for water splitting, the (La þ N)- and (Bi þ N)-codoped systems were the most suitable among these direct-bandgap doped systems for water splitting under visible-light irradiation without other sacrificial agents or surface decorations. Moreover, because of the s states of La and Bi atom contributed to the valance bandedge, the excited carriers became more delocalized and easier to move for enhanced efficiency. V. CONCLUSION

We have presented first-principles calculations of electronic and optical properties for La/Bi-doped and (La/Bi þ N)-codoped NaNbO3 and their relative band edges to redox electrodepotential of water splitting. These results would provide guidance for developing ternary metal oxides as visible-light response photocatalysts. We found that the PIB was formed in the bandgap in all the doped systems due to the Nb 4d orbital splitting induced by the dramatically enlarged O-Nb-O angles. Unlike the intermediate band, the PIB was not half-filled band; however, it played the same role as that of the IB. In other words, the PIB could make the wide

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FIG. 9. (Color online) A schematic illustration of relative positions of band edges for pure and doped NaNbO3 systems (the compact blue lines signified the PIB jointing with conduction band bottom).

bandgap semiconductors absorb visible light as long as it was degenerate or partially degenerate. The codoping of N atom in NaNbO3 did elevate the valence bandedge (more negative) without red-shifting the absorption edge, and repel the PIB split into two nondegenerate states. Comparing the relative positions of band edges with redox electrodepotential of water splitting and considering the width of bandgap in these systems, the (La þ N)- and (Bi þ N)-codoped NaNbO3 were the most suitable for water splitting under visible light irradiation without other sacrificial agents or surface decorations. Thanks to the high theoretical efficiency potential (63%) of the IB solar cells,41 we believe that (La þ N)- and (Bi þ N)-codoped NaNbO3 would be promising visible-light-driven photocatalysts for water splitting.

ACKNOWLEDGMENTS

First-principles calculations were performed at Center for Computational Science, CASHIPS. This work was supported by Natural Science Foundation of China (Grant Nos. 10874183 and 11074255), the Ministry of Science and Technology of China (No. 2011CB3021033), and Hundred Talent Program of Chinese Academy of Sciences. 1

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