Optical Transition and Photocatalytic Performance of d1 Metallic

Article pubs.acs.org/JPCC

Optical Transition and Photocatalytic Performance of d1 Metallic Perovskites Yingtao Zhu,† Ying Dai,*,† Kangrong Lai,†,‡ Zhujie Li,† and Baibiao Huang† †

School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China Department of Physics, Changji University, Changji 831100, People’s Republic of China

‡

ABSTRACT: Electronic structure and optical transition of three d1 metallic oxides SrNbO3, SrVO3, and CaVO3 are theoretically investigated employing conventional density functional theory and partially self-consistent GW calculations. To evaluate the visible light absorption, the matrix elements for direct transitions between band edge states are studied. Our results indicate that among the three inversion symmetry structures electron direct transition in the visible light region can only occur in SrNbO3, which is ascribed to different parity of band edge wave functions due to the mixing of Sr d states with Nb eg states. In addition, the effective mass of photogenerated carriers in SrNbO3 with isotropic characteristic is the smallest, which implies that the photogenerated carriers can transfer to the surface reaction sites more easily with less recombination. Therefore, SrNbO3 should be of better photocatalytic performance. The present work may be beneficial to exploring the series of metallic perovskite photocatalysts.

1. INTRODUCTION Recently, photocatalysts have attracted more and more attention due to their potential applications in energy production and environmental remediation. Although TiO2 is a representative semiconductor photocatalytic material,1−5 the large band gap (∼3.2 eV6) limits its optical absorption only in the UV range about 5% of the whole solar photon spectrum, which hampers its applications in the visible spectral region. Over the past decade, numerous studies have been performed to search for efficient photocatalysts with visible light activity by various strategies, i.e., metal and/or nonmetal doping,7−14 solid solution structures,15−18 metal particles decorating on surfaces (i.e., noble-metal/TiO 2 (SrTiO 3 ), 19−23 Ag/AgX (X = Cl,Br,I)24−26), semiconductor heterogeneous structures,27−30 and so forth. In the meantime, it is also one of the important issues in the photocatalysis fields to exploit new dopant-free materials with desired band structure for visible light photocatalytic activity.31,32 Traditionally, it is regarded that metals cannot act as photocatalysts because of the inevitable photogenerated carrier recombination without significant electric field. However, very recently, Xu et al.33 discovered that the red metallic d1 oxide SrNbO3 (SNO) can be used as an efficient photocatalyst under the visible light region toward photodegradation of methylene blue and overall water splitting assisted by appropriate sacrificial elements. It was pointed out that the electron transition from the partially occupied Nb t2g states to the fully unoccupied states is responsible for the visible light absorption, and the photocatalytic activity is attributed to its high carrier mobility by means of a simple first-principles calculation. However, there is no systemic theoretical investigation on the photocatalysis mechanism for this kind © 2013 American Chemical Society

of materials except a calculation for SNO in the work of Xu et al. This raises some interesting issues worthy of further investigations: (1) Considering that SNO has an inversion symmetry structure, the optical transition between d states seems to be forbidden according to the transition selection rules,34,35 so why does the SNO sample show experimentally photocatalytic activity under visible light? (2) The effective mass of photogenerated electrons and holes may be one of the dominant factors in determining the carrier transfer from bulk to the surface reactive site, which is an important process in the photocatalytic reaction,36,37so how about them in such d1 metallic oxides? (3) Are there any other d1 metallic oxides that could exhibit the same characters and properties? To answer and understand these issues, we select the three d1 metallic oxides SrNbO3 (SNO), SrVO3 (SVO), and CaVO3 (CVO), as shown in Figure 1, for the investigation in the present work. Because in the perovskite family the cubic perovskite structure SVO with 3d1 configuration (V4+) has been extensively researched due to its potential application in multiferroic devices, solid fuel cell, and high Tc superconductivity,38,39 the orthorhombically distorted structure CVO, which has 3d1 configuration and displays metallic properties too, also deserves to be examined to distinguish it from SNO and SVO. We comparatively investigate the optical transition and carrier effective mass of SNO, SVO, and CVO in the frame of first-principles density functional theory (DFT) to provide a theoretical understanding of the photocatalytic Received: December 9, 2012 Revised: February 24, 2013 Published: February 26, 2013 5593

dx.doi.org/10.1021/jp3121116 | J. Phys. Chem. C 2013, 117, 5593−5598

The Journal of Physical Chemistry C

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Figure 1. Geometrical structure for (a) SNO, (b) SVO, and (c) CVO. The red, green, and steel blue spheres represent O, Sr, and Ca, respectively. Nb and V are at the center of octahedral configurations.

performance of d1 metallic perovskite materials and guidance for exploiting new types of photocatalysts.

the R point, being 3-fold degenerate. For the upper fully unoccupied bands (CB), the lowest states are both at the Γ point appearing to be 2-fold degenerate. The bandwidth of B1 for SNO is larger than that of SVO, accounting for its smaller effective masses of photogenerated carriers, which can also be identified in the effective mass calculations as follows. In addition, the lowest unoccupied band in CB for SVO along the Γ−X path exhibits a flat characteristic, also indicating that the effective mass of the photogenerated electrons in SVO is larger than that in SNO along the [100] direction. In local distorted structure CVO as shown in Figure 1c, the number of bands near the Fermi level is more than that of SNO and SVO (shown in Figure 2c), implying higher conductivity character in CVO. Moreover, to check the validity of conventional DFT calculations on the electronic structure of “strong-correlated” SNO, SVO, and CVO systems, the G0W0 and GW0 calculations are also performed because of their better treatment of exchange-correlation effects. The corresponding calculated band structures are shown in the middle and right panels of Figure 2, respectively. It can be seen that except for the fact that gaps between VB and B1 (B1 and CB) in the three materials become a little larger the band characters (such as widths and topology) of B1 change little compared to the results from conventional DFT in the left panels. Therefore, the effective masses of photogenerated carriers are derived from the conventional DFT results following. Furthermore, as depicted in Figure 2, there are three possible band-edge electron transitions in these systems: (I) transition from states at the Fermi level to the empty states of the three partially occupied B1 bands with the lowest energy of about 1.4 eV and 0.85 eV for SNO and SVO, respectively, which are larger than that for CVO (0.36 eV); (II) transition from states at the Fermi level to the lowest states in fully unoccupied bands, where the lowest transition energies are about 2.31, 2.44, and 1.67 eV in SNO, SVO, and CVO, respectively; (III) transition from VB to Fermi level with the lowest transition energy out of the visible light region. Herein, we just consider the (I) and (II) processes for the visible light absorption. For process (I), the electrons are excited to the upper level in the partially filled B1 band under light irradiation, and the photogenerated electrons may easily relax to the lower states and recombine with the photogenerated holes immediately, which can be speculated that the (I) electron transition may have little contribution to the visible light photocatalytic activity. Specially, SVO and CVO, in which the band edge potential difference is smaller than the redox potential of water splitting, cannot be utilized to split water for H2 production. Subsequently, it is worth investigating whether the electron transition can occur in process (II). Traditionally, the electron

2. CALCULATION DETAILS All the spin-polarized DFT calculations were performed using the Vienna ab initio Simulation Package (VASP).40,41 The generalized gradient approximation (GGA) with Perdew− Burke−Ernzerhof (PBE)42 functional was adopted for the exchange-correlation potential. In addition, to check the validity of conventional DFT calculation on the electronic structure of “strong-correlated” systems, the non- and partially selfconsistent GW (G0W0 and GW0) methods were adopted.43 The projector augmented wave potentials and the cutoff energy of 400 eV were used. The convergence threshold for selfconsistent-field iteration was set at 10−5 eV, and all the geometric structures were fully optimized until all the components of the residual forces were smaller than 0.01 eV/ Å. A Gaussian smearing with width of 0.05 eV was employed to accelerate the electronic self-consistent convergence. We adopted Monkhorst−Pack k-point meshes of 10 × 10 × 10 for cubic structure SNO and SVO perovskites, and the relaxed lattice constants for bulk SNO and SVO in our calculations are 4.03 and 3.87 Å, respectively, shown in Figure 1, which are in good agreement with previous experimental44,45 and theoretical investigations.46,47 Of them, the crystal structure of CVO is distorted perovskite type, which belongs to the orthorhombic lattice group (Pbnm). For the distorted perovkite type CVO with optimized constant a = c = 5.38 Å and b = 7.60 Å, we adopted Monkhorst−Pack k-point meshes of 10 × 6 × 10. For the GW calculations, we constructed maximally localized wannier functions (MLWF) and obtained the band structures employing an interpolation scheme. The convergence accuracies of spread functionals for both gauge-invariant and nongauge-invariant parts were set to 10−10, and the calculation of MLWF was accomplished in the VASP2WANNIER90 interface.48 3. RESULTS AND DISCUSSION 3.1. Band Structure and Optical Absorption. First, we examine the band structures of SNO, SVO, and CVO using traditional DFT, G0W0, and GW0 methods shown in Figure 2. The band structures of the three systems can be divided into three parts, which are labeled as CB, B1, and VB, respectively. It can be easily seen that all of the three perovskite structures with d1 electrons display metallic properties with the Fermi level going through the B1 bands. For the SNO and SVO (in Figure 2(a) and (b)), the B1 parts are composed of three dispersive bands, and the highest unoccupied states in B1 both occur at 5594



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Figure 2. Calculated band structure of (a) SNO, (b) SVO, and (c) CVO along the certain high-symmetry direction. The Fermi level is set at zero, and the arrows represent the optical transitions. The red × symbol labels the forbidden optical transition. The left, middle, and right panels are the results at PEB, G0W0, and GW0 levels, respectively.

the optical transition matrix elements at all the k points (20 × 20 × 20 Monkhorst−Pack mesh including Γ point) for B1 and the lowest unoccupied CB bands are zero, which means that the electron direct transitions are not allowed under visible light and the forbidden optical transitions are labeled in Figure 2, left panel (with the red × symbol). Thus, the visible light absorption should be very weak under the lattice vibrations. Our results are in accord with the observations in optical conductivity experiment, in which the optical conductivity under visible light irradiation is very low, being attributed to the

excitation from O 2p states to d states in transition metal oxide photocatalysts, such as TiO2, clearly obeys the electron transition selection rules. However, in the present materials with inversion symmetry, the B1 and CB bands are mainly consisted of d states, and then, the electron transition in process (II) seems plausibly to be impossible because all the band edge states have the same parity. To obtain a more detailed understanding of the visible light absorption, the calculations on the optical transition matrix of the three materials at special k points are performed. In SVO and CVO, 5595



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d−d transition.49 However, the optical transition matrix elements away from Γ points in SNO appear to be nonzero, with the magnitude of about 0.1−0.3 au for the B1−CB transition, which is responsible for the visible light absorption experimentally observed by Xu et al.33 Thus, the efficiency of visible light absorption in SNO seems to be much better than that in SVO and CVO, and of course, we must acknowledge that only visible light absorption does not guarantee satisfactory photocatalytic activity. To examine the origin of the discrepancy in the transition matrix elements between SNO and SVO (or CVO), the projected density of states (DOS) plots of SNO, SVO, and CVO are presented in Figure 3. In SNO, the VB bands are

which is also depicted in their band structure plots. In addition, different from SNO, the Sr states in SVO lie energetically higher than that in SNO and do not mix with the V d states at the CB band edge. In the CVO system, due to the distortion of the orthorhombic structure, the V d states without a large degenerate splitting are found to be dominant character in the B1 bands, which differ from that in SNO and SVO. Furthermore, more states appear in the vicinity of the Fermi level, indicating its high conductivity character. Similar to SVO, the Ca d states of CVO are also pushed to the high-energy part of the CB. Further inspecting the B1 and CB band edge, it is found that the band edge electronic structure difference between SNO and SVO (or CVO) is that the Sr 3d states hybridize with Nb 3d states in SNO, and Sr 3d states in SVO (or Ca 3d states in CVO) deeply locate in the CB without hybridization with V 3d states near the CB band edge. As all three materials contain inversion symmetry and the electronic− dipole operator is of odd parity, strong optical transitions are only allowed between two states of opposing parity. We speculate that the appearance of Sr d states at the CB band edge mainly consisting of eg states lead to different parity of the CB wave functions compared to the B1 wave functions for the special k points away from the Γ point, and the optical transition can occur, although the transition matrix elements are not very large in SNO. However, the same parity of the B1 and CB wave functions for all special k points in SVO and CVO forbids the strong optical transition in the visible light regime. 3.2. Effective Mass of Photogenerated Electrons and Holes. To shed light on the photocatalytic performance of the three perovskite materials, the effective masses of holes (m*h ) and electrons (me*) along various directions have been calculated by fitting parabolic functions to the CB minimum and B1 maximum of them, according to eq 1 ⎛ d 2E ⎞−1 m* = ±ℏ ⎜ 2k ⎟ ⎝ dk ⎠ 2

(1)

where k is the wave vector, and Ek is the energy corresponding to the wave vector k. Herein, we do not adopt the point on the band at the Fermi level in the hole effective mass calculations, considering that the holes can immediately relax to the B1 maximum after electron excitation. On the basis of the symmetry of the cubic system for SNO and SVO, we adopt three directions along [100], [110], and [111]. The results are summarized in Table 1. According to the least energy principle,37 there will be more electrons (or holes) in the lowermost of CB (or the uppermost of B1) upon light radiation, referring to heave electrons (or holes), thus the heave electrons (or holes) will be the preferred channel. It can be seen that the electron effective masses of SNO (me*) are much smaller than that of SVO (m(1) e *) along all three directions, indicating more dispersive band characterization in SNO. For the SNO, the effective mass of ∼0.40 me (me is the free-electron mass), is much smaller than that of TiO2 (∼1.0 me). Moreover, the values of me* in SNO are almost equivalent along every direction, which may benefit the electron transfer to the surface. For CVO, the photogenerated electron effective mass is simulated by fitting a parabolic function to the band at X points about 0.4 eV above the Fermi level in B1. It is found that the electron effective masses along X−R, U−X, and X−S directions are all about 1.0 me and along X−T, X−Z, and X−Y directions are about 1.25, 1.37, and 1.64 me, respectively. It also can be seen that the band along the Γ−X direction appears very

Figure 3. DOS calculated at the PBE level for (a) SNO, (b) SVO, and (c) CVO (red for the O p orbital, pink for the Sr or Ca d orbital, blue for t2g, and dark yellow for eg, whereas the other colors are for V d orbitals). The vertical dot dash lines represent the Fermi level, EF.

dominantly composed of O 2p states. Some of the Nb 3d occupied states sufficiently disperse within the O 2p states, implying the covalent property of the Nb−O bonds. However, only a few Sr-related electronic states appear in the O 2p valence band, which demonstrates ionic interaction between Sr and NbO6 octahedral group. Considering the Nb−O octahedral structure, the octahedral crystal field leads to a splitting of the degenerate Nb 3d states into t2g (dxy, dxz, dyz) and eg (dz2, dx2y2), with the partially occupied t2g states lying below the eg states. The B1 bands mainly consist of t2g states and the eg states locate essentially at the bottom of CB. Moreover, distinguishing from SrTiO3,50 the Sr 3d states of SNO mainly distributing in CB get mixed with Nb 3d states at the CB band edge. With the same cubic crystalline symmetry as SNO, the SVO has similar electronic properties compared to that of SNO (see Figure 3(a) and 3(b)). However, it is noted that the bandwidth of the t2g states in SVO is smaller than that in SNO, 5596



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Table 1. Effective Mass of Electron (me*) and Hole (mh*) for SrNbO3 and SrVO3 in Units of Free-Electron Mass (me) Obtained from Parabolic Fitting to the CB Minimum and B1 Maximum along Three Direction in Reciprocal Space

and Xiangchao Ma for discussions and also thank the National Supercomputer Center in Jinan for providing high performance computation.

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direction systems SrNbO3

SrVO3

m(1) e * m(1) h * m(2) h * m(3) h * m(1) e * m(2) e * m(1) h * m(2) h * m(3) h *

[100]

[110]

[111]

0.396 −3.614 −0.273 −0.273 --a 0.256 −4.284 −1.339 −1.339

0.398 −1.040 −0.297 −0.370 1.011 0.341 −2.229 −1.880 −1.322

0.397 −0.525 −0.525 −0.256 0.509 0.509 −1.803 −1.803 −1.563

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a The band along the [100] direction is very flat implying the large effective mass.

flat, indicting the large effective mass. Similar to the case for SVO, the effective masses for photogenerated electrons display anisotropic characterization. Additionally, the anisotropic characterization of effective mass is not altered in G0W0 and GW0 calculations. Because of the larger effective mass than SNO and their effective mass anisotropic characterization, we infer that the photogenerated carriers in SVO and CVO may easily recombine, suppressing their photocatalytic activity compared to SNO.

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SUMMARY We have studied the electronic structures and optical properties of three d1 metallic perovskite oxides SNO, SVO, and CVO to understand optical transition and photocatalytic performance in SNO and explore the possibility of photoactivity in other similar characterized perovskite oxides. Our analysis on optical transitional matrix elements shows that SNO can absorb visible light although the magnitude is not very large. In SVO and CVO, the optical transition matrix is zero, implying that strong direct transition absorption is impossible. According to the DOS analysis, the hybridization of Sr 3d states with Nb eg states may be primarily responsible for the absorption of the visible light in SNO but not in SVO and CVO. In addition, the effective mass calculations for the three materials indicate that the photogenerated carrier transfer ability of SNO is higher than SVO and CVO due to its relatively small effective mass of electrons and isotropic charge distribution. Thus, SNO possesses the better visible light photocatalytic response in d1 metallic perovskite materials.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (973 program, 2013CB632401), National Science foundation of China under Grant 11174180, the Fund for Doctoral Program of National Education 20120131110066, and the Natural Science Foundation of Shandong Province under Grant number ZR2011AM009. We thank Meng Guo 5597



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