Electronic, Magnetic and Photocatalytic Properties in (Fe, Ni)-Codoped SrTiO3 with and without Oxygen Vacancies: a First-principles Study①

WANG Yue-Qin LIU Yin ZHANG Ming-Xu MIN Fn-Fei

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Electronic, Magnetic and Photocatalytic Properties in (Fe, Ni)-Codoped SrTiO3with and without Oxygen Vacancies: a First-principles Study①

WANG Yue-Qina, bLIU Yina②ZHANG Ming-XuaMIN Fan-Feia

a(232001)b(232001)

The enhanced magnetic and photocatalytic properties of (Fe, Ni)-codoped SrTiO3with and without oxygen vacancies are investigated using the first-principles calculations based on the density functional theory pluscalculations. It is revealed that the structure phase transition associated with O vacancy imposes significant influence on magnetic and optical properties. The results show that the Ni oxidation state in (Fe, Ni)-codoped SrTiO3is about 2+, which is different from that of 4+in Ni monodoped SrTiO3in previous experimental investigations. The presence of O vacancy leads to a semiconductor-half-metal transition in codoped SrTiO3. The (Fe, Ni)-codoped SrTiO3without O vacancy produces an enhanced magnetization and induces a giant magnetic moment of 3μB, while a relatively small magnetic moment of 0.36 μBis generated in (Fe, Ni)-codoped SrTiO3with O vacancy. The origin of the large enhancement of magnetic moment in (Fe, Ni)-codoped SrTiO3without O vacancy was ascribed to the reduced hybridization in Fe–O bonds and the enhanced hybridization in Ni–O bonds, which modulated antiferromagnetic spin structure. The dispersion of the conduction bands and valence bands of codoped SrTiO3is enhanced after codoping, which benefits the photocatalytic performance. Furthermore, the (Fe, Ni)-codoped SrTiO3shows a remarkable red-shift of absorption spectra edge and induces a strong optical absorption in the visible light region, indicating that it could be taken as a potential candidate for photocatalytic materials.

SrTiO3, electronic structure, magnetic, photocatalytic activity, o vacancy;

1 INTRODUCTION

Strontium titanate (SrTiO3) is a typical multiferroic material with high dielectric constant, high dispersion rate and good thermal stability, which has been widely used in multilayer ceramic capacitors and dynamic random access memory devices.Moreover, the SrTiO3is a semiconductor with wide band gap (~3.25 eV)[1], which is similar to that of TiO2. Thus, the SrTiO3is widely considered as one of the promising photocatalysts for solving environ- mental problems, in particular, for the photocatalytic water splitting and the removal of organic con- taminants[2-5]. Layered metal oxides, such as TiO2, ZnO, SrTiO3, BaTiO3, KTaO3and Sr2TiO4, have been reported to be highly active photocatalysts under light irradiation[6-10]. The lack of visible light utilization of photocatalysts has limited their prac- tical applications because all of them are generally wide band gap semiconductors and only respond to ultraviolet (UV) light. Therefore, great efforts have been undertaken to extend their optical absorption capacity into visible light region of the solar irradiation spectrum. Both theoretical predictions and experimental results have been demonstrated that the cationic or anionic species doped layered metal oxides such as SrTiO3or TiO2is an effective way for extending the photoactive region to the visible light[11-15].

Recently, SrTiO3or TiO2doped with various transition metals, especially magnetic 3-impurities (Fe, Co, Mn, Cr, Ni), has been extensively investi- gated, with the aim of improving magnetoelectric coupling effect and extending the photoactivity to visible light region[14, 16-18]. In the past decades, the enhanced ferromagnetism has been reported in the substitution of Ti4+ions by the magnetic elements such as Fe4+, Mn4+, Co2+and Ni2+[19-21]. In addition, the evidence has revealed that the magnetic impurity atoms and the intrinsic lattice defects in SrTiO3may be the possible sources of magnetization[22, 23]. Transition metal doped SrTiO3has been also intensely studied as efficient visible light-induced photocatalysis used in photocatalytic water split- ting[24, 25]. Xie et al. reported that the photocatalytic activity for the degradation of RhB in Fe-doped SrTiO3was much higher than that of pure SrTiO3[26]. Niishiro et al. demonstrated that the photocatalytic activity can be effectively improved by doping Ni into SrTiO3[27]. Li et al. reported that La/Ni codoped SrTiO3exhibited a higher photocatalytic activity under visible light irradiation than SrTiO3mono- doped with La or Ni[28]. Much other research works have also demonstrated that the effects of different types of dopant metal atoms on modifying the elec- tronic structure, magnetic and optical properties of SrTiO3photocatalysis are different[29-32]. However, the electronic mechanism for the enhanced magne- tism and improved photocatalytic performance in (Fe, Ni)-codoped SrTiO3is lack. Thus, a detailed investigation on the magnetic and photocatalytic properties of (Fe, Ni)-codoped SrTiO3is needed.

In this paper, we have performed a theoretical calculation on (Fe, Ni)-codoped SrTiO3using a DFT method. The effects of (Fe, Ni)-codoped on the electronic structure, magnetic and photocatalytic properties of SrTiO3have been studied. The calcu- lated results may serve as a prediction study and provide a theoretical foundation in improving the photocatalytic activity of photocatalysts with perovskite structure.

2 COMPUTATIONAL METHOD

All calculations were carried out by using the Cambridge serial total energy package (CASTEP)[33]within the MS 8.0 package based on the density functional theory (DFT) level using periodic boun- dary conditions and plane-wave expansion of the wave function.The interaction between nuclei and electrons is approximated with Vanderbit ultra-soft pseudo potential (USPP) and the Perdew-Burke- Ernzerhof (PBE) is taken as the exchange-correla- tion potential in the generalized gradient approxi- mation (GGA).In order to describe the localized transition, we adopt the on-site repulsioneff= 2.3, 4.0 and 6.4 eV on the Ti-3, Fe-3and Ni-3states as in other studies before[34, 35], respectively, and thevalues were not specified. The basis was treated with Sr 424652, Ti 3242, Fe 3642, Ni 3842and O 2224. The Brillouin zone intergrations are performed with 8×8×8 and 3×3×3 Monkhorst-Packpoint mesh for pure SrTiO3and (Fe, Ni)-codoped SrTiO3, respectively. All the calculations were carried out spin-polarization with a kinetic energy cutoff of 380 eV, and the convergence criteria for total energy and displacement are 1 × 10-5eV/atom and 10-3?, respectively.

In order to model SrTiO3crystal structure with doping and vacancy configurations, we built a 2×2×2 periodic supercell based on the optimized primitive cell (Fig. 1). The supercell contains 40 atoms with 8 Sr atoms, 6 Ti atoms, 24 O atoms, 1 Fe atom and 1 Ni atom, and the dopant concentration for Fe or Ni is 12.5%. Codoping and vacancy struc- tures can be treated by replacing two Ti atoms by one Fe atom and one Ni atom and removing oxygen atom, respectively. Because the transition metal tends to substitute the B-site in SrTiO3, we only consider the B-site (Ti) doping in our calcula- tions[36].

Fig. 1. Supercell structures of (a) pure SrTiO3and (b) the possible codoping structure

3 RESULTS AND DISCUSSION

3. 1 Electronic structure

As shown in Fig. 1, there are three possible configurations (S1-3) for the (Fe, Ni)-codoped SrTiO3. Both the anti-ferromagnetic (AFM) and ferromagnetic (FM) ordering of Fe and Ni atoms in our calculations are considered, because the electronic structure depends upon the magnetic configuration. To determine the ground state of (Fe, Ni)-codoped SrTiO3, the total energies of both AFM and FM configurations are calculated and the different Fe and Ni oxidation states are considered. The calculated total energies of different co-doping structures are shown in Table 1. For simplicity, the lowest total energy of codoping structures is set to zero. As shown in Table 1, the (Fe4+, Ni2+)-codoped AFM S3 configuration is considered to be the ground state as it possesses the lowest total energy. It is noticed that the AFM state is more energetically stable than FM in the same structure configuration. Previous experimental studies have shown that the Ni oxidation state of Ni monodoped SrTiO3(SrTi0.97Ni0.03O3) within low doping concentration is 4+[37]. It is worth noting that the calculated total energy of (Fe4+, Ni2+)-codoped SrTiO3structure is lower than that of (Fe4+, Ni4+)-codoped SrTiO3structure, which indicates that the estimated Ni oxidation state in SrTi0.75Fe0.125Ni0.125O3is 2+with high doping concentration. The presence of Fe4+ions and high doping concentration are the main reasons for the decrease of Ni oxidation state[38]. As shown Table 1, the energetically stable AFM state of (Fe, Ni) codoped SrTiO3is (Fe4+, Ni2+) > (Fe3+, Ni2+) > (Fe4+, Ni4+). The formation energyfis also calculated to estimate the stability of the (Fe4+, Ni2+)-codoped SrTiO3system.

wheredopedandpureare the total energies of codoped SrTiO3and pure SrTiO3, respectively.Ti,FeandNiare the total energies of Ti, Fe and Ni atoms, respectively. The calculated formation energies of the three configurations (S1-3) are 9.95, 9.93 and 9.85 eV, respectively. The AFM spin ordering S3 configuration is considered to be the most stable. For simplicity, our following discussion is focused on the ground state (AFM S3) within (Fe4+, Ni2+) codoping.

Table 1. Total Energy of Different Structural and Magnetic Configurations of SrTi0.75Fe0.125Ni0.125O3

It is also worthy of noting that the charge in (Fe4+, Ni2+)-codoped SrTiO3model is non-neutral (charge state of 2－). In addition, the charge-neutral state in the (Fe4+, Ni2+)-codoped model can be created by introducing one oxygen vacancy. To investigate the (Fe, Ni) codoping effect on the electric structure of SrTiO3, the crystal structure parameters are calculated. To clarify the effect of charge on the electronic structure, we took into account the consideration of co-doped model without oxygen vacancy (SrFe0.125Ti0.75Ni0.125O3) and with oxygen vacancy (SrFe0.125Ti0.75Ni0.125O2.875). The calculated lattice constants for pure SrTiO3are=== 3.939 ?, which agree well with the experimental results of 3.906 ?[39]. After codoping, there is a lattice expansion in doped SrTiO3, and the lattice constants for SrFe0.125Ti0.75Ni0.125O3and SrFe0.125Ti0.75Ni0.125O2.875are as follows:== 3.941,= 3.955 ? and== 3.959,= 3.990 ?. After doping, the lattice distortion induces a structure phase transition from the cubic phase to the tetragonal phase. The previous studies have revealed that the lattice distortion and phase transition are the main reasons for charge separation, and play an important role in increasing the photocatalytic activity of the layered oxides[40, 41]. Therefore, the phase transition in (Fe4+, Ni2+)-codoped SrTiO3can contribute to the improving of photocatalytic activity.

The calculated atomic charge population and bond population for pure and codoped SrTiO3are listed in Tables 2 and 3, respectively. In Table 2, there is electron charge density redistribution near the impurity Fe and Ni atoms after doping. Especially, the presence of O vacancy has great influence on the net charge distribution. The net charges of the Ti*atoms near the Fe and Ni impurity atoms have a little increase, while the net charges of O(1)*and O(2)*have a little decrease, implying that the interaction between Ti*and O(1)*or O(2)*is weakened after doping. The net charges of Sr*atoms near the impurity atoms is decreased after doping. In particular, the net charges of the Fe and Ni impurity atoms are much smaller than that of the Ti atom, which results in that the Fe–O or Ni–O covalent bond has weaker length than Ti–O. In Table 3, for pure SrTiO3, the bond populations of Ti–O(1)/O(2) and Sr–O(1)/O(2) are 0.63 and –0.04, which indicate that the former is a strong covalent bond, while the latter is an ionic one. The bond populations of Ti–O(1) and Ti–O(2) are decreased after doping. Particularly, the bond population of the Ti–O(1) bond is larger than that of Ti–O(2) in the SrFe0.125Ti0.75Ni0.125O2.875configuration. This implies that the TiO6octahedron is strongly distorted due to the presence of O vacancy. Because the O vacancy in the TiO2layer leads to a set of five Ti–O bands to be shortened, the strength of Ti–O bonds is intensified. For Sr–O bond, the bond populations of Sr–O(1) and Sr–O(2) in the SrTiO3perovskite layers are increased after doping, indicating that the ionicity of Sr–O(1) and Sr–O(2) is weakened after doping. In particular, the bond populations of Sr*–O(1)*and Sr*–O(2)*bonds near the Fe and Ni impurity atoms are larger than that of the Sr–O(1) and Sr–O(2) bonds, so Sr*–O(1)*and Sr*–O(2)*bonds have weaker ionicity than Sr–O(1) and Sr–O(2). For doped SrTiO3, the bond populations of Fe–O(1)*/O(2)*bonds for SrFe0.125Ti0.75Ni0.125O3and SrFe0.125Ti0.75Ni0.125O2.875are 0.38 and 0.40, respec- tively. This implies that the covalency of Fe–O(1)*/O(2)*bonds is intensified due to the presence of O vacancy. Moreover, the bond populations of Ni–O(1)*/O(2)*bonds are reduced in the presence of O vacancy, which is confirmed by the increased bond lengths of Ni–O(1)*/O(2)*. The bond populations of Fe–O(1)*/O(2)*bonds are larger than that of the Ni–O(1)*/O(2)*bonds, indicating that the covalency in Fe–O(1)*/O(2)*bonds is stronger than that of Ni–O(1)*/O(2)*. Because the strengths of Fe–O and Ni–O bonds are weaker than that of Ti–O, the structure stability of SrFe0.125Ti0.75Ni0.125O3and SrFe0.125Ti0.75Ni0.125O2.875is weakened after codoping.

Table 2. Charge Distribution Variation of Pure SrTiO3, SrFe0.125Ti0.75Ni0.125O3 and SrFe0.125Ti0.75Ni0.125O2.875

Table 3. Band Population and Bond Length Variation of Pure SrTiO3, SrFe0.125Ti0.75Ni0.125O3 and SrFe0.125Ti0.75Ni0.125O2.875. Here Ti*, Sr*, O(1)* and O(2)* Are the Atoms near the Impurity Fe and Ni Atoms

To further investigate the influence of electronic structure in (Fe, Ni)-codoped SrTiO3with and without oxygen vacancy, the density of states (DOS) of pure and codoped SrTiO3are calculated and shown in Fig. 2. For pure SrTiO3, the band gap obtained from our GGA+calculation is 2.05 eV, which is smaller than the experimental value of about 3.25 eV[1]. This is typically underestimated by DFT and also caused by the strong self-interaction of Ti-3states[9, 42]. Shen et al. Performed a GGA calculation of pure SrTiO3within the CASTEP package, and the band gap of SrTiO3was 1.85 eV[20]. It is found that our calculation performed with GGA+obtained a better band gap than that of GGA by other theoretical calculations. Therefore, a scissor approximation value of 1.2 eV is adopted in the optical calculation to compensate the underes- timation of the calculated band gap. As shown inFig. 2(b) and (c), for codoped SrTiO3, the density of conduction bands (CBS) shifts to the direction of CBSbottom, indicating that the Fe-3and Ni-3states play an important role in the CBSwith a range of 0～6 eV. As seen from Fig. 2(b), the SrFe0.125Ti0.75Ni0.125O3structure keeps the insulting band gap character while a reduced band gap is produced after codoping. The reduced band gap is attributed to the shift of the position for the bottom of CBStowards low energies. This narrowed band gap would have great influence on the photocatalytic properties of SrFe0.125Ti0.75Ni0.125O3. As seen from Fig. 2(c), the SrFe0.125Ti0.75Ni0.125O2.875shows half-metallic characteristic with the spin-up DOS is metallic at the Fermi level, while the spin-down DOS is insulating with a large band gap of 1.57 eV. The noticed feature is that the presence of oxygen vacancy induces the codoped SrTiO3to form half- metal band gap character. The calculated results demonstrate that the codoped SrTiO3exhibits a semiconductor-half-metal phase transition as the oxygen vacancy absent or not.

Fig. 2. Total electronic density of states (TDOS) of (a) pure SrTiO3(b) SrFe0.125Ti0.75Ni0.125O3and (c) SrFe0.125Ti0.75Ni0.125O2.875

3. 2 Magnetic properties

To investigate the magnetic property of (Fe, Ni)-codoped SrTiO3with and without oxygen vacancy, the magnetic moments of magnetic atoms are calculated. For the codoped SrTiO3without oxygen vacancy, the calculated magnetic moments of Fe4+and Ni2+are 3.46and –1.19B, respectively. The calculated magnetic moments of Fe4+and Ni2+in codoped SrTiO3with one oxygen vacancy are 2.42and –1.66B, respectively, which introduces a reduced magnetic moment. Obviously, the calcu- lated magnetic moments of Fe4+and Ni2+are smaller than the theoretical values (Fe4+: 4Band Ni2+: 2B). It is found that the pure SrTiO3is a nonmagnetic semiconductor, while the SrFe0.125Ti0.75Ni0.125O3is a magnetic semiconductor with a total magnetic moment of 3Band SrFe0.125Ti0.75Ni0.125O2.875is weak magnetic half-metallic with a total magnetic moment of 0.36B. The total magnetic moment of SrFe0.125Ti0.75Ni0.125O3(3B) is larger than that of the theoretical magnetic moment about 2B, implying that the magnetic properties of codoped SrTiO3without oxygen vacancy are greatly en- hanced. The total magnetic moment of SrFe0.125Ti0.75Ni0.125O2.875(0.36B) is much smaller than that of SrFe0.125Ti0.75Ni0.125O3, which can be ascribed to the strong hybridization produced in the presence of O vacancy[20]. In addition, the O atom near the Fe or Ni atom in the codoped SrTiO3without and with oxygen vacancy is magnetized, and the magnetic moments are 0.10/0.09and –0.09/–0.02B, respectively. The presence of mag- netic O atoms is associated with the hybridization with the O-2and 3states of metal atoms.

In order to further investigate the origin of magnetism in (Fe, Ni)-codoped SrTiO3with and without oxygen vacancy, the partial spin density of states (PDOS) is studied and the results are shown in Fig. 3. As shown in Fig. 3(a), for pure SrTiO3, the top of valance band (VB) consists of O-2states, and the CB is mainly composed of Ti-3states, and the contribution of O-2states is negligible. Because the spin-up and spin-down DOS of Fe and Ni atoms are exactly symmetrical, pure SrTiO3shows a non-ferromagnetic behavior. It can be observed that there is a strong hybridization between the Ti-3and O-2states, suggesting that the Ti-O bond is a strong covalent bond. As shown in Fig. 3(b) and (c), a net magnetic moment leads to the spin-up and spin-down DOS of Fe and Ni atoms unsymmetrical. After codoping, the DOS of Ti-3and O-2is decreased, and the density of 3states of Fe and Ni atoms has significant contribution to the DOS of (Fe, Ni)-codoped SrTiO3. As seen from Fig. 3(b), the strong hybridization between the 2states of O atom and 3states of Fe, Ni and Ti transition metal atoms in CB result in narrowing the band gap of SrFe0.125Ti0.75Ni0.125O3. As seen from Fig. 3(c), for SrFe0.125Ti0.75Ni0.125O2.875, the hybridization between Fe and O atoms are intensified, while the hybridi- zation between Ni and O atoms is reduced in the presence of O vacancy. The strong hybridization between Fe and O atoms reduces the spin-up magnetic moment of Fe atom. Meanwhile, the weak hybridization between Ni and O atoms enhances the spin-down magnetic moment of Ni atom, which explains the relatively small net magnetic moment of SrFe0.125Ti0.75Ni0.125O2.875. The spin-up Fe-3and O-2states at the Fermi level are excited into the band gap, which explains the origin of half-metallic characteristic in SrFe0.125Ti0.75Ni0.125O2.875. The origin of large enhancement of net magnetic moment in SrFe0.125Ti0.75Ni0.125O3was attributed to the reduced hybridization in Fe–O bonds and the enhanced hybridization in Ni–O bonds, which modulated antiferromagnetic spin structure. Fig. 4 displays the density of states of magnetic O atoms near the Fe and Ni atoms. As shown in Fig. 4, the density of O-2states of SrFe0.125Ti0.75Ni0.125O3is much different from that of SrFe0.125Ti0.75Ni0.125- O2.875, implying that the O vacancy has a significantinfluence on the DOS of magnetic O atoms. The previous study has con- cluded that the hybridization produces the magnetic O atoms and reduces the magnetic moments of impurity atoms[20]. For codoped SrTiO3, the density of O-2states of magnetic O atoms near the Fe impurity located in CB is larger than that of the O atoms near the Ni impurity, which indicates that the hybridization between Fe and O atoms is stronger than that between Ni and O atoms. In particular, the density of O-2states of magnetic O atoms near the Ni impurity located in CB are greatly decreased in the presence of O vacancy, see Fig. 4(b). This implies that the O vacancy reduces the hybridization between Ni and the nearest magnetic O atoms, which leads to the enhancement of magnetic moment of the Ni atom. In addition, as shown in Fig.4(b), the spin-up O-2electronic states of magnetic O atom near Fe are excited into the band gap, which explains the origin of half-metal electronic structure in the presence of O vacancy.

Fig. 3. Partial DOS of (a) pure SrTiO3(b) SrFe0.125Ti0.75Ni0.125O3and (c) SrFe0.125Ti0.75Ni0.125O2.875

Fig. 4. PDOS of the magnetic O atoms near Fe and Ni atoms of (a) SrFe0.125Ti0.75Ni0.125O3and (b) SrFe0.125Ti0.75Ni0.125O2.875

3. 3 Optical photocatalytic properties

The band structures of pure SrTiO3and codoped SrTiO3with and without oxygen vacancy are calcu- lated and presented in Fig. 5. There is a large spin- splitting in the CBSafter codoping, which is mainly ascribed to the exchange splitting of Fe-3states. Moreover, the presence of O vacancy increases the exchange splitting of Fe-3states at the bottom of CBS(Fig. 5(c)). We can notice that the CBSand VBSof SrFe0.125Ti0.75Ni0.125O3and SrFe0.125Ti0.75Ni0.125- O2.875are highly dispersive after codoping. As for the CBS, the average ionic radii of transition metal ions Fe4+(0.585 ?) and Ni2+(0.69 ?) are 0.6375 ? and larger than that of Ti4+(0.605 ?), thus making the Fe, Ni, unsubstituted Ti and O atoms more closely linked to each other, so the symmetry of TiO6octahedron has been broken after the substitution of Fe4+and Ni2+for Ti4+. The distorted structure leads to a stronger hybridization between the 3states of Fe (Ni and Ti) and O-2states, and therefore CBSbecome dispersive. As for the VBS, the dopant ions become overlapped within high doping concentration, which leads to an enhance- ment of the correlation movement between the transition metal atoms and O atoms, so the VBSare broadened and become dispersive. We can observe that the CBSand VBSof SrFe0.125Ti0.75Ni0.125O3are more dispersive than that of SrFe0.125Ti0.75- Ni0.125O2.875, which implies that the mobility of generated electrons and holes in SrFe0.125Ti0.75- Ni0.125O3is larger than that of SrFe0.125Ti0.75- Ni0.125O2.875. The high dispersion feature leads the electron-hole pairs to move more easily in SrFe0.125- Ti0.75Ni0.125O3and SrFe0.125Ti0.75Ni0.125O2.875, which is helpful for the enhancement of photocatalytic activities of (Fe, Ni) codoped SrTiO3system. In addition, the bottoms of CBSfor SrFe0.125- Ti0.75Ni0.125O3and SrFe0.125Ti0.75Ni0.125O2.875show considerable delocalization, inducing large mobility of generated electrons, and thus benefit the transport of electrons, which also contributes to the enhancement of photocatalytic property.

Fig. 5. Band structures of (a) pure SrTiO3(b) SrFe0.125Ti0.75Ni0.125O3and(c) SrFe0.125Ti0.75Ni0.125O2.875. The black solid and red dotted lines represent the spin-up band and spin-down band, respectively

To investigate the (Fe, Ni) codoping effect on the photocatalytic properties of SrTiO3, the optical absorption spectra of SrFe0.125Ti0.75Ni0.125O3and SrFe0.125Ti0.75Ni0.125O2.875are calculated, as shown in Fig. 6. In Fig. 6(a), the absorption spectra edges of SrFe0.125Ti0.75Ni0.125O3and SrFe0.125Ti0.75Ni0.125O2.875have obvious red-shift. The previous study has confirmed that the optical absorption of SrTiO3is enhanced due to the distortion structure[43]. The red-shift features in SrFe0.125Ti0.75Ni0.125O3and SrFe0.125Ti0.75Ni0.125O2.875systems are attributed to the cubic-to-tetragonal phase transition and the narrowed band gap after codoping. Fig. 6(b) displays the absorption spectra of pure and codoped SrTiO3in the visible regions. The new absorption peaks appear in the visible light regions (> 600 nm), which benefits for the enhanced photocatalytic activity of the codoped SrTiO3. The interesting feature is that the absorption coefficients of SrFe0.125Ti0.75Ni0.125O3and SrFe0.125Ti0.75Ni0.125O2.875are greatly enhanced in the wavelength region of 416～760 nm. In particular, the absorption coefficient of SrFe0.125- Ti0.75Ni0.125O3is larger than that of SrFe0.125- Ti0.75Ni0.125O2.875in the wavelength range of 416~600 nm, implying that the SrFe0.125Ti0.75- Ni0.125O3shows better photocatalytic performance than SrFe0.125Ti0.75Ni0.125O2.875, which probably lies in that the former keeps an insulating band gap character after codoping, while the latter is a half- metal. The semiconductor-half-metal phase tran- sition induces the decrease of photocatalytic activity of SrFe0.125Ti0.75Ni0.125O2.875.

Fig. 6. Absorption coefficient of pure SrTiO3and codoped SrTiO3with and without O vacancy in wavelength region (a) 100～1000 nm and (b) 390～760 nm

4 CONCLUSION

In conclusion, we have investigated the stability, electronic structure, enhanced magnetic and photo- catalytic properties of (Fe, Ni) codoped SrTiO3through the first-principles calculations. Compared with that of the Ni oxidation state, 4+, in Ni monodoped SrTiO3in the previous experimental studies, our calculated results show that the Ni oxidation state in (Fe, Ni) codoped SrTiO3within high codoping concentration is about 2+. The calculated results demonstrated that the structural stability of codoped SrTiO3with and without oxygen vacancy is weakened, and the phase transition cubic-to-tetragonal is produced after codoping. The structure phase transition associated with O vacancy imposes significant influence on magnetic behaviors and optical properties. There is a semiconductor- half-metal phase transition in the presence of O vacancy in codoped SrTiO3. The (Fe, Ni) codoped SrTiO3without and with O vacancy induces net magnetic moment of 3and 0.36 μB, respectively. The origin of the enhanced magnetic moment originates from the covalent effect in Fe–O and Ni–O bonds, which modulated the antiferromagnetic spin structure. It is revealed that the high dispersion of conduction bands and valence bands of codoped SrTiO3benefit the photocatalytic performance. In addition, the red-shift of absorption spectra edge and a new absorption appears in the visible region 416～ 800 nm are observed after codoping, suggesting that the (Fe, Ni) codoping is an effective way for improving the photocatalytic activity of SrTiO3with perovskite structure.

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8 November 2017;

6 March 2018

the National Natural Science Foundation of China (No. 51474011), the Postdoctoral Science Foundation of China (No. 2014M550337), the Key Technologies R&D Program of Anhui Province (No. 1604a0802122, 17030901091),and the academic funding project for the top talents of colleges and universities (No. gxbjZD14)

. Home Tel: +86-0554-6668643, Mobile: +86-15077951960, E-mail: yinliu@aust.edu.cn

10.14102/j.cnki.0254-5861.2011-1883