⇦Chinese Physics Letters, 2018, Vol. 35, No. 1, Article code 017101 Electronic Structure and Visible-Light Absorption of Transition Metals (TM=Cr, Mn, Fe, Co) and Zn-Codoped SrTiO$_{3}$: a First-Principles Study * Yue-Qin Wang(汪月琴)1,2, Yin Liu(刘银)1**, Ming-Xu Zhang(张明旭)1, Fan-Fei Min(闵凡飞)1 Affiliations 1School of Material Science and Engineering, Anhui University of Science and Technology, Huainan 232001 2School of Mechanics and optoelectronic Physics, Anhui University of Science and Technology, Huainan 232001 Received 13 October 2017 *Supported by the National Natural Science Foundation of China under Grant No 51474011, the Postdoctoral Science Foundation of China under Grant No 2014M550337, and the Key Technologies R&D Program of Anhui Province of China under Grant No 1604a0802122.
**Corresponding author. Email: yinliu@aust.edu.cn Citation Text: Wang Y Q, Liu Y, Zhang M X and Min F F 2018 Chin. Phys. Lett. 35 017101 Abstract First-principles calculations are performed on the influence of transition metal (TM=Cr, Mn, Fe, Co) as codopants on the electronic structure and visible-light absorption of Zn-doped SrTiO$_{3}$. The calculated results show that (Zn,Mn)-codoped SrTiO$_{3}$ requires the smallest formation energy in four codoping systems. The structures of the codoped systems display obvious lattice distortion, inducing a phase transition from cubic to rhombohedral after codoping. Some impurity Cr, Mn and Co 3$d$ states appear below the bottom of conduction band and some Fe 3$d$ states are located above the top of valence band, which leads to a significant narrowing of band gap after transition metal codoping. The enhancement of visible-light absorption are observed in transition metals (TM=Cr, Mn, Fe, Co) and Zn codoped SrTiO$_{3}$ systems. The prediction calculations suggested that the (Zn,Mn)- and (Zn,Co)-codoped SrTiO$_{3}$ could be the desirable visible-light photocatalysts. DOI:10.1088/0256-307X/35/1/017101 PACS:71.20.Nr, 73.22.-f, 78.20.-e © 2018 Chinese Physics Society Article Text Perovskite oxides are attractive candidates for photocatalytic materials, which have been widely used in solving the energy and environmental issues, such as water splitting, NO reduction, hydrogenolysis of hydrocarbons.[1,2] Strontium titanate (SrTiO$_{3}$) is one of them. However, its practical application has been restricted to the ultraviolet (UV) light ($\lambda < 365$ nm) due to the large band gap of SrTiO$_{3}$ ($\sim$3.25 eV).[3] Therefore, narrowing the band gap to extend the optical absorption edge has become one of the most important goals in visible-light-driven water splitting.[4-10] Recently, some new SrTiO$_{3}$ photocatalysts have been explored by transition metal substitution and carries doping in experiment, including Zn-doped SrTiO$_{3}$ and BaTiO$_{3}$,[11,12] (Mn,Fe,Co)-doped SrTiO$_{3}$,[13,14] Cr-doped SrTiO$_{3}$,[15] La and Nb-doped SrTiO$_{3}$,[16] etc. Zou et al.[12] have reported on the photocatalytic activity of Sr$_{1-x}$Zn$_{x}$TiO$_{3}$ ($x=2/3$ and 5/6) prepared by the simple sol-gel method and shows remarkable improvement in H$_{2}$ production efficiency compared with that of pure SrTiO$_{3}$. To further search for high photocatalytic activity materials, the A-site and B-site substitutional codoping are expected for visible light photocatalysts. In a previous experiment, Zhou et al.[14] have found that the transition metals (TM=Mn, Fe and Co) substitute Ti (B-site) in SrTiO$_{3}$ and NaTaO$_{3}$ and exhibit an enhancement of absorption activity in the visible-light region. To our knowledge, however, the electronic structure and optical absorption properties of (Zn, TM=Cr, Mn, Fe, Co)-codoped SrTiO$_{3}$ have not been reported. Therefore, the enhanced visible light absorption efficiency and photocatalytic performance are expected for (Zn, TM=Cr, Mn, Fe, Co)-codoped SrTiO$_{3}$. In the present work, the electronic structure and optical properties of Zn-doped and (Zn, TM=Cr, Mn, Fe, Co)-codoped SrTiO$_{3}$ are investigated using density functional theory (DFT). Because the Zn cation prefers to substitute the A-site[12] and the transition metal tends to substitute the B-site[14] in SrTiO$_{3}$, here we consider the Zn@Sr and TM@Ti codoping in our calculations. The mechanism of reduced band gap and the origin of enhanced photocatalytic performance have been studied. It is found that (Zn, TM)-codoped SrTiO$_{3}$ have better photocatalytic activity than that of Zn-doped SrTiO$_{3}$. The calculated results may be served as a prediction study and provide a theoretical foundation in improving the photocatalytic efficiency of SrTiO$_{3}$ with perovskite structure. All of the spin-polarized calculations were performed using the CASTEP[17] within the MS 8.0 software package based on DFT. The Perdew–Burke–Ernzerhof (PBE) parameterization is taken as the exchange-correlation potential in the generalized gradient approximation (GGA). The interaction between nuclei and electrons is approximated with the Vanderbit ultra-soft pseudo potential (USPP). To describe the localized transition, we adopt the on-site repulsion $U_{\rm eff}=2.3$ eV, 3.5 eV, 4.0 eV, 4.0 eV and 3.3 eV on the Ti-3$d$, Cr-3$d$, Mn-3$d$, Fe-3$d$ and Co-3$d$ states as other studies before, respectively.[18,19] The kinetic energy cutoff is chosen to be 380 eV. The $k$-point meshes of the Brillouin zone sampling for pure and doped SrTiO$_{3}$ were set at $8\times8\times8$ and $3\times3\times3$ based on the Monkhorst–Pack scheme, respectively. Both the lattice constants and atomic positions are fully relaxed until the residual forces were below 0.01 eV/Å.
cpl-35-1-017101-fig1.png
Fig. 1. The supercell model for doped SrTiO$_{3}$ showing the location of the dopants.
All the doped systems were constructed from a relaxed $2\times2\times2$ supercell based on the optimized primitive cell, and it is shown in Fig. 1. SrTiO$_{3}$ is a semiconductor photocatalyst in the typical perovskite structure with space group $Pm3m$, in which the center position is occupied by the Sr cation. The corner-sharing TiO$_{6}$ octahedra forms the skeleton crystal structure of SrTiO$_{3}$. To study the stabilities of different doped systems, the defect formation energy ($E_{\rm f}$) for the doped and codoped systems are calculated according to[20] $$\begin{alignat}{1} &E_{\rm f(Zn@Sr)}=E_{\rm (Zn@Sr)}-E_{\rm pure}-(\mu_{\rm Zn}-\mu_{\rm Sr}),~~ \tag {1} \end{alignat} $$ $$\begin{alignat}{1} &E_{\rm f(Zn@Sr\&TM@Ti)}=E_{\rm (Zn@Sr\& TM@Ti)}-E_{\rm pure}\\ &-(\mu_{\rm Zn}+\mu_{\rm TM}-\mu_{\rm Sr}-\mu_{\rm Ti}),~~ \tag {2} \end{alignat} $$ where $E$ is the total energy of bulk supercell, $\mu$ is the chemical potential of atom, and $\mu_{\rm TM}$ is the chemical potential of per transition metal atom in bulk material Cr, Mn, Fe and Co. The calculated defect formation energies of different doped systems are listed in Table 1. The larger the defect formation energy is, the more difficult substitutional codoping is.[14] The results show that (Zn,Mn) codoping is the most stable system because of the lowest defect formation energy.
Table 1. Defect formation energies $E_{\rm f}$ for different doped SrTiO$_{3}$ models.
Doped models Zn@Sr Zn@Sr&Cr@Ti Zn@Sr&Mn@Ti Zn@Sr&Fe@Ti Zn@Sr&Co@Ti
$E_{\rm f}$ (eV) 6.384 8.006 5.633 13.139 10.259
Table 2. Geometry parameters, unit cell volumes, average Mulliken charges and average bond lengths of pure and different doped SrTiO$_{3}$ TM are referred to the transition metal atoms Cr, Mn, Fe and Co, respectively.
Codped models Lattice parameters Volume (Å$^{3}$) Average Mulliken charge ($e$) Average bond length (Å)
$a/b/c$ (Å) $\alpha/\beta/\gamma$($^{\circ}$) Sr Ti O Zn Zn-O Ti-O TM-O
Pure STO 3.952 90 493.8 1.40 0.88 $-$0.76 1.976
Zn@Sr 3.940 90 489.5 1.35 0.89 $-$0.75 1.30 2.773 1.970
Zn@Sr&Cr@Ti 3.975 89.5 502.6 1.34 0.91 $-$0.74 1.25 2.667 1.991 2.018
Zn@Sr&Mn@Ti 3.955 89.6 494.9 1.33 0.93 $-$0.73 1.25 2.759 1.981 1.983
Zn@Sr&Fe@Ti 3.953 89.6 494.3 1.33 0.90 $-$0.74 1.31 2.770 1.982 1.983
Zn@Sr&Co@Ti 3.977 89.6 503.1 1.32 0.90 $-$0.74 1.26 2.843 1.995 2.009
The lattice parameters, volumes of supercell, average Mulliken charges and average bond lengths of pure STO and different doped STO are listed in Table 2. The calculated lattice constant of pure cubic structure SrTiO$_{3}$ is 3.952 Å, which is in good agreement with the experimental value of 3.906 Å.[21] For the Zn monodoped SrTiO$_{3}$, there is a slight lattice contraction with respect to pure SrTiO$_{3}$, which leads to the decrease of the volume while keeping the cubic perovskite structure. Because the electronic structure of doped systems are mainly associated with the radius of the substituting cations.[22] The lattice distortion is observed due to the Zn cation doping at the Sr site, which would induce an off-center displacement in the titled-TiO$_{6}$ octahedron to adjust the difference in radius between Zn cation (0.74 Å) and Sr host cations (1.18 Å). Therefore, the contraction of lattice constant and volume and the shortened Ti-O bond in Zn-doped SrTiO$_{3}$ can be attributed to the small radius substitution of Zn cation. However, for the Zn and transition metal atoms (TM=Cr, Mn, Fe, Co) codoped SrTiO$_{3}$, the lattice parameters and the volumes show a slight expansion with respect to pure and Zn-doped SrTiO$_{3}$. An increased Ti vacancy can be introduced and also lattice strain generated after TM with codopants doping Zn-SrTiO$_{3}$. This small lattice expansions in codoped SrTiO$_{3}$ systems are closely related to possible increased Ti deficiencies for Ti site substitutional doping and slight lattice distortion.[23] The structure of the codoped systems displays obvious lattice distortion, leading to a phase transition from cubic to rhombohedral after codoping. Mulliken charges of all atoms are redistributed after doping. The average Mulliken charge of Sr and O decreases by 3.57%–5.71% and 1.32%–3.94%, respectively, while the average Mulliken charge of Ti increases by 1.14%–5.68%. We can find that Zn and the transition metal atoms TM codoping have great influence on the distribution of the average Mulliken charge of Zn atom. For the case of substituting Sr with Zn and substituting Ti with transition metal cations TM (TM=Cr, Mn, Co), the calculated average Mulliken charges of Zn cations decline by 3.1%–3.8%, which imply that part of O-$2p$ electrons are transferred to nearby Zn cations. For the case of substituting Sr with Zn and substituting Ti with Fe cation, the calculated Mulliken charge of Zn increases by 0.8%. This slight change in Mulliken charge of Zn cation leads to the average bond length of Zn–O ionic bonds in (Zn,Fe) codoped SrTiO$_{3}$ (2.773 Å) having nearly no change with respect to that in Zn-doped SrTiO$_{3}$ (2.770 Å). The average bond lengths of TM–O in different doped SrTiO$_{3}$ systems are slightly larger than those of Ti–O bonds. The bond length is mainly determined by cations radius and electronegativity of bonded atoms.[24] The expansion in TM–O bond length can be explained by the fact that the ionic radii of transition metal cations TM (TM=Cr$^{4+}$, Mn$^{4+}$, Fe$^{4+}$ and Co$^{4+}$) are smaller than that of Ti$^{4+}$ cation and the electronegativity of doping cations (1.66, 1.55, 1.83 and 1.88 for Cr$^{4+}$, Mn$^{4+}$, Fe$^{4+}$ and Co$^{4+}$, respectively) is stronger than that of Ti$^{4+}$ (1.54).
cpl-35-1-017101-fig2.png
Fig. 2. Calculated TDOS of different doped SrTiO$_{3}$. Curves above and below the horizontal axis refer to the up-spin and down-spin DOS, respectively.
To investigate the influence of substitutional codoping on the electronic band structure of SrTiO$_{3}$, the total density of states (DOS) of pure and different doped SrTiO$_{3}$ structures are calculated and plotted in Fig. 2. The calculated indirect band gap of pure SrTiO$_{3}$ is 1.94 eV, close to the calculated value 1.73 eV by Zhou et al.[14] Due to the typical limitation of GGA, there is still essentially underestimation as compared with the experimental value of about 3.25 eV. For Zn-doped SrTiO$_{3}$ (Zn@Sr), the bottom of the conduction band (CB) moves towards the direction of low energy, resulting in a band gap narrowing of about 0.71 eV, which is responsible for the obvious red shift of the absorption edge for Zn-doped SrTiO$_{3}$ reported in experiment.[11] For Zn-Cr codoped SrTiO$_{3}$, there is nearly no change of the band gap compared with that of pure SrTiO$_{3}$. For (Zn, TM) (TM=Mn, Fe, Co) codoped SrTiO$_{3}$, some impurity states appear in the middle of the forbidden gap, resulting in a band gap narrowing of about 0.68 eV, 0.94 eV and 1.09 eV, respectively. These in-gap impurity states may act as the photo-excited electron–hole recombination center and benefit for the photocatalytic performance.
cpl-35-1-017101-fig3.png
Fig. 3. Calculated PDOS (a) pure, (b) Zn@Sr, (c) Zn@Sr&Cr@Ti, (d) Zn@Sr&Mn@Ti, (e) Zn@Sr&Fe@Ti and (f) Zn@Sr&Co@Ti for different doped SrTiO$_{3}$.
To further investigate how the substitutional codoping by transition metal cations modifies the electronic structure of SrTiO$_{3}$, the partial density of states (PDOS) of pure and different doped SrTiO$_{3}$ are shown in Fig. 3. For pure SrTiO$_{3}$, the valence band (VB) is dominated by O 2$p$ states, while the CB mainly consists of Ti 3$d$ states. For Zn-doped SrTiO$_{3}$ (Zn@Sr), the CB broadens with the mixing of Zn 3$p$ and O 2$p$ states, while the CB bottom consists of Ti 3$d$ states, which shifts toward the direction of low energy resulting in a band gap narrowing. For (Zn,Cr) codoped SrTiO$_{3}$ (Zn@Sr&Cr@Ti), it shows that the CB bottom dominantly consists of Cr 3$d$, Zn 3$p$ and O 2$p$ states, while the VB maximum is composed of the Cr 3$d$ and Ti 3$d$ states. The band gap is slightly reduced due to the appearance of Cr 3$d$ states below the bottom of CB. For (Zn, TM) (TM=Mn, Fe, Co) codoped SrTiO$_{3}$, some impurity states (Zn 3$p$, Mn 3$d$, Fe 3$d$ and Co 3$d$) are mixed with the edges of VB and CB, and most Mn 3$d$, Fe 3$d$ and Co 3$d$ are located in the band gap with respect to pure SrTiO$_{3}$, which can be ascribed to strong interaction between 3$d$ states of transition metal (Mn, Fe and Co) and 3$d$ states of Ti. Some impurity Cr, Mn and Co 3$d$ states appear below the bottom of CB and some Fe 3$d$ states are located above the top of VB, which lead to the band gap narrowing with respect to pure SrTiO$_{3}$. In particular, the substitutional codoping of (Zn,Fe)-SrTiO$_{3}$ introduces some impurity states above the top of VB (shown in Fig. 3(e)) resulting in a band gap narrowing, which could be responsible for the red shift of absorption edge in agreement with the previous experimental results reported in Fe-doped SrTiO$_{3}$.[25] On the basis of the electronic structure obtained above, we calculated the complex dielectric function $\varepsilon =\varepsilon_{1}+i\varepsilon_{2}$. The optical absorption coefficient $I(\omega)$ was estimated by $$\begin{alignat}{1} I(\omega)=2\omega \Big[\frac{(\varepsilon_1 (\omega)+\varepsilon_2(\omega ))^{1/2}-\varepsilon_1 (\omega )}{2}\Big]^{1/2},~~ \tag {3} \end{alignat} $$ where $\omega$ is the angular frequency of phonon, $\varepsilon_{1}$ and $\varepsilon_{2}$ are the real and imaginary parts of dielectric function, respectively. The absorption spectra of pure and different doped SrTiO$_{3}$ are calculated and shown in Fig. 4. Here we adopt a scissor shift of 1.31 eV in the optical property calculation to compensate for the underestimated band gap. As shown in Fig. 4, the obvious red shifts of absorption edge are produced in Zn/TM monodoped or (Zn, TM) (TM=Cr, Mn, Fe, Co) codoped SrTiO$_{3}$. It shows that pure SrTiO$_{3}$ can only respond in UV-light and shows no absorption activity in visible light region. For Zn-doped SrTiO$_{3}$, the slight narrowing of band gap is observed, because the CB bottom downward shifts can lead to the reduction of the phonon excitation energy from the VB to the CB, which induces an obvious red shift of the optical absorption edge while decreases the optical absorption in the UV-light region. For (Zn,Cr)-codoped SrTiO$_{3}$, the spin-down band gap is 2.22 eV, which is slightly larger than that of pure SrTiO$_{3}$, resulting in a reduction of absorption in the UV-light region. Meanwhile, the value of spin-up band gap (1.72 eV) is smaller than that of pure SrTiO$_{3}$, which induces a new absorption peak in the visible light region. For (Zn,Mn)- and (Zn,Co)-codoped SrTiO$_{3}$, the greatly reduced band gap due to some impurity electronic states excited into the forbidden gap indicates that the photo-generated electrons are excited easily from the VB to the CB, which induces a good optical absorption in the visible light region. For (Zn,Fe)-codoped SrTiO$_{3}$, the excitation of VB maximum into the gap and the relative larger band gap are comparable with those of (Zn,Mn) and (Zn,Co)-codoped SrTiO$_{3}$ systems, which leads to the fact that the intensity of absorption is weakened in the visible region. For better comparison, the inset of Fig. 4 shows the condensed plots in the visible-light region (390 nm–760 nm). We can observe that the (Zn, TM) (TM=Cr, Mn, Fe, Co) codoped SrTiO$_{3}$ shows significant enhancement of absorption efficiencies in the visible-light region with respect to that of pure and Zn-doped SrTiO$_{3}$. That is to say, the transition metal TM doping Zn-SrTiO$_{3}$ is an efficient way to extend the application of photocatalyst in the visible-light region. In addition, we can find that (Zn,Mn) codoped SrTiO$_{3}$ is the most promising visible-light photocatalyst in these doped systems. As shown in the inset plots of Fig. 4, its absorption edge is greatly enhanced to 706 nm with respect to Zn-doped SrTiO$_{3}$ (456 nm) and Mn-doped SrTiO$_{3}$ (545 nm).
cpl-35-1-017101-fig4.png
Fig. 4. Absorption of pure and different doped SrTiO$_{3}$. The inset shows the condensed plots (390 nm–760 nm).
In summary, we have calculated the electronic structure and photocatalytic properties of transition metals (TM=Cr, Mn, Fe, Co) and Zn codoped SrTiO$_{3}$ using the first-principles method. The results indicate that (Zn,Mn)-codoped SrTiO$_{3}$ is energetically favorable due to the lowest defect formation energy. The cubic-to-rhombohedral phase transition is produced after transition metal codoping. Some in-gap impurity 3$d$ states of transition metals lead to a significant band gap narrowing, and thus the red shifts and visible-light absorption are observed. The (Zn,Mn) and (Zn,Co)-codoped SrTiO$_{3}$ systems could be promising visible-light photocatalysts from our first-principles predictions. References Environmental Applications of Semiconductor PhotocatalysisEnhancing Charge Separation and Photocatalytic Activity of Cubic SrTiO 3 with Perovskite-Type Materials MTaO 3 (M=Na, K) for Environmental Remediation: A First-Principles StudyTheoretical design of highly active SrTiO3-based photocatalysts by a codoping scheme towards solar energy utilization for hydrogen productionBand gap engineering of SrTiO 3 for water splitting under visible light irradiationNew insights into assessing the favorable co-doping dopants with various co-doped cases for the band gap engineering of SrTiO3Band-Gap Engineering of NaNbO 3 for Photocatalytic H 2 Evolution with Visible LightA hybrid DFT based investigation of the photocatalytic activity of cation–anion codoped SrTiO 3 for water splitting under visible lightTowards high visible light photocatalytic activity in rare earth and N co-doped SrTiO 3 : a first principles evaluation and predictionHighly active SrTiO3 for visible light photocatalysis: A first-principles predictionSynthesis of Cr and La-codoped SrTiO 3 nanoparticles for enhanced photocatalytic performance under sunlight irradiationInfluence of Zn doping on the photocatalytic property of SrTiO3Preparation and photocatalytic activities of two new Zn-doped SrTiO3 and BaTiO3 photocatalysts for hydrogen production from water without cocatalysts loadingSolvothermal synthesis and visible light-driven photocatalytic degradation for tetracycline of Fe-doped SrTiO 3Effect of Metal Doping on Electronic Structure and Visible Light Absorption of SrTiO 3 and NaTaO 3 (Metal = Mn, Fe, and Co)Enhanced visible-light-driven photocatalytic degradation of tetracycline by Cr 3+ doping SrTiO 3 cubic nanoparticlesFirst-principles study of La and Sb-doping effects on electronic structure and optical properties of SrTiO 3Calibration of the DFT/GGA+U Method for Determination of Reduction Energies for Transition and Rare Earth Metal Oxides of Ti, V, Mo, and CeOxidation energies of transition metal oxides within the GGA + U frameworkStrengthening of γ-TiAl-Nb by short-range ordering of point defectsCrystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr 1−2 x Na x La x )TiO 3Multifold polar states in Zn-doped Sr 0.9 Ba 0.1 TiO 3 ceramicsNonlinear size dependence of anatase TiO2 lattice parametersThe Synthetic Effects of Iron with Sulfur and Fluorine on Photoabsorption and Photocatalytic Performance in CodopedEnhanced Photocatalytic Degradation of RhB Driven by Visible Light-Induced MMCT of Ti(IV)−O−Fe(II) Formed in Fe-Doped SrTiO 3
[1] Hoffmann M R et al 1995 Chem. Rev. 95 69
[2] Opoku F et al 2017 ChemistrySelect 2 6304
[3] Reunchan P et al 2013 J. Mater. Chem. A 1 4221
[4] Wang C et al 2014 Int. J. Hydrogen Energy 39 12507
[5] Chao Z et al 2015 Int. J. Hydrogen Energy 40 1343
[6] Li P et al 2014 Int. J. Photoenergy 2014 380421
[7] Modak B et al 2014 Phys. Chem. Chem. Phys. 16 24527
[8] Zhang C et al 2017 RSC Adv. 7 16282
[9] Wang C et al 2014 Solid State Commun. 181 5
[10] Tonda S et al 2014 Phys. Chem. Chem. Phys. 16 23819
[11] Wang G Y et al 2010 J. Fuel Chem. Technol. 38 502
[12] Zou J P et al 2012 Int. J. Hydrogen Energy 37 17068
[13] Li P et al 2014 RSC Adv. 4 47615
[14] Zhou X et al 2011 J. Phys. Chem. C 115 8305
[15] Cai F et al 2015 RSC Adv. 5 21290
[16] Yun J N et al 2010 Chin. Phys. B 19 017101
[17]Segall M et al 2005 Z. Kristallogr. 220 567
[18] Lutfalla S et al 2011 J. Chem. Theory Comput. 7 2218
[19] Wang L et al 2006 Phys. Rev. B 73 195107
[20] Li Y J et al 2011 Intermetallics 19 793
[21] Mitchell R H et al 2000 Phys. Chem. Miner. 27 583
[22] Guo Y Y et al 2015 Chin. Phys. B 24 127701
[23] Swamy V et al 2006 Appl. Phys. Lett. 88 243103
[24] Li X H et al 2012 Int. J. Photoenergy 2012 203529
[25] Xie T H et al 2008 J. Phys. Chem. C 112 9753