Investigation of the Preferential Doping Site and Regulating on the

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Investigation of the Preferential Doping Site and Regulating on the Visible Light Response and Redox Performance for Fe- and/or LaDoped InNbO4 Yanyong Song,† Zhebin Sun,‡ Yuhang Wu,† Zhanli Chai,† and Xiaojing Wang*,† †

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Inner Mongolia Key Laboratory of Chemistry and Physics of Rare Earth Materials, College of Chemistry and Chemical Engineering, Inner Mongolia University, Huhhot, Inner Mongolia, PR China ‡ Inner Mongolia Electric Power Research Institute, Huhhot, Inner Mongolia, PR China ABSTRACT: The preferential doping site, visible light response, and redox potential of Fe- and/or La-doped InNbO4 (INO) were investigated using first-principles density functional theory. Eight designed doping models, including Fe and/or La doping at In or/ and Nb sites of INO are constructed, respectively. It was found that Fe-doping and Fe,La-codoping to substitute In into an INO cell are energetically favorable, confirming that the steric hindrance plays a vital role for the selective doping site than the charge of the dopants. Fe doping always formed two impurity bands between the conduction and valence bands, originated from Fe 3d state, inferring the well visible light response. Furthermore, the presence of La has a specific regulation effects for Fe doping although the energy levels of the single La-doped models were completely similar to those of the undoped INO. The electron exchange between La and Fe dopants results in the significant interaction for codoping INO. Importantly, by doping La into INO cell, the redox potentials of Fe-doped INO could be well-regulated. The band potential moved to the more positive energy level of the models Fe-doped at Nb sites, while it shifted to the more negative level if Fe was doped at In site of La-INO. The present investigation may provide the guidance for the designative dopants to construct the photocatalyst with stable, visible response, and good redox performance.

1. INTRODUCTION The photocatalytic technology has been widely used in the field of energy and environment due to its advanced features of high efficiency, nonpolluting nature, and ease of operation.1,2 However, in practical, there still existed several significant bottlenecks, such as the quick recombination, poor reusable stability, and the narrowed sun light absorption. Element doping is considered as a promising tactics for these issues via improving the photoredox and inhibiting the recombination of the photocarriers.3,4 In recent years, there are vast literatures to research on metals and/or nonmetals doping as the most popular questions in semiconductor’s field.5−9 Generally, the effect of doping on photocatalytic process is very complex due to either the presence of many midgap states or the various doping contents and doping sites. Both the transfer of the carriers and the position of the energy band should be strongly affected by the doping modes of the foreign elements. Also, it should be dependent on the concrete photocatalytic process whether these midgap states play a role as recombination centers or interfacial transfer of carriers. Therefore, at the most fundamental level, to investigate the effluence based on the concentrations and location of the dopants is essentially significant in determining the electronic properties and quantum efficiency. However, it is very difficult to carefully control the location and concentration of the dopants in © XXXX American Chemical Society

experiment. It is very possible that the various preparation conditions may lead to the different doping modes.10 In this case, first-principle calculation based on DFT is a powerful way to disclose the electronic structure of the doped systems. Indium niobate, InNbO4, is a famous metal oxide which has attracted much attention due to its potential applications as photocatalysts, solid-state scintillators, thermophosphors, laserhost materials, luminescent materials, and energy storage and conversion devices.11−15 Wi et al. reported an excellent photocatalytic behavior of InNbO4, splitting H2 from pure water under visible-light irradiation.16 Hulme synthesized InNbO4 in connection with low-temperature dielectric studies.17 Zou et al.18 proposed that InNbO4 has a suitable band structure (Eg = 2.5 eV) to respond visible light irradiation. However, the photocatalytic efficiency of InNbO4 material is still not high enough under visible light due to its fast recombination rate of the photogenerated electron−hole pairs from its narrow band gap. Lim et al. found that doping with tetravalent Sn and divalent Ni ions can stabilize the wolframite lattice, leading to the red-shift of the absorption edge and the formation of a satellite peak below the absorption edge, respectively.19 Thus, the tactics through foreign element Received: May 11, 2018

A

DOI: 10.1021/acs.inorgchem.8b01287 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

discussed carefully with the 108-atom (2 × 2 × 2) supercell models for the Fe-, La-, and Fe,La-codoped INO. As shown in Figure 1, Fe- or

doping should be able to promote its quantum yield under solar light illustration. InNbO 4 crystallized in the wolframite (monoclinic) structure at ambient pressure (space group No. 13, P2/c, Z = 2) with two kinds of octahedral units, specifically NbO6 and InO6. These polyhedra form the separated infinite edge-sharing “zigzag” chains that run along the c direction. The InO6 chains are connected through NbO6 octahedral units to form the three-dimensional network.20 Thus, the foreign element doping was possibly located at A and/or B sites (A = In and B = Nb), which may result in the various performance. Consequently, to investigate the effect of the selective doping on the energy band structure may be special significantly. Fe(III) and La(III) have been identified as significantly doping ions to dope in different photocatalysts, such as TiO2,21 SnO2,22 ZnO,23 KNbO3,24 SrTiO4,25 and so on.26−29 The ionic radius of Fe3+ is 0.64 Å, Nb5+ 0.70 Å, In3+ 0.81 Å, and La3+ 1.06 Å, respectively, so doping Fe3+ and La3+ ions in the InNbO4 lattice is allowed in principle in terms of the radius and charge matching. However, the doping is sure to have the great various influences on the properties of materials due to the different radius and charge. Thus, a detailed knowledge of the related electronic structure is required. To this aim, the investigation of the priority for dopant’s radius or charge via Fe and/or La site-selective doping into InNbO4 may be very intriguing with the help of the first-principles simulation. In the present work, therefore, we exclusively reported the effect of Fe,La-doping on geometric, electronic, and optical properties of semiconductor InNbO4 within the framework of density functional theory. The expansive cell, formation energy, and band structure were calculated for the designed eight models of Fe- and/or La-doped InNbO4 system. The photocarrier transfer, the band gap narrowing, and the cooperative mechanism of Fe- and/or La-doped InNbO4 models were carefully investigated. The effects of the dopant’s charge and radius on the electronic structure and energy band were explored. Our main objective is to explore which one is the vital factor for the site-selection of dopants, steric hindrance or charge imbalance? Furthermore, we discuss the conjunction with the photocatalytic property and microstructure through site-selective doping to helpfully improve the quantum yield and solar light absorption for InNbO4 system.

Figure 1. Crystal structure of pure InNbO4 and metal-doped InNbO4. The green ball is Nb and the purple ball is In. M1 = Fe and M2 = La. La-substituted INO was constructed by replacing one of the eight (In3+ or Nb5+) ions with (Fe3+ or La3+) ion to simulate the 12.5 atom % doped levels, respectively, named as Fe@In-INO, La@In-INO, Fe@Nb-INO, or La@In-INO. For the Fe,La-codoped models, one of the eight (In or Nb atom) has been concurrently substituted by Fe and La atoms. This corresponds to a Fe or La content of x = 0.125, denoted as Fe,La@In-INO, Fe,La@Nb-INO, La@In,Fe@Nb-INO, and Fe@In,La@Nb-INO.

2. COMPUTATIONAL DETAILS

3. RESULTS AND DISCUSSION 3.1. Bulk Structural Parameters of INO. First, the astringency of plane-wave cutoff energy have been tested by optimizing lattice parameters and calculating the total energies of crystal cell at the different K point value of Brillouin zone (Figure 2). As seen, the total energy is decreased with the increase of the plane-wave cutoff energy. It inferred the system is relatively stable when the energy is close to 400 eV. Thus, the K point value was set up as to 3 × 2 × 3 to achieve the beautiful convergence effect. The maximum force of 0.01 eV/ Å, the maximum stress of 0.02 GPa, and the maximum

All calculations based on density function theory (DFT) were performed using CASTEP code,30 with a projector augmented wave pseudopotentials formalism.31 The Perdew−Burke−Ernzerhof (PBE) parametrization of the generalized gradient approximation (GGA) was adopted for the exchange and correlation functional.32 Plane wave functions are used as basis sets.33 The electron wave function was expanded in plane waves up to a cutoff energy of 200−400 eV in case of ultrasoft potential, while the valence shells are considered to be In4s24p64d105s25p1, Nb-4s24p64d45s1, Fe-3s23p63d64s2, and O-2s22p4, respectively. The geometries for all the compositions are optimized using a conjugated gradient technique by minimizing the Kohn− Sham energy function. The lattice parameters, as well as the atomic position relaxations were carried out until the forces and total energy converged within 0.015 eV/Å and 10−5 eV, respectively. The 2 × 2 × 2 supercell of InNbO4 (INO) has been built in which one supercell consists of eight indium, eight niobium, and 32 oxygen atoms, denoted as model INO(2 × 2 × 2). Hexagonal Bravais lattice has been used under periodic boundary conditions with a vacuum space of 18.81 Å along the z-axis to decouple the weakest periodic interactions. The cooperative effect of Fe,La bimetal codoping was

Figure 2. Total energy of INO (2 × 2 × 2) under the different Kpoint and cutoff values. B


Article

Inorganic Chemistry displacement of 5.0 × 10−4 Å were used in all calculations. Thereafter, a series of calculations based on the above conditions were performed. An optimized wolframite structure with space group of P2/c (ICSD 100695) for bulk INO was built with a single crystal cell of 8 oxygen atoms, 2 indium atoms, and 2 niobium atoms, and InO6 octahedrons form the layers by sharing the corner. The optimized lattice parameters a, b, c, and β are summarized in Table 1. These calculated lattice parameters and experimental values were in good agreement within a difference which is only about 2%.

As shown in Figure 4, the cell volumes and formation energies (Table 2) of Fe@In-INO are smaller than Fe@Nb-

Table 1. Experimental Values and Lattice Parameters a, b, c, and β experimental value30 calculated value31 this work

a (Å)

b (Å)

c (Å)

β (degrees)

5.188 5.225 5.222

5.796 5.923 5.898

4.802 4.946 4.919

90.03 91.33 91.11

Figure 4. Unit cell volume and doping atom radius of the different models.

Figure 3 shows the calculated electronic band structures and density of state of INO single crystal cell. It is divided into six parts: the orbit of Nb 4s, Nb 4p, O 2s, and In 4d, respectively, located in −54.33, −30.61, −16.16, and −12.23 eV. Both the valence band maximum (VBM) and the conduction band minimum (CBM) are located at the K-point, implying the intrinsic semiconducting property of INO with a direct K−K gap of 3.10 eV. The PDOS diagram shows that the valence band consists of O 2p and conduction band consists of Nb 4d and O 2p, located at −5.78 to −0.34 eV and 3.01−9.14 eV, respectively (Figure 3a). The calculated band gap is 3.10 eV (Figure 3b), which is closer to the experimental value (2.60 eV),20 less than the value (3.16 eV) in the report of Shi et al.34 In the existing research, there are greater deviation in the study of simulative band gap calculation, many researchers had discussed about that for it is own problem of computing method.35 3.2. Selective Sites of Fe Atom Doping into the Cell of INO. To investigate the best doping location of the Fe atom, whether is at the In site or the Nb site of INO, we calculated the formation energy, unit cell volume, and density of states for the designed Fe doped models. In fact, comparing the atoms radii with Fe, Nb, and In, the radius of Fe3+ (55.0 pm) is close to that of Nb5+ (64.0 pm) but noticeably smaller than that of In3+ (80.0 pm). Meanwhile, the charge of Fe3+ is equal to that of In3+ but smaller than that of Nb5+. Thus, we need to explore which is the most stable structure of the preferential doping site with radii or charges of dopant atoms.

INO when Fe atom was introduced to the cell of INO. It assigned that the removing of an In ion will create a larger space in the model, and then Fe could locate at this site with the smaller steric hindrance. Thus, the cell volumes and formation energies of Fe@In are smaller than Fe@Nb (Figure 4). Consequently, it proposed Fe3+ would be preferentially doped at the cation sites with the largest available space when it enters into the cell of INO. Furthermore, considering the system charge is unbalanced when Fe3+ions doped at Nb5+ site we try to introduce O vacancy in suitable site to balance the charge. Because there are two kinds of octahedrons (InO6 and NbO6) in the InNbO4 crystal lattice, we built the vacant models by removing an O atom from the octahedron of InO6 or NbO6, named VO1-INO and VO2-INO, respectively. It was found that although the cell volume of the model of O2 type point defect has slightly larger value its formation energy is lower than the model of O1 type point defect. It indicated that the model of VO2 vacancy is more stable. We further placed Fe atom into the position of the vacant O atom, named VO1-Fe@ Nb and VO2-Fe@Nb, respectively. It is found that the formation energy of the model of VO2-Fe@Nb is lower than that for the model of VO1-Fe@Nb. However, the formation energies of the two models with O vacancy are all far larger than those without O vacancy. Therefore, the influence on the formation energy from radius of the dopant ions is obviously more important than that from their charge. It provides us a

Figure 3. Band structure (a) and density of state (b) of INO. C


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Inorganic Chemistry Table 2. Cells Parameters and Formation Energies INO Fe@In-INO Fe@Nb-INO VO1-INO VO2-INO VO1-Fe@Nb VO2-Fe@Nb Ta@In-INO Ta@Nb-INO Bi@In-INO Bi@Nb-INO

a (Å)

b (Å)

c (Å)

β (degrees)

total energy (eV)

formation energy (eV)

10.44 10.40 10.39 10.46 10.46 10.50 10.52 10.50 10.50 10.48 10.52

11.80 11.72 11.76 11.70 11.83 11.91 11.89 11.86 11.92 11.90 11.95

4.92 4.86 4.90 4.92 4.92 5.00 5.02 5.02 5.02 5.05 5.02

91.12 90.91 91.38 91.36 90.84 90.98 92.20 91.12 90.98 91.18 91.04

−38913.20 −38215.31 −38218.86 −38474.39 −38476.31 −37780.79 −37780.82 −37488.51 −37495.63 −37502.63 −37505.28

2.44 7.62 4.86 2.95 11.74 11.70 1.50 3.11 1.50 7.58

Figure 5. (a) TDOS and (b) PDOS of INO, Fe@In-INO, and Fe@Nb-INO. For (a), the gray area represents the total state of InNbO4, and the blue and red curves represent the total states of Fe@In-INO and Fe@Nb-INO, respectively. For (b), the green curve represents O 2p state, the red curve represents total state, the blue curves represent Nb 4d state, the black curve represents In 5s states, and the brown curve represents Fe 3d state. The dashed lines represent the Fermi level.

conduction band (CB) dominantly consists of In 5s, Nb 4d, and O 2p orbitals. Valence band (VB) is composed of the mixture of Nb 4d and O 2p orbitals, while the impurity bands (IBs) is composed of Fe 3d orbitals. Obviously, when we pinned the Fermi level at the valence band maximum (VBM), the smaller band gap was observed for Fe@In-INO which was assigned to the small separation of Fe 3d orbitals. As Figure 6 shows, the electronic density around the Fe atom of Fe@InINO is slightly bigger than that of Fe@Nb-INO. The energy levels included all VBs and CBs, declined due to the doping of Fe atom both at Nb site and In site. By contrasting the Mulliken charge for two models of Fe@In and Fe@Nb (Table 3), it clearly indicates that (1) Fe atom gathered electrons in the two models and (2) the electron transfer to Fe is about 0.71 e in the model of Fe@In whereas it is about 0.56 e in the model of Fe@Nb. The results were assigned to the difference of the electronegativity which is 1.8 for Fe, 1.7 for In, and 1.6 for Nb, respectively. It means more easily lose electrons for In and more easily obtain electrons for Fe. Moreover, the substitution of either Fe@In or Fe@Nb all introduced two separated impurity states above the top of the VB and below the bottom of the CB, resulting in a band gap narrowing of 1.04 or 1.51 eV, respectively. As we know, InNbO4 crystallized with two kinds of octahedral units, specifically, NbO6 and InO6. Thus, whether Fe located at Nb or In sites, Fe 3d orbital will be split into two different energy levels (that is, eg and t2g) in octahedral coordination environment. In the case of Fe@In-INO, the available space is obviously bigger than that of Fe@Nb-INO, resulting in the

revelation that for the site-selective doping enough space is crucial whereas the charge balance is not so considerable. To further confirm the decision of the steric space on the preferred doping site, we simulated the doping models of Ta5+ (64 pm) ion which has the similar ionic radius and charge with Nb5+ (64 pm) ion. Furthermore, Bi5+ (76 pm) ions which has the same charge with Nb5+ and similar radius with that of In3+ (80 pm) was also used as the comparative calculation. The calculation condition is completely same as those of Fe in replacing of In or Nb atoms. As shown in Table 2, the formation energies of the doping at In site for both Ta and Bi are all smaller than those of the doping at Nb site. Obviously, both for Ta and Bi, the preferential doping site should be controlled by the available space when they enter into the INO cell. Conclusively, the doping site is determined by the steric hindrance. The total density of state (TDOS) and projected density of state (PDOS) of Fe-doped INO structures are shown in Figure 5, respectively. The zero energy value is set at the top of the valence band. In that case, it is easily to identify the band gap and the relative position of the states from the impurity atoms. Figure 5a clearly demonstrates that all of the dopants either at Nb site or In site lead to band gap narrowing and made the absorption edge obvious blue shift. The reduced band gap values are 1.04, and 1.51 eV for Fe@In-INO and Fe@Nb-INO, respectively. Furthermore, we can find compared with INO, the blueshifts mainly occur at the conduction band with a value of about 2.13 and 1.43 eV for Fe@In-INO and Fe@Nb-INO, respectively. The PDOS in Figure 5b indicates that the D


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Inorganic Chemistry

Figure 7. Comparing of the cell volume of the different models.

in the appointed positions to balance the less charge of La (3+) than that of Nb (5+), led to the bigger formation energy and the more distorted atom arranges. After comparing the properties of different La-doped models, we can demonstratively indicate that the most stable structure is the model of Ladoped at In site in INO. Thus, it is pointed out again that stereospecific blockade needs to be more considered than that the unbalanced charge in the doping system. The TDOS and PDOS of La@In-INO and La@Nb-INO structures are shown in Figure 8. The Fermi energy level locates in the top of VBM. As Figure 8a shows, there is nearly no change of the band position for La ions doping at either Nb site or In site with respect to pure INO. The band gap values are 3.11 and 2.98 eV by doping La at Nb and In sites, respectively. The PDOS in Figure 8b indicates that the CB dominantly consists of In 5s, Nb 4d, and O 2p orbitals. VB is composed of the mixture of Nb 4d and O 2p orbitals. Generally, the occupied orbitals of La ion almost locate at the deeper levels because La has more electrons than both In and Nb. As a result, we could only observe the slight contribution from La 5d orbital which located at the more negative site in VB and more positive site in CB (Figure 8b). Obviously, the energy band from La ions did not appear in both VB and CB. Therefore, the frontier band sites were not affected at all by La doping. Figure 9 shows the charge density difference around the La atom sites of La@In-INO and La@Nb-INO. Meanwhile the electronic densities of La@In and La@Nb were compared through quantifying that the value of Mulliken charges (Table 5). Combining these results we found the number of charge transfer is about 0.72 e from the model of La@In, more bigger than the number of La@Nb (0.58 e). It clearly indicates that La atom donated electrons to others in this system. 3.4. Fe,La-Codoped Models on the Framework of INO. Above all, it is identified that for the doping ions there is the priority to enter into the more available space site. Now we continue to explore the situation of Fe and La ions doped in INO at the same time. For this case, we have formulated two ways of doping Fe in the systems of La@In-INO and La@NbINO, respectively. For La@In-INO system, the doped positions of Fe atom have two selections because La ions had replaced the In ions. One is at the In site, and the other is at Nb. By simulation, we found the cell volumes of Fe doped at In or Nb sites are 604.61 and 610.56 Å3, respectively, all reduced on account of the

Figure 6. Charge density difference of (a) Nb layer and (b) In layer of the models of INO, Fe@In-INO, and Fe@Nb-INO. The red area represents the enrichment of electrons; the blue area represents the absence of electrons.

Table 3. Mulliken Charge of Fe Doped at Difference Sitesa Fe@In-INO Fe@Nb-INO

before

after

Δq

388.63 e 388.58 e

387.92 e 388.02 e

−0.71 e −0.56 e

“Before” means that the total charge of the undoped system without In or Nb atom. “After” means that the total charge of the doped system without Fe atom. Δq < 0 means that the system transfer electrons to Fe atom.

a

weak repulsive interaction between Fe 3d and the oxygen ligands. Consequently, it presented the smaller splitting gap of Fe 3d orbital in Fe@In-INO, comparing with Fe@Nb-INO. Anyway, the narrowing of band gap has resulted in the expansion of photoabsorption edge. Thus, we can conclude that Fe atoms doping at the In site is energetically favorable and is responsible for the observed visible light activity. 3.3. La Doped into the Cell of INO. For La, we carried out similar doping ways with the models of Fe doped in the cell of INO. The radius of La (164.0 ppm) is closer to In (162.6 ppm), compared with the radius of Nb (146.0 ppm). Meanwhile, the valence state of La is +3, same with In3+ in this system. From Figure 7, the cell volumes were expanded due to the doping of La ions with the bigger radius. Obviously, the cell volume becomes larger after doping at Nb site than that after doping at In. Comparing to the formation energies of La@In-INO, and La@Nb-INO (Table 4), it is known that the model of In substituted by La is more stable than the model of La in replacement of Nb. Again, it confirmed the suitable doping site will be located at the bigger allowed space. Because the charge is unbalanced in the model of La@Nb (La is 3+ and Nb is 5+), we created the vacant models of La doped at Nb sites in INO. Combined with the formation energies and cell parameters (Table 4), the model of La substituted at In site has two advantages: smaller cell volume and lower formation energy. The balanced charge models (VO1-La@Nb and VO2-La@Nb), that is, to remove an oxygen E


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Inorganic Chemistry Table 4. Cell Parameters and Formation Energies INO La@In-INO La@Nb-INO VO1-La@Nb VO2-La@Nb

a (Å)

b (Å)

c (Å)

β (degrees)

total energy (eV)


10.44 10.49 10.57 10.64 10.68

11.80 11.82 11.95 12.20 12.07

4.92 4.99 4.98 5.09 5.05

91.12 91.66 90.10 89.55 90.52

−38913.20 −38219.56 −38219.26 −37784.45 −37783.91

−3.59 5.43 6.29 6.83

Figure 8. (a) TDOS and (b) PDOS for InNbO4, La@In-INO, and La@Nb-INO. For (a), the blue, green and red curves represent the total states of InNbO4, La@In-INO, and La@Nb-INO, respectively. For (b), the green curve represents O 2p state, the red curve represents the total state, the blue curves represent Nb 4d state, the black curve represents In 5s state, the brown curve represents Fe 3d state, and light blue curve represents La 5d state. The dashed lines represent the Fermi level.

smaller radius of Fe ions. Furthermore, as expected the codoping of Fe and La ions at In sites (La,Fe@In-INO) results in the smaller volume. Simultaneously, the formation energy of La,Fe@In-INO (−1.03 eV) has smaller value compared with that of La@In,Fe@Nb-INO (4.06 eV). For La@Nb-INO system, Fe doped at either In or Nb sites; the formation energies are all increased, assigned to the available smaller space by removing Nb atom. Combined cell parameters with four models, the unit cells twist all bigger in the models of Fe@ Nb (Fe,La@Nb-INO and La@In,Fe@Nb-INO). Therefore, it indicated that the model of Fe doped at a bigger available space of In site is more stable and preferential than in the smaller space of Nb. The TDOS and PDOS of La@In-INO, La,Fe@In-INO, and La@In,Fe@Nb-INO are shown in Figure 10. The bands have a blueshift toward the lower energy level form La@In-INO to La@In,Fe@Nb-INO and La,Fe@In-INO (Figure 10a). It was assigned to the reduced total amount of electrons after substituting In or Nb atom with Fe atom. The PDOS in Figure 10b indicates that the CB dominantly consists of In 5s, Nb 4d, and O 2p orbitals, while the VB is composed of the mixture of Nb 4d and O 2p orbitals. It was obviously observed Fe 3d orbital is also split to the higher level t2g and lower level eg between the CB and VB due to the octahedron crystal field after Fe atom introduced into this system. Then, the band gaps of La@In,Fe@Nb-INO and La,Fe@In-INO are reduced to 1.08 and 1.54 eV. As what mentioned above, La atom donates electrons and Fe atom accepts these electrons. The net transfer charges (Table 7) is obviously reduced from the doped atoms to the system when Fe and La atoms simultaneously exist. It may be the result that the doped ions neutralize the charges number of donor and acceptor. For the model of La@Nb-INO, there are the larger formation energies and volumes because the small Nb was substituted by the large La. When Fe atom has been

Figure 9. Charge density difference of (a) Nb layer and (b) In layer of the models of INO, La@In-INO, and La@Nb-INO.

Table 5. Number of Charge Transfer of La@In-INO and La@Nb-INOa La@In-INO La@Nb-INO

before

after

Δq

388.63 e 388.58 e

389.35 e 389.16 e

0.72 e 0.58 e

“Before” means that the total number of the undoped system without In or Nb atom. “After” means that the total number of the doped system without La atom. Δq > 0 means that La atom transfer electrons to the system).

a

F


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Inorganic Chemistry

Figure 10. (a) TDOS and (b) PDOS of La@InNb-INO, La,Fe@In-INO and La@In,Fe@Nb-INO. For (a), the gray area represents the total state of La@In-INO, and the blue and red curves represent the total states of La,Fe@In-INO, and La@In,Fe@Nb-INO, respectively. For (b), the green curve represents O 2p state, the red curve represents total state, the blue curves represent Nb 4d state, the black curve represents In 5s state and the brown and light blue curves represent Fe 3d and La 5d states.

Table 6. Cell Parameters and Formation Energies La@In-INO La,Fe@In-INO La@In,Fe@Nb-INO La@Nb-INO La@Nb,Fe@In-INO La,Fe@Nb-INO

a (Å)

b (Å)

c (Å)

β (deg)

cell volume (Å3)

total energy (eV)


10.49 10.43 10.44 10.57 10.61 10.46

11.82 11.73 11.75 11.95 11.94 11.79

4.99 4.95 4.98 4.98 4.88 5.01

91.66 91.46 92.08 90.10 90.39 90.64

618.93 604.61 610.56 629.49 618.45 618.08

−38219.56 −37521.54 −37525.17 −38219.26 −37522.50 −37526.78

−3.59 −1.03 4.06 5.43 6.73 11.18

for the models of both Fe@In and Fe@Nb. Therefore, compared to that of Fe doped into the model of La@Nb-INO, the stability became worse for Fe@In,La@Nb-INO, and La,Fe@Nb-INO. The changed band level of Fe doped in the model of La@Nb-INO is shown in Figure 11, it indicated the CBM has a significant blueshift, but VBM kept unchanged. The reduced band gap values are 1.16 and 1.63 eV in the models of Fe@In,La@Nb-INO and Fe,La@Nb-INO, respectively. Similarly, Fe 3d orbital was split into the two level of the higher level t2g and lower level eg which appeared as the IBs between the CB and VB. Above all, the volumes and the formation energies were small for the model of Fe and La atoms codoped in INO. Meanwhile, the regulated effects of Fe ions doping are understandable if La ions existed in the system. The CB moved to the lower energy level, and band gap became

Table 7. Number of Charge Transfer of Fe and La in the Models of La,Fe@In-INO, La@In,Fe@Nb-INO, La@ Nb,Fe@In-INO, and La,Fe@Nb-INOa before La,Fe@In-INO La@In,Fe@Nb-INO La@Nb,Fe@In-INO La,Fe@Nb-INO

377.18 377.13 377.13 377.08

Δq

after e e e e

377.19 377.31 377.10 377.02

e e e e

0.01 0.18 −0.03 −0.06

e e e e

“Before” means that the total number of the undoped system without In or Nb atom. “After” means that the total number of the doped system without La atom. Δq > 0 means that La and Fe atom transfer electrons to the system.

a

introduced into In or Nb site, the cell volumes are reduced to 618.45 and 618.08 Å3, respectively (Table 6). Furthermore, the cells were twisted, while the formation energies were increased

Figure 11. (a) TDOS and (b) PDOS of La@Nb-INO, Fe@In,La@Nb-INO, and Fe,La@Nb-INO. G


Article

Inorganic Chemistry

Figure 12. Calculated VBM and CBM positions of Fe- and/or La-doped INO as compared with those of the corresponding experimental values of pure INO. The VBM and CBM values are given with respect to the SHE potential (V) and energy with respect to vacuum (eV).

site, then the CB level will move down. As a result, it cannot produce H2 with a lower reduced potential. Thus, the band potential edge and redox abilities of INO have different changes by Fe- and/or La-doping. Our simulation indicates that for Fe@In-INO the CB shifted down; thus, the band potentials are close to O2/H2O which means it cannot oxidize H2O to O2. Oppositely, the reducible ability do not change too much; thus, it could reduce H+ to H2, although the band gap decreased. Summarily, by site-selective Fe-doping, the band potential edges can be adjusted to shift down or up, thus controlling them to produce hydrogen or oxygen. Meanwhile, La-doping can adjust this reducibility or oxidizability to stronger, whereas it does really not influence on their band gap.

narrowed. Therefore, the codoped ways of Fe and La atoms produce the good regulation effects in this system. 3.5. Evaluation of the Band Potential and Redox Ability. To evaluate the redox ability of the metal ions doped in INO, the CBM and VBM of the Fe,La-doped INO together with the undoped INO, are depicted in Figure 12. Because DFT with local functions typically takes the Fermi energy level as a reference, further manipulation is required to predict the real position of VBMs and CBMs. However, the energy level positions, that we usually determine are based on the vacuum level. Therefore, we revise the position of VBMs and CBMs from the data origin of electronic states. The VBM values of pure INO with respect to the standard hydrogen electrode (SHE) potential are taken from the experimental values,36 and the CBM are from the sum of VBM and band gap of our calculation values. For the doped models, the CBMs were obtained from the DOS plots according to the relative positions as compared to that of pure INO, and the VBMs are obtained from the corrected band gaps. Compared with the SHE potential, INO has a weak reducing capacity with about 0.7 eV CBM lower than the H+/H2 potential. However, its VBM is more positive than O2/H2O potential, which is in excellent agreement with the experimental reports on the photocatalytic decomposition of water to produce oxygen.37 On the basis of the above standard of redox potential, we discuss and evaluate the redox activity of Fe- and/or La-doped INO system. (1) The band gaps are decreased by doping Fe element at In site, assigned to the formation of the IBs mainly from Fe 3d. Meanwhile, it can be seen the narrowed band gap originated from the upshift of VBM, whereas the CBM had nearly no changes when Fe doped at In sites. It is nearly close to the potential of O2/H2O; thus, such doping may inhibit the O2 production. (2) For Fe doped at Nb site, the CBM moves down obviously. Thus, the reduced ability was not changed, whereas the oxidized ability was increased significantly. Consequently, the oxidized performance was kept although the decreased band gap. (3) For La doping, it basically does not influence on the band gap, whereas it does influence the band potential site. If La doped at In site, then both valence and conduction band shift up; thus, it has increased reducibility and expanded bandgap, compared to Fe@In-INO. (4) For the codoping of Fe and La, the reducible property will be enhanced. If La atom located at Nb

4. CONCLUSION In this study, we presented a detailed simulation of Fe-, La-, and Fe,La-codoping into INO cell to explore the doping effect on the energetic, electronic, and redox properties by means of density functional theory. Through investigation, the formation energies of the designative dopants with the various radius and charge (La3+, Fe3+, Ta5+, aand Bi5+), it was determined that the higher priority for the selective doping site was always endowed to the less steric hindrance rather than the balanced charge. Thus, Fe-doped and Fe,La-codoped to substitute In into a INO cell are energetically favorable. In terms of the band structures, we found when Fe was doped into the INO cell, the two IBs of Fe 3d appeared between CB and VB, resulting in the ranges of photoresponse were expanded to the various degrees. However, the energy band from La ions did not appear in both VB and CB. Therefore, the energy levels of Ladoped models were completely similar to those of INO. By investigating the Fe,La-codoping models, it was proposed that codoping in INO can reduce the formation energies, compared with those of the separate Fe- or La-doping. It is confirmed the significant interaction between La and Fe dopants for codoping INO by analyzing the electrons exchange. Importantly, La atoms can take the regulation of the redox potentials in the systems of Fe-doped INO. When La atom is present, the band potential moved to the more positive energy level for Fe-doped at Nb sites of INO, while it shifted to the more negative level if Fe was doped at Nb sites. The present investigation has given the useful information about doping mechanism, energy band H


Article

Inorganic Chemistry

(14) Feng, H. Y.; Hou, D. F.; Huang, Y. H.; Hu, X. L. Facile synthesis of porous InNbO4, nanofibers by electrospinning and their enhanced visible-light-driven photocatalytic properties. J. Alloys Compd. 2014, 592, 301−305. (15) Zhang, X. F.; Quan, X.; Chen, S.; Yu, H. T. Constructing graphene/InNbO4 composite with excellent adsorptivity and charge separation performance for enhanced visible-light-driven photocatalytic ability. Appl. Catal., B 2011, 105, 237−242. (16) Zou, Z. G.; Arakawa, H. Direct water splitting into H2 and O2 under visible light irradiation with a new series of mixed oxide semiconductor photocatalysts. J. Photochem. Photobiol., A 2003, 158, 145−162. (17) Hulm, J. K. Dielectric properties and phase transitions of Cd2Nb2O7 and Pb2Nb2O7. Phys. Rev. 1953, 92, 504−505. (18) Zou, Z. G.; Ye, J. H.; Arakawa, H. Photophysical and photocatalytic properties of InMO4 (M = Nb5+, Ta5+) under visible light irradiation. Mater. Res. Bull. 2001, 36, 1185−1193. (19) Lim, S. T.; Kim, T. W.; Hur, S. G.; Hwang, S. J.; Park, H.; Choi, W.; Choy, J. H. Effects of p- and d-block metal co-substitution on the electronic structure and physicochemical properties of InMO4 (M = Nb and Ta) semiconductors. Chem. Phys. Lett. 2007, 434, 251−255. (20) Zou, Z. G.; Ye, J. H.; Arakawa, H. Structural properties of InNbO4 and InTaO4: correlation with photocatalytic and photophysical properties. Chem. Phys. Lett. 2000, 332, 271−277. (21) George, S.; Pokhrel, S.; Ji, Z. X.; Henderson, B. L.; Xia, T.; Li, L. J.; Zink, J. I.; Nel, A. E.; Madler, L. Role of Fe doping in tuning the band gap of TiO2 for the photo-oxidation-induced cytotoxicity paradigm. J. Am. Chem. Soc. 2011, 133, 11270−11278. (22) Aragón, F. H.; Coaquira, J. A. H.; Gonzalez, I.; Nagamine, L. C. C. M.; Macedo, W. A. A.; Morais, P. C. Fe doping effect on the structural, magnetic and surface properties of SnO2 nanoparticles prepared by a polymer precursor method. J. Phys. D: Appl. Phys. 2016, 49, 155002. (23) Wang, Y. S.; Thomas, P. J.; O’Brien, P. Optical properties of ZnO nanocrystals doped with Cd, Mg, Mn, and Fe ions. J. Phys. Chem. B 2006, 110, 21412−21415. (24) Kakimoto, K.; Masuda, I.; Ohsato, H. Ferroelectricity and solidsolution structure of KNbO3 ceramics doped with La and Fe. Key Eng. Mater. 2004, 269, 7−10. (25) Comes, R. B.; Sushko, P. V.; Heald, S. M.; Colby, R. J.; Bowden, M. E.; Chambers, S. A. Band-gap reduction and dopant interaction in epitaxial La, Cr and co-doped SrTiO3 thin films. Chem. Mater. 2014, 26, 7073−7082. (26) Liu, M.; Qiu, X.; Miyauchi, M.; Hashimoto, K. Energy-level matching of Fe(III) ions grafted at surface and doped in bulk for efficient visible-light photocatalysts. J. Am. Chem. Soc. 2013, 135, 10064−10072. (27) Yu, J. G.; Xiang, Q. J.; Zhou, M. H. Preparation, characterization and visible-light-driven photocatalytic activity of Fe-doped titania nanorods and first-principles study for electronic structures. Appl. Catal., B 2009, 90, 595−602. (28) Modak, B.; Ghosh, S. K. Exploring the role of La codoping beyond charge compensation for enhanced hydrogen evolution by Rh-SrTiO3. J. Phys. Chem. B 2015, 119, 11089−11098. (29) Dawson, J. A.; Tanaka, I. Local structure and energetics of Prand La-doped SrTiO3 grain boundaries and the influence on coreshell structure formation. J. Phys. Chem. C 2014, 118, 25765−25778. (30) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. First principles methods using {CASTEP}. Z. Kristallogr. - Cryst. Mater. 2005, 220, 567−570. (31) Francis, G. P.; Payne, M. C. Finite basis set corrections to total energy pseudopotential calculations. J. Phys.: Condens. Matter 1990, 2, 4395. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (33) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188−5192.

narrowing, and band potential shifting. It will provide the guidance to design the photocatalyst with stable, visible response and good redox performance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yuhang Wu: 0000-0002-4339-0753 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by National Natural Science Foundation of China (Grants 21777078 and 21567017), the Project of Research and Development of the Applied Technology for Inner Mongolia (Grants 201702112).

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REFERENCES

(1) Wang, S. L.; Li, J.; Wang, S.; Wu, J.; Wong, T. I.; Foo, M. L.; Chen, W.; Wu, K.; Xu, G. Q. Two-dimensional C/TiO 2 heterogeneous hybrid for noble-metal-free hydrogen evolution. ACS Catal. 2017, 7, 6892−6900. (2) Su, Y.; Lang, J. Y.; Li, L. P.; Guan, K.; Du, C. F.; Peng, L. M.; Han, D.; Wang, X. J. Unexpected catalytic performance in silent tantalum oxide through nitridation and defect chemistry. J. Am. Chem. Soc. 2013, 135, 11433−11436. (3) Liu, H.; Cao, Q.; Fu, L. J.; Li, C.; Wu, Y. P.; Wu, H. Q. Doping effects of zinc on LiFePO4 cathode material for lithium ion batteries. Electrochem. Commun. 2006, 8, 1553−1557. (4) Jang, B.; Hong, A.; Kang, H. E.; Alcantara, C.; Charreyron, S.; Mushtaq, F.; Pellicer, E.; Büchel, R.; Sort, J.; Lee, S. S.; Nelson, B. J.; Pané, S. Multi-wavelength light responsive Au/B-TiO2 janus micromotors. ACS Nano 2017, 11, 6146−6154. (5) Jang, D. M.; Kwak, I. H.; Kwon, E. L.; Jung, C. S.; Im, H. S.; Park, K.; Park, J. Transition-metal doping of oxide nanocrystals for enhanced catalytic oxygen evolution. J. Phys. Chem. C 2015, 119, 1921−1927. (6) Sekizawa, K.; Nonaka, T.; Arai, T.; Morikawa, T. Structural improvement of CaFe2O4 by metal doping toward enhanced cathodic photocurrent. ACS Appl. Mater. Interfaces 2014, 6, 10969−10973. (7) Lee, K. E.; Kim, J. E.; Maiti, U. N.; Lim, J.; Hwang, J. O.; Shim, J.; Oh, J. J.; Yun, T.; Kim, S. O. Liquid crystal size selection of largesize graphene oxide for size-dependent N-doping and oxygen reduction catalysis. ACS Nano 2014, 8, 9073−9080. (8) Zhang, R.; Fang, Y. Y.; Chen, T.; Qu, F. L.; Liu, Z.; Du, G.; Asiri, A. M.; Gao, T.; Sun, X. P. Enhanced photoelectrochemical water oxidation performance of Fe2O3 nanorods array by S doping. ACS Sustainable Chem. Eng. 2017, 5, 7502−7506. (9) Upadhyay, S.; Shrivastava, J.; Solanki, A.; Choudhary, S.; Sharma, V.; Kumar, P.; Singh, N.; Satsangi, V. R.; Shrivastav, R.; Waghmare, U. V.; Dass, S. Enhanced photoelectrochemical response of BaTiO3 with Fe doping: experiments and first-principles analysis. J. Phys. Chem. C 2011, 115, 24373−24380. (10) Su, Y. G.; Peng, L. M.; Guo, J. W.; Huang, S. S.; Lv, L.; Wang, X. J. Tunable optical and photocatalytic performance promoted by nonstoichiometric control and site-selective codoping of trivalent Ions in NaTaO3. J. Phys. Chem. C 2014, 118, 10728−10739. (11) Kako, T.; Ye, J. Photoinduced amphiphilic property of InNbO4 thin film. Langmuir 2007, 23, 1924−1927. (12) Tang, A.; Gu, L.; Zhang, H.; Zhao, Y.; Chen, H.; Zhang, H. Emission characteristics of Pr3+-doped InNbO4 used as a red-emitting phosphor for white LEDs. J. Adv. Mater. 2015, 17, 1243−1247. (13) Balamurugan, C.; Vijayakumar, E.; Subramania, A. Synthesis and characterization of InNbO4, nanopowder for gas sensors. Talanta 2012, 88, 115−120. I


Article

Inorganic Chemistry (34) Shi, H. F.; Zhou, C. P.; Zhang, C. L.; Ye, E. J. Anions (N,S) mono-doping and co-doping influences on electronic structures and optical properties of InNbO4. Int. J. Mod. Phys. B 2016, 30, 1650060. (35) Tian, F. H.; Liu, C. B. DFT description on electronic structure and optical absorption properties of anionic S-doped anatase TiO2. J. Phys. Chem. B 2006, 110, 17866−17871. (36) Zhang, X. F.; Quan, X.; Chen, S.; Yu, H. T. Constructing graphene/InNbO4 composite with excellent adsorptivity and charge separation performance for enhanced visible-light-driven photocatalytic ability. Appl. Catal., B 2011, 105, 237−242. (37) Zou, Z.; Ye, J.; Arakawa, H. Role of Ln in Bi2LnNbO7 (Ln: Y, rare earth): effect on band structure and photocatalytic properties. J. Phys. Chem. B 2002, 106, 517−520.

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