Anion–Anion Mediated Coupling in Layered Perovskite La2Ti2O7 for

Department of Materials Science and Engineering, College of Materials, Xiamen ... Band gap engineering of SnS 2 nanosheets by anion–anion codoping f...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Anion−Anion Mediated Coupling in Layered Perovskite La2Ti2O7 for Visible Light Photocatalysis Peng Liu,*,†,‡ Jawad Nisar,‡ Baisheng Sa,†,§ Biswarup Pathak,*,∥ and Rajeev Ahuja†,‡ †

Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden ‡ Condensed Matter Theory Group, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden § Department of Materials Science and Engineering, College of Materials, Xiamen University, 361005 Xiamen, China ∥ Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Khandwa Road, Indore, 452017, India ABSTRACT: Anionic−anionic (N−N, P−P, N−P, and C−S) mediated coupling can be introduced in the layered perovskite La2Ti2O7 structure for visible light photocatalysis. The anionic−anionic codoped La2Ti2O7 systems lower the band gap much more than their respective monodoping systems. Moreover, the electronic band positions of the doped systems with respect to the water oxidation/reduction potentials show that codoped (N−N, N−P, and C−S) systems are more promising candidates for visible-light photocatalysis. The calculated defect formation energy shows that the codoped systems are more stable than their respective monodoped systems.

1. INTRODUCTION Limited fossil fuel based energy and the related environmental effects are the biggest concern of this century. Therefore the biggest challenge is to find a source of energy which is clean, sustainable, and renewable. Hydrogen is reported to be the one of such promising alternatives that is clean and renewable. Hence the hydrogen production should be clean, and photocatalysis of water is the cleanest and safest way to produce hydrogen.1−3 For effective utilization of solar energy, the photocatalyst must meet some important criteria. First of all, the photocatalyst should have an optimal band gap of ∼2 eV for the effective utilization of visible solar light. Moreover, the conduction band edge position of the semiconductor must be located at a more negative potential than the hydrogen reduction potential (H+/H2) and the top of the valence band edge must be positioned more positively than the water oxidation potential (H2O/O2). The H2 and O2 evolution generally occurs on the photocatalyst surface. Therefore, the kinetics of the H2 and O2 evolution depends lot on the surface area of the photocatalyst. Therefore, the larger the surface area, the higher the activities. The kinetics of the H2 and O2 evolution even depends on the oxidizing (the difference between the valence band maximum and water oxidation potential) and the reducing power (the difference between the conduction band minimum and the hydrogen reduction potential) of the photocatalyst. Another important structural requirement is that the H2 and O2 evolution sites should be far from each other so that they (H2 and O2) do not have any chance of reacting back to produce water.4 Hence, the layered © XXXX American Chemical Society

structure materials have attracted much attention due to their layered structures which separate out the H2 and O2 evolution sites. Many metal oxides with layered structures, such as K 4 Nb 6 O 17 , 5 La 2 Ti 2 O 7 , 6 LiCa 2 Ta 3 O 10 , 7 Sr 2 Ta 2 O 7 , 8 and Ba5Ta4O159 show high photocatalytic activity under ultraviolet (UV) light. Among of all of these layered perovskites, La2Ti2O7 has received much attention because of its high photocatalytic activity and hyper valency of the La atom constructing the layered perovskite structure.10−12 However, the band gap of La2Ti2O7 (3.8 eV) is still high enough to absorb the visible region of solar light. Therefore band gap engineering might be a promising strategy for this compound to be visible light active photocatalyst. Doping of a foreign element (cationic or anionic) into a wide band gap semiconductor is a promising strategy for reducing their band gap. Hwang et al. showed12,13 that Fe3+ and Cr3+ doped La2Ti2O7 show visible light photocatalytic activity but their photocatalytic activities are far lower than the UV light activities in terms of H2 evolution rate. This is because such transition metal doping brings the partially filled 3d band of the metal in the band gap that reduces the band gap significantly, but such impurity states appearing in the band gap act as an electron−hole recombination center which reduces the photocatalytic efficiencies. Even though, the cationic/anionic monodoping reduces the band gap considerably but such Received: March 26, 2013 Revised: June 11, 2013

A

dx.doi.org/10.1021/jp402971b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

doping has some limitations.14 First of all, the desirable dopants may have only limited solubility or have sufficient solubility but produces deep donor/acceptor levels and monodoping associates with the spontaneous formation of compensating defects. To overcome such limitations, codoping is a promising strategy.15−17 It was firmly believed that two anionic dopants will repel each other and hence reduce the overall stability but recently Yin et al.18 showed that the holes mediated coupling between the anionic dopants not only reduces the band gap significantly but also improves their overall stability.19 In this work, we use the flat band potential to estimate the band position of semiconductor4,8 which is defined as potential at which there is no band bending in semiconductor. The flat band potential of La2Ti2O7 measured by Scaife20 is −0.43 eV (vs NHE). Thus, the bottom of the conduction band level and the top of the valence band potential of La2Ti2O7 are estimated to be −0.43 and 3.37 eV, respectively. The conduction band position of La2Ti2O7 is good enough with respect to the hydrogen reduction potential, but its valence band position is deep enough with respect to the water oxidation potential. So it is important to move the valence band edge (VBE) upward for the effective utilization of visible solar light. Therefore we choose to dope the anionic O-site of La2Ti2O7 with elements (C, N, P, or S) which have higher p-orbital energies than the oxygen so that the valence band edge can be pushed upward. Moreover such doping will either increase or decrease the total number of electrons to the system which will certainly have some effect toward the electronic structure and band edge position of La2Ti2O7. For this we have done the spin polarized calculations. The effect of anionic codoping is studied to see if any anionic mediated coupling could be introduced to improve their visible light photocatalytic activity. Hence, we have investigated the anionic (N−N, N−P, P−P, and C−S) codoped La2Ti2O7 systems for introducing the anionic mediated hole− hole coupling. We have also studied the relative stability of codoped systems with respect to their monodoped La2Ti2O7 by calculating their relative binding energies.

of 500 eV is used for the PBE and HSE06 method. The PAW potentials with the valence states 6s and 5d for La; 4s and 3d for Ti; 2s and 2p for O, N, and C; 3s and 3p for P. For doping cases, a 1 × 1 × 2 supercell with 88 atoms was used, where dopant complexes are substituting on the O site or Ti site. The absorption curves can be obtained from the imaginary part of the dielectric constant using DFT calculation.29

3. RESULTS AND DISCUSSION 3.1. Pristine La2Ti2O7. La2Ti2O7 possesses layered perovskite structure with monoclinic space group of P21 in room temperature. The GGA-PBE calculated lattice parameters of La2Ti2O7 are a = 7.7992 Å, b = 5.5952 Å, and c = 13.2511 Å which are close to the experimental values of a = 7.8121 Å, b = 5.5440 Å, and c = 13.010 Å respectively.30 Figure 1a shows the

Figure 1. (a) Crystal structure of bulk pristine La2Ti2O7 with TiO6 octahedra: La, Ti, and O atoms are represented by green, blue, and red spheres, respectively, and (b) band edge alignment of pure La2Ti2O7 is presented with respect to the water redox potentials.

2. COMPUTATIONAL DETAILS The first-principle calculations were performed using projected augmented wave (PAW) method,21 as implemented in the Vienna ab initio simulation package (VASP).22−24 The exchange-correlation interaction was treated in the level of the GGA using Perdew−Burke−Ernzerhof (GGA-PBE).25 The Brillion zone was integrated using Monkhorst−Pack generated sets of k-points.26 The calculations for electronic structures were performed using the Heyd−Scuseria−Ernzerhof (HSE06) hydride functional.27,28 The exchange-correlation contribution employed in the HSE06 functional is divided into short- and long-range parts; Hartree−Fock (HF) is mixed with PBE exchange-correlation in the short-ranged part. To avoid the expensive calculation of long-range HF, as well as enabling hybrid DFT calculations on metal elements for which conventional Hartree−Fock or global hybrid calculations are intractable, this term is replaced by long-range PBE exchangecorrelation. 1 3 HSE E XC = EXHF,sr, μ + E XPBE,sr, μ + E XPBE,lr, μ + ECPBE (1) 4 4 k-point mesh 3 × 5 × 1 was found to be sufficient to reach convergence for PBE calculations and 2 × 3 × 1 for the HSE06 calculations. In all calculations, the tolerance for energy convergence was set to 10−5 eV. A plane-wave cutoff energy

optimized crystal structure of layered perovskite La2Ti2O7 with TiO6 octahedra. The calculated band gap using the GGA-PBE method is 2.93 eV, which is lower than the experimental value of 3.8 eV but it is a well-known fact that DFT generally underestimates the band gap for metal oxides.31 Therefore, we have used hybrid functional HSE06 method to calculate the band gap of pristine La2Ti2O7 and the calculated band gap (3.86 eV) is very close to the experimental value of 3.8 eV.32 Thus from here on, we have always used HSE06 method for the electronic structure calculations. The electronic structure of pure La2Ti2O7 with the total and partial density of states are plotted and presented in Figure 2. It is observed that VBM is mainly dominated by the O 2p and La 4f states whereas CBM is mainly contributed by the Ti 3d states. The band position alignment (Figure 1) is done with respect to the water oxidation/reduction potential and it shows the valence band edge is deep enough (by 2.14 eV) with respect to the water oxidation potential. Figure 2 shows that the valence bad edge is mainly contributed by the anionic O 2p orbitals; therefore, anionic doping is considered for our detailed study. Thus, in the following sections, we discuss how the B

dx.doi.org/10.1021/jp402971b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

edge of S-doped La2Ti2O7 is moved upward by 0.32 and the CBM remains unchanged. Therefore, the effective band gap of S-doped La2Ti2O7 is reduced to 3.54 eV. The reason is that the sulfur 3p orbitals’ energy is 2.2 eV higher than the oxygen 2p orbitals;33 thus, the sulfur 3p orbitals are appearing just above the VBM edge. After doping S atoms in La2Ti2O7, a clean and narrow band gap is obtained. However, the band gap reduction is not good enough for the visible light photocatalysis. Therefore we wanted to dope with other anionic elements that are not isoelectronic to O atom and has higher p-orbital energy than the O 2p orbitals. For this, first we have considered N doping in the La2Ti2O7. The spin polarization calculation is done on N-doped La2Ti2O7 and it gives a total magnetic moment of +1.0 μB, which is mainly contributed by the N (+0.67 μB) atom. In the N doped La2Ti2O7, the N 2p creates some fully filled states below the Fermi level and some unoccupied empty states above the Fermi level (Figure 3b). Therefore, after N doping, the effective band gap of La2Ti2O7 is reduced to 1.79 eV, which is an ideal band gap for visible light photocatalysis. The donor/acceptors states appearing in the band gap are mainly contributed by the N 2p and O 2p orbitals.33 N-doping La2Ti2O7 shows p-type conductivity as the Fermi level shifted closer to the valence band edge. However ptype conductivity in the wide band gap metal oxide semiconductor is very unusual as the acceptor states appearing above the Fermi levels are thermodynamically unstable.34 The band edge alignment for the N-doped La2Ti2O7 is done (Figure 4) with respect to the water oxidation/reduction potential. Our

Figure 2. Calculated total and partial density of states of pristine La2Ti2O7 using the HSE06 functional. The vertical dashed line represents the Fermi level.

mono and co-anionic doping study can tune the VBM position to improve their visible light photocatalytic activities. 3.2. Monodoping in La2Ti2O7. In case of mono doping study, one of the O atoms in the 1 × 1 × 2 super cell of La2Ti2O7 is replaced by an anionic dopant (N, S, P, and C), which corresponds to the doping concentration of 1.6%. First, we have calculated the most stable site for each of the anionic (S, N, P, and C) dopants by comparing their relative energetics. Then we have investigated the effect of anionic monodoping on the electronic structure of La2Ti2O7. We have calculated the total magnetic moment of S-doped La2Ti2O7 system which is 0. This is because S is isoelectronic to O atom therefore the total number of electrons remain same in case of S doped La2Ti2O7 system. We have presented the density of states of S-doped La2Ti2O7 in Figure 3a. Because of the S 3p orbitals, the VBM

Figure 4. Electronic band edge positions with respect to the water reduction and oxidation potential levels for the pure and doped La2Ti2O7 systems. Red represents the CBM, and blue represents the VBM.

results show that, even though such doping reduces the band gap significantly, the impurity states appearing in the band gap might act as an electron−hole recombination center, which will definitely suppress the photocatalytic activities. Since P has one less valence electron than O, the substitution of P on the O site will also act as a single acceptor for P doping La2Ti2O7. The P-doping La2Ti2O7 gives a total magnetic moment of 1.0 μB where the maximum (0.51 μB) contribution comes from the P atom itself. The DOS of P-doped La2Ti2O7 is presented in Figure 3c. It shows that fully occupied/unoccupied states are appearing in the band gap which is very similar to the P monodoping on TiO2.19 It is because of the strong interactions between the P and nearby O atoms, which split the 2p orbitals electrons into donor levels closer to the VBM and acceptor states above the Fermi level. Therefore, after P doping, the effective band gap of La2Ti2O7 is reduced to 1.16

Figure 3. Calculated DOS and PDOS of (a) sulfur, (b) nitrogen, (c) phosphorus, and (d) carbon doped La2Ti2O7 systems using HSE06 functional. The vertical dash line indicates the Fermi level. C

dx.doi.org/10.1021/jp402971b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

In the case of N−N homodoping, the dopant complex will have two net holes in the codoping La2Ti2O7. The total and partial DOS of N−N codoped La2Ti2O7 are presented in Figure 5a. In the relaxed structure of N−N codoped La2Ti2O7, we find

eV (Table 1), but their band position alignment shows (Figure 4) that the conduction band position is more positive than the Table 1. Binding Energies (Eb) of Codoping and Band Gaps (Eg) of Different Doped La2Ti2O7 Systemsa PBE dopants pure S N P C C−S N−N N−P P−P

Eb (eV)

0.46 −0.01 0.28 0.51

HSE06 dopant−dopant distance (Å)

Eg (eV)

VBM shifts

CBM shifts

1.80 2.86 1.75 2.26

3.86 3.54 1.79 1.16 1.97 1.71 1.60 1.46 1.35

+0.32 +0.61 +1.51 +1.23 +2.04 +0.72 +2.26 +2.05

+0.00 −1.46 −1.17 −0.62 −0.07 −1.64 −0.1 −0.42

a

The respective values are calculated using two different functional (PBE and HSE06), and the shifting of VBM and CBM is tabulated for monodoping and co-doping systems.

water reduction potential level that is thermodynamically unfavorable for hydrogen evolutions. Since the energies of C 2p are much higher than the S 2p, O 2p, and N 2p orbitals but lower than the P 3p orbitals, C dopants might be a more suitable dopant than the others. Moreover it has two less valence electrons than the oxygen. Therefore, the calculated magnetic moment of C-doped La2Ti2O7 is +1.99 μB where the maximum contribution (1.29 μB) comes from the C atom. The total and partial DOS of C doped La2Ti2O7 are presented in Figure 3d. Here, we can see that the occupied C 2p (spin-up and spin-down) and unoccupied C 2p (spin-down) states are located below and above the Fermi level, respectively. For this, the effective band gap of C doped La2Ti2O7 is reduced to 1.97 eV. Though the valence band position is good enough for the water oxidation reaction, the conduction band position is too close to the water reduction potential level which is not good for hydrogen reduction reaction (Figure 4). 3.3. Codoping in La2Ti2O7. Generally mono anionic doping creates impurity states in the band gap (Figure 3) that might act as a electron−hole recombination center. Moreover, monodoping associates with the spontaneous formation of charge compensating defects in the system which can destabilize the system. In order to remove such unwanted impurity states, codoping is a very promising strategy. Here we choose to do anionic codoping at the two O atom sites, which are very close to each other. We did not consider the codoping between C−C and S−S since such codoped systems are not reported to be suitable for visible light photocatalysis.35 The relative binding energies of the codoping systems are calculated with respect to their monodoped systems using the following equation:18,36

Figure 5. Calculated total and partial DOS of coanionic (a) N−N and (b) P−P doped La2Ti2O7. The vertical dash line indicates the Fermi level.

that the distance between the two doped N atoms (2.86 Å) are far (Table 1) from the standard N−N single bond distance of 1.45 Å in hydrazine (NH2−NH2).37 Therefore we can clearly say that the anionic−anionic coupling is not happening between the doped N atoms. The total magnetic moment for N−N codoping La2Ti2O7 is +1.45 μB, in which two N atoms are mainly contributing with 0.59 and 0.60 μB, respectively. From Figure 5a, we can see there are some unoccupied states appearing above the Fermi level. This is because the anionic mediate coupling is not happening by the N−N homodoping which generally stabilizes such unoccupied impurity states appearing in the band gap.18,38 But such impurity states appearing in the band gap reduces the effective band gap of 3.86 to 1.60 eV. To verify the analysis of density of states, we plot the isosurface of the valence band maximum (VBM) and the conduction band minimum (CBM) of homo- doping La2Ti2O7 in Figure 6. For (N,N) codoping, the VBM is composed of N 2p orbitals and O 2p orbitals while the CBM is contributed by N 2p orbitals, O 2p orbitals, and Ti 3d orbitals. Those results are in agreement with the DOS analysis (Figure 5a). The calculated binding energy for (N, N) doped La2Ti2O7 is −0.01 eV (Table 1), which shows that N−N homodoping is not favorable over the N monodoped system. The P−P homodoping in La2Ti2O7 is very much similar to the N−N homodoping case, as in both cases it brings two net holes to the system. To understand the net effect of the holes, we have plotted the total and partial DOS in Figure 5b. Since the P 3p orbital has higher energy than O 2p, the valence band edge (Figure 5b) is shifted upward significantly (2.05 eV). From Figure 6c,d, it is clear that the VBM and CBM are mainly

ΔE b = E(X 0) + E(Y0) − E(X 0 + Y0) − E(La 2Ti 2O7 ) (2)

where E(La2Ti2O7), E(X0), E(Y0), and E(X0 + Y0) are the total energies of pure, two different dopants (C, N, P, or S), and codoped systems, respectively. So the positive binding energy means the strong coupling. The relative binding energies of the codoped systems are listed in Table 1. D

dx.doi.org/10.1021/jp402971b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

Figure 6. Orbital distribution of (a) the valence band maximum and (b) the conduction band minimum for the (N,N) codoped La2Ti2O7 and (c) the valence band maximum and (d) the conduction band minimum for (P,P) codoped La2Ti2O7.

composed of the 3p states of the P atoms. Unlike in monodoping, the codoping system gives a clean band gap of 1.35 eV, which would certainly reduce the recombination losses and enhance the photocatalytic activities. The valence band edge position for P−P homodoped (Figure 3) system is good in comparison to the P monodoped system for the water oxidation reaction. Moreover, the relative binding energy of 0.51 eV, reflects the strong coupling between the two P atoms. Thus such hole mediated coupling will be very effective in comparison to their respective monodoping as coupling removes the unoccupied empty states appearing on monodoping. The (N−P) doped system adds two net holes to the La2Ti2O7 system where one comes from the N atom and the other one from the P atom. The total and partial density states of the anionic N−P heterogeneous codoping system are plotted in Figure 7a. It shows that the fully filled states are appearing just above the valence band edge, therefore narrowing the band gap to 1.46 eV. For (N−P) codoping, the distance between the N and P atoms is 1.86 Å, much shorter than the two O atom distance (2.89 Å) of the parent system (La2Ti2O7). Therefore it shows strong coupling between the N 2p and P 3p orbitals. We have plotted the charge density of the VBM and the CBM of heterogeneous codoped La2Ti2O7 system in Figure 8. The strong couplings between the N and P atoms cause the VBM contributed by N 2p and P 3p hybrid orbitals. The CBM of (N,P) codoped La2Ti2O7 are composed by neighboring Ti 3d orbitals which is also shown in the density of state analysis. The valence band edge position of N−P codoped La2Ti2O7 systems is slightly higher than the water oxidation reaction level, while the conduction band edge is very good for water reduction reaction level. For the C−S codoping, the dopant C will add two holes to the system. The total and partial density of states of C−S doped La2Ti2O7 are plotted in Figure 7b, which show that fully filled states are generated above the VBM, reducing the band gap to 1.71 eV. From Figure 8c,d, the VBM is composed of C 2p and S 2p orbitals and the CBM is contributed by Ti 3d orbitals. Those results also are in agreement with the conclusion we get from the DOS analysis of (C, S) codoped La2Ti2O7 (Figure 7b). The coupling process between C and S atoms in La2Ti2O7

Figure 7. Calculated total and partial DOS of La2Ti2O7 with heterogeneous codoping of (a) (N−P) and (b) (C−S). The vertical dash line indicates the Fermi level.

causes the valence band edge shifted upward significantly by 2.04 eV which is similar with Yin’s research on TiO2.18 The alignment of C−S codoped La2Ti2O7 is present in Figure 3, which indicates it is good for visible light photocatalysis. Therefore, looking into the band edge alignment of any photocatalyst, we should be able to predict what kind of doping is going to be crucial for their higher photocatalytic activities. Sometimes, the band edge alignment of the pure system shows conduction band edge is close to the hydrogen reduction potential where as valence band edge is far from the water oxidation potential. So, in such cases, we should push the valence band edge upward not the conduction band edge downward. As the valence band edge is mainly composed of E

dx.doi.org/10.1021/jp402971b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

Δ + 2μLa(bulk) ≤ 2μLa ≤ 2μLa(bulk)

(3a)

Δ + 2μTi(bulk) ≤ 2μTi ≤ 2μTi(bulk)

(3b)

Δ + 7μo(gas) ≤ 7μo ≤ 7μo(gas)

(3c)

In case of the host elements, μLa and μTi are calculated form the pure bulk metals of La and Ti, while μo(gas) = 1/2μo2 is calculated with one O2 molecule centered in a 20 × 20 × 20 Å3 cubic box. For dopants, we consider the graphite, nitrogen gas, phosphorus, and sulfur as the C, N, P, and S reservoirs, respectively. Figure 9 shows the calculated formation energy for the doping systems. The positive value of the formation energy Figure 8. Orbital distribution of (a) the valence band maximum and (b) the conduction band minimum for the (N,P) codoped La2Ti2O7 and (c) the valence band maximum and (d) the conduction band minimum for (C,S) codoped La2Ti2O7.

anionic elements where as conduction band edge is mainly composed of cationic elements, therefore in such material anionic mono- or codoping will be very helpful for their further improvement. Therefore, we believe we can guide the experimentalist by looking at the band edge position that what kind of doping is going to be crucial for any photocatalyst. As we know when we dope anionic elements with higher porbital energy, then there is a shifting of valence band edge upward. Similarly the cationic band position can be tuned based on the metal’s d-orbital energy. We believe our such studies will not only give an idea about a particular metal’s role in a photocatalysts but also will give an idea about how the metals down the group or across the row (in the periodic table) can behave in a photocatalysts. Therefore such studies going to be very crucial and helpful for the understanding of the different metal’s role in the photocatalysis studies. 3.4. Defect Formation Energy for Doped La2Ti2O7. In order to search for the proper growth conditions, we have calculated the defect formation energy of the doping La2Ti2O7 systems. The defect formation energy ΔHf(D) of a defect of α atom can be defined as a function of the chemical potentials and the numbers of host and impurity atoms2,39 ΔHf (D) = E T(D) − E T(H ) + q(E F + E V ) −

Figure 9. Calculated defect formation energy of monodoped and codoped La2Ti2O7 as a function of oxygen chemical potential (μo).

increases with the chemical potential of the host atom because the vacancies are difficult to form under the host-rich condition. Figure 9 shows, the N monodoping is more favorable than the other monodoping which might be due to their similar atomic sizes. While, for the codoping system, (P−P) codoping is more energetically favorable than the other codoping. Moreover, at the O-poor condition, the P−P codoping is most favorable which could be due to the strong coupling between the P dopants. Therefore, the synthesis of N, S, P, and C doped La2Ti2O7 is very much possible in the O-poor condition where as synthesis of P−P, N−P, N−S, and N−C codoped La2Ti2O7 is only possible under the O-poor to poorer conditions. 3.5. Absorption Curves. We have plotted the optical absorption spectrum (Figure 10) of P−P, N−P, and C−S codoped La2Ti2O7 systems. This is because their band position alignment is good enough for the visible light photocatalysis (Figure 4). The optical spectra (Figure 10) of (P−P) show a peak at around 350 nm while the (N−P) and (C−S) show the absorption peak in the visible light region. Therefore, we show these codoped systems can harvest a longer wavelength of the visible light spectrum as compared to the pristine La2Ti2O7 for the visible light photocatalysis.

∑ nαμαelem α

(3)

Here ET(D) and ET(H), are the total energies of the systems with and without defects, respectively. Ev represents the energy of the VBM of the primitive supercell whereas Ef is the Fermi energy. Moreover, all the defect formation has been considered in neutral charge state therefore q = 0. nα is the number of atoms that have been added or removed. Here nα = 1 or −1 if an atom is added or removed respectively. μelement is the α chemical potential of the atom. In case of La2Ti2O7, The chemical potential of the elements La, Ti and O cannot exceed bulk gas those of the bulk μbulk La , μTi , and gaseous μO , respectively. element bulk element bulk elemlent Thus, μLa ≤ μLa , μTi ≤ μTi , and μO ≤ μgas O . When La2Ti2O7 is in equilibrium with reservoirs of La, Ti, and O, the sum of the chemical potentials of La, Ti, and O atoms must be equal to that of the bulk La2Ti2O7, i.e., 2μLa + 2μTi + 7μo = μLa2Ti2O7(bulk). The heat of formation of La2Ti2O7 is Δ = μLa2Ti2O7(bulk) − 2μLa(bulk) − 2μTi(bulk) − 7μo(gas). As the Δ value is negative for stable materials, therefore ΔμLa, ΔμTi, and Δμo must satisfy

4. CONCLUSIONS In conclusion, we have examined the effect of anionic monodoping (N, S, P, and C) and codoping on the geometrical F

dx.doi.org/10.1021/jp402971b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

(2) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746−50. (3) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B., Jr. Efficient Photochemical Water Splitting by A Chemically Modified n-TiO2. Science 2002, 297, 2243. (4) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253−78. (5) Kudo, A.; Sayama, K.; Tanaka, A.; Asakura, K.; Domen, K.; Maruya, K.; et al. Nickel-loaded K4Nb6O17 Photocatalyst in the Decomposition of H2O into H2 and O2: Structure and Reaction Mechanism. J. Catal. 1989, 120, 337−52. (6) Hwang, D. W.; Kim, H. G.; Lee, J. S.; Kim, J.; Li, W.; Oh, S. H. Photocatalytic Hydrogen Production from Water Over M-Doped La2Ti2O7 (M = Cr, Fe) under Visible Light Irradiation (λ> 420 nm). J. Phys. Chem. B 2004, 109, 2093−102. (7) Ikeue, K.; Mitsuyama, T.; Arayama, K.; Tsutsumi, A.; Machida, M. Effect of Heat Treatment on Local Structure and Photocatalytic Water Splitting Activity of Ni-loaded LiCa2Ta3O10. J. Ceram. Soc. Jpn. 2009, 117, 1161−5. (8) Kudo, A.; Kato, H.; Nakagawa, S. Water Splitting Into H2 and O2 on New Sr2M2O7 (M = Nb and Ta) Photocatalysts with Layered Perovskite Structures: Factors Affecting the Photocatalytic Activity. J. Phys. Chem. B 2000, 104, 571−5. (9) Otsuka, H.; Kim, K.; Kouzu, A.; Takimoto, I.; Fujimori, H.; Sakata, Y. Photocatalytic Performance of Ba5Ta4O15 to Decomposition of H2O into H2 and O2. Chem. Lett. 2005, 34, 822−3. (10) Kim, J.; Hwang, D.; Bae, S.; Kim, Y.; Lee, J. Effect of Precursors on the Morphology and the Photocatalytic Water-Splitting Activity of Layered Perovskite La2Ti2O7. Kor. J. Chem. Eng. 2001, 18, 941−7. (11) Wang, R.; Xu, D.; Liu, J.; Li, K.; Wang, H. Preparation and Photocatalytic Properties of CdS/La2Ti2O7 Nanocomposites under Visible Light. Chem. Eng. J. 2011, 168, 455−60. (12) Hwang, D. W.; Kim, H. G.; Lee, J. S.; Kim, J.; Li, W.; Oh, S. H. Photocatalytic Hydrogen Production from Water Over M-Doped La2Ti2O7 (M = Cr, Fe) under Visible Light Irradiation (λ > 420 nm). J. Phys. Chem. B 2005, 109, 2093−102. (13) Hwang, D. W.; Kim, H. G.; Jang, J. S.; Bae, S. W.; Ji, S. M.; Lee, J. S. Photocatalytic Decomposition of Water-methanol Solution over Metal-doped Layered Perovskites under Visible Light Irradiation. Catal. Today 2004, 93−95, 845−50. (14) Yan, Y.; Li, J.; Wei, S. H.; Al-Jassim, M. M. Possible Approach to Overcome the Doping Asymmetry in Wideband Gap Semiconductors. Phys. Rev. Lett. 2007, 98, 135506. (15) Niishiro, R.; Kato, H.; Kudo, A. Nickel and either Tantalum or Niobium-codoped TiO2 and SrTiO3 Photocatalysts with Visible-light Response for H2 or O2 Evolution from Aqueous Solutions. Phys. Chem. Chem. Phys. 2005, 7, 2241−5. (16) Ishii, T.; Kato, H.; Kudo, A. H2 Evolution from An Equeous Methanol Solution on SrTiO3 Photocatalysts Codoped with Chromium and Tantalum Ions under Visible Light Irradiation. J. Photochem. Photobiol. A 2004, 163, 181−6. (17) Ohno, T.; Tsubota, T.; Nakamura, Y.; Sayama, K. Preparation of S, C Cation-Codoped SrTiO3 and Its Photocatalytic Activity under Visible Light. Appl. Catal. A 2005, 288, 74−9. (18) Yin, W. J.; Wei, S. H.; Al. Jassim, M. M.; Yan, Y. Double-holemediated Coupling of Dopants and its Impact on Band Gap Engineering in TiO2. Phys. Rev. Lett. 2011, 106, 066801. (19) Long, R.; English, N. J. New Insights into the Band-gap Narrowing of (N, P)-codoped TiO2 from Hybrid Density Functional Theory Calculations. ChemPhysChem 2011, 12, 2604−8. (20) Scaife, D. E. Oxide Semiconductors in Photoelectrochemical Conversion of Solar Energy. Sol. Energy 1980, 25, 41−54. (21) Blochl, P. E. Projector Augmented-wave Method. Phys Rev B 1994, 50, 17953−79. (22) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558−61.

Figure 10. Calculated optical absorption spectra for pure, C−S, N−P, and P−P codoped La2Ti2O7 system.

and electronic structure of La2Ti2O7 using hybrid functional (HSE06) calculations. For monodoping systems, the band gap reduction can be achieved by an anionic dopant, which has higher p orbital energy than the O atom. However, the band alignment study shows that monodoping is not suitable for the hydrogen evolution reaction. Our codoping (N−N, P−P, N−P, and C−S) study on La2Ti2O7 systems show that the anionic− anionic mediated dopant−dopant (C−S, P−P, and N−P) coupling can reduce the band gap more effectively than their respective monodoped systems. Such doping not only reduces the band gap but also removes the unwanted impurity states that appear on the monodoping. Moreover, their band position is very good for the visible light photocatalysis. Our calculated optical absorption study also shows that such codoped (C−S, P−P, and N−P) La2Ti2O7 can harvest the boarder spectrum (visible) of solar light. The calculated binding energy indicates that the codoped systems are more stable than their respective monodoped systems. We believe our results will guide the experimentalist to study La2Ti2O7 with anionic−anionic codoping.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We would like to acknowledge the Swedish Research Council (VR), Swedish Energy Agency and Stiffelsen J. Gust Richerts Minne (SWECO) for financial support. J.N. is thankful to the Higher Education Commission (HEC) of Pakistan, P.L. also thanks the Chinese Scholarship Council (CSC). B.P. thanks the Council of Scientific Indian Research (CSIR), New Delhi for the funding. SNIC and UPPMAX are acknowledged for providing computing time.

■

REFERENCES

(1) Long, R.; English, N. J.; Prezhdo, O. V. Photo-induced Charge Separation across the Graphene−TiO2 Interface Is Faster Than Energy Losses: A Time-Domain Ab Initio Analysis. J. Am. Chem. Soc. 2012, 14238−14248. G

dx.doi.org/10.1021/jp402971b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

(23) Kresse, G.; Hafner, J. Ab Initio Molecular-dynamics Simulation of the Liquid-metal-amorphous-semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251−69. (24) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758−75. (25) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; et al. Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46, 6671−87. (26) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-zone Integrations. Phys. Rev. B 1976, 13, 5188−92. (27) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Erratum: “Hybrid Functionals Based on a Screened Coulomb Potential” [J. Chem. Phys. [bold 118], 8207 (2003)]. J. Chem. Phys. 2006, 124, 219906−1. (28) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207−15. (29) Liu, J. W.; Chen, G.; Li, Z. H.; Zhang, Z. G. Electronic Structure and Visible Light Photocatalysis Water Splitting Property of Chromium-doped SrTiO3. J. Solid. State Chem. 2006, 179, 3704−8. (30) Schmalle, H. W.; Williams, T.; Reller, A.; Linden, A.; Bednorz, J. G. The Twin Structure of La2Ti2O7: X-ray and Transmission Electron Microscopy Studies. Acta. Crystallogr. Sec. B 1993, 49, 235−44. (31) Godby, R. W.; Schloter, M.; Sham, L. J. Accurate ExchangeCorrelation Potential for Silicon and its Discontinuity on addition of an Electron. Phys. Rev. Lett. 1986, 56, 2415−8. (32) Hwang, D. W.; Lee, J. S.; Li, W.; Oh, S. H. Electronic Band Structure and Photocatalytic Activity of Ln2Ti2O7 (Ln = La, Pr, Nd). J. Phys. Chem. B 2003, 107, 4963−70. (33) Wei, S. H.; Krakauer, H.; Weinert, M. Linearized Augmentedplane-wave Calculation of the Electronic Structure and Total Energy of Tungsten. Phys. Rev. B 1985, 32, 7792−7. (34) Miyauchi, M.; Takashio, M.; Tobimatsu, H. Photocatalytic Activity of SrTiO3 Codoped with Nitrogen and Lanthanum under Visible Light Illumination. Langmuir 2003, 20, 232−6. (35) Nisar, J.; Pathak, B.; Wang, B.; Won, Kang, T.; Ahuja, R. Hole Mediated Coupling in Sr2Nb2O7 for Visible Light Photocatalysis. Phys. Chem. Chem. Phys. 2012, 14, 4891−7. (36) Nisar, J.; Wang, B. C.; Pathak, B.; Kang, T. W.; Ahuja, R. Moand N-doped BiNbO4 for Photocatalysis Applications. Appl. Phys. Lett. 2011, 99, 051909−3. (37) Shan, H.; Yang, Y.; James, A. J.; Sharp, P. R. Dinitrogen Bridged Gold Clusters. Science 1997, 275, 1460−2. (38) Liu, P.; Nisar, J.; Pathak, B.; Ahuja, R. Hybrid Density Functional study on SrTiO3 for visible light photocatalysis. Int. J. Hydrogen Energy 2012, 37, 11611−7. (39) Liu, P.; Nisar, J.; Ahuja, R.; Pathak, B. Layered Perovskite Sr2Ta2O7 for Visible Light Photocatalysis: A First Principles Study. J. Phys. Chem. C 2013, 117 (10), 5043−5050.

H

dx.doi.org/10.1021/jp402971b | J. Phys. Chem. C XXXX, XXX, XXX−XXX