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Density Functional Characterization of the Electronic Structure and Optical Properties of N-Doped, La-Doped, and N/La-Codoped SrTiO3 Wei Wei, Ying Dai,* Meng Guo, Lin Yu, and Baibiao Huang School of Physics, State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, P.R. China ReceiVed: March 21, 2009; ReVised Manuscript ReceiVed: May 20, 2009
We studied the electronic and optical properties of N-doped, La-doped, and N/La-codoped SrTiO3 by means of first-principles DFT calculations to explore the physical and chemical origin of the photocatalytic activity of these structures under visible light. Our results indicate that the introduction of La into SrTiO3 lattice is in favor of the incorporation of N and reduces the formation of oxygen vacancies in N/La-codoped SrTiO3 with La at Sr site, which is the most favorable structure with respect to both energetic and activity. In the codoped configuration, N 2p states are passivated and strongly mix with the O 2p valence band edge leading to a narrowed band gap, moreover, without the presence of recombination center due to donor-acceptor pair recombination. The narrowed band gap and absence of recombination center can give a reasonable explanation for the high photocatalytic activity of N/La-codoped SrTiO3 under visible light. This charge-compensated n-type/p-type dopants codoping method should be applicable for exploiting other efficient visible light-driven photocatalysts with wide band gap semiconductors. 1. Introduction Exploring new types of photocatalysts except TiO2 has become a crucial subject for energy source and environment science and technology. Cubic perovskite structure SrTiO3 as one of the promising photocatalytic candidates for TiO2 has been attracting more and more attentions due to its excellent photocatalytic performance capable of splitting water into H2 and O21-7 and decomposing organic compounds.8,9 However, similar to TiO2, because of its wide band gap (about 3.2 eV), SrTiO3 can only absorb a small portion of the solar spectrum in the ultraviolet (UV) light region (less than 5% of the solar flux incident at the earth surface lies in this region), which greatly restricts its energy conversion efficiency. For the purpose of improving the SrTiO3 photocatalytic efficiency, the most popular approach is to modify the energy band structure of SrTiO3 to shift its optical absorption edge from the UV region to the visible light region. Moreover, doping foreign elements into a semiconductor with wide band gap to create a new optical absorption edge is known to be one of the primary strategies for developing visible light-driven photocatalysts. Recently, various cation- or anion-doped SrTiO3 structures have been synthesized, and their corresponding activities under visible light have been investigated extensively.10-28 Among them, doping with N, La, or both N and La was found to be an efficient method to improve the visible light activity of SrTiO3.25-27 For example, experiments reported by Wang et al.25 showed that nitrogen doping can greatly improve the photocatalytic activity of SrTiO3 under visible light. Qin et al.26 reported that a suitable concentration of lanthanum dopant can greatly improve the photocatalytic activity of CoO/SrTiO3. However, although anion or cation doping can expand the optical absorption edge into visible light region which leads to visible light-response photocatalytic activity, it is not our eventually expected. In fact, cations modification can obviously reduce the photon conversion efficiency because there should present carrier recombination centers derived from the cation dopant and/or strongly localized * To whom correspondence should be addressed.
d states (transitional metal) within the band gap, which significantly reduce the carrier mobility. Our previous work involving the Cr-doped SrTiO3 system has also testified to the aforementioned fact.29 Anion-modified structures are deemed too instable since they are readily decomposed under light irradiation and thus present poor photocatalytic activity. In general, doping with wide band gap semiconductors has perhaps three disadvantages:30 (a) the desirable dopants have limited solubility, (b) the desirable dopants have sufficient solubility, but they produce deep levels, and (c) spontaneous formation of compensating defects. As a consequence, one should exploit other effective methods. By considering the band structure of the photocatalyst host and the chemical potential of the dopant, codoping may be a better choice. Wang et al.27 reported that the photocatalytic activity of N/La-codoped SrTiO3 under irradiation with a wavelength larger than 400 nm was 2.6 times greater than that of pure SrTiO3. They assumed that the high photocatalytic activity under visible light occurring in N/Lacodoped SrTiO3 was ascribed to a new absorption edge formed in the visible light region. Miyauchi et al.28 also reported a promising work about the photocatalytic activity of N/Lacodoped SrTiO3 experimentally, and they ascribed the high activity to the decrease of the oxygen vacancies, which may act as electron-hole pair recombination centers, because the charge balance can be maintained with N/La-codoping. Miyauchi et al. also provided some first-principles information of N/La-codoped SrTiO3 to demonstrate that codoping with La can reduce the formation of oxygen vacancies in nitrogen-doped SrTiO3. However, few theoretical studies describing explicitly the codoping synergistic effect and concerning the single Nand La-doped SrTiO3 structures have been reported, and the origin of high photocatalytic activity under visible light is not illuminated definitely. In the present work, we examined the microscopic electronic structure of SrTiO3 codoped with N and La to explore the synergistic effects of the dopants in detail on the defect formation energy, the energy band structure, and the photocatalytic absorption of this codoped SrTiO3 system by means of a first-principles density functional theory (DFT)
10.1021/jp902567j CCC: $40.75 2009 American Chemical Society Published on Web 07/23/2009
N-Doped, La-Doped, and N/La-Codoped SrTiO3
J. Phys. Chem. C, Vol. 113, No. 33, 2009 15047 TABLE 1: Defect Formation Energies for Various Doped SrTiO3 Structuresa defect formation energy N-doped La-doped N/La-codoped
5.21 -0.57,b 9.17c 2.78,d 13.72e
a The unit is eV. b Defect formation energy of substitutional La for Sr-doped SrTiO3. c Defect formation energy of substitutional La for Ti-doped SrTiO3. d Defect formation energy of N/La-codoped SrTiO3 with La at Sr site. e Defect formation energy of N/ La-codoped SrTiO3 with La at Ti site.
structure, we calculated the defect formation energy, Ef, according to the following equations and the results were listed in Table 1.
Ef ) EN-doped - (Eundoped - µO + µN) Figure 1. Bulk structure of cubic perovskite SrTiO3 codoped with N and La. The green, gray, and red spheres represent Sr, Ti, and O atoms, respectively. N and La atoms are labeled.
method. To obtain detailed insight, N- and La-doped SrTiO3 with La at different cationic sites (at Sr or Ti site) are also studied systematically. The process of N defect states passivating, which involves the reduction of oxygen vacancies, is investigated amply through the comparison of chemical environment and electronic properties between N-doped SrTiO3, Ladoped SrTiO3, and N/La-codoped SrTiO3 systems. Based on our calculations, the mechanism of band gap-narrowing is illuminated from the point of view of so-called donor-acceptor pair (DAP) recombination.31 The process of the codoping synergistic effect is specifically elucidated, which can provide some helpful theoretical information for exploiting effective photocatalysts. 2. Computational Details In the present work, we simulated the doping effects using a 2 × 2 × 2 repetition of the SrTiO3 unit bulk cell with 40-atom. We substituted a N atom for an O atom and a La atom for a Sr atom or a Ti atom in the SrTiO3 structure to model the N- or La-doping and N/La-codoping, respectively. To make it unambiguous, the codoped structure with La at the Sr site is displayed in Figure 1. We performed the first-principles DFT calculations using the quantum mechanical CASTEP32 package, in which the exchange correlation potential was described by a generalized gradient approximation (GGA)33 with Perdew-BurkeErnzerhof (PBE) scheme.34 Interaction between the valence electrons and ion core was substituted by an ultrasoft pseudopotential.35,36 Electronic wave functions were expanded in terms of a discrete plane wave basis set. The k space integrations were carried out utilizing the Monkhorst-Pack37 grid with 4 × 4 × 4 k points in the Brillouin zone of the SrTiO3 geometry, and the kinetic energy cutoff for the wave function expanding was 340 eV. Geometry optimization was carried out before singlepoint energy calculations, and the self-consistent convergence accuracy was set at 5 × 10-5 eV/atom. The convergence criterion of the largest force on atoms was 0.1 eV/ Å, the maximum displacement was 5 × 10-4 nm, and the stress was no more than 0.2 GPa, respectively. Electronic structures and total energies were calculated on the corresponding optimized crystal geometries. 3. Results and Discussion 3.1. Defect Formation Energies. To determine the energies required for doping N or La or codoping with N/La in the SrTiO3
Ef ) ELa-doped - [Eundoped - µSr(or µTi) + µLa] Ef ) EN/La-codoped - [Eundoped - µO - µSr(or µTi) + µN + µLa] where Eundoped, EN-doped, ELa-doped, and EN/La-codoped are the total energies of the SrTiO3 without doping and with N doping, La doping, and N/La codoping, respectively. µO and µN are the energies of O and N atom taken from the energies of molecular O2 and N2, respectively. µSr, µTi, and µLa are calculated from the bulk Sr, Ti, and La crystal, respectively. It is noticed from these aforesaid equations that the substitutional doping is energetically more favorable as the defect formation energy becomes smaller. From Table 1, because of the apparent smaller defect formation energy, it can be seen that La energetically prefers to substitute for Sr rather than Ti in the La-doped SrTiO3. This is reasonable because the ionic radius of La3+ (0.115 nm) is close to that of Sr2+ (0.113 nm) and largely different from that of Ti4+ (0.068 nm) resulting in the relative smaller formation energy for substituting La for Sr. Moreover, breaking of the covalent Ti-O bond requiring relative larger energy also manifests a relatively larger defect formation energy. Comparing the defect formation energy of N-doped SrTiO3 with that of N/La-codoped SrTiO3 with La at a Sr site, it can be seen that the codoped SrTiO3 structure can be obtained more easily than that for N-doped SrTiO3, which can be inferred from the presence of charge balance in this codoped structure. Moreover, by analyzing the formation energy, we conclude that the introduction of La is in favor of the incorporation of N into the SrTiO3 lattice as a consequence of charge compensation occurred between N and La dopants. So, the limited solubility of N dopant can be improved due to the incorporation of La. 3.2. Electronic Structure. The band structure of N/Lacodoped SrTiO3 with La at the Sr site (Figure 1), which is the energetically preferable configuration, is displayed in Figure 2c. For a comparison, those of undoped and N-doped SrTiO3 are also plotted (Figure 2, panels a and b, respectively). The band structure of undoped SrTiO3 displays a direct band gap about 1.86 eV at G (gamma point). Although the calculated band gap of the undoped SrTiO3 is underestimated compared with the experimental value (about 3.2 eV) due to the well-known limitation of GGA, the characteristics of the band structure as well as the relative variations of the band gap are expected to be qualitatively reasonable and reliable. A comparison of panels
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Figure 2. Band structures of (a) undoped SrTiO3, (b) substitutional N for O-doped SrTiO3, and (c) N/La-codoped SrTiO3 with La at Sr site. The dashed lines represent the highest occupied level.
Figure 4. (a) TDOS and (b) PDOS of N/La-codoped SrTiO3 with La at Ti site. The dashed lines represent the highest occupied level.
Figure 3. Projected DOS of (a) undoped SrTiO3, (b) substitutional N for O-doped SrTiO3, and (c) N/La-codoped SrTiO3 with La at the Sr site. The dashed lines represent the highest occupied level.
a and b in Figure 2 shows that some unoccupied acceptor states are introduced above the valence band resulting in a band gap narrowing of about 0.39 eV, while the conduction band shows no significant change. Here, we must acknowledge that the band gap-narrowing effect depends on N concentration in the oxide host, which has been proved in N-doped TiO2.38,39 In the present calculation, N-doped SrTiO3 shows a narrowed band gap rather than isolated N 2p states above the valence band, which can be ascribed to the high N concentration. Despite the fact that the nitrogen density is higher in the DFT calculations than that in experiment, electronic structure calculations surely give important information concerning the codoping effect, which is also pointed out in ref 28. The narrowed band gap may lead to the
reduction of photon transition energy and thus redshifts the optical absorption edge into the visible light region in N-doped SrTiO3. However, we must acknowledge that merely inducing visible light absorption does not guarantee satisfactory photocatalytic activity. It is pointed out that the N-doped SrTiO3 displays a p-type conductivity character to keep charge balance (shown in Figure 2b), whereas p-type conductivity in the oxide semiconductor with wide band gap is difficult to obtain, and these acceptor states are thermodynamically unstable.28 Consequently, nitrogen-doping inevitably introduces oxygen vacancies in the SrTiO3 structure as a consequence of electron transfer from the Ti3+ states derived from oxygen vacancies to the partially occupied N 2p states during the sample preparation process. Nevertheless, oxygen vacancies always play an undesirable role as the recombination center of photogenerated electron-hole pairs in the experiment, which badly decreases the photocatalytic activity. Therefore, the nitrogen (p-type dopant) related defect bands must be passivated by another n-type dopant for the intent of effective photocatalytic activity. Because the reducing ability of the SrTiO3 host is measured by the conduction band minimum, which is slightly above the
N-Doped, La-Doped, and N/La-Codoped SrTiO3
J. Phys. Chem. C, Vol. 113, No. 33, 2009 15049
Figure 5. Total DOS of (a) substitutional La for Ti-doped SrTiO3 and (c) substitutional La for Sr-doped SrTiO3. Projected DOS of (b) substitutional La for Ti-doped SrTiO3 and (d) substitutional La for Sr-doped SrTiO3. The dashed lines represent the highest occupied level.
hydrogen production level, the other n-type codoped dopant should not affect the conduction band of the SrTiO3 host significantly. La is one of the promising candidates chosen for codoping with N. The band structure character of N/La-codoped SrTiO3 with La at the Sr site shown in Figure 2c indicates that the bandwidth of the valence band is further expanded. The band gap narrows by about 0.53 eV compared with that of the undoped SrTiO3, which thus may lead to the photocatalytic activity under visible light. In this case, we can see that the valence band maximum increases greatly compared to the pure SrTiO3, whereas the change of conduction band is small indicating that the reducing ability is not reduced significantly. The large dispersion at the top of the valence band maximum also indicates that the N-derived defect bands are not too localized to limit the light absorption if the N/La concentration is reasonably large. Furthermore, Figure 2c also shows that the highest occupied level is pinned at the top of the valence band, which means that N/La codoping can keep the charge balance without the formation of undesired oxygen vacancies and eliminate the disadvantages aforementioned in the N-doped SrTiO3. To examine the origin of the electronic structure modification of the preferred codoped configuration, the projected DOS (density of states)plots of the N/La-codoped SrTiO3 with La at the Sr site are presented in Figure 3c. The projected DOS plots of undoped SrTiO3 and N-doped SrTiO3 are also plotted in Figure 3, panels a and b, respectively, as reference. For undoped SrTiO3, the upper valence band is dominantly composed of O 2p states and the conduction band Ti 3d states. Some Ti 3d states sufficiently disperse within the O 2p states illuminating the covalent property of the Ti-O bond. In addition, only a few Sr related electronic states appear in the O 2p valence band, which demonstrates ionic interaction between Sr and TiO6. It is noted that the bandwidth of O 2p states in the upper valence band is about 4.50 eV. After the introduction of substitutional N for O (Figure 3b), some N 2p partially occupied states are introduced above the valence band and overlap with O 2p states with the O 2p valence band expanded to be about 4.84 eV, which results in the band gap narrowing. Figure 3c shows that
the introduction of a La atom results in the O 2p valence band suffering a further expansion to be about 5.13 eV, whereas La related states have few contributions neither to the valence band nor to the conduction band due to the ionic interaction between La and TiO6. In this situation, partially occupied N 2p states are present as totally occupied. To keep charge balance, this N atom requires one more electron from the SrTiO3 lattice than an O atom does, so the N may act as a single acceptor. Meanwhile, the La atom releases one more electron than a Sr atom to the SrTiO3 lattice and may act as single donor. Spontaneously, the extra electron brought by La dopant pairs up with the unpaired N 2p electron in SrTiO3 lattice, namely, the electron on the donor level passivates the same amount of hole on the acceptor level, so that, this codoped system can still be of semiconductor character. Neither acceptor levels nor donor levels appearing within the band gap also confirm that the charge balance is maintained owing to the codoping of N and La, and thus the photocatalytic activity under visible light can be improved to an extensive degree. To obtain a more detailed characterization, the total DOS and projected DOS plots of N/La-codoped SrTiO3 with La at Ti site are also shown in Figure 4. For this codoped configuration, unlike that with La at the Sr site, both O 2p and N 2p orbitals are partially occupied. This is reasonable because the doped La as La3+ at a Ti4+ site means a La ion releases one less electron to the SrTiO3 lattice than each Ti ion does, which results in the unoccupied O 2p states just above the top of the valence band. In order to give a particular description, we also inspected the electronic characters of La in La-doped SrTiO3 systems with La substituting for different cations, i.e., Sr or Ti. The results are summarized in Figure 5. For substitutional La for Ti-doped SrTiO3 (Figure 5, panels a and b), the highest occupied level is pinned at the position slightly below the maximum of the valence band. La related states have little contribution neither to the conduction band nor to the valence band. From the projected DOS plots, it can be seen that after the incorporation of La, a hole is introduced, and this hole delocalizes at ambient O atoms. For substitutional La for Sr-doped SrTiO3 (Figure 5, panels c and d), the La3+ ion releases one more electron to the
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SrTiO3 lattice than each Sr2+ ion does, which induces some occupied Ti 3d donor states appearing just below the bottom of the conduction band. There may be a competitive effect between Ti and La with respect to electron transfer from the cations to O. Because the ionization energy of La is far smaller than that of Ti, valence electrons of La receive the preferential warrant transferring to O leading to the formation of Ti3+. From Figure 5d, it can be seen that this extra electron delocalizes on Ti atoms around La. When the N impurity is introduced, the electron in the Ti3+ states at the presence of La-doping will fill the N 2p states eliminating the formation of oxygen vacancies. The charge compensation process, Ti3+ (d1) change into Ti4+ (d0) and partially occupied N 2p states change into completely occupied, indicates a stabilizing effect.38 Subsequently, it can be deduced that there presents a donor-acceptor pair (DAP) recombination31 in the N/La-codoped SrTiO3 with La at Sr site, in which both acceptor levels and donor levels do not appear within the band gap resulting in the codoped SrTiO3 a tempting structure with a narrowed band gap and the recombination center of photogenerated carriers is suppressed. So the N/La-codoped SrTiO3 system possesses high photostability and high photocatalytic activity under visible light like reported in refs 27 and 28. 4. Summary The electronic and optical properties of N-doped, La-doped, and N/La-codoped SrTiO3 have been studied by means of firstprinciples DFT calculations. The results indicate that the optimal doping model is N/La-codoping with La at Sr site for both energetic and high photocatalytic activity under visible light, in which exists a DAP recombination and thus charge balance is kept. For this optimal codoped SrTiO3 system, oxygen vacancies associated with N-doping are not required and the overall energy cost for doping decreases compared with that of N-monodoping. Simultaneous introduction of N and La results in a band gap decreasing about 0.53 eV, which can be responsible for the high photocatalytic activity of the codoped SrTiO3 under visible light. In addition, no localized states appearing within the band gap indicates no formation of electron-hole recombination center and thus improves the energy conversion efficiency significantly. Acknowledgment. This work is supported by the National Basic Research Program of China (973 program, Grant No. 2007CB613302), National Natural Science Foundation of China under Grant No. 10774091, Natural Science Foundation of Shandong Province under Grant No. Y2007A18, and the Specialized Research Fund for the Doctoral Program of Higher Education 20060422023. References and Notes (1) Wrighton, M. S.; Ellis, A. B.; Wolczanski, P. T.; Morse, D. L.; Abrahamson, H. B.; Ginley, D. S. J. Am. Chem. Soc. 1976, 98, 277. (2) Lehn, J. M.; Sauvage, J. P.; Ziessel, R. NouV. J. Chim. 1980, 4, 62.
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