Research Article  Open Access
The Synthetic Effects of Iron with Sulfur and Fluorine on Photoabsorption and Photocatalytic Performance in Codoped
Abstract
The structural and electronic properties of ironfluorine (FeF) and ironsulfur (FeS) codoped anatase TiO_{2} are investigated by firstprinciples based on density functional theory. Our results show that the formation energy of codoped system is lower than that of singleelement doping, which indicates the synergic effect of codoping on the stability of the structure. Codopants introduced impurity gap states resulting in the electron transition energy reduction and thus the visible light absorption observed in the samples. It is concluded that FeS should be a better codoping pair because FeS codoping introduces extended impurity states resulting in stronger visible light absorption than that of FeF codoped compounds. This work gives understanding to the recent experiment and provides the evidence of choosing the more effective codopants in TiO_{2}.
1. Introduction
Titanium dioxide has been extensively studied as a promising photocatalyst due to its cheap, stable, and nontoxic characteristics. However, the use of TiO_{2} is limited by its wide band gap (~3.0 eV) which absorbs only ultraviolet light and accounts for just 5% of solar energy. Thus, modification of the electronic structure of TiO_{2} to enable the visible light absorption is of great importance [1–3]. An efficient way is doping pure TiO_{2} with metal [4–7] or nonmetal elements [8–12]. Metal elements diffused in the titanium lattice greatly enhance the visible light absorption because the impurity states introduced by the dopants lead to the visible light response of TiO_{2}. However, localized states appearing in the band gap of the host semiconductor often result in the recombination of photogenerated carriers and consequently result in lower photocatalytic activity. A possible way to improve the photocatalytic performance of doped TiO_{2} is to explore the cooperative effect by introducing more than one species of foreign elements to the host [13–16]. Appropriate choice of the codoping pair is the key factor. Recent experiment has reported that ironsulfur (FeS) codoped TiO_{2} exhibits quite high photoactivity under visible light illumination and is stable for longterm applications [17]. It is proposed that the doped Fe^{3+} ions can act as electron acceptor and efficiently prevent electronhole recombination. In addition, Liu et al. [18] found that ironfluorine (FeF) codoped TiO_{2} displays excellent photocatalytic activity under visible light irradiation. In the experiment, they speculated that the asprepared TiO_{2} samples match the anatase type, with no trace of rutile or brookite impurity being observed, and they also demonstrated that F could substitute for O and Fe could substitute for Ti in FeF codoped anatase TiO_{2}. Fluorine atom has one more valence electron than oxygen, and sulfur has the same outershell electron as oxygen atom; therefore, it is interesting to explore the origin of the high photocatalytic activity under visible light concerning these two kinds of codoped anatase TiO_{2}.
We should have comprehensive knowledge of singleelementdoped TiO_{2} before exploiting the cooperative effects of the codoped systems. For Fedoped TiO_{2}, a number of studies have been reported experimentally and theoretically. Liu et al. [19] demonstrated that Fedoped TiO_{2} shows visible light responses and diminished recombination rates of the photoexcited carriers. Fe at the 0.5 at.% level can significantly improve the photoactivity of TiO_{2} for both oxidation and reduction reactions [20]. As for the Fdoped TiO_{2}, recent experimental studies suggested that Fdoping neither causes any change in the adsorption edge nor affects the optical absorption of TiO_{2} but is beneficial to the crystalline anatase phase [21–27]. Umebayashi et al. [28] suggested that Sdoping causes the absorption edge of TiO_{2} to be shifted into lowerenergy region. The mixing of the S 3p states with VB increases the width of the VB itself and results in a decrease in the band gap due to Sdoping [29].
In this work, we examine the microscopic electronic structures of FeS and FeF codoped anatase TiO_{2} to explore the synthetic effects of the dopants by means of the firstprinciples density function theory (DFT) calculations. The defect formation energies are calculated to determine which configuration may be realized more easily in experiment. The codoping synergistic effect is specifically elucidated, and the corresponding related properties of FeS and FeF are compared to identify a better codoping pair. To obtain detailed insight, Fe, F, and S monodoped anatase TiO_{2} structures are also studied systematically. Our theoretical calculations may provide a comprehensive explanation for experimentally observed visiblelight photocatalytic activity in the metal and nonmetal codoped TiO_{2} and may offer some helpful theoretical information for exploiting new effective photocatalysts.
2. Computational Method
We carry out the spinpolarized density functional calculations of FeS codoped, FeF codoped, and singleelementdoped anatase TiO_{2} using the Vienna ab initio simulation package (VASP) [30, 31]. The PerdewWang 91 of generalized gradient approximation (GGA) is implemented to describe the exchange correlation function. The projectoraugmented wave (PAW) potential is used to represent the electronion interaction. The crystal lattice parameters are taken from previous calculations (, ), which are in agreement with the experimental values () [32]. We use a supercell containing 32 O atoms and 16 Ti atoms to model the bulk anatase TiO_{2}. In the codoped calculation models, one oxygen atom is replaced by a S (or F) atom and titanium atom by iron. In the monodoped TiO_{2}, we have just one O atom substituted by S (or F). The MonkhorstPack kpoint is set as in the Brillouin zone of the supercell, and we choose the planewave cutoff energy of 400 eV. All the atoms are fully optimized until the force on each atom is less than 0.1 meV.
3. Results and Discussion
3.1. Optimized Structure and Stability of Doped TiO_{2}
To investigate the relative stability of FeF and FeS codoped TiO_{2}, we calculate the defect formation energies. For comparison, the energies of F, Fe, and S monodoped TiO_{2} are also studied. The formation energies are calculated according to the following formulas: in which , , and are total energies of Fe, F(or S) monodoped and FeF (or FeS) codoped TiO_{2}, respectively. is the total energy of TiO_{2} without dopants. , , and are the chemical potential of Fe, Ti, F(or S), and O, respectively. It is commonly known that if the doped systems have smaller formation energies, it means that they are in a relatively stable phase. It should be also mentioned that formation energy is in connection with crystal growth circumstance. We simulate the corresponding Tirich and Orich conditions in our theoretical calculations. Under Tirich condition, is gotten from bulk Ti and is calculated according to the following formula: Here is energy of one formula unit of TiO_{2}. Under Orich, can be obtained from the groundstate energy of the O_{2} molecule (= 1/2 μ), while is fixed by condition (2).
In codoped TiO_{2} crystal lattice, dopants may substitute any host atoms however, not all of the configurations are stable. With the aim to find out the most stable configuration, different substitution sites according to the different distances between the two dopants are tested. Formation energy calculations imply that the structure with one Fe substituting for a Ti atom and simultaneously with one F (or S) at the first nearest neighboring (denoted as 1NN) O atom site is the most energetically favorable. We find that, in general, when the distances between the two dopants increase, the formation energies increase. This is probably because that the attraction forces between the anion and cation ions decrease when the two elements locate far from each other. Thus, it can be concluded that 1NN substitution is the most viable configuration in experimental process and then is chosen as our computational model hereafter (Figure 1).
The calculated formation energies of differentelementdoped TiO_{2} are summarized in Table 1, which suggests that (1) Fe monodoped, FeF and FeS codoped TiO_{2} are both energetically favorable under Orich condition, but unfavorable under Tirichcondition, (2) the formation of F monodoped TiO_{2} is thermodynamically favorable under both Tirich and Orich conditions, but S monodoped TiO_{2} is more stable under Tirich condition, which agrees with the former calculations [11], (3) the formation energy of FeF codoped TiO_{2} is much lower than both Fe and F monodoped TiO_{2}. For FeS codoping under Orich condition, the formation energy is smaller than S monodoped but larger than Fe monodoped TiO_{2}. The lower formation energies of codoping systems indicate cooperative effects of the different dopants and make the high photocatalytic semiconductor with higher dopants concentration more viable in experiment. Thus, we can conclude that codoped TiO_{2} samples have lower formation energies than monoelementdoped systems, which is due to the charge balance by incorporating both anion and cation ions. This effect is more obvious in FeF codoped TiO_{2} since the charge neutrality is well maintained relative to the undoped system, while the charge neutrality is not maintained so well for the FeS codoped TiO_{2}.

We further investigate the optimized structures of differentelementdoped TiO_{2}. For Fe monodoped TiO_{2}, the optimized OFe bond lengths (1.873 Å and 1.891 Å) are shorter than OTi bond lengths in pure TiO_{2} (1.930 Å and 1.973 Å). This is because that bond length is determined mainly by radius and electronegativity of bonded atoms. The electronegativity of iron (1.83) is stronger than that of titanium (1.54), and the ionic radius of iron (0.64 Å) is smaller than that of titanium (0.68 Å). In F monodoped TiO_{2}, the distance between F and Ti (2.000 Å) is longer than OTi bond length (1.930 Å). This can be ascribed to the fact that electronegativity of F (3.98) is stronger than that of O (3.44) while the radius of F^{} is a little bigger than that of O^{2−} (1.32). In Sdoped TiO_{2}, the STi bond lengths are 2.147 and 2.354 Å, which are much longer than the OTi bond lengths in pure TiO_{2} due to the bigger atom radius of S. As for FFe codoped TiO_{2}, the optimized FFe bond length is 1.966 Å, 1.9% of distortion compared with that of the pure TiO_{2}, which is smaller than the distortions of both FTi and FeO bond length (3.6% and 2.8%, resp.). SFe bond length (1.979 Å) in FeS codoped TiO_{2} is longer than that of FFe (1.966 Å), which is due to the bigger atom radius of S than that of F. The distortion of optimized SFe band length (2.5%) is smaller than that of both STi and FeO bond length (11.2% and 2.8%, resp.) in S and Fe monodoped TiO_{2}. The smaller distortion of codoped TiO_{2} is in connection with the smaller formation energy, and this should be ascribed to the synergetic effect of codoping.
3.2. Electronic Structures
To clarify how the dopants modify the electronic structure of TiO_{2}, we calculate the total density of states (DOS) and partial density of states (PDOS) of bulk TiO_{2} and doped TiO_{2} shown in Figure 2. For pure TiO_{2} (Figure 2(a)), the calculated band gap is 1.90 eV, which is consistent with the previous theoretical studies [33]. Although the theoretical band gap is smaller than the experimental value (3.2 eV) due to the wellknown shortcoming of GGA, it is reasonable to analyze the relative variations of the electronic structure without considering the exact band gap value. We can see from Figure 2(a’) that the top of the valence band (VBM) of pure TiO_{2} consists mainly of O 2p states, while the bottom of the conduction band (CBM) is dominated by Ti 3d states.
After substitution of fluorine for oxygen atom (Figure 2(b)), the Fermi level is pinned at the bottom of the conduction band which shows a donor character because of one more electron of F than that of host O. The PDOS (Figure 2(b′)) shows that most of F 2p states are delocalized in the lowerenergy range of VB and do not contribute to the band edge and may not lead to the absorption of visible light. Figure 2(c) shows that incorporation of iron into the lattice results in localized gap states. The Fermi level is pinned at the downspin orbit of the gap states, which shows halfmetallic character. Further projected density of states (PDOS) as shown in Figure 2(c′) predicates that the minority and majorityspin states within the gap are mainly attributed to Fe 3d. The impurity states induced above the VBM, below the CBM, and in the forbidden gap are beneficial to the visible light absorption. Figure 2(d) is the DOS of Sdoped TiO_{2}, and the VBM has a little shift. From the calculated PDOS of Figure 2(d′), the valence band is composed mainly of O 2p, and the conduction band is mainly Ti 3d. The localized states are generated by S 3p about 0.7 eV above the VBM relative to the undoped one. The excitation from these occupied S 3p states to conduction band might lead to a decrease of the photon excitation energy and induce more significant red shift of absorption, which is consistent with experimental absorption spectra measurements. However, the localized gap states in the middle of the forbidden band in Fedoped TiO_{2} provide recombination center of photogenerated electronhole pair, which is detrimental to the photocatalytic activity. The improvement of photocatalytic activity originating from visible light absorption is weakened by the increase of recombination of the carriers and eventually limits the great enhancement of photocatalytic activity.
For FeF codoped TiO_{2} (Figure 3(a)), localized impurity states are introduced between VBM and CBM. The PDOS in Figure 3(a′) demonstrates that three gap states mainly stem from Fe 3d orbital. The energies needed for electrons excitation from two occupied upspin states to the CBM are 1.6 and 0.9 eV, respectively, while the energy from downspin state to CBM is 0.5 eV. Hence, the electron transition from these impurity energy levels to the conduction band would lead to an obvious reduction of absorption energy. Our results give a good explanation for the experimentally observed red shift of absorption edge of the FeF codoped anatase TiO_{2} [18]. However, as the Fe monodoped structure, localized states introduced in the middle of the forbidden band can lead to the recombination of electrohole pair and thus do harm to the photocatalytic activity. Therefore, even though codoping with Fe and F may promote the incorporation of dopants into the TiO_{2} host lattice, it will not have pronounced enhancement in the photocatalytic activity compared with Fedoped TiO_{2}.
When Fe and S are introduced into TiO_{2} simultaneously, more obvious spin polarization can be observed in the band edge compared with monodoped TiO_{2} and localized gap states appear about 0.2 eV above the valence band with width of 0.4 eV and 0.2 eV below the conduction band with width of 0.6 eV (Figure 3(b)). The bandwidth decreases to 1.7 eV, 0.2 eV smaller than that of pure TiO_{2}, and the Fermi level is pinned in the gap states located below the conduction band due to the slight break of the charge neutrality by FeS codoping. Electron excitation from VBM to the gap states and CBM could lead to the visible light absorption as observed in experiment [17]. The PDOS shown in Figure 3(b′) indicates that the gap states above the valence band are the mixing of Fe 3d, O 2p, and S 3p orbitals while the states below the CBM are mainly hybridization of Fe 3d and O 2p. The formation energy reduction is found in both FeF and FeS codoped TiO_{2} which could enhance the solubility of dopants in the host lattice. Furthermore, the codoping of Fe and S produces extended states near the band edge and does not induce localized states in the center of the gap that often act as recombination centers. This electronic structure could enhance the visible light absorption and reduce the recombination of photogenerated electronhole pairs. Moreover, the impurity states near the band edge are more extended than those of monodoped and FeFdoped structures, which means that electron excitation between the valence band (conduction band) and impurity states can be more intense, and consequently the intensity of visible light absorption can be stronger. It should be mentioned that the mobility of the photocarriers in the impurity states is lower than that in the valence band of pure TiO_{2}; however, the impurity states near the band edge can also act as electron/hole traps, which reduces the recombination of photocarriers. Additionally, since the oxidation (reduction) power of photogenerated holes (electrons) in the gap states is reduced relative to that in the VB (CB) of pure TiO_{2}, one should accommodate a balance between the oxidation (reduction) power and visible light absorption of the photocatalysts.
Based on these analyses, we conclude that the FeS codoped TiO_{2} could possess the best photocatalytic activity under visible light irradiation among the Fe monodoped and FeF, FeS codoped structures.
4. Conclusions
We have examined the crystal structures and electronic and optical properties of FeF and FeS codoped anatase TiO_{2} based on DFT calculations. For comparison, Fe, S, and F singledoped TiO_{2} are also studied. Formation energy of the codoped system is much lower than that of the monoelement doping indicating the synergic effects of codopants on the stability of the doped structure. The calculated results indicate that the codoped atoms introduce impurity energy levels in the band gap mainly composed of Fe 3d states. Due to the less energy needed for an electron transition from the impurity energy levels to the conduction band bottom, codoped anatase TiO_{2} may show higher photocatalytic activity than the monodoped one under visible light, which may account for the experimentally observed phenomenon. However, FeF codoping introduced localized gap states which may result in visible light absorption but decline the photocatalytic activity. Compared with FeF codoping, FeS codoped TiO_{2} produces gap states near the band edge that are extended and may greatly enhance the visible light absorption and reduce the carrier recombination. Consequently, the photocatalytic performance under visible light of FeS codoped TiO_{2} is better than that of FeF codoped one, and FeS should be a better codoping pair.
Acknowledgments
This work is supported by the National Basic Research Program of China (973 program, 2007CB613302), the National Science Foundation of China under Grant nos. 11174180 and 20973102, and the Natural Science Foundation of Shandong Province under Grant no. ZR2011AM009.
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Copyright © 2012 Xiaohua Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.