International Journal of Photoenergy

Volume 2012 (2012), Article ID 203529, 7 pages

http://dx.doi.org/10.1155/2012/203529

## The Synthetic Effects of Iron with Sulfur and Fluorine on Photoabsorption and Photocatalytic Performance in Codoped

School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China

Received 10 August 2011; Accepted 8 September 2011

Academic Editor: Jiaguo Yu

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.

#### Abstract

The structural and electronic properties of iron-fluorine (Fe-F) and iron-sulfur (Fe-S) codoped anatase TiO_{2} are investigated by first-principles based on density functional theory. Our results show that the formation energy of codoped system is lower than that of single-element 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 Fe-S should be a better codoping pair because Fe-S codoping introduces extended impurity states resulting in stronger visible light absorption than that of Fe-F codoped compounds. This work gives understanding to the recent experiment and provides the evidence of choosing the more effective co-dopants 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 iron-sulfur (Fe-S) codoped TiO_{2} exhibits quite high photoactivity under visible light illumination and is stable for long-term applications [17]. It is proposed that the doped Fe^{3+} ions can act as electron acceptor and efficiently prevent electron-hole recombination. In addition, Liu et al. [18] found that iron-fluorine (Fe-F) codoped TiO_{2} displays excellent photocatalytic activity under visible light irradiation. In the experiment, they speculated that the as-prepared 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 Fe-F codoped anatase TiO_{2}. Fluorine atom has one more valence electron than oxygen, and sulfur has the same outer-shell 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 single-element-doped TiO_{2} before exploiting the cooperative effects of the codoped systems. For Fe-doped TiO_{2}, a number of studies have been reported experimentally and theoretically. Liu et al. [19] demonstrated that Fe-doped 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 F-doped TiO_{2}, recent experimental studies suggested that F-doping 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 S-doping causes the absorption edge of TiO_{2} to be shifted into lower-energy region. The mixing of the S 3*p* states with VB increases the width of the VB itself and results in a decrease in the band gap due to S-doping [29].

In this work, we examine the microscopic electronic structures of Fe-S and Fe-F codoped anatase TiO_{2} to explore the synthetic effects of the dopants by means of the first-principles 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 Fe-S and Fe-F 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 visible-light 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 spin-polarized density functional calculations of Fe-S codoped, Fe-F codoped, and single-element-doped anatase TiO_{2} using the Vienna ab initio simulation package (VASP) [30, 31]. The Perdew-Wang 91 of generalized gradient approximation (GGA) is implemented to describe the exchange correlation function. The projector-augmented wave (PAW) potential is used to represent the electron-ion 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 mono-doped TiO_{2}, we have just one O atom substituted by S (or F). The Monkhorst-Pack k-point is set as in the Brillouin zone of the supercell, and we choose the plane-wave 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 Fe-F and Fe-S 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 Fe-F (or Fe-S) 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 Ti-rich and O-rich conditions in our theoretical calculations. Under Ti-rich 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 O-rich, can be obtained from the ground-state 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 different-element-doped TiO_{2} are summarized in Table 1, which suggests that (1) Fe mono-doped, Fe-F and Fe-S codoped TiO_{2} are both energetically favorable under O-rich condition, but unfavorable under Ti-richcondition, (2) the formation of F mono-doped TiO_{2} is thermodynamically favorable under both Ti-rich and O-rich conditions, but S monodoped TiO_{2} is more stable under Ti-rich condition, which agrees with the former calculations [11], (3) the formation energy of Fe-F codoped TiO_{2} is much lower than both Fe and F mono-doped TiO_{2}. For Fe-S codoping under O-rich condition, the formation energy is smaller than S mono-doped but larger than Fe mono-doped 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 mono-element-doped systems, which is due to the charge balance by incorporating both anion and cation ions. This effect is more obvious in Fe-F 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 Fe-S codoped TiO_{2}.

We further investigate the optimized structures of different-element-doped TiO_{2}. For Fe mono-doped TiO_{2}, the optimized O-Fe bond lengths (1.873 Å and 1.891 Å) are shorter than O-Ti 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 mono-doped TiO_{2}, the distance between F and Ti (2.000 Å) is longer than O-Ti 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 S-doped TiO_{2}, the S-Ti bond lengths are 2.147 and 2.354 Å, which are much longer than the O-Ti bond lengths in pure TiO_{2} due to the bigger atom radius of S. As for F-Fe codoped TiO_{2}, the optimized F-Fe 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 F-Ti and Fe-O bond length (3.6% and 2.8%, resp.). S-Fe bond length (1.979 Å) in Fe-S codoped TiO_{2} is longer than that of F-Fe (1.966 Å), which is due to the bigger atom radius of S than that of F. The distortion of optimized S-Fe band length (2.5%) is smaller than that of both S-Ti and Fe-O bond length (11.2% and 2.8%, resp.) in S and Fe mono-doped 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 well-known 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 2*p* states, while the bottom of the conduction band (CBM) is dominated by Ti 3*d* 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 2*p* states are delocalized in the lower-energy 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 down-spin orbit of the gap states, which shows half-metallic character. Further projected density of states (PDOS) as shown in Figure 2(c′) predicates that the minority- and majority-spin states within the gap are mainly attributed to Fe 3*d*. 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 S-doped 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 2*p*, and the conduction band is mainly Ti 3*d*. The localized states are generated by S 3*p* about 0.7 eV above the VBM relative to the undoped one. The excitation from these occupied S 3*p* 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 Fe-doped TiO_{2} provide recombination center of photogenerated electron-hole 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 Fe-F 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 3*d* orbital. The energies needed for electrons excitation from two occupied up-spin states to the CBM are 1.6 and 0.9 eV, respectively, while the energy from down-spin 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 Fe-F codoped anatase TiO_{2} [18]. However, as the Fe mono-doped structure, localized states introduced in the middle of the forbidden band can lead to the recombination of electro-hole 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 Fe-doped 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 mono-doped 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 Fe-S 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 3*d*, O 2*p,* and S 3*p* orbitals while the states below the CBM are mainly hybridization of Fe 3*d* and O 2*p*. The formation energy reduction is found in both Fe-F and Fe-S 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 electron-hole pairs. Moreover, the impurity states near the band edge are more extended than those of mono-doped and Fe-F-doped 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 photo-carriers 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 Fe-S codoped TiO_{2} could possess the best photocatalytic activity under visible light irradiation among the Fe mono-doped and Fe-F, Fe-S codoped structures.

#### 4. Conclusions

We have examined the crystal structures and electronic and optical properties of Fe-F and Fe-S codoped anatase TiO_{2} based on DFT calculations. For comparison, Fe, S, and F single-doped TiO_{2} are also studied. Formation energy of the codoped system is much lower than that of the mono-element 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 3*d* 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 mono-doped one under visible light, which may account for the experimentally observed phenomenon. However, Fe-F codoping introduced localized gap states which may result in visible light absorption but decline the photocatalytic activity. Compared with Fe-F codoping, Fe-S 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 Fe-S codoped TiO_{2} is better than that of Fe-F codoped one, and Fe-S 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.

#### References

- J. Lu, Y. Dai, H. Jin, and B. Huang, “Effective increasing of optical absorption and energy conversion efficiency of anatase TiO
_{2}nanocrystals by hydrogenation,”*Physical Chemistry Chemical Physics*, vol. 13, no. 40, pp. 18063–18068, 2011. View at Publisher · View at Google Scholar - G. Shang, H. Fu, S. Yang, and T. Xu, “Mechanistic study of visible-light-induced photodegradation of 4-chlorophenol by TiO
_{2}-xNx $(0.021<\text{x}<0.049)$ with low nitrogen concentration,”*International Journal of Photoenergy*, vol. 2012, Article ID 759306, 9 pages, 2012. View at Publisher · View at Google Scholar - J. A. Byrne, P. A. Fernandez-Ibañez, P. S. M. Dunlop, D. M. A. Alrousan, and J. W. J. Hamilton, “Photocatalytic enhancement for solar disinfection of water: a review,”
*International Journal of Photoenergy*, vol. 2011, Article ID 798051, 12 pages, 2011. View at Publisher · View at Google Scholar - L. Jing, B. Xin, F. Yuan, L. Xue, B. Wang, and H. Fu, “Effects of surface oxygen vacancies on photophysical and photochemical processes of Zn-doped TiO
_{2}nanoparticles and their relationships,”*Journal of Physical Chemistry B*, vol. 110, no. 36, pp. 17860–17865, 2006. View at Publisher · View at Google Scholar · View at Scopus - S. Zhu, L. M. Wang, X. T. Zu, and X. Xiang, “Optical and magnetic properties of Ni nanoparticles in rutile formed by Ni ion implantation,”
*Applied Physics Letters*, vol. 88, no. 4, Article ID 043107, pp. 1–3, 2006. View at Publisher · View at Google Scholar · View at Scopus - L. F. Fu, N. D. Browning, S. X. Zhang, S. B. Ogale, D. C. Kundaliya, and T. Venkatesan, “Defects in Co-doped and (Co, Nb)-doped TiO
_{2}ferromagnetic thin films,”*Journal of Applied Physics*, vol. 100, no. 12, Article ID 123910, 2006. View at Publisher · View at Google Scholar · View at Scopus - A. di Paola, S. Ikeda, G. Marcì, B. Ohtani, and L. Palmisano, “Transition metal doped TiO
_{2}: physical properties and photocatalytic behaviour,”*International Journal of Photoenergy*, vol. 3, no. 4, pp. 171–176, 2001. View at Google Scholar · View at Scopus - R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, “Visible-light photocatalysis in nitrogen-doped titanium oxides,”
*Science*, vol. 293, no. 5528, pp. 269–271, 2001. View at Publisher · View at Google Scholar · View at Scopus - C. di Valentin, G. Pacchioni, A. Selloni, S. Livraghi, and E. Giamello, “Characterization of paramagnetic species in N-doped TiO
_{2}powders by EPR spectroscopy and DFT calculations,”*Journal of Physical Chemistry B*, vol. 109, no. 23, pp. 11414–11419, 2005. View at Publisher · View at Google Scholar · View at Scopus - T. Umebayashi, T. Yamaki, H. Itoh, and K. Asai, “Band gap narrowing of titanium dioxide by sulfur doping,”
*Applied Physics Letters*, vol. 81, no. 3, p. 454, 2002. View at Publisher · View at Google Scholar · View at Scopus - K. Yang, Y. Dai, B. Huang, and M. H. Whangbo, “Density functional characterization of the band edges, the band gap states, and the preferred doping sites of halogen-doped TiO
_{2},”*Chemistry of Materials*, vol. 20, no. 20, pp. 6528–6534, 2008. View at Publisher · View at Google Scholar · View at Scopus - V. K. Sharma, K. Winkelmann, Y. Krasnova, C. Lee, and M. Sohn, “Heterogeneous photocatalytic reduction of ferrate(VI) in UV-irradiated titania suspensions: role in enhancing destruction of nitrogen-containing pollutants,”
*International Journal of Photoenergy*, vol. 5, no. 3, pp. 183–190, 2003. View at Google Scholar · View at Scopus - L. Jia, C. Wu, Y. Li et al., “Enhanced visible-light photocatalytic activity of anatase TiO
_{2}through N and S codoping,”*Applied Physics Letters*, vol. 98, no. 21, Article ID 211903, 2011. View at Publisher · View at Google Scholar - M. Xing, Y. Wu, J. Zhang, and F. Chen, “Effect of synergy on the visible light activity of B, N and Fe co-doped TiO
_{2}for the degradation of MO,”*Nanoscale*, vol. 2, no. 7, pp. 1233–1239, 2010. View at Publisher · View at Google Scholar · View at Scopus - W. Wei, Y. Dai, M. Guo, L. Yu, and B. Huang, “Density functional characterization of the electronic structure and optical properties of N-doped, La-doped, and N/La-codoped SrTiO
_{3},”*Journal of Physical Chemistry C*, vol. 113, no. 33, pp. 15046–15050, 2009. View at Publisher · View at Google Scholar · View at Scopus - W. Wei, Y. Dai, M. Guo et al., “Codoping synergistic effects in N-doped SrTiO
_{3}for higher energy conversion efficiency,”*Physical Chemistry Chemical Physics*, vol. 12, no. 27, pp. 7612–7619, 2010. View at Publisher · View at Google Scholar · View at Scopus - V. M. Menéndez-Flores, D. W. Bahnemann, and T. Ohno, “Visible light photocatalytic activities of S-doped TiO
_{2}-Fe^{3+}in aqueous and gas phase,”*Applied Catalysis B*, vol. 103, no. 1-2, pp. 99–108, 2011. View at Publisher · View at Google Scholar - S. Liu, X. Sun, J. G. Li et al., “Fluorine- and iron-modified hierarchical anatase microsphere photocatalyst for water cleaning: facile wet chemical synthesis and wavelength-sensitive photocatalytic reactivity,”
*Langmuir*, vol. 26, no. 6, pp. 4546–4553, 2010. View at Publisher · View at Google Scholar · View at Scopus - Y. Liu, J. H. Wei, R. Xiong, C. X. Pan, and J. Shi, “Enhanced visible light photocatalytic properties of Fe-doped TiO
_{2}nanorod clusters and monodispersed nanoparticles,”*Applied Surface Science*, vol. 257, no. 18, pp. 8121–8126, 2011. View at Publisher · View at Google Scholar - W. Choi, A. Termin, and M. R. Hoffmann, “The role of metal ion dopants in quantum-sized TiO
_{2}: correlation between photoreactivity and charge carrier recombination dynamics,”*Journal of Physical Chemistry*, vol. 98, no. 51, pp. 13669–13679, 1994. View at Google Scholar · View at Scopus - D. Li, H. Haneda, S. Hishita, N. Ohashi, and N. K. Labhsetwar, “Fluorine-doped TiO
_{2}powders prepared by spray pyrolysis and their improved photocatalytic activity for decomposition of gas-phase acetaldehyde,”*Journal of Fluorine Chemistry*, vol. 126, no. 1, pp. 69–77, 2005. View at Publisher · View at Google Scholar · View at Scopus - D. Li, H. Haneda, S. Hishita, and N. Ohashi, “Visible-light-driven N-F-codoped TiO
_{2}photocatalysts. 2. Optical characterization, photocatalysis, and potential application to air purification,”*Chemistry of Materials*, vol. 17, no. 10, pp. 2596–2602, 2005. View at Publisher · View at Google Scholar · View at Scopus - A. Olasz, P. Mignon, F. Deproft, T. Veszprémi, and P. Geerlings, “Effect of the $\pi -\pi $ stacking interaction on the acidity of phenol,”
*Chemical Physics Letters*, vol. 407, no. 4–6, pp. 504–509, 2005. View at Publisher · View at Google Scholar · View at Scopus - J. K. Zhou, L. Lv, J. Yu et al., “Synthesis of self-organized polycrystalline F-doped TiO
_{2}hollow microspheres and their photocatalytic activity under visible light,”*Journal of Physical Chemistry C*, vol. 112, no. 14, pp. 5316–5321, 2008. View at Publisher · View at Google Scholar · View at Scopus - D. G. Huang, S. J. Liao, J. M. Liu, Z. Dang, and L. Petrik, “Preparation of visible-light responsive N-F-codoped TiO
_{2}photocatalyst by a sol-gel-solvothermal method,”*Journal of Photochemistry and Photobiology A*, vol. 184, no. 3, pp. 282–288, 2006. View at Publisher · View at Google Scholar · View at Scopus - D. Huang, S. Liao, S. Quan et al., “Preparation of anatase F doped TiO
_{2}sol and its performance for photodegradation of formaldehyde,”*Journal of Materials Science*, vol. 42, no. 19, pp. 8193–8202, 2007. View at Publisher · View at Google Scholar · View at Scopus - J. Tang, H. Quan, and J. Ye, “Photocatalytic properties and photoinduced hydrophilicity of surface-fluorinated TiO
_{2},”*Chemistry of Materials*, vol. 19, no. 1, pp. 116–122, 2007. View at Publisher · View at Google Scholar · View at Scopus - T. Umebayashi, T. Yamaki, H. Itoh, and K. Asai, “Band gap narrowing of titanium dioxide by sulfur doping,”
*Applied Physics Letters*, vol. 81, no. 3, p. 454, 2002. View at Publisher · View at Google Scholar · View at Scopus - T. Umebayashi, T. Yamaki, S. Yamamoto et al., “Sulfur-doping of rutile-titanium dioxide by ion implantation: photocurrent spectroscopy and first-principles band calculation studies,”
*Journal of Applied Physics*, vol. 93, no. 9, pp. 5156–5160, 2003. View at Publisher · View at Google Scholar · View at Scopus - G. Kresse and J. Furthmüller, “Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set,”
*Physical Review B*, vol. 54, no. 16, pp. 11169–11186, 1996. View at Google Scholar · View at Scopus - G. Kresse and J. Furthmüller, “Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set,”
*Computational Materials Science*, vol. 6, no. 1, pp. 15–50, 1996. View at Publisher · View at Google Scholar · View at Scopus - J. Zhu, Z. Deng, F. Chen et al., “Hydrothermal doping method for preparation of Cr
^{3+}-TiO_{2}photocatalysts with concentration gradient distribution of Cr^{3+},”*Applied Catalysis B*, vol. 62, no. 3-4, pp. 329–335, 2006. View at Publisher · View at Google Scholar · View at Scopus - K. Yang, Y. Dai, and B. Huang, “Understanding photocatalytic activity of S- and P-doped TiO
_{2}under visible light from first-principles,”*Journal of Physical Chemistry C*, vol. 111, no. 51, pp. 18986–18994, 2007. View at Publisher · View at Google Scholar · View at Scopus