International Journal of PhotoenergyVolume 2012 (2012), Article ID 398508, 8 pagesdoi:10.1155/2012/398508

Research Article

Enhanced Photoactivity of Fe + N Codoped Anatase-Rutile Nanowire Film under Visible Light Irradiation

Kewei Li,1,2 Haiying Wang,1,2,3 Chunxu Pan,³ Jianhong Wei,³ Rui Xiong,3,4 and Jing Shi³

¹Department of Communication Engineering, Chengdu Institute of Technology, Chengdu 611730, China²Intel-cec joint Lab, Institute of IC Mannufacturing Engineering, Chengdu 611730, China³Key Laboratory of Artificial Micro- and Nanostructures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China⁴Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan 430072, China

Received 9 January 2012; Revised 22 February 2012; Accepted 22 February 2012

Academic Editor: Baibiao Huang

Copyright © 2012 Kewei 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.

Rutile-anatase phase mixed codoped TiO₂ nanowires were designed and prepared by a two-step anodic oxidation method. The results of X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy confirm that the prepared codoped TiO₂ nanowires exhibit intimately contacted anatase-rutile heterostructure with the rutile content of 21.89%. The X-ray photoelectron spectroscopy measurements show that nitrogen and iron atoms are incorporated into the titania oxide lattice, and the UV-visible absorption spectra show that the codoping of iron and nitrogen atoms could extend the absorption to visible light region. The photocatalytic activities of all the samples were evaluated by photocatalytic degradation of methylene blue under visible light irradiation. The codoped sample achieves the best response to visible light and the highest photocatalytic activities. The enhancement of photocatalytic activity for codoped sample should be ascribed to the synergistic effects of codoped nitrogen and iron ions and the anatase-rutile heterostructure.

1. Introduction

Titanium oxide is a multifunctional material with a wide variety of potential applications including solar cell [1], photocatalyst [2], water splitting [3], and gas sensors [4]. However, the wide band gap of TiO₂ (3.2 eV for the anatase phase and 3.0 eV for the rutile phase) requires ultraviolet (UV) light for electron-hole separation, which is only 5% of the natural solar light [5]. It is of great significance to develop photocatalysts that can be used in visible light region to improve the photocatalytic efficiency. Numerous attempts have been made to optimize the band gap of TiO₂ by different doping schemes, for example, nonmetal doping (such as C, S, N, I [6–11]), metal doping (such as Fe, V, Cr, Nb [12–15]), and nonmetal-metal codoping (such as codoping [16], codoping [17], codoping [18], and codoping [19]), to improve the photocatalytic activity. As a result, it was found that the monodoping can generate the recombination center inside the TiO₂, which goes against the light-induced charge carriers’ migration to the surface [16, 20]. In compensated n-p codoping systems, the defect bands can be passivated and will not be effective as carrier recombination centers [16, 21] and the Columbic attraction between the n- and p-type dopants with opposite charge substantially enhances doping concentration [22]. Recently, noncompensated codoping was proposed by Zhu et al., for its distinctive merit of ensuring the creation of intermediate electronic bands in the gap region and enhancing photoactivity manifested by efficient electron-hole separation in the visible-light region [23].

It is well-known that powders consisting of anatase and rutile (a classical reference materials is Degussa P25, a mixture of 20% rutile, and 80% anatase) possess higher photoactivity than those of pure anatase. It was thought that the anatase-rutile heterojunction inside the TiO₂ was the main reason for enhanced photoactivity [24]. Deák et al. calculated the band structure of rutile and anatase and validated that both bands of rutile lie higher than those of anatase, so in mixed systems, the holes are accumulated in the former while the electrons in the latter [25]. As pointed out earlier, the charge separation will enhance the photochemical activity of mixed phase TiO₂ powders.

New interesting issue: does the noncompensated codoping with rutile-anatase mixing phase improve the visible light photocatalytic activity? It was reported that Fe-doped TiO₂ and N-doped TiO₂ exhibit enhanced visible light photocatalytic activity as compared to undoped TiO₂, while codoping is the noncompensated codoping, but the light photocatalytic activity is seldom reported. Thus, in this study, codoped TiO₂ nanowires with rutile-anatase mixed phase were designed and prepared. Compared with the undoped, Fe-doped and N-doped TiO₂, codoped TiO₂ gains the best photocatalytic activity under visible light irradiation and the mechanism of enhanced visible-light photocatalytic activity is discussed in detail.

2. Experimental Section

Titanium foils (0.6 mm thick, 99.5% purity, and cut in 1 cm × 2 cm) were used as the substrates for the growth of the TiO₂ nanowire arrays. The titanium sheets were cleaned by sonicating in 1 : 1 acetone and ethanol solution, followed by being rinsed with deionized water and dried in airstream. The anodization was carried out in a two-electrode electrochemical cell with a graphite sheet as the cathode at a constant potential 60 V. A DC power supply (WYK-6010, 0–60 V, and 0–10 A) was used to control the experimental current and voltage for 1 h. The electrolyte contained 0.5 wt% NH₄F, 5 mL H₂O, and 195 mL ethylene glycol. After anodization, the specimens were cleaned in 10% HCl by ultrasonic immediately for 10 minutes and dried in airstream. The anodization process was repeated for several times.

The samples were then rinsed by ethanol for 5 minutes, after which some of them were submerged in  mol/L Fe(NO₃)₃ solutions for 40 minutes for Fe doping. Postannealing in air at 700°C was employed to remove most of the organic and inorganic species encapsulated in the arrays and transform the amorphous titania to nanocrystalline TiO₂. Nitrogen doping was carried out by annealing the samples in ammonia atmosphere at 500°C. The reagents—acetone (CH₃COCH₃), ethanol (C₂H₅OH), ammonium fluoride (NH₄F), Fe(NO₃)₃, and ethylene glycol (C₂H₆O₂) were of analytical-grade without further purification.

The crystal structures of samples were characterized by X-ray diffraction (XRD, Bruker AXS D8 Advance diffractions) using Cu radiation. The surface morphologies of the nanowire arrays were observed by scanning electron microscopy (SEM, S-4800) and high-resolution transmission electron microscopy (TEM 2010FEF HRTEM, JEOL, Japan). The X-ray photoelectron spectroscopy (XPS) experiments were performed on a VG Multilab2000 spectrometer to obtain the information on chemical binding energy of the TiO₂ nanowires which was calibrated with the reference to the C 1s peak at 284.6 eV. The UV-visible absorption spectra were measured using a Cary 5000 UV-Vis-NIR spectrophotometer; BaSO₄ was used as a reflectance standard in a UV-visible diffuse reflectance experiment. The photocatalytic activities under visible light irradiation were evaluated by the degradation of methyl blue irradiated by a 450 W xenon lamp. In the process, a TiO₂ nanowire array with dimensions of 0.5 cm × 0.5 cm was immersed into 3 mL 10 mg/L methyl blue (MB) solution and placed below xenon lamp with 10 cm distance. The solution in the photoreactor was placed in dark for 30 minutes to reach the absorption-desorption equilibrium of the dye molecules on the sample surface.

3. Results and Discussions

Figure 1 shows the XRD patterns of the undoped, Fe-doped, N-doped, and codoped polycrystalline TiO₂ samples. Compared with the standard patterns of the anatase phase TiO₂ (JCPDS 21-1272) and the rutile phase (JCPDS 21-1276), no peaks of impurities are detected except the peaks of metal titanium coming from the substrates. The contents of rutile phase and anatase phase are calculated by the XRD results, using the method described by Zhu et al. [26] and listed in Table 1. The results show that the content of rutile phase in Fe-doped TiO₂ (19.66 wt%) is lower than that of the undoped TiO₂ (21.88 wt%), that is to say, the content of rutile decreases for the iron doping after the heating treatment, which indicates that the doping of iron may retard the transition from anatase to rutile. The content of rutile in the N-doped and codoped TiO₂ are 22.48 wt% and 21.89 wt%. The average crystallite sizes were calculated by Scherrer’s formula (,,  nm) and listed in Table 1, choosing the peaks at scattering angles of 25° and 27.5° for anatase and rutile, respectively. The average crystallite sizes for anatase phase in the undoped TiO₂ and samples doped with Fe, N, are 32.92 nm, 25.27 nm, 27.60 nm, and 27.78 nm, while the average crystallite sizes for rutile phase are 30.57 nm, 25.30 nm, 26.93 nm, and 28.78 nm, respectively. These results reveal that doping with Fe and N element will retard the growth of the crystallite sizes, which is similar to those described in [27, 28].

Table 1: The average crystallite size of anatase TiO₂ and the content of rutile, anatase for undoped, Fe-doped, nitrogen-doped, and codoped TiO₂ nanowires.

Undoped TiO₂
Fe-doped TiO₂
N-doped TiO₂
codoped TiO₂

Content of rutile (wt%)
21.88
19.66
22.48
21.89

Content of anatase (wt%)
78.12
80.34
77.52
78.11

Average crystallite size of anatase (nm)
32.92
25.27
27.60
27.78

Average crystallite size of rutile (nm)
30.57
25.30
26.93
28.78

Figure 1: X-ray diffraction patterns of undoped, Fe-doped, N-doped, and codoped TiO₂ nanowires.

Figures 2(a), 2(b), 2(c), and 2(d) present the illustrative top SEM images of typical undoped, Fe-doped, N-doped, and codoped TiO₂ samples. The right part of Figure 2(d) is the SEM image of cross section of the codoped sample. As it can be seen, the length and the diameter of all the samples are about 5 μm and 90 nm. The nanowires are arranged one by one closely, ensuring that light can be effectively scattered by the rutile phase in the mixing-phase films [29].

(a)

(b)

(c)

(d)

Figure 2: SEM images of (a) the undoped, (b) Fe-doped, (c) N-doped, and (d) codoped TiO₂ nanowires.

The crystalline structure of TiO₂ is further examined by HRTEM. The HRTEM images of a single codoped TiO₂ nanowire and the magnified local image in the white rectangle zone is shown in Figure 3. HRTEM observations reveal that the diameter of the nanoparticles is in a size range of 70–100 nm and TiO₂ nanoparticles can be discriminated and exhibit a perfect lattice. As shown in Figure 3, the clear anatase-rutile interface is confirmed by HRTEM, where the lattice spaces are measured to be 0.189 nm along the (200) plane of anatase phase and 0.248 nm along the (101) plane of rutile phase, respectively. The formation of intimate contact between anatase and rutile phases is realized so that the charge transfer between the two phases can occur smoothly [30]. The anatase-rutile interface is the active site for photocatalysis and is one of the dominate factors for the enhancement of the photocatalytic activity reported elsewhere [31].

Figure 3: The HRTEM images of codoped TiO₂ with magnified local image in the right.

To investigate the composition and the chemical state of ions, XPS measurements were performed. Figure 4(a) is the XPS survey spectrum of the sample. The sharp peaks at 284.7 eV, 458.60 eV, 530.15 eV are corresponding to the binding energy of C 1s, Ti 2p3/2, O1s, and the two weak photoelectron peaks at 710.41 eV and 396.00 eV correspond to Fe 2p3/2, and N 1s, respectively. The peak at 284.7 eV, in general, is a signal of adventitious elemental carbon as reported in other works [32, 33]. No other impurity peaks were observed, which reveals that the sample contains Ti, O, Fe, and N four elements.

(a)

(b)

(c)

Figure 4: XPS spectra of (a) survey spectrum, (b) N 1s, and (c) Fe 2p for codoped TiO₂.

Figure 4(b) is the N 1s core level spectra of codoped TiO₂. It is found that there are two peaks with similar intensity at the bonding energies of 396.04 eV and 400.20 eV, respectively. Generally, the peak at 396 eV reflects the formation of N–Ti–O bonds, indicating the substitution of N ion for O ion [34–36]. The peak at 400 eV can be assigned to molecular nitrogen bonded to the surface defects or the N atoms bonding to O sites in TiO₂, forming Ti–O–N bonds [37]. Figure 4(c) shows the high-resolution Fe 2p core level spectrum. As it can be seen, the binding energies of the Fe 2p XPS peaks are at 710.41 eV, indicating that the Fe element in the sample exists mainly in the +3 oxidation state [38]. The peak position of codoped TiO₂ exhibits a shift about 0.3 eV to lower energy compared with that in Fe₂O₃ (710.7 eV for Fe 2p3/2) [38]. The variation in the elemental binding energy due to the difference in the chemical potential and polarizability of Fe ions in codoped TiO₂ and Fe₂O₃ indicates that Fe³⁺ ion has incorporated into the TiO₂ lattice and Fe–O–Ti bonds form in the codoped sample [39]. The calculations of the N 1s core level spectrum and Fe 2p core level spectrum show that the N and Fe content is 3.15% and 1.09%, respectively.

Figure 5(a) illustrates the UV-vis absorption spectroscopy of the undoped, Fe-doped, N-doped, and codoped TiO₂. The undoped TiO₂ exhibits the characteristic spectrum of TiO₂ with its fundamental absorption sharp edge around 380 nm. Compared with the undoped TiO₂, the absorption edge is shifted toward visible light range for Fe-doped, N-doped, and codoped samples and a significant enhancement of absorption visible light range is observed in N-doped and codoped samples. As it can be seen from Figure 5(a), the spectrum of codoped sample has the largest red-shift. The local spectra (Figure 5(b)) indicate that the onset of the absorption spectrum appears at 384 nm, 394 nm, 396.3 nm, and 405 nm, for undoped, Fe-doped, N-doped, and codoped TiO₂, respectively. According to the equation , the band gaps of the undoped, Fe-doped, N-doped, and codoped TiO₂ are 3.23 eV, 3.15 eV, 3.13 eV, and 3.06 eV. Figure 5(c) shows the plots of versus deduced from Figure 5(a). The band gap of each samples is determined by fitting the absorption spectra data according to the equation ( is the absorption coefficient; is the photo energy; is a constant number; and is the absorption band gap energy). As it can be seen that the band gaps of the undoped, Fe-doped, N-doped, and codoped TiO₂ are 3.23 eV, 3.15 eV, 3.13 eV, and 3.06 eV, which are consistent with the results of Figure 5(b) and similar to those reported in [40, 41], but much higher than the values calculated by Romero-Gomez et al. [23]. The reason for this deviation may be that the doping content in experiments is much lower than that in the theoretical model.

(a)

(b)

(c)

Figure 5: UV-vis absorption spectroscopy of undoped, Fe-doped, N-doped, and codoped TiO₂ with the wavelength in the range of (a) 200–700 nm, (b) 330–450 nm, and (c) versus curves.

As to the origin of visible-light sensitivity in Fe, N and codoped TiO₂, the possible mechanism can be proposed. It is known that the band gap of TiO₂ can be tailored by doping with some nonmetal or metal elements. Herein, for the Fe-doped sample, the absorption edge red-shift could be understood by the band gap narrowing, which is the result of the induced sub-band-gap transition corresponding to the excitation of 3d electrons of Fe³⁺ to TiO₂ conduction band [41, 42]. In nitrogen doped sample, the doped N ions could induce an add-on shoulder on the edge of the valence band maximum and the localized N 2p states above the valence band [40]. On the other hand, it is known that N has a lower valence state than O so that the incorporation of N must promote the synchronous formation of oxygen vacancies for the charge equilibrium in TiO₂ [43]. Therefore, the band narrowing due to newly formed oxygen vacancies by N-doping in TiO₂ cannot be neglected. As a result, the visible light response for N-doped TiO₂ is attributed to both oxygen vacancies and the N 2p states. For codoped sample, compared with the N-doped TiO₂, the Fe element is introduced into the lattice of TiO₂ as the oxide state of Fe³⁺ by above discussion, so that Fe-doping is a p-type doping and can increase the oxygen vacancies induced by the hole doping of N, similar as [43]. Consequently, the codoped TiO₂ results in the best response to visible-light and the largest red-shift because of the synergistic effect of Fe and N codopant.

To evaluate the photocatalytic activities of the undoped, Fe-doped, N-doped, and codoped samples, experiments were carried out for MB degradation in an aqueous solution under visible light irradiation by insert a filter ( nm) between the Xe-lamp and the samples. Figure 6 shows the photocatalytic degradation curves of MB catalyzed by the samples. The photocatalytic activity is in the order of the codoped TiO₂ > N-doped TiO₂ > Fe-doped TiO₂ > undoped TiO₂. The codoped TiO₂ shows the best photocatalytic activity and leads to a removal of MB up to 57.42% in 4 hours.

Figure 6: Degradation curves of MB for the undoped, Fe-doped, N-doped, and codoped samples.

It has been reported that oxygen vacancy induced by N doping or self-doping plays an important role in the photocatalytic activity of TiO₂ catalyst by trapping the photoinduced electron and acting as a reactive center for the photocatalytic process [44, 45]. codoping on the photocatalytic activity in anatase-rutile mixed TiO₂ nanowires, p-type Fe doping can increase the oxygen vacancies in N-doped TiO₂ so that the photocatalytic activity of the sample is enhanced after introducing Fe³⁺ to form the codoped sample. It is known that the minimum of the conduction band of the rutile phase lies at higher in energy than that of the anatase phase by about 0.3-0.4 eV [25]. The Fe³⁺ energy level is just below the conduction band of anatase [41, 42]. The nitrogen doping could induce an add-on shoulder on the edge of the valence band maximum and the localized N 2p states above the valence band [40]. Therefore, after codoping, the anatase-rutile mixed TiO₂ nanowires have complex band lineup, similar with the band lineup of rutile and anatase [25]. This complex staggered alignment of the bands effectively enhances the charge separation with migrating holes accumulating in rutile while electrons in anatase, thus enhancing photocatalytic activity, accordingly.

4. Conclusions

In conclusion, TiO₂ nanowires with mixed rutile-anatase phase were synthesized by a two-step anodic oxidation method. Doping with nitrogen or iron could enhance the visible light absorption and the photocatalytic activity of TiO₂ nanowires. The codoped TiO₂ nanowire possesses the highest visible light absorption and the best photocatalytic activity for catalyzing the degradation of methylene blue. The increase of the quantity of O vacancy and the enhancement of the charge separation in the nanowires due to the complex staggered alignment of the bands leading by Fe and N codoping may be two of the reasons for the increase of photocatalytic activity in the codoped sample.

Acknowledgments

The authors would like to acknowledge the financial support from the National Basic Research Program (973 Program) (no. 2009CB939704 and no. 2009CB939705), the National Natural Science Funds (no. 51172166, 10974148, 61106005), National Science Fund for Talent Training in Basic Science (no. J0830310), and Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials (Hubei University).

References

C.-C. Wu, Y.-J. Zhuo, P.-N. Zhu, B. Chi, J. Pu, and J. Li, “Tunable fabrication of TiO₂ nanotube arrays with high aspect ratio and its application in dye sensitized solar cell,” Journal of Inorganic Materials, vol. 24, no. 5, pp. 897–901, 2009.

S. Sreekantan, R. Hazan, and Z. Lockman, “Photoactivity of anatase-rutile TiO₂ nanotubes formed by anodization method,” Thin Solid Films, vol. 518, no. 1, pp. 16–21, 2009.

Y. Liu, B. Zhou, J. Bai et al., “Efficient photochemical water splitting and organic pollutant degradation by highly ordered TiO₂ nanopore arrays,” Applied Catalysis B, vol. 89, no. 1-2, pp. 142–148, 2009.

M. H. Seo, M. Yuasa, T. Kida, J. S. Huh, K. Shimanoe, and N. Yamazoe, “Gas sensing characteristics and porosity control of nanostructured films composed of TiO₂ nanotubes,” Sensors and Actuators B, vol. 137, no. 2, pp. 513–520, 2009.

X. Chen and S. S. Mao, “Titanium dioxide nanomaterials: synthesis, properties, modifications and applications,” Chemical Reviews, vol. 107, no. 7, pp. 2891–2959, 2007.

X. Yang, C. Cao, K. Hohn et al., “Highly visible-light active C- and V-doped TiO₂ for degradation of acetaldehyde,” Journal of Catalysis, vol. 252, no. 2, pp. 296–302, 2007.

T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui, and M. Matsumura, “Preparation of S-doped TiO₂ photocatalysts and their photocatalytic activities under visible light,” Applied Catalysis A, vol. 265, no. 1, pp. 115–121, 2004.

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.

C.-C. Hu, T.-C. Hsu, and L.-H. Kao, “One-step cohydrothermal synthesis of nitrogen-doped titanium oxide nanotubes with enhanced visible light photocatalytic activity,” International Journal of Photoenergy, vol. 2012, Article ID 391958, 9 pages, 2012.

Y.-C. Tang, X.-H. Huang, H.-Q. Yu, and L.-H. Tang, “Nitrogen-doped TiO₂ photocatalyst prepared by mechanochemical method: doping mechanisms and visible photoactivity of pollutant degradation,” International Journal of Photoenergy, vol. 2012, Article ID 960726, 10 pages, 2012.

V. Štengl and T. M. Grygar, “The simplest way to iodine-doped anatase for photocatalysts activated by visible light,” International Journal of Photoenergy, vol. 2011, Article ID 685935, 13 pages, 2011.

S. Klosek and D. Raftery, “Visible light driven V-doped TiO₂ photocatalyst and its photooxidation of ethanol,” Journal of Physical Chemistry B, vol. 105, no. 14, pp. 2815–2819, 2002.

H. Kato and A. Kudo, “Visible-light-response and photocatalytic activities of TiO₂ and SrTiO3 photocatalysts codoped with antimony and chromium,” Journal of Physical Chemistry B, vol. 106, no. 19, pp. 5029–5034, 2002.

K. T. Ranjit and B. Viswanathan, “Synthesis, characterization and photocatalytic properties of iron-doped TiO₂ catalysts,” Journal of Photochemistry and Photobiology A, vol. 108, no. 1, pp. 79–84, 1997.

W. Li, Y. Wang, H. Lin et al., “Band gap tailoring of Nd³⁺-doped TiO₂ nanoparticles,” Applied Physics Letters, vol. 83, no. 20, pp. 4143–4145, 2003.

Y. Gai, J. Li, S.-S. Li, J.-B. Xia, and S.-H. Wei, “Design of narrow-gap TiO₂: a passivated codoping approach for enhanced photoelectrochemical activity,” Physical Review Letters, vol. 102, no. 3, Article ID 036402, 2009.

H. Liu, Z. Lu, L. Yue et al., “(Mo+N) codoped TiO₂ for enhanced visible-light photoactivity,” Applied Surface Science, vol. 257, no. 22, pp. 9355–9361, 2011.

J. Yang, J. Dai, and J. Li, “Synthesis, characterization and degradation of Bisphenol A using Pr, N co-doped TiO₂ with highly visible light activity,” Applied Surface Science, vol. 257, no. 21, pp. 8965–8973, 2011.

H. Gao, B. Lu, F. Liu, Y. Liu, and X. Zhao, “Photocatalytical properties and theoretical analysis of N, Cd-codoped TiO₂ synthesized by thermal decomposition method,” International Journal of Photoenergy, vol. 2012, Article ID 453018, 9 pages, 2012.

T. Umebayashi, T. Yamaki, H. Itoh, and K. Asai, “Analysis of electronic structures of 3d transition metal-doped TiO₂ based on band calculations,” Journal of Physics and Chemistry of Solids, vol. 63, no. 10, pp. 1909–1920, 2002.

K.-S. Ahn, Y. Yan, S. Shet, T. Deutsch, J. Turner, and M. Al-Jassim, “Enhanced photoelectrochemical responses of ZnO films through Ga and N codoping,” Applied Physics Letters, vol. 91, no. 23, Article ID 231909, 2007.

W. Zhu, X. Qiu, V. Iancu et al., “Band gap narrowing of titanium oxide semiconductors by noncompensated anion-Cation codoping for enhanced visible-Light photoactivity,” Physical Review Letters, vol. 103, no. 22, Article ID 226401, 2009.

P. Romero-Gomez, A. Borras, A. Barranco, J. P. Espinos, and A. R. Gonzalez-Elipe, “Enhanced photoactivity in bilayer films with buried rutile-anatase heterojunctions,” ChemPhysChem, vol. 12, no. 1, pp. 191–196, 2011.

P. Deák, B. Aradi, and T. Frauenheim, “Band lineup and charge carrier separation in mixed rutile-anatase systems,” Journal of Physical Chemistry C, vol. 115, no. 8, pp. 3443–3446, 2011.

K. Y. Jung and S. B. Park, “Anatase-phase titania: preparation by embedding silica and photocatalytic activity for the decomposition of trichloroethylene,” Journal of Photochemistry and Photobiology A, vol. 127, no. 1–3, pp. 117–122, 1999.

J. Zhu, W. Zheng, B. He, J. Zhang, and M. Anpo, “Characterization of Fe-TiO₂ photocatalysts synthesized by hydrothermal method and their photocatalytic reactivity for photodegradation of XRG dye diluted in water,” Journal of Molecular Catalysis A, vol. 216, no. 1, pp. 35–43, 2004.

J. Yu, M. Zhou, H. Yu, Q. Zhang, and Y. Yu, “Enhanced photoinduced super-hydrophilicity of the sol-gel-derived TiO₂ thin films by Fe-doping,” Materials Chemistry and Physics, vol. 95, no. 2-3, pp. 193–196, 2006.

W. Q. Peng, M. Yanagida, and L. Y. Han, “Rutile-anatase TiO₂ photoanodes for dye-sensitized solar cells,” Journal of Nonlinear Optical Physics and Materials, vol. 19, no. 4, pp. 673–679, 2010.

G. Liu, X. Yan, Z. Chen et al., “Synthesis of rutile-anatase core-shell structured TiO₂ for photocatalysis,” Journal of Materials Chemistry, vol. 19, no. 36, pp. 6590–6596, 2009.

T. Miyagi, M. Kamei, T. Mitsuhashi, T. Ishigaki, and A. Yamazaki, “Charge separation at the rutile/anatase interface: a dominant factor of photocatalytic activity,” Chemical Physics Letters, vol. 390, no. 4–6, pp. 399–402, 2004.

W. Ren, Z. Ai, F. Jia, L. Zhang, X. Fan, and Z. Zou, “Low temperature preparation and visible light photocatalytic activity of mesoporous carbon-doped crystalline TiO₂,” Applied Catalysis B, vol. 69, no. 3-4, pp. 138–144, 2007.

H. Irie, Y. Watanabe, and K. Hashimoto, “Carbon-doped anatase TiO₂ powders as a visible-light sensitive photocatalyst,” Chemistry Letters, vol. 32, no. 8, pp. 772–773, 2003.

J. L. Gole, J. D. Stout, C. Burda, Y. Lou, and X. Chen, “Highly efficient formation of visible light tunable TiO₂-xNx photocatalysts and their transformation at the nanoscale,” Journal of Physical Chemistry B, vol. 108, no. 4, pp. 1230–1240, 2004.

C. S. Gopinath, “Comment on "photoelectron spectroscopic investigation of nitrogen-doped titania nanoparticles",” Journal of Physical Chemistry B, vol. 110, no. 13, pp. 7079–7080, 2006.

C. Burda and J. Gole, “Reply to “comment on ‘photoelectron spectroscopic investigation of nitrogen-doped titania nanoparticles”’,” Journal of Physical Chemistry B, vol. 110, no. 13, pp. 7081–7082, 2006.

J. F. Molder, W. F. Stickle, P. E. Sobol, and K. D. Bomben, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, Minn, USA, 2nd edition, 1992.

C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder, and G. E. Muilenberg, Handbook of Xray Photoelectron Spectroscopy, PerkinElmer Corp., Physical Electronics Division, Eden Prairie, Minn, USA, 1979.

J. Li, J. Xu, W.-L. Dai, H. Li, and K. Fan, “Direct hydro-alcohol thermal synthesis of special core-shell structured Fe-doped titania microspheres with extended visible light response and enhanced photoactivity,” Applied Catalysis B, vol. 85, no. 3-4, pp. 162–170, 2009.

J. Wang, D. N. Tafen, J. P. Lewis et al., “Origin of photocatalytic activity of Nitrogen-doped TiO₂ nanobelts,” Journal of the American Chemical Society, vol. 131, no. 34, pp. 12290–12297, 2009.

J. Yu, Q. Xiang, and M. Zhou, “Preparation, characterization and visible-light-driven photocatalytic activity of Fe-doped titania nanorods and first-principles study for electronic structures,” Applied Catalysis B, vol. 90, no. 3-4, pp. 595–602, 2009.

W. Choi, A. Termin, and M. R. Hoffmann, “The role of metal ion dopants in quantum-sized TiO₂: correlation between photoreactivity and charge carrier recombination dynamics,” Journal of Physical Chemistry, vol. 98, no. 51, pp. 13669–13679, 1994.

K. Tan, H. Zhang, C. Xie, H. Zheng, Y. Gu, and W. F. Zhang, “Visible-light absorption and photocatalytic activity in molybdenum- and nitrogen-codoped TiO₂,” Catalysis Communications, vol. 11, no. 5, pp. 331–335, 2010.

T. Ihara, M. Miyoshi, Y. Iriyama, O. Matsumoto, and S. Sugihara, “Visible-light-active titanium oxide photocatalyst realized by an oxygen-deficient structure and by nitrogen doping,” Applied Catalysis B, vol. 42, no. 4, pp. 403–409, 2003.

I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara, and K. Takeuchi, “Role of oxygen vacancy in the plasma-treated TiO₂ photocatalyst with visible light activity for NO removal,” Journal of Molecular Catalysis A, vol. 161, no. 1-2, pp. 205–212, 2000.

I. Justicia, P. Ordejón, G. Canto et al., “Designed self-doped titanium oxide thin films for efficient visible-light photocatalysis,” Advanced Materials, vol. 14, no. 19, pp. 1399–1402, 2002.