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Cite this: DOI: 10.1039/c4dt01908c Received 25th June 2014, Accepted 1st August 2014 DOI: 10.1039/c4dt01908c
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F, Ca co-doped TiO2 nanocrystals with enhanced photocatalytic activity† Weiwei Fu,a Shuang Ding,a Ying Wang,a Lele Wu,a Daming Zhang,b Zhengwei Pan,c Runwei Wang,a Zongtao Zhang*a and Shilun Qiu*a
www.rsc.org/dalton
F, Ca co-doped TiO2 was synthesized by a facile one-step hydrothermal method. After doping with F, electrons can be simultaneously excited from valence band to the F doping energy level. The smaller crystal size caused by doping with Ca can exhibit more powerful redox ability and the efficient separation of photogenerated hole–electron pairs. Therefore, F, Ca co-doped TiO2 exhibited enhanced photocatalytic activity.
With industrialization and population growth in the past decades, environmental contamination by organic pollutants is becoming an overwhelming problem all over the world. One of the most promising ways to destroy toxic organic compounds is their photocatalytic oxidation over TiO2 materials.1–3 TiO2 exhibits a strong redox ability that can convert pollutants into CO2, H2O, and other small molecules.4,5 Although this process can occur in air at room temperature, the low catalytic activity, the difficulties associated with the recombination of holes (h+) and electrons (e−) and the efficient utilization of visible light have seriously hindered the large-scale application of TiO2 as a photocatalyst. Efforts have been focused towards designing novel photocatalysts by methods such as dye sensitization and heteroelement doping to improve the photocatalytic activity.6–9 One of the effective approach is to dope different elements into TiO2, including metal10–13 or nonmetal elements.14–16 The band gap that is responsible for the photoresponse can be narrowed by doping with nonmetallic elements such as N, C, F, and S.17–19 The separation of photo-excited electrons and holes, which play important roles in the photocatalytic activity, can be
a State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University, Changchun 130012, China. E-mail: [email protected]; Fax: (+86) 431-8516-8115 b State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China c Faculty of Engineering, University of Georgia, Athens, GA 30602, USA † Electronic supplementary information (ESI) available: The synthetic procedure, facilities information, TEM, FT-IR spectra, EDX spectra, UV-vis absorbance spectra and the photocatalytic activity of materials under broader-band UV-vis irradiation. See DOI: 10.1039/c4dt01908c
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enhanced by doping with metallic elements such as Fe, V, Nd, and La.20–23 However, TiO2 doped by a single element has not been found to meet practical requirements and co-doping with different elements may lead to better synergistic effects such as F-N co-doped TiO2,24 W-N co-doped TiO2 25 and Fe-F codoped TiO2.26 Compared to transition metals and rare earth metals, doping with alkaline earth metals, such as Ca and Sr, have not been studied much. Here, we achieved F, Ca co-doped TiO2 nanocrystals by the one-step hydrothermal reaction of calcium nitrate and titanium powder in HF solution. Compared to single F doping, the optimal level of F and Ca doping results in significant photocatalytic activity. The samples are crystalline; Fig. 1 shows the X-ray diffraction (XRD) patterns of F, Ca co-doped TiO2. There is no impurity phase in the XRD patterns of Fig. 1a because the doped fluorine ion easily entered the TiO2 lattice (fluorine ion radii are similar to oxygen ions) and simultaneously initiated the crystallization process.27,28 Fig. 1 also shows that increasing Ca2+ concentration resulted in an increase in the intensity and sharpness of the CaF2 peaks. This trend clearly indicates that
Fig. 1 XRD patterns of (a) F-TiO2 nanoparticles, (b) 0.5 atom % Ca doped F-TiO2, (c) 2 atom % Ca doped F-TiO2, (d) 5 atom % Ca doped F-TiO2, (e) 10 atom % Ca doped F-TiO2, (f ) 12 atom % Ca doped F-TiO2, (g) 15 atom % Ca doped F-TiO2, (h) 20 atom % Ca doped F-TiO2.
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it generated a large number of CaF2 with increasing Ca2+. CaF2 impurity phase is not detected in Fig. 1b. The reason may be that the Ca intercalated into the TiO2 lattice and only a little CaF2 was generated. ESI-Table 1 and 2† show the lattice parameters and the lattice distortion, respectively. The (101) interplanar distance (d101) was estimated from the XRD spectrum by using eqn (1): d101 ¼ λ=2 sin θ
ð1Þ
The dopant ion (Ca2+), with an ionic radius (1.06 Å) larger than Ti4+ (0.745 Å) but smaller than O2− (1.31 Å), can be either isomorphously substituted or interstitially introduced into the matrix to produce oxygen vacancies or interstitial Ti3+ ions, respectively. In ESI-Table 1,† compared with F-TiO2, F, Ca-TiO2 shows an increase in the a cell dimension and cell volume and a small decrease in the c cell dimension. This appears to confirm that the cation is certainly included within the anatase matrix. ESI-Fig. 1† shows that A (101) peaks of all F-Ca-TiO2 were shifted to lower degrees in comparison with F-TiO2, which is also attributed to the Ca doping. Moreover, it is noted that the size of Ca-F-TiO2 nanocrystals is smaller than that of F-TiO2 (ESI-Fig. 2†), indicating that Ca doping can inhibit the grain growth of TiO2. Fig. 2 shows the XPS spectra of F doped and F, 10 atom% Ca (nominal) co-doped TiO2 nanocrystals. Fig. 2a (2) shows that binding energies were shifted toward the lower energy side compared to those of the F singly-doped TiO2. Peng et al.29 reported that most references agreed on the lower binding energy of Ti4+ 2p in N-TiO2, in which the binding energy of Ti4+ 2p3/2 and Ti4+ 2p1/2 core levels can decrease by 0.5–2 eV. Thus, we assumed that Ca doping affected the chemical states of TiO2. In Fig. 2b, the peak at 347.7 eV is a representation of Ca2+ 2p3/2 of CaF2. Another peak at 352.3 eV
Fig. 2 XPS spectra of (a) Ti 2p of (1) F-TiO2, (2) 10 atom % Ca doped F-TiO2; (b) Ca 2p of 10 atom % Ca doped F-TiO2; (c) F 1s of (1) F-TiO2, (2) 10 atom % Ca doped F-TiO2; (d) O 1s of (1) F-TiO2, (2) 10 atom % Ca doped F-TiO2.
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suggested that Ca has fully integrated itself into the lattices of TiO2. Fig. 2c gives the F1s XPS spectra. The F1s region is composed of two contributions. In Fig. 2c (1), the two peaks located at 684.8 eV and 689.6 eV are attributed to the fluorine ions physically adsorbed onto the surface of TiO2 and the substitutional F-atoms in TiO2, respectively.30–34 The intensity of the peak located at 689.6 eV is weaker in Fig. 2c (2) than that in Fig. 2c (1). However, the width of the peak located at 684.8 eV in Fig. 2c (2) is larger than that in Fig. 2c (1). These facts can be attributed to Ca doping and high generation of CaF2. Fig. 2d shows the O 1s XPS spectra of F-doped and F, Cacodoped TiO2 samples. The F-TiO2 shows a band centered at 529.8 eV assigned to lattice O in TiO2 and a small contribution around 531.5 eV associated to OH groups at the surface. However, the main O 1s peak of F, Ca-codoping samples is strongly shifted to higher binding energies (ca. 530.05 eV). According to the report,25 we assumed that the shift attributed to the complex environment, in which the oxygen atoms near VTi sites are responsible for the lower binding energy tail of the peak, while the higher energy bump is related to the oxygen atoms that are first neighbors to the Ca impurity atoms. Fig. 3 shows the electron paramagnetic resonance (EPR) spectra of the F-TiO2 and Ca-F-TiO2 nanocrystals. As displayed in Fig. 3a, the signal at g = 2.072 is attributed to F doped into TiO2 because F− doping can convert some Ti4+ to Ti3+ by charge compensation.30–34 Fig. 3b shows six main hyperfine lines, which may be attributed to the quantum-size effect. The Ca doping inhibited the grain growth of TiO2, which resulted in the smaller average crystal size of the Ca-F-TiO2 (ESI-Fig. 2†). Therefore, the spectra of the F doped and Ca, F codoped TiO2 are significantly different.
Fig. 3 EPR spectra of (a) F-TiO2 nanocrystals, (b) 10 atom % Ca doped F-TiO2 nanocrystals.
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ESI-Fig. 5† shows UV-vis diffuse reflectance spectra of F-TiO2 and Ca doped F-TiO2 with different doping contents (molar ratios). Compared with F-TiO2, the absorption edge shifted toward blue wavelengths for all Ca doped F-TiO2, which was also attributed to the quantum-size effect. That results in a larger band gap and an increased rate constant for the charge transfer, which can result in improved photoefficiencies. To demonstrate the potential applicability of the Ca-F-TiO2 for the removal of contaminants from wastewater, we investigated their photocatalytic activity relative to that of a commercial photocatalyst (Degussa P25) and F-TiO2, by employing the photocatalytic degradation of Rhodamine B (RhB) as a test reaction. The characteristic absorption of RhB at λ = 553 nm was chosen to monitor the photocatalytic degradation process. ESI-Fig. 4† shows the absorption spectra of a solution of RhB at room temperature in the presence of different samples under visible light. A Xe lamp (300 W) with a 420 nm cut-off filter was used to ensure that only visible light illuminated the photocatalyst. It shows that 10 atom% Ca doped F-TiO2 (nominal) nanocrystals showed the best visible-light photoactivity for RhB degradation, and most Ca-F-TiO2 exhibited higher visible-light photoactivity than both F singly-doped TiO2 nanocrystals and P25. The relation between different dopant levels and the visible light photocatalytic activity indicates that calcium doping resulted in significant activity, and when the Ca doping content exceeds a certain point, excessive CaF2 coating on the surface of TiO2 would inhibit the adsorption of RhB and decrease the absorption of light, and thus decrease the photocatalytic activity. Fig. 4 shows the photocatalytic ability of the samples under UV light irradiation. From Fig. 4, it is clear that the dye is almost completely decomposed during the test of 10 atom% Ca doped F-TiO2 (nominal) nanocrystals after irradiation for 60 min, which indicates the high photocatalytic ability of 10 atom% Ca doped
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F-TiO2 (nominal) nanocrystals. In addition, the F, Ca-codoping TiO2 nanocrystals can be easily recycled by a simple filtration step. After three cycles of the photocatalytic degradation of RhB, the catalyst can decompose the dye completely within about 70 min, which indicates the excellent stability that is important for its practical application. Based on the results, we may ask why the Ca-F-TiO2 exhibits a higher RhB photodegradation capacity under both visible and UV light. It can be explained by considering several factors: (1) F-doping caused several beneficial effects, including the enhancement of surface acidity, creation of oxygen vacancies, and increase of active sites.35,36 In addition, the impurity caused by the F doping formed an impurity energy state (IES) (2.0 eV) below the CB of TiO2.37 The impurity level arising from the doping element can shorten the excitation path of electrons and reduce the apparent band gap. (2) The result of the smaller average crystal size implies more powerful redox ability due to the quantum-size effect. Reaction rate enhancement can be due to the increased migration of electrons and holes to the semiconductor surfaces which is possible in smaller particles, allowing their participation in the reaction and, thereby reduces the net electron–hole recombinations.1,38,39
Conclusions In summary, a new type of TiO2-based photocatalyst (CaF-TiO2) has been successfully prepared by codoping TiO2 with alkaline earth metal (Ca) and F using a simple hydrothermal method, which exhibits a higher photocatalytic activity than P25 and F-TiO2 under both visible light and UV light irradiation because of the smaller band gap from F doping and the effect of smaller crystal size from Ca doping. This implies that codoping with two foreign ions, metal and nonmetal, is a more efficient way to improve the photocatalytic activity of TiO2 than doping with just one type of ion.
Acknowledgements This work was supported by the National Natural Science Foundation of China (20841003, 20741001 and 21390394), and the New Century Outstanding Scholar Supporting Program.
Notes and references
Fig. 4 Changes in RhB concentration over the course of the photocatalytic degradation of RhB in the presence of various photocatalysts: (a) without any photocatalyst, (b) P25, (c) F-TiO2, (d) 2 atom % Ca doped F-TiO2, (e) 5 atom% Ca doped F-TiO2, (f ) 10 atom % Ca doped F-TiO2, (g) 12 atom % Ca doped F-TiO2, (h) 15 atom % Ca doped F-TiO2, (i) 20 atom % Ca doped F-TiO2 and under exposure to UV light.
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1 M. R. Hoffmann, S. T. Martin, W. Y. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69–96. 2 A. L. Linsebigler, G. Q. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735–758. 3 A. Mills and S. LeHunte, J. Photochem. Photobiol., A, 1997, 108, 1–35. 4 T. Nonami, H. Hase and K. Funakoshi, Catal. Today, 2004, 96, 113–118.
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5 S. Yanagida, G. K. R. Senadeera, K. Nakamura, T. Kitamura and Y. Wada, J. Photochem. Photobiol., A, 2004, 166, 75–80. 6 W. J. Youngblood, S. H. A. Lee, K. Maeda and T. E. Mallouk, Acc. Chem. Res., 2009, 42, 1966–1973. 7 F. E. Osterloh, Chem. Mater., 2008, 20, 35–54. 8 A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253–278. 9 C. Burda, Y. B. Lou, X. B. Chen, A. C. S. Samia, J. Stout and J. L. Gole, Nano Lett., 2003, 3, 1049–1051. 10 J. Li and H. C. Zeng, J. Am. Chem. Soc., 2007, 129, 15839– 15847. 11 W. Choi, A. Termin and M. R. Hoffmann, J. Phys. Chem., 1994, 98, 13669–13679. 12 J. H. Chen, M. S. Yao and X. L. Wang, J. Nanopart. Res., 2008, 10, 163–171. 13 A. Di Paola, E. Garcia-Lopez, S. Ikeda, G. Marci, B. Ohtani and L. Palmisano, Catal. Today, 2002, 75, 87–93. 14 O. Diwald, T. L. Thompson, T. Zubkov, E. G. Goralski, S. D. Walck and J. T. Yates, J. Phys. Chem. B, 2004, 108, 6004–6008. 15 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269–271. 16 T. Sano, N. Negishi, K. Koike, K. Takeuchi and S. Matsuzawa, J. Mater. Chem., 2004, 14, 380–384. 17 S. U. M. Khan, M. Al-Shahry and W. B. Ingler, Science, 2002, 297, 2243–2245. 18 J. C. Yu, J. G. Yu, W. K. Ho, Z. T. Jiang and L. Z. Zhang, Chem. Mater., 2002, 14, 3808–3816. 19 V. Pore, M. Ritala, M. Leskela, S. Areva, M. Jarn and J. Jarnstrom, J. Mater. Chem., 2007, 17, 1361–1371. 20 X. H. Wang, J. G. Li, H. Kamiyama, M. Katada, N. Ohashi, Y. Moriyoshi and T. Ishigaki, J. Am. Chem. Soc., 2005, 127, 10982–10990. 21 S. Klosek and D. Raftery, J. Phys. Chem. B, 2001, 105, 2815– 2819. 22 Y. H. Xu, C. Chen, X. L. Yang, X. Li and B. F. Wang, Appl. Surf. Sci., 2009, 255, 8624–8628.
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23 G. X. Cao, Y. G. Li, Q. H. Zhang and H. Z. Wang, J. Am. Ceram. Soc., 2010, 93, 25–27. 24 D. Li, H. Haneda, S. Hishita and N. Ohashi, Chem. Mater., 2005, 17, 2588–2595. 25 A. M. Márquez, J. J. Plata, Y. Ortega and J. F. Sanz, J. Phys. Chem. C, 2012, 116, 18759–18767. 26 Y. H. Zhang, F. Z. Lv, T. Wu, L. Yu, R. Zhang, B. Shen, X. H. Meng, Z. F. Ye and P. K. Chu, J. Sol-Gel Sci. Technol., 2011, 59, 387–391. 27 N. Todorova, T. Giannakopoulou, T. Vaimakis and C. Trapalis, Mater. Sci. Eng., B, 2008, 152, 50–54. 28 N. Todorova, T. Giannakopoulou, G. Romanos, T. Vaimakis, J. G. Yu and C. Trapalis, Int. J. Photoenergy, 2008, DOI: 10.1155/2008/534038. 29 F. Peng, L. Cai, L. Huang, H. Yu and H. Wang, J. Phys. Chem. Solids, 2008, 69, 1657–1664. 30 J. C. Yu, J. G. Yu, W. K. Ho, Z. T. Jiang and L. Z. Zhang, Chem. Mater., 2002, 14, 3808–3816. 31 D. Li, H. Haneda, N. K. Labhsetwar, S. Hishita and N. Ohashi, Chem. Phys. Lett., 2005, 401, 579–584. 32 S. Tosoni, O. Lamiel-Garcia, D. F. Hevia, J. M. Doña and F. Illas, J. Phys. Chem. C, 2012, 116, 12738–12746. 33 S. Tosoni, O. Lamiel-Garcia, D. F. Hevia and F. Illas, J. Phys. Chem. C, 2013, 117, 5855–5860. 34 Y. Ortega, O. Lamiel-Garcia, D. F. Hevia, S. Tosoni, J. Oviedo, M. A. San-Miguel and F. Illas, Surf. Sci., 2013, 618, 154–158. 35 H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng and G. Q. Lu, Nature, 2008, 453, 638–641. 36 D. Li, H. Haneda, S. Hishita, N. Ohashi and N. K. Labhsetwar, J. Fluorine Chem., 2005, 126, 69–77. 37 N. O. Gopal, H. H. Lo, S. C. Sheu and S. C. Ke, J. Am. Chem. Soc., 2010, 132, 10982–10983. 38 P. V. Kamat, Chem. Rev., 1993, 93, 267–300. 39 Z. Y. Liu, D. D. Sun, P. Guo and J. O. Leckie, Chem. – Eur. J., 2007, 13, 1851–1855.
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Cite this: DOI: 10.1039/c4dt01908c Received 25th June 2014, Accepted 1st August 2014 DOI: 10.1039/c4dt01908c
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F, Ca co-doped TiO2 nanocrystals with enhanced photocatalytic activity† Weiwei Fu,a Shuang Ding,a Ying Wang,a Lele Wu,a Daming Zhang,b Zhengwei Pan,c Runwei Wang,a Zongtao Zhang*a and Shilun Qiu*a
www.rsc.org/dalton
F, Ca co-doped TiO2 was synthesized by a facile one-step hydrothermal method. After doping with F, electrons can be simultaneously excited from valence band to the F doping energy level. The smaller crystal size caused by doping with Ca can exhibit more powerful redox ability and the efficient separation of photogenerated hole–electron pairs. Therefore, F, Ca co-doped TiO2 exhibited enhanced photocatalytic activity.
With industrialization and population growth in the past decades, environmental contamination by organic pollutants is becoming an overwhelming problem all over the world. One of the most promising ways to destroy toxic organic compounds is their photocatalytic oxidation over TiO2 materials.1–3 TiO2 exhibits a strong redox ability that can convert pollutants into CO2, H2O, and other small molecules.4,5 Although this process can occur in air at room temperature, the low catalytic activity, the difficulties associated with the recombination of holes (h+) and electrons (e−) and the efficient utilization of visible light have seriously hindered the large-scale application of TiO2 as a photocatalyst. Efforts have been focused towards designing novel photocatalysts by methods such as dye sensitization and heteroelement doping to improve the photocatalytic activity.6–9 One of the effective approach is to dope different elements into TiO2, including metal10–13 or nonmetal elements.14–16 The band gap that is responsible for the photoresponse can be narrowed by doping with nonmetallic elements such as N, C, F, and S.17–19 The separation of photo-excited electrons and holes, which play important roles in the photocatalytic activity, can be
a State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University, Changchun 130012, China. E-mail: [email protected]; Fax: (+86) 431-8516-8115 b State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China c Faculty of Engineering, University of Georgia, Athens, GA 30602, USA † Electronic supplementary information (ESI) available: The synthetic procedure, facilities information, TEM, FT-IR spectra, EDX spectra, UV-vis absorbance spectra and the photocatalytic activity of materials under broader-band UV-vis irradiation. See DOI: 10.1039/c4dt01908c
This journal is © The Royal Society of Chemistry 2014
enhanced by doping with metallic elements such as Fe, V, Nd, and La.20–23 However, TiO2 doped by a single element has not been found to meet practical requirements and co-doping with different elements may lead to better synergistic effects such as F-N co-doped TiO2,24 W-N co-doped TiO2 25 and Fe-F codoped TiO2.26 Compared to transition metals and rare earth metals, doping with alkaline earth metals, such as Ca and Sr, have not been studied much. Here, we achieved F, Ca co-doped TiO2 nanocrystals by the one-step hydrothermal reaction of calcium nitrate and titanium powder in HF solution. Compared to single F doping, the optimal level of F and Ca doping results in significant photocatalytic activity. The samples are crystalline; Fig. 1 shows the X-ray diffraction (XRD) patterns of F, Ca co-doped TiO2. There is no impurity phase in the XRD patterns of Fig. 1a because the doped fluorine ion easily entered the TiO2 lattice (fluorine ion radii are similar to oxygen ions) and simultaneously initiated the crystallization process.27,28 Fig. 1 also shows that increasing Ca2+ concentration resulted in an increase in the intensity and sharpness of the CaF2 peaks. This trend clearly indicates that
Fig. 1 XRD patterns of (a) F-TiO2 nanoparticles, (b) 0.5 atom % Ca doped F-TiO2, (c) 2 atom % Ca doped F-TiO2, (d) 5 atom % Ca doped F-TiO2, (e) 10 atom % Ca doped F-TiO2, (f ) 12 atom % Ca doped F-TiO2, (g) 15 atom % Ca doped F-TiO2, (h) 20 atom % Ca doped F-TiO2.
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it generated a large number of CaF2 with increasing Ca2+. CaF2 impurity phase is not detected in Fig. 1b. The reason may be that the Ca intercalated into the TiO2 lattice and only a little CaF2 was generated. ESI-Table 1 and 2† show the lattice parameters and the lattice distortion, respectively. The (101) interplanar distance (d101) was estimated from the XRD spectrum by using eqn (1): d101 ¼ λ=2 sin θ
ð1Þ
The dopant ion (Ca2+), with an ionic radius (1.06 Å) larger than Ti4+ (0.745 Å) but smaller than O2− (1.31 Å), can be either isomorphously substituted or interstitially introduced into the matrix to produce oxygen vacancies or interstitial Ti3+ ions, respectively. In ESI-Table 1,† compared with F-TiO2, F, Ca-TiO2 shows an increase in the a cell dimension and cell volume and a small decrease in the c cell dimension. This appears to confirm that the cation is certainly included within the anatase matrix. ESI-Fig. 1† shows that A (101) peaks of all F-Ca-TiO2 were shifted to lower degrees in comparison with F-TiO2, which is also attributed to the Ca doping. Moreover, it is noted that the size of Ca-F-TiO2 nanocrystals is smaller than that of F-TiO2 (ESI-Fig. 2†), indicating that Ca doping can inhibit the grain growth of TiO2. Fig. 2 shows the XPS spectra of F doped and F, 10 atom% Ca (nominal) co-doped TiO2 nanocrystals. Fig. 2a (2) shows that binding energies were shifted toward the lower energy side compared to those of the F singly-doped TiO2. Peng et al.29 reported that most references agreed on the lower binding energy of Ti4+ 2p in N-TiO2, in which the binding energy of Ti4+ 2p3/2 and Ti4+ 2p1/2 core levels can decrease by 0.5–2 eV. Thus, we assumed that Ca doping affected the chemical states of TiO2. In Fig. 2b, the peak at 347.7 eV is a representation of Ca2+ 2p3/2 of CaF2. Another peak at 352.3 eV
Fig. 2 XPS spectra of (a) Ti 2p of (1) F-TiO2, (2) 10 atom % Ca doped F-TiO2; (b) Ca 2p of 10 atom % Ca doped F-TiO2; (c) F 1s of (1) F-TiO2, (2) 10 atom % Ca doped F-TiO2; (d) O 1s of (1) F-TiO2, (2) 10 atom % Ca doped F-TiO2.
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suggested that Ca has fully integrated itself into the lattices of TiO2. Fig. 2c gives the F1s XPS spectra. The F1s region is composed of two contributions. In Fig. 2c (1), the two peaks located at 684.8 eV and 689.6 eV are attributed to the fluorine ions physically adsorbed onto the surface of TiO2 and the substitutional F-atoms in TiO2, respectively.30–34 The intensity of the peak located at 689.6 eV is weaker in Fig. 2c (2) than that in Fig. 2c (1). However, the width of the peak located at 684.8 eV in Fig. 2c (2) is larger than that in Fig. 2c (1). These facts can be attributed to Ca doping and high generation of CaF2. Fig. 2d shows the O 1s XPS spectra of F-doped and F, Cacodoped TiO2 samples. The F-TiO2 shows a band centered at 529.8 eV assigned to lattice O in TiO2 and a small contribution around 531.5 eV associated to OH groups at the surface. However, the main O 1s peak of F, Ca-codoping samples is strongly shifted to higher binding energies (ca. 530.05 eV). According to the report,25 we assumed that the shift attributed to the complex environment, in which the oxygen atoms near VTi sites are responsible for the lower binding energy tail of the peak, while the higher energy bump is related to the oxygen atoms that are first neighbors to the Ca impurity atoms. Fig. 3 shows the electron paramagnetic resonance (EPR) spectra of the F-TiO2 and Ca-F-TiO2 nanocrystals. As displayed in Fig. 3a, the signal at g = 2.072 is attributed to F doped into TiO2 because F− doping can convert some Ti4+ to Ti3+ by charge compensation.30–34 Fig. 3b shows six main hyperfine lines, which may be attributed to the quantum-size effect. The Ca doping inhibited the grain growth of TiO2, which resulted in the smaller average crystal size of the Ca-F-TiO2 (ESI-Fig. 2†). Therefore, the spectra of the F doped and Ca, F codoped TiO2 are significantly different.
Fig. 3 EPR spectra of (a) F-TiO2 nanocrystals, (b) 10 atom % Ca doped F-TiO2 nanocrystals.
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ESI-Fig. 5† shows UV-vis diffuse reflectance spectra of F-TiO2 and Ca doped F-TiO2 with different doping contents (molar ratios). Compared with F-TiO2, the absorption edge shifted toward blue wavelengths for all Ca doped F-TiO2, which was also attributed to the quantum-size effect. That results in a larger band gap and an increased rate constant for the charge transfer, which can result in improved photoefficiencies. To demonstrate the potential applicability of the Ca-F-TiO2 for the removal of contaminants from wastewater, we investigated their photocatalytic activity relative to that of a commercial photocatalyst (Degussa P25) and F-TiO2, by employing the photocatalytic degradation of Rhodamine B (RhB) as a test reaction. The characteristic absorption of RhB at λ = 553 nm was chosen to monitor the photocatalytic degradation process. ESI-Fig. 4† shows the absorption spectra of a solution of RhB at room temperature in the presence of different samples under visible light. A Xe lamp (300 W) with a 420 nm cut-off filter was used to ensure that only visible light illuminated the photocatalyst. It shows that 10 atom% Ca doped F-TiO2 (nominal) nanocrystals showed the best visible-light photoactivity for RhB degradation, and most Ca-F-TiO2 exhibited higher visible-light photoactivity than both F singly-doped TiO2 nanocrystals and P25. The relation between different dopant levels and the visible light photocatalytic activity indicates that calcium doping resulted in significant activity, and when the Ca doping content exceeds a certain point, excessive CaF2 coating on the surface of TiO2 would inhibit the adsorption of RhB and decrease the absorption of light, and thus decrease the photocatalytic activity. Fig. 4 shows the photocatalytic ability of the samples under UV light irradiation. From Fig. 4, it is clear that the dye is almost completely decomposed during the test of 10 atom% Ca doped F-TiO2 (nominal) nanocrystals after irradiation for 60 min, which indicates the high photocatalytic ability of 10 atom% Ca doped
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F-TiO2 (nominal) nanocrystals. In addition, the F, Ca-codoping TiO2 nanocrystals can be easily recycled by a simple filtration step. After three cycles of the photocatalytic degradation of RhB, the catalyst can decompose the dye completely within about 70 min, which indicates the excellent stability that is important for its practical application. Based on the results, we may ask why the Ca-F-TiO2 exhibits a higher RhB photodegradation capacity under both visible and UV light. It can be explained by considering several factors: (1) F-doping caused several beneficial effects, including the enhancement of surface acidity, creation of oxygen vacancies, and increase of active sites.35,36 In addition, the impurity caused by the F doping formed an impurity energy state (IES) (2.0 eV) below the CB of TiO2.37 The impurity level arising from the doping element can shorten the excitation path of electrons and reduce the apparent band gap. (2) The result of the smaller average crystal size implies more powerful redox ability due to the quantum-size effect. Reaction rate enhancement can be due to the increased migration of electrons and holes to the semiconductor surfaces which is possible in smaller particles, allowing their participation in the reaction and, thereby reduces the net electron–hole recombinations.1,38,39
Conclusions In summary, a new type of TiO2-based photocatalyst (CaF-TiO2) has been successfully prepared by codoping TiO2 with alkaline earth metal (Ca) and F using a simple hydrothermal method, which exhibits a higher photocatalytic activity than P25 and F-TiO2 under both visible light and UV light irradiation because of the smaller band gap from F doping and the effect of smaller crystal size from Ca doping. This implies that codoping with two foreign ions, metal and nonmetal, is a more efficient way to improve the photocatalytic activity of TiO2 than doping with just one type of ion.
Acknowledgements This work was supported by the National Natural Science Foundation of China (20841003, 20741001 and 21390394), and the New Century Outstanding Scholar Supporting Program.
Notes and references
Fig. 4 Changes in RhB concentration over the course of the photocatalytic degradation of RhB in the presence of various photocatalysts: (a) without any photocatalyst, (b) P25, (c) F-TiO2, (d) 2 atom % Ca doped F-TiO2, (e) 5 atom% Ca doped F-TiO2, (f ) 10 atom % Ca doped F-TiO2, (g) 12 atom % Ca doped F-TiO2, (h) 15 atom % Ca doped F-TiO2, (i) 20 atom % Ca doped F-TiO2 and under exposure to UV light.
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