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Influence of Fe3+ and Ho3+ co-doping on the photocatalytic activity of TiO2
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Materials Chemistry and Physics 106 (2007) 247–249
Influence of Fe3+ and Ho3+ co-doping on the photocatalytic activity of TiO2 Jian-Wen Shi a,b,∗ , Jing-Tang Zheng b , Yan Hu b , Yu-Cui Zhao b a
b
Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, Fujian 361003, China The State Key Laboratory of Heavy Oil, China University of Petroleum, Dongying, Shandong 257061, China Received 3 July 2006; received in revised form 3 May 2007; accepted 22 May 2007
Abstract The un-doping, single-doping and co-doping TiO2 nanoparticles have been prepared by sol–gel method with Ti(OC4 H9 )4 as a raw material and characterized by X-ray diffraction (XRD) and UV–vis absorption spectra, and their photocatalytic activities have been investigated by photocatalytic oxidation of methyl orange. It is found that Fe3+ -doping broadens the absorption profile, improves photoutilization of TiO2 , and then generates more electron–hole pairs. Ho3+ -doping restrains the increase of grain size, leads to crystal expansion and matrix distortion and retards the recombination of the photoexcited charge carriers. The photocatalytic activity of TiO2 co-doped with Fe3+ and Ho3+ ions is markedly improved due to the cooperative actions of the two dopants. © 2007 Elsevier B.V. All rights reserved. Keywords: TiO2 ; Photocatalysis; Doping; Iron; Holmium; Methyl orange
1. Introduction The photocatalytic oxidation of organic compounds by semiconductor catalysts such as TiO2 , ZnO and CdS has been extensively studied in the past decades [1–8]; of these TiO2 is widely used because of its optical and electronic properties, low cost, chemical stability and non-toxicity [4]. However, TiO2 utilizes only a very small fraction of the solar spectrum due to its band-gap energy, and the photocatalytic activity of TiO2 is limited by fast charge-carrier recombination and low interfacial charge-transfer rates of photogenerated carriers [24]. In order to improve its photocatalytic activity, many researches have been investigated. These results show that selective metal ion doping is one of the effective means. In the present, the investigations about doping elements focus mostly on transition metal ions doping [9–14]. Rare earth metals having incompletely occupied 4f and empty 5d orbitals often serve as catalyst or promote catalysis. Some results show that the photocatalytic activity of TiO2 can be improved because of the doping of some rare earth metals [15–18]. Co-doping with two kinds of ions have been investigated in recent years and activized the attention of most investigators
[19–21]. Wang et al. [22] found that under the irradiation of light with wavelengths longer than 400 and 290 nm, the photocatalytic activity of nitrogen and lanthanum co-doped SrTiO3 was 2.6 and 2 times greater than that of pure SrTiO3 . Zhang et al. [23] found that Pt deposited onto the Nb5+ /TiO2 increased the photodegradation activity of CHCl3 . These facts indicate that introducing two or more proper elements onto nanocrystalline TiO2 particles will improve the photocatalytic effect of TiO2 . Co-doping transition metal ions and rare earth metal into the nanocrystalline TiO2 may have a synergistic effect to increase the activity of TiO2 . In the work, Fe3+ and Ho3+ co-doping TiO2 was prepared by the sol–gel method. The photocatalytic activity was evaluated by photodegradation of methyl orange in solution. We find that the co-doping of Fe3+ and Ho3+ can obviously enhance the photocatalytic activity for the methyl orange degradation under UV irradiation compared with un-doping TiO2 and single-doping. The synergistic effect of two dopants that leads to the significant enhancement of photodegradation has also been discussed. 2. Experimental 2.1. Sol–gel synthesis of doping TiO2
∗ Corresponding author. Present address: Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, Fujian 361003, China. Tel.: +86 5468395024; fax: +86 5468395190. E-mail address: [email protected] (J.-W. Shi).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.05.042
A solution of Ti(OC4 H9 )4 , in anhydrous ethanol, was used as molecular precursor of TiO2 . In order to control the reaction kinetics, acetic acid was used as a chemical additive to moderate the reaction rate. The distilled water used for hydrolysis in solution with Ti(OC4 H9 )4 was added gradually
248
J.-W. Shi et al. / Materials Chemistry and Physics 106 (2007) 247–249
under magnetic stirring at room temperature. The molar ratio of these reactants was Ti(OC4 H9 )4 :H2 O:ethanol:acetic acid = 1:8:20:6. Then, the temperature was raised to 60 ◦ C and kept 6 h for obtaining gels. The gels were dried in a vacuum box at 80 ◦ C; the resulting dry gels were triturated in a mortar, and then were calcined in an oven at 500 ◦ C for 2 h to give TiO2 powder. Doping TiO2 nanoparticles were synthesized using almost the same method. The appropriate amount of Fe(NO3 )3 ·9H2 O or Ho(NO3 )3 ·6H2 O was dissolved in anhydrous ethanol, which was added to the distilled water prior to the hydrolysis of Ti(OC4 H9 )4 . For convenience, the samples were labeled as TF(X), TH(Y) for Fe3+ and Ho3+ doping TiO2 , and TF(X)H(Y) for Fe3+ and Ho3+ co-doping TiO2 , where X and Y referred to the nominal atomic concentration of Fe3+ and Ho3+ [25], respectively.
2.2. Characterization X-ray diffraction (XRD) patterns of all samples were obtained at room temperature with a Holand X’pert PROMPD diffractometer (Cu K␣ radiation, ˚ which operated at 45 kV and 40 mA. UV–vis absorption spectra λ = 1.5406 A) of samples were recorded on UV-3000 spectrophotometer. High pure BaSO4 was used as a standard reagent. The concentration of methyl orange in solution was measured with a spectrophotometer (DR/2500, America HACH Company).
2.3. Measurement of photocatalytic efficiency Fig. 1. XRD patterns of samples. The photoreaction was conducted in a 250-mL cylindrical vessel with a water-cooled quartz jacket. Irradiation was provided by a 500-W high-pressure mercury lamp with major emission at 365 nm, located in the center of the quartz jacket. A magnetic stirrer was equipped at the bottom of the reactor to achieve effective dispersion. Air was bubbled through the reaction solution from the bottom to ensure a constant dissolved O2 concentration. To assess the photocatalytic activity of doping TiO2 , pure TiO2 powder was also tested. The amount of TiO2 powder chosen was 2 g L−1 , the initial methyl orange concentration was 40 mg L−1 , and 250 mL methyl orange solution was used for the photocatalytic degradation every time. The temperature of the reaction solution was maintained at 30 ± 0.5 ◦ C. Some reactive solution was withdrawn at different intervals. The residual concentration of methyl orange was measured at 465 nm with a DR/2500 spectrophotometer.
summarized in Table 1. The crystallite size decreases because of the doping, which implies that Fe3+ or Ho3+ doping restrains the increase of grain size and refines crystallite size. The change of crystal parameters becomes great in the doping samples compared with un-doping implied crystal matrix that could be expanded. The matrix distortion increases in the order of TiO2 , TF(2), TF(1)H(1) and TH(2). The radii of Fe3+ , Ti4+ , and Ho3+ for coordination number 6 are 0.069, 0.074, and 0.0908 nm, respectively. According to Pauling’s principle, it is easy for Fe3+ ion to cooperate with the matrix of the TiO2 nanoparticles without causing much crystalline distortion. However, the substitution of Ho3+ into the matrix of TiO2 will affect the coordination number and distort the matrix. So the distortion of TH(2) is maximal, TF(2) is minimal and TF(1)H(1) appears between them in these doping samples.
3. Experimental results and discussion
3.2. UV–vis absorption spectra
3.1. X-ray diffraction
Fig. 2 shows the UV–vis absorption spectra of Fe3+ -doping, Ho3+ -doping, co-doping and un-doping TiO2 , respectively. Compared with the spectrum of pure titania, a tiny blue-shift of the absorption profile in the Ho3+ -doping TiO2 and the red-shift in the Fe3+ -doping TiO2 are clearly observed. The sample of TF(0.05)H(0.5) shows the same red-shift and the shifted degree appears between them.
X-ray diffraction measurements (Fig. 1) show that all samples prepared have the anatase structure. In the TH(2) samples, the broadening of X-ray diffraction peaks is observed obviously compared with un-doping sample. (1 0 3), (0 0 4) and (1 1 2) peaks of anatase are conjoint and result in a broad diffraction peak. Also (1 0 5) and (2 1 1) XRD peaks trend to conjoin together. The X-ray diffraction peaks of TF(2) samples are similar to un-doping sample. The broad degree of peaks in TF(1)H(1) appears between of them. No characteristic peak of iron oxide or holmium oxide is found in the XRD patterns implying either Fe3+ and Ho3+ ions are incorporated in the crystallinity of TiO2 , or iron oxide and holmium oxide are very small and highly dispersed [9]. The average grain size is calculated from the broadening of the (1 0 1) XRD peak of anatase using Scherrer’s equation: D = Kλ/(β cos θ), and the distortion of the TiO2 matrices was also estimated from the XRD spectra using the formula: ε = β/4tgθ. The particle characteristics for the samples used in this study are
3.3. Photocatalytic activity The photocatalytic activity of the samples was measured by the degradation of aqueous solution of methyl orange without concerning the degradation intermediates in detail. Before irradiating with a UV light, the samples were stirred in dark for half an hour. Detection results showed that the methyl orange concentration had negligible decrease caused by the slight absorption on photocatalysts surface, which indicated that there was no degradation in the absence of irradiation.
Table 1 XRD analysis result of samples Doping element
Un-doping Fe Ho Fe/Ho
Doping content (%)
0 2 2 1/1
Crystal size (nm)
18.2 13.0 12.5 12.8
Matrix distortion (%)
0.8595 1.2040 1.2501 1.2267
Crystal parameter a (nm)
c (nm)
V (nm3 )
0.3892 0.3891 0.3898 0.3884
0.8084 0.8260 0.8127 0.8486
0.1225 0.1251 0.1235 0.1280
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photoexcited charge carriers. So the photocatalytic activity of TiO2 co-doped with Fe3+ and Ho3+ is markedly improved.
4. Conclusions
Fig. 2. UV–vis absorption spectra of samples.
The sol–gel methods offer successful routes to synthesize doping titania catalysts. Fe3+ -doping broadens the absorption profile, improves photo utilization of TiO2 , and then generates more electron–hole pairs. Ho3+ -doping restrains the increase of grain size, leads to crystal expansion and matrix distortion and retards the recombination of the photoexcited charge carriers. The photocatalytic activity of TiO2 co-doped with Fe3+ and Ho3+ ions is markedly improved due to the cooperative actions of the two dopants. It may be expected that photocatalytic activity can be further improved by choosing proper dopants. Acknowledgement This research was financially supported in part by the Corporation of Shtar Science & Technology. References
Fig. 3. Degradation curves of methyl orange with irradiation time.
The evolutions of methyl orange photodegradation with irradiation time are presented in Fig. 3. In comparison with pure TiO2 , doping improves the photocatalytic activity of TiO2 . The co-doping of Fe3+ and Ho3+ can markedly enhance the photocatalytic activity compared with un-doping and single-doping TiO2 . Such an improvement implies that there is a synergistic effect in the catalytic activity when both Fe3+ and Ho3+ are co-doped into the nanocrystalline TiO2 particles. The red-shift of the absorption profile is observed obviously and the ultraviolet absorption is increased in the Fe3+ -doping TiO2 (Fig. 2). It shows that Fe3+ -doping improves photoutilization of TiO2 , generates more electron–hole pairs under photo irradiation, which helps to improve the photocatalytic activity of TiO2 . Crystal expansion and matrix distortion in crystal matrix of TiO2 for Ho3+ doping are observed obviously in Fig. 1 and Table 1. The expansion in the crystal matrix creates oxygen vacancies, which generates shallow energy states in the bottom of the conduction band and served as electron trap site in nanocrystalline TiO2 . Meanwhile, shallow energy states introduced by rare earth ion in the top valence band served as hole trap sites. The separation of the charge carriers is attributed to such trapping. Subsequently, the charge carriers transfer to the surface of photocatalyst and arose redox reactions. So the photocatalytic activity of doping TiO2 is promoted. When both Fe3+ and Ho3+ are co-doped into the nanocrystalline TiO2 particles, a cooperative operation will be produced. Fe3+ -doping broadens the absorption profile, improves photo utilization of TiO2 , and then generates more electron–hole pairs. Ho3+ -doping restrains the increase of grain size, leads to crystal expansion and matrix distortion and retards the recombination of the
[1] A.S. Topalov, D.V. Sojic, D.A. Molnar-Gabor, B.F. Abramovic, M.I. Comor, Appl. Catal. B: Environ. 54 (2004) 125. [2] B. Zielinska, J. Grzechulska, A.W. Morawski, J. Photochem. Photobiol. A: Chem. 157 (2003) 65. [3] D. Vionea, C. Minero, V.M. Maurinoa, E. Carlotti, T. Picatonotto, E. Pelizzetti, Appl. Catal. B: Environ. 58 (2005) 79. [4] L. Zhang, T. Kanki, N. Sano, A. Toyoda, Sep. Purif. Technol. 31 (2003) 105. [5] R. Wang, J.H. Xin, Y. Yang, H. Liu, L. Xu, J. Hu, Appl. Surf. Sci. 227 (2004) 312. [6] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energy Mater. Sol. Cells 77 (2003) 65. [7] C. Wang, H. Shang, Y. Tao, T. Yuan, G. Zhang, Sep. Purif. Technol. 32 (2003) 357. [8] Y. Bessekhouad, D. Robert, J.V. Weber, J. Photochem. Photobiol. A: Chem. 163 (2004) 569. [9] J.C.S. Wu, C.H. Chen, J. Photochem. Photobiol. A: Chem. 163 (2004) 509. [10] Y. Yang, X. Li, J. Chen, L. Wang, J. Photochem. Photobiol. A: Chem. 163 (2004) 517. [11] C. Wang, C. Bottcher, D.W. Bahnemann, J.K. Dohrmann, J. Nanoparticle Res. 6 (2004) 119. [12] B. Sun, E.P. Reddy, P.G. Smirniotis, Appl. Catal. B: Environ. 57 (2005) 139. [13] W. Zhang, Y. Li, S. Zhu, F. Wang, Catal. Today 93–95 (2004) 589. [14] J. Xu, M. Lu, X. Guo, H. Li, J. Mol. Catal. A: Chem. 226 (2005) 123. [15] Y. Xie, C. Yuan, Mater. Res. Bull. 39 (2004) 533. [16] Y. Xie, C. Yuan, Appl. Catal. B: Environ. 46 (2003) 251. [17] K.V. Baiju, C.P. Sibu, K. Rajesh, P. Krishna Pillai, P. Mukundan, K.G.K. Warrier, W. Wunderlich, Mater. Chem. Phys. 90 (2005) 123. [18] A.W. Xu, Y. Gao, H.Q. Liu, J. Catal. 207 (2002) 151. [19] S.Y. Chen, C.C. Ting, W.F. Hsieh, Thin Solid Films 434 (2003) 171. [20] H. Wei, Y. Wu, N. Lun, F. Zhao, J. Mater. Sci. 39 (2004) 1305. [21] P. Yang, Mengkai, D. Xu, D. Yuan, C. Song, S. Liu, X. Cheng, Opt. Mater. 24 (2003) 497. [22] J. Wang, S. Yin, M. Komatsu, T. Sato, J. Eur. Ceram. Soc. 25 (2005) 3207. [23] Z. Zhang, C. Wang, R. Zakaria, J.Y. Ying, J. Phys. Chem. B 102 (1998) 10871. [24] Z.H. Yuan, J.H. Jia, L.D. Zhang, Mater. Chem. Phys. 73 (2002) 323. [25] P. Yang, C. Lu, N.P. Hua, Y.K. Du, Mater. Lett. 57 (2002) 794.
Materials Chemistry and Physics 106 (2007) 247–249
Influence of Fe3+ and Ho3+ co-doping on the photocatalytic activity of TiO2 Jian-Wen Shi a,b,∗ , Jing-Tang Zheng b , Yan Hu b , Yu-Cui Zhao b a
b
Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, Fujian 361003, China The State Key Laboratory of Heavy Oil, China University of Petroleum, Dongying, Shandong 257061, China Received 3 July 2006; received in revised form 3 May 2007; accepted 22 May 2007
Abstract The un-doping, single-doping and co-doping TiO2 nanoparticles have been prepared by sol–gel method with Ti(OC4 H9 )4 as a raw material and characterized by X-ray diffraction (XRD) and UV–vis absorption spectra, and their photocatalytic activities have been investigated by photocatalytic oxidation of methyl orange. It is found that Fe3+ -doping broadens the absorption profile, improves photoutilization of TiO2 , and then generates more electron–hole pairs. Ho3+ -doping restrains the increase of grain size, leads to crystal expansion and matrix distortion and retards the recombination of the photoexcited charge carriers. The photocatalytic activity of TiO2 co-doped with Fe3+ and Ho3+ ions is markedly improved due to the cooperative actions of the two dopants. © 2007 Elsevier B.V. All rights reserved. Keywords: TiO2 ; Photocatalysis; Doping; Iron; Holmium; Methyl orange
1. Introduction The photocatalytic oxidation of organic compounds by semiconductor catalysts such as TiO2 , ZnO and CdS has been extensively studied in the past decades [1–8]; of these TiO2 is widely used because of its optical and electronic properties, low cost, chemical stability and non-toxicity [4]. However, TiO2 utilizes only a very small fraction of the solar spectrum due to its band-gap energy, and the photocatalytic activity of TiO2 is limited by fast charge-carrier recombination and low interfacial charge-transfer rates of photogenerated carriers [24]. In order to improve its photocatalytic activity, many researches have been investigated. These results show that selective metal ion doping is one of the effective means. In the present, the investigations about doping elements focus mostly on transition metal ions doping [9–14]. Rare earth metals having incompletely occupied 4f and empty 5d orbitals often serve as catalyst or promote catalysis. Some results show that the photocatalytic activity of TiO2 can be improved because of the doping of some rare earth metals [15–18]. Co-doping with two kinds of ions have been investigated in recent years and activized the attention of most investigators
[19–21]. Wang et al. [22] found that under the irradiation of light with wavelengths longer than 400 and 290 nm, the photocatalytic activity of nitrogen and lanthanum co-doped SrTiO3 was 2.6 and 2 times greater than that of pure SrTiO3 . Zhang et al. [23] found that Pt deposited onto the Nb5+ /TiO2 increased the photodegradation activity of CHCl3 . These facts indicate that introducing two or more proper elements onto nanocrystalline TiO2 particles will improve the photocatalytic effect of TiO2 . Co-doping transition metal ions and rare earth metal into the nanocrystalline TiO2 may have a synergistic effect to increase the activity of TiO2 . In the work, Fe3+ and Ho3+ co-doping TiO2 was prepared by the sol–gel method. The photocatalytic activity was evaluated by photodegradation of methyl orange in solution. We find that the co-doping of Fe3+ and Ho3+ can obviously enhance the photocatalytic activity for the methyl orange degradation under UV irradiation compared with un-doping TiO2 and single-doping. The synergistic effect of two dopants that leads to the significant enhancement of photodegradation has also been discussed. 2. Experimental 2.1. Sol–gel synthesis of doping TiO2
∗ Corresponding author. Present address: Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, Fujian 361003, China. Tel.: +86 5468395024; fax: +86 5468395190. E-mail address: [email protected] (J.-W. Shi).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.05.042
A solution of Ti(OC4 H9 )4 , in anhydrous ethanol, was used as molecular precursor of TiO2 . In order to control the reaction kinetics, acetic acid was used as a chemical additive to moderate the reaction rate. The distilled water used for hydrolysis in solution with Ti(OC4 H9 )4 was added gradually
248
J.-W. Shi et al. / Materials Chemistry and Physics 106 (2007) 247–249
under magnetic stirring at room temperature. The molar ratio of these reactants was Ti(OC4 H9 )4 :H2 O:ethanol:acetic acid = 1:8:20:6. Then, the temperature was raised to 60 ◦ C and kept 6 h for obtaining gels. The gels were dried in a vacuum box at 80 ◦ C; the resulting dry gels were triturated in a mortar, and then were calcined in an oven at 500 ◦ C for 2 h to give TiO2 powder. Doping TiO2 nanoparticles were synthesized using almost the same method. The appropriate amount of Fe(NO3 )3 ·9H2 O or Ho(NO3 )3 ·6H2 O was dissolved in anhydrous ethanol, which was added to the distilled water prior to the hydrolysis of Ti(OC4 H9 )4 . For convenience, the samples were labeled as TF(X), TH(Y) for Fe3+ and Ho3+ doping TiO2 , and TF(X)H(Y) for Fe3+ and Ho3+ co-doping TiO2 , where X and Y referred to the nominal atomic concentration of Fe3+ and Ho3+ [25], respectively.
2.2. Characterization X-ray diffraction (XRD) patterns of all samples were obtained at room temperature with a Holand X’pert PROMPD diffractometer (Cu K␣ radiation, ˚ which operated at 45 kV and 40 mA. UV–vis absorption spectra λ = 1.5406 A) of samples were recorded on UV-3000 spectrophotometer. High pure BaSO4 was used as a standard reagent. The concentration of methyl orange in solution was measured with a spectrophotometer (DR/2500, America HACH Company).
2.3. Measurement of photocatalytic efficiency Fig. 1. XRD patterns of samples. The photoreaction was conducted in a 250-mL cylindrical vessel with a water-cooled quartz jacket. Irradiation was provided by a 500-W high-pressure mercury lamp with major emission at 365 nm, located in the center of the quartz jacket. A magnetic stirrer was equipped at the bottom of the reactor to achieve effective dispersion. Air was bubbled through the reaction solution from the bottom to ensure a constant dissolved O2 concentration. To assess the photocatalytic activity of doping TiO2 , pure TiO2 powder was also tested. The amount of TiO2 powder chosen was 2 g L−1 , the initial methyl orange concentration was 40 mg L−1 , and 250 mL methyl orange solution was used for the photocatalytic degradation every time. The temperature of the reaction solution was maintained at 30 ± 0.5 ◦ C. Some reactive solution was withdrawn at different intervals. The residual concentration of methyl orange was measured at 465 nm with a DR/2500 spectrophotometer.
summarized in Table 1. The crystallite size decreases because of the doping, which implies that Fe3+ or Ho3+ doping restrains the increase of grain size and refines crystallite size. The change of crystal parameters becomes great in the doping samples compared with un-doping implied crystal matrix that could be expanded. The matrix distortion increases in the order of TiO2 , TF(2), TF(1)H(1) and TH(2). The radii of Fe3+ , Ti4+ , and Ho3+ for coordination number 6 are 0.069, 0.074, and 0.0908 nm, respectively. According to Pauling’s principle, it is easy for Fe3+ ion to cooperate with the matrix of the TiO2 nanoparticles without causing much crystalline distortion. However, the substitution of Ho3+ into the matrix of TiO2 will affect the coordination number and distort the matrix. So the distortion of TH(2) is maximal, TF(2) is minimal and TF(1)H(1) appears between them in these doping samples.
3. Experimental results and discussion
3.2. UV–vis absorption spectra
3.1. X-ray diffraction
Fig. 2 shows the UV–vis absorption spectra of Fe3+ -doping, Ho3+ -doping, co-doping and un-doping TiO2 , respectively. Compared with the spectrum of pure titania, a tiny blue-shift of the absorption profile in the Ho3+ -doping TiO2 and the red-shift in the Fe3+ -doping TiO2 are clearly observed. The sample of TF(0.05)H(0.5) shows the same red-shift and the shifted degree appears between them.
X-ray diffraction measurements (Fig. 1) show that all samples prepared have the anatase structure. In the TH(2) samples, the broadening of X-ray diffraction peaks is observed obviously compared with un-doping sample. (1 0 3), (0 0 4) and (1 1 2) peaks of anatase are conjoint and result in a broad diffraction peak. Also (1 0 5) and (2 1 1) XRD peaks trend to conjoin together. The X-ray diffraction peaks of TF(2) samples are similar to un-doping sample. The broad degree of peaks in TF(1)H(1) appears between of them. No characteristic peak of iron oxide or holmium oxide is found in the XRD patterns implying either Fe3+ and Ho3+ ions are incorporated in the crystallinity of TiO2 , or iron oxide and holmium oxide are very small and highly dispersed [9]. The average grain size is calculated from the broadening of the (1 0 1) XRD peak of anatase using Scherrer’s equation: D = Kλ/(β cos θ), and the distortion of the TiO2 matrices was also estimated from the XRD spectra using the formula: ε = β/4tgθ. The particle characteristics for the samples used in this study are
3.3. Photocatalytic activity The photocatalytic activity of the samples was measured by the degradation of aqueous solution of methyl orange without concerning the degradation intermediates in detail. Before irradiating with a UV light, the samples were stirred in dark for half an hour. Detection results showed that the methyl orange concentration had negligible decrease caused by the slight absorption on photocatalysts surface, which indicated that there was no degradation in the absence of irradiation.
Table 1 XRD analysis result of samples Doping element
Un-doping Fe Ho Fe/Ho
Doping content (%)
0 2 2 1/1
Crystal size (nm)
18.2 13.0 12.5 12.8
Matrix distortion (%)
0.8595 1.2040 1.2501 1.2267
Crystal parameter a (nm)
c (nm)
V (nm3 )
0.3892 0.3891 0.3898 0.3884
0.8084 0.8260 0.8127 0.8486
0.1225 0.1251 0.1235 0.1280
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photoexcited charge carriers. So the photocatalytic activity of TiO2 co-doped with Fe3+ and Ho3+ is markedly improved.
4. Conclusions
Fig. 2. UV–vis absorption spectra of samples.
The sol–gel methods offer successful routes to synthesize doping titania catalysts. Fe3+ -doping broadens the absorption profile, improves photo utilization of TiO2 , and then generates more electron–hole pairs. Ho3+ -doping restrains the increase of grain size, leads to crystal expansion and matrix distortion and retards the recombination of the photoexcited charge carriers. The photocatalytic activity of TiO2 co-doped with Fe3+ and Ho3+ ions is markedly improved due to the cooperative actions of the two dopants. It may be expected that photocatalytic activity can be further improved by choosing proper dopants. Acknowledgement This research was financially supported in part by the Corporation of Shtar Science & Technology. References
Fig. 3. Degradation curves of methyl orange with irradiation time.
The evolutions of methyl orange photodegradation with irradiation time are presented in Fig. 3. In comparison with pure TiO2 , doping improves the photocatalytic activity of TiO2 . The co-doping of Fe3+ and Ho3+ can markedly enhance the photocatalytic activity compared with un-doping and single-doping TiO2 . Such an improvement implies that there is a synergistic effect in the catalytic activity when both Fe3+ and Ho3+ are co-doped into the nanocrystalline TiO2 particles. The red-shift of the absorption profile is observed obviously and the ultraviolet absorption is increased in the Fe3+ -doping TiO2 (Fig. 2). It shows that Fe3+ -doping improves photoutilization of TiO2 , generates more electron–hole pairs under photo irradiation, which helps to improve the photocatalytic activity of TiO2 . Crystal expansion and matrix distortion in crystal matrix of TiO2 for Ho3+ doping are observed obviously in Fig. 1 and Table 1. The expansion in the crystal matrix creates oxygen vacancies, which generates shallow energy states in the bottom of the conduction band and served as electron trap site in nanocrystalline TiO2 . Meanwhile, shallow energy states introduced by rare earth ion in the top valence band served as hole trap sites. The separation of the charge carriers is attributed to such trapping. Subsequently, the charge carriers transfer to the surface of photocatalyst and arose redox reactions. So the photocatalytic activity of doping TiO2 is promoted. When both Fe3+ and Ho3+ are co-doped into the nanocrystalline TiO2 particles, a cooperative operation will be produced. Fe3+ -doping broadens the absorption profile, improves photo utilization of TiO2 , and then generates more electron–hole pairs. Ho3+ -doping restrains the increase of grain size, leads to crystal expansion and matrix distortion and retards the recombination of the
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