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Effects of I and F codoped TiO2 on the photocatalytic degradation of methylene blue
Desalination 249 (2009) 621–625
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Effects of I and F codoped TiO2 on the photocatalytic degradation of methylene blue Chen Wen a,⁎, Yu-Jie Zhu a, Takaki Kanbara b, Hua-Zhang Zhu a, Chang-Fa Xiao a a b
Institute of Material & Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China Tsukuba Research Center for Interdisciplinary Materials Science, University of Tsukuba, Tsukuba 3058573, Japan
a r t i c l e
i n f o
Article history: Accepted 30 January 2009 Available online 6 October 2009 Keywords: I–F-codoped TiO2 Photocatalytic activity Sunlight Methylene blue
a b s t r a c t Nanocrystalline I–F-codoped TiO2 was prepared by a sol–gel-impregnation method, using tetrabutylorthotitanate in a mixed NH4I–NH4F aqueous solution. The as-prepared TiO2 was characterized with UV–vis diffuse reflectance spectra, X-ray diffraction and nitrogen adsorption. The degradation of methylene blue (MB) over asprepared TiO2 in aqueous solution under simulated sunlight irradiation was remarkably enhanced by codoping with I and F. The effects of codoping and calcination temperature on the photocatalytic activity and microstructures were investigated. The photocatalytic activity of as-prepared I–F-codoped TiO2 was remarkably higher than that of pure, I-doped, and F-doped TiO2 when the molar ratios of I and F to Ti were kept in the value of 10. The influence of I–F-modification on the photocatalytic activity was discussed by considering the higher surface area, entire anatase phase, effective dopant content, and stronger absorbance of sunlight, corresponding to the higher quantum efficiency. In addition to a complete removal of color, the as-prepared TiO2 was simultaneously able to oxidize MB and small amounts of intermediates such as formic acid and phenol were detected. After prolonged sunlight irradiation some intermediates almost vanished, and MB appeared to be − 2− eventually mineralized to NH+ 4 , NO3 and SO4 . © 2009 Elsevier B.V. All rights reserved.
1. Introduction In order to solve global energy crises and an increase of serious environmental pollution, catalytic techniques have been applied in various fields. Photocatalysis is one promising method that has great potential for conversion of photon energy into chemical energy and decomposition of pollutants in air and water. Among these catalysts, TiO2 has been proved to be a competent photocatalyst for environmental applications because of its strong oxidizing ability, nontoxicity and long-term stability [1–5]. However, TiO2 with a band gap of 3.0–3.2 eV can be photoexcited under irradiation of UV light (λ<395 nm), which is only about 2–4% of sunlight [6]. Therefore, considerable effort has been made to increase the absorption of TiO2 in the visible region to improve its visible light response through various surface modifications such as doping of various metal or metal oxides [7–11]. Developing highly active photocatalysts with a visible light response to use sunlight is very important, because visible light composes the major part of the solar spectrum. Recently, a considerable number of studies have been made on the crystal structures and properties of the nonmetal-doped TiO2, and they revealed a new possibility of developing of sunlight-driven photocatalysts [12–17]. Asahi et al.[12] reported that nitrogen (N) doping in
⁎ Corresponding author. Tel.: +86 22 24528055; fax: +86 22 24528054. E-mail address: [email protected] (C. Wen). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.01.028
TiO2 shifted its optical absorption and enhanced the photocatalytic activity such as the photodegradation of methylene blue and gaseous acetaldehyde in the visible region of λ < 500 nm. According to the pioneering works of Asahi et al. some groups also investigated doping in TiO2 such as sulfur (S), carbon (C), bromine (Br), chlorine (Cl) and iodine (I) [18–21]. Yu et al. found that F doping caused red shift in the absorption edge and this doped TiO2 showed higher activity on the photocatalytic oxidation of acetone under UV light [15]. Lou et al. reported that Br and Cl codoping could reduce the band gap energy of TiO2 and enhanced the photocatalytic activity for H2 and O2 production in Na2CO3 solution under UV–visible light [20]. Hong et al. have succeeded in synthesizing I-doped TiO2, which was used as a visible light induced photocatalyst for the degradation of phenol under visible light irradiation [17]. In addition, codoping of N and F in TiO2 gave TiOxNyFz, which had a band gap at 570 nm and was shown to be effective for water oxidation [22]. It is clear that these nonmetal elements, like N, C, Br, Cl, F and I, have been proved to be beneficial dopants in TiO2 to narrow band gap. This means that the doping process is to elevate the valence band of TiO2 into a more negative position by dopant substitution. Methylene blue (MB), a common organic dye, was selected as target compound because MB is ubiquitously used and the removal of the dye from wastewaters has been an acute problem [23]. Li et al. found that during liquid phase photocatalytic degradation of MB under visible light irradiation (>420 nm), the as-prepared S-doped TiO2 exhibited much higher activity than pure TiO2 [18]. We here report that I–F-codoped TiO2, synthesized by a sol–gel-impregnation
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method, is active for the photocatalytic degradation of MB under simulated sunlight, UV and visible lights. The influencing factors of photoactivity and degradation products of MB are also described.
MS (Agilent 6890 N) equipped with a fused silica capillary column (DB-5, 60 m long, 0.32 mm i.d.). The column temperature was raised from 40 °C to 260 °C at 5 °C/min during the measurement.
2. Experimental 3. Results and discussion 2.1. Catalyst preparation 3.1. Characterization of codoped TiO2 All chemicals were of reagent-grade without further purification. Deionized water was used for all the experiments. The I–F-codoped TiO2 was prepared by a sol–gel-impregnation method. The mixture containing tetrabutylorthotitanate (10 mL) and absolute ethyl alcohol (40 mL) was agitated for 30 min and was added dropwise to a mixture of absolute ethyl alcohol (10 mL) and acetic acid (10 mL) with vigorous stirring for 2 h at room temperature. After aged for 5 h, the samples were dried at 70 °C for 24 h to remove water and alcohol and then the residues were ground to fine particles. The required amounts of NH4I, NH4F, and their mixture were dissolved in 10 mL deionized water, which were added to the particles, respectively, under continuous stirring at ambient temperature for 6 h to obtain different doping xerogels. Then molar ratios of I, F and both of them to Ti, which hereafter were designated as Ri, Rf and Rif, were 10 nominal atomic. After dried under vacuum at 70 °C for 10 h, the xerogels were calcined at 500, 600, and 700 °C in air for 2 h, respectively. We also prepared pure TiO2 without any dopants through the same procedure. 2.2. Catalyst characterization The Brunauer–Emmett–Teller (BET) surface areas of as-prepared TiO2 were determined from N2 adsorption isotherms at −196 °C using the Quantachrome NOVA 1000e method. UV–vis diffused reflectance spectra (UV–vis-DRS) of as-prepared TiO2 were obtained for the dry-pressed disk samples using a UV–vis spectrophotometer (JASCO V-570, Japan). BaSO4 was used as a reflectance standard in a UV–vis diffuse reflectance experiment. The X-ray diffraction (XRD) patterns of as-prepared TiO2 were recorded on a Bruker D8 GADD Xray diffractometer using Cu Kα radiation to determine the identity of any phase present and their crystallite size. 2.3. Photocatalysis reaction The photocatalytic activity experiments of as-prepared TiO2 for degradation of MB in solution were performed at ambient temperature using a 500 mL reactor. The catalyst (0.1 g) was dispersed in the reactor containing 100 mL of an aqueous solution of 10 mg/L MB, and air was bubbled at a flow rate of 100 mL/min before and during irradiation. The light source was a 250 W metal halide lamp as a simulated sunlight and total intensity reaching the solution was 13.4 ± 10 mW/cm2 for 310–550 nm. For experiments under UV and visible lights, a 30 W UV lamp in the range 310–400 nm and a 150 W tungsten lamp equipped with an optical cutoff filter (λ >420 nm) was separately used as light source. The integrated UV and visible light intensities were 1.54 ± 0.1 mW/cm2 for 365 nm and 0.56 ± 0.01 mW/ cm2 for 450 nm, respectively. The progress of photocatalytic degradation of MB was monitored by measuring the absorbance in a UV– visible spectrophotometer (Hitachi UV-2000, λmax = 665 nm). Inorganic ions in the effluents were analyzed using a DX-120 ion chromatograph (IC) equipped with an AS14 and a CS12A columns (eluents: 3.5 mM Na2CO3 + 1.0 mM NaHCO3 for anions, 20 mM methanesulfonic acid for cations). To identify the intermediates produced during the degradation of MB, after 100 mL of MB solution (50 mg/L) was oxidized for 20 h and filtrated, the solution was loaded into a Waters Sep-pakcartridge and the adsorbed intermediates were eluted with CH3CN. This step is called as solid phase extraction method (SPE). The solution was concentrated to about 0.1 mL with N2 purge for GC–MS measurement. The intermediates were examined with GC–
The specific surface areas of I–F–TiO2 with Rif = 10 calcined at 500, 600 and 700 °C were measured by BET method. The calculated surface area was 127 m2/g for 500 °C, 96 m2/g for 600 °C, and 54 m2/g for 700 °C, respectively. The greater surface area of I–F–TiO2 (500 °C) was attributed to the low calcination temperature. The UV–vis absorption spectra of as-prepared TiO2 with different R are shown in Fig. 1. As can be seen, a significant increase in the absorption at wavelengths shorter than 400 nm can be assigned to the intrinsic band gap absorption of TiO2 (about 3.2 eV). Compared to pure TiO2, the absorption spectra of I-doped, F-doped, and codoped TiO2 reveal not only the stronger absorption below ca. 400 nm region but also the red shifts of the absorption edge from 400 nm to the entire visible region. Especially, a stronger blue shift of I–F-codoped TiO2 with Rif = 10 in the UV light region is due to the well-known quantum-size effect for semiconductors [24], while its red shift in the visible light region is ascribed to the fact that doping with I and F can extend the absorption spectrum of TiO2 to the visible light range, narrow the band gap, and enhance the photocatalytic activity. This result is consistent with the Br and Cl codoping in TiO2 reported by Luo et al. [20]. It was found that only the doping of I atoms cause some significant shift in the fundamental absorption edge of TiO2 [17]. This conclusion is consistent with the calculated result for I-doped TiO2 reported by Liu et al., who claimed that the doped-I atoms affect the optical absorption property of TiO2 [25]. Huang et al. reported that F-doped in TiO2 converts some Ti4+ to Ti3+ by charge compensation [26]. However, the Ti3+ surface states form a donor level between the band gaps of TiO2, which would improve its visible light absorption, and also could induce a visible light response by the creation of oxygen vacancies (F and F+ centers) [15,16,26]. Several beneficial effects on visible light absorption as a synergetic effect of doped I and F atoms were produced. The absorption edge shifted toward longer wavelengths, indicating a decrease in the band gap energy of TiO2 and that more photogenerated electrons and holes could participate in the photocatalytic reactions.
Fig. 1. UV–vis absorption spectra of as-prepared TiO2 with different R calcined at 500 °C for 2 h. (a) TiO2; (b) I–TiO2 with Ri = 10; (c) F–TiO2 with Rf = 10; (d) I–F–TiO2 with Rif = 10.
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codoped TiO2. It can be seen that the calcination temperature influences the phase and the phase composition of TiO2 significantly. With increasing calcination temperature (from 500 °C to 600 °C), the peak intensities of anatase phase increase and the width of the diffraction peak becomes narrower, respectively, indicating that codoped TiO2 is well crystallized under the present preparation condition. In addition, it has also demonstrated that a small amount of rutile phase can be formed by being calcined at 700 °C, but the brookite phase was not observed. TiO2 generally contains three naturally occurring polymorphs: anatase, rutile, and brookite. Each structure exhibits different physical properties and has different applications [20]. Anatase TiO2 with lower agglomeration and smaller crystallite size usually shows higher photocatalytic activity than that of rutile TiO2 since rutile is normally prepared by calcination of anatase at high temperatures [27]. Hence these results imply that codoping with I and F in TiO2 not only suppresses the formation of any impurity phase but also prevents phase transition from anatase to rutile. 3.2. Photocatalytic activity of codoped TiO2
Fig. 2. X-ray diffraction (XRD) patterns of as-prepared TiO2. (A) TiO2 with different dopants calcined at 500 °C for 2 h; (B) I–F–TiO2 with Rif = 10 calcined at different temperatures for 2 h.
The crystalline phases of as-prepared TiO2 were determined by XRD, and the corresponding diffraction patterns are shown in Fig. 2. The percentage of rutile phase was calculated by the following equation [25]: Rutile = 1 = ð1 + 0:884 IA = IR Þ
ð1Þ
Anatase = 1−Rutile
ð2Þ
where IA and IR are the peak area for major anatase (101) and rutile phase (110), respectively. In Fig. 2(A), the doped TiO2 samples display only anatase phase with the peaks at 25.31°, 37.90°, 48.02° and 54.62°, respectively, which are generally considered more photoreactive, but pure TiO2 exhibits anatase (70%)–rutile (30%) mixture. The average crystallite size (d/nm) was estimated by applying the Scherrer formula (d = 0.89 λ/β cos θ; λ is the wavelength of X-ray radiation; β is the full width at half-maximum; θ is the diffraction angle) on the anatase (101) and rutile (110) diffraction peaks. The calculated d values are 10, 12, 20, and 51 nm for I-doped, I–F-codoped, F-doped, and pure TiO2, respectively. These results suggest that phase composition and crystallite size of TiO2 can be controlled by simply codoping with I and F in the synthetic process, making it extremely useful for the systematic study of the photocatalytic activity of TiO2. Fig. 2(B) indicates the effects of calcination temperature on phase structures of
To compare the photocatalytic activity of as-prepared TiO2, the degradation of MB as a test reaction was carried out, and the results are shown in Fig. 3. As can be seen, there was no obvious degradation of MB in the absence of catalyst under simulated sunlight irradiation including UV and visible lights, suggesting that direct photolysis of MB was negligible. However, the doped TiO2 samples show much better photocatalytic activity than pure TiO2. Furthermore, the I–F-codoped TiO2 is superior to the F-doped or I-doped TiO2 in both cases. The difference in the photocatalytic activity can be ascribed to higher surface area and additional stronger absorbance in the visible light range by doping with I and F. Similar results have also been observed for the F or I doping in TiO2 [15–17,25]. It is known that the photocatalytic activity of TiO2 is phase dependent. Anatase as a metastable phase exhibits the most activity in photocatalysis and the most stable rutile phase shows less activity or no activity at all [25]. Therefore, the higher photocatalytic activity of I–F-codoped TiO2 calcined at 500 °C is partially due to its high surface area and entire anatase phase. The degradation profile of MB on light source is often regarded as an important factor to distinguish if the degradation is driven really by light. Fig. 4 displays the degradation of MB under UV and visible lights
Fig. 3. Photocatalytic degradation of MB over as-prepared TiO2 with different dopants and calcined at 500 °C for 2 h under simulated sunlight irradiation. (○) without TiO2; (□) pure TiO2; (●) I–TiO2 with Ri = 10; (▲) F–TiO2 with Rf = 10; (■) I–F–TiO2 with Rif = 10; MB concentration = 10 mg/L; catalyst amount = 1 g/L.
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inner region of the photocatalyst to its surface, thus accelerating the separation of photogenarated electrons and holes. 3.3. Mineralization
Fig. 4. Effects of light source on the photocatalytic degradation of MB over as-prepared TiO2 with Rif = 10 and calcined at 500 °C for 2 h under UV and visible light irradiations. MB concentration = 10 mg/L; catalyst amount = 1 g/L.
in the presence of pure and codoped TiO2. It can be found that the photocatalytic activity of codoped TiO2 was higher than pure TiO2 under UV and visible lights. This indicates that doping with I and F makes an effective contribution to the activity of TiO2; it can be related to its absorption spectra and an entire anatase phase presented in Figs. 1 and 2, respectively. In view of these facts, it is easy to understand the increase in order of the degradation of MB over codoped TiO2 under simulated sunlight irradiation as shown in Fig. 3. The simulated sunlight induced degradation of MB over codoped TiO2 is shown in Fig. 5. It can be seen that, there was not a significant change in the degradation rate of MB as the calcination temperature was increased from 500 °C to 600 °C. The degradation rates were higher in the cases of codoped TiO2 calcined at 500 and 600 °C, compared to that of one calcined at 700 °C. This is due to the presence of higher surface area. It is well known from the literature that calcination temperature is an important factor that probably influences the crystallinity, morphology, and surface area of TiO2, which can clearly affect the photocatalytic activity [28,29]. Our previous research demonstrated higher photocatalytic activity for TiO2 with lower ratio of rutile to anatase [30]. The I–F-codoped TiO2 (500 °C) needs less time for photogenerated carriers to diffuse the
Fig. 5. Photocatalytic degradation of MB over I–F–TiO2 with Rif = 10 calcined at different temperatures under simulated sunlight irradiation. MB concentration = 10 mg/L; catalyst amount= 1 g/L.
To analyze the intermediates after the photocatalytic degradation of MB, a higher initial concentration of MB is necessary because both GC–MS and IC have different limits of quantitative determination. Therefore, 50 mg/L MB as initial concentration was selected during this experiment process. The photocatalytic degradation of MB leads to the conversion of heteroatoms nitrogen and sulfur into inorganic − 2− ions, such as NH+ 4 , NO3 and SO4 , respectively. Fig. 6 shows the timecourse curves of the evolution of inorganic ion during the degradation of MB over I–F–TiO2 under simulated sunlight irradiation. The 2− indicates a substantial mineralization evolution of NH+ 4 and SO4 degree with a non-nil formation rate. Thioether unit involved in the centered aromatic ring seems to undergo a direct oxidation from the oxidation state −2 to the highest final stable +6 one in SO2− 4 . The attached figure in Fig. 6 revealed that the degradation of MB increases with increase in irradiation time and it proceeds with a slow rate after 10 h irradiation. This may be due to the formation of intermediates and its competitiveness with parent MB molecules for photocatalytic degradation. About 95% MB was decolorized after illumination for 20 h. Moreover, after further extending over irradiation time (ca. increased in the effluent, and its 20 h), the concentration of SO2− 4 value reached ca. 10.58 mg/L, which indicated that ca. 62.57% of sulfur 2− from MB was converted into SO2− 4 . However, the quantity of SO4 released is lower than that of 16.91 mg/L expected from stoichiometry in the MB solution with the same concentration owing to the presence of the partial irreversible absorption between SO2− 4 and codoped TiO2. A similar observation in photodegradation of MB using TiO2 particles has been reported by Lachheb et al. [31]. As shown in Fig. 6, the mineralization of the nitrogen atoms contained in MB resulted in the formation of NH+ 4 , and with increasing irradiation time, the concentration of NH+ 4 increased in the effluent and it finally went up to 8.05 mg/L. Meanwhile, the NO− 3 accumulated gradually in the effluent; its concentration approached only 1.91 mg/L after the irradiation finished. Thus the NO− 3 was considered to be formed very slowly because its variation related to the concentration of NH+ 4 , which can be transformed subsequently into NO− 3 after being completely decolorized. In view of these facts,
Fig. 6. Evolution of inorganic ions in effluent under simulated sunlight irradiation during the degradation of MB over I–F–TiO2 with Rif = 10 calcined at 500 °C for 2 h. MB concentration = 50 mg/L; catalyst = 1 g/L.
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Fig. 7. Plausible degradation processes of MB over I–F–TiO2 with Rif = 10 calcined at 500 °C for 2 h.
we could deduce that MB should have been gradually decomposed to many fragments from the increase of inorganic ion concentration during the degradation process. Any intermediates generated were also analyzed by GC–MS and assigned by comparison with commercial standards. In the present experiment, the final products were detected at m/z = 46 and 94, corresponding to structures 1 and 2 in Fig. 7. However, the structures of other products such as a, b, and c have been separately reported by Gnaser et al. and Houas et al. based on their experimental results [23,32]. 4. Conclusions The I–F-codoped TiO2 was prepared by a sol–gel-impregnation method using tetrabutylorthotitanate as precursor. The I–F–TiO2 with Rif = 10 calcined at 500 and 600 °C was identified as being composed of a single anatase phase, demonstrating a higher sunlight induced catalytic activity for the degradation of MB compared with pure and single doped TiO2. The effective activity is ascribed to a synergetic effect of the doped iodine and fluorine atoms, which improved the UV–visible light absorption and led to greater crystallinity and smaller mean diameter in TiO2. The present work also provides useful information on understanding the mechanism of photocatalytic degradation of dyes and synthesizing a photocatalyst with higher activity. The photocatalytic degradation of MB over I–F–TiO2 could be successfully decolorized and degraded. Inorganic ions such as NH+ 4 , 2− NO− 3 and SO4 and other organic compounds such as formic acid and phenol were also observed, respectively, which means an effective mineralization degree. Research in this work is addressed to optimize the preparation of I–F-codoped TiO2 for its application in photocatalysis of water pollutants under sunlight irradiation. Acknowledgment This work was supported by the Science Foundation of Chinese Key Science and Technology 973 Subject (No. 2006CB708602). References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [2] C.H. Li, Y.H. Hsieh, W.T. Chiu, C.C. Liu, C.L. Kao, Study on preparation and photocatalytic performance of Ag/TiO2 and Pt/TiO2 phototocatalysts, Sep. Purif. Technol. 58 (2007) 148–151. [3] D.F. Ollis, C.S. Turchi, Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack, Environ. Prog. 9 (1990) 229–234.
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Effects of I and F codoped TiO2 on the photocatalytic degradation of methylene blue Chen Wen a,⁎, Yu-Jie Zhu a, Takaki Kanbara b, Hua-Zhang Zhu a, Chang-Fa Xiao a a b
Institute of Material & Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China Tsukuba Research Center for Interdisciplinary Materials Science, University of Tsukuba, Tsukuba 3058573, Japan
a r t i c l e
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Article history: Accepted 30 January 2009 Available online 6 October 2009 Keywords: I–F-codoped TiO2 Photocatalytic activity Sunlight Methylene blue
a b s t r a c t Nanocrystalline I–F-codoped TiO2 was prepared by a sol–gel-impregnation method, using tetrabutylorthotitanate in a mixed NH4I–NH4F aqueous solution. The as-prepared TiO2 was characterized with UV–vis diffuse reflectance spectra, X-ray diffraction and nitrogen adsorption. The degradation of methylene blue (MB) over asprepared TiO2 in aqueous solution under simulated sunlight irradiation was remarkably enhanced by codoping with I and F. The effects of codoping and calcination temperature on the photocatalytic activity and microstructures were investigated. The photocatalytic activity of as-prepared I–F-codoped TiO2 was remarkably higher than that of pure, I-doped, and F-doped TiO2 when the molar ratios of I and F to Ti were kept in the value of 10. The influence of I–F-modification on the photocatalytic activity was discussed by considering the higher surface area, entire anatase phase, effective dopant content, and stronger absorbance of sunlight, corresponding to the higher quantum efficiency. In addition to a complete removal of color, the as-prepared TiO2 was simultaneously able to oxidize MB and small amounts of intermediates such as formic acid and phenol were detected. After prolonged sunlight irradiation some intermediates almost vanished, and MB appeared to be − 2− eventually mineralized to NH+ 4 , NO3 and SO4 . © 2009 Elsevier B.V. All rights reserved.
1. Introduction In order to solve global energy crises and an increase of serious environmental pollution, catalytic techniques have been applied in various fields. Photocatalysis is one promising method that has great potential for conversion of photon energy into chemical energy and decomposition of pollutants in air and water. Among these catalysts, TiO2 has been proved to be a competent photocatalyst for environmental applications because of its strong oxidizing ability, nontoxicity and long-term stability [1–5]. However, TiO2 with a band gap of 3.0–3.2 eV can be photoexcited under irradiation of UV light (λ<395 nm), which is only about 2–4% of sunlight [6]. Therefore, considerable effort has been made to increase the absorption of TiO2 in the visible region to improve its visible light response through various surface modifications such as doping of various metal or metal oxides [7–11]. Developing highly active photocatalysts with a visible light response to use sunlight is very important, because visible light composes the major part of the solar spectrum. Recently, a considerable number of studies have been made on the crystal structures and properties of the nonmetal-doped TiO2, and they revealed a new possibility of developing of sunlight-driven photocatalysts [12–17]. Asahi et al.[12] reported that nitrogen (N) doping in
⁎ Corresponding author. Tel.: +86 22 24528055; fax: +86 22 24528054. E-mail address: [email protected] (C. Wen). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.01.028
TiO2 shifted its optical absorption and enhanced the photocatalytic activity such as the photodegradation of methylene blue and gaseous acetaldehyde in the visible region of λ < 500 nm. According to the pioneering works of Asahi et al. some groups also investigated doping in TiO2 such as sulfur (S), carbon (C), bromine (Br), chlorine (Cl) and iodine (I) [18–21]. Yu et al. found that F doping caused red shift in the absorption edge and this doped TiO2 showed higher activity on the photocatalytic oxidation of acetone under UV light [15]. Lou et al. reported that Br and Cl codoping could reduce the band gap energy of TiO2 and enhanced the photocatalytic activity for H2 and O2 production in Na2CO3 solution under UV–visible light [20]. Hong et al. have succeeded in synthesizing I-doped TiO2, which was used as a visible light induced photocatalyst for the degradation of phenol under visible light irradiation [17]. In addition, codoping of N and F in TiO2 gave TiOxNyFz, which had a band gap at 570 nm and was shown to be effective for water oxidation [22]. It is clear that these nonmetal elements, like N, C, Br, Cl, F and I, have been proved to be beneficial dopants in TiO2 to narrow band gap. This means that the doping process is to elevate the valence band of TiO2 into a more negative position by dopant substitution. Methylene blue (MB), a common organic dye, was selected as target compound because MB is ubiquitously used and the removal of the dye from wastewaters has been an acute problem [23]. Li et al. found that during liquid phase photocatalytic degradation of MB under visible light irradiation (>420 nm), the as-prepared S-doped TiO2 exhibited much higher activity than pure TiO2 [18]. We here report that I–F-codoped TiO2, synthesized by a sol–gel-impregnation
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method, is active for the photocatalytic degradation of MB under simulated sunlight, UV and visible lights. The influencing factors of photoactivity and degradation products of MB are also described.
MS (Agilent 6890 N) equipped with a fused silica capillary column (DB-5, 60 m long, 0.32 mm i.d.). The column temperature was raised from 40 °C to 260 °C at 5 °C/min during the measurement.
2. Experimental 3. Results and discussion 2.1. Catalyst preparation 3.1. Characterization of codoped TiO2 All chemicals were of reagent-grade without further purification. Deionized water was used for all the experiments. The I–F-codoped TiO2 was prepared by a sol–gel-impregnation method. The mixture containing tetrabutylorthotitanate (10 mL) and absolute ethyl alcohol (40 mL) was agitated for 30 min and was added dropwise to a mixture of absolute ethyl alcohol (10 mL) and acetic acid (10 mL) with vigorous stirring for 2 h at room temperature. After aged for 5 h, the samples were dried at 70 °C for 24 h to remove water and alcohol and then the residues were ground to fine particles. The required amounts of NH4I, NH4F, and their mixture were dissolved in 10 mL deionized water, which were added to the particles, respectively, under continuous stirring at ambient temperature for 6 h to obtain different doping xerogels. Then molar ratios of I, F and both of them to Ti, which hereafter were designated as Ri, Rf and Rif, were 10 nominal atomic. After dried under vacuum at 70 °C for 10 h, the xerogels were calcined at 500, 600, and 700 °C in air for 2 h, respectively. We also prepared pure TiO2 without any dopants through the same procedure. 2.2. Catalyst characterization The Brunauer–Emmett–Teller (BET) surface areas of as-prepared TiO2 were determined from N2 adsorption isotherms at −196 °C using the Quantachrome NOVA 1000e method. UV–vis diffused reflectance spectra (UV–vis-DRS) of as-prepared TiO2 were obtained for the dry-pressed disk samples using a UV–vis spectrophotometer (JASCO V-570, Japan). BaSO4 was used as a reflectance standard in a UV–vis diffuse reflectance experiment. The X-ray diffraction (XRD) patterns of as-prepared TiO2 were recorded on a Bruker D8 GADD Xray diffractometer using Cu Kα radiation to determine the identity of any phase present and their crystallite size. 2.3. Photocatalysis reaction The photocatalytic activity experiments of as-prepared TiO2 for degradation of MB in solution were performed at ambient temperature using a 500 mL reactor. The catalyst (0.1 g) was dispersed in the reactor containing 100 mL of an aqueous solution of 10 mg/L MB, and air was bubbled at a flow rate of 100 mL/min before and during irradiation. The light source was a 250 W metal halide lamp as a simulated sunlight and total intensity reaching the solution was 13.4 ± 10 mW/cm2 for 310–550 nm. For experiments under UV and visible lights, a 30 W UV lamp in the range 310–400 nm and a 150 W tungsten lamp equipped with an optical cutoff filter (λ >420 nm) was separately used as light source. The integrated UV and visible light intensities were 1.54 ± 0.1 mW/cm2 for 365 nm and 0.56 ± 0.01 mW/ cm2 for 450 nm, respectively. The progress of photocatalytic degradation of MB was monitored by measuring the absorbance in a UV– visible spectrophotometer (Hitachi UV-2000, λmax = 665 nm). Inorganic ions in the effluents were analyzed using a DX-120 ion chromatograph (IC) equipped with an AS14 and a CS12A columns (eluents: 3.5 mM Na2CO3 + 1.0 mM NaHCO3 for anions, 20 mM methanesulfonic acid for cations). To identify the intermediates produced during the degradation of MB, after 100 mL of MB solution (50 mg/L) was oxidized for 20 h and filtrated, the solution was loaded into a Waters Sep-pakcartridge and the adsorbed intermediates were eluted with CH3CN. This step is called as solid phase extraction method (SPE). The solution was concentrated to about 0.1 mL with N2 purge for GC–MS measurement. The intermediates were examined with GC–
The specific surface areas of I–F–TiO2 with Rif = 10 calcined at 500, 600 and 700 °C were measured by BET method. The calculated surface area was 127 m2/g for 500 °C, 96 m2/g for 600 °C, and 54 m2/g for 700 °C, respectively. The greater surface area of I–F–TiO2 (500 °C) was attributed to the low calcination temperature. The UV–vis absorption spectra of as-prepared TiO2 with different R are shown in Fig. 1. As can be seen, a significant increase in the absorption at wavelengths shorter than 400 nm can be assigned to the intrinsic band gap absorption of TiO2 (about 3.2 eV). Compared to pure TiO2, the absorption spectra of I-doped, F-doped, and codoped TiO2 reveal not only the stronger absorption below ca. 400 nm region but also the red shifts of the absorption edge from 400 nm to the entire visible region. Especially, a stronger blue shift of I–F-codoped TiO2 with Rif = 10 in the UV light region is due to the well-known quantum-size effect for semiconductors [24], while its red shift in the visible light region is ascribed to the fact that doping with I and F can extend the absorption spectrum of TiO2 to the visible light range, narrow the band gap, and enhance the photocatalytic activity. This result is consistent with the Br and Cl codoping in TiO2 reported by Luo et al. [20]. It was found that only the doping of I atoms cause some significant shift in the fundamental absorption edge of TiO2 [17]. This conclusion is consistent with the calculated result for I-doped TiO2 reported by Liu et al., who claimed that the doped-I atoms affect the optical absorption property of TiO2 [25]. Huang et al. reported that F-doped in TiO2 converts some Ti4+ to Ti3+ by charge compensation [26]. However, the Ti3+ surface states form a donor level between the band gaps of TiO2, which would improve its visible light absorption, and also could induce a visible light response by the creation of oxygen vacancies (F and F+ centers) [15,16,26]. Several beneficial effects on visible light absorption as a synergetic effect of doped I and F atoms were produced. The absorption edge shifted toward longer wavelengths, indicating a decrease in the band gap energy of TiO2 and that more photogenerated electrons and holes could participate in the photocatalytic reactions.
Fig. 1. UV–vis absorption spectra of as-prepared TiO2 with different R calcined at 500 °C for 2 h. (a) TiO2; (b) I–TiO2 with Ri = 10; (c) F–TiO2 with Rf = 10; (d) I–F–TiO2 with Rif = 10.
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codoped TiO2. It can be seen that the calcination temperature influences the phase and the phase composition of TiO2 significantly. With increasing calcination temperature (from 500 °C to 600 °C), the peak intensities of anatase phase increase and the width of the diffraction peak becomes narrower, respectively, indicating that codoped TiO2 is well crystallized under the present preparation condition. In addition, it has also demonstrated that a small amount of rutile phase can be formed by being calcined at 700 °C, but the brookite phase was not observed. TiO2 generally contains three naturally occurring polymorphs: anatase, rutile, and brookite. Each structure exhibits different physical properties and has different applications [20]. Anatase TiO2 with lower agglomeration and smaller crystallite size usually shows higher photocatalytic activity than that of rutile TiO2 since rutile is normally prepared by calcination of anatase at high temperatures [27]. Hence these results imply that codoping with I and F in TiO2 not only suppresses the formation of any impurity phase but also prevents phase transition from anatase to rutile. 3.2. Photocatalytic activity of codoped TiO2
Fig. 2. X-ray diffraction (XRD) patterns of as-prepared TiO2. (A) TiO2 with different dopants calcined at 500 °C for 2 h; (B) I–F–TiO2 with Rif = 10 calcined at different temperatures for 2 h.
The crystalline phases of as-prepared TiO2 were determined by XRD, and the corresponding diffraction patterns are shown in Fig. 2. The percentage of rutile phase was calculated by the following equation [25]: Rutile = 1 = ð1 + 0:884 IA = IR Þ
ð1Þ
Anatase = 1−Rutile
ð2Þ
where IA and IR are the peak area for major anatase (101) and rutile phase (110), respectively. In Fig. 2(A), the doped TiO2 samples display only anatase phase with the peaks at 25.31°, 37.90°, 48.02° and 54.62°, respectively, which are generally considered more photoreactive, but pure TiO2 exhibits anatase (70%)–rutile (30%) mixture. The average crystallite size (d/nm) was estimated by applying the Scherrer formula (d = 0.89 λ/β cos θ; λ is the wavelength of X-ray radiation; β is the full width at half-maximum; θ is the diffraction angle) on the anatase (101) and rutile (110) diffraction peaks. The calculated d values are 10, 12, 20, and 51 nm for I-doped, I–F-codoped, F-doped, and pure TiO2, respectively. These results suggest that phase composition and crystallite size of TiO2 can be controlled by simply codoping with I and F in the synthetic process, making it extremely useful for the systematic study of the photocatalytic activity of TiO2. Fig. 2(B) indicates the effects of calcination temperature on phase structures of
To compare the photocatalytic activity of as-prepared TiO2, the degradation of MB as a test reaction was carried out, and the results are shown in Fig. 3. As can be seen, there was no obvious degradation of MB in the absence of catalyst under simulated sunlight irradiation including UV and visible lights, suggesting that direct photolysis of MB was negligible. However, the doped TiO2 samples show much better photocatalytic activity than pure TiO2. Furthermore, the I–F-codoped TiO2 is superior to the F-doped or I-doped TiO2 in both cases. The difference in the photocatalytic activity can be ascribed to higher surface area and additional stronger absorbance in the visible light range by doping with I and F. Similar results have also been observed for the F or I doping in TiO2 [15–17,25]. It is known that the photocatalytic activity of TiO2 is phase dependent. Anatase as a metastable phase exhibits the most activity in photocatalysis and the most stable rutile phase shows less activity or no activity at all [25]. Therefore, the higher photocatalytic activity of I–F-codoped TiO2 calcined at 500 °C is partially due to its high surface area and entire anatase phase. The degradation profile of MB on light source is often regarded as an important factor to distinguish if the degradation is driven really by light. Fig. 4 displays the degradation of MB under UV and visible lights
Fig. 3. Photocatalytic degradation of MB over as-prepared TiO2 with different dopants and calcined at 500 °C for 2 h under simulated sunlight irradiation. (○) without TiO2; (□) pure TiO2; (●) I–TiO2 with Ri = 10; (▲) F–TiO2 with Rf = 10; (■) I–F–TiO2 with Rif = 10; MB concentration = 10 mg/L; catalyst amount = 1 g/L.
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inner region of the photocatalyst to its surface, thus accelerating the separation of photogenarated electrons and holes. 3.3. Mineralization
Fig. 4. Effects of light source on the photocatalytic degradation of MB over as-prepared TiO2 with Rif = 10 and calcined at 500 °C for 2 h under UV and visible light irradiations. MB concentration = 10 mg/L; catalyst amount = 1 g/L.
in the presence of pure and codoped TiO2. It can be found that the photocatalytic activity of codoped TiO2 was higher than pure TiO2 under UV and visible lights. This indicates that doping with I and F makes an effective contribution to the activity of TiO2; it can be related to its absorption spectra and an entire anatase phase presented in Figs. 1 and 2, respectively. In view of these facts, it is easy to understand the increase in order of the degradation of MB over codoped TiO2 under simulated sunlight irradiation as shown in Fig. 3. The simulated sunlight induced degradation of MB over codoped TiO2 is shown in Fig. 5. It can be seen that, there was not a significant change in the degradation rate of MB as the calcination temperature was increased from 500 °C to 600 °C. The degradation rates were higher in the cases of codoped TiO2 calcined at 500 and 600 °C, compared to that of one calcined at 700 °C. This is due to the presence of higher surface area. It is well known from the literature that calcination temperature is an important factor that probably influences the crystallinity, morphology, and surface area of TiO2, which can clearly affect the photocatalytic activity [28,29]. Our previous research demonstrated higher photocatalytic activity for TiO2 with lower ratio of rutile to anatase [30]. The I–F-codoped TiO2 (500 °C) needs less time for photogenerated carriers to diffuse the
Fig. 5. Photocatalytic degradation of MB over I–F–TiO2 with Rif = 10 calcined at different temperatures under simulated sunlight irradiation. MB concentration = 10 mg/L; catalyst amount= 1 g/L.
To analyze the intermediates after the photocatalytic degradation of MB, a higher initial concentration of MB is necessary because both GC–MS and IC have different limits of quantitative determination. Therefore, 50 mg/L MB as initial concentration was selected during this experiment process. The photocatalytic degradation of MB leads to the conversion of heteroatoms nitrogen and sulfur into inorganic − 2− ions, such as NH+ 4 , NO3 and SO4 , respectively. Fig. 6 shows the timecourse curves of the evolution of inorganic ion during the degradation of MB over I–F–TiO2 under simulated sunlight irradiation. The 2− indicates a substantial mineralization evolution of NH+ 4 and SO4 degree with a non-nil formation rate. Thioether unit involved in the centered aromatic ring seems to undergo a direct oxidation from the oxidation state −2 to the highest final stable +6 one in SO2− 4 . The attached figure in Fig. 6 revealed that the degradation of MB increases with increase in irradiation time and it proceeds with a slow rate after 10 h irradiation. This may be due to the formation of intermediates and its competitiveness with parent MB molecules for photocatalytic degradation. About 95% MB was decolorized after illumination for 20 h. Moreover, after further extending over irradiation time (ca. increased in the effluent, and its 20 h), the concentration of SO2− 4 value reached ca. 10.58 mg/L, which indicated that ca. 62.57% of sulfur 2− from MB was converted into SO2− 4 . However, the quantity of SO4 released is lower than that of 16.91 mg/L expected from stoichiometry in the MB solution with the same concentration owing to the presence of the partial irreversible absorption between SO2− 4 and codoped TiO2. A similar observation in photodegradation of MB using TiO2 particles has been reported by Lachheb et al. [31]. As shown in Fig. 6, the mineralization of the nitrogen atoms contained in MB resulted in the formation of NH+ 4 , and with increasing irradiation time, the concentration of NH+ 4 increased in the effluent and it finally went up to 8.05 mg/L. Meanwhile, the NO− 3 accumulated gradually in the effluent; its concentration approached only 1.91 mg/L after the irradiation finished. Thus the NO− 3 was considered to be formed very slowly because its variation related to the concentration of NH+ 4 , which can be transformed subsequently into NO− 3 after being completely decolorized. In view of these facts,
Fig. 6. Evolution of inorganic ions in effluent under simulated sunlight irradiation during the degradation of MB over I–F–TiO2 with Rif = 10 calcined at 500 °C for 2 h. MB concentration = 50 mg/L; catalyst = 1 g/L.
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Fig. 7. Plausible degradation processes of MB over I–F–TiO2 with Rif = 10 calcined at 500 °C for 2 h.
we could deduce that MB should have been gradually decomposed to many fragments from the increase of inorganic ion concentration during the degradation process. Any intermediates generated were also analyzed by GC–MS and assigned by comparison with commercial standards. In the present experiment, the final products were detected at m/z = 46 and 94, corresponding to structures 1 and 2 in Fig. 7. However, the structures of other products such as a, b, and c have been separately reported by Gnaser et al. and Houas et al. based on their experimental results [23,32]. 4. Conclusions The I–F-codoped TiO2 was prepared by a sol–gel-impregnation method using tetrabutylorthotitanate as precursor. The I–F–TiO2 with Rif = 10 calcined at 500 and 600 °C was identified as being composed of a single anatase phase, demonstrating a higher sunlight induced catalytic activity for the degradation of MB compared with pure and single doped TiO2. The effective activity is ascribed to a synergetic effect of the doped iodine and fluorine atoms, which improved the UV–visible light absorption and led to greater crystallinity and smaller mean diameter in TiO2. The present work also provides useful information on understanding the mechanism of photocatalytic degradation of dyes and synthesizing a photocatalyst with higher activity. The photocatalytic degradation of MB over I–F–TiO2 could be successfully decolorized and degraded. Inorganic ions such as NH+ 4 , 2− NO− 3 and SO4 and other organic compounds such as formic acid and phenol were also observed, respectively, which means an effective mineralization degree. Research in this work is addressed to optimize the preparation of I–F-codoped TiO2 for its application in photocatalysis of water pollutants under sunlight irradiation. Acknowledgment This work was supported by the Science Foundation of Chinese Key Science and Technology 973 Subject (No. 2006CB708602). References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [2] C.H. Li, Y.H. Hsieh, W.T. Chiu, C.C. Liu, C.L. Kao, Study on preparation and photocatalytic performance of Ag/TiO2 and Pt/TiO2 phototocatalysts, Sep. Purif. Technol. 58 (2007) 148–151. [3] D.F. Ollis, C.S. Turchi, Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack, Environ. Prog. 9 (1990) 229–234.
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