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Visible light-degradation of azo dye methyl orange using TiO2/β-FeOOH as a heterogeneous photo-Fenton-like catalyst Zhihui Xu, Ming Zhang, Jingyu Wu, Jianru Liang, Lixiang Zhou and Bo Lǚ
ABSTRACT In this study, a novel TiO2/β-FeOOH composite photocatalyst was synthesized by a hydrothermal method. X-ray diffraction, Fourier transform infrared spectrum, UV-vis diffuse reflectance spectra and scanning electron microscopy (SEM) were used to characterize the composite photocatalyst. The photocatalytic activity of the prepared composite photocatalyst was evaluated in a heterogeneous photo-Fenton-like process using methyl orange (MO) as target pollutant. The TiO2/β-FeOOH composites exhibited higher photocatalytic activity than pure β-FeOOH and TiO2 under visible-light irradiation. The enhanced photocatalytic activity can be ascribed to the formation of TiO2/β-FeOOH heterostructure, which plays an important role in expanding the photoactivity to the visible light region and in effectively prolonging the lifetime of photoinduced electrons and holes. Further investigation revealed that the 25TiO2/β-FeOOH composite synthesized with the TiO2/Fe3þ in a mole
Zhihui Xu Jingyu Wu Bo Lǔ̈ Department of Chemistry, College of Science, Nanjing Agricultural University, Nanjing 210095, China Ming Zhang Jianru Liang Lixiang Zhou (corresponding author) Department of Environmental Engineering, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China E-mail: [email protected]
ratio of 25:75 showed the highest catalytic activity. Key words
| heterogeneous catalyst, methyl orange, photo-Fenton, TiO2/β-FeOOH, visible-light degradation
INTRODUCTION Azo dyes comprise an important class of synthetic organic compounds, which are characterized by the presence of one or more azo bonds (N ¼ N). They represent approximately 50% of the worldwide dye production and are extensively used in a number of industries, such as textile and leather dyeing, food production, cosmetics, paper printing, and pharmaceutical industries, with the textile industry being the largest consumer (Sleiman et al. ). Estimates indicate that approximately 10–15% of the synthetic textile dyes used is lost in waste streams during manufacturing or processing operations (Shu & Chang ; Marco & José ). The effluents are strongly colored which not only created environmental and aesthetic problems, but also posed a great potential toxic threat to ecological and human health because most of these dyes are highly toxic and carcinogenic (Cicek et al. ). Due to the complex aromatic structure and stability of the azo dyes, traditional physical, chemical and biological methods for removal of these azo dyes are not efficient enough (Pagga & Drown ; Dutta et al. ; Netpradit et al. ; Khehra et al. ). Therefore, advanced oxidation processes (AOPs) have received special doi: 10.2166/wst.2013.475
attention because of their high oxidative power towards contaminants in wastewater, which is promoted by highly reactive hydroxyl radicals. Among the AOPs, the heterogeneous photo-Fenton-like reaction is a promising alternative for environmental remediation processes. The destruction of Acid Orange 7 (AO7) was investigated with the application of photo-Fenton-like processes, UV/Fe0/H2O2, (Kusic et al. ). The highest mineralization extent, 90.09% of total organic carbon (TOC) removal, was achieved within 60 min. Wu et al. () used schwertmannite/oxalate suspension for the photocatalytic degradation of methyl orange (MO) under UV irradiation. The degradation of MO was due to the attack of hydroxyl radicals generated on the surface of schwertmannite. A TiO2/UV/O3 system was used for the oxidation of Brilliant Red X-3B by Sun et al. (). A 92.77% degradation of Brilliant Red X-3B was achieved after 3 h of reaction. From the viewpoint of energy efficiency and conservation, the development of efficient visible light induced photocatalysts for the degradation of organic pollutants is an inevitable trend. Only a few works dealing with
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the use of visible light for the degradation of dyes have been reported to date (Chacón et al. ; Wang et al. ; Sajjad et al. ; Dong et al. ; Fan et al. ). A literature survey reveals that Akaganéite (βFeOOH) has a channels structure parallel to the c-axis (Parida , ; Yuan et al. ). This tunnel structure makes β-FeOOH an especially interesting material as a promising photo-Fenton catalyst in the heterogeneous system (Benz et al. ; Zhao et al. ). TiO2 is an excellent photocatalyst with high catalytic activity under UV irradiation. In order to extend the spectral response range of TiO2, a better approach is to combine TiO2 with other narrow band gap semiconductors to form composite photocatalysts. In the composite photocatalysts, the narrow band gap semiconductors can be excited by visible light irradiation, and the photogenerated electrons or holes will then be transferred to TiO2, which leads to efficient charge separation and enhances photocatalytic ability (Beydoun et al. ; Rana et al. ; Liu et al. ; Yang et al. ; Martha et al. ; Sahu & Parida ). The aim of this work was to prepare the novel TiO2/β-FeOOH composite photocatalysts with different content of TiO2, characterize them by a number of physico-chemical methods (X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), UV-vis diffuse reflectance spectra (DRS) and scanning electron microscopy (SEM)) and test their photocatalytic activities on the example of MO degradation under visible light irradiation in aqueous suspension in the presence of H2O2. That MO was chosen as the target pollutant in this paper is because MO is stable under ultraviolet (UV) light irradiation and a typical azo dye widely used in the textile industry.
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for 24 h, the obtained suspension was transferred into a teflon-lined stainless steel autoclave (200 mL). The sealed autoclave was maintained at 90 C for 8 h, and then cooled to room temperature naturally. The resulting sample was collected by centrifugation, washed several times with absolute ethanol and deionized water, and finally dried at 60 C for 24 h. By applying this procedure, five samples with a TiO2/Fe3þ mole ratio of 2.5:97.5, 10:90, 25:75, 40:60 and 50:50 in the initial suspensions (denoted as 2.5TiO2/β-FeOOH, 10TiO2/β-FeOOH, 25TiO2/β-FeOOH, 40TiO2/β-FeOOH and 50TiO2/βFeOOH) were prepared. The sample obtained by the same procedure but without addition of TiO2 was denoted as β-FeOOH. W
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Photodegradation procedures The photodegradation of MO was carried out in a XPA-7 photochemical reactor (Xujiang Electromechanical Plant, Nanjing, China). The temperature of reaction solutions was kept at 25 ± 2 C by cooling water circulation. A 500 W xenon lamp (or medium-pressure Hg lamp as the UV source) was used as an irradiation source of visible light (VL). The initial pH values of suspensions were adjusted to 4.5 with dilute sulfuric acid solution and sodium hydroxide solution. The initial concentration of MO and H2O2 were 80 and 300 mg L1, respectively, with a catalyst loading of 200 mg L1. At given irradiation time intervals, a small quantity of the suspension was taken and centrifuged to separate the catalyst particles from the suspension. The concentration of MO was determined by using a UV-vis spectrophotometer (Beijing Ruili Corp., UV-9100) at 464 nm. W
MATERIALS AND METHODS
Catalyst characterization
Catalyst preparation
The powder XRD patterns were recorded at a scanning rate of 4 min1 in the 2θ range of 10–80 using a Bruker D8 Advance instrument with Cu–Kα radiation (λ ¼ 1.5406 Å) at room temperature. The morphologies and nanostructures of synthesized products were further observed using a Hitachi S-3400N scanning electron microscope at the acceleration voltage 20 kV. FTIR measurements were performed on a Bruker Vector 22 FTIR spectrophotometer, with scanning from 4,000 to 400 cm1 using KBr pellets. The light reflectance properties were studied with a Cary50 UV-vis-NIR (near-infrared) spectrophotometer (Varian Australia PYT Ltd). W
All chemicals used in this work were of analytical grade, purchased from Shanghai Chemical Reagent Co., Ltd (China), and used without any further purification. Five TiO2/β-FeOOH materials, with the different content of TiO2, were obtained by deposition–precipitation method. 10.8116 g FeCl3·6H2O was added to a solution of 2.4000 g urea in 100 mL deionized water under permanent magnetic stirring for 30 min. Without adjusting the pH value, the required mass of TiO2 (Degussa P25) was added to the above mixed solution. After additional stirring
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RESULTS AND DISCUSSION Structural characteristics XRD analysis of photocatalysts Figure 1(a) exhibits the XRD patterns of TiO2 (Degussa P25), β-FeOOH and TiO2/β-FeOOH photocatalysts. The XRD pattern of the β-FeOOH sample matched the diffraction of tetragonal pure β-FeOOH (JCPDS card No. 341266) very well with cell constants of a0 ¼ 10.51 Å, b0 ¼ 10.51 Å and c0 ¼ 3.033 Å, and no impurity peak can be detected. It is known that Degussa P25, considered as one of the best photocatalysts, is also a mixture (Kolen’ko et al. ). As shown in Figure 1, the diffraction peaks at 2θ of 25.28 , 36.98 , 37.80 , 38.58 , 48.02 , 53.89 , 55.06 , 62.69 , 68.76 , 70.31 and 75.03 can be indexed to the characteristic peaks (101), (103), (004), (112), (200), (105), (211), (204), (116), (220) and (215) of anatase (JCPDS card No. 21-1272), respectively. Whereas, the other diffraction peaks of TiO2 sample can be indexed to the characteristic peaks of rutile (JCPDS card No. 21-1276). For the samples of 2.5TiO2/β-FeOOH, 10TiO2/β-FeOOH and 25TiO2/βFeOOH, several small peaks located at 2θ values of 11.84 , 26.72 , 35.16 and 55.90 suggest the appearance of βFeOOH phase, which was mixed with the anatase TiO2 phase and rutile TiO2 phase. In addition, negligible changes of all diffraction peak positions of anatase and rutile phase TiO2 compared with that of the TiO2 sample suggest that Fe3þ does not incorporate into the lattice of TiO2, but as βFeOOH deposits on the surface of TiO2. Furthermore, it can be observed that the diffraction peaks of β-FeOOH phase become slightly broader, which means that the βFeOOH particle size reduces. This result will be further testified and verified by SEM observation. However, the diffraction peaks of β-FeOOH phase almost disappeared in the 40TiO2/β-FeOOH and 50TiO2/β-FeOOH samples, which suggests that excess of TiO2 is not conducive to the formation of β-FeOOH crystalline. W
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FTIR analysis of photocatalysts Figure 1
The photocatalysts were analyzed by FTIR for further identification. As shown in Figure 1(b), the absorption peak at wavelength 1,639 cm1 was attributed to the O-H vibrations of absorbed H2O molecules or structural OH groups (Cheng & Zhao ; Tong et al. ). For the catalyst β-FeOOH, the absorption peaks at wavelength 852, 698, 646 and 496 cm1 were assigned to the vibration modes of the FeO6
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Structural characteristics of TiO2, β-FeOOH and 25TiO2/β-FeOOH: (a) XRD patterns (▪: Anatase; •: Rutile; ▾: Pure β-FeOOH); (b) Characteristic parts of the FTIR spectra; (c) UV-vis DRS.
coordination octahedron (Ristic et al. ; Cheng & Zhao ). The wide band between 1,000 and 400 cm1 of TiO2 sample was aroused by the stretching vibration of Ti-O (Wang et al. ). The most interesting feature is probably
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that the vibration modes of the FeO6 coordination octahedron gradually weakened and almost completely disappeared with the increase of TiO2/Fe3þ mole ratio in the initial suspensions. This phenomenon indicates that βFeOOH was not simply to cover the surface of TiO2, but likely to form a strong interaction (such as Fe–O–Ti bond) with TiO2 (Li et al. ). UV-vis DRS analysis of photocatalysts The UV-vis DRS of the TiO2, β-FeOOH and 25TiO2/βFeOOH photocatalysts are shown in Figure 1(c). As can be seen from Figure 1(c), the TiO2 (Degussa P25) sample displays photoabsorption properties in the UV light region, with an absorption edge at about 400 nm, whereas the βFeOOH photocatalyst has broader adsorption in the visible light region with an absorption edge at about 602 nm. The adsorption cut-off wavelength of the 25TiO2/β-FeOOH photocatalyst is about 624 nm. The energy band gap of 25TiO2/β-FeOOH photocatalyst can be estimated from the tangent line in the plot of the square root of KubelkaMunk function F(R) against photon energy (Gao et al. ). The tangent line, which is extrapolated to (F(R))1/2 ¼ 0, indicates the band gap is 1.99 eV. Such an energy band gap is smaller than that of the β-FeOOH photocatalyst (2.06 eV), possibly due to the strong interaction between βFeOOH and TiO2 in the composite photocatalyst. The formation of Fe–O–Ti bonds will overlap the conduction band of TiO2 and d orbital of Fe3þ (Li et al. ), and indicate the possibility of utilizing more visible light for photolysis. In addition, another interesting finding is that the UV-vis DRS of the 25TiO2/β-FeOOH composite photocatalyst is very similar to that of the β-FeOOH photocatalyst, which indicates that the β-FeOOH indeed was wrapped over the surface of TiO2, and either UV or visible light (in the test wavelength range) cannot penetrate the FeOOH layer into the TiO2 kernel. SEM analysis of photocatalysts Figures 2(a)–2(c) show the SEM images of TiO2, β-FeOOH and 25TiO2/β-FeOOH catalysts. It can be seen that TiO2 is characterized by a single morphology with an approximately globular structure and loose agglomeration. The observed average particles diameter of the TiO2 sample is about 0.24 μm. β-FeOOH catalyst exhibits a short-rod-like shape with an average width of about 0.25 μm and length of about 0.86 μm. However, the 25TiO2/β-FeOOH composite shows nest-like morphology with diameter range from two
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μm to more than a dozen μm. We can clearly see that smaller short-rod-like particles cover the surface of the nested structure. Based on the above discussion, the smaller short-rod-like particles should be β-FeOOH particles whose average diameters are about 0.31 μm. During the synthesis of composite, the titanium dioxide nanoparticles served as heterogeneous nuclei for the growth of β-FeOOH, while the existence of TiO2 nanoparticles also inhibited the growing up of β-FeOOH nanoparticles. This is in accordance with the XRD result. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis of 25TiO2/βFeOOH Further observation of the 25TiO2/β-FeOOH sample by TEM (Figure 2(d)) reveals that the short rods of β-FeOOH nanoparticles are attached to the surface of the TiO2 nanoparticles. Figures 2(e) and 2(f) illustrate the XPS spectra of Ti 2p and Fe 2p in the 25TiO2/β-FeOOH catalyst. The two peaks at 711.2 and 724.7 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively, corresponding to Fe3þ in the 25TiO2/β-FeOOH catalyst. Binding energy of Ti4þ in the 25TiO2/β-FeOOH composite was obtained at 458.8 and 464.6 eV, corresponding to Ti 2p3/2 and Ti 2p1/2, respectively. Photocatalytic degradation of MO Photocatalytic activity was evaluated by the degradation of MO. Figure 3(a) shows the time course of Ct/C0 of MO degradation over different photocatalysts under UV and VL irradiation (Ct and C0 are the concentration of MO solution at initial time and after irradiation at t time, respectively). As can be seen from the figure, the efficiency of MO photolysis over TiO2 under VL irradiation is small – during 120 min it degraded only 7.50% of the substrate, and the efficiency of MO photodegradation over TiO2 under UV irradiation increased to 34.7% within the same time. This result is easily understandable. Because TiO2 cannot be excited under visible illumination above 400 nm, which is confirmed from Figure 1(c). The results of MO degradation under UV and VL irradiation obtained using β-FeOOH photocatalyst are also presented. Previous studies (Zhao et al. ; Xu et al. ; Zhong et al. ) have shown that the heterogeneous photo-Fenton reaction was initiated by the formation of peroxide complex species with Fe (III) active sites on the catalyst surface. Then the surface iron complex experienced a cleavage under UV irradiation leading to the generation of Fe (II) complex.
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SEM images of different catalysts ((a) TiO2; (b) β-FeOOH; (c) 25TiO2/β-FeOOH), TEM image of 25TiO2/β-FeOOH (d), Ti 2p (e) and Fe 2p (f) XPS spectra of TiO2/β-FeOOH.
Since the Fe (II) complex was unstable, Fe (II) complex was oxidized by H2O2 and hydroxyl radical was generated at the same time. The hydroxyl radicals could thus attack the organic pollutants adsorbed on the surface of catalyst and lead to their degradation and mineralization. MO was photodegraded by 19.2 and 41.2% over β-FeOOH after 120 min under VL and UV irradiation, respectively. Although β-FeOOH can be excited under visible light, compared with TiO2 catalyst, the photodegradation efficiency of MO over β-FeOOH did not increase too much under either VL or UV irradiation. The reason may be that the photoinduced electrons and holes recombined quickly on the surface of β-FeOOH. However, in the case of 25TiO2/β-
FeOOH photocatalyst, the composite showed much higher photocatalytic activity than TiO2 and β-FeOOH under both UV and VL irradiation. MO was photodegraded by 86.3 and 97.2% over 25TiO2/β-FeOOH after 120 min under VL and UV irradiation, respectively. The enhanced photocatalytic activity can be ascribed to the formation of TiO2/β-FeOOH heterostructure, which plays an important role in the composite photocatalyst. When the 25TiO2/βFeOOH composite is under VL or UV irradiation, electrons (e) are excited to the conduction band of β-FeOOH, leaving equal holes (hþ) in the valence band. Then electrons will transfer from the conduction band of β-FeOOH to that of TiO2, the recombination of photoinduced carriers can be
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TiO2/β-FeOOH photocatalysts, the sample of 25TiO2/βFeOOH exhibited the highest photocatalytic efficiency. The lower photocatalytic activity of the 2.5TiO2/β-FeOOH and 10TiO2/β-FeOOH composites can be ascribed to two main reasons. Firstly, during the preparation of the composite photocatalyst, β-FeOOH grew on the surface of TiO2 such that a core-shell heterostructure might form. At a low mole ratio of TiO2/Fe3þ level, the outer layer of β-FeOOH might grow too thick, and the photoinduced electrons would need to migrate over a longer distance to the TiO2 kernel. More importantly, the higher content Fe3þ ions in the composite catalyst played the role of recombination sites to trap the photoinduced electrons and holes (Banic´ et al. ), and thus decreased the photocatalytic efficiency of TiO2/β-FeOOH catalysts. However, at a high mole ratio of TiO2/Fe3þ level, the higher contents of TiO2 in the composite catalysts was not conducive to the visible light absorbance, which leads to the low photocatalytic activity under visible light irradiation (Gao et al. ). Therefore, the photocatalytic activities of the 40TiO2/β-FeOOH and 50TiO2/β-FeOOH composites were also not high.
CONCLUSIONS
Figure 3
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Kinetic processes of MO photodegradation: (a) facilitated by TiO2, β-FeOOH and
25TiO2/β-FeOOH under UV or VL irradiation; (b) effect of TiO2/Fe3þ mole ratio on VL photocatalytic degradation of MO. Test conditions: catalyst dosage ¼ 200 mg L
1
1
, H2O2 initial concentration ¼ 300 mg L
, pH ¼ 4.5.
inhibited, and thus can prolong the life time of photoinduced carriers. Consequently, the photocatalytic activity of 25TiO2/β-FeOOH composite will be greatly improved. Apart from the method of photocatalyst preparation, one of the key factors that governs the efficiency of composite photocatalyst is content of component in the heterostructure (Banic´ et al. ; Liu et al. ). The effect of photodegradation of MO over TiO2/β-FeOOH catalysts with different TiO2/Fe3þ mole ratio under VL irradiation is illustrated in Figure 3(b). Compared with pure β-FeOOH, the TiO2/β-FeOOH catalysts exhibited a significant increase in the MO photodegradation efficiency. The photocatalytic efficiency of TiO2/β-FeOOH catalysts increased with increasing TiO2/Fe3þ mole ratio to 25:75, and then decreased with further increase of TiO2/Fe3þ mole ratio. It can be concluded that of all tested
In summary, photocatalyst TiO2/β-FeOOH was successfully synthesized by a simple hydrothermal route. The novel catalysts showed visible light photocatalytic activity in degrading MO, while the sample prepared with the TiO2/ Fe3þ mole ratio of 25:75 gave the best photocatalytic activity and was demonstrated to be more superior to the commercial Degussa P25 and the pure β-FeOOH. The enhanced visible light catalytic activity can be attributed to the formation of TiO2/β-FeOOH heterostructure, which plays a key role in overlapping the conduction band of TiO2 and d orbital of Fe3þ, in expanding the photoactivity to a wider visible-light range and in utilizing more visible light for the MO photodegradation, also in inhibiting the recombination of photo-excited electrons and holes. It would be of great promise for the industrial application of this catalyst with high visible-light catalytic efficiency to oxidize organic pollutants for wastewater treatment.
ACKNOWLEDGEMENT This study was financially supported by the National Natural Science Foundation of China (21077053, 40930738).
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REFERENCES Banic´, N., Abramovic´, B., Krstic´, J., Šojic´, D., Lončarevic´, D., Cherkezova-Zheleva, Z. & Guzsvány, V. Photodegradation of thiacloprid using Fe/TiO2 as a heterogeneous photo-Fenton catalyst. Applied Catalysis B: Environmental 107, 363–371. Benz, M., van der Kraan, A. M. & Prins, R. Reduction of aromatic nitrocompounds with hydrazine hydrate in the presence of an iron hydroxide catalyst II. Activity, X–ray diffraction and Mössbauer study of the iron oxide hydroxide catalyst. Applied Catalysis A: General 172, 149–157. Beydoun, D., Amal, R., Low, G. & McEvoy, S. Occurrence and prevention of photodissolution at the phase junction of magnetite and titanium dioxide. Journal of Molecular Catalysis A: Chemical 180, 193–200. Chacón, J. M., Leal, M. T., Sánchez, M. & Bandala, E. R. Solar photocatalytic degradation of azo-dyes by photoFenton process. Dyes and Pigments 69 (3), 144–150. Cheng, Z. W. & Zhao, D. N. Effects of experimental conditions on one-dimensional single-crystal nanostructure of β-FeOOH. Materials Chemistry and Physics 127, 220–226. Cicek, F., Ozer, D. & Ozer, A. Low cost removal of reactive dyes using wheat bran. Journal of Hazardous Materials 146, 408–416. Dong, Y. C., Dong, W. J., Cao, Y. N., Han, Z. B. & Ding, Z. H. Preparation and catalytic activity of Fe alginate gel beads for oxidative degradation of azo dyes under visible light irradiation. Catalysis Today 175 (1), 346–355. Dutta, K., Mukhopadhyay, S., Bhattacharjee, S. & Chaudhuri, B. Chemical oxidation of methylene blue using a Fentonlike reaction. Journal of Hazardous Materials 84, 57–71. Fan, J., Hu, X. Y., Xie, Z. G., Zhang, K. L. & Wang, J. J. Photocatalytic degradation of azo dye by novel Bi-based photocatalyst Bi4TaO8I under visible-light irradiation. Chemical Engineering Journal 179, 44–51. Gao, F., Chen, X. Y., Yin, K. B., Dong, S. A., Ren, Z. F., Yuan, F., Yu, T., Zou, Z. G. & Liu, J. M. Visible-light photocatalytic properties of weak magnetic BiFeO3 nanoparticles. Advanced Materials 19, 2889–2892. Gao, P., Liu, J. C., Zhang, T., Sun, D. D. & Ng, W. Hierarchical TiO2/CdS ‘spindle-like’ composite with high photodegradation and antibacterial capability under visible light irradiation. Journal of Hazardous Materials 229–230, 209–216. Khehra, M. S., Saini, H. S., Sharma, D. K., Chadha, B. S. & Chimni, S. S. Decolorization of various azo dyes by bacterial consortium. Dyes and Pigments 67, 55–61. Konlen’ko, Yu. V., Churagulov, B. R., Kunst, M., Mazerolles, L. & Colbeau-Justin, C. Photocatalytic properties of titania powders prepared by hydrothermal method. Applied Catalysis B: Environmental 54, 51–58. Kusic, H., Koprivanac, N. & Srsan, L. Azo dye degradation using Fenton type processes assisted by UV irradiation: a kinetic study. Journal of Photochemistry and Photobiology A: Chemistry 181, 195–202.
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|
68.10
|
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Li, J. X., Xu, J. H., Dai, W. L., Li, H. X. & Fan, K. N. Direct hydro-alcohol thermal synthesis of special core–shell structured Fe-doped titania microspheres with extended visible light response and enhanced photoactivity. Applied Catalysis B: Environmental 85, 162–170. Liu, L. F., Chen, F., Yang, F. L., Chen, Y. S. & Crittenden, J. Photocatalytic degradation of 2, 4-dichlorophenol using nanoscale Fe/TiO2. Chemical Engineering Journal 182–182, 189–195. Liu, Y. C., Zhong, H., Li, L. F. & Zhang, C. J. Temperature dependence of magnetic property and photocatalytic activity of Fe3O4/hydroxyapatite nanoparticles. Materials Research Bulletin 45, 2036–2039. Marco, S. L. & José, A. P. Decolorization of the azo dye reactive black 5 by Fenton and photo-Fenton oxidation. Dyes and Pigments 71, 236–244. Martha, S., Das, D. P., Biswal, N. & Parida, K. M. Facile synthesis of visible light responsive V2O5/N, S–TiO2 composite photocatalyst: enhanced hydrogen production and phenol degradation. Journal of Materials Chemistry 22, 10695–10703. Netpradit, S., Thiravetyan, P. & Towprayoon, S. Adsorption of three azo reactive dyes by metal hydroxide sludge: effect of temperature, pH, and electrolytes. Journal of Colloid and Interface Science 270, 255–261. Pagga, U. & Drown, D. The degradation of dye-stuffs. Part II. behaviour of dyestuffs in aerobic biodegradation tests. Chemosphere 15, 479–491. Parida, K. M. Studies of β-FeOOH. Chemical composition, microstructure and other characteristics of some synthetic akaganeite samples. Journal of Materials Science Letters 6, 1476–1478. Parida, K. M. Studies on β-FeOOH part 2 physico-chemical characteristics of thermally treated akaganeite (β-FeOOH). Journal of Materials Science 23, 1201–1205. Rana, S., Srivastava, R. S., Sorensson, M. M. & Misra, R. D. K. Synthesis and characterization of nanoparticles with magnetic core and photocatalytic shell: anatase TiO2– NiFe2O4 system. Materials Science and Engineering B 119, 144–151. Ristic, M., Music, S. & Orehovec, Z. Thermal decomposition of synthetic ammonium jarosite. Journal of Molecular Structure 744, 295–300. Sahu, N. & Parida, K. M. Photocatalytic activity of Au/TiO2 nanocomposite for Azo-Dyes degradation. Kinetics and Catalysis 53, 197–205. Sajjad, A. K. L., Shamaila, S., Tian, B. Z., Chen, F. & Zhang, J. L. Comparative studies of operational parameters of degradation of azo dyes in visible light by highly efficient WOx/TiO2 photocatalyst. Journal of Hazardous Materials 177 (1–3), 781–791. Shu, H. Y. & Chang, M. C. Decolorization effects of six azo dyes by O3, UV/O3 and UV/H2O2 processes. Dyes and Pigments 65, 25–31. Sleiman, M., Vildozo, D., Ferronato, C. & Chovelon, J. M. Photocatalytic degradation of azo dye metanil yellow:
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Z. Xu et al.
|
Visible light-degradation of azo dye using TiO2/β-FeOOH catalyst
optimization and kinetic modeling using a chemometric approach. Applied Catalysis B: Environmental 77, 1–11. Sun, J., Yan, X., Lv, K. L., Sun, S., Deng, K. J. & Du, D. Y. Photocatalytic degradation pathway for azo dye in TiO2/UV/ O3 system: hydroxyl radical versus hole. Journal of Molecular Catalysis A: Chemical 367, 31–37. Tong, G. X., Guan, J. G. & Zhang, Q. J. Goethite hierarchical nanostructures: glucose-assisted synthesis, chemical conversion into hematite with excellent photocatalytic properties. Materials Chemistry and Physics 127, 371–378. Wang, Z. H., Jiang, T. S., Du, Y. M., Chen, K. M. & Yin, H. B. Synthesis of mesoporous titania and the photocatalytic activity for decomposition of methyl orange. Materials Letters 60, 2493–2496. Wang, J., Zhao, G., Zhang, Z. H., Zhang, X. D., Zhang, G., Ma, T., Jiang, Y. F., Zhang, P. & Li, Y. Investigation on degradation of azo fuchsine using visible light in the presence of heat-treated anatase TiO2 powder. Dyes and Pigments 75 (2), 335–343. Wu, Y., Guo, J., Jiang, D. J., Zhou, P., Lan, Y. Q. & Zhou, L. X. Heterogeneous photocatalytic degradation of methyl orange in schwertmannite/oxalate suspension under UV irradiation.
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Environmental Science and Pollution Research International 19, 2313–2320. Xu, Z. H., Liang, J. R. & Zhou, L. X. Photo-Fenton-like degradation of azo dye methyl orange using synthetic ammonium and hydronium jarosite. Journal of Alloys and Compounds 546, 112–118. Yang, Z. P., Gong, X. Y. & Zhang, C. J. Recyclable Fe3O4/ hydroxyapatite composite nanoparticles for photocatalytic applications. Chemical Engineering Journal 165, 117–121. Yuan, Z. Y., Ren, T. Z. & Su, B. L. Surfactant mediated nanoparticle assembly of catalytic mesoporous crystalline iron oxide materials. Catalysis Today 93–95, 743–750. Zhao, Y. P., Hu, J. Y. & Chen, H. B. Elimination of estrogen and its estrogenicity by heterogeneous photo-Fenton catalyst β-FeOOH/resin. Journal of Photochemistry and Photobiology A: Chemistry 212, 94–100. Zhong, Y. H., Liang, X. L., Tan, W., Zhong, Y., He, H. P., Zhu, J. X., Yuan, P. & Jiang, Z. A comparative study about the effects of isomorphous substitution of transition metals (Ti, Cr, Mn, Co and Ni) on the UV/Fenton catalytic activity of magnetite. Journal of Molecular Catalysis A: Chemical 372, 29–34.
First received 30 April 2013; accepted in revised form 16 July 2013. Available online 24 October 2013
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Visible light-degradation of azo dye methyl orange using TiO2/β-FeOOH as a heterogeneous photo-Fenton-like catalyst Zhihui Xu, Ming Zhang, Jingyu Wu, Jianru Liang, Lixiang Zhou and Bo Lǚ
ABSTRACT In this study, a novel TiO2/β-FeOOH composite photocatalyst was synthesized by a hydrothermal method. X-ray diffraction, Fourier transform infrared spectrum, UV-vis diffuse reflectance spectra and scanning electron microscopy (SEM) were used to characterize the composite photocatalyst. The photocatalytic activity of the prepared composite photocatalyst was evaluated in a heterogeneous photo-Fenton-like process using methyl orange (MO) as target pollutant. The TiO2/β-FeOOH composites exhibited higher photocatalytic activity than pure β-FeOOH and TiO2 under visible-light irradiation. The enhanced photocatalytic activity can be ascribed to the formation of TiO2/β-FeOOH heterostructure, which plays an important role in expanding the photoactivity to the visible light region and in effectively prolonging the lifetime of photoinduced electrons and holes. Further investigation revealed that the 25TiO2/β-FeOOH composite synthesized with the TiO2/Fe3þ in a mole
Zhihui Xu Jingyu Wu Bo Lǔ̈ Department of Chemistry, College of Science, Nanjing Agricultural University, Nanjing 210095, China Ming Zhang Jianru Liang Lixiang Zhou (corresponding author) Department of Environmental Engineering, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China E-mail: [email protected]
ratio of 25:75 showed the highest catalytic activity. Key words
| heterogeneous catalyst, methyl orange, photo-Fenton, TiO2/β-FeOOH, visible-light degradation
INTRODUCTION Azo dyes comprise an important class of synthetic organic compounds, which are characterized by the presence of one or more azo bonds (N ¼ N). They represent approximately 50% of the worldwide dye production and are extensively used in a number of industries, such as textile and leather dyeing, food production, cosmetics, paper printing, and pharmaceutical industries, with the textile industry being the largest consumer (Sleiman et al. ). Estimates indicate that approximately 10–15% of the synthetic textile dyes used is lost in waste streams during manufacturing or processing operations (Shu & Chang ; Marco & José ). The effluents are strongly colored which not only created environmental and aesthetic problems, but also posed a great potential toxic threat to ecological and human health because most of these dyes are highly toxic and carcinogenic (Cicek et al. ). Due to the complex aromatic structure and stability of the azo dyes, traditional physical, chemical and biological methods for removal of these azo dyes are not efficient enough (Pagga & Drown ; Dutta et al. ; Netpradit et al. ; Khehra et al. ). Therefore, advanced oxidation processes (AOPs) have received special doi: 10.2166/wst.2013.475
attention because of their high oxidative power towards contaminants in wastewater, which is promoted by highly reactive hydroxyl radicals. Among the AOPs, the heterogeneous photo-Fenton-like reaction is a promising alternative for environmental remediation processes. The destruction of Acid Orange 7 (AO7) was investigated with the application of photo-Fenton-like processes, UV/Fe0/H2O2, (Kusic et al. ). The highest mineralization extent, 90.09% of total organic carbon (TOC) removal, was achieved within 60 min. Wu et al. () used schwertmannite/oxalate suspension for the photocatalytic degradation of methyl orange (MO) under UV irradiation. The degradation of MO was due to the attack of hydroxyl radicals generated on the surface of schwertmannite. A TiO2/UV/O3 system was used for the oxidation of Brilliant Red X-3B by Sun et al. (). A 92.77% degradation of Brilliant Red X-3B was achieved after 3 h of reaction. From the viewpoint of energy efficiency and conservation, the development of efficient visible light induced photocatalysts for the degradation of organic pollutants is an inevitable trend. Only a few works dealing with
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the use of visible light for the degradation of dyes have been reported to date (Chacón et al. ; Wang et al. ; Sajjad et al. ; Dong et al. ; Fan et al. ). A literature survey reveals that Akaganéite (βFeOOH) has a channels structure parallel to the c-axis (Parida , ; Yuan et al. ). This tunnel structure makes β-FeOOH an especially interesting material as a promising photo-Fenton catalyst in the heterogeneous system (Benz et al. ; Zhao et al. ). TiO2 is an excellent photocatalyst with high catalytic activity under UV irradiation. In order to extend the spectral response range of TiO2, a better approach is to combine TiO2 with other narrow band gap semiconductors to form composite photocatalysts. In the composite photocatalysts, the narrow band gap semiconductors can be excited by visible light irradiation, and the photogenerated electrons or holes will then be transferred to TiO2, which leads to efficient charge separation and enhances photocatalytic ability (Beydoun et al. ; Rana et al. ; Liu et al. ; Yang et al. ; Martha et al. ; Sahu & Parida ). The aim of this work was to prepare the novel TiO2/β-FeOOH composite photocatalysts with different content of TiO2, characterize them by a number of physico-chemical methods (X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), UV-vis diffuse reflectance spectra (DRS) and scanning electron microscopy (SEM)) and test their photocatalytic activities on the example of MO degradation under visible light irradiation in aqueous suspension in the presence of H2O2. That MO was chosen as the target pollutant in this paper is because MO is stable under ultraviolet (UV) light irradiation and a typical azo dye widely used in the textile industry.
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for 24 h, the obtained suspension was transferred into a teflon-lined stainless steel autoclave (200 mL). The sealed autoclave was maintained at 90 C for 8 h, and then cooled to room temperature naturally. The resulting sample was collected by centrifugation, washed several times with absolute ethanol and deionized water, and finally dried at 60 C for 24 h. By applying this procedure, five samples with a TiO2/Fe3þ mole ratio of 2.5:97.5, 10:90, 25:75, 40:60 and 50:50 in the initial suspensions (denoted as 2.5TiO2/β-FeOOH, 10TiO2/β-FeOOH, 25TiO2/β-FeOOH, 40TiO2/β-FeOOH and 50TiO2/βFeOOH) were prepared. The sample obtained by the same procedure but without addition of TiO2 was denoted as β-FeOOH. W
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Photodegradation procedures The photodegradation of MO was carried out in a XPA-7 photochemical reactor (Xujiang Electromechanical Plant, Nanjing, China). The temperature of reaction solutions was kept at 25 ± 2 C by cooling water circulation. A 500 W xenon lamp (or medium-pressure Hg lamp as the UV source) was used as an irradiation source of visible light (VL). The initial pH values of suspensions were adjusted to 4.5 with dilute sulfuric acid solution and sodium hydroxide solution. The initial concentration of MO and H2O2 were 80 and 300 mg L1, respectively, with a catalyst loading of 200 mg L1. At given irradiation time intervals, a small quantity of the suspension was taken and centrifuged to separate the catalyst particles from the suspension. The concentration of MO was determined by using a UV-vis spectrophotometer (Beijing Ruili Corp., UV-9100) at 464 nm. W
MATERIALS AND METHODS
Catalyst characterization
Catalyst preparation
The powder XRD patterns were recorded at a scanning rate of 4 min1 in the 2θ range of 10–80 using a Bruker D8 Advance instrument with Cu–Kα radiation (λ ¼ 1.5406 Å) at room temperature. The morphologies and nanostructures of synthesized products were further observed using a Hitachi S-3400N scanning electron microscope at the acceleration voltage 20 kV. FTIR measurements were performed on a Bruker Vector 22 FTIR spectrophotometer, with scanning from 4,000 to 400 cm1 using KBr pellets. The light reflectance properties were studied with a Cary50 UV-vis-NIR (near-infrared) spectrophotometer (Varian Australia PYT Ltd). W
All chemicals used in this work were of analytical grade, purchased from Shanghai Chemical Reagent Co., Ltd (China), and used without any further purification. Five TiO2/β-FeOOH materials, with the different content of TiO2, were obtained by deposition–precipitation method. 10.8116 g FeCl3·6H2O was added to a solution of 2.4000 g urea in 100 mL deionized water under permanent magnetic stirring for 30 min. Without adjusting the pH value, the required mass of TiO2 (Degussa P25) was added to the above mixed solution. After additional stirring
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RESULTS AND DISCUSSION Structural characteristics XRD analysis of photocatalysts Figure 1(a) exhibits the XRD patterns of TiO2 (Degussa P25), β-FeOOH and TiO2/β-FeOOH photocatalysts. The XRD pattern of the β-FeOOH sample matched the diffraction of tetragonal pure β-FeOOH (JCPDS card No. 341266) very well with cell constants of a0 ¼ 10.51 Å, b0 ¼ 10.51 Å and c0 ¼ 3.033 Å, and no impurity peak can be detected. It is known that Degussa P25, considered as one of the best photocatalysts, is also a mixture (Kolen’ko et al. ). As shown in Figure 1, the diffraction peaks at 2θ of 25.28 , 36.98 , 37.80 , 38.58 , 48.02 , 53.89 , 55.06 , 62.69 , 68.76 , 70.31 and 75.03 can be indexed to the characteristic peaks (101), (103), (004), (112), (200), (105), (211), (204), (116), (220) and (215) of anatase (JCPDS card No. 21-1272), respectively. Whereas, the other diffraction peaks of TiO2 sample can be indexed to the characteristic peaks of rutile (JCPDS card No. 21-1276). For the samples of 2.5TiO2/β-FeOOH, 10TiO2/β-FeOOH and 25TiO2/βFeOOH, several small peaks located at 2θ values of 11.84 , 26.72 , 35.16 and 55.90 suggest the appearance of βFeOOH phase, which was mixed with the anatase TiO2 phase and rutile TiO2 phase. In addition, negligible changes of all diffraction peak positions of anatase and rutile phase TiO2 compared with that of the TiO2 sample suggest that Fe3þ does not incorporate into the lattice of TiO2, but as βFeOOH deposits on the surface of TiO2. Furthermore, it can be observed that the diffraction peaks of β-FeOOH phase become slightly broader, which means that the βFeOOH particle size reduces. This result will be further testified and verified by SEM observation. However, the diffraction peaks of β-FeOOH phase almost disappeared in the 40TiO2/β-FeOOH and 50TiO2/β-FeOOH samples, which suggests that excess of TiO2 is not conducive to the formation of β-FeOOH crystalline. W
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FTIR analysis of photocatalysts Figure 1
The photocatalysts were analyzed by FTIR for further identification. As shown in Figure 1(b), the absorption peak at wavelength 1,639 cm1 was attributed to the O-H vibrations of absorbed H2O molecules or structural OH groups (Cheng & Zhao ; Tong et al. ). For the catalyst β-FeOOH, the absorption peaks at wavelength 852, 698, 646 and 496 cm1 were assigned to the vibration modes of the FeO6
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Structural characteristics of TiO2, β-FeOOH and 25TiO2/β-FeOOH: (a) XRD patterns (▪: Anatase; •: Rutile; ▾: Pure β-FeOOH); (b) Characteristic parts of the FTIR spectra; (c) UV-vis DRS.
coordination octahedron (Ristic et al. ; Cheng & Zhao ). The wide band between 1,000 and 400 cm1 of TiO2 sample was aroused by the stretching vibration of Ti-O (Wang et al. ). The most interesting feature is probably
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that the vibration modes of the FeO6 coordination octahedron gradually weakened and almost completely disappeared with the increase of TiO2/Fe3þ mole ratio in the initial suspensions. This phenomenon indicates that βFeOOH was not simply to cover the surface of TiO2, but likely to form a strong interaction (such as Fe–O–Ti bond) with TiO2 (Li et al. ). UV-vis DRS analysis of photocatalysts The UV-vis DRS of the TiO2, β-FeOOH and 25TiO2/βFeOOH photocatalysts are shown in Figure 1(c). As can be seen from Figure 1(c), the TiO2 (Degussa P25) sample displays photoabsorption properties in the UV light region, with an absorption edge at about 400 nm, whereas the βFeOOH photocatalyst has broader adsorption in the visible light region with an absorption edge at about 602 nm. The adsorption cut-off wavelength of the 25TiO2/β-FeOOH photocatalyst is about 624 nm. The energy band gap of 25TiO2/β-FeOOH photocatalyst can be estimated from the tangent line in the plot of the square root of KubelkaMunk function F(R) against photon energy (Gao et al. ). The tangent line, which is extrapolated to (F(R))1/2 ¼ 0, indicates the band gap is 1.99 eV. Such an energy band gap is smaller than that of the β-FeOOH photocatalyst (2.06 eV), possibly due to the strong interaction between βFeOOH and TiO2 in the composite photocatalyst. The formation of Fe–O–Ti bonds will overlap the conduction band of TiO2 and d orbital of Fe3þ (Li et al. ), and indicate the possibility of utilizing more visible light for photolysis. In addition, another interesting finding is that the UV-vis DRS of the 25TiO2/β-FeOOH composite photocatalyst is very similar to that of the β-FeOOH photocatalyst, which indicates that the β-FeOOH indeed was wrapped over the surface of TiO2, and either UV or visible light (in the test wavelength range) cannot penetrate the FeOOH layer into the TiO2 kernel. SEM analysis of photocatalysts Figures 2(a)–2(c) show the SEM images of TiO2, β-FeOOH and 25TiO2/β-FeOOH catalysts. It can be seen that TiO2 is characterized by a single morphology with an approximately globular structure and loose agglomeration. The observed average particles diameter of the TiO2 sample is about 0.24 μm. β-FeOOH catalyst exhibits a short-rod-like shape with an average width of about 0.25 μm and length of about 0.86 μm. However, the 25TiO2/β-FeOOH composite shows nest-like morphology with diameter range from two
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μm to more than a dozen μm. We can clearly see that smaller short-rod-like particles cover the surface of the nested structure. Based on the above discussion, the smaller short-rod-like particles should be β-FeOOH particles whose average diameters are about 0.31 μm. During the synthesis of composite, the titanium dioxide nanoparticles served as heterogeneous nuclei for the growth of β-FeOOH, while the existence of TiO2 nanoparticles also inhibited the growing up of β-FeOOH nanoparticles. This is in accordance with the XRD result. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis of 25TiO2/βFeOOH Further observation of the 25TiO2/β-FeOOH sample by TEM (Figure 2(d)) reveals that the short rods of β-FeOOH nanoparticles are attached to the surface of the TiO2 nanoparticles. Figures 2(e) and 2(f) illustrate the XPS spectra of Ti 2p and Fe 2p in the 25TiO2/β-FeOOH catalyst. The two peaks at 711.2 and 724.7 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively, corresponding to Fe3þ in the 25TiO2/β-FeOOH catalyst. Binding energy of Ti4þ in the 25TiO2/β-FeOOH composite was obtained at 458.8 and 464.6 eV, corresponding to Ti 2p3/2 and Ti 2p1/2, respectively. Photocatalytic degradation of MO Photocatalytic activity was evaluated by the degradation of MO. Figure 3(a) shows the time course of Ct/C0 of MO degradation over different photocatalysts under UV and VL irradiation (Ct and C0 are the concentration of MO solution at initial time and after irradiation at t time, respectively). As can be seen from the figure, the efficiency of MO photolysis over TiO2 under VL irradiation is small – during 120 min it degraded only 7.50% of the substrate, and the efficiency of MO photodegradation over TiO2 under UV irradiation increased to 34.7% within the same time. This result is easily understandable. Because TiO2 cannot be excited under visible illumination above 400 nm, which is confirmed from Figure 1(c). The results of MO degradation under UV and VL irradiation obtained using β-FeOOH photocatalyst are also presented. Previous studies (Zhao et al. ; Xu et al. ; Zhong et al. ) have shown that the heterogeneous photo-Fenton reaction was initiated by the formation of peroxide complex species with Fe (III) active sites on the catalyst surface. Then the surface iron complex experienced a cleavage under UV irradiation leading to the generation of Fe (II) complex.
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SEM images of different catalysts ((a) TiO2; (b) β-FeOOH; (c) 25TiO2/β-FeOOH), TEM image of 25TiO2/β-FeOOH (d), Ti 2p (e) and Fe 2p (f) XPS spectra of TiO2/β-FeOOH.
Since the Fe (II) complex was unstable, Fe (II) complex was oxidized by H2O2 and hydroxyl radical was generated at the same time. The hydroxyl radicals could thus attack the organic pollutants adsorbed on the surface of catalyst and lead to their degradation and mineralization. MO was photodegraded by 19.2 and 41.2% over β-FeOOH after 120 min under VL and UV irradiation, respectively. Although β-FeOOH can be excited under visible light, compared with TiO2 catalyst, the photodegradation efficiency of MO over β-FeOOH did not increase too much under either VL or UV irradiation. The reason may be that the photoinduced electrons and holes recombined quickly on the surface of β-FeOOH. However, in the case of 25TiO2/β-
FeOOH photocatalyst, the composite showed much higher photocatalytic activity than TiO2 and β-FeOOH under both UV and VL irradiation. MO was photodegraded by 86.3 and 97.2% over 25TiO2/β-FeOOH after 120 min under VL and UV irradiation, respectively. The enhanced photocatalytic activity can be ascribed to the formation of TiO2/β-FeOOH heterostructure, which plays an important role in the composite photocatalyst. When the 25TiO2/βFeOOH composite is under VL or UV irradiation, electrons (e) are excited to the conduction band of β-FeOOH, leaving equal holes (hþ) in the valence band. Then electrons will transfer from the conduction band of β-FeOOH to that of TiO2, the recombination of photoinduced carriers can be
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TiO2/β-FeOOH photocatalysts, the sample of 25TiO2/βFeOOH exhibited the highest photocatalytic efficiency. The lower photocatalytic activity of the 2.5TiO2/β-FeOOH and 10TiO2/β-FeOOH composites can be ascribed to two main reasons. Firstly, during the preparation of the composite photocatalyst, β-FeOOH grew on the surface of TiO2 such that a core-shell heterostructure might form. At a low mole ratio of TiO2/Fe3þ level, the outer layer of β-FeOOH might grow too thick, and the photoinduced electrons would need to migrate over a longer distance to the TiO2 kernel. More importantly, the higher content Fe3þ ions in the composite catalyst played the role of recombination sites to trap the photoinduced electrons and holes (Banic´ et al. ), and thus decreased the photocatalytic efficiency of TiO2/β-FeOOH catalysts. However, at a high mole ratio of TiO2/Fe3þ level, the higher contents of TiO2 in the composite catalysts was not conducive to the visible light absorbance, which leads to the low photocatalytic activity under visible light irradiation (Gao et al. ). Therefore, the photocatalytic activities of the 40TiO2/β-FeOOH and 50TiO2/β-FeOOH composites were also not high.
CONCLUSIONS
Figure 3
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Kinetic processes of MO photodegradation: (a) facilitated by TiO2, β-FeOOH and
25TiO2/β-FeOOH under UV or VL irradiation; (b) effect of TiO2/Fe3þ mole ratio on VL photocatalytic degradation of MO. Test conditions: catalyst dosage ¼ 200 mg L
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1
, H2O2 initial concentration ¼ 300 mg L
, pH ¼ 4.5.
inhibited, and thus can prolong the life time of photoinduced carriers. Consequently, the photocatalytic activity of 25TiO2/β-FeOOH composite will be greatly improved. Apart from the method of photocatalyst preparation, one of the key factors that governs the efficiency of composite photocatalyst is content of component in the heterostructure (Banic´ et al. ; Liu et al. ). The effect of photodegradation of MO over TiO2/β-FeOOH catalysts with different TiO2/Fe3þ mole ratio under VL irradiation is illustrated in Figure 3(b). Compared with pure β-FeOOH, the TiO2/β-FeOOH catalysts exhibited a significant increase in the MO photodegradation efficiency. The photocatalytic efficiency of TiO2/β-FeOOH catalysts increased with increasing TiO2/Fe3þ mole ratio to 25:75, and then decreased with further increase of TiO2/Fe3þ mole ratio. It can be concluded that of all tested
In summary, photocatalyst TiO2/β-FeOOH was successfully synthesized by a simple hydrothermal route. The novel catalysts showed visible light photocatalytic activity in degrading MO, while the sample prepared with the TiO2/ Fe3þ mole ratio of 25:75 gave the best photocatalytic activity and was demonstrated to be more superior to the commercial Degussa P25 and the pure β-FeOOH. The enhanced visible light catalytic activity can be attributed to the formation of TiO2/β-FeOOH heterostructure, which plays a key role in overlapping the conduction band of TiO2 and d orbital of Fe3þ, in expanding the photoactivity to a wider visible-light range and in utilizing more visible light for the MO photodegradation, also in inhibiting the recombination of photo-excited electrons and holes. It would be of great promise for the industrial application of this catalyst with high visible-light catalytic efficiency to oxidize organic pollutants for wastewater treatment.
ACKNOWLEDGEMENT This study was financially supported by the National Natural Science Foundation of China (21077053, 40930738).
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REFERENCES Banic´, N., Abramovic´, B., Krstic´, J., Šojic´, D., Lončarevic´, D., Cherkezova-Zheleva, Z. & Guzsvány, V. Photodegradation of thiacloprid using Fe/TiO2 as a heterogeneous photo-Fenton catalyst. Applied Catalysis B: Environmental 107, 363–371. Benz, M., van der Kraan, A. M. & Prins, R. Reduction of aromatic nitrocompounds with hydrazine hydrate in the presence of an iron hydroxide catalyst II. Activity, X–ray diffraction and Mössbauer study of the iron oxide hydroxide catalyst. Applied Catalysis A: General 172, 149–157. Beydoun, D., Amal, R., Low, G. & McEvoy, S. Occurrence and prevention of photodissolution at the phase junction of magnetite and titanium dioxide. Journal of Molecular Catalysis A: Chemical 180, 193–200. Chacón, J. M., Leal, M. T., Sánchez, M. & Bandala, E. R. Solar photocatalytic degradation of azo-dyes by photoFenton process. Dyes and Pigments 69 (3), 144–150. Cheng, Z. W. & Zhao, D. N. Effects of experimental conditions on one-dimensional single-crystal nanostructure of β-FeOOH. Materials Chemistry and Physics 127, 220–226. Cicek, F., Ozer, D. & Ozer, A. Low cost removal of reactive dyes using wheat bran. Journal of Hazardous Materials 146, 408–416. Dong, Y. C., Dong, W. J., Cao, Y. N., Han, Z. B. & Ding, Z. H. Preparation and catalytic activity of Fe alginate gel beads for oxidative degradation of azo dyes under visible light irradiation. Catalysis Today 175 (1), 346–355. Dutta, K., Mukhopadhyay, S., Bhattacharjee, S. & Chaudhuri, B. Chemical oxidation of methylene blue using a Fentonlike reaction. Journal of Hazardous Materials 84, 57–71. Fan, J., Hu, X. Y., Xie, Z. G., Zhang, K. L. & Wang, J. J. Photocatalytic degradation of azo dye by novel Bi-based photocatalyst Bi4TaO8I under visible-light irradiation. Chemical Engineering Journal 179, 44–51. Gao, F., Chen, X. Y., Yin, K. B., Dong, S. A., Ren, Z. F., Yuan, F., Yu, T., Zou, Z. G. & Liu, J. M. Visible-light photocatalytic properties of weak magnetic BiFeO3 nanoparticles. Advanced Materials 19, 2889–2892. Gao, P., Liu, J. C., Zhang, T., Sun, D. D. & Ng, W. Hierarchical TiO2/CdS ‘spindle-like’ composite with high photodegradation and antibacterial capability under visible light irradiation. Journal of Hazardous Materials 229–230, 209–216. Khehra, M. S., Saini, H. S., Sharma, D. K., Chadha, B. S. & Chimni, S. S. Decolorization of various azo dyes by bacterial consortium. Dyes and Pigments 67, 55–61. Konlen’ko, Yu. V., Churagulov, B. R., Kunst, M., Mazerolles, L. & Colbeau-Justin, C. Photocatalytic properties of titania powders prepared by hydrothermal method. Applied Catalysis B: Environmental 54, 51–58. Kusic, H., Koprivanac, N. & Srsan, L. Azo dye degradation using Fenton type processes assisted by UV irradiation: a kinetic study. Journal of Photochemistry and Photobiology A: Chemistry 181, 195–202.
Water Science & Technology
|
68.10
|
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Li, J. X., Xu, J. H., Dai, W. L., Li, H. X. & Fan, K. N. Direct hydro-alcohol thermal synthesis of special core–shell structured Fe-doped titania microspheres with extended visible light response and enhanced photoactivity. Applied Catalysis B: Environmental 85, 162–170. Liu, L. F., Chen, F., Yang, F. L., Chen, Y. S. & Crittenden, J. Photocatalytic degradation of 2, 4-dichlorophenol using nanoscale Fe/TiO2. Chemical Engineering Journal 182–182, 189–195. Liu, Y. C., Zhong, H., Li, L. F. & Zhang, C. J. Temperature dependence of magnetic property and photocatalytic activity of Fe3O4/hydroxyapatite nanoparticles. Materials Research Bulletin 45, 2036–2039. Marco, S. L. & José, A. P. Decolorization of the azo dye reactive black 5 by Fenton and photo-Fenton oxidation. Dyes and Pigments 71, 236–244. Martha, S., Das, D. P., Biswal, N. & Parida, K. M. Facile synthesis of visible light responsive V2O5/N, S–TiO2 composite photocatalyst: enhanced hydrogen production and phenol degradation. Journal of Materials Chemistry 22, 10695–10703. Netpradit, S., Thiravetyan, P. & Towprayoon, S. Adsorption of three azo reactive dyes by metal hydroxide sludge: effect of temperature, pH, and electrolytes. Journal of Colloid and Interface Science 270, 255–261. Pagga, U. & Drown, D. The degradation of dye-stuffs. Part II. behaviour of dyestuffs in aerobic biodegradation tests. Chemosphere 15, 479–491. Parida, K. M. Studies of β-FeOOH. Chemical composition, microstructure and other characteristics of some synthetic akaganeite samples. Journal of Materials Science Letters 6, 1476–1478. Parida, K. M. Studies on β-FeOOH part 2 physico-chemical characteristics of thermally treated akaganeite (β-FeOOH). Journal of Materials Science 23, 1201–1205. Rana, S., Srivastava, R. S., Sorensson, M. M. & Misra, R. D. K. Synthesis and characterization of nanoparticles with magnetic core and photocatalytic shell: anatase TiO2– NiFe2O4 system. Materials Science and Engineering B 119, 144–151. Ristic, M., Music, S. & Orehovec, Z. Thermal decomposition of synthetic ammonium jarosite. Journal of Molecular Structure 744, 295–300. Sahu, N. & Parida, K. M. Photocatalytic activity of Au/TiO2 nanocomposite for Azo-Dyes degradation. Kinetics and Catalysis 53, 197–205. Sajjad, A. K. L., Shamaila, S., Tian, B. Z., Chen, F. & Zhang, J. L. Comparative studies of operational parameters of degradation of azo dyes in visible light by highly efficient WOx/TiO2 photocatalyst. Journal of Hazardous Materials 177 (1–3), 781–791. Shu, H. Y. & Chang, M. C. Decolorization effects of six azo dyes by O3, UV/O3 and UV/H2O2 processes. Dyes and Pigments 65, 25–31. Sleiman, M., Vildozo, D., Ferronato, C. & Chovelon, J. M. Photocatalytic degradation of azo dye metanil yellow:
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Visible light-degradation of azo dye using TiO2/β-FeOOH catalyst
optimization and kinetic modeling using a chemometric approach. Applied Catalysis B: Environmental 77, 1–11. Sun, J., Yan, X., Lv, K. L., Sun, S., Deng, K. J. & Du, D. Y. Photocatalytic degradation pathway for azo dye in TiO2/UV/ O3 system: hydroxyl radical versus hole. Journal of Molecular Catalysis A: Chemical 367, 31–37. Tong, G. X., Guan, J. G. & Zhang, Q. J. Goethite hierarchical nanostructures: glucose-assisted synthesis, chemical conversion into hematite with excellent photocatalytic properties. Materials Chemistry and Physics 127, 371–378. Wang, Z. H., Jiang, T. S., Du, Y. M., Chen, K. M. & Yin, H. B. Synthesis of mesoporous titania and the photocatalytic activity for decomposition of methyl orange. Materials Letters 60, 2493–2496. Wang, J., Zhao, G., Zhang, Z. H., Zhang, X. D., Zhang, G., Ma, T., Jiang, Y. F., Zhang, P. & Li, Y. Investigation on degradation of azo fuchsine using visible light in the presence of heat-treated anatase TiO2 powder. Dyes and Pigments 75 (2), 335–343. Wu, Y., Guo, J., Jiang, D. J., Zhou, P., Lan, Y. Q. & Zhou, L. X. Heterogeneous photocatalytic degradation of methyl orange in schwertmannite/oxalate suspension under UV irradiation.
Water Science & Technology
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68.10
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2013
Environmental Science and Pollution Research International 19, 2313–2320. Xu, Z. H., Liang, J. R. & Zhou, L. X. Photo-Fenton-like degradation of azo dye methyl orange using synthetic ammonium and hydronium jarosite. Journal of Alloys and Compounds 546, 112–118. Yang, Z. P., Gong, X. Y. & Zhang, C. J. Recyclable Fe3O4/ hydroxyapatite composite nanoparticles for photocatalytic applications. Chemical Engineering Journal 165, 117–121. Yuan, Z. Y., Ren, T. Z. & Su, B. L. Surfactant mediated nanoparticle assembly of catalytic mesoporous crystalline iron oxide materials. Catalysis Today 93–95, 743–750. Zhao, Y. P., Hu, J. Y. & Chen, H. B. Elimination of estrogen and its estrogenicity by heterogeneous photo-Fenton catalyst β-FeOOH/resin. Journal of Photochemistry and Photobiology A: Chemistry 212, 94–100. Zhong, Y. H., Liang, X. L., Tan, W., Zhong, Y., He, H. P., Zhu, J. X., Yuan, P. & Jiang, Z. A comparative study about the effects of isomorphous substitution of transition metals (Ti, Cr, Mn, Co and Ni) on the UV/Fenton catalytic activity of magnetite. Journal of Molecular Catalysis A: Chemical 372, 29–34.
First received 30 April 2013; accepted in revised form 16 July 2013. Available online 24 October 2013
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