Microwave-assisted synthesis of hierarchical Bi7O9I3 microsheets for efficient photocatalytic degradation of bisphenol-A under visible light irradiation

Chemical Engineering Journal 209 (2012) 293–300

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Microwave-assisted synthesis of hierarchical Bi7O9I3 microsheets for efficient photocatalytic degradation of bisphenol-A under visible light irradiation Xin Xiao a,b, Rong Hao a, Xiaoxi Zuo a, Junmin Nan a,⇑, Laisheng Li a, Weide Zhang b,⇑ a b

School of Chemistry and Environment, South China Normal University; Key Lab of Theoretical Chemistry of Environment, Ministry of Education, Guangzhou 510006, PR China Nano Science Research Center, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" Hierarchical Bi7O9I3 microsheets are

synthesized using microwave route. " Photocatalyst is Bi7O9I3 microsheets. " Bi7O9I3 microsheets have visible-

light photocatalytic ability to bisphenol-A. " The photocatalysis of bisphenol-A over Bi7O9I3 microsheets is investigated.

a r t i c l e

i n f o

Article history: Received 18 May 2012 Received in revised form 30 July 2012 Accepted 31 July 2012 Available online 10 August 2012 Keywords: Bisphenol-A Bi7O9I3 Visible-light photocatalysis Microwave synthesis

a b s t r a c t A simple and energy-saving microwave heating route for rapid synthesis of Bi7O9I3 sheet-like hierarchical architectures has been demonstrated. The visible-light-induced photocatalytic performances of the prepared materials for the degradation of bisphenol-A (BPA, a known endocrine disrupting chemical) are studied systematically. Using Bi7O9I3 photocatalyst synthesized by 400 W microwaves heating 180 s and with a catalyst dosage of 1 g L1 in 20 mg L1 aqueous solution of BPA, a degradation percentage of 99% is obtained under visible-light irradiation for 60 min. The photocatalytic reaction follows pseudo first-order kinetics according to the Langmuir–Hinshelwood model, and the reaction rate constant of the optimal sample is over 16 times greater than that of the commercial Degussa P25 catalyst based on TiO2. In addition, two main intermediates are identified using liquid chromatography combined with mass spectrometry (LC–MS) technique, and subsequently, a simple and direct photodegradation mechanism is proposed. Furthermore, the as-synthesized photocatalysts exhibit a high mineralization capacity of BPA and good stability during the photodegradation reaction, suggesting a promising prospect in the practical application of the photodegradation of organic pollutants. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Bisphenol-A [2,2-bis(4-hydroxyphenyl)propane; BPA], an important raw organic chemical material, has been widely used in household and commercial products, including the lining of food cans, dental sealants, polycarbonate plastics, and epoxy resins. ⇑ Corresponding authors. Tel.: +86 20 39310255; fax: +86 20 39310187. E-mail address: [email protected] (J.M. Nan). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.07.142

However, it is also notably recognized as an endocrine disrupting chemical (EDC) and is highly toxic to aquatic organisms [1,2]. Thus, the removal of BPA in aquatic environment is a necessary topic for environmental protection [3,4]. So far, several methods have been developed to remove BPA from aqueous solutions, including physical absorption [5], filtration [6], microbial degradation [7], UV photolysis [8], ultrasonic degradation [9], electrochemical techniques [10], Fenton reactions [11], ozone oxidation [12], and photocatalytic degradation [13–16].

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Among these methods, photocatalytic degradation is a promising one due to its advantages for the removal of BPA such as high degradation and mineralization efficiency, low toxicity, and operating under ambient conditions [17–19]. Though the photocatalytic reaction of BPA by TiO2 has been extensively studied, the practical application of TiO2 is still limited due to its wide band gap energy (only effective under ultraviolet irradiation at k < 380 nm). What’s more, it is well known that the photocatalysts with nanosize are difficult to be separated completely from a slurry system after photocatalytic reaction owing to their small size, which seriously limits their practical application [20]. Therefore, the exploitation of novel, effective, visible-light-driven and easily reused photocatalysts is necessary to overcome these drawbacks. Recently, as a new family of promising photocatalysts, bismuth oxyhalides (BiOX, X = F [21], Cl [22], Br [23], and I [24]), have demonstrated remarkable photocatalytic activities due to their uniquely layered structures with an internal static electric field perpendicular to each layer, which can induce effective separation of photogenerated electron–hole pairs, and thus achieve a high photocatalytic performance. Other complex bismuth oxyhalides, such as Bi3O4Cl [25], Bi4NbO8Cl [26], PbBiO2Cl [27], Bi4O5I2 [28], and Bi5O7I [29], have been investigated and show good photocatalytic ability under visible light irradiation for the degradation of various organic contaminants. In our previous work [30], a novel hierarchical Bi7O9I3 micro/nano-architecture was synthesized using a facile oil-bath procedure, which showed high photocatalytic activity in the decomposition of phenol under visible light irradiation and could easily be recycled, suggesting it could be a promising photocatalyst in the treatment of wastewaters containing organic pollutants. However, until now, there have been very few studies on the synthesis of Bi7O9I3 materials and their photocatalytic properties for the degradation of organic pollutants. Microwave heating has been widely applied in chemical reactions and materials synthesis with several advantages compared with conventional heating, such as rapid heating, faster kinetics, homogeneity, higher yield, better reproducibility, and energy savings [31]. Previously, several bismuth-containing photocatalytic materials, such as Bi2O3 [32], Bi2S3 [33], Bi2WO6 [34], Bi2MoO6 [31], BiVO4 [35], BiFeO3 [36], and BiOBr [37] have been successfully synthesized by the microwave-assisted method. We synthesized hierarchical Bi7O9I3 microsheets through a novel and fast microwave heating route and subsequently studied their photocatalytic performance in removing BPA in aqueous solutions under visible light irradiation. In the present study, the morphology, structure and photoabsorption property of the as-synthesized hierarchical Bi7O9I3 microsheets were characterized, the optimal conditions of synthesis and photocatalytic degradation were explored, and a possible photocatalytic degradation mechanism was proposed.

2. Experimental 2.1. Materials and methods The BPA was purchased from Sinopharm Group Chemical Reagent Co. Ltd. Bismuth nitrate pentahydrate (Bi(NO3)35H2O) and potassium iodide (KI) were bought from Tianjin Kermel Chemical Reagent Co. Ltd. Ethylene glycol (EG) was obtained from Chinasun Specialty Products Co. Ltd. All chemicals were used as received without further purification. Distilled water was used to prepare the solutions in the experiments. In a typical synthesis, 0.485 g Bi(NO3)35H2O and 0.166 g KI were dissolved completely in 50 mL EG by stirring at room temperature using a 150 mL round flask as the container. Then, the reaction was performed in a microwave chemical reactor (MCR-3, at a fixed frequency of 2.45 GHz and a maximum output of 800 W,

Beijing Rui Cheng Wei Industry Equipment Co. Ltd., China) equipped with in situ magnetic stirring and a condensing apparatus. After completion of the reaction, the mixture was continuously stirred for 30 min until it reached room temperature; then, the precipitates were collected by centrifugation, washed several times with distilled water and ethanol, and finally dried in an oven overnight at 60 °C. The five typical samples synthesized under different microwave irradiation powers and reaction times are denoted as S1 (400 W, 180 s), S2 (240 W, 180 s), S3 (560 W, 180 s), S4 (400 W, 120 s), and S5 (400 W, 240 s), respectively. 2.2. Characterization and photocatalytic measurement The morphologies and microstructures of as-prepared samples were analyzed by a scanning electron microscope (SEM, JSM6510, JEOL, Japan). Crystalline phases of the obtained samples were identified by powder X-ray diffractometer (XRD, Y2000, Dandong, China) with Cu Ka as the radiation source for a 2h range of 10– 80°. The average crystallite size of catalysts was estimated using the Debye–Scherrer equation:

D ¼ Kk=b cos h

ð1Þ

where D is the average crystallite size (nm), K is a constant which is taken as 0.89 here, k is the wavelength of the X-ray radiation (nm), b is the full width at half maximum (FWHM) after subtraction of equipment broadening, and h is the Bragg angle of the peak. The UV–vis diffuse reflection spectra (DRS) were obtained using a UV–vis spectrophotometer (UV-3010, Hitachi, Japan). The band gap energy (Eg) of these samples was evaluated using the following equation [38]:

aðhv Þ ¼ Aðhv  Eg Þn=2

ð2Þ

where a, m, Eg and A are the absorption coefficient, light frequency, band gap energy, and a constant, respectively. Among them, n depends on the characteristics of the transition in a semiconductor. For BiOX, the value of n is 4 for their indirect transition [39,40]. Then band gap energies (Eg values) of BiOX can be thus estimated from a plot of (ahm)1/2 versus photon energy (ahm). The specific surface area was measured by nitrogen adsorption– desorption isotherms at 77 K according to the Brunauer–Emmett– Teller analysis (BET, ASAP 2020, Micromeritics, USA). A desorption isotherm was used to determine the pore size distribution using the Barrett–Joyner–Halenda (BJH) method. The photocatalytic degradation experiments were performed in a photochemical reactor (XPA-VII, Nanjing Xujiang Machine-electronic Plant, China), equipped with a 1000 W Xe lamp combined with a 420-nm cut-off filter as the light source. In each experiment, a certain amount of the as-synthesized Bi7O9I3 catalyst (varying from 0.2 to 2.0 g L1) was added to a 50 mL reaction solution containing BPA with various initial concentrations (ranging from 10 to 50 mg L1). Prior to irradiation, the solution with the catalyst was stirred for 1 h in the dark to allow the system to reach adsorption equilibrium. During the photocatalytic process, approximately 2.5 mL of the suspension was taken out at a specified time, and subsequently, the solids were removed from the solution using a 0.45 lm nitrocellulose filter and the filtrate were then identified using UV–vis spectroscopy (UV-1800, Shimadzu, Japan, k = 276 nm) to obtain the BPA concentrations in the solution. The total organic carbon concentration was measured by an automatic total organic carbon analyzer (TOC-V, Shimadzu, Japan). 2.3. Intermediates analysis The BPA and its intermediates in the solution were separated using high-performance liquid chromatography (HPLC, LC-10AT,

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Shimadzu, Kyoto, Japan) equipped with a C18 reverse phase column (5 lm, 4.6  250 mm) at 30 °C, with an injection volume of 20 lL. The mobile-phase composition was acetonitrile/water (80/ 20, v/v) at a flow rate of 1 mL min1. The LC–MS analysis was performed on a Surveyor liquid chromatography (Thermo, USA), equipped with a photodiode array (PDA) detector, and a LCQ DECA XP MAX mass spectrometer (Thermo Finnigan, USA) that consists of an ESI interface and an ion trap mass analyzer. The software for the control of the equipment and for acquiring and treating data is the Xcalibur 1.3 workstation. 3. Results and discussion 3.1. Characterization of catalysts 3.1.1. Surface morphologies It was observed that no deposition could be obtained if the microwave power was too low (<80 W) or if the reaction time was too short (<90 s). With increasing microwave power and irradiation time, the deposition appeared more quickly. However, if the power was very high (>640 W) or the reaction time too long (>300 s), the reaction became violent and was difficult to control. The morphologies and surface structures of the as-synthesized five samples were studied by SEM, as shown in Fig. 1. From the SEM observation, it is clearly shown that all samples predominantly consist of a large number of microsheets (Figs. 1A and C–F). The sheet size of the entire structure ranges from 2 to 10 lm and the thickness ranges from 100 to 500 nm. The highly magnified SEM images

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(Fig. 1B) show that the surfaces of the microsheets consist of many nanosheets that connect to each other, with thicknesses of approximately 10 nm, which eventually form special hierarchical micro/ nano-architecture. It is worth noting that the samples obtained by different microwave irradiation powers and reaction times have similar hierarchical structure morphologies, although higher irradiation powers or longer times seem to lead to thicker sheets. 3.1.2. Crystal structure The powder X-ray diffraction (XRD) patterns provide the crystal structure and phase information of the as-synthesized samples. As shown in Fig. 2, in the full-range of 10–80°, the diffractive patterns of the samples are quite similar but shift slightly to smaller diffraction angles compared with those of standard tetragonal structure for BiOI (JCPDS No. 73-2062). All the characteristic diffraction peaks of the as-synthesized samples then match well with those of Bi7O9I3 [30,41,42,28] at 2h at 28.7°, 31.5°, 36.8°, 45.2°, 49.2°, and 54.6°. What’s more, the yields of samples S1–S5 were measured to be 0.2714, 0.2697, 0.2754, 0.2752, and 0.2668 g, respectively, which is very close to the theoretical yields of 0.284 g. Then the mean crystallite sizes were estimated from the width of the peaks using the Scherrer equation (Eq. (1)). It was found that although a larger crystallite size could be formed with higher powers or longer times, the mean crystallite sizes of the five samples estimated from their peaks at 28.7° using the Scherrer equation were all less than 15 nm (see Table 1), which confirms that Bi7O9I3 nanocrystals with hierarchical structures were synthesized by this microwave heating route.

Fig. 1. (A and B) Low-magnification and high-magnification SEM images of the as-synthesized sample S1, (C–F) low-magnification SEM images of samples S2–S5.

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The values determined in this work are close to those reported in the literature [28,30]. These results indicate that the as-synthesized samples have suitable band gap energy for photocatalytic decomposition of organic contaminants under visible light irradiation.

Fig. 2. XRD patterns of the as-synthesized Bi7O9I3 samples.

Table 1 The synthesis condition, mean crystallite size, maximum optical absorption, estimated band-gap energy, BET surface area and apparent reaction rate constant of assynthesized samples. Sample Microwave power (W)

Reaction time (s)

Crystallite size (nm)

Abs. Eg Surface kapp (nm) (eV) area (min1) (m2 g1)

S1 S2 S3 S4 S5

180 180 180 120 240

9.6 10.3 12.1 10.5 14.2

592 582 584 583 584

400 240 560 400 400

2.24 2.26 2.28 2.30 2.27

53.96 50.20 52.16 43.65 48.43

0.08117 0.06593 0.06827 0.04399 0.06194

3.1.4. Formation mechanism Based on the SEM, XRD and DRS, it is reasonable to assume that the formation mechanism of the current Bi7O9I3 hierarchical microsheets is similar to the Bi7O9I3 synthesized by oil-bath heating in our previous work [30], namely, the formation consisted of an oxidation–reduction reaction under an open-to-atmosphere system following an aggregation and dissolution–recrystallization process. First, EG acts as the high-boiling point solvent, stabilizer and coordinating agent [43] to prevent the rapid hydrolysis of Bi(NO3)3, and the Bi7O9I3 nuclei were quickly formed under a high reaction temperature and a relatively high pH. Then, these crystal nuclei developed rapidly into nanosheets, and afterwards, the nanosheets aggregated into large sheet-oriented particles, which then underwent a dissolution–recrystallization process and grew into new two-dimensional microsheet structures. As a result, hierarchical Bi7O9I3 microsheets with numerous nanosheets standing on the surface were formed. Compared with the previous oil-bath heating method [30], the current method using microwave irradiation seems to have multiple advantages and features. First, the reaction rate of the microwave reaction is very fast and effective, particularly using a highly polarized solvent (EG) as the medium. In the current system, to obtain highly crystallized products, only 2–4 min is needed,

3.1.3. Photoabsorption property Fig. 3 shows the UV–vis absorption spectra of the as-synthesized samples. The five samples all exhibited strong photoabsorption in the ultraviolet to visible light range, where the optical absorption edges for samples S1, S2, S3, S4, and S5 were found to be 592, 582, 584, 583, and 584 nm, respectively. By using Eq. (2), the estimated band gap energies for samples S1–S5 were calculated to be approximately 2.24, 2.26, 2.28, 2.30, and 2.27 eV, respectively, as shown in the inset of Fig. 3 and included in Table 1.

Fig. 3. UV–vis diffuse reflectance spectra of the as-synthesized Bi7O9I3 samples. The inset shows the plots of (ahm)1/2 vs. photon energy (hm).

Fig. 4. (A) Degradation curves of BPA with an initial concentration of 20 mg L1 under visible light irradiation on (a) Blank (without catalyst), (b) Degussa P25 and (c) Bi7O9I3 microsheets (sample S1). (B) Photocatalytic degradation kinetics curves of BPA on Degussa P25 and samples S1, S2, S3, S4 and S5, respectively.

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Fig. 5. Effects of the initial concentration of (A) BPA solutions and (B) catalyst dosage on the photocatalytic performances of Bi7O9I3 microsheets (sample S1). Experimental conditions: reaction time = 60 min, catalyst dosage = 1 g L1 for BPA concentration experiments, and C0 = 20 mg L1 for catalyst dosage experiments.

which is fast and more economical than conventional oil-bath heating. Second, due to its rapid reaction, the Bi7O9I3 samples obtained by the microwave route are smaller in size and thinner than samples synthesized by the oil-bath method, which results in a larger specific surface area. As shown in Fig. S1A (Supporting Information), the isotherm of Bi7O9I3 microsheets (sample S1) is identified as type IV with a H3 hysteresis loop [44,45]. The specific surface area of the sample was calculated from N2 isotherms and was found to be as much as 53.96 m2 g1, which is much larger than that of our previous work using oil-bath heating method [30]. And the corresponding pore size distribution determined by using the BJH method reveals the mesoporous structure (BJH pore diameter = 3.8 nm) (Fig. S1B, Supporting Information). Then the specific surface area of sample S2, S3, S4 and S5 are estimated to be 50.20, 52.16, 43.65, and 48.43 m2 g1, respectively, using the same method. It is well-known that materials with high specific surface area and porous structrues often show excellent adsorptive and catalytic activities. Therefore, the as-synthesized Bi7O9I3 microsheets are reasonable to expect that good photocatalytic performance. 3.2. Photocatalytic degradation of BPA 3.2.1. Evaluation of the photocatalytic activity The time dependence of the removal ratio of BPA on the Bi7O9I3 microsheets under visible-light irradiation was investigated and compared with Degussa P25 (a commercial, highly active, mixedphase titania photocatalyst). Only the degradation curve of sample S1 is shown in Fig. 4A with that of Degussa P25 and without any

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Fig. 6. (A) TOC removal in the presence of Bi7O9I3 microsheets with an initial BPA concentration of 20 mg L1 and catalyst dosage of 1 g L1 under visible light irradiation. (B) Recycling properties of the Bi7O9I3 microsheet photocatalyst.

photocatalyst (Blank) due to the restriction of plotting, and the other samples have similar curve characteristics. It is clearly shown that BPA barely degraded without any photocatalyst (Blank), whereas the Bi7O9I3 microsheets exhibited a much higher activity than that with Degussa P25 under visible-light exposure. After irradiation for 60 min, only 23% of the BPA was removed with Degussa P25, while the removal efficiency of the as-synthesized sample S1 reached 99%, which demonstrates that the latter is an excellent visible-light-driven photocatalyst. The low visible light photocatalytic activity of Degussa P25 is attributed to its limited utilization of visible light. The reaction kinetics of BPA by Degussa P25 and samples S1–S5 were obtained by fitting the experimental data to the Langmuir– Hinshelwood model [46,47]. The equation used is as follows:

r¼

dC kr KC ¼ kr h ¼ dt ð1 þ KCÞ

ð3Þ

where r is the reaction rate, kr is the reaction rate constant, h is the surface coverage, K is the adsorption constant (in the dark), and C is the reactant concentration. When C is very small, the product KC is negligible with respect to unity, and the above equation can be described by pseudo first-order reaction kinetics. At the initial stage of photocatalytic reaction, the substrate concentration C = C0, the equation can be expressed as:

 ln



Ct C0



¼ kapp t

ð4Þ

where kapp (min1) is the apparent reaction rate constant. As shown in Fig. 4B, the linear relationship of ln(C0/Ct) vs. reaction time t indicates that the reaction can be described well by the pseudo first-or-

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der kinetics with high correlation coefficients (R > 0.985). The calculated kapp values for the Degussa P25 and the samples S1–S5 are 0.00494, 0.08117, 0.06593, 0.06827, 0.04399, and 0.06194 min1, respectively, which summarized in Table 1. These results show that sample S1 exhibits the maximum reaction rate constant for the degradation of BPA under visible light irradiation, which is more than 16 times greater than that of the Degussa P25. The high removal efficiencies of the as-synthesized samples of degrading BPA are ascribed to their smaller band gap energy and hierarchical structure characteristics with high specific surface area, porous structure and high surface-to-volume ratios. And the different removal efficiencies of the five samples were found to be primarily related to their surface area rather than the influence of microwave power or irradiation time of the reactions. Then, the sample S1 is used in the following experiments. 3.2.2. Effects of the initial conditions on photocatalytic degradation Different initial concentrations of BPA solutions, ranging from 10 to 50 mg L1, were used to observe the effect of varying the BPA concentration on the degradation efficiency. The results of their reaction rates are depicted in Fig. 5A, where it is clearly

shown that the lower the BPA concentration, the higher the efficiency of the BPA decomposition [48,49]. An explanation for this result is that as the initial concentration of BPA increases, more and more organic substances can be adsorbed on the surface of photocatalyst, thus the photogeneration of reactive oxygen species will be reduced since there is overloading of the active sites. In addition, increasing BPA concentration leads to an increase of the amount of incident photons which are absorbed by the pollutant molecules and never reach the photocatalyst surface [50,51]. In addition, the effect of the catalyst dosage on the degradation rate was also studied by varying the catalyst amounts from 0.2 to 2.0 g L1, and the results are illustrated in Fig. 5B. The degradation efficiency of BPA increased significantly as the catalyst concentration increased, from 0.2 to 1 g L1, and then decreased slightly with a further increase in the catalyst concentration. It is not difficult to understand that a lower catalytic activity was exhibited with a low catalyst dosage because of the smaller amount of catalytic active sites. However, if the dosage is much higher than the optimal value, an increase in the opacity and light scattering of the catalyst and a decrease in the number of catalytic surface active sites, likely caused by aggregation of the catalyst particles [52], will result in

Fig. 7. (A) LC–MS chromatograms of BPA degradation by Bi7O9I3 under visible-light irradiation. (B) Evolution of intermediates in BPA degradation during the photocatalysis process by calculating the peak areas of the extracted ion chromatography (EIC) of the product ions. (C–E) Mass spectra and corresponding proposed structures at m/z 227, 151 and 219 after a photoreaction of 40 min, respectively. (F) Photocatalytic degradation of BPA over Bi7O9I3 in the absence or presence of i-PrOH.

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the degradation efficiency of the BPA to no longer increase or even decline. Therefore, 1 g L1 was selected as the optimal catalyst dosage of the photocatalytic reaction. 3.2.3. TOC removal and catalyst recycle The concentration of total organic carbon (TOC) was chosen as a mineralization index to characterize the BPA degradation. The time independence of the TOC data in the BPA solution in the presence of the Bi7O9I3 catalyst under visible light irradiation is shown in Fig. 6A. It is observed that 89.1% of the TOC was eliminated after 60 min of irradiation, indicating that most of the BPA were mineralized in this process, which is important for the practical application of Bi7O9I3 microsheet photocatalysts to avoid secondary pollution. Moreover, it is well known that the separation of a photocatalyst from a suspended solution after a reaction is also important for its practical application. Due to the relatively larger size, the photocatalyst of the Bi7O9I3 microsheets has an advantage over other nanostructured catalysts, as the catalysts can be separated by simple filtration step. To investigate the recyclability, the assynthesized photocatalysts were collected after the photocatalytic reaction by filtration and reused in a photocatalytic reaction three times under the same conditions. As shown in Fig. 6B, the as-synthesized catalyst displayed good stability and maintained a high photocatalytic performance during the three reaction cycles. Additionally, XRD analysis and SEM observation on the Bi7O9I3 sample after the photocatalytic reaction, as indicated in Figs. S2A and S2B (Supporting Information), reveals that it structure and morphology remain intact. The results of the TOC removal efficiency and good stability indicate that the Bi7O9I3 microsheets synthesized by this facile method are promising in the practical application of photodegradation of pollutants. 3.3. Degradation intermediates of BPA Fig. 7A illustrates the LC–MS chromatograms of the BPA solution at different reaction times. It can be clearly observed that BPA (m/z 227, Figs. 7B and C), eluted at a retention time of 10.88 min, rapidly disappeared. Only two intermediates, at m/z 151 and 219, were observed by LC–MS analysis at a retention time of approximately 6.37 and 13.17 min, identified as methyl 4hydroxybenzoate (Fig. 7D) and hydroquinone (Fig. 7E), respectively. Furthermore, from Fig. 7A and B, it can also be observed that the intermediates at m/z 151 increased after the photoreaction, reaching its maximum concentrations after approximately 40 min and then rapidly decreased until almost completely disappearing after 240 min. The situation is similar at m/z 219, as seen in Fig. 7A. These results indicate that the two intermediates were produced during the photodegradation reaction and could then degrade continuously by the following photocatalytic processes. Many reports on the photocatalytic degradation of BPA using the TiO2 system suggested that BPA degradation primarily goes through demethylation and hydroxylation due to the generation of the important reactive species, hydroxyl radical (OH) [53–55]. However, in this visible-light-induced Bi7O9I3 system, no hydroxylated compound was found, which may be because the standard redox potential of Bi5+/Bi3+ (E0 = 1.59 V at pH = 0) is smaller than that of HO/OH (1.99 V) [56]. To provide evidence that OH is not the main active oxidative species involved in the photodegradation process, an additional experiment was performed in which isopropanol (i-PrOH) was added, which was used as the OH quencher in the photoreaction system. As shown in Fig. 7F, it was found that the photodegradation activity of BPA decreased slightly when iPrOH was added. Therefore, it is suggested that BPA photodegradation by Bi7O9I3 is dominated by direct holes oxidation, and thus, the degradation pathway of BPA is quite distinct from that oxidized

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by OH. This result is similar to the previous report on the degradation of BPA by Bi3.84W0.16O6.24 photocatalyst under simulated solar light irradiation [57]. Based on the aforementioned experimental results, a direct and simple pathway of BPA degradation by Bi7O9I3 is proposed: Bi7O9I3 can be efficiently stimulated to create electron–hole pairs under visible light irradiation due to its narrow band gap. Then, the photogenerated holes react directly with the BPA molecules adsorbed on the catalyst surface to form hydroquinone and methyl 4-hydroxybenzoate as the main degradation intermediates. And these intermediates were eventually mineralized to come into CO2 and H2O by the sequentially photocatalytic processes. 4. Conclusion In summary, visible-light-active Bi7O9I3 photocatalyst with sheet-like hierarchical architectures was successfully synthesized by using the unique heating mode of microwave route for the first time. The as-synthesized materials demonstrated an excellent visible-light-induced photocatalytic activity, high mineralization capacity and good stability for the degradation of BPA in aqueous solution. The optimal conditions of synthesis and photocatalytic degradation were investigated. The results of the LC–MS and the analyses of the photocatalytic mechanism suggest that direct holes are predominant in the degradation of BPA by Bi7O9I3 under visible light irradiation, and the main degradation intermediates are hydroquinone and methyl 4-hydroxybenzoate. Thus, the as-synthesized Bi7O9I3 photocatalyst is a very promising prospect in the treatment of organic pollutants in wastewater for industrial applications. Acknowledgement This study was financially supported by the National Natural Science Foundation of China (Contract No. 20977036). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2012.07.142. References [1] F.S. Vom Saal, C. Hughes, An extensive new literature concerning low-dose effects of bisphenol A shows the need for a new risk assessment, Environ. Health. Perspect. 113 (2005) 926–933. [2] L.N. Vandenberg, R. Hauser, M. Marcus, N. Olea, W.V. Welshons, Human exposure to bisphenol A (BPA), Reprod. Toxicol. 24 (2007) 139–177. [3] J.G. Hengstler, H. Foth, T. Gebel, P.-J. Kramer, W. Lilienblum, H. Schweinfurth, W. Völkel, K.-M. Wollin, U. Gundert-Remy, Critical evaluation of key evidence on the human health hazards of exposure to bisphenol A, Crit. Rev. Toxicol. 41 (2011) 263–291. [4] H. Melcer, G. Klecka, Treatment of wastewaters containing bisphenol A: state of the science review, Water Environ. Res. 83 (2011) 650–666. [5] F.M. Cao, P.L. Bai, H.C. Li, Y.L. Ma, X.P. Deng, C.S. Zhao, Preparation of polyethersulfone-organophilic montmorillonite hybrid particles for the removal of bisphenol A, J. Hazard. Mater. 162 (2009) 791–798. [6] Y. Zhang, C. Causserand, P. Aimar, J.P. Cravedi, Removal of bisphenol A by a nanofiltration membrane in view of drinking water production, Water Res. 40 (2006) 3793–3799. [7] G. Kabiersch, J. Rajasarkka, R. Ullrich, M. Tuomela, M. Hofrichter, M. Virta, A. Hatakka, K. Steffen, Fate of bisphenol A during treatment with the litterdecomposing fungi stropharia rugosoannulata and stropharia coronilla, Chemosphere 83 (2011) 226–232. [8] E.J. Rosenfeldt, K.G. Linden, Degradation of endocrine disrupting chemicals bisphenol A, ethinyl estradiol, and estradiol during UV photolysis and advanced oxidation processes, Environ. Sci. Technol. 38 (2004) 5476–5483. [9] Z.B. Guo, R. Feng, Ultrasonic irradiation-induced degradation of lowconcentration bisphenol A in aqueous solution, J. Hazard. Mater. 163 (2009) 855–860. [10] Y.H. Cui, X.Y. Li, G.H. Chen, Electrochemical degradation of bisphenol A on different anodes, Water Res. 43 (2009) 1968–1976.

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