BiOIO3 as a Novel Heterojunctional Photocatalyst with

Apr 6, 2017 - ... and the 20% BiOIO3-BiOBr0.75I0.25 sample possesses the optimal photoreactivity, which is 7.4, and 3.3 times higher than those of pur...
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Research Article pubs.acs.org/journal/ascecg

BiOBr0.75I0.25/BiOIO3 as a Novel Heterojunctional Photocatalyst with Superior Visible-Light-Driven Photocatalytic Activity in Removing Diverse Industrial Pollutants Chao Zeng,† Yingmo Hu,*,† and Hongwei Huang*,† †

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China S Supporting Information *

ABSTRACT: A series of novel heterojunctional photocatalysts BiOBr0.75I0.25/BiOIO3 were synthesized by a facile deposition− precipitation method for the first time. In contrast to pristine BiOIO3, the photoabsorption of BiOBr0.75I0.25/BiOIO3 composites in visible light region is greatly promoted. All the BiOBr0.75I0.25/BiOIO3 composite photocatalysts exhibit highly enhanced photocatalytic activity in decomposing bisphenol A under visible light (λ > 420 nm) illumination, and the 20% BiOIO3-BiOBr0.75I0.25 sample possesses the optimal photoreactivity, which is 7.4, and 3.3 times higher than those of pure BiOIO3 and BiOBr0.75I0.25. Moreover, the 20% BiOIO3BiOBr0.75I0.25 sample displays superior photocatalytic performance against diverse industrial contaminants and pharmaceuticals, including methyl orange, phenol, 2,4-dichlorophenol, chlortetracycline hydrochloride, and tetracycline hydrochloride. The enhancement of phototcatalytic activity is ascribed to the profoundly promoted transfer and separation of photoexcited charge carriers, which is verified by transient photocurrent response and photoluminescence emission. In addition, the photocatalytic mechanism over composite photocatalyst under visible light irradiation is systematically investigated by active species trapping experiment and •OH quantification experiment. This work may provide a new hint for fabrication of high-performance heterojunctions by combining the narrow-band gap and wideband gap semiconductors. KEYWORDS: Heterostrucure photocatalyst, BiOBr0.75I0.25, BiOIO3, Visible-light, Diverse industrial pollutants

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INTRODUCTION With the rapid development of global economy, environmental pollution is getting worse and worse, which seriously damages people’s health. Thereby, photocatalysis technology is developed and has been broadly recognized as a promising technology to curb the crises of environmental deterioration.1,2 The inherent properties of semiconductor photocatalysts used determine the efficiency of photocatalysis process. A wide-band gap (WBG) semiconductor usually possesses strong oxidation and reduction force, owing to its further energy band potentials away from Fermi level, nevertheless, it is confined by low utilization efficiency of visible light, which accounts for 48% of solar spectrum.3 On the other hand, a semiconductor with a narrow-band gap (NBG) can effectively absorb visible light, but it suffers from weak photoinduced redox ability. Fabricated by coupling WBG and NBG semiconductors, heterojunctional photocatalysts can set their advantages and overcome the shortcomings. For example, the heterostructure photocatalysts, Ag2O/TiO2, BiOI/BiOCl, and g-C3N4/BiVO4, all exhibit remarkably enhanced photocatalytic activity compared to the single-component semiconductors.4−6 So the heterojunctional © 2017 American Chemical Society

photocatalysts are expected to show appreciable photoabsorption and outstanding visible-light driving photocatalytic performance. BiOIO3, a novel noncentrosymmetrical bismuth-based material, is reported as an efficient semiconductor photocatalyst in the removal of gaseous and liquid pollutants.7,8 Benefiting from its unique crystal configuration, which consists of polar (IO3)− group and (Bi2O2)2+ layers, BiOIO3 exhibits excellent separation efficiency of photoinduced charge carriers.9 However, stuck with its relatively large band gap (∼3 eV), BiOIO3 cannot effectively respond to visible light. Fabrication of heterojunctional photocatalytst is a feasible strategy to ameliorate the visible light absorption of BiOIO3, of which the photocatalytic activity can be improved at the same time. As reported by many previous articles,10−12 bismuth oxyhalides (BiOX, X = Cl, Br, and I), a family of V−VI−VII layered ternary oxides, possess outstanding photocatalytic performance. Received: December 15, 2016 Revised: February 26, 2017 Published: April 6, 2017 3897

DOI: 10.1021/acssuschemeng.6b03066 ACS Sustainable Chem. Eng. 2017, 5, 3897−3905

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JEM-2100 electron microscopy to characterize the microstructure, respectively. The UV−vis diffuse reflectance spectra (DRS) were measured with a Varian Cary 5000 UV−vis spectrophotometer to study the optic property. The photoluminescence emission (PL) spectra were identified by a fluorescence spectrophotometer (Hitachi F-4600) using a xenon lamp as an excitation source. All of the abovementioned measurements were conducted at room temperature. Photocatalytic Activity. The photocatalytic activities of the asprepared composite photocatalysts were evaluated by photocatalytic degradation of diverse industrial contaminants, including MO (1 × 10−5 mol/L), phenol (10 mg/L), BPA (10 mg/L), 2,4-DCP (10 mg/ L), tetracycline hydrochloride (10 mg/L), and chlortetracycline hydrochloride (10 mg/L) under visible light (λ > 420 nm) illumination. 50 mg photocatalyst powder was ultrasonically suspended in 50 mL of pollutant aqueous solution. Prior to irradiation, the mixtures in quartz tubes were vigorously stirred in the dark for 30 min to get an adsorption−desorption equilibrium between catalyst and aqueous solution. After illumination, about 3 mL of suspension was taken from each quartz tube for every 30 min and centrifuged to obtain the supernatant liquids. The concentration and temporal absorption spectrum of pollutant aqueous solution was recorded on a Cary 5000 UV−vis spectrophotometer. Active Species Trapping Experiment and •OH Quantification Experiment. Several kinds of active species, including hydroxyl radical (•OH), superoxide radical (•O2−), and hole (h+), would be produced in the photocatalysis process. To identify the active species generated during the photocatalytic process, another BPA photodegradation experiment was conducted by adding 1 mM disodium ethylenediaminetetraacetate (EDTA-2Na), 1 mM benzoquinone (BQ) and 1 mM isopropyl alcohol (IPA) to scavenge h+, •O2−, and •OH, respectively. To further estimate the relative concentration of •OH, terephthalic acid (TA) was taken to react with •OH to produce 2hydroxyterephthalic acid, which is a highly fluorescent product. The amount 2-hydroxyterephthalic acid, which is proportional to the amount of •OH generated in a photocatalysis system, was quantitatively analyzed with a fluorescence spectrophotometer at 425 nm excited and at 315 nm after centrifugation. Photoelectrochemical Measurements. The photocurrent response and Mott−Schottky curve were performed on an electrochemical analyzer (CHI660E, Shanghai) equipped with a standard three-electrode system and using Na2SO4 (0.1 M) as electrolyte solution. A 300 W xenon lamp equipped with a 420 nm filter at 0.0 V is taken as the light source. The saturated calomel electrode (SCE) was employed as a reference electrode, and the platinum wires worked as a counter electrode. The working electrodes were BOBI, 20%-BOIOBOBI, and BOIO films coated on ITO.

In 2008, Huang et al. reported a new solid solution BiOBrxI1−x with distinctive visible light absorption and photocatalytic performance in photodegrading methyl orange (MO) under visible light irradiation.13 Since then, BiOBr0.75I0.25 draws intensive attention by virtue of its prominent photocatalytic capability in the decomposition of azo-dye.14 Given the benign energy band potential and visible light absorption, rose-red colored BiOBr0.75I0.25 may be a good choice for constructing a heterostructure to promote the absorption in the visible light region and photocatalysis performance of BiOIO3. In this work, a series of BOBI/BOIO (BiOBr0.75I0.25/ BiOIO3) heterojunction photocatalysts with different molar ratios of BOIO/BOBI (5%, 10%, 20%, 40%) were synthesized by a facile deposition−precipitation method for the first time. With increasing the amount of BOBI, the photoabsorption edge of BOBI/BOIO composite successively red shifts from 420 to 670 nm. The photocatalytic activity of BOBI/BOIO composites, compared with BOIO precursor, is greatly enhanced in degrading bisphenol A (BPA) under visible light (λ > 420 nm) illumination. In addition, the 20% BOIO-BOBI still exhibits appreciable photocatalytic activity in the decomposition of diverse industrial contaminants and pharmaceuticals, including methyl orange (MO), phenol, 2,4dichlorophenol (2,4-DCP), chlortetracycline hydrochloride, and tetracycline hydrochloride under the excitation of visible light (λ > 420 nm). The photocatalytic mechanism of BOBI/ BOIO composite photoctalyst is investigated systematically.

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EXPERIMENTAL SECTION

Preparation of the Photocatalyst. All reagents used were of analytical grade. BiOIO3 was synthesized by hydrothermal method. Typically, 2.91g of Bi(NO3)3·5H2O was dispersed in 10 mL of deionized water and 4 mL of HNO3 with ultrasonic to obtain a homogeneous suspension. 1g of I2O5 was ultrasonically dissolved in 10 mL of deionized water. Under magnetic agitation, the I2O5 aqueous solution was dropwise added to the above Bi(NO3)3 solution. After stirring for 30 min at room temperature, the resultant white suspension was transferred into a 50 mL Teflon-lined stainless steel autoclave, and then 6 mL of deionized water was added into the reaction still. The autoclave was heated at 180 °C for 24 h in an oven. After natural cooling, the white product was collected by filtration, washed with distilled water and ethanol, and then dried at 60 °C for 10 h. The obtained sample BiOIO3 was denoted as BOIO. The BiOBr0.75I0.25/BiOIO3 (BOBI/BOIO) heterojunction photocatalysts were prepared by a facile deposition−precipitation method. Representatively, 0.9701 g of Bi(NO3)3·5H2O and quantitive BiOIO3 precursor with different BiOIO3/BiOBr0.75I0.25 molar ratios of 5%, 10%, 20%, and 40% were ultrasonically dispersed in 20 mL of glycol. Stoichiometric amounts of KBr and KI were dissolved in 20 mL of deionized water, and then the KBr/KI aqueous solution was added into the above Bi(NO3)3·5H2O and BiOIO3 suspension under stirring drop by drop. After stirring for 2 h at ambient temperature, the suspension was centrifuged, washed, and then dried at 60 °C in a desiccator for 10 h. For convenience, the as-obtained samples with different BiOIO3/BiOBr0.75I0.25 molar ratios of 5%, 10%, 20%, and 40% are marked as 5% BOIO-BOBI, 10% BOIO-BOBI, 20% BOIO-BOBI, and 40% BOIO-BOBI, respectively. The pure BiOBr0.75I0.25 (named as BOBI) was fabricated as the above route without adding BiOIO3 precursor. Characterization. X-ray powder diffraction (XRD) was carried out on a Bruker D8 to investigate the crystallinity phase of the as-obtained samples. The X-ray photoelectron spectroscopy (XPS) was performed on a ESCALAB 250xi electron spectrometer to verify the surface chemical situation of samples. Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) images were recorded on Hitachi S-4800 field emission scanning electron microscope and

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RESULTS AND DISCUSSION The XRD patterns of BOIO (BiOIO3), BOBI (BiOBr0.75I0.25), and BOIO-BOBI (BiOIO3-BiOBr0.75I0.25) composite photocatalysts are displayed in Figure 1. The characteristic diffraction peaks of BOBI consist of peaks located at 25.1°, 31.6°, 32.2°, 46.1°, and 56.9°. On the basis of the standard dates of BiOBr (JCPDS #09-0393) and BiOI (JCPDS #10-0445) and previous papers13,14 about BiOBrxI1−x solid solution, it can be deduced that the pure BiOBr0.75I0.25 (BOBI) is synthesized. All of the diffraction peaks of pure BOIO (BiOIO3) are in accordance with the BiOIO3 standard data (ICSD # 262019). With increasing the addition amount of BOIO in the BOIO-BOBI (BiOIO3-BiOBr0.75I0.25) composites, the intensity of peaks corresponding to BOIO gradually ascends, and the corresponding peaks of BOBI can still be found. Thus, the as-obtained BOIO-BOBI composite samples are composed of the BOIO (BiOIO3) and BOBI (BiOBr0.75I0.25) two phases. X-ray photoelectron spectroscopy (XPS) is employed to analyze the element composition and chemical situation of each atoms of the as-prepared BOBI, 20% BOIO-BOBI, and BOIO 3898

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due to the low amount of BOIO in 20% BOIO-BOBI. The high-resolution XPS spectra for Bi 4f and O 1s of BOBI, 20% BOIO-BOBI, and BOIO are exhibited in Figure 2c,d. The band energies of peaks for Bi or O in BOBI and BOIO are different because the chemical condition of the bismuth or oxygen atoms in BOBI are different from that in BOIO.16 Furthermore, the peaks of Bi or O in 20% BOIO-BOBI locate in the middle of that in BOBI and BOIO. The results of XPS further confirm that BOIO and BOBI constitute the as-prepared composite photocatalysts. To investigate the morphology and microstructure of the asobtained samples, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) are carried out, as displayed in Figure 3. The insets in Figure 3a,d,g depict the schematic illustrations on morphology of BOBI, 20% BOIO-BOBI, and BOIO, respectively. The SEM and TEM images in Figure 3a,b exhibit that pure BOBI is composed of flower-like microspheres with a size of about 2 μm, which are constituted by uniform nanosheets. Figure 3c shows the HRTEM image of BOBI, where two adjacent fringes with distances of 0.858 and 0.310 nm can be separately indexed in the (001) and (102) plane of BOBI on the basis of JCPDS #09-0393 (BiOBr) and JCPDS #10-0445 (BiOI) card information. It can be found from Figure 3g,h that yhe pure BOIO sample is composed of irregular blocks with a diameter of 0.1−0.5 μm. In Figure 3i, the lattice fringes with intervals of 0.336 nm correspond to the (121) plane of BOIO (ICSD #262019). For a 20% BOIO-BOBI sample, it can be observed from Figure 3d,e that BOBI nanosheets surround the BOIO cores. According to Figure 3f, two sets of adjacent fringes with interplanar spacing of 0.312

Figure 1. XRD patterns of BOIO (BiOIO3), BOBI (BiOBr0.75I0.25), and BOIO-BOBI (BiOIO3-BiOBr0.75I0.25) composite photocatalysts.

samples, as shown in Figure 2. The main peaks of Bi 4f, O 1s, I 3d, Br 3d, and C 1s can be detected in the spectra of BOBI and 20% BOIO-BOBI. Compared with the above two other spectra, the spectrum of BOIO lacks peaks for Br 3d. The C peak with energy band at about 283.7 eV is credited to the residual carbon from the XPS instrument (Figure 2a). Figure 2b shows the high-resolution XPS spectra for I 3d of BOBI, 20% BOIOBOBI, and BOIO. For BOBI, two peaks at 619.6 and 630.9 eV can be observed, which can be attributed to I− 3d5/2 and I− 3d3/2. For BOIO, two peaks at 624.0 and 635.5 eV can be assigned to I5+ 3d5/2 and I5+ 3d3/2. In 20% BOIO-BOBI, there are four peaks centering at 619.5, 624.2, 630.8, and 635.6 eV, which can be ascribed to I− 3d5/2, I5+ 3d5/2, I− 3d3/2, and I5+ 3d3/2, respectively.15 The peak corresponding to I5+ is weak,

Figure 2. Typical XPS survey spectra (a) and high-resolution XPS spectrum for I 3d (b), Bi 4f (c), and O 1s (d) of BOBI, 20% BOIO-BOBI, and BOIO. 3899

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Figure 3. SEM, TEM, and HRTEM images for BOBI (a−c), 20% BOIO-BOBI (d−f), and BOIO (g−i). The insets in a, d, g depict the schematic illustrations on morphology of BOBI, 20% BOIO-BOBI, and BOIO, respectively.

n for BOIO and BOBI is 2. The Eg of BOIO and BOBI is estimated to be 2.87 and 2.02 eV, respectively, as shown in Figure 4b. The photocatalytic activities of the as-synthesized photocatalysts are first evaluated by the removal of BPA under visible light (λ > 420 nm) illumination. The photodegradation curves of BPA solution over the as-prepared samples are exhibited in Figure 5a. It can be found that BPA cannot be decomposed in the absence of photocatalyst. The photocatalytic activities of all BOIO-BOBI composite samples are stronger than that of pure BOIO and BOBI. The 20% BOIO-BOBI composite photocatalyst shows the best photocatalytic performance among all of the samples. The degradation efficiencies of BOIO, BOBI, and 20% BOIO-BOBI are 7.25%, 45.6%, and 85.2% after 120 min of visible light irradiation, respectively. The reaction kinetics of the BPA photodecomposition process are quantitatively calculated via the following pseudo-first-order equation:19

and 0.290 nm can be attributed to the (102) plane of BOBI and (002) plane of BOIO, respectively. Therefore, the 20% BOIOBOBI sample consists of BOIO and BOBI, and simultaneously the two phases tightly contact with each other, boosting the separation and transfer of photoinduced charges. The light harvesting ability is vital for the photocatalytic activity of semiconductor photocatalysts, which can be investigated by diffuse reflection spectroscopy (DRS). As shown in Figure 4a, one can deduce that the absorption edge of BOIO is about 399 nm, and meanwhile BOBI shows a wide response range of visible light, where the absorption edge extends to about 597 nm. The absorption edges of BOIO-BOBI composite samples all locate between that of BOIO and BOBI, and the photoresponse in visible region is continuously strengthened with the rise of the BOBI amount. Thus, it is believed that the photoabsorption of BOIO can be greatly improved by BOBI, which is conducive to the visible-lightdriven photocatalytic process. The band gap energies of BOIO and BOBI can be determined by the following Kubelka−Munk equation:17 αℏv = A(αℏv − Eg )n

ln(C0/C)=kappt

(2)

where kapp, t, C0, C are the rate constant (min−1), reaction time, initial BPA solution concentration (mol/L), and instantaneous BPA solution concentration (mol/L) at time t, respectively. The apparent rate constants of all the as-obtained samples are shown in Figure 5b. With increasing the addition amount of BOIO in the BOIO-BOBI composites, the reaction rate constant of as-prepared BOIO-BOBI composite samples

(1)

where ℏ, v, α, Eg, and A represent the Planck’s constant, photon frequency, optical absorption coefficient, photonic energy band gap, and a constant, respectively. Both BOIO and BOBI are indirect-transition allowed semiconductors,9,14,18 so the value of 3900

DOI: 10.1021/acssuschemeng.6b03066 ACS Sustainable Chem. Eng. 2017, 5, 3897−3905

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ascends first and reaches the maximum of 0.0149 min−1 at 20% BOIO-BOBI, which is 7.40, and 3.29 times higher than that of BOIO (0.0020 min−1) and BOBI (0.0045 min−1), and then declines with further increasing the amount of BOIO. To further test the photooxidation ability of the 20% BOIOBOBI heterojunction photocatalyst, diverse industrial contaminants and pharmaceuticals, including phenol, 2,4-DCP, MO, chlortetracycline hydrochloride, and tetracycline hydrochloride, which can cause tissue necrosis or reproductive abnormalities to humans,20 were taken as the photodegradation target. Figure 6a−e shows the temporal absorption spectra of phenol, 2,4DCP, tetracycline hydrochloride, chlortetracycline hydrochloride, and MO over the 20% BOIO-BOBI composite photocatalyst under visible light (λ > 420 nm) irradiation. One can see that all of the above-mentioned pollutants can be decomposed by 20% BOIO-BOBI, and the removal efficiencies are 12.5%, 7.53%, 17.6%, 16.7%, and 85.7% for phenol, 2,4DCP, tetracycline hydrochloride, chlortetracycline hydrochloride, and MO with 120 min visible light illumination, respectively, as shown in Figure 6f. These results validate the universally superior photooxidation capability in degrading diverse pollutants over 20% BOIO-BOBI, reflecting its great potential for practical application in environmental remediation. Photostability is pivotal for the practical application of semiconductor photocatalyst and can be evaluated by recycling experiments. As exhibited in Figure 7a, the photocatalytic activity for photodecomposing BPA over 20% BOIO-BOBI composite photocatalyst after four successive cycles did not show significant decline. Moreover, the XRD pattern and microstructure was almost unchanged after photoreaction, compared to the XRD pattern and microstructure before

Figure 4. DRS spectra for the BOBI, BOIO, and BOIO-BOBI composite samples (a) and band gap of BOBI and BOIO (b).

Figure 5. Photodegradation curves (a) and kinetic constants for degradation (b) of BPA solution over the as-prepared BOBI, BOIO, and BOIOBOBI composite samples under visible light (λ > 420 nm) illumination. Time-resolved absorption spectra of BPA for 20% BOIO-BOBI (c) and BOBI (d). 3901

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Figure 6. Temporal absorption spectra of phenol (a), 2,4-DCP (b), tetracycline hydrochloride (c), chlortetracycline hydrochloride (d), and MO (e) over 20% BOIO-BOBI composite photocatalyst under visible light (λ > 420 nm) illumination. Degradation rate of diverse pollutants after 120 min visible light irradiation (f).

20% BOIO-BOBI sample favors the charge transfer. In addition, the PL spectrum can indicate the recombination rate of photoinduced charge carrier, and high emission intensity corresponds to a high recombination rate.22 Figure 8b depicts the PL spectra and photoluminescence excitation (PLE) spectra of BOBI, BOIO, and BOIO-BOBI samples. Emission intensity of all the composite photocatalysts is lower than that of BOIO and BOBI. The 20% BOIO-BOBI sample possesses the lowest intensity among all of the samples, suggesting its possibly lowest recombination rate of electrons and holes. On the basis of above experiment results, it can be inferred that the heterojunction fabrication can facilitate the separation and inhibit the recombination of photoexcited charge carriers. To ascertain the main active species for photodecomposing BPA, active species trapping experiments are carried out with the addition of benzoquinone (BQ), isopropanol (IPA), disodium ethylenediaminetetraacetate (EDTA-2Na) to quench •O2−, •OH, and h+, respectively.23 As shown in Figure 9a, the addition of IPA has a slight effect on BPA degradation. However, the degradation efficiency of BPA is tremendously

photoreaction (Figures 7b and S1). These results indicate the high stability of BOIO-BOBI composite photocatalysts, boding for its promising applications in environmental remediation. The photocatalytic performance of semiconductor phototcatalyst is mainly determined by the separation and transfer efficiency of photoinduced charge carriers. Transient photocurrent response was taken to indicate the separation efficiency of the photogenerated e−/h+ pairs, and an enhanced current density corroborates an increase in separation efficiency.21 As shown in Figure 8a, compared with the pristine BOIO and BOBI, the 20% BOIO-BOBI electrode displays a distinctly larger photocurrent density, which is about 9.78 and 1.58 times higher than that of BOIO and BOBI, reflecting the great improvement of separation efficiency for 20% BOIO-BOBI. Moreover, the mechanically mixed BOIO-BOBI composite (MM-20% BOIO-BOBI sample) is synthesized and compared in photocurrent. It can be found from Figure S2 that the transient photocurrent response of 20% BOIO-BOBI is much stronger than that of MM-20% BOIO-BOBI, evidencing that the heterojunctional interface between the two phases in the 3902

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Figure 9. Photocatalytic degradation of BPA over 20% BOIO-BOBI alone and with adding different scavengers under visible light (λ > 420 nm) illumination (a). PL spectra of a TAOH solution over 20%BOIO-BOBI sample with visible light (λ > 420 nm) irradiation (b).

Figure 7. Cycling runs for photodecomposing BPA (a) and XRD pattern (b) before and after photocatalysis over 20% BOIO-BOBI composite photocatalyst under visible light (λ > 420 nm) irradiation.

TAOH at different irradiation time are very weak, supporting that •OH is not the active species in the decomposition process of BPA, which is in keeping with the results of active species trapping experiments. Mott−Schottky method was employed to identify the band structure of BOBI and BOIO electrodes. The flat band potential (Efb) can be determined by using the following equation:25

restrained after adding BQ or EDTA-2Na, evidencing that •O2− and h+ are crucial active species in the process of photodegrading BPA under visible light (λ > 420 nm) illumination. To further verify the photocatalytic mechanism, a terephthalic acid photoluminescence probing technique (TAPL) was employed to detect the •OH radicals, because highly fluorescent 2-hydroxyterephthalic acid (TAOH) can be produced via the reaction between terephthalic acid (TA) and •OH.24 As shown in Figure 9b, the characteristic peaks of

Figure 8. Transient photocurrent responses under visible light (λ > 420 nm) illumination (a) and PL spectra (b) of BOBI, BOIO, and BOIO-BOBI samples. 3903

DOI: 10.1021/acssuschemeng.6b03066 ACS Sustainable Chem. Eng. 2017, 5, 3897−3905

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ACS Sustainable Chemistry & Engineering κBT ⎞ 1 2 ⎛ = − − E E ⎟ ⎜ fb εε0ND ⎝ q ⎠ C2

(3)

where ε and ε0 are the dielectric constants of free space and the film electrode, C, ND, E, kb, T, and q represents the space charge capacitance, donor density, applied potential, Boltzmann’s constant, temperature, and electronic charge, respectively. As shown in Figure 10, with taking the intercept of the axis with Figure 11. Proposed photocatalytic mechanism over 20% BOIOBOBI composite photocatalyst under visible light (λ > 420 nm) irradiation.

potentials of O2/•O2− is −0.05 eV,20 so the electrons on the CB of BOIO can reduce O2 to be •O2− in the photocatalytic system. In addition, the VB potential of BOBI (0.65 eV) is lower than that of BOIO (2.07 eV), suggesting that the holes on VB of BOBI would not transfer to VB of BOIO. Because of the potentials of •OH/OH− (1.99 eV) and •OH/H2O (2.27 eV),20 •OH cannot be produced by the holes on the VB of BOBI. Therefore, the main active species in the process of photodegrading BPA under visible light (λ > 420 nm) illumination are h+ and •O2−. In addition, developing heterostructure can facilitate the separation and transfer of photoinduced charge carriers.

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CONCLUSION In summary, BiOBr0.75I0.25/BiOIO3 heterojunction was prepared by a facile deposition−precipitation method. Compared to pure BiOIO3, the light absorption and photocatalytic activity of BiOBr0.75I0.25/BiOIO3 composites are greatly improved. The 20% BiOIO3-BiOBr0.75I0.25 sample exhibits optimal photoreactivity in degrading BPA under visible light (λ > 420 nm) irradiation among all of the composite phototcatalysts. Besides, the 20% BiOIO3-BiOBr0.75I0.25 sample can efficiently decompose diverse industrial contaminants and pharmaceuticals, including MO, phenol, 2,4-DCP, chlortetracycline hydrochloride, and tetracycline hydrochloride. The transient photocurrent response and PL suggest that heterojunction fabrication facilitates the separation and inhibits the recombination of photoinduced charge carriers. The present work paves a new way to prepare heterojunctional photocatalysts that can decompose diverse industrial pollutants.

Figure 10. Mott−Schottky plots for BOBI (a) and BOIO (b) photoctalysts.

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potential values, the Efb of BOBI and BOIO is calculated to be −1.51 and 0.94 V versus a saturated calomel electrode (SCE), which is −1.27 V and −0.70 V versus a normal hydrogen electrode (NHE), respectively. Both BOBI and BOIO are regarded as n-type semiconductors because of the positive slope of the Mott−Schottky curve. For many n-type semiconductors, Efb is 0.1 V below the conduction band (CB) position (ECB).26 Consequently, ECB of BOBI and BOIO can be determined as −1.37 and −0.80 eV, and their valence band (VB) potentials (EVB) are 0.65 and 2.07 eV, respectively. According to the band structures of BOBI and BOIO, the proposed photocatalytic mechanism over 20% BOIO-BOBI composite photocatalyst under visible light (λ > 420 nm) irradiation is depicted in Figure 11. With a relatively narrow energy band gap (2.02 eV), BOBI can be excited by visible light and produce electron−hole pairs, meanwhile BOIO almost cannot respond to visible light because of its wide band gap (2.87 eV). The CB potential of BOBI (−1.37 eV) is more negative than that of BOIO (−0.80 eV), so the electrons on the CB of BOBI would transfer to the CB of BOIO. The redox

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03066. SEM image of 20% BOIO-BOBI after cycling runs for photodecomposing BPA. Transient photocurrent responses of 20% BOIO-BOBI heterojunction and mechanically mixed MM-20% BOIO-BOBI composite under visible light (λ > 420 nm) illumination (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yingmo Hu: 0000-0002-9839-0961 Hongwei Huang: 0000-0003-0271-1079 3904

DOI: 10.1021/acssuschemeng.6b03066 ACS Sustainable Chem. Eng. 2017, 5, 3897−3905

Research Article

ACS Sustainable Chemistry & Engineering Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (grant nos. 51672258, 51302251, and 51572246) and the Fundamental Research Funds for the Central Universities (2652015296).

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DOI: 10.1021/acssuschemeng.6b03066 ACS Sustainable Chem. Eng. 2017, 5, 3897−3905