Achieving Enhanced UV and Visible Light Photocatalytic Activity for

Research Article pubs.acs.org/journal/ascecg

Achieving Enhanced UV and Visible Light Photocatalytic Activity for Ternary Ag/AgBr/BiOIO3: Decomposition for Diverse Industrial Contaminants with Distinct Mechanisms and Complete Mineralization Ability Fang Chen,† Hongwei Huang,*,† Chao Zeng,† Xin Du,‡ and Yihe Zhang*,† †

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 ‡ Research Center for Bioengineering and Sensing Technology, Department of Chemistry & Biological Engineering, University of Science & Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: Heterojunction fabrication and noble metal deposition serving as efficacious means for promoting photocatalytic activity attract huge interests. Here, a series of ternary Ag/AgBr/BiOIO3 composite photocatalysts that integrate the above two aspects are prepared by in situ crystallization of Ag/AgBr on BiOIO3. The photocatalytic performance is first investigated by degrading MO with visible light and UV light irradiation. The results indicate that Ag/AgBr/BiOIO3 composites present strengthened photocatalytic activity compared with BiOIO3 and Ag/AgBr under both light sources. Distinct activity enhancement levels corresponding to different mechanisms with UV and visible light illumination are uncovered, which are closely related to the applied light source. The universal catalytic activity of Ag/AgBr/BiOIO3 is surveyed by decomposition of diverse antibiotics and phenols, including tetracycline hydrochloride, chlortetracycline hydrochloride, bisphenol A, phenol, and 2,4-dichlorophenol which discloses that this ternary heterojunction photocatalyst demonstrates unselective catalytic activity with universality. Importantly, Ag/AgBr/BiOIO3 displays a strong mineralization ability, completely decomposing BPA into CO2 and H2O. This work affords a new reference for designing heterojunction photocatalyst with multiple advantageous effect and powerful capability for environmental purification. KEYWORDS: Photocatalysis, SPR, BiOIO3, AgBr, Antibiotics, Phenols

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INTRODUCTION Photocatalysis with semiconductors, regarded as a green purification tactic, has raised concern due to its prominent abilities for energy generation and environmental purification.1,2 Various types of photocatalysts, like ZnO, TiO2, g-C3N4, and CdS, have been investigated widely. However, these traditional photocatalysts suffer from some drawbacks, like insufficient photoabsorption, photo/photochemical corrosion and high recombination ratio of holes and electrons, which largely confine their applications. Exploration of efficient visible-light-responsive photocatalytic © 2017 American Chemical Society

materials is the key to expand and promote the practical applications of semiconductor photocatalyst for environmental purification.3,4 Photocatalytic activity mainly has a correlation with the optical absorption and charge separation and transfer.5 Bismuth-based photocatalytic materials such as bismuth oxyhalides,6,7 bismuth tungstate,8−10 basic bismuth nitrate, or borate,11−13 etc., show Received: April 25, 2017 Revised: July 11, 2017 Published: July 24, 2017 7777

DOI: 10.1021/acssuschemeng.7b01259 ACS Sustainable Chem. Eng. 2017, 5, 7777−7791

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ACS Sustainable Chemistry & Engineering

Ag/AgBr/BiOIO3 ternary photocatalysts were fabricated by a simple in situ precipitation approach. A certain amount of BiOIO3 and AgNO3 were added in deionized water (25 mL). Next, 25 mL of KBr with KBr/BiOIO3 molar ratios of 10%, 20%, 40%, 80%, 150%, and 300% were dissolved in the above suspensions dropwise. They were stirred at ambient condition for 1 h, and then the composites were filtrated and dried completely. In accordance with the different KBr/BiOIO3 molar ratios, the composites were denoted as 10%Ag/AgBr/BiOIO3, 20%Ag/ AgBr/BiOIO3, 40%Ag/AgBr/BiOIO3, 80%Ag/AgBr/BiOIO3, 150%Ag/ AgBr/BiOIO3, and 300%Ag/AgBr/BiOIO3, respectively. The emergence of metallic Ag resulted from AgBr by the surrounding light reduction. The formation schematic diagram was shown in Scheme 1 Characterization. X-ray powder diffraction (XRD, Bruker D8) is applied to confirm the phase structure of BiOIO3 series products. The scanning electron microscopy (SEM, Hitachi) was employed to analyze the morphology. Meanwhile, the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, JEOL) were also used to analyze the microstructure. UV−vis diffuse reflectance spectra (DRS, Varian Cary 500) was implemented to survey the optical properties. The surface composition for a series of as-synthesized photocatalysts were studied by the X-ray photoelectron spectroscopy. The photoluminescence (PL) spectra (Hitachi, Japan) are employed to further investigate the charge recombination rate. Photocatalytic Experiment. The degradation of diverse contaminants and antibiotics, including methyl orange (MO, 2 × 10−5 mol/L), tetracycline hydrochloride (10 mg/L), chlortetracycline hydrochloride (10 mg/L), bisphenol A (BPA, 10 mg/L), phenol (10 mg/L), and 2,4-dichlorophenol (2,4-DCP, 10 mg/L) were conducted to inspect the photocatalytic properties of Ag/AgBr/BiOIO3 with visible light (500 W Xe lamp with 420 nm cutoff filters) or UV light (300 W mercury lamp) irradiation. Twenty-five milligrams of photocatalysts were ultrasonically dispersed in contaminant aqueous solution (50 mL). After stirring for 30 min in darkness, the adsorption−desorption balance was achieved. In the experiment of degradation, about 3 mL of suspension was extracted at each 1 min or half an hour. Then, the supernatant was obtained by centrifugation and investigated by recording the absorption spectra of pollutants. Active Species Trapping Test. For revealing the active species produced in the degradation reaction, the trapping agents, including isopropanol, ethylene diaminetetraacetic acid disodium, and p-benzoquinone were used for trapping hydroxyl radicals, holes, and superoxide radicals, respectively.31 During the photocatalytic process of MO, the different trapping agents (the amount of quenching agents was 1 mmol) were dissolved in the MO solution. The next procedures are the same as the aforementioned MO photodegradation processes. Photoelectrochemical Measurements. The electrochemical impedance spectra and photocurrent response measurements were conducted on an electrochemical system (CHI-660E, Chenhua Instruments Co) with the 0.1 mol/L Na2SO4 solution as electrolyte. The as-prepared sample films on ITO glass and saturated calomel electrode are the working electrode and reference electrode, respectively. Platinum wire was used as the counter electrode. The measurement was performed under simulated solar light irradiation (300 W xenon bulb). The applied voltage is 0.0 V in this measurement of transient photocurrent. To prepare the working electrodes of Ag/AgBr, BiOIO3 and Ag/AgBr/BiOIO3, 15 mg of the samples were completely dispersed in 1 mL of ethanol. Subsequently, the obtained slurry was drop-coated on a 20 × 40 mm ITO glass and exposed to air for 24 h to be dried.

great promise for their special layered structure. Among them, BiOIO3, a novel bismuth-based material, possesses superior photocatalytic activity for photodegradation of azo-dyes, NO, and Hg removal.14−17 Particularly and very lately, our group discovered the photo and piezoelectric-induced molecular oxygen activation can be realized by BiOIO3.18 The layered crystal configuration of BiOIO3 is composed of (Bi2O2)2+ layers and interbedded (IO3)− anions. The arrangement of polar (IO3)− groups in BiOIO3 results in polarized electric fields and noncentrosymmetrical crystal structure. All of these structural characteristics can greatly facilitate the separation of photoinduced charges and then endow BiOIO3 with outstanding photocatalytic activity.19 Nevertheless, BiOIO3 only responds to UV light attributed to its relatively wide band energy (3.0−3.3 eV). To reinforce the light absorption of BiOIO3, many strategies (e.g., doping, metal deposition, surface modification and heterostructure construction) have been achieved.20−23 Among them, developing heterojunction can not only enhance the light response of photocatalyst but also depress the recombination of photoinduced charge carriers.24,25 For another, silver-species-containing photocatalysts exhibit superior photoabsorption in the visible region as a result of Ag surface plasmonic resonance (SPR).26,27 Ag/TiO2 and Ag/AgBr were demonstrated showing high visible-lightresponsive photocatalytic activity.28,29 Besides, Ag/AgBr/AgIO3 composites present noteworthy photoactivity for the degradation of MO with irradiation of visible light.30 Consequently, considering the structural benefits of BiOIO3, it is available to simultaneously combine heterojunction and SPR effect to strengthen the photocatalytic performance of BiOIO3. Herein, the Ag/AgBr/BiOIO3 ternary heterojunctions were synthesized by a simple in situ crystallization approach. The photocatalysis activity of as-obtained photocatalysts are tested via decomposition of methyl orange (MO), phenols and antibiotics separately with visible light and ultraviolet light (UV) illumination. In comparison with the Ag/AgBr and BiOIO3, Ag/AgBr/BiOIO3 photocatalysts show higher photocatalytic properties with a complete mineralization capability. Remarkably, it is found that Ag/AgBr/BiOIO3 shows diverse photocatalytic mechanisms under visible light and UV light illumination, and the photoactivity enhancement level is closely related to the applied light source. Besides, the ternary Ag/AgBr/BiOIO3 heterojunction photocatalysts are characterized systematically by multiple techniques.

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EXPERIMENTAL SECTION

Synthesis of the Photocatalyst. Analytical-grade chemicals were used without further disposal. BiOIO3 precursor was prepared via a typical hydrothermal approach. First, I2O5 (0.501 g) and Bi(NO3)3· 5H2O (1.455 g) were dissolved in deionized water (25 mL) and 4 mL of concentrated HNO3 with magnetic stirring for 35 min. Second, a Teflon-lines autoclave (50 mL) containing the above solution was used to complete hydrothermally treatment for 20 h at 180 °C. Then, by filtration and washing alternately with distilled ethanol and water for 3 times, BiOIO3 precursor were finally collected and dried at 60 °C for 6 h.

Scheme 1. Schematic Illustration of Fabrication Process for the Ag/AgBr/BiOIO3 Composites

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RESULTS AND DISCUSSION Structure, Microstructure, and Composition. To study the crystallographic structure of samples, XRD patterns of a series of Ag/AgBr/BiOIO3 ternary composites, BiOIO3, and Ag/AgBr and are shown in Figure 1. The diffraction peaks of

composites, their XRD patterns apparently change when increasing the molar ratio of KBr to BiOIO3. The peaks at 27.6°, 30.9°, and 37.9° correspond to the characteristic peaks of BiOIO3, AgBr, and Ag, which are highlighted by red, purple, and blue stripes, respectively. With the increase of the ratio of KBr to BiOIO3, the intensity of peaks for BiOIO3 gradually decrease, and on the contrary, the peaks intensity for AgBr and Ag continuously increases. The peaks of Ag are not obvious for 10%, 20%, and 40% Ag/AgBr/BiOIO3 samples, which is probably attributed to the high dispersity and low content of Ag. The above results demonstrate that the ternary Ag/AgBr/BiOIO3 composites are obtained by this in situ deposition reaction, and the crystalline phase of BiOIO3 is not severely effected with the introduction of Ag/AgBr. The chemical state and surface element compositions of 150% Ag/AgBr/BiOIO3 are inspected via X-ray photoelectron spectroscopy. As depicted in Figure 2a, it consists of Ag, Br, O, Bi, I, and C elements. The Bi 4f XPS spectra are showed in Figure 2b. The two peaks centering at 164.4.0 and 159.0 eV can be separately attributed to the Bi 4f5/2 and Bi 4f7/2 of Bi3+ ions. The O 1s high-resolution XPS spectrum is presented in Figure 2c. The peaks located at 530.34 and 531.21 eV are attributed to the Bi−O bonds and the hydroxyl groups (−OH) absorbed on the surface, respectively.23 As exhibited in the Ag 3d spectrum (Figure 2d), it consists of two sets of peaks. The binding energies of 367.8, 368.6, 373.7, and 374.6 eV match with Ag+ 3d5/2, Ag 3d5/2, Ag+ 3d3/2, and Ag 3d3/2 of Ag0, respectively. The XPS results verify that the as-prepared 150%Ag/AgBr/BiOIO3

Figure 1. XRD patterns for BiOIO3, AgBr, and Ag/AgBr/BiOIO3 samples.

Ag/AgBr and BiOIO3 coincide with the standard data of Ag (JCPDS no. 1-1164), AgBr (JCPDS no. 1-950) and BiOIO3 (ICSD no. 262019), respectively. With respect to the Ag/AgBr/BiOIO3

Figure 2. (a) XPS survey spectra and high-resolution XPS spectra for (b) Bi 4f, (c) O 1s, and (d) Ag 3d of 150%Ag/AgBr/BiOIO3. 7779


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Figure 3. SEM images for (a) BiOIO3, (b) Ag/AgBr, (c) 40%Ag/AgBr/BiOIO3, and (d) 150%Ag/AgBr/BiOIO3. (e) Schematic illustration for morphology evolution for BiOIO3, Ag/AgBr, 40%Ag/AgBr/BiOIO3, and 150%Ag/AgBr/BiOIO3 products.

contains Ag+ and Ag0.2,32,33 Both the XPS and XRD results are consistent. The existence of metallic Ag is attributed to the photoreduction by the surrounding light in the preparation process. The microstructure, morphology, and element distribution of as-synthesized samples are analyzed by TEM, HRTEM, SEM, and EDX-mapping. As depicted in Figure 3a, the pristine BiOIO3 displays a flake structure with smooth surface and width of 0.2−2 μm and thickness of ∼10 nm. Figure 3b shows the SEM image for Ag/AgBr. The size of AgBr particles is 1−2 μm, and the nanoparticles observed on the surface of AgBr are Ag. As depicted in the SEM images of Ag/AgBr/BiOIO3 composites (Figure 3, panels c and d), one can obviously observe that the products are composed of Ag/AgBr particles and BiOIO3 flakes, and the morphology of BiOIO3 does not show obvious changes. Figure 3e depicts the schematic illustration on the formation of the Ag/AgBr/BiOIO3 heterojunction, which can give a direct and clear picture of the synthetic process. The TEM image of BiOIO3 is exhibited in Figure 4a, which confirms the flake structure of BiOIO3. As presented in Figure 4b, the HRTEM pattern of BiOIO3 displays the lattice fringe with a distance of 0.33 nm, in line with the (121) plane of BiOIO3. TEM images of 150%Ag/AgBr/BiOIO3 of before and after irradiation under the transmission electron beam within extremely short time are presented in Figure 4, panels c and d. The particles in the highlighted area show obvious contraction after electron beam irradiation, which can be ascribed to the decomposition from

AgBr to Ag with eletron beam irradiation. It also accounts for appearance of Ag in the Ag/AgBr/BiOIO3 composites.33 Figure 4 (panels e−h) displays the EDX-mapping of the 150%Ag/AgBr/ BiOIO3 sample. It can be observed that Ag, Br, Bi, and O uniformly distributed across the Ag/AgBr/BiOIO3. The EDX of the 150% Ag/AgBr/BiOIO3 sample was performed to identify the phases of Ag, AgBr, and BiOIO3 in the Ag/AgBr/BiOIO3. As exhibited in Figure 5, the signal of Au resulted from the instrument. The EDX result provides solid evidence that the three phases of Ag, AgBr, and BiOIO3 coexist in the composite photocatalyst. The specific surface area of photocatalysts is surveyed by the BET adsorption−desorption method. It can be found that the surface area of 150%Ag/AgBr/BiOIO3 (5.7 m2/g) shows no big difference with that of pure BiOIO3 (6.2 m2/g) and Ag/AgBr (1.2 m2/g), which would not contribute to the photocatalytic performance enhancement of composites. Photoabsorption Properties and Band Gap. To inspect the optical absorption properties of Ag/AgBr, BiOIO3, and Ag/AgBr/BiOIO3 composites, they are examined by UV−vis diffuse reflection spectroscopy. The absorption edge of primary BiOIO3 locates at approximately 400 nm as exhibited (Figure 6a), and its bandgap is 3.1 eV, implying that pristine BiOIO3 can hardly absorb visible light. On the contrary, the Ag/AgBr sample shows effective photoabsorption in the visible range, and its absorption edge is about 500 nm. As the Ag/AgBr content increases, the absorption edges of Ag/AgBr/BiOIO3 composites gradually 7780


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Figure 4. (a) TEM and (b) HRTEM images of BiOIO3 sample. TEM images of 150%Ag/AgBr/BiOIO3 sample (c) before and (d) after electron beam irradiation. (e−h) EDX mapping of 150%Ag/AgBr/BiOIO3.

Figure 5. SEM and EDX images for the 150%Ag/AgBr/BiOIO3 composite.

red-shifts to the visible light region. Meantime, an absorption peak centering around 550 nm in the visible light range is observed, corresponding to the Ag surface plasmon resonance absorption.30

Band gap energies for BiOIO3 and Ag/AgBr can be determined based on the following Kubelka−Munk (KM) equation: αhγ = A(hγ − Eg )n /2 7781

(1) DOI: 10.1021/acssuschemeng.7b01259 ACS Sustainable Chem. Eng. 2017, 5, 7777−7791

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Figure 6. (a) UV−vis diffuse reflectance spectra for the BiOIO3, Ag/AgBr, and Ag/AgBr/BiOIO3 composites. (b) The determined band gaps of the BiOIO3 and Ag/AgBr.

Figure 7. (a) Photocatalytic degradation curves of MO and (b) apparent rate constants for degrading MO over the BiOIO3, Ag/AgBr, and Ag/AgBr/ BiOIO3 composite photocatalysts under UV light illumination. Temporal absorption spectra over (c) BiOIO3 and (d) 150%Ag/AgBr/BiOIO3.

here γ, h, α are the photon frequency, Planck’s constant, and optical absorption coefficient, respectively. Eg and A represent the band gap energy and a constant, respectively. The n value relies on the semiconductor type. The electronic transition for BiOIO3 and Ag/AgBr is indirect-allowed, so the n is 4. The band gaps of the pure BiOIO3 and Ag/AgBr are 3.00 and 2.30 eV, respectively (Figure 6b). Photodegradation of MO under UV Light Irradiation. The photocatalytic properties of BiOIO3, Ag/AgBr, and

Ag/AgBr/BiOIO3 composites are assessed by choosing MO as a representative pollutant for degradation.34,35 Because pure BiOIO3 principally responds to UV light, MO degradation experiments of pure BiOIO3, Ag/AgBr, and the composite samples were first conducted under UV light irradiation, and their photocatalytic degradation curves are displayed in Figure 7a. It is clear that the photocatalytic activities of all the as-obtained composite photocatalysts are better than that of BiOIO3. But, 7782


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Figure 8. (a) Photocatalytic degradation curves of MO and (b) apparent rate constants for degrading MO over the BiOIO3, Ag/AgBr, and Ag/AgBr/ BiOIO3 composite photocatalysts under visible light illumination. Temporal absorption spectra over (c) BiOIO3, (d) Ag/AgBr, and (e) 150%Ag/AgBr/ BiOIO3. (f) Photocatalytic degradation curves of 2,4-DCP over the pure BiOIO3, Ag/AgBr, and 150%Ag/AgBr/BiOIO3 composite photocatalysts under visible light illumination. Temporal absorption spectra of 2,4-DCP over (g) BiOIO3 and (h) Ag/AgBr. 7783


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Figure 9. Temporal absorption spectra of (a) 2,4-DCP, (b) phenol, (c) BPA, (d) tetracycline hydrochloride, and (e) chlortetracycline hydrochloride over the 150%Ag/AgBr/BiOIO3 photocatalyst under visible light (λ > 420 nm) irradiation. (f) Degradation efficiency of diverse contaminants under visible light irradiation.

only 150%Ag/AgBr/BiOIO3 that removes 87.7% of MO after 4 min UV light irradiation outperforms Ag/AgBr. As displayed in Figure 7b, the photodegradation rate constant for BiOIO3, Ag/AgBr, and 150%Ag/AgBr/BiOIO3 is 0.128, 0.642, and 0.697 min−1, respectively. Namely, the photoactivity of 150% Ag/AgBr/BiOIO3 is separately 5.4 and 1.1 times higher than that of BiOIO3 and Ag/AgBr. The temporal absorption spectra of MO for BiOIO3, 150%Ag/AgBr/BiOIO3, and Ag/AgBr are exhibited in Figure 7 (panels c and d) and Figure S1. It confirms that MO is gradually destroyed by these catalysts. Photodegradation of MO and 2,4-DCP with VisibleLight Illumination. Figure 8a displays photodegradation curves of MO with visible light (λ > 420 nm) illumination. BiOIO3 shows a neglectable degradation efficiency due to its poor visiblelight absorption. Distinct from UV-light degradation, all the

composite photocatalysts exhibit remarkably enhanced degradation capability than pristine samples with visible light illumination. 150%Ag/AgBr/BiOIO3 still shows the best photoreactivity among the photocatalysts, where the degradation efficiency reaches approximately 92% within 0.5 h, far exceeding that of BiOIO3 (1.3%) and Ag/AgBr (18%). To make a quantitative comparison, the photodegradation rate of these samples was determined via the following formula:23 ln(C /C0) = −kappt

(2)

Here t is time of the reaction. C and C0 represent the MO concentration at t and the initial MO concentration, respectively. kapp represents the apparent rate constant. As displayed in Figure 8b, the apparent rate of BiOIO3, Ag/AgBr and 20%, 40%, 80%, 7784


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Figure 10. Photocatalytic degradation efficiency and TOC percentage of BPA over the 150%Ag/AgBr/BiOIO3 heterojunction photocatalyst under visible light illumination.

Figure 11. (a) Recycling degradation curves and (b) degradation efficiency for degrading MO over 150%Ag/AgBr/BiOIO3 under visible light illumination. (c) XRD patterns of 150%Ag/AgBr/BiOIO3 before and after photoreaction. (d) XPS spectra of 150%Ag/AgBr/BiOIO3 before and after photoreaction.

activity of as-obtained photocatalysts, a degradation experiment toward 2,4-dichlorophenol (2,4-DCP) was carried out. It can be found from Figure 8f that the 150%Ag/AgBr/BiOIO3 possesses the highest photocatalytic performance among the three catalysts. Figure 8 (panels g and h) and Figure 9a show the absorption spectra over BiOIO3, Ag/AgBr, and 150%Ag/AgBr/ BiOIO3. The characteristic absorption band of 2,4-DCP gradually decreases with an increase in irradiation time, and the 2,4-DCP concentration catalyzed by 150%Ag/AgBr/BiOIO3 shows the fastest decline, confirming the best photocatalytic

150%, and 300%Ag/AgBr/BiOIO3 are calculated to be 0.015, 0.361, 0.903, 0.554, 1.856, 2.736, and 2.604 h−1, respectively. The 150%Ag/AgBr/BiOIO3 shows the highest photocatalytic efficiency, which is about 180 and 7.5 times that of the pure BiOIO3 and Ag/AgBr, respectively. The instantaneous absorption spectra of MO over BiOIO3, Ag/AgBr and 150%Ag/AgBr/BiOIO3 are provided in Figure 8 (panels c−e). The concentration of MO solution gradually declines with increasing the illumination time, demonstrating the decomposition of MO molecules. For further confirming the consumedly reinforced photocatalytic 7785


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Photodegradation of Diverse Industrial Pollutants and Antibiotics. To eliminate the dye sensitization effect and further confirm the universally photocatalytic capability of the 150%Ag/ AgBr/BiOIO3 composite photocatalyst, diverse stubborn contaminants, including tetracycline hydrochloride, chlortetracycline hydrochloride, colorless phenol, and bisphenol A (BPA), are selected as the target with the visible light (λ > 420 nm) illumination. Figure 9 (panels a−e) shows the temporal absorption spectra of chlortetracycline hydrochloride, tetracycline hydrochloride, BPA, phenol, and 2,4-DCP over 150%Ag/ AgBr/BiOIO3. 150%Ag/AgBr/BiOIO3 can efficiently decompose these pollutants and antibiotics. After 5 h of illumination, the removal efficiencies are 47.8%, 40.0%, 20.0%, 34.2%, and 31.4% for chlortetracycline hydrochloride, tetracycline hydrochloride, BPA, phenol, and 2,4-DCP, respectively (Figure 9f), confirming the nonselective photocatalytic activity. Mineralization Ability of Ag/AgBr/BiOIO3 Composite Photocatalyst. To investigate the mineralization ability of 150%Ag/AgBr/BiOIO3 composite photocatalyst, the total organic carbon (TOC) of BPA as a typical stubborn contaminant was analyzed. Figure 10 displays the TOC data and the degradation efficiency of BPA solution over 150%Ag/AgBr/ BiOIO3 composite for 0, 2, and 4 h irradiation. As revealed, both the photodegradation efficiency and mineralization ratio of organic carbon of BPA after 4 h of visible light irradiation is 70.9% and 71.0%, respectively. It indicates that all the degraded BPA is decomposed into CO2 and H2O, manifesting the complete mineralization ability of the Ag/AgBr/BiOIO 3 composite photocatalyst. Stability of Ag/AgBr/BiOIO3 Composite Photocatalyst. The cycling experiment is implemented to study the stability and recyclability of the modified 150%Ag/AgBr/BiOIO3 catalyst for MO degradation. As exhibited in Figure 11 (panels a and b), the original MO degradation efficiency is 95.4% within 1.5 h. After three times of cycles, the MO degradation ratio is still 93.5%, implying that the excellent photocatalytic activity of the 150% Ag/AgBr/BiOIO3 is well kept. To further verify the stability of as-obtained samples, XPS and XRD after photoreaction were conducted. The XRD has no obvious change (Figure 11c) compared with that of 150%Ag/AgBr/BiOIO3 before the photocatalytic test. The Ag XPS spectrum of the 150%Ag/ AgBr/BiOIO3 after photoreaction is displayed in Figure 11d. Through the comparison of the XPS spectra before and after photoreaction, it can be found that the peak intensity corresponding to metallic Ag did not increase, demonstrating that no obvious decomposition for AgBr into metallic Ag occurred during the photodegradation process. It is likely ascribed to the efficient separation of holes and electrons in Ag/AgBr/BiOIO3, which then rapidly participate in the reaction. These evidence apparently demonstrate that the 150%Ag/AgBr/ BiOIO3 composite photocatalyst has a high stability against to photo/photochemical corrosion. Charge Separation, Reactive Intermediate, and Photocatalytic Activity Enhancement Mechanism. Photocurrent is a valid technique to reveal the charge separation and transfer dynamics that take a crucial role in the photodegradation process.35 Figure 12a shows the transient photocurrent responses of pure BiOIO3, Ag/AgBr, and 150%Ag/AgBr/ BiOIO3 with visible light illumination. Compared with pristine BiOIO3 and Ag/AgBr samples, the photocurrent intensity for 150%Ag/AgBr/BiOIO3 is markedly strengthened, demonstrating the promoted charge separation. The electrochemical impedance spectra (EIS) were implemented to reflect the

Figure 12. (a) Comparison of transient photocurrent responses and (b) EIS Nynquist plots and the proposed equivalent circuit and the fitted values (insets of b) for pure BiOIO3, Ag/AgBr, and 150%Ag/ AgBr/BiOIO3 under UV−vis light irradiation. (c) PL spectra of pure BiOIO3, Ag/AgBr, and Ag/AgBr/BiOIO3 composites under the excitation of 245 nm.

performance. Compared to the UV-light degradation, the composite catalysts present evidently higher activity enhancement with irradiation of visible-light degradation, which means the different photocatalytic mechanisms under different light sources (UV and visible light). 7786


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Figure 13. Photocatalytic degradation of MO over (a and b) 150%Ag/AgBr/BiOIO3, (c and d) Ag/AgBr, and (e and f) BiOIO3 samples alone and with different scavengers under irradiation of UV light.

In this work, RCT is approximated 2407, 2368, and 913 Ω for pristine BiOIO3, Ag/AgBr, and 150%Ag/AgBr/BiOIO3 samples, respectively, suggesting that the 150%Ag/AgBr/BiOIO3 composite possesses the smallest charge transfer resistance. The above result further affirms that formation of Ag/AgBr/BiOIO3 heterojunction benefits charge transfer.36 Photoluminescence spectroscopy (PL) was employed to further investigate the charge separation rate. Generally, the low photocatalytic capability corresponds to the high fluorescence intensity, which is caused by high bulk recombination.37 As revealed by Figure 12c, the series of emission peaks of BiOIO3, Ag/AgBr, and 20%, 80%, 150%, and 300%Ag/AgBr/BiOIO3 samples locate at approximately 390 nm with excitation of 245 nm light. The 150%Ag/ AgBr/BiOIO3 sample displays the weakest emission intensity,

interfacial transfer efficiency of photoinduced electrons and holes. The Nyquist plots of BiOIO3, Ag/AgBr, and 150%Ag/ AgBr/BiOIO3 with light irradiation are shown in Figure 12b. Generally, a small radius represents a high electrons and holes transfer efficiency. Compared with pure BiOIO3 and Ag/AgBr, it can be found that the 150%Ag/AgBr/BiOIO3 composite shows a relatively smaller radius, revealing that the interfacial charge transfer efficiency has been obviously increased. As displayed in the inset of Figure 12b, the EIS data were fitted by an equivalent circuit to obtain a quantitative understanding. Here, Q, R, RCT, Cdl, and RΩ represent the constant-phase element, the reaction resistance, the charge transfer resistance, the electric double layer capacitor, and the ohmic resistance, respectively. Generally, the arc diameter is positively correlated with the RCT. 7787


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Figure 14. Photocatalytic degradation of MO over (a and b) 150%Ag/AgBr/BiOIO3, (c and d) Ag/AgBr, and (e and f) BiOIO3 samples alone and with different scavengers under irradiation of visible light (λ > 420 nm).

applied potential, respectively. ND is donor density. C is the charge capacitance. ε0 and ε indicates the film and the dielectric constants. As exhibited in Figure S2, BiOIO3 is a n-type semiconductor because of the positive slope. To calculate the x-intercept, the flat band potentials (Ffb) of BiOIO3 are determined to be −0.62 eV relative to the saturated calomel electrode (SCE), amounting to −0.38 eV versus the normal hydrogen electrode (NHE). Because the flat potential is 0.1−0.3 eV lower than the conduction band position,16 the conduction band (CB) level of BiOIO3 is calculated as −0.68 eV and the valence band (VB) position of BiOIO3 is estimated to be 2.32 eV. The trapping experiments were implemented to explore the main active species in the photodegradation reaction. P-benzoquinone (BQ), isopropanol (IPA) and ethylene diaminetetraacetic acid

implying the most excellent separation of photoinduced holes and electrons. The above results may give a good explanation for why the 300%Ag/AgBr/BiOIO3 sample shows the highest photoabsorption in all the photocatalysts, but its performance is not the best. Compared with light absorption, charge separation efficiency probably takes a dominant role in reinforcing photocatalytic performance.38 To inspect the band structure, the Mott−Schottky method was employed for calculating the flat band (Efb) potentials:39 κ T⎞ 1 2 ⎛ = ⎜E − Efb − B ⎟ 2 εε0ND ⎝ q ⎠ C

(3)

herein q, T, κb, Efb and E are the electronic charge, the temperature, Boltzmann’s constant, the flat-band potential, and 7788


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ACS Sustainable Chemistry & Engineering disodium (EDTA-2Na) are utilized to capture superoxide radicals (•O2−), hydroxyl radicals (•OH), and holes (h+) in the experiments of MO photodegradation, respectively.17 For the 150%Ag/AgBr/BiOIO3 composite, as presented in Figure 13 (panels a−d), with the addition of BQ, the photodegradation of MO is seriously inhibited under the UV light illumination. It can be concluded that the •O2− plays a greater part in the photodegradation test. Compared with the trapping experiments with illumination of visible-light for Ag/AgBr sample, under the UV light irradiation, the degradation of MO is remarkably reduced by adding p-benzoquinone (BQ) and slightly inhibited by adding ethylene diaminetetraacetic acid disodium (EDTA-2Na). It appears that •O2− is the primary active species and h+ plays as a subordinate role. As shown in Figure 13 (panels e and f), for pure BiOIO3 sample with illumination of UV light, h+ and •O2− dominate the degradation process. Under visible light irradiation, h+, •O2−, and •OH all have influence on the photocatalytic process (Figure 14, panels a and b), implying that •OH, •O2−, and h+ are active species for the photocatalytic degradation. For Ag/AgBr sample, as the shown in Figure 14 (panels c and d), hydroxyl radicals (•OH) have little influence on the photodegradation. By contrast, the degradation rates of MO are significantly reduced when h+ and •O2− have been trapped, revealing that •O2− and h+ are the primary active species. As seen from Figure 14 (panels e and f), BiOIO3 cannot be activated by visible-light, resulting in almost no active species. On the basis of the experimental results above, the photocatalytic mechanisms for Ag/AgBr/BiOIO3 under both light sources are separately proposed, as depicted in Figure 15. It is reported from the previous study that the CB of BiOIO3 comprises I 5p, O 2p, and Bi 6p orbitals, and the VB mainly consists of O 2p orbitals.23 As exhibited in Figure 15a, under the excitation of UV-light, both BiOIO3 and AgBr can produce electron−hole pairs. The CB of AgBr is −0.30 eV. The CB of BiOIO3 is −0.68 eV, which is more negative than that of AgBr, thus the electrons of BiOIO3 would migrate onto the CB of AgBr and then transfer to Ag. The more positive potential of BiOIO3 favors the transfer of the holes from BiOIO3 to the VB of AgBr. The electrons would reduce O2 to •O2−. Because the VB of AgBr (2.00 eV) is very close to or lower than the redox potentials of •OH/H2O (2.27 eV) and OH−/•OH (1.99 eV), the oxidation ability of holes is insufficient to oxidize OH− to •OH. As only •O2− functions as active species, Ag/AgBr/BiOIO3 composite thereby displays moderately improved photocatalytic performance with UV light illumination.40,41 Figure 15b exhibits the speculated photocatalytic mechanism for Ag/AgBr/BiOIO3 with illumination of visible light. BiOIO3 barely absorbs visible light on account of its wide band gap (3.0 eV), and it almost does not produce holes and electrons. Ag/AgBr/BiOIO3 displays drastically reinforced photocatalytic performance in contrast to the pure samples under visible light. Synergistic effect of Ag nanoparticles SPR effect and fabrication of heterojunction are the main reasons. Because of the SPR effect of the metallic Ag, it can respond to visible light (550 nm) to produce holes and electrons. The photoinduced electrons on the Fermi level (0.4 eV) of Ag could be motivated to excited states, and transfer to the CB of BiOIO3 and AgBr, leaving positive charges. In order to recover the primal state, Ag needs to accept electrons from the VB of BiOIO3. The above factors can increase charge carrier lifetimes and depress the recombination of holes and electrons.40 In addition, the near-field resonant energy transfer of Ag could accelerate the production and separation of hole−electron pairs in BiOIO3. The VB of BiOIO3 is 2.32 eV,

Figure 15. Schematic diagrams for different charge-transfer mechanisms of 150%Ag/AgBr/BiOIO3 under the irradiation of (a) UV light and (b) visible light (λ > 420 nm).

which is more positive than that of •OH/H2O (2.27 eV) and OH−/•OH (1.99 eV),39 hence OH− can be oxidized to •OH by holes. At the same time, the electrons and holes can be activated on AgBr with a relative narrow band gap (2.30 eV). The ECB and EVB of AgBr are separately −0.30 and 2.00 eV. The maximum energy (2.95 eV) of the visible light can effectively excite the AgBr semiconductor, and the photogenerated electrons in VB of AgBr can jump into a higher band energy level.40−44 Together with the electrons transferred from Ag, it results in that more electrons react with O2 and a large amount of superoxide radicals (•O2−) are produced, and O2 is reduced to •O2− under the action of the electrons from the CB of BiOIO3. Therefore, benefiting from the synergistic effect of metallic Ag SPR effect and charge transfer in heterojunction, •O2, •OH, and h+ all can be produced as active species. As a result, consumedly enhanced photocatalytic performance is achieved in the Ag/AgBr/BiOIO3 heterojunction with visible light irradiation.42−44

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CONCLUSIONS In conclusion, ternary Ag/AgBr/BiOIO3 heterojunction photocatalytic materials were successfully fabricated by a readily achievable in situ precipitation approach. Compare with the pristine BiOIO3, the Ag/AgBr/BiOIO3 composite photocatalyst exhibits moderately (5.4 and 1.1 times superior than BiOIO3 and Ag/AgBr) and significantly (180 and 7.5 times superior than BiOIO3 and Ag/AgBr) strengthened photocatalytic activity with UV and visible light illumination, respectively, which corresponds to different photocatalytic mechanisms. With illumination of UV light, charge transfer happens between bands and •O2− serves as the only active specie. In contrast, •O2−, •OH, and h+ are all produced and play important roles with illumination of visible light because of charge transfer and the SPR effect of metallic Ag, contributing to the consumedly 7789


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(9) Tian, Y. L.; Chang, B. B.; Lu, J. L.; Fu, J.; Xi, F. N.; Dong, X. P. Hydrothermal Synthesis of Graphitic Carbon Nitride-Bi2WO6 Heterojunctions with Enhanced Visible Light Photocatalytic Activities. ACS Appl. Mater. Interfaces 2013, 5, 7079−7085. (10) Tian, J.; Sang, Y. Y.; Yu, G. W.; Jiang, H. D.; Mu, X. N.; Liu, H. A Bi2WO6-Based Hybrid Photocatalyst with Broad Spectrum Photocatalytic Properties under UV, Visible and Near-Infrared Irradiation. Adv. Mater. 2013, 25, 5075−5080. (11) Huang, H. W.; Li, X. W.; Wang, J. J.; Dong, F.; Chu, P. K.; Zhang, T. R.; Zhang, Y. H. Anionic Group Self-Doping as a Promising Strategy: Band-Gap Engineering and Multi-Functional Applications of HighPerformance CO32‑ Doped Bi2O2CO3. ACS Catal. 2015, 5, 4094−4103. (12) Huang, H. W.; He, Y.; Lin, Z. S.; Kang, L.; Zhang, Y. H. Two Novel Bi-Based Borate Photocatalysts: Crystal Structure, Electronic Structure, Photoelectrochemical Properties and Photocatalytic Activity under Simulated Solar Light Irradiation. J. Phys. Chem. C 2013, 117, 22986−22994. (13) Huang, H. W.; He, Y.; Li, X. W.; Li, M.; Zeng, C.; Dong, F.; Du, X.; Zhang, T. R.; Zhang, Y. H. Non-Centrosymmetric Bi2O2(OH)(NO3) as a Desirable [Bi2O2]2+ Layered Photocatalyst: Strong Intrinsic Polarity, Rational Band Structure and {001} Active Exposing Facets CoBenefiting for Robust Photooxidating Capability. J. Mater. Chem. A 2015, 3, 24547−24556. (14) Wang, W. J.; Huang, B. B.; Ma, X. C.; Wang, Z. Y.; Qin, X. Y.; Zhang, X. Y.; Dai, Y.; Whangbo, M. H. Efficient Separation of Photogenerated Electron-Hole Pairs by the Combination of A Heterolayered Structure and Internal Polar Field in Pyroelectric BiOIO3 Nanoplates. Chem. - Eur. J. 2013, 19, 14777−14780. (15) Huang, H. W.; He, Y.; He, R.; Jiang, X. X.; Lin, Z. S.; Zhang, Y. H.; Wang, S. C. Novel Bi-based Iodate Photocatalysts with High Photocatalytic Activity. Inorg. Chem. Commun. 2014, 40, 215−219. (16) Dong, F.; Xiong, T.; Sun, Y. J.; Zhang, Y. X.; Zhou, Y. Controlling Interfacial Contact and Exposed Facets for Enhancing Photocatalysis via 2D-2D Heterostructure. Chem. Commun. 2015, 51, 8249−8252. (17) Qi, X. M.; Gu, M. L.; Zhu, X. Y.; Wu, J.; Long, H. M.; He, K.; Wu, Q. Fabrication of BiOIO3 Nanosheets with Remarkable Photocatalytic Oxidation Removal for Gaseous Elemental Mercury. Chem. Eng. J. 2016, 285, 11−19. (18) Huang, H. W.; Tu, S. C.; Zeng, C.; Zhang, T. R.; Reshak, A. H.; Zhang, Y. H. Macroscopic Polarization Enhancement Promoting Photoand Piezoelectric-Induced Charge Separation and Molecular Oxygen Activation. Angew. Chem., Int. Ed. 2017, DOI: 10.1002/anie.201706549. (19) Nguyen, S. D.; Yeon, J.; Kim, S. H.; Halasyamani, P. S. A New Polar Oxide Material with a Large SHG Response that Contains Two Lone-Pair Cations and Exhibits An Aurivillius-Type (Bi2O2)2+ Layer. J. Am. Chem. Soc. 2011, 133, 12422−12425. (20) Wang, W. J.; Cheng, H. F.; Huang, B. B.; Liu, X. L.; Qin, X. Y.; Zhang, X. Y.; Dai, Y. Hydrothermal Synthesis of C3N4/BiOIO3 Heterostructures with Enhanced Photocatalytic Properties. J. Colloid Interface Sci. 2015, 442, 97−102. (21) Feng, J. W.; Huang, H. W.; Yu, S. X.; Dong, F.; Zhang, Y. H. A Sself-Sacrifice Template Route to Iodine Modified BiOIO3: Band Gap Engineering and Highly Boosted Visible-Light Active Photoreactivity. Phys. Chem. Chem. Phys. 2016, 18, 7851−7859. (22) Xiong, T.; Zhang, H. J.; Zhang, Y. X.; Dong, F. Ternary Ag/AgCl/ BiOIO3 Composites for Enhanced Visible-Light-Driven Photocatalysis. Chin. J. Catal. 2015, 36, 2155−2163. (23) Yu, S. X.; Huang, H. W.; Dong, F.; Li, M.; Tian, N.; Zhang, T. R.; Zhang, Y. H. Synchronously Achieving Plasmonic Bi Metal Deposition and I−Doping by Utilizing BiOIO3 as the Self-Sacrificing Template for High-Performance Multifunctional Applications. ACS Appl. Mater. Interfaces 2015, 7, 27925−27933. (24) Xia, Y. X.; Lv, K. L.; Tang, D. G.; Li, M.; Li, Q. Superiority of Graphene over Carbon Analogs for Enhanced Photocatalytic H2Production Activity of ZnIn2S4. Appl. Catal., B 2017, 206, 344−352. (25) Long, M. C.; Hu, P. D.; Wu, H. D.; Cai, J.; Tan, B. H.; Zhou, B. X. Efficient Visible Light Photocatalytic Heterostructure of Nonstoichiometric Bismuth Oxyiodide and Iodine Intercalated Bi2O2CO3. Appl. Catal., B 2016, 184, 20−27.

enhanced photocatalytic performance. Moreover, Ag/AgBr/BiOIO3 shows high photo-oxidation performance for decomposing MO, tetracycline hydrochloride, chlortetracycline hydrochloride, bisphenol A (BPA), phenol, and 2,4-dichlorophenol (2,4-DCP) under visible light illumination, and its strong mineralization ability is unearthed. This study paves a new way for development of high-performance ternary composite photocatalytic materials for environmental applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01259. Temporal absorption spectra of MO over Ag/AgBr and Mott−Schottky curve of BiOIO3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-010-82332247. *E-mail: [email protected]. ORCID

Hongwei Huang: 0000-0003-0271-1079 Yihe Zhang: 0000-0002-1407-4129 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was jointly supported by the National Natural Science Foundations of China (No. 51672258 and No. 51572246), the Fundamental Research Funds for the Central Universities (2652015296).

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Achieving Enhanced UV and Visible Light Photocatalytic Activity for

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