Ag and tubular photo-reactor

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Fabrication of 3D sponge@AgBr-AgCl/Ag and tubular photo-reactor for continuous waste water purification under sunlight irradiation Weihang Kong, Shilin Wang, Di Wu, Cuirong Chen, Yusheng Luo, Yuetian Pei, baozhu Tian, and Jinlong Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.9b02575 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 24, 2019

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ACS Sustainable Chemistry & Engineering

Fabrication of 3D sponge@AgBr-AgCl/Ag and tubular photo-reactor for continuous waste water purification under sunlight irradiation Weihang Kong,†,# Shilin Wang,†,# Di Wu, † Cuirong Chen, † Yusheng Luo,† Yuetian Pei†, Baozhu Tian,*,† Jinlong Zhang† †Key

Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research

Center, Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science & Technology, 130 Meilong Road, Shanghai, 200237, China.

ACS Paragon Plus Environment

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Abstract: Semiconductor photocatalysis has a promising prospect in the remediation of organic pollutants in environmental waste water. At present, the main bottlenecks of relative researches lie in how to more effectively improve the sunlight activity of photocatalysts and realize their reutilization. Herein, we assembled AgBr-AgCl/Ag on the skeletons of melamine sponge (MS) by immobilizing Ag nanowires (NWs) on MS skeletons, transferring Ag into AgBr-AgCl, and further reducing part Ag+ into metallic Ag. The microstructures of MS@AgBr-AgCl/Ag were characterized by the means of scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). It was found that AgBr-AgCl/Ag NWs were successfully loaded on the skeletons of MS to form 3D structure and there existed a composite structure between AgBr and AgCl. The transient photocurrents and electrochemical impedance analyses revealed that AgBr-AgCl composite structure effectively decreased the impedance of charge carrier transfer and consequently MS@AgBr-AgCl/Ag NWs displayed much higher photocurrent than MS@AgBr/Ag NWs and MS@AgCl/Ag NWs. The photocatalytic activities of the samples were evaluated by degrading antibiotic sulfadiazine (SD) and five organic dyes (fuchsin basic, methyl orange, acid orange 7, malachite green and rhodamine B). Beneficial from the excellent visible-light response and charge carrier separation properties, MS@AgBr-AgCl/Ag NWs displayed the optimal photocatalytic activity among these samples. Radical trapping experiments demonstrated that •O2− and h+ are the main reactive species responsible for organic contaminant degradation. Based on the results of high performance liquid chromatography-mass spectra (HPLC-MS), the possible structures of the main intermediates towards SD degradation were proposed. By filling MS@AgBr-AgCl/Ag NWs into glass tubes and connecting them together, we fabricated a tubular fixed-bed photoreactor for continuous waste water purification. Overall, this work provides a reference for constructing visible light driven photocatalysts and fixedbed photoreactors used for the environmental waste water purification under sunlight irradiation.

Keywords: AgBr-AgCl/Ag; Melamine sponge; 3D structure; Fixed-bed photoreactor; Photocatalytic degradation; Dye; Antibiotics


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INTRODUCTION Along with the rapid industrialization, a large quantity of effluents containing various organic contaminants such as herbicides/pesticides, organic dyes, and antibiotics, etc., have been discharged into water sources.1 For instance, more than 1 × 106 tons of dyestuff are produced annually and about 10−15% carcinogenic and mutagenic dyes are discharged into the aquatic ecosystem, causing serious human diseases such as cancer, lung diseases, and so on.2,3 Antibiotics have been widely used to destroy the disease-causing bacteria. However, the accumulation of antibiotics in the environment can disrupt ecosystem equilibrium and induce bacterial resistance against drugs.4 Because of the high toxicity and difficulty to biological treatment, these organic pollutants have become a tremendous threat to human health and ecological system.5−10 As a green strategy to remedy the organic pollutants in environmental waste water, TiO2-based photocatalysis has attracted extensive attention in last decades.11,12 However, TiO2 only absorbs UV light because of its wide band gap and usually displays very low sunlight utilization efficiency.13−15 To overcome this problem, the researchers developed many strategies such as coupling with other narrow band semiconductor, doping with metals or nonmetals, forming Z-sheme structure, sensitizing with noble metal by surface plasmon resonance (SPR), and so on.16−30 In 2008, AgCl/Ag plasmonic photocatalyst was fabricated to degrade methyl orange dye by Huang’s group.23 From then on, silver halide-based plasmonic photocatalysts (AgX/Ag, X = Cl, Br, and I) have received increasing research attention owing to their superior visible-light activity in degrading the organic pollutants of waste water.22,32−39 Normally, the photocatalytic performances of AgX/Ag plasmonic photocatalysts heavily rely on their morphological structures. Therefore, most of the studies have been focused on constructing AgX/Ag with different morphologies and investigating the influence of morphology on photocatalytic activity.22,31−48 So far, AgX photocatalytic materials with various morphologies such as cubes,33−36 nanowires,40,41 porous structures,42,

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microspheres,44 and cubic cage,51 have been

successfully prepared.


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There still exist two bottlenecks that restrain the application of AgX/Ag plasmonic photocatalysts in waste water remediation: One is the low separation efficiency of photogenerated charge carriers; the other is the recovery of AgX/Ag photocatalyst. To solve the first problem, AgBr/AgCl/Ag and AgBr/AgI/Ag heterostructure/solid solution have been fabricated. 34,42,48−52 It was found that the hybrid photocatalysts showed higher activity than the single AgX/Ag in terms of organic degradation. In the case of photocatalyst reuse, the traditional centrifugation and filtration are often associated with serious catalyst loss and high energy consumption.53 Immobilizing AgX/Ag on the 2D substrates not only decreases the effective surface area but also increases the diffusion distance of organic molecules to AgX/Ag film, resulting in low purification efficiency of waste water. Magnetic separation is an effective strategy of separating the nano-sized AgX/Ag photocatalysts,54 similar to the magnetic separation of other photocatalysts.55−58 However, it usually needs complicated synthetic processes. Recently, Ge et al. assembled AgCl/Ag nanowires in a polymer sponge to form 3D AgCl/Ag NW networks, which provides the an effective and low-cost strategy to realize the continuous degradation of organic contaminants in waste water.53 Herein, we assembled AgBr-AgCl/Ag nanowires (NWs) on the skeletons of melamine sponge (MS) by immobilizing Ag NWs on MS skeletons, transferring Ag NWs into AgBr-AgCl, and further reducing part Ag+ into metallic Ag. In the hybrid structure, Ag nanoparticles (NPs) act as the sensitizer to absorb visible light; AgBr and AgCl play the roles of separating photogenerated electrons and holes; 3D MS was used to load AgBr-AgCl/Ag to realize the reuse of photocatalyst. The microstructures and optical property of MS@AgBr-AgCl/Ag were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–vis diffuse reflectance spectra (UV–vis DRS). The separation efficiency of photogenerated charge carriers was explored by transient photocurrent and electrochemical impedance analyses. The photocatataltic degradations of antibiotic sulfadiazine (SD) and five organic dyes (fuchsin basic, methyl orange, acid orange 7, malachite green and rhodamine B) were employed as the probe


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reactions to evaluate the photocatalytic activities of MS@AgBr-AgCl/Ag and the control photocatalysts. Thereinto, the main intermediates produced during SD degradation were analyzed by high performance liquid chromatography-mass spectra (HPLC-MS). On the basis of UV–vis DRS, Mott–Schottky analyses, radical trapping experiments, the degradation mechanism of organic pollutants over MS@AgBr-AgCl/Ag was proposed. To realize the continuous degradation of organic pollutants in water, we designed a fixed-bed photoreactor by filling the MS@AgBrAgCl/Ag sponge into several glass tubes and further connecting them together. With this tubular photoreactor, the degradations of SD and five organic dyes were performed.

EXPERIMENTAL SECTION Preparation of Ag NWs and MS@AgBr-AgCl/Ag NWs Ag NWs were synthesized by a modified polyol reduction method using AgNO3 as Ag source and glycerol as reduction agent, similar to the previous report.59 In a typical procedure, 5.8 g PVP (K30) was added into a 250 mL flask containing 190 mL glycerol and the mixture was stirred at 110 °C until PVP was totally dissolved. After the temperature was dropped to 50 °C, 1.58 g AgNO3 was added into the above solution, followed by addition of a mixture solution of 10 mL glycerol and 0.5 mL H2O containing 59 mg NaCl. Under N2 atmosphere, the above solution was heated to 210 °C. At this moment, the solution turned from pale white to brown and grey. Then, the suspension was dropped to room temperature and washed with ethanol and water for two times by centrifugation. Finally, the obtained Ag NWs were dispersed in water with a concentration of 5 mg/L. MS@AgBr-AgCl/Ag NWs was prepared by following procedures: 5 mL Ag NWs dispersion (5 mg/mL) was dropped on a piece of MS (3 cm × 6 cm × 0.3 cm) and dried at 70 °C. Subsequently, the Ag NWs-loaded sponge was further heated at 200 °C for 30 min to form the welding spots among Ag NWs, denoted as MS@Ag NWs. Under gentle stirring, MS@Ag NWs were dipped into 100 mL aqueous solution containing 8.0 mg NaCl, 12.0 mg NaBr, 113 mg Fe(NO3)3•9H2O, and 200 mg


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PVP(K-30). After reacted for 90 min in dark, the obtained MS@AgBr-AgCl NWs was washed with water for 3 times and dried at 70 °C. Finally, the MS@AgBr-AgCl NWs was put into water and irradiated by a 300 Xe lamp for 30 min to generate Ag0 on the surface of MS@AgBr-AgCl NWs, denoted as MS@AgBr-AgCl/Ag NWs. MS@AgBr/Ag NWs and MS@AgCl/Ag NWs were prepared with the same procedures except for only using NaCl (15 mg) or NaBr (26 mg) as the precursor in the process of Ag transformation.

Characterization The morphology of the samples was observed by a TESCAN NOVA3 scanning electron microscopy (SEM), operated at 15 kV. The X-ray diffraction (XRD) patterns were collected with a Rigaku D/max 2550 VB/PC X-ray diffractometer, using Cu Kα radiation (λ = 0.154056 nm). The Xray photoelectron spectra (XPS) were analyzed with a Perkin-Elmer PHI 5000 Versaprobe system using Al K1 radiation. The shift of the binding energy from relative surface charging was adjusted by assuming the binding energy of the adventitious C1s at 284.8 eV. The UV-vis diffuse reflectance spectra were analyzed with a SHIMADZU UV-2450 spectrometer. The actual Ag content in MS@AgBr-AgCl NWs was measured by an Agilent 725-ES inductively coupled plasma atomic emission spectrometer (ICP-AES). The main intermediates produced during SD degradation were analyzed by LTQ XL high performance liquid chromatography-mass spectra (HPLC-MS) made by THERMO manufacturer.

Photoelectrochemical measurement The transient photocurrent, electrochemical impedance, and Mott-Schottky analyses were conducted on a Zahner electrochemical work station equipped with a three electrode system, using photocatalyst sample as the a working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode as the reference electrode. Herein, the working electrode was prepared by coating 1.0 mg photocatalyst sample on fluorine-doped SnO2 glass with an active area of 1.5 cm2, similar to our previous report.60 The transient photocurrents were tested in 0.5 M Na2SO4 aqueous solution, using a 500 W halogen tungsten lamp (λ > 420 nm) as the light source. The electrochemical


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impedance spectra (EIS) were tested in the aqueous solution of 25 mM {K3[Fe(CN)6]}, 25 mM K4[Fe(CN)6] and 0.1 M KCl. The Mott-Schottky curves were measured in 0.5 M Na2SO4 aqueous solution without light irradiation.

Photocatalytic activity measurements Here, we selected the antibiotic sulfadiazine (SD) and five organic dyes (fuchsin basic, methyl orange, acid orange 7, malachite green and rhodamine B) as the model organic contaminants in waste water to investigate the photocatalytic activities of the photocatalyst samples. The photocatalytic degradation reactions were conducted using a home-made reactor and a 500 W halogen lamp with an ultraviolet cut-off filter (λ ≥ 420 nm) was employed as the light source to drive the photocatalytic reaction. For each test, a piece of MS@AgX/Ag sponge was put into a quartz tube containing 50 mL SD aqueous solution (10 mg/L) or dyes aqueous solution (10 mg/L). Before light irradiation, the tube was kept in dark for 30 min to make the SD or dyes reach adsorption‒desorption equilibrium on the surface of samples. At certain time intervals, the residual concentrations of SD and dyes were measured by High Performance Liquid chromatography (HPLC) and UV-vis spectrophotometer, respectively. To realize the continuous degradation of organic contaminants in water, we designed a fixed-bed photoreactor by filling the MS@AgBr-AgCl/Ag sponge into several glass tubes and further connecting them together. By a peristaltic pump, the polluted water was continuously injected to the fixed-bed tubular reactor, in which the pollutant was degraded under sunlight irradiation (Scheme 1). To make sure the sufficient contact between the organic pollutant and the photocatalyst, the flowing rate of organic pollutant solution is 200 mL/h.


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Scheme 1. Schematic diagram of continuously degrading organic pollutant aqueous solution via the home-made tubular reactor under sunlight irradiation.

RESULTS AND DISCUSSION Synthtic route, formation mechanism, and morphology

Scheme 2. Synthetic routes of MS@AgCl/Ag NWs, MS@AgBr/Ag NWs, and MS@AgBrAgCl/Ag NWs. Scheme 2 schematically illustrates the synthetic routes of MS@AgCl/Ag, MS@AgBr/Ag and MS@AgBr-AgCl/Ag. Firstly, the Ag NWs were fixed onto the skeleton of MS and immobilized by thermal welding treatment at 200 °C. Through the heat treatment, Ag NWs not only form welding


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joints together but also combine with the MS frameworks. Then, the Ag NWs were oxidized into AgCl, AgBr, or AgBr-AgCl NWs by Fe3+ ions. In this process, the Cl− (or Br−) ions not only served as the chloride (or bromide) source for the growth of AgCl (or AgBr) NWs but also as the activation agents to trigger the replacement reaction. The standard redox potential of Ag+/Ag couples is +0.80 V (vs. SHE), which is higher than that of Fe3+/Fe2+ (+0.771 V).40 Normally, metallic Ag cannot be oxidized to Ag+ ions by Fe3+. However, in the presence of Cl− or Br− ions, the redox potentials of Ag/AgX (vs SHE) decrease to +0.223 V for Ag/AgCl and +0.007 V for Ag/AgBr,40,41 both of which are lower than that of Fe3+/Fe2+ pair (+0.771 V). Hence, the electrons generated from Ag NWs migrate

to Fe3+ ions to form Fe2+, and then Ag atoms were in-situ oxidized to AgCl and AgBr. When the Cl− and Br− ions are coexistent, the AgBr will be preferentially produced because the potential difference between E0Fe3 + /Fe2 + and E0AgBr/Ag (+0.764 V) is higher than that of E0Fe3 + /Fe2 + and E0AgCl/Ag (+0.548 V). Next, Cl− ions will take part in the reaction as soon as Br− ions are consumed, leading to the formation of AgCl. As a result, the composite semiconductor AgBr-AgCl will be produced. Finally, MS@AgBrAgCl was excited by visible light to form metallic Ag0 on the surface of MS@AgBr-AgCl to obtain MS@AgBr-AgCl/Ag.


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Figure 1. SEM images of (A) Ag NWs, (B) MS@Ag NWs, (C) MS@AgCl/Ag NWs, (D) MS@AgBr/Ag NWs, and (E, F) MS@AgBr-AgCl/Ag NWs. Element mapping images of MS@AgBrAgCl/Ag NWs: (G) Ag; (H) Cl; (I) Br. The morphologies of the synthesized samples were observed by scanning electron microscopy. As displayed in Figure 1A, the as-prepared Ag NWs possess a smooth surface and their diameter is ~ 90 nm. After heat treatment, it appeared some welding spots on the junctions of Ag NWs, which can fix the Ag NWs and prevent their exfoliation from MS (Figure 1B). Further treated by Fe(NO3)3 and halide salts, the surface of Ag NWs became rough and appeared many nanoparticles (Figure 1C−E), implying the successful transformation from silver to silver halide. From the element mapping images of MS@AgBr-AgCl/Ag in a small region (Figure 1G‒I), it can be seen that Ag, Cl and Br display uniform distribution in MS@AgBr-AgCl/Ag. From the EDS results (Figure S1), the element percentages of Ag, Br, and Cl in AgBr-AgCl/Ag were determined to be 52.25%, 27.61%, and 20.14%, respectively. The actual Ag content in MS@AgBr-AgCl NWs was measured by ICP-AES to be 28.2 wt.%, near to the loading Ag percentage (29.4 wt.%).

Crystalline structure and surface composition

Figure 2. XRD patterns of (a) MS, (b) MS@Ag NWs, (c) MS@AgCl/Ag NWs, (d) MS@AgBr/Ag NWs, and (e) MS@AgBr-AgCl/Ag NWs.


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Figure 2 shows the XRD patterns of MS, MS@Ag NWs, MS@AgCl/Ag NWs, MS@AgBr/Ag NWs, and MS@AgBr-AgCl/Ag NWs. Compared to melamine sponge (MS) (Figure 2a), MS@Ag NWs appears four peaks at 2θ = 38.1°, 44.4°, 64.6°, and 77.4° (Figure 2b), ascribed to the facecentered cubic Ag (111), (200), (220), and (311) planes, respectively (JCPDS Card, No. 65-2871).61 As shown in Figure 2c, MS@AgCl/Ag NWs presents three strong diffraction peaks at 2θ = 27.8°, 32.2°, 46.2°, matched with to the AgCl (111), (200), (220) planes, respectively (JCPDS Card, No. 311238). Similarly, MS@AgBr/Ag NWs presents four strong diffraction peaks at 2θ = 31.0°, 44.3°, 55.0°, and 73.3°, which can be indexed to AgBr (200), (220), (222), and (420) plane reflections (JCPDS Card, No. 06-0438), respectively. For sample MS@AgBr-AgCl/Ag NWs, it simultaneously displays the diffraction peaks of AgCl and AgBr, indicating the formation of AgBr and AgCl composite. The sample MS@AgCl/Ag NWs, MS@AgBr/Ag NWs, and MS@AgBr-AgCl/Ag NWs do not show the obvious Ag characteristic diffraction peak, which is due to the low content and low crystallinity of Ag NPs. To identify the formation of Ag in the process of photo-reduction, we synthesized AgCl/Ag, AgBr/Ag and AgBr-AgCl/Ag by respectively photo-reducing AgCl, AgBr, and AgBr-AgCl NPs in the absence of melamine sponge. As shown in Figure S2, all of AgCl/Ag, AgBr/Ag and AgBr-AgCl/Ag clearly displayed the Ag (111) diffraction peaks at 2θ = 38.1°, indicating the generation of metallic Ag in the three samples.


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Figure 3. XPS spectra of as-prepared (a) and photocatalytic reaction-used (b) MS@AgBr-AgCl/Ag NWs: (A) Survey XPS spectra; (B) Ag 3d high resolution spectra; (C) Cl 2p high resolution spectra; (D) Br 3d high resolution spectra. The elemental composition and chemical status of MS@AgBr-AgCl/Ag were analyzed by XPS technique. The survey spectra show that both the as-prepared and used MS@AgBr-AgCl/Ag samples contain Ag, Cl, Br, O, and N elements (Figure 3A). Herein, the N 1s belongs to the N in melamine sponge, while the O 1s may be derived from the adsorbed oxygen. In Figure 3B, the Ag 3d5/2 and Ag 3d3/2 peaks can be fitted to two sets of peaks, in which the set of peak at 367.3 eV and 373.3 eV is assigned to Ag+ in AgCl-AgBr while that at 368.1 eV and 374.0 eV belongs to metallic Ag.33,54 According to the peak areas in XPS, the relative contents of metallic Ag0 and Ag+ in the total Ag were calculated to be 10.2 at% and 89.8 at%, respectively. By comparing the peak areas of Ag+ and Ag0 in the as-prepared and used MS@AgBr-AgCl/Ag samples, it can be seen that after the photocatalytic reaction, the relative peak area of metallic Ag0 increased, demonstrating that part Ag+ in AgBr-AgCl has been transferred into metallic Ag0 in the photocatalytic reaction. For the Cl 2p spectra (Figure


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3C), the peaks at 197.4 eV and 199.0 eV are correspond to Cl 2p3/2 and Cl 2p1/2, respectively.33,62 In the case of Br 3d spectra (Figure 3D), the two peaks at 67.8 eV and 68.9 eV are attributed to Br 3d5/2 and Br 3d3/2, respectively.54 On the basis of the peak areas and sensitivity factors of Br and Cl elements, their relative contents were calculated to be 52.5 at% for Br and 47.5 at% for Cl, respectively. The above XPS results further confirmed the chemical structures of MS@AgBrAgCl/Ag. Based on the peak areas in XPS, the relative content of metallic Ag0 in the total Ag of MS@AgCl/Ag was determined to be 13.3%, which is higher than that in MS@AgBr/Ag (8.1%) (Calculation details see Supporting Information). This result indicates that Ag+ in AgCl is easier to be reduced into Ag than that in AgBr.

Absorption spectra and band gaps

Figure 4. (A) UV-Vis DRS spectra of (a) MS, (b) MS@AgCl/Ag NWs, (c) MS@AgBr/Ag NWs, and (d) MS@AgBr-AgCl/Ag NWs. The insert is plot of (αhv)1/2 versus photon energy (hv) for MS@AgBr/Ag NWs (black), MS@AgBr-AgCl/Ag NWs (red), and MS@AgCl/Ag NWs (blue). (B,C)


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Mott-Schottky curve of (B) AgCl NWs and (C) AgBr NWs. (D) Conduction band and valence band of AgCl and AgBr estimated by Mott-Schottky measurements and absorption spectra. The light absorption properties of the different samples were analyzed by UV‒Vis diffuse reflectance spectroscope. As shown in Figure 4A, the absorption edges of AgCl, AgBr-AgCl and AgBr were estimated to be 365 nm, 420 nm, and 455 nm. For a composite semiconductor, it usually exhibits an absorption edge between the absorption edges of the two semiconductors.11,63−64 For instance, Yan et al. reported that the absorption edge of anatase‒rutile composite regularly red-shifts with increasing the rutile content in the composites.64 It should be mentioned that the absorption of AgBr-AgCl composite results from the absorption superimposition of AgBr and AgCl but not the real electron transmission from the VB of AgBr-AgCl to its CB. According to the optical absorption properties, the band gap energy of the a semiconductor can be calculated by the Kubelka-Munk equation:65 αhv = A(hv−Eg)n/2, where α, hv, A, and Eg are absorption coefficient, photon energy, constant, and band gap energy, respectively. n value is determined by the optical transition type of a semiconductor (n = 1 for direct transition and n = 4 for indirect transition). There exist indirect transition for AgCl and AgBr,66,67 thus the band gaps of MS@AgCl/Ag, MS@AgBr/Ag, and MS@AgB-AgCl/Ag were determined to be 3.38 eV, 2.75 eV, and 2.98 eV, respectively, using the plot of (αhv)1/2 versus photon energy (hv) (Figure 4A). All of the samples MS@AgCl/Ag, MS@AgBr/Ag, and MS@AgB-AgCl/Ag exhibit a broad absorption peak around 530 nm, ascribed to the SPR absorption of Ag NPs. Interestingly, the SPR absorption intensities of the three samples display a decline trend in the order of MS@AgCl/Ag, MS@AgBr-AgCl/Ag, and MS@AgBr/Ag, which is in connection with the size and number of Ag NPs produced during light reduction. Since the solubility of AgCl (1.9 × 10−3 at 25°C) is more higher than that of AgBr (1.4 × 10−4 at 25 °C),68 AgCl can release more Ag+ ions when compared with AgBr. As a result, more and bigger Ag NPs will be produced on AgCl surface than on AgBr surface. Since the CB and flat band potentials are very near,69,70 the CB potentials of AgCl and AgBr can be determined by Mott−Schottky analysis. By extrapolating the Mott−Schottky plots at different frequencies,71,72 the CB potentials of AgCl and AgBr were determined to be −0.67 eV and −0.57 eV, respectively (Figure 4B and C). On the basis of


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band gaps and CB potentials, the VB potentials of AgCl and AgBr were calculated to be 2.71 eV and 2.18 eV, respectively (Figure 4D).

Photoelectrochemical properties

Figure 5. (A) Transient photocurrents and (B) Nyquist plots of EIS spectra of (a) MS@AgBr/Ag NWs, (b) MS@AgCl/Ag NWs, and (c) MS@AgBr-AgCl/Ag NWs. The photoelectrical properties of MS@AgBr/Ag, MS@AgCl/Ag and MS@AgBr-AgCl/Ag were analyzed by the transient photocurrents under intermittent visible light irradiation. Among the three samples, MS@AgBr-AgCl/Ag exhibits the highest photocurrent (Figure 5A), implying that MS@AgBr-AgCl/Ag possesses the highest photoelectric efficiency. Electrochemical impedance analysis is an effective technique to analyze the dynamics of the charges in the interfacial and bulk region of the semiconductors.73,74 As displayed in Figure 5B, MS@AgBr-AgCl/Ag displays the smallest semicircle diameter in the Nyquist plots of EIS spectra, suggesting that MS@AgBr-AgCl/Ag


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owns the smallest impedance for charge carrier transfer. As is well known, the photoelectric efficiency of a semiconductor is related to the production and transfer of photo-generated charge carriers. Thus, the diversity of the above samples in transient photocurrent should result from their differences in light absorption and electrochemical impedance. Since the impedances of MS@AgCl/Ag and MS@AgBr/Ag are very similar, their difference in transient photocurrents should be attributed to the fact that MS@AgCl/Ag can more effectively respond to visible light. In the case of MS@AgCl/Ag and MS@AgBr-AgCl/Ag, although the visible light absorption of MS@AgBrAgCl/Ag is weaker than that of MS@AgCl/Ag, the hetero-junction structure between AgBr and AgCl can effectively decrease its impedance. As a result, the transient photocurrent of MS@AgBr-AgCl/Ag is higher than that of MS@AgCl/Ag.

Photocatalytic activity and mechanism


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Figure 6. (A) Photocatalytic degradation curves of SD and (B) fitted kinetic curves of SD degradation over the different photocatalysts: (a) MS@AgBr/Ag NWs, (b) MS@AgCl/Ag NWs, and (c) MS@AgBr-AgCl/Ag NWs. (C) Cycling degradation curves and (D) fitted kinetic curves of SD over as-prepared MS@AgBr-AgCl/Ag NWs (a‒c) and recovered MS@AgBr-AgCl/Ag NWs (d and e). (E) Photocatalytic degradation curves and (F) fitted kinetic curves of SD over MS@AgBr-AgCl/Ag NWs in the presence of different radical trapping agents: (a) BZQ (4 mM), (b) EDTA (4 mM), (c) TBA (4 mM), and (d) No scavenger. Figure 6A and B show the degradation curves and fitted kinetic curves of sulfadiazine (SD) under visible light irradiation over the different photocatalysts, respectively, where C0 represents the concentration of SD after the adsorption‒desorption equilibrium and C represents the SD concentration after photocatalytic reaction for a certain time. From Figure 6A, it can be seen that MS@AgBr-AgCl/Ag exhibits the highest photocatalytic activity among the three photocatalysts. After visible light excitation for 60 min, the degradation rates of SD over MS@AgBr/Ag NWs, MS@AgCl/Ag NWs and MS@AgBr-AgCl/Ag NWs reached 60%, 83%, and 93%, respectively. To accurately compare the photocatalytic activity of these photocatalysts, the kinetic curves between ln(C0/C) and reaction time were fitted. As displayed in Figure 6B, all the fitted kinetic curves exhibit well linearity and high regression coefficients (R2 > 0.99), suggesting that the photocatalytic degradation of SD over these samples follows pseudo-first-order kinetics.75,76 The rate constant for MS@AgBr-AgCl/Ag NWs reaches 0.059 min−1, which is nearly 3 and 1.5 times as high as that of MS@AgBr/Ag NWs (0.02 min−1) and MS@AgCl/Ag NWs (0.04 min−1), respectively. Combing the photoelectrical measurements, we can deduced that the superior photocatalytic activity of MS@AgBrAgCl/Ag should be attributed to both its excellent visible light response ability and high charge transfer efficiency. Since the photo-stability of a photocatalyst has a huge impact to its practical applications, we further investigated the photo-stability of MS@AgBr-AgCl/Ag by cycle degradation experiments. As displayed in Figure 6C and D, the activity of MS@AgBr-AgCl/Ag NWs exhibits an evident decline from the first run to the third cycle and the rate constant decreases gradually from 0.058 min−1 to 0.045 min−1 (2nd cycle) and 0.028 min−1 (3rd cycle). Correspondingly, the XRD peak


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intensities of AgCl and AgBr diffraction peaks display a dramatic decrease within three cycles (Figure S3), which is because part AgCl and AgBr have been converted into metallic Ag after the photocatalytic reaction (Figure 3B). To recover the activity of MS@AgBr-AgCl/Ag, we retreated the used MS@AgBr-AgCl/Ag sample with NaCl, NaBr, and Fe(NO3)3 solution. It was found that the rate constant was recovered from 0.028 min−1 to 0.049 min−1 (Figure 6D). From the cycle experiment (Figure 6C and D) and XRD pattern (Figure S3), it can be confirmed that AgCl and AgBr can be reproduced and the activity also can be recovered by chemical recovery treatment. In the photocatalytic reaction, several reactive species, such as superoxide radical (•O2−), hole (h+), and hydroxyl radical (•OH), participate in the degradation of organic pollutants. To investigate the reactive species responsible for the degradation of SD over MS@AgBr-AgCl/Ag, a series of degradation experiments with different radical trapping agents (radical trapping experiments) were carried out. Here, tert butyl alcohol (TBA), benzoquinone (BZQ), and EDTA-2Na were used to scavenge •OH, •O2−, and h+, respectively. From the variation of SD degradation curves (Figure 6E), it can be deduced that both •O2− and h+ are the reactive species for the degradation of SD. Moreover, we also investigated the kinetics of SD degradation in the presence of different scavengers. As shown in Figure 6F, the degradations of SD in the presence of these scavengers also follow pseudo-first-order kinetics. The rate constant in turn decreases from 0.059 min−1 (no scavenger) to 0.012 min−1 (BZQ), 0.040 min−1 (EDTA), and 0.047 min−1 (TBA). Therein, •O2− and h+, respectively, play the primary and secondary roles in SD degradation. h+ can oxidize Cl− (or Br−) into •Cl (or •Br), which further degrades SD. The degradation rate of SD almost has no change in the presence of TBA, implying that •OH does not take part in SD degradation. The high performance liquid chromatography-mass spectrometry (HPLC-MS) was employed to analyze the intermediates produced in the SD degradation process. Figure S4A shows the HPLC spectrogram of SD solution after photocatalytic reaction for 30 min over MS@AgBr-AgCl/Ag NWs under visible light irradiation. There exist nine peaks in the HPLC spectrogram, implying that there are eight main intermediates except for the reactant SD. The Mass spectra (MS) of the corresponding components at different HPLC retention time in HPLC spectrogram were displayed in Figure S4B‒J. In Figure S4H, the compound at the retention time of


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5.52 min has a mass-to-charge ratio of 251, corresponding to the reactant SD. According to the massto-charge ratios of molecular ion peaks, the eight degradation intermediates can be classified into five types, noted as Product A, B, C, D and E, respectively. According to the organic reaction principles, their possible molecular structures are deduced and listed in Table 1. Table 1 Proposed molecular structure and HPLC retention time of SD and nine intermediates produced in the process of SD degradation over MS@AgBr-AgCl/Ag NWs Elemental

Deduced molecular

Retention time

composition

structure

(min)

Reactant

C10H10N4O2S

5.52

Product A

C10H10N4

1.96, 2.07, 2.73

Product B

C10H9ClN4

2.44

Product C

C10H9BrN4

2.58, 3.43

Product D

C10H9ClN4O2S

8.97

Product E

C10H9BrN4O2S

9.56


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Figure 7. (A) Photocatalytic degradation curves and (B) fitted kinetic curves of MG, AO7, MO, RhB, and FB over MS@AgBr-AgCl/Ag NWs. In addition to antibiotic SD, the photocatalytic degradation of organic dyes malachite green (MG), acid orange 7 (AO7), methyl orange (MO), rhodamine B (Rh-B), and fuchsin basic (FB) over MS@AgBr-AgCl/Ag NWs were also investigated. As shown in Figure 7A, after visible light irradiation for 60 min, the degradation rates of AO7, MG, and FB reached nearly 100%, while those of MO and Rh-B are about 85%. The fitted kinetics curves (Figure 7B) indicate that the photocatalytic degradations of all the five dyes follow pseudo-first-order kinetics. The rate constants for AO7, MG, MO, FB, and Rh-B are 0.115 min−1, 0.086 min−1, 0.054 min−1, 0.051 min−1, and 0.043 min−1, respectively. Since the decolorization of dyes always does not mean its mineralization, we tested the total organic carbon (TOC) concentration of the FB degradation solution. As shown in Figure S5, FB can be totally degraded within 60 min, while the TOC of the reaction solution only decreases ~8% after degradation for 5 h. This result means that the decolorization rate of FB is far faster than its mineralization rate, similar to the previous reports.77−79


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Figure 8. (A) Photocatalytic degradation curves and (B) fitted kinetic curves of SD solution at different initial pHs over MS@AgBr-AgCl/Ag NWs. (C) Photocatalytic degradation curves and (D) fitted kinetic curves FB solution at different initial pHs over MS@AgBr-AgCl/Ag NWs. We further investigated the effect of pH (3–11) on the photocatalytic degradations of SD and FB over MS@AgBr-AgCl/Ag NWs. From the degradation curves (Figure 8A) and fitted kinetic curves (Figure 8B) of SD at different initial pHs, it can be seen that the activity of SD degradation increases with the increase of pH from 3 to 11. The HPLC-MS analysis indicates that the main intermediates of SD degradation include the chlorinated and brominated products. As displayed in Figure S6, H+ will be produced during the step 1 and step 3. According to the chemical equilibrium, excessive H+ is not conducive to the degradation reaction of SD when pH < 7. On the contrary, excessive OH− is conducive to the degradation reaction of SD when pH > 7. Therefore, the activity of photocatalytic degradation of SD over MS@AgBr-AgCl/Ag NWs increases with the increase of pH. As displayed in Figure 8C and D, MS@AgBr-AgCl/Ag NWs exhibits the higher photocatalytic activity for FB degradation at neutral condition (pH 7) than at acidic (pH 3 or 5) and alkaline conditions (pH 9 or 11). The isoelectric points (IEPs) of AgCl and AgBr are 5.3 and 5.5, respectively.80 When the pH is lower than their IEPs, the surface of AgBr-AgCl/Ag NWs would be positively charged, which is not


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favorable to the adsorption of cationic dye FB. At pH 7, the surface of AgBr-AgCl/Ag NWs would be negatively charged, beneficial to the adsorption of FB. When pH is increased to 9 and 11, FB gradually changes from cation to neutral molecule, resulting in a low adsorption of FB on the surface of AgBr-AgCl/Ag NWs. Therefore, the photocatalytic activity declined in alkaline condition. The photocatalytic activity at pH 11 is higher than that at pH 9, which is because FB would be partially decomposed at high alkaline condition.81 Thus, the low initial FB concentration would lead to higher apparent photocatalytic activity.82

Figure 9. (A,B) Energy level diagrams of AgCl/Ag before (A) and after (B) visible light irradiation. (C) Photocatalytic degradation mechanism of organic pollutants over MS@AgBr-AgCl/Ag NWs under visible light irradiation.


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To deeply understand the photocatalytic mechanism of MS@AgBr-AgCl/Ag NWs for organic degradation, we investigated the effect of Ag SPR on the photocatalytic activity of MS@AgBrAgCl/Ag NWs. Compared to MS@AgBr-AgCl, MS@AgBr-AgCl/Ag exhibits evidently improved activity for the degradations of SD and five organic dyes (Figure S7), indicating that Ag plays an important role for the improvement of visible light activity. When a noble metal contacts with a semiconductor, a Schottky barrier will be produced at their interface because of their different work functions, thus the electrons would migrate from the component with lower work function to the one with higher work function to realize the equilibration of Fermi level.83 Since AgCl (φAgCl = 4.8 eV) has a higher work function than Ag (φAg = 4.25 eV),84,85 the Fermi energy level of AgCl would be lower than that of Ag. Consequently, the electrons would transfer from Ag to AgCl until the two systems attain equilibrium and form a new Fermi level (Figure 9A). Similar to AgCl, AgBr can also accept the electron from Ag to equilibrate the Fermi level (φAgBr = 5.3 eV) 83. Under visible light irradiation, the electromagnetic field of light induces the collective coherent oscillation of the free electrons of Ag (SPR effect), leading to the separation of charge carriers around the particle surface and forming dipole oscillation along the direction of light electric field.86 According to the literatures,87, 88 the energy level of metal SPR-excited electron is usually 1.0~4.0 eV higher than its Fermi level. Consequently, the excited electrons possess enough energy to transfer from Ag to the conduction band (CB) of AgCl (Figure 9B) and AgBr. Thus, the Ag particles play the important role for improving the visible light activity of MS@AgBr-AgCl/Ag NWs by acting as a photosensitizer. Based on the above experiment results and analyses, the degradation mechanism of organic pollutants over MS@AgBr-AgCl/Ag was proposed and illustrated in Figure 9C. Under visible light irradiation, the electron-hole pairs in Ag NPs are produced and further separated by Ag SPR effect. Subsequently, the excited electrons are transferred to AgCl and AgBr CB, Meanwhile, the holes on Ag would transfer to the valence bands (VB) of AgBr and AgCl. Moreover, AgBr also can absorb

visible light to produce electrons and holes. The electrons on AgCl and AgBr CB would react with O2 to produce •O2− to degrade organic contaminants, while the holes on AgBr and AgCl VB would directly oxidize organic contaminants or react with Br− and Cl− to produce •Br and •Cl to


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degrade organic contaminants. This can explain why •OH has not been detected in the radical trapping experiments. Since MS@AgBr-AgCl/Ag can effectively separate the photogenerated electrons and holes, MS@AgBr-AgCl/Ag exhibits higher photocatalytic activity than MS@AgBr/Ag and MS@AgCl/Ag.

Figure 10. (A) Continuous degradation of FB via the tubular reactor under sunlight irradiation. (B) Enlarged photo of the tubular reactor. (C) Control experiment via the tubular reactor without light irradiation. (D, E) Photos of the original FB solution (D) and the FB solution degraded by the tubular reactor (E). (F, G) Photos of the original MG solution (F) and the MG solution degraded by the tubular reactor (G). Here, we further investigated the continuous degradation of SD and five organic dye solutions using the home-made tubular reactor under sunlight irradiation. Taking FB as an example, from Figure 10A‒E and video 1 (Supporting information), it can be seen that the purple FB solution becomes colorless after it flew out of the tubular reactor under sunlight irradiation. In contrast, the color of the outflowing solution almost had no change when the sunlight was sheltered by a box (Figure 10C) and video 2 (Supporting information). The above experiment indicates that the removal of FB is ascribed to the photocatalytic degradation but not adsorption. After sunlight irradiation for 80 min, 250 mL FB solution was successfully degraded, demonstrating that the tubular reactor with MS@AgBr-AgCl/Ag NWs can be used for the continuous degradation of organic pollutants in waste


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water. As shown in Figure 10F‒G and Video 3‒Video 10), MG, MO, AO7, and Rh-B solutions can also be degraded into a colorless solution with the tubular reactor under sunlight irradiation.

CONCLUSIONS TiO2-based photocatalysis still has two shortages—low sunlight utilization efficiency and difficulty to reuse, which severely blocks its application in organic waste water remediation. In this work, we prepared a spongy photocatalyst MS@AgBr-AgCl/Ag by immobilizing Ag nanowires (NWs) on MS skeletons, transferring Ag NWs into AgBr-AgCl, and further reducing part Ag+ into metallic Ag. A serious of analyses revealed that the Ag nanoparticles can response to the whole visible light spectrum while the composite structure between AgBr and AgCl can effectively separate the photogenerated electrons and holes. Consequently, MS@AgBr-AgCl/Ag NWs exhibits the excellent photocatalytic activity for degrading antibiotic sulfadiazine and five organic dyes (fuchsin basic, methyl orange, acid orange 7, malachite green and rhodamine B). It was revealed that •O2−, h+, •Cl and •Br are the reactive species responsible for the degradation of organic contaminants over MS@AgBr-AgCl/Ag NWs. By filling MS@AgBr-AgCl/Ag NWs into glass tubes and connected them one by one, we constructed a fixed-bed photoreactor. With this photoreactor, the antibiotic sulfadiazine and five organic dyes solutions were continuously degraded. We believe that this work provides a valuable reference to design sunlight driven photocatalysts and fixed-bed photoreactors used for environmental waste water purification.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Tel./Fax: +86-21-64252062. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


25


# Weihang

Page 26 of 38

Kong and Shilin Wang contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work has been supported by the National Natural Science Foundation of China (21573069, U1862112), Shanghai Municipal Science and Technology Major Project (2018SHZDZX03) and the Programme of Introducing Talents of Discipline to Universities (B16017).

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Fabrication of 3D sponge@AgBr-AgCl/Ag and tubular photo-reactor for continuous waste water purification under sunlight irradiation.


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Ag and tubular photo-reactor

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