Rapid Photocatalytic Decolorization of Methyl Orange under Visible

Jul 21, 2017 - Reactive species such as •O2–, h+, and e– generated through this photocatalytic process have played key roles in decolorization w...
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Research Article pubs.acs.org/journal/ascecg

Rapid Photocatalytic Decolorization of Methyl Orange under Visible Light Using VS4/Carbon Powder Nanocomposites Ruquan Cai,† Baogang Zhang,*,† Jiaxin Shi,† Min Li,† and Zhen He*,‡ †

School of Water Resources and Environment, Key Laboratory of Groundwater Circulation and Evolution, China University of Geosciences Beijing, Ministry of Education, 303# Jiaoyi Building, 29# Xueyuan Road, Haidian District, Beijing 100083, China ‡ Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, 403 Durham Hall, Blacksburg, Virginia 24061, United States S Supporting Information *

ABSTRACT: Photocatalytic decolorization of the dye compounds represents a promising approach for textile wastewater treatment, and thus, development of new photocatalysts will be of strong interest. Herein, a novel photocatalyst based on transition metal chalcogenides vanadium tetrasulfide (VS4) was prepared via a simple one-step hydrothermal synthesis, and the synthesized VS4/carbon powder nanocomposites exhibited excellent photocatalytic decolorization efficiency under visible light irradiation. A significantly high methyl orange (MO) decolorization rate of 13.4−19.8 mg L−1 h−1 was achieved. Reactive species such as •O2−, h+, and e− generated through this photocatalytic process have played key roles in decolorization with phthalic acid as a product. The optimal catalyst dosage was determined to be 1.0 g L−1. The decolorization rate was enhanced with the increase in initial MO concentration, and an acidic condition favored the degradation. The VS4/carbon powder nanocomposites could maintain the MO decolorization efficiency above 90% in four consecutive reused cycles. The results of this study have collectively demonstrated a promising photocatalyst that encourages further investigation and development for dye decolorization in textile wastewater. KEYWORDS: Vanadium tetrasulfide (VS4), Photocatalytic decolorization, Methyl orange (MO), Nanocomposites, Visible light

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INTRODUCTION Wastewater generated by the textile industries is known to contain a considerable amount of untreated dyes.1 Some dyes are highly toxic, carcinogenic, and mutagenic, and can be bioaccumulated along the food chain.2,3 Thus, removing dyes before discharging those textile wastewaters is critically important to protecting natural water bodies and our environment. Dye wastewater can be treated via biochemical and/or physicochemical methods.4 Although biological treatment is usually cost-effective, the treatment efficiency and removal rate is generally low, likely due to the recalcitrant characteristics of dye compounds to microbial degradation.5 Physico-chemical treatments, such as advanced oxidation processes (AOPs),6 are more powerful and faster.7 AOPs are an effective method to remove biorefractory organics from wastewaters and have been applied to treat dye compounds in wastewater.8 In general, there are three major types of AOPs applied to treat dye compounds, Fenton (H2O2/ Fe2+) processes, Fenton-like processes, and photocatalytic processes.9 Fenton processes use chain reactions of H2O2 and Fe2+ to generate hydroxyl radicals (•OH) that have a strong oxidizing ability and can break down dye compounds. Despite the advantages such as low cost, high dye degradation efficiency, and relatively simple operation, Fenton processes © 2017 American Chemical Society

have major drawbacks of requirement of low pH range, generation of a large amount of iron sludge, and rapid decay of catalyst.10 To assist or improve Fenton processes, Fenton-like processes have been developed by incorporating UV, O3, and/ or photoelectric effects, for example, forming a photo-Fenton (H2O2/Fe2+/UV) process. The dye degradation efficiency of the photo-Fenton process can be increased by the Fe3+ photoreduction that generates new •OH and regenerates Fe2+ ions under UV light, and Fe2+ ions can further react with H2O2.11 Photocatalytic processes use light energy to drive chemical reactions via photocatalysis; in this process, O2 is adsorbed on the surface of pholocatalysts by accepting electrons, and converted to •O2−, which may later interact with H2O to generate •OH.12,13 Among the various AOP technologies, semiconductor photocatalysis has emerged as a promising technology because of potential energy advantages by using either natural sunlight or artificial indoor illumination to drive oxidation.14 Numerous photocatalytic materials have been discovered and developed, such as titanium dioxide (TiO2) and zinc oxide (ZnO), tin Received: April 13, 2017 Revised: May 27, 2017 Published: July 21, 2017 7690

DOI: 10.1021/acssuschemeng.7b01137 ACS Sustainable Chem. Eng. 2017, 5, 7690−7699

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ACS Sustainable Chemistry & Engineering

studied to confirm the function of the added carbon powders in the VS4/CP nanocomposites. Characterization. Compositions of the VS4/CP nanocomposites and VSx were analyzed by energy dispersive X-ray (EDX) on a JEOL JAX-840 scanning electron microscope (SEM) operating at 20 kV (JEOL JAX-840, Hitachi Limited, Japan). Zeta potentials were measured by zeta potential instrument (Zetasizer Nano ZS, Malvern, UK). X-ray diffraction (XRD) measurements were performed using a Rigaku-D/MAX-PC 2500 X-ray diffractometer with Cu Kα (λ = 1.5405 Å) as the radiation source operating at 40 Kv and 200 mA (Rigaku-D/MAX-PC 2500, Rigaku, Japan). The XRD patterns were analyzed by using Jade 6 software. The transmission electron microscope (TEM) images were obtained on a JEOL TEM system operating at 200 kV (JEM-2100, Hitachi Limited, Japan). X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Kratos XSAM-800 spectrometer with an Mg Kα radiator (XSAM-800, Kratos, U.K.). Photocatalytic Test. The photocatalytic performance of the synthesized photocatalysts was examined by the photocatalytic decolorization of MO in an aqueous solution. In the experiment, 0.05 g of the VS4/CP nanocomposites was added to a 50 mL MO aqueous solution with an initial MO concentration of 10 mg L−1 and pH of 5. Prior to irradiation, the suspension was mixed with a magnetic stirrer in a dark box for 1 h to obtain the adsorption/ desorption equilibration for the photocatalysts. The mixing by stirring was also continuously applied throughout the photocatalytic decolorization process. The solution was irradiated by a 300 W xenon lamp (CEL-HXF300E, CEAULIGHT, China) passing through a 420 nm cutoff filter, which was used as a visible illuminant of the photocatalytic reaction. A circulating cooling water bath was employed to eliminate the thermal effect derived from the light, and the temperature of the MO solution was maintained at 25.0 °C. Every 5 min, 4 mL of the solution was collected and then filtered using a 0.22 μm Millipore membrane to remove the VS4/CP nanocomposites particles for MO measurement, and a typical cycle lasted 0.5 h. Photocatalytic tests were also conducted with phenol (40 mg L−1) as the target in a 4-h operation with the same initial conditions as a previous study.21 Several factors affecting MO decolorization such as pH, catalyst dosage, dye concentration, and irradiation condition were evaluated for the VS4/CP nanocomposites. The pH of the MO solution was adjusted to 3, 5, 7, and 9, by adding sulfuric acid or sodium hydroxide and monitored by using a pH meter (S500, Mettler Toledo, USA). The synthesized photocatalysts were also submerged in the target solution under the selected pH for 1 h, after which they were used for photocatalytic decolorization with an initial MO concentration of 10 mg L−1 and pH of 5 to examine the stability of the developed photocatalysts at different pH values. The catalyst dosage was adjusted to 0.5, 1.0, 1.5, and 2.0 g L−1. The dye concentration was varied at 5, 10, 15, and 20 mg L−1. Three light sources were used as the irradiation conditions for the VS4/CP nanocomposites, namely, visible light, UV light, and sunlight (natural light that was directly obtained in the sunshine from 1 to 4 pm). Visible light (artificial light source) was provided by a xenon lamp with a UV cutoff filter (UVIRCUT420, CEAULIGHT, China), while UV was supplied from xenon lamp by using a UV reflection sheet (UVREF, CEAULIGHT, China). Functional scavenger compounds were added to the MO solution to remove the corresponding reactive species during the photocatalytic decolorization process with a catalyst dosage of 1.0 g L−1, an initial MO concentration of 10 mg L−1, and pH of 5 under visible light irradiation:22 Fe(II)-EDTA of 0.1 mM was used to remove H2O2,23 sodium oxalate (Na2C2O4) with a concentration of 0.5 mM was employed to remove h+, Cr(VI) of 0.05 mM was used to remove e−,24 isopropanol of 5 mM was used to remove •OH, and 4-hydroxy2,2,6,6-tetramethylpiperdinyloxy (TEMPOL) of 2 mM was used for •O2− removal.25 Variations of MO decolorization under each condition were used to understand the mechanisms of photodegradation. In the stability test, four consecutive cycles were performed, and the corresponding decolorization efficiencies were recorded to examine

oxide (SnO2), cubic zirconia (ZrO2), cadmium sulfide (CdS), and other oxide sulfide semiconductor.15 Despite good decolorization effects, those catalysts have a relatively low catalytic efficiency or rate, resulting in the demand for a large amount of catalysts to achieve desired results. In addition, some of them can only be used under UV light irradiation, and this will affect application prospects of those catalysts. Vanadium ion doping is one of the best alternatives to enhance the separation efficiency of electron−hole pairs and the visible light-driven photoactivity,16 suggesting that vanadium can function as an essential element in photocatalysts. To develop photocatalysts toward practical applications especially under visible light irradiation, metal chalcogenides were studied.17 Those materials have been used in Li-ion batteries, optics, photocatalysis, energy storage, and conversion.18 Vanadium tetrasulfide (VS4) is a metal chalcogenide that has received relatively little attention from the scientific community. The past research of VS4 focused on its applications as an anode material for Li batteries, which can have a high charge capacity after 100 cycles (e.g., initial: 1105 mA h g−1; 100 cycles: 954 mA h g−1).18 VS4 has also been studied for visiblelight-driven photocatalytic water splitting with effective hydrogen production.17 Although VS4 exhibits catalytic abilities for different applications, there have not been any studies exploring its photocatalytic decolorization of dyes under visible light. Furthermore, the synthesis of pure VS4 has not been clearly demonstrated because of experimental difficulties and the existence of nonstoichiometric phases.18 Carbonaceous materials with π-electron conjugative structures can be utilized as templates to form hybrids to enhance the absorption in the visible spectrum region during VS4 preparation,17−19 while these conjunctions and benefits for VS4 have not been fully revealed. In this study, we used a simple one-step hydrothermal method to synthesize VS4/carbon powder (VS4/CP) nanocomposites and experimentally investigated the feasibility of the synthesized VS4/CP nanocomposites for photocatalytic decolorization of methyl orange (MO) under visible light irradiation. The key factors that could affect the dyes decolorization including pH, catalyst dosage, and dye concentration were systematically investigated.20 To examine the stability of the photocatalysts, the VS4/CP nanocomposites photocatalytic decolorization process was repeated for at least four consecutive cycles. A scavenger study and detection of reactive species generation was performed to determine the photocatalytic decolorization mechanism of the developed photocatalyst.

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METHODS AND MATERIALS

Synthesis of VS4/CP Nanocomposites. To synthesize the catalyst, 0.448 g of carbon powder was dispersed in 30 mL of deionized (DI) water using ultrasonic oscillation for about 0.5 h to form a homogeneous solution. Then, 0.9 g of sodium orthovanadate and 2.5 g of thioacetamide were added gradually to the above solution; after continuous stirring for 30 min, the solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated to 170 °C for 12 h. Once the autoclave was sufficiently cooled, the nanocomposite products were centrifuged (at 4000 rpm), washed with DI water and ethyl alcohol several times to remove water-soluble ions, and then dried in a vacuum drying oven at 60 °C for 6 h. The obtained nanocomposite product was black VS4 powder containing 5 wt % carbon.18 Synthesis was also performed in the same way with the same kinds and amounts of reagents except the addition of carbon powders to prepare vanadium chalcogenide (VSx). Its physical−chemical characteristics and photocatalytic performance were also comparatively 7691

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ACS Sustainable Chemistry & Engineering the stability and reusability of the VS4/CP nanocomposites with a catalyst dosage of 1.0 g L−1, an initial MO concentration of 10 mg L−1, and pH of 5 under visible light irradiation; the result was compared with that of the prepared VSx. Each test during the whole photocatalytic process was repeated three times, and the mean results were reported. Analytical Methods. The concentration of MO was measured by a UV−visible spectrophotometer at 464 nm (DR 5000, HACH, USA).26 Total oxidation ability was determined by spectrophotometry as previously reported.26 The MO solution sample was added to the mixture of potassium iodide (KI) and ammonium molybdate, shaken quickly, and monitored by a UV−visible spectrophotometer (DR 5000, HACH, USA) at 352 nm for H2O2 determination. The hydroxyl radical was determined by using the dimethyl sulfoxide (DMSO) method.27,28 To investigate the changes in the molecule and structural characteristics of MO with photocatalytic decolorization, the fullwavelength scan was monitored by a UV−visible spectrophotometer (DR 5000, HACH, USA).29 To determine the decolorization products, gas chromatography/mass spectrometry (GC/MS, Trace GC-DSQ, Thermo Fisher, TR-35MS, 30 m × 0.25 mm × 0.25 μm) analysis was performed.26 In the GC/MS measurement, 100 mL of the treated samples were extracted by 20 mL of methylene dichloride (chromatogram grade), and the extraction solutions without water were confirmed by anhydrous sodium sulfate dehydrated; then, the samples were put in a Termovap Sample Concentrator (HGC-24A, Hengao, CHN) to blow-dry. According to the previous studies,30 ultrapure helium was used as the carrier gas with a constant flow rate of 1.0 mL min−1. The oven temperature was programmed from 50 °C for 4 min, then increased at a ramp rate of 15 °C min−1 to 280 °C and held for 3 min. MS was operated under the following conditions: transfer line, 220 °C; ion source, 220 °C, and electron energy, 70 eV.

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RESULTS AND DISCUSSION Characterization of VS4/CP Nanocomposites. The synthesized composites were characterized for structure and composition. The uniformly distributed nanoparticles were observed on the carbon powder matrix with a size of 50−100 nm as shown in the TEM images (Figure 1A and B). Their lattice spacing was about 0.56 nm, well consistent with the VS4 nanostructures reported previously.18 Elemental mapping of the composites revealed higher element counts of vanadium, sulfur, and carbon in the deeper color region, combined with elemental carbon (Figure 1C), and the stoichiometric ratio of S to V was determined as 22:5.4 (or 4.07:1). The EDX results of the developed composites suggested the components of C, V, and S, with a S/V ratio of 4; the Si element could be derived from the immersible silicon lithium-drift detector (Figure 2A, Figure S1, Supporting Information). The XRD pattern of the VS4/CP nanocomposites showed the diffraction peak corresponding to the (110) plane at 15.8° [JCPDS No. 072-1294] (Figure 2B), demonstrating the monoclinic phase of nanoparticles,31 while the peak of carbon at 16.7° and peak of sulfur at 22.8° were assigned to the (020) and (101) diffraction, respectively [JCPDS No. 024-1206],18 compared with the XRD pattern of VSx (Figure S2A, Supporting Information). XPS was employed to probe the electronic structures of the VS4/CP nanocomposites (Figure 2C, D, and E). The overall XPS spectra of VS4/CP nanocomposites showed the detected characteristic peaks of S, C, and V elements, suggesting the successful removal of water-souble ions after hydrothermal synthesis (Figure S3). The highest C 1s peak centered at 284.4 eV corresponded to the C−C bond, and the fitted peak observed at 287.8 eV indicated the presence of a trace amount of oxygen-containing functional groups, suggesting that the hydrothermal method was efficient to remove these kinds of functional groups.18 The S 2p core level analysis confirmed the

Figure 1. (A) and (B) TEM images and (C) elemental mapping of the VS4/CP nanocomposites.

existence of the S22− species,17 as the peaks at 162.7 and 163.9 eV can be indexed to S 2p3/2 and S 2p1/2, respectively, for the S22− dimer. The shift of these two peaks (about 0.4 eV) to the higher binding energy direction could be explained by the formation of a C−S bond,17,32 indicating the chemical interaction between VS4 and carbon powders, compared with the S 2p spectrum of VS x (Figure S2B, Supporting Information). The V 2p peak located at about 515.7 and 523.1 eV could be ascribed to binding energies of V 2p3/2 and V 2p1/2, respectively, suggesting that V 2p was the characteristic of V4+.18 The shift of V 2p to the lower binding energy direction referring to the V 2p spectrum of the prepared VSx (Figure S2C, Supporting Information) indicated that V received some electrons from its neighbors through electronic effect, which would promote electron transfers during the photocatalytic process. XPS analysis of VSx showed that the component of prepared vanadium chalcogenide was a mixture of VS4 and VSx (x = 2−4) (Figure S2B and S2C, Supporting Information).33 7692

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Figure 2. (A) EDX, (B) XRD pattern, and (C), (D). and (E) XPS spectra of the VS4/CP nanocomposites.

All these results confirmed the successful synthesis of VS4 with the benefits of adding carbon powders in the VS4/CP nanocomposites. Photocatalytic MO Decolorization by VS4/CP Nanocomposites. A photocatalytic test was performed by the photocatalytic decolorization of a MO aqueous solution.34 Clearly, the addition of the VS4/CP nanocomposites under visible light irradiation led to significant decolorization of MO dye (98.8 ± 0.9%) in 30 min, while the MO solution without the VS4/CP nanocomposites had a decolorization efficiency of 3.8 ± 1.2% (Figure 3). This demonstrated that the prepared VS4/CP nanocomposites were an effective photocatalyst for photocatalytic decolorization of MO dye. In the absence of illumination (dark condition), the VS4/CP nanocomposites resulted in 16.3 ± 2.2% of MO decolorization, indicating that an adsorption reaction occurred. When a different light source, UV light, replaced visible light, the decolorization efficiency of MO decreased to 67.0 ± 3.9% due to a relatively narrow bandgap of 1.0 eV for VS4 according to density functional theory (DET).18 This exhibited the advantage of the VS4/CP nanocomposites because visible light has more application

Figure 3. Time history of MO decolorization efficiency under different conditions with 30 min operation.

prospects than UV light. Even under sunlight, the VS4/CP nanocomposites achieved 70.1 ± 3.3% decolorization efficiency of MO. The VSx under visible light irradiation resulted in only 44.0 ± 1.4% MO decolorization, demonstrating that vanadium could realize separation of electron−hole pairs while carbon 7693

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ACS Sustainable Chemistry & Engineering Table 1. Photocatalytic Decolorization of MO in Aqueous Solution Catalyzed by Different Photocatalysts Time (h)

MO concentration (mg L−1)

Catalyst dosage (g L−1)

Light source

Decolorization efficiency (%)

Decolorization rate (mg L−1 h−1)

Photoreactor

Refs

graphene oxide/ TiO2 composites CdS/TiO2

3

12

1.0

visible light

36.0

1.4

glass beaker

37

2

10

1.0

visible light

95.0

4.8

38

CuFe2O4/AgBr VS4/CP nanocomposites Cu/ZnO Co2TiO4 nanoparticles Se-ZnS nanocomposites VS4/CP nanocomposites ZnO

2.7 0.5

20 10

1.0 1.0

visible light visible light

88.2 98.8

6.5 19.8

4 0.7

20 5

1.0 0.2

UV light UV light

88.0 100

4.4 7.1

glass beaker with a cooling water cycle system glass beaker beaker with a circulating cooling water bath glass beaker glass beaker

2.7

13.1

4

UV light

95.5

4.6

glass beaker

0.5

10

1.0

UV light

67.0

13.4

3

10

2.0

sunlight

95.0

3.2

This study 43

4

20

1.0

sunlight

92.0

4.6

beaker with a circulating cooling water bath beaker with a circulating water jacket quartz conical flask

6 0.5

50 10

0.6 1.0

sunlight sunlight

60.0 70.1

5.0 14.0

glass beaker glass beaker

45 This study

Photocatalyst

g-C3N4/graphene oxide aerogel TiO2 VS4/CP nanocomposites

39 This study 40 41 42

44

C3N4/graphene oxide aerogel of 4.6 mg L−1 h−1, and TiO2 of 5.0 mg L−1 h−1 in a beaker with a circulating water jacket or quartz conical flask, respectively.43−45 The photoreactor setup can affect the decolorization efficiency (Table 1), so the common photoreactor (beaker with a circulating cooling water bath) has been used in photocatalytic reactions. Those results have demonstrated the effective and efficient photocatalytic MO decolorization by the VS4/CP nanocomposites with a high photocatalytic decolorization rate. Catalytic Mechanisms by VS4/CP Nanocomposites. The photocatalytic process can generate reactive species such as •OH, H2O2, •O2−, e−, and h+ through eqs 1−4, and some of them may be responsible for the MO decolorization on the VS4/CP nanocomposites.46

material played a role in the nucleation to promote photocatalytic activity. The added carbon powders increased MO decolorization efficiencies through strengthening visible light absorption by reducing the bandgap of VS4 and weakening the recombination of photogenerated holes and electrons,35 thereby improving the dispersibility of VS4. Additionally, the VS4/CP nanocomposites also exhibited satisfactory performance of photocatalytic degradation of phenol (Figure S4, Supporting Information), with removal efficiency of 49.7 ± 1.2% within 4 h; this was comparable with the results using the FeWO4@ZnWO4/ZnO photocatalyst21 and thus further confirmed the excellent photocatalytic activities of the VS4/ CP nanocomposites in pollutants degradation. Compared to other photocatalysts for decolorization of MO in aqueous solutions under visible light such as graphene oxide/ TiO2 composites and CdS/TiO2, the VS4/CP nanocomposites catalysts achieved comparable decolorization efficiency but had significant advantages in short reaction time, low catalyst dosage, and high MO decolorization rate, as shown in Table 1, where the data were normalized based on previous studies.36 For example, to achieve 88% decolorization for MO, only 0.5 h was required for the VS4/CP nanocomposites, much shorter than 2 h using CdS/TiO2 and 2.7 h using CuFe2O4/AgBr under visible light in the similar glass beaker with a cooling water cycle system. 37 Particularly, the calculated MO decolorization rate was 19.8 mg L−1 h−1 for the VS4/CP nanocomposites, significantly higher than 4.8 mg L−1 h−1 for CdS/TiO 2 , 1.4 mg L −1 h −1 for graphene oxide/TiO 2 composites, and 6.5 mg L−1 h−1 for CuFe2O4/AgBr.37−39 Although the VS4/CP nanocomposites had lower decolorization efficiency of MO than Cu/ZnO, Co2TiO4 nanoparticles, or Se-ZnS nanocomposites under UV light in glass beakers,40−42 its MO decolorization rate of 13.4 mg L−1 h−1 was much higher than that of those photocatalysts, with 4.4 mg L−1 h−1 for Cu/ ZnO, 7.1 mg L−1 h−1 for Co2TiO4 nanoparticles, and 4.6 mg L−1 h−1 for Se-ZnS nanocomposites. Likewise, despite a lower MO decolorization efficiency under sunlight, the VS4/CP nanocomposites had a significantly higher decolorization rate of 14.0 mg L−1 h−1 than the reported ZnO of 3.2 mg L−1 h−1, g-

O2 + e− → •O2−

(1)

•O2− + H+ → •OOH

(2)

•OOH + e− + H+ → H 2O2 −

(3) −

O2 + H 2O2 → •OH + OH + O2

(4)

The measured total oxidation ability in the photocatalytic decolorization system performed by the VS4/CP nanocomposites was about 3.3 ± 0.3 mg L−1, higher than that obtained from VS4 (1.5 ± 0.2 mg L−1), and it also had a much higher level of H2O2 (11.0 ± 0.2 μM) and slight increase in •OH (1.88 ± 0.2 μM) than those obtained for VSx (3.5 ± 0.3 mg L−1 of H2O2 and 1.8 ± 0.2 μM of •OH). To further examine the mechanism of photocatalytic MO decolorization by the VS4/CP nanocomposites, several scavenger compounds were added to the MO aqueous solution to remove the corresponding reactive species, compared with the photocatalytic reaction without scavengers (Figure 4). Addition of TEMPOL, sodium oxalate, and Cr(VI) significantly inhibited the photocatalytic decolorization by the VS4/CP nanocomposites with MO decolorization efficiency decreased to 18.4 ± 2.3%, 53.4 ± 2.8%, and 21.2 ± 3.1%, respectively, suggesting that •O2−, h+, and e− could have played important roles in the photocatalytic decolorization of MO. Functions of 7694

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in the GC/MS scheme were also listed as a cracking reaction, and recombination could take place during the detection process at a high temperature, consistent with the results of the previous MO study.50 The dye molecules were broken down into the compounds with smaller molecular weights after 30 min of operation, for example, indole. Some derivatives of benzene, such as phthalic acid, were also detected. This confirmed the destruction of the MO molecule and the generation of the intermediates.26 Additionally, the species of intermediates detected here were more complex than results obtained by Li et al.,51 probably due to the effective destruction of MO as well as the bonding between the intermediates. Effects of Operating Factors. In order to evaluate the effects of catalyst dosage, the experiment was conducted by varying the VS4/CP nanocomposites catalyst dosage from 0.5 to 2.0 g L−1, with the initial MO of 10 mg L−1 and pH of 5 under visible light irradiation. As shown in Figure 6A, the decolorization efficiency of MO increased from 82.1 ± 2.1% to 98.8 ± 0.9% with increasing the catalyst dosage from 0.5 to 1.0 g L−1. However, a further increase in the catalyst dosage to 1.5 and 2.0 g L−1 did not result in further enhancement of the decolorization efficiency (97.3 ± 1.7% at 1.5 g L−1 and 95.7 ± 1.3% at 2.0 g L−1). The improved removal efficiency with more catalysts was likely due to the increased number of active sites that caused an increase in the number of reactive species, benefiting the decolorization of MO.52 Overdose of the catalysts could decrease the removal efficiency when a large amount of catalysts shields the light and then an increase in the turbidity of the suspension, resulting in a decrease in the photoactivated volume of MO solution.52,53 This light attenuation effect was established through a linear correlation between kinetics constant k and catalyst dosage:

Figure 4. MO decolorization efficiency by the VS4/CP nanocomposites in the presence of different scavengers under visible light irradiation.

•O2− and e− were crucial to this process with some auxiliary effects from h+ as MO decolorization might happen through both oxidation by active oxidants and reduction by electrons.47,48 Similar observations were also obtained in the photocatalytic disinfection experiments with the main contribution from •O2− and the complementary role of h+.22 The addition of Fe(II)-EDTA and isopropanol hardly inhibited the MO decolorization (Figure 4), with MO decolorization efficiencies of 95.2 ± 1.3% and 99.0 ± 2.9%, respectively. Thus, the roles of H2O2 and •OH in the photocatalytic decolorization of MO by the VS4/CP nanocomposites could be minor, consistent with the findings in a previous study of photocatalytic disinfection.22 The change in the MO molecule during photocatalytic decolorization by the VS4/CP nanocomposites was also monitored. The azo group of MO was destroyed gradually as the peak at 464 nm showing the n−π* transition of the azo group weakened over time and eventually disappeared through photooxidation, confirming the achievement of color removal (Figure 5). Another peak at 270 nm from the π−π* transition

k = 18.52[VS4 /CP nanocomposites] − 0.307R2 = 0.722 (5)

This assumption had also been reported previously.54 The optimized catalyst dosage of 1.0 g L−1 was selected for the following experiments. Moreover, the basis of optimum catalyst dosage is found to be dependent on the initial solute concentration, and 10 mg L−1 of MO was frequently employed during photocatalytic decolorization.39,44,45 Thus, this selected dosage was significantly less than that applied for MO decolorization by other photocatalysts (Table 1), retaining the significant advantage of the VS4/CP nanocomposites catalyst. The initial concentration of MO dye in a given photocatalytic reaction can affect its removal. This was examined by varying the initial MO concentration from 5 to 20 mg L−1, with pH of 5 under visible light irradiation. As expected, the low concentrations of 5 or 10 mg L−1 led to great decolorization efficiency of 99.6 ± 1.9% or 98.8 ± 0.9% (Figure 6B). The MO decolorization efficiency decreased to 68.5 ± 1.8% or 66.2 ± 1.6% when the initial MO concentration increased to 15 or 20 mg L−1. However, the MO decolorization rate per unit mass of the VS4/CP nanocomposites catalyst increased significantly with the enhancement of initial MO concentration. When the initial MO concentration was increased from 5 to 10 mg L−1, the MO decolorization rate was nearly doubled from 10.0 to 19.8 mg L−1 h−1. A further increase in the initial MO concentration also improved the MO decolorization rate to 20.6 mg L−1 h−1 (initial MO concentration of 15 mg L−1) and then to 26.5 mg L−1 h−1 (initial MO concentration of 20 mg L−1). Sufficient contact between MO and the generated reactive

Figure 5. UV−vis spectra evolution during the photocatalytic decolorization of MO using the VS4/CP nanocomposites.

of benzene rings disappeared quickly, while a new peak at 248 nm appeared accordingly, indicating that the relevant structures had been progressively destroyed with the generation of new compounds. The newly appeared peaks also became weaker during the operation, implying the gradual mineralization of MO.49 GC/MS results revealed the changes in the molecule and structural characteristics of MO (Table 2). The compounds in the original samples were similar to the results obtained in our previous MO study.26 Organics reflected by appeared peaks 7695

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ACS Sustainable Chemistry & Engineering Table 2. Organic Compounds in Original and Processed Solutions after 30 min Operation Original sample

Processed sample

Thiocarbamic acid

√a

−b

Butanoic acid Cyclopentane 3,5-Dimethylbenzaldehyde thiocarbamoylhydrazone 2,3-Dichloro-2-methyl- Propanal

√ √ √

− − −

√

Cycloheptanone Phthalic acid Pyrazine

√ √ √

Organic compounds

a

Original sample

Processed sample

Organic compounds

Original sample

Processed sample

Phthalic acid

√

−

−

√

√ √ √

√ − −

− − −

√ √ √

−

Decane Heptadecanoic acid 1-(3′,4′-Dichlorophenyl)-3phenylimidazolidinone Cyclic octaatomic sulfur

−

√

−

√

− − −

Methanol Indole 1,3,5-Triazine

− − −

√ √ √

2-methylbenzoic acid 2-butanone Ethanethioamide 2- Chloropropionic acid 2,2′Sulfinyldiethanol Phthalic acid Cyclopentane 1H-Purine

− − −

√ √ √

Organic compounds

Detected in the sample. bNot detected.

also been observed with photocatalytic decolorization of MO by TiO2.55 The solution pH is another key factor to both the generation of reactive species and chemical forms of MO, thereby affecting the MO decolorization. Different initial pH (3, 5, 7, 9) was examined, with an initial MO concentration of 10 mg L−1 under visible light irradiation. It was observed that the decolorization efficiency of MO decreased with the increase in pH (Figure 6C). An acidic condition of pH 3 or 5 led to higher MO decolorization efficiency of 99.7 ± 0.2% or 98.8 ± 0.9%. A slight decrease in color removal efficiency (94.3 ± 2.5%) was observed under a neutral condition (pH 7), while the lowest MO decolorization efficiency of 88.6 ± 1.4% was obtained under an alkaline condition (pH 9). The acidic condition favored the decolorization process catalyzed by theVS4/CP nanocomposites because a high concentration of H+ would facilitate the generation of reactive species according to eqs 2, 3, and 4. In addition, MO is likely to change into a quinine form under the acidic condition, which is ionized.41,43 H+ ions interact with the azo linkage, which is particularly susceptible to be electrophilic attacked by reactive species, decreasing the electron densities at the azo group.53 Oxidative powers of the photogenerated reactive species are lowered with increasing the pH, and MO is in the azo form in neutral and alkaline media. The generated reactive species tended to be consumed by an oxygen evolution reaction in a photocatalytic water splitting process instead of MO decolorization through eq 656 2H 2O + 4h+ → O2 ↑ +4H+

(6)

The effects of adsorption by the VS4/CP nanocomposites under dark conditions for MO removal were also investigated under different pH. MO removal efficiencies increased with the decrease in pH, as surfaces of VS4/CP nanocomposites were positively charged, which favored the adsorption of azo dyes,57 with a point of zero charge for the VS4/CP nanocomposites of 4.8 that was indicated by the measured zeta potentials. The highest adsorption decolorization efficiency was less than 20%, indicating that dark adsorption of dye on the developed VS4/ CP nanocomposites only showed very slight effects on MO decolorization. In addition, the decolorization efficiencies obtained by the VS4/CP nanocomposites and prepared VSx submerged in the target solution under the tested pH showed rare differences to their performances without presoaking in the same solution as presented in Figure 3 and Figure S5 (Supporting Information), confirming that the developed photocatalysts were fairly stable in the tested pH range.58

Figure 6. Operating factors studied for MO decolorization by VS4/CP nanocomposites: (A) catalyst dosage, (B) MO concentration, and (C) pH.

species could be realized under higher initial MO concentrations, making the use of reactive species more effective and thus improving MO decolorization rates. Similar principles had 7696

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e− could have played important roles in the photocatalytic decolorization of MO, while the contributions of H2O2 and •OH were minor. The tests of operating factors showed that the optimal catalyst dosage was identified as 1.0 g L−1, increasing the initial MO concentration improved the MO removal rate per mass of catalysts, and an acidic condition would favor MO decolorization. The VS4/CP nanocomposites had a relatively stable performance in four consecutive reused cycles. This work has provided a promising route to increase our understanding of VS4/CP nanocomposites toward its further development and application to dye decolorization.

Stability and Reusability. The stability and reusability of the VS4/CP nanocomposites catalyst was examined with four consecutive cycles. As shown in Figure 7, only a slight decrease

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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.7b01137. Figure S1: TEM image of VS4/CP nanocomposites. Figure S2: XRD pattern and XPS spectra of the VSx. Figure S3: Overall XPS spectra of VS4/CP nanocomposites. Figure S4: Photocatalytic activity toward the degradation of phenol under visible light irradiation by VS4/CP nanocomposites. Figure S5: Decolorizations of VS4/CP nanocomposites and VSx with initial MO concentration of 10 mg L−1 and pH of 5. Figure S6: XRD of VS4/CP nanocomposites after four consecutive cycles with visible light irradiation. (PDF)

Figure 7. MO decolorization by VS4/CP nanocomposites in four consecutive cycles with visible light irradiation.

was observed with all the MO decolorization efficiency maintained above 90%. The XRD spectra after four continuous cycles with visible light irradiation were nearly identical to its zero irradiation level (Figure S6, Supporting Information), suggesting that the structure of the VS4/CP nanocomposites was hardly destroyed during photocatalytic decolorization. Little leakage of vanadium in the aqueous solution was detected by inductively coupled plasma mass spectroscopy (less than 1% of the vanadium mass loading). These have demonstrated that the VS4/CP nanocomposites were stable and robust, when compared with several existing catalysts. For example, a considerable reduction in photocatalytic activity of ZnO was observed, with only 52% of MO being decolorized after three cycles of usage.59 Another catalyst, BaTiO3@g-C3N4, accomplished 67−76% of the MO decolorization after three consecutive runs.60 In the VS4/CP nanocomposites, the added carbon powders played an important role in the nucleation,18 strengthening the visible light absorption of the photocatalyst and promoting photoproduction carrier separation and transfer effectively.35 A presumable photocorrosion effect could also lead to the declining trend in MO decolorization, but the close contact between VS4 and carbon powder in the VS4/CP nanocomposites might inhibit the photocorrosion. Sulfur and carbon also formed chemical bonds, thus representing long-lasting photostability. Such a structure between metal and carbon was also reported for ZnO-graphene composites in which graphene exhibited a protective effect for ZnO.59

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 10 8232 2281. Fax: +86 10 8232 1081. E-mails: [email protected], [email protected] (B. Zhang). *Tel.: +1 540 231 1346. Fax: +1 540 231 7916. E-mail: [email protected] (Z. He). ORCID

Baogang Zhang: 0000-0002-0060-503X Zhen He: 0000-0001-6302-6556 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was supported by the National Natural Science Foundation of China (NSFC) (No. 41672237).

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REFERENCES

(1) Lachheb, H.; Puzenat, E.; Houas, A.; Ksibi, M.; Elaloui, E.; Guillard, C.; Herrmann, J. M. Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania. Appl. Catal., B 2002, 39, 75−90. (2) Patil, B. N.; Naik, D. B.; Shrivastava, V. S. Photocatalytic degradation of hazardous Ponceau-S dye from industrial wastewater using nanosized niobium pentoxide with carbon. Desalination 2011, 269, 276−283. (3) Wei, S. T.; Hu, X. L.; Liu, H. L.; Wang, Q.; He, C. Y. Rapid degradation of Congo red by molecularly imprinted polypyrrolecoated magnetic TiO2 nanoparticles in dark at ambient conditions. J. Hazard. Mater. 2015, 294, 168−176. (4) Mu, Y.; Rabaey, K.; Rozendal, R. A.; Yuan, Z. G.; Keller, J. Decolorization of azo dyes in bioelectrochemical systems. Environ. Sci. Technol. 2009, 43, 5137−5143.

CONCLUSIONS In this study, the VS4/CP nanocomposites were prepared by a simple one-step hydrothermal synthesis method as a novel photocatalyst for dye decolorization. Both the morphology and structure of the synthesized photocatalysts proved the formation of nanostructured VS4 carbon composites. The feasibility of this photocatalyst for MO decolorization had been examined with exhibiting great photocatalytic decolorization efficiency under visible light irradiation. Compared with the other photocatalysts for decolorization of MO in aqueous solution, the VS4/CP nanocomposites had significant advantages of short reaction time and high MO removal rate per mass of catalysts. The scavenger study suggested that •O2−, h+, and 7697

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Research Article

ACS Sustainable Chemistry & Engineering (5) Quan, X.; Zhang, X.; Xu, H. In-situ formation and immobilization of biogenic nanopalladium into anaerobic granular sludge enhances azo dyes degradation. Water Res. 2015, 78, 74−83. (6) Kuriechen, S. K.; Murugesan, S.; Raj, S. P.; Maruthamuthu, P. Visible light assisted photocatalytic mineralization of Reactive Red 180 usingcolloidal TiO2 and oxone. Chem. Eng. J. 2011, 174, 530−538. (7) Sapawe, N.; Jalil, A. A.; Triwahyono, S. One-pot electro-synthesis of ZrO2−ZnO/HY nanocomposite for photocatalytic decolorization of various dye-contaminants. Chem. Eng. J. 2013, 225, 254−265. (8) Oturan, M. A.; Aaron, J. J. Advanced Oxidation Processes in water/wastewater treatment: principles and applications. A Review. Crit. Rev. Environ. Sci. Technol. 2014, 44, 2577−2641. (9) Lucas, M. S.; Peres, J. A. Decolorization of the azo dye Reactive Black 5 by Fenton and photo-Fenton oxidation. Dyes Pigm. 2006, 71, 236−244. (10) Jia, Z.; Zhang, W. C.; Wang, W. M.; Habibi, D.; Zhang, L. C. Amorphous Fe78Si9B13 alloy: An efficient and reusable photo-enhanced Fenton-like catalyst in degradation of cibacron brilliant red 3B-A dye under UV-vis light. Appl. Catal., B 2016, 192, 46−56. (11) Subramanian, G.; Madras, G. Introducing saccharic acid as an efficient iron chelate to enhance photo-Fenton degradation of organic contaminants. Water Res. 2016, 104, 168−177. (12) Pan, D. Y.; Jiao, J. K.; Li, Z.; Guo, Y. T.; Feng, C. Q.; Liu, Y.; Wang, L.; Wu, M. H. Efficient separation of electron-hole pairs in graphene quantum dots by TiO2 heterojunctions for dye degradation. ACS Sustainable Chem. Eng. 2015, 3, 2405−2413. (13) Tripathy, N.; Ahmad, R.; Song, J. E.; Ko, H. A.; Hahn, Y. B.; Khang, G. Photocatalytic degradation of methyl orange dye by ZnO nanoneedle under UV irradiation. Mater. Lett. 2014, 136, 171−174. (14) Gong, D. G.; Highfield, J. G.; Ng, S. Z. E.; Tang, Y. X.; Ho, W. C. J.; Tay, Q.; Chen, Z. Poly tri-s-triazines as visible light sensitizers in titania-based composite photocatalysts: promotion of melon development from urea over acid titanates. ACS Sustainable Chem. Eng. 2014, 2, 149−157. (15) Karunakaran, C.; Dhanalakshmi, R. Semiconductor-catalyzed degradation of phenols with sunlight. Sol. Energy Mater. Sol. Cells 2008, 92, 1315−1321. (16) Zhang, M.; Lu, D. D.; Zhang, Z. H.; Yang, J. J. Enhancement of visible-light-induced photocurrent photocatalytic activity of V and N codoped TiO2 nanotube array films. J. Electrochem. Soc. 2014, 161, H416−H421. (17) Guo, W. W.; Wu, D. F. Facile synthesis of VS4/graphene nanocomposites and their visible-light-driven photocatalytic water splitting activities. Int. J. Hydrogen Energy 2014, 39, 16832−16840. (18) Rout, C. S.; Kim, B. H.; Xu, X.; Yang, J.; Jeong, H. Y.; Odkhuu, D.; Park, N.; Cho, J.; Shin, H. S. Synthesis and characterization of patronite form of vanadium sulfide on graphitic layer. J. Am. Chem. Soc. 2013, 135, 8720−8725. (19) Bao, N.; Hu, X. D.; Zhang, Q. Z.; Miao, X. H.; Jie, X. Y.; Zhou, S. Synthesis of porous carbon-doped g-C3N4 nanosheets with enhanced visible-light photocatalytic activity. Appl. Surf. Sci. 2017, 403, 682−690. (20) Nguyen, T. D.; Phan, N. H.; Do, M. H.; Ngo, K. T. Magnetic Fe2MO4 (M:Fe, Mn) activated carbons: Fabrication, characterization and heterogeneous Fenton oxidation of methyl orange. J. Hazard. Mater. 2011, 185, 653−661. (21) Wang, F. Z.; Li, W. J.; Gu, S. N.; Li, H. D.; Liu, X. T.; Wang, M. Z. Fabrication of FeWO4@ZnWO4/ZnO heterojunction photocatalyst: synergistic effect of ZnWO4/ZnO and FeWO4@ZnWO4/ ZnO heterojunction structure on the enhancement of visible-Light photocatalytic activity. ACS Sustainable Chem. Eng. 2016, 4, 6288− 6298. (22) Jiang, D. L.; Li, J.; Xing, C. S.; Zhang, Z. Y.; Meng, S. C.; Chen, M. Two-dimensional CaIn2S4/g-C3N4 heterojunction nanocomposite with enhanced visible-light photocatalytic activities: interfacial engineering and mechanism insight. ACS Appl. Mater. Interfaces 2015, 7, 19234−19242. (23) Zhang, L. S.; Wong, K. H.; Yip, H. Y.; Hu, C.; Yu, J. C.; Chan, C. Y.; Wong, P. K. Effective photocatalytic disinfection of E. coli K-12

using AgBr-Ag-Bi2WO6 nanojunction system irradiated by visible light: The role of diffusing hydroxyl radicals. Environ. Sci. Technol. 2010, 44, 1392−1398. (24) Chen, Y. X.; Yang, S. Y.; Wang, K.; Lou, L. P. Role of primary active species and TiO2 surface characteristic in UV-illuminated photodegradation of Acid Orange 7. J. Photochem. Photobiol., A 2005, 172, 47−54. (25) Huang, H. W.; He, Y.; Du, X.; Chu, P. K.; Zhang, Y. H. A general and facile approach to heterostructured core/shell BiVO4/ BiOI p-n lunction: room-temperature in situ assembly and highly boosted visible-light photocatalysis. ACS Sustainable Chem. Eng. 2015, 3, 3262−3273. (26) Wang, Z. J.; Zhang, B. G.; Borthwick, A. G. L.; Feng, C. P.; Ni, J. R. Utilization of single-chamber microbial fuel cells as renewable power sources for electrochemical degradation of nitrogen-containing organic compounds. Chem. Eng. J. 2015, 280, 99−105. (27) Li, Y. L.; Zhang, B. G.; Borthwick, A.; Long, Y. L. Efficient electrochemical oxidation of thallium (I) in groundwater using borondoped diamond anode. Electrochim. Acta 2016, 222, 1137−1143. (28) Li, H. N.; Zhu, X. P.; Ni, J. R. Inactivation of Escherichia coli in Na2SO4 electrolyte using boron-doped diamond anode. Electrochim. Acta 2010, 56, 448−453. (29) Jantawasu, P.; Sreethawong, T.; Chavadej, S. Photocatalytic activity of nanocrystalline mesoporous-assembled TiO2 photocatalyst for degradation of methyl orange monoazo dye in aqueous wastewater. Chem. Eng. J. 2009, 155, 223−233. (30) Zhang, C. P.; Li, M. C.; Liu, G. L.; Luo, H. P.; Zhang, R. D. Pyridine degradation in the microbial fuel cells. J. Hazard. Mater. 2009, 172, 465−471. (31) Xu, X.; Gao, Z.; Cui, Z.; Liang, Y.; Li, Z.; Zhu, S.; Yang, X.; Ma, J. Synthesis of Cu2O Octadecahedron/TiO2 quantum dot heterojunctions with high visible light photocatalytic activity and high stability. ACS Appl. Mater. Interfaces 2016, 8, 91−101. (32) Qian, W. W.; Gao, Q. M.; Zhang, H.; Tian, W. Q.; Li, Z. Y.; Tan, Y. L. Crosslinked polypyrrole grafted reduced graphene oxidesulfur nanocomposite cathode for high performance Li-S Battery. Electrochim. Acta 2017, 235, 32−41. (33) Mohan, P.; Yang, J.; Jena, A.; Shin, H. S. VS2/rGO hybrid nanosheets prepared by annealing of VS4/rGO. J. Solid State Chem. 2015, 224, 82−87. (34) Wang, D.; Zhang, J.; Luo, Q.; Li, X.; Duan, Y.; An, J. Characterization and photocatalytic activity of poly(3-hexylthiophene)-modified TiO2 for degradation of methyl orange under visible light. J. Hazard. Mater. 2009, 169, 546−550. (35) Zhu, G.; Li, X. L.; Wang, H. Y.; Zhang, L. Microwave assisted synthesis of reduced graphene oxide incorporated MOF-derived ZnO composites for photocatalytic application. Catal. Commun. 2017, 88, 5−8. (36) Huang, W. T.; Chen, J. F.; Hu, Y. Y.; Chen, J.; Sun, J.; Zhang, L. H. Enhanced simultaneous decolorization of azo dye and electricity generation in microbial fuel cell (MFC) with redox mediator modified anode. Int. J. Hydrogen Energy 2017, 42, 2349−2359. (37) Chen, C.; Cai, W.; Long, M.; Zhou, B.; Wu, Y.; Wu, D.; Feng, Y. Synthesis of visible-light responsive graphene oxide/TiO2 composites with p/n heterojunction. ACS Nano 2010, 4, 6425−6432. (38) Yang, G.; Yang, B.; Xiao, T.; Yan, Z. One-step solvothermal synthesis of hierarchically porous nanostructured CdS/TiO2 heterojunction with higher visible light photocatalytic activity. Appl. Surf. Sci. 2013, 283, 402−410. (39) Zhao, Y. L.; Lin, C. P.; Bi, H. J.; Liu, Y. G.; Yan, Q. S. Magnetically separable CuFe2O4/AgBr composite photocatalysts: Preparation, characterization, photocatalytic activity and photocatalytic mechanism under visible light. Appl. Surf. Sci. 2017, 392, 701−707. (40) Fu, M.; Li, Y.; Wu, S.; Lu, P.; Liu, J.; Dong, F. Sol-gel preparation and enhanced photocatalytic performance of Cu-doped ZnO nanoparticles. Appl. Surf. Sci. 2011, 258, 1587−1591. (41) Ramezani, M.; Hosseinpour-Mashkani, S. M. Controlled synthesis, characterization, and photocatalytic application of Co2TiO4 nanoparticles. J. Electron. Mater. 2017, 46, 1371−1377. 7698

DOI: 10.1021/acssuschemeng.7b01137 ACS Sustainable Chem. Eng. 2017, 5, 7690−7699

Research Article

ACS Sustainable Chemistry & Engineering

under simulated sunlight irradiation. J. Alloys Compd. 2015, 622, 1098−1104.

(42) Ahluwalia, S.; Prakash, N. T.; Prakash, R.; Pal, B. Improved degradation of methyl orange dye using bio-co-catalyst Se nanoparticles impregnated ZnS photocatalyst under UV irradiation. Chem. Eng. J. 2016, 306, 1041−1048. (43) Khatamian, M.; Fazayeli, M.; Divband, B. Preparation, characterization and photocatalytic properties of polythiophenesensitized zinc oxide hybrid nanocomposites. Mater. Sci. Semicond. Process. 2014, 26, 540−547. (44) Tong, Z.; Yang, D.; Shi, J.; Nan, Y.; Sun, Y.; Jiang, Z. Threedimensional porous aerogel constructed by g-C3N4 and graphene oxide nanosheets with excellent visible-light photocatalytic performance. ACS Appl. Mater. Interfaces 2015, 7, 25693−25701. (45) Ong, S. A.; Min, O. M.; Ho, L. N.; Wong, Y. S. Comparative study on photocatalytic degradation of mono azo dye acid orange 7 and methyl orange under solar light irradiation. Water, Air, Soil Pollut. 2012, 223, 5483−5493. (46) Liang, J. L.; Shan, C.; Zhang, X.; Tong, M. P. Bactericidal mechanism of BiOI-AgI under visible light irradiation. Chem. Eng. J. 2015, 279, 277−285. (47) Troupis, A.; Triantis, T. M.; Gkika, E.; Hiskia, A.; Papaconstantinou, E. Photocatalytic reductive-oxidative degradation of Acid Orange 7 by polyoxometalates. Appl. Catal., B 2009, 86, 98− 107. (48) Zhang, B.; Wang, Z.; Zhou, X.; Shi, C.; Guo, H.; Feng, C. Electrochemical decolorization of methyl orange powered by bioelectricity from single-chamber microbial fuel cells. Bioresour. Technol. 2015, 181, 360−362. (49) Lu, X. F.; Ma, H. R.; Zhang, Q.; Du, K. Degradation of methyl orange by UV, O3 and UV/O3 systems: analysis of the degradation effects and mineralization mechanism. Res. Chem. Intermed. 2013, 39, 4189−4203. (50) Fang, Z.; Song, H. L.; Yu, R.; Li, X. A microbial fuel cell-coupled constructed wetland promotes degradation of azo dye decolorization products. Ecol. Eng. 2016, 94, 455−463. (51) Li, S. H.; Zhao, Y.; Chu, J.; Li, W. W.; Yu, H. Q.; Liu, G. Electrochemical degradation of methyl orange on Pt-Bi/C nanostructured electrode by a square-wave potential method. Electrochim. Acta 2013, 92, 93−101. (52) Kansal, S. K.; Singh, M.; Sud, D. Studies on photodegradation of two commercial dyes in aqueous phase using different photocatalysts. J. Hazard. Mater. 2007, 141, 581−590. (53) Li, H.; Lei, Z.; Liu, C.; Zhang, Z.; Lu, B. Photocatalytic degradation of lignin on synthesized Ag-AgCl/ZnO nanorods under solar light and preliminary trials for methane fermentation. Bioresour. Technol. 2015, 175, 494−501. (54) Gong, H.; Chu, W. Photodegradation of Sulfamethoxazole with a Recyclable Catalyst. Ind. Eng. Chem. Res. 2015, 54, 12763−12769. (55) Yan, T. J.; Yuan, R. S.; Li, W. J.; You, J. M. Origin of superior photocatalytic activity in rutile TiO2 hierarchical microspheres: The dominate role of “microcavity” structure. Appl. Catal., A 2014, 478, 204−210. (56) Montoya, J. F.; Bahnemann, D. W.; Salvador, P.; Peral, J. Catalytic role of bridging oxygens in TiO2 liquid phase photocatalytic reactions: analysis of H216O photooxidation on labeled Ti18O2. Catal. Sci. Technol. 2017, 7, 902−910. (57) Sajjad, A. K. L.; Shamaila, S.; Tian, B. Z.; Chen, F.; Zhang, J. L. Comparative studies of operational parameters of degradation of azo dyes in visible light by highly efficient WOx/TiO2 photocatalyst. J. Hazard. Mater. 2010, 177, 781−791. (58) Li, F.; Sun, S.; Jiang, Y.; Xia, M.; Sun, M.; Xue, B. Photodegradation of an azo dye using immobilized nanoparticles of TiO2 supported by natural porous mineral. J. Hazard. Mater. 2008, 152, 1037−1044. (59) Tayyebi, A.; Outokesh, M.; Tayebi, M.; Shafikhani, A.; Sengor, S. S. ZnO quantum dots-graphene composites: Formation mechanism and enhanced photocatalytic activity for degradation of methyl orange dye. J. Alloys Compd. 2016, 663, 738−749. (60) Xian, T.; Yang, H.; Di, L. J.; Dai, J. F. Enhanced photocatalytic activity of BaTiO3@g-C3N4 for the degradation of methyl orange 7699

DOI: 10.1021/acssuschemeng.7b01137 ACS Sustainable Chem. Eng. 2017, 5, 7690−7699