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A new complementary catalyst and catalytic mechanism: Ag2MoO4/Ag/AgBr/GO heterostructure Yu-Yang Bai, Feng-Rui Wang, and Jin-Ku Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01265 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 4, 2016

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Industrial & Engineering Chemistry Research

A new complementary catalyst and catalytic mechanism: Ag2MoO4/Ag/AgBr/GO heterostructure Yu-Yang Bai, Feng-Rui Wang, Jin-Ku Liu

*

Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237 P.R. China

ABSTRACT: Ag2MoO4/Ag/AgBr/GO heterostructure photocatalyst is efficaciously designed and equipped. The Ag2MoO4/Ag/AgBr/GO heterostructure exhibits excellent photocatalytic property. The tetracycline hydrochloride (TC-HCl) is degraded completely within 75 min by Ag2MoO4/Ag/AgBr/GO heterostructure under 350 W Xe lamp cutting-off the UV light, in which the photocatalytic rate is 1.36 times higher than Ag2MoO4/Ag/AgBr composite. The photocatalytic activity is highly heightened owing to the enhanced charge separation and topical SPR of Ag0. In addition, GO in Ag2MoO4/Ag/AgBr/GO heterostructure further enlarge the delocalization of the charge transfer compared with Ag2MoO4/Ag/AgBr. After recycling eight times, the photocatalytic rate of Ag2MoO4/Ag/AgBr/GO only debases by 4.5 %. Experimental results indicate the Ag2MoO4/Ag/AgBr/GO heterostructure has great potential application for dealing with TC-HCl residues in water.

*

Corresponding author; E-mail: [email protected] 1

ACS Paragon Plus Environment


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1. INTRODUCTION Over past years, the presence of antibiotic residues in wastewater and their harmful effects on living ecosystems and human life have attracted worldwide attention. Tetracycline hydrochloride (TC-HCl) is a typical antibiotic which is widely used in bacterial infection. The blowdown of TC-HCl into the living environment through different pathways: pharmaceutical industry, hospital, and excretion of human and livestock.1 There are various techniques to handle the tetracycline residues, such as carbon adsorption, microbial degradation, and electrolysis. However, the semiconductor photocatalytic degradation is an ideal method2,3 for its high efficient and low toxigenicity, producing CO2 and H2O as end products of degradation.4,5 Recently, researches on the degradation of TC-HCl by semiconductor have been concerned,6-8 but most of semiconductors just absorb UV light for the degradation. Therefore, looking for the desirable photocatalysts with remarkable visible light absorption is very necessary. Among semiconductor photocatalysts, silver/silver halide (Ag/AgX, X = Cl, Br, I) and silver molybdate (Ag2MoO4) based compounds have been considered as potential high-efficient photocatalysts and photosensitive material in recent years.9-13 Ag/AgBr heterostructure exhibits strong visible light absorption14-16 owing to the surface plasmon resonance (SPR).17-20 Unfortunately, the instability of these photocatalysts greatly limited their practical applications. As we all known, the AgBr can be resolved to Ag0 and then oxidized to Ag2O under the UV and visible irradiation very easily. The inevitable and uncontrolled photocorrosion become a main restriction for the Ag/AgBr application. This phenomenon not merely destroys the structure of Ag/AgBr heterostructure but also 2


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reduces the light energy absorption of Ag/AgBr heterostructure, which may become the Achille’s heel of this photocatalyst. Sequentially it influences the photocatalytic activity and stability of Ag/AgBr heterostructure. To enhance the stability of Ag-based photocatalysts, some support materials such as CNTs,21-23 bentonite,24 polyacrylonitrile nanofiber,25,26 attapulgite,27 and g-C3N428 have been used. These researchers conclude that support materials worked as electron trap can change the charge transmission to restrain the charge recombination, reducing the photocorrosion of Ag/AgBr heterostructure. Thereby, the photocatalytic activity and stability of Ag/AgBr heterostructure get corresponding rise. Graphene Oxide (GO) which contains many different oxygen-containing groups (carboxyl, hydroxyl) is an excellent graphene derivative.29 It has been considered as an excellent catalyst support and promoter for its large specific surface, carrier mobility,30 and high optical transmittance.31 To date, various GO-based materials like TiO2-GO,32,33 AgX-GO,34,35 Ag3VO4-GO,36 Ag2CO3-GO,37 Ag3PO4-GO,29,38,39 Ag2CrO4-GO,40 have been designed. Most of these studies applied it in the organic dyes degradation and showed excellent stability and photocatalytic activity.40 However, the GO-based photocatalysts for tetracycline degradation have not been reported. Herein, we prepared Ag2MoO4/Ag/AgBr/GO semiconductor photocatalyst via a facile precipitation method and have applied in degradation of TC-HCl for the first time. Compared

with

Ag2MoO4/Ag/AgBr

heterostructure

photocatalyst,

Ag2MoO4/Ag/AgBr/GO semiconductor photocatalyst showed substantial increase in the photocatalytic activity and photostability for TC-HCl under visible light. Moreover, the 3



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photocorrosion of the Ag/AgBr/Ag2MoO4 semiconductor photocatalyst was discussed and found that GO sheets can act as an electron acceptor, inhibiting the photocorrosion. The present work will extend the studies of GO-based photocatalysts and produce a new photocatalyst for TC-HCl degradation. At the same time, we come up with a new idea about utilize the complementary materials to design a novel efficient catalyst.

2. EXPERIMENTAL SECTIONS 2.1. Synthesis of Ag2MoO4/Ag/AgBr/GO Hybrid Heterostructure. GO was synthesized with the method reported by Hummers. Materials of GO was from natural graphite powder (325 mesh, 99.995%). The GO was synthesized by the chemical

exfoliation

of

graphite.41

pre-oxidized

The

Ag2MoO4/Ag/AgBr/GO

heterostructure was self-assembled via precipitation method in the dark environment. Typically, 20 mg GO was added into 20 mL deionized water to obtain a GO aqueous suspension (1 mg mL−1) by sonicating for 2 h. 0.9 g of AgNO3 was added to GO suspension and stirred for 30 min to get a homogeneous suspension. Then, adding 0.55 g of Na2MoO4 to the mixture above, and stirred for 30min. Subsequently, cautiously added, dropwise, 12 mL 0.5 M KBr solution to the mixture, stirred continually for 6 h.

To

obtain the metallic Ag, A UV light (8 W; UV Pen-ray) was used as the luminous energy to illuminate the suspension for 10 min. After several times of washing with deionized water and centrifugation, Ag2MoO4/Ag/AgBr/GO heterostructure was collected, and then dried the products in desiccator at 55 °C. In order to investigate the optimal GO loading amount, samples with different GO amounts of 1 % (defined as BG 1), 1.5 % (BG 1.5), 2 % (BG 2)

4


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and 3 % (BG 3) in Ag2MoO4/AgBr heterostructures were prepared. GO content can be seen in Table S1. 2.2. Characterizations. The crystal structures of Ag2MoO4, Ag2MoO4/AgBr and Ag2MoO4/Ag/AgBr/GO heterostructure were studied by X-ray powder diffraction (XRD, Shimadzu XD-3A diffractometer). The micromorphologies of products were researched by the scan electron microscopy (SEM, Hitachi, S-4800) and transmission electron microscopy (TEM, Hitachi-800). The surface composition of Ag2MoO4/Ag/AgBr/GO heterostructure was investigated by X-ray photoelectron spectroscopy (VG Escalab MK II). The optical properties of Ag2MoO4/Ag/AgBr/GO heterostructure were measured by UV-Vis spectroscopy (Shimadzu, UV-2450) and Fourier transform infrared spectroscopy (FT-IR, Shimadzu, IRPrestige-21). The surface area was conducted by V-Sorb 2800 analyzer of specific surface area. Raman spectrum was studied by a laser-Raman spectrometer (Iuvia Reflerx). The total organic carbon (TOC) was tested by the total organic carbon analyzer (TOC-CPN, ELEMENTAR, Liqui TOC). 2.3. The photocatalytic Degradation Experimerent. To evaluate the photocatalytic properties of Ag2MoO4/Ag/AgBr/GO heterostructure, the tetracycline hydrochloride (TC-HCl) solution (40 mg/L) was chosen as the degradation agent and using 350 W Xe lamp as light source (the wavelength > 420 nm). 0.1 g Ag2MoO4/Ag/AgBr/GO catalyst powders were added into 50 mL TC-HCl aqueous solution.

The

suspensions

were

stirred

for

30 minutes

in

dark

to

reach

absorption-desorption equilibrium before irradiation. At interval of 30 min, the 5



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photocatalyst powders were removed and clear solutions were collected by centrifuging the reacted suspension. The concentrations of TC-HCl was evaluated by the UV-Vis spectrophotometer (Shimadzu, UV-2600), and the decline of typical absorption peak at 356 nm was monitored the photocatalytic efficiency. The degradation efficiency (%) was calculated as follows: Degradation (%) = (C/C0) ×100 %

(1)

Where C0 is the original concentration of TC-HCl solution, and C is the concentration of organic dyes solution at time t.

3. RESULTS AND DISCUSSION 3.1. Micromorphologies and Structures.

6


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Figure 1. The SEM (a) and TEM (b) images of the Ag2MoO4/Ag/AgBr/GO heterostructure, (c) The overlaying element mapping images of Ag2MoO4/Ag/AgBr/GO heterostructure, and the element mapping images of (d) Ag, (e) Br, (f) Mo.

The micromorphologies and structures of the products were studied by SEM and TEM technique. The research results (Figure 1a) showed that the Ag2MoO4/Ag/AgBr particles were uniform sphere, and the mean diameter was 300 nm. The GO sheets and the Ag2MoO4/Ag/AgBr particles were intimately intermixed and could be identified clearly. The typical TEM images (Figure 1b) showed that the Ag2MoO4/Ag/AgBr heterostructure dispersed over the GO sheet, which were uniform sphere with a mean diameter of 150 nm. The

scanning

transition

electron

microscopy

(STEM)

image

of

Ag2MoO4/Ag/AgBr/GO heterostructure and element mapping images were shown in Figure 1c-1f. Ag, Br and Mo were all distributed in the sample, indicated the successful formation of Ag2MoO4/Ag/AgBr heterostructure. As shown in Figure 1c, Mo main dispersed in the interior of the sample, while Br main dispersed in the edge of the signal. The weaker edge signal of Mo element and the stronger edge signal of Br element meant that the Ag2MoO4 has been covered by the Ag and AgBr in small part. 7



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Figure 2. (a) XRD patterns (b) Raman spectra of BG 2 and GO

The XRD patterns of the pure Ag2MoO4 crystal, and Ag2MoO4/Ag/AgBr/GO heterostructure with different scales of GO were showed in Figure 2a. All of the diffraction peaks of Ag2MoO4 and Ag2MoO4/Ag/AgBr/GO heterostructure were matched with the cubic phase Ag2MoO4 (JCPDS file, No: 08-0473). We could also identify cubic-phase AgBr crystal (JCPDS file, No: 06-0438) by the peaks at 30.96°, 44.35°, 55.04°, and 73.26°. The characteristic diffraction peaks of GO were invisible for the low content of GO. The fact that hardly impurity peaks exist verified the high purity of the resultant products. The diffraction intensity of Ag2MoO4 changed as the GO introduced, while the peak structure remained the same. The Raman spectra (Figure 2b) were used to prove the present of GO in the heterostructure. The two peaks at the 1350 cm-1 and 1600 cm-1 appearing in Raman spectrum belong to D band and G band of GO, which were derived from the breathing modes of sp2 atoms in rings and the stretching of the sp2-hybridized C-C bonds in GO sheets.42 Furthermore, the peaks around 758 cm-1 and 872 cm-1 were match with Ag2MoO4 phase of the silver molybdate, the diffraction peak at 355 cm-1 was too low to observe clearly.43 The Raman spectra and XRD further confirmed the existence of components in Ag2MoO4/Ag/AgBr/GO heterostructure. 8


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The X-ray photoelectron spectrum (XPS) also proved the presence of Ag, O, Mo, C and Br on the surface of Ag2MoO4/Ag/AgBr/GO heterostructure (Figure S1). The binding energies of Ag 3d, C 1s, O 1s, Mo 3d and Br 3d were recorded (Figure S1a). Figure S1b displayed the Ag 3d spectra, which exhibited two peaks on 367.60 and 373.60 eV of Ag 3d5/2 and Ag 3d3/2. XPS Peak was used to further analyze the form of the Ag, which were split up into four peaks. Peaks on 367.62 and 373.64 eV were corresponding to Ag+ while peaks at 374.00 and 368.32 eV belong to Ag0, 44 suggesting the existence of Ag0 in the Ag@Ag2MoO4-AgBr

heterostructure.

In

Figure

S1c,

the

O

1s

spectra

of

Ag2MoO4/Ag/AgBr/GO heterostructure could be divided into three peaks. Peak locating at about 531.2 eV was attributed to O-Mo species,13 and peaks at 532.2 eV and 532.6 eV were related with O-C and O-C=O.37 C 1s spectra of Ag2MoO4/Ag/AgBr/GO was present in Figure S1d, it could be divided into four peaks, C-C bond at 284.62 eV, C-O bond at 287.18 eV, C=O bond at 288.10 eV, and O-C=O bond at 290.11 eV.39 In Figure S1e, two peaks at 232.62 and 235.69 eV were agree with the position of Mo.13 The position of Br 3d emerged on 67.78 and 68.73 eV, which coincided with Br 3d5/2 and Br 3d3/2.45 The XPS results indicated that the Ag2MoO4/Ag/AgBr particles were loaded on GO sheets successfully. The UV-Vis spectra were showed in Figure S2a. From the UV-Vis spectra, absorption edge of Ag2MoO4/Ag/AgBr/GO heterostructure had no significant shift compared with Ag/AgBr/Ag2MoO4 heterostructure. It indicated that GO was not incorporated in the Ag/AgBr/Ag2MoO4 heterostructure but as the free carbon, which just extend the background absorption but not a changed absorption edge. Therefore, the band 9



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gap energy was also remained the same as before.

38

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With the increase of GO in

heterostructure, the absorption intensity also gradually increased. It may due to the changed electron transmission of π → π* of GO and n → π* of the semiconductors and GO. 46, 47 The change of electron transmission also avoided the recombination of charge carriers, thus improving the photocatalytic performance of the heterostructure. For crystalline semiconductors, the optical absorption can be calculated by Equation 2: αhν = A (hν - Eg)

n/2

(2)

Where α, ν, A, and Eg stand for absorption coefficient, light frequency, proportionality constant and band gap, separately. After calculation, the Eg values of Ag/AgBr/Ag2MoO4 and Ag2MoO4Ag/AgBr/GO were 3.11 and 3.06 eV, respectively. The Kubelka-Munk plot of Ag/AgBr/Ag2MoO4, and Ag/AgBr/Ag2MoO4/GO were shown in Figure S2b. Figure S2c was the FT-IR spectra of the samples. The peak at 3380 cm-1 was due to the O-H stretching vibration of hydrogen bonding. The peak around 1650 cm-1 was corresponding to the H-O-H group. The stretching vibration peak of O-Mo-O was shown at 830 cm-1.48 Peak around 1740 cm−1 was due to the C=O stretching of COOH groups. C-O bond was at 1035 cm-1. The absorption at 1405 cm-1 may be assigned to C-OH groups.49 In Figure S2d, the BET surface area of Ag/AgBr/Ag2MoO4/GO heterostructure was 4.48 m2/g while the BET of Ag/AgBr/Ag2MoO4 was only 1.04 m2/g. It can be inferred that the larger surface area of Ag/AgBr/Ag2MoO4/GO heterostructure provided more active sites and enhanced the photocatalytic activity. 3.2. Photocatalytic Properties.

10


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Figure 3. (a), (b) The photocatalytic degradation for TC-HCl. (c) TOC of the irradiated TC-HCl by the BG2 heterostructure. (d) Dynamic simulation of the catalysis.

The photocatalytic activities of the Ag/AgBr/Ag2MoO4/GO photocatalysts with different GO content were investigated by the degradation of TC-HCl.50 The photodegradation rate of TC-HCl under the different gradation time were shown in Figure 3a. A blank test was carried out to survey the self-degradation of TC-HCl, and result showed that there was almost no loss of the TC-HCl concentration. BG 0 was Ag/AgBr/Ag2MoO4 heterostructure photocatalysts. About 73.8% of TC-HCl was decomposed after 75 min. All of the Ag/AgBr/Ag2MoO4/GO heterostructure photocatalysts exhibited better photocatalytic performance compared with pure Ag/AgBr/Ag2MoO4 heterostructure. Especially, the BG 2 (GO content 2 %) showed the 11



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best photocatalytic activity among these Ag2MoO4/Ag/AgBr/GO heterostructure photocatalysts. Within 75 min, the TC-HCl has been degraded completely by BG 2. This phenomenon indicated that GO content 2 % was the optimal ratio. Compared with Ag2MoO4/Ag/AgBr photocatalyst, the photocatalytic activity of Ag2MoO4/Ag/AgBr/GO was enhanced by 36.2 %. Moreover, from Figure 3b we found that after the introduction of metallic Ag, the photocatalytic activity was improved slightly. The suspension contained the photocatalysts and organic matter solution was stirred for 30 min to reach adsorption-desorption balance before exposed to the light. The UV-Vis absorption of TC-HCl was at 356 nm, which can be the typical peak to monitor the diminished TC-HCl solution concentration along with degradation time changed. Meanwhile, the concentration of TOC was also measured. As showed in Figure 3c, at the end of photocatalytic process, the TOC of solution was 6 %, which indicated that the TC-HCl have been mineralized exhaustively by BG 2 heterostructure. Moreover, the TOC result also prove that the TC-HCl was degraded in to CO2 and H2O. The degradation kinetics of TC-HCl was verified by the first order reaction kinetics, which we know as the Langmuir-Hinshelwood (L-H) model.51 ln (C0/C) = kt

(3)

Where C0 is original TC-HCl concentration, Ct is TC-HCl concentration at time t, and k is the first-order rate constant. The curves of ln (C0/C) versus t were presented in Figure 3d. There was an obvious linear relation between ln (C0/C) and t, which meant that the reaction was a typical first-order reaction. Obviously, the Ag/AgBr/Ag2MoO4/GO (GO content 2 %) heterostructure was the best in the degradation of TC-HCl. 12


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3.3. The photostability of Ag/AgBr/Ag2MoO4/GO heterostructure.

Figure 4. The photocatalytic activities of BG for TC-HCl in different recycle runs: (a) BG 0, (b) BG 2.

The photostability of the Ag2MoO4/Ag/AgBr/GO heterostructure was evaluated repeatedly eight times by degrading the TC-HCl solution. The experiments result showed in Figure 4. After eight cycles, the BG 2 showed better stability than BG 0. The degradation rate of the Ag/AgBr/Ag2MoO4 heterostructure reduced by 18.5 %, while the degradation rate of the Ag/AgBr/Ag2MoO4/GO heterostructure only reduced by 4.5 %. Moreover, the XRD spectra of BG 0 and BG 2 after four cycles were showed in Figure S3. There was a distinct diffraction peaks of Ag0 appeared in Ag/AgBr/Ag2MoO4 heterostructure, indicating that Ag particles had generated during the degradation of TC-HCl

process.

However,

no

obvious

Ag0

peaks

were

detected

from

Ag/AgBr/Ag2MoO4/GO heterostructure. That was to say, the Ag/AgBr/Ag2MoO4 heterostructure showed remarkable photocorrosion phenomenon after eight cycles. Recycle runs also revealed that Ag/AgBr/Ag2MoO4/GO heterostructure have excellent stability by the introduction of GO for the suppression of photocorrosion. To make sure the active species of Ag/AgBr/Ag2MoO4/GO heterostructure in the degradation of TC-HC, triethanolamine (TEOA, 10 mM) and isopropanol (IPA, 20 mM) 13



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were used to trap photo induced holes (h+) and hydroxyl radicals (·OH). When TEOA was added into the reaction system, the catalytic rate showed great deactivation (Figure S4). Simultaneously, the photocatalytic efficiency in the reaction system after the addition of IPA also decreased from 100 % to 57.2 % in 75 min. Above the results, the photo induced holes (h+) and hydroxyl radicals (·OH) all played a vital role in the degradation of TH-HCl. 3.4. The mechanism of photocatalysis.

Figure 5. Schematic diagram of the photocatalytic mechanism.

Figure 5 presented the degradation mechanism of Ag2MoO4/Ag/AgBr/GO heterostructure. In Ag2MoO4/Ag/AgBr/GO heterostructure, the Ag, AgBr and Ag2MoO4 dispersed on the GO sheet and contacted intimately. The valence band (VB) of AgBr is 2.16 eV, and the conduction band (CB) is -0.45 eV.12 While the VB and CB of Ag2MoO4 is 3.11 eV and -0.27 eV.12 When AgBr and Ag2MoO4 is irradiated by light, the electrons in the VB of AgBr and Ag2MoO4 are excited, then the excited electrons leap into CB of AgBr and Ag2MoO4, leaving the holes in the VB. As can be seen, the CB location of Ag2MoO4 is lower than AgBr, and the VB location of Ag2MoO4 is also lower than that of AgBr. For close contacted AgBr and Ag2MoO4, electrons on the CB of Ag2MoO4 can 14


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transfer to VB of AgBr. Then the transferred electrons and holes (h+) in the VB of AgBr recombined each other. The photoginduced holes left in the VB of Ag2MoO4 can degrade TC-HCl to harmless products. Meanwhile, some of the photoinduced electrons in the CB of AgBr reacted with DO (Dissolved Oxygen) to produce superoxide radical anions (·O2-) and active hydroxyl radical (·OH), ·O2- and ·OH could decompose organic pollutant. The SPR effect of nano-silver on the surface of Ag2MoO4-AgBr can promote the absorption of light, which also enhance the photocatalytic efficiency. The reactive groups trapping experiments confirmed the mechanism of photocatalysis. However, when the Ag2MoO4-AgBr heterostructure was loaded with Ag nanoparticles, the electrons in the CB of Ag2MoO4-AgBr heterostructure would easily transfer to the silver nanoparticles, and lead to the Ag2MoO4-AgBr heterostructure have been decomposed to Ag0 easier under long time irradiation, which result in the photocatalysis performance and stability reduced obviously. Act as an acceptor, GO was reduced during the photocatalytic reaction, promoting the formation of reduced GO. Reduced GO which acted as a transmission medium in Ag/AgBr/Ag2MoO4/GO heterostructure was in favor of the transmission of photoinduced electrons. With the specific π-conjugated structures of reduced GO, electrons on reduced GO would shift quickly. The quick transfer of photoinduced electron accelerated the formation of more electrons in the Ag2MoO4 and AgBr.29 On the other hand, the high BET surface of GO offered more active sites, further enhance the photocatalytic performance. The experiment of photocatalytic properties (Figure 3) and stability (Figure 4) also reveal that GO can enhanced

photocatalytic

performance

and

the

photocatalytic

15


stability

of


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Ag2MoO4/Ag/AgBr/GO heterostructure. 3.5. Degradation of TC-HCl. The TC-HCl was widely applied in medicine, due to its lower cost and higher antimicrobial activity. Nowadays, the pharmaceutical factories are spread all over the world, its emissions of wastewater easy to cause environmental pollution. Moreover, their self-degradation capacity was so weak in ecosystem that photodegradation with visible light source was one of the appropriate solution. The Ag2MoO4/Ag/AgBr/GO heterostructure photocatalyst is an ideal method to deal with the TC-HCl, and it can be fixed from a contact oxidation pond to catalyze by solar light. In experiments, the initial TC-HCl solution concentration of 20 mg/L could be decomposed to 0 mg/L within 75 min through prolonging the photodegradation experiment. The total organic carbon (TOC) test in front showed that there was no organic matter resided in the TC-HCl solution, proving that the TC-HCl was degraded in to CO2 and H2O. The degradation pathway of TC-HCl could be illustrated as Figure S5.

4. Conclusion In summary, a high efficiency and stability photocatalyst: Ag2MoO4/Ag/AgBr/GO heterostructure are prepared through a facile precipitation method. The excellent photocatalytic performance of Ag2MoO4/Ag/AgBr/GO heterostructure is mainly attributed to the enhanced charge separation. Moreover, GO act as an acceptor, which prevent Ag+ change to Ag0, improving the stability of photocatalyst. These complementary materials make the Ag2MoO4/Ag/AgBr/GO heterostructure has excellent photocatalytic activity and stability. The TC-HCl is degraded completely under visible 16


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light

irradiation

within

75

min,

suggesting

the

great

application

of

the

Ag2MoO4/Ag/AgBr/GO heterostructure in the TC-HCl disposal, which can be used to deal with the tetracycline residues in water.

SUPPORTING INFORMATION The content of GO in the samples; the XPS spectra of Ag2MoO4/Ag/AgBr/GO heterostructure; the UV-Vis spectrum, Kubelka-Munk plot, FT-IR spectra and BET spectrum of Ag2MoO4/Ag/AgBr/GO heterostructure; XRD spectra of BG 0 and BG 2 after recycled four times; reactive specie trapping experiments of Ag2MoO4/Ag/AgBr/GO heterostructure; pathway of the intermediate products for TC-HCl photodegradation. These materials are supplied in the supporting information.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation China (Grant 21341007) and Shanghai Key Laboratory of Multiphase Materials Chemical Engineering (MMCE2015002).

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