Insight into MoS2 Synthesis with Bio-photoelectrochemical

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Insight into MoS2 Synthesis with Bio-photoelectrochemical Engineering and Their Application in Levofloxacin Elimination Libin Zeng, Xinyong Li, Shiying Fan, Mingmei Zhang, Zhifan Yin, Moses O. Tade, and Shaomin Liu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00524 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 22, 2018

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ACS Applied Energy Materials

Insight into MoS2 Synthesis with Bio-photoelectrochemical Engineering and Their Application in Levofloxacin Elimination Libin Zenga, Xinyong Lia,b*, Shiying Fana, Mingmei Zhanga, Zhifan Yina, Moses Tadéb, Shaomin Liub a

State Key Laboratory of Fine Chemicals, Key Laboratory of Industrial Ecology and

Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China b

Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth,

WA 6845, Australia

*Corresponding author: Tel: +86-411-8470-7733; Fax: +86-411-8470-8083; E-mail: [email protected]

1

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Abstract Biosynthesis of nanomaterials is an emerging technology in recent decades ascribed to its unique "greener" route and higher energy efficiency. It is superior to the traditional physicochemical synthesis processes, in which the hazardous intermediates generating or high energy-consuming are often inevitable but remain a significant obstacle. In this work, a coupling system based on photo-driven microbial fuel cell (MFC) was constructed to controllably synthesize different sizes of MoS2 nanomaterials in situ. By virtue of MFC producing electricity as a driving force, the MoS42- ions could be reduced to MoS2 nanoparticles. Impressively, photo-excited electrons produced from polydopamine coating TiO2 nanotubes (PDA/TiO2 NTs) electrode under visible light irradiation (> 420 nm), could also be utilized online to facilitate MoS2 nanoparticles growth effectively. Interestingly, MoS2 material was further cultivated on PDA/TiO2 substrate and then biologically modified MoS2/PDA/TiO2 electrodes were easily obtained, which exhibited unique hydrophilic behavior (14.74°), and bio-electrocatalytic performance for effectively promoting antibiotics complete removal in the MFC and photoelectrocatalytic (PEC) cooperative system. Thus, we believe such the obvious advantages of constructed photo-boosted MFC system could provide an environmentally benign pathway to synthesize nano-structured electrode materials, and create new opportunities for diverse pollutants removal in situ.

Keywords:

Bio-synthesis;

Electron

transfer;

Photo-microbial

2


fuel

cell;

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Photoelectrocatalytic; Radical groups

3



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1. Introduction In recent decades, human beings are continuously facing intractable crises of the residues of antibiotic and rapid increase of resistant genes even super-bacteria in water environment and ecosystem1. One of the best long-term promising routes to address these hazards is rational design of the highly effective photocatalytic materials based photoelectrocatalytic (PEC) system, which stands out as an increasing potential in the application of environmental remediation.2-3 Among diverse photocatalyst systems, molybdenum disulfide (MoS2) has receiving extensive research enthusiasm as an important member of two dimensional (2D) functional material due to its inherent physical-chemical properties and thickness dependent band gap feature (1.3-1.9 eV).4 In recent years, especially its morphology effect5-6 and rich redox electrochemical characteristics endowed it excellent catalytic activity for widespread applications in energy conversion, storage devices and pollutants mineralization. In addition, MoS2 has been considered as a promising electrocatalytic material for lithium-ion batteries (LIBs)7-8, oxygen reduction reaction (ORR)9-10, hydrogen evolution reaction (HER)11-13 and wastewater purification14-15, due to its exposing many edge active sites and showing high electron transfer rate.12,

16

Nonetheless, up to now, a variety of physical and chemical methods, such as kinds of exfoliation17-20, electrochemical14, hydro/solvothermal21-26, chemical vapor deposition (CVD)27-28 and hot-injection method29, have been developed to synthesize MoS2 materials. Despite several new methods14,

30-31

are being positively explored to

fabricate MoS2 nanomaterials, while they usually suffer from high energy-consuming 4


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or environmentally unfriendly. While in recent years, bio-modified catalysts could display admirable electron transfer characteristics.32-34 Therefore, exploring efficient optimization of green bio-approaches for fabricating MoS2 nanoparticles is a challenging yet urgently needed work. In comparison to the aforementioned strategies, bioelectrochemical system (BES) for the construction of nanomaterials has been attracting growing interest because of its advantage of environment-friendly and energy-saving. Among all kinds of BES reactors, microbial fuel cell (MFC) could not only continuous recycle energy from wastewater35-37, but also possess huge reduction ability from exoelectrogens.38-39 Belcher and his co-workers demonstrated that bio-facilitated synthetic method could be as a robust platform for the construction of electrode materials.40 For one of the big advantages of MFC generating electricity, Gyenge et al. reported that the graphene anode could be robustly and effectively exfoliated by the electricity generated from air cathode MFC.34 Meanwhile, the precious metals (Ag, Pd and Au) could be better to obviously selectively recover from contaminants through the synergistic effect of microorganisms.41-43 Recently, bio-fabrication has been significantly focused on the recovery of metal sulfides (MSx). Yu et al. indicated that the FeS nanomaterials were successfully recycled from wastewater in bioconversion system in situ by naphthol green B and thiosulfate.44 Meanwhile, Macaskie and his co-workers ingeniously put forward that ZnS quantum dots also could be bio-synthesized in a mental remediation system with the help of sulfate reducing bacteria.45 Crucially, the preparation routes of these MSx were just at room temperature and environmentally benign. It would be 5



extremely meaningful to synthesize 2D MoS2 nanosheets and develop MoS2 based electrode via biotechnology for harvesting improved catalytic performance. However, to the best of our knowledge, no work has been reported to incorporate biological/MFC means into the fabrication of 2D MoS2 nanoparticles. Based on these, the exploitation of photo-driven MFC coupling with photoelectrocatalytic cell (PMFC-PEC), to enhance antibiotics elimination accompanied by the preparation of nanomaterials simultaneously, is becoming more meaningful. Herein, we performed a PMFC-PEC system to synthesize highly effective MoS2 nanosheets and MoS2/PDA/TiO2 NTs electrode for the catalytic degradation of antibiotics, which was consisted of PDA modified TiO2 NTs as photo cathode and active exoelectrogens as bio-anode. And the effects of reaction time and visible light irradiation on the MoS2 catalyst preparation process were further investigated to optimize the synthetic conditions, also MoS2/PDA/TiO2 NTs electrode was constructed in-situ as photo-anode for PEC system. Experimental characterizations further confirmed that this developed composite electrode exhibited more edge active sites and excellent catalytic activity. The mechanism of LEV degradation by PMFC-PEC system was verified by remarkably promoted LEV removal rate and improved electronic paramagnetic resonance (EPR) signals. This work presented here can provide a better understanding of biosynthesis route to design new materials for pollutants elimination and energy catalysis.

2. Experimental section 6


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The detailed chemicals, the preparation of PDA decorated TiO2 NTs electrode, the instrument parameters for characterizing catalysts and electrochemical measurement were presented in the Supporting Information.

2.1 MFC configuration and operation An air-cathode MFC (running for 2 years) was set up to enrich a new double chamber MFC inoculum. In this special constructed MFC reactor for MoS2 synthesis, the anode electrode was filled with porous carbon felt fragements (2 stripes, 4 cm × 2 cm × 1 cm each cut into small chips) linked to a graphite rod (diameter, 8 mm), and the cathode electrode was PDA/TiO2 NTs(1 cm × 2 cm). The MFC was made of anode chamber (30 mL), cathode chamber (15 mL) and a cation exchange membrane (5 cm * 5 cm). The anode medium was referred to our former work.46 The pH of the medium was 7.0 and the organic substrate was 3 mM CH3CH2ONa solution. Each time, the anolyte was deoxygenated by bubbling N2 for 10 min prior to use. The duration of each anode feeding cycle was each day. The catholyte was 0.1 mM (NH4)2[MoS4] solution (solvent (V %) = H2O/C2H5OH = 9:1) as the sole electron acceptors in the cathode chamber. Prior to use, electrolyte solution was also saturated with nitrogen gas to remove residual oxygen. And the reactor was connected with 1 kΩ external resistor. The reactive route for the formation of MoS2 nanosheets could be elaborated in detail (Scheme 1).

2.2 Biofabrication of MoS2 nanosheets After the anode potential of the constructed dual-chamber MFC approached to

7



stability (0.472 mV vs SCE), which suggested this MFC possessed the best bio-activity. After that, the cathode chamber would be replaced with the fresh electrode and catholyte. Then the cathode electrode was subjected to visible light irradiation in order to synthesize MoS2 materials (λ> 420 nm, 33 mW/cm2; the distance between the electrode and the center of the light source, 10 cm). The visible light-driven PDA/TiO2 electrode could generate the photo-induced charge-carrier following by incurring efficient interfacial electron transfers over the composites, cooperated jointly with the electrons coming from MFC anode microorganisms to reduce the MoS42- to MoS2 simultaneously and spontaneously. It could be observed that with the reaction time extending, the solution of cathode chamber gradually became light yellow, and the samples were collected from the cathode chamber at different time stages (2 h interval). After filtered with 0.22 µm inorganic filter membrane, the obtained filtrate was kept in a refrigerator at 4℃ for further analysis. Subsequently, the cathode chamber was replenished with deionized water several times for washing the chamber, then the fresh MoS2 precursor solution was charged again. Meanwhile, under the dark situation, the process was the same to the aforementioned, except the sampling time was changed to one day each time. All experiments were carried out at room temperature, and the anolyte was aerated N2 flow for 10 min at least.

2.3 Building of photo-MFC coupled with photoelectrochemical system In order to evaluate the catalytic activity of MoS2/PDA/TiO2 NTs electrode, it was 8


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used as the photo-anode in photoelectrochemical cell (PEC), which came from the cathode chamber of photo-MFC (PMFC) after it has run for 6 h. Meanwhile, the PMFC was not only used to synthesize MoS2 nanosheets, also served as a bias for PEC reactor. The model antibiotic levofloxaxin (LEV, 10 mg/L) was chosen as the electron acceptor with 0.01 M Na2SO4 in PEC cell. In this coupling PMFC-PEC reactor, a 10 Ω external resistor was used to link between the photo-cathode of PMFC and the photo-anode of PEC. The anode and saturated calomel electrode (SCE) of PMFC were also connected to the cathode (Pt wire) and SCE of PEC, respectively. After running this coupling system, the effluent from PEC cell was collected every 30 min. The LEV degradation rate was measured by UV-vis spectrophotometer at the wavelength of 286 nm. To illustrate the effect of biological synergy on the catalytic activity of the electrode, the layered MoS2 nanosheets were deposited on PDA/TiO2 electrode by photo-reduction of 0.1 mM (NH4)2[MoS4] solution for 6 h, which the obtained electrode was as a control electrode and named MoS2/PDA/TiO2-1. The following catalytic experiments were consistent with the above-mentioned.

3. Results and discussion The morphology and elemental components of the effluent solution from MFC photo-cathode, were investigated by field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectra (EDX), respectively. In Figure S1, the flake-forming materials could be found in this sample. Meanwhile, the element mapping signals of Mo and S were also clearly displayed in this selected range, 9



implying that photo assisted MFC could successfully produce MoS2 nanosheets. To further verify this result, transmission electron microscopy (TEM) was applied to inspect its crystal structure. Multi-sizes layered structure were exhibited in Figure 1a. The high resolution TEM image (Figure 1b) explained that the interlayer distance of its lattices were 0.254 and 0.309 nm, corresponding to the spacing for (102) and (004) crystal plane of MoS2 (JCPDS 37-1492), respectively. The two dimensional MoS2 nanosheets could be gained by MFC route, and it would provide rich edge sites47, which could serve as catalytic active sites for pollutants removal. In order to optimize the synthesis parameters of MoS2, the main influencing factors (reaction time and visible light) were further investigated in MFC synthetic system. In this bio-synthesis system, the potential of anode (0.472 V vs SCE) was used as the unified standard to assess the performance of MFC (Figure S2a). A simple UV-vis spectroscopy detection method was used to evaluate the visible light effect on synthetic phenomenon. As shown in Figure 2, the yield of MoS2 nanoparticles was too slow to be time-consuming under dark condition (Figure 2a). Surprisingly, under visible light driven condition (Figure 2b), this MFC powerfully accelerated MoS42converting to MoS2 particles, which shortened the synthesis time to several hours. The main reason came from the values of current to the bio-cathode chamber (Figure S2b), which the current was 0.486 mA higher than that in dark (0.105 mA). Also, from the inset image, it was found that the brownish yellow solution became more and more clearly as time elapsed, which belonged to the feature of MoS2 solution. Therefore, it revealed the role of visible light in reducing the energy barrier of MoS2 10


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generation. For analysis and identification of the chemical bonds on MoS2 nanoparticles, FT-IR spectra showed new peaks at 570 and 920 cm-1 compared with the MoS42- precursor solution (Figure S3), which was ascribed to Mo-S bands.48 Afterwards, photoluminescence (PL) method combined with low-temperature electronic paramagnetic resonance (EPR) technique were carried out to reveal the best synthesis time under visible light irradiation. For different reaction time (Figure 2c), the increasing intensity of PL appeared with the MFC running time from 0 to 8 h. The samples taken at 6 h and 8 h presented the strongest PL intensity, suggesting that the synthesized MoS2 nanosheets were stacked each other and their edge sites would be hiden (as shown in the results of SEM). In Figure 2d, the low-temperature (100 K) EPR spectra showed a prominent EPR signal at g = 2.003, which appeared in the 2, 4, 6 and 8 h samples, corresponding to the typical signal of MoS2 material.49 Also, the signal of 6 h sample was stronger than that of others, explaining that 6 h derived MoS2 nanoparticles might display the maximum edges exposure. To further explain the above time-dependent hypothesis, the morphology of the different time MoS2 samples were examined by FE-SEM. In Figure 3, the layered structure MoS2 could be found in all range of time, but their size gradually expanded with increasing the photo excitation time, even new grown MoS2 nanosheets further resulted in stacking and clogging after 8 h visible light irradiation, which was consistent with the results of PL and low EPR measurements. Based on these optimize conditions, 6 h was selected as the following MoS2 material preparation condition and subsequent object for study. 11



To explore the synthesis mechanism (Scheme 1), the open circuit voltage (OCV) of MFC and pH change of cathode chamber were analyzed. In Figure S2 (c-d), no matter under visible light or in dark, the pH values were increased then decreased, which were ascribed to the maintained pH balance of anode (pH = 7.0) and cathode chamber (pH = 4.5), and H+ continuous generation from anode microbial metabolism (CH3COO- + 2H2O → 2CO2 + 7H+ + 8e-) after the first pH balance, respectively. It indicated that acidic environment facilitated MoS42- to be reduced to MoS2. Meanwhile, the photo-electrons could be induced by PDA/TiO2 electrode under visible light illumination, coupled with bio-generated electrons to noticeably accelerate the reaction ([MoS4]2- + 2e- → MoS2 + 2S2-), arousing the OCV of the whole MFC increasingly. These results demonstrated that the combination of bio/photo-electrons not only strikingly promoted MoS2 growth, but also improved the electricity production performance of MFC. Except the MoS2 nanoparticles synthesized in MFC system, a new MoS2/PDA/TiO2 electrode was constructed at the same time. In Figure S4a, small PDA nanoparticles adsorbed on the edges of TiO2 nanotubes, which acted as an excellent visible light photosensitizer for the TiO2 substrate material.50 After PDA/TiO2 electrode under visible light driven in MFC system, the surface of tubes became extremely rough because of MoS2 nanoparticles formed (Figure S4b). More importantly, the deposition of MoS2 materials could enhance the surface area of PDA/TiO2, which would be explained by the LEV absorption measurements. The biosynthesized MoS2 was further confirmed by XRD characterization. In Figure S5a, new diffraction peaks 12


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at 2θ = 9.3° and 32.4° were formed in MoS2-1 and MoS2-2, which corresponded to the (002) plane shifted to a low angle51 and (100) of MoS2, respectively. Furthermore, the only one peak 14.4° could be found in the MoS2/PDA/TiO2 electrode, corresponding to the (002) plane of MoS2 (JPCDs no. 37-1492). These results could further illustrate that MoS2 nanoparticles and MoS2 based electrode were successfully synthesized by bio/photo electrons synergetic effect. The surface chemical states of as-prepared electrodes were collected by X-ray Photoelectron Spectroscopy (XPS) in detail. N 1s and C 1s peaks were found in PDA decorated electrode (Figure S6), indicating that organic polymer was successfully coated on the TiO2 nanotubes. For MoS2/PDA/TiO2 electrode, the substrate TiO2 could be confirmed by the peaks of Ti 2p and O 1s signals (Figure 4a-b). The binding energies of Mo 3d were observed at 232.32 and 229.05 eV (Figure 4c), which were attributed to Mo 3d3/2 and Mo 3d5/2 orbits of the Mo4+ state, respectively. Besides, Mo6+ also existed corresponding to 233.32 and 236.21 eV, which might be due to the oxidation of partial MoS2 particles.52 Combined with S 2p peaks (Figure 4d), the S2could be proved by 226.12, 163.08 and 161.83 eV, which were ascribed to S 2s, 2p1/2 and 2p3/2, respectively, and the results were similar to the previously reported case14. After comparison with TiO2, PDA/TiO2 and MoS2/PDA/TiO2 electrodes (Figure 4 (e-f)), the introduction of PDA molecules made the binding energies of Ti 2p and O 1s slightly shift to low field, demonstrating that TiO2 could get electrons from PDA under visible light illumination. Subsequently, with the formation of MoS2 nanosheets in bio-electrochemical cooperative system, Ti 2p and O 1s were shift to higher value, 13



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indicating that MoS2 also promoted the orbits of Ti 2p and O1s to lose electrons, and there was a strong electronic interaction between MoS2, PDA and TiO2. The free electrons could be generated by visible light through exciting electrons to conduction band (CB) of PDA/TiO2, and holes left on that of valence band (VB) and then captured by ethanol solvent. The photo-electrons combined with bio-electrons could possess enough capability to reduce MoS42- to MoS2 nanoparticles. To further elucidate photo-generated carrier separation and recombination of this constructed electrode, PL experiment was carried out. In Figure S7, PL signals of MoS2/PDA/TiO2 electrode obviously exhibited a relatively lower PL intensity than those of both PDA/TiO2 and TiO2, which demonstrated that bio-synthesized MoS2 could markedly accelerate the charges separation under photo-driven condition (Table 1). To investigate the catalytic performance of as-prepared samples, these obtained electrodes were assembled into a photo-anode materials in PEC reactor of PMFC-PEC coupled system to evaluate their catalytic performance. Figure 5 simultaneously indicated the degradation of the target pollutant LEV catalytic performance.

The

catalytic

activity

of

electrodes

(TiO2,

PDA/TiO2

and

MoS2/PDA/TiO2) was inspected in MFC-PEC system for LEV removal. For comparison, the dark condition and self-photolysis of LEV were also carried out (Figure 5a). It was showed that nearly no LEV was degraded and the self-photolysis was less than 11%. Meanwhile, it was found that though the catalytic activity of PDA-modified electrode exhibited no big difference with the bare TiO2, they showed 14


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50% higher removal efficiency compared with the PEC reactor alone. While MoS2/PDA/TiO2 catalyst displayed excellent LEV degradation performance, as the LEV was nearly completely removed (~100%) within 150 min. In addition, the degradation process of LEV by MFC-PEC system was fitted well with the pseudo-first order reaction (Figure 5b), and their kinetic constants were 8.5×10-5, 7.2×10-4, 4.4×10-3, 3.4×10-3 and 2.4×10-2 min-1, respectively. Interestingly, the best degradation performance with ~100% removal within 90 min in PMFC-PEC system (Figure 5c). The kinetic constant reached to 5.2×10-2 min-1 (Figure 5d), which was 2.17 times than that of MFC-PEC system. At the same time, the photocatalytic activity of MoS2/PDA/TiO2 electrode was also investigated without PMFC assistance. Figure S8 (a-b) showed that this route only removed ~80% LEV within 180 min, which indicated that the assistance of MFC/PMFC strikingly promoted the catalytic activity towards LEV. Compared with non-bio-assisted synthesized electrode (Figure S8(c)), the bio-facilitated method presented excellent catalytic performance for LEV elimination. These results comprehensively explained that bio-synthesis MoS2 decorated PDA/TiO2 performed striking catalytic performance especially with the PMFC-PEC synergistically coupling effect. Combined with the previous work (Table 1), the hybrid electrode possessed excellent catalytic activity in LEV removal especially in this constructed PMFC-PEC system. Based on the aforementioned analysis and EPR results, a possible catalytic mechanism for LEV degradation was proposed (Figure 6). Under visible light irradiations, the molecular PDA antennas could be then excited firstly based on the 15



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synergistic effect through the presence of the π–π* electron transition53 followed by injection the excited electrons into the conductions band of TiO2 NTs to incur effective charge spatial separations and interfacial transfers. In this cathode chamber MFC system, visible light induced-holes based on the PDA/TiO2 NTs electrode ((PDA/TiO2-h+)) were captured by bio-electrons (e-bio) producing from the metabolism of exoelectrogens in the anode (reactions (1)-(3)). Meanwhile, MoS42ions were reduced to MoS2 by electrons in cathode chamber (reaction (4)). The above reactions effectively prevented the recombination of photo-carriers and facilitated the formation of hydrophilic MoS2/PDA/TiO2 catalyst (the wetting angle: 14.74°, Figure S9), which indicated the obtained electrode would be better in contact with aqueous contaminants. Moreover, it largely improved the electricity generation of MFC, which could be properly used to generate a micro-electric field for PEC reactor. In order to improve the electrons/holes separation of photo-anode in PEC system, a PMFC-PEC system was constructed for LEV degradation. In PMFC-PEC coupled system, photo-excited

electrons

over

the

MoS2/PDA/TiO2

NTs

electrode

(MoS2/PDA/TiO2-e-pho) in PEC system were transferred to trap photo-generated holes on PDA/TiO2 NTs electrode (PDA/TiO2-h+) in the cathode of MFC by the external circuit. Then, the left holes (MoS2/PDA/TiO2-h+) could mineralize pollutants and remaining electrons could reduce MoS42- ions to MoS2 nanoparticles (reactions (5)-(11)). The photo-excited holes (MoS2/PDA/TiO2-h+) could react with H3O+ to produce •OH radicals (reaction (7)), which displayed high oxidation ability for LEV removal (reaction 11). Meanwhile, the absorbed oxygen molecule on MoS2/PDA/TiO2 16


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surface (MoS2/PDA/TiO2-(O2)abs) would react with the photo-electrons or bio-electrons ((MoS2/PDA/TiO2-e-pho)/e-bio) to produce •O2- radicals (reaction (8)), and then after series of reactions, the pollutant LEV could be indirectly mineralized (reactions (9)-(11)). In this cooperated system, doubled electron-hole pairs under illumination in different systems were both effectively hindered, resulting in the production of MoS2 and the degradation of LEV. It was interesting that the electrical energy produced in MFC was in situ providing a bias for PEC cell to better degrade LEV. In contrast, PEC cell helped MoS42- reduce to MoS2 nanoparticles on PDA/TiO2 NTs electrode and improved the electricity output from MFC reactor. ୣ୶୭ୣ୪ୣୡ୲୰୭୥ୣ୬ୱ

CH3COO- + 2H2O ሱۛۛۛۛۛۛۛۛۛۛሮ 2CO2 + 7H+ + 8e-bio

(1)

௛௩

(PDA/TiO2)sur + ሱሮ (PDA/TiO2-h+) + (PDA/TiO2-e-pho)

(2)

(PDA/TiO2-h+)sur + e-bio →(PDA/TiO2) + h‫’ݒ‬

(3) ௛௩

(PDA/TiO2)sur + [MoS4]2- + 2e-bio/(PDA/TiO2-e-pho) ሱሮ (MoS2/PDA/TiO2) + 2S2 ௛௩

(MoS2/PDA/TiO2)sur ሱሮ (MoS2/PDA/TiO2-h+) + (MoS2/PDA/TiO2-e-pho)

(4) (5)

(MoS2/PDA/TiO2-e-pho) + (PDA/TiO2-h+) → (MoS2/PDA/TiO2) + (PDA/TiO2) + h‫’’ݒ‬ (6 ) (MoS2/PDA/TiO2-h+) + H3O+ → (MoS2/PDA/TiO2-•OH) + H+ + H2O

(7)

(MoS2/PDA/TiO2-(O2)abs) + (MoS2/PDA/TiO2-e-pho)/e-bio → (MoS2/PDA/TiO2-•O2-) (8) (MoS2/PDA/TiO2-•O2-) + H+ → (MoS2/PDA/TiO2-•HO2) (MoS2/PDA/TiO2-e-pho)/e-bio

+

(MoS2/PDA/TiO2-•HO2)

(MoS2/PDA/TiO2-•OH)

+ 17


(9) +

H+

→ OH-


(10) (MoS2/PDA/TiO2-•OH) + LEV → (MoS2/PDA/TiO2) + CO2 + H2O + ••• (11) To further confirm the aforementioned the proposed mechanism, EPR technique was carried out to investigate reactive oxygen species (ROS) in PMFC-PEC system. In Figure 7, the ROS of MoS2 aqueous solution and MoS2/PDA/TiO2 electrode were explored, respectively. Taking •OH radicals for example, the effect of reaction time on •OH radicals strength was investigated to further elucidate the best reaction time for the fabrication of MoS2 nanosheets. In Figure 7a-b, they all displayed obviously •OH radicals signals, and 6 h derived MoS2 showed the higher response of •OH radicals than others with DMPO solution in effective time range, which indicated that bio-synthesized MoS2 catalyst could produce more abundant •OH radicals under visible light irradiation, and it also was consistent with the previous optimization time (6 h) on MoS2 synthesis combined with SEM and PL analysis. For MoS2/PDA/TiO2 electrode, both characteristic signals of •OH and •O2- radicals were detected over the surface of this composite electrode (Figure 7c-d), which could help explain that the formation of •OH and •O2- radicals as main ROS groups attacked the pollutant molecules.

4. Conclusion In summary, a feasibility strategy was firstly developed by using a facial PMFC platform to fabricate MoS2 nanomaterial and MoS2/PDA/TiO2 electrode for 18


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photo-driven bioelectrocatalytic degradation of LEV. Based on the optimization of synthesis time and visible light effect, 6 h and visible light assistance were selected as the ideal parameters for the fabrication of MoS2 nanomaterial. Especially, the developed PMFC-PEC system effectively facilitated the separation of doubled electron/hole pairs in PEC and PMFC system, respectively, and also could serve as a bias for boosting PEC performance. Moreover, the optimized biosynthesis routes, the enhanced ROS groups, the incorporation of dual-electrons (e-bio/e-pho) route, as well as improved hydrophilic capability could be responsible for the improved catalytic degradation of LEV with a removal rate of 5.2×10-2 min-1. Thus, we believe the designed PMFC system will provide a better environmentally pathway to develop new nano-structured electrode materials and be used in widespread field for wastewater treatment and energy storage.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21577012), the Major Program of the National Natural Science Foundation of China (No. 21590813), the Key Project of the National Ministry of Science and Technology (No. 2016YFC0204204), the Program of Introducing Talents of Discipline to Universities (B13012), and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education.

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ASSOCIATED CONTENT Supporting Information The chemicals and supplementary experiment, characterization and electrochemical parameters, are presented in Supplementing Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table Captions Table 1. Comparison of various methods for MoS2 fabrication.

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Table 1

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Figure Captions Scheme 1. Schematic illustration of fabricating MoS2 in the microbial fuel cell assisted with visible light irradiation. Figure 1. (a) TEM image of bio-synthesized MoS2 nanosheets, (b) HETEM image of as-prepared MoS2 sample. Figure 2. UV-Vis spectra of the reaction solution from cathode chamber varying from different time under dark (a) and visible light irradiation (b), inset: optical images for effluent solution; (c) Photoluminescence spectra of visible light assisted biosynthesizing MoS2 nanoparticle recorded at an excitation wavelength of 360 nm; (d) Low-temperature EPR spectra of time-based effluent at the condition of 100 K. Figure 3. SEM images of bio-assisted synthesized MoS2 nanosheets under different time: (a) and (b) 2 h, (c) and (d) 4 h, (e) and (f) 6 h, (g) and (h) 8 h. Figure 4. High resolution X-Ray photoelectron spectra (XPS) survey spectra of the composites: (a-d) Ti 2p, O 1s, Mo 3d and S 2p regions for the surface of the MoS2/PDA/TiO2 NTAs electrode, respectively; (e) Ti and (f) O from TiO2, PDA/TiO2 and MoS2/PDA/TiO2 electrodes, respectively. Figure 5. (a) The variation of Ct/C0 of LEV by different conditions under simulated sun light irradiation, Where C0 (mg/L) and Ct (mg/L) are the initial LEV concentration and LEV concentration at time t, all the bias was provided by MFC. (b) The variation of ln(C0/Ct) of LEV by different processes; (c) The variation of Ct/C0 of LEV by PMFC-PEC system and the variation of ln(C0/Ct) of LEV (d) by using MoS2/PDA/TiO2 electrode. 31



Figure 6. Possible catalytic mechanism of this electrode on LEV. Figure 7. EPR spectra of OH radical adduct trapped by DMPO in PMFC-PEC system: (a) 6 h with or without DMPO-OH and (b) DMPO–OH trapped in different synthesis time for MoS2 solution under visible light irradiation; (c) DMPO–OH formed in irradiated aqueous and (d) DMPO–O2 formed in irradiated methanol dispersions for bio-synthesizing MoS2/PDA/TiO2 electrode.

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Scheme 1

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Graphical Table of Contents

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Insight into MoS2 Synthesis with Bio-photoelectrochemical

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