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Recyclable Nanoscale Zero Valent Iron Doped g-C3N4/MoS2 for Efficient Photocatalytic of RhB and Cr(VI) Driven by Visible Light Xiu Wang, Mingzhu Hong, Fuwei Zhang, Zanyong Zhuang, and Yan Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01024 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016

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ACS Sustainable Chemistry & Engineering

Recyclable Nanoscale Zero Valent Iron Doped g-C3N4/MoS2 for Efficient Photocatalytic of RhB and Cr(VI) Driven by Visible Light

Xiu Wanga,b, Mingzhu Honga,b, Fuwei Zhanga,b, Zanyong Zhuanga,b*, Yan Yua,b* a

Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), Fujian Province University, China

b

College of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350108, China *Corresponding author. Fax: +86 591 22866534; E-mail address: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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Abstract Photocatalytic materials for environmental remediation of organic pollutions and heavy metals require not only a strong visible light response and high photocatalytic performance, but also the regeneration and reuse of catalysts. In this work, a ternary hybrid structure material of Nanoscale Zero Valent Iron(Fe0) doped g-C3N4/MoS2 layered structure (GCNFM) was synthesized by a facile strategy.

Compared

with

the

pure

GCN,

GCNM

and

Fe-GCN,

the

photodegradation efficiency of the GCNFM toward the RhB and Cr(VI) under visible light are considerably enhanced to 98.2% for RhB and 91.4% for Cr(VI), respectively. In addition, the reaction rate constants (KRhB and KCr) of GCNFM are much higher than those of GCN, GCNM and Fe-GCN. attributing to that Fe0 and MoS2 composited with GCNM promotes the separation of photogenerated electron-hole pairs. Moreover, with the loading of MoS2 and/or Fe0, the holes could displace the ·O2- as the main reactive oxygen specie in GCN. GCNFM maintains an efficient degradation ability to both the RhB and Cr(VI) after several cycles, in spite that normally Fe0 will be consumed and deactivated with the reduction proceeding as previously reported. It suggests that the photogenerated electrons, in response, can reduce the Fe(III)/Fe(II) to Fe0, inducing a regeneration and reuse of Fe0. We anticipate this work can provide a good example for the design of efficient, visible light driven and recyclable photocatalysts for environmental remediation of both the organic pollution and 2


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heavy metals.

Keywords: Photocatalysis, Recyclable, Nanocomposite, Water treatment, Fe0

Introduction Excessive discharge of industrial wastewaters into the environment has posed a great problem worldwide. Either the organic pollutants (e.g. Phenol or RhB) or the heavy metal ions (e.g. Cr(VI), Cd(II), and Hg(II)) from wastewaters will induced serious harms to the environment and humanity.1-3 Worse still, most of the industrial wastewaters contain both the heavy metals irons and organic pollutants at the same time. Therefore, the rational design of materials to achieve the simultaneous removal of these two pollutants becomes an urgent case. In past decades, much attention has been focused on the photocatalytic strategy for the removal of environmental contaminants,4 and various kinds of semiconductor materials (e.g. TiO2, CdS, WO3, ZnO and Fe2O3) have been developed as active catalysts for the photodegradation of organic pollutants.5-7 Graphitic carbon nitride (g-C3N4), a metal-free semiconductor with band gap of 2.7 eV, has drawn tremendous attention for photocatalytic applications, as it can adsorb the blue light and possess high thermal and chemical stability.8-10 Nevertheless, just like many other semiconductor materials, g-C3N4 (GCN) exhibits weak photocatalytic activity due to the poor light absorption performance and the high recombination probability of photogenerated

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electron-hole pairs.11-14 Generally, the co-catalyst, has been extensively investigated to composite with GCN, providing the reduction or oxidation active sites, trapping the charge carries, and suppressing the recombination of photogenerated electron-hole pairs.15-17 MoS2 composited with GCN, in particular, show enhanced photocatalytic performance toward the organic species.18 Theoretically, the photoreduction techniques also appear to be applicable to the environmentally metal remediation, by which the metal ions could be removed from solutions by reduction to a less toxic lower oxidation state or to the metal which could be recovered.2,19 However, in our preliminary experiments, both the GCN and MoS2-GCN exhibit weak activity of heavy metal ions reduction. Because in the GCN and GCNM system, electrons are even more important in reduce O2 that produces more superoxide radicals (·O2−) than in reduce heavy metal ions. In contrast, recently, nanoscale zero valent iron (Fe0) has received much attentions for treating the wastewaters containing the heavy metals because of its large specific surface area, high reaction activity, and strong reductive power.20 An implication from the above researches is that, a composition of GCNM with the Fe0 might have an ability to achieve the photodegradation of organic pollutants, and as well as a reduction of the heavy metals. In addition, the electrons exciting from the valence band (VB) to the conduction band (CB) of GCN could transfer to Fe nanoparticles because of its excellent electronic conductivity,21 which could promote photogenerated electron-hole pairs separation, thus improve the photocatalytic efficiency.

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Moreover, normally, Fe0 will be consumed and oxidized to Fe(II) and/or Fe(III) during the metals reduction. However, it is possible that, the photogenerated electrons can reduce the Fe(III)/Fe(II) to Fe0, which means a regeneration and reuse of Fe0. Inspired by the above ideas, in this work, a ternary hybrid structure material of Fe0 doped GCN/MoS2 layered structure network (GCNFM) was synthesized by a facile strategy. Our results showed that, as-synthesized GCNFM can achieve a simultaneous and efficient degradation of RhB and reduction of Cr(VI) under visible light irradiation. The regeneration and reusability of GCNFM were examined. Finally, combining with the research of reactive oxygen species monitored by EPR (electron paramagnetic resonance) technique, a possible photocatalytic mechanism of GCNFM was discussed.

Experimental section Materials All chemical, including the urea (CON2H4), ammonium molybdate ((NH4)6Mo7O24·4H2O), thiourea (CN2H4S), ferricchloride (FeCl3·6H2O), sodium borohydride (NaBH4),

potassium dichromate (K2Cr2O7, 99.9%

purity),

rhodamine B (RhB), PEDOTPSS, sodium nitrate (NaNO3), t-BuOH, EDTA-2Na, Benzoquinone (BQ) and ethanol are of analytical grade and were purchased from Xilong Chemical Co., Ltd (Guang dong, China). All chemicals were used as received. Preparation

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The g-C3N4 (GCN) nanosheets were prepared by heating 5.0 g of urea in an alumina combustion boat under nitrogen gas flow (10 ml/min) at 550 oC for 4 h, following by heating at 550 oC for another 4 h in air.22,23 The obtained product was collected and ground into powder. After that, GCN monolayer films were synthesized by using the chemical exfoliation method. As-prepared GCN (3 g) was mixed with 100 mL of methanol in a 250 mL beaker under vigorous stirring for 3 h at room temperature. Then, the mixture was slowly injected into 500 mL of deionized water under ultrasonic treatment. The resulting suspension was centrifuged at 5000 rpm for 5 min to remove the unexfoliated GCN, and then, the final suspension dried at 80 oC in air overnight. Finally, the resulting products were GCN monolayer films. Layered MoS2 modified GCN nanoparticles (GCNM) were synthesized by a hydrothermal method: As-prepared GCN (1 g) was dispersed in 30 mL of deionized water with 0.1236 g of ammonium molybdate ((NH4)6Mo7O24·4H2O) and 0.2665 g of thiourea under sonication.24 Then, the mixture was transferred into a Teflon-lined autoclave and treated at 210 oC for 24 h. The products were centrifuged, and then dried in an oven at 85 oC for 24 h. To verify a formation of MoS2 in such method, the pure MoS2 was also prepared by the same conditions without in the absence of GCN. After that, as-prepared GCNM monolayer films (2 g) was added to 100 mL of 0.02 M FeCl3·6H2O solution under stirring. The suspension was sonicated for 10 min, followed by stirring period at room temperature for 30 min. After that, 100 mL of 0.04 M freshly prepared NaBH4

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solution was quickly added to the suspension to reduce the Fe3+ under nitrogen gas flow, followed by continuously stirring for 30 min. To remove excess NaBH4 and unbound iron nanoparticles,25 the obtained nanohybrid materials were centrifuged, and washed thoroughly with deionized water. The obtained samples were Fe0 doped GCN integrated with MoS2 film powders (GCNFM). The Fe0 doped GCN was also prepared by the same conditions in the absence of MoS2. Characterization The crystal structure of samples was determined with Panalytical X’pert MPD X-ray Diffractometer (XRD). Fourier transform infrared (FTIR) spectra of all the catalysts were recorded by using a TJ270-30A infrared spectrophotometer (Tianjin,China). The morphology of the samples was examined by filed emission scanning electron microscope (SEM, Philips XL30). UV-vis diffuse reflection spectroscopy

(DRS)

was

performed

on

a

Hitachi

UV-3010

UV-vis

spectrophotometer. X-ray photoelectron spectrometer (XPS, PHI 5000 Versa Probe)

was

exerted

to

the

surface

analysis

of

the

GCNFM.

The

photoluminescence (PL) spectra and the time-resolved fluorescence decay spectra of the photocatalysts were measured on the Varian Cary Eclipse spectrometer with excitation wavelength of 325 nm. The transient photocurrents were measured with an electrochemical analyzer (CHI660B, CHI Shanghai, Inc.). EPR spectra were recorded at room temperature by using a Bruker A-300-EPR X-band

spectrometer.

BET

specific

surface

areas

calculated

by

the

Brunauer-Emmett-Teller (BET) equation in the relative pressure range (P/P0) of

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0-1. Photoelectrochemical measurement The working electrode was prepared as follows: 10 mg of the sample powder was mixed with 20 uL of PEDOTPSS, 100 uL of water under ultrasonic treatment. As-obtained mud was uniformly dropped onto a substrate of 1×1 cm2 ITO glass, which was dried at 100 oC for 2 h in N2 atmosphere. The photocurrent measurements were completed in a standard three-electrode system (using the prepared electrode, Pt wire, and Ag/AgCl as the working electrode, counter electrode, the reference electrode, respectively).18 The light source was a 300 W Xe lamp, and the electrolyte was a 0.3 M Na2SO4 solution. Photocatalytic activity measurement The photocatalytic activity of the photocatalyst was performed by the degradation of RhB and Cr(VI), using a Xe lamp (500 W) equipped with a UV cut off filter (λ> 420 nm) as the irradiation source. In a typical photocatalytic experiment, Photocatalyst (30 mg) was totally dispersed in a mixed aqueous solution of RhB (50 mL 20 ppm) and K2Cr2O7 (50 mL 20 ppm), and then the suspension was stirred for 30 min in the dark to ensure absorption-desorption equilibrium, after which the reaction suspension was irradiated for 120 min under visible light. At certain time intervals, 3.0 mL of the suspension was withdrawn, and centrifuged to remove the particles, the concentration of RhB and Cr(VI) were analyzed by recording the absorbance using a UV-vis spectrophotometer.

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Results and discussion Characterization of samples

Figure 1. a) XRD patterns and b) FTIR spectra of MoS2, GCN, GCNM, Fe-GCN and GCNFM.

X-ray diffraction studies were used to investigate the structure of as-synthesized photocatalysts. Fig. 1a shows the XRD patterns of MoS2, GCN, GCNM, Fe-GCN and GCNFM. For the pure polymeric GCN, the two distinct diffraction peaks at 13.2º and 27.4º are in good agreement with GCN (JCPDS 87-1526) as reported,8,13 corresponding to (100) and (002) diffraction of the graphitic materials, respectively. For the pure MoS2, all the diffraction peaks of Mo-based samples can well be indexed to be the hexagonal phase of MoS2 (JCPDS 37-1492).26 However, no obvious diffraction peaks of MoS2 were observed from the XRD patterns of GCNM. It can be attributed to the small amount of MoS2 on GCN, confirmed by EDS and XPS analysis in the following. For Fe-GCN or GCNFM sample, a weak peak at 44.6º was observed, in line with the diffraction peak from Fe0.27 No signals of iron oxides were observed, indicating

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that the content of other valence iron is below the detection limit. In addition, the diffraction peaks assigned to GCN among the three samples remained unchanged, suggesting that the structure of g-C3N4 does not change before and after the loading of MoS2 and Fe. FTIR spectra were further used to analyze the functional groups on MoS2, GCN, GCNM, Fe-GCN and GCNFM (Fig. 1b). For the sample of GCN, there exist a broad peak at 3000-3500 cm-1 assigned to the stretching vibration of O-H bands and N-H components.28 The GCN spectrum contains several major bands between 1200-1600 cm-1, which can be assign to the stretching vibration of aromatic C-N heterocycles containing either trigonal N-(C)3 or bridging C-NH-C units, suggesting the formation of C-N-C bonds. In addition, the peak at 806 cm-1 could be ascribed to the breathing mode of s-triazine ring8. It is worth noting that, GCN, GCNM, Fe-GCN and GCNFM samples had similar adsorption bands, showing the main characteristic peaks of GCN. No new functional groups were formed in the GCNFM hybrids, which indicate that the structure of GCN do not been seriously destroyed after the introduction of MoS2 and direct borohydride reduction of ferric. These results were in good accordance with the XRD analysis. The morphologies of the samples were further characterized by the SEM observations. Fig. 2 shows the SEM images of GCN, MoS2, GCNM and GCNFM, respectively. As shown in Fig. 2a, the pure GCN is composed of irregular thin crumpled nanosheets with large size. Differently, the pure MoS2 has a sheet-like

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structure with a much smaller size as illustrated in Fig. 2b. Fig. 2c shows the SEM image of GCNFM, wherein Fe/MoS2 co-catalysts are present as irregular nanoparticles loading on the GCN surface. The corresponding EDS spectrum analysis of GCNFM is displayed in Fig. 2d. The GCNFM sample consists of C, N, Mo, S and Fe elements, well agreeing with the chemical composition of GCNFM. In addition, the evaluated contents of MoS2 and Fe is 11.2 wt% and 5.6 wt%, respectively. The corresponding elemental mapping analysis of C, N, S Mo and Fe are shown in Fig. 2e-j. The results clearly show that, the Fe and MoS2 are well dispersed on the surface of GCN.

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(d)

Figure 2. SEM images of a) GCN, b) MoS2 and c) GCNFM; d) EDS analysis of GCNFM; e) a low resolution image of GCNFM and f-i) corresponding elemental mapping images of C, N, S, Mo and Fe in plane e.

Fig. 3a-b shows the UV-vis diffuse reflection spectra of as-prepared GCN, GCNM, Fe-GCN and GCNFM samples. It can be found that, the pure GCN has an absorption edge at about 460 nm, corresponding to band gap at 2.7 eV as previously reported.9,13 After the loading of MoS2, the GCNM sample shows the similar absorption edge with slightly enhanced absorption to the visible region. 12


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As for Fe-GCN, the absorption edge shows a slight red shift compared with the GCNM. Once both the MoS2 and Fe0 were introduced, the absorption edge of GCNFM samples at 471 nm (corresponding to a band gap at 2.63 eV) has a red shift by 0.07 eV compared to that of the pure GCN. Namely, the light absorption of the GCNFM composite significantly moves to visible light range, particularly after the loading of Fe0. The results indicate that, the composite samples may be able to absorb more visible light and thus exhibit improved catalytic activity.

Figure 3. a) UV-vis diffuse reflection spectra (DRS); b) plots of (F(R)E)1/2 versus energy (E); c) PL emission spectra; and d) the transient photocurrent responses of pure GCN, GCNM, Fe-GCN and GCNFM samples, respectively. Each sample were measured for five on-off cycles of intermittent irradiation under visible light

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The photoluminescence (PL) emission spectrum is also provided to verify the transfer behavior of charge carriers in pure GCN, GCNM, Fe-GCN and GCNFM samples (Fig. 3c). Theoretically, the higher PL intensity means more efficient carriers participate in the recombination, on the contrary, the lower PL intensity means more carriers participate in photocatalytic process.14,15 The PL intensity of the samples follows an order of GCN > GCNM > Fe-GCN > GCNFM. It indicates that, the recombination of charge carriers of GCNFM will be suppressed by using MoS2 and/or Fe0. The transient photocurrent responses analysis further confirms the above PL analysis. As shown in Fig. 3d, the transient photocurrent responses of the samples follow an order of GCN < GCNM < Fe-GCN < GCNFM. In particular, under the same conditions, the photocurrent response of GCNFM is greatly improved, about 5 times as high as that of GCN. These results confirmed the superior charge transfer and recombination inhibition in the GCNFM composite. The chemical composition and chemical states of GCNFM was further investigated by X-ray photoelectron spectrum (XPS). As shown in Fig. 4, the chemical binding energies at approximately 288.2, 398.8, 229.1, 161.8, 711.5 and 532.1 eV for C 1s, N 1s, Mo 3d, S 2p, Fe 2p, and O 1s,29,30 indicating the presence of C, N, Mo, S, Fe and O in the GCNFM, respectively. High resolution spectra of C 1s, N 1s, Mo 3d, S 2p and Fe 2p are shown in Fig. 4b-f. The C 1s spectra (Fig. 4b) could be fitted to three peaks at 284.6, 285.4 and 288.2 eV, which are attributed to pure graphitic sites in the carbon nitride matrix C-C bonds, sp2 hybridized

14


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carbon atoms bonded to N in an aromatic ring C-NH2 and sp2 hybridized C atoms in the triazine, respectively. The N 1s XPS signal in Fig. 4c could be deconvoluted into three peaks located at 398.5, 399.1, and 400.5 eV, which could be assigned to sp2 C-N=C, sp3 N-C3 and C-NH2 functional group, respectively.29 Fig. 4d presents the high-resolution XPS spectra for Mo 3d. The two peaks at approximately 229.1 and 232.8 eV can be assigned to the binding energies of Mo 3d5/2 and Mo 3d3/2, indicating the existence Mo4+ species in MoS2.30 The S 2p peaks (Fig. 4e) located at 161.6 and 163.0 eV are in accordance with S 2p3/2 and S 2p1/2, showing the typical binding energies of S2- of MoS2.31 Therefore, the XPS results confirm that the MoS2 have been successfully introduced onto the GCN surface. In addition, the Fe 2p spectra (Fig. 4f ) have the typical peaks at 706.4 eV (Fe 2p3/2) and 720.2 eV (Fe 2p1/2), while no signals for iron oxides were found. This indicated that, the iron presenting on the GCNM surface is mainly in zero valent state, which well agrees with the XRD analysis of the GCNFM. As shown in Fig. S1 in Supporting Information, the specific surface area of each sample (GCN, GCNM, Fe-GCN and GCNFM) is determined by the N2 adsorption/desorption analysis. The hysteresis loops for samples all followed the H3 IV type. The specific surface area of GCN, GCNM, Fe-GCN and GCNFM is about 28.79, 39.38, 28.63 and 37.5 m2 g-1, respectively. An increasing of the BET specific surface areas of GCNM can be due to the incorporation of layered MoS2 nanoparticles, which might bring more active adsorption/catalysis sites.

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Figure 4. a) X-ray photoelectron spectroscopy (XPS) survey spectra; and high resolution spectra of b) C 1s, c) N 1s, d) Mo 3d, e) S 2p and f ) Fe 2p of the GCNFM.

Photocatalytic activity The simultaneous photodegradation of Cr(VI) (with UV adsorption peak at 356 nm) and RhB (with UV adsorption peak at 557 nm) was carried out to evaluate the photocatalytic activities of the samples. Fig. 5a-b and Fig. S2 in Supporting Information shows the reduction of Cr(VI) and decolorization of 16


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RhB as a function of time by different kinds of photocatalysts (GCN, GCNM, Fe-GCN and GCNFM) under visible light irradiation, wherein only 59.7%, 81.4% and 84.1% of RhB molecules could be decomposed within 120 min, respectively. Inspiringly, the GCNFM displays superior photocatalytic activity, with 98.2% of RhB molecules were degraded in just 120 min. It was also found that, the photodegradation efficiency of Cr(VI) can fast increase from 11.4% for GCN, to 55.6% for GCNM, 83.2% for Fe-GCN to 91.4% for GCNFM, respectively. In addition, the blue shift of the maximum absorption in the UV-vis spectra demonstrates that the de-ethylation reaction of Rh B exists in the degradation process. Fig. 5a-b show the relationship between C/C0～t of GCN, GCNM, Fe-GCN, GCNFM and sample without any catalyst, where C and C0 is the concentration of Cr(VI) (or RhB) at time t and t0, respectively. Our results show that, the sample without any catalyst basically did no change with time under the visible light. In other words, the direct photocatalysis of RhB and Cr(VI) can be ignored in the absence of photocatalysts. Before the photocatalytic reaction, we also test the efficiency of materials in the dark. As shown in Fig. 5a, the photocatalytic degradation effect of RhB with all photocatalysts in the dark can basically be negligible. It indicates that, the photocatalysts themselves have no or much weak ability toward the degradation of RhB. However, in darkness experiments, the reduction ability of Cr(VI) of Fe-GCN and GCNFM is slightly higher than that of GCN and GCNM (Fig. 5b), as the Fe0 have a better reducibility and easily

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reduce Cr(VI) to Cr(III). Interestingly, under the visible light, the degradation of GCNFM toward RhB and Cr(VI) is considerably enhanced as compared with GCN, Fe-GCN and GCNM nanocomposite. An implication from these results is that, the loading of Fe0 and MoS2 both improve the photocatalytic efficiency of GCN, especially for the reduction of the Cr(VI). Quantitative investigation of the reaction kinetics of Cr(VI) and RhB photodegradation by the as-prepared photocatalysts was also performed (Fig. 5c-d). The experimental data were fitted by the pseudo-first-order kinetic model ln(C0/C) = Kt, where the value of rate constant K is equal to the corresponding slope of the fitting line. Linear relationships were obtained as depicted in Fig. 5c-d, indicating that the Cr(VI) and RhB photodegradation process can be fitted by the pseudo-first-order model well. As shown in Fig. 5e, GCNFM has the highest degradation rate constant KRhB (0.032 min-1), about 4.6, 2.5 and 2.0 times higher than that of pure GCN (0.007 min-1), GCNM (0.013 min-1) and Fe-GCN ( 0.016 min-1), respectively. Moreover, the value of rate constant KCr of GCNFM is 0.021 min-1, about 20, 2.5 and 1.5 times higher than that of pure GCN (0.001 min-1), GCNM (0.006 min-1) and Fe-GCN (0.014 min-1), respectively. In short, the GCNFM exhibits a dramatically enhancement on photocatalytic activity upon identical conditions than GCN and GCNM.

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Figure 5. a-b) C/C0 and c-d) kinetics for degradation of RhB and Cr(VI) of the as-prepared photocatalysts and without catalyst; and the degradation rate constant for e) the mixture systems with RhB and Cr(VI) and f ) for a single pollutant system containing only the RhB (or Cr).

To further probe the underlying mechanism, the photocatalytic activity of RhB (or Cr) treated alone by GCNFM was evaluated (Fig. S3 in Supporting Information). As presented in Fig. 5e-f, the KRhB in absence of Cr(VI) is about 19



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0.017 min-1, much smaller than 0.032 min-1 in RhB-Cr(VI) system. Similarly, the KCr in absence of RhB is about 0.013 min-1, smaller than 0.021 min-1 in RhB-Cr(VI) system. In other words, the presence of Cr(VI) (or RhB) clearly promote the degradation rate of RhB (or Cr). The EPR technique was also used to detect the radicals in reaction systems. As shown in Fig. 6a, in GCN under visible light irradiation, EPR signals assigned to superoxide radical (·O2−) was observed, and no obvious signals of hydroxyl radical (·OH) can be found. An existence of ·O2− confirms the results from previous studies that, the photogenerated electrons in CB of g-C3N4 (-1.19 eV vs. NHE) is negative enough to reduce the O2 to generate the ·O2− (O2/·O2−= −0.33 eV vs. NHE)32. Since part of the electrons will be consumed to form the ·O2-, it leads to a diminished ability of electrons to reduce the Cr(VI). After the loading of MoS2 and Fe0, the ·O2- signal of GCNM, Fe-GCN and GCNFM become weaken and even disappeared. The phenomenon was further verified by the active species trapping experiments (by using t-BuOH, EDTA-2Na and benzoquinone (BQ) help to capture the ·OH, holes and ·O2−, respectively). As shown in Fig.6b-e, the ·O2− could be the main oxidative species for GCN, in line with the EPR analysis. After the loading of MoS2 and/or Fe0, it shows that, the hole replacing of ·O2- becomes the main reactive oxygen specie in GCNM, Fe-GCN and GCNMF. All these results demonstrate that, a transferring of electron to MoS2 and Fe0 not only help to suppress the recombination of electron-hole pair, but also suppress the consumption of the electrons to form

20


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the ·O2-. The both effect could promote the photocatalytic activity of GCNFM.

Figure 6. a) DMPO spin-trapping EPR spectra of all samples methanol dispersion for DMPO-O2− and aqueous dispersion for DMPO-OH, the photocatalytic degradation efficiency of Rh B in b) GCN, c) GCNM, d) Fe-GCN and e) GCNFM system with various scavengers.

Regeneration and recycle of GCNFM In order to achieve the application of nanoadsorbents, the recycle of adsorbents should be considered. As shown in Fig. 7, the adsorption/desorption cycles were repeated five times. In the process, the catalyst is recycled and washed by ethanol, then is dried in vacuum. Firstly, the degradation efficiency of GCNFM toward RhB will only be cut by 2% after five cycles, indicating that the GCNFM maintain an efficient ability to the degradation of organic species. In comparison, the degradation efficiency of Cr(VI) are dropping from 91.4 % to 73.1 % in the first cycle. A slight decrease in the adsorption capacity might be mainly because of the loss of Fe0 during reducing Cr(VI), wherein a part of Fe0 is released into the solution in the form of Fe(II) and Fe(III). However, what should 21



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be noted is that, generally, Fe0 will be consumed, and hard to be recycled. However, in this work, there still exist 73.1 % after the five cycles, indicating the partial recovery of the iron in this work. It can also be speculated that, a possible way to suppress the loss of metal Fe is by adding a small amount of ferrous ions into the solution during the photoredction of Cr6+. We will try to explore more details in the future work.

Figure 7. Five degradation cycles of RhB and Cr(VI) by GCNFM under visible light irradiation.

The underlying mechanism of photocatalytic As shown in Fig. 8, the underlying mechanism of the photodegradation for RhB and Cr(VI) by GCNFM was proposed. It has been held that, under visible light irradiation, the electrons (e-) will be excited from the VB to the CB of GCN, producing the holes (h+) in the VB. Normally, these charge carriers are likely to recombine and only a fraction of electrons could participate in the photocatalytic reaction, resulting in a low activity of catalyst.33 22


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Figure 8. Schematic drawing illustrating synthetic route and the mechanism of charge separation and photocatalytic process over GCNFM photocatalysts under visible light irradiation.

The role of MoS2 When GCN was composited with MoS2 and irradiated with the visible light, the electrons can be excited from the CBs of both MoS2 and GCN monolayers (Eqs. 1). Generally, the excited electrons in GCN could transfer to MoS2 to promote the separation of electron-hole pairs, since the CB level of GCN is more negative than that of MoS2.34 What should be noted is that, as indicated by Wang et al,35 although the band alignment promotes the separation of electron-hole pairs, a built-in polarized field between g-C3N4 and MoS2 sheets might suppress the carriers separation. Our experiments (Fig. 5) show that, the photocatalysis activities of GCNM are higher than the pure GCN. It indicates that, the band alignment effect is in dominant place to promote the carriers separation and thus lead to the enhanced activity of GCNM.

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g - C3 N 4 /MoS2 + hv → e -CB + h +VB

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(1)

The role of Fe0 When GCN was composited with Fe0, as indicated by the PL and the transient photocurrent responses analysis (Fig. 3c-d), the electrons could also transfer to Fe0 nanoparticles because of the excellent electronic conductivity of Fe0. In addition, as shown in Eqs.2-3, the Fe(II) or Fe(III) (producing by the partial oxidation of Fe0) could also capture the electron. The consumption of the electron during the redox cycles of Fe0 further helps to accelerate the separation of electron-hole pairs.

Fe(III) + e -CB → Fe(II)

(2)

Fe(II) + 2e -CB → Fe 0

(3)

The reaction potentials and the energy level diagrams As aforementioned above, the photogenerated electrons in CB of g-C3N4 was negative enough to reduce the O2 to generate ·O2− (Eqs. 4). In this case, the RhB can be oxidized by photogenerated holes (h+) holes and/or ·O2- (Eqs. 5). However, the consumption of electrons in this process will diminish the ability of Cr(VI) degradation. O 2 + e - → ⋅O 2(4)

Rh B + h +VB /⋅ O 2- → simple molecules → CO 2 + H 2O (5) 24


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As shown in Fig. 6, a transferring of electron to MoS2 and Fe0 in GCNFM not only help to suppress the recombination of electron-hole pair, but also suppress the consumption of the electrons to form the ·O2-. As aforementioned, the increased separation efficiency of charge carriers is the main reason for the enhanced performance of GCNM. According to Eqs. (6), the electrons in CB of MoS2 (-0.2 eV vs. NHE) was negative enough to reduce the Cr(VI) to Cr(III) (E0 = 1.35 V vs. NHE). Meanwhile, the reduction potential of Cr2O72-/Cr(III) is more positive than Fe2+/Fe(s) (E0=-0.44 vs. NHE) and Fe3+/Fe2+ (E0=0.77 V vs. NHE). Hence, Fe0 and Fe(II) also could easily reduce Cr(VI) to Cr(III) according to Eqs. (7-8).

3e -CB + Cr(VI) → Cr(III)

(6)

3Fe 0 + 2Cr(VI) → 3Fe(II) + 2Cr(III)

(7)

3Fe(II) + Cr(VI) → 3Fe(III) + Cr(III)

(8)

The synergy effect between the degradation of RhB and Cr The control experiments in Fig. 5f also indicates that, there exist the synergistic effect on the reduction of Cr(VI) and the oxidation of RhB. The consumption of the electrons (or holes) will help to accelerate the separation of electron-hole pairs, suppressing the recombination of carriers.

Conclusion In summary, a ternary hybrid structure material of Fe0 doped GCN/MoS2 layered structure network (GCNFM) was successfully synthesized by a facile strategy.

Compared

with

the

pure

GCN,

GCNM

25


and

Fe-GCN,

the


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photodegradation ability of the GCNFM toward the RhB and Cr(VI) under visible light irradiation are considerably enhanced, wherein 98.2% of RhB and 91.4% of Cr(VI) can be removed under sunlight irradiation for 120 min. In particular, GCNFM have the highest value of reaction rate constant KRhB, which is about 4.6, 2.5 and 2.0 times higher than that of pure GCN, GCNM and Fe-GCN, respectively. KCr of GCNFM is 0.021 min-1, which is about 20, 2.5 and 1.5 times higher than that of pure GCN (0.001 min-1), GCNM (0.006 min-1) and Fe-GCN (0.014 min-1), respectively. Moreover, GCNFM still maintain an efficient degradation ability toward both the RhB and Cr(VI). It can be attributed that, Fe0 composited with GCNM could promote photogenerated electron-hole pairs separation to improve the photocatalytic efficiency, while the photogenerated electrons in response can reduce the Fe(III)/Fe(II) to Fe0. Moreover, with the loading of MoS2 and/or Fe0, the holes could displace the ·O2- as the main reactive oxygen specie in GCN. A regeneration and reuse of Fe0 can help to support long-term application of GCWFM in water treatment. The findings in this work can provide a good example for the design of efficient, visible light driven and recyclable photocatalysts for environmental remediation of both the organic pollution and heavy metals.

Acknowledgments The authors gratefully acknowledge the financially support from the National Natural Science Foundation of China (NO. 51102047, 51472050 and 51402295).

Supporting Information 26


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The Supporting Information is available free of charge on the ACS Publications website at DOI: ****, including the specific surface area analysis, and the UV-vis spectra showing the degradation of Cr(VI) and RhB under different conditions.

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Recyclable Nanoscale Zero Valent Iron Doped g-C3N4/MoS2 for Efficient Photocatalytic of RhB and Cr(VI) Driven by Visible Light

Xiu Wanga,b, Mingzhu Honga,b, Fuwei Zhanga,b, Zanyong Zhuanga,b*, Yan Yua,b* a

Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), Fujian Province University, China

b

College of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350108, China *Corresponding author. Fax: +86 591 22866534; E-mail address:[email protected]; [email protected]

For Table of Contents Use Only

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The recycle and reuse of Fe0 in GCNFM under visible light irradiation supports the sustainable and long-term application of catalyst.

33


MoS2 for

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