Recyclable Nanoscale Zero Valent Iron Doped g ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Recyclable Nanoscale Zero Valent Iron Doped g‑C3N4/MoS2 for Efficient Photocatalysis of RhB and Cr(VI) Driven by Visible Light Xiu Wang,†,‡ Mingzhu Hong,†,‡ Fuwei Zhang,†,‡ Zanyong Zhuang,*,†,‡ and Yan Yu*,†,‡ †

Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), Fujian Province University, Fuzhou, Fujian Province 350108, China ‡ College of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350108, China S Supporting Information *

ABSTRACT: Photocatalytic materials for environmental remediation of organic pollution 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 a 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 is 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, which is attributed to the fact that Fe0 and MoS2 composited with GCNM promote 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 species in GCN. GCNFM maintains an efficient degradation ability to both RhB and Cr(VI) after several cycles, in spite of the fact that normally Fe0 will be consumed and deactivated with the reduction proceeding as previously reported. This suggests that the photogenerated electrons, in response, can reduce the Fe(III)/Fe(II) to Fe0, inducing 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 organic pollution and heavy metals. KEYWORDS: Photocatalysis, Recyclable, Nanocomposite, Water treatment, Fe0

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probability of photogenerated electron−hole pairs.11−14 Generally, the cocatalyst 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, showed 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 reducing O2 that produces more superoxide radicals (•O2−) than in reducing heavy metal ions. In contrast, recently, nanoscale zerovalent iron (Fe0) has received much attention for treating wastewaters containing heavy metals

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 harm to the environment and humanity.1−3 Worse still, most of the industrial wastewaters contain both the heavy metal 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 a band gap of 2.7 eV, has drawn tremendous attention for photocatalytic applications, as it can adsorb blue light and possess high thermal and chemical stability.8−10 Nevertheless, just like many other semiconductor materials, gC3N4 (GCN) exhibits weak photocatalytic activity due to the poor light absorption performance and the high recombination © 2016 American Chemical Society

Received: May 11, 2016 Revised: May 23, 2016 Published: June 9, 2016 4055

DOI: 10.1021/acssuschemeng.6b01024 ACS Sustainable Chem. Eng. 2016, 4, 4055−4063

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) XRD patterns and (b) FTIR spectra of MoS2, GCN, GCNM, Fe-GCN, and GCNFM. 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 °C for 24 h. The products were centrifuged, and then dried in an oven at 85 °C for 24 h. To verify formation of MoS2 by such a method, the pure MoS2 was also prepared under the same conditions in the absence of GCN. After that, as-prepared GCNM monolayer films (2 g) were added to 100 mL of 0.02 M FeCl3·6H2O solution under stirring. The suspension was sonicated for 10 min, followed by stirring at room temperature for 30 min. After that, 100 mL of 0.04 M freshly prepared NaBH4 solution was quickly added to the suspension to reduce the Fe3+ under nitrogen gas flow, followed by continuous 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 structures of samples were determined with a Panalytical X’pert MPD X-ray diffractometer (XRD). The 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 with a field 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 spectrometry (XPS, PHI 5000 Versa Probe) was applied 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 an 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-300EPR X-band spectrometer. BET specific surface areas were calculated using the Brunauer−Emmett−Teller (BET) equation in the relative pressure range (P/P0) 0−1. Photoelectrochemical measurement. The working electrode was prepared as follows: 10 mg of the sample powder was mixed with 20 μL of PEDOTPSS and 100 μL of water under ultrasonic treatment. Asobtained mud was uniformly dropped onto a substrate of 1 × 1 cm2 ITO glass, which was dried at 100 °C for 2 h in a N2 atmosphere. The photocurrent measurements were completed in a standard threeelectrode system (using the prepared electrode, Pt wire, and Ag/AgCl as the working electrode, counter electrode, and reference electrode,

because of its large specific surface area, high reaction activity, and strong reductive power.20 An implication from the above research is that the composition of GCNM with Fe0 might have the ability to achieve the photodegradation of organic pollutants, and also the reduction of 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 improving the photocatalytic efficiency. 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 a 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 the EPR (electron paramagnetic resonance) technique, a possible photocatalytic mechanism of GCNFM was discussed.

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EXPERIMENTAL SECTION

Materials. All chemicals, 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. 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 °C for 4 h, followed by heating at 550 °C for another 4 h in air.22,23 The obtained product was collected and ground into a 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 was dried at 80 °C 4056


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ACS Sustainable Chemistry & Engineering 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, and 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. X-ray diffraction studies were used to investigate the structure of as-synthesized photocatalysts. Figure 1a shows the XRD patterns of MoS2, GCN, GCNM, FeGCN, and GCNFM. For the pure polymeric GCN, the two distinct diffraction peaks at 13.2° and 27.4° 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. This can be attributed to the small amount of MoS2 on GCN, confirmed by EDS and XPS analysis in the following. For the Fe-GCN or GCNFM sample, a weak peak at 44.6° was observed, in line with the diffraction peak from Fe0.27 No signals of iron oxides were observed, indicating 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 gC3N4 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 (Figure 1b). For the sample of GCN, there exists 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 and 1600 cm−1, which can be assigned 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 the s-triazine ring.8 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 indicates that the structure of GCN has 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. Figure 2 shows the SEM images of GCN, MoS2, GCNM, and GCNFM, respectively. As shown in Figure 2a, the pure GCN is composed of irregular thin crumpled nanosheets with large size. Differently, the pure MoS2 has a sheet-like structure with a much smaller size as illustrated in Figure 2b. Figure 2c shows the SEM image of GCNFM, wherein Fe/MoS2 cocatalysts are present as irregular nanoparticles loading on the GCN surface. The corresponding EDS spectrum

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

analysis of GCNFM is displayed in Figure 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 are 11.2 and 5.6 wt %, respectively. The corresponding elemental mapping analyses of C, N, S Mo, and Fe are shown in Figure 2e−j. The results clearly show that the Fe and MoS2 are well dispersed on the surface of GCN. Figure 3a−b shows the UV−vis diffuse reflection spectra of asprepared 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 a band gap at 2.7 eV, as previously reported.9,13 After the loading of MoS2, the GCNM sample shows a similar absorption edge with slightly enhanced absorption to the visible region. 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 the 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. 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 (Figure 3c). Theoretically, the higher PL intensity means more efficient carriers participate in the recombination; on the contrary, the 4057


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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 was measured for five on−off cycles of intermittent irradiation under visible light.

lower PL intensity means more carriers participate in the photocatalytic process.14,15 The PL intensity of the samples follows the order GCN > GCNM > Fe-GCN > GCNFM. This 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 Figure 3d, the transient photocurrent responses of the samples follow the order GCN < GCNM < FeGCN < 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 were further investigated by X-ray photoelectron spectroscopy (XPS). As shown in Figure 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 1s29,30 indicate 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 Figure 4b−f. The C 1s spectra (Figure 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 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 Figure 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 groups, respectively.29 Figure 4d presents the highresolution 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 of Mo4+ species in MoS2.30 The S 2p peaks (Figure 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 (Figure 4f) have 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 the zerovalent state, which well agrees with the XRD analysis of the GCNFM. As shown in Figure S1 in the 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. 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. Figure 5a−b and Figure S2 in Supporting Information show the reduction of Cr(VI) and decolorization of 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 degraded in just 120 min. It was also found that the photodegradation efficiency of Cr(VI) can quickly increase from 11.4% for GCN, to 55.6% for GCNM, to 83.2% for Fe-GCN, and to 91.4% for GCNFM, respectively. In addition, the blue shift of the maximum absorption in the UV−vis spectra demonstrates that the deethylation reaction of Rh B exists in the degradation process. 4058


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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.

fitted by the pseudo-first-order kinetic model ln(C0/C) = Kt, where the value of the rate constant K is equal to the corresponding slope of the fitting line. Linear relationships were obtained as depicted in Figure 5c−d, indicating that the Cr(VI) and RhB photodegradation process can be fitted by the pseudo-first-order model well. As shown in Figure 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 the 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 FeGCN (0.014 min−1), respectively. In short, the GCNFM exhibits a dramatic enhancement in photocatalytic activity compared to GCN and GCNM under identical conditions. To further probe the underlying mechanism, the photocatalytic activity of RhB (or Cr) treated alone by GCNFM was evaluated (Figure S3 in the Supporting Information). As presented in Figure 5e−f, the KRhB in the absence of Cr(VI) is about 0.017 min−1, much smaller than 0.032 min−1 in the RhBCr(VI) system. Similarly, the KCr in the absence of RhB is about 0.013 min−1, smaller than 0.021 min−1 in the RhB-Cr(VI) system. In other words, the presence of Cr(VI) (or RhB) clearly promotes the degradation rate of RhB (or Cr).

Figure 5a−b show the relationship between C/C0 ∼ t of GCN, GCNM, Fe-GCN, GCNFM, and sample without any catalyst, where C and C0 are the concentration of Cr(VI) (or RhB) at time t and t0, respectively. Our results show that the sample without any catalyst basically did not change with time under 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 Figure 5a, the photocatalytic degradation effect of RhB with all photocatalysts in the dark can basically be negligible. This indicates that the photocatalysts themselves have no or very 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 (Figure 5b), as the Fe0 has a better reducibility and easily reduces Cr(VI) to Cr(III). Interestingly, under 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 (Figure 5c−d). The experimental data were 4059


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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).

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

from previous studies that the photogenerated electrons in CB of g-C3N4 (−1.19 eV vs NHE) are 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−, this leads to a diminished ability of electrons to reduce the Cr(VI).

The EPR technique was also used to detect the radicals in reaction systems. As shown in Figure 6a, in GCN under visible light irradiation, EPR signals assigned to superoxide radical (•O2−) were observed, and no obvious signals of hydroxyl radical (•OH) can be found. The existence of •O2− confirms the results 4060


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ACS Sustainable Chemistry & Engineering After the loading of MoS2 and Fe0, the •O2− signals of GCNM, Fe-GCN, and GCNFM become weakened and even disappear. The phenomenon was further verified by the active species trapping experiments (by using t-BuOH, EDTA-2Na, and benzoquinone (BQ) to help to capture the •OH, holes, and • 2− O , respectively). As shown in Figure 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 species in GCNM, Fe-GCN, and GCNMF. All these results demonstrate that a transferring of electrons to MoS2 and Fe0 not only helps to suppress the recombination of the electron−hole pair, but also suppresses the consumption of the electrons to form the •O2−. Both effects could promote the photocatalytic activity of GCNFM. Regeneration and recycle of GCNFM. In order to achieve the application of nanoadsorbents, the recycle of adsorbents should be considered. As shown in Figure 7, the adsorption/

GCNFM maintains an efficient ability to degrade the organic species. In comparison, the degradation efficiency of Cr(VI) drops 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 be noted is that, generally, Fe0 will be consumed, and will be 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. Underlying mechanismof photodegradation. As shown in Figure 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 Role of MoS2. When GCN was composited with MoS2 and irradiated with 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 (Figure 5) show that the photocatalysis activities of GCNM are higher than the pure GCN. This indicates that the band alignment is the dominant effect to promote the carriers separation and thus lead to the enhanced activity of GCNM.

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

g‐C3N4 /MoS2 + hv → e−CB + h+VB

desorption cycles were repeated five times. In the process, the catalyst is recycled and washed with ethanol, and then is dried in vacuum. First, the degradation efficiency of GCNFM toward RhB will only be cut by 2% after five cycles, indicating that the

0

(1) 0

Role of Fe . When GCN was composited with Fe , as indicated by the PL and the transient photocurrent responses analysis (Figure 3c−d), the electrons could also transfer to Fe0 nanoparticles because of the excellent electronic conductivity of

Figure 8. Schematic drawing illustrating the synthetic route and the mechanism of charge separation and the photocatalytic process over GCNFM photocatalysts under visible light irradiation. 4061


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ACS Sustainable Chemistry & Engineering Fe0. In addition, as shown in eqs 2 and 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)

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 species 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.

Reaction potentials and the energy level diagrams. As aforementioned above, the photogenerated electrons in CB of gC3N4 were negative enough to reduce the O2 to generate •O2− (eq 4). In this case, the RhB can be oxidized by photogenerated holes (h+) and/or •O2− (eq 5). However, the consumption of electrons in this process will diminish the ability of Cr(VI) degradation. •

O2 + e− → O2 −

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01024. Specific surface area analysis, and the UV−vis spectra showing the degradation of Cr(VI) and RhB under different conditions (PDF)

(4)

•

Rh B + h+VB/ O2 − → simple molecules → CO2 + H 2O (5)

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As shown in Figure 6, a transferring of electron to MoS2 and Fe0 in GCNFM not only helps to suppress the recombination of the electron−hole pair, but also suppresses the consumption of the electrons to form •O2−. As aforementioned, the increased separation efficiency of charge carriers is the main reason for the enhanced performance of GCNM. According to eq 6, the electrons in CB of MoS2 (−0.2 eV vs NHE) were 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 and 8. 3e−CB + Cr(VI) → Cr(III)

(7)

3Fe(II) + Cr(VI) → 3Fe(III) + Cr(III)

(8)

AUTHOR INFORMATION

Corresponding Authors

*Fax: +86 591 22866534; E-mail address: [email protected]. *Fax: +86 591 22866534; E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 51102047, 51472050, and 51402295).

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3Fe0 + 2Cr(VI) → 3Fe(II) + 2Cr(III)

ASSOCIATED CONTENT

REFERENCES

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Synergy effect between the degradation of RhB and Cr. The control experiments in Figure 5f also indicate that there exists 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.

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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, and Fe-GCN, the photodegradation ability of the GCNFM toward the RhB and Cr(VI) under visible light irradiation is 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 has 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 maintains an efficient degradation ability toward both the RhB and Cr(VI). This can be attributed to the fact that 4062


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