Recyclable Carbon Nanofibers@Hierarchical I-Doped Bi2O2CO3

Unique hierarchical photocatalytic carbon nanofibers (CNFs)@I-doped Bi2O2CO3–MoS2 membranes were designed and fabricated and applied for efficient ...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 2676−2683

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Recyclable Carbon Nanofibers@Hierarchical I‑Doped Bi2O2CO3−MoS2 Membranes for Highly Efficient Water Remediation under VisibleLight Irradiation Jundie Hu, Dongyun Chen,* Najun Li, Qingfeng Xu, Hua Li, Jinghui He, and Jianmei Lu* College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, No. 199, Ren’ai Road, Suzhou 215123, China S Supporting Information *

ABSTRACT: Unique hierarchical photocatalytic carbon nanofibers (CNFs)@I-doped Bi2O2CO3−MoS2 membranes were designed and fabricated and applied for efficient water remediation under visible-light irradiation. First, I-doped Bi2O2CO3 nanosheets were uniformly decorated on the surface of CNFs. The introduction of iodine element resulted in narrowing of the bandgap of Bi2O2CO3 because CO32− was partly replaced by I−, thereby enhancing the absorption intensity of the photocatalyst to visible light and improving its photocatalytic efficiency. Subsequently, thinner MoS2 nanoflakes were also modified on the surface of the CNFs and in the gaps between the I-doped Bi2O2CO3 nanosheets using a hydrothermal method; this modification was beneficial to electronic transmission and prevented the rapid recombination of photogenerated electron− hole pairs. This new photocatalytic CNFs@I-doped Bi2O2CO3−MoS2 nanocomposite exhibited excellent photodegradation ability for eliminating the refractory pollutant Rhodamine B (RhB) from wastewater. Complete degradation of 50 mL of RhB (1.0 × 10−5 M) was achieved using 50 mg of CNFs@I-doped Bi2O2CO3−MoS2 within 5 min. Moreover, the photocatalyst membrane was stable and recyclable after multiple runs. All of these factors demonstrate the potential application of this photocatalyst in the removal of RhB from wastewater. KEYWORDS: Photocatalyst membrane, Water remediation, Hierarchical, Visible-light irradiation, Recyclable

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INTRODUCTION The treatment of water contamination remains a critical issue worldwide. Wastewater is mostly produced from the emission of organic pollutants (such as phenol, acid red, methylene blue, or RhB from printing, textiles, food, and factories) and seriously affects the environment and humanity.1−4 Therefore, the degradation of organic pollutants is imperative. Over the past few decades, many traditional techniques and methods have been developed for the degradation of organic pollutants from wastewater (such as adsorption,5,6 chemical oxidation,7,8 and biodegradation9,10). However, several disadvantages (incompletely, secondary pollution, etc.) limit the industrial application of these methods. In the past few years, a photocatalytic strategy11−14 for the degradation of dye wastewater has attracted considerable attention. Bi-based semiconductors, such as flower-like Bi 2 WO 6 , 1 5 − 2 1 Bi2MoO6,22,23 BiVO4,24−26 Bi2O2CO3,27−29 and BiOX (X = Cl, Br, I),30,31 have been widely investigated because of their high photocatalytic activity, especially for the photodegradation of RhB. However, pure photocatalyst nanoparticles still cannot fully meet the requirements for practical application in the © 2017 American Chemical Society

degradation of RhB because of their wide bandgaps and the rapid recombination of photogenerated electron−hole (e−−h+) pairs. Nevertheless, the construction of heterojunctions,32,33 surface modification of semiconductor photocatalysts,34,35 and element doping36,37 can be used to enhance their photocatalytic activity. Li and co-workers prepared spherelike g-C3N4/BiOI heterojunctions for photocatalytic degradation of RhB.30 This new photocatalyst displayed enhanced photocatalytic activity; RhB (100 mL, 10 mg L−1) could be degraded completely within 40 min, and its efficiency was 35% higher than that of pure BiOI. Wu and co-workers synthesized the p-MoS2/n-rGO heterostructure successfully, which showed significant photocatalytic activity for the hydrogen evolution reaction because the p-MoS2/n-rGO junction greatly enhanced the charge generation and suppresses the charge recombination.38 Qian and co-workers prepared roselike I-doped Bi2O2CO3 microspheres with enhanced visible light response, which due to the doping of I− could narrow its bandgap,29 and the DFT Received: November 16, 2017 Revised: December 8, 2017 Published: December 30, 2017 2676

DOI: 10.1021/acssuschemeng.7b04270 ACS Sustainable Chem. Eng. 2018, 6, 2676−2683

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Schematic Illustration of the Fabrication of CNFs@IBOC−MoS2 Membranes and Their Use for the Photodegradation of RhB under Visible-Light Illumination

Characterization. Scanning electron microscopy (SEM) (Hitachi S-4700) coupled with X-ray energy-dispersive spectroscopy (EDS) were used to measure the morphology of the samples. The structure, element, and crystal lattice of the samples were examined by transmission electron microscopy (TEM) (Tecnai G220), high-resolution transmission electron microscopy (HRTEM), and TEM energy-dispersive X-ray (EDX). X-ray diffraction (XRD) (X’Pert-Pro MPD) was used to investigate the crystallographic structure of the products. Xray photoelectron spectroscopy (XPS) analysis was performed in an X-ray photoelectron spectrometer (ESCALAB MK II) using Al Kα radiation as the exciting source. A UV−vis spectrophotometer (CARY50) was used to measure the ultraviolet−visible (UV−vis) absorbance spectra of samples, thereby calculating the band gaps of the products. A fluorescence spectrophotometer (FLS920) was used to record the photoluminescence (PL) spectra of the samples with an excitation wavelength of 370 nm. Electrochemical measurements and electrochemical impedance spectra (ESI) measurements were conducted with a CHI 660B electrochemical system (Shanghai, China) according to the literature. Fabrication of CNFs@I-Doped Bi2O2CO3. CNFs membranes were easily prepared by the method of electrospinning and calcination (see Supporting Information).40 I-doped Bi2O2CO3 was modified on the surface of CNFs by a simple hydrothermal method according to a previously described process with some modifications as follows: 0.23 g of Na2CO3 and 0.8 g of bismuth citrate were dissolved in a mixed solution of deionized water (30 mL) and ethylene glycol (6 mL) and stirred for 30 min to obtain a transparent solution. Then, 0.6 g of NaI was added to the transparent solution and further stirred for 2 h at room temperature. Subsequently, the precursor suspension and 50 mg of CNFs were transferred into a 50 mL Teflon-lined stainless autoclave and heated at 160 °C for 24 h. After the reaction, the reacted mixture was collected with tweezers and washed with deionized water and ethanol for three times, respectively, and then dried in a vacuum oven at 60 °C for 6 h. The obtained product was labeled as CNFs@IBOC. I-doped Bi2O2CO3 was synthesized under the same conditions without adding CNFs and labeled as IBOC. Bi2O2CO3 microflowers can also be modified on the CNFs by the same method without adding NaI, labeled as CNFs@BOC.

calculations support the above result. On the basis of the excellent performance of I-doped Bi2O2CO3, Rhodamine B and Cr(VI) could almostly be degraded within a short time under visible-light irradiation (λ> 400 nm). However, the recovery of powder photocatalysts is difficult, and repeated use is inconvenient; therefore, immobilization of the photocatalyst on a suitable carrier is integral.39,40 Herein, a new recyclable photocatalytic nanocomposite, CNFs@I-doped Bi2O2CO3−MoS2 membranes, was fabricated using a simple two-step hydrothermal method and successfully used to degrade RhB under visible-light irradiation (Scheme 1). First, I-doped Bi2O2CO3 nanosheets were decorated on the surface of CNFs, which were obtained by the calcination of polyacrylonitrile nanofibers. The morphology of the CNFs@Idoped Bi2O2CO3 consisted of many leaves grown on branches, which resulted in a large specific surface area. The doping of iodine not only narrowed the bandgap of Bi2O2CO3 but also enhanced the absorption of visible light. Second, thinner MoS2 nanoflakes were also introduced, which improved the efficiency of electronic transmission and prevented the rapid recombination of photogenerated e−−h+ pairs. In this way, we immobilized the photocatalyst I-doped Bi2O2CO3 and MoS2 on the CNF membranes, which could enhance the photocatalytic effect and is convenient for recycling use. Then, the manufactured CNFs@I-doped Bi2O2CO3−MoS2 nanocomposites were used to degrade RhB under visible-light irradiation and could be recycled several times. For comparison, photocatalytic nanocomposites without iodine, CNFs@ Bi2O2CO3−MoS2, were also synthesized and exhibited a completely different morphology.

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EXPERIMETAL SECTION Materials. Polyacrylonitrile (PAN, MW = 150,000), bismuth citrate, sodium molybdate (Na2MoO4), and RhB were purchased from Sigma-Aldrich. Sodium iodide (NaI) and sodium carbonate (Na2CO3) were purchased from Adamas; N,N-dimethylformamide (DMF), thioacetamide (TAA), and ethylene glycol were purchased from Sinopharm Chemical Reagent Co., Ltd. (China), and deionized water was used throughout the experiments. All of the chemicals were used without further purification. 2677

DOI: 10.1021/acssuschemeng.7b04270 ACS Sustainable Chem. Eng. 2018, 6, 2676−2683

Research Article

ACS Sustainable Chemistry & Engineering Identically, Bi2O2CO3 microflowers were prepared in this way without adding NaI and CNFs and labeled as BOC. Fabrication of CNFs@I-Doped Bi2O2CO3−MoS2. First, 24.2 mg of Na2MoO4 and 19.4 mg of thioacetamide were dissolved in 30 mL of deionized water and underwent ultrasound treatment for 30 min; then, the mixture and CNFs@IBOC were transferred to a 50 mL Teflon-lined stainless autoclave and heated at 200 °C for 16 h. Finally, the product was collected with tweezers and washed with deionized water and ethanol for three times, respectively, and then dried in a vacuum oven at 60 °C for 6 h.41 The obtained product was labeled as CNFs@IBOC−MoS2. If CNFs@IBOC was substituted by CNFs@BOC, CNFs@BOC−MoS2 was obtained. Photocatalytic Evaluation. The photocatalytic activity of the samples was evaluated by the degradation of RhB under visible light irradiation; a xenon lamp (300W, simulated sunlight) was chosen as the visible light source. The temperature of the reaction solution was kept at 25 °C over the whole process. Then, 50 mg of photocatalysts were added to 50 mL of RhB aqueous solution (1 × 10−5 M) in a beaker (the photocatalysts were in a very small plastic cup that has holes on the bottom of it to prevent the photocatalyst membrane from being damaged); then, the suspensions were stirred in the dark for 30 min to reach the adsorption− desorption equilibrium. Afterward, a xenon lamp vertically placed outside the beaker was turned on, and at a given time interval, a 3 mL suspension was taken from the reaction suspension. Subsequently, the suspensions were measured with the UV−vis spectrophotometer at a wavelength of 554 nm. The concentration changes were described by C/C0, where C0 is the initial concentration of RhB and C is the remaining concentration of RhB. Recycling Experiment of the RhB Degradation. For the recycling property and stability of these photocatalysts to be evaluated, photocatalyst membranes were collected with tweezers after this reaction, thoroughly washed with deionized water and ethanol, and placed in another fresh RhB solution. Then, the photocatalyst CNFs@IBOC−MoS2 was continuously used for five cycles to degrade the RhB solution under the same conditions.

Figure 1. SEM images of CNFs@BOC−MoS2 (a, b), CNFs@IBOC (c, d), and CNFs@IBOC−MoS2 (e, f); photograph of the CNFs@ IBOC−MoS2 membrane (e, inset). SEM images of the pure BOC, IBOC, CNFs, and CNFs@BOC are provided in Figure S1.

Bi2O2CO3 nanosheets, acting as bridges among the IBOC nanosheets (Figure 1e and f). In addition, the inset in Figure 1f clearly illustrates that the MoS2 nanoflakes were considerably thinner than those of IBOC. For the structure of these nanocomposites to be further elucidated, TEM and HRTEM analyses were performed. The TEM image (Figure 2a) and HRTEM image (Figure 2b) of the CNFs@BOC−MoS2 reveal that BOC was firmly loaded on the CNFs and was enfolded by layered MoS2 nanosheets, which was beneficial for electronic transmission and visible light absorption. Figure 2c presents a TEM image that shows the even growth of the CNFs@IBOC−MoS2, IBOC nanosheets, and MoS2 nanoflakes on the surface of the CNFs. The clear

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RESULTS AND DISCUSSION Morphology and Structure. Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and highresolution TEM (HRTEM) were performed to analyze the morphology and microstructure of CNFs@BOC−MoS2, CNFs@IBOC, and CNFs@IBOC−MoS2. The SEM images of CNFs@BOC−MoS2 in Figure 1a−b demonstrate that the spherical photocatalyst BOC was modified on the surface of the CNFs and wrapped by MoS2 thin nanosheets similar to a string of pearls with diameters of ∼2−3 μm. Surprisingly, when the catalyst BOC was doped with iodine to increase its absorbance intensity against visible light, substantial morphological changes were observed, as shown in Figure 1c and d. Uniform sheet loading of I-doped Bi2O2CO3 on the CNFs was observed, which greatly increased the surface area of the photocatalyst, thereby improving its photocatalytic efficiency. The magnified SEM image of CNFs@IBOC (insetin Figure 1d) indicates that the thickness of the I-doped Bi2O2CO3 nanosheets was ∼10− 20 nm; all of the nanosheets were the same size and were connected. Thinner MoS2 nanosheets were observed on the outside of the CNFs and in the voids between the I-doped

Figure 2. TEM images of CNFs@BOC−MoS2 (a, b). TEM (c) and HRTEM (d) images of CNFs@IBOC−MoS2. 2678

DOI: 10.1021/acssuschemeng.7b04270 ACS Sustainable Chem. Eng. 2018, 6, 2676−2683

Research Article

ACS Sustainable Chemistry & Engineering lattice fringes of the selected area (marked by the red circle in Figure 2c) in the HRTEM image (Figure 2d) imply the high crystallinity of the CNFs@IBOC−MoS2. The lattice spacings of 0.62 and 0.27 nm correspond to the (002) plane of MoS2 and the (110) crystal facet of I-doped Bi2O2CO3, respectively.29,41 SEM and EDS mapping images of CNFs@IBOC−MoS2 are presented in Figure 3. We can conclude that Bi, I, C, Mo, O,

Figure 3. SEM image of CNFs@IBOC−MoS2 (top left) and SEM EDS mapping images of Bi (a), I (b), C (c), Mo (d), and S (e).

and S were uniformly attached to the surface of the CNFs and the IBOC nanosheets and thin MoS2 nanoflakes were successfully loaded onto the CNFs. TEM coupled with EDS was also performed, and the results are presented in Figures S2 and S3, further supporting these conclusions. Phase and Composition. X-ray diffraction (XRD) patterns of CNFs@IBOC−MoS2 nanocomposites, CNFs@IBOC, the pure IBOC nanosheets, and pure MoS2 microflowers are presented in Figure 4. The samples containing I-doped

Figure 5. XPS spectra of CNFs@IBOC−MoS2 (a) and highresolution XPS spectra of C 1s (b), O 1s (c), I 3d (d), Bi 4f and S 2p (e), and Mo 3d and S 2s (f).

ing to the CNFs O=C−O and C=O, respectively. The binding energies of O 1s (Figure 5c) at 531.1, 532.9, and 533.8 eV were associated with Bi−O bonding, carbonate ions, and adsorbed H2O on the surface, respectively. The peaks at 630.0 and 618.7 eV were assigned to I 3d3 and I 3d5 (Figure 5d). Bi 4f7/2 and Bi 4f5/2 peaks were observed at 159.3 and 164.6 eV (Figure 5e), and S 2p3/2 and S 2p1/2 peaks from MoS2 were observed at 162.6 and 163.8 eV, respectively. Figure 5f shows the binding energies of Mo 3d5/2 and Mo 3d3/2 at 228.7 and 232.9 eV, respectively; the presence of Mo6+ was also detected at 236.1 eV. In addition, the binding energy of S2S was observed at 226.4 eV. In comparison, the XPS of CNFs@BOC−MoS2 was also provided in Figure S4. Optical Properties. Ultraviolet−visible (UV−vis) diffuse reflectance spectroscopy (DRS) was used to investigate the optical properties of the as-prepared samples, and the results are presented in Figure 6a. The pure IBOC nanosheets exhibited an absorption edge at ∼450 nm; however, the BOC microflowers showed an absorption edge at ∼360 nm. This finding indicates that the IBOC nanosheets exhibited a wider range of visible light absorption. However, the intensity of optical absorbance was weakened after the IBOC nanosheets were modified on the surface of the CNFs, likely because the IBOC content was low. After MoS2 was loaded on the CNFs@ IBOC, the CNFs@IBOC−MoS2 nanocomposite had an absorption edge at ∼500 nm, whereas the CNFs@BOC− MoS2 exhibited an absorption edge at ∼400 nm, further demonstrating that the absorption over visible light of the CNFs@IBOC−MoS2 was stronger. The bandgap energies of the prepared samples were calculated from Tauc plots ((αhv)2 = A(hv − Eg)), as shown in Figure 6b. The Eg values of the BOC, IBOC, and CNFs@IBOC−MoS2 were approximately 3.12, 2.66, and 2.24 eV, respectively. Comparison of these Eg values indicates that the CNFs@IBOC−MoS2 exhibited visible-

Figure 4. XRD patterns (a) and enlarged view of the 2θ = 24−39° diffraction region (b) of the synthesized samples.

Bi2O2CO3 can be assigned to the tetragonal phase of Bi2O2CO3 (JCPDS card No. 41-1488) with no other diffraction peaks detected, demonstrating the purity and good crystallinity of these products. The 2θ peaks at 30.2° and 32.7° in the XRD patterns of CNFs@IBOC−MoS2, CNFs@IBOC, and IBOC correspond to the (013) and (110) planes of Bi2O2CO3, respectively (Figure 4b).41 The XRD pattern of the MoS2 microflowers contains three diffraction peaks at 13.8°, 33.1°, and 58.5°, which were also observed in the XRD pattern of CNFs@IBOC−MoS2 but with weakened intensity. We deduced that the MoS2 content loaded on the CNFs was too low. X-ray photoelectron spectroscopy (XPS) was used to further examine the chemical state and elemental composition of the CNFs@IBOC−MoS2 membranes, especially the presence of I, as observed in Figure 5.29 The presence of C, O, I, Bi, Mo, and S were apparent in the survey of CNFs@IBOC−MoS2 (Figure 5a). The C 1s XPS spectra (Figure 5b) show three characteristic peaks at 284.9, 284.7, and 286.3 eV correspond2679

DOI: 10.1021/acssuschemeng.7b04270 ACS Sustainable Chem. Eng. 2018, 6, 2676−2683

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ACS Sustainable Chemistry & Engineering

7a, the pure CNFs without the catalyst showed a negligible effect for RhB degradation, which indicates that the CNFs did

Figure 6. UV−vis DRS absorption of as-prepared samples (a), plots of (αhν)1/2 vs photon energy of BOC, IBOC, and CNFs@IBOC−MoS2 (b), photocurrent transient (c), and EIS spectra (d) of the as-obtained samples under visible-light irradiation.

Figure 7. Photocatalytic degradation of RhB (1 × 10−5 M) using different photocatalysts (a), UV−vis DRS spectra of RhB solution photodegraded by CNFs@IBOC−MoS2 (b), cycling of CNFs@ IBOC−MoS2 photocatalyst for RhB degradation (c), and UV−vis DRS spectra of RhB solution photodegraded by CNFs@BOC−MoS2 (d).

light-driven photocatalytic ability because of the small bandgap of the IBOC nanosheets. Electrochemical Properties. The electrochemical properties of the samples were analyzed based on their photocurrent responses and electrochemical impedance spectroscopy (EIS) spectra, as shown in Figure 6c and d. The photocurrent tests were performed according to the procedures described in the literature (see Supporting Information). As observed in Figure 6c, upon visible-light irradiation, the photocurrent rapidly increased and then reached a plateau. As expected, the current density of IBOC was 5× greater than that of BOC, which demonstrates that the separation efficiency of photogenerated charge carriers and electronic transmission efficiency of IBOC were higher than those of BOC. Interestingly, the current density increased when the IBOC nanosheets were loaded on the surface of the CNFs, thus demonstrating that the CNFs not only acted as carriers but were also beneficial to the electronic transmission. Similarly, the addition of MoS2 nanoflakes greatly improved the electron transport efficiency, thereby improving the photocatalytic efficiency. EIS spectra (Figure 6d) were further used to evaluate the electrochemical characteristics. The arc radius of the Nyquist plot is used to evaluate the magnitude of the electrochemical impedance. The arc radius of IBOC was smaller than that of BOC, which indicates that IBOC exhibited good separation and transfer efficiency of e−−h+ pairs. The introduction of CNFs reduced the arc radius, indicating that the CNFs were conducive to the transmission of electrons. As expected, the CNFs@IBOC−MoS2 nanocomposites exhibited the smallest electrochemical impedance, indicating that the separation and transfer efficiency of e−−h+ pairs of this material were the highest, indirectly implying that the CNFs@IBOC− MoS2 exhibited the best photocatalytic effect. PL spectroscopy of the as-prepared samples (Figure S5) was also performed to proof the above conclusion. CNFs@IBOC−MoS2 showed an obviously decreased emission signal than that of CNFs@IBOC and IBOC, indicating that there is a lower radiative recombination of photogenerated e−−h+ pairs for CNFs@ IBOC−MoS2. Photocatalytic Performance for RhB Degradation. The photocatalytic performance as well as the stability and reusability of the as-prepared samples were evaluated using the RhB photodegradation reaction. As demonstrated in Figure

not exhibit catalytic activity and that the RhB was stable in aqueous solution under visible-light irradiation. The order of photocatalytic activity for these photocatalysts can be summarized as follows: CNFs@IBOC−MoS2 > CNFs@ IBOC > IBOC−MoS2 > IBOC > CNFs@BOC−MoS2. Complete degradation of RhB was achieved within 5 min using the CNFs@IBOC−MoS2 membranes, which was much more efficient than using the CNFs@IBOC (∼8 min), pure IBOC powder (11 min), and the CNFs@BOC−MoS2 (only ∼50% was degraded within 15 min). For comparison, the physical mixture of CNFs, IBOC, and MoS2 was also tested, however, the effect of this physical mixture showed no significant effect, as shown in Figure S6. The efficient photocatalytic performance of the CNFs@IBOC−MoS2 is attributed to the narrowed bandgap of IOBC and its large specific surface area; in addition, the introduction of MoS2 and CNFs was beneficial for the enhancement of charge-carrier separation. UV−vis DRS spectra of the RhB solution photodegraded by the CNFs@IBOC−MoS2 and CNFs@ BOC−MoS2 are presented in Figure 7b and d, respectively. The RhB aqueous solution changed from pink to colorless, as observed in the inset of Figure 7b. The major absorption peak of RhB is 554 nm; however, with the absorption band shifting toward the blue region with increasing time, we deduced that the degradation of the RhB solution is a de-ethylation process and that the ethyl groups were removed one-by-one from RhB.42 The reusability of the CNFs@IBOC−MoS2 was also investigated by performing five consecutive batch reaction tests. As observed in Figure 7c, the degradation efficiency was not significantly reduced, which demonstrates the stability and reusability of the CNFs@IBOC−MoS2. Photocatalytic Mechanism for RhB Degradation. A schematic of the mechanism for the photocatalytic degradation of RhB by CNFs@IBOC−MoS2 under visible-light irradiation is presented in Scheme 2. According to the literature, the CB and VB of MoS2 were −0.09 and 1.81 eV, respectively.44 The VB XPS of I-doped Bi2O2CO3 was checked, and the results show that the VB of IBOC is 1.50 eV; therefore, the CB of 2680

DOI: 10.1021/acssuschemeng.7b04270 ACS Sustainable Chem. Eng. 2018, 6, 2676−2683

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ACS Sustainable Chemistry & Engineering

Scheme 2. Photocatalytic Mechanism of the Photodegradation of RhB over CNFs@IBOC−MoS2 Membranes under VisibleLight Irradiation

IBOC was calculated to be −1.16 eV, as shown in Figure S7. The p−n junction will be formed once p-type MoS2 and n-type IOBC contacted, and band offset between the two semiconductors occurred; then, the p−n junction reaches an equal Fermi level (Ef).45,46 MoS2 and IOBC were both be excited to produce e−−h+ pairs under visible-light irradiation (eq 1).27 However, the photogenerated electrons in the conduction band (CB) of MoS2 will transfer to the CNFs and then transfer to the CB of IBOC; the separated electrons easily accumulated in the CB of IBOC; then, to prevent the recombination of e−−h+ pairs, importantly, the CB potential of IBOC was negative enough to reduce the O2 to generate ·O2− (O2/·O2− = −0.33 eV vs NHE) (eq 2).43 Furthermore, the photogenerated holes in the VB of IBOC will transfer to the VB of MoS2; surprisingly, the h+ remaining in the VB of MoS2 exhibited strong oxidative power and could participate in the photocatalytic oxidation reaction, and RhB could be degraded directly (eq 3).27,28,43 Therefore, this mechanism (summarized in eqs 1−3) enabled the CNFs@IBOC−MoS2 photocatalysts to realize highly efficient degradation of RhB under visible-light irradiation, and this material could be easily reused many times. −

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+

IBOC−MoS2 + hv → e + h

(1)

O2 + e − → ·O2 −

(2)

RhB + h+/· O2 − → products

(3)

Bi2O2CO3 and its large specific surface area, which is beneficial for the absorption of visible light. More importantly, the membrane of CNFs@IBOC−MoS2 was stable and recyclable after multiple runs. These findings demonstrate the potential of CNFs@IBOC−MoS2 membranes for the degradation of RhB from wastewater.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b04270. Synthesis of CNFs, electrochemical measurements, SEM images, TEM EDX mapping images, EDX, XPS spectra, PL spectra, and degradation of RhB and VB XPS spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hua Li: 0000-0002-0982-5573 Jianmei Lu: 0000-0003-2451-7154 Notes

The authors declare no competing financial interest.

CONCLUSIONS In summary, hierarchical CNFs@IBOC−MoS2 nanocomposites were successfully synthesized using a two-step hydrothermal method. The prepared CNFs@IBOC−MoS2 photocatalyst exhibited excellent photocatalytic activity for the degradation of RhB under visible-light irradiation; 50 mL of RhB (1.0 × 10−5 M) could be completely degraded within 5 min by 50 mg of CNFs@IBOC−MoS2. For comparison, CNFs@BOC−MoS2 was also successfully synthesized using the same method; however, only 50% of the RhB solution could be degraded within 15 min using this photocatalyst. We attribute the high photocatalytic efficiency of CNFs@IBOC−MoS2 to the introduction of I, which can narrow the bandgap of

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the National Key R&D Program of China (2017YFC0210901, 2017YFC0210906), National Natural Science Foundation of China (51573122, 21722607, 21776190), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (17KJA430014, 17KJA150009), Science and Technology Program for Social Development of Jiangsu (BE2015637), and the project supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). 2681

DOI: 10.1021/acssuschemeng.7b04270 ACS Sustainable Chem. Eng. 2018, 6, 2676−2683

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ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.7b04270 ACS Sustainable Chem. Eng. 2018, 6, 2676−2683