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Flexible Electrospun Carbon Nanofiber/Tin(IV) Sulfide Core/Sheath Membranes for Photocatalytically Treating Chromium(VI)-Containing Wastewater Yunlei Zhong, Xun Qiu, Dongyun Chen, Na-Jun Li, Qing-Feng Xu, Hua Li, Jinghui He, and Jian-Mei Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10241 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

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ACS Applied Materials & Interfaces

Flexible

Electrospun

Core/Sheath

Carbon

Membranes

for

Nanofiber/Tin(IV) Photocatalytically

Sulfide Treating

Chromium(VI)-Containing Wastewater Yunlei Zhong, Xun Qiu, 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, Suzhou, 215123, China.

Keywords: electropun carbon nanofiber; nanofiber membrane, CNF@SnS2, visible light-driven photocatalyst, hexavalent chromium wastewater

Abstract

We report an efficient method for fabricating flexible membranes of electrospun carbon nanofiber/tin(IV) sulfide (CNF@SnS2) core/sheath fibers. CNF@SnS2 is a new photocatalytic material which can be used to treat wastewater containing high concentrations of hexavalent chromium (Cr(VI)). The hierarchical CNF@SnS2 core/sheath membranes have a threedimensional macroporous architecture. This provides continuous channels for the rapid diffusion of photoelectrons generated by SnS2 nanoparticles, under visible light irradiation. The visible light (λ>400 nm) driven photocatalytic properties of CNF@SnS2 are evaluated by the reduction of water-soluble Cr(VI). CNF@SnS2 exhibits high visible light-driven photocatalytic activity,

ACS Paragon Plus Environment

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because of its low band gap of 2.34 eV. Moreover, CNF@SnS2 exhibits good photocatalytic stability and excellent cycling stability. Under visible light irradiation, the optimized CNF@SnS2 membranes exhibit a high rate of degradation of 250 mg/L aqueous Cr(VI), and can completely degrade the Cr(VI) within 90 min.

1. Introduction Industrial activity has resulted in increased heavy metal pollution, which is a serious environmental problem. 1-3 Unlike organic pollutants, heavy metals are hard to be biodegraded in the environment.

4

Hexavalent chromium (Cr(VI)) can be released into the environment by

industrial processes such as leather tanning, electroplating, wood preservation, paint manufacturing, metal plating, steel manufacturing, and chromate production.

5-6

Due to its high

toxicity, hexavalent chromium (Cr(VI)) is classified as a kind of potential carcinogen substance for human beings.

7-8

The concentration of Cr(VI) in wastewater should be controlled in

acceptable levels before release, to protect potable water supplies and public health.9 The World Health Organization (WHO) recommends that a maximum safe concentration of Cr(VI) in potable water is 50 µg/L.10 Various research efforts have been attempted to address this issue, but without any great breakthroughs to date.

11-14

How to efficiently and economically treat

chromium wastewater has attracted considerable academic and industrial interests. Various methods, such as ion exchange, electrochemical precipitation, reverse osmosis, and adsorption, have been used to remove hexavalent chromium from wastewater,

15-18

These

conventional methods usually require large amount of reducing agents, which is also often harmful to the environment, as well as being uneconomical. In contrast to these methods, the reduction of water-soluble hexavalent chromium by photocatalysts is relatively simple, economical, environmentally friendly, and efficient,

19

as the catalysts are reusable and exploit


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solar energy.

20

Unwanted byproducts are not released during the treatment of water-soluble

hexavalent chromium.

21

Therefore, the reduction of aqueous hexavalent chromium by

semiconductor photocatalysts is considered to be a promising method to handle water-soluble hexavalent chromium. As we all know, TiO2 has been the most studied semiconductor photocatalyst, because of its excellent light activity, chemical stability, and low cost.

22-23

Nevertheless, TiO2 is not excited by visible wavelengths, which comprise about 46% of the solar spectrum due to its broad band gap (3.2 eV).

24

Alternative visible light-responsive

semiconductor photocatalysts that more efficiently use solar energy are therefore required.

Scheme 1. Schematic illustration of the synthesis of the CNF@SnS2 membranes, and their application in reducing Cr(VI) to Cr(III).


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Semiconducting metal sulfides are typically photo-responsive to shortwave visible and near infrared wavelengths,

25

so are considered to be Promising semiconductor photosensitive

substance with broadband gap and sunlight-activated photocatalysts. Among them, CdS has a band gap of about 2.42 eV,

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and is currently receiving much attention. However, CdS is

detrimental to human health and the environment, because of its toxicity. non-toxic and chemically stable in acidic or neutral media.

28

27

In contrast, SnS2 is

The band gap of SnS2 (2.08‒2.44

eV) is suitable for use as an efficient visible light-driven photocatalyst.

27

SnS2 can reportedly

reduce Cr(VI) under visible light irradiation more efficiently and rapidly than biodegradation methods.

29

SnS2 nanocrystals have also been shown to adsorb Cr(III).

29

However, the practical

application of SnS2 materials is limited by their small surface area, difficulties with dispersing, and susceptibility to aggregation and light corrosion. 30 Many functional materials have been adhered to the surfaces of other more easily separated materials such as magnetite (Fe3O4) nanoparticles and fibers.31-32 This approach has yielded good results for removing pollutants. Carbon fibers have been widely used as adsorbent and photocatalysts carrier,

33-34

for the removal of organic species.

35

This is because of their

excellent electronic liquidity, concentrated pore distribution, and small fiber diameter.

36, 37

Carbon fibers also exhibit excellent stability in the presence of highly acidic or alkaline media and at high temperatures, due to their strong sp2-hybridized structure.38-40 Thus, modifying a carbon-based material with a SnS2 nanostructure should avoid SnS2 aggregation, and yield good photocatalytic efficiency and cycling stability. 41 In this study, we report an efficient for fabricating flexible membranes of electrospun carbon nanofiber/tin(IV) sulfide (CNF@SnS2) core/sheath fibers. Flexible self-standing CNF membranes with a three-dimensional (3D) fiber network and excellent structural stability are


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obtained by electrospinning and subsequent carbonization, as shown in Scheme 1. SnS2 nanoparticles are then uniformly anchored on the CNFs, to yield hierarchical nanostructures. This prevents the SnS2 nanoparticles from aggregating during the photocatalytic degradation cycles. The conducting CNF cores can quickly and efficiently transport photoelectrons produced by the SnS2 sheath, under visible light irradiation (λ>400 nm). The electrospun CNF@SnS2 core/sheath membranes exhibit excellent cycling stability and more efficient photocatalytic efficiency for the reduction Cr(VI) than the component CNFs membranes and SnS2. The CNF@SnS2 membranes are therefore a potential photocatalyst for degrading Cr(VI) in wastewater under visible light.

2. Experimental section 2.1 Reagents All reagents were used without further purification. Tetramethyl orthosilicate (TMOS), SnCl4·5H2O, thioacetamide (TAA) and other chemicals were of analytic grade and purchased from Sinopharm Chemical Reagents Co., Ltd. Polyacrylonitrile (PAN) (Mw = 150 000 g mol−1) was purchased from Sigma-Aldrich. 2.2 Fabrication of CNF@SnS2 Core/Sheath Membranes PAN nanofiber membranes were prepared by electro-spinning technique. Firstly, 1 g PAN was dissolved completely in 9 mL N,N-dimethylformamide (DMF). Then, the mixture was transferred to a 5 mL plastic syringe for electrospinning (voltage: 20 kV, injection rate: 0.2 mm min−1). In order to obtain CNF membranes, the PAN membranes were carbonized at 500 °C for 2 h under an inert atmosphere with a heating rate of 2 K min−1. CNF@SnS2 core/sheath membranes with different SnS2 loadings were then prepared by chemical bath deposition (CBD).


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Briefly, a certain amount of SnCl4·5H2O (145, 290, or 580 mg) and thioacetamide (120 mg) were dissolved in ethyl alcohol (40 mL) under ultrasound conditions. The CNF membranes (50 mg) were

then

immersed

in

the

above

solution,

which

was

then

transferred

to

a

polytetrafluoroethylene-lined stainless steel autoclave and heated in a homogeneous reactor at 120 °C for 6 h. CNF@SnS2 membranes prepared using 145, 290, and 580 mg of SnCl4·5H2O are denoted respectively CNF@SnS2-1, CNF@SnS2-2, and CNF@SnS2-3. By controlled trial, pure SnS2 was also synthesized by the same method. 2. 3. Photocatalytic reduction of Cr(VI) Cr(VI) mainly exists as Cr2O72- in water, so potassium dichromate solution was used to model Cr(VI)-containing wastewater. In order to study the influence of solution pH on catalytic efficiency, CNF@SnS2 (50 mg) was added to Cr(VI) solutions (50 mg/L) at pH values of 2, 4, or 8. These solutions were stirred and then irradiated with a Xe lamp (300 W, λ>400 nm). Ultraviolet-visible spectrum was recorded at regular intervals using a spectrophotometer, until constant spectra were obtained. The durations (min) required for complete reduction were compared, to assess the effect of solution pH. Comparison experiments were also carried out. Specifically, CNF@SnS2 membranes, SnS2, or CNF membranes (50 mg) were added to Cr(VI) solutions (50 mg/L) at pH 2. Each Cr(VI) solution was then irradiated with a Xe lamp (300 W, λ>400 nm) for 90 min under stirring. After irradiating by visible light, the Cr(VI) contents of the solutions were then analyzed by UV-vis spectrophotometry, to compare the degradations. 2. 4 Removal of total chromium To qualitatively investigate the visible light catalytic ability of the CNF@SnS2 membranes, Cr(VI) solutions (50 mL) of different concentrations (20, 50, 100, 150, 200, 250, or 300 mg/L)


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were poured into transparent glass bottles, and CNF@SnS2 (50 mg) was added to each. The mixtures were irradiated with a Xe lamp (300 W, λ>400 nm) for 90 m in under stirring. After reaction, the concentrations of unreduced Cr(VI) in the solutions were determined using a colorimetric method, with 1,5-diphenylcarbazide spectrophometry.42, 43 To study the cycling performance of the CNF@SnS2 membranes in treating Cr(VI) solutions, CNF@SnS2 (50 mg) (loop three times) was added to Cr(VI) solution (50 mg/L, 50 ml), and the resulting mixture was stirred for 90 min. After reaction, the Cr(VI) contents of the solutions were analyzed by UV-vis spectrophotometry, to compare the degradations. Characterization Transmission electron microscopy (TEM; Hitachi H600) and scanning electron microscopy (SEM; Hitachi S-4800) coupled with X-ray energy dispersive spectroscopy (EDS) were used to observe the morphology, structure, and size of the CNF@SnS2 and its components. X-ray diffraction (XRD; X’ Pert-Pro MPD), thermogravimetric analysis (TGA; Netzsch TG209) and X-ray photoelectron spectroscopy (XPS; Axis Ultra HAS) were used to investigate the effect of the CNF and SnS2 contents of CNF@SnS2 on its structural properties. The optical properties were determined by UV-visible diffuse reflectance spectroscopy (UV-vis DRS, Shimadzu UV3600).

3. Results and discussion 3.1 Morphology and Structure of the CNF@SnS2 Membranes As can be seen in Figure 1, both the obtained PAN nanofibers and CNFs have uniform diameter of around 300‒400 nm and smooth surfaces.

It was noting that the connected


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nanofibers with 3D microstructure could provide a larger surface area which is benefit for further modification of photocatalytically active nanomaterials.

Figure 1. SEM images of an electrospun PAN nanofiber membrane a) and CNF membrane b). In this study, low-temperature CBD was used to grow crystalline SnS2 nanosheets on CNFs insitu. CNF@SnS2 membranes prepared using 145, 290, and 580 mg of SnCl4·5H2O are denoted CNF@SnS2-1, CNF@SnS2-2, and CNF@SnS2-3, respectively. Figure 2 shows that the SnS2 nanosheet loading on the CNF nanofibers increases with increasing SnCl4 concentration used in the preparation. Small SnS2 nanosheets are evenly coated on the surface of CNF@SnS2-1 membranes (Figure 2a and b), which was prepared at SnCl4 low concentration. Increasing the SnCl4 concentration results in thin SnS2 nanosheets with curled shapes growing vertically on the nanofiber surface (Figure 2c and d). The TEM images of Figure S2 (Supporting Information) also shows that SnS2 nanosheets are evenly coated on the surface of CNF@SnS2-2 membranes. As shown in Figure 2e and f, serious aggregation occurred and thick layer SnS2 nanosheets were observed after further increasing the SnCl4 concentration. The reasons of this phenomenon may due to the rapid nucleation of SnS2 at high concentration. The surface area of CNF@SnS2 will rapidly decrease under these conditions. As shown in Figure S1 (Supporting Information), pristine SnS2 nanosheets would aggregate to form a larger spherical nanoflowers if without CNF


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membrane. Therefore, the presence of CNF matrix could effectively prevent aggregation of the SnS2 nanosheets.

Figure 2. Low- (left) and high- (right) magnification SEM images of the a, b) CNF@SnS2-1, c, d) CNF@SnS2-2, and e, f) CNF@SnS2-3 membranes. A SEM image and elemental maps of the CNF@SnS2-2 membrane are shown in Figure 3. The SEM image in Figure 3a shows thin SnS2 nanosheets with curled shapes uniformly coated on the nanofiber surfaces. Figure 3b, c, and d shows elemental maps of C, S, and Sn, respectively. The uniform color intensity further confirms the SnS2 coating on the CNFs. At the same time, the SEM-EDX of CNF@SnS2 membranes also prove above results in Figure S3 (Supporting information).


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XPS spectra were collected to detect the surface composition and chemical states of the CNF@SnS2-2 membrane, and are shown in Figure 4. The XPS survey spectrum in Figure 4a shows the expected Sn, S, and C elements in CNF@SnS2-2. The XPS peaks (Figure 4b) at 486.6 and 495.1 eV correspond to the Sn3d5/2 and Sn3d3/2 states, respectively. The peak at 162.6 eV in Figure 4c corresponds to the S2p3/2 state of S22- moieties. The peak at 284.8 eV in Figure 4d corresponds to the C1s state. XPS spectra were also collected to determine the surface composition and chemical states of the CNF membrane and SnS2, and are shown in Figure S4 (Supporting information). By contrast, these results further indicate the SnS2 coating on the CNFs.

Figure 3. SEM image of CNF@SnS2-2 a), and elemental maps b, c and d), of the CNF@SnS2-2 membrane.


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Figure 4. XPS survey spectrum a) and Sn 3d b), S 2p c), and C 1s d) spectral regions for the CNF@SnS2-2 membranes. Figure 5a shows XRD patterns of the CNF@SnS2-2 membrane, SnS2, and the CNF membrane. All diffraction peaks of SnS2 can be assigned to hexagonal SnS2 (JCPDS Card no. 23-0677).42-43 Peaks at 2θ of 15, 28, 32, 42, 50, 53, and 61° in the XRD patterns of CNF@SnS2-2 and SnS2 correspond to the (001), (100), (002), (003), (110), (111), and (200) planes of SnS2, respectively. The presence of these peaks in the pattern of CNF@SnS2-2 indicates the SnS2 coating on the CNFs. SnS2 and the CNF and CNF@SnS2-2 membranes were also characterized by TGA (Figure 5b). The CNF membranes exhibit an approximate 95 wt.% weight loss in the range 100‒700 °C, which is attributed to the loss of C from the CNF membranes. After modification with SnS2, the CNF@SnS2-2 membranes exhibit an approximate 85 wt.% weight loss. Thus, SnS2 constitutes approximately 10 wt.% of the CNF@SnS2-2 membranes. SnS2 exhibits a 30 wt.% loss from 100 to 700 °C.


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Figure 5. XRD patterns of the CNF membranes, SnS2, and CNF@SnS2-2 membranes a); TGA curves of the CNF membranes, SnS2, and CNF@SnS2-2 membranes b); UV-vis DRS spectra of the CNF membranes, SnS2, and CNF@SnS2-2 membranes c and d). UV-vis DRS spectra were then recorded and converted to absorption spectra, to study the absorption properties of SnS2 and the CNF and CNF@SnS2-2 membranes (Figure 5c). Figure 5c shows that SnS2 absorbs in the visible region. CNF@SnS2-2 also absorbs across a wide wavelength range, and is capable of harvesting visible light. The Tauc approach36 can be used to determine the band gap energy (Eg) of CNF@SnS2-2, and the result is shown in Figure 5d. The Eg values of SnS2 and CNF@SnS2-2 obtained from Figure 5d are approximately 2.01 and 2.34 eV, respectively. The band gap of CNF@SnS2-2 is slightly higher than that of SnS2. Thus, CNF@SnS2-2 exhibits visible light driven photocatalytic ability, because of the small band gap of SnS2.


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Figure 6. Photocatalytic reduction of 50 mL of 50 mg/L Cr(VI) solution under visible light irradiation, in the case of 50 mg of CNF@SnS2-2 at pH of 2, 4, and 8. Removal efficiency of 50 mg/L Cr(VI) solution during 20 min of irradiation a). UV-vis absorption spectra of Cr(VI) solution during 20 min of irradiation at pH 2 b), 4 c), and 8 d). pH is an important parameter affecting photocatalysis. Figure 6 shows the effect of pH on the photocatalytic reduction of Cr(VI) by the CNF@SnS2-2 membranes. The photocatalytic reduction of aqueous Cr(VI) at low pH is much higher than at high pH. At high pH, Cr(OH)3 readily precipitates and deposits on the CNF@SnS2-2 surface, which decreases the photocatalytic activity.


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Figure 7. Reduction of 50 mg/L Cr(VI) a); UV-vis absorption spectra of Cr(VI) in the presence of the CNF@SnS2 membranes b), CNF membranes c), and SnS2 d). The CNF membranes, SnS2, and CNF@SnS2-2 membranes were then used to degrade 50 mg/L Cr(VI) solution over 90 min, under stirring. The results are shown in Figure 7a. SnS2 degrades Cr(VI) at high rate in solution over 30‒100 min. The Cr(VI) concentration achieves equilibrium after 90 min. The Cr(VI) removal efficiency reaches 96.3% after 90 min. However, the Cr(VI) concentration reaches equilibrium after 20 min when treated with the CNF membranes, but only 30% of Cr(VI) is adsorbed from the 50 mg/L Cr(VI) solution. When treated with the CNF@SnS2-2 membranes, 90% of Cr(VI) is removed within 10 min, and equilibrium is achieved after 40 min. 100% of Cr(VI) is removed from the 50 mg/L Cr(VI) solution after 40 min. UV-vis absorption spectra of 50 mg/L Cr(VI) solutions treated with the CNF@SnS2-2 and CNF membranes and SnS2 are shown in Figure 7b, c, and d, respectively. The CNF@SnS2-2 membranes (Figure 7b) more efficiently reduce Cr(VI) than the CNF membranes (Figure 7c) and SnS2 (Figure 7d). The UV-vis absorption spectra in Figure 7 show the characteristic absorptions


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of Cr(VI) at 255 and 355 nm, and their intensities decrease more quickly in the presence of the CNF@SnS2-2 membranes (Figure 7b), than in the presence of the CNF membranes (Figure 7c) and SnS2 (Figure 7d). Figure 7c shows that the strong Cr(VI) absorptions at 255 and 355 nm change little after 90 min, in agreement with Figure 7a. Thus, the Cr(VI) concentration does not significantly change in Figure 7c. Figure 7b shows that the intensities of these absorptions decrease quickly, and that equilibrium occurs after 10 min. However, no absorption of Cr(VI) at 255 and 355 nm is observed after 50 min (Figure 7d). These results are in agreement with the degradation of Cr(VI), and further confirm the good degradation ability of the CNF@SnS2-2 membranes.

Figure 8. Photocatalytic reduction of 50 mL of 50 mg/L Cr(VI) solution under visible light irradiation, in the presence of 50 mg of CNF@SnS2-2 at pH 2. UV-vis absorption spectra of Cr(VI) by the CNF@SnS2-2 membranes during the first a), second b), and third c)degradation cycles d).


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UV-vis absorption spectra showing the cycling performance of the CNF@SnS2-2 membranes are shown in Figure 8a, b, and c. After three cycles, the 50 mg/L Cr(VI) solution is completely degraded within 90 min. The efficiency of CNF@SnS2-2 during the second and third cycles (Figure 8b and c) is slightly lower than in the first (Figure 8a), but is still excellent. The results of Figure 8d also illustrate this point. Thus, the CNF@SnS2-2 membranes exhibit better removal efficiency and cycling performance in degrading Cr(VI) from wastewater, than SnS2 and the CNF membranes.

Figure 9. Cr(VI) removal efficiencies of 50 mg of CNF@SnS2-2 exposed to visible light for 90 min at pH 2, for different initial Cr(VI) concentrations. Figure 9 shows the photocatalytic reduction efficiencies of 50 mg of CNF@SnS2-2 membranes exposed to visible light for 90 min, for different initial Cr(VI) concentrations. The CNF@SnS2-2 membranes clearly reduce Cr(VI) with a high efficiency. The photocatalytic efficiencies for 20, 50, 100, 150, 200, and 250 mg/L Cr(VI) solutions are all near 100%. At a Cr(VI) concentration of 300 mg/L, the catalytic efficiency is still in excess of 95%.


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A schematic of the mechanism for the photocatalytic reduction of Cr(VI) is shown in Scheme 2. Photoelectrons and holes are generated in the CNF@SnS2-2 membranes, under irradiation by visible light (Equation 1). The major reactive H+ are generated via the oxidation reduction of H2O (Equation 2). CrO42- reacts with photogenerated photoelectrons and H+, producing Cr3+ and H2O as shown in Equation 3. CNF@SnS2 + hν → e− + h+

(1)

H2O + 2h+ → 1/2O2 + 2H+

(2)

CrO42- + 8H+ + 3e− →Cr3+ + 4H2O

(3)

Scheme 2. Schematic illustration of the reaction mechanism for Cr(VI) removal. 4. Conclusion CNF@SnS2 membranes were prepared for treating wastewater containing high Cr(VI) concentrations. The CNF@SnS2 membranes have clear advantages over SnS2 nanoflowers, because of their easily and cost-effectively prepared materials, and their flexible, self-standing, macroporous architecture with excellent structural stability. Hierarchical nanostructures are formed by uniformly anchoring SnS2 nanoparticles on the CNF membranes. This prevents


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aggregation of the SnS2 nanoparticles in the process of the photocatalytic degradation of Cr(VI). These synergistic effects improve the visible light-driven ability of the optimized CNF@SnS2 membranes, which exhibit favorable degradation of 250 mg/L Cr(VI) solution, Cr(VI) removal efficiencies of up to 100%, and excellent cyclical stability. These results indicate that the CNF@SnS2 membranes are promising for treating wastewater containing high Cr(VI) concentrations.

Supporting Information Supporting information is available from the http://pubs.acs.org or from the author.

Corresponding Author *Jianmei Lu. E-mail: [email protected]. *Dongyun Chen. E-mail: [email protected].

Acknowledgements We gratefully acknowledge the financial support provided by National Natural Science Foundation of China (21336005, 21301125, and 51573122), Natural Science Foundation of the Education Committee of Jiangsu Province (15KJB150026), Environmental Protection Research Foundation of Suzhou, and Suzhou Nano-project (ZXG2013001, ZXG201420).


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We report an efficient and scalable process for fabricating flexible membranes of electrospun carbon nanofiber/tin(IV) sulfide (CNF@SnS2) core/sheath fibers. The hierarchical CNF@SnS2 core/sheath membranes have a three-dimensional macroporous architecture. This provides continuous channels for the rapid diffusion of photoelectrons generated by SnS2 nanoparticles, under visible light irradiation (λ>400 nm). CNF@SnS2 also exhibits good photocatalytic stability and excellent cycling stability. CNF@SnS2 accelerated the photocatalytic process of hexavalent chromium (Cr(VI)) due to these advantages.

Flexible Electrospun Carbon Nanofiber/Tin(IV) Sulfide Core/Sheath Membranes for Photocatalytically Treating Chromium(VI)-Containing Wastewater

Yunlei Zhong, Xun Qiu, Dongyun Chen,* Najun Li, Qingfeng Xu, Hua Li, Jinghui He and Jianmei Lu*


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Tin(IV ... - ACS Publications

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