BiOI Immobilized on Flexible Electrospun

Dec 21, 2017 - treatment. KEYWORDS: Electrospun nanofibers, Flexibility, g-C3N4/BiOI heterojunction, In situ synthesis, Visible photocatalytic activit...
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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Heterojunction of g‑C3N4/BiOI Immobilized on Flexible Electrospun Polyacrylonitrile Nanofibers: Facile Preparation and Enhanced Visible Photocatalytic Activity for Floating Photocatalysis Xuejiao Zhou, Changlu Shao,* Shu Yang, Xiaowei Li, Xiaohui Guo, Xiaoxiao Wang, Xinghua Li,* and Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, Key Laboratory of UV-Emitting Materials and Technology, Northeast Normal University, Ministry of Education, 5268 Renmin Street, Changchun 130024, People’s Republic of China S Supporting Information *

ABSTRACT: The heterojunction of g-C3N4/BiOI was uniformly immobilized on electrospun polyacrylonitrile (PAN) nanofibers via a facile in situ synthesis at room temperature. X-ray photoelectron spectra showed that both the C and N 1s peaks of PAN/g-C3N4/BiOI nanofibers shifted to higher binding energies as compared to those of PAN/g-C3N4 nanofibers suggesting electron transfer from g-C3N4 to BiOI during the formation of heterojunctions. The enhanced photocurrent densities and significant decrease in photoluminescence intensity confirmed the effective charge separation at the heterojunction interfaces in PAN/g-C3N4/BiOI nanofibers. The efficient separation of the electron−hole pairs and their strong absorption in the visible region results in superior photocatalytic activities in the degradation of Rhodamine B (RhB) dyes and toxic Cr(VI) ions under visible-light irradiation versus PAN/g-C3N4 and PAN/BiOI nanofibers. Moreover, the filmlike PAN/g-C3N4/BiOI nanofibers have ultralong one-dimensional nanostructures and flexible self-supporting structures that can be used as useful floating photocatalysts. They can float easily on liquid and are directly reused without separation during the photocatalytic reaction. These results demonstrate that flexible PAN/g-C3N4/BiOI nanofibers with high photocatalytic activity and excellent reusability have potential in wastewater treatment. KEYWORDS: Electrospun nanofibers, Flexibility, g-C3N4/BiOI heterojunction, In situ synthesis, Visible photocatalytic activity

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∼1.8 eV) is one of the most suitable semiconductors to form the heterojunction with g-C3N4.9 The heterojunction of gC3N4/BiOI enables photoinduced electrons from g-C3N4 to quickly transfer to the more positive conduction band (CB) of BiOI because of their strong internal electric field.11 This can induce effective separation of photogenerated electron−hole pairs to achieve high photocatalytic activity. Recently, many kinds of g-C3N4/BiOI heterojunctions and their nanostructures have been prepared and investigated because nanostructured photocatalysts with higher quantum yield and specific surface areas can improve the photocatalytic efficiency better than bulk material.9,12 However, nanostructured heterojunctions of gC3N4/BiOI have poor dispersion in water and easily aggregate because of their higher surface energy.13,14 This leads to a remarkable reduction in their photocatalytic activities. Nanostructured heterojunctions of g-C3N4/BiOI also suffer some drawbacks as photocatalysts especially in the separation and recycling of the suspended small nanoparticles. This may not

INTRODUCTION Semiconductor-based photocatalysis technologies in the arena of environmental remediation and solar energy conversion have been extensively studied.1 Graphite carbon nitride (g-C3N4) is a new metal-free semiconductor photocatalyst that has recently been widely applied in the degradation of dye pollutants, the removal of toxic heavy metal ions, and the photocatalysis for hydrogen evolution during water splitting.2 It offers a suitable electronic structure (2.7 eV), high thermal and chemical stability, low cost, and excellent optical and photoelectrochemical properties.3 However, the photocatalytic activity of pure gC3N4 is limited by the high recombination rate and low charge mobility rate of the photogenerated electron−hole pairs.4 Thus, many efforts have been made to reduce the recombination of photogenerated electron−hole pairs during photocatalytic reactions by constructing heterojunctions between g-C3N4 and another semiconductor with a suitable band potential including metallic oxide (TiO2, ZnO), metal sulfide (CdS, MoS2), or oxyhalide (BiOCl, BiOBr, BiOI), etc.5−10 Of these semiconductors, bismuth oxyiodide (BiOI) with the well-matched band potentials, unique layered structure, and the stronger photoresponse in the visible-light region (the band gap © XXXX American Chemical Society

Received: October 16, 2017 Revised: December 13, 2017 Published: December 21, 2017 A

DOI: 10.1021/acssuschemeng.7b03760 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

according to the literature.21 Next, 2.5 mmol of g-C3N4 powder was dissolved in 11 mL of N,N-dimethylformamide (DMF) and then further exfoliated by ultrasonic treatment for 1 h followed by 2.5 mmol of Bi(NO3)3·5H2O and 1.2 g of polyacrylonitrile (PAN) powders. After magnetic agitation for 48 h, the homogeneous precursor solution of PAN, g-C3N4, Bi(NO3)3, and DMF was obtained for electrospinning. A voltage of ∼10 kV and a distance of ∼16 cm were applied between the syringe tip and the counter electrode that was covered by aluminum foil. The nitrate-free PAN/g-C3N4 nanofibers were similarly prepared. Synthesis of the PAN/g-C3N4/BiOI Nanofibers. PAN/g-C3N4/ Bi(NO3)3 nanofibers (10 mg) were dipped into 100 mL of KI aqueous solution (0.3 mM) for 10 min, as seen in Scheme 1. The color of the

satisfy large-scale practical applications. For a solution to these problems, the immobilization of nanostructured heterojunctions of g-C3N4/BiOI on an appropriate support might be a promising approach. Until now, different dimensional nanostructured materials have been used as supports for photocatalysts such as zerodimensional nanoparticles and two-dimensional nanofilms.15,16 However, the nanoparticle supports with a nanoscale size are easily lost during photocatalytic reaction and separation.17 This may cause secondary pollution. On the other hand, using twodimensional nanofilms as supports may dramatically reduce the interfacial contact between photocatalysts and pollutants because of their lower specific surface area.14,18 This also decreases photocatalytic performance. In contrast, one-dimensional nanofibers with a high surface area and high length−diameter ratios have high potential in overcoming these problems.19,20 In particular, electrospun polyacrylonitrile (PAN) nanofibers with an ultralong onedimensional nanostructure and good flexibility are a promising support for immobilizing the nanostructured heterojunctions of g-C3N4/BiOI. The PAN nanofibers have several advantages as a support: (1) The flexible PAN-based nanofibers have an open three-dimensional macroscopic structure and excellent hydrophobicity. This can minimize agglomeration and improve separation of the nanostructured heterojunctions of g-C3N4/ BiOI in water for practical applications. (2) The PAN-based nanofibers have a higher special surface area than nanofilms. This improves the exposed active sites of the nanostructured photocatalysts and further enhances the activity in photocatalytic reactions. (3) More importantly, PAN-based nanofibers with a lower density makes them easily float on a liquid exposing them to light. This improves the irradiation efficiency of the light and maximizes their photocatalytic properties. However, there are no reports describing the immobilization of nanostructured heterojunctions of g-C3N4/BiOI on onedimensional nanofibers. On the basis of the above consideration, heterojunctions of gC3N4/BiOI were uniformly immobilized on the supports of electrospun PAN nanofibers via a soft and facile in situ synthesis. First, the PAN/g-C3N4/Bi(NO3)3 nanofibers were prepared by electrospinning a precursor that contained PAN, gC3N4, and Bi(NO3)3. Thereafter, the PAN/g-C3N4/Bi(NO3)3 nanofibers were bathed in an aqueous solution of potassium iodide at room temperature for in situ synthesis of BiOI on the composite nanofibers. The PAN/g-C3N4/BiOI nanofibers were well-characterized with strong interactions in the heterojunction via possible chemical bonding between BiOI and gC3N4. The PAN/g-C3N4/BiOI nanofibers had a markedly enhanced photocatalytic performance via the degradation of dye pollutants (Rhodamine B, RhB) and toxic heavy metal ions [Cr(VI)] via visible-light irradiation. The in situ synthesis of the heterojunction promoted the transfer of photoinduced electrons in the photocatalytic reaction. These results offer a novel and rational method for the immobilization of nanoscale photocatalysts on flexible supports. The obtained selfsupporting photocatalysts could float easily and do not require separation for reuse in water purification and energy conversion.

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Scheme 1. Schematic Diagram of PAN/g-C3N4/BiOI Nanofibers Synthesis

PAN/g-C3N4/Bi(NO3)3 nanofibers changed from white to yellow indicating that the BiOI nanostructures were immobilized in situ on the PAN/g-C3N4 nanofibers. The obtained PAN/g-C3N4/BiOI nanofibers were washed with distilled water three times and dried at 60 °C for 8 h. The PAN/BiOI nanofibers were obtained by dipping PAN/Bi(NO3)3 nanofibers into aqueous KI. Detailed characterization of the products is found in the Supporting Information.

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RESULTS AND DISCUSSION Morphology and Structure Analyses. The surface morphologies of the as-prepared samples were studied with scanning electron microscopy (SEM). The PAN/g-C3N4 nanofibers exhibit ultralong one-dimensional nanostructures without severe agglomeration (Figure 1A). This is because the bulk g-C3N4 can be exfoliated into smaller nanostructures (∼160 nm) through the ultrasonic exfoliation in precursors (Figure S1). The surface of the PAN/g-C3N4/Bi(NO3)3 nanofibers does not significantly change compared to that of the PAN/g-C3N4 nanofibers indicating that Bi ions can be uniformly dispersed in PAN/g-C3N4/Bi(NO3)3 nanofibers (Figure S2). After impregnation in the aqueous KI, the clear lamellar nanostructures were broadly dispersed on the PAN/gC3N4/BiOI nanofibers (Figure 1C). Similar lamellar nanostructures were also observed on PAN/BiOI nanofibers by impregnating the PAN/Bi(NO3)3 nanofibers into aqueous KI (Figure 1E). Meanwhile, the surface color of the PAN/g-C3N4/ BiOI and PAN/BiOI nanofibers both changed to yellow (inset of Figure 1C,E) versus the white PAN/g-C3N4 nanofibers (Figure 1A inset). This is attributed to the loading of BiOI nanostructures on the nanofibers. Notably, all of these PANbased composite nanofibers still exhibited high flexibility and macroscopic filmlike structures. This may help design a new floating photocatalyst that does not need to be separated from wastewater during photocatalysis. In addition, energy-dispersive X-ray (EDX) analysis showed that C, N, and O all existed in all the PAN-based composite nanofibers (Figure 1B,D,F). The Bi

EXPERIMENTAL SECTION

Synthesis of the PAN/C3N4/Bi(NO3)3 Nanofibers. All reagents were purchased from Beijing Chemicals (Beijing, China) and used without further purification. First, the g-C3N4 powder was prepared B

DOI: 10.1021/acssuschemeng.7b03760 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 2. (A) Cross-sectional TEM and (B) corresponding HRTEM images of the PAN/g-C3N4/BiOI nanofibers. FFT pattern of BiOI nanostructures in the PAN/g-C3N4/BiOI nanofibers (B inset). (C) Cross-sectional STEM image and corresponding mappings of PAN/gC3N4/BiOI nanofibers: (D) C element, (E) N element, (F) Bi element, (G) O element, and (H) I element. The two groups of parallel lines indicate representative composite nanofibers contained in the g-C3N4.

nanofibers. This further suggests that the BiOI samples with the smaller nanostructures could be easily immobilized on the PAN/g-C3N4/BiOI nanofibers. This was beneficial for the formation of uniform heterojunctions of g-C3N4/BiOI. The X-ray diffraction (XRD) patterns of pure g-C3N4 powders, PAN/g-C3N4 nanofibers, PAN/g-C3N4/BiOI nanofibers, and PAN/BiOI nanofibers are shown in Figure S3. The g-C3N4 powders have two characteristic peaks near 13.2° and 27.4°, corresponding to the periodic array of the in-plane tri-striazine and the interplanar stacking of aromatic systems.24,25 These are perfectly indexed as the (1 0 0) and (0 0 2) diffraction planes of the graphite-like g-C3N4, respectively.26 However, characteristic diffraction peaks of g-C3N4 were not observed in PAN/g-C3N4 or PAN/g-C3N4/BiOI nanofibers, probably because of the relatively low content of g-C3N4 in the these nanofibers. After impregnation in the KI aqueous solution, there were sharp diffraction peaks for BiOI nanostructures. This clearly illustrates the good crystallinity of the PAN/g-C3N4/BiOI and PAN/BiOI nanofibers. The diffraction peaks of the BiOI nanostructures in PAN/g-C3N4/ BiOI or PAN/BiOI nanofibers are perfectly indexed as tetragonal BiOI (JCPDS 73-2062).11 The characteristic signal peaks of the Fourier transform infrared (FTIR) spectrum are the most direct and effective evidence to confirm the presence of g-C3N4 in PAN/g-C3N4/ BiOI nanofibers. The PAN/g-C3N4 and PAN/g-C3N4/BiOI nanofibers show typical stretching vibration modes for the CN heterocycles from the g-C3N4: 1319, 1567, and 1638 cm−1 (Figure 3A).27 The characteristic peaks at 809 cm−1 belong to the stretching vibration mode of the s-triazine ring unit derived from the g-C3N4.28 The characteristic absorption peaks at 2246, 1453, 1235, and 1736 cm−1 in all samples originated from the PAN supports; these were attributed to the vibrations of the cyano groups, carbon−hydrogen, carbon−carbon, and carbon− oxygen bonds, respectively.14 The prominent peak at 539 cm−1 was attributed to the Bi−O stretching mode in the PAN/BiOI nanofibers.29 Notably, the characteristic peak derived from

Figure 1. SEM images of (A) PAN/g-C3N4, (C) PAN/g-C3N4/BiOI, and (E) PAN/BiOI nanofibers. EDX spectra of samples (B) PAN/gC3N4, (D) PAN/g-C3N4/BiOI, and (F) PAN/BiOI nanofibers.

and I elements were only seen in PAN/g-C3N4/BiOI (Figure 1D) and PAN/BiOI (Figure 1F) nanofibers. This further indicated that the BiOI nanostructures were immobilized in situ on PAN/g-C3N4 or PAN nanofibers at room temperature. A cross-sectional transmission electron microscopy (TEM) image was used to further study the microstructure of PAN/gC3N4/BiOI nanofibers. The g-C3N4 nanostructures partially exposed in the nanofibers can be seen on their surface (Figure 2A), and the small dark spots/sheets are easily seen in/on the PAN/g-C3N4/BiOI nanofiber. This indicates the uniform dispersion of BiOI nanostructures immobilized on the nanofiber. A high-resolution transmission electron microscopy (HRTEM) image of PAN/g-C3N4/BiOI nanofibers was obtained from the area marked with a rectangle in Figure 2A; this displays clear lattice fringes with a stacking distance of 0.325 nm (Figure 2B). This corresponds to the (0 0 2) lattice spacing of g-C3N4.22 The clear lattice fringes have a lattice spacing of 0.28 nm; this matched the spacing of the (1 1 0) crystal planes of tetragonal BiOI nanostructures.9 Moreover, the fast Fourier transform (FFT) pattern shows strong reflections associated with (1 1 0) planes demonstrating that the BiOI nanostructures are stacked along the (1 1 0) direction enforced by van der Waals interactions (Figure 2B inset).23 These observations imply that the g-C 3 N 4 and BiOI nanostructures are present in the PAN/g-C3N4/BiOI nanofibers to form the g-C3N4/BiOI heterojunctions. The scanning TEM (STEM) and elemental mapping images of a representative individual PAN/g-C3N4/BiOI nanofiber contained in g-C3N4 are shown in Figure 2C−H. The Bi, O, and I elements derived from the BiOI nanostructures are welldistributed on the selection area of PAN/g-C3N4/BiOI C

DOI: 10.1021/acssuschemeng.7b03760 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 3. (A) FTIR spectra of the PAN/g-C3N4, PAN/g-C3N4/BiOI, and PAN/BiOI nanofibers. (B) Enlarged view of FTIR spectra of PAN/gC3N4/BiOI and PAN/BiOI nanofibers.

Figure 4. (A) XPS full-spectra, and (B) C 1s and (C) N 1s core-level spectra for the as-synthesized samples of PAN/g-C3N4, PAN/g-C3N4/BiOI, and PAN/BiOI nanofibers. (D) Bi 4f, (E) I 3d, and (F) O 1s core-level spectra for PAN/g-C3N4/BiOI and PAN/BiOI nanofibers.

CNHx, and N(C)3 groups, respectively.21,27 However, the typical peaks of g-C3N4 noted above for PAN/g-C3N4/BiOI nanofibers obviously shifted to higher binding energies of 289.6, 399.1, 400.7, and 402.6 eV after the BiOI nanostructures are loaded. In addition, the XPS spectra of Bi 4f, I 3d, and O 1s for PAN/g-C3N4/BiOI and PAN/BiOI nanofibers were also given in Figure 4D−F, respectively. The splitting between Bi 4f7/2 and Bi 4f5/2 are both 5.3 eV for PAN/g-C3N4/BiOI and PAN/BiOI nanofibers.23 This suggests the normal state of Bi3+ in PAN/g-C3N4/BiOI and PAN/BiOI nanofibers (Figure 4D). The binding energies of the Bi 4f, I 3d, and O 1s peak for PAN/ g-C3N4/BiOI nanofibers all shift to lower binding energies versus PAN/BiOI nanofibers. Some previous studies have suggested that the internal electrical field could strongly affect the local binding energy of the g-C3N4 and BiOI in the heterojunction.11,29,33 The FTIR data indicate strong interactions between g-C3N4 and BiOI molecules. Thus, when they are in contact with each other and form heterojunctions, the electrons could transfer from g-C3N4 to BiOI because the Fermi level of g-C3N4 is higher than that of BiOI.34 The transfer and depletion of surface electrons from gC3N4 will form a positive charge region on the g-C3N4. This will increase the binding energy of C and N during XPS testing. On the contrary, the negative charge region on BiOI will simultaneously reduce the binding energy of Bi, I, and O.

vibration of the Bi−O bond in the PAN/g-C3N4/BiOI nanofibers was shifted to a larger wavenumber (Figure 3B). This probably derives from the structural distortion of the [Bi2O2]2+ slabs in the PAN/g-C3N4/BiOI nanofibers.9,29 These results suggest that an interfacial interaction between the gC3N4 and BiOI might exist in the PAN/g-C3N4/BiOI nanofibers. This would enhance the photocatalytic activity. XPS Analysis of the Samples. The bonding configuration and chemical composition of the as-fabricated samples were further analyzed by X-ray photoelectron spectroscopy (XPS). Figure 4A shows the scanned XPS spectra of PAN/g-C3N4, PAN/g-C3N4/BiOI, and PAN/BiOI nanofibers. As compared to pure PAN/g-C3N4 nanofibers, some new peaks derived from Bi and I elements were seen in PAN/g-C3N4/BiOI and PAN/ BiOI nanofibers. This further demonstrates that the BiOI nanostructures exist in PAN/g-C3N4/BiOI and PAN/BiOI nanofibers. The binding energy of the C 1s peak at 284.6 eV is usually used to calibrate the XPS measurements.30 The highresolution XPS spectra of C and N 1s are presented in Figure 4B,C, respectively. The peaks at 287.6 and 397.6 eV were both attributed to the cyano group derived from the PAN supports.31,32 The typical binding energies of g-C3N4 are clearly seen at 288.6, 398.9, 400.4, and 401.8 eV for the PAN/g-C3N4 nanofibers. These correspond to the NCN, CNC, D

DOI: 10.1021/acssuschemeng.7b03760 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 5. (A) Photocatalytic degradation and (B) kinetic linear simulation curves of RhB over the as-synthesized samples. (C) Effects of scavengers and (D) photocatalytic mechanism scheme for the photodegradation of RhB with PAN/g-C3N4/BiOI nanofibers under visible-light irradiation (λ > 400 nm, I = 150 mW cm−2). (E) Photocurrent spectra of as-synthesized samples under visible-light irradiation and their corresponding (F) PL spectra.

The photocatalytic activity of the prepared samples was evaluated via degradation of RhB as a model pollutant in an aqueous solution under visible light. The UV−vis absorption spectra in Figure 5A indicate that all samples could reach saturated adsorption equilibrium within 30 min in the dark. Under visible light, the self-degradation of RhB dye could be neglected.28 The degradation efficiency of RhB could reach 98.0%, 68.7%, and 42.0% after irradiation for 90 min in the presence of PAN/g-C3N4/BiOI, PAN/BiOI, and PAN/g-C3N4 nanofibers, respectively. The PAN/g-C3N4/BiOI nanofibers exhibit higher photocatalytic activity than PAN/BiOI or PAN/g-C3N4 nanofibers. For further determination of the heterojunction effects in PAN/g-C3N4/BiOI nanofibers, a physical mixture of g-C3N4 powder and BiOI nanostructures was studied as a control. The corresponding mass of g-C3N4 and BiOI that exist in PAN/gC3N4/BiOI nanofibers was calculated via thermogravimetric analysis in the Supporting Information (Figure S4). As expected, the efficiency of RhB degradation over the PAN/gC3N4/BiOI nanofibers (98.0%) was also significantly enhanced relative to a physical mixture (76.9%) of g-C3N4 powder and BiOI nanostructures. As compared to the suspended powders in the physical mixture, the effective separation of electron− hole pairs at the heterojunction interface between g-C3N4 and BiOI in PAN/g-C3N4/BiOI nanofibers can enhance the photocatalytic activity. The apparent first-order rate constant

This will lower their binding energies. Hence, the shifts of the binding energies for the heterojunction suggest that there is an effective charge transfer at the heterojunction interface. This might make charge separation easier during photocatalysis. Photocatalytic Performance. The optical properties of PAN/g-C3N4, PAN/BiOI, and PAN/g-C3N4/BiOI nanofibers were seen in UV−vis diffuse reflectance spectroscopy (DRS) analysis. The PAN/g-C3N4 and PAN/BiOI nanofibers exhibit an absorption edge at ∼450 and 630 nm in the visible-light range, respectively (Figure S5). The band gap energies of PAN/g-C3N4 and PAN/BiOI nanofibers are described by the following equation:35−37 αhν = A(hν − Eg )n /2

(1)

where α, h, A, and Eg are the absorption coefficient, photon energy, a constant, and the band gap of semiconductor, respectively. The value of n depends on the characteristics of the transition in the semiconductor (g-C3N4, n = 1; BiOI, n = 4). As shown in Figure S6, the band gap energies of g-C3N4 and BiOI are 2.7 and 1.94 eV, respectively. Of note, the optical absorption intensity of PAN/g-C3N4/BiOI nanofibers is higher than that of PAN/g-C3N4 and PAN/BiOI nanofibers. This might be due to the overlapping absorption characteristics of the heterojunction. E

DOI: 10.1021/acssuschemeng.7b03760 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 6. (A) Photocatalytic degradation and (B) kinetic linear simulation curves of Cr(VI) over the as-synthesized samples. (C) Illustration of floating photocatalysis process for the degradation of RhB and the photoreduction of Cr(VI). (D) Cycling test and reusability of PAN/g-C3N4/BiOI nanofibers under visible-light irradiation.

was significantly higher than that of PAN/g-C3N4 and PAN/ BiOI nanofibers. This demonstrates that a more effective separation of photoinduced electron−hole pairs and faster interfacial charge transfer occurs on the heterojunction of PAN/g-C3N4/BiOI nanofibers. In addition, PL spectroscopy shows that the emission peak intensity of PAN/g-C3N4/BiOI nanofibers significantly decreased relative to that of PAN/gC3N4 nanofibers (Figure 5F). This further suggests that the recombination of photoinduced electron−hole pairs was greatly inhibited by the interfacial charge transfer between g-C3N4 and BiOI nanostructures. Hence, the effective charge separation at the heterojunction interface on PAN/g-C3N4/BiOI nanofibers underlies their higher photocatalytic activity relative to PAN/gC3N4 and PAN/BiOI nanofibers. Cr(VI) is toxic and is found in industrial wastewater. It is a serious threat to human health and is a model to evaluate the ability of the photocatalyst to transfer electrons. Thus, the photocatalytic activities and stability of our photocatalysts were also evaluated by reducing Cr(VI) ions converted into low-toxic Cr(III) in aqueous solution under visible-light irradiation. The self-oxidation of the K2Cr2O7 aqueous solution as well as the Cr(VI) adsorbed on the surface of the photocatalyst can be neglected in Figure 6A. The efficiencies of Cr(VI) removal for PAN/g-C3N4, PAN/BiOI nanofibers, and PAN/g-C3N4/BiOI nanofibers were 24.6%, 58.5%, and 98.5% at 150 min under visible-light irradiation, respectively. The UV−vis absorption spectra of Cr(VI) over PAN/gC3N4/BiOI nanofibers show that the intensity of the primary peak at 540 nm decreased with increasing irradiation time (Figure S7). After only 150 min, the Cr(VI) had been completely reduced in an aqueous solution of K2Cr2O7. Moreover, kinetic analysis of photocatalytic reduction of Cr(VI) offers a better comparison on their photocatalytic activities (Figure 6B). The first-order rate constant of PAN/gC3N4/BiOI nanofibers in the reduction of Cr(VI) was 0.0261 min−1. This was 15.4-fold higher than that of PAN/g-C3N4

of the photocatalytic reaction can evaluate the degradation rate of the photocatalyst as follows:38,39 ln(C0/C) = K appt

(2) −1

Here, Kapp is the apparent first-order rate constant (min ), and C0 is the initial concentration (mg L−1) of RhB. Terms C (mg L−1) and t (min) are the concentration of RhB and the corresponding visible-light irradiation time, respectively. Figure 5B shows that the apparent first-order rate constants of PAN/gC3N4, PAN/g-C3N4/BiOI, and PAN/BiOI nanofibers for decomposing RhB under visible light were 0.0043, 0.0399, and 0.0140 min−1, respectively. The first-order rate constant of PAN/g-C3N4/BiOI nanofibers was 9.3-fold higher than that of PAN/g-C3N4 nanofibers and about 2.8 times that of the PAN/ BiOI nanofibers. These results indicate that the PAN/g-C3N4/ BiOI nanofibers had better photocatalytic activity in the degradation of RhB. Thus, we conclude that the formation of a high-efficiency heterojunction in PAN/g-C3N4/BiOI nanofibers plays a key role in enhancing the interfacial charge transfer. Radical trapping was designed to study the contributory active species in the photocatalysis reaction (Figure 5C). The details of trapping experiments and the possible mechanism of the decomposing RhB were evaluated in the Supporting Information. The results showed that the degradation of RhB is mainly driven by the participation of h+ radicals (Figure 5D). These h+ rapidly transfer to the g-C3N4 and directly react with adsorbed RhB dye to produce final degradation products. Thus, the effective charge separation in PAN/g-C3N4/BiOI nanofibers enhances the photocatalytic activities. Photoluminescence (PL) spectroscopy and photocurrent measurements are tools to study photoinduced interfacial charge transfer processes in PAN/g-C3N4/BiOI nanofibers. Figure 5E shows the photocurrent measurements for PAN/gC3N4/BiOI, PAN/BiOI, and PAN/g-C3N4 nanofibers. The photocurrent intensity of the PAN/g-C3N4/BiOI nanofibers F

DOI: 10.1021/acssuschemeng.7b03760 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering nanofibers (0.0017 min−1) and 5.1 times that of the PAN/BiOI nanofibers (0.0051 min−1). Obviously, the PAN/g-C3N4/BiOI nanofibers exhibit higher photocatalytic activity for removing Cr(VI) than PAN/g-C3N4 and PAN/BiOI nanofibers. This indicates that the formation of heterojunctions of g-C3N4/BiOI on PAN nanofibers was crucial to their enhanced photocatalytic performance. Furthermore, the efficiency of Cr(VI) removal by PAN/gC3N4/BiOI nanofibers was significantly enhanced relative to a purely physical mixture of g-C 3 N 4 powder and BiOI nanostructures (64.4%). As compared to the suspended powders in a physical mixture, the effective separation of electron−hole pairs at the heterojunction interface between gC3N4 and BiOI in PAN/g-C3N4/BiOI nanofibers might result in obviously enhanced photocatalytic activity. More importantly, the heterojunction of g-C3N4/BiOI immobilized on the PAN nanofibers not only overcomes the unfavorable aggregation, but also avoids the loss of powder during photocatalysis. These results were consistent with the conclusions of the degradation of RhB and further indicate that PAN/g-C3N4/BiOI nanofibers have excellent photocatalytic properties in the degradation of RhB and the removal of Cr(VI) ions. Furthermore, discussions of the mechanism for the enhanced photocatalytic reduction of Cr(VI) by PAN/gC3N4/BiOI nanofibers are shown in the Supporting Information (Figure S8). Recyclability of PAN/g-C3N4/BiOI Nanofibers. We also studied the stability of the photocatalytic activity over multiple recycling experiments. The nanofibers could float on the surface of the solution to overcome challenges with subsidence and can maximize the absorption of visible light during photocatalysis (Figure 6C). After the photocatalytic reaction, the PAN/g-C3N4/BiOI nanofibers could be removed directly from the liquid for the next cycle. The PAN/g-C3N4/BiOI nanofibers have no obvious decrease in photocatalytic activity during recycling (Figure 6D). Additionally, there was no appreciable change in their overall morphologies and constructions via SEM and XRD analysis of the PAN/gC3N4/BiOI nanofibers after the photocatalytic reaction (Figure S9). As compared to the powdery photocatalysts, the excellent stability of the PAN/g-C3N4/BiOI nanofibers is mainly attributed to their flexible self-supporting network structure, which benefits their reuse and overcomes challenges in agglomeration, subsidence, and loss during photocatalytic reactions.

without separation from the wastewater. This facilitates highefficiency utilization and industrialization. This work offers new insights into the design and fabrication of flexible selfsupporting photocatalysts based on g-C3N4-based heterojunctions with high efficiency and practical performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03760. Characterization, photocatalytic and photocurrent measurements, trapping experiments, discussion of the photocatalytic mechanisms, SEM images, XRD patterns, UV−vis diffuse reflection spectra, TG curves, BET nitrogen adsorption/desorption isotherms, and EDX analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 8643185098803. *E-mail: [email protected]. Phone: 8643185098803. ORCID

Changlu Shao: 0000-0002-5024-3268 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors sincerely acknowledge Dr. Hancheng Zhu for support in the characterization of samples. The present work is supported financially by the National Natural Science Foundation of China (51572045, 51272041, 61201107, 11604044, and 91233204), the National Basic Research Program of China (973 Program) (2012CB933703), the 111 Project (B13013), the Natural Science Foundation of Jilin Province of China (20160101313JC), the Fundamental Research Funds for the Central Universities (2412017FZ009, 2412017QD007, 2412016KJ017), and the China Postdoctoral Science Foundation (2017M610188).

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REFERENCES

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CONCLUSIONS In summary, the heterojunction of g-C 3 N 4 /BiOI was successfully immobilized in situ on electrospun PAN nanofibers via a facile impregnation method at room temperature. The PAN/g-C3N4/BiOI nanofibers markedly enhance the photocatalytic activities during degradation of RhB dye and removal of toxic Cr(VI) ions under visible-light irradiation compared to PAN/g-C3N4 and PAN/BiOI nanofibers. These improved photocatalytic activities are attributed to effective charge transfer across the heterojunction interface as made evident by the increased photocurrent densities and the significant decrease in PL intensity studies combined with XPS and FTIR analysis. The PAN/g-C3N4/BiOI nanofibers have macroscopic network structures and flexible self-supporting structures that allowed them to easily float on the liquid and maximize the absorption of visible light for photocatalysis. Notably, these flexible floating photocatalysts can be directly reused, i.e., G

DOI: 10.1021/acssuschemeng.7b03760 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acssuschemeng.7b03760 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX