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Preparation of electrospun ZnS-loaded hybrid carbon nanofiberic membranes for photocatalytic applications
Powder Technology 298 (2016) 1–8
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Preparation of electrospun ZnS-loaded hybrid carbon nanofiberic membranes for photocatalytic applications Hua Chen a, Guohua Jiang a,b,c,⁎, Weijiang Yu a, Depeng Liu a, Yongkun Liu a, Lei Li a, Qin Huang a, Zaizai Tong a,b,c, Wenxing Chen a,b,c a b c
Department of Materials Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, PR China National Engineering Laboratory for Textile Fiber Materials and Processing Technology (Zhejiang), Hangzhou 310018, PR China Key Laboratory of Advanced Textile Materials and Manufacturing Technology (ATMT), Ministry of Education, Hangzhou 310018, PR China
a r t i c l e
i n f o
Article history: Received 20 February 2016 Received in revised form 25 April 2016 Accepted 12 May 2016 Available online 14 May 2016 Keywords: Nanocarbon fibers Electrospun Oxidation Photocatalytic
a b s t r a c t In this paper, graphene oxide/polyacrylonitrile (GO/PAN) composite nanofibrous membranes were firstly prepared by an electrospinning method. Then, the GO/PAN composite nanofibrous membranes were further transferred to graphene oxides/carbon nanofibrous membranes (GO-CNFs) by a calcination treatment. Following a solvothermal treatment, zinc sulfide (ZnS) nanopartciles were covered onto GO-CNFs to form electrospun carbon nanofibrous membranes loaded with GO/ZnS (GO/ZnS-CNFs). The composition and microstructure of GO/ZnSCNFs were characterized by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman and UV–vis diffuse reflectance spectra analysis. Due to the synergistic effect between photocatalytic activity of ZnS and excellent adsorption capacity of GO-CNFs, the resultant GO/ZnS-CNFs exhibited excellent photocatalytic activity for oxidation of p-aminotoluene and phenol under mild conditions. The resultant hybrid carbon composite membranes offer the significant advantages, such as low dosage, high catalytic activity, easy recycling and excellent stability. © 2016 Elsevier B.V. All rights reserved.
1. Introduction As known, carbon materials have been widely studied as support for catalytic applications [1,2]. Carbon nanofibers (CNFs) were a new class of flexible materials with high mechanical strength, superior electroconductibility and excellent corrosion resistance. Meanwhile, they could supply a large surface area, which is critical for nanostructure-based catalytic materials [3]. In our previous study [4], we studied activated polyacrylonitrile carbon nanofibers with BiOBr and AgBr decorating for photocatalytic degradation of rhodamine B. The photocatalytic ability was dramatically improved due to the synergistic effect between photocatalytic activity of BiOBr and AgBr, and excellent adsorption capacity of CNFs. However, these composite CNFs were easy to be broken under high-speed stirring due to their low mechanical strength. Graphene oxide (GO) comprises graphene sheets with oxygen-containing functional groups on the basal planes and the edges. It can be fabricated in large quantities at a low cost [5]. The oxygen-containing functional groups of GO increase the interlayer distance between adjacent graphene sheets and interaction strength between graphene sheets and CNFs matrix, thus, enable catalytic nanoparticles to be adsorbed on graphene sheets as well, achieving composite CNFs with higher mechanical strength [6]. ⁎ Corresponding author. E-mail address: [email protected] (G. Jiang).
http://dx.doi.org/10.1016/j.powtec.2016.05.017 0032-5910/© 2016 Elsevier B.V. All rights reserved.
As one of typical II–IV binary semiconductor materials, zinc sulfide (ZnS) has received considerable concern due to its excellent properties. According to the previous reports, ZnS exists in two main crystalline forms: the cubic sphalerite phase and the hexagonal wurtzite phase [7]. Both phases are direct and large band-gap semiconductors at room temperature (3.7 eV for the cubic sphalerite phase and 3.8 eV for the hexagonal wurtzite phase), regarded as good candidates for light-emitting materials, electroluminescent devices, photoluminescent devices, photocatalysts, solar cells, optical sensors, lasers, and optical recording materials [8–16]. Recently, researchers have focused on the synthesis of ZnS composites through combined ZnS with other nanostructured carbon materials to form the delocalized conjugated materials. These composite materials are well matched with the photocatalysts in energy level and an intensive interface hybrid effect emerges between these materials, causing rapid charge separation and slow charge recombination in the electron-transfer process [17]. p-Aminotoluene (p-toluidine) is a colorless crystal with high toxicity. Although it can be oxidated in acetonitrile by hydrogen peroxide, its rate is a very slow [18]. Using ozone as oxidizing agent to oxide paminotoluene, the yield of aromatic products does not exceed 15% [19]. Phenol is one of the most toxic, and is used widely in petrochemical, chemical, and pharmaceutical industries. The presence of phenol in aqueous environments presents serious problems due to its toxicity, persistence in the environment, and bioaccumulation [20]. Therefore, the removal of phenol by environmentally friendly is a major
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consideration for current research. To pursue a high efficiency method oxide p-aminotoluene and phenol is attract considerable interest in organic synthesis and environmental protection. The photo-catalytic oxidation is an environmentally ‘benign’ or ‘green’ synthesis, since it consumes low-energy photons and occurs under atmospheric pressure and at room temperature [21]. In this paper, graphene oxide/polyacrylonitrile (GO/PAN) composite nanofibrous membranes were firstly prepared by an electrospunning method. Then, the GO/PAN composite nanofibrous membranes were further transferred to graphene oxides/ carbon nanofibrous membranes (GO/CNFs) by a calcination treatment. Following a solvothermal treatment, zinc sulfide (ZnS) nanoparticles were covered onto GO/CNFs to form electrospun carbon nanofibrous membranes loaded with GO/ZnS (GO/ZnS-CNFs). The catalytic activity of the resultant GO/ZnS-CNFs was investigated in oxidation of paminotoluene and phenol by a photochemical method. The recycling use of the catalyst was also of concern. The redox experiment was carried out without chemical additives and poisonous by-products under mild reaction conditions. The mechanism of oxidation was discussed. 2. Experimental 2.1. Materials Polyacrylonitrile (PAN, M w = 150.000) power, N, N′dimethylformamide (DMF, AR), zinc acetate (ZnC2H6O2·2H2O, AR) and sodium sulfide (Na2S·9H2O, AR) were purchased from aladdin Co. Ltd. (China). 2.2. Preparation of GO/ZnS-CNFs Typically, the GO solution was firstly fabricated on the basis of a modified Hummer's method [22]. Then, 1.0 g PAN, 0.1 g GO and 0.5 g zinc acetate were dissolved in 10 mL DMF by rapid stirring for 12 h to obtain a homogeneous solution. About 8 mL of the solution was placed in a 10 mL syringe. The syringe was placed in a syringe pump that maintained a solution feeding rate of 0.08 mm/min. The distance between the needle tip and collector was 12 cm, and the voltage was set at 15 kV. Subsequently, the sample was annealed at 550 °C for 3 h in pure nitrogen with a heating rate of 2 °C min−1 to obtain intermediate products. The intermediate products were put into 6 mM NaS solution and transferred to a 80 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 6 h. Finally, the product was dried at 60 °C for 6 h to obtain GO/ZnS-CNFs membranes. The mass fraction of ZnS on GO/ ZnS-CNFs membranes is about 13.24% that calculated by EDS analysis. 2.3. Photocatalytic conversion of 4-aminotoluene to 4-nitrotoluene The photocatalytic activity of GO/ZnS-CNFs membranes were detected through the photocatalytic oxidation of 4-aminotoluene in the mixed solution which the volume rate of water to acetonitrile is 3:2. The photocatalytic oxidation of 4-aminotoluene experiment was conducted in a photochemical reaction equipment with a 500 W metal halide lamp as the light source. In every experiment, 2 × 5 cm of GO/ZnSCNFs membrane was added into 10 mL of 4-aminotoluene solution (0.002 mol·L−1). The concentrations of 4-aminotoluene after an appropriate irradiation time were measured by HPLC technique. The mobile phase for HPLC was a mixture of water and acetonitrile in a ratio of 3:2 (v/v). 2.4. Characterization The structures and crystal phase of the as-prepared samples were analyzed with a SIEMENS Diffractometer D5000 X-ray diffractometer (XRD) with Cu Kα radiation source at 35 kV, with a scan rate of 4° s−1 in the 2θ range of 5–80°. X-ray photoelectron spectroscopy (XPS) data were obtained with an obtained with an ESCALab220i-XL electron
spectrometer from VG Scientific using 300 W Al Kα radiation. The ULTRA-55 field emission scanning electronmicroscopy (FE-SEM) at an accelerating voltage of 10 kV and JSM-2100 transmission electron microscopy (TEM) was used to characterize the morphology of the as-prepared samples. Raman spectra were recorded using a micro-Raman spectroscopy (TriVista TR557 Princeton Instruments). The Agilent high performance liquid chromatography (HPLC) 1100 was used to confirm the products in the reaction. The concentration of organics was measured by UV1901PC UV–vis spectrophotometer (Shanghai Aucy Technology Instrument CO.,LTD, China). 3. Results and discussion The schematic representation for preparation of GO/ZnS-CNFs membranes is shown in Scheme 1. The GO/PAN composite nanofibrous membranes are firstly prepared by a electrospunning method. Then, the GO/PAN composite nanofibrous membranes are further transferred to GO-CNFs by a calcination treatment. Fig. 1A shows the electrospun nanofibers after calcination treatment are the homogeneous nanofibrous mats with reveal bulge structure. Apparently, a large number of pores are observed on the surface of as-prepared samples, as shown in Fig. 1B. These pores are formed due to the release of gases pyrolysis of polymer matrix during the calcination process [23,24], which are beneficial for absorbing the organic molecules onto the catalytic active sites on the surface of composite nanofibers. Following a solvothermal treatment, zinc sulfide (ZnS) nanoparticles are covered onto GO-CNFs to form electrospun carbon nanofibrous membranes loaded with GO/ZnS (GO/ZnS-CNFs). Fig. 1C and D are the SEM images of GO/ZnS-CNFs membrane at different magnifications. The microcosmic structure of the membrane is maintained after solvothermal reaction and the ZnS nanoparticles with a diameter at about 10 nm exhibit a good dispersity on the surface of nanofibers. The energy dispersive X-ray spectroscopy (EDS) verify the existence of C, O, Zn and S elements in GO/ZnS-CNFs membrane samples (Fig. 1E), and atom ratio of S/Zn closed to 1: 1 (inset in Fig. 1E) suggests that nanoparticles distributed in composite membrane are ZnS. The S, Zn, C, and O edge mappings are shown in Fig. 1F-I, respectively. It can be seen that the orange color (assign to S), red color (assign to Zn), white color (assign to C) and green color (assign to O) are uniformly distributed in the sample of GO/ZnS-CNFs membranes. The resultant GO/ZnS-CNFs membranes are expected to present outstanding mechanical flexibility, enabling them to maintain the original dimensions after folding and crushing (Fig. 2). Apparently, the strategy has shown excellent versatility and scalability for potentially large-scale fabricating lightweight graphene-based carbon nanofibrous products by employing specific textured matrices. The high mechanical flexibility may be contributed to the including of GO nanosheets, which have numerous oxygen-containing functional groups and presents the high specific surface area and crumpled surface nanofibers with curling edges, could play a beneficial role in enhancing interfacial interaction with the GO and the carbon nanofibrous matrix [25]. To further obtain the microscopic morphology and structure information, the transmission electron microscopy (TEM) analysis of GO/ ZnS-CNFs has been performed. As shown in Fig. 3A, a light-colored layer on the surface of nanofibers can be observed. Further magnification of surface structure of nanofibers, the light-colored layer is consisted of many nanosheets, indicating that the GO is successfully incorporated onto the nanofibers, as shown in Fig. 3B. Furthermore, no clear interface between the GO nanosheets and the CNFs can be founded, indicating that the junction at the edges of GO and CNFs might be linked by strong interaction [26]. Besides, many nanoparticles also can be observed in the nanofibers. The crystalline nature of these nanoparticles is further investigated by high resolution TEM (HRTEM) (Fig. 3C). The lattice fringes with a d-spacing of 0.313 nm, 0.192 nm, 0.164 nm are contributed to the (111), (220), (311) crystal planes of ZnS [27]. Three distinct diffraction rings are seen from the
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Scheme 1. The schematic representation for preparation of GO/ZnS-CNFs composite membrane.
selected area electron diffraction (SAED) pattern in inset of Fig. 3C, which can be indexed to (111), (220), and (311) crystal planes of ZnS. The XRD patterns of as-prepared sample are shown in Fig. 3D. The asprepared sample (blue curve) has a sharp diffraction at 2θ value of ca. 13o which is consistent with the pure GO diffraction peak (green
curve). The other distinctive peaks located at around 28.6, 47.4, 56.3 can be indexed to (111), (220), and (311) crystal planes of ZnS (JCPDS No. 05-0566) [28], which is consistent with EDS and HR-TEM analysis. The above results confirm that the target GO/ZnS-CNFs composite membranes are successfully prepared.
Fig. 1. SEM images of GO-CNFs sample at a magnification of 10 K (A) and 20 K (B); SEM images of GO/ZnS-CNFs sample at a magnification of 10 K (C) and 20 K (D); EDS (E) and elemental mappings (F\ \I) of GO/ZnS-CNFs sample.
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Fig. 2. Flexibility testing of GO/ZnS-CNFs sample upon 100 bending cycles (A–D); flexibility and dimensional retention after deformation of GO/ZnS-CNFs sample (E and F).
Raman spectroscopy is employed to study and characterize the microstructure of carbon materials. As showed in Fig. 4A, the Raman spectrum of GO-CNFs displays two prominent peaks at 1355 and 1581 cm−1, corresponding to the well-documented D and G bands, respectively. The similar D and G bands also can be detected in GO/ZnSCNFs product. The ID /IG intensity ratio of GO-CNFs and GO/ZnS-CNFs are 1.19 and 1.20 respectively. The close ID /IG intensity ratio implies the negligible changes of graphitic structure in both samples [29], as shown in Fig. 4B. To analyze the chemical composition of the prepared products and to identify the chemical status of all elements in the samples, X-ray photoelectron spectroscopy (XPS) analysis was carried out. The XPS spectrum of the GO/ZnS-CNFs membranes exhibit prominent peaks of Zn,
S, O and relatively feeble peaks of C, as shown in Fig. 5A. The Fig. 5B shows the high-resolution XPS spectrum of Zn 2p. The two peaks at 1022.6 and 1043.7 eV of Zn 2p indicate the presence of Zn2 + in the product [29]. Fig. 5C is the high-resolution XPS spectrum of S 2p, which can be fitted as two peaks with binding energies at 162.1 and 168.9 eV, indicating the presence of the S2− [30]. Fig. 5D shows the carbon single/double bonds (C–C/C = C, peak 1, 284.6 eV) and oxygencontaining functional groups dominated by hydroxyl and epoxy groups (C–OH/C–O–C, peak 2, 285.8 eV) with relatively small amounts of carboxylic (COO, peak 3, 289.3 eV) groups [31]. The absorption edge and band-gap energies of the as-prepared GO/ ZnS-CNFs membrane were determined by UV–vis diffuse reflectance spectra. It can be founded that GO/ZnS-CNFs membranes exhibit the
Fig. 3. TEM images (A and B), HR-TEM image (C, SAED pattern in inset) and XRD patterns (D) of GO/ZnS-CNFs sample.
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Fig. 4. Raman spectra of GO-CNFs and GO/ZnS-CNFs samples (A) and the D and G peaks areas of GO-CNFs and GO/ZnS-CNFs samples (B).
strongest absorption under the UV light region. The band gap energy of the GO/ZnS-CNFs membranes is estimated to be 3.41 eV, as shown in Fig. 6A. The stronger absorption and the smaller band gap are contributed to the enhancement of photocatalytic activity of the as-prepared products [32,33]. The valence band (VB) of GO/ZnS-CNFs composite membranes measured by XPS valence spectra is 2.67 eV (Fig. 6B). Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves were used to investigate the porous structure of the as-prepared products. As shown in Fig. 6C, the as-prepared products exhibit the Type IV isotherms characteristic of mesoporosity [34]. The Brunauer-Emmett-Teller specific surface areas, mean pore diameters and total pore volumes of the as-prepared products are summarized in Fig. 6D. The GO/ZnS-CNFs exhibits the highest specific surface area at 22.31 m2/g and the lowest mean pore diameter at 40.86 nm. However, The GO-CNFs have the lower specific surface area at 9.76 m2/g and the larger mean pore diameter at 63.02 nm which are close to the values of GO-PAN products before calcination treatment. After introduced the
ZnS nanoparticles into composite membrane, the specific surface area of products is significantly increased. The high specific surface area could play a beneficial role in enhancing the capacity of adsorption during the catalytic reaction. The 4-aminotoluene is a colorless organic compound with shiny flake crystals. It is toxic and slightly soluble in water, alcohol, ether, benzene and hydrochloric acid. Besides, the 4-aminotoluene is a strong methemoglobin-forming agent, and can stimulate the bladder and urethra, which will cause hematuria [35–37]. 4-Nitrotoluene has low toxic and it is an important intermediate of dyes, medicine and plastic [38]. For investigation the catalytic activity of GO/ZnS-CNFs membrane, the catalytic conversion of 4-aminotoluene to 4-nitrotoluene has been carried out under UV irradiation and room temperature. The products during the photocatalytic oxidation were determined by high performance liquid chromatography (HPLC). As revealed in Fig. 7A, the relative retention time (RRT) of chromatographic peaks of pure 4-aminotoluene (black curve) and 4-nitrotoluene (red curve) are located at 2.53 min
Fig. 5. XPS spectra of GO/ZnS-CNFs (A); Zn 2p XPS data (B); S 2p XPS data (C) and C 1 s XPS data (D).
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Fig. 6. UV/vis diffuse reflectance spectra (A); valence band of the GO/ZnS-CNFs (B) and Brunauer–Emmett–Teller (BET) surface analysis and pore size distribution data (C and D) of GO– PAN, GO–CNFs and GO/ZnS-CNFs hybrid nanofibers.
Fig. 7. HPLC of 4-aminotoluene solutions after UV irradiation over the GO/ZnS-CNFs sample (A); the percent conversions for oxidation of 4-aminotoluene over ZnS, GO-CFs, GO/ZnS-CNFs and GO/ZnS-CNFs without UV irradiation (B); reusability of GO/ZnS-CNFs for oxidation of 4-aminotoluene (C) and trapping experiment of active species during the photocatalytic reaction (D).
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and 4.13 min, respectively. After adding a piece of GO/ZnS-CNFs membrane into the reactive solution, a distinctive peck at 4.1 min corresponded to the 4-nitrotoluene can be observed under UV irradiation for 1 h. And the chromatographic peak (2.53 min) of 4-aminotoluene is almost disappeared, which indicates the 4-aminotoluene has been successfully transformed into 4-nitrotoluene. However, with absence of GO/ZnS-CNFs membrane, the chromatographic peak (2.53 min) of 4aminotoluene is almost no change and no peaks around 4.1 min can be founded, which indicating the important role of GO/ZnS-CNFs membrane. For analysis the photocatalytic activity, the conversions percent for catalytic oxidation of 4-aminotoluene over pure ZnS, GO-CFs and GO/ZnSCNFs are tested under the same conditions. As depicted in the Fig. 7B, the conversions percent are about 80%, 5% over pure ZnS and GO-CNFs, while it can be further increased up to 90% over GO/ZnS-CNFs, which is due to the synergistic effect between GO-CNFs and ZnS. The GO/ZnS-CNFs is still keeping the high efficiency for photocatalytic oxidation after 5 cycles (Fig. 7C). The excellent reuse performance results from good binding property between GO, CNFs and ZnS and its outstanding mechanical flexibility. To investigate the plausible reaction mechanism for oxidation of 4-aminotoluene over GO/ZnS-CNFs membranes, the active species during photocatalytic reaction are detected. The hydroxyl radicals (•OH), superoxide radical (•O–2), and holes (h+) were investigated by adding 1.0 mM isopropyl alcohol (IPA) (a quencher of ·OH), p-benzoquinone (BQ) (a quencher of •O–2), and triethanolamine (TEOA) (a quencher of h+), respectively [34]. The method was similar to the former photocatalytic activity test. As shown in Fig. 7D, the photocatalytic conversion of 4-aminotoluene decreased obviously with the addition of TEOA, BQ and IPA. It indicates that •OH, •O–2 and h+ participated in the conversion of 4-aminotoluene to nitrotoluene. It is worth noting that the percent conversion of 4aminotoluene is only 18.1% after addition of TEOA, which is far lower than that with no adding any quenchers. Therefore, it can be concluded that the h+, •O–2 and •OH are the most important active species of the GO/ZnS-CNFs products in aqueous solution for photocatalytic oxidation of 4-aminotoluene. Furthermore, the conservation is higher (~90%) than that from the sum of the all quenchers indicating the presence of synergistic effect of these active species. UV irradiation generates several reactive species such as h+, •O–2 and •OH that cooperate in the photo-catalytic conversion of 4-aminotoluene. The oxidation-reduction potential of these reactive species is lower in a basic solution than in an acidic solution. In this study, 4-nitrotoluene is main product by photo-catalytic reaction of 4-aminotoluene. The possible reason is that •OH radicals can be scavenged by OH–. The reaction between •OH and OH– is given by [39]:
•
1 2O2
OH þ OH− → • O− 2 þ H2 O
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Thus, the direct oxidation pathway will be the dominant route for photo-catalytic conversion of 4-aminotoluene. The as-prepared GO/ZnS-CNFs products were further tested as catalyst for removal of phenol under a 500 W lamp to provide UV light (λ ≤ 400 nm). In a typical experiment, the original absorption peak of phenol is centered at 270 nm. After introducing GO/ZnS-CNFs into the phenol solution, the peak at 270 nm is decreased with increasing the irradiation time. After irradiation about 60 min, the peak is almost disappeared completely (Fig. 8A). Various authors have reported the photocatalytic oxidation of phenol as the model compound [40]. Three major aromatic intermediates were detected: ortho-dihydroxybenzene, para-dihydroxybenzene, and 1.4-benzoquinone. All of these species were identified in previous studies [20,41]. Moreover, the pseudofirst-order kinetics can be used to evaluate the kinetic reaction rate of the current catalytic reaction. The kinetic rate constant kapp = 3.0 × 10−2 s− 1 can be calculated from the rate equation ln(Ct/ C0) = −kapp t, where C0 is the initial concentration of phenol and Ct represents the concentration of phenol at the time t, as shown in Fig. 8B. Based on the results mentioned above, a possible photocatalytic mechanism of the GO/ZnS-CNFs membrane is proposed. The band gap energy and potential of edges of the valence band for the GO/ZnSCNFs membrane are 3.41 and 2.67 eV which has proved in Fig. 6. So the conduction band for the GOs/CNFs/ZnS membrane is − 0.74 eV. Under the UV light irradiation, the photogenerated h+, •O–2 and •OH are in their valance band and conductance band, respectively. In a weak basic condition (4-aminotoluene solution), •OH radicals can be scavenged by OH– to form •O–2. The direct oxidation pathway will be the dominant route for oxidation of the 4-aminotoluene in the water system. In the case of photocatalytic conversion of phenol, the pH of solution is acidic or near neutral. The photogenerated h+, •O–2 and •OH are involved in the photocatalytic reaction to form various aromatic intermediates. Moreover, the resultant hybrid carbon composite membranes offer the significant advantages, such as low dosage, high catalytic activity, easy recycling and excellent stability. 4. Conclusions A highly flexible nanofibrous composites membrane (GO/ZnS-CNFs) has been prepared by a convenient way combing of electrospinning, calcination and solvothermal treatments. Due to the synergistic effect between photocatalytic activity of ZnS and excellent adsorption capacity of GO-CNFs, the resultant GO/ZnS-CNFs exhibited excellent photocatalytic activity for oxidation of 4-aminotoluene and phenol under mild conditions. The h+, •O2– and •OH are the main active species of the GOs/CNFs/ZnS products in aqueous solution for photocatalytic conversion of phenol under UV light irradiation. The direct oxidation pathway
Fig. 8. Time-dependent changes of the phenol solution spectrum with GO/ZnS-CNFs (A); and the first order reaction curve for photocatalytic conversion of phenol (B).
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will be the dominant route for oxidation of the 4-aminotoluene due to •OH radicals can be scavenged by OH– to form •O–2. The resultant hybrid carbon composite membranes offer the significant advantages, such as low dosage, high catalytic activity, easy recycling and excellent stability. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51373155, 51133006) and “521 Talents Training Plan” in Zhejiang Sci-Tech University (ZSTU). References [1] H. Chen, G. Jiang, W. Yu, D. Liu, Y. Liu, L. Li, Q. Huang, Z. Tong, Electrospun carbon nanofiberic coated with urchin-like ZnCo2O4 nanosheets as a flexible electrode material, J. Mater. Chem. A 4 (2016) 5958–5964. [2] H. Chen, G. Jiang, L. Li, Y. Liu, Q. Huang, T. Jiang, X. Du, Facile fabrication of highly flexible graphene paper for photocatalytic reduction of 4-nitrophenol, Bull. Mater. Sci. 5 (2015) 1457–1463. [3] Z. Li, M.S. Akhtar, B. Yang, Supercapacitors with ultrahigh energy density based on mesoporous carbon nanofibers: enhanced double-layer electrochemical properties, J. Alloys Compd. 653 (2015) 212–218. [4] G. Jiang, Z. Wei, H. Chen, X. Du, L. Li, Y. Liu, Q. Huang, W. Chen, Preparation of novel carbon nanofibers with BiOBr and AgBr decorating for photocatalytic degradation of rhodamine B, RSC Adv. 5 (2015) 30433–30437. [5] J. Zhu, L. Zhu, Z. Lu, L. Gu, S. Cao, X. Cao, Selectively expanding graphene oxide paper for creating multifunctional carbon materials, J. Phys. Chem. C 116 (2012) 23075–23082. [6] H. Hsu, C. Wang, Y. Chang, J. Hu, B. Yao, C. Lin, Graphene oxides and carbon nanotubes embedded in polyacrylonitrile-based carbon nanofibers used as electrodes for supercapacitor, J. Phys. Chem. Solids 85 (2015) 62–68. [7] S. Park, C. Jin, H.W. Kim, C. Lee, Fabrication and luminescence properties of In2O3capped ZnS nanowires, J. Alloys Compd. 509 (2011) 6262–6266. [8] X. Gao, N. Zhuo, C. Liao, L. Xiao, H. Wang, Y. Cui, J. Zhang, Industrial fabrication of Mn-doped CdS/ZnS core/shell nanocrystals for white-light-emitting diodes, Opt. Mater. Express 10 (2015) 2164–2173. [9] Y. Yu, X. Gao, C. Liao, Y. Cut, J. Zhang, Electroluminescent characteristics of Mndoped CdS/ZnS core/shell nanocrystals, Chinese J. Inorg. Chem. 31 (2015) 859–900. [10] L. Wang, S. Huang, Synthesis and photoluminescent properties of ZnS:Cd nanoparticles and their phase-transferred nanocomposite with polyvinylpyrrolidone, Compos. Interfaces 2 (2015) 75–84. [11] J.K. Cooper, S. Gul, S.A. Lindley, J. Yano, J. Zhang, Tunable photoluminescent core/ shell Cu+-doped ZnSe/ZnS quantum dots codoped with Al3+, Ga3+, or In3+, ACS Appl. Mater. Interfaces 7 (2015) 10055–10066. [12] J.-Y. Park, D.-Y. Choi, K.-J. Hwang, S.-J. Park, S.-D. Yoon, Y.-H. Yun, X.G. Zhao, H.-B. Gu, I.-H. Lee, Synthesis of ZnS microspheres by template-free hydrothermal method for photocatalytic reaction, J. Nanosci. Nanotechnol. 15 (2015) 5224–5227. [13] C. Chang, K. Huang, J. Chen, K. Chu, M. Hsu, Improved photocatalytic hydrogen production of ZnO/ZnS based photocatalysts by Ce doping, J. Taiwan Inst. Chem. Eng. 55 (2015) 82–89. [14] C. Chen, H. Wu, B. Lin, Evaluating the development of high-tech industries: Taiwan's science park Original Research Article, Technol. Forecast. Soc. Chang. 73 (2005) 452–465. [15] C. Zheng, B. Yao, X. Duan, S. Bai, J. Li, T. Dai, C. Li, Y. Pan, Cr2+:ZnS saturable absorber passively Q-swithed Ho:LuVO4 laser, Chin. Phys. Lett. 32 (2015) 215–223. [16] J. Kaur, M. Sharm, O.P. Pandey, Structural and optical studies of undoped and copper doped zinc sulphide nanoparticles for photocatalytic application, Superlattice. Microst. 77 (2015) 35–53. [17] S. Jiang, G. Tang, Y. Ma, Y. Hu, L. Song, Synthesis of nitrogen-doped graphene-ZnS quantum dots composites with highly efficient visible light photodegradation, Mater. Chem. Phys. 151 (2015) 34–42. [18] M. Croston, J. Langston, R. Sangoi, K.S.V. Santhanam, Catalytic oxidation of p-toluidine at multiwalled functionalized carbon nanotubes, Int. J. Nanosci. 1 (2002) 277–283.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Preparation of electrospun ZnS-loaded hybrid carbon nanofiberic membranes for photocatalytic applications Hua Chen a, Guohua Jiang a,b,c,⁎, Weijiang Yu a, Depeng Liu a, Yongkun Liu a, Lei Li a, Qin Huang a, Zaizai Tong a,b,c, Wenxing Chen a,b,c a b c
Department of Materials Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, PR China National Engineering Laboratory for Textile Fiber Materials and Processing Technology (Zhejiang), Hangzhou 310018, PR China Key Laboratory of Advanced Textile Materials and Manufacturing Technology (ATMT), Ministry of Education, Hangzhou 310018, PR China
a r t i c l e
i n f o
Article history: Received 20 February 2016 Received in revised form 25 April 2016 Accepted 12 May 2016 Available online 14 May 2016 Keywords: Nanocarbon fibers Electrospun Oxidation Photocatalytic
a b s t r a c t In this paper, graphene oxide/polyacrylonitrile (GO/PAN) composite nanofibrous membranes were firstly prepared by an electrospinning method. Then, the GO/PAN composite nanofibrous membranes were further transferred to graphene oxides/carbon nanofibrous membranes (GO-CNFs) by a calcination treatment. Following a solvothermal treatment, zinc sulfide (ZnS) nanopartciles were covered onto GO-CNFs to form electrospun carbon nanofibrous membranes loaded with GO/ZnS (GO/ZnS-CNFs). The composition and microstructure of GO/ZnSCNFs were characterized by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman and UV–vis diffuse reflectance spectra analysis. Due to the synergistic effect between photocatalytic activity of ZnS and excellent adsorption capacity of GO-CNFs, the resultant GO/ZnS-CNFs exhibited excellent photocatalytic activity for oxidation of p-aminotoluene and phenol under mild conditions. The resultant hybrid carbon composite membranes offer the significant advantages, such as low dosage, high catalytic activity, easy recycling and excellent stability. © 2016 Elsevier B.V. All rights reserved.
1. Introduction As known, carbon materials have been widely studied as support for catalytic applications [1,2]. Carbon nanofibers (CNFs) were a new class of flexible materials with high mechanical strength, superior electroconductibility and excellent corrosion resistance. Meanwhile, they could supply a large surface area, which is critical for nanostructure-based catalytic materials [3]. In our previous study [4], we studied activated polyacrylonitrile carbon nanofibers with BiOBr and AgBr decorating for photocatalytic degradation of rhodamine B. The photocatalytic ability was dramatically improved due to the synergistic effect between photocatalytic activity of BiOBr and AgBr, and excellent adsorption capacity of CNFs. However, these composite CNFs were easy to be broken under high-speed stirring due to their low mechanical strength. Graphene oxide (GO) comprises graphene sheets with oxygen-containing functional groups on the basal planes and the edges. It can be fabricated in large quantities at a low cost [5]. The oxygen-containing functional groups of GO increase the interlayer distance between adjacent graphene sheets and interaction strength between graphene sheets and CNFs matrix, thus, enable catalytic nanoparticles to be adsorbed on graphene sheets as well, achieving composite CNFs with higher mechanical strength [6]. ⁎ Corresponding author. E-mail address: [email protected] (G. Jiang).
http://dx.doi.org/10.1016/j.powtec.2016.05.017 0032-5910/© 2016 Elsevier B.V. All rights reserved.
As one of typical II–IV binary semiconductor materials, zinc sulfide (ZnS) has received considerable concern due to its excellent properties. According to the previous reports, ZnS exists in two main crystalline forms: the cubic sphalerite phase and the hexagonal wurtzite phase [7]. Both phases are direct and large band-gap semiconductors at room temperature (3.7 eV for the cubic sphalerite phase and 3.8 eV for the hexagonal wurtzite phase), regarded as good candidates for light-emitting materials, electroluminescent devices, photoluminescent devices, photocatalysts, solar cells, optical sensors, lasers, and optical recording materials [8–16]. Recently, researchers have focused on the synthesis of ZnS composites through combined ZnS with other nanostructured carbon materials to form the delocalized conjugated materials. These composite materials are well matched with the photocatalysts in energy level and an intensive interface hybrid effect emerges between these materials, causing rapid charge separation and slow charge recombination in the electron-transfer process [17]. p-Aminotoluene (p-toluidine) is a colorless crystal with high toxicity. Although it can be oxidated in acetonitrile by hydrogen peroxide, its rate is a very slow [18]. Using ozone as oxidizing agent to oxide paminotoluene, the yield of aromatic products does not exceed 15% [19]. Phenol is one of the most toxic, and is used widely in petrochemical, chemical, and pharmaceutical industries. The presence of phenol in aqueous environments presents serious problems due to its toxicity, persistence in the environment, and bioaccumulation [20]. Therefore, the removal of phenol by environmentally friendly is a major
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consideration for current research. To pursue a high efficiency method oxide p-aminotoluene and phenol is attract considerable interest in organic synthesis and environmental protection. The photo-catalytic oxidation is an environmentally ‘benign’ or ‘green’ synthesis, since it consumes low-energy photons and occurs under atmospheric pressure and at room temperature [21]. In this paper, graphene oxide/polyacrylonitrile (GO/PAN) composite nanofibrous membranes were firstly prepared by an electrospunning method. Then, the GO/PAN composite nanofibrous membranes were further transferred to graphene oxides/ carbon nanofibrous membranes (GO/CNFs) by a calcination treatment. Following a solvothermal treatment, zinc sulfide (ZnS) nanoparticles were covered onto GO/CNFs to form electrospun carbon nanofibrous membranes loaded with GO/ZnS (GO/ZnS-CNFs). The catalytic activity of the resultant GO/ZnS-CNFs was investigated in oxidation of paminotoluene and phenol by a photochemical method. The recycling use of the catalyst was also of concern. The redox experiment was carried out without chemical additives and poisonous by-products under mild reaction conditions. The mechanism of oxidation was discussed. 2. Experimental 2.1. Materials Polyacrylonitrile (PAN, M w = 150.000) power, N, N′dimethylformamide (DMF, AR), zinc acetate (ZnC2H6O2·2H2O, AR) and sodium sulfide (Na2S·9H2O, AR) were purchased from aladdin Co. Ltd. (China). 2.2. Preparation of GO/ZnS-CNFs Typically, the GO solution was firstly fabricated on the basis of a modified Hummer's method [22]. Then, 1.0 g PAN, 0.1 g GO and 0.5 g zinc acetate were dissolved in 10 mL DMF by rapid stirring for 12 h to obtain a homogeneous solution. About 8 mL of the solution was placed in a 10 mL syringe. The syringe was placed in a syringe pump that maintained a solution feeding rate of 0.08 mm/min. The distance between the needle tip and collector was 12 cm, and the voltage was set at 15 kV. Subsequently, the sample was annealed at 550 °C for 3 h in pure nitrogen with a heating rate of 2 °C min−1 to obtain intermediate products. The intermediate products were put into 6 mM NaS solution and transferred to a 80 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 6 h. Finally, the product was dried at 60 °C for 6 h to obtain GO/ZnS-CNFs membranes. The mass fraction of ZnS on GO/ ZnS-CNFs membranes is about 13.24% that calculated by EDS analysis. 2.3. Photocatalytic conversion of 4-aminotoluene to 4-nitrotoluene The photocatalytic activity of GO/ZnS-CNFs membranes were detected through the photocatalytic oxidation of 4-aminotoluene in the mixed solution which the volume rate of water to acetonitrile is 3:2. The photocatalytic oxidation of 4-aminotoluene experiment was conducted in a photochemical reaction equipment with a 500 W metal halide lamp as the light source. In every experiment, 2 × 5 cm of GO/ZnSCNFs membrane was added into 10 mL of 4-aminotoluene solution (0.002 mol·L−1). The concentrations of 4-aminotoluene after an appropriate irradiation time were measured by HPLC technique. The mobile phase for HPLC was a mixture of water and acetonitrile in a ratio of 3:2 (v/v). 2.4. Characterization The structures and crystal phase of the as-prepared samples were analyzed with a SIEMENS Diffractometer D5000 X-ray diffractometer (XRD) with Cu Kα radiation source at 35 kV, with a scan rate of 4° s−1 in the 2θ range of 5–80°. X-ray photoelectron spectroscopy (XPS) data were obtained with an obtained with an ESCALab220i-XL electron
spectrometer from VG Scientific using 300 W Al Kα radiation. The ULTRA-55 field emission scanning electronmicroscopy (FE-SEM) at an accelerating voltage of 10 kV and JSM-2100 transmission electron microscopy (TEM) was used to characterize the morphology of the as-prepared samples. Raman spectra were recorded using a micro-Raman spectroscopy (TriVista TR557 Princeton Instruments). The Agilent high performance liquid chromatography (HPLC) 1100 was used to confirm the products in the reaction. The concentration of organics was measured by UV1901PC UV–vis spectrophotometer (Shanghai Aucy Technology Instrument CO.,LTD, China). 3. Results and discussion The schematic representation for preparation of GO/ZnS-CNFs membranes is shown in Scheme 1. The GO/PAN composite nanofibrous membranes are firstly prepared by a electrospunning method. Then, the GO/PAN composite nanofibrous membranes are further transferred to GO-CNFs by a calcination treatment. Fig. 1A shows the electrospun nanofibers after calcination treatment are the homogeneous nanofibrous mats with reveal bulge structure. Apparently, a large number of pores are observed on the surface of as-prepared samples, as shown in Fig. 1B. These pores are formed due to the release of gases pyrolysis of polymer matrix during the calcination process [23,24], which are beneficial for absorbing the organic molecules onto the catalytic active sites on the surface of composite nanofibers. Following a solvothermal treatment, zinc sulfide (ZnS) nanoparticles are covered onto GO-CNFs to form electrospun carbon nanofibrous membranes loaded with GO/ZnS (GO/ZnS-CNFs). Fig. 1C and D are the SEM images of GO/ZnS-CNFs membrane at different magnifications. The microcosmic structure of the membrane is maintained after solvothermal reaction and the ZnS nanoparticles with a diameter at about 10 nm exhibit a good dispersity on the surface of nanofibers. The energy dispersive X-ray spectroscopy (EDS) verify the existence of C, O, Zn and S elements in GO/ZnS-CNFs membrane samples (Fig. 1E), and atom ratio of S/Zn closed to 1: 1 (inset in Fig. 1E) suggests that nanoparticles distributed in composite membrane are ZnS. The S, Zn, C, and O edge mappings are shown in Fig. 1F-I, respectively. It can be seen that the orange color (assign to S), red color (assign to Zn), white color (assign to C) and green color (assign to O) are uniformly distributed in the sample of GO/ZnS-CNFs membranes. The resultant GO/ZnS-CNFs membranes are expected to present outstanding mechanical flexibility, enabling them to maintain the original dimensions after folding and crushing (Fig. 2). Apparently, the strategy has shown excellent versatility and scalability for potentially large-scale fabricating lightweight graphene-based carbon nanofibrous products by employing specific textured matrices. The high mechanical flexibility may be contributed to the including of GO nanosheets, which have numerous oxygen-containing functional groups and presents the high specific surface area and crumpled surface nanofibers with curling edges, could play a beneficial role in enhancing interfacial interaction with the GO and the carbon nanofibrous matrix [25]. To further obtain the microscopic morphology and structure information, the transmission electron microscopy (TEM) analysis of GO/ ZnS-CNFs has been performed. As shown in Fig. 3A, a light-colored layer on the surface of nanofibers can be observed. Further magnification of surface structure of nanofibers, the light-colored layer is consisted of many nanosheets, indicating that the GO is successfully incorporated onto the nanofibers, as shown in Fig. 3B. Furthermore, no clear interface between the GO nanosheets and the CNFs can be founded, indicating that the junction at the edges of GO and CNFs might be linked by strong interaction [26]. Besides, many nanoparticles also can be observed in the nanofibers. The crystalline nature of these nanoparticles is further investigated by high resolution TEM (HRTEM) (Fig. 3C). The lattice fringes with a d-spacing of 0.313 nm, 0.192 nm, 0.164 nm are contributed to the (111), (220), (311) crystal planes of ZnS [27]. Three distinct diffraction rings are seen from the
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Scheme 1. The schematic representation for preparation of GO/ZnS-CNFs composite membrane.
selected area electron diffraction (SAED) pattern in inset of Fig. 3C, which can be indexed to (111), (220), and (311) crystal planes of ZnS. The XRD patterns of as-prepared sample are shown in Fig. 3D. The asprepared sample (blue curve) has a sharp diffraction at 2θ value of ca. 13o which is consistent with the pure GO diffraction peak (green
curve). The other distinctive peaks located at around 28.6, 47.4, 56.3 can be indexed to (111), (220), and (311) crystal planes of ZnS (JCPDS No. 05-0566) [28], which is consistent with EDS and HR-TEM analysis. The above results confirm that the target GO/ZnS-CNFs composite membranes are successfully prepared.
Fig. 1. SEM images of GO-CNFs sample at a magnification of 10 K (A) and 20 K (B); SEM images of GO/ZnS-CNFs sample at a magnification of 10 K (C) and 20 K (D); EDS (E) and elemental mappings (F\ \I) of GO/ZnS-CNFs sample.
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Fig. 2. Flexibility testing of GO/ZnS-CNFs sample upon 100 bending cycles (A–D); flexibility and dimensional retention after deformation of GO/ZnS-CNFs sample (E and F).
Raman spectroscopy is employed to study and characterize the microstructure of carbon materials. As showed in Fig. 4A, the Raman spectrum of GO-CNFs displays two prominent peaks at 1355 and 1581 cm−1, corresponding to the well-documented D and G bands, respectively. The similar D and G bands also can be detected in GO/ZnSCNFs product. The ID /IG intensity ratio of GO-CNFs and GO/ZnS-CNFs are 1.19 and 1.20 respectively. The close ID /IG intensity ratio implies the negligible changes of graphitic structure in both samples [29], as shown in Fig. 4B. To analyze the chemical composition of the prepared products and to identify the chemical status of all elements in the samples, X-ray photoelectron spectroscopy (XPS) analysis was carried out. The XPS spectrum of the GO/ZnS-CNFs membranes exhibit prominent peaks of Zn,
S, O and relatively feeble peaks of C, as shown in Fig. 5A. The Fig. 5B shows the high-resolution XPS spectrum of Zn 2p. The two peaks at 1022.6 and 1043.7 eV of Zn 2p indicate the presence of Zn2 + in the product [29]. Fig. 5C is the high-resolution XPS spectrum of S 2p, which can be fitted as two peaks with binding energies at 162.1 and 168.9 eV, indicating the presence of the S2− [30]. Fig. 5D shows the carbon single/double bonds (C–C/C = C, peak 1, 284.6 eV) and oxygencontaining functional groups dominated by hydroxyl and epoxy groups (C–OH/C–O–C, peak 2, 285.8 eV) with relatively small amounts of carboxylic (COO, peak 3, 289.3 eV) groups [31]. The absorption edge and band-gap energies of the as-prepared GO/ ZnS-CNFs membrane were determined by UV–vis diffuse reflectance spectra. It can be founded that GO/ZnS-CNFs membranes exhibit the
Fig. 3. TEM images (A and B), HR-TEM image (C, SAED pattern in inset) and XRD patterns (D) of GO/ZnS-CNFs sample.
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Fig. 4. Raman spectra of GO-CNFs and GO/ZnS-CNFs samples (A) and the D and G peaks areas of GO-CNFs and GO/ZnS-CNFs samples (B).
strongest absorption under the UV light region. The band gap energy of the GO/ZnS-CNFs membranes is estimated to be 3.41 eV, as shown in Fig. 6A. The stronger absorption and the smaller band gap are contributed to the enhancement of photocatalytic activity of the as-prepared products [32,33]. The valence band (VB) of GO/ZnS-CNFs composite membranes measured by XPS valence spectra is 2.67 eV (Fig. 6B). Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves were used to investigate the porous structure of the as-prepared products. As shown in Fig. 6C, the as-prepared products exhibit the Type IV isotherms characteristic of mesoporosity [34]. The Brunauer-Emmett-Teller specific surface areas, mean pore diameters and total pore volumes of the as-prepared products are summarized in Fig. 6D. The GO/ZnS-CNFs exhibits the highest specific surface area at 22.31 m2/g and the lowest mean pore diameter at 40.86 nm. However, The GO-CNFs have the lower specific surface area at 9.76 m2/g and the larger mean pore diameter at 63.02 nm which are close to the values of GO-PAN products before calcination treatment. After introduced the
ZnS nanoparticles into composite membrane, the specific surface area of products is significantly increased. The high specific surface area could play a beneficial role in enhancing the capacity of adsorption during the catalytic reaction. The 4-aminotoluene is a colorless organic compound with shiny flake crystals. It is toxic and slightly soluble in water, alcohol, ether, benzene and hydrochloric acid. Besides, the 4-aminotoluene is a strong methemoglobin-forming agent, and can stimulate the bladder and urethra, which will cause hematuria [35–37]. 4-Nitrotoluene has low toxic and it is an important intermediate of dyes, medicine and plastic [38]. For investigation the catalytic activity of GO/ZnS-CNFs membrane, the catalytic conversion of 4-aminotoluene to 4-nitrotoluene has been carried out under UV irradiation and room temperature. The products during the photocatalytic oxidation were determined by high performance liquid chromatography (HPLC). As revealed in Fig. 7A, the relative retention time (RRT) of chromatographic peaks of pure 4-aminotoluene (black curve) and 4-nitrotoluene (red curve) are located at 2.53 min
Fig. 5. XPS spectra of GO/ZnS-CNFs (A); Zn 2p XPS data (B); S 2p XPS data (C) and C 1 s XPS data (D).
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Fig. 6. UV/vis diffuse reflectance spectra (A); valence band of the GO/ZnS-CNFs (B) and Brunauer–Emmett–Teller (BET) surface analysis and pore size distribution data (C and D) of GO– PAN, GO–CNFs and GO/ZnS-CNFs hybrid nanofibers.
Fig. 7. HPLC of 4-aminotoluene solutions after UV irradiation over the GO/ZnS-CNFs sample (A); the percent conversions for oxidation of 4-aminotoluene over ZnS, GO-CFs, GO/ZnS-CNFs and GO/ZnS-CNFs without UV irradiation (B); reusability of GO/ZnS-CNFs for oxidation of 4-aminotoluene (C) and trapping experiment of active species during the photocatalytic reaction (D).
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and 4.13 min, respectively. After adding a piece of GO/ZnS-CNFs membrane into the reactive solution, a distinctive peck at 4.1 min corresponded to the 4-nitrotoluene can be observed under UV irradiation for 1 h. And the chromatographic peak (2.53 min) of 4-aminotoluene is almost disappeared, which indicates the 4-aminotoluene has been successfully transformed into 4-nitrotoluene. However, with absence of GO/ZnS-CNFs membrane, the chromatographic peak (2.53 min) of 4aminotoluene is almost no change and no peaks around 4.1 min can be founded, which indicating the important role of GO/ZnS-CNFs membrane. For analysis the photocatalytic activity, the conversions percent for catalytic oxidation of 4-aminotoluene over pure ZnS, GO-CFs and GO/ZnSCNFs are tested under the same conditions. As depicted in the Fig. 7B, the conversions percent are about 80%, 5% over pure ZnS and GO-CNFs, while it can be further increased up to 90% over GO/ZnS-CNFs, which is due to the synergistic effect between GO-CNFs and ZnS. The GO/ZnS-CNFs is still keeping the high efficiency for photocatalytic oxidation after 5 cycles (Fig. 7C). The excellent reuse performance results from good binding property between GO, CNFs and ZnS and its outstanding mechanical flexibility. To investigate the plausible reaction mechanism for oxidation of 4-aminotoluene over GO/ZnS-CNFs membranes, the active species during photocatalytic reaction are detected. The hydroxyl radicals (•OH), superoxide radical (•O–2), and holes (h+) were investigated by adding 1.0 mM isopropyl alcohol (IPA) (a quencher of ·OH), p-benzoquinone (BQ) (a quencher of •O–2), and triethanolamine (TEOA) (a quencher of h+), respectively [34]. The method was similar to the former photocatalytic activity test. As shown in Fig. 7D, the photocatalytic conversion of 4-aminotoluene decreased obviously with the addition of TEOA, BQ and IPA. It indicates that •OH, •O–2 and h+ participated in the conversion of 4-aminotoluene to nitrotoluene. It is worth noting that the percent conversion of 4aminotoluene is only 18.1% after addition of TEOA, which is far lower than that with no adding any quenchers. Therefore, it can be concluded that the h+, •O–2 and •OH are the most important active species of the GO/ZnS-CNFs products in aqueous solution for photocatalytic oxidation of 4-aminotoluene. Furthermore, the conservation is higher (~90%) than that from the sum of the all quenchers indicating the presence of synergistic effect of these active species. UV irradiation generates several reactive species such as h+, •O–2 and •OH that cooperate in the photo-catalytic conversion of 4-aminotoluene. The oxidation-reduction potential of these reactive species is lower in a basic solution than in an acidic solution. In this study, 4-nitrotoluene is main product by photo-catalytic reaction of 4-aminotoluene. The possible reason is that •OH radicals can be scavenged by OH–. The reaction between •OH and OH– is given by [39]:
•
1 2O2
OH þ OH− → • O− 2 þ H2 O
7
Thus, the direct oxidation pathway will be the dominant route for photo-catalytic conversion of 4-aminotoluene. The as-prepared GO/ZnS-CNFs products were further tested as catalyst for removal of phenol under a 500 W lamp to provide UV light (λ ≤ 400 nm). In a typical experiment, the original absorption peak of phenol is centered at 270 nm. After introducing GO/ZnS-CNFs into the phenol solution, the peak at 270 nm is decreased with increasing the irradiation time. After irradiation about 60 min, the peak is almost disappeared completely (Fig. 8A). Various authors have reported the photocatalytic oxidation of phenol as the model compound [40]. Three major aromatic intermediates were detected: ortho-dihydroxybenzene, para-dihydroxybenzene, and 1.4-benzoquinone. All of these species were identified in previous studies [20,41]. Moreover, the pseudofirst-order kinetics can be used to evaluate the kinetic reaction rate of the current catalytic reaction. The kinetic rate constant kapp = 3.0 × 10−2 s− 1 can be calculated from the rate equation ln(Ct/ C0) = −kapp t, where C0 is the initial concentration of phenol and Ct represents the concentration of phenol at the time t, as shown in Fig. 8B. Based on the results mentioned above, a possible photocatalytic mechanism of the GO/ZnS-CNFs membrane is proposed. The band gap energy and potential of edges of the valence band for the GO/ZnSCNFs membrane are 3.41 and 2.67 eV which has proved in Fig. 6. So the conduction band for the GOs/CNFs/ZnS membrane is − 0.74 eV. Under the UV light irradiation, the photogenerated h+, •O–2 and •OH are in their valance band and conductance band, respectively. In a weak basic condition (4-aminotoluene solution), •OH radicals can be scavenged by OH– to form •O–2. The direct oxidation pathway will be the dominant route for oxidation of the 4-aminotoluene in the water system. In the case of photocatalytic conversion of phenol, the pH of solution is acidic or near neutral. The photogenerated h+, •O–2 and •OH are involved in the photocatalytic reaction to form various aromatic intermediates. Moreover, the resultant hybrid carbon composite membranes offer the significant advantages, such as low dosage, high catalytic activity, easy recycling and excellent stability. 4. Conclusions A highly flexible nanofibrous composites membrane (GO/ZnS-CNFs) has been prepared by a convenient way combing of electrospinning, calcination and solvothermal treatments. Due to the synergistic effect between photocatalytic activity of ZnS and excellent adsorption capacity of GO-CNFs, the resultant GO/ZnS-CNFs exhibited excellent photocatalytic activity for oxidation of 4-aminotoluene and phenol under mild conditions. The h+, •O2– and •OH are the main active species of the GOs/CNFs/ZnS products in aqueous solution for photocatalytic conversion of phenol under UV light irradiation. The direct oxidation pathway
Fig. 8. Time-dependent changes of the phenol solution spectrum with GO/ZnS-CNFs (A); and the first order reaction curve for photocatalytic conversion of phenol (B).
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will be the dominant route for oxidation of the 4-aminotoluene due to •OH radicals can be scavenged by OH– to form •O–2. The resultant hybrid carbon composite membranes offer the significant advantages, such as low dosage, high catalytic activity, easy recycling and excellent stability. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51373155, 51133006) and “521 Talents Training Plan” in Zhejiang Sci-Tech University (ZSTU). References [1] H. Chen, G. Jiang, W. Yu, D. Liu, Y. Liu, L. Li, Q. Huang, Z. Tong, Electrospun carbon nanofiberic coated with urchin-like ZnCo2O4 nanosheets as a flexible electrode material, J. Mater. Chem. A 4 (2016) 5958–5964. [2] H. Chen, G. Jiang, L. Li, Y. Liu, Q. Huang, T. Jiang, X. Du, Facile fabrication of highly flexible graphene paper for photocatalytic reduction of 4-nitrophenol, Bull. Mater. Sci. 5 (2015) 1457–1463. [3] Z. Li, M.S. Akhtar, B. Yang, Supercapacitors with ultrahigh energy density based on mesoporous carbon nanofibers: enhanced double-layer electrochemical properties, J. Alloys Compd. 653 (2015) 212–218. [4] G. Jiang, Z. Wei, H. Chen, X. Du, L. Li, Y. Liu, Q. Huang, W. Chen, Preparation of novel carbon nanofibers with BiOBr and AgBr decorating for photocatalytic degradation of rhodamine B, RSC Adv. 5 (2015) 30433–30437. [5] J. Zhu, L. Zhu, Z. Lu, L. Gu, S. Cao, X. Cao, Selectively expanding graphene oxide paper for creating multifunctional carbon materials, J. Phys. Chem. C 116 (2012) 23075–23082. [6] H. Hsu, C. Wang, Y. Chang, J. Hu, B. Yao, C. Lin, Graphene oxides and carbon nanotubes embedded in polyacrylonitrile-based carbon nanofibers used as electrodes for supercapacitor, J. Phys. Chem. Solids 85 (2015) 62–68. [7] S. Park, C. Jin, H.W. Kim, C. Lee, Fabrication and luminescence properties of In2O3capped ZnS nanowires, J. Alloys Compd. 509 (2011) 6262–6266. [8] X. Gao, N. Zhuo, C. Liao, L. Xiao, H. Wang, Y. Cui, J. Zhang, Industrial fabrication of Mn-doped CdS/ZnS core/shell nanocrystals for white-light-emitting diodes, Opt. Mater. Express 10 (2015) 2164–2173. [9] Y. Yu, X. Gao, C. Liao, Y. Cut, J. Zhang, Electroluminescent characteristics of Mndoped CdS/ZnS core/shell nanocrystals, Chinese J. Inorg. Chem. 31 (2015) 859–900. [10] L. Wang, S. Huang, Synthesis and photoluminescent properties of ZnS:Cd nanoparticles and their phase-transferred nanocomposite with polyvinylpyrrolidone, Compos. Interfaces 2 (2015) 75–84. [11] J.K. Cooper, S. Gul, S.A. Lindley, J. Yano, J. Zhang, Tunable photoluminescent core/ shell Cu+-doped ZnSe/ZnS quantum dots codoped with Al3+, Ga3+, or In3+, ACS Appl. Mater. Interfaces 7 (2015) 10055–10066. [12] J.-Y. Park, D.-Y. Choi, K.-J. Hwang, S.-J. Park, S.-D. Yoon, Y.-H. Yun, X.G. Zhao, H.-B. Gu, I.-H. Lee, Synthesis of ZnS microspheres by template-free hydrothermal method for photocatalytic reaction, J. Nanosci. Nanotechnol. 15 (2015) 5224–5227. [13] C. Chang, K. Huang, J. Chen, K. Chu, M. Hsu, Improved photocatalytic hydrogen production of ZnO/ZnS based photocatalysts by Ce doping, J. Taiwan Inst. Chem. Eng. 55 (2015) 82–89. [14] C. Chen, H. Wu, B. Lin, Evaluating the development of high-tech industries: Taiwan's science park Original Research Article, Technol. Forecast. Soc. Chang. 73 (2005) 452–465. [15] C. Zheng, B. Yao, X. Duan, S. Bai, J. Li, T. Dai, C. Li, Y. Pan, Cr2+:ZnS saturable absorber passively Q-swithed Ho:LuVO4 laser, Chin. Phys. Lett. 32 (2015) 215–223. [16] J. Kaur, M. Sharm, O.P. Pandey, Structural and optical studies of undoped and copper doped zinc sulphide nanoparticles for photocatalytic application, Superlattice. Microst. 77 (2015) 35–53. [17] S. Jiang, G. Tang, Y. Ma, Y. Hu, L. Song, Synthesis of nitrogen-doped graphene-ZnS quantum dots composites with highly efficient visible light photodegradation, Mater. Chem. Phys. 151 (2015) 34–42. [18] M. Croston, J. Langston, R. Sangoi, K.S.V. Santhanam, Catalytic oxidation of p-toluidine at multiwalled functionalized carbon nanotubes, Int. J. Nanosci. 1 (2002) 277–283.
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