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Free Channel Formation around Graphitic Carbon Nitride Embedded in Porous Polyethylene Terephthalate Nanofibers with Excellent Reusability for Eliminating Antibiotics under Solar Irradiation Dandan Qin, Wangyang Lu, Zhexin Zhu, Nan Li, Tiefeng Xu, Gangqiang Wang, and Wenxing Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02800 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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Industrial & Engineering Chemistry Research

Free Channel Formation around Graphitic Carbon Nitride Embedded in Porous Polyethylene Terephthalate Nanofibers with Excellent Reusability for Eliminating Antibiotics under Solar Irradiation Dandan Qin, Wangyang Lu*, Zhexin Zhu, Nan Li, Tiefeng Xu, Gangqiang Wang, and Wenxing Chen*

National Engineering Lab for Textile Fiber Materials & Processing Technology (Zhejiang), Zhejiang Sci-Tech University, Hangzhou 310018, China. E-mail: [email protected], [email protected]

ABSTRACT: The sublethal antibiotics content in the environment brings added urgency to degrade antibiotics for preventing the growth of antibiotic resistance and the public health crisis. Herein, we adopted a rational and intriguing strategy for constructing a porous supported-photocatalyst. A solution containing graphitic carbon nitride (g-C3N4), polyethylene glycol (PEG) and polyethylene terephthalate (PET) was electrospun, and followed by a post-processing to remove the PEG, obtainning a porous nanofibers (g-C3N4@PET). Meanwhile, the g-C3N4@PET morphology and composition were determined by a series of analysis techniques. The g-C3N4/PET exhibit several favorable characteristics, namely, (i) easy to create more active sites due to the formation of pores throughout the nanofibers and successful embedding of 1

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g-C3N4, (ii) interconnected channel is benefit for catalysts-antibiotics contact and light absorption, (iii) possesses high photocatalytic performance and reusability, avoiding the reunion and sedimentation of pure g-C3N4. This approach has an enormous potential for loading powder catalysts in real applications.

1. INTRODUCTION

The large application of antibiotics to prevent diseases in humans and livestock has led to the presence of sublethal antibiotics content in the environment. This situation increases the threat of developing a stronger resistance to antibiotics in every region of the world.1,

2

Recently, Long et al.3 found that antibiotics are equivalent to an

indirect bacterial mutagen, which could impose the selective challenge and accelerate resistance mutations. According to a World Health Organization report, the invention of new antibiotics is entails high costs, significant time-consumption and low margins and is therefore incapable of keeping up with the emergence of antibiotic resistance.4 Worse yet, a bleak report estimates that the number of people dying from antimicrobial resistance will climb to 10 million from 700 000 by 2050.5 As a consequence of this, it is an urgent demand to overcome the challenge of antibiotic resistance not only by using antibiotics only when essential, but also by degrading antibiotics. On the other hand, common antibiotics treatments present some drawbacks. For example, physical adsorption is limited due to the difficulty of disposing of the sludge and the microbial method is extraordinarily time-consuming.

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Although the advanced oxidation technology is highly efficient, it does not work without the addition of an oxidizer. In this context, the development of a rational approach characterized by high performance and low energy consumption is desired. Currently, photocatalysis has attracted the interest of numerous scholars dealing with problems related to chemical synthesis, environmental purification6 and energy generation. This process possesses a tremendous potential as it can utilize the inexhaustible solar energy through an adequate photocatalyst.7 Nowadays, graphitic carbon nitride (g-C3N4) is being greatly considered on account of its high stability, visible light response, multiple preparation methods and the structure that is easy to tailor.8-10 Its practical implementation is, however, still restricted because of the easy aggregation and rapid recombination of photoexcited electrons (e-) and holes (h+). Enormous efforts have been devoted to address the above-mentioned problems.11-15 Despite some advances, a straightforward method for the improvement of actual applications of g-C3N4 is still missing, mainly because of the difficulty of recycling the material used in nanoscale photocatalysts, which is in turn related to the sedimentation. Such a challenge concerns all powder catalysts and is still difficult to tackle.

So far, loading the powder catalysts on other substrates is a common technology employed to improve performance, dispersity, recyclability and stability.16-18 Loading may be achieved via diversified synthesis approaches, such as adsorption, chemical vapor deposition,19 chemical bonding,20 and impregnation.21 Nevertheless, they are far 3



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from those multifaceted requirements containing high photocatalytic activity, fascinating loading fastness, high reusability, a simple preparation method, etc. Apart from optimizing the preparation methods, selecting and building a novel carrier to replace those commonly used, such as heavy metals, metal oxides,22, 23 and silicates,24 is another area of passionate research. Noble metals promote the separation and transfer of electrons and holes, but are expensive and cause immense pressure on the environment. Thus, polymers with extraordinary advantages such as high yield, low weight and low cost have attracted increasing attention in the scientific community.2527

Yoon et al.28 dispersed TiO2 on polystyrene fibers with controlled wettability. The

high surface area of the supports is extremely desired as it can improve light absorption and the interaction between the reactants and active catalytic sites. In this view, nanofibers or carbon fibers have been regarded as candidates for loading nanoscale or powder catalysts on account of their high specific surface area. Lee et al.29 prepared a highly reactive TiO2 coating on polymer nanofibers. To date, the electrospinning technology has been widely performed to develop continuous polymer nanofibers,30-32 however, the powder catalyst inside fibers is still not work in photocatalytic reactions due to the lack of exposure.33 For this reason, the introduction of pores or channels into the entire nanofibers is desired as it can enhance light penetration, accelerate the mass transfer and make the best of active radicals.34 Shui et al.35 prepared porous nanofibers containing a metal-organic framework for catalytic application. Today, hierarchical porous nanofibers are usually constructed through 4


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phase separation during electrospinning or post-treatment including the selective removal of a component via dissolution or carbonization.30,

36, 37

However, the

formation of porous nanofibers by phase separation is quite inconvenient. Besides, the carbon nanofibers obtained via carbonization of electrospun nanofibers become black, preventing the catalysts inside the fibers from absorbing light.

Herein, we demonstrate a distinctive and simple method for the preparation of gC3N4-loaded, highly porous nanofibers. To synthesize the porous nanofibers, we chose polyethylene terephthalate (PET) as a support, and polyethylene glycol (PEG) as a porogen. The preparation procedure involved electrospinning a blending solution containing g-C3N4, PEG and PET a subsequent post-treatment process. The electrospun nanofibers (g-C3N4/PEG/PET) were immersed in a water bath and heated to 60°C for 24 h, until the PEG was completely removed from the fibers. The removal of PEG is a pivotal step in the development an interconnected pore structure that leads the g-C3N4 to be successfully embedded by the porous nanofibers and allows easy access to active sites via efficient mass transfer. The obtained porous nanofibers (gC3N4@PET) were characterized by various analysis technologies, which showed a uniformly porous structure with a high surface, significantly increasing a lot of active photocatalytic sites and the accessibility for substrates. Additionally, the porous nanofibers g-C3N4@PET exhibited high photocatalytic performance, which can rival that of the freestanding g-C3N4, and it also possessed an intriguing reusability in the degradation of antibiotics using solar irradiation. Finally, we proposed a probable 5



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mechanism for antibiotics degradation and a possible SQX degradation pathway by the g-C3N4@PET under solar irradiation. This synthesis method could provide a new idea to load powder or nanoscale catalysts on fibers, with enormous potential in the fields of catalysis, filtration, drug delivery, tissue engineering, and sensing.

2. EXPERIMENTAL SECTION

2.1. Materials. PET was purchased from Zhejiang Guxiandao Industrial Fiber Co., Ltd. 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) was obtained from Fluorochem, Ltd. Potassium iodide (KI),isopropanol (IPA) and dimethyl pyridine N-oxide (DMPO) come from Hangzhou Gao Jing Fine Chemical Co., Ltd. and Tianjin Wing Tai Chemical Co., Ltd., respectively. Other reagents, such as urea, polyethylene glycol (PEG 20000), sulfaquinoxaline (SQX), sulfachloropyridazine, sulfamerazine, sulfadiazine , sulfamethoxydiazine, methylene blue , rhodamine B, sodium dodecyl sulfonate (SDS), p-benzoquinone (BQ) and p-chlorophenol , were received from Aladdin Chemical Co., Ltd. Acetonitrile (Sigma–Aldrich), formic acid (J&K chemical Inc.) and methanol (Merck) all belonged to chromatographic grade for ultraperformance liquid chromatography . All other chemicals were analytical reagents and were used as received without further purification. Water used throughout the experiments was ultrafiltrated by Milli-Q Advantage A10 (Millipore).

2.2. Photocatalyst preparation. g-C3N4 was synthesized through thermal decomposition of urea, as described by Dong et al.38 g-C3N4 (0.08 g) and PET (0.8 g) 6


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were added into HFIP (8 g) using vigorous magnetic stirring at ambient temperature until a uniform suspension was obtained. The g-C3N4/PEG/PET electrospinning solutions were then prepared by dissolving PEG into the above suspension. And the PEG/PET weight ratios were varied as follows: 2%, 5%, 10%, 20%, 30%. The obtained g-C3N4/PEG/PET electrospinning solutions were all a faint yellow viscous liquid. What’s more, the solution viscosity increased with the PEG content. In addition, as the PEG/PET weight ratio reached to 30% or more, the PEG dissolving became difficult. PEG/PET and PET electrospinning solutions were also prepared through the same procedure. Each electrospinning solution was then loaded into a 10 mL syringe for the electrospinning process, carried out with the same operating parameters for the different solution. The conditions adopted were: flow rate of 1 mL/h, 15 kV applied voltage and a collecting distance of 21 cm. The PET, PEG/PET and g-C3N4/PEG/PET nanofiber films were collected and the PEG/PET and gC3N4/PEG/PET were immersed in a 100 mL water bath at 60°C for 24 h. Water was changed every hour. Finally, we obtained the T-PET and g-C3N4@PET porous nanofibers.

2.3. Characterization. Sample morphologies were observed by scanning electron microscopy (SEM), performed on a Field emission scanning electronic microscope (FESEM, ULTRA-55, 3 kV). Transmission electron microscopy (TEM) images were taken on a JEOL JEM-2010 transmission electron microscope, 200 kV. The surface area and pore size distribution (PSD) of the relevant samples were determined by the 7



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Brunauer–Emmett–Teller (BET) method at a nitrogen adsorption apparatus (Micromeritics, ASAP2020HD88). Sample composition was analyzed via a Fouriertransform infrared spectrometer (Thermo Nicolet 5700), UV-Vis spectrometer (HITACHI, 1J1-0015), two-dimensional X-ray diffractometer (Bruker, D8 Discover) with Cu Kα radiation, and thermogravimetric analysis (Mettler Toledo, Switzerland) carried out from 30°C to 800°C at 10°C/min in air atmosphere. The photoluminescence spectra were operated in a fluorescence spectrophotometer (Hitachi, F-4600). The electronic band structure was monitored by electron paramagnetic resonance (Bruker, A300).

2.4. Photocatalytic experiments. Photocatalytic reactions were performed in a SUN-Q-Light photoreactor (Xe-1-BC, U.S.A.).17 SQX was selected as a model antibiotic. An aqueous solution of SQX (2×10−5 mol/L, 30 mL) containing the photocatalyst (30 mg) was photoirradiated under sunlight, 200 rpm, as shown in Figure S1. Before solar irradiation, the SQX solution (containing photocatalysts) had been kept in the dark for 2 hours to reach adsorption–desorption equilibrium. During the experiments, about 1 mL of solution was collected at fixed time intervals and was filtered by a 0.22 µm membrane filter to remove impurities. The SQX concentration was measured via high performance liquid chromatography (Acquity BEH C18 column, 1.7 µm, 2.1×50 mm2, Waters). The mobile phase was 20% acetonitrile and 80% water, containing 0.1% formic acid. All the photocatalytic tests were performed in duplicate. To evaluate the stability and reusability of the photocatalysts, after each 8


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cycle, take the photocatalysts out and then add it into another new SQX solution to perform other photocatalytic experiments under the same conditions.

3. RESULITS AND DISCUSSION

3.1. Characterization. Figure 1 exhibits the FESEM micrographs of the gC3N4@PET nanofibers obtained by the post-treatment for the electrospun gC3N4/PEG/PET nanofibers with different PEG contents. It is evident that the number and diameter of pores on the fiber surface increased with increasing PEG contents in the electrospinning solution. During post-processing, the PEG present in the fibers was removed via dissolution in water, resulting in the formation of pores.39, 40 The pores provide many active sites that can ensure the contact between the reactants and the g-C3N4 catalyst in the fibers.35, 36 The Figure 1 shows that despite the high amount of pores obtained with a 30% PEG concentration, many fibers were bonded to each other. In addition, the electrospinning process for the g-C3N4/PEG/PET solution containing 30% PEG is fairly difficult due to the high viscosity of the solution. Thus, we chose g-C3N4/PEG/PET nanofibers containing 20% PEG as the main study object. The morphologies of T-PET, g-C3N4/PEG/PET, g-C3N4@PET nanofibers are presented in Figure 2 (A–F). The surface of g-C3N4/PEG/PET nanofibers is smooth, while T-PET and g-C3N4@PET exhibit many uniform pores, which resulted from the removal of PEG. Furthermore, Figure 2 (G, H), obtained by high-resolution FESEM observations, shows a distinctly porous structure in the cross section of g-C3N4@PET nanofibers. In other words, pores are also present inside the fibers, creating channels 9



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that may be used by the reactants to reach the photocatalytic active sites.36 In general, the porous structure of fibers increases the specific surface area, which is beneficial to the photocatalytic reactions.

TEM images of g-C3N4/PEG/PET and g-C3N4@PET nanofibers are displayed in Figure 3. Many white domains can be clearly seen in the g-C3N4@PET nanofibers, indicating that there are many pores in the internal fibers. Furthermore, it can be observed that pore size can be assigned to a mesopores structure (2–50 nm). The PEG plays a vital role as a porogen for the formation of the uniform pore structure. Additionally, there exists some dark blocks in the fibers, due to the catalyst (g-C3N4) inside the fibers, highlighted by dotted boxes. Meanwhile, it is obvious that there are some white domains among the dark blocks. In other words, the pores and g-C3N4 are linked together, creating channels that enable adequate catalysts-antibiotics contact and light adsorption.41 The results are in accordance with those of the FESEM. More importantly, it can be found that the PEG domains in fibers are continuous and easy to remove in a water bath, leading to the generation of the external and internal pores.42

The N2 adsorption and desorption isotherms and Barrett–Joyer–Halenda measurements of g-C3N4@PET, T-PET and g-C3N4/PEG/PET nanofibers are shown in Figure 4. The three isotherm curves all exhibit type III behavior, ascribed to the weak adsorbent–adsorbent interaction.43 The hysteresis loop between the adsorption and desorption branches indicates the presence of mesopores in the g-C3N4@PET nanofibers. The formation of a mesoporous structure is attributed to the g-C3N4 10


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pores,36 the channels generated after removing PEG, and the slit-shaped pores resulting from the disordered nanofibers.44 Meanwhile, the narrow hysteresis loop suggests the existence of slit-shaped pores in the g-C3N4/PEG/PET nanofibers deriving from the disordered fibers.45 Compared to g-C3N4/PEG/PET, the BET surface area of g-C3N4@PET increased dramatically from 3.35 m2/g to 7.01 m2/g, which is presumably ascribed to the formation of g-C3N4 pores and fiber pores following PEG removal. In addition, the PSD plot of g-C3N4@PET consists of two peaks with pores widths of ~10 and ~30 nm. This is in accordance with the result of pure g-C3N4, as shown in Figure S2. These features further confirm that g-C3N4@PET possesses a mesoporous structure. The BET results are consistent with the TEM observations.

The bulk composition of relevant samples was confirmed by FTIR, as shown in Figure 5. The band at ~1730 cm−1 represents the stretching vibration of C=O in PET.27 It is obvious that the peak at 2871 cm−1 originates from the stretching vibration of –CH2– in PEG.46 Compared with the spectrum of g-C3N4/PEG/PET, the characteristic absorption peak of g-C3N4@PET at 2871 cm−1 is rather weak. This indicates that the PEG initially present in g-C3N4/PEG/PET has been almost removed completely by the post-processing, which is consistent with the result of Figure S3. Although there were still trace amounts of residue PEG in the g-C3N4@PET nanofibers, the post-treatment failed to remove them because of the entanglement between PEG and PET molecular chains. Furthermore, the g-C3N4 possessed the main 11



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absorption bands at 3100, 1200–1600 and 810 cm−1.47-49 The characteristic peaks of gC3N4 can be also observed in the spectrums of g-C3N4/PEG/PET and g-C3N4@PET. Thus, the g-C3N4 exists in g-C3N4/PEG/PET and g-C3N4@PET nanofibers.

Thermogravimetric analysis was also performed to study the composition and stability of related samples. The results are shown in Figure 6. The degradation of PEG started at ~170°C because of oxidation in the air atmosphere.50 Compared with the curve relative to g-C3N4/PEG/PET, that of g-C3N4@PET showed only two massloss steps, which further confirmed there was hardly any PEG in the g-C3N4@PET nanofibers. Namely, the PEG in the g-C3N4/PEG/PET had been almost completely removed, in agreement with the FTIR results. What’s more, the first mass-loss observed for g-C3N4/PEG/PET reached ~20%, which corresponded to the PEG content in the electrospinning solution. Meanwhile, the obtained g-C3N4@PET exhibited a good thermal stability.

UV-vis diffuse reflectance spectrums (DRS) results show that g-C3N4@PET has a much stronger absorption than that of g-C3N4/PEG/PET (Figure S4). It can be concluded that the porous structure of g-C3N4@PET is beneficial to light absorption due to light scattering and penetration, and the exposure of g-C3N4. Therefore, gC3N4@PET can absorb visible light and has a good photocatalytic performance. Furthermore, the photoluminescence spectrum reveals the photoinduced charges migration, transfer and separation. Figure S5 shows that g-C3N4@PET, gC3N4/PEG/PET and g-C3N4 all possess a broad luminescence peak centered at ~450 12


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nm.51 This peak can be ascribed to the recombination of photogenerated charges. Moreover, the charge combination rate of g-C3N4@PET can be slightly inhibited, which might be due to the good dispersity of g-C3N4 in the porous fibers. In addition, the in-situ electron paramagnetic resonance (EPR) tests of g-C3N4@PET, g-C3N4 and g-C3N4@PET were also carried out, as shown in Figure S6. They all display one single Lorentzian line with a g value of 2.0034 ascribed to the generation of unpaired electrons on π-conjugated CN aromatic rings.52 However, it can be significantly observed that the EPR signal of g-C3N4@PET is greatly enhanced in comparison to gC3N4/PEG/PET, presumably due to the porous structure, which enhances light absorption and the exposure of g-C3N4 in the fibers and consequently favors the photochemical generation of more radical pairs in the photocatalyst. In addition, the Lorentzian line of g-C3N4@PET is equivalent to that of pure g-C3N4. Hence, the formation of pores is very important for photocatalysis.

The two dimension (2-D) X-ray patterns are displayed in Figure 7. The expansion of the spectra of PET and PEG/PET illustrates that the two samples are amorphous. After post-processing, T-PET and g-C3N4@PET both crystallize and show an isotropic diffraction pattern. Three obvious preferred orientations ((010), (-110), (100)) can be observed and can be ascribed to the peaks at 17.6°, 23.0°, and 25.6°, respectively.53 More importantly, the presence of g-C3N4 in g-C3N4/PEG/PET and gC3N4@PET can be directly observed from Figure 6(D–G), because g-C3N4 has an obvious diffraction peak at 27.4°, indexed as the (002) preferred orientation.54-56 13



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Based on the FESEM, TEM, BET, FTIR, TG, DRS, PL, 2D-XRD analyses, we proposed that PEG in g-C3N4/PEG/PET can be almost completely removed through post-treatment in a water bath, resulting in the formation of porous g-C3N4@PET nanofibers. The porous structure not only creates effective channels for reactants reaching the active sites, but also promotes sunlight absorption and mass transfer.

3.2. Photocatalytic Results and Analysis. The photocatalytic performances of the as-obtained samples for degrading SQX in aqueous solution were evaluated under sunlight radiation. As seen from Figure S7, the photocatalytic performance of gC3N4@PET was enhanced by the increase of PEG content in the electrospinning solution, which is accordance with the results of Figure 1.The photocatalytic activity is improved with the increase of porosity in fiber due to the generation of more active sites. However, because of the difficulty of electrospinning a solution with 30% PEG content and having studied the morphology of the as-prepared fibers, we chose gC3N4@PET as the optimum photocatalysts, which was obtained from electrospinning g-C3N4/PEG/PET containing 20% PEG versus to PET. The photocatalytic performance of the reference samples is shown in Figure 8. There is no obvious adsorption or degradation of SQX in the dark and in presence of g-C3N4@PET, and only a weak degradation of SQX can be seen under solar irradiation without gC3N4@PET. Furthermore, the photocatalytic activity of g-C3N4@PET is on a par with that of the pure g-C3N4, which is attributed to the generation of pores, and is consistent with the result of in-situ EPR (Figure S6). The pores created many active 14


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sites that not only promote the transfer of reactants and products, but also make full use of the photoinduced electrons and holes. Although the photocatalytic activity of gC3N4@PET was weak a little, due to the incomplete exposure of g-C3N4 in porous PET nanofibers, the reunion and sedimentation of pure g-C3N4 is very unsatisfactory. Herein, loading powder catalysts is rather important in actual application.

Additionally, we investigated the influence of pH on the photocatalytic activity of g-C3N4@PET. The results are shown in Figure S8. The SQX can be removed completely under acidic, neutral, and alkaline conditions via different irradiation times, which depends on the photocatalytic activity of g-C3N4. For comparison, the photocatalytic performance of g-C3N4@PET was measured in the presence of various inorganic or organic compounds. As seen from Figure S9, no significant inhibition can be noticed in the presence of the inorganic compounds. Only a slight suppression is found when urea or SDS is present in the SQX solution. In short, the results illustrate that g-C3N4@PET possesses a high photocatalytic performance in a wide range of pH values and high background. To confirm the photocatalytic performance of g-C3N4@PET for other antibiotics and dyes, relevant tests were carried out and the results are shown in Table S1. The removal rates for various substrates all reach 90% after different reaction times in the presence of g-C3N4@PET. Furthermore, according to our previous study, the final products of SQX degradation are biodegradable small molecular acids.17 Thus, g-C3N4@PET has a good photocatalytic activity for various 15



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antibiotics and dyes, and possesses a potential practical application for the removal of antibiotics.

3.3. Stability of the Photocatalysts. To evaluate the stability and reusability of gC3N4@PET, recycling tests were performed for degrading SQX under solar irradiation. In each cycle, the g-C3N4@PET was reused. The time required for SQX photodegradation, measured for ten consecutive cycles, is displayed in Figure 9. No distinct loss of activity can be noticed. Furthermore, the morphology of g-C3N4@PET after ten cycles was recorded by FESEM (Figure S10) and no significant changes were observed. In addition, the composition and structure of the g-C3N4@PET after ten reactions were not transformed, as shown in Figure S11. All these results indicate the excellent reusability and long-term stability of the catalyst, which successfully avoids the reunion and sedimentation of pure g-C3N4.

3.4. Mechanisms of Photocatalysis. The photocatalytic mechanism of gC3N4@PET was studied through trapping tests and EPR technique. The investigation of active species during the photodegradation process was performed by dissolving different scavengers and the results are shown in Figure 10. It can be discovered that the SQX degradation is obviously inhibited by the addition of BQ, which indicates that •O2‾ radicals are the main active species.57 However, the addition of KI or IPA as the h+ and •OH trapping agents in the photocatalytic system both resulted in a slight decrease in degradation rate.58 Thus, the h+ and •OH also play a role in the SQX photodegradation. The EPR spectra of DMPO-•OH and DMPO-•O2‾adducts on the g16


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C3N4@PET nanofibers are displayed in Figure 11. We can distinctly observe the characteristic peaks of DMPO-•O2‾, while a weak signal DMPO-•OH can be observed, which further indicates that only a small quantity of •OH are formed. In Figure 12, we propose a possible mechanism for antibiotics photodegradation over g-C3N4@PET. With the formation of pores in the internal and external g-C3N4@PET nanofibers, more active sites are generated. Under solar irradiation, the valence band electrons of g-C3N4 in the as-prepared g-C3N4@PET porous nanofibers are excited to their conduction band, and holes are formed in the valence band. The •O2‾ radicals are generated because the photogenerated electrons are trapped by O2. The •O2‾ radicals possess strong oxidation capacity59 and can oxidize the antibiotics, thus playing a dominant role in degrading the antibiotics. A secondary pathway, the holes with a strong oxidation capacity can directly oxidize the antibiotics. In addition, the holes can oxidize the adsorbored H2O molecules onto g-C3N4 to OH• with an outstanding oxidation capacity.60 What’s more, the OH• was not the dominant active radical, which can avoid the degradation of the supporter PET.61, 62

3.5. SQX Degradation Pathway. The degradation intermediates of SQX by gC3N4@PET under solar irradiation were examined by UPLC Synapt G2-S HDMS in the both positive and negative ion modes. The detailed parameters of intermediates are shown in Table S2 and S3. Temporal UPLC spectra profiles during the photocatalytic degrading SQX was displayed in Figure 13. Peak A was the SQX, and the other four peaks (B-E) were SQX intermediates. After the solar irradiation 17



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reaction, the peak A gradually decreased, while the peak B increased and reached the top at 100 min, and the maximum of C, D, and E appeared at 60, 100, 100 min. The SQX can be completely decreased after 150 min solar irradiation. Therefore, the possible SQX degradation pathway by g-C3N4@PET under solar irradiation was proposed, as shown in Figure S12. At first, a hydroxylated product E (SQX-OH)63 was formed, and then the product D was generated, probably due to the cleavage of S—N bond.64, 65 Besides, another generated product N1-(quinoxalin-2yl) benzene-1,4diamine (product B) was resulted from that SO2 was probably extruded from SQX,66 it can be speculated that the cleavage of C—N bond resulted in the SQX degradation. Finally, most intermediates were transformed to biodegradable small acids (P1-P3), sulphate ions and nitrate ions67.

4. Conclusions

In summary, a porous g-C3N4@PET nanofiber was prepared in a simple and novel approach based on electrospun g-C3N4/PEG/PET and a post-treatment in which the PEG contained in the fibers was dissolved in water, leading to the generation of pores. g-C3N4@PET showed a favorable photocatalytic performance that can challenge the freestanding g-C3N4, and fascinating reusability for degrading antibiotics under solar irradiation. The porous structure was demonstrated to promote the generation of active sites, which arises from the interconnected channels in the fibers. Moreover, the presence of pores is beneficial for mass transfer and light absorption, and thus provides a new method for loading powder catalysts to improve dispersity and 18


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reusability. In addition, this preparation method might be envisaged to be significant for other areas, such as filtration, catalysis and drug deliver.

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FIGURES

Figure 1. Comparision of the surfaces of g-C3N4@PET fabricated by electrospinning solution containing different PEG versus to PET: 2% (A, B) ; 5% (C, D); 10% (E, F); 20% (G, H) and 30% (I, J).

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Figure 2. FESEM images of T-PET (A, B), g-C3N4/PEG/PET (C, D), g-C3N4@PET (E, F) and cross section of g-C3N4@PET (G, H).

Figure 3. TEM images of g-C3N4/PEG/PET (A, B) and g-C3N4@PET (C, D). 21



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Figure 4. N2 sorption isotherms of g-C3N4@PET, T-PET and g-C3N4/PEG/PET. The inset shows the pore size distribution of above samples by Barrett–Joyer–Halenda measurements.

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Figure 5. FTIR spectrum of g-C3N4, PET, PEG, g-C3N4/PEG/PET, g-C3N4@PET.

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Figure 6. Thermogravimetric analysis for g-C3N4, PET, g-C3N4@PET, gC3N4/PEG/PET and PEG.

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Figure 7. 2D-XRD patterns of (A) PET, (B) PEG/PET, (C) T-PET, (D) gC3N4/PEG/PET, (E) g-C3N4@PET, (F) g-C3N4, (G) integral of the XRD patterns of above samples. 25



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Figure 8. Photocatalytic degradation of SQX (2 × 10−5 mol/L) with different catalytic conditions under solar irradiation, pH 5.

Figure 9. Time repeated processes of using the g-C3N4@PET for degradation of SQX (2 × 10−5 mol/L) under solar irradiation, pH 5.

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Figure 10. Photocatalytic degradation of SQX (2 × 10−5 mol/L) by g-C3N4@PET (1 g/L) in the presence of different scavengers under solar irradiation, pH 5.

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Figure 11. DMPO spin-trapping EPR spectra (A) aqueous solution, (B) methanol solution, in the presence of g-C3N4@PET photocatalysts (1 g/L) under solar irradiation, [DMPO] = 10 mM, pH 5.


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Figure 12. The proposed mechanism of degrading antibiotics in presence of g-C3N4@PET over solar irradiation.

Figure 13. Temporal UPLC spectra profiles during the photocatalytic degradation of SQX (4 × 10−5 mol/L) by g-C3N4@PET over solar irradiation obtained from UPLC Synapt G2-S HDMS, obtained 5 peaks (A-E).


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.? Photocatalytic reactor pictures, N2 sorption isotherms with the pore size distribution, FTIR spectrum, UV–vis diffuse reflectance absorption spectrum, photoluminescence spectra, EPR spectra, related photocatalytic performance tests, FESEM images, FTIR spectrum, UV–vis diffuse reflectance absorption spectrum, photoluminescence spectra, EPR spectra, XRD pattern, possible photocatalytic degradation pathway, and degradation intermediates of SQX. AUTHOR INFORMATION Corresponding Author Wangyang Lu, Tel. & Fax: +86-571-8684-3611. E-mail: [email protected]. Wenxing Chen, Tel. & Fax: +86-571-8684-3611.E-mail: [email protected]. Funding Sources This work was supported by the National Natural Science Foundation of China (No. 51133006), the Public Technology Application Research Project of Zhejiang Province (NO. 2015C33018 and 2016C33019). Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 51133006), the Public Technology Application Research Project of Zhejiang Province (NO. 2015C33018 and 2016C33019).


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