nanomaterials

Article Facile 3D Nitride Integrated Electrospun Nanofibrous Membranes for Purging Organic Pollutants

1,2,3, 1, 1 1 Dai-Hua Jiang †, Pei-Chi Chiu †, Chia-Jung Cho , Loganathan Veeramuthu , Shih-Huang Tung 2, Toshifumi Satoh 3 , Wei-Hung Chiang 4 , Xingke Cai 5 and Chi-Ching Kuo 1,*

1 Institute of Organic and Polymeric Materials, Research and Development Center of Smart Textile Technology, National Taipei University of Technology, Taipei 10608, Taiwan; [email protected] (D.-H.J.); [email protected] (P.-C.C.); [email protected] (C.-J.C.); [email protected] (L.V.) 2 Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan; [email protected] 3 Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan; [email protected] 4 Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan; [email protected] 5 Institute for Advanced Study, Shenzhen University, Shenzhen 518060, Guangdong, China; [email protected] * Correspondence: [email protected]; Tel.: +886-2-27712171 (ext. 2407) These authors contributed equally to this work. †  Received: 25 August 2019; Accepted: 24 September 2019; Published: 27 September 2019 

Abstract: Elegant integration of three-dimensional (3D) (BN) into the porous structure of a polymer nanofiber (NF) membrane system results in a surface with enhanced absorption capacity for removal. Various BN-based applications were designed and developed successfully, but BN-based absorption systems remain relatively unexplored. To develop a reusable absorption strategy with high removal efficiency, we used a composite of 3D BN and polyacrylonitrile (PAN) to prepare a NF membrane with a porous structure by using electrospinning and spray techniques (BN-PAN ES NFs). The removal efficiency of the 3D BN NF membrane was higher than that of a pure carbon NF membrane. Water pollutants, such as the dyes Congo red (CR), basic yellow 1 (BY), and rhodamine B (Rh B), were tested, and the absorption ratios were 46%, 53%, and 45%, respectively. Furthermore, the aforementioned dyes and pollutants can be completely eliminated and removed from water by heating because of the high heat resistance of 3D BN. The membrane can be recycled and reused at least 10 times. These results indicate that BN-PAN ES NFs have can be used in water purification and treatment for absorption applications, and that they can be reused after heat treatment.

Keywords: boron nitride; electrospun fibers; removal of pollutants; 3D structure

1. Introduction Growth of industrial activity and increasing water usage (inevitable water utilities) around the world have led to the release of various pollutants, such as toxic heavy metals and organic pollutants into the aquatic environment, particularly in developing countries. Organic pollutants include phenols, dyes, pesticides, detergents, insecticides, herbicides, and other persistent organic pollutants. Organic pollutants are a major concern because of their potential mutagenicity, carcinogenicity, teratogenicity, and high bioaccumulation. In addition to biological and human hazards, organic pollutants permanently damage land and rivers. Therefore, recycling and reusing water are becoming increasingly important

Nanomaterials 2019, 9, 1383; doi:10.3390/nano9101383 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 1383 2 of 15 for sustainable utilization of water resources and protection of the environment [1–5]. Many treatment methods are currently available for removing organic pollutants from water, such as reverse osmosis [6], adsorption [6–9], chemical precipitation [10], coagulation [11,12], electroplating [13], oxidation–reduction [14], and ion exchange [15,16]. Various water treatment methods use adsorption because it is economical, universal, and easy to operate [1–5]. Adsorption-based water treatment methods use materials with a high surface/volume ratio or special manufacturing methods to achieve remarkably high removal of organic pollutants. Electrospinning is an easy, versatile, and inexpensive technique to produce nanometer-scale fibers for assembling various functional nanofibers (NFs) [17–19]. We successfully prepared various ES polymer NFs for different applications, such as sensing of pH levels [20], temperature [20,21], NO level [22], and heavy metal ions [23,24]. Cleaning toxic pollutants by using a high surface/volume ratio of ES NFs is an important subject. Some studies have suggested that conducting heterogeneous photocatalysis by using zinc oxide (ZnO) NFs is a new promising technique for cost-effective treatment of organic pollutants and transformation of hazardous substances into their benign forms. In general, heterogeneous photocatalysis by using ZnO NFs involves developing smart approaches to reduce the harmful effects of highly toxic and recalcitrant pollutants. Some research has focused on the fabrication of ES hybrid NFs or composite ES NFs and their applications in the photodegradation of different organic pollutants that are discharged into wastewater in textile and other industries [25,26]. As a synthetic material, boron nitride (BN) has several well-defined crystallographic structures, such as cubic BN (c-BN), hexagonal BN (h-BN), rhombohedral BN (r-BN), and wurtzite BN (w-BN). BN has many special properties, such as low density, low conductivity, electrical insulating properties, excellent mechanical properties, a high surface area, extremely high resistance to oxidation, high chemical stability, a low thermal expansion coefficient, a high melting point, and high thermal conductivity [27–29]; hence, it has been widely used as an electrical insulator and in heat resistant materials for decades because it is a good conductor of heat as well as a good electrical insulator. High-quality h-BN crystal prepared at 2100 ◦C by using a nickel-molybdenum solvent was found to be a promising deep ultraviolet light emitter [30]. r-BN was detected during the synthesis of h-BN. r-BN was reported to be formed during the conversion of c-BN to h-BN [27]. h-BN is a two-dimensional (2D) layered material that is stable at 1500 ◦C in air and does not react with most chemicals. h-BN has been morphed to manufacture products for various applications, including BN powders, one-dimensional (1D) nanotubes (BNNTs), 1D BN NFs, 2D nanosheets, 2D BN porous belts, three-dimensional (3D) particles, and 3D BN structures. Several studies on 3D BN structures have successfully combined the properties of BN materials and the stability of 3D nanostructures to produce materials with high adsorption capacity and stable cycle. Many potential applications, including as a photocatalyst [31,32], catalytic supports [33,34], filtration [35,36], and organic pollutant adsorption [37,38] have been demonstrated. In a study by Dan Liu’s et al presents direct synthesis of 3D BN structures by using a simple thermal treatment process. A 3D BN structure consists of an interconnected flexible network of nanosheets. The typical adsorption/desorption results demonstrated that the maximum specific surface area of the as-prepared samples was 1156 m2 g 1, · − and the total pore volume was approximately 1.17 cm3 g 1. The 3D BN structure displayed high · − adsorption rates and large capacities for organic dyes in water without any other additives because of its low density, high resistance to oxidation, high chemical inertness, and high surface area. Notably, 88% of the starting adsorption capacity was maintained after 15 cycles. These results indicated that the potential application of 3D BN structures as an environment-friendly material for water purification and treatment [29]. However, thus far, no study has combined the high surface-to-volume ratio of ES NFs and the porous structure of 3D BN for application in water purification and treatment. In view of aforementioned advantages, we propose that 3D BN is a suitable material to be used in absorption, and it can be combined with ES NFs. First, we produced ES hybrid or composite ES NFs using BN-PAN materials. After absorbing dyes and pollutants from water, the NFs can be reused after subjecting them to heat treatment because 3D BN has high heat resistance. Our study is Nanomaterials 2019, 9, 1383 3 of 15 the first to demonstrate the adsorption application of materials prepared by using ES. In this study, we combined the 3D BN structure and ES NFs to prepare high adsorption material with the novel electrospinning spray coating composite method and demonstrated the dye scavenging applications as a proof of concept.

2. Experimental Section

2.1. Materials

Polyacrylonitrile (PAN, 98%, Mw = 150,000), basic yellow (BY, C28H31ClN2O3) and Congo Red (CR, C H N Na O S , 35%) were purchased from Sigma-Aldrich, St. Louis, MO, USA. 32 22 6 2 6 2 ≥ Dimethylformamide (DMF), C3H7NO (98%), ethanol (95%), and methanol (HPLC grade) were purchased from ECHO Chemical Co., Miaoli, R.O.C. Boron Oxide (B2O3) and (CH4N2O) were prchased from J.T.Baker-Avantor Chemical Co., HsinChu, R.O.C. Rhodamine-B (Rh B, C28H31ClN2O3) was purchased from Acros Organics Co., Beijing, China.

2.2. Preparation of Electrospinning Nanofibers (NFs) and Polymer Solution The PAN solution was placed in a 5 mL syringe. The PAN was first dissolved in the solvent, N, N-dimethyl formamide (DMF), to make it into a polymer solution which has a concentration of 12 wt%. The feed rate of the solution was 1 mL h 1. The metallic needle was connected to a high-voltage power · − supply, and a piece of aluminum foil was placed 20 cm below the tip of the needle to collect the NFs. The spinning voltage was set at 12.9 kV. After collection time of 10 min, Non-woven electrospun PAN NF was collected on aluminum foil. All experiments were carried out at room temperature and at a relative humidity of about twenty percent.

2.3. Synthesis of Three-Dimensional Boron Nitride Precursor

In a typical synthesis, boron trioxide (B2O3) and urea (CH4N2O) with 1:10, 1:5, and 1:1 molar ratios were respectively mixed in 3 mL methanol under stirring to form a clear colorless solution, which was prepared according to the reported procedures [29].

2.4. Regulation of Spray Coating The different molar ratios of BN and urea (1:10, 1:5, and 1:1) were prepared and utilized as a precursor for the spray coating process to form 3D BN. The sample was placed 20 cm below the tip of the spray nozzle to do spray coating. The pressure supply was about 1 kg cm 2, and the flow rate was · − controlled at about 0.5 mL h 1. The spraying times of all the samples were 10, 30, and 60 s, respectively. · − 2.5. Process of Electrospun Boron Nitride PAN Nanofiber(NF) Membrane We followed two different mechanisms to manufacture the BN ES NFs in our research which is illustrated in Figure1. The spray coating collecting time of all sample were fixed as 30 seconds. The collecting time of the electrospun fibers was about 10 min. In this part, we compared two techniques of the spray coating to collect the boron nitride precursor, and they were respectively (1) direct spraying and (2) electrospinning and spraying composite technology. Then, the prepared samples were kept in the heating oven to dry, and we used the SEM to observe the distribution of BN precursor on the NF membrane. One of the methods was direct spray coating. We took the as-spun PAN NF membrane as the substrate and used the spray jet to spray the BN precursor solution on the surface of the membrane. Then, the prepared samples were kept in the heating oven to dry. Nanomaterials 2019, 9, 1383 4 of 15 Nanomaterials 2019, 9, x FOR PEER REVIEW 4 of 15

Figure 1. TheThe process process to to produce produce the the boron boron nitride nitride (BN) (BN)-- polyacrylonitrile polyacrylonitrile (PAN) (PAN) electrospinning ( (ES)ES) polymer nanofibersnanofibers (NFs):(NFs): (a) spray spray-coating-coating BN precursor to PAN NFs.NFs. ( b) directly spray coating BN precursor and electrospinning PANPAN NFsNFs atat thethe samesame time.time. 2.6. Process of Electrospinning 3D Boron Nitride Carbon NF Membrane 2.6. Process of Electrospinning 3D Boron Nitride Carbon NF Membrane Electrospun carbon NFs (ECNFs) were prepared by electrospinning PAN solution followed by Electrospun carbon NFs (ECNFs) were prepared by electrospinning PAN solution followed by stabilization and carbonization. In the stabilization process, the PAN NF membrane was placed in the stabilization and carbonization. In the stabilization process, the PAN NF membrane was placed in quartz boat and heated at a rate of 1 C min 1 from 30 C to 280 C and held there for 1 h in a constant the quartz boat and heated at a rate◦ of· 1 °C·− min−1 from◦ 30 °C ◦to 280 °C and held there for 1 h in a flow of air. Carbonization was carried out by heating the stabilized PAN NF membrane in a constant constant flow of air. Carbonization was carried out by heating the stabilized PAN NF membrane in flow of nitrogen/hydrogen (10% hydrogen) gas to 800 C at a rate of 5 C min 1 and maintained at a constant flow of nitrogen/hydrogen (10% hydrogen)◦ gas to 800 °C ◦at · a rate− of 5 °C ·min−1 and 800 C for 2 h. In the stabilization treatment process of electrospinning PAN NFs, the BN precursor maintained◦ at 800 °C for 2 h. In the stabilization treatment process of electrospinning PAN NFs, the also become porous BN nanosheets with a 3D structure at the same time. During the heat treatment BN precursor also become porous BN nanosheets with a 3D structure at the same time. During the process, the urea is decomposed at high temperature, releasing a number of gaseous species including heat treatment process, the urea is decomposed at high temperature, releasing a number of gaseous CO2, HCNO, N2O, H2O, and NH3. The escape of gases (gas bubbles) acts as fugitive templates of the species including CO2, HCNO, N2O, H2O, and NH3. The escape of gases (gas bubbles) acts as fugitive templates of the final porous structure. The white crystal powders were heated in a tube furnace at 800 °C for 2 h under nitrogen/hydrogen (10% hydrogen) gas flow, and a black sample was produced.

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final2.7. C porousharacterization structure. of 3D The Boron white Nitride crystal Carbon powders NF wereMembrane heated in a tube furnace at 800 ◦C for 2 h under nitrogenThe/ hydrogen structure (10% of the hydrogen) sample gas was flow, observed and a black with sample a Hitachi was H produced.-7100 (operating at 100 kV) 2.7.transmission Characterization electron of 3Dmicroscope Boron Nitride (TEM) Carbon (Hitachi NF Membrane H-7100, Tokyo, Japan), and scanning electron microscopic (SEM) (Hitachi TM-3000, Tokyo, Japan) images were obtained with a cold-field emission scanningThe structure electron of microscope the sample was(HR- observedSEM) (Hitachi with a Hitachi S-4800, H-7100Tokyo, (operating Japan) equipped at 100 kV) with transmission energy- electrondispersive. microscope The X-ray (TEM) photoelectron (Hitachi H-7100, spectra Tokyo, (XPS) Japan),(ESCALAB and scanning250, England) electron were microscopic collected (SEM)on an (HitachiESCA Lab TM-3000, MKII X- Tokyo,ray photoelectron Japan) images spectrometer were obtained using with non a-monochromatized cold-field emission Mg scanning-Ka X-ray electron as the microscopeexcitation source. (HR-SEM) (Hitachi S-4800, Tokyo, Japan) equipped with energy-dispersive. The X-ray photoelectron spectra (XPS) (ESCALAB 250, England) were collected on an ESCA Lab MKII X-ray photoelectron2.8. Dye Removal spectrometer Test using non-monochromatized Mg-Ka X-ray as the excitation source.

2.8. DyeDye Removal solutions Test (CR, BY, and Rh B) of different concentrations were prepared by dissolving appropriate amounts of CR into deionized water (DI water), respectively. In a typical adsorption of a CRDye experiment, solutions 3 × (CR, 3 cm BY, of andthe 3D Rh BN B) NF of dimembranefferent concentrations was added to were 100 mL prepared CR aqueous by dissolving solution appropriate(10 mg/L), and amounts under stirring, of CR into and deionized UV-vis absorption water (DI spectra water), were respectively. recorded Inat adifferent typical time adsorption intervals of a CR experiment, 3 3 cm of the 3D BN NF membrane was added to 100 mL CR aqueous solution to monitor the process× at 496 nm. The adsorption isotherm was obtained by varying the initial CR (10concentration. mg/L), and underThe adsorption stirring, and studies UV-vis of absorptionRh, B, and spectraBY were were similar recorded to those at di offferent CR except time intervals for the todetection monitor wavelength the process difference at 496 nm. (553 The nm adsorption for Rh B and isotherm 412 nm was for obtainedBY). by varying the initial CR concentration. The adsorption studies of Rh, B, and BY were similar to those of CR except for the detection3. Results wavelengthand Discussion difference (553 nm for Rh B and 412 nm for BY).

3. Results and Discussion 3.1. Chemical and Structural Characterization of Morphology 3.1. ChemicalThe morphology and Structural was Characterizationmodified by sele ofcting Morphology suitable solutions during ES. Figure 2 shows the cold The field morphology emission scanning was modified electron by selecting microscope suitable and solutions energy dispersive during ES. spectrometer Figure2 shows (FE the-SEM) cold fieldimages emission of PAN scanning ES NFs with electron different microscope magnifications and energy for dispersive 12 wt% concentration. spectrometer As (FE-SEM) seen in imagesFigure of2, PANno beads ES NFs were with observed different magnifications on the surface for or 12 layer wt% concentration. of the PAN NFs As seen membrane. in Figure 2 Stable, no beads PAN were NF observedmembrane on generation the surface can or layerbe achieved of the PAN by regulating NFs membrane. parameters Stable and PAN environmental NF membrane factors. generation Figure can 2 beshows achieved that the by regulatingdiameters of parameters all NFs in and the environmentalPAN NF membrane factors. are Figure similar2 shows and the that aver theage diameters diameter of allof the NFs NFs in theis about PAN NF540 membranenm. are similar and the average diameter of the NFs is about 540 nm.

Figure 2. Field emission scanning electron microscope and energy dispersive spectrometer (FE-SEM) Figure 2. Field emission scanning electron microscope and energy dispersive spectrometer (FE- image of pure PAN ES NFs with 10,000 magnification. SEM) image of pure× PAN ES NFs with 10,000× magnification. Spray coating were adjusted with different spraying times of 10, 30, and 60 s, to improve the fiber Spray coating were adjusted with different spraying times of 10, 30, and 60 s, to improve the quality. The collecting time of electrospinning is about 10 min. In this study, we used direct spraying fiber quality. The collecting time of electrospinning is about 10 min. In this study, we used direct spraying to collect the BN precursor. Then, the collected sample was placed in the heating oven to dry, and we used the FE-SEM to observe distribution of the BN precursor on the NF membrane

Nanomaterials 2019, 9, 1383 6 of 15

Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 15 to collect the BN precursor. Then, the collected sample was placed in the heating oven to dry, and we(Figure used S1, the Supporting FE-SEM to information). observe distribution The FE-SEM of the image BN precursor shows the on different the NF spray membrane coating (Figure times S1,of theSupporting BN ES NF information). membrane. In The the FE-SEM spray coating image showsprocess, the the di sprayingfferent spray time coating was settimes at 10, of30, the an BNd 60 ES s. AnNF air membrane. jet was used In the to sprayspray coatingthe precursor process, solution, the spraying and it timewas wascollected set at on 10, the 30, surface and 60 s.of Anthe airNFs. jet wasFigure used S1a to shows spray the the low precursor-magnification solution, image and itwhen was collectedfewer BN onsheets the surfacewere placed of the on NFs. the FigurePAN NFs. S1a Weshows observed the low-magnification that BN was thinner image when and fewer smaller BN than sheets the were other placed materials. on the Figure PAN NFs. S1c We shows observed that numerousthat BN was BN thinner sheets and were smaller placed than on thethe otherNF membrane, materials. Figureand evidently, S1c shows the that BN numerous sheets connect BN sheets and werestack placedon each on other the NFand membrane, attach the PAN and evidently,NFs to form the a BN large sheets BN film connect on the and PAN stack NF on membrane. each other The and successiveattach the PANstacking NFs of to BN form stru a largectures BN with film increased on the PAN spray NF coating membrane. time Theof 60s successive was clearly stacking evidenced of BN withstructures Figure with S1e increased,f. Such thick spray films coating apparently time of 60slose was their clearly appealing evidenced flexibility with Figureand turn S1e,f. fragile Such under thick higherfilms apparently spray time. lose Therefore, their appealing we concluded flexibility that and 30 s turn was fragilethe most under suitable higher spraying spray time.time for Therefore, the BN weES NF concluded membrane that in 30 the s was spray the coating most suitable process. spraying time for the BN ES NF membrane in the spray coatingIn our process. study, we compared two techniques of spray coating to collect the BN precursor, namely directIn spraying our study, as wewell compared as electrosp twoinning techniques and spraying of spray coatingcomposite to collect technology the BN (Fig precursor,ure 1). Figure namely 3 shdirectows sprayingthe SEM asimages well asof electrospinningspray coating with and different spraying spraying composite techniques. technology Fig (Figureure 3a1,b). show Figure the3 SEMshows images the SEM of direct images spraying. of spray As coating shown with in Figure different 3a, spraying the BN precursor techniques. covered Figure the3a,b NF show surface. the Furthermore,SEM images ofwe direct observed spraying. the NF As membrane shown in layer Figure and3a, noted the BN that precursor the BN precursor covered the could NF not surface. seep intoFurthermore, the middle we or observed bottom of the the NF NF membrane membrane. layer The and second noted technique that the BNused precursor composite could technology, not seep electrospinninginto the middle, orand bottom spraying of the to prepare NF membrane. the BN ES The NF second membrane. technique In this used method, composite the coating technology, was directlyelectrospinning, sprayed and on spraying the NFs to prepareduring the elect BNrospinning ES NF membrane. to produce In this the method, PAN NF the coating membrane. was Simultaneously,directly sprayed on the the BN NFs precursor during electrospinning solution was to sprayed produce on the the PAN NFs NF and membrane. the BN Simultaneously, precursor was collectedthe BN precursor along with solution the PAN was NFs. sprayed By using on the this NFs method and the to manufacture BN precursor the was BN collected ES NF membrane, along with the BN PAN precursor NFs. By usingcan be this placed method in the to middle manufacture or at the the bottom BN ES NFof the membrane, PAN ES NF the membrane. BN precursor These can resultsbe placed are in shown the middle in Fig orure at the 3c bottom,d. Notably of the, PAN the BN ES NF ES membrane. NF membranes These prepared results are using shown the in electrospinningFigure3c,d. Notably, and spraying the BN composite ES NF membranes technique prepared are better using because the NFs electrospinning protect and keep and BN spraying inside thecomposite NF membrane technique layers. are better because NFs protect and keep BN inside the NF membrane layers.

Figure 3. 3.FE-SEM FE-SEM images images of directive of directive spray coating spray coatingprocess: process(a) low magnification,: (a) low magnification, (b) high magnification; (b) high magnificationelectrospinning; electrospinning and spraying composite and spraying process: composite (c) low magnification, process: (c) ( lowd) high magnification, magnification. (d) high magnification.

To obtain a high-quality 3D BN structure, we used varying ratios of the BN precursor in spray coating, as shown in Figure S2. In this study, we used boron trioxide (B2O3) and urea (CH4N2O) as

Nanomaterials 2019, 9, 1383 7 of 15

To obtain a high-quality 3D BN structure, we used varying ratios of the BN precursor in spray Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 15 coating, as shown in Figure S2. In this study, we used boron trioxide (B2O3) and urea (CH4N2O) as precursor materials in the precursor solution. B2O3 and CH4N2O were stirred in molar ratios of 1:10, precursor materials in the precursor solution. B2O3 and CH4N2O were stirred in molar ratios of 1:10, 1:5, and 1:1 in 3 mL methanol to obtain a clear, colorless solution. To compare the effect of different 1:5, and 1:1 in 3 mL methanol to obtain a clear, colorless solution. To compare the effect of different ratios of the BN precursor, we can observe the distribution is very different from low-magnification ratios of the BN precursor, we can observe the distribution is very different from low-magnification SEM images, as shown in Figure S2a,c,e, and they indicated that boron oxide and urea were mixed in SEM images, as shown in Figure S2a,c,e, and they indicated that boron oxide and urea were mixed the ratios of 1:10, 1:5, and 1:1. In the precursor synthesis process, the areas of the BN precursor film in the ratios of 1:10, 1:5, and 1:1. In the precursor synthesis process, the areas of the BN precursor film increased with the increase in the boron oxide ratio, which led to a higher ratio of boron oxide to urea increased with the increase in the boron oxide ratio, which led to a higher ratio of boron oxide to urea toto form form the the BN BN precursor precursor crystal. crystal. FigureFigure4a 4a show show the the SEM SEM images images of of the the pure pure PANPAN NFNF membrane,membrane, and the inset inset of of Figure Figure 4a4a shows shows thethe exterior exterior of of the the PAN PAN NF NF membrane. membrane. InIn FigureFigure4 4a,a, wewe observeobserve that the NF NF membrane membrane appears appears white white andand shiny shiny before before it is it subjectedis subjected to the to heatthe heat treatment treatment process process and theand surface the surface of the of PAN the NFs PAN is NFs smooth. is Thesmooth. average The diameter average ofdiameter the PAN of NFs the isPAN approximately NFs is approximately 540 nm. The 540 PAN nm. NFThe membranePAN NF membrane transforms intotransforms carbon NF into membrane carbon NF when membrane it is subjected when it is to subjected heat treatment, to heat astreatment, shown in as Figure shown4b. in TheFigure inset 4b. of theThe Figure inset4 bof showsthe Figure the exterior4b shows of the the exterior carbon of NF the membrane, carbon NF whichmembrane, appears which black appears and is black not shiny.and Theis carbonnot shiny. NF The surface carbon is coarser NF surface than is that coarser of the than PAN that NF, of and the thePAN average NF, and diameter the average of the diameter carbon NFsof isthe approximately carbon NFs 340 is approximately nm. The diff erences 340 nm. between The differences the PAN between and carbon the PAN NFs are and attributable carbon NFs to are the long-chainattributable polymer to the structure long-chain dehydrogenation polymer structure and dehydrogenation cyclization which and form cyclization an orderly which row-column form an cyclicorderly structure. row-column After cyclic crossing structure. the carbonization After crossing temperaturethe carbonization of 625 temperature◦C, the PAN of 625 NFs °C , ultimatelythe PAN undergoNFs ultimately carbonization undergo and carbonization a subsequent and denitrification a subsequent process. denitrification process.

FigureFigure 4. 4.FE-SEM FE-SEM images images ofof ((aa)) purepure PANPAN NFNF and (b) pure carbon carbon NF NF with with high high magnification. magnification.

Next,Next, we we investigated investigated thethe eeffectsffects ofof didifferentfferent molar ratios on on BN BN after after heat heat treatment, treatment, and and the the distributiondistribution is is presented presented in in Figure Figure5 a,c,e;5a,c,e; they they respectively respectively indicatedindicated that boron boron oxide oxide and and urea urea were were mixedmixed in in the the ratio ratio ofof 1:10, 1:5, 1:5, and and 1:1. 1:1. It is It alre is alreadyady revealed revealed that thatthe spray the spray coated coated precursors precursors can lead can leadto the to the film film formation formation b/w b the/w theNF NFlayers layers [39]. [ 39The]. Theareas areas of the of BN the precursor BN precursor film increased film increased with the with theincrease increase in in the the amount amount of of boron boron oxide. oxide. The The result result is presented is presented in Figure in Figure 5a,c,e.5a,c,e. According According to the to thedynamic dynamic templating templating approach, approach, byproducts byproducts of of the the condensation condensation of of the the solid solid framework framework evolve evolve as as nanobubblesnanobubbles that that act act as as fugitive fugitive templates templates ofof thethe finalfinal porous structure structure [28] [28].. Different Different molar molar ratios ratios of of thethe BN BN precursor precursor in in the the synthesized synthesized 3D 3D BN BN structures structures are are shown shown in Figurein Figure5. We 5. We find find that that samples samples with BNwith molar BN ratiosmolar ofratios 1:10 of and 1:10 1:5 and have 1:5 ahave typical a typical 3D porous 3D porous structure. structure. When When boron boron oxide oxide and and urea urea bind, thebind, samples the samples with BN with molar BN ratios molar of ratios 1:10 andof 1:10 1:5 and had 1:5 excess had excess urea on urea the on white the BNwhite crystal, BN crystal, which which formed theformed fugitive the templates. fugitive temp As shownlates. As in Figureshown5 ina, thereFigure are 5a, fewer there 3Dare BN fewer porous 3D BN structures porous onstructures the samples on withthea samples BN molar with ratio a BN of molar 1:10,which ratio of may 1:10, cause which low may effi causeciency low in efficiency the absorbance in the test.absorbance The sample test. The with thesample BN precursor with the molar BN precursor ratio 1:1 hadmolar less ratio leftover 1:1 had urea less because leftover most urea of because the urea most was boundof the withurea boronwas oxidebound to formwith aboron larger oxide white to crystal.form a larger Therefore, white during crystal. the Therefore, heat treatment during process,the heat ureatreatment decomposed process, at urea decomposed at high temperature to release gaseous species to form the 3D BN porous structure and there was less dynamic templating. The sample with the BN precursor molar ratio of 1:1 had a large 3D BN structure because of binding in the BN precursor stack. At this point, we decided that

Nanomaterials 2019, 9, 1383 8 of 15 high temperature to release gaseous species to form the 3D BN porous structure and there was less dynamicNanomaterials templating. 2019, 9, x FOR The PEER sample REVIEW with the BN precursor molar ratio of 1:1 had a large 3D BN structure8 of 15 because of binding in the BN precursor stack. At this point, we decided that the BN precursor molar the BN precursor molar ratios of 1:5 are the best option to be utilized in the dye test. Corresponding ratiossurface of 1:5 elemental are the best analyses option of to the be pureutilized carbon in the and dye 3D test. BN Corresponding NF membranes surface were conducted elemental analyses using ofenergy the pure-dispersive carbon and X-ray 3D (EDX) BN NF spectrum, membranes as shown were in conducted Figure S3. using Show energy-dispersivethe EDX of pure PAN X-ray carbon (EDX) spectrum,NFs and as BN shown-PAN in composite Figure S3. ES Show NFs, the respectively. EDX of pure The PAN results carbon indicate NFs and that BN-PAN the pure composite carbon NF ES NFs,membrane respectively. consists The of results elements indicate C and that O, theas shown pure carbon in Figure NF membraneS3a. In particular, consists the of signal elements of C C is and O,strong. as shown The inEDX Figure spectrum S3a. Inin particular,Figure S3b indicates the signal that of the C is 3D strong. BN NF The membrane EDX spectrum consists inof elements Figure S3b indicatesC, O, B, that and the N. 3D In BN particular, NF membrane the elem consistsental analyses of elements represent C, O, B, different and N. Inareas particular, of the the BN elemental ES NF analysesmembrane, represent and we diff canerent clearly areas distinguish of the BN ESthe NFareas membrane, with BN and and carbon we can structures. clearly distinguish We can conclude the areas withthat BN we and successfully carbon structures. prepared We a 3D can BN conclude NF membrane that we successfullyby using the prepared electrospinning a 3D BN and NF spraying membrane bycomposite using the electrospinningmethod. and spraying composite method.

Figure 5. FE-SEM images of BN structure synthesized with different precursor ratio of B O and Figure 5. FE-SEM images of BN structure synthesized with different precursor ratio of B2O23 and3 CH N O with different molar ratio. 1:10 (a) low magnification, (b) high magnification; 1:5 (c) low CH4 42N2O with different molar ratio. 1:10 (a) low magnification, (b) high magnification; 1:5 (c) low magnification,magnification, (d ()d high) high magnification; magnification; 1:1 1:1 (e(e)) lowlow magnification,magnification, ( (ff)) high high magnification. magnification.

Nanomaterials 2019, 9, 1383 9 of 15

Nanomaterials 2019, 9, x FOR PEER REVIEW 9 of 15 To improve our understanding of the porous structure of our prepared fibers, additional SEM and TEM imagesTo improve are measured.our understanding Figure of6 athe shows porous the structure FE-SEM of our image prepared of the fibers, 3D BNadditional porous SEM structure. When boronand TEM oxide images and are urea measured. were binding, Figure 6a the shows samples the FE with-SEM theimage BN of molar the 3D ratio BN porous of 1:5 structure. had excess urea When boron oxide and urea were binding, the samples with the BN molar ratio of 1:5 had excess urea on the whiteon the BNwhite crystal, BN crystal, which which formed formed fugitive fugitive templates.templates. The The TEM TEM images images in Figure in Figure 6b,c clearly6b,c clearly show thatshow 2D that BN 2D nanosheets BN nanosheets stack stack to to form form the the 3D3D porous structures. structures. XPS XPSwerewere recorded, recorded, and the and the chemicalchemical states states of B andof B Nand elements N elements were were investigated investigated using using XPS. XPS. Figure Figure S4 shows S4 shows typical typical B1s, N1s, B1s, N1s, O1s, andO1s, C1s and spectra, C1s spectra, with corresponding with corresponding binding binding energies energies of 190.8, of 190.8, 398.4, 398.4, 531, and531, 284 and eV, 284 respectively. eV, respectively. These values are very close to the previously reported values of BN layers with BN3 and These values are very close to the previously reported values of BN layers with BN3 and NB3 trigonal NB3 trigonal units [28,40,41]. From the rational consecutive optimization done with a mechanism, units [28,40,41]. From the rational consecutive optimization done with a mechanism, spray coat time, spray coat time, and BN precursor ratio, we prepared the 3D BN NF membrane with a good porous and BNarchitecture. precursor ratio, we prepared the 3D BN NF membrane with a good porous architecture.

Figure 6.Figure(a) FE-SEM 6. (a) FE-SEM image image of theof the 3D 3D BN BN porous porous structure, (b (,bc), cthe) the transmission transmission electron electron microscope microscope (TEM) images(TEM) images of the of 2D the BN 2D nanosheetsBN nanosheets stack stack form form the 3D 3D porous porous structures structures on different on different places. places.

3.2. Dye3.2. Test Dye of Test CR, of BY, CR, and BY, RhBand RhB Dye solutions (CR, BY, and RhB) of different concentrations were prepared by dissolving Dye solutions (CR, BY, and RhB) of different concentrations were prepared by dissolving appropriate amounts of CR, BY, and Rh B in deionized water. In a typical adsorption of CR appropriateexperiment, amounts 3 × of3 cm CR,2 of BY, the and as-prepared Rh B in deionizedBN sample water.(496 nm) In was a typical added adsorption to 100 mL CR of CRaqueous experiment, 2 3 3 cmsolutionof the (10 as-prepared mg/L) und BNer stirring, sample and (496 UV nm)-visible was (UV added-vis) to absorption 100 mL CR spectra aqueous were solutionrecorded (10at mg/L) × under stirring,different andtime UV-visibleintervals to monitor (UV-vis) the absorption process. The spectraadsorption were isotherm recorded was obtained at different by varying time intervals the to monitorinitial the process.CR concentration. The adsorption The adsorption isotherm studies was of obtainedBY and Rh byB were varying similar the to initialthose of CR CR concentration. except The adsorptionfor the difference studies in of detection BY and Rhwavelengths B were similar (412 nm to for those BY and of 553 CR nm except for Rh for B). the CR, di anff erenceanionic indye, detection is regarded as a primary toxic pollutant in water resources. UV-vis absorption spectroscopy was used wavelengthsat the (412maximum nm for absorption BY and wavelength 553 nm for of Rh CR B). (496 CR, nm) an to anionic estimate dye, the isadsorption regarded kinetics as a primary in an toxic pollutantaqueous in water CR solution resources. (Figure UV-vis 7a). BY, absorption a textile dye spectroscopy (cationic), regarded was usedas a primary at the toxic maximum pollutant absorption in wavelengthwater of resources, CR (496 was nm) chosen to estimate as a typical the organic adsorption waste. kinetics The initial in BY an concentration aqueous CR in solution water was (Figure 50 7a). BY, a textilemg/L, dyeas shown (cationic), in Figure regarded 7b. The characteristic as a primary absorption toxic pollutant of BY at in412 water nm was resources, chosen to wasmonitor chosen as a typicalthe organic adsorption waste. process. The Rh initial B dye is BY commonly concentration used tracer in waterin determining was 50 the mg flow/L, asand shown transport. in Rh Figure 7b. B dyes fluoresce and can thus be detected easily and inexpensively by using fluorometers. Rh B dye The characteristic absorption of BY at 412 nm was chosen to monitor the adsorption process. Rh B finds extensive roles in biotechnology applications, such as fluorescence microscopy, flow cytometry, dye is commonlyand fluorescence used correlationtracer in determining spectroscopy. the UV- flowvis absorption and transport. spectroscopy Rh B dyes was used fluoresce at the and can thus bemaximum detected absorption easily and wavelength inexpensively of Rh B (553 by usingnm) to fluorometers.estimate the adsorption Rh B dyekinetics finds in a extensive10 mg/L roles in biotechnologyaqueous Rh applications, B solution, as such shown as in fluorescence Figure 7c. In microscopy, the Figure 7a– flowc, we cytometry, can clearly observe and fluorescence the correlation spectroscopy. UV-vis absorption spectroscopy was used at the maximum absorption wavelength of Rh B (553 nm) to estimate the adsorption kinetics in a 10 mg/L aqueous Rh B solution, as shown in Figure7c. In the Figure7a–c, we can clearly observe the absorption-intensity of dye solutions (CR, BY, and RhB) without NF membrane is sustained, directing dye solutions is not degradable in the air condition for 180 min. Nanomaterials 2019, 9, x FOR PEER REVIEW 10 of 15 absorptionNanomaterials-intensity2019, 9, 1383 of dye solutions (CR, BY, and RhB) without NF membrane is sustained, directing10 of 15 dye solutions is not degradable in the air condition for 180 min.

Figure 7. UV-vis absorption spectra of (a) the Congo Red (CR) aqueous solution (10 mg/L), (b) the basic Figure 7. UV-vis absorption spectra of (a) the Congo Red (CR) aqueous solution (10 mg/L), (b) the yellow (BY) aqueous solution (50 mg/L), (c) the Rh B aqueous solution (10 mg/L). basic yellow (BY) aqueous solution (50 mg/L), (c) the Rh B aqueous solution (10 mg/L). Figure8 shows the UV–vis absorption spectra and the removal e fficiency of the pure carbon Figure 8 shows the UV–vis absorption spectra and the removal efficiency of the pure carbon NF NF membrane in aqueous solutions of CR, BY, and Rh B. After 180 min, the carbon NF membrane membrane in aqueous solutions of CR, BY, and Rh B. After 180 min, the carbon NF membrane removed approximately 13% of the CR, 30% of the BY, and 16% of the RhB, as shown in Figure8. removed approximately 13% of the CR, 30% of the BY, and 16% of the RhB, as shown in Figure 8. In In this study, we employed 3D BN NF membrane aiming to cohesive improvement with adsorption this study, we employed 3D BN NF membrane aiming to cohesive improvement with adsorption capacity and higher surface/volume ratio. Figure9a–c show the adsorption spectra of the 3D BN capacity and higher surface/volume ratio. Figure 9a–c show the adsorption spectra of the 3D BN NF NF membrane, and that the absorption of all dye samples is proportional to the time. Notably, the membrane, and that the absorption of all dye samples is proportional to the time. Notably, the differences between the pure carbon and 3D BN NF membranes can be investigated by comparing differences between the pure carbon and 3D BN NF membranes can be investigated by comparing adsorption rates, the absorption ratios of CR, BY, and RhB are 46%, 53%, and 45%, respectively. By adsorption rates, the absorption ratios of CR, BY, and RhB are 46%, 53%, and 45%, respectively. By contrast, the 3D BN NF membrane demonstrated efficient absorption compared with the pure carbon contrast, the 3D BN NF membrane demonstrated efficient absorption compared with the pure carbon NF membrane because of the improved surface to volume ratio aided with formation of porous NF membrane because of the improved surface to volume ratio aided with formation of porous structured nanosheets. Comparing Figure8c to Figure9c, it was observed that the BN modified sample structured nanosheets. Comparing Figure 8c to Figure 9c, it was observed that the BN modified performed much better for RhB aqueous solutions. This means that the porous architecture of 3D BN

Nanomaterials 2019, 9, x FOR PEER REVIEW 11 of 15 Nanomaterials 2019, 9, 1383 11 of 15 sample performed much better for RhB aqueous solutions. This means that the porous architecture of 3D BN for BN modified sample enhanced the absorption ability, comparing with pure carbon NF for BN modified sample enhanced the absorption ability, comparing with pure carbon NF membrane. membrane. Compared with Figure 9b,c, a dramatically drop at 150 min is observed in Figure 9c. This Compared with Figure9b,c, a dramatically drop at 150 min is observed in Figure9c. This may have may have been caused by different adsorption abilities between 3D BN NFs and different dye been caused by different adsorption abilities between 3D BN NFs and different dye solutions. The solutions. The slope of the absorption vs. time have a little bit different, but the final result of the slope of the absorption vs. time have a little bit different, but the final result of the absorption rate absorption rate is almost the same. Figure S5 shows that the absorption ratios of Rh B researched at is almost the same. Figure S5 shows that the absorption ratios of Rh B researched at 90%, indicating 90%, indicating almost full complete removal of dye over time (450 min). Furthermore, in our study, almost full complete removal of dye over time (450 min). Furthermore, in our study, we immersed we immersed our sample to the dye solution to absorb the dye and then removed the dye by heating our sample to the dye solution to absorb the dye and then removed the dye by heating to 500–800 C. to 500–800 °C . Most dyes were carbonized and degraded by heating to such high temperature. ◦So, Most dyes were carbonized and degraded by heating to such high temperature. So, we removed it out, we removed it out, and the membrane could be reused again. As shown in Figure S6, the membrane and the membrane could be reused again. As shown in Figure S6, the membrane can be recycled and can be recycled and reused at least 10 times. Our interesting results suggests that simple optimized reused at least 10 times. Our interesting results suggests that simple optimized blending ratio of BN blending ratio of BN precursors on PAN nanofibers can produce desirable high adsorption capacity precursors on PAN nanofibers can produce desirable high adsorption capacity using the combined using the combined electrospinning and spray coating technique. electrospinning and spray coating technique.

Figure 8. The removal efficiency of pure carbon NF membrane towards (a) CR, (b) BY, (c) Rh B Figure 8. The removal efficiency of pure carbon NF membrane towards (a) CR, (b) BY, (c) Rh B aqueous solutions. aqueous solutions.

Nanomaterials 2019, 9, 1383 12 of 15 Nanomaterials 2019, 9, x FOR PEER REVIEW 12 of 15

Figure 9.9. TheThe removalremoval e ffiefficiencyciency of of the the 3D 3D BN NFBN membraneNF membrane in (a) CR,in (a (b) )CR, BY, (cb)) RhBY, B aqueous(c) Rh B solutions.aqueous solutions. 4. Conclusions 4. ConclusionIn this study, we successfully fabricated a PAN NF membrane with a 3D BN porous structure by using a simple electrospinning and spraying composite technique. After heat treatment, the 3D BN with In this study, we successfully fabricated a PAN NF membrane with a 3D BN porous structure a porous structure located within PAN NFs was successfully framed. By using sensible optimization, by using a simple electrospinning and spraying composite technique. After heat treatment, the 3D we determined that the optimal parameters of the precursor are a molar ratio of 1:5 and a spraying BN with a porous structure located within PAN NFs was successfully framed. By using sensible time of 30 s. Furthermore, the prepared BN-PAN ES NF membrane showed a higher adsorption ability optimization, we determined that the optimal parameters of the precursor are a molar ratio of 1:5 and of the tested dyes from water ascribed to the 3D BN nanosheets successful embedment onto NFs a spraying time of 30 s. Furthermore, the prepared BN-PAN ES NF membrane showed a higher with good porous architecture. The developed 3D BN material has better porous structure, excellent adsorption ability of the tested dyes from water ascribed to the 3D BN nanosheets successful adsorption towards anionic, cationic, and fluorescent dyes, and is lightweight. These credible features embedment onto NFs with good porous architecture. The developed 3D BN material has better make the 3D BN ES NF membranes suitable for water purification, and they have a greater wide range porous structure, excellent adsorption towards anionic, cationic, and fluorescent dyes, and is lightweight.of potential applications, These credible such features as in filters, make energy the storage, 3D BN hydrogen ES NF membranes storage, and suitable capacitors. for water purification, and they have a greater wide range of potential applications, such as in filters, energy storage, hydrogen storage, and capacitors.

Nanomaterials 2019, 9, 1383 13 of 15

Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/9/10/1383/s1, Figure S1: FE-SEM images of BN/PAN ES NFs with different spray coating time, the BN ES nanofiber membrane with the spray coating time. Figure S2: FE-SEM images of BN structure about the different ratio of the BN precursor, and the B2O3 and CH4N2O were respectively mixed with different molar ratio. Figure S3: The EDX of pure PAN carbon NFs and BN-PAN composite ES NFs. Figure S4: The X-ray photoelectron spectra (XPS) of BN-PAN composite ES NFs. Figure S5: The removal efficiency of the 3D BN NF membrane in Rh B aqueous solutions after 450 mins. Figure S6: The membrane can be recycled and reused for at least 10 times (the plot of the PL intensity vs time for the 10 cycles). Author Contributions: For this research articles, the individual contributions of authors: conceptualization, W.-H.C. and C.-C.K.; resources, X.C.; methodology, validation, investigation, and writing—original draft preparation, D.-H.J., and P.-C.C.; formal analysis, C.-J.C. and L.V.; writing—review and editing, S.-H.T. and T.S.; supervision and project administration, C.-C.K. Funding: This work was supported by the Ministry of Science and Technology, Taiwan (Contracts: MOST 106-2221-E-027 -119 -MY3, MOST 105-2221-E-027-134- and MOST 104-2113-M-027-007-MY3). Conflicts of Interest: The authors declare no conflict of interest.

References

1. Abdeen, Z.; Mohammad, S.G. Study of the Adsorption Efficiency of an Eco-Friendly Carbohydrate Polymer for Contaminated Aqueous Solution by Organophosphorus Pesticide. Open J. Org. Polym. Mater. 2014, 4, 16–28. [CrossRef] 2. Abdolali, A.; Guo, W.; Ngo, H.H.; Chen, S.; Nguyen, N.; Tung, K.; Guo, W. Typical lignocellulosic wastes and by-products for biosorption process in water and wastewater treatment: A critical review. Bioresour. Technol. 2014, 160, 57–66. [CrossRef][PubMed]

3. Mohammadi, M.; Sabbaghi, S. Photo-catalytic degradation of 2,4-DCP wastewater using MWCNT/TiO2 nano-composite activated by UV and solar light. Environ. Nanotechnol. Monit. Manag. 2014, 1, 24–29. [CrossRef] 4. Tran, V.S.; Ngo, H.H.; Guo, W.; Zhang, J.; Liang, S.; Ton-That, C.; Zhang, X. Typical low cost biosorbents for adsorptive removal of specific organic pollutants from water. Bioresour. Technol. 2015, 182, 353–363. [CrossRef][PubMed] 5. Rafatullah, M.; Sulaiman, O.; Hashim, R.; Ahmad, A. Adsorption of methylene blue on low-cost adsorbents: A review. J. Hazard. Mater. 2010, 177, 70–80. [CrossRef][PubMed] 6. Luo, Y.; Guo, W.; Ngo, H.H.; Nghiem, L.D.; Hai, F.I.; Zhang, J.; Liang, S.; Wang, X.C. A review on the occurrence of micropollutants in the aquatic environment and their fate and removal during wastewater treatment. Sci. Total Environ. 2014, 473, 619–641. [CrossRef][PubMed] 7. Barakat, M.; Barakat, M. New trends in removing heavy metals from industrial wastewater. Arab. J. Chem. 2011, 4, 361–377. [CrossRef] 8. Mohan, D.; Sarswat, A.; Ok, Y.S.; Pittman, C.U. Organic and inorganic contaminants removal from water with biochar, a renewable, low cost and sustainable adsorbent—A critical review. Bioresour. Technol. 2014, 160, 191–202. [CrossRef][PubMed] 9. Stackelberg, P.E.; Gibs, J.; Furlong, E.T.; Meyer, M.T.; Zaugg, S.D.; Lippincott, R.L. Efficiency of conventional drinking-water-treatment processes in removal of pharmaceuticals and other organic compounds. Sci. Total Environ. 2007, 377, 255–272. [CrossRef][PubMed] 10. Oller, I.; Malato, S.; Sánchez-Pérez, J. Combination of Advanced Oxidation Processes and biological treatments for wastewater decontamination—A review. Sci. Total Environ. 2011, 409, 4141–4166. [CrossRef] 11. Cao, P. RETRACTED ARTICLE: Full-scale study of lime enhanced coagulation for organic pollutants removal. In Proceedings of the 2010 2nd Conference on Environmental Science and Information Application Technology, ESIAT 2010, Wuhan, China, 17–18 July 2010; pp. 589–591. 12. Krupi´nska,I. The Impact of the Oxidising Agent Type and Coagulant Type on The Effectiveness of Coagulation in the Removal of Pollutants from Underground Water with An Increased Content of Organic Substances. J. Environ. Eng. Landsc. Manag. 2016, 24, 70–78. [CrossRef] 13. Wang, Z.; Ye, K.-W.; Yang, Z.; Xia, Z.-T.; Luo, Q.-Q.; Wan, X.-K.; Shi, H.-X. Magnetic field assisted electrocatalytic oxidation of organic pollutants in electroplating wastewater. J. Cent. South Univ. 2012, 19, 1679–1684. [CrossRef] Nanomaterials 2019, 9, 1383 14 of 15

14. Von Gunten, U. Ozonation of drinking water: Part I. Oxidation kinetics and product formation. Water Res. 2003, 37, 1443–1467. [CrossRef] 15. Feng, Y.; Yang, S.; Xia, L.; Wang, Z.; Suo, N.; Chen, H.; Long, Y.; Zhou, B.; Yu, Y. In-situ ion exchange electrocatalysis biological coupling (i-IEEBC) for simultaneously enhanced degradation of organic pollutants and heavy metals in electroplating wastewater. J. Hazard. Mater. 2019, 364, 562–570. [CrossRef][PubMed] 16. Weng, Y.; Weng, Z.; Liang, Z.; Lyu, H.; Zhuang, Z.; Yu, Y. Mesoporous Ca–Mn–O as an efficient scavenger toward organic pollutants and heavy metals: Ion exchange provoking ultrafast Fenton-like reaction based on the synergy of alkaline earth/transition metals. J. Mater. Chem. A 2018, 6, 9528–9538. [CrossRef] 17. Babel, A.; Li, D.; Xia, Y.; Jenekhe, S.A. Electrospun Nanofibers of Blends of Conjugated Polymers: Morphology, Optical Properties, and Field-Effect Transistors. Macromology 2005, 38, 4705–4711. [CrossRef] 18. Chen, Y.Y.; Kuo, C.C.; Chen, B.Y.; Chiu, P.C.; Tsai, P.C. Multifunctional polyacrylonitrile-ZnO/Ag electrospun nanofiber membranes with various ZnO morphologies for photocatalytic, UV-shielding, and antibacterial applications. J. Polym. Sci. Part B Polym. Phys. 2015, 53, 262–269. [CrossRef] 19. Kuo, C.-C.; Lin, C.-H.; Chen, W.-C. Morphology and Photophysical Properties of Light-Emitting Electrospun Nanofibers Prepared from Poly(fluorene) Derivative/PMMA Blends. Macromology 2007, 40, 6959–6966. [CrossRef] 20. Kuo, C.-C.; Tung, Y.-C.; Chen, W.-C. Morphology and pH Sensing Characteristics of New Luminescent Electrospun Fibers Prepared from Poly(phenylquinoline)-block-Polystyrene/Polystyrene Blends. Macromol. Rapid Commun. 2010, 31, 65–70. [CrossRef][PubMed] 21. Chiu, Y.C.; Chen, Y.; Kuo, C.C.; Tung, S.H.; Kakuchi, T.; Chen, W.C. Synthesis, morphology, and sensory applications of multifunctional rod-coil-coil triblock copolymers and their electrospun nanofibers. ACS Appl. Mater. Interfaces 2012, 4, 3387–3395. [CrossRef] 22. Chen, L.N.; Chiu, Y.C.; Hung, J.J.; Kuo, C.C.; Chen, W.C. Multifunctional Electrospun Nanofibers Prepared from Poly ((N-isopropylacrylamide)-co-(N-hydroxymethylacrylamide)) and Their Blends with 1, 2-Diaminoanthraquinone for NO Gas Detection. Macromol. Chem. Phys. 2014, 215, 286–294. [CrossRef] 23. Chen, L.-N.; Kuo, C.-C.; Chiu, Y.-C.; Chen, W.-C. Ultra metal ions and pH sensing characteristics of thermoresponsive luminescent electrospun nanofibers prepared from poly(HPBO-co-NIPAAm-co-SA). RSC Adv. 2014, 4, 45345–45353. [CrossRef] 24. Wang, W.; Yang, Q.; Sun, L.; Wang, H.; Zhang, C.; Fei, X.; Sun, M.; Li, Y. Preparation of fluorescent nanofibrous film as a sensing material and adsorbent for Cu2+ in aqueous solution via copolymerization and electrospinning. J. Hazard. Mater. 2011, 194, 185–192. [CrossRef][PubMed] 25. Panthi, G.; Park, M.; Kim, H.-Y.; Lee, S.-Y.; Park, S.-J. Electrospun ZnO hybrid nanofibers for photodegradation of wastewater containing organic dyes: A review. J. Ind. Eng. Chem. 2015, 21, 26–35. [CrossRef] 26. Zhang, Z.; Shao, C.; Li, X.; Wang, C.; Zhang, M.; Liu, Y. Electrospun nanofibers of p-type NiO/n-type ZnO heterojunctions with enhanced photocatalytic activity. ACS Appl. Mater. Interfaces 2010, 2, 2915–2923. [CrossRef][PubMed] 27. Sachdev, H.; Haubner, R.; Nöth, H.; Lux, B. Investigation of the c-BN/h-BN phase transformation at normal pressure. Diam. Relat. Mater. 1997, 6, 286–292. [CrossRef] 28. Chunyi, Z.; Yoshio, B.; Chengchun, T.; Hiroaki, K.; Dimitri, G. Large-Scale Fabrication of Boron Nitride Nanosheetsand Their Utilization in Polymeric Composites withImproved Thermal and Mechanical Properties. Adv. Mater. 2009, 21, 2889–2893. 29. Liu, D.; Lei, W.; Qin, S.; Chen, Y. Template-Free Synthesis of Functional 3D BN architecture for removal of dyes from water. Sci. Rep. 2014, 4, 4453. [CrossRef][PubMed] 30. Kubota, Y.; Watanabe, K.; Tsuda, O.; Taniguchi, T. Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure. Science 2007, 317, 932–934. [CrossRef] 31. Zhou, C.; Lai, C.; Zhang, C.; Zeng, G.; Huang, D.; Cheng, M.; Hu, L.; Xiong, W.; Chen, M.; Wang, J.; et al. Semiconductor/boron nitride composites: Synthesis, properties, and photocatalysis applications. Appl. Catal. B Environ. 2018, 238, 6–18. [CrossRef] 32. Sheng, Y.; Yang, J.; Wang, F.; Liu, L.; Liu, H.; Yan, C.; Guo, Z. Sol-gel synthesized hexagonal boron nitride/titania nanocomposites with enhanced photocatalytic activity. Appl. Surf. Sci. 2019, 465, 154–163. [CrossRef] Nanomaterials 2019, 9, 1383 15 of 15

33. Cao, Y.; Maitarad, P.; Gao, M.; Taketsugu, T.; Li, H.; Yan, T.; Shi, L.; Zhang, D.; Zhanga, D. Defect-induced efficient dry reforming of methane over two-dimensional Ni/h-boron nitride nanosheet catalysts. Appl. Catal. B Environ. 2018, 238, 51–60. [CrossRef] 34. Wang, Y.; Shi, L.; Lu, W.; Sun, Q.; Wang, Z.; Zhi, C.; Lu, A.-H. Spherical Boron Nitride Supported Gold-Copper Catalysts for the Low-Temperature Selective Oxidation of Ethanol. ChemCatChem 2017, 9, 1363–1367. [CrossRef] 35. Low, Z.-X.; Ji, J.; Blumenstock, D.; Chew, Y.-M.; Wolverson, D.; Mattia, D. Fouling resistant 2D boron nitride nanosheet – PES nanofiltration membranes. J. Membr. Sci. 2018, 563, 949–956. [CrossRef] 36. Zhang, Y.; Shi, Q.; Liu, Y.; Wang, Y.; Meng, Z.; Xiao, C.; Deng, K.; Rao, D.; Lu, R. Hexagonal Boron Nitride with Designed Nanopores as a High-Efficiency Membrane for Separating Gaseous Hydrogen from Methane. J. Phys. Chem. C 2015, 119, 19826–19831. [CrossRef] 37. Pang, J.; Chao, Y.; Chang, H.; Li, H.; Xiong, J.; Zhang, Q.; Chen, G.; Qian, J.; Zhu, W.; Li, H. Silver Nanoparticle-Decorated Boron Nitride with Tunable Electronic Properties for Enhancement of Adsorption Performance. ACS Sustain. Chem. Eng. 2018, 6, 4948–4957. [CrossRef] 38. Yu, S.; Wang, X.; Pang, H.; Zhang, R.; Song, W.; Fu, D.; Hayat, T.; Wang, X. Boron nitride-based materials for the removal of pollutants from aqueous solutions: A review. Chem. Eng. J. 2018, 333, 343–360. [CrossRef] 39. Nagarajan, S.; Belaid, H.; Pochat-Bohatier, C.; Teyssier, C.; Iatsunskyi, I.; Romero, L.E.C.; Balme, S.; Cornu, D.; Miele, P.; Kalkura, N.S.; et al. Design of Boron Nitride/Gelatin Electrospun Nanofibers for Bone Tissue Engineering. ACS Appl. Mater. Interfaces 2017, 9, 33695–33706. [CrossRef][PubMed] 40. Lei, W.; Portehault, D.; Dimova, R.; Antonietti, M. Boron Carbon Nitride Nanostructures from Salt Melts: Tunable Water-Soluble Phosphors. J. Am. Chem. Soc. 2011, 133, 7121–7127. [CrossRef][PubMed] 41. Lei, W.; Portehault, D.; Liu, D.; Qin, S.; Chen, Y. Porous boron nitride nanosheets for effective water cleaning. Nat. Commun. 2013, 4, 1777. [CrossRef]

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