Keywords

1 Introduction

Electrospinning technology is a technology for preparing polymer micro–nano-fibers [1,2,3]. This technology is simple and easy to operate and has high production efficiency. A thin-film composite of continuous nanoscale fibers can be prepared from a polymer solution. The electrostatic spinning device is mainly composed of three parts: high voltage power supply, spinning head, and receiving screen. Under the action of electric field force, the charges on the surface of the droplet at the tip of the spinning needle are concentrated and repel each other, and the droplet is elongated to form a Taylor cone [4, 5]. After spraying out, it solidifies in the air and finally forms disorderly arranged nanofibers on the receiving device. Under the impetus of the stepping pump, the spinning continued, and the fine stream ejected from the needle was continuously deposited on the receiving screen and finally formed a film product woven by nanofibers [6, 7]. In recent decades, the research on electrospinning has mainly focused on the development of electrospinning nanofiber raw materials, multi-component polymer electrospinning, and electrospinning jet instability models.

Electrospinning technology has many outstanding advantages, such as no complicated and expensive equipment and instruments, and lower experimental costs. In addition, the operation of electrospinning is simple and easy, and the applicable raw materials are widely free from harsh requirements. The most important thing is that the prepared fibers are at the nanometer level, and the average fiber diameter is generally between tens and hundreds of nanometers. Therefore, the resulting film has the characteristics of controllable fiber diameter [8], high tensile strength [9, 10], and very large surface area [11,12,13,14,15,16,17]. Therefore, these advantages make the electrospun nanofiber film have a wide range of potential applications in many fields, including tissue engineering [18], drug sustained-release [19], nanosensors [20], energy applications [21], biochips and catalyst loading.

At present, the construction of nanostructures of various materials (such as polymers, inorganics, and multi-component composites) has been achieved through electrospinning technology [22,23,24], which has shown great abilities in the fields of catalysis, drug carriers, and filtration potential [25, 26]. Liu et al. have successfully prepared a new composite film based on silver nano-particles modified polyvinyl alcohol/polyacrylic acid/carboxyl functionalized graphene oxide (PVA/PAA/GO-COOH@AgNPs) as Efficient Dye Photocatalyst Materials for Wastewater Treatment [27]. Panaitescu et al. processed modified poly(3-hydroxybutyrate) by using nanocellulose and plasma technology as a candidate material for food packaging applications [28]. Rauwel et al. introduced several nanomaterials in the book “Application and Behavior of Nanomaterials in Water Treatment”, such as graphene/CNT and nanostructured Prussian blue hybrid nanocomposites, controlled growth of LDH films and rod-shaped MnO nanocomposites, and so on. It is applied to sewage treatment, and a good purification effect is obtained [29]. In addition, the fiber material shows good adsorption capacity due to its microporous structure. This chapter provides a comprehensive review of the preparation process of various electrospun fiber materials and applies these composite nanofiber materials to the fields of adsorption, catalysis, and air filtration. It also provides challenges and prospects to inspire more exciting developments in the future.

2 Preparation of Electrospinning Nanocomposite Membrane and Its Adsorption and Degradation of Organic Dyes in Wastewater

The preparation of electrospun nanocomposite membranes has been a hot topic in many fields, such as chemistry, materials science, and nanotechnology. In this section, we will focus on the application of different types of polymer nanofilms in the adsorption and degradation of organic dyes: (i) bioinspired polydopamine sheathed nanofibers containing carboxylate graphene oxide nanosheet for high-efficient dyes scavenger, (ii) preparation of TiO2 nanoparticles modified electrospun nanocomposite membranes toward efficient dye degradation for wastewater treatment, (iii) polydopamine-coated electrospun poly(vinyl alcohol)/poly(acrylic acid) membranes as efficient dye adsorbent with good recyclability, (iv) fabrication and highly efficient dye removal characterization of beta-cyclodextrin-based composite polymer fibers by electrospinning, and (v) self-assembled AgNP-containing nanocomposites constructed by electrospinning as efficient dye photocatalyst materials for wastewater treatment.

2.1 Bioinspired Polydopamine Sheathed Nanofibers Containing Carboxylate Graphene Oxide Nanosheet for High-Efficient Dyes Scavenger

Firstly, Xing et al. successfully prepared a new type of hierarchical bioinspired nanocomposite materials of poly(vinyl alcohol)/poly(acrylic acid)/carboxylate graphene oxide nanosheet@polydopamine (PVA/PAA/GO-COOH@PDA) by electrospinning technique, thermal treatment, and polydopamine modification [30]. The obtained composite membranes are composed of polymeric nanofibers with carboxylate graphene oxide nanosheets, which are anchored on the fibers by the heat-induced crosslinking reaction. The preparation process demonstrates an eco-friendly and controllable manner. These as-formed nanocomposites were characterized by various morphological methods and spectral techniques. Due to the unique polydopamine and graphene oxide containing structures in composites, the as-obtained composite demonstrates well-efficient adsorption capacity toward dye removal, which is primarily due to the specific surface area of electrospun membranes and the active polydopamine/graphene oxide components [31,32,33]. In addition, it should be noted that the adsorption capacities of the as-obtained PVA/PAA/GO-COOH@PDA nanocomposites on MB show better performance than two other used dyes. The main reason for the difference can be speculated to the matched strong π–π stacking and electrostatic interactions between nanocomposites and MB molecules. At the same time, the abundant amino and hydroxyl groups in the PDA surface give more adsorbent activity points for dye molecules, which demonstrate the tailored strategy to improve the absorption performance (Figs. 1 and 2).

Fig. 1
figure 1

Reproduced with permission from Ref. [30]. Copyright 2017, American Chemical Society

Schematic illustration of the fabrication and dye adsorption of PVA/PAA/GO-COOH@PDA nanocomposites by electrospinning and thermal treatment.

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Fig. 2
figure 2

Reproduced with permission from Ref. [30]. Copyright 2017, American Chemical Society

Adsorption kinetics curves of as-prepared PVA/PAA/GO-COOH@PDA nanocomposites on MB (a, b), RhB (c, d), and CR (e, f) at 298 K.

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Therefore, the present work is expected to open a new avenue for the design and preparation of eco-friendly electrospun composites loaded with functional GO and nanoparticles, which could enhance the practical application in wastewater treatment by using functionalized composite nanofibers materials.

2.2 Preparation of TiO2 Nanoparticles Modified Electrospun Nanocomposite Membranes Toward Efficient Dye Degradation for Wastewater Treatment

Firstly, Hou et al. prepared nano-polyvinyl alcohol/polyacrylic acid/carboxyl functionalized graphene oxide nanocomposite films by using electrospinning technology [34]. The composite membrane prepared by electrospinning technology is composed of polymer nanofibers with carboxyl functionalized graphene oxide sheets, which are anchored to the fibers through a thermally induced crosslinking reaction. TiO2 nanoparticles are deposited uniformly and uniformly on the surface of the resulting composite film. The preparation process of the nano-composite membrane is simple, green, and environmentally friendly, and it is easy to operate and control. The prepared composite membranes exhibited an effective photocatalytic ability for dye degradation, which was mainly attributed to the specific surface area of electrospun membranes and the photoactivity of TiO2 nanoparticles. In addition, the composite membrane reported here is easy to regenerate, which indicates potential large-scale applications in wastewater treatment and dye removal (Figs. 3, 4 and Table 1).

Fig. 3
figure 3

Reprinted with permission from Ref. [34]. Copyright 2017, Elsevier Ltd

Schematic illustration of the fabrication of PVA/PAA/GO-COOH@TiO2 nanocomposite by electrospinning and thermal treatment.

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Fig. 4
figure 4

Reprinted with permission from Ref. [34]. Copyright 2017, Elsevier Ltd

Photocatalytic kinetics curves of as-prepared PVA/PAA/GO-COOH@TiO2 nanocomposite on CR (a and b), RhB (c and d), and MB (e and f) at 298 K.

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Table 1 Kinetic parameters of PVA/PAA/GO-COOH nanocomposite and PVA/PAA/GO-COOH@TiO2 nanocomposite for CR, RhB, and MB (e and f) degradations and removal at 298 K (experimental data from Fig. 4)
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At the same time, the author also studied the photocatalytic performance of the prepared PVA/PAA/GO-COOH@TiO2 nanocomposite membrane to three model dye solutions (CR, RhB, and MB). The degradation program currently studied was characterized by placing the obtained PVA/PAA/GO-COOH@TiO2 nanocomposites in different aqueous dye solutions [35,36,37,38,39,40]. In addition, this catalytic experiment was measured and repeated three times. The degradation kinetics experiments of the prepared PVA/PAA/GO-COOH@TiO2 nanocomposites were carried out. The results are shown in Fig. 4. This work is expected to open up a new way for the design and preparation of environmentally friendly electrospun composite materials containing functional GO and nanoparticles. The use of functional composite nanofiber materials can enhance the practical application of wastewater treatment.

2.3 Polydopamine-Coated Electrospun Poly(Vinyl Alcohol)/poly(Acrylic Acid) Membranes as Efficient Dye Adsorbent with Good Recyclability

Inspired by the characteristics of adhesion proteins in marine mussels, dopamine, which is a catecholamine, can self-polymerize under alkaline reaction conditions and form a polydopamine (PDA) film on almost all types of substrates [41, 42]. The PDA coating can form a highly stable polymer layer on the target surface and shows special adhesion in the presence of residual catechol groups on the PDA layer, which helps to further react with appropriate molecules; it also provides the possibility for customized PDA coatings for various applications [43]. For example, PDA membranes are often used as modifiers to improve the hydrophilicity and reactivity of target substrates (such as clay [44], polystyrene nanofibers [45], and even polytetrafluoroethylene [46]). In particular, Gao et al. prepared a PDA-functionalized graphene hydrogel in a one-step process and found that the PDA-coated graphene hydrogel has good adsorption capacity for heavy metals and organic dyes in wastewater [47]. Recently, nanoparticles decorated with PDA (such as Fe3O4 [48] and natural zeolite [49]) have also been synthesized and used to remove various pollutants. However, considering the above-environmental issues, we should focus on the use of polydopamine as an adsorbent to remove dye contaminants in water for more research.

Yan et al. prepared a free-standing polyvinyl alcohol/polyacrylic acid (PVA/PAA) film with polydopamine (PDA) coating based on electrospinning and self-polymerization of dopamine [50]. The preparation process is simple, green, controllable, and low energy consumption, and there are no strict restrictions on the reaction conditions. Thanks to the high specific surface area of electrospun membranes and the rich “adhesive” functional groups of polydopamine, the membranes produced exhibited effective adsorption properties for methyl blue, with an adsorption capacity of up to 1147.6 mg g−1. Moreover, compared with other nanoparticle adsorbents, the prepared free-standing membrane has high flexibility and is easy to handle and recycle, and the most important thing is easy to elute and regenerate so that it has potential applications in wastewater treatment prospect (Figs. 5 and 6).

Fig. 5
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Reprinted with permission from Ref. [50]. Copyright 2015, Elsevier Ltd

Schematic illustration of the preparation of PVA/PAA@PDA membranes and their applications for dye adsorption.

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Fig. 6
figure 6

Reprinted with permission from Ref. [50]. Copyright 2015, Elsevier Ltd

Adsorption isotherms (a and b) and the corresponding Langmuir plot (c), Freundlich plot (d) for MB adsorption on PVA/PAA@PDA-15 membrane.

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2.4 Fabrication and Highly Efficient Dye Removal Characterization of Beta-Cyclodextrin-Based Composite Polymer Fibers by Electrospinning

As a class of cyclic oligosaccharides, cyclodextrins have hollow cones, which have external hydrophilicity and internal hydrophobicity. This special structure has many special physical and chemical properties. It can selectively bind small organic molecules in aqueous solution, and the formed inclusion complex has different degrees of stability [51,52,53]. Therefore, cyclodextrins and its derivatives are widely used in medicine, food, chemical engineering materials, especially wastewater treatment [54,55,56,57,58]. For example, Li et al. prepared a material based on cyclodextrins to remove malachite green [59]. The adsorption results are in accordance with the Langmuir model, and the maximum adsorption capacity reaches 91.9 mg/g. Yilmaz et al. synthesized two polymers based on β-cyclodextrin with the aid of 4, 4′-methylene-bis-phenyldiisocyanate (MDI) or hexamethylene diisocyanate (HMDI) [60]. These materials can remove azo dyes and aromatic amines, and the main adsorption mechanism is the host–guest interaction. At present, some cyclodextrin-based fiber systems by electrospinning have been reported. For example, Cui et al. described the use of plasma-treated polyethylene oxide-β-cyclodextrin nanofibers to enhance antibacterial activity [61]. Celebioglu et al. demonstrated electrospinning of polymer-free nanofiber structures formed from inclusion compounds between hydroxypropyl-β-cyclodextrin and vitamin E [62]. The prepared vitamin E-containing fiber web provides enhanced photostability of sensitive vitamin E by inclusion complexes even after exposure to UV light.

Guo et al. prepared a new type of composite fiber adsorption material composed of ε-polycaprolactone (PCL) and β-cyclodextrin-based polymer (PCD) by electrospinning, used to solve the increasing pollution of azo dye problem [63]. More importantly, PCD was chosen because of its infinite long chain and cavity structure. Due to a large number of free cyclodextrin pores on the fiber surface, the resulting membrane helps to form more host–guest interactions. Therefore, according to previous reports [64], its excellent selective adsorption capacity can be foreseen. During the electrospinning process, long-chain polymer molecules may occupy some cyclodextrin cavities, which is not conducive to the interaction between the host and the guest and even leads to the reduction of poor adsorption. However, our PCL/(n%) PCD composite fiber has countless holes, which can guarantee selective adsorption capacity. Therefore, it is obvious that the PCL/(n%) PCD composite fibers obtained can exhibit significant adsorption capacity for azo dyes and have host–guest interactions. Moreover, the introduction of a β-cyclodextrin polymer can effectively improve the mechanical strength and stability of the membrane. This shows that the obtained composite material has great potential, which can help the problem of azo dye pollution in wastewater treatment (Figs. 7, 8 and 9).

Fig. 7
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Reprinted with permission from Ref. [63]. Copyright 2019, MDPI Ltd

Schematic illustration of preparation and application in dye removal of PCL/(n%)PCD composite fibers by electrospinning.

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Fig. 8
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Reprinted with permission from Ref. [63]. Copyright 2019, MDPI Ltd

SEM images and diameter distribution histograms of neat PCL fibers (a), PCL/(10%)PCD(b), PCL/(20%)PCD(c), PCL/(30%)PCD (d), PCL/(40%)PCD (e), and PCL/(50%)PCD (f).

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Fig. 9
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Reprinted with permission from Ref. [63]. Copyright 2019, MDPI Ltd

Kinetic adsorption of MB (a, b) and 4-aminoazobenzene (c, d).

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2.5 Self-assembled AgNP-Containing Nanocomposites Constructed by Electrospinning as Efficient Dye Photocatalyst Materials for Wastewater Treatment

Graphene oxide (GO)-based nanocomposite fibers have attracted great interest due to their adjustable dispersibility, large oxygen-containing functional groups, and special chemical modification and preferred reaction positions/sites [65]. Liu et al. have successfully prepared a new composite film based on silver nanoparticles modified polyvinyl alcohol/polyacrylic acid/carboxyl functionalized graphene oxide (PVA/PAA/GO-COOH@AgNPs) [27]. The new composite membrane utilizes the advantages of graphene oxide (GO) nanocomposites and many novel advantages of electrospun fibers. At the same time, the Ag nanoparticles produced by the reduction of ascorbic acid solution to AgNO3 are firmly fixed in the nanometer using hydrogen bonding and electrostatic interaction with the surface of the fiber. First, due to its chemical and mechanical properties and large specific surface area, the prepared PVA/PAA/GO-COOH system was selected as the matrix material for electrospinning [66]. Secondly, compared with conventional organic solvents, deionized water is used as a solvent for preparing electrospinning solutions, which has the characteristics of low cost and environmental friendliness. Most importantly, Ag nanoparticles show that they can achieve good stability between the flexible surface of the fiber and the effective GO nanosheets and function under visible light. The powerful π–π force in the GO sheet can make various dyes in water have strong adsorption force. In addition, the fixed carboxyl groups in GO nanosheets can generate strong electrostatic interactions through their highly negatively charged characteristics, which can promote the diffusion and enrichment of target dyes. For the degradation of MB, the prepared PVA/PAA/GO-COOH@AgNPs nanocomposite membranes still have significant catalytic activity even after eight catalytic degradations at room temperature. Therefore, the author provides a green and novel method to prepare highly efficient dye photocatalytic wastewater treatment materials (Fig. 10).

Fig. 10
figure 10

Reprinted with permission from Ref. [27]. Copyright 2018, MDPI Ltd

Schematic illustration of the fabrication of PVA/PAA/GO-COOH@AgNPs nanocomposites by electrospinning and thermal treatment.

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3 Preparation of Electrospun Nanofiber and Characterization of Catalytic Performance

3.1 Scalable Fabrication of Nanoporous Carbon Fiber Films as Bifunctional Catalytic Electrodes for Flexible Zn-Air Batteries

With the rapid development of flexible and wearable optoelectronic devices, there is an urgent need to use flexible high-density energy storage devices as the power source [67]. Recently, people have made tremendous efforts to develop flexible lithium-ion batteries and supercapacitors. However, due to the low energy density of the batteries and the limited cycle life, it has become a research difficulty [68]. Because metal–air batteries are said to have high energy capacity [69], they are the next generation of promising wearable optoelectronic products for energy storage devices. Of particular concern is that zinc–air batteries have received extensive research and development attention due to their high energy density, low cost, and high safety [70]. However, most cathodes currently used in zinc–air batteries are bulky and rare. Meet the specific requirements of flexible Zn-air batteries. In addition, the development of highly efficient dual-functional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen release reaction (OER) of flexible rechargeable zinc–air batteries remains a huge challenge [71], although important progress has been made recently used in traditional zinc–air batteries [72] (Figs. 11, 12 and 13).

Fig. 11
figure 11

Reprinted with permission from Ref. [73]. Copyright 2016, Wiley-Blackwell

a Schematic representation of the fabrication procedure toward the NCNF. b Photographs of the resultant flexible NCNF. c Chemical structure of PI polymer. SEM images of d the pristine PI film and e, f NCNF-1000. g, h HRTEM, i STEM images and corresponding elemental mapping images of C, O, N of NCNF-1000.

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Fig. 12
figure 12

Reprinted with permission from Ref. [73]. Copyright 2016, Wiley-Blackwell

a CV curves of NCNF-1000 and Pt/C, in O2-saturated (solid line) and N2-saturated (dotted line) 0.1 M KOH. b LSV curves of different catalysts for ORR in O2-saturated at 1600 rpm. c LSV curves of NCNF-1000 for ORR at different rotating speeds and the inset is K-L plots of NCNF-1000 at different potentials including the calculated number of electron transfer (n) per O2. d LSV curves of different catalysts for OER at 1600 rpm in 0.1 M KOH. e Tafel slopes derived from (d). f LSV curves of different catalysts for both ORR and OER in 0.1 M KOH at 1600 rpm (scan rate 5 mV s−1). The catalyst loading was 0.1 mg cm−2 for all catalysts.

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Fig. 13
figure 13

Reprinted with permission from Ref. [73]. Copyright 2016, Wiley-Blackwell

a Polarization and power density curves of the primary Zn-air batteries with different catalysts. b Galvanostatic discharge curves of the primary Zn-air battery with NCNF-1000 as a catalyst at different current densities, which was normalized to the area of air–cathode. c Schematic representation of the rechargeable Zn-air battery. d Charge and discharge polarization curves. e Galvanostatic discharge–charge cycling curves at 10 mA cm−2 of rechargeable Zn-air batteries with the NCNF-1000 and Pt/C as catalyst, respectively. f Photograph of a blue LED (≈3.0 V) powered by two liquid Zn-air batteries with the NCNF-1000 air–cathode connected in series.

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Liu et al. found that newly developed nanoporous carbon nanofiber films (NCNFs) with the large specific surface areas are flexible and show that they are excellent when used as air cathodes in liquid Zn-air batteries used in ambient air [73]. The performance has a high open-circuit voltage (1.48 V), maximum power density (185 mW cm−2), and energy density (776 Wh kg-1) and has a large specific surface area (1249 m2 g−1), high electrical conductivity (147 S m−1), moderate tensile strength (1.89 MPa), and tensile modulus (0.31 GPa). They show excellent performance for ORR (initial potential = 0.97 V vs RHE; limiting current density = 4.7 mA cm−2) and OER (initial potential = 1.43 V vs RHE, potential = 1.84 V @ 10 mA cm−2) dual-function electrocatalytic activity.

3.2 Carbon Nanofiber-Supported PdNi Alloy Nanoparticles as Highly Efficient Bifunctional Catalysts for Hydrogen and Oxygen Evolution Reactions

Currently, much attention is focused on developing efficient and clean renewable energy technologies to increase energy demand and alleviate environmental problems [74,75,76]. As we all know, the production of hydrogen through water electrolysis is a key strategy to overcome these energy challenges [77,78,79]. However, the water decomposition half-reactions, namely the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), have a high overpotential and are high energy [80, 81]. An effective electrocatalyst is essential to enhance the electrochemical water-splitting performance. In particular, precious metal catalysts (such as Pt and RuO2) have been used as the most advanced electrocatalysts for HER and OER, respectively [82,83,84]. However, the high cost and low global reserves of these metals limit their wide practical application. In addition, a catalyst capable of driving both HER and OER is very needed, which is a basic requirement as a high-efficiency energy conversion device in water decomposition. Unfortunately, it is relatively difficult to develop effective, durable, and cost-effective bifunctional catalysts for HER and OER. Note that highly efficient bifunctional catalysts with high HER and OER activity are still highly desired. Chem et al. prepared carbon nanofiber-loaded PdNi alloy nanoparticles, a highly efficient dual-function catalyst for hydrogen and oxygen release reactions [85].

A new class of the PdNi alloy structure was prepared on CNFs by electrospinning and subsequent carbonization. The as-synthesized PdNi/CNFs, which have a low Pd loading, exhibit superior catalytic activity, and good stability in both the HER and OER and are thus a promising alternative bifunctional electrocatalyst. The highly efficient catalytic performance of the PdNi/CNFs is attributed to the synergistic effects of the PdNi alloy and the properties of the CNF substrate. The combination of these factors enhances the catalytic activity and is expected to enable the realization of cost-effective hydrogen and oxygen generation. Therefore, the catalyst developed in this work could be suitable for use in various water-splitting applications (Figs. 14, 15 and 16).

Fig. 14
figure 14

Reprinted with permission from Ref. [85]. Copyright 2017, Elsevier Ltd

a, c, e FE-SEM and b, d, f TEM images of the Pd/CNFs (a, b), Ni/CNFs (c, d) and PdNi/CNFs-1:2 (e, f). The insets are the corresponding HRTEM images.

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Fig. 15
figure 15

Reprinted with permission from Ref. [85]. Copyright 2017, Elsevier Ltd

HER electrocatalysis in 0.5 M H2SO4. a Polarization curves and b corresponding Tafel plots of the Pd/CNFs, Ni/CNFs, PdNi/CNFs-1:2 and commercial Pt/C catalyst; c Histograms of overpotentials at j = 10 mA cm−2 for the catalysts of Pd/CNFs, Ni/CNFs, PdNi/CNFs-1:2 and commercial Pt/C; d The linear fit of the capacitive currents of the catalysts versus the scan rates, inset in (d) is the electrochemical cyclic voltammograms of Pd/CNFs at potential scanning rates from 20 mV s−1 to 200 mV s−1; e EIS of the Pd/CNFs, Ni/CNFs and PdNi/CNFs-1:2 at the potential of 0.1 V versus RHE (the insets show the equivalent circuit of the fitted curves and the full EIS spectrum of the Ni/CNFs); f chronoamperometric response (j-t) curve of PdNi/CNFs-1:2 at a constant voltage of 50 mV versus RHE (the insets show a digital image of the H2 bubbles formed on the PdNi/CNFs-1:2 membrane during the electrocatalytic process).

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Fig. 16
figure 16

Reprinted with permission from Ref. [85]. Copyright 2017, Elsevier Ltd

OER and HER electrocatalysis in 1 M KOH. a Polarization curves and b corresponding Tafel plots of the Pd/CNFs, Ni/CNFs, PdNi/CNFs-1:2 and commercial RuO2 catalyst for OER; c Polarization curves and d corresponding Tafel plots of the Pd/CNFs, Ni/CNFs, PdNi/CNFs-1:2 and commercial Pt/C catalyst for HER in 1 M KOH electrolyte; e EIS of the Pd/CNFs, Ni/CNFs and PdNi/CNFs-1:2 at the potential of 1.60 V versus RHE (the inset shows the equivalent circuit of the fitted curves); f chronoamperometric response (j-t) curve of PdNi/CNFs-1:2 at a constant potential of 1.60 V versus RHE (the inset shows a digital image of the O2 bubbles formed on the PdNi/CNFs-1:2 membrane during the electrocatalytic process).

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3.3 Preparation of Palladium Nanoparticles Decorated Polyethyleneimine/Polycaprolactone Composite Fibers Constructed by Electrospinning with Highly Efficient and Recyclable Catalytic Performances

In recent years, in the context of rapid technological and economic development, available water sources are increasingly scarce, and people have paid extensive attention to the protection and purification of water. Hazardous organic waste is the main source of water pollution [86,87,88,89,90,91,92]. The conversion of harmful organic chemicals into harmless or low-toxic compounds under mild conditions has become an extremely important research field [93,94,95]. As we all know, 4-nitrophenol (4-NP) is widely used in the synthesis of dyes and drugs and is one of the chemical products commonly used in the chemical industry [96].

Nano-sized palladium nanoparticles show high catalytic activity due to the tendency of aggregation and have serious limitations in the field of catalysis. A solid substrate with a large specific surface area is an ideal carrier for palladium nanoparticles. Wang et al. successfully designed and prepared polyethyleneimine/polycaprolactone/Pd nanoparticles (PEI/PCL@PdNPs) composite catalyst by electrospinning and reduction method using PEI/PCL electrospun fiber as the carrier [97]. A large number of pit structures increase the specific surface area of electrospun fibers and provide an active site for the loading of palladium particles, so the added PEI component effectively adjusts the microscopic morphology of PEI/PCL fibers. The obtained PEI/PCL@PdNPs catalysts for the reduction of 4-nitrophenol (4-NP) and 2-nitroaniline (2-NA) showed very effective, stable, and reusable catalytic performance. It is worth mentioning that the reaction rate constant of 4-NP catalytic reduction is 0.16597 s−1. Therefore, we have developed a highly efficient catalyst with potential application prospects in the field of catalysis and water treatment (Fig. 17 and 18).

Fig. 17
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Reprinted with permission from Ref. [97]. Copyright 2019, MDPI Ltd

Preparation process of polyethyleneimine/polycaprolactone/Pd nanoparticles (PEI/PCL@PdNPs) composite and its catalytic performances.

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Fig. 18
figure 18

Reprinted with permission from Ref. [97]. Copyright 2019, MDPI Ltd

UV absorption curves of 4-NP and 2-NA before and after adding NaBH4 aqueous solution (a, d); catalytic reduction of 4-NP and 2-NA with PEI/PCL@PdNPs composite (PEI:PCL, w/w, 35:65) and photographs of 4-NP and 2-NA solution after reduction (b, e); the linear relationship of the reduction process (c, f).

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The stability and recyclability of the catalyst are another aspect of evaluating the catalyst. Therefore, it is very important and necessary to explore the repetitive catalytic capabilities of PEI/PCL@PdNPs catalysts for 4-NP and 2-NA, as shown in Fig. 19. The catalytic efficiency of PEI/PCL@PdNPs catalysts after repeated catalytic reduction of fresh 4-NP and 2-NA systems for 8 cycles was 95% and 92%, respectively. The above results indicate that the PEI/PCL@PdNPs catalyst has high catalytic activity, stability, and recyclability. Referring to the previous literature, the catalytic efficiency has hardly decreased, which is attributed to the adhesion of organic matter on the catalyst surface and the loss of palladium particles on the catalyst surface during the washing of PEI/PCL@PdNPs with ethanol and ultrapure water [98]. This work provides new research clues for the preparation of composite materials loaded with PdNPs and ideal metal particle-based carriers.

Fig. 19
figure 19

Reprinted with permission from Ref. [97]. Copyright 2019, MDPI Ltd

Recyclability test of PEI/PCL@PdNPs catalyst for the reduction of 4-NP (a) and 2-NA (b).

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4 Hierarchical Electrospun Nanofibers Treated by Solvent Vapor Annealing as Air Filtration Mat for High-Efficiency PM2.5 Capture

In the past few years, environmental issues have received extensive attention due to their danger to human health and the environment [99,100,101]. Particulate matter (PM) is a very complex mixture of pollutants, with very fine particles and small droplets [102]. PM2.5 is particularly harmful because these small particles can penetrate the lungs and bronchi [30, 103,104,105,106,107]. In an indoor environment, these particles can be filtered through ventilation or central air-conditioning system, and the personal protection of outdoor personnel is not good, because most commercial masks have low PM2.5 removal efficiency because of their small diameter particles. Therefore, how to effectively and conveniently remove these PM pollutants from the air has become an urgent and challenging problem for researchers. For example, designing and developing high-efficiency filter materials that can efficiently remove PM2.5 particles are the central focus: evidence-based, high porosity, high accumulation of filter fibers. When the fiber diameter is small and the fiber diameter is large, the removal efficiency of PM2.5 is higher. As we all know, electrospinning is a relatively simple method for producing continuous fibers with nanometer and submicron diameters. The fibers prepared by electrospinning have high porosity, fine pore size, staggered pore structure, small pore size, and controllable diverse structure and thickness, which make them ideal for air filtration materials.

Huang et al. introduced a new type of high-efficiency air filter mat that can be used for outdoor protection [108]. The nanocomposite of the new high-efficiency air filter was successfully made of poly (ε-caprolactone)/polyethylene oxide (PCL/PEO) using electrospinning technology and solvent vapor annealing (SVA). The SVA treatment gives the fiber surface a wrinkle effect and enhances the PM2.5 capture ability of the protective mask. This nano-wrinkled air filter mat can effectively filter PM2.5 under severe pollution conditions (PM2.5 particle concentration is higher than 225 mg m−3), and the removal efficiency is 80.01%. Field tests have shown that the air filter mat has a high PM2.5 removal efficiency under dense fog. Compared with commercial masks, the manufactured SVA-treated PCL/PEO air filter pads have a simpler, greener, and more environmentally friendly preparation process and have excellent degradation characteristics, with a wide range of potential applications and high filtration efficiency (Figs. 20, 21 and 22).

Fig. 20
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Reprinted with permission from Ref. [108]. Copyright 2019, Springer Ltd

A schematic illustration of the preparation and filtration of the obtained electrospun nanowrinkled air filtration mat.

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Fig. 21
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Reprinted with permission from Ref. [108]. Copyright 2019, Springer Ltd

SEM images of the prepared electrospun PCL/PEO nanofibers (a, Sample 1) and SVA treatment at different time intervals: b one day; c 2 days; d 3 days; e 4 days; f 5 days.

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Fig. 22
figure 22

Reprinted with permission from Ref. [108]. Copyright 2019, Springer Ltd

Schematics of the air filtration mat that captured PM2.5 by airflow (a) and working state with mask filtration (b); c PM2.5 filtration curves for the air filtration mat (Sample 1) with different volumes of spinning precursor solution; d PM2.5 removal efficiency plots for primary fibers and SVA-treated fibers.

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5 Conclusions and Remarks

In summary, as a simple and widely used technology, electrospinning can satisfy people's desire to quickly prepare nanofiber materials from a wide range of materials. By studying the composition, structure, porosity, surface, and fiber orientation of nanofibers, fiber properties can be selectively tailored for various applications. Although nanofibers have broad application prospects in heterogeneous catalysis and biomedical research, they still face a series of challenges, such as low dispersibility and uncontrollable degradability. With the precise control of the nanofiber microstructure, the preparation of surface-active nanofiber sponges will provide the possibility of developing flexible and recyclable catalytic systems. As a scaffold polymer nanofiber for tissue regeneration, its design, composition, and structure still need to be further optimized in clinical applications. Currently, the preparation of nanofibers is still in the laboratory research stage, the fiber output is low, mass production cannot be achieved, and industrialization is difficult to achieve. Although the electrospinning method is simple and can prepare a variety of nanofibers with different compositions, different structures, and different arrangements, there are still many challenges. For example, the nanofibers prepared by electrospinning cannot obtain filaments separated from each other, and the yield is low, and the strength Low, which limits the application. At present, people have made some breakthroughs in these problems. By simulating the complex spatial distribution of natural tissue, the three-dimensional scaffold has functional grading in terms of composition, arrangement, porosity, and pore size, which will make it better for tissue regeneration applications. It is believed that in the near future, electrospinning technology will be fully developed, and eventually from laboratory to industry, from basic research to clinical application.