Rational design of electrospun nanofibrous materials for oil/water emulsion separation

Abstract

The frequent oil spill accidents and ever-increasing oily wastewater discharged from our daily life and industries greatly threaten the ecological system as well as human health and cause heavy economic loss. To address this problem, developing advanced separation materials towards efficient oil/water emulsion separation is a critical way and has currently risen as a research focus. Electrospun nanofibrous materials with tunable wettability, controllable pore structure, diverse dimensions, and good connectivity hold great promise for meeting the requirements of emulsion separation applications. Herein, we present a comprehensive overview on the recent research regarding the design and fabrication of electrospun nanofibrous materials with special wettability and optimized porous structure for emulsion separation. We firstly introduce the emulsion separation mechanism, followed by the design principles of nanofibrous filtration materials. We subsequently discuss the preparation of electrospun nanofibrous materials beginning with selective wettability tunability (i.e., hydrophobic–oleophilic, hydrophilic–oleophobic, and switchable properties) and porous structure optimization (i.e., 2D and 3D porous structures). Based on the dimensions, nanofibrous materials can be categorized into nanofibrous membranes and nanofibrous aerogels. For each type, representative studies will be highlighted, focusing on the relationship between the synergy of the selective wettability and porous structure and emulsion separation performance. Finally, the existing challenges and future perspectives in the burgeoning field are provided.

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Jichao Zhang

Jichao Zhang received his MS degree from the College of Textiles in Donghua University in 2015. He is now a PhD candidate in Prof. Bin Ding's group. His current research interests mainly focus on bio-inspired surfaces with special wettability, and nanofibrous oil/water separation materials.

Yang Si

Yang Si received his BS from Sichuan University, China in 2009. After that he earned his PhD degree from Donghua University, China in 2015. Currently, he is a Full Professor in the Innovation Center for Textile Science and Technology in Donghua University. His research mainly includes antibacterial bioprotective materials, water disinfection materials, and nanofibrous aerogels.

Bin Ding

Bin Ding received his BS from Northeast Normal University, China, in 1998 and his MS from Chonbuk National University, Korea, in 2003. After that he earned his PhD degree from Keio University, Japan, in 2005. Currently he is a Full Professor in the College of Textiles in Donghua University and awarded as “Changjiang Scholar” by Ministry of Education, China. His research mainly focuses on oil/water separation materials, flexible ceramic nanofibers, waterproof and breathable materials, filtration materials, nanofibrous aerogels, etc. He has already published more than 300 SCI papers and has been authorized more than 50 patents.

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1. Introduction

1.1. Background

Frequent offshore oil production and marine transportation have caused serious leakage of oil into the ocean, which has not only brought about a disastrous impact on the ecosystem but led to enormous economic loss.^1–5 For instance, the 2010 Gulf of Mexico oil blowout resulted in 4.9 million barrels of precious oil going into the sea and contaminated an area of 3200 km².^6–8 Besides, the increasing amount of oily wastewater released from our daily life and oil-related industries (e.g., textiles, leather, food, and metal finishing) has already become one kind of major pollutants aggravating the freshwater shortage.^9–11 The environmental and economic concerns have emphasized the great need for technologies and materials for oil/water separation. Oil/water mixtures can be classified into three types according to the dispersed phase size: free oil/water mixtures (>150 μm), oil/water dispersions (20–150 μm), and oil/water emulsions (<20 μm).¹² Free–floating and dispersed oil can be easily removed by absorbents. In contrast, separating emulsified oil/water mixtures has been confirmed to be the most tough and challenging because of the robust oil/water interface and adsorbed interface-active component.^13,14 Traditional techniques including centrifuges, coalescers, sedimentation, and adsorption have limitations of time-consuming and complex operational processes, and are not applicable for oil/water emulsion separation.¹⁵ Recently, filtration technology has gained much attention for the separation of oil/water emulsions due to its easy operation, low energy cost, and cost-effectiveness.^16–19 Unfortunately, most filtration materials, including phase inversion membranes,^20–24 metal sintered materials,²⁵ and porous substrate (metal mesh,^26–28 textile,²⁹ and sponge³⁰)-based materials, are often limited by low permeation flux, poor oil/water selectivity or complicated fabrication processes.^31–33 Therefore, it is of great importance to create advanced effective materials for oil/water emulsion separation.

1.2. Mechanisms of oil/water emulsion separation

An emulsified oil/water mixture is a colloidal dispersion, in which a tiny quantity (<10%) of liquid (e.g., water or oil) in the form of small droplets is suspended in an immiscible liquid (e.g., oil or water).^9,34–36 In this binary mixture system, the droplet is known as the dispersed phase, and the other liquid is the continuous phase.³⁷ Thus, there are two major tasks for filtration materials during separation, i.e., dispersed phase interception and continuous phase permeation.

1.2.1. Dispersed phase interception. The removal of the dispersed phase from a oil/water emulsion is quite different from the filtration of rigid solid particles because emulsified droplets can deform and squeeze through pores less than their sizes.³⁸ To stop the dispersed phase from passing through a porous material, the size exclusion effect of the porous material should function synergistically with the wettability.^39,40 The wettability of a liquid on a porous material can be expressed by the liquid contact angle, which determines the intrusion pressure (P_i) of the liquid, the maximum pressure that the porous material can withstand without letting the liquid flow through.⁴¹ The intrusion pressure can be calculated by the Young–Laplace equation:where γ_L represents the surface tension of the liquid, θ is the intrinsic contact angle of the liquid on a flat surface, and r_p is the pore radius. A porous material could repel a certain amount of liquid when the intrinsic contact angle of the given liquid is more than 90°. If the intrinsic contact angle is less than 90°, the liquid will permeate the material pores spontaneously. Thus, the separation of oil and water can depend on the intrusion pressure difference between the oil and water. More specifically, removing the oil from an oil/water mixture can be achieved by a filtration material with hydrophobicity and oleophilicity because of its opposite intrusion pressure for oil and water. A hydrophilic–oleophobic filtration material can be employed to remove water from an oil/water mixture.⁴² For the removal of emulsified droplets, the selectively wettable materials should also be endowed with a pore size (2r_p) less than or comparable to the diameter of the emulsified droplets (2r_d).^39,43 It should be noticed that the pore size distribution also influences the interception capacity. Uniform pores with suitable size should be across the whole porous material. If there is a wide distribution of pore size, most of the droplets will firstly penetrate the larger pores (path of lower resistance), resulting in poor selectivity (Fig. 1e and f).

Fig. 1 Schematics showing four underliquid wetting states: (a) underliquid Young's state, (b) underliquid Wenzel's state, (c) underliquid transitional state, and (d) underliquid Cassie's state. (e) With uniform pore size, it is possible to acquire entire interception by a porous material. (f) With nonuniform pores, the largest pore size determines the selectivity.

Notably, four wetting states (i.e., Young's state, Wenzel's state, the transitional state, and Cassie's state) describe a liquid on a solid surface, and demonstrate that the wettability is dependent on the chemical composition and surface morphology.^44,45 The Young state relates the wettability of a liquid on an ideal smooth surface to the chemical composition. The Wenzel state introduces the effect of roughness into the wettability of a solid surface and puts forward that the rough surface could lead to an amplification of wetting and anti-wetting behaviors. Lyophilic materials will be more lyophilic with an increasing degree of roughness, while lyophobic materials will become more lyophobic when the roughness is enhanced. However, for some porous surfaces, it is difficult for the cavities to be wetted by a liquid and a large amount of air can be entrapped in them, thus giving a composite interface. In this state, the Cassie model is more applicable. Besides, a transitional state exists between the Wenzel state and the Cassie state, in which the liquid may partly intrude into the valleys of the surface rough structure. When separating oil/water emulsions using filtration materials with selective wettability, the separation process will be achieved on an oil–water–solid three-phase interface. For example, if a hydrophobic–oleophilic material is used for separating water-in-oil emulsions, the entire cavity between the microstructures will be wetted by the oil, resulting in a trapped oil layer underneath the water droplet. When treating emulsified oily water using a hydrophilic–oleophobic material, the air in the rough surface will be fully replaced with water. Hence, the underliquid wetting states (i.e., underliquid Young's state, underliquid Wenzel's state, underliquid transitional state, and underliquid Cassie's state) on solid surfaces could be more in line with the oil/water separation situation (Fig. 1a–d).^46,47 Due to the different contact areas of the dispersed droplets and solid surface in these four states, the adhesion properties of dispersed droplets to solid surfaces differ sharply and decrease in the order: underliquid Young's state > underliquid Wenzel's state > underliquid transitional state > underliquid Cassie's state. Therefore, the adhesive forces between dispersed droplets and filtration material surfaces can be finely modulated by adjusting the degree of droplet penetration into the surface microstructures, which provides an effective way to improve the antifouling performance of filtration materials.

1.2.2. Continuous phase permeation. A continuous phase flowing through a porous material can be considered as a viscous flow, which could be expressed by the Hagen–Poiseuille equation:⁴⁸where J is the permeation flux, ε is the porosity, ΔP is the driving pressure, μ is the liquid viscosity, d is the material thickness, and τ is the tortuosity. Based on the thickness dimension, porous filtration materials can have two basic forms: 2D membranes and 3D blocks. For either of them, an increase in pore size could promote the permeability of the continuous phase, but too large pores may lead to the decline of the separation efficiency. Meanwhile, a well-connected porous structure with high porosity allows the continuous phase to pass through the materials with a low flow resistance. Hence, an ideal emulsion separation material must (i) possess the opposite affinity to water and oil, (ii) possess a suitable pore size to make a good trade-off between selectivity and permeability, and (iii) have as large a porosity as possible.

1.3. Electrospinning and electrospun materials

Electrospinning is considered to be a variant of the electrostatic spraying process, in which a liquid droplet is electrified to form a jet, and then it undergoes vigorous whipping and splitting motion to generate nanoscale continuous fibers. After decades of development, electrospinning technology has been an effective and simple method in fabricating nanofibers with controllable chemical compositions and diverse fibrous structures.⁴⁹ For example, electrospinning has been utilized to synthesize nanofibers from over 120 kinds of polymers with various surface energy.^50–53 Organic/inorganic hybrid nanofibers can be obtained by loading additives into the polymeric spinning solution or combining with post-treatment. By combination with sol–gel chemistry, a wide variety of inorganic nanofibers have also been prepared. Besides, electrospinning holds powerful structure tunability that could tailor the nanofibrous structures from nanotexture on a single nanofiber, to porous structure among fibers, to macroscopic bulk structure (e.g., 1D yarn, 2D membranes, and 3D aerogels).⁵⁴ Additionally, nanofibrous materials usually combine other outstanding advantages of excellent pore connectivity and high porosity.⁵⁵ Therefore, the past several years have witnessed a booming expansion of research on the preparation of nanofibrous materials with selectively wettable surfaces (trending from fixed wettability to switchable/responsive wettability) and diverse pore structures (trending from 2D porous structure to 3D porous structure) for effective oil/water emulsion separation. Fig. 2 summarizes some important milestones that have been achieved in nanofibrous emulsion separation material systems.^56–66

Fig. 2 Timeline of some important milestones in the development of nanofibrous emulsion separation materials.^56–66

This review aims to present a systematic overview of the current developments of electrospun nanofibrous materials with selective wettability and optimized porous structure for oil/water emulsion separation (Fig. 3). First, we focus on the design and fabrication of selectively wettable nanofibrous materials, which mainly include the introduction of basic raw materials with appropriate surface energy and generic strategies to roughen the nanofiber surfaces. Then, we highlight some representative methods for regulating 2D and 3D porous structures by electrospinning. Based on the dimension, nanofibers can be assembled into separation materials with two basic forms: 2D membranes and 3D aerogels. The application of these two types of nanofibrous materials for emulsified oil purification and oily wastewater remediation is subsequently summarized with the associated advantages and disadvantages. To conclude, the existing challenges and future perspectives facing nanofibrous emulsion separation materials are also discussed.

Fig. 3 Electrospun nanofibrous materials for oil/water emulsion separation. The nanofibers could be of special wettabilities, such as hydrophobic and oleophilic, hydrophilic and oleophobic, and responsive/switchable, and could be well-tailored with 2D and 3D porous structures.

2. Fabricating nanofibrous materials with selective wettability surfaces

As is well-known, the combination of an appropriate chemical composition and surface roughness is crucial to the preparation of selectively wettable materials. Therefore, two steps, i.e., the selection of materials (electrospinnable polymers, additive agents and so on) with suitable surface energy for electrospinning, and the generation of hierarchical roughness on nanofibers, are involved. This section focuses on the summary and discussion of recently used raw materials for selectively wettable nanofibers, along with strategies to roughen the nanofiber surfaces.

2.1. Electrospinnable polymer materials

Polymers with appropriate surface energy and good spinnability can be used to fabricate selectively wettable nanofibrous materials by a simple direct electrospinning technique. The spinnable polymer requires a suitable molecular weight and can be completely dissolved in some solvents. Generally, the electrospinning of a solution of spinnable low-surface-energy polymers could achieve nanofibrous materials with a hydrophobic–oleophilic property. Also, polymers with high surface energy are selected for fabricating hydrophilic–oleophobic nanofibrous materials.

2.1.1. Hydrophobic–oleophilic polymers. Polymers with low-surface-energy groups, such as –CH₂, –CH₃, –CF₂, –CF₂H, and –CF₃, are the preferential materials in the fabrication of hydrophobic–oleophilic nanofibers by direct electrospinning. Hence, the polymers can be categorized into two classes: non-fluorinated and fluorinated polymers.

Polystyrene (PS), a synthetic aromatic polymer with low-surface-energy owing to its phenyl group, has been widely used for the preparation of hydrophobic–oleophilic nanofibers by direct electrospinning.^67–76 However, the major problem associated with PS nanofibrous materials is their weak mechanical strength and poor integrity due to the brittle nature of the fibers. To overcome this issue, some rubber-like polymer monomers (e.g., butyl acrylate and dimethylsiloxane) were incorporated into PS systems, and the nanofibers derived from synthesized copolymer poly(styrene-co-butyl acrylate) (PS-co-PBA) and poly(styrene-co-dimethylsiloxane) (PS-co-PDMS) showed good hydrophobicity (water contact angles (WCAs) >130° and oil contact angles (OCAs) of about 0°) and excellent flexibility as well as good mechanical integrity.^77–79 Aromatic polyimide (PI) is a step ladder polymer with good hydrophobicity–oleophilicity, and it has been widely used in the filtration field due to the good chemical resistance, superior thermo-oxidative stability, and excellent mechanical properties. However, the insoluble nature in most organic solvents hinders its direct electrospinning. An effective way is the introduction of flexible linkages (e.g., siloxane) into the PI macromolecules to improve its solubility.⁸⁰ For example, Tian et al. copolymerized a flexible linkage, 1,3-bis(3-aminopropyl)tetramethyldisiloxane, into the PI skeletal chain, and then they used dimethylformamide (DMF) as a solvent for direct electrospinning.⁸¹ The resultant PI nanofibers demonstrated hydrophobic and superoleophilic properties with an OCA of 0.9°. The intriguing wettability of PI nanofibers can remain under harsh conditions, such as sodium chloride (NaCl), hydrochloric acid (HCl), sodium hydroxide (NaOH), methylene blue, and sodium dodecylbenzenesulfonate solutions.⁸² Notably, the combination of electrospinning of a poly(amic acid) (PI precursor) solution and thermal imidization provides an indirect pathway to prepare PI nanofibers.⁸³ Besides, aromatic polymers for the fabrication of hydrophobic–oleophilic nanofibers by direct electrospinning also include polysulfone (PSF),^84,85 polyetherimide,⁸⁶ poly(butylene terephthalate) (PBT),⁸⁷ polyurethane (PU),⁸⁸ and polyetherimide poly(disperse orange 3) (PDO 3).⁶⁶ Some aliphatic polymers, such as syndiotactic polypropylene (SPP),^89,90 poly(methyl methacrylate) (PMMA),^91,92 poly(vinyl chloride) (PVC),⁷³ and poly(vinyl butyral) (PVB),⁹³ can also be directly electrospun into hydrophobic–oleophilic nanofibers with WCAs of above 100°.

Recently, some low-surface energy biodegradable polymers, including biodegradable polyesters (e.g., polylactide (PLA), poly(glycolic acid) (PGA), poly(ε-caprolactone) (PCL), poly(lactic acid-co-glycolic acid) (PLGA), and poly(hydroxybutyrate) (PHB)) and cellulose derivatives (e.g., cellulose acetate (CA) and ethylcellulose), have attracted a great deal of attention for fabricating hydrophobic–oleophilic surfaces because of their sustainability and environmental friendliness.^94–104 For example, Zhang et al. fabricated PLA nanofibers by electrospinning a solution of PLA in a DMF/dichloromethane mixture, and the resultant electrospun PLA products showed a good hydrophobic–oleophilic property with a WCA of 133° and an OCA of 0°.¹⁰⁰ As another example, Ma et al. electrospun nanofibers of CA from a mixed solvent of dimethylacetamide (DMAc) and acetone.¹⁰⁵ Due to the acetyl groups on the glucose rings, the resultant CA nanofibrous membrane showed a hydrophobic property with a WCA of 124°.

Apart from non-fluorinated polymers, fluorocarbon-based polymers with multiple carbon–fluorine bonds have also been explored to fabricate hydrophobic nanofibrous materials. Derived from the high electronegativity of fluorine atoms and the high bond dissociation energy of the C–F bond, fluoropolymers usually possess excellent physicochemical stability. Polyvinylidene fluoride (PVDF)^106–109 and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP)¹¹⁰ are the most often used fluoropolymers for the preparation of hydrophobic–oleophilic nanofibers. The obtained electrospun products feature remarkable properties such as excellent chemical resistance, high mechanical strength, and good thermal stability. Notably, polytetrafluoroethylene (PTFE) as an extremely chemically stable polymer is attracting vigorous attention in the preparation of porous filtration membranes, and the PTFE membrane can maintain its surface morphology and chemical composition in harsh conditions. However, the outstanding chemical stability of PTFE also presents a major challenge in its direct electrospinning. Fortunately, Qing and co-workers reported a robust method for the fabrication of PTFE nanofibers.¹¹¹ Briefly, PTFE nanoparticles (NPs) were dispersed in a carrier polymer poly(vinyl alcohol) (PVA) aqueous solution for direct electrospinning. Then the resultant PVA/PTFE NP nanofibers were sintered in air under a proper temperature to decompose PVA and weld PTFE NPs into continuous nanofibers. The resultant electrospun PTFE nanofibers exhibited hydrophobicity and oleophilicity with a WCA about 150° and an OCA of 0°.

2.1.2. Hydrophilic–oleophobic polymers. To fabricate nanofibrous filtration materials for only allowing water to flow through freely, a simple method is to directly electrospin polymers with a hydrophilic–oleophobic property. However, few researchers have successfully prepared hydrophilic–oleophobic nanofibers; in theory, oil-repellent surfaces combining extremely low surface energy usually repel water, because the water surface tension is always higher than that of oils.¹¹² Inspired by fish scales that enable the fish to live in oil-contaminated water,⁴⁶ researchers have recently proposed a biomimetic way to achieve nanofibrous membranes with hydrophilicity–oleophobicity, i.e., hydrophilicity–underwater oleophobicity, by utilization of high-surface-energy polymers.^40,113,114 Generally, these polymers possess plenty of polar groups, such as hydroxyl, carboxyl, amino, acylamino, and nitrile.

To date, lots of synthetic polymers have been successfully electrospun for the preparation of hydrophilic–underwater oleophobic nanofibers. Polyamide (PA) and polyacrylonitrile (PAN) are the two most attractive polymers for the preparation of hydrophilic–underwater oleophobic nanofibrous materials because of their high-surface-energy and water insolubility. For instance, Lin et al. electrospun nanofibers of a semi-aromatic PA, poly(trimethyl hexamethylene terephthalamide) (PA6(3)T).¹¹⁵ Because of the numerous amide groups, the resultant PA6(3)T nanofibers were hydrophilic in air with a WCA of 63.9° and underwater oleophobic with an underwater oil contact angle (UWOCA) of 115.4°. The powerful hydrogen bonds between polymer macromolecules endowed the PA6(3)T nanofibers with excellent structure stability in water. In another study, Liang et al. fabricated hydrophilic PAN nanofibers with a WCA of about 18° by electrospinning.¹¹⁶ The abundant hydrogen bonds and dipole–dipole interactions between polymer molecules rendered the PAN nanofibers good structural stability in an aquatic environment. In other work, researchers further improved the hydrophilicity of the PAN nanofibers to superhydrophilicity by post-treatment, such as hydrolysis treatment and heat treatment. For example, Zhang et al. increased the hydrophilicity of PAN nanofibers by basic hydrolysis treatment after electrospinning.¹¹⁷ Because plentiful amounts of hydrophilic groups (hydroxyl, carbonyl, and amine groups) were generated on the fiber surfaces after nitrile reacted with NaOH, the PAN nanofibers changed from hydrophilic to superhydrophilic. Lim et al. demonstrated the thermally-induced hydrophilic enhancement of PAN nanofibers.¹¹⁸ The PAN nanofibers were heated in air at 200 °C to convert the abundant nitrile groups into amide and carboxyl groups, leading to highly hydrophilic PAN nanofibers with a WCA of 0°.

From a practical point of view, the nanofibers derived from water-soluble polymers, including PVA, poly(vinyl pyrrolidone) (PVP), poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), poly(ethylene oxide) (PEO), polyacrylamide (PAM), poly(ethylene glycol) (PEG), poly(ethylene imine), and zwitterionic polymers, could not be used for oil/water separation. An effective method to overcome this issue is to crosslink the water-soluble polymers after electrospinning. For example, Zhu and co-workers electrospun PVA/PAA aqueous solution, followed by thermal crosslinking of the composite nanofibers.¹¹⁹ Because a significant number of ester linkages were formed between hydroxyl groups of PVA and carboxyl groups of PAA, the obtained PVA/PAA nanofibers possessed hydrophilicity and good underwater durability. Based on the same principle, Fan et al. used glutaraldehyde as a crosslinker to stabilize the hydrophilic PVA layer, and the resultant membrane showed a promising hydrophilic—underwater oleophobic property in a complex environment (e.g., acids, alkalis, and salts).¹²⁰

2.1.3. Switchable wettability polymers. Recently, polymers with switchable wettability that are able to oscillate between two extreme wetting states (i.e., hydrophobic–oleophilic and hydrophilic–oleophobic) have drawn a great deal of attention owing to their great potential of realizing on-demand oil/water separation in response to external stimuli, such as heat,^121–126 pH,^63,92,127 gas,⁶⁴ light,^128,129 and even multiple stimuli.^130,131 It is worth mentioning that the wettability varies in a small range on a polymer flat membrane, and the inherent roughness of electrospun materials can greatly enlarge the transition range of surface wettability, which is beneficial for oil/water separation.

Poly(N-isopropylacrylamide) (PNIPAAm) is an extensively studied thermo-responsive polymer, but suffers from easy solubility in water (Fig. 4a). Recently, a PNIPAAm segment has been often introduced into a hydrophobic polymer system (e.g., PMMA and PS) to improve the underwater durability.¹³² For instance, Li and co-workers fabricated temperature-sensitive nanofibers by the direct electrospinning of copolymer PMMA-b-PNIPAAm solution.¹²⁵ The PMMA in the glassy state could endow the resultant nanofibers with good stability underwater, and PNIPAM rendered the nanofibers thermo-responsive wettability. Briefly, PNIPAM will be swollen in water below the lower critical solution temperature (LCST), endowing PMMA-b-PNIPAAM nanofibers with hydrophilicity with a WCA of 0° and underwater superoleophobicity with an OCA of 153°, while the PNIPAAm could collapse above the LCST, resulting in hydrophobicity with a WCA of 130° and underwater oleophilicity with an OCA of 37° (Fig. 4b and c). Besides, the synthesized PMMA-b-PNIPAAM nanofibers could reversibly switch between opposite wetting states by heating and cooling.

Fig. 4 (a) Polymer with thermo-responsiveness. (b) Diagram showing its thermo-responsive mechanism. Reproduced with permission from ref. 126. Copyright 2018, American Chemical Society. (c) The PMMA-b-PNIPAAm nanofibers showing hydrophilicity and underwater oleophobicity below the LCST, and showing hydrophobicity and underwater oleophilicity above the LCST. Reproduced with permission from ref. 125. Copyright 2015, Royal Society of Chemistry. (d) A summary of common molecules with a pH-responsive property. (e) Schematic explanation about the pH-responsiveness of PMMA-b-P4VP nanofibers. (f) pH-triggered reversible water wettability transition of PMMA-b-P4VP nanofibers. Reproduced with permission from ref. 63. Copyright 2015, American Chemical Society. (g) Some common polymers that are sensitive to CO₂. (h) Schematic showing the CO₂-responsiveness of PMMA-co-PDEAEMA nanofibers. (i) The PMMA-co-PDEAEMA nanofibers exhibiting gas-switchable wettability. Reproduced with permission from ref. 64. Copyright 2015, Wiley-VCH.

Polymers with carboxyl or pyridine groups, such as poly(2-vinylpyridine) (P2VP), and poly(4-vinylpyridine) (P4VP), generally show a pH-responsive switchable wetting property because of the pH-triggered configurational changes of molecules (Fig. 4d). Li and co-workers synthesized a PMMA-b-P4VP copolymer, followed by electrospinning of it into nanofibers.⁶³ The protonation and deprotonation of pyridine groups induced by the solution pH values (e.g., 3 or 7) could regulate the chain conformation (e.g. extended or collapsed) of the P4VP segments. The electrostatic repulsion derived from protonated pyridyl groups could make the P4VP segment extend to water, leading to hydrophilic–underwater oleophobic wetting behavior. The collapsed conformation of the P4VP segment enables the PMMA segments to control the access of water, resulting in a hydrophobic–oleophilic property (Fig. 4e). The intriguing switchable wettability can be reversible for 5 cycles in response to the solution pH values (Fig. 4f).

Gas triggers featuring easy addition or removal in large scale operation have evoked much attention in industrial applications. The gas can be CO₂, NH₃, O₂, and H₂. CO₂ as an inexpensive, non-toxic, and non-flammable gas has become the most widely used gas trigger. CO₂-switchable polymers usually contain tertiary amine groups, and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) and poly(N,N-diethylaminoethyl methacrylate) (PDEAEMA) are the most utilized polymers due to their commercial availability. For example, Che and co-workers synthesized a PMMA-co-PDEAEMA copolymer by a radical copolymerization reaction, and then it was electrospun into continuous nanofibers (Fig. 4g).⁶⁴ The resultant PMMA-co-PDEAEMA nanofibers could switch the oil/water wettability by utilizing CO₂ as the trigger because of the protonation and deprotonation effects of amine groups with CO₂ and N₂ (Fig. 4h). The protonated macromolecular chains derived from the interaction of tertiary amine groups and CO₂ in water displayed a stretched conformation, leading to hydrophilicity–underwater oleophobicity for the nanofibers. Once the CO₂ was completely removed, the PMMA-co-PDEAEMA molecules recovered to their hydrophobic–oleophilic aggregate state. The nanofibers could reversibly switch between the two opposite wetting states by alternating CO₂ and N₂ stimulation (Fig. 4i).

2.2. Surface modification agents

As mentioned above, nanofibrous materials with oil/water selective wettability can be obtained by the selection of appropriate spinnable polymers for direct electrospinning. However, the limited types of these polymers would seriously restrict the diversity of chemical compositions and hinder further wettability improvement. Electrospinning of a blend of a spinnable host polymer and non-spinnable additives with desirable surface energy or loading additives on host polymer fiber surfaces by post-treatment will greatly widen the selection range of materials, representing an effective way to address this issue. In this section, we will summarize the commonly utilized non-spinnable agents for wettability tunability.

2.2.1. Hydrophobic–oleophilic agents.
(1) Organosilicons. The common organosilicons for hydrophobic–oleophilic surfaces mainly contain polysiloxanes and organosilanes. Polydimethylsiloxane (PDMS) is a commonly used polysiloxanes with low-surface-energy to fabricate hydrophobic surfaces because of its outstanding chemical stability and good anti-abrasive property.¹³³ For example, Mi et al. deposited a layer of PDMS on cobalt-doped SiO₂ nanofibers.¹³⁴ Compared with the pristine hydrophilic SiO₂ nanofibers, the resultant SiO₂/PDMS nanofibers showed robust hydrophobicity with a WCA of about 150°. Interestingly, the hydrophobic property can be well maintained even in liquid nitrogen and a hot environment of 200 °C (Fig. 5a). As another example, a layer of PDMS was well anchored on the PAN nanofiber surface by blade coating and a subsequent polymerization process. The resultant PAN/PDMS nanofibers showed hydrophobic surfaces with a WCA of 141°. Besides PDMS coatings, several other kinds of polysiloxanes have also been employed to develop hydrophobic nanofibers, such as amino-silicone oil (ASO) and polymethylhydrosiloxane (PMHS). For example, Sheng et al. prepared hydrophobic electrospun PAN nanofibers by blade coating with ASO.¹³⁵ The wettability of the PAN nanofibers was transformed from hydrophilicity to hydrophobicity with a WCA of 140°. In another study, Zhu et al. directly anchored PMHS onto PU nanofibers, and the WCA significantly increased from 108.8° to 130.2°.¹³⁶

Fig. 5 (a) SiO₂ nanofibrous materials modified with PDMS. Reproduced with permission from ref. 134. Copyright 2018, Elsevier. (b) Hydrophobization of PVA nanofibers by silane grafting. Reproduced with permission from ref. 137. Copyright 2015, American Chemical Society. (c) CA nanofiber modified with F-PBZ by in situ polymerization. Reproduced with permission from ref. 138. Copyright 2012, Royal Society of Chemistry. (d) Tailoring the wettability of PVDF–HFP nanofibers by CNTs. Reproduced with permission from ref. 139. Copyright 2016, Elsevier. (e) Schematic showing the modification of PS nanofibers by PDA and thiol. Reproduced with permission from ref. 140. Copyright 2012, Royal Society of Chemistry. (f) Synthetic procedures for the preparation of BN nanotube nanofibers. Reproduced with permission from ref. 141. Copyright 2017, Royal Society of Chemistry.

Polysiloxanes could be partly filled into the pores among nanofibers other than only enwrapping the fiber surface during the modification process, resulting in the deterioration of the pore connectivity. Organosilanes as small molecules are grafted on the fiber surface to generate a hydrophobic–oleopilic functional layer, thereby having no effect on the porous structure, but changing the wetting behaviors. Up to now, a series of organosilanes (e.g., methoxysilane, ethoxysilane, and chlorosilane) has been utilized for improving the surface hydrophobic–oleophilic property of nanofibers. The most commonly used organosilanes for hydrophobic–oleophilic modification are summarized in Table 1. It is worth mentioning that the increase of the length of the aliphatic chain could enhance the surface hydrophobicity. For example, Wang et al. prepared hydrophobic–oleophilic TiO₂ nanofibers by grafting hexyltrichlorosilane (C6) and trichlorooctadecylsilane (C18) monolayers respectively.¹⁴² The morphology of the treated TiO₂ nanofibers was not changed, and the wettability changed from hydrophilicity to hydrophobicity–oleophilicity. Due to the longer aliphatic chain of trichlorooctadecylsilane, the C18–TiO₂ nanofibers showed a WCA of 105° higher than that of the C6–TiO₂ nanofibers (95°). Besides, the replacement of the hydrogen atoms by fluorine atoms in the aliphatic chain could also enhance the hydrophobicity, but the excessive density of fluoroalkylsilanes on the fiber surfaces may lead to oleophobicity. For instance, 1H,1H,2H,2H-perfluorododecyltrichlorosilane (C12F21)-grafted TiO₂ nanofibers showed a higher WCA than that of TiO₂ nanofibers with a dodecyltrichlorosilane (C12) monolayer.¹⁴² Interestingly, Xu et al. improved the hydrophobicity of crosslinked PVA nanofibers by assembling a layer of 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (C10F17) through a condensation reaction between hydroxyl groups of PVA and Si–O alkyl groups of organosilanes (Fig. 5b).¹³⁷ The WCA of the modified PVA nanofibers increased by 30°, achieving a promising superhydrophobic–oleophilic property. However, the C10F17–TiO₂ nanofibers prepared by Wang and co-workers were lyophobic to both water and oil.¹³¹ The reason may be the higher areal density of –CF₃ groups on TiO₂ nanofibers than that of the crosslinked PVA nanofibers.

Table 1 Common organosilanes used to improve the hydrophobicity–oleophilicity of nanofibrous surfaces

Organosilanes	WCA	OCA	Chemical structures	Ref.
1H,1H,2H,2H-Perfluorodecyltrimethoxysilane	148–160°	0–8°		137 and 143
Hexadecyltrimethoxysilane	>140°	—		144
1H,1H,2H,2H-Perfluorooctyltriethoxysilane	139–153°	0°		145–150
Vinyltrimethoxysilane	>145°	—		151
Methyltriethoxysilane	>150°	0°		152
Octyltriethoxysilane	>150°	—		151
Vinyltriethoxysilane	>135°	—		151
Methyltrichlorosilane	150.4°	0°		153
Hexyltrichlorosilane	∼140°	0°		142
Dodecyltrichlorosilane	∼141°	0°		142
Hexadecyltrichorosilane	∼143°	0°		142
Octadecyltrichlorosilane	144.7–163.1°	0°		142, 154 and 155

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(2) Polymers. Commonly used polymers for hydrophobic–oleophilic modification mainly include polymers with long alkyl chains and fluorinated polymers. For example, Kaplan et al. synthesized novel poly(lactic acid-co-glycerol monostearate) (PLA–PGC18) copolymers with long aliphatic side chains (i.e., stearic acid), and then added them into PLA solution for electrospinning.¹⁵⁶ During the fiber formation process, the low-surface-energy polymer PLA–PGC18 held more mobility to migrate to the surface, while PLA tends to remain at the core. As a result, the synthesized PLA/PLA–PGC18 nanofibers exhibited a significant increase in hydrophobicity with a WCA of about 150° higher than pure PLGA nanofibers with a WCA of 110°. Polymers with active groups can be used as coating agents to improve the hydrophobicity of nanofibers. For example, Zhao et al. produced highly hydrophobic CA electrospun nanofibers by grafting a layer of ECO, a hyperbranched polymer with long chains of hydrocarbons, on fiber surfaces.¹⁵⁷ The blocked isocyanate crosslinkers were firstly anchored onto the CA nanofiber surface as an interlayer, and then ECO covalently reacted with them by heat treatment. Benefiting from the hydrocarbon chains of ECO, the pristine hydrophilic CA nanofibers exhibited high hydrophobicity with a WCA of 132.2°.

Thermosetting polymers with 3D crosslinked networks are also efficient for switching the wettability of nanofibers to hydrophobicity–oleophilicity by in situ polymerization. For example, fluorinated polybenzoxazine (PBZ) with low surface energy and good chemical resistance has been widely used to render nanofibers stable hydrophobic–oleophilicity.¹⁵⁸ Shang et al. prepared a crosslinked PBZ functional layer on CA nanofibers by in situ polymerization (Fig. 5c).¹³⁸ Briefly, they firstly dipped the CA nanofibers into an acetone solution of a trifunctional benzoxazine monomer. After subsequent heat treatment, benzoxazine was polymerized into PBZ and formed a Mannich bridge crosslinked structure. The as-prepared CA nanofibers showed a remarkable transition from hydrophilicity to hydrophobicity with a WCA of 136°.

(3) Carbon materials. Carbon materials have drawn tremendous attention in the fabrication of hydrophobic–oleophilic surfaces due to their low surface energy, good chemical inertness, and excellent mechanical stability. In the past decades, carbon has been extended from conventional charcoal and graphite to nanostructured forms (e.g., carbon nanotubes (CNTs),^139,159,160 graphene nanoplatelets (GNPs),¹⁶¹ carbon black (CB),¹⁶² and carbon soot^163,164). CNTs are a typical 1D carbon material with a hollow cylindrical structure. Shon and co-workers demonstrated the preparation of PVDF–HFP/CNT composite nanofibers by blend electrospinning (Fig. 5d).¹³⁹ The CNTs were dispersed in a well-aligned fashion on/inside the PVDF–HFP nanofibers and the WCA of the resultant nanofibers increased from 149° to 158.5° as the CNT concentration increased to 5 wt%. GNPs as a 2D nanomaterial possess nanoscale thickness and diameter ranging from submicron to micron. Tijing et al. utilized GNPs to modify PVDF–HFP nanofibers by blend electrospinning.¹⁶¹ Due to the hydrophobic nature of GNPs, the WCA of the PVDF–HFP/GNP nanofibers exhibited a value of 163°, which was higher than that of pristine PVDF–HFP nanofibers (142°). Carbon soot is a product from the combustion of carbon-based materials and possesses extremely high hydrophobicity. It was found that the WCA was about 166° on the surface of carbon soot film. Compared with other nanostructured carbon, carbon soot is a cheap and easily gained material. Recently, it has been widely used as a hydrophobic additive for the wettability regulation of nanofibers. For example, Lei et al. incorporated candle soot into PVDF nanofibers by electrospinning PVDF solution dispersed with soot particles.¹⁶⁴ The obtained PVDF/candle soot nanofibers achieved an improved hydrophobic property with a high WCA of 162°.
(4) Others. The recently reported low-surface-energy agents also involve some organic small molecule compounds (thiols, aliphatic acids, and surfactants) which consist of a long hydrophobic chain ending with active groups, as summarized in Table 2. For example, Li et al. uniformly deposited a polydopamine (PDA) active layer on PS nanofibers, and then the resultant PDA-coated PS nanofibers were directly used as a platform to be firmly grafted with undecanethiol by a Michael reaction.¹⁴⁰ Consequently, the resultant nanofibers showed promising hydrophobicity with a WCA of 136° (Fig. 5e). Similarly, Almasian et al. firstly functionalized PAN nanofibers by coating with PDA, and then a layer of nonylphenol ethoxylate nonionic surfactant was grafted onto the nanofiber surfaces.¹⁶⁵ The resultant nanofibers exhibited a superhydrophobic property with a WCA of above 150°.

Table 2 Common surfactants, thiols, and aliphatic acids used to produce hydrophobic–oleophilic nanofibrous surfaces

Types	Agents	WCA	OCA	Ref.
Thiols	Undecanethiol	136°	0°	140
	Hexadecylthiol	>146°	0°	166 and 167
	1H,1H,2H,2H-Perfluorodecanethiol	∼155°	∼0°	168
Surfactants	Nonylphenol ethoxylate	>150°	0°	165
Aliphatic acids	Cyclohexanecarboxylic acid	>150°	0°	169

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Boron nitride (BN) is an important hydrophobic nitride ceramic and possesses four crystal structures. Hexagonal BN, analogous to graphite with a layered structure, has been a promising candidate for hydrophobicity tunability due to the good thermal and chemical stability. For example, Hao et al. used PAN nanofibers as templates to create a thin layer of hexagonal BN on them by atomic layer deposition (ALD) of pre-ceramic polyborazine. The resultant PAN/polyborazine nanofibers were annealed under an Ar flow to transform the precursors into dense BN nanotubes forming into PAN/BN hybrid nanofibers, and then heated under NH₃ to remove PAN, leading to formation of BN nanotube nanofibers (Fig. 5f).¹⁴¹ The authors demonstrated that the obtained BN nanofibers exhibited a superior water repellent property with a WCA of about 150° and high oleophilicity with an OCA of 0°. The PAN/BN hybrid nanofibrous membrane would be expected to be hydrophobic–oleophilic.

2.2.2. Hydrophilic–oleophobic agents.
(1) Amphiphilic copolymers. Amphiphilic copolymers consisting of hydrophilic polymer segments (e.g., zwitterionic polymers, PEO, poly((2-methacryloyloxy)ethylphosphorylcholine), and poly(methoxy poly(ethylene oxide)methacrylate)) and hydrophobic segments (e.g., PMMA, PLA, PCL, and PS) have been extensively used to increase the hydrophilicity of nanofiber surfaces. Because of their amphiphilic characteristic, these polymers are usually used to hydrophilize hydrophobic host polymers (e.g., PVDF, PHB, SEBS, PLA, and PCL) by blend electrospinning.^98,170–173 The idea is that the low-surface-energy hydrophobic segments could drive the small-molecule copolymer out-migration to the fiber–air interface and then act as anchors to fix on the host polymers, while the hydrophilic segments are useful for improving the hydrophilicity.^174–177

For example, Govinna et al. introduced two kinds of zwitterionic polymer-based copolymers, i.e., poly((methyl methacrylate)-ran-sulfobetaine methacrylate) (P(MMA-ran-SBMA)) and poly((methyl methacrylate)-ran-sulfobetaine-2-vinylpyridine) (P(MMA-ran-SB2VP)), into PVDF nanofibers by blend electrospinning.¹⁷⁰ During the fiber formation process, the smaller molecular weight and amphiphilic character of the copolymers made them easily accumulate on the fiber surface. Meanwhile, the zwitterionic polymer segments extended upon contact with the aqueous medium, while the PMMA segments anchored in the PVDF matrix to prevent the copolymer from leaching out in water. As a result, the as-prepared PVDF/PMMA-r-SBMA and PVDF/PMMA-r-SB2VP nanofibers both showed significantly enhanced surface hydrophilicity. Triblock copolymers were also investigated to increase the hydrophilicity of the nanofibers. For example, Kurusu et al. incorporated a triblock copolymer PEO–PPO–PEO (F127) into hydrophobic styrene-b-ethylenebutylene-b-styrene (SEBS) nanofibers to regulate the hydrophilicity by blend electrospinning.¹⁷¹ During the electrospinning process, F127 molecules could migrate and aggregate onto the nanofiber surface for its incompatible property with the SEBS matrix. As a result, PPO segments anchored the F127 molecular chain on the hydrophobic SEBS matrix and brush-like PEO segments extended to contact with water. This ternary copolymer changed the wettability of SEBS nanofibers from hydrophobicity with a WCA of 139° to superhydrophilicity with fast water spreading as well as a WCA below 5° (Fig. 6a).

Fig. 6 (a) The hydrophilization of hydrophobic SEBS nanofibers by modification with F127. Reproduced with permission from ref. 171. Copyright 2015, American Chemical Society. (b) PAN/HPEI nanofibers modified with PDA. (c) The enhanced hydrophilicity of PAN/HPEI nanofibers by coating with PDA. Reproduced with permission from ref. 178. Copyright 2018, Elsevier. (d) Electrospun nanofibers modified by TiO₂ through the LBL assembly method. (e) TEM image showing a layer of TiO₂ deposited on the PSEI fiber surface. Reproduced with permission from ref. 179. Copyright 2009, Wiley-VCH.

(2) Natural polymers. Natural polymers obtained from living organisms (wood, crustacean shells and so on) usually possess lots of oxygen-rich hydrocarbon hydrophilic groups and intrinsic underwater durability. Besides, these polymers also feature promising biocompatibility and non-toxicity. However, the direct electrospinning of natural polymers is seriously limited by low productivity and poor fiberizability.

Natural polymers have been blended with synthetic polymers for electrospinning, leading to the development of natural polymer-based hydrophilic nanofibers. Chitosan is a deacetylated form of chitin with amino and hydroxyl groups. Blend electrospinning of chitosan has been used to improve the hydrophilicity of many synthetic polymer nanofibers, such as PA66,¹⁸⁰ PLA,¹⁸¹ and PLGA¹⁸² nanofibers. Silk is a protein polymer with amino or hydroxyl groups at the side chains. Guo et al. utilized silk to hydrophilize PBT nanofibers by blend electrospinning.⁸⁷ In comparison to pristine PBT nanofibers with a WCA of about 125°, the introduction of silk significantly increased the hydrophilicity, leading to a WCA of 18° for the PBT/silk nanofibers.

Another way of using natural polymers to promote nanofiber hydrophilicity is to introduce natural polymers on the fiber surface by a coating strategy. Dopamine is a neurotransmitter released by the human brain for carrying signals between cells. It contains lots of polar groups (e.g., catechol and ethylamino groups), and possesses a similar molecular structure to dihydroxyphenylalanine derived from mussel foot proteins, which provides robust adhesion ability of mussels on marine substrates. Inspired by the phenomena, a growing number of researchers have developed PDA coating layers for promoting the hydrophilicity of nanofibers. For instance, Wang and co-workers used dopamine as the monomer to fabricate PDA-coated nanofibers. By simple immersion of PAN/hyperbranched polyethyleneimine (HPEI) nanofibers into an aqueous solution of dopamine hydrochloride and Tris-buffer, thin PDA shells were synthesized on the fibers. The as-prepared PAN/HPEI/PDA nanofibers showed superhydrophilicity with a water spreading time of less than 1.2 s (Fig. 6b and c).¹⁷⁸ Based on the same principle, Mosinger et al. prepared hydrophilic electrospun PS nanofibers. After they coated a layer of PDA on the PS nanofibers through in situ polymerization, the hydrophobic PS nanofiber surfaces (a WCA of about 130°) were changed into highly hydrophilic surfaces with a WCA below 5°.¹⁸³ Polysaccharides are the most abundant polymers in nature, which have gained lots of attention in filtration and biomedical applications. Recently, polysaccharide nanocrystals have been incorporated onto electrospun nanofiber surfaces to tailor the surface energy. Here, the electrospun nanofibers employed can be PVDF–HFP,¹⁸⁴ CA,¹⁸⁵ and PAN¹⁸⁶ nanofibers. By virtue of the polysaccharide modification, the pristine nanofibers demonstrated a great improvement of hydrophilicity with a fast water spreading time.

(3) Oxide ceramics. Oxide ceramics are materials that consist of metal or nonmetal elements and exist as compounds of oxides. The strong ionic and covalent bonding render oxide ceramics excellent resistance to high temperatures and various chemicals. Recently, they have been widely used to hydrophilize nanofibers.

Direct utilization of nanostructured oxide ceramics by blend electrospinning is a common method to promote the hydrophilicity of nanofibers. For example, tourmaline as a hydrophilic crystalline material possesses lots of silanol groups. Tijing et al. hydrophilized PU nanofibers by incorporation of tourmaline by blend electrospinning.¹⁸⁷ In comparison to the pristine PU nanofibers with a WCA of 125.2°, the PU/tourmaline nanofibers showed a WCA of 13° as the concentration of tourmaline increased to 5 wt%. Similar results were reported for electrospun nanocomposites of PAN with SiO₂. The hydroxyl groups on SiO₂ effectively influence the interactions between the PAN/SiO₂ nanofibers and water, leading a faster spreading rate than PAN nanofibers.¹⁸⁸ Other oxide ceramics, such as TiO₂, ZnO, ZrO₂, and hydroxyapatite have also been electrospun in nanofibers to improve the hydrophilicity. However, the blend electrospinning method showed a limited influence on improving the hydrophobicity of highly hydrophobic nanofibers because most of the introduced oxide ceramics could agglomerate in the interior of the fiber matrix. In view of this, many researchers have applied a surface coating treatment to anchor oxide ceramics on fiber surfaces. For example, the group of Rutledge reported a layer-by-layer (LBL) assembly method for enhancing the hydrophilic property of electrospun polymer nanofibers, such as PS, PMMA, and poly(dimethylsiloxane-b-etherimide) (PSEI) nanofibers (Fig. 6d).¹⁷⁹ The polymeric nanofibers were firstly treated with low-pressure air plasma to generate negatively charged groups on their surface. Then they were alternately dipped in polyhedral oligosilsesquioxane (POSS-NH³⁺) and TiO₂ aqueous dispersions to adsorb positively charged POSS-NH³⁺ molecules and negatively charged TiO₂ to enhance the hydrophilicity (Fig. 6e).

Another attractive method to incorporate oxide ceramics into nanofibers is based on an inorganic precursor, involving two key procedures: (i) fabrication of hybrid nanofibers by electrospinning a solution of the polymer carrier and the inorganic precursor, and (ii) calcination of the hybrid nanofibers under a gas flow. When an inert gas (e.g., Ar and N₂) was selected as the gas flow, the carbon chain polymer carrier could be carbonized, leading to the formation of carbon/oxide ceramic hybrid nanofibers. For example, Lu et al. firstly electrospun a mixture of preceramic precursor polyureasilazane and carbon precursor PAN into nanofibers.¹⁸⁹ During the subsequent calcination process, polyureasilazane was transformed into silicon oxycarbide (SiCO) together with the carbonization of PAN. Thanks to the abundant surface silanol groups of SiCO, the synthesized C/SiCO nanofibers exhibited good superhydrophilicity. When the hybrid nanofibers were calcined in air, the polymeric carrier will decompose, leading to the formation of hydrophilic oxide ceramic nanofibers. For example, Yang et al. fabricated SiO₂ nanofibers by calcining PVA/tetraethyl orthosilicate (TEOS) hybrid nanofibers in air.⁶¹ The as-prepared SiO₂ nanofibers displayed superamphiphilicity in air with both a WCA and OCA of about 0°. Notably, due to the large aspect ratio of the fibers (larger than 1000), the SiO₂ nanofibers can be easily bent and folded without any cracks, which is significant for oil/water separation. In addition to SiO₂, a variety of other oxide ceramics have also been successfully prepared as hydrophilic nanofibers by this method, including TiO₂, ZrO₂, ZnO, BaTiO₃, LiCoO₂, NiFe₂O₄, and LiFePO₄.^190–196

2.2.3. Approaches to roughen smooth surfaces. The theoretical background has revealed that a sufficient hierarchical micro/nano-structure is needed for a superwettable surface. However, the intrinsic roughness on electrospun nanofibrous materials is not enough to bring about superwettability due to its single scale. Therefore, a common strategy is to construct nanoscale rough topography on fiber surfaces, and many important methods involving particle-decoration, in situ growth, etching, and phase separation are used.
(1) Particle-decoration method. This method involves the introduction of additional nanoparticles onto nanofiber surfaces for creating nanotextures, which includes two major routes: fabrication of polymer/particle composite nanofibers during electrospinning (i.e., colloid electrospinning), and incorporation of nanoparticles after electrospinning by post-treatment (dip coating, surface self-assembly and so on).

Colloid electrospinning is the simplest strategy to prepare composite nanofibers with hierarchical rough surfaces. For example, Li and co-workers prepared hierarchical organic/inorganic hybrid nanofibers by electrospinning of PVDF/octadecyltrichlorosilane-capped SiO₂ NP/DMF colloid solution.¹⁵⁴ The SiO₂ NPs contributed to a multiscale hierarchical structure on the PVDF nanofibers, making the wettability change from hydrophobic to superhydrophobic with a WCA of 163.1°. By adjusting the number of particles in the spinning solution, the density of nano-protrusions on the fibers can be easily controlled, and thus the wettability of the nanofibers can be regulated. For example, Ge et al. controlled the surface morphology of electrospun PAN/SiO₂ NP products by the content of SiO₂ NPs.⁶⁵ When the concentration of SiO₂ NPs increased from 1 to 4 wt%, the surfaces of the PAN/SiO₂ NP products exhibited increased nanoscale roughness, leading to a significant improvement of the UWOCA from 155° to 162°. Because colloid electrospinning could incorporate nanoparticles in both the surface and interior of the fiber matrix, nanoparticles with poor dispersity may deteriorate the fiber mechanical property. Besides, the encapsulation of nanoparticles within polymer fibers limits their application in roughness construction. Surface treatment of the nanofibers is an effective route to overcome this issue, which refers to immersion of the pristine fibers into a nanoparticle suspension, and then the particles could be adsorbed on the fiber surfaces by electrostatic forces, hydrogen bonding or other interactions. For example, Wang et al. prepared hierarchical nanofibers through electrostatic surface assembly (Fig. 7a).¹⁹⁷ After grafting of positively charged polyethyleneimine (PEI) on the hydrolyzed PAN (HPAN) nanofibers, the negatively charged SiO₂ NPs homogeneously wrapped around the obtained HPAN/PEI nanofibers by electrostatic forces. The resultant hybrid nanofibers exhibited nanoscale protrusions on the fiber surfaces (Fig. 7b and c). Similar work was reported by Lee et al.,¹⁹⁸ who firstly introduced a cationic surfactant into PVDF–HFP solution, followed by electrospinning of it into PVDF–HFP nanofibers with positive charge. After immersion of them into a dispersion of negatively charged SiO₂ NPs, a layer of SiO₂ NPs was firmly absorbed on the PVDF–HFP nanofiber surfaces by strong electrostatic attraction, leading to a hierarchical rough structure on the fiber surfaces. Song et al. introduced Fe₃O₄ NPs with an electropositive property on the PAA surface by electrostatic attraction, resulting a magnetic hierarchical rough surface.¹⁹⁹

Fig. 7 (a) Preparation of hierarchical nanofibers by the particle-decoration method. SEM images of the PAN nanofibers before (b) and after (c) modification by SiO₂ NPs. Reproduced with permission from ref. 197. Copyright 2018, Elsevier. (d) Schematic of in situ growth of BiOBr NPs on TiO₂ nanofibers. SEM images of TiO₂ nanofibers (e) and a dense coating of BiOBr NPs homogeneously wrapping around TiO₂ nanofibers (f). Reproduced with permission from ref. 200. Copyright 2018, Royal Society of Chemistry. (g) Schematic diagram of the formation of nanogrooves on PI nanofibers by the etching method. SEM images of (h) PAA/PS nanofibers with a smooth surface and (i) PI nanofibers with nanogrooves. Reproduced with permission from ref. 201. Copyright 2014, American Chemical Society.

(2) In situ growth method. The in situ growth method encompasses various construction routes, such as hydrothermal treatment, chemical vapor deposition (CVD), electroless plating, and successive ionic layer adsorption and reaction (SILAR). It allows for the gradual growth of nanocrystals with controllable morphology and uniform dispersity on smooth nanofiber surfaces. The chemical bonding energy between the created nanoscale coatings and nanofibers ensures a robust rough surface, which is conducive to practical applications.

Hydrothermal treatment is a commonly used route to synthesize nanomaterials, including crystal growth and transformation, phase equilibrium, and nanocrystal formation. When the hydrothermal process is combined with nanofibers, various metal oxides with different morphologies (e.g., sphere, sheet, and rods) can be constructed on fiber surfaces. For example, Chang et al. demonstrated a hydrothermal treatment to grow a layer of ZnO nanorods on PI nanofibers.²⁰² The ZnO NPs as seeds were first deposited on PI nanofibers by dip coating, and then the resultant nanofibers were immersed in an aqueous solution of zinc nitrate hexahydrate and hexamethylenetetramine under high temperature for hydrothermal growth. As a result, the PI nanofibers showed a rough surface with a dense layer of ZnO nanorods, which was beneficial for superwettability. The CVD method is another advanced synthetic route for the growth of nanocrystals onto a fibrous substrate through chemical interactions of precursors at high temperature. Compared with the hydrothermal method, CVD avoids the solvent removal process and consequently forms a more uniform coating on nanofiber surfaces. For instance, Park et al. uniformly anchored CNT arrays on the surface of carbon nanofibers by CVD.¹⁵⁹ Briefly, carbon/Fe₃O₄ NP hybrid nanofibers were fabricated by electrospinning of PAN/Fe(Acc)₃ solution and a subsequent heat treatment process. Then the metal oxides acted as the catalyst for the formation of CNTs on the carbon nanofiber surfaces by a CVD process under a gas mixture (Ar/H₂/C₂H₂). The fabricated fibers exhibited increased surface roughness, displaying a superhydrophobic property with a WCA of 155°, much higher than that of the smooth carbon nanofibers. As another example, Zeng et al. reported a thermal CVD method to make graphene sheets grow homogeneously and densely along every carbon nanofiber, thereby resulting in an obvious difference in WCA between pristine carbon nanofibers and graphene nanofibers (130° for the former one, but 173° for the latter rougher one).²⁰³

The above two routes require the growth of nanocrystals under harsh operational conditions (high temperature and high pressure), thus putting a limit on scalable application. Very recently, the SILAR technique, which relies on ionic reaction under normal temperature and pressure conditions to generate nanocrystals on nanofiber surfaces, has triggered much attention. For example, Cai et al. anchored a layer of BiOBr NPs onto TiO₂ nanofibers by SILAR (Fig. 7d).²⁰⁰ Briefly, TiO₂ fibers were firstly submerged into Bi(NO₃)₃ aqueous solution to absorb Bi³⁺, and then the treated nanofibers were soaked into KBr aqueous solution to form insoluble BiOBr on the TiO₂ surface through the chemical interaction between Bi³⁺ and Br⁻. After multiple cycles of alternately dipping the TiO₂ nanofibers into the two ionic liquids, BiOBr NPs uniformly adhered to the TiO₂ nanofibers, leading to nanoscale roughness on the fiber surfaces (Fig. 7e and f). Similar work was reported by Ma et al., who anchored Fe³⁺/phytic acid coordination complex nanoparticles on PI nanofibers.²⁰⁴ As to the electroless plating process, the metal ions are first suspended in a chemical bath, and when a reducing agent is added, the chemical reaction begins to yield metal nanoparticles on the fiber surfaces. For example, Li et al. demonstrated an electroless plating process to construct nanoscale roughness on PAN nanofibers.¹⁶⁶ PAN nanofibers were firstly aminated to generate amine and imine groups for full adsorption of [Ag(NH₃)₂]⁺. Then glucose as a reducing agent was added in the solution to synthesize Ag NPs on the PAN nanofiber surfaces. Usually, the created roughness can be adjusted by tuning the electroless plating time.

(3) Etching method. The etching method means the removal of an unprotected component from the pristine multicomponent fibers by a chemical or physical process, such as decomposition, dissolution, and sublimation. If the aforementioned two roughening methods are designed based on the concept of “addition”, the etching method can be considered as the opposed design philosophy of “subtraction”.

For example, Guo and co-workers introduced nanogroove structure onto PI nanofibers by the etching method (Fig. 7g).²⁰¹ During the electrospinning of PAA/PS solution, the PS molecular chains were well extended, and aggregated with each other, and then distributed in the PAA/PS fibers along the axial direction. After subsequent thermal imidization, the PS was decomposed, leaving numerous nanogrooves on the PI fiber surfaces (Fig. 7h and i). The resultant PI nanofibers showed a WCA of 134°. In contrast, PI smooth nanofibers only had a WCA of 129°. As another example, Zhang et al. successfully synthesized hierarchical PAN nanofibers by the selection of PVP as a sacrificial template through a dissolution-based etching route.²⁰⁵ The PAN/PVP composite nanofibers were firstly electrospun, and then the fibers were washed with water to dissolve the PVP, enabling the formation of a porous skin on the PAN nanofibers. In another study, Liu et al. used terephthalic acid (PTA) as the sublimating agent to decorate electrospun carbon nanofibers with macropores.²⁰⁶ The PAN/PTA hybrid nanofibers were firstly electrospun. Then PAN was converted into carbon and PTA sublimed by a calcination process under an argon gas flow. The resultant carbon nanofibers exhibited uniformly distributed tiny pores on their surfaces.

(4) Phase separation method. In this method, a pristine homogenous phase loses its original equilibrium and turns into a new multiphase system that consists of a polymer phase and a liquid phase by the differences of polarity and intermolecular forces. After the liquid phase is removed, roughened polymer fibers could be obtained. This method is also based on the concept of “subtraction”, in which no further post-treatment is needed.

For instance, Lin et al. demonstrated a phase separation method to construct various microstructures (protrusions, grooves, and pits) on PS nanofibers (Fig. 8b–d).²⁰⁷ During the fiber formation process, the water vapor in the air as a non-solvent diffused into the fluid jet and mixed with the solvent, and the solution in the jet then became an unstable thermodynamic system. Combining it with the rapidly decreased temperature of the fluid jet, phase separation could be triggered, leading to the generation of a polymer-rich phase and a solvent-rich phase in the solution jet. After solvent evaporation, the polymer-rich region solidified and formed into the polymeric matrix, while the solvent-rich region evolved into nanopores, resulting in roughed nanofiber surfaces (Fig. 8a). Notably, the microstructures of PS fibers could be finely tuned by the polymer molecular weight, solvent composition, and solution concentration. Based on the same principle, Zhu et al. incorporated submicron pores onto PMMA/CdTe quantum dot nanofibers. During the electrospinning of PMMA/CdTe/chloroform, the evaporation of chloroform significantly lowered the ambient temperature, leading to the condensation of water vapor onto the jet. After water evaporation, rough porous surfaces were successfully gained on the nanofibers, resulting in an increased WCA of 151.2°.²⁰⁸

Fig. 8 (a) Schematic showing the formation process of roughened electrospun nanofibers by phase separation. SEM images of the roughened PS fibers formed from various weight ratios of tetrahydrofuran : DMF in solvent: (b) 1 : 4, (c) 3 : 2, and (d) 5 : 0. Reproduced with permission from ref. 207. Copyright 2012, Royal Society of Chemistry.

3. Fabricating nanofibrous materials with optimized 2D and 3D porous structures

The porous structure of nanofibrous materials greatly influences the transport behaviors of emulsified droplets and the continuous phase during separation. The most common nanofibrous materials are single-component nanofibers with randomly arranged 2D nonwoven morphology, of which the simple porous structure is very limited in obtaining high separation performance. Through years of endless efforts, tremendous developments have been achieved in finely tailoring the arrangement structure of nanofibers by electrospinning system design and post-processing treatment, which endows nanofibrous materials (i.e., nanofibrous membranes and aerogels) with rational 2D and 3D porous structures, thereby providing a promising approach for achieving a maximum permeability/selectivity trade-off in filtration applications. In this section, we will introduce recently developed methods of regulating 2D and 3D porous structures.

3.1. Methods for regulating 2D porous structure

3.1.1. Multi-spinneret electrospinning. Multi-spinneret electrospinning exploits an electrospinning system with multiple spinnerets to fabricate membranes with bi-component and multi-component nanofibers. By virtue of the structure characteristics (e.g., diameter and shape) of different component fibers, the membrane porosity and pore size can be synergistically regulated (Fig. 9a). For example, Zhang et al. utilized a multi-spinneret electrospinning system to cause thin PAN nanofibers (with a diameter of ∼270 nm) and thick PSU microfibers (with a diameter of ∼1.3 μm) to uniformly assemble into an integrated membrane.²⁰⁹ Because the thin nanofibers facilitated the formation of small pore size and microfibers were beneficial for reducing the packing density, the PAN/PSU composite nanofibrous membrane could possess a fascinating structure with both a small pore size and high porosity through tuning the jet ratio of PAN/PSU. Another study was demonstrated by Gao et al., in which they fabricated a membrane composed of PAN nanofibers and PAN microspheres by multi-spinneret electrospinning.²¹⁵ The interwoven PAN nanofibers constituted a robust nanofibrous skeletal framework, in which the microspheres were uniformly inserted, thus enlarging the space among PAN fibers. Compared with the PAN nanofibrous membrane, the PAN nanofiber/microsphere membrane showed a higher porosity without increased pore size, suggesting its potential for rapid matter transfer under similar interception capacity. As a common and facile method for tuning the membrane structure, multi-spinneret electrospinning has been applied in industry. Yet there are still challenges of homogeneous assembly among different fibrous components. Further optimization of the electrospinning equipment (e.g., needle configuration, needle numbers, and interneedle spacing) and spinning parameters is a promising method to enhance the structural uniformity.

Fig. 9 Schematic showing the method for regulating the 2D porous structure of nanofibrous membranes. (a) Multi-spinneret electrospinning method. Reproduced with permission from ref. 209. Copyright 2016, American Chemical Society. (b) Template-assisted electrospinning method. Reproduced with permission from ref. 210. Copyright 2007, Wiley-VCH. (c) Electrospinning/netting method. Reproduced with permission from ref. 211. Copyright 2017, Wiley-VCH. (d) Sequential electrospinning method. Reproduced with permission from ref. 212. Copyright 2018, Wiley-VCH. (e) Filtration deposition method. Reproduced with permission from ref. 213. Copyright 2011, American Chemical Society. (f) Interfacial polymerization method. Reproduced with permission from ref. 214. Copyright 2015, Elsevier.

3.1.2. Template-assisted electrospinning. Different from the conventional electrospinning process, which relies on a flat collector, template-guided electrospinning with a patterned collector offers a means of externally controlling the nanofiber deposition. The macroscopic organization of nanofibers can be tuned by the design of the collector, thereby consequently leading to tailoring the membrane porous structure (e.g., pore size and shape) (Fig. 9b).^{210,216–218} For instance, Song et al. fabricated PVB nanofibrous membranes with different pore sizes by electrospinning nanofibers on metal meshes with various mesh numbers.⁹³ During the fiber collection, the nanofibers preferentially deposited on the electroconductive metal wires rather than in the interspaces among the wires, leading to a patterned structure similar to the template collector. As the mesh number increased, the pore size of the resultant nanofibrous membrane could be enlarged. In another study, by virtue of metal ring collectors with metal needles, Li et al. prepared a biomimetic nanofibrous membrane with radially aligned nanofibers and small pore size. This intriguing structure allowed emulsified droplets to directionally move to the intersection of nanofibers under a wettability gradient and gravity.²¹⁹ Besides, uniaxially aligned arrays of nanofibers could also be obtained under certain conditions such as the utilization of a collector pattened with a linear air gap.²²⁰ However, template-assisted electrospinning is not able to fabricate highly patterned nanofibers with a large membrane thickness because of the electrostatic repulsion between the deposited fibers and the incoming fibers. As a result, the ability to fabricate patterned nanofibrous membranes with submicron pore size is still a challenge.

3.1.3. Electrospinning/netting. Electrospinning/netting is considered to be a variation of the electrospinning process. By virtue of phase separation of tiny charged solution droplets in an electric field, it enables one-step preparation of a 2D “nanonet” with nanoscale diameters (20–50 nm) and a topological Steiner-tree (Fig. 9c).²¹¹ This unique product can significantly decrease the membrane pore size without increasing the membrane thickness, which is beneficial for enhancing the membrane permeability. The formation of a nanonet requires the spinning solution parameters (e.g., electrical conductivity, viscoelastic property, and surface energy) to be within a certain range. Besides, the ambient parameters (e.g., temperature and relative humidity) also have a great influence on the formation of the nanonet. Until now, lots of polymers have succeeded in fabricating a nanonet, including hydrophobic polymers (e.g., PVDF and PU) and hydrophilic polymers (e.g., PAN, PVDF-g-PAA, PA6, PA66, PA56, PVA, PAA, CS, and cellulose).²²¹ For instance, Zuo et al. fabricated a PU nanofiber/net membrane by using an ionic liquid (DMAc/LiCl) as a solvent through electrospinning/netting. The complexing action of Li⁺ and DMAc molecules could effectively mobilize Cl⁻, which then functioned as charge carriers to improve the charge capacity of the solution, thus increasing the formation probability of nanonets. The resultant PU nanonets endowed the PU membrane with a pore size of only 0.4 μm, which is much smaller than that of common PU nanofibrous membranes (6–12 μm).²¹¹ Besides hydrophobic polymer systems, Wang et al. fabricated a hydrophilic PA6 nanofiber/net membrane by finely tailoring the ratio of polymer and solvent.²²² Due to the small pore size (∼83 nm) of the PA6 nanonet, the resultant PA6 membrane exhibited a small pore size (<0.2 μm) within a thin thickness (∼10 μm). In another report, Liu et al. investigated the formation of PAN nanonets at different relative humidity. They found that the PAN charged droplets can fully phase-separate at a slower speed during the electrospinning/netting process under a relative humidity of 35%. Higher relative humidity could rapidly dissipate the charge, and thus the formation of the nanonet was severely suppressed. The resultant PAN nanofiber/net membranes formed under 35% relative humidity possessed the integrated characteristics of a small pore size (<0.3 μm) and high porosity (∼93.9%).²²³ Also, Cheng et al. reported a PVDF-g-PAA nanofiber/net membrane by electrospinning/netting, and the membrane pore size was about 0.32 μm.²²⁴ Up to now, the 2D “nanonets” derived by electrospinning/netting are all polymer-based materials, and it is still challenging to prepare inorganic nanonets, which are potentially important in liquid filtration under harsh conditions (high temperature, acids, alkalis, and salts).

3.1.4. Sequential electrospinning. This method refers to the direct electrospinning of nanofibers onto a porous nanofibrous substrate, providing the capacity to fabricate multilayer nanofibrous membranes with asymmetrical porous structure (pore size, porosity and so on) on each surface (Fig. 9d). Thus, the synthesized multilayer membranes show great potential in the simultaneous optimization of permeability, selectivity, and mechanical strength.²²⁵ For example, Wang and co-workers fabricated a bilayer nanofibrous membrane consisting of a thin hydrophilic PVA nanofibrous coating layer and PAN nanofibrous bottom layer by sequential electrospinning.²²⁶ Then the PVA nanofiber layer was melted through exposure to water vapor and crosslinked in glutaraldehyde water/acetone solution to form a dense film on the support layer. The resultant PVA/PAN membranes demonstrated bottom-up hierarchical networks. It should be noted that the porous structure of the PVA coating layer can be easily controlled by tuning the deposition time and the crosslinking degree of PVA nanofibers. Similarly, a fascinating report by Raza et al. presented a sequential electrospinning method to deposit PEO/PEGDA nanofibers onto a PAN/PEG nanofibrous membrane. Then they utilized UV-irradiation to in situ crosslink the PEGDA to decrease the pore size of the PEO/PEGDA layer, thereby forming a nanofibrous membrane with asymmetrical pore size.²²⁷ Sequential electrospinning is the most simple and effective approach to tune the 2D porous structure in multilayer membranes. However, the porosity among nanofibers may be deteriorated in the following cross-linking process, which is not beneficial for liquid permeation during oil/water separation.

3.1.5. Filtration deposition. This method means to filtrate a suspension of 1D nanomaterials (with a diameter of several nanometers) onto an electrospun membrane, leaving a layer of 1D nanomaterials with nanoscale thickness on the electrospun substrate (Fig. 9e). This method as a complement of sequential electrospinning significantly increases the physicochemical diversity of coating layers in multilayer membranes. Currently, a variety of ultrafine nanofibers, including biomass fibers (e.g., cellulose nanocrystals and chitin nanofibers), and inorganic fibers (e.g., CNTs and metal nanowires), are employed as the building blocks of coating layers.^213,228,229 For instance, Cao et al. prepared a bilayer membrane with a PAN nanofibrous membrane as a substrate and jute cellulose nanocrystals (with fiber diameter ranging from 3 to 10 nm) as the building blocks of the coating layer by the filtration deposition method.²²⁹ The synthesized PAN/cellulose nanocrystal membrane demonstrated nanoscale pore size and a thin thickness in the coating layer. A similar process was also realized by depositing ultrafine chitin nanofibers with diameters ranging from 15 to 20 nm on an electrospun PAN nanofibrous membrane.²¹³ The resultant membrane possessed an average pore size of ∼20 nm and main pore size distribution below 50 nm. As another example, Tian and co-workers fabricated a multilayer structured membrane by utilization of an electrospun PAN membrane as the support layer and CNTs as the coating layer.²³⁰ The resultant PAN/CNT membrane possessed a narrow pore size distribution and a small average pore size of 0.25 μm, which was more uniform and smaller than the PAN nanofibrous substrate with an average pore size of 0.8 μm. However, it should be noted that 1D nanomaterials with length in the sub-micron range could infuse into the nanofibrous substrate, leading to a discontinuous coating layer and partly blocked pore channels.

3.1.6. Interfacial polymerization. Interfacial polymerization refers to two highly reactive polyfunctional monomers reacting at the interface between two immiscible liquids, resulting in a polymeric film with a thin thickness (from 10 nm to several micrometers) and small pore size at the interface of the two phases. When the two-phase interface is close to the nanofibrous membrane surface, the synthesized polymeric film could act as a coating layer to adhere to the nanofibrous support, leading to a multilayer membrane (Fig. 9f).²³¹ The porous structure and surface morphology of the coating layer can be well controlled by adjustment of the concentration of monomers, reaction time, and solvent type. Notably, the top layer nanofibers in the nanofibrous substrate can be embedded into the coating layer to create transmission channels between the fiber and polymer matrix, thus facilitating liquid transportation.²³² As a typical example, Obaid et al. developed a thin PA film on a PSF nanofibrous membrane by the interfacial polymerization of m-phenylenediamine and 1,3,5-benzenetricarbonyl chloride, thereby optimizing the membrane structure.^214,233 Notably, when utilizing electrospun nanomaterials as a platform for interfacial polymerization, the high porosity of the nanofibrous membrane could provide more adhesion points for the formation of the polymeric film, which made it more uniform. However, the considerable porous structure may also lead to defects of the polymerized film, which means that the active layer tends to be damaged under filtration and decreases the selective performance.

3.2. Methods for regulating 3D porous structure

The above methods could flexibly regulate the 2D porous structure in nanofibrous membranes. Despite their outstanding potential in filtration and separation, the major problem associated with membranes still exist, such as easy pore blockage. More recently, strategies of tailoring the 3D porous structure of nanofibrous materials have come into sight, leading to the formation of 3D nanofibrous aerogels. Compared with nanofibrous membranes, nanofibrous aerogels can offer much stereo storage room for droplet retention, thereby effectively avoiding sharp blocking of transport channels by emulsified droplets and greatly prolonging the service life. Meanwhile, nanofibrous aerogels possess ultrahigh porosity (>90%), which is beneficial for continuous phase permeation. Generally, there are two methods for tuning the 3D porous structure of nanofibrous aerogels, i.e., physical template assembly and bottom-up self-assembly.

3.2.1. Physical template assembly. Template assembly of 3D nanofibrous networks refers to the employment of an existing 3D block substrate as a template, followed by the introduction of nanofibers to tailor its interior frame (Fig. 10a). Commercially available sponges have been extensively utilized as a 3D physical template. For instance, Lv et al. recently reported a physical template assembly process to fabricate an elastic 3D aerogel by the insertion of SiO₂ nanofibers into a poly(melamine formaldehyde) sponge.²³⁴ Numerous minor pores derived from interwoven SiO₂ fibers were created in the sponge, so the intrinsic large pores in the pristine sponge decreased quickly from about 110 to 23 μm. Meanwhile, the aerogel maintained a high porosity (>97.8%) (Fig. 10b–e). Because the structure features of the sponge played a key role in the arrangement of the nanofibers, the porous structure of nanofibrous aerogels would not be flexibly tuned for fixed main frameworks (most sponges with a pore size of 100–500 μm). Therefore, the nanofibrous aerogels synthetized by this method may show a great challenge for removal of submicron or nanoscale emulsified droplets.

Fig. 10 (a) The regulation of the 3D porous structure of a nanofibrous aerogel by physical template assembly. (b) An optical photograph and (c–e) SEM photographs of the as-prepared nanofibrous aerogel. Reproduced with permission from ref. 234. Copyright 2017, Wiley-VCH.

3.2.2. Bottom-up self-assembly. Bottom-up self-assembly is an economical and efficient method to regulate the 3D porous structure of nanofibrous materials through homogenizing nanofiber dispersions, freeze drying to remove the solvent, and crosslinking the fibrous framework. By carefully controlling the dispersion concentration, fiber dispersity, freezing conditions, and the character of the solvent and fiber, the 3D porous structure (pore size, pore volume, and pore morphology) can be readily regulated, thus overcoming the limitations of the previous method.⁶² Meanwhile, the nanofibrous aerogels fabricated by this method usually possess hierarchical pore morphologies, thus making them perfect candidates for separation applications, even for separation of oil/water microemulsions. Besides, various shapes (e.g., cubes, cones, cylinders, and tubes) could be easily obtained by designing molds, which is significant for the fabrication of filter modules. For example, Si et al. fabricated a cellular structured nanofibrous aerogel by self-assembly of PAN and SiO₂ nanofibers.⁶² Benzoxazine was employed as a crosslinking agent to stabilize the 3D fibrous scaffold (Fig. 11a). The major cellular pores in the aerogels originated from the non-planar solvent crystals and the minor cellular pores were derived from the bonding of the tangled PAN and SiO₂ fibers (Fig. 11c–e). Such a unique hierarchical cellular structure provided numerous microchannels, enabling liquid to pass through easily (Fig. 11f). Moreover, the simplicity and good controllability of the bottom-up self-assembly method ensured that nanofibrous aerogels with various sizes and shapes could be facilely designed and fabricated without second processing (Fig. 11b). Notably, in order to ensure the mechanical integrity of the fibrous aerogel during the filtration process, the crosslinking process in this method should be optimized according to the application environment (such as acids, alkalis, and salts) to avoid aerogel disintegration into suspensions of short fibers upon contact with filtrates.

Fig. 11 (a) The regulation of a 3D porous structure nanofibrous aerogel by bottom-up self-assembly. (b) A digital image showing the moldability of nanofibrous aerogels. (c–e) SEM images with different magnifications of the obtained nanofibrous aerogel. (f) Schematic of the levels of hierarchy in three dimensions. Reproduced with permission from ref. 62. Copyright 2014, Nature Publishing Group.

4. Applications of electrospun nanofibrous materials in emulsion separation

As discussed in the previous sections, electrospun nanofibrous materials as one of the hottest nanomaterials possess remarkable features, such as controllable chemical compositions, tunable porous structure, diverse dimensions, and good pore connectivity, which make them a promising candidate for emulsified oil purification and oily wastewater remediation. In this section, we will focus on new and typical nanofibrous materials applied in these fields.

4.1. Oil purification

Oil purification is a process that removes emulsified water droplets from oily matter, which requires the electrospun nanofibrous materials to have hydrophobic–oleophilic chemical compositions and appropriate pore size. Generally, these nanofibrous materials can be classified into two types.

4.1.1. Hydrophobic–oleophilic nanofibrous membranes. Since the fabrication of hydrophobic–oleophilic nanofibrous membranes for oil purification through the direct electrospinning of low-surface-energy polymers (PP,^89,90 PVAc,²³⁵ PVP,²³⁵ and PVDF–HFP¹¹⁰) was reported by Chase et al., considerable efforts have been devoted to the synthesis of filtration membranes in such a way. However, these membranes generally have quite a few drawbacks like poor antifouling performance and unsatisfactory separation performance due to the insufficient roughness and large pore size.

Over the past few years, taking inspiration from the lotus leaf, a variety of biomimetic superwettable nanofibrous materials have been developed by the introduction of nanoscale roughness structure. According to wettability theory, hierarchical roughness structure can transfer the Wenzel state of the pristine nanofibrous membrane to the Cassie state, thus leading to an outstanding anti-water-adhesion property.²³⁶ For example, Huang et al. reported a superhydrophobic–superoleophilic nanofibrous membrane by construction of a fluorinated polybenzoxazine (F-PBZ)/Al₂O₃ NP functional layer on silica nanofiber surfaces through in situ polymerization (Fig. 12a and b).⁶⁰ The introduction of Al₂O₃ NPs created a hierarchical rough structure on the membrane, which remarkably enhanced the selective wettability and rendered the membrane superhydrophobic–superoleophilic with a WCA larger than 160° and an OCA approaching 0° (Fig. 12c). When a water-in-petroleum ether emulsion was poured onto the SiO₂/F-PBZ/Al₂O₃ NPs membrane, only oil could permeate through the membrane, while the water droplets were intercepted by the membrane, yielding a permeation flux of 892 L m⁻² h⁻¹ (Fig. 12d and e). The trapped oil layer in the hierarchical rough structure effectively prevented contact between the underoil water droplet and membrane surface, leading to an outstanding reusability without flux decline upon 10 cycles (Fig. 12g). Besides, the membrane showed prominent thermal stability (Fig. 12f), which is crucial to practical applications.

Fig. 12 (a) Schematic of the preparation of and (b) SEM image of the SiO₂/F-PBZ/Al₂O₃ NP nanofibrous membrane. The inset shows a water droplet on the membrane. (c) Digital photograph showing the selective wetting behavior for oil (red) and water (blue). (d) Schematic illustration of the separation of a water-in-oil emulsion by the SiO₂/F-PBZ/Al₂O₃ NP membrane. (e) Digital images and optical micrographs of the feed and filtrate. (f) WCAs of the SiO₂/F-PBZ/Al₂O₃ NPs under various temperatures. (g) Permeation flux of the F-PBZ/Al₂O₃ membrane for a water-in-oil emulsion within 10 cycles. Reproduced with permission from ref. 60. Copyright 2013, Royal Society of Chemistry.

The aforementioned membranes are ineffective for nanoscale emulsions (with a droplet size below 1 μm) because of their micron pore size. To address this problem, Kang et al. demonstrated a sequential electrospinning method to fabricate a thin layer of PDO 3 nanofibers on a highly porous PLC nanofibrous membrane.⁶⁶ After subsequent light irradiation, the PDO 3 nanofibers were partly fused together to reduce the pore size of the coating layer, while the PCL nanofiber layer remained intact. Thus, this membrane exhibited a bilayer architecture with nanopores in the PDO 3 coating layer and micron pores in the PCL substrate (Fig. 13a–c). Meanwhile, by virtue of the low-surface-energy composition of PDO 3, the obtained PDO 3/PCL membrane possessed highly hydrophobic and superoleophilic properties with a WCA of 141° and an OCA of 0° (Fig. 13d). Utilizing this membrane, the author demonstrated the removal of nanoscale water droplets (with a diameter ranging from 3 to 300 nm) from a series of emulsions (i.e., water-in-isooctane, water-in-petroleum ether, and water-in-hexane), exhibiting high separation efficiencies (>99.96%) and satisfactory oil fluxes (14 165 ± 354 L m⁻² h⁻¹ bar⁻¹ for surfactant-free emulsions and 912 ± 79 L m⁻² h⁻¹ bar⁻¹ for surfactant-stabilized emulsions). Notably, although the PDO 3/PCL membrane possessed a small pore size, it also demonstrated high permeation fluxes. The reason was that this membrane possessed a thin coating layer and a porous bottom layer, and such a unique structure could effectively decrease the transport distance for oil.

Fig. 13 (a) Schematic of PDO 3 nanofibers, tailoring the membrane pore size by light irradiation. (b and c) Microscopic structure of the PDO 3/PCL membrane after exposure to light. (d) Wettability of the membrane toward water and oil. (e) Procedure for the synthesis of a biomimetic PVDF multilayer nanofibrous membrane by sequential electrospinning. (f) SEM images showing the section morphology of the PVDF multilayer membrane. (g) Separating water-in-oil emulsions without (up) and with (down) a surfactant. Reproduced with permission from ref. 66. Copyright 2017, Royal Society of Chemistry. Reproduced with permission from ref. 237. Copyright 2019, Royal Society of Chemistry.

More recently, growing attention has been paid to synchronous regulation of the wettability and porous structure. For instance, Jiang and co-workers presented a filtration coating strategy to form a thin layer of hydrophobic CNTs on a PAN nanofibrous substrate.²²⁸ The introduction of CNTs not only created nanotexture on PAN fiber surfaces but endowed the PAN/CNT multilayer membrane with nanoscale pore size. Thus, the PAN/CNT membrane was endowed with Cassie wetting behavior for oil droplets at the solid/water/oil interface, which is significant for enhancing the antifouling performance. The nanopores of the coating layer significantly increased the size sieving capacity of the membrane. In consequence, the developed PAN/CNT membrane was used to treat a nanosized water-in-chloroform emulsion (with a diameter ranging from 0.3 to 1 μm), achieving a high separation efficiency up to 99.2% and an oil flux of 12 000 L m⁻² h⁻¹. However, this membrane usually suffers from poor interlayer integrity between the inorganic and organic layers due to the incompatibility, so the inorganic layer tends to detach from the polymeric substrate after a long period of use. To overcome this issue, Zhang and co-workers constructed an all-polymer multilayer membrane consisting of a taro leaf-like hierarchical PVDF thin layer on a porous PVDF nanofibrous membrane through regulating the jet mode of sequential electrospinning (Fig. 13e).²³⁷ The obtained polymeric membrane displayed an asymmetrical cross-sectional structure with submicron pores on the top layer and micron pores on the substrate layer as well as good structure integrity (Fig. 13f). Besides, benefiting from the synergy of the biomimetic hierarchical surface and the oleophilic fluorinated polymeric matrix, the PVDF membrane demonstrated a superwettable property with a WCA above 150° and an OCA of 0°. Thanks to the synergy of oil/water selectivity and submicron pores, the PVDF multilayer membrane can be used for separating micrometer- and nanometer-size oil droplets from multiple surfactant-free and surfactant-stabilized water-in-oil emulsions, including water-in-diesel, water-in-isooctane, water-in-petroleum ether, and water-in-cyclohexane emulsions (Fig. 13g).

4.1.2. Hydrophobic–oleophilic nanofibrous aerogels. Benefiting from the surface chemical composition control and porous structure construction, the separation performance of 2D nanofibrous membranes has been remarkably improved. However, this high performance can only keep stable in the initial stage of the separation process, because water is denser than most oils, and thus water will settle below the oil and form a water layer that prevents oil from penetrating the porous material surface. Although some filtration units can reduce droplet accumulation and enhance the separation stability, they have some limitations such as complex assembly processes.²³⁸ Recently, 3D porous nanofibrous aerogels have drawn much attention owing to their unique characteristics, such as ultrahigh porosity, high inter surface area, and stereo sieving manner, which endow them with great potential for droplet storage, thereby leading to a continuous separation process for a long time. As a typical example, Si and co-workers prepared a hierarchical cellular structured nanofibrous aerogel with a superhydrophobic–superoleophilic property by bottom-up self-assembly (Fig. 14a).²³⁹ The electrospun PAN nanofibers and SiO₂ nanofibers were applied to construct a 3D macroporous skeleton, while SiO₂ NPs were introduced to build nanoscale roughness (Fig. 14b–e). Fluoric benzoxazine with low surface energy was used as a hydrophobizing agent and crosslinking agent to increase the stability of the 3D fibrous skeleton. The resultant nanofibrous aerogel demonstrated integrated properties of selective superwettability with a WCA of 162° and OCA of 0° and stable 3D porous structure with ultrahigh porosity (>98%) (Fig. 14f). When treating water-in-oil emulsions (i.e., water-in-petroleum ether, water-in-gasoline, and water-in-diesel), the emulsified water droplets were easily intercepted in these long channels, resulting in high separation efficiency. Its outstanding pore connectivity with high porosity ensured that the oil searched for substitute routes penetrating the aerogel even when some channels were blocked by water droplets, resulting in high permeability for long term service. As a result, the as-prepared nanofibrous aerogel could achieve an intriguing oil flux of 8140 L m⁻² h⁻¹ and a high separation efficiency (Fig. 14g). Meanwhile, the nanofibrous aerogel showed excellent antifouling performance, indicating a robust performance for real emulsion separation (Fig. 14h). For 3D nanofibrous aerogels, a simple in situ pumping method could realize continuous oil purification (Fig. 14i).

Fig. 14 (a) Schematic of the preparation of the hydrophobic–oleophilic nanofibrous aerogel. (b) A digital photograph of the nanofibrous aerogel on a massive scale of 2.5 L. (c–e) SEM images showing the microscopic structure of the nanofibrous aerogel. (f) Digital photo of the selective permeability of water and oil on the surface of the nanofibrous aerogel. (g) Separation of water-in-oil emulsions using the aerogel, and the corresponding optical microscopy photographs of the emulsions and filtrates. (h) Variation of the oil flux and flux recovery during filtration cycles. (i) Photograph showing a device for continuous oil collection from water-in-oil emulsions. Reproduced with permission from ref. 239. Copyright 2015, American Chemical Society.

4.2. Oily wastewater remediation

When treating emulsified oily wastewater, it is reasonable for filtration materials to let water pass through freely and repel oil completely. Recently, researchers have successfully developed a variety of nanofibrous materials with a hydrophilic–oleophobic (i.e., hydrophilic–underwater oleophobic) property and suitable porous structure for treating oily wastewater.

4.2.1. Hydrophilic–oleophobic nanofibrous membranes. The prevailing strategy for the preparation of hydrophilic–underwater oleophobic nanofibrous membranes is the direct electrospinning of high-surface-energy materials. For instance, Zang and co-workers prepared core–shell structured nanofibers by electrospinning of CA/PMAA solution. During the fiber formation process, phase separation induced PMAA as a hydrophilic shell and CA as a supporting core.²⁴⁰ The subsequent heat treatment induced ester linkages between PMAA and CA, thus increasing the underwater durability of the PMAA shell. Due to the strong affinity of PMAA to water, the resultant CA/PMAA nanofibrous membrane possessed superhydrophilicity–underwater superoleophobicity with a WCA of 0° and UWOCA of 155°. Attributed to the synergistic effect of the promising wettability and small pore size (∼2 μm), the membrane could selectively separate micron oil droplets (n-hexane, petroleum ether, hexadecane, and diesel) from emulsified oily water, exhibiting a high separation efficiency. The pore size of the CA/PMAA membrane could be tuned by the pH value of the emulsions. The enlarged membrane pore size could greatly improve the water flux. Note that the CA/PMAA membrane demonstrated a high underwater oil-adhesion property despite its superwettability, which may lead to poor antifouling performance and a short lifetime. Besides, this membrane exhibited a large pore size above 1 μm, which limited its applications in treating nanoscale emulsions.

Currently, impressive progress has been made on the design and development of high-performance separation membranes to overcome the aforementioned problems, which are usually designed based on two criteria: (i) construction of a roughness structure on fiber surfaces to endow the membrane with an underwater Cassie state, and (ii) regulating the pore size in a suitable range to intercept nanosized droplets. For example, Zhang et al. fabricated a hierarchical TiO₂ nanofibrous membrane through the combination of electrospinning and hydrothermal treatment.²⁴¹ Briefly, a PVP/TiO₂ hybrid nanofibrous membrane was first prepared by electrospinning, and then it was converted into a pure TiO₂ nanofibrous membrane by calcination in air (Fig. 15a). After a subsequent hydrothermal process, TiO₂ nanorods densely grew on the nanofiber surfaces, resulting in pine-branch structured hierarchical fibers and a small pore size (Fig. 15b). The resultant hierarchical TiO₂ nanofibrous membrane possessed a superhydrophilic–underwater superoleophobic property with a WCA of 0° and a UWOCA of about 160° (Fig. 15c). Because the entrapped water in the hierarchical roughness structure significantly decreased the contact area between the oil droplet and the membrane surface, the synthesized TiO₂ membrane showed an ultralow oil adhesive force of nearly 0 μN (Fig. 15d), which was beneficial for the antifouling performance. Consequently, the as-prepared hierarchical TiO₂ membrane could allow treating nanoscale oil (petroleum ether, toluene, hexadecane, isooctane, gasoline, and diesel)-in-water emulsions (with an oil droplet diameter ranging from 41.1 to 313.5 nm) with water fluxes above 35 L m⁻² h⁻¹. More than 99% of oil droplets were removed from the water (Fig. 15e). Also, this membrane showed an excellent recyclable property, which may meet the requirements for practical applications. Interestingly, the TiO₂ membrane showed higher water fluxes under UV illumination, which suggested that UV-induced hydroxyl groups on the TiO₂ nanofiber surface could make the TiO₂ membrane more superhydrophilic. It should be noted that once oil permeated through the membrane, it would be contaminated and must be rinsed with organic solvents (e.g., ethanol) to recover the wettability. To avoid oil contamination completely, zwitterionic polymers were employed to be incorporated on the membrane surface for emulsion separation due to their superior electrostatic binding with water molecules. For example, Zhang et al. developed a zwitterionic nanohydrogel (ZN)-grafted PVDF-g-PAA nanofibrous membrane by the combination of electrospinning of PVDF-g-PAA dilute solution and inverse microemulsion polymerization.²⁴² The resultant PVDF-g-PAA/ZN membrane was composed of interpenetrated nanofibers and microspheres, demonstrating a lotus-leaf like hierarchical surface. Thanks to the hierarchical roughness and hydrophilic polymeric matrix, the PVDF-g-PAA/ZN membrane showed superhydrophilic and underwater oleophobic properties with a WCA of 0° and a UWOCA greater than 150° for various oils. Because the zwitterionic polymer exhibited outstanding binding affinity to water, even when the PVDF-g-PAA/ZN nanofibrous membranes were fouled by oils in air, the spread oils can shrink into a droplet and then detach from the membrane surface once immersed in water. By virtue of these integrated features and small pore size, the synthesized PVDF-g-PAA/ZN nanofibrous membrane could remove lots of nanoscale emulsified oil from oily wastewater. This membrane was believed to show stable separation performance after long term use.

Fig. 15 (a) Formation procedures and (b) SEM image of the hierarchical TiO₂ nanofibrous membrane. (c) WCA and UWOCA of the hierarchical TiO₂ membrane. (d) Adhesive force–distance curves between diesel droplets and the TiO₂ membrane underwater. (e) A separation device and images of the emulsion before and after separation. Reproduced with permission from ref. 241. Copyright 2017, Royal Society of Chemistry.

4.2.2. Hydrophilic–oleophobic nanofibrous aerogels. The integrated features of large specific surface areas, ultrahigh porosity, 3D interconnected channels, and easy access to large thicknesses (>1 cm) have made nanofibrous aerogels favorable candidates for oil purification. More recently, nanofibrous aerogels with a hydrophilic–underwater oleophobic property have also gained tremendous interest in emulsified oily wastewater remediation. Xiao et al. developed a GO sheet/CA nanofiber nanofibrous aerogel with ultrahigh porosity (>99%) by bottom-up self-assembly.²⁴³ The GO sheets adhered to CA nanofibers while the CA nanofibers acted as bridges to connect several GO sheets in turn, forming an open-cell geometry structure. Aiming to endow the nanofibrous aerogel with hydrophilicity–underwater oleophobicity and small pore size, the GO/CA nanofibrous aerogel was compressed at a certain strain and then modified with PDA and PEI. The resultant GO/CA/PDA/PEI nanofibrous aerogel demonstrated excellent capability for the separation of emulsified oily water (n-hexane-in-water emulsion), exhibiting a high separation efficiency (>99%) and a superb water flux of more than 19 580 L m⁻² h⁻¹ bar⁻¹ (several times higher than commercial membranes).

5. Conclusions and perspectives

In this review, we focus on an overview of the recent progress on electrospun nanofibrous materials for emulsion separation by summarizing the interface wettability tunability from hydrophobic–oleophilic to hydrophilic–oleophobic, architecture regulation from 2D porous structure in membranes to 3D porous structure in aerogels, and their promising applications in emulsified oil purification and oily wastewater remediation. Although tremendous advances have been achieved, several challenging problems still exist.

First, more quantitative research is needed to guide scientists to design nanofibrous filtration materials. For example, it is still less well known how emulsified droplets dynamically interact with the pore channel during separation. Second, the surface physicochemical structure of nanofibers can be readily destroyed by external mechanical forces and chemical contamination, which makes the nanofibrous materials lose selectivity. Future trends in this field may see an expansion towards self-healing nanofibrous materials for their easy and automatic restoration of selective wettability after damage. Third, most nanofibrous materials reported to date are effective to treat oil/water emulsions derived from oils (e.g., petroleum ether, toluene, hexane, chloroform, hexadecane, cyclohexane, isooctane, gasoline, and diesel) with low viscosity (<10 mPa s⁻¹), but cannot work well in treating highly viscous oil and oily wastewater. Therefore, it is highly desired to develop nanofibrous materials for separating emulsified high viscosity oil/water mixtures with high efficiency. Fourth, in the real world, an oil/water emulsion is a highly complicated system due to the inevitable introduced contaminants, such as microorganisms, inorganic salts, and dyes. Therefore, scientists are required to develop multifunctional nanofibrous materials. Finally, from an environmentally friendly and sustainable perspective, researchers need to consider the reclamation of industrial and daily waste, for example, food packaging, cigarette ends, and food residue, as raw materials for designing nanofibrous separation materials.

Despite the many challenges involved, despite many challenges are involved. Future development of electrospun nanofibrous materials will be achieved by multidisciplinary cooperation of materials science, surface chemistry, and engineering. We believe that with additional efforts, an exciting future in the design and fabrication of nanofibrous emulsion separation materials will emerge.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (no. 223201900081 and 17D310107), the National Natural Science Foundation of China (no. 51773033, 2191101289, and 51925302), and the Shanghai Municipal Science and Technology Committee of Shanghai Outstanding Academic Leaders Plan (18XD1400200).

Notes and references

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