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Abstract

Engineering surface morphologies of nanofibers has been attracting significant consideration in numerous fields and applications. Among different methods of generating nanofibers, electrospinning is the most widely adopted technique owing to the ease of forming nanofibers with an extensive range of properties and its exceptional advantages, such as the variety of shapes and sizes, as well as the adaptable porosity of nanofiber webs. The branched structure is considered one of the most attractive structures for scientific researchers due to its outstanding properties (e.g., high-specific surface area and extremely tiny diameters of branched nanofibers). Therefore, this work is the first one that summarizes the strategies and methods, reported so far, of producing branched nanofibers of different materials. The material types, formation mechanisms, characterizations, and applications of the branched nanofibers generated through different techniques will be discussed in detail in this study. We believe this work can be served as an important reference for the preparations, strategies, and applications of the branched nanofibers.

Introduction

Nanomaterials such as nanofibers, nanotubes, nanocomposites, nanoparticles, nanorods, and molecular self-assembled materials have received a great deal of interest in recent years. Nanomaterials feature a variety of unique electrical, mechanical, chemical, and other properties that have resulted in a scientific revolution in the development of new devices and materials.^1,2,3 Most nanomaterials have been formed by synthetic bottom-up methods and were discontinuous objects in many cases, leading to difficulties in their alignment, assembly, and processing for desirable applications.⁴ In contrast, nanofibers which are fibers with diameters in the nanometer range can be formed easily.⁵

Nanofibers can be generated by different techniques such as template synthesis, phase separation, drawing, wet spinning, and self-assembly.^6-10 However, most of these methods have disadvantages because occasionally procedures are not appropriate for some polymers, are not manageable, and have no adaptation over the orientation and diameter of fibers.

Since the late 1990s, the popularity of nanofibers has been rising as evident from expanding the number of publications. Nanofibers have unique properties (e.g., small diameter of fibers,^11,12 high specific surface area,^13,14 flexibility,^15,16 good pore structures,^17,18 ease of functionality,^19,20 low density,^21,22 excellent mechanical properties,^23,24 and exceptional adjustability^25,26). Therefore, they can be used in different areas such as energy harvesting,^27-29 antibacterial applications,^30,31 tissue engineering,^32,33 biomedical applications,^34-37 self-cleaning surfaces,^38,39 sensors,^40,41 catalyst,^42,43 filtrations,^44,45 and food packaging.^46,47

Since the surface morphology of nanofibers (e.g., porous fibers,^48-50 crimped fibers,^51,52 cactus-like nanofibers,⁵³ wrinkled fibers,^54,55 grooved fibers,^56-59 rough fibers,^49,60 tree-like fibers,^61,62 ribbon fibers,^63,64 butterfly wings fibers,^65,66 and beaded fibers,^67,68 etc.) can enhance or alter the properties and applications of nanofibers, branched nanofibers provide outstanding specific surface area,⁶⁹ tiny diameters,^70,71 excellent biological properties,⁷² good piezoelectricity,⁷³ large porosity,⁶⁹ outstanding mechanical properties,⁷³ good thermal stability,⁷⁴ excellent thermal insulation⁷⁴ and so on hence have multifunctional capabilities. Therefore, the branched structure has shown great potential in various applications such as filtration,⁶⁹ energy harvesting,⁷³ oil-water separation,⁷⁰ and energy storage.⁷⁵

Since 2004, the number of studies on branched nanofibers has been increasing. However, up to now, there is no review paper about this structure. To the best of our knowledge, this is the first work comprehensively and systemically reviews the importance of the branched structure.

The objective of this review is to shed light on the significance of branched nanofibers owing to their outstanding properties, resulting in their ability to be used successfully in multiple applications.

This review summarizes the materials, preparation methods, characterizations, and applications of the branched nanofibers. We believe this study can serve as a good reference for the production, characterizations, and applications of the branched nanofibers from different materials.

Electrospinning’s fundamental concept

As an increasingly extensive nanofibers formation method, electrospinning is an effective and adaptable method for producing fibers with diameters ranging from a few nanometers to some micrometers.^76-78 The gathering of these fibers forms electrospun nanofiber nonwoven webs.

The discovery of this technique dates back to 1934 when fibers were spun from cellulose acetate/acetone solution by Anton Formhals. Electrospun nanofibers have unique properties such as high porosity, high surface area, and excellent mechanical, chemical, and electrical properties, resulting in making them a favorable demand for researchers.

Electrospinning involves electrification of a liquid droplet to generate a jet, and subsequent stretching and elongation of jet to form fiber(s). The electrospinning device contains three major components: reservoir (including syringe pump, syringe, and syringe needle), conductive collector (stationary or rotating), and high-voltage power supply which can be either alternating current or direct current as demonstrated in Figure 1.^48,79

Figure 1.

Schematic diagram of the electrospinning apparatus with inset of Taylor cone.⁴⁸

A polymer solution or melt is extruded through a tinny nozzle to form a liquid droplet as a result of surface tension. When the voltage is applied between the needle and collecting electrode, an electric field will be produced resulting in accelerating and elongating an electrically charged polymer jet to form Taylor cone at the needle syringe top, subsequently ejecting the jet from the cone toward the collector. When the jet flying toward the collector, it stretches and whips while solvent(s) evaporates resulting in forming fibers on the collector.⁸⁰

It is worth mentioning that the structure of electrospun nanofibers and their properties can be affected by (I) the working parameters of electrospinning process represented by solution parameters (molecular weight,^54,81 polymer concentration,^56,82 viscosity,^83,84 surface tension,^56,85 and conductivity^86,87), ambient parameters (temperature and relative humidity),^49,88 and processing parameters (applied voltage,^76,89,90 flow rate,^76,91 distance between the tip of the needle and the collector (DTC),^76,92 diameter of the needle,^76,93 and collector type^94-96), (II) constituents of composite or hybrid nanofibers,^97-99 (III) post-processing treatments such as heat treatment, chemical treatment which enable the production of fibers without the need to change the spinning parameters, leading to modify the surface morphology and properties of materials,^100-103 and (IV) electrospinning types which are grouped mainly to electrospinning based on basic needle and needleless electrospinning.^80,104,105

Generating of the branched nanofibers

The branched structure has been attracting the attention of researchers due to its outstanding properties. Therefore, this structure has been serving in different fields, such as oil cleanup, air filtrations, energy harvesting, sensors, scaffolds, energy storage, and other industries. Here, the materials, formation methods, and properties of the branched nanofibers are reviewed (Table 1).

Table 1.

Summary of the previously reported materials, formation methods, properties, and applications of the branched nanofibers.

Materials	Formation methods	Properties	Applications	Ref
Te	Electrospinning and GDR	Excellent sensing performance toward NO₂	Sensors for NO₂	¹⁰⁶
pI/PAN blend	Electrospinning	Strong hydrophobicity	Nano-coating, nano composite, nano reinforcement, nano medicine, and drug release	¹⁰⁷
TiO₂@SnO₂	Electrospinning and hydrothermal treatment	Large surface area, excellent cycling stability, and good rate capacity	Anode for lithium-ion batteries	¹⁰⁸
TiO₂/Carbon	Electrospinning and an alkali hydrothermal treatment	Large surface area, 1D structure, and flexible feature	Anode for SIBs and some other flexible electronic devices	¹⁰⁹
NBT/C@MoS₂	Electrospinning and hydrothermal treatment	Large surface area and enhance the electronic conductivity	Anode for sodium-ion batteries	¹¹⁰
TiO₂	Electrospinning and an alkali hydrothermal treatment	Excellent photocatalytic performance	Photocatalytic water splitting, energy storage, solar cells, and electronics	¹¹¹
PVDF	Electrospinning	Low cost, high specific surface area, good pore size, and tiny fiber diameter	Air filtration	⁶⁹
PVDF	Electrospinning	Low cost, high crystallinity, supreme F(β), excellent mechanical properties, and tiny fiber diameter	Energy harvesting	⁷³
PVDF	Electrospinning	Low cost, outstanding specific surface area, small pore size, and tiny fiber diameter	Oil-water separation	⁷⁰
Polyaniline	Chemical oxidative polymerization	Uniform nanofibers diameters and interconnected nanofibers	Chemical sensors or actuators, gas-separation membranes and neuron devices	¹¹³
Polyaniline	Solid-state reaction	Adjustable roughness and diameter of nanofibers	Chemical sensors or actuators, gas-separation membranes and neuron devices	¹¹⁴
Polyaniline	Solid-state reaction	Good processability and unique structure	Chemical sensors, electrode material, and novel conductive fillers in nanocomposites	¹¹⁶
Alumina	Calcination	High aspect ratio and interconnected nanofibers	Filler for highly thermoconductive materials.	⁷⁴
Carbon nanofibers	CVD	High yield, low operation temperature, high purity, and easy control of carbon nanostructure	Energy storage	⁷⁵
TiO₂	Interface-assisted fabrication approach	Enhanced photocatalytic performance	Nanoscale electronic and photonic devices	¹²⁰
Kevlar	Supercritical drying of 3D networks	Compressive strength and high heat insulation	Optics, insulation, biosensing, and acoustics	⁷²
α--MnO₂	Reflux method	Tiny branches	Information storage, optical computing, and microanalysis, gas sensors, and nanoscaled optoelectronic devices	¹²⁴

Formation methods

Branched nanofibers have been generated by different methods which are electrospinning, chemical oxidative polymerization, solid-state reaction, calcination, chemical vapor deposition method (CVD), interface-assisted synthesis, supercritical drying, and reflux.

Electrospinning

The widest method used for generating branched nanofibers is electrospinning owing to its outstanding advantages as mentioned in the previous section. Herein, various studies about generating electrospun branched nanofibers are reviewed.

Park et al.¹⁰⁶ described the synthesis of branched tellurium (Te) hollow nanofibers by the combination of electrospinning and galvanic displacement reaction (GDR) and demonstrated their excellent sensing performance toward nitrogen dioxide (NO₂) at room temperature (Figure 2). The morphologies (smooth or branched) and wall thickness of hollow nanofibers were precisely controlled by adjusting the electrolyte composition. They found that the branched Te hollow nanofibers show excellent sensing performance in terms of greater sensitivity, and faster response/recovery times toward NO₂ compared to smooth Te hollow nanofibers.

Figure 2.

(a) Schematic illustration of GDR of nickel nanofibers to form smooth or branched Te hollow nanofibers: electrospun Ni nanofiber (I) is galvanically displaced by Te (II). As the reaction continues, branched Te hollow nanofibers (III) or smooth Te hollow nanofibers (IV) are produced depending on the concentration of HTeO₂. SEM images of electrospun Ni nanofibers (b) and branched Te hollow nanofibers synthesized from 0.5 (c) and 1.0 (d) mM HTeO₂⁺. Smooth Te hollow nanofibers synthesized from 10 mM HTeO₂⁺ (e). Inset images display low magnification images of Te nanofibers.¹⁰⁶

Konno et al.¹⁰⁷ fabricated core/shell-like structured branched nanofibers by a phase separation process using simple polymer blend polyacrylonitrile (PAN) and polyimide (PI) solutions with electrospinning. They found that pI/PAN blend ratios (from 50/50 to 80/20) provided core/shell-like branched nanofiber structures. Moreover, they revealed that the applied voltage during the electrospinning process and the PAN content created different branching ratios in the blended nanofibers.

Zhu et al.¹⁰⁸ prepared hierarchically heterostructured titanium dioxide (TiO₂)@ stannic oxide (SnO₂) core-branch nanofibers by a combination of electrospinning and hydrothermal treatment (Figure 3(a)). The hydrothermally grown SnO₂ nanocubes have solid contact with the TiO₂ nanofibers, approaching the morphology of a string of beads (Figures 3(b)–(d)). Consequently, SnO₂ nanocubes on the surface of the TiO₂ nanofibers have the ability to provide a larger surface area for lithium insertion/extraction which could lead to large reversible capacity, while the backbone of TiO₂ nanofibers can sustain the structure stability during the cycling.

Figure 3.

(a) Schematic diagram of the synthesis process of hierarchically heterostructured TiO2@SnO2 core-branch nanofibers. SEM images of b) 10 wt% TiO₂@SnO₂ nanofibers, c) 20 wt %TiO₂@SnO₂ nanofibers, d) 30 wt %TiO₂@SnO₂ nanofibers.¹⁰⁸

Wang et al.¹⁰⁹ reported a rational design of free-standing branched TiO₂/carbon nanofibers generated via electrospinning and an alkali hydrothermal treatment method. The as-prepared branched TiO₂/carbon nanofibers are well performed with a flexible feature, large surface area, and 1D structure which are beneficial for pseudocapacitive contribution as well as ion and electron transfer (Figure 4). The in situ fabrication provides the materials with strong structural stability, as well as carbon can increase the electrical conductivity of the branched TiO₂/carbon nanofibers electrode and suppress the agglomeration of TiO₂ nanoparticles.

Figure 4.

(a) Schematic illustration of the synthesis process of the branched TiO₂/carbon nanofibers. b, c) SEM images of branched TiO₂/carbon nanofibers with different magnifications, respectively.¹⁰⁹

Wang et al.¹¹⁰ also demonstrated the design and synthesis of a novel architecture of expanded molybdenum disulfide (MoS₂) nanosheets grown on nitrogen-doped branched TiO₂/carbon nanofibers (NBT/carbon@MoS₂) by using electrospinning subsequently with hydrothermal treatment (Figure 5). Their design has many advantages. i) The branched TiO₂/carbon nanofibers can not only function as the mechanical backbone and conductor medium to enhance the electronic conductivity of MoS₂ but also afford a large surface area to grow MoS₂; ii) the TiO₂ in the NBT/carbon @MoS₂ nanofibers can limit sulfur dissolution issue during cycling, whereas the doping of nitrogen can facilitate the transfer of both Na+ ions and electrons; iii) the contribution of extrinsic pseudocapacitance also enhances the kinetics.

Figure 5.

(a) Schematic illustration of the preparation process of NBT/carbon @MoS₂ nanofibers and NT/carbon @MoS₂ nanofibers. SEM images of (b,c) nitrogen-doped TiO₂/carbon nanofibers, (d,e) nitrogen-doped branched TiO₂/carbon nanofibers, (f,g) NT/carbon @MoS₂ nanofibers, and (h,i) NBT/carbon @MoS₂ nanofibers at different magnifications.¹¹⁰

Nasir et al. ¹¹¹ prepared a 1D nanofiber of TiO₂ by electrospinning and formed a branched structure on the fiber by following the alkali hydrothermal method (Figure 6). The CVD was adopted, first to load g-C₃N₄ QDs then nanoflakes of tin (II) selenide (SnSe₂) on the branched fiber of TiO₂, and make a strong heterojunction. The composite exhibited excellent photocatalytic performance by producing hydrogen about 2375 μmol g⁻¹.h⁻¹ having a quantum efficiency of HER more than 16% at 420 nm. The photoluminescence, time decay fluorescent spectra, and photoelectrochemical results approved that SnSe₂ not only reduces the charge recombination by increasing the transfer of electrons but also provides an active site for hydrogen production as a co-catalyst.

Figure 6.

(a) Schematic illustration of the synthesis method for hierarchically branched TiO₂ nanofibers/Cn/SnSe₂. FESEM Images b) uniform and smooth as-electrospun TiO₂ nanofibers; c) hierarchical branched TiO₂ fiber after alkali hydrothermal treatment; d) branched TiO₂ fiber after decorating graphitic-carbon nitride quantum dots (g-C₃N₄ QDs) on it, inset is the magnified image of the same sample; e) the hairy structure having g-C₃N₄ QDs and also SnSe₂ nanoflakes, inset is the magnified image of the same sample in which nanoflakes can be seen on branches.¹¹¹

Zaarour et al.⁶⁹ fabricated a polyvinylidene fluoride (PVDF) branched nanofiber web with a diameter of less than 50 nm via one-step electrospinning (Figure 7). The branched structure was produced by adding a Tetrabutylammonium chloride (TBAC) to 10% PVDF/(ACE/DMF) solution at the solvent ratio of 1:2 with an applied voltage of 18 kV, tip to collector distance of 18 cm, flow rate of 1.5 mL/h, and RH of 10%. The formation mechanism of the branched structure was attributed to the presence of TBAC which could be dissolved easily in the PVDF solution and enhances the electrical conductivity of the solution. When the density of the jet’s charges exceeds a certain threshold value, the electric forces overcome the surface tension resulting in the schism of the jet. Moreover, the TBAC decreased the forces between the PVDF molecules thanks to its space steric structure and accordingly serving for the schism of the jet. The results proved that the branched structure increases significantly the specific surface area and reduces the pore size of the nanofiber webs, resulting in enhancing the ability for catching the small particles. In other words, this structure has the ability to serve successfully in the field of air filtration.

Figure 7.

(a) Schematic diagram illustrating the electrospinning process of branched fibers. SEM images of (b) 15% PVDF/(ACE/DMF), (c) 10% PVDF/(ACE/DMF), (d) 10% PVDF/(ACE/DMF)-TBAC.⁶⁹

Zaarour et al.⁷³ also electrospun directly branched PVDF nanofibers with a diameter of less than 40 nm. The branched structure was formed by adding 0.2 mol/L TBAC to 8% PVDF/(THF/DMF: 1/2) solution at the relative humidity of 10%. The results showed that the branched structure has high crystallinity, supreme β phase content (F(β)), and excellent mechanical properties. Therefore, this structure can be used successfully in the field of energy harvesting.

Zaarour et al.⁷⁰ also generated branched PVDF nanofibers directly with a diameter of less than 35 nm via one-step electrospinning. The branched structure was formed by adding 0.2 mol/L TBAC to 8% (THF/DMF: 1/2) PVDF at a temperature of 20°C, relative humidity of ∼10%, applied voltage of 18 kV, and flow rate of 1.5 ml/h. The results of N₂ physical adsorption-desorption isotherms showed that the branched PVDF nanofibers sorbent has an outstanding specific surface area of 70.98 ± 8.45 m²/g. As a result, this structure can be served successfully in the field of oil cleanup.

Chemical oxidative polymerization

This method is often classified as a kind of polycondensation since chain growth is complemented by the formation of low-molecular products. It is used for the synthesis of polymeric (oligomeric) materials from different classes of monomers.¹¹²

Li et al.¹¹³ demonstrated a practical and simple route to the synthesis of branched polyaniline nanofibers with diameters between 60 and 90 nm by chemical oxidative polymerization of aniline in a surfactant gel, which was obtained using a mixture of hexadecyltrimethylammonium chloride (C₁₆TMA), acetic acid, aniline, and water at −7°C (Figure 8). Moreover, they investigated the effects of the concentration of C₁₆TMA, dopant, and solvent on the morphologies of polyaniline. They found that there is a positive relationship between the concentration of C₁₆TMA and the lengths of the polyaniline branches, while dopant has no obvious influence on the morphologies of polyaniline. Furthermore, the solvent used has a big effect on the diameter of branched nanofibers.

Figure 8.

SEM images of dendritic polyaniline nanofibers synthesized with 0.3 mol/L C₁₆TMA at −7°C in acetic acid: (a, b) at low magnification and (c) at high magnification. 2. (d) SEM image of dendritic polyaniline nanofibers synthesized with 0.3 mol/L C₁₆TMA at −7°C in ethanol solution.¹¹³

Solid-state reaction

The solid-state chemical reaction is considered an efficient, simple, surfactant-free, and convenient chemical reaction method that has been widely used to prepare various nanomaterials. Since the liquid monomer aniline forms solid salts with mineral acids, for example, aniline hydrochloride (mp. 196–198°C), it is possible to use room temperature solid-state polymerization of a solid anilinium salt to produce polyaniline nanomaterials.¹¹⁴

Du et al.^114,115 demonstrated a facile solid-state mechanochemical route for branched polyaniline nanofibers without using any surfactants, organic acid, and additional templates. They prepared branched polyaniline nanofibers with a solid-state reaction simply between the monomer and oxidants (Figure 9(a)–(d)). They found that the formation of branched nanofibers is possibly related to the mechanochemical oxidation polymerization process and the linear nature of the polyaniline molecule chains. It is worth mentioning that the structure of products depends on the rate of nucleation and growth of the ultimate products in the solid-state reaction.

Figure 9.

(a) TEM image and (b) SEM image of the products after 7 days.¹¹⁵ (c) TEM image and (d) SEM image of the products obtained after reaction for 24 h at 25°C.¹¹⁴ (e) TEM image and (f) SEM image of the products obtained after grinding for 40 min.¹¹⁶

Zhou et al.¹¹⁶ synthesized branched polyaniline nanofibers with diameters of 15–40 nm and lengths about several hundred nanometers using iron(III) chloride (FeCl₃) as the oxidant by solid-state reaction method. The effects of reaction time and concentration of the oxidant (Figure 9 (e)–(f)). They studied the effect of the reaction time and concentration of the oxidant on the morphology of products. The results showed that there is no relationship between the reaction time and the morphology of products, while the concentration of the oxidant has little effect on it.

Calcination

Calcination means heating a solid chemical compound to high temperatures in the absence or limited supply of air or oxygen to remove volatile substances or impurities and/or to incur thermal decomposition.¹¹⁷

Shimazaki et al.⁷⁴ fabricated a branched alumina nanofiber filler by the calcination of an alumina precursor, aluminum isopropoxide, adsorbed onto a paper filter (Figure 10).

Figure 10.

(a) SEM image of alumina nanofiber filler with a branched structure. (b) Cross-sectional SEM images of nanocomposites prepared by curing epoxy resin with sheet-shaped filler and (c) branched filler dispersed in resin.⁷⁴

The branched nanofiber filler is advantageous as the constituent nanofibers are interconnected with each other and have a high aspect ratio. This leads to an increase in the number of junctions among nanofibers resulting in yielding more thermoconductive path in the composite. Moreover, alumina is used a lot as an industrial material for thermoconductive filler owing to its high aspect ratio and its high thermoconductivity, which allows the creation of more thermoconductive path in the resin at a lower filler concentration. They report a high thermal conductivity of the polymer composite containing the branched nanofiber filler. They found that the branched structure of the nanofiber filler is effective for highly thermoconductive materials.

CVD

CVD is a vacuum deposition technique used to generate high-performance, high quality, and solid materials. Generally, this method is used in the field of semiconductor industry.¹¹⁸

Among the different techniques that have been applied for the synthesis of carbon nanofibers and carbon tubes, catalytic CVD appears the most promising because of low operation temperature, high yield, high purity, and easy control of carbon nanostructure by adjusting the process parameters. Generally, the CVD method requires a catalyst in the form of small particles on a solid support. The choice of a metallic catalyst, such as nickel, cobalt, or iron has been shown to influence the growth and morphology of the generated carbon nanomaterials.

Tao et al.⁷⁵ reported the synthesis of multi-branched carbon nanofibers with a porous structure using the catalyst Cu doped with the alkali-element (Li, Na, or K) via a simple thermal CVD method (Figure 11). They found that alkali-element doping plays a key role in this synthesis process.

Figure 11.

(a) Shows low magnification and (b) the high-magnification FESEM images of as-prepared carbon nanofibers. (c) The FESEM image of carbon nanofibers grown from a catalyst particle. (d) The TEM image of carbon nanofibers grown from the catalyst particle, the inset is the corresponding SAED pattern of the catalyst particle. (e) shows a high-magnification TEM image of carbon nanofibers, and (f) the high-resolution TEM image of the carbon nanofibers.⁷⁵

Interface-assisted synthesis

This method is a powerful emerging tool for the synthesis of conducting polymers on a large scale. Interface-assisted synthesis strategies offer effective nanostructure control in a confined two-dimensional space.¹¹⁹

Wang et al.¹²⁰ employed water–dichloromethane interface-assisted hydrothermal technique to grow rutile TiO₂ nanowires on electrospun anatase TiO₂ nanofibers, using highly reactive titanium tetrachloride (TiCl₄) as a precursor. The water–dichloromethane interface inhibited the generation of rutile nanowires in the water phase, however, promoted the selective radial growth of densely packed rutile nanowires on anatase nanofibers to produce a branched heterojunction (Figure 12). The length and density of rutile nanowires could be readily regulated by maneuvering reaction parameters. The formation mechanism of the branched heterojunction involved (1) the entrapment of rutile precursor nanoparticles at the water–dichloromethane interface, (2) the growth of rutile nanowires on anatase nanofibers via Ostwald ripening through the scavenging of interface-entrapped rutile nanoparticles. The heterojunction formed at anatase nanofiber and rutile nanowire improved the charge separation of both under ultraviolet excitation, as verified by photoluminescence and surface photovoltage spectra.

Figure 12.

(a) Scheme for the formation process of hyperbranched TiO₂ heterostructures. b-d) SEM image of different samples: (b) low magnification image of the sample which preheated for 2h (AR1); (c) high-magnification image of sample AR1; (d) low magnification image of the sample which preheated for 4h (AR2); (e) high-magnification image of sample AR2.¹²⁰

Supercritical drying

Supercritical drying is also called critical-point drying, supercritical extraction, and supercritical lyophilization. It is a method for transforming liquid in a substance into gas in the absence of capillary stress and surface tension. This technique is usually used to convert gels into aerogels. It can be said that this process is performed to replace the liquid in a material with a gas and isolate the solid component from the material without destroying the material’s delicate nanostructured pore network.^121,122

Li et al.⁷² prepared biomimetic aramid nanofibers aerogels made from branched aramid nanofibers derived from Kevlar with the structure replicating articular cartilage by supercritical drying of 3D networks held together by hydrogen bonds. Thanks to the branching morphology of the nanofibers, the 3D nanoscale networks with high interconnectivity and extensive percolation can be got (Figure 13). This structure-based aerogel exhibited high porosity, good thermal stability, high special surface area, and excellent thermal insulation.

Figure 13.

Scanning electron microscopy images of branched aramid nanofiber aerogels showing the effects of different exchange solvents: (A) deionized water, (B) no exchange solvent, (C) ethanol, and (D) n‐heptane.⁷²

Reflux

Reflux is a method involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations. It is also used in chemistry to supply energy to reactions over a long time.¹²³

Zheng et al.¹²⁴ synthesized pure multiple branched α -MnO₂ nanofibers based on the use of MnO⁻⁴ to oxidize Mn²⁺ under a weak acidic condition through a simple refluxing in the absence of surfactants or catalysts as templates (Figure 14). The results showed that these nanofibers were produced by two-step epitaxial coalescence of small primary nanofibers. Here in, the small nanofibers were merged through the (1 1 0) plane to procedure larger monocrystalline nanofibers, which then attached one by one through the two of (1 1 0) and (2 1 1) planes, resulting in the formation of multiply branched structures.

Figure 14.

TEM images of the samples prepared for different reaction periods (a) 1 h; (b) 3 h; (c) 5 h; (d) 10 h.¹²⁴

Applications of the branched nanofibers

Based on the special characters of branched nanofibers, they have received much consideration for use in various applications. Herein, different applications of branched nanofibers are reviewed.

Energy harvesting

Energy harvesting (also known as energy scavenging or power harvesting) is defined as capturing various amounts of energy from different naturally occurring energy sources, collecting them, and storing them for later use.¹²⁵ Electrospun branched piezoelectric nanofibers have shown great potential in the field of energy harvesting.

Zaarour et al.⁷³ proved that the branched PVDF structure has supreme F(β), high crystallinity, and excellent mechanical properties. Furthermore, the PENG based on the branched PVDF nanofiber web showed outstanding electrical outputs of ∼2.6 V and ∼3.1 μA due to its high friction area, high F(β), and small diameters (Figure 15). Since PVDF branched structure has many advantages, it has the ability to serve successfully in the field of energy harvesting.

Figure 15.

(a) Schematic diagram of the PENG. (b) Digital photo of the actual PENG. (c) Voltage output generated by the PENG based on different samples. (d) Current output generated by the PENG based on different samples.⁷³

Air filtration

Air filtration is a technique used widely to remove particles from an air stream owing to its relative ease and flexibility. Filters fabricated from nanofibers are the best choice for air filtration due to their excellent filtration capacity and improved service life in real operating environments.¹²⁶ Therefore, branched nanofibers have been shown great potential in the field of air filtration.

Zaarour et al.⁶⁹ found that PVDF branched nanofiber web with a diameter of less than 50 nm showed great potential in the field of air filtration. The results showed that the branched structure enhances significantly the specific surface area and decreases the pore size of the nanofiber webs, resulting in improving the ability for trapping the small particles. Moreover, the extremely tiny diameter of branched nanofibers (<10 nm) increases the van der Waals attractive forces between nanofibers and particles. As a result, the filtration performance of air filters can be enhanced according to the ultra-low penetration air standard (the level of U15) (Figure 16).

Figure 16.

(a) Filtration efficiency, (b) pressure drop, and (c) quality factor of electrospun webs at different basis weights of webs. (S1, S2, S3 represent beaded-free nanofibers, beaded nanofibers, branched nanofibers, respectively).⁶⁹

Oil cleanup

Oil spills pose a significant hazard to the environment in general, and the marine ecosystem in particular. This pollution is due to human activity such as releasing oil to coastal waters or oceans from drilling rigs and wells, offshore platforms, tankers, etc. Studies proved that sorbents fabricated from nanofibers are the best choice for oil cleanup owing to their high specific surface area.¹²⁷ Therefore, branched nanofibers are in great demand for researchers interested in the field of oil cleanup.

Zaarour et al.⁷⁰ studied the oil absorption capacity of PVDF sorbent based on the branched structure. This sorbent showed an outstanding oil absorption capacity of 93 ± 3.2 g/g, 122 ± 7.3 g/g, and 149 ± 10.1 g/g for olive oil, motor oil, and silicon oil, respectively, owing to its high specific surface area (Figure 17). These results are ascribed to the small diameter of fibers generated and the existence of branches with a tiny diameter (∼5 nm) which allows ensnaring oil droplets between them.

Figure 17.

(a) Specific surface area of samples M1, M2, and M3. (b) The nitrogen adsorption isotherms of the samples M1, M2, and M3. c) Pictures of oil absorption. (I) 50 mL of a water-oil mixture (1:1) without sorbent, (II) during absorption, (III) during draining. d) Oil absorption capacities of the samples M1, M2, and M3. (M1, M2, M3 represent beaded-free nanofibers, beaded nanofibers, branched nanofibers, respectively).⁷⁰

Energy storage

Energy storage is defined as capturing the energy generated at one time for use at a later time to reduce disparities between energy production and energy demand. Devices that store energy are called batteries or accumulators.^128,129

Tao et al.⁷⁵ prepared successfully branched carbon nanofibers with porous structure via a simple thermal CVD method. They used their design as polarized electrodes, as a result, electrochemical double-layer capacitors with a remarkable specific capacitance of ca. 297 F/g was obtained with 6 M KOH as the electrolyte. They found that carbon nanofibers have great potential in energy storage applications.

Meng et al.¹³⁰ electrodeposited branched PANI nanofibers with a diameter of 100 nm on Ti mesh substrate and molybdenum trioxide (MoO₃₎ nanobelts with a width of 30–700 nm were obtained by the hydrothermal reaction method in an autoclave. To improve the electrochemical performance of the electrode nanomaterials, a redox-active electrolyte containing 0.1 M Fe^2+/3+ redox couple was adopted. As a result, the PANI electrode showed a great capacitance of 3330 F g⁻¹ at 1 A g⁻¹ in 0.1 M Fe2+/3+/0.5 M H₂SO₄ electrolyte. The as-assembled asymmetric supercapacitor (ASC) achieved an outstanding energy density of 54 Wh kg⁻¹ at a power density of 900 W kg⁻¹. Furthermore, it displayed high cycle stability, and its capacitance even improved to 109% of the original value after 1000 charge-discharge cycles (Figure 18).

Figure 18.

(a) Galvanostatic charge/discharge curves of ASC at different current densities; (b) specific capacitance of ASC in different electrolytes; (c) capacitance retention and the columbic efficiency of 1000 cyclic test; and, (d) electrochemical impedance spectroscopy spectra before and after cycling test.¹³⁰

Wang et al.¹⁰⁹ prepared hierarchically branched TiO₂/carbon nanofibers through a facile in situ synthesis route including electrospinning followed by an alkali hydrothermal treatment.

The branched TiO₂/carbon nanofibers’ electrode delivers a stable capacity of 283.5 mAh g⁻¹ at a current density of 200 mA g⁻¹ after 1000 cycles and can still achieve 204.1 mAh g⁻¹ even when the current density is raised to a very high rate of 2000 mA g⁻¹ (Figure 19). They found that the free-standing hierarchically branched TiO₂/carbon nanofibers can be served as anode materials for sodium-ion batteries (SIBs) and some other flexible electronic devices.

Figure 19.

Electrochemical performance of the BT/carbon nanofibers anode: (a) CV curves at a scan rate of 0.5 mV s⁻¹. (b) Discharge/charge voltage profiles at a current density of 200 mA g⁻¹ (c) Cycling performance at 200 mA g⁻¹ and corresponding coulombic efficiency. (d)–(e) Charge/discharge profiles and rate performance obtained at various current densities.¹⁰⁹

Wang et al.¹¹⁰ fabricated NBT/carbon@MoS₂ nanofibers using electrospinning followed by hydrothermal treatment. They found that the NBT/carbon@MoS₂ nanofibers deliver not only ultralong cycle stability (448.2 mAh g⁻¹ after 600 cycles at 200 mA g⁻¹) but also superior rate performance (258.3 mAh g⁻¹ at 2000 mA g⁻¹) (Figure 20). Therefore, the NBT/carbon @MoS₂ nanofibers can be used as promising anode materials for high-performance SIBs.

Figure 20.

Sodium storage performance of NBT/carbon @MoS₂ nanofibers. (a) CV curves at a scan rate of 0.5 mV s−1, (b) galvanostatic charge/discharge voltage profiles at a current density of 200 mA g−1, (c) cycling performance of NBT/carbon @MoS₂ nanofibers in comparison with the NT/carbon @MoS₂ nanofibers at a current density of 200 mA g−1, (d) charge/discharge profiles, and (e) rate performance obtained at various current densities.¹¹⁰

Zhu et al.¹⁰⁸ synthesized successfully hierarchically branched TiO₂@SnO₂ nanofibers with tunable TiO₂/SnO₂ mass ratios via electrospinning followed by hydrothermal treatment. Their results showed the TiO2@SnO2 core-branch nanofibers with 30 wt% TiO2 exhibit both good rate capacity (378.3 mAh g⁻¹ at a current density of 2.0 A g⁻¹) and excellent cycling stability (453.3 mAh g⁻¹ after 100 cycles at a current density of 500 mA g⁻¹), suggesting they can be used as promising anodes for high-performance lithium-ion batteries (Figure 21).

Figure 21.

(a) Cycling performances of the ST1, ST2, and ST3 electrodes at a current density of 500 mA g⁻¹ for 100 cycles. (b) Rate performances of the ST1, ST2, and ST3 electrodes at different current densities from 0.1 to 2 A g⁻¹. (ST1, ST2, and ST3 represent composites consisting of 8 wt%, 18 wt%, and 33 wt% of TiO2, respectively.¹⁰⁸

Photocatalytic

This phenomenon is known as photocatalysis. Photocatalysis is a phenomenon, in which an electron-hole pair is generated on exposure of a semiconducting material to light. Photocatalysts are materials that alter the rate of a chemical reaction on exposure to light.¹³¹

Wang et al.¹²⁰ provided a facile way to achieve controlled growth of rutile TiO₂ nanowires over anatase nanofibers. The liquid–liquid interface promotes entrapment of precursor nanoparticles on the surface of nanofibers to yield ordered hyperbranched TiO₂ heterostructures. Such unique heterostructures display higher photocatalytic performance in comparison with pure anatase TiO₂ nanofibers and can be easily recycled. Branched TiO₂ heterostructures show improved photocatalytic performance in comparison with the mixture of anatase TiO₂ nanofibers and rutile nanowires, because of the synergistic effect by anatase/rutile heterojunction. Moreover, such branched TiO₂ heterostructures are expected to open new opportunities for the fabrication of nanoscale electronic and photonic devices owing to their intrinsic complexity and dimensionality.

Nasir et al.¹¹¹ reported the fabrication of g-C₃N₄ QDs on the branched TiO₂ fiber by a CVD method. The pure SnSe₂ nanoflakes were also grown over this composition by the CVD method. They proved that SnSe₂ nanoflakes were grown successfully and the synergetic effect of SnSe₂ as a co-catalyst effectively decreases the charge recombination and provides an active site for the photocatalytic hydrogen performance (Figure 22).

Figure 22.

(a) Photocatalytic hydrogen performance of sample carbon nitride (CN), hierarchical branched TiO₂ fiber after alkali hydrothermal treatment (HBT), HBTCN, HBT/CN/SnSe₂, error bars are based on average data of three readings, (b) Cyclic test of the hydrogen performance of sample HBT/CN/SnSe₂ up to 30 h.¹¹¹ (16).

Heat resistance

Aerogels are acknowledged for low thermal conductivity, high porosity, large specific surface area, and low density. These properties make them favorable for applications that required thermal insulation. The performance of hydrogels can be enhanced when they are in the nanoscale. Consequently, the branched structure has shown great potential in this field.

Li et al.⁷² reported on biomimetic branched aramid nanofibers aerogels with the structure replicating articular cartilage, prepared by supercritical drying of 3D networks held together by hydrogen bonds. The aerogels exhibited high porosity with an average open pore size of 21.5 nm and a correspondingly low specific density of 0.0081 g/cm³. In addition, the aerogels showed a high compressive strength of 825 kPa at a strain of 80%. Because of the unique aramid chemistry of the parent nanofibers, aramid aerogels combine low thermal conductivity of 0.026 W/m•K with high thermal stability up to 530°C, which is unusually high for polymeric and composite materials of any type, opening a broad range of applications from electronics to space travel (Figure 23).

Figure 23.

Thermal conductivity of branched aramid nanofiber aerogels. (a) Thermal conductivity of branched aramid nanofibers aerogel 55 wt%. (b) Thermal conductivity of branched aramid nanofibers aerogel 50 wt%. (c) Thermal conductivity of branched aramid nanofibers aerogel 45 wt%. (d) Relationship between the concentrations of branched aramid nanofibers and Brunauer–Emmett–Teller method thermal conductivity in which the q″ of the Y-axis in (A), (B), and (C) mean the heat flux density. (e) branched aramid nanofibers aerogel was baked with a lighter flame for 5 min f, (g) Side view of aerogel.⁷²

Other applications

Branched nanofibers can also be used in many applications such as nano-coating, nano reinforcement, nanocomposite, nanomedicine, drug release, sensors for NO₂, scaffolds, electronics, chemical sensors or actuators, gas-separation membranes, and neuron devices.^{72,106,107,113,114,116,130}

Summary and future outlook

This paper reviews the materials, strategies, and applications of the branched nanofibers. A brief introduction of the branched nanofibers, material types, formation methods, characterizations, and applications of the branched fibers are discussed in detail. Based on our discussion, it can be concluded that:

1. A wide variety of materials can be used for producing a branched structure such as polyaniline, aramid, alumina, PI, PAN, TiO2/carbon, carbon, TiO2, a-MnO₂, Te, TiO₂@SnO₂, PANI, PVDF.

2. Branched nanofibers can be obtained by different methods (electrospinning, chemical oxidative polymerization, solid-state reaction surfactant, calcination, CVD, interface-assisted synthesis, supercritical drying, and reflux).

3. Branched nanofibers have amazing properties (e.g., high surface area, tiny diameter, good piezoelectricity, excellent biological properties, outstanding mechanical properties, large porosity, excellent thermal insulation, and good thermal stability).

4. Branched nanostructure can serve successfully in many fields based on the material type (e.g., energy harvesting, air filtration, oil-water separation, energy storage, photocatalytic, nano-coating, nanocomposite, nano reinforcement, nanomedicine, scaffolds, drug release, and sensors for NO₂, electronics).

In recent years, the branched structure has shown brilliant attainments in fundamental understanding and technological enhancements. About the future applications of branched structure, some vital issues need to be addressed:

1. Studying the effect of the formation methods on the branches degree and properties of the branched nanofibers.

2. Exploring the relationships between the material type and the branches degree as well as properties of the branched nanofibers.

3. Innovating new methods that can increase the branches degree of the nanofibers, resulting in enhancing their property.

4. Finding new applications of the branched nanofibers.

In summary, the branched structure of nanomaterials has the chance to be one of the promising demands of commercial companies. We hope that our dream about the golden age of the branched nanofibers will come to fruition soon.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Bilal Zaarour

References

Tahir

Sagir

Asiri

. Nanomaterials: Synthesis, Characterization, Hazards and Safety. Amsterdam, Netherland: Elsevier, 2021.

Tanabe

. Nanostructured Materials for Solar Cell Applications. NanoMater 2022; 12: 26.

Krishnamoorthy

. Novel Nanomaterials. London, UK: IntechOpen, 2021.

Baig

Kammakakam

Falath

. Nanomaterials: a review of synthesis methods, properties, recent progress, and challenges. Mater Adv 2021; 2: 1821–1871.

Haider

. Electrospinning and Electrospraying: Techniques and Applications. London, UK: IntechOpen, 2019.

Parmar

Patel

Bhatia

. Membrane Techniques for the Preparation of Nanomaterials. Emerging Technologies for Nanoparticle Manufacturing. Berlin, Germany: Springer, 2021, pp.349–361.

Jiang

Han

, et al. Ultrafast synthesis for functional nanomaterials. Cell Rep. Phys. SCI 2021; 2: 100302.

Yang

Pan

Sun

, et al. Fabrication of polymer composite fibers embedding ultra-long micro/nanowires. Nanomater. 2021; 11: 939.

Gao

Wang

Liu

, et al. High mechanical performance based on the alignment of cellulose nanocrystal/chitosan composite filaments through continuous coaxial wet spinning. Cellul. 2021: 1–14.

10.

Pan

Leng

Deng

, et al. Recording the self-assembly behavior of nanomaterials directed by hydrogen bonding. Cryst Growth Des 2021; 21: 2187–2195.

11.

Zhu

Zaarour

Jin

. Fabrication of perfect CMCS/PVA nanofibers for keeping food fresh via an in situ mixing electrospinning. Mater Res Express. 2019;7: 015008–015017.

12.

Dai

Liu

Zhang

, et al. Electrospinning polyacrylonitrile/graphene oxide/polyimide nanofibrous membranes for high-efficiency PM2. 5 filtration. Sep Purif Technol 2021; 276: 119243.

13.

Zhu

Zaarour

Jin

. Unexpectedly high oil cleanup capacity of electrospun poly (vinylidene fluoride) fiber webs induced by spindle porous bowl like beads. Soft Mater.2019; 17:410–417.

14.

Wang

, et al. Advanced electrospun hydrogel fibers for wound healing. Composites B: Eng, 223. 2021: 109101.

15.

Zhao

Zhang

, et al. Lightweight and flexible electrospun polymer nanofiber/metal nanoparticle hybrid membrane for high-performance electromagnetic interference shielding. NPG Asia Mater. 2018; 10: 749–760.

16.

Xue

Zhang

Zheng

, et al. Flexible piezoelectric device directly assembled through the continuous electrospinning method. Smart Mater Structures 2021; 30: 045006.

17.

Ameer

Kasoju

. Strategies to tune electrospun scaffold porosity for effective cell response in tissue Engineering. J Funct Biomater 2019; 10: 30.

18.

Zhou

Gong

, et al. Waterborne electrospinning of fluorine-free stretchable nanofiber membranes with waterproof and breathable capabilities for protective textiles. J Colloid Interf Sci 2021; 602: 105–114.

19.

Al-Enizi

Zagho

Elzatahry

. Polymer-based electrospun nanofibers for biomedical applications. Nanomaterials. 2018; 8: 259.

20.

Alturki

. Rationally design of electrospun polysaccharides polymeric nanofiber webs by various tools for biomedical applications: a review. Int J Biol Macromolecules 2021; 184: 648–665.

21.

García-Mateos

Ruiz-Rosas

Rosas

, et al. Controlling the composition, morphology, porosity, and surface chemistry of lignin-based electrospun carbon materials. Front Mater 2019; 6.

22.

Tang

Wang

, et al. Understanding the cellular responses based on low-density electrospun fiber networks. Mater Sci Eng C 2021; 119: 111470.

23.

Mochane

Motsoeneng

Sadiku

, et al. Morphology and properties of electrospun pcl and its composites for medical applications: a mini review. Appl Sci 2019; 9: 2205.

24.

Liu

Peng

, et al. Electrospinning preparation of perylene-bisimide-functionalized graphene/polylactic acid shape-memory films with excellent mechanical and thermal properties. New J Chem 2021; 45: 772–779.

25.

Herrero-Herrero

Gómez-Tejedor

Vallés-Lluch

. PLA/PCL electrospun membranes of tailored fibres diameter as drug delivery systems. Eur Polym J 2018; 99: 445–455.

26.

Meng

Gao

, et al. An oil‐contamination‐resistant PVP/PAN electrospinning membrane for high‐efficient oil-water mixture and emulsion separation. J Appl Polym Sci 2021; 138: 50043.

27.

Zaarour

Zhu

Huang

, et al. A review on piezoelectric fibers and nanowires for energy harvesting. J Ind Textiles 2021; 51: 297–340.

28.

Guan

, et al. Breathable, washable and wearable woven-structured triboelectric nanogenerators utilizing electrospun nanofibers for biomechanical energy harvesting and self-powered sensing. Nano Energy 2021; 80: 105549.

29.

Zaarour

. Enhanced piezoelectricity of PVDF nanofibers via a plasticizer treatment for energy harvesting. Mater Res Express. 2021; 8: 125001.

30.

S-M

Tiwari

, et al. ZnO/Ag nanoparticles incorporated multifunctional parallel side by side nanofibers for air filtration with enhanced removing organic contaminants and antibacterial properties. Colloids Surf A: Physicochemical Eng Aspects 2021; 621: 126564.

31.

Topuz

Kilic

Durgun

, et al. Fast-dissolving antibacterial nanofibers of cyclodextrin/antibiotic inclusion complexes for oral drug delivery. J Colloid Interf Sci 2021; 585: 184–194.

32.

Serafin

Murphy

Rubio

, et al. Printable alginate/gelatin hydrogel reinforced with carbon nanofibers as electrically conductive scaffolds for tissue engineering. Mater Sci Eng C 2021; 122: 111927.

33.

Zhang

Ouyang

, et al. Electroactive electrospun nanofibers for tissue engineering. Nano Today 2021; 39: 101196.

34.

Zare

Davoodi

Ramakrishna

. Electrospun shape memory polymer micro-/nanofibers and tailoring their roles for biomedical applications. Nanomaterials. 2021; 11: 933.

35.

Dai

, et al. Chitosan/polydopamine layer by layer self-assembled silk fibroin nanofibers for biomedical applications. Carbohydr Polym. 2021; 251: 117058.

36.

Pandey

Pharmaceutical and biomedical applications of cellulose nanofibers: a review. Environ Chem Lett 2021: 1–13.

37.

Venault

Chin

Y-T

Maggay

, et al. Poly(vinylidene fluoride)/poly(styrene-co-acrylic acid) nanofibers as potential materials for blood separation. J Membr Sci 2022; 641: 119881.

38.

Karagoz

Kiremitler

Sarp

, et al. Antibacterial, antiviral, and self-cleaning mats with sensing capabilities based on electrospun nanofibers decorated with ZnO nanorods and Ag nanoparticles for protective clothing applications. ACS Appl Mater Inter. 2021; 13: 5678–5690.

39.

Chen

Z-C

Chang

T-L

Liou

D-S

, et al. Fabrication of a bio-inspired hydrophobic thin film by glutaraldehyde crosslinking electrospun composite self-cleaning nanofibers. Mater Lett 2021; 298: 129975.

40.

Korotcenkov

. Electrospun metal oxide nanofibers and their conductometric gas sensor application. part 1: nanofibers and features of their forming. Nanomaterials. 2021; 11: 1544.

41.

Abdel Rahman

Greish

Mahmoud

, et al. Fabrication and characterization of cellulose acetate-based nanofibers and nanofilms for H2S gas sensing application. Carbohydr Polym 2021; 258: 117643.

42.

Yan

Huang

Zhang

, et al. Facile synthesis of bimetallic fluoride heterojunctions on defect-enriched porous carbon nanofibers for efficient ORR catalysts. Nano Lett. 2021; 21: 2618–2624.

43.

Khandavalli

Sharma-Nene

Kabir

, et al. Toward optimizing electrospun nanofiber fuel cell catalyst layers: polymer-particle interactions and spinnability. ACS Appl Polym Mater 2021; 3: 2374–2384.

44.

Cui

, et al. Multistructured electrospun nanofibers for air filtration: a review. ACS Appl Mater Inter 2021; 13: 23293–23313.

45.

Mamun

Blachowicz

Sabantina

. Electrospun nanofiber mats for filtering applications-technology, structure and materials. Polymers. 2021; 13: 1368.

46.

Aman Mohammadi

Ramezani

Hosseini

, et al. Electrospun antibacterial and antioxidant zein/polylactic acid/hydroxypropyl methylcellulose nanofibers as an active food packaging system. Food Bioproc Technology, 14 2021: 1529–1541. DOI: 10.1007/s11947-021-02654-7.

47.

Min

Sun

Zhou

, et al. Electrospun pullulan/PVA nanofibers integrated with thymol-loaded porphyrin metal−organic framework for antibacterial food packaging. Carbohydr Polym, 270 2021: 118391.

48.

Huang

Thomas

. Fabricating porous poly (lactic acid) fibres via electrospinning. Eur Polym. J 2018; 99: 464–476.

49.

Zaarour

Zhu

Huang

, et al. Controlling the secondary surface morphology of electrospun pvdf nanofibers by regulating the solvent and relative humidity. Nanoscale Res Lett 2018; 13: 285–296.

50.

Wang

Zeng

, et al. Fabrication of Electrospun polymer nanofibers with diverse morphologies. Molecules. 2019; 24: 834.

51.

Zaarour

Zhu

Huang

, et al. A mini review on the generation of crimped ultrathin fibers via electrospinning: Materials, strategies, and applications. Polym Adv Tech 2020; 31: 1449- 1462.

52.

Pavlova

Bagrov

Monakhova

, et al. Tuning the properties of electrospun polylactide mats by ethanol treatment. Mater Des 2019; 181: 108061.

53.

Zaarour

Zhu

Huang

, et al. Fabrication of a polyvinylidene fluoride cactus-like nanofiber through one-step electrospinning. RSC Adv. 2018; 8: 42353–42360.

54.

Zaarour

Zhu

Jin

. Controlling the surface structure, mechanical properties, crystallinity, and piezoelectric properties of electrospun PVDF nanofibers by maneuvering molecular weight. Soft Mater. 2019; 17: 181–189.

55.

Damato

Puhl

Ziemba

, et al. Exploring the effects of electrospun fiber surface nanotopography on neurite outgrowth and branching in neuron cultures. PLOS One 2019; 14: e0211731.

56.

Zaarour

Zhang

Zhu

Jin

X. Y.

Huang

Maneuvering surface structures of polyvinylidene fluoride nanofibers by controlling solvent systems and polymer concentration. Textile Res J 2019; 89: 2406–2422.

57.

Zhang

Weng

Liu

, et al. Facile fabrication and characterization on alginate microfibres with grooved structure via microfluidic spinning. R Soc Open Sci 2019; 6: 181928.

58.

Walsh

Karperien

Dabiri

SMH

, et al. Fiber-based microphysiological systems: a powerful tool for high throughput drug screening. Microphysiological Syst 2019; 3: 14.

59.

Zhang

Zaarour

Zhu

, et al. A comparative study of electrospun polyvinylidene fluoride and poly (vinylidenefluoride-co-trifluoroethylene) fiber webs: mechanical properties, crystallinity, and piezoelectric properties. J Eng Fibers Fabr 2020; 15: 1–8.

60.

Nakashima

Fukushima

Hyuga

. Fiber template approach toward preparing one-dimensional silica nanostructure with rough surface. Adv Powder Technology 2021; 32: 1099–1105.

61.

Xiao

Wang

Zhu

, et al. Development of tree-like nanofibrous air filter with durable antibacterial property. Separation Purif Technology 2021; 259: 118135.

62.

Liu

Wang

, et al. Preparation and characterization of novel thin film composite nanofiltration membrane with PVDF tree-like nanofiber membrane as composite scaffold. Mater Des 2020; 196: 109101.

63.

Stanishevsky

Wetuski

Yockell-Lelièvre

. Crystallization and stability of electrospun ribbon- and cylinder-shaped tungsten oxide nanofibers. Ceramics Int 2016; 42: 388–395.

64.

Guo

Wang

, et al. Fabrication and photocatalytic activity of LaFeO 3 ribbon‐like nanofibers. J Chin Chem Soc 2020; 67: 990–997.

65.

Zheng

Gao

Jiang

. Directional adhesion of superhydrophobic butterfly wings. Soft Matter 2007; 3: 178–182.

66.

Hastuti

Kusumaatmaja

Kartini

. fabrication of electrospun multi-walled carbon nanotube/tio2 nanofiber. In: Key Engineering Materials 2021. Stafa-Zurich, Switzerland: Trans Tech Publications Ltd., 2021, pp.67–73.

67.

Korycka

Mirek

Kramek-Romanowska

, et al. Effect of electrospinning process variables on the size of polymer fibers and bead-on-string structures established with a 23 factorial design. Beilstein J Nanotechnology 2018; 9: 2466–2478.

68.

Samanta

Thangapandian

Srivastava

, et al. Frustrated crystallization behavior of poly (ethylene oxide) in electrospun core-shell nanofibers and beads. Fibers Polym. 2021: 1–12.

69.

Zaarour

Tina

Zhu

Jin

. Branched nanofibers with tiny diameters for air filtration via one-step electrospinning. J Ndustrial Textiles, 152808372092377 2020. DOI: 10.1177/1528083720923773.

70.

Zaarour

Zhu

Jin

. Direct fabrication of electrospun branched nanofibers with tiny diameters for oil absorption. J Dispersion Sci Technology, 42 2020: 2085–2091. DOI:10.1080/01932691.2020.1798779

71.

Zhang

Zhen

Liu

, et al. Branched polyethylene glycol/polypropylene micro-nanofiber nonwovens for fast liquid planar transmission. J Engineered Fibers Fabrics 2019; 14: 155892501985079. DOI: 10.1177/1558925019850798.

72.

, et al. Biomimetic nanoporous aerogels from branched aramid nanofibers combining high heat insulation and compressive strength. SmartMat 2021; 2: 76–87.

73.

Zaarour

Zhu

Jin

. Direct generation of electrospun branched nanofibers for energy harvesting. Polym Adv Tech 2020; 31: 2659–2666.

74.

Shimazaki

Hojo

Takezawa

. Preparation and characterization of thermoconductive polymer nanocomposite with branched alumina nanofiber. Appl Phys Lett 2008; 92: 120.

75.

Tao

Zhang

, et al. Synthesis of multi-branched porous carbon nanofibers and their application in electrochemical double-layer capacitors. Carbon 2006; 44: 1425–1428.

76.

Zaarour

Zhu

Jin

. Maneuvering the secondary surface morphology of electrospun poly (vinylidene fluoride) nanofibers by controlling the processing parameters. Mater Res Express. 2019; 7: 015008–015017.

77.

Liu

Wang

, et al. Electrospun jets number and nanofiber morphology effected by voltage value: numerical simulation and experimental verification. Nanoscale Res Lett 2019; 14: 310.

78.

Zaarour

Zhu

Jin

. A review on the secondary surface morphology of electrospun nanofibers: formation mechanisms, characterizations, and applications. ChemistrySelect 2020; 5: 1335–1348.

79.

Wan

. Introduction to Nanofiber Materials. Cambridge, England: Cambridge University Press, 2014.

80.

Xue

Dai

Xia

Electrospinning and electrospun nanofibers: methods, materials, and applications. Chem Rev 2019; 119: 5298–5415.

81.

Park

. Effect of molecular weight on electro-spinning performance of regenerated silk. Int J Biol Macromolecules 2018; 106: 1166–1172.

82.

Hossain

Gong

Rigout

. Effect of polymer concentration on electrospinning of hydroxypropyl-β-cyclodextrins/PEO nanofibres. J Textile Inst 2016; 107: 1511–1518.

83.

Ramakrishna

Fujihara

Teo

W-E

, et al. An Introduction to Electrospinning and Nanofibers. Singapore: World Scientific, 2005.

84.

Wang

Hashimoto

. Impact of entanglement density on solution electrospinning: a phenomenological model for fiber diameter. Macromolecules. 2016; 49: 7985–7996.

85.

Ryšánek

Benada

Tokarský

, et al. Specific structure, morphology, and properties of polyacrylonitrile (PAN) membranes prepared by needleless electrospinning; forming hollow fibers. Mater Sci Eng C 2019; 105: 110151.

86.

Tomasula

De Sousa

, et al. Electrospinning pullulan fibers from salt solutions. Polymers. 2017; 9: 32.

87.

Liu

Huang

Jin

. Electrospinning of grooved polystyrene fibers: effect of solvent systems. Nanoscale Res Lett 2015; 10: 237–246.

88.

Haider

Kang

I-K

. A comprehensive review summarizing the effect of electrospinning parameters and potential applications of nanofibers in biomedical and biotechnology. Arabian J Chem 2018; 11: 1165–1188.

89.

Jian

Zhu

Jiang

, et al. Nanofibers with diameter below one nanometer from electrospinning. RSC Adv. 2018; 8: 4794–4802.

90.

Meng

Zheng

Xin

. Surface morphologies of electrospun polystyrene fibers induced by an electric field. Textile Res J 2019; 89: 3850–3859.

91.

Acik

Cansoy

Kamaci

. Effect of flow rate on wetting and optical properties of electrospun poly(vinyl acetate) micro-fibers. Colloid Polym Sci 2019; 297: 77–83.

92.

Ray

Chen

S-S

C-W

, et al. A comprehensive review: electrospinning technique for fabrication and surface modification of membranes for water treatment application. RSC Adv. 2016; 6: 85495–85514.

93.

Kuchi

Harish

Reddy

. Effect of polymer concentration, needle diameter and annealing temperature on TiO2-PVP composite nanofibers synthesized by electrospinning technique. Ceramics Int 2018; 44: 5266–5272.

94.

Alfaro De Prá

Ribeiro-do-Valle

Maraschin

, et al. Effect of collector design on the morphological properties of polycaprolactone electrospun fibers. Mater Lett 2017; 193: 154–157.

95.

Zaarour

Zhu

Huang

, et al. Enhanced piezoelectric properties of randomly oriented and aligned electrospun PVDF fibers by regulating the surface morphology. J Appl Polym Sci 2019; 136: 47049–47056.

96.

Zhu

Zaarour

Jin

. Direct generation of electrospun interconnected macroporous nanofibers using a water bath as a collector. Mater Res Express. 2020;7: 015082.

97.

Jiang

Chen

Duan

, et al. Electrospun nanofiber reinforced composites: a review. Polym Chem 2018; 9: 2685–2720.

98.

Yusof

Shamsudin

Abdullah

, et al. Electrospinning of carboxymethyl starch/poly(L-lactide acid) composite nanofiber. Polym Adv Tech 2018; 29: 1843–1851.

99.

Pascariu Dorneanu

Cojocaru

Samoila

, et al. Novel fibrous composites based on electrospun PSF and PVDF ultrathin fibers reinforced with inorganic nanoparticles: evaluation as oil spill sorbents. Polym Adv Tech 2018; 29: 1435–1446.

100.

Sun

Cheng

, et al. Electrospun fibers and their application in drug controlled release, biological dressings, tissue repair, and enzyme immobilization. RSC Adv. 2019; 9: 25712–25729.

101.

Polaskova

Peer

Cermak

, et al. Effect of thermal treatment on crystallinity of poly(ethylene oxide) electrospun fibers. Polymers. 2019; 11: 1384.

102.

Ghosal

Chandra

, Electrospinning over solvent casting: tuning of mechanical properties of membranes. Sci. Rep 2018; 8: 5058.

103.

Schneider

Facure

MHM

Chagas

PAM

, et al. Tailoring the surface properties of micro/nanofibers using 0d, 1d, 2d, and 3d nanostructures: a review on post‐modification methods. Advanced Materials Interfaces 2021;8: 2100430.

104.

Mahmoudifard

Vossoughi

Soleimani

. Different types of electrospun nanofibers and their effect on microfluidic-based immunoassay. Polym Adv Tech 2019; 30: 973–982.

105.

Han

Steckl

. Coaxial electrospinning formation of complex polymer fibers and their applications. ChemPlusChem 2019; 84: 1453–1497.

106.

Park

Jung

Zhang

, et al. Branched tellurium hollow nanofibers by galvanic displacement reaction and their sensing performance toward nitrogen dioxide. Nanoscale 2013; 5: 3058–3062.

107.

Konno

Kishi

Tanaka

, et al. Core/shell-like structured ultrafine branched nanofibers created by electrospinning. Polym J 2014; 46: 792–799.

108.

Zhu

Jan

Zan

, et al. Hierarchically branched TiO 2 @SnO 2 nanofibers as high performance anodes for lithium-ion batteries. Mater Res Bull 2017; 96: 405–412.

109.

Wang

Yang

Wang

, et al. In situ fabrication of branched tio 2/c nanofibers as binder‐free and free‐standing anodes for high‐performance sodium‐ion batteries. Small 2019; 15: 1901584.

110.

Wang

Yang

Wang

, et al. Controllable design of mos 2 nanosheets grown on nitrogen‐doped branched tio 2/c nanofibers: toward enhanced sodium storage performance induced by pseudocapacitance behavior. Small 2020; 16: 1904589.

111.

Nasir

Yang

Ayub

, et al. Tin diselinide a stable co-catalyst coupled with branched TiO2 fiber and g-C3N4 quantum dots for photocatalytic hydrogen evolution. Appl Catal B: Environ 2020; 270: 118900.

112.

Sapurina

Shishov

. Oxidative polymerization of aniline: molecular synthesis of polyaniline and the formation of supramolecular structures. New Polymers Special Applications 2012; 740: 272.

113.

Zhang

. Synthesis of dendritic polyaniline nanofibers in a surfactant gel. Macromolecules. 2004; 37: 2683–2685.

114.

X-S

Zhou

C-F

Mai

Y-W

. Facile synthesis of hierarchical polyaniline nanostructures with dendritic nanofibers as scaffolds. The J Phys Chem C 2008; 112: 19836–19840.

115.

X-S

Zhou

C-F

Wang

G-T

, et al. Novel solid-state and template-free synthesis of branched polyaniline nanofibers. Chem Mater 2008; 20: 3806–3808.

116.

Zhou

C-F

X-S

Liu

, et al. Solid phase mechanochemical synthesis of polyaniline branched nanofibers. Synth Met 2009; 159: 1302–1307.

117.

Luo

Yuan

Sun

, et al. Effect of calcination temperature on the humidity sensitivity of TiO2/graphene oxide nanocomposites. Mater Sci Semiconductor Process 2019; 89: 186–193.

118.

Park

J-H

Sudarshan

. Chemical Vapor Deposition. Washington, USA: ASM international, 2001.

119.

Chondath

Menamparambath

MM.

. Interface-assisted synthesis: a gateway to effective nanostructure tuning of conducting polymers. Nanoscale Adv. 2021; 3: 918–941.

120.

Wang

Zhang

Shao

, et al. Rutile TiO2 nanowires on anatase TiO2 nanofibers: a branched heterostructured photocatalysts via interface-assisted fabrication approach. J Colloid Interf Sci 2011; 363: 157–164.

121.

Hatami

Viganó

Innocentini Mei

, et al. Production of alginate-based aerogel particles using supercritical drying: experiment, comprehensive mathematical model, and optimization. J Supercrit Fluids 2020; 160: 104791.

122.

Mißfeldt

Gurikov

Llsberg

, et al. Continuous supercritical drying of aerogel particles: proof of concept. Ind Eng Chem Res 2020; 59: 11284–11295.

123.

Krell

. Handbook of Laboratory Distillation. Amsterdam, Netherlands: Elsevier, 1982.

124.

Zheng

Cheng

Bao

, et al. Multiple branched α-MnO2 nanofibers: a two-step epitaxial growth. J Cryst Growth 2006; 286: 156–161.

125.

Sezer

Koç

. A comprehensive review on the state-of-the-art of piezoelectric energy harvesting. Nano Energy 2020: 105567.

126.

Yin

, et al. Electrospun nanofibers for high-performance air filtration. Composites Commun 2019; 15: 6–19.

127.

Shi

H-G

S-L

Cheng

J-B

, et al. Multifunctional photothermal conversion nanocoatings toward highly efficient and safe high-viscosity oil cleanup absorption. ACS Appl Mater Inter 2021; 13: 11948–11957.

128.

Olabi

Onumaegbu

Wilberforce

, et al. Critical review of energy storage systems. Energy 2021; 214: 118987.

129.

Koohi-Fayegh

Rosen

. A review of energy storage types, applications and recent developments. J Energ Storage 2020; 27: 101047.

130.

Meng

Xia

, et al. Electrodeposited polyaniline nanofibers and moo3 nanobelts for high-performance asymmetric supercapacitor with redox active electrolyte. Polymers. 2020; 12: 2303.

131.

Ameta

Solanki

Ameta

, et al. Photocatalysis. Advanced Oxidation Processes for Waste Water Treatment. Amsterdam, Netherlands: Elsevier, 2018, pp.135–175.

A comprehensive review on branched nanofibers: Preparations,strategies,and applications

Abstract

Introduction

Electrospinning’s fundamental concept

Generating of the branched nanofibers

Formation methods

Electrospinning

Chemical oxidative polymerization

Solid-state reaction

Calcination

CVD

Interface-assisted synthesis

Supercritical drying

Reflux

Applications of the branched nanofibers

Energy harvesting

Air filtration

Oil cleanup

Energy storage

Photocatalytic

Heat resistance

Other applications

Summary and future outlook

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References