Electrospun mesoporous fibers

Electrospinning is a common technique for fabricating nanofibers which exhibits large surface area to mass. This makes it attractive in applications such as sensors, photocatalytic degradation, energy storage and energy conversion where large surface area is able to improve sensitivity and performance. In the push for better performance through higher surface area, researchers are looking into the fabrication of electrospun mesoporous fibers.

Electrospun inorganic fibers often demonstrate mesoporous structure as they are made from an assembly of nanocrystals. Titanium dioxide (TiO₂) is commonly investigated for use as a photocatalytic agent and in solar cells applications. Thus various techniques have been designed to construct electrospun mesoporous TiO₂ fibers. The simplest way of producing mesoporous TiO₂ is by mixing TiO₂ precursor such as titanium isopropoxide [Mondal et al 2014] or tetrabutyl titanate (TBT) [Li et al 2012] to a polymer material (polyvinylpyrrodidone) followed by electrospinning. Using a solution of V₂O₅ powder and poly(vinylpyrrodidone) (PVP), mesoporous vanadium pentoxide nanofibers has been fabricated after annealing at 500 °C in air for 1 h. The mesoporous fibers were found to exhibit excellent Li-ion storage capacity [Yu et al 2011].

TEM images of TiO2 following sintering of PVP/TBT composite nanofibers at 700°C [Li et al. The Scientific World Journal. 2012; 2012: 154939. doi.org/10.1100/2012/154939. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

In many cases, a surfactant is added to the solution to facilitate formation of mesoporous fibers. Although electrospinning and sintering of some TiO₂ precursors are able to form mesoporous fibers, electrospinning of PVP/tetrabutyl orthotitanate was shown to give rise to nonporous fibers after sintering. With the addition of P123 as a structure directing agent into the solution for electrospinning, mesoporous TiO₂ fibers can be produced after sintering[Wang et al 2012]. Song et al (2009) was also able to construct mesoporous ZnO-SnO₂ fibers with the addition of P123. The surface areas of mesoporous ZnO-SnO₂ and ZnO-SnO₂ fibers was found to be 156 m²g^-1 and 40 m²g^-1 respectively, demonstrating the effectiveness of mesoporous structure in increasing the surface area. P123 has also been used in the construction of mesoporous tungsten oxide fibers from electrospinning of tungsten precursor of W(OC₂H₅)₆ and PVP [Nguyen et al 2012]. A cationic surfactant, cetyltrimethyl ammonium bromide (CTAB) has been used successfully to construct meosporous PVP/SiO₂ by adding the surfactant to the solution for electrospinning [Taha et al 2012].

A bicomponent system has been used to generate mesoporous fibers by selective removal of one of its components. Teng et al (2012) fabricated mesoporous carbon nanofibers by electrospinning phenolic resin precursor as carbon source with additions of F127, PVP and tetraethyl orthosilicate (TEOS). Porosity of the carbon nanofibers were controlled by varying the amount of TEOS added to the mixture which was removed using 10 wt% HF aqueous solution. Carbon nanofibers with the highest porosity (from solution with the highest concentration of TEOS) demonstrated greatest adsorption capacity for dye molecules.

Similar to adding surfactants, foaming agents may also be added to facilitate the formation of mesoporous fibers. Foaming agents added to the solution will decompose into vapor phases during electrospinning and the escaping vapors will create homogeneous and thoroughly mesoporous fibers. Hou et al (2015) used foaming agent, diisopropyl azodiformate (DIPA) to mix into a solution of polyvinylpyrrolidone (PVP) and butyl titanate (TBOT). CTAB and paraffin oil was later added to the solution for the formation of titanium dioxide (TiO₂) mesoporous tubes. Following sintering, the mesoporous TiO₂ tubes were tested for its photocatalytic activity for the evolution of hydrogen. The mesoporous TiO₂ tubes were able to exhibit significantly higher photocatalytic activity at a rate of 499 µmol g^-1.h^-1 compared to P25 nanoparticles which was at 198 µmol g^-1.h^-1. The photocatalytic stability in the mesoporous TiO₂ tubes was also greater than P25 nanoparticles with no noticeable drop in its activity after 3 cycles of recovery and re-use.

(a) Schematic illustration for the formation of mesoporous TiO2 hollow fibers via the foaming assisted electrospinning. (b) A typical SEM image of the calcined products under a low magnification. (c-e) Typical SEM images of the calcined products under higher magnifications and different views. (f) A representative XRD pattern of the calcined products. [Hou et al. Scientific Reports 2015; 5: 15228. This work is licensed under a Creative Commons Attribution 4.0 International.]

Using sacrificial material and foaming agents are two different concepts of forming pores in electrospun fibers. It is therefore possible to combine these two techniques to generate more porous fibers. Mao et al (2017) used both polymethyl methacrylate (PMMA), CaCO₃ as pore-forming agents in the construction of porous carbon nanofibers. As the pore forming mechanism from PMMA and CaCO₃ differs, this result in two distinct pore type. PMMA is random mixed into the polyacrylonitrile (PAN) which is the carbon precursor, and due to its molecular chain, it tends to form surface pores with gully like shape after sintering. CaCO₃ are powders and when sintered, it decomposes into carbon dioxide and in the process, forms small spherical pores inside the fibers. As the amount of CaCO₃ added increases, the specific surface area also increase. However, beyond an optimum amount, the specific surface area drops. This is due to agglomeration of CaCO₃ particles as shown by presence of macropores in the fiber.

Mesoporous electrospun fibers may also be made from polymeric fibers. Gupta et al (2009) was able to construct porous nylon-6 electrospun fibers using a salt additive. The salt, GaCl₃, is a Lewis acid which interacts with the Lewis base sites (CdO groups) on the nylon-6 chains. This interferes with the hydrogen bonding and disrupts crystallization. GaCl₃ was subsequently removed from the electrospun fibers by soaking the membrane in water for 24hrs. Pores were formed throughout the fibers from the vacated salt. Mesoporous electrospun fibers may form when more volatile solvents are trapped beneath a skin layer. Zaarour et al (2018) showed that electrospun polyvinylidene fluoride (PVDF) dissolved in a mxture of acetone (ACE) and N,N-dimethylformamide (DMF) forms mesopores in its core when electrospun in high humidity environment (relative humidity greater than 40%). At low humidity of 2%, solid core fibers were electrospun. At high humidity, water condensation on the surface of the fiber during eletrospinning causes rapid precipitation of PVDF and forming a skin layer. This traps pockets of solvent in its core which aggregated until there is sufficient pressure to break free from the surface of the fiber, leaving behind evenly distributed pores.

(Cross-sectional SEM images of PVDF fibers electrospun at different levels of RH: (a) 2%, (b) 22%, (c) 42%, and (d) 62% [Zaarour et al 2018].

Phase separation may be used to create mesoporous fibers by mixing a non-solvent into the polymer solution and electrospinning. Abolhasani et al (2022) introduced water as a non-solvent for PVDF solution. By varying the amount of water, the liquid-liquid phase separation process can be controlled to attain different degrees of porosity at the fiber core. While the core of the electrospun fibers were porous, the outer surface of the fiber is smooth due to the relatively small amount of water added. The porous electrospun PVDF fibers were found to exhibit a β-phase of up to 92% while a solid core electrospun fiber showed β-phase of about 77%.

(a-d) Representative histograms of PVDF nanofiber diameter for K0 (PVDF only), G0.1 (PVDF with 0.1% graphene), B0.1 (PVDF with 0.1% graphene and 3.4 vol% water added to solution), and S0 (PVDF with 5.4 vol% water added to solution), with their respective (e-h) cross-sectional SEM images of the nanofibers. The respective random networks of the nanofibers are presented in ref. 10 with the insets showing the surface of each nanofiber. The scale bars are 300?nm, 1 µm, and 100?nm for the cross-section, fibers and inset images, respectively. PVDF, poly(vinylidene fluoride); SEM, scanning electron microscopy [Abolhasani et al 2022]

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Content [Fiber Physical Structure]