Introduction to inorganic nanofibers by electrospinning

Electrospinning has been used in the production of inorganic nanofibers by using its precursor solutions. To make the spinning process easier, polymers are often added to the solution which are removed following the sintering process. In some cases, while it is possible to electrospin inorganic precursors without the aid of a carrier polymer, having a carrier polymer is still preferred. Ramlow et al (2022) compared the resultant inorganic silicon carbonitride fibers from pyrolysis of electrospun polysilazane and polysilazane/polyacrylonitrile (PAN) fibers. The fibers from electrospun polysilazane/PAN have a smaller diameter than polysilazane. This may be due to the much lower concentration needed to obtain smooth electrospun fibers when PAN is added to the polysilazane solution. To get smooth electrospun fibers, pure polysilazane solution requires a concentration of 60 wt% while the mixture of polysilazane/PAN solution only requires 17.5 wt%. The presence of PAN may generate defects in the form of pores after pyrolysis although depending on the specific application, the pores may have the benefit of further increasing the surface area of the resultant fiber. The availability of precursors for many inorganic materials has resulted in the fabrication of a wide range of inorganic nanofibers using electrospinning. While there are different techniques for producing electrospun nanofibers such as needle-based electrospinning and needleless electrospinning, the mechanical performance of the resultant inorganic nanofibers are expected to be similar. This has been shown to be the case for electrospun TiO₂ nanofibers [Vahtrus et al 2015]. However, given the greater stability of the solution jet for needle-based electrospinning, dimensional and mechanical consistency is better compared to needleless electrospinning [Vahtrus et al 2015]. Taking advantage of the high surface area of nanofibers and its form, new concepts and applications are been tested.

Most inorganic electrospun fibers are constructed from the sintering of its precursors. Certainly, sintering parameters such as temperature and heating rate will have an effect on the fiber form and material phase. Typically, the fibers undergo three phases during the sintering process. Phase one involves the removal of residual solvents and water vapors from the fibers. Second phase is where actual fiber shrinkage takes place as the organic materials are removed, polymerization, condensation and structural relaxation proceeds. Depending on the materials and temperature, phase three is where the inorganic material enters the glass transition stage [Shendokar et al 2008]. For production of inorganic fibers, most sintering ends at phase two. Preparation of most precursor solutions includes the addition of a carrier polymer, usually polyvinyl pyrrolidone or polyvinyl alcohol, to facilitate the formation of fibers during electrospinning. These polymers are typically soluble in water and ethanol, the same solvents used in dissolution of the salt for preparation of the precursor solution. These organic polymers will be removed during the sintering process. Jian et al (2022) showed the effect of sintering temperature on zinc oxide (ZnO) doped with 1.5 wt.% lanthanum (La) nanofibers. ZnO and La salt were blended with polyacrylonitrile (PAN) solution to form the precursor solution. With increasing sintering temperature, the ZnO crystallinity increases and this potentially reduces the recombination centers of photogenerated electrons and holes and hence increases photocatalytic efficiency. As the sintering temperature increases from 500 °C to 700 °C, the surfaces of the ZnO/La nanofibers become rougher as the PAN decomposes. However, at a sintering temperature of 700 °C, agglomeration of the oxide particles can be seen and this reduces the overall photocatalytic efficiency. Therefore, the highest photocatalytic efficiency was recorded for ZnO/La nanofibers sintered at 600 °C with 84% Rhodamine B decomposition in 510 min. There are some precursors that does not require a companion carrier polymer for electrospinning into fibers. Loccufier et al (2019) prepared an electrospinnable silica sol using tetraethyl orthosilicate (TEOS). To create the electrospinnable sol, a series of steps were needed to eventually form loosely cross-linked molecules to achieve the desired viscosity for electrospinning. However, even with optimized solution property, a stable electrospinning jet was only maintained for 2 to 5 minutes.

Electrospun inorganic nanofibers are generally made out of nano-size grains or crystals. The size of the grains would affect its specific surface area and this may in turn affect its ability to perform as sensors and catalyst. By varying the heating temperature, the grain size may be altered. Li et al (2012) showed that with TiO₂, calcination of electrospun polyvinyl pyyrolidone/tetrabutyl titanate (PVP/TBT) at temperature of 500°C, 600°C and 700°C for 3 hours gave rise to increasing grain size. This results in a drop in its specific surface area which in turn reduces its photocatalytic activity. Chen et al (2019) constructed first transition-metal (Ti, Mn, Co, Ni and Zn) oxide nanofibers by electrospinning and tested their photocatalytic efficiencies. Ti, Mn, Co, Ni and Zn precursors were mixed with polyvinyl pyrrolidone (PVP) for electrospinning. Following calcination of the electrospun fibers, TiO₂ and NiO nanofibers comprised of nanoparticles that were uniform and compact with an average diameter of 10 and 20 nm, respectively. However, the sizes of Mn₂O₃, Co₃O₄ and ZnO nanoparticles that form the fibers were relatively large, with sizes of 80-100, 60-80, 50-75 nm, respectively. They attributed these differences in the solubility of the respective salt in the PVP solutions. The precursors of TiO₂ and NiO nanofibers were clear and transparent, while the other three precursors are homogeneous emulsions. During calcination, numerous nucleation sites were formed for TiO₂ and NiO and these draw upon surrounding atoms to the crystal nucleus. For Mn₂O₃, Co₃O₄ and ZnO, particles were already present in the solution as evident by emulsion state. These act as ready nucleation sites during calcination and their rapid growth form larger nanoparticles. ZnO, TiO₂ and NiO nanofibers exhibit excellent photocatalytic efficiency and high cycling ability to methylene blue (MB) under ultraviolet (UV) irradiation.

TEM images of (a) the PVP/TBT composite nanofibers and TiO2 nanofibers obtained at (b) 500°C, (c) 600°C and (d) 700°C, respectively. [Li et al 2012 This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Inorganic crystals often form different phases depending on the molecular packing. For TiO₂, rutile and anatase phases are the most commonly encountered and manufactured depending on the applications. The former is mainly for coating and cosmetics purpose due to its high refractive index and UV absorption cross-section and the latter is for catalysis as it is chemically and optically active. A simple method directing the phase formation and transformation is by controlling the calcination temperature. In electrospun TiO₂, calcination temperature of less than 400°C produces mainly anatase phase while at temperature more than 600°C, rutile phase dominates [Song et al 2013]. Further study on the effect of annealing temperature on the TiO₂ phases were carried out by Secundino-Sanchez et al (2019). In their annealed electrospun fibers, annealing temperature of less than 600°C gave pure anatase phase. At annealing temperature less than 800°C, mixed anatase and rutile phase was obtained and pure rutile phase was obtained at temperature between 800°C and 1000°C. During thermal treatment, lost of oxygen was evident and the vacancies due to this oxygen loss has been suggested to drive crystalline phase transformation. Radiative band red shift from 2.56 to 1.32 eV was recorded as the TiO₂ nanofibers transformed from anatase to rutile phase with increasing treatment temperature.

The formation of crystals with specific facets orientation is influenced by parameters such as chemical used and pH of the precursors. Less known is the influence of the precursor processing method such as the use of electrospinning in controlling material phase and crystal facets orientations. Dai et al (2009) showed that with electrospinning, anatase TiO₂ nanocrystals with exposed {001} facets can be fabricated. Electrospinning plays a critical role in this formation due to the small diameter of the nanofibers which allows moisture in the air to diffuse into the core for complete hydrolysis for Ti(OiPr)₄ at a relative constant rate. Using other processing methods such as direct injection of the same solution into acid medium or forming cast films, the same crystals cannot be achieved.

Beyond crystal phase and orientation, the interaction of the precursor solution with the environment during electrospinning and subsequent calcination also lead to physical changes in the resultant fiber. From a single orifice nozzle, electrospun hollow inorganic nanofibers have been fabricated. In SnO₂hollow fibers produced from electrospinning, it has been proposed that Kirkendall effect and surface diffusion during the calcination process has driven this formation [Xia et al 2012].

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].

Fibers with surface projections may be constructed to increase surface area. Since inorganic fibers are used in many applications where higher surface area is desirable, researchers have shown the feasibility of using hydrothermal process to grow projections on the electrospun fibers. Fan et al (2019) produced electrospun ZnO nanofibers with two different type of surface projections. ZnO nanofibers were first produced by annealing of electrospun Zn(CH₃COO)₂·2H₂O/polyvinylpyrrolidone (PVP) fibers. A reaction solution was prepared by mixing hexamethylenetetramine (HMTA) and zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) followed by the addition of ZnO. Hydrothermal reaction of this mixture produces fire cracker like surface. When the ZnO nanofibers were sonicated prior to hydrothermal treatment, flower like projections were produced.

SEM images of as-prepared samples: (a) ZnO with fire cracker like surface in low magnification, (b,c) ZnO with fire cracker like surface at high magnification, (d) ZnO with flower like projections in low magnification and (e,f) ZnO with flower like projections at high magnification [Fan et al 2019].

Additives may be added to the precursor solution for electrospinning such that multiple reactions may take place during the annealing process to form hybrid structures. Wang et al (2019) constructed an electrospun membrane with dendritic hybrid architecture comprising of carbon nanotubes (CNTs) with nickel sulfide nanoparticles (NS) encapsulated inside, grown from the surface of porous electrospun N-doped carbon nanofibers (CNFs). The multi-stage process to construct this membrane starts with electrospinning of polyacrylonitrile (PAN) containing nickel acetate to form a fibrous membrane. During annealing, PAN was converted into porous N-doped carbon fibers which served as a reducing agent to convert Ni salt into Ni nanoparticles. Thiophene introduced during annealing, decomposes into H₂S and gaseous hydrocarbons. The hydrocarbons served as the carbon source and the catalytic effect of Ni nanoparticles encouraged growth of carbon nanotubes (CNTs) epitaxially out of the CNFs. The Ni nanoparticles reacted with H₂S to form nickel sulfides nanoparticles inside the CNTs. Pan et al (2019) fabricated CuCo₂O₄nanoparticles@N-carbon nanofiber (CuCo₂O₄NPs@N-CNFs) film by electrospinning of their precursors blend followed by carbonization/oxidation processes. The precursor solutions for forming carbon and CuCo₂O₄ was prepared separately. In this case, polyacrylonitrile (PAN) and polyvinylpyrrolidone (PVP) was used as the precursor for carbon and Cu(CH₃COO)₂ and 4.0 mmol Co(CH₃COO)₂ were the precursors for CuCo₂O₄. These were then mixed together and stirred for a few hours before electrospinning. The resulting hybrid electrospun mat was carbonized and CuCo₂O₄ nanoparticles were formed during the same sintering process. The resultant CuCo₂O₄NPS@N-CNFs undergo a room-temperature in situ sulfurization by immersing into 2.0 m Na₂S solution for a couple of hours. The CuCo₂S₄ NSs@N-CNFs) films showed remarkable bifunctional catalytic performance (Ej= 10 (OER) - E1/2 (ORR) = 0.751 V) with excellent mechanical flexibility.

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.]

Sintering and annealing of electrospun inorganic precursors fibers are often conducted in a thermal convection furnace. This is usually a slow process as the heat is transferred from the heating element to the sample. Khishigbayar et al (2017) explored the use of microwave to convert electrospun polycarbosilane (PCS) to SiC fibers. Their result showed that the random and isotropic distribution of electrospun SiC fiber membrane was able to facilitate microwave adsorption. A temperature of over 800 °C was needed for conversion of PCS to SiC fibers. Using microwave radiation (2.45 GHz), temperature of 1000 °C was recorded on the electrospun PCS membrane. The temperature increases with higher membrane thickness due to greater microwave absorption. However, beyond a thickness of 1.44 mm, the radiation temperature remained constant at 1600 °C which represents a saturation of microwave absorption.

Electrospun fibers may be incorporated with catalytic additives for further processing. Vapor phase transport (VPT) uses catalysts embedded on a substrate to initiate and grow nanostructures off the substrate. Islam et al (2022) electrospin AuCl₃/PAN nanofibres followed by carbonization to form Au nanoparticles (AuNPs)-decorated carbon nanofibres. The presence of AuNPs function as the catalyst for VPT growth in a furnace. A mixture of graphite powder and ZnO powder was used as the source for VPT growth in a furnace. The growth in the diameter and length of the nanostructures increases linearly with the furnace temperature. It is only at temperature of 1000 °C that microstructure starts to project from the nanofibers surface with gold nanoparticle droplets at the tip of many ZnO projections. In VPT, it is the metal vapor atoms that bind onto the catalyst. Therefore, higher temperature would increase the formation and movement of the metal vapor atoms and facilitate crystal growth. The same VPT condition yielded similar projections from In₂O₃ and SnO₂.

Illustration of the fabrication process with electrospinning, carbonization to obtain carbon fibers with surface catalyst and vapor phase growth of metal oxide nanowires around the nanofibres [Islam et al 2022].

Many inorganic materials are able to participate in electron transfer and this property has seen them used in many energy applications. Several electrospun inorganic nanofibers have been tested for use in dye-sensitized solar cell (DSSC). Perhaps the most frequently electrospun nanofiber for this application is based on TiO₂. Unfortunately, energy conversion efficiency of electrospun TiO₂ based DSSC is still relatively low at less than 7% [Fujihara et al 2007, Jin et al 2012]. Carbon nanofibers are commonly produced in laboratory using electrospun polyacrylonitrile (PAN) as its precursor. Electrospun carbon fibers have been investigated for use as electrode materials for energy storage [Mao et al 2013]. In lithium-ion batteries, electrospun nanofibers may be used as anode materials. To improve its performance, other electrochemically active metallic particles have been incorporated into electrospun carbon fibers.

Sensors are another application where inorganic electrospun fibers have found potential use. High surface area of nanofibers meant that the sensor would be very sensitive to stimulants. a humidity sensor [Khoshaman 2011]. Such sensors are very useful in high temperature environment due to its stability at those conditions [Ding et al 2010, see Introsensor.html]. For resistivity sensor, high surface area alone is not sufficient for its good performance. Fiber continuity is also important for transmission of electrons for faster response and recovery. Chen et al (2013) [see gasresistivitysensor.html] compared the performance of electrospun Co-doped SnO2 nanofibers and nanosphere for detection of methane. Although nanosphere has a larger surface area, nanofiber based sensor showed better gas response, higher saturated detection concentration, and quicker response-recovery time.

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