Inorganic composites from Electrospun fibers

A material may be incorporated with a secondary material to form a composite with enhanced properties. Conventional composites are mostly about improving mechanical properties. However, there are now many composites that are focused on imparting other functional properties to the material. Many inorganic materials exhibit interesting functional properties such as photocatalysis and piezoelectricity. To further improve its properties, a secondary material may be added to form an inorganic composite. Having the material in the form of nanofibers instead of free nanoparticles has the advantage of preventing aggregation and having a high surface area.

The mechanical properties of inorganic composites may be improved by adding suitable strengthening material. Abdo et al (2019) examined the mechanical properties enhancement of magnesium matrix reinforced with electrospun carbon or titanium oxide (TiO₂) nanofibers. The inorganic composite is formed by mixing the formed electrospun carbon or TiO₂ nanofibers into magnesium powder followed by sintering and compaction to fuse the metal powder. Compressive tests showed that optimum addition of TiO₂ nanofibers into the matrix improves the hardness (up to 64%) and ultimate compressive strengths (up to 12%). However, the use of carbon nanofibers (CNFs) in Mg matrix slightly reduces its mechanical properties. Such contrasting results may be due to better interfacial bonding between TiO₂ nanofibers and magnesium while the decrease in strength when CNFs is added is due to poor interfacial bonding.

To enhance photocatalytic activity of inorganic material in visible light, dopants may be added to reduce the energy band gap. Samadi et al (2012) doped electrospun ZnO nanofibers with multi-walled carbon nanotube (MWCNT) and the doped ZnO demonstrated photocatalytic activity in visible light while pure ZnO nanofibers were inactive. Al-Enizi et al (2023) constructed ZnO/CdS nanofibers by electrospinning precursors of ZnO mixed with CdS nanoparticles (NPs). As ZnO is inactive in the visible light region, the addition of CdS which has a narrow-band gap was able to tune the ZnO band gap to harvest visible light. Production of H₂ was carried out by placing the membrane in an aqueous solution containing Na₂SO₃/Na₂S under light irradiation. Comparison was made with ZnO only nanofibrous membrane and CdS NPs. The ZnO/CdS composite nanofibrous membrane was able to produce up to 820 µmolh^-1 g^-1. This is much greater than ZnO nanofibrous membrane and CdS NPs which produced 115 µmolh^-1 g^-1, and 180 µmolh^-1 g^-1 respectively. Superior performance of the ZnO/CdS composite nanofibrous membrane was probably due to the high surface area of the nanofibrous membrane, absorption efficiency of visible light and the low photogenerated electron-hole recombination rate. Zhong et al (2016) constructed flexible membranes of electrospun carbon nanofiber/tin(IV) sulfide (CNF/SnS₂) core/sheath fibers for the purpose of wastewater treatment. SnS₂ is able to generate photoelectrons under visible light irradiation due to its low band gap of 2.34 eV and this was shown to reduce water-soluble Cr(VI). Dai et al (2010) fabricated a layered composite inorganic fibrous membrane containing Pt nanoparticles. First, TiO₂ nanofibers were formed by sintering its electrospun precursor fibers. This is then coated with polyvinyl pyrrolidone (PVP) stabilized Pt nanoparticles followed by another layer of SiO₂ nanoparticles. The presence of SiO₂ nanoparticles was found to stabilise the Pt nanoparticles and prevent their aggregation during sintering at temperatures above 350 °C. Catalytic activity of the composite was determined using methyl red for the model reaction. It was shown that even with the SiO₂ sheath over the Pt nanoparticles layer, methyl red molecules were able to penetrate through the porous sheath layer for catalytic reaction.

In sensor applications, inorganic composite often performs better than pure inorganic material. The presence of a second ion may have several effects on the main inorganic compound. The additional ion may inhibit the grain growth of the main inorganic compound. Reduced grain size would increase the surface area hence providing more adsorption sites. Having the second ion may also influence the valence band and electron-hole recombination. Fan et al (2021) constructed an acetone sensor using electrospun inorganic nanofibers. They compared the performance of In₂O₃/NiO composite nanofibers with pure NiO nanofibers. The In₂O₃/NiO composite nanofibers response to 50 ppm acetone was more than 10 times greater than pure NiO nanofibers. The minimum detection limit of In₂O₃/NiO composite nanofibers was 10 ppn while pure NiO nanofibers was 100 ppb. The improved performance of In₂O₃/NiO composite nanofibers over pure NiO nanofibers was attributed to smaller grain size and adjustment of the heterojunction. NiO is a p-type semiconductor while In₂O₃ is a n-type metal oxide. The difference in the Fermi energy level of NiO and In₂O₃ encourages the electrons from the conduction band of In₂O₃ to be transferred to the valence band of NiO. This creates a hole depletion layer on NiO resulting in an increase of the resistance. Reaction of acetone gas on the surface of In₂O₃ causes more electrons to be transferred from the In₂O₃ side to the NiO which further widens the hole depletion layer and increasing the resistance. Such changes in the resistance of the composite nanofibers is translated into a higher acetone-sensing response.

Distribution of the materials in the composite may improve its performance for specific applications. In sensors, the material responsible for interacting with the environment may be better if it is closer to the surface of the nanofibers. Sui et al (2017) showed that using foaming agent, they are able to encourage the migration of TiO₂ nanoparticles to the surface of carbon nanofibers. To prepare this composite, a homogeneous solution of polyacrylonitrile (PAN) and betaine-TiO₂ mixtures (12 wt%) were dissolved in N,N-dimethylformamide (DMF) with betaine as the foaming agent. Electrospinning of this mixture give rise to PAN nanofibers with well dispersed TiO₂ and betaine. During carbonization, the gas produced by decomposition of betaine would push the TiO₂ nanoparticles towards the surface. With higher carbonization temperature, the amount of TiO₂ nanoparticles on the surface increases. However, when the temperature reaches 1000 °C, TiO₂ particles were agglomerated.

Schematic illustrations of the synthetic pathways of TiO2@CNFs composites [Sui et al 2017]

Hydrothermal post treatment has been used to create inorganic composite fibrous network with a functional layer on the surface. Depending on the treatment process and material, various secondary structures has been formed on the underlying electrospinning derived inorganic fibers. Zhang et al (2019) used hydrothermal post-treatment to grow NiO Nanosheets@ on TiO₂ nanofibers prepared by electrospinning. Construction of the NiO Nanosheets@ is by dipping the TiO₂ nanofibers sheet in NiSO₄·6H₂O and hexamethylenetetramine (HMT) followed by sintering.

(a) Low- and (b) high-magnification SEM images of the TiO2 fibers. The inset figure shows the TiO2 electrode. (c) Low- and (d) high-magnification SEM images of the NiO/TiO2 composite film. (e and f) TEM images of the NiO/TiO2 composite. (g) HRTEM and (h) SAED images of NiO NS. [Zhang et al 2019]

Kim et al (2021) demonstrated the importance of having a hydrophilic surface for uniform dispersion of nanoparticles fabricated by hydrothermal treatment on the surface of carbon nanofiber (CNF). To construct hydrophilic CNF, a small amount of Fe- and N-doped graphene nanoplates (FeN@GnP) are added to carbon nanofibers prepared by the carbonization of electrospun polyacrylonitrile (PAN) fibers. FeN@GnP was sonicated to form a suspension before PAN is added to form PAN/FeN@GnP solution for electrospinning. Electrospun carbon nanofiber (CNF) membrane was found to be hydrophobic with a surface contact angle of 143°. For CNF/FeN@GnP membrane, the surface has a hydrophilic contact angle of 42°. Loading of the CNF/FeN@GnP membrane with Co₃O₄ nanoparticles using hydrothermal synthesis method showed uniform dispersion of Co₃O₄ nanoparticles over the surface of the membrane. In contrast, the same loading process on the hydrophobic CNF membrane saw the presence of Co₃O₄ aggregates.

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