TiO₂ nanofibers via electrospinning

TiO₂ is one of the most widely investigated inorganic oxide produced through electrospinning due to its properties and relative ease of production using precursors. TiO₂, rutile and anatase phases are the most commonly encountered and manufactured depending on the applications. The former is mainly for coating and cosmetics purposes 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]. Li et al (2012) showed that with TiO₂, calcination of electrospun polyvinyl pyrrolidone/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. While it is widely accepted that the anatase phase is what gives TiO₂ its photocatalytic property, Li et al (2022) suggest that a small amount of rutile phase in a majority anatase phase will give the electrospun TiO₂ fiber the highest photocatalytic performance. In their research, the interfacial temperature between anatase and rutile phase occurs at 700 °C instead of 600 °C reported by others. While it is mainly anatase phase when annealed at 700 °C, a small amount of rutile phase can be found. Li et al (2022) hypothesized that the two phases in close proximity to each other forming an anatase-rutile heterojunction resulted in the highest photocatalytic action compared to pure anatase phase TiO₂ from lower annealing temperature. Since anatase and rutile phase occupy different bands, the electron generated from the anatase conduction band (CB) would migrate to the rutile phase CB and the corresponding hole from the valence band (VB) of rutile phase to the VB of anatase phase. This improves the separation efficiency of the photogenerated electron-hole pair and reduces its recombination.

Mesoporous structure has a higher surface area compared to solid fibers and this makes it ideal for applications such as sensors, photocatalytic degradation, energy storage and energy conversion. Electrospinning and sintering of some TiO₂ precursors are able to form mesoporous fibers, however, 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].

Using electrospinning, TiO₂ doped with other salt may be produced. These salts may be added with the TiO₂ precursor solution for electrospinning. Doped TiO₂ has been found to be very useful in improving its sensor applications. Qi et al (2008) showed that KCL-doped TiO₂ nanofibers demonstrated impedance variance of more than four orders of magnitude in the range of 11% to 95% relative humidity over pure TiO₂ nanofibers. Li et al (2008) also reported improved impedance variance when TiO₂ nanofibers were doped with up to 30% LiCl.

Many applications of TiO₂ are based on its photo-generated electron-hole pair where the hole in the valence band of TiO₂ participated in further reaction. However, rapid recombination of the electron-hole pair reduces the activity of the TiO₂. A common strategy is to introduce an alternative material to transfer the photo-generated electron away so that the holes are given sufficient time for further reactions. Koozekonan et al (2021) tested the UV protection ability of electrospun polyacrylonitrile (PAN) loaded with multi-walled carbon nanotubes (MWCNT) and TiO₂ nanoparticles and both. Electrospun PAN fibers alone do not offer UV protection and 1% loading of either MWCNT or TiO₂ nanoparticles would offer good protection. However, to achieve excellent protection, more than 10% loading of either MWCNT or TiO₂ nanoparticles needs to be added. Higher concentration of TiO₂ needed to offer excellent UV protection is probably due to recombination of electrons and hole pairs. When MWCNT and TiO₂ are added together for electrospinning in PAN, the resultant composite nanofibers require only 1% of the additives to achieve excellent UV protection. MWCNT, being conductive, was able to transfer the photo-generated electron from the TiO₂ away and prevented immediate recombination of the electron-hole pair. This leaves behind the hole in the valence band of the TiO₂ for reaction.

The introduction of oxygen vacancies in electrospun derived TiO₂ nanofibers through H₂ plasma treatment has been shown to improve photocatalytic performance. Li et al (2022) showed a 30% improvement in photocatalytic degradation of methyl orange solution in plasma-treated TiO₂ nanofibers compared to the same nanofibers without plasma treatment. With greater number of oxygen vacancies, more electrons are trapped. This facilitates the separation of photogenerated electron-hole pairs. The oxygen vacancies may participate in the photocatalytic degradation by converting O₂ to reactive O₂^-.

Hydrothermal post treatment has been used to create inorganic composite fibrous networks with a functional layer on the surface. Depending on the treatment process and material, various secondary structures have 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]

Inorganic nanofibers such as TiO₂ require sintering of its electrospun precursor material. Since there will be further weight loss from the precursor material form, feasible commercial application of such inorganic nanofibers would require the development of mass production methods. Nealy et al (2020) used an alternating current (AC) on a free surface electrospinning setup to electrospin a solution of titanium(IV) n-butoxide (Ti(OBu)₄)/ polyvinylpyrrolidone (PVP), with alternating current (AC)-voltages up to 40 kV rms at 60 Hz was used. The polymer solution was fed into a shallow dish-like electrode where the AC high voltage was applied. The collector was a PTFE plastic mesh placed about 50 cm above the electrode. Dish-like electrodes with diameters from 10 to 25 mm were tested. With smaller diameter electrodes, an increase of fiber bundling was observed which is not desirable. The feed-rate of the solution was adjusted to support a stable generation of fibers. Assuming the precursor fibers were converted to TiO₂, the production rate of TiO₂ nanofiber from one electrode is about 5.2 gh^-1. A 5 - 7 cm thick fluffy layer of the fibers can be collected using this setup.

(a) AFES experiment schematics and photos of the flows of precursor fibers with x1.5 TiO2/polymer yield mass ratio generated (mass of TiO2 generated per unit mass of polymer carrier) on (b) 25 mm and (c) 12.5 mm diameter electrode at 29.5 kV rms AC voltage; and with (d) x0.5 and (e) x2.5 TiO2/Polymer yield mass ratio generated using 25 mm diameter electrode at the same AC voltage [Nealy et al 2020].

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