We wondered how precursor used as an iron source, depending on whether it includes hydrate, would affect fibres' morphologies. With this viewpoint, we fabricated electrospun nanofibres using iron chloride with hydrate and without hydrate. Besides, we wondered how their photocatalytic performances would change based on morphological differences. Microstructural features of as-synthesized PAN/Metal composite nanofibers are presented in figure 2. SEM micrographs show that PAN/Metal composite nanofibers collected on the aluminium foil were randomly distributed to form a web of fibres. PAN/Metal composite nanofibers produced by iron(III) chloride hexahydrate salt as an iron source are presented in figures 2a-b. The average diameters of PAN/Metal composite nanofibers produced using hydrated iron salt are 340 nm, as seen in figure 2a-b. The average diameters of PAN/Metal composite nanofibers formed by using anhydrated iron salt, which is wider than those produced by hydrated iron salt, are 532 nm, as seen in figure 2c-d.
The PAN/Metal composite nanofibers fabricated by electrospinning are subsequently calcined to transform into metal oxide nanofibers and remove polymer components at 500 °C temperature for 2 hours. The micrographs belonging to transformed nanofibers after annealing are given in figure 3, and drastic changes in nanofibers are seen in the images. After the heat treatment, the diameter of the electrospun nanofibers inherently decreases due to the evaporation of the solvent and polymer. The average diameter of fabricated α-Fe2O3 nanofibers is approximately 150 nm and 240 nm for Hyd-α-Fe2O3 and AnHyd-α-Fe2O3, respectively. Both nanofibers also have a shrinkage ratio of %55 relative to their uncalcined forms. It should be noted that the shrinkage ratio of nanofibers due to the loss of the polymer after heat treatment is identical to each other, but their morphological feature is not. Their morphological differences are obviously seen when looking at STEM images of related nanofibers presented in figure 4. The named AnHyd-Fe2O3 nanofibers consist of smaller grain, low porosity, and strictly form, whereas named Hyd-Fe2O3 nanofibers consist of bigger grain and higher porosity.
It is important to note that the size of the grains and porosity ratio in the iron oxide nanofibers changes by using metal salts as an iron source. As a result, it is observed that the used iron source to fabricate porous α-Fe2O3 nanofibers, concerning whether metal salt contains hydrate or not, has an important influence on the final morphological characteristics of α-Fe2O3 nanofibers. Araujo and co-researchers reported the avarage fibres’ diameters of electrospun Fe2O3 was 360 nm and diameter distribution ranging from 200 to 900 nm calcinated at 800 °C by using Iron (III) nitrate nonahydrate and polyvinylpyrrolidone [53]. Another work executed by Petrovicovà and coworkers reported that the diameters of hematite fibres calcined for 2 hours at 600 °C varied between 120 and 500 nm using iron (II) acetate and polyacrylonitrile (PAN) as precursors [54]. In that study, the smaller average diameter of hematite fibres could be fabricated after a lower calcination temperature.
Phase analysis of the produced electrospun α-Fe2O3 nanofibers crystals with highly and slightly porous was carried out using an X-ray diffractometer (XRD) and obtained characteristic phase patterns of samples are shown in Fig 5. All of the observed diffraction peaks of produced porous nanofibres are well convenient with JCPDS file data (JCPDS card no. 33–0664), which is a trigonal hematite phase (α-Fe2O3, space group: R-3c) with lattice parameters of a = b = 0.503 nm and c = 1.373 nm. It was observed that there are no peaks belonging to another phase. On the other hand, It was observed that the (110) reflections created from electrospun Hyd-Fe2O3 nanofibers are more intense than those created from electrospun AnHyd- Fe2O3 nanofibers.
The absorption/reflection characteristics and the calculated optical bandgap of produced electrospun α- Fe2O3 nanofibers have been presented in figure 6a-d. The absorption spectrum curve of electrospun α- Fe2O3 nanofibers as a function of wavelength are given in Figure 6b, derived from transforming the diffused reflectance of nanofiber powders into the absorbance by utilizing the Kubelka-Munk (K-M) Reflectance Theory [55, 56]. To determine the optical band gap values of electrospun α- Fe2O3 nanofibers, the Tauc relation below is used [57,58].
αhν = A(hν-Eg)n (1)
In the equation given above, α is the extinction coefficient and corresponds to F(R) in the Kubelka-Munk Reflectance Theory. The other variables of h, ν, A, and Eg in the equation are Planck constant (J.s), light frequency (s-1), proportionality constant, and optical bandgap of the material (eV), respectively. Some recent studies [59, 60] report that the band type of the hematite has a direct allowed transition band. The bandgap values of produced electrospun α-Fe2O3 nanofibers were determined from the plot of (F(R)hν)2 vs hν via extrapolating the straight portion to the energy axis at α=0 for allowed direct transition as shown in figure 6c and 6d. The band gap value of AnHyd-Fe2O3 nanofibers was calculated to be 1.90 eV. For Hyd-Fe2O3 nanofibers, that value was found to be 2.02 eV, which was slightly wider than that belonging to AnHyd-Fe2O3 nanofibers.
The valence and conduction band positions of the produced electrospun α-Fe2O3 nanofibers can be calculated using the following empirical relation. [61]:
ECB = χ – Ee – ½ Eg (2)
EVB = ECB + Eg (3)
where χ is the absolute electronegativity of the semiconductor, which is related to the first ionization energy and atomic electron affinity for constituent atoms of the semiconductor. The details of the empirical relation above could be found in the literature to determine absolute electronegativity [62, 63]. The value of absolute electronegativity for hematite (Fe2O3) is 5.88 eV [64]. Ee is the energy of free electrons on the hydrogen scale (4.5 eV). ECB, EVB, and Eg conduction band potential, valence band potential, and semiconductor bandgap, respectively. Hence, ECB and EVB were calculated to be 0.37 eV and 2.39 eV, respectively, for electrospun Hyd-Fe2O3 nanofibers, whose optical bandgap value was found to be 2.02 eV via Tauc approximation, and the results are depicted in figure 6.
It is well known that the degradation of chemical pollutants in aqueous solutions are emanated from highly reactive free radicals. The reactive oxygen species (ROS) [65], such as superoxide anion radical (•O2-), singlet oxygen (1O2), hydrogen peroxide (H2O2) and hydroxyl radical (•OH), are produced in an aqueous media by induced reactions for decontamination of wastewater [66]. In the heterogeneous photocatalytic process, the reactive oxygen species are produced by photoinduced electrons (e-)-holes (h+) pairs in the presence of a photocatalyst. [67, 68]. In that work, photogenerated electron-hole pairs are generated by electrospun α-Fe2O3 nanofibers utilising the visible region of the solar spectrum. The following factors specify the destiny of photogenerated electrons [69]:
a) The photogenerated electrons are trapped to produce OH radicals by H2O2 added because it acts as an electron acceptor:
Fe2O3 → Fe2O3 (ecb-, hvb+) (4)
H2O2 + ecb- → OH- + OH• (5)
b) The photogenerated electrons are trapped to create Fe2+ by the surface Fe3+ in the following reactions:
Fe3+ + ecb- → Fe2+ (6)
Fe2+ + H2O2 → Fe3+ + OH- + OH• (7)
The generated reactive oxygen species cause the degradation of organic dye molecules by transforming them into less harmful or harmless products via photocatalytic α- Fe2O3 nanofibers in the presence of H2O2, which takes place by the two factors above. Additionally, it is important to consider that the •OH radicals can be generated by the photolysis of H2O2 under light irradiation. Fig. 7 shows a schematic illustration of the simplified free radical generation (•OH) mechanism and photocatalytic degradation of the MO and RhB dyes.
The degradation rate of pollutants in aquatic media is assigned through monitoring changes in their absorption characteristics as a function of time. Briefly, the light absorption changes by a species in solution are dependent on its absorptivity and concentration according to the Beer-Lambert Law. It is said that the absorbance's maximum values should gradually decrease depending on the undegraded dye concentration remaining in the solution because of the degradation of organic molecules during photocatalytic reactions. With this perspective, the degradation rate of dye as a function of time in any photocatalytic process can easily be calculated by Beer-Lambert Law, given by the following equation [70] :
A= ε l c (8)
where A is absorbance, ε is molar absorption coefficient (M -1cm-1), l is the optical path length (cm) of light in the medium, and c is the molar concentration of the absorbing species in the medium.
We examined photocatalysis of electrospun nanofibers fabricated by two different iron salts of precursors against rhodamine B (RhB) and methyl orange (MO), respectively, exhibiting different ionic characteristics. The absorption spectrum changes (in a and b) obtained from heterogeneous photocatalysis and their related transformed concentration vs time graphics (c) are represented in figures 8 and 9. When looking at the absorptivity of electrospun nanofibers after stirring for 1 hour in the dark, electrospun Hyd-Fe2O3 nanofibers showed higher adsorption towards different characteristics of dyes relative to the AnHyd- Fe2O3 nanofibers. In the case of the photocatalysis of RhB, AnHyd-Fe2O3 nanofibers degraded %56 of the dye in 2 hours while Hyd-Fe2O3 nanofibers degraded all of the dye molecules in the aquatic media. In the photocatalytic degradation experiment of methyl orange (MO), electrospun Hyd-Fe2O3 nanofibers with highly porous destroyed all MO dye molecules in 90 minutes, whereas Hyd- Fe2O3 nanofibers could degrade as low as %13 of dye in 120 minutes. The electrospun Hyd- Fe2O3 nanofibers, which were smaller in diameter and highly porous, exhibited higher photocatalytic performances against both dyes according to electrospun AnHyd-Fe2O3 nanofibers. Also, the electrospun Hyd-Fe2O3 nanofibers were better for degrading MO than for degrading RhB.
The efficient removal of model dyes (RhB and MO) through heterogeneous photocatalytic processes in this work could have resulted from several incorporated sophisticated effects. First, the morphology of the fibres owns orientated nanograins with a few ten nanometers in size and is highly porous, which could enhance the charge transfer mechanism and facilitate electron-hole pair separation. The shorter charge transfer pathway causes faster e−-h+ charge transfer towards the surface before their recombination happens, leading to an enhanced photodegradation rate [71]. The observed enhancement in the photocatalytic activity of the electrospun Hyd-Fe2O3 nanofibers might be due to its multiple light scattering processes leading to reduced e−-h+ recombination rate in the porous architecture. The highly porous architecture can increase light-harvesting efficiency by scattering enhancement and trapping [72].