Synthesis, characterization, and catalytic applications of hematite (α-Fe₂O₃) nanoparticles as reusable nanocatalyst

Iron (III) chloride (FeCl₃), 4-NPD and sodium borohydride were obtained from Sigma-Aldrich. The air dried rhizome of C. rotundus was obtained from local suppliers in Yeongchon, South Korea.

Dried C. rotundus rhizome was crushed to make a fine powder, and 5 g of it was placed in 200 ml of deionized milli-Q water. The broth solution was then boiled for 10 min, filtered through a 0.2 mm filter and stored at 4 °C until needed.

Initially, 5 ml sample of C. rotundus extract was added to 50 ml of the prepared 2 mM iron (III) chloride (FeCl₃) solutions and sonicated for 2 h at 60 °C. The resulting nanoparticles were purified by centrifugation in a Beckman Coulter's Avanti J-E centrifuge (USA) at 10 000 rpm for 20 min.

The ultraviolet–visible (UV–Vis) spectra of the synthesized α-Fe₂O₃ NPs were recorded using a UV–Vis spectrophotometer (Optizen 3220, double beam). Fourier transform infrared (FTIR, JASCO spectrophotometer) spectroscopy was performed to determine the bioactive molecules present in the C. rotundus extract that may account for reduction and stabilization of nanoparticles. The size and morphology of the nanoparticles were analyzed by field emission scanning electron microscopy (FE-SEM coupled with an energy dispersive x-ray microanalysis (EDS) detector, Hitachi S4200, Japan) and transmission electron microscopy (TEM, FEI Tecnai G2 F20 ST FE-TEM). The chemical compositions of the α-Fe₂O₃ NPs were analyzed by EDS attached to the transmission electron microscope. Crystal structure identification was achieved by x-ray diffraction (XRD) using Cu-Kα radiation (PANalytical X'Pert MRD model). In addition, a phase study was performed by Raman spectroscopy using XploRA Plus, HORIBA Scientific with a TE air cooled CCD detector. The weight loss % of the nanoparticles were performed by thermogravimetric analysis (TGA) coupled with differential thermal calorimetry (DTA, SDT-Q600 V20.5 Build 15) from room temperature to 700 °C under a N₂ atmosphere at a heating rate of 10 °C min⁻¹. The magnetic properties were measured by vibrating sample magnetometry (VSM, Lake Shore Cryotronics, Inc., Idea-VSM, model 662).

The photocatalytic experiment was carried out using 15 W Ultraviolet fluorescent lamp. 100 mg of α-Fe₂O₃ nanoparticles were added to 200 ml aqueous solution of CR (10 mg l⁻¹). The solution was continuously stirred in the dark for 1 h to maintain adsorption of CR onto α-Fe₂O₃ nanoparticles. Then the solution was exposed to 15 W ultraviolet fluorescent lamp at room temperature. Degradation of CR aqueous solution was monitored using UV–Vis spectroscopy by collecting the samples at regular intervals.

The reduction of 4-NPD was examined as a model reaction to confirm the catalytic activity of the synthesized α-Fe₂O₃ NPs. The catalytic reaction was monitored by UV–Vis spectroscopy. In these experiments, 2.0 ml of 0.1 mM 4-NPD was mixed with 1.0 ml of 0.1 M NaBH₄ in a quartz cuvette, and the spectrum was measured. After obtaining the spectra, 3.0 mg of the hematite nanoparticles was then added to the above reaction mixture as a catalyst. After adding the α-Fe₂O₃ NPs, the UV–Vis absorption spectra were recorded at 40 s intervals over the scanning range, 200–600 nm, at room temperature (25  ±  2 °C).

The UV–visible spectra of α-Fe₂O₃ NPs synthesized using aqueous C. rotundus extracts showed continuous absorption in the visible region, but small differences in the displayed UV range. The α-Fe₂O₃ NPs showed strong absorption in the UV range compared to FeCl₃ (figure 1(a)). Similar UV–visible spectra were obtained using the previously reported methods [40, 41].

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Figure 1(b) shows the FTIR spectra of the as-synthesized α-Fe₂O₃ NPs. In the spectrum, the vibration band at 3463 cm⁻¹ was ascribed to the O–H stretching and a vibration band at 1631 cm⁻¹ was assigned to C=C stretching [42] due to capping by bioactive molecules present on the surface of the iron nanoparticles. The strong absorption bands at 580 and 474 cm⁻¹ were assigned to the Fe–O group, which is typical for α-Fe₂O₃ [43] due to metal oxygen stretching vibration modes, and were not observed in the plant extract.

The synthesized α-Fe₂O₃ NPs were highly crystalline with XRD peaks corresponding to the face-centered cubic (fcc) phase of metallic iron. All the XRD peaks were strong, narrow and sharp without the presence of other impurity peaks indicating that the as-synthesized product was well crystallized. The major XRD peaks at 24.14°, 33.24°, 35.61°, 40.81°, 49.47°, 54.09°, 57.62°, 62.46°, and 64.13° 2θ were assigned to the (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (0 1 8), (2 1 4) and (3 0 0) planes, respectively (figure 1(c)). The products were indexed to pure α-Fe₂O₃ NPs with a rhombohedral structure, which is in agreement with the reported [21] values (hematite, JCPDS no.33-0664). The high purity indicates that the as-synthesized α-Fe₂O₃ NPs are good for catalytic applications. Similar XRD peaks to those reported previously were observed [44, 45].

TGA was carried out to determine if the precursor had been converted completely to the desired metal oxide from room temperature to 700 °C. The mass loss occurs in the following three steps: a weight loss of 2.5% in the first step (100 °C) due to the removal of adsorbed moisture and other volatile matter in the nanoparticles; a weight loss of 13.1% in the second step (200 °C), indicating the decomposition of capping agents and other biomolecules present in the nanoparticles; and a weight loss after 250 °C (20.1%) due to the adsorbed oxygen species [46]. The final residual mass was found to be 64.3%, which confirmed the presence of α-Fe₂O₃ NPs (figure 1(d)). The differential scanning calorimetry (DSC) thermogram revealed heat variations associated with an exothermic and endothermic reaction. The endothermic peaks observed at 138.5 °C and 300.7 °C, were assigned to thermal decomposition of the bioactive molecules present in the plant extract.

The surface morphology of the prepared α-Fe₂O₃ NPs was examined by SEM (figure 2(a)), which suggested that the particles were slightly irregular in shape and size. The porous structure formed by the network of spherical nanoparticles interlinked over a rhombohedral material [47] was also evident from the images. The TEM images of the nanoparticles showed that the particles have both spherical and rhombohedral morphologies [48] and monodisperse in nature (figure 2(b)). The mean grain size was found to be approximately 60 and 80 nm. Hematite nanoparticles with few agglomerates were visible in the TEM images [49]. The elemental composition assigned by EDS revealed a strong signal at ~0.7 and 6.2 keV for iron and 0.2 keV for oxygen (figure 2(c)) due to surface plasmon resonance. The typical absorption peaks for C and Cu was assigned to the carbon coated copper grid used for sample preparation.

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To further clarify the phase of the rhombohedral particles, figure 3 shows the Raman spectrum of α-Fe₂O₃ NPs. The α-Fe₂O₃ NPs showed three strong peaks around 222, 281, and 399 cm⁻¹, and two weak bands at 483 and 610 cm⁻¹. The peaks at 281 cm⁻¹ and 399 cm⁻¹ in the Raman spectrum of iron are related to the stretching (Fe–O) mode between two Fe and O atoms [50]. The positions and intensities of these bands are in good agreement with the previously reported data for face-centered α-Fe₂O₃ NPs [51, 52].

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To understand the magnetic behavior of the synthesized α-Fe₂O₃ NPs, magnetic hysteresis measurements were carried out in an applied magnetic field at room temperature. The values of saturation magnetization (M_s), remanent magnetization (M_r), and coercivity (H_c) are shown in figure 4(a). The magnetization increased almost linearly with increasing applied magnetic field up to 5000 Oe. The magnetization measurements of the α-Fe₂O₃ NPs exhibit hysteresis loops with the saturation magnetization (M_s), remanent magnetization (M_r) and coercivity (H_c) of 10.01 emu g⁻¹, 1.03 emu g⁻¹ and 200 Oe, respectively, suggesting that the rhombohedral α-Fe₂O₃ NPs are ferromagnetic in nature.

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The catalytic activity was first measured using NaBH₄ for the reduction of 4-NPD. The reduction reaction was monitored by UV–Vis spectroscopy. The intensity of the color of 4-NPD changed after adding an aqueous solution of NaBH₄. This color change was attributed to the formation of the phenolate ion. The phenolate ion showed an absorption peak at 400 nm, which remained intact in the presence of NaBH₄ for 30 min, as shown in figure 4(b). This suggests that no reduction occurred in the absence of the catalyst. This might be due to the high kinetic barrier between the mutually repelling negative ions of phenolate ion and borohydride ion [52]. After the addition of α-Fe₂O₃ NPs (1.0 mg ml⁻¹), the reduction of 4-NPD was completed within 12 min, as shown in figure 4(c). The intensity of the absorption peak at 400 nm decreased and finally disappeared, giving rise to a peak at 300 nm, indicating the reduction of 4-NPD to 1,2,4-benzenetriamine. The rate constant, which was calculated by plotting ln(A_t/A₀) as a function of time for the reduction of 4-NPD, was found to be 7.0  ×  10⁻³ min⁻¹, as shown in figure 4(d).

The decolorization performance of the synthesized α-Fe₂O₃ NPs were tested on CR dye. Figure 5(a) shows the adsorption spectra of the CR solution in the presence of α-Fe₂O₃ NPs at different time intervals. The main absorption peak of CR centered at 500 nm before and after irradiation. When the fluorescence lamp was turned on, the main peaks decreased with the increased irradiation time, revealing that the CR solution was decomposed in the present system within 25 min. Figure 5(b) shows the photocatalytic degradation rate of CR.

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After the reaction, the catalyst was recovered by an external magnetic field. The residues were then washed with dichloromethane and dried at 80 °C. The recovered catalyst was then reused in the same reduction reaction under identical conditions three times without any significant loss.