Facile Fabrication of Three-Dimensional Fusiform-Like α-Fe2O3 for Enhanced Photocatalytic Performance

α-Fe2O3 fusiform nanorods were prepared by a simple hydrothermal method employing the mixture of FeCl3·6H2O and urea as raw materials. The samples were examined by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and UV–vis diffuse reflectance spectra (UV–DRS). Its visible-light photocatalytic performances were evaluated by photocatalytic decolorization methylene blue (MB) in visible light irradiation. It was found that pure phase α-Fe2O3 nanorods with a length of about 125 nm and a diameter of 50 nm were successfully synthesized. The photocatalytic decolorization of MB results indicated that α-Fe2O3 nanorods showed higher photocatalytic activity than that of commercial Fe2O3 nanoparticles—these are attributed to its unique three-dimensional structure and lower electron-hole recombination rate.


Introduction
Environmental pollution has severely threatened human survival and prevented social development. However, semiconductor photocatalysis is regarded as a latent approach to solving current environmental issues [1][2][3]. Recently, the use of visible light and semiconductor photocatalysts to promote the degradation of environmental pollutants has attracted more and more attention [4,5]. In addition, available semiconductor photocatalysts (such as TiO 2 and ZnO) are usually limited by either low efficiency in utilizing visible light or a high charge recombination rate. Hence, alternative strategies have been put forward to enhance their photocatalytic activity under visible light.
Among various metal oxide nanostructures, the scientific community has paid more attention to three-dimensional iron oxide and hydroxyl oxide nanostructures due to their inherent magnetic, morphological and phase-dependent features; they are applied in many fields such as biomedical treatment, water treatment and gas sensors. Hematite (α-Fe 2 O 3 ) is considered to be one of the main forms of pure phase iron oxide, which is able to maintain the highest thermodynamic stability possible; thus it is usually used as a sensitizer for wide-bandgap semiconductors [6][7][8][9][10][11]. α-Fe 2 O 3 has aroused great attention as a consequence of its abundant availability, environmental compatibility and very stable corundum structure [5,12,13]. However, its low conductivity, short carrier diffusion length and relatively high potential, limit the saturation current and current development potential [14]. α-Fe 2 O 3 is prepared by methods such as chemical vapor deposition (CVD) [15][16][17], spray pyrolysis [18], hydrothermal methods [19], and precipitation [20]. Shape, size, surface structure and microstructure are the main factors which affect the chemical and physical properties of nanomaterials. Recently, different morphology types of α-Fe 2 O 3 have been extensively studied. For instance, nanocrystals [21,22], polyhedral nanoparticles [23], nanorods [24], nanoribbons [25], nanotubes [26], nanostructured microspheres [27,28], hollow nanostructures [29,30] and nanoplates [31] are used to heighten the photocatalytic performance of α-Fe 2 O 3 . Cha et al. reported on the synthesis of α-Fe 2 O 3 nanorods with efficient photocatalytic and magnetic properties [32]. Hao et al. synthesized single-crystalline α-Fe 2 O 3 nanoplates, exhibiting excellent photocatalytic properties towards RhB and weak ferromagnetic behavior [33]. Chen et al. synthesized α-Fe 2 O 3 crystals with nanoparticle, nanotube, and nanorod-like morphologies by employing a facile hydrothermal method and examined their photocatalytic activity [34]. However, there are few reports on the use of hematite with special spindle morphology as photocatalysts. Liu et al. synthesized porous fusiform-Fe 2 O 3 (hematite) by hydrothermal synthesis assisted by a simple surfactant sodium dodecyl sulfate (SDS) [35].
Herein, we report a facile route to prepare α-Fe 2 O 3 nanorods. The spindle β-FeOOH nanorods are firstly obtained via a water bath treatment of aqueous solution containing FeCl 3 ·6H 2 O, urea and polyethylene glycol-2000. α-Fe 2 O 3 nanorods are prepared by the calcination of β-FeOOH at 400 • C for 2 h and their photocatalytic activity is explored by degradation of the pollutant methylene blue (MB). Compared to commercial Fe 2 O 3 nanoparticles, the α-Fe 2 O 3 nanorods showed higher photocatalytic properties towards MB in visible light irradiation.

Synthesis of α-Fe 2 O 3 Nanorods
In this work, a facile hydrothermal method was used to obtain α-Fe 2 O 3 nanorods . In a typical procedure, 4 g of FeCl 3 ·6H 2 O, 1 g of urea and 2 g of polyethylene glycol-2000 were dissolved in 70 mL distilled water under vigorous stirring. After stirring, the resulting mixture was heated to 85 • C in a water bath for 2 h. Then, the mixture was separated by means of a centrifuge at 8000 rpm/min and washed sequentially with distilled water and ethanol repeatedly. The β-FeOOH precursor (P85) was obtained and sintered at 400 • C for 2 h in the air in a pipe furnace; then it was cooled down to obtain the final α-Fe 2 O 3 nanorod (P85-1) sample.

Characterization
The phase composition of the samples was characterized by means of X-ray diffraction (XRD) (X'Pert Pro, PANalytical) operaing at 40 kV and 40 mA with Cu-K α radiation (λ = 1.5406 Å). The morphology and structure of as-prepared samples were observed by HRTEM (JEM-2100) with an acceleration voltage of 200 kV. Carbon-coated copper grids were used as the sample holders. SEM was carried out using a Hitachi S-4800 instrument operating at 5 kV. FT-IR of the samples were collected with a PE Spectrum One B IR spectrometer. UV-DRS were determined by a UV-vis spectrophotometer (Shimadzu UV-2550).

Photocatalytic Experiments
The photo degradation experiments were performed in a quartz reactor (using a small magneton for stirring) containing 40 mL (10 mg/L) of MB solution and 0.1 g of catalyst.
During the process of photocatalysis, all other lights were insulated. The high-pressure Xenon lamp (150 W, GYZ220, China) was used as a visible-light source, which was placed at about 10cm from the reactor. A 410 nm cut off filter was placed above the reactor to cut off UV light; the average light intensity was 50 mW/cm 2 . Prior to irradiation, the suspension was kept in the dark under stirring for 60 min in order to ensure the establishing of an adsorption/desorption equilibrium. At given time intervals, 4mL of aliquots were collected from the suspension and immediately centrifuged and analyzed by means of recording variations of the maximum absorption band (664 nm) of MB using a UV-visible spectrophotometer (UV 2550, Shimadzu). Figure 1 demonstrates the typical XRD patterns of the precursor β-FeOOH (P85) and α-Fe 2 O 3 (P85-1). From Figure 1a, there are peaks at 11.83 • , 16.74 • and 26.85 • , and so on, which are in good agreement with the JCPDS file of β-FeOOH (JCPDS 34-1266) [36,37]. As is shown in Figure 1b [27,38]; this is consistent with the XRD results of commercial Fe 2 O 3 nanoparticles ( Figure S1 in Supplementary Materials). In addition, characteristic peaks of impurities could not be observed; this indicates the phase transition from β-FeOOH to α-Fe 2 O 3 . The augmented peak sharpness in Figure 1b indicates that α-Fe 2 O 3 is well crystallized. ensure the establishing of an adsorption/desorption equilibrium. At given time intervals 4mL of aliquots were collected from the suspension and immediately centrifuged and analyzed by means of recording variations of the maximum absorption band (664 nm) o MB using a UV-visible spectrophotometer (UV 2550, Shimadzu). Figure 1 demonstrates the typical XRD patterns of the precursor β-FeOOH (P85 and α-Fe2O3 (P85-1). From Figure 1a, there are peaks at 11.83°, 16.74° and 26.85°, and so on, which are in good agreement with the JCPDS file of β-FeOOH (JCPDS 34-1266) [36,37 As is shown in Figure 1b 1010) and (220) facets of α-Fe2O3 nanopolyhedrons, respectively, which is consistent with the JCPDS file of α-Fe2O3 (JCPDS 33-0664) [27,38]; this is consistent with the XRD results of commercial Fe2O3 nanoparticles ( Figure S1 in Supplementary Materials). In addition, characteristic peaks of impurities could not be observed; this indicates the phase transition from β-FeOOH to α-Fe2O3. The augmented peak sharpness in Figure 1b indicates that α-Fe2O3 is well crystallized. The morphology of the samples was detected by SEM. Figure 2 demonstrates the SEM images of the precursor P85, sample P85-1 and commercial Fe2O3 nanoparticles Figures 2a and Figure S2 (Supplementary Materials) show the images of the precursor which clearly demonstrate that the nanorods were of a length of about 200 nm and a diameter of about 60 nm. The surface of the nanorods was smooth and the smooth-surfaced particles were similar in size. Figure 2b,c shows the images of P85-1 which had changed following sintering, from a fusiform shape to irregular rods. The rod-shaped particles were polymerized, with a length of about 125 nm and a diameter of 50 nm. Figure 2d shows the images of commercial Fe2O3 nanoparticles were near-spherical. It can be concluded from Figure 2 that Fe2O3 nanoparticles with differen The morphology of the samples was detected by SEM. Figure 2 demonstrates the SEM images of the precursor P85, sample P85-1 and commercial Fe 2 O 3 nanoparticles. Figure 2a and Figure S2 (Supplementary Materials) show the images of the precursor, which clearly demonstrate that the nanorods were of a length of about 200 nm and a diameter of about 60 nm. The surface of the nanorods was smooth and the smoothsurfaced particles were similar in size. Figure 2b,c shows the images of P85-1, which had changed following sintering, from a fusiform shape to irregular rods. The rod-shaped particles were polymerized, with a length of about 125 nm and a diameter of 50 nm. Figure 2d shows the images of commercial Fe 2 O 3 nanoparticles were near-spherical. It can be concluded from Figure 2 that Fe 2 O 3 nanoparticles with different morphologies were prepared under different experimental conditions, and the prepared Fe 2 O 3 exhibited a more regular fusiform-like structure.  The size and microstructure of the prepared β-FeOOH and α-Fe2O3 samples were further examined with TEM in Figure 3a,c. Furthermore, Figure 3b demonstrates a lattice fringe of 0.74 nm corresponding to the (110) facet of β-FeOOH, which further confirmed that P85 is β-FeOOH. Besides this, the distance of 0.35 nm of α-Fe2O3 could clearly identify lattice spacing, which corresponds to interplane distances of (012) plane in Figure 3d, consistent with the XRD results [27,38]. The size and microstructure of the prepared β-FeOOH and α-Fe 2 O 3 samples were further examined with TEM in Figure 3a,c. Furthermore, Figure 3b demonstrates a lattice fringe of 0.74 nm corresponding to the (110) facet of β-FeOOH, which further confirmed that P85 is β-FeOOH. Besides this, the distance of 0.35 nm of α-Fe 2 O 3 could clearly identify lattice spacing, which corresponds to interplane distances of (012) plane in Figure 3d, consistent with the XRD results [27,38].

Results and Discussion
The infrared spectrum of β-FeOOH in Figure 4a demonstrates that the precursor P85 showed absorption peaks at 3440 cm −1 and 1600 cm −1 , 850 cm −1 , 700 cm −1 . Of these, the peaks at 3440 cm −1 and 700 cm −1 showed strong absorption, and the peak at 3440 cm −1 corresponded to symmetric and anti-symmetric stretching vibrations of O-H group. The peak at 1600 cm −1 corresponded to the bending vibration of O-H bond, while the other two peaks at 850 cm −1 and 700 cm −1 corresponded to the stretching vibration of Fe-O bond [30,38]. Figure 4b demonstrated that the strong absorption peaks of α-Fe 2 O 3 only emerge at 508 cm −1 and 480 cm −1 . In addition, the two stretching vibration peaks for Fe-O at 3440 cm −1 and 1600 cm −1 only appeared as weak absorption peaks, which is consistent with commercial Fe 2 O 3 results ( Figure S3 in Supplementary Materials). This is because the O-H bond absorption had disappeared completely, which indicated that the precursor β-FeOOH had transformed to α-Fe 2 O 3 [23,38,39]. The infrared spectrum of β-FeOOH in Figure 4a demonstrates that the precursor P85 showed absorption peaks at 3440 cm −1 and 1600 cm −1 , 850 cm −1 , 700 cm −1 . Of these, the peaks at 3440 cm −1 and 700 cm −1 showed strong absorption, and the peak at 3440 cm −1 corresponded to symmetric and anti-symmetric stretching vibrations of O-H group. The peak at 1600 cm −1 corresponded to the bending vibration of O-H bond, while the other two peaks at 850 cm −1 and 700 cm −1 corresponded to the stretching vibration of Fe-O bond [30,38]. Figure 4b demonstrated that the strong absorption peaks of α-Fe2O3 only emerge at 508 cm −1 and 480 cm −1 . In addition, the two stretching vibration peaks for Fe-O at 3440 cm −1 and 1600 cm −1 only appeared as weak absorption peaks, which is consistent with commercial Fe2O3 results ( Figure S3 in Supplementary Materials). This is because the O-H bond absorption had disappeared completely, which indicated that the precursor β-FeOOH had transformed to α-Fe2O3 [23,38,39].  UV-Vis diffuse reflectance was measured through ultraviolet and visib absorption technology. It can be seen from Figure 5 that the commercial Fe2O3 sh narrow absorption of visible light with an edge that occurred at around 450 nm. H compared with the commercial Fe2O3 and the P85 precursor, the photo absorptio of the prepared α-Fe2O3 nanorods showed a more obvious redshift from 450 nm UV-Vis diffuse reflectance was measured through ultraviolet and visible light absorption technology. It can be seen from Figure 5 that the commercial Fe 2 O 3 showed a Nanomaterials 2021, 11, 2650 6 of 9 narrow absorption of visible light with an edge that occurred at around 450 nm. However, compared with the commercial Fe 2 O 3 and the P85 precursor, the photo absorption edge of the prepared α-Fe 2 O 3 nanorods showed a more obvious redshift from 450 nm to 550 nm, so that the optical absorption of α-Fe 2 O 3 nanorods was significantly stronger in visible-light regions [4,10,40]. Furthermore, the increase of light absorption range was conducive to the enhancement of photocatalytic activity. In addition, the UV-Vis diffuse reflectance was further combined and the band gap diagram was calculated via Tauc plot ( Figure S4   UV-Vis diffuse reflectance was measured through ultraviolet and visibl absorption technology. It can be seen from Figure 5 that the commercial Fe2O3 sh narrow absorption of visible light with an edge that occurred at around 450 nm. H compared with the commercial Fe2O3 and the P85 precursor, the photo absorptio of the prepared α-Fe2O3 nanorods showed a more obvious redshift from 450 nm nm, so that the optical absorption of α-Fe2O3 nanorods was significantly stron visible-light regions [4,10,40]. Furthermore, the increase of light absorption ran conducive to the enhancement of photocatalytic activity. In addition, the UV-Vis reflectance was further combined and the band gap diagram was calculated via Ta ( Figure S4 in Supplementary Materials). Compared with the band gap of com Fe2O3 (2.2 eV), the band gap (1.95 eV) of P85-1 was significantly shorter, indicati P85-1 has a stronger light utilization efficiency and enhanced photocatalytic abilit Methylene blue (MB) is a highly significant dye and has been extensively app industrial production, which inevitably pollutes the environment. Therefo Methylene blue (MB) is a highly significant dye and has been extensively applied in industrial production, which inevitably pollutes the environment. Therefore, the photocatalytic performance of prepared α-Fe 2 O 3 was evaluated by degradation of MB under visible light irradiation. Before the photocatalytic process, the solution containing MB and catalysts were agitated to reach the adsorption equilibrium in the dark. Figure 6 demonstrates the photocatalytic evaluation curve of the prepared α-Fe 2 O 3 under visible light. The figure clearly shows the variation of MB solution concentrations with degradation time and the fitting result of degradation kinetics. With the extension of light time, the MB solution concentration gradually decreased and the degradation process conformed to a first-order kinetic model. After 60 min of light, the degradation rate of MB of α-Fe 2 O 3 reached 83%, which is better than that of commercial Fe 2 O 3 and the P85 precursor. Moreover, the catalystfree condition was also used as a comparative experiment and its degradation rate was only about 25%. This further confirms that the prepared α-Fe 2 O 3 exhibits remarkably high photocatalytic activity, which is consistent with the results on photocurrent ( Figure S5 in Supplementary Materials). The repeated degradation experiments of α-Fe 2 O 3 (P85-1) showed a slight decrease in MB degradation rate after four cycles, indicating that it had high stability ( Figure S6 in Supplementary Materials). Besides this, there were no significant differences in the XRD spectra of α-Fe 2 O 3 before and after four cycles of use, which further proves its stability ( Figure S7 in Supplementary Materials). experiment and its degradation rate was only about 25%. This further confirms that the prepared α-Fe2O3 exhibits remarkably high photocatalytic activity, which is consistent with the results on photocurrent ( Figure S5 in Supplementary Materials). The repeated degradation experiments of α-Fe2O3 (P85-1) showed a slight decrease in MB degradation rate after four cycles, indicating that it had high stability ( Figure S6 in Supplementary Materials). Besides this, there were no significant differences in the XRD spectra of α-Fe2O3 before and after four cycles of use, which further proves its stability ( Figure S7 in Supplementary Materials).

Conclusions
In conclusion, a simple one-step hydrothermal method was used to prepare three-dimensional fusiform-like α-Fe2O3, which was applied to photocatalytic degradation of MB. It could be seen through SEM and TEM that the three-dimensional fusiform-like α-Fe2O3 was about 125 nm in length and 50 nm in diameter. In addition, the prepared α-Fe2O3 showed excellent degradation efficiency of MB solution under visible light illumination, which reached 83% after 60 min sunlight illumination. This is far superior to traditional commercial Fe2O3 and further proves α-Fe2O3's excellent photocatalysis performance. As a result, α-Fe2O3 has a wider visible light absorption edge, which promotes the improvement of photocatalytic activity, thereby showing better degradation performance. Furthermore, the as-prepared samples exhibited more advantages such as being low cost, environmentally friendly and low risk. Therefore, this work provides a promising and constructive theoretical support for solving problems such as energy shortages and environmental pollution in the future.

Conclusions
In conclusion, a simple one-step hydrothermal method was used to prepare threedimensional fusiform-like α-Fe 2 O 3 , which was applied to photocatalytic degradation of MB. It could be seen through SEM and TEM that the three-dimensional fusiform-like α-Fe 2 O 3 was about 125 nm in length and 50 nm in diameter. In addition, the prepared α-Fe 2 O 3 showed excellent degradation efficiency of MB solution under visible light illumination, which reached 83% after 60 min sunlight illumination. This is far superior to traditional commercial Fe 2 O 3 and further proves α-Fe 2 O 3 's excellent photocatalysis performance. As a result, α-Fe 2 O 3 has a wider visible light absorption edge, which promotes the improvement of photocatalytic activity, thereby showing better degradation performance. Furthermore, the as-prepared samples exhibited more advantages such as being low cost, environmentally friendly and low risk. Therefore, this work provides a promising and constructive theoretical support for solving problems such as energy shortages and environmental pollution in the future.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.