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

Abstract

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.

1. 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 TiO₂ 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-Fe₂O₃) 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-Fe₂O₃ 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-Fe₂O₃ 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-Fe₂O₃ 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-Fe₂O₃. Cha et al. reported on the synthesis of α-Fe₂O₃ nanorods with efficient photocatalytic and magnetic properties [32]. Hao et al. synthesized single-crystalline α-Fe₂O₃ nanoplates, exhibiting excellent photocatalytic properties towards RhB and weak ferromagnetic behavior [33]. Chen et al. synthesized α-Fe₂O₃ 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-Fe₂O₃ (hematite) by hydrothermal synthesis assisted by a simple surfactant sodium dodecyl sulfate (SDS) [35].

Herein, we report a facile route to prepare α-Fe₂O₃ nanorods. The spindle β-FeOOH nanorods are firstly obtained via a water bath treatment of aqueous solution containing FeCl₃·6H₂O, urea and polyethylene glycol-2000. α-Fe₂O₃ 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 Fe₂O₃ nanoparticles, the α-Fe₂O₃ nanorods showed higher photocatalytic properties towards MB in visible light irradiation.

2. Experimental Section

2.1. Synthesis of α-Fe₂O₃ Nanorods

In this work, a facile hydrothermal method was used to obtain α-Fe₂O₃ nanorods_. In a typical procedure, 4 g of FeCl₃·6H₂O, 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-Fe₂O₃ nanorod (P85-1) sample.

2.2. 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).

2.3. 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/cm². 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).

3. Results and Discussion

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, the diffraction peaks at 24.0°, 33.0°, 35.5°, 40.7°, 49.3°, 54.0°, 57.6°, 62.3°, 63.9°, 71.8° and 75.3° were attributed to (012), (104), (110), (113), (024), (116), (018), (214), (300), (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. 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 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 different morphologies were prepared under different experimental conditions, and the prepared Fe₂O₃ 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 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 visible light absorption technology. It can be seen from Figure 5 that the commercial Fe₂O₃ showed a narrow absorption of visible light with an edge that occurred at around 450 nm. However, compared with the commercial Fe₂O₃ and the P85 precursor, the photo absorption edge of the prepared α-Fe₂O₃ nanorods showed a more obvious redshift from 450 nm to 550 nm, so that the optical absorption of α-Fe₂O₃ 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 in Supplementary Materials). Compared with the band gap of commercial Fe₂O₃ (2.2 eV), the band gap (1.95 eV) of P85-1 was significantly shorter, indicating that P85-1 has a stronger light utilization efficiency and enhanced photocatalytic ability.

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-Fe₂O₃ 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-Fe₂O₃ 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-Fe₂O₃ reached 83%, which is better than that of commercial Fe₂O₃ and the P85 precursor. Moreover, the catalyst-free condition was also used as a comparative 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).

4. 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.