Evaluation of Photocatalytic Activity and Electrochemical Properties of Hematite Nanoparticles

Abstract

The symmetric nano morphologies, asymmetric electronic structures, and as well as the heterojunctions of the developed photocatalytic systems perform a vital role in promoting light absorption, separation of electron and hole pairs and charge carrier transport to the surface when exposed to near-infrared (NIR) light. In this present work, we synthesized hematite (α-Fe₂O₃) nanoparticles (NPs) by a facile hydrothermal method and studied their structural, optical, and photocatalytic properties. Powder X-ray diffraction (XRD) confirmed the rhombohedral phase of the α-Fe₂O₃ NPs, and Fourier transform infrared spectroscopy (FT-IR) was used to investigate symmetric and asymmetric stretching vibrations of the functional groups on the surface of the catalysts. The optical bandgap energy was estimated to be 2.25 eV using UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and scanning electron microscopy (SEM) images indicated sphere like morphology. The oxidation and reduction properties of α-Fe₂O₃ NPs were analyzed by cyclic voltammetry (CV). The α-Fe₂O₃ NPs were utilized for the degradation of methylene blue (MB) dye under natural sunlight. The experimental results demonstrate that the degradation efficiency was achieved at 33% in 2 h, and the pseudo-first-order rate constant was calculated to be 0.0033 min⁻¹.

1. Introduction

Advances in the field of nanotechnology over the past few decades have enabled researchers to develop and synthesize nanomaterials with unique physicochemical properties for photocatalytic applications. Among these nanomaterials, metal-oxide-based nanoparticles have attracted great attention due to their excellent physiochemical properties, which are largely different from their bulk counterpart. Nanomaterials have become a fascinating class of materials that are highly sought after for various useful applications. Incorporating nanoscale materials into catalyst research generates new ideas for the development of catalyst science. Nanocatalysts have high activity and selectivity, allowing them to improve the rate and yield of catalytic reactions. The photodegradation of organic pollutants is significantly increased when metal oxide nanoparticles are used as catalysts. Additionally, the catalytic performance of metal oxides depends not only on their composition but also on their structure, phase, shape, size, particle size, and the particle agglomeration of the dispersed sample. Advancement in industrialization, leading to toxic by-products, has altered the environment by unleashing a distinct variety of toxins and emissions of hazardous gases into the atmosphere [1,2,3]. Therefore, conventionally implemented strategies such as immobilization, biological and chemical oxidation, and incineration have been largely employed to treat several types of organic and toxic industrial contaminants [4,5]. Nanomaterials (NMs), due to their nanoscale sizes, have augmented a new prospect for their several industrial and environmental applications, including sewage treatment and removal of hazardous contaminants [6,7,8]. Confronting the subject of treatment of hazardous contaminants from the environment, NMs delivering unique optical, magnetic, or electrical properties become a fundamental key for environmental remediation [9]. Therefore, the distinct physicochemical material characteristics that were unattainable in the conventional bulk matrix drew several research scholars and scientists to elevate their attention to the field of nanoscience and technology [10,11,12]. Metal oxide nanoparticles such as TiO₂, ZnO, SnO₂, and ZrO₂ and their combination with other metals/metal oxides are effective photocatalysts in water treatment processes, including pharmaceutical and dye-containing effluents [10,11,12,13,14,15,16,17]. Among all metal oxide NPs, iron oxide nanoparticles (Fe₂O₃ NPs) have received much attention for organic pollutants degradation and biomedical applications, etc., due to their low cost, low toxicity, and unique magnetic properties [18,19]. Iron oxide can exist in different polymorphs, including magnetite (Fe₃O₄), wustite (FeO), maghemite (γ-Fe₂O₃), and hematite (α-Fe₂O₃), of which hematite is thermodynamically stable phase under ambient conditions [20,21,22]. Synthesis of α-Fe₂O₃ NPs was reported by using various methods [23,24,25,26,27,28]. Hematite (α-Fe₂O₃) is a metal-oxide-based semiconductor which has been widely investigated in the field of photocatalysis, including photocatalytic water splitting, CO₂ reduction, and organic pollutants degradation. It has a narrow bandgap energy of 2.1 eV, which allows light absorption in the visible region up to 570 nm, as well as suitable VB and CB edge position for water splitting, CO₂ reduction, and dye degradation. In addition, it exhibits several favorable properties, including earth abundance, nontoxicity, facile synthetic accessibility, and high physicochemical stability [29]. Therefore, in the present work, α-Fe₂O₃ nanoparticles were synthesized by hydrothermal route, and their photocatalytic and electrochemical activities were studied.

2. Experimental Procedure

2.1. Materials

For the hydrothermal synthesis of α-Fe₂O₃ NPs, the following chemicals were used without any further purification. Analytical-grade ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O), ammonia (NH₃) solution, and absolute ethanol were purchased from Sigma-Aldrich.

2.2. Synthesis of α-Fe₂O₃ Nanoparticles

For the hydrothermal synthesis of α-Fe₂O₃ NPs [20], an iron source, Fe (NO₃)₃·9H₂O (1.8 g), was dissolved in 20 mL of distilled water to form a homogeneous solution. Subsequently, an ammonia solution was added dropwise to the above reaction solution under constant stirring for 30 min to adjust the pH value to 9.0. Subsequently, the reaction mixture was transferred to a 25 mL Teflon-lined stainless-steel autoclave. The autoclave was then heated at 150 °C for 10 h in a heating oven and naturally cooled to room temperature. Finally, the resulting product was washed several times with distilled water followed by ethanol and dried at 80 °C in a drying oven.

2.3. Study of Photocatalytic Activity

The photocatalytic degradation performance of α-Fe₂O₃ NPs was evaluated using MB dye as a model pollutant. In a typical photocatalytic degradation reaction, 40 mg of the catalyst powder was dispersed in 60 mL of a solution containing 10 ppm MB dye and stirred for 30 min. Before starting the photocatalytic reaction, the reactor was kept in the dark and stirred for 30 min to ensure the adsorption/desorption equilibrium. During this time, MB dye molecules were adsorbed on the surface of the α-Fe₂O₃ NPs. Subsequently, the reactor was exposed to natural outdoor sunlight, and 5 mL aliquots were taken from the reaction mixture every 30 min interval. The reaction mixture was then centrifuged, and the absorbance of the supernatant was measured by UV–Vis spectroscopy at the maximum absorption wavelength of the MB dye (λ_max = 662 nm). The percentage of degradation efficiency was estimated by Equation (1)

$$\mathrm{Degradation} (\%)=\frac{{\mathrm{C}}_0-\mathrm{C}}{{\mathrm{C}}_0}\times 100$$

(1)

where C₀ and C denote the absorbance initially before exposure to sunlight and after exposure time t, respectively.

2.4. Study of Electrochemical Activity

The electrochemical activity of α-Fe₂O₃ NPs was evaluated using a three-electrode system. A α-Fe₂O₃ drop-casted GEC acted as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as the reference electrode. A 0.5 M Na₂SO₄ solution was used as an electrolyte for electrochemical activity studies. The potential was applied to the working electrode with respect to the Ag/AgCl reference electrode in the range of −2.0 to 2.0 at a scan rate of 20 mV/s. The working electrode was fabricated as follows: First, the GCE surface was polished using 0.5 µm alumina slurry, followed by rinsing with deionized water and drying at room temperature. Then, 20 mg of the catalyst powder was dispersed in a mixture of ethanol (0.5 mL) and Nafion (5 wt%) solution, followed by sonication for 30 min. This is known as ink or slurry solution. Then, 15 µL of the ink solution was drop-casted onto the surface of the GEC and allowed to dry at room temperature. Electrochemical impedance measurements were performed using a Versa STST MC impedance spectrometer in the frequency range of 1 Hz to 1 MHz.

2.5. Characterization

The structural properties of the prepared sample were investigated by X-ray diffraction analysis using the Rigaku Multiflex diffractometer (Japan). The surface morphology and chemical composition of the samples were examined by SEM using the EI-Quanta FEG 200F instrument (Thermo Fisher Scientific, USA), which was connected to an EDX detector. The chemical structure and surface functional groups were identified using the FT-IR instrument (Perkin Elmer, USA). The optical characteristic of the sample was explored using the Cary 50 UV–Vis spectrophotometer (Varian Technologies, USA).

3. Results and Discussion

3.1. Structural Analysis

XRD analysis was carried out for α-Fe₂O₃ NPs in the 2θ range of 20–80°, as shown in Figure 1. XRD results reveal that the prepared α-Fe₂O₃ NPs have a rhombohedral structure with a space group of R-3c, and the diffraction characteristic peaks are indexed as (012), (104), (110), (113), (024), (116), (018), (214), (300), (1010), and (220) planes corresponding to α-Fe₂O₃. The data were well corroborated with the literature data JCPDS card No. 84-0311 [30]. Furthermore, the appearance of sharp, narrow, and highly intense peaks in the spectrum indicates the crystalline nature and purity of the synthesized α-Fe₂O₃ sample. The average crystallite size calculated from the three most intense peaks using the Scherrer formula, Equation (2). The average crystal size was found to be 26.2 nm.

$$\mathrm{D}=\frac{\mathsf{\kappa }\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}$$

(2)

where κ denotes the shape factor, which is taken as 0.9, D refers to the average crystallite size, λ is the wavelength of the CuK_α X-ray radiation = 0.15406 nm, and β and θ refer to the full width at half maximum intensity of the peaks and the Bragg angle, both measured in radians, respectively. The average crystal size was calculated from the Williamson–Hall (W-H) plot, as shown in Figure 1b. The W-H equation is given in Equation (3).

$$\mathsf{\beta }\cos \mathsf{\theta }=\frac{0.9\mathsf{\lambda }}{\mathrm{D}}+4\mathsf{\epsilon }\sin \mathsf{\theta }$$

(3)

The average crystalline size was calculated to be 26.18 nm of the α-Fe₂O₃ NPs. The crystallite sizes calculated from the Scherrer formula and W-H plot were approximately the similar for the α-Fe₂O₃ NPs. Furthermore, Figure 1c,d shows the ball-and-stick model and polyhedral model of the rhombohedral crystal structure of α-Fe₂O₃, respectively, where the arrangement of Fe³⁺ and O²⁻ ions in α-Fe₂O₃ can be seen. Moreover, corundum structured and crystallized in the trigonal R-3c space group. Fe³⁺ was bonded to six equivalent O²⁻ atoms to form a distorted face, edge, and corner-sharing FeO₆ octahedron mixture. The angles of the corner-sharing octahedral tilt range from 48–60°. Three Fe-O bond lengths were shorter (1.98 Å) and three were longer (2.13 Å). O²⁻ was bonded to four equivalent Fe³⁺ atoms to form a trigonal pyramid with distorted edges and corner-sharing OFe₄ [19].

3.2. FT-IR Analysis

FT-IR analysis was used to understand the chemical structure of the sample as shown in Figure 2. The weak peak that appeared at 2364 cm⁻¹ can be attributed to the CO₂ asymmetric stretching vibration present in the atmosphere. The band at 1396 cm⁻¹ can be attributed to the -OH in-plane deformation of the Fe-OH bond. The band at 901 cm⁻¹ corresponds to the stretching vibration of the Fe-O-Fe bond [31,32]. The bands at 555 and 469 cm⁻¹ can be assigned to the Fe-O-Fe bending vibration of α-Fe₂O₃ NPs [33].

3.3. Morphology

SEM and EDX were performed to evaluate the morphology and chemical composition of the α-Fe₂O₃ NPs. Figure 3a–c shows the SEM micrographs of the α-Fe₂O₃ NPs. Figure 3a shows the presence of fine grains (b and c) with a spherical shape, with some agglomeration. The average particle size (diameter) was found to be around 150 nm, using ImageJ software (Figure 3d). Figure 3e shows the EDX spectrum of α-Fe₂O₃ NPs, which detected the peaks of Fe and O, resulting in α-Fe₂O₃ NPs with high purity. The EDS images of the α-Fe₂O₃ NPs are shown in Figure 4. The figure shows that Fe and O are evenly distributed, confirming the homogeneity of the sample. Additional peaks for Cl were found in the EDX spectra due to the presence of impurities in the Fe (NO₃)₃·9H₂O starting material. Furthermore, the Fe:O atomic ratio of α-Fe₂O₃ NPs, obtained from the SEM-EDS analysis, is estimated to be approximately 2:3, which is very closely consistent with the theoretical weight ratio (2:3) of Fe₂O₃.

3.4. Dynamic Light Scattering (DLS)

The hydrodynamic particle size distribution of the α-Fe₂O₃ NPs was estimated using dynamic light scattering (DLS). Figure 5 shows the DLS size distribution plot of the α-Fe₂O₃ NPs dispersed in DW. According to the histogram, the average hydrodynamic particle size was found to be 141 nm.

3.5. Optical Properties

UV–Vis spectroscopy was performed to evaluate the absorbance and bandgap of the α-Fe₂O₃ NPs, as shown in Figure 6a,b. The maximum absorption was observed at 560 nm. Based on these results, α-Fe₂O₃ NPs can be used in photocatalytic MB dye degradation that requires visible-light absorption. The band gap energy was calculated by extrapolating the line from the from ${\left(\mathsf{\alpha }\mathrm{h}\mathsf{\nu }\right)}^{1/2}$ versus $\mathrm{h}\mathsf{\nu }$, as shown in Figure 6b. The band gap was calculated to be 2.25 eV, which is in good agreement with an earlier report [34].

3.6. Electrochemical Property Study

The cyclic voltammetry curve of the α-Fe₂O₃ NPs is shown in Figure 7. The observed anodic and cathodic peaks represent the oxidation and reduction reactions associated with the interconversion of Fe⁰ and Fe³⁺ species in the solution, respectively. The presence of double anodic peaks at −0.39 V and 0.32 V might be due to the oxidation of Fe⁰ to Fe²⁺ and then to Fe²⁺ to Fe³⁺ at the applied potential range and pH of the solution. However, the cathodic peak corresponding to the conversion of Fe³⁺ to Fe²⁺ for the reverse reduction reaction almost gets diminished due to the irreversible reaction with the formation of a solid electrolyte interface layer [35]. The Nyquist plot of the electrochemical impedance spectra (EIS) of the synthesized α-Fe₂O₃ NPs is shown in Figure 7b. The Nyquist plot was drawn by imaginary (Z″) versus real (Z′) impedance. The Figure 7b inset shows the equivalent circuit diagram. This study shows the kinetics of the charge transfer between the active electrode and the electrolyte interface. The presence of the semicircle with a small diameter in the higher frequency region indicates the redox reaction, and a linear slope in the lower frequency region in the plot indicates the diffusion-controlled process at the electrode–electrolyte interface. Since the semicircle is quite small, the reaction is kinetically facile, and the ionic conduction and electrolyte diffusion over the porous structure of the electrode is also clearly evident.

3.7. Photocatalytic Activity Study

The photocatalytic activity of the α-Fe₂O₃ NPs for the degradation of the methylene blue dye under sunlight irradiation was evaluated. The degradation of the MB dye irradiated by natural sunlight was observed every 30 min. The intensity of the peak at 662 nm decreased in the presence of the catalyst, complete degradation was observed at 120 min, and the photocatalytic activity is shown in Figure 8a. The photodegradation efficiency of the α-Fe₂O₃ NPs, as shown in Figure 8b, indicates that 33% of the dye decomposed after 2 h of irradiation. The rate of degradation of MB dye without the catalyst (blank test) and with catalyst is shown in Figure 8c; the photodegradation reaction data can be described well by pseudo-first-order reaction kinetics, given by Equation (4), as shown in Figure 8d.

$$\ln \left(\frac{{\mathrm{C}}_0}{\mathrm{C}}\right)={\mathsf{\kappa }}_1\mathrm{t}$$

(4)

where k₁ is the pseudo-first-order reaction rate constant, and C₀ and C are as defined previously in Equation (1). The value of k₁ obtained is 0.0033 min⁻¹, and the half-life of the reaction, calculated using t_1/2 = 0.693/k₁, is 120 min. Table 1 provides a comparison of the photodegradation performances of α-Fe₂O₃ NPs using different methods for various pollutants under different conditions.

When α-Fe₂O₃ absorbs sunlight equal to or greater than the bandgap energy, electrons in the valence band are excited to the conduction band, resulting in the generation of holes in the valence band and electrons in the conduction band. Oxygen and water molecules were adsorbed onto the surface of the α-Fe₂O₃ catalyst. Positive holes in the valence band oxidize OH^- or water at the catalyst surface to produce ^•OH radicals, which are extremely strong oxidants (the oxidation potential of the ^•OH radical is 2.80 V). The hydroxyl radicals subsequently oxidize the MB dye and produce CO₂ and H₂O (Figure 9) [36,37,38]. The electrons on the catalyst surface are rapidly captured by molecular oxygen adsorbed on the catalyst surface and are reduced to form superoxide radical anions (O₂^•−; the oxidation potential of the superoxide radical anion is −2.4 V). Superoxide radicals react with the MB dye to produce CO₂ and H₂O, as shown in Equations (5)–(8), respectively.

$${\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{h}\mathsf{\nu }\to {\mathrm{Fe}}_2{\mathrm{O}}_3({\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}+{\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+})$$

(5)

$${\mathrm{O}}_2+{\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}\to {\mathrm{O}}_2^{-.}$$

(6)

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}\to {\mathrm{H}}^{+}+\mathrm{H}{\mathrm{O}}^{.}$$

(7)

$$\mathrm{M}\mathrm{B}+{\mathrm{O}}_2^{-.} \mathrm{and} {\mathrm{HO}}^{.}\to \mathrm{decompositon} \mathrm{products}$$

(8)

Table 1. Comparison of the photocatalytic degradation of various dyes using various α-Fe₂O₃ nanostructures.

S.No.	Catalyst/ Morphology	Contaminant (s)	Lamp Power and Irradiance	Light	% Removal	Pseudo-First-Order Rate Constant	References
1	Porous Fe2O3 nanorods	Rhodamine B (RhB) eosin B, Methylene blue (MB), p-nitrophenol, Methylene orange (MO)	500 W Xe lamp	Simulated solar light	86% 83% 23% 17% 13%	0.0131 min−1	[39]
2	α-Fe2O3 nanowires	RhB	350 W Xenon lamp	420 nm cut-off filter	85%	-	[40]
3	Porous Fe2O3 nanotubes	RhB	Xenon lamp	λ ≥ 420 nm	99%	0.282 min−1	[41]
4	Cubic Fe2O3 Disc Fe2O3	RhB	12 Philips TL 8w/54-7656 bulb lamps	-	-	0.005 min−1 0.042 min−1	[42]
5	Mesoporous spindlelike Fe2O3	RhB	Mercury and tungsten mixed light lamp (OSRAM, 250W, including UV and visible light)	UV and visible light	95%	-	[43]
6	Ultrathin α-Fe2O3 nanosheets	Bisphenol S (BPS)	A 300 W xenon lamp (PE300BF)	420 nm cut-off filter	90%	0.0164 min−1	[44]
7	1D α-Fe2O3 nanobraids 1D α-Fe2O3 nanoporous	Congo red (CR)	400 W metal halide lamp	λ ≥ 365 nm	91% 90%	-	[45]
8	Porous α-Fe2O3 nanorods	MB	250 W halide lamp	420 nm cut-off filter	95%	1.04 × 10−2 min−1	[46]
9	α-Fe2O3 hollow sphere	Salicylic acid	-	UV light	-	-	[47]
10	α-Fe2O3 hollow spindles	Phenol	high-pressure Hg lamp (500 W, Nanjing Stonetech)	UV irradiation (high-pressure Hg lamp is 365 nm after filtering)	10%	-	[48]
11	α-Fe2O3 mesoporous	Salicylic acid	high-pressure Hg lamp	UV irradiation	95%	-	[49]
12	α-Fe2O3 hollow microspheres	Salicylic acid	high-pressure Hg lamp (300 W)	UV light	58%	-	[50]
13	α-Fe2O3 hollow microspheres assisted solvothermal method	RhB	300 W Xe lamp	400 nm cut-off filter	98%	-	[51]
14	α-Fe2O3	Rose Bengal	200 W tungsten lamp	-	98%	1.57 × 10−2 min−1	[52]
15	α-Fe2O3	H2S	Xe-lamp light source (Oriel, New-port Stratford, Stratford, CT) of intensity 450 W	cut-off filter (>420 nm)	-	-	[53]
16	α-Fe2O3 dendrites, αFe2O3 nanospindles, α-Fe2O3 nanorods, α-Fe2O3 nanocubes	RhB	2 mW UV source	(λ = 365 nm)	82% 83% 84% 84%	0.322 min−1 0.589 min−1 0.8505 min−1 0.876 min−1	[54]
17	α-Fe2O3 nanoparticles	RhB	500 W xenon lamp	420 nm cutoff filter	52%	-	[55]
18	1D α-Fe2O3 microrods 1D α-Fe2O3 nanorods	RhB	300 W xenon lamp	λ > 420 nm	-	0.00977 min−1 0.148 min−1	[56]
19	α-Fe2O3 oblique α-Fe2O3 truncated nanocubes	RhB	300 W Hg lamp	λ = 365 nm	59%	-	[57]
20	α-Fe2O3 microflowers, α-Fe2O3 nanoparticles α-Fe2O3 nanospindles	RhB	500 W xenon lamp	420 nm cut-off filter	98% 94% 91%	-	[58]
21	flowerlike α-Fe2O3 nanostructures	RhB	250 W high-pressure Hg lamp	UV irradiation	59%	-	[59]
22	α-Fe2O3 hollow core/shell hierarchical nanostructures	Phenol	high-pressure Hg lamp	UV irradiation	60%	-	[60]
23	α-Fe2O3 nanoparticle	MB	outdoor sunlight	-	33%	0.0033 min−1	Present work

Table 1. Comparison of the photocatalytic degradation of various dyes using various α-Fe₂O₃ nanostructures.

S.No.	Catalyst/ Morphology	Contaminant (s)	Lamp Power and Irradiance	Light	% Removal	Pseudo-First-Order Rate Constant	References
1	Porous Fe2O3 nanorods	Rhodamine B (RhB) eosin B, Methylene blue (MB), p-nitrophenol, Methylene orange (MO)	500 W Xe lamp	Simulated solar light	86% 83% 23% 17% 13%	0.0131 min−1	[39]
2	α-Fe2O3 nanowires	RhB	350 W Xenon lamp	420 nm cut-off filter	85%	-	[40]
3	Porous Fe2O3 nanotubes	RhB	Xenon lamp	λ ≥ 420 nm	99%	0.282 min−1	[41]
4	Cubic Fe2O3 Disc Fe2O3	RhB	12 Philips TL 8w/54-7656 bulb lamps	-	-	0.005 min−1 0.042 min−1	[42]
5	Mesoporous spindlelike Fe2O3	RhB	Mercury and tungsten mixed light lamp (OSRAM, 250W, including UV and visible light)	UV and visible light	95%	-	[43]
6	Ultrathin α-Fe2O3 nanosheets	Bisphenol S (BPS)	A 300 W xenon lamp (PE300BF)	420 nm cut-off filter	90%	0.0164 min−1	[44]
7	1D α-Fe2O3 nanobraids 1D α-Fe2O3 nanoporous	Congo red (CR)	400 W metal halide lamp	λ ≥ 365 nm	91% 90%	-	[45]
8	Porous α-Fe2O3 nanorods	MB	250 W halide lamp	420 nm cut-off filter	95%	1.04 × 10−2 min−1	[46]
9	α-Fe2O3 hollow sphere	Salicylic acid	-	UV light	-	-	[47]
10	α-Fe2O3 hollow spindles	Phenol	high-pressure Hg lamp (500 W, Nanjing Stonetech)	UV irradiation (high-pressure Hg lamp is 365 nm after filtering)	10%	-	[48]
11	α-Fe2O3 mesoporous	Salicylic acid	high-pressure Hg lamp	UV irradiation	95%	-	[49]
12	α-Fe2O3 hollow microspheres	Salicylic acid	high-pressure Hg lamp (300 W)	UV light	58%	-	[50]
13	α-Fe2O3 hollow microspheres assisted solvothermal method	RhB	300 W Xe lamp	400 nm cut-off filter	98%	-	[51]
14	α-Fe2O3	Rose Bengal	200 W tungsten lamp	-	98%	1.57 × 10−2 min−1	[52]
15	α-Fe2O3	H2S	Xe-lamp light source (Oriel, New-port Stratford, Stratford, CT) of intensity 450 W	cut-off filter (>420 nm)	-	-	[53]
16	α-Fe2O3 dendrites, αFe2O3 nanospindles, α-Fe2O3 nanorods, α-Fe2O3 nanocubes	RhB	2 mW UV source	(λ = 365 nm)	82% 83% 84% 84%	0.322 min−1 0.589 min−1 0.8505 min−1 0.876 min−1	[54]
17	α-Fe2O3 nanoparticles	RhB	500 W xenon lamp	420 nm cutoff filter	52%	-	[55]
18	1D α-Fe2O3 microrods 1D α-Fe2O3 nanorods	RhB	300 W xenon lamp	λ > 420 nm	-	0.00977 min−1 0.148 min−1	[56]
19	α-Fe2O3 oblique α-Fe2O3 truncated nanocubes	RhB	300 W Hg lamp	λ = 365 nm	59%	-	[57]
20	α-Fe2O3 microflowers, α-Fe2O3 nanoparticles α-Fe2O3 nanospindles	RhB	500 W xenon lamp	420 nm cut-off filter	98% 94% 91%	-	[58]
21	flowerlike α-Fe2O3 nanostructures	RhB	250 W high-pressure Hg lamp	UV irradiation	59%	-	[59]
22	α-Fe2O3 hollow core/shell hierarchical nanostructures	Phenol	high-pressure Hg lamp	UV irradiation	60%	-	[60]
23	α-Fe2O3 nanoparticle	MB	outdoor sunlight	-	33%	0.0033 min−1	Present work

4. Conclusions

Hematite (α-Fe₂O₃) NPs were successfully synthesized using a facile hydrothermal method. From powder XRD analysis, the as-synthesized α-Fe₂O₃ was found to have a rhombohedral phase with high crystallinity. The FT-IR spectrum indicated the presence of a α-Fe₂O₃ phase in the synthesized sample. The SEM images indicate the formation of an aggregated spherelike morphology. DLS indicated that the synthesized hematite particles dispersed in DW were stable with an average hydrodynamic size of 141 nm. The photocatalytic activity of the as-synthesized α-Fe₂O₃ NPs was also studied for the model contaminant MB dye under natural sunlight irradiation, and it was found that 33% of the dye could be decomposed after 2 h of radiation exposure at natural pH without introducing oxidants.