Metal oxide nanomaterials are a varied category of materials that have a wide range of applications such as water purification, environmental remediation, energy, personal care products, and medical technology [1-7]. This category of materials, due to their wide band gap and high photosensitive nature, can be used as a photocatalyst for the photocatalytic degradation of organic dyes [8-11].
There are various kinds of iron oxide including hematite (α-Fe2O3), maghemite (c-Fe2O3) and wustite (FeO) [12, 13], among which α-Fe2O3 has features such as band gap 2.1 eV , non-toxicity, being cheap, and good chemical stability in aqueous solutions [15, 16]. Also, it has varied applications in water splitting, cement industry, gas sensing, solar energy conversion, lithium-ion battery production, water purification, and pigmentation which all of these have caused increasing attention to the production of these nanoparticles [17-19]. Up to now, several synthetic methods have been developed to produce the α-Fe2O3 nanoparticles, including hydrolysis of iron salt, sol-gel, and hydrothermal synthesis [20-24]. Among these methods, the green sol-gel method supply products with a low cost, simple work-up, and high efficiency [25-27]. So, this method can be efficient and appropriate for the synthesis of nanoparticles like Fe2O3.
One of the main causes of environmental problems is the effluent of factories, such as textile factories, that are released into the environment. In the world, over 10,000 different colors and pigments are produced that 60–70% of them are synthetic azo dyes used in the textile factories [28-30]. Therefore, enter of synthetic colors like azo dyes in the rivers can cause serious risks for the living organisms such as aquatics, plants and human beings . Hence, over the last few years, the researchers have paid more attention on removing the synthetic dyes from the environment. In this work, the photocatalytic activity of α-Fe2O3-NPs has been evaluated for the degradation of Congo red dye in the presence of visible light irradiation. FT-IR, XRD, FESEM, and TEM are the techniques that were used for characterization of α-Fe2O3 NPs. The chemical structure of dye is observed in Scheme 1 and characteristics of it are listed in Table 1 .
Arabic gum (AG) was obtained from a local health food store. FeCl3, as the iron source, was purchased from daijung (Darmstadt, Korea) and used without further purification. The FT-IR spectra were measured on a Jasco 6300 FT-IR spectrometer (KBr disks). The structural properties of synthesized nanoparticles were investigated by X-ray powder diffraction (XRD) pattern on an X’Pert-PRO advanced diffractometer using Cu (Kα) radiation (wavelength: 1.5406 Å) at 40 kV and 40 mA at room temperature in the range of 2θ from 20º to 80º. The external structure of the sample was determined by a Jasco 6300 FT-IR spectroscopy. The FT-IR spectrum was collected between the wavenumber of 400 and 4000 cm-1. The particle size and morphology of the sample were analyzed by Transmission electron microscopy (Philips CM30) and field emission scanning electron microscopy (Zeiss EVO 18, Germany). The diffuse reflectance UV–Vis spectroscopy (DRS) of the sample was recorded by a UV–Vis spectrophotometer (Shimadzu, UV-2550, Japan) using BaSO4 as a reference.
Synthesis of α-Fe2O3 nanoparticles using Arabic gum
At first, in order to prepare an Arabic gel, 0.3 g of the Arabic gum (AG) was dissolved in 40 mL of deionized water and stirred for 120 min at 75°C. FeCl3 solution (2 mmol of FeCl3 in 5 mL of deionized water) was added to the gel. Then, the sample container was put in a sand bath and the temperature of the sand bath was fixed at 75 ºC with stirring for 12 h to obtain a black color resin. After that, the dry gel was annealed at 600°C for 4 h to obtain a black powder of α-Fe2O3.
Photocatalytic dye degradation
Dye degradation experiments were carried out in a batch mode photoreactor. The irradiation source was a fluorescent lamp (λ >400 nm, 80 W, Pars, Iran) installed above the batch photoreactor.
The photocatalytic degradation of Congo red (CR) dye was carried out by various dosages of the sample (0.03, 0.04, 0.05 and 0.06 g) in 50 mL solution of dye (20 mg/L) at room temperature. The effect of initial dye concentration on the photocatalytic dye degradation was studied. For this purpose, 0.05 g of α-Fe2O3 was added to 50 mL of different dye concentrations (10, 20, 30 and 40 mg/L). The solute on samples was withdrawn from the reaction medium at regular time intervals. The catalyst was separated from the solution using a centrifuge. The change of absorbance at a maximum wavelength (λmax= 499 nm) of dye was monitored by UV–Vis spectrophotometer. The effect of visible light irradiation on the removal of dyes was surveyed. The percentage of the photocatalytic degradation of dye checked out by the following equation,
Where A0 is the initial absorbance and A is the final absorbance, at λmax = 499 nm.
RESULTS AND DISCUSSION
Characterization of α-Fe2O3-NPs
The FT-IR spectra were recorded in the solid phase using the KBr pellet technique in the range of 400-4000 cm-1. Fig. 1 shows the FT-IR spectrum of the sample calcined at 600°C for 4 h. The spectrum displays two characteristics broad bands at 437-440 cm-1 and 537–541 cm-1 corresponding to Fe-O stretching. The highest band at 594 cm-1 corresponds to intrinsic stretching vibrations of Fe↔O, and the lowest band at 474 cm-1 is related to the bending vibration of O↔Fe↔O .
Fig. 2 shows the X-ray diffraction patterns of the sample synthesized by AG and calcined at 600°C for 4 h. As shown in Fig. 2, the characteristic diffraction peaks at 2θ are 24.0, 32.98, 35.54, 40.86, 43.39, 49.51, 54.06, 57.42, 62.35, 63.85, 69.52, 71.89, 75.39, 77.70, and 78.70 which can be assigned to (012), (104), (110), (113), (202), (024), (116), (018), (214), (300), (208), (1010), (220), (306), and (223) planes, respectively. All the diffraction peaks were readily indexed to a pure rhombohedral phase of α-Fe2O3 (JCPDS Card no. 72-0469) with a=b = 5.041 Å and c = 13.757 Å. The crystallite size of α-Fe2O3 NPs was determined from the full width at half maximum (FWHM) of the XRD pattern by the Scherrer formula:
D = 0.9λ ∕ β cos θ
where D is the crystallite size (nm), λ is the X-ray wavelength of Cu Kα = 0.154 nm, β is the full width at half maximum of the peak, and θ is the Bragg angle . Using the above method we obtained an average size of 19 nm for α-Fe2O3 NPs.
The FESEM image shows the particle size and external morphology of the α-Fe2O3 NPs (Fig. 3). According to the result of this analysis, nanoparticles of α-Fe2O3 produced with this method have a relatively uniform spherical shape in nano-size.
The TEM image in Fig. 4 confirms the morphology and particle size of the α-Fe2O3 NPs. As shown in Fig. 4, the α-Fe2O3 NPs have spherical morphology and average particle size is about 45-50 nm.
The band gap of α-Fe2O3 nanoparticles was measured by diffuse reflectance spectroscopy (DRS). The band gap energy was calculated by a reflectance technique by exerting the Tauc theory . The results show that the band gap of α-Fe2O3 nanoparticles is roughly 2 eV (Fig. 5). Therefore, it is certified that the α-Fe2O3 nanoparticles are suitable photocatalyst in a visible-light region.
Effect of visible light irradiation and α-Fe2O3 NPs catalyst
The photocatalytic activity of α-Fe2O3 NPs on degradation Congo red dye was evaluated under the following state: α-Fe2O3 nano photocatalyst under visible light irradiation, α-Fe2O3 nano photocatalyst under dark and visible light irradiation without α-Fe2O3 NPs. As can be seen in Fig. 6, simultaneous applying of α-Fe2O3 NPs with visible light irradiation has led to degradation of 90% of Congo red dye within 90 min. When the degradation of dye was followed in the presence of catalyst without visible light, adsorption of dye was attained 45% of Congo red.
The effect of photocatalyst dosage
For checking the effect of the photocatalyst amount on decolorization, the different dosages of α-Fe2O3 NPs were used. In this experiment, 0.03- 0.06 gr of the catalyst was applied in 20 mg/L concentration of Congo red and at a constant time of 90 min. Fig. 7 shows the photocatalyst effect of α-Fe2O3 dosage on decolorization of dyes percentages for 90 min.
The effect of initial dye concentration
In order to survey the changes of initial dye concentrations on degradation performance, various concentrations between 10 to 40 mg/L were exploited and other operating factors were kept fixed. The relationship between initial dye concentration and degradation efficiency at 0.05 g of the photocatalyst is demonstrated in Fig.8. It was remarked that the rate of photocatalytic degradation decreases as the initial concentration of Congo red dye enhances from 10 to 40 mg/L in 90 min.
The effect of time on the degradation of Congo red dye
The UV-Vis spectra of the Congo red were studied for the following degradation procedure in the presence of α-Fe2O3 NPs as the photocatalyst at diverse time gap under the visible irradiation. The maximum absorption peak of C.R is found at 499 nm that clearly decreases in intensity with raising irradiation time (Fig. 9). Approximately 90% of C.R is degraded in 90 min.
The reusability of the photocatalyst
The reusability of the photocatalyst was measured for the degradation of C.R dye under the determined reaction conditions. In this test, the catalyst was separated from the solution using a centrifuge, and washed with distilled water before using in the subsequent run. As demonstrated in Fig. 10, the nanocatalyst retained usefully and was reused four times with a good degradation performance. So, α-Fe2O3 NPs maintain in a good decolorization efficiency and stability.
In this work, we have reported the green synthesis of α-Fe2O3 NPs using Arabic gum (AG) as a biopolymer template by the sol-gel method. This method has many advantages such as non-toxicity, low cost, simple work-up, compounds uniformity, and high efficiency. The XRD results showed a pure rhombohedral phase after heat treatment at 600°C for only 4 h. Also, the results of FESEM and TEM confirmed the synthesis of nanoparticles with the spherical shape in nanometer size. The photocatalytic activity of the α-Fe2O3 nanoparticles was studied for degradation of Congo red dye under visible light irradiation. The results showed that 90% of Congo red dye was degraded within 90 min. The nano-catalyst can be recycled four times with no significant loss of photocatalytic activity.
This work was supported by the “Kordestan Cement Co., Kordestan, Bijar, Iran” and the “Atlas Pood Co., Tabriz, Iran”.
CONFLICTS OF INTEREST
The authors declare that they have no conflict of interest.