Synthesis, characterization and photocatalytic properties of Iron oxide nanoparticles synthesized by sol-gel autocombustion with ultrasonic irradiation

INTRODUCTION

Dyes are an important class of chemicals which are widely used in several industrial processes such as leather, textile and printing industries. The release of the dyes in the effluents of these industries may cause major invironmental problems. Most of these dyes are hazerdous material because they can cause cancer, genetic mutation or other effects[1]. Various treatment processes and techniques have been used to remove the dye pollutants from contaminated water[2-5]. Photocatalytic degradation of organic pollutants over semiconductors has been extensively studied for more than 30 years[6]. Heterogenous photocatalysis is one the most successful methods for removal of dye pollutants in wastewater[7]. Hematite (α-Fe₂O₃), as an n-type semiconductor (Eg=1.8-2.2) and the most thermodynamically stable species of iron oxides, has been widely applied and studied in recent years[9-11]. Wang has evaluated pure and gold-deposited iron oxide aerogels for the photocatalytic degradation of disperse blue 79 dye in water, which is an azo dyestuff popularly seen in the textile wastewater [12]. Sajjadi and Goharshadi have fabricated highly monodispersed crystalline hematite with similar cube shape via ultrasound assisted hydrothermal techniques which was used for photo degradation of three organic dyes, ractive black 5(Rb5), methylene blue(MB) and rhodamine B (RhB)[13]. Poros hematite has been applied for treatment As(v) from aqeous solution as adsorbant [14]. Hematit is nontoxic, cheap and good resistant to corrosion[15]. Various methods can be used for the preparation of α-Fe₂O₃ with high crystallinity. α-Fe₂O₃ nanoparticles are immoblized on different substrates. Li and coworkers have prepared a series of Fe₂O_3/Al₂O₃ samples by sol-gel method  using ferric nitrate, aluminium nitrate and glycol as starting material[12]. Xu and coworker reported alumina-dispersed α-Fe₂O₃ (Fe₂O₃/alumina) which is not  more photoactive than the unsupported α-Fe₂O₃ [17].

In this work, α-Fe₂O₃ nanoparticles are synthesized with sol gal autocombustion method in combination with ultrasonic irradiation. This technique is convenient and cheap. A paste of α-Fe₂O₃ nanoparticls with terpinol and ethyl celloluse is prepared and a glass slide is coated with this paste. In this condition, photocatalyst nanoparticles are immobilized on glass slide as heterogenous photocatalyst  providing the possibility of easy and fast reuse of catalyst. Photocatalytic degradation of Reactive Red 4 in oxygenated solution is used to determine the photocatalytic activity. Finally, the kinetic order and constant rate of RR4 degradation are determined.

EXPERIMENTAL

Materials

The precursor material, iron (III) nitrate nonahydrate, Fe(NO₃)₃.9H₂O was obtained from Merck Chemical company. Ethyl alcohol and glycine were supplied by Sigma-Aldrich Chemical Co. Distilled water was used throughout.

Synthesis of iron oxide nanoparticles

Iron oxide powders were synthesized from sol-gel auto combustion by ultrasonic irradiation method. 0.01 mol of Fe(NO₃)₃.9H₂O (4.102 g) was dissolved in 20 ml distilled water by stirring on hotplate at 90^oC. Then, 0.0025 mol glycine (C₂H₅NO₂) 0.190 g was added to solution to chelate metal ion. An ammonia solution (21%) was added to adjust the pH value to 9.5. The clear solution was refluxed 2 h at 100 ^oC to get complex in N₂ atmosphere. Then, solution was placed in ultrasonic irradiation bath at 40 ^oC for 45 minutes. This solution was heated at 175^oC until solutions were evaporated slowly and the viscous gels were formed. Gel was transferred to cruse and heated at 90^oC for 13 h. Then, product was calcined at 675⁰C for 4h.

Coating on glass using nanoparticles pastes by Doctor Blade method

To stabilize iron oxide nanaoparticles on substrate, first, we prepared a paste of iron oxide nanoparticles by adding terpinol, ethyl ethyl cellulose, acetic acid, ethnol and double distilled water under certain process that showed in flow chart (Fig. 1). Then, paste was used for coating on glass by Doctor Blade method, the area of the photo catalyst coated on glass was 11 cm².The photo catalyst coated glass was air dried, and sintering at 300 ^oC in air to remove residual solvent and any organic compounds to improve the contact between the film and the substrate.

Characterization

FT-IR absorption spectrum of sample after calcination was obtained using KBr disks on a FT-IR 6300. Powder X-ray diffraction analysis was performed on a D8 Advance, BRUKER diffractometer in the diffraction angle range 2θ=5-80° with Cu Kα radiation (1.5406 A°). Diffuse reflectance spectra (DRS) were collected with a V-670, JASCO spectrophotometer and transformed to the absorption spectra according to the Tauc relationship. Field emission scanning electron microscopy (FESEM) images was taken on a Hitachi, S-4160 scanning electron microscope. The concentration of azo dyes solutions was determined using its absorbance by a Cary 500 UV–Vis spectrophotometer.

RESULTS AND DISCUSSIONS

XRD analysis

X-ray diffraction was used to characterize the phase and crystal property of calcined iron oxide nanoparticles synthesized by sol-gel auto combustion with ultrasonic irradiation. The diffraction peaks of the sample (Fig. 2) match well with the standard pattern of Rhombohedral hematite, syn (JCPDS card no. 33.0664), where the diffraction peaks at 2θ values of 27.77, 38.4, 41.5, 47.7, 57.9, 63.5, 73.8 and 75.7 can be ascribed to the reflections of (012), (104), (110), (113), (024), (116), (214) and (300), respectively. No peaks for any impurities are observed, indicating high purity of the product. The average crystallite size of Fe₂O₃ calculated from X-ray line broadening using Scherrer’s equation was found to be about 36.7 nm.

FESEM characterization

The morphology of the sample was investigated by FESEM. Fig. 3 indicates that the sample prepared by sol-gel autocombustion with ultrasonic irradiation consists of spherical particles and the effect of sintering is observed. It clearly shows that the diameter of particles is 76 nm.

FTIR analysis

The FTIR spectrum was used to identify the functional groups on the surface of the Fe₂O_3. Fig. 4 represents the IR spectrum of sample after calcination. The peak at 3440 cm⁻¹arises from stretching vibration of O−H and the peak located at around 1624 cm⁻¹can be ascribed to bending vibrations of adsorbed water molecules on surface of nanoparticles. The weak band at 1127cm⁻¹ can be assigned to residue FeOOH in structure. The observed peak at 914cm⁻¹ can be related to stretching vibration of C−O group. The peaks at 544 and 477.4 cm⁻¹arises from Fe−O−Fe vibrations.

DRS analysis

UV–Vis diffuse reflectance spectra of Fe₂O₃ prepared by sol-gel autocombustion with ultrasonic irradiation were collected. It can be clearly seen from Fig. 5 that absorbance of Fe₂O₃ is in the range of 200–850 nm indicating that the band edge absorption has been broadened to visible light region. UV–Vis absorption spectroscopy method has been extensively applied to investigate the optical properties of various kinds of optoelectronic.UV–Vis spectrum of Fe₂O₃was taken as a function of wavelength. The optical band gap of Fe₂O₃ was calculated using Tauc relation αhv= A (hv–Eg)ⁿ, where α is the absorption coefficient, A is a constant and n is an index with the values 1/2, 3/2,2 and 3, depending on the nature of electronic transition, where n=1/2 for direct band gap semiconductors. An extrapolation of the linear region of plot (αhv)²vs hv gives the value of  the optical band gap Eg as shown in Fig.6.The measured band gap of iron oxide was found to be 2.11 eV.

Photocatalytic studies of Fe₂O₃

Photocatalytic activity of Fe₂O₃ was investigated through Reactive Red 4 (RR4) degradation under UV-light irradiation. Fig. 7 shows the chemical structure of RR4. For immobilization Fe₂O₃ nanoparticles, the paste of Fe₂O₃ prepared and then a thin layer of paste was coated on glass slide with doctor Blade method as mentioned in section 2.3. To determine dye degradation percentage by photo catalyst, first standard solutions with different concentrations of RR4 were prepared and their UV-Vis spectra were recorded (inserted in Fig. 8). Then, the graph of absorbance in maximum wavelength (λ_max=534 nm) vs concentration was plotted and the equation of the most fitted line was obtained (Fig. 8).This equation was used to determine the concentration of RR4. In the following a RR4 solution with a concentration of 10.4 ppm was exposed at O₂ gas for 30 min. The immobilized photocatalyst was placed in a petri dish contained 25 of above dye solution. The mixture of RR4 and immobilized photocatalyst was stirred in the dark condition to allow the mixture to establish sorption and desorption equilibrium of dye molecules by the photo catalyst nanoparticles. The UV-Vis absorbance spectrum of RR4 solution after dark condition showed a minor change. Then, petri dish was exposed under UV light irradiation for a certain time and after each time UV-Vis spectrum was recorded. The degradation process continued till 135 min UV light irradiation. Fig. 9 shows absorption spectra of RR4 degradation using iron oxide nanoparticles prepared by sol-gel autocombustion with ultrasonic irradiation under different irradiation time. Degradation rate was calculated using the following equation:

where C_o is the initial concentration of dye and C_t is the concentration of pollutant at time t. The degradation percentage of RR4 in the presence of Fe₂O₃ photocatalystis was about 52% (Fig. 10). Photocatalytic reaction kinetic can be expressed by the Langmuir–Hinshelwood(L–H) model,

ln(C_t/C₀)=K_appt

where k_app is the apparent pseudo first order reaction rate constant and t is the reaction time. A plot of ln(C/C_o) vs t will yield a slope of k_app. The rate constant of degradation of RR4 in the presence of Fe₂O₃ under UV light irradiation equals to 0.005 min^-1 (Fig.11).

Mechanism of photocatalytic degradation

In the presence of UV light irradiation, an electron in Fe₂O₃ is excited, producing conduction band electron (e^-_CB) and valence band holes (h⁺). The photogenerated holes at the surface of Fe₂O₃ can react with H₂O to produce OH^. radicals, which are strong oxidants capable of oxidizing many organic pollutants, resulting in degradation of pollutants. The photogenerated electron can either recombine with the positive holes or react with available O₂ to produce super oxide ions (O2̇^-) that subsequently produce hydroproxyl radicals which in turn may react further to form H₂O₂ and more OH^. radicals.

CONCLUSION

α-Fe₂O₃ nanoparticles were successfully synthesized using sol-gel autocombustion method with ultrasonic irradiation. The property of sample was characterized by XRD, FESEM, FTIR and DRS. α- Fe₂O₃ NP coated on glass successfully degraded 52% oxygenated RR4 solution in 135 min under UV light irradiation.The RR4 degradation kinetic in the presence of α- Fe₂O₃ NP stabilized on glass followed Langmuir-Hinshelwood model and pseudo first order rate constant was about 0.005 min^-1.

ACKNOWLEDGEMENTS

The authors wish to thank the University of Isfahan for financially supporting this work.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.