synthesis of iron oxide nanoparticles and study of its

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We have synthesized Iron oxide (α-Fe2O3) nano particles by hydrothermal method .... hypotheses have been presented for the origin of blue, green and yellow ...
ISSN: 0973-7464 Vol. XVIII: No. 1 & 2 SB Academic Review 2012 : 108-116

SYNTHESIS OF IRON OXIDE NANOPARTICLES AND STUDY OF ITS OPTICALPROPERTIES Paulose Thomas and Abraham K E Department of Physics, S B College, Changanacherry. Abstract Ultrafine iron oxide nanoparticles have been prepared through hydrothermal method. The structural and morphological nature of samples is characterized by XRD, SEM and HRTEM analysis. The optical characterization is carried out by UV-Visible reflectance spectroscopy and room temperature photoluminescence analysis. The band gap obtained in the present study is 2.22 eV. The possible spectral transition for PL emissions is discussed in photoluminescence analysis. Introduction Iron oxides are chemical compounds composed of iron and oxygen. Iron (III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. Fe2O3 can be obtained in various polymorphs. In the main ones, α and γ, iron adopts octahedral coordination geometry. That is, each Fe centre is bound to six oxygen ligands. αFe2O3 has the rhombohedral structure and is the most common form. It occurs naturally as the mineral hematite which is mined as the main ore of iron. Nowadays iron oxide powders are widely used in industry for various applications: magnetic data storage materials, catalysis, inorganic pigments and precursor for ferrite synthesis. The hydrothermal synthesis is widely used for producing materials with interesting properties [1, 2]. The present study undertaken is to prepare iron oxide nanostructure particles by hydrothermal method and characterize it structurally and optically. Materials and methods We have synthesized Iron oxide (α-Fe2O3) nano particles by hydrothermal method using Iron nitrate and Hexamine as starting materials purchased from Merck India Ltd. In the first step, iron nitrate (1.1414 g) is dissolved in 100ml of distilled water and Hexamine (0.4906g) solution is then added to the above solution drop wise with constant stirring which results in reddish brown precipitation. The solution is kept in a Teflon jar inside an autoclave. It is then heated in a furnace for 6 hours at 200°C. The solution is allowed to settle and washed with distilled water and ethanol several times. It is finally dried at 80°C at 5 hours which leaves brown coloured Iron oxide nano particles in powder form. 108

ISSN: 0973-7464 Vol. XVIII: No. 1 & 2 SB Academic Review 2012 : 108-116

Results and discussions X-ray diffraction analysis

Fig1. X-ray diffraction analysis spectrum of α-Fe2O3 nanoparticles. The above Fig1 shown above represents the X-ray diffraction spectrum of α-Fe2O3 nanoparticles. The (h,k,l) value of the each peaks are marked on the spectrum. The XRD study confirms the formation of Rhombohedra system of α-Fe2O3 nano particles. The d-values of the Bragg Peaks in the XRD pattern of the powdered sample and corresponding JCPDS (card no. 89-0598) value are tabulated in Table1. It is observed that d-values match well with the standard values. The average crystallite size of as prepared α-Fe2O3 nanoparticles is obtained from Scherer’s formula and is of 19nm.

Tables1: XRD powder diffraction data for α-Fe2O3 nano particles.

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Determination of crystallite size (D) using Scherer’s formula The average crystallite size of the α-Fe2O3 nano particle is estimated from the X-ray line broadening using Scherer’s formula, D = kλ/βcosθ (1) Where, k = 0.94 λ = 1.54 A0 β = FWHM (in radians) θ = Bragg’s diffraction angle The crystallite size of the particles are shown in table -2. Table 2 crystallite size calculated from Scherer’s formula.

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SEM analysis A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that can be detected and that contain information about the sample’s surface topography and composition. From Fig 2 shows the SEM image of α-Fe2O3 nanoparticles; it reveals that most of the samples are agglomerated and hence the exact morphological shape of as prepared α-Fe2O3 nanoparticles could not identified from SEM micrographs.

Fig 2 SEM images of α-Fe2O3 nanoparticles TEM analysis Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device. The TEM analysis revealed that the shape, size, d-spacing, (hkl) values and crystallinity of the samples. Fig. 3 represents the HRTEM images of α-Fe2O3 nanoparticles. The morphology of the sample is found to be rhombohedra. From the HRTEM analysis, we have obtained the average particle size of the prepared sample as 28nm which is in close agreement with the X-ray diffraction results. The SAED 111

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pattern shows the crystal planes which clearly match with the crystal planes identified from X-ray diffraction analysis.

Fig 3 (A, B, C) represents HRTEM images of α-Fe2O3 nanoparticles and D represents the SAED pattern. FTIR analysis Fourier transforms infrared spectroscopy (FTIR) analysis is a technique that provides the information about the chemical bond and molecular structure of materials. Fig 4 represents the FTIR spectrum of α-Fe2O3 nanoparticles in the frequency range 400-4000cm-1. There is a strong broad band observed at 3099.31cm-1 which may be attributed to O-H stretching vibrations. The IR spectra of the product display one peak at around 782.92cm-1 which is attributed to the Fe-O vibrations of á-Fe2O3 . The α-Fe2O3 and γ-Fe2O3 exhibit two or three peaks between 500 and 800 cm-1 [36], which are different from Fe3O4. In the case of α-Fe2O3 , the bands appearing at 1641.50 cm-1 can be attributed to the angular deformation of water H-OH [7]. The band observed at 1364.25cm-1 reveals the C-H in plane bend vibrations. 112

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Fig 4. The FTIR spectrum of α-Fe2O3 . UV-visible reflectance spectroscopy The UV-Visible reflectance spectrum of α-Fe2O3 nanoparticle is represented in the Fig 5.

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Fig 5 UV-Visible reflectance spectrum of α-Fe2O3 nanoparticles.

Fig 6 determination of energy band gap from Tauc Plot. A Tauc plot is used to determine the optical band gap. It is a convenient way of studying the optical absorption spectrum of a material pioneered by J.Tauc. A Tauc plot shows the quantity hν (the energy of the light) on the X-axis and the quantity (αhν)n on the Y-axis, where á is the absorption coefficient of the material. The exponent n has a value ½ for indirect transitions and 2 for direct transitions. The resulting plot has a distinct linear regime which denotes the onset of absorption. Thus extrapolating this linear region to the X-axis yields the energy of the optical bandgap of the material The Tauc plot of α-Fe2O3 nanoparticles is shown in the Fig 6. Extrapolating this linear region to the X-axis, the bandgap energy was found to be 2.22 eV. Photoluminescence spectrum Photoluminescence (PL) is a process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons in another wavelength. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon.

Figure.7.PL Analysis of α-Fe2O3 114

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The room temperature photoluminescence spectrum of α-Fe2O3 nanoparticles with excitation wavelength (λexc) at 360nm is presented in Fig. 5. Three emission peaks (λem) are observed in the spectrum at 431 nm, 535 nm, and 646 nm. The strong blue or yellow band emission at 535 nm and 645 nm were attributed to deep levels or traps state emission resulting from the recombination of a photogenerated hole with a singly ionized charge state of the specific defects [8-11]. The emission peaks at 431 nm and 535 nm can be represented by the atomic transitions of iron vacancies. The peak at 646 nm corresponds to the atomic transitions of oxygen vacancies. Various hypotheses have been presented for the origin of blue, green and yellow emissions such as singly ionized oxygen vacancies, interstitial cations, oxygen antisite and metal vacancies [9]. However, we can identify the peaks at 431 nm and 535 nm as the spectral transitions 2D0→2P and 1D0→3P of oxygen atom. Similarly, the peak at 646 nm represents the transition 4P0→4D of iron atom. Conclusions We have successfully synthesized α-Fe2O3 nanoparticles by hydrothermal method. The basic characterization confirms that the prepared samples are α-Fe 2 O 3 nanoparticles. The XRD reveals the crystal structure of α-Fe2O3 nanoparticles. We have calculated the d-values from XRD data which is in good agreement with the JCPDS data card. The crystal planes are also identical compared with the JCPDS data card. The grain size of α-Fe2O3 nanoparticles are calculated by Scherer’s formula.The morphology of as prepared α-Fe2O3 nanoparticles was analyzed by SEM and HRTEM. The direct ban gap of the sample is found to be 2.2eV from UV-Vis reflectance spectroscopy. The molecular vibrations of α-Fe 2O 3 nanoparticles is obtained From FTIR analysis. PL spectroscopy the photoluminescence emission is observed around λem = 431 nm, 535 nm and 646 nm with excitation λexc = 360nm. The peaks at 431 nm and 535 nm represent the spectral transitions 2D0→2P and 1D0→3P of oxygen atom. The peak at 646 nm represents the transition 4P0→4D of iron atom.

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