Magnetite Fe₃O₄ nanoparticles synthesis by wet chemical reduction and their characterization

Figure 1 shows the EDAX spectrum of synthesized Fe₃O₄ nanoparticles. The EDAX data of weight % of as synthesized Fe₃O₄ nanoparticles are; Fe: 74.5% and O: 25.5%. The EDAX spectrum shows only Fe (iron) and O (oxygen) elements without any impurities. Hence, the EDAX analysis states that the as synthesized Fe₃O₄ nanoparticles are in perfect stoichiometry.

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The FTIR spectrum of dispersed Fe₃O₄ nanoparticles in de-ionized water is shown in figure 2. The peak at 3414.89 cm⁻¹ are due to O–H stretching vibration arising from hydroxyl groups from the water on nanoparticles. The absorption peaks at 2926.22, 2860.65, 1628.61, 1385.22 and 1020.54 cm⁻¹ are due to de-ionized water used as solvent. The 695.29 and 468.80 cm⁻¹ absorption peaks corresponds to the Fe–O bond vibration of Fe₃O₄ nanoparticles. These peaks values nearly matches with the reported values of Kim et al [15].

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Figure 3 shows the powder XRD pattern of synthesized Fe₃O₄ nanoparticles. The diffractions peaks could be indexed as that of cubic structure of Fe₃O₄ phase having lattice parameter a = 8.39 Å. The lattice parameter is in good agreement with the standard data (JCPDS: 19-0629). The XRD did not show any other phases such as FeO, Fe₂O₃, etc. The particles size was calculated from the XRD data using Scherrer's equation [16]

$$\begin{eqnarray}&&D=\frac{k\lambda }{\beta {\rm cos} \theta },\end{eqnarray} \tag{ 1 }$$

where D is particle size, k is the grain shape factor taken as unity contemplating that the particles are spherical in shape, λ is the incident x-ray wavelength of Cu–Kα radiation and θ is the Bragg's angle, β is the broadening of diffraction line measured at half maximum intensity (radians). The determined particle size came out to be 7.20 nm.

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The grain size and the micro strain in the as-synthesized Fe₃O₄ nanoparticles were estimated using the XRD data by Hall–Williamson relation [16]

$$\begin{eqnarray}&&\frac{\beta {\rm cos} \theta }{\lambda }=\frac{K}{L}+\frac{4\varepsilon {\rm sin} \theta }{\lambda }\cdot \end{eqnarray} \tag{ 2 }$$

The Hall–Williamson's equation incorporates the Scherrer's formula of grain size and the micro strain term. Here β is full width at half maximum of the diffraction peaks, which is expressed as a linear combination of the contributions from the strain () and grain size (L). In contour to the equation (2), the plot of (βcosθ)/λ versus 4sinθ/λ is a straight line. Figure 4, in which the reciprocal of an intercept on (βcosθ)/λ axis gives the average grain size and the slope, gives the residual strain. The grain size determined from the intercept comes out to be ∼6.58 nm. This grain size value is in good agreement with the value obtained from the Scherrer's equation. The residual strain obtained from the slope of the plot is 4.31 × 10⁻³ for the as-synthesized Fe₃O₄ nanoparticles. The positive value of the residual strain for the Fe₃O₄ nanoparticles indicates it to be tensile strain.

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Figure 5 shows the SEM images of the as-synthesized Fe₃O₄ nanoparticles that reveals the surface topography. The SEM image of figure 5(a) clearly indicates that the products are composed of spheres having uniform flower-like features. The magnified image shown in figure 5(b), of the sphere surface clearly shows that the entire sphere surface is covered by flower-like architecture, which is built from several nano-sheets. The magnified SEM image, figure 5(b), reveals that the flowers are formed of nano-sheets on the spherical surfaces having voids.

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The morphology and size study of the as-synthesized Fe₃O₄ nanoparticles were done by high resolution transmission electron microscopy (HRTEM). The samples used for HRTEM observations were prepared by dispersing the particles in de-ionized water under sonication of 30 min, then placing a drop of the dispersion onto a copper grid coated with a layer of amorphous carbon. Figure 6 shows the HRTEM images of Fe₃O₄ nanoparticles at two different magnifications. The HRTEM images divulge that the particles are spherical in shape corroborating the SEM observation. The images also shows that the synthesized Fe₃O₄ nanoparticles are distributed uniformly and spheres are interconnected with each other. From this micrograph it is noticed that the particles have an average size of about 8 nm. This result is in accordance with the results obtained from the XRD.

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Figure 7 shows the optical absorbance spectrum of the as-synthesized Fe₃O₄ nanoparticles at ambient temperature. The clear colloid obtained after 30 min sonication of Fe₃O₄ nanoparticles dispersed in de-ionized water was used as a sample and only de-ionized water was used as reference. The absorbance spectrum shows the absorbance is in the visible range of the wavelength. The absorption peak is at 370 nm, while the absorption edge lies between 375 and 650 nm.

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Magnetic properties of the as-synthesized Fe₃O₄ nanoparticles sample was studied with the help of VSM. The variation of magnetization (M) versus the applied magnetic field (H) for the Fe₃O₄ nanoparticles measured at 10 and 300 K temperature is shown in figure 8. In both the cases, the magnetization increases with increasing magnetic field and saturates at higher magnetic fields. From this figure it is worth noting that the coercive field and remanence magnetization is very small (approximately zero). These are characteristics of super paramagnetic particles. At higher magnetic fields all the particle's magnetic moments align themselves along the applied field direction giving rise to saturation of the magnetization. The low temperature (10 K) M–H curve reveals that the value of saturation magnetization (M_s) is 26.03 emu g⁻¹ and the remnant magnetization (M_r) and coercivity field are very small (approximately zero). The M–H curve for 300 K reveals that the saturation magnetization (M_s) is 19.19 emu g⁻¹ and the remnant magnetization (M_r) and coercivity field is also about zero.

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The variation of magnetization as a function of temperature for the as-synthesized Fe₃O₄ nanoparticles is shown in figure 9 for two different applied magnetic fields, (a) 10 000 Oe and (b) 500 Oe. The temperature variation was from 4 to 320 K. The figure 9 for both the applied magnetic fields shows that the magnetization value decreases slowly with increasing temperature. The magnitude decrease of magnetization with temperature is more in case of applied magnetic field of 10 000 than 500 Oe. This magnetization behavior is in line to natural behavior of magnetic samples.

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The TG curve for Fe₃O₄ nanoparticles is shown in figure 10. The TG curve was measured from room temperature to 1124 K in the inert N₂ atmosphere at heating rate of 5 K min⁻¹. In this temperature range the sample showed continuous weight loss as the temperature increased from room temperature to 1124 K. The complete weight loss was divided into four steps viz room temperature to 499 K, 500 to 734 K, 735 to 934 K and 935 to 1124 K.

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The activation energy of the Fe₃O₄ nanoparticles in the four different steps of thermal decompositions such as: step-I from room temperature to 499 K, step-II from 500 to 734 K, step-III from 735 to 934 K and step-IV from 935 to 1124 K of TG curve was calculated using Broido [17] and Piloyan–Novikova (P–N) [18] relations. The values of activation energy of four decomposition steps calculated using the two different relations are tabulated in table 1.

The values of the activation energy for four steps determined by two different methods are in good agreement with each other.