Structural and Morphological Study of Fe₂O₃ Nanoparticles

 

S. A. Ansari¹*, A. Azam² and A.H. Naqvi¹

¹Centre of Excellence in Materials Science (Nanomaterials), Department of Applied Physics,

Aligarh Muslim University, Aligarh, India

²Centre of Nanotechnology, King Abdulaziz University, Jeddah, KSA

*Corresponding Author E-mail: sajid009alichem@gmail.com

ABSTRACT:

Nanoscale magnetic particles show an interesting area of study and they find applications in various fields. In the present study the magnetic nanoparticles of Fe₂O₃ were synthesized through sol-gel method and were characterized by using the X-ray diffraction analysis (XRD), Atomic Force Microscopy (AFM), FTIR and Particle Size Analyzer at room temperature. XRD analysis confirms that all the samples exhibit single phase hexagonal structure and excludes the presence of any secondary phase. The average crystallite size calculated from XRD data was found to be between 18-30 nm for the samples sintered at temperature ranging from 100ºC to 1000ºC. Tapping mode 3-D AFM image of Fe₂O₃ nanoparticles shows different shapes and sizes of nanoparticles aggregates. The nanostructure tend to form rather than  homogeneous aggregates having pyramidical structure, which consist of parallel chains between 10-30 nm in breadth with different orientations. The roughness measurements of the samples showed that the surface is little bit smooth with some cracks. FTIR study showed that the bands near 473 cm^-1 and 540 cm^-1 were characteristic absorptions bands of Fe–O bond for Fe₂O₃ nanoparticles. Particle size Analyzer data showed that the average particle size is 19.82 nm for as-synthesized Fe₂O₃ nanoparticles.

 

 

 

1. INTRODUCTION:

In the past few decades, porous materials have been used in many fields, such as filter, catalyst, cell, support, optical   material etc^1-3. In general, porous materials can be classified into three types depending on their pore diameters, namely, microporous (<2   nm), meso- or transitional porous (2-50 nm), and macroporous (>50 nm) material, respectively⁴. Currently, the mesoporous material have attracted growing research interest and have great impact in the application   of catalysis, separation, adsorption and sensing due to their special structural features such as special surface area and   interior void^{2, 5-8}. Magnetic nanoparticles show remarkable new phenomena such as superparamagnetism, high field irreversibility, high saturation field, extra anisotropy contribution or shifted loop after field cooling. These phenomena arise from finite size and surface effects that dominate the magnetic behaviour of individual nanoparticles⁹.  From the point of view of applied research, iron (III) oxide in all its form is one of the most commonly used metal oxides with various applications in many environmental and industrial fields.

 

Iron oxides are components of several ores used for the production of iron and steel, geologically and archeologically important earth-sample, mineral as well as extraterrestrial materials. Due to their hardness, catalytic activity, surface resistivity and the other exceptional (magnetic, optical, electronic) properties, they are used as abrasives, polishing agents, catalysts, gas sensors, pigments, photoanodes for photoelectrochemical cells or contrast agents in magnetic resonance imaging. They also play an important role in the production of ferrites and in the magnetic prospecting of archeological areas^10-11.The existence of amorphous Fe₂O₃ and four polymorphs (alpha, beta, gamma, epsilon) is well established¹². The most frequent polymorphs, the hexagonal corundum structure “alpha” and cubic spinel structure “gamma”, have been found in nature as hematite and maghemite minerals. The other polymorphs, the cubic bixbyite structure “beta” and orthorhombic structure “epsilon”, as well as nanoparticles of all forms, have been synthesized and extensively investigated in recent years⁸. The magnetic iron oxides, magnetite (Fe₃O₄) and maghemite (c-Fe₂O₃), define a strategic class of chemically stable materials exhibiting numerous and important industrial, technological and environmental applications. In this paper we report the preparation of Fe₂O₃ nanoparticles using sol-gel method and correlate the properties of the obtained nanoparticles as a function of various parameters.

2. EXPERIMENTAL DETAILS:

In the present investigation we have prepared nanoparticles of hematite α-Fe₂O₃ and maghemite (γ-Fe₂O₃) by the sol-gel method and studied the parameters that control their size. We have prepared these particles by a slightly modified method for greater simplicity. 200 ml (0.1M) of iron nitrate Fe (NO₃)₃. 9H₂O (Aldrich 98%) was used as a precursor solution, and was gelated by using 800 ml of mono hydrated citric acid (Aldrich 98%) solution (0.05 to 0.2M) as ligand molecules, and singly distilled water as the solvent. The iron solution was added to the citric acid solution drop wise under vigorous stirring. The solution was then heated to a temperature of 70ºC, with vigorous stirring until the gel was formed and the contained water was evaporated. The gel was dried at 100ºC for 16 hours. The dried gel obtained was ground for 30 minutes. Finally, Fe₂O₃ nanopowder is formed and this powder is annealed at different temperatures and characterized by XRD and various techniques.

 

Fig.1. XRD patterns of Fe₂O₃ nanoparticles at different temperature

 

3. RESULTS AND DISCUSSIONS:

3.1. XRD analysis of Fe₂O₃ nanoparticles

XRD patterns of Fe₂O₃ nanoparticles annealed at different temperatures have been shown in Fig.1. The data revealed that both maghemite (γ-Fe₂O₃) and hematite (α-Fe₂O₃) particles are present in the sample. The two major XRD peaks were obtained at 2θ = 33.2˚ and 35.7˚. The other peaks were observed at 2θ = 41˚, 49.55˚, 54.25˚ and 62.55˚. The relative intensities of the two major peaks at low temperature (100ºC) are approximately same which suggests that α-Fe₂O₃ is the major phase at low temperature range in the sample. The lattice constants so obtained for this set of α-Fe₂O₃ nanoparticles are a = b = 5.0356 Å and c = 13.7489 Å. All the remaining samples annealed below the temperature of 250^oC gave a similar pattern, which confirms that they are all predominantly of the α-Fe₂O₃ phase. However the samples annealed at a higher temperature (300-1000˚C) exhibit a definite change in the ratios of the two major peaks at 33.2 and 35.7^o as shown in Fig. 1. Both the major peaks are still at the same positions but the intensities of the highest and second highest peaks have been reversed. The peak at 35.7^o is now the most intense while the peak at 33.2^o is second highest peak suggesting the existence of pure gamma phase. We believe that this is an indication that both gamma and alpha phases are present in these samples prepared at higher temperature, the amount of the gamma phase is probably small and hence the smaller peaks are not visible. So the contribution from the hematite are clearly visible at 33.2^o, they overlap with those from the maghemite at 35.7^o thus leading to the observed peak intensity. This is an indication that with the increase in annealing temperature (250-400^oC) structural changes occur that allow the formation of the gamma phase along with the alpha phase.

 

Average crystallite size D for different specimens was calculated from the main peaks using Scherrer’s formula:

D = 0.9λ/β cosθ

Here λ is the x-ray wavelength (Cu Kα = 1.5418Å), β is the full width at half maximum of the peak and θ is the peak position¹³. Using the above method we have obtained an average crystallite size which comes out to be in the range 18-30 nm shown in Fig. 2. It can be also observed from Table 1 that the crystalline size of Fe₂O₃ increased from 18 nm to 30 nm when the annealing temperature increased from 100 to 1000 C°.

 

Fig. 2. Variation in crystal size at different annealing temperature

 

Table 1. The size of the crystal corresponding to annealing temperature shown in the table

Annealing temperature in ˚C	Crystal Size(nm)
100	18.2
300	26.7
500	25.8
800	29.4
1000	29.5

 

3.2. Atomic Force Microscope:

The surface morphology of Fe₂O₃ nanoparticle was analyzed by using the Atomic Force Microscope (AFM; Innova SPM, Veeco) in tapping mode shown in figure 3. Commercial etched silicon tips as scanning probes with typical resonance frequency of 300Hz (RTESP, Veeco) were used. The microscope was placed on a pneumatic antivibration desk, under a damping cover. The processing was conducted using the SPM Lab software and the measurements were realized in the ranges from 2000×2000 nm to 100×100 nm with the resolution of 300×300 pixels. Iron oxide samples for AFM analyses are necessary to be spread on a suitable surface since they are of a powder character. Synthetically prepared mica (Structure Probe, Grade V-4, USA) has been chosen as the ground. Its topmost layers are ripped off straight before spreading the sample in order to achieve an atomically smooth and clean surface. The iron oxide powder is dispersed in water by using of ultrasonication at 140 W for 3 minutes. Depending upon the type of sample, a small addition of sodium hexametaphosphate solution can be applied to inhibit the agglomeration tendencies of ultrafine particles. 20 μl of liquid containing the dispersed nanoparticles are taken using a micropipette immediately after ultrasonication. The dispersed Fe₂O₃ particles are spread on the preheated mica and put into a drying oven to evaporate water.

 

Fig. 3. AFM images of Fe₂O₃ nanopowder

Tapping mode 3D AFM image of Fe₂O₃ nanopraticles show different shaped and sizes of nanoparticles aggregates (Fig.3). The nanostructure tend to form rather homogeneous aggregates with pyramidical structure, which consist of parallel chains between 10-30 nm in breadth with different orientation. The roughness measurement of the samples showed that the surface is little bit smooth with some cracks. This aggregated pyramidical shaped morphology of the nanoparticles may be due to the magnetic interaction between the nanoparticles behaving as magnetic dipoles.

 

Fig. 4. FTIR spectra of Fe₂O₃ nanopowder

 

3.3. Particle Size Analyzer:

By using the instrument particle size analyzer analyze the sample from 1 nm to10,000  nm (10 micrometer). There is a special condition for this instrument that the sample should always be in suspension or emulsion form, more stable, more dilute suspension gives better result. A laser light (He- Ne laser) used as probe, the wavelength of this laser light is 632.8 nm. The suspension solution of Fe₂O₃ nanoparticles are prepared and  then solution was taken in a cuvette  and inserted then  run the program after completing the program the results  are shown in the figure5:

 

Fig. 5. Particle Size spectra from the Nanophox

 

3.4. FTIR Analysis:

FTIR spectra were recorded in KBr matrix in the range 400-4000 cm^-1 in solid phase using Perkin-Elmer FTIR Spectrophotometer as shown in figure 4. The broad band observed at 3436 cm^-1 was attributed to νO-H stretching of water of hydration and crystallization νO-H hydroxyl linked to metals. The band appearing at 1632 cm^-1 was attributed to the angular deformation of water δH-OH, in general, associated to water of hydration. The absorption band at 685 and 558.86 cm^-1 in the curve is attributed to the bending vibrations of the Fe-O in α-Fe₂O₃¹⁴.

 

4. CONCLUSION:

We have successfully synthesized nanoparticles of Fe₂O₃ by simple sol-gel method. For microstructural studies we have used X-ray diffraction, FTIR, AFM and Particle size analyzer techniques. XRD data shows that the nanoparticles of Fe₂O₃ are all in single phase and exhibit no other impurity phase. Average crystal size has been calculated using Scherer’s formula and it has been found between 18-29 nm that is also in the agreement of the data taken with the help of Particle Size Analyzer. From the XRD data we have concluded that until 400ºC the possibilities of γ-Fe₂O₃ phase is more while more than 400ºC α-Fe₂O₃ nanoparticles are confirmed. An IR spectrum of Fe₂O₃ nanoparticles show more than two absorption bands but at around 580 cm^-1 indicate the presence of γ-Fe₂O₃. The surface images of Fe₂O₃ nanoparticles have been taken out by using Atomic Force Microscopy in tapping mode. The resultant images show the conical surface morphology, which confirms the atomic and lattice structure of Fe₂O₃ nanoparticles.

 

5. ACKNOWLEDGEMENTS:

Authors are grateful to the Council of Science and Technology (CST), Govt. of UP, India for financial support in the form of Center of Excellence in Materials Science (Nanomaterials).

 

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Received on 29.06.2011        Modified on 10.07.2011

Accepted on 21.07.2011        © AJRC All right reserved