Synthesis and Characterization of Iron Oxide Nanoparticles by Solid ...

J Clust Sci (2010) 21:11–20 DOI 10.1007/s10876-009-0278-x ORIGINAL PAPER

Synthesis and Characterization of Iron Oxide Nanoparticles by Solid State Chemical Reaction Method Hassan Karami

Received: 9 October 2009 / Published online: 29 December 2009 Ó Springer Science+Business Media, LLC 2009

Abstract The iron oxide nanoparticles were synthesized by solid state chemical reaction method. The synthesized powders were characterized by XRD, SEM, EDAX and TG-DTA techniques. An average grain size of 10–20 nm for Maghemite and 80–85 nm for Hematite was calculated using XRD line broadening and SEM. The effect of different parameters such as annealing temperature, milling time, Fe?3:Fe?2 concentration ratio have been investigated on the particle size and phase formation. Heat treatment of the produced powders in which Fe?3:Fe?2 ratio equal to 2:1, resulted in tetragonal (Maghemite) and rombohedral (Hematite) structures at 300 and 600 °C, respectively. It was found that by changing Fe?3:Fe?2 ratios from 2:1 to 1:2, hematite phase and from 2:1 to 1:1, Maghemite phase were formed at 300 °C. In addition with increasing milling time, the iron oxide particle size decreases, but there was no changing in the kind of phase. Keywords Solid state

Iron oxide Nanoparticles Hematite Maghemite

Introduction Nanoparticles of magnetic oxides have attracted great interest in recent years because of their unique physical and chemical properties. There are two well-known crystalline types of Fe2O3: Maghemite (the c phase) and hematite (the a phase). Because of their industrial importance in preparing magnetic recording materials [1], pigments [2], catalysts [3], sorbent for removal of heavy metals from water and H. Karami (&) Nano Research Laboratory, Department of Chemistry, Payame Noor University (PNU), Abhar, Zanjan, Iran e-mail: [email protected]

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soil [4] and gas-sensitive materials [5], nanocrystalline iron oxide have been of considerable interest in recent years. Iron oxide nanoparticles synthesized by different methods such as coprecipitation [6], sol–gel [7], combustion processes [8], microemulsion [9] and electrochemical processes [10]. However, the methods reported have the disadvantages of the need to use expensive organic precursors as starting materials, or particle aggregations during high temperature oxidation in the air. Development of new, convenient and large-scale synthesis methods for iron oxide nanoparticles is still a major challenge. Mechanochemical processing is a novel method to synthesize nanostructured materials. The solid state chemical reaction method is a kind of mechanochemical method in which a chemical reaction tacks place during grinding solid precursors. It is an organic-solvent free process and thus ecologically cleans. In this work, iron oxide nanoparticles were synthesized by solid state chemical reaction because of simplicity, low cost and high yield scale. The goal of the present work is to carry out a study on the effect of different parameters such as heat treatment, milling time, Fe?3:Fe?2 ratios on kind of formed phase of iron oxide and its particle size.

Experimental Materials and Equipments Iron(II) chloride tetrahydrate (FeCl24H2O), iron(III) chloride hexahydrate (FeCl36H2O) and potassium chloride and other regents were all of analytical grade and used without purification. The crystal structures were identified by a powder X-ray diffractometer (XRD, ˚ ). The XRD Patterns of Philips PW-1840) employing Cu Ka radiation (k = 1.5418 A nanoparticles were verified by comparing with the JCPDS data. The morphology and chemical composition of the synthesized iron oxide nanoparticles were observed by scanning electron microscopy/energy dispersive X-ray analysis (SEM/ EDAX, Philips XL-30). The combined thermogravimetry and differential thermal analysis (TG-DTA, Rheometeric STA-1500) was performed at a scan rate of 10 °C/min from room temperature to 1,000 °C.

Preparation of Iron Oxide Nanoparticles by Solid State Chemical Reaction Method For the synthesis of iron oxide nano-particles as solid phase, mixed powders of FeCl36H2O (1.35 g), FeCl24H2O (0.50 g) and KCl (3.9 g) were ground in a mortar at room temperature for 30 min. The mixture obtained from grinding was a yellow paste. To the mortar was added KOH powder (1.22 g), followed by grinding for another 30 min at room temperature. During the KOH addition and the subsequent grinding, a significant amount of heat and some vapour was given off in the first few minutes. The product was washed with the distilled water, treated in an ultrasonic

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bath for 15 min and centrifuged (3,500 rpm) for 15 min. This process was repeated several times until no Cl- ion could be detected in the centrifugate. The product was then dried in vacuum at 50 °C and became brown nano-powder (Maghemite). By increasing the temperature, the Maghemite (c-Fe2O3) transferred to hematite (a-Fe2O3). Flowchart of preparation process is shown in Fig. 1.

Fig. 1 Flowchart of synthesis process of iron oxide nanoparticles

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Results and Discussion Mechanism of the Fe2O3 Nanoparticles Formation The iron oxide particles are produced by the following solid-state reaction: FeCl2 4H2 OðsÞ þ 2FeCl3 6H2 OðsÞ þ 8KOHðsÞ ! 8KCl þ Fe3 O4 þ 14H2 O 4Fe3 O4 þ O2 ðgÞ ! 6c-Fe2 O3 c-Fe2 O3 ! a-Fe2 O3 : The solid state chemical reaction method is a kind of mechanochemical method in which a chemical reaction tacks place during grinding solid precursors. The formation of salt by-product (KCl), provide an effective driving force for the small particles. The growth of the particles is inhibited by the by-salts produced in the solid–solid state reaction. In other words, KCl and several water molecules are produced in the current reaction. The precipitation of these salts leads to formation of walls surrounding the nano-particles to keep them from growing [11].

Characterizations Iron Oxide Nanoparticles The Influence of Heat Treatment on Phase and Particle Size From the XRD pattern, the average particle sizes are obtained from the most intense peaks of (311) and (104) in Maghemite and hematite by using the Debye–Scherer equation, respectively. The particle size of synthesized samples was measured from the peak broadening were in good agreement with the values obtained by SEM. Comparison of the XRD patterns with the JCPDS data confirms the samples are Maghemite with tetragonal structure. The XRD patterns and SEM images of the obtained samples after heat treatment at different temperatures are shown in Fig. 2. The observed diffraction peaks at (111), (220), (311), (400), (440), and (511) agree well with the tetragonal structure of Maghemite (c-Fe2O3) at 50 and 300 °C (JCPDS file No 25-1402). Since Fe ions are more sensitive to oxidation: Fe2þ þ O2 ! Fe3þ : Even at low temperature surface Fe ions readily oxidize in air which will lead to the formation of Maghemite at the surface [12]. By increasing the temperature, the phase of Maghemite transferred to hematite at 600 and 800 °C. The diffraction peaks at (012), (104), (110), (113), (024), (116), (122), (214), (300), (119), (220), (306), (134), and (226) attributed to hematite (a-Fe2O3) formation with the rhombohedral structure (JCPDS file No 24-0072). The average grain size of iron oxide particles was estimated according to the Scherrer’s equation where h is the diffraction angle of the peak of the tetragonal phase and b is the full width at half maximum of the peak (in radian), which is calibrated by high purity silicon. Summary of these results is shown in Fig. 2

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Fig. 2 XRD pattern of the iron oxide nanoparticles annealed at different temperatures: a 50 °C, b 300 °C, c 600 °C, d 800 °C; the effect of temperature on particles size (as inserted curve); scanning electron micrograph of the iron oxide nanoparticles annealed for 2 h at different temperatures

(as inserted curve). Average particles sizes for samples prepared at low temperature (Maghemite) were calculated 10–20 nm. Sample heating from 100 to 800 °C causes to change Maghemite into Hematite and particles sizes increase up to 85 nm

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Fig. 3 EDAX spectra of synthesized iron oxide nanoparticles

D¼

0:9k : b cos h

ð1Þ

Figure 2 (inserted curve) shows that by increasing the temperature, the iron oxide particle sizes became larger. The size growing and changing phase of formed iron oxide nanoparticles were confirmed by the SEM images (Fig. 2). The chemical composition of iron oxide nanoparticles were analyzed by EDAX and are shown in Fig. 3. The results of quantitative analysis reveal that the O/Fe atomic ratio of the iron oxide nanoparticles is 1.515 which is compatible with the theoretical O/Fe atomic ratios of Fe2O3 (1.50). Figure 4 shows DTA–TGA curves of the samples treated at 50 °C. There is an endothermic peak under 200 °C with a 5.15% weight loss which corresponds to the desorption of the water physically adsorbed on the iron oxide surface. In addition, there is an exothermic peak at 400–530 °C which related to Maghemite to hematite phase transition without any weight loss on the DTA curve. The Influence of Milling Time The synthesized samples at different milling times were analyzed by XRD patterns. XRD patterns showed that the particles sizes decreased but there was no change in kind of iron oxide phase. SEM images of synthesized iron oxides at different milling time in Fig. 5 are shown that by increasing the milling time, the particles size decreased. The XRD results were well agreement with SEM images.

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Fig. 4 DTA–TGA curves of the samples treated (Fe?3:Fe?2 ratio 2:1) at 50 °C

Fig. 5 Scanning electron micrographs of the iron oxide nanoparticles annealed at 300 °C for 2 h at different milling time

The synthesized sample at milling time of 60 min was analyzed by DTA–TGA. The obtained curves were similar to Fig. 4. The obtained results showed that two endothermic peaks observed in DTA curve under 200 °C which attributed to desorption of water and CO physically adsorbed on the oxide surface with a 10.2% weight loss on TGA curve. An obvious exothermic peak appears between 400 and 530 °C. This peak is related to Maghemite to hematite phase transition without any weight loss on the TGA curve. The third endothermic peak at 760 °C corresponded to decomposition hematite to wustite (FeO) without any weight loss and the last endothermic peak at 900–1,000 °C attributed to transition of hematite to magnetite with a 4.65% weight loss. The Influence of Fe?3:Fe?2 Ratios on Kind of Phase of Iron Oxide The effect of Fe?3:Fe?2 ratios on kind of formed phase were investigated by preparing the samples B, B1, B2 in which Fe?3:Fe?2 ratios were 2:1, 1:1, 1:2,

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Fig. 6 XRD patterns and scanning electron micrographs of the iron oxide nanoparticles annealed for 2 h at 300 °C for samples B, B1 and B2

respectively. The obtaining of c-phase (Maghemite) for B, B1 and a-phase (hematite) for B2 samples were confirmed by XRD patterns in Fig. 6. So the hematite can be synthesized through applying the Fe?3:Fe?2 ratios (1:2) at lower temperature (300 °C). The reason may be that by increasing Fe?2, the tendency to

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occupy of octahedral positions by Fe?2 was raised and hematite phase with a hexagonal structure with a rombohedral in its centre was formed [13, 14]. The SEM images in Fig. 6 show that the hematite with smaller grain size and more homogeny distribution could be obtained by changing Fe?3:Fe?2 ratios from 2:1 to 1:2 at lower temperature (300 °C). Figure 7 is DTA–TGA curves of the B1 and B2 samples. In B1, an endothermic peak observed in DTA curve under 100 °C which attributed to desorption of water physically adsorbed on the oxide surface with a 5.7% weight loss on TGA curve. Also there are two exothermic peaks on DTA curve. The first peak is a weak exothermic peak which appeared between 400 and 500 °C and assigned to a direct transition of Maghemite to hematite without any weight loss and the second

Fig. 7 DTA–TGA curves of samples B1 (a) and B2 (b)

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exothermic peak at 860 °C assigned to a transition hematite to magnetite without any weight loss. As it is obvious in Fig. 7, in B2 sample, an endothermic peak observed in DTA curve under 100 °C which attributed to desorption of water physically adsorbed on the oxide surface with a 1.61% weight losing. Also an exothermic peak which corresponded to direct transition of Maghemite to hematite at 390 °C that is lower than B and B1 and it agrees well with XRD patterns. Finally, comparing of the proposed method by other synthesis methods for iron oxides such as hydrothermal, precipitation, hydrolysis methods shows that the presented methods have some advantages such as small size of final particles, solvent-free, short synthesis time and reliable and easy controlling [15, 16].

Conclusion Maghemite nanoparticles were synthesized by solid state chemical reaction method successfully. According our study, Maghemite nanoparticles were obtained, if Fe?3:Fe?2 ratios were 2:1 and 1:2 at 300 °C, respectively. By changing Fe?3:Fe?2 ratios from 2:1 to 1:1, the needed temperature for forming the hematite nanoparticles decrease from 600 to 300 °C. Acknowledgement We gratefully acknowledge the support of Abhar Payame Noor University Research Council, throughout these research experiments.

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