J Inorg Organomet Polym DOI 10.1007/s10904-009-9276-6
Soft Template Synthesis of Super Paramagnetic Fe3O4 Nanoparticles a Novel Technique Sharif Ahmad Æ Ufana Riaz Æ Ajeet Kaushik Æ Javed Alam
Received: 24 January 2009 / Accepted: 17 April 2009 Ó Springer Science+Business Media, LLC 2009
Abstract The present study reports a facile technique for the synthesis of crystalline super paramagnetic nano ferrite (Fe3O4) particles using diethyl amine as a soft template. The spectral properties of Fe3O4 nanoparticles were characterized by UV–visible and Fourier Transform Infrared (FTIR) spectroscopies while the crystalline structure and particle size was estimated using X-Ray diffraction (XRD) as well as transmission electron microscopy (TEM) techniques. The super paramagnetic behavior of Fe3O4 nanoparticles was determined using vibrating sample magnetometer (VSM) at 300 K. The results of the studies revealed that this technique could be adopted to synthesize agglomerate free super paramagnetic Fe3O4 nanoparticles which may find potential application in the filed of biosensor and corrosion protective coatings. Keywords Nanostructures Soft template Magnetic structure X-ray diffraction Diethyl amine Magnetic properties
1 Introduction Lately academic and technological research has been diverted towards one dimensional nanostructured magnetic materials owing to their extensive applications in fabricating nanodevices [1–4]. Among the various magnetic nanostructured materials, ferrite (Fe3O4) nanoparticles S. Ahmad (&) U. Riaz A. Kaushik J. Alam Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia University, New Delhi 110025, India e-mail: [email protected]
U. Riaz e-mail: [email protected]
have attracted much interest because of their unique properties [5–9]. Nanosized Fe3O4 particles have been widely used as recording material, pigments, electro photographic developer , mineral separation , and efficient heat transfer applications  and in cancer therapy . It has been reported that the magnetic behavior of Fe3O4 nanoparticles changes with the particles size. Therefore the synthesis and processing of these nanoparticles has become the topic of major concern . A variety of methods have been adopted to synthesize Fe3O4 nanoparticles, including the reduction of hematite by CO/CO2 , c-ray radiation , co-precipitation from the solution of ferrous/ferric-salt mixture in alkaline medium , hydrolysis , sol–gel technique  and oxidation of Fe(OH)2 by H2O2 . These procedures suffer from the inability to control the size of Fe3O4 nanoparticles because the large surface area, high surface energy as well as high magnetization that result in aggregation and cluster formation. Recently template-confined methods have also been adopted to develop nano crystalline Fe3O4 nanoparticles . One of the major drawbacks of this technique is that the complete removal of template is a tedious task which eventually affects the purity of the nanoparticles . Hence, a surfactant-free template method is expected to be of great significance for the synthesis of agglomerate free nanomaterials. Till date no literature is available on the synthesis of Fe3O4 nanoparticles using diethylamine . The present paper reports a novel soft template technique to synthesize agglomerate free super paramagnetic Fe3O4 nanoparticles. The nanoparticles were characterized by spectral, morphological as well as magnetic studies. The crystalline nature and phase purity of Fe3O4 nanoparticles was characterized using XRD and was further confirmed by transmission electron microscopy (TEM). The particle size of
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Fe3O4 was estimated to be around 23 nm with the help of Scherrer equation while the atomic force microscopy (AFM) image of Fe3O4 film deposited on (ITO) exhibited a uniform porous morphology of non-agglomerated Fe3O4 nanoparticles (about 30 nm) with average roughness of *20 nm. The vibrating sample magnetometer (VSM) study confirmed the super paramagnetic behavior of Fe3O4 nanoparticles. It was observed that the Fe3O4 nanoparticles synthesized by this method exhibited least agglomeration having reasonably good magnetic as well as morphological properties.
2 Experimental Procedure 2.1 Materials Ferrous chloride, ferric chloride and diethyl amine were purchased from S.D.Fine (Bombay) India, were of analytical grade and have been used for the preparation of Fe3O4 nano particles. 2.2 Synthesis of Fe3O4 Nanoparticles Ferrous chloride (3 9 10-2 M) and Ferric chloride (6 9 10-2 M) solutions (mole ratio 1:2) were prepared in 250 mL beaker separately and were mixed with the help of magnetic stirrer at room temperature (30 °C). Diethyl amine solution (5 N, 5 ml) was then added drop wise and the pH of the solution was adjusted to about 11. The stirring of the solution was continued for the 5 h until the stable black particles were formed. The magnetic behavior of the particles was monitored by the applying the magnetic field at the bottom side of conical flask. Finally the ultra fine precipitate of Fe3O4 particles were filtered washed several times with water and methanol. The ferrite powder was then dried in vacuum oven at 100 °C for 72 h to ensure complete removal of water.
3 Characterization 3.1 Spectral Analysis Fourier Transform Infrared (FTIR) spectra of the nano particles were taken in dried KBr powder on Perkin-Elmer spectrometer model 1750. 3.2 Morphological Analysis X-ray diffractograms were recorded on X-ray diffractometer model Philips W3710 using copper Ka radiations. Transmission electron microscopy was used for particle
size confirmation. Transmission electron micrographs were taken on Morgagni 268-D TEM, FEI, USA. The sample was prepared by placing a drop of the nanoparticles on carbon-coated copper grid and subsequently drying in air, before transferring it to the microscope operated at an accelerated voltage of 120 kV. Atomic force microscopy (AFM) of Fe3O4 nanoparticles film (dispersed in methanol) onto ITO substrate was carried out using Vieco spectrophotometer. 3.3 Magnetic Measurements The magnetic measurements were performed using Vibrating Sample Magnetometer Model E.G & G Princetonat 300 K by applying a field of 1000 A/m at a frequency of 16 Hz.
4 Results and Discussions According to the thermodynamics of Fe3O4 synthesis, a complete precipitation of Fe3O4 takes place at controlled pH, while maintaining a molar ratio of Fe3?:Fe2? 2:1. Therefore the control of size, shape and composition of nanoparticles depends on medium used. At room temperature, when diethylamine is added into the solution containing Fe2? and Fe3? with pH value about 11, Fe2? and Fe3? are converted into hydroxide compounds as shown in Scheme 1. With the rise of temperature, the hydroxide compounds crystallize to Fe3O4 slowly. The growth of the nanoparticles in the organic solvent under optimum synthetic conditions usually takes place by the formation of tiny crystalline nuclei in the medium, followed by crystal growth . In order to prevent the nanoparticles from oxidation as well as from agglomeration, Fe3O4 nanoparticles were synthesized in diethylamine, Scheme 1. The utilization of diethylamine to synthesize the magnetic nanoparticles was found to be simpler, more tractable and efficient with appreciable control over composition and even the shape of the nanoparticles. During this synthesis diethyl amine acted as a soft template to develop uniform and extremely fine dispersion of nanocrystals of Fe3O4 and it was easier to completely remove this template after the reaction. Moreover it was also possible to reprecipitate fine nanocrystals from the solvent easily. 4.1 Morphological Properties The X-ray diffraction pattern, depicted in, Fig. 1, corresponds to Fe3O4 nanoparticles. (JCPDS–International center diffraction data, PDF cards 3-864 and 22-1086). The reflection peak at 2h = 35.60° corresponds to the spinel phase of Fe3O4. The diffractions peaks of the magnetic
J Inorg Organomet Polym Scheme 1 Formation of Fe3O4 nanoparticles
(radiant) K is the constant. According to the b values of the Fe3O4 (311) and Fe3O4 (440) peaks, the estimated particle size was calculated to be about 25 nm. The broadening of the reflection peak also indicates the formation of the ultrafine nanoparticles.
4.2 Transmission Electron Microscopy
Fig. 1 X-ray diffraction pattern of Fe3O4 nanoparticles
nanoparticles were measured to be 2h = 30.10° (d = 0.297 nm), 35.42° (d = 0.253 nm), 43.20° (d = 0.209 nm), 53.48° (d = 0.171 nm) and 56.94° (d = 0.162 nm). These data are in good agreement with that of the Fe3O4 nanoparticle . The particle size Fe3O4 nanoparticles has been estimated using Scherrer equation: D¼
Kk b cos h
as where D is the particle size, k is the X-ray wavelength (nm), h is Bragg angle, b is the excess line broadening
TEM micrographs of Fe3O4 nanoparticles are shown in Fig. 2a. The TEM images reveal that Fe3O4 nanoparticles are similar in shape and appear to be uniformly dispersed leading to the formation of a self-organized network like morphology. The particles size observed in the TEM micrograph is in accordance with the crystallite size estimated by the Scherrer equation (*25 nm). Moreover, the TEM diffraction ring (digital diffraction pattern, Fig. 2b) shows that the synthesized Fe3O4 nanoparticles exhibit a crystalline state . The two dimensional and three dimensional AFM images, Fig. 3, of Fe3O4 nanoparticles show that nanoparticles are strongly adhered on the substrate with porous morphology exhibiting a least agglomerated state. The average surface roughness of Fe3O4 nanoparticles film was estimated *20 nm. The AFM studies confirm that the film of Fe3O4 nanoparticles are strictly interconnected by the mutual interaction among the nanoparticles due to the presence of high surface area and dipole–dipole interaction.
J Inorg Organomet Polym Fig. 2 a TEM of Fe3O4 nanoparticles b digital diffraction pattern
4.3 FT-IR Analysis The FTIR spectrum of Fe3O4 nanoparticles, Fig. 4, exhibits an absorption peak at 3,440 cm-1, which is the characteristic peak of OH stretching vibration and is also indicative of the presence of some amount of ferric hydroxide in Fe3O4 [27, 28]. The two distinct absorption peaks at 565 at 421 cm-1 are attributed to the vibrations of Fe2?–O2- and Fe3?–O2- respectively . Interestingly the peaks related to diethyl amine (shown in inset Fig. 4) are not observed in the FT-IR spectrum of Fe3O4 confirming the complete removal of the template. The sharp and high intense peak appears at 565 cm-1 demonstrates the high degree of crystallinity of the Fe3O4 nanoparticles. The characteristic absorption bands therefore confirm the presence of presence of spinel structure Fe3O4. 4.4 UV–Visible Spectra The UV–visible spectrum of Fe3O4 nanoparticle as shown in Fig. 5. In case of magnetic nanoparticles, the UV absorption band is observed in the region 330–450 nm which originates primarily from the absorption and scattering of light by magnetic nanoparticles and is in accordance with the previous literatures [30, 31].The high absorption band at 410 nm indicate the formation of a least agglomerated nanosize particles. The absence of any additional peaks related to diethylamine (which is observed in the region of 250–300 nm as shown in inset) confirms that the Fe3O4 nanoparticle were not encapsulated by the later and it only acted as a soft template. 4.5 Magnetic Properties Fig. 3 a and b Two dimensional atomic force microscopy image of Fe3O4 nanoparticles
The magnetic properties of the Fe3O4 nanoparticles have been measured with the help of the VSM. The applied
J Inorg Organomet Polym Fig. 4 FT-IR spectrum of Fe3O4 nanoparticles (inset FTIR spectrum of diethylamine)
applied field H (between -8000 Oe and ?8000 Oe) for ?8000 Oe nanoparticles. The VSM curve reveals that there is a hysteresis for the Fe3O4 nanoparticles. The coercive force is small (Hc = 116.35 Oe), in the comparison to the reported the coercive force is 500–800 Oe for the bulk magnetic particles [33, 34], however, it does not approach to zero, thereby exhibiting a super paramagnetic behavior [35, 36].On increasing the applied field from the 0 to 8000 Oe, the magnetization increases sharply; M is nearly saturated at about 2000 Oe. For the Fe3O4 nanoparticles, Ms is around 35.76 emu/g while the reported value, Ms is 84 emu/g for the bulk Fe3O4 particles and 65 emu/g for Fe3O4 nanoparticles [36, 37]. The measured magnetization of nanoparticles was found to be considerably lower than the values measured from bulk magnetite.
Fig. 5 UV–visible absorption spectrum of Fe3O4 nanoparticles (inset UV–visible spectrum of diethylamine)
magnetic field H dependence of the magnetization M can be described by Langevin equation  as M ¼ MsðCothy 1=YÞ; and Y ¼ mH=kBT where Ms is the saturation magnetization of the nanoparticles, m the average magnetic moment of an individual particle in the sample and kB the Boltzemann constant. The Fig. 6 shows the plots of the magnetization M versus
Fig. 6 Vibrating sample magnetometer curve of Fe3O4 nanoparticle at room temperature
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Fig. 7 Cyclic voltammograms of Fe3O4/ITO as a function of scan rate
4.6 Electrochemical Characterization CV spectra of Fe3O4/ITO, Fig. 7 shows that the magnitude of current increases and peak potential shifts to lower value corresponding to ITO electrode. These results suggest that Fe3O4 nanoparticles promote electron transfer due to uniform nano porous dispersion on the electrode surface as indicated by the AFM studies. This can also be attributed to the induced magnetisation of Fe3O4 nanoparticles domains on applying potential that results in enhanced magnitude of current as compared to that of bare ITO. It can be noticed that Fe3O4/ITO electrode undergoes a reversible electron transfer with electrode surface.
5 Conclusion A facile technique was adopted to synthesize super paramagnetic Fe3O4 nanoparticles using diethyl amine as a soft template. The broad absorption band *410 nm in UV–visible spectra and a sharp band at 600 cm-1 in FTIR spectra further confirmed the formation of Fe3O4 nanoparticles. AFM analysis revealed that Fe3O4 nanoparticles exhibited an average particle size of 25 nm having a non-agglomerated nature and porous morphology. The VSM analysis revealed a superparamagnetic behavior of Fe3O4 nanoparticles with small coercive force (116 Oe) and small magnetization (35.6 emu/g) as compared to bulk Fe3O4. The results of these studies suggest that the soft template method can be used for the controlled synthesis of Fe3O4 nanoparticles which may find potential applications in the field of biosensor and corrosion protective coatings.
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