Materials Chemistry and Physics 132 (2012) 196–202
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Fe3 O4 inverse spinal super paramagnetic nanoparticles Obaid ur Rahman, Subash Chandra Mohapatra, Sharif Ahmad ∗ Materials Research Lab, Department Of Chemistry, Jamia Millia Islamia, New Delhi 110025, India
a r t i c l e
i n f o
Article history: Received 15 June 2011 Received in revised form 3 November 2011 Accepted 14 November 2011 Keywords: Ferrite nanoparticles Superparamagnetism VSM and EPR
a b s t r a c t The present article reports an energy efficient method for the synthesis of superparamagnetic ferrite (Fe3 O4 ) nanoparticles (10–40 nm) and their annealing effect on the morphology, size, curie temperature and magnetic behavior at 50, 300, 400 and 500 ◦ C. The synthesized nanoparticles were characterized by various spectroscopic techniques like FT-IR and UV–visible. The crystalline structure and particle size were estimated through solid phase as well as the liquid phase using XRD, TEM and DLS techniques. Superparamagnetic behavior of nanoparticles was confirmed by VSM. The EPR study reveals that the main feature of X-Band solid state EPR spectrum has strong transition at geff ∼ 3.23 (2100G) and a relatively weak transition at geff ∼ 2.05 (3300G). The later transition further confirms the super paramagnetic nature of these nano ferrites. The activation energy and order of weight losses of nano ferrites were found to be: 39.6 KJ mol−1 and 0.21 orders (600–800 ◦ C), respectively, analyze with the help of TGA while the specific ˚ were determined by Quanta chrome BET instrument. surface area (23.1 m2 g−1 ) and pore size (9 A) © 2011 Elsevier B.V. All rights reserved.
1. Introduction Among the various nanostructure materials, metal oxide nanoparticles are the important class of materials as their optical, magnetic and electrical properties find a wide range of high tech applications [1]. Fe3 O4 nanoparticles are common ferrite with an inverse cubic spinal structure. These class of compounds exhibit unique electrical and magnetic properties due to the transfer of electrons between Fe2+ and Fe3+ on octahedral sites [2]. Fe3 O4 nanoparticles have been the subject of intense interest because of their potential applications in several advance technological areas due to their promising physical and chemical properties. Generally, these properties depend on the size and structure of particles [3,4]. Fe3 O4 nanoparticles find wide applications in the field of biomedical, as anticancer agent [5,6], corrosion protective pigments in paints and coatings [7]. The magnetic atoms or ions in such solid materials are arranged in a periodic lattice and their magnetic moments collectively interact through molecular exchange fields, which give rise to a long-range magnetic ordering. Among all iron oxide nanoparticles, Fe3 O4 represent the most interesting properties due to of its unique structure i.e. the presence of iron cations in two valence states, Fe2+ , Fe3+ on tetrahedral and octahedral sites with an inverse cubic spinel structure. The coercivity and remenance values for the super paramagnetic nano Fe3 O4 nanoparticles have been found to be zero by the earlier reported methods [8]. Presently, cell labelling strategies find application of
∗ Corresponding author. Tel.: +91 11 26827508x3268; fax: +91 11 2684 0229. E-mail address: sharifahmad
[email protected] (S. Ahmad). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.11.032
superparamagnetic ferrite either through conjugating the magnetic nanoparticles to the cellular surface of the stem cell or by internalization of the particles into the cell. Superparamagnetic ferrite can work in both of these ways, since the potential to manipulate their surface chemistry is plentiful and their sizes along with other attributes promote their successful uptake into cells. The superparamagnetic nano ferrites also interface well with MRI technology. The use of superparamagnetic particles play a crucial role in the diagnostic imaging modality technique finds application in the study of stem cell [9]. Karaoglu et al. report the poly ethylene glycol (PEG) assisted hydrothermal route to study the influence of the hydrolyzing agent on the properties of PEG-iron oxide (Fe3 O4 ) nano composites and Köseoglu et al. reports the investigation on the structural and magnetic properties of Mn0.2 Ni0.8 –Fe2 O4 nanoparticles synthesized by a PEG assisted hydrothermal rout [10,11]. Guobin Ding et al. reports the development and characterization of a magnetic micellar nanocarrier based on the amphiphilic copolymer [methoxy poly (ethylene glycol)-poly(d,l-lactideco-glycolide)] MPEG-PLGA and magnetite (Fe3 O4 ) nanoparticles, and discuss its potential for double-targeted hydrophobic drug delivery [12]. Cheng-Hao Liu et al. reports the development of a reusable, single-step system for the detection of specific substrates using oxidase-functionalized Fe3 O4 nanoparticles (NPs) as a bienzyme system and using amplex ultrared (AU) as a fluorogenic substrate [13]. Phadatare et al., reports The PEG assisted NiFe2 O4 nanoparticles for the possible biomedical applications such as magnetic resonance imaging, drug delivery, tissue repair, magnetic fluid hyperthermia, etc. [14]. Literatures available on the synthesis of nanoferrite using (PEG) based polyol methods as reported by Karaoglu et al., [10] the
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PEG assisted hydrothermal route for PEG–Fe3 O4 nanocomposite, solvothermal method, thermal decomposition method, etc. [11,15,16]. However, scant literature available on the synthesis of ferrite nanoparticles using ethylene glycol at low temperature. Here in, we have simply used the ethylene glycol (EG) of low molecular weight (58 amu) which may be attributed to the synthesis of ferrite nanoparticles at reasonably low temperature as compared to those of reported in literatures. While most of the earlier reported synthesis involve high temperature (>200 ◦ C) and long time (6–48 h), we have developed a novel efficient method for the synthesis of ferrite nanoparticles involving minimum possible temperature and time (50 ◦ C, 4 h) using EG as solvent, stabilizer and reducing agent. The formation of the nanoparticles in the polyol at pH 13 occurs through the nucleation of tiny crystallites followed by the growth of stable nano crystalline particles. The utilization of ethylene glycol in the synthesis of the magnetic nanoparticles was not only found to be easier and efficient but also has appreciable control over the composition and shape of the nanoparticles. The extraction of nanoparticles from the reaction mixture was found to be simpler owing to the greater miscibility of EG in water and methanol. The manuscript also discussed the effect of annealing temperature on the morphology and magnetism of nanoferrite. 2. Experimental 2.1. Materials All the chemicals were used of analytical grade. Ferrous sulphate heptahydrate (Merck India), ethylene glycol (EG), ammonium hydroxide (SD fine chemicals Pvt. Ltd., India) and hydrogen peroxide (Fisher Scientific, India) were used as received. 2.2. Synthesis of Fe3 O4 nanoparticles Ferrous sulphate heptahydrate (3 g, 1.08 × 10−2 M) and ethylene glycol (50 ml) were taken in a 250 ml three necked flask, fitted with a reflux condenser. The solution was stirred under nitrogen atmosphere for 30 min, followed by drop wise addition of 20 ml 0.5% aqueous solution of 50% H2 O2 . The reaction mixture was heated at 50 ◦ C for 4 h under continuous stirring. The pH of the reaction was maintained at pH 13 by occasional addition of 25% aqueous ammonia solution. The progress of reaction was visually monitored. The nanoparticles formed were dialyzed for 24 h and purified repeatedly by magnetic field separation, decantation and redispersion. Finally, the ultra fine precipitate of Fe3 O4 nanoparticles was filtered and washed several times with water and methanol. The ferrite powder was then dried under vacuum at 50 ◦ C for 72 h, then Fe3 O4 nanoparticles were annealed at 300, 400, and 500 ◦ C.
197
Fig. 1. X-ray diffraction pattern of nanoferrite.
16 Hz. Thermo gravimetric analysis (TGA/DTA) of the synthesized nanoparticles was done on EXSTAR TG/DTA 6000 under nitrogen atmosphere at a heating rate of 20 ◦ C min−1 . Activation energy and order of decomposition were determined using freeman Carrol and coat’s plots methods [17,18], by plotting the graph. log(dw/dt) T −1 Vs log wr log wr where dw, dt, wr and T are the changes in mass, time, dw and temperature in Kelvin, respectively. The plot was found to be linear with intercept at x axis and the slope (−ve or +ve sign) are found to be characteristic of physical and chemical reaction of nano ferrites. Activation energy and order of decomposition of nanoferrite was determined on the basis of the calculated slope and intercept, respectively [17,18]. The EPR spectrum was recorded using Varian E-109 spectrometer operating in the X-band (9.5 GHz) at 25 ◦ C. 3. Result and discussions 3.1. Synthesis Super paramagnetic ferrite nanoparticles were synthesized in situ through the precipitation of Fe3 O4 nanoparticles using metal salt precursor (FeSO4 ) in EG found to be very simple cost-effective and eco-friendly. 3.2. X-ray diffraction (XRD) studies
2.3. Characterization XRD were obtained on Philips X-ray diffractometer (Model Philips W3710) using copper K␣ radiation. TEM studies were carried out using electron microscope (Model Philips Morgagni 268) operating at 80 kV at AIIMS, New Delhi, India. The light scattering experiments were performed on a Photocor FC. All the measurements were done at a scattering angle of 90◦ at temperature 20 ◦ C. BET surface area experiment was carried out by Quantachrome instrument (model NOVA 2000e USA) under liquid nitrogen environment to determine the surface area and pore size of Fe3 O4 nanoparticles. FT-IR spectra were measured on Perkin-Elmer spectrometer (Model 1750). UV–visible spectra were recorded in aqueous medium on Perkin Elmer Lambda (Model EZ-221). The magnetic measurements were performed on vibrating sample magnetometer (VSM) (model Lake Shore’s new 7400 series) at 27 ◦ C by applying a field of 1000 A m−1 at a frequency of
The XRD pattern of the prepared and annealed (300, 400 and 500 ◦ C) Fe3 O4 nanoparticles samples are shown in Fig. 1. All the diffraction peaks can be indexed to an inverse spinel structure of Fe3 O4 nanoparticles, which is in good agreement with the literature value (JCPDS Card No. 19-0629). The strong and sharp peaks suggested that Fe3 O4 crystals are highly crystalline. However, the broadening in the reflection peaks was due to the particles size at nano domain. All XRD patterns show diffraction peak at 2 = 35.70◦ corresponds to the spinel phase of Fe3 O4 nanoparticles. The XRD peaks of magnetic nanoparticles at 2 = 30.22◦ , 43.52◦ , 57.43◦ and 63.11◦ were found to be in good agreement with those of previously reported 2 values of Fe3 O4 nanoparticles and matches well with the JCPDS Card No.19-0629. The average crystallite size of nanoparticles was calculated from the lower full-width-at-halfmaximum (FWHM) of (3 1 1) diffraction reflection using Scherrer’s equation: D = 0.9/ˇ cos , where D is the particle size, is the X-ray
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Fig. 2. Transmission electron micrographs of nanoferrite at: (a) 50 ◦ C, (b) 300 ◦ C, (c) 400 ◦ C and (d) 500 ◦ C, respectively.
wavelength (nm), is Bragg’s angle; ˇ is the excess line broadening (radiant), and the particle size of Fe3 O4 nanoparticles was found in the range of 20–25 nm using Scherer equation. The intensity of all diffraction peaks at 2 = 35.70◦ are followed the decreasing of intensity due to formulation more crystalline phase particles in all annealed Fe3 O4 nanoparticles. The estimated particle size at 300 ◦ C was estimated as ∼21 nm [19,20]. 3.3. Transmission electron microscopy (TEM) studies The TEM specimens were prepared by dispersing the powdered compounds in double distilled water and sonicating them for 45 min. One drop of the sonicated solution was placed on to the wax coated copper grid and then dried in air. Fig. 2 reveals that Fe3 O4 nanoparticles are spherical in shape and particles size is in accordance with the crystallite size as estimated in previous Section 3.2 by the Scherrer’s equation (20–25 nm). However, Fe3 O4 nanoparticles are found to be well interconnected with least agglomeration, this may perhaps be due to the high surface charge onto Fe3 O4 nanoparticles and magneto dipole interaction. The increase in the annealing temperature enlarged the size of the primary particle, as expected from the crystallization phenomenon (Fig. 3.). These values were 20, 15, 30 and 35 nm for the annealed samples at 50, 300, 400 and 500 ◦ C, respectively. Interestingly when the temperature is increasing, crystallite size also increases but at 300 ◦ C has some deviation which would be uniformly distributed nanoparticles with almost no agglomeration that may be attributed to improve the electronic properties for desired application [21].
obtained as 45–60, 20–30, 55–60, 60–70 nm as shown Fig. 4(a–d). DLS studies also confirmed the range and size of the particles annealed at 50, 300, 400 and 500 ◦ C analyzed by TEM experiment as discussed in Section 3.3. However, in case of DLS studies the hydration sphere was found to be more for Fe3 O4 nanoparticles annealed at 50, 400 and 500 ◦ C than 300 ◦ C by an average of 20–30 nm. The higher value of average size (compared to TEM) obtained in DLS originates from the fact that DLS measures the hydrodynamic radii of the particles, which include the solvent layer at the interface. As discussed in previous section size of annealed Fe3 O4 particle size at 400 and 500 ◦ C is 30 and 35 nm, respectively, the larger particle at 400 and 500 ◦ C is due to the aggregation of the nanoparticles in water. So aggregation in water results larger hydration sphere and contributed to larger particle size [22].
3.4. Dynamic light scattering (DLS) studies Powdered sample of Fe3 O4 nanoparticles were dispersed in double distilled water with the help of sonicator and the light scattering experiments of sample were performed on a Photocor FC. DLS images show that the average sizes of the Fe3 O4 nanoparticles were
Fig. 3. Particle size variation with the annealing temperature.
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Fig. 4. Size distribution plot of Fe3 O4 nanoparticles at: (a) 50 ◦ C, (b) 300 ◦ C, (c) 400 ◦ C and (d) 500 ◦ C.
3.5. BET
3.7. UV–visible spectroscopic studies
The average surface area of the Fe3 O4 nanoparticles was found to be 23.14 m2 g−1 and pore size of the nanoparticles comes out to ˚ Hence the synthesized nanoparticles is porous in nature. be 9 A.
The UV–visible spectrum of Fe3 O4 nanoparticles (Fig. 6) shows an absorption band in the region of 330–450 nm, which originates primarily from the absorption and scattering of UV radiation by magnetic nanoparticles, which is in accordance with the previously reported literature [27]. The absorption band at 330 nm indicates the formation of nano sized particles.
3.6. FT-IR spectroscopic studies FT-IR spectra of annealed samples were recorded at room temperature depicted in Fig. 5(a–d). In Fig. 5a, the characteristic M–O sharp peak observed at 585 cm−1 . This can be attributed to the high degree of crystalline nature of the Fe3 O4 nanoparticles [23,24]. In Fig. 5a, the peaks at 3413 cm−1 is ascribed to the stretching vibrations of hydrogen-bonded surface water molecules and hydroxyl groups. The peak at 1619 cm−1 is assigned to the O–H bending [23–26]. Moreover, the peak at 1000 cm−1 is attributed to vibrational band of C–O bond for pure ethylene glycol [10], the peak at 750 cm−1 may be due to the deformation bending of mono substituted –C–C– of ethylene glycol, which may be present at the surface as impurity. In case of samples annealed at 300, 400 and 500 ◦ C the sharp doublet is observed at 585 cm−1 . And peaks at 3413 cm−1 and 1619 cm−1 have been vanished this can be attributed for the complete removal of surface water molecule, forming highly crystalline Fe3 O4 nanoparticles. This is in consistent with that of XRD results as discussed in Section 3.2.
Fig. 5. FT-IR spectra of nanoferrite.
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Fig. 8. Variation of magnetization with the annealing temperature. Fig. 6. UV–visible spectrum of nanoferrite in water.
3.8. Vibrating sample magnetometer (VSM) studies The magnetic properties of Fe3 O4 nanoparticles were characterized by vibrating sample magnetometer (VSM, Model Lake Shore’s new 7400 series) at 300 K, using magnetization M and applied field H can be described by Langevin equation [28]: M = Ms (Cothy − 1), and Y =
mH kT
where Ms is the saturation magnetization of nanoparticles, ‘m’ is the average magnetic moment of individual nano particle in the sample and k, the Boltzmann constant [29]. Fig. 7 shows the plots of the magnetization ‘M’ versus applied field ‘H’ (between −2000 Oe and +2000 Oe) of these prepared Fe3 O4 samples were obtained after different annealed temperature. The VSM curve reveals the formation of a hysteresis loop for the Fe3 O4 nanoparticles, with zero coercivity and remanance values, which exhibits superparamagnetic behavior of Fe3 O4 nanoparticles. On increasing the applied field from 0 to 1000 Oe, the magnetization ‘M’ increases sharply; and becomes nearly saturated at about 1000 Oe. It was found that all the samples have strong magnetic responses to a varying magnetic field. The hysteresis loops showed smooth change of magnetization with applied field. Fig. 8 depicts the saturation of magnetization values of Fe3 O4 samples increased with the annealing temperature and further decreases with the increase in annealing temperature. Fig. 9 indicates that the prepared Fe3 O4 samples posse’s magnetic properties which are dependent on size and morphology of the nanoparticles. As we change the annealing temperature, particle size of nanoparticles also changes and surface area to volume
o
50 C o 300 C o 400 C o 500 C
ratio changes and hence the magnetization changes. The Fe3 O4 nanoparticles obtained from non annealed sample possessed a saturation magnetization (Ms ) of 65.4 emu g−1 , while the annealed Fe3 O4 superstructures obtained after 300 ◦ C, 400 ◦ C and 500 ◦ C had Ms values of merely 76.8 emu g−1 , 61.2 emu g−1 and 43.3 emu g−1 , respectively. While the reported Ms Value, is 84 emu g−1 for the bulk Fe3 O4 particles and 80.7 emu g−1 for Fe3 O4 nanoparticles [27,28,30]. As we increase the sintering temperature, the beginning magnetization increases from 65.4 emu g−1 to 76.8 emu g−1 . However, the magnetization decreases as we increase the sintering temperature 300, 400 and 500 ◦ C from 76.8 emu g−1 to 61.2 emu g−1 to 43.3 emu g−1 , respectively. It is very interesting to note that the magnetization of ferrite nanoparticles follow the curie law of magnetization, in the case of annealing curie temperature increase in between 300 and 400 ◦ C after that curie temperature decrease vice versa with the saturation of magnetization value as it is evident from Fig. 8. In present work, the value for the saturation of magnetization (Ms ) is found to be higher than those of other earlier reported work [29,31]. Very significant and promising effect observes on the Ms due to the annealing of Fe3 O4 nanoparticles at optimum temperature 300 ◦ C. At this temperature, the nanoparticles are uniformly distributed with almost no agglomeration and further correlated with XRD, TEM and DLS studies as discussed in Sections 3.2–3.4, respectively. 3.9. Thermo gravimetric analysis The thermal analysis of nanoferrite shows initially a negligible weight loss (0.4%) in the region of 90–150 ◦ C, supported by DTA endothermic peak, which confirms the loss of hydrogen bonded water molecule present at the surface of nano ferrite. A second small weight loss (2%) occurred in the temperature range 150–300 ◦ C along the second DTA endothermic peak, which is 90
M (emu/g)
80
76.8
M(emu/g)
70
65.4
60
61.2
50 40 30
30
20
-1500 -1000 -500
0
5 500
1000 15 500 2000
H (O e)
20
43.3
M(emu/g)
35
Av. Parcle size(nm)
15
10 0 50
300
400
500
Temperature(oC) Fig. 7. vibrating sample magnetometer curve of Fe3 O4 nanoparticles at room temperature.
Fig. 9. Variation of magnetization with annealing temperature and particle size.
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state of iron, Fe2+ , is also paramagnetic due to full spin lattice relaxation characteristic of orbitally degenerate. EPR of Fe2+ can be observed at very low temperatures (∼4 K) [34]. While the low intensity peak observed at geff ∼ 2.05 (3300G) can be attributed to the super paramagnetic nature of nano ferrite. The room temperature EPR spectrum is found to be corroborated with literature value of super paramagnetic ferrite nanoparticles [35]. 4. Conclusion
Fig. 10. Thermo gravimetric analysis of Fe3 O4 nanoparticles.
probably due to the removal of trapped water molecules from the lattice. The order of decomposition reaction and activation energy is found to be 0.56 and 17.34 kJ mol−1 , respectively. Further weight loss of 2.1% is observed in the range 650–800 ◦ C with a DTA exothermic peak, the decomposition event can be explained by Eq. (I). Fe3 O4
,−1/2O2
−→
(650−800 ◦ C
3FeO
(I)
For Eq. (I), the order of decomposition reaction and activation energy are found to be 0.2 and 39.5 kJ mol−1 , respectively. Total weight loss is about 4.5%, which is due to the phase transition from Fe3 O4 to FeO and found to be equal to 1/2 O2 [32], because FeO is thermodynamically stable above 570 ◦ C in phase diagram of the Fe–O system [33]. So the TGA and DTA analysis can be corroborated with each other in terms of kinetics and activation energy of nano ferrite as shown in Fig. 10. 3.10. The EPR spectrum The EPR spectrum of nanoferrite was taken at room temperature (298 K). The Fe3+ (3d5 ) are 6 S ground state with long spin-lattice relaxation times characteristics of S-state ion, their EPR is generally observed at room temperature. Fe3+ has geff ∼ 2. However, the Fe3+ ions often experience relatively large crystal field effects such that the separation between the three kramer’s doublet can exceed 0.3 cm−1 in the energy of the X-band microwave. In such fine structure more often EPR lines are observed at low fields corresponding to geff 2. Fig. 11 shows peak at geff ∼ 3.23 (2100G) indicating that Fe3+ transition from −1/2 → +1/2. The lower valance
A single step synergistically energy conserved facile technique has been adopted to synthesize the super paramagnetic Fe3 O4 nanoparticles using ethylene glycol as solvent, stabilizer and reducing agent. The broad absorption band at 330 nm in UV–visible spectra and a sharp band at 585 cm−1 in FT-IR spectra in addition to XRD, TEM and DLS studies further confirm the formation of Fe3 O4 nanoparticles. The VSM analysis reveals that the saturation magnetization values of Fe3 O4 samples increased with the increase in annealing temperature to 300 ◦ C and further decreases beyond this temperature. In other words, prepared Fe3 O4 samples possess size and morphology dependent magnetic property. The magnetization follows a unique pattern that as we change the annealing temperature particle size of nanoparticles changes and surface area to volume ratio also changes, henceforth the magnetization changes. Very significant and promising effect observes on the Ms due to the annealing of Fe3 O4 nanoparticles at optimum temperature 300 ◦ C. At this temperature, the nanoparticles are uniformly distributed with almost no agglomeration. It is very interesting to note that the magnetization of ferrite nanoparticles follow the curie law of magnetization. The EPR peak at geff ∼ 2.05 (3300G) also confirms the super paramagnetic nature of the nanoferrite along with higher super paramagnetic property in comparison to earlier reported nano ferrites. The structure and mechanism of nano ferrite is being established by TGA analysis. Due to high demand and application of nanoferrite in medical sciences, it would be a new challenge for our super paramagnetic nanoferrite, so further studies of nano ferrite from medical point of view will be performed in near future. Acknowledgements Authors are thankful to UGC Delhi, India for financial support UGC-BSR Meritorious fellowship and to UGC – Dr. D.S. Kothari post doctoral fellowship (No. F.4-2/2006(BSR)/13-215/2008(BSR) and for SAP program in department of chemistry, Jamia Millia Islamia for providing TGA/DTA facility. Authors are also thankful to Dr. Tokeer Ahmad, Department of Chemistry, Jamia Millia Islamia, New Delhi for providing the BET facility for surface area study.
Intensity(a.u.)
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1000
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3000
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Magnetic Field [G] Fig. 11. EPR spectrum of Fe3 O4 nanoparticles in solid state at microwave frequency 9.5 GHz.
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