Synthesis and Characterizations of Magnetite

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Synthesis and Characterizations of Magnetite Nanocomposite Films for Radiation Shielding

Sayed M. Badawy,1 A. A. Abd El-Latif2 National Center for Clinical and Environmental Toxicology, Faculty of Medicine, Cairo University, Cairo, Egypt 1

2

Physics Department, Faculty of Science of Al Areish, Sues Canal University, Egypt

Polyvinyl alcohol (PVA) films containing magnetite Fe3O4 nanoparticles have been prepared by co-precipitation method for use in gamma ray shielding and protection. Characterizations of the magnetite/PVA nanocomposite films were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis spectroscopy, and magnetization measurements. TEM images showed that the synthesized magnetite particles had about 6–11 nm dimensions. Optical study’s results revealed that the optical energy band gaps of thin films range between 1.82 and 2.81 eV at room temperature using UV–visible absorption spectroscopy. The saturation magnetization (MS) value measured by vibrating sample magnetometer VSM was found to be 8.1 emu/g with superparamagnetic nature. The radiation shielding properties such as linear attenuation coefficients (l) and half-value thickness (HVT) for the magnetite nanocomposite films have been obtained experimentally for different photon energies. The results imply that these nanocomposites films are promising radiation shielding C 2015 materials. POLYM. COMPOS., 00:000–000, 2017. V Society of Plastics Engineers

INTRODUCTION Exposures to high-energy or ionizing radiation may result in cell mutation, cancer, and death. Numerous studies have reported the use of a variety of shielding materials for attenuation or absorption of the undesired radiations [1–5]. Lead and other high-Z materials have been employed to attenuate high-energy radiations such as X-rays and c rays. However, the radioprotective gear made from these This article was published online on 31 July 2015. An error was subsequently identified. This notice is included in the online and print versions to indicate that both have been corrected 17 February 2017. Correspondence to: S.M. Badawy; e-mail: [email protected] Contract grant sponsor: Aljouf University; contract grant number: 232/34. DOI 10.1002/pc.23660 Published online in Wiley Online Library (wileyonlinelibrary.com). C 2015 Society of Plastics Engineers V

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materials is heavy and bulky, both of which often are unwanted features for most applications [1]. Nanomaterials dispersed in a polymer matrix can be used to design effective radiation shields. The attenuation is obtained by a combination of the nanoparticles and the polymers. Polymer-based composites are particularly interesting candidates as radiation shielding materials for varied reasons. They are lighter than their metal counterparts, and can be processed to achieve effective shielding for radiations associated with specific industry [6]. The polymer nanocomposites combine the excellent functional properties of nanoparticles with desired properties of host polymers [7–10]. These improvements concern mechanical properties, surface properties, dimensional stability, thermal stability, chemical stability, antimicrobial properties, photocatalytic, optical and electrical properties. The incorporation of only a few percent of nanosized particles can make dramatic property changes [7–10]. The surface area, the number of dangling bond atoms, and unsaturated coordination on the surface of the nanoparticles lead to interface polarization and multiple scattering which are useful for absorption of electromagnetic radiations [11–15]. The most common methods for synthesis of iron oxide NPs include co-precipitation, thermal decomposition, hydrothermal synthesis, microemulsion, sonochemical synthesis, and sonochemical synthetic route electrochemical synthesis [16–18], laser pyrolysis techniques [19], microorganism or bacterial synthesis [20, 21]. The co-precipitation technique is probably the simplest and most efficient chemical pathway to obtain iron oxides (Fe3O4 or cFe2O3). This method consists of mixing ferric and ferrous ions in a 2:1 molar ratio in highly basic solutions at room temperature or at elevated temperature [22]. Recently polymer-functionalized iron oxide NPs are receiving more and more attention, due to the fact that polymer coatings provide the advantage of increasing the repulsive forces to balance the magnetic and van der Waals attractive forces acting on the NPs. Moreover, polymerfunctionalized iron oxide NPs have been extensively

investigated due to interest in their unique physical or chemical properties [16]. This work is aimed at characterization of magnetite Fe3O4 nanocomposite films prepared by co-precipitation method with PVA for applications in radiation protection and absorption of gamma radiation. The most important quantity characterizing the penetration and diffusion of gamma radiation in extended matter is the attenuation coefficient (l) which depends on the photon energy (E), atomic number (z), and composition of the medium. Hence, the linear attenuation coefficient (l) and the halfvalue thickness (HVT) were evaluated for magnetite Fe3O4 nanocomposite films for different gamma energies. MATERIALS AND METHODS Synthesis of Magnetite Nanocomposite Films Magnetite–PVA nanocomposite was prepared by introducing mixture of 10 ml of 2 M FeSO4 and 40 ml of 1 M Fe2(SO4)3 into 400 ml of 5% PVA and 0.6 M NH4OH solution with stirring. The reacting solution was kept at a constant pH of 12 throughout the course of precipitation with the addition of 3 M NH4OH. The solution was poured into Petri dishes, 15 ml/dish, and allowed to dry to form the films by casting under ambient temperature for 1 day in a dark room. Iron oxide nanocomposite films, in the range of 0.84–1.22 mm thickness, were obtained. The calculated content of the Fe3O4 in the composite films is 0.185 g/g (the total content of iron is 0.14 g/g). Characterizations of Magnetite Nanocomposite Films X-ray diffraction pattern was recorded in the continuous scanning mode at room temperature on an EMPYREAN Diffractometer system operated at 45 kV and a current of 30 mA using Cu tube with Cu Ka1 radiation ˚ ). The diffraction intensities were recorded (k 5 1.5406 A from 4 8 to 80 8, in 2h angles in a step of 0.026 8 with scan-step time 19 s. The morphological analysis of magnetite nanoparticles was analyzed using transmission electron microscopy (JEOL JEM-2100). Magnetization measurements were performed at room temperature using a vibrating sample magnetometer VSM (Lakeshore 7410). The absorbance UV–vis spectroscopy measurement of pure PVA and magnetite nanocomposite thin films (0.01 mm) was recorded on APEL spectrophotometer (model PD-303 UV) operated with wavelength accuracy of 62 nm. Thin film of 0.01 mm was measured by UV– vis spectroscopy in a wavelength ranging from 320 to 860 nm. Gamma Spectroscopy Technique Gamma spectroscopic measurement was performed with scintillation detector NaI (TI). A Leybold Cassy lab, multichannel analyzer (model Pocket-CASSY 524058) 2 POLYMER COMPOSITES—2017

was used for the measurements. The detector was shielded with a 5 cm thickness lead well. The radiation was confined to a narrow beam by a lead collimator having a small hole. Three sources of gamma radiation have been used: 60Co, 137Cs, and 22Na. Each source is housed in its own lead container. Increasing the thickness of the magnetite nanocomposite films for attenuation of gamma rays up to 6.53 mm thickness was obtained by adding multiple films (0.84–1.22 mm) with each other. RESULTS AND DISCUSSION Synthesis of Magnetite Nanocomposite Films The co-precipitation method consists of mixing ferric and ferrous ions in a 2:1 molar ratio in highly basic solutions at room temperature in the presence of polyvinyl alcohol solutions. The chemical reaction of Fe3O4 formation may be written as Eq. 1. Fe21 1 2Fe31 1 8OH2 ! Fe3 O4 1 4H2 O

(1)

Complete precipitation of Fe3O4 is expected at a pH 12, with a stoichiometric ratio of 2:1 (Fe31/Fe21) in a nonoxidizing oxygen environment [23]. Magnetite (Fe3O4) is not very stable and is sensitive to oxidation. Magnetite is transformed into maghemite (c Fe3O4) in the presence of oxygen (Eq. 2). 4Fe3 O4 1 O2 ! 6Fe2 O3

(2)

Some synthesis methods suggest the presence of both magnetite and maghemite in the resulting preparations [24] and the final iron oxide composition is very often intermediate between magnetite (Fe3O4) and maghemite (c-Fe2O3), due to the oxidation of the particles during the synthesis [22]. In this work, the formation of magnetite is confirmed by optical absorbance spectrum shown in Fig. 1. Magnetite shows thermally induced electronic transitions assigned to intervalence charge transference; for this reason, it exhibits an absorption in the visible and near-IR region [25]. Maghemite is an insulator and does not present any absorption in the near-IR region. The oxidation process of magnetite to maghemite has been monitored by the loss of optical absorption in the near-IR region [25]. X-Ray Diffraction Figures 2 and 3 display the XRD patterns of magnetite nanoparticles and magnetite/PVA nanocomposite film, respectively. The diffraction peaks at 30.07, 35.60, 43.18, 57.40, and 63.03 8 h corresponding to 022, 113, 004, 115, and 044 Bragg reflection, respectively, are in accordance with the standard XRD card (reference code: 98-0158742) of magnetite. The XRD peaks matched crystal planes of magnetite Fe3O4 with a cubic crystal structure. DOI 10.1002/pc

FIG. 3. XRD pattern of magnetite nanocomposite thin film. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 1. UV–vis absorption spectra for pure PVA and magnetite nanocomposite thin films. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Most of the time, it is difficult to identify magnetite from maghemite [26], given that their diffraction spectra are very similar [27]. Although magnetite nanoparticles are encapsulated by PVA, PVA has no evident effect on the crystal structure of iron oxide. The Sherrer diffraction formula was used to estimate the crystalline domain size (d): d5

kk b cos h

where, d is the crystallite size (nm), k is shape factor which usually takes a value of is the X-ray wavelength; (CuKa1 5 1.5406

(3) the so-called about 0.94, k ˚ ), b is the A

FWHM of the main diffraction peak, and h is the diffraction angle corresponding to the maximum intensity peak in the XRD pattern (35.60 8). The crystallite size estimated from the FWHM of the (113) peak of iron oxide using the Scherrer’s equation is about 14.2 nm in magnetite nanocomposite films. Transmission Electron Microscopy Figure 4 shows the TEM micrographs of the magnetite/PVA nanocomposite films which reflected that most of the nanoparticles possessed spherical shapes. The nanoparticles incorporated into the polymer matrix seem to be fairly dispersed within a smooth polymer matrix. It can be seen that the particle size of magnetite nanoparticles prepared by the co-precipitation method in the presence of PVA are found to be in the range 6–11 nm, which is approximately the size calculated by the Scherrer formula. Optical Transmission Spectroscopy

FIG. 2. XRD pattern of magnetite nanoparticles. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary. com.]

DOI 10.1002/pc

UV–vis absorption spectrum of the magnetite/PVA nanocomposite thin film, presented in Fig. 1, is compared with UV–vis absorption spectrum of pure PVA film. No significant absorption peaks are observed for the pure PVA film [28]. From the UV–vis spectra of magnetite nanocomposite thin film, it is clear that the absorbance increases with a decrease in wavelength and a sharp increase in absorption at wavelengths near to the absorption edge of the threshold wavelength for onset of absorption, indicating the presence of the optical band gap in the material. The energy corresponding to this determines the band gap of the semiconductor material [29]. The optical absorption edge is significantly shifted toward longer wavelength with the presence of the Fe3O4 nanoparticles [30]. The absorption cut-off is taken as the POLYMER COMPOSITES—2017 3

FIG. 6. Plotting (hm a)2 as a function of photon energy hm for direct transition. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

aðmÞ 5 ao ðhm2Eg Þn =hm

FIG. 4. TEM micrograph of magnetite nanocomposite thin film.

wavelength at which the optical absorption edge begins to increase abruptly. The extinction spectrum of magnetite particles shows the near-infrared charge-transfer band. The insulator maghemite Fe2O3 should have no absorption in the nearIR region [25]. Only the spectra of magnetite (Fe3O4) and wustite (FeO) show finite absorption in the near-IR region while the spectra of other iron oxides, e.g, maghemite, show almost no absorption beyond 700 nm [31]. Davis and Mott (1970) gave an expression for the absorption coefficient, a(m), as a function of photon energy (hm) for indirect and direct transition through Eq. 4 [32]:

(4)

where a(m) 5 A/L is the absorption coefficient of the sample, A is the absorption, L is the thickness of the cell, ao is a constant related to the extent of the band tailing, Eg is the optical band gap energy (eV) calculated from Planck–Einstein relation, where the wavelength is ranging from 320 to 860 nm. The exponent n 5 1/2 for allowed direct transition, while n 5 2 for allowed indirect transition. By plotting (hm a)1/2 and (hm a)2 as a function of photon energy hm and by extrapolating the linear regions of these curves to (hm a)1/2 5 0 for indirect transition and (hm a)2 5 0 for direct transition as shown in Figs. 5 and 6, respectively, we obtain the values of optical energy band gaps for indirect Eg1 and direct Eg2 transition which are 1.82 and 2.81 eV, respectively, which agrees with the value of data reported for magnetite [30, 33]. This may be attributed by the broadening valence band due to electrons of Fe atoms. The metallization will cause the decreasing energy band gap. As a consequence, the electrons only need small energy to move from the valence band to the conduction band [30]. The values of both direct and indirect energy band gaps of the iron oxide nanocomposite film classify this sample as a semiconductor. The energy band gap of the semiconductors is between 0 and 3 eV [33, 34]. Magnetic Properties

FIG. 5. Plotting (hm a)1/2 as a function of photon energy hm for indirect transition. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Figure 7 shows the magnetization versus magnetic field loop of the magnetite/PVA nanocomposite films. Development of saturated loop confirms the magnetic nature of the samples. The magnetic properties of the films were evaluated using a VSM at room temperature. The magnetization curve of magnetite nanocomposite films shows a typical superparamagnetic behavior at room temperature without any hysteresis loop since the remanence and coercivity are negligible. The saturation magnetization Ms at room temperature was 0.4176 memu/ cm2 (8.1 emu/g) which is much smaller than that of pure magnetite nanoparticles [35, 36]. There have been several DOI 10.1002/pc

TABLE 1. Linear attenuation coefficients and half value thickness for each source. Source

E in MeV

l, cm21

HVT, cm

Na-22 Cs-137 Co-60 Na-22 Co-60

0.511 0.662 1.17 1.275 1.33

0.813 0.692 0.404 0.320 0.216

0.852 1.000 1.715 2.165 3.208

I=Io 5 expð2lxÞ ) logðI=Io Þ 5 0:4343 lx

FIG. 7. The magnetization vs magnetic field loop of magnetite nanocomposite thin film.

reports on the decrease in Ms values of composites as magnetite nanoparticles are incorporated into a nonmagnetic polymer matrix [37–40]. Linear Attenuation Coefficients and Half Value Thickness The fraction of photons that interact with the shielding medium per centimeter of shielding (linear attenuation coefficients of gamma rays, l) in the magnetite/PVA nanocomposite films for different sources 60Co, 137Cs, and 22Na have been calculated graphically from the attenuation curves by using the following equation.

FIG. 8. Attenuation for different gamma-ray energies with increasing thickness of magnetite nanocomposite thin film. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary. com.]

DOI 10.1002/pc

(5)

where Io is the primary photons per second, I is the photons per second which pass normally through the shielding medium containing N atoms/cm3, l is the linear attenuation coefficient, and x is the thickness of the absorber. The half value thickness (HVT) is the absorber thickness required to reduce the intensity of the incident radiation to half its initial value. The following relationship exists between the half value thickness (HVT) and the linear absorption coefficient: HVT 5 ln2=l 5 0:693=l

(6)

Figure 8 illustrates the exponential attenuation for different gamma-ray energies and shows that the transmission increases with increasing of the gamma-ray energy and decreases exponentially with the increasing of the thickness of the magnetite nanocomposite films. Table 1 shows the source and photon energy, linear attenuation coefficients, and half value thickness. The linear attenuation coefficient, l, and the half-value thickness of the magnetite nanocomposite films were evaluated for different gamma ray energies. The linear attenuation coefficients depend on the gamma ray energy which have larger value at lower energies and decrease rapidly toward higher energies, in reverse of half value thickness, as shown in Fig. 9. As the beam energy increases, the magnetite nanocomposite films attenuate fewer photons than the lower energy photons. The lower energy photons are more easily stopped in the attenuating material, so there are fewer photons reaching the detector. As the energy of the photons increase, they are able to penetrate the attenuator materials more deeply, resulting in a higher HVT. Table 2 shows the radiation shielding properties of the magnetite/PVA nanocomposite in the present work compared to the pure PVA [41, 42] and lead as a reference [42, 43] at photon energy of 0.662 MeV. The magnetite/ PVA nanocomposite shows better values when compared with pure PVA. The pure PVA has shielding ability of nearly 8% of that of lead, whereas the magnetite/PVA nanocomposite has shielding ability of nearly 59% of that of lead, in terms of the linear attenuation coefficient. However, the magnetite nanocomposite exhibits better properties than lead in terms of lightness and flexibility. The heaviness of magnetite nanocomposite is only 16% of lead. POLYMER COMPOSITES—2017 5

value of the magnetic permeability, which interact with the electromagnetic fields in the radiation [44, 45]. CONCLUSION

FIG. 9. Linear attenuation coefficient, l, and the half-value thickness, HVT, of magnetite nanocomposite thin films at different gamma-ray energies. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

The enhanced radiation shielding performance of the magnetite/PVA nanocomposite films implies that these nanocomposites films are promising radiation shielding materials. Shielding materials are known to control the radiation through the interaction mechanism [1]. There are three main mechanisms of photon interactions: (i) photon scattering (elastic or inelastic), (ii) photoelectric effect, and (iii) pair production. The mechanism of photon scattering, which refer to the reflections at various surfaces or interfaces in the shield, requires the presence of a large surface area or interface area in the shield [41]. The surface area, the number of dangling bond atoms, and unsaturated coordination on the surface of the nanoparticles lead to interface polarization and multiple scattering which are useful for shielding of radiations [11–15]. For significant absorption of the electromagnetic radiation by the shield, the shield should have electric and/or magnetic dipoles, such as Fe3O4 or other materials having a high

Characterizations of polyvinyl alcohol (PVA) films containing magnetite Fe3O4 nanoparticles for use in gamma ray shielding and protection have been investigated. The XRD peaks matched crystal planes of magnetite Fe3O4 with a cubic crystal structure. Transmission electron microscopy (TEM) images showed that the synthesized iron oxide particles had about 6–11 nm. Optical study’s results revealed that the optical energy band gaps of thin films classify this sample as a semiconductor. The magnetization curve of iron oxide nanocomposite films shows a typical superparamagnetic behavior at room temperature and that the saturation magnetization Ms is much smaller than that of pure magnetite nanoparticles due to incorporation of the magnetite nanoparticles in a nonmagnetic polymer matrix. The enhanced radiation shielding performance of magnetite nanocomposites films implies that these nanocomposites films are promising radiation shielding materials. REFERENCES 1. S. Nambiar and J. Yeow, ACS Appl. Mater. Interf., 4 (11), 5717 (2012). 2. A.E. Abdo, M.A.M. Ali, and M.R. Ismail, Radiat. Phys. Chem., 66, 185 (2003). 3. C. Zeitlin, S.B. Guetersloh, L.H. Heilbronn, and H. Miller, Nucl. Instrum. Methods Phys. Res., Sect. B, 252, 308 (2006). 4. M. Tajiri, Y. Tokiya, J. Uenishi, M. Sunoka, and K. Watanabe, Med. Phys., 80, 391 (2006). 5. D.Y. Stewart, P.F. Harrison, B. Morgan, and Y. Ramachers, Nucl. Instrum. Meth. Phys. Res., Sect. A, 571, 651 (2007). 6. R.C. Singleterry Jr. and S.A. Thibeault, Materials for LowEnergy Neutron Radiation Shielding; technical report NASA/TP-2000-210281; National Aeronautics and Space Administration: Washington, D.C., 2000. 7. S.M. Badawy, Green Process Synth., 3(3), 229, (2014). 8. S.M. Badawy, Green Process Synth., 3(6), 463(2014). 9. W. Zhang, J. Zeng, L. Liu, and Y. Fang, J. Mater. Chem., 14, 209 (2004). 10. V.K. Tiwari, T. Shripathi, N.P. Lalla, and P. Maiti, Nanoscale, 4, 167 (2012).

TABLE 2. Radiation shielding properties and heaviness of the magnetite/ PVA nanocomposite in comparison with the pure PVA and lead as a reference at photon energy of 0.662 MeV.

Materials Polyvinyl alcohol (PVA) Lead Magnetite/PVA nanocomposite

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Density (g/cm3)

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