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γ Synthesis, characterization and magnetic properties of irradiated and non irradiated magnetite nano powders

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https://www.eduzhai.net International Journal of M aterials and Chemistry 2013, 3(5): 106-111 DOI: 10.5923/j.ijmc.20130305.04 Synthesis, Characterization and Magnetic Properties of γ-irradiated and Unirradiated Magnetite Nanopowders M. Khairy Chemistry Department, Faculty of Science, Benha University, Benha, Egypt Abstract Nano-sized magnetite (Fe3O4) nanoparticles were prepared using dry and wet chemica l methods in presence of surfactants as capping agents. The samples were characterized by X-ray diffraction, FT-IR, thermal analysis (DTA and TG), Electron microscopy (SEM) as we ll as (TEM), dynamic laser scattering analyzer (DLS) and vibrating sample magnetometer (VSM ) techniques. The X-ray diffraction pattern show cubic spinel crystal structure for all samples. Pa rtic le size in the range of 8 - 55 n m is obtained. The effect of preparation method and γ- irradiation process on the magnetic properties of prepared samples was studied and discussed. All samples show soft-magnetic behavior with much lo wer coerciv ity and much higher saturation magnetization. The coercive force (Hc), saturation magnetizat ion (Bs), remanent induction (Br) and the rat io of remanent induction to saturation magnetizat ion (Br/ Bs) are found to be size and shape dependent. The saturation magnetization value lies between 20.5 and 64.5 emu/g. The magnetic properties are exp lained by electron hopping mechanis m between Fe2+ and Fe3+ -ions. The use of magnetite nanoparticles in preparation of ferrofluid was investigated. The ferroflu id stability increases with decreasing the particle size. Keywords Nanostructures, Magnetite, VSM, Gamma Irradiation, Magnetic Properties 1. Introduction Over the past few years, controlling the size and shape of metal o xides has attracted significant interest because the shape and size of these materials have significant influence on their chemical and physical properties[1-4]. Magnetite (Fe3O4), as one kind of important transition metal o xides, is of particular importance because of both its unique properties including magnetic properties, chemical stability, biocompatibility, and lo w to xicity and potential applications in magnetic recording and separation, catalyst, photocatalyst, pigments, ferrofluids, magnetic resonance imaging (MRI), drug delivery, etc.[5-8]. There are several methods h ave been dev elo ped t o synthesize Fe3 O4 n anopart icles[9]. Ho wever, the shape controlled synthesis of Fe3O4 in the nanometer regime has limited success. The size and morphology of nanoparticles are two important characteristic influencing their electrical, optical and magnet ic p rop ert ies[10-13]. The d ifferen ce between nano and bulk materials has immense theoretical and technological importance. The way nanoparticles are synthesized may determine their morphological uniformity and size distribution; these conditions become one of the key challenging issues in nanoscience and nanotechnology[13]. * Corresponding author: moh_khairy3@yahoo.com (M. Khairy) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved Most common ly, Fe3O4 nanoparticles are prepared v ia co-precipitating ferrous and ferric ions in aqueous solution[14]. Ho wever, the corresponding chemical reactions do occur very fast, just promptly on mixing reactants, and this makes it very difficult to control the crystallization process. As a consequence, the magnetic nanoparticles of the resulting iron o xides tend to exhib it relatively poor size uniformity and crystallinity, and this feature may limit their use in many technological ap p licatio n s . Ionizing rad iation such as gamma ray has frequently been used in studies of the physical properties of crystalline solids[15]. St ructural defects can be introduced by ionizing radiation. Severe d isruption of the lattice of a crystalline solid is possible, with the formation of a large number of defects. In this context, in the present investigation, dry and wet chemical methods have been employed to synthesized Fe3O4 powders with an average particle size down to several nanometers. The XRD, FTIR, SEM and TEM techniques were used to characterize the structure, purity, and the size of the nanoparticles. The magnetic properties were evaluated by vibrating sample magnetometer (VSM) measurement. The magnetization curves of these nanocrystals have been tested and analyzed. The relationship between preparation methods and the magnetic p roperties of resulting materials identified via the above methods was discussed. The influence of gamma ray on the magnetic properties of the synthesized nanoparticles was examined. The investigated Fe3O4 nanoparticles were d ispersed into aqueous citric acid International Journal of M aterials and Chemistry 2013, 3(5): 106-111 107 to obtain nanofluids and their stabilities were tested. 2. Experimental 2.1. Materials Analytical pure ferrous nitrate (Fe(NO3)2·4H2O), Ferric nitrate (Fe(NO3)3·9H2O), ferrous oxalate dihydrate (FeC2O4.2H2O), ammoniu m hydro xide (NH4OH, 28– 30% of ammon ia), 1-adamantanecarboxy lic acid (ACA, 99%), and cetylpyridiniu m bro mide (CPB) were purchased from Aldrich and used for Fe3O4 synthesis. 2.2. Synthesis of Magnetite Nanoparticles The magnetite samples were prepared using both thermal decomposition and wet chemical methods in the presence and absence of surfactants as follows: (a) Wet chemical method Fe3O4 nanoparticles were synthesized according to the coprecipitation method. A typical preparation process is described as following: a definite weights of Fe(NO3)2·4H2O (21.4 g) and Fe(NO3)3·9H2O (28.9 g) were dissolved in distilled water in presence of 10 mmol surfactant (50 ml) with a n itrogen protection. After the solution had been bubbled with nitrogen for 30 min , 0.1 mol/ L NH4OH solution was introduced slowly into the system to adjust the pH above 11. The system was continuously bubbled with nitrogen for 20 min to remove o xygen. The formed black precipitates were collected fro m the solution by centrifugation, washed with distilled water several t imes, and dried at 90℃ fo r 8 h. The co mposition of Fe3O4 was verified by KMnO4 titration method[16]. The results showed a ratio of Fe3+: Fe2+ agrees with the expected 2:1 ratio within 2%.The samp les were denoted as S for the sample prepared without surfactant and as SCPB and SACA for the samples prepared using CPB and ACA surfactants, respectively. (b) Thermal decomposition dry method For the dry method, 10 g of ferrous oxalate dihydrate was thermally decomposed, at low o xygen partial p ressure, for 6 h at T=773 K[17]. The obtained product was then milled in a planetary ball mill. The milling was performed in a closed container with a hardened-steel vial of 120 ml volu me and 80 hardened-steel with a diameter of 10 mm at ambient temperature. The milling intensity was 200 rp m and the ball-to-powder weight ratio of 20:1 was chosen. The milling process was carried out fo r one hour. The sample is denoted as Sd. All dry magnetite samples were kept in desiccators over calciu m chloride to avoid the o xidation to maghemite. The prepared samples were irradiated by γ-ray source using a 60Co gamma cell (60Co gamma cell 2000 Ci with a dose rate of 1.5 Gy/s (150 rad/s) at a temperature of 30℃. Each samp le was subjected to a total final dose of 1x105 Gy (10 Mrad). The irradiated samp les were d istinguished fro m the unirradiated ones by adding an asterisk beside the symbol of the irrad iated samples, e.g. Sd*. For preparing aqueous based ferrofluids, one gram fro m each one of γ-irrad iated and unirradiated magnetite samples was suspended in 10 ml o f 50 wt% aqueous citric acid and heated at 90oC for 90 minutes under vigorously mechanically s tirrin g . 2.3. Instrumentation and Measurements Thermal analysis (DTA and TG), XRD, FT-IR, SEM and TEM techniques were employed to characterize thermal stability, structure and morphology of the investigated magnetite samp les. The thermal analysis was recorded using Schimad zu DT-50 thermal analy zer with samples of about mg in a static air at mosphere and at heating rate 10 K/ min. The thermograms showed thermal stability for all samples over a temperature range of 300 - 673 K. Phase identification of the prepared samples was carried out at roo m temperature by X-ray diffract ion (XRD) patterns using a Philips diffractometer PW 1710 with Cu-Kα irradiat ion (λ=0.15405 nm). FT-IR spectra were performed by FT-IR spectrophotometer model Perkin-Elmer 599 using KBr pellet technique. Scanning electron microscopy (SEM) and Transmitt ion electron microscopy (TEM) were taken using electron microscope models JEM-5200 Joel and Joel 2010 respectively. The volume-average diameter and size distribution of Fe3O4 magnetic nanoparticles in ethanol we re measured by dynamic light scattering (DLS) using Malvern HPPS5001 laser particle-size analyzer at 25℃. Magnetic measurements were measured at roo m temperature using a vibrating samp le magnetometer (VSM; 9600-1 LDJ, USA) in a maximu m applied field o f 15 kOe. Sedimentation method was used to test the ferrofluid stability of the studied samples. The sedimentation under gravity was carried out as described by Thies-Weesie et al.[18]. Weighted amounts of the ferroflu ids were poured into cylindrical glass tubes of 1 cm d iameter and 15 cm length and carefully closed. The tubes were vertically immersed in a water bath, placed on a heavy marb le table to min imize v ibrations. The equip ment was placed in a thermostatic room to keep the temperature of the sedimentation dispersions constant at 25.0(0.1℃). 3. Results and Discussion 3.1. Characterization of the samples X-ray diffraction patterns of the synthesized Fe3O4 samples are shown in Fig. 1. A ll the observed peaks agree well with the corresponding reported JCPDS file (19-629) for the magnetite spinel structure. The crystallites sizes, DXRD, were estimated by using the Scherrer's equation[19], based on (311) peak: DXRD = 0.9 λ / β cosθ where λ is the X-ray wave length, θ the Bragg’s angle and β is the pure full width of the diffraction line at half of the maximu m intensity. DXRD values obtained lie in the range of 8 - 55 n m, Table 1. The irrad iated samples also exh ibit the same crystalline phase with crystallite sizes in the same order with that found 108 M . Khairy: Synthesis, Characterization and M agnetic Properties of γ-irradiated and Unirradiated M agnetite Nanopowders for unirradiated ones. d c with random shapes. This reflects the role of surfactants as capping agent in controlling the growth rate of various faces of Fe3O4 crystal. The SEM of irradiated samp les show surface morphologies almost similar to that observed for unirradiated ones. This reflects the insignificant influence of the formed lattice defects, during the irradiat ion process, on the morphologies of the magnetite specimens. Intensity (a. u.) b a 20 30 40 50 60 70 80 2θ (degree) Figure 1. XRD patterns of Fe3O4 samples: a)S b) Sd c) SCPB d) SACA Table 1. Particle size and FT-IR data of γ-irradiated and unirradiated nano magnetite samples samples Particle size (T EM), nm Particle size υο υt (XRD), nm cm-1 cm-1 S 52 S* 47 55 384 560 51 388 564 S CPB 27 (diameter) 38 391 568 S * CPB 23 (diameter) 33 395 571 Sd Sd* 19 17 16 13 399 571 401 575 S ACA S * ACA 9 (diamet er) 8 (diamet er) 10 405 578 8 408 583 Figure 2. SEM micrographsof: a)S b) Sd c) SCPB d) SACA e)S* f) Sd* g) SCPB* h) SACA* Figs. 2a-e shows the scanning electron micrographs (SEM ) of γ- irrad iated and unirrd iated samples. The morphology of the particles is found to depend on both the type of surfactant and the preparation method used. The SEM images of S and Sd show surface morphology consisting of nearly spherical like structure. On the other side, the images of SACA and SCPB reveal spread of somehow aggregates of granular part icles Figure 3. TEM micrographs of: a)S b) Sd c) SCPB d) SACA e)S* f) Sd* g) SCPB* h) SACA* The morphologies of Fe3O4 part icles were further investigated by TEM , Fig. 3. The TEM images confirm clearly the formation of nano scale magnetite particles. Both of the shape and particle size were also found to depend on the preparation method. The TEM images of S and Sd samples show that the Fe3O4 particles consist of agglomerated shapes of deformed spheres with an average diameter of 52 and 19 n m, respectively. Usually, spherical shapes are formed when the nucleation rate per unit area is isotopic at the interface between the Fe3O4 magnetic nanoparticles. Minimization of the surface free energy by reduction of total surface area/volu me, results in the equivalent growth rate along different d irections of the nucleation because the sphere has the smallest surface area per unit volume o f any shape. The TEM images for the samples prepared in presence of surfactants SACA and SCPB (Figs. 3(c,d,g,h)) show particles with nano rods structure with diameter of 9 n m and length of 0.69 μm as well as 27 nm and length of 3 μm, respectively. These nano rods are a good evidence of ˝oriented attachment 'growth, in which the individual particles were aligned like a wall, whereas the second layer of bricks were about to be put on the first. The difference between the crystallite sizes estimated by the Sherrer’s formu la and that found by TEM images (Table 1) is mainly attributed to the different approach of two techniques. In XRD, we determine the mean particle size using a shape factor of 0.9. Th is factor is depended on the shape of the particle used and is not definite determined. Therefore, any change in the shape factor will cause a change in the particle size calculated. The analysis of the particles using dynamic light scattering (DLS) showed that the magnetite particles are well dispersed International Journal of M aterials and Chemistry 2013, 3(5): 106-111 109 in ethanol, and the mean particle sizes obtained agree with those found by XRD and the TEM results for the spherical particles of S and Sd samples. On the other hand, the DSL analysis of SACA and SCPB with nano rod morphology showed particle sizes of ~12 and ~ 42 n m, respectively, which agree well with that found by XRD and d iffer than that found by TEM. This is attributed to that the DSL results show the hydrodynamic d imension fro m measuring the diffusion coefficient of the particle[20]. This coefficient is not only dependent on the mass of the particle, but also the shape and the surface chemistry of the particles because these parameters affect the part icle-solvent interactions, and therefore, the Brownian mot ion of the particles. For all the investigated samples, the FTIR spectra showed two main vibrat ion bands for the iron-oxygen on both octal and tetrahedral positions (νO) and (νt) at 384-408 c m-1 and at 560-583 cm-1[21], respectively. The peak positions of these modes vary with the particle size , Table 1. A princ ipal e ffect of both finite sizes of nanoparticles is producing from the breaking of a large number of bonds for surface atoms, resulting in the rearrangement of localized electrons on the particle surface. As a result, the surface bond force constant increases as Fe3O4 is reduced to nanoscale dimensions, so that the absorption bands of IR spectra shift to higher wave numbers, as shown in our samples. The IR spectra of irradiated samples showed slight shift in band positions than that found for unirradiated ones. This may be attributed to the lattice defects producing in the magnetite after irradiation p ro cess . bulk Fe3O4 (Bs= 92 emu/g)[22]. This decrease obtained in saturation magnetization value Bs can be understood on the basis of the increase occurring in the magnetically dead layer thickness (nonmagnetic or weakly magnetic interfaces) with the decreasing the crystallite size. W ith decreasing the particle size, the surface to volume ratio increases and in turn the magnetically dead layer fraction increases. Moreover, the magnetic spins of the surface molecu les of nano particles are disordered due to the incomplete coordination, which may also be a reason for lower saturation magnetizat ion. Generally, it can be said that the total magnetization of the sample decreases with decreasing particle size due to the increase in the magnetic integral[23-25] and ultimately reaches the super paramagnetic state, when each particle acts as a big spin with suppressed exchange interaction between particles. These might be the reasons for the magnetic mo ment quenching and consequently for the decrease in the saturation magnetization. 3.2. Magnetic Properties The basic property of any magnetic material is the relation between flu x density and field strength. i.e. the B-H loop. For our samples, hysteresis loops with a normal (S-shape) type are observed at room temperature, Fig. 4 (a, b). The size and shape of the hysteresis curve are of considerable practical importance. The area within a loop represents a magnetic energy loss[22, 23]. This energy loss is defined as heat that is gratitude with in the magnetic specimen and is capable of rising the temperature. The areas obtained for all sa mples are found to be small, and the loops are thin and narrow which is a specific criteria fo r soft ferrite. There is almost immeasurable coercivity at roo m temperature indicating that the prepared Fe3O4 samples are super paramagnetic. This super paramagnetic behavior means that the thermal energy can overcome the anisotropy energy barrier of a single Fe3O4 nano particle, and the coercivity and remanence of Fe3O4 nano particles are zero in the absence of external field. Saturation magnetic flu x density (Bs), remnant magnetic flu x density (Br) and the ratio of remanent induction to saturation magnetization (Br/Bs) were determined fro m B-H loops and listed in Table 2. Fro m wh ich it can be seen that the magnetic properties are size dependent. The coercivity Hc increases with increasing the crystallite size. The saturation magnetizat ion Bs ranges from 25.9 - 64.5 emu/g wh ich are in all cases far fro m the reported values for Figure 4a. Hysteresis curves at room temperature for unirradiated samples, a)S b) SCPB c) Sd d) SACA b. Hysteresis curves at room temperature for irradiated samples, a)S* b) SCPB * c) Sd * d) SACA * The magnetization of γ-irradiated samples shows the same behavior as that found for unirradiated ones with a significant change in the magnetic parameters, Table 2. It is obvious that the magnetic values for irradiated specimens are less than that of unirradiated ones. These changes may be attributed to a change occurring in the Fe2+/Fe3+ ratio after irradiation process, according to the following interaction [26]: γ + Fe2+ ↔ Fe3+ + e The obtained results may be also understood on the basis of crystal structure of Fe3O4. Magnetite (Fe3O4) is a ferrimagnetic iron o xide having cubic inverse spinel structure with o xygen anions forming an FCC closed packing and iron (cat ions) located at the interstitial 110 M . Khairy: Synthesis, Characterization and M agnetic Properties of γ-irradiated and Unirradiated M agnetite Nanopowders tetrahedral sites and octahedral sites[22]. The electron can hop between Fe2+ and Fe3+ ions in the octahedral sites at room temperature imparting half metallic property to magnetite. The magnetic mo ment of the unit cell co mes only fro m Fe2+ ions with a magnetic mo ment of 4μB[27]. Therefore, the decrease in Fe2+/Fe3+ ratio after irradiation process causes the decrease observed in Bs values for irradiated samples. Table 2. Magnetic properties of the nano magnetite samples Sample DXRD (nm) Hc (Oe) Br (emu/g) Bs (emu/g) Br/Bs ×100 S 55 114 1.03 S* 51 105 1.11 64.5 2.02 56.1 1.96 SCPB SCPB* 38 33 102 78 0.73 0.59 43.4 2.12 35.1 1.68 magnetic properties. Usually, they show superparamagnetic behavior considering each particle as a thermally ag itated permanent magnet in the carrier liquid. In the presence of a magnetic field, H, the magnetic mo ment (μ) of the particles will try to align with the magnetic fie ld direction leading to a macroscopic magnetizat ion of the liquid. The magnetizat ion, M, of the liquid behaves as the magnetization of a paramagnetic system. The stability of the ferrofluids prepared under the same conditions is found to depend on each of the particle size and morphology, Tab le 3. The samples with spherical part icles (S and Sd) show higher stability than that of the other samples. Moreover, for the same morphology, the stability is found to increase with decreasing the particle size. The γirradiated samples show less stability than that of unirradiated ones. This is attributed to the smaller magnetic values of the irradiated specimens as compared with that of unirradiated ones. Sd 16 77 0.33 Sd* 13 69 0.28 SACA 10 50 0.27 SACA* 8 36 0.21 34.0 0.97 28.2 0.95 25.9 1.04 20.5 1.02 3.3. Aqueous Ferro Fl ui d A ferrofluid is a colloidal suspension of suitably coated magnetite particles in a liquid med iu m having unusual properties due to the simu ltaneous flu id mechanic effects and magnetic effects. Its wide spread applicat ions are in the form of seals to protect high speed CD drives , as rotary shaft seals, for imp roving performance of audio speakers, in oscillation damp ing and position sensing[28], etc. A stable aqueous ferrofluid fo r our samples was prepared by dissolving the prepared nano particles in citric acid (used as dispersant) to form a stable aqueous dispersed magnetite particles, and simultaneously provide functional groups on the particle surface that can be used for further surface derivatization. The behavior of ferrofluids is mainly determined by their 4. Conclusions The results show that Fe3O4 nanoparticles can be produced in the sizes range fro m 8 to 55 n m by dry and wet chemical methods. The sizes and the morphologies of magnetite particles are affected by the operational parameters. The magnetic propert ies of nanosized ferrites show an apparent superparamagnetic behavior and the magnetization is affected by γ- irradiation process. The magnetite particles have saturation magnetizat ion values (Bs) ranging between 20.5 and 64.5 emu/g, wh ich are far fro m the reported values for bulk Fe3O4 (Bs = 92 emu/g). The magnetic parameters (Bs, Br and Hc) decrease with decreasing the particle size due to the existence of spin canting. Stable suspensions of magnetite nanoparticles in water (water-based ferrofluids) are prepared using citric acid as a dispersant. The stability of ferrofluids increases with decreasing the particle size of magnetite. Table 3. Sedimentation data of ferrofluids sample Fe3O4 Sd Sd* S S* SACA SACA* SCPB SCPB* Time (h) Height (cm) 0 9.8 9.8 9.8 9.8 9.8 9.8 9.8 9.8 2 9.7 9.6 8.4 8.2 6.5 6.3 5 4.8 12 9.2 9 4.1 3.9 3.1 2.7 3 2.8 26 8.3 7.9 2.2 2.1 1.5 1.2 1 0.8 40 1 0.6 0.2 0.1 0.1 0.1 0 0 50 0 0 0 0 0 0 International Journal of M aterials and Chemistry 2013, 3(5): 106-111 111 141-145. REFERENCES [13] A. L. Andrade, M . A. Valente, J. M . F. Ferreira, J. D. 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