Sains Malaysiana 37(4)(2008): 389-394 Size-controlled Synthesis and Characterization of Fe 3 O 4 Nanoparticles by Chemical Coprecipitation Method (Sintesis Saiz Terkawal dan Pencirian Nanozarah Fe 3 O 4 Melalui Kaedah Kepemendakan Kimia) CHIA CHIN HUA, SARANI ZAKARIA, R. FARAHIYAN, LIEW TZE KHONG, KIEN L. NGUYEN, MUSTAFFA ABDULLAH & SAHRIM AHMAD ABSTRACT Magnetite (Fe 3 O 4 ) nanoparticles have been synthesized using the chemical coprecipitation method. The Fe 3 O 4 nanoparticles were likely formed via dissolution-recrystallization process. During the precipitation process, ferrihydrite and Fe(OH) 2 particles formed aggregates and followed by the formation of spherical Fe 3 O 4 particles. The synthesized Fe 3 O 4 nanoparticles exhibited superparamagnetic behavior and in single crystal form. The synthesis temperature and the degree of agitation during the precipitation were found to be decisive in controlling the crystallite and particle size of the produced Fe 3 O 4 nanoparticles. Lower temperature and higher degree of agitation were the favorable conditions for producing smaller particle. The magnetic properties (saturation magnetization and coercivity) of the Fe 3 O 4 nanoparticles increased with the particle size. Keywords: Chemical coprecipitation; ferrite; magnetite; nanoparticles ABSTRAK Nanozarah magnetit (Fe 3 O 4 ) telah disintesis dengan menggunakan kaedah pemendakan kimia. Nanozarah Fe 3 O 4 terbentuk melalui proses pelarutan-penghabluran. Semasa proses pemendakan, zarah-zarah ferihidrit dan Fe(OH) 2 membentuk agregat and diikuti dengan pembentukan zarah-zarah magnetit yang berbentuk sfera. Nanozarah-nanozarah magnetit yang terbentuk menunjukkan sifat superparamagnet dan berhablur tunggal. Suhu sintesis dan darjah pengacauan semasa proses pemendakan memainkan peranan penting dalam pengawalan saiz hablur dan saiz zarah nanozarah Fe 3 O 4 . Suhu rendah dan darjah pengacauan yang tinggi merupakan keadaan yang sesuai untuk menghasilkan zarah dengan saiz yang kecil. Sifat kemagnetan tepu dan koersiviti bagi nanozarah Fe 3 O 4 didapati meningkat dengan peningkatan saiz zarah. Kata kunci: Ferit; kepemendakan kimia; magnetit; nanozarah enzyme in a magnetic lab-on-a-chip (Bilková et al. 2006), bio-marking (Sharma et al. 2006), drug carrier (Park et al. 2004), MRI contrast agent and cell-tracking (LaConte et al. 2005), hyperthermia treatment (Ito et al. 2004) and DNA analysis through sensitive colorimetric methods (Tartaj et al. 2006). Other applications of magnetic nanoparticles are selective metal removal (Ngomsik et al. 2005), magnetically stabilized fluidized bed reactor (Hou & Williams 2002) and magnetic fluids for optical devices (Horng et al. 2001). Methods for preparing Fe 3 O 4 nanoparticles include wet chemical coprecipitation of ferrous and ferric ions with ammonia in aqueous solution, in micro-emulsion, or on the partial oxidation of ferrous hydroxide gels, and spray pyrolysis. Recent reports revealed the trend of using coprecipitation (Gokon et al. 2002; Tartaj et al. 2003; Jeong et al. 2004; Yu & Chow 2004; Lee et al. 2005)as the preferred method, though other methods are also employed such as reduction precipitation method (Qu et al. 1999) and partial oxidation of aqueous hydroxide gel (Sugimoto 2000). INTRODUCTION Magnetic nanoparticles possess unique physiochemical, magnetic, and optical properties that are of great importance in diverse applications (Huber 2005). Cobalt, iron, and iron oxide such as maghemite and magnetite nanoparticles have been widely considered as the most suitable materials for production of these particles due to their superparamagnetic property. Iron also has lower magnetocrystalline anisotropy, which means that much larger iron nanoparticles can still be superparamagnetic at a given temperature compared to cobalt. Additionally, iron (and its oxides) is relatively non- toxic compared to the magnetic particles of cobalt and nickel, therefore making it attractive to be tailored into nano-sized particles for biomedical (in vivo) applications. The ability to synthesize nano-sized magnetic particles has opened up the possibility of their uses for a diverse range of applications. These include various in-vitro and in-vivo bio-applications such as selective protein separation (Bucak et al. 2003) and protein digestion by immobilized
Transcript
Sains Malaysiana 37(4)(2008): 389-394
Size-controlled Synthesis and Characterization of Fe3O4 Nanoparticles by Chemical Coprecipitation Method
(Sintesis Saiz Terkawal dan Pencirian Nanozarah Fe3O4Melalui Kaedah Kepemendakan Kimia)
CHIA CHIN HUA, SARANI ZAKARIA, R. FARAHIYAN, LIEW TZE KHONG, KIEN L.NGUYEN, MUSTAFFA ABDULLAH & SAHRIM AHMAD
ABSTRACT
Magnetite (Fe3O4) nanoparticles have been synthesized using the chemical coprecipitation method. The Fe3O4nanoparticles were likely formed via dissolution-recrystallization process. During the precipitation process, ferrihydriteand Fe(OH)2 particles formed aggregates and followed by the formation of spherical Fe3O4 particles. The synthesizedFe3O4 nanoparticles exhibited superparamagnetic behavior and in single crystal form. The synthesis temperature andthe degree of agitation during the precipitation were found to be decisive in controlling the crystallite and particle sizeof the produced Fe3O4 nanoparticles. Lower temperature and higher degree of agitation were the favorable conditionsfor producing smaller particle. The magnetic properties (saturation magnetization and coercivity) of the Fe3O4nanoparticles increased with the particle size.
Keywords: Chemical coprecipitation; ferrite; magnetite; nanoparticles
ABSTRAK
Nanozarah magnetit (Fe3O4) telah disintesis dengan menggunakan kaedah pemendakan kimia. Nanozarah Fe3O4 terbentukmelalui proses pelarutan-penghabluran. Semasa proses pemendakan, zarah-zarah ferihidrit dan Fe(OH)2 membentukagregat and diikuti dengan pembentukan zarah-zarah magnetit yang berbentuk sfera. Nanozarah-nanozarah magnetityang terbentuk menunjukkan sifat superparamagnet dan berhablur tunggal. Suhu sintesis dan darjah pengacauan semasaproses pemendakan memainkan peranan penting dalam pengawalan saiz hablur dan saiz zarah nanozarah Fe3O4. Suhurendah dan darjah pengacauan yang tinggi merupakan keadaan yang sesuai untuk menghasilkan zarah dengan saiz yangkecil. Sifat kemagnetan tepu dan koersiviti bagi nanozarah Fe3O4 didapati meningkat dengan peningkatan saiz zarah.
Kata kunci: Ferit; kepemendakan kimia; magnetit; nanozarah
enzyme in a magnetic lab-on-a-chip (Bilková et al. 2006),bio-marking (Sharma et al. 2006), drug carrier (Park et al.2004), MRI contrast agent and cell-tracking (LaConte etal. 2005), hyperthermia treatment (Ito et al. 2004) and DNAanalysis through sensitive colorimetric methods (Tartaj etal. 2006). Other applications of magnetic nanoparticles areselective metal removal (Ngomsik et al. 2005),magnetically stabilized fluidized bed reactor (Hou &Williams 2002) and magnetic fluids for optical devices(Horng et al. 2001).
Methods for preparing Fe3O4 nanoparticles include wetchemical coprecipitation of ferrous and ferric ions withammonia in aqueous solution, in micro-emulsion, or on thepartial oxidation of ferrous hydroxide gels, and spraypyrolysis. Recent reports revealed the trend of usingcoprecipitation (Gokon et al. 2002; Tartaj et al. 2003; Jeonget al. 2004; Yu & Chow 2004; Lee et al. 2005) as the preferredmethod, though other methods are also employed such asreduction precipitation method (Qu et al. 1999) and partialoxidation of aqueous hydroxide gel (Sugimoto 2000).
INTRODUCTION
Magnetic nanoparticles possess unique physiochemical,magnetic, and optical properties that are of greatimportance in diverse applications (Huber 2005). Cobalt,iron, and iron oxide such as maghemite and magnetitenanoparticles have been widely considered as the mostsuitable materials for production of these particles due totheir superparamagnetic property.
Iron also has lower magnetocrystalline anisotropy,which means that much larger iron nanoparticles can stillbe superparamagnetic at a given temperature compared tocobalt. Additionally, iron (and its oxides) is relatively non-toxic compared to the magnetic particles of cobalt andnickel, therefore making it attractive to be tailored intonano-sized particles for biomedical (in vivo) applications.
The ability to synthesize nano-sized magnetic particleshas opened up the possibility of their uses for a diverserange of applications. These include various in-vitro andin-vivo bio-applications such as selective protein separation(Bucak et al. 2003) and protein digestion by immobilized
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In the coprecipitation method, a stoichiometricmixture of ferrous and ferric ions serves as the iron source.It has been shown that by adjusting the pH, ionic strength,and temperature, it is possible to control the mean size ofthe particles over one order of magnitude. In this study,attentions have been given to study the effect oftemperature and degree of agitation on the particle sizeduring the precipitation. In addition the magnetic propertiesof the particles have also been investigated.
MATERIALS AND METHODS
All the chemicals were of reagent grade and used withoutfurther purification. Ferrous chloride tetrahydrate (FeCl2 •4 H2O), ferric chloride (FeCl3), hydrochloric acid (HCl),and sodium hydroxide (NaOH) were purchased fromAldrich. Distilled water was deoxygenated by bubblingwith N2 gas for 15 minutes prior to use.
FeCl2 • 4 H2O and FeCl3 at 1:2 molar ratio weredissolved into a round bottom flask which contains 200ml of deoxygenated distilled water and kept at desiredtemperature. While vigorously stirring the reaction mixture,the stoichiometry amount of NaOH was added into thesolution. Nitrogen gas was kept passing through thesolution during the experiment to prevent the oxidation ofFe2+ in the system in order to maintain the stoichiometryratio between Fe2+ and Fe3+. Afterward, the resulting blackprecipitate in the flask was separated by placing onto apermanent magnet to accelerate the settling. The clear saltsolution was decanted. The precipitate was washed throughseveral cycles with acetone and distilled water and thendried in oven at 35°C for 24 hours. Table 1 summarizesthe conditions for the samples preparation. No furthercalcinations was conducted for all the samples due to theeasily oxidation behavior of Fe3O4 at high temperature.
The crystallographic analysis was performed using !Siemens diffractometer D5000 with CuK
! radiation (" =
1.5418 Å). The sample was continuously scanned in the2# range of 25-70°. The mean crystallite size of thenanoparticles was determined using the Debye-Scherrer’sformula (Huang & Tang 2005)
Dhkl = 0.9 " / $ cos # (1)
where Dhkl is the mean crystallite size, $ is the broadeningof full width at half maximum intensity (FWHM) of the mainintense peak (311) in radian, # is the Bragg angle, and " isthe X-ray wavelength. The interplanar spacing (dhkl) andthe lattice constant (a0) were determined by Equation (2)and (3), respectively.
dhkl ="#2 sin . (2)
a d h k lhkl02 2 2= + + . (3)
A Philips Model CM12 TEM, operating at 120 keV,was used to study the morphology of the samples. Thesamples were dispersed into the acetone solution (90%)with ultrasonic treatment for 10 minutes and a drop ofsuspension was added onto a carbon coated copper grid. Itwas then left to dry in ambient temperature at least 30minutes and then kept in the desicator for characterization.The magnetic properties of the sample were measuredusing a computerized vibrating sample magnetometer(VSM) at 25°C with Microprocessor-9500, LDJ electronics.
RESULTS AND DISCUSSION
MECHANISM OF THE FORMATION OF Fe3O4
Prior to the precipitation of Fe3O4 from Fe2+ and Fe3+ saltsmixtures, two separate experiments were carried out in analkaline solution to observe the precipitation of Fe2+ andFe3+in order to determine the type of compounds formedfrom each salt. In the case of Fe2+ salt, a grey precipitateFe(OH)2 was formed when NaOH was added. It was wellknown that Fe(OH)2 formed at pH > 7 by the hydroxylationof the ferrous ions under anaerobic conditions (Jolivet etal. 2004). In this experiment, the precipitate changed fromgrey to dark brown immediately when it was suspended inan acetone solution due to the oxidation of Fe(OH)2. Theoxidation resulted in the formation of ferric oxides oroxyhydroxides (!-FeOOH and !-Fe2O3) (Misawa et al.1974; Miyamoto 1976; Enomoto et al. 1996). The darkbrown precipitate was then examined using an electronmicroscope (Figure 1(a)). The examination suggested thatthe precipitate was mainly goethite (!-FeOOH).
TABLE 1. Crystallite parameter and average particle size of Fe3O4 prepared at various conditions
Sample Temperature Stirring Lattice Crystallite Average (ºC) rate (rpm) size, D (nm) constant, physical size,
Figure 1(b) showed the formation of ferrihydrite whenNaOH was added into the Fe3+ solution. The X-raydiffraction pattern shown in Figure 2 confirmed thisformation. Ferrihydrite is a poor ordered ferricoxyhydroxide, and it is thermodynamically unstable andreadily transforms to more stable crystalline compounds,such as goethite (!-FeOOH) and hematite (!-Fe2O3)(Cornell & Schneider 1989; Schwertmann et al. 1999).
During the preparation of Sample 1, a small amountof the suspension was collected at different time intervals,and dispersed in an acetone solution (90%). The dispersionwas then ultrasonicated prior to the preparation of thesample for the electron microscope analysis. The formationprocess of Fe3O4 was demonstrated in Figure 1(c)-(f). It
could be seen that the initial samples, which were collectedat 3 min and 15 min intervals (c) & (d), consisted of alarge number of acicular particles (!-FeOOH), which wereformed from Fe(OH)2. The amount of !-FeOOH was thengradually disappeared, and the dissolution completed after~30 minutes (Figure 1 (e) & (f). The formation of sphericalFe3O4 particles likely occurred by the aggregation of theferrihydrite and the Fe(OH)2 particles. The microscopicstudies showed that the spherical Fe3O4 particles graduallygrew bigger and became darker. This observation wasconsistent with the previous studies reported that theformation of Fe3O4 involved the dissolution-recrystallization process. Fe2+ adsorbed onto ferrichydroxide and followed by the structural rearrangement
FIGURE 1. Transmission electron micrographs i) !%FeOOH and ferrihydrite formed separately in alkaline solution 1 (a) &(b) and ii) Formation of Fe3O4 nanoparticles at 25°C and 400 rpm 1 (c)-(f)
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due to the electron transfer of Fe2+and Fe3+ throughoverlapping d-orbital (Belleville et al. 1992; Tronc et al.1992; Wang et al. 1998).
EFFECT OF SYNTHESIS TEMPERATURE
The XRD results (Figure 3) indicated that all the sampleswere in spinel structure with face-centered cubic phaseaccording to the standard JCPDF file. No impurities weredetected from the samples. The result indicated that theincrease in the temperature increased the intensity of theBragg peaks. The crystallite size of the particles wasincreased from 10.3 nm to 13.2 nm when the temperatureincreased from 25 to 100°C, suggesting that at hightemperature the precipitate particles possessed highercrystallinity, higher lattice constant and larger grain size.This effect was complement with the electron micrographsas shown in Figure 4(a)-(c), where the average particlessize was increased from 10.9±1.5 nm to 14.3±1.8 nm, whentemperature increased from 25 to 100°C.
It was well known that an elevated temperaturepromoted the hydrolysis reaction and the dehydration offerrite precursor, and at room temperature the formationof Fe3+ oxides was significant retarding the formation ofmagnetite (Wang et al. 2005). A previous study alsoreported that when the temperature increased the volumeof the liquor would expand and resulting in a reduction ofsupersaturation, which diminished the nucleation rate andpromoted the growth rate of the particles (Bhattacharya etal. 2002). It was noticeable that both the crystallite sizeand particle size were approximately equal, confirmingthat the particles had a single-crystal structure.
EFFECT OF STIRRING SPEED
The XRD (Figure 2) and TEM (Figure 3) results indicatedthat when the stirring speed increased from 400 rpm to1000 rpm, the crystallite size and the average size of thesample were decreased from 10.3 nm to 9.0 nm and10.9±1.5 nm to 8.8±1.2 nm, respectively. The reason ofthis reduction was suggested to be the anomalous diffusionof particles at higher degree of agitation reduced the growthkinetics of the particles, and resulted in the smaller sizedparticles.
MAGNETIC PROPERTIES
The details and the linear relationships between the particlesize, the saturation magnetization (Ms) and the coercivity(Hc) were showed in Table 2. All the samples possesssuperparamagnetic behavior and have lower saturationmagnetization values than the bulk Fe3O4 (~92 emu/g)(Cornell & Schwertmann 2003). This difference in thesaturation magnetization values could be attributed to thesurface order / disorder interaction of the magnetic spinmoment and the disturbance in the spinel structureinversion as a result of Laplace pressure (Nedkov et al.2004).
FIGURE 2. X-ray diffraction pattern of ferrhydrite fromFe3+ in an alkaline solution
FIGURE 3. XRD diffraction patterns of the Fe3O4 nanoparticles (Sample 1, 2, 3, and 4)
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CONCLUSION
Magnetite (Fe3O4) nanoparticles have been synthesizedusing the chemical coprecipitation method. The crystalliteand average particle size of the Fe3O4 nanoparticlesincreased with the synthesis temperature, and decreasedwith the increase of stirring speed. These two parameterscan be used in producing Fe3O4 nanoparticles with desiredparticle size. The saturation magnetization and coercivityof the particles increased with the crystallite and particlesizes.
ACKNOWLEDGEMENT
Authors are thankful to the Universiti Kebangsaan Malaysiaand Scientific Advancement Grant Allocation (STGL-009-2006) for the financial support to carry out this work.
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Chia Chin Hua, Sarani Zakaria, R. Farahiyan, Liew Tze Khong,Mustaffa Abdullah & Sahrim AhmadSchool of Applied PhysicsFaculty of Science and TechnologyUniversiti Kebangsaan Malaysia43600 Bangi, Selangor D.EMalaysia
Kien L. NguyenAustralian Pulp and Paper InstituteDepartment of Chemical EngineeringMonash UniversityClayton VIC 3800Australia
Received : 19 February 2008Accepted : 6 March 2008
