J. Phys. Chem. C 2007, 111, 10175-10180
10175
Low-Temperature Crystallization of Barium Ferrite Nanoparticles by a Sodium Citrate-Aided Synthetic Process Soon-Gil Kim, Wei-Ning Wang, Toru Iwaki, Akihiro Yabuki, and Kikuo Okuyama* Department of Chemical Engineering, Graduate School of Engineering, Hiroshima UniVersity, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan ReceiVed: December 1, 2006; In Final Form: March 15, 2007
This article describes the synthesis of barium ferrite nanoparticles at low crystallization temperatures using a sodium citrate-aided process. Monodispersed barium ferrite amorphous nanoparticles were synthesized by the formation of metal-citrate complexes at pH 10, followed by hydrolysis at 100 °C. The mean particle diameter was 4.7 nm with a specific surface area of 137.8 m2/g. This is the first time that sodium citrate has been used as a chelating agent in the synthesis of barium ferrite nanoparticles. Sodium citrate plays two important roles in the process: it allows the homogeneous mixing of two metal cations in the as-synthesized barium ferrite amorphous nanoparticles, and it retards particle growth via the formation of surface citrate complexes, inhibiting the agglomeration of the nanoparticles. The amorphous precursor nanoparticles were transformed into a hexagonal structure by calcination at elevated temperatures (500-750 °C) in air. XRD patterns showed that the amorphous phase of the nanoparticles was completely transformed to the hexagonal phase after calcination at 600 °C for 100 min, with no intermediate phase evident. This crystallization temperature was lower than previously reported crystallization temperatures. Crystallization behavior was examined using thermal analysis and FTIR measurements. Particle size, measured from SEM images, was increased from 35 to 130 nm by elevating the calcination temperature from 600 to 750 °C. Barium ferrite nanoparticles calcinated at 600 °C had high magnetic properties: the coercivity and saturation magnetization values were 3580 Oe and 43 emu/g, respectively.
Introduction Because of their ferromagnetic properties, the use of ferrites in a variety of electronic, magnetic, and catalytic applications has been widely investigated.1 Typically, ferrites exhibit two types of structural symmetries (i.e., cubic ferrites and hexagonal ferrites). Cubic ferrites have the general formula MFe2O4, where M is a bivalent ion such as Mn2+, Ni2+, Fe2+, Co2+, or Mg2+. All cubic ferrites crystallize in spinel structures that are magnetically soft and easily magnetized and demagnetized.2,3 Hexagonal ferrites such as BaFe12O19 are known to be a permanent magnets. Because of their high Curie temperature, very large magnetocrystalline anisotropy, high coercivity, and high magnetization, hexagonal ferrites are used in magnetic recording materials.4 The magnetic properties of these particles can be controlled by adjusting the particle size, size distribution, morphology, and microstructure.5 Recently, magnetic nanoparticles have attracted attention because of their potential use in a variety of applications such as high-density magnetic recording media. In this study, nanosized barium ferrite particles were synthesized using a liquid-phase process.6,7 Many methods for the synthesis of barium ferrite have been developed, such as sol-gel, coprecipitation, microemulsion, hydrothermal, aerosol, and glass crystallization.4,7-11 These processes yield magnetic particles with high crystallinity and high magnetic properties. However, these require high calcination temperatures and prolonged calcination times for crystallization into the hexagonal phase. Hard-agglomerated particles with large diameters and irregular morphologies typically result. * Corresponding author. E-mail:
[email protected]. Tel: +8182-424-7716. Fax: +81-82-424-7850.
For example, Liu et al. used a coprecipitation method to synthesize barium ferrite nanoparticles. Crystallization of coprecipitated particles occurred at temperatures exceeding 800 °C, and the calcinated particles were larger than 100 nm with hard agglomeration, despite the use of a sodium chloride matrix to protect against particle growth.7 Glass crystallization is a good method for producing magnetic nanoparticles with sizes ranging from 8 to 18 nm. However, the magnetic values of these particles are extremely low (40 Oe), even after being calcined at approximately 580 °C for 1 h.11 The Pechini method produces a bulklike gel consisting of homogeneous mixed-metal cations.4,12 The most important factor in the low-temperature crystallization of these bulklike gels is the ratio of the metal cations to the chelating agent.13 Using this method to synthesize barium ferrite, Yu and Liu13 used a molar ratio of Ba2+/Fe3+/ citrate equal to 1:12:19 to produce a single hexagonal phase of crystals after calcination at 650 °C for 5 h. However, TEM results showed that the particles were hard agglomerated with diameters in the submicrometer order. Agglomeration is an undesirable outcome of barium ferrite crystallization using calcination. Low calcination temperatures and short calcination times reduce agglomeration, instead producing well-dispersed nanoparticles. In this study, a novel chemical synthesis of barium ferrite nanoparticles using calcination at low temperatures was described. For the first time, sodium citrate, instead of citric acid, was used as the chelating agent in the synthesis of barium ferrite nanoparticles. Sodium citrate played two important roles in the process: it allowed the homogeneous mixing of two metal cations (Ba2+, Fe3+) in the as-synthesized barium ferrite
10.1021/jp068249b CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007
10176 J. Phys. Chem. C, Vol. 111, No. 28, 2007 amorphous nanoparticles, and it retarded particle growth via the formation of surface citrate complexes, thereby inhibiting the agglomeration of the nanoparticles. The reaction between metal salts and sodium citrate was though to yield metal cationcitrate complexes under basic conditions (pH >7).13 Subsequently, refluxing of the complexes was required to hydrolyze the complexes and yield nanoparticles. Sodium hydroxide may also facilitate the formation of the colloidal dispersion by creating a negative surface charge on the particles.14,15 At this stage, the formed nanoparticles were believed to have good uniformity of cations at the atomic level. The purpose of this study was to synthesize amorphous barium ferrite nanoparticles with uniformly mixed atomic arrangements. It was hypothesized that the calcination temperature required for the crystallization of these barium ferrite nanoparticles would be lower than the temperature required for the formation of bulk phases. The effects of calcination temperature on particle size, morphology, crystallinity, and magnetic properties were systematically examined. The novel synthesis described in this article may be directly scaled up for the production of barium ferrite nanoparticles with high homogeneity. Experimental Section Materials. Barium ferrite amorphous nanoparticles were synthesized by a sodium citrate-aided process using ferric nitrate (Fe(NO3)3‚9H2O, >99.9%), barium chloride (BaCl2‚2H2O, >99.0%), and sodium citrate (Na3C6H5O7‚2H2O, >99.0%) in the presence of sodium hydroxide (NaOH, >97%). All chemicals were purchased from Kanto Chemical (Tokyo, Japan) and were used without additional purification. The molar ratio of Ba2+/Fe3+/citrate was fixed at 1:12:13. The total concentration of BaCl2 (0.027 M) was constant in this study. Synthetic Procedure of Citrate-Protected Barium Ferrite Nanoparticles. Aqueous solutions of 0.7 M ferric nitrate, 0.7 M barium chloride, and 0.7 M sodium citrate were prepared by dissolving the salts in pure water. A 120 mL aliquot of ferric nitrate solution was mixed with 10 mL of barium chloride solution by continuous magnetic stirring in a round-bottomed flask. Sodium citrate solution (130 mL) was then added to the mixture of ferric nitrate and barium chloride. The total volume of the solution was 260 mL. Metal citrate complexes were formed by the addition of NaOH (9 N) to the mixture until the pH value was 10. The resulting mixture was refluxed at 100 °C for 3 h under an Ar atmosphere to avoid the formation of impurities such as BaCO3. After complete hydrolysis, the resulting red-brown colloidal suspension was cooled to room temperature. The precipitants were then separated by centrifugation and redispersed in absolute ethanol; this separation process was repeated several times. The precipitants were dried in an oven at 60 °C for 24 h. The dried powders calcinated at temperatures between 500 and 750 °C for 100 min in air at a heating rate of 100 °C/min. Characterization. The initial pH of the mixed solution was checked using a pH meter (D-21, Horiba, Japan). Crystal structure analyses were done using an X-ray diffractometer (XRD) (RINT-2200V, Rigaku, Japan) with Cu KR radiation (λ ) 1.5408 Å). The morphologies of the as-synthesized and the calcinated barium ferrite particles were observed by fieldemission scanning electron microscopy (FE-SEM) (S-5000 and 5200, Hitachi, Japan). The morphologic characteristics of the composite particles were confirmed using transmission electron microscopy (TEM, HF-2000, Hitachi, Tokyo, Japan) at 200 kV. Thermal gravimetric analysis (TG) and differential thermal
Kim et al. analysis (DTA) (TG-DTA 6200, Seiko Instruments Inc., Japan) were performed. During the TG-DTA measurements, approximately 14 mg of sample was placed in a platinum crucible in the temperature range of 50-900 °C under flowing air (200 mL/min) at a heating rate of 10 °C/min. The chemical species of the resulting barium ferrite powder were analyzed by means of a Fourier-transform infrared (FTIR) spectrophotometer (Prestige-21, Shimadzu Corporation, Japan). The FTIR measurements were carried out at room temperature using the conventional KBr pellet technique. The surface area was evaluated using nitrogen gas adsorption with the BrunauerEmmett-Teller method (BET) (Monosorb MS-21, Quantachrome Inst.). Magnetic hysteresis measurements of the barium ferrite powder were carried out using a sample magnetometer (VSM) (BHV-35, Riken Denshi, Japan) with a field strength of up to 10 kOe at room temperature. For VSM measurements, the powder was dispersed in paraffin wax, and the powderwax composites were put into a cylindrical cell. Results and Discussion Nanoparticle Formation and Particle Size Evolution by Increasing Calcination Temperature. Figure 1 shows the morphologies of the barium ferrite nanoparticles observed by FE-SEM. The mean particle sizes were determined by measuring more than 200 particles for each sample in the SEM images. The mean particle sizes for the as-synthesized and calcinated barium ferrite nanoparticles at 600 (b), 700 (c), and 750 °C (d) were 4.7 ( 0.8, 35.4 ( 5.99, 91.6 ( 31.5, and 129 ( 23.8 nm, respectively. Caruntu et al. investigated the mechanism by which spinel ferrite nanoparticles are formed using metal chlorides and sodium hydroxide in the presence of diethylene glycol.14 Mixing these precursors caused the immediate precipitation of metal hydroxyl-diethylene glycol complexes. The complexes underwent nucleophilic substitution reactions when the temperature was increased. The diethylene glycol on the particle surface was easily exchanged for oleic acid, which was added to ensure the termination of crystal growth. On the basis of this scenario, it was assumed that the reaction mechanism for the formation of barium ferrite nanoparticles was as follows:
The chelating reaction between barium chloride and citrate occurred under acidic conditions (pH
