Single crystalline CoFe2O4 nanoparticles with high coercivity were prepared via a one-step hydrothermal method. The shape and size of the nanocrystals (in the range of 20–100 nm) can be controlled by varying synthesis parameters such as the concentration of NaOH and CTAB. X-ray diffraction and Raman spectra analysis confirmed that all the as-synthesized nanoparticles have a face centered cubic spinel crystal structure. HRTEM observation of particles shows interlayer spacing 0.48 nm of (111) lattice planes. A coercive force up to 5.0 kOe and saturation magnetization of 73 emu/g was achieved at room temperature for the 40 nm CoFe2O4 nanoparticles.
Magnetic nanoparticles have been widely investigated because of their potential application in exchange-coupled nanocomposite magnets, magnetic data storage, magnetic fluids, and biomedicine.1–5 Nanoparticle size, shape, crystal structure, and composition are often related to nanoparticle magnetism such as anisotropy, magnetization, coercivity, and ordering temperature.6–11 CoFe2O4 has been studied widely due to its interesting magnetic properties such as high anisotropy and coercivity, excellent chemical stability, and good mechanical hardness. Many advances to synthesize magnetic nanostructures have been made using a variety of chemical and physical approaches.12–16 A number of efforts have been made to increase coercivity in cobalt ferrite. The maximum coercivity of 9.3 kOe at room temperature reported to date in thin films deposited on SiO2 single crystal substrate that was due to the small grain size (50 nm) and lattice strain in the films.17 The large coercivity of 4.6 kOe was reported in CoFe2O4 nanoparticles synthesized using seed-mediated growth method.18 High coercivity of 9.4 kOe was reported in oleic acid capped CoFe2O4 nanoparticles with low magnetization of 7.1 emu/g.19
In this work, we report the synthesis of single crystalline ferromagnetic CoFe2O4 nanoparticles through a one-step hydrothermal method, which was employed along with the surfactants to control morphology, size, and composition of the nanoparticles. This method is suitable for mass production of magnetic CoFe2O4 nanoparticles.
In a typical synthesis, FeCl3.6H2O (1.08 g, 4 mmol), CoCl2 (0.26 g, 2 mmol), NaOH (0.8 g, 20 mmol), and cetyltrimethylammonium bromide (CTAB) (1.0 g, 2.75 mmol) were dissolved in 7 ml deionized (DI) water by intensive stirring till a homogeneous solution was obtained. Subsequently, a 14 ml of ethylenediamine (EDA) was added dropwise into the above mentioned aqueous solution. After stirring for 30 min, the slurry mixture obtained was transferred into a Teflon-lined stainless steel autoclave. This autoclave was kept in a furnace and heated at 200 °C for 10 h. After cooling to room temperature, the black powder consisting of cobalt ferrite nanocrystals was separated from reaction fluid by centrifugation at 6000 rpm for 15 min. The powder was collected then redispersed in 30 ml ethanol using an ultrasonic bath. The sample was centrifuged once again at 6000 rpm for 15 min and the ethanol was discarded. This purification step was repeated two more times.
The transmission electron microscopy (TEM) images were recorded on a JEOL 1200 EX electron microscope at an accelerating voltage of 120 kV. HRTEM images were obtained with Hitachi H-9500 with an accelerated voltage of 300 kV. Powder X-ray diffraction (XRD) spectra were recorded on a Rigaku Ultima IV diffractometer with a Cu Kα X-ray source. The Raman spectra measurements were carried out using DXR Raman microscope (Thermo Scientific) with a laser (532 nm and 5 mW) excitation source. Magnetic measurements of the metallic samples were performed using a Quantum Design MPMS magnetometer. Samples for magnetic characterization were prepared by depositing a drop of the final ethanol dispersion on a silicon substrate evaporating the solvent at room temperature and by hardening randomly oriented CoFe2O4 powders in epoxy.
RESULTS AND DISCUSSION
CoFe2O4 nanocrystals with different size and shape were synthesized via the hydrothermal route. Various combinations of NaOH and CTAB were studied at fixed precursor concentration and fixed reaction temperature. We first studied the effect of surfactant addition on size and shape of the nanocrystals. The surfactant CTAB is cationic and is responsible for the rod shape assembly of nanoparticles. Upon the ionization, CTAB constitutes the CTA+ ion that forms the pair with negatively charged Fe–OH–Co, thus constituting the crystal growth initializers.20
Fig. 1 shows the TEM images for the samples prepared using different concentration of NaOH and CTAB by the hydrothermal method. Figs. 1(a)–1(b) show typical CoFe2O4 nanoparticles synthesized using 12 mmol and 20 mmol NaOH without adding CTAB into the solution mixture. The particles shown in Fig. 1(a) have different shapes with a broad particle size distribution. Fig. 1(b) shows a narrow size distribution of the nanoparticles with diameters of 20–40 nm. Fig. 1(c) shows the typical TEM image of ∼40 nm CoFe2O4 nanoparticles prepared with 2.75 mmol of CTAB and 20 mmol of NaOH. When 5.5 mmol of CTAB was used, we observed a mixture of nanoparticles with diameter 10–30 nm and shuttle like nanorods with diameter of 40–50 nm and length of 60–100 nm as shown in Fig. 1(d). Fig. 1(e) shows the HRTEM image of representative nanoparticles from Fig. 1(c). The nanoparticle is single crystalline and has a lattice fringes of 0.48 nm spacing of the (111) planes in inverse spinel structured face centered cubic (fcc) CoFe2O4.
The crystal structure of nanoparticles was further characterized using XRD. Fig. 2 shows the XRD patterns of the CoFe2O4 nanocrystals prepared by varying the concentration of NaOH and CTAB. It shows the characteristic (220), (311), (222), (400), (422), (511), (440), and (533) diffraction peaks of CoFe2O4 phase with fcc crystal structure (Figs. 2(a)–2(c)). The average crystallite size of CoFe2O4 nanoparticles was estimated from x-ray diffraction patterns using Scherrer formula (d = 0.9 λ/β cos θB).The average crystallite size values determined from XRD patterns for the samples shown in Figs. 2(a)–2(c) were ∼28 nm, ∼38 nm, and ∼42 nm, respectively, which are in good agreement with the particles size observed by TEM (Figs. 1(a), 1(c), and 1(d)) that further confirm CoFe2O4 nanoparticles are single crystalline. We observed the presence of α-Fe2O3 phase along with CoFe2O4 phase (Fig. 2(a)) in the XRD pattern when 12 mmol of NaOH (Fig. 1(a)) was used in the reaction. When NaOH amount was increased to 20 mmol, the final product obtained was pure CoFe2O4 phase without the α-Fe2O3 impurity as seen in Figs. 2(b) and 2(c).
The information on the coordination of the metal ions in the nanocrystals was obtained from the Raman spectroscopy. CoFe2O4 also has the inverse spinel structure like Fe3O4 with cubic symmetry.21–23 According to previous studies, the low-frequency vibrations (below 600 cm−1) are attributed to the motion of oxygen around the octahedral lattice site whereas the higher frequencies are attributed to oxygen around tetrahedral sites.24 In this work, the intense band in the Raman spectra at 682 cm−1 is characteristic of the tetrahedral site and the band at 470 cm−1 is due to Co2+ at octahedral sites (Fig. 3).
Fig. 4(a) shows hysteresis loop of CoFe2O4 nanoparticles measured at 300 and 5 K, using an applied field of 5 T. The maximum coercivity of 5.0 kOe and a saturation magnetization of 73 emu/g were obtained at 300 K for the 40 nm particles (Fig. 1(c)). At 5 K, the coercivity of 17.7 kOe and saturation magnetization of 79.7 emu/g were obtained, which can be attributed to the increase in magnetocrystalline anisotropy at low temperatures. The value of squareness ratio (Mr/Ms) of 0.66 indicates that the system consists of randomly oriented equiaxial particles with cubic magnetocrystalline anisotropy. The presence of Co2+ ions on the octahedral sites of the spinel structure is the primary reason for the strong anisotropy of cobalt ferrite.25 Furthermore, the larger anisotropy in nanosized materials compared to bulk materials is expected due to the additional contributions from surface anisotropy and size-related effects.26 It has been reported that the critical size of single-domain CoFe2O4 particles is about 40 nm from both the high switching field obtained by the magnetic torque analysis and the relatively high coercive field obtained for CoFe2O4 particles.18 It is therefore expected that CoFe2O4 particles about 40 nm can have high coercivity. The method of synthesis applied in the present work favors good crystallinity and magnetic ordering, resulting in a high saturation magnetization. Fig. 4(b) shows the variation of coercivity and magnetization as a function of temperature. Both the coercivity and the magnetization for the 40 nm particle size (Fig. 1(c)) decreases with temperature.
In summary, single crystalline CoFe2O4 nanoparticles were prepared using a simple hydrothermal method. The size and shape of the CoFe2O4 nanocrystals were found to be dependent on the reaction conditions such as NaOH concentration and CTAB surfactant concentration. A maximum saturation magnetization of 73 emu/g and coercive force of 5.0 kOe were obtained at 300 K, which is the highest reported via one step hydrothermal method to our best knowledge. An in-depth evaluation of the influence of the bases on the morphology, structure, chemical composition, and magnetic properties of the nanocrystals is provided. This work sheds new light on the design of magnetic nanoparticles with improved properties through the hydrothermal method. The obtained results also suggest the possibility to optimize magnetic property of CoFe2O4 nanoparticles with controlled magnetic alignment for magnetic energy and data storage applications.
This work has been supported by the U.S. DoD/ARO under grant W911NF-11–1–0507, and the Center for Nanostructured Materials and Characterization Center for Materials and Biology at the University of Texas at Arlington.