Figure shows a typical XRD pattern of the as-fabricated sample (reaction for 24 h). All diffraction peaks can be indexed to a simple cubic lattice (FCC), and the positions along with relative intensity of peaks match well with standard CoFe2O4 powder diffraction database (JCPDS File No. 22-1086), indicating that the as-obtained products have an Fd3m cubic spinel structure. According to the Scherrer equation, the average crystallite size which is calculated based on the XRD pattern (311) is approximately 8 nm. In fact, the XRD patterns of the others (reaction for 12 and 36 h) are similar to it (not shown). More accurately, XPS was used to determine its composition. Figure and show the high-resolution XPS spectra of 2p Co and Fe, respectively. The peak at 780.8 eV is from Co2p3/2, with a shake up satellite at 785.9 eV, while the peak at 797.2 eV is caused by Co2p1/2, with a satellite peak at 803.0 eV. The presence of those two peaks and the highly intense satellites near them is consistent with the presence of Co2+ in the high-spin state. All the Fe2p spectra generally show a main peak at a binding energy (BE) of around 710.3 eV, accompanied by a satellite line visible at a BE of around 718.3 eV, only indicative of the presence of Fe3+ cations. Further quantitative analysis finds that the atomic ratio between Co and Fe is about 1:2, which is compatible with the data of XRD.
The as-synthesized CoFe2O4 MSs for a XRD pattern, and XPS spectra b Co2p and c Fe2p
The SEM images of the as-synthesized samples obtained at different reaction times are illustrated in Fig. and . It is clear that both of the products have uniform spherical shapes. The average size of the CoFe2O4 MSs is ~220 nm [solvothermal treated for 12 h (Fig. )]. However, when extending the reaction time to 24 h, the average size increases to ~330 nm (as revealed in Fig. ). As shown in the corresponding TEM images in Fig. and , these micrometer-sized CoFe2O4 spheres [~220 nm (Fig. ) and ~330 nm (Fig. ) in diameter, respectively] can clearly be seen. Additionally, an individual sphere is not a single microparticle but the assemblies of small CoFe2O4 nanoparticles (the diameter of ~8 nm), in which the size of primary nanoparticles is in excellent agreement with the XRD results.
SEM/TEM images of products prepared at different reaction time: a/c 12 h, b/d 24 h
To obtain a better understanding of the formation and evolution of CoFe2
MSs along with the reaction time, that the reaction duration was further extended to 36 h was carried out. The MSs were evacuated resulting in octahedral-shaped CoFe2
particles, which are confirmed by the SEM image (Fig. ) and TEM image (Fig. ). As a consequence, a proposed mechanism of formation and evolution of the CoFe2
MSs is sketched in Fig. . It is well known that PVP can selectively absorb on a certain crystal facet of the as-prepared primary building blocks such as nanoparticles, nanosheets, nanoplates, nanorods, and so on [20
]. In our experimental system, PVP surfactant contributes not only to preventing these primary building blocks from entropy-driven random aggregation but also to controlling the formation of the regular geometry. The formation and evolution of MSs seems to be as follows: at first, this CoFe2
phase undergoes consequent nucleation and growth around the entire surface stabilized by PVP. In the subsequent process, driven by the minimization of the total energy of the system, the small primary CoFe2
nanoparticles aggregated together to form three-dimensional (3D) spheres. Ostwald ripening occurred under solvothermal conditions, resulting in the formation of CoFe2
spheres from small grows into larger due to isotropic growth. The octahedral shape of the product obtained by reaction for 36 h may be explained by combination of thermodynamic aspects of crystal growth and the selective adsorption model of surfactants on different crystallographic facets [24
]. Moreover, the structures may be deduced that development of the (111) facets was handicapped by the adsorbed PVP surfactant on these facets. Of course, convincing explanation on the mechanism of MSs is our further work.
a SEM/b TEM images of studied samples with 36 h reaction time
Schematic illustration of the formation and evolution of CoFe2O4 MSs along with the reaction time
The magnetic behavior of CoFe2
MSs is very important for practical applications. The room-temperature magnetization curves (Fig. , ) display the two relatively high Ms values of 41.2 (taken from 220 nm) and 55.2 emu/g (330 nm), respectively. Both values are, however, somewhat lower than the bulk value (71.2 emu/g) [18
]. The hysteresis loop shows essentially no coercivity (HC
) for the smaller size samples (220 nm) and negligible value of 27 Oe (larger size specimens, a diameter of 330 nm), suggesting that improved coalescence of the crystallites in the nanostructures results in increased magnetic coupling and higher magnetization. According to the results of XRD and TEM observations, the average size of primary crystals is about 8 nm, smaller than the SP critical size of CoFe2
]. So it is reasonable that the self-assembled cobalt ferrite MSs reveal SP behavior even though their sphere size exceeds 200 nm. The stabilization of the MSs in distilled water thanks to the surfaces capped of hydrophilic PVP. Slight agitation can bring the MSs to back into the aqueous solution when the magnet is removed (Fig. ) although these magnetic MSs can be completely separated from the solution when the solution is subjected to an external magnetic field within minutes, as shown in Fig. . It can be obviously seen that the CoFe2
MSs have rapid magnetic response ability at room temperature, as well as, highly monodispersed, and biocompatible.
Figure 5 RT magnetization curves for the of CoFe2O4 MSs with different reaction time: a 12 h, b 24 h (The inset shows enlarged magnetic hysteresis loops at low applied fields); photographs of CoFe2O4 MSs dispersion in a vial, c without magnetic field, d with magnetic (more ...)