Magnetite hollow spheres: solution synthesis, phase formation and magnetic property
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- Sun, Q., Ren, Z., Wang, R. et al. J Nanopart Res (2011) 13: 213. doi:10.1007/s11051-010-0020-5
Polycrystalline magnetite hollow spheres with diameter of about 200 nm and shell thickness of 30–60 nm were prepared via a facile solution route. For the reaction, ethylene glycol (EG) served as the reducing agent and soldium acetate played the role of precipitator. In addition, polyvinylpyrrolidone (PVP) served as a surface stabilizer. The morphologies and structures were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The intermediate products at different stages were also studied to shed light on the evolution of phase formation. It revealed that the hollow structure formed via self-assembly of nanocrystallites (about 15 nm) using sodium acetate as mild precipitator. Evidences further pointed out that the Ostwald ripening process well explained the growth mechanism of the hollow structure. Magnetization measurements showed that the coercivity of magnetite hollow spheres at low temperature is about 200 Oe and the saturation magnetization is about 83 emu g−1, roughly 85% that of the bulk phase, close to the value of its solid counterpart. In addition, a freezing transition was observed at 25 K.
KeywordsMagnetite hollow spheresPhase formationMagnetic property
As a useful functional material, magnetite (Fe3O4) has wide application potentials in diverse areas such as ferrofluid (Raj et al. 1995; Butter et al. 2002; Hong et al. 2006), magnetic data storage materials (Zeng et al. 2002), devices (Dadarlat et al. 2008; Ge et al. 2008; Zhang et al. 2008) and environment protection materials (Oliveira et al. 2004). Moreover, due to its excellent biocompatibility, magnetite is also an ideal candidate for biomedical applications (Doyle et al. 2002; Hung et al. 2007; Zhang et al. 2009).
In the past few years, motivated by the belief that novel properties of nanomaterials can be gained by rationally tuning their size, shape, as well as the way they are fabricated (Puntes et al. 2001; Sun and Xia 2002; Wang et al. 2005; Franger et al. 2007; Guo et al. 2007; Wang et al. 2007), magnetite nanostructures such as nanoparticles, nanowires, and other novel nanostructures have been synthesized via various approaches (Ling et al. 2004; Zhong et al. 2006; Kovalenko et al. 2007; Aldea et al. 2009; Zhang and Zhang 2009). In particular, nano-sized hollow magnetite has attracted great interest because of its properties such as low density, selective permeability, and large specific area, etc. (Li et al. 2006; Cong et al. 2008; Deng et al. 2008; Huang et al. 2009). However, among all these previous works, expensive complex polymers, toxic chemical reagents, or sacrificial templates were needed to obtain magnetite hollow nanostructures. Besides, these methods still face shortcomings including low productivity and rigorous reaction conditions. Developing simple and efficient route to synthesize magnetite hollow structures remains as a challenge in wet chemical synthesis.
Hence, in this work, a facile solvothermal method was developed to synthesize submicron-sized magnetite hollow spheres. The novelty of this work could be characterized by its convenience and environmental friendliness. Besides, the phase formation mechanism was investigated. It was found that the presence of sodium acetate was crucial to the formation of pure magnetite phase and its concentration was important to the morphology evolutions from solid spheres to their hollow counterparts or to the self-assembly chains of solid particles.
The X-ray diffraction (XRD) patterns of the as-prepared products were recorded on an analytical X’Pert Pro MPD X-ray diffractometer equipped with Cu-Kα radiation (λ = 0.15409 nm). The morphologies and structures were characterized by a Hitachi S-4800 scanning electron microscope (SEM) with a cold field emission gun and by a JEOL JEM-2100F transmission electron microscope (TEM) and high-resolution TEM (HRTEM) operated at 200 kV. The specimens for the SEM studies were prepared by dispersing the powder samples on the silicon substrates, while that for TEM and HRTEM investigations were obtained by dispersing of the as-prepared products in ethanol solution and then drops of the suspension were placed on copper grids and dried in air. Magnetic properties of the powder sample, ~5.06 mg, were measured by a superconducting quantum interference device (SQUID) magnetometer (Quantum Design).
Formation of magnetite hollow spheres
To understand the phase formation of magnetite and the mechanism for the morphology evolution from solid particles to hollow spheres, experiments of time evolutions were carried out by the addition of low concentration of NaAc (0.24 M), which is 16.8 mmol in 70 mL of the “initial solution in procedure 2 of the synthesis process illustrated in Fig. 1. Furthermore, reactions without NaAc and with high concentration of NaAc (0.72 M), about 50.4 mmol in 70 mL of the “initial solution”, were also conducted as controlled experiments to clarify the phase formation mechanism of magnetite hollow spheres.
EG serving as a reducing agent is well discussed in the polyol process (Sun and Xia 2002; Zhong et al. 2006; Wang et al. 2008a, b). The ferrous ion might be the result of the reducing reaction between ferric iron and EG. To confirm the reducing property of EG in our experiment, we substituted of EG with water as solution and only Fe2O3 was obtained as final products.
Discussions on growth mechanism
By the studies presented above, the addition of low concentration of NaAc (0.24 M) is the key to produce the hollow spheres of magnetite. It is noticeable that the diameter of solid particles increases sequentially from about 50 nm (Fig. 4b) to 100–120 nm (Fig. 4c), and eventually reaches 200 nm as diameter of hollow spheres (Fig. 3) as prolonging reaction time from 5, 8 to 16 h with reaction temperature of 200 °C. Due to the sintering of several solid spheres with increase in the time of the reaction, solid aggregation spheres with diameter about 120 nm formed with sintering of several smaller solid spheres (i.e. 50 nm). In addition, for the hollow spheres, the size of nanocrystallites decreases inwardly from outer surface, i.e., from over 15 nm on the outer surface down to about 10 nm in the interior (shown in Fig. 3b). Furthermore, the large nanocrystallites (~15 nm) is packed more densely on the surface than those in the interior (~10 nm), as shown in Fig. 3b. From the energy point of view, it evidences a larger surface cohesive energy, which in turn favors the dissolution of the inner small nanocrystallites. The above discussions suggest that an inside-out mass transportation occurs and that the large nanocrystallites on the outer surface grow at the expense of the smaller one inside. This phenomena was similar to inside-out Ostwald ripening which was first described by Ostwald in 1896 (Ostwald 1900) concerning the growth of large precipitates at the expense of smaller precipitates. Besides, similar phenomena were also observed in other hollow structures of a wide range of materials obtained via the Ostwald ripening (Lou et al. 2006; Teo et al. 2006; Ottaviano et al. 2009).
Based on the experiment results and discussions above, we believe that the formation of the magnetite hollow spheres experienced a two-step process. First is the sintering process, which could be observed as the diameter increase of the spheres as shown in Fig. 4b and c with reaction time increase from 5 to 8 h. During this process, the sintering process is the main reason for the diameter increase from 50 to 120 nm. Second, with reaction time prolonging to 16 h, the magnetite hollow structure with larger nanoparticles on surface and smaller ones inside appeared which can attributed to the inside-out Ostwald ripening. Thus, the main reason for the formation of magnetite hollow spheres was the inside-out Ostwald ripening.
On the contrary, high concentration of NaAc (0.72) led to the formation of magnetite solid spheres without obvious size increase or hollow structure emerges which means the formation mechanism of magnetite hollow spheres and well-crystallized solid particles were different according to the concentration of NaAc.
Polycrystalline Fe3O4 hollow spheres were synthesized by a facile hydrothermal method. PVP was added as a surface stabilizer and both of EG and sodium acetate (NaAc) were essential for the formation of the magnetite hollow spheres. To be specific, EG was reducing agent, and NaAc played the role of precipitator controlling the release rate of OH− which is necessary for the formation of the hollow structure. The growth mechanism of the magnetite hollow spheres was suggested by the Ostwald ripening effect, which was verified by the time evolution experiments. Magnetization measurements show that the saturation magnetization, Ms = 82.8 emu g−1 at 5 K is about 85% of the bulk value and the coercivity is 200 Oe. From this point of view, the magnetite hollow spheres are promising for a potential application as carriers for drug targeting.
This work was supported by the National Natural Science Foundation of China (Nos. 50671003, 50971011 and 10874006), Beijing Natural Science Foundation (No. 1102025), the National Basic Research Program of China (Nos. 2009CB939901 and 2010CB934601), the Program for New Century Excellent Talents in University (NCET-06-0175) and Research Fund for the Doctoral Program of Higher Education of China (20091102110038).