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Review on the Synthesis and Applications of Nanomaterials
Recently, Fe3O4 nanomaterials have attracted tremendous attention because of their favorable electric and magnetic properties. Fe3O4 nanostructures with various morphologies have been successfully synthesized and have been used in many fields such as lithium-ion batteries (LIBs), wastewater treatment, and magnetic resonance imaging (MRI) contrast agents. In this paper, we provide an in-depth discussion of recent development of Fe3O4 nanomaterials, including their effective synthetic methods and potential applications.
Nanomaterials have been attracting great attention owing to their excellent electrical, optical, magnetic, and catalytic properties. It is well known that the phases, sizes, and morphologies of nanomaterials have great influence on their properties and potential applications; thereby, the controlled synthesis of nanostructured materials with novel morphologies has recently received much attention [1–3]. As a kind of conventional magnetic material, Fe3O4 nanomaterials have been used in many fields because of their unique electric and magnetic properties [4, 5]. Several novel and effective methods have been developed to synthesize Fe3O4 nanomaterials with various shapes, such as nanorods, nanotubes, and hierarchical superstructures [6–10]. Fe3O4 nanomaterials have superior properties and great potential applications in the fields of lithium-ion batteries, wastewater treatment, and drug delivery [11–15].
Until now, several overviews of the literature on the Fe3O4 nanomaterials have been reported to keep the readers abreast of the rapid development. For example, a review by Yang’s group has focused on the synthesis, growth mechanism, and applications of Fe3O4 nanomaterials . Nevertheless, many successes on the synthesis, properties, and applications of Fe3O4 nanomaterials have been continually reported in the last few years; thereby, it seems timely to review the development of Fe3O4 nanostructures.
Herein, we provide an update on currently available methods for the synthesis of Fe3O4 nanomaterials with various morphologies; in spite of that, some important and original findings reported earlier are also included. The unique properties, potential applications, and future prospects of Fe3O4 nanostructures have also been discussed.
2. Synthesis of Fe3O4 Nanomaterials
Generally, the intrinsic shape of nanocrystal is dominated by the crystalline structure of initial seed, and the final shape is governed by the subsequent growth stage through delicate control of external factors (e.g., kinetic energy barrier and templates) . Fe3O4 has a cubic inverse spinel structure based on Fd-3m space group . The lattice constant is . In the unit cell, as shown in Figure 1, the oxygen ions form an fcc closed packing, and the iron ions occupy interstitial tetrahedral sites and octahedral sites, symbolized as , in which (tetrahedral positions) is occupied by Fe3+ ions and (octahedral sites) is occupied by eight Fe2+ ions and eight Fe3+ ions.
2.1. 0D Fe3O4 Nanomaterials
0D Fe3O4 nanomaterials have been widely studied due to their current and promising applications. It is known that the physicochemical properties and potential applications of Fe3O4 nanomaterials are strongly influenced by their sizes; moreover, Fe3O4 nanomaterials tend to aggregate because of the strong magnetic dipole-dipole attractions between the crystals and the large surface energy . Thus, many efforts have been devoted to prepare Fe3O4 nanomaterials with controlled size and well-defined surface property .
After the Sugimoto group have fabricated monodisperse Fe3O4 nanoparticles in 1980, various methods have been developed for the synthesis of Fe3O4 nanoparticles with narrow size distribution and good dispersity. Sun and Zeng have synthesized 4 nm Fe3O4 nanoparticles via the high-temperature reaction of Fe(acac)3 in the phenyl ether with 1,2-hexadecanediol, oleic acid, and oleylamine. “Seed-mediated” growth method is used to make larger nanoparticles, and Fe3O4 nanoparticles (3–20 nm) can be obtained by changing the quantity of seeds . The Gao’s group have used a solvothermal method to synthesize Fe3O4 nanoparticles with a mean diameter of 25 nm . In the synthesis, [Zn(CO3)2(OH)6], accepting Fe2+ precipitates through –OH, can prevent the agglomeration of Fe2+ precipitates, and superparamagnetic Fe3O4 nanoparticles can be sequentially obtained. Compared to other successes [11, 28], the as-synthesized Fe3O4 nanoparticles with good dispersity have not been coated by other substances (e.g., silica and polymer) and can keep their naturally properties. Other routes, including coprecipitation method, reverse micelle method, and high temperature liquid phase method, have also been explored to fabricate Fe3O4 nanoparticles with different diameters [29–32].
Besides spherical nanoparticles, 0D Fe3O4 nanomaterials with other morphologies have been prepared, such as octahedron [19, 33, 34], dodecahedron , and cube [35–37]. Based on the literature , the shape of the particle is closely related to the crystallographic surfaces that enclose the particle. As to Fe3O4, the relative surface energies are in the order of owing to the distances between these three faces and coordination number with neighboring atoms [39, 40]. Therefore, the growth rate of (111) plane is quicker than that of other planes, and the octahedral shapes would be the thermodynamically favored morphology according to the Wulff theorem. For example, Zhang et al. have presented a simple method for octahedral Fe3O4 nanoparticles, in which tetracosane is the reaction media, oleylamine is the surfactant and the reducing agent, and Fe(OA)3 is the precursor . As shown in Figure 2(a), the octahedrons have a size of nm. The octahedrons can also self-assemble into oriented superstructures due to their anisotropic shapes (Figure 2(b)). As discussed above, the (110) facet has the highest surface energy, so there is much difficulty to fabricate magnetite nanocrystals enclosed by (110) plane. Nevertheless, Li and coworkers have prepared rhombic dodecahedral (RD) Fe3O4 nanocrystals via a microwave-assisted route in the presence of ionic liquid (IL) [C12Py]+[ClO4]− . In the synthesis, ILs change the surface condition of the Fe3O4 nanocrystals, and HMT/phenol adsorbed on (110) planes is benefitcial for the crystal growth along  direction; thus, RD Fe3O4 nanocrystals enclosed by twelve (110) flakes can be obtained. Moreover, if the nuclei are bounded by (100) planes, cubic Fe3O4 nanomaterials can be formed .
2.2. 1D and 2D Fe3O4 Nanomaterials
Although it is difficult to prepare anisotropic Fe3O4 nanocrystal because of its cubic spinel structure, anisotropic nanocrystals can be obtained by using templates or surfactants to control the growth rate on different crystal planes. More recently, 1D magnetic nanomaterials, such as nanotubes, nanorods, and nanowires, have become a pressing need for their potential applications in lithium-ion batteries and field emission displays . Particularly, tubular Fe3O4 nanostructures have stimulated extensive efforts owing to their well-defined magnetic states. It was reported that some conventional methods (i.e., template-assisted method) are not benefitcial for the formation of single-crystalline nanotubes; thereby, some novel methods have been explored [9, 42–44]. Geng et al. have applied proteins from egg albumin as nanoreactors for the fabrication of single-crystalline Fe3O4 nanotubes. A flake structure is prepared with the assistance of egg albumin, and then Fe3O4 nanotubes are prepared from the flake-like precursors based on a “rolling-up” mechanism. Additionally, nanorods and nanowires have been successfully synthesized [43, 45–48]. For example, Zhang and coworkers have prepared single-crystalline Fe3O4 nanowires with large aspect ratio by a one-step sol-gel process; Wang et al. have reduced -Fe2O3 nanowires to Fe3O4 nanowires under H2 and Ar2 at 400–900°C via a V-S process; chemical vapor deposition (CVD) method has also been used to synthesize 1D Fe3O4 nanomaterials.
2D Fe3O4 nanomaterials, such as nanorings and nanoflakes, have also attracted much attention to their special properties. Jia et al. have synthesized Fe3O4 nanorings by the reduction of hematite in the presence of phosphate and sulfate . In the synthesis, compared with (001) plane, and have stronger adhesion to the (110) and (100) planes; therefore, the capsule crystals have a tendency to grow along the  direction and the following dissolution process also takes place along the  direction. Finally, Fe3O4 nanorings can be formed. Zhu group have prepared Fe3O4 nanosheets by oxidizing Fe substrates in acidic solution in a hot plate at 70°C . Fe3O4 nanoplates have been synthesized by reducing -Fe2O3 nanoplates in the presence of PVP. In the experiment, PVP can selectively coordinate with (111) facet of -Fe2O3, which reduces the growth rate along the  direction, resulting in nanoplates bounded by the (111) planes. Finally, -Fe2O3 nanoplates could be transformed to Fe3O4 with the shape and size being unchanged. Other strategies (i.e., hydrothermal and solvothermal methods) have been developed to fabricate Fe3O4 nanoprisms [15, 51, 52].
2.3. Fe3O4 Hierarchical Superstructures
Recently, many research efforts in nanoscience have been devoted to the self-assembly of nanoscale building blocks into 2D and 3D hierarchical superstructures, which could prevent the agglomeration of nanomaterials and supply more tunable and unique properties . In addition, the “superparamagnetic limit,” that is, the conflict between reducing the magnetic energy barrier and decreasing the size, restrains the development of Fe3O4 nanomaterials . To some extent, Fe3O4 hierarchical superstructures could overcome this limit, and some researchers concerned about the synthesis of self-assembled Fe3O4 superstructures.
As shown in Figure 3(a), 1D chainlike arrays of hollow Fe3O4 nanospheres have been prepared by aging preassembled Fe nanoparticles in aqueous solution . The formation mechanism is proposed based on the nanoscale Kirkendall effect: Fe nanoparticles are firstly self-assembled into chain-like structure and solid Fe spheres are then gradually oxidized into Fe3O4 hollow nanospheres. Other self-assembled Fe3O4 chains have also been synthesized [10, 54].
The Wang group have successfully synthesized a hierarchical and porous structure of Fe3O4 hollow submicrospheres with Fe3O4 nanoparticles via a solvothermal method . As displayed in Figure 3(b), Fe3O4 submicrospheres are built from Fe3O4 nanoparticles with diameters of 20–30 nm. The formation mechanism can be attributed to reduction and Ostwald ripening: Fe2O3 submicrospheres are firstly synthesized; hematite is then reduced to magnetite and an incomplete layer consisted of Fe3O4 nuclei is formed on the solid Fe2O3 surfaces; a Fe2O3-Fe3O4 core-cell structure is formed in the presence of 1,6-hexanediol; finally, Fe3O4 hollow submicrospheres are obtained through Ostwald ripening process.
Hierarchical flower-like Fe3O4 superstructures have been wildly researched [22, 55–57]. Zhong et al. have reported the synthesis of flower-like Fe3O4 superstructures by an ethylene-glycol-(EG-)mediated self-assembly process. Han and coworkers have prepared flower-like Fe3O4 under 80°C in the absent of any surfactant or organic solvent (Figure 3(c)). Ultrasound-assisted hydrothermal route has also been used to fabricate Fe3O4 hierarchical flower-like microspheres.
Many other Fe3O4 hierarchical superstructures with special morphologies have been synthesized. For example, Fe3O4 microspheres assembled by tetrahedral nanocrystals , porous hollow Fe3O4 beads constructed with rod-like nanoparticles , and nanoparticles-assembled Fe3O4 dendritic patterns .
3. Potential Applications of Fe3O4 Nanomaterials
3.1. Lithium-Ion Batteries (LIBs)
Lithium-ion batteries are regarded as the most promising rechargeable energy storage technology due to the increasing applications of portable electronic devices and transportations. In order to obtain high power and energy density, Fe3O4 nanomaterials have been extensively explored as LIB anode materials for their high theoretical capacity (900–1000 mA·h·g−1), low cost, environmental benignity, and special properties [59, 61]. For example, single-crystalline mesoporous Fe3O4 nanorod exhibits a high reversible capacity of 843.5 mA·h·g−1 after 50th cycle at 0.1 C; furthermore, the nanorods have superior electron transport ability, which makes them highly attractive for the potential application as LIB anode materials . However, the high surface area of nanomaterials may cause secondary reactions such as electrolyte decomposition between electrode and electrolyte and form thick solid electrolyte interphase (SEI) films on the electrode surface . Fortunately, it was found that surface modifications could partly solve these problems . Carbon-coated Fe3O4 nanospindles can increase the electronic conductivity of electrodes leading to thin and uniform SEI films, but also stabilize the obtained SEI films; thereafter, the Fe3O4-C composites are excellent anode materials for highly efficient LIBs with high reversible capacity, high rate capability, and enhanced cycling performance. Li group have reported monodisperse Fe3O4/C core-shell spheres, chains, and rings with tunable magnetic properties based on structural evolution from eccentric Fe2O3/poly(acrylic acid) core-shell nanoparticles . The possible formation mechanism is shown in Figure 4. Compared with the Fe3O4/C core-shell spheres, the chains and rings exhibit higher reversible capacity and better cycling stability. Several other ways have been used to form Fe3O4-C composites [30, 64, 65]. For instance, porous carbons or mixing graphene layers are impregnated with Fe3O4 precursor; Fe3O4 NPs and carbon are simultaneously formed from a precursor with high surface area and porosity.
3.2. Wastewater Treatment
In recent years, wastewater treatment has attracted considerable attention because clean water is vital to the human and because of a variety of key industries . The development of nanoscience opens a novel and effective way for the wastewater treatment. Many groups have used Fe3O4 nanomaterials to treat heavy metal ions and organic pollution. Nanostructured Fe3O4 microspheres (NFMSs) with a large specific surface area (135.9 m2·g−1) can remove toxic Cr6+ from polluted water, and it is found that 1 g NFMSs remove 43.48 mg Cr6+ ions at room temperature . Fe3O4 nanomaterials have also been successfully used as catalysts to remove organic pollutions, such as xylenol orange, phenol, and aniline, from wastewater [68–70].
3.3. Other Applications
Fe3O4 nanomaterials have been applied in many other fields, including metal chemosensor , magnetorheological elastomer , SERS spectroscopy , magnetic resonance contrast agent , catalyst [74, 75], drug delivery , and magnetic resonance imaging (MRI) contrast agents .
In conclusions, recent synthetic efforts have led to the formation of Fe3O4 nanomaterials with various morphologies. In spite of the exciting new development, the application of Fe3O4 nanomaterials in industry is still in its infancy. However, with the progress in the fundamental understanding of the physics and chemistry in the Fe3O4 nanomaterials, we foresee that novel properties and applications will be demonstrated in the not-so-distant future.
This work is financially supported by the Natural Science Foundation of Henan Department of Science & Technology (no. 112300410224 and no. 13A150813) and the Natural Science Foundation of Nanyang Normal University (no. 2X2010014).
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