Journal of Nanomaterials

Volume 2013 (2013), Article ID 902538, 7 pages

http://dx.doi.org/10.1155/2013/902538

Review Article

Review on the Synthesis and Applications of Nanomaterials

Xiaodi Liu,¹ Zhiguo Zhong,² Yufeng Tang,¹ and Bingyu Liang¹

¹College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang, Henan 473061, China

²Center of Analysis and Testing, Nanyang Normal University, Nanyang, Henan 473061, China

Received 17 January 2013; Accepted 16 February 2013

Academic Editor: Jianmin Ma

Copyright © 2013 Xiaodi Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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.

1. Introduction

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 [16]. 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) [17]. Fe₃O₄ has a cubic inverse spinel structure based on Fd-3m space group [18]. 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.

Figure 1: Crystal structure of cubic inverse spinel Fe₃O₄ ((green ball) Fe³⁺ (blue ball) Fe²⁺ (red ball) O).

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 [24]. Thus, many efforts have been devoted to prepare Fe₃O₄ nanomaterials with controlled size and well-defined surface property [25].

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 [26]. The Gao’s group have used a solvothermal method to synthesize Fe₃O₄ nanoparticles with a mean diameter of 25 nm [27]. 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 [6], and cube [35–37]. Based on the literature [38], 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 [19]. 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]⁻ [6]. 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 [100] 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 [36].

(a)

(b)

(c)

Figure 2: (a) TEM image of self-assembled monolayer Fe₃O₄ nanocrystals; (b) SEM image of assembled multilayer Fe₃O₄ nanocrystals; (c) SEM image of RD Fe₃O₄ nanocrystals; the inset is model of RD enclosed by twelve (110) planes [6, 19].

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 [41]. 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 [49]. 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 [001] direction and the following dissolution process also takes place along the [001] 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 [50]. 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 [111] 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 [53]. 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 [9]. 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 [20]. 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].

(a)

(b)

(c)

Figure 3: (a) TEM images of the chainlike arrays of Fe₃O₄ hollow nanospheres; (b) SEM image of the hierarchial porous Fe₃O₄ hollow submicrospheres; (c) SEM and TEM images of the flower-like Fe₃O₄ superstructures [20–22].

The Wang group have successfully synthesized a hierarchical and porous structure of Fe₃O₄ hollow submicrospheres with Fe₃O₄ nanoparticles via a solvothermal method [21]. 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 [58], porous hollow Fe₃O₄ beads constructed with rod-like nanoparticles [59], and nanoparticles-assembled Fe₃O₄ dendritic patterns [60].

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 [46]. 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 [62]. Fortunately, it was found that surface modifications could partly solve these problems [23]. 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 [63]. 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.

Figure 4: Schematic illustration of the synthesis of Fe₃O₄/C core-shell spheres, chains, and rings [23].

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 [66]. 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 [67]. 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 [71], magnetorheological elastomer [72], SERS spectroscopy [57], magnetic resonance contrast agent [73], catalyst [74, 75], drug delivery [76], and magnetic resonance imaging (MRI) contrast agents [77].

4. Conclusions

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.

Acknowledgments

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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