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Fe₃ O₄ Based Nanostructures

Magnetite (Fe₃ O₄ ) and Fe₃ O₄ based nanostructure composites with pseudocapacitance, high theoretical capacity, environmental friendliness and low cost, are extensively applied to electromagnetic wave absorption [106], LIBs [3, 12, 15], biotechnological devices [107, 108], and supercapacitor [4, 109]. Fe₃ O₄ nanoparticles is one of the high-performance anodes in electrochemical devices. Unfortunately, Fe₃ O₄ nanostructures still face some problems, such as a severe volume variation (~ 200%) during the insertion and extraction of Li ⁺ and relatively low electrical conductivity, posing negative influence on the cycling stability [19, 110, 111]. From research results, we found that the structure and morphology of Fe₃ O₄ have a high influence on the electrochemical performance of Fe₃ O₄ and Fe₃ O₄ based composites.

Synthesis and Characterization

Recently, nanostructure engineering is demonstrated as a highly-effective approach to obtain improved electrochemical performance of Fe₃ O₄ and Fe₃ O₄ based composites. Therefore, various nanostructures including 0D nanoparticles [112], 1D nanorods/wires [113, 114], 2D nanoflakes/sheets [115, 116], 3D hierarchical/porous architectures [117, 118], and hybrid nanostructures of iron oxides [16] are proposed. The electrochemical performance of Fe₃ O₄ nanostructures can be optimized by rational design of their morphology, composition, porosity and surface characteristics.

Solution phase synthetic method is a facile and rapid way to obtain Fe₃ O₄ based nanostructures, because of the associative advantages, such as low synthesis temperature (always below 250 °C), easy control of morphology via adjusting hydrothermal conditions (e.g., PH, density of reactant and dosage of active agent, etc.). Solution phase synthetic method includes solvothermal synthesis [119], thermolysis [120], co-precipitation [121], sol–gel process [122, 123], micro-emulsion [124], etc. Simultaneously we compare the pros and cons of these methods.

Solvothermal Synthesis

Solvothermal synthesis, which reacts in a special closed reaction vessel, is one commonly used methods for synthesizing Fe₃ O₄ . In a hermetic environment, it is a facile method using aqueous solution as reaction medium at high temperature and high-pressure hermetic environment. Fe₃ O₄ nanostructures with various morphologies (0D, 1D, 2D and 3D) were synthesized applying this approach.

An et al. [119] obtained the Fe₃ O₄ /graphene nanowires by solvothermal synthesis and calcination with FeCl₃ , NH₄ VO₃ and graphene as precursors. Phase transition of Fe₃ O₄ /VO_x (FVO) after annealing was confirmed by XRD. For the XRD pattern of sample without annealing, all diffraction peaks of FVO and graphene decorated FVO correspond to FeVO₄ ·1.1H₂ O. For the XRD pattern of sample after annealing, there are no peaks of any vanadium oxides. And the inductive coupled high frequency plasma (ICP) result indicated that the molarity ratio of Fe and V is ~ 0.94:1, confirming the existence of amorphous vanadium oxide.

Mu et al. [125] reported dispersed Fe₃ O₄ nanosheets on carbon nanofiber by combing the electrospinning and solvothermal process. In this work, Fe₃ O₄ nanosheets are uniformly attached on the surface of carbon nanofiber with the diameter of about 500 nm.

Fe₃ O₄ nanoparticle with high specific surface area via FeCl₃ and organic solvent ethanolamine (ETA) as precursors is reported by Wang et al. [126]. In this preparation, ETA is critical factor for compounding Fe₃ O₄ nanoparticles with high specific surface area, and Fe ³⁺ is gradually reduced to Fe ²⁺ by ETA during dissolution process, demonstrating that Fe ²⁺ increased as the increase of ultrasonication time. The ratio of ETA and FeCl₃ has a large impact on the nanoscale grain size and specific surface area of Fe₃ O₄ . And the results showed that the grain size of 20–40 nm is achieved with 60 mL ETA and 6 mmol FeCl₃ . When the amount of ETA is 80 mL, smaller nanoparticles (5–10 nm) are obtained.

Another representative work is reported by Chen et al. [127], in which graphene nanosheets decorated with Fe₃ O₄ nanoparticles (USIO/G) were synthesized using a facile solvothermal process. For the synthesis of USIO composite decorated with reduced graphene oxide (RGO), is used FeCl₃ ·H₂ O as precursor, then NaHCO₃ and L-ascorbic acid were added to form USIO/G. In this process, L-ascorbic acid was oxidized to dehydroascorbic acid (DHAA) by some of Fe ³⁺ , which were reduced to Fe ²⁺ . Formation process of USIO/G is schematically shown in Fig. 9. The Fe₃ O₄ nanoparticles with uniform distribution, which are beneficial for electrical conductivity of graphene, mitigation of volume expansion of Fe₃ O₄ , and facilitating Fe₃ O₄ particles into the electrolyte.

Reprinted with Permission from [127]. Copyright, Royal Society of Chemistry

Schematic illustration of the formation process of USIO/G.

Xiong et al. [128] a kind of hierarchical hollow Fe₃ O₄ (H-Fe₃ O₄ ) microspheres prepared by controlled thermal decomposition of iron alkoxide precursor. In a classical reaction, ethylene glycol (EG) serves as reduction reagent that partly reduces Fe ³⁺ to Fe ²⁺ with sodium acetate (NaAc), and polyvinylpyrrolidone (PVP) [128]. For this synthesis, PVP served as a surface stabilizer, which has important role in the formation and transformation of hollow interiors.

With the development of solvothermal synthesis, it emerging as an efficient method with the advantages of low energy consumption, little reunion and easy to control shape, etc. Chen et al. [129] synthesized poly (acrylic acid) (PAA)-entangled Fe₃ O₄ nanospheres by a facile solvothermal method. In their synthesis, the ethylenediamine is crucial to the controlling of the uniformity of nanospheres, and the PAA molecules served as carbon source that transforms into the carbon matrix by heating treatment in inert atmosphere. As shown in SEM image of the prepared C-Fe₃ O₄ nanospheres, very uniform spherical particles with a diameter of 150–200 nm are synthesized. Observed from SEM images in Fig. 10a, the nanospheres contain small irregular particles, and have a relatively rough surface. In the control experiment without ethylenediamine (EDA), the synthesized particles are much less uniform with a wider size distribution of 100–500 nm, allowing the formation of nanospheres with smaller size.

Reprinted with Permission from [130]. Copyright, Wiley–VCH

아 SEM images of the Fe₃ O₄ nanospheres synthesized with ethylenediamine (EDA). The inset of (a ) shows a SEM image with higher magnification. (Reprinted with Permission from [129]. Copyright, American Chemical Society.). Physicochemical characterization of the octahedral Fe₃ O₄ nanoparticles. b SEM image showing the octahedral geometry of the iron oxide nanoparticles.

Co-precipitation

Due to its high cost-effectiveness, environmental friendliness, and facile synthesizing protocol, co-precipitation is a general approach for Fe₃ O₄ nanoparticles. Thus, in iron based rechargeable battery systems, Fe₃ O₄ nanomaterials are especially suitable for large-scale electrochemical applications to solve the energy requirement of the modern society.

Li et al. [121] proposed Fe₃ O₄ polyhedron as LIBs anode for alkaline secondary batteries by a co-precipitation. Annealing temperature makes a high effect on the physical and electrochemical performance of Fe₃ O₄ nanomaterials. The 700 °C-annealed Fe₃ O₄ exhibited a higher electrochemical performance, such as a higher specific discharge capacity of 604.2 mA h g ⁻¹ with a charging efficiency of 83.9% at 120 mA g ⁻¹ . Ooi et al. [130] demonstrated octahedral Fe₃ O₄ nanoparticles using a facile solvothermal route. Scanning electron microscope (SEM) image of Fe₃ O₄ nanoparticles is shown in Fig. 10b, which depicts that octahedral Fe₃ O₄ nanoparticles with an average length of 93 ± 18 nm were prepared by the hydrothermal method, showing a roughly Gaussian size distribution. Then, the crystal structure of octahedral nanoparticles can be further evaluated by HRTEM, and the composition of the bulk sample was further characterized by XRD and X-ray photo electron spectroscopy (XPS).

Thermolysis

The thermolysis is small monodisperse magnetic nanocrystals synthesized by organic metal compounds in high boiling point solvents containing stabilizing agent. Previous Organic metal bodies include metal acetylacetone compounds, metal cupferron, or metal Carbonyl compounds, and usually choose fatty acids, oleic acid, or hexadecyl amine as a surfactant. Zhang et al. [120] reported ultrafine Fe₃ O₄ nanocrystals uniformly encapsulated in two-dimensional (2D) carbon nanonetworks through thermolysis of Fe(C₅ H₇ O₂ )₃ precursor at 350 °C under vacuum, which named as 2D Fe₃ O₄ /C nanonetworks. This facile process using low-cost precursor proposed a green approach for preparing Fe₃ O₄ /carbon composite. Additionally, compared with the reported Fe₃ O₄ /carbon composites, the particle size of Fe₃ O₄ is controllable and a size of ∼ 3 nm can be obtained.

Benefitting from synergistic effects of carbon nanonetworks with excellent electrical conductivity and ultrafine Fe₃ O₄ particles with uniform distribution, high reversible capacity, excellent rate capability and superior cyclability at the voltage of 0.01–3.0 V (vs. Li/Li ⁺ ) are obtained. Nanoparticles with unique iron oxide (Fe₃ O₄ ) cores and zinc oxide (ZnO) shells were prepared by Jaramillo et al. [131]. Fe₃ O₄ nanoparticle synthesized through a thermolysis method using Fe(C₅ H₇ O₂ )₃ as organic metal body presoma, triethylene glycol as surface active agent, and core–shell Fe₃ O₄ @ZnO nanoparticles were successfully synthesized using straightforward methodologies. The structural and optical properties of the materials were characterized using a combination of X-ray diffraction, electron microscopy, and light spectroscopy. Importantly, the purity of the core and shell phases in the Fe₃ O₄ @ZnO nanoparticles was confirmed by both XRD and TEM, and the ZnO shell was shown to increase the transparency of the core–shell nanoparticles relative to the single-component Fe₃ O₄ nanoparticles. Zhang et al. [132] demonstrated a high crystalline Fe₃ O₄ -graphene composite by one-step reaction of thermolysis. And they demonstrated that the attachment of iron-organic complex with graphene oxide (GO) sheets can facilely result in magnetic graphene composites via a time-dependent calcination process.

Sol–Gel Process

The specific method is using the metal alkoxide, metal mineral compound or a mixture of the above two substances to hydrolysis and polymerization, uniform gel gradually, then condense into a transparent gel, however, after drying and heating, finally the oxide ultrafine powders was received. Tang et al. [122] prepared nanostructured magnetite thin film by sol–gel method using inexpensive iron (II) chloride precursor. Fe₃ O₄ nanoparticles were prepared at 300 °C, however, α-Fe₂ O₃ is generated when temperature increased to 350 °C, and this result restricts its applications. Xu et al. [123] proposed magnetite nanoparticles by virtue of sol–gel process combined with annealing in vacuum at 200–400 °C using nontoxic and low-cost ferric nitrate. In their study, Fe₃ O₄ nanoparticles with various sizes can be synthesized facilely through varying the annealing temperature.

Micro-emulsion Method

Micro-emulsion is composed of two mutual miscibility of liquid mixture of thermodynamic stability and isotropy dispersion, one of these or two kinds of liquid called micro area, and fixed by interface layer of the surfactant molecules. The key factors controlling the reaction solution contain concentration, pH value, reaction time and temperature. Micro-emulsion as a rapid expansion of new technology possesses many advantages. For example, high purity and uniform particle size distribution molecular dopant was synthesized at low temperature and simple reaction process. But there are also some shortcomings, for instance, the reaction system mostly contains organic solvents, which leads to high cost, pollution of environmental health and long reaction time. The prepared Fe₃ O₄ nanoparticles have excellent catalytic performance for the synthesis of quinoxaline in different solvents. Novel core–shell magnetic Fe₃ O₄ /silica nanocomposites with triblock-copolymer grafted on their surface (Fe₃ O₄ @SiO₂ @MDN) were successfully synthesized by combining sol–gel process with seeded aqueous-phase radical copolymerization approach [133]. The Fe₃ O₄ @SiO₂ @MDN microspheres were synthesized in following three steps. Firstly, the initial magnetic Fe₃ O₄ microspheres were synthesized by a solvothermal reaction. Then a sol–gel process was utilized to prepare silica coated Fe₃ O₄ microspheres (Fe₃ O₄ @SiO₂ ), and a thin amorphous silica layer was formed on Fe₃ O₄ microspheres. Afterward, the Fe₃ O₄ @SiO₂ microspheres were modified by 3-(methacryloxypropyl) trimethoxysilane (MPS). Finally, the triblock copolymer was fabricated by aqueous phase radical copolymerization reaction among MPS, divinylbenzene (DVB) and N-Vinyl-2-pyrrolidone (NVP) on the surface of Fe₃ O₄ @SiO₂ . The magnetic Fe₃ O₄ particles with narrow size distribution have nearly spherical shape and smooth surface. Li et al. [124] reported hexagonal and triangular monodisperse Fe₃ O₄ nanosheets by a two-step microemulsion solvothermal approach, in which the uniform Fe₃ O₄ nanoparticles are prepared and then these hydrophobic nanocrystals are dispersed in a uniform microemulsion environment as “seeds” for further re-growth through a secondary solvothermal process. In the first step, near-spherical monodisperse 7–8 nm Fe₃ O₄ nanoparticles were formed through a kinetically controlled process. In the second step, the formation of anisotropic Fe₃ O₄ nanosheets is a thermodynamically controlled process and all the exposed surfaces of the triangular and hexagonal nanosheets are (111) crystal planes, which have the lowest surface energy for FCC Fe₃ O₄ .

Other Methods

Physical methods are also significant ways to prepared Fe₃ O₄ nanostructure for anode of LIBs. Several advantages, such as good crystallization, fine-tuned particle size, and high purity of products are highlighted in recent literatures. But these methods usually demand advanced and expensive equipment, result in a higher cost, poor dispensability of particles dispersion, and agglomeration of nanostructures. For instance, Du et al. [109] fabricated activated carbon (AC)-Fe₃ O₄ nanoparticles asymmetric supercapacitor, and Fe₃ O₄ nanostructure was prepared by microwave method. The precursor, FeSO₄ ·7H₂ O and NH₃ ·H₂ O mixed solution, was heated in microwave oven. The black precipitate was separated by magnet and washed repeatedly with DI water. The resulted microstructural properties of prepared nanoparticle were characterized by nitrogen adsorption (Quantachrome NOVA 2000), XRD and SEM [109]. Chen et al. [127] synthesized graphene nanosheets decorated with ultra-small Fe₃ O₄ nanoparticles (USIO/G). Seo et al. [134] reported an integrated usage of magnetic particles in microalgal downstream processes, specifically microalgal harvesting and lipid extraction through one-step aerosol spray pyrolysis and applied in microalgal harvesting and serial microalgal lipid entrapment. TEM/EDS, XPS, and FT-IR analysis suggested that the cationic and lipophilic functionalities arose from not fully decomposed PVP, due to the short residence time in the reactor. Kang et al. [135] proposed Fe₃ O₄ nanocrystals confined in mesocellular carbon foam (MSU-F–C) by a “host–guest” approach and applied it as LIBs anode. In this study, a precursor of Fe(NO₃ )₃ ·9H₂ O is impregnated in MSU-F–C having uniform cellular pores with a diamter of ~ 30 nm, followed by heating treatment at 400 °C for 4 h in argon (Ar) atmosphere. Fe₃ O₄ nanocrystals with size of 13–27 nm were fabricated inside the pores of MSU-F–C. The existance of the carbon most likely allows the reduction of some Fe ³⁺ to Fe ²⁺ ions by a carbothermoreduction process. The physical performance and pore structure of MSU-F–C and Fe₃ O₄ -loaded composites were characterized with nitrogen sorption, and the composites have high capacities of ∼800–1000 mA h g ⁻¹ at 0.1 A g ⁻¹ (∼0.1 C rate), high rate capability and good cycling performance.

Application

Fe₃ O₄ possesses lots of unique properties, and is highly promising for applications in LIBs and supercapacitors [136,137,138,139,140,141,142]. Table 4 summarizes some applications.

Li-Ion Batteries

Due to conversion reaction of Fe₃ O₄ during charge/discharge process and other advantages, the Fe₃ O₄ is usually studied and applied as LIBs anode [143,144,145,146,147]. For TMOs, they have higher theoretical capacity (~ 500–1000 mA h g ⁻¹ ) than conventional graphite (about 372 mA h g ⁻¹ ). Furthermore, Fe₃ O₄ has superior conductivity compared with other transition metal oxides. Thus, it is well-focused by recent studies. It has been reported that composite electrodes with graphene have high performance due to their large surface area, high electrical conductivity and adaptive or flexible structure for high reliability. Qiu et al. [3] reported a kind of composite anode composed of ultra-dispersed Fe₃ O₄ nanoparticles (3–8 nm) and RGO sheet. It has excellent cyclic performance (624 mA h g ⁻¹ for up to 50 charge/discharge cycles at a current density of 0.1 A g ⁻¹ ), and good specific capability (624 and 415 mAh g ⁻¹ at 0.1 and 2.4 A g ⁻¹ , respectively) for LIBs. The obtained Fe₃ O₄ /RGO exhibited high and ultrastable Photo-Fenton activity (Fig. 11).

Reprinted with Permission from [3]. Copyright, Elsevier B.V

The charge/discharge curve of Fe₃ O₄ /RGO composites (a ) and mechanically mixed Fe₃ O₄ /RGO composites (M-Fe₃ O₄ /RGO) (b ) electrodes at constant current densities of 0.1 A g ⁻¹ . Cycling performance of Fe₃ O₄ /RGO composites and M-Fe₃ O₄ /RGO composites electrode at constant current densities of 0.1 A g ⁻¹ (c ). Rate capability of Fe₃ O_4/ RGO composites and physically mixed Fe₃ O₄ /RGO composites at the current densities between 0.1 A g ⁻¹ and 2.4 A g ⁻¹ (d ). Nyquist plots of the electrodes of Fe₃ O₄ /RGO sheet and mechanically mixed Fe₃ O₄ /RGO composites. All of the measurements were conducted using a voltage window of 0.01–3.0 V (e ). Schematic representation of the electrochemical reaction path on the Fe₃ O₄ /RGO composites (f ).

Pyrolyzed carbon is also a good “companion” for Fe₃ O₄ anodes. Apart from the facile protocol, porous structure formed by pyrolysis always exhibits high specific capacity of Fe₃ O₄ composite anode. Wang et al. [12] reported hollow N-doped Fe₃ O₄ /C nanocages with hierarchical porosities by carbonizing polydopamine-coated PB nanocrystals as LIBs anode (Fig. 12). The specific capacity of N-doped Fe₃ O₄ /C nanocages is ~ 878.7 mA h g ⁻¹ after 200 cycles at a current density of 200 mA g ⁻¹ , much higher than that of N-doped Fe₃ O₄ /C derived from pure PB (merely 547 mA h g ⁻¹ ). It is also desirable to design anisotropic structure of Fe₃ O₄ nanoparticles with carbon coated layer. Zhang et al. [19] reported a kind of carbon-coated Fe₃ O₄ nanospindles derived from α-Fe₂ O₃ nanospindles with length of about 500 nm and an axis ratio of ~ 4. Following by a hydrothermal synthesis method with glucose, the obtained LIBs anode delivered a high reversible capacity of ~ 745 mA h g ⁻¹ at C/5 and ~ 600 mA h g ⁻¹ at C/2.

Reprinted with Permission from [12], Copyright, Elsevier Ltd

아 CVs of the HPHNF during the first three cycles at 0.2 mV s ⁻¹ , b Galvanostatic charge/discharge profiles of the HPHNF electrodes for the 1st, 50th, 100th, 150th and 200th cycle at a specific current of 200 mA g ⁻¹ . c Cycling performance of the HPHNF nanocomposites, N-doped Fe₃ O₄ /C nanocomposites and graphite at a specific current of 200 mA g ⁻¹ . d Coulombic efficiency of HPHNF.

The most impressive work towards this field is probably the mesoporous iron oxide nanoparticle clusters with carbon coating reported by Lee et al. [148]. After a few cycles, the formation of SEI greatly enhanced the stability of interface between electrode and electrolyte. Electrochemical test exhibited a high specific capacity of 970 mA h g ⁻¹ for LIBs.

Supercapacitors

Fe₃ O₄ is a highly promising candidate for supercapacitor electrode because of its relatively high electrical conductivity, fast reversible redox reaction, low cost and eco-friendly nature [149,150,151,152]. Similar to batteries, high performance supercapacitors also require two factors:large specific surface area and long-term stability. Those two features usually were achieved by building some porous structures and carbon coated layers. Fe₃ O₄ nanoparticle with a high specific surface area was synthesized by Wang et al. [126] using a bottom up approach. Ferric chloride was firstly sonicated with ethanolamine and then processed through a solvothermal reaction. The obtained active nanomaterials showed a specific surface area of 165.05 m ² g ⁻¹ and a specific capacitance of 207.7 F g ⁻¹ at 0.4 A g ⁻¹ .

Also, highly dispersed Fe₃ O₄ nanosheets on 1D CNFs is reported by Mu et al. [125]. The Fe₃ O₄ /CNFs composites showed a higher specific capacitance than pure Fe₃ O₄ in 1 M Na₂ SO₃ . To further enlarge the specific capacitance and cycle stability, hierarchically porous carbon spheres with Fe₃ O₄ using as supercapacitors exhibited high capacitivity of 1153 F g ⁻¹ at 2 A g ⁻¹ and high specific capacitance of 514 F g ⁻¹ at 100 A g ⁻¹ . In addition, the assembled asymmetric supercapacitor with double-shelled hollow carbon spheres and Fe₃ O₄ , has excellent cycling stability (96.7% retention after 8000 cycles) and high energy density (17–45 Wh kg ⁻¹ ) at a power density of 400–8000 W kg ⁻¹ [4].