Zinc ferrite composite material with controllable morphology and its applications

Materials Science & Engineering B 224 (2017) 125–138

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Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb

Zinc ferrite composite material with controllable morphology and its applications ⁎

Ming Qina, Qin Shuaia, , Guanglei Wuc, Bohan Zhengd, Zhengdong Wange, Hongjing Wub,

MARK ⁎

a

Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, PR China c Institute of Materials for Energy and Environment, State Key Laboratory Breeding Base of New Fiber Materials and Modern Textile, College of Materials Science and Engineering, Qingdao University, Qingdao 266071, PR China d School of Materials Science and Engineering, Chang’an University, Xi’an 710064, PR China e Center of Nanomaterials for Renewable Energy (CNRE), State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Zinc ferrite Chemical synthesis Modification Application

ZnFe2O4 is an attractive material due to its unique properties and various applications. A large volume of works on the synthesis of ZnFe2O4 have been reported such as mechanochemical synthesis, co-precipitation method, sol-gel auto-combustion method, electrospinning method, hydrothermal/solvothermal method, and spray drying. The synthetic methodologies have significant influence on the properties, morphologies and structures of the ZnFe2O4. Moreover, with the Fe3+ and Zn2+ substituted by different metal ions, the properties of ZnFe2O4 can be altered. Besides, the zinc ferrite composites with controlled morphologies and structures have been investigated. Owing to the enhanced magnetic, electrical and catalytic properties of the zinc ferrite composites, extensive applications of zinc ferrite composites have been achieved. In this review, we summarized the synthetic methods as well as the modifications of the ZnFe2O4. The review also dealt with applications of the ZnFe2O4 and its composites in the fields of sensors, photocatalysis and lithium ion batteries, etc.

1. Introduction In recent years, zinc ferrite has been widely investigated due to its unique magnetic [1], electrical properties [2], microwave absorption [3] and photocatalytic properties [4]. However, with the improvement of synthetic method and the combination of novel functional materials, zinc ferrite with controllable morphology and structure or the zinc ferrite composite can be produced which dramatically improve the application of the zinc ferrite materials. Zinc ferrite is a traditional spinel ferrite and the structure is shown in Fig. 1. Zn2+ ions occupy in the tetrahedral A-sites whereas the Fe3+ ions occupy in the octahedral B-sites, thus the zinc ferrite is a normal spinel structure. In the normal spinel structure, the magnetic moment is arranged in the opposite direction and the size is equal, so that the magnetic moment of the ferrite is counteracted and the magnetism is not displayed. When the Zn2+ ions partially occupy in octahedral Bsites caused by the changing of the temperature the intermediate spinel ferrite is obtained and exhibits ferrimagnetism. The variation in properties of ZnFe2O4 is not only influenced by size effect but also due to the incorporating of the foreign ions and novel

⁎

functional materials. When modifying the zinc ferrite with the introduction of metallic ions, the ion dimension, site preference of the substituent ion, extent of substitution and synthetic method, etc. have a significant influence on the distribution of Zn2+, Fe3+ and the foreign cation over the tetrahedral and octahedral sites, which results in the variation of the magnetic and electrical properties. Moreover, large volume of works on the ZnFe2O4 composites has been reported due to the enhanced performance on the different applications. Considering both of the types of the composite materials and the morphology and structure affect the properties of zinc ferrite composites, the ZnFe2O4 composites are briefly concluded based on the types of the composite materials in this review. The major purpose of this review is to summarize the synthesis, modification and applications of ZnFe2O4 in recent years. 2. Synthetic methods and their influence on properties of zinc ferrite The properties of the materials mainly depend on the different synthetic methods, so it is necessary for us to discuss the ways to

Corresponding authors. E-mail addresses: [email protected] (Q. Shuai), [email protected] (H. Wu).

http://dx.doi.org/10.1016/j.mseb.2017.07.016 Received 14 May 2017; Received in revised form 14 July 2017; Accepted 26 July 2017 0921-5107/ © 2017 Elsevier B.V. All rights reserved.

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Fig. 1. The crystal structure of zinc ferrite in which the

represent the Zn2+, the

represent the Fe3+ and the

represent the O2−.

Table 1 The synthetic methods for different morphologies of ZnFe2O4 and its composites and their application fields. Materials

Precursors

Synthetic method

Morphologies

Application

Ref.

ZnFe2O4 ZnFe2O4

Zn(CH3COO)2·2H2O, Fe(NO3)3·9H2O, PVP FeCl3·6H2O, ZnCl2, NH4·Ac, EG

Electrospinning Solvothermal

Anode materials for LIBs Microwave absorber

[14] [32]

ZnFe2O4 ZnFe2O4 ZnFe2O4 Co3O4/ZnFe2O4

n-pentanol, Zn(NO3)2, H2C2O4, TAB, cyclohexane, FeSO4 Zn(NO3)2·6H2O, Fe(NO3)3·9H2O, dextrin ZnSO4·7H2O, FeSO4·7H2O, EG, oxalic acid Zn(NO3)2·6H2O, Fe(acac)3, DMF, H2BDC, Co3O4 nanocages Zn(CH3COO)2·2H2O, FeSO4, Na3C6H5O7·2H2O, ammonia

Microemulsion Spray drying Spray drying Solvothermal

Nanofibers/nanorods Hollow nanospheres/ nanosheets Nanorods Yolk-shell Porous nanorods Hollow Starfish-Shaped

Ethanol sensor Anode materials for LIBs Anode materials for LIBs Electrode material for supercapacitor Gas sensors

[33] [35] [36] [51]

Cyclohexane, EG, CTAB, oxalic acid,ZnSO4·7H2O, FeSO4·7H2O Zn(NO3)2·6H2O, Fe(NO3)3·9H2O, CH3COONa·3H2O, glucose, EG, AgNO3, Na2HPO4·12H2O, ethanol Zn(NO3)2·6H2O, Fe(acac)3, PVP, H2BDC, DMF

In situ graft copolymerization Solvothermal and in situ precipitation One-step carbonization of Zn/Fe MOFs Solvothermal

Anode materials for LIBs

[55]

Photocatalyst for 2,4DCP degradation Anode materials for LIBs

[62] [63]

Acetone sensors

[67]

Photo-Fenton degradation of dyes

[71]

ZnO/ZnFe2O4

ZnFe2O4@C CMSs@ZnFe24/ Ag3PO4 ZnO/ZnFe2O4@ carbon ZnFe2O4 ZnFe2O4

Zn(NO3)2·6H2O, Fe(NO3)3·9H2O, urea, glycerol, isopropanol Zn(NO3)2·6H2O, Fe(NO3)3·9H2O/(NH4)2Fe(SO4)2, ZnSO4, H2C2O4/FeCl2·4H2O, ZnCl2, urea, C6H8O6/Zn (NO3)2·6H2O, Fe(NO3)3·9H2O, glucose

A mild two-step method

Hydrothermal

Inner hollow microspheres surrounded by outer nanosheets 3-Dimensional cuboid nanowhiskers Core-shell Hierarchical ball-in-ball nanospheres Hierarchical yolk-shell microspheres Nanoparticles/nanorods/nanoflowers/hollow microspheres

[52]

thiourea fuel additive at 1:1.2 and 1:2 molar ratios. Yadav et al. [6] prepared the ZnFe2O4 nanoparticles by sol-gel autocombustion method with the starch as the fuel. A further annealing process at temperature of 400, 600, and 800 °C was displayed to obtain the ZnFe2O4 nanoparticles. The result depicted the crystallite size and lattice parameter increased with a higher annealing temperature. The variation of the annealing temperature also resulted in the change of dielectric and electrical properties. Sutka et al. [7] synthesized ZnFe2O4 at three different combustion conditions, (i) xerogel in chamotte crucible and muffle oven; (ii) xerogel in chamotte crucible and open air; (iii) xerogel in the form of a thick layer (1 mm) in open air. The pure ZnFe2O4 was obtained only under the third condition due to the appropriate oxygen content inside the reaction environment. Despite the advantages discussed above, there still some drawbacks exist such as the xerogel fabricated by the heating of the solution is easy to crack and the agglomeration of the zinc ferrite powders during the synthetic process.

synthesize ZnFe2O4. ZnFe2O4 can be synthesized by various synthetic methods such as mechanochemical synthesis, co-precipitation method, sol-gel auto-combustion method, electrospinning method, hydrothermal/solvothermal method, and spray drying. And the methods for the fabrication of ZnFe2O4 will be briefly introduced in Table 1.

2.1. Sol-gel auto-combustion method Sol-gel auto-combustion method was briefly concluded as the following steps: metal nitrate and fuel mixed in solution to form metal complexes; solution containing the metal complexes forming xerogel by sol-gel process; the combustion of xerogel in the air. Compared to the traditional sol-gel method, the method is a combination of the sol-gel method and self-propagating synthesis, which makes it own advantages such as controlling the purity and particle size of the target product due to the heat released by the oxidation of organic complexing agents. The types of complexing agent, pH, annealing time, and temperature will affect the yield of the final product. Mahmoudzadeh et al. [5] investigated the different kinds of fuel additives at different molar ratios to the influence of the zinc ferrite. Glycine, urea and thiourea were chosen as the fuel additives at three different molar ratios to metal nitrates of 1.2:1, 2:1, and 3:1. The result demonstrated that the fuel additives and its molar ratios to metal nitrates affected not only the microstructure but also the crystallite size of the ZnFe2O4. The pure ZnFe2O4 was obtained at the condition of

2.2. Co-precipitation method Co-precipitation method is a widely used wet chemical synthesis method. The mixed of each component can be carried out at the molecular or atomic level that means we can control the doping content precisely. The nanopowders produced by the method have several 126

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extended from the tip of the cone. In this way, the polymer filaments with nanometer diameter can be produced. Ponhan et al. [13] prepared ZnFe2O4 nanofibres by electrospinning method using polyvinyl pyrrolidone (PVP) and Zn and Fe nitrates as the precursor solution. A further heat treatment of 500, 600 and 700 °C to the as-prepared ZnFe2O4/PVP composite nanofibres was carried out to obtain the pure ZnFe2O4 nanofibres. The crystallite size and lattice parameter increased with the increment of the calcination temperature. The variation in calcination temperature resulted in the different magnetic properties, e.g., the samples calcined at 500 and 600 °C presented a superparamagnetic behavior but the paramagnetic behavior of the ZnFe2O4 samples was observed at a calcination temperature of 700 °C. Teh et al. [14] synthesized ZnFe2O4 nanofibers and nanorods through electrospinning method. The ZnFe2O4 nanofibers and nanorods were produced by adjusting the content of PVP and the nanofibers showed a better electrochemical performance than that of nanorods. Problems such as low production efficiency, unstable consistency of ZnFe2O4 nanofibers are also needed to be solved.

advantages such as the small particle size, high purity, and simple preparation process. A typical reaction progress is performed as following steps: metal salts are taken in stoichiometric amounts and are dissolved in solvent followed by the addition of a precipitating agent to adjust the pH and a solid product is obtained and then the further heat treatment helps to forming the crystal structure. The concentration of the solution, the temperature, and the pH are the key factors of the formation of the final particles. Yang et al. [8] fabricated ZnFe2O4 nanoparticles by alcohol-water co-precipitation method. Several variables in the experiment such as the crystallizing temperature, proportion of alcohol to water were investigated. The result showed that the crystallite size changing with the variation of the proportion of alcohol to water and the minimum diameter of the sample was obtained with a proportion of alcohol to water of 2:6. The author also studied the relationship between crystallizing temperature and the grain size. Lee et al. [9] prepared ZnFe2O4 nanoparticles by co-precipitation method at different pH values varying from 3 to 12. The sodium hydroxide solution was used to adjust the pH of the solution. The XRD pattern depicted that the pure ZnFe2O4 nanoparticles was obtained with a pH of 8–12. Familiar work but more accurately controlling of the pH was carried out by Lee et al. [10]. The sodium hydroxide solution was replaced by the aqueous buffer solutions whose pH ranged from 6 to 12. With the help of the buffer solutions, the author discovered that the pure ZnFe2O4 nanoparticles were obtained with a pH of 7–12. Although the co-precipitation method can be carried out at a low temperature with simple process, the co-precipitation is difficult to wash and filter, more importantly, the agglomeration of the zinc ferrite powders during the synthetic process result in the formation of nonuniform zinc ferrite particles.

2.5. Hydrothermal/solvothermal method Hydrothermal method is a chemical reaction in a sealed container with water as a solvent at high temperature and high pressure. Hydrothermal method can provide a special physical and chemical environment which cannot be obtained under the condition of atmospheric pressure. Similarly, in a solvothermal synthesis process the organic compounds are used as a solvent, using the principle of hydrothermal synthesis to gain the nanocomposite oxide material. Reactant concentration, pH value and hydrothermal time are the critical factors for the formation of the final product [15–30]. Rameshbabu et al. [31] used a typical hydrothermal method for the formation of the ZnFe2O4 with the help of the surfactant ethylamine. The precursor was calcined at different temperature of 300 and 600 °C for 10 h to produce the ZnFe2O4 nanoparticles. The crystallite sizes increased from 21 to 28 nm with a higher annealing temperature. The saturation magnetization decreased from 12.9 to 9.10 emu/g with the increment of the annealing temperature was also observed. ZnFe2O4 hollow nanospheres and nanosheets were synthesized through solvothermal synthesis by Yan et al. [32]. ZnFe2O4 hollow nanospheres was prepared by adding the NH4·Ac as protective agent whereas the ZnFe2O4 nanosheets was produced with the addition of the K·Ac as protective agent. The influence of the reaction time, temperature and reactant concentration to the final product was systematically discussed. The magnetic properties study showed that the saturation magnetization and coercivity value of Zn ferrite nanosheets was much smaller than that of ZnFe2O4 hollow nanospheres. The long reaction time limits the application of this method. Besides, the invisible reaction process only allows the researchers to adjust the reaction parameters after the characterizations of the zinc ferrite.

2.3. Mechanochemical synthesis method The mechanochemical synthesis method is a method for preparing nano-compound by the interaction between different metal elements or metal oxides by high-energy ball milling. High yield and simple synthetic process can be achieved by the method. The formation of the final products is in correlation with several variations such as the ball milling temperature, time and ball-to-powder mass ratio. Marinca et al. [11] synthesized ZnFe2O4 nanoparticles via reactive milling at different reaction times of 4, 8, 12 and 16 h. The author found that the ZnFe2O4 nanoparticles was obtained after 8 h of milling, crystallite size decreased with the increment of the milling time but lattice parameter exhibited opposite trend, i.e., the lattice parameter increased with a longer milling time. The magnetization studies reflected the saturation magnetization was also changed with the variation of the milling time. The influence of the annealing temperature to the properties of ZnFe2O4 nanoparticles was studied by Mozaffari and Masoudi [12]. The ZnFe2O4 was synthesized and then annealing at different temperature of 150, 300, and 800 °C for 5 h in air. The crystallite sizes increased from 13 to 29 nm with a higher annealing temperature. The magnetization was also changed due to the cation distribution changing in octahedral B-site and tetrahedral A-site of ZnFe2O4 nanoparticles caused by the different annealing temperature. However, this method for the formation of zinc ferrite suffers from the problems that long reaction time is required and impurities are easy to introduce into the zinc ferrite. Besides, the long reaction time also results in the large energy consumption.

2.6. Microemulsion method Microemulsion method is the process which uses two mutually insoluble solvent to form a uniform emulsion with the help of surfactant, then the solid phase precipitation from the emulsion to form the nanopowder with a certain particle size. Zhu et al. [33] produced ZnFe2O4 nanorods using cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/n-pentanol and oxalic acid microemulsion. The ZnFe2O4 nanorods was obtained with a calcination of the precursor ZnFe2(C2O4)3 at 500 °C for 3 h. The porous ZnFe2O4 nanorods was applied to the ethanol sensor and showed an excellent sensing performance. Ahn and Choi [34] prepared ZnFe2O4 nanoparticle by microemulsion method. The aerosol OT/water/iso-octane inverse microemulsion was used to obtain the ZnFe2O4 nanoparticle. The sample with a lattice

2.4. Electrospinning method Electrospinning is a kind of special fiber manufacturing process, polymer solution or melt jet spinning in a strong electric field. Under the action of the electric field, the liquid droplets in the syringe needle will be changed from spherical to conical, and the filaments can be 127

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constant of 8.43 Å and average particle size of 8.4 nm was concluded from the XRD pattern. The maximum saturation magnetization of 54 emu/g was observed at 5 K. Mössbauer hyperfine spectra study demonstrated the superexchange interaction between iron ions in tetrahedral and octahedral sites. Superparamagnetic behavior of the sample was observed. The emulgator used in the formation of zinc ferrite is difficult to handle, it is also a threat to the environment.

impurity phase was formed. The particles size increased with increasing the Bi amount and could be observed from Fig. 2. The decrement of the saturation magnetization was also observed due to the replacement of the non-magnetic Bi3+ ion to the Fe3+ ion. Plenty of works have been done to search the transition element substitution for Fe to the influence of the ZnFe2O4. The phenomenon that the facile incorporated of the Bi ion into the lattice of ZnFe2O4 was due to the comparable size of the ion to the Fe. ZnFe2-xCrxO4 (0 ≤ x ≤ 2) was obtained through the sol-gel autocombustion method by Borhan et al. [38]. They found that with the increment of the Cr ion, crystallite size and lattice constant decreased. At the same time, saturation magnetization decreased with a higher ratio of the Cr ion was owing to the Cr3+ paramagnetic ion preferentially occupied octahedral B sites in which the magnetic moment of the sublattice suffered a decrement. Abu-Dief et al. [39] synthesized ZnFe2-xCrxO4 (0 ≤ x ≤ 0.3) using solvothermal method. Surprisingly, different from former result, they discovered that the lattice parameter had no dependence on Cr doping amount. The size of the samples decreased with increasing of Cr-doping content, at the same time, the increase in anisotropy, the decrease in coercivity and saturation magnetization was also observed. ZnFe2-xMoxO4 (0 ≤ x ≤ 0.3) was synthesized using sol-gel autocombustion method by Heiba et al. [40]. The XRD characterization confirmed the single-phase cubic structure of the samples and the fact that the lattice parameter of pure ZnFe2O4 with 8.4476 Å decreased with the addition of the Mo. The decrement of the saturation magnetization was also observed due to the replacement of the non-magnetic Mo3+ ion to the Fe3+ ion. Rare earth elements have good paramagnetic properties, the magnetic moment is less affected by the environment, and so it can further improve the magnetic properties of ferrite materials and improve the stability of the material. Due to the large radius of rare earth elements, the substitution takes place only in the octahedral sites and thus changing the saturation magnetization and increasing the crystallite size. Shinde et al. [41] synthesized ZnNdxFe2-xO4 (0 ≤ x ≤ 0.03) using co-precipitation method and studied the influence of the Nd3+ substitution. The decrease of lattice parameter with the increase of the Nd3+ was observed not owing to the Nd3+ entering into the lattice but the formation of isolating ultra-thin layer. The resistivity increased due to the Nd3+ substitution for the octahedral (B) sites Fe3+ which decreased the electrons hopping between the Fe3+ and Fe2+. The dielectric loss also decreased with the increase of the Nd3+ ions. Rahimi-Nasrabadi et al. [42] synthesized ZnFe2O4 using the sol-gel method with the help of different kinds of surfactants and the La3+ doped ZnFe2O4 in which the content of La was 0.05. They discovered that the size and the structure of ZnFe2O4 varied with the changing of the surfactant. Furthermore, with the incorporation of the La3+, the lattice parameter decreased from 8.443 to 5.5203 Å and the saturation magnetization also decreased at the same time. Masoudpanah et al. [43] also investigated the La substitution to the influence of the structure and magnetic properties of ZnFe2O4. ZnFe2-xLaxO4 (0 ≤ x ≤ 0.2) was synthesized by sol-gel auto-combustion method. The crystallite size decreased from 21 to 10 nm with increase of La3+ substitution and the lattice parameter increased from 0.8441 to 0.8532 nm with the La3+ additions due to the larger ionic radii of the La3+ ions. With the increase of the La, the saturation magnetization of the ZnFe2xLaxO4 firstly increase and then decrease and reached to the maximum of 7.6 emu/g when x = 0.05.

2.7. Spray drying The basic principle of the spray drying method is to spray the liquid into very fine droplets through the atomizer. The heat exchange and material exchange are carried out on the basis of the uniform mixing of the drying medium and the droplet, so that the solvent is vaporized or the molten material is solidified. A uniform size and high purity of yield can be obtained. Won et al. [35] fabricated yolk-shell structured ZnFe2O4 powders by spray drying method. The spray-drying process was carried out with the reactant of Zn and Fe nitrates and dextrin. The precursor was calcined at 350, 400, and 450 °C for 3 h to gain the final product. The samples were used as the electrode materials for the lithium-ion batteries. Among them, the ZnFe2O4 powders calcined at 450 °C exhibited higher electrochemical performance than others. Porous ZnFe2O4 nanorods were synthesized by Mao et al. [36]. The spray drying process was carried out with the reactant of Zn and Fe sulfate and oxalic acid, ethylene glycol and cyclohexane. The precursor was calcined at 450 °C for 5 h. ZnFe2O4 microrods with an average diameter of 0.5–1 μm was obtained and an excellent electrochemical performance as the electrode materials of lithium-ion battery was also observed. The cost of the spray drying device is relatively high and the low thermal efficiency during the synthetic process also restricts its application. Furthermore, the different synthetic conditions of zinc ferrite are summarized in Table 2. 3. The different modification technologies to the influence on the properties of zinc ferrite Compared to the pure zinc ferrite material, the modified ZnFe2O4 possesses more interesting properties. Nowadays, enormous works have been reported about the modified ZnFe2O4 material. Among them, most of the materials are modified by ion substitution and forming the composite with other substances. The specific contents will be discussed below. 3.1. Ion substitution Different types of ions can incorporate into the lattice of the ZnFe2O4. Owing to the variation properties of substituted ions such as the ionic radius, and magnetism, the ZnFe2O4 with different sizes, saturation magnetization can be prepared. Here, we will discuss the influence of ion substitution to the properties of zinc ferrite. 3.1.1. Ion substitution for the Fe Fe3+ is a kind of ion with magnetic moment of 5 μB. In a spinel structure of ZnFe2O4, the Fe3+ prefers to occupy in the octahedral Bsites. Once the substituted ions replace the Fe3+ in octahedral B-sites, owing to the decrease of the magnetic moment, most of the materials will suffer a decrease in the saturation magnetization. The effect of Bi doping on the structural and magnetic properties of ZnFe2O4 was investigated by Shoushtari et al. [37]. ZnFe2-xBixO4 (0 ≤ x ≤ 0.15) was formed and with the increase of the Bi the lattice constant increased because of the larger radius of Bi cation (1.03 Å) than the iron cation (0.67 Å). The XRD showed that when the concentration of Bi reached to a critical value, e.g., the x > 0.15, the

3.1.2. Ion substitution for the Zn In a spinel structure of ZnFe2O4, the non-magnetic Zn2+ prefers to occupy in the tetrahedral A sites. With the addition of the foreign ion, the cation distribution between tetrahedral (A) and octahedral (B) will be changed which leads to the variation of structural, electrical and magnetic properties. 128

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Table 2 The different synthetic conditions of zinc ferrite. Synthetic method

Reactant proportion

Additive

Influence factors

Calcining process

Product Size

Refs.

Sol-gel auto-combustion method

Fe(NO3)3·9H2O; Zn (NO3)2·4H2O 2:1 Fe(NO3)3·9H2O; Zn (NO3)2·4H2O 2:1 Fe(NO3)3·9H2O; Zn (NO3)2·4H2O 2:1

Glycine/urea/thiourea as fuels

pH; additive species and ratio

500 °C for 2 h with heating rate of 2 °C/min

11–38 nm

[5]

Starch as fuels

pH; additive species; calcination temperature

400, 600 and 800 °C for 2 h

4.8–12 nm

[6]

Citric acid as fuels

pH; external conditions of the auto-combustion reaction

300 °C for 3 h

∼20 nm

[7]

ZnCl2; FeCl3·6H2O 1:2 ZnCl2; FeCl3·6H2O ZnCl2 1.4 g; FeCl3·6H2O 5.6 g

Alcohol

pH; ratio of alcohol and water; calcination conditions pH pH

500–900 °C for 2 h

63.1–110.8 nm

[8]

650 °C for 6 h 650 °C for 6 h

– –

[9] [10]

ZnO; α-Fe2O3 1:2

–

350 °C for 4 h

20 ± 2 nm

[11]

Metallic Zn and Fe powders 1:2

Double distillated water

Milling time; calcining process Milling time; calcination temperature

150, 300, and 800 °C for 5 h in air with a heating rate of 10 °C/min

13, 15, and 29 nm

[12]

Fe(NO3)3·9H2O 0.02 mol; Zn (NO3)2·4H2O 0.01 mol Zn(CH3COO)2·2H2O; Fe (NO3)3·9H2O 1:2

DMF; PVP; acetic acid; ethanol

Calcination temperature

19, 20 and 26 nm

[13]

DMF; PVP; acetic acid; ethanol

Ratio of PVP versus metallic precursor

500, 600 and 700 °C for 2 h in air with a heating rate of 5 °C/min 450 °C for 2 h with a heating rate of 0.5 °C/min

11(3) nm

[14]

Fe(NO3)3·9H2O; Zn (NO3)2·4H2O 2:1 ZnCl2 1 mmol; FeCl3·6H2O 2 mmol

Ethylamine as the surfactant

Surfactant; calcination temperature

300 °C and 600 °C for 10 h

21 nm, 28 nm

[31]

Ethylene glycol; NH4·Ac; KAc

Reaction time; reaction temperature; protective agent species

–

90 nm; 250 nm

[32]

Zn(NO3)2 0.05 mol/L; FeSO4 0.1 mol/L Metallic salts Fe(III)/Zn(II) 2:1

CTAB; oxalic acid; cyclohexane; n-pentanol Aerosol OT; Na2CO3; iso-octane

Solvent species; calcining process Solvent species; calcining process

500 °C for 3 h

5–10 nm

[33]

About 330 °C

8.4 nm, 8.8 nm

[34]

Fe(NO3)3·9H2O; Zn (NO3)2·4H2O Total of 0.1 mol/L ZnSO4·7H2O (5.75 g); FeSO4·7H2O (11.05 g)

Dextrin

Addition of dextrin; calcination temperature

Between 350 and 450 °C for 3h

–

[35]

Oxalic acid; ethylene glycol; cyclohexane

Calcining process

In argon atmosphere for 5 h at 450 °C

–

[36]

Co-precipitation method

Mechanochemical synthesis method

Electrospinning method

Hydrothermal/ solvothermal method

Microemulsion method

Spray drying

– –

cation A. However, all the particle sizes were decreased compared to the pure ZnFe2O4 in which the particle size of the Cu0.2Zn0.8Fe2O4 was decreased to 12.3 nm which reached to the minimum among the materials. Khan et al. [48] used the rare earth element Tb3+ to substitute the 2+ Zn ions and discussed the magnetic properties. With the increase of the Tb3+ the lattice parameter also increased, this was owing to the larger ionic radii of Tb3+. The dielectric constant, dielectric loss, coercivity, and the saturation magnetization were all decreased with the substitution of terbium.

Wiriya et al. [44] synthesized Zn1-xMnxFe2O4 nanoparticles (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) by hydrothermal method and studied the magnetic properties. The crystallite sizes were decreased with increasing Mn content and the lattice parameters were increased which was due to the replacement of Zn2+ (r = 0.740 Å) by a larger ionic radius of Mn2+ (r = 0.830 Å). The coercivity and the magnetization of Zn1-xMnxFe2O4 nanoparticles was decreased with increasing Mn content but the maximum value of magnetization was with x = 0.6 due to the existence of the maximal net magnetic moment. Kurian et al. [45] fabricated MnxZn1−xFe2O4 (x = 0.0, 0.25, 0.5, 0.75 and 1.0) by sol gel method. With the addition of the Mn, the lattice parameter decreased constantly due to the smaller ionic radius of the Mn. Zn1-xCuxFe2O4 (x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) were synthesized by Manikandan et al. [46]. The lattice parameter decreased with the increment of the Cu which was caused by the smaller ionic radius of the substituted Cu. However, the increase of particle sizes with a higher Cu content was due to the coalescence and agglomeration and the ion distribution in the tetrahedral (A) and octahedral (B) site. The saturation magnetization increased from 1.638 to 58.58 emu/g and Zn1xCuxFe2O4 become ferromagnetic with x ≥ 0.2. A series of A0.2Zn0.8Fe2O4 ferrites with A = Co, Ni and Cu were prepared through co-precipitation by Boudjemaa et al. [47]. The lattice parameter was found not directly correlated with the ionic radius of

3.2. Nanocomposites Nanocomposites refer to two or more than two kinds of solid phase at least one dimension of the nanoscale composite material. The introduction of nano-phase can greatly improve the physical and chemical structure of the matrix, thus greatly changing the various functional properties of nanocomposite materials, so that the thermal, optical, electrical, magnetic and other properties are optimized. 3.2.1. Metal oxide The combination of ZnFe2O4 and metal oxides has been widely investigated and with the addition of different kinds of metal oxides such as TiO2, ZnO, Co3O4 and Fe2O3, the enhanced magnetic, electrical and 129

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Fig. 2. FESEM images of ZnFe2-xBixO4 and the increased size with the increase of the Bi amount (reprinted with permission from Ref. [37], copyright 2016 Elsevier).

ZnFe2O4. Meanwhile, the paramagnetism, larger coercivity and high squareness the composite behaved proved it a potential application in magnetic devices. Double-shell composites consisting of inner ZnO hollow microspheres surrounded by outer ZnFe2O4 nanosheets were successfully synthesized by Li et al. [52]. The structural characterization of ZnFe2O4-decorated ZnO heterostructures was shown in Fig. 4. The ZnO/ZnFe2O4 heterostructure possessed a higher BET surface of 53.8 m2/g than that of ZnO with 13.7 m2/g. The gas senor behavior over different kinds of volatile organic compound gases was investigated and the composite showed a pleasant response toward acetone. Moreover, the response and recover times of the ZnO/ZnFe2O4 were less than that of only ZnO or ZnFe2O4. The author also discussed the gas-sensing mechanism. With a narrow bandgap (1.9 eV), good chemical stability and suitable band position in which its conduction band edge lies above that of α-Fe2O3, ZnFe2O4 could be an ideal semiconductor to form a heterojunction with α-Fe2O3. Furthermore, the valence band edge of ZnFe2O4 is also at a more negative value than that of α-Fe2O3, hence a type-II band structure will form which could improve the separation of photogenerated electron-hole pairs, leading to increased energy conversion efficiency [53]. Highly-oriented Fe2O3/ZnFe2O4 in 1-D nanocolumnar arrays with different thickness of the ZnFe2O4 overlayer (6, 12 and 18 nm) was prepared by Luo et al. [54]. The photoelectrochemical (PEC) tests were carried out and the Fe2O3/ZnFe2O4 with a ZnFe2O4 overlayer of 12 nm had a better PEC performance over the others. Photoluminescence test proved that the cause of the better PEC performance of the Fe2O3/

photocatalytic properties ZnFe2O4 composites can be produced. Moreira et al. [49] prepared two different types of ZnFe2O4 and TiO2 nanocomposites which was ZnFe2O4/TiO2 (ZnFe2O4 over the TiO2 surface) and TiO2/ZnFe2O4 (TiO2 over the ZnFe2O4 surface). The methyl orange (MO) photocatalytic degradation experiment was carried out under the UV–Vis and the TiO2/ZnFe2O4 showed the best photocatalytic efficiency over any other materials in this experiment. The 10 ppm MO was totally removed in 10 min by TiO2/ZnFe2O4. The photocatalytic experiment under visible light was also implemented and the TiO2/ZnFe2O4 behaved a satisfactory photocatalytic degradation efficiency of 65.1% after 120 min of irradiation. ZnFe2O4/TiO2 nanocrystals were successfully fabricated by a two-step process by Li [50]. Due to the narrower band gap of ZnFe2O4 of 1.86 eV compared to the TiO2 of 3.2 eV, when the visible light worked as incident photon, the ZnFe2O4 was easier excited by the photons to generate electrons and holes, the transfer of electrons form ZnFe2O4 to TiO2 would lead to the effectively separated of electrons and holes thus enhanced the photoresponse of TiO2 nanotube arrays in visible region (see Scheme 1). Starfish-shaped porous Co3O4/ZnFe2O4 hollow nanocomposite was fabricated by Hu et al. [51] for the first time. The morphology and structure of the hollow nanocomposites was studied by TEM and shown in Fig. 3. Although the saturation magnetization reduced from 52.8 emu/g of ZnFe2O4 to 27.5 emu/g of the Co3O4/ZnFe2O4 at 1.8 K that was caused by the decrease of the ZnFe2O4 component and the Co3O4/ZnFe2O4 size effect, the composite exhibited larger coercivity and high squareness than that of ZnFe2O4. The author considered this phenomenon was caused by the underlying surface and interface exchange coupling effects as well as the unique structure of Co3O4/ 130

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Scheme 1. Possible pathway of the photoelectron transfer excited by visible light irradiation for ZnFe2O4/TiO2 (reprinted with permission from Ref. [50], copyright 2011 American Chemical Society).

dispersibility in aqueous solution, and the ability to couple the electroactive particle and unique optical properties, and the synergistic effect is generated by combining with the multifunctional nanoparticles that can make the composite materials with different properties. Wang et al. [56] synthesized N and S co-doped graphene supported hollow ZnFe2O4 nanosphere composites (ZnFe2O4/NSG) and used it as anode materials of the lithium-ion batteries. The synthetic process of the composites was a two-step hydrothermal method and briefly introduced in the Fig. 5. There was no impurity phase in the XRD patterns suggested that the composites remained spinel structure with the incorporation of N, S and graphene. The large BET surface of the composites with 101.6 m2/g was due to the addition of graphene. The electrochemical performance test was also carried out. Reduced graphene oxide-ZnFe2O4 composites (RG/ZF) with different mass ratios of mRG/ZF (0.1, 0.2, 0.4 and 0.6) were prepared through co-precipitation method by Shen et al. [57]. The photocatalytic efficiency toward rhodamine B was investigated and the RG/ZF with a mRG/ZF = 0.4 displayed a better performance over others. Although the saturation magnetization of the RG/ZF decreased due to the addition of the nonmagnetic RGO, the photocatalyst RG/ZF still could be removed

ZnFe2O4 was the facile charge separation resulting from the Fe2O3/ ZnFe2O4 heterojunction. 3.2.2. Carbon material In recent years, carbon materials such as graphene oxide and carbon nano tube (CNT) have been used to incorporate with other materials due to the excellent properties of the carbon materials. Vast works on the combination of the carbon materials and ZnFe2O4 has been reported and is discussed ahead. Qu et al. [55] synthesized 3D cuboid structured ZnFe2O4@C nanowhiskers using in situ graft polymerization method. The mass loss curve of TG testing showed that the content of carbon was around 7.7 wt%. The fact that there was no peak corresponding to carbon on the XRD patterns implied that the carbon was amorphous which was also proved by the adsorption peaks in the Raman spectra. The author considered the better performance of ZnFe2O4@C to be the influence of the unique morphology and nano-structure. Graphene oxide composite materials have been widely researched and applied in recent years, mainly based on the fact that graphene oxide possesses many excellent properties, such as easy synthesis, good

Fig. 3. TEM image of (a) Co3O4/ZnFe2O4 hollow nanocomposites, (b) a single ZnFe2O4 nanotube, (c) High resolution TEM lattice image of a ZnFe2O4 nanoparticle, and (d) SAED patterns of the ZnFe2O4 nanotube (reprinted with permission from Ref. [51], copyright 2015 American Chemical Society).

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Fig. 5. Schematic illustration of the synthesis of NSG nanosheets (a) and ZnFe2O4/NSG composites (b) (reprinted with permission from Ref. [56], copyright 2017 Elsevier).

pot microwave assisted self-assembly method by Mady et al. [59]. Different amount of GO (20, 40, 80 and 100 mg) was used to synthesize the nanocomposites and the photocatalytic activity of the nanocomposites was test on three kinds of organic dye methylene blue (MB), rhodamine B (RhB), and methyl orange (MO). An Ag/ZnO/ZnFe2O4 porous and hollow nanostructure which was marked as p-Ag/ZnO/ZnFe2O4 and h-Ag/ZnO/ZnFe2O4 respectively was prepared by Wu et al. [60]. The h-composite possessed a larger BET surface area of 63.51 m2/g than that of p-composite of 38.29 m2/g, which implied that the h-composite would have a better adsorption performance. Photocatalytic degradation of MB test was carried out and the h-composite revealed a higher degradation efficiency and rate over others. The author also discovered that Ag content had an important influence on the photocatalytic activity. Ag3PO4 turns out to be the highest in quantum efficiency and has been widely used for organic pollutants degradation under visible light irradiation which makes it a promising way to reduce Ag consumption without sacrificing their high photocatalytic performances [61]. Chen et al. [62] synthesized core-shell carbon microspheres (CMSs) @ ZnFe2O4/Ag3PO4 composite and studied the magnetism and photocatalytic activity of the composite. The ratio of carbon microspheres and ZnFe2O4 was constantly 2:1 but with a changing content of Ag3PO4 with 0.75, 3 and 12, respectively. The saturation magnetization of composite with a mass ratio of 2:1:3 decreased to 0.87 emu/g compared to the pure ZnFe2O4 of 4.08 emu/g that was ascribed to the addition of the non-magnetic carbon microspheres and Ag3PO4. The CMSs@ ZnFe2O4/Ag3PO4 with a mass ratio of 2:1:3 possessed a better photocatalytic performance on the decomposition of 2, 4-dichlorophenol compared to composite with other mass ratio. The possible photocatalytic mechanism was discussed and shown in Fig. 6. The photocatalytic mechanism learned from the Fig. 6 could be summarized as follows: first of all, the combination of ZnFe2O4 and Ag3PO4 could separate the photoinduced electrons and holes effectively by the electrons and holes transfer due to the matching CB and VB of ZnFe2O4 and Ag3PO4; secondly, because of the more negative CB position of ZnFe2O4 as well as the more positive VB position of Ag3PO4 than the standard redox potentials, the hydroxyl radical (OH%) could be formed during the electrons and holes transfer process from various sources and enhanced the photocatalytic efficiency significantly; finally, the addition of CMSs

Fig. 4. Structural characterizations of ZnFe2O4-decorated ZnO heterostructures: (a-c) SEM images at different magnification, (d, e) TEM images, (f) HRTEM image, (g) scanning TEM image, and (h, i) elemental mapping images (reprinted with permission from Ref. [52], copyright 2015 American Chemical Society).

with the help of the external magnetic field. Owing to their unique characteristics such as small size, relatively large specific area, hollow and layered structure, CNTs have excellent electrical, mechanical and thermal properties, and can be used as molecular wires, nanometer semiconductor materials, catalyst carrier, molecules absorbent and the near field emission material. Dang et al. [58] prepared CNTs/ZnFe2O4 composites with different CNTs content (0, 0.5, 1, 3, 5 and 100) by hydrothermal. The XRD pattern reflected that the crystalline was not changed with the addition of the CNTs. The photocatalytic H2 production activity was test and the highest H2 evolution rate of 18 μmol·g/h was achieved with the CNTs content of 1 wt%, the photocatalytic stability of the composite was also observed. ZnFe2O4/MWCNTs were prepared through hydrothermal process by Chen et al. A higher photocatalytic activity of the composite was observed, the phenomenon was attributed to the photocatalytic degradation instead of the simple adsorption on the surfaces of the MB. 3.2.3. Multiple composite The incorporation of zinc ferrite with more than one material also has drawn increasing attention. Vast works on the combination of zinc ferrite with metal oxides and carbon materials, silver species and carbon materials, inorganic-organic composite have been reported recently. Ag-ZnFe2O4@r-GO nanocomposites were synthesized through one132

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aniline was prepared by Li et al. [64]. The author found that the electrical conductivity increased with the increment of aniline which was due to the electrical conductivity of the polyaniline. The magnetic properties of the composite with 50% of aniline additive amount were studied and the saturation magnetization decreased constantly with the addition of the TiO2 and polyaniline. For all this, the magnetic photocatalyst still could be removed with the external magnetic field. Another kind of organic-inorganic composite TCPP/ZnFe2O4@ZnO nano hollow sphere composite was synthesized by Rabbani et al. [65]. Similar conclusion that the saturation magnetization decreased from 76.29 emu/g of ZnFe2O4 to 26 emu/g of TCPP/ZnFe2O4@ZnO was caused by the addition of the non-magnetic ZnO and TCPP. Even so, the composites still exhibited high photocatalytic activity over methylene blue and 4-nitrophenol.

Fig. 6. Possible pathway of the photoinduced electron/hole transfer and the formation of free radicals excited by visible light irradiation in the CMSs@ZnFe2O4/Ag3PO4 photocatalytic system (reprinted with permission from Ref. [62], copyright 2015 Elsevier).

4. The application of the ZnFe2O4 and its composites provided the reaction sites for the formation of active free radical, the CMSs itself also worked as the medium of the photoinduced carriers’ migration. Similarly, Chen et al. [63] synthesized hierarchical ball-in-ball ZnO/ ZnFe2O4@carbon nanospheres. The ball-in-ball Zn/Fe-MOFs precursor was prepared and a further heat treatment at 500 °C under different gas atmosphere (N2, air) was displayed to obtain the desired product (500 N) and contrastive product (500 A). The morphology and structure was studied and shown in Fig. 7. Both the composite 500 N and 500 A was used as anode materials for LIBs and 500 N showed a better electrochemical performance over the 500 A and other materials which could be attributed to the special nanosized structure and the high specific surface area. ZnFe2O4/TiO2/polyaniline with different additive amounts of

Due to the superior properties such as the gas-sensitivity, narrow band gap, good cycle stability and large capacitance of the ZnFe2O4 and its composites, the materials have been applied in many fields such as the gas sensor, semiconductor photocatalyst and the cathode materials of the lithium ion battery. Numerous works have been reported on the application of ZnFe2O4 its composites. It will be discussed below. 4.1. Sensor In recent years, gas-sensing properties and electrochemical sensing properties of the ZnFe2O4 nanoparticle has been investigated for many times. The ZnFe2O4 nanoparticle shows excellent selectively and high sensitivity to some gases like acetone, ethanol vapor, etc. The gassensing properties of the ZnFe2O4 nanoparticle can be promoted by Fig. 7. XRD pattern (A), SEM image (B), TEM image (C) and HRTEM (D) of ZnO/ZnFe2O4@carbon calcined under N2 atmosphere (reprinted with permission from Ref. [63], copyright 2016 Elsevier).

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Fig. 8. (a) Response transient of the gas sensor based on porous ZnFe2O4 nanospheres to 30 ppm acetone and ethanol at 200 °C, (b and c) the dynamic response curve and recovery curve of the porous ZnFe2O4 nanospheres to 30 ppm acetone, (d and e) displaying three periods of response-recovery curve to 30 ppm acetone and ethanol, respectively (reprinted with permission from Ref. [66], copyright 2016 Elsevier).

of the external magnet. Different morphologies of ZnFe2O4 were synthesized through hydrothermal method by Dhiman et al. [71]. Zinc ferrite nanoparticles, porous nanorods, nanoflowers and hollow microspheres were synthesized using different precursors. The morphologies of the samples were studied by FESEM and shown in Fig. 9. The photocatalytic efficiency to the Safranine-O and Remazol Brilliant Yellow was investigated and the result showed that the ZnFe2O4 nanorods had a better photocatalytic performance for both of the dyes, and the fact that the photocatalytic activity had a order of ZnFe2O4 porous nanorods > nanoparticles > nanoflowers > hollow microspheres was obtained. Borhan et al. [72] fabricated ZnFe2-xCrxO4 (x = 0, 0.25, 0.5, 1.0, 1.50 and 2.0) with an average particle size varying between 72.2 and 56.5 nm. The photocatalytic activity under UV light was investigated and the composite ZnFe0.5Cr1.5O4 showed better photodegradation efficiency to Orange I which reached to 92.8% in 45 min. The photocatalytic activity under visible light was also tested and the photodegradation efficiency to Orange I was 49% at the same reaction time. Core-shell ZnFe2O4/ZnS nanocomposites having a ferromagnetic behavior with medium magnetization of 35 emu/g was obtained by Yoo et al. [73]. The ZnFe2O4/ZnS nanocomposites possessed better photocatalytic efficiency to MO over others in first 150 min but was exceeded by ZnS after 150 min. The author thought the cause of the phenomenon was the much large surface-to-volume ratio of the small particle size of ZnS. ZnFe2O4 nanoparticles and conjugated polymer (CPVC) from the dehydrochlorination of polyvinyl chloride (PVC), i.e., ZnFe2O4/CPVC was synthesized by Xu et al. [74]. The ZnFe2O4/CPVC had a better photocatalytic activity to the photocatalytic reduction of aqueous Cr (VI) over the ZnFe2O4 nanoparticles, the decrease of Cr (VI) concentration increased from 23.2% to 55.8% with the addition of the CPVC. Yao et al. [75] fabricated ZnFe2O4-C3N4 hybrid and used the composite to investigate the photocatalytic degradation of Orange II under visible light irradiation in which the H2O2 was used as an oxidant. The hybrid with a mass ratio of ZnFe2O4-C3N4 (2:3) had better performance over others, and the initial concentration of H2O2 also had a significant influence to the photocatalytic performance. H2O2 with concentration of 0.1 M showed the best photocatalytic performance and further increase of H2O2 concentration would lead to the decrease of degradation rate which was attribute to the %OH radicals scavenging and the formation of H2O%.

controlling the morphology and structure. Zhou et al. [66] synthesized porous ZnFe2O4 nanospheres and found the ZnFe2O4 nanospheres displayed an excellent selectively to the acetone. The response time to 30 ppm acetone at 200 °C was ∼9 s but with a rather long recovery time of ∼272 s (see Fig. 8). The author also discovered that the ZnFe2O4 nanospheres could response to a lower acetone concentration of 800 ppb which was possibly due to the unique porous structure. Ghosh et al. [67] discussed the ZnFe2O4 particle size to the influence on the H2 and H2S sensing characteristics. Different sizes of ZnFe2O4 particle were obtained through controlling the ball milling time. After 10 h ball milling, ZnFe2O4 particle with size of ∼6.51 nm was obtained and showed the higher gas-sensing properties to H2 and H2S over others. The response to the 200 ppm H2 increased from ∼38% of unmilled sample (14.6 nm) to ∼52% of 10 h ball milling samples, the response to the 200 ppm H2S also increased from ∼75% to ∼82%. Hierarchical ZnFe2O4 yolk-shell microspheres were prepared by Zhou et al. [68] and showed high sensitivity to the acetone. The gas sensing properties study revealed that the material possessed a higher response to the gases at the temperature of 200 °C and even could response to a low acetone concentration of 500 ppb. The study also proved the longterm stability of the ZnFe2O4 yolk-shell microspheres based gas sensor. Core-shell structure polypyrrole-ZnFe2O4 was prepared by Shahnavaz et al. [69]. Cyclic voltammetric study showed that the composite had a better performance to the oxidation of glucose which was due to the addition of the polypyrrole. After the optimization of the sensor, the detection limit was 0.1 mM and a linear response to the glucose whose concentration ranged from 0.1 to 8 mM with sensitivity of 145.36 μA·cm2/mM was achieved. They further synthesized ZnFe2O4/reduced graphene oxide modified glassy carbon electrode [70]. The composite with 30% of graphene showed a better performance to the oxidation of glucose. The amperometric detection implied that the detection limit of 1.2 μM could be reached, beyond that the fact that the sensor displayed an excellent linear response to the glucose whose concentration ranged from 0.1 to 12.5 mM with sensitivity of 1689.6 μA·cm2/mM could be observed. 4.2. Photocatalytic materials Spinel ZnFe2O4 possesses a narrow bandgap of ∼1.9 eV and electronic structure which makes it a promising material in the photocatalytic field. Besides, due to the magnetic property of the ZnFe2O4, the materials can be easily removed from the solution with the addition 134

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Fig. 9. FESEM images of ZnFe2O4 nanostructures: (a) nanoparticles, (b) porous nanorods, (c) nanoflowers, (d) hollow microspheres, and their photocatalytic activities (e) Safranine-O, (f) Remazol Brilliant Yellow (reprinted with permission from Ref. [71], copyright 2016 Elsevier).

capacities were stable at ∼1200 mAh/g after the first discharge and charge process. The rate capacity was 1238 mAh/g with a specific current of 100 mA/g and decreased with the increase of the specific current, surprisingly the rate capacity was increased to ∼1500 mAh/g, which was ascribed to the synergistic effect of the presence of nanosized ZnFe2O4 particles and the continuous carbon network. The data could be learned from Fig. 10.

4.3. Lithium ion batteries Numerous works have been reported on the ZnFe2O4 as electrode materials for the lithium ion batteries (LIB). The excellent electrochemical properties such as the high specific capacity, good cycling performance, good rate capability, and high reversible specific capacity that make it potential electrode materials for the LIBs. Moreover, with the addition of materials which exhibit excellent electrochemical properties the composite displays a better performance. Hollow spherical ZnFe2O4 was prepared through hydrothermal method by Guo et al. [76]. ZnFe2O4 with particles size between 10 and 20 nm was obtained. The reversibly capacity over 900 mAh/g was achieved. With the help of the XRD characterization and SAED patterns, the lithium storage mechanism was discussed and the result showed that 9 lithium ions were stored per formula of ZnFe2O4 in the initial discharge. ZnFe2O4 nanoparticles with different amount of S doping were synthesized by Nie et al. [77]. Electrochemical measurements were carried out and the data showed that the composite exhibited higher discharge capacity after 60 cycles at a current density of 100 mA/g than that of ZnFe2O4 nanoparticles that meant the S doping increased the cycling stability. The rate capacities were also increased and the composite with 1.26 wt% S doping displayed the best properties among the composites. Jiang et al. [78] fabricated ZnFe2O4/carbon nanocomposites in which ZnFe2O4 nanoparticles were encapsulated within the continuous carbon network. The composite with a ZnFe2O4 content of 79.3% exhibited the best performance over others. The discharge and charge

4.4. Other fields Beside the applications discussed above, the ZnFe2O4 can also be used in other fields. ZnFe2O4 is a promising material for supercapacitor due to its low toxicity, high specific surface area, low resistance and fascinating electrochemical behavior. Vadiyar et al. [79] fabricated porous ZnFe2O4 nano-flake and the ZnFe2O4 was used as the electrode material for supercapacitor which showed an excellent performance. ZnFe2O4 can further be used as the microwave absorption material [80–90]. The core-shell ZnFe2O4@SiO2 hollow microspheres/reduced graphene oxides composites were prepared by Zhang et al. [91]. An enhanced microwave absorption property including the maximum reflection loss value and the absorption bandwidth was achieved. Shah et al. [92] investigated the ferromagnetic ZnFe2O4 containing glass ceramics applied for the hyperthermia treatment of cancer. The result demonstrated the bioactive glass ceramics could bond to the living tissues.

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Fig. 10. (a) Cyclic voltammetry profiles of ZnFe2O4/carbon nanocomposites for the first seven cycles between 0.005 and 3 V at a scan rate of 0.1 mV/s, (b) charge/discharge profiles of the ZnFe2O4/C nanocomposites electrode for the first 10 cycles between 0.005 and 3 V at specific current of 100 mA/g, (c) rate capability of ZnFe2O4 (79.3 wt%)/carbon nanocomposites electrode tested under different current rates, (d) cycling performance of ZnFe2O4 (79.3 wt%)/carbon nanocomposites tested at specific current of 200 mA/g (0.2 C) for 430 cycles (reprinted with permission from Ref. [78], copyright 2016 American Chemical Society). Ser. 217 (2010) 012108. [2] A. Sutka, M. Stingaciu, D. Jakovlevs, G. Mezinskis, Comparison of different methods to produce dense zinc ferrite ceramics with high electrical resistance, Ceram. Int. 40 (2014) 2519–2522. [3] S. Tyagi, H.B. Baskey, R.C. Agarwala, V. Agarwala, T.C. Shami, Synthesis and characterization of microwave absorbing SrFe12O19/ZnFe2O4 nanocomposite, Trans. Indian Inst. Metals 64 (2011) 607–614. [4] P.P. Hankare, R.P. Patil, A.V. Jadhav, K.M. Garadkar, R. Sasikala, Enhanced photocatalytic degradation of methyl red and thymol blue using titania-alumina-zinc ferrite nanocomposite, Appl. Catal. B 107 (2011) 333–339. [5] G. Mahmoudzadeh, S.A. Khorrami, S.S. Madani, M. Frounchi, The influence of different fuel additives at different molar ratios on the crystallite phase formation process, structural characteristics and morphology of dispersed zinc ferrite powder by sol-gel auto-combustion, J. Ceram. Process. Res. 13 (2012) 368–372. [6] R.S. Yadav, J. Havlica, J. Masilko, J. Tkacz, I. Kuřitka, J. Vilcakova, Anneal-tuned structural, dielectric and electrical properties of ZnFe2O4 nanoparticles synthesized by starch-assisted sol-gel auto-combustion method, J. Mater. Sci.: Mater. Electron. 27 (2016) 5992–6002. [7] A. Sutka, G. Mezinskis, M. Zamovskis, D. Jakovlevs, I. Pavlovska, Monophasic ZnFe2O4 synthesis from a xerogel layer by auto combustion, Ceram. Int. 39 (2013) 8499–8502. [8] G.Q. Yang, B. Han, Z.T. Sun, L.M. Yan, X.Y. Wang, Preparation and characterization of brown nanometer pigment with spinel structure, Dyes Pigments 55 (2002) 9–16. [9] H. Lee, J.C. Jung, H. Kim, Y. Chung, T.J. Kim, S.J. Lee, S. Oh, Y.S. Kim, I.K. Song, Effect of pH in the preparation of ZnFe2O4 for oxidative dehydrogenation of nbutene to 1,3-butadiene: correlation between catalytic performance and surface acidity of ZnFe2O4, Catal. Commun. 9 (2008) 1137–1142. [10] H. Lee, J.C. Jung, H. Kim, Y. Chung, T.J. Kim, S.J. Lee, S. Oh, Y.S. Kim, I.K. Song, Preparation of ZnFe2O4 catalysts by a co-precipitation method using aqueous buffer solution and their catalytic activity for oxidative dehydrogenation of n-butene to 1, 3-butadiene, Catal. Lett. 122 (2008) 281–286. [11] T. Marinca, I. Chicinas, O. Isnard, V. Pop, Structural and magnetic properties of nanocrystalline ZnFe2O4 powder synthesized by reactive ball milling, Optoelectron. Adv. Mater. 5 (2011) 39–43. [12] M. Mozaffari, H. Masoudi, Zinc ferrite nanoparticles: new preparation method and magnetic properties, J. Supercond. Nov. Magn. 27 (2014) 2563–2567. [13] W. Ponhan, E. Swatsitang, S. Maensiri, Fabrication and magnetic properties of electrospun zinc ferrite (ZnFe2O4) nanofibres, Mater. Sci. Technol. 26 (2010) 1298–1303. [14] P.F. Teh, Y. Sharma, S.S. Pramana, M. Srinivasan, Nanoweb anodes composed of one-dimensional, high aspect ratio, size tunable electrospun ZnFe2O4 nanofibers for lithium ion batteries, J. Mater. Chem. 21 (2011) 14999–15008. [15] G. Wu, Y. Cheng, Y. Ren, Y. Wang, Z. Wang, H. Wu, Synthesis and characterization of γ-Fe2O3@C nanorod-carbon sphere composite and its application as microwave absorbing material, J. Alloys Compd. 654 (2015) 346–350. [16] H. Wu, G. Wu, L. Wang, Peculiar porous α-Fe2O3, γ-Fe2O3 and Fe3O4 nanospheres: facile synthesis of electromagnetic properties, Powder Technol. 269 (2015) 443–451. [17] G. Wu, Y. Cheng, F. Xiang, Z. Jia, Q. Xie, G. Wu, H. Wu, Morphology-controlled synthesis, characterization and microwave absorption properties of nanostructured 3D CeO2, Mater. Sci. Semicon. Process. 41 (2016) 6–11.

5. Conclusions Tremendous research on the ZnFe2O4 has been reported in recent years due to the superior properties of the ZnFe2O4. Based on the different synthetic method, ZnFe2O4 with different morphologies, structures and properties can be synthesized. Furthermore, with the substitution for the Fe and Zn the properties such as lattice parameter, saturation magnetization, may be transformed immensely. Besides, abundant composites of zinc ferrite have been synthesized. The combination of ZnFe2O4 with other materials can obtain the product with enhanced properties. The ZnFe2O4 and its composites are widely used in different fields such as gas sensor, semiconductor photocatalyst, and the cathode materials of the lithium ion battery, etc. Although significant advances have been made in the substitution of Zn2+ or Fe3+ in the zinc ferrite, there still remain some problems. Majority of the researches focus on the point that Zn2+ or Fe3+ replace only by a single element, however, the substitution of Zn2+ or Fe3+ by two or more metal ions has not been reported yet, i.e., materials with formula of Zn1x-yAxByFe2O4 or ZnFe2-x-yAxByO4 has not been investigated. Besides, few researches about the substitution for Zn2+ or Fe3+ at the same time have been reported, that means the materials with formula of Zn1xAxFe2-yByO4 are lack of enough research. Thus, future research about the substitution of zinc ferrite can not only base on the newer ions but also multi ions substitution. As for the composite of zinc ferrite, large volume of works on the ZnFe2O4 composites are based on the combination of zinc ferrite with the inorganic materials including metal oxide, graphene and the superposition of the above materials, in other words, the combination of zinc ferrite with organic materials have not been investigated adequately. As is known, the organic-inorganic hybrid materials possess excellent properties from both of inorganic component and organic component, such as high strength, high temperature resistance, toughness and good processability. Thus, it’s necessary to study more about the zinc ferrite based organic-inorganic hybrid materials. References [1] J.P. Singh, R.S. Payal, R.C. Srivastava, H.M. Agrawal, P. Chand, Effect of thermal treatment on the magnetic properties of nanostructured zinc ferrite, J. Phys.: Conf.

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