Synthesis of various magnetic nanostructures and the microwave characterizations 6

Moreover, the effect of enhanced saturation magnetization of as-synthesized zinc ferrite particles on the microwave absorption performance was investigated.. the practical synthesis, mix

Chapter 6 Synthesis of Zn-ferrite nanoparticles with high saturation magnetization and their microwave absorption property 6.1 Introduction

An optional method to extend the Snoek’s limitation is to enhance the saturation magnetization, as introduced in Section 1.1.3 In this chapter, Zn-ferrite nanoparticles were synthesized by thermal decomposition method by doping Zn element into Fe3O4 The as-synthesized Zn-ferrite particles showed much higher saturation magnetization than Fe3O4 The origin of the unusual high saturation magnetization was also investigated in this chapter Moreover, the effect of enhanced saturation magnetization

of as-synthesized zinc ferrite particles on the microwave absorption performance was investigated

Over the past decades, magnetic spinel ferrites [M(II) Fe(III)2O4; M represents Co,

Mn, Ni, Zn or Fe, etc.] have attracted considerable research interest for their wide range of technological applications in magnetic recording, microwave technology, catalytic and biomedical fields.[1-5] Spinel has a face-centred cubic structure with the oxide anions arranged in a cubic closed-packed lattice The metal cations fill either the tetrahedral or octahedral interstices, resulting in normal structure when M occupies the tetrahedral (A) sites and inverse structure when M occupies the

octahedral (B) sites Also, intermediate cases exist where the cations distribute at both sites with a description of (M1-xFex)[MxFe2-x]O4, where the round and square brackets refer to A sites and B sites respectively x stands for the inversion degree, defined as the fraction of A sites occupied by Fe3+ cations Fig 6.1 shows the inverse spinel structure Fe3O4 with half of the Fe atoms at tetrahedral sites being occupying by M atoms Magnetically, the origin of net moment in a unit formula (u.f.) of the spinel structure is from the arithmetic difference of the magnetic moments at A site (upwards) and at B-sites (downwards).[6] Hence the cation distribution exerts a decisive

influence on the magnetic properties of spinel ferrites,[7-9] such as blocking temperature, magnetization as well as AC magnetic susceptibility The control of cation distribution provides a means to tailor their properties Theoretically, the preferable distribution of various metal cations in spinel ferrites could be predicted with taking the crystal field stabilization energy and ionic radius into the consideration.[10] The results based on a normalized ion energy method[11] showed that Fe2+, Co2+ and Ni2+ tend to locate at B sites to form inverse spinel structure, while

Zn2+ and Mn2+ tend to locate at A sites to form normal spinel structure However, in

Fig 6.1 Schematic illustration of inverse spinel structure of Fe 3 O 4 , in the case, half of Fe atoms at tetrahedral sites are replaced by M atoms

the practical synthesis, mixed distribution of M cations on both A and B sites is always detected when the particle sizes enter into nano regime[12,13] or an additional dopant is introduced.[14] Besides, the non-stoichiometric composition of spinel ferrites is found to be an influential factor that induces the inversion degree.[15]

As is known to all, bulk ZnFe2O4 is a typical representative of normal spinel structure,

in which all nonmagnetic Zn2+ cations and magnetic Fe3+ cations are located at A sites and B sites, respectively, leading to paramagnetism at room temperature,[16,17] while partial inversed Zn ferrites always possess ferrimagnetic property Some researchers[18-20]have reported that Zn ferrites show a room temperature saturation magnetization (Ms) ranging from several to 80 emu/g Among the available methods for synthesis of Zn ferrite particles,[21-26] such as high temperature calcination, ball milling, coprecipitation, combustion, sol-gel, hydrothermal and thermal decomposition routes, we could find that calcination and ball milling methods are favourable to produce high crystalline nanostructure, but agglomeration or large size distribution would limit the applications Comparatively, chemical syntheses are preferred due to the ease of better control over the shape and size Moreover, chemical methods tend to fabricate non-stoichiometric composition Zn ferrites The insufficient amount of Zn atoms is easy to induce large inversion degree as well as tremendous magnetism Thereof, hydrothermal route[27] and coprecipitation process[28] could be used to produce Zn ferritenanoparticles, of which the reversion degree is over 0.7,

resulting in large room temperature saturation magnetization (Ms) around 80 emu/g Most of the studies[29,30] focused on the ferrimagnetism of Zn ferrite particles with ultra-small sizes because the cation distribution was also found to be related with nanoregime However, the surface disorder usually gets stronger with decreasing particle size and reduces the magnetization.[31] In this work, large size Zn ferrite particles in bulk regime are preferred to shed some light on the relationship between the cation distribution and the ferrimagnetic property

Spinel ferrites are of great interest in the electromagnetic applications because they can absorb electromagnetic radiation in microwave bands.[32] Compared with the traditional Zn ferrite, as-synthesized particles exhibit promising magnetic property, which is attractive to investigate their performance of radar absorption To date, there were very few reports available regarding the radar absorption by using pure zinc ferrite particles Srivastava et al.[33] reported the permeability spectrum of zinc ferrite prepared from high temperature calcination process in 1970s Although high room temperature Ms was observed, the resonant frequency appeared at only several tens megahertz And the inhomogeneous particle sizes limited the application From the results revealed by Yan and Li et al.,[34,35] we can see that the shape and size of zinc ferrite nanoparticles are influential to the microwave absorption performance, however, the saturation magnetization of the reported zinc ferrite was lower than bulk

Fe3O4 (~90 emu/g)[36] Referring to the size- and shape- controllable synthesis of

spinel ferrites, thermal decomposition route has been successfully employed in the synthesis of Fe3O4 crystals with an excellent uniformity in particle shape and size.[37,38] This method has been successfully used to synthesize very uniform Fe3O4particles with size from 6 nm to 430 nm, as introduced in Chapter 5 The size and shape control as well as the composition control over as-synthesized Zn-ferrite by thermal decomposition method were also investigated in this chapter

6.2 Experimental results

6.2.1 Synthesis and characterizations on large size Zn-ferrite nanoparticles

6.2.1.1 Effect of the molar ratio of Zn precursor to Fe precursor on the composition and morphology

Uniform Zn ferrite nanoparticles were prepared via thermal decomposition of iron and zinc precursors in the solvent of benzyl ether For all experiments, the same amounts

of oleic acid (28 mmol) and benzyl ether (20 mL) are used In the composition control synthesis, the concentration of Fe(acac)3 was fixed at 0.6 M The molar ratio of Zn/Fe precursors was adjusted from 0 to 1.25 As listed in Table 2.2, the as-synthesized samples were named as ZF0 to ZF7 Actually, ZF0 is pure Fe3O4 without any zinc dopant Variations on the morphology of as-synthesized samples could be observed from SEM images as shown in Fig 6.2(a-f) When the amount of zinc precursor is around 4 mmol or less, octahedral shape particles are formed, such as samples ZF1

and ZF2 With increasing the zinc precursor to 6 mmol (ZF3) and 8 mmol (ZF4), well facet polyhedral crystallites are formed When 10 mmol of zinc precursor is used, nonuniform particles including huge cubes and some small ones are obtained, as shown for the sample ZF5 Based on our previous study on the formation

mechanism of octahedral magnetite particles,[34] we have learnt that oleic acid acts

as both reducing agent and stabilizer in the synthesis process and the precursor/surfactant ratio is crucial to the morphology Further increase of zinc precursor will result in an insufficiency in the surfactant, which makes it hard to stabilize all formed nucleus homogenously That is why the sample ZF6 seems irregular and rough Hence, there is a limitation set by the ratio of precursor to surfactant Only if the ratio is less than 0.85, Zn ferrite nanoparticles with defined shapes could be obtained This could be further verified by sample ZF7, which was prepared by using precursors amounting to 27 mmol We could find many

Fig 6.2 (a-f) SEM images of as-synthesized samples All the scale bars stand for

200 nm.

heterogeneous particles included in the SEM image, as shown by Fig 6.3

The formation of various shapes may rely on the ratio of precursor/surfactant, which

is the only modified parameter in the experiment Hence, the mechanism is proposed

as following based on the observed results The previous works on the shape control

of Au,[39] In2O3[40] as well as FeO[41] particles have indicated that the selective absorption of surfactant on which surface is mostly dependent on the surface energy The high-index crystallography planes usually possess higher surface energy As a result, the particles tend to be surrounded by low-index planes, such as the {111},

{110} and {100} planes in face-centered cubic structured materials In Chapter 4, the

Fig 6.3 SEM image of sample ZF7, which are synthesized with using 12 mmol of Fe precursors and 15 mmol of Zn precursors.

Fig 6.4 Schematic drawings for different shapes of Zn ferrite nanoparticles (A) Octahedron formed at low precursor/surfactant ratio (0.42-0.58); (B) polyhedron (truncated octahedron) formed at medium ratio (0.58-0.72) and (C) cube formed at high ratio (0.72-0.78) When the ratio is over 0.78, irregular particles will be obtained The precursors include Fe(acac) 3 and Zn(acac) 2

precursor/surfactant ratio is found to be decisive to the octahedron-shape of Fe3O4nanoparticles The particle shapes are closely related to these crystallographic planes that enclose the particles The octahedral shape has eight faces enclosed by {111} planes and the cubic shape has six faces enclosed by {100} planes The schematic drawings were shown in Fig 6.4 to present three different shapes, including octahedron, polyhedron and cube, which were displayed by as-synthesized Zn ferrite nanoparticles The polyhedral shape in this work is a kind of truncated octahedron The excellent stabilizing function of oleic acid is due to the existence of carboxylic group, as reported,[42] which always binds to certain crystal faces with a nonpolar tail group and hinders the growth in the direction normal to the bound faces The surface energy is a key factor for the selective adsorption of stabilizer The {111} planes possess the lowest surface energy,[43] hence the octahedron is much easier to form compared with the cube shape In our work, octahedral zinc ferrite nanoparticles are observed when the precursor/surfactant ratio is below 0.58 This is in coincidence with Yang’s[44] point that the presence of excess oleic acid will facilitate the growth

of <100> over <111> direction, resulting in the formation of octahedral nanoparticles When the ratio is above 0.72, oleic acid tends to stabilize on the surface of (100) rather than (111), leading to a faster growth in the <111> direction and forming cubic nanoparticles The polyhedral nanoparticles are formed as the ratio of precursor/surfactant is within the range of 0.6 to 0.75, resulting from a competence

between the growth of <100> and <111> direction during the formation of zinc ferrite nanoparticles Although the shape induced performance is not in the scope of this study, the proposed formation mechanism of varying shapes is expected to be helpful

to the further application of the developed method

It is worth noting that the increase of used zinc precursor will not give rise to the obtained particle size It is the amount of Fe precursor that is decisive to the particles size When the amount of iron precursor is adjusted to be 12 mmol, all the produced samples are with similar sizes above 100 nm, which are in bulk size regime Based on our observation, Zn(acac)2xH2O, as a subordinate precursor, does affect the synthesis process in two manners The one is the effect on the ratio of precursor to surfactant, resulting in a morphology variation as explained above The other is the effect on the ratio of Zn to Fe precursors, which further induces different compositions of as-synthesized zinc ferrite nanoparticles The yielded composition of as-synthesized samples were detected by EDS and confirmed through the ICP-MS, as listed in Table 2.2 There exists a discrepancy between the starting ratio of Fe to Zn precursors and the final ratio of iron to zinc concentrations in all samples Obviously, the as-synthesized zinc ferrite nanoparticles are nonstoichiometric, given as a formula of

ZndFe3-dO4, where d presents the atomic content of zinc atoms As listed in Table 2.2, the d value increases gradually with the amount of zinc precursor, reaches a saturation value of d ca 0.52, as indicated by EDS results of sample ZF5, ZF6 Even if the zinc

precursor is increased to 15 mmol (ZF7), the d value is no more than 0.53

6.2.1.2 Investigation on the mechanism for high room temperature magnetization of as-synthesized Zn-ferrite nanoparticles

The crystallographic information of the as-synthesized zinc ferrite nanoparticles with different compositions were studied by XRD (Fig 6.5a) All diffraction peaks match better with the standard Fe3O4 diffraction data (JCPDS no 88-0135) than zinc ferrite

database, this may due to the low doping concentration of Zn atoms Although the zinc dopant concentration varies between 0 and 0.527, the pure spinel cubic structure

is shown by all the samples According to Rietveld refinements of XRD patterns, the obtained lattice constants increase gradually with the zinc dopant concentrations, reach a maximum value when Zn dopant content is 0.522 for sample ZF5 The as-observed irregular shape and poor crystallinity of sample ZF6 and ZF7 may account for the abnormal decrease of lattice constant with increasing Zn dopant contents

Fig 6.5 (a) Typical XRD patterns and (b) magnetic hysteresis loops of Zn ferrite samples.

We further investigated the magnetic properties of as-synthesized zinc ferrite nanoparticles The room temperature magnetic hysteresis (M-H) loops were collected and the saturation magnetization (Ms) values were recorded The M-H loops of some typical samples are shown in Fig 6.5b Actually, sample ZF0 is pure Fe3O4 without any zinc dopant, while ZF2 presents zinc doped ferrite samples Sample ZF0, which shows a normal magnetization of bulk Fe3O4, i.e 83.5 emu/g, while samples with number from ZF2 to ZF5 exhibit extra higher Ms Sample ZF4 with the composition

of Zn0.468Fe2.532O4 shown a maximal Ms of 110 emu/g Empirical analysis leads us to assume that this high magnetization of zinc dopant ferrite nanoparticles is mainly caused by the nonstoichiometric structure, as it could lead to a redistribution of iron atoms in the tetrahedral sites and octahedral sites Hence, a question is raised here, i.e how do the zinc and iron atoms distribute in the spinel structure? To settle this question, the Mössbauer spectra were employed to acquire more details on magnetic structure of as-synthesized zinc ferrite samples

Mössbauer spectra recorded at ambient condition, as displayed in Fig 6.6a The presence of two sextets in all samples confirms that the room temperature ferrimagnetism is shown not only by pure Fe3O4 (ZF0) also by Zn doped ferrite samples As is known to all, Fe3O4 owns an inverse spinel structure,[45] in which half

of Fe3+ cations occupy tetrahedral (A) sites, while all of the Fe2+ cations and the other half of the Fe3+ cations occupy octahedral (B) sites, resulting in a formula structure of