Synthesis of various magnetic nanostructures and the microwave characterizations 5

g., magnetite and maghemite comprise another important class of nanostructured materials, which have widespread applications as diverse as environmental remediation, magnetic recording a

Chapter 5 Size controllable synthesis of octahedral Fe3O4 nanoparticles

and the microwave absorbing properties

5.1 Introduction

For Fe-based alloys, the flake-like structure is effective to improve the microwave absorption performance The most commonly used method to synthesize Fe-based alloys particles is the high energy ball milling process, however, only flake-like structure could be obtained via this method To investigate more on the effect of particle shape of magnetic materials on the microwave absorption performance, we further choose spinel ferrites as our studying materials Compared with Fe-based alloys, spinel ferrite possesses relative high resistivity and moderate saturation magnetization When look back to the Snoek’s law as following

(μi− 1)ƒr = 2

3γ4πMs (1.10 in Chapter 1)

we could learn that the moderate saturation magnetization of spinel ferrite will lead to

a much lower Snoek’s limitation, which means that the resonance frequency of spinel ferrites is very low, usually below 1 GHz.[1] To enhance its resonance frequency for microwave applications, we should enhance its saturation magnetization or induce shape anisotropic field into the particles As a typical member in spinel family, Fe3O4 was firstly synthesized in this work for the study on microwave absorption

performance

One of the challenges for modern chemists and materials scientists is to control and manipulate the shapes of materials in the nanometer scale, as different shapes of the nanostructures can introduce novel electronic, optical, or magnetic properties, compared with their spherical counterparts.[2-4] Substantial progress has been made

on the shape-controlled synthesis of semiconductor nanocrystals Several typical nonspherical examples such as triangles, rods, cubes, arrows and tetrapods[5-8] have been reported Magnetic iron oxide nanoparticles (e g., magnetite and maghemite) comprise another important class of nanostructured materials, which have widespread applications as diverse as environmental remediation, magnetic recording and bimolecular tagging, imaging, and sensing.[9-12] Compared to semiconductor and metallic nanocrystals, magnetic nanoparticles with nonspherical shapes demonstrate more appealing anisotropic magnetic properties.[13] Apart from the most common spherical shape, magnetic iron oxide nanoparticles with cubic, tetra-pod, tubular, triangular, ring/tube-like, octahedral and pyramidal shapes have been fabricated.[14-17]

Despite the efforts described above to enrich the library of the shapes of iron oxide nanostructures, the size range of the products obtained through any single route is usually limited To the best of our knowledge, there were few reports on synthesizing nonspherical iron oxide particles with sizes ranging from sub-10-nm up to several

hundred nanometers through a single route It is known that the superparamagnetic limit for magnetite is ∼20 nm.[18,19] Most organic-phase synthesis methods reported previously have mainly focused on the size control of 4 to 20 nm from the thermal decomposition processes of iron acetylacetonate or iron oleate.[20]

In this chapter, we are going to investigate a facile synthesis of single crystalline octahedron-shaped magnetite (Fe3O4) nanoparticles bound with {111} planes through

a thermal decomposition route The particle size can be readily tuned from 6nm to ∼

430 nm with narrow size distributions (σ <10%) without any seed-mediated growth or size sorting processes The investigation on the thermal decomposition method is important in our work Firstly, this method allows us to prepare Fe3O4 particles with different saturation magnetization values by controlling the particle size Secondly, it has been a versatile method to obtain other spinel ferrite particles In later chapter, Zn ferrite particles with similar spinel structure as Fe3O4 will also be introduced Moreover, this method is suitable for large scale synthesis; hence we could investigate the as-synthesized Fe3O4 and Zn-ferrite particles as microwave absorbing materials in

my work

5.2 Experimental results

The reaction kinetics was studied by monitoring the change in the magnetic moment

of the reaction solution during the synthesis process To trace the progress of the reaction, aliquots of the solution were sampled using a syringe As shown in Fig 5.1,

the magnetic moment remained relatively unchanged for 5 min at 280 ℃ before a small increase was observed Another sharp increase in the magnetic moment was detected when the reaction was allowed for 35 min A similar two-step increase in magnetic moment was also observed when iron oxide nanoparticles were formed during the heating-up process of the iron-oleate complex.[21] The two increases in magnetic moment correspond to two processes, the formation of intermediate monomers and the growth of nanoparticles The morphology for some typical samples was shown in Fig 5.2 to further evidence the two processes When the reaction time was 0 min at 280 ℃, no particles formed Only some dark spots were observed in the SEM image While some uncrystallized and agglomerated powder (so called the monomer) was observed in the sample at 5 min (280 ℃), which may account for the first step increase of magnetic moment of reaction solution With the reaction

Fig 5.1 The change of the magnetic moment of the reaction solution versus reaction time at 280 ℃ Two steps of increases in magnetic moment at 5 min and

35 min are indicated

underwent for 35 min at 280 ℃, well crystallized and self-assembled particles were formed, which led to the second step increase of magnetic moment With the reaction

time increasing to 60 min, the particles size got a bit larger, resulting in a further enhancement of magnetic moment of reaction solution, which was consistent with the observation shown by Fig 5.1a The detailed growth mechanism is not completely understood at this stage However, the above experimental observations strongly suggest the importance of the heating profile in the fabrication of shape-controlled

Fe3O4 nanoparticles

nanoparticles

We have also investigated the formation mechanism of the octahedral Fe3O4

nanoparticles through the study on different experiment parameters, such as the molar ratio of precursors to surfactant and the concentration of surfactant

Fig 5.2 The morphology observed in SEM images of reaction solution sampled at

280 ℃ with different reaction time: 0 min, 5 min, 35 min and 60 min The scale bars in all the images stand for 500 nm

5.2.2.1 Effect from the molar ratio of precursors to surfactant

As listed in Table 2.1, the concentration of oleic acid was fixed at 0.8 M to shed some light on the effect of iron precursor on the formation of Fe3O4 nanoparticles The particles size turned out to increase from 6 nm to ~ 430 nm when the amount of iron

precursor was adjusted from 8 mmol to 24 mmol When the amount of iron precursor exceeded 40 mmol, nonmagnetic red-colored particles were obtained, although a small portion of black powder could be separated magnetically XRD analysis indicates that the red colored particles are in hematite phase (JCPDS no 33-0664), and SEM images reveal that the particles have undefined faceted structures, as shown

in Fig 5.3 The magnetically separated particles are cube-shaped magnetite particles with an average size of ∼ 200 nm (Fig 5.4) When the amount of iron precursor was reduced to less than 4 mmol, no particles but gel-like products with dark brown color were obtained These findings indicate that to obtain the well-defined octahedron-shaped nanoparticles with a pure magnetite phase, the molar ratio between

Fig 5.3 XRD pattern (A) and typical SEM image (B) of the hematite nanoparticles synthesized by using 40 mmol of Fe(acac) 3 The inset picture shows the red color of the hematite nanoparticle dispersion

precursor and surfactant must be within a specific range, that is, 0.6 - 0.2 When the ratio is larger than 0.6, the amount of Fe2+ reduced from Fe3+ is not

enough to form the stoichiometric magnetite phase It has been suggested and partially proven experimentally that the trace amount of CO, H2, and carbon produced

by the thermal decomposition of the iron-oleate complex is responsible for the reduction of Fe3+ to Fe2+.[21] We therefore speculate that oleic acid serves as both the reducing and capping agent in this synthesis,[22] and the precursor/surfactant ratio must be less than 0.6 for the purpose of enough reduction of Fe3+ to Fe2+

5.2.2.2 Effect from the concentration of surfactant

In the synthesis the concentration of surfactant in the reaction solution must be larger than 0.35 M Irregular-shaped particles with decreasing sizes were obtained when the surfactant was continuously reduced from 0.35 to 0.07 M while keeping other parameters unchanged, as displayed by Fig 5.5 A similar trend has been found for the synthesis of cubic MnFe2O4, which has a similar structure as magnetite.[23] Hou et al have also found that oleic acid facilitates the growth of <100> over the <111> direction, which leads to the formation of FeO truncated octahedron nanoparticles.[24] Here in our synthesis, the presence of excess oleic acid facilitates the final formation

Fig 5.4 TEM image of Fe 3 O 4 nanocubes magnetically separated from the product synthesized by using 40 mmol of Fe(acac) 3

of faceted octahedron-shaped structures

various sizes

Fig 5.6A presents a typical transmission electron microscopy (TEM) image of the as-synthesized magnetite nanooctahedra with an average size of 53 nm and standard deviation of 5.8% The length between two opposite vertices was taken as the particle size, as shown in the schematic representation of an octahedron-shaped particle (Fig 5.6B) Particles with parallelogram projection shapes were taken for the particle size measurement to ensure reliable statistics, as shown in Fig 5.7 Fig 5.6C - E presents particles with different projection shapes dried on a copper grid, namely, hexagon, rectangle, and parallelogram, respectively Schematic drawings of the corresponding projections of a perfect octahedron are also included, which closely resemble the TEM observations The corresponding high-resolution TEM (HRTEM) images are

50 nm 5 nm

Fig 5.5 TEM images of the as-obtained Fe 3 O 4 nanoparticles by reducing the concentration of oleic acid: A) 0.35, B) 0.19, C) 0.10 and D-E) 0.07 M

shown in Fig 5.6F - H The lattice spacing in Fig 5.6F and G is measured at 0.295 and 0.298 nm, which are close to the standard d spacing of {220} at 0.297 nm for the cubic spinel-structured magnetite The lattice spacing in Fig 5.6G is measured at 0.487 nm, which is close to the d spacing of {111} at 0.485 nm On the basis of the

Fig 5.6 (A) Typical TEM image of 53 nm Fe 3 O 4 nanooctahedra (B) Schematic 3D model of one octahedron-shaped nanoparticle TEM and HRTEM images of 53 nm

Fe 3 O 4 nanooctahedra with different projection shapes: (C, F) hexagonal (zone axis:

<111>), (D, G) rectangle (zone axis: <112>, and (E, H) parallelogram (zone axis:

<110>) Scale bars: (A) 100 nm; (C - E) 20 nm; and (F - H) 2 nm.

Fig 5.7 Illustration of particle size measurement As the orientation of the octahedron-shaped particles can easily lead to the wrong estimation of particle size, the length between two opposite vertices for parallelogram-shaped projection was taken as the particle size, as indicated by the red lines

morphology characterizations described above, it is concluded that the as-obtained nanoparticles are single crystalline magnetite nanooctahedra, with their eight faces

enclosed by {111} planes, as shown in Fig 5.6B

The average particle size of the magnetite octahedra can be readily tuned from several hundred to sub-10 nm simply by adjusting the amount of iron precursor while keeping other parameters unchanged, as demonstrated by the images in Fig 5.8 For example,∼430 nm particles were obtained when 24 mmol of Fe(acac)3 was used, as shown in a typical scanning electron microscopy (SEM) image (Fig 5.8A) The inset image presents a single perfect octahedron-shaped particle with one of its {111} planes facing upward As the amount of precursor was gradually reduced from 20 to 8

Fig 5.8 SEM (A, B) and TEM (C - E) images of the as-synthesized Fe 3 O 4

nanooctahedra with different average sizes by adjusting the concentration of precursor [Fe(acac) 3 ]: (A) ∼ 430 nm (inset: SEM image of one single

octahedron-shaped particle); (B) 114 nm; (C) 21 nm; (D) 18 nm; and (E) 8 nm (F) TEM image of 6 nm-sized spherical nanoparticles Scale bars: (A) 2 μm (inset: 100 nm); (B) 1 μm; and (C - F) 50 nm

mmol, magnetite octahedra with average particle sizes of 114, 21, 18, 8, and 6 nm were obtained It should be noted here that the octahedron geometry of the nanoparticles is gradually lost as the particle size decreases to less than 10 nm, and the

6 nm-sized nanoparticles are almost perfectly spherical in shape The as-synthesized particles have remarkably narrow size distributions, as indicated in the size histograms in Fig 5.9

The crystallographic information of the as-synthesized nanooctahedra with different sizes was studied using X-ray diffraction (XRD), as summarized in Fig 5.10 The positions and relative intensities of all diffraction peaks match well with the standard magnetite diffraction data (JCPDS no 19-0629) The crystalline sizes calculated from

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Particle Size (nm)

0.0 0.1 0.2 0.3 0.4

35 40 45 50 55 60 65 70 75

Particle Size (nm)

0.0

0.1

0.2

0.3

0.4

12 14 16 18 20 22 24 26 28

Particle Size (nm)

0.0 0.1 0.2 0.3 0.4 0.5

Particle Size (nm)

0.0 0.1 0.2 0.3 0.4

Particle Size (nm)

0.0 0.1 0.2 0.3 0.4 0.5

Particle Size (nm)

Fig 5.9 Size Distribution histograms for different sized nanooctahedra: (A) 114.3±12.2 nm; (B) 53.3±3.0 nm; (C) 21.3 ±1.1 nm; (D) 17.5±0.9 nm; (E) 7.5±0.6

nm and (F) 5.7±0.5 nm