Synthesis of various magnetic nanostructures and the microwave characterizations 7

The synthesis process reported here is so called chemical reduction method, which is used to convert -Fe2O3 to Fe3O4 with the morphology being preserved.. The developed method aims to o

Chapter 7 Synthesis and microwave absorption of Fe3O4 particles with

various structures by chemical reduction route 7.1 Introduction

Chapter 6 presented that the resonance frequency and the permeability of as-synthesized Zn-ferrite were higher than those of Fe3O4 Hence the enhanced saturation magnetization could extend the Snoek’s limitation Another optional method to extend the Snoek’s limitation is to induce the shape anisotropic field into

Fe3O4 particles The shape effect on the Snoek’s law could be described as following:

(μi− 1)fr = 1

2γ4πMs(Hha

H ea)1/2 (1.11 in Chapter 1) The value on the right side would be extended due to Hha> Hea In this case, the permeability of Fe3O4 may be high at relative high frequency range (GHz range) In this chapter, we succeeded in obtaining uniform Fe3O4 particles with different shapes and studied the effect of various shapes on the resonance frequency The synthesis process reported here is so called chemical reduction method, which is used to convert

-Fe2O3 to Fe3O4 with the morphology being preserved

For several decades, shape control over iron oxide nanocrystals is one of the most interesting topics because their physical and chemical properties can be manipulated through variations on their morphology and size Unique electron-transport behavior

was shown by Fe3O4 nanowires.[1] Very high specific capacity (~ 749 mA·h·g-1 at C/5 and ~ 600 mA·h·g-1 at C/2) was exhibited by carbon coated Fe3O4 nanospindles when used as an anode material for Li-Ion batteries.[2] Room temperature magnetoresistance as high as ~ 1.2% was observed in MgO/Fe3O4 core-shell nanowires.[3] Relative high luminescence and very strong magnetic resonance T2* effect was displayed by quantum dot capped Fe3O4 nanorings.[4] Extra high coercive fields of 76.5 ( 1.5) mT was detected in Fe3O4 tube arrays.[5] With these intriguing properties reported, the fabrication of various shapes of iron oxides, especially Fe3O4

particles, attracts more and more attentions So far, many synthesis methods have been developed for synthesis of Fe3O4 particles, such as sol-gel in reverse micelles,[6] hydrothermal,[7] co-precipitation[8] and thermal decomposition.[9,10] However, these methods tend to form Fe3O4 nanoparticles with isotropic shapes Recently,

Fe3O4 with hollow structures (rings, tubes ad capsules, etc.) and 1-dimensional structures (wires, rods and spindles, etc.) were successfully synthesized by template method[11] - dehydration or reduction of premade isotropic -FeOOH or -Fe2O3

particles.[5,12] Compared with dehydration, reduction of premade -Fe2O3 seems to

be a simple and effective method Usually, the reduction of -Fe2O3 involves an annealing process in reductive atmosphere at elevated temperatures (typically in the range of 300-500 ℃) This annealing treatment of nanostructured materials may result in undesirable aggregation and sintering In this work, we have developed a

chemical reduction method, which allows the reduction process to be carried out in organic solvent The developed method aims to obtain pure Fe3O4 phase after the reduction with preserving the morphology of premade -Fe2O3 template In this study, two kinds of reducing agents, i.e oleic acid and H2-involved gas (5%H2-95%Ar gas mixture), are used, and their effects on the reduction process are investigated The developed reduction method is suitable for massive production, which makes it possible to investigate the microwave absorption performance of as-reduced Fe3O4

particles with various structures

Since the shape of formed Fe3O4 nanoparticles is most related with that of premade

-Fe2O3 template, it is important for us to prepare -Fe2O3 with various shapes prior

to the reduction process Unlike Fe3O4, the existing methods are more effective to form high-quality -Fe2O3 particles with varied shapes -Fe2O3 nanobelts, nanowires and nanoflakes could be produced by thermal oxidation of iron in oxygen atmosphere[13,14] or by calcination of -FeOOH in air.[15] -Fe2O3 nanotubes and nanorods could be formed by template method[16,17] as well as hydrothermal method.[18] -Fe2O3 nanorings, nanodiscs and capsules could also be prepared by hydrothermal method.[19-21] Based on the references, the hydrothermal route is found to be versatile and able to synthesize -Fe2O3 nanoparticles of different shapes, such as rings, tubes, capsules, rods as well as discs Hence, on the purpose to enrich the library of the shapes of Fe3O4 nanostructures, the hydrothermal method was

employed in the current study As an extension work of our previous report,[22] ammonium phosphate (NH4H2PO4) was chose as additives to facilitate the hydrolysis

of FeCl3 to produce Fe3+ ions and control the growth of -Fe2O3 nanoparticles Besides the -Fe2O3 rings and tubes reported by the previous works, single-crystalline

-Fe2O3 rods with tunable sizes were also developed by adjusting the ratio of [Fe3+]/[H2PO4-] To the best of my knowledge, it is the first report on the synthesis of

-Fe2O3 rings, tubes and rods by one single route It is worthy to mention that the sizes of developed rods can be well controlled by adjusting the concentration of starting materials The formation mechanism on the variety of as-prepared -Fe2O3particles by hydrothermal method is also described in this chapter

7.2 Experimental results

7.2.1 Synthesis of -Fe2O3 with various shapes by hydrothermal treatment

7.2.1.1 Mechanism on the formation of -Fe2O3 nanoparticles with different morphology

In this work, large-scale -Fe2O3 nanoparticles with various shapes and sizes can be prepared via a facile hydrothermal treatment The formation of -Fe2O3 nanocrystals starts from the -Fe2O3 monomers generated by the hydrolysis of Fe3+ ions at 220 ℃

in the presence of phosphate ions The formed monomers possess very high surface energy and tend to aggregate rapidly The aggregation process is affected greatly by

the concentration of Fe3+ ions as well as the phosphate ions, resulting in -Fe2O3 of different shapes Similar mechanism has been already revealed by some previous reports regarding to the hollow nanocrystals, such as nanorings and nanotubes.[19,22,23] For the formation of nanorings, -Fe2O3 monomers aggregate to form a disk first, followed by a subsequent ‘etching’ of the -Fe2O3 nanodisks by phosphate ions at the central part, leading to ring-structure particles formed In our experimental results, when the concentrations of Fe3+ and H2PO4- ions are 5 mM and 0.72 mM, 150 nm disks would be formed if the heating period at 220 ℃ is around 10 h; while 154 nm rings would be obtained if the heating period at 220 ℃ is prolonged

to 48 h, as shown in Fig 7.1 The SEM image in Fig 7.1b shows a mixture of disks and freshly formed rings, which is a kind of intermediate product before the final

formation of nanorings The morphology evolution of formed particles from disk to ring is consistent with the previous reports.[19] However, the mechanism (why the aggregation of -Fe2O3 monomers forms disk at the middle stage) is still not well understood Although we know that molar ratio of Fe3+ to H2PO4- ions is crucial to this problem, no specific answer could be given so far Nevertheless, researchers have

Fig 7.1 SEM images of (a) -Fe 2 O 3 disks (10 h at 220 ℃); (b) mixed product of disk and rings (20 h at 220 ℃) and (c) -Fe 2 O 3 rings (48h at 220 ℃) The scale bars for all the images stand for 200 nm

tried to explain the dissolution of the hematite particles by the following reactions:

Fe2O3 + 6 H+ 2 Fe3+ + 3 H2O (7.1)

Fe3+ + xH2PO4- [Fe(H2PO4)x]3-x (7.2)

According to Eq (7.2), the formation of [Fe(H2PO4)x]3-x will consume Fe3+ ions and

lower the concentration of Fe3+ in the aqueous solution Then the decomposition of

Fe2O3 to Fe 3+ ions is forced by the lack of Fe3+ ions to reach a thermodynamic

equilibrium state, as indicated by Eq (7.1) The central part of as formed disk is rich

of H2PO4- because of the aggregation of -Fe2O3 monomers starts from there, leading

to a final ring-structure after the dissolution process

The above mechanism is also valid for the formation of -Fe2O3 tubes With using

different concentrations of FeCl3 and NH4H2PO4, as listed in Table 2.3, the

aggregation of -Fe2O3 monomers forms spindle-like crystals first, as revealed by our

previous study,[22] and followed by a dissolution along the central axis As reported,

for -Fe2O3 tubes fabricated by this hydrothermal route, the central axis is always

along the c axis of trigonal Fe2O3, corresponding to [0 0 1] crystal orientation The

preferential growth along [0 0 1] direction may be dominated by the selective

adsorption of phosphate ions on different crystal facets of Fe2O3 other than (0 0 1)

facets As investigated by Jia et al.,[23] the adsorption capacity and affinity of (0 0 1)

plane to phosphate ions are much lower than that of other planes, such as (1 1 0), (0 1

2) and (1 0 4) planes, originating from the absence of singly coordinated hydroxyl

groups on (0 0 1) plane In other words, the attachment of phosphate ions on the facet will hinder the growth in the direction normal to the facet And the density of attached phosphate ions, depending on the molar ratio of phosphate ions to iron precursor, is very important to the morphology of formed -Fe2O3 particles The function of phosphate ions in the employed hydrothermal route is similar with that of oleic acid and oleylamine used in the thermal decomposition method for synthesis of shaped metal oxides.[24,25]

7.2.1.2 Shape controllable synthesis of -Fe2O3 nanoparticles

As a key factor that influences the morphology of as-synthesized -Fe2O3nanocrystals, the molar ratio of iron precursor to phosphate ions, i.e [Fe3+]/[H2PO4-], was adjusted in this work Besides the repeated work on the synthesis of -Fe2O3 rings and tubes by following the previous works, we further developed -Fe2O3 balls and rods, as shown in Fig 7.2 The concentrations of used materials are listed in Table 2.3 From Table 2.3, we further observed that 74 nm -Fe2O3 rings and 70 nm

-Fe2O3 tubes were produced with using the same ratio of [Fe3+]/[H2PO4-] but different concentrations of iron precursors Due to their similar outer diameters, tubes with longer sizes could be seen as elongated rings This observation may reveal that (a) the morphology of -Fe2O3 nanoparticles is mainly controlled by the ratio of [Fe3+]/[H2PO4-]; while (b) the size of -Fe2O3 nanoparticles is mainly dependent on the concentration of iron precursor when the ratio of [Fe3+]/[H2PO4-] is fixed to keep

the morphology This finding is further supported by the experimental work on the synthesis of -Fe2O3 rods with different sizes

7.2.1.3 Size controllable synthesis of -Fe2O3 rods

For the synthesis of -Fe2O3 rods,the ratio of [Fe3+]/[H2PO4-] was adjusted to be 20:0.36 The concentration of iron precursor was tuned by adding different amount of distilled water, as demonstrated by Table 2.3 The results indicated that the size of as-synthesized -Fe2O3 rods could be successfully controlled by only adjusting the concentration of precursor A trend that higher concentrations of the iron precursor lead to larger sizes of -Fe2O3 rods was observed Referring to the size control of as-obtained -Fe2O3 rods, a maximum length of 120 nm was reached However, the uniformity and quality is poor as shown by the SEM images in Fig 7.3a Some fragments of -Fe2O3 rods, as pointed out by the arrowheads, could be found The

Fig 7.2 SEM images of -Fe 2 O 3 with different shapes: (a) 117 nm -Fe 2 O 3 balls; (b) 74 nm -Fe 2 O 3 rings; (c) 70 nm -Fe 2 O 3 tubes and (d) 98 nm -Fe 2 O 3 rod The scale bars for all the images stand for 200 nm

TEM images in Fig 7.3b indicate that some hollow capsules and broken ones are involved in the sample of 120 nm -Fe2O3 rods Hence, there exists a maximal size limitation for as-prepared -Fe2O3 rods by the hydrothermal method with employing FeCl3-NH4H2PO4 system When the concentration of iron precursor decreased, we can obtain shorter but high-quality rods, as shown in Fig 7.4a&b The size and size distribution of as-synthesized -Fe2O3 particles was obtained by counting 80 to 100 particles in the SEM images The statistical results were also listed in Table 2.3 The average value of outer diameter is adopted to name the sample Take 98-rod as an example, the outer diameter is 98 nm with a deviation less than 8 nm

As far as I know, it is the first time that -Fe2O3 rods with tunable sizes via hydrothermal method are reported To learn more about the structure of as-prepared

-Fe2O3 rods, HRTEM images and SAED patterns are acquired The results reveal a perfect single crystal structure for all -Fe2O3 rods with different sizes The lattice spacing in Fig 7.4c is measured at about 0.253 nm, which is close to the standard d

Fig 7.3 (a) SEM images of 120 nm -Fe 2 O 3 rods and (b) TEM images of capsules and broken ones involved in as-prepared 120 nm -Fe 2 O 3 rods

spacing of {1 1 0} at 0.252 nm for the hematite The SAED patterns and HRTEM analyses reveal that the nanorods grow along [0 0 1] (c axis), as labeled in the image The oriented growth of -Fe2O3 rods along [0 0 1] direction further illustrates the weak adsorption affinity of (0 0 1) phosphate ions onto (0 0 1) plane of trigonal hematite Different from the previously reported hollow structure, 98 nm-rods, 61 nm-rod as well as 55 nm-rods are solid This may be due to the high ratio of [Fe3+]/[H2PO4-] and the low concentration of phosphate ions in the aqueous solution, resulting in insufficient phosphate ions for the dissolution process, as displayed by Eq (7.2)

Fig 7.4 SEM images of (a) 61 nm Fe 3 O 4 rods and (b) 55 nm Fe 3 O 4 rods; (c) the HRTEM image for as-synthesized -Fe 2 O 3 rod and the corresponding SAED pattern (inset)

7.2.2 Chemical reduction of -Fe 2 O 3 to Fe 3 O 4 nanoparticles

7.2.2.1 Effect of reducing agent (oleic acid) on the reduction process

For the reduction process, trioctylamine (TOA) with a boiling point above 365℃ was chosen as the solvent, and oleic acid was used as reducing agent To investigate the effect of oleic acid on the phase transformation from -Fe2O3 to Fe3O4 in the current work, a set of experiments was applied to 74 nm -Fe2O3 rings firstly The experimental conditions were listed in Table 7.1 With using 100 mg Fe2O3 rings as starting material, the amount of oleic acid was adjusted from 0.5 g to 2 g, then to 3.5 g, corresponding to as-obtained sample B, C and D To be noted that the reduction process for these three samples was under pure Ar gas flow, indicating that no other reducing agent but only oleic acid was used After phase identification by XRD,

Table 7.1 Reduction conditions for phase transformation from -Fe 2 O 3 to Fe 3 O 4

Note: For each batch of experiments, 100 mg of 74 nm -Fe 2 O 3 rings dispersed

in 35 mL TOA was used as starting materials (so called sample A)

sample B was examined to be a mixture of hematite and magnetite, as shown in Fig 7.5 Most of as-reduced particles in sample B were broken into pieces The saturation magnetization of sample B (only 50.9 emu/g) was relatively low due to the existence

of hematite phase, as recorded by the magnetic hysteresis loops in Fig 7.6 With enhancing the amount of oleic acid to 2 g, an improvement could be observed based

on the XRD and SEM results No hematite phase could be found for sample C and fewer broken particles were shown compared with sample B, and the magnetization was enhanced as well These results allow us to speculate that oleic acid acts not only

as reducing agent but also as capping agent The function of oleic acid as reducing agent is trying to break down the particles to finish the reduction from Fe3+ to Fe2+;

Fig 7.5 SEM images of (a) 74 nm -Fe 2 O 3 rings, i.e sample A; and as reduced samples: (b) sample B; (c) sample C; (d) sample D The scale bars on these images stand for 200 nm (e) The photo image of two samples, the one labeled with letter

‘A’ is for sample A dispersed in TOA, the other one with ‘T’ is for transparent solution obtained after reduction process when the ratio of oleic acid to -Fe 2 O 3

rings is adjusted to be 29:1 The color of sample B, C and D seems the same, as shown by the inset photo in figure (c) (f) The XRD patterns for different samples

the other function as capping agent is responsible for protecting the particles from being broken during the redox reaction The multifunction of oleic acid has been investigated in other chemical reactions, such as the synthesis of Fe3O4 nanoparticles via thermal decomposition method.[26,27] Compared with these reference works, the refluxing temperature in the current study is much higher (350 ℃ versus 280 ℃ or

290 ℃) As revealed by Dieste et al.,[28] the higher the temperature, the less C=O

double bonds of oleic acid could be detected due to the decomposition of carboxyl

group When the temperature reaches 430 ℃ or above, no C=O bonds could be detected because of the complete decomposition of carboxyl group This also means stronger reducibility of oleic acid but weaker stability as capping agent at higher temperatures, because more CO, H2 and C will be produced as a result of the decomposition of carboxyl group.[29] Our first concern is the phase conversion from

-Fe2O3 to Fe3O4 Hence, we set the refluxing temperature of the reduction process at

a relative high value, i.e 350 ℃ At this temperature, the oleic acid still performs multifunction but with a relative strong reducibility Temperature higher than 350 ℃ will damage the magnetic stirrer severely

Fig 7.6 The M-H loops of as-reduced samples B, C and D